Power partial-discard strategy to obtain improved performance for simulated moving bed chromatography

Journal of Chromatography A, 1529 (2017) 72–80

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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

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Power partial-discard strategy to obtain improved performance for simulated moving bed chromatography Ji-Woo Chung a,1 , Kyung-Min Kim b,c,1 , Tae-Ung Yoon b , Seung-Ik Kim b , Tae-Sung Jung d , Sang-Sup Han d , Youn-Sang Bae a,b,∗ a

Department of Integrated Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, Republic of Korea Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, Republic of Korea c Creative Future Laboratory, KEPCO Research Institute, 105 Munji-ro, Yuseong-gu, Daejeon, Republic of Korea d Clean Fuel Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 20 July 2017 Received in revised form 1 October 2017 Accepted 15 October 2017 Available online 7 November 2017 Keywords: Partial-discard Simulated moving bed (SMB) chromatography Purity Recovery Performance parameters

a b s t r a c t A novel power partial-discard (PPD) strategy was developed as a variant of the partial-discard (PD) operation to further improve the separation performance of the simulated moving bed (SMB) process. The PPD operation varied the flow rates of discard streams by introducing a new variable, the discard amount (DA) as well as varying the reported variable, discard length (DL), while the conventional PD used fixed discard flow rates. The PPD operations showed significantly improved purities in spite of losses in recoveries. Remarkably, the PPD operation could provide more enhanced purity for a given recovery or more enhanced recovery for a given purity than the PD operation. The two variables, DA and DL, in the PPD operation played a key role in achieving the desired purity and recovery. The PPD operations will be useful for attaining high-purity products with reasonable recoveries. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The simulated moving bed (SMB) is composed of multiple columns with a sequence of inlet and outlet nodes. The countercurrent flow between the adsorbent and fluid is simulated by switching of inlet and outlet nodes at suitable time intervals. Compared to batch chromatography, high productivity and low eluent consumption can be obtained in the SMB process because of countercurrent operations as well as continuous production of products [1–5]. The SMB technology was developed in the late 1950s and has mostly been applied to large-scale separations in the petrochemical and sugar industries. Nowadays, it is also applied to the separation of chiral drugs and their enantiomers in the pharmaceutical industry [6–8]. Especially in the field of pharmacy, ultrapure products are necessary to develop safer and more successful drugs that meet strict requirements for new drugs [9]. Since chiral stationary phases usually have very low selectivity (1.1–1.4), many trials are undertaken for developing more efficient strategies to attain high-purity

∗ Corresponding author at: Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul, Republic of Korea. E-mail address: [email protected] (Y.-S. Bae). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.chroma.2017.10.079 0021-9673/© 2017 Elsevier B.V. All rights reserved.

products from chiral enantiomers [10]. Various operating strategies have been developed to improve the separation performance of the SMB. Some examples are as follows. The concept of varying the feed flow rate within a switching period was introduced as the PowerFeed (PF) operation [3] and later more detailed applications were proposed, such as partial-feed, outlet streams swing (OSS), intermittent SMB, partial port-closing, and SimCon operations [2,11–14]. A non-synchronous shift of the inlet/outlet nodes was introduced in the VariCol operation [15]. The variation of the feed concentration during a switching period was also introduced in the ModiCon operation [4]. The addition of a chromatographic column at the feed inlet node was suggested in the FeedCol operation to introduce pre-separated feed into the SMB [16]. Re-feeding of fractions of the products into the feed was proposed as the Backfill operation [17]. In our previous study, a concept of discarding parts of the products was introduced as the partial-discard (PD) strategy. Using this PD operation, the purities could be successfully enhanced by discarding contaminated parts of products although the recoveries deteriorated [9]. To complement this PD operation, concepts of recycling the discarded portions were suggested as the fractionation and feedback SMB (FF-SMB) and recycling partial-discard (RPD) operations. In these operations, parts of the discarded prod-

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Previously, the PF operation was proposed to improve the performances of the SMB by varying the flow rates within a switching period [3]. We reasoned that the concept of varying flow rates could further enhance the performances of PD operation. Therefore, in this study, we propose a PPD operation where the flow rate of the discard stream (Qk,discard ) is different from the flow rate of extract or raffinate product stream (Qk,product ). For this operation, we introduced two variables, the discard length (DL) and the discard amount (DA). The discard length has been already defined in the PD operation as the ratio of the discard period to the entire switching period (tsw ). Here, the discard period for the extract product is the initial stage (tini ) while that for the raffinate product is the last stage (tlast ). In this study, the discard amount is newly defined as the ratio of the amount discarded to the total effluent amount from outlet node. These variables can be expressed mathematically as follows: discard length (%) = (extract) Fig. 1. Internal concentration profiles of the normal four-zone SMBs at the beginning and at the end of a switching period.

tini × 100 tsw

(raffinate)

tlast × 100 tsw

(1a) (1b)

discard amount (%) = (extract)

tini × QE,discard × 100 tini × QE,discard + (tmid + tlast ) × QE,product

(raffinate)

Fig. 2. PD and PPD operations for a four-zone SMB [9]. For PD, Qk,product = Qk,discard (k = R or E). For PPD, Qk,product = / Qk,discard (k = R or E).

ucts can be stored in reservoir tanks and recycled to the feed node at a suitable time [18,19]. However, these operations require additional valves and a reservoir for precise control; thus, the resulting process is quite complicated. In this study, as another supplementing strategy for the PD operation, the power partial-discard (PPD) strategy was developed to improve the separation performance. This PPD operation is similar to the PD operation but varies the flow rates of the discard streams by controlling the discard amount (amount of discarded portion) and discard length (time duration of the discard stage). To investigate the effect of the PPD strategy on the purity and recovery of SMB process, a four-zone SMB process with two columns per zone was chosen. The separation performances of PPD operations were compared with those of the PD operation and the normal SMB. 2. Principle of the power partial-discard (PPD) strategy In the normal SMB, most impurity components are observed at the initial stage of the extract node and the last stage of the raffinate node in a switching period (Fig. 1). Therefore, the PD operation was proposed to obtain high-purity products by simply discarding the contaminated portion for each product node [9,19]. As displayed in Fig. 2, a portion of the extract product was discarded during the initial stage of a switching period (tsw ) while a portion of the raffinate product was discarded during the last stage. In conventional PD, the flow rate of the discard stream (Qk,discard ) was identical to the flow rate of the extract or raffinate product stream (Qk,product ).

tlast × QR,discard × 100 tlast × QR,discard + (tini + tmid ) × QR,product

(2a)

(2b)

In the PPD operation, the total effluent amount from outlet node is set to be identical to those of the PD and normal SMB operations for suitable comparison. In other words, the mean flow rates of effluent streams are the same for normal SMB, PD, and PPD operations. Therefore, for the PPD operations, we can vary the discard flow rate (Qk,discard ) and product flow rate (Qk,product ) by controlling the DA and DL values. As displayed in Fig. 3, these PPD operations can be categorized into two types: 1) PPD with a fixed DL (PPDL); 2) PPD with a fixed DA (PPDA). For simplicity, Fig. 3 only considers the PPD operation at the extract node. Fig. 3b and c show two different cases for the PPDL operation when the DL is fixed at 20%. If DA is set to be larger than DL (e.g., DA = 35%), a higher QE,discard is needed compared to that of the conventional PD. To attain the same mean flow rates of effluent streams with PD, a lower QE,product is required in this case. In the same way, if DA is set to be smaller than DL (e.g. DA = 5%), a lower QE,discard as well as a higher QE,product are needed compared to those of the conventional PD. Fig. 3d and e present two different cases of PPDA operation where DA is fixed at 20%. If DL is set to be larger than DA (e.g. DL = 35%), a lower QE,discard is required compared to the PD. To obtain identical mean flow rates of effluent streams with PD, a higher QE,product is needed in this case. In the same way, if DL is set to be smaller than DL (e.g. DL = 5%), a higher QE,discard as well as a lower QE,product are required compared to that of the PD. In this study, the flow rates of zones 2 and 3 were fixed at the same values as those in the normal SMB and PD. However, the flow rates of zones 1 and 4 were changed to control the Qk,discard and Qk,product values. The PPD operations will be explained in further detail in Section 3.2 with equations. 3. Models and methods 3.1. Column model In this study, the equilibrium-dispersive model was used as the column model [20]. This model is known to accurately describe a homogeneously packed column [21]. It is based on the following assumptions. First, equilibrium between the bulk liquid phase

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Fig. 3. Variations of discard flow rate (QE,discard ) and product flow rate (QE,product ) at the extract node for four different cases of PPD operations: (a) Normal PD operation that has the same product flow rates with normal SMB operation; (b) PPDL operation with a higher DA (DA = 35%); (c) PPDL operation with a lower DA (DA = 5%); (d) PPDA operation with a higher DL (DL = 35%); (e) PPDA operation with a lower DL (DL = 5%). For PD and PPDL operations, a fixed DL (20%) is used and for PD and PPDA operations, a fixed DA (20%) is used. The solid lines indicate the flow rates of normal SMB operation.

and the adsorbed phase is achieved instantly. Secondly, the effects of axial dispersion and mass transfer resistance are lumped as an apparent axial dispersion coefficient. Thirdly, the axial dispersion coefficients are the same for all components and for all columns in the SMB unit [20,22]. In this model, the differential mass balance is expressed as follows [9]: 2

∂Ci ∂C ∂ Ci uL 1 − ε ∂qi , Dap = +u i + = Dap 2 N ε 2 ∂t ∂z ∂t ∂z

(3)

Here, Ci and qi refer to the bulk liquid-phase and adsorbed-phase concentrations of component i, respectively; u is the interstitial velocity of the liquid-phase; ε is the total porosity; Dap is the apparent axial dispersion coefficient; L is the column length and N is the number of theoretical plates of a column. Since the equilibrium-dispersive model assumes fast adsorption time, masstransfer effects are negligible and a sufficiently large number of theoretical plates is applied [21,23]. In this study, the following linear adsorption equilibrium was assumed for all the SMB operations [9]: qi = H i Ci Here, Hi is the Henry isotherm coefficient of component i.

(4)

The following initial and boundary conditions were applied to each column [9]: att = 0,C i = qi = 0

(5)

Ciin

(6)

∂Ci = 0 ∂z

(7)

at z = 0, Ci = at z = L,

Here, Ci in is the liquid-phase concentration of component i at the inlet of a zone. 3.2. Node model for PPD operations To simulate the SMB operations, a node model for the SMB configuration is needed in addition to the column model described above. For the PPD operations, we used the following node model, which combines the mass balance equations at the inlet and outlet nodes, displayed in Fig. 2. (a) Desorbent node: Q4 + QD = Q1

(8)

out Ci,4 Q4

(9)

=

in Ci,1 Q1

(b) Extract node:

J.-W. Chung et al. / J. Chromatogr. A 1529 (2017) 72–80 Table 1 System and operating parameters for all the SMB operations in this study [9].

*Initial stage (discard period) Q 1 − Q 2 = Q E,discard

(10)

out in Ci,1 = Ci,2 = Ci,E,discard

(11)

*Middle and last stages (product period) Q 1 − Q 2 = Q E,product

(12)

out Ci,1

(13)

=

in Ci,2

= Ci,E,product

(c) Feed node: Q2 + QF = Q3 out Ci,2 Q2

(14)

+ Ci,F QF =

in Ci,3 Q3

(15)

(d) Raffinate node: *Initial and middle stages (product period) Q 3 − Q 4 = Q R,product

(16)

out Ci,3

(17)

=

in Ci,4

= Ci,R,product

*Last stage (discard period) Q 3 − Q 4 = Q R,discard

(18)

out in Ci,3 = Ci,4 = Ci,R,discard

(19)

In these equations, Qj is the flowrate of zone j (j = 1, 2, 3, or 4); QD and QF are the desorbent and feed flow rates; Qk,discard and Qk,product (k = E or R) are the discard and product flowrate; Ci,k,product and Ci,k,discard are the effluent concentrations of species i (i = A or B) through outlet node k (k = E or R) during product and discard out and C in are the concentrations of components i at the periods; Ci,j i,j outlet and inlet of zone j; and Ci,F is the concentration of component i of the feed. In all the SMB operations, the positions of inlet and outlet nodes were switched in the direction of the liquid flow after a switching period (Fig. 1). These shifts can be modeled by updating the initial and boundary conditions at the beginning of each switching period. A simulation package, gPROMS Processbuilder 1.1.0, was used to simulate the periodic discontinuities caused by the discrete switching of nodes and a 1st order Backward Finite Difference Method (BFDM) was used to solve derivatives [24]. 3.3. Performance parameters In this study, the effect of the PPD strategy on two important SMB performance parameters (purity and recovery) was investigated. For the PD and PPD operations, the performance parameters were defined as: Purity (%):

 tsw

(Extract)

 tsw tini

tini

CA,E,product dt

CA,E,product dt +

 tsw tini

 tini +tmid (Raffinate)

 tini +tmid 0

0

(Extract)

QE,product ×

(Raffinate)

CB,E,product dt

 tsw tini

 tini +tmid

 tini +tmid 0

0

CA,E,product dt

CA,F × QF × tsw QR,product ×

× 100

CB,R,product dt

CA,R,product dt +

Recovery (%):

75

CB,R,product dt

× 100 (21)

× 100

CB,R,product dt

CB,F × QF × tsw

(20)

(22)

× 100

(23)

System and operating parameters Ci,F (Feed concentration) i = A,B (g/L) SMB configuration D (column diameter) (cm) L (column length) (cm) ε (total porosity) N (number of plate) H1 H2 Q1 (Flow rate of zone 1) (mL/min) Q2 (Flow rate of zone 2) (mL/min) Q3 (Flow rate of zone 3) (mL/min) Q4 (Flow rate of zone 4) (mL/min) tsw (Switching time) (s)

Here,t sw = t ini + t mid + t last

2.0 2-2-2-2 (8 column) 1.0 10.0 0.6667 1000 1.2 1.0 22.51 18.88 19.88 16.88 24.8

(24)

In the above equations, A and B are the more retained and less retained components, respectively. The performance parameters were calculated for a switching period at cyclic steady-state conditions. In this study, the cyclic steady-state condition was defined as the state of less than 0.001% change between the performance profiles in two consecutive switching periods.

4. Results and discussion As already discussed in our previous study [9], the PD operation produces significantly higher purity than the normal SMB although it results in considerable losses in recovery. This is because the contaminated portions in product streams are discarded. Therefore, the PD operation would be an efficient strategy for obtaining highvalue products by achieving extremely high purities. In this study, we would like to further enhance the purity or recovery by two different types of PPD operations. In this study, to compare the performances of the PPD operations with those of normal SMB and PD operations, the system data and operating parameters were taken from a previous study, as summarized in Table 1 [9]. It should be noted that the operating point was selected based on the triangle theory. For the PD operations, the same system and operating parameters as those of the normal SMB were used, with the exception of discarding a portion of the product during the initial or last stage of a switching period. For the PPD operations, the same system and operating parameters as those of the PD operations were used, with the exception of varying the effluent flow rates from extract and raffinate nodes within a switching period. As summarized in Table 2, various runs were designed for the PD and PPD operations. The simulations for the PD operations (PD(1)–PD(7)) were performed as standards to be compared to the PPD operations (PPDL(1)–PPDL(6) and PPDA(1)–PPDA(6)). As already explained in Section 2 and Fig. 3, PPDL and PPDA indicate the PPD operation with a fixed discard length (DL = 20%) and the PPD operation with a fixed discard amount (DA = 20%), respectively. In this study, 20% was arbitrarily chosen as the standard condition of DA and DL for investigating the effect of each variable. The same DA and DL values were applied to both the extract and raffinate products in each simulation run for PD and PPD operations. It should be noted that the PD or PPD strategies were applied to the extract product at the initial stage of a switching period and to the raffinate product at the last stage of a switching period (Fig. 2).

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Table 2 Various runs for PD SMB and PPD SMB operations.

Discard length (%) Discard amount (%)

Discard length (%) Discard amount (%)

Discard length (%) Discard amount (%)

Run PD(1)

Run PD(2)

Run PD(3)

Run PD(4)

Run PD(5)

Run PD(6)

Run PD(7)

5 5

10 10

15 15

20 20

25 25

30 30

35 35

Run PPDL(1)

Run PPDL(2)

Run PPDL(3)

Run PPDL(4)

Run PPDL(5)

Run PPDL(6)

20 5

20 10

20 15

20 25

20 30

20 35

Run PPDA(1)

Run PPDA(2)

Run PPDA(3)

Run PPDA(4)

Run PPDA(5)

Run PPDA(6)

5 20

10 20

15 20

25 20

30 20

35 20

Fig. 4. Effect of DA in the PPDL operations on the performance parameters: (a) purity and (b) recovery.

4.1. Effect of discard amount (DA) for power partial-discard with fixed discard length (DL) As a type of PPD operations, the PPDL varied the DA while keeping the DL fixed. Fig. 4 shows the effect of DA on the purity and recovery of PPDL operations (PPDL(1)–PPDL(6)) when DL is fixed at 20%. It should be noted that when the DA is also 20% (PD(4)), the PPDL operation becomes identical to the PD operation because DL and DA are the same at this condition. As shown in Fig. 4, the PPDL operations including the PD operation show significantly higher extract and raffinate purities than normal SMB for all DA values although these operations show losses in recovery. As the DA increases, both the extract and raffinate purities increased (Fig. 4a).

These improvements were expected from Figs. 1 and 2 since the higher value of DA can remove larger amounts of impurities. However, with the increase in DA, recoveries decreased steadily (Fig. 4b) because greater amounts of products were also removed as well as impurities. Since the purpose of this study was to improve the separation performance of the conventional PD operation, we will now focus on the comparisons between PD and PPD operations. When higher DA values were applied (PPDL(4)–PPDL(6)) compared to the DA value (20%) of the PD operation, both the extract and raffinate purities were enhanced but the corresponding recoveries deteriorated (Fig. 4). This can be mainly explained by the increased amount of discard in the case of the higher value of DA. Additionally, this can be further analyzed by the comparison of internal concentration profiles of PD (PD(4)) and PPDL with a high DA (PPDL(6)), as displayed in Fig. 5. Interestingly, the internal concentration profiles for PPDL(6) were sharper than those for PD(4). This arises from the increase in the zone 1 flow rate (Q1 ) during the initial stage of a switching period (discard period for extract node) as well as the decrease in the zone 4 flow rate (Q4 ) during the last stage of a switching period (discard period for raffinate node) in order to attain a higher DA value. As a result of the sharper internal profiles, at the beginning of the product period for the extract, the tail of the raffinate (impurity) concentration profile for PPDL(6) is almost pushed away from the extract outlet node (Fig. 5a). Similarly, at the end of the product period for the raffinate, the front of the extract (impurity) concentration profile for PPDL(6) is observed to have just arrived at the raffinate outlet node (Fig. 5b). These may have contributed to the enhanced purities for the PPDL operations with high DA values. On the other hand, at the beginning of the product period for the extract, the height of the extract (product) concentration profile for PPDL(6) was lower than that for PD(4) (Fig. 5a). Likewise, at the end of the product period for the raffinate, the height of the raffinate (product) concentration profile for PPDL(6) was lower than that for PD(4) (Fig. 5b). These may support the deteriorated recoveries for the PPDL operations with high DA values. When lower DA values were used (PPDL(1)–PPDL(3)) compared to the DA value (20%) of the PD operation, the recoveries were enhanced although the purities decreased (Fig. 4). The main reason would be the reduced amount of discard because of applying the lower DA values. This can be further explained by the comparison of internal concentration profiles of PD (Run PD(4)) and PPDL with a low DA (Run PPDL(1)). As shown in Fig. 5, the internal profiles for PPDL(1) are broader than those for PD(4). This originated from the decrease in the zone 1 flow rate (Q1) during the initial stage of a switching period (discard period for extract node) as well as the increase in the zone 4 flow rate (Q4) during the last stage of a switching period (discard period for raffinate node) in order to attain a lower DA value. At the beginning of the product period for the extract as well as the end of the product period for the raffinate,

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Fig. 6. Comparisons of transient effluent concentration profiles between PPDL and PD operations at the extract node during a switching period at the steady-state.

Fig. 5. Comparisons of internal concentration profiles between PPDL and PD operations: (a) at the beginning of the product period for the extract; (b) at the end of the product period for the raffinate.

PPDL(1) had higher impurity concentrations than PD(4) (Fig. 5a and b). These may have led to the deteriorated purities for the PPD operations with low DA values. On the other hand, the height of the product concentration profile for PPDL(1) was higher than that for PD(4) either at the beginning of the product period for the extract or at the end of the product period for the raffinate. These may support the improved recoveries for the PPDL operations with low DA values, compared to those of the normal PD operation. Fig. 6 shows the transient effluent concentration profiles during a switching period. Here, the effluent profiles at the extract node were presented as a represented case. This figure explains more clearly the behaviors of the performance parameters of the PPDL operations. As DA increases (PPDL(6)), the effluent concentration profile for impurity component (raffinate) was getting lower, which supports the reason of the enhancement in the purity (Fig. 4a). However, with increasing DA values (PPDL(6)), the effluent concentration profile for product component (extract) was also getting lower, which supports the deterioration of the recovery (Fig. 4b). 4.2. Effect of discard length (DL) for power partial-discard with fixed discard amount (DA) As the other type of PPD operations, the PPDA fixed the DA but varied the DL. Fig. 7 presents the effect of DL on the purity and recovery of the PPDA operation (PPDA(1)–PPDA(6)) when DA was fixed at 20%. It should be noted that when DL is also 20% (Run PD(4)),

Fig. 7. Effect of DL in the PPDA operations on the performance parameters: (a) purity and (b) recovery.

the PPDA operation was the same as the PD operation. As displayed in Fig. 7, the PPDA operations including the PD operation exhibited considerably higher purities than that of normal SMB in spite of losses in recoveries. When higher DL values were used (PPDA(4)–PPDA(6)) compared to the DL value (20%) of the PD operation, both the extract and raffinate purities improved but the recoveries decreased slightly (Fig. 7). This cannot be easily explained because the discard amount was fixed for the PPDA operations. Therefore, to explain this, the

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Fig. 8. Comparisons of internal concentration profiles of PPDA and PD operations: (a) at the beginning of the product period for the extract; (b) at the end of the product period for the raffinate.

internal concentration profiles of PPDA with a high DL (PPDA(6)) were compared with those of normal PD (PD(4)), as shown in Fig. 8. At the beginning of the product period for the extract, the internal concentration profiles for PPDA(6) shifted towards the right side compared to those for PD(4) (Fig. 8a). This was attributed to the movement of internal profiles following the flow stream for a longer time owing to the increase in the discard length (35%). It should be noted that, for PPDA(6) and PD(4), the extract products were obtained after passing 35% and 20% of a switching period, respectively. Therefore, at the beginning of the product period for the extract, the tail of the raffinate (impurity) concentration profile for PPDA(6) was almost pushed away from the extract outlet node. This may explain the enhanced extract purities for the PPD operations with high DL values. On the other hand, at the beginning of the product period for the extract, PPDA(6) had a slightly lower extract (product) concentration than PD(4). This may have led to the slight decrease in the extract recoveries for the PPDA operations with high DL values. At the end of the product period for the raffinate, the internal concentration profiles for PPDA(6) shifted towards the left side compared to those for PD(4) (Fig. 8b). This originated from the movement of internal profiles following the flow stream for a shorter time owing to the increase in the discard length (35%). It should be noted that, for PPDA(6) and PD(4), the raffinate products were obtained at the initial 65% and 80% of a switching period,

respectively. Hence, at the end of the product period for the raffinate, the front of the extract (impurity) concentration profile for PPDA(6) had just arrived to the raffinate outlet node. This may have led to the improved raffinate purities for the PPD operations with high DL values. On the other hand, at the end of the product period for the raffinate, the height of the raffinate (product) concentration profile for PPDA(6) was slightly lower than that for PD(4). This may explain the slight deterioration in the raffinate recoveries for the PPDA operations with high DL values. When lower DL values were used (PPDA(1)–PPDA(3)) compared to the DL value (20%) of the PD operation, both the extract and raffinate purities deteriorated but the recoveries improved slightly (Fig. 7). To explain this, the internal concentration profiles of PPDA with a low DL (PPDA(1)) and normal PD (PD(4)) were compared, as displayed in Fig. 8. At the beginning of the product period for the extract, the internal concentration profiles for PPDA(1) shifted towards the left side compared to those for PD(4) (Fig. 8a). This arose from the movement of internal profiles following the flow stream for a shorter time (5% of a switching period) owing to the decrease in the discard length (5%). Hence, at the beginning of the product period for the extract, the raffinate (impurity) concentration of PPDA(1) was higher than that of the PD(4). This may have led to the decreased extract purities for the PPD operations with low DL values. On the other hand, PPDA(6) had a slightly higher extract (product) concentration than PD(4) at the beginning of the product period for the extract. This may have contributed to the slight increase in the extract recoveries for the PPDA operations with low DL values. At the end of the product period for the raffinate, the internal concentration profiles for PPDA(1) shifted towards the right side compared to those for PD(4) (Fig. 8b). This was due to the movement of internal profiles following the flow stream for a longer time (95% of a switching period) because of the decrease in the discard length (5%). Therefore, at the end of the product period for the raffinate, PPDA(1) showed a higher concentration of extract component (impurity) compared to PD(4). This may explain the decreased raffinate purities for the PPD operations with low DL values. On the other hand, PPDA(1) showed a higher concentration of raffinate (product) than PD(4) at the end of the product period for the raffinate. This may explain the slight improvement in the raffinate recoveries for the PPDA operations with low DL values. Fig. 9a–c show the transient effluent concentration profiles during a switching period for Run PPDA(1), Run PD(4), and Run PPDA(6), respectively. Here, the effluent profiles at the extract node were shown as a represented case. A remarkable point in Fig. 9 is that the product period starts at a different time within a switching period for each run. These figures explain more clearly the behaviors of the performance parameters of the PPDA operations. As DL increases (PPDA(6)), the amount of impurity component (raffinate) obtained during the product period was getting smaller, which supports the improvement in the purity (Fig. 7a). However, with increasing DL values (PPDA(6)), the amount of product component (extract) obtained during the product period was also getting smaller, which explains the slight decrease in the recovery (Fig. 7b). 4.3. Purity vs. recovery Fig. 10 summarizes the performances of PPDL and PPDA with those of PD and normal SMB. The behaviors of purity vs. recovery were almost the same for extract and raffinate products. In the case of PD operations, as the DL (or DA) increased, purities increased significantly although recoveries decreased considerably. For PPDL operations, with the increase in DA, the purities improved slightly but recoveries decreased significantly. This may be due to the lower product concentration profiles near the product nodes (Figs. 5 and 6) as well as the increased discard amount. For

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Fig. 10. Plots of purity vs. recovery for PPDL, PPDA, PD, and normal SMB: (a) performances of extract products; (b) performances of raffinate products. The thick lines indicate the operating conditions where PPD shows superior recovery to that of PD for achieving the same purity.

In the case of PPDA operations, when the DL of PPDA was lower than DA (20%), lower recoveries were obtained for the same purities as that of PD. However, when the DL was higher than DA (20%), higher recoveries could be attained for the same purities as that of PD or higher purities could be achieved for the same recoveries. These show that PPDA or PPDL operations can be used for improving the purity or recovery of PD while maintaining the recovery or purity, respectively. Therefore, we can select suitable DA and DL values depending on the target purity and recovery. A rigorous optimization study may be required as a future work for more accurate comparisons among different operation modes and an easier selection of DL and DA for specific purity and recovery [25,26]. Fig. 9. Transient effluent concentration profiles at the extract node during a switching period at the steady-state: (a) Run PPDA(1), (b) Run PD(4), and (c) Run PPDA(6).

PPDA operations, with the increase in the DL, the purities improved significantly but recoveries deteriorated slightly. The fixed DA as well as the lower product concentrations near the product nodes (Figs. 8 and 9) may have led to the very small changes in recoveries. Since the purpose of this study was to further improve the performances of PD operation, the comparisons between PPD and PD operations are important. In the case of PPDL operations, when DA was higher than DL (20%), lower recoveries were obtained for the same purities as that of PD. However, when the DA of PPDL was lower than DL (20%), higher recoveries can be obtained for the same purities as that of PD or higher purities can be attained for the same recoveries as that of PD.

5. Conclusions Previously, we have suggested a partial-discard (PD) strategy, which significantly improves the purity of a SMB process by discarding the contaminated portions in product streams. However, this PD operation results in considerable losses in recovery. In this study, we developed a novel power partial-discard (PPD) strategy to further improve the purity or recovery of PD operations by introducing two variables: the discard length (DL) and the discard amount (DA). In PPD operations, the discard flow rate (Qk,discard ) is different from the product flow rate (Qk,product ). These two flow rates can be changed by controlling the DL and DA values. We categorized the PPD operations into two types: 1) PPD with a fixed DL (PPDL); 2) PPD with a fixed DA (PPDA). For PPDL operations,

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with increase in the DA, the purities improved slightly but recoveries decreased significantly. For PPDA operations, with increase in the DL, the purities improved considerably but recoveries deteriorated slightly. When the DA was lower than the DL in PPDL operations, higher purities or recoveries could be obtained for the same recoveries or purities as those of PD. When the DL was higher than the DA in PPDA operations, higher purities or recoveries could be attained for the same recoveries or purities as those of PD. These indicate that PPD operations can be used for enhancing the purity or recovery of the normal PD by adjusting two key variables, the DL and DA. The proposed PPD strategies will provide valuable methods for producing high-purity products in the SMB process while reducing the recovery losses from discarding some portions of the products. Acknowledgments We would like to acknowledge the financial support from the R&D Convergence Program (CRC-14-1-KRICT) of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology). This work was also supported by In-house Research and Development Program of the Korea Institute of Energy Research (KIER) (B7-2437-03). References [1] M. Mazzotti, G. Storti, M. Morbidelli, Optimal operation of simulated moving bed units for nonlinear chromatographic separations, J. Chromatogr. A 769 (1997) 3–24. [2] Y. Zang, P.C. Wankat, SMB operation strategy-partial feed, Ind. Eng. Chem. Res. 41 (2002) 2504–2511. [3] Z. Zhang, M. Mazzotti, M. Morbidelli, PowerFeed operation of simulated moving bed units: changing flow-rates during the switching interval, J. Chromatogr. A 1006 (2003) 87–99. [4] H. Schramm, M. Kaspereit, A. Kienle, A. Seidel-Morgenstern, Simulated moving bed process with cyclic modulation of the feed concentration, J. Chromatogr. A 1006 (2003) 77–86. [5] Y. Xie, B. Hritzko, C.Y. Chin, N.-H.L. Wang, Separation of FTC-ester enantiomers using a simulated moving bed, Ind. Eng. Chem. Res. 42 (2003) 4055–4067. [6] M. Juza, M. Mazzotti, M. Morbidelli, Simulated moving-bed chromatography and its application to chirotechnology, Trends Biotechnol. 18 (2000) 108–118. [7] M. Schulte, J. Strube, Preparative enantioseparation by simulated moving bed chromatography, J. Chromatogr. A 906 (2001) 399–416. [8] A. Rajendran, G. Paredes, M. Mazzotti, Simulated moving bed chromatography for the separation of enantiomers, J. Chromatogr. A 1216 (2009) 709–738.

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