(63a) Preliminary Technical Feasibility Study Into Effect of Withdrawing Spent Caustic Stream From Existing Activated Sludge Treatment Plant

Our olefins plant produces olefins (ethylene, propylene, butylenes) as well as aromatics (Benzene, Toluene, and Xylene, or BTX) by thermally cracking naphtha in the presence of dilution steam. The plant employs an activated sludge (A/S) wastewater treatment plant to treat a combination of the following three major wastewater streams: the spent caustic stream (SCS) from the cracked gases scrubbing unit, the blowdown stream (DSG) from the dilution steam generator used to produce steam for the thermal cracker, and the dilution water stream (DWS) consisting of rainwater, equipment cleaning water, fire alarm testing and etc. As part of our waste minimization policy, a new plan is afoot to divert the SCS completely to a caustic soda recovery plant, thereby not only significantly reducing the total influent flow to the A/S plant but also changing the wastewater characteristics. This leads to a necessity to conduct a technical feasibility study into the corresponding effects on the performance of the A/S plant. In practice it is also necessary to know how the SCS flow diversion should be implemented in order to ensure smooth withdrawal of SCS during the transition period.

Simultaneously, the feasibility of total elimination of the DWS and furthermore the possible inclusion of wastewater from a nearby HDPE plant in the near future are to be investigated. To smoothly accomplish the above-mentioned series of planned changes, a series of preliminary experimental studies at the lab-scale level (using batch reactors and continuous reactors) have been carried out to study the A/S process performance under different scenarios. Based on the experimental data, a dynamic math model of the lab-scale A/S process is also developed and substantiated. In the next step the extension of the math model to the existing A/S plant will be carried out whilst taking into the difference in residence time distribution in the lab-scale and full-scale aeration tanks. Then the validated model will be used to predict the A/S plant performance in response to the planned withdrawal of SCS and DWS streams as well as the subsequent inclusion of the HDPE plant stream. What will be reported here is the lab-scale experimental results and the corresponding math model substantiated by these experimental results.