Study Of Regeneration Technique For Spent Caustic Liquid From Ethylene Cracking

Spent caustic liquid from ethylene cracking is the waste caustic liquid produced from the caustic scrubbing of cracking gas in ethylene plant. Presently, there've been no new breakthroughs in comprehensive utilization of the waste caustic liquid from ethylene cracking both domestically and internationally. Based on the principles of less amount, non-harmful and resource-producing for treatment of waste materials, the oil-removed caustic scrubbed waste liquid of ethylene cracking gas is studied here. The purpose is to develop the caustification technology for regeneration of the remained caustic (NaOH, Na2CO3, etc.) from ethylene cracking spent caustic liquid, so that the regenerated caustic liquid can be recovered for using in the caustic scrubbing system of ethylene cracking gas. At the same time the contamination of it to the environment is eliminated and the waste material is turned to useful one.After caustification regeneration of the spent caustic liquid from ethylene cracking, the caustification conversion ratio of Na2CO3, can be over 75%, and the caustification ratio of the regenerated caustic liquid can keep over 85%. The caustification efficiency has no obvious drop after repeated caustification and absorption. The Na2S, Na2S2O3 and Na2SO4 and oil have little influence on the caustification process. Meanwhile the white mud can be repeatedly regenerated and used.Under the same total alkalinity, compared with fresh caustic liquid, the viscosity, density and surface tension of the regenerated spent caustic liquid varies little. The influence of its physical changes on the hydrodynamics and resistance to mass transfer in the caustic scrubber can be neglected. After repeated regeneration and usage, the absorbing performance of the spent ethylene caustic liquid doesn't vary much. It can meet the requirement of absorbing the acid gas such as CO2 in the cracking gas. Under the same total alkalinity, the absorbing rate of the regenerated spent caustic liquid (or mass transfer factor) is directly related with the OH concentration. The content of Na2S and Na2CO3 in the regenerated spent caustic liquid has negative influence on the absorption rate. So the collection of Na2S in the regenerated spent caustic liquid shall be avoided in every way and the caustification conversion ratio of the spent caustic liquid shall be increased as much as possible.