Treating, tail gas

Another variation of the Selectox process can be used with the Beavon process in tail gas treating. The hydrogenated Claus tail gas stream is sent to a Selectox reactor. Overall recoveries of up to 98.5% are possible. Use of Beavon/Selectox, however, typically costs more than use of Superclaus. 

There are many processes used in tail-gas treating. The Sulfreen and the Cold Bed Absorption (CBA) processes use two psirallel reactors in a cycle, where one reactor operates below the sulfur dew point to absorb the sulfur while the second is regenerated with heat to recover molten sulfur, tiven though sulfur recoveries with the additional reactors are normally 99-99.5% of the inlet stream to the Claus unit, incineration of the outlet gas may still be required. 

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... 

Tail gas cleanup is required because a well-designed Claus plant with three catalytic stages and fresh catalyst will recover only 95-97% of its feed sulfur (8), which is not generally sufficient to meet current emission standards. In addition, feed impurities and catalyst aging will reduce overall recovery in some plants to about 92% just before catalyst changeout. Therefore, tail-gas cleanup is required. Tail-gas treating processes are generally classified as follows ... 

Tail Gas Cleanup Process Efficiency - Required process efficiency depends on applicable emission regulations. Low-efficiency processes result in up to 99.0-99.5% overall sulfur recovery when combined with the Claus plant and include the Sulfreen, SNPA/Haldor-Topsoe, CBA, IFP, and Beavon Mark II processes. High-efficiency tail-gas treating processes can achieve overall sulfur recoveries of 99.8% and above under ideal conditions. These include the Beavon Mark I, SCOT, Trencor, and Wellman-Lord processes. 

The primary problem with the systems that have been developed to utilize lower feed sulfur concentrations is the reduced efficiency of sulfur recovery. This effect is not so apparent in lost sulfur revenues as it is in the increased size and cost of the downstream tail-gas treating unit required to minimize the sulfur emissions from the overall sulfur-treatment section. 

The overall problem, from sulfur in a raw gas through sulfur recovery and tail gas treating, has historically been addressed by a case study approach. In this approach, each plant section is optimized based upon predetermined subsystem interfaces. A preferred approach, rather, is to evaluate the entire system analytically, allowing the interfaces between the subprocesses to float, to determine the overall cost-effective,approach. In this technique, compromises are taken in each of the unit processes so that the total system operates more effectively. 

A further question arises on the need for the exceptionally high purity tail-gas treating processes. The energy demands of these systems are excessive, compared with less efficient systems. 

Indeed, more pollutants may be emitted in the production of energy to replace the energy used for tail-gas treatment, than are recovered by the more efficient tail-gas treating processes. 

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... 

The sulfur plant of the energy refinery, as contrasted to other sulfur plant applications, will have a high concentration of carbon dioxide. This CO2 will have a significant impact, both chemically and thermally, on the sulfur plant operation and downstream tail-gas treating. Energy demands and/or sulfur emissions may be adversely affected, relative to known operations. 

Figure 2 Low-pressure-drop reactors used for tail-gas treating.

Sulfur Production and Tail Gas Treating 30 Tons per Operating Day 6 - — — 1 ... 

Water treatment Tail-gas treating Mercury capture C02 capture... 

The treated gas had to meet typical pipeline gas specifications, that is, less than 4 ppm vol. H2S and less than 2.0 vol.% CO2. The add gas liberated is sent to conventional two-stage Claus units followed by a Sulfreen tail gas treating unit. Figure 10.4 illustrates the original amine sweetening units process flow-scheme. There are two identical trains, each equipped with two high-pressure absorbers and two regenerators. 

Since the conversion limit even in improved Claus plants is not readily raised above about 97%, the emphasis in emission control has now passed to using tail gas-treating plants to attain overall conversion efficiencies of 99% or more. The economics of control at Claus plants are probably more favorable than in any other case requiring control of dilute gas streams (i.e., those containing less than about 2—3% sulfur dioxide). 

P. H. Berben, et ai, "Deactivation of Claus Tail-Gas Treating Catalysts", in Catalyst Deactivation, eds. Delmon Froment, Elsevier, Amsterdam, 1987, pp. 303-316. 

A somewhat similar but apparently more successful Beavon-Stretford type process is the LOW-CAT process promoted by ARI Technology. The French tail-gas treating process marketed by Instilut Francaise du Petrole (IFP) also cannot be made to work in a predictable fashion. 

Generally, Claus plants are not considered for very small amounts of sulfur (less than 10-20 t/d) because the unit cost is extremely high at these small sizes. For this size range, processes normally considered for tail-gas treating become economically practical for the entire sulfur recovery operation. 

Final Recovery—Tail-Gas Treating. Given that Claus plants achieve only about 98% sulfur recovery, and given the stringent sulfur emissions in the oil and natural gas industries, numerous processes have been developed to recover sulfur from the tail-gas of Claus plants. Table 2-5 lists the prominent tail-gas treating processes and licensors. 

Although these technologies are generally called tail-gas treating processes, they also can process the entire acid gas if the H2S concentration is extremely low or if the total amount of sulfur is quite low (generally less than 10-20 t/d). 

Tail-gas treating processes are generally unnecessary in coal gasification plants that require less than 95% overall sulfur recovery/reduction (26). A well-designed acid gas removal system can easily and effectively remove most of the sulfur from coal gas, and can readily attain this standard with a Claus plant. 

The ammonia and other volatile nitrogen compounds are then removed from the coal gas by water scrubbing. To assure over 98% sulfur removal and recovery, the trace amounts of COS in the coal gas are converted to H S via catalytic hydrolysis. The HjS is then removed from the coal gas by the conventional Dow GAS/SPEC ST-1 methyldiethanolamine (MDEA) process and the is con-verted to valuable liquid sulfur by the conventional Claus process with MDEA-based tail gas treating. 

The hot raw gas leaving the gasifier is cooled by generation of high pressure steam. Fly ash is then hot/dry removed by ceramic filters. Tlie gas is then water scrubbed to remove ammonia and remaining trace solids. To assure high overall sulfur removal, the trace amounts of COS in the gas are converted to H2S by catalytic hydrolysis. The H2S is removed and converted to valuable by-product sulfur by conventional acid gas removal with Claus and tail gas treating. 

As shown, elemental sulfur is produced by the reversible reaction between SO2 and H2S. COS and CS2 appear in small amounts, but even traces of these compounds are hard to remove in tail-gas treating units. Ammonia comes in with the sour-water stripper off-gas. In the Claus process, it is thermally decomposed into nitrogen and water. 

Tail-gas treating units (TGTU) bring the total sulfur recovery up to >99.9%. Most tail-gas treating processes send the tail gas to a hydrotreater, which converts all sulfiir-containing compounds (SO2, SO3, COS, CS2 and Sx) into H2S. In the SCOT process, offered by Shell Global Solutions, the H2S is aborbed by an amine and returned to the front of the Claus fiimace. In the LO-CAT process, offered by Merichem, H2S is air-oxidized to sulfur in an aqueous solution containing a chelated iron catalyst. 

As stated earlier, carbonyl sulfide and carbon disulfide hydrolyze fairly readily at temperatures in the range of 600° to 7(X)°F in the presence of water vapor and an active Claus catalyst. It is therefore advantageous to design the first catalytic converter for operation at this temperature level if high conversion efficiency is required and if substantial quantities of caibonyl sulfide and carbon disulfide are present. However, this results in inefficient operation of the first converter with respect to the Claus reaction, and installation of a third converter may be desirable to compensate for this loss in efficiency. If air pollution conirol regulations require high conversion efficiency, it is usually economical to use only two catalytic converters, and then remove residual sulfur compounds in a tail gas treating unit. 

Most of the Claus plant tail gas treating processes that have achieved commercial status can be categorized into three basic types (I) sub-dewpoint Claus processes in which a higher conversion efficiency is obtained for the basic Claus reaction by operating the final catalyst bed(s) of the system at a very low temperature, i.e., below the dew point of sulfur in the gas stream (2) direct oxidation processes in which the Claus process section of the plant is oper-... 

Kettner, R.. Luebcke, T., and Stemfels, E. A., 1988, Experience with the MODOP Tail Gas Treating Process, paper presented at 38th Canadian Chemical Engineering Conference, Edmonton, Canada, October 4. 

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