Sulfur process, Wellman-Lord

Regenerable absorption processes have also been developed. In these processes, the solvent releases the sulfur dioxide in a regenerator and then is reused in the absorber. The WelLman-Lord process is typical of a regenerable process. Figure 11 illustrates the process flow scheme. Sulfur dioxide removal efficiency is from 95—98%. The gas is prescmbbed with water, then contacts a sodium sulfite solution in an absorber. The sulfur dioxide is absorbed into solution by the following reaction ... 

Since the Claus process by itself removes only about 90% of the hydrogen sulfide in the gas stream, the Beaven, SCOT, or Wellman-Lord processes are often used to further recover sulfur. In the Beaven process, the hydrogen sulfide in the relatively low concentration gas stream from the Claus process can be almost completely removed by absorption in a quinone solution. 

The S02 from the Wellman-Lord process or any other concentrated S02 stream (e.g. gases from copper smelting) can be oxidized to S03 to produce sulfuric acid as discussed in Chapter 6. 

Wellman-Lord The most widely used regenerable flue-gas desulfurization process. The sulfur dioxide is absorbed in sodium sulfite solution in a wet spray scrubber, forming sodium bisulfite ... 

Recycle processes convert the sulfur species in the tail gas to a single compound which is captured, concentrated, and returned to the Claus plant. Examples are the SCOT (10), Trencor (11), and Wellman-Lord (8,12) processes. The first two recover and recycle H2S while the Wellman-Lord process recovers and recycles SO2. 

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. 

In the Wellman-Lord process, sodium sulfite is used to capture the sulfur dioxide. The sodium bisulfite thus formed is later heated to evolve sulfur dioxide and regenerate the sulfite scrubbing material. The sulfur dioxide-rich product stream can be compressed or liquefied and oxidized to sulfuric acid, or reduced to sulfur. 

The catalyst is necessary to ensure that the components react with reasonable speed, bnt unfortnnately, the reaction does not always proceed to completion. For this reason, two or three stages are nsed, with sulfur being removed between the stages. For analysts it is valuable to know that carbon disulfide (CS2) is a by-product from the reaction in the high-temperature furnace. The carbon disulfide can be destroyed catalytically before it enters the catalytic section proper. Generally, the Clans process may remove only about 90% of the hydrogen sulfide in the gas stream, and as already noted, other processes, such as the Beaven process, SCOT process, or Wellman-Lord process are often used to recover additional sulfur. 

Figure 25.4 shows a typical sulfur recovery plant based on the Claus process. The tail gas from the Claus reactors may be further processed to remove any remaining sulfur compounds. Combined H2S removal efficiencies of 99.5-99.99 percent are achievable.20 This may be done by low-temperature Claus-type solid-bed processes (e.g., the Sulfreen process), wet-Claus absorption/oxidation processes (e.g., the Clauspol 1500 process), or hydrogenation of the off-gas to form H2S for recycle (e.g., the SCOT process). Residual sulfur compounds in the tail gas are then incinerated to S02. The residual S02 in the oxidized tail gas may be scrubbed by any of several processes (e.g., the Wellman-Lord process) before being vented to the environment. It is feasible to bring the H2S content of... 

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

The Wellman-Lord process uses the effective sodium sulfite-sodium bisulfite equilibrium to capture sulfur dioxide from flue gases [40] (Eqs. 3.26 and 3.27). 

Wellman—Lord process. This is a regenerable-sorbent process, producing SO2 gas, which can be sold for industrial uses. It uses a solution of sodium sulfite (Na2S03), which absorbs SO2 and becomes a sodium bisulfite solution (NaHSOs). The sodium bisulfite is then decomposed in a forced circulation evaporator, releasing the SO2 at sufficiently high concentration to be compressed and sold as SO2 gas, or used for producing elemental sulfur or sulfuric acid. ... 

The third class of control systems may use any of the sulfur dioxide control systems among those used commercially are the Haldor Topsoe (5, 80), Wellman-Lord (27, 07), and Chiyoda (27, 07, 87) systems. The circumstances are generally highly favorable for recovery processes that produce a stream of concentrated sulfur dioxide, since this can be recycled to the Claus plant. The application of processes that produce sulfuric acid or solid wastes will be dictated only by peculiar local circumstances. 

The adaptability of this sulfur dioxide reduction technology to a feed gas containing 100% sulfur dioxide (dry basis) will be demonstrated at the D. H. Mitchell Station of the Northern Indiana Public Service Co. (NIPSCO) at Gary, Indiana (8). In this application the process will be combined with the Wellman-Lord sulfur dioxide recovery process to provide a complete flue gas desulfurization system for a 115-MW coal-fired boiler in a project jointly funded by NIPSCO and the Environmental Protection Agency. 

Recent Experience of the Wellman-Lord Sulfur Dioxide Recovery Process... 

The first commercial application of the Wellman-Lord process was at the Olin Corp. sulfuric acid regeneration plant in Paulsboro, N. J. This plant regenerates 700 tons of sulfuric acid/day and handles a variety of spent acids. The sulfur dioxide content of the tail gas varies as the blends of these acids change. 

At Chiba, Japan, the Japan Synthetic Rubber Co. has two steam boilers with evaporation rates of 130 metric tons/hr, each equipped with Wellman-Lord sulfur dioxide recovery units. The load to each scrubber is essentially constant, but sulfur dioxide concentration varies from 400 to 2,000 ppm because of the varying quantities of waste process gas from the plant. 

The Wellman-Lord process can be a significant factor in helping domestic power plants to meet the air pollution abatement requirements of the Clean Air Act of 1970. To show its applicability to the utilities industry, Davy Powergas Inc. is building a demonstration installation at the Dean H. Mitchell Station of Northern Indiana Public Service Co. in Gary, Ind. When completed, it will consist of a Wellman—Lord sulfur dioxide recovery unit connected to an Allied Chemical Co. sulfur dioxide-to-sulfur reduction process to produce elemental sulfur. Davy Powergas guarantees emissions of 200 ppm by volume or less of sulfur dioxide at this facility. 

Processes that have been successful in other industrial applications presently are being studied for inclusion in the Wellman-Lord process. This would allow the system to operate as a closed loop with total sodium and sulfur dioxide recovery and would eliminate eflluent streams requiring treatment. Further test work is necessary, however, before including another idea into the commercially proved process. 

At chemical sites producing sulfuric acid, further reduction in SOj emissions of power plants is possible using the Wellman-Lord process in which pure SOj is recovered and recycled to the production process. The power plant thus supplies starting materials to the chemical production plant, and becomes an integral part of it. In the... 

An economic study by an independent engineering firm, commissioned by Union Carbide, compared the CANSOLV FGD process to five other commercial processes. Ibe processes compared were the co-current wet limestone, jet bubbler, countercurrent wet limestone, dry lime, and Wellman-Lord processes. The study showed that the economics for the CANSOLV process can be very favorable when compared with limestone processes in higji sulfur applications (Hakka et al., 1991). 

I en percent of the regenerator offgas is purified in a Wellman-Lord unit, and the concentrated sulfur dioxide from this unit is added to the remaining 90% of the gas which is then processed in a BSRP system. The Wellman-Lord process is described in Chapter 7. The BSRP system consists of a Beavon Sulfur Removal (BSR) unit to convert all sulfur compounds to hydrogen sulfide (see Chapter 8) followed by a Stretford unit to convert the hydrogen sulHde to elemental sulfur (see Chapter 9). 

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