AN AUTOMATIC PRECISION D. P. HARRISON
J. W. H A L L
T h e study of catalytic reactions has always been burdened with a discouraging number of experimental difficulties. The goals of precision and ease of experimentation have long seemed mutually- inconsistent. I n many ways this has had unfortunate effects on catalytic research for all but the most determined investigators. New ideas are fragile and must be subjected to experimental test with the greatest speed and ease of manipulation. Cumbersome apparatus and imprecise techniques can easily quell the tentative enthusiasm of a n investigator and actually discourage the intelligent testing of a new idea. I n the last several years techniques and new items of equipment have been introduced which can be applied as a means of reducing the effort involved in obtaining precise data on catalytic systems. With such a goal in mind we have recently constructed and used an automatic microreactor system which has proved unusually amenable to the rapid and easy accumulation of data on reaction kinetics, catalyst behavior, catalyst activity studies, and catalyst comparisons. I t is possible with this system to obtain such data with little or no attention by an operator. The purpose of this paper is to describe the characteristics of the automatic microreactor system and provide specifications so that others may construct similar units should they desire.
Previous Work
Over the past 120 years, beginning with the first discovery of catalytic phenomena, research workers have shown great ingenuity in devising useful procedures and equipment for observing catalysis. I t has, however, only recently been possible to employ such delx e s as continuous stream analyzers, automatic samplers, and other control devices which are so numerous and readily obtainable today. One need consider, therefore, only '
18
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
H. F. RASE
equipment of rather recent origin that has been described in the literature, for it is from these that many ideas have been borrowed and combined with our own to produce the automatic precision microreactor system. W. Keith Hall and associates (2, 3) have described in various articles a microcatalytic technique in which a slug of reactants and products could be passed through the reactor and then a chromatograph, or a continuous flow reaction could be used and the chromatograph employed as an intermittent sampling device. Their ideas culminated in a semiautomatic microreactor which employed a doser system operated by means of solenoid valves. Pozzi and Rase (5) have examined the entire problem of precision in catalytic experimentation with respect to reactor types and arrangements. These previous studies served as the background for development of the automatic precision microreactor. The ideas of the previous investigators were combined hvith new ideas and new equipment now available to produce the final design described. General Description
The purposes and operating characteristics of each portion of the microreactor system are discussed in the following section. This will be followed by detailed specifications of all major items of equipment. The accompanying lists and drawings should prove helpful in constructing a similar apparatus. The reactor system is designed for maximum reaction conditions of 800" C . and 1500 p.s.i.g. To attain the high operating pressure, however, a pressure controller would have to be added and the mass flow meter replaced by another model designed for high pressure operation. The liquid feed system is limited by the "0" rings in the sample valve to a maximum temperature of 160' C . Reactor. T h e reactor (R-1 Figure 2), a physically small yet most significant par't of the apparatus, is
Figure 2. Flow diagram of the reactor system
constructed from standard stainless steel fittings. These fittings can be purchased in tubing sizes from through 1 inch, and many different reactor configurations can readily be constructed without resort to shop labor. Pozzi and Rase (5) have discussed the use of reactors of this type in precision studies requiring isothermal conditions. Figure 3 depicts a typical arrangement for a catalytic reactor. The particles of the catalyst are contained by quartz cloth. This cloth is easily cut using a cork borer, and it is installed by placing it on the shoulder of the union and positioning the tubing so that there remains approximately 0.01 to 0.015 inch between the tubing and the shoulder. Catalysts may be charged to form a packed-bed or a fluidized bed. Fluidized systems on the very small scale here suggested are easily operable and have the advantages of complete mixing of catalysts and reactants. Temperature and parti21 pressure gradients within the catalysts are eliminated and operating at low conversion assures precise data. If the fluidized system is used, the experimenter must make brief studies with an open-top reactor or a glass model to determine ideal reactor sizes and flow rates for the best fluidization characteristics. Noncatalytic reactions may also be studied by employing tubing and fitting arrangements of proper size and length. Generally, Vl6-inch tubing is used to conduct the feed in and out of the reactor sand bath shown in Figure 4, and a major portion of reactor volume is contained in a larger tubing section. By referring to Figure 4, it is seen that the reactor and connecting tubing are designed for easy removal from the sand bath (B-I). This bath consists of a stainless steel vessel containing sand which is fluidized by preheated air entering at the bottom. The entire sand bath is contained in an insulated area composed of four and one-half inches of high-alumina refractory followed 20
INDUSTRIAL A N D ENGINEERING
CHEMISTRY
304 8.8. TUBING,
'/s IN. 0.0.
'
REDUCING UNION, % X ?J% IN. REFRASll CLOTH
304 S.S. SFAMLESS 'TUBING, %. IN. 0.0. X '/s
IN. I.D.
Figure 3. Microreactor assembly
H. F. Rase is Projes.ror oj Chemical Engineering, and D . P. Harrison is a Graduate Student in the Department of Chemical Engineerang at the Universitj of Texas. J . W . Hall is Associate Professor of Chemical Engineering ut t h e LTniversity of Arizona. T h e authors express their appreciation to J . W . Roper f o r aid i n the design and construction !of the apparatus, and to the Humble Oil and Rejhing Co. and the h'ational Science Foundation jor support of projects associated with this reactor system.
AUTHORS
by one inch of magnesia block insulation (see Figure 1). Heat is supplied to the reactor by the fluidized sand which contacts the electrically heated sides of the bath. The heater is controlled by the temperature recordercontroller (TRC-1). Because of the excellent agitation of the fluidizing sand, overall heat-transfer coefficients of approximately 100 BTU/(hr.)(sq. ft.)(” F.) are obtained. The fluidized air is preheated by a second electric-resistance heater which is set manually by a variable transformer. I n practice the preheater is used to bring the fluidizing air temperature to roughly 10 degrees below the reaction temperature. The preheater thus serves to give a n approximate temperature setting while the controlled heater is used for fine ad.justment. Three iron-constantan thermocouples are inserted along the sand bath. The center thermocouple (TRC1) is connected to the temperature recorder-controller. The upper and lower thermocouples (TI-1-1 TI-1-3) are connected to a multipoint temperature indicator (TI-1) along with other thermocouples. Unlike a reactor immersed in an oil bath, the reactor can be removed and another previously made-up reactor rapidly installed without the messiness usually associated with removing apparatus from oil baths. All one need do is blow off the small amount of sand adhering to the tubing prior to opening the reactor. The ease of removing and installing reactors makes it possible to accomplish such changes with negligible temperature upsets. Feed System. The reactor was planned primarily for reactions in the gaseous phase, but either liquid or gaseous feeds can be used as well as combinations of liquid and gas. The feed system was designed for high precision metering of gases and liquids. Small errors in the flow quantities can cause large errors in experimental results because of the inherently low flow rates employed in microreactor systems. Accordingly, a positive-displacement pump for measuring and pumping liquid feed was selected that would provide highly accurate though variable feed rates. The pump consists of a positive-displacement cylinder operated by a synchronous motor through a gear train with gear ratios providing ten feed rates of from 2 to 30 cc. per hour. The pump cylinder is arranged vertically so that liquid feed may be loaded into the pump without trapping air or other gases. The 90-cc. capacity is adequate for a number of tests, and the pump may be readied for recharging by manually cranking the piston down to the initial position. A limit-switch stops the pump automatically when it reaches the end of its travel. Gaseous feed is supplied froni commercial cylinders, pressure from which is reduced by a pressure-regulating valve (PCV-1). The gas from the cylinders is conducted through pretreatment vessels which are shown in Figure 2 for the case of hydrogen feed. With hydrogen a deoxygenating catalyst unit is used to remove small traces of oxygen by palladium catalyst, and the water formed is then adsorbed in a silica-gel dryer.
The purified hydrogen passes first through a rotameter (FI-1) and then through a thermal conductivity cell (CR-1). The thermal conductivity cell is used for sensing reactor effluent when the product is not being sampled by the chromatograph and is contained in an air bath (Air Bath I, Figure 2) which is maintained at a temperature sufficient to keep all the feed materials in the vapor state. Air, electrically heated and controlled by a variable transformer is used as the heating medium. In addition to this primary use, the air, in combination with the fluidizing air, serves as a purge to sweep the vented reaction mixture from the system. The liquid feed, the flow rate of which is already measured by the setting of the feed pump, enters the air bath and is vaporized in an enlarged portion of the tubing, I t then mixes with the gaseous portion of the feed, the flow rate of which is controlled by a metering valve (V-1) and measured by a flow transducer (FI-2). A stainless steel micron filter (F-1), up-stream from the metering valve, serves to remove any particles which would tend to clog the valve ports. By maintaining a back pressure on the vaporization tube, any pulsations due to uneven vaporization, which are common with low flow rates, can be eliminated. The combined feed passes through a four-way manual selector valve (V-2). I t is possible with this valve to direct the feed to the vent while steady-state temperature conditions are being attained in the reactor. When the operator is ready to start a run, he simply changes the position of the selector valve to send the feed directly to the reactor (R-1). Sampling System. The reactor effluent passes through a stainless steel micron filter (F-2) and then enters a solenoid-actuated, nitrogen-operated valve (V-3). Referring to Figure 5, the effluent enters the valve at B when the valve is in the normal position and flows through the sample loop from C to F and is then discharged through G and passes to the sensing side of a thermal conductivity cell (CR-1-2) in Air Bath-1. During this time the helium from the chromatograph enters the valve at D and then flows directly back to the chromatograph through E. The sample loop size is easily changed to fit the particular reaction system. I n the sample or energized position, helium enters the valve at D and sweeps the entrapped reactor product from the sample loop into the chromatographic column and thermal conductivity cell (CR-1-1) through E. When sampling is occurring, the reactor effluent passes directly through the valve from D to A and into the sensing side of the thermal conductivity cell (CR-1-2), and then to the vent. The electrical system for the valve is arranged so that the solenoid may be actuated either automatically on a predetermined time schedule or nianually at the discretion of the operator. A cylinder of nitrogen a t about 40 p.s.i.g. is used as the actuating gas and has proved entirely satisfactory. The “0”-rings in the valve have given much improved service when they are lubricated lightly with silicone lubricant. The stainless-steel micron filter was installed ahead of VOL. 5 7
NO. 1
JANUARY
1965
21
......... ..........
.- ........
FLUIDIZING
Figure 4. Microrcactar and sand bath assembly
conductivity of the reaction stream is continuously obtained. I n the case illustrated in Figure 2, the hydrogen gas feed is used as the reference gas. The constant monitoring of the reaction product provides a highly sensitive check on upsets in the-xystem which would not ordinarily be ascertainable until some time after the results from the chromatograph were analyzed. As an example, if pulsations, either sudden or cyclical, should occur due to some uneven vaporization in the liquid feed system or to upsets in the gaseous feed system, marked changes in the thermal conductivity would result. This has proved a sensitive indicator of even the smallest upsets. Prior to introducing feed into the reactor system the combined feed can be passed through the thermal conductivity cell and the existence of steadystate flow conditions ascertained. If the unliiely combination of two feeds with identical thermal conductivity existed, such use of the cell would not be possible. Control System. One of the unique and decidedly useful features of this microreactor is the automatic control system. The reactor temperature is controlled by the temperature recorder-controller (TRC-1). This controller can either be manually set at a predetermined temperature or can be automatically programmed through a steady temperature rise or a series of temperature increases followed by temperature plateaus. In this manner it is possible automatically to program a series of temperature tests for the reactor which will require little or no attention by the operator. The sampling cycle is automatically controlled by a synchronous motor-driven, cam-operated timer (8-1
Table 11). This is a multiswitch instrument, and, in addition to automatic sampling, other switches are utilized in providing automatic chromatographic attenuation and automatic activation of the recorder chart. The automatic on-off operation of the chart drive is simply a useful method for compacting the otherwise lengthy chromatograms to more convenient s u e and a means for saving chart paper. The type of automatic attenuation needed is dependent upon the reaction being studied. Section 3, Figure 7, shows that a single-pole, double-throw toggle switch is provided for selecting either automatic or manual attenuation. With this switch in the manual position any attenuation may be selected simply by use of the attenuation selector on the chromatograph. When the selector switch is in the automatic position, the attenuation range is controlled in a predetermined time sequence by the automatic timer cams. Two manual multithrow, single-pole switches are utilized so that an exact attenuation may be selected from high- and lowsensitivity ranges. The high-sensitivity range provides 1, 2, 4, 8, 16 attenuation and is suitable for those components present in amounts ranging from trace impurities up to approximately 5%. The low-sensitivity range provides 32. 64, 128, 256, or 512 attenuation for those components present in larger amounts. The sequence of attenuation, of course, is highly dependent on the reaction being studied. It is selected by preliminary runs in which manual attenuation is employed in order to find the optimum for each peak. By having the two attenuation ranges, it is possible to study a
Figure 7. Wiring diagram for nutmati> sampling,
. ..
TABLE I.
EQUIPMENT SPEC1FlCATlONS
__
Item No I
B-1 D-1 F-1 F-2
N a m e and Manufacturer Sand Bath Dept. Shops Silica Gel Dryer Dept. Shops Micron Filter Hoke, Inc. Micron Filter Hoke, Inc.
Service
Spec$cations See Figure 4
Water Removal from Feed
9 ” section of 2 “ sch. 40 pipe filled with silica gel
Must be periodically replaced or regenerated
Dust Removal from Feed
Series 540, Model S5412231 Series 540, Model S5412231
303 S S housing with 316 SS filter element; 2-5 micron filter range; NPT (Fem)connections
Entrained Catalyst Fines Removal from Reactor Effluent Preheat Fluidizing Air
H-I
Preheater
P- 1
Liquid Feed Pump Dept. Shops
Meter and Pump Liquid Feed
R-1
Reactor
Microreactor
R-2
Deoxo Unit Engelhard Industries Liquid Reservoir Dept. Shops Metering Valve Hoke, Inc.
Oxygen Removal from Feed
Pressure, 50 p.s.i. ; capacity, 5 c.f.h.
Liquid Feed Storage and Charge
650-ml. capacity
Manual Control of Gaseous Feed Manual By-Pass and Purge of Microreactor
Model 2PY 280
T-1
v-1 v-2
Selector Valve Republic Mfg. co.
v-3
Sampling Valve Loe Engineering Co. Solenoid Valve Hoke, Inc.
v-4
Mixing Tee Crawford Fitting Co. Vaporization Tube
303 SS housing with 316 SS filter element; 2-5 micron filter range; l/8’’ NPT (Fem) connections
l’/~ ft. of 1/2” 304 SS pipe wrapped with 40 ft. of 1 6 - , ga. nichrome wire
72-r.p.m. synchronous motor; 10 nonslip gear ratios; feed rates, 2-30 cc. per hr. See Figure 3 and text
,
Described by M’ebb, Dallas, and Campbell (6). Now built and sold by Pressure Products, Inc., Hatboro, Pa, Swagelok fittings manufactured by Crawford Fitting Co., Cleveland, Ohio
~
section of 3” sch. 40 304 SS pipe with SS plate welded over ends
5I/2‘’
I
Automatic Sampling of Reactor Effluent Automatic By-Pass and Purge of Reactor
1 1 1
1
4-way valve with 0.d. tube connections; silicone rubber 0rings Model L-208-8V equipped with Model LVO-200H T energizer Series 95, Model S95.4133
Mix Gas and Vapor Feed
Swagelok ‘/a’’ 316 SS all tube tee
Furnish Surface and Volume for Vaporization of Liquid Feed
4” section of SS tubing
catalytic system, for example, which begins a t high initial activity and then declines to a much lower activity. Low-sensitivity operation during the early stages of such a study followed by high-sensitivity operation will yield the most desirable peak heights. An automatic elapsed time indicator-controller (8-2, Table 11) is employed to extend the automatic features of the control system. This unit will actuate on a preset time schedule, the liquid feed pump (P-1), the variable transformer connected to the fluidizing-air preheater, and the three-way solenoid valve (V-4). I n practice, the duties of this timer-controller are integrated with the programmed temperature controller so that the catalyst may be pretreated automatically with various gases a t some planned temperature profile. As a n example, hydrogenation catalyst must be reduced with hydrogen a t high temperature prior to use. This is a time consuming process that is most conveniently controlled automatically. I n addition to these major control features, various 24
Remark3
-I-
Constant Temperature Bath for Microreactor
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
l/4”
1
316 SS with I/16’’ orifice, 1 O stem point and vernier handle; NPT (Fem) connections Teflon sealing surface
12 sire 2-4 Viton A O-rings form sealing surfaces. See Figure 5 for flow details Universal construction with 316 SS body, IYPT (Fem) connections, explosion-proof housing, and high temperature solenoid coils
0.d. 304
indicator lights and an elapsed time indicator (8-3, Table 11) are included for providing the operator with further aid in making trouble-free studies. Equipment Specifications
Tables I and I1 along with the flow diagram (Figure 2) and the perspective drawing (Figure 1) are presented to provide the necessary data for interested readers desiring to construct a similar microreactor system. The tables give sufficient information for ordering all the items of equipment. Smaller features such as tubing and ordinary valves are not included since these are generally stock items. I t should be noted, however, that all tubing and valves in the feed and product sections are constructed of 304 or 316 stainless steel. Assessment of Automatic Microreactor
A similar microreactor was employed by J. W. Hall and Rase (4) and the presently described, improved reactor system is currently being used on a study involving benzene hydrogenation.
