*4.6. Pseudo-SMB*

Pseudo-SMB is a new SMB technology mainly used to separate ternary mixtures. The process can be seen as a combination of the true moving bed and simulated moving bed. There are two main steps. The first step is similar to the TMB process, where the ternary mixture (A, B and C) is injected into the inlet and the desorbent (D1) is also injected, with the aim of separating component B, which has an intermediate affinity for the desorbent. The second step is similar to the SMB process, and closes the device with only the desorbent (D2) injected from the inlet in order to collect the components A and C, respectively. In the process of separating ternary mixtures, the pseudo-SMB is relatively easy to operate and has advantages for small-scale ternary separation [26,56,91–93].

## *4.7. Outlet Swing Stream (OSS) SMB*

Gomes et al. [94] proposed an unusual SMB in which the flow rates of zones II and III were kept constant based on the conventional SMB, and the flow rates of each outlet (extract, raffinate and desorbent) were artificially manipulated to dynamically adjust the flow rates of zones I and IV for the separation operation. It can effectively improve the product purity and reduce the desorbent consumption [26].

#### *4.8. Backfill-SMB (BF-SMB)*

To improve the separation and chromatography performance of conventional SMB, Kim et al. [95] proposed a strategy called backfill-simulated moving bed (BF-SMB). A part of the product is refilled into the SMB from the feed node or intermediate node as a feed to simulate a TMB-like effect, enriching the main components near the product extraction node, thereby improving the separation of the SMB performance. This strategy can effectively improve product purity without compromising recovery and desorbent consumption [17,95].

#### *4.9. SMB Cascades*

Complex separation tasks such as the separation of ternary or more multiple mixtures can be handled by properly connecting multiple SMBs working in succession. This process is called SMB cascades, also known as tandem SMB. It is generally operated using two (or more) consecutive SMB units. A ternary (or multiple) mixture is fed from the inlet into the first SMB unit, and after separating a single material the remaining mixture is introduced into the second SMB unit for further separation. If the initial feed has only three mixtures, two SMB cascades will separate them all; if there are more than three, a third SMB unit will need to be passed through, and so on. Since each unit is independent of the others, it is possible to set independent parameters without unit interfering. After the cascade operation, the separation performance is greatly improved compared to the conventional SMB. However, it is important to note that the more SMBs are cascaded, the more diluted the sample becomes, and thus the productivity is reduced. Therefore, the idea of bypass SMB, as mentioned before, can be used for proper priming to ensure sample purity [26,96,97].

#### *4.10. SimCon*

Song et al. [98] proposed a novel SMB strategy called SimCon. Under the constraints of the maximum allowable pressure or flow rate, the feed flow and product flow are simultaneously controlled. This operation can make the flow and pressure fluctuations in the column as small as possible to improve the performance of the SMB. The SimCon operation consists of three steps. In the first step, when the desorbent is injected, only the raffinate port is opened. In the second step, all inlet and outlet ports are opened, which is consistent with the conventional SMB process and is called an intermediate step. In the final step, only the raffinate port is closed, and the other ports are opened [17]. Experimental data have confirmed that, compared with conventional SMB, SimCon operation can increase the product purity by 3.2%, the recovery rate by 3.1%, the productivity by 0.9 g/L/h, and the desorbent consumption by 0.04 L/g [98]. The separation performance and process cost can be effectively optimized by the SimCon strategy.

In conclusion, changing the different feeding or operating modes can effectively improve the separation efficiency, reduce the solvent consumption, or increase the product purity; a brief description of three main variants is listed in Table 1. Through these changes, while improving the performance of the equipment, SMB can also handle more challenging separation tasks, improve flexibility, or make the operation simple.


**Table 1.** A summary and comparison of three SMB variants.

#### **5. Conclusions**

In this paper, three significant types of SMB variants were introduced and analyzed. First, modifications of the conventional SMB process based on zone structure changes were reviewed. In most zone variants, the separation performance and process economy could be improved by simplifying the operating zone construction. Secondly, gradient variants of the SMB process were investigated, in which SMB's performance was effectively enhanced by introducing concentration, temperature, or pressure gradients with a result of altering the adsorption behavior of each zone. Finally, the SMB variants with different feed or operation modes were researched. This revealed that the separation performance could be adjusted by using ModiCon, VariCol and PowerFeed modes, alone or in combination, or choosing new SMB technologies such as ISMB, SSMB, Pseudo-SMB, OSS, BF-SMB, SMB cascades, and SimCon. According to the literature review and analysis results, it can be concluded that: (1) The use of new SMB technology or the combination of ModiCon, VariCol, and PowerFeed modes have a promising application in the future. (2) Multi-component separation by using new SMB technology will also be an important and challenging research direction. (3) The combination of the new SMB methods and multi-objective optimization (MOO) strategy can effectively improve the separation performance of SMB, which will simplify the process design and provide valuable guidance for practical industrial applications.

**Author Contributions:** Conceptualization, X.Z. and Y.L.; validation, Y.L., J.L. and A.K.R.; investigation, X.Z. and Y.L.; writing—original draft preparation, X.Z.; writing—review and editing, Y.L. and A.K.R.; supervision, Y.L., J.L. and A.K.R.; funding acquisition, Y.L. and A.K.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by [NSERC Canada], grant number [RP438568]; National Natural Science Foundation of China, grant number [22268031]; Natural Science Foundation of the Inner Mongolia Autonomous Region, grant number [2021BS02003]; Basic Research Funding for Universities Directly Under Inner Mongolia Autonomous Region, grant number [JY20220212].

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors express their great thanks for the support from the NSERC Canada (Grant No. RP438568), National Natural Science Foundation of China (Grant No. 22268031), the Natural Science Foundation of the Inner Mongolia Autonomous Region (Grant No. 2021BS02003), and the Basic Research Funding for Universities Directly Under Inner Mongolia Autonomous Region (Grant No. JY20220212).

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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