Catalytic Steam Cracking of a Deasphalted ... - ACS Publications

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Catalytic Steam Cracking of a Deasphalted Vacuum Residue Using a Ni/K Ultradispersed Catalyst Fredy A. Cabrales-Navarro* and Pedro Pereira-Almao Department of Chemical and Petroleum Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive N.W., Calgary, Alberta T2N 1N4, Canada ABSTRACT: Catalytic steam cracking (CSC) of heavy hydrocarbons is seen as an alternative for further improvement upon conventional thermal cracking performance. In this work, upgrading of an industrial deasphalted vacuum residue via CSC was assessed in a bench-scale pilot plant resembling a visbreaking unit. The performance of a 400 ppm of Ni and 300 ppm K ultradispersed catalyst (UDC) formulation previously used for CSC of vacuum residue was evaluated for this nonasphaltene containing fraction. Reactivity experiments were conducted at temperatures within 435−445 °C and liquid hourly space velocities (LHSV) of 3−5.5 h−1 and operating pressure of 300 psig. A preliminary reactivity evaluation using isotopic water spanning temperatures between 423 and 445 °C was carried out to determine the conditions at which water splitting was occurring. Finally, lumped kinetic modeling including asphaltenes generation in the process was evaluated, and results were compared with previously reported thermal cracking experiments. Operating variables (T, LHSV) were found to have similar effects on the reactivity, as in thermal cracking for CSC of DAO. Even though water splitting was evidenced at temperatures above 430 °C, no significant improvement in the physical bulk properties of the liquid products was obtained for the catalytic experiments using the current formulation. This is attributed to the high degree of condensation reactions triggered at the range of temperatures evaluated. A global activation energy for the conversion of DAO (560 °C+) of 175 kJ/mol and a modeling error of 4.23% were determined. Asphaltenes generation was evidenced at a similar extent as that of thermal cracking from a kinetic point of view.

1. INTRODUCTION Extra-heavy oils and bitumen are characterized by having considerable amounts of asphaltene compounds that make difficult their processing under conventional refinery schemes. In this way, the implementation of deasphalting units, where asphaltene molecules are removed from the hydrocarbon fraction producing a deasphalted oil (DAO) stream, has become popular in the industry, demanding processes for upgrading these nonasphaltene containing heavy hydrocarbons, for which an alternative technology involving innovative ultradispersed catalysts and steam activation, such as catalytic steam cracking (CSC), may have a very important role. Catalytic steam cracking of heavy hydrocarbon fractions is a novel technique developed by Pereira-Almao et al.,1 in which a combination of conventional thermal cracking and mild hydrocracking processes was implemented. A bimetallic ultradispersed catalyst (UDC), typically combinations of alkali metals (such as K) and transition metals (such as Ni), is incorporated in the feedstock of interest in the form of unsupported particles at a submicronic scale. This bimetallic Ni/K catalyst has two functionalities: water splitting and hydrogenating. Hydrogen incorporation is much lower than in conventional hydroconversion processing. However, the process conditions are much less severe; thus, not only the capital cost to implement the process is lower, but also the operational expenses. These advantages could make it economically attractive. In 2001, Pereira-Almao et al.2 carried out a complete evaluation of the technical benefits and economic potential of this technology for Venezuelan extraheavy oil from the Orinoco belt and concluded that this upgrading technology was feasible in both aspects for the Venezuelan oil industry scenario. One very important © 2017 American Chemical Society

advantage of this technology compared to conventional thermal cracking (visbreaking) is that the conversion level can be considerably increased, which reflects in higher distillates content, while maintaining the stability of the product at an acceptable value.3−5 The following mechanism of reaction, resulting from a combination of steam reforming and thermal cracking reactions, has been proposed for the upgrading of heavy oil using ultradispersed catalysts3,5,6 Thermal Cracking R − R n′ → R n. + R n′ .

(1)

Catalytic Dissociation of Water CAT

H2O ⎯⎯⎯→ H. + OH.

(2)

Free Radicals Saturation CAT

R. + R n′ . + 2H. ⎯⎯⎯→ R − H + R n′ − H

(3)

Oxidation/Reforming CAT

R n′ . + 2OH. ⎯⎯⎯→ R n′− 1 + CO2 + H2

(4)

Condensation R n′ . + R. → R n′ − R n′ , R − R

(5)

Received: November 13, 2016 Revised: February 1, 2017 Published: February 6, 2017 3121

DOI: 10.1021/acs.energyfuels.6b03004 Energy Fuels 2017, 31, 3121−3131

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Energy & Fuels

Figure 1. Catalyst incorporation unit − Skid.

field upgrading of heavy and extra-heavy oils using CSC with ultradispersed catalysts are proposed, all of them aiming to reach Canadian pipeline transportability conditions, which are 19 API, 350 cSt at the carrier’s reference temperature, and 1 wt % olefins (1-decene equivalent). The said schemes have the advantage of using CSC instead of other more expensive processes, such as hydrocracking. One key characteristic of the invention is that the CSC process is applied to nonasphaltene containing fractions such as VGO and Deasphalted Oil (DAO), instead of directly to the residual fractions, such as VR, which allows one to go deeper in terms of conversion while having a stable final product. This is based on the fact that, even for CSC, processing of the asphaltenes in the reaction is usually the limiting step of the process in terms of stability. This technology development has highlighted the advantages of CSC as a feasible process in field upgrading schemes due to its relative simplicity, which motivates the continuation of research in this area, especially when nonasphaltene containing hydrocarbons are used as feedstock, since there is limited information available in the literature related to their reactivity and kinetics. This work targets the evaluation of the catalytic steam cracking route using a Ni/K catalyst formulation for upgrading a deasphalted vacuum residue obtained from a bitumen upgrading facility located in Northern Alberta, Canada. In the first preparation developed by Pereira-Almao,1 the catalyst is incorporated as a microemulsion. In this preparation, the microemulsion is decomposed to form metal oxides that remain suspended as particles in the hydrocarbon. A completely continuous preparation procedure was followed in this work using a Skid preparation unit. The main objective of the work is to assess the capabilities of this process when asphaltenes are not present in the feedstock as compared to the discussed previous works where the investigated materials were VRs or VGO. Additionally, this catalytic approach is compared to a baseline case of thermal cracking upgrading carried out in a

From the mechanism of reaction, the effect of the bimetallic catalyst on the different reactions is observed. The alkali catalyst (K) promotes reaction 2, increasing the production of hydrogen free radicals that are used in reaction 3 for the saturation of the hydrocarbon free radicals obtained from the thermal cracking reaction. The OH radicals also react with the hydrocarbon free radicals by means of a typical reforming process to produce carbon dioxide and hydrogen. These two steps are mainly promoted by the Ni metal catalyst. Hydrocarbon free radicals reacting with themselves according to the condensation reaction (eq 5) to form asphaltenes and coke is highly undesirable. Trujillo-Ferrer3 applied the catalytic steam cracking technology for the upgrading of Athabasca vacuum gas oil (VGO), in order to evaluate the applicability and the performance of the process to this particular hydrocarbon fraction from the Alberta Oil Sands reserves. With his work, Trujillo-Ferrer validated previous findings from the evaluations with Venezuelan extraheavy oil. It was possible to reach higher conversion levels than with thermal cracking by increasing the severity of the process, while keeping a steady value of the microcarbon content, which indicates lower tendency of the product to form coke by inhibition of the condensation reactions taking place during thermal cracking as well as lower gas yields. Fathi and PereiraAlmao5 studied the upgradability of an Arabian light vacuum residue (ALVR) at high space velocities, ranging from 5 to 10 h−1, using CSC. They were able to obtain a relative increase of 13% in conversion of the residual fraction with catalytic steam cracking at the asphaltenes stability limit using the K−Ni ultradispersed catalyst. In this study, the capability of the catalyst to dissociate water and to provide hydrogen for the process was corroborated using isotopic water. In the patent entitled “Systems and Methods for Catalytic Steam Cracking of Non-asphaltene Containing Heavy Hydrocarbons” by Nexen Energy ULC,6 three different schemes for 3122


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Figure 2. Schematic of the experimental setup for reactivity tests.

previous work,7 highlighting the benefits of a catalytic route for DAO upgrading with nonsupported catalysts from reactivity, product quality, and kinetic points of view.

reactor. A general scheme of the reactivity test unit is shown in Figure 2. Details of the experimental setup were provided in a previous work.9 2.3. Characterization Techniques. 2.3.1. Feedstock and Liquid Products. Common heavy oil characterization techniques were used to characterize the feedstock after catalyst incorporation as well as the liquid product samples collected from each experiment. Inductively coupled plasma (ICP) was used for the determination of metals concentration within the UDC formulation. DAO samples were prepared for analysis using microwave-assisted digestion following the ASTM D7455 norm. Subsequently, digested samples were analyzed following the ASTM D7691-05 method in a Thermoelectron IRIS Intrepid II spectrometer. Product distribution based on boiling point was carried out following the ASTM D7169-05 norm using an Agilent 6890N chromatograph as described by Carbognani et al.10 The P-value stability index as described by Di Carlo and Janis11 was used as reference to determine the state of peptization of asphaltenes. This method has been previously used to analyze samples of VR CSC and DAO thermal cracking.5,7 Density at a temperature of 15.6 °C was determined in a digital densitometer, Rudolph Research Analytical model DDM2911, as explained in more detail elsewhere.12 This density was used later on to determine the API following the definition in standard norm ASTM-D287-292. The viscosity at 60 °C for the feedstock and 25 °C for the liquid products after reaction was measured in a Brookfield viscometer model DV-II+ assembled with a TC-502 water recirculation system for temperature control. Hydrocarbon type distribution - saturates/aromatics/resins/asphaltenes (SARA) analysis was performed using pentane as the solvent for asphaltenes precipitation following the modification to the standard norm developed by Carbognani et al.13 for bitumen analysis. Solid contents were quantified by vacuum filtration of a 1:50 (w/w) solution of DAO after reaction and CHCl3 (99% Aldrich) in a 47 mm Millipore filter unit provided with 0.45 μm nylon membranes from Pall Corporation, as explained by Galarraga et al.14 The mass of solids after filtration was corrected by subtracting the theoretical amount of catalyst incorporated (Ni: as NiO and K as K2O). 2.3.2. Gas Samples. Gas chromatography analysis was conducted inline to quantify the light hydrocarbons and permanent gases generated for each experiment in an SRI Instruments chromatograph model 8610 C. Details of the specifications of the equipment were provided elsewhere. 9 A Pfeiffer OmniStar Quadrupole Mass Spectrometer (QMS) was used to analyze the gas streams from thermal and catalytic steam cracking reactions carried out using normal

2. EXPERIMENTAL METHODS 2.1. Catalytic Feed Preparation. An industrial deasphalted vacuum residue (DAO) obtained from a bitumen upgrading facility located in Northern Alberta, Canada, was used as feedstock. The material comes from solvent deasphalting of a mixture of unconverted vacuum residue from a thermally cracked DAO plus virgin vacuum residue. For the incorporation of the ultradispersed catalyst into the hydrocarbon feed, a Skid preparation unit available in the Catalysis for Bitumen Upgrading Group (CBU) in the Schulich School of Engineering at University of Calgary was used. The first step in the preparation of the catalytic feed was to adjust the viscosity of the deasphalted vacuum residue by using gasoline as diluent. This dilution was needed, since the equipment does not have the capability of heating up the feedstock, which is immobile at ambient conditions. Then, diluted DAO was mixed with the aqueous solutions of the corresponding metals that need to be incorporated, with nickel acetate and potassium hydroxide (KOH) being the precursor salts selected for incorporation of Ni and K particles. As illustrated in Figure 1, the first precursor solution added was the potassium hydroxide and then nickel acetate. This was to neutralize naphthenic acids present in the oil by reacting with KOH and producing naphthenic salts that are known to act as natural surfactants that help emulsification of the water in oil.8 The target concentration for the K and Ni was 400 and 300 ppm, respectively. The water-in-oil transient emulsion was subsequently passed through a high temperature reactor operating at 370 °C to decompose the metal precursors into metal oxides. Next, the stream was sent to a hot separator where water and diluent were separated from the oil, leaving the ultradispersed catalyst particles suspended in the DAO. The gaseous streams were passed through a condenser to liquefy water and diluent and then to a cold separator where the mixture of diluent and water was collected and gases produced in the decomposition of the precursor salts were separated. The gas stream was bubbled in an aqueous KOH solution trap to sweeten it before directing it to ventilation. 2.2. Reactivity Test Unit. Reactivity experiments were performed in a bench-scale pilot plant equipped with an up-flow open tubular 3123


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Energy & Fuels as well as isotopic water (H2O18 - 50% enriched in O18). As described by Fathi and Pereira-Almao,5 this benchtop equipment consists of a heated inlet system to prevent condensation of water and light hydrocarbons in the capillary gas inlet tube, a PrismaPlus mass spectrometer, a dry-compressing diaphragm vacuum pump, and a HiPace turbopump. The software enables scanning of atomic masses ranging from 1 to 300 amu as well as data processing. As shown in Figure 2, the QMS was connected to the gas outlet after the backpressure valve for continuous evolved gas analysis. For this work, the interest was to detect transfer of O18 coming from the splitting of the isotopic water molecule to the carbon radicals generated by thermal cracking to form isotopic CO2 as explained in detail in Section 2.4.1. 2.4. Experimental Plan. 2.4.1. Evidence of Water Splitting with Isotopic Water. In order to corroborate water splitting promoted by the Ni/K UD catalyst, a mixture of 50% isotopic water H2O18 and 50% demineralized water was used in a continuous catalytic steam cracking run. As explained in the gas characterization procedure, a QMS was coupled to the gas stream to monitor the signals of interest corresponding to the following compounds: H2 (m/e = 2), CO2 (m/e = 44), CO16O18 (m/e = 46), and CO218 (m/e = 48). The pilot plant was started using DAO as feedstock without any catalyst added, at a LHSV of 2 h−1 until a reaction temperature of 423 °C was reached. LHSV was defined for all experiments at pumping conditions as expressed in eq 6. Then, a stabilization period of 2 h was allowed. Feedstock was switched to DAO with Ni/K UDC (DAO+CAT), keeping the same experimental conditions and allowing enough time to purge all feedlines and displace the DAO present in the reactor (approximately 140 mL in feed lines and reactor). Subsequently, labeled water was added into the water reservoir and a 30 min purge time was allowed to evacuate the pure demineralized water from the feedlines. A data collection time of 30 min was allowed at this temperature. Next, the reaction temperature was increased up to 430 and 437 °C, always ensuring 30 min of data collection once the temperature was stable. Finally, LHSV was increased to 3 h−1 and temperature up to 440 °C, and reaction data was collected for 30 min. During this last step, labeled water was depleted and regular demineralized water was poured into the water reservoir to continue the experiment. Finally, the pilot plant underwent a systematic shutdown procedure at the end of the experiment.

Figure 3. Reaction pathways for kinetic modeling of DAO catalytic steam cracking. suggested by the authors, the following sequence of the activation energies was imposed on the mathematical modeling in order to incorporate chemical considerations within the complex mathematical algorithm: 2.5.1. Sequence. The activation energies of thermal cracking of heavier lumps are lower than lighter ones. Furthermore, a sequence was imposed on the activation energies in such a way that the activation energy of conversion of a lump to lighter products is higher than that to heavier products. In addition, constraints on the kinetic constant of gas production were imposed in such a way that gas production proceeds more readily from heavier lumps (k7 > k12 > k16).

3. RESULTS AND DISCUSSION 3.1. Catalytic Feedstock Preparation. In order to collect the amount of feedstock required to complete the experimental plan, three different batches of DAO+CAT were prepared in the skid preparation unit. For each batch, catalyst concentration was measured by ICP analysis to guarantee that an adequate concentration of particles close to the set target values has been incorporated into the feedstock. As presented in Table 1, where the absolute error (AE) of each metal concentration with respect to the target value is

LHSV [h−1] Flow of DAO + CAT at 140 °C and 300 psig [mL /h] = Reactor Volume [mL] (6) 2.4.2. Reactivity Experiments. For the catalytic evaluation of the DAO reactivity, the Ni/K catalyst matrix was incorporated into the feed using a skid preparation unit as explained in Section 2.1, targeting a concentration of 300 ppm of Ni and 400 ppm of K. As in previous works,3,4 a 5% wt of water was used. A total of three different temperatures were evaluated; these are 435, 440, and 445 °C. Three liquid hourly space velocities (LHSV) between 3 and 5.5 h−1 defined at pumping conditions (140 °C and 300 psig) were conducted at each temperature. In this way, a total set of 9 conditions that would allow having enough data to conduct a kinetic study were collected. The pressure of the tests (300 psig) was the same for all the evaluated conditions. In order to compare the reactivity at each condition, the conversion of hydrocarbons that boil above 560 °C defined in eq 7 was used:

Table 1. Catalyst Concentration in DAO+CAT Target Batch 1 Batch 2 Batch 3 a

RE - Ni [%]

K [ppm]

RE - K [%]

300 282 312 254

N/A 6.0 4.0 15.3

400 402 433 377

N/A 0.5 8.25 5.8

RE: relative error.

reported, the majority of the actual measured catalyst concentrations were close to the expected target values, with an average relative error of 6.7%. This proves the reliability and reproducibility of the results obtained in the skid unit when operated for different times for the incorporation of UDC particles. After validation of accurate catalyst concentrations on the different batches, all prepared materials were mixed together in a mixing tank and the resulting blend was fully characterized. Table 2 presents a summary of the characterization of the feedstock once it was processed in the skid preparation unit as well as the characterization of unprocessed DAO. As illustrated in the table, incorporation of the catalyst particles has a slight effect on the bulk properties of the feedstock. The main reason

Conversion %wt HC (560 °C + )Feed − %wt HC (560 °C + )Product = %wt HC (560 °C + )Feed × 100%

Ni [ppm]

(7)

2.5. Kinetic Modeling. Lump kinetic modeling including asphaltenes generation during the CSC process was done following the procedure discussed elsewhere for thermal cracking of the same feedstock under similar levels of steam.7 A schematic of the reaction pathways considered for kinetic modeling is shown in Figure 3. As 3124


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should be observed when isotopic water is used. A schematic of water splitting and O18 transference from isotopic water to CO2 is presented in Figure 4. Since the experimental setup was pressurized with industrial N2 before reaching the actual temperature of reaction, in the beginning of the run the concentration of reaction gases was very low, increasing gradually as they were produced from the CSC process. This increase in concentration during the experiment was reflected in an increase in the intensity of the signals measured by the QMS over time. As illustrated in Figure 5, where the QMS signals for normal CO2 (m/e = 44) and labeled CO2 (m/e = 46 and 48) are shown, which are directly related to the concentration of each species in the gas phase inside the unit, no change of any of the relative concentrations of any of the three species with respect to the others was observed at the initial temperature of 423 °C, using regular demineralized water. As mentioned previously, the increase of all signals at the same rate is due to the decrease of the N2 concentration in the unit. Similarly, no relative change was observed when the catalytic feedstock was incorporated either with regular water or with isotopic water at the same reaction conditions. All signals increase at the same rate up to 8 h of reaction due to the increasing total concentration of reaction gases and decreasing concentration of N2 in the unit. When the temperature was increased to 430 °C, a noticeable increase in the CO2 (m/e = 46) signal was observed. In the same way, the trend of the signal corresponding to CO2 (m/e = 48) increased as well at a temperature of 437 °C. When plotting the ratios of CO2 (m/e = 46) and CO2 (m/e = 48) with respect to CO2 (m/ e = 44) as depicted in Figure 5, the incorporation of isotopic oxygen atoms in the formed CO2 molecules can be easily observed due to the sharp slope change of the plot. This allow us to affirm that, for CSC of DAO using a Ni/K UDC formulation, water splitting starts occurring at temperatures above 430 °C. This is in agreement with previous results reported by Fathi et al.5 for catalytic steam cracking of ALVR where similar temperature levels were needed for water splitting to take place. The H2 and CO2 molar concentrations calculated from the GC analysis carried out at each condition allow one to verify the performance of the catalyst in a quantitative manner under the conditions of our tests. As seen in Figure 6, there is a constant increase in the hydrogen concentration in the gas phase as the reaction proceeds compared to thermal cracking. For CO2, its concentration increases from catalytic conditions C2 (423 °C and LHSV 2 h−1) to C4 (440 °C and LHSV 3

Table 2. DAO Feedstock Properties before and after Catalyst Incorporation Analysis

DAO

DAO+CAT (Ni/K)

API Gravity Microcarbon Residue [% wt] Viscosity @ 60 °C [cP] Viscosity @ 100 °C [cP] SARA Saturates [% wt] Aromatics [% wt] Resins [% wt] Asphaltenes-C5 [% wt] Cut Yields Naphtha (28−190 °C) [% wt] Kerosene (190−260 °C) [% wt] Diesel (260−343 °C) [% wt] LVGO (343−453 °C) [% wt] HVGO (453−560 °C) [% wt] Vacuum Residue (560 °C+) [% wt]

6.0 12.95 5339 300 4.0 75.0 20.2 0.7 0.2 2.7 5.3 7.0 17.2 67.7

6.7 14.41 3436 N/A 3.6 76.3 18.7 1.4 2.4 2.6 5.1 6.6 17.4 66.0

for the change in properties, such as API gravity and viscosity, is related to a certain amount of the gasoline used as diluent that remains in the feedstock after separation; SimDist results indicate it is on the order of 2.2%. An increase in MCR (after subtracting the theoretical amount of catalyst particles), and a marginal increase of asphaltenes and a decrease of resins is related to the thermal treatment at 370 °C required to decompose the precursor salts during catalyst incorporation, that even though it was very short in time, had a certain effect on the bulk physical properties. 3.2. Evaluation of Water Splitting. According to the water dissociation and oxidation/reforming reactions that are theoretically taking place during CSC processing (eqs 1 and 2), the catalyst would promote formation of hydrogen free radicals and also CO2 and H2 when compared to a merely thermal noncatalytic reaction used as the benchmark. In addition, when using isotopic water (H 2 O 18 ), water splitting can be corroborated by detecting transfer of O18 from the isotopic water to form either CO2 (m/e = 46), where only one O18 is present in the newly formed CO2 molecule, or CO2 (m/e = 48), where two O18 are present in the CO2 molecule. In this way, an overall increase in the H2 and CO2 signals should be observed for the catalytic process as compared to the thermal cracking reaction if regular demineralized water is used. In the same way, an increase in the relative signal of CO2 (m/e = 46) and CO2 (m/e = 48) with respect to normal CO2 (m/e = 44)

Figure 4. Mechanism of H2O splitting by K+. 3125


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Figure 5. Mass spectrometry analysis for catalytic steam cracking of DAO using O18 isotopic water.

h−1). This agrees with the MS results where transference of isotopic oxygen O18 from the isotopic water splitting was observed at T ≥ 430. This supports QMS results in a quantitative manner to affirm that the Ni/K UDC is indeed carrying out water splitting providing hydrogen to the reaction media. The main findings of these results are that a very specific window of operating conditions is needed in such a way that the UDC can actively dissociate water under the CSC processing of DAO. 3.3. Reactivity Analysis. Figure 7 shows the product distributions vs conversion for the CSC conditions explored in this work, compared with the thermal cracking baseline reported elsewhere.7 As observed, the product distribution for

CSC followed the same trends as in thermal cracking, with a significant decrease of the heavier cut DAO (560 °C+), a slight increase of distillates and naphtha, and a slight increase of the LVGO cut with respect to the feedstock. The main difference is that severity ranges attained in CSC are above the 43.3% conversion level achieved for thermal cracking at the evaluated operating conditions, reaching a maximum conversion of 51.4%. Additionally, a considerable yield of around 15% of asphaltenes was achieved at the most severe condition. This behavior suggests that the presence of the UD catalyst does not perform any hydrocarbon cracking function that would lead to an increase of conversion; this parameter is merely controlled by thermal cracking reactions. The function of the UDC is 3126


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way, the attainable conversion level is controlled in the same way as for thermal cracking and the increased conversions obtained were a result of the severe operating conditions needed to provide the adequate conditions for the UDC to catalytically dissociate water (T > 430 °C) Table 3 summarizes the reactivity results for all the conditions evaluated in this work as well as one thermal cracking condition at a conversion level of 43.3% reported in the work of Cabrales-Navarro and Pereira-Almao.7 The range of conversions of the 560 °C+ hydrocarbons achieved for the CSC reactions at the temperature and LHSV combinations explored in this work spanned from 37.1% to 51.4%. The limit for further increase on thermal cracking conversion beyond 43.3% is the stability of the asphaltenes, which was reported to be at the commonly accepted limit of P-value = 1.15. Operation below a P-value of 1.15 is not recommended, since asphaltenes deposition and coke formation inside the reactor and along the connecting lines would start to occur. A higher P-value for the CSC route would allow the increase of the severity of the process, producing in that way more valuable products. When carrying out the P-value analysis, a considerable presence of solid particles was observed in the samples, which made difficult its determination. These particles are at least in part agglomerates of catalyst particles, which would be removed by water washing and desalting in an industrial operation. In order to overcome this issue, removal of CHCl3 insoluble material was carried out using vacuum filtration in order to clean the sample with further removal of the solvent via rotoevaporation and subsequent removal of remaining solvent traces by placing the sample in a vacuum oven for 12 h at 50 °C. Due to the limited amount of cleaned sample, only the Pvalue at the limit value of 1.15 was done, as reported in Figure 8. When making a direct comparison of this parameter for thermal and catalytic processes at similar conversion levels (Thermal, C8 and C11), no improvement in the stability of the product was observed. At the higher levels of conversion reached under CSC processing, the stability of the liquid product was compromised, with P-values within the range 1.00 to 1.15. These results contrast with previous findings, where the

Figure 6. H2 and CO2 comparison for thermal tracking and catalytic steam cracking. Ci: Catalytic conditions.

Figure 7. Product distribution for thermal cracking and catalytic steam cracking.

solely to provide hydrogen for hydrocarbon radicals saturation and an eventual enhancement of the product quality. In this

Table 3. Summary of Experimental Conditions and Whole Liquid Product Characterization for Catalytic Steam Cracking of DAO Experiment Analysis

Thermal

C5

C6

C7

C8

C9

C10

C11

C12

C13

Temperature [°C] LHSV [h−1] Pressure [psig] Water content [% wt] HC Mass Balance [%] Conversion HC (560 °C+) [%] P-value

423 2 300 5 98.9 43.3

435 4 300 5 98.0 37.1

435 3.5 300 5 98.1 46.7

435 3 300 5 96.9 48.3

440 4.5 300 5 96.0 42.7

440 4 300 5 98.2 46.2

440 3.5 300 5 97.6 51.4

445 5.5 300 5 93.7 44.2

445 5 300 5 96.2 44.6

445 4.5 300 5 99.9 46.1

1.15

SARA

11.9 62.6 11.4 14.1

Pv > 1.15 11.8 67.4 8.3 12.5

8.1 16.54 2802 0.23

9.1 17.40 1981 0.17

Saturates [% wt] Aromatics [% wt] Resins [% wt] Asphaltenes-C5 [% wt] API Gravity MCR [% wt] Viscosity @25 °C [cP] Solids [% wt]

1< Pv < 1.15 13.2 64.7 7.2 14.0 9.4 18.02 1499 0.35

1< Pv < 1.15 13.4 63.6 7.2 15.9 9.6 18.37 1383 0.43 3127

1.15 13.2 65.3 7.2 14.3 8.6 18.38 1499 0.56

1< Pv < 1.15 12.8 64.5 7.3 15.4 10.0 18.15 1010 0.35

1< Pv < 1.15 14.4 61.5 7.1 17.0 10.1 18.57 832 0.61

1.15 14.1 59.3 13.1 13.6 9.0 17.20 1034 0.35

1< Pv < 1.15 14.3 63.1 7.2 15.4 9.5 17.91 1036 0.48

1< Pv < 1.15 13.9 62.7 5.6 17.8 10.3 18.75 1096 0.51


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Figure 9. Comparison of solid contents on the whole liquid products from catalytic steam cracking and thermal cracking.

This is due to at least two facts. First, the catalyst particles per se increase the solid content by acting as a solid seeds, getting encapsulated by polar compounds (resins and asphalthenes), as can be derived from the increased microcarbon content of the feed with catalyst, which passes from 12.95% to 14.57%. In total, this contribution could be sufficient for the difference with respect to thermal cracking. Second, the condensation reactions usually have high activation energies that result in their promotion at high temperatures. In this order of ideas, the experimental conditions needed for catalytic functions to take place with regard to water splitting have a counterproductive effect on the overall performance of the process. Another factor to take into account is the characteristics of this particular DAO that has a high content of aromatic hydrocarbons and comes industrially from a sequential thermal cracking/solvent deasphalting recycling loop that makes it much more prone to produce insoluble compounds and asphaltenes as compared to a virgin DAO. 7 Further investigations of catalyst formulations to address the challenges of this particular feedstock should be tasked. An interesting approach would be to develop a catalyst formulation able to dissociate water and hydrogenate at lower temperatures in a range where thermal cracking still occurs, but condensation reactions are minimized. Lastly, it is perhaps interesting to notice in Figure 9 that the trend of solid content increases with severity and seems to turn exponential (expectably for coke formation) whereas the catalysts test does not seem to provide a definitive trend, perhaps indicative of a rather invariable presence of solids less related to the reaction process. Comparing the behavior of important properties related to the quality of the whole liquid product, linear trends were obtained for the plot of the logarithm of the measured viscosity and MCR as a function of conversion with coefficients of correlation of 0.9689 and 0.9082, as illustrated in Figure 10. These are fairly good for experimental data collected at a pilot plant level. When comparing the slopes of the linear correlation obtained for each property as a function of conversion and with values reported for thermal cracking (Viscosity: −0.1013; and MCR = 0.1026), we can see that the slope obtained for the viscosity plot under CSC conditions (slope = −0.1017) is practically the same as that of thermal cracking. This corroborates that, for DAO, the extent of reduction of viscosity is driven mainly by conversion regardless of the type of

Figure 8. Microscope images of P-value for products gathered at 440 °C via catalytic steam cracking.

stability of the product was improved, allowing the process to reach higher levels of conversion, and consequently producing more valuable lighter hydrocarbons.4,5 However, the feedstock used in the previous reports was virgin vacuum residue from Arabian light oil, substantially different from the partially cracked DAO feedstock of the present tests. Possibly, the catalytic route has a better performance for asphaltene containing hydrocarbons, where the nature of this heavy and highly reactive fraction benefits CSC processing. In the same way, other indicators of the extent of the condensation reactions taking place where the catalyst could act as an inhibitor, such as asphaltenes content, are within similar levels at isoconversion. Also, no reduction in gas yield as a function of conversion was observed for the explored range of operating conditions. Undisclosed results suggest that UDC can act as seeds for coke formation and asphaltenes agglomeration if not operated at the appropriate conditions for vacuum residue processing. Nonetheless, no previous works in the area of CSC of DAO using UD catalyst have been found to confirm the same behavior with this feedstock. Additionally, even though the UDC is not promoting a significant enhancement of the bulk physical properties of the products, the chemistry of CSC is taking place and does not promote a detrimental effect. Based on the discussed results, the evidence indicates that the functionality of the UDC producing hydrogen from water splitting, attributed to the K particles, is working appropriately. However, its hydrogenating capabilities to saturate the hydrocarbon free radicals minimizing condensation reactions, which are attributed to the Nickel particles, are not taking place to an extent that could bring significant enhancement in product quality. Presumably, the submicronic particles are not in an active state or are rapidly poisoned by the formation of insoluble compounds due to the high severity needed for the process. As presented in Figure 9, where the content of CHCl3 insoluble materials as a function of conversion is compared for both CSC and TC, a marked difference in the trends is observed for both processes. At similar conversion levels, the amounts of solids present in the liquid product for the CSC reactions are more than twice as high as those produced via TC. 3128


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Energy & Fuels

Figure 12. Average absolute error by lump for kinetic modeling of catalytic steam cracking of DAO.

purposes. A very good fitting of the Arrhenius equation was obtained with coefficient of correlation close to 1 for all reaction pathways. The global activation energies of conversion of the DAO (560 °C+), HVGO, and LVGO lumps were 175, 178, and 188 kJ/mol. Also, the activation energies of a single lump have an increasing trend for lighter compared to heavier ones. For example, for DAO (560 °C+), the activation energy of reaction 2 is lower than that for reaction 3, and the activation energy for reaction 3 is lower than that for reaction 4, and so on. Production of the gas lump proceeds more readily from heavier lumps with k7 > k12 > k16. Under these circumstances, all the constraints implemented in the mathematical modeling were satisfied. The reactivity of the DAO (560 °C) lump at the explored operating conditions for CSC showed a marked selectivity toward production of HVGO, followed by Asp-C5 and gases. The kinetic constants toward these three lumps are 2 orders of magnitude higher for HVGO and Asp-C5 production and 1 order of magnitude higher for gas production. This contrasts with the extrapolated values for thermal cracking without UDC, where the selectivity of DAO (560 °C) is more distributed toward all the other lumps. Very importantly, the kinetic constant for production of Asp-C5 is lower for CSC. An advantageous catalytic route would, besides increasing the product quality, inhibit this reaction step while favoring the others, producing in this way higher amount of desirable products, preferably within the VGO range, while keeping the produced asphaltenes content as low as possible. Additionally, HVGO and LVGO are much more reactive toward the production of lighter compounds under CSC processing conditions. In this order of ideas, taking into consideration that the mechanism of conversion under CSC is driven by thermal cracking reactions and that there are evident differences between the kinetic parameters obtained from the extrapolated values of the thermal cracking model developed at temperatures up to 423 °C, it can be implied that there is a change of the reaction mechanism at high temperatures (T > 430 °C) combined with high LHSV (>3 h−1).

Figure 10. MCR and viscosity profile vs conversion of HC (560 °C+).

properties. Conversely, the slope obtained for the MCR plot (0.0813) is 21% lower than the one obtained for thermal cracking, which would be an indication of a superiority of CSC to form products less prone to coking. 3.4. Kinetic Study. Kinetic modeling of the catalytic steam cracking of the deasphalted vacuum residue, including asphaltenes generation, was successfully conducted following the procedure developed by Cabrales-Navarro and PereiraAlmao.7 A very good fitting between the experimental and modeled data was obtained, as illustrated in Figure 11, where a

Figure 11. Predicted model composition vs experimental composition for kinetic modeling of catalytic steam cracking of DAO.

coefficient of correlation of 0.9841 was achieved for the plot of modeled compositions vs experimental results. A global average absolute error (GAAE) of 4.23% was obtained, which is lower than the modeling errors obtained for thermal cracking. Considering the modeling errors for each lump as presented in Figure 12, the highest absolute errors were obtained for the lighter lumps (naphtha and gas) and for the heavier ones (DAO (560 °C+) and Asp-C5, which also agrees with the thermal cracking modeling. Table 4 presents the kinetic parameters for kinetic modeling of CSC as well as kinetic constants at 435 °C extrapolated for the kinetic modeling of thermal cracking for comparison

4. CONCLUSIONS It was found that the selection of the operating conditions is of paramount importance for catalytic steam cracking of deasphalted vacuum residue to have catalytic activity. The capabilities of the Ni/K ultradispersed catalyst as a water dissociation agent at high reaction temperatures (T > 430 °C) were evidenced. However, these severe conditions combined with the characteristics of the feedstock limit the correct performance of hydrogenating functions due to the increased 3129


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Energy & Fuels Table 4. Estimated Kinetic Parameters for Upgrading of DAO via Catalytic Steam Cracking TC

Catalytic Steam Cracking (CSC) Kinetic Constants [h−1]

Reaction r1 r2 r3 r4 r5 r6 r7 r8 r9 r10 r11 r12 r13 r14 r15 r16 l

a

DAO (560 C+) → Asp-C5 DAO (560 C+) → HVGO DAO (560 C+) → LVGO DAO (560 C+) → Diesel DAO (560 C+) → Kerosene DAO (560 C+) → Naphtha DAO (560 C+) → Gas HVGO → LVGO HVGO → Diesel HVGO → Kerosene HVGO → Naphtha HVGO → Gas LVGO → Diesel LVGO → Kerosene LVGO → Naphtha LVGO → Gas AAE [%] kglobal

Ea

Ln A

435 °Ca

435 °C

440 °C

445 °C

[kJ/mol]

[A in h−1 ]

1.2435 0.7690 0.7536 0.4157 0.1418 0.3490 0.4055 0.6229 0.1905 0.3337 0.1895 0.2626 0.3154 0.6165 0.3103 0.1354

1.0832 2.2328 0.0169 0.0177 0.0153 0.0144 0.2556 2.3934 0.7436 0.6494 0.4751 0.2205 0.5560 0.4570 0.9195 0.1599 4.57 3.6359

1.4015 2.8828 0.0207 0.0217 0.0188 0.0178 0.3190 2.9410 0.9195 0.8054 0.5915 0.2831 0.6875 0.5685 1.1492 0.2128 3.58 4.6823

1.6890 3.3177 0.0253 0.0266 0.0231 0.0219 0.3913 3.6028 1.1320 0.9967 0.7350 0.3621 0.8480 0.7044 1.4313 0.2818 4.52 5.4948

188 168 170 172 175 177 180 173 178 181 185 210 179 183 187 240

32.00 29.28 24.79 25.25 25.52 25.89 29.23 30.24 29.89 30.33 30.60 34.11 29.73 30.29 31.69 38.89 GAAE[%]

r2 0.9923 0.9740 1.0000 1.0000 1.0000 1.0000 0.9996 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 1.0000 4.23

Kinetic constants extrapolated from thermal cracking kinetic parameters reported elsewhere.

Catalysis for Bitumen Upgrading. Also, the contribution of facilities from the Canada Foundation for Innovation, the Institute for Sustainable Energy, Environment and Economy, the Schulich School of Engineering and the Faculty of Science at the University of Calgary are greatly appreciated.

production of coke. Nonetheless, no significant enhancement of product quality was observed with the current formulation at the same conversion levels obtained for thermal cracking. Product distributions obtained followed the same trend as in thermal cracking. Thus, UDC per se does not induce hydrocarbon cracking. In addition, as for thermal cracking, liquid product stability seems to be the major constraint to reach increased conversions. Operation beyond the 43% HC 560 °C+ conversion would destabilize products from the presence of the generated asphaltenes and would promote undesirable condensation reactions regardless of processing with the current catalyst formulation. In this way, the impact that an optimized catalyst formulation would have on product quality and consequently the added-value this might add to current thermal cracking technologies motivates the development of new catalyst formulations at more moderate conditions for this particular feedstock. From the kinetic point of view, a change to the mechanism was observed under the studied high severity conditions. The DAO (560 °C+) fraction is more selective toward production of HVGO compared to TC. Moreover, HVGO and LVGO fractions were determined to be much more reactive under CSC operating conditions.

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REFERENCES

(1) Pereira-Almao, P.; Marzin, R.; Zacarias, L.; Cordova, J.; Carrazza, J.; Marino, M. Steam conversion process and catalyst. U.S. Patent No. US5885441, 1999. (2) Pereira-Almao, P.; Flores, C.; Zbinden, H.; Guitian, J.; Solari, R. B.; Feintuch, H.; Gillis, D. Aquaconversion technology offers added value to E. Venezuela synthetic crude oil production. Oil Gas J. 2001, 99 (20), 79−85. (3) Trujillo-Ferrer, G. Thermal and Catalytic Steam Reactivity Evaluation of Athabasca Vacuum Gasoil. M.Sc. Thesis, University of Calgary, Calgary, 2008. (4) Fathi, M. M. Comparative Upgrading of Arab Light Vacuum Residuum via Aquaprocessing and Thermal Cracking. Ph.D. Thesis, University of Calgary, Calgary, 2011. (5) Fathi, M. M.; Pereira-Almao, P. Catalytic Aquaprocessing of Arab Light Vacuum Residue via Short Space Times. Energy Fuels 2011, 25 (11), 4867−4877. (6) Pereira-Almao, P.; Trujillo-Ferrer, G.; Peluso, E.; Galarraga, C.; Sosa, C.; Algara, C. S.; Lopez-Linares, F.; Carbognani-Ortega, L. A.; Zerpa-Reques, N. G. Systems and Methods for Catalytic Steam Cracking of Non-Asphaltene Containing Heavy Hydrocarbons. U.S. Patent No. US20130015100A1, 2013. (7) Cabrales-Navarro, F.; Pereira-Almao, P. Reactivity and Comprehensive Kinetic Modeling of Deasphalted Vacuum Residue Thermal Cracking. Energy Fuels 2016, submitted for publication. (8) Poteau, S.; Argillier, J.-F.; Langevin, D.; Pincet, F.; Perez, E. Influence of pH on Stability and Dynamic Properties of Asphaltenes and Other Amphiphilic Molecules at the Oil−Water Interface. Energy Fuels 2005, 19 (4), 1337−1341. (9) Cabrales-Navarro, F. Reactivity Study and Kinetic Modeling of Deasphalted Oil Upgrading via Thermal and Catalytic Steam Cracking. Ph.D. Thesis, University of Calgary, Calgary, 2016. (10) Carbognani, L.; Lubkowitz, J.; Gonzalez, M. F.; Pereira-Almao, P. High Temperature Simulated Distillation of Athabasca Vacuum

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fredy A. Cabrales-Navarro: 0000-0002-0076-2115 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), NexenCNOOC Ltd, and Alberta Innovates-Energy and Environment Solutions (AIEES) for the financial support provided through the NSERC/NEXEN/AIEES Industrial Research Chair in 3130


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Energy & Fuels Residue Fractions. Bimodal Distributions and Evidence for Secondary “On-Column” Cracking of Heavy Hydrocarbons. Energy Fuels 2007, 21 (5), 2831−2839. (11) Di Carlo, S.; Janis, B. Composition and visbreakability of petroleum residues. Chem. Eng. Sci. 1992, 47 (9), 2695−2700. (12) Ortega, L. C.; Rogel, E.; Vien, J.; Ovalles, C.; Guzman, H.; Lopez-Linares, F.; Pereira-Almao, P. Effect of Precipitating Conditions on Asphaltene Properties and Aggregation. Energy Fuels 2015, 29 (6), 3664−3674. (13) Carbognani, L.; Gonzalez, M. F.; Pereira-Almao, P. Characterization of Athabasca Vacuum Residue and Its Visbroken Products. Stability and Fast Hydrocarbon Group-Type Distributions. Energy Fuels 2007, 21 (3), 1631−1639. (14) Galarraga, C. E.; Pereira-Almao, P. Hydrocracking of Athabasca Bitumen Using Submicronic Multimetallic Catalysts at Near InReservoir Conditions. Energy Fuels 2010, 24 (4), 2383−2389.

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