Shift Conversion in Partial Oxidation Plants

The raw synthesis gases from partial oxidation of heavy hydrocarbons and coal differ mainly in two aspects from that produced from light hydrocarbons by steam reforming. First, depending on the feedstock composition, the gas may contain a rather high amount of sulfur compounds (mainly H2S with smaller quantities of COS) second, the CO content is much higher, in some cases in excess of 50%. The sulfur compounds (Section 4.3.1.4) can be removed ahead of the shift conversion to give a sulfur-free gas suitable for the classical iron HTS catalyst. In another process variant the sulfur compounds are removed after shift conversion at lower concentration because of dilution by C02. The standard iron catalyst can tolerate only a limited amount of sulfur compounds. With a sulfur concentration in the feed 100 ppm sulfur will be stored as iron sulfide (Eq. 87)  

FeS also catalyzes the shift reaction, but its activity is only half that of Fe,04 [592]-[594], In principle the catalyst can tolerate up to 500 or 1000 ppm H2S, but with a considerable loss of mechanical strength, which is additionally affected by other contaminants in the gas, such as soot and traces of formic acid. For this reason the so-called dirty shift catalyst is used in this case, which was originally introduced by BASF [639]. This cobalt-molybdenum-alumina catalyst [603], [630], [640]-[644] is present under reaction conditions in sulfidized form and requires for its performance a sulfur content in the gas in excess of 1 g S/m3. Reaction temperatures are between 230 and 500 °C. COS is not hydrolyzed on dirty shift catalysts, but may be removed in the subsequent sour-gas removal stage using the Rectisol process. Separate hydrolysis on alumina based catalysts is possible at temperatures below 200 °C [603], 

Equations to describe reaction rate measurements on dirty shift catalyst are given in [648], [649], 

Irrespective of the catalyst type used, the high initial carbon monoxide concentrations mean that the reaction must generally be performed in steps, with intermediate cooling. But it has also been reported that the CO content can be reduced from 50 to 0.8 % in a single step in a large hydrogen plant by using a quasi-isothermal reactor (e.g., the Linde spiral-wound reactor). 

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