The Effect of Interference Techniques on Fischer-Tropsch Product Distributions.

Abstract

Novel reaction engineering designs, also referred to as interference techniques, have been shown to improve the yield of heavier hydrocarbons in Fischer-Tropsch (FT) product distributions. This dissertation explores two such techniques: distributed syngas feeding and azomethane co-feeding. An FT reaction model is first developed as an aid in simulating and understanding the experimental results presented herein. This model also explores whether the rate constants governing the FT polymerization reaction depend upon the length of the growing surface hydrocarbon chain. A distributed syngas feeding strategy was employed experimentally by using two series plug flow reactors and feeding syngas into the entrances of both. The results from these experiments are compared against those from a normal feeding strategy in which syngas was fed only to the entrance of the first series reactor. A distributed feeding strategy is counterproductive to generating heavier product distributions when complete or near-complete CO conversion is effected within the first series reactor, leading to a decrease in C5+ hydrocarbon selectivity of over 60%. However, effecting only incremental CO conversion within the first series reactor leads to an improvement in C5+ hydrocarbon selectivity of up to 30% using a distributed feeding strategy. This result represents a significant finding in that the weight of an FT product distribution was increased simply by altering the location of the inlet syngas. Azomethane co-feeding was meant both to demonstrate a proof of concept in adding a recycled stream of activated methane to an FT reactor and to ascertain the occurrence of methyl termination steps in FT. Azomethane co-feeding experiments generated lighter product distributions compared to experiments in which deionized water was co-fed as a control – α values for C8-C13 hydrocarbons were 0.69-0.70 for the water co-feeding experiments and 0.64-0.65 for the azomethane co-feeding experiments. However, the azomethane co-feeding experiments generated product distributions with greater paraffinic content than the corresponding water co-feeding experiments. These results demonstrate the likelihood that methyl termination steps take place on the surface of iron FT catalysts. Further exploration must be performed to find conditions in which a recycled, re-activated methane stream can improve heavy hydrocarbon selectivity.