ROTARY CONCENTRIC-TUBE DISTILLING COLUMN Charles
B. Willingbarn,
Vincent
A.
Sedlak, and Frederick
D. Rossini
NATIONAL BUREAU OF STANDARDS, WASHINGTON, D. C.
James W. Westhaver UNITED STATES PATENT OFFICE, WASHINGTON, D. C.
This report describes the assembly and testing of a rotary concentric-tube distilling column, in which the rectifying section i s an empty annular space 0.043 inch (1 -09mm.) in width formed b y the inside surface of a stationary outer cylinder and the outside surface of a rotating, closed, inner cylinder. The latter is 2-93 inches (7.44 cm.) in outside diameter and 23.0 inches (58.4 cm.) in length. Results are reported for the number of equivalent theoretical plates (at total reflux) and the pressure drop as determined in experiments made at atmospheric pressure, at speeds of rotation
from 0 to 4000 r.p.rn., and a t throughput values from 600 to 4700 ml. (liquid) per hour. Typical results for a speed of rotation of 4000 r.p.m., calculated per meter length of rectifying section, are as follows: throughput, 3000 ml. (liquid) per hour; pressure drop, 2.0 mm. Hg; number of theoretical plates, 62; holdup, 17.5 ml. (liquid). The calculated value of the efficiency factor (throughput divided b y holdup per equivalent theoretical plate) for the foregoing conditions is about ten times the best values previously reported for other rectifying columns.
OTH theoretically and esperinientally, open tuhes of small diameter, concentric tubes with a sinal1 annular space, or parallel plates with a small space bet,iveen them ran be made t o yield high separating efficiencies in distillation a t suffi-
Tlie prv-cnt report dcscribts the apparatus and givw an account of the espc.rinit.nti and results obtained in 1946 on the rotary concmtric-tube distilling column.
B
cient,ly lon. valuer; of throughput (3, 7 , 9, 12, 14, 1.7, 17;). The theory ( 1 7 .shoii-=that t h e separating efficiency is improvcd tiv making the folloiving changes in t,he three important variable decreasing the distance between opposite interior n-alls of t h e rectifying section (or t h e diameter, if the rectifying scrtion is a cylinderical t ut)e decreasing the throughput,, and increasing the diffusion coefficient in the gas phase. Decrease in the spacing (or diameter) has already heen carried about as far as is practicable. Decreasing the throughput heloiv t h e already lovi values i.- impract,ical. I n a static apparat'us for a given t.eniperature and composition the diffusion coefficient, of t.he molecules in thc gas phase is substantially constant,. Accordingly, in order t,o effect, some improvement in separating efficiency, t h e authors designed an apparatus in which the diffusion coefficient of t h e molecules in t h e gas phase could be increased: T h e gas phase iyas sent into turbulence by rotat'inr the inner closed cylinder in a concentric-tube rectifying section. The original apparatus had a clearance of 0.085 inch (2.16 mm.! tietween the n-alls of t,he rectifying sectmion,which via3 3 inrhw in ' diamet,er; it was designed in t h e latter part, of Dwc~mhc~r 1042 and was assembled in t h e laboratory in May 1943. The preliminary series of tests, complet.ed in 1943, yic,ldrd the following promising results for a speed of rotat,ion of 4000 r.p.m., calculated per meter length of rectifying section: 63 equivalrsnt theoret,ical plates (at tot,al reflux) a t a throughput of 1200 nil. (liquid) per hour, and 32 equivalent theoretical plates (at total reflux) a t a throughput of 3000 ml. (liquid) per hour. The holdup per meter length of rect,ifying section was calculated to be about 18 nil. (liquid') at the higher t'hroughput,. Before the experiment,s were continued, the rotating inner cylinder was made 1&ger by plating on nickel and then machining down so that, t,he clea'rance bet'ween the walls of the rectifying space was approximately half the original value. Because of the pressure of other work, coupled mith the removal of the distillation equipment t o a new laborat,ory, t8heexperiments on the rot,ary concentric-tube distilling column were not, resumed until February 1946. 1,
APPARATUS
Thc, aswnihly of tho apparatus is shown in Figure 1. The rect ifging qcavtion of this rotary concentric-tube distilling column is the empty annular space 0.013 inch (1.09 mm.) in width, formed by thrs insidc >urfare of the stationary outer cylinder and t,he outside surfacc of thr, rotating closed inner cylinder, which is 2.93 inches (7.41 mi. 1 in outside diameter and 23.0 inches (58.4 cm.) in length. Figurc. 2 shows the details of t,he Pyres head and condenser n-ith the manually operated sampling valve, and Figure 3 shows the details of the Pyres pot wit,h t h r re-entrant tube for sampling. TESTING THE COLUMN
Thrb niistur
2O.i
be dewribed as the number of equivalent theoretical platr,x through which the mat,erial being fract,ionated passes in unit time. For this calculation the values of holdup were t,aken t o be those given in Table 11. For the rotary concentric-tube column the efficiency factor changes relatively little with throughput at a given speed of rotation but increases markedly n-ith spcctl of rotation. The curves in Figure 9 marked d-1000, 9-2000, A-3000, and A-1000 show the same results with the efficiency factor as a func4000 tion of t#hroughput> for four different speeds of rotat,ion. r.p.m. the value of t'he efficiency factor is about, 10,000 hour-', approsimately constant with throughput. Figure 9 s h o w al-n the values of the efficiency factor for other rectifying section; as r e p o r t d in the references cited.
In conclusion, the following points ai'e noted: 1. The experimental performance of the rotary concentrictube distilling column appears t o conform to the theory as cloPely as can be expected. 2. The rotation of the inner tube appears to overcome the rivulet channeling of the liquid reflux n-hich occurs at, zero or near zero speeds. 3. The excess (overcalculated) number of theoretical plates observed experiment,ally in the region of turbulent flow of vapor, although not expected, appears to have a reasonable esplnnation on t,he basis of eddy diffusion in the film of liquid reflux. 4. The rotary concentric-tube distilling column has low v:llueh of pressure drop per unit throughput n-hich may be quite adviultageous for distillations a t lon- pressure.
(1) Baker, R. H., Barkenbus, C., and Roswell, C. -4., ISD. 1:s~;. CHEXf., -4S.kL. ED., 12,468 (1940). (2) Bragg, L. B., IXD. ESG. CHEJf., 33, 279 (1941). (3) Bragg, L. B., ISD. ESG. C H E l f . , . ~ N A L . ED.,11, 283 (1889). ( 4 ) Brandt, P. L., Perkins, R. B., Jr., and Halverson, L. K., Oil Gas J . , 45, 86 (1946). ( 5 ) Bruun, J. H., and Faulconer, IT. B. X i , , ISD. ESG. CHEY., -1s.t~. ED., 9, 192 (1937). ( 6 ) Collins. F. C.. and Lantz. V..Ibid.. 18. 673 (1946). ( 7 ) Donnell, C. K., and Kennedy, R. Xi., Proc. A m . Petloleitm Inst., 26 (III), 23 (1946). (9) Friedman, S. J., and Miller, C . O., ISD.EXG.CHEY..33, S85 (1941). 1!4) Hall, 6 . -I., and Palkin, S., IND.EXG. CHmr., .~I,.IL. ED., 14, 807 (1942). (10) Kuhn, IT., Helz. C h m . A c t a , 25, 252 (1942). 111) Lecky, H . S., and Emell, R . H., IND.ESG. CHEM., .Ira~. I,:u., 12, 544 (1940). f 12) Naragon, E. .4., and Lewis, C. J.,Ibid., 18,448 (1946). ( 1 3 ) Podbielniak, TT. J., Ibid., 13, 639 (1941). 114) Rose, d.,ISD. ENG.CHEM., 28, 1210 (1936). 15) Selker, SI. L., Burke, R. E.. and Lankelma, H. P., ISD. ESG. C H E U . , - k i h L . ED.,12,352 (1940). (16) Taylor, G. I., Pioc. Roy. Soc., A151, 494 (1935). 17) TTesthaver, J. IT.. ISD.ESG. CHEM.. 34, 126 (1942). (IC;) Kesthaver, J. IT., J . Research S a t l . Bur. Standards, 38, 169 (1947). (1:)~ 11-hitmore, R. C. Fenske 11. It., Quiggle, D., Bornstein, H., Carney, T. D., Lawroski. S., Popkin, .4.H., Wagner, R. B., Wheeler. V~ R., and Whitaker, 6.S., J . Am. Chem. S O C . , 62. 795 (1940). f.20) Willingham, C. B., and Rossini, F. D.. J . Research S a t ! . B U T . .Standards, 37, 15 (1946); R.P. 1724. ~I
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PRESESTED a i p a r t of t h e T h i r t e e n t h Annual Chemical Engineering Symposium of the Division of Industrial a n d Engineering Chemistry. AYERIC.AN C H E M I C ASOCIETY. L This work was performed as p a r t of .4merican Petroleiim I n s t i t u t e Research Project 6 a t t h e Xational Bureau of S t a n d a r d s on rhe analyai., purification, a n d properties of hydrorarbons.
EFFlCIENT PAC.KING FOR RECTIFYING COLUMNS H. 0.M c M a h o n A R T H U R D. LITTLE, INC., C A M B R I D G E 42. M A S S
HE work described in this paper had as its objective thc tlesign of a small portable liquid air distillation plant, IThicli could be used t o supply the oxygen requirements of a bomber crew during lengthy flights at, high altitudes. One of the problems associated with this project was t o produce a rectifying column, the height and diameter of which ~ i - t r eto be kept t o :I minimum. Furt.her requirements were very light weight and Ionheat capacity to minimize the time required to cool the column to liquid air temperatures. The scope of the present paper is limited to a description of the column packing finally chosen as being the most, suitable for the air distillation application.
T
D E S C R I P T I O N OF P A C K I N G
Preliminary tests indicated t h a t quarter-inch ceramic Berl saddles were more efficient as column packing than any of the
otlic*r ('oninion materials.
;\c.c-oidingly, it \vas assumed that saddlcv .tamped from thin sheet mcbtal or ivire cloth ivoultl retain the advant,agcous properties of ceramic saddlei and would have. the additional advantages of low w-eight and small hrat capacity. This proved t o be true, and, after several different -izes of saddles from several different materials had h e m staniptd out and tested, a packing \T-as finally obtained which had considerably better H.E.T.P. (height equivalent to a theoretical plate'i at substantially higher throughputs than did the quartc~rinch ceramic saddles. The saddles finally settled upon x e r c stamped from squares of ', inch, 100 X 100 mesh wire cloth. T h e material was I x brass, and the wire diameter was 0.0045 inch. It is not to be assumed that, this part.icular wire diameter, mesh, and .saddle size necessarily constitute the hest choice. Becausc of t>ho
