Ind. Eng. Chem. Res. 1994,33, 2157-2160
2157
SEPARATIONS Separation of Isomeric Dimethylnaphthalene Mixture in Supercritical Carbon Dioxide by Using Zeolite Yoshio Iwai,’ Hirohisa Uchida, Yasuhiko Mori, Hidenori Higashi, Tomohiro Matsuki, Takeshi Furuya, and Yasuhiko Arai Department of Chemical Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan
Koji Yamamoto Glastic Corporation, 4321 Glenridge Road, Cleveland, Ohio 44121
Yutaka Mito Polymer & Chemical Technology Laboratory, Kobe Steel, Ltd., Hyogo 651-22, Japan
The adsorption separation of the isomeric dimethylnaphthalene (DMN) mixture of 2,6- and 2,7DMN in supercritical carbon dioxide was performed. The experiment was carried out by using various zeolites as the adsorbent at the temperature 308.2 K and the pressures 12.0,14.8, and 19.8 MPa. It was found that a suitable zeolite exists for the separation of the DMN mixture and that the zeolite can adsorb 2,7-DMN selectively. It shows the possibility for the adsorption separation of the DMN mixture in supercritical carbon dioxide.
Introduction Separation methods for aromatic compounds in coal liquefaction products have received considerable attention recently for use for the raw materials of fine chemicals. Dimethylnaphthalene (DMN) as one of the aromatic compounds has many industrial applications. In particular, 2,6-DMN is a raw material for high-quality synthetic fibers or functional resins. However, 2,6-DMN is present in isomeric DMN mixtures obtained from coal liquefaction, and it is difficult to separate 2,6-DMN from the DMN mixtures by using conventional methods, such as distillation and solvent extraction. Yamamoto et al. (1992) tried to synthesize 2,6-DMN in supercritical carbon dioxide with zeolites as the catalyst, and the method was found to be better than the others. However, a DMN mixture synthesized by the method contains about equal amounts of 2,6- and 2,7-DMN. Therefore, it is necessary to separate the DMN mixture. Moreover, it is desirable to separate the DMN mixture under the supercritical condition in order to simplify the synthetic process. In a previous study (Iwai et al., 1993),as the fundamental data to study the feasibility of separation, the solubilities of the mixture of 2,6- and 2,7-DMN in supercriticalcarbon dioxide at 308.2 and 318.2 K were measured. According to the result, the crossover pressures of 2,6- and 2,7-DMN were found to be very close. Therefore, it is very difficult to separate the isomeric DMN mixture by retrograde crystallization (Johnston et al., 1987). Thus it is necessary to consider the new separation method under the supercritical condition. For example, Lin et al. (1991) studied the separation of m-xylene and ethylbenzene on silicalite in supercritical carbon dioxide, and Tan et al. (1990) studied the separation of xylene isomers on silicalite in supercritical and gaseous carbon dioxide. ~
* To whom correspondence should be addressed.
In the present study, the adsorption separation of the isomeric DMN mixture of 2,6- and 2,7-DMN extracted in supercritical carbon dioxide was studied by using various zeolites. Zeolites are often used to selectively adsorb chemicals from liquids, but they were used in the supercritical fluid because the supercritical fluid has rather liquidlike properties. In order to examine the effect of the pressure on the adsorption separation, the experiment was performed at a fixed temperature and various pressures by using a most effective zeolite for the separation of the DMN mixture. The main object of this study is to investigate the possibility for the adsorption separation of the isomeric mixture under the supercritical condition.
Experimental Section Equipment and Procedures, A flow-type apparatus was used in this study. The apparatus is shown schematically in Figure 1. From a gas cylinder (l), carbon dioxide was supplied and liquefied through a cooling unit (3). The liquefied carbon dioxide was sent to a preheater (7) by a high-pressure liquid chromatography pump (4). When carbon dioxide passed through the preheater, it became a supercritical fluid. A preequilibrium cell (8) and an equilibrium cell (9) were used. The preequilibrium cell was equipped to obtain sufficient equilibrium conditions. It was made of SUS316, and its inner diameter, height, and volume were 30 mm, 150 mm, and about 100 cm3, respectively. The equilibrium cell was made of SUS304,and its inner diameter, height, and volume were 30 mm, 170 mm, and about 120 cm3, respectively. Solid solute was packed into the cells with glass beads to prevent channeling. The adsorption cell (10) was connected to the outlet of the equilibrium cell. It was made of SUS316, and ita inner diameter, height, and volume were 20 mm, 120 mm, and about 40 cm3, respectively. Zeolite pellets (about 20 g) were packed into the adsorption cell. The preheater, preequilibrium cell, equilibrium cell, and
0888-5885/94/2633-215~$O4.50/0 0 1994 American Chemical Society
2158 Ind. Eng. Chem. Res., Vol. 33, No. 9, 1994 14
14
14
Q Q Q,
----------------
U
I
I
15
Figure 1. Flow-type apparatus: (1) gas cylinder;(2) dryer;(3) cooling unit; (4) pump; (5) stopper; (6) safety valve; (7) preheater; (8) preequilibrium cell; (9) equilibrium cell; (10) adsorption cell; (11) filter; (12) U-shaped glass tube; (13) gas meter; (14) Bourdon gauge; (15) water bath; (V1,3-8) stop valve; (V2) back-pressure regulator; (V9)expansion valve.
t
n
1
Y
801
8
A
601
the expansion valve was removed and trapped by using pure carbon dioxide through valves V7 and V8 after closing valves V4, V5, and V6. The amount of trapped solutes was determined by weight. A gas chromatograph with a flame ionization detector (Shimadzu Co. GC-8A)was used for analyzing the composition of trapped solutes. The adsorption amounts of 2,6- and 2,7-DMN were calculated from the mass balance by using the trapped amounts of 2,6- and 2,7-DMN and the bulk concentrations of 2,6- and 2,7-DMN in supercritical carbon dioxide, namely, the solubility data of 2,6- and 2,7-DMNin supercritical carbon dioxide obtained by Iwai et al. (1993). Materials. Reagent-grade 2,6- and 2,7-DMN (supplied by Wako Pure Chem. Ind., Ltd.) were used. Gas chromatographic analysis indicated that their purities were more than 99% and 98%, respectively. After impurity components in the DMN mixture were extracted with supercritical carbon dioxide by the apparatus, the remaining high-purity components were used for the experiments. Four types of zeolite pelleb (Tosoh Co. HSZ series, 320NAD, 320HOD, 330HUD, 620HOD) were used as the adsorbent. Their properties are shown in Table 1. Before use, the zeolites were dried at 393 K for 24 h. Highpurity carbon dioxide (more than 99.9 %, Sumitomo Seika Co.) was used as received.
Results and Discussion The saturated adsorption amount Q is defined as follows:
0 . U - s
308.2K 12.0 MPa . /
,
I
.
I
,
where q2,6 and q2,7 are the amounts of 2,6- and 2,7-DMN adsorbed, respectively and wzeois the amount of packed zeolite. The separation coefficient of adsorption K is defined as follows:
I
adsorption cell were immersed into a water bath (15) at an experimental temperature which was controlled within f O . l K. Valve V7 was closed, and valves V4, V5, and V6 were opened to introduce supercritical carbon dioxide into the preequilibrium cell, equilibrium cell, and adsorption cell. When supercritical carbon dioxide passed through the preequilibrium cell and equilibrium cell, supercritical carbon dioxide was in contact with solid solutes under equilibrium pressure. The equilibrium pressure was measured by a Bourdon gauge (14) calibrated against a strain pressure gauge (accuracy *0.3% 1. When supercritical carbon dioxide saturated with solutes passed through the adsorption cell, the solutes were adsorbed by the packed zeolite. The supercritical carbon dioxide containing solutes was decompressed through an expansion valve and then introduced into a U-shaped glass tube (12) cooled in an ice bath. In the tube, gaseous carbon dioxide and solid solutes were separated. Usually 0.1-1.0 g of solutes was trapped in a run, and the flow rate of carbon dioxide was adjusted to be about 3.3 cm31s. The volume of carbon dioxide was measured by a wet gas meter (13). Any small amount of solutes remaining in the tubing and
where y2,6 and y2,7 are the solubilities of 2,6- and 2,7-DMN in supercritical carbon dioxide under the experimental condition, respectively. The solubility data of Iwai et al. (1993) were used for y2,6 and y2,7. The values of K and Q obtained experimentally at 308.2 K and 14.8 MPa are shown in Table 2. The experiments were performed more than two times for each zeolite. The reproducibility of K was within l o % , and the reproducibility of Q was within 20%. This table indicates that the zeolite 620HOD has no effect on the separation of the DMN mixture. The other zeolites (320NAD, 320HOD, 330HUD) can adsorb 2,7-DMN selectively and have an effect on the separation of the DMN mixture. In particular, the highest separation efficiency is obtained when the zeolite 320NAD is used. Figures 2-4 show the time change (represented by the flow amount of C02) of composition of the trapped solutes for each pressure using the zeolite 320NAD. Experimental breakthrough curves for each pressure using the zeolite 320NAD are also shown in Figures 5-7. In these figures, n2,6and n2,7 are the trapped amounts of 2,6- and 2,7-DMN, respectively, and ncOz is the flow amount of CO2. These figures indicate that almost pure 2,6-DMN can be obtained in the extract in the initial stage of adsorption for each pressure and that the breakthrough times are shorter at higher pressures (14.8 and 19.8 MPa). The breakthrough behaviors at 14.8 and 19.8 MPa are almost same, and this is possibly because the densities of supercritical carbon
Ind. Eng. Chem. Res., Vol. 33, No. 9,1994 2159 Table 1. Properties of Four Types of Zeolites. name
type
320NAD 320HOD 330HUD 620HOD
NaY
HY US-Y H-mordenite
SiOz/Al2Os (mole ratio) (dry basis) 5.6 5.6 5.8 15.2
NazO (wt
pore vol. (cmS/g) 0.35-0.5 0.4-0.6 0.4-0.6 0.35-0.4
5%) (drybasis) 12.5 4.3 0.6 0.55
sp. surf. area (m2/g) 600 550 500 300
pore size
(A) -8 -8 -8 7
'Quoted from catalog of Tosoh Co. (HSZseries, Tosoh Co., Yamaguchi 746, Japan).
Table 2. Separation Coefficient of Adsorption, K,and Saturated Adsomtion Amount, 8, at 308.2 K and 14.8 MPa name K Q (g/g of zeolite) 320NAD 2.1 0.049 320HOD 1.7 0.070 330HUD 2.0 0.046 620HOD 1.0 0.006
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3
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I Y
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d
-8 0.004
I
0
t x
I
0.006
1 308.2 K 12.0 MPa
0
60
-
0
w
1
,
I
2
.
I
4
nc02 [moll
Figure 5. Breakthrough curves for zeolite 320NAD at 308.2 K and
t
308.2 K 14.8 MPa
0
I
I
1
2
3
nc02 [moll
12.0 MPa. (n2,e)Amount of 2,6-DMN trapped; (n2.7) amount of 2,7DMN trapped; (nco,)flow amount of COz; (0) experimental data of 2,6-DMN for run 1;(0) experimental data of 2,6-DMN for run 2; (A) experimental data of 2,6-DMN for run 3; ( 0 )experimental data of 2,7-DMN for run 1; (1) experimental data of 2,7-DMN for run 2; (A) experimental data of 2,7-DMN for run 3; (- - -) case of no zeolite for 2,6-DMN; (- -) case of no zeolite for 2,7-DMN.
-
Figure 3. Time change of composition in the trapped solutes for zeolite 320NAD at 308.2 K and 14.8 MPa. Symbols are the same as in Figure 2.
1001
-
7
I
I
1
c
Y
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:0.002
2
0
2C
3
20
A
0
0
A
- . -
308.2 K 14.8 MPa I
I
1
2 nc02
I 3
[moll
Figure 6. Breakthrough curves for zeolite 320NAD at 308.2 K and 14.8 MPa. Symbols are the same as in Figure 5.
liquid phase was also performed for comparison of separation efficiency. Octane (supplied by Nacalai Tesque, Inc., its purity is more than 98%) was used as the solvent. The DMN mixture was added in liquid octane. The mole fraction of each DMN in liquid octane was set to be the same as that in the condition of supercritical carbon dioxide at 308.2 K and 14.8 MPa, i.e., y2,6 = 3.8 X 103and y2,7 = 5.5 X lo3. The volume of octane was 50 mL, and the weight of added zeolite 320NAD was 1g. The composition of the solution was measured after 48 h at 308.2 K and atmospheric pressure, and the adsorption amounts of 2,6and 2,7-DMN were calculated from the mass balance. The values of K and Q obtained experimentally are also shown
2160 Ind. Eng. Chem. Res., Vol. 33, No. 9,1994
, -
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- U b U - - U
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method will be probably one of the possible separation methods for not only the DMN mixture but also other structural isomeric mixtures in the industrial separation process. Further experiments and discussion on the separation mechanism at the supercritical condition will need to be carried out in future work. An effective separation process of isomeric mixtures will be designed based on the more detailed results. Acknowledgment The authors gratefully acknowledge the financial support in part provided by the Grant-in-Aid for Scientific Research on Priority Areas (Supercritical Fluid 224,1993, 04238106), The Ministry of Education, Science and Culture, Japan. Nomenclature K = separation coefficient of adsorption n2,6,n2,7= amount of 2,6- and 2,7-DMNtrapped, respectively, mol nco, = flow amount of C02, mol Q = amount of saturated adsorption, gig of zeolite @,6,q2,7 = amount of 2,6- and 2,7-DMN adsorbed,respectively, g
12.0 14.8 19.8 in liquid octane
1.6 2.1 2.2
1.4
0.120 0.049 0.076 0.170
in Table 3. The experiment was performed two times, and the reproducibilities of K and Q were within 5 % .This table indicates that the high separation efficiency and low adsorption amounts of DMN are obtained in the supercritical carbon dioxide compared with liquid octane. Conclusions In this work, the adsorption separation of the isomeric DMN mixture of 2,6- and 2,7-DMN in supercritical carbon dioxide was performed. The experiment was carried out by using various zeolites as the adsorbent at the temperature 308.2 K and the pressures 12.0, 14.8, and 19.8 MPa. It was found that a suitable zeolite exists for the separation of the DMN mixture and that the zeolite can adsorb 2,7-DMN selectively. High separation efficiency is also obtained at higher pressures. The possibility of the adsorption separation of the DMN mixture in supercritical carbon dioxide was shown in this study. This
y2.6, y2,7 = solubilities of 2,6- and 2,7-DMN in supercritical carbon dioxide under the experimental condition, respectively, mole fraction w,, = amount of packed zeolite, g Literature Cited Iwai, Y.; Mori, Y.; Hosotani, N.; Higashi, H.; Furuya, T.; Arai, Y.; Yamamoto, K.; Mito, Y. Solubilities of 2,6-and 2,7-Dimethylnaphthalenes in Supercritical Carbon Dioxide. J. Chem. Eng. Data 1993, 38 (4),509. Johnston, K. P.; Barry, S. E.;Read,N. K.; Holcomb,T. R. Separation of Isomers Using Retrograde Crystalliition from Supercritical Fluids. Znd. Eng. Chem. Res. 1987,26 (ll), 2372. Lin, W.-F.; Tan, C.4. Separation of m-Xylene and Ethylbenzene in Gaseous Carbon Dioxide. Sep. Sci. Technol. 1991,26 (12),1549. Tan, C.-S.; Tsay, J.-L. Separation of Xylene Isomers on Silicalite in Supercritical and Gaseous Carbon Dioxide. Znd.Eng. Chem.Res. 1990,29 (3), 502. Yamamoto, K.; Mito, Y.; Nozoe, I. Proceedings of Fifty-Seuenth Annual Meeting of the Society of Chemical Engineers, Japan (SCEJ);SCEJ Tokyo, 1992;p 132 (Part 11).
Received for review March 9, 1994 Accepted June 15,1994' Abstractpublishedin Advance ACSAbstracts, July 15,1994.
