REFORMING OF ACETONE-BUTANOL-ETHANOL MIXTURE FOR H2 PRODUCTION: THERMODYNAMIC AND EXERGY ANALYSIS

Abstract

Recently, the gap between demand and supply of energy has been increased due to exorbitant consumption of non renewable energy sources all over the world. These energy sources are finite in quantity and release harmful contaminated gaseous emissions in the environment which also affect the health of all living creatures. To overcome these concerns, numerous efforts by multitudinous researchers for the advancement of novel and sustainable economical energy source are being made in the form of a great substitution as biorenewable source to ensure energy security with high efficiency. In past decades, H2 from biomass derived energy sources as feedstock is getting much attention for fuel cell operation to generate electricity. H2 can be produced by many processes viz. thermochemical water splitting, pyrolysis, coal gasification, hydrocarbon reforming, etc. Biorenewable energy sources or biofuels (methanol, butanol, ethanol, propane, glycerol, biogas, etc.) can be obtained as primary energy sources for the production of H2 as clean energy fuel by using fermentation of various feedstocks such as lignocellulosic biomass materials (sugarbeets, wheat, sugarcane, corncob, agricultural wastes, cornstalk, barley straw, switch grass, etc.), aqueous phase liquids of biomass pyrolysis, and micro-algae. For fermentation, Clostridium strain, is the most popular strain, and is thus used to ferment a variety of substrates (glucose, sucrose, xylose, lactose, and starch) and ligno-cellulosic biomass materials to obtain ABE (acetone-butanol-ethanol) mixture. ABE mixture contains 60 wt% water and remaining 40 wt% is ABE (Acetone:Butanol:Ethanol = 3:6:1) mixture in terms of mass ratio. Many separation techniques are used to recover butanol, acetone, and ethanol from fermentation broth such as distillation, adsorption, liquid-liquid extraction, reverse-osmosis, thermopervaporation, gas stripping etc. Amongst them, distillation is very popular and well-established unit operation, and therefore, was used by many chemical process plants to separate the components from this mixture. Generally, four components are separated by a series of four distillation columns. The first column is used to remove approximately 99.5 wt% acetone at operating pressure of 0.7 atm. Due to complex nature, butanol and ethanol obtained from ABE form azeotropes with water. Therefore, the other columns recover the rest butanol, ethanol, and water from the mixture which need high investment cost, and high operating cost because of high energy requirement. This high cost is the challenge for the researchers to innovate an optimum and economic alternate way. As an alternative, use of butanol-ethanol mixture, and acetone-butanol-ethanol mixture as obtained can be the better options. To the best of our knowledge, studies on the reforming of ABE mixture has not yet been done, and thus no information is available in the literature. ii Thus, the present study is focused on the utilization of these mixtures as biorenewable energy sources for the production of hydrogen to achieve the economic and environmental benefits. Steam reforming, dry reforming, oxidative steam reforming or autothermal reforming, etc. are several fuel processing technologies for the conversion of biorenewable sources to hydrogen. Among these, steam reforming has been chosen for the present study because it provides higher percentage of hydrogen as compared to other processes. Objectives of present research work are formulated as given below: Thermodynamic and Exergy Analysis of following biomass fermentation based Biofuels for efficient, environmental friendly and economic production of H2 by steam reforming: (i) Acetone, butanol, and ethanol as individual fuels, (ii) Butanol-ethanol (B–E) mixture as a fuel For this biofuel, oxidative steam reforming has also been studied. (iii) Acetone-butanol-ethanol-water mixture as a fuel Lastly, the comparative evaluation of these fuels has been done with respect to several parameters namely, yield of H2 , process conditions, carbon formation, steam to fuel molar ratio, thermal and exergy efficiencies, etc. First, steam reforming of acetone, butanol, and ethanol individually has been performed for the sake of completeness and comparison. Thereafter, the butanol-ethanol (B–E) mixture with water, is studied as a renewable biofuel for the production of clean energy carrier hydrogen by steam reforming process (SRB–E). The butanol–ethanol water mixture (BE mixture) contains 8.66 mole of butanol, 2.32 mole of ethanol and 89.02 mole of water. If butanol and ethanol mixture is considered as a renewable biofuel, then it consists of 78.87 mol% butanol and 21.13 mol% ethanol or in this renewable fuel butanol and ethanol are approximately in 80:20 ratio on molar basis. Likewise, water is about 8 times this renewable fuel on molar basis. The thermodynamic analysis of steam Reforming of B–E mixture is carried out by Gibbs free energy minimization method. The thermal and exergy efficiencies for the process are investigated to exploit the potential of B–E mixture for hydrogen production. For performance evaluation, the variational trends of moles of products (H2, CO, CO2, CH4, and carbon) are studied at equilibrium as a function of temperature (573–1473 K), pressure (1–10 atm), steam/fuel molar feed ratio (0–12) for composition of B–E mixture (50 to 90% B). For mixture (90% B), the maximum production of H2 (9.56 mol per mol of fuel) is achieved at 973 K temperature, 1 atm iii pressure, molar feed ratio of 12. Methane and carbon formation are negligible at high temperature (> 873 K) and molar feed ratio (> 5) for all B–E compositions. Energy required per mol of H2 is 50.77 kJ/mol for mixture (90% B) and is lower than that for steam reforming of butanol. The thermal efficiency is 70.07%, close to maximum for mixture (90% B), which is higher than butanol (69.89%), and ethanol (68.49%). For 90%B mixture, exergy efficiency (48.58%) is also comparable with that of butanol (48.69%) and ethanol (46.15%). This study proposes direct use of B-E mixture as a renewable fuel for H2 production. Further, the mixture of acetone-butanol-ethanol-water contains 5.24mol of acetone, 8.21 mol of butanol, 2.20 mol of ethanol, and 84.35 mol of water, respectively. In this biorenewable fuel; acetone, butanol, and ethanol are present approximately in the molar ratio of 33:52:15. A thermodynamic equilibrium analysis on steam reforming of this mixture has been performed to produce H2 by Gibbs free energy minimization method. The effect of process variables such as temperature (573-1473K), pressure (1-10atm), and steam/fuel molar feed ratio (FABE=5.5-12) have been investigated on equilibrium compositions of products, H2, CO, CO2, CH4 and solid carbon. The best suitable conditions for maximization of desired product H2, suppression of CH4, and inhibition of solid carbon are 973 K, 1 atm, steam/fuel molar feed ratio=12. Under these conditions, the maximum molar production of hydrogen is 8.35 with negligible formation of carbon and methane. Furthermore, the energy requirement per mol of H2 (48.96 kJ), thermal efficiency (69.13%), exergy efficiency (55.09%), exergy destruction (85.36 kJ/mol), and generated entropy (0.286 kJ/mol.K) have been achieved at same operating conditions. Generally, steam reforming (SR) is the most widely used for the production of hydrogen as it provides high yield of H2. However, SR processes are highly endothermic and so it is operated at relatively high temperature. Consequently, a large amount of heat from the external source is required to drive the process. High energy requirement directly influences the production cost. Another drawback is that the SR process suffers from severe catalytic deactivation due to the formation of carbon during the reaction which also lowers the H2 production efficiency. The problem of carbon formation can be analyzed in either of the two ways- one by introducing oxidant like O2 to the feed and other by developing coke resistant catalysts. The addition of O2 to the feed of SR process results in the oxidation of hydrocarbon fuel which is an exothermic process. This process does not require an external heat source and results in negligible carbon formation but at the expense of low hydrogen yield. Therefore, in order to reduce the external energy consumption and to achieve energy self sufficient system, oxidative steam reforming of butanol-ethanol mixture as fuel has also been explored in the present work to produce H2 by iv using Gibbs free energy minimization method. The effects of pressure (1-10 atm), temperature (573-1473 K), steam/fuel molar feed ratio (fO1= 9 and 12), O2/fuel molar feed ratio (fO2=0-3), and B-E mixture compositions (50-90%B) on equilibrium compositions of H2, CO, CO2, CH4, and carbon are performed. The maximum H2 yield (65.46% for fO2=0, and 58% for fO2=0.75) has been achieved at fO1=9, 90% B mixture, 1 atm, and 973 K. The yields of CO, CO2, and CH4 with respect to maximum H2 are 53.39%, 44.38%, and 2.23% for fO2=0, and 45.68%, 53.27%, and 1.05% for fO2=0.75, respectively. Energy required per mol of H2, thermal and exergy efficiencies for the process are also evaluated. The results of steam reforming and oxidative steam reforming of butanol-ethanol mixture in terms of maximum hydrogen production have been compared. It is found that the overall performance of oxidative steam reforming is less than that of steam reforming of butanol-ethanol mixture. The production of hydrogen and thermal efficiency of reformer are found less in oxidative steam reforming of B-E mixture (OSRB-E) as compared to its steam reforming. Further, the evaluation of various fuels such as methanol, glycerol, acetone, butanol, ethanol, butanol-ethanol mixture, acetone-butanol-ethanol mixture has been considered for comparison. It is our view that SR-ABE process is efficient, economical and environment friendly, and utilizes water rich ABE mixture as a renewable fuel for H2 production. The process utilizes ABE mixture as produced during fermentation and avoids the use of costly processing in the train of separation units.