Development of an azeotropic distillation scheme for purification of tetrahydrofuran

FluidPhaseEquilibria, 52 (1989) 161-168 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

DEVELOPMENT OF AN AZEOTROPIC DISTlLLATION PURIFICATION OF TETRAHYD ROFURAN

161

SCHEME FOR

Te Chang and T. Thomas Shih ARC0 Chemical Company 3801 West Chester Pike

Newtown Square, Pennsylvania U.S.A.

19073

ABSTRACT Tetrahydrofuran (THF) is produced by the dehydrocyclization of 1,4-butanediol. The key step in purification is breaking of the THFlwater azeotrope. Separation schemes based on azeotropic distillation are considered in this paper. The paper presents a good example of a typical industrial application of phase equilibria that uses a combination of data from the literature, UNIFAC estimations, and experimental measurements. Process development work included selection of entrainers, measurement and evaluation of phase equilibrium data, and the use of a commercial process simulator to model proposed schemes. Normal pentane was found lo be the best entrainer of the several evaluated. Purification schemes based on n-pentane are compared with a conventional distillation method. INTRODUCTION

Recently, ARC0 Chemical Company announced and began construction of a 75-millonlb/year butanediol and derivatives plant in Texas. The process makes 1,4-butanediol (BDO) from ally1 alcohol which in turn is produced by the catalytic isomerization of propylene oxide (Brooks, 1987). Tetrahydrofuran (THF) is one of the major BDO derivatives which will also be produced. THF has been widely used as a solvent and in recent years has been developed as a feed stock for synthetic high polymers. High purity and low water content are required for these applications. Selection of an efficient THF purification scheme, therefore, is critical to product quality and process economics. THF is produced by the dehydrocyclization of 1,4-butanediol in the presence of an acid catalyst. The reaction product contains 80 wt % THF and 20 wt % water. In addition, about 4000 ppm of organic impurties are also produced. THF and water form a minimum boiling azeotrope containing 5.8 wt% of water at atmospheric pressure. The key step in the separation is breaking of the THF/water azeotrope. In addition, both light and heavy impurities must also be removed. A number of methods are available to break the THF/water azeotrope, such as extractive distillation, azeotropic distillation, low/high pressure distillation, adsorption, salt or caustic addition, and crystallization. Distillation methods are preferred by industry. Tanabe et al. (1978), Yoshida et al. (1978), Coates (1981), and Stock and Tse (1982) used a two-step sequential distillation under different pressures (low followed by high pressure). Yamada et al. (1987) suggested the use of 1,4-butanediol as an extractive distillation solvent. Bente (1957) added n-pentane to THF/water and then fractionated the upper organic layer. Azeotropic distillation has not been reported for the dehydration of THF. However, this distillation technique has been commonly used for the dehydration of alcohol using entrainers such as benzene and ethers. Black and Ditsler (1972 and 1980) suggested that azeotropic distillation using n-pentane for ethanol dehydration has operating cost advantages over other distillation techniques. In this paper, we examine the feasibility of azeotropic distillation for the dehydration of THF. The development work discussed includes selecting entrainers for the system, measuring necessary vapor-liquid equilibrium data, establishing VLE and LLE models, and 0378-3812/89/$03.50

0 1989 Elsevier Science Publishers B.V.

162 Table 1. Potential Entrainers for the Azeotropic Distillation of THF

Entrainer

b.p.l OC

Azeotrope with water wt% water b.p.l OC

n-Pentane n-Hexane MTBE Acrolein I-Chloropropane Isopropylethylether THF (0 psig) (200 psig)

36 69 55 53 46.6 61.9 66 181

1.4 5.6 4.0 2.6 2.2 2 5.8 13.9

I at atmospheric

pressure

1 at25

34.6 61.6 52.6 52.4 44 51.8 64 163.5

Mutual solubility” wt% water wt% entrainer in entrainer in water 0.011 0.013 2.9 -

0.004 0.001 5.2 100

: OD -

a0

o C

constructing a computer process simulation model to optimize operating variables. The results from two azeotropic distillation schemes are compared with a conventional low/high pressure distillation scheme. ENTRAINER SELECTION Azeotropic distillation is accomplished by introducing a third component, an entrainer, to the column to form a new azeotrope with one of the original constituents. This new azeotrope can be a ternary azeotrope. It can also be heterogeneous or homogeneous. The new azeotrope should have a lower boiling temperature than the original minimum boiling azeotrope. Based on this criterion, a number of potential entrainers were identified. Table 1 lists their azeotropic data and mutual solubilities with water. An entrainer which forms a heteroazeotrope with water is preferred because it can be easily recovered by decantation. Other desirable characteristics for a satisfactory entrainer as described by Treybal (1980) are that it be low cost, readily available, chemically stable, nontoxic, noncorrosive, have low heat of vaporization, low freezing point, and low viscosity. Among the entrainers screened, n-pentane, n-hexane and methyl tert-butyl ether (MTBE) all form heteroazeotropes with water and meet the above requirements. The data indicate that the n-hexanelwater azeotrope boiling point is too close to that of the THF/water azeotrope. The boiling temperatures of the n-pentanelwater and MTBElwater azeotropes are both significantly lower than the THF/water azeotropic temperature. Therefore, n-pentane and MTBE were selected for further evaluation. The water content of the entrainerlwater azeotrope is an indicator of the water-removing capability of the entrainer. Therefore, MTBE which forms an azeotrope at 4 wt% water, may be better than n-pentane, which forms an azeotrope at 1.4 wt% (Table 1). However, consideration must also be given to entraineriwater mutual solubilities. The relatively high solubility of water in MTBE (2.9 wt% at 25 “C) could offset its high water-removing capability, while the high MTBE solubility in water implies that an entrainer recovery system would be required. The solubility of n-pentane in water is very low. Therefore, the loss of n-pentane would be low and the wastewater may be disposed of more easily. All these considerations have to be further developed using ternary LLE data. PHASE EQUILIBRIA Since azeotropic distillation involves highly nonideal systems, accurate vapor-liquid and liquid-liquid equilibrium data are necessary to characterize the distillation system. Modem commercial process simulators use packaged multicomponent VLE models to carry out trayto-tray calculations in a distillation simulation. Activity coefficient models based on the Wilson, NRTL, or UNIQUAC equations are commonly employed. These models require

163

only binary interaction parameters and have been proven useful for ternary and multicomponent systems. Binary equilibrium data are, therefore, particularly important. In this work, the NRTL equation was used with binary parameters derived from both VLE and LLE data. The NRTL equation was selected since ARC0 Chemical has an established NRTL parameter data base, and any additional work should add to that. Also, a large amount of NRTL parameters are readily available from the DECHEMA databooks (Gmehling et al., 1978; Sorensen and Arlt, 1979). In the early development stage, the UNIFAC model was useful for predicting binary VLE for the missing pairs in the data base. After the actual experimental data became available, NRTL parameters were re-regressed. The NRTL parameters used in this work are listed in Table 2. Binary LLE Binary LLE data for the n-pentane/water and MTBE/water systems were collected and regressed by Sorensen and Arlt (1979) at several temperatures. The reported NRTL parameters were further reduced to fit the following equation as a function of temperature: Agi j/RT(K) = a, j + bi j/T(K)

(1)

Binary VLE For THFlwater system, the NRTL parameters recommended by Gmehling et al. (1978) from among the many binary data sets available were adopted. Binary VLE data for n-pentane/THF and MTBE/THF were initially calculated by the UNIFAC equation. Later, experimental data were generated at 760 mm Hg using a modified Othmer still (Chang and Shih, 1989). Figure 1 shows the relative volatilities of n-pentane and MTBE to THF as a function of THF content in the liquid phase. The NRTL equation fit the data well. The UNIFAC predictions are not quite as good. Both systems exhibit weak positive deviations. is between 1.5 and 1.2. The low relative volatility throughout the composition The aMTBE/THF range means that a significant number of stages are required to separate the entrainer (MTBE) from the product (THF) in the azeotropic distillation column. However, for n-pen&me, the ~~~~~~~~ranges from 4.5 to 1.5. The high O!at n-pentane infinite dilution ensures an easy separation between n-pentane and THF at the bottom of the azeotropic column. Ternary LLE LLE data for the n-pentane/THF/water and MTBEITHFlwater systems were obtained experimentally for evaluating the decantation process and verifing the phase equilibrium model based on binary data. The experimental data were generated by the saturation method at 25 OC and the cloud point method at room temperature (Chang and Shih, 1989). Figures 2 and 3 show the LLE diagrams for the MTBEITHFlwater and n-pentane/THFlwater systems, respectively. The phase envelopes calculated by the model are also presented for comparison. Two solid straight lines are drawn on these diagrams, representing the compositions of all mixtures of entrainer with crude THF and with the THF/water azeotrope. The line for crude THF goes through the two phase region for both the ternary LLE systems. Therefore, addition of entrainer and then decantation to remove significant water prior to distillation is a process option to consider. A more favorable phase separation is obtained for n-pentane than for MTBE. The THFlwater azeotrope - n-pentane line goes through the two phase region showing it is possible to produce a THF/water phase containing less water than the azeotrope composition by the addition of n-pentane to crude THF. The follow-up distillations would be for recovery of n-pentane and dry THF from the two phases. MODELING OF AZJZOTROPIC DISTILLATION

SCHEMES

Based on the above phase equilibrium data, the separation of MTBE and THF is relatively

164 Table 2. NRTL Parameters

Binary system

a12

b 12

a21

b 21

a12

THF( I)-Water(2) n-Pentane(l)-Water(2) n-Pentane(l)-THF(2) MTBE(l)-Water(2) MTBE(l)-THF(2)

0 -8.9486 0 -9.8447 0

460.8194 4569.7674 83.3950 3111.1661 342.4420

0 11.9010 0 16.1565 0

868.1004 -635.7478 142.4740 -3606.5912 -225.2689

0.4522 0.2 0.47 0.2 0.3

Fig. 1. Relative Volatilities of n-Per&me and h%TBE to THF at 760 mm Hg.

Fig. 2. MTBE/THF/Watcr LLE at 25 ‘C.

Fig. 3. n-PentaneiTHFIWater LLE at 25 ‘C.

165

diffkult. Hence, MTBE was eliminated from further consideration, leaving only n-pentane. Two azeotropic distillation schemes based on the latter were then studied using the ASPEN PLUS’” process simulator. These schemes are described below. Concentrator/Azeotropic

Column

The crude THF is fed to a concentrator which contains 15 theoretical stages including the condenser and reboiler (Figure 4). The purpose of the concentrator is to bring the crude THF close to the THF/water azeotrope, taking water from the column bottom. The concentrator overhead is fed to an azeotropic column which produces dry THF as the bottom product. The azeotropic column overhead vapors are condensed to provide two liquid phases, a n-pentane phase and an aqueous phase. The aqueous phase is drawn off for direct disposal. It could also be recycled to the concentrator for organics recovery, if necessary, but this was not simulated. The n-pentane phase is returned as reflux to the overhead. The number of theoretical stages and the column operating conditions were adjusted to obtain a product THF containing less than 0.1 ppm of n-pentane and 100 ppm of water. The azeotropic column was found optimal with 24 theoretical stages. Both columns were designed to operate at atmospheric pressure. Operating conditions and material balances are given in Tables 3 and4. A minimum reflux can be estimated based on the water content in the n-pentanelwater azeotrope. A reflux ratio (R/D) of 71.3 (reflux/product = 4.46) which is close to the minimum reflux (70.4) was used in the calculation. High reflux increases product contamination with n-pen&me but reduces water content. Bottom product quality is very sensitive to the feed tray location. The optimum feed location was found at 14th tray from the top including the overhead decanter. Four distillation columns will be involved in the full purification scheme including lights and heavies columns. The steam requirement for water removal is estimated to be 1.2 lb steam/lb THF. There is an additional operating cost for the lights and heavies columns. DecanterIAzeotropic

Column

The crude THF is mixed with n-pentane at a solvent to feed ratio of 0.25 and fed to a decanter or settler (see Figure 5). The aqueous phase is drawn off and sent to a stripper for recovering THF and n-pentane. The THF-pentane organic phase containing only about 2 % water is fed to an azeotropic column of 30 theoretical stages at atmospheric pressure. This azeotropic distillation column removes n-pentane and remaining water overhead and recovers dry THF as the bottom product. The product THF contains less than 0.1 ppm of n-pentane and less than 1 ppm of water. The overhead vapor condensate splits into two liquid phases, a n-pen&me phase and an aqueous phase, in an overhead decanter. The aqueous phase rate is small (2.6 lb/100 lb THF) and is sent to a stripper for THF and n-pentane recovery. Some of the n-pentane phase is recycled to the decanter stage but most of is returned to the column as reflux. In this operation, a reflux ratio (R/D) of 96.6 (reflux/product = 1.85) was used. More trays (30 stages) are needed than in the previous scheme (24 stages) to separate n-pentane from THF because the feed contains about 23 % n-pentane. However, less reflux is required due to less water in the feed, which saves energy. The stripper contains 8 theoretical stages including the condenser and reboiler. The overhead is about 80% THF (close to the crude THF feed). Water is removed from the bottom for disposal. As before, two additional columns are needed for lights and heavies removal. Table 3 provides the distillation operating conditions. Table 4 shows the resulting material balances. Compared to the first scheme, a decanter has been added. The number of columns for water removal is the same, but the columns are smaller. The steam requirement for water removal is significantly less at about 0.5 lb/lb THF.

166 Table 3. Operating Conditions for n-Pentaae Azeotropic Diitillation Schemes Concentrator/Azeo. Cal. Azeo. Col. Concentrator

Column

No. of theor. trays Feed tray (from top) RID Pressure, psia (top) (btm) Temperature, ‘C (top) (btm) Heat dutyl, Mbtu/hr Reboiler Condenser 1 Basis: (1) 20 MMlb THF/yr

Decanter/Azeo. Azeo. Col.

Stripper

Cal.

15 8 1.0 14.7 15.5 64.0 101.5

24 14 71.3 14.7 16.0 34.3 68.5

30 16 96.6 14.7 16.3 34.3 69.2

1.1 14.7 15.1 64.7 100.8

1280 -1210

2270 -2150

1320 -1170

155 -78

(2) 8000 hr/yr

Table 4. Material Balances for n-Pentane Azeotropic Distillation Schemes

Stream no. 00@@0@0@ Component, wt% (ppm) Case 1 - Concentrator/Azeotropic THF 80 Water 20 n-Pentane Total Flow, Ib/hr 3128

Column 100

94.12 5.88

469

Case 2 - DecanterIAzeotropic Column THF 80 12.70 Water 20 87.29 n-Pentane (95) Total Flow, lb/hr 3128 667

2659

100 (39) (0.06) 2500

0.04 99.96 (37) 159

74.13 1.92 23.95 3377

loo (7) (0.004) 2500

0.05 99.94 (39) 67

0.28

(0.1)

99.71 810

561

0.01

ioo’

Table 5. Comparisons of THF Purification Schemes1 Scheme

No. of separation Equipment

steps

Cost, $MM

Operating Cost, $M/year Steam Solvent Loss Total ’ Basis: (1) 20 MMlb THF/yr

Concentrator/ Azeo. Cal.

Decanter/ Azeo. Cal.

Low/High Pressure

4

5

4

3.2

3.4

3.1

148 20 168

97 20 117

120

(2) 8000 hr/yr

120

79.96 19.98 0.06 106

167

THFProduct

Li;hts ®

at

CrudeTH~

=

,,

~

8

,,

DriedTHF

Wastewater

Heavies

Fig. 4. The ConcenU'ator/Azeotropic Distillation of THF with n-Pcntanc.

®

(~ I rudeTHF

~antelecycie~°Ivent

Decanter

~

~

~ Lights

~iI THFProduct

,

,

'

,

I I I

i I i f

1 i

Wastewater

Fig. 5. Thc Dccantcr/Azcotropic D ~ f i o n

13.9%Water

~1

pslg CrudeTHF

L

. . . . . . . .

.J

~f Heavies

of THF with n-Pcntane.

Lights

5.8%water

Wastewater

--

DriedTHF

THFProduct

rl

DriedTHF Heavies

Fig. 6. The Conventional Low/High Pressure Distillation Scheme.

168

COMPARISON OF DISTILLATION

PROCESSES

The economics of the two azeotropic distillation schemes are compared with a conventional low/high pressure distillation scheme (Figure 6) in Table 5. The low/high pressure distillation takes advantage of the effect of pressure on the azeotropic composition (Table 1). In the first column, operating near atmospheric pressure, the azeotropic mixture containing 5.8 wt % water is taken overhead while pure water is removed at the bottom. The overhead from the first column is then fed to a second column operating at about 200 psig where the azeotropic composition shifts to 13.9 wt% water. Since the azeotrope is minimum boiling, the remaining water is taken overhead allowing dry THF to be produced at the bottom. The overhead is recycled to the first column. Both columns have 15 theoretical stages. Comparing the three cases, four separation steps are required for the conventional and the concentrator/azeotropic column schemes to remove water, lights and heavies, while five separation steps are required for the decanter/azeotropic column scheme. Equipment cost, based on a 20 MM lb/year THF plant, is slightly higher for both the azeotropic distillations, while operating cost is higher for the concentrator/azeotropic column scheme. Overall, npentane azeotropic distillation is competitive with low/high pressure distillation, but has no advantage. CONCLUSIONS Phase equilibrium models formulated from binary VLE and LLE data predict reasonably well for the ternary LLE systems of MTBElTHFlwater and n-pentane/THFlwater. Normal pentane is effective for the dehydration of crude THF in an azeotropic distillation while MTBE is inferior due to the difficulty of its separation from THF. THF purification schemes based on n-pentane azeotropic distillation are comparable to a conventional low/high pressure scheme but offer no technical or economic edge. ACKNOWLEDGMENT The authors wish to thank Mr. Ken Gramo for his assistant in generating experimantal data and Mr. Ron Jamieson for his valuable suggestions in writing this paper. REFERENCES Bente, P. F. Jr., 1957, U.S. Patent 2,790,813 “Safe Recovery of THF Vapors”, April 30. Black, C., 1980. “Distillation Modeling of Ethanol Recovery and Dehydration Processes for Ethanol and Gashol”, CEP Sept., 78. Black, C and D. E. Ditsler, 1972, “Dehydration of Aqueous Ethanol Mixtures by Extractive Distillation”, Advan. Chem. Ser., 115, 1. Brooks, K., Chemical Week, April 1.5, 1987, pp. 15-16. Chang, T. and T. T. Shih, 1989, J. Chem. Eng. Data, to be submitted. Coates, J. S., 1981, U.S. Patent4,257,961 “Purification ofTHF”, May 24. Gmehling, J., U. Onken and W. Arlt, 1978, “Vapor-Liquid Equilibrium Data Collection”, DECHEMA Chemistry Series, Vol. I. Stock. A. M and W. S. Tse. 1982. U.S. Patent 4.348.262 “Refining THF”. Seo. 7. Sorensen, J. M. and W. Arlt, 197b. “Liquid-Liqhid Equilibrium Data Colldctidn”, Binary Systems. DECHEMA Chemistry Data Series, Vol. V. Tanabe Y., J. Toriya, M. Sato and K. Shiraga, 1978, U.S. Patent 4,093,633 “Process for Preparing THF”. June 6. Treybei, R. k., 1980, “Mass-Transfer Operations”, 3rd ed., McGraw-Hill Book Company, New York. Yamada T. and K. Yamaguchi, 1987, U.S. Patent 4.665.205 “Process for Preparing. THF”. Mav 12. Yoshida, S., M. Miyajima, M. Ito, N. Kato and T Saitom, 1978, Japanese Pat&t 53-39427 “Method for Dehydrating Water-Containing THF”, October 21.