Study On Deeply Oil-removal And Desulfurization Of Spent Caustic From Ethylene Plant

Key issues existing in the present treating technologies for spent caustic from ethylene plant, such as no economic benefits, incomplete treating, and environment pollution, can be resolved using a combined process of oil- removal--causticization--desulfurization, which can effectively recovery useful components, get valuable by-product ZnS and regenerated caustic. In this paper, laboratory and pilot studies on the combined process were conducted, and the results can provide reliable technical support for the industrial applications of this process. Detailed research results are as follows:1. Activated carbon adsorption can remove oil from spent caustic, at the space velocity 0.100L / (kg ? min), oil removal percentage could reach to 25% or more, and the throughput can reached 350L/kg. Ceramic membrane filtration is useful for oil removal from spent caustic, oil removal percentage is up to 28%. In the lower-flux and lower trans-membrane pressure, the membrane fouling rate is slower, throughput per unit area can reached to 400L/m2. A method of alternating oxidizer - hydrochloric acid - alkali-based for the contaminated membrane cleaning is effective, but costs high and generates waste liquid. The activated carbon adsorption for oil-removal from spent caustic for industrial production processes is suggested.2. For laboratory scale, optimizied operating conditions for the desulfurization process were determined as follows: reaction temperature 80～85℃,n(ZnO)∶n(Na2S)molar ratio 0.9:1, reaction time 150min. Under the conditions above, the S2- removal percentage and ZnS purity are higher than 88% and 95%, respectively.3. The desulfurization process could be described by the kinetic equation: The predicted Na2S conversions are in good agreement with the the experimental ones.4. The pilot results show that the combined process of quartz sand filter--PP cotton filter -- activated carbon adsorption is quite effective for oil removal. The throughput with oil removal 55 percent reached to 200L/kg activated carbon. In the conditions of: n(CaO):n(Na2CO3) molar ratio 1.1:1, causticization temperature 85℃, causticization time 150min, Na2CO3 conversion is above 75%, and in the conditions of: n(ZnO):n(Na2S) molar ratio 0.9:1, desulfurization temperature 85℃, desulfurization time 150min,the desulfurization percentage and ZnS purity are higher than 88.5% and 96%, respectively.5. The economic analysis results show that the process with throughput 20000m3/a costs 320 million$ of total investment, generates 9.768 million $/a of the profit.