Design of Still Heads for Batch Fractionation in Laboratory Columns -
The Vapor Take-Off System A. R. RICHARDS, Trinidad Leaseholds Limited, Pointe-&-Pierre, Trinidad, B. W. I.
cooled reflux. When the system is extended to modes of operation other than total reflux the product is usually collected from the liquid reflux system, a technique which increases the reflux holdup. This may be avoided by using a vapor-phase product drawoff (1, 2, S ) , and the auxiliary condenser and control systems described beloJy make use of this principle.
Low-holdup reflux regulators for batch distillation are described in which the product is removed in the vapor phase. Possible designs are discussed with reference to glass models, and detailed designs are given for still heads of general application. Details of heads applied to 25-mm. Stedman packed columns show how these designs may be modified to suit special requirements.
Types of Auxiliary Condenser and Control Systems I n the closed system shown in Figure 1, 2, a second condenser has been added and so arranged that condensate collected in this second condenser cannot return to the column. I n such a system the rate of condensation on the cooling surface of the auxiliary condenser determines the rate of product removal. The rate of heat transfer depends on the supply of product to the condenser, the difference in temperature between the vapor and condensing surface (At), and the material and design of the condenser. Figure 1 , 3to 11,indicates in diagrammatic form the various methods of controlling vapor take-off which are described below. The merits and demerits of these systems and their operating characteristics are summarized in Table I. Since the reflux holdup is the same for all these systems, the term “holdup” in the table refers to run-down line holdup. Performance tests on models of these condensers (designs 3 t o 11) were undertaken in order to study their behavior with reference to the following fundamental factors :
D
URIKG the past few years experimental and theoretical work in connection with batch fractionation has served to emphasize the importance of the effect of holdup. Considerable ingenuity has been demonstrated in the design of still heads to eliminate reflux holdup in order that full advantage may be taken of the fine fractionation achieved by modern low-holdup column packings. This paper is concerned with further refinements in the design of still heads suitable for the control of batch fractionation on laboratory columns. -4column fitted with an overhead condenser of the type shown in Figure 1, may be operated on total reflux with a minimum of reflux holdup. If the condenser is so designed that only the latent heat is removed from the condensing vapors and the reflux is returned to the column a t its boiling point, the liquid film will be of minimum thickness and none of the packing will be used as a heat exchanger to warm up
The holdup must be a minimum. The response t o the control must be rapid.
TABLE I. CHARACTERISTICS OF COSDESSER AND CONTROL SYSTEMS System 3
Response* Product Rate Stability Variables Which Increas- of ProdM a y Possibly Control Decreasing ing uct R a t e Affect Stability Vapor rate, stopcock Quick Quick Excel- Small p r e s s u r e in vapor line lent changes in reflux system
4
Water rate, stopcock in condenser water inlet
5
Water temperature, Fair mixing of hot and cold water t o condenser inlet Water level, cocks in Poor condenser i n 1 e t and outlet or constant-level device Condensate 1 e v e 1, uick. lag in constant-level deresponse of vice or stopcock c o l u m n on product line condition
8 9
Fair
Top
Poor
Poor
temperature, cooling water pressure and temperature H o t and cold water temperature and pressures, top temperature Water temperature and rate, top temperature
Fair
Poor
Fair
Poor
Quick
Excellent
Water rate and temperature, Jop temperature in 8 and stopcock grease in Y
Run-down Holdup Nil
Nil
Behavior on Total Possible Suitability for ReEux Contamination Laboratory Use Excellent Grease from Cnsuitable owing t o stopcock stopcock in vapor line; see. however Bruun ( 8 ) or Bush and Schwartz ( 4 ) Response slow and None Unsuitable, poor recondenser must sponse, needs conbe lagged tinuous adjustment
Nil
Response fair and condenser must be lagged
None
Si1
Response fair and condenser must be lagged
Kone
High with low rate and vice versa
Tends t o incorporate rundown holdup in column holdup
Stopcock grease in 9
Ease of control makes 9 attractive in spitr of contaminatioii and holdup
From mercury seal on total reflux None
Has limited appliration: Bee. however. Fenske‘s design ( 6 )
10
Removable condensing surface
Quick
Quick
Good
Water rate and top temperature
Xi1
Requires mercury seal
11
Gas blanket
Quick
Quick
Good
Water temperature and pressure, partially self-stabilieing towards t o p temperature changes
Si1
Excellent
_. 649
Unsuitable, too many auxiliary controls for water temperature, poor response Unsuitable, poor response
Most satisfactory
650
INDUSTRIAL AND ENGINEERING CHEMISTRY
GENERAL ARRANGEMENT
t
AUXILIARY SYSTEMS
o@@‘@
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FIGURE 1 The control must be positive and stable. The system must permit of operation on total reflux. The use of sto cocks or mercury seals should be avoided when possible, particufarly where they may come into contact Kith ;gasoline vapors.
Design Details The performance tests indicate that designs 9 and 11 are -the most suitable for laboratory control, but before the construction of this equipment is described .other factors which affect the design details must be considered. I n the first place it is a n advantage if the auxiliary equipment can be made small and compact enough to be supported only by the column, so as to avoid She necessity for expansion joints which may either cause holdup or give rise to leaks. Such heads can he changed very easily if broken or if the operation of the still is to be changed, but in order to do this it is necess a r y to remove the product through the head and not from the side of the column. The use of vapor-tight connections be-tween the head and the column is unnecessary providing the distillation takes place at atmospheric pressure. Condensers of the type shown in Figure 2, 12, are in constant use for total-reflux work and for certain organic reactions such as nitrations, since a stirrer and thermometer can be easily acaommodated. However, in the latest designs, it has been found possible to use ground joints which can be wateraooled and can be protected from product
Vol. 14, No. 8
vapors, which might attack the lubricant, by the use of an air blanket. If the reflux is overcooled and returned to the column substantially below its boiling point, the top few plates are used as a heat exchanger and there is a loss of efficiency. As a consequence devices other than “drippers” for measuring reflux rates have been avoided. Reflux rates and ratios can most accurately be determined by preliminary experiments with pure compounds in which the power required by the still pot and column heaters can be compared with the pressure drop across the column when the top and bottom rates are the same. I n such experiments holdup is of no consequence and several devices may be used which otherwise would not be permissible. Finally, the location of the thermometer is of some importance. Top temperature is not, in the writer’s experience, a satisfactory criterion of purity when close fractionations are being dealt with and as a consequence temperature records to within 0.2” C. are usually sufficient. Unless a reboiler of the Cottrell type is used batchwise or in conjunction with a large column, the holdup (about 10 ml.) associated with it seriously affects the sharpness with which a cut may be made, and very often it will be found that the Cottrell temperature lags below the top temperature. It is possible to fabricate reboilers with a holdup of less than 1ml., provided they are located within the column itself and utilize the main condenser for cooling and the column vapor for heating, but as they are of limited use it is not considered necessary to describe them here. Moreover, in a column with fifty or more plates very little difference in temperature can be detected in the top few plates (unless the reflux is overcooled) and little improvement in the vapor liquid equilibrium can be obtained by running the product through a reboiler. Therefore in order to obtain a true top temperature it is better t o suspend the thermometer or thermocouple in the vapor a t the top of the column and arrange a suitable guard so that the liquid reflux does not flow over the thermometer bulb or thermocouple tip.
Constructional Details The simplest head based on design 9 is shown in Figure 2, 13. An eight-turn coil of 3-mm. inside diameter Pyrex tubing forms the main reflux condenser. The vapor line, A , and the water
J
1
I
“ U FIGURE 2
August 15, 1942
ANALYTICAL EDITION
inlet line, B , are accommodated axially as well as a thermometer which is not shown in the figure. It is difficult as a consequence to make the outside diameter of the condenser coil sufficiently small to enter a column 2.5 cm. (1 inch) in diameter. However, if such heads are required for columns smaller than 2.5-cm. (1inch) diameter, they may be fabricated from tinned copper tubing. The product condenser consists of a 5-cm. (2 inch) length of tube flattened in order t o reduce the holdup. It is better to draw this flat tube from previously flattened thick-walled tubin than to flatten tubing of the correct size and wall thickness. !‘his tube is enclosed in an ordinary water jacket, the water outlet of which is fused to the water inlet, B , of the main condenser. This water tube serves as a support which maintains the relative positions of the main and auxiliary condensers. It is important that the thermometer and vapor tube should not touch the main condenser. The head is rested in the top of the column with the stopcock closed, and water circulated through the condensers. The column is brought into operation and total reflux conditions are established by condensation on the main coil condenser. In order to remove product from the still a small amount of air is then withdrawn through the stopcock. Product vapor is withdrawn from below the main condenser via the vapor tube into the auxiliary condenser where it condenses. If the stopcock is closed the auxiliary condenser first fills rapidly and then the liquid level will continue to climb slowly to the top of tube A until total reflux is established. The stopcock may now be opened and the level of the product in the auxiliary condenser will fall until the exposed condenser surface is just sufficient to condense a volume of vapor which corresponds to the product demand. This condition may be maintained throughout the distillation, the product rate being controlled by the stopcock the setting of which is facilitated by filing a short scratch around the key barrel starting from the bore hole. If a small amount of air remains in the auxiliary condenser when the head is primed it may slow down the action of the auxiliary condenser by blanketing the surface or even seal off the condenser completely by remaining trapped in the head at the top of tube A . This can be obviated and priming facilitated by sealing a side tube and stopcock to the bend, C, in the vapor tube. The bell on the collecting end of the vapor tube must have a terminal diameter sufficient to reduce the velocity of the entrant vapor appreciably below the vapor velocity in the column; otherwise vortex rings travel down the outside of the vapor tube and draw air from above the main condenser, which can then pass up the vapor tube and blanket the auxiliary condenser. With a narrow-mouthed vapor tube this phenomenon persists even if the vapor tube projects as much as 20 cm. (8 inches) below the main condenser or if the top of the condenser system and the space between the condenser and column are plugged. Glass wool packed below the main condenser but above the mouth of tube A will keep the air from the auxiliary system but the bell mouth is the most satisfactory solution of the problem. A development of this head is shoxn in Figure 3, 14, which has been successfully used for the delivery of constant volume (10-ml.) samples in the fractionation analysis of gasolines. The main condenser, A , 14, is of a design similar to that shown in 12. The heat transfer through the cooling water is partly by convection and partly due to forced circulation, and although it would appear that such a design would not be so satisfactory as one in which complete circulation obtains it has been found suitable in practice. In slightly larger models it would be preferable to run the water inlet line, B , down inside the water space to the bottom of the condenser. Both surfaces of this main condenser may be in contact with the vapor as far as the hole, C, which pierces the water annulus, but above this hole air is trapped between the condenser and column wall and this serves to protect the lubricant of the ground joint from product vapors. The inner surface of the condenser is, of course, available above the equalizing hole. The product vapor is removed through a bellmouthed tube and condensed in a small auxiliary condenser, so that the operation of priming the head is the same as for the design shown in 13. A thermometer is suspended from the closed end of the extension tube, D,so that the bulb lies beside the bell and the stem is not in contact with the condenser. Under these conditions the bulb records the vapor temperature, and not the temperature of the cooled reflux. The pressure inside the still is controlled via the tube, E , which is in connection with a manostat assembly. After passing the product-rate control cock, F , the liquid product is collected in the jacketed receiver G , \Thich is protected against loss by the flattened condenser, H. The product siphons intermittently from this receiver at intervals determined by the control-cock setting. If the still pressure is widely different from atmospheric this form of delivery is not suitable since although an auxiliary receiver below the siphon
651
U 0 FIGURE 3
connected by an equalizing line to tube E would enable the siphon to operate, the difficulty attendant on changing such receivers renders the head unsuitable. For pressures around atmospheric a rubber connection is made from the side tube, J , to one side of a manometer which is used t o indicate the pressure drop across the packing. In order to simplify the drawing both the thermometer and equalizing tube from D to the top of G are omitted. Figure 3, 15, shows a general-purpose head based on design 11. The bayonet-type main condenser, thermometer extension, vapor tube, and pressure-control outlet are similar to those shown in 14. [Aniline point thermometers are suitable for these types of heads since the main condenser assembly only obscures the unetched part of the stem and a few of the lower graduations.) The vapor outlet leaves the thermometer extension, A , 15, and communicates via a short capillary tube with the side of the small chamber, B. The head of this chamber carries a very small bayonet-type condenser for condensing product, while the lower end is sealed t3 a short capillary liquid-seal tube, D. Sealed into the chamber is the permanent gas inlet, C. The auxiliary bayonet condenser should not reach to the bottom of B, since for very high reflux ratios it is better to use the walls of the lower part of B as the condensing surface. The liquid seal, D, communicates with a jacketed receiver through a tube wide enough to prevent siphoning, and with the pressure control inlet, E , through a flattened protecting condenser, F . The equalizing line from A is connected to the bottom of F opposite the entrance of the seal, D,but to simplify the diagram both it and the thermometer are again omitted. On total reflux the head is full of permanent gas (air) except for the lower end of the main condenser. The head is primed by removing a small amount of air from B through tube C,’. The product vapor rises up inside the vapor tube with a slight “pumping” action for 1or 2 seconds until the tube is warmed up, and, if sufficient air has been removed from B , the product vapor condenses on the lower walls of B and the condensed liquid fills the seal. The bayonet condenser is drawn to a point so that Lvhen the air blanket is slowly raised there is no marked discontinuity in the increase in condensation. When At is large the lower part of tube B may be lagged with felt or cotton wool in order to give a more sensitive control of low product rates. I t has been found possible to keep a constant product rate of 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
6.52
ml. per hour for 48 hours, since the rate is not controlled by narrow openings which may become blocked with grease or, as may happen in gasoline fractionation, by traces of moisture. The auxiliary systems of both designs 14 and 15 may of course be used satisfactorily with simple main condensers as in Figure 1, 2.
The flexibility of these systems of vapor take-off may be denionstrated by two other modifications which have proved useful for special applications.
A closed tube may be inserted axially through the condenser in design 15 and the upper end of this tube brought out through the thermometer support tube above the va or line and sealed into the permanent gas inlet line. The tip ofthis closed tube is located close to the thermometer bulb in the product vapor so that an increase in “top” temperature will cause the air in the closed compensating tube to expand and, as a consequence, drive down the blanket in chamber B. If the volume of air in the compensator is adjusted (by the addition of a few drops of oil) it is possible to oppose the increase in At due to a rise in top temperature by a reduction in available condensing surface, so that the product rate remains substantially constant. It is sometimes an advantage in precise fractionation to increase the reflux ratio as the cut point is approached. Design 9 may be modified to do this automatically, as shown in 16 in diagrammatic form. The vapor line communicates directly to a vertical condenser which is emptied intermittently by a siphon. As product condenses in the lower part of this condenser-receiver,
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the product level rises and cuts off part of the available condenser surface. As a result there is a progressive decrease in product rate as the distillation proceeds until the siphon is primed and the cycle repeated. The volume of the cut depends on the volume of the receiver up to the bend in the siphon, the final product rate on the residual condenser surface above the top of the siphon, and the initial product rate on the total condenser surface. It is possible therefore to design such a head to meet any specified conditions of distillation. If the to temperature is increasing throughout the run there will be a increase in product rate unless the water temperature is raised or the area of condenser remaining above the siphon progressively reduced by an air blanket in the water jacket.
SEW
Acknowledgment Thanks are due to Trinidad Leaseholds Limited for permission t o publish this laboratory application of still-head design.
Literature Cited (1) Arthur, P., and Nickolls, C. L., IND. ENG.CHEX, A N A L . ED., 13, 356 (1941). (2) Bruun, J. H., Bur. Standards J . Research, 7, 851 (1931). (3) Bruun, J. H., IND.ENG.CHEM.,SNAL. ED., 7, 359 (1935). (4) Bush, M.T., and Schwarts, A. M., Zbid., 4, 138 (1932). (5) Whitmore, F. C., and Lux, A. R., J . Am. Chem. Soc., 54, 3448 (1932).
An Oil Manometer-Manostat to Control Column Throughput S. A. HALL AND SARIUEL PALKIN, Bureau of Agricultural Chemistry a n d Engineering, Washington, D. C.
T
HE importance of control of distillation rate (through-
put) for optimum column performance is well recognized. Control of the heat input to the still pot by rheostat or Variac alone, except over short periods of time, has been found undependable because of the marked effect, especially a t low throughput, of even small variations in line voltage on the total heat supply t o the assembly. This is particularly true where tall columns are used and where the column insulation is dependent upon heat compensation. [For short columns (not over 90 cm., 3 feet) in which column and still pot are vacuum-jacketed, such as Podbielniak type (4),Variac control is probably much more dependable.] Any serious departure from t h e optimum throughput of a given column (especially a packed column) by increase or decrease of rate may seriously impair the column efficiency (HETP); if t h e rate is too fast, it may cause flooding; if too slow, it may fail to supply adequate reflux. The latter condition is especially likely t o result where “product” or “take-off” is maintained a t a constant value by a stopcock setting, or some similar means is used for reflux ratio control.
Manostatic control of heat input to the still pot, by taking advantage of the relationship of column pressure drop t o distillation rate, has been used by Othmer (2), Rossini and Willingham (5), and Selker, Burk, and Lankelma (6). The manostatic liquid (mercury or other conductor) provides the switch mechanism for opening and closing the control circuit to the still-pot heater. Mercury, as the manostatic liquid, can be used advantageously only with columns having appreciable pressure drop. With recent developments in columns having very low pressure drop, mercury is too heavy and lighter liquids, such as are used in the control described by Selker et al. ( B ) , are necessary. When a two-fluid manostatic system is used and the conducting manostatic liquid is an aqueous salt solution, use of the control device is restricted to column operation at atmospheric pressure because of the excessive volatility a t reduced pressure of the water in the manostat. The manostatic control described in this paper utilizes oil alone (mineral oil of very low vapor pressure) as the manostatic liquid. The movement of the meniscus, as a result of
TABLE I. THROUGHPUT
Test No.
r -
Min.
1 26 2 32 92 3 4 132 a Pressure drop as b Obtained at still
-
Difference between No. of Minimum and Total Time Throughput Maximum Elapsed Determinattonsb Throughput
ThroughputA p N0.aMax. AV. Min. Max. Cc./hour M m . of oil Hours 28 30 16 35 301,’~ 291/2 32 2 42 37 40 43 72 100 96 49 52 17 146 139 measured by distance between upper and lower levels of manometric fluid head by counting drops a t varying intervals during test.
Cc./hou 29 34 46 22
9 10 8 14
Deviation from Average Throughput Cc./hour % -4.5 *14.8 *El
*13.5 4.2
-4
i
*7
f
5.0
