high temperature hydrogen sulfide and carbonyl sulfide removal with manganese oxide (mno) and iron...

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  • Ind. Eng. Chem. Res. 1993,32, 139-149 139

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    High Temperature H2S and COS Removal with MnO and FeO on yA1203 Acceptors

    J. Peter Wakker,? Albert W. Gerritsen, and Jacob A. Moulijn* Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

    A process has been developed to remove H2S and COS at high temperatures (675-1075 K) from fuel gases produced by a coal gasification plant. The process uses MnO or FeO on y-A1203 acceptors which can be regenerated after sulfidation with a gas containing steam. The acceptors should be used in a fixed bed or moving bed reactor. It is important to know what influence the raw fuel gas composition of a coal gasification plant and operating temperature and pressure have on the sul- fidation behavior of the acceptors. Experiments are carried out in a laboratory setup to determine these influences. After an initial deactivation the acceptors can be used over 400 sulfidation-re- generation cycles. Water adversely affects the capacity, whereas CO favors the capacity due to the watewas shift reaction. Trace compounds can influence the capacity negatively as, e.g., HC1, whereas hydrocarbons do not influence acceptor performance. An optimum between capacity and stability is reached a t a operating temperature of 875 K. An increasing pressure does not influence the sulfidation reaction, but the water-gas shift reaction may be positively influenced.

    Introduction The composition of a raw fuel gas produced by a coal

    gasifier can vary over a wide range, depending upon the type of gasifier (fixed bed, fluidized bed, entrained bed) and the type of coal used. The main compounds produced by a gasifier are CO, H2, COz, HzO, N2, H2S, and COS. There are large differences in gas composition. The hy- drogen concentration varies from 29 to 40 vol %, the carbon monoxide concentration even from 16 to 65 vol % , the carbon dioxide concentration between 1 and 31 vol % , water between 1 and 20 vol %, hydrogen sulfide plus carbonyl sulfide between 0.1 and 1.5 vol %, and methane (and traces of other hydrocarbons) between 0 and 10 vol % (Brainard, 1982a,b; Miles, 1976; Nelson, 1979a-c; Schilling et al., 1979; Schlinger and Richtor, 1980; Shap- pert, 1983; Synthesis Fuel Associates, 1983). The fuel gas can be used to produce electricity, as a synthesis gas or as a reducing gas. In this paper we mainly consider the use of the fuel gas to produce electricity in a coal gasification combined cycle (CGCC) power plant. Before the fuel gas can be used it must be cleaned. A wide variety of existing commercial and well-developed gas processing systems is available for the removal of sulfur, ammonia, and other undesirables from the fuel gas. In general these are low

    Present address: Akzo Chemicals bv., P.O. Box 10,7400 AA Deventer, The Netherlands.

    temperature processes involving the use of various liquids with either an organic or aqueous base. Consequently, the gas must be cooled before admission to the scrubbing towers. However, when gas cleaning is carried out at high temperatures, the overall efficiency can be improved and lead to lower kilowatt-hour prices. In high temperature gas cleaning the following steps can be distinguished: dust removal, desulfurization, removal of halogens, removal of alkali metals and other trace compounds, and the removal of nitrogeneous compounds.

    A number of proceases for high temperature regenerative H2S removal is currently under investigation. The most important ones use zinc ferrite (e.g. Ayala and Marsh, 1991), zinc titanate (e.g. Gangwal et al., 1989) and copper based sorbents (Desai and Brown, 1990). The main dis- advantages of these processes are the limited applicability and the regeneration route. When these sorbent materials are used in highly reducing gases, as, e.g., the case with the Shell gasification process, the metal oxide is reduced into the metallic state, or metal carbides are formed. This results in loss of strength and capacity of the sorbent.

    Regeneration of these sorbents is carried out with an oxygen/nitrogen mixture. The regeneration is a highly exothermic reaction, and therefore only low oxygen con- centrations can be used. SOz is formed in low concen- trations. Further, the regeneration is complicated by sulfate formation. Due to the oxidative character of the regeneration, inevitable hydrogen consumption takes place

    0888-5885/93/2632-0139$04.00/0 0 1993 American Chemical Society

  • 140 Ind. Eng. Chem. Res., Vol. 32, No. 1,1993

    during the next sulfidation. This hydrogen can be better used to produce electricity.

    Two processes which possibly can produce elemental sulfur during regeneration are the mixed metal oxide process (Anderson and Hill, 1988) and iron oxide on silica (Wal, 1987), but both processes have not been proven yet.

    The process discuased in this paper can overcome several of the disadvantages of the above processes. The acceptors tested are able to remove H2S from the fuel gas to levels less than 20 ppm according to the following reaction: MeO/7-A1203 + H2S e

    Me = Mn or Fe (1) It has been proven by Wakker (1992) that the active compound in this reaction is a metal aluminate, MeA1204 (Me = Mn or Fe), making steam regeneration of the sul- fided acceptor material possible. Also other reactions occur, e.g., the water gas shift reaction and COS formation and capture. COS may be removed by direct reaction with the acceptor, or may be converted to H a which reacts with the acceptor according to reaction 1. The performance of the acceptors is strongly dependent upon gas composition and process variables such as temperature. Especially the water concentration is expected to have a strong influence on the acceptor performance as water shifts the equilib- rium in reaction 1 to the left. Therefore, the acceptor can be regenerated with a gas containing steam and H2S is formed back in a high concentration.

    Moreover, the gas produced by a coal gasifier can contain a number of compounds which may deactivate the acceptor or influence the acceptor performance negatively by blocking active sites. Among these compounds are heavy metals, HC1, HF, NH3, and HCN, and hydrocarbons. Because it is impractical to add heavy metals in low con- centrations to a synthetic gas mixture, one best can observe the influence of heavy metals by carrying out experiments under realistic conditions. These experiments are de- scribed elsewhere by Wakker and Gerritsen (1990b). The expected HCl concentrations for the Shell and Texaco gasification processes, fed with Australian Wambo coal, are 600 and 500 ppm and the HF concentrations 224 and 188 ppm, respectively. Wambo coal only contains 0.05 wt % Cl and 0.01 wt % F on a dry base (Alderliesten et al., 1990). Tests done with HC1 and hydrocarbons will be discussed in this paper as well.

    Experimental Section Acceptor Preparation. The acceptor material is pre-

    pared by wet impregnation. The carrier (7-A1203, AKZO/Ketjen 001-1.5E, ground and sieved, particle di- ameter 0.25-0.42 mm, surface area 260 m2/g) is added to the impregnation solution (3.3 mL of solution/g of carrier) and shaken a few times during impregnation. The im- pregnation time is about 16 h. For preparation of a manganese containing acceptor, a 2 M manganese acetate (Merck, pa.) solution is used. An iron containing acceptor is prepared from an 1 M iron(II1) ammonium oxalate (Riedel de Hiihn, p.a.) solution. After impregnation the material ie filtered, dried at room temperature during 5-6 h, and calcined in static air a t 575 K during 60 h. The manganese is now present as Mn02 or Mn203, and iron, as Fez03 or Fe30,. The last step in preparation is reduc- tion, carried out in situ in the reactor a t reaction tem- perature to obtain the active metal aluminate, MnAl2O4 or FeA1201. The manganese containing acceptors consisted of 7.10-8.23 wt % Mn on 7-A120, and the iron containing acceptor of 4.64 wt % Fe on 7-Al2OP Fresh acceptors have a surface area of 180-200 m2/g and an average pore di- ameter of 4-5 nm.

    MeS/y-A1203 + H20

    Instrumental De


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