Eng$nyring Process development
I
IN DISTILLATION COLUMNS OPERATING AT REDUCED PRESSURE
MARY MYLES, JULIAN FELDMAN, IRVING WENDER, SYNTHETIC LIQUID FUELS BRANCH, BUREAU
T
AND
MILTON ORCHIN
OF MINES, BRUCETON,
PA.
was measured and contiolled T h i s work was undertalten to study the effect of pressure by B manometer connectrd on the efficiency of paelced distillation columns. The paclcas to compare the pera t one end to the pot through ings used were Rerl saddles, tripleand single-turn helices, fornianre of different types the adapter, J , and a t the glass spheres, and a commercial PIeli-grid column packing. of packings commonly used other end to the top of the Fractionating efficiency, throughput, and holdup were in laboratory d i s t i l la t i on rolumn through D. >Ierdetermined at various reflux rates and at various pressures rury was used as a manomc o 1u m n s . E f f i c i e n c y , eter liquid. T h e operating from 20 to 760 m m . In general, effiaiencies decreased with throughput (reflux rate), and pressure was measured and decreasing pressure at operating pressures below about liquid holdup were detercontrolled by a Podbielniak 200 nim. The fixed I-Ieli-grid packing was superior to all mmed for each packing in a manostat actuating a the loose packing tested and of the latter, glass spheres t h r o t t I e d - d o wn solenoid standard 1 - i n c h r o l u m n gave the best results. Although efficiency decreased with which maintained the desired The packings tested were 1/4preasure to *0.5 mm of increasing throughput for the Heli-grid packing, the remch Berl saddles, triple-turn mercury. Heat to the pot verse effect was observed with most other paclcings. was supplied by a Glah-Col glass helicps I/4 inch in diamThe results of the investigation indicate that columns heating mantle which wah eter, single-turn glass filled with the loose paokings studied should be operated at controlled by a Variac An lielices 1/1 inch in diameter, internal bare-wire coil in t h r close to maximum throughput capacity, and that these pot provided a hot spot, in glass spheres 0.143 inch in packings may operate at highest separating power at the boiling liquid and ensured diameter, and commerrial pressures of about 200 m m . smooth ebullition. H e l i - g r i d packing. Theae The void mace of each tvue packings were used because of packing b a s determiyiQd f~om the weight, and density of the paeking and the volume of t h r of their popularity and availabilit,y. Tests were conducted a t packed section. The void space values of the various packings atmospheric pressure, and a t pressures of 150, 100, 50, and xre shown in Table I. 20 mm. of mercury. n-Heptane and methylcyclohexane test mixture was used for atmospheric evaluations. n-Dodecant. TEST PROCEDURE and cyclohexylc~yclopentanetest mixture ( 5 ) was used a t reduced DETERMINATION OF CoLmix EFFICIENCY. Efficiencies at pressures. atmospheric pressure were determined by using the n-heptanrAPPARATUS methylcyclohexane test mixture with an a value of 1.083 ( 7 ) The equipment used in the tests consisted of a column 25 For reduced pressure measurements the n-dodecane-ryc.lo?im. in diameter and 3.5 feet long, filled with the various packhexylcyclopentane mixture (6) was used. ings, a pot, a condenser, a sampling apparatus, and various instruments for controlling the operation. A diagram of the Five hundred milliliters of test mixture were boiled in a 1-litel arrangement is shown in Figure 1. flask. When the vapors reached the reflux head the heat w t i 5 The column was made of g1a.ss and was vacuum-jacketed. gradually increased and the column slowly hooded. Aftei The vacuum jacket contained a metal radiation shield, G , and flooding, the heat was gradually reduced and adjusted so that an expansion bellows, F . A perforated glass plate, E , was the desired differential pressure was secured. In this way, thc sealed in above the packing section (constructed by the W. J. packings mere wet from the bottom up and did not have a charire Podbielniak Co., Chicago, Ill.). In order to pack the column; l o dry out. The mixture was allowed to reflux for a t least 4 the reflux head, A , was removed and the column closed with a hours a t the desired pressure differential (back pressure). Vapor st,opper and turned upside down. The column was then filled samples were removed via sampling device L (Figure I ) and liquid Ivith mineral oil, and the packing to be investigated was dropped feed from the pot via sampling device K. These sampling through the oil. To ensure even distribution, each piece of systems were rinsed with 0.5-ml. portions of the sample before packing was dropped singly. The removable conical wire screen samples were taken for analysis. Overhead vapor samples support, H , was put in place by squeezing i t t,hrough the narrower were taken a t the end of the test periods by raising the valvr, H, portion of the column. It then rested on the tapered shoulder with a solenoid for 1-second periods a t 60-second interval., at, the bottom of the packed Recbion. The column was then while L was maintained a t the same pressure as the head ot the inverted to its normal position, and the oil was drained and comcolumn. Efficiencies were then determined by the Ii'enske plet,elp removed by refluxing with acetone. The acetone wss equation ( 6 ) , using the appropriate value for relative v01atilit~. drained and the packing carefully dried with air. This procedure was used with each type of random packing, but in addition a second method of packing was used with glass spheres. In this method, the beads were dropped individually COLUMNS TABLE I. VOIDSPACEIN PACKED into the dry, inverted column wit.h continuous tapping on the outside with a wooden block. This resulted in a denser packin Void Sliam. Type of Packing 5% arrangement and is referred to hereafter as tightly packef spheres. Heli-grid 80 (9) Berl saddles. '/r-inch uorcelain 57.5 For purposes of comparison, a commercial Heli-grid column, 92.9 25 mm. in diameter and 3 feet long, was included in the test 84.6 work. 42.8 48.6 Vapor temperature was measured by means of a thermocouple, Cy, in the reflux condenser. Pressure drop through the packing
HE purpose of this stud?
1452
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1453
.
Figure 2.
Effect of Reflux Rate on Efficiency at Atmospheric Pressure
ferential. From these curves, reflux rates a t any back premure could be interpolated. DETERMINATION OF HOLDUP. A general method for determining dynamic holdups is to determine the change in pot concentration when a solution of an involatile solute is distilled in a volatile solvent. Fenske used stearic acid in bensene to determine holdups a t atmospheric pressure. In the present work, stearic acid in dodecane was found t o be suitable for determining holdup in vacuum column distillation. When heated in a one-plate still
Figure 1. -4diabatic Column for Testing Distillation Efficiencies of Loose Packing Materials
DETERMINATION OF REFLUXRATE. Reflux rates (boil-up Tates) were determined by means of a rate device, Z (Figure l), placed between the pot and the column ( 3 ) . The values for the reflux rates were obtained at each system pressure by measuring the weight of reflux liquid returning to the flask in an interval of time (usually 0.25 to 0.5 minute) short enough not to disturb the equilibrium in the column. The sample container was kept a t the same pressure as the distillation pressure. Rates were determined under vacuum using n-dodecane as the liquid, and atmospherically using n-heptane. Curves were obtained by plotting, for each packing a t each overhead prpssure, the reflux rate value against the pressure dif-
Figure 3.
Effect of Reflux Rate on Efficiency at 150-Mm. Mercury Pressure
1454
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 6
36
TABLE 11. HOLDCP OF TESTC o L m i N Packing Hell-grid Spheres (tightly packed) Spheres (loosely packed) Single-turn helices Berl saddles Triple-turn helices a
Holdup, Grams of Dodecsne, at 50 100 150 740 nim. mm. mm. mm. mrn. 12-85 26-73 30-78 35-82 41-52O 37-53 27-38 30-42 33-48 36-54 33-54 23-36 25-39 27-46 23-49 24-53 . . . 34-61 17-33 . . . 31-42 30-44 23-30 24-33 15-21 25-33 9-17 ... 24-26
32
20
28
24
Grams of toluene. Spheres, loosely
20
a t a pressure of 30 mm. of mercury, less than 0.6% of the stearic acid, in a 5% weight mixture, was found in the distillate.
-+ 16
In order to secure a large change in pot concentration, 200 grams of a 7 % by weight solution of stearic acid in dodecane were used. The column was started as usual, flooded, and allowed to reflux for about 1 hour. A small sample was then removed from the pot, through the takeoff tube. The sample was weighed and titrated with approximately 0.1 N sodium hydroxide. Eighty milliliters of isopropyl alcohol were added to each sample to dissolve the sodium stearate that was formed during the titration. Holdups were determined a t the desired distillation pressures and reflux rates for each packing. Plots of the holdup versus back pressure gave parallel straight lines for the different pressures. The range of holdups from minimuni to maximum reflux rates a t each pressure is given in Table IT.
Triple-turn
0
~~
200
Figure 5.
helices
400 R A T E , 6G0R0A M S C 8 i020 PER 1,000 HOUR
l,POO
1,400
Effect of Reflux Rate on Efficiency a t 50-Mm. 31Iercury Pressure
flus which are the mean of the sloncst rate a t which reflux a t the condenser waR observed and the highest rate just below the flood rate. This reflux rate was secured by taking half the sum of the rates a t minimuni reflux and maximum reflux. This method of expression was chosen because the more familiar practice of plotting plates against one half the flood rate could not be used in some cases, inasmuch as the authors could not get any reflux with some columns a t one half the flood rate. The curves of Figure 7 are especially noteworthy. These show that with all packings, greater separating power of components is achieved in the pressure range 150 to 200 mm. than a t pressures below or above this value. This point needs further experimental verification, because data in the pressure range 150 to 750 nim. are lacking and the data a t 760 mm. were secured A ith the heptane-cyclohexane mixture, while all other points represent data obtained R ith the dodecane-cyclohexylcydopentane mixture. The heights equivalent t o a theoretical plate or H.E.T.P. values for the various packings a t each of thc pres-
48
40
- 32 +
24
16
0
12
LOO
400 600 800 1,000 R A T E , G R A M S Ci2 PER HOUR
1,200
1,400
20
Figure 4.
Effect of Reflux Rate on Efficiency a t 100-Mm. Mercury Pressure 16
RESULTS A h D DISCUSSION
The pressure drops for each type of packing a t various reflux rates and various pressures were determined and are shown in Table 111. The lowest pressure drop a t all rates and all operating pressures was secured with the three-turn helices; the glass spheres gave the highest pressure drop a t all pressures and all reflux rates. The latter fact would make the spheres unsuitable for tall columns where the large drop in presaure would increase the temperature necessary in the pot. The performance of the packed columns operating a t various pressures and rates is summarized in Figures 2 to 6. In Figure 7. the total number of plates, ( n 1) derived from the Fenske equation, is plotted against distillation pressure a t rates of re-
+
-+
12
8
0
200
Figure 6.
400 600 800 1,000 R A T E , G R A M S C l e P E R HOUR
1,200
1,400
Effect of Reflux Rate on Efficiency a t 20-Mm. Mercury Pressure
INDUSTRIAL AND ENGINEERING CHEMISTRY
lune 1951
1455
1,60C
1.40C
1,eoc a
2 I.CQC h > 80C
z
w
2 6OC w Y
40C
eoc
4FT3zEE3 T r i p l e - t u r n helices
0
100
200
T300 O T A L 400 PRESSURE 500
600
700
800
Figure 7. Efficiency of Different Packings as Function of Distillation Pressure at Average Reflux Rates
0
Figure 8.
Efficiency Factors of Different Packings
as Function of Distillation Pressure at Average
Reflux Rates
CONCLUSIONS
The Heli-grid packing is the most efficient of the packings investigated in this study. It is superior a t all pressures studied (20 to 760 mm.). Of the random packings studied, the glass spheres have the lowest H.E.T.P. at all pressures. The H.E.T.P. of the Heli-grid packing increases with increasing reflux rate a t all pressures. However, the H.E.T.P. of the glass spheres packing decreases with increasing rate at all pressures. The H.E.T.P. of single-turn helices and Berl saddles also decreases slightly with increased rate a t all reduced pressures, while the H.E.T.P. value with triple-turn helices remains essentially the same with increased rate a t all pressures. There are indications that there is a pressure, probably around 200 mm., which is optimum for securing the maximum separating efficiency with all types of packing used.
.L
a
sures studied are given in Table Is’ for operations a t maximum separating power and a t mean rates. Berg and Popovac reported ( 1 ) that a 2-foot 1-inch column packed with ’/S-inch helices and operating a t reflux rates just below flooding gave efficiencies varying from eleven to fourteen plates at pressures from 20 mm. to atmospheric. They considered this scatter to be within experimental error and concluded t h a t column performance is independent of operating pressure. Although this condition may hold for the particular column tested, the authors feel that columns more sensitive to operating variables such as throughput and reflux ratio would follow the trend indicated in these experiments and predicted by ’ Byron, Bowman, and Coull (4). Struck and Kinnev ”~ (10) comoared the efficiencies of a 0.75inch column when operated with several different packings and showed that there was a small change in efficiency with operating pressure from TABLE 111. PRESSURE DROPAND REFLUX RATES FOR VARIOUSPACKINGS 10 mm. t o atmospheric with a maximum in effiAT DIFFERENT PRESSURES ciency at about 100 mm This is more or less (Grams per hour) in agreement with the authors’ work, even though 20Mm. 50 M m . 100 Mm. 150 Mm. Atm. the standards of comparison are different. Struck Packing Min. Max. Min. illax. Min. Max. Min. Max. Min. Max. and Kinney used the same reflux rate throughout Heli-grid 5 P.D. ... ... 28 5 28 6 27 1 27 for comparison, whereas the authors selected R.R. . . . . . 1200 1650 650 1600 550 1100 500 5500 P.D. 6 26 a 32 35 6 Loose spheres 5 32 3 27 what they believed t o be more nearly equivalent 900 700 1150 1000 1400 R.R. 450 700 500 125 2500 31 3.5 Tight spheres P.D. 30 30 4 3.5 4 35 5 25 conditions. The selection of equivalent condi620 900 800 1170 1000 1300 . . . 380 720 R.R. ... tions is, no doubt, open t o debate, for such factors 2 Berl saddles P.D. 8 1 2 8 ... ... 2 8 8 R.R. 450 800 600 1100 . . . . . . 900 2200 2600 4800 as vapor velocity, mass velocity, surface tension, Three-turn 1 5 1 5 ... P.D. 1 ... 5 1 helices R.R. 450 900 800 1400 . . . . . . 1000 2200 1200 1600 and viscosity all affect efficiency and are modified 14 . . . Single-turn 2 14 3 P.D. , . . 3 2 18 by the temperature, pressure, and pressure drop helices , 970 1200 . . . 1450 2000 2400 450 900 R.R. of the distillation operation. The over-all efficiency of a fractionating column TABLEIV. MINIMUMH.E.T.P. VALUESFOR PACKIXGS (INCHES) does not depend on “resolving power” alone but Threeis also dependent on reflux rate and holdup. The HeliLoose Tight Berl Single Turn grid Spheres Spheres Saddles Helices Helices “efficiency factor” used by Bragg ( 2 ) and Podbiel20 mm.a c2.0 2.3 2.3 4.2 3.8 9.2 niak (8) is accepted as a measure of over-all ... 3.2 At mean rate a t 20 rnm. 3.2 5.2 4.1 9.2 50 mm.n 1.1 2.0 2.2 4.2 3.8 9.2 column efficiency. The efficiency factor is ob2.6 4.7 4.0 9.2 At mean rate a t 50 mm. 1.4 2.3 tained by dividing the throughput in grams per 100 m m Q 0.71 1.7 1.9 ... ... ... A t mean rate a t 100 mm. 1.2 2.5 2.6 ... ... ... hour by the holdup per theoretical plate in grams 150 mm.a 0.55 1.4 1.6 3.2 3.1 7.7 At mean rate a t 150 mm. 1,2 2.2 2.2 3.8 3.6 7.7 per plate, and may be expressed in terms of number Atma 0.36 2.2 ... 4.7 4.2 7.0 of theoretical plates per unit time. Figure 8 At mean rate a t atm. 2.4 3.0 ... 4.7 4.2 7.0 relates change in efficiency factor at the mean rate, At maximum separating power. with distillation pressure. .
I
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1456
LITERATURE CITED
Vol. 43, No. 6
Fenske, M. R., Ibid., 24, 482 (1932) Griswold, 35i 247 (1943). Podbielniak, W. J., Ibid., 34, 126 (1942). Podbielniak, W. J., private communication. (10) Struck, R. T., and Kinney, C. R.,IND. ENG.C H m f . , 42, 77 (6) (7) (8) (9)
(1) Berg, L., and Popovac, D. O., Chem. Eng. Progress, 45, 683 (1949). (2) Bragg, L. B., IND.ENG.CHENI., ANAL. ED.,11, 283 (1939). (3) Ibid., 15, 290 (1943). (4) Byron, E. S., Bowman, J. R., and Coull, J., IND.ENG.CHEM., 43, 1006 (1951). (5) Feldman, J., Myles, M., Wender, I., and Orchin, M., Ibid., 41, 1032 (1949).
J.3
(1950). RECEIVED October 4, 1950. Presented before the Division of Petroleum Chemistry a t the 116th Meeting of the AMBRICANCHEMICALSOCIETY, Atlantic City, N . J.
for Adsorption
Enggiring ROWSS
t Streams
development I
R. L. HOPKINS U. S. BUREAU OF MINES, BARTLESVILLE, OKLA.
of oil fractions of any desired size and of providing for direct observation of the oil to solvent ratio a t all times. This simplifies determination of the “break point” between the paraffincycloparaffin and aromatic portions by observation of the sudden increase in oil to solvent ratio and by the appearance of ma-
T h e continuous stripper described was developed to facilitate large scale Iaboratory adsorption operations in which pentane, benzene, and isopropyl alcohol are used and must subsequently be removed from the sample. Solvent removal is accomplished with this device as the effluent emerges from the column. This results in great saving of time and laboratory space over that rea quired for batch stripping, and the sample is less subject to damage through prolonged heating and oxidation.
r24-40 15-35
f
% A-7
T
HE Diesel fuel studies a t the Bureau of Mines required separation of severa1 hundred samples by adsorption in the course of a year. Most of these were approximately 1200 ml. in volume and ranged in boiling point from 150’ t o 400 O C. The direct displacement technique ( 1 ) previously used in this laboratory proved inadequate for a program of this scope because of interference in the adsorption pattern by the highly viscous naphthenes, whichresulted inpoorseparation, and because of the desorbent costs. It was therefore necessary to resort t o a technique involving dilution of the sample with pentane (9). It was realized t h a t removal of solvent by batch distillation of each separate cut from such a large number of samples would be very costly in time, labor, and working space. I n addition, the samples would be subjected t o comparatively high temperatures for long periods, which might result in thermal changes in the samples. To adapt the dilution technique t o this program, it became advisable to devise an apparatus for continuous and complete removal of the solvent from the sample as it emerged from the adsorption column. This process of continuous stripping also has theadvantage of permitting collection
5
3 mm O.D.
25 mm. 0.D.
1
35mmOD
,250
W. H E A T E R
Bc 7
m m . 0.0.
! 25 mm. 0.D
?12 mm O.D.
z-! -
0
0
MILLIMETERS 1
2
3 4 INCNES
5
6
Figure 1. Vapor Heated Solvent Stripper
