Desulfurization sulfur recovery

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. 

Although the Claus catalytic conversion is a highly efficient process as presently employed in sulfur recovery plants the continuing efforts to reduce sulfur emissions to atmosphere demand that the last possible ounce of efficiency be squeezed from the process. Whether further small but critical improvements in the already high sulfur recovery efficiency can be achieved by more fine tuning of the converters and their catalyst charge remains to be seen. What cannot be accomplished in the catalytic converters will be achieved in the tail gas desulfurization processes. 

These developments and similar tail gas desulfurization processes based on the Claus such as the Bumines Citrate (43) IFP (44) and Clean Air (45) processes are simply methods for forcing the Claus redox reaction of the upstream converter units further to completion. Thus they increase sulfur recovery from H2S and... 

Reductive Tail Gas Treatments. It was largely as a result of the effort to achieve better than 99% recovery that the reductive tail gas desulfurization processes (46) were developed in the 1970 s. The two main methods are the Beavon Sulfur Removal (BSR) (47) and the Shell Claus Off-Gas Treatment (SCOT) (48) processes. Both of these processes are now widely used as tail gas desulfurization units on sulfur recovery plants and can readily achieve point source emission levels below 250 ppm and below 100 ppm if necessary to meet regulatory standards. 

In the regenerator, coke deposited on the catalyst is partially burned to form carbon monoxide in order to reduce iron tetroxide and to act as a heat supply. In the desulfurizer, sulfur in the solid catalyst is removed and recovered as molten sulfur in the final recovery stage. 

The equipment of an all-out purification unit is certainly less complex than that of the selective type. However, since the combined removal of the sulfur components and the entire CO2 would leave the acid gas with so low a sulfur load that sulfur recovery in a Claus unit would not be possible any more, this arrangement requires a considerable effort to desulfurize the offgases sufficiently to conform to environ mental legislation and to make sulfur recovery cost-effective. 

As already mentioned at the beginning of the chapter on gas cleaning, the oxidative processes do not eliminate the acid gas component by a reversible method and release it again as the solvent is regenerated. Rather, a reaction takes place in the wash liquor so that the absorbed gas component is obtained in some other form. This is of particular interest for selective desulfurization of raw gases and fcH H2S removal from acid gases to facilitate sulfur recovery on the one hand and. 

Biocatalytic desulfurization of diesel fuel Sulfur recovery using oxygen-enriched air California smog control Zero emissions from a THF plant Volatile organic compound (VOC) abatement—thermal incineration, catalytic incineration, or adsorption, for ozone control... 

Beside the above-mentioned catalysts used for the production processes of ammonia, hydrogen, urea and other important inorganic chemicals, some other catalysts might be used during some accessorial processes. They are N2 production catalysts, CO selective oxidation catalysts, sulfur recovery catalysts, CO2 dehydrogenation catalysts, molecular sieve desiccants and de-poison catalysts such as desulfurization, dechlorination, and dearsenization, etc. 

Advanced Hot Gas Desulfurization. Hot gas desulfurization has already been discussed in the acid gas removal portion of this section. Advanced systems are being developed for air-blown IGCC applications. Acid gas removal and sulfur recovery are two separate processes for conventional systems. 

The overall sulfur recovery is 97% for low-sulfur coal and 99% for high-sulfur coal. The clean desulfurized coal gas is saturated with hot water for nitrogen oxide (NOJ control, and then preheated before being burned in a GE frame 7E combustion turbine. The turbine firing temperature is 1,985°F. The turbine combustors have been specially designed to burn the 265 Btu/scf (dry) gas. The addition of steam to the gas achieves a total water vapor content of about 25 vol% before combustion. The combustion turbine generates about 65 MW of electricity. 

Coal gasification does not consume limestone for sulfur recovery. Limestone- or hme-based desulfurization systems produce at least 1.4 pounds of CO2 per pound of sulfur removed. The dry desulfurization systems such as fluidized bed combustion, spray dry scrubbers, and sorbent injection, require large amounts of excess limestone or lime and therefore produce 3-5 pounds of COj per pound of sulfur removed. 

The feed air preheats to about 1,300°F in a gas-fired heater. The gasifier is designed to operate at 20 atmospheres pressure and to convert only about 50-70% of the coal to fuel gas. Therefore, recovery of a large amount of char in the hot raw coal gas for direct combustion is necessary. VEW now appears to favor dry sulfur recovery fi-om the coal gas via a circulating fluidized bed of limestone. Alternatively, VEW is considering designs with no coal gas desulfurization, thereby increasing the amount of sulfur dioxide in the flue gas fi om the pulverized boiler. 

FIGURE 2.8 Demonstration of sulfur recovery from a long term saturation test performed by feeding 10 seem of simulated biogas through a concentrated 20% enzyme solution for lOOh. Data shows the transient profile for the outlet H2S concentration, whereas the inset shows a picture of the yellow sulfirr precipitate. These results highlight the potential of this enzymatic desulfurization technology to also recover elemental sulfur as a valuable byproduct. 

The SASOLI sulfur recovery plant initially used a conventional Stretford unit to desulfurize Rectisol regeneration off-gas. Operational problems in the SASOL I Stretford plant led to a decision to switch to the Sulfolin process in the SASOL I, SASOL II, and SASOL in plants. The following problems were encountered in SASOL I with the Stretford process ... 

Because hydrocarbon feeds for steam reforming should be free of sulfur, feed desulfurization is required ahead of the steam reformer (see Sulfur REMOVAL AND RECOVERY). As seen in Figure 1, the first desulfurization step usually consists of passing the sulfur-containing hydrocarbon feed at about 300—400°C over a Co—Mo catalyst in the presence of 2—5% H2 to convert organic sulfur compounds to H2S. As much as 25% H2 may be used if olefins... 

Emissions control systems play an important role at most coal-fired power plants. For example, PC-fired plants sited in the United States require some type of sulfur dioxide control system to meet the regulations set forth in the Clean Air Act Amendments of 1990, unless the boiler bums low sulfur coal or benefits from offsets from other highly controlled boilers within a given utiUty system. Flue-gas desulfurization (FGD) is most commonly accomphshed by the appHcation of either dry- or wet-limestone systems. Wet FGD systems, also referred to as wet scmbbers, are the most effective solution for large faciUties. Modem scmbbers can typically produce a saleable waUboard-quaUty gypsum as a by-product of the SO2 control process (see SULFURREMOVAL AND RECOVERY). 

Sulfur Dioxide Emissions and Control. A substantial part of the sulfur dioxide in the atmosphere is the result of burning sulfur-containing fuel, notably coal, and smelting sulfide ores. Methods for controlling sulfur dioxide emissions have been reviewed (312—314) (see also Air POLLUTION CONTROL PffiTHODS COAL CONVERSION PROCESSES, CLEANING AND DESULFURIZATION EXHAUST CONTROL, INDUSTRIAL SULFURREMOVAL AND RECOVERY). 

Minor and potential new uses for ammonium thiosulfate include flue-gas desulfurization (76,77), removal of nitrogen oxides and sulfur dioxide from flue gases (78,79), converting sulfur ia hydrocarbons to a water-soluble form (80), and converting cellulose to hydrocarbons (81,82) (see Sulfur REMOVAL AND RECOVERY). 

Calcium carbonate is finding increasing use in flue gas desulfurization. This appHcation by a variety of engineering processes traps the sulfur—oxygen compounds produced in the combustion of coal (qv) (see Coal conversion process Exhaust contdol, industrial Sulfurremoval and recovery). 

In the United States calcium carbide-based acetylene is mainly used in the oxyacetylene welding market although some continues to be used for production of such chemicals as vinyl ethers and acetylenic alcohols. Calcium carbide is used extensively as a desulfurizing reagent in steel and ductile iron production allowing steel mills to use high sulfur coke without the penalty of excessive sulfur in the resultant steel (see Sulfurremoval and recovery). Calcium cyanamide production continues in Canada and Europe (see Cyanamides). 

Recovery of iodine hy action of sulfur dioxide on aqueous sodium iodate Hydrogenation of vegetable oils with gaseous hydrogen Desulfurization of gases by scrubbing with aqueous ethauolamiues... 

Newer secondary recovery plants use lead paste desulfurization to reduce sulfur dioxide emissions and waste sludge generation during smelting. Battery paste containing lead sulfate and lead oxide is desulfurized with soda ash to produce market-grade sodium sulfate solution. The desulfurized paste is processed in a reverberatory furnace. The lead carbonate product may then be treated in a short rotary furnace. The battery grids and posts are processed separately in a rotary smelter. 

A procedure for immobilization of a P. stutzeri UP-1 strain using sodium alginate was reported [133], This strain does not perform sulfur-specific desulfurization, but degrades DBT via the Kodama pathway. Nevertheless, the report discussed immobilization of the biocatalyst cells in alginate beads with successful biocatalyst recovery and regeneration for a period of 600 h. However, the immobilized biocatalyst did decrease in specific activity, although the extent of loss was not discussed. The biocatalyst was separated after every 100 h of treatment, washed with saline and a boric acid solution and reused in subsequent experiment. The non-immobilized cells were shown to loose activity gradually with complete loss of activity after four repeat runs of 20 hour each. The report does not mention any control runs, which leaves the question of DBT disappearance via adsorption on immobilized beads unanswered and likewise the claim of a better immobilized biocatalyst. 

Desulfurization of other diesel feedstocks from Total Raffinage was also reported by EBC. In these studies, different engineered biocatalysts were used. Two different middle distillate fractions, one containing 1850 ppm sulfur and other containing 650 ppm sulfur, were tested. R. erythropolis sp. RA-18 was used in one experiment and was reported to desulfurize the diesel from 1850 to < 1200ppm sulfur within 24 hours. On the other hand, it removed sulfur from a middle distillate with 650ppm sulfur to below 200 ppm sulfur [222], Various Pseudomonas strains were also tested in this study and reported to remove less amounts of sulfur. A favorable characteristic of the Pseudomonas strains is their inability to form stable emulsions, which can be useful trait for product recovery. 

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