Advanced Chemical Engineering

Reforming of liquid fuels to hydrogen is being considered to enable hydrogen-powered fuel cells to be used to generate remote power. For example, the military is interested in using hydrogen fuel cells to replace conventional batteries, which have a low power density and a short lifetime. Reforming, then, could be used to transform military fuels to hydrogen to power fuel cells. Autothermal reforming is one means for converting liquid fuels to hydrogen. In this process, the liquid fuel is reacted with oxygen and water to produce hydrogen. The overall reaction involves multiple reactions. Assuming that isooctane (2,2,4 trimethyl pentane) is the fuel, the overall reaction scheme can be written: C8H18 + 12.5 O2  8 CO2 + 9 H2O C8H18 + 8 H2O  8 CO + 17 H2 C8H18 + 8 CO2  16 CO + 9 H2 C8H18 + 16 H2O  8 CO2 + 25 H2 CO + H2O  CO2 + H2 Your assignment is to model the reforming of isooctane in a packed-bed reactor. Reaction Kinetics Pacheco, et. al. (Pacheco, 2003) fit experimental data for a proprietary Pt/CeO2 catalyst to obtain reaction kinetics for each of these reactions. The reaction rate laws they used are shown in Table 1, where the reaction order corresponds to that shown above, and the rate law constants are given in Table 2. 2 Table 1. Reaction rate laws for all reactions involved in isooctane reforming r1 = k1Pic8PO2 ( )         ++++ − = 2 22 88 222 2 3 28 2 5.2 2 2 2 1 / / COCO HH i Ci C HOHOH i C HOH CO H PPKPKPKPK KPPPP P k r         = − 283 2 2 2 2833 1 iC CO CO H iC CO PPK PP PPkr ( )         ++++ − = 2 22 88 222 42 4 2 2 28 5.3 2 4 1 / 4 / COCO HH i Ci C HOHOH i C HOH CO H PPKPKPKPK KPPPP P k r ( )         ++++ − = 2 22 88 222 2 522 2 5 5 1 / / COCO HH i Ci C HOHOH CO HOH CO H PPKPKPKPK KPPPP P k r Table 2. Kinetic parameters for all reactions involved in isooctane reforming Parameter Pre-exponential factor or KTR Activation energy and heat of adsorption (kJ/mol) k1 (mol/gcat/s/bar2 ) 2.58E+08 166 k2 (mol bar0.5/gcat/s) 2.61E+09 240.1 k3 (mol/gcat/s/bar2 ) 2.78E-05 23.7 k4 (mol bar0.5/gcat/s) 1.52E+07 243.9 k5 (mol/gcat/s/bar) 1.55E+01 67.1 KH2O (dimensionless) 1.57E+04 HH2O= 88.7 KH2 (dimensionless) 0.0296 (TR=648 K) HH2= -82.9 KCO (dimensionless) 40.91 (TR=648 K) HCO= -70.65 KiC8 (dimensionless) 0.1791 (TR=823 K) HiC8= -38.28 Note: For H2, CO, and iC8, K is found from:               − − = R R T TTR H KK R 11 exp For H2O, K is found from: TRHKK )//exp( o −= R Note that the equilibrium constants are calculated assuming reaction stoichiometry for methane, not iso-octane. For example, K2 is the equilibrium constant for the following reaction: C8H18 + H2O  CO + 3 H2