Efficiency of Fractional Distillation Columns ALLEN C. BUCK1 Cleveland. Ohio INTRODUCTION
T
HE importance of fractionaldistillation asa research tool '.1s constantly increasing. An important factor in connection with laboratory fractional distillation columns with which students should be acquainted is the measurement of their efficiency. Aside from its application to the practical operation of a column a statement of the efficiency of a column usually provides a convenient description of its effectiveness, without recourse to drawings or a lengthy discussion of its construction. The data required for measurement of the efficiency of columns arescattered throughout thechemical literature. The purpose of this article is the collection and presentation of the directions for the application of these data to the measurement of the efficiency of fractional distillation columns. A simple laboratory column with a total condensation variable takeoff still head and special still pot is shown in Figure 1. The labeled parts of the column may be conveniently referred to in connection with the following directions for measuring the efficiency of a column.
THERMOMETE
"\
A n
STILL HEAD
PACKED SECTION--\
II II
PROCEDURE
Efficiency of a fractional distillation column is a descriptive term usually meaning the total number of theoretical plates under some set of operating conditions. Generally the efficiency of a column is measured while it is operating at equilibrium under total reflux; i. e., the takeoff rate is zero. Takeoff rate is defined as the rate of taking off distillate, expressed as ml. of liquid per hour. The maximum efficiency is determined by operating the column a t a variety of reflux rates until the maximum number of theoretical plates is given by a particular reflux rate. Reflux rate is defined as the rate of liquid descending the column, expressed as ml. of liquid per hour. Several binary mixtures of compounds have been used for measuring the efficiency of columns. A mixture of n-heptane and methyl cyclohexane is frequently used, and the procedure described here concerns this mixture. The method, in brief, consists of refluxing a mixture of pure n-heptane and methyl cyclohexane through a column a t total reflux until equilibrium is reached. Samples of the mixture are simultaneously removed from the still head and still pot takeoffs for measurement of index of refraction. With the aid of a mathematical equation involving the molar composition of the samples, which is obtained from the index of
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Present address: Sherwin Williams Company, Western Reserve University. Cleveland. Ohio.
x
STILL POT
DRIPPER
STILL
/%'?&FF
xJf
refraction data, the efficiency of the column is readily calculated. The detailed directious for measuring the efficiency of a fractional distillation column are as follows: Preparation of Pure n-Heptune. Nearly pure nheptane may he obtained from the California Chemical Company. It is purified by treatment with chlorosulfonic acid. A mixture of n-heptane and 15 per cent by weight of chlorosulfonic acid is s t i m d in a roundbottom flask for 15 hours a t room temperature. The upper n-heptane layer is decanted from the mixture, washed with water and aaueous sodium carbonate solu-
tion until neutral, refluxed over sodium hydroxide pellets, and finally distilled from sodium wire through an efficient column. A middle fractiou is collected as pure n-heptane and should have the following physical c o n ~ t a n t s : ~ pdO 1.3878; d y 0.6839; b. p. 760 mm. 98.4OC. Preparation of Pure Methyl Cyclohexane. Eastman Kodak's "~ractical" made of methyl is . cyclohexane . treated with cold concentrated sulfuric acid, washed with water a d aqueous sodium carbonate solution until neutral and distilled from sodium wire through an efficientcolumn. A middle fractiou is collected as pure methyl cyclohexane and should have the following physical constants :2 Ny 1.4232; dtO 0.7693; 100.8"C. b. p. ,ea,,. Preparation of a Mixture of n-Heptane and Methyl Cyclohexane. A mixture of one part n-heptaue and two parts methyl cyclohexane by volume is prepared and should have a refractive index a t 20°C. of about 1.4150. However, this value is not critical. Establishment of Equilibrium Conditiuns. The colurnn to be tested should be clean and free of all organic material. Approximately 50 ml. of the mixture of nheptane and methyl cyclohexane is placed in the special still pot (cf. Figure 1) and the mixture vaporized at a known reflux rate under total r&ux conditions through the column for a t least eight hours. The reflux rate is obtained by counting the drops of liquid returning to WROMILEY AND QUIGGLE. "Vapor-liquid equilibria of hydro-
the still pot from the column and is expressed as ml. of liquid per hour. Collecting Samplesfor Refractive Index Measurements. After the column is in equilibrium, approximately 0.5ml. samples of liquid are simultaneously removed from the still head takeoff and still pot takeoff (cf. Figure 1). The indexes of refraction are measured at 20°C. This procedure is repeated for a variety of reflux rates, proceeding from a high to a low reflux rate, and the data are collected under the headings shown in Table 1.3 TABLE 1 EPWICIBNCY TRSTS
Rctllic Role
Cc./hr.
Re*rodiu$ lndcr Still pot Still head
.Uol Prodion n-Hcplour Still head Still pot N
Determination of Mol Fraction of n-Heptane in Still Head and Pot. Bromiley and Quiggle2have measured the index of refraction of known mixtures of n-heptane and methyl cyclohexane and their results are tabulated in Table 2. TABLE 2
Mol Froition of n-HePlonr
carbon mixtures." Ind. Eng. Cham., 25, 1 1 3 6 8 (1933).
A graph should be prepared from these data plotting the mol fraction on n-heptane on the abscissa and the index of refraction on the ordinate. The mol fraction of n-heptane for each pair of samples simultaneously taken from the still head and still pot may be obtained from this graph. These values should be recorded in the proper place in Table 1. Calculation of the Eficiency of a Column. The efficiencyof the column, N, at the various reflux rates listed in Table 1, is obtained from the following equation:'
where N is the number of theoretical plates; R has the valueof 1.0726a 4 ; xo is the mole fractionof n-heptane in the still head; x, is the mol fraction of n-heptane in the still pot. If x, is greater than xo they are interchanged in the above formula. The maximum efficiency is obtained from the reflux rate which gives the highest value of N in Table 1. More recently Lecky and Ewe115 have plotted the (Continued on page 492) a
SELKBE, BURK,AND LANKELMA, "An efficient low holdup
laboratory column," Ind. Eng. Chem., Anal. Ed., 12, 352-5 1.3900 1.3950 1.4000 1.4050 1.4100 1.4150 1.4200 (1940). Index of Refraction BEATTY AND CALINGABRT, "Vap~l-liquid equilibrium of hydrocarbon mixtures," Ind. Eng. Chcm., 26, 504-8 (1934). FIGURE Z.-Cunv~ orr INDEX on REPRACTION (AT 20°C.) on AND EWELL, "Spiral screen packing for efficient labon-HBpraN~.METHYLCYCLOHEXANE VS. THEORBTICAL PLATES. LECKY VERTICALSCALEHAS NO PARTICUL~R ZERO POINT,EACH ratory fractionating columns," Ind. Eng. Chern., Anal. Ed., 12, 544-7 (1940). Drvrsron MERELYREP~ESENTING Two T~eo~Bnwu. PLATE^.
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492
JOURNAL OF C/iEMICAL EDUCATION
copper tubing with molten lead and then cutting off the calculated lengths. The lead in the upper portion was then remelted to permit the insertion of eyebolts (with nuts attached) and final adjustrrent of the weight to proper value. The sizes corresponding to the various weights are given in Table l. DEMONSTRATION
Boyle's Law: Following an explanation of the difference between force and pressure, the increase in pressure in pounds per square inch caused by each weight was calculated at the blackboard by dividing the known weight by the area of the piston. To these pressures were added 15 pounds per square inch (atmospheric pressure plus the constant pressure resulting from the weight of the plunger). The volume within the cylinder was adjusted to 100 m!. The weights were then carefully lowered to the piston top by means of the cable and pulley. The volume corresponding to each weight was added to the table at the blackboard. The product of the pressure and the volume was computed. Actual results along with the approximate length of the weights are given in Table 1. TABLE I
No.
Length
Weight
(In.)
(Lbs.)
0 I 2 3
1'/,
5
18 1/ 101
P", -
!!' A 0
2 1/ . 4 5/.
•
1II/I
(MI.)
I'V (Pou'lds X MI.)
100
1500
Vol. P-Po+P",
15 [6.82 18.86 21. 47
3.04
1.82
6.44 10.81
3.86 6.47
16.06 25.63
9.62
24.62
15.3.';
30.35
9.
1514
80
1509 1503 1477
7'
60 50
piston, the volume was adjusted to approximately 70 m!. (room temperature). The water jacket was then placed over the syringe. The rubber tubing from the upper side ann was led to the drain while that from the lower side ann was connected to a 2-liter reservoir of ice water. A pinch damp on the latter was used to control the flow of ice water. The reservoir of ice water was elevated above the level of thf water jacket. By opening the pinch damp the water jacket was filled with ice water and sufficient water was allowed to pass through the apparatus to assure the attainment of a temperature close to O°C. The volume of gas within the syringe was then noted. By removing the rubber tubing from the ice water reservoir and lead· ing it to the drain, while the tube from the upper side ann was connected to a stearn generator, the water jacket was filled with steam and the temperature of the gas raised to approximately 100°C. It is well to remove the weight during temperature changes to rr.inimize leakage. While temperature equilibriwn was being established the volume at 100°C. was calculated at the blackboard. The actual volume was then ob· served. If necessary, satisfactory results can be obtained by using hot and cold water from service taps. In this case the temperature can be read by allowing tb. water, after it has passed through the water jacket, to drain into a beaker placed in the sink. The thennom· eter can be placed in the beaker to detennine the hot and cold temperatures. Actual results of the two pro· cedures are given in Table 2.
I
l!j18
TABLE 2
Po .. pressure in Ibs./sq. in. with no weights on piston. P", .. pressure in Ib~./sq. in. due to applied weil{ht. P
.. total pressure
Charles' La.w:
Oll
_ T (0c.)--.. 1 Z
gas.
With Nwnber :) weig-ht on
the
I.
2.
Icc Water--8teaw Hot Water-Cold 'Vater
o 16
..
000
v,
V.
\.,
(Cal.)
WbsJ
62 60
85
6,
-
83
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EFFICIENCY OF FRACTIONAL DISTILLATION COLUMNS (Conlinutdj,om page 476) index of refraction of mixtures of n-heptane and methyl cyclohexane vs. theoretical plates (Figure 2), frol11 which one can obtain the efficiency of a column directly from the index of refraction data which have been collected in Table I. The indexes of refraction of each pair of samples taken simultaneously from the still
head and still pot are located 011 the curve in Figure 2. Horizontal lines from these two points on the curve to the ordinate give the number of theoreticai plates corresponding to each index of refraction. The difference between these two values, minus one theoretical platt" for the still pot, gives the efficiency of the column.
TAKE YOUR PICK ;'lli~torical experience hus shown tlwt the destrlLction and elirnination of a foreign rUitionality is not i'L the lea~f, (J,pai,ut the laws of life, provided that desf.rlLction and elimination nre co,nplvted."-'Fernpr IJest. Nazi historian
"We hold these truths to be ,-self-evident, that all ,nen are created equal, that they t,re endowed by their Creator with certain unalienable Rights, that amoll,g these are LiJe, Liberty, and the lu"rsuit oj 1I"{J{Jincss."-The Declarar,ion of Indepentlence
