A process synthesis approach to investigate the effect of the probability of chain growth on the efficiency of Fischer-Tropsch synthesis

To evaluate a process at the early stage of its development, one must be able to perform simple calculations to compare all the alternatives. This must be done on a reasonably realistic basis, so that one can make credible decisions; however, the process should not be so detailed and laborious that it becomes too "expensive" to perform for alternatives that can ultimately be discarded. A methodology for performing these calculations has been developed, and this will be illustrated with a case study on Fischer - Tropsch synthesis (FTS). The effect of designing a Fischer-Tropsch (FT) process targeting a particular value of the probability of chain growth (a) on the overall carbon efficiency has been studied using simplified FTS flowsheet models. Two process configurations-namely, the once-through and recycle processes-have been compared, and it is observed that, for a fixed production rate of liquid fuels at 100% CO conversion, the carbon efficiency for the process with a recycle stream is higher than that of the once-through process for all values of a. However, if the aim is to maximize diesel production by hydrocracking the waxes, it has been determined that an optimal a value should be sought to reduce the cost of hydrocracking very heavy waxes. The incorporation of wax hydrocracking in the two processes reduces the carbon efficiency at all a values beyond ∼0.7, thereby making it uneconomical to produce very-long-chain hydrocarbons.