Question

The formation of styrene via the dehydrogenation of ethylbenzene is a highly endothermic reaction. In addition, ethylbenzene may decompose to benzene and toluene and also may react with hydrogen to form toluene and methane:

 

This process is presented in Appendix B as Project B.3 From the information given in Appendix B, determine the following:

1. The single-pass conversion of ethylbenzene

2. The overall conversion of ethylbenzene

3. The yield of styrene

Suggest one strategy to increase the yield of styrene, and sketch any changes to the PFD that this strategy would require.

Answer 1

(a) Single pass conversion of ethylbenzene

 

(b) Overall conversion of ethylbenzene

Ethylbenzene entering process (stream 1) =   121 kmol/h

Ethylbenzene leaving process (stream 21, 22, 23 & 26) = 0.05 + 0 + 0 + 0.10 = 0.15 kmol/h

(c) Yield of styrene

Moles of ethylbenzene required to produce styrene (Stream 21) = 120.03 kmo1/h

Moles of ethylbenzene fed to process (stream 1) = 121 kmol/h

Moles of ethylbenzene leaving process (Streams 21 + 26) = 0.15 kmol/h

 

Possible strategies to increase the yield of styrene (not necessary based on the yield above but these will also increase the single pass conversion) are:

(i) Increase steam content of reactor feed this pushes the desired equilibrium reaction to the right.

(ii) Increasing the temperature also pushes the equilibrium to right but increases benzene and toluene production.

(iii) Remove hydrogen in effluent from each reactor this will push the equilibrium of the desired reaction to the right and reduce the production of toluene from the third reaction. This will require the use of a membrane separator, shown in the following diagram.