Ind. Eng. Chem. Res. 2002, 41, 3131-3138
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Monolith Reactor for the Dehydrogenation of Ethylbenzene to Styrene Wei Liu, William P. Addiego, and Charles M. Sorensen Corning Incorporated, Science and Technology, Sullivan Park, Corning, New York 14831
Thorsten Boger* Corning GmbH, Abraham-Lincoln-Strasse 30, D-65189 Wiesbaden, Germany
Honeycomb-shaped monolithic catalysts offer an attractive solution to the long-standing problem of balancing the reactor pressure drop with mass transfer in fixed-bed reactors. This approach is discussed in detail in this paper based on experimental and modeling data for the dehydrogenation of ethylbenzene to styrene. An extruded monolithic catalyst was found to be stable and active for the ethylbenzene dehydrogenation reaction. The pressure drop across the monolithic catalyst bed is about 20 times less than that of a packed bed of conventional 1/8 in. (3.2 mm) catalyst beads. The low pressure drop nature of a monolithic catalyst makes it feasible to convert conventional radial flow reactors into simpler, axial flow reactor designs. Modeling results show that additional process performance improvements can be realized through novel reactor designs that take advantage of monolithic catalysts. Introduction Optimizing the balance between the reactor pressure drop and mass transfer has been a long-standing challenge in the design of fixed-bed catalytic reactors. A minimal pressure drop is desired because the selectivity and yield for certain reactions are improved and because capital and operating costs can be reduced when smaller, more efficient reactors are employed. Low pressure drop often requires large catalyst pellet sizes, but for efficient intraparticle mass and heat transfer, small catalyst size is needed. Maximization of the benefits of both variables cannot be met at the same time with conventional packed-bed technology. Fluidized beds have been used by industry to take advantage of small catalyst particle sizes at low pressure drop. However, the fluidized bed is not suitable for all reactions because of the expenses associated with catalyst attrition and loss, complex hydrodynamics, and additional hardware and operational issues relative to fixed beds. Alternatively, packed beds utilizing a radial flow design have been used to conduct low pressure drop operation with relatively small catalyst beads. One key disadvantage of these reactors is their relatively poor volumetric efficiency. As a result, although fluidizedbed and radial flow reactors are successfully used in commercial processes, more reliable and efficient reactor technologies are always desired to overcome the inherent problems in the existing processes. The monolithic reactor is presented in this paper as a new technology to dramatically improve the process performance of dehydrogenation reactions, which are favored by low pressure. Data and example cases using the dehydrogenation of ethylbenzene to styrene will be shown. This type of nonoxidative dehydrogenation reaction is endothermic, and heat management inside the * To whom correspondence should be addressed. Tel.: +49 611 7366 168. Fax: +49 611 7366 143. E-mail: BogerT@ corning.com.
reactor is another critical aspect of reactor design. Monolithic catalysts permit novel reactor designs where heat integration and the overall process performance can be improved, especially for retrofits of existing radial flow reactors. Reactor for Dehydrogenation of Ethylbenzene to Styrene The dehydrogenation of ethylbenzene to styrene is a major industrial process used to produce valuable commodity chemicals such as polystyrene and synthetic rubber.
ethylbenzene T H2 + styrene + EB ST ∆H (+124.3 kJ/mol) (1) As much as 20 × 106 tons/year of styrene monomer is produced worldwide. Although this process was commercialized in the 1930s, research and development of new catalysts, reactor designs, and process routes has been continuously pursued to achieve process improvement.1-13 The nonoxidative dehydrogenation of ethylbenzene to styrene is endothermic, reversible, and equilibrium-limited. The formation of styrene is favored by high temperatures and low pressures. Typical reaction conditions are 550-650 °C and 0.3-1.0 bar. Steam is often used to further reduce the partial pressures as well as to provide heat for the reaction, to stabilize the catalyst, and to minimize coke formation.3,13 Because of the low pressures required, the pressure drop across the catalyst is critical and radial flow reactors are applied in most industrial processes. Figure 1 shows a schematic of a commonly used two-stage radial flow reactor system. Radial flow reactors are operated adiabatically. Because dehydrogenation is endothermic, the gas temperature drops significantly across the first reactor. Thus, the reacting stream needs to be reheated before entering the second reactor.
10.1021/ie010730v CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002
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sharply increases with a decrease of the catalyst particle size, dp,eq.
ηg(1 - B)2 Fg(1 - B) 2 ∆p ) 150 u + 1.75 u0 0 L 3d 2 3d B
p,eq
B
(2)
p,eq
The development of catalyst pellets having new shapes that allow lower pressure drop has been reported.7 However, modification of the catalyst pellet size and shape is an incremental improvement that does not fully solve the fundamental problem of high pressure drop with packed catalyst pellets. Advantages of Monolithic Catalysts
Figure 1. Schematic of a two-stage radial flow reactor apparatus for dehydrogenation of ethylbenzene to styrene.
Compared to a unidirectional, axial flow packed-bed reactor occupying the same vessel, the radial flow reactor reduces the pressure drop by providing a large cross-sectional area for the gas flow to pass through. However, there are several drawbacks associated with the radial flow reactor: (1) The gas distribution and collection chambers occupy a large fraction of the overall reactor volume, about 30% or more. Thus, the reactor space utilization efficiency is low. (2) The large fraction of free space increases the extent of unselective thermal reactions. (3) The gas stream must change its flow direction by 90°, which can cause poor axial gas distribution over the packed bed and problems with catalyst attrition at the edge. (4) Expensive reactor internals are used to hold the catalyst bed, and there are mechanical reliability issues with these related to the stresses of high-temperature operation. (5) A large temperature gradient can develop along the catalyst bed. Typical per pass conversion of ethylbenzene across both reactors is about 60-70% at a selectivity of 9396% to styrene. The main byproducts are toluene and benzene. A high one-pass conversion over the reactor is very important to the overall process economics because it reduces the costs related to separating and recycling the unconverted ethylbenzene. The conversion can be increased by raising the reaction temperature. However, this causes other undesirable side effects such as a loss in selectivity and a high catalyst deactivation rate. One can consider using more than two reactors in series with inter stage heating,8 but in most cases this is not economic because of the increased capital cost. A key limitation to the current reactor technology is the fact that the catalyst is available only in the form of pellets. Catalyst beads with a diameter of 1/8 in. (3.2 mm) are commonly used. Although it would be desirable to use catalyst pellets with a smaller size to improve catalyst utilization and selectivity, the accompanied increase in the pressure drop is a major practical restriction. From the well-known Ergun equation, we can see that the pressure drop, ∆p, across a packed bed
There are other applications beyond the chemical manufacturing industry in which a gas-phase reaction is conducted over a solid catalyst and where a minimal pressure drop is absolutely critical. The most prominent examples are the automotive catalyst converter and the removal of NOx from flue gases in power plants by selective catalytic reduction. In both cases are honeycomb-shaped monolithic catalysts, the technology of choice because of their unique combination of high reaction efficiency and low pressure drop. These applications have demonstrated that monolithic catalysts can be manufactured in large quantities to provide reliable solutions for both large and small reactors. Monolithic catalysts comprise an array of parallel, narrow channels with the wall being catalyzed. Typically, the channel opening and catalyst wall thickness are varied independently, while for a conventional packed pellet bed, the catalyst size is the only major parameter to work with. A fundamental reason for lower pressure drop in monoliths is that under practical conditions the flow in the channels is laminar whereas it is turbulent in packed pellet beds. In addition, the monolithic catalyst presents straightforward channels that reduce flow resistance. The pressure drop, ∆p, across a monolithic bed can be described by eq 3.
∆p f0 ηg ) u + L 2 d 2 0 B h
ζ Fu2 2 g 0 usually negligible
(3)
In eq 3, the second term, being proportional to the square of the velocity, is negligible in almost all gasphase applications because of laminar flow conditions. The thickness of the catalyst wall does not explicitly show up, giving an extra degree of freedom for a catalyst designer to optimize the catalyst for a maximum in utilization and selectivity. The effectiveness factor for ethylbenzene dehydrogenation over 1/8 in. (3.2 mm) commercial catalyst pellets is reported to be low, about 0.5-0.7, indicating that the catalyst is not fully utilized.14 Data reported in the literature suggest that reducing the catalyst size can result in an improvement to both activity and selectivity.13,14 Thus, two major advantages of monolithic catalysts for the dehydrogenation process are (1) low pressure drop and (2) reduced heat/mass-transfer resistance inside the catalyst. Table 1 lists the pressure drop and characteristic heat- and mass-transfer dimensions of two monolithic catalysts in comparison to a 1/8 in. (3.2 mm) catalyst bead. The pressure drop across a 55 cell/ in.2 (8.5 cell/cm2) monolithic catalyst module based on
Ind. Eng. Chem. Res., Vol. 41, No. 13, 2002 3133 Table 1. Comparison of a Monolithic Catalyst to a Catalyst Bead packed-bed properties
monolith 1
monolith 2
catalyst bead
void fraction wall thickness, mm
0.5 1
0.5 0.5
cell density, cell/in.2 (cell/cm2) pressure drop,a bar/m characteristic mass/heat-transfer dimension,b mm
55 (8.5)
220 (35.2)
0.5 3.2 (diameter) N/A
0.021 0.5
0.085 0.25
0.437 0.8
a The pressure drop is calculated based on the following reaction and fluid properties: reactor pressure of 0.7 bar, temperature of 870 K, cross-sectional bed area of 12.6 m2, mass flow rate of 15.0 kg/s, linear velocity of 6.8 m/s, viscosity of 3.20 × 10-5 kg/(m‚s), and gas density of 0.177 kg/m3. b Characteristic length ) surfaceto-volume ratio.
eq 3 is about 0.02 bar/m at a superficial gas linear velocity of 6.8 m/s. In contrast, the pressure drop calculated from eq 2 for a catalyst bed packed with 1/8 in. (3.2 mm) beads at the same void fraction is 0.44 bar/ m, which is about 20 times the pressure drop over the monolithic catalyst bed. The characteristic heat- and mass-transfer dimensions are 0.5 mm for the monolithic catalyst and 0.8 mm for the 1/8 in. (3.2 mm) bead. Clearly, low pressure drop and enhanced heat and mass transfer are realized with the monolithic catalyst at the same time. The calculated pressure drop value for conventional catalyst pellets also explains why a radial flow reactor is the only practical solution with this type of catalyst. Monolithic Catalysts for Ethylbenzene Dehydrogenation From the above discussions, we see that monolithic catalyst technology is ideal for application to the styrene process. Recently, Corning Inc. has developed an extrusion method to make iron oxide based monolithic catalysts.15 Iron oxide promoted with potassium is an active catalyst formulation for the dehydrogenation of ethylbenzene. Extrusion batches for the monolithic catalyst were prepared using R-Fe2O3 mixed with appropriate amounts of K2CO3, (NH4)6Mo7O24, Ce2(CO3)3, and MgCO3 or CaCO3, to yield compositions based on their respective oxides as listed in Table 2. The extrusion batch compositions were calculated based on the analyzed equivalent oxide of the salts. Catalyst precursors were dry-blended in a tubular mixer with certain organic binders. After dry mixing, organic emulsions were added with water and mulled to form a plastic dough. Additional plasticizers and lubricants were also added sequentially during mulling. The extrusion dough was then homogenized to ensure a thorough
Figure 2. Extruded iron oxide monolithic catalysts with 100 cells/ in.2 (15.5 cell/cm2) and 0.635 mm wall thickness.
mixing of solid and liquid components. After extrusion, the monolithic modules were dried at 80 °C and calcined at 850 °C for 6 h. Monolithic catalysts have been extruded with square channels of 100, 200, and 400 cell/ in.2 (15.5, 31, and 62 cell/cm2, respectively) and with respective web thicknesses of 0.64, 0.38, and 0.18 mm. The new method does, of course, allow for the extrusion of monoliths with lower cell densities and other web thicknesses.15 In this study, only experimental results with the 100 cell/in.2 (15.5 cell/cm2) monoliths are discussed. Figure 2 shows a photograph of the 100 cell/in.2 (15.5 cell/cm2) sample. Calcined samples were characterized for surface area, porosity, pore size distribution, and A-axis crush strength. The monolithic catalysts have a BET surface area of 3-4 m2/g, are macroporous with a mean pore size of 330-380 nm, and have >50% porosity. The monolithic modules were very strong, exhibiting an A-axis crushing strength (i.e., force applied parallel to cell walls) of 9-14 MPa after calcination. The 100 cell/in.2 monolithic catalysts were tested for catalytic performance using an integral packed-bed reactor. The reactor consisted of a stainless steel tube within which a 1-in.-diameter quartz tube was inserted to support the monolithic catalyst. A catalyst sample of 50 cm3 volume, 2.5 cm diameter by 10.0 cm length, was used. At the same packing volume, catalysts A and B (Table 2) were weighed as 49.96 and 59.12 g, respectively. The outer surface of the monolithic part was
Table 2. Monolithic Catalyst Composition for Ethylbenzene Dehydrogenation general composition
actual composition of oxide wt %
component
batch component
range of oxide wt %
function/promoter
catalyst Aa
catalyst Ba
Fe2O3 K2O CeO2 MoO3 CaO MgO
oxide carbonate carbonate ammonium molybdate carbonate carbonate
25-80 10-35 0-5 0-3 0-3 0-10
activity activity activity selectivity chemical/mechanical stability
72 16 4 1 0 7
79 11 4 3 2 1
a Monolithic physical properties are as follows: geometry ) 100 cells/in.2 (15.5 cell/cm2), wall thickness ) 0.5-0.7 mm, channel width )1.8-2.0 mm. A-Axis crushing strength: 11-12.4 MPa. BET surface area: ∼4 m2/g. Packing density: 1.0-1.2 g/cm3.
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Figure 3. Conversion of ethylbenzene, selectivity to styrene, and yield for catalysts A (left) and B (right) during a 50 h startup time period.
smoothed by sanding so that it fitted tightly inside the 2.5 cm quartz tube reactor. Sections above and below the monolith were packed with 60 mesh silicon carbide (SiC) particles to distribute gas flow uniformly. The SiC particles above the bed were separated from the monolithic catalyst by a layer of quartz wool to prevent them from falling into the monolithic channels. The uppermost portion of the reactor tube above the SiC particles was fully filled with inert R-alumina balls to preheat the reactants to reaction temperature prior to contacting with the catalyst. Two thermocouples were placed at the top and bottom of the monolithic catalyst to monitor the reaction temperature. The reactor furnace was operated quasi-isothermally, and the reaction was conducted at atmospheric pressure. One comparative test with monolith A, crushed and sieved to 40-60 mesh (420-250 µm), was conducted. In this test 25 cm3 of the crushed particle (28.44 g) was diluted with 25 cm3 of SiC particles (average particle size: 250 µm). The resulting catalyst bed height was about 10 cm. Additional packing of the same SiC particles only were placed on the top and bottom of the catalyst bed at 3 cm height each, to achieve uniform gas distribution. The thermocouple and preheating bed loading were identical with those for experiments with the monolithic catalyst. Reagent-grade ethylbenzene and deionized water were delivered to the top of the reactor by two liquid pumps to provide the appropriate steam-to-ethylbenzene ratio. The liquids were mixed and vaporized in the reactor preheating zone. The reaction effluent was cooled, and the liquid product and off-gas were then separated. Liquid hydrocarbons were separated from water and measured by gas chromatography. Two catalyst compositions, termed A and B, were tested under conditions typical for commercial ethylbenzene dehydrogenation. The composition of these catalysts is summarized in the last two columns of Table 2. Catalyst stability was examined first using an inlet temperature of 605 °C, a pressure of 1 bar, a liquid hourly space velocity (LHSV) based on the packing volume of 0.48 h-1, and a steam-to-oil ratio (S/O) of 2.3 by weight. Figure 3 shows variation of the conversion, selectivity, and styrene yield during the first 50 h on stream; both catalysts showed stable behavior. For styrene catalysts, it is known that a certain time period is needed to stabilize the catalyst after startup while the catalyst composition equilibrates under reaction conditions. The conversion over both catalysts slightly
Figure 4. Conversion of ethylbenzene, selectivity to styrene, and yield for catalyst A at different reaction temperatures.
increases with time, and for catalyst A, a slight decrease in the selectivity is observed, which may be a result of increased conversion. Catalyst A shows higher conversion than catalyst B. Such an activity difference is consistent with the catalyst composition shown in Table 2. Catalyst A contains more potassium oxide than catalyst B, while it is generally believed that potassium oxide promotes the dehydrogenation activity.13 Monolithic catalyst A maintains its physical integrity after the reaction testing. In Figure 4 the temperature dependence on the reaction performance is shown for this catalyst. Reaction conditions are the same as those described above except for temperature. In accordance with what is expected from chemical equilibrium and kinetics, conversion increases with increasing temperature. The increase is concomitant with a decrease in the selectivity due to the acceleration of undesired parallel reactions. The catalyst was tested under various conditions for about 1 week, and no apparent deactivation was observed. After the reactor was unloaded, the monolithic catalyst still maintained its physical integrity. These preliminary results confirm that the monolithic catalysts prepared in this work have reasonable catalytic activity. To further understand monolithic catalyst performance, the reaction results of catalyst A in different forms are compared in Table 3. Because catalyst A in monolithic form has a packing density different from that in the particle form, the comparison is made on the basis of weight hourly liquid space velocity (WHSV). To take into account the differences in reaction conditions, e.g., the temperatures, the catalyst activity is calculated based on the following kinetics equations.
Ethylbenzene disappearance rate: rEB )
k1(PEB - PH2PST/KP)
(4)
PEB + (80PST)2
[ (
k1 ) a0(118) exp -
)]
Eapp 1 1 R T T0
KP ) K0 exp
(∆H RT )
(5)
(6)
with T0 ) 895 K, Eapp ) 276 kJ/mol, and ∆H ) 124.3 kJ/mol. Kinetics are based on reactor temperatures that are an average of the bed inlet and outlet values.
Ind. Eng. Chem. Res., Vol. 41, No. 13, 2002 3135 Table 3. Comparison of Catalyst A Activity in Different Forms (Reaction Pressure ) 1 atm) catalyst form
WHSV, h-1
H2O/EB, M
bed temp, °C
EB convn, %
selectivity to ST, %
phenylacetylene content, %
activity
monolitha particleb
0.42 0.42
13.4 13.4
608 603
76.1 72.7
90.7 93.5
0.0160 0.0104
1.8 1.7
a
Catalyst A in monolithic form. b Catalyst A was crushed into 40-60 mesh (420-250 µm) and diluted with SiC particles.
Table 4. Comparison of the Monolithic Catalyst Activity with Literature Data 1/
catalyst A
8
in. catalyst pelleta
reaction condition
2.2
2.6
2.3
2.2
temperature, °C H2O/EB molar ratio LHSV EB conversion, % ST selectivity, %
605 6.7 0.48 66.7 93.3
587.8 13.4 0.48 68.2 93.4
593 9.3 0.48 64.8 92.3
593 12.2 0.48 68.1 92.4
a 1/ in. commercial catalyst produced by United Catalysts Inc. 8 reported.8
In the last column of Table 3, the intrinsic activity of catalyst A, a0, in the monolithic form is compared to that in the crushed form. The higher conversion reported in Table 3 for the monoliths is due to the higher temperature, whereas the lower selectivity is a result of the inverse relationship between the conversion and selectivity, as will be discussed later. Similar values of 1.8 and 1.7 are found for the catalyst activity, suggesting that mass- and heat-transfer resistances inside the monolithic catalyst are minimal and that the catalyst effectiveness factor is very high. In contrast, it is reported in the literature that commercial catalyst pellets show only about 50% of the activity of crushed catalyst particles.14 With the kinetics expressions, we are able to compare the catalyst activity of monoliths with the activity reported for commercial catalyst pellets. Such a comparison of the relative activity is given in Table 4. Variations in reaction conditions are considered with the help of the kinetic modeling. Table 4 shows that monolithic catalyst A has an activity comparable to a commercial catalyst pellet measured in laboratory testing. It should be mentioned that catalyst A also has a much higher activity than the monolithic catalyst disclosed in the previous patent.16 Selectivity is another important performance factor, and we have found that it strongly correlates with the ethylbenzene conversion. The effect of reaction conditions on selectivity appears to be secondary. A practical method for a rough assessment of the selectivity of a catalyst is to plot it against the conversion. This allows a comparison of the selectivity of different catalysts at the same conversion level. Figure 5 shows such a plot with data for the monolithic catalysts A and B together with literature data. From the figure, it can be concluded that the monolithic catalyst selectivity is comparable to that of the conventional bead catalyst. In summary, a monolithic catalyst having activity and selectivity comparable to those of commercial catalyst pellets has been successfully prepared. It is worth noting that the focus of the present catalyst preparation work was directed toward showing feasibility for a broad range of extrudable catalyst compositions and not toward optimization of the catalyst composition itself. It is believed that monolithic catalyst activity and selectivity can be significantly improved by optimizing
Figure 5. Monolithic catalyst selectivity in comparison with literature data from refs 2 and 7-9. Conditions: LHSV ) 0.48-1 h-1, S/O ) 1.2-2.4 by weight, p ) 1 bar, and T ) 590-630 °C. Table 5. Process Conditions Used for the Case Study unit capacity, tons/h reactor inlet temperature, °C steam/oil ratio, w/w reactor inlet pressure, bar dimensions of the radial flow reactor, m inner diameter of the catalyst bed outer diameter of the catalyst bed inner diameter of the reactor vessel length of the catalyst bed
40.0 616 1.4 0.5 1.7 3.2 4.0 10.2
the catalyst composition. Based on modeling results, further activity improvements are possible. Retrofitting Radial Flow Reactors to Axial Flow In the previous section, it was shown that it is possible to make a catalyst having monolithic geometry that is suitable for the dehydrogenation of ethylbenzene to styrene. In this section, novel reactor concepts based on monolithic catalysts will be discussed and further comparisons with bead catalysts will be made. All data presented in this section are generated from reactor simulations using a commercial process simulator. In the simulations it was assumed that the monolithic catalyst activity on the basis of the catalyst weight is the same as that of the conventional catalyst pellet, though a higher monolithic catalyst activity is likely obtained because of enhanced catalyst effectiveness. Monolithic catalyst selectivity was also assumed to be the same as those of pellets. Essentially, the present discussion focuses on the reactor design rather than the catalyst material itself. A case study of the pressure drop was conducted by simulating a commercial reactor having a feed capacity of 40 tons/h, mainly consisting of ethylbenzene. Table 5 lists key process parameters used in the simulation. The radial flow reactor dimensions are 1.7 m inner diameter of the catalyst bed, 3.2 m outer diameter of the bed, 4.0 m inner diameter of the reactor vessel, and 10.2 m catalyst bed height. The reactor feed temperature, pressure, steam/oil ratio, and LHSV are 616 °C, 0.5 bar, 1.4 by weight, and 0.8 h-1, respectively. The LHSV is based on the catalyst bed volume and not the
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Table 6. Case Study on the Pressure Drop of Monoliths vs Conventional Packed Pellet Beds property
monolith C
monolith D
commercial pellet A
commercial pellet B
wall thickness or pellet diameter, mm cell density, cells/in.2 (cells/cm2) void fraction, % packing density, kg/m3 LHSV,b h-1 flow rate (ave), kg/m3‚h φ/φcylinderc θc
1.5 25 (3.9) 49 1000 0.62 880 1 0.5-0.7
1.05 50 (7.8) 49 1000 0.62 880 0.7 0.65-0.82
3 (cylindrical) NA 33 1300 0.8 680 1.0 0.5-0.7
3.5 “shaped”a NA 33 1300 0.8 680
