Strategies to reduce the extent of product dilution in a tandem simulated-moving-bed process for ternary separation without loss of throughput

•Product dilution in tandem SMB increases energy consumption for its post-processing.•We devised strategies for reducing product dilution in tandem SMB comprising R1 & R2.•Use of different column lengths in R1 & R2 units was effective in reducing dilution.•Placing more columns in zones containing both edges of product band reduced dilution.•Using the above methods together maximized the effectiveness without throughput loss. One of highly-efficient separation processes for recovery of a target product in the pharmaceutical and fine chemical industries is a simulated-moving-bed (SMB) process. Although an SMB can ensure high separation capability and high throughput, it always has a problem of product dilution, which increases the energy consumption for post-processing of SMB product streams. This issue is even more important in a tandem SMB, which uses two subordinate SMB units in series for implementing a ternary-separation task. To address this issue, we sought to develop effective strategies for reducing the extent of product dilution in a tandem SMB without loss of throughput. For this task, a standing-wave-design (SWD) method was used to find a clue to reducing the extent of a tandem-SMB product dilution, which revealed the following results. First, for the considered issue, it could be effective to increase the column length of the SMB unit under the influence of column-efficiency limiting factor while decreasing the column length of the other SMB unit accordingly for the purpose of keeping total bed volume constant. It could also be effective to place more columns in the zones containing both edges of solute band for a target component. The two aforementioned strategies (called “S1” and “S2” respectively) were applied to the tandem SMB for recovery of neoagarotetraose from neoagarooligosaccharides. It was found that when the S1 method was utilized, the extent of product dilution in the tandem SMB was reduced by 56%. Furthermore, if the S1 and S2 methods would be used together, the extent of the tandem-SMB product dilution could be reduced by up to 85%. It is thus expected that the proposed strategies can contribute to a substantial improvement in the energy efficiency for post-processing of product stream from a tandem SMB process for ternary separation.