As indicated in Figure 10.3, the objective of the sulfur recovery process is to convert H2S to elemental sulfur. Sulfur recovery takes place in a series of two steps: Claus Process and SCOT Process [3]. In the Modified Claus Process, partial combustion of H2S takes place to generate SO2 that is reacted with the remaining H2S to recover sulfur as elemental sulfur. The Modified Claus Process, once-through burner operation, works only with acid gases that contain more than 50% H2S by volume. In the process, hydrogen in H2S is converted to H2O. The second stage, the SCOT Process, functions as a tail gas clean-up operation to remove the sulfur compounds produced in the side reactions of the Claus Process, i.e., carbonyl sulfide (COS) and carbon disulfide (CS2).
[3] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 283-290.
Figure 10.4 shows the configuration of the multi-step Modified Claus Process that includes two kinds of reactors: a burner reactor and a converter reactor. In the burner reactor, H2S is burned with compressed air to SO2 and H2O. Two critically important variables of the burner reactor are the oxygen to H2S ratio and the reactor temperature. The O2/H2S ratio needs to be one-third of the stoichiometric ratio for complete combustion of H2S. The significance of the O2/H2S will be discussed further in the next section. The temperature in the burner reactor must be maintained typically at 1850°F to make sure that any ammonia present in the feed gas is completely destroyed to protect the catalysts in the converter reactor. The effluent gas from the burner reactor is cooled to 450°F (above the dew point of S) in the waste heat boiler as it enters the converter reactor for catalytic conversion of H2S and SO2 to elemental sulfur and water. The converter effluent is introduced into a condenser unit to obtain elemental sulfur as a liquid product. Small quantities of S produced in the burner reactor may also be recovered after the waste heat boiler. Typically, three sets of converter-condenser units in series are needed to achieve 95% recovery of S in the Modified Claus Process.
Figure 10.5 shows the principal reactions in the Modified Claus Process in the burner and converter-reactor sections. In the burner, H2S is partially oxidized to produce H2O and SO2. In the reactor converter, the burner product SO2 reacts with the remaining H2S to produce elemental sulfur (the intended product in the sulfur recovery process) along with the side product water. Ideally, the final products should consist only of elemental sulfur and water with no H2S or SO2 present. The only way to achieve the intended product mix is to control the O2/H2S ratio in the burner. As can be seen, the stoichiometric ratio of O2/H2S for complete conversion of H2S to SO2 is 3/2 which would effectively convert all H2S to SO2. In order to reserve part of the feed H2S to react with the burner product so that no H2S or SO2 remains in the final product from the converter, the O2/H2S ratio should be controlled at 1/3 of the stoichiometric ratio, that is (1/3)(3/2)=1/2. As a self-check exercise, explain with chemical equations why the desired oxygen/hydrogen sulfide ratio in the feed to the Burner in the Claus Process should be 1/3 of the stoichiometric oxygen/hydrogen sulfide ratio (for complete combustion of hydrogen sulfide). The answer to this exercise is given at the end of the lesson.
As seen in Figure 10.5, the side reactions in the burner produce COS and CS2 which cannot be converted in the catalytic reactions that take place in the converter reactor. Therefore, a tail gas clean-up process, or SCOT Process, is needed to reduce the concentration of these side products to less than 20 ppm by volume in the outlet.
Figure 10.6 illustrates how the SCOT Process is integrated with the Claus Unit to convert COS, CS2 and any remaining SO2 by reacting with H2 in the catalytic reactor back to H2S to be recycled to the Claus Unit to close the loop. The hydrogenating catalysts used in SCOT contain nickel or tungsten on alumina support, and the reaction takes place at 480-570°. By coupling Claus and SCOT processes, more than 99% of sulfur entering the Claus unit can be recovered as elemental sulfur to be sold as a refinery product.