Rectisol process

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. 

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. ... 

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. 

The raw gas is purified in the Lurgi Rectisol process (cold methanol scrubbing) where various by-products (eg tar naphthas) and impurities (eg CO2 and H2S) are removed. 

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. 

Rectisol process - [CATALYSTS - SUPPORTED] (Vol 5) -carbon dioxide recovery [CARBON DIOXIDE] (Vol 5)... 

The soot-free gas that leaves the carbon removal scrubber then goes through the following steps Desulphurization (the Rectisol process is often used), CO Shift, C02 Removal and Methanation (see Figure 5.21 and Figure 5.22). These purification steps will be discussed in more detail in later sections. 

In 2003 the Rectisol process is offered for licensing by both companies for the removal of C02, H2S, HCN, 0 6 (benzene), COS and gum-forming hydrocarbons from syngas and fuel gas. In 2003, over 70 units were in operation or under construction1. 

FeS also catalyzes the shift reaction, but its activity is only half that of Fe,04 [592]-[594], In principle the catalyst can tolerate up to 500 or 1000 ppm H2S, but with a considerable loss of mechanical strength, which is additionally affected by other contaminants in the gas, such as soot and traces of formic acid. For this reason the so-called dirty shift catalyst is used in this case, which was originally introduced by BASF [639]. This cobalt-molybdenum-alumina catalyst [603], [630], [640]-[644] is present under reaction conditions in sulfidized form and requires for its performance a sulfur content in the gas in excess of 1 g S/m3. Reaction temperatures are between 230 and 500 °C. COS is not hydrolyzed on dirty shift catalysts, but may be removed in the subsequent sour-gas removal stage using the Rectisol process. Separate hydrolysis on alumina based catalysts is possible at temperatures below 200 °C [603],... 

The physical solvents methanol (Rectisol process), sulfolane (Sulfinol Process) and N-rnethylpyrrolidone (Purisol) are preferentially used in the treatment of partial oxidation gases and will be described separately in the following section. 

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-... 

Enrichment of acid gases such as H2S and CO2 is a critical step in coal gasification because hydrogen sulfide needs to reach a sufficiently high concentration to be effectively treated by the conventional Claus unit. Currently the proven process for separating the H2S and CO2 from H2 is the Rectisol process which is exuemely expensive but it produces a H2S-rich acid gas mixture that can be further handled by the inexpensive, established Claus process to recover elemental sulfur and reduce SOx emissions. 

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. 

The Rectisol process uses cold methanol at a temperature of about -70 to -30 °C. Methanol s selectivity for H2S over C02 at these temperatures is about 6,52 a little lower than that of Selexol at its usual operating temperature. However, the solubilities of H2S and COS in methanol, at typical process operating temperatures, are higher than that in Selexol and allow for very deep sulfur removal (<0.1ppmv). 

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