Ind. Eng. Chem. Process Des. Dev. 1981,20, 326-331
326
Sosna. M. C.; Kondukov, N. B. Inzb. f k . Z b . 1968, 75, 73. T h . 0.M.: ~ o s h k o v .V. D. "Procedlngs of the USSR Symposium on Processes yith EluMized Beds" (in Russlan), Gostechnika USSR,Moscow, 1957 (cited in Mika, 1977). Wen, C. Y.; Yu, Y. H. AIChE J. 1966. 12, 610. Wen, C. Y., Yu, Y. H. Chem. Eng. Prog. Symp. Ser. 1966, 62, 100.
Received for reuiew May 16, 1980 Accepted October 9,1980
Supplementary Material Available: Measured values of minimumfluidization velocities of corundum, limestone,
lime, and brown coal ash and mean relative deviations of the minimum fluidization velocities computed by eq 5-12 and eq 16-18 from the exDerimental values at temperatures 20-890 "c(6 pages). Ordering information is given on any current masthead page.
Coke Formation and Deactivation of the Catalyst in the Reaction of Propyfene on Calcined NaNH,-Y Bernd E. Langner Universitat Essen, Institut fur Technische Chemie, 4300 Essen, West Germany
The deactivation of the catalysts during the reaction of propylene on NaH-Y zeolites by coke formation has been investigated as a function of time on stream at dlfferent reaction temperatures using different experimental methods. The results show that at temperatures below 300 O C deactivation is caused by strongly adsorbed compounds in the pore system of the catalyst. Above 300 O C deactivation is caused by blocking the entrances of the pores at the exterior surface with macromolecular material. These compounds may be formed via oligoaromatic compounds which are built up in the pores of the catalyst and are able to migrate to the pore mouths at the exterior surface, where they can accumulate according to their high coking tendency.
Introduction The reactions of hydrocarbons on zeolites and other alumosilicates are accompanied by the deposit of coke on the surface of the catalyst which decreases the reaction rate with time on stream. Although the kinetics of coke formation in cracking reactions have been thoroughly investigated (Voorhies, 1945; Pachovsky et al., 1973; Beeckman and Froment, 1979; Ozawa and Bischoff, 1968b), the mechanism of deactivation is unknown so far. While IR spectroscopic measurements show that deactivation may be due to the destruction of active hydroxyl groups in the catalysts (Eisenbach and Gallei, 1979), kinetic measurements show that deactivation may be caused by a change of the transport properties of the catalyst, because coke and coke precursors fill the pores or block the entrances to the pores of the catalysts leading to a decrease of diffusion of the reactants to the active site (Butt, 1972). Studies about the chemical nature of coke and coke precursors (Langner, 1980; Langner and Meyer, 1980; Venuto and Hamilton, 1967; Eberly et al., 1966) demonstrate that the deposits on the catalyst surface consist mainly of alkylated fused aromatics and hydroaromatics. The important role of polyaromatic compounds in the deactivation of cracking catalysts is confirmed by a study of Appleby et al. (1962), which clearly demonstrates that polycyclic aromatic compounds possess a high tendency for coke formation. So the reaction between aromaticsvia carbenium ions-has been proposed as the mechanism for coke formation. On the other hand, radiotracer investigations of the reaction of a mixture of paraffins and aromatics have shown that both compounds take part in coke forming reactions in equal amounts (Walsh and Rollmann, 1977; Walsh and Rollmann, 1979). Furthermore, it could be established that dienes also possess a high tendency for coke formation. So a mechanism has been proposed in which Diels-Alder additions between dienes 0196-4305/81/1120-0326$01.25/0
or between dienes and olefins are responsible for the formation of oligocyclics (Langner and Meyer, 1980). Not only the organic reactions leading to coke are discussed in different ways, but also the results concerning the place where these reactions take place are conflicting. The great influence of the dimensions of the pores of different zeolite catalysts on the selectivity of coke formation (Rollmann, 1977; Rollmann and Walsh, 1979) indicates that the inner structure determines the extent of coke formation. This is supported by the decrease of active hydroxyl groups during deactivation (Eisenbach and Gallei, 1979). Furthermore, analyses of coke formed in the reaction of ethylene on NaH-Y zeolites provide evidence that the number of fused rings in the polyaromatics is limited by the dimensions of the pore system of the catalyst (Venuto and Hamilton, 1967). However, there are other results which demonstrate that a major part of coke formation occurs at the exterior surface of the catalyst particles. From X-ray measurementa and electron microscopic studies it was concluded that coke is distributed in the interstices between the particles of a cracking catalyst (Haldeman and Botty, 1958). This is confirmed by investigations of the cracking reactions of cumene on H-mordenite (Butt et al., 1975; Butt, 1976), which clearly show that the deactivation of the catalyst is caused by the increase of diffusional resistances between the particles of a catalyst pellet. Also, the high molecular weight of the graphite-like structure (Haldeman and Botty, 1958),which would not fit into the narrow pores of the cracking catalyst, supports the assumption that a great part of coke is formed on the outer surface of a catalyst particle. These contradictory results indicate that coke formation is one of the least understood phenomena in catalytic cracking. As it is known that most reactions in catalytic cracking proceed via olefinic intermediates (Poutsma, 1976), the deactivation in the reaction of propylene on 0 1981 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 20,No. 2, 1981 327
calcined NaNH4-Y was chosen as a model reaction for the formation of coke in hydrocarbon reactions on zeolites. It could be expected that measurements concerning the amount of coke, its carbon/hydrogen ratio, and the loss of pore volume of the catalyst during the reaction combined with the analyses of heptane extracts of used catalysts would provide more information about the origin of coke formation in catalytic cracking.
Experimental Section Catalysts. The catalyst base was a commercial NaY zeolite (Linde 30-200) which was exchanged with a 0.2 N ammonium nitrate solution. For details, see Langner (1980). The exchange degree x has been denoted as NaNH4(x)-Y. In most of the experiments the exchange degree was 26%. Apparatus and Procedure. The reactions were carried out in an apparatus described elsewhere (Langner, 1980). Prior to the reaction, the catalysts were activated by calcination in a stream of dry nitrogen (1L/h) for 17 h. In most of the experiments the calcination temperature was 350 "C. Then the reaction mixture of nitrogen (5.5 L/h) and propylene (0.5 L/h) was passed over the catalyst. The anaylses of the reaction products were made by gas chromatography (Langner, 1980). After different times on stream the flow of propylene was stopped and the catalyst was purged with nitrogen for h at the reaction temperature to remove loosely adsorbed hydrocarbons from the catalysts. The pore volumes of the spent catalysts were determined by a conventional BET apparatus with nitrogen at -196 "C. Prior to the adsorption measurements, the catalysts were kept under vacuum (lod torr) at 150 "C for 1h. The relative pore volume is expressed as the ratio of grams of adsorbed nitrogen/gram of dry catalyst (xlm). The measurements show that in all experiments the adsorption isotherms were of the Langmuir type with a constant amount of adsorbed nitrogen between 70 and 350 torr. So the values at a pressure of 150 torr were taken for the determination of the pore volumes. The pore volume of a fresh catalyst was 0.29 g of N2/g of cat., which is in excellent agreement with published measurements (Breck, 1974). The amount of coke formed during the reaction and the carbon/hydrogen ratio was analyzed by burning off the deposits on the catalyst in oxygen at about 800 "C. Because of the evolution of water from the hydroxyl groups of the zeolites at these high temperatures a gravimetric determination of the hydrogen content of the coke failed. Therefore, carbon dioxide and water have been indirectly determined by a volumetric method described by Enterman and van Leuven (1972). The errors of this method were 1-4% for the amount of coke and 4-8% for the carbon/hydrogen ratio. To analyze the soluble part of the deposits a calibrated procedure was strictly applied. As it could be presumed that a great part of the residue was adsorbed within the pore system of the zeolite, the zeolite structure was destroyed by ball-milling for 17 h. By this treatment the relative pore volume of a fresh catalyst decreases from 0.29 g of N2/g of cat. to less than 0.03 g of Nz/g of cat. Then the ball-milled catalysts were extracted for 3 h with a measured volume of n-heptane under reflux. The pale yellow extracts were separated in a high-pressure liquid chromatograph with a UV detector (Knauer, Type FR 30) on a 25-cm column of Lichrosorb (5 pm) with n-hexane as the fluid phase. The different fractions were analyzed by UV spectroscopy and compared with reference substances (benzene, naphthalene, anthracene, phenantrene, diphenyl,
1
2
3 Eme on Stream
i
6
5
Ih I
Figure 1. Conversion of propylene as a function of time on stream at different reaction temperatures (for details see Experimental Section). 0.3
1 0
:
2W'C
0 : 250'C A
i
x :
i
35o*c
4oov
*--Lsos 1
2
3 Time on Stream
5
L
I
h
6
1
Figure 2. Relative pore volume of the spent catalysts as a function of time on stream at different reaction temperatures (for details see Experimental Section).
pyrene) or recorded spectra given by Clar (1964). Results The Change of Conversion and Porevolume of the Catalyst during Time on Stream. The reaction of propylene on NaH-Y results in the formation of cracking products, predominantly C4-and C5-hydrocarbonswith a high degree of saturated and branched compounds. Hydrogen transfer reactions also lead to a considerable amount of propane in the reaction mixture, the formation of which shows distinct induction periods during the time of reaction. These induction periods and the mechanism leading to the cracking products have been discussed in a previous paper (Langner, 1980) for the reaction of butene on NaH-Y. Figure 1 shows the dependence of the conversion of propylene with time on stream a t different reaction temperatures. It can be seen that the deactivation of the catalyst is strongly dependent on the reaction temperature. At low temperatures the conversion rapidly decreases during the first 2 h to one-half of the initial value, whereas at 400 "C the conversion is nearly constant during the same amount of time, and deactivation takes place only after 3 h time on stream. The decrease of the activity of the catalyst is closely related to a loss of pore volume. In Figure 2 the loss of adsorption capacity for nitrogen of spent catalysis is represented. At a reaction temperature of 200 "C the pore volume rapidly decreases in the beginning of the reaction and then remains nearly constant. At 250 "C a similar curve is obtained, but there is still a free pore volume of more than 30% of the fresh catalyst
328
Ind. Eng. Chem. Process Des. Dev., Vol. 20,No. 2, 1981
Table I. Influence of the Acidity of the Zeolite on the Pore Volume after the Reaction with Propylene exchange pore v01.F pore V O ~ . , ~ degree, % g of N,/g of cat. g of N,/g of cat. 0.233 0.136 0.090
6 26 70
.-15CDc
!
21
0
:
200T
0
i
250°C
A
i
350%
I
i
LOOT
0.162 0.100 0.088
calcination temp, "C
pore V O ~ . , ~ g of N,/g of cat.
350 400 500 550
0.136 0.147 0.166 0.182
a 350 'C, 4 h time on stream; calcination temperature, 200 'C, 6 h time on stream; calcination temper350 "C. 350 'C, 4 h time on stream; exchange ature, 350 "C. degree, 26%.
after 6 h time on stream, although conversion (Figure 1) has fallen down to one-tenth of the initial value. A t reaction temperatures above 300 "C the loss of pore volume is slower in the beginning of the reaction, but in contrast to the low-temperature experiments the curve continuously decreases until the adsorption capacity is totally destroyed by coke formation. A further increase of temperature then, again, accelerates the loss of pore volume with time on stream. Although a comparison of Figure 1 and Figure 2 shows that there is a close connection between the loss of activity and the loss of pore volume by fouling; there is a surprisingly high conversion at high reaction temperatures after long times on stream, even through the pore volume of the catalyst is nearly zero. These results are in contrast to those of Ozawa and Bischoff (1968a),whoe found no loss of surface during the deactivation in the reaction of ethylene on an alumo-silicate catalyst. But it has to be taken into consideration that they had very low coke levels in their experiments. Table I shows that the loss of pore volume is dependent on the number of active sites. Therefore, an increase of the exchange degree of the zeolite results in an acceleration of coke-forming reactions. For example, after the reaction of the low exchanged NaNH4(6)-Y exhibits a pore volume which is about twice as much as for the high exchanged NaNH4(70)-Y. Furthermore, if the catalyst is calcined at temperatures higher than 350 "C prior to the reaction, the rate of coke formation decreases, leading to the relatively high value for the pore volume of the spent catalyst which has been calcined at 550 "C. These results suggest that Bronsted sites take part in coke-forming reactions, as it is known that Bronsted sites are partially destroyed by thermal treatment a t high temperatures. Amount of Coke Formed and its Carbon/Hydrogen Ratio. It could be expected that the amount of carbon deposita on the catalyst is closely related to the loss of pore volume. This is confirmed by Figure 3, which indicates that at reaction temperatures below 300 "C the formation of carbon on the catalyst is nearly completed after 4 h time on stream, while at higher temperatures considerable coke formation takes place even after 6 h. Furthermore, an evaluation of the amount of coke necessary to destroy the whole pore volume of the zeolite based on the values of Figure 2 and Figure 3 (after 6 h) demonstrates that at a reaction tempeature at 450 "C about 32% carbon on the catalyst would totally destroy the adsorption capacity while the same calculation leads to only 22% for a reaction temperature of 200 "C. The degree of unsaturation of the residues on the catalysts can be expressed by the wbon/hydrogen ratio. The
2
1
3 l i m e on Streom
6
5
1
I
hj
Figure 3. Amount of coke deposit as a function of time on stream at different reaction temperatures (for details see Experimental Section).
2.0
05
t I
1
4 200
250
300
Reaction Temperature
350
LW
150
['C I
Figure 4. Carbon/hydrogen ratio of the deposita and the maximum amount of propane during the reaction (time on stream for the C/H ratio: 6 h).
high amount of paraffinic hydrocarbons and the lack of compounds which are more unsaturated than monoolefii in the reaction mixture should result in a carbon/hydrogen ratio of the residue greater than 0.5, the value for the propylene feed. Figure 4 demonstrates that this is valid for all reaction temperatures. But the degree of unsaturation is strongly dependent on the reaction temperature leading to C/Hvalues between 0.7 at 200 "C and 1.8 at 450 "C after 6 h time on stream. As shown for propane, the degree of unsaturation is closely related to the amount of paraffins in the reaction mixture (Figure 4). At high reaction temperatures the carbon/hydrogen ratio significantly changes with times on stream, while at reaction temperatures below 300 "C the degree of unsaturation remains nearly constant during the period of the experiments (Figure 5). Coke Analyses. To get information about the chemical nature of coke and coke precursors at different reaction temperatures, heptane extracts of ball-milled, used catalysts have been analyzed by means of high-pressure liquid chromatography. Again, the results strongly depend on the reaction temperature, so the soluble portion of coke and coke precursors amounts to about 35 mg of coke/g of catalyst for the catalyst at 200 "C and less than 3 mg of coke/g of catalyst for the 400 OC experiment after 6 h time on stream. These values correspond to about 25% respectively less than 2% of the total amount of the residue analyzed by burning off. The low solubility of the coke produced at high temperatures indicates that nearly the whole residue consists of macromolecular, insoluble com-
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981 329
I
- 1 p 2.0
n" c
8 2
1.5
T = 200'C
Q. ..
Q%.
-
-z
." 6
g
1.0-
0.5
e
xe
1
/\I
c
4 1
2
3
1
Time on Stream
I
5 h
6
I
1
1 = LOOT
18
IS
12
9
Retentiomolum
3
6 [ ml
I
T = 400 'C
0.5
4 1
2
3
Tlme on Stream
L
5
f
6
L hI
Figure 5. Carbon/hydrogen ratio and the amount of propane in the reaction mixture as function of time on stream at reaction temperatures of (a) 400 "C and (b) 200 "C.
pounds, whereas at low reaction temperatures a considerable part of the coke consists of soluble material of low molecular weight. The differences between the coke samples a t different reaction temperatures are demonstrated even more clearly by their high-pressure liquid chromatograms represented in Figure 6. The separation was made with a silica column, which separates the compounds according to their aromaticity (Suatoni, 1979). Figure 6 demonstrates that the soluble part of the coke formed at 400 "C results in a well-resolved chromatogram with distinct relatively sharp peaks, while for coke form the low-temperature experiment there are obtained only poorly resolved broad overlapping maxima. Furthermore, the chromatograms show that in the low-temperature coke extract the baseline is reached after a retention volume of about 10 mL, while there is a long tailing to a retention volume of more than 200 mL for the 400 "C deposit. For additional analyses the fractions of the chromatograms have been collected and analyzed by UV spectroscopy. In the 400 "C chromatogram alkylated benzenes, naphthalene, phenanthrene, pyrene, and possibly anthracene derivatives could be identified by comparison with reference compounds. Furthermore, ring systems of five fused rings have been detected by comparison with recorded spectra (Clar, 1964). The retention volumes of these compounds are included in Figure 6. These results demonstrate that the coke extracts from the experiments a t high reaction temperatures consist of fused-possibly alkylated-aromatic systems with up to about 6 rings. The long tailing in the chromatograms is a hint for compounds with a high molecular weight. This is in agreement with the low solubility of the coke formed at high temperatures. On the other hand, for the coke extracts from the lowtemperature experiments only broad absorption maxima between 220 and 270 nm are obtained. These W spectra resemble the spectra of the so-called conjunct polymers which are obtained in the alkylation of olefins in sulfuric acid and hydrogen fluoride (Miron and Lee, 1963). So it may be suggested that the coke precursors formed on
18
15
12 9 Retentiomolum
6 Iml 1
3
Figure 6. High-pressure liquid chromatograms of heptane extracta of the coke from the experiments at 200 and 400 "C.
zeolites at low reaction temperature have a simliar structure to the conjunct polymers, namely cyclic polyolefinic hydrocarbons with a high proportion of conjugated double bonds. Discussion As the decrease of the conversion in the reaction of propylene on calcined NaNH,-Y proceeds similarly to the decrease of the adsorption capacity of the catalyst, it may be assumed that the blockage of the pore system of the zeolite is the reason for the deactivation. Nevertheless, there are considerable differences for coke formation a t different reaction temperatures. These differences are pronounced (i) by the rapid decrease of the pore volume of the catalyst to a nearly constant value a t low temperature, while at high temperatures the pore volume nearly linearly decreases until the adsorption capacity is zero; (ii) by the carbon/hydrogen ratio of the coke formed, which increases with increasing temperature and is significantly dependent on time on stream only a t high reaction temperatures; (iii) by the amount of the heptane-soluble part of the coke, which at low reaction temperature is more than 10 times greater than at high reaction temperatures; and (iv) by the chemical structure of this soluble part of coke, which consist of fused aromatics with 1to about 6 rings at high reaction temperatures, while at low reaction temperatures the UV spectra of the extracts resemble those of so-called conjunct polymers-cyclic olefins and dienes formed in the acid catlayzed alkylation of olefins (Miron and Lee, 1963). All these differences can be explained only by a change in the reaction mechanism of coke formation with increasing reaction temperature. At temperatures below 300
330
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 2, 1981
"C, coke precursors must be formed which are strongly adsorbed at the active sites within the catalyst. As the conversion falls to about one-tenth of the initial value after 6 h time on stream at a reaction temperature of 250 "C; although the available pore volume only decreases to one third of the fresh catalyst, it may be assumed that only such pores or such parts of the pores which contain enough active sites are blocked by coke formation. This is confirmed by the effect of the exchange degree and the calcination temperature of the catalyst on the pore volume after 6 h time on stream. Furthermore, there is only little diffusion of the coke precursors between the pores and from the pores to the outer surface. This means that the dimensions of the coke formed are mainly limited by the dimensions of the pore structure of the zeolite. This limitation of the size is shown by the relatively high amounts of heptane-soluble material, which can be extracted from the catalyst from low-temperature experiments and by the lack of a long tailing in the chromatograms. Because of the low mobility of the coke precursors, only little mass exchange can take place in the pore system. This implicates a small reaction rate for hydrogen transfer reactions leading to a relatively low aromaticity and low C/Hratio of the coke, which remained unchanged with time on stream. The lower value of the final pore volume at the experiments at 250 "C as against the 200 "C experiment may be attributed to reaction products-presumably paraffinic dimers, which are loosely bounded to the catalyst and can desorb only at reaction temperatures of 250 "C. At reaction temperatures above 300 "C the extent of hydride transfer reactions and the diffusion of the coke precursors is much greater, resulting in a high proportion of paraffins in the reaction mixture and a high aromaticity of the coke precursors. At these temperatures the aromatics can diffuse into the whole pore system and to the exterior surface of the catalyst. This extensive mobility may account for the continuous destruction of the whole pore volume of the catalyst. As in the experiments at low temperatures the size of the formed aromatics should be limited by the pore dimensions of the catalyst. Indeed the soluble part of the deposits contains predominantly aromatics with up to about 6 fused rings, but the little solubility of the coke and the long tailing in the chromatograms point to the macromolecular character of most of the coke. These compounds, however, can be formed only at the exterior surface of the catalysts because of the limited pore size of the zeolites. Therefore, it may be suggested that the ultimate-perhaps graphite like-coke grows at the exterior surface (including the pore mouths) by reaction of those coke precursors, which are formed within the pores of the zeolite and which migrate to the exterior. I t is known that oligoaromaticcompounds have a high tendency for coke formation (Appleby et al., 1962), so that a coke forming reaction of the compounds detected, like anthracene, pyrene, or other aromatics to coke, may proceed at catalytic sites on the exterior surface, especially on stacking faults at the pore mouths of the zeolite crystallites. Furthermore, the growth to polymers is favored, since desorption from the surface becomes less and less probably with increasing molecular weight. So the deactivation of the catalyst proceeds from the exterior surface by blocking the entrances of the pores of the zeolite. But there apprears to be a considerable mobility of the coke even on the exterior surface at high reaction temperatures, as the reaction rate is not zero for a nearly totally destroyed adsorption capacity, measured by adsorption of nitrogen. The very low proportion of the extractable compounds of coke formed at high reaction temperatures demonstrates
that the pores of the zeolite are filled only to a little extent. Therefore, the amount of coke on the catalyst is not limited by its pore volume. This is supported by the evaluated high values of coke necessary for the total destruction of the pore volume in the high-temperature experiments as against the low-temperature experiments. This mechanism for coke formation at high reaction temperatures is able to account for the seemingly contrary results found in previous papers, which on the one hand show that coke formation occurs on the exterior surface between the ultimate particles (Haldeman and Botty, 1958; Butt et al., 1975) and on the other hand point out the close connection between the pore dimensions of the inner structure of the catalyst and coke selectivity (Walsh and Rollmann, 1977; Walsh and Rollmann, 1979). As for coke formation on the exterior surface the formation of oligoaromatic coke precursors within the zeolite is a prerequisite, the dimensions of the pore system play an important role for the maximum size of these aromatics. But the tendency for coking reactions increases with the number of fused rings in the aromatic system (Appleby et al., 1962), leading to low coke selectivity for coke formation on those zeolites, which do not favor the formation of highly condensed systems because of the dimensions of their pores. This is supported by the high amounts of monoaromatics and the low coking tendency in the reaction of lower olefins on a ZSM-5 catalyst (Anderson et al., 1979) in which deactivation of the catalysts is slow, as only monoaromatic compounds are formed within the pore system, which possess a very low tendency for coke formation on the exterior surface and can rapidly desorb from the surface. These results suggests that the formation of oligoaromatic coke precursors inside the pore system is an important prerequisite for coking a t the outer surface. Nevertheless, the organic reactions leading to coke at the outer surface were unknown hitherto. It may be speculated that cycloadditions via charge transfer complexes between oligoaromatics play an important role in these reactions. Conclusions The investigations on coke formation in the reaction of propylene on calcined NaNH4-Y have shown that the deactivation of the catalyst proceeds by two different mechanisms according to reaction temperature. At reaction temperatures below 300 "C the deactivation is caused mainly by the strong adsorption of compoundspresumably cyclic olefins and dienes-in the pores of the zeolite, which fill the active part of the pore system. At reaction temperatures above 300 "C, however, aromatization of the oligocyclic compounds by hydrogen transfer reactions can take place and the aromatics formed are able to diffuse to the exterior surface of the catalyst. The high coking tendency of these oligoaromatic compounds is responsible for their growth to macromolecular, graphite-like species on the exterior surface of the catalyst, probably starting from the pore mouths of the zeolite crystals where they block the entries of the pores. Therefore, the coking tendency is strongly dependent on the size of the aromatic systems which migrate to the exterior surface. This results in a strong dependence of the selectivity of coke formation on the dimension of the pore system of the catalyst. Acknowledgment The author wishes to thank Mrs. A. Schriider for valuable technical assistance. Literature Cited Anderson, J. R.; Foger. K.; Mole, T.; Rajadhyska, P. A.; Sanders, J. V. J. Catal. 1979, 38, 114.
Ind. Eng. Chem. Process Des. Dev. 1981, 20, 331-339 Appleby, W. G.; Gibson, J. W.; Good,G. M. Ind. Eng. Chem. Process Des. Dev. 1062, 7, 102. Beeckman, J. W.; Froment, G. F. Ind. Eng. Chem. Fundam. 1070, 78, 245. Breck, D. W. “Zeolite Molecular Sieves”, Why: New York, 1974; p 614. Butt, J. B. Adv. Chem. Ser. 1072, No. 709, 259. Butt, J. 8.; Delgado-Diaz, S.; Muno. W. E. J. Cafal. 1075, 37, 158. Butt, J. B. J. Cafal. 1076. 47, 190. Clar. E. “Polycycllc Hydrocarbons”, Vol. I and 11. Academic Press: London, 1964. Eberly, P. E.; Klmberlln, C. N.; Mliler, W. H.; Drushel, H. V. Ind. fng. Chem. Process Des. Dev. 1066, 5, 193. Elsenbach, 0.;Gallei, E. J. Cafal. 1070, 56, 377. Enterman, W.; van Leuven, H. C. E. Anal. Chem. 1072, 44, 589. Haldeman, R. G.; Bow, M. C. J. Phys. Chem. 1058, 63, 489. Langner, B. E. J. Cafal. 1060, 65, 416. Langner, B. E.; Meyer, S. In “Catalyst Deactivation, Proceedings of the International Symposium”, Delmon, B.; Froment, G. F., Ed.; Elsevier: Amsterdam, 1980 p 91. Mlron, S.; Lee, R. J. J . Chem. fng. Data 1063, 8 , 150.
331
Ozawa, Y.; Blschoff, K. Ind. fng. Chem. Process Des. Dev. 1068r, 7 , 72. Ozawa, Y.; Blschoff, K. Ind. Eng. Chem. Process Des. Dev. 1068b, 7, 67. Pachovsky, R. A.; Best, D.; Wojciechowskl, B. W. Ind. fng. Chem. Process Des. Dev. 1073, 72, 254. Poutsma, M. L. ACSh4onogr. 1076, No. 777. Rollmann, L. D. J. Catel. 1077, 47, 113. Rollmann. L. D.; Walsh. D. E. J. Cafal. 1070. 56, 139. Suatoni, J. C. In ”Chromatography In Petroleum Analyses”, Altgelt, K. H.; Gouw, T. H., Ed.; Dekker: New York, 1979; p 121. Venuto, P. B.; Hamilton, L. A. Ind. fng. Chem. Prod. Res. Dev. 1067, 6. 190. Voorhles, A. Ind. fng. Chem. 1945, 37, 318. Walsh, D. E.; Rollmann, L. D. J. Catal. 1077, 49, 369. Walsh, D. E.; Rollmann, L. D. J. Catal. 1070, 58, 195.
Received for review July 21, 1980 Accepted November 24. 1980
UNIFAC Parameter Table for Prediction of Liquid-Liquid Equilibria Thomas Magnussen DECHEMA, Postfach 97 01 46, Frankfurt am Main 97, Wesf Germany
Peter Rasmussen and Aage Fredenslund” Insfltuftef for Kemlteknik, Danmarks Tekniske H0jskole, 2800 Lyngby, Denmark
A UNIFAC groupinteraction parameter table especially suited for prediction of liquid-liquid equilibria at temperatures between 10 and 40 O C has been developed. A total of 512 binary parameters representing the interactions between 32 different groups have been determined on the basis of approximately 100 binary and 300 ternary liquid-liquid equilibrium data sets. The parameters were estimated so that the reported mole fractions are represented as well as possible. The mean absolute deviation between experimental and predicted equilibrium composition is 2 mol % . Care was taken to reduce problems associated with false solutions to the liquid-liquid equilibrium criterion.
Introduction Liquid-liquid equilibria (LLE) have in recent years gained increased interest in chemical technology. Due to the rising cost of energy, new separation processes based on extraction are becoming more attractive than before. Also, it may be feasible to operate known processes at new conditions, necessitating checks for liquid-phase stability at various points on the flow sheet. Thus, the need for calculating and predicting LLE compositions has very much increased. In principle, LLE compositions may be calculated using any model for the excess Gibbs energy. Unfortunately-due to our lack of understanding of the behavior of liquids-it is not possible to quantitatively predict multicomponent LLE from binary data only or to predict LLE compositions using model parameters based on vapor-liquid equilibrium (VLE) data. This is shown in Table I. If six UNIQUAC binary interaction parameters are fitted individually to each of the 17 ternary LLE data sets used as test mixtures by Sarensen et al. (1979a,b), the overall mean absolute deviation between experimental and calculated LLE compositions is 0.5 mol %. When the ternary LLE compositions are predicted using binary LLE and VLE data only, the deviation is 3.7 mol %. When the UNIFAC parameter table by Skjold-Jerrgensen et al. (1979), which is based on VLE data is used, the deviation is 9.2 mol % . These predictions are clearly unsatisfactory compared to the individual fits.
Table I. Absolute Mean Deviation (Mole %) between Experimental and Calculated Mole Fractions for 17 Ternary LLE Test Systems (Sorensen et al., 1979) UNIQUAC
0.48
UNIQUAC UNIQUAC
individual f i t t o each of the 1 7 systems “common parameters” prediction from binaries
UNIFAC UNIFAC
LLE parameters VLE parameters
1.73 9.22
1.0 3.65
In order to predict multicomponent LLE as well as possible with the presently available models for the excess Gibbs energy, it is necessary to base the model parameters on binary and ternary LLE data. However, since the number of components of interest in LLE applications is very large, the data needed for obtainiig these parameters are often not available. The purpose of this article is to present a UNIFAC group-contribution parameter table especially suited for the prediction of LLE. We have used available LLE data (Sarensen and Arlt, 1979; see also Sarensen et al., 1979a,b) to calculate group-interaction parameters. These may be used to predict LLE compositions in mixtures for which no data are available, as long as the mixtures in question can be constructed from groups for which parameters are available. The UNIFAC model is that described in detail by Fredenslund et al. (1975,1977) and by Skjold-Jargensen et al. (1979). The same model is used in this work; only 0 1981 American Chemical Society
