The production of maleic anhydride by the air oxidation of benzene was studied using a vanadium pentoxide catalyst [Chem Eng Sci, 43, 1051 (1988)]. The reactions that occur are C6H6 + 9/ 2 O2 −−→ C4H2O3 + 2 CO2 + 2 H2O, C4H2O3 + 3 O2 −−→ 4 CO2 + H2O, C6H6 + 15 2 O2 −−→ 6 CO2 + 3 H2O Because these reactions were carried out in excess air, volume change with reaction can be neglected, and the reactions can be written symbolically as a pseudo-first-order reaction sequence: A k1 −−→ B k2 −−→ C A k3 −−→ D where A = benzene, B = maleic anhydride, C and D = unwanted products (H2O, CO2 ). The corresponding pseudo specific reaction rates, ki are (in dm3 /kg cat/sec): k1 = 4280 exp[ -12,660/T(K)], k2 = 70,100 exp[-15,000/T(K)], and k3 = 26 exp[-10,800/T(K)]. At 848 K, k1 = 1.5 × 10−3 , k2 = 1.46 × 10−3 , k3 = 7.65 × 10−5 . These reactions are carried out isothermally in both a CSTR and a PBR. Benzene enters the reactor at a concentration of 0.01 mol dm−3. The total volumetric flow rate is 0.0025 dm3 sec−1.

 (a) Which reactions will dominate at low temperatures and which will dominate at high temperatures? For the sake of comparison, assume that 848 K is a moderate temperature.

 (b) For a catalytic weight of 50 kg, determine the exit concentrations from a “fluidized” CSTR at 848 K. (Ans: CB = 0.0003 mol dm3 )

 (c) What is the selectivity of B to C and of B to D in the CSTR?

 (d) Plot the concentrations of all species as a function of PBR catalyst weight (up to 10 kg) assuming isothermal operation at 848 K.

 (e) What feed conditions and reactor or combinations of reactors shown in Figure 6-3 would you use to maximize the production of maleic anhydride? 

(f) How would your results in part (d) change if pressure drop were taken into account with α = 0.099 kg cat−1 in the PBR? Make a plot similar to that in part (d) and describe any differences.