Combination Batch and Continuous Fractionation Column S. T. KlGUCHl C.
I
F. Broun 6
N R E C E N T years, the pilot scale column hss become an incr-ngly lmportant tool in process development work. The column is used in pilot plant studies to prepare feed stocks and to purify products. It is used to evaluate crude oile and petroleum fractions, to prepare sample lots of chemicals, and to get process design information. These applications sometimes require a bat& distillation, sometimes a continuous distillation. This paper describes a single multipurpose column that can be used for both contiououa and batcb distillations. The column is designed to process a variety of feed stock8 over a wide range of operating conditions. The need for Helribility in pilot scale distillation columns has been well summarized by Carpenter and HelWig (8). The eapscity of the unit for batcb distillations ranges from 5 to 17 gallons. Nominal feed rates for cantinnous distillations range from 0.5 to 10 gallons per hour. The column can be operated at pressures ranging from 5 mm. of mercury absolute to Mx) pounds per aqnare inch m d at temperaturesfrom 10Dto750aF. ”
Column I s Adaptable l o Batch or Continuous and Pleasure or Vacuum Operation
A Schematic flow diagram is shown in Figure 1, and photographs of the unit are shown in Figures 2 and 3. The unit consists uf a feed section, 3-inch-diameter packed column, dual semice reboiler, condenser, reflux mechanism, and product accumulators. Suitahle automatie controls are provided so that one technician can eaaily operate the unit. In general, the instrumentation follows the principles used for commercial columns, but, in some cases, special controls better adapted to pilot scale work have been employed. In the design of the column, s p e d attention was given to frequent disassembly of the unit. Frequent shutdowns were anticipated for repairs, cleanup, inspection, and cllssging the packing or height of packing. In addition, precautions were taken to minimiee leakage. This is especially important in s m a l l scale plants to ensure good material balances. Welded-line construction is used wherever poeaible, and smooth pipe bends are used in preference to angletype elbows to minimize presmre drop and line joints. Accurately machined stainless steel weld unions am employed in all process lines, since commercial high pressure unions were found to be unsatisfactory for vacuum service. Flangea of the ring joint type are used between column sections. The de* of all vessels is in accordance with the ASME Code for Unfired F’resaure Vessele. To combat m a i o n in the unit, all the major process lines and vessels are made of 5% chmme steel, or Type 304, or Type 347 stninless ateel. Although the chief concern is C O R O S ~ O ~from high sulfur crudes, the general use of stainless steel construction is recommended to prevent scale formation. Scale from mild steel can eventually plug control valves or lodge on valve seats and prevent tight shutoffs. July 19s4
Co., Alhonbm, Calif.
All motors and heaters are operated on 110 volts except the reboiler heaters. T h e e are operated a t 220 volts. As a general rule, the use of 110 volts is confined to loads below 1 kw. Stabilized voltage of 115 volts is supplied to all of the adiabatic heaters. This is advantageous because once eq iilibrium is egtabliabed no further adjustments or automatic controls are required. Batch Operation. I n a typical batch run, the charge is introduced into the reboiler with the transfer pump. Heat ia applied to the charge by m h s of Calmd electric hestem wound mound the reboiler. For quick s t a r t u p , , a total of 10 kw. can be supplied to the reboiler. When the c h r g e reaches its initial boiling point, the reboiler wall heaters ?e turned off and the diatillation is conducted using only the heaters wound around the bottom section of the boiler. The. boilup rate is automatically controlled by a differential pressure controller that actuates one of the bottom heater elements. Column operation is made adiabatic by manually adjusting electrical heaters wound around each column section. Overhead vapors are condensed and the condensate flows by gravity to a timer-controlled reflux splitter. Reflux is returned to the column by gravity flow. The product stream ia subooaled before it flows to dual receiving t a n k s or to graduated receivers. For distillations other than atmwpheric, a liquid seal is maintained in the product line by a liquid-level controller imtalled on the reHux splitter gage glaas. The distillation is continued until the liquid in the reboiler is reduced to ahont 0.5 gallon or until the specified products m e produced. When water ia present in the charge, it is trapped and removed at the water t r a p o u t . section at the bottom of the column before the distillation is attempted. Continuous Operation. Feed from the surge tank is metered to two preheaters in series by means of a Zenith pump, The temperature of the feed leaving the laat preheater is automatically controlled by a temperature controller. Overhead vapors are condensed and Bow to the automatic reflux splitter. ReHux is returned to the column by gravity or i t can be pumped back with a Zenith pump. Product from the reflux splitter is subcwled before it flows to the dual receiving tanks. The bottoms temperature or any other column temprature may be antomatically controlled by means of a second temperstnre controller. Bottoms are pumped from the reboiler to dual bottoms receiving tanks with B Zenith pump. The bottoma s r e subcooled by a cooler located in the pump discharge line. The reboiler liquid level is automatically controlled by a Tbermocap type liquid-level controller. During the NU, the differentialpressure recorder is used to trace the internal behavior of the
column. Pressure Regulation. For operation a t presaures above atmospheric, the prenaure ia automtically controlled in one of two ways, depending on whether or not noncondensables are present. If noncondensables are p-nt or added for pressure control purposes, the preasure is controlled by regulating the waste gaa
I N D U S T R I A L A N D EN 0 I N E E R 1 N 0 -C H E HIS T R H
13a
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
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8 1364
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 46,No. 7
PILOT PLANTS vent from the condenser, The amount of light ends so released is usually measured with a wet-gas test meter. If noncondensables are absent, the pressure is controlled by regulating the flow of coolant to the condenser coil. On vacuum runs, the column vacuum is automatically controlled by a Precision Micro-Set manostat. This manostat provides accurate control and has the flexibility necessary for pilot plant work. Column Performance Tests Prove Ease of Maintenance and Operating Efficiency
The pilot scale column has been used to fractionate many kinds of complex mixtures. It has proved to be easy to operate, t o require little maintenance, and to give excellent and reproducible separations. The following test data illustrate the capacity and efficiency of the column for both batch and continuous operation. Test Conditions. The column was packed with 0.24 X 0.24 inch protruded-metal packing, to a height of 100 inches. This packing is made by the Scientific Development Co. The test mixture was n-heptane and methylcyclohexane. The n-heptane was a pure grade obtained from Phillips Petroleum Co. The methylcyclohexane also was a highly purified material supplied by Dow Chemical Co. The refractive indices a t 20" C. were 1.3878 for the heptane and 1.4230for the methylcyclohexane. An Abbe refractometer maintained a t 20' i 0.1" C. was used to analyze the test samples, and the composition of the samples was determined by using the refractive index data of Bromiley and Quiggle ( 1). Batch Operation. Efficiency tests were conducted a t total refluy and atmospheric pressure with the packing initially unflooded and then preflooded. The test procedure consisted of charging to the reboiler about 15 gallons of the test mixture that contained approximately 20 mole % n-heptane. The desired boilup rate was then set and automatically conCrolled by means of the differential pressure controller. Tests with the unflooded packing were started a t the lowest boilup rates. The rates were then increased until the fiood point was reached. The point of complete flooding was detected by watching for liquid buildup in the reflux splitter gage glass. Tests with the packing preflooded were conducted starting a t the highest rates and working down to the lowest rates. The column was brought to adiabatic operating conditions and maintained at steady conditions for 4 hours before samples were removed. Then 5-mL samples were withdrawn at 1-hour intei-vals until the refractive indices showed no change for a t least 3 hours. Overhead samples were taken from the reflux return line and bottoms samples were taken from the trap-out section just beloiv the packing. The sample taps were flushed each time before actual samples were removed. Boilup rates were evaluated by measuring the time required to collect a known volume of reflux in the trap-out section. Blank tests on the trap-out section indicated negligible condensation as a result of imperfect adiabatic conditions. Results. Test resulh covering the useful operating range of the column are summarized in Table I. The Underwood-Fenske equation and an O( of 1 0 7 were used to compute the number of theoretical platep. The relationship between theoretical plates and boilup rates is shoa n graphically in Figure 4. The number of theoretical plates in the 3-inch-diameter column varies from 60 a t low boilup rates t o 40 near the flood point. The decrease in efficiency a t high liquid rates may be attributed to a disproportionate decrease in active packing surface area, This is because the increase in column holdup and vapor rate is not offset by an increase in active contact surface area. For comparison, the data of Heinlein (4)and Scientific Development Go. (6) for 2- and 4-inch-diameter columns are also shown in Figure 4. Little difference in efficiency is noted between the 3- and kinchJuly 1954
Figure 2.
Over-all View of
Fractionation Column
diameter columns below boilup rates of 10 gdlons per hour. The column was repacked during the tests, and no effect of variations in packed density was found. The packing technique recommended by Cannon (2) was therefore Considered satisfactory for protruded packing. A study of triangular data points in Figure 4 reveals that preflooding has little effect on the efficiency of the packing for the hydrocaybon system being tested. This is a valuable feature of protruded packing, especially for pilot plant work, since i t reduces the time required to carry out a distillation. The relationship between boilup rates and pressure drop is shown in Figure 5. Data of Heinlein ( 4 ) and Scientific Development Co. (6) for 2- and 4-inch-diameter columns are included for comparison. A straight-line relationship is obtained when log coordinates are used. This plot shows that the maximum
Table 1.
Column Efficiency at Total Reflux
n-Heptane-methylcyclohexane test mixture 0 . 2 4 X 0 . 2 4 inch protruded packing Packed height 100 inches Boilup Rate Gal./hr. Mole/hr.
~ p , Inches of Water
n-Heptane Concn., Mole % Top Bottom
Theoretical Plates
Packing Not Preflooded
Packing Preflooded 11.4 6.3 7.8 3.8
0.664 0.366 0.454 0.221
15.5 4 7 2
INDUSTRIAL AND ENGINEERING CHEMISTRY
79.6 88.6 85.1 92.4
18.4 17.6 17.3 15.5
38 53.5 49 62
1365
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Samples for analyses were taken hourly from the feed tank, column feed zone, and overhead and bottoms stream. Liquid samples were taken a t all points except the column feed zone. Vapor samples mere taken at this point because of the difficulty of collecting a good liquid sample. The sampling device used t o collect the vapor sample consisted of l/a-inch stainless steel tubing closed a t the end and slotted along its length. This tubing was located in the packing, 35 inches above the bottom of the packed section with the slot turned down. The vapor samples were removed slowly a t a rate of 2' mi. per minute to minimize entrainment. Results. Typical results for the continuous distillation runs are summarized in Table 11. The McCabe-Thiele method was used to compute the number of theoretical plates, and the composition of the column feed zone sample was used to ascertain the actual feed plate location on the McCabe-Thiele diagram. Flexibility in Preheater Selection Is Feature of Feed Section
Figure 3.
Auxiliary Equipment and Control Panel
loading is 12.5 gallons per hour for the 3-inch column, based on the n-heptane-methylcyclohexane test mixture. Continuous Operation. Efficiency studies n.it'h feed int,roduced continuously to the column were made with the same packed column used for the batch runs. The column was operated a t at,mospheric pressure and with both a rectifying and stripping sect,ion. Feed a t slightly below its bubble point temperature was charged to the column 31 inches from t,he bottom of the packed section. Reflux was reheated to slightly below its bubble point before it n-as returned to the top of the column. A feed rate of about 1.30 gallons per hour was ueed, and the vapor load was held constant during the run by controlling the reboiler heat input with a differential-pressurr controller. About 3 hours were required to line out the unit', and a minimum of 3 additional hours were required to ensure steady-state conditions and to get accurate material balance checks. The column was operated under st,eady-state conditions for 2 hours before samples were removed. The runs n-ere continued until the refractive indices of the samples showed no further change and good material balance checks were obtained.
Table II.
The feed section consists of a calibrated feed surge tank, transfrr pump, feed metering pump, and feed preheaters. Tank. The feed tank has an operating capacity of 10 gallon and is fabricated of 5% chrome steel. The tank is provided with a steam jacket that is used to handle viscous materials. A calibrated gage glass allows visual observation of the liquid level. Pumps. A Viking rotary pump is used to transfer stock into the feed surge tank for continuous runs or directly into the reboiler for batch runs. The pump has a rated capacity of 3.5 gallone per minute and can also be used to recirculate the charge stock to prevent stratification. An air-motor drive allows the pumping rate to be adjusted easily. The feed pump is a Zenith metering unit. It consists of a '/d-hp. explosionproof motor, Graham variable speed transmission, and gear pump. Five sizes of interchangeable precision gear pumps are used to cover a range of 0 to 10 gallons per hour. The pumps have close t'olerances. .4 200-mesh screen strainer is installed in the suction line to keep foreign particles out .of the pump. The Zenith unit provides a smooth, accurately measured flow and a range of rates, and it allows changes in flow rate to be made quickly and easily. Pumps and lines in the feed system are provided with steam tracing for ease of handling viscous materials. The steam loads are small and steam traps are not provided. Condensate and excess steam are vented to the drain. Preheaters. Construction of the preheaters used to heat the feed for continuous distillation runs, is shown in Figure 6. Two of these preheaters are used in series to control the heating curve of the fecd. Each preheater consists of a '/y-inch stainless steel process coil cast in an electrically-heat'ed aluminum block. Each block contains two 1-kw. Calrod electric heaters. One heater circuit in each block is controlled by a Variac variable transformer. Either of the two Calrod heaters may be placed on T'ariac control by means of a three-position selector switch. This flesibility in heater selection allows a quick change-over in case one
Typical Results of Continuous Feed Operation n-Heptane-methylcyolohexane test mixture 0.24 X 0.24 inch protruded packing Packed height 100 inches
Overhead n-Heptane Concn., % Feeda n-Heptane Mole/ Charge plate concn., % hr. 56.6 0.0141 28.2 30.1 65.0 47.1 31.0
a
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66.9 50 28.7
84.0 72.8 39.5
0.0194 0.0159 0.0605
Bottoms n-Heptane Mole/ concn., % hr. 23.1 57.6 40.6 20.5
0.067 0.0535 0.0631 0.0310
Moles Reflux h4ole Product 29.2 19.4 19.4
7.9
Mole Redux
Theoretical Plate
0.412 0.376 0.308 0.401
27 23 31 23
Feed plate vapor composition.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
of the heaters burns out during a run. This type of heater provides excellent temperature control of the feed, it allows uniform temperatures to be established quickly in the aluminum block, and it permits the coils to be cleaned easily. I n the second preheater, the Calrod element on Variac control is connected to an automatic temperature controller. This controller maintains a constant feed temperature a t the outlet of the preheater. The controller operates with a resistance thermometer sensing element with a range of 0" to 1000" F. Since the heater load exceeds the controller contact rating, an Ebert mercury plunger relay is used in the heater circuit. Individual switches and ammeters are supplied in all of the heater circuits. The ammeters are used to indicate heater performance, but they are not used for control. Control of the preheaters is accomplished by using the graduated dial readings on the Variacs. Santocel powdered insulation is used around the top and sides to insulate the aluminum blocks, aiid 2 inches of Kaylo block insulation is used to insulate and support the bottom of the block in the heater casing. Two a/,o-inch stainless steel thermocouple wells are provided in each block to measure the block temperatures. One of these is located 1 inch from the top, and the other is located l s / 4 inches from the bottom, Except for special heater studies, only the top thermocouple is connected to the temperature indicator. Nichrome wire heaters are wound around the feed transfer line to compensate for heat losses. The heater is controlled by a Variac to maintain the feed entering the column a t the same temperature as the feed leaving the preheaters. In this way, excessive superheating of the feed is avoided in the preheater. The detailed construction of the heater is similar to that for the column heaters. Column Design Permits Disassembly and Alteration of Unit
The column is fabricated of 3-inch stainless steel pipe, Schedule 40, Type 304. To provide for flexibility, the column is built in sections 3 feet long. Pressure-tight junctions are achieved by means of ring-joint flanges welded to the sections. Three sections are bolted together to form a column with a packed height of 8.5 feet. This height corresponds to about 40 to 60 theoretical plates, the equal of most common commercial columns. Multiple feed inlet connections are installed near the top and middle of each section. For superfractionation work, three additional 8ec-
4
Figure 4.
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July 1954
1
6
7
8 9 IO II I2 I3 BOILUP R I T E , GALLONS PER HOUR
14
I5
16
Effect of Boilup Rate on Theoretical Total reflux Atmospheric pressure 0.24 X 0.24 inch protruded packing Preflooded, Cannon (2) V Preflooded, Kiguchi A Unflooded, Kiguchi Preflooded, Cannon (2)
e added to increase the total column height to 18 feet, or the equivalent of 100 plates. The entire column is supported from the bottom to allow for free expansion upon heating. Liquid Distributors. Liquid distributors are not ordinarily used in the column, but ells attached to both the feed and reflux line direct the liquid onto the center of the packing. Tests with the protruded type of packing show that uniform liquid distribution is attained 5 to 6 inches below the top surface of the packing in a 3-inch column. Packing. Several types of packing are available for pilot scale columns. The packing used for the tests described in the first part of this paper, was 0.24 X 0 24 inch, Type 316 stainless steel protruded packing with 0.40 X 0.37 inch hole size. This packing was first developed by Cannon ( 2 ) . I t is very useful for pilot scale columns, since equilibrium conditions are generally established without the need for preflooding, and columns can b e packed easily and uniformly. This is accomplished by letting the packing drop into the column with a free fall of a t least 3 feet. The packing is supported in each column section by a 60" cone made of 8-mesh stainless steel wire screen. The cone i s screwed to the face of the bottom flange. A retainer screen is installed on the top of the column to prevent carry-over of packing into the condenser in case of column upset. One disadvantage of protruded metal packing is its greater fouling tendency compared to packing such as Berl saddles. This is no problem for clean storks For dirty stocks such a s crudes, the column is flushed regularly LTith solvents such as hot o-dichlorobenzene or Arochlor. This procedure helps to keep the packing relatively clean and increases the period between major cleanups or packing replacements. Adiabatic Jacket. To obtain adiabatic conditions, the column is wrapped with Nichrome wire heaters and covered with 3 l / 2 inches of Kaylo insulation. Individual heaters rated a t 500 watts are wound around each 3-fOot column section to facilitate maintenance of the proper temperature gradient along the column. These heaters are independently controlled by separate Variacs. This control over short sections is especially desirable for precise distillations. The heater for the top section also supplies heat to the overhead vapor outlet line. Heat is added to this line to prevent condensation of vapors ahead of the condenser. The heater for the bottom section also supplies heat to the gage glass on the trap-out tray. Extra capacity is provided in all these heaters so that the same windings map serve to heat the unit quickly on start-ups. Nichrome wire beaded with fish-spine insulators is used for the heaters. The insulated wire is wound around a sheet aluminum jacket. X 1/4-inch air-space separates this aluminum jacket from the column, and a second aluminum jacket is placed over the heaters to improve the heat distribution. Ironconstantan thermocouples are welded to the center of each column section. Similar thermocouples are attached to the inner aluminum jacket opposite the corresponding wall thermocouples. Adiabatic conditions are established manually by adjusting the Variacs to bring corresponding thermocouples within 1" F. of each other. This is done easily with the help of a calibration chart that relates the Variac setting to column temperature. Trap-Out Tray. A special trap-out section is installed a t the bottom of the column. This is a 4inch chimney tray equipped with acalibrated gage 17 10 I9 20 glass. Suitable valves are provided around the section so that liquid may be trapped and removed Plates from the gage glass. This section has several applications. For batch distillations, the tray serves to trap and remove water present in the charge. For column performance studies, the
INDUSTRIAL AND ENGINEERING CHEMISTRY
1367
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Figure
5.
Relation between Boilup and Pressure Drop
n-Heptane-inethylcyclohexane test mixture 0.24 X 0.24 inch protruded packing
boilup rates are readily evaluated by measuring the time required to collect a given volunie of reflux in the tray. Samples of bot,tom reflux may be removed from the section when the efficiencies of various packing materials are tested.
To CacilitiLte interchange of the heater that is being controlled. a €our-outlet receptacle is provided for the lower heaters. Thr: top receptacle is connected to the controller circuit. Hence, if the controlled heater should burn out (luring a run, one of the other heaters can be plugged into this receptacle immediately u.ithout interrupting the run. Two adiabatic heaters are provided for t,he 18-inch section. These heaters prevent condensation of reboiler vapors on thc: walls of the reboiler. Each heat,er is controlled by a Variac. One heater extends from the 4-inch vapor outlet t o the top of the reboiler and down around t,he gate glass. The second heater covers the walls of the 18-inch section above the reboiler leg. .4total of 4 ' i 2 inches of Kaylo insulation is \Trapped around the reboiler t o reduce heat losses. Boilup Control. A duorange recording flow controller is used to measure and control the boilup rate on batch runs. The instrument is usually not used for control purposes on continuous runs. The controller is actunted by means of a mercury manometer nhich measures t'he differential pressure across the packing. The column is designed to operate oyer a differential pressure range of 2 to 10 inches of water, but measurement of differentials to 50 or 60 inches of miter are occasionally required when the column is flooded. T o cover the entire range satisfactorily, two high pressure range chambers are manifolded to a single loiv pressure range chamber behind the controller. .A suitable valve arrangement is provided so that either range chamber can be readily placed in service. One of the range chambers has a range of 0 to 10 inches of uat,er; the other has a range of 0 to 100 inches of water. A single chart calibrated from 0 to 10 is used for both chambers. The nianometer taps are taken across the top of t'he reboiler and tho condcnser. h lrnoclcbaclr condenser is located on the reboiler tap to knock back condensable vapors. The manonieter lines are kept filled with nitrogen.
Liquid-Level Control System in Reboiler I s Adapted for Small Vessels
The reboiler is a 20-inch length of 18-inch stainless steel pipe, Schedule 40, Type 303, set vertically, xit,h a 10-inch piece of &inch pipe welded to the bot,toni. The 6-inch bottom section serves many purposes. It, reduces holdup and line-out time for continuous dist,illations, permits better control of the reboiler heat input, serves as the reboiler under cont,inuous distillations, and is useful for distilling batch charges to yield residues as loiv as 0.5 gallon without the rielc of burning out the heaters. The 18-inch-diameter pipe is closed a t both ends wit,h weld caps. -4 4-inch flanged vapor coniiection and a '/a-inch reflux return line are provided at the t'op of the reboiler. -4 calibrated, steel enclosed gage glass is provided to allow visual observation of the liquid level in the reboiler. The maximum batch charge capacity is about 17 gallons. The nominal holdup for continuous distillation is about 1 gallon. Heaters. Three Calrod electric heaters, 2 kw., 220 volts, are interwound on the loner half of t,he 18-inch section. They are used for quick heat, and they operate at full capacity without Variac control. Individual switches are provided so that each heater may be operated separately, and separate ammeters are connected to each heater t o indicate burned out heaters and general heater operation. Four Calrod electric heaters, 1 kw., 220 volts, are interwound on the 6-inch reboiler leg. These heaters are used to control the boilup rate on both continuous and batch distillations. Three of the heaters operate a t full capacity through individual switches, but the fourth heater is controlled by a Variac. The operation of this heater is further regulated by either a temperature controller or a differential-pressure controller. A three-position selector switch is provided to select either controller. 1368
I
, L-.
Figure 6.
Preheater
The air output from the flow controller actuates a Mercoid pressure switch. This, in turn, operates one of the reboiler leg heaters. It has been found that this control scheme enables the column differential pressure to be held within 1.5 to 3% of the control point. Temperature Control. A temperature controller is used to control the column temperature for continuous distillations. The sensing element can be located at one of many points on the column to control any desired temperature. Safety Cutoff. A high temperature limit controller is provided to protect, the reboiler heaters in case the reboiler should go dry. This instrument is of the millivoltmeter t,ype and is actuated by a Chromel-Alumel thermocouplc. The instrument is relatively inexpensive and yet off em dependable and trouble-frce performance. The controller is usually set t o operate from 25' to 100" F
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
PILOT PLANTS above the maximum normal reboiler wall operating temperature. For batch runs, the controller summons the operator a t the end of the run when only a small residue is left. For continuous runs, the controller warns the operator of any failure that results in a low liquid level in the reboiler. Liquid-Level Control. The reboiler liquid level is automatirally controlled by a Thermocap relay that works in conjunction with a solenoid operated valve, Zenith pump, and pneumatic pressure control valve. The operating principle of this control system is indicated in Figure 1. Bottoms are pumped from the reboiler with a Zenith pump which is operated continuously. The Thermocap relay operates a solenoid valve on the pump discharge line. When the reboiler level is low, the solenoid valve remains cloaed, and bottoms are recycled through the control valve, aftercooler, and pump. When the level rises above the desired control point, the Thermocap operates to open the solenoid valve. This allows product to flow to the bottoms receivers. At the same time, the pneumatic control valve closes because of a drop in line pressure, so that the pump can develop sufficient head to transfer product into the receiver. Aftercooler. An aftercooler is provided on the pump discharge recycle line to cool the bottoms. Since the bottoms are reciroulated through the pump whenever the solenoid valve is closed, this recycle stream cools the pump. This is especially advantageous when operating a t high bottoms temperatures. Thermocap Operation. The Thermocap detects minute changes in the electrical capacjty due to liquid movement near a fixed electrode. E o direct liquid contact is involved. The electrode designed for this service consists of a 26-gage bare Nichrome wire, insulated with glass and inserted in the reboiler gage glass. A Conax packing gland is used to seal the glass electrode. Bare Sichrome Kire is extended from the electrode to the Thermocap input post. This wire should be less than 3 feet in length and should be allowed to hang freely in the air. The Thermocap relay provides simple on-off control action a t the instrument power outlet receptacle. In the present app l i c a t i o n , t h e solenoid operated valve is plugged into this receptacle. Recirculation C o n t r o l . R e c i r c u l a t i o n of t h e bottoms is governed by a diaphragm control ' v a l v e . The valve is operated by a p n e u m a t i c proportionalpressure controller with a range of 0 to 800 pounds p e r s q u a r e i n c h gage. Viscous materials are excluded from the Bourdon tube pressure element by means of a diaphragm seal on the controlled pressure piping, and dibutyl phthalate is used as the sealing liquid for the Bourdon tube. The pressure control setting on the controller is not critical so long as i t exceeds the reboiler pressure. Figure 7. Automatic This level control scheme Reflux Splitter is well adapted for use on small vessels. It operates A. Reflux line 8. Product line to maintain a liquid level F. Swinging funnel within '/e inch of the con1. Condensate inlet line trol point. E x t r a n e o u s S. Solenoid P. Plunger holdup is eliminated, and I
July 1954
the electrode occupies very little space in the vessel. Since no moving parts are involved, the electrode can be tightly sealed to prevent leakage. The method works equally well for vacuum service or pressure operation, and the control method is applicable to both conductive and nonconductive liquids. Alternative Level Control. An alternative liquid-level control scheme may be set up whereby the Thermocap is employed to regulate the reboiler heaters instead of the solenoid valve. In this case, the bottoms pump is set for constant bottoms withdrawal, and a constant reboiler level is maintained by cont,rolling the reboiler heat input with the Thermocap relay. Automatic Reflux Splitter Maintains Selected Reflux Ratio
Overhead vapors are condensed in a total condenser. To attain maximum flexibility, the condenser is constructed of a coil of '/%-inchstainless-steel tubing placed in a 6-inch jacketed shell, as indicated in Figure 1. The condenser may be operated using the coil alone, jacket alone, or both the coil and the cooling jacket. Operation with the coil alone results in better column pressure control when the control depends on the flow of coolant thiough the condenser. Circulation of coolant through both the coil and cooling jacket provides a maximum surface for vapor condcnsation under high loads, and when the jacket alone is used, this type of condenser avoids appreciable subcooling of the condensate. A separate aftercooler is provided on the overhead rundown line to subcool the over-head product as it drains to the receivers. The condenser is manifolded so that either cooling tower water, hot water, brine, or air may be used as coolant. A three-way valve is used on the inlet manifold to direct coolant to either the coil or jacket or to both the coil and the cooling jacket. Valves are also provided so that liquid can be drained from the condenser jacket whenever the coil alone is used. A sight flow indicator is installed on the inlet manifold to allow a visual check on the flow. Iron-constantan thermocouples are provided in both the inlet and outlet line to check the performance of the condenser. Reflux Regulator. A working drawing of the reflux device used for adjusting and maintaining a constant reflux ratio is shown in Figure 7. Condensate flows by gravity into the swinging funnel. In the vertical position, the funnel discharges condensate into outlet A t o s u p p l y r e f l u x to the column. When t h e funnel is tipped, the flow i s d i v e r t e d a c r o s s the baffle t o outlet B , which leads t o t h e overhead tanks. The position of the funnel is controlled by a solenoid operated plunger. The operating cycle of the solenoid is regulated by a cycle timer. The on-off s c h e d u l e of t h e timer determines the reflux ratio, which can be set to a maximum ratio of 1 2 0 t o 1 . T h e solenoid is a normally closed, high temperature coil. To keep the solenoid coil as cool as possible, a '/*-inch copper tubing air line is connected to Figure 8. Removable the coil housing. Thermocouple Fitting
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT A second scheme for supplying reflux is possible when a constant ratio of reflux t o feed is desired. For this operation, a Zenith pump is used to supply reflux instead of the reflux splitter. This pump is driven by the same transmission as the feed pump, but appropriate pump sizes and driving gears are used t o get the desired flow ratios. Excess condensate from the condenser that is not pumped as reflux is allowed to overflow across the baffle in the reflux splitter into the overhead run-down line, Product Accumulators. Dual receiving tanks of 1- and 5gallon capacity are provided for both the overhead and bottom products. The tanks are fabricated of 5% chrome steel. Calibrated gage glasses are provided on each tank for visual observation of the liquid level. On critical runs, however, yields are determined by weighing the cuts. Jackets are built around the receiving tanks to keep the products cool or hot, depending on the nature of the products. The jackets on the overhead receivers are manifolded so that either brine or cooling water may be circulated t o cool the products. T h e jackets on the bottoms receivers are manifolded so that either steam or cooling water may be circulated. Steam is provided for heating viscous stocks to maintain good fluidity so that the receivers may be drained easily. Extensive Instrumentation Ensures Praper Column Pressure Control
The column operating pressures are controlled in one of two ways, depending on the amount of noncondensables present. Where noncondensables are absent, the pressure is controlled bT7 regdating the flow of coolant to the condenser. For this control, the coolant is circulated to the condenser coils, and the jacket is usually by-passed. When large volumes of noncondensables are present, the pressure is controlled by venting the waste gas from the condenser. Instrumentation for pressure control by either method consists of a pressure transmitter, recording-receiver pressure controller, and miniature diaphragm operated control valve. Separate control valves are installed on the condenser coolant manifold and the waste gas vent line. These miniature control valves are specially designed by the Research Instruments Co. to handle small flows encountered in pilot plant work. Transmitter. A pressure Transaire transmitter with R 40pound-per-square-inch range span is used to transmit column pressures t o the pressure controller. This transmitter is of the force balance type with a sensitivity of 0.1 pound per square inch. Multiple range springs are used t o cover a total range of 40 to 600 pounds per square inch absolute. The working span of 40 pounds per square inch may be adjusted anywhere within this range. This working span corresponds to a pressure change that produces a change of 3 to 15 pounds per square inch in the transmitted air pressure. The output from the transmitter is connected to a recording pressure controller. A chart with a range of 0 t o 40 pounds per square inch is used on this controller t o match the transmitter range span. In this wag, a calibration at any point within the range span fixes the whole scale of the chart. Column Seal. For distillations conducted a t pressures other than atmospheric, a liquid seal is required in the overhead ploduct Pine. The necessary seal is obtained by maintaining liquid level in the reflux splitter gage glass by means of a Tbermocap unit and a solenoid valve on the run-down line The pressure in the product receivers is maintained slightly below column pressure.
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Vacuum System. Vacuum is applied to the column, condenser, and overhead receivers by either a 4.5 or 13 cubic-foot-perminute vacuum pump. A 1-inch pipe, Schedule 40, is used for the vacuum header lines. A 20-gallon vacuum surge tank is installed in the vacuum system to minimize vacuum surges. The column vacuum is automatically controlled by a Precision MicroSet manostat. A 4-inch-diameter, baffle-type cold trap is installed ahead of the vacuum pump t o trap any condensable vapors. A mixture of dry ice and alcohol is generally uscd as the coolant in the cold trap. Several tvpes of vacuum indicators are used to measure the vacuum. Over the range of 0 to 20 mm. of mercury, three types of absolute vacuum gages ai? used. These include a Wallace and Tiernan, a Dubrovin, and a McLeod vacuum gage. For routine work, only the Wallace and Tiernan and the Dubrovin gages are used. For operations a t 20 to 760 mm. of mercury, a Meriam absolute pressure mercury manometer is used. This type of manometer is convenient since it is independent of any variations in the barometric pressure Grove V series, seat seal valves are used on the main vacuum lines. These valves are dpsigned with O-ring seals and contain no other valve packing. The valves have performed satisfactorily down to 1 mm. of mercury. Miscellaneous Equipment. VALVES. To minimize leakage throughout the unit, Kerotest packless diaphragm globe valves are used almost exclusively on the process line. These valves are constructed of Type 302 stainless steel with either Teflon or stainless steel seats. The Teflon seats are for service below 460" F., while the stainless steel seats are for service above 450' F. PRESSURE RELIEF. T o avoid the possibility of leakage with spring loaded relief valves, rupture disks are used for protection against excessive pressures. TEVPERITURE IVSTRUMENTS. Important column tcmperatures are recorded continuously with a four-point temperature recorder. The recorded points are the overhead vapors and the feed and bottoms temperatures. A 70-point electronic temperature indicator is used to measure temperatures throughout the unit. The rapid response of this instrument permits readings to be taken quickly. THERMOCOUPLE WELL. Special removable iron-constantan thermocouples, constructed as shown in Figure 8, are used to measure temperatures a t all the critical points. The thermocouple is welded to the end of a s / or~ 1I8-inch stainless step1 tubing t o promote a sensitive measuring junction. The removable well is sealed in place by means of a compression cone fitting. This eliminates the troublesome leak problem encountered with threaded joints under high temperatures and vacuum service. Literature Cited (1) Bromiley, E. C., and Quiggle, D., Ind. Eng. Chem., Anal. Ed., 25, 1136 (1934). (2) Cannon, >I. R., IND.ENG.CHEM.,41, 1953 (1949). (3) Carpenter, J. K., and Helwig, R. W., Ibid., 42, 5 i l (1950). (4) Heinlein, A. C., Manning, R. E.. and Cannon, M . R., Chem. Eng. Progr., 47, 344 (1951). (6) Scientific Development Co , State College, Pa., Bull. 12 (1952). RECEIVED for reviev May 9, 1953. QCCEPTED March 23, 1954. Presented before the Division of Petroleum Chemistry at the 123rd Neeting of the AMIRICAS C H E m c h L SOCIETY. Los Angeles, Calif.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 7
