Fischer-Tropsch

Fischer-Tropsch synthesis: process considerations based on performance of iron-based catalysts

Abstract

Alternative reactor configurations and the utilization of a low-alpha versus high-alpha iron-based catalyst are compared for the Fischer-Tropsch synthesis based on actual catalyst performance. The use of coalderived synthesis gas with a low H2/CO ratio requires a judicious balance between the rates of the Fischer- Tropsch and water-gas shift reactions. The hydrocarbon space-time yield is considerably increased by using either a reactor with recycle or a series of reactors with low syngas conversion per pass instead of a singlepass reactor with high syngas conversion. The production of gasoline and diesel-range hydrocarbons by downstream processing of the Fischer-Tropsch reaction products involves both hydrocracking and oligomerization. The difference between processing the products from a low-alpha and a high-alpha ironbased catalyst lies merely in the relative sizes of the hydrocracking and oligomerization reactors required. In addition, it is advantageous to use a low-alpha catalyst from the viewpoint of Fischer-Tropsch slurry reactor operation.

Table of Contents

Introduction4

Related Studies5

Study on an iron-manganese Fischer-Tropsch synthesis catalyst prepared from ferrous sulfate5

Fischer-Tropsch synthesis6

Study on iron-manganese catalysts for Fischer-Tropsch synthesis8

Catalyst preparation8

Catalyst Characterization9

Analysis & Procedures10

Hydrocarbon Productivity10

Alternative Reactor Configurations12

Comparison of Low- And High-Alpha Catalysts14

Operation of Slurry Reactor with Low-Alpha Catalyst16

Conclusions17

References18

Fischer-Tropsch synthesis: process considerations based on performance of iron-based catalysts

Introduction

The production of liquid fuels from coal is a desirable goal to reduce dependence on crude oil imports. Indirect coal liquefaction is a promising approach to achieve this goal. Coal is first gasified to produce synthesis gas, which is a mixture of carbon monoxide and hydrogen. The synthesis gas is then converted to hydrocarbons by the Fischer-Tropsch synthesis (FTS). Advanced fluidized coal gasification processes produce a synthesis gas with a low H2/CO ratio, ~0.67 (Yang, 2005, pp. 105). The direct processing of this gas in the FTS eliminates the need for an additional step (water-gas shift) to increase the H2/CO ratio. The inherent water-gas shift activity possessed by iron FTS catalysts allows the direct processing of low-H2/CO-ratio synthesis gas in a slurry reactor without excessive coking of the catalyst. Moreover, the slurry reactor is superior to the multi-tubular bed reactor in terms of temperature control. The water-gas shift (WGS) reaction occurs simultaneously with the production of hydrocarbons during FTS over iron-based catalysts. These two reactions are

For a feed gas having an H2/CO ratio of 0.67, the maximum obtainable conversion of CO without the WGS reaction is only 33%. This illustrates the desirability of a high rate for the WGS reaction. However, a high WGS reaction rate could convert a larger amount of CO (source of carbon) to CO2 rather than the desired hydrocarbon products. However, from the mass balance point of view, the production of CO2 parallels the production of H2 and potentially increases the rate of reaction (1) so that it is not a loss, as the CO and CO2 are exchangeable through the WGS reaction.

Thus the relative extents of the FTS and WGS reactions need to be optimized for the maximum production of hydrocarbons. In this study the performance of an iron-based ...