Standard Designs for Pilot Unit Equipment JOHN A . RIDGWAY,
JR.
Pati American Refining Corporation, Texas C i t y , T e x .
'Fo
is from the vien-point that thew tools Ivarrant considcration in t,heir own right, rather than as component parts of integrated assemblies, thal this paper is pre~rntetl.
expedite pilot research it is necessary to divorce, in so far as possible, the detailed design of equipment
from the research problem proper. To attain this objective there should be available in the research laboratory a file of standard well proved designs for the equipment types normaIly encountered in pilot installations. I t is from the viewpoint that these designs warrant consi,deration in their own right, rather than as component parts of integrated assemblies, that this paper is prepared. Specific items discussed are heating and heat transfer units, sampling and metering devices, and details of reactor design.
I
iK ISDUSTRIAL research, the length of an investigation is
one of the factors that may have a major effect on its financial success. Unnecessarily extended studies increase the direct cost of the research, delay return on the investment, and, frequently more important, may result in final commercialization only after a less favorable competitive situation has developed. Serious delays, particularly in pilot research, are frequently encountered in setting up suitable equipment. Some of these are inherently unavoidable as they are concerned with the process development itself. Others, however, may be encountered in the design, procurement, or fabrication of equipment to perform standard well defined unit operations. These latter may be minimized by (1) maintaining as complete an inventory, as the laboratory size and policy allows, of the various types of equipment normally employed in pilot operations and (2) supplementing this inventory by a file of standard well proved designs for those types of equip.merit not readily stocked. The development of these standard designs is a problem separate from any specific process study. The units of equipment represented by these designs are the tools of pirot research and it
THERMOWELL
BEADED RESISTANCE WIRE / ,WRCELAIN
/
/
HEATER LEAD SUPPORT TUBE
'INTER~AL PIPE
JACKET"G
Figure 1.
TUBE
nPE
High Temperature Heater
HEATING AND HEAT TRANSFER
Electrical Heater---High Temperature. The first item to bo presented is equipment designed for heating a stream of fluid to a maximum temperature of about 1400' I?. It was desircd tha,t the heater be electrical, that provisions be made for automatic t'emperature control, and that it be compact, light, and easily constructed. ,4s a solut,ion, tthe unit shown i n Figure 7 was developed. This heat,er consists essentially of t\vo pipes mount,ed one within the other, sized so that there is but a minimum of annular clearance, and wit'h a spiral groove cut in the outside surface of the internal pipe so that' a path for the passage of liquid is provide,? in the annular space b e h e e n the pipes. The end of the internal pipe is closed and the two pipes are connected by a welded junction. Unions or flanges can be used in place of welding and may be desirable depending on the installation. However, if it is possible to remove the heater from the equipment, it is frequently more convenient t o keep on hand a spare heater and to send the used heater to the shop for cleaning or for whatever maintenance work may be required. A design in which a welded joint between the inner and outer tube was formed at both ends so that the inner assembly did not have a floating end was considered. Such equipment has not been employed as it was feared that differential expansion of the two tubes might result in rupture of one of the welds. Heat is provided by a \Tinding of beaded resistance wire on the outside of the assembly. To prevent sagging of the wire and to improve heat transfer from the wire to the pipe, the latter was grooved to fit the beads. T o further assuro that the wire would remain in position, a layer of Baldwin-Hill No. 6 high tempeiature insulation cement was applied as a mud directly over the winding and allowed to dry before installing the remainder of the insulation. Although not shown in detail, the leads for this resistancc winding were brought out to terminal block5 through tubes welded tangentially to the outside of the jacketing pipe. This provision is important in guarding againat the development of a loose winding which promotes shorts and failure of the heating element,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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in the effluent stream was employed in temperature measurement and a peened thermocouple was installed a t the mid-point of the heater for use in adjusting temperatures before introducing the charge. Although this particular design has not been used with temperatures above 650 F., h'gher temperatures can be reached as limited by the characteristics of the cartridge heater emp oyed. Temperature control has been quite satisfactory with this installation even though the metal bar used in the differential expansion system is subject to changes in ambient temperature. Heat Exchangers. There was required a compact heat exchanger that could be constructed readily a t low cost. A solution that has been found t o be satisfactory is shown in Figure 3. The exchanger is constructed by winding a pipe coil around a mandrel formed by a second short length of pipe and enclosing the entire assembly in a third larger piece of pipe as shown I n assembling the unit, the mandrel pipe is welded to one of the end plates so that fluid is prevented from flowing through it. By correctly choosing the pipe sizes, clearance can be maintained a t a low figure so that the coil baffles the annular space between the internal and the jacketing pipe. The coil and the baffled annular space thus form two parallel spiral passages for the streams undergoing heat exchange. O
METAL BAR
Figure 2.
Low Temperature Heater
Temperature control was accomplished by connecting the heater circuit to a Burling temperature controller head mounted as shown. The differential expansion of the inner assembly and of a porcelain tube, shown installed, actuated a microswitch in the controller head. This type control has an advantage over one dependent on the temperature of the exit stream in that, in case flow is accidentally stopped, overheating of the unit does not occur. As an aid in setting the controller, it is well to have an indicating light installed in parallel with the heater so that it can be ascertained readily whether the heater is on or off. Temperature measurement is provided by a thermocouple inserted through the porcelain tube into a thermowell in the plugged end of the internal pipe. This point will give R godd measurement of the stream exit temperature if it is properly locatedthat is, if the end of the thermowell is a short distance beyond the end of the resistance winding so that the direct effect of heat input from that winding on the temperature of the well is minimized. Since this thermowell is purposely located to reflect the temperature of the fluid being heated, it does not furnish a reliable indication of heater temperature in the absence of flow. Accordingly, a separate temperature point for use in setting the temperature of the heater before starting the charge is desirable and may be supplied by locating a thermocouple in the porcelain tube near the center of the heating section. Heaters of this design have been employed in this plant for several years. Satisfactory performance has been obtained when designing the resistance coil for as high as 4 watts per square cm. and with heat transfer rates, based on internal area of the jacketing pipe, as high as 14,000 B.t.u. per hour per square foot assuming 100% heater efficiency. The efficiency depends on the effectiveness of the insulation and, although precise data have not been obtained, values above 75% can be anticipated. These units have been used in stme operations where coking was encountered but, a t least in small installations, it has always been possible either to burn out the deposits or to dismantle and mechanically clean the unit. This feature, incidentally, makes the heater of utility in studying fouling phenomena. Temperature control during on-stream periods has been * 2 O a t the 1000° F. level. The only disadvantage observed with this design was in short cycle operations where there was some difficulty in quickly lining out the temperature. Electrical Heaters-Low Temperature. While the heater just described can be used in all temperature ranges, the use of the relatively expensive resistance wire winding is unnecessary except in the high ranges. When operating a t temperatures below about 650' F., the more cheaply constructed unit shown in Figure 2 has proved satisfactory. The same basic design was employed with the exception that a cartridge heater, rather than an external resistance winding, was used as the source of heat. Temperature control in this case was achieved by mounting the controller externally and using a metal bar located outside the jnsulation, rather than the internal porcelain tube, as the expansion reference. An auxiliary thermowell
MANDREL PIPE
R E COIL
Figure 3.
J A C ~ T I N G RPE
Heat Exchanger
The performance of these exchangers has been entirely satisfactory. Welded construction was used throughout as i t was deemed cheaper to replace the entire exchanger than to employ a more expensive design that would allow cleaning in the few instances where such might be necessary. SAMPLING AND METERING
Proportioning Sampler. I n connection with one pilot operation, means were required for obtaining a representative sample of a low pressure gas stream that varied both in volume and composition. The solution developed for this problem is shown in Figure 4. The stream was metered by a wet test gas meter; this meter was connected by a pulley and belt arrangement so that it rotated a cam which in turn actuated a mercoid switch. With this arrangement, the switch made and broke an electrical circuit a specified number of times per unit volume of gas passing through the meter. Only 8 small torque was required to turn the cam and it was found that this did not affect the accuracy of the meter. The mercoid switch was connected in turn with a three-way solenoid valve which alternately, as the circuit was made and broken, connected the sampling mechanism to air pressure and to the atmosphere. The sampling mechanism proper consisted of two chambers
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Figure 4.
Proportioning Sampler
sepaiated by a flexible diaphragm with one of the chambers fitted with check valves so that a cyclic movement of the diaphragm caused a pumping action that transferred gas from the line to the sample drum. Movement of the diaphragm x a s effected bv air piessure on the discharge stroke and by spring action, when air pressuie was released, on the suction stroke. The quantity of gas discharged per cycle was controlled by stroke length which in turn was adjusted by a stop limiting the return on the suction stroke. This adjustable stop was provided by mounting a valve stem and bonnet awembly as shomm.
qzFA
I10 v.
'ii
ARK
I
Figure 5 .
Liquid Displacement Aleter
Vol. 41, No. 5
as a precaution against discharge pressure affecting discharge volume, a coridition that, would be anticipated, as ii reault of gas compressibility, if displacement was not complete. The sampling device was employed for about a year and performed iu R sat'isfactorymanner. Diaphragm failurr: was the only difficulty encountered anti this was not serious as the condition of the diaphragm could be readily checked by observing the oil reservoir when the air pressure was on; leakagc. was indicated by continuous bubbling of gas through the reservoir. Liquid Displacement Meter. In pilot installations involving recycle streams, some method is required for measuriug the volume of those streams. Flow indicators such as orifice meters ('NU be employed but, are not, sat'isfactor!when very small flows are involved. require integrat,ion if total quantit!. of flow is the desired value, and musi, be corrected for changes in characieristics of the streain being metered. h pair of gage tarikc. also can be used either with manual or automatic control. 'LYic large holdup of such a system is objectionable in its effcct, on the time requirement for lining out, the pilot unit. .S. third solution to this problem is the positive displacement niet,er. One desigt-i t h a t has been evolved for such a meter is shown in Figure 5. The measuring unit, in this syst'em is a cylinder contairling i i reciprocat,ing pist.on. The stream being measured is dirwted alternately to the two ends of the cylinder by means of a four-way solenoid valve. The valve is connected to a relay system which is act,uated by contact between t,he piston and the (,I(+ trode of a spark plug installed as shown. Khen ~ h c ~ piston completes a stroke in one direction, it contart,?; an electrode and causes t,he relay to reverse the solenoid valve. This cyclic operation continues: as lorig as there is flov. The total flow is recorded by art electrical counter connected in parallel with the solruoid valve. When electronic units such as those manufactured by Arthur E. LaPine & Company were used as the primary relays in the system, a m e k i n g accuracy of =tO.lyo wae obtained in operations with gas oils at flow rates of 0.26 to 1.5 gallons per hour. A unit of t'his type can be employed vith a liquid thpt is nonconductive and has some lubricat'irig propcrties. By replacing t h cylindcr-piston arrangement by a bellows, satisfact>ory RUG service can be obtained with a liquid not possessing such lubricating properties. Alt'hough not described in this paper, a, third design incorporating a four-n.ay snap-act,ion valve actuat,ed by the movement of N bellows has been cmployed with conduct'ive nonluhrieating liquids such as anhydrous hydrogen fluoride. MISCELLANEOUS REACTOR DESIGNS
B glass-walled vessel waj mounted on the discharge line of the pump and sufficient oil was introduced so that when the diaphragm was in the discharge position the pump chamber was completely full of oil and this vessel partially so. The quantity of oil used mas small enough that the composition of the gas sample pumped was not perceptibly changed by absorption. The oil served the twofold purpose of sealing the valves against leakage and, by filling voids, ensuring complete displacenient of gas on the discharge stroke. This latter item \vas of importance
Reactor Closure. The problem of obtaining convenient, con^pact, readily-available reactor closures is always present. Of t,hs numerous designs that have been employed, no single one is suitable for all applications. However, one that has been forrnti to be exceptionally versatile is illustrated in Figure 6. This type closure has been used for high pressure work but h a 5 generally received little attention in low pressure applicalioiis. In this closure, a plug is provided that is substantially smaller than t,he react,or in which it,is inst#altlledbut with the hottom of t,he
May 1949
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INDUSTRIAL AND ENGINEERING' CHEMISTRY
plug enlarged so that there is only a small clearance between it and the reactor walls. For convenience in installation, it is well to provide stops, such as might be formed by small beads of weld metal on the inside of the reactor, so that the plug does not fall into the vessel. Packing is placed in the annular space between the plug and the reactor and a packing retaining ring screwed in place. lnternal pressure serves to tighten the packing, but it is usually necessary, when pressures are low and the packing material stiff, to supplcment this force by external means. For this purpose, the tightening ring shown has been provided. By screwing down on this ring, the plug is pulled up and the packing compressed. In this operation, the plug must be prevented from rotating. By slotting the top of the plug or drilling holes to fit a spanner wrench, means for backup can be obtained.
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were included in the design. This was provided by winding a copper tube around the stuffing box, soldering it n place, and filling the interstices of the coil with more solder. By this means a solid structure was built up which allowed rapid transfer of heat from the box to the cooling fluid. The feature of internal cooling is of particular value in those cases where a packing characterized by low heat conductivity is employed. When using such a packing with external cooling only, heat removal by conduction through the packing to the cooling coilseisinadequate, high temperatures are developed, and shaft scoring and disintegration of the packing result. By using internal cooling in which friction heat is removed directly from the shaft, satisfactory performance is attained with materials such as shredded Teflon which are characterized by low heat conductivity.
HOLES FOR SPANNER WRENCH
/
\
GRIOUND JOINT RING COOCING COIL
SOLDER RlNO
GLAND OIL OR
LANTERN RI~G
+
LEAKAGE
stop
Figure 6.
Reactor Closure
This closure has been used on reactors as small as 1.5 inch iri diameter and has been found satisfactory for service a t temperatures as high as 1100" F. whm aluminum foil wrapped asbestos packing such as Durametallic type D is used. However, it is better adapted for installations of larger size and is particularly useful in lower temperature operations where a plastic packing such as Durametallic B-66 (shredded Teflon) can be employed. In connection with this Teflon packing, it is good practice to use a material that is not plastic in nature for the first and last rings of the packing to prevent extrusion of the Teflon. Stuffig Boxes. As in the case of reactor closures, numerous designs of stuffing boxes for stirring type reactors have been used. Of these, one in particular (Figure 7) combines features that have made it highly useful in pilot operations. The features in question are first, the use of a double packing with an interpacking vent and secondly, the use of internal cooling of the shaft. The double packing with an interpacking vent allows collection of leakage so that the condition of the packing can be observed readily and corrections can be made for such leakage in obtaining a material balance for the unit. There was considered an alternate double packing design in which a single extended stuffing box was used with packing and lantern rings installed in the following order: (1) primary seal packing, ( 2 ) gland oil lantern ring, (3) additional primary seal packing, (4)interpacking vent larmtern ring, and ( 5 ) secondary seal packing. This was not employed as it was feared that a single gland would uot be effective in compressing a packing of the length required. Internal cooling of the shaft was attained by employing a holl~wshaft in which was inserted a loosely fitting tube. Cooling water Kas introduoed through this tube and flowed out through the annular space between it and the hollow shaft thus effecting heat removal. As a precaution, means for evternal cooling also
PULLEY COOLING W COOLING WATER IN
Figure 7.
Stuffing Box
This design packing gland has been utilized for about a year with a reactor operating under pressures as high as 2500 pounds per square inch gage. It has performed with equally satisfactory results whether the external cooling coil was in service or not. DISCUSSION
In this paper, no attempt has been made to present an exhaustive survey of units of pilot equipment. Rather, a few items were considered for which the need frequently arises in pilot installations and for which designs, proved effective through use, were available. By employing these designs, some degree of standardization of equipment has been attained which has been beneficial in expediting pilot research in the author's laboratory. They have been described in the hope that they will prove useful in similarly expediting the work of other laboratories engaged in this field. ACKNOWLEDGMENT
The author desires t o acknowledge the cooperation of R. G. Weist, A. J. Moon, and other members of the organization in the development of these designs and of P. L. Brandt in reviewing the manuscript. RECEIVEDFebruary 8. 1940.
