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 3 Reactor Model and Simulation – Base Case The base case reactor design is to be based on the following reaction conditions: Reactor temperature – 700oC Reactor pressure – 5 psig Catalyst size – 0.51 mm H2O/C (mol/mol in the feed) - 1.43 O2/C (mol/mol in the feed) – 0.42 O2 is supplied as air No pressure drop Isooctane molar flow rate = 0.00269 kmol/hr Assume that there are no internal pore diffusion limitations and no external heat or mass transfer limitations. The desired production rate of hydrogen is 0.033 kmol/hour. Report the mass of catalyst needed to achieve this production rate and the concentration profile in the reactor. Discuss the trends seen in the profiles. Reactor Model – Adiabatic Next you should consider the same reactor, but operating adiabatically rather than isothermally. Assume the inlet temperature is 700oC. Other than the change in heat transfer mode, you may make the same assumptions as you did for the base case. Report the mass of catalyst needed to achieve the desired production rate and the concentration and temperature profiles in the reactor. Reactor Model – Optimized Finally, you should optimize the reactor to achieve the desired hydrogen production rate with a minimum of carbon monoxide production. You should use your adiabatic reactor model. You may change as many of the base case variables as you want, although you should keep in mind the feasibility of those changes. You should report the same information you reported for the base case and adiabatic models, and should also discuss the trends you noted while doing your optimization study (i.e. how different parameters affected hydrogen production rate and CO concentration).