The separation of butanediol and propanediol by simulated moving bed

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The separation of butanediol and propanediol by simulated moving bed Ming-Tsai Liang a,∗, Chih-Hsiung Lin a, Pei-Ying Tsai a, Hsiang-Ping Wang b, Hou-Peng Wan c, Tzu-Yueh Yang c a b c

Dept. Chem. Eng., I-Shou University, Kaohsiung City 840, Taiwan, ROC Shiny Chem. Ind. Co., Yeong An Industrial Zone, Kaohsiung, Taiwan, ROC Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 19 June 2015 Revised 13 October 2015 Accepted 1 December 2015 Available online xxx Keywords: Butanediol Propanediol Simulated moving bed

a b s t r a c t This work studies the process development for the purification of PDO (1, 3-propanediol) by SMB (simulated moving bed). The propylene glycol produced by hydrogenation of glycerin generally contains BDO (1, 4-butanediol), which has a similar boiling point to PDO. In this study, Mitsubishi SP70 is used as the adsorbent for the SMB to separate BDO and PDO. Three series of experiments with 10, 100, and 200 g/L of feeding concentration, with an equal weight of BDO and PDO, were conducted to investigate the influence of feeding concentration on separation. The series of experiments was conducted at constant flow rates, and the switching time for the rotating valves was the only changed variable. Also, Langmuir adsorption isotherms, axial dispersion and mass transfer coefficients for BDO and PDO were independently evaluated through single column chromatography. From experimental results using 10 g/L of feed concentration, it is found that separation results can be well predicted by the Triangle theory as the dead volume of the SMB is assumed as 12% of the empty column. The optimized separation from this series of experiments is 100% pure BDO and 92.56% pure PDO, the productivity of the SP70 is 0.101 KKD (kg/kg/day), and the amount of water recycling is 300 L/kg. The obtained system’s parameters, including the dead volume, are then used for the ASPEN simulation. It is shown that the simulated results from ASPEN can reasonably fit all experimental results. This confirms that the obtained parameters for the SMB are accurate, and the optimized operating conditions and the scale-up design can be accurately developed through simulation experiments in the future. The methods for measuring and estimating the parameters used in this study can also work as examples for the process development of SMB. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Since the global production of biodiesel has increased greatly in the last two decades, crude glycerin is in oversupply [1]. There is a need to use glycerin as the starting material for the production of bio-based materials. It is well known that hydrogenolysis of glycerin can produce propylene diol [1–4], which can be used as the starting material to obtain di-, tri-, and tetra-propylene glycol for the cosmetic and painting industries. However, various alcohols are also produced as by-products in hydrogenolysis, such as ethylene glycol, butanediol, pentadiol, and hexanediol. To obtain propylene diol, flash distillation is first applied to separate ethylene glycol, propylene diol, and butane diol as the top product. Further separation of ethylene glycol from the top product could be finished by azeotropic distillation or the chromatographic process [5–8]. Therefore, the separation of BDO (1, 4butanediol) and PDO (1, 3-propanediol) becomes an unsolved issue

∗

Corresponding author. Tel.: +886 7 6577711. E-mail address: [email protected], [email protected] (M.-T. Liang).

in obtaining propylene diol. Although vacuum distillation, azeotropic distillation, and reactive distillation are proposed to separate the BDO and PDO [5,6], the boiling points of BDO and PDO are 230 °C and 211 °C, respectively, and they are too close to be economically separated by distillation. Chromatographic separation is then proposed [7,8]. SMB (Simulated moving bed) is a sophisticated continuous chromatography [9–12]. It can effectively operate the chromatographic process by continuously feeding the feedstock solution. Compared with batch chromatography, the productivity of an adsorbent is largely enhanced more than tenfold, while consumption of the solvent and water is significantly reduced to 10% [13]. The SMB was originally invented for the separation of petroleum chemicals in the 1960 s and further applied to the sugar industry in the late 1970 s. In the 1990 s, the pharmaceutical industry widely and quickly applied the SMB to chiral separation and purification for active pharmaceutical ingredients [9–12]. Rodrigues et al. already published a book to introduce and review the fundamentals of simulated moving bed technology [12]. The SMB has already proven its maturity and cost-effectiveness in various industries. The application of SMB in

http://dx.doi.org/10.1016/j.jtice.2015.12.003 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: M.-T. Liang et al., The separation of butanediol and propanediol by simulated moving bed, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.12.003

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Fig. 1. Illustration of the flow direction of liquid and solid in an SMB.

the biorefinery for the production of bio-based materials is naturally expected [14], such as the separation of lactic acid and acetic acid [15], and the recovery of ionic liquid from the biomass hydrolyzated sugar liquors [16]. For the separation of BDO and PDO, Dowex V493 and Mitsubishi SP700 were tested in a simulated moving bed with an open-loop design and with an additional and separated zone for the regeneration of resin [8]. The design is generally called CIP-SMB, cleaning in position SMB. The open-loop design will significantly dilute the feedstock solution, and the separated zone for the regeneration of an adsorbent will reduce the productivity of resin. A separated zone for regeneration is normally required for poor desorption of the selected adsorbent. The open-loop design diminishes the fourth section in a traditional SMB, which can increase the productivity of the adsorbent. However, the effluent of raffinate will be significantly diluted, and more energy consumption is required in the downstream concentration. Mitsubishi SP70 is an FDA approved resin with easier desorption than that of SP700. Therefore, the design of an additional zone for regeneration could be diminished to enhance the productivity of the adsorbent. In this study, Mitsubishi SP70 was used as the adsorbent for a traditional four-section SMB to separate the BDO and PDO. Also, the adsorption isotherms, the axial dispersions, and the mass transfer coefficients for BDO and PDO in packed columns with SP70 resin were investigated and confirmed by fitting the experimental results to the prediction by ASPEN simulation. The obtained adsorption isotherms and the confirmed parameters for the mass transfer provide useful information for the scale-up design and optimized operating conditions.

2. The SMB and its parameters

retention component B will be carried by the solid to the left and the effluent from the port of extract. For linear adsorption, neglecting axial dispersion and mass transfer resistance between the solid and liquid phase, the relative volumetric flow rates of liquid to the solid in Sections 2 and 3 must satisfy the Triangle theory first proposed by Morbidelli et al. [17]:

KA < m2 ,

Fig. 1 illustrates the configuration and flow direction of liquid and solid for a traditional SMB used in this study. The SMB has four sections, and each has two columns. Section 1 is the regeneration section for the adsorbent; Section 2 is the desorption section; Section 3 is the adsorption section; and Section 4 is the regeneration section for the desorbent. The liquid effluent from the fourth section is recycled to the first section, and the recycling design is called a closed-loop design. The liquid flow rate in each section of the SMB is controlled by four HPLC pumps installed at ports of desorbent, feed, extract, and recycle. The movement of the solid is fulfilled by constantly switching the ports of inlets and outlets to the next column, and the flow rate of the solid can be controlled by setting the time period for switching the ports. For a binary feedstock solution, the weak retention component A will be flushed by the liquid desorbent to the right, as seen in Fig. 1, and the effluent from the port of raffinate, and the strong

(1)

where mj is the relative volumetric flow rate in Section j and is defined as:

mj =

Q j tsw − VC εt − V jD

(2)

VC (1 − εt )

where Qj is the liquid flow rate in Section j, tsw is the switching time of valves, VC is the volume of the empty column, ε t is the total porosity of the packed column, and VD j is the dead volume in Section j. In this study, the dead volume in each section is assumed to be a constant and treated as a parameter, which can be evaluated by fitting a series of experimental results with theoretical predictions. In order to regenerate the adsorbent and to recycle the desorbent, the relative volumetric flow rates in Sections 1 and 4 are confined as:

KA > m4 ;

KB < m1

(3)

For nonlinear adsorption with equilibrium and non-dispersion conditions, the criteria for setting the operational conditions were also derived for the Langmuir isotherm by Morbidelli et al. [17]. On the phase plane (m2 , m3 ), the separable operating conditions were confined into a deformed triangle plotted by three lines, which are:

⎧  KA − α (1 + bAC f,A ) m2 + bAC f,A α m3 = α (KA − α ) wr line ⎪ ⎪ ⎪  ⎨ KA − α (1 + bAC f,A ) m2 + bAC f,A KB m3 = KB (KA − KB ) wb line √ 2 ⎪ √ ⎪ ⎪ ⎩m = m + KB − m2 ra curve 3

2.1. The SMB

m3 < KB

2

(4)

bA C f,A

where Cf is the feed concentration, and α is the large root for the following quadratic equation:









1 + bAC f,A + bBC f,B X 2 − KA (1 + bBC f,B ) + KB (1 + bAC f,A ) X − KA KB = 0

(5) and b is the parameter in the Langmuir isotherm: qi =

1+

KiCi i

biCi

(6)

where q is the concentration of the solute in a solid phase. The parameters K and b in the Langmuir adsorption isotherms for each component can be independently found by a series of pulse tests or frontal chromatography with different concentrations. The triangles defined by Eqs. (1) and (4) are illustrated in Fig. 2. For Eq. (1), a right triangle is obtained for the diluted feed, and an asymmetrical deformed triangle connected by points b-w-r-a in Fig. 2 is drawn

Please cite this article as: M.-T. Liang et al., The separation of butanediol and propanediol by simulated moving bed, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.12.003

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3

calculated by the following equation [10]:

⎧  t 2 R ⎪ ⎪ N = 5.54 Symmetric peak ⎪ ⎨ w0.5

2  tR ⎪ w0.1 ⎪ N = 41.7 Asymmetric peak ⎪ ⎩ 1.25 + b0.1

a KB

m3

r w

a0.1

where w is the peak wildness at the baseline, and a and b are the left and right wildness at 10% of the peak height. The linear velocity of the desorbent inside a packed column is calculated as:

b K A m2

u=

Fig. 2. Illustration of the Triangle theory for linear and nonlinear isotherms.

for the concentrated feed. Although the triangle theory provides nice instructions for setting the operating conditions, only model base experiments provide detailed information for the separated results. For nonlinear adsorption and non-equilibrium conditions, the operating condition for the separation of A and B by the SMB is dependent on the mass transfer coefficient between the solid and the liquid phases, the axial dispersion coefficients, and the adsorption isotherms. From the mass conservation, the governing equations for chromatography can be derived as [18]:

⎧ ∂C ∂q ∂C ⎪ ⎪ εe + (1 − εe )(1 − εP ) + εe (u − uS ) ⎪ ⎪ ∂t ∂t ∂x ⎪ ⎨ ∂q ∂C ∂ 2C − uS (1 − εe )εP = εe D 2 −(1 − εe )(1 − εP )uS ⎪ ∂x ∂x ∂x ⎪ ⎪ ⎪ ⎪ ∂q ∂q ⎩ − uS = −k(q − f (C )) ∂t ∂x

(7)

where u and us are the linear velocity of the desorbent and adsorbent,

εe and εp are the inter-particle porosity and intra-particle porosity, D

is the axial dispersion coefficient, x is the coordinated distance, t is the evolution of time for the chromatography and f(C) is the adsorption isotherm for the solute. If the dispersion and mass transfer resistance are considered and the porosity of the packed column cannot be treated as a lumped parameter, Eq. (4) can only be solved by the numerical method [18]. Commercial software, such as ASPEN Chromatography and Chromwork, also provide simulation for the traditional SMB and nontraditional SMB. In this work, the parameters are carefully estimated and used for the ASPEN Chromatography to fit the experimental results from a traditional four-zone SMB by adjusting the dead volume. Afterwards the confirmation of parameters, scaleup design, optimized operating conditions, and optimized design for the SMB can be accurately and simply developed by the simulation experiments.

2.2. Measurement of mass transfer coefficient and axial dispersion The mass transfer coefficient in this study is found by applying the van Deemter equation [10]:

HETP = A +

G + Bu u

(8)

where A, B, and G are the parameters, and HETP is the height of the equivalent theoretical plate and calculated by:

HETP =

LC N

(10)

(9)

where LC is the length of the packed column, and N is the number of theoretical plates, which is found by observing the peak broadening in a chromatogram. The number of theoretical plates can be

4Q

(11)

εe π DC2

where DC is the diameter of the packed column. The mass transfer coefficient can be correlated to the parameter B in van Deemter’s equation as:



keff

k =2 1 + k

2

εe dP 1 1 − εe 6 B

(12)

where dP is the diameter of the resin, which is 0.24 mm for the SP70, and k is defined as:

k =

1 − εe

εe

[εP + (1 − εP )K]

(13)

It is reported that the decrease in the mass transfer coefficient on the separable operating conditions’ mapping on the phase plane will further shrink the triangle [19]. The axial dispersion coefficient is assumed to be a parameter independent of the concentration in this study and found by fitting the breakthrough curve with an approximate solution derived by Lapidus and Amundson in displacement chromatography [20]. The approximate solution with an infinite mass transfer rate is expressed as:

⎧ ⎨

⎛

1 C = 1 − erf⎝ Cf 2⎩

x−



ut

1−εt

1+ ε t

⎞⎫ ⎬ K ⎠ ⎭

4Dt 1−ε 1+ ε t K

(14)

t

By this approximate solution, displacement chromatography can be conducted and fitted by Eq. (14) through adjusting the parameter K and the dispersion coefficient D. 3. Experiment and single column chromatography 3.1. Analysis and the packed columns To analyze the content of BDO and PDO in a solution, an HPLC with an RI detector is used. A packed column from Thermo C18 (4.6 × 150 mm) is used and the flow rate of DI water is set as 1.0 mL/min. A 20 μL sample loop is used for the calibration, and the corresponding factors for BDO and PDO are 2.264 × 106 and 2.076 × 106 , respectively. It is observed that the retention times for PDO and BDO are 2.05 and 3.45 min, respectively, which implies that C18 can also be used as an adsorbent for the separation of BDO and PDO. However, the separation will not be economically feasible using the C18 as the adsorbent. BDO and PDO are supplied by Shiny Chem. Ind. Co. with 99.5% purity. The SP70 is purchased from Mitsubishi Co. The unpacked resin is washed and immersed overnight in methanol with 99.5% purity. It is then vacuum dried and moistened with DI water before being packed into the columns. Two packed columns, with different sizes, are used in this study. A stainless HPLC column 1.0 × 25 cm is used for the investigation of the adsorption isotherm. An in-house design of a PVC column 2.64 × 14.24 cm is used for the SMB and for measuring the axial dispersion, mass transfer coefficients and the interparticle and intraparticle porosities. Eight PVC columns packed with SP70 for the SMB are tested for their homogeneity by displacement chromatography using PDO as

Please cite this article as: M.-T. Liang et al., The separation of butanediol and propanediol by simulated moving bed, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.12.003

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Fig. 5. Series of breakthrough curves for BDO at different concentrations. Fig. 3. The breakthrough curves and fitted curves for Eq. (14).

Fig. 4. Series of breakthrough curves for PDO at different concentrations.

the indicator. In the SMB, each column is installed with a multiport valve from Valco Co., and an additional multiport valve is installed to guide the liquid flow at the port of extract. A check valve is also installed for each column to prevent the back flow of liquid when switching the multiport valve to simulate the movement of a solid. The tubing is 1/8” and made of TPFE from Valco Co. HPLC or prepHPLC pumps are supplied by Hanbon Science and Technology Co. In the ports of raffinate and extract, back pressure regulators, with 50 psi and 250 psi, are installed to stabilize the flow rate and the pressure of the SMB system.

3.2. Column porosity and axial dispersion coefficient A PVC column, packed with SP70, is used to find the interparticle and intraparticle porosities and the axial dispersion coefficient. The column is connected to an HPLC/RI system and eluted by 4 mL/min of DI water. Three aqueous solutions with 0.05 M of BDO, PDO, and ZnCl2 are prepared. ZnCl2 is treated as the non-retention indicator to measure the porosities and the dead volume of the HPLC system. Without connecting the PVC column, displacement chromatography of the ZnCl2 solution is conducted to measure the dead volume. The obtained breakthrough curve, without connecting the PVC column, is then fitted by Eq. (14) with negligible adsorption. An imaged porosity is then found as 0.048 to fit the breakthrough curve, as shown in Fig. 3. The symbols in Fig. 3 are the experimental results and the solid lines are the fitted results. The obtained imaged porosity is then used to calculate the dead volume for the HPLC/RI system. It is found that the dead volume and the dead time for the elution system are 3.69 mL and 0.92 min, respectively.

After connecting the PVC column to the HPLC/RI system, displacement chromatography of aqueous solutions of ZnCl2 , BDO, and PDO is conducted. The chromatograms are shown in Fig. 3. On these chromatograms, the time course of elution is deducted by the dead time 0.92 min. For ZnCl2 , the fitting of the breakthrough curve, as shown in Fig. 3, can provide the total porosity of the packed column by assuming a zero for the adsorption constant. It is found that the total porosity of the packed column is 0.824. For SP70, the water content of the resin is provided by the vendor as 55∼65%. Therefore, it is reasonable to assume that the intraparticle porosity is 0.60. Accordingly, the interparticle porosity is calculated as 0.56, and the retention time for the non-retention ZnCl2 is 15.93 min, which is also exactly the inflection point in the breakthrough curve. This represents a loose packing, which may be due to the wet resin being packed by a dry method. During packing of the eight PVC columns, an equal weight of resin is individually prepared for each column and gradually loaded into the column to assure packing homogeneity, which is further confirmed by the displacement chromatography for the eight columns. After finding the porosity, the fitting of breakthrough curves for BDO and PDO can be finished by adjusting the adsorption constant and the axial dispersion coefficient. Accordingly, it is found that the adsorption constants for BDO and PDO are 6.80 and 2.55, respectively, and the axial dispersion coefficient for PDO and for BDO are exactly the same and found as 5.5 × 10−7 m2 /s. The obtained axial dispersion coefficient is then used for the ASPEN Chromatography. 3.3. Langmuir adsorption isotherms Instead of using PVC columns, a stainless column 1.0 × 25 cm is applied to investigate the adsorption isotherms. A series of displacement chromatography is conducted with different concentrations, as shown in Figs. 4 and 5. If the concentration of solute is increased, early breakthrough is observed. This implies that the adsorption isotherm belongs to the Langmuir type. It is also found that the breakthrough curve can be fitted by Eq. (14), if the concentrations of BDO and PDO are lower than 50 g/L. Above 50 g/L, the fitting becomes poor. It is presumed that a shockwave could occur during the elution of the concentrated solution. Therefore, the experimental results for concentrations higher than 50 g/L are discarded when calculating the adsorption isotherm. For each displacement chromatography technique, an adsorption constant and an axial dispersion coefficient can be found by fitting with Eq. (14). By multiplying the obtained adsorption constant by the concentration, the adsorbed amount of solute on the stationery phase can then be found. From the series of displacement chromatography, an adsorption isotherm can be illustrated. In this study, the Langmuir adsorption model is selected to fit the adsorption isotherm. By minimizing the sum of the square of the difference between the experimental and calculated results, the parameters for

Please cite this article as: M.-T. Liang et al., The separation of butanediol and propanediol by simulated moving bed, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.12.003

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5

12 10

m3

8 6 4

Cfeed = 10 g/L

2 0

the Langmuir equation are calculated. Fig. 6 depicts the experimental results as symbols and the fitted adsorption isotherm as solid lines, and the Langmuir equation can be expressed as:

qPDO

qBDO

8.507CBDO = 1 + 0.044CBDO

(15)

Eq. (15) implies that the linear adsorption constants for PDO and BPD are 2.674 and 8.507, respectively. It is also noted that the slope of the adsorption isotherm decreases with increasing concentration. Therefore, a decrease in the adsorption constant is expected if displacement chromatography is conducted at 3.8 g/L for PDO and 4.5 g/L for BDO, as illustrated in Fig. 3. 3.4. Mass transfer coefficient In order to find the parameters for the van Deemter Equation, a series of pulse tests was conducted for BDO and PDO. By changing the flow rate of the eluent, the peak in the chromatogram shifts and deforms. Eqs. (9) and (10) are then used to evaluate the number of the theoretical plate N and the height of equivalent theoretical plate HETP. Tables 1 and 2 summarize the peak parameters for the calculation of N and HETP at different flow rates for PDO and BDO, respectively. The linear velocity of the desorbent in the column is also calculated using Eq. (11) and listed in the tables. Accordingly, the van Deemter equation for BDO and PDO is established, as shown in Fig. 7, and the bottom row in Tables 1 and 2 also shows the parameters for the van Deemter equation. Accordingly, the mass transfer coefficients for PDO and BDO can be calculated by Eq. (12) as 6.3 × 10−5 and 5.53 × 10−5 cm/s, respectively.

15

HETP (cm)

12

BDO

9 6 PDO

3 0

0

3

6 9 v (cm/min)

12

15

Fig. 7. Curve fittings of van Deemter equation for BDO and PDO.

2

4

6

m2

8

10

12

Fig. 8. The triangle theory and the mapping of the first series of experiments.

Fig. 6. The Langmuir adsorption isotherms for BDO and PDO.

2.674CPDO = ; 1 + 0.021CPDO

0

4. Separation by SMB and prediction from ASPEN chromatography 4.1. Separation by SMB with diluted feed Based on the triangle theory, a series of experiments is designed for separation by SMB. The first series of experiments is conducted for feeding at 10 g/L with equal weights of BDO and PDO. For the Langmuir isotherm, the right triangle on the phase plane of (m2 , m3 ) described by Eq. (1) is deformed and shrunk. Fig. 8 shows two triangles on the phase plane of (m2 , m3 ) according to the obtained isotherms in Eq. (15). The right triangle was plotted by Eq. (1), and the deformed triangle was plotted by Eq. (4). The series of experiments is set with fixed flow rates in each section, and the only changed variable is the time of the switching valve. The flow rates for the desorbent, extract, feed, raffinate, and recycle are designed as 9.0, 6.0, 3.0, 6.0 and 9.0 mL/min. The series of switching times for rotating the multiport valve is designed as those listed in Table 3. After a steady state is established, the effluents collected at the extract and raffinate are submitted for HPLC/RI analysis to determine the concentration of BDO and PDO for each switching time; the purity at the extract and raffinate is calculated according to the following equations:

PBDO =

E CBDO E E CPDO + CBDO

;

PPDO =

R CPDO R R CPDO + CBDO

(16)

where P is the purity, and the superscripts E and R represent the extract and raffinate. The purities at the extract and raffinate are also calculated and listed in Table 3. It is observed that the experiments conducted with switching times ranging from 9 to 11 mins can be accounted for as experiments with separable operating conditions. By conducting a series of experiments with changed switching times and fixed flow rates, Yu and Ching showed the details to optimize the operational conditions for an SMB with an approximated Langmuir isotherm [21]. In this study, the Langmuir adsorption equation is already confirmed, and the deviation from the ASPEN simulation is explained by the dead volume compensation, as described in Eq. (2). Although a dead volume for each section can be physically measured before the SMB construction, several uncertainties still remain unsolved for model simulation, such as uncertainty in adsorption, equilibrium kinetics, axial dispersion coefficients, porosities and inhomogeneity of the packed columns. Sa Gomes et al. further used the switching time compensation to perfectly fit the experimental results with the model simulation [22]. Before designing, Sa Gomes et al. carefully measured the dead volume for the SMB and treated it as 11.5% of the empty column volume and further compensated the deviation by 3.1% of the switching time to perfectly fit

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M.-T. Liang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–8 Table 1 The number of the theoretical plate for PDO at different flow rates. Q(mL/min)

tR (min)

a0.1 (min)

b0.1 (min)

w0.1 (min)

u(cm/min)

N

HETP (cm)

1.0 13.50 6.30 10.09 983.3 2.274 9.92 1.51 1.5 8.83 4.49 7.57 723.5 3.41 7.61 1.97 2.0 6.47 3.42 6.34 585.4 4.55 5.90 2.54 3.0 4.13 2.31 4.89 432.0 6.82 4.07 3.68 4.0 2.98 1.66 4.15 348.9 9.10 2.92 5.14 5.0 2.14 1.31 3.69 300.0 11.37 1.88 7.99 Parameters for van Deemter equation: A = –3.258 cm; B = 0.903 min; G = 6.549 cm2 /min

Table 2 The number of the theoretical plates for BDO at different flow rates. Q(mL/min)

tR (min)

a0.1 (min)

b0.1 (min)

w0.1 (min)

u(cm/min)

N

HETP (cm)

1.0 26.40 14.74 22.84 2254.7 2.27 7.35 2.04 2.0 12.50 7.93 14.79 1362.9 4.55 4.05 3.70 3.0 7.62 5.19 11.76 1016.7 6.82 2.40 6.26 4.0 5.37 3.67 9.95 817.3 9.10 1.64 9.16 5.0 4.02 2.74 9.15 713.4 11.37 1.04 14.42 Parameters for van Deemter equation: A = –8.307 cm; B = 1.841 min; G = 14.284 cm2 /min

Table 3 Purities and recovery at extract and raffinate from the SMB experiments. Conc. of feed (wt. %)

tsw (min)

Concentration (wt. %) Raffinate

1

10

20

5.0 6.0 6.5 7.0 8.0 9.0 10.0 11.0 12.0 5.0 6.5 7.0 7.5 8.0 8.5 9.0 5.0 6.0 7.0 8.0 10.0

Purity

Remark

Extract

PDO

BDO

PDO

BDO

PDO

BDO

0.005 0.001 0.001 0.004 0.017 0.026 0.025 0.025 0.024 0.021 0.107 0.195 0.23 0.247 0.253 0.25 0.031 0.191 0.431 0.466 0.485

0.009 0.009 0.008 0.007 0.005 0.005 0.002 0.007 0.013 0.065 0.043 0.047 0.077 0.109 0.14 0.161 0.089 0.088 0.236 0.329 0.436

0.021 0.026 0.026 0.024 0.008 0.001 0.000 0.000 0.001 0.233 0.141 0.056 0.025 0.01 0.005 0.004 0.488 0.299 0.063 0.017 0.005

0.016 0.016 0.018 0.02 0.02 0.023 0.023 0.018 0.011 0.19 0.211 0.204 0.178 0.138 0.115 0.092 0.414 0.394 0.234 0.154 0.057

0.357 0.1 0.111 0.364 0.773 0.839 0.926 0.781 0.649 0.244 0.713 0.806 0.749 0.694 0.644 0.608 0.258 0.685 0.646 0.586 0.527

0.432 0.381 0.409 0.455 0.714 0.958 1.000 1.000 0.917 0.449 0.599 0.785 0.877 0.932 0.958 0.958 0.459 0.466 0.788 0.901 0.919

the experimental results from a lab-scale SMB [22]. In this study, uncertainties and dead volumes from tuning and fitting are lumped into a parameter denoted as dead volume to approximately fit the experimental results from a lab-scale SMB. The approximated dead volume may not be applied for scale-up SMB, yet it helps to confirm thermodynamic and transport parameters found in this study. In order to evaluate the dead volume for the SMB, the operating conditions for the first series of experiments are mapped onto the (m2 , m3 ) phase plane with or without considering the dead volume for the SMB, as shown in Fig. 8. The squares in Fig. 8 represent the mapping without considering the dead volume, and the solid diamonds are the mapping considering the dead volume being equal to 12% of the empty column. It is noted that the increase in switching time will shift the coordinates of mapping up and right. From Fig. 8, it is also observed that the squares do not fit the experimental results. If compared with Table 3, the separable switching times are 9.0–11.0 min, yet the mapping without considering the dead volume predicts that the separable switching times are 8–10 mins. If the dead

Pure R Pure E and R Pure E

volume is assumed to be 12% of the empty column, the mapping shifts to the left and the mapping for the separable switching time locates exactly onto the deformed triangle defined by Eq. (4). Accordingly, the dead volume is reasonably assumed as 12% of the empty column. Table 3 shows that if the switching time of the valves is set at 10 mins, the purity of BDO is 100%, and the purity of PDO is 92.59%. This implies that the recovery of PDO is near 100%. Therefore, the optimized separation is designed as that conducted at 10 mins of switching time. Since the apparent density of SP70 is reported as 0.685 kg/L, the productivity of SP70 is 0.101 KKD (kg/kg/day), and the amount of recycling water is 300 L/kg. Further decreasing the switching time to 2.0 min can simply increase the productivity by five times as 0.505 KKD and reduce the water recycling down to 60 L/kg if the operating pressure is still below the SMB designed pressure. It is also noted from Fig. 8 that both 9.0 and 11 mins of switching time are located exactly inside the separable region defined by the equilibrium adsorption, yet the experimental results slightly deviate from the prediction. Azevedo and Rodrigue demonstrated that

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12

Cfeed = 100, 200 g/L

10

m3

8 6 4 2 0

0

2

4

6

m2

8

10

12

Fig. 9. The triangle theory and the mapping for the second and the third series of experiments. Fig. 10. The change in purity with the switching time at 10 g/L of feed.

the separable triangle region defined by the equilibrium theory will be shrunk after considering the mass transfer resistance [23]. Therefore, further shrinkage of the deformed triangle in Fig. 8 is expected, and the exclusion of experiments with 9.0 and 11.0 mins of switching time from the separable region will also be expected. Accordingly, the existence of mass transfer resistance will lead to the reduction of separation performance. 4.2. Separation by SMB with concentrated feed and prediction from ASPEN chromatography The productivity of an SMB can be increased by simply increasing the feed concentration. Two series of experiments with concentrated feed are tested. One is at 100 g/L and the other is at 200 g/L. Both are composed of equal weights of BDO and PDO. Both series are also conducted at fixed flow rates as that for the first series of experiments. The experimental results are also listed in Table 3. It is observed that no complete separation can be accomplished for these two series of experiments. Fig. 9 plots two triangles according to Eq. (4) for the second and the third series of experiments and shows the mapping for the experimental operational condition considering the dead volume found for diluted feed. In Fig. 9, the deformed triangle plotted by solid lines represents feed at 200 g/L, and the dashed line is that at 100 g/L. The symbols of a circle and cross represent the concentration of 200 g/L and 100 g/L, respectively. For the experiments with switching time of less than 6.0 min, m2 is less than zero and the mapping is not shown in Fig. 9. It is observed from Fig. 9 that all operating conditions are not located inside the deformed triangles. According to the isotherms in Fig. 6, the adsorbed amounts of BDO and PDO gradually level off as the concentration increases up to 50 g/L. Therefore, the triangle is dramatically shrunk, and the control of the separation becomes difficult. Fortunately, the experimental results can still be predicted by solving Eq. (7). The operating conditions for the first series of experiments were submitted for the simulation by ASPEN Chromatography considering the dead volume as 12% of the empty column. The change in purity for the extract and raffinate with the switching time from the ASPEN simulation is illustrated in Fig. 10. The experimental results in Table 3 are also shown in Fig. 10. The red solid squares are for BDO and the green solid triangles are for PDO. The solid lines represent the simulation results from ASPEN chromatography, the red line is for BDO and the green line is for PDO. It is observed that the prediction reasonably fits the experimental results. This implies that all parameters including the mass transfer coefficient, the axial dispersion co-

Fig. 11. The change in purity with the switching time at 100 g/L of feed.

Fig. 12. The change in purity with the switching time at 200 g/L of feed.

Please cite this article as: M.-T. Liang et al., The separation of butanediol and propanediol by simulated moving bed, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.12.003

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efficient, the adsorption isotherms, and the porosities are accurately measured and the dead volume of the SMB is reasonably estimated. By the same parameters, the prediction for experiments with concentrated feed is also plotted in Figs. 11 and 12. It is observed that the prediction can also predict the experimental results for the concentrated feed. It is also noted that the 10 min of switching time with 200 g/L of feed in Fig. 12 does not fit the prediction. The inappropriate prediction could be due to the occurrence of flooding at the raffinate. Therefore, the concentration at the extract will be extremely diluted, and the concentration derived from the calibration curve could lead to uncertainty. The good fitting for the concentrated feed implies that the adsorption isotherms expressed in Eq. (13) are accurate and acceptable. 5. Conclusion In this work, the separation of BDO and PDO is conducted in a traditional four-section SMB with SP70 as the adsorbent. The system’s parameters for simulation are also established. By using single column chromatography, the Langmuir adsorption isotherms and the axial dispersion and mass transfer coefficients for BDO and PDO are found. By comparing the experimental results with diluted feeding to the Triangle theory, the dead volume for the SMB is evaluated. The obtained parameters can then fit the experimental results with feeding concentrations up to 200 g/L. This work provides system parameters for the separation of BDO and PDO. It will provide useful information for future simulation experiments to find the optimized operating condition and the scale-up design. Acknowledgments Financial support from the Ministry of Science and Technology of Taiwan and the Bureau of Energy, Ministry of Economic Affairs, Taiwan, ROC (103-D0108) is acknowledged. References [1] Johnson DT, Taconi KA. The glycerin glut: options for the value-added conversion of crude glycerol resulting from biodiesel production. Environ Progress 2007;26:338–48. [2] Nakagawa Y, Shinmi Y, Koso S, Tomishige K. Direct hydrogenolysis of glycerol into 1, 3-propanediol over rhenium-modified iridium catalyst. J Catal 2010;272:191–4.

[3] Stankowiak, A., Franke, O., Appel, J., Buehring, D., Waxhsen, O., Method for producing 1, 2-propanediel by hydrogenolysis of glycerin, US Appl. Pub. 2011/71323 A1, Mar. 24, 2011 [4] Kitamura S, Su-enaga T, Ikenaga N, Miyake T, Suzuki T. Steam reforming of glycerin using Ni-based catalysts loaded on CaO–ZrO2 solid solution. Catal Lett 2011;141:895–905. [5] Berg L. Separation of propylene glycol from 1,2-butanediol by azeotropic distillation, US patent 5423955, Jun. 13, 1995 [6] Kalagias P. Processes for isolating or purifying propylene glycol, ethylene glycol and products produced therefrom, US patent 8143458 B2, Mar. 27, 2012 [7] Dabagov NS, Balandin AA. Chromatographic separation of polyhydric alcohols on resins of KU-2 type. Communication 1. Development of a quantitative method of analysis. Russ Chem Bull 1966;15:1259–65. [8] Hilaly AK, Sandage RD, Soper JG. Separation of a mixture of polyhydric alcohols, US patent Appl. Pub. 2009/0120878 A1, May 14, 2009 [9] Rajendran A, Paredes G, Mazzotti M. Simulated moving bed chromatography for the separation of enantiomers. J Chromatogr. A 2009;1216:709–38. [10] Schmidt-Traub H, editor. Preparative chromatography of fine chemicals and pharmaceutical agents. KGaA, Weinheim: Wiley-VCH Verlag GmbH & Co.; 2005. [11] Subramanian G, editor. Chiral separation techniques – A practical approach. 2nd ed. KGaA, Weinheim: Wiley-VCH, Verlag GmbH & Co.; 2001. [12] Rodrigues AE, Pereira C, Minceva M, Ribeiro AM, Ribeiro A, Silva M, Graca N, Santos SC. Simulated moving bed technology, principles, design and process applications. 1st ed. Butterworth-Heinamann; 2015. [13] Grinter T. The development of an environmentally sustainable process for radafaxine. Green Chemistry in the Pharmaceutical Industry. Dunn PJ, Wells AS, Williams MT, editors. WILEY-VCH; 2010. [14] Chin CY, Wang N-HL. Simulated moving bed technology for biorefinery applications. Separation and Purification Technologies in Biorefineries. Raswamy S, Huang HJ, Mamarao BV, editors. JOHN WILEY and Sons; 2013. [15] Lee HJ, Xie Y, Koo YM, Wang N-HL. Separation of lactic acid from acetic acid using a four zone SMB. Biotechnol. Progress 2004;20:179–92. [16] Mai NL, Nguyen NT, Kim J-I, Park H-M, Lee S-K, Y-M Koo. Recovery of ionic liquid and sugars from hydrolyzated biomass using ion exclusion simulated moving bed chromatography. J Chromatogr A 2012;1227:67–72. [17] Migliorini C, Mazzotti M, Morbidelli M. Continuous chromatographic separation through simulated moving beds under linear and nonlinear conditions. J Chromatogr A 1998;827:161–73. [18] Pais LS, Loureiro JM, Rodrigues AE. Chiral separation by SMB chromatography. Sep Purif Technol 2000;20:67–77. [19] Rodrigues AE, Pais LS. Design of SMB chiral separations using the concept of separation volume. Sep Sci Technol 2004;39:245–70. [20] Wankat PC. Rate-controlled separations. Amsterdam, The Netherlands: Kluwer; 1990. [21] Yu HW, Ching CB. Optimization of a simulated moving bed based on an approximated Langmuir model. AIChE J. 2002;48:2240–6. [22] Sá Gomes P, Zabkova M, Zabka M, Minceva M, Rodrigues AE. Separation of chiral mixtures in real SMB units: the FlexSMB-LSRER. AIChE J. 2010;56:125– 142. [23] Azevedo DCS, Rodrigue AE. Design of a simulated moving bed in the presence of mass-transfer resistances. AIChE J. 1999;45:956–66.

Please cite this article as: M.-T. Liang et al., The separation of butanediol and propanediol by simulated moving bed, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.12.003