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Jalama, Kalala

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University of Johannesburg Institutional Repository

  Abstract— The equilibrium composition of biofuel formed by cracking of a modeled vegetable oil has been predicted using the system Gibbs energy minimization. In particular the effects of temperature and feed dilution by inert on the equilibrium product selectivities have been studied. Methane, aromatics and small amounts of H2, CO and CO2 were predicted in the 350 – 600oC range. The increase in temperature decreases the CH4, aromatics and CO2 molar proportion in the equilibrium product. Feed dilution slightly increases the carbon selectivity to CO in the 350-450oC range, decreases the carbon selectivity to CO2 and has no significant effect on the carbon selectivity to CH4 and aromatics.  Index Terms— Cracking, equilibrium product, vegetable oil   I. INTRODUCTION NERGY from renewable sources is currently among the most researched topics around the globe because of the challenges that the world is facing. These challenges include high energy demand, especially in fast developing regions, depletion of fossil fuel reserves and environmental challenges such as CO2 emissions which contribute to the global warming and climate change.  Biofuel produced from plant-derived biomass such vegetable oils is considered “CO2 neutral” since they are from plant sources which typically remove CO2 from the atmosphere, and release the same amount when burnt [1]. Vegetable oils can undergo processes such as transesterification, cracking, steam reforming, etc. in order to produce biofuel. Taufiqurrahmi and Bhatia [1] have recently reviewed the catalytic cracking of edible and non-edible oils for the production of biofuels. They have indicated that catalytic cracking is one of the most efficient methods to produce biofuel, especially biogasoline, by cracking of vegetable oil in the presence of suitable catalyst.     Although various operating conditions have been reported in literature, very little is known on the equilibrium composition of the formed biofuel under different cracking conditions. This information is very useful for process  Manuscript received July 23, 2012; revised August =10, 2012. This work has benefited from the financial support of the Dean’s office of the Faculty of Engineering and The Built Environment, University of Johannesburg.  K. Jalama is with the Department of Chemical Engineering in the School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, Johannesburg 2028, South Africa  (Phone: +27-11-559-6157; fax: +27-11-559-6340; e-mail: kjalama@uj.ac.za).  design and development. This study aims at predicting the equilibrium composition of biofuel formed by vegetable oil cracking. In particular the effect of temperature and feed dilution by inert on the equilibrium product selectivities will be studied.  II. METHODOLOGY     The prediction of the equilibrium product composition for vegetable oil cracking process was performed with ChemCad 6.4 simulation package. The feed to the cracking reactor was chosen as 1 kmol/h of vegetable oil without and with various degrees of dilution by an inert gas. The effect of temperature on the equilibrium product was studied in the 350 – 600oC range and oil to inert molar ratios of 1:1, 1:10 and 1:100 were used to determine the effect of feed dilution on the equilibrium product.   The selected process pressure was 1 bar throughout this study. The vegetable oil was modelled by triolein and the cracking process was modelled by Gibbs free energy minimization in ChemCad. The Gibbs reactor model is based on the principal that at chemical equilibrium the total Gibbs energy of the system has its minimum value. By attempting to minimize the total energy of the system, individual equilibrium constants are not considered. Rather, the possible reaction species are noted, and the distribution of these species is established using a general mathematical technique to give a minimum free energy for the system [2]. The selected possible components in the predicted equilibrium product included H2, CO, CO2, alkanes (C1 to C15), olefins (C2 to C15), cyclic hydrocarbons, aromatic compounds, light ketones, alcohols, carboxylic acids and solid carbon. Except for the solid carbon, all the selected components with their physical and chemical properties were available in ChemCad 6.4 components database. The solid carbon was added to the ChemCad components database and was modelled as graphite. The graphite heat capacity polynomial coefficients determined by Butland and Maddison [3] were used in the simulation. The enthalpy and Gibbs energy of formation for the solid carbon were entered as 0 kJ/mol.                                                                                                     Vegetable Oil to Biofuel by Cracking: Equilibrium Product Selectivities K. Jalama EProceedings of the World Congress on Engineering and Computer Science 2012 Vol II WCECS 2012, October 24-26, 2012, San Francisco, USAISBN: 978-988-19252-4-4 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCECS 2012 III. RESULTS AND DISCUSSION A. Effect of temperature     The effect of temperature on vegetable oil cracking equilibrium product composition is summarized in table 1.  Complete conversions for triolein were predicted in the 350-600oC range and the major products were methane (56.9 to 62 mol.%) and aromatics (19.1 to 22.3 mol.%) with a combined proportion accounting for ca. 76 to 84 mol.% of the total product. The remaining proportion was mainly  TABLE I VEGETABLE OIL CRACKING EQUILIBRIUM PRODUCT COMPOSITION AS FUNCTION OF TEMPERATURE Components Chemical formula 350oC 400oC 450oC 500oC 550oC 600oCHydrogen (H2) 0.1112 0.2741 0.5996 1.1902 2.1743 3.6942Carbon monoxide (CO) 6.8394 12.9617 17.7263 19.6706 20.1752 20.2029Carbon dioxide (CO2) 7.8850 4.3775 1.6440 0.5001 0.1539 0.0520Water (H2O) 0.0061 0.0076 0.0073 0.0060 0.0047 0.0036Triolein (C57H104O6) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Coke C0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Alkanes 62.8087 61.5219 60.3259 59.4046 58.3746 56.9164Methane (CH4) 62.7772 61.4885 60.2908 59.3677 58.3362 56.8771Ethane (C2H6) 0.0313 0.0333 0.0350 0.0367 0.0383 0.0392N-propane (C3H8) 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001N-butane (C4H10) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-pentane (C5H12) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-hexane (C6H14) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-heptane (C7H16) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-octane (C8H18) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-nonane (C9H20) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-decane (C10H22) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-Undecane (C11H24) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-dodecane (C12H26) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-tridecane (C13H28) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-tetradecane (C14H30) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000N-pentadecane (C15H32) 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 0.0000 0.0000 0.0000 0.0000Olefins 0.0004 0.0013 0.0034 0.0083 0.0181 0.0359Ethylene (C2H4) 0.0004 0.0012 0.0033 0.0080 0.0175 0.0348Propylene (C3H6) 0.0001 0.0002 0.0003 0.0006 0.00111-butene (C4H8)1,3-butadiene (C4H6)1-pentene (C5H10)1-hexene (C6H12)1-heptene (C7H14)1-octene (C8H16)1-nonene (C9H18)1-decene (C10H20)1-undecene (C11H22)1-dodecene (C12H24)1-tridecene (C13H26)1-tetradecene (C14H28)1-pentadecene (C15H30)Cyclic hydrocarbons 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Cyclopentene (C5H8) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Cyclopentane (C5H10) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Cyclohexene (C6H10) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Cyclohexane (C6H12) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Cycloheptene (C7H12) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Cycloheptane (C7H14) 0.0000 0.0000 0.0000 0.0000 0.0000 0.00000.0000 0.0000 0.0000 0.0000 0.0000 0.0000Aromatics 22.3493 20.8560 19.6935 19.2202 19.0992 19.0949Benzene (C6H6) 19.3317 18.6104 17.9544 17.7893 17.8756 18.0266Toluene (C7H8) 2.8490 2.1436 1.6729 1.3839 1.1883 1.0408Ethylbenzene (C8H10) 0.0105 0.0075 0.0057 0.0046 0.0038 0.0033O-xylene (C8H10) 0.0381 0.0235 0.0155 0.0111 0.0084 0.0066M-xylene (C8H10) 0.0858 0.0507 0.0321 0.0222 0.0164 0.0125P-xylene (C8H10) 0.0341 0.0203 0.0130 0.0090 0.0067 0.00510.0000 0.0000 0.0000 0.0000 0.0000 0.0000Short chains alcohols, ketone and carboxylic acids 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Methanol (CH4O) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Ethanol (C2H6O) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Propanol (C3H8O) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Acetone (C3H6O) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Acetic acid (C2H4O2) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Acrylic acid (C3H4O2) 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000Total 100 100 100 100 100 100Products Product molar composition [%] Proceedings of the World Congress on Engineering and Computer Science 2012 Vol II WCECS 2012, October 24-26, 2012, San Francisco, USAISBN: 978-988-19252-4-4 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCECS 2012 accounted for by H2, CO and CO2. Traces of water and light olefins (ethylene and propylene) were also predicted. No solid carbon, alcohols, ketones and carboxylic acids were predicted. The data show that the increase in temperature decreases the CH4, aromatics and CO2 formation at equilibrium. For example at 350oC compositions of ca. 62.8% for CH4 , 22.35% for aromatics and 7.9% for CO2 were predicted compared to ca. 56.9%, 19.1%, 0.052% for CH4, aromatics and CO2 respectively at 600oC. On the other hand CO and H2 have been predicted to increase with the increase in temperature. The aromatics can be condensed as liquid fuel and the remaining gas mixture can be used as combustible or can undergo subsequent transformation to, for example, produce syngas with H2/CO ratios of ca. 2/1 suitable for Fischer-Tropsch process to produce liquid fuel. Fig. 1 shows the selectivity of the carbon in the vegetable oil to CO, CO2, CH4 and aromatics as a function of temperature. The selectivity to CH4 almost remains constant throughout the entire selected temperature range while that of aromatics and CO2 decreases with temperature and becomes temperature insensitive from 500oC and above. The carbon selectivity to CO increases with the temperature and also becomes temperature insensitive from 500oC and above. The predicted data suggest that temperature from 500oC and above must be used to avoid CO2 formation. At these temperatures, the carbon selectivities to CH4 and aromatics all combined account for 90%.  Fig. 1. Carbon selectivity to CO, CO2, CH4 and aromatics B. Effect of feed dilution by inert     For an industrial application the vegetable oil cracking will require a catalyst that will be carried by an inert gas. For this reason, this section reports the effect of an inert gas on the vegetable oil cracking. N2 was selected as inert and the effects of feed dilution on carbon selectivity to CO, CO2, CH4 and aromatics are respectively presented in tables 2 to 5.  TABLE II EFFECT OF FEED DILUTION ON CO SELECTIVITY (CARBON BASED %) AS FUNCTION OF TEMPERATURE N2: Oil ratio  Temperature [oC]       350 400 450 500 550 600 1:1   3.239 5.796 8.617 9.934 10.345 10.465 10:1   3.669 6.910 9.242 10.149 10.410 10.486 100:1   5.997 8.964 10.127 10.422 10.495 10.515   TABLE III EFFECT OF FEED DILUTION ON CO2 SELECTIVITY (CARBON BASED %) AS FUNCTION OF TEMPERATURE N2: Oil ratio  Temperature [oC]       350 400 450 500 550 600 1:1   3.642 2.363 0.953 0.294 0.090 0.030 10:1   3.427 1.807 0.641 0.187 0.057 0.019 100:1   2.264 0.780 0.199 0.052 0.015 0.005   TABLE IV EFFECT OF FEED DILUTION ON CH4 SELECTIVITY (CARBON BASED %) AS FUNCTION OF TEMPERATURE N2: Oil ratio  Temperature [oC]       350 400 450 500 550 600 1:1   29.265 29.716 30.174 30.256 30.030 29.554 10:1   29.386 29.966 30.293 30.256 29.941 29.370 100:1   29.924 30.369 30.384 30.070 29.456 28.488   TABLE IV EFFECT OF FEED DILUTION ON AROMATICS SELECTIVITY (CARBON BASED %) AS FUNCTION OF TEMPERATURE N2: Oil ratio  Temperature [oC]       350 400 450 500 550 600 1:1   63.853 62.124 60.253 59.507 59.517 59.914 10:1   63.517 61.316 59.821 59.398 59.572 60.086 100:1   61.814 59.885 59.286 59.447 60.012 60.949       A slight increase in carbon selectivity to CO was predicted with the increase in feed dilution in the range of 350 and 450oC. No significant effect of dilution on the carbon selectivity to CO was noted at temperatures of 500oC and above (table 2). Feed dilution decreases the loss of carbon through CO2 formation as indicated by the decreases in selectivity to CO2 at higher dilutions (table 3). No significant effect of feed dilution was predicted for the carbon selectivity to CH4 (table 4) and aromatics (table 5).  Proceedings of the World Congress on Engineering and Computer Science 2012 Vol II WCECS 2012, October 24-26, 2012, San Francisco, USAISBN: 978-988-19252-4-4 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCECS 2012 IV. CONCLUSION Cracking of vegetable oil in the 350 – 600oC temperature range was predicted to mainly produce CH4 and aromatics and small amounts of H2, CO and CO2. The increase in cracking temperature decreases the CH4, aromatics and CO2 molar proportion in the equilibrium product. Feed dilution slightly increases the carbon selectivity to CO in the 350-450oC range, decreases the carbon selectivity to CO2 and has no significant effect on the carbon selectivity to methane and aromatics.  REFERENCES [1] N. Taufiqurrahmi,  S. Bhatia, “Catalytic cracking of edible and non-edible oils for the production of biofuels”, Energy Environ. Sci., vol. 4, 2011, pp. 1087-1112 [2] ChemCad 6 Help and References manual, Chemstations, Inc. 11490,Westheimer  Road, Suite 900, Houston, Texas 77077 [3] A.T.D. Butland,  R. J. Maddison, “The specific heat of graphite and evaluation of measurements”, J. Nuclear. Mat., vol. 49, 1973/74, pp. 45-61   Proceedings of the World Congress on Engineering and Computer Science 2012 Vol II WCECS 2012, October 24-26, 2012, San Francisco, USAISBN: 978-988-19252-4-4 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online)WCECS 2012

Vegetable oil to biofuel by cracking : equilibrium product selectivities

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Vegetable oil to biofuel by cracking : equilibrium product selectivities

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