(479c) Diafiltration Cascades Increase Solute Recovery and Purity

The development of more selective separation processes is necessary to meet society’s growing materials and energy demands. Improved solute selectivity, a problem typically addressed through the development of more selective materials, can also be achieved through staged processes.

This presentation focuses on the use of diafiltration cascades to improve the recovery and purity of target solutes. To begin, we explore the design of a module in which a diafiltrate is introduced uniformly along the length of the system. The analysis is experimentally validated, and the system is shown to possess three regimes (i.e., diluting, constant, and concentrating) that describe the concentration of the feed solution over the length of the module. The analysis is extended to examine multi-staged diafiltration cascades with two unique configurations based on the connectivity of the stages. Namely, a rectifying configuration uses the permeate and retentate from the previous stage as the feed and diafiltrate, respectively, to the subsequent stage. On the other hand, the stripping configuration uses the retentate of one stage as the feed for the subsequent stage and the permeate as the diafiltrate. For cascades that operate at a constant diafiltrate to feed flow ratio, it can be shown that the stripping configuration reduces the diafiltrate necessary to remove low molecular weight impurities from impermeable molecules. Alternatively, the rectifying configuration can improve the fractionation of solutes with similar size and charge. The analysis is completed with a discussion on how asymmetric cascades (i.e., each stage has unique diafiltrate to feed flow ratios) can improve the recovery and purity of solutes relative to systems that operate at constant diafiltrate to feed ratios.