Rectisol, carbon dioxide removal

Figure 5.28. How diagram of rectisol process for selective hydrogen sulfide removal, followed by carbon dioxide removal.189...

Figure 5.28 shows a flow scheme of the Rectisol process for selective hydrogen sulfide removal, followed by carbon dioxide removal.189 H2S and C02 can be removed respectively, but two absorbers and two regenerators are required. The chief drawback of the Rectisol process is its complex scheme and the need to refrigerate the solvent due to its low boiling point, which results in high capital and operating costs. 

If the gas contains large amounts of sulfur, it is quite often the practice to remove this sulfur before the carbon monoxide-shift operation. There are a number of sulfur removal processes, namely, amine scrubbing, hot potassium carbonate scrubbing, and the Rectisol process. Ash removal and carbon removal usually are carried out before the gas is treated for sulfur or carbon dioxide removal and before the shift operation. 

Cold methanol has proven to be an effective solvent for acid gas removal. Cold methanol is nonselective in terms of hydrogen sulfide and carbon dioxide. The carbon dioxide is released from solution easily by reduction in pressure. Steam heating is required to release the hydrogen sulfide. A cold methanol process is Hcensed by Lurgi as Rectisol and by the Institute Francaise du Petrole (IFP) as IFPEXOL. 

The elemental sulfur is removed by conventional technology. The gases are purified by the Lurgi Rectisol process which uses a low temperature methanol wash to remove H2S, COS and CO2. The acid gas stream is then passed to a Stretford unit which is preferred to the Claus unit because of the high percentage of carbon dioxide in the stream. Sulfur in the stack gas would be removed by conventional flue gas desulfurization techniques and the sulfur would then remain as sulphite sludge and not be recovered as elemental sulfur. 

Sodium carbonate is a less costly absorbent, but also less efficient. Other acid gas removal techniques use physical absorption of carbon dioxide in organic solvents, such as the dimethyl ether of polyethylene glycol, or methanol (the Selexol or Rectisol processes). For effective absorption these require high gas pressures. Regeneration of solvent is done by pressure letdown plus some air stripping (sparging of air through the solvent). 

Raw coal is cru ed and clastified into three sizes. The finest portion is used in the power plant, which has four boilers, h having a capaaty of 160 tons of steam per hour. A total of about 7 million cu ft per hr of raw gas is supplied at 25 atm from nine Luigi,fixed-bed gasifiers. A Linde air-liquefaction plant supplies about 70 tons per hr of oxygen for the gasification. Carbon dioxide and sulfur compoimds are removed in a Rectisol plant. 

The gas is then treated to remove sulfur, using any of a number of different processes, including dry iron oxide, Sulfinol, or Rectisol. It is then shift converted and carbon dioxide is removed (Figure 11). 

Currently, there are more than 100 commercial plants employing the Rectisol process for the treatment of synthesis gas or town gas streams to remove carbon dioxide and hydrogen sulfide. 

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