Ind. Eng. Chem. Res. 1988,27, 139-143
139
Study of an Adsorption Process Using Silicalite for Sulfur Dioxide Removal from Combustion Gases Sai V. Gollakota and Colin D. Chriswell* Ames Laboratory, Fossil Energy Program, Iowa State University, Ames, Iowa 50011
An adsorption process using silicalite, a hydrophobic molecular sieve, was investigated for the removal of sulfur dioxide from combustion gases. Silicalite, which is a polymorph of silicon dioxide, was found to remove SO2from simulated stack gases to give SOz-free effluents. Silicalite adsorbs sulfur dioxide in the presence of major components of stack gases such as nitrogen, oxygen, carbon dioxide, and water vapor. This process has the potential to reduce the sulfur dioxide content of hot combustion gases before they enter the turbine in a direct coal-fired power system. The sulfur dioxide adsorption capacity of silicalite was determined at temperatures from 25 to 800 "C and pressures from 1to 21.4 atm. The breakthrough curves were modeled based on the equation of Bohart and Adams. T h e adsorption characteristics were evaluated as a function of various parameters which are necessary for the design of a n adsorber. T h e adsorption-desorption properties of silicalite were compared with those of active carbon and a zeolite.
A major portion of the coal consumed in the United States is burned in utility and industrial boilers for generating electrical energy. Advanced direct coal-fired turbines, which have the potential to provide more economical power-generating systems, are being developed (Moore, 1985). The presence of various contaminants such as particulates, sulfur and nitrogen compounds, and trace metals in combustion gases is deleterious to the operation of the direct coal-fired turbine power systems. Thus, methods are being developed to clean up the coal-derived gas steams present a t high temperatures and high pressures for efficient operation of the turbines. As compliance with sulfur dioxide emission regulations is a major requirement due to the New Source Performance Standards (NSPS) established in 1971, and as no viable process is presently available for high-temperature/high-pressure removal of SOz,the present work focuses on SOz control of postcombustion gases prior to the turbine. Several throwaway processes (e.g., limestone/lime scrubbing) and recovery processes (e.g., catalytic oxidation/reduction) have been developed over the past 2 decades for desulfurization of stack gases from oil- and coal-fired power plants (Slack, 1971; Sulfur Dioxide Processing, 1975; Engdahl and Rosenburg, 1978). Among them, the adsorption processes have some unique advantages; e.g., regeneration of adsorbent usually has minimum energy requirements, a gas adsorber design is relatively simple compared to the design of a chemical reactor, and the waste disposal problems are usually minimum. A few adsorption processes such as the PuraSiv S process (Collins et al., 1974) and the Westvaco activated carbon process (Brown et al., 1972) have been well developed for SO2 removal at low temperatures. These processes, which have been primarily developed to meet clean air standards, usually require cooling of stack gases for efficient removal of SOZ. Thus, they may not be suitable for SOz removal directly from hot combustion gases typically present a t 1000-1400 O F and 5-30 atm. Much attention has recently been focused on evaluating new adsorbents such as silicalite, calcium silicates, and zinc ferrite for hot gas desulfurization. For example, Yang and Shen (1979) have studied the reaction of calcium silicates with SOz present in hot gases. A regenerative solid sorbent, zinc ferrite, is being evaluated for high-temperature removal of hydrogen sulfide from coal gas to be fed to molten carbonate fuel cells (Grindley et al., 1985). Silicalite, which is a hydrophobic molecular sieve, possesses unique physical and adsorption properties. This
material, which is a polymorph of silicon dioxide, is very stable to heat; it will degrade only slowly at temperatures higher than 1100 "C. It is also stable to most mineral acids. Furthermore, it has greater stability than other SOz adsorbents, such as carbon, under oxidative conditions required for adsorbent regeneration. It has been found that silicalite selectively adsorbs molecules smaller than its pore size, which is about 6 A. Thus, sulfur dioxide, which has a diameter of about 3.7 A (Wright, 1979), is likely to be strongly adsorbed on silicalite. As silicalite is composed entirely of silica, it does not have the ion-exchange properties typical of other conventional molecular sieves such as aluminosilicate zeolites. The aluminosilicate zeolites have limited stability in acidic environments due to susceptibility of aluminum to acid attack (Wright, 1979). Silicalite contains essentially no aluminum or cation sites. Because of the lack of strong field gradients due to aluminum or exchangeable cations, which in conventional molecular sieves serve as hydrophilic sites, silicalite exhibits a hydrophobic character (Flanigen, 1980). Commercial silicalite does contain small amounts of defect hydroxyl groups and is formulated with a nonhydrophobic binder. The defect sites and the binder have been found to result in only an insignificant accumulation of water (Klein, 1982). Thus, silicalite was found to have utility in a number of applications such as the removal of chloroform fr
