*2.1. One-Column SMB*

One-column SMB was firstly developed by Wankat et al. [34,35] as a one-column chromatography with multiple tanks like a four-zone SMB cycle. As shown in Figure 4, the main principle is a four-step cycle. In the first step, the feed stream and the solution in tank 2 are fed into the column, the less retained component (raffinate) is collected at the output port, and meanwhile tank 1 is filled. In the second step, tank 1 is fed into the column and tank 4 is filled. Then, in the third step, fresh desorbent and the solution of

tank 4 enter into the column, while the more retained component (extract) is collected and tank 3 is filled. In the final step, tank 3 and tank 2 work in the same way.

**Figure 4.** Schematic diagram of one-column SMB.

At present, there are few studies on this kind of SMB variant, and the one-column system exhibits obvious advantages and disadvantages. Specifically, it is cheap and flexible, and has the advantage if frequent desorbent changes are required. However, there is also a clear disadvantage that, due to the use of storage tanks, there is a mixing process during the whole operation and switching processes, which reduces the separation efficiency.

#### *2.2. Two-Zone SMB*

Two-zone SMB is proposed by Lee et al. [36]; its structure is shown in Figure 5. In this system, only the separation zones (zones II and III of the conventional SMB) are reserved, implying that the eluent will directly enter zone II, and the feed mixture will enter zone III. In the nth switching, the second half of the light component will move towards the end of zone III with the liquid phase and finally leave the column, while the heavy component moves backward with the solid phase and remains in zone II. At the *n* + 1st switching, the first column of the original zone II moves to the end of the zone III and becomes the last column of the zone III, so that the heavy component previously retained in zone II enters into zone III and leaves the column, while the first half of the adsorbed light component will immediately flow out from the exit, so that the two components can be collected separately.

**Figure 5.** Schematic diagram of Two-zone SMB: (**a**) The end of the previous switch. (**b**) The beginning of the next switch.

Wankat et al. [37] designed another two-zone SMB using a two-step process, combining zones I and II of the conventional SMB into a new zone I, and zones III and IV into a new zone II. At first, the feed stream is introduced between zone I and zone II, while some desorbent circulates from zone I to zone II, and the remaining desorbent is sent to the tank. In the second step (no feed), the fresh desorbent and the desorbent in the tank are used to produce the product. The raffinate and extract products are collected from zone I and zone II, respectively. At the end of the second step, all the ports are switched and the whole operation process is repeated.

In conclusion, the two-zone SMB has the advantage of low cost and is more economical due to the isolation of two regeneration zones. In addition, relatively high purity can be achieved from this simplified equipment. For example, in Lee's work [36], compared to the conventional SMB, the two-zone SMB improved the purity and recovery of the fructose-rich product from 0.78% and 4.11% to 15.67% and 15.87%, respectively. As a result, the separation cost was reduced due to the low material consumption and simple column arrangement. Moreover, there still exist obvious disadvantages: (1) Although the port switch of two-zone SMB is similar to that of conventional four-zone SMB, it cannot achieve the countercurrent movement of the solid and liquid phases, so it is not available for continuous operation. (2) The purity and recovery of the two-zone SMB is lower than that of the four zone SMB, owing to the simplification. (3) The final purity cannot be easily increased by increasing the number of columns in each zone.

#### *2.3. Three-Zone SMB*

The three-zone SMB is the most studied mode among these variants; the schematic diagram is shown in Figure 6. In the typical three-step process, there is no desorbent cycle loop so the liquid phase regeneration zone is isolated. The desorbent enters from the end of zone III, and the binary mixture is fed between zones I and II. Finally, the heavy fraction (extract) exits between zones II and III, and the light fraction (raffinate) exits from the front end of zone I [38–41].

**Figure 6.** Standard three-zone SMB with three-step.

#### 2.3.1. Three-Zone SMB without Zone IV

Wang et al. [42] designed an open-loop three-zone SMB in which zone IV is isolated. Then, this system is applied for the separation of Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) using a 1-1-2 column configuration, as shown in Figure 7. Zone I is completely independent and is used for elution. Zone II and zone III work as purification and adsorption sections, respectively. The mixture is separated in zone III, and the raffinate product EPA is obtained at the outlet (R port). The fluid phase is withdrawn in zone II to improve the separation of the components adsorbed in zone III. The eluent is withdrawn in zone I, and the extract product DHA is collected at the outlet (E port) to realize the regeneration of the column. Under the control of the automatic system, zones I to III move continuously along the fluid phase direction for one column length at each switching time. The results showed that, in the 2-2-2 mode, the purity of both DHA and EPA reached 99% and the recovery was close to or at 100%. The solvent consumption was 1.11 L/g, which was significantly lower compared with 1.46 L/g in the 1-1-2 mode [42]. In consequence, this kind of variant could improve both the separation performance and the economical efficiency of SMB.

**Figure 7.** Schematic diagram of three-zone SMB (with 1-1-2 column distribution).

## 2.3.2. Three-Zone SMB without Zone I

Wei et al. [43] developed a three-zone SMB without zone I, which can simultaneously achieve the high purity and low desorbent consumption. For instance, zone I is isolated, and the desorbent directly enters zone II. During the switching time, due to the column switching, the part of the solution that retains more components partially enters zone IV and continues to move forward as the extract. The other part is at the first column in zone II, as the raffinate, and is converted from a discontinuous to continuous liquid stream under the action of the desorbent. The extractive residue and extractables were collected from zone III and zone IV, respectively.

#### 2.3.3. Port Variant

Lee et al. [44] proposed a new three-zone SMB system called three-port SMB (TT-SMB). Actually, it is a combination of the above two three-zone operations. In the first step, the extract port is closed, then the desorbent and feed solution are injected into the inlet of zones I–IV and the feed node between zones II and III, respectively. When the extract port is closed, the solution from zone III is the raffinate. In the second step, the raffinate port is closed and the desorbent is supplied to the inlet of zone II. Since there is no raffinate port, the stream from zones I to IV is the extract solution. The results showed that the product purity was generally improved by 1–4%, the recovery was generally improved by 0.8–4.8%, and the productivity was increased by up to 13.8 g/L/h using TT-SMB compared to conventional SMB.

#### 2.3.4. Other Variants

Wankat et al. [39] put forward two ways to improve the operation of the three-zone SMB, namely "partial withdrawal" and "partial feeding". The "partial withdrawal" mode is shown in Figure 8. In the first step, the obtained raffinate is recovered. In the second step, the desorbent is recycled during the switching time. In the third step, the raffinate is recovered and the cycle repeats. The "partial feed" mode is shown in Figure 9. Compared to the other three-zone SMB, this system only has a pulse feed in the second step during the switching time and no feed is introduced in the first and third steps. With the same feeding method, the three-zone SMB improved the recovery by up to 8.87% and the purity by up to 7.82% compared to the four-zone SMB.

**Figure 8.** Schematic diagram of the three-zone SMB ("partial withdrawal" operation).

**Figure 9.** Schematic diagram of the three-zone SMB ("partial feed" operation).

In summary, compared to the conventional four-zone SMB, the separation performance in areas such as purity, recovery and eluent consumption of the three-zone SMB is inferior due to the open-loop structure and the diluted raffinate stream. However, this zone modification has several advantages: (1) Adsorbent consumption and the required equipment units such as valves and pumps are reduced. The productivity of the three-zone SMB becomes higher due to the saved amount of adsorbent. (2) The operation of three-zone SMB is relatively simple. (3) The productivity, desorbent efficiency, product purity and recovery can be improved by introducing the partial feed or partial withdrawal operation, which effectively overcome the drawbacks of the three-zone SMB.

#### 2.3.5. Applications for Three-Zone SMB

The application of three-zone SMB in biological separation was investigated by Keβler et al. [45] and Kim et al. [46,47]. Keβler et al. studied the separation of IgG/lysozyme mixtures and the purification of dimeric BMP-2 from multicomponent mixtures by using the three-zone SMB with a concentration gradient. The results showed that solvent consumption was significantly reduced, and productivity was improved compared to the conventional SMB. In Kim's work, guanine and cytosine were successfully isolated from nucleotides with the three-zone SMB method. The final product purity of cytosine and guanine were achieved at 95% and 90%, respectively. Another work of Kim et al. used three-zone SMB to isolate immunoglobulin Y (IgY) from eggs, and the final IgY purity of 98% was obtained. The above research results show the excellent separation performance and development potential of three-zone SMB for bio-separation.

The application of three-zone SMB in enantiomeric drug separation was investigated by Cunha et al. [48] Two enantiomers (L-PZQ and D-PZQ) of praziquantel (PZQ) were successfully isolated using three-zone SMB. At least one enantiomer with high purity and productivity could be obtained. However, there are few research works focusing on the enantiomeric drug separation by using the three-zone SMB, indicating that the variant still has a great research potential in this area.

Yao et al. [49], Pangpromphan et al. [50,51] and Nam et al. [52] investigated the application of three-zone SMB in the food separation process. Yao et al. constructed the first asynchronous three-zone SMB for the separation of vanillin and syringaldehyde. They finally obtained relatively high product purity and effectively improved the feed flow rate. Pangpromphan et al. successfully separated Alpha-Tocopherol and Gamma-Oryzanol in rice bran oil using three-zone SMB. A mathematical model of adsorption kinetics was constructed and the corresponding operating conditions were optimized. Optimization results showed that the final purity of both products was quite high. The above examples reveal that three-zone SMB is a very effective technology for the separation of food ingredients, and meanwhile possesses high product purity and great modification potential.

#### *2.4. Bypass SMB*

Rajendran et al. [53] reported a new operation mode based on conventional four-zone SMB, as shown in Figure 10. The feed solution enters the system between zones II and III, and the extract and raffinate are recovered between zones I and II and between zones III and IV, respectively. After the separation and purification, the feed streams of the binary mixture are bypassed to the extract and raffinate streams with a certain volume. In this way, the desired product purity can be obtained by conjunctively purifying and mixing.

**Figure 10.** Schematic diagram of bypass SMB.

The advantages of bypass in SMB are high operating flexibility and good selectivity, which makes it suitable for cases where the purity of the target product is not strictly required. However, since this SMB mode is currently only targeted at producing specific products, its application range is narrow and the productivity is not significantly improved compared to the conventional SMB. Therefore, there are few research works and applications at present.

#### *2.5. SMBs with More Than Four Zones*

Generally, conventional four-zone SMBs only can handle the binary mixtures' separation task. To separate multi-mixtures, more zones need to be added to break through the limitations in terms of zone variants. The following is a brief description of the five-zone SMB and the nine-zone SMB.

A five-zone SMB is a closed loop with multiple chromatographic columns in series, generally equipped with two inlets (feed and desorbent ports) and three outlets (extractant 1, extractant 2, and extractive residue). The three inlets are assigned to low-affinity substance A, medium-affinity substance B and high-affinity substance C. Usually, the inlet is between zone III and zone IV, and low-affinity substance A is collected from the raffinate port (between zone IV and zone V), while high-affinity substance C and medium-affinity substance B are collected at the extract 1 port (between zone I and zone II) and extract 2 port (between zone II and zone III), respectively. For example, Mun [54] and Xie et al. [55] have successfully separated multiple components by designing and using a five-zone SMB with high yields and purity. The nine-zone SMB can be regarded as a five-zone SMB in parallel with the conventional four-zone SMB, and the whole system forms two closed loops with

bypass stream. In Wooley et al.'s work [56], a nine-zone SMB was applied to extract two sugars from the biohydrolysis product with a purity close to 100% and a recovery of 88%.
