Variable-Control Stillhead for Laboratory Columns S. C. ROTHMANN, 6 Gates Place, Charleston, W. Va.
T
HE apparatus herein described was developed largely
from practical experience and research in analyzing the various synthetic solvent derivatives of pentane in the laboratory of the Sharples Solvents Corporation at Belle, W. Va. Data as to these products are found in Tables I and I1 of Clark (3). I n the manufacture of these pentane derivatives, a rigid a k m e n t between control laboratory and the plant is essential. Rather stringent periodic checks and analyses were run on samples brought into the control laboratory during the various stages of the process. since all stages of the process are interdependent, the analysis of samples was carried ~ ~ ~ of ~ accuracy. The on frequently with a W ~ F J X Idegree bulk of these analyses involved Engler distillation runs, comPleX fraCtiOnatiOnS, Simple Volumetric titrations, Specific gravity measurements, and other miscellaneous control tests. I n the search department, a greater degree of accuracy Was desired and the fractionation methods of the control labora-
FIGURE1. DIAGRAM OF STILLHEAD tory were found to be inadequate for obtaining plant and sales specification data. One of the most desired specifications in solvent or lacquer formulation is obtained from the study of the boiling ranges. There are two possible methods: simple Engler distillation using no fractionation, and more complete fractional distillation using a high rate of reflux, repeating the fractionation until constant results are observed where an exact quantitative analysis is desired. By repeating fractionations several times, a good separation between substances of close boiling points-e. g., carbon tetrachloride, boiling point 76" C., and benzene, boiling point 80" C,-can be effected.
I n attempting to make exact quantitative separations, various types of columns were used and found to present manifold difficulties in design, control, and operation. The most obvious difficulty in using the ordinary columns was that of obtaining accurate thermometer readings a t the vapor product take-off which would not be subject to changes resuiting from the close proximity of the cold reflux as i t returned from the side arm, As most commercial stillheads made no provision for the correction of this difficulty, the writer set out to design a stillhead that would not only remedy this condition but also would lend itself to variable control and further increase the efficiency of the column. From the available literature of the past ten years (1-35), the writer found that, whereas an unusually large amount of excellent work had been accomplished in designing equipment for laboratory fractional distillation with especial emphasis on columns and their design, little attention has been given to designing adequate stillhead control equipment. Because there is a marked similarity in the essential requirements for stillheads and for laboratory columns, the writer drew freely upon this literature for theoretical and practical guidance. Many features of column design can be and some are incorporated in this design for an efficient stillhead. The primary function of a fractionating column is to enrich the volatile components in the vapors arising from a still and to effect their separation by allowing them to interact with condensate originally produced by the partial condensation of the first vapors evolved. Efficiency is measured in terms of separating power of a definite unit of column length, while capacity is the measure of the quantity of vapor and liquid that can be passed countercurrently to each other in the column without priming or loading it. Efficiency is proportional to a factor expressing the time of contact between liquid and vapor. Differences in composition of the liquid phase in equilibrium determine whether a fractional separation of the mixture can be made, If there is no obvious difference, separation cannot be effected; if the difference is great, the separation is easy. The most effective method of obtaining a partial separation by fractional distillation is by passing the vapor through a countercurrent scrubber in which it is washed or scrubbed by liquid refluxed down the scrubber. The sharpness of separation under these conditions is dependent upon intimacy of contact of vapor and liquid in the column, thermal insulation of the column, and ratio of reflux to distillate. Too great a hold-up of reflux in the column materially decreases the sharpness of cuts. This difficulty can be remedied by making adequate provision for the free return of liquid and the free entrance of vapors into the column. The essential control of reflux usually depends wholly upon constancy of wall temperature obtained by controlling loss of heat through the walls. Efficiently controlled temperatures of the vapors leaving the top of the column are also of utmost importance in attempting fractionation where the regulation of the ratio of ref3ux to distillate is desired. A cooling agent functions here either to increase or decrease the refluxing action upon the vapors. The wide difference of opinion on the relative efficiencies of laboratory columns is believed to result from using such columns under different conditions of temperature, rate of distillation, etc.
338
September 15,1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
The trend in column designing today is to produce all the refluxing or condensation at the top of the column in order to obtain the benefit of the total quantity of reflux through the entire length of the column.
DESCRIPTION OF APPARATUS This design for a variable control stillhead embodies universally known principles which, however, have not been heretofore utilized in a similar manner. The underlying principle of this apparatus consists in providing for reflux by partial condensation of vapors and for complete and easy control of the reflux ratio. Partial condensation of the vapor has the same effect on its composition as has redistillation. The combination of both partial condensation and rectification with simple distillation, here used, affords an efficient means for separating volatile liquids. The stillhead consists of the following essential parts: heavywalled Pyrex jacket, dephlegmator tower, partial condenser, fluid control stopcock, controlled va or take-off, and other miscellaneous accessories. The jacket shown in the illustration is made of heavy-walled Pyrex tubing 5 cm. (2 inches) in diameter and 17.5 cm. high. It is rounded a t the top advisedly to afford an ideal deflecting surface for the vapors. To cut down the radiation loss and diminish the draft effects in order to make this jacket as nearly adiabatic as possible, the exterior can be lagged with magnesia lagging or pipe covering, asbestos tape or twine, or the exterior can be covered with ordinary electrician’s tape and coated with varnish afterwards. If a greater degree of accuracy is desired, the exterior can be silvered or, better yet, encased in an air-tight jacket. The de hlegmator tower A is made of ordinary 2-cm. Pyrex tubing agout 24 cm. in height. T o allow a free flow of vapor from the column the sides of this tower A are perforated with small orifices, E, about 1 mm. in diameter. These orifices are easily made by heating spots on the sides of the tower and puncturing these heated areas with a needle. The orifices below the condenser line, G, are intentionally made a little larger, 2 or 3 mm. in diameter, to permit the evolution of a greater amount of vapor for washing through the reflux collected in the lower part of the jacket D, which for all practical purposes serves as a secondary scrubber. De hlegrnator A is enlar ed somewhat a t I , providing for a fluif catch chamber t o tafe care of the excessively large surges of reflux returning to the column. The lower portion of the tower-that is, the part entering the fractionating column-is tapered somewhat to serve as a drip indicator and further perforated with larger vent holes, K , through which the vapor flows unhampered by the reflux returning to the column. This provision is made to eliminate the possibility of flooding the tower by the trapping of vapor. The joint assembly a t J fits securely into the top of the ordinary laboratory fractionating column and can be fitted with any convenient standard interchangeable ground-glass connection. The gasket cushioned flanged ends devised by Othmer (92) may be substituted advantageously here. The joint as shown in the illustration is about a No. 15 size. For effective partial condensation of the vapor, a large area of condensing surface is provided for by the partial condenser C which has a condensing medium entrance at F and outlet a t B. This condenser C is blown from ordinary soft glass tubing about 4 mm. in internal diameter. For liquids boiling below 70’ C.-for example, amylene, isoamylene, or the pentanes-water is used as the condensing medium. A delicate re ulation of the water supply can be obtained by using a needle v3ve or the ordinary laboratory screw clamp. For high-boiling liquids, kerosene or a regulated amount of air can be passed through the condenser instead of water. To indicate the temperature of vapor as product a glass-jacketed Fisher organic thermometer L fitted with a standard interchangeable ground-glass joint M is fixed in the thermometer welt N . The vapor take-off 0 is made of ordinary heavy-walled glass tubing of about 2-mm. bore to avoid excessive hold-up a t its junction with the thermometer well. Q represents a one-way ground-glass stopcock, preferably of the hollow t e to allow for expansion changes a t high temperatures. To stationary member of stopcock Q is attached a calibrated protractor chart P over which moves a needle, attached to the male member by means of a V grooved inside the handle of the male member. With this arrangement, taken from Leslie and Geniesse ( I C ) , the cock can be opened accurately to any desired degree in the regulation of the vapor flow. In order to minimize vapor leakage a t the take-off 0, it is advantageous to use a stopcock which cannot be dislodged
8
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339
easily under normal operating conditions. An excellent stopcock for this purpose is the one designed by Evans (11). He writes: “A hole is ground in the protruding end of the plug, in such a position that a small glass collar may be held firmly a ainst the shell of the stopcock by means of a rather s t i i iece of piano wire. The area of contact of the shell and small colrar is $ound and greased to permit ease of manipulation and the other ace of the collar made concave to cause a small amount of tension in the wire ” The vapor take-off R can be connected to a product condenser of the Liebig t pe. To avoid loadin of the stillhead by the accumulation ofytoo much reflux a t thefower portion or scrubber, adequate drain orifices are located at H and S. These orifices are pushed inward to drain off the excess liquid collected. The reflux take-off B leads to a standard three-way ground-glass stopcock T,which serves either as a re ulator of the flow of reflux or as a sample take-off. With &ght revisions of the ap aratus, T can be arranged to charge reflux either back to the coirnn or back to the original still as desired
CONCLUSION A rather crude working model of this apparatus without the later refinements was experimented with in the Sharples laboratory and was found to function very well in meeting the problem presented. The present design offers a number of experimental possibilities which the writer hopes t o develop and elaborate upon later. With a few slight revisions, it may be utilized in the distillation of isomers, under reduced pressure. Aside from the control advantages which it offers, insuring uniform rate of distillation, it also possesses advantages in simplicity of design and construction, and absence of metals, corks, and rubber stoppers, thus eliminating corrosion or solvent problems. It also makes possible an even and uniform distribution of the reflux over the top of the column filling, which is conducive to the efficient operation of a relatively short fractionating column. ACKNOWLEDGMENT The writer wishes to acknowledge valuable aid and suggestions received from L. H. Clark, H. A. Bohall, and R. F. Fieck, all of the Sharples Solvents Corporation, who greatly encouraged him by their very willing cooperation.
LITERATURE CITED (1) Brown and Caine, “Studies in Distillation. I,” Part 2, Trans. Am. I n s t . Chem. E n g . , August, 1928. (2) Calingaert, G., and Huggins, F. E., Jr., IND.ENG.C H ~ M 16, ., 584 (1924). (3) Clark. L. H., Ibid.. 22. 439 (1930). (4) Clarke, H. T., and Rahrs, E. H.,‘Ibid., 15, 349 (1923). (5) Ibid., 18, 1092 (1926). (6) Cooper, C. M., and Fasce, E. V., Ibid., 20, 420 (1928). (7) Davis, W. S., and Dougherty, J. P., Ibid., Anal. Ed., 2, 193 (1932). (8) Doran, C. A,, Ibid., 5,101 (1933). (9) Eddy, C. W., Ibid., 4, 198 (1932). (10) Egloff, G., andLowry, C. D., Jr., IND. ENG.C H ~ M21,920 ., (1929). (11) Evans, R. N., Ibid., 24, 856 (1932). (12) Hausbrand, “Principles and Practices of Industrial Distillation,” Wiley, 1928. (13) Hill, J. B., and Ferris, 8. W., IXD. ENG.CHEIM., 19,379 (1927). (14) Leslie, E. H., and Geniesse, J. C., Ibid., 18, 590 (1926). (15) Lewis, W. K., Ibid., 14, 492 (1922). (16) Lewis, W. K., and Robinson, C. S., Ibid., 14, 481 (1922). (17) Loveless, A. W. T., Ibid., 18, 826 (1926). (18) McCullough, R., and Gittings, L. D., Ibid., 22, 584 (1930). (19) Morrell, J. C., and Egloff, G., Ibid., 19, 1292 (1927). (20) Nason. E. H.. Ibid.. 15. 1188 (1923). (21j O t h e r , D. F.,Ibid., 20, 743 (i928)’. (22) Ibid., 22, 322 (1930). (23) Ibid., Anal. Ed., 4, 232 (1932). (24) Peters, W. A., J. IND. ENO.C E ~ M14, . , 476 (1922). (25) Peters, W. A., Jr., Ibid., 15, 402 (1923). (26) Ibid.. 15, 734 (19239. (27) Peters, W. A., and Baker, T., Ibid., 18, 69 (1926). (28) Podbielniak, W. J., Ibid., Anal. Ed., 3, (1931). (29) Ibid., 5, 119 (1933). (30) Podbielniak, W. J., and Brown, G. G., Ibid., 21,773 (1929).
ANALYTICAL EDITION
Vol. 5 , No. 5
Ed., 3, 138 (1931).
(34) Wilson, M. M., and Wooster, F. J., IND.ENQ.CHEM.,21, 592 (1929). (35) Young,‘S., “Distillation Principles and Processes,” Macmillan, 1922.
Leipzig, 1928.
RECEIVED May 8, 1933
Robinson, C. S., “Elements of Fractional Distillation,”
McGraw-Hill. 1922. Schwartz, A. M., and Bush, M. T., IND.ENQ.CHEM.,Anal.
Thomann, Kurt, “Destillieren und Rektifizieren,” Otto Spamer,
Method for Determining the Dustiness of Coal and Coke A. R. POWELL AND C. C. RUSSELL,The Koppers Research Corporation, Pittsburgh, Pa.
T
HE treatment of s o l i d
bottom slide is 61 cm. above the Dustless coal and coke for domestic fuel have fuels to allay dust a t the bottom and serves to collect the been a subject of iniense interest in the sale of time of delivery, and also d u s t a f t e r t h e fuel has been fuel. The rapid growth of interest and use of dropped. Guides are placed so to prevent dust after the fuel fuel treatment to allay dust led to the introduction that the slides move easily. A dries out in the householder’s of many materials for this purpose. The drawer 30.5 em. deep is located bin, was introduced several years ago. A rapid growth of interest at the bottom of the apparatus, necessity of a quantitative method was soon and provides a convenient means by fuel merchant a n d p r o recognized for the determination of the dustiness of removing the fuel after the ducers ensued, r e s u l t i n g i n a of solid fuels and also for comparison qf the very general adoption of some test is completed. The entire eficacy of the various materials suggested for a p p a r a t u s is made of fairly form of fuel treatment. Calcium treatment. A method and apparatus were heavy sheet metal and so conchloride was one of the materials s t r u c t e d that it is dust-tight. suggested for the purpose, but consequently developed. The top slide is made of sheet a variety of other materials has While the test is empirical and results obsteel and the bottom slide of also been proposed and offered tained with treated fuels must be compared with rather thin sheet polished on one to the trade. Spray manufacblanks of the untreated fuel sampled at the same side-for example, stainless steel turers have also interested themtime, the usefulness has been proved through use or chromium-plated brass. The selves in developing equipment inside surface of the apparatus for the small coal yard as well over a period of years. Data obtained by this is smooth from the top to the as for larger producers and dismethod are presented. lower slide. tributors. The Drinciple of fuel t r e a t METHODOF MAKINGDETERMINATIONS ment i&olvek spraying with a solution or emulsion of some material which has the property of holding dust permanently The top compartment of the box, made by the insertion of the to the individual pieces of fuel, or to agglomerate the dust so upper slide, holds the sample of fuel to be tested. A sample of that it will not rise when the fuel is handled. Calcium chlo- about 25 kg. is placed in this compartment and the cover closed It has been found more satisfactory to weigh the sample ride solutions are used because of the deliquescent properties tightly. as taken rather than try t o obtain exactly 25 kg. The lower of that salt. Other materials, such as oil, modified waste (polished) slide, which has been brushed clean, is then placed in sulfite liquor, emulsions, etc., are also utilized. Any material the guide extension in position for insertion. When ail is in readiness for the test, the up er slide is withdrawn with one quick used for fuel treatment must be both effective and cheap. motion, thus allowing the Fuel t o drop into the drawer. ExThe problem of treating fuel resolves itself into selection actly 6 seconds later, as indicated by a stop watch, the lower of a material which will allay dust, selection of a satisfactory slide is inserted and allowed to stand undisturbed for 2 minutes. method of application, and determination of the quantity of At the end of that time the slide is removed and the dust which has settled on it is carefully brushed into a weighing bottle and material necessary to make the fuel substantially dustless. weighed on an analytical balance. The weight of dust obtained It is evident that some method of quantitative determina- is reported in gram8 per metric ton of fuel. tion of the dustiness of fuel both before and after treatment In addition to the above procedure, it has been found desirable is necessary to make adequate comparisons of the effective- in some cases to determine not only the dust which will settle ness of various materials, quantities of treating agent, and in 2 minutes but also that which will settle in 10 minutes. This is accomplished by the use of two slides at the bottom. Both methods of application. The Koppers Research Corporation slides are inserted together 5 seconds after the fuel is dropped, has developed a method which has been in use since 1928, and 2 minutes later the top one is withdrawn. The second slide and, although it is empirical, it is believed that the results is withdrawn 8 minutes later. The ratio of the two values gives a size-characteristic of the dust and is important in determining obtained are satisfactory. the efficacy of various treatments.
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DESCRIPTION OF APPARATUS The method involves dropping a weighed quantity of fuel into a closed container and allowing the dust so raised to settle on a polished metal plate so that it can be removed
The dust tests should be made in a room free of strong drafts, so that no dust will be blown out of the sample or apparatus, and so that the settled dust can be transferred to the weighing bottle without loss.
and weighed. The apparatus consists of a box 1.52 meters high and 45.7 em. square, arranged with a cover and having two horizontal slides inserted from the side. The top slide is 30.5 cm. below the top and, when inserted, makes a compartment in which the fuel to be tested is placed. The
I n sampling fuel for the dust test, extreme care must be taken so that the normal amount of dust will be retained. It is obvious that the fuel can be tested as sampled with the
SAMPLING
