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Application of Bench-Scale Tests in Understanding a Commercial Fluidized-Bed Reactor Operation J. J. Zhou*,† and S. M. Lee‡ Eli Lilly and Company, Lilly Corporate Center, DC 3127, Indianapolis, Indiana 46285, and Solutia Inc., F.M. 2917, P.O. Box 711, Alvin, Texas 77512
Small-scale fluidized-bed reactors are widely used by both industry and academy. Well-planned tests can generate useful information about commercial reactor operation. However, there are limitations to which results from laboratory-scale reactors can be applied to commerical-scale reactor operation. In this paper, fluidized-bed acrylonitrile reactors are used to demonstrate the similarities and differences between laboratory and commercial reactors. Laboratory reactor performance is compared with the performance of commercial reactors. For fluidized-bed reactors, it is recommended to study reaction kinetics and reactor hydrodynamics separately. Introduction Fluidized beds are widely used in the chemical and petrochemical industries for making products, drying particles, and sizing solids. Most commercial fluidizedbed reactors have diameters ranging from 4-10 m. Because interruption of the commercial reactors’ operation is not desirable for economical reasons, the opportunity to do tests in these units is usually limited. To circumvent this problem, many companies operate smaller bench- or pilot-scale fluidized-bed units for process optimization or improvement, troubleshooting, and catalyst testing and qualification. In addition to the difference in scales, pilot- or benchscale units usually have different configurations for gas feeding and gas-solids separation. Most of the small units are not operated under the same conditions. For example, compared to commercial reactors, most pilotand bench-scale units are operated in a different fluidization regime because of much lower gas velocity. Although these small units are very important tools for better understanding commercial operations, caution must be exercised when applying results from pilot and bench-scale units to commercial-scale units. In this paper, acrylonitrile (AN) production is used to demonstrate how to apply data from pilot-scale units to commercial reactors. The typical operation of both AN bench- and commercial-scale units is introduced. The differences between AN commercial reactors and pilot reactors are discussed. Some results from benchscale units are compared with the results from commercial-scale units. AN is commercially produced by ammoxidation of propylene. The reaction is carried out in the presence of ammonia and oxygen, using a metal oxide catalyst in fluidized-bed reactors. The main reaction for AN production is
2CH2dCHCH3 + 2NH3 + 3O2 f 2CH2dCHCN + 6H2O (1) * To whom correspondence should be addressed. Tel.: (317) 277-0008. Fax: (317) 276-1403. E-mail:
[email protected]. † Eli Lilly and Company. ‡ Solutia Inc. Tel.: (281) 228-4354. Fax: (281) 228-4168. E-mail:
[email protected].
The most important byproduct of the process is hydrogen cyanide (HCN). The reaction for HCN production is
CH2dCHCH3 + 3NH3 + 3O3 f 3HCN + 6H2O (2) Other main byproducts include carbon oxides (CO and CO2), nitrogen oxide (NO), acrolein (ACR), and acetonitrile. Typically, reactors are operated under 2 atm pressure and 480 °C temperature. There are several catalysts that are used to produce AN in commercial operations. The yield/selectivity can be very different among these catalysts. Yield and selectivity are typically calculated using the composition of the reactor exhaust gas measured by online gas chromatography. The choice of catalyst is mainly determined by the economics, which is a combination of catalyst cost, cost structure of the raw materials, market demand for products and byproducts, and raw material utilization. Solutia Inc. is a leading manufacturer of AN with 10 commercial units of various capacities. It also has four bench-scale units. MAC 3, a proprietary AN catalyst developed by Solutia Inc., is used in all of its commercial- and bench-scale reactors. MAC 3 is a very active catalyst and highly selective to make AN. Other advantages of using MAC 3 are high HCN yield, low ammonia burning, and low NOx formation in reactors. The Sauter mean particle size of fresh MAC 3 catalyst is about 45 µm, and the particle density is 1500 kg/m3. The catalyst is typical Geldart A particles. System Description 1. Bench-Scale Reactors. A schematic of a typical AN bench-scale unit is shown in Figure 1. The reactor can be made of 2 in. pipe. A sintered-metal or ceramic plate is usually used as a gas distributor. There are several screens in the reactor for prevention of slugging and for bubble destruction. A sintered-metal pipe can be used to separate entrained catalysts from exhaust gases. The temperature and pressure at the windbox, catalyst bed, freeboard, and exhaust gas line are measured. Propylene (C3H6) and ammonia (NH3) from bottles are fed along with air into the windbox where they are mixed before they go through a gas distributor and into
10.1021/ie030752c CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004
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in the plant. A control valve at the exhaust gas line can be used to control the reactor pressure. The exhaust gases then go to recovery and purification systems to obtain AN and HCN products. The catalyst in Solutia Inc.’s reactors is routinely sampled and shipped to the laboratory for testing. In addition to chemical and physical properties, the catalyst samples are tested in bench-scale units to evaluate the performance of commercial reactors. Samples of reactor makeup fresh MAC 3 catalyst are also tested in bench-scale units. The bench-scale units are also used for troubleshooting and process improvement. Similar to all of the other AN manufacturing processes, Solutia Inc. commercial reactors are typically operated at a superficial gas velocity of about 0.6 m/s, a temperature of 480 °C, and a freeboard pressure of 2 atm.
Figure 1. Schematic of the bench-scale AN reactor.
Figure 2. Schematic of the AN commercial reactor.
a reactor where an AN catalyst was preloaded. Mass flowmeters and control valves are used to control the feed rates at preset values. The temperature of the catalyst bed is maintained by heating wires wrapped outside the reactor wall. The product gas leaving the catalyst bed enters an expanded freeboard. A filter installed in the freeboard removes catalyst solids from exhaust gases. A control in the exhaust gas line regulates the reactor pressure at a desired value. A gas chromatograph (GC) analyzes the composition of exhaust gases. The exhaust gas goes through a converter where it is cleaned before being vented. The typical superficial gas velocity is 0.1 m/s. The typical freeboard pressure is about 2 atm. 2. Commercial Reactors. Solutia has 10 commercial AN reactors with internal diameters of 4.5-9 m and a height of 15.2 m. A typical commercial AN reactor is illustrated in Figure 2. Preheated air is fed into the reactor through a lower gas sparger. C3H6 and NH3 are preheated and mixed before they are fed into the reactor through an upper sparger. Both air and hydrocarbon spargers are immersed in preloaded catalyst particles. Cyclones are used to remove catalyst particles from exhaust gases before they leave the reactor. The collected catalyst particles are returned to the bottom of the reactor through cyclone diplegs. Cooling coils control bed temperatures. Water is fed to the cooling coils, where steam is produced and sent to other operations
Results and Discussion The key factors that control the performance of fluidized-bed reactors can be classified into two categories: reaction kinetics related and hydrodynamics related. Factors affecting reaction kinetics include temperature, pressure, air/propylene ratio, and ammonia/ propylene ratio. Density, particle size distribution, fraction of fines (defined as particles of less than 44 µm), and superficial gas velocity have direct impacts on reactor hydrodynamics. The same catalysts are usually used in bench- or pilot-scale reactors and commercial reactors. Therefore, the influence of the difference in catalyst is not discussed. In this paper, the discussions are focused on gas-solids flow patterns and key operating parameters such as temperature and pressure in fluidized-bed reactors. 1. Gas-Solids Flow. It is obvious that commercial AN reactors are much larger than bench-scale reactors for both internal diameter and height. The wall effect in bench-scale reactors is much more significant for the gas-solids flow pattern. As described above, commercial AN reactors are usually operated at a superficial gas velocity of 0.6 m/s, while the superficial gas velocity in bench-scale units is much lower, at 0.1 m/s. The fluidization regime calculation for MAC 3 indicates that commercial AN reactors are operated in a turbulent fluidization regime, in which stable bubbles do not exist. Bench-scale AN reactors are operated in a bubbling-bed regime, which is very different from that of commercial reactors. Limited by the size, bench-scale reactors do not usually have an internal solids recirculation system. As a consequence, they cannot be operated at high gas velocities as in commercial reactors. Screens in benchscale reactors also prevent them from being operated at high gas velocity. To break stable bubbles and prevent slugging, as shown in Figure 1, screens are usually used in benchscale reactors. The proper screen sizes and locations let the bench-scale reactor closely replicate the conversions of C3H6 and NH3 and the yields of AN and HCN in commercial reactors. However, because commercial reactors are operated in a different fluidization regime, the impact of the physical properties of catalysts (such as particle size, size distribution, and particle density) on the performance of small reactors can be misleading when used to predict the impact of these particle properties on commercial reactors. The influence of the superficial gas velocity on the reactor performance can also be very different between bench-scale and commercial reactors.
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Figure 3. Influence of the amount of fines on the reactor performance.
Figure 3 compares the impact of the fraction of catalyst fines on the AN yield (defined as the propylene/ AN selectivity multiplied by propylene conversion) in laboratory and commercial reactors. Not surprisingly, the performance of commercial reactors is more dependent on the fraction of fines than the laboratory units. Increasing the amount of catalyst fines in commercial reactors can usually improve the AN yield significantly. In laboratory reactors, which are operated at much lower superficial gas velocity, there is only a slight increase in the AN yield when the fraction of fines is increased. Similarly, the impact of the particle density on the reactor performance is less significant in laboratory reactors. When the laboratory data are applied to the operation of commercial reactors, it is very important to understand the difference in hydrodynamics between the reactors of different scales. As described earlier, commercial AN reactors have internal cyclones that do not exist in bench-scale reactors. Also, because of the difference in superficial velocity, the gas-solids flow pattern near the bottom of the commercial reactors is completely different from that in the bench-scale reactors. As a result, it is very difficult to use bench-scale units to evaluate the gas feed systems in commercial reactors. In the laboratory unit, all of the reactants (air, propylene, and ammonia) are premixed before being fed through the sintered-metal distributor. In commercial reactors, for safety reasons, air, NH3, and C3H6 are fed into the reactors separately. NH3 and C3H6 are introduced into the reactor at several feet above the air distributor. As a consequence, there are areas which are O2 rich and also areas under a reducing environment. The impacts on the catalyst performance and catalyst lifespan are usually difficult to evaluate in bench-scale units. It is also difficult to evaluate the performance of a catalyst that needs to be quickly regenerated under an oxygen-rich environment in a small reactor that does not have multiple gas feed distributors. It was found that laboratory units require twice as much gas resident time than the fixed-bed units for the same propylene conversion, while the required gas residence time for commercial reactors is about twice that of laboratory units. This means that the laboratory unit is only half as efficient as the fixed-bed unit, while the commercial unit is even worse, only one-fourth as efficient as the fixed-bed unit. Theoretically, a longer resident time promotes more decomposition of AN to HCN and COx. It was found that for AN reactors the lesser the amount of catalyst required to achieve the
desired conversion, the higher the AN selectivity and yield will be. The AN selectivity in fixed-bed reactors is approximately 4% higher than that in the laboratory units, which is also 4% higher than that in the commercial units. A main reason for the difference in the AN selectivity is the difference in gas-solids contact time. The effects of the hydrodynamics on the contact efficiency or the AN selectivity are also different when comparing a laboratory reactor with the commercial unit. A series of internal cyclones are generally used in commercial reactors to separate catalyst particles from product gases before they exit the reactors. Solids are returned to the bottom of the reactors through diplegs. Wang et al.1 found that solids in diplegs for the primary cyclones may drag substantial amounts of product gas to the bottom of the reactors. For a catalytic reaction, that gas-solids contact time is usually critical. Also, the product gas recirculation with catalyst solids may significantly reduce the product yield because of much longer gas-solids contact time. Also, because trickle valves in diplegs do not seal well, for the second and third stage cyclones, feed gas may bypass the catalyst bed by going through the cyclone diplegs. This can reduce the conversion of feed gases, thus reducing the product yield. However, cyclone vendors claimed that the impact was negligible in commercial reactors. More study is needed before a firm conclusion can be reached. Because pilot- and benchscale reactors usually have a very small diameter, it is not practical to have internal cyclones. Therefore, the impact of internal cyclones on the gas flow and reactor performance cannot be evaluated in small reactors. Internals such as heat-transfer coils in commercial reactors may also influence the hydrodynamics of gas flow. These internals may break big gas bubbles and solids clusters if they exist, which may improve the performance of commercial reactors. The internals also may reduce the effective reactor diameter. The capacity of AN reactors is found to be related more to the effective reactor diameter than to the diameter of the reactor vessel. The gas distribution system plays a major role in the solids flow pattern near the bottom of the reactor, where most of the reactions occur.2 Because oxidizing and reducing environments can influence the catalyst performance, the location and distribution of solids from cyclone diplegs are very important to the reactor performance. The internals such as heat-transfer coils can also influence the solids flow pattern. In commercial AN reactors, where the superficial gas velocity is much higher, solids are carried upward by gases to several series of cyclones, where solids are separated and returned to the bottom of the reactors. The solid recirculation makes the whole catalyst bed very homogeneous. In contrast to the commercial unit, in the bench or pilot scale reactors, where superficial gas velocity is much lower, solids usually segregate. It was found that big and heavy solids remain at the bottom, while the very small particles are pushed and remain in the upper bed. Solids segregation in laboratory reactors has two negative impacts. First, the loss of catalyst fines in the bulk of the bed makes it unfeasible to study the impact of fines on the reactor performance. Second, the finer particles at the upper bed, which has less oxygen than
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Figure 4. AN yield vs time.
the rest of the bed, can be reduced more easily. The reduced catalyst has a very low AN yield. The impacts were found to be worse over time. Figure 4 illustrates the catalyst deterioration in reactors of both laboratory and commercial reactors. In laboratory reactors, the catalyst activity drops significantly after several days of continuous operation. In addition to reduction, coking is another very common problem for many heterogeneous reaction catalysts such as AN catalysts. As a result, regeneration of the catalysts is required. In the commercial AN reactors, there is an area, called the autozone, between the bottom air distributor and the upper HC distributor where it is oxygen rich. Its main functions are (1) to reoxidize the returning catalyst and (2) to burn coke from the surface of catalyst particles. This keeps the catalyst in commercial reactors very active over a long period of time. Lacking an autozone in laboratory reactors, the catalyst deteriorates much faster. Both solids segregation and the lack of the regeneration zone are responsible for most of the performance deterioration in laboratory operation over time. It is even worse when the fraction of fines is high in the catalyst. This brings up two important aspects. First, any study on long-term effects cannot be carried out in the laboratory reactor. Second, the selection of a time period during which data are obtained for the laboratory unit is very critical. For many catalysts, it takes hours to reach a steady state; meanwhile, the catalyst may deteriorate quickly after hours of continuous operation. Simply using time average data without paying enough attention to the catalyst performance over time can be very dangerous. In summary, there are significant differences in solids concentration profiles and gas-solids contact between bench- or pilot-scale and commercial reactors. The performance of commercial AN reactors is usually much more sensitive to the catalyst physical properties. Simply taking the results of the catalyst life, catalyst deterioration, and impact of particle properties on the reactor performance obtained in bench- or pilot-scale reactors and applying them to commercial reactors can be very misleading. When the laboratory data are applied to the operation of commercial reactors, it is very important to understand the difference in hydrodynamics between reactors of different scales. 2. Air/Propylene Ratio. As shown in Figure 5, the influence of the air/propylene ratio in feed gases on propylene conversion is the same in both the laboratory and commercial AN reactors. The air/propylene ratio was found to have an almost identical effect on the AN
Figure 5. Influence of the air/propylene ratio on the propylene conversion.
Figure 6. Influence of the ammonia/propylene ratio on the propylene/AN selectivity.
selectivity and the HCN selectivity regardless of the difference in the flow regime and gas backmixing between laboratory and commercial reactors. 3. Ammonia/Propylene Ratio. As illustrated in Figure 6, when the ammonia/propylene ratio is increased, the propylene/AN selectivity first increases. Then the selectivity slightly decreases when the ammonia-to-propylene ratio is further increased. The impact of the ammonia/propylene ratio on the propylene/ AN selectivity is very similar between laboratory and commercial reactors. The locations of peaks in the propylene/AN selectivity are very close as well. Figure 6 also shows that the propylene/AN selectivity is usually higher in laboratory reactors than in commercial reactors. It is believed that gas backmixing is one of the main reasons for the lower propylene/AN selectivity in commercial reactors. As discussed earlier, another factor contributing to the lower propylene/AN selectivity is that some of the product gas is dragged down by recirculating catalyst solids in the internal cyclones. As shown in Figure 7, the propylene/HCN selectivity is proportional to the ammonia/propylene ratio. The diagram also shows relatively higher HCN selectivity in the commercial unit. This is due to (1) higher reaction pressure in the commercial unit and (2) more AN decomposing to HCN because of longer resident time and higher freeboard temperature in the commercial reactor. ACR is another main byproduct of AN production. It is the main impurity that is produced in AN reactors. AN consumers usually have limitations on how much ACR in AN is acceptable. As shown in Figure 8, ammonia is very effective in reducing ACR production
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Figure 7. Influence of the ammonia/propylene ratio on the propylene/HCN selectivity.
Figure 9. Influence of the reactor freeboard pressure on the propylene/AN selectivity.
Figure 8. Influence of the ammonia/propylene ratio on the propylene/ACR selectivity.
Figure 10. Influence of the reactor bed temperature on the propylene/AN selectivity.
in reactors because of the fact that ACR can react with ammonia rapidly to form AN in the catalyst bed. Under the same ammonia/propylene ratio, the propylene/ACR selectivity in laboratory reactors is the same as that in commercial reactors. 4. Pressure. In addition to reaction kinetics, gas and solids flow patterns are also influenced by the pressure and temperature in fluidized-bed reactors.3 For commercial fluidized-bed reactors, the reactor pressure is usually referred to as the freeboard pressure or the pressure that is measured near the top of the reactors. However, for tests in pilot- or bench-scale reactors by AN producers, the reactor pressure can be referred to as the freeboard pressure, windbox pressure, or pressure measured in the solids bed. Obviously, tests with constant windbox pressure are different from tests with constant freeboard pressure because of the difference in bed pressure drops. With different definitions for the reactor pressure, comparing the results from bench- or pilot-scale reactors to commercial reactors can be misleading. For AN reactors, it was found that pressure has a negative impact on the AN selectivity and a positive effect on the HCN selectivity. Pressure can make the AN molecules more difficult to detach from the sites on the catalyst and also more difficult to diffuse from the inner pores to the outside. As a consequence, more AN decomposition is experienced at higher reactor pressure. The HCN selectivity is found to be higher at higher reactor pressure. Figure 9 demonstrates the impact of the reactor pressure on the propylene/AN selectivity. The selectivity for both laboratory and commercial reactors decreases with increasing reactor pressure. That is why all AN
producers try to operate AN reactors at low pressure. However, lower operating pressure means lower reactor capacity at the same superficial gas velocity. Economically, there is an optimum operating pressure. The optimum is different for different AN catalysts. Most AN reactors’ operating pressure is between 16 and 22 psig. However, even under the same freeboard pressure, for bench- or pilot-scale reactors, the pressure near the bottom where most fast reactions happen is usually different from that of commercial reactors because of the difference in the height of the solids bed. Therefore, the optimum reactor pressure for bench- or pilot-scale reactors can be different from that of commercial reactors. The reactor pressure in AN commercial reactors is mainly determined by the pressure drops of their downstream processes. As a consequence, there is not much freedom to decrease the reactor pressure to improve the reactor performance. The above information also suggests that when the performances between the laboratory and commercial units are compared, we need to make proper yield and selectivity adjustments if their reaction pressures are significantly different. 5. Temperature. As shown in Figure 10, the propylene/AN selectivity in both laboratory and commercial reactors increases to a maximum when the bed temperature is increased, and then the selectivity decreases with further increases in the reactor bed temperature. This is because, at higher temperature, the propylene to AN reaction is faster, while AN decomposition is faster too. The temperature under which the propylene/ AN selectivity reaches a maximum in laboratory reactors is very close to that of commercial reactors. Figure 11 shows how the reactor bed temperature influences the propylene to HCN yield. Contrary to the
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reactors, however, the purge gases may have a significant impact on gas flow patterns. Unfortunately, people working on small-scale reactors often ignore the impact.
Figure 11. Influence of the reactor bed temperature on the propylene/HCN selectivity.
propylene/AN selectivity, the propylene/HCN selectivity first decreases with increasing reactor bed temperature until a minimum is reached and then increases with increasing bed temperature. The propylene/HCN selectivity in both laboratory and commercial reactors reaches the minimum value at a similar temperature. The opposite trend of AN and HCN selectivity with respect to the bed temperature as mentioned above is understandable because reactions (1) and (2) compete against each other. Higher propylene/AN selectivity usually means lower propylene/HCN selectivity. For AN production in bench- or pilot-scale reactors, the rate of heat loss can be higher than the rate of heat generation; therefore, reactor walls are usually heated to maintain the reactor temperature. For commercial AN reactors, cooling coils are used to remove the reaction heat and maintain the reactor temperature. Therefore, temperature profiles in the small reactor can be very different from those in the commercial reactor. Also, gas and solids in the small reactor contact hot wall surfaces, while there are cold surfaces in the commercial reactor. The impact of the location and configuration of the cooling coils on the reactor performance cannot be explained by bench or pilot studies. It is important to keep the whole catalyst particle bed at the desired reaction temperature for good performance. Besides poor performance, a low reaction temperature can also promote coke formation on the catalyst. For commercial fluidized-bed reactors, feed gases are preheated. Besides, high in-reactor solid recirculation plus the heat generation in the regeneration zone can easily raise the temperature at the HC distributor to the required reaction temperature. It is important to preheat the feed mixture to near the reaction temperature before it contacts the catalyst to minimize problems such as catalyst coking. However, the heated mixture of ammonia and propylene feed gases should be fed into the catalyst bed quickly to avoid undesirable reactions. 6. Others. Purge gases are usually used to keep solids from plugging pressure taps and keep observation windows clear. For commercial reactors, the amount of purge gas, a fraction of the total gas flow in the reactors, is usually too small to have a significant impact on the reactor performance. In small pilot- and bench-scale
Conclusions Well-designed tests in small-scale laboratory fluidized-bed reactors can be very useful to get a better understanding of the factors affecting commercial reactors. However, because of the difference in the reactor configuration and dimension, pressure and temperature control, and gas-solids flow pattern, there are limits on what can be learned from small-scale reactor studies. As an example, for AN processes, laboratory studies on the impact of the superficial gas velocity, gas residence time, and fraction of fines in the reactor can be misleading. Catalyst deterioration cannot be characterized in laboratory reactors either. However, laboratory reactor studies on the influence of gas composition on the yield of AN and byproducts and the impact of the reactor pressure and temperature on the reactor performance can be applied quantitatively to commercial reactors. Because it is impractical to perform all of the studies in commercial reactors and there is a need to have a better understanding of the operation of commercial fluidized-bed reactors, the best approach to small-scale fluidized-bed studies is to separate the tests on chemical reaction kinetics and hydrodynamics.3 All of the tests related to reaction kinetics can be done in a small fixedbed reactor. Meanwhile, gas-solids flow patterns can be studied separately in a cold-flow fluidized-bed reactor. Although many parameters such as the influences of pressure and temperature on gas-solids flow patterns will not be properly studied, data in the literature such as work by Knowlton4 can be used to help understand the issues. This approach makes the complicated task relatively easy. Once the results in reaction kinetics and reactor hydrodynamics are available, there are some models5 that can be very helpful in understanding the impact of operating parameters on the performance of commercial reactors. Literature Cited (1) Wang, S. J.; Geldart, D.; Beck, M. S.; Dyakowski, T. A Behavior of a Catalyst Powder Flowing Down in a Dipleg. Chem. Eng. J. 2003, 77 (1-2), 51-56. (2) Rowe, P. R. A Model for Chemical Reaction in the Entry Region of a Gas Fluidized-Bed Reactor. Chem. Eng. Sci. 1993, 48, 2519-2524. (3) Kashkin, V. N.; Lakhmostov, I. A.; Zolotarskii, V. S.; Noskov, A. S.; Zhou, J. J. Studies on the Onset Velocity of Turbulent Fluidization for Alpha-Alumina Particles. Chem. Eng. J. 2003, 91, 215-218. (4) Knowlton, T. M. Fluidization, Solids Handling, and Processing. In Pressure and Temperature Effects in Fluid-Particle Systems; Yang, W.-C., Ed.; Noyes Publications: Westwood, NJ, 1999; Chapter 2. (5) Thompson, M. L.; Bi, H.; Grace, J. R. A Generalized Bubbling/Turbulent Fluidized-Bed Reactor Model. Chem. Eng. Sci. 1999, 54, 2175-2185.
Received for review October 7, 2003 Revised manuscript received December 15, 2003 Accepted December 17, 2003 IE030752C
