Energy & Fuels 1994,8,1304-1309
1304
A Novel Laboratory Scale Short Contact Time Batch Reactor System for Studying Fuel Processes. 1. Apparatus and Preliminary Experiments He Huang, William H. Calkins,*and Michael T. Klein Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received April 11, 1994@
Study of the chemical reactions involved in the initial stages of coal liquefaction was carried out in a novel laboratory scale batch reactor capable of operation up to 450 "C and 17 MPa (2500 psig) under well-defined contact times from a few seconds to 30 min or longer, This report covers the design, construction, and use of this apparatus for studying coal liquefaction and other fuelrelated processes. The characteristics of the reactor system and its operation include the following: (1)The reaction mixture a t ambient temperature is driven through a preheater into a reactor using high-pressure gas. Both preheater and reactor are in a sand bath at reaction temperature prior to the start of the reaction. (2) The reaction mixture is then agitated by gas bubbles introduced through the mixture from the bottom of the reactor. ( 3) The process is terminated (quenched) in a similar way by blowing the reaction mixture through a precooler into a receiver with high pressure gas. This reactor is particularly effective for running coal liquefaction and the hydroprocessing of coal-derived vacuum resids, when kinetic measurements at very short reaction times are required. The reactor is simple in design and can be conveniently operated in the laboratory. It has the potential t o augment the widely used tubing bombs or small autoclaves for laboratory studies of many high-pressure, high-temperature reactions.
Introduction In many high-pressure and high-temperature reactions, important information concerning the chemical and physical processes occurring can be obtained through study of the reactions at the very early stages (the first few seconds to 2-3 min), before complicating secondary reactions occur. The major challenge to do this is to bring the system up to the reaction temperature and pressure in a time frame less than that required for the physical and chemical processes t o occur. It is also important to agitate the reactor contents to maintain uniformity of temperature and concentration, particularly in reactions of slurries or other heterogeneous systems. Coal liquefaction and other fuel processes, where free radical reactions are involved at temperatures in the range of 350-450 "C, are excellent examples of processes where the necessity for study of the reactions at very short contact times and low conversions has been recognized. Neavel showed that coal undergoes significant changes in a few minutes at temperatures as low as 350 "C.l Whitehurst, Mitchell, and Farcasiu in their book on coal liquefaction point out the importance of and discuss experiments aimed at studying the process at short contact times to understand coal liquefaction.2 In most laboratory high-pressure equipment used t o study high-temperature, high-pressure fuel processes, for example, tubing bombs or autoclaves, the heat-up and cool-down times of the massive equipment required t o hold the pressure are long compared to the times Abstract published in Advance ACS Abstracts, October 1, 1994. (1)Neavel, R. C. Fuel 1976,55, 237. (2) Whitehurst, D. D.; Mitchell, T. 0.; Farcasiu, M. Coal Liquefuction: The Chemistry and Technology of Thermal Processes, Academic Press: New York, 1980. @
i 0'
0
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600
400
800
1000
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Figure 1. Representative temperature-time profile for a 20 cm3 tubing-bomb reactor containing 10 g of tetralin.
involved in the reactions themselves. A representative temperature profile for a typical 20 cm3 tubing-bomb reactor containing 10 g of tetralin is shown in Figure 1. The long heat-up and cool-down times involved obscure fundamental interpretation of the initial rates and reaction pathways and do not allow practical process studies to be made at very short reaction times. Various researchers have devised systems to avoid these problems. Cassidy and co-workers3used a magnetically stirred batch autoclave which was preheated and charged with solid and liquid reactants with a hydraulically-operated ram. Samples were taken via a series of valves located at the base of the autoclave into a quench system. Kinetic data were obtained on Australian brown and bituminous coals, sampling at 15 s intervals over the first 1.5 min and less frequently thereafter. (3) Cassidy, P. J.; Jackson, W. R.; Larkins, F. P.; Louey, M. B.; Rash, D.; Watkins, I. D. Fuel 1989,68,32.
0887-0624/94/2508-1304$04.50/0 0 1994 American Chemical Society
A Novel Laboratory Scale Batch Reactor System
An alternative approach is to use a flow system. McPherson et al.4 charged a packed-bed microreactor with a mixture of coal and y-alumina and pumped tetralin through the reactor a t various temperatures and pumping rates. Kandiyoti and co-workers5passed a stream of tetralin through a bed of coal and examined the products formed as a function of time. While these flow systems have been important in isolating the initial products of the liquefaction reaction, the data resulting therefrom are not directly translatable into reaction kinetics from a conventional liquefaction reactor. Provine and co-workers devised a system to follow coal liquefaction in the first 10 s to 3-5 min of the reaction using a flow system. The solvent, catalyst, etc. are brought up to the desired flow rate and temperature, and the coal or other fuel material is injected batchwise into the solvent stream. Sample aliquots are then taken at intervals for analysis and rate measurements. Such a system and the results of coal liquefaction kinetic measurements are described in references 6 and 7. While it is capable of providing accurate kinetic data at very short contact times, the experimental equipment is very complex and expensive, and the time required to make a series of kinetic runs is substantial. In addition, a large amount of solvent is required per run. If changes in solvent, catalyst, and reaction conditions are to be studied, the time and cost required to make a meaningful series of kinetic runs with various coals are considerable. This paper describes the design, construction, and operation of a novel short contact time batch reactor (SCTBR)system including blow case, preheater, reactor, cooler, and receiver for studying the conversion kinetics of direct coal liquefaction and other fuel processes at reaction times from a few seconds to 30 min or more. The apparatus is brought up to reaction temperature before the reaction mixture is injected, and the reactants are blown through the preheater and into the preheated reactor in less than 0.3 s with high-pressure gas. Measurements and calculations have shown that reaction temperature is achieved during the injection. Agitation of the reaction mixture is achieved by injection of gas at the bottom of the reactor, and the degree of agitation is measured and controlled by the exit gas flow rate. This reaction system is simple to operate and can be used t o make kinetic runs at times as short as 10 s and longer and many reaction variables such as solvent type, coal type, concentration, catalyst type, and many other variables can be screened quickly and easily in the laboratory. Experimental Apparatus and Operation Apparatus. The key components of the reactor system are illustrated in Figure 2. The batch reactor was constructed of 3/4 in. 0.d. 316 stainless steel tubing of approximately 12 in. in length with wall thickness of 0.433 in. The preheater consisted of about 21 R of a (4) McPherson, W. P.; Foster, N. R.; Hastings, D. W.; Kalman, J. R.; Okada, K.; Heng, S. Fuel 1986,64,454. ( 5 ) Gibbins, J. R.; Kandiyoti, R. Fuel 1991, 70, 909. (6) Huang,H.; Provine, W. D.; June, B.; Jacintha, M. A,; Rethwisch, D. G.; Calkins, W. H.; Klein, M. T.; Dybowski, C. R.; Scouten, C. G. Proc. 7th Int. Conf. Coal Sei. 1993,1 , 266. (7)F'rovine, W. D.; Jung, B.; Jacintha, M. A.; Rethwisch, D. G.; Huang, H.; Calkins, W. H.; Klein, M. T.; Scouten, C. G.; Dybowski, C. R. Catal. Today 1994, 19, 409.
Energy & Fuels, Vol. 8, No. 6,1994 1305
lit-
Thermocouple
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Figure 2. Short contact time batch reactor (SCTBR)for studying fuel processes at high pressures and high temperatures. coiled l/4 in. 0.d. 316 stainless steel tubing with wall thickness of 0.035 in. In operation, both the empty preheater and reactor are immersed in a sand bath and brought up to the desired reaction temperature prior to the start of reaction. Using high-pressure gas, the reaction mixture at ambient temperature is driven into the reactor from a small blow case through the preheater. To determine whether plug flow occurred through the preheater tubing, an empty bomb which was identical in dimensions to the reactor was substituted for the reactor and a valve was inserted at the entrance of the bomb. A series of experiments was carried out in which different amounts of the tetralin were blown into the bomb with 1000 psig of nitrogen through the preheater tubing from the blow case (see Figure 3) with the system at ambient temperature. The valves were closed within approximately 2 s from the time of injection. The bomb and the blow case were then depressurized and the amounts of tetralin in each were determined. The results are summarized in Table 1. Approximately 9093 wt % of the tetralin were recovered in the bomb. Only a few drops (less than 0.5 g) were found in the preheater tubing. The same amounts of the tetralin were retained in the blow case regardless of the amount of the tetralin charged, suggesting that most of the holdup occurred in the blow case itself. If there had been a breakthrough of the gas through the tetralin (not plug flow) during the transport through the preheater, the high and constant percentage recovery would probably not have been observed. The injected reaction mixture in the reactor is agitated by gas bubbles introduced from the bottom of the reactor. The degree of agitation is controlled by the exit gas flow rate from the top of the reactor and the configuration of the gas orifices. In the case where the agitating gas is also a reactant (e.g., hydrogen in hydroprocessing), the gas bubbles provide very high contact or interfacial surface between the gas and the liquid or slurry reaction mixture. At a selected time, the reactor contents are driven out of the reactor through a precooler and into a cold receiver with highpressure gas. Both receiver and precooler are immersed in a water bath or other cooling medium. Cooling of the product mixture to about 25 "C is achieved during the transport.
Huang et al.
1306 Energy & Fuels, Vol. 8, No. 6, 1994
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Figure 3. SCTBR system for studying fuel processes from Table 1. Material Balance in Reactor, Preheater Tubing, and Sample Reservoir (Tetralin; lo00 psig of 25 "C) feed (g) recovery (g) (%)
remainder in sample reservoir (g) (%)
unrecovered (g) (%)
29.5 26.8 90.8 1.9 6.4 0.8 2.7
33.2 30.9 93.1 2.1 6.3 0.2 0.6
Temperature in lhe Reactor
40.1 37.3 93.0 2.1 5.2 0.7 1.7
Further details of the reactor system are shown in Figure 3. The heating bath is a Techne IFB-52 industrial fluidized sand bath, which maintains a reaction temperature of 1 2 "C. The approximately 30 cm3 reactor is capable of containing 17 MPa (2500 psig) pressure up to 550 "C. The tubing used for the precooler was identical to that used for the preheater. Since a gas (e.g. hydrogen or nitrogen) is bubbled through the reaction mixture under pressure and out through a letdown valve, a small water-cooled condenser above the reactor is necessary to avoid loss of solvent or other low boiling components. For operability, a disengaging space above the condenser before the let-down valve is also required. Reaction Temperature Profile. A typical temperature-time profile for a liquefaction run of Illinois No. 6 coal in tetralin at 390 "C is shown in Figure 4. At the time of injection, the reactor temperature dropped by 5-8 " C . This was recovered as the reactor returned to the sand bath temperature within 30 s. The rapid response and stable temperature profile are due t o the small quantity of the reaction mixture relative to the massive preheater. With this essential design feature, the thermal transient is entirely focused on the reactants as they proceed through the preheater. Temperature drop at injection can be reduced by longer preheater tubing. Sample Recovery. Sample recovery is defined as the ratio of the amount of the material driven into the receiver to that charged into the blow case. Surface
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40
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Figure 4. Representative temperature-time profile for a typical short coal liquefaction run (Illinois No. 6 coal; T:C= 8:l; 390 "C; 1000 psig of Nz).
wetting causes a hold-up of material when the reactants are driven through the reactor system. The overall recovery of material from a reactor run depends on the surface areas of these equipment pieces and the viscosity or fluidity of the sample plug. The preheater and precooler represent the bulk of the surface involved. On the one hand, the longer the preheater, the smaller the temperature drop on injection. On the other hand, the longer the preheater, the lower the overall sample recovery. This necessitates a design trade-off between the temperature drop on injection and the degree of overall material recovery from the reactor. For the process streams studied thus far, recoveries have varied from 65 to 90 wt %, depending on the viscosity of the process stream. Clearly some surface wetting will occur and 100% recovery of the reaction mixture is not practical. This means that, for mechanistic studies, analytical methods must be available t o follow the course of the reaction with aliquots as is done with many other kinetic experiments. It is critical, however, that the aliquots be representative of the reactor contents. This was shown to be the case in coal liquefaction and resid conversion experiments by com-
A Novel Laboratory Scale Batch Reactor System
paring the products discharged into the receiver with those charged to the reactor at ambient temperatures. When a higher material recovery is required, the first sample can be supplemented with solvent washes of the system. The additional quantities of the reaction mixture can be recovered by solvent removal in a rotovapor. Of course, the material recovered in a second wash has not experienced exactly the same thermal history as the first sample and is therefore not useful for kinetic data. Cleaning System. The reactor system is cleaned in place by a series of suitable solvent washes. The number of washes and type of solvent used are selected based on the materials and processes being studied. The wash effectiveness can be monitored by analysis of the wash streams. For heterogeneous mixtures, such as involved in coal liquefaction and coal-derivedvacuum resid hydroprocessing, the wash effectiveness was followed through analysis of both the soluble reactant concentration and the particulate concentration in the wash streams. For example, tetralin was a suitable solvent in these cases and about five 30 cm3 washes were required to obtain about 99% sample recovery. Fluid Flow and Heat Transfer within the Preheater. The concept of the SCTBR system is to bring up both preheater and reactor to the desired reaction temperature prior t o the start of the reaction. This eliminates the long time involved in heating the reactor itself and provides heat to the preheater tubing for transfer to the process stream. The transport of the process stream from the blow case into the reactor through the preheater occurs very rapidly (in about 0.3 s) in turbulent plug flow. The rapid heat up is due to both the large amount of heat contained in the preheater tubes and the gradient developed in the axial direction that provides the efficient transfer of this heat under turbulent flow conditions. The time of the transport of the process stream (plug) through the preheater tubing was determined by temperature measurements in the reactor using a thermocouple attached to a high speed Linseis (Series 2000) strip chart recorder. The response time of the thermocouple in the reactor, which is the time between the start of the injection and the beginning of the temperature change, was determined by the recorder a t a chart speed of 100 cndmin. It includes the time delay of the thermocouple ( t d 0.38 s) and the transport time of the plug ( t t r ) through the preheater tubing into the reactor. Mathematically,
tr = t, + t,, Multiple tests including coal-tetralin, resid-tetralin, and tetralin alone indicate that the response time ( t r ) of the thermocouple in the reactor is 0.65 f 0.03 s. Using eq 1,the measured transport time of the plug (ttr) through the preheater tubing from the blow case into the reactor is about 0.3 s, which is in agreement with fluid flow calculations (0.31 s, see part 2 of this series). A n example of the operation of the SCTBR system is demonstrated using 30 g of tetralin. The relevant experimental conditions involved were 1000 psig of nitrogen pressure, reaction temperature 400 "C and ambient temperature 20 "C. The temperature of the tetralin stream reached 392 "C when the entire stream was in the reactor.
Energy & Fuels, Vol. 8, No. 6, 1994 1307 The large axial gradient in wall temperature developed along the preheater tubing during this injection is the key to the efficient heat-up. For example, if the sensible heat of the preheater was transferred to the process stream uniformly along the length of the preheater, i.e., without the axial gradient in wall temperature, the process stream could only reach 353 "C, according to an overall "integral" energy balance. This would leave the sample 39 "C below the experimentally observed temperature. The low heat-transfer coefficient between the fluidized sand and the preheater (0.340.68 kW*m-2*oC-1 or 60-120 Btwh-1-ft-2*"F-1)8and the very short residence time (0.3 s) make inconsequential the heat transfer from the fluidized sand to the preheater tubing and process stream as a source of the additional heat-up. Quantitatively, the amount of this heat transfer in the short transfer interval is less than 0.63-1.26 kJ, accounting for only 3-7% of total required heat to bring 30 g of tetralin from 20 to 392 "C. Rather, the axial gradient allows for the early portions of the preheater tube to fall well below the "uniform transfer'' temperature of 353 "C, and other later portions t o remain at, essentially, the bath temperature. This allows for essentially complete heat up as follows. As the first increment of tetralin proceeds through the preheater tubing, the temperature of the plug rises from ambient (e.g., 20 "C) at the entrance closely approaching the reaction temperature (e.g., 400 "C) at the exit. Thus the temperature gradient for heat transfer, AT, at the entrance is much higher than that at the exit. Each incremental amount of the tetralin experiences different temperature gradients along the preheater. Thus, the temperature gradients for heat transfer along the preheater tubing are functions of both the position along the preheater and the sequence of tetralin increments going through the preheater. The result is that more heat is extracted from the preheater at the entrance, where the temperature gradient is larger, and less is extracted at the exit. The net effect is that more heat is extracted overall from the preheater than that calculated from a simple heat balance. In conclusion, this preliminary analysis indicates that the rapid response and stable temperature-time profile of Figure 4 are not only due t o preheating both the comparatively massive reactor and preheater t o the reaction temperature prior to the start of the reaction (this allows the thermal transient to be focused only on the reaction mixture), but also due to the axial temperature gradient developed along the preheater. This allows more sensible heat to be extracted from the preheater a t the entrance portion of the preheater and leaves a high -temperature zone near the exit for the final approach to the reaction temperature. Mathematical treatment of the momentum and heat transfer within the preheater are described in part 2 of this series. The same analysis can be applied to the precooler during the quenching process.
Results and Discussion Coal Liquefaction. Liquefaction runs of the Illinois No. 6 coal (C) in tetralin (T, the H-donor solvent) at very ( 8 ) Summers, P. H. Presented at The 48th Annual Convention of the Wire Association International, Inc., St. Louis, MO, Oct. 17, 1978.
Huang et al.
1308 Energy & Fuels, Vol. 8, No. 6, 1994
E:
Table 2. Coal Liquefaction Runs (Illinois No. 6; TIC = 8; 1000 psig of Nz) run no. DOE DOE DOE DOE DOE DOE DOE DOE DOE DOE DOE DOE DOE DOE
001 012 014
015 016 017 018 020 021 019 013 011 009 010
T,"C
t , min
recovery, wt %
15 390 385 384 385 386 385 387 387 385 390 390 390 390
30.00 0.17 0.33 0.50 0.75 1.00 2.00 3.00 5.00 7.50 10.00 15.00 30.00 60.00
76 85 81 75 86 82 86 92 89 88 81 89 85 77
Time, min
20
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(9) Huang, H; Wang, K.; Calkins, W. H.; Klein, M. T. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel C h e n . 1994,39 (31, 741.
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Resid derived from Pittsburgh bituminous coal
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Figure 5. Conversion kinetics of the Illinois No. 6 coal liquefaction in tetralin at 390 "C (T:C = 8:l;1000 psig of Nz). short reaction times are summarized in Table 2. Samples could be taken at as short a reaction time as 10 s with confidence that the desired reaction temperature during this interval was substantially achieved. Recovery of the reaction mixture was about 80-90 wt %. No difficulty was encountered in running the reaction mixture through the reactor system with T/C weight ratio up to 2:l. Conversion was determined by measuring the ash content of the coal liquefaction residue using thermogravimetric analysis on aliquots.9 The conversion kinetics of the Illinois No. 6 coal liquefaction in tetralin at 390 "C is shown in Figure 5 . It will be noted that there is very rapid conversion of the coal in the first minute of the liquefaction process. This is apparently due to solvent extraction of the soluble components of the coal by the donor solvent. After about 1 min, a steady conversion of the coal at 390 "C was observed up to at least 50% conversion at 1 h. Hydroprocessing of Coal-Derived Resids. The hydroprocessing reactions of coal-derived distillation resids in tetralin under hydrogen pressure with and without a catalyst of Ni/Mo on alumina were tested in the reactor as an example of heterogeneous mixtures. Binary mixtures of resid (R) in tetralin (T) with R/T ratios from 1:2 to 1 5 proved easy to feed. The lower R/T ratios gave lower sample recovery (as low as 65%) due to the higher viscosity of the feed material. Thermogravimetric determination of the ash content provided an estimate of the conversion of two resids, one derived from Pittsburgh bituminous coal and the other from Wyodak-Anderson subbituminous coal.1°-12 The
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A Novel Laboratory Scale Batch Reactor System Conclusions A novel laboratory scale short contact time batch reactor (SCTBR) capable of operation up to 450 "C and 17 MPa (2500 psig) that provides samples at well defined contact times from a few seconds to 30 min or longer was designed, constructed, and operated. The rapid heat-up of the sample occurs because of the prior heating of the metal preheater tubing and batch reactor and because of the rapid and efficient transfer of the sensible heat from the preheater t o the process stream. Agitation is maintained by bubbling gas through the reactor. The degree of agitation is controlled by the exit gas flow rate.
Energy & Fuels, Vol. 8, No. 6, 1994 1309
This SCTBR reactor was shown t o be suitable for the study of fuel processing such as coal liquefaction, coalderived resid hydroprocessing, and model compound reactions.
Acknowledgment. The support of various portions of this work by the Department of Energy under DEFG22-93PC93205 and by CONSOL Inc. under DOE subcontract DE-AC22-89PC898893 is gratefully acknowledged. Additional funds for purchase of thermal analysis equipment was provided by the University of Delaware.
