Vacuum Distillation development
VARIABLES IN PACKED COLUMN DISTILLATION AT SUBATMOSPHERIC PRESSURES
MAX S. PETERS*
M. R. CANNON
AND
THE P E N N S Y L V A N I A STATE COLLEGE, STATE COLLEGE, P A .
T
H E modern chemist and chemical engineer encounter many problems which involve the separation of one material from another by distillation a t reduced pressures. The industrial application of vacuum distillation is widespread. It is used commercially in the purification of many chemical compounds such as glycerol, hydrogen peroxide, turpentine, and fatty acids. Vacuum distillation has also been employed for a number of years by the petroleum and coal tar industries for the separation of many of their products. Distillation a t reduced pressures can often be used to separate mat,erials which form azeotropes a t atmospheric pressure or to obtain an increased relative volatility. I t is of particular value in separating many materials which decompose or polymerize a t their atmospheric boiling temperatures. Despite the wide use of vacuum distillation, thcrc has been little information published concerning the effect of reduced pressure on the important variables such as efficiency, pressure drop, and maximum allowable Jrelocity. In this work the term efficiency corresponds to an expression of the total number of theoretical plates or transfer units in the column. The effect of reduced pressure on the efficiency of packed columns has been considered in several recent articles ( 1 , 2 , 7 , 1 1 , 14, 19) and conflicting results have been indicated by the different investigators. Berg and Popovac ( 1 ) investigated the effect of reduced pressure on the efficiency of a rectification column packed with l/ginch stainless steel Fenske helices using n-octane-toluene as the test mixture. They reported that the column efficiency was independent of pressure over a pressure range of 20 to 760 mm. of mercury absolute. Hawkins and Brent ( 7 ) also have reported t h a t column efficiency is independent of operating pressure. They conducted their experiments on 50 X 50 mesh stainless steel spiral screen packing and 4-mm. Raschig rings using chlorobenzene-ethylbenzene as the test mixture. They did their work a t operating pressures betweeii 20 and 760 mni. of mercury absolute. Lloyd ( 1 1 ) has theorized that the efficiency of a packed column should decrease as operating pressure is reduced, because of the fact that the decrease in temperature causes the viscosity of the liquid to increase. Struck and Kinney (19) irivevtigated the effect, of diminished pressure on the efficiency of fractionating columns packed with Raschig rings, Fenske stainless steel helices, Cannon protruded nickel packing (hole size A ) , and McMahon wire-gauze saddles. They found that a t a constant boilup rate, the efficiency increased as pressure was decreased and reached a maximum value a t pressures of 50 t o 100 mm. of mercury. Myles, Wender, Orchin, and Feldman ( 1 4 ) have reported that the efficiency of fractionating I Present address, Department of Chemical Engineering, Unirersity of Illinois, Urbana, Ill.
columns packed with Heligrids, glass spheres, single-turn helices, or Berl saddles showed a maximum value a t an operating prcssure of 200 mm. of mercury. On the basis of theoretical reasoning, Bowman ( 3 ) has predicted that there should be a pressure range in which the efficiency of a fractionating column has a maximum value. SCOPE OF WORK
The purpose of this work has been to investigate the problcms and variables concerned in vacuum distillation. These factors have been examined from both a theoretical and experimental viewpoint, and the results have been presented in such a manner that t,hey can be applied t o practical vacuum distillation problems. The effects of reduced pressure on column efficiency, pressure drop, and maximum allowable velocity have been determined in this work for the following packings: 0.16 X 0.16 inch stainless steel protruded packing (hole size B ) ; 0.24 X 0.24 inch stainless steel protruded packing (hole size B); 0.25-inch stainless steel 100-mesh screen McMahon packing; and 0.25-inch ceramic Berl baddles. Table I shows the properties of these packings. The pressure range from 10 to 735 mm. of mercury Ivas examined and tests were carried out a t total reflux and a t finite reflux. All the packings were tested over a range of reflux rate from as low as possible to the flooding rate. Two different test mixtures (n-decane-tmns-Decalin and chlorobenzene-ethylbenzene) were used. DESCRIPTIOK O F EQUIPnlENT
All of the experimental work r a s carried out, in a 2-inch diameter column. The column was insulated and wrapped with a compensating heat,ing wire which, TTith t,he aid of t,hermocouples, permitted t.he column to be set for adiabatic operation. A graduated sight glass was placed in the liquid-runback line leading from the base of the column t o the still pot. This sight glass ITas used for determining reflux rates a t the bottom of the column by closing a valve beneath the sight glass and determining the time for a certain volume of the column runback to collect. The condensation rat,e at the top of the column n-as determined by a heat balance around the condenser. The packing \?-assupported by a screen cone inserted with t'he point upxard. The liquid distributor plate was similar to the type described by Heinlein, Manning, and Cannon (9). Dowtherm vapor vias used for heating the still pot and the amount of heat input was controlled by a nianostat which could be set to maint,ain any desired column pressure drop. -411 the finite reflux runs m-ere made with t'he distillate being continuously returned to the still pot,. The product takeoff rate was determined by direct measurement and the rate was set by means of a needle valve located a t the takeoff point. 1452
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1952
TEST MIXTURES
An extensive literature search was carried out in order to find a good test mixture to use for experimental work in vacuum distillation. After careful consideration of the test mixtures that could be used for vacuum distillation work, it was decided that the
1453
trans-Decalin obtained from the distillation was 1.46919 which agrees with the value of 1.4692 reported by Myers ( I S ) . The high-grade chlorobenzene and ethylbenzene obtained from the Dow Chemical Co. were redistilled in the 100-plate column with a condenser pressure of 300 mm. of mercury. The refraotive index (nsop] of the purified chlorobenzene was 1.52462 as compared to 1.52460 re orted by Timmermans (do). The refractive index of the purifier; ethylbenzene was 1.49592 which agrees with the value of 1.49587. reported by Doss ( 3 ) . OPERATING PROCEDURE
Before each series of runs was started, the column was prcflooded t o make certain the packing was completely wetted. It was found that the column was at equilibrium after 6 hours of operation a t any one rate. Accordingly, rcadings mere spaced 6 hours or more apart. The compositions of the top and bottom samples were found by means of refractive index (ng) determinations on a five-place Valentine refractometer. The bottoms composition for the various total reflux runs using the n-decane-trans-Decalin test mixture averaged about 5 mole % n-decane. For the finite reflux runs, the bottoms composition averaged approximately 30 mole % n-decane. With the chlorobenzene-ethylbenzenc test mixture, the composition of the bottonis for the total reflux runs was about 13 mole % chlorobenzene. The operating procedure for the finite reflux runs was the same as that described above. After the column had been flooded, the desired boilup rate was set by means of the pressure drop monostat and the reflux ratio was set by means of the l/s-inch needle valve. The column was run under steady conditions for a t least 6 hours between readings t o make certain that equilibrium was reached.
1.09'
I
ABSOLUTE Figure 1.
I
I 1.0
PRESSURE,mm. Hg
Effect of Pressure on Kelative Volatility
0.9 w
v)
0
I
This mixture has a sufficiently high boiling point to permit usc of ordinary tap water as the condensing agent over t h e pressure range of 10 t o 760 mm. of mercury. The mixture has a relative volatility that is constant over the entire concentration range a t each pressure (6, 19). Accurate information is available as to the value of relative volatility a t different pressures (5, 1 9 ) . Analysis of the composition of the mixture can be made by refractive index determinations, and the refractive index spread over the concentration range is such as t o permit very accurate composition determinations ( 1 8 ) . The individual components of the mixture are readily available and can be purified with little difficulty. The test mixture chlorobenzene-ethylbenzene was also used in this work t o make several check runs. The relative volatility for this mixture does not change with composition and is fairly constant with pressure change ( 6 ) . Accurate information relating the refractive index and composition for this mixture a t 25' C. is available ( 6 ) . Figure 1 shows the effect of pressure on relative volatility for the two systems used in this work. The n-decane was purchased as 95 mole yon-decane. This was purified by percolating it slowly through 28- to 200-mesh silica gel packed to a height of 7 feet in an 18-mm. glass column. The n-decane was then distilled in a column containing 100 theoretical plates. The distillation was carried out at 60 mm. of mercury. of the purified n-decane was 1.41193as The refractive index found on a five-place Valentine refractometer. This compares very favorably to the value of 1.4119 reported by Struck (18) and 1.4120 reported by Myers ( I S ) . Pure trans-Decalin was obtained by distillation of a mixture of trans- and cis-Decalin in the 100-plate column using a condenser pressure of 30 mm. of mercury. The refractive index of the pure
(nv)
