Rectisol plant

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. 

In the expansion vessel DA of a Rectisol plant (installation for physical gas cleaning), whose flow sheet is shown in Fig. 9.49, CO2 is separated from methanol saturated with CO2 by lowering the pressure. The separated CO2 is fed into the vessel FA. The methanol, which is again saturated because pressure is lower (this implies, of course, a CO2 content lower than initially) is discharged in a controlled way from DA. The control maintains a fllUng level of about 40 % in the vessel... 

Sulfur is recovered from the sour gas streams from the Rectisol plants using the Claus process followed by the Sulfreen process for tailgas cleanup. 

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 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. 

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. 

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. 

To reach a better CO conversion, it is possible to add a low-temperature shift reactor, which increases the CO2 capture rate (see also Fig. 10.3). If both clean CO2 for storage and clean hydrogen for fuel cell applications are required, a combination of a C02-capture plant (e.g., absorption with Rectisol) and a PSA plant is necessary. If only pure hydrogen is required, a PSA unit would be sufficient (and is standard practice), but the C02 stream would be contaminated by impurities, such as H2, N2 or CO, which have to be removed for geological storage. 

The synthesis gas from the waste heat recovery is rich in acid gases and is passed to a "Rectisol" or "Selexol" plant for their removal. During this process small amounts of hydrogen, carbon monoxide, methane, nitrogen and argon are dissolved in the solution and lost from the system in the acid gas stream. 

The "Rectisol" or "Selexol" plant of the "selective" type absorbs both H S and C0 , but in the regeneration section of the plant the H S and CO are separated to provide two streams, one or which is CO, virtually free from H S, and the other is of a suitable CO2/H2S ratio to be processed in a conventional sulfur recovery plant. 

Rectisol A process for removing sulfur compounds from gas mixtures resulting from the partial oxidation of hydrocarbons, based on physical absorption in methanol at low temperatures. Originally developed in 1951 by Lurgi Gesellschaft fur Warmetechnik for the SASOL coal gasification plant... 

The Rectisol process [667], [707], [711]-[715] seems to be the prime choice in partial oxidation plants. The process, invented by Lurgi and developed further by Linde, operates with chilled methanol, a cheap and readily available solvent, in which carbon dioxide, hydrogen sulfide and carbonyl sulfide (COS) are readily soluble at low operating temperatures of below - 30 °C. The Henry absorption coefficient for H2S is about six times higher than for C02-... 

At high syngas pressure, physical solvent-based processes become increasingly attractive, for example, the Rectisol and Selexol processes. There are more than 55 Selexol plants worldwide, treating natural and syngas.51 The Selexol process solvent is a mixture of dimethyl ethers of polyethylene glycol, and has the formulation... 

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