Richard D. Campbell I Fractional Distillation University of Iowa

Iowa City I A laboratory experiment

Distillation is a common method for purification of organic liquids and a limited number of low-melting solids. In common laboratory practice, distillation is a classic art, highly formalized, but offering ample occasion and medium for self-expression. Unfortunately, much of this practice tends to orua- mentality rather than utility.

The chemist usually desires to recover the largest possible quantity of pure product. However, maximum purit,y and maximum recovery are mutually incompat- ible. Thus, the distillation is a compromise of several practical considerations:

The purity of product required. The product must be extremely pure if it is to be used for elemental or spectroscopic analysis, or as a reagent in quantitative work. The product usually is sat,isfactory as a syn- thesis intermediat,e if it contains substantial (10-25%) amou11t.s of impurities. These considerat~ions are tempered by the nature of impurities and the manner in which thry might i~lterfere with the use of thr product.

The quantity needed. Small quantit,ies are required for analyses. Microtechniques in analytical pro- cedures make it possible to do all of the needed ana- lytical determinations with less than a gram of ma- terial. For preparative work however, the maximum quantity of product obtainable is desired, provided the level and nature of impurities will not decrease the yield in the subsequent synthetic step.

The time available. If a high degree of purity of a difficultly purified substance is the goal, a substantial expenditure of time is expected. Preparation of quantities of intermediates, on the other hand, seldom justifies elaborat,e and tedious purification. In this instance, there is the alternative of preparing larger quantities (in single runs or multiple runs) rather than attempting to recover a maximum percentage of the product.

The equipment available. In choosing among avail- ahle kinds of distillation apparatus the factors usually considered are appropriate size, good control of pot and column heating, control of ebullition, column with good vapor-liquid contact but with holdup lim- ited, receivers for collecting separate fractions, and vacuum control for reduced pressure distillation. Common faults include the use of apparatus much more elaborate than necessary, failure to obtain the performance of which the apparatus is capable, or expecting performance of which the apparatus is incapable.

Laboratory distillation technique becomes critical in fractional distillation in which two volatile components are to be separated. I t becomes much more critical if

the fractional distillation is to he performed under reduced pressure. This experiment demonstrates in a practical way the critical factors involved. The experiment is designed to demonstrate to the student the performance characteristics of common distillation apparatus, as well as the fundamentals of laboratory distillation.

It is assumed that the student has performed simple distillations and fractional distillation experiments of the kind found in familiar laboratory manuals (I).

Several graphical treatments (2-5) have been used to express operation of a distillation column and factors which affect the enrichment obtained. Three common graphical methods are the temperature-composition diagram, the vapor-liquid equilibrium diagram, and the enthalpy-composition diagram. All these are for binary systems. multiple component systems are much more difficult to treat (2). Most laboratory distillations are satisfactorily discussed in terms of bi- nary systems. The vapor-liquid equilibrium behavior of ideal and non-ideal binary solutions is discussed by Daniels (3).

The temperature-composition diagram (Figures 1 and 2) is easiest for students to understand, but is unsatisfactory for treatment of operating condit,ions other than total reflux (complete vaporization and t,otal condensation a t each theoretical plate).

I I

60 I 0 50 100

MOLE % CHLOROFORM

Figure 1. Temperature-composition diagram for chloroform and benzene

348 / Jovrnol of Chemicd Education

The enthalpy-composition diagram provides a eom- pletely rigorous treatment for any binary system in an adiabatic column. This is the Ponchon method (4, 6). The required data for only afew binary systems are available, however.

I I I I I 0 2 0 4 0 6 0 8 0 100

MOLE % CARBON TETRACHLORIDE

Figure 2. Temperature-comporition diagram for carbon tetroshloride ond benzene.

The McCabe-Thiele Treatmenl

The vapor-liquid equilibrium diagram is used in the MeCabeThiele treatment of distillation (4, 6, 7). In this treatment it is assumed that the molal heat of vaporization of mixtures of varying composition is eon- stant. This assumption of constant molal overflow results in slight deviations for binary systems only if association in the liquid state occurs (e.g., ethanol- water system). No error results for most organic mixtures. The MeCahe-Thiele treatment is otherwise as rigorous as the enthalpy-composition treatment. Its advantages are (a) availability of physical data for binary systems, (b) simplicity of material balance equations, (c) ease of handling reflux ratios, and (d) ease of application to batch distillation.

In t.he MeCabe-Thiele treatment the following definitions are used:

A = mare volatile component (e.g., chloroform or carbon tetra- chloride)

B = less volatile oomponcnt (ex. , benzene) z = mole fraction of A in liquid phase (as per cent) y = mole fraction of A in vapor phase (as per cent) L = moles of liquid flowing down column V = moles of vapor flowing up column D = males of distillate withdrawn at column head

The vapor-liquid equilibrium data (8) are plotted, x vs. y, giving the equilibrium curve. Any point on this curve represents an equilibrium vaporization. Thus, equilibrium vaporization of a liquid composed of equimolar amounts of chloroform and benzene (x = 50%) would give vapor containing 66 mole per cent of chloroform (y = 66%; see Vigure 3) .

The x = y line is drawn for reference. A point on this line can represent total vaporization or total con- densation. The latter change occurs in a distillation

column at total reflux. Thus, if the vapor with y = 66Y0 is totally condensed the liquid composition resulting is x = 66%. Therefore the step indicated in Figure 3 represents one theoretical plate at total reflux, that is, one equilibrium vaporization and one total eondensa.tion.

Operoting Condition

A material balance on the distillation column is given in equation (1). .kt total reflux, D = 0, so L = V and it is seen that

L + D = V (1)

total condensation occurs. A material balance on component A at any point in the column is given by:

LA + Da = VA or s L + zoD = yV ( 2 )

(where x, is distillate composition). Thus, x = y is the operatiug line at total reflux. When D Z 0, then x = y and equation (2) becomes the operating line.

lves: Rearrangement of (2) g'

Reflux ratio R i~ defined as

R = L / D (4)

Combining (1) and (3) one obtains ( 5 ) :

Using the definition (4) in (5) one obtains (6)

In this form, the operatiug line is seen to be a straight line with slope R/(R + 1) and intercept x,/(R + 1) . The slope of the operating line is set by the observable and controllable reflux ratio. In practice, it is eon- venient to locate the point at which the operating line intersects the x = y line. This point is x, = .x = y. as

80t EOUlLlBRlUM

VAPORIZATION //

Figure 3. McCabe-Theile diagram for chloroform and benzene. Unifr ore male per cent.

Volume 39, Number 7, July 1962 / 349

seen from equations (1) and (2). This point and the intercept y = xD/(R + 1) define the operating line.

The complete graphical analysis of a sample dis- tillation is shown in Figure 4. The significance of various points and lines is indicated. The analysis permits determination of any one of several factors: required reflux ratio, number of theoretical plates or stages of a given column, number of theoretical plates required for a given separation, or separation possible with a given column and reflux ratio.

1001 1 I ,

EOUILIBRIUM

DIST ILLATE I - 1

0 4 0 6 0 8 0 X

Figure 4. MsCabe-Theile diagram for a typical fractionating column at reflux ratio of flvo.

Application to Batch Dirtillation

In batch distillation (B), the pot composition does not remain constant during the course of the distillation. As the more volatile component A is removed more rapidly than B, the pot composition is enriched in B. Thus as the distillation proceeds, the following changes occur gradually and simultaneously: the mole fraction of A in the distillate decreases, the mole fraction of A in the pot decreases, the temperature of the distillation in- creases, and the difference in composition across the column will either increase or decrease (or both in the sequence named). Thus, for example, a liquid con- taining 50% of component A is distilled in a three plate column (four plate distillation) a t R = 20 to give a distillate containing 94% of A (Fig. 5). As the distillation proceeds the pot composition approaches 40% of A. If uniform operating conditions have been maintained, the fresh distillate has a composition of 90% of A. The accumulated distillate has an ap- proximate composition of 92y0 of A. If the distillation were continued until the pot composition reached 30Y0 of A, the fresh distillate would have the composition of 81% of A. A second distillate fraction collected during this second stage of the distillation would have an approximate composition of 86% of A. A simple ma.terial balance on one component (10) can be used to estimate quantities of distillates on residues. The results are summarized on Table 1, and diagrammed in Figure 5.

350 / Journal of Chemical Education

Table 1. Batch Distillation of Chloroform-Benzene

Mole % Moles of Wt of A liouid liouid

Initial, pot 50 1 .00 1 0 3 . 8 g Intermedmte, pot 40 0 .81 79.7 Distillate 1 92 0 . 1 9 24.1 Final, pot residue 30 0.66 62.0 Distillate 2 86 0 .14 17.7

.. Figure 5. McCabe.Theile diagram for o typical bobh distillation of chlwoform and benzene. Reflux ratio is 20. See Table I. Operoting lines - ; initial column condition ; intermediate _i Rnal .--,

Evaluation of Column and Operating Conditions

A binary system must he selected for distillation, to evaluate the column and operating conditions. The components of the binary system must be miscible in all proportions. Binary systems forming constant- boiling mixtures are unsuitable. Refractive index is a property which is useful for determining the composi- t,ion of mixtures. Therefore a substantial differential between refractive indexes is desired. Suitable char- acteristics are found in the benzene-chloroform and benzene-carbontetrachloride systems.

Standard refractive indexes of the pure solvents must be determined initially. From these data, a straight- line graph is drawn, plotting refractive index (ordinate) against composition in mole per cent (abscissa). The composition of any mixture of the two components can be determined by measuring its refractive index, and then reading the corresponding composition from the graph. Care must be taken to determine all refractive indexes at the same temperature, since a variation of 1°C can result in a 2 mole error in composition, or more.

For a column with less than a six-plate r a h g ex- pected, the chloroform-benzene mixture is suitable. Boiling points differ by 19.2'C. The refractive indexes differ by 0.057. A four-place refractometer (prism type) permits readings corresponding to *0.2 mole %.

For columns with plate ratings of five to fifteen, the carbontetrachloride-benzene system is suitable. The

boiling points differ by 3.5'C. The refractive indexes ( k ) Record all drop count rates (roflux rate and distillate in

differ by 0.040, permitting ready determination of mole per etc.). per cent to +0.3. This procedure provides data for initial operation

A starting composition is decided upon. This will only. slight modification in sampling would allow one usually be 2@35% of the more volatile component. to follow the course of batch distillation. A mixture is made up with the approximate composi- tion reauired. (Calculate the volumes of CC14 and Experimental Results CeH6 required for a 25-75 mole per cent mixture.) A sample of this mixture is retained in a sample vial labelled P-1. This liquid is placed in the pot of the still. The column is brought to equilibrium a t total reflux and a t the selected pressure. The boil-up rate is gradually increased, and the reflux drop rate from the column head is counted. When flooding occurs, the boil-up rate is recorded (drops of reflux per minute). The boil-up rate is decreased so that flooding no longer occurs. Equilibrium is established again a t a boil-up rate which is a chosen fraction of the flooding rate. When the column is a t equilibrium, one milliliter of dis- tillate is collected in a small clean vial labelled D-1. Oneratine conditions are recorded.

The column evaluations were carried out according to the procedure described. The evaluations were per- formed by students of two classes of the Organic Prep- arations course in our department. Equipment used was that which is ordmarily available for upper division laboratory courses.

The results of the analysis applied to different columns are listed in Tables 2 and 3. All of these data were obtained using the following operating conditions: atmospheric pressure, total reflux, and the boil-up rate approximately one-half of the flooding rate. The binary system employed was either chloroform-benzene or carbontetrachloride-benn as indicated. -

Column operation is then changed to a specific reflux ratio. This observed ratio is the ratio of reflux Table 2. Columns a t Total Reflux

drop count to distillate drop count. Equilibrium is Length, attained as rapidly as possible. A sample of distillate diameter

Column (cm) Plates Systema HETP is collected in a samnle vial labelled 0 - 2 . The still is immediately stopped and a sample of the pot is with- %,"p,;;$mn 49 X 1 . 3 2 . 8 A 17.1

40 X 2 . 5 2 . 7 B 14.8 drawn into a sample vial labelled P-2. 3 2 x 1 ~ 4 R A fi 7 .- , . . . . ..

The refractive indexes of the four samples are meas- 23 x 1 . 6 2 .8 B 8 .2 ured carefully at the same temperature a t which the 20 4 B 5

25 6 B 4 . 2 calibration graph was determined. The graph is used GI,,, beads 29 1 1 . 7 B 2.5 to convert the refractive indexes into mole per cent Glass helices 31 11 B 2.8

30 composition. This gives figures for pot and head g$z\g band 58 15 B 2.0 43 B 1 . 4

composition a t total reflux and at operating reflux ratio. a Binary system A is chloroform-benzene, B is carbon-tetra-

chloride-benzene. The McCabe-Thiele diagram is drawn UD for use as

follows:

(a) The z = y line is drawn. This is the operating line a t total reflux.

(b) The vapor-liquid equilibrium curve is drawn up (S), using mole % data.

(c) The pot and head compositions me plotted on the z = y line and labelled (P-1, H-I, P-2 and H-2).

(d) The operating line a t specified reflux ratio R is plotted to intersect the z = y line a t the head composition H-2 and with a slope of R/(R + 1).

(e) A "stepped" line is drawn betweon the z = y line and the equilibrium line, starting a t P-1 and ending st H-1. This "stepped" line is labelled "total reflux."

(f) A "stepped" line is drawn between the operating line and the equilihnurn hne, starting a t P-2 and ending a t H-2. This "stepped" line is labelled with the reflux ratio (R = 5, or R = 10, etc.).

(g) The plate rating is determined by counting the "steps" in (e) and (j). The plate rating of the column is one plate less than the count in ( e ) and in (f), since the vaporization in the pot and condensation in the head accounts for the one theoretical plate.

(h) Column dimensions are measured: packing height, column inside diameter, packing size or mesh, and packing type. Addi- tional data if desired: column void space (fill packed section with solvent from buret), column vapor space (measure volume or weight of solvent which drains from packed section of column after complete filling), and estimate or calculate vapor-liquid surface area.

(i) From plate rating (or theoretic plate number TP) and pecking height (PH) calculate height equivalent to a theoretical d a t e (HETP = P H I T P ) a t total reflux and a t reflux ratio R. ' ( j ) kecord all observed temperatures (heating bath, pot, column jacket, head jsrket, and head interior).

The theoretical plate ratings of the columns show the expected trend. The Vigreaux columns are not con- vincingly better than the simple open column. At less than ideal conditions commonly employed in laboratory distillation-viz., low pressure, low reflux ratio, high boil-up rate-the use of such columns is little improve- ment over simple distillation.

The HETP ratings determined for the Todd column (packing: wire spiral wound on center rod) and for the Stedman packed column were not as good as the HETP ratings for the columns packed with heads and with helices. In distillations in which column holdup of liquid is an important consideration, column efficiency (HETP) is somewhat counterbalanced in these cases. Holdup becomes a serious problem with the helices and the Stedman packing. Glass beads appear to be superior to other packing in most respects for packed columns.

The p~rformance of the zig-zag column is excellent, with the HETP value of 2 cm. The holdup is small and liquid-vapor surface area is small. Apparently the controlled turbulence in both phases is responsible for the column efficiency One disadvantage is a peculiar flooding effect which occurs a t the bends with moder- ately high vapor rates.

The spinning band column was found to have the smallest HETP. The apparatus employed was the Piros-glover Microstill. This apparatus did not afford ready control of the reflux ratio. Low hold-up and

Volume 39, Number 7, July 1962 / 351

high flooding rate make this column very useful for sharp fractionation of small to medium quantities of liquid.

Operating conditions were varied in the analysis of some columns to determine the sensitivity of the columns to these conditions. The conditions evaluated were pressure, boil-up rate and flooding, and reflux ratio.

The effect of boil-up rate was evaluated with the Strdman column, with the glass helices packed column, and with t,he glass beads packed column (see runs 7-12, 2.5, 26, 35-38). In each of the columns the approximate flooding rate was determined. The boil- up rate mas gradually increased. At the flooding rate, the vapor velocity was sufficiently high to support (visibly) the refluxing liquid, resulting in unstable downward flow: a sharp increase in pressure differential across the column has been observed (11). In runs 7-10, 25, and 37 the boil-up rate was maintained just

below the flooding rate. The boil-up rate was held a t one-half to one-fourth the flooding rate in runs 11, 26, and 35. This resulted in as much as a two-fold increase

.in plate rating. Thus, the efficiency of packed columns is very sensitive to incipient flooding.

This sensitivity of columns to boil-up rate is a major factor in the effect of changing the operating pressure, or changing the reflux ratio.

The effect of operating pressure was observed in four of the columns. Generally (but somewhat erratically) a decrease in operating pressure diminished column efficiency. This observation is most obvious in runs 29-34 with the zig-zag column. A decrease in operating pressure is suggested by some sources to improve separation. Flooding, or any related instability of liquid flow which prevents equilibration, decreases column efficiency. A change in pressure from one atmosphere to one-half atmosphere lowers (12) the boil-up rate at which flooding occurs by 10-30%.

Table 2. Evaluation of Operating Conditions

Length Diam. Binarya Theoretical Reflux HETP Column or packing (om) ( 4 system Pressure plated ratio (cm)

Modified Vigreaux Hask 32 1.3 A 1 4.8 6.7 (vac. jacket) 2 . P Op.= 12.8

Todd column 20 0.5 B 1 4 5 2 OD. 10 ~.

Stedman (heated jacket) 25 B 1 6 4.2 5'/4 10 4.8

B 0.33 3 8.3 3 10 8.3

B 0.23 3 8 .3 1 5 25

B 0.26 7 3.6 5.5 10

Vigreaux (vac. jacket) 23 1.6 B 1 2.8 OP. 8.2 40 2.5 B 1 2.7 14.8

2.0 5 20 40 2.5 B 0.26 2.3 17.4

1 .4 5 28.5

Open 48 1.3 A 1 2.8 17.1 2.6 5 18.5

48 1.3 A 1 2.7 17.8 2.6 5 18.5

Glass beads 29 1.3 B 1 11.7 20 4.7 5

A 1 7 3.2 5 4 5

A 1 4.8 20 4.3 10

Zig-zag 30 B 1 15 2 12 10

B 0.53 4.5 3.5 5

B 0.26 2 1.8

Glsss helices 31 1.3 B 1 11 10 151

B 1 6 59 5

B 0.53 4.7 2.5 5

B 0.26 6.8 7.6 5

Spinning band 58 0.3 B 1 43 1.4

a ChC18-Benzene indicated by A; CC1,Benzene by B. Determined graphically assuming R = 0. ' Limit of R is 5. d For column only. Pot accounts for one TP. R u n s 5, 6, 11, 12 with boil-up rate approximately one-fourth Hooding rate. Runs 7-10 with boil-up rate just below Hooding rate. 1 Estimated. Observed ratio was 5.

3.5 plates determined if R = 15.

352 / Journal o f Chemical Education

Vapor velocities vary widely in laboratory distillation. In rull 22 the open column vapor velocity was 7.7 cm/sec. For a vacuum distillation of nitrobenzene at 10 mm and 8 5 T , in the same apparatus the vapor velocity would be 580 cm/sec. Efficient vapor-liquid equilibration cannot be expected.

The column analyses were carried out employing the same vapor-liquid equilibrium diagram a t all pressures employed. I t is assumed that 110 change in the vapor- liquid curve occurs with a change in pressure. A sim- ple calculation shows the size of error thus introduced. For the calrulation, a pressure change from one atmos- phere to one-half atmosphere is considered. Relative volatility is calculated by:

log a = AHLTz - Tr)

2.3RTlTs

Substituting from Troutons rule H,=22T where T is the average temperature (TI + Tz)/2 then:

log a = 22T(T9 - TI) _ 22(T2 - TI) -

2.3RTZ 2.3RT

The change in the pressure results in a decrease in T of 20°C (c.g., benzene, carbon tetrachloride). At 80°C this represeuts an increase in log w of 6%. The error in the number of plates determined for a given column will be 6y0 as well, since the number of plates required for a given enrichmeut is an exponential function of m (9) :

The reflux rat,io was varied in the operatiou of most of the columns. The observed effect in chauging t,he column operatiou from total reflux to a finite ratio (R = 5-20) consisteutly decreased the number of theoretical plates calculated by a small amount (e.g., runs 22-28). In all cases, the enrichment of the dis- tillate was less at a finite reflux ratio thau at total reflux. Column evaluations are usually carried out a t total reflux aud distillations are not. Thus, compara- tive columu plate ratings alone provide an incomplete guide for selection of laboratory distillation apparatus.

Three cousiderat,ions are recognized in the effect of reflux ratio upon column performance. First, a change in reflux ratio changes the enrichment (or separation) which is obtained by a given number of theoretical plates. This is the result of a change in the slope of the operating line. A minimum reflux ratio is readily determined. I11 Figure 4 it can be seen that in order to obtain a distillate containing 80% CHC13 from a liquid containing 25% CHC13, the reflux ratio can be no less thau 5 , regardless of the number of theoretical plates available for the separation.

Secondly, a change in reflux ratio changes the vapor and liquid rates in the column. The columns are undoubtedly sensitive to these changes. The effects are, however, difficult to nvaluat,e.

4 third problem is the observation of the reflux ratio. If the column and columu head are not adia- batic, spurious reflux occurs in the columu, and the operating reflux ratio is differeut from the ratio of ob- served drop counts. This was obviously the case in nu1 36. The observed reflux ratio was 5. The minimum reflux ratio possible for the observed separation is 6.5. The actual reflux ratio operating in the column most probably is 10-15. This is due to heat loss from the column or distillation head. This is illustrated in the case of the zig-zag column. The observed reflux ratio was 5 in run 30. The "true" reflux ratio could have been no less than 7, to give an operating line which would be possible even with an infinite number of plates. The reflux ratio was estimated as 9.5 to give an ap- propriate operating line with a plate rating comparable with that determined a t total reflux. The observed reflux ratios and "true" reflux ratios are noted on the tables.

columns under such reflux conditions, careful measure- ment of the reflux ratio is necessary.

Conclusions

ks a result of this experiment, some pertinent rec- ommendations for distillation procedure are obvious:

(a) Vacuum distillatiou is generally unsatisfactory for critical separation, in which boiling points are not widely separated.

(b ) Reflux ratio in fractioual distillation can be maintained profitably above 20 in critical distillation.

( c ) Boil-up rate is critical in distillation and should be maintained at one-half to one-fourth of the flooding rate.

Literature Cited

(1) ADAMS, R., and J o n x s o ~ , J. R. , "Labboretory Experiments in Organic Chemistry," 4th ed., Maomillan Co., New York, 1949, pp. 37-53; COLEMAN, G. H., WAWZONEK, S., AND BUCKLES, R. E., "Laboratory Manual of Organic Chemistry," Prentice-Hall, New York, 1949, pp. 13-14.

(2) PERRY. J. H., "Chemiesl Engineers' Handbook" 3rd ed., MeGrsw-Hill Book Co., New York, 1950, Chap. 9.

(3) DANIELB, F., "Outlines of Physical Chemistry," John Wiley and Sons, Ine., New York, 1948, pp. 208-16.

(4) BROWN, G. G., "Unit Operations," John Wiley & Sons, New York, 1950, Chap. 23 ff.

(5) PONCHON, M., Tech. Modeme, 13,30 (1921). (6) MCCABE, W. L., AND TKIELE, E. W., Ind. Eng. Chem., 17,

605 (1925). 171 PERRY. J. H.. on. oil.. DD. 5914 . , . ... i s j ib id . , pp. 573-5. (9) WIBERG, K. B., "Laboratory Technique in Organic Chemis-

try," McGrew-Hill Book Co., New York, 1960, pp. 4G57. (10) PERRY, J. H., op. Cil., p. 338, example 2. (11) SRERWOOD, T. K., SAIPLEY, G. H., AND HOLLOWAY, F. A. L.,

Ind. Eng. Chem., 30,765 (1938). (12) PERRY, J. H., op. cit, pp. 683-6.

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