Sour Gas, Shift

The shift from carbon monoxide to carbon dioxide generally occurs in two steps - first a High Temperature Shift Conversion and then a Low Temperature shift conversion. In some cases the two steps may be combined in one isothermal or adiabatic step called Medium Temperature Shift Conversion. When the feed gas to the CO conversion is not desulfurized, the CO conversion is called Sour Gas Shift and a special type of sulfur-resistant catalyst is used166. 

Modern WGS catalysts can be divided into four classes, namely, HTS, LTS, sour gas shift, and precious-metal-containing catalysts. Iron oxide or iron-chromium mixed oxides promote the WGS in the 350-450 °C range and are modified by MgO or ZnO for good sulfur resistance and mechanical strength. The second type of WGS catalysts are mixed copper-zinc oxides which promote WGS in... 

Depending on the operating conditions, three different types of WGS processes are applied [232] [303] [391]. High-temperature shift (300— 500°C) over a robust catalyst is used for primary conversion. Medium temperature shift (200—330 C) is used for special purposes. Low-temperature shift (185—250 C) is used to achieve maximum conversion. Sour gas shift (350 C) is used to operate under high sulphur eonditions and low H2/CO (raw coal gas, etc)... 

For methanol synthesis, the raw gas leaving the gasification unit requires significant additional treatment. Once the gas has been cooled (generally in a direct contact scrubber in which particulate matter is also removed), COS is hydrolyzed over a suitable catalyst, H2S is removed to a large extent, the gas composition is shifted across a sour gas shift converter + H2 reaction promoted over a... 

Once the raw has been scrubbed for soot removal, H2s is removed and CO is shifted across a cobalt molybdenum sour gas shift catalyst to adjust the H20/C0/C02 ratio. Finally, excess C02 is removed and the gas may be compressed (if required) and then processed in a conventional methanol synthesis loop. The processing steps as just described yield a syn that is approximately stoichiometric in nature (i 1.0) althou considerably more concentrated in CO than C02. 

Obviously, the end-of-pipe measures can fix tbe problem temporarily, but not remove tbe cause. Sometimes tbe problem is shifted or masked into another one. For this reason, an end-of-pipe solution should be examined from a plantwide viewpoint and beyond. For example, sour-gas scrubbing by chemical absorption may cut air pollution locally, but involves tbe pollution created by the manufacture of chemicals elsewhere. In this case, physical processes or using green (recyclable) solvents are more suitable. The best way is tbe reduction of acid components by changing the chemistry, such as for example using a more selective catalyst. 

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],... 

The commercial Co-Mo catalysts operate in the temperature range 250-350 °C and at pressures from atm to 40 bar. The typical process conditions for a Texaco partial oxidation process that generate syngas from heavy oil which use sour Co-Mo WGS catalyst are shown in Table 4.3. Three Co-Mo catalysts beds are used. The syngas from partial oxidation reactor contains 0.25% of H2S. The inlet CO concentration of 46% is reduced to 1% at the exit of third bed. However, the Co-Mo catalyst converts H2S and CO into COS. Hence, COS hydrolysis has to be performed after the water-gas shift reaction. However, if we... 

B. Liu, Q. Zong, P. P. Edwards, F. Zou, X. Du, Z. Jiang, T. Xiao, H. AlMegren, Effect of titania addition on the performance of C0M0/AI2O3 sour water gas shift catalysts under lean steam to gas ratio conditions, Ind. Eng. Chem. Res. 51 (2012) 11674-11680. 

A distinction is normally made between clean gas conversion where the CO shift conversion unit is preceded by a gas purification stage to remove the major part of the sulfur components and any higher hydrocarbons that may be present in the gas, and sour gas conversion which accepts the untreated coal gas as a feed. A special type of the latter is what has been termed raw gas conversion, where considerable quantities of high boiling hydrocarbons are not eliminated from the raw gas but are left to pass the CO shift conversion stage. 

The process diagram shown here is used for such gas condensates whose sour gas content is as least as high as their ammonia content. The NH3/CO2/H2O system exhibits different boiling properties at different pressures. The so-called maximum temperature azeotrope lines, which are defined so that the CO2/NH3 ratios are the same in both phases when the liquid and gas phases are in equilibrium, are shifted in the direction of higher ammonia mixes as pressures are increased. 

Wang Y, Lang X, Fan S (2013) Hydrate capture CO2 liom shifted synthesis gas, line gas and sour natural gas or biogas. J Energy Chem 22(l) 39-47... 

---