Ind. Eng. Chem. Process Des. Dev. 1984, 23, 242-250
242
kL = rate constant of coking by multilayer coke, h-I atm-’ PA, PB = partial pressures of A and B, respectively, atm q = total coke content of catalyst, mg of coke/g of cat. S = active site t = operation time, h Greek Letters a’, a2 = deactivation constants, mg of coke/g of cat.
p , PI, pZ = deactivation constants, g of cat./mg of coke y = pc
deactivation constant, h-’
= stoichiometric factor, mol/mg of coke Registry No. C,7440-44-0.
Literature Cited Anderson. R. B.; Whitehouse, A. M. Ind. Eng. Chem. 1061, 53, 1011. Beeckman, J. W.; Froment, 0.F. Chem. f n g . Scl. 1080, 35, 805. Butt, J. B.; Rohan, D. M. Chem. Eng. Scl. 1068, 23. 489. Cooper, 8. J.; Trlmm, D. L. "Proceedings, International Symposium on Catalyst Deacthratlon, Antwerp”; Elsevier: Amsterdam, 1980 p 63. Dumez, F. J.; Froment, G. F. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 291. Froment, G. F. Proc. Int. Congr. Catal. 6th, London 1976, 10
Froment, G. F. “Proceedings, International Symposium on Catalyst Deactlvatlon, Antwerp”; Elsevler: Amsterdam, 1980 p 1. Froment, G. F.; Blschoff. K. B. Chem. Eng. Scl. 1061, 16, 189. Froment, G. F.; Blschoff, K. B. Chem. Eng. Scl. 1962, 17, 105. Krlshnaswamy, S.; Kittrell, J. R. Ind. Eng. Chem. Process D e s . Dev. 1978, 17, 200. Krlshnaswamy, S.; Kittrell, J. R. Ind. Eng. Chem. Process D e s . Dev. 1979, 18, 399. Ozawa, Y.; Blschoff, K. B. Ind. Eng. Chem. Process Res. Dev. l988a 7 , 67. Ozawa, Y.; Bischoff, K. 8. Ind. Eng. Chem. Process Res. Dev. 1068b 7 , 72. Prater, C. D.; Lago, R. M. Adv. Catal. 1056, 8 , 372. Relff, E. K. Jr.; Kittrell, J. R. Ind. Eng. Chem. Fundam. 1060, 19, 126. Romero, A.; Bilbao, J.; Gonzalez-Velasco, J. R. Ind. Eng. Chem. Process Des. D e v . 1081, 2 0 , 570. Rudershausen, C. 0.; Watson, C. C. Chem. Eng. Scl. 1054, 3 , 110. Takeuch. M.; Ishige, T.; Fukumuro, T.; Kubota, H.; Shlmdo, M. Kagaku Kogaku 1966, 4 , 387. Voorhies, A. Ind. Eng. Chem. 1945, 3 7 , 318.
Receiued for review November 29, 1982 Revised manuscript receiued May 9, 1983 Accepted June 20, 1983
Tubing Bomb Coal Liquefaction Technique Peter S. Maa;
Richard C. Neavel, and Lonnie W. Vernon
Exxon Research and Englneerlng Company, Baytown, Texas 77520
An experimental coal liquefaction test procedure is described in which coal, solvent, and hydrogen are reacted in a small, agitated batch reactor (tubing bomb) that is rapkay heated In a sand bath. After completion of the desired reaction tlme, the reactor is quench-cooled, and solM and IlquM reaction products are removed using cyclohexane as a wash medium. Conversion of coal to cyclohexane soluble material is reproducible and correlates well with the conversion to 538 OC (1000 O F ) minus boiling point liquids obtained by larger scale process units. The principal data obtained with the tubing bomb technique are: hydrogen consumption (molecular hydrogen and hydrogen transferred from the solvent), gas make (H,S, COX,and hydrocarbon gases), water make, liquid yiekls, solid residue (cyclohexane insolubles), and conversion. Details of the apparatus, experimental procedure, sample analysis procedures, and data workup are described. Experimental results for a bituminous, a subbituminous coal and a lignite are shown to illustrate the technique.
Introduction Since 1968, investigators of coal conversion processes at the Baytown, Texas laboratory of Exxon Research and Engineering Company have effectively employed a small reactor system referred to as a “tubing bomb” by Neavel (1976). The system, though conceived independently, is in some respects similar to that described by Curran et al. (1967). Through the years since 1968, it has been modified and improved, and has been proven to be a versatile tool. It was originally developed to replace large, slow-heating autoclaves used to study coal liquefaction reactions, and has since been employed in a broad range of investigations where relatively small samples are adequate and where a rapid heat-up of reactants is desirable. In the coal liquefaction area, tubing bombs have been used to investigate the effects of variations in feedstocks, solvents, and process conditions. They have also been employed to study reactions of pure compounds as models of liquefaction systems by Aczel et ai. (1979) and by Vernon (1980). Catalytic processes and noncatalytic processes have been studied in a variety of liquid and gaseous media, including pyrolytic reactions in inert atmospheres. 0196-4305/84/1123-0242$01.50/0
The objective of this paper is to provide to the scientific and technical community a detailed description of the procedures that are employed in the use of tubing bomb reactors to investigate the liquefaction of coal in a solvent or vehicle. We believe that the utility and versatility of the system has withstood the test of time and that our experiences could be usefully passed on to other investigators. Because some other investigators are already using similar systems, we believe that comparability of results will be enhanced if procedures are most similar. The description of procedures that follows is quite detailed. Though some shortcuts can be employed, we have found that they invariably lead to results that do not compare favorably to those obtained when all of the specified steps are carefully followed. We caution the reader, therefore, that, based on our experience, one assumes a real risk of deriving erroneous conclusions should the procedures not be followed carefully.
Experimental Apparatus The apparatus used for a tubing bomb liquefaction experiment includes a tubing bomb setup, a gas charging system, a gas collection system, a top load balance with 0 1984 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Voi. 23, No. 2, 1984 243 STOPPER
UEIISTRUT
Abbreviations AC AFCT AMP DPCS F Fi FR LPS MA MPSB PI PT SCR SPC
TLC
TIMER SPC
S
I ,
TB TB H Ti TLC TR
TBH
L
1
WATER OUT
Tw
Air Cylinder Agitation Frequency Control Timer Ammeter Digital Pressure Calibration System Air Filter Flow Indicator Flow Regulator Low Pressure Shutdown Milli-Ammeter Multi-Point Swltch Box Pressure Indicator Pressure Transducer Silicone Control Rectifier Set Point Controller Solenoid Valve Tubing Bomb Tubing Bomb Holder Temperature Indicator Temperature Limit Controller Temperature Rworder Thermoweil
WATER BATH
WATER
"
Figure 1. Tubing bomb setup.
a tubing bomb stand, a tubing bomb preparation stand, a tubing bomb vise,and other conventional apparatus used for coal preparation such as crusher, sample splitter, vacuum oven, etc. Only equipment which was devised and constructed in our research laboratory will be discussed in detail below. Tubing Bomb Setup. The tubing bomb setup is shown schematically in Figure 1. The setup consists of six tubing bombs, a tubing bomb holder, an agitation system, a rope and pulley system, a sand bath, a water bath and a control panel. Tubing Bomb. The tubing bomb is a miniature liquefaction autoclave. It is made of 316 stainless steel tubing and Swagelok fittings. Several different configurations have been tried and wed in the past six years. The most effective configuration is shown in Figure 2. This bomb has a total volume of about 36 cm3 and a working pressure of 204 atm. The body of the tubing bomb consists of a horizontal tube with caps at both ends and a small diameter vertical extension tube (stem) welded in the middle of the horizontal tube. The horizontal tube has two preset ferrules to hold nuts and caps together. Both nuts are placed on the bomb during ita construction and are not removable once the ferrules are set. Both caps are removable to allow for charging the reactants at the start of experiments and to facilitate cleaning the bomb at the end of experiment. The life of a tubing bomb is determined by how the ferrule is set. If the ferrule is not set correctly, the bomb will leak and this is difficult to correct later. Therefore, the procedure for presetting the ferrule will be discussed in the experimental procedure. The extension tube is used for pressure measurement and for gas charging and collection. The extension tube is connected to a union tee to allow a pressure transducer or a 2.5 in. diameter pressure gauge to be attached to the bomb to monitor the bomb pressure. A side tube with a needle valve and a male quick connector provides easy connection of the extension tube to a manifold for pres-
NOTE: ALL TUBING 55 316 SEAMLESS
QUICK CONNECT
(032cmXO07sm)
1/4"r 0 0WELD% 3
i
(0 64cm X 0 0 9 4 41'
(10 2"
WELD
3/8(1x 0 065 (O95cm X 0 1 7 4
4'l I10 2cm)
II
P4"
I
NVT
CAP
(1 91 cm X 0 3 4
t
Figure 2. Tubing bomb.
surization with reaction gas, for drying the bomb with nitrogen gas, or for gas collection. The needle valve provides a capability for releasing the gas slowly during depressuring. The length and diameter of the stem are
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Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984
fp TR
T i l TIC
10 79 cm)
10 GAUGE
(127"
'
15 1"
CLAMP WIDTH = 3 . 4 " ( 1 91 c m )
Figure 3. Tubing bomb holder.
Size of Heater Beaded Electrical K W Volts Amps Gauge Length Controller No. of Heaters Heater Nichrome V
To Air Cylinder
r-4
u, To Tubing Bomb Holder Figure 4. Pin.
designed to minimize heating of the pressure measuring device beyond 49 O C and to provide a low dead volume. The lowest portion of this stem is made of larger tubing (0.95 cm) than the remainder for a stronger support of the upper extension portion, and more importantly, to allow quick draining of slurry from the stem back to the bomb to avoid liquid carryover during gas collection. Tubing Bomb Holder. Normally, a set of six tubing bombs is used a t the same time. The bombs are fixed in a tubing bomb holder, which is shown schematically in Figure 3. The holder consists of two racks and a vertical extension. Each rack is provided with six clamps (see Figure 3) to hold three tubing bombs. The extension is provided with a horizontal bar, a handle, and a hole. The horizontal bar is provided to secure the extension tubes of the tubing bombs with a metal wire or clamp. The hole allows the holder to be connected to the agitation system by a pin. A schematic diagram of the pin is shown in Figure 4. Agitation System. A single-ended, double acting air cylinder is controlled by two solenoid valves which in turn are controlled by a frequency timer. The frequency of the timer can be adjusted between 0 and 150 cycles per minute. The stroke of the piston of the air cylinder is about 5 cm. A U-shape metal clip is welded to the lower end of the piston of the air cylinder which can hold the tubing bomb holder by a pin as described above. The upper end of the air cylinder is tied to a rope and pulley system. Note that it is not necessary to mount the cylinder rigidly and the agitation is in an up-and-down manner instead of con-
5
230
23.8
t 12
91'
SCR
3, parallel
(28 ml
Figure 5. Sand bath.
ventional rotary nature as in other autoclave operations. Rope and Pulley System. A pulley is fixed in two wheels which can travel in a unistrut to provide horizontal movement of the tubing bomb holder between the sand bath and water bath. A nylon rope attached to the air cylinder and run through the pulley is used to control the height of the tubing bomb holder in the sand bath and water bath. Sand Bath. The sand bath is a heated, air fluidized sand bed. A schematic diagram with dimensions is shown in Figure 5. The fluidizing air comes through a porous steel plate at the bottom of the sand bath. In case of loss of air, the heater is automatically turned off by a low pressure switch, set at 0.14 atm (2 psi), in the air line. An expanded top is provided to minimize any loss of sand. Three beaded electrical heaters surround the vessel, and are "mudded" in. The sand bath is provided with J-type thermocouples located at four different bed depths in a common thermowell. The temperatures at those four depths are continuously recorded. Three thermocouples are located at the middle position (no. 3 in Figure 5) of the sand bath: one is connected to the controller and SCR to control the temperature of the sand bath; the second one is connected to a limit controller to prevent temperature runaway; and the third one is connected to the temperature indicator and recorder. A multi-point switch box allows a digital temperature indicator to show temperatures at any of the four depths at any time. The sand bath temperature can be controlled to be k0.6 "C by maintaining the bath slightly above the minimum fluidization velocity. The sand bath can also be heated up linearly at a rate of 0.6-1.7 "C/min from any lower temperature to a higher temperature by adjusting the power input to the heaters. The maximum operating temperature is 538 "C. Control Panel. Most of the instruments used in the control panel have been described above and are shown in Figure 1. They can be grouped by their functions for easy identification. Thme appearing within the dotted line in Figure 1 are not accessible from the front of the panel; reaction pressure, DPCS-1, DPCS-2; air flow to sand bath,
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 2, (5.1"
1984
245
j"
b
FW CYLlNDER GAS
13 1/2"
CONNECT QUICK
Figure 6. Gas charging system. (I 91"
YiPIPE
c1 IS
FEMALE
QUICK CONNECT
UJ! Figure 8. Tubing bomb weighing stand.
G 8 , 8"
DRA N TO 3,NK
CILlNLlER
CB2 ! " l
Figure 7. Gas collecting system.
FI, FR, LPS; temperature of sand bath, AMP, MA, TLC, SPC, TI, MPSB, TR, SCR; agitation, AFCT; residence time, timer. Gas Charging System. The gas charging system is shown schematically in Figure 6. This system has a gas cylinder connected to a manifold. The manifold consists of a pressure regulator (PR), a pressure indicator (PI),a safety valve, a vent, 6.4 mm (W.T. 1.24 mm) stainless steel tubing with eight ports, a pressure transducer, and a digital pressure calibration system. Each port is provided with a valve and a female quick connector to fit the tubing bomb. The pressure calibration system provides accurate pressure measurement (f0.14 atm) and the vent is used for venting excess gas. This system is housed in a hood for safety and can be used to charge any gas or gas mixture desired for the liquefaction. It provides a constant pressure charge for all the bombs connected at the same time. Hydrogen gas is commonly used in the liquefaction study and for calibrating the volume of the bomb. Gas Collecting System. The gas collecting system is shown schematically in Figure 7. It consists of two 1.91-cm diameter pipes and six glass collecting bulbs-three 500mL bulbs and three 250-mL bulbs. the 500-mL bulbs are connected to the two pipes with tygon tubing and the 250-mL bulbs are connected to the two pipes with plastic quick-connects to facilitate removal for gas analysis. The system is filled with water from the one end of the lower pipe until the water comes out from one end of the upper pipe. The other end of the upper pipe is connected to a U-tube which is connected, in turn, to a female quick connector for attaching the tubing bomb. The other end of the lower pipe is directed to a graduated cylinder for water volume measurement. A trough with drain is mounted underneath the lower pipe to contain water spills. Tubing Bomb Weighing Stand. The tubing bomb weighing stand, made of aluminum, is shown schematically in Figure 8. The base of the stand is made to fit the pan of a top loading balance. The vertical extension provides support for the stem of the tubing bomb. The body of the bomb is seated against the stops in the base. The whole
f
I15 3cmI 6"
t
Figure 9. Tubing bomb preparation stand. BOX
/ F?% WELDED TO STAND) GATE
I II
Figure 10. Tubing bomb vise.
system is surrounded by a plexiglass draft shield. Tubing Bomb Preparation Stand. A tubing bomb preparation stand is shown schematically in Figure 9. This stand is constructed of wood and allows six tubing bombs to be held firmly while charging the reactants. With one side of the tubing bomb capped and inserted into the hole, it is convenient to charge the solid or liquid reactants into the bomb from the other end. When the bomb is not in use, it is advisable to seat it in the stand to avoid accidental dropping of the pressure transducer. Tubing Bomb Vise. The tubing bomb vise is shown schematically in Figure 10. The vise consists of a cut off ll/sin. box wrench, a piece of metal with a hole cut in the upper end to fit a tubing bomb, a gate to close the hole, and a foot to locate the bomb. This vise holds the bomb with a 30" slope, thereby preventing the slurry from dripping out when the cap is removed. The wrench is to facilitate holding the cap while an open-end wrench is used on the nut.
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Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 2, 1984
Experimental Procedure The tubing bomb experimental procedure can be divided into the following steps: A, preparation of coal samples; B, preparation of the tubing bombs; C, charging the tubing bombs; D, running the test; E, collecting the product gas; F, cyclohexane wash of slurry; G, preparation of product sample; H, distillation of cyclohexane decant. Each of these steps is discussed in detail in the following paragraphs. Step B is required when a new bomb is used. Steps C, D, E, and F are required for every run. Step G is required if hydrogen consumption calculation is needed. In Step H, cyclohexane-soluble liquid is prepared for analyses such as sulfur,nitrogen and oxygen contents, and for other product characterizations. Step A. Preparation of Coal Samples. The 3-g coal sample used in each tubing bomb test must be representative of the original batch of coal to ensure that the yields obtained in each bomb at the same condition are comparable. The coal sample has to be well preserved to ensure the yields obtained at different times at the same condition are comparable. Approximately 1to 3 kg of the bulk coal sample is dried to about 2 wt % moisture and ground to -60 or -100 mesh particle size (preferably under an inert gas blanket). The crushed coal is riffled to produce several samples of about 454 g each. The 454-g samples are each reduced to 30-g samples through the use of a 16-tube rotary sample splitter. Each sample is placed in a 4-02 bottle with caps loosely set. The bottles are evacuated in a vacuum desiccator for 1h, then purged with Np The caps are then tightened and sealed with plastic electrical tape. Before charging the tubing bomb, one bottle is dried with the cap removed in a vacuum oven at 93 "C and -30 mmHg for over 8 h. The oven is purged with Nz, and the bottle tightly capped and sealed with electrical tape (or the sample is kept in a desiccator under vacuum). Step B. Preparation of the Tubing Bombs. A new bomb must be leak tested with H2 at the expected operation pressure and heated up to remove the coating material on the tube prior to initial weighing. The volume of each bomb must be calibrated. The following procedures are generally used to prepare the tubing bombs. (1) Preset the ferrule of a tubing bomb. Machine smooth both ends of the tubing bomb. Put the nut, rear ferrule, front ferrule, and Swagelok ferrule pre-swaging tool (1210 series) in position on the tubing bomb and then finger tighten the nut on the pre-swaging tool. Make a vertical ink mark through the nut and bomb as a reference. Use these marks to tighten the nut about 1 and turn while the preswaging tool is in a vise. Loosen the nut, and make sure the front ferrule is rotatable but not removable by hand. If the ferrule is removable, add an additional l/ls turn. (2) Each bomb is pressure tested at 100 atm with hydrogen for 30 min. (3) Measure the volume of each bomb twice. Take the average value. It is done by charging with 650 psig Hz and then discharging Hz into the gas collection system (see section F) to obtain the volume of H2. The bomb volume is then calculated. (4) Charge about 10 g of hydrogenated creosote oil (or other appropriate solvent) to the new bomb, heat up in the sand bath for about 2 h, and then clean up the bomb with methy ethyl ketone (MEK). Dry well and record the tare weight of the bomb. Step C. Charging the Tubing Bombs. The weight charged into a bomb has to be precise because solvent to coal ratio and Hz charge on weight of coal are all related. The gas charging system will ensure a constant pressure charge for each bomb. The following procedures are generally followed to charge the bombs: (1)Take the
weight of the clean, dry bomb. (2) Take the weight of the bomb plus "no-bind". Do not put in more than 0.05 g of "no-bind". Apply it carefully only on the middle of the threads of the caps. ("No-bind" is a colloidal copper compound used to prevent seizing and galling of threaded surfaces caused by high temperatures and heavy loads.) (3) Tighten the cap at one end of the bomb; leave the other end open. Seat the bomb on a stand. (4) Add weighed amount of solvent to the bomb. (5) Add weighed amount of coal to the bomb. (6) Tighten the cap at the other end of the tube using the tubing bomb vise. ( 7 ) Take the weight of the bomb with its contents. Compare weight 7 minus weight 2 and weight 4 plus weight 5 as a cross-check. (8) Pressurize the bomb with hydrogen from the gas charging system; record the pressure and the temperature. Calculate the weight of H2 added from the pressure. (9) Take the weight of the bomb no-bind solvent coal + Hz. (10) Compare weight 9 minus weight 7 with the calculated result from weight 8 as a cross-check. Step D. Running the Test. The six prepared tubing bombs are hung on the bomb holder which is then fitted to the agitation system. The tubing bomb holder is then placed in the sand bath at a predetermined temperature and agitation (commonly 120 cyclelmin) is begun. The pressure indicators are insulated at their base by aluminum foil. A timer is used to indicate residence time. It has been determined that the heat-up period for the tubing bombs to reach 450 OC (a commonly employed temperature) is about 3 min. To compensate for this, the bombs are not quenched until 3 min after they are lifted out of the sand bath. Thus residence time is defined as the time from initial placement in the sand bath to the time the bombs are lifted from the bath. During the experiment, the temperature and pressure are recorded for various time intervals. The time-average temperature is defined as the reaction temperature. The pressure is an important indicator of whether the bomb is leaking or not. A sudden sharp drop indicates a leaking bomb. At the completion of the reaction, sand is blown from the wall of the bomb after lifting it from the sand bath. Three minutes after the bombs are taken out of the sand bath, they are quenched in the water bath, and then washed with acetone and dried. The pressure of the bomb at room temperature is recorded. The bomb is weighed and the weight is compared with the weight obtained before the reaction. They should be identical. The U-tube liquid trap is weighed and connected to the tubing bomb. The valve is opened slowly to collect the gas by water displacement (see step F, collecting the product gas). Ambient temperature and pressure are recorded. The volume of displaced water is measured, using as many gas bottles as necessary (Figure 7 ) ,and simultaneously a gas sample is collected. After gas analysis, the weight of product gas can be calculated from its volume. The weight of the gas can also be determined by difference from the weight of the bomb plus contents before and after depressuring (plus the weight of the trapped liquid, if any). With proper opening of the valve (slowly!), there should be no liquid in the trap, however. One bomb per run is kept for product analysis, i.e., by mass spectrometry, infrared spectroscopy, etc. (see Preparation of Product Sample). For the remaining bombs, the procedure in step F is followed for the cyclohexane wash. Step E. Collecting the Product Gas. The product gas collection from each tubing bomb is done to measure gas volume and to collect gas samples for analysis. The gas sample for analysis is collected in a 250-mL bulb. However, the water displacement method can result in
+
+
+
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984
absorption of water-soluble gases such as H2S and COz. When H2S determination is critical, the gas from one of the bombs is collected in a Teflon-coated aluminum bag for analysis; the other two are collected by water displacement to measure the gas volume. With both methods, providing a time for equilibration of the gases of the whole bomb after releasing to atmospheric pressure is critical. It has been found that the initial outgoing gases are disproportionately rich in light components such as H2, CHI. The following steps are followed for water displacement method. (SeeFigure 7 for the position of valves.) (1)Make sure there is no leak in the system and then connect the tubing bomb to the U-tube. (2) Isolate glass bulbs GB2, GB3 and fill up bulb GB1 for analysis. Collect the remaining gas in 500-mL glass bulbs. Allow the system to equilibrate by leaving valves between the glass bulbs open for 5 min and then isolate GB1. Collect the total displaced water and measure the volume in a graduated cylinder. Open the bomb slowly during the collection of the gas. (3) Close the tubing bomb valve and also valves 1and 3. (4) Repeat step 1 for other tubing bombs except do not fill up bulb GB1. (5) Follow step 2, but this time collect the gas sample in GB2 isolating GB1 and GB3. For the next tubing bomb use GB3, isolating GB1 and GB2. (6) After gas collection is finished for three tubing bombs, disconnect GB1, GB2, and GB3 from the system. Then replace these bulbs with another set of bulbs and repeat the above steps to collect the gases from the remaining 3 bombs. Step F. Cyclohexane Wash of Slurry. The cyclohexane insolubles give the best correlation with the bottoms obtained by vacuum distillation having a 538 "C (1000 O F ) cut point. However, the cyclohexane to slurry ratio has to be precisely defined in order to obtain reproducible results. Three alternative methods have been used: (A) Liquids are first decanted from the slurry recovered into a centrifuge tube, and then the residual solids are washed 10 times with cyclohexane at a constant cyclohexane/solid ratio. (B) The whole slurry is washed at a cyclohexane/slurry ratio of 10/1 (weight ratio) 10 times. (C) The whole slurry is washed at a cyclohexane/slurry ratio of 10/1 (weight ratio) 5 times. The quantity of cyclohexane insolubles obtained from method A is less than method B which is less than method C. In method A, the decanted liquids contain heavy components (viz., cyclohexane insolubles). Methods B and C will precipitate those insolubles giving more bottoms. Method B is used when heavy liquids (bp 260-538 "C) are used as a liquefaction solvent. Method C is used for routine liquefaction studies. The detailed procedures are described below. The contents of tubing bomb are transferred to a tared, 90-mL, heavy wall centrifuge tube. Residual contents of the bomb are rinsed with cyclohexane into the same tube. The bomb is dried with an N2 sweep; the washing and drying are continued until the weight of the individual bomb matches with its initial tare weight within 0.1 g. The weight is recorded. Cyclohexane is added to the centrifuge tube to a 10 to 1 cyclohexane to slurry ratio (by weight). The mixture is stirred with a rod for 2 min and then centrifuged at 2000 rpm for 15 min to form two layers. The upper layer is decanted to a container. The cyclohexane washing is repeated 4 times. The contents of the centrifuge tube are dried in a nitrogen-purged oven at 100 "C for 30 min to remove part of the cyclohexane and then in a vacuum oven at 100 "C for 16 h. The centrifuge tube with the insolubles is weighed. To the weight of the insolubles, add the weight of any cyclohexane insolubles on the rod and spatula. The
247
bomb is washed with MEK to reduce the weight of bomb to within 0.02 g of original tare weight and the weight is recorded. The weight of any material recovered by MEK is also added to the weight of cyclohexane-insolubles. The dried cyclohexane insolubles from each tube are analyzed for moisture, ash, and SO3 in the ash. Step G. Preparation of the Product Sample. A special sample undiluted with cyclohexane is required if the product is to be analyzed. It is very difficult to obtain a homogenous sample from the slurry for product analysis. We found the most reproducible results to be obtained by separating the liquid from the solid by centrifugation, submitting both samples for analysis, and then combining the results by knowing the ratio of liquid and solid in the slurry. The following steps are followed to prepare the product sample. As much as possible of the bomb contents is transferred to a tared centrifuge tube without using any solvent and weighed. The tube is centrifuged at 2000 rpm for 15 min to form two layers. The upper layer is decanted and the centrifuge tube with solid layer is weighed. The liquid weight is calculated by difference. Aliquots of the liquid and solid samples are submitted for analyses. We routinely submit 0.1 g of liquid and 0.2 g of solid for mass spectrometer analysis in order to assess hydrogen transfer from the solvent. Residual liquid and solid after sampling are recombined and a cyclohexane wash is carried out as described in step F. Cyclohexane soluble liquids can be separated from cyclohexane for product quality study or chemical analyses by distilling off the cyclohexane.
Sample Analysis and Data Workup The most important yield data from coal liquefaction are hydrogen consumption, gas make, water make, solid residue, and liquids. The tubing bomb technique gives a direct measure of gas make, molecular (gaseous) hydrogen consumption, solid residue, and water make (Karl Fischer method or mass spectrometer analysis) and gives an indirect measure of solvent hydrogen consumption, water make (oxygen balance or pressure measurement), and liquids. The degree of sample analysis and data workup depend on the objective of the run. The sample analyses required and the calculations for each product yield are discussed in detail below. The required sample preparations and analyses for each of the six bombs are tabulated in Table I for .reference. Hydrogen Consumption. In a donor solvent liquefaction study, hydrogen is consumed from molecular (gaseous)hydrogen and solvent hydrogen. The molecular hydrogen consumption is defined as the difference between molecular hydrogen charged and molecular hydrogen remaining after reaction. It can be caculated as follows
H2charge (atm) X bomb volume available for H2 (cm3) X (mol ~ t ) ~ z ) / ( 8 2 . 0 5 7 ( catm m ~ g-mol-' K-l) X temp at H2charge (K))
wt of hydrogen charged (g) = (pressure of
where bomb volume available for H2 (cm3) = empty bomb volume (cm3) - (wt of coal(g))/density of coal (g/cm3) - (wt of solvent(g))/(density of solvent (g/cm3)) w t of hydrogen out (9) = (mol % H2in product gas X volume of gas (cm3) X 1 atm X (mol wt),,)/82.057 (cm3 atm g-mol-l K-l) X temp at gas collection (K)
248
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984
Table I. Sample Preparations and Analyses for Each Bomb
w t % on coal. The precision of volume measurement = run no. 1
2 bomb no.
1
transducer to measure pressure gauge t o measure pressure gas volume gas analysis Karl Fischer for water mass spectrometer for water pressure change method for water mass spectrometer sample cyclohexane wash ofslurry ash analysis for Cyclohexane insoluble cyclohexane-soluble liquid by distillation
2
3
X xa
x
4
6
X
x x x x x x x a x x x x x x x
Xb
Xb
X
X
X
X Xc Xc
5
X X X
X X
Xc Xc
X X X X X
X
X
a If aluminum bag is used for collecting the gas, the volume of the gas is not measured. Optional. If Karl Fischer method is used for H,O determination, then this bomb is not avzilable for this test.
The mol % Hz in product gas is usually measured by gas chromatography. molecular H2 consumption (wt % on coal) = (wt of hydrogen in - wt of hydrogen out)/wt of coal X 100 The solubilities of molecular hydrogen in solvent at room temperature are neglected. In the pressure range of H2 charged (up to 7 MPa), less than 1.2 wt % of the total hydrogen charge is dissolved in the solvent if there is no vaporization of the solvent as reported by Simenick et al. (1977). This is much below measurement limits. The precision of measurement = fO.l wt % on coal. The solvent hydrogen consumption is calculated by the difference of donatable hydrogen (D.H.) in solvent charged and donatable hydrogen remained in the solvent after reaction. If Tetralin is used as solvent, the solvent hydrogen consumption is the difference of donatable hydrogen in Tetralin charged and donatable hydrogen in Tetralin remaining after reaction. Isomerization of Tetralin into methylindan is not counted in this calculation. This compensates approximately for any tetralin that might be produced from coal during liquefaction. Gas Make. The gas make consists of CO, COz, HzS, methane, ethane, ethylene, propane, and propylene; gas composition is determined by gas chromatographic analysis. Heavier (C4to C,) gases are also determined in the gas analysis by the gas chromatographic method; their quantity is added to the liquid yield instead of gas make. Since we know the volume of the gas, the temperature at which the gas is collected, the mol % of each species, and the molecular weight, the gas make can be calculated as follows wt of ith gas (8) = (partial pressure of the ith
component (atm) x vol (cm3) x (mol wtIi)/82.057 (cm3 atm g-mol-' K-') X temp (K) wt % of ithgas
on coal = (wt of ith gas)/(wt of coal)
gas make (wt % on coal) = HzS + CO,
x 100
+ (C, - C,)
The precision of measurement of the ith component = f0.2
&2O cm3. The gas volume determined by the water displacement method results in the water-soluble gas species being dissolved in the water. For low sulfur coals ( propane and ethane >> ethylene, propane >> propylene. Water Make. The water make can be 'determined by one of the following four methods: (1)Karl Fischer, (2) oxygen balance, (3) pressure change method, and (4) mass spectrometer analysis. The detailed methods and their accuracy are discussed below. Karl Fischer Method. Products are recovered from one of the triplicate bombs with three 80-mL washes of methyl ethyl ketone (MEK). When the Karl Fischer method is used, only two bombs are left for cyclohexane wash to determine solid residue. The combined products are centrifuged, and the decant, which contains the water, is analyzed by the Karl Fischer technique as described by Mitchell and Smith (1948) to determine the HzO content. Because the MEK will absorb moisture from the air, a blank MEK wash (240 mL) of the bomb is carried out to simulate the time required to wash a bomb. Since MEK is the solvent of choice, a modified Karl Fischer reagent is used in the analysis which has the following composition: 84.7 g of iodine in 920 mL of pyridine and 30 mL of methanol with 45 mL of liquid sulfur dioxide. If the washing is carried out in a drybox, only the initial H20in MEK has to be corrected. The calculation is shown below
HzO on coal = [(water in MEK decant water in MEK blank)(ppm) X amount of solution (cm3)]/wt of coal (g) X density g/cm3
wt %
The precision of measurement = *1.5 wt % on coal. Oxygen Balance. The oxygen contents in feed coal and in products such as carbon monoxide, carbon dioxide, solid residue, and cyclohexane (CH) soluble liquid can be determined. The oxygen availabe for making water can be calculated according to the following formula OH20
= Ocoal ( O C O + OCOs + Osolid
residue
+ ocyclohexane-soluble
liquids)
The oxygen content in coal, solid residue, and cyclohexane-soluble liquids are determined by neutron activation analysis as reported by Hamrin et al. (1975). Each sample is determined 5 times and the average number is used. Carbon monoxide and carbon dioxide are determined in gas chromatographic analysis. In the calculation, the wt % on coal of each component and oxygen content in that component are needed to determine the wt % oxygen of that component based on coal. The precision of measurement = f l wt 70 on coal. Pressure Change Method. The total pressures at reaction temperature and after cooling down are measured by the precision pressure transducer to *2 psi. The vapor
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984 t
ZXKJ
2003
I I/ i II
TOTAL PRESSURE AT 840.F
f I
TOTAL PRESSURE AT W'F
840-F, 40 min.
I W p h k Coal 810V. 40 min. I .6 tekolin lo Coal
249
' TOTAL PRES~LRE AT W+ Big
%aLionik
G*F, 40 min.
1 .6 tetmlin to Coal
1 .6 tetrolin to Coal
1%
BOMB PRESSURE, Psis
Toto1 R c m e U Doom Tamprotue
1
Toto1 R e w e at Room Temprotme
1Dx)
"
30
k 0
I
I
l
in
20
30
RESIDENCE TIME, MINUTES
a
l
40
o
0
I
I
IO
20
I
UI
I
I
I
IO
20
30
C
RESIDENCE TIME, MINUTES
40
30
RESIDENCE TIME, MINUTES
Figure 11. Reaction pressures for coals of various ranks: (a) Illinois; (b) Wyodak (c) Texas Lignite.
pressure contribution of the solvent at the reaction temperature has to be experimentally estimated in advance. The water and gas pressure at reaction temperature is the difference between total pressure and solvent vapor pressure. One can use the water and gas pressure at reaction temperature to calculate the water and gas pressure at 373 K according to the ideal gas law and then use the total gas pressure at room temperature to calculate the gas pressure at 373 K according to the ideal gas law. The water pressure at 373 K is the difference between the water and gas pressure at 373 K and the gas pressure at 373 K. The water pressure at 373 K is then used to calculate the weight of water and wt % of H20 based on weight of coal. The accuracy of this method is limited by the uncertainty of the vapor pressure of the coal-derived liquid and the reactor volume changes at room temperature and reaction temperature. It is reproducible but is not very accurate. At best, it is a semiempirical estimation method. The precision of estimate is generally no worse than fl w t % on coal. Mass Spectrometer Analysis. The w t 5% H20 in the product slurry can be determined by mass spectrometer. In general, the w t % determined is lower than the amount determined by oxygen balance. Sampling uniformity may be the problem. Solid Residue. The weight of cyclohexane-insolubles is determined by direct weighing. The SO3-free ash can be used to obtain an additional estimate of the wt % solid residue through an ash balance procedure. w t % solid residue (as wt % coal) = [ash in coal (SO3
free)]/ [ash in solid residue (SO3 free)] The w t % solid residue obtained from ash balance should agree with the solids obtained by weight balance to within 2 w t %.
Low rank coals require SO3free ash analysis since they commonly have a high alkaline earth content and will retain SO3 during combustion. If SO3-free ash balances are not used solid residues 3-5 w t % high will be calculated for low rank coals. For bituminous coals residue calculations will be within 0.2 w t % even if the SO3-freebasis is not used. The average value of all the six numbers (ash
balance and mass balance) is reported as the solid residue yield. In a particular run, if the ash balance shows higher solid residue than weight balance, it may be due to the loss of slurry during the cyclohexane wash. If the weight balance shows a higher residue than ash balance, it may be due to the tubing bomb not being cleaned enough before running the bombs so that the contaminating material gets into the slurry during liquefaction. Liquids. In the tubing bomb experiment, the liquid yield is obtained indirectly from calculation according to the following formula
+
liquids = 100 H2 consumption (wt 5%) (gas make + water make + solid residue) (wt %) In many experiments, the purpose is to compare the liquids for the same coal run at different conditions to see the extent of the process improvement. In this case the water determination (which is fairly independent of process conditions) is not critical and the liquids can be calculated as follows
+
liquids plus water = 100 H2consumption (gas make + solid residue) (wt %) or
liquids plus water = 100 - (gas make
+ solid residue) (wt %)
In some cases, the coal conversion (100 - solid residue, as percent of coal) is used for comparison at different conditions. Liquids can also be obtained directly from the decant by distilling off most of the cyclohexane and determining any cyclohexane remaining in the liquids by mass spectrometer. This method gives the least reproducible result because during distillation certain light materials may be lost. The precision of the estimation of liquids is the accumulation of errors of the other measurements (hydrogen consumption, gas make, water make, and solid residue).
250
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984
Table 11. Comparison of Liquefaction Yields for Illinois, Wyodak Coals and Lignitea o
coal Illinois No. 6, bituminous gas make
cox
H,S C,-C, water ( b y oxygen balance) liquids solid residue H, consumption molecular H, solvent H,
Wyoming subbituminous
Yields, Wt DAF Coal 8.5 13.8 2.5 8.9 0.5 0.1 5.5 4.8 7.2 8.7 44.7 42.4 2.8 0.6 2.2
35.9 44.5 2.8 0.5 2.3
Bituminous
A
Subbituminous
0
Lignite
Texas lignite
/
12.1 7.2 0.1 4.8 10.1 41.1 39.3 2.5 0.5 2.0
a Standard conditions for tubing bombs are 840 'F, 4 0 min residence time, 650 psig H, charge, and 1.6 Tetralini coal.
The precision of estimation is between f l and rt3 wt % depending on which form of liquids or conversion is considered and how precisely the water make is measured. Experimental Results The yields from three coals, a bituminous coal (Illinois No. 6 seam), a subbituminous coal (Wyoming)and a lignite (Texas) are shown in Table 11. The liquefaction reaction was carried out a t 450 "C, 40 min residence time, about 1500 psig (see Figure 11) H2 partial pressure (about 3 wt % of molecular hydrogen charge based on weight of coal) and 1.6 Tetralin to coal ratio. Figure 12 shows the comparison of conversion to cyclohexane-solubles (plus gases and water) in the tubing bomb to conversion to 1000 O F (538 OC) minus material in a small pilot plant. From the parity plot, it is evident that the cyclohexane wash with the procedure described in method (c) of step F gives a good correlation with pilot plant conversion measured by product distillation to a 538 "C cut point. Recommendations for Future Improvements The bottleneck of the tubing bomb operation is the cyclohexane wash step in which the slurry has to be transferred from the bomb to the centrifuge tube. This results in the possibility of losing material and is time consuming. If the tubing bomb itself could be used as a centrifuge tube, the reproducibility could be improved and time saved. Water displacement method in collecting the product gas results in the loss of water soluble gases. Dry methods of gas collection which gives precise volume measurement would give a better gas make value. The yield of cyclohexane insolubles correlates well with the yield of +538 OC material by distillation for certain coals at certain liquefaction conditions. The accuracy of this correlation depends on the coal, solvent, and lique-
TUBING BOMB CYCLOHEXANE CONVERSION (Wt. '10 DAF Coal)
Figure 12. Parity plot of pilot plant 538 OC conversion vs. tubing bomb cyclohexane conversion for coals of various ranks.
faction conditions employed. A microdistillation method for coal slurry which has very low liquid holdup and presents no sampling problem would be valuable. This may expand the usefulness of the tubing bomb experiments beyond just a screening tool. The liquid yields could then be correlated in a kinetic model to give prediction at other liquefaction conditions.
Acknowledgment The authors acknowledge the assistance of the following Exxon Research and Engineering Company personnel for technical help and valuable suggestions during development of the tubing bomb procedure. Barry C. Deane and Gerald R. Bookmyer helped to design the tubing bomb unit. Thomas Aczel and Herschel J. Karchmer have developed many of the analytical methods for tubing bomb analyses. Karl W. Plumlee, J. Lloyd Sutterby, Michael A. Gibson, Lavanga Veluswamy, and Syamal Poddar have made valuable suggestions during the course of the development of the technique. The authors thank the Exxon Research and Engineering Company for allowing us to publish this manuscript. Registry No. Tetralin, 119-64-2;hydrogen sulfide, 7783-06-4; carbon oxide, 12795-06-1; water, 7732-18-5.
Literature Cited Aczel. T.; Gorbaty, M. L.; Mea, P. S.; Schlosberg, R. H. Fuel 1979, 56. 228. Curran, G. P.; Struck, R. T.; Gorln, E. Ind. Eng. Chem. Fmcess D e s . D e v . 1967, 6 , 166.
Hamrln, C. H., Jr.; Maa, P. S.; Chyl, L. L.; Ehmann, W. D. Fuel 1975, 54, 70. Mitchell, J., Jr.; Smith, D. M. "Aquametry, Appllcaton of the Karl Flscher Reagent to Quentitatlve Analyses Involving Water"; Vol. 5, Intersclence: New York, 1948. Neavel, R. C. Fuel 1976, 55, 237. Slmnick, J. J.; Lawson, C. C.; Lln, H. M.; Chao, K. C. A I C H E J . 1977, 23(4) 469.
Vernon, L. W. Fuel 1980, 59,
102.
Received for review June 13, 1980 Revised manuscript received August 8, 1982 Accepted August 2, 1983
