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4-methoxyphenol mixture by stripping crystallization
Journal of Industrial and Engineering Chemistry 18 (2012) 963–968
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Separation of the catechol/4-methoxyphenol mixture by stripping crystallization Lie-Ding Shiau *, Shu-Li Zeng Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kweishan, Taoyuan 33302, Taiwan
A R T I C L E I N F O
Article history: Received 18 May 2011 Accepted 26 September 2011 Available online 4 February 2012 Keywords: Crystallization Distillation Catechol 4-Methoxyphenol
A B S T R A C T
This work presents a novel separation scheme, stripping crystallization (SC), to separate the catechol/4methoxyphenol mixture. While operated at a triple-point condition, in which the liquid mixture is vaporized and crystallized simultaneously due to the three-phase equilibrium, SC combines distillation and crystallization to produce pure crystals. Experimental results demonstrate the feasibility of applying SC to purify catechol in the catechol/4-methoxyphenol mixture. However, purifying 4-methoxyphenol by SC in the catechol/4-methoxyphenol mixture is rather difficult. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Separation and purification play an important role in a chemical manufacturing process. Separation and purification processes typically account for 40–70% of both capital and operating costs in the chemical industry, and their proper application can significantly reduce costs and increase profits [1,2]. Among the various separation techniques, crystallization is extensively adopted in various industrial applications and exceeded in scope only by distillation processes [3,4]. Crystallization has received renewed interest in recent years, owing to the rising demand for pure intermediates/products in the pharmaceutical and chemical industries [5,6]. Hybrid distillation/melt crystallization processes are also widely used to separate isomer mixtures [7,8]. Our laboratory has recently developed stripping crystallization (SC), also referred to as distillative freezing (DF), to separate the mixtures with close boiling temperatures, including mixed xylenes [9–11], diethylbenzene isomers [12], and benzene/cyclohexane mixtures [13]. While operated at a triple-point condition, in which the liquid mixture is vaporized and crystallized simultaneously due to the three-phase equilibrium, SC combines distillation and crystallization to produce pure crystals. Adequately controlling temperature and pressure during the operation allows SC to form pure crystals, and liquid phase and vapor phase of mixtures. SC can be continued until the liquid phase is eliminated, and only pure crystals remain in the feed. Thus, unlike conventional crystallization, filtration or centrifugation is unnecessary to separate the solid crystals from the mother liquor since no mother liquor is present
* Corresponding author. Tel.: +886 3 2118800x5291; fax: +886 3 2118700. E-mail address: [email protected] (L.-D. Shiau).
with the pure crystals. Additionally, crystal washing is unnecessary since no impurities are adhered on the crystal surfaces at the end of the operation [9]. Separation of the catechol/4-methoxyphenol mixture is critical in the manufacturing of phenol derivatives [14]. Due to the close boiling temperatures (catechol: bp = 245 8C and 4-methoxyphenol: bp = 243 8C), separating the catechol/4-methoxyphenol mixture by conventional distillation is extremely difficult. Therefore, this work investigates the feasibility of SC in the separation of the catechol/4-methoxyphenol mixture. 2. Principles of SC SC operational principles can be explained by referring to the phase diagrams [9]. Fig. 1(a) illustrates the solid-liquid equilibrium (SLE) and vapor–liquid equilibrium (VLE) for catechol (A) and 4methoxyphenol (B) at a normal pressure (P = 1.013 105 Pa) [15– 18]. Notably, the catechol/4-methoxyphenol liquid mixture exhibits nonideal solution behavior. The eutectic point lies at T = 36 8C and XB = 0.71. This figure also plots the SLE based on the ideal liquid solution for comparison. As the pressure is reduced, SLE is assumed to remain nearly the same, while VLE moves downward. For instance, Fig. 1(b) illustrates the low pressure phase diagram at P = 133 Pa, which is lower than the triple-point pressure of pure catechol and higher than the triple-point pressure of pure 4-methoxyphenol. This diagram shows the three-phase state with pure catechol solid, and liquid phase and vapor phase of mixtures at T = 77 8C. Fig. 1(c) illustrates the low pressure phase diagram at P = 10.7 Pa, which is lower than the triple-point pressure of pure catechol or pure 4-methoxyphenol. This diagram shows two threephase states. The first three-phase state has pure catechol solid, as
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.09.006
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the range of 0 < X B < 0:71 and 4-methoxyphenol crystals in the range of 0:71 < X B < 1.
Notation C P; jðSÞ C P; jðLÞ C P; jðVÞ
DC P; j HS; j HL; j HV; j DHm; j DHv; j Ln Mj P P sat j R RE RS Sn T T m; j T tri; j Vn Xj X W; j
heat capacity of solid phase for j-component (J/mol K) heat capacity of liquid phase for j-component (J/mol K) heat capacity of vapor phase for j-component (J/mol K) change in the heat capacity from solid phase to liquid phase for j-component (J/mol K) enthalpy of the solid of j-component (J/g) enthalpy of the liquid of j-component (J/g) enthalpy of the vapor of j-component (J/g) heat of melting for j-component (>0) (J/mol) heat of vaporization for j-component (>0) (J/mol) mass of the liquid phase out of stage n (g) molecular weight of j-component (>0) (g) pressure (Pa) saturated pressure of the liquid of j-component (Pa) ideal gas constant (8.314 J/mol K) experimental recovery ratio (dimensionless) simulated recovery ratio (dimensionless) mass of the solid phase out of stage n (g) temperature (K) melting temperature of j-component (K) triple point temperature of j-component (K) mass of the vapor phase out of stage n (g) mole fraction of j-component in liquid phase (dimensionless) weight fraction of j-component in liquid phase X M X W; j ¼ X M jþXj M (dimensionless) A
Yj Y W; j
A
B
A
B
two adjustable parameters for the activity coefficients in the Wilson equation (dimensionless) Subscript 0 in at in in
ln½ðX A Þn ðg A Þn ¼
DHm;A
1
1 Tn
T m;A DC p;A T m;A ln R Tn R
þ
DC P;A T m;A T n R
Tn (1)
At pressures of 1 atm and lower, the assumption of ideal gases normally incurs little error. As SC is operated at low pressures, VLE of catechol and 4-methoxyphenol in stage n can be described by [19,20]: ðY A Þn Pn ¼ ðX A Þn ðg A Þn ðPAsat Þn
(2)
ðY B Þn P n ¼ ðX B Þn ðg B Þn ðPBsat Þn
(3)
Due to the nonideality of liquid solutions, the activity coefficients are evaluated by the Wilson equation as [19,20]: lnðg A Þn ¼ ln½ðX A Þn þ LAB ðX B Þn LAB LBA þ ðX B Þn ðX A Þn þ LAB ðX B Þn LBA ðX A Þn þ ðX B Þn
(4)
B
Greek letters gj activity coefficient of j-component in liquid phase (dimensionless) LAB , LBA
f n N
The SC process is simulated in a series of N equilibrium stage operations shown in Fig. 2. Alternatively, this figure is also an abstract representation of a batch SC process in a single vessel. Each stage is operated at a three-phase equilibrium. The temperature of each stage is chosen to meet the requirements of: T n1 T n ¼ DT for n ¼ 1; 2; :::; N. The vapor formed in each stage is condensed to the liquid and removed, while the solid and the liquid formed in each stage enter the next stage. When SC is applied to produce catechol (A) crystals from the catechol/4-methoxyphenol liquid mixtures in the range of 0 < X B < 0:71, each stage is maintained at a three-phase state with pure catechol crystals, as well as the liquid and vapor phases of mixtures. SLE of catechol in stage n is described by the van’t Hoff equation as [19,20]
B
mole fraction of j-component in vapor phase (dimensionless) weight fraction of j-component in vapor phase Y M Y W; j ¼ Y M jþYj M (dimensionless) A
3. SC simulation process
the feed the end of the experiment stage n the final stage
well as liquid phase and vapor phase of mixtures at T = 42 8C on the right portion of the figure. The other three-phase state has pure 4methoxyphenol solid, as well as liquid phase and vapor phase of mixtures at T = 43 8C on the left portion of the figure. Thus, SC provides a potential method to produce both catechol crystals in
lnðg B Þn ¼ ln½ðX B Þn þ LBA ðX A Þn LAB LBA ðX A Þn ðX A Þn þ LAB ðX B Þn LBA ðX A Þn þ ðX B Þn
(5)
where LAB and LBA denote two adjustable parameters. Additionally, we have ðX A Þn þ ðX B Þn ¼ 1
(6)
ðY A Þn þ ðY B Þn ¼ 1
(7)
When Tn is specified, Eqs. (1)–(7) constitute a set of seven equations that can be solved simultaneously for seven unknowns – Pn, ðX A Þn , ðX B Þn , ðg A Þn , ðg B Þn , ðY A Þn and ðY B Þn for n ¼ 1; 2; :::; N. Thus, the three-phase equilibrium conditions during the SC operation can be determined. Notably T0 denotes the triple-point temperature of the mixture feed determined by Eq. (1). The physical properties for catechol and 4-methoxyphenol needed in the SC simulation are taken from The Design Institute for Physical Property Data (DIPPR) [21]. The nonideality of catechol/4methoxyphenol liquid solution is accounted for by incorporating the activity coefficients from the Wilson equation [15–18]. The final SC operation should be stopped above the eutectic temperature to avoid the cocrystallization of catechol and 4-methoxyphenol.
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Fig. 1. Phase diagram of catechol (A) and 4-methoxyphenol (B) at (a) P = 1.013 105 Pa (—— represents the solid–liquid equilibrium calculated based on the ideal solution); (b) P = 133 Pa; (c) P = 10.7 Pa.
The total material balance in stage n can be expressed as: Sn1 þ Ln1 ¼ Sn þ Ln þ V n
(8)
As SC is applied to produce catechol crystals in each stage, 4methoxyphenol only exits in the liquid and vapor phases. The material balance of 4-methoxyphenol in stage n can be expressed as: Ln1 ðX W;B Þn1 ¼ Ln ðX W;B Þn þ V n ðY W;B Þn
(9)
As the latent heat released in forming catechol crystals is removed by vaporizing portions of liquid mixtures at each stage, the energy balance in stage n is described by: Sn1 ðHS;B Þn1 þ Ln1 ðHL Þn1 ¼ Sn ðHS;B Þn þ Ln ðHL Þn þ V n ðHV Þn
(10)
where ðHL Þn and ðHV Þn denote the enthalpy of the liquid mixture and the vapor mixture in stage n, respectively. Thus, ðHL Þn and ðHV Þn can be defined as: ðHL Þn ¼ ðX W;A Þn ðHL;A Þn þ ðX W;B Þn ðHL;B Þn
(11)
ðHV Þn ¼ ðY W;A Þn ðHV;A Þn þ ðY W;B Þn ðHV;B Þn
(12)
For convenience, it is assumed that HL; j ¼ 0 at T tri; j for jcomponent. Then, ðHS; j Þn , ðHL; j Þn and ðHV; j Þn in stage n can be determined as: ðHS; j Þn ¼
1 ½DHm; j ðT tri; j Þ þ Mj
Z
Tn T tri; j
C P; jðSÞ dT
(13)
L.-D. Shiau, S.-L. Zeng / Journal of Industrial and Engineering Chemistry 18 (2012) 963–968
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V1
S0 L0 (XA)0 (XB)0
Vn-1
1
n- 1
T1 P1
Tn-1 Pn-1
Vn
Sn-1 Ln-1 (XA)n-1 (XB)n-1
n Tn Pn
Vn+1
Sn Ln (XA)n (XB)n
VN
n+1
N
Tn+1 Pn+1
TN PN
SN
T0 P0 Fig. 2. Simulated SC operation where each stage is operated at a three-phase equilibrium state.
1 ðHL; j Þn ¼ Mj
ðHV; j Þn ¼
"Z
#
Tn
T tri; j
C P; jðLÞ dT
# " Z Tn 1 DHv; j ðT tri; j Þ þ C P; jðVÞ dT Mj T tri; j
(14)
(15)
Incorporating Eqs. (11)–(15) into Eq. (10) allows one to obtain Sn, Ln and Vn by solving Eqs. (8)–(10) simultaneously for n ¼ 1; 2; :::; N. Thus, the crystals and liquid remaining in each stage during the SC process can be determined. Notably, ðX W;A Þn , ðX W;B Þn , ðY W;A Þn and ðY W;B Þn can be calculated directly from the previously determined ðX A Þn , ðX B Þn , ðY A Þn and ðY B Þn . Similar simulation equations can be derived when SC is applied to produce 4-methoxyphenol (B) crystals from the catechol/4methoxyphenol mixture in the range 0:71 < X B < 1. 4. Experimental The experimental assembly consists of a 100 cm3 sample container in a 750 cm3 glass reactor, as shown in Fig. 3. The glass reactor is immersed in a constant temperature bath. In the beginning of the experiment, 20 g of melt mixture is placed in the sample container. Pressure in the glass reactor is then lowered using a mechanical vacuum pump (GLD-210B, ULVAC). Next, the vapor is condensed using a glass condenser circulated with the cooling water. Additionally, the reactor pressure is controlled by adjusting the valve connected to the vacuum pump. Finally, the temperature is controlled by adjusting the constant temperature bath. The feed is prepared by mixing catechol (Sigma–Aldrich, >99% purity) and 4-methoxyphenol (Sigma–Aldrich, >99% purity) in different ratios. The sample temperature is then lowered gradually
with the increasing time. The cooling rate typically starts at 2 8C/ min and then decreases gradually to 0.5 8C/min at the end. An experiment is generally finished within 50–100 min. During the experiment, a temperature probe (type K, thermocouple) is positioned in the center of the liquid feed, and a pressure gauge (Micro-ion plus, Granville-Phillips) is connected to the glass reactor. As the sample temperature in the chamber is lowered, the pressure is reduced by controlling the vacuum pump. A series of three-phase equilibrium conditions are achieved by controlling the temperature and pressure based on the simulation results. Thus, the liquid mixture is vaporized and solidified simultaneously due to the three-phase equilibrium. The SC experiment here is stopped at the lowest pressure attainable in the apparatus, depending on the corresponding temperature. Finally, the sample remained in the vessel is weighed, and its composition is analyzed by HPLC. HPLC (Spectra System, Thermo Separation products) consists of an ERC-3415a degasser, a Spectra System P1000 pump, a Spectra System AS1000 automatic sample injector and a UV 6000LP detector. The detection wavelength is set at 280 nm UV absorbance. The injection volume is 20 mL, and the flow rate is 1.0 mL/ min. Supelco Discovery HS C18 column (250 mm 4.6 mm, 5 mm particle size) is used for the separation. Mobile phases consist of water with 1% acetic acid (solvent A) and acetonitrile with 1% acetic acid (solvent B). Mobile phases are then filtered through a Millipore 0.22 mm membrane filter before use. Additionally, the composition of the mobile phases starts with 0% solvent B, increases to 31% solvent B at 10.5 min and then reaches 100% solvent B at 15 min. Ethanol (99.5%) is used as a solvent to prepare the catechol/4-methoxyphenol mixture solution for calibration in the quantitative analysis of the mixture composition at 280 nm UV absorbance.
Fig. 3. Schematic diagram of the experimental apparatus for the SC operation with the features: (1) vacuum pump, (2) condenser, (3) constant temperature bath, (4) thermocouple, (5) pressure gauge, (6) temperature controller, (7) glass reactor, (8) sample container.
L.-D. Shiau, S.-L. Zeng / Journal of Industrial and Engineering Chemistry 18 (2012) 963–968
1.00
700
(49%) (55%) (55%) (56%)
0.95
1.00
600
P
300
X W,B (72%)
XW,B
XW,A
400
P(Pa)
XW,A
feed 3
20
(87%) (90%) (92%)
(76%)
(73%)
feed 5
0.94
P
12
feed 2 (76%)
0.80
0.92
200
XW,A
8
(88%)
feed 1
0.75
16
P(Pa)
500
(72%)
0.85
X W,B
(77%) (75%)
100
0.90
4
feed 4
0 30
40
50
60
70
80
0.88
90100
o
T( C)
In the first part, SC is applied to produce catechol (A) crystals from the catechol/4-methoxyphenol mixture in the range of 0 < X B < 0:71. The SC simulation equations are solved for three feeds: feed 1 (20 g with X W;A ¼ 0:80), feed 2 (20 g with X W;A ¼ 0:85) and feed 3 (20 g with X W;A ¼ 0:90). Fig. 4 displays the calculated curve, P(T), by solving Eqs. (1)–(7) to simulate the SC operating conditions in the production of catechol crystals. Thus, as the temperature is decreased during the SC experiments, the corresponding pressure for the three-phase equilibrium is depicted by P(T). The experimental recovery ratio of catechol (RE ) is defined as: (16)
where ðX W;A Þ0 denotes the initial purity of the mixture feed, ðX W;A Þf represents the experimental final purity of the product at the end of the experiment, W f refers to the final weight of the product including the crystals and the remaining liquid obtained at the end of the experiment, and L0 is the weight of the liquid mixture feed initially. As some liquid may remain along with the final crystals at the end of SC, the simulated purity of the final product, including the final crystals and the remaining liquid, is defined as: SN þ LN ðX W;A ÞN SN þ LN
(17)
The simulated recovery ratio of catechol (RS ) is defined as: SN þ LN ðX W;A ÞN L0 ðX W;A Þ0
36
40
44
48
52
0 56
T( C)
5. Results and discussion
W f ðX W;A Þf L0 ðX W;A Þ0
32
o
Fig. 4. Comparison of experimental and simulation results for separation of catechol from the catechol-enriched mixture (&, initial purity for feed 1, , simulated final purity for feed 1, &, experimental final purity for feed 1; *, initial purity for feed 2, , simulated final purity for feed 2; *, experimental final purity for feed 2; ~, initial purity for feed 3; , simulated final purity for feed 3; ~, experimental final purity for feed 3).
RS ¼
24
0.98
0.96
(63%) (61%)
0.90
X W;A ¼
28
(70%)
XW,A
RE ¼
967
(18)
where SN , LN and ðX W;A ÞN denote the crystals, liquid and weight fraction of catechol in the final stage based on the simulation
Fig. 5. Comparison of experimental and simulation results for separation of 4methoxyphenol from the 4-methoxyphenol-enriched mixture ((&, initial purity for feed 4; , simulated final purity for feed 4; &, experimental final purity for feed 4; *, initial purity for feed 5; , simulated final purity for feed 5; *, experimental final purity for feed 5).
results. Notably, only pure catechol crystals are formed based on the simulations. Fig. 4 plots the simulated final purity of the product versus the final operating temperature, X W;A ðTÞ, as three curves, i.e. one for each feed. The starting point of each curve represents the feed purity and the initial SC operating temperature, while the ending point of each curve refers to the simulated product purity and the final SC operating temperature. The number in the parenthesis next to the ending point of each curve represents the simulated recovery ratio. Experimental results are compared with the simulation results. Each data point represents the experimental final purity of the product versus the final operating temperature for an experimental run. The number in the parenthesis next to each data point represents the experimental recovery ratio. Simulation results suggest that feed 1 can be purified from X W;A ¼ 0:80 to X W;A ¼ 0:823 with RS ¼ 76% as SC is operated from 95 8C to 59 8C. Two experimental runs indicate that feed 1 is purified from X W;A ¼ 0:80 to X W;A ¼ 0:90 0:905 with RE ¼ 61% 63%. Similarly, simulation results suggest that feed 2 can be purified from X W;A ¼ 0:85 to X W;A ¼ 0:886 with RS ¼ 72% as SC is operated from 97 8C to 56 8C. Two experimental runs indicate that feed 2 is purified from X W;A ¼ 0:85 to X W;A ¼ 0:965 0:972 with RE ¼ 55% 56%. Simulation results suggest that feed 3 can be purified from X W;A ¼ 0:90 to X W;A ¼ 0:945 with RS ¼ 70% as SC is operated from 101 8C to 55 8C. Two experimental runs indicate that feed 3 is purified from X W;A ¼ 0:90 to X W;A ¼ 0:992 0:997 with RE ¼ 49% 52%. Thus, the final experimental X W;A exceeds the simulated value for various feeds. However, the experimental recovery ratio is always lower than the simulated recovery ratio. In the second part, SC is applied to produce 4-methoxyphenol (B) crystals from the catechol/4-methoxyphenol mixture in the range 0:71 < X B < 1. The SC simulation equations are then solved for two feeds: feed 4 (20 g with X W;B ¼ 0:90) and feed 5 (20 g with X W;B ¼ 0:95). Fig. 5 displays the calculated curve, P(T), to simulate the SC operating conditions during the production of
L.-D. Shiau, S.-L. Zeng / Journal of Industrial and Engineering Chemistry 18 (2012) 963–968
968
a
O
H
operated at the three-phase equilibrium. However, the threephase equilibrium might not always be achieved during the experiments; (b) the physical properties and the Wilson parameters in the simulations are taken from the literature for a normal pressure and a certain temperature range. However, the SC experiments are operated at a low pressure and occasionally beyond the applicable temperature range; (c) liquid inclusion might occur in crystal growth during the SC operation; (d) some liquid among crystals might not be removed entirely upon completion of the experiments. If the data of the physical properties and the Wilson parameters in the studied temperature and pressure range are available, the true three-phase equilibrium conditions can be determined by the proposed model. The temperature and pressure of the SC experiments can then be carefully controlled to ensure that the three-phase equilibrium is always achieved during the experiments. Thus, experimental results should be more consistent with simulation results.
1.355Å
0.972Å
1.420Å
O H b
1.111Å
H
H C
1.402Å
H
1.355Å
O
O
Fig. 6. Chemical structures of (a) catechol; (b) 4-methoxyphenol.
4-methoxyphenol crystals. This figure also plots the simulated final purity of the product versus the final operating temperature, X W;B ðTÞ, as two curves, i.e. one for each feed. Simulation results suggest that feed 4 can be purified from X W;B ¼ 0:90 to X W;B ¼ 0:908 with RS ¼ 88% as SC is operated from 49 8C to 41.5 8C. Two experimental runs indicate that feed 4 is purified from X W;B ¼ 0:90 to X W;B ¼ 0:905 0:907 with RE ¼ 75% 77%. Similarly, simulation results suggest that feed 5 can be purified from X W;B ¼ 0:95 to X W;B ¼ 0:963 with RS ¼ 87% as SC is operated from 52 8C to 41.4 8C. According to Fig. 5, the model predicts a lower X W;B with a higher RS if SC is stopped at a higher temperature. Three experimental runs indicate that feed 5 is only purified from X W;B ¼ 0:95 to X W;B ¼ 0:951 0:953 with RE ¼ 72% 76%. Thus, the final experimental X W;B is lower than the simulated value for various feeds. Moreover, the experimental recovery ratio is also lower than the simulated recovery ratio. According to Fig. 6, catechol has a smaller planar structure while 4-methoxyphenol has a larger non-planar configuration. Therefore, smaller catechol might be incorporated into the larger 4-methoxyphenol crystal lattice, leading to 4-methoxyphenol crystals with a lower purity. However, it is difficult for larger 4methoxyphenol to be incorporated into the smaller catechol crystal lattice, leading to catechol crystals with a higher purity. This finding is consistent with that in which the final experimental X W;A exceeds the simulated value while the final experimental X W;B is lower than the simulated value. Experimental results indicate that further purifying 4-methoxyphenol from the mixture is difficult since the SC operable temperature ranges only from 56 8C to 36 8C. However, further purifying catechol from the mixture is relatively easy since the SC operable temperature ranges from 104.6 8C to 36 8C. The experimental recovery ratio is generally lower than the simulated recovery ratio in the purification of either catechol or 4methoxyphenol by SC. The discrepancy between the simulated and experimental results can be attributed to the following reasons: (a) Simulation results are obtained based on the assumption that each stage is
6. Conclusions This work presents a novel separation scheme, stripping crystallization (SC), to separate the catechol/4-methoxyphenol mixture. In principle, SC is operated at the three-phase equilibrium, in which the liquid mixture is vaporized and crystallized simultaneously. Thus, SC combines distillation and crystallization to produce pure crystals by controlling temperature and pressure. A model is also developed to simulate the three-phase equilibrium and direct the SC experiments. Experimental results indicate that SC can be used efficiently to further purify catechol from the catechol/4-methoxyphenol mixture. However further purifying 4methoxyphenol from the catechol/4-methoxyphenol mixture is difficult. In contrast with conventional crystallization, filtration and crystal washing is unnecessary since no mother liquor is adhered on the crystal surfaces upon completion of the operation. Acknowledgments The financial support of National Science Council of Taiwan and Chang Gung Memorial Hospital is greatly appreciated. References [1] J.L. Humphery, G.E. Keller, Separation Process Technology, McGraw-Hill Book Co., New York, 1997. [2] P.C. Wankat, Separation Process Engineering, 2nd ed., Prentice Hall, NJ, 2007. [3] R.W. Rousseau, Crystallization, Encyclopedia of Separation Technology, vol. 1, Wiley Interscience, 1997. [4] L.A. Cisternas, C.M. Vasquez, R.E. Swaney, AIChE J. 52 (2006) 1754. [5] K.S. Kwok, H.C. Chan, C.K. Chan, K.M. Ng, Ind. Eng. Chem. Res. 44 (2005) 3788. [6] S.W. Lin, K.M. Ng, C. Wibowo, Comput. Chem. Eng. 32 (2008) 956. [7] D.A. Berry, K.M. Ng, AIChE J. 434 (1997) 1751. [8] M.B. Franke, N. Nowotny, E.N. Ndocko, A. Gorak, J. Strube, AIChE J. 54 (2008) 2925. [9] L.D. Shiau, C.C. Wen, B.S. Lin, Ind. Eng. Chem. Res. 44 (2005) 2258. [10] L.D. Shiau, C.C. Wen, B.S. Lin, AIChE J. 52 (2006) 1962. [11] L.D. Shiau, C.C. Wen, B.S. Lin, AIChE J. 54 (2008) 337. [12] L.D. Shiau, K.F. Liu, S.M. Jang, S.C. Wu, J. Chin. Inst. Chem. Eng. 39 (2008) 59. [13] L.D. Shiau, C.C. Yu, Sep. Purif. Technol. 66 (2009) 422. [14] R.E. Kirk, D.F. Othmer, Encyclopedia of Chemical Technology, vol. 4, John Wiley & Sons, Inc., New York, 1991. [15] M.J. Lee, Y.K. Chang, H.M. Lin, C.H. Chen, J. Chem. Eng. Data 42 (1997) 349. [16] S.M. Hwang, M.J. Lee, H.M. Lin, Fluid Phase Equilib. 172 (2000) 183. [17] M.J. Lee, F.L. Wu, H.M. Lin, Ind. Eng. Chem. Res. 40 (2001) 4596. [18] H.M. Lin, Y.H. Chou, F.L. Wu, M.J. Lee, Fluid Phase Equilib. 220 (2004) 71. [19] J.M. Prausnitz, R.N. Lichtenthaler, E.G. de Azevedo, Molecular Thermodynamics of Fluid-phase Equilibria, Prentice-Hall Inc., NJ, 1999. [20] J.M. Smith, H.C. van Ness, M.M. Abbott, Introduction to Chemical Engineering Thermodynamics, McGraw-Hill Book Co., Singapore, 2001. [21] NIST Standard Reference Database 11: DIPPR data compilation of pure compound properties. Version 5.0, sponsored by The Design Institute for Physical Property Data (DIPPR) of the American Institute of Chemical Engineers, copyright by The American Institute of Chemical Engineers, 1985.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Separation of the catechol/4-methoxyphenol mixture by stripping crystallization Lie-Ding Shiau *, Shu-Li Zeng Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kweishan, Taoyuan 33302, Taiwan
A R T I C L E I N F O
Article history: Received 18 May 2011 Accepted 26 September 2011 Available online 4 February 2012 Keywords: Crystallization Distillation Catechol 4-Methoxyphenol
A B S T R A C T
This work presents a novel separation scheme, stripping crystallization (SC), to separate the catechol/4methoxyphenol mixture. While operated at a triple-point condition, in which the liquid mixture is vaporized and crystallized simultaneously due to the three-phase equilibrium, SC combines distillation and crystallization to produce pure crystals. Experimental results demonstrate the feasibility of applying SC to purify catechol in the catechol/4-methoxyphenol mixture. However, purifying 4-methoxyphenol by SC in the catechol/4-methoxyphenol mixture is rather difficult. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Separation and purification play an important role in a chemical manufacturing process. Separation and purification processes typically account for 40–70% of both capital and operating costs in the chemical industry, and their proper application can significantly reduce costs and increase profits [1,2]. Among the various separation techniques, crystallization is extensively adopted in various industrial applications and exceeded in scope only by distillation processes [3,4]. Crystallization has received renewed interest in recent years, owing to the rising demand for pure intermediates/products in the pharmaceutical and chemical industries [5,6]. Hybrid distillation/melt crystallization processes are also widely used to separate isomer mixtures [7,8]. Our laboratory has recently developed stripping crystallization (SC), also referred to as distillative freezing (DF), to separate the mixtures with close boiling temperatures, including mixed xylenes [9–11], diethylbenzene isomers [12], and benzene/cyclohexane mixtures [13]. While operated at a triple-point condition, in which the liquid mixture is vaporized and crystallized simultaneously due to the three-phase equilibrium, SC combines distillation and crystallization to produce pure crystals. Adequately controlling temperature and pressure during the operation allows SC to form pure crystals, and liquid phase and vapor phase of mixtures. SC can be continued until the liquid phase is eliminated, and only pure crystals remain in the feed. Thus, unlike conventional crystallization, filtration or centrifugation is unnecessary to separate the solid crystals from the mother liquor since no mother liquor is present
* Corresponding author. Tel.: +886 3 2118800x5291; fax: +886 3 2118700. E-mail address: [email protected] (L.-D. Shiau).
with the pure crystals. Additionally, crystal washing is unnecessary since no impurities are adhered on the crystal surfaces at the end of the operation [9]. Separation of the catechol/4-methoxyphenol mixture is critical in the manufacturing of phenol derivatives [14]. Due to the close boiling temperatures (catechol: bp = 245 8C and 4-methoxyphenol: bp = 243 8C), separating the catechol/4-methoxyphenol mixture by conventional distillation is extremely difficult. Therefore, this work investigates the feasibility of SC in the separation of the catechol/4-methoxyphenol mixture. 2. Principles of SC SC operational principles can be explained by referring to the phase diagrams [9]. Fig. 1(a) illustrates the solid-liquid equilibrium (SLE) and vapor–liquid equilibrium (VLE) for catechol (A) and 4methoxyphenol (B) at a normal pressure (P = 1.013 105 Pa) [15– 18]. Notably, the catechol/4-methoxyphenol liquid mixture exhibits nonideal solution behavior. The eutectic point lies at T = 36 8C and XB = 0.71. This figure also plots the SLE based on the ideal liquid solution for comparison. As the pressure is reduced, SLE is assumed to remain nearly the same, while VLE moves downward. For instance, Fig. 1(b) illustrates the low pressure phase diagram at P = 133 Pa, which is lower than the triple-point pressure of pure catechol and higher than the triple-point pressure of pure 4-methoxyphenol. This diagram shows the three-phase state with pure catechol solid, and liquid phase and vapor phase of mixtures at T = 77 8C. Fig. 1(c) illustrates the low pressure phase diagram at P = 10.7 Pa, which is lower than the triple-point pressure of pure catechol or pure 4-methoxyphenol. This diagram shows two threephase states. The first three-phase state has pure catechol solid, as
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.09.006
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the range of 0 < X B < 0:71 and 4-methoxyphenol crystals in the range of 0:71 < X B < 1.
Notation C P; jðSÞ C P; jðLÞ C P; jðVÞ
DC P; j HS; j HL; j HV; j DHm; j DHv; j Ln Mj P P sat j R RE RS Sn T T m; j T tri; j Vn Xj X W; j
heat capacity of solid phase for j-component (J/mol K) heat capacity of liquid phase for j-component (J/mol K) heat capacity of vapor phase for j-component (J/mol K) change in the heat capacity from solid phase to liquid phase for j-component (J/mol K) enthalpy of the solid of j-component (J/g) enthalpy of the liquid of j-component (J/g) enthalpy of the vapor of j-component (J/g) heat of melting for j-component (>0) (J/mol) heat of vaporization for j-component (>0) (J/mol) mass of the liquid phase out of stage n (g) molecular weight of j-component (>0) (g) pressure (Pa) saturated pressure of the liquid of j-component (Pa) ideal gas constant (8.314 J/mol K) experimental recovery ratio (dimensionless) simulated recovery ratio (dimensionless) mass of the solid phase out of stage n (g) temperature (K) melting temperature of j-component (K) triple point temperature of j-component (K) mass of the vapor phase out of stage n (g) mole fraction of j-component in liquid phase (dimensionless) weight fraction of j-component in liquid phase X M X W; j ¼ X M jþXj M (dimensionless) A
Yj Y W; j
A
B
A
B
two adjustable parameters for the activity coefficients in the Wilson equation (dimensionless) Subscript 0 in at in in
ln½ðX A Þn ðg A Þn ¼
DHm;A
1
1 Tn
T m;A DC p;A T m;A ln R Tn R
þ
DC P;A T m;A T n R
Tn (1)
At pressures of 1 atm and lower, the assumption of ideal gases normally incurs little error. As SC is operated at low pressures, VLE of catechol and 4-methoxyphenol in stage n can be described by [19,20]: ðY A Þn Pn ¼ ðX A Þn ðg A Þn ðPAsat Þn
(2)
ðY B Þn P n ¼ ðX B Þn ðg B Þn ðPBsat Þn
(3)
Due to the nonideality of liquid solutions, the activity coefficients are evaluated by the Wilson equation as [19,20]: lnðg A Þn ¼ ln½ðX A Þn þ LAB ðX B Þn LAB LBA þ ðX B Þn ðX A Þn þ LAB ðX B Þn LBA ðX A Þn þ ðX B Þn
(4)
B
Greek letters gj activity coefficient of j-component in liquid phase (dimensionless) LAB , LBA
f n N
The SC process is simulated in a series of N equilibrium stage operations shown in Fig. 2. Alternatively, this figure is also an abstract representation of a batch SC process in a single vessel. Each stage is operated at a three-phase equilibrium. The temperature of each stage is chosen to meet the requirements of: T n1 T n ¼ DT for n ¼ 1; 2; :::; N. The vapor formed in each stage is condensed to the liquid and removed, while the solid and the liquid formed in each stage enter the next stage. When SC is applied to produce catechol (A) crystals from the catechol/4-methoxyphenol liquid mixtures in the range of 0 < X B < 0:71, each stage is maintained at a three-phase state with pure catechol crystals, as well as the liquid and vapor phases of mixtures. SLE of catechol in stage n is described by the van’t Hoff equation as [19,20]
B
mole fraction of j-component in vapor phase (dimensionless) weight fraction of j-component in vapor phase Y M Y W; j ¼ Y M jþYj M (dimensionless) A
3. SC simulation process
the feed the end of the experiment stage n the final stage
well as liquid phase and vapor phase of mixtures at T = 42 8C on the right portion of the figure. The other three-phase state has pure 4methoxyphenol solid, as well as liquid phase and vapor phase of mixtures at T = 43 8C on the left portion of the figure. Thus, SC provides a potential method to produce both catechol crystals in
lnðg B Þn ¼ ln½ðX B Þn þ LBA ðX A Þn LAB LBA ðX A Þn ðX A Þn þ LAB ðX B Þn LBA ðX A Þn þ ðX B Þn
(5)
where LAB and LBA denote two adjustable parameters. Additionally, we have ðX A Þn þ ðX B Þn ¼ 1
(6)
ðY A Þn þ ðY B Þn ¼ 1
(7)
When Tn is specified, Eqs. (1)–(7) constitute a set of seven equations that can be solved simultaneously for seven unknowns – Pn, ðX A Þn , ðX B Þn , ðg A Þn , ðg B Þn , ðY A Þn and ðY B Þn for n ¼ 1; 2; :::; N. Thus, the three-phase equilibrium conditions during the SC operation can be determined. Notably T0 denotes the triple-point temperature of the mixture feed determined by Eq. (1). The physical properties for catechol and 4-methoxyphenol needed in the SC simulation are taken from The Design Institute for Physical Property Data (DIPPR) [21]. The nonideality of catechol/4methoxyphenol liquid solution is accounted for by incorporating the activity coefficients from the Wilson equation [15–18]. The final SC operation should be stopped above the eutectic temperature to avoid the cocrystallization of catechol and 4-methoxyphenol.
L.-D. Shiau, S.-L. Zeng / Journal of Industrial and Engineering Chemistry 18 (2012) 963–968
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Fig. 1. Phase diagram of catechol (A) and 4-methoxyphenol (B) at (a) P = 1.013 105 Pa (—— represents the solid–liquid equilibrium calculated based on the ideal solution); (b) P = 133 Pa; (c) P = 10.7 Pa.
The total material balance in stage n can be expressed as: Sn1 þ Ln1 ¼ Sn þ Ln þ V n
(8)
As SC is applied to produce catechol crystals in each stage, 4methoxyphenol only exits in the liquid and vapor phases. The material balance of 4-methoxyphenol in stage n can be expressed as: Ln1 ðX W;B Þn1 ¼ Ln ðX W;B Þn þ V n ðY W;B Þn
(9)
As the latent heat released in forming catechol crystals is removed by vaporizing portions of liquid mixtures at each stage, the energy balance in stage n is described by: Sn1 ðHS;B Þn1 þ Ln1 ðHL Þn1 ¼ Sn ðHS;B Þn þ Ln ðHL Þn þ V n ðHV Þn
(10)
where ðHL Þn and ðHV Þn denote the enthalpy of the liquid mixture and the vapor mixture in stage n, respectively. Thus, ðHL Þn and ðHV Þn can be defined as: ðHL Þn ¼ ðX W;A Þn ðHL;A Þn þ ðX W;B Þn ðHL;B Þn
(11)
ðHV Þn ¼ ðY W;A Þn ðHV;A Þn þ ðY W;B Þn ðHV;B Þn
(12)
For convenience, it is assumed that HL; j ¼ 0 at T tri; j for jcomponent. Then, ðHS; j Þn , ðHL; j Þn and ðHV; j Þn in stage n can be determined as: ðHS; j Þn ¼
1 ½DHm; j ðT tri; j Þ þ Mj
Z
Tn T tri; j
C P; jðSÞ dT
(13)
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V1
S0 L0 (XA)0 (XB)0
Vn-1
1
n- 1
T1 P1
Tn-1 Pn-1
Vn
Sn-1 Ln-1 (XA)n-1 (XB)n-1
n Tn Pn
Vn+1
Sn Ln (XA)n (XB)n
VN
n+1
N
Tn+1 Pn+1
TN PN
SN
T0 P0 Fig. 2. Simulated SC operation where each stage is operated at a three-phase equilibrium state.
1 ðHL; j Þn ¼ Mj
ðHV; j Þn ¼
"Z
#
Tn
T tri; j
C P; jðLÞ dT
# " Z Tn 1 DHv; j ðT tri; j Þ þ C P; jðVÞ dT Mj T tri; j
(14)
(15)
Incorporating Eqs. (11)–(15) into Eq. (10) allows one to obtain Sn, Ln and Vn by solving Eqs. (8)–(10) simultaneously for n ¼ 1; 2; :::; N. Thus, the crystals and liquid remaining in each stage during the SC process can be determined. Notably, ðX W;A Þn , ðX W;B Þn , ðY W;A Þn and ðY W;B Þn can be calculated directly from the previously determined ðX A Þn , ðX B Þn , ðY A Þn and ðY B Þn . Similar simulation equations can be derived when SC is applied to produce 4-methoxyphenol (B) crystals from the catechol/4methoxyphenol mixture in the range 0:71 < X B < 1. 4. Experimental The experimental assembly consists of a 100 cm3 sample container in a 750 cm3 glass reactor, as shown in Fig. 3. The glass reactor is immersed in a constant temperature bath. In the beginning of the experiment, 20 g of melt mixture is placed in the sample container. Pressure in the glass reactor is then lowered using a mechanical vacuum pump (GLD-210B, ULVAC). Next, the vapor is condensed using a glass condenser circulated with the cooling water. Additionally, the reactor pressure is controlled by adjusting the valve connected to the vacuum pump. Finally, the temperature is controlled by adjusting the constant temperature bath. The feed is prepared by mixing catechol (Sigma–Aldrich, >99% purity) and 4-methoxyphenol (Sigma–Aldrich, >99% purity) in different ratios. The sample temperature is then lowered gradually
with the increasing time. The cooling rate typically starts at 2 8C/ min and then decreases gradually to 0.5 8C/min at the end. An experiment is generally finished within 50–100 min. During the experiment, a temperature probe (type K, thermocouple) is positioned in the center of the liquid feed, and a pressure gauge (Micro-ion plus, Granville-Phillips) is connected to the glass reactor. As the sample temperature in the chamber is lowered, the pressure is reduced by controlling the vacuum pump. A series of three-phase equilibrium conditions are achieved by controlling the temperature and pressure based on the simulation results. Thus, the liquid mixture is vaporized and solidified simultaneously due to the three-phase equilibrium. The SC experiment here is stopped at the lowest pressure attainable in the apparatus, depending on the corresponding temperature. Finally, the sample remained in the vessel is weighed, and its composition is analyzed by HPLC. HPLC (Spectra System, Thermo Separation products) consists of an ERC-3415a degasser, a Spectra System P1000 pump, a Spectra System AS1000 automatic sample injector and a UV 6000LP detector. The detection wavelength is set at 280 nm UV absorbance. The injection volume is 20 mL, and the flow rate is 1.0 mL/ min. Supelco Discovery HS C18 column (250 mm 4.6 mm, 5 mm particle size) is used for the separation. Mobile phases consist of water with 1% acetic acid (solvent A) and acetonitrile with 1% acetic acid (solvent B). Mobile phases are then filtered through a Millipore 0.22 mm membrane filter before use. Additionally, the composition of the mobile phases starts with 0% solvent B, increases to 31% solvent B at 10.5 min and then reaches 100% solvent B at 15 min. Ethanol (99.5%) is used as a solvent to prepare the catechol/4-methoxyphenol mixture solution for calibration in the quantitative analysis of the mixture composition at 280 nm UV absorbance.
Fig. 3. Schematic diagram of the experimental apparatus for the SC operation with the features: (1) vacuum pump, (2) condenser, (3) constant temperature bath, (4) thermocouple, (5) pressure gauge, (6) temperature controller, (7) glass reactor, (8) sample container.
L.-D. Shiau, S.-L. Zeng / Journal of Industrial and Engineering Chemistry 18 (2012) 963–968
1.00
700
(49%) (55%) (55%) (56%)
0.95
1.00
600
P
300
X W,B (72%)
XW,B
XW,A
400
P(Pa)
XW,A
feed 3
20
(87%) (90%) (92%)
(76%)
(73%)
feed 5
0.94
P
12
feed 2 (76%)
0.80
0.92
200
XW,A
8
(88%)
feed 1
0.75
16
P(Pa)
500
(72%)
0.85
X W,B
(77%) (75%)
100
0.90
4
feed 4
0 30
40
50
60
70
80
0.88
90100
o
T( C)
In the first part, SC is applied to produce catechol (A) crystals from the catechol/4-methoxyphenol mixture in the range of 0 < X B < 0:71. The SC simulation equations are solved for three feeds: feed 1 (20 g with X W;A ¼ 0:80), feed 2 (20 g with X W;A ¼ 0:85) and feed 3 (20 g with X W;A ¼ 0:90). Fig. 4 displays the calculated curve, P(T), by solving Eqs. (1)–(7) to simulate the SC operating conditions in the production of catechol crystals. Thus, as the temperature is decreased during the SC experiments, the corresponding pressure for the three-phase equilibrium is depicted by P(T). The experimental recovery ratio of catechol (RE ) is defined as: (16)
where ðX W;A Þ0 denotes the initial purity of the mixture feed, ðX W;A Þf represents the experimental final purity of the product at the end of the experiment, W f refers to the final weight of the product including the crystals and the remaining liquid obtained at the end of the experiment, and L0 is the weight of the liquid mixture feed initially. As some liquid may remain along with the final crystals at the end of SC, the simulated purity of the final product, including the final crystals and the remaining liquid, is defined as: SN þ LN ðX W;A ÞN SN þ LN
(17)
The simulated recovery ratio of catechol (RS ) is defined as: SN þ LN ðX W;A ÞN L0 ðX W;A Þ0
36
40
44
48
52
0 56
T( C)
5. Results and discussion
W f ðX W;A Þf L0 ðX W;A Þ0
32
o
Fig. 4. Comparison of experimental and simulation results for separation of catechol from the catechol-enriched mixture (&, initial purity for feed 1, , simulated final purity for feed 1, &, experimental final purity for feed 1; *, initial purity for feed 2, , simulated final purity for feed 2; *, experimental final purity for feed 2; ~, initial purity for feed 3; , simulated final purity for feed 3; ~, experimental final purity for feed 3).
RS ¼
24
0.98
0.96
(63%) (61%)
0.90
X W;A ¼
28
(70%)
XW,A
RE ¼
967
(18)
where SN , LN and ðX W;A ÞN denote the crystals, liquid and weight fraction of catechol in the final stage based on the simulation
Fig. 5. Comparison of experimental and simulation results for separation of 4methoxyphenol from the 4-methoxyphenol-enriched mixture ((&, initial purity for feed 4; , simulated final purity for feed 4; &, experimental final purity for feed 4; *, initial purity for feed 5; , simulated final purity for feed 5; *, experimental final purity for feed 5).
results. Notably, only pure catechol crystals are formed based on the simulations. Fig. 4 plots the simulated final purity of the product versus the final operating temperature, X W;A ðTÞ, as three curves, i.e. one for each feed. The starting point of each curve represents the feed purity and the initial SC operating temperature, while the ending point of each curve refers to the simulated product purity and the final SC operating temperature. The number in the parenthesis next to the ending point of each curve represents the simulated recovery ratio. Experimental results are compared with the simulation results. Each data point represents the experimental final purity of the product versus the final operating temperature for an experimental run. The number in the parenthesis next to each data point represents the experimental recovery ratio. Simulation results suggest that feed 1 can be purified from X W;A ¼ 0:80 to X W;A ¼ 0:823 with RS ¼ 76% as SC is operated from 95 8C to 59 8C. Two experimental runs indicate that feed 1 is purified from X W;A ¼ 0:80 to X W;A ¼ 0:90 0:905 with RE ¼ 61% 63%. Similarly, simulation results suggest that feed 2 can be purified from X W;A ¼ 0:85 to X W;A ¼ 0:886 with RS ¼ 72% as SC is operated from 97 8C to 56 8C. Two experimental runs indicate that feed 2 is purified from X W;A ¼ 0:85 to X W;A ¼ 0:965 0:972 with RE ¼ 55% 56%. Simulation results suggest that feed 3 can be purified from X W;A ¼ 0:90 to X W;A ¼ 0:945 with RS ¼ 70% as SC is operated from 101 8C to 55 8C. Two experimental runs indicate that feed 3 is purified from X W;A ¼ 0:90 to X W;A ¼ 0:992 0:997 with RE ¼ 49% 52%. Thus, the final experimental X W;A exceeds the simulated value for various feeds. However, the experimental recovery ratio is always lower than the simulated recovery ratio. In the second part, SC is applied to produce 4-methoxyphenol (B) crystals from the catechol/4-methoxyphenol mixture in the range 0:71 < X B < 1. The SC simulation equations are then solved for two feeds: feed 4 (20 g with X W;B ¼ 0:90) and feed 5 (20 g with X W;B ¼ 0:95). Fig. 5 displays the calculated curve, P(T), to simulate the SC operating conditions during the production of
L.-D. Shiau, S.-L. Zeng / Journal of Industrial and Engineering Chemistry 18 (2012) 963–968
968
a
O
H
operated at the three-phase equilibrium. However, the threephase equilibrium might not always be achieved during the experiments; (b) the physical properties and the Wilson parameters in the simulations are taken from the literature for a normal pressure and a certain temperature range. However, the SC experiments are operated at a low pressure and occasionally beyond the applicable temperature range; (c) liquid inclusion might occur in crystal growth during the SC operation; (d) some liquid among crystals might not be removed entirely upon completion of the experiments. If the data of the physical properties and the Wilson parameters in the studied temperature and pressure range are available, the true three-phase equilibrium conditions can be determined by the proposed model. The temperature and pressure of the SC experiments can then be carefully controlled to ensure that the three-phase equilibrium is always achieved during the experiments. Thus, experimental results should be more consistent with simulation results.
1.355Å
0.972Å
1.420Å
O H b
1.111Å
H
H C
1.402Å
H
1.355Å
O
O
Fig. 6. Chemical structures of (a) catechol; (b) 4-methoxyphenol.
4-methoxyphenol crystals. This figure also plots the simulated final purity of the product versus the final operating temperature, X W;B ðTÞ, as two curves, i.e. one for each feed. Simulation results suggest that feed 4 can be purified from X W;B ¼ 0:90 to X W;B ¼ 0:908 with RS ¼ 88% as SC is operated from 49 8C to 41.5 8C. Two experimental runs indicate that feed 4 is purified from X W;B ¼ 0:90 to X W;B ¼ 0:905 0:907 with RE ¼ 75% 77%. Similarly, simulation results suggest that feed 5 can be purified from X W;B ¼ 0:95 to X W;B ¼ 0:963 with RS ¼ 87% as SC is operated from 52 8C to 41.4 8C. According to Fig. 5, the model predicts a lower X W;B with a higher RS if SC is stopped at a higher temperature. Three experimental runs indicate that feed 5 is only purified from X W;B ¼ 0:95 to X W;B ¼ 0:951 0:953 with RE ¼ 72% 76%. Thus, the final experimental X W;B is lower than the simulated value for various feeds. Moreover, the experimental recovery ratio is also lower than the simulated recovery ratio. According to Fig. 6, catechol has a smaller planar structure while 4-methoxyphenol has a larger non-planar configuration. Therefore, smaller catechol might be incorporated into the larger 4-methoxyphenol crystal lattice, leading to 4-methoxyphenol crystals with a lower purity. However, it is difficult for larger 4methoxyphenol to be incorporated into the smaller catechol crystal lattice, leading to catechol crystals with a higher purity. This finding is consistent with that in which the final experimental X W;A exceeds the simulated value while the final experimental X W;B is lower than the simulated value. Experimental results indicate that further purifying 4-methoxyphenol from the mixture is difficult since the SC operable temperature ranges only from 56 8C to 36 8C. However, further purifying catechol from the mixture is relatively easy since the SC operable temperature ranges from 104.6 8C to 36 8C. The experimental recovery ratio is generally lower than the simulated recovery ratio in the purification of either catechol or 4methoxyphenol by SC. The discrepancy between the simulated and experimental results can be attributed to the following reasons: (a) Simulation results are obtained based on the assumption that each stage is
6. Conclusions This work presents a novel separation scheme, stripping crystallization (SC), to separate the catechol/4-methoxyphenol mixture. In principle, SC is operated at the three-phase equilibrium, in which the liquid mixture is vaporized and crystallized simultaneously. Thus, SC combines distillation and crystallization to produce pure crystals by controlling temperature and pressure. A model is also developed to simulate the three-phase equilibrium and direct the SC experiments. Experimental results indicate that SC can be used efficiently to further purify catechol from the catechol/4-methoxyphenol mixture. However further purifying 4methoxyphenol from the catechol/4-methoxyphenol mixture is difficult. In contrast with conventional crystallization, filtration and crystal washing is unnecessary since no mother liquor is adhered on the crystal surfaces upon completion of the operation. Acknowledgments The financial support of National Science Council of Taiwan and Chang Gung Memorial Hospital is greatly appreciated. References [1] J.L. Humphery, G.E. Keller, Separation Process Technology, McGraw-Hill Book Co., New York, 1997. [2] P.C. Wankat, Separation Process Engineering, 2nd ed., Prentice Hall, NJ, 2007. [3] R.W. Rousseau, Crystallization, Encyclopedia of Separation Technology, vol. 1, Wiley Interscience, 1997. [4] L.A. Cisternas, C.M. Vasquez, R.E. Swaney, AIChE J. 52 (2006) 1754. [5] K.S. Kwok, H.C. Chan, C.K. Chan, K.M. Ng, Ind. Eng. Chem. Res. 44 (2005) 3788. [6] S.W. Lin, K.M. Ng, C. Wibowo, Comput. Chem. Eng. 32 (2008) 956. [7] D.A. Berry, K.M. Ng, AIChE J. 434 (1997) 1751. [8] M.B. Franke, N. Nowotny, E.N. Ndocko, A. Gorak, J. Strube, AIChE J. 54 (2008) 2925. [9] L.D. Shiau, C.C. Wen, B.S. Lin, Ind. Eng. Chem. Res. 44 (2005) 2258. [10] L.D. Shiau, C.C. Wen, B.S. Lin, AIChE J. 52 (2006) 1962. [11] L.D. Shiau, C.C. Wen, B.S. Lin, AIChE J. 54 (2008) 337. [12] L.D. Shiau, K.F. Liu, S.M. Jang, S.C. Wu, J. Chin. Inst. Chem. Eng. 39 (2008) 59. [13] L.D. Shiau, C.C. Yu, Sep. Purif. Technol. 66 (2009) 422. [14] R.E. Kirk, D.F. Othmer, Encyclopedia of Chemical Technology, vol. 4, John Wiley & Sons, Inc., New York, 1991. [15] M.J. Lee, Y.K. Chang, H.M. Lin, C.H. Chen, J. Chem. Eng. Data 42 (1997) 349. [16] S.M. Hwang, M.J. Lee, H.M. Lin, Fluid Phase Equilib. 172 (2000) 183. [17] M.J. Lee, F.L. Wu, H.M. Lin, Ind. Eng. Chem. Res. 40 (2001) 4596. [18] H.M. Lin, Y.H. Chou, F.L. Wu, M.J. Lee, Fluid Phase Equilib. 220 (2004) 71. [19] J.M. Prausnitz, R.N. Lichtenthaler, E.G. de Azevedo, Molecular Thermodynamics of Fluid-phase Equilibria, Prentice-Hall Inc., NJ, 1999. [20] J.M. Smith, H.C. van Ness, M.M. Abbott, Introduction to Chemical Engineering Thermodynamics, McGraw-Hill Book Co., Singapore, 2001. [21] NIST Standard Reference Database 11: DIPPR data compilation of pure compound properties. Version 5.0, sponsored by The Design Institute for Physical Property Data (DIPPR) of the American Institute of Chemical Engineers, copyright by The American Institute of Chemical Engineers, 1985.