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I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T'R Y
have been applied successfully in fluidized solids systems. Heat balances also can be employed, but they arelimited t o pilot plants wherein heat losses from the part of the system involved are small compared t o the heat contents of the flowing streams. Where impurities, such as carbon, are continuously deposited and removed from the circulated solids, balances can be made on these impurities. For example, in fluid catalytic cracking, catalyst rates &refrequently calculated using the rate of carbon burning in the regenerator and the change in carbon concentration on the catalyst occurring during regeneration. Heat Transfer and Temperature Control. As has been pointed out, heat transfer throughout fluidized solids beds is at a high rate. This is due to the extreme turbulence of the fluidized solids bed which results in rapid transfer of heat between solids and gas, and transfer of solids from one part of the bed to another. Moreover, the tremendous number of contacts of fluidized solids particles with metal walls which occur because of the turbulence of the bed result in high rates of transfer of heat between the particles and the walls. Film coefficients of over 300 B.t.u. per hour per square foot per F. have been obtained under certain conditions with fluidized solids dense beds. These high heat transfer coefficients may be utiliz8d effectively in the addition or removal of heat in a fluidized solids system. A reactor may be provided with a jacket within which a heat transfer medium may be circulated, condensed, or vaporized. Tubes or coils also may be installed withinfluidizedsolidsbeds for transfer of heat. Furthermore, heat transfer can be accomplished effectively in fluidized solids beds by the direct injection of a gas, vaporizable liquid, or combustible gas or liquid. I n circulating solids systems, as illustrated by Figure 2, large quantities of heat are transferred efficiently by circulating solids from one vessel to the other. For example, in fluid catalytic cracking, catalyst is heated t o high temperatures by burning off carbon in the regenerator, and this hot catalyst, transferred t o the reactor inlet, provides the heat necessary for heating and vaporizing the oil fed, and provides the endothermic heat of reaction.
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I n many pilot unit operations carried out at high temperatures, it is necessary t o make up for heat losses by providing extraneous heat to the system. This is done frequently by winding sections or all of the unit with insulated electric resistance wire, or by using strip heaters or Calrod heaters. Gas or condensing liquid in jackets also is used frequently. It is possible also t o design for adiabatic operation wherein careful compensation is made for heat losses. I n this type of design, the vessel is covered with about 2 inches of insulation, and electric heating elements are placed on the outside of this insulating layer preferably over a light-gage metal jacket. Additional insulation then is applied on top of the heating elements. Thermocouples are installed in the plane of the heating elements and also directly opposite each of these couples on the metal wall of the vessel. The amount of power provided for the heating elements then is adjusted so that the temperature difference between each pair of thermocouples is essentially zero. Using the methods and information presented in this paper, it should be possible t o design successfully a pilot plant for almost any type of fluidized solids operation. Fluid catalytic cracking plants have been discussed in the literature ( I , 3, 7 ) , and these descriptions should be helpful in arriving at a practical final design. LITERATURE CITED (1) Anon., Petroleum Processing, 2 , 518 (1947). (2) Blanding, F. H., and Roetheli, B. E., Oil Gas J., 45, No. 41,
84 (1947). (3) Carlsmith, L. E., and Johnson, F. E., IND. ENG.CHEM.,37, 451 (1945). (4) Daniels, L. S., Petroleum Refiner,25, No.9,109 (1946). ( 5 ) Murphree, E. V., Brown, C. L., Gohr, E. J., Jahnig, C. E., Martin, H. Z., and Tyson, c. W., Trans. Amer. Inst. Chem. Engrs., 41, No. I , 19 (1945). (6) Murphree, E. V., Gohr, E. J., and Kaulakis, A. F., tech. paper, Pacific Chemical Exposition (1947). (7) Trainer, R.P., Alexander, N. W., and Kunreuther, F., IND.ENQ. CHEM.,40,175 (1948). RECEIVED April 26, 1948.
LANTS
Polymerization Units for Thermosetting Resins F.E.R E E S E
AND
ELI P E R R Y
MONSANTO CHEMICAL C O M P A N Y , SPRINGFIELD, M A S S .
P i l o t plant units for thermosetting resins (phenolics, aminoplasts, and alkyds) must be capable of handling a wide variety of resins and t h e design must allow flexibility in operation. Extraneous features are undesirable, however, because the installations may become so complicated as t o require highly trained personnel. Most thermosetting resin reactions arecarried out batchwise in kettles. Separate kettle installations are recommended for liquid and l u m p resins. Kettle design involves size, material of construction, agitation, heat transfer, openings and valves, and safety. T h e proper choice of the condenser, receiver, instrumentation, services, gasketing, general layout, and auxiliary equipment will govern t h e over-all usefulnes of the units.
R
ESINS can be classified into two general groups: thermoplastic and thermosetting. Thermoplastics are fusible and soluble whereas thermosetting resins become permanently infusible and insoluble on the application of heat. The production of the two groups of resins depends very much on the different effects of heat. This article deals only with thermosetting resins. I n general, the production of these resins involves the controlled application of heat t o bring about solution, chemical reaction, dehydration, and, finally, rapid cooling t o prevent overreaction. The installation of a pilot plant unit for the production of liquid or lump thermosetting resins presents a unique problem. A pilot plant resin kettle is rarely, if ever, installed for one particular resin with the expectation t h a t it will be dismantled following completion of the project. Rather, it is installed for
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INDUSTRIAL AND ENGINEERING CHEMISTRY
a particular class of resins and is expected t o be a n all-purpose picce of reaction equipment in which may be prepared a n y and all resins of a given type for present and future development work. Based on their own experiences in the design, layout, installation, and operation of pilot plant units for the manufacture of phenolic, aminoplast, and alkyd resins, the authors are presenting the major problems x+hich ale encountered together with their recommendations. It is beyond the scope of this paper t o deal exhaustively with a n y of the items or to discuss any one of the resin types in detail, and only a general survey is attempted. Although many of the comments are applicable t o production scale equipment, the subject is treated entirely from the point of view of pilot plant operation. KETTLES
t o give poor mixing unless more thaii one agitator is used. The optimum a g i h t o r assembly would be one which would have a variable spced drive, interchangeable anchor or turbo-type agitat,ors and adjust.able baffles. Practical considerations make such a n arrangement impossible; a compromise is necessary, dependent on t,he type of resin under consideration. I n multiple kett,le installations it is common pract,ice to equip some kettles with single speed anchor agitators and others with variable.speed t,urbo agitators. Lump resins (approximately 100% solids:) require anchor-type agitators t o scrape the sides and to give mixing with the high viscosities encountered. Baffles other than the thermometer well are impractical. The power consumption is large and sturdy gear reducers are needed. A typical anchor agitator for a 10-gallon lcetile has a speed of 60 r.p.m. and requires a 0.5 h.p. motor; a 100-gallon kettle requires a 10 h.p. motor TTith a n agitator speed of 40 r.p.in. For liquid resins which generally have low viscosit,ies at, reaction kmperatures a turbo agitator can be used. Turbo agitated kettles are equipped with baffles placed 90 degrees apart and extending the full height of the straight side wall. The impeller diameter should be one quarter to one half the ket,tle diameter. A typical turbo agitator on a 10-gallon kettle requires a 0.5 h.p. motor and should have a variable speed range of 60 t o 600 r.p.m.; for a 100-gallon kettle a 6 h.p. motor and a speed range of 0 t o 350 r.p.m. are recommended. With both types of agihtors overdesign is the rule t o allow maximum flexibility. The length of the agitator shaft must be kept t o a minimum 1 o avoid whipping and subsequent bending of the shaft. T o obtain good agitation, t'he shaft of the agitator must approach the bottom of the vessel, but above the top of the kettle close coupliiig of the gear reducer is recommended. Frequently, whip can be eliminated by using slower speeds or thicker shafts. With thermosetting resins, resin build-up and difficult'y in cleaning when changing from one type of resin t o another make the USG of a n internal bearing for the shaft a t the bottom of the vessel undesirable. The top of the kettle must, have a variety of openings. T h e manhole on a 100-gallon kcltlc should bc large enough to allow a
Most thermosetting resins are processed batchv ise in kettles. A condenser, a receiving tank, instrumentation, and auxiliary equipment are required for efficient operation. All equipment must be suitable for full vacuum t o facilitate control of the reaction and to permit removal of water or solvents at reduced temperatures. Aside from a small factor of safety for reactions which get out of control, kettles need not stand positive pressure above t h a t normally encountered when operating a vented vessel at atmospheric pressure. Therefore, resin kettles need only be designed for 20 pounds per square inch gage pressure. Good translation from laboratory t o plant units can be obtained with the use of two different size kettles for pilot plant development work. A IO-gallon kettle provides a significant stepup from glassware scale for the confirmation of variables and the discovery of any factors which were not apparent in the laboratory. Another kettle of capacity between 75 and 175 gallons is desirable for larger scale production: t o bring t o light a n y problems caused by a n approximate tenfold scale-up; t o uncover problems which may arise because of continuous and repeated operation; and to provide material for sales development. For general pilot plant work, kettles should br constructed of corrosion resistant material Metal salts have a n effect on the reactivity of alkyd resins, and aminoplasts must moct rigid color specifications. T o allow for the preparation of certain specialty resins, phenolic kettles also should be constructed of corrosion resistant material, although most phenolic reactions can be carried out successfully in iron. Stainless steel (Type 304 or 347) is preferable because of its chemical resistance, strength, and hardness. h-iLlcel is less resistant and is too soft t o allow a thorough job of chipping the ma11 scale formed during inany reactions. h glass lining for mineral acids or a highly resistant type of stainless steel (Type 316) for organic acids is required for specific reactions, b u t such materials of constiuction are of less value as general-purpose equipment becauw of brittleness oi softness and difficultv in fabrication and repair. Kettles constructed of iron aic definitely unsatisfactory because of limited application. Corrosion characteristics for plant design can be found most economically and conveniently by laboratory iathei than by pilot plant service tests. Agitation is of great significance in pilot planting thermosetting resins because of its effect on the reaction rate and on heat transfer. Therefore, in the usual design of kettles the height is one Figure 1. Sampling Device for a Liquid a t I t s Boiling Point t o two times the diameter since taller vessels tend
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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iii the bottom. Because any pockets will collect resin which will harden on the continued application of heat, all bottom valves connected directly to the kettles should be of the flush bottom type. For lump resins, a large, quickopening type of dump valve is required because the high viscosity resins must be removed before they set-up in the kettle. The valve usually consists of a large diameter flat mctaf disk, hinged a t one point and forced flush up against the bottom of the vessel by a clamping bar. For dumping, the disk is allowed to fall I€# 7open by removing the clamping bar and the - , resin flows freely from the kettle. Flush bottom valves for liquid resins are of the conventional type, with the stem usually rising into the kettle for ease of cleaning. The valve seat, stem, and handle must be removable as a unit from t h e kettle, and the kettle bottom itself should never serve as a seat for the valve. Often a flush bottom valve for liquid resins is mounted in the center of the disk-type full-opening valve required for lump resins so t h a t the kettle can be used interchangeably. A quick-opening plugcock should be provided in the line from the kettle immediately follom ing the liquid resin dump valve (Figure 2 ) . This plugcock is useful since flush bottom valves have a tendency to leak. The plugcock also aids in obtaining a good vacuum and regulating the flow of product. Liquids and solids can be charged through openings in the top of the kettle by gravity or by means of a pump. It is convenient also t o charge material by vacuum either through the top or bottom of the kettle; this technique has been used on finely divided solid material but finds its most general application for liquids. Pilot plant kettles are used for a wide variety of materials so t h a t all equipment must be designed with maximum accessibility for cleaning. For thermosetting resins, the cleaning problem DRUN +SEW€.? must receive special attention because it is possible to obtain a kettle full of insoluble, infusible Figure 2. Process Piping Diagram for Thermosetting Resin K e t t l e material if the reaction gets out of hand. Kettle walls should be smooth, and unnecessary iecesses or auxiliary fixtures must be avoided within the vessel. person t o enter for repair and maintenance work. For the samc For lump resins, the large disk-type discharge valve is especially reason, one must be able t o rcach into the 10-gall6n kettle. Two valuable for cleaning through the bottom of the vessel. sight glasses are required, one t o accommodate a light and one As with all chemical equipment some type of safety relief t o through which the interior of the kettle may be viewed. T o save prevent the build-up of dangerous pressures in the kettle must be space, the viewing glass can be incorporated into the manhole provided. The choice is usually between a pop valve or a rupture cover. The manhole may be used also for charging raw material, disk. -4 rupture disk is preferable (mandatory in the case of and frequently only the sight glass is removed for this purposv lump resins) because the orifice in a pop valve is subject t o obbecause it has fewer bolts. Openings must be provided for the struction by resin. The disks should be selected from corrosion large vapor line t o the condenser, the smaller liquid return line, resistant materials and frequent inspections are necessary since a.thermometer well, and a pressure tap. Provision should be made for two extra lines for charging liquids: one of these lines the metal must be paper-thin for kettles which operate at near atmospheric pressure, For many resin pilot plant kettles a should be an entity in itself, with a vertical nozzle into the kettle; rubber stopper will serve as a satisfactory substitute for a rupture the other can be teed-off of the condensate return line. Samples disk. Calculations show t h a t the area which is blocked off by a can be taken through the separate vertical liquid charging line. No. 11 stopper will provide more than adequate protection for a For sampling from boiling liquids, a gate valve in series with a 150-gallon kettle. A stopper has t h e advantage of sturdy eonstuffingbox can be used t o give a vapor-tight locker which prestruction, of being easily replaceable, and of blowing a t a very lorn vents fumes from gctting out into the room. Figure 1 shows pressure. the construction of such a sampler. Sample lines through the Kettles for general pilot plant work must be equipped with a kettle jacket are desirable but clog up frequently and may be a source of contamination. For small kettles, samples may be jackct for temperature control. Internal coils are difficult t o clean, repair, and operate. One may heat with hot water, steam, taken through the bottom valve, but for kettles with a capacity circulating oil, Dowtherm, or electric strir, heaters. If moderate above 50 gallons such a procedure is laborious as the bottom temperatures are needed (approximately 175 C. maximum) valve is one floor below the operating level. Most kettles are emptied by gravity through a discharge valve steam is usually pieferable. The high pressure steam can be
4
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4
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INDUSTRIAL AND ENGINEERING CHEMISTRY
reduced t o give any temperature between 100" and 175" C.; t h e range of 40" t o 110" C. can be covered by the use of a circulating hot water system. Cold wat,er would be used for cooling and all three operations then permit the use of the same jacket,. Often higher temperatures are desired or lower jacket, pressures art: required so t,hat Dowtherm or circulating hot oil must be used in the jacket. Dowtherm is preferable t o oil because it can be used as a condensing vapor or as a liquid. Both have t'he disadvantage of being objectionable when there is a leak in the closed heating system. I n addition, for high temperatures the heating media must have some source of heat supply other than plant steam. For example, gas, oil, or electricity are possibilities. Cnfortunately, all introduce a fire and explosion hazard which is not present in the area when steam is used. Because most resin processing involves the use of infla.mmable material, these allernative primary heat sources must be maintained a t a safe distance from the unit. There is also the factor of the oil and Dowt,herm becoming viscous when used as cooling media, since it is impossible to use water int'erchangeably in the same jacket with them. Instead of installing the elaborate system required for oil and Dowtherm, elect,ric strip heat,ers can be used directly on the ket'tle malls. A jacket is still required for cooling water. A s the strip heaters must be mounted directly on the kettle, some of the area becomes unavailable for jacket'ing. T h e heaters may be mounted on the bottom of the vessel, on the walls, or on a portion of the side n-alls between split jackets. If a split jacket system is used, a flexible connection is required between the jackets t o provide for expansion and contraction. Electric heaters represent, a greater fire hazard than do other methods of heating. If heaters are not mounted correctly, it is possible for materials within the kettle to exceed their autoignition temperatures. Banks of heaters should be mounted horizont.ally (not vertically) with individual controla for each bank. No heater should be utilized unless it covers a portion of the kettle which is in contact with liquid. X t'hin copper sheath covering the complete ket,tle wall is useful in obtaining equal dist,ribution of electrical heat. Independent of the final choice of heating medium any kettle jacket must be protected b y means of a pop valve set a t the rated jacket pressure. CONDENSERS
As stated previously, condensing equipment is required for handling water or solvent vapors which are distilled from the kettle. This equipment should be inst'alled t o allow the return of the condensat'e t o the kettle or the collection of the condensate in a receiving t,ank. A relatively crude approximat'ion for condenser size in a pilot plant installation is t h a t the square feet of condensing area should equal numerically one half the gallons of working capacity of t,he kettle. Only in the most unusual cases will this calculation result in a n undersize unit. The material of construction of t,he condenser will be decided when a decision is made on the kettle as they both should be of the same material. The construction of the condenser will be dependent on the type of resin which is t o be made. A Jvater tube condenser is used when high condensing capacity is required and a cool condensat,c is not required. If it is desirable to obtain a cool distillate t o aid phase separation in the condensate, a ,vapor tube condenser should be used. The condenser may be built t o operate in either manner. I n those cases where two or more kettles are t o be installed, one of each type of condenser is convenient. The t8ypeof condenser is influenced also by available headroom as a Tyater tube condenser can be mount,ed horizontally whereas the vapor tube type is usually nionnt,ed in a vertical or inclined position. I n the design of condensers for pilot plant use there is little point i n going t o the expense of providing expansion wrinkles or floating heads since they are necessary only in large size units. I n t'he manufacture of certain resins, particularly some phe-
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nolics, plugging of the condenser will occur from sublimation of material into the condenser or from operating errors allowing foaming of the batch into the condenser. Provision may be made for blowing steam through the condenser t o aid in its cleanout. I n cases where it is anticipated t h a t this trouble will occur frequently, the vapor tube condenser is preferable as it lends itself more readily to cleanout than the water tube type. ,411 condensate from the condenser should be led through a sight box before returning to the kettle or collection in the receiver. Observation of the condensate flow is a valuable index of the progress of the reaction as it will indicate the presence and the vigor of boiling in the kettle. One type of sight box which has proven very satisfactory is a square box with round sight glasses on the front and back. The condensate enters through the side over a drip lip and leaves a t the bottom. From the top there is a connection to the vacuum and to the vent systems for equalization of pressures. KETTLE RECEIVERS
Condensate which is not returned to the kettle is collected in a receiving tank at atmospheric pressure or under vacuum. For ordinary operation a single receiver per kettle is sufficient. 4 receiver should have approximately 75y0of the capacity of the kettle it serves. While this may seem large, i t must be remembered that the receiver acts as a knockout tank during vacuum distillation. The overdesign allows also for flexible operation. The tank may be constructed of plain steel and should be equipped with a calibrated gage glass. Almost any shape tank is satisfactory but the use of a horizontal cylindrical tank of diameter approximately one half its length allows a reasonably sensitive calibration on the gage glass along Tvith a large decrease in vapor velocity t o reduce entrainment. Two bottom connections are desirable b u t they can tee out of a common line from the tank; one line goes to the sever and the other empties into drums. I n this connection, the elevation should be such as t o allow adequate room for placing containers under the tank and headroom for working personnel. Three lines lead into the top of the tank: one for condensate, one for vacuum, and one for venting. The tanks receive only volatile liquid or small amounts of entrained solids so that handholes are sufficient for cleaning.
PIPING The piping arrangement of a pilot plant ketfle is the controlling factor in allowing efficient utilization of the kettle without long training periods and the use of highly trained personnel. Extra care used in the piping layout \Till pay dividends in ease of operation. I n one's zeal to obtain a flexible pilot plant unit, it is easy to obtain a Christmas tree effect for which a book of operating instructions is needed. The goal toward which the layout should be directed is simplicity, clarity, minimum number of valves, and the proper placement of lines and valves. I n the case of multiple kettle installations, all kettles should be installed with identical layouts of process and service piping. A diagrammatic sketch of the minimum process piping requifements is shown is Figure 2. The service piping setup is dependent on plant practices and the facilities available. However, i t is desirable t o standardize on a service piping header for supplying steam, hot water, cold water, etc., t o the kettle jacket with a definite arrangement of the lines entering it. The use of a color code for the service lines and valves is recommended also. The size of the pipes is usually a matter of rule-of-thumb on pilot plant kettles. The size should be exaggerated because of the excessive demands which will be encountered in some experimental work. The writers have rarely, if ever, found it necessary t o decrease the size of piping but have often found it desirable t o increase it. I n most cases, it is desirable to use gate valves or plugcocks i n
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the process piping because of their free-flow and their resistance to plugging. The selection of valves for the scrvice piping can follow standard piping practice. The material of construction of the process piping and valves up to and including the valve regulating the flow of condensate t o the receiver (Figure 2) should be ide"ica1 with the material of construction of the kettle and condenser. The material of construction of the piping t o the receiving tank from the condensate valve above i t should correspond t o that of the receiver. The type of fittings-screwed, screwed flanges, or welded flanges-for the process piping is t o some extent a matter of preference but also is dictated by the material of construction. For example, screwed stainless fittings over 1-inch pipe size are extremely difficult t o make t o a tight joint. It will be found that hydrocarbons and some alcohols will leak readily throtigh screwed connections. Furthermore, it will be necessary t o break connections frequently on pilot plant installations and the remaking of a tight joint with screwed fittings becomes increasingly difficult. Welded flange fittings, particularly in pipc sizes 1 inch and over, are the most universally satisfactory type but piping layout and expense may obviate their use. VACUUM SYSTEM
Few resins are made without the use of vacuum dehydration or reflux a t some phase of the reaction. Even in the case of alkyds i t is advantageous t o be able t o apply vacuum t o the kettle. The most satisfactory source of vacuum where steam pressure is available will be a steam ejector. If a steam supply is not available vacuum pumps will suffice, but the tendency of resinous materials t o enter the vacuum system where they polymerize causes high maintenance costs. A vacuum of 29 inches of mercury is required. I n most cases a ' vacuum i n excess of 22 inches of mercury is not used, but the higher vacuum is desirable in the processing of certain phenolic lump resins. The capacity of the vacuum system t o be used will vary with the kettle size and the care used in sealing the kettle against leaks. Because of the frequent opening and closing of many openings on the kettle, gasket seals often are not ideal and large quantities of noncondensable gases must be removed continuously. A two-stage steam ejector having a capacity of 2.5 cubic feet per minute of free air (30 inches of mercury absolute pressure and 70" F.) a t 29 inches of mercury vacuum is satisfactory for kettle installations up to 250 gallons. It is advisable t o so pipe the kettle t h a t all vapors from the kettle must go through the condensate receiver before entering the vacuum system as is shown in Figure 2 . This arrangement allows the receiver t o act as a knockout drum for any entrained material and ensures that, in the case of a foam-over, the main lines will not be filled with resin, causing the shutdown of other equipment which may be on the same vacuum system. T o ensure t h a t the pipes selected for the vacuum system are of sufficient size at least 1-inch lines should be provided for the path of noncondensables for kettles u p t o 50 gallons and 1.5-inch lines for kettles above 50 gallons. INSTRUMENTATION
Minimum instrumentation requirements for a pilot plant resin kettle consist of a batch temperature recorder, a n on-off recordercontroller for the hot fluid circulating system, and a throttlingtype vacuum recofder-controller. For special applications, the instrumentation can be expanded, but for well over 99% of the pilot plant work on thermosetting resins this basic arrangeqent is satisfactory. The vacuum t a p from the kettle should go directly into a trap to catch entrained material. The outlet from the trap should go t o a sight box filled with glass wool so t h a t a n operator can check
2043
visually whether any material is being pulled through the lines. It will be necessary t o clean out sight boxes at infrequent intervals. Only with this positive visual guard can one be assured t h a t the internal parts of the vacuum controller are not going to become inoperative because of resin build-up. Since a n instrument is subject t o error, a positive check on the vacuum in t h e kettle is obtained by providing a manometer which also is connectcd t o the sight box. I n case of instrument failure, operations can be continued by adjusting the manual vacuum by-pass and the vent line. I n plant size equipment, the contents of a kettle are heated and cooled slowly so t h a t the lags due t o the thermometer well and heat-sensitive element are unimportant in the measurement of the batch temperature. Hence, the size of the well or bulb is not critical. For pilot plant scale equipment, special attention is required in fabricating thermometer wells and choosing t h e temperature sensitive elements because the contents of the kettle can be heated and cooled so rapidly t h a t the lags due t o the heat capacities of the well and element and the heat transfer through the well become of extreme importance for good process control. The thermometer well must be of minimum size commensurate with its required structural strength. For 10-gallon kettles this implies the exclusive use of thermocouples rather than vapor bulbs. For 100-gallon kettles vapor bulbs are usually satisfactory, but tests are nccessary t o establish that the temperature measurement lag is negligible on any new installation. For small size glass-lined equipment a vapor bulb may prove feasible since it can be established, at times, t h a t the major lag exists i n the rate of heat transfer through the walls of the vessel. A U X I L I A R Y EQUIPMENT
I n addition t o the kettle installation itself there are numerous pieces of auxiliary equipment which are needed for a complete resin pilot plant. Some of these pieces are adjuncts t o t h e kettle itself, some are for subsequent handling of the resin folloving reaction, and some are for the handling of by-products. Resins of the alkyd type require an inert atmosphere i n t h e kettle t o keep oxygen from the contents. The inert gas being bubbled through the batch also assists in agitation. It has been found on pilot plant kettles t h a t sparge rings used t o introduce gas a t the bottom of a kettle plug easily because of their small size, and a more satisfactory arrangement consists of 0.25-inch pipes running down the inside wall of the kettle at 90 degree intervals ending in a n open tee a t the bottom of the straight side. I n case of plugging, these lines can be removed and cleaned easily. Provision should be made for the flushing of these lines with water following caustic washing of the kettle. It is advisable t o use corrosion resistant piping (such as stainless Type 347) for these inert gas systems to avoid carrying contaminating metal salts into the kettle. I n many resin reactions there are vapors being carried out of the kettle which must not be discharged t o the atmosphere. A small section of unpacked pipe with a water spray head in t h e top and suitable inlet and outlet connections is satisfactory for scrubbing vent gases. The runs of pipe t o this washer should hr, kept as short as possible t o avoid plugging of the lines by subliming material. A small bleed of steam into the vent line through a Penberthy-type ejector will aid in keeping the lines open and will provide a slight vacuum in the kettle t o prevent fumes from escaping from the kettle a t poorly sealed openings. The material of construction of the scrubbing system should be corrosion resistant t o avoid high maintenance costs and unpleasant breakdowns of the system in the middle of runs due t o pinholing. At pipe bends plugged tees should be used instead of elbows to allow easy and rapid cleanout of the lines in case of stoppage. Some condensed vapors will separate into two phases and it will be desirable t o use a phase separator in the line from the conden-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Figure 3.
View of Pilot Plant Resin Kettles from Operating Floor
ser. This phase separator should be built v i t h the lower take-off arm adjustable. While it is possible to calculate a fixed position for this arm for a given set of conditions, it will be found that vaiiations in condensate temperature, flow rates, raw materials, and formulation will give conditions requiring varying settings. This phase separator may be designed as a portable unit which can be installed in the condensate system Tyhen needed and removed when not in use. This approach is desirable in cases where the unit is little used since it simplifies the process piping. If the final product from the pilot plant kettle is a lump resin, it must be emptied from the kettle while hot (usually into metal pans) and then cooled quickly t o its biittle state. If pans are used, it is desirable to rack these pans into a movable cait as they are filled with hot resin so that they may be moved easily t o another location for cooling to rooin temperature. The size of the pans will be dependent on the allowable cooling time for the resin, but the depth of the trays shouId not exceed 1.5 inches. The selection of the proper type of pump for the liquid resin handling is quite important. I n general, it has been found that a gear pump is the most desirable unit for this norlr because of its higher discharge preswre and 4onfer specd rwulting in fewer aa(1king problems and less heat build-up in the casing. hlthougli a t least one stainless pump should be available for corrosive applicationr, iron pumps can handle the majoritv of the ~ o r bek cause the corrosion characterisfirs of resin solutions ofttln ar mild. If the resin is of the liquid-resin type, it is oidinarily ncces to carry out a clarifying filtration on the product. The fi selection should be made on the premise that most resin filtrations are a clarifjing operation lather than the removal of a high percentage of solids. A jacket on the filter capable of carrying 40 to 60 pounds per square inch gage steam is a desirable feature when viscous mateiials ale being handled. Plate and frame, vertical leaf, and horizontal leaf filteir all ha\? been used on resin filtrations with good results. Holy-ever, for pilot plant operatio the horizontal plate filter is believed t o be the most satisfacto t\ pe. I n this latter type the precoat bed and the filter cake u not slip if the filtering presanie is released as often happens in pilot plant work, the holdup is small as the filter may be blown QUt through the bottom leaf; cleaning and assembling of the filter are simple; and jacketing is relatively easy. In the case of both pumps and filters the units should be portable to allow flexibility of operation. A11 such equipment should be mounted on individual dollirs fh
Vol. 40, No. 11
Large quantities of distillates are obtained frequently which contain recoverable solvents, generally alcohols. -1small distillation unit should be provided for this solvent recovery. The unit should be of stainless steel throughout with facilities for performing azeotropic distillations involving phase scparation in the distillak as is cncountered LTith butanol rectification. The condcnser should be of the vapor-t,ubc type and the interior of the still pot or reboiler should he accessible for cleaning as the distillates from the resin reaction usually contain small amounts of dissolved material which build up in the pot. A batch still with a 6-inch packed column xi11 have sufficient capacity for normal pilot, plant work. PACKING AND GASKETS
Gaslieting for ket,tles and auxiliary equipment prescnts no special. difficulty. For work a t atmospheric base gasket (Garlock No. 7021) will prove sealing becomes a difficult problem, rubber, neoprene, Perbunan, Hycar, or Teflon coated mat,erial can be used depending on the temperature and type of solvent' present. For large flanges under moderate vacuum or small flanges under high vacuum, a metal ridge (approximately one half the thickness of the gasket) wclcled around the inner diameter of the flange has proved its value in holding the gasket in place. This technique is especially useful for the softer materials. For a top-entering agitator where the shaft is in contact with volatile vapors rather than solid or dissolved resin, the packing problem is not serious. Wax flax or dry flax impregnated with a suitable plasticizer can be used. Periodic lubrication is required because of the leaching action of solvents. If color is unimportant or the product is going to be filtered, graphited asbestos is satisfactory. The choice of the proper rnatcrial for the packing glands of pumps represents a difficult problem; its solution is not entirely satisfactory. Many thermosetting resins will harden to insoluble, infusible masses even a t room temperature so that, in time, even the most pliable packing becomes stiff and rigid. Under these conditions tightening the bolts on the gland-follower will not be effective in reducing leaks, and repacking is necessary. Pumps niay have to be repacked twice a month. Even this Pchcdule assumes a thorough flushing of the pumps with suitable solvent, after using. The technique of keeping the stuffing box free by using a lantern ring and forcing a liquid into the ring a t a pressure slightly a b o w that of the pump can be used if a liquid, with lubricating qualities which d l not contaminate the product, can be found. Dry flax has been used for punips where good color is required. Wax and other comnion organic lubricants are useless because of the leaching action of solvents and the subsequent difficult,y in relubricating the packing. Grapliiled asbestos is valuable for general use. The problem of contamination by the graphite is negligible especially as most resin solutions are filtered through diatomaceous beds t o remove impurities of colloidal size. The use of Teflon for pump packings for thermosetting materia,ls probably will not be of any special value because the problem is not, one of inertness but rather of the build-up of insoluble, infusible material.
I N D U S.TR I A L A N D E N G I N E E R I N G C H E M I S T R Y
November 1948
2045
ELECTRI CAL EQUIPMENT
From an electrical standpoint resin kettle installations and auxiliary resin handling equipment must be classified as Class I, Group D, hazard areas because of the volatile and inflammable solvents used in resin processing. The extent of the area surrounding the kettle to be included in this classification will be a function of the type of ventilation available and the auxiliary installations for handling solvents. The usual precautions in grounding equipment and care in avoiding static electrical discharges in solvent handling are assumed. Pilot plant installations invariably involve more resin spillage and open handling than plant installations where specific units are installed which have closed systems. Consequently, much more caution must be exercised. BUILDING
AND
Figure 4.
View of Pilot Plant Resin Kettles from Ground Level
SAFETY
It is difficult t o be specific about the proper general layout ~ O resin kettle installations since each will be an individual problem dependent on the building and space available. The attention given the layout, however, will determine the ease of operation of the equipment. All auxiliary equipment as well as the kettle itself should have a t least 2 feet of free space clearance from other equipment. This does not include the working area in front of the kettle. The kettle should be on a second floor level and should not have the lower €ip of the manhole more that 12 inches from the floor level t o allow easy charging of solid material from bags. The manhole cover itself should point toward the open working area by the kettle. Sufficient working area must be provided for the storage of dry material t o be charged and one or two drums of liquid raw materials which may be required. A small workbench for records, samples, and test reagents and a sink should be provided as part of the working area. The flooring on the operating level should be solid to prevent spillage of material onto persons below; the excessive open handling of pilot plant materials encourages this possibility. The pilot plant kettle should discharge on the ground level so that the finished product will be on the shipping floor. I t is advantageous to carry out filtration and resin handling operations on this same level where the disposal and drainage of spillage are {em of a problem. If a t all possible, trench drains followed by catch basins should be provided. Figures 3 and 4 show a typical pilot plant installation consisting of several kettles and auxiliary equipment. Adequate ventilation must be provided because all types of resins involve raw materials and finished products that are toxic or inflammable. On both the charging floor and the dumping floor it is advisable to have cross ventilation. A Toof ventilation
I
fan should be provided for intermittent use as needed in cases of emergency. One must resign oneself to the inevitable set-up batches which necessitate chipping of the kettle. Therefore, one final word should be said, particularly to those who have had little contact with industrial safety problems. I n entering resin kettles, all standard precautions for entering tanks and closed vessels must be followed including locked agitator switches, blanked charging lines, rope harnesses, and outside observation of the man in the kettle. The necessity for gas masks, rubber clothing, and goggles will be dictated by the conditions at hand. The explosive hazard from solvent-containing gels should not be overlooked. SUMMARY
Two factors are of utmost importance in the design of pilot plant units for thermosetting resins: Provision for carrying out a multiplicity of resin reactions. Overcapacity design. Since a pilot plant unit comes into being in the early stages of a development, it often is not realized that most of the equipment discussed in this paper will become essential as a company expands its interests within a given thermosetting resin field. Although in the original installation it may be advisable t o stop short in the provision of certain items, their possible future need should be remembered. The need for overcapacity design of condensers, agitator drives, piping, and service facilities also is related t o the future demands which will be made on the kettle when new laboratory developments will require pilot plant work. These demands are unpredictable and are best described in what should be the keynote of the entire project-prepare for the extreme. RECEIVED April 26, 1948
