Rectisol

Rectisol®, independently developed by Linde and Lurgi, is a physical acid gas removal process using an organic solvent (typically methanol) at subzero temperatures, and characteristic of physical acid gas removal (AGR) processes, it can purify synthesis gas down to 0.1 ppm total sulfur, including hydrogen sulfide (H₂S) and carbonyl sulfide (COS), and carbon dioxide (CO₂) in the ppm range. It is also able to remove impurities such as hydrocarbons, ammonia, and hydrogen cyanide, consistent with its origin as a treatment for syngas from the Lurgi gasifier which contains these kinds of impurities.

The Rectisol process uses a cheap and easily available, non-proprietary solvent, and is flexible in process configuration. On the other hand, refrigeration is required, associated with significant capital and operating costs, and high vapor pressure of methanol causes solvent losses.

A simplified flow scheme of the Rectisol® process provided by technology vendor Linde is displayed in Figure 1. This is a configuration intended for dual removal of sulfur gases and CO₂ in separate fractions, resulting in a pure CO₂ product and an H₂S/COS enriched Claus gas fraction.

Commercial scale Rectisol® units are operated world-wide for the purification of hydrogen, production of ammonia, production of syngas for methanol synthesis, and the production of pure carbon monoxide and oxogases.

Due to the physical nature of the process, high pressure and high sour gas concentrations are particularly favorable. Rectisol® is therefore frequently used to purify shifted, partially shifted or unshifted gas downstream from residue oil, coal, or lignite gasification processes. Due to the low operation temperature, Rectisol® is also favorable for cryogenic downstream processes such as liquid nitrogen wash, cryogenic recovery of carbon monoxide and oxogas.

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