A Comparison of Caustic Soda and Soda Ash as Chemical Media for SO2 Recovery in Oilfield Steam Generators

Abstract

Caustic soda and soda ash were used as alkalinity sources in the liquid scrubbing of SO2 in flue gases produced from crude-oil-fired steam generators used in thermally enhanced oil recovery. Caustic soda was used more efficiently, allowing the SO2 scrubber to run at a lower pH with a higher SO2 removal capacity. Overall economics favored the use of caustic soda over soda ash. Introduction This study compares caustic soda with soda ash for scrubbing sulfur dioxide in flue gas streams from stream generators used in thermally enhanced oil recovery. The data apply to many other applications as well. Reaction products were measured, and the scrubber chemistry was identified. Mass balances were made by varying chemical feed rates to determine SO2 recovery efficiencies and economics.While distinct differences were observed between caustic soda and soda ash, both the SO2 recovery and the economics favor the use of caustic soda, even though its cost per pound is greater. This is attributable primarily to differences in chemical-reaction kinetics and handling characteristics of the two chemicals. Scrubber Chemistry On a purely stoichiometric basis, 1 lbm caustic soda is equivalent in alkalinity to 1.33 lbm soda ash. However, observed chemical consumption rates are nonstoichiometric. In reality, more than 2 lbm soda ash are necessary to do the job of 1 lbm caustic. To understand this phenomenon, scrubber chemistry must be analyzed. Reactions in a system using a aqueous medium to scrub SO2 from flue gases, alkali must be added to maintain the desired degree of SO2 removal. At the point of alkali addition, several reactions can occur, depending on which alkali is used. A simplified overview of these reactions would be NaOHSO2 ---- Na2SO3, NaHSO3,....................(1) and Na3CO3SO2-------- Na2SO3, NaHSO3, CO2.............(2) In reality, the chemistry and much more complex. The actual scrubbing, or SO2 removal, is carried out with ionic species formed from the above highly soluble sodium salts.The chemistry of scrubbing and the ionic species present depend on which alkali is used. Once the present depend on which alkali is used. Once the alkali enters the scrubbing system, it reacts quickly according to Ionic Reactions 3 or 4, depending on whether caustic soda or soda ash is used: Caustic Soda OH + HSO3 ----------->SO3 = + H20.......(3) Soda AshCO3 =+2HSO3 ----------->2SO3 = +CO2 ......(4) JPT P. 751