994
Ind. Eng. C h e m . Res. 1990,29, 994-1003
Menzel, T.; Kantorek, H. J.; Franz, K.; Buchholz, R.; Onken, U. Zur Str6mungsstruktur in Airlift-Schlaufenreaktoren. Chem.-Zng.Tech. 1985,57,139. Menzel, T.; Jiiger, W. R.; Ewald, G.; Onken, U. Lokale fluiddynamische Parameter in Blasensiiulenreaktoren bei hoheren Fliissigkeitsviskositiiten. Chem.-Zng.-Tech. 1989,61, 70. Miyauchi, T.; Shyu, C. N. Flow of Fluid in Gas Bubble Columns. Kagaku Kogaku 1970,34, 958. Miyauchi, T.; Furusaki, S.; Morooka, S.; Ikeda, Y. Transport Phenomena and Reaction in Fluidized Catalyst Beds. Adu. Chem. 275. Eng. 1981,..11, Prandtl, L. Uber die ausgebildete Turbulenz. 2. Angew. Math. Mech. 1925,5 , 136. Reichardt, H. Vollstiindige Darstellung der turbulenten Geschwindigkeitsverteilung in glatten Rohren. 2. Angew. Math. Mech. 1951,31,208. Riquarts, H. P. Stromungsmechanische Modellierung von Blasensaulenreaktoren. Chem.-1ng.--Tech. 1982,54,770.
Rustemeyer, U.; Pauli, J.; Menzel, T.; Buchholz, R.; Onken, U. Liquid-Phase Mixing Model for Bubble Columns in Consideration of the Hydrodynamics. Chem. Eng. Technol. 1989,in press. Schogerl, K.; Liibbert, A.; Korte, T.; Diekmann, J. Mefkechnik zur Charakterisierung von Gas/Fl&sigkeitsreaktoren. Chem. --Ing. -Tech. 1985,57,641. Sekizawa, T.; Kubota, H.; Chung, W. C. Apparent Slip Velocity With Recirculating Turbulent Flow in Bubble Columns. J.Chem. Eng. Jpn. 1983,16, 327. Serizawa, A.; Kataoka, I,; Michijoshi, I. Turbulence Structure of Air-Water Bubbly Flow. Znt. J. Multiphase Flow 1975,2, 221. Ueyama, K.;Miyauchi, T. Properties of Recirculating Turbulent Two Phase Flow in Gas Bubble Columna. MChE J. 1979,25,258. Zlokarnik, M. Verfahrenstechnik der aeroben Abwasserreinigung. Chem.-1ng.-Tech. 1982,54,939.
Received f o r review September 6, 1989 Accepted January 5, 1990
Percolation and Blocking in Supported Liquid-Phase Catalysts: Styrene Catalyst as a Particular Case Ewald Wicke and Achim Bartsch* Znstitut fur Physikalische Chemie der Uniuersitdt Miinster, SchloDplatz 4,0-4400 Munster, FRG
Supported liquid-phase catalysts (SLPCs) provide promising possibilities for heterogenization of homogeneous catalysis. On the other hand, there are catalysts with liquid components at reaction conditions that have a well-established and technically important tradition, e.g., the catalysts for SO2 oxidation and for styrene production. The consistent results of a reaction rate maximum with increasing liquid loading of the porous support led t o the development of different theories; those that prevailed were based on the blocking concept. Recent developments of percolation theory set this concept on the more general basis of mathematical models. The styrene catalyst, containing a melt of KOH/K2C03 as promoter in a bulky framework of magnetite crystal needles, represents a special kind of a SLPC. At the reaction temperature of 600 " C , the transport behavior of gases in the porous structure and the distribution of the melt (blocking effects) were investigated by stationary diffusion measurements. A number of problems, e.g., instability of the melt a t 600 "C, were solved successfully. Measurements with different potassium contents in the support provided the basis for process modeling in a single-pellet and in a fixed-bed reactor. 1. Supported Liquid-Phase Catalysts 1.1. General Information. SLP (supported liquid phase) catalysts are catalysts whose active components are dispersed on an usually porous support material and are either totally or at least partially liquid under reaction conditions. In view of the sites of reaction, there are three groups of SLP catalysts to be distinguished: Group I: homogeneous reaction in the volume of the liquid phase; examples, transition-metal complexes as catalytic centers in a solvent with low vapor pressure under reaction conditions (heterogenized homogeneous catalysis), Vz05/ KzS2O7melt for SOp oxidation (Villadsen and Livbjerg 1978). Group 11: reaction at the surface of the liquid phase; examples, liquid metals (alkali metals, tin, zinc) (Kenney, 1975). Group 111: reactions at the phase boundaries liquid phase/solid framework; this occurs when the liquid as well as the solid contains active components and/or promoters; example, styrene catalysts on the basis Fe304/KOH. Special advantages of the fine dispersion of the catalyst on a porous support are the large gaslliquid exchange area,
*Presentaddreas: Technische UniversiGt Hamburg-Harburg, Verfahrenstechnik IV, Eipendorfer Str. 40, D-2100Hamburg 90, FRG. 0888-5885/90/2629-0994$02.50/0
short diffusion paths in the liquid phase, and the performance of the reaction in a fixed bed reactor that can easily be operated in chemical engineering. A further advantage is the ease of separation of the gaseous reaction products from the liquid catalyst phase. As recent investigations have shown (Ohlrogge 1988), it seems to be possible to apply SLP catalysts in a fluidized-bed reactor, too. There also occur some specific problems of the SLPC, such as, e.g., loss of liquid phase by conveyance of vapor with the reaction gas flow due to the small, but still perceptible, vapor pressure of the liquid. Some important examples for application of SLP catalysts are listed in Table I. For future developments, the SLP technique offers promising applications of coordinatively unsaturated transition-metal complexes as heterogenized catalysts. Above all, the usually low working temperatures and the high selectivities, especially for reactions as complicated as oxo synthesis and a large number of hydrocarbon transformations (Parshall, 1980; Henrici-Oliv6 and Oliv6, 1977; Collmann and Hegedus, 1980), are outstanding features of these catalysts used in a solvent with low vapor pressure-one of the best known is the Wilkinson catalyst (Osborn et al., 1966). Hydroformylation (Rony, 1969; Gerritsen et al., 1980), isomerization (Acres et al., 1966; Rony, 1975),and hydrogenation of hydrocarbons are some 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 995 Table I. Technical Applications of SLP Catalysts reaction catalyst (+ support) CuCl promoted with rare earth and alkali oxychlorination chlorides (Deacon catalyst) oxidative chlorination of hydrocarbons same as above oligomerization of alkenes HSP04/P206 SOz oxidation V'20E./K2S207
references Kenney (1975),Villadsen and Livbjerg (1981 Allen and Clark (1971),Dotaen 1974) Villadsen and Livbjerg (1978) Grydgaard et al. (1978),Livbjerg et al. (1974, 1976),Simecek (1970) Wainwright and Forster (1979) 'Komiyama and Inoue (1977)
'
o-xylene ethylene
--
phthalic acid anhydride acetaldehyde
___----
-
VzOs + alkali PdCl2/CuCl2 (aq sol.)
Mass Transfer Film Porous System with Liquid Loading
Figure 1. Steps of transport processes in an isothermal reaction at a SLP Catalyst (Hesse, 1982). Mass transport (1) a c r m the external m a s transfer f h ,(2)in the free pore space of the support, (3) across the gas/liquid interface, (4)within the liquid film, (5)blocking of narrow pore sections by the liquid phase.
examples of reactions of transition-metal complexes in SLP catalysts. 1.2. Reaction Rate and Extent of Liquid Loading. The reaction behavior of an SLP catalyst is determined by a number of steps in transport processes that are different from the usual supported catalysts (Hesse, 1982). As shown in Figure 1,the reaction components first pass through the mass-transfer film at the external surface of the catalyst pellet and then diffuse into the free pore space up to the gas/liquid boundary and, in most cases, into the liquid phase. The reaction products are leaving the supported catalyst in the reverse direction. The "internal" transport processes, Le., steps 2-4, depend strongly on the extent of liquid loading, Le., on the ratio q = VL/Vp (= "loading") where VL is the volume of liquid charged into the support and Vp is its pore volume. As a rule-but not in either case-the pore structure can be considered as a rigid one; thus, shrinking or swelling processes are dispensable (an exception is the Fe304/KOH catalyst with high alkali contents; see section 2.5). The higher the extent of loading, the stronger are the small pores blocked for gas transport by the liquid phase. Therefore, the loading exerts two opposite influences on
1
.
0
the rate of chemical conversion: On the one hand a high loading provides a large amount of catalyst, but on the other the pores become more and more clogged. Essential investigations of the dependence of the extent of conversion on the loading have been carried out by Rony (1969, 1975), taking as an example the hydroformylation of propylene with RhCl(CO)(PPh& in butyl benzyl phthalate solution on SiO, as catalyst. Independently of the residence time, Rony found a maximum of conversion in the loading range 0.4 Iq I0.6. A similar behavior was observed by Heinrich (1979),Kretschmer (1980), and Sulistyo (1983) at other SLPC systems. Heinrich investigated the oxidation of ethylene to acetaldehyde with palladium(Wtrifluoracetate complex as a catalyst and copper(I1) acetate and trifluoracetate as promoter in benzoic acid ethyl ester as a solvent. A maximum of conversion always was attained at q = 0.5, see Figure 2a, independently of the types of glass frit pellets used as porous supports (from Schott, Mainz, FRG, specifications D2,D3,and D4 with pore radii of 35 f 10 pm, 15 f 5 pm, and 6 f 1.5 pm, respectively). The same behavior is displayed by the technical support material Ed 19/2 (Alumosilica,Heraeus, Hanau, mean pore radius 3 pm). When hydrogenating ethylene with H2PtC&/SnC1,in phosphorus acid tributyl ester as a solvent on glass frit pellets as a support, Kretschmer found that the maximum conversion is also in the range 0.4Iq 5 0.6, Figure 2b. Similar results were obtained by Sulistyo on the hydroformylation of ethylene at the rhodium complex HRh(CO)(PPh3), in butyl benzyl phthalate as a solvent, Figure 2c. The SLP catalysts for these investigations were prepared by soaking the porous pellets with a*mixtureof low-volatile catalyst solution and a highly volatile solvent-for instance, chloroform or methanol-by alternating evacuation and repressing with an inert gas. Subsequently, the volatile solvent was removed by slow evaporation into an inert
7
la)
I
L
Ob
qd2 0:C d6 R8
q-
Figure 2. Conversion in the liquid phase and reaction rate vs liquid loading, q, for (a) ethylene oxidation (Heinrich, 1979), (b) ethylene hydrogenation (Kretschmer, 1980), and (c) ethylene hydroformylation (Sulistyo, 1983) at different SLP catalysts (see text).
996 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
carrier gas and cautious evacuation. This procedure guaranteed a uniform distribution of the catalyst solution within the porous pellets. Such distribution cannot be expected when in bimodal pore systems the microporous part is filled up first or when by the method applied for charging the pellets an external shell only is soaked with the liquid. This may have occurred in the investigations of Shaw et al. (1988), who filled the solvent of the catalyst salts (water) by condensation of water vapor from the feed flow-in the presence of He as a carrier gas-into microporous pellets ( r = 5 nm), provided before with the (dry) catalyst salts. They found the rate maximum at a liquid loading of q = 0.1 only, presumably because He remained enclosed within the pellets during this mode of charging. The first attempt to understand the rate maximum (Rony, 1968) was based on the model of a cylindrical dead-end pore, the wall of which is covered with a liquid film in the beginning (i.e., at small q) and is then filled up with liquid from the end of the pore with increasing q. As long as the film thickness only increases with increasing q, the reaction rate was expected to grow, but to decrease again when the liquid begins to fill up the cross section of the pores. Accordingly, a maximum conversion (or reaction rate) with increasing loading, i.e., an optimal value, qW, was foreseen. Thus, the model described the behavior qualitatively, but it turned out to be unsatisfactory with regard to quantitative results. Abed and Rinker (1973) thought that the maximum conversion is due to the fact that the reaction rate in the SLP catalyst is increasingly limited by diffusion at growing loading. They investigated the influence of the loading upon the pore diffusion by measuring the effective diffusion coefficients of a technical supported catalyst (Girdler T-708) loaded with ethylene glycol. Thereby they found a simple relation between loading and permeability (q): \k = 9,(1 - q ) 2
(1) where ‘kois the permeability of the unloaded support. The permeability is defined as the ratio of the effective diffusion coefficient, Deff,of a gas couple used for the measurements within the pore structure to the diffusion coefficient, DG,of the same gas couple in the free gas phase: ‘k = Defl/Dc (Den measured at gas pressures where no Knudsen diffusion is involved). Thus, eq 1can be written as
Deff = DG*,(~
(2) The authors assumed that a similar context is valid for the diffusion in the liquid phase and completed eq 2 as follows: D*er = DGqo(1 - qI2 + DLH\koq2 (3) (1)’
Here DL is the diffusion coefficient of the minority component of the reaction mixture in the liquid phase and H is its Henry constant. By introducing D*sffinto the calculation of the effectiveness factor or the Thiele modulus, the authors obtained a context between loading and reaction rate that yielded a rate maximum. In this model, which has recently been improved by Datta et al. (Datta and Rinker, 1985; Datta et al., 1985a,b),the position of the maximum on the q axis, however, depends on the extent of pore diffusion influence without liquid loading. In the case where this influence is small and the solubility of the reaction components in the liquid phase, i.e., Henry’s constant in eq 3, is large, the calculated position of the maximum shifts up to q = 1 (Datta et al., 1985b) in that case. the diffusion in the liquid phase is of main importance for the effective diffusion coefficient in eq 3). The ad hoc statement of eq 3 is indeed not convincing. Assuming the liquid would be distributed over the walls
of the pores, one had to start with a model of parallel filmlike diffusion in the gas and the liquid phase in direction of the pore axis:
D*eff= D ~ q o ( 1 9) + DLHqoq
(4)
A model of this type, however, cannot lead to the factor (1 - q ) 2 , which has been found by measurements of gas diffusion in the free pore space, eq 2. Therefore, it is obvious that the assumption of a uniform liquid film along the walls of the pore is not correct. The same conclusion has been drawn by Livbjerg et al. (1976) when they investigated the V2O5/K2SzO,SLP catalyst for oxidation of S02-the technically most important SLP catalyst at present-in order to examine the model of Abed and Rinker. The results contradicted the predictions of the model and led to the idea that the melt will form plugs and clusters in narrows of the pore structure with increasing loading and thereby will block certain ranges of this structure for the reaction. By means of microprobe records and electron microscope pictures, clusters of the melt of ca. 25-pm size could be made visible (Grydgaard et al. (1978), between which finely dispersed melt provides the main amount of catalyst surface for the reaction. 1.3. Percolation and Blocking in the Pore Structure. The concept of blocking of the pore structure by liquid plugs was supported further by diffusion measurements (Hollwedel-Gropmann, 1978; Wicke and Hesse, 1984) performed in a gas pressure range of 0.01-1 bar at glass frits of specification D4, which were loaded by different amounts of squalane. The measurements were carried out by means of the ortho-hydrogen-para-hydrogen method in a diffusion-reaction cell (Hugo and Wicke, 1968). The results for the relative permeabilities q(q)/\ko dependent on the loading are illustrated for two measured probes in Figure 3. The values decrease monotoneously with increasing loading approximately after eq 1, but they tend to zero already at q = 0.8. By means of the relations 4 9 ) = to(1
- 4)
\k = t X
(5)
(6)
where t ( q ) and to are the porosities at loading q and q = 0, respectively, and X is the labyrinth factor (A = 1/7, where T E tortuosity), the relative labyrinth factor can be calculated by (7) Its value decreases with increasing loading and also tends to zero at q = 0.8, Figure 3b. Thus, the permeability of the porous structure is disappearing when its porosity still has finite values. As shown by Hugo (1974), such a behavior can be expected generally in the case of sintered materials and of compacts with porous structure. With increasing pressure and decreasing porosity, one connection between neighboring pore elements after the other becomes closed, whereby the whole network of pores disintegrates more and more into single subnets, largely isolated from one another. A statistical consideration led Hugo to the relation X = ( 5 P - 1)/4
(8)
between the labyrinth factor and the remaining porosity, with the parameter values 0.75 I m I1; in these values, specialities of the pore structure are affected. According to eq 8, the limiting (or “critical”) porosities, where X 0, are in the range 0.12 5 c, I 0.2.
-
Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 997
01
02
03
:-Lo-.
OL
05
0.6
0.7
, 08
Figure 4. Scheme of a Bethe network (or Bethe 'tree") with coordination number 2 = 3.
0.21
t
'b'
\ q-
01 0.2 a3 O.L 0.5 0.6 0.7 0.8 Figure 3. Relative values of the permeabilities (a) and labyrinth factors (b) on two samples (0,0)of D4 glass frits vs loading with squalane (Hollwedel-GroBmann,1978).
Monte Carlo calculations of gas diffusion through porous model systems-disordered assemblages of spheres or sponge structures-resulted in similar values of the limiting porosity (Nakano and Evans, 1983). Broadbent and Hammersley (1957) were the first to point out, when establishing the theory of percolation, that mass transport through media that are dispersed by chance can occur only above a minimum value of porosity, the so-called percolation threshold. Within the scope of this theory, a network model, where from each branching point the same number (2) of connections lead to neighboring nodes, is often applied (Mohanty et al., 1982; Reyes and Jensen, 1985). Figure 4 demonstrates such a so-called Bethe network with the coordination number Z = 3. The calculation of the percolation threshold is easy to perform by statistically disordered stoppages of an increasing number of nodes and yields the limiting value of the porosity-Le., the fraction of nodes not yet stopped-for disappearing permeability: tc = l / ( Z - 1) (9) As a comparison between experimental and computer results of the permeability of different pore systems shows (Mohanty et al., 1982), the coordination numbers of the Bethe networks, which have to be chosen to fit the behavior of the model systems to that of real pore systems, are in the small range 4 I 2 I7. After eq 9 this corresponds to critical porosities of 0.17 I t, I0.33 and is in relatively good agreement with the former results from the Hugo equation (eq 8). By means of these conclusions, it is possible now to refer to the questions in section 1.2. According to percolation theory, one has to consider that the limiting porosity is the sum of the remaining hollow spaces in the pore structure that are totally enclosed by liquid plugs and clusters
(isolated pores) and are therefore not accessible from the outside. Thus, only the part - 'c = eo(qc - q ) (10) of the porosity that is left behind at liquid loading [t(q) = t o ( l - q ) ] is accessible to the reaction components from outside. qc in eq 10 is the loading that leaves open only the limiting porosity, t, = to(l - q,). The reduction of the accessible pore volume-and thus of the available gas/ liquid exchange interface-is the actual cause for the tendency of the reaction rate to decrease with increasing liquid loading. Therefore, one can formulate for the reaction rate r a d q c - q)? (11) wherefrom a maximum results at q(=qopt) = q c / ( y + 1). Since an exponent of y i= 0.5 is to be expected (Mohanty et al. 1982) for the increase of the accessible pore volume (from zero at q = q,) with decreasing q, one gets qopt = 2/3qc. The critical limiting value of the liquid loading is in the range 0.7 < q, < 0.9 (see, e.g., Figure 3); thus, the range 0.3 < qopt < 0.6 is to be expected from eq 11, in agreement with the measurements (e.g., Figure 2). Hence, the characteristic blocking parameter in this model is the limiting loading for disappearing permeability of the porous structure, which can be determined most reliably. It should be noted, moreover, that the blocking model and eq 11need a correction that is, however, usually small. The percolation theory and the Bethe network refer to the interior of the pore system, not to a layer immediately beneath the external surface when the blocking is diminished by breaking off effects of the porous framework at the border to the exterior. The thickness of this border layer of increased accessibility is expected to equal a few pore diameters only and will contribute usually only little to the overall reaction rate. Hesse and Hoffmeister (1987) have chosen a different possibility to determine the effect of blocking. They measured the reduction of the BET surface with increasing loading by adsorption of N2 at 77 K (where the liquid loading is frozen). The reduction of the surface can be described sufficiently well by S(q) = So(l- q)" with 1 C n < 2 and is identified with the gas/liquid exchange area accessible for the reaction. This yields for the reaction rate r 0: q(1 - 4)" (12) which results in a maximum at q (=qopt) = 1/(1+ n),Le., in the range 0.3 < qopt < 0.5.
998 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
Which one of the methods presented here for considering the blocking process is to be preferred in general cannot be decided at present. Nevertheless, it may be stated that the method of diffusion measurements is practicable for large groups of different systems and can easily be adapted to different testing conditions. Some examples will be given in the following sections, dealing with investigations on the catalyst for production of styrene from ethylbenzene. 2. Investigations on Styrene Catalysts Based on FeJOl (Magnetite) 2.1. General Information. Due to its worldwide capacity of about 12 X lo6 tons/year (1980), styrene is one of the mmt important substances of chemical industry and is, except for the polymerization to polystyrene, the basis for many other products (Lieb and Hildebrand, 1982). It is produced almost exclusively by dehydrogenation of ethylbenzene: PhCH2CH3 PhCH=CH2 + H2 (1) which runs with high selectivity for catalysts based on magnetite, Fe304,at temperatures of about 600 “C. Byproducts are small amounts of benzene, toluene, and other hydrocarbons down to C2H4 and CH4 as well as CO and C02 (from reactions with added steam). The aim of investigations on the styrene catalyst was and always is a further increase of selectivity as well as of activity and operating time and, simultaneously, a decrease in the working temperature of the catalyst. To intensify research, the BMFT (Bundesminister fur Forschung und Technologie, Bonn, FRG) had initiated a cooperation between several working groups at universities and at BASF AG within the framework of a project of “SLP catalysts”. Our group at Munster dealt with the problem of “diffusion transport” within this project (Bartsch, 1988), and in the following the methods of investigation and the results are presented. Since dehydrogenation of ethylbenzene is a reaction with volume increase, the equilibrium can be shifted in favor of the styrene yield by adding a diluting gas. Usually steam is applied for dilution and, simultaneously, for introducing part of the heat necessary for the endothermic reaction. Furthermore, steam delays the coking of the catalyst and keeps the oxidation state of iron in the range of optimum selectivity (Lee, 1973): ++
Due to recrystallization processes and coking, catalysts from pure magnetite become quickly deactivated. The recrystallization can be inhibited by adding chromium oxide as a structural promoter, and the addition of potassium (in the forms of KOH and K2CO3) leads to a “self-regeneration” of the catalyst by promoting the gasification of the coke depositions by steam. Moreover, the small electronic work function of the potassium leads to a strong increase of the activity of the iron oxide (Lee, 1973, Figure 2) as well as to the neutralization of acid centers of the catalyst. The formation of benzene is thereby reduced and the formation of styrene-of toluene as well-is promoted (Shibata and Kiyoura, 1969). Due to the high activity of the potassium-promoted catalysts, the reaction rate is often inhibited by pore diffusion, as has been demonstrated by Lee (1973) with catalysts of different grain sizes and by Mocerarov et al. (1974) with catalysts of different porosity. The potassium promoter, however, contributes to the deactivation of the styrene catalyst, too. Microprobe re-
Reactor Entrance Reactor Exlt Figure 5. Composition of KOH/K2C03 mixture in the pellets (wt %, mean values) along the catalyst bed (Mrop, 1983).
cords of the cross section of a catalyst pellet and potassium analysis along a fixed bed reactor made by Lee (1973) and by MroP (1983) revealed that the potassium moves with increasing time of use: in the single pellet it accumulates in the center; along the reactor bed it becomes enriched at the end. By careful grinding of aged catalysts (time of use 20000 h), Lee (1973) prepared samples from the center and from the external surface of the pellet, both showing just small activity in a differential reactor. The samples taken from the center had a K20 content of 20-30% by weight, whereas the samples taken from the margin were nearly free of potassium (the fresh catalyst had a K20 content of 4.2%). The active region of an aged catalyst is therefore located in a small zone between the margin and center of the pellet. One reason, among others, for the redistribution of the potassium is certainly the fact, as pointed out recently by MroP (1987),that styrene reacts faster with steam under development of C02 than ethylbenzene does. As a consequence, the partial pressure of COzincreases toward the center of the pellets and toward the end of the reactor. Furthermore, at reaction temperatures of about 600 OC, KOH is noticeably volatile (vapor pressure 0.1 mbar (Landolt-Bornstein, 1962)). This leads to a separation and an enrichment of solid K2C03in the regions of higher C02 partial pressures, i.e., in the center of the pellets and toward the end of the reactor. According to this, potassium compounds change in nature along the catalyst bed, see Figure 5. In the entrance section of the reactor, the preponderant compound is KOH, while toward the end the fraction of K2C03increases more and more. The effect (Mrop, 1987) that the C02 partial pressure, being higher at the end of the reactor, shifts the equilibrium hydroxide/carbonate toward K2C03
e H8°
K2C03
KOH
(111)
operates in the same direction. The question of whether the styrene catalyst is an SLPC, i.e., if there is a melt in the catalyst under reaction conditions or not, has to do with the potassium compounds only. As seen in the phase diagram, Figure 6, a mixture of KOH/K2C03up to a carbonate content of 60% is liquid at 600 “C, the typical temperature for styrene synthesis. Therefore, the alkali should be a melt, at least at the reactor entrance. The phase diagram, however, is based on samples free of water. An essential decrease of the melting point could be achieved by the high steam content in the reaction mixture-water-containing KOH shows decreases of the melting point of some 100 deg (Landolt-Bornstein, 1962)-it is possible, therefore, that KOH/K2C03mixtures with more than 60% K2C03are liquid, too. 2.2. Determination of Pore Structure Parameters of the Styrene Catalyst. When permeability, 9,and mean radius of the transport pores, P, are determined by
Ind. Eng. Chem. Res., Vol. 29, No. 6,1990 999 are the cross-sectional area and thickness of the diskshaped sample. c is the total concentration of the gas mixture 1, 2; x: and x! are the mole fractions of as 1 in the upper and the lower mixing chamber; and D, is the Knudsen diffusion coefficient of gas 1. The quantity a = 1 - (M2/M1)1/2 considers the influence of the different molecular weights of the two gases. Equation 13 is fitted to the measured values of the diffusion fluxes J1vs P with a small computer by nonlinear regression; herefrom the values of the permeability and of the Knudsen diffusion coefficient, DY,are obtained. With DF = 2/3iW1
I
0
20
60
LO
K,CO,
80
100
(Wt.-%l
Figure 6. Phase diagram of the KOH/K2COBsystem (Janz and Tomkins, 1983): (e) range of melt at 600 "C. lb21
1
AP-0
L!
2
2(+11
(4
c
*r
0
uognetc
WIWI
VWWm
(b) Figure 7. Experimental setup: (a) diffusion/reaction cell with gas streams, schematic; (b) total arrangement for stationary diffusion measurements and in situ reaction (Bartsch, 1988).
diffusion measurements, the liquid phase has to have the same distribution in the porous structure as under reaction conditions. Due to the high working temperature of 600 "C and to the catalytic activity of the samples to be investigated for the o-H2-pH2 conversion, it was not possible to use the o-H2-p-H2method in this case. Therefore, the nowadays mostly practized method of stationary countercurrent diffusion of two gases that are inert in the measuring system was used (Wicke and Kallenbach, 1941). The coie of this system is shown schematically in Figure 7a. Gases 1and 2 are guided through two mixing chambers along the faces of a disk-shaped porous sample (hatched in Figure 7). The diffusive flux,J1,of component 1across the sample is determined by analysis of one of the emerging gas flows, e.g., 2 (+l). Measurements were carried out under different pressures in the range of about 70 to loo0 mbar and were evaluated by (Evans, et al., 1961)
where the symbols have the following meanings: D12is the binary diffusion coefficient of the gas pair 1, 2; F and L
( c i j , is the mean molecular velocity), the Knudsen coefficient yields the second pore structure parameter, i.e., the pore radius of the transport pores. Since the Knudsen terms in eq 13 represent small additional quantities only, measurements along a rather broad pressure range are necessary to obtain fairly reliable values of DT and thus of the mean radii (from about 10 measuring points within the range 70 mbar IP I1000 mbar, the P values were obtained with an uncertainty of ca. &lo%). The first testings with the gas pair CH,/N2 showed, however (Bartsch, 1988), that each alteration of the gas pressure changed the distribution of the melt in the pore structure. In order to obtain reproducible results, first the measurements therefore were done over the range of temperatures in question under constant pressure, and then the measuring system was cooled down beneath the melting point of the KOH/K2C03 mixture before the pressure was raised up to the next level. This method too, however, yielded no reproducible results within the range of the reaction temperature of about 600 "C. As it turned out, the K&O3 permanently lost C02 in the N2/CH, flow, and as a consequence, the permeability of the sample continuously changed during the measurement. This could be avoided by using COz instead of CH, as the measurng gas, combined with N2 or Ar. At the same time, this procedure was useful to solidify the KOH part of the melt to KzCO3. In order to make sure that the pore structure parameters obtained in this way correspond to reaction conditions, the diffusion apparatus was modified in such a way that the catalyst could be treated by reaction with a mixture of water and ethylbenzene inside the diffusion measuring cell itself. 2.3. Experimental Arrangement. The apparatus (Bartsch, 1988) used for the stationary diffusion measurements and in situ reaction is presented in Figure 7. Its main parts are the supply of measuring gases and pressure regulation; gas analysis, pressure measurement, and control; diffusion cell and reactor; data record and evaluation system; and vaporizer and cooler for the reaction mixture. This open flow system has the advantage that long-term alterations of the pore system can be perceived, too. The flow rates of the measuring gas fluxes are adjusted by thermal mass flow controllers v, and v2. The gas pressure is kept constant by means of an electronic system (P) which consists of a pressure transmitter (piezo-resistive semiconductor) and a regulating valve with the pertinent amplification and control electronics. Thus,it was possible to adjust the pressure in the range 70-1000 mbar over long times to f l mbar at gas flow rates of 5-6 mL/s (STP). The pressure difference (AP)between the two mixing chambers is kept constant at nearly zero by adjusting the precision needle valves, N1 and N2, with the help of the differential manometer, D1 (MKS Baratron), and is sur-
1000 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
rDlnml
(a)
U
lcm
rpInml
Figure 9. Distribution of pore radii, determined by Hg porosimetry: (a) original catalyst sample (BASF),(b) pressed in the sample holder after crushing.
(bl
Figure 8. Diffwion cell: (a) cross section with sample (marked by black), (b) view to the construction with the gas guiding tubes.
veyed continuously by a separate digital multimeter. The deviations from zero were underneath AP = fO.O1 mbar. The COPcontent in the nitrogen or in the argon flow is analyzed by nondispersive IR spectroscopy (URAS 3E, Hartmann & Braun, Frankfurt). The gas pressure inside the URAS apparatus is determined by means of the differential manometer, D2, in connection with D3, which measures the pressure difference against vacuum (C0.1 mbar), i.e., the total gas pressure. In order to comply with the measuring condtions-20-600 "C, 70-1000 mbar-the diffusion cell illustrated in Figure 8 has been developed. It is made from heat-resistant high-grade steel and consists of two flanges between which a special fine steel ring as a sample holder is pressed by means of fine steel screws. Both flanges, each with one-half of the sample holder, make up the two mixing chambers above and underneath the sample. Each chamber is equipped with three pipe lines (inner diameter 4 mm), staggered by 60°, which serve for delivery and outlet of the gas fluxes as well as for connection with the differential manometer, D1. At the front of the sample holder as well as on the sides of the flanges facing the holder, there are special cutting edges installed which are kept reliably tight with Cu disks as sealing material, even if temperature cycles occur. The samples, consisting of catalyst material that had been crushed before, were directly pressed into the holder by means of a pressing tool made from tempered and polished fine steel (pressure between 10 and 50 bar). By this procedure, the powder was pressed to the same apparent density as that of the original sample supplied by BASF
AG. Tests to glue the original sample into the holder have failed, because no cement has been found that could overcome the extreme changes of temperature. The measured data, necessary for determining the stationary diffusion flux of a single measurement, were programmed into a small computer (HP-86b, Hewlett-Packard) via a digital voltmeter with a scanner (10 channels; PREMA, Mainz) for evaluation and storage. To control the scanner and to collect and evaluate the data after eq 13, a BASIC program was used. During in situ reaction at the catalyst samples in the holders, the components of ethylbenzene and water were taken out of their storages by means of peristaltic pumps which precisely kept constant the adjusted flow rate of the liquid (order of magnitude 0.001 mL/s). The two fluxes were connected and were led into an evaporator heated to 160 "C, see Figure 7. From there the vapor mixture ethylbenzene/water (x1/7 by volume) passed through the %port ball valves, H1 and H2, shifted accordingly,through both mixing chambers of the diffusion cell, heated to 600 "C, along the faces of the catalyst sample. Subsequently the mixture after reaction passed through the 3-port ball valve, H3,which had also been adjusted to "reaction", into a cooler where the vapors of the reaction componenh and the products were condensed and were available for analysis. 2.4. Results of the Measurements. In order to check how far the pore structure remained unchanged during the process mentioned above-to mortar the catalyst material and to press it into the holder-the surface area (onepoint BET method) and the pore size distribution (Hg porosimetry, PM== 800 bar) of a catalyst sample processed in this way were determined and compared with the values of an untreated specimen. As the results illustrated in Figure 9 show (we are grateful to Dr. Ambach and to Dr. Briicker of BASF AG, Ludwigshafen, for carrying out the measurements), there is no noticeable influence of this preparation method on the pore size distribution and on the porosity. Also the values of the surface area, 5.5 and 5.8 m2/g, respectively, show a negligible difference only. The pore size distribution points to a bimodal pore system
Ind. Eng. Chem. Res., Vol. 29, No. 6,1990 1001 2Lh Reaction
CO, 02
N2/C02 rlpml
103
1 1
t lhl
-
Figure 10. Alterations of the permeability, \k, and of the mean radius, F, of the transport pores after different pretreatmentsof the catalyst samples (details see text).
with macropores of about 0.1 pm and mesopores of about or even below 0.01 pm. The macropores are formed obviously by the fiber structure of the Fe304needle crystals (crystal shape of the basic material goethite) which are used for the styrene catalyst. Furthermore, it was essential to test how far the pore structure parameters of the original catalyst samples, that had been run with the reaction already at BASF AG, had been altered through this method of preparation. For that purpose the catalyst was subjected to different procedure steps and after each step the pore structure parameters were determined by diffusion measurements. The results are illustrated in Figure 10. It shows the changes of the permeability and of the mean radius of the transport pores after four consecutive steps of procedure as a function of time. A catalyst sample pressed into the holder was investigated first at ambient temperature with the gas pair N2/C02(result (1)in Figure 10). Afterwards it was heated up to 600 "C in N2/C02 atmosphere, and the diffusion measurements were repeated at this temperature (result (2)). The increase of temperature changed the permeability and the mean pore radii only slightly, and after a subsequent tempering time of ca. 20 h at 600 "C under N2/C02 atmosphere, a third measurement yielded the same parameter values (results (3)). Now a period of reaction followed, in order to bring the sample up to a state corresponding to reaction conditions. To this end a vapor mixture of ethylbenzene/water = 1/7 was led in situ over the catalyst for 24 h with a flow rate of 1 mL/s at 600 "C and 1bar of total pressure. Random tests of the condensate by IR spectroscopy showed 20% conversion. The KOH melt that had been formed from K2C03and HzO during the reaction period was now fixed in the pore structure (resolidified to K2C03) by a 3-h treatment with a flow of C02 at 600 "C (third processing step, Figure 10). The subsequent diffusion measurement (result (4)) showed in comparison with measurement ( 3 ) strong changes of the pore structure parameters. The KOH melt formed during the reaction period had obviously locked narrows and smaller transport pores, which resulted in an increase of the mean radius of the pores still effective for gas diffusion transport and in a decrease of the permeability of the sample. Subsequently the sample was left under a N2/C02flow for several hours at 600 "C, and the diffusion measurement ( 5 ) carried out afterward showed little changes only compared to (4). This series of measurements demonstrates that the potassium promoter in the sample that had been prepared by crushing and pressing was no longer in the same state of distribution in the pore structure as after its use for reaction at BASF AG but could be redistributed to an at
least similar state by a renewed interval of reaction in situ. Therefore, a reaction period is to be inserted prior to diffusion measurements on this SLP catalyst. Subsequently, the distribution of the melt can be fixed by treatment with COz. Further series of measurements indicated that a reaction period of 24 h is sufficient for establishing a stationary distribution of the melt in the pore structure and that a repetition of the reaction phase-for liquefying-and treatment with C02 for refixing the potassium promoter changes its distribution in the pore structure to a negligible extent only. 2.5. Measurements on Samples with Different Potassium Contents. The method developed so far turned out to be a reliable procedure to determine the pore structure parameters under reaction conditions. As the distribution of the potassium in the catalyst over longer periods of operation changes not only within the single pellets but also along the catalyst bed, as described above, this method was employed for diffusion measurements on samples with different potassium contents. The influence of this content on the values of \k and P under reaction conditions should thereby be determined, in order to get information for modeling the transport processes in the pellets in different sections of the catalyst bed. From the permeabilities, and the porosities measured at the same time, the labyrinth factors were calculated in order to find out whether the decrease of the permeability would be caused mainly by a decrease of the labyrinth factor. This then would point to the formation of dead-end pores and of liquid clusters that block direct diffusion paths. The porosity was measured by vacuum soaking of the pressed samples with ethylbenzene before the f i t reaction period and later on again after finishing the diffusion measurements. The samples for the measurements were prepared in such a way that their pore structure-prior to the addition of potassium-had the same apparent density as the original catalyst samples without potassium, Le., 1.67 g/mL. For this purpose, an always equal quantity of powder from mortared catalyst, free of potassium, was mixed with different additions of KzC03 (made of mortared, granular K2CO3, pure, Merck) and this mixture was pressed into the cylindrical cavity of the annular sample holders. The additions of K2C03powder were chosen in such a way that after transformation to KOH the mixtures contained KOH contents of 10,20,30, and 40 wt % in relation to the total weight of the sample. (The weight percent values therefore mean KOH contents, while the samples themselves have been made with K2C03due to easier handling.) Anyway, during reaction a relation of carbonate to hydroxide according to the reaction conditions will be established, independently of the basic material. The technical-grade styrene catalyst contains about 1 2 wt% KOH. In addition to the four potassium-containing samples, one sample free of potassium was prepared. In order to remove moisture and to get the same starting conditions, the five holders with the samples were annealed together in a separate fine steel reactor at 300 "C under COP. Subsequently, the pore structure parameters of each sample were determined by diffusion measurements at ambient temperature with the gas pair Ar/C02, and the porosities were measured. The results of these measurements are indicated in Figure 11 by Ybeforen. As expected, the values of permeability, mean pore readius, porosity, and labyrinth factor decrease with increasing KOH content at equal apparent density of the pore structure. (The sample with 40% KOH had ex-
1002 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
3. Conclusions OLfC
’
/ before L.
. . ; - j O‘
afterwards
!,O0.1 - 00
h
%KOH
O0
10
20
30 % KOH
d) c) Figure 11. Changes of characteristic transport parameters with increasing KOH contents in the catalyst samples “before”a reaction period and “afterwards? (a) permeability, (b) mean radius of the transport pores, (c) open porosity, (d) labyrinth factor.
panded so largely during the annealing that parts of it had broken out of the sample holder. A sample with such a high potassium content is obviously no longer stable enough mechanically.) After the porosity measurements, the samples were reinstalled in the fine steel reactor and heated to 600 “C under a flow of argon. Afterward a longer reaction period (3.5 days) and a 7-h treatment with COz-both at 600 “C-were inserted. Subsequently, the samples were cooled under COz to room temperature and placed in the diffusion apparatus. The results of the diffusion measurements, carried out at room temperature, and of the following porosity measurements are indicated in Figure 11 by “afterwards”. As Figure l l a shows, the porosity diminishes with increasing KOH content, stronger after the reaction period than before. This results from the redistribution of the melt during the reaction phase at 600 “C and corresponds to the blocking effect characteristic for SLP catalysts. The increase of the mean pore radius after reaction, Figure llb, also points to this effect, as preferably narrow pores are blocked, and thus the mean value of the radii of the still open transport pores rises. As a comparison with Figure l l d reveals, the stronger decrease of the permeability after reaction is due only to the stronger decrease of the labyrinth factor, whereas the porosity, Figure llc, scarcely shows an effect of the reaction, i.e., of the liquefaction of the potassium promoter in the meantime. Therefore, it is to be assumed that the blocking effect of the melt noticeably raises the number of dead-end pores. Regarding the sample with a KOH content of 30%, the values of q , P, and t in Figure 11are evidently too high. This obviously indicates a beginning of the expansion effect that breaks up the sample with 40% KOH. In the values of the labyrinth factor, Figure l l d , these effects of \k and t largely compensate each other.
3.1. Comparison with Typical SLP Catalysts. The first group of SLP catalysts presented in section 1, Le., porous supports loaded with catalytically active liquid solutions, can be taken as “typical” SLPC. This group is characterized by (i) a decrease of the permeability down to zero before the loading fills up all hollow spaces in the porous system (percolation threshold, Figure 3) and by (ii) a pronounced maximum of reaction rate with increasing loading (Figure 2). The percolation threshold becomes apparent in the behavior of the styrene catalyst as well; an extrapolation of the curves marked “afterwards” in Figure lla,d points to a loading of 35% KOH for vanishing permeability, and at this loading the porosity is still rather high, as Figure l l c shows. (Due to the expansion effects of the porous structure, mentioned above, a more quantitative evaluation is not possible.) With regard to the reaction rate maximum, however, the styrene catalyst behaves differently. As Lee (1973) found, the activity of Fe30, increased steeply with small additions of KzO and attains a limiting value at a coverage of about 1 monolayer of K+ on the surface. The limiting reaction rate remains constant at higher additions of KzO and decreases slowly only at rather high KzO admixtures (up to an amount of about 15 monolayers). The quick attainment of a limiting value at small loadings already corresponds to the nature of the potassium as a surface promoter to Fe304-group I11 SLP in section l-instead of a liquid phase with homogeneous catalysis in the volume. It remains obscure, however, why higher KzO admixtures don’t noticeably diminish the access of the ethylbenzene to the Fea04surface in the investigation of Lee. 3.2. Valuation of the Diffusion Measurements. The results on styrene catalyst samples with different potassium contents presented here can be taken as a basis for modeling the catalyst pellets in different sections of the reactor. In spite of the effort necessary in calculation, this procedure seems to be worthwhile, as considerable changes of the permeability and of the pore diffusion effects come into play by redistributions of the potassium melt in the pore structure of single pellets (Figure 11) and by the nonuniform distribution of the potassium promoter along the catalyst bed. Accordingly, one should consider in modeling the local variations of the diffusion transport conditions in order to approach the actual situation. The investigations have shown that stationary diffusion measurements on porous catalysts with and without liquid loading can be performed under difficult testing conditions and also in direct context with the reaction. An important result, which could not be foreseen, was the fact that the liquid phase-at least in the case dealt with here-could be solidified without noticeable changes of its distribution in the pore structure. Up to now the method of stationary diffusion measurements used to be considered as complicated and a waste of time. By means of modern electronic devices for measuring, adjusting, and evaluating, however, it can be simplified in many cases to a routine method, giving information on the distribution of the liquid in the pore structure of SLP catalysts-and on changes of this distribution-that can scarcely be obtained by other methods.
Acknowledgment Financial support of the project by BMFT is gratefully acknowledged. Our special thanks are due to the members
Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 1003 of the BASF group for trustful cooperation and multiple assistance.
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Received for reuiew September 6, 1989 Revised manuscript receiued February 26, 1990 Accepted March 9, 1990
