(575ax) An Investigation of the Effect of Mass Transfer on the Product Selectivity In Fischer-Tropsch Synthesis

Despite many years of research in the area of Fischer-Tropsch kinetics there is still little agreement on the basic reaction models for this process. Any investigation attempting to achieve this goal is hampered by the very high number of products produced in this reaction system. This is addressed by the use of generalized product distribution model such as the Schulz Flory alpha distribution parameter and the hydrocarbon class distributions.

In this paper we will attempt to tackle one portion of the field that has been somewhat less than comprehensively covered. This research attempts to understand the impact of the mass transfer on the product distribution between the olefin and paraffin product ranges.

A CSTR is used to investigate the influence of mass transfer on the kinetics and selectivity of Fischer-Tropsch synthesis (FTS) with a supported cobalt catalyst. As the final aim of the research is to propose an intrinsic kinetic model for FTS, a CSTR was selected because of its obvious advantages in deducing kinetic models when is compared to a micro PFR.

One of the more interesting resulting presented in this paper is the effect of mass transfer, as affected by variations in the stirring speed, on the olefin to paraffin (O/P) product ratio. This behavior exhibited itself as a varying over a lower stirring range, and then once the mass transfer limitations were overcome at a set value, the ratio stabilized at all higher speeds.

A further aspect of the presented research lies in the development of a model relating the turbulence in the tank with the outer mass transfer and consequently the kinetics. This work will help us to better understand the mechanism of FTS and the influence of outer mass transfer on the selectivity.

The tests were run at conditions of 20bar, a temperature of 190 and with a range of stirring speeds from 50Rpm to 1500Rpm and with H2 to CO ratios from 1:1 to 4:1.