Rectisol

The cmde product from the gasifier contains CO2 and H2S, which must be removed before the gas can be used to produce chemicals. The Rectisol process is used to remove these contaminants from the gas. This is accompHshed by scmbbing the product with cold methanol which dissolves the CO2 and H2S and lets the H2 and CO pass through the scmbber. The H2S is sent to a Claus sulfur plant where over 99.7% of the sulfur in the coal feed is recovered in the form of elemental sulfur. A portion of the clean H2 and CO are separated in a cryogenic distillation process. The main product from the cryogenic distillation is a purified CO stream for use in the acetic anhydride process. The remaining CO and hydrogen are used in the methanol plant. 

The Rectisol process is more readily appHcable for acid gas removal from synthesis gas made by partial oxidation of heavy feedstocks. The solvents used in Purisol, Fluor Solvent, and Selexol processes have low vapor pressures and hence solution losses are minimal. Absorption systems are generally corrosion-free. 

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 German Lurgi Company and Linde A. G. developed the Rectisol process to use methanol to sweeten natural gas. Due to the high vapor pressure of methanol this process is usually operated at temperatures of -30 to -100°F. It has been applied to the purification of gas 1 plants and in coal gasification plants, but is not used commonlv natural gas streams. 

Important processes commercially used are the Selexol, the Sulfinol, and the Rectisol processes. In these processes, no chemical reaction occurs between the acid gas and the solvent. The solvent, or absorbent, is a liquid that selectively absorbs the acid gases and leaves out the hydrocarbons. In the Selexol process for example, the solvent is dimethyl ether of polyethylene glycol. Raw natural gas passes countercurrently to the descending solvent. When the solvent becomes saturated with the acid gases, the pressure is reduced, and hydrogen sulfide and carbon dioxide are desorbed. The solvent is then recycled to the absorption tower. Figure 1-1 shows the Selexol process. ... 

Fischer Tropsch technology is best exemplified by the SASOL projects in South Africa. After coal is gasified to a synthesis gas mixture, it is purified in a rectisol unit. The purified gas mixture is reacted in a synthol unit over an iron-based catalyst. The main products are gasoline, diesel fuel, and jet fuels. By-products are ethylene, propylene, alpha olefins, sulfur, phenol, and ammonia which are used for the production of downstream chemicals. 

Two other components, methanol and benzene, were included in this study. Methanol is important in processes using Rectisol Systems for C02 removal prior to methanation. Benzene was considered in order to determine the effect of aromatics on catalyst activity and potential carbon formation. 

MeOH is included because of use of the Rectisol system is expected. 

Benzene was included in order to study the effect of aromatics on the catalyst in the event of catalytic sulfur removal rather than use of the Rectisol system. 

Benzene. Benzene is ordinarily scrubbed out by a Rectisol system before the methanators. However, if a different H2S removal system were used, benzene could pass through the system and then hydrogenate, plug up the catalyst pores, or reform. Benzene was therefore included in this poison study. 

Consequently, two semicommercial pilot plants have been operated for 1.5 years. One plant, designed and erected by Lurgi and South African Coal, Oil, and Gas Corp. (SASOL), Sasolburg, South Africa, was operated as a sidestream plant to a commercial Fischer-Tropsch synthesis plant. Synthesis gas is produced in a commercial coal pressure gasification plant which includes Rectisol gas purification and shift conversion so the overall process scheme for producing SNG from coal could be demonstrated successfully. The other plant, a joint effort of Lurgi and El Paso Natural Gas Corp., was operated at the same time at Petrochemie Schwechat, near Vienna, Austria. Since the starting material was synthesis gas produced from naphtha, different reaction conditions from those of the SASOL plant have also been operated successfully. 

Residual C02 Content. The feed gas to Rectisol gas purification contains 29-36 vol % C02 depending on the rate of shift conversion. The rate of C02 to be washed out will be determined by the requirements of methane synthesis and by the need to minimize the cost of Rectisol purification. 

Catalyst Poisons. It is well known that sulfur, chlorine, etc. are strong poisons for nickel catalyst. Chlorine was not detectable in the synthesis gas downstream of the Rectisol in the SASOL plant. The total sulfur content of this gas—in the form of H2S, COS, and organic sulfur components—averaged 0.08 mg/m3 with maximum values of 0.2 mg total sulfur/m3. 

These tests demonstrated that the Lurgi Rectisol process provides an extremely pure synthesis gas which can be charged directly to the metha-nation plant without problems of sulfur poisoning of the nickel catalyst. However, in order to cope with a sudden sulfur breakthrough from Rectisol as a result of maloperation, a commercial methanation plant should be operated with a ZnO emergency catchpot on line. 

Scrubber system, Rectisol (S/G), steam/gas ratio. Shift... 

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