From Environmental Management: Wastewater and Groundwater Treatment

Petroleum refineries, electric power plants, sulfide-based metal smelters, and other SO2-emitting industries worldwide are under increasing regulatory pressure to reduce their sulfur emissions. To date, non-regenerative fluegas-desulfurization (FGD) methods involving lime or limestone plus scrubbing with caustic soda have been the predominant SO2-removal technology, especially in power plants and refineries. However, regenerative FGD methods involving liquid-phase absorption are seeing increasing application.

Various regenerative approaches, which use organic or inorganic solvents for SO2 absorption, are becoming more competitive vis-à-vis conventional caustic scrubbing. One already well-established example is the sodium sulfite process, also known as the Wellman-Lord process, with more than 30 commercial units worldwide.

More recently, regenerative processes employing other chemicals have been developed. These newer technologies are easy to operate, and are expected to have lower investment and operating costs compared to their predecessors. The new processes allow reduction of SO2 levels in the treated gas to below 100 ppmv.

Capital costs for regenerative processes are higher than for non-regenerative caustic scrubbing, due to the need for additional equipment for solvent regeneration. On the other hand, a lesser chemical consumption results in lower operating cost. All in all, the economic viability of regenerative processes is critically dependent on factors such as solvent cost, the solvent’s SO2-holding capacity, the method of solvent regeneration, and regeneration heat duties. Here, we offer a detailed, model-based analysis to aid the selection of optimal solvents for regenerative SO2 recovery. Three options are studied:
• Physical solvents, such as polyglycol ethers
• Regenerant solutions buffered by inorganic salts
• Regenerant solutions buffered by aqueous diamines

In the last-named two options, the role of the buffer is to accommodate the acidity that arises when SO2 dissolves in water; this buffering allows the process units to have high cyclic capacity and absorption efficiency.

© 2014 Chemical Engineering

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