As stated in the previous post, the purpose of the fixed bed reactor is to react essentially all of the pentenes so that we don't have to separate them from the isooctane product. In order for that to happen, the feed stream must have an excess of hydrogen with respect to the total pentenes. Let's look at the composition of the CSTR effluent stream (4).
The hydrogen molar concentration is greater than the sum of the pentene concentrations. Since each pentene mole requires one hydrogen mole, the feed stream has enough hydrogen to complete the reaction.
Make your own luck
What could be done if there were not enough hydrogen in the stream? There are several design variables to explore.
The options below will increase the amount of reaction in the CSTR which will lower the pentenes in steam 4.
In addition to those options, increasing the CSTR reactor pressure will have two desired effects. First, it will increase the hydrogen in solution. This effect might be enough to result in an excess of hydrogen in the CTSR effluent. Second, the increased hydrogen concentration in the CSTR liquid will increase the reaction rate, thereby reducing the amount of unreacted pentenes in a manner similar to the five options above.
Can more hydrogen be added to the fixed bed reactor?
Yes, but adding more hydrogen will make a two phase stream in the reactor. I try to avoid this type of reactor if possible because of the additional scaleup issues. The main issue is the fraction of the catalyst that is wetted by the liquid. This variable will change, sometimes dramatically, from laboratory to commercial scale.
Raising the pressure of the fixed bed reactor above that of the CSTR in order to increase the dissolved hydrogen is a possibility. However, that option is probably not economical because it would add a pump between the CSTR and fixed bed reactor and it would involve a two-level hydrogen compression system.