1. Field of the Invention
This invention relates to fluidizable zinc titanate materials in a particle size range of 50 to 400 .mu.m, which have high chemical reactivity for reduced sulfur species such as H.sub.2 S, COS, and CS.sub.2, high sulfur capture capacity, and good regenerability. Most importantly, this invention involves preparing zinc titanate materials with high attrition resistance, comparable to commercial fluid catalytic cracking (FCC) catalysts.
2. Discussion of the Background
Coal represents our largest fossil energy. The efficiency of converting this stored chemical energy to commonly used electrical energy can be improved significantly if the coal is first gasified and the resulting hot fuel gas is oxidized in either a turbine or a fuel cell. The projected thermal efficiency for such a system can be as high as 50%, compared to 30% to 35% for conventional coal combustion systems.
One of the major problems encountered in this approach is the presence of sulfur in coals, which is converted primarily to H.sub.2 S and to some extent to COS and CS.sub.2 during gasification. During combustion of the fuel gas in a turbine, H.sub.2 S oxidizes to SO.sub.2, which is a precursor to "acid-rain." In addition to its toxicity to the environment, higher concentrations of H.sub.2 S (several thousand ppm) are also detrimental--the gas is corrosive to turbine blades, other equipment, and instruments and adversely affects the performance of molten carbonate fuel cells due to sulfur poisoning of electrodes. Therefore, the H.sub.2 S concentration level in the fuel gas must be on the order of a few ppm or less.
Well-known processes for removing H.sub.2 S from fuel gas are wet processes operated within a liquid phase, for example, an amine solution. These processes, which operate at room temperature or relatively low temperatures, require cooling of the fuel gas and therefore impose a severe thermal penalty. Furthermore, the costs associated with these wet techniques for treatment of process-generated wastewater are relatively high. Therefore, to improve the process performance it would be desirable to have an easily regenerable sorbent for removal of reduced sulfur species at high temperature. Good regenerability would decrease not only the cost of the sorbent but also the costs associated with frequent loading and unloading of the reactors and the costs associated with disposal of used sorbent.
Earlier sorbent development work focused on iron oxide. U.S. Pat. No. 4,089,809, assigned to W. L. Farrior, disclosed a solid adsorbent consisting of iron oxide supported on silica for removal of H.sub.2 S from hot gaseous mixtures. Adsorbent was prepared in the form of pellets by admixing Fe.sub.2 O.sub.3, silica, and a suitable binder and tested in a fixed-bed reactor. No mention is made of sorbent suitable for fluidized-bed operation. Also, although it is claimed that the sorbent thus prepared has slightly better mechanical strength than one prepared by Shultz et al. (U.S. Pat. No. 3,579,923) on fly ash support, no results have been reported on the crush strength of the reacted pellets. Furthermore, no mention has been made of the nature of the fuel gas tested. It is widely known that highly reducing fuel gas leads to excessive weakening of iron oxide sorbents as a result of formation of iron carbide and reduction of iron oxide into metallic form. Moreover, the chemical equilibrium conditions impose a limit on iron oxide as to its efficacy in reducing H.sub.2 S levels down to a few ppm levels, depending on the temperature and water vapor content of coal gas. For example, at 550.degree. C. with 20% water vapor in gas, theoretically iron oxide sorbents can reduce H.sub.2 S levels down to no less than 361 ppm.
Studies at the U.S. Department of Energy's Morgantown Energy Technology Center (DOE/METC) and elsewhere revealed that zinc oxide by itself can reduce H.sub.2 S levels of fuel gas down to a few ppm level. In fact, a U.S. patent was granted to Institut Francais du Petrole of France (U.S. Pat. No. 4,088,736) in which a zinc oxide sorbent supported on silica and/or alumina was claimed. This sorbent was prepared by admixing zinc oxide, alumina, and a metal oxide, making a paste by adding water; extruding the paste into the desired shape; and allowing the extrudates to indurate between 500.degree. and 1000.degree. C. The sorbent was used in fixed beds. While, it has been disclosed that the process could be carried out in fluidized or moving beds, a sorbent material suitable for fluidized-bed application was neither prepared nor claimed.
Although the above patent discloses a process for desulfurization of a fuel gas, the gas composition used in testing the sorbents shown in embodied examples is not realistic. The gas containing 16.6% CO, 16.6% H.sub.2, 33.3% N.sub.2, and 33.3% H.sub.2 O is mildly reducing compared to actual gasifier gases such as Texaco or Shell. Despite the mild reducing nature of the gas tested, data shown in Example 8 of the patent indicate a 10% to 20% drop in crush strength and a 5% to 10% increase in bulk density, both indicating a deterioration in sorbent structure. The pure ZnO sorbents (supported on an inert support) have been known to lose zinc due to the reduction by CO and/or H.sub.2 present in the fuel gas thereby resulting in zinc vaporization which further leads to sorbent decrepitation.
Later sorbent development activities focused on the development of zinc ferrite sorbent, which was prepared by mixing equimolar amounts of zinc oxide and iron oxide. U.S. Pat. No. 4,769,045, assigned to Thomas Grindley of the U.S. Department of Energy, disclosed a process for removing H.sub.2 S from coal gasifier gas using a fixed bed of cylindrical extrudates of zinc ferrite. Although the fixed bed of zinc ferrite extrudates exhibited satisfactory performance in terms of bringing down the H.sub.2 S levels of coal gas to a few ppm levels, it suffered many limitations, including poor temperature control during highly exothermic regeneration of sulfided sorbent, unsteady state operation, and a nonuniform regenerator off-gas composition. Furthermore, high reaction temperatures (550.degree. C. or higher) and/or highly reducing fuel gases led to sorbent decrepitation due to excessive reduction of iron oxide and zinc vaporization as discussed earlier.
To overcome sorbent weakening problems, AMAX conducted a study to develop a method to produce durable zinc ferrite sorbents for fixed-bed applications. A U.S. Pat. (No. 4,732,888) was granted to AMAX, Inc., in 1988 that disclosed a recipe for producing durable zinc ferrite sorbents. Testing of the best AMAX zinc ferrite formulations at DOE/METC and at the Research Triangle institute (RTI) under a variety of operating conditions showed that higher temperatures (&gt;600.degree. C.) and/or highly reducing fuel gas led to sorbent degradation for the reasons outfined above. The sorbent degradation was mainly due to "chemical" transformations rather than by mechanical stresses as noted previously. (See R. Gupta and S.K. Gangwal, "Enhanced Durability of Desulfurization Sorbents for Fluidized-Bed Applications," Topical Report to DOE/METC, Contract No. DE-AC21-88MC25006, June 1991, NTIS No. NTIS/DE 91002090.)
In addition to their fixed-bed applications, zinc ferrite sorbents were also developed for fluidized-bed systems. Fluidized-bed hot-gas desulfurization systems overcome some of the major limitations of fixed beds and offer much better gas-solid contact efficiency, better temperature control, and much greater flexibility in design alternatives for continuous sorbent circulation between absorption (sulfidation) and regeneration reactors, thereby leading to high desulfurization efficiencies. However, the sorbent needed for fluidized-bed reactors must be highly attrition resistant in order for it to withstand stresses induced by rapid temperature swings, chemical transformations, and fluidization and transport. Testing of a number of zinc ferrite sorbents in a bench-scale fluidized-bed reactor at DOE/METC and RTI indicated that, like fixed-bed pellets, the application of fluidizable zinc ferrite sorbent particles was also limited to a maximum temperature of 550.degree. C. and to moderately reducing fuel gases containing at least 1 5% water vapor because of "chemical" attrition resulting from excessive reduction of iron oxide and zinc vaporization. (See R. Gupta and S. K. Gangwal, "Enhanced Durability of Desulfurization Sorbents for Fluidized-Bed Applications," Topical Report to DOE/METC, Contract No. DE-AC21-88MC25006, June 1991, NTIS No. NTIS/DE 91002090.)
Extensive kinetic and bench-scale testing of various candidate sorbents conducted to find alternatives to zinc ferrite sorbent that can be used at high temperatures with severely reducing gases showed that a zinc titanate sorbent can be used up to 750.degree. C. and in the presence of severely reducing gases. The zinc titanate sorbent contains ZnO and TiO.sub.2 in a suitable molar ratio (typical range being 0.8 to 2.0). Only ZnO is the reactive component of the sorbent while TiO.sub.2 provides stability to the sorbent by preventing zinc vaporization. A series of zinc titanate sorbent pellets with different ZnO to TiO.sub.2 molar ratios were investigated in fixed beds. (See Gangwal et al., "Bench-Scale Testing of Novel High-Temperature Desulfurization Sorbents," Final Report to DOE/METC, No. DOE/MC 23126-2662, 1988, NTIS No. NTIS/DE 89000935.)
The use of zinc titanate compounds for selectively removing H.sub.2 S at a temperature of 450.degree. to 600.degree. C. was not new. In a series of U.S. patents granted to Phillips Petroleum Company (the two most relevant are Nos. 4,313,820 and 4,725,415), a process was disclosed for removal of H.sub.2 S from gas streams. Also disclosed in these patents was the role of zinc titanate as a catalyst for a number of applications, including hydrodesulfurization for conversion of organic sulfur compounds into H.sub.2 S and its subsequent absorption by the catalyst, hydrotreating, catalytic reforming, catalytic hydrodesulfurization and denitrogenation, hydrocracking, oxidative dehydrogeneration, and selective hydrogenation. These inventions, however, dealt mainly with process studies in fixed-bed reactors demonstrating the use of zinc titanate as a catalyst material, using pellets of a size of 20 to 40 mesh. This particle size is obviously unsuitable for fluidized-bed applications. The H.sub.2 S removal temperatures were around 400.degree. C. In these patents, no sorbent material inventions were claimed and no procedures were described to produce fluidizable materials with high attrition resistance. Furthermore, no results were reported on the durability of these materials under a simulated fuel gas environment.
U.S. Pat. No. 4,977,123, assigned to Flytzani-Stephanopolous and Jothimurugesan of the Massachusetts Institute of Technology, disclosed a method for preparing mixed metal oxide sorbents, including zinc titanates, and catalysts in a pellet form suitable only for use in fixed bed reactors. The method consists of preparing a citrate precursor of the desired metal oxide, calcining it for 4 to 8 hours at 550.degree. to 850.degree. C., crushing and screening the calcined material to less than 210 .mu.m size, adding 2 to 7% of clay binder (typically bentonite), making a paste and extruding it through a die to produce pellets of a desired size and shape, drying the extrudates, and finally recalcining them for 2 to 6 hours at 650.degree. to 850.degree. C. Thus, this process, requires eight to ten complex steps. Production of commercial quantities of zinc titanate (typically required for 100- to 200-MW integrated gasification combined cycle [IGCCI plants) using this complex technique is not believed to be economically viable. Furthermore, the invention describes a method to prepare sorbents for fixed-bed applications only. Methods to prepare attrition-resistant materials suitable for fluidized-bed applications are not mentioned. Also, this patent disclosure does not provide any data on either the short- or long-term chemical reactivity and mechanical strength of zinc titanate compounds produced by this invention. To the best of our knowledge, the zinc titanate materials prepared using the disclosed technique were never even tested in a simulated fuel gas environment, not to mention the real fuel gas. Therefore, it is not known whether the zinc titanate materials manufactured using this technique will have desirable properties in a real system.
Also, it is not certain, due to the complex nature of the manufacturing process, to what extent the physical and chemical properties of materials produced batch-wise can be reproduced. In fact, later elaborate studies of this invention have shown that the material preparation technique is not reproducible. (See S. Lew, "High-Temperature Sulfidation and Reduction of Zinc Titanate and Zinc Oxide Sorbents," Ph.D. Thesis, Massachusetts Institute of Technology, 1990). If one were to use commercially available powders of zinc oxide and titanium oxide, this process cannot be used to produce fluidizable particles having a satisfactory value of mechanical strength even by crushing and screening. This is primarily because the particle size required in the intermediate step (after complexation) is between 63 to 210 .mu.m. If these larger particles were used as raw materials, the resulting product may have a highly non-uniform zinc and titanium distribution in the sorbent matrix. This nonhomogenity in Zn and Ti distribution can lead to zinc vaporization and, in turn, to very poor attrition resistance. Furthermore calcination is probably the biggest contributor to the sorbent cost. Calcining twice in this process and that to for an excessive amount of time (6 to 14 hours) will substantially add to material cost and make it commercially unacceptable.
Finally, in theory, particles in the correct size range for fluidized beds could be produced by crushing and screening the extrudates produced by the above technique. However, this is not practical due to two reasons. Crushing and screening will produce angular particulates with sharp edges which would be subject to high attrition as has been observed. For example, see R. Gupta and S.K. Gangwal, "Enhanced Durability of Desulfurization Sorbents for Fluidized Bed Application", Topical Report to DOE/METC, Contract No. DE-AC21-88MC25006, June 1991, NTIS No. NTIS/DE91002090. Also crushing and screening of the zinc titanate extrudates will give extremely low yield in the desired particle size range. Thus if the extrudates prepared by this technique were crushed and screening, the resulting particles would be unsuitable for fluidized bed operation. Furthermore, we have observed that crushing of calcined zinc titanate extrudates primarily leads to a flaky weak material not particles.
Attempts have been made to improve the attrition resistance of zinc titanate materials for fluidized-bed applications. In U.S. Pat. No. 4,477,592, assigned to Arthur Aidag, a process of catalytic reforming of a cyclopentane-containing organic feedstock using a zinc titanate catalyst was disclosed. A hydrogelling step was added in the manufacturing process to impart additional attrition resistance to the catalyst, which was used in a transport reactor type system. This hydrogelling step involved dispersion of a finely powdered (2 to 10 .mu.m) zinc titanate in a suspension of c-alumina monohydrate with the addition of nitric acid to form a hydrosol, which was then dried, calcined at 648.degree. C. for 2 hours, and finally crushed and screened to produce a 420- to 1190-.mu.m size catalyst to be used in a transport reactor. The attrition resistance of the catalyst thus prepared was claimed to be an order of magnitude better than the material prepared without hydrogelling. However, the increase in attrition resistance occurred at the cost of significantly reduced catalyst capacity due to reduced zinc titanate content. The maximum zinc titanate content claimed for the hydrogel material was only 50% by weight. Also, suitability of the hydrogel zinc titanate material was demonstrated only as a reforming catalyst and not as a, high-temperature desulfurization agent. Furthermore, the significant number of complex wet processing steps required in hydrogelling the zinc titanate would significantly increase the cost of the material, thus reducing its commercial viability.
Attempts were also made to produce zinc titanate materials in a granular form as aerogels suitable for a catalyst support primarily in polymerization and, to a lesser extent, in hydrogenation and isomerization applications. See U.S. Pat. Nos. 4,619,908 and 4,717,708, assigned to Cheng et al. of Stauffer Chemical Company. The preparation of these aerogels consisted of hydrolyzing zinc and titanium-containing solvents to produce a gel, then contacting the gel with an extraction fluid at supercritical conditions, and finally drying the gel. No specific mention of a zinc and titanium combination is made in the examples included in the patent. Also, because in the invention, materials were produced primarily for catalyst support, no specific mention of either particle size or attrition resistance is made. The manufacturing process involved a series of complicated, hard to reproduce and control, steps which are unlikely and expensive for commercial-scale manufacturing. The material has high surface area and very high pore volume and is only suitable as a catalyst. Very high pore volumes are known to lead to materials of poor strength, which would be very unsuitable in fluidized-bed reactors.