**1. Introduction**

The simulated moving bed (SMB) concept was proposed by Broughton et al. of UOP in 1961 for the separation of xylene isomers in the petrochemical field. It was then gradually applied to the sugar industry and chiral drug resolution areas [1–4]. The typical SMB system is developed based on the True Moving Bed (TMB), which involves several fixedbed chromatographic columns. It divides into four zones by four inlet and outlet ports (feed, raffinate, desorbent, and extract). Different from the TMB process, the counter-current movement of the solid phase towards the fluid phase (as shown in Figure 1) is achieved by the simultaneous synchronous switch of four streams [5–14]. Therefore, the problems associated with the movement of the solid phase can be solved, such as particle attrition, bed voidage variation, unstable flow rate, and bed expansion.

An illustration of the SMB process is shown in Figure 2. The feed stream containing both strongly adsorbed (heavy) and weakly adsorbed (light) components enters the system between zones II and III. With the column switching, the heavy component moves backward into zone II with the solid phase, while the light component is desorbed by the eluent and moves forward into zone III with the liquid phase, thus achieving the separation purpose. Therefore, the II and III zones are normally called the separation zone, where the operating conditions are set such that the two components move counter currently. After

**Citation:** Zhang, X.; Liu, J.; Ray, A.K.; Li, Y. Research Progress on the Typical Variants of Simulated Moving Bed: From the Established Processes to the Advanced Technologies. *Processes* **2023**, *11*, 508. https:// doi.org/10.3390/pr11020508

Academic Editors: Elio Santacesaria, Riccardo Tesser and Vincenzo Russo

Received: 12 December 2022 Revised: 3 February 2023 Accepted: 6 February 2023 Published: 8 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that, the heavy component is desorbed from the solid phase in zone I, which makes the solid phase regenerate, so zone I is also called the solid phase regeneration zone, while the light component is adsorbed in zone IV, which regenerates the liquid phase, so zone IV is the liquid phase regeneration zone [15–17]. For efficient operation, the solid and liquid phase regeneration zones are typically operated in co-current mode by setting the operating parameters appropriately.

**Figure 1.** Schematic diagram of counter-current movement.

**Figure 2.** Schematic diagram of Simulated Moving Bed.

SMB technology has its specific advantages for binary systems and complicated systems whose components have similar properties and as such are difficult to separate by traditional methods. In recent years, with the development of the economy and the progress of science and technology, the industrial requirements for energy consumption, solvent consumption, separation efficiency, product specifications, and flexibility of process control become higher and more stringent. As a result, some new SMB modes have been successively proposed. For example, the Varicol system based on the non-synchronous switching of inlet/outlet ports proposed by Ludemann-Hombourger improves the performance of SMB within a cycle, making the operation more flexible and the requirement for the number of columns significantly reduced [18,19]. The SMB process with internal flow rate changes realizes the redistribution of each component through the change of flow rate. This technology can save solvent consumption, but the process control becomes complicated [20]. Gradient SMB systems include the introduction of concentration, temperature, and pressure gradient. The introduction of a gradient condition can realize the reallocation of each composition, improve the efficiency of separation, and reduce the solvent consumption; however, the implementation process is complicated, with a need to ensure the synchronicity of the switch and the gradient change [21–23]. In addition, the sequential simulated moving bed (SSMB) developed in recent years [24] divides a switching of the traditional SMB process into three steps, and each step presents a different

operation mode. SSMB has shown excellent performance and great potential in terms of separation effect, process control, energy consumption and solvent consumption.

After years of research and development, diverse SMB-based variants have emerged with different operating modes and application areas [25,26]. This paper mainly focuses on an investigation of SMB variants and divides these into zone variant, gradient variant and feed or operation variant. The framework structure of this review is shown in Figure 3. According to the literature review and analysis, a detailed introduction and comparison were completed, and the separation mechanism, switching modes, advantages and disadvantages and applications of each SMB variant were summarized. Finally, this work can provide practical suggestions and references for SMB research works and industrial applications (intended for both the expert and novice), meanwhile putting forward the application prospect and future development direction.

**Figure 3.** The framework structure of review.

#### **2. Zone Variant**

As the conventional four-zone SMB technology is relatively mature, researchers attempted to change the zone partition or structure to make the equipment simpler, meanwhile improving the separation performance (purity, productivity, etc.) or reducing operating costs [27–29]. The so-called zone variant is to reduce non-essential functional zones to combine or delete one or several areas of the four-zone SMB. Usually, the separation zones (zone II and zone III) are retained, and zone I (solid phase regeneration zone) or zone IV (liquid phase regeneration zone) are modified. However, the reduction of the regeneration zone means that the solid and liquid phases cannot be adequately regenerated and recycled, leading to problems such as increased desorbent consumption [30–33]. In the following sections, zone variant will be investigated from four aspects: one-column SMB, two-zone SMB, three-zone SMB and bypass SMB; furthermore, their advantages, disadvantages, and applications are analyzed, respectively.
