Znd. Eng. Chem. Res. 1990,29, 2089-2093
2089
SEPARATIONS Purification of Terephthalic Acid by Crystal Aging Pamela M. Brown, Michael Marquering, and Allan S. Myerson* Department of Chemical Engineering, Polytechnic University, 333 Jay Street, Brooklyn, New York 11201
The rate of removal of 4-carboxybenzaldehyde (4-CBA) from terephthalic acid (TPA)crystals during crystal aging was determined in both aqueous solution and 90% acetic acid (10% water by volume). It was found experimentally that over 50% of the 4-CBA could be removed from TPA crystals aged 2 h a t 493 K in 90% acetic acid. I t was also found that purification occurs more rapidly in 90% acetic acid than in water a t the same aging conditions. A crystal aging and purification mechanism based on the oxidation of 4-CBA to TPA in the solution and periodic growth and dissolution due to temperature fluctuations was developed and used to estimate the purification rate. Estimated purification levels were within the experimental error of 15% in most cases.
Introduction and Background Terephthalic acid (TPA)is a major commodity chemical. Worldwide production has increased at an annual rate of 13% for the last 5 years. It is a crystal with two carboxylic acid groups in the para position on a benzene ring. Due to its symmetry, it produces long-chained poly(ethy1ene terephthalate) (PET), a polyester, when reacted with ethylene glycol via a condensation reaction. Plastic containers and fibers are two of the many uses of PET. Commercially, the majority of TPA is produced by the air oxidation of p-xylene in acetic acid, promoted by cobalt and bromine catalysts. The impurities present in terephthalic acid are generally intermediates, oxidation byproducts, and catalyst. The difference between the intermediate 4-carboxybenzaldehyde (4-CBA) and TPA is that CCBA has an aldehyde group rather than a carboxylic acid group para to the other carboxylic acid group on the benzene ring. 4-CBA is so similar chemically and physically to TPA that it is one of the most difficult contaminants to remove and, unfortunately, is probably one of the most deleterious. The aldehyde group does not undergo the condensation reaction with ethylene glycol and acts as a chain terminator in the PET polymerization. This results in fibers that break easily as well as a slower rate of polymerization. Crystal aging refers to all changes that occur after nucleation and growth. The aging of TPA has been studied for a number of years by Myerson and co-workers (Gaines and Myerson, 1982,1983; Saska and Myerson, 1985,1987; Myerson and Saska, 1984; Brown and Myerson, 1989). These studies demonstrated that amorphous, globular TPA particles will transform into needle crystals when suspended in their own saturated solution at temperatures ranging from 353 to 493 K. Purity was found to improve substantially with respect to both 4-CBA and the catalyst cobalt. Scanning electron microscope (SEM) photographs of aged crystals revealed that rod-shaped faceted particles had nucleated and grown on the original globular TPA (Figure 1). The nucleation and growth indicate that a finite supersaturation must have existed at some point during the aging. A small temperature decrease (less than *All correspondence should be sent to A. S. Myerson. 0S88-5SS5/90/2629-2089~02.50/0
1K) could result in crystal growth due to the rapid change of solubility with temperature at aging temperatures (Bemis et al., 1980). Similarly, a small temperature increase could result in dissolution. These temperature fluctuations are unavoidable due to the cycling nature of temperature controllers. A transformation of habit from globular to faceted needles would result from a series of dissolution and growth cycles since at low supersaturation all nucleation and growth will be in the faceted needle form. A mechanism of crystal aging based on temperature fluctuations would allow prediction of the rate of aging, if accurate growth and dissolution kinetics were available, along with the temperature history of the system and solubility data. In order to test this aging mechanism, Myerson and co-workers (Brown and Myerson, 1989; Brown, 1989; Marquering, 1989) performed experiments to determine the crystal growth and dissolution kinetics of TPA in water and 90% acetic acid as a function of temperature using the zero-time derivative method of Garside et al. (1982). Details of the experimental procedure and data analysis are given in Brown (1989) and Marquering (1989) for water and 90% acetic acid, respectively. The crystal growth and dissolution kinetics are expressed in terms of the relationships growth
d(C - C,,,)/dt = -k,A(C - CSat)g
(1)
d(C,,t - C)/dt = -kdA(Csat - C ) d
(2)
dissolution Dissolution is considered to be a diffusion-controlled process (Garside et al., 1982), so the order of dissolution, d , is considered to be 1. Experiments were conducted at temperatures ranging from 298 to 313 K and the results fit to an Arrhenius type equation. The results for growth and dissolution in both water and 90% acetic acid solutions appear in Table I. The estimated error in k d and k, is *lo%. The coefficient of correlation was found to be 0.991 for k d and 0.999 for k,. Brown and Myerson (1989) performed aging experiments using deuterated TPA (d-TPA) as a tracer. Satu0 1990 American Chemical Society
2090 Ind. Eng. Chem. Res., Vol. 29,No. 10,1990 Filter-xtal Collection
To High Pressure N Tank
c 1x1
dTPAynjection Valve
D-TPA xtals stored here
during preheat
Temwrature
I
8'
LE U
I
RTD Tube
Figure 1. SEM photograph showing spindle growth on amorphous TPA particle. Reprinted with permission from Saska and Myerson. Copyright 1987 AIChE. Table I. Experimental and Curve-Fit Dissolution Rate Coefficients kdaand k,b
Water Curve Fit: kd = 103.7658 exp(-2920.99/!4?) 298 0.0064 0.0058 313 0.0093 0.0095 325 0.014 0.013 328 0.017 0.019 353 0.030 0.027
Water Curve Fit: k, = 9.761 218 298 323 333 343
Heater Magnetic Stirrer
I
k,
kd
temp, K exptl curve fit exptl curve fit Acetic Acid Curve Fit: kd = 24.228 exp(-2217,2/T) 303.2 0.014 0.016 . 313.2 0.21 0.020 323.2 0.028 0.025 333.2 0.036 0.031 343.2 0.035 0.038 353.2 0.038 0.046 363.2 0.058 0.054
Acetic Acid Curve Fit: k, = 2.12 313.2 323.2 333.2 343.2 353.2
:
X
X
exp(2830.2/T) 0.178 0.18 0.14 0.13 0.094 0.10 0.085 0.081 0.064 0.064 exp(2695.96/T) 0.785 0.782 0.40 0.406 0.32 0.316 0.25 0.250
O In [ (g of TPA/(cm2 of P A - m i n ) ](100 g of solution/g of [(g of TPA/(cm2 of TPA-min)]100 g of solution/g of TPA)g.
rated solutions of TPA were prepared at aging temperatures of 413-473 K. Crystals of d-TPA were injected into the well-mixed saturated solution. The temperature as a' function of time was recorded. Samples of solution and crystals were withdrawn periodically, and the percentage of d-TPA in the solution and the crystals was determined by using 13C NMR. The change in the percentage of dTPA was used as a measure of the extent of aging. A mechanism of crystal aging based on crystal growth and dissolution due to small temperature fluctuations was used to estimate the aging rate. This estimation employed the experimentally determined growth and dissolution kinetics as well as the temperature vs time data. A comparison of the experimentaland calculated results (Brown, 1989; Marquering, 1989) showed that while the deviation was large in some cases, the general trend indicated that temperature fluctuations can account for the crystal aging rate.
TITANIUM REACTION VESSEL
Figure 2. Aging apparatus. Reprinted with permission from Brown and Myerson. Copyright 1989 AIChE.
It has been demonstrated (Brown, 1989; Marquering, 1989) that impure TPA can be purified during crystal aging. It is the purpose of this work to experimentally examine the rate of purification of TPA by crystal aging and relate it to a mechanism based on the oxidation of 4-CBA to TPA, crystal dissolution, crystal growth, and temperature fluctuation. Experimental Apparatus and Procedure 1. Determination of the Rate of Removal of 4-CBA from TPA Crystals during Aging of Impure TPA. The apparatus employed to determine the rate of crystal purification appears in Figure 2. The vessel was constructed of titanium to resist corrosion and was designed for temperatures up to 525 K and pressures up to 22 atm. A magnetic stirrer was used for mixing. An OMEGA PID temperature controller was utilized for heating, temperature control, and temperature measurement. A saturated solution of impure TPA at 413,433,453,473, or 493 K was prepared by adding the required amount of TPA to a solution and overheating by 10 K. The system was allowed to cool to the saturation temperature, and then impure TPA crystals from the same batch were injected into the reaction vessel. Samples of solution and crystals were withdrawn periodically at aging temperatures using a Pall porous metal filter. The concentrations of 4-CBA in the crystals and solution were determined by using reversephase high-pressure liquid chromatography (HPLC). Details of the experimental procedure are given in Brown (1989) and Marquering (1989). 2. Determination of the Rate of Oxidation of 4-CBA to TPA in Solution. A literature survey (Hendricks et al., 1978) revealed that the rate of oxidation of 4-CBA to TPA was first order with respect to 4-CBA and was also dependent on the concentration of cobalt and dissolved oxygen. The concentration of oxygen and cobalt was assumed to be constant. Thus, the rate of oxidation during aging was assumed to fit an equation of the form [4-CBA] = [4CBAIo exp(-k,,t) (3) The oxidation kinetics were determined by preparing a
Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990 2091
I 0
20 30 Tlme (hours)
10
T ' Exptl. Rwulta
-
40
60
0 0
10
6
413K
saturated solution of impure TPA at aging temperatures and periodically withdrawing solution. The change in the concentration of CCBA with time was determined by using reverse-phase HPLC. 3. Calculation of the Purification of TPA by Crystal Aging. It is assumed that the dominant driving force in the crystal aging process is temperature fluctuations. These temperature fluctuations cause local dissolution of TPA (during temperature increase) and crystal growth (during temperature decrease). Evidence of this mechanism can be seen in Figure 1 (Saska and Myerson, 1987) which shows a new rod shaped T P A crystal growing out of the initial "boulder like" TPA particle. The purification of TPA that occurs during aging can be accounted for in the following way. During a dissolution (higher temperature) period, a portion of the crystal dissolves. The dissolved material includes the impurities. The main impurity, 4-CBA, reacts in solution to form TPA, thus lowering the impurity concentration in solution. During the crystal growth phase (lower temperature), the TPA crystal grows. The distribution of the 4-CBA impurity is determined by a distribution coefficient between the phases. However, since, the 4-CBA concentration has declined due to reaction, both of these factors aid the purification. A calculation based on this mechanism was performed. A monodisperse population of TPA particles is assumed. This is reasonable since at the low supersaturation employed, little or no nucleation is likely to occur. In addition, calculations with the model have shown that results are insensitive to small changes in the total surface area. Experimentally determined crystal growth and dissolution kinetics are used along with solubility vs temperature data and observed temperature fluctuations. The concentration of 4-CBA in solution vs time due to the oxidation of the 4-CBA was calculated by using the experimentally determined kinetics relation (Brown, 1989; Marquering, 1989): water d[4-CBA]/dt = -58.4 exp(-3531.35/2')[4-CBA]= k0,[4-CBA] (4) 90% acetic acid d[4-CBA]/dt = -26.993 exp(-3116.24/T)[4CBA] = kox[4-CBAl (5) The 4-CBA concentration in the newly grown TPA crystals was obtained by using the distribution coefficients of Fujita et al. (1968) and Robinson (1981).
Results and Discussion The experimentally determined concentrations of CCBA in the crystals as a function of aging time in aqueous so-
20
30
26
T-433K
-
Dlat. Coal. '.*.Roblnoon Dbt. Cod. Amplltude-O.PK, Porlod -e4 mlnutw
+Fullta
Figure 3. Purification of TPA in water at 413 K.
16 Tlme (hours)
'.*.
Exptl R*8uItl +Fullto Dlat. Co.1. Roblnaon Dllt. Amplltude-O.7K. Perlod 7 0 mlnutw
Cool.
Figure 4. Purification of TPA in water at 433 K.
0
6
10
16
20
Tlme (hours)
T- 473K ' Exptl Raaulta
..*.
+Fullta 0181. Coal. Roblnaon Dlat. Coal Amplltude-O.BK, Perlod- 80 mlnutea
Figure 5. Purification of TPA in water at 473 K.
lution are presented in Figures 3-5, as well as the predicted values using the distribution coefficients determined by both Fujita et al. (1968) and Robinson (1981). It can be seen that between 413 and 473 K substantial purification occurs during aging with the rate of purification increasing as the temperature increases. At 413 and 473 K, the model using the Fujita distribution coefficient ( K = 0.1) is a better predictor of the experimental results. The predicted results and the experimental results are within the experimental error of f15% in most cases. At 433 K, the purification model using the Robinson distribution coefficients modified to account for the different rate of change of saturation concentration with temperature in aqueous solution ( K = 0.41 to 0.67, depending on the rate of the temperature change) is a better predictor of the experimental results with the experimental and calculated results again within 15% in most cases. There is no obvious explanation for why one distribution coefficient works better in the model than the other at different temperatures. One distribution coefficient may actually be correct or other errors in the model may be negated, such as the assumptions made or the rates of oxidation, dissolution, or crystal growth. The experimental and predicted results for the concentration of 4-CBA in the solution and crystals as a function of aging time in 90% acetic acid are presented in Figures 6-11. Here the Robinson distribution coefficient (K = 0.07-0.20 depending on the rate of temperature change) and the Fujita et al. distribution coefficient (K = 0.10) both predict the rate of purification within the experimental error of 15% in most cases. I t has been shown that crystal aging is a viable and predictable method for purifying TPA. It can be seen that the rate of purification increases as the temperature increases and that purification occurs more rapidly in 90%
2092 Ind. Eng. Chem. Res., Vol. 29, No. 10, 1990
0
30
20
10
50
40
eo
Tlme (hours)
OL-----I
+ Exp.
Exp 1 0
+-
1-FuJlte
10
5
15 20 Tlme (hours)
3
Exp 1-Aobln8on
T-473K
25
30
Exp 1
Figure 10. Purification of TPA in acetic acid a t 473 K.
35
'
0; 0
05
1.5
1
0
EXP 2
2
25
Tlme (hours) T-4Q3K
* Exp. Moblneon
+ Exp. 1-FuIIte
6
+FuJlte Dlst Coef. Roblnson Diet Cos1 AmPlltude-O.BK, Perlod d o mlnutee
T-433K a
4
*
' Exptl Result8
Figure 6. Purification of TPA in acetic acid a t 413 K.
0
2
1
u Exp. P-FuJlte 4 Exp. Pdoblnson Amplltude-0 ZK, PsrloddO mln
Exp 2
I -
0
T-413K
+ EXP
Exp 1
Exp 2-Fullte 4 Exp 2doblnson Amplltuds-O.4K. Perlod-BO mln %
0
Exp 2
AmplItude-3
Figure 7. Purification of TPA in acetic acid a t 433 K.
l-Fullte
* Exp
ldoblnson
Exp P-FuJlta 4 Exp P-Roblneon Amplltude-1 OK, Porlod- BO mln, Exp 1
EK, Perlod- BO mln, Exp 2
Figure 11. Purification of TPA in acetic acid a t 493 K
0
2
1
3
4
5
Tlme (hours) T-463K *
Exp. 1
4- Exp. l-FuJlte
0
Exp. 2
u. Exp. 2-FuJlta
C Exp. ldoblnsoo
4 Exp P-Roblneon AmplltUd64).WY, PerloddO mlnute8
Figure 8. Purification of TPA in acetic acid a t 453 K.
in 90% acetic acid and the rate of dissolution is also more rapid in 90% acetic acid, although the rate of crystal growth was found to be more rapid in aqueous solution at a given temperature. Thus, the observation that purification occurs more rapidly in 90% acetic acid may be due to the more rapid oxidation and dissolution in 90% acetic acid. It should also be noted that the temperature history in the vessel plays a strong role in the rate of purification. Rapid temperature oscillations with a large amplitude (but not so large as to cause nucleation or amorphous crystal growth) will result in the largest amount of purification. This is because a small temperature period and large amplitude causes the most interaction between the solution and crystal due to maximum dissolution and growth cycles. Acknowledgment
A portion of this work was supported by the National Science Foundation through Grant 8508387. P. M. Brown was partially supported during this work by the James Lago Fellowship. Nomenclature I
01
0
1
2
1
3
4
5
u 6 7
Tlme (hours)
0
1-473K Exp 1 Exp 2
i- Exp l-FuJlta
* Exp
1-Roblnson
Exp Pfullte 4 Erp 2-Ronlnaon Amplltude4.QK, Perlod-BO mlnutaa
Figure 9. Purification of TPA in acetic acid a t 473 K.
acetic acid than in aqueous solution at a given temperature up to 473 K. Below 473 K, TPA is more soluble in 90% acetic acid, but above 473 K, TPA is more soluble in water. Experimentally, 4-CBA was found to oxidize more rapidly
A = crystal surface area per volume solution (cm2of TPA/100 g of solution) C = TPA solution concentration (g of TPA/100 g of solution) [4-CBA] = 4-CBA solution concentration (g of 4-CBA/100 g of solution) k,, = oxidation rate constant (h-l) kd = dissolution rate constant [(g of T P A / ( c m 2 of TPA. min))(100 g of solution/g of TPA)d] k , = crystal growth rate constant [(g of TPA/(cm2 of TPA.min))(lOOg of solution/g of TPA)g]
T = temperature (K)
t = time (h) d/dt = differential with respect to time
I n d . Eng. Chem. Res. 1990,29, 2093-2100
Superscripts and Subscripts d = dissolution order g = crystal growth order 0 = zero time sat = saturation Registry No. 4-CBA, 619-66-9; TPA, 100-21-0; acetic acid, 64-19-7.
Literature Cited Bemis, A. G.; Dindorf, J. A.; Horward, B.; Samans, C. Phtalic Acids and Other Benzene Polycarboxylic Acids. Kirk-Othmer Encyclopedia of Chemical Technology, -. 3rd ed.; Wiley: New York, i g s o ; v ~ i .i i , p 732. Brown. P. M. Crvstal Aeine of Tereohthalic Acid. Ph.D. Dissertation; Po1ytech;lic Unyveisity, Ne; York, 1989. Brown, P. M.; Myerson, A. S. Crystal Aging of Terephthalic Acid. AIChE J . 1989, 35 (lo), 1749. Fujita, Y.; Takeda, A.; Tanaka, T. Behavior of 4-Formyl Benzoic Acid in Terephthalic Acid Synthesis. Gen. Chem. SOC.Diu. Pet. Chem. Present. 1968, 13 (4), 85-87. Gaines, S.; Myerson, A. S. Removal of Impurities Through Crystal Aging. AIChE Symp. Ser. 1982, 78, (No. 215), 42.
2093
Gaines, S.; Myerson, A. S. The Agglomeration and Aging of Terephthalic Acid Particles. Part. Sci. Technol. 1983, 2 , 409. Garside, J. L.; Giblaro, G.; Tauare, N. S. Evaluation of Crystal Growth Kinetics from a Desupersaturation Curve Using Initial Derivatives. Chem. Eng. Sci. 1982, 37, 1625. Hendricks, C. F.; Van Beek, H. C. A,; Heertjes, P. M. The Kinetics of the Autooxidation of Aldehydes in the Presence of Cobalt (11) and Cobalt (111) Acetate in Acetic Acid Solutions. Ind. Eng. Chem. Prod. Res. Deu. 1978, 17 (3), 260-265. Marquering, M. W. Crystal Aging of Terephthalic Acid in 90% Acetic Acid Solution. M.S. Thesis, Polytechnic University, New York, 1989. Myerson, A. S.; Saska, M. Formation of Solvent Inclusions in Terephthalic Acid Crystals. AIChE J . 1984, 30, 865. Robinson, P. A. Partition Coefficients of Terephthalic Acid. Ph.D. Dissertation, University of Strathclyde, Glasgow, Scotland, U.K., 1981. Saska, M.; Myerson, A. S. Polymorphism and Aging in Terephthalic Acid. Cryst. Res. Technol. 1985, 20, 201. Saska, M.; Myerson, A. S.Crystal Aging and Crystal Habit of Terephthalic Acid. AIChE J . 1987, 33, 848.
Received for review January 18, 1990 Accepted June 5, 1990
Facilitated Transport of COSthrough an Immobilized Liquid Membrane of Aqueous Diethanolamine Asim K. Guha, Sudipto Majumdar, and Kamalesh K. Sirkar* Department of Chemistry and Chemical Engineering, Center for Membranes and Separation Technologies, Stevens Institute of Technology, Castle Point, Hoboken, New Jersey 07030
Permeabilities and separation factors for the CO2-N2 system have been experimentally determined for facilitated transport and separation through an immobilized liquid membrane ( E M ) of an aqeuous solution of diethanolamine over a wide range of COz partial pressures. Facilitated COz transport in such a system has been modeled, and the set of coupled diffusion-reaction equations has been numerically solved. Experimentally obtained COZ-N2 separation factors at 230-516 cmHg total pressures compare very well with model predictions over the whole range of COz partial pressures. The effects of membrane thickness and downstream COz partial pressure have been numerically studied. Improved knowledge and estimates of various physicochemical parameters will substantially improve the model capability for predicting species permeability.
Introduction Acid gas treatment for removal of COz and H2S is of major industrial importance. The increased demand for acid gas treating and the increase in the cost of purification by conventional processes suggest a need for energy-efficient and selective gas treating technology (Astarita et al., 1983). Membrane gas separation can become a viable alternative because of its inherent simplicity, ease of control, compact modular nature, and great potential for lower cost and energy efficiency. These attractive features have stimulated significant research in the field of gas separation using polymeric as well as liquid membranes. Further, using the principle of facilitated transport, it is possible to devise highly permselective membranes (Ward, 1972). Facilitated transport of gases is important from physiological as well as engineering considerations. A number of review articles has been published on this subject in recent years (Schultz et al., 1974; Smith et al., 1977; Kimura et al., 1979; Way et al., 1982; Meldon et al., 1982; Matson et al., 1983; Sengupta and Sirkar, 1986; Noble et al., 1988). Quite a few studies have examined C02transport through an immobilized liquid membrane (ILM) im-
* To whom all correspondence should be addressed. 0888-5885/90/2629-2093$02.50/0
pregnated with carbonate and bicarbonate solutions (Enns, 1967; Ward and Robb, 1967; Otto and Quinn, 1971; Suchdeo and Schultz, 1974; Donaldson and Quinn, 1975; Kimura and Walmet, 1980; Jung and Ihm, 1984; Bhave and Sirkar, 1986). Apart from the carbonate/bicarbonate carrier, another group of chemicals that can facilitate COz transport is comprised of amines. In particular, aqueous solutions of diethanolamine (DEA) have long been used for COz absorption in a variety of industrial gas-cleaning processes (Astarita et al., 1983). The high COz capacity is generally attributed to the formation of carbamate (Danckwerts, 1979):
C 0 2 + R2NH
& R,NCOO- + H+ k-Am
R2NH + H++ R,NH,+ with the overall reaction being COS+ 2RzNH R,NCOO-
+ R2NHz+
(1) (2)
(3)
Here DEA is represented as RzNH. Reaction 1 is the rate-limiting step for this overall reaction. Thus, at the high-pressure side of the liquid membrane, absorption is enhanced by the forward reaction and the carrier is consumed. Similarly, at the low-pressure side, the carrier is 0 1990 American Chemical Society
