A Study On Causticization-Crystallization Combined Technology Of Spent Caustic From Ethylene Plant

Ethylene spent caustic is the waste caustic liquid produced from the causticscrubbing of cracking gas in ethylene plant, its direct discharge will pollute theenvironment seriously. At present, the general disposal method is making theminnocuous, and then discharging. There have been no new breakthroughs in thecomprehensive utilization of spent caustic both domestically and internationally.A new causticization—Crystallization Combined Technology was proposed in thispaper, which is in the comprehensive utilization of spent caustic with the oil-removedspent caustic as the research object. At first, convert Na₂CO₃ into NaOH bycausticization reaction for the regenerating recycling of spent caustic;then purify theNa2S from spent caustic by crystallizing. In this way Na₂S can be reclaimed from thespent caustic and the Na₂S concentration reduced in spent caustic, ensuring theconversion rate of causticization reaction. At the same time, the acid gas absorptionexperiments have been done to confirm the acid gas absorption performance of caustic,and the absorption capability between regenerated caustic, fresh alkaline andSuper-Alkaline in absorption tower have been compared. The regenerate experiment ofwhite mud that is the product of causticization reaction has also been done to research theregeneration conditions.The results show that: â‘  The conversion rate of Na₂CO₃ is more than 90% whenreaction temperature is about 80℃, reaction time more than 90 min and stoichiometricratio is about 1.3. The results of absorption experiments show that the absorptionperformance of regenerated caustic is very good as its absorption rate is better than thatof Super-Alkaline in absorption tower, then the regenerated caustic can satisfy therequirement of comprehensive utilization. â‘¡ The recovery rate of Na₂S can reach 60%and the purification of Na₂S crystal can reach 30%, on the conditions of quantity of watervaporization is about 42%, crystal temperature is about 15℃, and crystal time is about 40min. â‘¢ CaO content can reach more than 97% when the regeneration temperature ofwhite mud exceeds 900℃.