Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
the chains are extended can be as large as several tens of microns in diameter and up to 1 pm thick. The main growth mechanism is a simultaneous crystallization and polymerization process. In the copolymer crystals the comonomer units are incorporated as defects in the POM lattice. Acknowledgment
Contributions of data to this paper by Dr. K. Hertwig, Dr. M. Rodriquez-B., Dr. A. Lucke, and Dr. G. Lieser are greatly acknowledged by the authors. In addition, the authors acknowledge financial support by the Deutsche Forschungsgemeinschaft and the Fonds der chemischen Industrie.
263
Hermann, H. D., Weissermei, K., Makromol. Chem., 94,91 (1966). Jaacks, V., Adv. Chem. Ser., No. 91,371 (1969). Jenkins, S. H., Puderson, I. O., U S . Patent 2964500 (1960) (Du Pont de Nemours 8. Co); Chem. Abstr., 5 5 , 7907 (1961). Kern, W., Inauguraldissertation, Universitat Freiburg, 1932. Kern, W., Cherdron, H., Jaacks, V., Angew. Chem., 73, 177 (1961). Kuilmer, K., Fischer, E., Weissermei, K., D. P. 1 124703 (1959) (Farbwerke Hoechst AG): Chem. Abstr., 56. 15675 11962). Mateva, R., WGner, G., Lieser, G., J . Polym’. Sci: Polym. Lett. Ed., 1 1 , 369 (1973). Plesch, P. H., Westermann, P. H., J . folym. Sci. Polym. Symp., 16, 3837 (1968). Staudinger, H., Liithy, M., Helv. Chim. Acta, 8, 41 (1925). Staudinger, H., Johner, H., Signer, R., Mie, G., Hengstenberg, J., Natunvissenschaffen, 15, 379 (1927). Staudinger, H., Kern, W., in “Dehochmolekularen organischen Verbindungen”, p 224 ff, Springer, Berlin, 1932. Weissermei, K., Fischer, E., Gutweiier, K., Hermann, H. D., Cherdron, H., Angew. Chem., IS,512 (1967). Wilski, H., Makromol. Chem., 150, 209 (1971).
Literature Cited Burton, W. K., Cabrera, N., Frank, F. C., Phil. Trans. R . SOC.London, Ser. A , 243, 299 (1951). Droscher, M., Lieser, G., Reimann, H., Wegner, G., Polymer, 16, 497 (1975). Drhcher, M., Hertwig, K., Reimnn, H.,Wegner, G., Makromol. Chem., 177, 1695 (1976a). Droscher: M., Hertwig. K., Rodriguez-B., M., Wegner, G., Makroml. Chem., 177,2793 (1976b).
Received for review May 25, 1979 Accepted August 1, 1979 This paper was presented at the Symposium “Worldwide Progress of the Petro-, Organic and Polymer Chemical Industries”, ACS/ CSJ Chemical Congress, April 1979, Honolulu, Hawaii.
Economical p-Xylene and Ethylbenzene Separated from Mixed Xylene Maomi Seko,’ Tetsuya Mlyake, and Koji Inada Asahi Chemical Industry Co., Ltd., 1-2, Yurakucho l-chome, Chiyoda-ku, Tokyo, Japan
A new adsorption process of separating p-xylene and ethylbenzene from mixed xylene has been developed, namely displacement chromatography utilizing improved zeolite adsorbent of high selectivity and new desorbent. In this process, the selectivity and adsorption capacity of zeolite are optimized by selecting the crystal forms, exchanged cations, and moisture content. This process enables us to separate xylene mixtures of p-, m-, and o-xylene and ethylbenzene into p-xylene, ethylbenzene, and the rest by utilizing three zeolite columns, contrary to the conventional adsorption separation processes, in which the simulated moving bed columns and rotary valves are utilized. pand m-xylenes are separated at the outlet of the main adsorption column where an ethylbenzene adsorption zone is located in the middle of p-xylene and m-xylene peaks. The ethylbenzene-containing zone with m- or p-xylene is again separated respectively into each component by utilizing two additional adsorption columns so that ethylbenzene is recovered as a product. Another characteristic of this process is the low fuel consumption to recover the desorbent from products because of its high relative volatility to xylenes. Since the separation is carried out in a form of displacement chromatography, recovered xylene products contain relatively small amounts of desorbent, which also reduces the fuel cost of distillation.
Introduction
p-Xylene, which is a raw material for the synthesis of terephthalic acid, is separated from mixed xylene of p - , m-, o-xylene and ethylbenzene supplied from naphthacrackers, reformers, or the disproportionation process of toluene. Mixed xylene is separated by distillation, crystallization, and extraction of the HF-BF3 complex of mxylene or selective adsorption. Also, xylene separations are usually carried out industrially with an isomerization process which is to convert m-,o-xylene, and ethylbenzene into p-xylene. The selective adsorption process by zeolite adsorbent is generally considered the most economical process among many industrial separation processes. However, the conventional adsorption process is unable to separate ethylbenzene due to the small selectivity between xylene isomers and ethylbenzene is separated by a huge distillation
column before the adsorption process. Asahi Chemical Industry Co., Ltd., has successfully developed a new adsorption process by applying a unique chromatographic principle and an improved zeolite adsorbent. The Asahi process can separate p-xylene and ethylbenzene as products at a low separation cost. The conventional adsorption process of mixed xylene is based upon so-called “elution chromatography” as shown in Figure 1,where the xylene adsorption band in the separation column is developed with a large amount of desorbent, and the selectivity between different xylene isomers is relatively small so that the adsorption band has to migrate a long distance to have a sufficient separation. This requires a large amount of zeolite adsorbent, and xylene isomers are diluted by a large amount of desorbent utilized for elution. High fuel consumption is required to distill the desorbent. Figure 2 shows the conventional
00 19-7890/79/ 12 18-0263$0 1,0010 @ 1979 American Chemical Society
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Table I. Comparison of t h e Elution and Displacement Chromatography -
item applicable process adsorption capacity % utilization of zeolite product concentration fuel consumption for recovering desorbent equipment
Af6 Mixture
D
elution low selectivity low low low high
displacement high selectivity high high high low
simulated moving bed, a multicompartment column and rotary valves
Derorbent D
D
D
B+D
Figure 1. Elution chromatography.
single fixed bed column with ordinary valves
A,/B Mixture
Derorbent D
D
D
D
B+D
Figure 3. Asahi’s displacement chromatography. t
1$
A+B D A+D
‘\
O n Off Valve
Derorbent D
BtD
l D i
L A
Figure 4. Asahi process. Figure 2. Conventional xylene separation process.
adsorption process using zeolite. The column is separated into many compartments, each of which is connected to a rotary valve. The inlet for feed xylene or desorbent or the outlet for products is successively moved from one compartment to the next in accordance with the advance of the adsorption band through the column. This operation, called the simulated moving bed process, is required along with complicated equipment and high investment. Asahi Process Contrary to the conventional process, the migration distance of the xylene adsorption band is small in the Asahi process because of high selectivity between xylene isomers. As shown in Figure 3, each isomer of mixed xylene is separated as a sharp peak by migrating a short distance, so that the separation of isomer is completed before the adsorption band diffuses and is diluted with desorbent. This type of operation is called “displacement chromatography” where the desorbent may not substantially migrate ahead of the adsorption band during migration. The typical separation column used in the Asahi process is shown in Figure 4 . The column is an ordinary cylin-
drical vessel with simple valves a t the top and bottom of the column. The operation and the apparatus are simple, and investment cost is low. Table I shows the major differences between the Asahi process based on displacement chromatography and the conventional process based on elution chromatography. As described before, high selectivity of zeolite adsorbent is essential to realize displacement chromatography. In displacement chromatography, the separation cost is low because of low migration distance, high xylene adsorption capacity per unit weight of zeolite, high isomer concentration in the product, and the saving of the fuel consumption to recover the desorbent. Characteristics of Zeolite In the Asahi process, the characteristics of the adsorbent are improved to provide for efficient separation of xylene. The total amount of separation, W (milligrams of isomer A per gram of zeolite) from a mixture of A and B is expressed by Spedding’s (1955) formula
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
265
I
10
-
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I
I
20
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I
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Figure 5. Amount of separation vs. Selectivity. 200
=i 50
r
-
-
08
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12
14
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Radius of Exchanged Cation ( A )
I
Figure 7. Acidity of zeolite vs. radius of exchanged cation.
t I 0.5
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1.5
Radius of Exchanged Cation (AI
Figure 6. Adsorption capacity vs. radius of exchanged cation.
01
02
03 0405
1
Acidity i m m o l l g Zeolite)
where No is the molar fraction of A in the feed, KBAis the selectivity of A over isomer B, KBA= [(A),/(B),l/[(A)B/ (B)J, ( ), is the concentration in zeolite phase (milligrams per gram of zeolite), ( is the concentration in the outside solution (milligrams per milliliter), and X, is the total amount of adsorption of xylene (milligrams per gram of zeolite). Figure 5 shows the total amount of separation, W, in terms of selectivity, KBA,and the total of amount of adsorption, X,, when No is 0.5. As shown in Figure 5, W increases proportionally with KBAwhen KBAis in the range of 1 to 10. It is essential for an efficient separation to have high selectivity, KBA,and high amount of adsorption of xylene, X,. Amount of Adsorption of Xylene The amount of adsorption of xylene in the zeolite is influenced by the pore size in the zeolite and, especially for the separation of xylene which has a large molecular diameter, it is essential to use large pore size crystals of natural Faujasite, Zeolite X, or Zeolite Y. The adsorption amount of xylenes is dependent on the ionic radius of the exchanged cation as shown in Figure 6. The adsorption amount is larger for the zeolites exchanged with monovalent small cations. Selectivity The selectivity and adsorption capacity of zeolite are influenced by the physical properties of zeolite and the adsorbed isomer. Dependent upon the production method of zeolite, the following are found most influential for the performance of xylene separation: (1)exchanged metal cation; (2) the ratio of SiOz/Al2O3of zeolite; (3) moisture content; (4)desorbent. 1. Exchanged Metal Cations. The acidity of zeolite is measured to investigate the selectivity and other physical
Figure 8. Selectivity vs. acidity of zeolite.
properties of the cation-exchanged zeolite. As shown in Figure 7 , the total acidity of zeolite (the sum of Bronsted and Lewis acid points) measured by titration with n-butylamine and Hammett indicator (Methyl Red) has a strong correlation with the ionic radius or valence of exchanged cation. Generally crystalline zeolite shows strongly acidic properties of a Bronsted acid due to the polarization of adsorbed water molecules in a strong electrostatic field between the exchanged cations and A104anion (Ward, 1968). When a Bronsted acid of zeolite is dehydrolyzed, the zeolite acts as a Lewis acid. Figure 7 shows that the smaller the ionic radius or the larger the valencies of exchanged cation, the stronger is the acidity of the zeolite. The relation between the selectivity of zeolites of p- and m-xylene and exchanged metal cations is shown in Figure 8. This shows the strong relation between the selectivity and the acidity of zeolite. The lower the acidity of zeolite, the higher is the selectivity for p-xylene. This is explained by strong affinity between the acidic zeolite and the most basic isomer of m-xylene which has the strongest electron donating characteristic (Kilpatrick and Luborsky, 1953) among xylenes. In this way, the selectivity of the xylene isomer is predicted by the acidity of zeolite exchanged by the specific cation. 2. SiOz/A120,. The acidity of zeolites, as shown in Figure 9, decreases when the ratio of Si02/Al,03 increases. High SiOz/A1203ratio reduces the adsorptive site of A104and increases the strength of the electrostatic field of each site. Figure 9 shows that the reduction of AlO, sites more seriously influences reduction of the acidity of zeolite. The selectivity of p - over m-xylene, KmXPX,is proportional to the ratio of SiOz/Al2O3as shown in Figure 10.
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1
1. Suitable Desorbent
oX+mX
I
031
3. Weak Desorbent
2. Strong Desorbent
1
2
3 4 5 6 Molar Ratio of SiO,IAY,O,
7
8
Figure 11. Elution curves and desorbing power.
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p X l m X Mixture
, 1
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4
5
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7
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8
Molar Ratio of S i O , / A I , O ,
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pi( process.
Figure 10. Selectivity vs. molar ratio of SiOz/A1203.
Therefore, it is preferable to increase the ratio of Si02/ A1203to improve p-xylene selectivity as long as the crystalline structure of zeolite is maintained. In the Asahi process, the selectivity of the xylene isomer is improved by adjusting the ratio of SiO2/Al2O3. 3. Moisture Content. The moisture content of zeolite is adjusted by the manufacturing condition of zeolite such as calcination time and temperature. The lower the moisture content, the lower acidity has zeolite, which increases the selectivity of KmXPXand the amount of adsorption of xylene. This is explained from the fact that the polarized water molecule in the zeolite works as a Bronsted acid. In the Asahi process the selectivity of p over rn-xylene is improved to 6-8 from the conventional zeolite of 2-4. 4. Desorbent. In order to carry out an effective separation of xylene, it is essential to select the suitable desorbent which maintains the stationary elution curve of the xylene adsorption band during the migration through the column. This enables one to obtain maximum product yield from both ends of the adsorption band. Important properties required for the desorbent are as follows. (1)The desorbent should not reduce the selectivity and adsorption capacity of zeolite, when it is mixed with xylene. (2) The desorbent should have a suitable desorbing power to xylene adsorbed in the zeolite. (3) The desorbing power should not be reduced over the wide range of xylene concentration, especially at the boundaries of the band where the concentration of xylene is low. (4)The relative volatility of desorbent to xylene should be large enough so that it is easily recovered by distillation. ( 5 ) The desorbent should be chemically stable in the operational condition.
Among these characteristics, the appropriate strength of desorbing power is most important to produce the stationary elution curve of the xylene adsorption band. When the desorbing power is appropriate, the distribution of the concentration of xylene in the adsorption band which is identified by the elution curve is symmetrical. In this case, the maximum amount of products is obtained from both front and rear boundaries of the adsorption band. As shown in Figure 11,when the desorbing power of the desorbent is stronger than an appropriate value, the rear part of the elution curve has a sharp boundary, while the front part diffuses too widely to obtain the product in an appropriate concentration. On the other hand, when the desorbing power is weaker than an appropriate value, the front boundary is sharp while the rear end of the elution curve diffuses widely so that a sufficient concentration of the product would not be obtained from the rear boundary. Most hydrocarbons of desorbent tend to lose desorbing power at the rear end of the band in which xylene concentration is low. Consequently this results in the abnormal and long tailing of the xylene adsorption band. In this point of view, it is important tb investigate the desorbing power of the desorbent in the various concentrations of xylene.
Chromatographic Operation and Optimization of the Process The operation of the Asahi process is shown in Figure 12. Mixed xylene is supplied to the adsorption column packed with the zeolite adsorbent, and the adsorption band of xylene is formed at a length of to l/lo of that of the column. The desorbent is supplied from the top of the column to develop the adsorption band. Such a procedure is repeated with appropriate intervals so that the xylene
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
A
Product
Recycle t o Feed
Product
Desorbent
Xylene
I
2
\
B
'
D
C
To Sub Column
Product
Figure 15. Chromatography of mixed xylene.
Figure 13. Chromatography of p - and rn-xylene.
.'
'
Product or To Isomerisation
267
mX.oX
Column
*\
.4
6
8
10
12
14
16
1
Isomerization Process
Migration Distance (m)
Figure 14. Productivity and concentration of xylene vs. migration distance.
adsorption band and the desorbent band are alternately formed in the separation column. The feed of xylene and the desorbent are easily switched by the ordinary valves at the top of the column. When the xylene band migrates, xylene isomers in the adsorption band start to isolate from each other and accumulate to both ends of the band where the concentration of isomer increases steadily. The length of the middle part of the adsorption band, where mixed isomers remain, becomes shorter. The concentration of each separated isomer near the boundaries gradually increases as the band migrates. Figure 13 shows the typical elution curve of m- and p-xylene separation by the Asahi process. m-Xylene and p-xylene accumulate a t the front or rear boundaries, respectively, of the adsorption band, and all mixed xylene is almost separated to each isomer. The middle part of the elution curve consists of the mixture of the feed composition, and transient zones of p- or m-xylene. The volume of zeolite is selected to have minimum length of the unseparated zone of the band by adjusting the flow rate of feed and the migration distance.
Productivity Figure 14 is the relation between the productivity and the migration distance of a band. The productivity of product xylene (tons) per unit time (day) and unit volume of zeolite adsorbent (m3) will decrease with an increase of the migration distance, whereas the concentration of separated xylene isomer increases as the band migrates. Therefore, the optimum distance of migration is selected to minimize the total cost of separation. Separation of Ethylbenzene Figure 15 shows the typical elution curve of the Asahi process where feedstock contains four major isomers of xylene, namely p-, m-, o-xylene, and ethylbenzene. At the
Figure 16. Block flow sheet of Asahi process.
outlet of the separation column, the sharp ethylbenzene peak appears between t h e g - and m-xylene peaks, due to the high selectivity, KmXP . In the commercial operation, the length of the column is designed so that the front end of the p-xylene band barely touches the rear end of the m-xylene band a t the outlet of the adsorption column after migration. The effluent is divided into four fractions, namely A, B, C, and D fractions, as shown in Figure 15. Fraction A contains only m- and 0-xylenes. Fraction B consists of m-, o-xylenes, and ethylbenzene. Fraction C consists of ethylbenzene and p-xylene. Fraction D only consists of p-xylene that is obtained as a product after removing desorbent. A typical flow diagram of the Asahi process is shown in Figure 16. This is one of the most economical process to produce p-xylene and ethylbenzene simultaneously and the sole process to separate ethylbenzene by using zeolite. Feed xylene is mixed together with the xylene obtained in the isomerization process before supplying to the main separation column. The xylene band in the column is developed with the desorbent. Four successive fractions, A, B, C, and D, which are equivalent to those in Figure 15, are collected at the outlet of the column. Each fraction except fraction D is sent to the isomerization process, subcolumn I or subcolumn 11, respectively, after recovering the desorbent by distillation. m- and o-xylene and ethylbenzene in fraction B are separated in subcolumn I, where xylene is sent to the isomerization process. p-Xylene and ethylbenzene in fraction C are also separated in subcolumn 11. Both effluents are collected as products. The size of subcolumns I and I1 is relatively small due to the limited flow of each isomer. The advantage of the Asahi process is to recover all of the ethylbenzene in the mixed xylene so that the capacity
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
Table 11. Economics of t h e Asahi Process ~~
plant capacity, tonsiyear purity of products, % operating hours, days /year feed composition, %
p-xylene ethylbenzene total m-xylene o-xylene p-xylene ethylbenzene
adsorbent, tons effluent xylene concentration, % (av) kg of feed xylene/kg of product (pX + EB) kg of adsorbent/kg of product ( p X + EB) fuel consumption, kcal/kg of product (PX + EB)
70 000 15 340 8 5 340 99.5 330 41 21 20 18 170 55 1.095 0.00039 2.2 x 103
of the isomerization process can be minimized. It is of course possible to recover only p-xylene as a product by a slight modification of this process; however, it may be less feasible from the total economy of the process. Toluene, one of the byproducts of the isomerization process, has the same retention time as ethylbenzene, and C8-saturates have the same retention time as rn-xylene. Both byproducts are separated by subsequent distillation. Advantages of Asahi Process Major advantages of Asahi Process are summarized as follows. 1. Zeolite adsorbent required per unit production rate is approximately one-half of the conventional process, because of the high selectivity.
2. The heat consumption of the process is about one-half of the conventional process, partly because of the high product concentration in the effluent. 3. Investment cost is low because simple column structure as well as the process is realized by displacement chromatography. 4. Ethylbenzene is recovered as a product so that the fixed cost incurred to p-xylene is reduced. This also helps to reduce the load to the isomerization process. Table I1 shows the economics of the Asahi process. In the Asahi process, in a plant producing 70000 tons of p-xylene and 15340 tons of ethylbenzene per year (99.5% purity), the volume of zeolite is about 50% of the conventional process and the fuel consumption is estimated also to be about half of the conventional process. This new separation process will have a wide application for xylene separation, not only installing new capacity but also modifications or upgrading of existing plants to add to production capacity for p-xylene and ethylbenzene. Literature Cited Kilpatrlck, M., Luborsky, F. E., J . Am. Chem. Soc., 7 5 , 577 (1953). Speddlng. F. H., Powell, J. E., Sue, H. J., J . Am. Chem. Soc., 77,6125 (1955). Ward, J. W., J . Catal., 10, 34 (1968).
Received f o r reuiew April 6 , 1979 Accepted April 23, 1979 Presented at the Symposium on “World-Wide Progress of the Petro-, Organic, and Polymer Chemical Industries”, Division of Industrial and Engineering Chemistry, ACS/CSJ Chemical Congress, Honolulu, Hawaii, April 1979.
The Role of Chemisorption in Fischer-Tropsch Synthesis Vladimir Ponec’ and Willem A. van Barneveld Gorlaeus Laboratoria, Rijksuniversiteit Leiden, 2300 RA Leiden, The Netherlands
The relation between the activity in Fischer-Tropsch synthesis and chemisorption of reaction components and intermediates is being discussed. I t is shown that CO adsorption plays a key role in these reactions. Metals which do not dissociate CO easily are almost inactive in Fischer-Tropsch reactions. Metals which dissociate CO easily but form too stable oxides (and carbides) are also inactive. The just-mentioned conditions locate in the periodic table the activity for Fischer-Tropsch synthesis of hydrocarbons in the group of metals: Fe, Co, Ni, Ru, and Rh. Analysis of the available experimental information shows that carbon formed by CO dissociation is, after partial hydrogenation, a chain growth initiating and propagating species.
Introduction A causal relation between adsorption and activation of molecules in a catalytic process was already suggested by Faraday in the last century. Since then numerous attempts were undertaken to gain more information on this relation for various catalytic reactions. The relation chemisorption-catalysis has two very important aspects: (a) Which parameter characterizing the solid catalyst or chemisorption on this solid should be related to the catalytic activity? (b) Which particles must be adsorbed during the process and which may eventually react from the gas phase? Question b is often the key to the solution of the first question. The problem has been attacked many times by using formal kinetics, but experience shows that kinetics are usually not unambiguous in giving the answer. Much more reliable are chemisorption measurements upon cata0019-7890/79/1218-0268$01.00/0
lytic reactions running on the surface being investigated (33). Also mechanistic studies using isotopes or various surface science methods are a better source of information on this problem than formal kinetics alone (see, e.g., symposia on these problems (10, 28)). As far as question a is concerned, refer to the following. Many attempts were made since about 1950 to correlate the catalytic activity of metals with one single parameter characterizing the metal. None of these correlations was satisfying and sometimes even the correlation parameter itself raised many questions (22) as, for example, the socalled &character of the metallic bond, once very popular among chemists. On the other hand, following the old idea of Sabatier that a chemisorption complex must be neither too stable nor unstable, various authors found a correlation between the catalytic activity and one or another parameter characterizing the chemisorption bond strength of re@ 1979 American
Chemical Society
