Nitrogen Removal from Oil: A Review - Energy & Fuels (ACS

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Nitrogen Removal from Oil: A Review Glaucia H. C. Prado,* Yuan Rao, and Arno de Klerk Department of Chemical and Materials Engineering, University of Alberta, 9211 116th Street, Edmonton, Alberta T6G 1H9, Canada ABSTRACT: The selective removal of nitrogen-containing compounds from oil and oil fractions is of interest because of the potential deleterious impact of such compounds on products and processes. Problems caused by nitrogen-containing compounds include gum formation, acid catalyst inhibition and deactivation, acid−base pair-related corrosion, and metal complexation. A brief overview of the classes of nitrogen compounds found in oil is provided. The review of processes to remove nitrogen from oil emphasizes studies that investigated denitrogenation of industrial feedstocks, such as refinery fractions, heavy oils, and bitumens. The main topics covered are hydrotreating, liquid−liquid phase partitioning, solvent deasphalting, adsorption, chemical conversion followed by separation, and microbial conversion. Chemical conversion processes include oxidative denitrogenation, N-alkylation, complexation with metal salts, and conversion in high-temperature water. There are many processes for denitrogenation by separation of the nitrogen-rich products from oil without removing the nitrogen group from the nitrogencontaining compounds. As a consequence, most of these processes are viable mainly for removal of nitrogen from low-nitrogencontent oils, typically with 350 °C) and fluid catalytic cracking catalysts (>480 °C), are only inhibited by nitrogen bases. At high temperatures, the equilibrium constant of the acid−base reaction is smaller, and a larger proportion of the acid sites are available to perform acid catalysis. The reaction rate in the presence of nitrogen bases is nevertheless lower than in the absence of nitrogen bases. For example, about 100 °C higher temperature was required to achieve the same conversion over a hydrocracking catalyst operated at 6 MPa H2 pressure with 6 kPa NH3 partial pressure compared to 0 kPa NH3.4 (c) Basic nitrogen-containing compounds can also react with acids present in the oil, such as carboxylic acids, acids formed during processing, or acids added to the process. The products from the acid−base reactions are heavier and may exhibit different phase behavior. Precipitation of organic salts formed by the acid−base reactions can cause fouling of process equipment. At higher temperatures, the acids can be liberated from the acid−base pairs, and in the presence of water, this may lead to localized corrosion. For example, severe localized corrosion of the atmospheric distillation unit (ADU) was reported following the processing of oil that contained amine chloride salts, which released hydrochloric acid during distillation.5 Received: October 24, 2016 Revised: December 6, 2016

A

DOI: 10.1021/acs.energyfuels.6b02779 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (d) The metals most commonly reported in crude oil are Ni and V; some oils also contain meaningful (>10 μg·g−1) amounts of other metals, such as Fe.6 Although metals in oil are not exclusively found in metalloporphyrin structures, nitrogen plays an important role in metal coordination. Fouling of process equipment and catalysts occurs as a result of deposition of metals during processing, and in this respect nitrogen is indirectly responsible for the processing problems attributed to metals in oil. (e) In catalytic processes, nitrogen-containing compounds competitively adsorb with other compounds. Such competitive adsorption limits the access of other compounds to the catalyst surface, thereby reducing the rate of conversion of such compounds. For example, nitrogen-containing compounds compete with sulfurcontaining compounds for active sites during hydrotreating, causing hydrodesulfurization to be less efficient when nitrogen-containing compounds are present in the feed.7,8 In view of the inefficiencies and challenges caused by nitrogen-containing compounds, it is desirable to remove nitrogen-containing compounds from oil, and it is preferable to do so before refining. While several studies can be found in the literature dealing with sulfur removal, much less has been reported on nitrogen removal from oil. These served as motivations for the present review of nitrogen removal. Studies involving model oils are included in the review, but because of our specific interest in real industrial feedstocks, emphasis is placed on denitrogenation of refinery fractions as well as straight-run heavy oils and bitumens.

Table 1. Nitrogen Contents of Selected High-NitrogenContent Oils oil bitumen bitumen bitumen bitumen heavy crude oil heavy oil (coalderived) heavy oil medium oil heavy oil heavy oil heavy oil bitumen bitumen bitumen

nitrogen content (wt %)

ref

Canada, AB, Cold Lake Canada, AB, Athabasca Canada, AB, Peace River Canada, AB, Athabasca Republic of Tatarstan, Ashal’cha China, Shengli

0.34 0.40−0.56 0.51 0.66 0.63

10 10 10 11 12

0.50

13

United States, OK, Shovel-tum United States, CA, Ventura United States, CA, San Ardo United States, CA, Gato Ridge United States, CA, Oxnard United States, CA, Edna United States, UT, Sunnyside United States, UT, Vernal

0.37

14

0.41

14

0.91

14

0.67

15

0.88 1.23 0.96

15 15 15

1.18

15

location

The most prominent nitrogen-containing classes in crude oil are pyridine derivatives and pyrrole derivatives, including porphyrins, which have a tetrapyrrole macrocycle (Figure 1).

2. CLASSES OF NITROGEN-CONTAINING COMPOUNDS IN OIL The nitrogen content of crude oils is usually less than 1 wt % N. In about 90% of crude oils the nitrogen content is N2 > N2O1 ≈ N1O3 > N1O4.20 B


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type of technology depends to a large extent on the boiling range and properties of the feed material. To facilitate discussion, a process flow diagram of a generic packed-bed hydrotreating process is shown in Figure 3. The

In a study of Arabian crude oils, nitrogen-containing compounds were identified using a combination of gas chromatography coupled with mass spectrometry (GC−MS), high-performance liquid chromatography (HPLC), infrared spectroscopy, and UV−vis spectroscopy.21 Some of the lowermolecular-mass compounds with multiple heteroatoms that were identified (Figure 2) were amides and hydroxyquinolines

Figure 3. Process flow diagram of a generic packed bed hydrotreating process.

Figure 2. Examples of nitrogen-containing compounds with more than one heteroatom that were identified in crude oil samples.21

liquid feed and part of the hydrogen is combined and preheated to the reactor inlet temperature. Hydrogen is always present in excess. The packed reactor contains a hydrotreating catalyst, and hydrotreating is accompanied by a temperature increase. Hydrogen quench, which is the injection of cold hydrogen, is employed for heat management and to replenish hydrogen that was consumed by hydrotreating. The reactor product is cooled and phase-separated to recover the hydrotreated liquid. The gas contains mainly hydrogen but also gaseous products produced during hydrotreating, such as ammonia due to HDN and hydrogen sulfide due to HDS. The gas is cleaned to remove the other gases from the hydrogen before it is recycled. Some of the non-hydrogen compounds remain in the recycle gas, and to avoid buildup of compounds that are not efficiently removed during gas cleaning, some of the recycle is purged. In many industrial hydrotreating processes, multiple catalyst beds and even multiple reactors in series might be employed. The catalyst loading diagrams can be complex, and usually more than one type of hydrotreating catalyst is loaded. The start-of-run inlet temperature (250−350 °C), pressure (1.5−15 MPa), space velocity (0.7−12 h−1), and hydrogen-toliquid feed ratio (250−6000 normal m3·m−3) all depend on the feed material and extent of hydrotreating required.26 These are just typical ranges. For deep HDN of high-nitrogen-content oils, such as coal-derived oils, more severe conditions might be required. For example, hydrotreating of creosote distillate to produce a distillate blending material for diesel fuel is industrially performed at 280−380 °C and 18.5 MPa.27 Most aspects of the process shown in Figure 3 are common to hydrotreating processes in general. However, when the feed material has higher nitrogen content and HDN is the primary objective, the following aspects of the hydrotreating process will likely be affected: (a) More severe operating conditions will be required. (b) Because of the reaction requirements of HDN (discussed in section 3.2), it is likely that HDA will increase concomitantly with HDN. Both HDN and HDA are very exothermic reactions. The adiabatic temperature increase during hydrotreating would therefore be higher. The heat

(N1O1), pyrrolecarboxylic acids (N1O2), imidazoles (N2), and caffeines (N4O2). The N2 species may also include compounds with more than one pyridine ring according to a different study.22 Despite the diversity of compound classes, the literature is in agreement that species with a single nitrogen atom (N1) are the most abundant in crude oil and that the dominant classes of nitrogen-containing compounds are pyrroles and pyridines.

3. HYDROTREATING Industrially, catalytic hydrodenitrogenation (HDN)23,24 is the most commonly applied method for removal of nitrogen from oil. Hydrotreating is the “base case” technology against which all other methods of nitrogen removal are compared. Any alternative nitrogen removal technology must demonstrate a clear advantage over HDN before it will be considered for industrial use. Hydrotreating is not a selective nitrogen removal process. Hydrodemetalation (HDM), hydrodesulfurization (HDS), hydrodeoxygenation (HDO), hydrodearomatization (HDA), and olefin saturation all take place in parallel with HDN. Of the most abundant heteroatoms in oils (S, N, and O), nitrogen is actually the most difficult to remove by hydroprocessing.25 Pyridinic nitrogen removal is particularly challenging because it requires 1 double bond and one single bond to be broken for denitrogenation to occur; in comparison only two bonds need to be broken for desulfurization of thiophenic sulfur and deoxygenation of furanic oxygen. 3.1. Hydrotreating Process Technology. There are approximately 30 different hydrotreating processes than can be licensed.16 An overview of some of these process technologies can be found in the literature.26 Hydrotreating processes operate on the same principles even though there are differences in catalysts, reactor technology, heat management, product workup, and gas loop design. The selection of a specific C


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Figure 4. Heterocyclic ring-opening pathways during HDN in the presence of an acid−base pair as illustrated by the conversion of indole.

Figure 5. Substitution reaction leading to denitrogenation during HDN in the presence of H2S, as illustrated with the product shown in Figure 4, i.e., 2-ethylaniline.

to be sp3-hybridized, which in turn requires ring saturation by hydrogenation. Heterocyclic ring opening by either nucleophilic substitution or Hofmann degradation is facilitated by the presence of an acid−base pair, H+B− (Figure 4). An increase in the HDN rate in the presence of H2S, which supplies H+ and HS− as acid− base pair, has been noted in many studies.29 Irrespective of the pathway, it leads to ring opening, yielding the same products that contain amine and olefinic groups. The steps involved in denitrogenation of alkylamines over sulfided base-metal catalysts were explored by Prins et al.,30 who found that denitrogenation occurred through substitution of the alkylamine by H2S to form the alkanethiol and NH3. When an amine group is bonded to an sp3-hybridized carbon and when H2S is present as an acid−base pair, the acid−base pair that is eliminated is not H2S, but NH3. The elimination leaves the thiol group (B = SH) attached to the molecule from which the nitrogen was removed as ammonia. This is illustrated in Figure 5. A detailed description of HDN chemistry focusing on homogeneous catalysis to explore single-site HDN catalysis can be found in the recent work by Bachrach et al.31 3.3. Hydrodenitrogenation Catalysts. The catalysts commercially used for hydrotreating processes are tungsten and molybdenum sulfides (active components) supported on alumina, and their properties are modified by adding cobalt or nickel (hydrogenation promoters). Even though both catalysts can remove both nitrogen and sulfur, Co/Mo is more selective for sulfur removal while Ni/Mo is more selective for nitrogen removal. This is the case because Ni/Mo catalysts have higher hydrogenation activity than Co/Mo catalysts. Although Ni/W

management of the process design must make provision for this. (c) The H2 consumption will increase as the nitrogen content of the feed increases. Although this seems obvious, the increase is not directly proportional to the nitrogen content because the extents of all of the hydrotreating reactions are affected by the more severe conditions required for HDN as well as the mechanism of hydrodenitrogenation (discussed in section 3.2). (d) The gas cleaning step in the gas loop for H2 recycle will have to deal with an increased amount of NH3. 3.2. Hydrodenitrogenation Chemistry. The chemistries of nitrogen and sulfur removal during the hydrotreating process differ from each other, even though similarities can be found. In both sulfur and nitrogen removal, alkyl substituents in heterocyclic compounds provide steric hindrance, which decreases heteroatom removal from ring structures. Nitrogen is more difficult to remove in hydrotreating than sulfur. This is the case because ring hydrogenation is not required for sulfur removal while ring saturation is an obligatory step for nitrogen removal.28 Many studies with nitrogen model compounds have been conducted to understand the reaction mechanism for hydrodenitrogenation. In heterocyclic nitrogen-containing compounds, the C−N bonds cannot be cleaved directly through hydrogenolysis. Denitrogenation over hydrotreating catalysts proceeds through heterocyclic ring opening as an intermediate step, either by nucleophilic substitution or Hofmann-type degradation.29 In both instances, scission of the C−N bond to form the ring-opened intermediate requires the adjacent carbon D


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showed that nitrogen conversion was increased by increasing the temperature and decreasing the liquid hourly space velocity (LHSV) for both boron- and phosphorus-containing catalysts.41−43 In comparison with alumina, other metal oxide supports usually result in higher HDN activities but are more difficult to prepare with high surface areas. The main research activities consist in developing new preparation procedures with the objective of increasing the surface area and structural stability. In a recent study, ZrO2 was prepared with and without the chelating ligand ethylenediaminetetraacetic acid (EDTA), and its hydrodenitrogenation activity was evaluated using heavy gas oil from Athabasca bitumen. It was found that the nitrogen conversion was higher for Ni/Mo/Meso-Zr with EDTA than for Ni/Mo/Meso-Zr without EDTA. In addition, Ni/Mo/ Meso-Zr with EDTA presented equal nitrogen conversion as Ni/Mo/Al2O3 at 395 °C. It was explained that EDTA provides an increase in active metal dispersion, a decrease in metal− support interactions, and an increase in nickel sulfidation temperature.46 Ni/W supported on citric-treated Y zeolite mixed with a titania−silica composite catalyst was used to study the removal of basic and nonbasic nitrogen compounds from Liaohe Chinese coker gas oil.47 As for conventional hydrotreating catalysts, total nitrogen removal was improved by increasing the pressure and temperature and decreasing the LSHV. Unlike heavy gas oil treated with Ni/Mo or Co/Mo catalyst, higher removal of basic nitrogen was achieved compared with nonbasic nitrogen.45 Zeolites have been used as catalyst supports because they contain strongly acidic protons in their crystalline lattices, resulting in high carbonium ion activity. Hydrotreatment of heavy gas oil from Athabasca bitumen using Ni/W, Ni/Mo, and Co/Mo catalysts supported on zeolitic silica−alumina and its hydrodenitrogenation was compared with that of the catalysts supported on commercial alumina. It was observed that the catalysts supported on zeolitic silica−alumina promoted 2-fold higher nitrogen removal than the commercial supports.48 Metal chloride catalysts were studied as an alternative for hydrodenitrogenation of heavy California gas oil. It was found that NiCl2 supported on silica−alumina presented higher denitrogenation activity than commercial Ni/W supported on silica−alumina and Co/Mo supported on alumina. It was explained that addition of 5% methylene chloride to the system results in HCl formation, which allows the catalyst to remain in its chloride form rather than its sulfide form. HCl can then react with NiCl2 to form H+(NiCl3)−, which is a Friedel−Crafts catalyst that possess hydrogenation and cracking activities. If water is present in the system, NiCl2 can react with water to form H+(NiCl2OH)−, which is another Friedel−Crafts catalyst. Because an insoluble solid was obtained for the reactions at low temperatures, it is suggested that nitrogen compounds are removed in the form of NH4Cl through the formation of a carbonium ion by the interaction between the HCl and the catalyst.49 Generally the presence of CO and CO2 in the gas feed is negative, resulting in lower HDN conversion. Some examples are given to illustrate the point. Hydrotreatment of gas oil in the presence of CO2 using a Co/Mo/Al2O3 catalyst resulted in 64 wt % nitrogen removal, compared with 86 wt % when CO2 was not co-fed into the reactor.50 The presence of CO during hydrocracking of bitumen using Co/Mo/Al2O3, Mo/Al2O3, and FeSO4/SiO2 catalysts also decreased nitrogen removal from

has a higher denitrogenation rate than Ni/Mo, Ni/W is more expensive than Ni/Mo, so Ni/Mo is more often used in oil refineries for nitrogen removal.16 Review papers on this topic are available.23,24 The nitrogen removal efficiency and kinetic parameters depend on the feed, and many studies can be found in the literature.32−36 Nevertheless, an investigation with eight different kinds of heavy oils with different properties has revealed that nitrogen removal using commercial catalysts is inversely proportional to compounds containing three or more aromatic rings. It is explained that these compounds inhibit hydrodenitrogenation by competing with nitrogen compounds for catalytic sites as well as hydrogen (since hydrogenation is a required step for hydrodenitrogenation).37 Since the nitrogen content of heavy oils is high and its removal is achieved with higher pressure and higher temperature, hydrotreating becomes an expensive process for heavy crude oil, and more active catalysts have been studied with the purpose of achieving high nitrogen removal with less energy and hydrogen consumption. The majority of the research consists of the use modifiers of supports or new supports to replace alumina. The main supports studied are silica−alumina, zeolites, mesoporous silica, metal oxides, and mixed oxides,38 while the main alumina support modifiers studied are fluorine, boron, and phosphorus, 39,40 even though boron and phosphorus were also used to modify Ni/Mo/Al2O3 itself.41−44 The direction of these developments makes sense considering the heterocyclic ring-opening chemistry (Figure 4), which involves an acid−base pair. Hydrodenitrogenation of heavy gas oil and coker kerosene distillates, both derived from Athabasca bitumen, has shown that the use of hydrogenation promoters such as Ni and Co in MoO3 supported on alumina increased nitrogen removal from both feeds. For example, 36% of the nitrogen was removed from coker kerosene when the unpromoted catalyst was used, while 50% of the nitrogen was removed over the promoted catalyst. Furthermore, nonbasic nitrogen compounds had higher removal rates than basic nitrogen compounds, and nitrogen removal was increased by increasing the ratio of hydrogenation promoter to Mo up to a maximum of 0.6.45 Many explanations for the improvement in nitrogen removal by addition of hydrogenation promoters have been proposed in the literature, such as the monolayer theory, pseudointercalation theory, and remote control theory.38 However, it is clear from the heterocyclic ring-opening chemistry (Figure 4) that hydrogenation is a prerequisite for ring opening, and increasing the hydrogenation power of a catalyst would facilitate this. Fluorine, boron, and phosphorus have been added to the alumina support or the catalyst itself because they increase the extremely strong acid sites of the catalyst. By substituting some Al by B3+, both Brønsted and Lewis acidic sites can be formed in the presence of water, which increases nitrogen removal.39 On the other hand, the higher nitrogen conversion obtained by phosphorus addition is attributed to geometrical changes caused by phosphorus. Phosphorus causes an increase in the number of stacks of the active site (MoS2) that are more accessible to bulky molecules, consequently increasing nitrogen removal. Addition of 1.7 wt % boron and 2.7 wt % phosphorus to Ni/Mo/Al2O3 catalyst increased the nitrogen conversion during hydrotreating of heavy gas oil from Athabasca bitumen from 61.9 to 78.0 wt % and from 61.9 to 78.4 wt %, respectively.44 Optimized process conditions as well as kinetic parameters were determined in a number of studies, which E


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Energy & Fuels Table 2. Denitrogenation of Real Industrial Feedstocks by Hydrotreating feed

process conditions

catalyst

gas oil from bitumen

NiMo/Al2O3 + 2.7 wt % P

gas oil from bitumen

NiMo/Al2O3 + 1.7 wt % B

heavy gas oil from Athabasca bitumen

NiMo/Al2O3 + 1.7 wt % B


NiMo/Al2O3 + 2.7 wt % P

coker kerosene from Athabasca bitumen

Mo/Al2O3

coker kerosene from Athabasca bitumen

NiMo/Al2O3 and CoMo/Al2O3


NiMo/Meso-Zr


NiMo/Meso-Zr (EDTA)


NiMo/Al2O3


NiMo/SBA-15

Liaohe Chinese coker gas oil

NiW/CYCTS


NiMo/zeolite−alumina−silica

heavy California gas oil

NiCl2/alumina silica


NiW/silica−alumina


CoMo/alumina

straight-run gas oil

CoMo/Al2O3


CoMo/Al2O3


CoMo/Al2O3


CoMo/Al2O3


CoMo/Al2O3

9.4 MPa LSHV: 0.5 h−1 400 °C 9.4 MPa LSHV: 0.5 h−1 400 °C 8.79 MPa LSHV: 1 h−1 385 °C 8.79 MPa LSHV: 1 h−1 385 °C 13.9 MPa LSHV: 2 h−1 320 °C 13.9 MPa LSHV: 2 h−1 320 °C 8.8 MPa LSHV: 1 h−1 395 °C 8.8 MPa LSHV: 1 h−1 395 °C 8.8 MPa LSHV: 1 h−1 395 °C 8.8 MPa LSHV: 1 h−1 395 °C 6.0 MPa LSHV: 1 h−1 420 °C 6.89 MPa LSHV: 1 h−1 422 °C 6.89 MPa LSHV: 0.5 h−1 427 °C 6.89 MPa LSHV: 0.5 h−1 427 °C 6.89 MPa LSHV: 0.5 h−1 427 °C 6.2 MPa LSHV: 2 h−1 360 °C 6.2 MPa LSHV: 2 h−1 360 °C 5.0 MPa LSHV: 1 h−1 330 °C 5.0 MPa LSHV: 1 h−1 330 °C 5.0 MPa LSHV: 1 h−1 330 °C F

nitrogen conversion/removal (%)

ref

−

97.0 wt % N converted

41

−


41

−


44

−


44

−

36% N removed

45

−

50% N removed

45

−

∼40% N converted

46

−

∼50% N converted

46

−

∼50% N converted

46

−

∼20% N converted

46

−

80% N converted

47

−

89% N removed

48

5 vol % methylene chloride

∼95% N removed

49

−

∼85% N removed

49

−

∼75% N removed

49

−

86 wt % N removed

50

906.14 mLCO2/h

64 wt % N removed

50

−

∼92% N removed

52

3.13 wt % ethyl decanoate

∼79% N removed

52

1.16 wt % propanoic acid

∼83% N removed

52

co-feed


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Energy & Fuels bitumen.51 Although these observations were explained in terms of the water gas shift reaction, the decrease in H2 partial pressure and competitive adsorption appear to be more plausible explanations. The presence of different oxygenated compounds during hydrotreating of straight-run gas oil using a CoMo/Al2O3 catalyst was also investigated.52 The impact of oxygenates on HDN depended on the oxygen-containing functional group. It was found that 2-propanol, cyclopentanone, anisole, and guaiacol did not inhibit nitrogen removal, as the nitrogen removal was similar to when no oxygenate was added to the gas oil. However, ethyl decanoate and propanoic acid inhibited nitrogen removal from gas oil. Analogous observations were reported by Leckel,53 and the results could be explained by the preferential adsorption of oxygenates on either the metal sites or the support material. Carboxylic acids, and the acids derived from esters, preferentially adsorb on metal sites, suppressing the hydrogenation activity, which would explain the decrease in HDN activity in the presence of such oxygenates. Alcohols and carbonyls preferentially adsorb on the support material, which would explain their lack of impact on HDN. Some examples of hydrodenitrogenation of real feedstocks with different catalysts and conditions can be found in Table 2.

Table 3. Solubility Parameters of Selected NitrogenContaining Compounds55 δ (MPa1/2)

piperidine pyridine quinoline acridine pyrrolidine pyrrole indole carbazole aniline

20.2 21.8 21.4 22.6 20.4 21.6 22.1 23.5 22.5

two liquid phases is not governed by just the difference in the solubility parameters. Compounds that form an irregular solution may be soluble in each other despite large differences in their solubility parameters. 4.1. Liquid−Liquid Extraction Technology. A process flow diagram for a generic multistage liquid−liquid extraction process is shown in Figure 6. The steps involved in the process

4. LIQUID−LIQUID PHASE PARTITIONING Phase partitioning of nitrogen-containing compounds between oil and a second immiscible liquid phase forms the basis for many processes based on liquid−liquid extraction. Liquid− liquid extraction exploits the differences in the Hildebrand solubility parameters (δ) of the bulk oil, nitrogen-containing compounds, and immiscible liquid used as second liquid phase for extraction. Processes based on “pure” liquid−liquid extraction, which does not rely on a chemical interaction between the nitrogen-containing compounds and the molecules in the extracting liquid, can be described using regular solution theory, as the term was originally defined.54 The liquid-phase preference for a nitrogen-containing compound with solubility parameter δN would then depend on its difference with respect to the solubility parameters of the two phases. For selective extraction of the nitrogen-containing compounds, the solubility parameter of the extracting liquid, δext, compared to the oil, δoil, must be such that eq 1 preferably holds true: |δ N − δext| < |δ N − δoil| < |δext − δoil|

compound

Figure 6. Process flow diagram of a generic multistage liquid−liquid extraction process.

are mixing of the two liquid phases to facilitate liquid−liquid extraction, phase separation of the two liquid phases, and recovery and recycling of the solvent that was used for extraction. The number of equilibrium stages and the method of contacting depend on the liquid-phase partitioning, extraction requirements, mass transport rate, and physical properties of the liquid phases. Various types of liquid−liquid extractors are available.56 Operating parameters such as mixing intensity, mixing time, and temperature can be manipulated to improve the mass transport. The choice of solvent is critical, and it is affected by a number of considerations. First, and the most obvious, is the nature of the nitrogen-containing compounds to be extracted, which would determine whether extraction is performed on the basis of polarity, acid−base character, or some other chemical property of the solvent.57 Second, but by no means secondary, is the recoverability of the solvent. The efficiency of solvent recovery affects the operating cost of the process. Solvent can be lost through solubility in the oil, incomplete liquid−liquid separation, and incomplete recovery from the extracted nitrogen-containing compounds. Other properties of the solvent, such as volatility, oil miscibility and density difference with respect to the oil, might be critical in solvent selection. Third is the purpose of the nitrogen removal. The degree of

(1)

Equation 1 is a mathematical expression of the phrase “like dissolves like”. Most organic compounds have solubility parameters in the range 14−29 MPa1/2. Exceptions are lowerdensity apolar molecules, such as propane, which have lower values, and high-density polar molecules, such as methanol, which have higher values. Nitrogen-containing compounds tend to have solubility parameters that are more typical of polar compounds (Table 3).55 To put these values into perspective, the Hildebrand solubility parameter of Athabasca bitumen is 18.25 MPa1/2.28 Mixed solvents can in principle be used to obtain a solvent system with different solubility parameter and characteristics than those of the pure solvents. However, this would complicate subsequent solvent recovery and recycling (section 4.1). Liquid−liquid extraction can also exploit chemical interactions to solvate the nitrogen-containing compounds in the extracting liquid. In such cases the extraction solution would be termed an “irregular” solution, and the partitioning between the G


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Energy & Fuels Table 4. Denitrogenation of Real Industrial Feedstocks by Solvent Extraction feed

solvent

type of solvent

distillates from Battle River coal liquid

methanol + water

organic + water

distillates from Wandoan coal liquid

methanol + water

organic + water

straight-run diesel

methanol + water + acetic acid + complexing agent

organic + water + acid

Fushun shale oil

phosphoric and sulfuric acids

acid

Fushun shale oil

phosphoric and acetic acids

acid

catalytically cracked diesel oil

acetic acid

acid

vacuum gas oil

acetic acid

acid

gasoline

HCl (1 mol L−1)

acid


triethylmethylammonium acetate and triethylmethylammonium butyrate

ionic liquids

nitrogen removal and/or the recovery of the nitrogencontaining compounds might also affect the solvent choice. The main solvents used for nitrogen removal from crude oil are organic solvents,58,59 acidic solvents,60−63 and ionic liquids.64−70 Examples of denitrogenation of oils, excluding model oils, by liquid−liquid extraction using different types of solvents can be found in Table 4. A special class of solvents that have been investigated only with model oils are deep eutectic solvents.71,72 Deep eutectic solvents are a new class of ionic liquids. What differentiate these solvents from ionic liquids are their chemical properties, since they have similar physical properties. A deep eutectic solvent is a eutectic mixture of a Lewis or Brønsted acid or base containing a variety of anions and cations, while an ionic liquid is composed of one type of discrete anion and cation.73 4.2. Solvents Used for Liquid−Liquid Extraction. 4.2.1. Polar Solvents. A mixture of methanol and water was used to extract nitrogen compounds from middle distillates of Battle River and Wandoan coal liquids. Nitrogen extraction increased with increasing amount of methanol in the mixture. The percentage of nitrogen extracted from Battle River coal liquids was ∼50% and from Wandoan coal liquids was ∼34%. Because of the polarity of the solvents, phenolic compounds were also extracted, which lowered the selectivity of nitrogen compounds extracted.58 Metallic ions were added as complexing agents in a methanol/water/acetic acid mixture for the removal of basic nitrogen compounds in straight-run diesel. It was found that the highest basic nitrogen removal (90.8%) was achieved using undisclosed metallic ions as complexing agents at room temperature and 3 min reaction time.59 It was surprising to find that there were very few solubility data available for aromatic nitrogen-containing compounds in common polar solvents such as water and ethanol (Table 5).74−77 This is the case despite a significant body of literature

process conditions 30 °C 10 min 30 °C 10 min 30 °C 3 min 60 °C acid:oil = 1:1 20 min 60 °C acid:oil = 1:1 20 min 20 °C 80% acetic acid 10 min room temperature 90% acetic acid 15 min room temperature 20 min

nitrogen conversion/removal

ref

∼50% N extracted

58

∼34% N extracted

58

90.8% basic N removed

59

97.43% basic N removed, 38.2% total N removed

60

90.71% basic N removed, 30.5% total N removed

60

∼78% basic N removed

61

63% basic N removed

62


63

30% N removed

70

Table 5. Solubilities of Nitrogen-Containing Compounds Listed in Table 2 in Ethanol and Water at 20−25 °Ca solubility in polar solvents compound

ethanol

water

ref

quinoline pyrrole indole aniline

− − 35.85 g/100 g of ethanol −

6191 mg/L 55.685 g/L − 3.65 g/100 g of water

74 75 76 77

a

Based on a solubility data search using the Reaxys database.

on the toxicity of nitrogen-containing heterocyclic compounds, which was developed as part of the synthetic fuels programs in the United States in the 1970s and 1980s.78 Process information from the Char-Oil-Energy-Development (COED) coal liquefaction process indicates that the aqueous product from the first stage of coal pyrolysis contained 0.05 wt % N and had a pH of 3.6 and that the aqueous product from the second stage of coal pyrolysis contained 0.93 wt % N and had a pH of 8.8.79 Although ammonia is often a significant contributor to the nitrogen content of such process water streams, not all of the nitrogen was due to ammonia. There is consequently a clear gap in the basic data for liquid−liquid extraction using the most common of solvents, water. Considering the impact that partitioning of nitrogencontaining compounds between the organic and aqueous phase can have on water toxicity, it is surprising that so little fundamental work has been reported. 4.2.2. Acidic Solvents. The use of acidic solvents has the objective of removing primarily basic nitrogen compounds from crude oil by an acid−base reaction that precedes extraction. As basic nitrogen compounds possess a lone pair of electrons, they are Lewis bases. Acid−base reactions with Lewis acids result in the formation of complexes. Acid−base reactions with Brønsted acids result in the protonation of the nitrogen to form salts of H


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Energy & Fuels

liquids.67 On the other hand, ionic liquids containing bistriflimide and chloride salts of copper and iron selectively removed nitrogen compounds in the presence of sulfur compounds in a model oil. The selectivity was improved by using such ionic liquids supported on monolithic silica.68 The validation of such processes with real feeds is of extreme importance, as petroleum-derived oils are complex mixtures and the presence of other compounds and differences in physical properties compared with model oils may affect nitrogen removal. For example, in a study with several ionic liquids containing imidazole or quaternary ammonium groups, denitrogenation was successfully achieved by removal of quinoline, indole, and carbazole from a model oil. However, when some of the ionic liquids were evaluated for nitrogen removal from straight-run gas oil, some of them failed to remove nitrogen-containing compounds. The highest nitrogen removal was 30% using triethylmethylammonium butyrate and triethylmethylammonium acetate ionic liquids.70 The main limitations regarding the use of ionic liquids include limited reuse, high energy consumption, loss of hydrocarbons, and the need for several extraction cycles to achieve reasonable denitrogenation. It was recommended that research should focus on methods to overcome such problems before ionic liquids can be considered for nitrogen removal by industry.69 Another limitation not cited would be the physical state of the ionic liquid when it is used for the real feed extraction. For example, the ionic liquid could turn into a solid or dissolve in a real feed. 4.2.4. Deep Eutectic Solvents. The use of deep eutectic solvents for crude oil nitrogen removal has recently been investigated. They are an alternative to ionic liquids as they have the same basic properties, such as low volatility and existence in the liquid phase over a wide range of temperature, but with lower cost, lower toxicity, and simpler synthesis. They are composed of a salt combined with a hydrogen-bond donor or a complexing agent. Like ionic liquids, deep eutectic solvents may have their properties changed by manipulation of the salt and hydrogen-bond donor/complexing agent combination.71 In the first study reported,72 choline chloride and phenylacetic acid deep eutectic solvents presented the highest extraction from model oil: 99.2% of pyridine and 98.2% of carbazole were removed. It was explained that the denitrogenation performance is dependent on the acidity of the hydrogen-bond donor component, as high acidity causes low affinity with basic nitrogen compounds and excessive acidity leads to chemical reactions instead of physical separation.

the pyridinium ion. Salts of the pyridinium ion are usually water-soluble and readily extracted into an aqueous phase. An acid will be a good solvent for nitrogen removal if it has high acid strength and low volatility, is water-soluble for easier recovery, and can form reversible bonds with nitrogen compounds. Even though formic acid is a stronger acid than acetic acid, it is not commonly used because it is very unstable.61 The use of acetic acid for removal of basic nitrogen compounds has been reported for shale oil,60 catalytically cracked diesel oil,61 and vacuum gas oil.62 Process parameters for denitrogenation of catalytically cracked diesel oil and vacuum gas oil by acetic acid were determined, and the highest basic nitrogen removal was ∼78% for catalytically cracked diesel oil61 and 63% for vacuum gas oil.62 Even though acetic acid in combination with phosphoric acid and formic acid could remove 90.71% and 88.65% of basic nitrogen compounds, respectively, from shale oil, the highest removal (97.43%) was achieved with phosphoric acid and sulfuric acid in a ratio equal to 17:3.60 It is interesting to note that nonbasic nitrogen compounds were also extracted along with basic nitrogen compounds by aqueous acetic acid in the study by Qi et al.61 The explanation given by the authors was that weak bases such as nonbasic nitrogen compounds reacted with strong acids, which resulted in their extraction by the acid. Following on these observations, it was suggested to employ strong acids, such as hydrochloric acid, for removal of nonbasic nitrogen compounds.61 In the study by Feng,63 aniline and quinoline were the main compounds extracted from gasoline with hydrochloric acid at concentrations less than 1 mol·L−1, while indole was the main compound extracted when the hydrochloric acid concentration was 6 mol·L−1. The author did not provide any explanation for the removal of nonbasic nitrogen compounds with hydrochloric acid. It was claimed that the removal of pyrrolic compounds from shale oil was achieved by acidification with strong acids, such as sulfuric acid, because of polymerization.80 Addition reactions of pyrrolic compounds take place under acidic conditions,1,81 and acid-catalyzed addition, rather than extraction, could potentially also explain the removal of neutral nitrogen-containing compounds in the work of Qi et al.61 and Feng.63 Whatever the correct explanation is, it is important to understand the mechanism involved in nitrogen removal, since polymerization taking place in a liquid−liquid extraction process will affect its design and operation. 4.2.3. Ionic Liquids. Ionic liquids are composed of heterocyclic organic cations and organic or inorganic anions. They have low vapor pressure, are nonflammable, and exist in the liquid phase over a wide range of temperatures. Different cation/anion combinations provide unique properties of ionic liquids, which can be tailored to facilitate the extraction of nitrogen-containing compounds.64 The majority of denitrogenation studies involving ionic liquids have been reported for model oils in order to investigate the chemistry and selectivity. For example, dicyanamide-based ionic liquids were effective in removing carbazole and pyridine from a model oil,65 while zinccontaining ionic liquids were effective in removing quinoline, acridine, and indole from a model oil.66 Model oils containing not only nitrogen compounds but also sulfur compounds were also investigated. Acidic ionic liquids were able to remove 93.8% of carbazole and 97.8% of pyridine in a model oil that also contained sulfur compounds. In this study, 93.8% of the sulfur compounds were also removed by the acidic ionic

5. SOLVENT DEASPHALTING Solvent deasphalting is a commonly employed process in oil refining for processing of heavy oil fractions, and it operates on the principle of liquid−solid phase partitioning. In a solvent deasphalting process, a light paraffinic solvent is mixed with the oil to change the solubility parameter and solubility characteristics of the bulk liquid phase. Molecules in the oil that exceed the solubility threshold or that become immiscible because the Gibbs free energy of mixing becomes positive, will form a separate phase. In solvent deasphalting, the second solid phase that is formed is called the asphaltenes fraction. The nature of the solvent and method used to induce liquid− solid phase partitioning affects the yield and composition of the asphalthenes fraction that is precipitated from the oil. Using a light paraffinic solvent to create the driving force for precipitation causes the asphaltenes fraction to contain I


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Energy & Fuels

The types of adsorbents that have been reported in the literature for nitrogen compound removal are silica gel,8,63,83−85 activated alumina,63,86,87 activated carbon,87−92 metal−organic frameworks,93,94 ion-exchange resins and modified polymers,97−106 and zeolites107−109,114,110,111 as well as a number of diverse materials such as solid acids and modified silica− aluminas.115,116 A summary of the nitrogen removal from real petroleum-derived feed materials with each of the adsorbents is presented in Table 6. 6.1. Adsorption Technology. Adsorption technology processes usually consist of at least two vessels containing adsorbents in parallel to enable continuous operation (Figure 8). This is necessary because adsorption technologies have two modes of operation: adsorption and regeneration. During the adsorption mode of operation, the oil feed is passed over the adsorbent, and the nitrogen-containing compounds are removed by adsorption onto the adsorbent. The nitrogen content of the oil at the outlet of the adsorption vessel is lower than that in the feed and remains constant until breakthrough occurs. Breakthrough occurs when the adsorption capacity is sufficiently consumed that the remaining capacity is insufficient to achieve the same extent of nitrogen removal. Not all of the adsorbent capacity is used when breakthrough occurs because adsorption is not instantaneous and part of the adsorbent is only partially loaded. The adsorption kinetics determines how much of the adsorption capacity remains at breakthrough, with faster adsorption kinetics causing less unused capacity to remain at breakthrough. Once breakthrough is reached, the adsorbent must be regenerated. During the regeneration mode, the nitrogen-containing compounds on the adsorbent are desorbed. The method of desorption depends on the nature and strength of the adsorption as well as the nature of the adsorbent. The most common methods of regeneration are by temperature and chemical reaction. Physisorption can be reversed by increasing the temperature. When the temperature is increased, the physisorbed nitrogen-containing compounds are desorbed and can then be removed by washing the adsorbent with a stripping liquid. Chemisorption usually requires a chemical reaction to remove the chemisorbed compounds. The reaction can either displace the adsorbed materials, such as regeneration of acidic support materials with a stronger acid, or involve destruction of the adsorbed material, such as complete oxidation of the compound by reaction with hot air. The ease of regeneration affects both the amount of time required for regeneration and the cost of regeneration. In the simplest configuration, shown in Figure 8, the first vessel is used for adsorption while the adsorbent in the second vessel is being regenerated. Irrespective of the adsorption technology selected and the adsorbent used, adsorption processes are better suited for removing nitrogen-containing compounds from oils with low nitrogen content than for bulk nitrogen removal for oils with high nitrogen content. For example, to remove the nitrogencontaining compounds from oil containing 1 wt % N using an adsorbent with a capacity of 2.4 molN·kg−1 and very fast adsorption kinetics, at least 0.3 kg of adsorbent/kg of oil is required. In a continuous process, the amount of adsorbent that is required is determined by both the nitrogen content of the oil and the time it takes to regenerate the adsorbent. Using the same example as before, if it takes 4 h to regenerate the adsorbent, the amount of adsorbent required in each vessel is at least 1.2 kg per kg·h−1 of oil flow rate.

molecules that on average have a higher heteroatom content, are more polar in nature, and have a higher molecular mass than the bulk oil. As a consequence, nitrogen-containing compounds partition between the liquid and solid phases in such a way that the solid phase (asphaltenes fraction) is enriched in nitrogen-containing compounds (Figure 7).82 The oil that remains in the liquid phase, after solvent recovery, has a lower nitrogen content than the oil feed from the solvent deasphalting process.

Figure 7. Partitioning of nitrogen-containing compounds during solvent deasphalting.82

From Figure 7, it should be clear that solvent deasphalting is not capable of deep nitrogen removal from oil. For example, if heavy oil containing 1 wt % N is solvent-deasphalted so that the asphaltenes yield is 20 wt %, the deasphalted oil will still have around 0.7 wt % N.

6. ADSORPTION Adsorption processes employed for the removal of nitrogencontaining compounds from oil use a solid material, the adsorbent, to retain the nitrogen-containing compounds as the oil flows over the adsorbent. The adsorbent material is selected so that the nitrogen-containing compounds would be attracted and retained by interactions with the surface of the adsorbent. Compounds can be adsorbed on the surface by chemical or physical interactions. When there is a chemical interaction (chemisorption), a chemical bond is formed between the adsorbent and the compound, and the electronic configuration of the compound is changed. When the compound is retained on the surface by a physical interaction (physisorption), such as through van der Waals or electrostatic forces, the compound maintains its identity and no new bonds are formed. The type of interaction affects the strength of retention and the selectivity. Chemisorption is generally more selective because it makes use of the chemical identity of the nitrogen-containing compounds, and the compounds are retained more strongly. Although this is advantageous during adsorption, it also makes it more difficult to remove the adsorbed compounds after adsorption, when the adsorbent is regenerated for reuse. The main parameters relevant to adsorption studies are adsorption kinetics, adsorbent capacity, selectivity, adsorbent regenerability, adsorbent lifetime, and cost. Different types of adsorbents for denitrogenation of oil have been reported and were recently reviewed by Laredo et al.7 The maximum adsorption capacity reported for any of the materials tested was in the range 2−3 mol of N/kg of adsorbent. J


Review

Energy & Fuels Table 6. Denitrogenation of Real Industrial Feedstocks by Adsorption type of adsorbent silica gel

feed

adsorbent

light cycle oil gasoline diesel (tar sands, Canada) Diesel (Rota, Spain) SK SRGO Coker gas oil

activated carbon

Japanese SRGO

MAXSORB-II

Japanese SRGO

MGC-B

Japanese SRGO

OG-20A

metal−organic frameworks

SRGO and LCO

MIL-101(Cr)

ion-exchange resins

Arabian straight-run fuel oil

HPQ

polymers

Iranian SRGO

PGMA-TENF

bitumen-derived HGO

PGMA-TENF

bitumen-derived LGO

PS-NN-DNF

bitumen-derived LGO

PGMA-NN-TriNF

bitumen-derived LGO

PGMA-NN-TENF

bitumen-derived LGO

PGMA-NN-TENF

bitumen-derived LGO

PS-NN-TENF

bitumen-derived HGO

PGMA-NN-TENF

bitumen-derived LGO

PGMA-DAP3-TENF

bitumen-derived LGO

P3-ON-TENF

gasoline commercial diesel oil

CrY

commercial diesel oil

CuY

alumina zeolites

K

process conditions room temperature room temperature room temperature 30 min room temperature 30 min room temperature 1000 bbl/day room temperature 1000 bbl/day 0.14 MPa 30 °C 0.14 MPa 30 °C 0.14 MPa 30 °C 403 K for 12 h remaining time at 298 K LSHV: 0.8 h−1 313 K 15 min room temperature gas oil:polymer = 8:1 room temperature 1h 15 wt % polymer 22 °C 400 rpm 24 h oil:polymer = 4:1 22 °C 400 rpm 24 h oil:polymer = 4:1 22 °C 400 rpm 24 h oil:polymer = 4:1 20 °C 200 rpm 48 h oil:polymer = 1:5 20 °C 200 rpm 48 h oil:polymer = 1:5 50 °C 200 rpm 48 h oil:polymer = 1:5 22 °C 400 rpm 48 h oil:polymer = 4:1 22 °C 200 rpm 24 h oil:polymer = 4:1 room temperature ambient pressure room temperature ambient pressure


ref

97.5% N removed 98.22% basic N removed 99.6% N extracted

8 63 83

99.7% N extracted

83

82% N removed

84

86% N removed

84

77% N removed

88

41% N removed

88

43% N removed

88

90% N removed

94

41% N removed

98

50% N removed

100

6.7% N removed

101

9.4% N removed

102

14.4% N removed

102

11.2% N removed

102

11.4% N removed

103

0.8% N removed

103

5.3% N removed

103

19% N removed

105

14.6% N removed

106

99.29% basic N removed 56.58% N removed

63 107

60.16% N removed

107


Review

Energy & Fuels Table 6. continued type of adsorbent

methyl viologen aluminosilicate

feed

adsorbent


ZnY


NaY


CuY

Anqing naphtha

13X impregnated with H3PO4

Cold Lake bitumen

Chabazite clay

FCC diesel oil

Co-MCM-41


Ti-HMS

commercial light oil straight-run light gas oil light cycle oil

other solid acids

process conditions

lubricant base oil

solid acid

room temperature ambient pressure room temperature ambient pressure room temperature ambient pressure room temperature 20 °C feed velocity 6 h−1 400 °C 1h 50% zeolite ambient pressure room temperature 30 min ambient pressure room temperature 298 K 24 h 298 K 24 h 298 K 24 h 110 °C 1 wt % solid acid 30 min


ref

37.41% N removed

107

23.72% N removed

107

99.8% N removed

108

84.5% N removed

109

74% N removed

110

33.46% N removed

113

>90% N removed

114

86% N removed

115

65% N removed

115

72% N removed

115

87% basic nitrogen removed

116

silica gel before hydrotreating was able to reduce the nitrogen content from 635 to 16 ppm while the sulfur and aromatic contents remained unchanged.8 Pretreatment of middle distillates by silica gel was commercially implemented by SK Corporation of Korea in 2002 to produce ultralow-sulfur diesel. The plant can process 1000 barrels/day, and 70−86% nitrogen removal can be achieved depending on the feed (Table 6). Pretreatment with silica gel to remove nitrogen-containing compounds resulted in deeper desulfurization. The pretreated product after hydrotreating contained 9−60 ppm sulfur, whereas the hydrotreated product without feed pretreatment contained 53−360 ppm sulfur.84 Improved hydrodesulfurization subsequent to nitrogen removal was also reported by others.7,8 6.2.2. Activated Alumina. The use of activated alumina for nitrogen adsorption has been mainly studied with model diesel oils containing sulfur, nitrogen, and aromatic compounds. It has been demonstrated that activated alumina has more adsorption capacity for nitrogen than other compounds present in the model fuel oil. The mechanisms responsible for the higher selectivity toward nitrogen compounds involve electrostatic interactions and acid−base interactions.86,87 Basic nitrogen compounds present higher adsorption with acidic activated alumina, while more acidic nitrogen compounds present higher adsorption with lower-acidity activated alumina.87 Alumina was able to remove 99.29% of basic nitrogen from gasoline.63 6.2.3. Activated Carbon. The denitrogenation efficiency of adsorption using activated carbon depends on its surface area as well as its surface chemical composition. Nitrogen removal from straight-run gas oil (SRGO) using three different types of activated carbon has shown that nitrogen removal is higher when the oxygen content of the activated carbon is increased (Table 6).88 In a later study, it was found that nitrogen

Figure 8. Simple adsorption process with two vessels. One vessel is in adsorption mode, and one vessel is being regenerated.

6.2. Adsorbents for Denitrogenation. 6.2.1. Silica Gel. Silica gel removes nitrogen-containing compounds by physisorption. Since silica gel does not contain strong adsorptive sites, the attraction between the adsorbent and polar nitrogen compounds is by hydrogen bonding, which facilitates silica regeneration.85 Silica gel has therefore also been employed to preconcentrate nitrogen-containing compounds for analysis, where denitrogenation of the oil is not the only objective. Mushrush et al.83 used silica gel to remove nitrogen from 22 finished diesel fuels from different parts of the world, and 97.8− 99.7% organic nitrogen extraction was achieved. Nitrogen compounds were subsequently desorbed and analyzed by gas chromatography−mass spectrometry (GC−MS). The most abundant compounds in the diesel fuels were pyridines (60%), quinolines and tetrahydroquinolines (26%), and indoles and carbazoles (10%). Silica gel appeared to be quite selective for nitrogencontaining compounds, despite the fact that it only physisorbed these compounds. Pretreatment of light cycle oil (LCO) by L


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Energy & Fuels

basic nitrogen-containing compounds through the acid−base reaction between the basic nitrogen compounds and the acidic groups. For example, it was reported that a high-acidity resin was able to remove only basic nitrogen compounds from hydrotreated jet fuel, diesel fuel, and vacuum gas oil fractions derived from shale oil.97 The use of ion-exchange resins for removal of neutral nitrogen-containing compounds has been reported for Arabian straight-run fuel oil.98 Around 41% of the neutral nitrogen compounds could be extracted from straight-run fuel oil by using an ion-exchange resin containing a pyridinium chloride functionality. The resin was regenerated by washing with pentane and methanol. A total of 78% of the nitrogencontaining compounds could be extracted in a three-step extraction process (i.e., the extraction was repeated three times). The success in removing neutral nitrogen compounds was explained in terms of hydrogen bonding between the chloride ion and the neutral nitrogen-containing compounds, such as carbazole, which acted as proton donors.98 However, it should be noted that pyridinium halide perhalide in pyridine was reported to be a good halogenation agent for indole,81,117 and the removal by the pyridinium chloride-containing resin might also have involved halogenation. Modification of polymers with π-acceptor molecules in principle enables adsorption of pyrrolic nitrogen-containing compounds through complex formation as well as pyridinic nitrogen-containing compounds through Lewis acid−base interactions.99 Employing this approach enabled 50% nitrogen removal (and 5% sulfur removal) from an Iranian straight-run gas oil using 2,4,5,7-tetrafluorenone (TENF) as the π acceptor and poly(glycidyl methacrylate) (PGMA) as the polymer support.100 When the same material was employed for nitrogen removal from bitumen-derived heavy gas oil, only 6.7% of the total nitrogen was removed; the majority of the nitrogen compounds adsorbed were pyrrolic.101 Somewhat better nitrogen removal was achieved when 2,4,7-trinitro-9-fluorenone (TriNF) was employed as the π acceptor instead of TENF.102 An investigation into the impact of the polymer support on nitrogen removal by TENF concluded that hydrophilic polymers with higher surface area, such as TENF-modified PGMA, are more selective and efficient for nitrogen removal when sulfur and aromatic compounds are present in real feeds.103 In a different study,104 hydrophilic polymers achieved higher nitrogen removal from diesel fuel compared with hydrophobic polymers, which led to the same conclusion. The linker length was also investigated as a parameter that may affect nitrogen removal, and it was found that the linker length had little effect on nitrogen adsorption apart from steric hindrance.105 Polymers with high internal phase emulsion achieved 14.6% nitrogen removal from bitumen-derived gas oil.106 6.2.6. Zeolites. Different types of zeolites were studied for adsorptive denitrogenation of fuels/oils. Adsorptive denitrogenation of diesel oil using modified NaY zeolite achieved 23.72− 60.16% basic nitrogen removal (Table 6). The highest denitrogenation was achieved with higher-valence metal ions.107 CuY zeolite could reduce the nitrogen content of commercial diesel oil from 83 to 0.1 ppm by π-complex formation between nitrogen compounds and the zeolite.108 13X zeolite impregnated with phosphoric acid was able to remove 84.5% of nitrogen from naphtha.109

adsorption is higher when the activated carbon contains oxygenated compounds that release CO during temperatureprogrammed desorption, while CO2-liberating groups inhibit nitrogen adsorption by activated carbon. Addition of COliberating groups can be achieved by manipulating the oxidation process of the activated carbon.89 The adsorption of nitrogen compounds from LCO was also higher when wood-based activated carbon oxidized by (NH4)2S2O8 was used, compared with the nonoxidized activated carbon. When the oxidant concentration increased from 0 to 15%, the nitrogen adsorption capacity increased by 18%. A study with model fuel oil revealed that quinoline adsorption was favored by carboxylic acid groups and lactone groups present in the activated carbon, while indole and carbazole adsorption were favored by carboxylic anhydride groups and phenolic groups.90 A commercial activated carbon (Maxsorb I) oxidized by ammonium persulfate was also used to understand the mechanism of oxygenate compounds in activated carbon. The results showed that the surface of activated carbon containing high oxygen content would interact with the nitrogen compounds by acid−base and hydrogen-bonding interactions, while the surface containing low oxygen content would interact by π−π interactions, which are weaker.91 The use of natural materials to prepare activated carbons for nitrogen removal has also been investigated.92 Chemical activation of palm oil shell, coconut shell, olive stone, apricot stone, and almond shell produced adsorbents with high surface area, high density, high carbon content, and low ash content. 6.2.4. Metal−Organic Frameworks. Metal−organic frameworks (MOFs) are composed of inorganic and organic materials and contain two components. The first component is a metal ion or a cluster of metal ions, while the second component is an organic linker. The organic linker can be selected to enable nitrogen removal by acid−base interactions, coordination bond formation, π complexation, hydrogen bonding, and van der Waals forces. The main merits when using MOFs as adsorbent materials are their high porosity and tunable chemical properties, while the main demerits are their instability at high temperature and in basic media. The use of MOFs for adsorption and removal of nitrogen-containing compounds was recently reviewed by Ahmed and Jhung.93 Only a few studies have reported the use of MOFs with real industrial feed materials. For example, straight-run gas oil and its mixtures with light cycle oil have been denitrogenated with MOF MIL-101(Cr), and the nitrogen removal was around 90%. Regeneration studies showed that the adsorbent could be used for at least 280 cycles while keeping almost the same nitrogen removal capacity.94 It was reported that a functionalized MOF was able to remove both indole and quinoline from model oils.95,96 The success in removing nonbasic indole is attributed to hydrogen bonding between the neutral nitrogen and sulfonic or carboxylic groups on the MOF. Since MOF preparation is time-consuming and high consistency is difficult to achieve, other compounds containing sulfonic groups could possibly be used for the removal of nonbasic nitrogen compounds in real industrial feed materials, as discussed in the next subsection. 6.2.5. Ion-Exchange Resins and Modified Polymers. The types of nitrogen-containing compounds that will be adsorbed by ion-exchange resins and modified polymeric materials depend on the nature of the functional groups on the organic substrate. The use of acidic groups is efficient for the removal of M


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Energy & Fuels

not eliminate the nitrogen group from the compound. When the nitrogen content in the oil product is low, the derivatized product can be treated as a waste product after separation from the oil. This approach is viable because there is a small loss of material. In any process that employs derivatization to facilitate separation, the bulk of the material that is removed from the oil is not nitrogen but rather carbon and hydrogen that form part of the nitrogen-containing compound. In cases where the nitrogen content is high, it is important also to have a strategy to liberate nitrogen from the hydrocarbon matter so that the hydrocarbon matter can be returned to the oil. Three steps are required (Figure 9): step 1 is the derivatization reaction; step 2

Zeolites are also used in catalytic cracking of oils, and denitrogenation can be achieved by adsorption of nitrogen compounds in the zeolite catalyst under cracking conditions. Modified natural chabazite was able to remove 74% of the nitrogen from catalytic cracking of bitumen.110 In a later study with model compounds it was suggested that under cracking conditions nonbasic nitrogen is converted into basic nitrogen, which is then adsorbed on the acidic chabazite zeolite.111 Heteroatom-containing mesoporous molecular sieves have been studied for denitrogenation of oils. In a first study it was observed that Co-MCM-41 molecular sieve was more effective in removing basic nitrogen from model oil compared with MCM-41. However, when they were used in commercial diesel oil, MCM-41 mesoporous molecular sieve presented higher removal of basic nitrogen (68.22%) than Co-MCM-41 (65.91%).112 In a later study, it was found that the content of Co influences the absorption capacity of the mesoporous molecular sieves. For example, the absorption capacity of basic nitrogen from fluidized catalytic cracking (FCC) diesel oil was 9.11 mg of nitrogen/g for Co-MCM-41 and 7.36 mg of nitrogen/g for MCM-41. Nevertheless, the absorption capacity of Co-MCM-41 decreases with increasing Co content. It was explained that Co enhances nitrogen absorption due to physical absorption and π complexation but that a high Co content promotes the formation of Co3O4 on the channel surface, which blocks active sites and consequently decreases nitrogen absorption. The total nitrogen removal from FCC diesel oil achieved was 33.46%.113 Ti-HMS mesoporous molecular sieve could remove more than 90% of the nitrogen from diesel, where the adsorption process was spontaneous and exothermic. In addition, adsorption occurs in the pores of the zeolite.114 6.2.7. Other Materials. Amorphous silica−alumina modified with methyl viologen dichloride (C12H14N2+Cl2−) was investigated for denitrogenation and desulfurization of model oils as well as light oils.115 While both sulfur and nitrogen could be removed from model oils, only denitrogenation was achieved for the real feedstocks. It was explained that the presence of aromatic hydrocarbons in the light oils suppressed sulfur adsorption by the methyl viologen aluminosilicate, while nitrogen adsorption was achieved as a result of the lower ionization potential of nitrogen compounds, which easily formed a charge-transfer complex with the adsorbent. The adsorbent could be regenerated by toluene washing.115 A nonspecified solid acid selectively removed nitrogen compounds from lubricating base oil. Nitrogen compounds are known to be detrimental to the oxidation stability of base oils, while sulfur has the opposite effect. The solid acid was able to remove 87% of the basic nitrogen compounds and only 13% of the sulfur compounds.116

Figure 9. Block flow diagram for nitrogen removal from oils with high nitrogen content.

is the separation, which becomes more efficient as a result of the derivatization; and step 3 is the extrusion of the nitrogen from the nitrogen-containing compounds so that the hydrocarbon fraction can be recovered as denitrogenated oil. If step 3 is not performed, the loss of material can become excessive, and the method is not useful for high-nitrogen-content oils. To illustrate the point, if the average molecular formula of the nitrogen-containing compounds in oil is C12H9N (carbazole), when 1 wt % N is removed, 12 wt % of the oil is removed. 7.1. Oxidation. Separation of nitrogen compounds from oil by oxidation relies on the principle that oxidized nitrogen compounds can be more efficiently extracted with polar solvents that are immiscible with oil than the unoxidized nitrogen-containing compounds. The main challenge in this process is to selectively oxidize nitrogen compounds, since oxidation of other compounds present in the oil may result in the formation of heavier products that are undesirable. Two common modifications of simple oxidative denitrogenation (ODN) using oxidants such as air, hydrogen peroxide, and organic peroxides are found. The first modification involves the use of a catalyst, either to assist with the decomposition of peroxides (R−O−OH) into free radicals (R−O· + ·OH) or to lower the activation energy of the transition state during oxidation. The second modification is the use of UV light to increase the oxidation rate by free radical formation. Only a few studies of oxidation of industrial oils for nitrogen removal have been reported, and in those instances oxidative desulfurization and oxidative denitrogenation are performed in tandem. For example, the oxidation process described by Guth and Diaz118 uses a gaseous mixture of oxides of nitrogen to oxidize sulfur and nitrogen in fuel oils, which are then extracted with methanol. The selectivity of the process is improved by pretreating the oil to remove other compounds that can also be oxidized and form coke.118 In the study by Conceiçaõ et al.119 nitrogen compounds in a gas oil were selectively oxidized by a mixture of ethanoic acid and hydrogen peroxide, and the oxidized compounds were removed in the aqueous phase. When hydrogen peroxide is mixed with an organic acid, it generates the corresponding

7. CHEMICAL CONVERSION FOLLOWED BY SEPARATION The chemical conversion of nitrogen-containing compounds has one of two objectives: either to derivatize the nitrogencontaining compounds to improve subsequent removal or to eliminate the nitrogen functional group to produce a nitrogencontaining product that can easily be removed, such as by forming NH3 or NOx. Hydrogenation, which was discussed as a separate topic (section 3), belongs to the latter group of chemical conversions. Derivatization of nitrogen-containing compounds usually employs a reagent that is capable of addition or substitution reactions to produce a chemically modified product but does N


Review

Energy & Fuels Table 7. Denitrogenation of Real Industrial Feedstocks by Oxidation feed

oxidant

separation process

hydrotreated VGO

peracetic acid

acetic acid extraction

Japanese LGO

t-BuOOH + 16 wt % MoO3/Al2O3

adsorption by silica gel

straight-run LGO

H2O2 + AcOH

acetonitrile−water azeotropic mixture extraction

commercial light oil H2O2 + AcOH


light cycle oil


H2O2 + AcOH

oxidation conditions 60 °C 30 min atmospheric pressure WHSV: 30 h−1 70 °C atmospheric pressure 3h 70 °C atmospheric pressure 3h 70 °C atmospheric pressure 3h

nitrogen conversion/ removal

ref

90% N removed

121

94.1% N removed

122

43.7% N removed

123

58.2% N removed

123

62.1% N removed

123

Table 8. Denitrogenation of Real Industrial Feedstocks by Oxidative Photoirradiation feed

polar solvent

commercial light oil

H2O

straight-run light gas oil

H2O

light cycle oil

H2O


H2O + 30% H2O2

straight-run light gas oil

H2O + 30% H2O2

light cycle oil

H2O + 30% H2O2

vacuum gas oil

acetonitrile

process conditions 50 36 50 36 50 36 50 36 50 36 50 36 50 10

peroxy acids of the carboxylic acids, which can convert pyridinic nitrogen to N-oxides.120 Other compounds such as hydrocarbons, olefins, ketones, and phenols remained constant after oxidation and separation of Brazilian petroleum gas oil. Another process to achieve the same utilized nonaqueous organic peroxide as the oxidant will generate acetic acid as a byproduct, in which oxidized nitrogen compounds would be soluble without the need for further polar solvent extraction.121 By this method, the nitrogen and sulfur contents of hydrotreated vacuum gas oil (VGO) were reduced by 90% and 73%, respectively. The results of this process and other processes for oxidative denitrogenation are given in Table 7.121−123 During oxidative desulfurization of light gas oil, it was also observed that nitrogen compounds were oxidized and removed from the oil.122 The oxidant employed was tert- butyl hydroperoxide (t-BuOOH) in the presence of 16 wt % MoO3/Al2O3 catalyst. Oxidized compounds were further removed from the oil by adsorption on silica gel. The nitrogen content was reduced from 13.5 to 0.8 ppm. A study with model compounds revealed that indole was the most reactive compound, followed by quinoline, acridine, and carbazole. Even though it was not possible to identify which types of compounds were extracted from the oil, the results suggested

°C h °C h °C h °C h °C h °C h °C h


ref

70% N removed

124

48% N removed

124

23% N removed

124

93% denitrogenation yield

124


124


124

>90% denitrogenation yield

125

that polymeric materials formed during the reaction contained nitrogen and oxygen.122 Oxidation of nitrogen and sulfur compounds present in three different light oils were successfully achieved by hydrogen peroxide and acetic acid followed by extraction with an acetonitrile−water azeotropic mixture.123 When tert-butyl hydroperoxide was employed as the oxidant, the most difficult nitrogen compound to denitrogenate was carbazole. In a variation on the standard autoxidation process, photoirradiation was employed as means to increase the efficiency. The light oil was mixed with acetonitrile or water, and sulfur and nitrogen compounds present in the oil were transferred to the polar phase and then photoirradiated with UV light to form highly polarized compounds that were not miscible with the low-polarity oil. The oxidized nitrogencontaining compounds were removed in the form of an NO3− ion that was soluble in the polar phase. This process was investigated with model compounds (aniline, indole, and carbazole)124 and industrial oil feedstocks.124,125 Carbazole was the nitrogen compound that was the most difficult to denitrogenate by photoirradiation. It was explained that the photodecomposition of carbazole is “suppressed by tripletenergy transfer from photo-excited carbazole to ground-state double-ring aromatic hydrocarbons.” However, denitrogenation of light oils is increased by adding hydrogen peroxide into the O


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Energy & Fuels oil/water mixture to increase the oxidant power. Nevertheless, autoxidation with photoirradiation in acetonitrile presented higher denitrogenation than in water/hydrogen peroxide solvent.124 Shiraishi et al.125 presented how photoirradiation could be incorporated into the conventional refining process, as this process could also remove both nitrogen and sulfur compounds from oils. Denitrogenation yields of some industrial oils are presented in Table 8. Other kinds of oxidants and catalysts have been investigated for oxidation of nitrogen model compounds. For example, oxidation of pyridine and its derivatives by H2O2 catalyzed by a polyoxometalate intercalated into a layered double hydroxide modified with tris(hydroxymethyl)aminomethane (trisLDHLa(PW11)2) was successfully achieved at room temperature. Further denitrogenation of model oil containing quinolines was achieved within 40 min at 75 °C.126 Natural and reduced limonite were also used as catalysts for the oxidation of quinoline by H2O2 and formic acid. Reduced limonite provided higher oxidation (90% conversion after 360 min) because Fe2+ could react with H2O2/formic acid via Fenton-like mechanism.127 The effect of nitrogen compounds on oxidative desulfurization for different catalysts has also been reported in the literature. For example, the presence of pyridine and pyrrole decreased the removal rate of thiophene during oxidation with H2O2 over Ti-containing molecular sieves.128 In contrast, quinoline and indole did not affect oxidative desulfurization. In the case of benzothiophene and 4,6-dimethyldibenzothiophene, the presence of carbazole decreased their removal rate. By applying an excess of oxidant, the authors concluded that the decrease in the removal rate is not due to the competition for H2O2 but rather the adsorption of nitrogen compounds on the catalyst active sites.128 The same explanation was forwarded for a decrease in oxidative desulfurization of model compounds by V2O5/Al2O3 and V2O5/TiO2 catalysts performed in the presence of quinoline, indole, and carbazole.129 The presence of indole and quinoline also decreased the reaction rate of oxidation of sulfides by H2O2 catalyzed by H3+nPMo12−nVnO40 supported on silica-encapsulated γ-Fe2O3 nanoparticles.130 7.2. N-Alkylation. Alkylation as a reaction has been investigated for denitrogenation of light oils131 and vacuum gas oil.132 The nitrogen compounds are methylated and then removed by precipitation. The alkylating agents used were CH3I and AgBF4. Nitrogen compounds are then precipitated as N-methylated tetrafluoroborates. Denitrogenation of light oils and vacuum gas oil was successfully achieved by this method (Table 9), and it was accompanied by alkylation of sulfur and aromatics.131,132 Studies with model compounds revealed that aniline, pyridine, quinoline, and carbazole were removed as precipitates of their corresponding methylated products, while indole and pyrrole were removed as insoluble polymerized materials. In addition, denitrogenation of basic nitrogen compounds and compounds containing alkyl substituents was more effective than that of nonbasic nitrogen compounds. This is because denitrogenation by this method depends on the electron density on the nitrogen atom, which is increased by alkyl substituents.131 Bromoacetic acid (CH2BrCOOH) was also explored as a reagent for N-alkylation. Basic nitrogen compounds could be removed from crude oil by an alkylation denitrogenation method with bromoacetic acid. In the work of Zhang et al.,133 polar pyridinium bromide salts were obtained by reacting

Table 9. Denitrogenation of Real Industrial Feedstocks by N-Alkylation nitrogen conversion/ removal

alkylating agent

alkylation conditions


CH3I/AgBF4

light gas oil

CH3/AgBF4

light cycle oil

CH3/AgBF4

vacuum gas oil

CH3/AgBF4

30 °C 11 h N2 atmosphere 30 °C 11 h N2 atmosphere 30 °C 11 h N2 atmosphere 30 °C 11 h N2 atmosphere

feed

ref

83 wt % N removed

131

96 wt % N removed

131

94 wt % N removed

131

93 wt % N removed

132

pyridine derivatives with bromoacetic acid using ethyl acetate as a solvent. The polar bromide salts were used as substrates for the production of indolizines. Salt formation resulted from alkylation at the nitrogen atom instead of the acid−base reaction with the carboxylic acid group (Figure 10). Since the

Figure 10. Reaction between pyridine and bromoacetic acid.

polar bromide salts were insoluble in ethyl acetate, basic nitrogen compounds in oil could be removed by reacting them with bromoacetic acid to form insoluble salts. The salts could then be separated from the oil by filtration or liquid−liquid extraction. Mehranpour et al.134 also obtained insoluble salts by reacting quinoline derivatives with bromoacetic acid, even though the reaction conditions were slightly modified. Quinoline and pyridine derivatives could be recovered later by hydrolyzing the salts in a basic environment. As presented in the work of Bogardus and Higuchi,135 the carbon−nitrogen bond of N-(4hydroxy-3,5-dimethylbenzyl)pyridinium bromide was cleaved during its hydrolysis under basic conditions to generate pyridine and 4-hydroxymethyl-2,6-dimethylphenol, and the rate of the hydrolysis was increased by decreasing the pH of the solution. The procedure described above is suitable for basic nitrogen compounds only, since it is a nucleophilic substitution alkylation reaction and nonbasic nitrogen compounds are not good nucleophiles. Nevertheless, nonbasic nitrogen compounds can react with bromoacetic acid if basic conditions are provided. Pyrrole and carbazole were successfully converted into 1pyrroleacetate sodium salt and N-carbazolylacetate sodium salt, respectively, when reacted with bromoacetic acid under basic conditions.136,137 Since oils contain basic and nonbasic nitrogen compounds and hydrolysis of quaternary ammonium derivatives of tertiary amines is achieved under basic conditions,135 the removal of nonbasic nitrogen compounds by this procedure P


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Energy & Fuels

nitrogen could be removed by this method, suggesting that other factors are involved in nitrogen removal by CuCl2 rather than a Lewis acid−base reaction. Following this approach, 95% of the CuCl2 could be recovered and regenerated. A study with model compounds also revealed that nitrogen removal was selective, i.e. the sulfur and hydrocarbon contents remained constant after the complexation process with CuCl2.139 It was found that the nitrogen removal efficiency with CuCl2 could be increased by the addition of some water during the process.140 The denitrogenation of heavy coker gas oil derived from Athabasca bitumen was increased from ∼20% to 60% by increasing the water content from ∼3.5% to 9%. It was explained that copper chloride dissolves in water, which improves the contact between the salt and components at the oil−water interface. The method was selective for nitrogen removal, as the sulfur content of the oil was not meaningfully affected. Metallic ion/complexing extraction was also studied for straight-run diesel. The authors did not specify what kinds of salt were used for the study; nevertheless, 90.8% of the total nitrogen could be removed from straight-run diesel.141 Hydrated iron(III) chloride adsorbed on attapulgus clay was suggested by Rayner-Canham and Dickerhoof142 to denitrogenate petroleum distillates by complexation. Amines were successfully complexed by iron chloride−clay suspended in toluene, while quinoline and pyrazole complexed with iron chloride were retained on the surface of the clay only when pentane was used as a solvent for the complexation process.142 7.4. Conversion in High-Temperature Water. Two important reviews have discussed the use of water as a reagent at high temperature. The first review, in two parts,143,144 considered the role of water in reactions that are relevant to the transformation of organic material under geological conditions to ultimately produce fossil fuels. Akiya and Savage145 provided a systematic study of the different roles of water in conversion chemistry. These reviews covered the chemistry of water over a wide temperature range, which included supercritical conditions. The critical point of water is at 374 °C and 22 MPa. 7.4.1. Subcritical Water. The dissociation constant of water increases with temperature, reaching a maximum at around 250−275 °C, which makes this an interesting temperature range for subcritical water chemistry. In a series of papers,146−148 the conversion of pyridine and its derivatives was investigated by keeping the pyridines and water at reaction conditions of 250 °C and high pressure for several days. Although some conversions were observed, the nitrogen group was not eliminated from the pyridines. Most pyrroles and indoles were not denitrogenated by water when kept at 250 °C for 5 days, the exception being 2,5-dimethylpyrrole.149 About half of the products from the reaction of 2,5-dimethylpyrrole and water were denitrogenated ketones. Arylamines were not denitrogenated by water at 250 °C, but in the presence of phosphoric acid, significant conversion to phenols was found.150 It was further found that hydroxydeamination (the Bucherer reaction) did not take place in the presence of NaHSO3 and water, despite the severe reaction conditions.150 At a reaction temperature of 350 °C, which is sufficiently high for thermal cracking to become significant, the conversion of nitrogen-containing compounds in the presence of water was higher. Denitrogenation of various nitrogen heterocycles was found at 350 °C but was not observed at 250 °C.151

would require two steps: (i) the removal of basic nitrogen compounds by selective alkylation with bromoacetic acid and (ii) the removal of nonbasic nitrogen compounds by alkylation with bromoacetic acid under basic conditions. 7.3. Complexation with Metallic Salts. Basic nitrogen compounds can be removed from oil by complexation with metallic salts. This is because the metal ions are Lewis acids (electron-pair acceptors) and basic nitrogen compounds are Lewis bases (electron-pair donors). The Lewis acid−base reaction results in the formation of an organometallic complex. Examples of denitrogenation by complexation using metal salts can be found in Table 10.63,138−141 Table 10. Denitrogenation of Real Industrial Feedstocks by Metal Complexation feed

metal salt

gasoline

20 wt % FeCl3

gasoline

20 wt % SnCl2

shale oil

CuCl2

LGO from shale oil

CuCl2

LGO from coal liquid

CuCl2

pyrolysis product from Athabasca bitumen Athabasca HGO

CuCl2

straight-run diesel

CuCl2 + 8.6 wt % H2O not specified

process conditions


ref

room temperature 20 min room temperature 20 min 65 °C 45 min 65 °C 2h 65 °C 2h 65 °C 2h


63


63

∼85% N removed

138

88% N removed

139

87.3% N removed

139

63.4% N removed

139

65 °C 2h salt:oil = 0.2 salt:oil = 1.0 3 min 20−30 °C

60.0% N removed

140

90.8% N removed

141

Aqueous solutions of SnCl2 and FeCl3 were mixed with gasoline, and basic nitrogen compounds were successfully removed (98.89% for FeCl3 and 95.72% for SnCl2) by complexation with the metal halides.63 The use of metal salts for nitrogen removal from shale oil by complexation was investigated.138 Different solids (transition metal halides, sulfates, acetates, nitrates, phosphates, and fluoroborates) were screened for their removal of pyridine and its derivatives. Copper chloride (CuCl2) demonstrated high affinity for all pyridine compounds, including 2,6-dimethylpyridine, where the nitrogen is sterically crowded. In addition, it was found that the Lewis acidity of the metal is not the only factor affecting nitrogen complexation. The crystal structure of the formed solid affects the nitrogen complexation, since pyridine has to permeate into the solid crystal, which affects the capacity. More than 85% of the nitrogen content was removed by treating shale oil with CuCl2 at 65 °C for 45 min. CuCl2 could be regenerated by water washing followed by evaporation. Synthetic crude oils from various sources, shale oil, coal liquid, and oilsands bitumen, were also reacted with CuCl2 with the purpose of removing nitrogen compounds.139 The results revealed that not only the total nitrogen content decreased but also 80.6−91.2% basic nitrogen and 29.8−68.3% nonbasic Q


Review

Energy & Fuels 7.4.2. Supercritical Water. The high diffusivity of supercritical fluids has been explored for heavy oil upgrading, and studies using supercritical water for removal of nitrogen from oil are available in the literature. The reason cited for the use of water specifically is that water’s dielectric constant can be changed with temperature and pressure, which increases the miscibility with organic compounds and thereby the rate of reaction.152 Data on the change in properties of water can be found in the review by Akiya and Savage.145 The challenge with interpreting any of the literature on supercritical water conversion is that the temperature is already in a range where thermal cracking takes place. Distinguishing between thermal cracking reactions and those caused by water is difficult, noting that water also plays a moderate role by suppressing reactions such as free radical recombination. Denitrogenation studies with supercritical water were initiated using model compounds with the objective of understanding the chemical reaction mechanism. Various pyridines, pyrroles, N-oxides, and oxime derivatives were studied.153−157 The model compounds were reacted with supercritical water with or without catalysts (ZnCl2 and FeO3).153,154 The reaction mechanism differs for each model compound and process parameter. Briefly, the main denitrogenation reactions for isoquinoline and quinoline involve carbon oxidation, decarboxylation,153 and C−N hydrolysis preceded by hydrogenation and hydrocracking of heterocycles.154 In addition, ZnCl2 increases the reactivity of less reactive species.153 In the absence of other reagents, water alone was not efficient at converting pyridine and pyrrole derivatives, and very small amounts of denitrogenated compounds were found after reaction.155,156 The N-oxide and oxime derivatives were more reactive.157 Yuan et al.158 studied catalytic denitrogenation of quinoline in supercritical water with oxygen. Oxygen was supplied into the system while hydrogen was generated in situ by supercritical water. The catalyst was the sulfide NiMo catalyst. By varying the operational conditions, they achieved an 85% reduction in nitrogen, and the denitrogenation pathway involved three reactions: oxidation of hydrocarbons, the water gas shift reaction, and hydrodenitrogenation, where the water gas shift reaction and hydrodenitrogenation were catalyzed by the NiMo catalyst.158 The patents U.S. Patent 4,594,141159 and EP 0671454 (A2)160 describe the use of supercritical water for nitrogen removal by using it in conjunction with olefin/halide compounds and high-pressure carbon monoxide, respectively. The olefins were used in conjunction with supercritical water because they increase the cracking rate of organic saturated materials, while CO facilitates the transfer of hydride ions for hydrogenation. The claim of both patents was proved with model compounds. On the other hand, patent WO 2009085436161 used crude oils with different properties to prove denitrogenation with supercritical water in the absence of any other additive. The results are presented in Table 11. The reactions that take place when the oils are reacted with supercritical water are thermal cracking, steam reforming, water gas shift, demetalization, and desulfurization.

Table 11. Denitrogenation of Real Industrial Feedstocks by Supercritical Water feed

process

heavy oil

batch

crude oil

continuous

crude oil

continuous

process conditions 400 °C 30 min 20 vol % diluent 400 °C 23.4 MPa 400 °C 23.4 MPa


ref

62.5% N removed

161

57.3% N removed

161

43.75% N removed

161

microorganisms occurs aerobically. First, oxygenases insert oxygen into the substrate to yield hydroxylated intermediates, after which ring cleavage occurs. Quinoline, indole, and pyrrole are readily degraded by microbial attack, while carbazole is the most difficult one to degrade. It is believed that the difficulty in degrading carbazole arises from the fact that its degradation starts with angular deoxygenation, which is difficult to achieve.162 Because biodenitrogenation results in ring destruction, the main disadvantage of this method is that it promotes the loss of the fuels energetic value, and a current focus of this area involves genetic modification of microorganisms to obtain ones that can selectively remove nitrogen without degrading the carbon skeleton.162,163 In the study of Sugaya et al.,164 quinoline was added to Arabian light crude oil, and it was degraded by Comamonas sp. TKV3-2-1. Quinoline was completely converted into watersoluble 2-hydroquinoline in 30 min. In addition, the strain maintained its denitrogenation activity for 7 days, and more than 99% of the cells could be recovered by centrifugation. Biodenitrogenation studies with real feedstocks have rarely been reported, and a summary is presented in Table 12. One example is the use of the HY9 strain to remove nitrogen from diesel oil. It was found that addition of the surfactant Tween80 increased the removal of nitrogen from 12.9% to 19.3%. It was explained that the surfactant improves the solubility of diesel oil in nutrients.165 Heavy gas oil derived from Brazilian petroleum was submitted to biodenitrogenation with Rhodococcus erythropolis ATCC 4277. The oil was mixed with water at different concentrations, and the nitrogen reductions were 43.2%, 40.2%, and 31.2% for the systems containing 40%, 20%, and 60% heavy gas oil, respectively. On the other hand, there was no nitrogen removal for the system containing only heavy gas oil. High dispersion of cells in the reaction medium containing 40% heavy gas oil and aggregated cells in the medium containing only oil were observed, which explains the higher denitrogenation achieved when the oil concentration was 40%.166

9. CONCLUDING REMARKS A considerable diversity of processes have been proposed as denitrogenation strategies for the removal of nitrogencontaining compounds from oil. The processes can be divided into two broad categories: (a) Conversion processes that extract nitrogen from the nitrogen-containing compounds to form products that can readily be separated from the denitrogenated oil. In these processes, most or all of the hydrocarbon matter that was associated with the nitrogen remain in the oil. The most widely applied industrial process in this

8. MICROBIAL CONVERSION Different strains of microorganisms have been studied for denitrogenation of oil. Even though denitrogenation pathways differ for each compound and for different strains, there are some similarities. The degradation for the majority of R


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Energy & Fuels

and return the hydrocarbon matter to the oil (Figure 9) become important considerations before industrial application can be considered. There appears to be little literature on the elimination of nitrogen from the nitrogen-rich products obtained from denitrogenation of oil. Judging by the current state of the literature on the topic, removal of nitrogen from real industrial feedstocks with high nitrogen contents appears to be viable only by hydrotreating. Most of the alternative processes for denitrogenation rely on the removal of nitrogen-containing compounds without eliminating the nitrogen from these compounds. When nitrogen is not eliminated from the molecule and the hydrocarbon matter is rejected with the nitrogen, the loss of material from the oil increases as the nitrogen content of the oil increases. The loss of material from an oil containing 1 wt % N can easily be of the order 10−20 wt % of the oil, even for a process with high selectivity for removing nitrogen-containing compounds from the oil. There is a gap in the literature with respect to processes for eliminating the nitrogen from nitrogenrich products obtained by denitrogenation of oil.

Table 12. Denitrogenation of Real Feedstocks by Biodenitrogenation feed

strain

diesel oil

HY9

diesel oil

HY9

heavy gas oil

R. erythropolis ATCC 4277

heavy gas oil


heavy gas oil


heavy gas oil


process conditions 35 °C 125 rpm 4 days 0 g Tween 80 35 °C 125 rpm 4 days 0.175 g Tween 80 28 °C 200 rpm 18 h 40% HGO in water 28 °C 200 rpm 18 h 20% HGO in water 28 °C 200 rpm 18 h 60% HGO in water 28 °C 200 rpm 18 h 100% HGO


ref

12.9% N removed

165

19.3% N removed

165

43.2% N removed

166

40.2% N removed

166

31.2% N removed

166

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +1 780-655-3437. Fax: +1 780-492-2881. E-mail: [email protected]. ORCID 0% N removed

Arno de Klerk: 0000-0002-8146-9024

166

Notes

The authors declare no competing financial interest.

■

category is hydrotreating (section 3), which eliminates the nitrogen as ammonia. The other two processes that could potentially belong to this category are chemical conversion under thermal cracking conditions (section 7.4) and microbial conversion (section 8). (b) Separation processes that remove the nitrogen-containing compounds from the oil, so that the remaining oil is denitrogenated. In these processes the hydrocarbon matter associated with the nitrogen in the nitrogencontaining compounds is removed from the oil. The bulk of the processes for nitrogen removal by separation are in this category. The viability of applying these processes industrially depends on two factors: first, the nitrogen content of the oil, and second, the fate of the nitrogencontaining compounds that were removed from the oil. When the oil has a low nitrogen content, typically

Nitrogen Removal from Oil: A Review - Energy & Fuels (ACS

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