Technology
Desulfurization process uses electrodiaiysis The water splitting section consists of two identical parallel trains, each of which contains three stages. A recirculation tank with each stage is vented to limit sulfur dioxide in the acid feed to the water splitters to its saturation concentration. Vented sulfur dioxide is combined with that from a succeeding stripper operation. Acid solution from the splitters, consisting of a sodium sulfate solution satuA lot of stack gas desulfurization pro- rated with sulfur dioxide, is transferred to cesses have been developed in recent a stripper, where the solution is heated by years to clean up the gases generated by direct injection of steam. Sulfur dioxide electrical power plants burning high-sul- from the stripper, combined with the fur coaL For the most part, they have been sulfur dioxide from the recirculation variations on a couple of basic themes. A tanks, then can be further processed to new process is now being groomed for the sulfur or sulfuric acid or can be liquecompetition that brings with it a new fied. Bottoms from the stripper consist of a technology. The process is being developed by Al- nearly pure solution of sodium sulfate. lied Chemical. It makes use of Allied's Most of it is recycled to the absorber, but developments in bipolar membrane sulfate must be removed in an amount technology applied in an electrodiaiysis equivalent to that formed by oxidation in technique. Development has progressed the absorber. This is done by feeding through small-scale pilot plant operation. some of the bottoms to an evaporator And a full-scale pilot plant of the elec- where water is removed. Sodium sulfate trodiaiysis portion of the process is ex- isfilteredand dried, and it is likely to be pected to be starting up within the next fairly pure and readily marketable, Allied says. Sodium values lost with the sulfate few months. A description of the process and a recap are made up through addition of sodium of the work to date was presented late last carbonate to the absorber scrubbing somonth in Chicago at the 174th National lution. Allied isn't saying much for now about Meeting of the American Chemical Society. Dr. Kang-Jen Liu, Dr. K. Nagasu- the bipolar membranes themselves. The bramanian, and Dr. F. P. Chlanda of Al- electrodiaiysis cells, however, make use of lied's corporate chemical research center, two types of membranes, cation-permeMorristown, N.J., point out the advan- able and bipolar, to separate a cell into tages of the process: Power consumption acid and base compartments. When a diis low; internal flexibility enables the rect current is passed across the electroprocess to operate under varying operat- diaiysis stack, hydrogen and hydroxyl ions ing loads; and the economics at this point are generated at the bipolar membranes. appear highly favorable. Maintaining electroneutrality in the acid The process is a regenerative type. and base compartments, cations migrate Sulfur dioxide is removed from the flue from acid to base compartments through gas by scrubbing the gas with a highly al- the cation-permeable membrane, Liu kaline solution of sodium sulfide/sodium points out. Experimental results on the system, hydroxide/sodium carbonate. In the absorber, sulfur dioxide is converted to so- Liu says, show that an excellent current dium bisulfite. Sodium sulfate also is efficiency of 85% is obtained. And, he says, formed in the absorber as a result of oxi- since the potential drop is only about 1.6 dation of sulfite species, the research volts per cell unit, the system consumes workers explain. less than half the energy required for a Spent absorber solution is filtered to conventional electrolysis acid/base genremove particulate and cooled to about eration process. To make economic comparisons, Allied 110° F before being fed into the electrodiaiysis section for regeneration. The so- has used studies of limestone scrubbing (a lution feeds both acid and base compart- throwaway-type process) and Davy ments of the water splitters. There the Powergas sodium scrubbing (a regenerasolution, consisting mostly of bisulfite, is tive process making sulfuric acid and soconverted to a basic solution of sulfite/ dium sulfate) made by Tennessee Valley hydroxide for recycle to the absorbers, Authority and PEDCo Environmental and to a solution of sulfurous acid from Specialists Inc., adjusted to January 1977 which concentrated sulfur dioxide is eas- costs. ily regenerated. The basis for the economic evaluation
New method based on bipolar membrane technology uses less power than other gas cleanup processes, operates under varying operating loads
Allied Chemical's Kang-Jen Liu works with small-scale electrodiaiysis system
is a moderate size (500 Mw) power plant located in the Midwest, burning highsulfur (3.5%) coal and operating at a load factor of 80%. On this basis, Allied finds that annual operating costs for its process before by-product credits is 2.52 to 3.34 mills per kwh. With by-product credits, the operating costs are 1.66 to 2.49 mills per kwh. Allied has worked out a comparison of its process with the other two on the basis of sulfuric acid production with its and the Davy Powergas processes and on-site sludge disposal for the limestone scrubbing process. Itfindsthat capital cost for limestone scrubbing would run $52.80 to $62.50 per kw; for the Davy Powergas process $69.60 to $96.76 per kw; and for its membrane process $41.08 to $63.75 per kw. A further plant-by-plant comparison of operating costs also was made. Average capital for the three processes was $26.2 million for the Allied process, $41.6 million for the Davy Powergas, and $28.8 million for the limestone scrubbing. Average annual operating cost on this basis came to 2.08 mills per kwh for the Allied process, 3.65 mills per kwh for the Davy Powergas, and 3.21 mills per kwh for limestone scrubbing. • Sept. 19, 1977 C&EN
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