E PILOT PLANT
GENERAL-PURP
Additional equipment is described that can be added to the pilot plant as required for special operations such as metering small quantities of gaseous or liquid reaction promoters into the feed btock ahead of the reaction zone; pretreating the feed with various agents; removing water from the feed; withdrawing used catalyst and adding fresh catalyst at intervals; and srrubbing the product with a medium such as caustic soda solution for purification purposes or for removal of corrosive materials ahead of the product handling system. If a fixed-bed operation is being carried out in which catalyst activity is degraded by carbon or sulfur deposition on the catalyst, and it is desired to regenerate the catalyst in situ, auxiliary equipment also can be installed for periodic removal of these materials from the catalyst by controlled oxidation with metered quantities of air in mixture with an inert gas. Such regeneration equipment (8) is of straight forward design and will not be described here.
P i l o t plant equipment that has been used for stud)-ing a wide variety of catalytic and processing operations in petroleum refining research and development is described. Flexibility is obtained by making piping alterations and inserting various reactor vessels into a generalized pilot plant layout. Special auxiliary equipment is described.
I
X I’ETROLEULI research there is frequent occasion to study various catalytic conversions of hydrocarbon stocks under conditions t h a t will allow, on a relatively sinall scale, accurate measurements of product yields and quality and of catalyst life in a continuous operation. In these operations it is usually desired to cover a considerable range of the common operating variables such as temperature, pressure, and time of contact with t h e catalyst. An unusual diversity of operations from the standpoint of both the types of reactions studied and the feed stocks being investigated is encountered in petroleum processing. The equipment described in this report was designed for general use in this field. In view of its general utility in petroleum refining research, the features of the equipment may have application in other fields of research and development. The general purpose pilot plant is based on the permanent installation of charge and product handling systems suitable for diverse operations, whereas reaction vessels and other auxiliary equipment arc installcd to satisfy the requirements of the specific problem under investigation. Substantial economies can be realized in this manner when it is necessary to study a multiplicity of operations each of which can be conducted with the same type of feed and product handling systems. For reactions involving granular catalysts or treating agents, a jacketed fixed-bed reactor vessel can be inserted into the equipment system. Provision is made for applying heat to the reactor, and for rcmoving heat in the case of exothermic reactions. If liquid catalysts or liquid treating agents are being studied, the fixed-bed reactor is replaced with either a turbomixer type reactor, employing a power driven stirrer operating through a packing gland, or a jet reactor, in which the hydrocarbon or an emulsion of the catalyst and the hydrocarbon is recycled through a mixing orifice. I n this case the reactor usually is followed with a vessel for gravity separation of the immiscible hydrocarbon-catalyst mixture, from which the catalyst is returned to the reactor system. These reactors might be replaced with a jiggler type reactor for contacting vaporized hydrocarbons with powdered solid catalysts in the fluidized state. However, the special instrumentation and handling requirements for fluidized solids are rather complicated and, in general, it is preferable to employ a separate unit designed specifically for this work. The fluidized solids technique has been discussed fully in the literature ( 7 ) ,and u7ill not be covered in this paper.
SCOPE OF OPERATIORS
The pilot plant equipment described has been uscd success~ully in processing a variety of hydrocarbon stocks derived from petroleum, ranging from light fractions such as propane, butanes, and pentanes t o heavy lube oil distillate fractions. The principal feed stocks, however, have been various naphtha fractions boiling in the gasoline range between 100 O and 400’ F. Typical processing operations for which the equipment has been used include: polymerization and copolymerization of light olefins such as propene, butenes, pentenes, and hexenes, employing various liquid catalysts; alkylation of isobutane with light olefins such as butenes and pentenes, employing sulfuric acid catalyst; alkylation of aromatics with olefins, employing both liquid and solid catalvsts; isomerization of pentanes and hexanes, employing a metal halide catalyst supported on a porous solid earlier with promoters; conversion of olefins to alcohols employing a n acid catalyst; catalytic hydrogenation of naphthas, polymers, and lube oil fractions over solid catalysts; and treating of naphtha fractions with such agents as bauxite and aqueous caustic soda. GENERAL PRACTICES
In the design and operation of a pilot plant, it is desirable to duplicate the important conditions of plant scale operation. However, because of practical difficulties, duplication of all phases of plant scale operation is seldom achieved ( 4 ) . For cxample, it generally is infeasible t o duplicate linear velocities through a catalyst bed without violating other desired conditions, except in the case of processes that will employ quite shallow beds of catalyst in the commercial plant. Full duplication of the plant conditions is usually not required, the function of pilot plant study being only to obtain adequate technical information and engineering data t o serve as the basis for design of a full scale plant, unit.
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seal all instrument lines, pressure gages, and relief valves in the contact area with a 1 L/ mercury leg. It is good practice t o install a OR GAS m E x u small disengaging drum ahead of pumps and motor valves in critical locations t o minimize fouling from scale. These drums usually give less trouble than do strainers, which tend t o clog in some instances. However, the combination of a knockout pot followed by a strainer has proved highly satisfactory. A full use of instrument control is desirable in pilot plant operations. Automatic control is fully as important in small scale operations as i t is in the commercial plants. Pneumatic tvDe instruments are usuallv t o be preferred over the electrical type since the former involve no difficulty for use in hazardous areas. Air-operated motor valves give good servicc. I n general, pressure controllers are considerablv less exDensive than temDerature controllers and can be used advantageously for Figure 1. Flow Sheet of General-Purpose Pilot Plant controlling temperature by holding constant pressure on a boiling heat transfer liquid in the jacket of a line or reaction vessel. This application is described later in the discussion of the equipment illusThe pilot plant need not necessarily be a replica of the commercial trated in Figure 3. plant t h a t may subsequently be built. For example, a n entirely All hot vessels and lines should be adequately insulated. different form of liquid-liquid contacting might be employed in a plant scale unit than is used in the pilot equipment; or, in a n Suitable insulating materials for various temperature ranges include these: Johns-Manville sponge felt (laminated asbestos) operation calling for a n excess of a gas such as hydrogen t o be in100" t o 700" F . ; magnesia 100" t o 700" F.; Johns-Manville troduced into the reactor system, the excess gas undoubtedly Superex composition, covered with sponge felt, 700" t o 1200" F. would be recovered and recycled in a plant scale operation while For cold operations, either cork or hair felt is suitable for temin a pilot operation it probably would be more practical t o use the peratures from +30° to -30" F. The required thickness of ingas on a once-through basis and discard the excess. sulation varies with temperature level. For operation a t 1000" to Vl'hile carbon steel is employed in most plant construction, in 1200" F., 2 inches of Superex composition covered with 2 inches of pilot scale construction i t is, in many cases, desirable t o use alloy either sponge felt or magnesia are recommended. materials such as KA2S t o permit more severe operating conditions than will be used commercially, or t o minimize fouling at critical points, or t o prolong the life of the equipment. The use of extra PILOT PLANT CHARGE SYSTEM strong pipe in the construction of the pilot plant is also desirable A flow sheet for the general-purpose pilot plant is shown in not only t o provide large pressure tolerances b u t t o minimize Figure 1. I n Figures 2 through 9, additional items of equipment damage to the small pipe and fittings during construction and are shown representing: alternate reactor vessels which can be maintenance. It is also good practice in pilot plant construction interchanged with the simple fixed-bed reactor unit shown in the to back-weld threaded pipe connections and fittings in the ingeneral flowsheet; and other auxiliary equipment t h a t can be terest of eliminating leakage. I n the pilot equipment, Hastelloy installed t o perform specific functions on a permanent basis or as metal can be used t o advantage as valve trim or in other strategic the occasion demands. locations when extremely corrosive conditions are encountered. I n the general plan shown in Figure 1, the hydrocarbon charge Other nonferrous materials and alloys also have special applicastock is pumped from barrel storage, using a small gear pump, into tions. the twin charge drums. The charge drums are manifolded so that Relief valves must be used in keeping with safety requirements. one can be refilled while the contents of the other are being A spring-loaded type relief valve has general utility. Difficulty metered t o the unit. The drums are of welded pipe construction, is sometimes experienced in eliminating leakages through these and each drum is jacketed a s shown in the drawing, with a spiral valves, particularly in vapor service when the valve setting is in the upper pipe connection t o permit expansion between the rather close to the operating pressure. Such leakages present a drum shell and the jacket. Heat is supplied t o the jacket serious problem in pilot plant operations, where accurate mathrough steam and water connections t o develop a substantial terial balances on a small scale are required. Methods of pump suction pressure when gasoline-and-lighter hydrocarbons minimizing this leakage problem include connecting the relief are being charged t o the reactor system. When higher than atvalve discharge to a location in the process system t h a t is operated mospheric temperatures are maintained in the charge drums, some at a lower pressure, so t h a t any leaking material is recycled. I n temperature reduction occurs as the oil flows through the piping conventional installations where the relief valve discharges to a t o the pump suction. As a result the temperature in the pump is vent, a bubbler glass containing water can be installed on the well below the bubble temperature and even pumping rates are relief valve discharge so that any leakage can be immediately obmaintained. Difficulty is frequently encountered in pumping served and corrective measures taken promptly. I n locations and liquids a t their bubble temperature because of vapor binding in services where relief valve release and leakage can be unusually the pump. The 150-pound steam is supplied t o the jacket under serious, it may be desirable t o install dual relief valves in parallel, instrument control so t h a t a constant pressure is maintained on each preceded by a block valve. This arrangement permits rethe charge drum. Each drum is fitted with calibrated clearmoval and overhauling of one relief valve without interrupting vision gage glasses, and has a capacity of about 15 liters. T h e operation of the pilot plant. When highly corrosive chemicals are charge drums are approved for operation at pressures up t o 300 being processed, such as hydrogen chloride gas, it is desirable t o 00
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a t various point,s through the vessel. The tip of the thermopounds per square inch and a t tempera---.----._ _ _ _ - _ _ _ _ _ _ _couple tube is centered through the grid platc. If a highly exothermic reaction such as hydrogenation is being tures u p t o 400" F. NATURAL conducted, it is necessary to make provision for removal of the The feed stock is GAS heat released if a satisfactory temperature pattern is t o be obcharged t o the reCONDENSER tained. The quantity o l lieat released is dependent on the type actor system with a of hydrogenation process being carried out a,nd the feed stock plunger-type proporcomposit,ion. Heat removal can be accomplished conveniently by tioning pump. A using a jacketed reactor vessel, and placing t'he electrical heating Hills-McCanna Type element,s on the outer ~vallof the jacket,. This type of reactor is UMD-2F pump is illustrated in Figure 2; this vessel can be substituted for the unsuitable for this servjacketed reactor shown in Figure 1. Inside the jacket is placcd a ice. The charge rate liquid of appropriate boiling point. Heat is removed from the is usually in the range catalyst bed b y causing the liquid to boil in thc jacket. The of 0.5 t o 4.0 liters quantity of heat removed can be controlled within the desired per hour. The feed limits by superimposing pressure on the liquid in the jacket,. Inert, passes through a pregas or natural gas pressure, under instrument control, is used for heater and through the this purpose. As indicated in Figure 2, t h e vapors pass from t,he reactor vessel. top section of the jacket t o an external water-cooled condenscr; Either a n upflow the vapors are condensed and drain back t o the bottom of thc or a downflow type jacket through an external line. Other variations can be used, of reactor can be 4 such as a n internal coil instead of the jacket. If it should be deused; the choice deFigure 2. Fixed-Bed Reactor sired to provide a temperature pattern corresponding to several pends on the operawith Jacket for Heat Removal plant react,ors in series, the jackel arrangement can be sectionaltion being conducted. ized t o give several independent jacket units in series. Upflow is effective for I n the case of processes that are carried out a t a moderate temavoiding channeling in a liquid-phase process and is paiticuperature level of 200" t,o 300" F. and in which a relatively low larly desirable in a process that tends t o deposit sludge on heat of reaction is involved, it is convenient t o dispense with the catalyst. Downflow usually is somewhat more convenient f o y electrical heating and to use the type of heating equipment illus-. a vapor-phase process, particularly when side reactions tend to lrated in Figure 3. Isomerization processes fall in this category. produce small quantities of high boiling material. Domnflow is A s shown in Figure 3, t,he jacket of a standard reactor is coildesirable in a mixed-phase operation i o avoid the accumulation nected to a n external vessel containing a suitable boiling liquid, of liquid in the reaction vessel. such as n-butane or n-pentane, in a closed system. The liquid in this vessel, or boiler, is heated by means of an internal steam coil. VARIOUS REACTOR s Y s m u s The vaporized liquid flows from the top section of the boilcr An important factor in reactor design is temperature control. through a vapor line and enters the reactor jacket a t the top. Tho The most desirable type of heat control is influenced by: the temvapor condenses on the jacket ~ ~ a l larid s , t8hecondensed liquid perature level a t which the operat'ion is carried out; sources of flows by gravity back to the bottom section of t8heboiler. The heat or power that' are available; the degree of heat release or boiler system is mnintained undcr constant, regulat,ed pressuw heat absorpt,ion due to the exothermic or endothermic nature of the chemical reaction; and whether i t is desired t o conduct an isothermal or a n adiabatic type of operation. When preheat temperatures of 700 O to 1500e F. are required, improved temperature cont'rol is obtained by using a preheater coil immersed in a n electrically-heated lead bath (6). Molten salt' baths ( 6 ) can be used for higher temperatures, and low melting metal alloys, boilPREHEA ZER ing mercury, or boiling liquids such as Dowtherni ( 3 )can be used RZACTOR for lower temperatures. The reactor heating arrangement shown in Figure 1 is particularly applicable for carrying out an endothermic reaction such as hydroforming under isothermal conditions, in t,he region of 800' FEED / N to 1000" F. Heat input to the preheater and the reactor is provided, in this instance, by electrigal heating elements. Four separate electrical circuits a.rc cmployed for the rcactor in order that the heat input may be varied in each one-quarter section of the catalyst bed. The separate heating circuits allow additional heat input t o compensate for the heat absorbed due to chemical reaction as the feed moves progressively through the,catalyst bed. If close temperature control is desired, it is usually necessary to employ an instrument t o regulate the electrical power supplied to the heater winding. A sat,isfactory instrument for this purBOILER pose is the Wheelco Capacitrol. The reactor shown in Figure l is provided with a capped connection a t the top for charging granular catalyst and a flanged bottom t o allow removal of the used catalyst. The catalyst is supported on a removable grid plate. The catalyst charge t o this reactor usually is in the range of 500 t o 1000 ml. A 0.25-inch outside diameter stccl thermocouple tube extends downward through the length of the reactor Figure 3. Boiler System for Heating to allow spacing of t,hermocouples for t,emperature measurements Jacketed Reactor ?
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April 1949
by a n instrument that actuates a motor valve on the steam condensate. The pressure on the boiler liquid in turn fixes the temperature in the reactor jacket. A system of this kind provides excellent reactor temperature control since excellent heat transfer rates are provided. Additional connections can be made from the boiler t o a preheater vessel to provide preheating requirements If heat losses a t reactor blind flanges are undesirable, the blind flange can be fitted with a cup-shaped jacket which can be tied separately into the boiler heating system. The system should be well insulated, and the various connecting lines should be made as short as practicable. Smooth pipe bends are to be preferred to angle type fittings so as to minimize pressure drop in the boiler system. These jacketed reactors heated by condensing vapor can be used for temperatures as high as 700' F. Dowtherm A (9)is a suitable liquid t o use as a boiling heat transfer medium for temperatures above 600" F. I n the case of these higher temperature operations where only moderate heats of reaction are encountered, good temperature control can be obtained with a jacketed reactor by circulating a liquid such as Dowtherm through the jacket while heating the Dowtherm in an external electrical or gas-fired furnace. Examples of appropriate methods for supplying heat to reactor vessels over various temperature ranges are: Temp.,
I?.
Heat Light hydrocarbon boilers or circulating water Steam under controlled pressure Boiling hydrocarbon or Dowtherm Boiling mercury Molten lead Molten salts or direct electrical windings
Room temperature-212 212-350
.
350-700 700-950 700-1500 1200-1780
In pilot plant operations involving the use of liquid catalysts, a reactor suitable for general use is the jacketed pressure vessel fitted with a motor-driven stirrer t h a t is illustrated in Figure 4. The reactor assembly shown in Figure 4, with minor piping changes, can be substituted for the fixed-bed reactor shown in Figure 1 and thus be used in conjunction with the remainder of the pilot plant equipment. The reactor vessel is of welded steel construction, with a dished bottom and flanged top, and has an inside diameter of approximately 10 inches. The motor-driven stirrer is a rotor-stator assembly turning a t about 900 r.p.m., and is shafted through a stuffing box in the flanged head. The vessel is approved for operation at pressures up to 500 pounds per square inch and a t temperatures up t o 400" F. The vessel is jacketed for temperature control. For elevated temperatures, steam or a mixture of steam and water is admitted to the jacket under manual control. Low temperature operation, down to about IO" F., can be accomplished by evaporating propane in the jacket. The feed inlet line and a thermocouple well pass through the flanged head and extend to a point immediately above the stirrer blades. Connections are provided a t the bottom of the vessel for the withdrawal of used catalyst or for the addition of fresh catalyst or promoter materials from pressured blowcases. A vapor space is maintained in the vessel above the point of product
sT'RR31
FEED l N 7
7 ROOUCT
CATALYST RErURN
Figure 4.
Mixing Type,of Reactor
865 PRODUCT
CATALYST ADDITION CASF
RECYCLE PUMP
Figure 5.
Jet Type Reactor
take-off and inert gas pressure can be superimposed for maintaining the reactants in the liquid phase. The vapor space prevents fouling of the stirrer packing gland by the catalyst. The reactor product flows to a separate vessel fitted with a level indicator, where any catalyst carry-over settles out and gravitates back t o the reactor through an underconnecting line. The reactor and the separator also are connected by a line on the vapor side for pressure equalization. The catalyst inventory in the reactor vessel usually is about 2000 ml. Catalyst holdup in the settling system normally is about 800 ml. The product holdup in the separator amounts t o about 1000 ml. The hydrocarbon products flow from the separator to the product handling facilities. Another type of reactor that finds application in the Contacting of liquid hydrocarbons with liquid catalysts, particularly when prolonged, intimate contacting of the reactants and catalyst is desired, is the jet-type recycling unit shown in Figure 5 . This reactor, which has been found particularly useful in carrying out reactions involving the alkylation of isoparaffins with olefins in the presence of an acid catalyst, has been described fully in a previous article ( 2 ) . I n Figure 5 the hydrocarbon feed enters th3 bottom of the reactor through a mixing jet and a loose hydrocarbon-catalyst emulsion is established. Midway in the reactor emulsion is withdrawn and recycled a t a high rate by means of a centrifugal pump through the same mixing jet at the bottom of the vessel. The upper section of the vessel serves as a settling zone, and hydrocarbon products are withdrawn a t the top. Provisions are made for withdrawing used catalyst by means of a small trap box in the emulsion recycle line, for adding fresh catalyst from a pressured blowcase, and for recycling hydrocarbon product if such should be desired. PRODUCT RECOVERY SYSTEM
Whichever type of reactor is employed, the product recovery facilities follow the pattern indicated jn Figure 1. The total product from the reactor passes through a small disengaging drum for the elimination of any suspended solid materials such as entrained catalyst or scale and then through a water-jacketed condenser or cooler. The product then passes t o a gas separator vessel where liquid and vapor are separated. The vapor passes overhead through a motor valve that holds a constant pressure on the reactor, cooler, and separator. This valve releases the vapor at a uniform rate for measurement through a wet test gas meter operating a t substantially atmospheric pressure: Suitable gas sampling apparatus is installed either before or after the meter. Spot, composite, or total gas samples are taken as the occasion demands. If the reactor is being operated at a rather high pressure and it is required to effect the gas separation a t a rela-
INDUSTRIAL AND ENGINEERING CHEMISTRY IF----
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CAPACITY DRUM
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Figure 6.
Gas Metering System
tively low pressure t'o obtain the desired degree of separation, an additional back-pressure motor valve is installed between the reactor and the separator vessels so that a stepwise reduction in pressure is provided. The liquid product flows from the bot,toin of the separator vessel to calibrated twin product receiving drums that are used alternately. The product is withdrawn from the receiving drums into suitable sampling or storage containers. If it is desired, dist,illation equipment can be incorporated in the product syst'em for fractionation of the liquid product on a continuous basis. However, it' usually is more convenient to recover the total liquid product and make the desired product work-ups on representative samples in separat,e batch distillation equipment. Facilities also can readily be included in the recovery system for caustic neutralizing or scrubbing the product. AUXILIARY EQUIPMENT
In the following paragraphs are described several items of equipment that can be optionally included in the pilot plant design shown in Figure l for the purpose of performing a specific function. Gas Metering System. I n a number of pilot unit operations it becomes necessary t o meter gaseous materials into the reactor system, usually at a point immediately ahead of the feed inlet t o the reactor chamber. When relatively large volumes of gas are being handled, as in typical hydrogenation or hydroforming operations, metering is best accomplished with a standard orificetype recording flow rate controller. Rotameters are suitable for measuring gas or liquid rates over a rather wide range, but are usually not desirable for metering very low gas rates of about 5 nil. per minute or less. When extremely small quantities of gases in the category of catalyst promoters are to be metered, special equipment is required. These small gas rates can be measured by obtaining the pressure drop across a calibrat,ed system of very tiny orifices or capillary tubes ( 1 ) . However, trouble is sometimes experienced with fouling of the capillaries with foreign particles in this type of apparatus. Anot,her apparatus suitable for injecting small quantities of gases into the reactor system is shown in Figure 6. The apparatus as shown includes a mercury seal system on all instrument connections, pressure gages, and relief valves; hence i t is suitable for handling corrosive materials, such as hydrogen chloride. I n Figure 6 the gas t o be metered flows from cylinder storage through a motor valve which maintains a gas supply a t constant pressure in a capacity drum downstream. The drum pressure is set a t about 50 t o 100 pounds per square inch above that of the pilot unit reactor. The gas passes from the capacity drum through a small knockout trap to the metering system, where it is first bubbled through a layer of white oil in a clear-vision gage glass. I n normal operation, valve B is closed and the gas flow rate is controlled manually wit'h needle valve A, which admits the gas to t'he reactor system. A small Hoke needle valve has been found skt'able for this service. The observable bubble rate affords the operator an estimation of
Vol. 41, No. 4
the rate of gas flow, as well as assurance that a positive flow is occurring. Located beyond the bubbler i s a calibrated mercury displacement gage for periodically measuring the rate of gas flow. Ga,s-saturated oil also can be used in this gage in place of mercury when extremely low rates are being measured. I n normal operation, valve C is open. To measure the gas rate, valve C is temporarily closed so that the gas flows against the liquid in the calibrated gage. The time required for the gas t o displace a given volume of liquid in the gage is measured with a stop watch. Valve C is then reopened. From the measureinenis and the calibration, the rate of gas flow is readily determined, While valve C is closed, the liquid rising in the overflow pot pushes gss ahead of it into the reactor system so that the rate of gas flow t o the reactor continues uninterrupt'ed during the rate-checking operation. Product Scrubber. In certain pilot plant operations it is desired to wash the product with aqueous caustic soda, water, or other agents for treating or purification purposes. For example, if anhydrous hydrogen chloride in limited quantities is being injected into the reactor system in a certain process, it would be desirable to remove the hydrogen chloride from the product by caustic scrubbing ahead of the recovery system in the interest of both reducing corrosion in the subsequent equipment and simplifying the product handling procedure. A scrubbing vessel suitable for such a caustic washing operation is vhovc-n in Figure 7. This vessel would be installed between the product, cooler and the liquid-gas separator in the general pilot plant assembly shown in Figure 1. The scrubbing vessel in Figure 7 is flanged at both ends, and is packed with ceramic rings to afford improved contacting. A mercury seal a t the bottom of the vessel is provided to prevent the incoming product from contacting the caustic in the inlet lines since deposition of sodium chloride would occur in t,hese pipes as a result of poor mixing of the spent caustic with the caustic inventory of the vessel. A U-tube arrangemenb in the inlet piping confines the mercury t o the bottom of the scrubber and prevents backflow in t'he incoming piping. Thc sight glass containing the mercury provides a means of indicating the rate of flow of incoming product. Caustic solution is withdrawn from the system through valve C and fresh caustic for I'Oplacement is pumped into the vessel from storage, using a small
T TO O SEPARATOR SEPARATOR
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CAUSTIC LEVEL
7r
PACKED SCRUBBING COLUMN
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y
PRODUCTIN
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Figure 7.
~
, Caustic Scrubber for Keactor Product
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April 1949
gear pump. The caustic level is set at a fixed point in a clearvision gage glass located immediately above the vessel. Maintaining the caustic level a t this point in the gage glass results in establishing a constant caustic inventory in the system when caustic is withdrawn and replaced during operation and thereby provides correct hydrocarbon material balances. The product entering the scrubber bubbles through the mercury and passes through a riser at the bottom of the vessel whph elitends up t o the grid plate supporting the ceramic packing. A weep hole is located in this riser a t its base. The line for withdrawing caustic during operation is located below the packing support plate and fresh replacement caustic is added through this same connection. I n adding fresh caustic solution, the caustic pump is started and a suitable constant pressure of caustic in the connecting line is established by adjusting valve I3 in the caustic recycle line. Then valve A is cracked open and the caustic solution is slowly admitted t o the vessel until the desired caustic level in the upper gage glass is obtained. The product flows from the scrubber to the separator vessel. The scrubber operates under the same back pressure as is maintained on the separator by the pressure control instrument. When the caustic scrubber is used, a separate motor valve on the reactor outlet must be employed t o provide the desired reactor operating pressure. Feed Drying Still. I n processing operations involving catalysts such as aluminum chloride, it is essential that the hydrocarbon feed stock be bone-dry if accurate measurements of catalyst consumption are to be obtained. I n continuous plant scale operations, hydrocarbon stocks transferred directly from distillation units t o other processing units are satisfactorily dry. However, pilot plant stocks that have been sampled from plant operations and stored in drums or other containers pick up varying quantities of moisture from exposure and storage conditions. The most effective means of removing small quantities of dissolved water from hydrocarbon stocks is distillation, which will reduce the water content t o a much lower value than is obtainable with desiccants such as calcium chloride. The dissolved water exhibits volatility characteristics approximately comparable to those of propane. Distillation equipment that has been used with hydrocarbon stocks for providing a water-free feed stock for pilot plant operations is shown in Figure 8. The still proper consists of a steel shell of about 5-gallon capacity, heated with a steam coil, and a 3-inch diameter column about 8 feet long and packed with ceramic rings. The still is charged from storage drums and is
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CONDENSER
867
operated batchwise in 4-t o 8-hour cycles under total reflux conditions. A constant pressure of natural gas under instrument control is maintained on the top of the column. The hydrocarbon is distilled overhead at a fairly rapid rate. The overhead is condensed in a water-jacketed coil and flows into the disengaging drum illustrated in Figure 8. Any water present is gradually concentrated a t this point and collects in a look glass located below this drum. The hydrocarbon level in the accumulator drum is several feet above the point where reflux enters the column, thereby permitting the hydrocarbon t o flow by gravity from the disengaging drum through a rotameter t o the distillation column as reflux. The boil-up rate in the still is indicated by the flow rate through this rotameter. A small bleed of natural gas is added t o the vapor line a t the top of the column and leaves the system continuously from the top of the disengaging drum. This bleed gas passes through a water bubbler which gives a visual indication of the rate of gas discard. A portion of the water from the hydrocarbon feed is swept out with this gas. By employing this bleed gas, the operating pressure on the still is held a t about 20 pounds per square inch gage, thereby permitting the contents of the still to be subsequently transferred t o the pilot plant charge drums without the use of another pump and without admitting moisturecontaining air to the system.
CHECK VALVE
L.
%
ORRQSIVE LIOUID
p Figure 9.
Pumping Arrangement for Corrosive Liquids
Pumping Corrosive Liquids. I n some processing operations i t may prove necessary t o pump liquids that are highly corrosive and difficult to handle in ordinary pumps. I n Figure 9 is illustrated a n arrangement whereby pumping can be accomplished without contact between the pump and the corrosive liquid being handled. This operation also has application when difficult pump packing problems are encountered. The pump operates under a n oil seal which in turn is sealed off from the corrosive material being pumped by a mercury layer in a U-tube connection. The pump pulse is transmitted through these liquid seals t o a special alloy check valve operating on the corrosive liquid that is being transferred. At intervals, a small auxiliary pump is used t o supply required make-up oil for sealing the primary pump. LITERATURE CITED
(1) Bloch, H. S., Hoffman, A. E., Oleszko, T. J., and Chenicek, J. A., presented before the Division of Petroleum Chemistry a t the CHEMICAL SOCYITY,New 108th Meeting of the AMERICAN York, N. Y. ( 2 ) Bowerman, E. W., and Filbert, B. M., IND. ENG.CHEM.,40,542-4 (1948). ( 3 ) Chem. & Met. Eng., staff article, 51, 118-19 (May 1944). (4) Edgeworth-Johnstone, R., Truns. Inst. Chem. Engrs. (London), 17, 129-36 (1939). (5) Johnson, W. B., and Nagle, W. M., IXD. ENG.CHEM.,39, 971-4 (1947). (6) Metals Handbook, American Society for Metals, p. 311-22 (1939).
(7) Murphree, E. V., Gohr, E. J., and Kaulakis, A. F.,J. Inst. Petroleum, 33, 608-20 (1947). (8) Yule, L. T., and Bennett, R. B., IND.ENG.CHEM.,40, 1995-8 (1948).
*rw+ Figure 8.
Still for Drying Feed
RECEIVED December 30, 1948.
