Water Gas Shift
In syngas processes where hydrogen production is maximized, the use of a water gas shift reactor is typically utilized. These reactors shift a portion of the CO content in the syngas to CO2 and additional H2 via the water gas shift reaction given below, which is exothermic.
CO + H2O = CO2 + H2
The water gas shift reaction is favored at lower temperatures and higher steam content. There are four common types of water gas shift reactors used depending on the syngas conditions: High Temperature Shift Conversion (HTSC), Medium Temperature Shift Conversion (MTSC), Low Temperature Shift Conversion (LTSC), and Sour Gas Shift Conversion (SGS).
A HTSC is utilized for a syngas stream that does not contain any appreciable amounts of sulfur. The syngas stream typically enters the reactor between 600°F and 800°F, and passes through a bed of copper promoted iron-chromium catalyst. The use of a HTSC can produce a syngas stream with a CO content of approximately 2.5% on a dry basis.
A MTSC is also utilized for a syngas stream that does not contain any appreciable amounts of sulfur. The syngas stream typically enters the reactor between 325°F and 625°F, and passes through a bed of copper zinc alumina catalyst. The use of a MTSC can produce a syngas stream with a CO content of approximately 0.5% on a dry basis.
A LTSC is typically utilized downstream of a HTSC to further reduce the amount of CO in the syngas and to maximize the production of hydrogen. The LTSC typically utilizes a Zn-Cu catalyst and it is easily poisoned by sulfur and chloride compounds. The syngas typically enters the LTSC at 350°F to 500°F. The use of a HTSC followed by a LTSC can produce a syngas stream with a CO content of approximately 0.2% on a dry basis.
A SGS process is used for syngas streams that contain high levels of H2S. This is commonly used in the gasification processes where the feedstocks are not desulfurized prior to entry into the gasifier. The SGS typically utilizes a bed of cobalt molybdenum catalyst. The syngas typically enters the SGS reactor at 450-500°F.