Rectisol units

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. 

In the gasifiers the sulphur present in the coal is converted to H S which is scrubbed out, together with the C0 in the downstre Rectisol unit. The effluent stream from this unit (typically 1.5 % HpS in CO2) is treated in a homogeneous catalytic process in which tne H S is converted to high grade elemental sulphur. The catalytic liquor consists essentially of an alkaline vanadium solution. In the... 

A selective Rectisol unit with methanol of about - 30 °C as solvent is used to remove H2S and COS (together with some C02) to less than 0.1%. The removed sulfur-rich... 

The Rectisol process, developed by Lurgi, is the most widely used physical solvent gas treating process in the world. More than 100 Rectisol units are in operation or under construction worldwide. Its most prevalent application is for deep sulfur removal from syngas that subsequently undergoes catalytic conversion to such products as ammonia, hydrogen, and Fischer-Tropsch liquids. 

A selective Rectisol unit as shown in Fig. 2.10 in conjunction with slip stream CO shift conversion is not only more flexible but more cost-effective as well. It will be preferable above all to purify gases from high-temperature coal gasification whose low CO2 contents in the raw gas normally do not require the sulfur content in the Claus gas to be increased by reabsorbing H2S. 

Figure 2.17 shows the arrangement of such a Rectisol unit with a preabsorber in which the above-mentioned pollutants are washed out by methanol. The spent methanol is regenrated via extraction with water and subsequent two-stage distillation. The extraction stage recovers the hydrocarbons and the distillation stages remove the other pollutants. 

The reabsorber ensures that all the sulfur entering the Rectisol unit is concentrated in about 10% of the CO2 absorbed from the raw gas. The acid gas has an H2S content of 5 vol. %. The heat-regenerated methanol is fed to the absorber trimming stage. 

The purified gas from the Rectisol unit is mixed with the gas from the purge gas reforming unit and with the hydrogen separated from the purge gas in the PSA unit. The mixture so obtained - some 592 (XX) m /h - constitutes the finished syngas with a stoichiometric ratio of about 2.02. A turbocompressor driven by a steam turbine compresses it from 24.5 to 73 bar before it is delivered to the... 

The acid gas from the Rectisol unit and the CO2 and H2S removed in the CLL unit are jointly fed to a Concat unit to convmt the sulfur components of the gas into 94wt.% sulfuric acid (Sect. 5.2). )Wth very few exceptions, the Concat offgas containing less than 1 ppm of H2S and less than 100 ppm of SO2 can be discharged to the atmosphere everywhere in the world without any af-... 

Another application of the Sulfolin process is at the Rheinbraun AG HTW coal gasification plant near Cologne, West Germany (Heisel, 1989). The raw synthesis gas is purified in various steps, and eventually the H2S, and part of the CO2, are removed in a non-selective Rectisol wash. The H2S/CO2 stream from the Rectisol unit is treated using the Sulfolin process. 

A Rectisol unit designed for the production of synthesis gas and carbon dioxide, which are suitable as feeds to a urea plant, has been described by Linde AG and Lurgi GmbH (1992). The material balance for this type of plant and a summary of utility requirements based on a 1.000 tons/day ammonia unit are provided in Table 14-19. 

The Great Plains Rectisol unit consists of two identical absorption and regeneration trains with a common naphtha extraction and methanol recovery train. The basic flow scheme is... 

Hochgesand (1968, 1970) and Kriebel (1989) present essentially identical flow diagrams for a selective Rectisol unit treating high-pressure partial oxidation gas. See Figure 14-18 for details. 

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