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Challenge
While the extraction and distillation steps involved in this system are individually fairly straightforward for Koch Modular engineers, there were challenges with the concurrent optimization of both operations. The extraction step involved in this process removes organic acids from an aqueous stream using an extraction solvent, whereas the distillation step allows for the separation of the solvent and organic acids so that the solvent can be recycled back into the extraction column. Pilot tests were performed for both steps but expected results were initially not obtained. More specifically, the published VLE data provided by the client did not support the distillation pilot test results. A recovered solvent overhead product was expected to be at 50 ppm of organic acids at most; however, pilot plant results were indicating levels of around 200 ppm instead. As a result, the pilot extraction column was operated with a recycled solvent higher in organic acids concentration and the required raffinate specification could not be met.
SOLUTION
Koch Modular engineers overcame these inconsistencies by generating new VLE data and subsequently performing pilot tests producing reliable results and optimizing the extraction and distillation steps. The pilot extraction set-up involved a 3” diameter x 60-stage SCHEIBEL® Column with standard impeller diameters. The solvent was spiked with varying amounts of organic acids to assess the conditions in which the target raffinate in the extraction column could be achieved. Based upon the performance during pilot plant testing, a solvent-to-feed ration of 3.5 was determined, though operating at a lower ratio still achieved essentially the same raffinate concentration for organic acids. Ultimately, a 60-stage agitated SCHEIBEL® Column was chosen for the extraction column due to the rapid and easy phase separation observed during the pilot tests. A distillation column consisting of structured packing and internals was designed for the solvent recovery. Additionally, Koch Modular engineers suggested operating the distillation column under vacuum as a better separation is achieved with fewer stages and reduced energy (steam) consumption.
To briefly review the process taking place, the aqueous feed stream containing organic acids enters an extraction column operating with an extraction solvent. The extraction column bottoms product is the treated water which is sent to a neutralization drum off module. The extraction column top product is an organic acids and extraction solvent mixture which is sent into a distillation column. The distillation bottoms product consists of organic acids which are sent off to a reboiler and a spent acid cooler. The distillation top product is the recovered extraction solvent which is sent to the solvent condenser and accumulator, and eventually recycled back into the extraction column.
RESULTS
Koch Modular provided two modules containing the extraction and distillation columns. At heights of 72’ and 82’, these modules were two of the largest delivered to date. The system was delivered with a performance guarantee to provide a treated water product containing ≤1100 ppmw organic acids. With the bulk of the organic acids removed from the aqueous feed stream, the treated water product is then neutralized off module and transported to a biological wastewater treatment plant where the final traces of acid are removed.
The installation of both modules was successful and the system has been running ever since with no reported issues.