Claus process efficiency

In the second section, unconverted hydrogen sulfide reacts with the produced sulfur dioxide over a bauxite catalyst in the Claus reactor. Normally more than one reactor is available. In the Super-Claus process (Figure 4-3), three reactors are used. The last reactor contains a selective oxidation catalyst of high efficiency. The reaction is slightly exothermic ... 

Hydrosulfreen A process for removing sulfur compounds from the tail gas from the Claus process. It combines the Sulfreen process with an upstream hydrolysis/oxidation stage, which improves efficiency and optimizes the emission control. Developed jointly by Lurgi and Societe National Elf Aquitaine, and installed in 1990 in the Mazovian Refining and Petrochemical Works, near Warsaw, Poland. See also Oxysulfreen. 

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 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 efficiency of the Claus Process, long the means of conversion of H2S to sulfur, has been increased through improvements in reaction furnace, catalyst bed and computerized feed composition control leading to recovery efficiencies in excess of 98%. Recent development of a Claus Process under pressure may yield further important improvements. Add on tail gas clean-up processes have further reduced plant effluent in response to environmental protection requirements. 

Furnace temperatures have also been shown to be important in controlling the formation of COS. While COS has little effect on the downstream Claus catalyst efficiency its presence in the gas stream leads to higher loading of the reductive tail gas clean-up processes (e.g. SCOT, BSR, see environment) or to higher SO2 emissions in the stack gas. The recent developments regarding the control of its formation in the front end furnace are thus a significant contribution to the improvement of environmental quality control. 

Recent developments in Claus converter efficiency can be divided into two categories improvements in process operating technique and improvements in catalyst management. 

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. 

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... 

Extent of deactivation is a basic criterion determining the efficiency of industrial catalysts. In the case of alumina catalysts widely used in the Claus process the main reason for their deactivation is sulfation (refs. 1,2) The poisoning by sulfates leads to a reduction in life of Claus catalysts, which usually does not exceed ilixee years ... 

The next four papers concentrate on another emission source, hydrogen sulfide from Claus units. The Claus process has been used for several years to remove hydrogen sulfide from petroleum refinery waste gases. However, current environmental concerns require a more efficient recovery of the sulfur values. Some of the new technology to improve Claus plant efficiencies is discussed in these chapters. 

Chemical solvents, e.g., alkanolamines, or physical solvents may be used as concentrating processes from which desorbed H2S can be fed to a Claus process and may also be used to deplete CO2 selectively when a raw gas has too high a C02 H2S ratio for a Claus plant to handle efficiently. The initial thermal stage (950-1,250 °C) of Claus processes oxidizes H2S to SO2, the latter then oxidizing more of the former and producing predominantly S2 species, as predicted by Fig. 1 ... 

The Shell Oil Sulfinol-M process removes sulfur (mostly as H2S) from the coal gas. This process uses a mixture of MDEA and a physical solvent. It has advantages over processes that use only physical solvents because it avoids refrigeration, flash gas recompression, and large electricity requirements (36). A Claus process then converts the H2S to by-product liquid elemental sulfur. A Shell Oil SCOT system processes the Claus tailgas for higher sulfur recovery efficiency. The overall sulfur recovery is about 99%, leaving less than 120 ppmv of total sulfur in the purified coal gas. The low sulfur levels in the coal gas allow low stack temperatures without acid dew point corrosion. 

The literature describing the theoretical as well as design and operational aspects of the Claus process is quite voluminous. In view of this extensive coverage, the following discussion will be directed primarily toward current technology on the design and operation of plants that provide high efficiency sulfiir recovery and low emission of sulfiirous pollutants to the atmosphere. 

The catalyst used in the Claus process is normally either granular natural bauxite or alumina shaped into pellets or balls. For high-efficiency plants, an alumina catalyst of high activity is usually preferred. Resistance to attrition and to the relatively high temperatures during activity restoration procedures or rejuvenation are also important catalyst properties. Furthermore, since the Claus process is operated at low pressure (S-12 psig), the catalyst shape must be such that an excessive pressure drop is not incurred at typical design space velocities. 

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-... 

As shown in Figure 8-1, the equilibrium conversion of H2S to sulfur increases with decreasing temperature in the moderate temperature region, and continues to decrease as the temperature is reduced below the sulfur dew point, approaching 100% at a temperature of about 2S0°F. Conventional Claus process catalyst beds are maintained at a temperature well above the sulfur dew point to avoid the deposition of liquid sulfur on the catalyst, but this precludes the attainment of the high efficiencies possible at lower temperatures. 

Oxidation catalysts were among the first to be described and then developed industrially. Because of the energy evolved, oxidation processes were originally known as catalytically induced combustion. Some of the earliest catalytic oxidation reactions used commercially are shown in Table 4.1. This list could also include the Deacon and the Claus processes, which were described in Chapter 2. Subsequently, nitric acid and formaldehyde were produced on a large scale by catalytic oxidation processes. In most early processes, once a reasonable eatalyst had been developed, production was limited only by demand and the availability of efficient equipment. 

A disadvantage of the hydrocarbon—sulfur process is the formation of one mole of hydrogen sulfide by-product for every two atoms of hydrogen in the hydrocarbon. Technology for efficient recovery of sulfur values in hydrogen sulfide became commercially available at about the same time that the methane—sulfur process was developed. With an efficient Claus sulfur recovery unit, the hydrocarbon—sulfur process is economically attractive. 

Raw material usages per ton of carbon disulfide are approximately 310 m of methane, or equivalent volume of other hydrocarbon gas, and 0.86—0.92 ton of sulfur (87,88), which includes typical Claus sulfur recovery efficiency. Fuel usage, as natural gas, is about 180 m /ton carbon disulfide excluding the fuel gas assist for the incinerator or flare. The process is a net generator of steam the amount depends on process design considerations. 

The production of COS in the front end reaction furnace presents special problems since sulfur in this form may be difficult to remove in the downstream catalytic beds under conditions that are optimal for the Claus redox reaction between H2S and SO COS (and CS2) were known to be generated from hydrocarbon impurities carried over in the acid gas feed thus the efficiency of the up-stream sweetening process became an important factor. The reaction of CO2, a common constituent of the acid gas feed, with H2S and/or sulfur under furnace temperature conditions has also been shown to be an important source of COS. 

Among the most effective of the modifications to Claus operating procedure is accurate temperature control of the catalyst beds. Gamson and Elkins (27) in the early 1950 s showed that equilibrium sulfur conversion efficiencies in the catalytic redox reaction rise dramatically as operating temperatures are lowered toward the dewpoint of sulfur. While some highly efficient subdewpoint Claus type processes are now in use the bulk of sulfur production from H2S still requires that the converters be operated above the dewpoint. Careful control of converter bed temperature has, however, contributed to improved efficiencies. This has in large part resulted from better instrumentation of the Claus train and effective information feed back systems. 

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. 

Using essentially the same sub-dewpoint Claus reaction principle the Cold Bed Absorption (CBA) process of Amoco (47) achieves the same level of tail gas desulfurization. The low temperature high efficiency swing converters can be in line rather than as a tail gas clean up add on unit. 

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