MULTIPLE AUTOMATED MICROUNITS C A R L D . A C K E R M A N , A N T H O N Y A N D R I C H A R D A. W R I G H T
B. H A R T M A N
Gulf Research @ Dezelopment Go., Pittsburgh, Pa.
A group of four microscale units for fixed-bed catalytic reactions is described. Each unit is capable of independent, continuous operation with liquid feed rates of 3 to 45 ml. per hour a t pressures up to 300 atm. Gas flow rates are 2 to 84 liters per hour. Better data are the result of careful design using high quality hardware. This includes data presentation with digital readout in engineering units. The application of grouped multiple units with automatic control systems and automated stream analyzers produces economy in manpower and equipment.
N THE SEARCH
for new and better fixed-bed catalysts, pre-
I liminary tests are required to evaluate potential catalyst performance. Since the results of these tests will determine future more expensive testing on larger units, preliminary testing must be conducted accurately. This goal can be achieved on microscale units that give results comparable to those obtained on larger scale units. The use of microscale units is justified by the following: A minimum amount of charge stock and test catalyst is required. “Isothermal” reaction data are more nearly isothermal. With small amounts of combustibles in an area, safety is improved. Microscale size produces economies in housing space requirements. Multiple units can be clustered together and automated, producing economies in manpower and equipment requirements.
Equipment Features
The systems used to control flow, pressure, and level, measure fluid density, and weigh products are described by Ackerman and Hill (1966). These variables are accurately measured by electronic differential pressure cells. The analog signals from these cells are indicated with four-digit resolution by using a digital voltmeter. In applying these control and measuring systems to this scale of unit, the reduction or elimination of liquid product holdup is important. The internal volume of commercially available differential pressure cells has been reduced to as little as 1.5 ml., as described by Ackerman and Hill (1966). The precision control valves are especially designed for minimum holdup and high flow rangeability. All process liquid transfer lines are 0.020-inch i.d. All process gas transfer lines are 0.062-inch i.d. [ l A : MPRODUCT iC CHARGE REACTOR
Process Features
As shown in Figure 1, representing one unit of the four, pressured metering fluid is flow-controlled into a displacer vessel. This fluid forces a seal piston to expel the charge stocks from the displacer vessel into the reactor. Continuous operation is maintained by dual displacer vessels. The gas charge that is flow-controlled joins the liquid charge prior to entering the reactor. The fixed catalyst-bed reactor consists of a j / ~ ~ - i n ci.d. h by 9/16-inch 0.d. stainless steel tube 35 inches long. Catalyst volume can be varied from 5 to 25 ml. Close clearance rods inserted above the catalyst reduce holdup and improve feed preheating. The reactor can be heated to temperatures up to 100Oo F. The reaction products are phase-separated at reactor pressure. Liquid holdup in the separator is controlled at 0.5 ml. or less by throttling liquid flow to the debutanizer. The gas phase flowing from the separator is throttled to maintain pressure on the separator and reactor. The debutanized liquid product flows into equipment used to determine the specific gravity automatically and accumulate the product for automatic weighing. The gas streams from the phase separator and debutanizer are recombined and flow through a gas chromatograph sampling manifold to a wet-test meter. 476
l&EC PROCESS D E S I G N A N D DEVELOPMENT
ANALYSIS R
R
A group of four units is described, each capable of accurate testing of 5 to 25 ml. of fixed-bed catalyst. Continuous liquid feed to each unit can be varied from 3 to 45 ml. per hour at pressures up to 300 atm. Gas flowing at 2 to 84 standard liters per hour joins the liquid feed and passes over the catalyst bed. Better data are economically obtained by automation of multiple units with high quality hardware. Sufficient automation has been used to reduce manpower requirements to one operator per shift for the four units. Some of the concepts used in building these units are believed to be unique.
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1
HEATER DISPLACER
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150 PSI
I
LIQUID
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PRESSURED METERING FLUID SUPPLY
Simplified
flow
PRODUCT 4 1AUTOMATIC
diagram for one
1
microunit
LIOUID CHARGE STOCK TO REACTOR A
LRY
Figure 2. Liquid feed system for flow rates from 3 to 45 ml. per hour at pressures up to 4500 p.s.i.
Liquid Feed System. Fi of the liquid feed system. 1 flows from a common 1500pressure nitrogen. Flow to e differential pressure, and re( enters the 400-ml. displacer \ a magnetic piston to expel Total feed input is measure, pressure signal. The mag"' switches used to warn the OF the limit of travel. Dual d continuous reactor feed. T h pressure cell, and displacers environment far accurate feed A displacer vessel is rechar;yed manually, using low pressure nitrogen to force the liquid charge from an 800-ml. supply reservoir into the displacer. This causes the piston to expel the metering fluid out of the displacer vessel to an 8-liter low pressure reservoir. The re'charged displacement vessel is manually brought to operatirlg pressure by the metering fluid bypass line. The feed displacer seal pist,on assembly is shown in Figure 3. The Teflon "slipper" firs ove!c the O-ring and prevents the 0ring from rolling. Contamincadon of the liquid charge by the metering fluid appears to bc2 low (less than 0.1'%). Actual contamination levels have no!t been measured, since the metering fluid is compatible with the liquid charge and catalysts being studied. The effect o'f a leak would be indicated by an error in unit weight balaiice, provided the specific gravity of the metering fluid is differ€:nt from that of the feed stock. A seal piston is also necessary in the high pressure metering fluid reservoir to prew:nt nitrogen absorption in the metering fluid. Absorbed riitrogen will desorb, because of pressure drop over the meti:ring fluid flow control system, and cause metering error. A magnetically tipped extension rod connected to the niitnn ~ . ~ ~ ~ . ~~.... . ~activates . switches on the high pressure reservoir recharging pump. The performance of this control and measuring- system is good. Liquid flow rate can be read to 0.01 ml. per hour, with flow control maintained to f0.02 ml. per hour variation. The liquid feed system is calibrated at fixed operating pressures by flowing directly from the feed system to the phase separator. The liquid is collected at atmospheric pressure as it flows from the phase separator level control valve. The liquid feed system is linear at flow rates of 3.0 to 45.0 ml. per hour. Liquid feed system calibration is necessary at operating pressures, because of the effect of pressure on metering fluid viscosity (Kauzel, 1965). This feed system is cheaper than individual high pressure pumping systems for each unit. Thermastating and feed rate changes are easier. Changing feed stocks requires only cleaning of the displacer vessels and transfer lines, which can be easily removed from the system. Spare replacement displacers also minimize down time. Charge Gas System. The charge gas is metered with a capillary and flow controlled at reactor pressures. Variations in reactor pressure control influence the performance ~~~~
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for example, is normally controlled to within f l p.3.i. by a control valve in the gas-phase exit line from the phase separator. Charge gas flow control precisian to the reactor is ~ t 0 . 2 5 7of~ flowmeter ranges. A calibration curve is required for the charge gas metering capillary. Investigation of the capillary entrance and exit pressure drop shows that when hydrogen gas is flowing, this effect can produce a 4% error in the flow rate compared to Weymouth's equation for gases (Brawn,
1950). Reactor and Heater System assem hly is shown in Figure 4.
Figure 3.
Displocer vessel seal piston assembly A. 8.
c. D.
E.
Stoinlex steel keeper Teflon slipper sea1 Viton O-ring 1-inch diameter magnet Teflon guide
2-ZONE
BRONZE BLOCK
HEATER C O R E - ~ ' I . D , x 3"O.D.x 36" LONG
m7 LTER DISCS
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INSULATION CASE
Figure 4. Heater-reactor assembly for testing 5 to 25 ml. of fixed-bed catalyst a t isothermal coriditions The assembly is designed for easy remVi11 of the reactor from the heater core. The top closure (1lot shown) is a J--J s i \ , 9 , :--L -.-. 2 - > ~ ~ - 2. x a u u a ~ u 18 n 16 I I I L L L cuucu p u g urillled through and tapped to receive a '/&nch 0.d. compression-type tube fitting. The bottom exit from the reactor is '/#-inch 0.d. tubing for receiving compression-typefittings. Various lengths of removable deadman rods are supplied, depending upon catalyst charge desired. The reactor wall to deadman clearance is 0.005 inch. This close clearance gives minimum holduu and imuroves feed oreheatinr. Porous metal disks with 20-micron pore size are used to support the catalyst and protect exit lines against plugging. The heating system for the reactor consists of a two-zone annular bronze black. Each block zone is heated externally by a 1000-watt capacity heater. Heat flows into the bronze block to the reactor located in the center of the block. Temperatures are controlled to 1 0 . 5 ' up to 1000° F. by one thermocouple located longitudinally and radially in the center of each block zone. The block-zone temperature is scanned every 55 seconds and heat turned on automatically as required. A second block-zone thermocouple identically located presents temperature at the control panel in units of degrees Fahrenheit. Readability is 0.1" F. No temperature measurement within the reactor is made, because of reactor size. Catalyst conditions are very nearly isothermal, because of the mass ratio of catalyst to reactor and bronze block. Block temperatures are used to represent catalyst conditions. Debutanizer Design. The design of the debutanizer is shown in Figure 5. One S/ls-inch 0.d. tube is used as a combination rebailer, distillation column, and condenser. The condenser consists of tracing 20 inches of the top of the b/ls-inch 0.d. tube with '/cinch 0.d. capper tubing. Heat transfer cement is used to bond the tubes together. Eight inches of the bottom of the Z/,s-inch 0.d. tube are wrapped with electric heating wire rated far 100 watts. A siliconcontrolled rectifier is used to control power input to this reboiler section. Thermocouples are mounted on the outside tube wall for temperature measurement and control.
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477
( 7 DEBUTANIZER GAS . The debutanizer is packed with protruded
0.16-inch metal packing (Cannon, 1949). A U-tube seal with siphon break is installed at the bottom of the debutanizer, positioned so that the reboiler operates as a wetted-wall surface, thus further reducing liquid holdup. The debutanizer is insulated with rigid, closed-cellurethane insulation. Debutanizer temperature is automatically controlled, using skin thermocouples attached above the feed point. Freon refrigerant is regulated to control this temperature. Rehailer heat input is on manual control. However, the control devices can be readily changed to provide automatic temperature control below the feed point. Reboiler heat can be regulated automatically to control stripping temperature with manual control of Freon refrigerant to the condenser.
4 5"
D.D. x 0.02D" IO WALL TUBE 53" LONG-PACKED WITH PROTRUDED METAL PACKING
6
-
110 VOLT SUPPLY REBOILER
DEBUTAN WED LIQUID PRODUCT
Figure 5. Debutanizer designed for flexible control and minimum liquid holdup
Debutanizer performance bas been determined by material balance and chromatographic analysis of gas and liquid products. A 20 ml. per hour liquid feed consisting of 62.5% hexane and heavier, 11.5% pentanes, 14% butanes, and 6% propane and lighter was 99% debutanizer with a loss of 0.8% of the pentane in the gas product.
DIGITAL LIQUID
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i GAS CHROMATOGRAPH INITIATION
PRODUCT GAS COUNTERS SWITCHING
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PUNCHED T A P E L I S T E R TAPE IDENTIFICATION
Figure 6.
PRODUCTS DRAIN
Timed sequence circuitry
Liquid Product Analysis. The stahilized liquid flows into equipment designed to determine the specific gravity automatically, accumulate and weigh the product, and drain the product into bottles at speciiied time intervals. Specific gravity and product weighing are done using electronic differential pressure cells. The design for these systems is identical to that described by Ackerman and Hill in 1966, except that smaller precision bore product weighing tubes are used. Tubes of 3/g- and 3/n-inch i.d. give the best flexibility for this scale of unit. Product weighing with the 3/8-inch bore tubes has a resolution of 0.01 gram. The 3/r-inch bore weighing tube is limited to 0.1 gram by the system resolution. The specific gravity system also has smaller internal volume than that described by Ackerman and Hill in 1966. This system is accurate to &O.OOt specific gravity unit over a range of 0.6500 to 0.8500 as compared to +0.0005 specific reoarted hv Ackerman gravitv, unit for thv lareer scale svstem , and €iill (1966). This loss of accuracy is due to increasedI noise i:hat is believed to be caused by gas bubbles. and meterAnalysis. Gas.Product . . . . The sampling, analysis, , l , ~ l ~ ~ ing ot the recomblned gas Streams are also nanalsa aulumatically. A process gas chromatograph is attached to the units through a Stream sampling manifold. As now adjusted, the chromatograph can service up to five preselected streams each hour. The chromatographic unit is equipped with a stream selection panel to select desired streams and an electronic integrator which integrates each peak and presents its output data to a lister and a tape punch. The punch and lister record the retention time and peak area of each component as well as identification information. The punched tape is then taken to a tape-to-card converter and the cards are processed on an off-line computer. The product gas streams are analyzed once every hour. Chromatographic analyses for hydrogen, hydrogen sulfide, and saturated hydrocarbons from methane through penta nes are made. Companent analysis can range from 0.02 to 100% with 0.01% resolution. The gas stream from each unit is measured . . by an inte; grating wet-test meter to 0.03-liter resalutlon. lntegrarea readings of gas volume are presented every hour at the control panel far manual recording. ~~~
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A more detailed description of the automated gas sampling and chromatographic equipment is presented by Ackerman (1965). Control Circuitry and Data Presentation
Figure 7. Instrument console 478
I&EC PROCESS D E S I G N A N D D E V E L O P M E N T
., -1 All automatic control operations of me unnb aic ~ U V C L L K U by a master control digital clock (Figure 6). The digital clock starts the chromatographs every hour and enters identification information on punched and printed tapes: chromatograph number, time and date information, and the stream being analyzed.
mounted on the instrument console. Each variable to be recorded is provided with a zero adjustment so it can be positioned at any location on the recorder chart. The recorded points from each unit are then positioned to fall in a vertical line on the chart and a high alarm contact is provided far each unit. Any unit point upon going into an alarm condition will light a signal light and ring an alarm hell, which will continue to ring until acknowledged by the operator. The lamp remains on until the condition is corrected by the operator. Each data point recorded by the operator is wired through a unit selector switch to a digital voltmeter. The analog signal to be presented on the digital voltmeter is adjusted to read directly in engineering units by means of zero and span adjustments. Four-digit resolution is displayed on the digital voltmeter. Thermocouple input signals are also linearized in the ranee between 0' and l000O F. and n r e s m t d dirertiv
T o improve manpower and equipment utilization, four separate testing units are clustered together (Figure 8). The
electronic transmitters for pressures, levels, and gas flow meas.~~ the ~~~. background are the constant temperature iiremmt ln cabinets with thi: debutanizers mounted on top, and on the left is a gas chronnatograph. The floor space used is 18 X 30 feet. This arraqgement of major equipment gives a compact . . . .. instailanon, minimizes tubing. .runs, and saves steps for the one operator (per shift). The grouping concept also enables sharing of gas supplies, chromatographs, metering fluid supply system, and control console instrumentation. A more detailed ~~~~~~~~~~~
Figure 8.
Arrangement of rnoior equipment
Counters are used to integrate the flow rate pulses from fmea 2nrl r nmrll.ll.+ 0119-..._. mp+p..Q nllniirltp mllntprc _" " .___ _ _ " _ _ _~ " r..l_.l are provided so that the previous hour's integrated data are available on one counter while the other cou nter integrates the pulses. The digit clock will, at the start L,f a new hour, -__.I ....l"o"t^ zero the counter not in use and switch the sisLLou this zeroed counter. This method gives the totalized readings for these streams for I-hour periods. Liquid product flow into the specific gravity and liquid product weighing equipment is stopped by the clock at the beginning of every hour for a 10-minute period. This enables the operator to obtain accurate data on specific gravity and the accumulated liquid product for the pievious hour. Surge bottles store the continuing flow during this 10-minute period. The liquid product that has accumulated in the weighing equipment can be automatically drained at preselected 2-hour time intervals or a multiple of 2 hours. Draining starts 10 minutes after the start of the selected hour and continues until 30 minutes after the .hour. At this time draining valves are closed and surge bottle valves are opened. The digital clock controls this timing. The digital clock sounds an alarm at the start of each hour. This alerts the operator to record data from all units. A 10-minute time period is allotted for manual data recording of charge gas rate, reactor pressure and temperatures, debutanizer temperatures, product specific gravity, and hourly accumulated quantities of liquid feed, liquid product, and gas product on each unit. Alarm scanning and trend recording of the important process variables are provided by a 12-point millivolt recorder iirn.irl _ _l___
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recently by Ackerman (1965). Conclusions
Semiautomated microunits can be built for flexibility with continuous operation at elevated pressures. High quality hardware is available that gives good precision in process control and measurements. The extent of automation presented in this paper enables efficient operation of four separate microunits built in one group. Other degrees of automation are possible, depending upon requirement. Litertllure Cited Ackerman, C. D., Chem. Eric. Proq. 61, 67 (1965). K., ISA J . 13,45 (1966). Ackerman, C. D., Hill .I. Brown, G. B., "Unit Operations," p. 143, Wiley, New York, 1950. Cannon, M. R., Ind. E q . Chcm. 41, 1953 (1949). Kouzel, B., Hydrocarbon Procm. Pefrol.Rlfner 44, 120 (1965). RECEWED for review October 24, 1966 ACCEPTED March 28, 1967
Division of Petroleum Chemistry, 152nd Meeting, ACS, New York, N.Y., September 1966.
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