Modified Claus Process Applied to Natural Gas for Sulfur Recovery
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MODIFIED CLAUS PROCESS APPLIED TO NATURAL GAS FOR SULFUR RECOVERY Nicuşor VATACHI, Viorel POPA University “Dunărea de Jos” of Galati, ROMANIA Abstract: Approximately 25% of the natural gas being brought into production from new sources requires H2S removal and disposal. Consequently, sulfur removal processes will play an increasingly larger role in future gas processing. As late as 1950, over half of the world sulfur supply came from “voluntary producers,” that is, companies whose principal purpose was to produce elemental sulfur. Now, these producers furnish less than 5% of the world’s supply and “involuntary producers,” primarily petroleum refineries and natural gas plants, are the major source of the element. The most common method of converting H2S into elemental sulfur, is the Claus process or one of its modifications. Unfortunately, the exit stream from Claus plants usually cannot meet environmental emission requirements, and, consequently, a tail gas cleanup unit (TGCU) is often employed to eliminate the last of the sulfur compounds. The most commonly used processes are Shell Claus off Gas Treating (SCOT), SUPERCLAUS, and cold-bed adsorption (CBA). This paper describes Claus and tail gas cleanup processes. Keywords: sulfur recovery, gas processing, environmental protection , modified Claus process, tailgas cleanup processes. configurations are presented: straight-through and split flow: 1. INTRODUCTION - the straight-through process is the preferred and simplest. It can process feed streams Because approximately 25% of the natural that contain more than 55 mol% H2S; with gas being brought in to production from new air or acid gas preheat, and can process 30 to sources requires H2S removal and disposal, 55 mol% H2S in the feed; sulfur removal processes will play an - the split-flow configuration can process increasingly larger role in future gas processing. feeds that contain 5 to 30 mol% H2S. The Currently only two methods are available for straight-through process provides the dealing with large quantities of H2S: disposal of highest sulfur-recovery efficiency. the gas by injection into underground Unfortunately, the exit stream from Claus formations and conversion of the H2S into a plants usually cannot meet environmental usable product, elemental sulfur. emission requirements, and, consequently, a tail However, more commonly, H2S is converted gas cleanup unit (TGCU) is often employed to into elemental sulfur, much of which goes into eliminate the last of the sulfur compounds. sulfuric acid production. The most commonly used processes are As late as 1950, over half of the world’s Shell Claus off Gas Treating (SCOT), sulfur supply came from “voluntary producers,” SUPERCLAUS, and cold-bed adsorption that is, companies whose principal purpose was (CBA). to produce elemental sulfur. Now, these producers furnish less than 5% of the world’s 2. PROPERTIES OF SULFUR supply and “involuntary producers,” primarily petroleum refineries and natural gas plants, are The thermo physical properties of sulfur are the major source of the element. unusual. For a complete understanding of the The most common method of converting Claus conversion process, a brief discussion of H2S into elemental sulfur is the Claus process or the relevant properties is necessary. one of its modifications. Two modified 1024 Sulfur vapor exists as Sx, where x can have values from 1 through 8. Figure 1 shows the 3. CLAUS SULFUR RECOVERY distribution of sulfur-vapor species as a function PROCESS of temperature. All Claus units involve an initial combustion step in a furnace. The combustion products then pass through a series of catalytic converters, each of which produces elemental sulfur. The Claus process consists of the vapor- phase oxidation of hydrogen sulfide to form water and elemental sulfur, according to the overall reaction: 3⎛⎞3 33H 222SO+→ HOS +⎜⎟x (1) 2 ⎝⎠x The above overall reaction does not represent the reaction mechanism or show intermediate steps. In practice, the reaction is carried out in two steps: 3 ⎯⎯→ Fig.1. Sulfur-vapor species as a function HS22+ O←⎯⎯ HO 2+ SO2 (2) 2 of temperature. ⎯⎯→ ⎛⎞3 22H 22SSO++←⎯⎯ HO 2⎜⎟ Sx (3) ⎝⎠x Note that at lower temperatures, S8 dominates, but as the temperature rises S8 The first reaction is a highly exothermic converts toS6, and finally to S2. The sequence is combustion reaction, whereas the second is a not unexpected, because increased temperature more weakly exothermic reaction promoted by a means increased energy for the molecules, catalyst to reach equilibrium. which leads to the breakup of the clusters. The Figure 2 shows the equilibrium conversion formation of sulfur clusters has a very obtained for H2S into elemental sulfur by the pronounced effect on the physical properties Claus reaction. Kohl and Nielsen (1997) state that have a significant effect on processing that the unusual shape of the equilibrium curve operations, notably viscosity(fluid flow) and is caused by the existence of different sulfur heat capacity (heat transfer). species at different reaction temperatures. They point out that at a sulfur partial 100 S to sulfurS to 2 80 60 Percent conversionH of 40 0 205 425 650 870 1100 1315 1540 Temperature, oC Fig.2. Equilibrium conversion of hydrogen sulfide to sulfur 1025 pressure of 0.05 bar and temperatures below produce steam in a waste-heat boiler. Both 370°C, the vapor is mostly S6 and S8, but at the reactions take place in the furnace - boiler same partial pressure and temperatures over combination, and the gases exit the waste-heat approximately 540°C, S2 predominates. This boiler in the range of 260 to 343°C, which is shift in species causes the equilibrium constant above the sulfur dew point, so no sulfur in the reaction to shift from a downward slope condenses in the boiler. to an upward slope, as shown in Figure 2. This The combustion furnace-boiler is followed behavior has a significant effect on the by several catalytic reactors in which only the operation of the Claus process. The melting second reaction takes place because all the O2 point of amorphous sulfur is 120°C, and its has been consumed in the furnace. Each normal boiling point is 445°C. Figure 2 shows catalytic reactor is followed by a condenser to that the maximum conversion to sulfur by remove the sulfur formed. The gas is cooled to reaction (1) is obtained at temperatures near the 149 to 204°C in the condenser to remove melting point of sulfur, but to maintain sulfur in elemental sulfur. The condenser generally the vapor state, relatively high temperatures are achieves cooling by heat exchange with water to required. Consequently, if the catalytic produce low-pressure steam. Vapor that leaves converters are to operate under conditions in the condenser is at the sulfur dew point, so the which the sulfur does not condense on the gas is reheated before passing to the next catalyst, they cannot operate at optimum converter to prevent sulfur deposition on the equilibrium conversion. This is the reason for catalyst. A combustion-furnace flame having a series of converters, with the sulfur temperature of 927°C should be maintained product withdrawn from the reacting mixture because the flame is not stable below this value. between converters. Withdrawing the sulfur The straight-through configuration cannot be product causes the reaction (3) to shift to the used at H2S concentrations below 55%, because right, which results in more sulfur product. the feed gas heating value is too low. Figure 3 and Figure 4 show simplified flow Concentrations low as 40% are acceptable if the diagrams for the two common configurations, air or acid gas is preheated. straight through and split flow, respectively. In For H2S concentrations in the range 25 to the straight-through configuration, the first 40%, the split flow configuration Fig.4. can be reaction takes place in a combustion furnace utilized. In this scheme, the feed is split, and operating near ambient pressure (0.2 to 0.6 bar). one third or more of the feed goes to the furnace The air flow rate is adjusted to react with one and the remainder joins the furnace exit gas third of the H2S, along with any other before entering the first catalytic converter. combustibles, such as hydrocarbons and When two thirds of the feed is bypassed, the mercaptans. The H2S reaction is exothermic combustion air is adjusted to oxidize all the H2S 3 (24.000 kJ/m , at 25°C at 1 atm) and is used to to SO2, and, consequently, the necessary flame Fig.3. Straight-through Claus unit. 1026 Fig.4. Split flow Claus unit. Typically two thirds of feed bypasses the reaction furnace. temperature can be maintained. SUPERCLAUS is an example of selective oxidation for final sulfur removal. The process The split-flow process has two constraints: as described involves a slightly modified two- 1. Sufficient gas must be bypassed so that stage Claus unit followed by a third-stage the flame temperature is greater than catalytic reactor to oxidize the remaining H2S to approximately 927°C; elemental sulfur. Two reactors use the standard 2. Maximum bypass is two thirds because Claus catalyst, whereas the third reactor one third of the H2S must be reacted to form contains the selective oxidation catalyst. The SO2. If air preheating is used with the split-flow Claus unit itself is operated with a deficiency of configuration, gases with as little as 7% H2S can air so that the gas that exits the second reactor be processed. Generally, the sulfur recovery in contains 0.8 to3 vol% H2S. Sufficient air is the conventional plants discussed above varies added to this exit gas to keep the oxygen level from 90 to 96% for two catalytic converters. It in the 0.5 to 2 vol% range. increases to 95 to 98% for three catalytic The mixture then goes to the third reactor, converters.