Ind. Eng. Chem. Res. 1993,32, 2812-2819
2812
Development of A Zero-Emissions Sulfur-Recovery Process. 2. Sulfur-Recovery Process Based on the Reactions of HzS and COSat High Temperature Gavin P. Towlert and Scott Lynn’ Department of Chemical Engineering, University of California at Berkeley, Berkeley, California 94720
When HzS is heated above 600 OC in the presence of COZ,elemental sulfur and synthesis gas are formed. A sulfur-recovery process using this chemistry is as follows: HzS is heated in a hightemperature reactor in the presence of COZand a suitable catalyst. The primary products of the overall reaction are SZ,CO, Hz, and HzO. Small amounts of COS, S02, and CSZalso form. Rapid quenching of the reaction mixture to roughly 600 “C prevents loss of SZduring cooling. Carbonyl sulfide is removed from the product gas by hydrolysis back to COZand H2S. Unreacted COZand HzS are removed from the product gas and recycled t o the reactor, leaving a gas consisting chiefly of HZ and CO, which recovers the hydrogen value from the HzS. This process is economically favorable compared to the existing sulfur-recovery technology and allows emissions of sulfurcontaining gases to be controlled t o very low levels.
Introduction When hydrogen sulfide is heated to temperatures above 600 OC in the presence of COZ,the conversion of H2S to elemental sulfur is much greater than in the absence of C02. The first paper in this series (Towler and Lynn, 1993) describes the thermochemistry and kinetics of this system and suggests that a sulfur-recovery process based on this chemistry would have several advantages relative to the Claus process. This paper gives a detailed description of such a process. A flowsheet is presented and important aspects of the design are discussed in more detail. The first sulfur-recovery process based on the H2S/COz system at high temperature was that proposed by Bowman (1991). While studying thermal cycles for hydrogen sulfide splitting,Bowman observed the same experimental effect described in the first paper in this series. Bowman patented a process based upon his thermochemical calculations, but these were somewhat incomplete and did not include an analysis of either the kinetics or the reaction mechanisms of the system. It is therefore possible to develop a process that is significantly better than that proposed by Bowman by exploiting the kinetics and other factors described in the previous papers in this series. No other report of a study of this chemistry was found in the literature.
Process Description The base-case flow diagram for the Berkeley ZeroEmissions Sulfur Processis shown in Figure 1. This section gives a description of the main features of the base-case process. The process is then divided into three sections, and a flowsheet mass balance, for a basis of 50 tonneslday sulfur production, is presented for each. The first stage in the process is a standard absorbed stripper loop for recovering HzS from the hydrocarbon gas. The “sour” fuel gas can be any HzS-containing gas, e.g., refinery gas, natural gas, or coal gas. The gas is contacted with a suitable solvent that absorbs all of the HzS and the desired amount of COZfrom the gas, leaving + Current address: Centre for Process Integration, Department of Chemical Engineering, U.M.I.S.T., P.O.Box 88, Manchester M60 lQD, United Kingdom.
0888-5885/93/2632-2812$04.O0lO
a treated fuel gas stream that is sulfur-free to the extent required by specifications. Typically, the HzS content is reduced to roughly 160 ppm if the sweetened gas is to be used as refinery fuel gas, but in some cases stricter limits must be met, depending on local legislation. In the event that the fuel gas contains no COZ,a make-up stream is introduced later in the process. The selection of solvent and design of the absorber to meet these criteria are well understood, and design equations can be found in standard texts such as Kohl and Riesenfeld (1985), which was used in developing the flowsheets presented below. A typical solvent would be a solution of an ethanolamine in water; for example the flowsheet given below uses a 2.0 kmol/m3 solution of diethanolamine (DEA). The “rich” solution containing dissolved H2S and COZ is heated in the first stripper, regenerating the solvent and producing a gas stream containing the desired proportion of HzS and C02, saturated with water vapor. This gas is cooled to remove as much water as possible since Hz0 adversely affects the reaction equilibrium, as discussed the first paper in this series (Towler and Lynn, 1993). This cooling is normally accomplished by having a few extra plates at the top of the stripper column, with cold water circulating over them; the water is cooled in an external cooler, as shown in Figure 1. If the cooler uses cooling water as coolant, then the gas temperature can be lowered to roughly 40 O C (or lower, depending on climate), which corresponds to a water vapor concentration of 7.4 mol % at 1-atm pressure. It is advantageous to reduce this water concentration, for reasons discussed in the first paper in this series (Towler and Lynn, 1993). The stripper may therefore be run at higher pressure than 1atm. The optimum ratio of H2S to COz in the reactor feed gas depends on a number of factors. Firstly, the reactor yield (moles of S/mole of feed) is maximized when the feed gas is roughly 70 mol % HzS, dry basis-this minimizes the process recycle. Increasing the amount of COz in the reactor feed increases ita heat capacity and hence the hightemperature heat requirement of the process; however, increasing the amount of HzS increases the amount of HzS to be recycled, and hence the second stripper heat requirement (since the heat of reaction between HzS and the solvent is usually greater than that between COZand the solvent). The trade-off between these effects depends on the relative availability and cost of high- and low0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32,No. 11, 1993 2813 Furnace Make-up
BFW Steam
Sweet"
Reactor Section
First AbsorberHydrolyzer Stripper Loop Figure 1. Base-case flowsheet for the Berkeley Zero-Emissions Sulfur Process.
temperature heat within the process, and is something that would be addressed at the detailed design stage. A typical feed composition might be expected to be roughly 60mol% H2S, dry basis, as used in the flowsheet presented below. The gas mixture is then sent to the reactor section, which consists of the high-temperature furnace and associated heat exchangers. The object of the reactor section is to heat the gas and allow it to come to equilibrium at a high temperature, and then quench it rapidly to a temperature a t which the kinetics of the important reactions are very slow, so that these reactions do not have time to reequilibrate a t a lower temperature. There are several methods for achieving this quench. The greatest quench rates (9000-100 OOO K/s) are achieved by spraying a stream of cold water into the gas; however, this does not permit recovery of heat from the gas and may encourage the formation of SO2 by promoting the reverse of the Claus reaction. Quench rates of 5000-60 000 K/s can be achieved by cooling the gas in a waste-heat boiler using pressurized boiling water as coolant (as used in the thermal cracking of ethylene). The steam thus produced can be used as a heat source for the process strippers. Alternatively, the reactor-exit gas may be cooled by countercurrent heat exchange with the incoming gas, giving quench rates of roughly 2000-10 000 K/s. It was found experimentally that a quench rate of roughly 1000 K/s is adequate to prevent loss of elemental sulfur during cooling (Towler and Lynn, 1993),so any of these methods would be suitable. The arrangement of heat exchangers around the furnace is thus determined in part by the quench requirement and partly by heat-recovery needs and again depends on the relative availability of high- and low-temperature heat. Process energy requirements and heat recovery are discussed below. The thermochemical analysis presented in the first of these papers showed that the process performance is improved by running the furnace at higher temperatures. In practice this is limited by the availability of materials that will withstand the corrosive, sulfur-containing environment at high temperatures. The choice of materials for the furnace is discussed below: materials are identified that can withstand temperatures up to lo00 OC; however, the furnace would probably be kept at 950 O C for safety purposes.
Second Absorber-
Stripper Loop
The gas leaving the furnace contains C02, HzS, CO, H2, COS, H20, elemental sulfur, and traces of SO2 (
