Sulfur plants, Claus

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. 

M. R. Gray, "The Profitability of Oxygen Combustion Air Use In Claus Sulfur Plants," M. Eng. Thesis, University of Calgary, 1980. 

Chen, J. K., Chow, T. K., Cross, F., Hull, R. L., and Watson, D., 1995, SURE process for Claus sulfur plant oxygen enrichment, paper presented at Sulphur 95, Abu Dhabi, United Arab Emirates, October. 

Gray, M. R., and Svrcek, W. Y., 1981, Oxygen Use in Claus Sulfur Plants, Gas Conditioning Conference Proceedings, Univ. of Oklahoma, Norman, OK. 

The Stretford Process sweetens and also produces sulfur. It is good for low feed gas concentrations of H2S. Economically, the Stretford Process is comparable to an amine plant plus a Claus sulfur recovery plant. Usually, the amine/Claus combination is favored over Stretford for large plants. Stretford can selectively remove H2S in the presence of high CO2 concentrations. This is the process used in the coal gasification example in the Introduction. 

Claus sulfur recovery plant, 23 601 Clay, 5 640. See also Clays in FCC catalysis, 11 681 as filler, 11 312... 

Process Alternatives. Process alternatives for sulfur recovery are shown schematically in Figure 2. The choice of either elemental sulfur or sulfuric acid will depend on economics and markets related to each plant location. Elemental sulfur may be produced by gas-phase oxidation (the Claus process) or liquid-phase oxidation (e.g., the Stretford process). Stretford units were described in Section 1 and are well discussed in the literature (1, 2> 5) Claus sulfur recovery efficiency is usually less than required by current air emission standards. Therefore, some form of tail-gas treating is required. Sulfuric acid may be produced by the well-known contact process (6). This process is licensed by a number of firms, each of which has its own... 

The generation of the required reducing gas is very expensive because natural gas or low sulfur oil are used. Both of these fuels are in short supply and do not offer long-term solutions to the problem. However, in certain industrial processes, like petroleum refineries, a reducing gas could be readily available. Also, if a Claus sulfur recovery plant existed on-site, the concentrated SO2 stream could be sent to the Claus plant where it would mix with the H2S containing gas streams. Final adjustment of the H2S S02 ratio would be necessary. If the overall sulfur balance were favorable, the need for a reducing gas could be avoided. Either of these options could make the use of a recovery process economically attractive for industrial applications. 

Nowhere is this effect more important than in the sulfur plant. In the sulfur recovery section, the air flow to the Claus reactor must be carefully proportioned to the sulfur flow in the feed even slight variations cause significant penalty in the sulfur plant efficiency and increased load on the tail-gas treating... 

Excess oils and hydrocarbons in the sulfur plant feed can readily darken the sulfur, minimizing its sales value. Ammonia can cause fouling of the Claus catalyst beds. Cyanides have a tendency to polymerize, causing significant problems with the formation of Prussian blue. Organic forms of sulfur are not so readily reacted in low-temperature Claus beds and may cause problems in achieving the desired sulfur plant efficiency. Addi-... 

Paskall (25) has recently reviewed the various modifications to the Claus process that result in optimum sulfur recovery efficiency. Overall plant conversion efficiencies in the range of 97% were considered to be the upper limit at the beginning of the 1970 s (26). While this is a very respectable conversion efficiency for an industrial process the unrecovered 3% in a 2,000 tonne/d sulfur plant represents 60 tonnes/d of sulfur lost, mainly to atmosphere as 120 tonnes/d of SO2. Modifications to the four stage Claus converter train however, can raise overall conversions to over 98.5% thus halving the sulfur loss to the plant tail gas. This either reduces environmental impact or the load on tail gas desulfurization units that will be discussed later. 

Once removed from the raw gas, the question arises as to what should be done with the acid gas. If there is a large amount of acid gas, it may be economical to build a Claus-type sulfur plant to convert the hydrogen sulfide into the more benign elemental sulfur. Once the H2S has been converted to sulfur, the leftover carbon dioxide is emitted to the atmosphere. Claus plants can be quite efficient, but even so, they also emit significant amounts of sulfur compounds. For example, a Claus plant processing 10 MMSCFD of H2S and converting 99.9% of the H2S into elemental sulfur (which is only possible with the addition of a tail gas clean up unit) emits the equivalent of 0.01 MMSCFD or approximately 0.4 ton/day of sulfur into the atmosphere. Note that there is more discussion of standard volumes and sulfur equivalents later in this chapter. 

For small acid gas streams, Claus-type sulfur plants are not feasible. In the past, it was permissible to flare small amounts of acid gas. However, with growing environmental concerns, such practices are being legislated out of existence. 

Processes of this type are most frequently used for pro-cessir tail gases from Claus sulfur recovery plants, wiiich are operated to contain the correct stoichiometric proportions of hydrogen sulfide and sulfur dioxide [791. The other type of direct converaon process uses an alkaline solution (often sodium carbonate) containing one or more mild oxidizing agents that oxidize hydrogen sulfide to sulfur and are then themselves regenerated by oxidation with air [80]. 

It s hard to do much harm to sulfur plant catalyst without first causing excessive pressure drop. If you suspect reduced recovery because of lost catalyst activity, check the temperature rise across each reactor. For example, for a three-stage Claus unit ... 

While this type of "indirect reheat" exchanger is a fine way to expedite sulfur plant start-ups, it does very little to improve conversion of H2S/SO2 to liquid sulfur. For the 50-T/D Claus unit, a bypass to direct reheat gas to the first fixed bed reactor was installed. Figure 5-5 illustrates the location of the bypass. 

Claus Sulfur Recovery Process, At the sulfur plant, H2S is combined with sour-water stripper off-gas and sent to a Claus unit. Invented in 1881 by Carl Freidrich Claus/ almost every refinery in the world uses some version of this process to convert H2S into elemental sulfur. A simplified version of Claus-reaction chemistry is shown in Figure 26. 

After the regenerator pressure profile and the reflux drum pressure are established, the reflux drum temperature, is fixed. If the acid gas is to be fed to a Claus suliiir plant, the reflux drum temperature should be minimized consistent with the available cooling medium temperature since water contained in the acid gas is detrimental to sulfur plant performance. For air cooling, an approach of 20°F is common, while a t> pical approach for cooling water would be in the range of 10°F. 

Molten salt from the reducer is next processed to convert sulfides back to carbonates for recycle to the absorber. This is accomplished in a regeneration column, which operates at about 800°F and uses a mixture of carbon dioxide and water vapor to displace hydrogen sulfide gas from the molten salt. The hydrogen sulfide-rich gas stream from this step is fed directly into a Claus type sulfur plant. Work on the process was terminated after a small demonstration unit developed mechanical problems, including plugging of a mist eliminator at the absorber outlet and corrosion in some lines carrying hot molten salt. 

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