Pilot Plants - American Chemical Society

lnst r umentat ion for Pilot Plants Part of the Panel Discussion on Pilot Plants Presented before the Division of Petroleum Chemistry, 123rd Meeting, American Chemical Society

Clyde Berg Union Oil Co. o f California

Pilot Plant Instrumentation

C. H . McIntosh The Texas Co.

G. P. Masologites The Atlantic Refining Co.

Specialized Recorders and Controllers on Pilot Units

Recorders and Controllers in Pilot Unit Instrumentation

ILOT plants and modern process development methods are eliminating years of trial and error in the creation of new products and the perfection of new processing techniques. The fundamental objective of pilot plant operations is to prove the correctness of deductions arrived at from the advanced knowledge of all fields of science integrated and quantitatively evaluated to create the new or improved processing technique. The rapid advancements in technology have made this blending together of knowledge from various fields to create a new process more and more intricate a t the same time that the possibilities of achievement have become more and more promising. I n the course of pilot plant studies, critical process temperatures, pressures, flow rates, reflux ratios, pH values, solids movement, and liquid level must be controlled and maintained in a manner representative of the commercial plant process in the course of creation. Recording instruments are a must for exacting control over critical variables and for providing a record of the process variables in a pilot plant.

What may be considered a negligible amount of corrosion in commercial operations may subject pilot plant valves and measuring elements to much magnified and possibly serious fouling effects. Thermocouples. Thermocouples are widely used in pilot plant operations for measuring temperature; they consist of two dissimilar wires welded together at one end. Figure 1 shows a bare wire-hot junction thermocouple with a high pressure insulating seal which is frequently used in pilot plant equipment where good sensitivity and quick response are desired. Similar responsiveness can be retained by using a so-called pencil-type thermocouple, also illustrated in Figure 1, wherein one of the thermocouple elements is in the form of a tube with the other metallic component passing inside it. Where a thermocouple well is employed in pilot plant operations, careful consideration should be given to the possible errors due to heat conduction along the metal wall of the thermocouple protecting well. Figure 2 is a cross section of a pilot plant reactor where this problem of heat conduction along the thermocouple well was solved by a longitudinal tube running the full length of the reactor and through which multiple thermocouples of fine wire gage covered with braided glass insulation were introduced. The problem of errors in thermocouple readings due to radiation effects are accentuated when measuring high temperature gas streams in pilot plant operations. A diagram of a thermowell construction utilizing concentric radiation shields to minimize erroneous temperature readings in spite of low gas velocities a t the entrance to a pilot plant catalytic reactor is given in Figure 3. Resistance Thermometers and Radiation Pyrometers. Resistance thermometers provide a very sensitive element for temperature measurement where considerable accuracy over a rather narrow temperature range is desired. Where temperatures in the range of 800" to 3200"F. must be measured, a radiation unit employing a, thermocouple or thermopile may be used. A pilot

P

Fundamenfals of Pilot Plant Control Follow Those of Commercial Installations

Pilot plant instrumentation and control equipment have objectives similar to those of commercial installations but must contend with a number of additional complicating factors. These include: 1. A wide range of process conditions must be explored to establish optimum conditions in the new process. 2. Since pilot plants operate at reduced capacities, available dimensions for installation of measuring elements frequently limit or prevent the use of conventional commercial equipment and may re uire special design. 3. &e surface-to-volume ratio employed in pilot plant operations is considerably higher than that of the commercial units.

1836

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 45, No. 9

PILOT PLANTS

4

plant installation using a radiation type of measuring element is shown in Figure 4. Volume Displacement Meters. Fluid flow is one of the most important variables measured iu pilot plant operations and may Direct measurements tit2 measured by direct or inferred means. include such items as reciprocating piston meters, constant displacement pumps, oscillating disk meters, nutating disk meters, water-sealed gas meters, water-sealed gasholders, and diaphragm gas meters. Rate-of-Flow Meters. Orifices are the most widely used method of rate of flow measurement. Where flow quantities are extremely small, a capillary tube may be substituted for the orifice installation. I n this case, of course, the pressure differential response characteristic becomes linear. A diagram of a typical pilot plant orifice installation with pipeline taps is given in Figure 5 . Venturi tubes are sometimes used where large flows must be measured; however, where dust and solids accumulation constitute a problem, Venturi tubes offer advantages for even small installations. Differential Pressure Meters. Differential pressure meters are generally employed with an orifice or other dynamic measurement of fluid flow for translation of the pressure differential into a specific measurement. Although glass tube manometers are frequently used as indicating meters in pilot plant work, instrument control generally employs an actuating device, such as a differential pressure transmitter or indicator. Many types of commercial differential pressure units are available. The newer force-balance transmitters are of particular interest in pilot plants hecause of their small size and volume displacement. Seals and Purges. Seals and purges for differential flowmeters or differential pressure meters are particularly important in pilot plant operations where a condensable vapor or a dusty gas is being metered. To prevent condensation in the pressure transmission line, the pressure tap line must either be provided with a constant gas purge or be completely filled with fluid and equipped with a condenser or seal pots a t the orifice location to ensure constant fluid level in each of the pressure tap lines. The purge operation is carried out by passing a small flow of inert gas-too small to upset the flow measurement-into the system through each manometer leg. Rotameters are commonly used to meter the flow of purge gas or liquid. The addition of a

installation. A unit of this type is utilized in Figure 8. Where dusty conditions must be contended with, the pressure tap should be one half inch in diameter or larger and slope u p to

REACTOR FEED

RAUATIW WSS MEATING COILS

TMERYOCOUPLES

THERMOCOUPLE RAUATIW M L D REACTOR PRODUCT

2 f O D.2t.1 I 25-12 T U B I N

Figure 2. Pilot Plant Reactor Thermocouple Installation

u"l

THERMOCOUPLE

P S PIPE CONTAINING TUERMDCWPLE LEADS

OCOUPCE ION SHIELD

Figure

3.

Radiation Shield in Pilot Plant Thermocouples

avoid ready fouling. Where conditions are particularly severe, a porous plug may be inserted at the entrance to the pressure tap line to prevent access of dust. The location of a transmitter immediately adjacent to the orifice also provides a convenient method for minimizing problems with pressure tap fouling. SPECIFIED LENGTH Rotameters. An important type of meter used in pilot plant work is a rotameter in which a float is supported in the flowing stream. This meter is of the area type employing a constant differential pressure but variable orifice area, indicated by variation of the N P T MOUNTING ATTACHMENT float position in a tapered glass or plastic tube. Liquid Level Measurement. Liquid level measureFigure 1. Bare Wire Pencil-Type Thermocouple ment is an important part of most processes and can be Type Thermocouple measured in pilot plant operations either by direct or indirert means. Direct means of liauid level measuresmall differential pressure controller is convenient to maintain ment include gage glasses, floats, and buoyancy type displaceconstant purge gas rates in spite of variations in system pressure ment units as well as weighing methods. An example of a as is illustrated in Figure 6. Under conditions of elevated presfloat-type liquid level unit for control of liquid levels in sure it becomes preferable to utilize a capillary metering element pilot plants is given in Figure 9. Figure 10 shows a and a high pressure differential controller as shown in Figure 7. torque tube unit for liquid level measurement and control in Certain types of differential pressure controllers can be provided which a more elaborate control element is included such as with an internal flow restriction, yielding a package unit for purge automatic reset and variable adjustable proportioning range. September 1953

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

1837

ENGINEERING AND PROCESS DEVELOPMENT Another inferential method of measuring hydrostatic head is t o measure the pressure necessary to force air down a pipe until it bubbles from the bott'om a t the desired reference point. A pilot plant installation of this method ,on a gasholder is illustrated in Figure 12. Pilot plants often use electrical methods for measurement. Actual conduction through the liquid is a common method. An electronic method empIoying condenser plates is also employed, and occasionally a photocell is utilized to measure liquid level by looking through a gage glass. Solids Level Measurement. The level of solids may be measured by several different methods. A dynamic method, analogous to that of torque tube level control, was developed by the Union Oil Co.; it employs a grid suspended in a moving solids bed actuating a torque tube. This met,hod of level indicabiori is OIL 1 t t D 10 V U I M A C C continuous but requires movement of the solids bed (Figure 13). Figure 4. Pilot Plant lnstallafion of Radiation Pyrometer Other methods of solids indication may be employed, such as diaphragm actuation or electronic methods. Flow Ratio ControI. Flow ratio is important $ORIFICE in a variety of pilot plant process operat,ions,including dist,illation,solvent extraction, and blending. Flow ratio may be either controlled by direct, or inferred means. Any combination of liquid flowmeters with transmission to a common HIGH PRESSURE LOW PRESSURE control unit may be used for this purpose. Mu1t.iple-orifice installat'ions with common or interrelated control can be used. Figure 14 illustrates the use of automatic blending equipment in a catalytic cracking pilot plant employing a liquid level control device to add automatically the necessary fresh feed in the 1500 Le. VOGT O V A L FLG. U N I O N No, 2 7 0 3 - REMOVE I N N E R L I P FROM charge to the reactor. Figure 15 illustrates a F E M A L E F A C E TO PROVIDE S P A C E FOR ORIFICE PLATE AND GASKETS. direct type of flow ratio control utilized in a solvent ext.raction pilot plant wherein the two cylinders of a double-acting positive displaceGASKETS ment pump are set to deliver fluid at the desired Figure 5. Pilot Plant Orifice Installation ratio. Types of Automatic Control Action. I n the simpler processes which have little lag besides the demand side capacity lag, the control problem is easy, and an elementary controller will do 511 A displacement type of liquid level transmit.ting unit incorpoexcellent job. When transfer lag and dead time are involved t'o rat& the pneumatic force-balance principle, which minimizes an appreciable extent, a more complicated cont'rol mechanism is motion of the float, is shown in Figure 11. Differential pressure transmitters are commonly used for liquid le.r.el indication. Mercury manometer units with electric transmission are frequently used in this service.

-

CE

F L A N G E W I T H F L A N G E TAPS

SURGE C H A M B E R S A - FLOWING Llcauio HEAVIER e - TFLOWING H A N PURGE Liauio LIGHTER

-+-------r

0-

.

9--

L1 2 0

THAN

PURGE

- - A _ - -

EGULATORS ORIFICE UNIONS

RECORDING

Figure 6.

1838

Purge System in Pilot Plant Operations

Figure 7.

Purge System for Pilot Plant Operations


Vol. 45, No. 9

PILOT PLANTS CONTROLLER CASE

7

/TORQUE

TUBE UPPER T A N K TAP

K N I F E EDGE ASSEMBLY

DISPLACER CHAMBER

DISPLACER

LOWER T A N K TAP'

Figure 10. Masoneilan Torque Tube Level Controller .*

Figure 8. Purge System for Pilot Plant Operations

LIQUID LEVEL

necessary. The more refined types of controller initiate corrective action in accordance with the direction and amount of deviation and the rate of change of the deviation. The methods of automatic control most generally used today may be classified into one of the following main types: Two position Proportional position Floating control Proportional plus reset Proportional plus rate response Proportional plus reset plus rate response Proportional plus reset plus inverse derivative or rate response

STANDARD FLANGE

MOORE PACKLESS LEXIBLE SHAFT

Frequency Response Technique Will Establish Control Problems

The pilot plant provides an unsurpassed o p portunity for determination of process control characteristics by experimental methods. Both the semiempirical method of Ziegler-Nichols ( 3 ) and the frequency response technique ( 1 ) can be used for determining the nature and Moore Product Co. Float-Type Liquid Level Control for Pilot Plants character of control problems. The ZieglerNichols method is adequate for moRt pilot plant instrumentation problems. However, the frequency response technique is the most refined of the experimental methods. The Ziegler-Nichols method can readily be applied in an existing pilot plant control circuit with no additional equipment. It involves procurement of a so-called signature curve by making a stepwise change in the final control element. The reaction EXTENSION

Figure

9.

/RESTRICTION ORIFICE I

W

A

I

R PURGE

WATER SEAL

Figure 1 1. Minneapolis-Honeywell Liquid Level Controller with Pneumatic Transmission September 1953

CONTROL VALVE

Figure 12.

Pilot Plant Liquid Level Pneumatic Transmission


1839

I

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KX production forum

I

Equipment and DesiProducts that are inclined to develop “skins” may best be dried with wet air

by David E P

k

operator said that Tthe semiworks material in the pans of the E

dryer looked l i e “elephant skin.” He had used a well-chosen expression. The dyestuff which had heen put in the pans of the atmospheric shelf dryer as a paste was now, after 24 hours, a wrinkled, leathery mass. When the operator stuck a bar into it, he could lift the elephant skin, disclosing a wet paste beneath the hardened surface. Day6 of additional drying finally gave a hard, solid of most unaatisfactory quality. It could he pulveri d , of course, but it was not the soft, friable wwder that was desired.

Figure 1.

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*IILrl‘

fi.l

liun 7 9 h B

-

temperature speci6ed for this dyestuff (70’ C.) gave no trouble and were completed in 24 hours. This was something different, however, in that a standardizing agent which had been incorporated in the hatch seemed to give it an unusual property of forming a more or less impermeable crust through which the water could not escape. Various expedients had been tried to overcome the dficulty. Different temperatures did not solve the problem. Neither did the use of thin layers in the pans. The latter scheme reduced the capacity per square foot of pan area without eliminating the toughness of the final product, which adhered tenaciously to the pans. It was known that the drying of other materials that caw-harden on the surface or develop surface cracks can he controlled by decreasing the rate of evaporation while accelerating the rate a t which moisture m u s e s t o the surface. It was suggested there fore that both these effects could be achieved by increasing the humidity of the circulating air. With a higher partial pressure of water vapor in the air, the driving force causing evapora-

t

tion (differencebetween vapor pressure’ of the water on the surface and partial pressure in the air) would be reduced. This should allow more time for the water to d8use from the interior of the layer on the pan into the surface. Another advantage t o be expected from using air of higher humidity was an increase in the wet-bulh temperature of the air. Since water in eontact with air tends t o maintain a temperature corresponding t o the w e t bulb temperature of that air, it was reasoned that the material in the pan would rise in temperature. This should increase the rate of diffusion, which is a function of the hatch temperature. To try out the idea, the next batch was started with dryer vents closed as tightly as possible. No improve ment was observed, probably because the actual increase in humidity was not very great. The next step waa to put a pan of water on one of the shelves so that additional water would be available for humidifying the air. The results were somewhat better than before, but there was still casehardening of the product. Finally, a small steam line was

Rate of Drying a t 70” C.

A = relative humid+, 5% relatin humidity, 30%

The product being handled was one that was t o be produced in a plant operation as soon as possible. The equipment that would be available for drying was an atmospheric shelf dryer similar t o the one in the semiworks. It was therefore important t o solve the problem as quickly as possible. Case hardening of the surface of materials being dried had not heen a common phenomenon in this plant. Most materials that were dried a t the September 1953

80

90

100

110

I20

Tmperalmm,

Figure 2.

I30

140

OF

I50

160

I

Abbreviated Humidity Chart

INDUSTRIAL A N D ENGINEERING CHEMISTRY

83 A

PILOT PLANTS

\

"DIRECT "DERIVATIVE

,

TIME-CONSTANT

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S l A B l L I T V O B T A I N E D wlln VCRV bARI)OW l H R O T l L l N G RbNGE (WIT*OUT R E S E l OR O F R I v A T I v E I

LlQUlD L E V E L

\ IIII I F

nYP0THETK;AL CURVES SHOWING APPLICATION OF 'DIRECT' AND "INVERSE" DERIVATWO ON PROCESS CONTROL

Figure 17.

Applications of Inverse and Direct Derivative in Process Control

poor performance of experimental automatic control systems. The control valve is an integral part of the over-all instrument control loop, and proper operating characteristics of the control valve are as important as any other part of the control system. In pilot plant operation there is frequent need for alteration of valve port sizes to maintain operating characteristics consistent with altered process conditions. The more frequent error in pilot plant valve selection is that of oversizing. When this condition exists, controller sensitivity is penalized. Commercial types of flow control valves should not be employed unless specifically designed for flow rates used in pilot plants. Generally, the valve port leakage or valve port clearness will be excessively large. For small flow quantities pilot plant valves should be of the direct-acting singleseated type, actuated by a pneumatic diaphragm or piston. It is important that actual flow capacity characteristics of the pilot plant valve be readily and a c c u r a t e l y e v a l u a t e d and that means be available to change the internal port when p r o c e s s i n g c o n d i t i o n s a r e altered.

Pilot Plant Controls Utilize Most Types of Power Units

Power units are mechanisms that furnish the power for operating the control element in response to controller impulse. Pilot plants utilize all the important types, which include pneumatic diaphragm units, pneumatic or hydraulic cylinders, electric motors, and electric solenoids. Pilot plant control valves are frequently powered by a pneumatic diaphragm motor where pressure is applied to one side of the diaphragm and the force on the diaphragm is resisted by compression of a spring on the other side. Diaphragm motors are also employed to operate dampers, rotary plug valves, Variacs, and other pilot plant equipment. Where accurate control is necessary, a position controller is incorporated with the diaphragm motor or cylinder. Pilot plant valves often require positioners for good performance when operating under fouling conditions or at low flow rates with small valve stem travel.

CYCLE T I M E R

AIR iUPPLY

1

Careful Selection and Installation of Valves Aids Operation

I n pilot plant operations the control valve is subject to wide extremes of temperature, pressure, and f o u l i n g . Selection of the proper size valves for pilot plant operations is difficult because of the unusual capacity range in which they frequently operate and lack of knowledge of the flow characteristic of the valves involved. Incorrect sizing of pilot plant valves perhaps more than any other single factor contributes to

September 1953

AIR SUPPLYFigure 18. Cycle Timer in Solids Flow Pilot Plant


1841

ENGINEERING AND PROCESS DEVELOPMENT Figures 19 and 20 are nomograph8 for determining the required valve area for operation on liquids and gases as in the capacity range of pilot plant operations. The valve area travel chart in Figure 21 refers to six different types of pilot plant valve ports:

001

00.)

1. 2. 3. 4. 5. 6.

003

0 02

001

0001

P

2

0005

5

5 D

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0 0003

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0 002

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.,

03

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0.2

-

0 001

00007 01

OOOOI

=

c - 8

Chisel point (wedge) plug, Chisel point plug, l4 inch KO.2 taper pin No. 0 taper pin No. 3-0 taper pin S o . 5-0 taper pin

inch

The detailed construction of these direct-acting valve plugs and seats is shown in Figure 22. These valve seats and plugs are normally installed in check valve bodies (Figure 2 3 ) 1%hich are available from manufacturers in carbon steel or any of the stainless alloys, although bar stock construction may be employed under special conditions. A packing gland construction suitable for use on these assembled pilot plant valve bodies with the adapter for use with a conventional pistonactuated or diaphragm-actuated valve top works is shown in Figure 24. The procedure for sizing a pilot plant valve is as follows:

Liquids Determine the following characteristics of the liquid: 1. Liquid density, HzO = 1 2. Allowable pressure drop across the pilot plant valve, pounds per square inch 3. Flow rate desired in gallons per hour From the nomograph given in Figure 19, determine the required valve area From the valve area travel chart given in Figure 21, select the valve port design having the required area a t the desired valve travel Figure 22 will give the valve port construction corresponding to that selected from the valve area travel chart Gases

00003

1. Determine the value of the specific heat ratio, k , for the sga or gas mixture being handled 2. Determine the pressure ratio,

0000)

Figure 19.

1842

Pilot Plant Valve Design for Liquids


PZ

E’

desired for operating conditions of the valve. Also, calculate the value of B , given by the relationship

Vol. 45, No. 9

PILOT PLANTS

I

Recorders and Controllers in Pilot Unit Instrumentation

_yv

E. R. ROTH AND G. P. MASOLOGITES Th. Allontie R..Rning Co., ?hh&d.lphb, Pa.

w . ;

HAT d e t e r m e s the extent and choice of instrumentation for pilot units? For the most part, the same factors that influence commercial instahtione. Pilot Units are instrumented to operate at- nearly automatically as possible. Automatic features reduce manpower needs, improve time efficiency, and sid in the securing of accurate experimental data. Choice is based on what instrumentation gives the utmost in dependability and doea the job beat. Experience has shown that the most trouble-free operation canes from the use of conventional instruments, wherever possible. This is tempered by the unique problems that are met in pilot plant and laboratory work.

timing and automatic reset adjustments. Derivative corrective action has not been necessary providing the measuring lag is small and the proper control valve w epeoified. Reset adjustment, however, is an easential feature. Temparature Recording and Control. Tower top temperatures are controlled by the reflux ratio which is varied by flow of coolant through internal reflux coils. The temperature wntrol point in usually in the tower overhead vapor. In the case of a stabilizer tower the control point is in the coolant discharge line. The temperature control instrwnent serves to illustrate our choice of the throttling temperature wntroller. T b is the electronic null-balance potentiometer with pneumatic throttling action which Use8 the thermocouple for its primary m e w i n g element. The electronic potentiometer bas completely replaced the bulb or thermometer-type controller for our pilot unit work. The electronic instrument allows operation of several continuoua fractionating towers in series with a minimum of attention. This WBB previously very difficult with the bulbtype controller. In pilot unit operation the primary measuring element must recogniee a change in the variable being measured at B rate in accord with the capacity lag or inertia of the process aystem. The thermocouple 8 8 a measuring element satisfies this requirement whereaa the bulb or thermometer element does not. Advantages other than rapid response of the electronic controller are the flexibility of the primary element (the thermocouple) as to distance from the controller and e m of replacement. Ease of range change is another desirable feature. On-off temperature control of coil-in-lead heating baths, rea+

Thorough Study of Inakumenhlion in Design Stagea Pays Dividends

Figure 1 is a simplified flow diagram of a 5-barrel-per-day catalytic reforming pilot unit. Primary considerations are the control of operating variables and the measurement and control of materials charged to and discharged from the unit. Heat and material balances readily obtained and not obscured by frequent upsets ensure reliable operating and design data. Thorough study and consideration of instrumentation in the first design stages pay8 dividends in the form of month, high efficiency operation. From Such a careful initial study of the flow sheet, the measurement and control problems are defined. Through conferences between the pilot unit and engineering aections the limits and extent of control are set. The equipment to recard or indicate necessary operating and design data is now also outlined. The unit under review (Figure 1)has performed extremely well on a 24hour continuous schedule for several years, and the quality of data has been very gatifybg. From the To meter 8 stack start this unit hss been opersted by only one man per shift and his eole attention is all that is required in all phaaes of the operation of the unit. This unit was on test 16 hours after b&sl s t a n u p and has averaged a 96% time efficiency over the past 3 years. Several years ago two operations per shift would have been assigned to the unit without question and 15 yeara ago three operators per shift would have been required. Pressure Rewrding and Control. All fractionating tower pressures are controlled by panel-mounted, p r e s s u r e recorder controllers. These and other pressure controllers are conventional pneumatic throttling controllers complete with subpanels that allow switching from automatic to manual operation. All throttling controllers, both preseure and temperature, employ pneumatic Figure 1. systems and are complete with propor-

I

September 1953

T

I$

.

A

met r stpd

temperature flow pressure

Iliquid

level

Flow Diagram of Reforming Unit

INDUSTRIAL A N D ENGINEERING CHEMISTRY

1845

P ron VALVE AT I M I E R T

AREA-VALVE TRAVEL M I A FOR PILOT PLANT VALVES

-

__

ChT no s490

--- --

-

Figure 23. Pilot Plant Control Valve Bodies

--

Vogt 1500-lb. forged piston check valve body used with diaphragm pointplug and valve seat

lAP $CAT I N S E l l

Figure 24. Standard Valve Top Works Adapter for Pilot Plant Valve

steel to be chisel insert

VALVE BODY

' I l l l l i 0.1

0.2

0.3

0.4

0.1

0.6

VALVE TRAVEL- INCHES OPEN

-@.

Figure 21 I

'.,

boards have been well thought out and permit the freedom of accessibility to instruments necessary for good operation. literature Cited '1) Caldwell, W. I., Taylor Technol., 4, KO.2 (1951). ( 2 ) Ziegler, J. G., and Kichols, N. B., Truns. Am. SOC.Mech. Engrs., 64, 759 (1942). RECEIVED for review April 15 1953 AdCEPTED

CHISEL POINT PLUO

VALVE SEAT INSERT CHISEL POINT PLUG

VALVE SEAT I N S E R T

B

A

T A P E R PIN TAPER P I N

VALVE SEAT INSERT

C

1844

June 2 2 , 1953.

VALVE SEAT INSERT

TAPER P I N

D

VALVE SEAT I N S E R T

E Figure 22.


Pilot Plant Control Valves Vol. 45, No. 9

PILOT PLANTS

Recorders and Controllers in Pilot Unit Instrumentation E. R. ROTH AND G. P. MASOLOGITES T h . Atlantic Refining Co., Philadelphia, Pa.

W

I B

HAT determines the extent and choice of instrumentation for pilot units? For the most part, the same factors that influence commercial installations. Pilot Units are instrumented to operate as nearly automatically as possible. Automatic features reduce manpower needs, improve time efficiency, and aid in the securirig of accurate experimental data. Choice is based on what instrumentation gives the utmost in dependability and does the job best. Experience has shown that the most trouble-free operation comes from the use of conventional instruments, wherever possible. This is tempered by the unique problems that are met in pilot plant and laboratory work. Thorough Study of Instrumentation in Design Stages Pays Dividends

Figure 1 is a simplified flow diagram of a 5-barrel-per-day catalytic reforming pilot unit. Primary considerations are the control of operating variables and the measurement and control of materials charged to and discharged from the unit. Heat and material balances readily obtained and not obscured by frequent upsets ensure reliable operating and design data. Thorough study and consideration of instrumentation in the first design stages pays dividends in the form of smooth, high efficiency operation. From such a careful initial study of the flow sheet, the measurement and control problems are defined. Through conferences between the pilot unit and engineering sections the limits and extent of control are set. The equipment to record or indicate necessary operating and design data is now also outlined. The unit under review (Figure 1) has performed extremely well on a 24-hour continuous schedule for several years, and the quality of data has been very gratifying. From the To meter a stack start this unit has been operated by only one man per shift and his sole attention is all that is required in all phases of the operation of the unit. This unit was on test 16 hours after initial start-up and has averaged a 96% time efficiency over the past 3 years. Several years ago two operations per shift would have been assigned to the unit without question and 15 years ago three operators per shift would have been required. Pressure Recording and Control. All fractionating tower pressures are controlled by p a n el - m o u n ted, p r e s s u r e recorder controllers. These and other pressure controllers are conventional pneumatic throttling controllers complete with subpanels that allow switching from automatic to manual operation. All throttling controllers, both pressure and temperature, employ pneumatic systems and are complete with proporSeptember 1953

tioning and automatic reset adjustments. Derivative corrective action has not been necessary providing the measuring lag is small and the proper control valve is specified. Reset adjustment, however, is an essential feature. Temperature Recording and Control. Tower top temperatures are controlled by the reflux ratio which is varied by flow of coolant through internal reflux coils. The temperature control point is usually in the tower overhead vapor. In the case of a stabilizer tower the control point is in the coolant discharge line. The temperature control instrument serves to illustrate our choice of the throttling temperature controller. This is the electronic null-balance potentiometer with pneumatic throttling action which uses the thermocouple for its primary measuring element. The electronic potentiometer has completely replaced the bulb or thermometer-type controller for our pilot unit work. The electronic instrument allows operation of several continuous fractionating towers in series with a minimum of attention. This was previously very difficult with the bulb-type controller. I n pilot unit operation the primary measuring element must recognize a change in the variable being measured at a rate in accord with the capacity lag or inertia of the process system, The thermocouple as a measuring element satisfies this requirement whereas the bulb or thermometer element does not. Advantages other than rapid response of the electronic controller are the flexibility of the primary element (the thermocouple) as to distance from the controller and ease of replacement. Ease of range change is another desirable feature. On-off temperature control of coil-in-lead heating baths, reac-

liquid l e v e l Figure 1.

Y

Flow Diagram of Reforming Unit


1845

ENGINEERING AND PROCESS DEVELOPMENT tors, and the like is accomplished b y thermocouple instrumciii such as the millivoltmeter pyrometer and the multipoint glavanometer-potentiometer. The pyrometer controllers are specified as the three-position type, with the uppermost contact serving as a safety cutoff in case of excessive temperature rise. The sixpoint galvanometer-potentiometer is a unique instrument that serves well when six on-off control points are required. On-off control as practiced is really two-position control Quantities and vessel mass are small in pilot unit work, and full on-off operation results in excessive cycling of temperatures. For an electrical heating application 60 to 90% of the requiietl poller is kept full “on,” arid only the balance of the electrical requirement is controlled. Further practice dictates that the thermocouple be placed where it Fill most quickly sense a temperature change-in the molten lead of a lead bath, for example, or near an external strip heater on a vessel surface. Level Control. Most of the level controllers on Atlantic Refining pilot units are the nonindicating ball-float type with throttling pneumatic action This instrument is usually mounted on the blind flange of an external float chamber with a torque tube extended through the flange to the air pilot. Occasionally, this instrument is mounted on a directly conneeted flanged n o z ~ l eof a tower for interface control. I n standard tower reboileis the float, sight glass, and connecting piping are electrically lagged and insulated. The same temperature must be held in the level s ~ s t e mas exists in the reboiler to prevent vapor condensation and thermal circulation of liquid. Figure 2 shows the ball-float level controller holding a level i n the high pressure separator of the irforming unit. The instruFigure 2.

High Pressure Gas Separator with Liquid Level Control

Figure 3.

1846

Low Flow Control Installation

Figure 4.

Automatic Level Control for Low Flow Unit


Vol. 45, No. 9

PILOT PLANTS ~

-

number of points recorded on a multipoint instrument is generally held to eight. The electronic null-balance potentiometer bas simplified thermocouple installation. With pyrometer and galvanometer type inetrumeuts the length and gage of lead wire is important, and sll wire connections must be carefully made. The high impedance and greater sensitivity of the electronic inatnunents makes the lead wire installation leae critical. Atlantic Refining specifies m e lead wire and uses a crimping tool for field connectione, although a smaller gage wire could be run and a simpler type of connection &e. The central switch panel makes a good terminal for tbermocouples. At one time binding poet terminal boards, external to the panel, were used and d&d points were brought to a rotary switch. We still do not have complete flexibility, but the keyawiteh panel serves its purpose well and belpa keep leads in order. The plug-and-jack type of connector has been field proved and if probably an anmer, at least if electronic instruments are used. A few plug-in-jack systems have been used, and more w e planned where complete flexibility justifies the higher cost. Panel Boards. The central panel board allows the operator to perfom hi duties in an orderly and efficient manner. By group ing all controls a t a central point, the operator can most quickly see the over-all performance of a unit. He can readily correct upseta because all controls are at his Iingertips. Safety of operation is also a prime consideration and should not be overlooked or da-emphasiaed. Figure 7 shows the general deaign of our panel boards. For thia pilot unit, the a u t o t r d o r m e r s for manually adjusting adiabatic electric heating circuita are also mounted on the instnunent panel. Ample lighting is a part of the panel de& and a convenient data table if provided. When more antotransformem are required than can be convenientlymountedontine~ntpsnel,aseparate transformer panel if provided. An important feature is the provision of pilot

lights for each transformer 80 that the operator can tell at a glance which circuits are “on.” This is particnlarly helpful at change of shifts for 24-hour continuous operation. Figure 8 shows a special type of central panel board. These panel boards were constructed for the high pressure laboratory and have instruments for individual high pressure cells grouped together. Flexibility is attained by mounting each instrument on a separate plate of a standard sise-in this case 2 feet square. The field of high pressure instrumentation is a highly specialized one. Commercial instrumentation, however, is available for recording pressures to 200,000 pounds per square inch and recording and controlling preaeures to 50,000 pounds per square inch. Easy Operation and Accumh Data Jus* Instrumentation COS^

The investment in recording and control equipment is small when compared to the cost of operating a pilot unit. For example, the cost, installed, of the in&rument systems and panel board of the catalytic reforming unit described was 20% of the total cast for the unit. This amounts to leae than the charges for 1 month’s operation of the unit-a small price to pay for tronblefree operation and accurate experimental data. In general, total installed instnunentation costs for pilot unita at Atlantic ReIiningvarybetweenalowof lO%andahighof ZO%forunitscosting about S30,oOO and 160,000. This percentage may range as high as 40% for bench scale units. These figures are for new instrnmenta and would be decreased considerably as the instruments became available for re-use. Experience over the past few years has shown these costa to be justified. A unit that previously required two mea per shift to operate now requires only one. In spite of this reduction in manpower, more test r u n s are produced and better data are obtained. Rwnrrao for review Apdl 15. 1858.

A C O ~ J ~ U~ ~Daa. S

ma.

Specialized Recorders and Controllers on Pilot Units C. H. MCINTOSH The T a m s Co., P w t Arlhmur, Tax.

T

HE chemical engineer in petroleum refining is often con-

fronted with the problem of obtaining from small scale units data suitable for the design of larger u n i t s or for piloting the operation of plant scale equipment, It if neceassry, therefore, to obtain scourate and dependable data from small units where continuous operation becomes more and more difficult as the size if decreased. One way of improving the quality of the data obtained is by the u8e of automatic control equipment to maintain steadystate conditions in a continuous proceea for reasonable lengths of time. However, the use of commercial instruments on pilot plants often requires that the engineer modify the conventional application or design special devices to obtain the desired control of operations. Small scale equipment introduces many problems not normally encountered in larger units. In many investigations, optimum conditions lie close to a region where the unit becomes inoperable. In such casea accurate control of process variables becomes extremely important, as demonstrated by the operation of pilot equipment for thermally cracking hydrocarbons. Under normal

September 1953

conditions an increase of 10’ F. in the temperature of the heater coil containing the flowing hydrocarbons may cause an immediate shutdown because of decomposition resulting in coke formation as contrasted with a normal run of several days’ duration. The size of the pilot unit sometimes introduces problem not normally encountered in larger units. An example lies in the use of bleed gsa in the meter taps of fluid solids systems ( 9 ) to prevent plugging. The volume of bleed gas is so great, particularly in high pressure operations, that it becomes a major component of the total gas stream leaving the system. Similar problems occur in the use of orifice meters for small rates of flow. Orifice dimensione become so small that accurate measurements are very difficult to obtain because of partisl or complete plugging of the orifice with tiny particles of coke or scale. Vessels are often too small for the installation of conventional liquid level controllers and require external float chambers designed to minimize liquid holdup. Pressure controllers require small valves with positive shutoff and often must hold high pressures at very small flow rates. These are just a few of the many requirements impoeed by small scale units.


1849

ENGINEERING AND PROCESS DEVELOPMENT

Figure 7.

Pilot Unit Panel Board

for smooth operation. Figure 3 shows such a low flow application. Both the mercury manometer chamber instrunlent and the diaphragm transmitter were tried on small units, and the latter was found to be the instrument for this low flow problem. The marked decrease in holdup volume for the diaphragm transmitter and the greatly increased speed of response through the elimination of the mass of the mercury are believed to be responsible for its success. The control valve with a low enough coefficient and sufficient range was developed in conjunrtion with a manufacturer of splinetype valves. Suitable control was realized only when a valve with a C, coefficient as lox- as 0.000063 was developed. This coefficient is equivalent to a water flow of four to five drops per minute through a wide open valve a t 1 pound per square inch pressure drop. The range of such valves in the system under conditions of good control varies from only 10/1 to 15/1 of maximum to minumum flow. 1i-e find that valve positioners are essential. Control valves between the C, I anges of 0.004 and 2 are made up in Atlantic Refining’s shops, ‘4standard forged steel body and the desired tapered needle trim are assembled with pneumatic diaphragm operators. Over a period of time a number of sets of valve trim have been made, and it is a simple matter to change a valve port size. The throttling valves in any well-controlled system must be properly engineered and constructed, with full appreciation of fabrication details and process application. They are essentially an instrument in themselves, particularly in low flow work. Level Control. The low range diaphragm differential pressure transmitter has also been applied to level control in high pressure, low flow applications. Figure 4 shows the elements of such equipment. The high pressure side of the “cell” consists of small bore tubing and a chamber filled completely with a reference fluid. The over-all height of the high pressure side is equivalent to the 20-inch water range for which the transmitter is adjusted. The low pressure side is connected to the bottom of the sight glass which serves as the liquid receiver. In this particular case, the liquid level is held t o plus or minus l/s-inch a t flow rates of 100 cc. per hour. A special level control technique is shown in Figure 5. We always strive to utilize commercially available instruments, but in this case improvisation was necessary. The problem was to

1848

control the interface of a n oil-solvent system a t an elevated pressure. The difficulty lay in the facts that the gravibies of the two liquids were similar and subject to some variation and that’ the lower layer held considerable suspended solids. Our ansrer utilized the marked difference in electrical conductivities of the two liquids. The upper layer (extract) was a nonconductor, whereas tlic conductivity of the raffinate u-as very high. By inserting a probe insulated from the vessel body and bringing wires from the probe and vessel t o a conductivity controller, the interface level could be detected. When t’helevel of the raffinate rose and touched the probe, the controller circuit closed. The control contacts operated a solenoid valve in a regulated air supply to a diaphragm ?ontrol valve in t,he extract line, The rate of ciontacting was frequent enough to maintain the diaphragm valve intermediate to a fully opened or closed position. Indicating and Recording PotentiometersThermocouple Systems. The temperature indica,toi and recorder are the stethoscope oi a pilot, unit operator. With important pressures, flows, levels, and temperatures held constant by controllers, the success of the pilot unit test run depeiids largely on see&& reIiaI)Ie and sufficient temperat,ure readings.

Figure 8.

High Pressure Laboratory Panel Board

Only the most important points are recorded, but suffivient couples are run t o the indicator a t the panel board to answer any doubtful operating question and to secure ample design information. A typical central key-switch box is shown in Figure 6 with accompanying recorder and potentiometer. Only reasonably fast recording speeds, usually 12-second full scale travel, are specified. Matching chart speeds are 2 t o 4 inches per hour. The


Vol. 45, No. 9

PILOT PLANTS

a

number of points recorded on a multipoint instrument is generally held to eight. The electronic null-balance potentiometer has simplified thermocouple installation. With pyrometer and galvanometer type instruments the length and gage of lead wire is important, and all wire connections must be carefully made. The high impedance and greater sensitivity of the electronic instruments makes the lead wire installation less critical. Atlantic Refining specifies 20-gage lead wire and uses a crimping tool for field connections, although a smaller gage wire could be run and a simpler type of connection made. The central switch panel makes a good terminal for thermocouples. A t one time binding post terminal boards, external to the panel, were used and desired points were brought to a rotary switch. We still do not have complete flexibility, but the keyswitch panel serves its purpose well and helps keep leads in order. The plug-and-jack type of connector has been field proved and is probably an answer, at least if electronic instruments are used. A few plug-in-jack systems have been used, and more are planned where complete flexibility justifies the higher cost. Panel Boards. The central panel board allows the operator to perform his duties in an orderly and efficient manner. By grouping all controls a t a central point, the operator can most quickly see the over-all performance of a unit. He can readily correct upsets because all controls are a t his fingertips. Safety of operation is also a prime consideration and should not be overlooked or de-emphasized. Figure 7 shows the general design of our panel boards. For this pilot unit, the autotransformers for manually adjusting adiabatic electric heating circuits are also mounted on the instrument panel. Ample lighting is a part of the panel design and a convenient data table is provided. When more autotransformers are required than can be conveniently mounted on theinstrument panel, a separate transformer panel is provided. An important feature is the provision of pilot

lights for each transformer so that the operator can tell a t a glance which circuits are “on.” This is particularly helpful a t change of shifts for 24-hour continuous operation. Figure 8 shows a special type of central panel board. These panel boards were constructed for the high pressure laboratory and have instruments for individual high pressure cells grouped together. Flexibility is attained by mounting each instrument on a separate plate of a standard size-in this case 2 feet square. The field of high pressure instrumentation is a highly specialized one. Commercial instrumentation, however, is available for recording pressures to 200,000 pounds per square inch and recording and controlling pressures t o 50,000 pounds per square inch. Easy Operation and Accurate Data Justify Instrumentation Costs The investment in recording and control equipment is small when compared to the cost of operating a pilot unit. For example, the cost, installed, of the instrument systems and panel board of the catalytic reforming unit described was 20% of the total cost for the unit. This amounts to less than the charges for 1 month’s operation of the unit-a small price to pay for troublefree operation and accurate experimental data. In general, total installed instrumentation costs for pilot units a t Atlantic Refining vary between a low of 10% and a high of 20% for units costing about $30,000 and $60,000. This percentage may range as high as 40% for bench scale units. These figures are for new instruments and would be decreased considerably as the instruments became available for re-use. Experience over the past few years has shown these costs to be justified. A unit that previously required two men per shift to operate now requires only one. I n spite of this reduction in manpower, more test runs are produced and better data are obtained. RECEIVED for review April 15, 1963.

ACCEPTED June 23, 1963.

Specialized Recorders and Controllers on Pilot Units C. H. MCINTOSH The Texos Co., Port Arthur, rex.

T

HE chemical engineer in petroleum refining is often con-

fronted with the problem of obtaining from small scale units data suitable for the design of larger units or for piloting the operation of plant scale equipment. It is necessary, therefore, to obtain accurate and dependable data from small units where continuous operation becomes more and more difficult as the size is decreased. One way of improving the quality of the data obtained is by the use of automatic control equipment to maintain steadystate conditions in a continuous process for reasonable lengths of time. However, the use of commercial instruments on pilot plants often requires that the engineer modify the conventional application or design special devices to obtain the desired control of operations. Small scale equipment introduces many problems not normally encountered in larger units. I n many investigations, optimum conditions lie close to a region where the unit becomes inoperable. I n such cases accurate control of process variables becomes extremely important, as demonstrated by the operation of pilot equipment for thermally cracking hydrocarbons. Under normal

September 1953

conditions an increase of 10’ F. in the temperature of the heater coil containing the flowing hydrocarbons may cause an immediate shutdown because of decomposition resulting in coke formation as contrasted with a normal run of several days’ duration. The size of the pilot unit sometimes introduces problems not normally encountered in larger units. An example lies in the use of bleed gas in the meter taps of fluid solids systems (3) to prevent plugging. The volume of bleed gas is so great, particularly in high pressure operations, that it becomes a major component of the total gas stream leaving the system. Similar problems occur in the use of orifice meters for small rates of flow. Orifice dimensions become so small that accurate measurements are very difficult to obtain because of partial or complete plugging of the orifice with tiny particles of coke or scale. Vessels are often too small for the installation of conventional liquid level controllers and require external float chambers designed to minimize liquid holdup. Pressure controllers require small valves with positive shutoff and often must hold high pressures a t very small flow rates. *Theseare just a few of the many requirements imposed by small scale units.


1849

ENGINEERING AND PROCESS DEVELOPMENT Small Scale Operations Require Special Methods to Make the Instrument Fit the Job

The object of this paper is to present a few examples of controlling and recording instruments in pilot plant operations. These examples were selected to illustrate applications that are not conventional in commercial units and to show some of the methods that have been devised to make the instrument fit the job. 115V RESISTANCE

1 VARIABLE AUTOTRANSFOQMER

THERMOCOUPLE

Figure 1.

Temperature Controller

Temperature. Many pilot plants operate at high temperatures (800' to 1050" F.), and heating with electric nindings is convenient. Electrical windings may be used for supplying heat to molten metal or salt preheaters ( 1 ) or fractionating towers, for eliminating heat losses from adiabatic reactors and transfer lines, and many other purposes. Figure 1 shows a method of temperature control for electric Tvindings. -4 variable autotransformer (usually oil-immersed) is employed to set the power level just above that required t o maintain the desired temperature. -41~0, a small resistance is in series with the winding. The recording temperature controller operates a relay which shorts out this small resistance causing a small change in the current flowing through the winding. Proper sizing of this series resistance will result in very good temperature control. A large change in the desired temperature will require readjustment of the autotransformer and possibly resizing of the series resistance. Pressure. Pressure is nornially maintained in the laboratory with conventional pressure controllers and air-diaphragm valves, direct-operated spring or wsight-loaded valves, or external and

HED

ro

internal pilot-operated valves. I n some installations, hen-ever, other arrangements must be made. An example is found in high temperature hydrocarbon vapors where coking conditions exist. Coke formation in valve seats prevents proper seating of diaphragm valves and can also freeze the valve stems where throttling controllers may not piovide enough motion. A valve with a twisting closing action is preferable for this type of service. I t is also desirable that the valve be in constant motion. The combination of these actions pievents the valve stem from freezing and also giinds the coke particle^ from the valve seats. h control aiid operating mechanism of this type is illustrated in Figure 2. I ratchet wheel is attached to the valve stem. h motor-driven gear nil1 either open or close the valve depending upon the position of a relay operated pair1 on the ratchet. A pressure gage with an electrical contact opeiatei: a mercury tube relay which engages either the opening or the closing paa 1. The control range of this drvice is usually sufficient 111 pilot plant operations wheie the valve seat size and stem shapr are changed as necessary to handle the Auid a t the operating piessure. It should be mentioned that an air-driven mechanism with the desired valve motion is also available. Rate of Flow. Special flow controllers are often required in pilot plant operations. For example, FiguJ e 3 shows a proportional flow controller which is used as an automatic samplei. A wet-test meter is emplo~ed to meter the gas stream A ratchet wheel opeiating a microswitch is attached to the revolving pointei on the meter. This microsaitch completes a &volt circuit that operates an electric automobile fuel pump, which pumps watei from the sample bottle. The reversing switch has been removed from the pump so that it makes only one stroke each time the gas meter closes the microswitch. The amount of water pumped from the gas sample bottle is thus proportional to the total gas flow. The bubbler trap provides a visual indication of satisfactory operation of the pump and prevents the samples from back mixing with the main gas stream. The pump stroke is adjustahl~ so that the desired sample size can be obtained regardless of the test period length. Since it is possible, in Some cases, for the meter to stop rotating with the circuit energized, a high resistance holding relay or similar device should be placed in the circuit to prevent discharging the battery. This device has proved to be a very dependable proportional flow controller. Fluid Level. Figure 4 shows an example of a liquid levcl controller designed to reduce liquid holdup to a minimum. In this case the total holdup is only 40 cc. This controller employs a transparent sight glass and a photoelectric switch. The liquid meniscus diffracts the light beam away from the phototube. This interruption causes an elertronic relay to operate a solenoid valve Tvhich in turn opens a diaphragm valve in the liquid dran off

VALVE

PRESSURE

GAGE

ECCENTRIC

Figure 2 . 1850

X O i DING RELAY

Pressure Controller

Figure 3.

Proportional

Flow Controller-Gas


Sampler

Yoi, 45, No. 9

PILOT PLANTS line. I n many cases a solenoid valve in the drawoff line would be all that is required, but a diaphragm valve has given better service a t high pressures. The best results have been obtained with this instrument on clear liquids, such aa naphthas, and where misting of the sight glass does not occur.

Q

ELECTRONIC

LlGHr SOURbS

streams. This deviceemploys an aluminum float from which is suspended a magnetic rod. This rod is encased in a nonmagnetic housing. A contact box is clamped on the housing, so that the balanced pointer may follow the travel of the pole at the end of the magnet. The electrical contacts may be employed to operate a suitable valve in the liquid drawoff line. A small vibrator is attached to the box to prevent sticking of the electrical contacts. During a start-up of the pilot unit, the level controllers are employed to hold tower levels. As operation continues and the unit approaches equilibrium conditions it is desirable to reduce the liquid holdup in the unit and pump directly from the bottom of the towers. The electrical contacts are simply removed from the box and the level box serves as an indicator. An additional advantage is that the level box may be adjusted vertically over a range of about 12 inches. This device thus serves a dual role of an adjustable level controller and/or level indicator.

r I II

I BALL FLOAT AT INTERFACE

I

/

I I I

I I

I

YENr

Figure 4.

Liquid Level Controller

-4 115 V

-7 I

II

However, this device may also be used for dark liquids, provided they are free-flowing and do not stain the sight glass excessively. I n this service the light source and the phototube are mounted at right angles to the sight glass. The dark liquid actually blots out the light, and it is not necessary to depend on diffraction as was done for clear liquids.

ASS

OD

HOUSING

!

u i -----_----------

L

J

Figure 6.

Liquid interface Level Controller

Another liquid level controller, shown in Figure 6, has been modified to serve as an interface controller in a solvent extraction column which handles very viscous liquids. A float is connected through a packing gland to an external arm. This arm has an adjustable counterbalance so that the float weight can be adjusted for the liquids involved. Raising the float closes a mercury switch which opens a solenoid valve in the liquid drawoff line. Very viscous oils cause the float to stick and respond very slowly. This problem was solved by placing an electromagnet in the circuit. The electromagnet is energized a t the same time as the solenoid valve in the drawoff line. This magnet jerks the level arm, pulling the float down into the less viscous extract layer. The float will then rise back to its normal position and again actuate the mercury switch. Thus, the liquid is withdrawn as a series of spurts, rather than as a steady stream, and float sticking is avoided.

MAGNET

TO REGULATOR

Figure 5. Liquid Level Controilerindicator

Another type of level controller is sketched in Figure 5. The design of this controller fits the special needs of the continuous fractionating towers on some pilot units with internal recycle September 1953

I

Timing and Sampling Cycles Run Smoothly with Automatic Controls

Time Cycle Controllers. Many laboratory units involve a fixed sequence of timed operations. Rapid and uniform switching from one period t o another is best handled by an automatic controller. Several types of time cycle controllers are available for the operation of pilot plant equipment. Adjustable sequence timers have proved to be very convenient in such service. Figure 7 pictures a controller unit with four basic timed periods for operating a cyclic catalytic pilot unit. The timers operate relays that in turn actuate the desired operations. The operations performed ( 2 )are briefly as follows:


1851

ENGINEERING AND PROCESS DEVELOPMENT

P

Figure 8.

Figure

7.

Time-Cycle Controller

1. Process Start charge pump Start proportional sampler 2 . Process purge Start steam (vater) purge pump 3 . Regeneration Open valve on regeneration air line Start regeneration gas sampler (for carbon determination) 4. Regeneration purge Start steam (water) purge pump Any period or any circuit in a period may be manually controlled even while the timers are running. Automatic Sampler. Complete automatic control of pilot plant operations also requires automatic control of sampling. A cyclic unit as just described also requires that the sampling equipment be governed by the time cycle controller. A suitable proportional gas sampler was described in Figure 3. Figure 8 shows the electrical hookup of a constant rate automatic gas sampler. The function of the sampler is to operate the pump which removes water from the gasholder. Gas is thus pulled through an absorption train, through the bubbler trap, arid into the gasholder.

Automatic Gas Sampler

The time cycle controller operates a relay that completes the circuit to the pump only during the period when the sampling is desired. During this same period another relay opens the circuit to the solenoid valve on the water refill line to prevent filling the gasholder during the sampling period. After the sampling period is complete the solenoid on the water line is energized so that the gasholder fills with water. When the holder is filled, electrical contacts in the top are shorted. An electronic relay then opens the circuit to the solenoid valve on the water supply, preventing further flow. These applications, although covered very briefly because it was desired to illustrate several different types of installations, point out that pilot plant operations often present many control problems to the engineer. Quite often suitable commercial instruments are not available and the engineer must use his ingenuity to devise modifications of existing controllers or design and build the necessary equipment to accomplish his objective. Acknowledgment

The assistance of A. R. VanderPloeg and R. F. Huhndorff in reviewing and commenting on this paper is gratefully acknowledged. The help of -4.W. Ellsworth, who prepared the figures, is also appreciated. Literature Cited (1) Carpenter, J. K., and Helwig, R. W., IND.ENG.CHEW,42, 572 (1950). ( 2 ) Huhndorff, R. F., Z'bid., 41, 1300 (1949). (3) Murphree, E. V., Gohr, E. J., and Kaulakis, A. F., J. Inst. Petroleum, 33, 608-20 (1947). RECEIVED for review April 15, 1953.

ACCEPTED June 23. 1953.

THE END

SYMPOSIUM ON PILOT PLANTS

1852


Vol. 45, No. 9

Pilot Plants - American Chemical Society

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