Catalytic Hydrocracking of Fresh and Used Cooking Oil - Industrial

Aug 17, 2009 - Hydrocracking of vegetable oils is a prominent technology for the production of biofuels. This work compares the product yields and qua...
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Ind. Eng. Chem. Res. 2009, 48, 8402–8406

Catalytic Hydrocracking of Fresh and Used Cooking Oil Stella Bezergianni,* Spyros Voutetakis, and Aggeliki Kalogianni Chemical Process Engineering Research Institute (CPERI), Centre for Research & Technology Hellas (CERTH), 6th km Harilaou-Thermi Road, Thermi-Thessaloniki, Greece

Hydrocracking of vegetable oils is a prominent technology for the production of biofuels. This work compares the product yields and quality of hydrocracking fresh and used cooking oil under nominal operating conditions. Cracking, heteroatom removal and saturation reaction mechanisms are evaluated for both feedstock types and for three typical hydrocracking temperatures. The assessment of both feedstocks indicates that they are both suitable for high diesel yields with smaller kerosene/jet and gasoline/naphtha yields. As temperature increases, diesel selectivity increases for both feedstock types. However, the used oil feedstock exhibits higher kerosene/jet and naphtha selectivity at low temperatures (350 °C) and lower at the highest hydrocracking temperature (390 °C). 1. Introduction During the past decade national and international legislation has promoted the use of biofuels as an alternative source of transportation energy, since their production enhances sustainability and economic growth. Biodiesel is the most common biofuel employed in Europe. Biodiesel production in particular is worthy of continued study and optimization of production procedures because of its environmentally beneficial attributes and its renewable nature.1 Its production (mostly via transesterification) is mainly based on raw vegetable oil.2 Vegetable oil is produced from oil-based crops (rapeseed, soy-bean, palm, sunflower, etc.) which give moderate yields per hectare. However, there are several considerations associated with the existing biofuels production processes. The main byproduct glycerin via the transesterification method is both an economic but also an environmental consideration. Furthermore, large investments for biodiesel production units are required in order to ensure high efficiency.3 The most important consideration though is the price and availability of vegetable oil, since its cost might reach up to 75% of the total biodiesel production cost.4 The use of waste cooking oil for the production of biofuels can compensate to the former consideration since used (fried) vegetable oil, collected from restaurants and/or homes, costs at least 2-3 times cheaper than virgin vegetable oils.5 Several transesterification techniques and different types of catalysts have been employed in order to explore used cooking oil as a feedstock for biodiesel production. Alkali-catalyzed transesterification of a single step4-6 or of a two-step process7 gives high yields at moderate methanol/oil ratios and mild temperatures. Another interesting technology is based on heterogeneous solid catalyst-based transesterification8-10 which employs more environmentally benign catalysts and is effective for used cooking oil feedstocks, but requires higher temperatures. Enzymatic-catalysis-based transesterification exhibits significant yields at moderate operating conditions11-14 and shows significant potential. An alternative technology for biofuels production technology, which employs the existing infrastructure of petroleum refineries, is the catalytic hydroprocessing of vegetable oil.15,16 This technology has already several industrial applications.17-19 The biodiesel produced from hydrotreated vegetable oils has better * To whom correspondence should be addressed. Tel.: +30-2310498315. Fax: +30-2310-498380. E-mail: [email protected].

fuel properties than the biodiesel produced via transesterification. In addition, the use of biodiesel from hydroteated vegetable oils improves engine fuel economy, implying that this technology has a significant potential.15 Hydroprocessing of raw vegetable oil-heavy vacuum gas oil mixtures has been explored by employing hydrotreating20 and hydrocracking21 catalysts at nominal operating conditions. Hydrocracking of used cooking oil has also been studied as a potential process for biofuels production.22 This paper involves the investigation and comparison of raw and used cooking oil as hydrocracking feedstocks for biodiesel production. In particular, the effect of reactor temperature on product yields and quality is studied for both feedstocks. 2. Methodology For this study a small-scale pilot plant hydroprocessing unit of CPERI/CERTH was employed. This hydroprocessing unit has been employed for hydrotreating (HDS, HDN) and hydrocracking of various feedstocks, both of fossil and biobased origin. It mainly consists of a feed system, a fixed-bed reactor system and a product separation system, as schematically depicted in Figure 1; however, the unit is described in more detail in the author’s previous work.21 The main feedstock is mixed with high pressure hydrogen and enters the fixed bed reactor where the feed molecules undergo hydrotreating and/or hydrocracking reactions. The product exits the reactor in a mixed gas-liquid phase and is cooled before it enters a high pressure-low temperature separator, where the gas and liquid phases separate. For the evaluation of the hydrocracking effectiveness, both feed and product analysis is performed. The total liquid product and feedstock characterization involves several measurements including simulated distillation (Agilent 6890N-GC), density

Figure 1. Simplified schematic diagram of CPERI/CERTH hydroprocessing pilot plant.

10.1021/ie900445m CCC: $40.75  2009 American Chemical Society Published on Web 08/17/2009

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(Anton-Paar DMA 4500), sulfur and nitrogen (Antek 5000), carbon (CHN LECO 800), and hydrogen (Oxford Instruments NMR MQA 7020). Once total carbon, hydrogen, sulfur, and nitrogen weight percent are determined, the oxygen concentration is indirectly determined, assuming it is the only significant element contained in the product. The gaseous product is analyzed offline via a Hewlett-Packard 5890 Series II-GC, equipped with two detectors: a thermal conductivity detector (TCD) and a flame ionization detector (FID). The TCD is used for the analysis of H2, CO, CO2, O2, N2, and H2S while the FID is used for CH4 and C2-C6, hydrocarbons. For all experiments in this study, the same commercial hydrocracking catalyst was employed. The catalyst was presulphided according to the catalyst provider’s recommended procedure. Furthermore, in order to maintain constant catalyst activity, DMDS (di-methyl-di-sulfide) and TBA (tetra-butylamine) were added to achieve a specific sulfur and nitrogen concentration in each feedstock (∼2 wt % S and ∼700 wppm N). Each experiment (condition) was considered complete when the reactions reached steady state, usually after 5-6 days on stream. This was verified by monitoring the product density daily. Once the product density was stabilized, the individual effects of each experiment were considered stable and the study complete. The product collected during the last day of each study was analyzed in detail, as it represented that particular condition. In order to analyze the effectiveness of hydrocracking reactions, hydrocracking conversion is utilized, which is defined as the percentage of the heavy fraction of feed which has been converted to lighter products during hydrocracking: conversion(%) )

feed360+ - product360+ × 100 feed360+

(1)

where feed360+ and product360+ are the weight percent of the feed and product respectively which have a boiling point higher than 360 °C. Furthermore, in order to measure the hydrocracking effectiveness toward the production of a particular product instead of other products, the measure of selectivity is employed. Selectivity can be defined for different products (for example, diesel, gasoline, etc.) based on the boiling point range which defines these products. The selectivities of diesel, kerosene/jet, and naphtha production are defined in the following equations: diesel selectivity(%) )

product180-360 - feed180-360 × 100 feed360+ - product360+ (2)

kero/jet selectivity(%) )

naphtha selectivity(%) )

product170-270 - feed170-270 × 100 feed360+ - product360+ (3) product40-200 - feed40-200 × 100 feed360+ - product360+ (4)

where feed360+ and product360+ are the weight percent of the feed and product, respectively, which have a boiling point higher than 360 °C, feed180-360 and product180-360 are the weight percent of the feed and product, respectively, which have a boiling point between 180 and 360 °C (diesel molecules), feed170-270 and product170-270 are the weight percent of the feed and product, respectively, which have a boiling point between 170 and 270 °C (kerosene/jet molecules), and feed40-200 and product82-200

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Table 1. Properties of Fresh and Used Cooking Oil

3

density (kg/m ) S (wppm) N (wppm) H (wt %) C (wt %) O (wt %) refractive index bromine index

fresh cooking oil

used cooking oil

891.4 0.9 0.69 11.62 76.36 12.02 1.45513 49.2

896.6 38 47.42 11.62 76.74 11.6355 1.45511 46.60

are the weight percent of the feed and product, respectively, which have a boiling point between 40 and 200 °C (naphtha molecules). 3. Results For this study, a series of experiments was conducted aiming to identify the effects of utilizing fresh versus used commercial cooking oil feedstocks for hydrocracking in order to produce hybrid biofuels. The fresh cooking oil was a conventional commercial cooking oil (sunflower oil), while the used cooking oil was obtained mostly from local restaurants as well as households after extensively being used for frying. The comparison of the fresh and used cooking oil is presented via Table 1. From the comparison of the characteristics of the two oils in Table 1, it is apparent that the fresh cooking oil is not too different from the used cooking oil. The used cooking oil density is slightly higher than that of the fresh cooking oil, as cooking oil undergoes thermolytic, oxidative, and hydrolytic reactions.23 Interestingly, the used cooking oil has higher sulfur and nitrogen content, which are most likely caused by the hydrolysis and oxidation of existing sulfur and nitrogen compounds contained in food items that were fried. Another small difference is observed in the bromine index between the two oils, which shows higher values for fresh cooking oil over the used one. This indicates that fresh cooking oil contains a slightly higher number of unsaturated bonds over the used one. This observation is in agreement with literature,24 according to which used cooking oil exhibits polarity which increases upon repetitive frying. The two types of cooking oil were used as feedstocks in the hydroprocessing pilot plant unit of CPERI/CERTH (Figure 1). Two experimental runs were conducted in order to study the two feedstocks. Both experimental runs employed the same commercial hydrocracking catalyst and were conducted at identical conditions, i.e. reactor temperatures 350, 370, and 390 °C, system pressure 2000 psig (13789.5 kPa), liquid hourly space velocity (LHSV) 1.5 h-1, and H2-to-liquid feed ratio (H2/oil) of 6000 scfb (1068 nm3/m3). 3.1. Product Yields. The hydrocracking temperature is the most dominant operating parameter which defines catalyst performance as well as catalyst life. In this study three typical hydrocracking temperatures (350, 370, and 390 °C) are assessed in terms of hydrocracking activity as well as product yields and qualities. The comparison of the product yields (naphtha/gasoline and diesel) resulting from hydrocracking of the used and fresh cooking oil is given in Figures 2 and 3, respectively, for the three reactor temperatures studied. The product yields are determined by the simulation distillation data of the different products, considering the boiling range of naphtha/gasoline (40-200 °C) and diesel (180-360 °C). Diesel production is mostly favored as it is easily observed by comparing the two figures at all reactor temperatures. Furthermore, the naphtha/

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Figure 2. Gasoline yield of hydrocracking fresh and used cooking oil at three different temperatures. All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

Figure 3. Diesel yield of hydrocracking fresh and used cooking oil at three different temperatures. All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3). Table 2. Conversion and Selectivities of Hydrocracking Fresh and Used Cooking Oil at Three Different Temperaturesa 350 °C

conversion diesel selectivity kero/jet selectivity naphtha selectivity

370 °C

390 °C

used

fresh

used

fresh

used

fresh

72.62 98.12 6.21 3.04

80.37 98.87 5.57 2.24

73.97 94.30 8.66 7.24

78.41 95.31 8.67 6.12

81.88 92.04 20.04 12.17

85.09 91.04 22.24 12.68

a All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

gasoline yield increases with temperature, as higher temperatures favor cracking and therefore the production of lighter molecules, while diesel yields are certainly not favored. Interestingly used cooking oil exhibits higher naphtha/gasoline yields at moderate temperatures, while fresh cooking oil exhibits higher diesel productivity at all temperatures. This difference in naphtha/ gasoline and diesel productivities is up to a point expected as used cooking oil contains molecules of higher polarity, which enable the ion-exchange on the catalyst surface and, therefore, the overall cracking activity. The hydrocracking effectiveness, as it is differentiated for the two feedstocks, can also be identified by comparing the conversion and product selectivities in Table 2. The conversion and product selectivities are calculated from the distillation data

of each feed and product by employing eqs 1-4. With respect to conversion, hydrocracking is more severe for the fresh cooking oil at all reaction temperatures, which is expected because the fresh cooking oil contains a higher degree of saturated molecules and is also slightly lighter in terms of density than the used cooking oil feedstock. This shows that the amount of large molecules (with boiling point > 360 °C) which is converted to smaller and more useful molecules (with boiling point < 360 °C) is higher for fresh cooking oil. Furthermore conversion increases with temperature for both feedstocks considered, which is also excepted as temperature favors cracking reactions that mainly define conversion. Regarding product selectivities, the comparison differs for each product. Diesel selectivity is decreasing as temperature increases, which is expected as increasing temperature causes a higher degree of cracking reactions which leads to cracking not only feedstock molecules but also diesel molecules into lighter ones. On the other hand kerosene/jet and naphtha selectivies increase for higher temperatures due to the aforementioned effect. The two different trends are observed for both feedstocks and were also reported in the literature.21 Diesel selectivies for the two feedstocks do not show any appreciable difference at any hydrocracking temperature. However, in the case of the kerosene/jet and naphtha selectivity, the used oil feedstock shows higher values at low temperatures (350 °C) and lower values at the highest hydrocracking temperature (390 °C). 3.2. Heteroatom Removal. Even though sulfur and nitrogen are contained in insignificant amounts in the two feedstocks (see Table 1), sulfur (DMDS) and nitrogen (TBA) additives are artificially added in the feedstock to regulate catalyst activity, a typical procedure in hydroprocessing pilot plants. Besides cracking of heavy molecules to lighter ones, heteroatom removal (mainly sulfur, nitrogen, and oxygen) is also a significant measure of the overall hydrocracking effectiveness, as heteroatoms are not desired in the final products. The extent of heteroatom removal is expressed in Figure 4 as the percentage of the sulfur, nitrogen, and oxygen contained in the feed which has been removed during hydrocracking of each feedstock. Oxygen on the other hand is naturally contained as the feedstocks consist of esters, ketones, etc. Among the three elements, nitrogen is the most easily removed for both feedstocks, with its removal percent being over 99.5% for all cases. Sulfur is also effectively removed for both feedstocks, but used cooking oil exhibits a slightly higher extent. This difference is clearly seen in Table 3 with the product sulfur always being below 300 wppm in the case of used cooking oil, while it exceeds 400 wppm for fresh cooking oil. Sulfur removal improves significantly for increasing temperatures in the case of used cooking oil, exhibiting a similar trend with conversion. Finally, in terms of oxygen removal both feedstocks are performing similarly, even though the fresh cooking oil contains less oxygen initially. 3.3. Saturation. Another important reaction mechanism of all hydroprocessing processes is the saturation of double bonds, which enables the cracking reactions to take effect. Both used and fresh cooking oil contain a large amount of double bonds, indirectly indicated by the bromine index of the two feedstocks (Table 1). Hydrocracking of the two oils enables a high degree of saturation, as it is clearly depicted in Table 4, with fresh cooking oil showing the smaller bromine index values. Interestingly, however, the bromine index increases with increasing hydrocracking temperature for both cases of used and fresh cooking oil. This implies that saturation is not favored by

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Figure 4. Heteroatom (sulfur, nitrogen, and oxygen) removal percent via hydrocracking of used and fresh cooking oil at three hydrocracking temperatures. All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3). Table 3. Heteroatom (Sulfur, Nitrogen, and Oxygen) Removal Percent via Hydrocracking of Used and Fresh Cooking Oil at Three Hydrocracking Temperaturesa

Table 5. Comparison of Carbon-to-Hydrogen (C/H) Ratio of Hydrocracking Feed and Products at Three Different Hydrocracking Temperatures Using Fresh and Used Cooking Oila

product

used cooking oil fresh cooking oil

S (wppm) N (wppm) O (wt %) S (wppm) N (wppm) O (wt %)

feed

350 °C

370 °C

390 °C

27070 551.70 12.58 26690 642.4 11.57

278.6 1.95 1.86 1090 0.09 0.01

156.8 1.31 0.57 431.5 0.02 0.66

109.2 0.10 0.47 1140 0.08 -0.11

All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

Used Fresh

Feed

350 °C

Product 370 °C

390 °C

6.50 6.53

5.71 5.73

5.79 5.72

5.81 5.77

a All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

4. Conclusions

a

Table 4. Comparison of Bromine Index of Hydrocracking Feed and Products at Three Different Hydrocracking Temperatures Using Fresh and Used Cooking Oila product

used fresh

feed

350 °C

370 °C

390 °C

49100 47400

158.2 19.3

224.4 56.1

425 167

a All experiments were performed at P ) 2000 psig (13789.5 kPa), LHSV ) 1.5 h-1, and H2/oil ) 6000 scfb (1068 nm3/m3).

temperature, which can be expected as saturation is a competing reaction mechanism to the cracking one. The saturation extent can also be observed via the carbonto-hydrogen ratio (C/H). In Table 5 the C/H ratio of both feedstocks and their corresponding products are given, for the three different hydrocracking temperatures, where it is observed that the C/H ratio is decreased for both used and fresh cooking oil feedstocks. However, the hydrocracking products of the used cooking oil have a slightly higher C/H ratio, compared to the products of the fresh cooking oil. Moreover, the C/H ratio increases with hydrocracking temperature, indicating that temperature does not favor saturation.

Hydrocracking of vegetable oils is a prominent process for the production of hybrid biofuels. This work regards both fresh and used vegetable cooking oils for hydrocracking via a commercial catalyst, considering three typical hydrocracking temperatures. Both feedstocks are able to give good product yields and quality, while exhibiting preference to diesel production. Nevertheless, diesel production is not favored by hydrocracking temperature, which increases the production of lighter products (kerosene/jet and naphtha/gasoline). In terms of heteroatom removal, nitrogen is the most easily removed element in both used and fresh cooking oil. Sulfur however is more easily removed from used cooking oil while oxygen is removed adequately for both used and fresh cooking oil. Finally a large degree of saturation is achieved via hydrocracking of both oils, which decreases however with temperature, as saturation reactions compete with the cracking ones. Literature Cited (1) Refaat, A. A.; Attia, N. K.; Sibak, H. A.; El Sheltawy, S. T.; El Diwani, G. I. Production optimization and quality assessment of biodiesel from waste vegetable oil. Int. J. EnViron. Sci. Technol. 2006, 5, 75. (2) Panorama of Energy - Energy statistics to support EU policies and solutions, KS-76-06-604-EN-N. Eurostat statistics books; European Union Publications: Luxembourg, 2007. (3) Knothe, G.; Van Gerpen, J.; Krahl, J. The biodiesel handbook; AOCS press: Campaign, IL, 2005.

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cooking oils by transesterification using lipase immobilized onto a novel microporous polymer. Bioresour. Technol. 2009, 100, 1983. (15) Huber, G. W.; Corma, A. Synergies between bio- and oil refineries for the production of fuels from biomass. Angew. Chem., Int. Ed. 2007, 46, 7184. (16) Stumborg, M.; Wong, A.; Hogan, E. Hydroprocessed vegetable oils for diesel fuel improvement. Bioresour. Technol. 1996, 56, 13. (17) Neste oil corporation. http://www.nesteoil.com (accessed August 2007). (18) HPinnovations. Process incorporates renewables as part of refining operations. Hydrocarbon Process. 2006, 85, 33. (19) Hayashi, H. Development of bio-hydrofined diesel fuel. 14th Annual Fuels and Lubes Asia Conference, Grand Hyatt, Seoul, Korea, March 5-7, 2008, p 1. (20) Huber, G. W.; O’Connor, P.; Corma, A. Processing biomass in conventional oil refineries: Production of high quality diesel by hydrotreating vegetable oils in heavy vacuum oil mixtures. Appl. Catal. A: Gen. 2007, 329, 120. (21) Bezergianni, S.; Kalogianni, A.; Vasalos, I. A. Hydrocracking of vacuum gas oil-vegetable oil mixtures for biofuels production. Bioresource Technol. 2009, 100, 3036. (22) Bezergianni, S; Kalogianni, A. Hydrocracking of Used Cooking Oil for Biofuels Production. Bioresource Technol. 2009, 100, 3927. (23) Nawar, W. W. Chemical Changes in Lipids Produced by Thermal Processing. J. Chem. Educ. 1984, 61 (4), 229–302. (24) Guesta, F. J.; Sanchez-Muniz, C.; Polonio-Garrido, S.; Varela-Lopz, A. R. Thermoxidative and Hydrolytic Changes in Sunflower Oil Used in Frying with a Fast Turnover of Fresh Oil. J. Am. Oil Chem. Soc. 1993, 70, 1069.

ReceiVed for reView March 18, 2009 ReVised manuscript receiVed July 21, 2009 Accepted August 1, 2009 IE900445M