G .
M. COOKE
A VERSATILE CONTINUOUS PILOT STILL Increasingly, laboratorzes are faced with the problem of preparing large quantities of experimental liquids. Of course, such quantities can be prepared i n batch stills or plant-scale rgfinery units, but both of these methods have disadvantages. Batch stzlls require considerable space and the range of charge size is limited. R d n e r y units are relatively inJexible and involve conszderable cost when adapted to produce streams 01 Less than commercial volume. T h e unzt descrzbed h e r e j l l s the gap between batch laboratory stills and commercial reJnery units--it can prepare or modify pilot plant feed stocks in 100- to 10,000-gallon quantities. complete, atmospheric and vacuum still is suitThis able for a wide variety of uses, including crude reduction, feed preparation, and specialty running. It is all electric and semiautomatic, and can evaporate up to 50% of a crude oil feed entering two towers-at 70 pounds per hour in one tower having a diameter of 5 ' / 2 inches, and at 40 pounds in the other having a diameter of 4l/4 inches. Low-condenser and high-reboiler holdup is used, as suggested (6). Nominal capacity is 5 barrels per day at about 50% evaporation, but the pumps can handle up to 25 barrels if only light skimming is done. Rates are reduced under vacuum. The pilot still is versatile. The towers can be operated in series or parallel, either or both at atmospheric pressure or under vacuum, and each can be fed from the top, middle, or bottom. A flash evaporator is also included. Economy of operation is an important aspect. By including automatic temperature and rate controllers, plus some novel devices, the plant can be operated by one man. Thus, cost of feed stock preparation is cut almost in half. The still is based on two large fractionators ( 1 3 E ) which are all-glass towers having a 54-inch packed section and inside diameters of 110 and 140 mm. These large units have not been described in the literature but laboratory sizes of the same design have ( 3 ) . The towers, enclosed in glass-fiber jackets for both heat compensation and protection, have a feed point built in at the top just below the liquid divider, and another at the middle. Bottom feed is accomplished by pumping directly into the reboilers which are made of stainless steel. The towers, packed with either 0.24or 0.16-inch protruded stainless steel packing ( Z ) , give the equivalent of 10 to 15 theoretical plates at total reflux.
Even though complicated, the still can be operated by one man. instument panel, not shown, is in the chart on page 49 VOL.
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This is without distributors which are helpful in large diameter packed towers (5). The number of theoretical plates can be increased by adding another section above the main tower, or it can be decreased by operating the tower partially packed. Reflux ratio is adjustable by setting the timer (dE) which is individually supplied for each tower. The solenoid magnets which lift the valves are 220-volt coils made for a standard commercial valve (79E). However, the magnets are operated at 110 volts, and even then, they become hot when used at low reflux ratios. They could be cooled by using a finned housing and air jets, but it is just as easy to surround them with small tin cans filled with dry ice. High capacity condensers ( 73E) normally supplied with the fractionators can reflux full boil-up of pentanes with 35' F. cooling medium. SYith - 30" F. refrigerant, large quantities of butane can be refluxed. Dry gas is normally vented from atmospheric topping of crudes, but for operating at dry-ice temperatures (-100' F.), high capacity traps are available. The traps, supplied on special order, consist of coils of large copper tubing surrounded by dry ice. O n the other hand, partial condensation is obtained by thermostating temperature of the exit water at a point higher than normal. This technique is useful in a series operation where the overhead cut from the larger tower is too large for the smaller tower to handle. The operation can be kept in balance by venting additional light ends from the first condenser and thus decreasing load on the second tower. Cylindrical reboilers (73E), one fitted to the bottom of each tower, are of stainless steel. Each is equipped with immersion heaters as well as skin heaters which are made of special asbestos and glass fabric similar to those for the towers but having a heat density of 15 watts per square inch, Each reboiler has a feed point at the top, a draw-off point a t the bottom, a sight glass for automatic control of liquid level, plus a loop of tubing mounted through the wall and connected to the water supply for quenching the distillation quickly. The reboilers are supplied on special order. Each tower is equipped with a product cooler (73E), which includes a sidearm for take-off control. The cooler coil is manifolded to both service water and the - 30" F. refrigerant. Flash Evaporator
The flash evaporator, of stainless steel tubing, 8 inches long and 6 inches in diameter, has a disengaging section of disks and donuts mounted above it. Feed enters the lower section tangentially to assist in separating vapor from liquid by centrifugal force. Bottoms are drawn off through a recycle system in the bottom, and vapor passes overhead through a 2-inch pipe. Liquid level is controlled by a stainless steel "sight glass.') Three heaters (8E) are wound concentrically around the vessel, without touching its surface, and the whole unit is cast into an aluminum block, 21 inches long and 8 inches in diameter. The block is then insulated with 48
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2-inch magnesia pipe covering and protected by a sheetaluminum shell. Uniform heating is maintained by carefully spacing the heaters and by the aluminum metal surrounding the vessel. A stepless controller (22E) using a saturable core reactor operates all three heaters together. The sensing point is a thermowell cast into the block, and feed temperatures are taken at a tee just ahead of the tangential entry. Heat Supply for Feed Evaporation
Three transformers, each with a 15-kv.-amp. rating, supply 220 and 110 volts to the distribution box for the entire unit. Feed for each tower is heated by two cast-aluminum block heaters (a preheater connected directly to a main heater). Two are used to reduce over-all size of each piece and to permit better control. Like heaters for the flash evaporator, they are insulated with 2-inch magnesia pipe covering and protected with a sheetaluminum shell. The main preheater consists of two 2500-watt elements ( 8 E ) in high temperature sheaths, wound into coils, 4 I i ~ inches in inside diameter, and mounted inside a 20-turn coil of stainless steel tubing having an outside diameter of ' 1 2 inch. Inside diameter of the coil is 6 inches and a 2-inch space separates its turns. Center of the casting is empty out to a diameter of 4 inches, but except for a 3/8-inch tubing connection at each end used for injecting cooling air when needed, it is closed at the top and bottom. A thermocouple well of tubing, inch in diameter, is incorporated in each end. Construction of the main heater is similar to that of the preheater, except that its coil is 3 / 8 inch in outside diameter. It is heated by three 5000-watt heaters (8E). The feed preheater and main heater for the smaller tower have a total of 2500 and 4500 watts, respectively, and are similarly arranged in series. The coils in both preheaters are of stainless steel tubing, l / q inch in outside diameter. The last heater in each aluminum block is controlled by a temperature controller (22E), using a saturable core reactor and a sensing thermocouple in the thermowell a t the outlet end. With this arrangement, feed temperature can be controlled closely, even though rate may vary, and the operator is relieved of attention to this important detail. All other heaters are set manually at a steady heat input through variable autotransformers and are normally left untouched during the run. Pumping System
Except where temperatures exceed 150' F., pumps (5E) with special Teflon vacuum packing and nylon A U T H O R G. M . Cooke is in the Research Department of the Imperial Oil, Ltd., Sarnia, Ont., Canada. Originally emjloyed as a Pilot Plant Supervisor, he has, since 1950, been engaged in developing improved apparatus and techniques for distillation, crude oil assay, and pilot plant equipment.
gears are used. .For high tewperature s-5 carbon gears are available, but they have been found v u y fragile. The pumps easily maintain sufficient pressure gradient for vacuum service. For r a v i n g foreign matter which could damage the pumps or interrupt the run, double sintered bronze filters arranged in parallel are used in front of each feed pump. The -feed pump is driven by a variable speed d.c. hotor (77E) enclmd in an air-purged box. A hydraulic transmission (21E) is coupled to the output shaft of the motor, and the pump is mounted on the transmission. Both a variable speed motor and variable speed transmission are used. When pumping thin liquids through the heaters, at high speed against a high pressure drop, full motor speed is necessary; hut, at low speed against low pressure, adequate volume is delivered. For viscous liquids, low pump speed is needed but the motor can not develop full power at low r.p.m.; hence it must be speeded up and the transmission (27E) must be adjusted. The combination gives good flexibility. Pumps (5E) and variable transmissions (77E) with 16:l gear box are provided for handling the overhead product from No. 1 tower either to tankage or to No. 2 tower, and for similarly handlig the overhead product from No. 2 tower. Usually liquids run from the overhead product line to tankage by gravity, but not always. Sometimes, such a large overhead fraction is taken that it must be
assisted b y the pump. I t must be .wntinuously pumpassisted when operating under vacuum in order to overcome the atmospheric pressure gradient to the tanks. Therefore, automatic liquid level detectors are installed in the sidearm of each rundown cooler. These level detectors of the simple light-beam type are adapted from a commercial product (7E). A long Aoat of foam glass rises in the arm with the liquid level. When the light beam is interrupted, a fixed resistance is removed from the tachometer circuit of the pump motor, causing it to speed up about 25%. When the float drops from the path of the beam, the lower pumping rate is restored. The pump is set manually to a speed just below that required to maintain take-off, making this a simple on-off control. I t is dependable and requires littk attention. A different method is used to pump from the two reboilers and from the flash evaporator, because these liquids are at their boiling point. Variable drives (17E)are used, but speed is automatically controlled by a capacitance sensing device which detects level. The sensor for the rebodem is different from that in the flash evaporator. The sight glasses attached to the reboilers are standard */,-inch borosilicate glass sight glasses mounted with Teflon ferrules in right-angle fittings (ZE). Inside each glass is a float made of foam glass and wrapped in aluminum foil. A specially designed (75E)relay is mounted nearby
NO. I TOWER, 110 MM. 1.0.
NO. 2 IOWER, 110 MM. 1.0. 10TAL EVAPORAIOR HEN, 27,COO W A m IOIAl REBOILER HEAT, 15,000 WAHS
..... .....I U RECORDER
.,
Tha still is omatile. T h towcrs can be opcrntcd i n smm or pmallcl, either or both at obMsphnic prmwc or undn vacuum, and each can be fed from th6 top, middle, or b o t h VOL S A
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with its sensing loop at the point where the level control is desired. The output from the relay circuit is designed to replace the output of the tachometer on the motor (77E); thus, when the float rises near the sensing loop, the motor gradually speeds up the pump until full speed is attained. Before this, the level will have been established because the pump has more than adequate capacity. As a full proportioning control, this combination performs well at atmospheric pressure and accomplishes, with practically no attention at all, what is normally a tedious and time-consuming job. The reboiler pumps, mounted so as to be fully immersed in the liquid, were adapted from a commercially supplied model (7E). The flange was reduced to 3*/, inches in diameter and mounted directly on a suitable face plate in the reboiler bottom. The pump must be protected from rust during idle periods. Because of hot service, the face gasket and the neoprene O-rings on the shaft are replaced by similar ones made of Viton A (3E). This is a new fluorocarbon elastomer useful in this service at temperatures up to 600" F. The pump, so mounted, has no suction l i e and hence much of the trouble associated with pumping out of a vacuum is eliminated. Vacuum operation is further simplified as shown in the flow diagram: Oil is taken in by the pump and forced against the back pressure regulator ( I O E ) which is set to open at about 20 p.s.i.a. Thus, when the pump is turning at a speed slightly higher than that necessary to hold the regulator open, a simple solenoid valve connected to a tee in the high pressure leg (5 p.s.i.g.) can be opened to remove liquid from the system under vacuum. In vacuum service, the relay in the level sensor (75E) is used to open the take-off valve in order to maintain the reboiler level. The flexible diaphragm in the regulator (7OE) must be changed to Viton if used at temperatures above about 350' F. A by-pass needle valve can be used to increase the control range. This device has reduced to a simple operation continuous removal of hot product from a vacuum system. To the author's knowledge, this arrangement has not been used before. Similar pump installations are used for all liquid products when running under vacuum, but for the flash evaporator a different liquid level detector is used. It was also developed in these laboratories. The flash evaporator handles very hot and often viscous material. Therefore, instead of vitreous sight glasses, a Type 316 stainless steel tube, 1 inch in outside diameter, was installed. Inside the tube, a foam glass float, protected by foil wrapping, carries a soft-iron core, 3 inches long and '/a inch in diameter. On the outside of the stainless tube is wound a differential transformer whose secondary develops a d.c. signal as the float enters the windings. It is separated from the a x . component by a simple filter circuit and measured with a milliammeter. The signal rises to a peak as the float approaches the center of the coil and then declines. To prevent the float from beiig lost by rising through the transformer, a stop is installed which prevents the float from rising more than half way out of the top end. 50
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
EAR PUM?
I
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WWCl
Rnnomng 6oiling l i p 2 from thc m u m r y r h . Continuow rcmowl of hot product hns bun rcdwtd to a simple oprrofion
Thus, when the signal has passed through the maximum and fallen to a steady value, the level is above the set point. If the signal fluctuates at or below the maximum, the level is at or below the set point. In much' of the feed and product system where pressures are below 200 p.s.i.g. and temperatures below 150' F., the new Zytel (3E) tubing and nylon tube fittings (77E) are used. They reduce considerably the initial cost and the labor necessary in making changes in lines where stainless steel was previously used. Vocuum Supply
For vacuum sexvice, a large mechanical pump (6E) is connected through a surge tank having a capacity of about 2 cubic feet, to a manifold through which either or both of the two towers can be connected. Under such conditions, the vent line is shut off. Good vacuum valves for this service are available (9E). They are very small for their throat area and employ neoprene diaphragms. Vacuum is measured by the signal from a new type of vacuum gage (73E)which uses the boiling point of a pure compound to indicate pressure in the system (7). The boiling point can be recorded directly in millivolts on the strip chart recorder or it can be recorded on a special chart calibrated in millimeters of mercury absolute pressure. The strip chart method is satisfactory because converting degrees Fahrenheit. to millimeters of mercury is simple. The vacuum controller (72E) is a transistorized package designed for use with the vacuum gage (73E).
A sensing element (77E) in the thermowell beside the thermocouple sends a signal to the controller where it is amplified and used to operate a relay. The relay opens and closes the power circuit to an electrically operated air bleed near the pump. A manual bleed is first set to hold a pressure just below that desired for operation. Then the set point is selected by turning a dial on the controller so that the automatic bleed is opened below that point. This allows the pressure to rise to the selected level where it is cut off again. This type of on-off control is satisfactory for all pressure levels from atmospheric down to a few millimeters of mercury. Sensitivity of the controller is adequate over the whole range. Refrigerant Supply I
A 10-hp. compressor, capable of absorbing 30,000 B.t.u. per hour, refrigerates an insulated tank containing 250 gallons of 50y0ethylene glycol inhibited with Nacap (20E). The refrigerant is maintained at -30' F. A pump and recycle system containing a throttling valve circulates cold glycol to the unit. This supply has been adequate for all normal operations. Accessories
The still is larger than those recently described (4, 7), and occupies a 12 by 24 foot room, two stories high, with a stairwell cut into the upper floor. The stairwell opening is large enough to permit towers and condensers to extend into the second story: thus, condensers and smaller upper parts can be easily manipulated, and from the stairway itself, the back of the unit is easily accessible. For versatility and flexibility of arrangement, the still is mounted on a rack of Cantruss (76E). All electric devices except pump servos and capacitance sensors are in front of the operator. A large panel board contains the variable autotransformers for manual heat control as well as the automatic temperature controllers and reflux ratio timers. Also on the board is a 16-point recorder (14E) which makes a permanent temperature record for all thermocouples. An adjacent panel contains two rotameters-one to indicate feed rate for each still. This panel also contains pressure gages for the feed heaters, and water valves for coolant supply to the condensers. Outside of the building, against the wall adjacent to the still, are six domestic fuel tanks each with an electric level gage which can be read inside the building. Two are used as feed tanks, four as product tanks, and each is insulated and steam-traced. A nylon gear pump, similar to that used for emptying laundry tubs and mounted on the ground near the tanks, is manifolded so it can fill or empty any tank or circulate between any two tanks. For loading runs of more than about 1000 gallons, an underground plastic pipe which can be connected directly to a tank car on a railway spur line near the building is used instead of barrels. In this area which is normally safe but may become temporarily unsafe because of hydrocarbon gases and vapors, considerable effort has been made to conform
with Class 1, Group D, Division 2, of the Canadian Electric Code and the U. S . National Electric Code, Exposed electric motors are explosion-proof models, and those which are not explosion-proof are enclosed in airpurged boxes. The main panel board comprises one wall of a large air-purged box and carries a safety light signal. Electrical devices outside the box employ only enclosed mercury switches. An automatic carbon dioxide fire extinguishing system, with an alarm and sensor on each floor, is connected to all doors and windows so that it will operate automatic closing devices. After the.alarm sounds, release of carbon dioxide is delayed for 5 seconds to permit exit of personnel. Then, simultaneously with release of the gas, an interlock with the power line cuts off electricity. Power can also be cut off manually simply by pushing a button outside the building, and carbon dioxide also can be released manually if necessary. SUGGESTED READING (1) Anderson, R. C., Cooke, G. M., Kenyon, L. C., Mathiasen, R.L., Rev. Sci. Znstr. 32,780 (1961). (2) Cannon, M. R., Bull. 12, Scientific Development Co., Box 795 State College, Pa. (3) Cooke, G. M., Jameson, B. G., Anal. Chem. 27, 1798 (1955). (4) Kiguchi, S. T., IND.ENG.CHEM.46, 1363 (1954). (5) Manning, R. E., Cannon, M. R., Zbid., 49,347 (1957). (6) Rose, A., Williams, T. J., Z6id., 47,2284 (1955). (7) Ward, C. C., Schwartz, F. G., Petrol. Processing 5,164 (1950). EQU I PM ENT (1 E) Consolidated Electrodynamics Corp., Pasadena, Calif. ; pump for 14-in (CMS) still. (2E) Crawford Fitting Co., Cleveland, Ohio; Swagelok fittings. (3E) E. I. du Pont de Nemours & Co., Inc.; Zytel (nylon resin) and Viton A. (4E) Eagle Signal Corp., Moline, Ill.; Flexopulse timer, 0-120 sec. (5E) Eco Engineering Co., Newark, N. J.; Gearchem pump, Series 700. (6E) Edwards High Vacuum (Can.), Ltd., Burlington, Ont. ; mechanical pump 28-450. (7E) Electronics Corp. of America, Photoswitch Div., Cambridge Mass. ; light-beam level detector. (8E) General Electric Co.; Calrod heaters Cat. No. C4A283; 5000 watts. (9E) Grinnel Co. of Canada, Ltd., Toronto, Ont.; vacuum valves. (1OE) Grove Regulator Go., Oakland, Calif. ; back pressure regulator 90-W. (11E) The Imperial Brass Mfg. Co., Chicago, Ill.; nylon tubing and fittings. (12E) Geo. Kelk, Ltd., Don Mills, Ont.; vacuum controller built for Imperial Oil, Ltd. (13E) H. S. Martin & Son, Evanston, 111.; Sarnia Mark I1 fractionators, reboilers, tensimeter, coolers, and high capacity condensers. (14E) Minneapolis-Honeywell Regulator Co. ; Brown recorder, 153X 65. (15E) Niagara Electron Laboratories, Andover, N. Y. ; Thermocap relay. (16E) Northern Electric Co., Montreal, P. Q.; Cantruss rack. (17E) Servo-Tek Products Co., Hawthorne, N. J.; 1/2-hp. d.c. variable speed motor, Series 300. (18E) E. H. Sargent & Co., Chicago, Ill.; Thermistor sensing element. (19E) The Skinner Chuck Co., New Britain, Conn.; V-5 type valve. (20E) R. T. Vanderbilt Co., Inc., New York, N. Y.; Nacap inhibitor. (21E) Vickers, Inc., Detroit, Mich. ; hydraulic transmission, S/, hp. (22E) West Instrument Co., Chicago, Ill. ; Gardsman JS stepless controller. VOL. 5 4
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