Distillation Analysis - Analytical Chemistry (ACS Publications)

Distillation Analysis R. T.

Leslie and E. C. Kuehner, National Bureau of Standards, Washington, D. C.

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includes work which was published from September 1961 to September 1963 and which has come to the attention of the reviewers. Some of it was actually published earlier but escaped the review published in ;Ipril 1962. The object has been t o summarize briefly all the developments in the knowledge of distillation which can be classified as fundamental or technological with fundamental importance and which can assist in analyses or separations. It will necessarily overlap, t o some extent, some technological reviews (15'4, 16A, 19A, 2 l A , 26-4-98.4). The reviewers are inclined to agree with the remark of J. R. Fair (15.4) that fractional distillation may not be "glamorous" but is still "exciting." HIS REVIEW

FUNDAMENTAL INVESTIGATIONS

General papers on the theory of distillation deal with the thermodynamics ( I A , 44A) and principles of separation which use equilibrium between vapor and liquid or solid phases ( 5 A , 23A), the analogy between distillation and extraction (49iz),the general problems of distillation of multicomponent systems (8A), and some of the calculations which can be made from the temperature of the vapor (40'4). Other papers of the fundamental type concern the laws of vapor and liquid flow in rectifying columns (?'A), the effect of heat transfer between phases ( S I A ) , response to transient disturbances in rate of flow of vapor ( 4 9 ) or reflux (Sil), and changes in the nature of fluid flow from laminar through turbulent flow to emulsification (29A, 42A). It is appropriate to note a t this point t h a t efficiency is reported to increase when emulsification occurs (47ii) and that optical reflectivity has been used to study foam density and interfacial area ( 10.4). Mass transfer has received the attention appropriate to its importance. Such aspects have been considered as transfer in bubble formation (ZOA,4511) and in condensation, evaporation, and diffusion (%?a). It has been shown that heat transfer can upset the application of conventional theories of mass transfer (2,l). The resistance to mass transfer between vapor and liquid ha5 been reported for some conventional packings (22'4, 4K1) and calculation

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of transfer coefficients by the penetration theory and by the boundary theory have been compared ( 1 1 A ) . The enthalpies and entropies of activation for condensation and vaporization processes are related to the coefficient of condensation (24A). The effect of reflux ratio on the coefficient of mass transfer has been treated mathematically (6.4). Somewhat more concerned with the study of the technique of distillation are papers which discuss the time required for a distillation (26=2), the effects of flow and hold up of liquid in the column on the rate of approach to equilibrium (46-4), and the effect of factors such as reflux ratio (I8.4), takeoff (12.4), and pressure drop (33.1)on the efficiency. A method of calculating the pressure drop in packed columns a t flooding conditions was developed (304). Several papers report that preflooding increases the efficiency of smoothsurfaced packings such as Raschig rings but has only a small effect on wound or woven packings which tend to be self-wetting (13Aj37A). d study of vertical, wetted-wall stills has shown that heat transfer occurs when the film ruptures (34.4)) that the temperature of the vapor is uniform as far as the interface of the vapor and the liquid (l4*4),that fractionation in such stills improves with increased reflux ratio (50d), decreased total pressure (50.4), and increased vapor velocity which tends to rupture the film (48.4). The latter observation is supported by the fact that, for open tubes, the graph of H T G against Reynolds numbers gives two straight lines with a change of slope a t about 1500 (%A). The slopes of the lines were found to be independent of other changes such as column heat loss, composition of the test mixture, and changes of pressure. Others have observed that the H T U of such columns is given by aZReSc where I is the diameter of the tube, Re the Reynolds number, Sc the Schmidt number, and a is a ronstant which varies with pressure and shows a minimum a t 380 torr (32-4). The mass transfer in such columns is controlled by the vapor film and by the mechanism of condensation and evaporation ( 1 7 A ) . The kinetics of distillation in systems which contain crystals, droplets (35L1, JlA4), or fogs (,9.4), and the effect of the presence of an inert gas (3&1) have been treated mathematically.

UNUSUAL METHODS OF SEPARATION BASED ON EQUILIBRIUM OF VAPOR AND LIQUID

These methods are useful in themselves, and they suggest new methods. Thus, a patent reveals t h a t the halogen deuteride in natural halogen hydride can be dissociated by radiation of the proper wavelength without affecting the hydride and the deuterium can be removed by chemical reaction ( 1 6 B ) . The patent points out that the method is not limited to this application. The elimination of traces of impurities by radiation during distillation is another possibility. Strong P-ray emitters have been placed in still columns to eliminate volatile material which can be polymerized by radiation ( 7 B ) . It has been suggested that selective infrared heating should permit the isolation of components. Hydrogen as a carrier gas would move distillate with slow pulsations toward a source of infrared radiation of selected wavelengths and the effect should be amplified as the result of the pulsations until significant separation occurred ( d B ) . There is some difference of opinion about the effect of electric and magnetic fields on boiling points. A change of *0.lo C. was reported to occur when a n electric field of 0 to 30 kv./cm. was imposed on boiling benzene, carbontetrachloride, methanol, ethanol, or isopropanol ( l 4 B , 1 8 B ) , b u t no such rise was noted by other investigators ( l B , 15B). It may be appropriate to recall that a n effect on the composition of positive binary azeotropes of a magnetic field has been reported ( I l B ) and noted in this review in 1958. A change in the rate of flow of a viscous, electrically conducting gas between long, coaxial, rotating cylinders has been observed when a n electrical or a magnetic field was imposed ( 8 1 B ) . It has also been observed that the vertical vibration of columns in which liquids and gases are in contact can result in the formation of bubbles of gas in the liquid ( 4 B , i B ) . This action could point to improvement in the contact of liquid with vapor in distillation columns. Other methods resemble gas liquid chromatography. A mixture of propane, butane, and isobutane can be enriched in propane by passing it through a column containing a solid packing with a nonvolatile, liquid coating (6B). Steam distillation of phenols through

packing coated with glycerol, cholesterol, or silicon oil is said t o cause separation

(8B)' Various types of cycling of boilup have been reported Defore and have appeared again ( I O B , f 2 B ) . The pulsing of pressure has been found t o aid efficiency a n d . capaci1,y (17B). Heat pumping devices have been used to alter the quantity of reflux in a column locally (QB, 19B), and i t has been observed t h a t distillat ion is sometimes more effective under r.onadiabatic than adiabatic conditions ( I SB). The selective adsorption of binary vapor mixtures is recommenced for the separation of low-boiling azeotropes (20B). A general discussion of the separation of liquid mixtures by dir3tillation includes t h e treatment of membrane distillation and thermal diffusion (SB). LABORATORY STILLS A q D ACCESSORIES

Automatic fraction collectors are always of interest. One of those reported responds to a change in dielectric constant (S7C), another t o mass ( S I C ) , and a third t o change in temperature of distillate with time ( I S C ) . A review of t h e field of fraction collectors of all kinds has also been published (SSC). Still heads for special purposes include a modification for opeoation a t -30' C. using a solution of CaCI2as the means of heat transfer and liquid air as refrigerant (I"), a reflux control by partial condensation a t a pressure between the vapor pressure of the lowest- and highest-boiling constituents (S4C), one using partial condensation which is particularly adapted to removing components present in very small quantities ( I C ) , and a manually operated type suitable for semimicio- or macrowork (SOC). Articles describe glass film-rectification stills for operatisn from temperatures below 0' C. (22C) t o 165' C. (f5C). A series of publications describes a number of different types of glass stills and accessories which are commercially available (SSC). A versatile bubblecap unit ( 7 C ) and a multiple tube still (232) both of metal can also be purchased. A multipurpose apparatus useful for distillation especially in organic work ( I S C , SSC), and a column packed with glass beads for petroleum, etc. (2C) have been described. There is also a general discussion of laboratory columns ( d S C ) , a review of glass a p r a r a t u s for laboratory separations (%?C), and a brief description of a laboratory for the distillation of food pioducts (29C). Microdistillation apparatus has been designed which can be operated with reflux ( I I C ) ,with h i g i boiling materials (SC), a t reduced pressure (f2C),with steam (such as Kjeldahl analyses or

separation of organic acids) (24C), for particulate samples from t h e atmosphere (SC), or for distillation and diffusion (2SC). To regulate the boilup of stills automatically, a mercury thermoregulator or capacity switch operated by a thermometer in the distillate a t the head of t h e column can be used to proportionate the heat supply to t h e pot (6C),or a n electrically heated thermistor (semiconductor) located in t h e stream of vapor entering the bottom of the column can be used t o sense the flow of vapor and actuate a motor-driven voltage regulator for varying the heat input (24C). There are also a number of special types of vaporizers for laboratory stills which are useful (SSC). An automated laboratory still has been described which has a means of controlling t h e reflux rate t o 5% and of controlling the composition of distillate from binary mixtures by varying the rate of take-off. The composition is monitored by dielectric properties and the reflux rate h y counting drops photoelectrically (8C). .\nother automated still makes use of thermocouples for sensing the operations (9C). h semiautomatic unit suitable for 5 to 100 ml. (4C) samples distilling a t -20' t o -100" F. has been constructed. More or less automatic stills for the analysis of petroleum (SOC), for the determination of the 50, 90, and 9591, overpoints of samples of petroleum from plant lines (27C), and a statistical method of predicting the composition of fractions of petroleum which can be used in automatic analysis ( I S C ) have been reported. A protection device for electrically heated glass stills (SBC), a n automatic cutoff device for distillation (IOC), and a n aluminum shield for heating mantles (19C) add to the safety of laboratory distillation. The efficiencies of 1 1 ~ - and 1/4-inch mesh cylinders called Rorad rings were found to be high even without preflooding ( f S = 1 ) ,and a mesh packing in the shape of hollow hemispheres with four tooth-shaped cuts is recommended ( 2 I C ) . Somewhat surprisingly it has been found that pouring hot solder through packed columns improves the heat transfer b u t does not increase the pressure drop (S5C). TESTING OF STILLS

A laboratory exercise has been outlined which uses benzene-chloroform as the test mixture and the difference in temperature between the top and the bottom of the column as the means for stepping off stapes on the vapor-liquid equilibrium curve ( I 2 D ) . The use of a test mixture of 2,3-dimethylpentane and 2-methylhexane has been shown to result, a t 500 theoretical plates, in d a t a

of the same standard deviation as that obtainable with n-heptane and 2,2,4trimethylpentane a t 225 plates ( 4 0 ) . Small concentrations of radioactive C136as the tracer element in mixtures of dichloroethane in benzene or of chlorobenzene in ethylbenzene have been recommended. For the first mixture, log CY = (52.8/T) - 0.1091, and for the second, log ct = (36.78/T) - 0.04491 where CY is the ratio of volatilities (130). A discussion of the advantages of using dilute solutions as test mixtures is now available in the collected report of the Brighton symposium (SD). It has been shown that log CY can be expressed as a bx cx2 dx3 for mixtures of bx cx2 for associated liquids, as a polar liquids, as a bx for nonpolar liquids with different molecular shapes or volumes, and is constant for nonpolar liquids with similar volumes and shapes (SO). The use of a single formula is recommended for expressing t h e Smoker equations (IOD). I t has also been recommended t h a t the Obolentzev-Frost equation should be used in place of t h e Fenske equation unless there is total wetting of the packing and unless only small samples are removed for test ( 5 0 ) . For small glass Raschig rings or nire spirals, the maximum number of theoretical plates which can be obtained by varying the boilup rate remains approximately constant for a range of reflux rates ( 7 D ) . This does not agree with the findings of others. T h e minimum reflux ratios recommended are so/l for fine packings and 25/, for coarse ones ( 7 D ) . A study has been made of the effect of various factors on the efficiency of columns packed with helices. Empirical equations have been developed to relate efficiency with diameter of the column, certain parameters of the packing, and feed conditions ( 1 4 0 ) . A method to determine the composition of te.t mixtures in columns bv ronductometric means has been studied (GO). The method does not require the removal of samples. A study of the change in temperature required to produce flooding in a column a t different pressure drops shows that such curves are parallel for homologous series of constituents in mixtures. The separation of binary mixtures is most effective when the column is operated a t the pressure drop where the divergence of the temperature-flooding curves of the constituent. i. greatest because their properties differ most in this range (SO). The behavior \Tas investigated of the relative volatility of binary mivtures which qhon limited mutual solubility such as TiC14-Fe2C16and SiClr-FeeCIG. I t was found that the results could be expressed by a A , B = a ,deal X ( B ) nhere a , d e n 1 is obtained by Raoult'. lawand X(BI is the mole fraction of the .lightly soluble component in the saturated solution of the

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other ( l l D ) . It has been found t h a t the addition of butylcellosolve to a mixture of n-octane and ethylcyclohexane increased their relative volatility more than t h e addition of 2-propanol ( 9 0 ) . A general index of the power of fractional distillation t o separate a series of hydrocarbons can be expressed in terms of relative volatilities ( I D ) . EXTRACTIVE DISTILLATION

The separations by extractive distillation will be touched on in the alphabetical order of one of the materials in t h e mixtures. Acetal containing alcohols and/or water can be purified by extractive distillation with polyhydric alcohols, glycol ethers, polyhydric amines, or alcohol amines. The crude acetal is passed into the lower third of the column, and the extractive agent enters the top a t a volume ratio of 4 t o 1 (8E). Butadiene and isoprene have been removed from a mixture of hydrocarbons containing four and five carbon atoms by extractive distillation with a mixture of 85% acetonitrile and 15% water @E). Butadiene can be separated from acetylene with furfural (4E) or ffom traces of vinylacetylene by furfural containing 770 of water ( 1 0 E ) . F o r separating butadiene from hlydrocarbons containing 4 carbon atoms, ethylcellosolve is better than chlorex or furfural and t h a t operation under conditions which produce emulsions greatly increased the effectiveness ( 6 E ) . T h e relative volatilities of benzene or of cyclohexane and 2,3-dimethylpentane are effected only slightly by the presence of (C4H9)3Nor Cl(CRCC1F)z C1, and these agents have been found to be ineffective for separating the mixtures (16E). The recovery of 91.6% of the 2-ethylbutanol from a mixture containing only 42% of the material has been accomplished by using 3 volumes of 2,3-dihydroxypropane as the extracting agent and operating a t 200 torr. The product was pure (6B). A patent reveals t h a t the separation of paraffin, olefin, and diene hydrocarbons containing 5 carbon atoms is greatly improved by using ethylenediamine and pyridine or its homolog as entrainers (1E). Mono-olefins have been separated from diolefins and paraffins by using dimethylformamide. T o reduce polymerization the mixture was diluted with a n inert hydrocarbon having one more carbon atom than the unsaturated material b u t boiling lower than the amide (26B). Olefins have been separated from paraffins using agents of the sulfolane type (%?E, B E , 2 7 E ) . The volatility of aromatic hydrocarbons can be lowered bv using the asphaltic fraction of crude oil. Nonaromatic materials can be distilled off (28E). Mixtures of 2- and 3-methyl-1-butanol, which should be a nearly ideal system,

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have been separated by distillation with 2,3-dichloro- or 2,3-dibromo-l-propanol. These agents were the most effective of 26 compounds tried. The average relative volatility was increased from 1.078 to 1.2 by using 92 mole % of extracting agent. A column which is especially adapted to continuous extractive distillation is described in connection with this work (14E). Phosphorus trichloride can be separated from azeotropic mixtures with paraffin hydrocarbons by extractive distillation with sulfolanes containing not fewer than 12 carbon atoms. I n the presence of dibutylsulfolane the separation factor of PC13 and 2,4-dimethylpentane is 1.42 ( f 9 E ) . This is especially interesting because it has been reported t h a t cyclic hydrocarbons can be purified by azeotropic distillation with PC13. Cyclic and acyclic hydrocarbons are also separated by extractive distillation with agents consisting of mixtures of furfural or phenol with various glycols in a range of compositions (17E, f 8 E ) . A patent reveals t h a t trimethylamine can be recovered from mixtures with methanol, mono-, di-, and trimethylamine, and water by extractive distillation with water in a 60-plate column a t 225 lb./sq. in. by proper recycling (15E). Another patent reveals that 70% vinyl acetate and 30% methanol can be separated into constituents which are 99% pure using water. The water is fed t o the top and serves to condense the vapors as well as to entrain them ( 2 1 E ) . The effects of various added components on the relative volatilities of mixtures have been studied to select promising agents for extractive distillation. A semimicromethod using GLC (gas liquid chromatography) has been devised for testing the effectiveness of such agents ( 2 9 E ) , and the effect of aniline on t h e separation factors of the following systems was determined using GLC: benzene and hexane; methylcyclohexane and toluene, heptane, or octane; octane and toluene (26E). The effect on the relative volatilities of hydrocarbons with five carbon atoms of such polar solvents as methyl nitrate and cyanide, triethyleneglycolbutyrate, aniline, phenylcyanide, and methylformamide was found to be small as was predicted from tests using GLC (QE). Methylformamide increases the relative volatility of isopentane and pentane 4.7 fold, isopentane and ethylene 2.8 fold, and ethylene and isoprene 2.2 fold (2OE). The effect has been calculated on t h e relative volatilities of hydrocarbons containing five carbon atoms of adding methylcyanide alone or with 10 or 15y0of water and of methylformamide alone or with 13 mole yo of water. Activity coefficients a t infinite dilution have been determined by ebulliometry for Cs-hydrocar-

bons ( I d E ) ,and activity coefficient ratios were determined for pentane and 1pentene with 15 entrainment liquids using GLC retention time (YE). When polar solvents are used with hydrocarbons in extractive distillation the amount of solvent carried over can be decreased by injecting 1.5% of water into the column below the condenser (SOE). For mixtures of three components one of which is minor and not very soluble in the other constituents, a solvent can be added to the stripping section of the column to remove this constituent and thereby prevent its separation intoasecond liquid phase (24E). A laboratory still made of standard glass parts which is suitable for pilot plant work ( S E ) ,and a laboratory still suitable for continuous azeotropic, extractive, or reactive distillation has been described ( 1 f E ) . rl system for the control of extractive distillation columns is based on the computation of the internal reflux ( I S E ) . AZEOTROPIC DISTILLATION

A review of azeotropic distillation with 51 references (44F) and a volume of collected d a t a have published recently @OF). Contributions to the knowledge of azeotropism have varied widely in nature. A method of predicting the tendency to form azeotropes is based on the ,4 and B parameters in the modified Redlich-Kister expression for G E (excess Gibbs free energy). When these parameters are calculated from experimental data, the magnitude of A and B determines whether a n azeotrope exists and the sign of A determines the type (72F). The composition of azeotropes at different temperatures and pressures can be determined analytically and the observation has been made that most binary azeotropic mixtures are equimolecular at t h e temperature at which the vapor pressure of both components are equal (Y7F). An empirical relation, Po 1 1 2 = a (To 4- 2.0309) 4- 3.5471, can be used to correlate the pressure and the boiling points of a n azeotropic mixture (79F). The relation applies t o binary or ternary systems. A study of the evaporation rates of a number of binary systems of hydrocarbons and alcohols showed that the rate is higher for a positive azeotrope and lower for a negative azeotrope than for the pure components (14F). A qualitative method for predicting the location and the range of a ternary azeotrope by connecting the three binary azeotropic points on the ternary diagram has been The raman demonstrated ( 1 R F ) . spectra of the system of benzene and methanol at 20' C. indicate a complex of C6Hs2 CH30H. In the system of benzene and cyclohexane, a flat maximum in the spectrum was observed

over a range of compcsition ( I F ) . The method of Lecat for estimating azeotropic d a t a for mixtures of homologous series was modified by considering molecular interaction in solution (76F). It is possible to predict the azeotropic range of mixtures containing methanol from t h e difference between the heat of vaporization of t h e riethanol and the second component and the dielectric constant of the second material. T h u s for nonpolar compounds whose dielectric constants were 1.8 t o 2.6 (compared with 33.7 for methmol a t 20' C.)> azeotropes occur for differences in heats of vaporization ranging from -1000 t o +3000 kcal./mol. For material with dielectric constants from 3 to 12, t h e range of difference in heat of vaporization decreases to --200 t o f2500. When the dielectric constants are 18 t o 28, the range of difference in heat of vaporization is only 300 to 1000 (49F). Binary systems of accltone with hydrocarbons containing five carbon atoms have been found t o dtviate increasingly from ideality in the order: dienes, olefins, paraffins ( 4 3 F ) . Hexylene glycol, aniline, and f L rfural were found to eliminate the azeotrope of 2,4dimethylpentane and benzene a t 760, 400, and 250 torr. About 33 mole yo of hexylene glycol was required while 20yo of furfural or aniline was sufficient. A change in the curvature of the graphs of relative volatilities a t low and high concentrations of the solvent can be attributed ti, a tendency to immiscibility of components (63F). It has been found ths t the composition of binary azeotropes of organic acids, RCOOH, with water can be expressed B or in in weight per cent by A log P mole yo by Ct D u here P is pressure in torr, and t is the t ?mperature in "C. T h e value of the cons,ants A , B , C, and D a r e : for R = H, 9.731, 49.464, -, - ; for R = ethyl, 2.623, 10.006, 0.073, 3.31; and for R == propyl, 7.071, - 1.890, 0.046, 0.53 (7SF). I n the same way,, the addition oi sulfuric acid or calcium chloride to the azeotrope of HC1 and water causes a change in composition. More than 40% of sulfuric acid or %yoof calcium chloride causes t h e azeotrope t o disappear ( 4 I F ) . Graphical studies of heterazeotropic system of ryclohexandj cyclohexanone, and water showed t h a t the effectiveness of heterazeotropic rectification was equivalent to a vacuum rectification a t 40 torr without the water. Graphical methods for calculati i g heterazeotropic rectification proresstss (1%') and a general method of calculating the concentration and the amount of liquid and vapor phases in the distillation of ideal multicomponent systems in the presence of a n inert gas such as steam (39F) have been developed. For the study of binary and ternarj azeotropes, a n apparatus consisting of two ebullio-

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meters and a means of maintaining constant pressure t o 1 0 . 5 torr has been described (58F). A method for determining the composition of positivenegative azeotropes has been described in detail ( I O F ) . D a t a on binary azeotropes are given in Table I. The data on the other azeotropic systems appearing in the period of this review are given in various forms so that it is not easily tabulated. A more or less alphabetical order using the first letter of the name of t h e constituent nearest the beginning of t h e alphabet has been followed in t h e following lists. Where the d a t a are available, the boiling point of the azeotrope is given in "C. followed by the composition in per cent of the constituent used for the order of listing, type, and the pressure in torr a t which the data were determined. Ternary Homoazeotropes. =Icetone, chloroform, a n d carbon tetrachloride (results of a s t u d y of this system are presented as a triangular rectification diagram) ( 2 4 F ) ; aniline, phenol, and n-tridecane (184.45; 48.5, 33.5, 18.0; pos.-neg.; 600) (169.37; 47.0, 33.7, 19.3; pos.-neg.; 500) (161.71; 46.7, 33.5, 19.8; pos.-neg.; 400) ( 6 d F ) ; butanol, dioxane, and toluene (101.8; 20.7,44.3, 30.0; pos.; 760); butylacetate, 35.2, 27.4, butanol, and water ( - ; 37.4; double and triple azeotrope; 760) ( 2 5 F ) ; ethylene glycol, phenol, and 3-picoline (186.41; 15.9, 67.7, 16.4; saddle; 760) (5OF); formic acid 1,2dichloroethane, and water (- ; 35.2, 27.4, 37.4; double and triple azeotrope; 760); (7OF); nitromethane, methylpropionate, and methylcyclohexane (nonazeotropic) (75F). Quaternary Homoazeotropes. Acetic acid, ethylbenzene, nonane, a n d pyridine ( - ; 17, 18, 38, 27; saddle; 760). [With some compositions of t h e system, a n abnormal d r o p in temperature occurs a n d fractions of varying composition are obtained. An explanation is given (9F, I O F ) . ] Separations obtained by azeotropic distillation will be listed in the alphabetical order used for t h e d a t a on azeotropes. iicetone has been separated from isopropanol by taking off a side stream of alcohol-water azeotrope ( 5 7 F ) . It has also been separated effectively from methylcyclohexane by using 90% water and distilling a t 720 torr (26F). Acetic acid can be dried by adding furfural to form the azeotrope with water (69F). It has been separated from formic acid by using chloroform (45F). 1,3-Butadiene and 1-butene separate when distilled with methylamine (XIF ) . Pure benzene and toluene are obtained from hydrogenated materials by using water and acetone or methanol ( 1 7 F ) . A good graphical

method for calculating the parameters of the azeotropic distillation of benzene and ethanol has been published (46F). Cyclopentane and 2,2-dimethylbutane were separated from gasoline with aniline using a 15-plate column ( I S F ) . A reaction mixture of esters was dehydrated with a n organic azeotroping agent (748'). Ethylbenzene has been separated from technical xylene using ethylcyclohexane and a fractionating column of 25 theoretical plates (29F). Aromatic hydrocarbons such as oxylene can be separated from petroleum products with methanol (66F). Better separation of solvent mixtures such as halogenated hydrocarbons, cyclic hydrocarbons, and alcohols by azeotropic distillation can be obtained if some of the distillate is returned to the column a t a level different from that at which it is withdrawn ( S F ) . Water aids the separation of light aromatic. hydrocarbons from pyrolyzed material (32F). A study of t h e effect of dimethylformamide on mixtures of 2-methyl-2butene and isoprene showed t h a t the relative volatilities change from 0.86 without additive to 1.27 with 50 mole yo and t o 1.50 with 80 mole yo of additive. Higher concentrations cause a decrease in relative volatility (6'7F). The dehydration of pyridine with benzene or carbontetrachloride can be done conveniently in a laboratory still with a settling trap from which one of the phases can be returned to the column ( 6 4 F ) . The still can also be used for dehydration of ethyldihydroxymalonate using chloroform, benzene, or toluene. Some ethyloxomalonate resulted (65F). It is also adapted to the preparation of ethyloxalate using benzene for dehydration (65F). Azeotropic dehydration 1% ith benzene can be used to produce 5-methyl-3-carbethoxyisoxazole from ethylacetylpyruvate and hydroxylamine (65F). A modified DeanStark trap automatically collects and drains water in azeotropic distillation (2F). Steam distillation has been used to obtain n,n-diethyltoluamide using a column with 10 t o 12 theoretical plates operated a t 40 to 75 torr pressure (73F). It has been found that steam distillation of mixtures of o-cresol and ortho-

CH3Ce,H40CH2CO2HO-C13--OCH~COZH with chlorobenzene produces a high yield of o-cresol ( 5 d F ) . Steam distillation of lactic acid flowing over a conical sieve produces a good grade of acid (71F ) . I n binary azeotropic systems with low mutual solubilities, d a t a can be predicted by Paster's bilateral formulation if enough of the properties of the pure components are known. The validity of the method was shown with the water-acetic acid system (54F). The methods of rectifying such systems can VOL.

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Table 1.

Binary Hornoozeotropes Azeotropic data ?.P., Comp., C. 9% Pressure .- A 38.6 88 15 Atm. 103.80 0.3 400 Torra 148.94 13.2 760 Torr". 182.93 86.2 760 Torr 174.20 85.4 600 Torr 167.42 84.2 500 Torr 400 Torr 159.43 85.4 i60 Torr 185.67 58.1 177.21 57 7 600 Torr 500 Torr i i o .73 57 .o 400 Torr 168 14 56 6 760 Torra 175 31 57 5 GE = 140 cal./mole GE = 74 cal./mole GE = 71 cal./mole

B Ref. Type Max. Vinyl chloride 4F n-octane Min. 59F Pionane Min. 60F Min. n-Tridecane 62F Min. n-Tridecane 62F n-Tridecane Min. 62F n-Tridecane Min. 62F Max. Phenol 62F Max. Phenol 62F Max. Phenol 62F Max. Phenol 62F Max. n-Undecane 61F p-Cymene 27F Mesitylene 27F 27 F Pseudocumene b %-Paraffin hydrocarbons 26 F 760 Torr Min. 116 .€? 85.7 Styrene 62F 9 0 . 1 Torr Min. 88.3 Styrene 62F 6W 50 Torr Min. 62F 81 .o 46. 3c Styrene 760 Torr Max. 45.1 104.6 Toluene 16F SOF 25.8 31.86 Pentane Min. 200 Torr 22F 98,72 18.35 Water 1-18 Atm. Cyclohexane 48F 1-18 Atm. Propanol 48F 1-18 Atm. Methylethyl ketone 48F 760 Torr 7.4 Cyclohexane Min. 32.2 760 Torr 105.5 Toluene 693.2 Torr 47d 56f Ethylpropylether 64.5 Max. 41" 760 Torr Methylethylthioether 66.6 SgF Min. 760 Torr Titanium tetrachloride 87 55F 105 1-18 Atm. Methylettiylketone 48F 1-18 .Atm. Propanol 48F Non. 76F Toluene Non. 760 Torr Diprop ylether 36 F Non. 760 Torr 56f Diethylthioether Min. 760 Torr 135.1 m-Xylene 55f Min. 760 Torr 134.5c 63F p-Xylene Max. 70F Water Min. 70f l12-Dichlorethane Min. Quinoline 34F Quinaldine 34F Min . 760 Torr 30.0 84.6 58f Methanol Min. 760 Torr 6F 94.6 Isopropyl borate 760 Torr 11F 107.2 93' m-X y 1ene 760 Torr 11F 92' p-Xylene 107.1 Non . 11F 760 Torr o-Xylene Non. 76F Dioxane Min. 760 Torr 75F 101.2 44.0 Toluene Kon. Quinaldine 34F Min. 7-Methyl quinoline Max. 760 Torr 33,30 Potassium chloride Min. 760 Torr 84.9 29.6 Hexane 42F Min. 25.75 760 Torr 51.5 Isoprene 42F Min. 760 Torr 24.657.6 Trimethyl ethylene 42F Min. 8 Torr 25f 760 30.7 Isoprene Min. 760 Torr 23 F 6.5 27.6 2-Methyl-1-butene Min. 760 Torr 25 F 10 33 2-Methyl-2-butene Min. 760 Torr 23F 1.6 4.0 3-Methyl-1-butene Min. 23F 760 Torr 8.5 26.8 1-Pentene Min. 68F 750 Torr 15.4 37.2 Dichloromethane Min. 760 Torr 8F 23.1 99.3 Water Min. 75F 760 Torr 60.5 81 . 7 Methyl nitrate Max. 76F 760 Torr 11.5 79.3 Methyl propionate Max. 57f 760 Torr 102-106 32-36 Water Son. 75F Nitromethane Min. 760 Torr 7F 66.2 Water 99 Min. 6F 760 Torr' 4.35 _ _ -2 Water Min. 6.2F 83.1 760 Torr 180,56 n-Tridecane Min. 62F 600 Torr 82.3 172.24 n-Tridecane Min. 62F 82.1 500 Torr 165.87 n-Tridecane Min. 400 Torr 62F 81.8 162.20 n-Tridecane b 88F n-Paraffin hydrocarbons i Methanol 40F i Ethanol 40F i Propanol 40f i Butanol Max. 70.2 760 Torr 147 o-Xylene 1,1,2,2-Tetrachloroethane k 31F m-Xylene 1,1,2,2-Tetrachloroethane b 28F n-Paraffin hvdrocarbons m-Xylidine Disagrees with Horsley GE decreases with increase in boiling point of azeotrope. a Also determined a t a range of 400 to 760 Torr. . in pressure decreases concentration of alcohol. 0 Mixed dimers or RM,C12 . = 1 . 4 4 2 8 ~ ~f ~Decrease ( 1 9 F ) . R I = 1 . 3 9 7 1 ) ~ ~* ~RI postulated. * The change in relative volatility with composition was greater for systems of methyl formate with paraffins and dienes The lower pressure limit of azeotropism was also determined (51F2). 9 Gives change of aaeotropic than with olefins and dienes. composition with pressure. IC Some evidence for an azeotrope a t 99.4 mole per cent m-xylene. A Ammonia Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Aniline Acetic acid Acetic acid Acetic acid Acetic acid Acetone Acrolein Benzene Benzene Benzene Butanol Butanol Chloroform Chloroform Chloroacetyl chloride Cyclohexane Cyclohexane Dioxane Ethylene trichloride Ethylene trichloride Ethylene glycol Ethylene glycol Formic acid Formic acid Isoquinoline lsoquinoline Isoprene Isopropanol Isobutanol Isobutanol Isobutanol Isobutanol Isobutanol Lepidine Lepidine Magnesium chloride Methyl formate* Methy lformate* Methylformateh Methanol Methanol Methanol Methanol Methanol Methanol Methylacrylic acid Methylcyclo hexane Methylcyclohexane Methylhydrazine Methyl propionate Nitric acid Propionic acid Phenol Phenol Phenol Phenol +Toluidine Toluene Toluene Toluene Toluene

60R

ANALYTICAL CHEMISTRY

%

2;

4%

be classified either as producing distillate containing less of the volatile component than the heterazeotrope or producing distillate identical with it (3%'). SUBLIMATION

Ak study of the sublimation of naphthalene showed that the mechanisms of transfer of heat and mass can be classified by ra iges of pressure : (1) between 740 and 60 torr, transfer is by the boundary process of molecular diffusion; (2) at 40 t o 0.1 torr, it becomes a disordered molecular transfer ; (3) from 0.1 to 0.07 torr, a molecular viscous regime appears; and (4) at Knudsen numbers greater than 0.04, the transfer is by undisturbed molecular motion to and from t3e exchange surface (16G). I t has heen noted that plates are formed in a g;as stream during bhe formation of a turbulent' boundary layer (15G). The velority of movement of sublimation fronts in laminar flow of incompressible gas near a critical velocity can be calculated (18G). A study of the p-dichlorobenzene-p-dibromobenzene system showed that solid solutions of 9.9, 29.9 50.0, and 70.1 mole yoof the rhloro-compound were in equilibrium with vapoi. containing 66.7, 87.1, 91.2, and 95.8 mole % of the same constituent a t 50' C. For p-dichlorobenzene and p-bromochlorobenzene the compositions were 10.0, 29.9, 49.9, and 69.9 mole 7o in the solid and 21.9, 59.1, 79.8, and 80.3 in the vapor (SG). T h e solid-vapor equilibrium of CdTe ( I S G ) , the constitution of the vapor in equilibrium with solid PuF4 ( I G ) , and the rates of sublimat,ion of the GeS2 and GeS (17G) have SO been investigated. The in teres ting observation has been made that multicomponent films of stoichiometric composition can be obtained by beam evaporation in vacuo (12G). Another so newhat curious observation is that urea and thiourea immediately crystallize as adducts when they are sublimed c'nto surfaces of adductable material s i c h as cetane in decalin. They condense in t'heir usual forms on surfaces of nonadductable material (5G). Crystalline films of CdS are obtained by slow evaporation in vacuo. The rate of evaporation can be slowed by adding alumina crystals (4G). For sublimation or distillation of materials which melt near their boiling points, a n arrangement has been patented for keeping i.he condensate in contact with the valiors so that it does not solidify. The device has been adapted to handling mixtures of T a c k NbCls. %rCI,-HfCI,, and PC16-A1C13 (IOG). Another patent uses circulating cold water t o rc'more sublimed mnitro-p-toluidine ( 9 G ) . Other patents dewrihe apparatus for sublimation (2G,

6G, 11G). A device of convenient design for laboratory sublimation (8G, 14G) and a precision fractionating apparatus (SSC) can be purchased. A method of analysis of small quantities of materials such as mixtures of UFs, CIFs, CIS and CIF in which the constituents have distinctly different vapor pressures is based on very slou evaporation or entrainment in a stream of gas ( 7 G ) . hlixtures of anthracene with anthraquinone (19G),all the isomers of aminobenzoic acid with o-nitrophenol (19G), hydroquinone with pyrocatechol (2OG), pyrogallol with phloroglucinol (ZOG), o-phthalic with terephthalic acid (WlG), and a- \\ith p-benzene, dicarboxylic acid (2IG) were separated by sublimation Homever mixtures of hydroquinone with resorcinol (2OG), pyrocatechol 1%ith resorcinol (WOG), aminobenzoic acid with p- and m-nitrophenol (19G), vanillin with isovanillin (21G),and a - and p-naphthol (21G) were only partially separated. ?-Benzenedicarboxylic acid is difficult to separate from the a-isomer by sublimation (21G). V A C U U M A N D MOLECULAR DISTILLATION

Approximate and precise methods for calculating the number of transfer units required for the vacuum rectification of binary mixtures are available ( 1 9 H ) . Tests with 2,2.4-trimethyIpentane and toluene a t 760, 500, 200, and 50 torr show that the height of a theoretical transfer unit in wetted-wall columns does not change much with pressure in the laminar or turbulent flow range ( 2 H ) . The effect has been studied of flow rate, channel diameter. gas phase turbulence, physical properties of the components, and pressure on efficiency in film rectification ( 8 H ) . Experiments Rith the distillation of a mixture of phenol and cresol in a packed column indicate that efficiency is independent of pressure a t 5 to 50 torr. I t decreased with f l o ~rate and the pressure drop u as not proportional to the square of the flow rate divided by the density of the vapor as is usually found for turbulent flow ( I I H ) . Rotating columns and others operating at pressures from 1 t o 20 torr (IOU).a glass film-evaporator (I?"), a vacuum evaporator for radioactive materials heated by electron bombardment ( S H ) ,and a barrel-size pilot plant still suitable for operation a t 0.05 t o 50 torr ( 5 H ) have been described. For larger operations a t reduced pressure, columns have been made in which a series of vapor ( 1 2 " ) or liquid pumps ( I H ) on a single shaft through t h e column reduce t h e pressure in stages. =Z glass take-off device for vacuum TX ork ( 1 8 H ) and a thermostatically operated valve located in the vacuum line which decreases bumping (411) are simple but useful accessories.

A number of general discussions of techniques of molecular distillation with technological applications are available (6H,9 H , 15H, 16H,2 S H ) . A vacuum fractionator has been described (21H , 2 2 H ) which is somewhat similar to the brushing still of Perry and Cox ( I S H ) . hnother somewhat similar still has a rotating, externally heated borosilicate glass cylinder with a stationary condenser consisting of concentric cones. The condensate forms on the under side of t h e cones and drains t o form a countercurrent moiement of liquid and vapor (24H). Another has a heated inner cylinder over which liquid flows and is spread by a rotating spiral ( 7 H ) . Other described apparatus includes a high vacuum still for high melting materials ( l C H ) , a still readily converted from high vacuum fractionation to molecular distillation (2OH), and a convenient laboratory device for sublimation or molecular distillation, available commercially (8G). APPLICATION OF DISTILLATION T O ANALYSES, SEPARATIONS, A N D SPECIAL PROBLEMS

Analyses. Detailed directions are given for t h e distillation of t h e following systems for t h e purpose of analysis: volatile acids [from their salts with steam (94Z)I; small amounts of arsenic [for polarographic analysis ( 4 9 Z ) ] ;traces of Br2 (0.1 p.p.m.) [from large amounts of C12 for electrometric titration (1191)]; fluorine compounds [from ores with steam (2OZ)I; [macro amounts by steam distillation for subsequent titration (16Z)I; moisture [from powders for near infrared spectrometry ( l S Z ) ] ; benzene and xylene [for assay distillation of commercial toluene (92Z)I; carbon dioxide [directly from solution (701-721, lO5Z)l or resulting from decarboxylation (681;. 69Z, 4Z)]; protein [from milk with steam ( 1 0 7 1 ) ] ; and selenium [with HCI and 13r2 followed by reduct,ion to the colloidal state with SnC12 for photometric determination (58Z)l. The compositions of cuts from the distillation of petroleum are related to the composition of their pyrolysis products (7.51). Special Problems. Rectification columns have been used t o produce chemical reactions such as the preparation of ChJ, C2H5Br, and CH3*ksOsSa2 (731) and to study reaction rates such as aniline with benzoin (821) or the rearrangement' of fatty acid anhydride (541). Evaporative resistance is a sensitive measure of the purity of monolayers (601). Vaporization has also been used to determine molecular weight by differential ebulliometry (IOdZ), by carrier vapor in heterazeotropic systems (901), and by isothermal distillation (44Z)adapted to microquantities of petroleum oils VOL. 36, NO. 5, APRIL 1964

61 R

(351) and

(38I , 481).

other organic substances

Separations, Inorganic. Distillation in various guises is more widely used t h a n is generally realized for t h e purification of inorganic elements and compounds. Some inorganic materials are difficult to distill because relatively high temperatures such as the electric arc (631, 71) or the solar furnace (10%) are required. However, a high degree of separation can sometimes be achieved by distillation of inorganic materials (261,51). Distillation of alloys in the arc during examination of the spectra has been discussed theoretically (631, 7Z). The ion microscope has been used t o study evaporation of R-hfo and W-Ta systems. Mo takes part in the crystal growth of the probe a t all temperatures but Ta does not. The difference is attributed to a difference in atomic diameters (831). A curious observation has been made that zinc vapor sho\$s zero thermal accommodation in flowing through a borosilicate glass tube (911). Papers and patents which describe distillations of inorganic materials are listed alphabetically below. Mercury and water have been omitted because they have received enough attention to constitute special fields. It is hoped that some of these references will be of help to others wiith similar problems. Arsenic distilled [as the element in vacuum or inert gas (rl21) or with Se from metallurgical dusts (36Z) ] ; aluminum [as h l C & from FeC13 (211), as the element by passing vapors of NaC1 through the molten metal ( I I Z ) , or as the element from impure alloys (871)l; antimony [as sulfide from ores (1021)l ; barium [grain-by-grain evaporation of the titanate (761)]; boron [as BSHsfrom other boranes with Lewis base @ S I ) , as B5Hg a t reduced pressure and with heat t o decompose impurities (531), as borates in very small traces using CH30H and H3P04 (741), as H3B03 by esterification with CH,OH (771)]; cadmium [as the element in vacuo to reach a purity of 99.99994% ( S I ) , as the element in a 10- to 18-plate column of quartz at reduced pressure or in Ar (can be used for Hg or Zn) (IQZ)]; fluorine [as the element in a vacuum-jacketed stainless steel still with a capacity of 0.5 lb. (QQZ)]; germanium [as halide (2061, 671), as halide discussed theoretically (also Si, Ti, Sn, and B) (181),as halide in small amounts from HC1 solutions (QZ), or as hydride (Z7Z)I; hydrofluoric acid [as solution in polyethylene still under reduced pressure to produce semiconductor grade acid (881)] ; iron [as element in vacuo to remove P, 0, hs. S, Sn, Cu, A h , and minor elements (291, 85Z), and cobalt as chelates of 8-diketone (12Z)I; lanthanum [as oxides in a solar furnace to remove cerium

62 R

ANALYTLCAL CHEMISTRY

(27Z)I; lead [as element at 600" C. to remove Zn (66Z),Cu, and other trace contaminants (4ZZ)I; potassium [as element from sodium by molecular distillation (641), as chloride from contaminants (5Z) ; molybdenum [as chlorides or oxychlorides to remove Wo (129Z)I: magnesium [as element in vacuo t o separate MgZ4, MgZ5, and MgZGusing a n alloy with Ga (a for MgZ4/i\Igz6is 1.034 =t 0.003) (84Z)l; manganese [as the element by sublimation in vacuo (114I)l; phosphorus [as t h e element in inert atmosphere (112Z)l; ruthenium [as tetraoxide with oxidizing acid (uses RulOG as tracer) (65I)], radioactive materials [as elements in vacuo (studied Hg, In, and Cd) (%I), cf. (191)]; silver [as chloride from mixture with NaCl to remove Pb, Zn, Cu, Fe, Ca, and Mg (1271)]; sulfur [as the element from traces of Se using packed or plate columns (281, 801); silicon [as silane (171), as SiHC13, or as SiC1, (compared distillation \+ith other methods) (IZ), as element bv zone melting combined with distillation (891) or as SiHC13 from which the element %as subsequently grown as crystals (371)1; titanium [as oxide, sulfide (344,or as TiC14to free it from TiOC1, (IlSZ)]; tellurium [as the element by multiple distillation ( I l O I ) , 1 1 1 Z ) I ; thorium [as oxide to purify the element ( Z Z Z ) ] ; zinc [as the element from Pb (231, 301, S l l ) , as the element for purification (591), as the element to obtain very high purity (99.99995%) (141, 451),as halide (a compound of the element with the halide is suspected which disproportionates when cool) (511), or as the element from a fused salt bath (1091)l; uranium [as Uf6 from BrF5 in a small packed column (451)] ; zirconium [as halide with P0C13 under pressure to remove Hf, Cb, Ta (781), as the element in a n electronbeam furnace @I)]. Separations, Organic. References to the separation a n d analysis of petroleum have been omitted because t h e field is covered by the Review of Petroleum Technology (29.4) and other literature devoted exclusively to petroleum. Anthracene [95y0 pure from crude material containing naphthalene and other constituents by using tandem columns of 21 and 45 plates (15Z)bI; ammonia [from water (pressure, vapor flow rate, and reflux rate had little effect on the efficiency but ab01 e 99.9% or below 15% S H 3 the efficiency fell sharply) (391)l; ethyltoluene [isomers separated using 40 theoretical plates (621)]; carbon diovide [from cyclohesylamine carbonate by azeotropic distillation with HzO a t 4.5 to 5 atm. pressure (961) ] ; 2,5-dimethylparaffins [from reaction products in 1-meter spinning band column (%9Z)] ; terephthalate esters [from crude misture

by adding high boiling acid and distilling in a stream of CH30H vapor (861)l; bis(2-ethylhe~y1)phthalate and sebacate [from commercial plasticizers (for diffusion pumps) (46Z)l: essential oils [by exhaustive steam distillation (971)l; fatty acids [by hydrolysis of fats with steam a t 450 to 500" F. combined with distillation (47Z), from synthetic mixtures (QSZ)]: CISfatty acid methylesters [by GLC and extractive distillation (40Z)I; glycerides of fatty acids [by molecular distillation (6Z)I; 2 furfuraldehyde [from residues by modified steam distillation (651)l;guaiacol [from methylated pyrocatechol by distillation a t 5 to 20 torr pressure through 15 to 18 theoretical plates ( I l S Z ) ] ; glycols [from products of hydrogenolysis of xylitol at 5 to 10 torr pressure (8Z)I; a and 8 ionones [from reaction mistures by vacuum distillation (521)]; micelles of sunflower oil [from naphtha solvent in a stream of COz (IOZ)]; methylchlorosilanes [from mixtures of silanes (calculated the optimum reflux ratio and number of plates) (571. S l Z ) ] ; lactic acid [from mistures by steam distillation (93Z)I; H C X [polymer-free by distilling with a stabilizing acid as a carrier (1201)] ; o-nitroanisole [purified by modified methanol distillation (561)l; cyclohesanone [from cyclohexanol (gives vapor pressure and calculates that a 1 : 1 mixture can be separated by distillation a t 80" C. to give 99% cyclohexanone) (411)] ; organic solvents [from absorbent by steam distillation (1081)1; optically active paraffin hydrocarbons and alcohols [from reaction mixtures (fill)]; rice bran oil ' [from crude material by molecular distillation ( I O l Z ) ] ; synthetic organic moderatedreactor coolant [by flash distillation ;% ](1)' nonionic surfactants [by molecular distillation (1001)] ; styrene [from residues of reactions used in the synthesis of styrene from ethylbenzene ( I I S Z ) ] ; irradiated tributylphosphate [by molecular distillation (641, 961)] ; triethylenediamine [from piperazine by adding alkyl- or hydrogenated-hydrocarbon, or other addatives boiling between 135" and 180" C. (431)l; p-xylene [from reaction products of tert-butylation of mixtures with mxylene to produce a high-purity product (501)I. VAPOR-LIQUID EQUILIBRIA

As usual, a large amount of work has been devoted to predicting, correlating, and testing data for consistency. The conditions which data must meet to be used in calculations involving the Margules and the Scatchard-Hamer equations have been discussed (39J). These conditions are also good means of testing data. The method of Redlich and Kister for verification of data based

on vapor-liquid equilibria was studied for nine systems of considerably different types (95). ?'he application of the Gibbs-Duhem eq iation for testing the thermodynamic consistency of vapor liquid equilibrium has been investigated ( 4 4 . Thermodynamic consistency tests have been proposed based on visual methods (585) and on heats of mixing ( 4 4 5 ) . Fifteen methods for testing the d a t a of binary mixtures of nonelectrolytes have been reviewed (525). Data can also be evaluated from the total pressure and composition of binary liquid mixtuies ($75). D a t a for binary and normtl paraffin hydrocarbons can be calculated by equations involving boiling point, critical pressure, and temperature, and the vapor pressure of the higher boiling constituent (275). The methods of prl2dicting equilibria of binary hydrocarbon systems are numerous. Among them are equations of state ( 5 6 5 ) , a n ideal K value dependent on critical constants and corresponding states ( 5 4 4 , a K which depends on composition (875), a perturbation on Raoult s Law (assuming

t h a t the condensed phase is ideal and t h a t the vapor phase obeys a virial equation of state) ( 8 4 4 , a method of calculating Van Laar constants for mixtures of polar with nonpolar constituents (215), a method using the activity coefficients and fugacities of pure components ( 6 6 J ) , and a n empirical method without experimental d a t a (1135). For nonideal binary systems, the equilibria can be calculated from the differential of the van der K a a l s equation and the dependence of the differential heats of vaporization on the temperature (68J, 2QJ). A method has been proposed for calculating the activity of a solution from the properties of pure components and ultrasonic d a t a (305). Activity coefficients of constituents have been calculated by applying statistical thermodynamics to a lattice picture of the liquids (1105). An estimate of the mole fraction of a constituent in the vapor can be obtained from the product of the change of mole fraction of one constituent in the vapor with pressure and the difference between

the total pressure and the vapor pressure of the other constituent in the pure state a t a given temperature ( 1 3 0 J ) . For correlations in the critical region, equations using reduced states have been used ( 6 5 5 ) and the effect on tests for thermodynamic consistency when one component is above its critical temperature has been discussed (15). For binary mixtures which are only partly miscible, a basic equation for thermodynamic consistency can be given (1015), a n integration method can be used for calculation (1145), or the Margules and the ScatchardHamer equations can be used with a simplified method of solving for the A and B values (1045). The effect of association of molecules ( 6 4 5 ) and of isotopic substitutions (884 on equilibrium has been analyzed and a means of correlating the vapor-liquid equilibria of azeotropic mixtures has been developed (128J). A nomograph (315) and a graphical method called zygography (125J) can be used to present much data compactly. An empirical method of predicting the

Table II. Vapor-Liquid Equilibria

Binary Mixtures A

Acetic. acid Acetic acid Acetic. anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetic anhydride Acetone Acetone Acetone Benzene Benzene Benzene Benzene Benzene Benzene Benzene Benzene Benzene Benzene Benzene 1,3-Butadiene Butane 2-Butanone 2-Butanone Cadmium Cadmium Carbon monoxide Carbon tetrachloride Carbon tetrachloride Carbon tetrachloride Chloroform Cylohexane Cyclohexane Cyclotiexane Cyclohexane Cyclohexanone Diborane 1,2-Dichloroethane Diethylamine Diethylene glycol Diethylene glycol

B Propionic acid Waterb Acetic acid Cyclohexane Methylene diacetate Pyridine Acetone Acrolein Chloroform Pyridine n-Alcoholsc Cyclohexane Ethyl acetate Ethylene Ethyl etherd n-Hexane n-Hexanee n-Hexaned Methyl cyclohexane Tetrachloroethylene Trichloroethylene Chloroprene Nitrogen Ethylacetate 2-Propanol Antimony Zincb Methane Butanol Ethanol lt1,2-Trichloroethane 2-Propanol Ethyl etherd 2-Met hylpent anol-4 Pyridine Toluene Cyclohexanolf Hydrazine 1,1,2-Trichloroethane Triethylamine o-Xylene Toluene

Ref. lOSJ 118J 41 J 41J 41J 41J 123J

45J

129J 41J 106J 109J 1lJ 63J 48F 26 J 86 J 485 103J 47F 85J

$.E 7 2J

72J 98J 1 2 J , 18J 1165

36J S6J 485 72J 48F 91 J 41J 14J 19J 19J

A

B

Dimethyl formamide Di henyl Etgane Ethanol Ethylacetate Ethylacetate Ethylene chloride Ethylene glycol Furfurol 3-Heptene 3-Heptene Hydrazine Hydrazine Isoprene Isoprene Isoprene Isoprene Methanol Methyl acetate Methylene chloride Methylene chloride Methyl formamide Phenol Phosphorus oxychloride Phosphorus pentachloride Phosphorus trichloride Propylene Sulfur Terphenyls tert-Butanol Tin Titanium tetrachloride Titanium tetrachloride Titanium tetrachloride Triethylamine

Methyl formate Dip henyloxide Propane Toluene Toluene p-Xylene Kerosine Water Water@ Toluene +Heptane Dimethylhydrazine (uns) Water a-Isoamylene 6-Isoamylene 7-Isoamylene trans-Pi erylene Met hylgr mate 2-Propanol Methylethylketone Water Water sec-Butylbenzene Silicon tetrachloride Silicon tetrachloride

Ref.

Silicon tetrachloride Water Selenium High boiling material Water Zinc Carbon tetrachloride Silicon tetrachloride Trichloroacetyl chloride Water

96 J

$E

67J 11J 11J 1S2J 78J 117J 126J 126J 8 3J l8lJ 225 22J 225 22J 60J 7 2J 61J 61J

SSJ 119J 76J 76J 765 65J 16J, 16J 8J

rJ

93J, 18OJ 122J 122J 55J 51J

4:;

105J 106J

(Continued)

VOL. 36, NO. 5 , APRIL 1964

63 R

Table II.

Vapor-liquid Equilibria" (Continued) Ternary Mixtures A B C Ref. Acetic acid Water Chloroform 1OJ Acetic acid Water Formic acid 7OJ Ammonia Sulfur dioxide Water lllJ Ammonium fluoride Ammonium acid fluoride Water 76J Benzene Carbon tetrachloride hlethanolh 36J Benzene Carbon tetrachloride Ethanolh 36 J Benzene Carbon tetrachloride Propanol* 36 J Benzene Carbon tetrachloride Butanol* 36 J Benzene Chloroform Methylacetate 73J Benzene Chloroform Methylethylketone 53J Benzene Cyclohexane Propand 68J Benzene Cyclohexane Trichloroethylene 9dJ Benzene n-Hexane Methylcyclopentane ZJ Benzene n-Hexane Ethylenediamine 6dJ Butane Ethane Pentane 34J, 67J Butane Methane Xitrogen Cadmium Silver Zinc Chloroform Methanol Ethylacetate 74J Dibenzyl n-Hexadecane Phenanthrene5 60J Ethane Methane Xitrogen 13J Ethane Methane Hydrogen 13J Ethanol Citral Water lO7J Ethylbenzene n-Octane Cellosolve 71J Ethylene oxide Ethylene cyanohydrin Hydrocyanic acid Helium hlethane Nitrogen 4:: n-Heptane 3-Heptane Toluene 126J Hydrazine Water Dimethyl hydrazine (uns) 8dJ Methylene chloride Methylethylketone Water 61J Titanium tetrachloride Phosphorus oxychloride Vanadium oxychloride 77J Quaternary System B C D Ref. 4 Ethanol Benzene n-Hexane hlethylcyclohexane ZJ Ethanol Benzene n-Hexane Methylcyclopentane Ethane Methane Hydrogen Nitrogen 1-Butene Furfural Isobutane Water 42J Kot included in table are ammonium complex systems ( 6 9 J )and methyl esters of fatty acids with 6 to 18 carbon atoms in the parent acid chain (Q5.J). * At high pressure. In critical region. At 1 to 18 atm. pressure. The relative volatility changes with addition of propanol. I Possibly azeotropic. 0 At low pressure. * Predicted from date of binary mixtures. 100 Torr.

%::

48::

6

effect of pressure on binary mixtures is based o n d a t a obtained a t atmospheric pressure ( f I 2 J ) . It has been pointed out that constants for liquid-vapor equilibria should fall on a straight line when plotted logarithmically against l/T°K.

(124J).

The equilibrium diagrams of ternary systems can be predicted from d a t a for the binary systems which are involved (404 or by a derivative form of the Gibbs-Duhem equation (I O8J). For ternary systems in which two components are pseudoideal but each forms a nonideal system with the third constituent, it is possible to predict the activity coefficients if the RedlichKister constants of corresponding binary mixtures are known ( I O U ) . By thermodynamic methods (8OJ) or by a simultaneous solution of DuhemMargules equations for each liquid phase ( 8 f J ) ,it is possible to deal with ternary mixtures having two liquid phases. For quaternary systems, a three suffix Scatchard-Hamer equation has been developed ( 3 8 J ) . Prediction of vapor liquid equilibria for multicomponent systems is generally based on d a t a from simpler systems (23J, 25J, 15J, 1$3J, 127J). A system in which I

64 R

ANALYTICAL CHEMISTRY

constituents react has been studied ( S S J ) . Problems of flash distillations can be solved by integration methods

The mixtures for which the vaporliquid equilibria have been reported are arranged alphabetically in Table 11.

(34. Tests have been made comparing the d a t a on methanol-water mixtures using the Gellespie still, the Ramalho method, and a n apparatus using a glass spring manometer. The Gellespie still gave good agreement with calculated values ( 2 4 5 ) . Static and dynamic methods were compared using acetone and chloroform. The dynamic method of Swietoslawski gave the better results (2OJ). Apparatus and methods have been described which are specially suited to the study of Ca hydrocarbons (354, to systems in which the equilibrium ratio is high ( I Y J ) , to multicomponent systems (49J, 89J, OOJ), and to high boiling mixtures such as Cd-Zn at various pressures (12OJ). A method using simple distillation has been applied to the system composed of acetic acid, ethylbenzene, and styrene in which the analyses of the vapor was done chemically or by using styrene tagged with CI4 ( I S I J ) , and an apparatus has been described in which equilibrium and heat of vaporization d a t a can be determined simultaneously ( 1OOJ).

SEPARATION O R CONCENTRATION OF ISOTOPES BY DISTILLATION

Distillation or processes of exchange between vapors and liquids or solids are effective methods for concentrating isotopes. It is possible, too, that such concentration sometimes occurs unintentionally and may have effects of which the user of the material is unaware. Thus it has been reported that the volatility ratios between ordinary pjopanol and isopropanol with isotopic substitutions are as follows: 0'8, 1.0021; D in the radical, 1.0090; D in the hydroxyl, 1.0155; C13 substitution, 1.0012 (36K). In the evaporation of methane the volatility ratio of the ordinary to the C" substituted compound is 1.0246 (26K, 3 2 K ) . The isotopic effect on vapor pressure has been explained by van der Waals forces through vibrational excitation and vibrational - translational - rotational coupling ( 3 5 K ) . Other concentrations or separations which have been obtained by distillation or sublimation are listed alphabetically.

boronlo by rectification of BF3 ( 2 5 K ) or exchange between BF3 in the vapor with BF3 dissolved ..n phenol ( 2 7 K ) ; products of the irradiiition of gold with 150 m.e.v. protons (:Hglg5,Pt188,Irig2, 0 ~ 1 8 5 , Re1*3) by sublimation (2SK); hydrogen isotopes as the element ( 6 K , 16K, 1 Q K , 21K, % K ) , as hydrides ( S S K ) , or as azeotroyes of strong acids with water (34K); h.rypton82s86as the element a t low temperature ( 1 S K ); lithium6 by molecular distillation of the metal (17K, 2 2 K ) ; nitrogen15 by exchange between NO and HKOI in chemical reaction and exchange (12K) or by distillation with the water azeotrope (14K); nitrogcnl5 and oxygen1* simultaneous by exchange of NO wit’h solutions in liquid HC1 (26K) or CHI (1OK); oxygenlR by azeotropes of strong acids with water ( 1 8 K ) ; ~ilicon28-2~~~0 as SiH4 (separation as SiF, was ineffective) ( 1 1 K ) ; tritium in tritium labeled formaldehyde (SOK); titanium isotope^^^-^" as TiC14 (28K); heavy water by recti5 cat,ion of ordinary water (24K, 2 Q K ) ,by exchange between water and H2S or Hz, or exchange between ammonia and H2 with subsequent conversion of the D2 to heavy water (15K, 2OK); uranium isotopes as cF6under centrifugai force ( 1 K ) . The ratios of vapor pressure of the following isotopes have also been reported: .1r36/r1r40 (4K,5 K , T K ) ; C14H4iC13H4/C12113D ( 9 K ); KeZ0/Nez2 ( 2 K , S K , 7 K ) ; N214/S14N15( 5 K ) , ~ ~ 1 4 0 , w 5 ~ (YK) 1 4 0 ; 0 2 1 6 / 0 1 6 0 1 8 ( 5 ~; ) So216,.S021* ( 8 K , YK). LITERATURE CITED

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~

”

VOL. 36, NO. 5 , APRIL 1964

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\---I

Testing of Stills

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