ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT Physical Description and Appearance The cabinet model Titrilog (Figure 9 ) is housed in a floor mounted case 6G inches high, 22.5 inches wide, and 18.5 inchee deep. The strip chart recorder is mounted at eye level. Directly below it is the control panel. The portable model (Figure 10) consists of the Titrilog, 11 X 22 X 12 inches high and weighing about 50 pounds, the portable recorder, 10 X 13 X 17 inchep high, and the filter unit, 6 X 13 X 11 inches high. All units of the portable Titrilog can be transported easily in the trunk compartment of an automobile.
Figure 10.
Literature Cited
Portable Titrilog Model 26-1 03
(1) Austin. R. R., Am. Gas Assoc., Proc., 31, 505-16 (19491.
existing range. .4 laboratory dilution apparatus has indicated the feasibility of testing concentrations of hydrogen sulfide up to
100%. Preliminary work, with some promise, has been conducted toward using the Titrilog to determine sulfur in liquid samples by combustion of the sample and determining sulfur dioyide in the combustion gases. Gaseous samples have similarly been burned when interfering substances such as olefin are present. The interfering compound is burned to carbon dioxide and the sulfur to sulfur dioxide. .The principle of continuous electrolytic generation of titrating agent a t a potentiometric balance point may be applicable to the generation of other titrants, or t o the continuous titration of liquid samples. Keither of these applications has as yet been investigated.
(2) Austin, R. R., Percy, L. E., and Escher, E. E., Gas, 26, No. 6, 47-63, NO. 8, 33-8 (1950) (3) Austin, R.R., Turner, G. Ti., and Percy, L. E., Instrztments, 22, 588 (1949). (4) Eckfeldt, E. L. (to Leeds & Sorthrup Co.), U.S. Patent 2,821,671 (Dec. 16, 1952). (5) Reitmeier, R. E. (to The Girdler Corp.), Ibid., 2,405,872 (Aug. 13, 1946)). (e) Schaeffer, P. A., Briglio, A , , Jr., and Brookman, J. A,, Jr., Anal. C h a . , 20, 1008-14 (1948).
(7) Sease, J. W., Lee, T., Holzman, G., Swift, E. H., and Nieman, C., Ibid., pp. 431-4, (8) Washburn, R. W., and .lustin, R. R., Pioc. Natl. Air Pollution Symposium 1949, 1, 89-76 (1951). RECEIVED for reyiew September 7, 195.3.
~ C E P T E D February
17, 1954.
tomat ic Ilastrumentatio
enc
nits E. R. ROTH The
Aflanfic Refining Co., Philadelphia, fa.
Properly limited in its application, the bench scale unit i s an important and reliable laboratory tool. This i s particularly true when industrial control instrumentation is used to full advantage. The type of unit described finds wide use for cafalytic research studies, catalyst evaluations, catalyst life determinations and product yield, and distribution data for a variety of problems. A broad range of gas and liquid feed rates are controlled at system pressures up to 1000 pounds per square inch and reactor temperatures up to 1050' F. Gas rates of 1 to 16 standard cubic feet per hour of hydrogen are controlled and recorded, and liquid feed rates of 30 to 600 milliliters per hour are maintained. Liquid level control and recording is a feature that has been added with excellent results.
TYPICAL schemat,ic flow diagram is shown in Figure 1. The unit consists essentially of a tubular reactor and its furnace, a controlled gas feed system, a low flow proportioning pump for liquid feed, and a gas separator with liquid level and system pressure control. Gas sample train and gas metering devices are necessary adjuncts to the unit. Safety devices vary from unit to unit, and may only protect against excessive furnace
1428
temperature or be so complete as t o shut the unit down and keep a protective stream of hydrogen passing over the catalyst. The piping, fittings, valvee, and vessels are engineered for high pressure hydrogen application. A view of one of the unita i s shown in Figure 2. Actually this is one of two units operated by one operator per shift on a 24 hour continuous basie. All pneumatic control instruments are dual recorder-controllers.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
PRQCESS INSTRUMENTATION The bench scale unit is an assembly of equipment suitable for laboratory installation and designed for the smallest flows that meet the requirements of operability, reproducibility of data, and adequate samples for evaluations. The bench unit complements other laboratory facilities and does not usually replace the pilot unit.
n2
SUPPLY
(SYSTEM
PRESSIJRE
P L U S 100 P S I
)
comparison with hand-operated units of former years. Details on controls for temperature, pressure, gas, and liquid are presented. A short discussion of low flow control valves is included. The degree of control that can be attained on bench scale units is demonstrated by the charts shown in Figure 3. Included are charts for flow pressure, and liquid level from a dual unit setup with dual control instruments. The conditions during the runs shown were system pressures of 500 pounds per square inch gage, flow rates of 6 standard liters per minute hydrogen and 200 milliliters per hour liquid feed for each unit.
L J
0
1
FLOW PRESSURE L l O U l D LEVEL
Figure 1. Flow Diagram of Bench Scale Unit The concern of this paper is equipment operability and reproducibility of data. Hand operation of a small unit, particularly when dealing with micro flows on a continuous basis requires special techniques and experience. Even then, and assuming the absence of annoying mechanical faults, proper manipulation and good data are by no means assured from operator to operator with hand control. The instrumentation of a continuous flow catalyst evaluation unit is described that practically mass produces accurate evaluations. Runs of several hours’ duration are repeatedly completed with only about a ‘/a to of one operator’s time. This discussion is valuable, because it shows how successfully standard industrial instruments can be applied to the small flows encountered in the laboratory. The measure of success is demonstrated by the number of units that have been put into operation and by the number of acceptable runs turned out in
Figure 2. July 1954
Figure 3. Instrument Chart Records Flow Control
Gas. The essentials of the hydrogen flow control system are a
regulator for keeping the hydrogen supply 75 to 100 pounds per equare inch above system pressure, a metering orifice of reliable calibration, a sensitive and accurate differential pressure primary element with recording and controlling means, and a throttling control valve with a C, coefficient (gallons per minute of water that will flow ihrough a valve a t 1 pound per square inch pressuredrop) of 0.00063 to 0.000063 and a flow rate rangeability of a t least 10 to 1. The capillary orifice devised for low flow work is shown in Figure 4. A number of capillary tubes provide the necessary pressure drop, The oonstruction shown makes it easy to assemble as many tubes in a bnndle as are believed necessary. Capillary tubes are merely pinched o f f if, on calibration, the flow is too great. The capillary tubes used are stainless steel, 0.007- to 0.019-inch inside diameter, with the bulk of the needs satisfied by using 0.017-inch inside diameter X 0.050-inch outside diameter tubing in lengths of 2.25 inches. Details of construction are given in Figure 5. A pressure drop of 100 to 200 inches of water is specified for all capillarysorifices a t the maximum desired flow. The 200-inch range applies to the lower flow rates. The pressure drop is kept high to prevent reversal of signal and loss of control when slight upsets occur. For example, total volume of a typical bench Continuous Bench Scale Unit Showing Automatic Control Panel scale unit, including tubing, is approxi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
1429
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT mately 700 cubic centimeters. Even though the flow control valve is put on the downstream side of the orifice to minimize volume each side of the orifice, pressure fluctuations of the system cause considerable cycling of the flow rate and system pressure. Operating a t 1 standard liter per minute, or less, of hydrogen. a 1% change in system pressure involves an appreciable time for coming back to control pressure. For such small units and minute flows, it is imperative to avoid any condition that causes wild cycling if standard control instruments are to be used to advantage. Calculations show the flow to be streamlined through the capillary tubes, ranging from about 300 to 1500 Reynold's number over the flow range. However, the application of fluid mechanics equations to small-bore tubing is of questionable value and accuracy in this case. Experience shows the entrance and exit conditions of the tubing t o be important, particularly for viscous flow. T h a t is, if either end of a single capillary is partially crimped or closed, the calibration is affected. The best approach to sizing a capillary orifice is through experience Figure 4. for various bores, lengths, and flow ranges. For the example illustrated in Figures Capillary 4 and 5 , the calibration shows the orifice Orifice flow range to be 3 to 14 standard liters per minute of hydrogen a t 10 t o 100 inches of water pressure drop, respectively, and system conditions of 85" F. and 600 pounds per squaie inch gage; At these conditions, then, a single 2.25-inch length of O.Oli-inch bore tubing is equivalent to 1.3 standard liters per minute maximum flow.
DIA.
DIA.
S T A I N L E S S STEEL
STD. 1/4" 0.D. 59/60 JOINT
NUT L A \
,
I3/8" J
y
ICCL
'0.050*
O.D.
x
0.017" 1.0. B R O N Z E Cl., I L L A R I E S A S
REO'D
Figure 5. Details of Capillary Orifice
1430
Either the minimum or maximum flow rate desired is specified, and a capillary is built t o meet the one single condition. The complete calibration curve is found by calibrating in place in the unit a t pressure levels desired. The importance of using the newer-type differential pressure transmitter with its low volume and null-balance principle instead of a mercury-chamber instrument is apparent from the above discussion. The mercury-chamber instrument requires a fluid displacement well in excess of the volume of the orifice system. This high volume, together with the high inertia of the mercury, makes operat,ion with such an inst'rument difficult. Standard differential pressure receivers (recording and cont,rolling) are used for all gas flows. Proportional band and reset are always specified. As mentioned previously, control valves with a C, coefficient of 0.00063 t,o 0.000063 cover the range of flow for these bench scale units. It is hard to visualize how small the clearances of the plug and seat are for such valves. When initially considering automatic control for bench scale units some years ago, only one manufacturer of control valves was found who could supply what was want,ed. Even then, ir was some lime before the manufacturer's techniques were perfected so that satisfactory valve trim w i e supplied. For hand control a fine tapered. needle is necessary. A 1/8-inch pipe size valve, operated essentially closed, is capable of throttling the flow. However, the valve adjustment available is only "closed, or more closed." The type of trim used is shown in Figure 6. Both seat and plug are Stellite, and the minute clearances are obtained by hand lapping to a finished fit. The valve is characterized by a scratch of varying depth along the plug length, and the assembled valve is calibrated. Proper guiding of the plug avoids binding. I n time, wear is sufficient under conditions of operation t o lose control. The flow curve for the valve flattens out more and more and the throttling characteristics approach those of an open and shut valve. This occur8 over a period of about a year. Liquid. Liquid flow rates are controlled by adjustable stroke proportioning pumps. I n order to maintain constant feed rates varying from 30 to 600 milliliters per hour, the pump must be of special design. The factors involved are high discharge pressure ( t o 1000 pounds per square inch at lorn rat,es) and sufficient length and frequency of stroke, together with a highly dependable check valve, t o ensure a constant, high efficiency liquid rate. Light nonlubricating hydrocarbons must be handled; the stroke or pump rate must be adjustable while running; and the drive transmission and plunger packing must be reliable and well designed for continuous uninterrupted service. The pumps are assembled to operate n-ith 0- t o l'/ls-inch stroke a t 60 strokes per minute. The recommended minimum stroke length is inch which gives 25 milliliters per hour in operation. Figure 7 shows the plunger design and a few of the details including packing arrangement. The stepped plunger allows low rates a t reasonable stroke lengths, because the extension of the plunger fills most of the displacement, volume. As to packing, a number of schemes \i-ould probably suffice, but the one s h o w has proved the best. A pump Figure 6. Trim of this design is now commercially for Low Flow Conavailable. trol Valves (HamFigure 8 is an over-all view of the mel-Dah1 Co.)
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46,No. 1
PROCESS INSTRUMENTATION REAMED HOLEQO01"
N.F.(12 THDS. PER INCH)
( 2 4 THDS. PER INCH) \
i l ' l l l IIIllIl
i
1
PUMP B O D Y Figure 7. Details of Liquid Proportioning Pump PumD Packing Inside Outside pump assembly. The various ure 9. An adjustable range No. Diameter, Diameter, Thickness, parte are readily distinguishMaterial Required Inches Inches Inches differential pressure transmitter is used with the range able. The piping arrangeLarge end 0.320 0.812 311 6 ment is designed to operate set for 0 to 20 inches of B Teflon 3 0.310 0.812" 1/16 with a glycerol leg with a C Aluminum 2 0.312 0.812" 0.029 water. A reference leg with 0.260 0.750 3/16 Small end 8 Teflon 2 connection for venting arid fill chamber is sized as to @ Teflon 3 0.247 0.750" 1/16 recharging. T h e c h e c k height for this range. The 2 0.250 0.75O5 0.029 @Aluminum valves are an important item sight-glass receiver is installed aTolerance -4-0.001. in low flow pumping. The -0,000. a i t h its center point a t about check valves fabricated have ' / p the height of the referKel-F inserts for seats with ence leg. The control instrument is a duplicate of the flow receiver1/18-inch port diameter and 7/,2-inch diameter balls. recorder-controller, Pressure Control. The primary considerations for good pressure control seem to be the pressure control valve and a steady Temperature Control. The control of temperature in the reactor is a critical requirement although elaborate controls flow control system. The aspects of control valve design have been mentioned above, and, together with a valve positioner for have not been necessary. The reactor chamber containing the catalyst is inserted in a tubular aluminum bronze furnace which positive stroking, pose no problem. The pressure control inis 2l/8 inches outside diameter X l l b /inches ~ ~ inside diameter X strument is a staudard recorder-controller M ith proportional band and reset. If the flow control system operates smoothly, 24 inches long. Beaded heating wire, to the extent of 3000 the system pressure is held constant to very close limits. Liquid Level Control. One of the automatic control features GAS T O S Y S T E M that materially helps the operator is constant liquid level. As PRESSURE VALVE liquid products are condensed and accumulated, automatic control keeps a constant level to within inch in the sightv glass receiver. The time saving is achieved by leveling the unit &a it is about to be tested knowing that the level is constant during the test period. The installation for liquid level control is shown in Fig-
a
8
DIFFERENTIAL PRESSURE T R A N S M I T T E R ( 2 0 " RANGE1
Figure 9. Schematic Installation of liquid Level Control
Figure 8. liquid Proportioning Pump Assembly (HillsMcCanna Base and Microadjustment) July 1954
watts, i s wrapped around the furnace and covered with a porcelain cement. This assembly is put into a sheet metal jacket and packed with insulation. If the electrical heating wires are divided into three circuits top, middle, and bottom sections of the furnace, on-off control of the top circuit only is sufficient for good temperature control. The middle and bottom circuits are adjusted with autotransformers. There are other types of furnaces that might be used-lead bath jacketed, for examplebut the ease of reactor removal with the metal blocktype furnace is highly desirable. Mechanical Considerations. The one aspect to stress is hox to avoid leaks. A small leak on a bench scale unit can waste a lot of time and valuable effort. The design of these units requires either light wall tubing and compression fittings, or heavy wall tubing and the high pressure 59" cone, 60" seat-type fitting. Wherever possible, joints are
INDUSTRIAL AND ENGINEERING CHEMISTRY
1431
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT avoided and welded or brazed connections arc specified. No simple pipe fittings are used. I n general, the heavy wall tubing and high pressure fitting combination has proved most satisfactory from an operating viewpoint. Another consideration for good operation is to avoid dirt because of the small clearance control valves. Only stainless steel tubing and fittings are used, but nevertheless, precautions must be taken. Once a unit is amembled, it is highly advisable t o remove sections a few a t a time and Tyash them before the initial start-up. This is true even if filters are installed in the lines. Once all metal chips and dirt’ are removed, smoot,h operation is assured.
warranted. However, it is logical that bench scale units will be expanded in the future and be made more automatic, including means for automatic analyses and data processing. What ha5 been presented here is a prior step to such push button units. Acknowledgment
Credit is due the personnel who have constructed and worked n i t h these units, as each in his own field has contributed to the successful operation of the units. Particular credit is given the Instrument Division of The Atlantic Refining Co. for the design of thc capillary restrictoru.
Conclusions
References
Automatic control of bench scale unit3 speeds up research efforts. The particular unit described completes several teat runs per day, depending on the durat,ion of the test period. Operating conditions are quickly set and held. The data are consistently reliable, with weight balances checking to an average of 96%. The cost of a single unit, complete as described, is approximately $8000. Instrumentation accoinits for slightly more than half this amount. However, the cost per test run is appreciably reduced when one operator per shift handles two units. The unit described is a simplified laboratory tool. Larger units are equipped to recycle gas and liquid streams and also to fractionate feed and products. Such small scale equipment may be added, but to date more elaborate installiitions do not seem
JOHN F. BISHOP AND
RALPH
(1) Clark, E. L., Golden, P. L., Whitehouse, A. II.,and Storch, H. H., ISD. ESG. CHEX.,39, 1555 (1947). ( 2 ) Grote, H. W., Hoekstra, J., and Tohiasson, G. T., Ibid., 43, 545 (1951). (3) Henrigues. H. J., Ibid., 39, 1564 (1947). (4) Rockwell, R. A,, Instruments, 25, 1074 (1952). (5) l t o t h , E. It., and hIasologites, G. P., ISD. EXG.CHEM.,45, 1845 (1953). (6) Siiepcevich, C. XI., and Brown, G. G., “A Small Scale Plant for Studying Catalytic Reactions at High Pressures and Temperatures,” h.S.31.E. Petroleum Jleohanical Engineering Conference, Tulsa, Okla., 1951. (7) Swezey, F. H., Chem. Eng., 54, 131 (October 1947). (8) Wolfson, 11.L., Pelipetz, 11.G., Damick, A. D . , and Clark, E. L., I S D . E N G . CHESf., 43, 536 (1951). RECEIVED for r e v i e v September 7, 1953.
ACCEPTEDl l a r c l i 23, 19.54,
S. WHITE
Beckman Instruments, Inc., Fullerton, Calif.
For many processes where purity and color in the end products are of prime importance, continuous colorimetric methods of quality control are required. Instrumentation providing this control must be capable of recording extremely reliable, accurate, continuous measurement under u variety of adverse environments. The Beckman flow colorimeter monitors and records plank stream color by passing a light beam through a known amount 04 process fluid and appropriate color filters. A phototube detects the amount of light transmitted at the desired color and feeds this information to a remotely positioned amplifier and recorder. Operating controls are also at this location. Substantial savings in production costs are made possible with this instrument through continuous, uniform product quality control.
W
IDESPREAD use of continuous process production for an
expanding list of applications in our industrial economy has focused attention on the growing need for continuous process instrumentation. Fortunately, many very recent advances in scientific instrumentation have combined to make possible the development of equipment t o satisfy this need. Because of the splendid cooperation between industry and instrument manufacturers, excellent industrial instruments have been developed that today are earning their cost many times over. Since the field is a large one, this discussion is restricted t o a eingle phase of industrial instrumentation. One of the most interesting and widely useful types of continuous process instrumentation is continuous colorimetric analysis provided by a flow colorimeter. This industrial instrument analyzes process fluids by passing a light beam of controlled wave length through the process fluid and continuously observing its 1432
optical transmission characteristics. For many processes where accurate control of purity, color, or chemical composition are of prime importance, continuous colorimetric methods of quality control are required. I n addition, there are a vast number of industrial applications requiring less accuracy where colorimetric process control prosrides Iyatchdog protection for color or turbidity. What are the basic requirements for a good industrial flow colorimeter? From a practical standpoint, such an instrument must provide reliable, accurate, continuous measurements under a variety of adverse environments. What does this mean? It means that a successful instrument must be virtually troublefree in operation. The design must be extremely simple and mechanically rugged. Ready accessibility for servicing is fundamental. It is of utmost importance that the instrument be unaffected by environmental moisture and temperature. I n many
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 46, No. 7
