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W. J. CERVENY and W. C. PFEFFERLE' Research Department, Standard Oil Co. (Ind.), Whiting, Ind.
An Isothermal Catalytic-Conversion Unit For conversion processes where small-scale operation is desired -here is a most useful test unit for catalyst screening CATAI.YsT-SDREENINC STUDIES are often limited in accuracy by the ability to control and measure reaction temperatures. Small temperature changes can cause large changes in catalyst activity. In highly endothermic or exothermic reactions, much heat must be transferred through the catalyst bed if large temperature gradients are to be avoided. When such exothermic reactions as hydrogenation take place in a conventional single-heater flow reactor, individual temperature increments in the catalyst bed often exceed the mean bed temperature by 150' F. Radial temperature gradients are large. The conventional method of approximating isothermal conditions involves a diluted catalyst bed in a long multiple-heater reactor. This technique is awkward, time-consuming, and not suitable for small scale studies of short duration. Therefore, a new isothermal reactor was designed for screening hydrogenation catalysts. The program required that the reactor hold axial and radial temperature gradients to less than 10' F., permit rapid and reproducible temperature settings, have a total volume of less than 50 ml., hold up less than 20 ml. of liquid, and be easy to fill, empty, and 1 Present address, Baker & Co., Newark, N. J.
clean. To meet the minimum ternperature gradient, heat transfer between the catalyst bed and isothermal reservoir was maximized by using a thin annular bed and a thin-walled reactor immersed in a molten-metal bath. The small reactor minimizes the catalyst volume and has a simple closure. Other features were a constant-feed positive-displacement pump, a bubble counter for metering hydrogen input, a surge-free product-recovery system, and a continuous product stabilizer.
Apparatus Hydrogen from storage is supplied at 3000 p.s.i. from a manually set metering valve, measured by a bubble counter, and passed through a purification train. Liquid feed is metered by a pump. Reactants pass through a short preheating tube, downward through the reactor, both immersed in a molten-met a1 bath. The products pass to a small liquid diverter and are reduced to atmospheric pressure by a back-pressure control valve. For analysis, the products are separated in a glass stabilizer into a gaseous fraction and a liquid fraction. Hydrogen measurement and purity are important. A pressure regulator reduces the hydrogen pressure from 5000
to a constant line pressure 300 p.s.i. above operating pressure for the bubblecounter metering valve. Hydrogen is purified after the bubble counter by passing through three vessels to remove carbon dioxide, oxygen, and water. Figure 1 on the next page shows two typical calibrations of the bubble counter. For accurate results, the counter should be kept at constant temperature. A Ruska single-stroke positive-displacement pump is used for metering the liquid feed. To permit quick changes from one feed to another, two 250-ml. cylinders are piped separately to the preheating tube. The rate at which the piston displaces the feed can be altered by changing pick-off gears. Because catalyst volume is small, flow surges inherent in reciprocating pumps cause serious variation in reaction rate. The controlled-temperature moltenmetal bath consists of a 4-inch Schedule 40 pipe, 20 inches long. A bafRe '/16 inch thick and 14 inches long is centered lengthwise in it. A thermowell 18 inches long is attached to the baffle. Two instrument-controlled heaters regulate the bath temperature; one is enough to maintain temperature, but both are required for quick heating. An immersed air coil permits the bath to be cooled rapidly. The molten OUTLET H 2
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Flow through this unit is similar to that obtained in conventional bench-scale flow units
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Bubble counter ( 7 1 consists of a high-pressure transparent Jerguson gage, a light source, a photocell, and an impulse counter on the instrument panel. The vertically mounted carbon-steel gage is partially filled with water containing an antioxidant. Each hydrogen bubble emerging from the tube in the axial center of the gage breaks the beam of light focused above the tube, and the photocell sends an impulse to the counter VOL. 50, NO. 9
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Figure 1. In these typical calibrations of the bubble coun710 ter, curves and open points 5 represent original calibrations; solid points are checks after % 705 six months. Little change in 2 Q calibration occurred a
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metal is a mixture of 18% cadmium, 32% lead, and 50% tin; it melts at 380' F. Pulverized charcoal is placed on the bath to suppress oxidation. An airdriven stirrer provides circulation, directed into essentially vertical motion by the baffle. Isothermal reactor, closure, and thermowell. To the open end of the reactor is welded a tubing-to-pipe adapter. The closure, a tubing plug through which the thermowell tube (0.132-inch 0.d. X 0.098-inch i.d.) is inserted and welded is secured by a standard high-pressure nut. The catalyst bed, a cylinder about 5 inches long, fills the '/S-inch annulus between the thermowell and the reactor wall. Before the catalyst is charged, a a/4-inch layer of quartz wool and a a / r inch layer of glass beads are placed in the bottom of the reactor. The thermo-
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Figure 2. These curves show the catalyst temperature profiles for two hydrogenation runs of a 50-50 mixture of Tetralin and dodecane
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well and closure are inserted to a point l / d inchabove theclosed positionand alumina and catalyst successively added, as shown in the adjoining cross section, through a paper funnel. The paper funnel is torn away, granular material is blown from the cIosure surfaces, and the reactor is sealed. A hypodermic-type thermocouple, 0.065 inch in diameter, is inserted into the thermowell. The diverter, a 50-ml. pressure cylinder filled with glass beads to reduce void volume, separates liquid and gaseous products to prevent the liquid from entering lines to the instruments that measure and control pressure. The back-pressure control valve (Fisher Governor, Type 530) is operated by a 0- to 5000-p.s.i. pressure-recorder controller. I t is mounted with its inlet downward, rather than horizontally, to permit gaseous and liquid products to pass through smoothly in small increments instead of large surges. A Heise 0- to 8000-p.s.i. gage measures reaction messure. The high-pressure equipment, tubing, and fittings are suitable for 3000 p.s.i. or higher. Critical parts are of stainless steel. Rupture disks are located on the pump, inlet and outlet lines to the reactor, and elsewhere, to protect against over-pressure. A reinforced concrete cell houses high-pressure components separate from the control area and personnel. Valves in the cell are set before a run or remotely controlled by extensions through the cell wall. Position of the thermocouple in the well is controlled by a cable that extends into the control area. Performance
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Reactor is 0.25-inch Schedule 40 seamless pipe, 15 inches long, 0.09-inch wall, 27-mi. volume. Preheating and outlet tubes are 0.25 inch 0.d. and 0.083 inch i.d. 1256
The unit has performed outstandingly during 15 months of almost continuous operation. In catalyst-screening studies, an average of 5 runs was made per 8-hour period. Temperature gradients have been well within the design limits; and weight balances have averaged 99%. Figure 2 shows temperature profiles in two typical screening runs hydrogenating a 50-50 mixture of Tetralin and dodecane over 10 ml. of catalyst at a liquid rate of 90 mi. per hour. A t 20% conversion of the Tetralin to Decalin, the
INDUSTRIAL AND ENGINEERING CHEMtSTRY
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catalyst temperature duplicated the bath temperature except for a 1' F. rise toward the bottom of the bed. At 65% conversion, the maximum temperature in the bed was 5' above that of the bath, but the largest temperature increment was 2' F. from the mean bed temperature; the heat of reaction is calculated to be about 100,000 B.t.u./ hour/cu. foot of catalyst-enough to raise the temperature about 270' F. under adiabatic conditions. As conversion rises, more of it occurs in the top part of the catalyst bed. At 100% conversion, the rate of heat generation can easily exceed 1,000,000 B.t.u./ hour/cu. foot in that part of the bed. Even under these extreme conditions, temperature increments do not exceed 27' F. When such severe conditions are encountered, catalyst dilution with inert material is recommended. The possibility of channeling and uneven contacting in the catalyst bed has been considered but no evidence has been found. Good flow distribution through the bed is indicated by Tetralin conversions of 98% at velocities as high as 9 volumes of feed per hour per volume of catalyst. The selected catalyst particle size of 60 to 100 mesh probably helps prevent channeling. The many advantages of this unit recommecd it for use in many types of conversion processes where small-scale operation is desired. Processing Equipment Berkeley Division, Beckman Instruments, Inc., Richmond, Calif., light source, Model 454. Fisher Governor Co., Marshalltown, Iowa, control valve, Type 530. Heise Bourdon Tube Co., Inc., Newton, Conn., Heise gage, reaction pressure of 0 to 8000 p.s.i. Jerguson Gage and Valve Co., Burlington, Mass., Jerguson gage. Ruska Instrument Gorp., Houston, Tex., single-stroke positivedisplacement pump. Literature Cited (1) Shields, S. E., Dewey, P. W. [to Standard Oil Co. (Ind.)J, U. s. Patent,
pending. RECEIVED for review January 3, 1958 ACCEPTEDApril 12, 1958
