290
Ind. Eng.
Chem. Fundam. 1986, 25, 290-292
EXPERIMENTAL TECHNIQUES A New Pump for Experimental Recycle Reactors Jale F. Akyurtlu,+ Daniel J. Curtln,t Igor N. Svlatdavsky,* and Warren
E. Stewart'
Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706
A gradientless reactor, employing a novel centrifugal pump, has been developed for measurements of gas-solid chemical kinetics. The pump is noncontaminating, is operable at high pressures (3-67 atm) and temperatures (up to 810 K), and provides high recycle rates at moderate pressure differentials. The system has been used to study the kinetics of competltive hydrogenation of benzene and toluene.
Introduction Kinetic expressions for the design of industrial catalytic reactors are best obtained from reaction rate measurements in a gradient-free system. Recycle reactors are most suited for this purpose because they allow readily measurable total conversions with a well-defined state in the reaction zone. Interphase heat and mass transfer are also facilitated by the high recirculation velocities used. Two types of reactors have been used in recycle systems: spinning baskets (Brisk et al., 1968; Carberry, 1964; Gillespie and Carberry 1966; Tajbl et d., 1966) and fixed-bed reactors (Perkins and Rase, 1958; Berty et al., 1969; Brown and Bennett, 1970; Chambers et al., 1965; Garanin et al., 1967; Hanson and Benson, 1973; Kipennan, 1971; Korbach and Stewart, 1964; Korneichuk et al., 1969; Livbjerg and Villadsen, 1971; Mahoney, 1974; Timoshenko et al., 1969). These systems are reviewed and compared by Jankowski (1978) and Sunderland (1976). Fixed-bed reactors are preferable because the gas flow through the catalyst bed is well-defined and can be calculated from preliminary experiments. Temperature control and mounting of the catalyst are simpler, and the dynamic balancing problems of the spinning basket are avoided.
A New P u m p Four types of pumps were considered here for circulating the recycle stream: (1)centrifugal pump externally driven through a sealed shaft, (2) centrifugal, totally enclosed pump driven by an induction motor, (3) centrifugal, totally enclosed pump driven by a magnetic linkage, and (4) positive-displacement pump driven by a bellows-sealed mechanical linkage. Types 2-4 all require bearings within the reactor, with consequent, problems of bearing wear or reactor contamination by lubricants. Type 1seems to be the most trouble *To whom all correspondence should be addressed. Present address: Department of Chemical Engineering, University of Connecticut, Storm, CT 06268. ARC0 Ventures Co., Box 2600, Dublin, CA 94566. *Department of Nuclear Engineering, University of Wisconsin, Madison, WI 53706.
*
0196-4313/86/1025-0290$01.50/0
free because the moving parts can be serviced easily and the use of external bearings will prevent contamination. We chose type 1 in preference to the commercially available type 3 to minimize the risk of poisoning our platinum hydrogenation catalysts. The pump unit (Figure 1) was constructed at the Physical Sciences Laboratory at the University of Wisconsin. It is made of 316 stainless steel and can operate at temperatures up to 810 K with pressures from 3 to 67 atm. The pumping element is a closed impeller with straight radial vanes, having an outer diameter of 8.00 in. The impeller housing is sealed by a gold-plated metal O-ring. The impeller shaft has two seals. The primary or high-pressure seal is a noncontacting kinetic wedge seal, manufactured by Crane Packing Co. The seal stator is made of titanium carbide lapped to a spherical contour, with spiral grooves approximately 200 pin. deep. The seal rotor is made of tungsten carbide and is flat to within 6OOO A. The minimum pressure required for the separation of the primary seal faces is approximately 3 atm. A continuous leak is maintained through the seal, thus preventing migration of contaminants into the reactor. The primary seal has a water jacket for cooling during high-temperature operation. Circulation of distilled water through the jacket provides a symmetrical temperature gradient in the seal housing to minimize thermal distortion and to protect the Viton O-rings used on the stator seal. The operating temperature limit for the primary seal is 480 K. A secondary, low-pressure graphite-metal seal is provided to enclose the main product outlet. This seal is continuously lubricated with a fluorosilicone oil by a gravity feed system. Two exit ports are provided along the shaft: a gas sampling port (4) above the primary seal and the main product outlet (18) between the seals. The shaft is hollow down to the secondary seal to minimize heat loss from the reaction system. Two thrust bearings support the large axial force exerted by the system pressure. A precision radial bearing, mounted directly above the thrust bearings, ensures accurate alignment of the primary seal. 0 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25, No. 2, 1986 291 Table I. Data for a Recvcle Reactor ExDeriment" ~~
~
hydrogen benzene cyclohexane toluene methylcyclohexane total
~
fresh feed mole fraction 0.8570 0.0 0.0715 0.0 0.0715 Loo00
reactor inlet mole fraction 0.8564 0.0052 0.0666 0.0085 0.0633
1.oooO
reactor mean partial press., atm 3.7482 0.0247 0.2978 0.0420 0.2806 4.39
reactor exit mole fraction 0.8564 0.0054 0.0664 0.0089 0.0629 1.oooO
~
~~~
production rates, mol/(h.cm2) 2.22 x 104 2.79 x 10-5 -2.79 X lo6 4.61 x 10-5 -4.61 x 10-5
"Gas temperatures: reactor inlet, 595.2 K; reactor outlet, 597.0 K; reactor mean 596.1 K. Flow rates: fresh feed, 1.625 mol/h; recycle stream, 20.8 X 1.625 = 33.8 mol/h. Catalytic surface area: 327 cm2. Reactor
l
+
41
Feed
Rotameter
Impdlrr Houring
Tranrducar
Exit Stream
Figure 2. Setup for pump calibration.
::i Pump speed: 4000 rpm
100
3000rprn
d 50
40
Figure 1. New recycle pump: (1)impeller housing, (2)impeller inlet (reactor outlet), (3) gold O-ring, (4)exit port to the gas chromatograph, (5)impeller and shaft assembly, (6)lower impeller and seal housing, (7)primary seal assembly, (8)viton O-ring, (9)lubricated seal housing, (10)hypodermic needle, (11) bearing housing, (12) thrust bearings, (13)lubricating oil reservoir, (14)V-belt, (15)pulley, (16)bearing plate, (17)bearing spacer, (18)exit port between the seals, (19)water cavity, (20)water inlet, (21)water outlet, (22)radial bearing, (23)motor, (24)frame and casters, (25)adjusting and stabilizing screw, (26)impeller outlet (reactor inlet).
The pump is dynamically stable up to 5000 rpm. A direct current motor and V-belt drive are used. A cylindrical oven, mounted on top of the impeller housing, was used to heat the reactor-pump unit to reaction temperatures. The oven wall and top were heated uniformly by resistance windings.
Calibration of the Pump and Recycle Loop The pump and recycle loop were calibrated in preliminary runs with nitrogen to allow calculation of recycle rates in the catalytic experiments. The calibration arrangement is shown in Figure 2; the rotameter and pressure transducer were removed for the catalytic experiments. A trap was used in all experiments to prevent stray
t
0
2
6
4
8
IO
12 14 16
capo c ity ,m3/s ( x I 07 Figure 3. Pump characteristic curves calculated from eq 1.
particles from getting into the pump. The pressure rise across the pump was represented by the dimensionless relation P-z - PI p~~~
-
1
+
6.0 4.8 x 1 0 5 ( ~ / ~ 0 3 ) 2
(1)
which fits the pressure differences with a relative standard deviation of 15%. Characteristic curves calculated from eq 1are shown in Figure 3. The pressure drop (pi- p l ) across the reactor system was expressed as a quadratic function of flow rate Q, in the manner of Ergun's equation (Bird et al., 1960). The recycle flow rate Q for each catalytic experiment was calculated by equating this pressure-drop function to p z - p1 of eq 1.
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Chem. Fundam., Vol. 25, No. 2,
1986
Table 11. Differences between the Interfacial and Bulk Gas Compositions component hydrogen benzene cyclohexane
104(Ayy,/y,b) component 0.047 toluene 9.37 methylcyclohexane -1.10
104(AYz/Ytb) 10.4 -1.85
Operating Experience The pump was used in a reaction system for hydrogenation of benzene and toluene and dehydrogenation of their reaction products on platinum wire catalyst. Reactor pressures from 4.4 to 25 atm, temperatures from 600 to 672 K, and recycle ratios up to 22 were investigated. Selectivity studies were conducted, rather than absolute rate measurements, because there was a continuous deactivation of the catalyst by trace impurities in the reactants (Akyurtlu, 1975; Akyurtlu and Stewart, 1978). The equipment was constructed to be operable from 3 atm reactor pressure to 67 atm. However, the leakage through the primary seal increases rapidly with pressure. At high pressure, large feed rates are required to offset this leakage. This fact established the maximum operating pressure of 25 atm used during the experimental program. The minimum operating pressure for the primary seal was given by the manufacturer to be 3 atm at room temperature. However, since the effect of elevated temperatures on the operation of the seals was not known, a minimum pressure of 4.4 atm was chosen for the experimental design. The results of the study, except for the pump design, have been reported by Akyurtlu (1975) and Akyurtlu and Stewart (1978). The pump performed well, once continuous lubrication was installed to prevent adhesion of the carbon seal. For future users of this type of pump, we suggest using more efficient seals, such as those now available from Ferrofluidics, Inc. Such seals would permit operation down to subatmospheric pressures. Table I gives results for a typical run in the kinetic study mentioned above (Akyurtlu, 1975; Akyurtlu and Stewart, 1978). The effectiveness of the recycle arrangements is evident in the close agreement of the reactor inlet and outlet conditions. Table I1 shows the relative mole fraction differences between the bulk gas state and the catalyst surface under the experimental conditions of Table I. These values are calculated from the j factor correlation of Yoshida et al. (1962) as used by Bird et al. (1960). Clearly, mass-transfer resistance is negligible a t these conditions. The calculated temperature difference between the bulk gas state and the catalyst surface is only 0.03 K. Thus, the heat-transfer resistance is also negligible a t these conditions. Discussion The pump demonstrated here has several attractive features: (1)The bearings are outside the recycle system. Catalyst contamination by lubricant vapors is thus
avoided, and the bearings are not subjected k~the reaction atmosphere. Differential thermal expansion of the bearing parts is also minimized. (2) The shaft drive allows a wide range of power inputa and permits high recirculation rates even at low-density conditions. With modified seals, the operating region could be extended to atmospheric pressure or below. (3) The pump is self-contained, not part of the reactor. This permits greater flexibility in the reactor configuration and better measurement and control of the reaction conditions. A more uniform state can be achieved in the catalyst bed, by conducting the necessary heat transfer and mixing outside the reaction vessel. Acknowledgment This work was supported by the National Science Foundation through Grants ENG 70-01220 and CPE 8308748. Nomenclature D = diameter of impeller, m N = angular velocity of im eller, rps p = system pressure, N/m f) Q = volumetric throughput of the pump, m3/s y = mole fraction in gas phase Ay = difference in mole fractions between the catalyst surface and the bulk gas state Greek Letters M p
= viscosity, kg/(ms)
= density of gaseous mixture in the pump, kg/m3
Subscripts i = component i
b = bulk Literature Cited Akyurtlu, J. F. PhD. Thesis, University of Wisconsin, Madison, WI, 1975. Akyurtiu, J. F.; Stewart, W. E. J. Catal. 1978, 5 7 , 101. Berty, J. M.; Hambrick, J. 0.;Maione, T. R.; Ullock, T. S. AIChE Meeting, New Orleans, LA, 1969, Prepr. 42 E. Bird, R . B.; Stewart, W. E.; Lightfoot, E. N. "Transport Phenomena"; Wiiey: New York, 1960. Brisk, M. L.; Day, R. L.; Jones, M.; Warren, J. B. Trans. Inst. Chem. Eng. 1988, 46, T3. Brown, C. 8.; Bennett, C. 0. AIChE J. 1970, 76, 817. Carberry, J. J. Ind. Eng. Chem. 1984, 56, 39. Chambers, R. P.; Dougharty, N. A.; Boudart, M. J. Catal. 1985, 4, 625. Garanin, V. I.; Kurkchi, U. M.; Minachev, Kh. M. Kinet. Katal. 1987, 8 , 605. Giiiespie, B.; Carberry, J. J. Ind. Eng. Chem. fundam. 1968, 5 , 164 Hanson, F. V.; Benson, J. E. J. Catal. 1973, 37, 471. Jankowski, H. Chem. Tech. (Leipzig) 1978, 30, 441. Kiperman, S . L. Int. Chem. Eng. 1971, 7 1 , 513. Korbach, P. F.; Stewart, W. E. Ind. Eng. Chem. fundam. 1984, 3 , 24. Korneichuk, G. P.; Streitsov, 0. A.; Andrusenko, Yu. G. Kinet. Kafal. 1969, 7 1 , 784. Livbjerg, H.; Villadsen, J. Chem. Eng. Sci. 1971, 26, 1495. Mahoney, J. A. J. Catal. 1974, 32, 247. Perkins. T. K.; Rase, H. F. AIChE J . 1958, 4 , 351. Sunderiand, P. Trans. Inst. Chem. Eng. 1978, 54, 135. Tajbl, D. G.; Simons, J. B.;Carberry, J. J. Ind. Eng. Chem. fundam. 1968, 5, 171. Timoshenko, V. I.; Buyanov, R. A.; Proshin, 0. I.Kinet. Katal. 1989, 10, 681. Yoshida, F.; Ramaswami, D.;Hougen, 0. A. AIChE J. 1982, 8 , 5.
Received for review January 30, 1984 Revised manuscript received May 20, 1985 Accepted June 14, 1985
