VAPOR-LIQUID EQUILIBRIUM GF BENZENE-BIPHENYL by MICHAEL WALDICHUK -0- a T H E S I S Submitted I N P a R T I a L F U L F I L
GF
BENZENE-BIPHENYL
BY
MICHAEL WALDICHUK
-0-
A thesis submitted in partial fulfilment of
the requirements for the degree of
MASTER OF ARTS
IN THE DEPARTMENT
OF
CHEMISTRY
-0-
THE UNIVERSITY OF BRITISH COLUMBIA
1 OCTOBER, 1950. ' THE UNIVERSITY OF BRITISH COLUMBIA VANCOUVER, CANADA
CHEMISTRY
September 30, 1950.
To Whom It May Concern:
This is to certify that the thesis entitled
"Vapour-Liquid Equilibrium of Benzene-Biphenyl" by
Mr. Michael Waldichuk measures up to the required
standards of the Master's thesis in this Department.
Yours truly,
-ew ABSTRACT
A comprehensive survey has been made of vapor liquid equilibrium apparatuses in the light of historical development. Two of the better types of units have been chosen and built for this research. The vapor liquid equilibria of benzene-n-butanol and benzene-biphenyl were determined on the Gillespie-Fowler equilibrium still. Thermo• dynamic consistency of the results was checked with the van
Laar and Margules integrations of the Gibbs Dubem equation.
Results obtained on the benzene-n-butanol system appear to Con4 form to theory within experimental error. However, those of benzene-biphenyl show very little consistency. Temperature-
Composition diagrams were drawn for both systems and follow the general form for non-azeotropic mixtures; ACKNOWLBDGEMEIJT
The author wishes to express his appreciation of the constructive criticism as well as encouragement given by Dr. L.W. Shemilt under whose supervision this research was carried out. Thanks are also due Mr. Wm. Pye, who carried out the glassblowing on the Gillespie-Fowler equilibrium still along with other smaller projects; and to Mr. A. Werner, who gave helpful suggestions and hints in the distillation and general purification of the compounds.
Acknowledgement is also made of the National Research Council Summer Scholarship which aided financially in the continuation of this research for the months May to September of 1949 and for May of 1950. TABLE OF CONTENTS
TITLE PAGE
I Introduction 1
II Theoretical" Discussion 7
Gibbs Duhem equation Van Laar Equations Margules Equations Other thermodynamic Treatments of Vapor-liquid equilibria.
Ill Historical Development of the Equilibrium Apparatus 24
A. Liquid Recirculating Type
1. Early Work
Determinations of Biling Temperatures Internal Heaters Cottrell Pump Vapor Trap
2. Modern Equilibrium Stills
The Othmer equilibrium still Chilton's equilibrium still Still of Scatchard, Raymond and Gilmann
3. Latest Developments in Equilibrium Stills
Flash chamber Gillespie Still with vapor disengagement chamber Fowler modification of the Gillespie still
4. Specialized Stills
Stills for partially miscible binary mixtures Equilibrium stills for high vacua Equilibrium stills.r'for low temperature
B. Vapor Recirculating Type
Early Forms Constant temperature type of recirculating apparatus
C. Other Methods of Vapor-liquid equilibrium Deter• minations Bomb method Dynamic Plow Method
Apparatus
A. Vapor Recirculating Apparatus
High vacuum system Purification of mercury Main Equilibrium apparatus Circulating pump Commutator for circulating pump Temperature control Constant temperature baths Stirring equipment Heating, temperature measurement and control Constant Temperature air Thermomostats Constant Temperature Oil bath
B. Liquid Recirculating Apparatus
Modifications of Fowler still
C. Refractometer
D. Platinum Resistance thermometer
Materials
A. Benzene
1. Initial Purification
2. Distillation
3. Fractional Recrystallization
4. Check on purity
(a) Refractive index
(b) Freezing point
(c) Density
B. Butanol
C. Biphenyl
1. Recrystallization
2. Distillation (a) Determination of conditions
(b) Biphenyl still developed
Pressure control and measurement Efficiency of Column Improvements for tne still Distillation Procedure
3. Check on Purity
VI ExperimentaluProcedures
A. Determination of Refractive Index-Composition Curves
Benzene-butanol Benzene biphenyl
B. Vapor-liquid equilibrium determinations on the Gillespie-Fowler still
C. Determinations on the vapor recirculation
apparatus
VII Results
A. Benzene-Butanol
B. Benzene-Biphenyl
VIII Discussion of Results
A. Benzene-Butanol
B. Benzene-Biphenyl
IX Bibliography TABLES
1. Physical data for Benzene from the literature. 66-7.
2. Physical data for Butanol from the literature. 69.
3. Freezing point data for biphenyl from the literature. 8l.
4. Refractive Index - Composition data for benzene- butanol at 20°C. .90.
5. Experimental yapor-liquid equilibrium data for
benzene-butanol at atmospheric pressure. 91*
6. Vapor pressure data for benzene. 93.
7. Activity coefficients of benzene and n-butanol from experimental vapor-liquid equilibria. 93. 8. Theoretical activity coefficients of benzene and n-butanol. 9J?. 9. Refractive Index-Composition data for benzene- biphenyl at 70QC. ' 9.6.
10. Smoothed Refractive Index-Composition data for ben• zene -biphenyl at 70°C. 97.
11. Experimental vapor-liquid equilibrium data for benzene-biphenyl at atmospheric pressure. 98.
12. Smoothed values for the vapor-liquid equilibrium of benzene-biphenyl. 99.
13. Activity coefficients of benzene and biphenyl from experimental data. 101.
14. Theoretical activity coefficients of benzene and biphenyl. 101., ILLUSTRATIONS
1. Vapor pressure - composition diagrams of two component systems.
2. Equilibrium still designed by Brown in 18*79.
3. Carveth's apparatus (18*99). 4. Zawidski's apparatus (1900).
5. Cottrell's apparatus with an air lift device (1919).
6. SwietoslawskiTs boiling point apparatus. (1925).
7. Still of Sameshima incorporating a vapor trap (1918). 8. Original Othmer Still (1928).
9. Othmer still of 1932.
10. New, improved Othmer Still (194S).
11. Apparatus of Carey and Lewis (1932).
12. Still of Rogers, Knight and Choppin (1947).
13. Chilton's still (1935). 14. Apparatus of Scatchard, Raymond and Gilmanvx.
15. Equilibrium still of ilones, Schoenborn and Colburn (1943).
16. Gillespie's equilibrium still (1946).
17. Othmer's refined still modified by Smith and Bonner (1949).
IS. Equilibrium still of Williams (1947).
19. Equilibrium still of Perry and Fuguilt (1947). 20. Still of Gordon and Benson for low temperature equilibria (1946).
21. Apparatus of Rosanoff, Lamb, and Briethut (1909).
22. High vacuum system.
23. Main equilibrium apparatus.
24. Commutator arrangement. 25. Circulating apparatus.
26. Circuit diagram of temperature control units for air baths.
27. Constant temperature baths in section and stirring unit.
28. (a) Circuit diagram of electronic control unit for constant temperature in oil bath.
26% (b) Finger-type thermor.egulator.
29. Fowler-Gillespie equilibrium still.
30. (a) Benzene still.
30. (b) Theimer vacuum adapter.
31. Freezing point apparatus.
32. Cooling curve for benzene.
33. Vapor pressure curve for biphenyl.
34. Semi micro still.
35. Biphenyl still.
36. Pressure regulator for vacuum distillation.
37. Cooling curve for biphenyl.
38. Refractive index - Composition cux^ve for benzene - n-butanol at 20OC.
39. Experimental vapor liquid equilibrium curve for benzene- n-butanol.
40. Vapor pressure curve for n-butanol.
41. Vapor pressure curve for benzene.
42. Plot of log activity coefficients vs mole fraction in liquid for benzene-butanol.
43. Temperature-composition diagram for benzene-butanol at atmospheric pressure.
44• Refractive index-composition curve for benzene-biphenyl at 70°C
45• Experimental vapor-liquid equilibrium curve for benzene- biphenyl . 46. Plot of log activity coefficients mole fraction in liquid for benzene-biphenyl.
47. Temperature-composition diagram for benzene-biphenyl.
PLATES
I Vapor Recirculation equilibrium apparatus.
II Fowler-Gillespie Equilibrium Still.
III Benzene still.
IV Biphenyl still. 1
CHAPTER I
INTRODUCTION
Vapor liquid equilibrium data are of great impor• tance in the fields of distillation, extraction, and other contact processes. A knowledge of such data is one of the fundamental requirements in the quantitative design calcul• ations for columns in fractional distillation. However, the experimental work required to gain this knowledge has been found complicated, often requiring elaborate equipment, and very seldom reproducing the results of previous researchers.
In recent years, there has been an outburst of experimental work and published papers on such determinations. Many authors have done a great deal of research into the theoretical aspects of vapor liquid equilibrium. This theoretical approach has been stimulated by the need for certain mathematical expressions which will relate the thermodynamic properties of compounds to the vapor-liquid equilibrium. Researchers in this field have felt that eventually some relations should be obtained which would virtually eliminate the necessity of extensive experimental work to determine the vapor liquid equilibrium of a system. Up to the present, certain fundamen• tal thermodynamic expressions have been developed and utilized for the effective extension of common physical data of some non-ideal systems.
Vapor liquid equilibrium data consist essentially of the composition of the liquid and vapor phases of a system when they are in equilibrium. The x - y diagram (x is the mole fraction of the more volatile component in the liquid phase and y is the mole fraction of the more volatile compon•
ent in the vapor phase) is very common in books on distillation as well as In published papers on vapor liquid equilibria.
Experimental difficulties in obtaining these data are many.
They usually stem from the nature of the apparatus used in the determinations. There have been many types of apparatuses developed for equilibrium determinations, but fundamentally they can be classified under two main groups. There is the vapor recirculation type which is ordinarily employed under
constant temperature and in an evacuated system. The other
is the liquid recirculation type mainly used for equilibrium
determinations under atmospheric pressure. Many researchers feel that the former is superior for accurate determinations
because it eliminates most of the common faults attributed
to the liquid recirculating stills. Since the latter is
essentially a distillation type of apparatus, it suffers from
refluxing of the vapors in the region between the boiler and
the vapor section, entrainment of droplets of liquid in the
vapor, flashing of the more volatile component when the cold
condensate returns to the hot boiler, and superheating of the
system initially. However, the apparatus can usually be built
very simply involving very few mechanic'al parts, and deter•
minations are readily made. On the other hand, the vapor
recirculation type of apparatus poses many problems in con•
struction and operation. Both types of units will be dis•
cussed fully in a later section. 3
The system, benzene-biphenyl chosen in this work, has very little practical application. It is one, however, that has considerable theoretical interest. A complete thermodynamic evaluation of the system could throw considerable
light on certain concepts in the theories of solutions. The
two components of the system are so vastly different in their
physical properties and critical constants that certain workers in the field of solutions have taken a keen interest
in the study of benzene-biphenyl. Others have experimented
with the system from the point of view of statistical thermo•
dynamics because of the difference in size of the two compon•
ent molecules. The earliest work on the system by Tyrer in
1910 (132) was done in connection with the density of different
composition of the components at 25°C. This was followed by
the research of Washburn and Read who determined the eutectic
point in 1915 (13#) and the elevation of the boiling point in
1919 (139). In 1921 Gehloff (39) did what was probably the
first thermodynamic evaluation of the system when he deter•
mined the heat of solution.
Some time elapsed before any further work was
carried out on benzene-biphenyl. Warner, Scheib and Svirbely
(136) in 1934 carried out determinations on the solubility of
biphenyl and benzene. A short period later in 193&, Gilmann
and Gross (44) took an interest in the system from the point
of view of ideality. They determined the vapor pressure of
benzene over benzene-biphenyl solutions and found that the
system obeys Kaoult's Law within experimental error. In a paper published in 194$ (130) Tompa discusses the thermo• dynamics of the benzene-biphenyl system as evaluated with
Guggenheim's formulae (52). These formulae are based on the lattice model, and in their application to the system it is assumed that the biphenyl molecule occupies twice the volume of the benzene molecule. Experimental determinations were also made by Tompa and the results were shown to conform to the statistical thermodynamic picture. One of the latest references to the system was made in a paper read to the
Manchester Section of the Chemical Society in October, 1949, by Ward and Brooks (134)• They measured the viscosities of the system at 5°C intervals of temperature and at different
compositions. Their results showed that the mixture is ideal
and gives nearly a straight line plot for ITL V against 1. T
However, they made no mention of the early work carried out
on the viscosity of the system (66). Since they published no values, it was not possible to check the earlier results.
Work on the benzene- n- butylalcohol system has been
considered only of secondary importance in this research.
Vapor-liquid equilibrium data for it was obtained primarily as a check on the equilibrium still (Gillespie-Fowler) built towards the latter part of the investigations. More intensive work was carried out by Emerson and Cundill (30), who inves• tigated the system in their research for the B.A.Sc. degree.
The system has not been previously studied for its vapor liquid equilibrium, and the results obtained here proved to be of considerable interest. N-Butanol is the first straight chain alcohol of the aliphatic series forming no azeotrope with benzene, but the behaviour does not appear to deviate too far from that of similar mixtures. Results appear to be consistent within experimental error when checked thermo-
dynamically. On the other hand, data obtained for the benzene- biphenyl system appear to have no thermodynamic consistency whatsoever. Although the x-y curve for the latter system
seems reasonable in appearance, a plot of the logarithm of
the activity coefficient against the mole fraction in the liquid phase gives very little indication of a smooth curve.
An effort has been made to evaluate the system thermodynamic•
ally; but since constants in both the van Loar and Margules
integrations of the Gibbs Duhem equation are dependent on
experimental values, no brief is held for its validity.
The purpose of the research described here is two•
fold. An endeavour has been made to check better forms of the
two types of vapor-liquid equilibrium apparatus,one against
the other, and to check for thermodynamic consistency of
results in each case by an application of at least one of the
integrated forms of the Gibbs Duhem equation. Also equil•
ibrium data for the system, benzene-biphenyl, was sought for
purposes of theoretical consideration. Unfortunately, some
of the work originally planned has been unsuccessful up to the
point to which the investigations were carried. The vapor
recirculation apparatus requires further modification and
improvement to give the desired results. The Gillespie-Fowler
equilibrium still cannot be considered as the best type of unit for the determination of-the vapor-liquid equilibrium of benzene-biphenyl. However, those results which were obtained
are given as fully as possible along with their treatment.
In addition, a comprehensive survey of the literature is
given to aid those who may continue studies along the lines
of vapor liquid equilibrium. Much of the material is very
descriptive and deals largely with apparatus construction.
This was felt necessary for a successful continuation of the work as well as for any reasonable reproduction of results. 7
CHAPTER II
THEORETICAL DISCUSSION
Yapor-liquid equilibrium has been treated theoret• ically at great length in recent years. This has been found necessary for any kind of thermodynamic evaluation of systems where involved experimental determinations can be avoided.
Especially in the field of fractional distillation where quan• titative results are necessary for column design calculations has it been the case.
To overcome the complexity of the quantitative relationships involved in determining the vapor liquid equil• ibrium, certain basic thermodynamic relations have been devel• oped (48) (72). These relations apply in all cases, but in most of these there are unknown factors which limit their usefulness until simplifying assumptions are made. The applicability of the thermodynamic relation may be due to the limitations of these simplifying assumptions. However, even in such a case the relationships serve as valuable criteria for estimating the normal behaviour of a system as well as a
check on the thermodynamic consistency of experimental results.
If we consider a system of two miscible components,
Dalton's Law of partial pressures holds true at ordinary pressures, i.e.,
A P-L + P2 = P (1)
Most mixtures, however, do not obey Raoult's Law, which states
A See the end of discussion for an explanation of nomenclature. 8 that "the fractional lowering of the vapor pressure of the solvent is equal to the mole fraction of the solute in solut• ion", or stated symbolically,
?1 - xxPi0 (2)
A system obeying Raoult's Law can be considered as ideal, and deviations from ideality may be due either to the vapor phase, the liquid phase or both. These deviations can be both physical and chemical in nature. The important factors believed to be involved are that molecules have volume and that they exert forces upon each other which may be attract•
ions or repulsions. Actual chemical effects may also be
involved especially where the components are ehemioally dis•
similar, e.g., belonging to different homologous series. It has been shown experimentally that substances, similar chem•
ically deviate only slightly from Raoult's Law.
Non-ideal systems exhibit either negative or positive deviations depending on whether the molecules of one
substance tend to lower or raise the escaping tendency of the
other oomponent molecules. Positive deviation is the most
common, and from Figure 1 it can be seen that it results in
the partial pressure of the components as well as the total pressure of the system b«ir» Raoult's Law. As shown in the diagram, the total vapor pressure may reach a maximum at a certain concentration. This is usu• ally the case if the deviation is great and the vapor pressures of the two components do not differ greatly. Under such cir• cumstances, there exists a certain concentration at which the A MOLE. FRACTION B FIG ta -MINIMUM BOtUNG, SYSTEM DOTTED UNC3 BtPRtatNT *l DEAL CURVES A MOLE. FRACTION E> B R^.lb.-MMIMUM BOIUMq SYSTEM F\a I - VAPOR PRELSSURE: COMPOSITION DIAGRAMS OF TWO COMPONENT SYSTEMS total vapor pressure will be equal to the atmospheric pressure at a lower temperature than at any other concentration. In other words, the solution has a minimum boiling point at this concentration. Where the total vapor pressure curve exper• iences a minimum corresponding to a concentration at which the solution exerts a pressure equal to atmospheric at a temper• ature higher than for either of the components, we have a maximum boiling mixture. Systems having either of these properties are known as constant boiling or azeotropio mixtures, Gibbs-Dubem Equation For the general case of a system in which transfer of material takes place, any extensive property such as free energy, F, will depend not only on pressure and temperature but also on the mass of each component present. Thus F = f (T,P,ni,n2, —-) and the general differential expression for dF can be written as follows: dF =/e>F\dF^dT + /-3F\dP V (-3FVLm + / 3 FWg+~ v5"T7Pini,n2r.— V^y/T,nlfn2 VaaJ P,T,n2—p,T,: ^^FNdT + feF\dP + TfarniW (3) T n T,P V¥ /P,ni,n2---\dP/T,n1,n2 Vd i/T,P The partial differential/"^ F \ is known as the partial molar free energy and is often written as F'i for convenience. It has also been defined as the chemical potential ju.t . Now, at constant temperature and pressure we obtain the expression, dF = /Uldnt (4) It can be shown that the chemical potential with respect to a change in one component is independent of the amount of that component provided its relative concentration is constant. Hence 4 can be integrated with u (5> The integration constant drops out when there is no change in concentration. FromEuler's Theorem, (6) Since the free energy, F, is an extensive property of the phase and, therefore, a homogeneous function of the first degree in , our expression reduces to (7) Equation 7 is known as the Gibbs-Duhem equation (72). When applied to changes in composition at constant temperature and pressure, it is rigorously exact. Where only the temperature varies over a small range, it may be approximately valid. When the mass of each component present is constant, F - f (P,T) and (8) By definition, H - E + PV (9) F - H - TS (10) Combining 9 and 10, F • E + PV - TS (11) Differentiating 11 generally, dF = dE + PdV + VdP - TdS - SdT (12) From the first and second laws of thermodynamics, dE = TdS - PdV (13) Substituting 13 into 12, dF - VdP - SdT (14) Comparing equations 8 and 14, we obtain T ll5) (H)T" We have already defined ML "(MN (16) If we have only one oomponent in two phases we can have only one degree of freedom, as determined by the Phase Rule, and we can write M - dF 7 dn (17) Rearranging and integrating, we obtain yj&.n = JdF ju. n - F (18) If we have only one mole of component,F =^ and 15 can be written in the following form for a mole of an ideal gas: by* = dF = VdP = RT dP (19) Integrating 19 we obtain at constant temperature M - RT ln P2 + K ( 20) FT In order that this equation apply to the general case of any non-ideal gas, a new property, fugaoity f, is defined such that dyU. = RT df (21) If the lower limit of integration is taken as fugaeity in some standard state denoted by f°, we can integrate 21 as follows: (22) Equation 7 can be expressed as n, djLA, + nxd yu^ + —- = o and since n^,n2> are proportional to the mole fractions 2-1»^2»"" ' xAju, + y^fX% + = 0 (23) Dividing through hy dxi we obtain xi f^/M + x2 (^tt±S + = 0 ( 24) But from the definition of fugacity we can write dyW = RT d inf for one mole of gas at constant temperature. Therefore, + + M • *2 0 (25) which is a more useful form of the Gibbs-Duhem equation. Here the fugacity can be considered as the "ideal" partial pressure and is identical with the partial pressure for conditions under which the gas laws hold. The ratio f can be defined as the activity, a, and a the activity coefficient "tf i x Then a-, = fi ; a9 - f 2 ( 26) fj 2 Y. = :i V* = a2 ( 27) xl x2. Combining ( 26) and ( 27) " ; V - f? x fl° l * f2°x2 (28) If the vapors.behave ideally f 1 » PX ; f2 = P2 and f1°= Pi° ; f2°= P2° where Pi° and P2° are the 13 actual pressures of the pure components. Under such conditions we can write X - Pi > - P? • I1 ' Fp~xi (29) From Dal ton's Law, Pi - yiP * ?2 = y2p (?Q) Combine 29 and 30 to obtain 1 x^Pp ; xf^o (51) These expressions for the activity coefficients are of great value in the treatment of vapor liquid equilibria. They give directly the deviation factors from Raoult»s Law, which when plotted on semilog paper against mdle. fraction of one of the components in the liquid phase reveal characteristic curves if the data are thermodynamically consistent. If we can assume that there is ideal behavious in the vapor phase, by virtue of equation 28 we can express the fug- acity of a component in terms of the activity coefficient, pressure of the pure component and its mole fraction. Thus fl « h*i°*l * f2 = *'2P2°X2 (32) Then substituting these values into 23 Tn 0 e ( ~S&r\ R° \ and Y ^ (Ln P-THerms are zero since P^ and P2° are constants at constant temperature, and we are left with x + + l(a&v'O ^(3^) + x23x2 + 0 If we consider a two component system and that x2 <= (1-x^) and dx2 = -dx^, our expression reduces to xiJBM.) = x2 (ikL] (33) V 3%. /p,r V 3}U V This form of the Gibbs Duhem equation expressed in terms of activity coefficients is of immediate value in studying experimental data on vapor-liquid equilibrium. It relates the slopes of the plots of logarithms c$. activity coefficients against the composition of the liquid phase. However, since the magnitudes of such slopes are difficult to obtain, there have been a number of solutions advanced for this differential equation. At least two of these solutions will be considered here since they are to be used later in the treatment of the data. Van Laar Equations J. J. van Laar (68), in a thorough study of the thermodynamics of binary mixtures derived, semi-empirically, solutions to the Gibbs Duhem equation. These expressions have proved to be very useful in treatment of vapor liquid equil• ibrium data. They have been modified and rearranged by var• ious authors, especially by Carlson and Colburn (13), Gilliland et al (43) and White (142). The derivation of the solution will not be attempted here since it becomes considerably in• volved. However, the general form of the equations will be 15 shown and they can be readily proved to be integrations of the Gibbs-Duhem equation. Carlson and Colburn give the van Laar equations in the symmetrical forms log tf, " A (54) (l+ Axi V (35) When xi = 0 and x2 = 1 log ^, = A , log K = 0 and ^ = 1 When XT_ = 1 • and x2 = 0 log ^= E> , log >j, » G and = 1 Thus the constants A and B in the equations can be obtained from experimental log $ vs composition plot when the curves are extrapolated to x^ = 0 and xi = 1. It is assumed that these two experimental values of log % are very nearly correct in order to check the thermodynamic consistency of the remainder. The fact that and ^ are equal to 1 when xj_ and x2 are equal to 1, respectivelyj satisfies the limiting condition that Rabult's law holds for a component whose concentration approa• ches 10G mole peroent. Other qualitative checks on experimental data can be readily indicated from the peculiar properties of equations 34 and 35• When xi = 0.5 log % » A g A - AB2 ~ (36) ' /i,+ A\2. IA •+ B)2 TA+BJ? ^ BV B2 log 1 = B , - B • ^ = A2B -? (37) X A + -B\2 (A + B) 2 (A + B)2 ^ 57 A* Divide (36) by B and (37) by A and the two can be equated, log £ - AB . = log \ 1 (A •+ B)2 I (38) Now if A •» B AB =1 and as A and B differ the ratio (A + B)2 4~ decreases, e.g., when A = 2B AB - 2B2 m 2 (A + B)2 fgZ 9 From 38, it can be seen that the half way value on one curve is approximately equal to one quarter of the end value on the other curve. Thus, at x^ = 0.5, if the curve of log 2(, vs x± should be lower, it will have a higher end value than the curve log #t vs xi. Equations 34 and 35 can also be written " Ax22 . (39) + T x 2 (x2 | l) l0e - BXI2 (xx + B x2)Z (40) To show that these satisfy the Gibbs-Duhem equation, they can be differentiated remembering that xi - l-x2 and that dx^ = dx2 2 xl (tUtS - xi{-2Ax2 ( A xi+x^ (k-l\- 2Ax2 ] x 2 x • o( -2BXl (x +B x>3(B-l)- 2Bxl ] Simplifying ~ ? * \ 1 "2A X2 " 2- xlx2 ) xi = 2 W^' ^y..?,A....iv f*^ + V ^ ' I fa • BxVJ \ ljXl 2 (42) Now, if we multiply both the numerator and denomin• ator of (42) by A3 we obtain 2 2 2A fxi x2 + 3x lx2 B 3x2 \ / A xi + x2>3: which is identical to (41). Hence the van Laar equations satisfy the Gibbs-Duhem partial differential equation. Gilliland et al (43) employ very similar expressions for the van Laar equation, but corrected for temperature, i.e., log If, - 13B// TX (1 + A xiV (43) v x2o / log = AB/T (44) X1; If (43) is solved for B, B = T (l + Ax]V iog<, (43A) . x2' - then the expression for B can be substituted into (44) and one can solve for A. % x + (44A) A = (4i + -Aasi^ 2 y lolog*g ^ , A =/xif 10* Tk (44B) With any set of accurate data of activity coeffic• ients corresponding to a certain concentration, the constant in the above equations can be evaluated. Usually dependable values can be obtained from the azeotropic composition and the whole vapor liquid equilibrium curve can be evaluated. White (142) rearranged the van Laar equations some what again and showed that they could be used as straight- line forms as follows: (logK,)-!/^^^.^ (W) 1 2 1 2 (log i,)- / - B / * ^1/2 (46) These equations were applied to the system 1-butanol-xvater < > 5 by Smith and Bonner (113) who plotted (log $( )'~ " •' against anc xl/x2 * (i°S^)~^"^ against X2* The check on the exper- xl V imental results was not near as close as in the log 0 against x plot. Margules Equations. Margules (76) found a solution to the Gibbs Duhem equation by integrating it in terms of a pair of exponential series. He derived the constants of one of the equations from those of the other by applying equation 33. Thus the two series expressions obtained for the activity coefficients of the components in a binary mixture were as follows: + cx log o, = ax2 + bxg 2 log a^i + b^2 + clx^ US) These expressions are substituted into equation 33 and we obtain the following two expressions: 2 xi (d-£n V. ) - -xi d £n V, = -(axi + 2bx2_x2 +-30X3X2) (49) 1 1 :L 2 x2 (JI&JQSIA = -x2 d £N = -(a x2+2b x1X2+3c x2x1 )' (50) V "d^-L - '/ • dxx If the coefficients of 49 and 50 are evaluated in such a way that the constant terms in each case are equal, the coefficients of the first power of the mole fraction terms are equal and so on for the higher power terms, we can put both expressions completely in terms of x2(from x2=l-x;j_) and equate. 2 2 a - ax2 + 2bx2 - 2bx2 + 3cx^ - 3cx2^ x 1 1 2 1 1 2 1 = a x2 + 2b x2 - 2b x2 + 3c x2 - 6c x2 + 3c x2^ Collecting terras for each power of x2 2 3 a + (-a+2b)x2 + (-2b+3c)x2 - 3cx2 1 1 1 2 (a +2b +3c )x2 + (-2b!-6cl)x2 + 3clx2 Equate coefficients a =0 (i) -a + 2b = al + 2bl + 3c1 (ii) 1 -2b + 3c = -2b - 6C1 (iii) -3c = 3cl (iv) Solutions obtained are as follows: cl = -c From (iii) 2b = 2tA - 3c Substitute value for b into (ii) -a + 2b1 - 3c = a1 + 2b1 + 3c1 Therefore, -a = a1 and a1 = 0 From (iii) b1 = 2b + 3c 2 Substituting values for the constants into 47 and 4#, we obtain 2 + .." Irxi, = bx2 .CX2^ (51) JLrdir* bxi2 + 1 CX!2 - cx-^ (52) Although the equations have two independent constants only- one point is needed to evaluate both of these. If more data are available it is convenient to plot ^' vs. xn , and Jlr\\x vs. x2; and if the Margules equation, agrees with the XT* data straight lines should be obtained. The slope of the two lines should be just equal to the constant c, while the intercept can be used for evaluating the constant b. Since the constants of the Margules equation are a function of the temperature, if one experimental point is used to evaluate them, they should be suitable for other compositions at the same temperature assuming the equations to apply. At other temperatures, however, additional experimental data are required. Carlson and Colburn (15) slightly revised the constants of the Margules equation in order to utilize the terminal values of the log vs. x plot as constants. With the constants A and B representing the same values as the corresponding symbols in the van Laar equation, the Margules two-term equations can be written as follows: 2 log tf, = (2B-A)x2 + 2(A-B)x23 (53) 2 log (2A-B)X1 + 2(B-A)x13 (54) where (2B-A) = b and 2(A-B) = c in 51 and 52. It can be readily seen that at xi = 0, log #f = A and log 0^= 0; at xi 85 1, log tf, = 0 and log ^= B. An interesting feature of these equations is that at x^ = 0.5, log ^1 = B/4 and log = A regardless of the values of A and 4 B. When A = B the Morgules equations become identical with those of van Laar. As the value A departs from unity, the B two sets of equations represent increasingly different curves. Other Thermodynamic Treatments of Vapor Liquid Equilibria There have been a considerable number of other theories put forth in the literature in recent years on the thermodynamic evaluation of systems. These will be only briefly mentioned because they have not been used to any extent in this research. Scatchard and Hamer (109) extended the methods of van Laar to give the activity coefficients in terms of molar volumes and volume fractions of the components. Scatchard [108) proposed a thermodynamic relation in which all the constants represent physical properties for systems where the change of entropy on mixing is the same as that for an ideal mixture. Some time later, Scatchard, Wood and Mochel (111) concluded, after extensive work on three systems formed by: ... - . .• • - - - • 22 benzene, cyclohexane and carbon tetrachloride, that-there was no agreement between the theoretical and experimental values of the constant in the Scatchard equation. Redlich and Kister (99)in a recent paper discuss the examination of experimental equilibrium data to efficiently evaluate the thermodynamic properties of nonelectrolyte solutions. Their main concern was the design of columns from laboratory data which represents the mole fractions x of the liquid and y of the vapor in equilibrium as functions of the temperature at constant pressure. They appeal to certain equations of state in their evaluations and present a very comprehensive study of the assumptions involved. Gilmont et al (45) take a somewhat new approach in the determination of activity coefficients. They utilize the relative volatility which is independent of composition and total pressure, being the ratio of the vapor pressure of the pure components at constant temperature. A symmetrical form of power series was applied for the relative volatility as a function of composition. 23 NOMENCLATURE A = Arbitrary constant in van Laar and Margules equations. Equal B = Arbitrary constant in van Laar and Margules equations. Equal to log at x2 = 0. a al ) b b| )= Arbitrary constants in series integrations leading to c c )' Margules equations. E = internal energy. F = free energy. f - fugacity. f° = fugacity in the Standard State. ^ = activity coefficient. H = enthalpy, n = moles of component. P = total pressure (atmospheric), mm Hg. ^1^2 ~ Partial pressures of components, mm Hg. 0 Pl°>P2 = vapor pressures of pure components, mm Hg. R m gas constant. S = entropy. T = absolute temperature. U= F = ^FT = chemical potential (partial molar free energy). V = volume. x = mole fraction in liquid. y = mole fraction in vapor in equilibrium with x. Subscripts 1 = low-boiling component (benzene). 2 = high-boiling component (n-butanol or biphenyl). CHAPTER III HISTORICAL DEVELOPMENT OF THE EQUILIBRIUM APPARATUS A. Liquid Recirculating Type By far the largest proportion of vapor-liquid equilibrium units used are the distillation types which in their more advanced form are liquid recirculating. One of the earliest investigators in the field of vapor liquid equilibrium was Brown. In a number of papers published in 1881 (8) he made a critical survey of stills up to that time. At the turn of the century, Sydney Young (LM">) made an ex• cellent criticism of equilibrium stills in his classic book on distillation, "Distillation Principles and Processes" pub• lished in 1903 and subsequently revised in 1922. Only very recently, a comprehensive treatise was written on the evo• lution of this type of still by R.T. Fowler (35). An en• deavour will be made here to cover the development of equil• ibrium stills according to the new devices introduced to improve the accuracy of determinations. The vapor recircul• ating type of apparatus will be discussed as well as some of the specialized types. 1. Early Work After making a complete survey of equilibrium stills up to his time, Brown designed an apparatus which he describes in a paper in 1879 (7). The still is of the dynamic distillation type where the composition of the liquid continuously changes. It is illustrated in Figure 2 which shows a copper vessel A with a long neck surrounded by an outer jacket leading to a condenser and condensate receiver B. The neck of the vessel has numerous holes through which the vapors pass into the outer jacket. This arrangement keeps the neck at the temperature of the vapors and prevents con• densation and reflux. The vapors finally condense in the condenser-and enter receiver B from which a sample can be removed for analysis. Brown felt that by having a large quantity of liquid in the boiler compared to that distilled off he could obtain reasonable composition values of the vapor in equilibrium with the liquid at any given concentration. In 1898 Lehfeldt (71) introduced what he felt was an improve• ment and made the boiler smaller. The chief difficulty with this type of apparatus is, of course, that there is no pro• f- vision to prevent super heating and, as a result, accurate temperature measurement cannot be obtained. Furthermore, it is a case of straight distillation with a continuous weakening of the liquid in the more volatile component. Analysis has to be made of successive fractions and a cal• culation must be resorted to for an estimate of equilibrium values. Determination of Boiling Temperatures The first serious attempt to determine the boiling temperatures of the liquid and vapors in equilibrium was made by Carveth in 1899 (16),. Figure 3 shows some of the details of the apparatus. A large bulb contained a fairly large volume of solvent and solute. Glass beads were added to pre• vent superheating. Into the flask were fitted, by means of a rubber stopper, a thermometer, condenser, and a bulb-shaped tube also containing a thermometer. In determinations, the bulb-shaped tube was withdrawn from the liquid and turned so that the small funnel was clear of the drip tip of the con• denser. The liquid was boiled and its temperature was noted on ttiermometer 1. Then the bulb was turned so' that the con• densed vapor was allowed to drip into the funnel and onto the bulb of thermometer 2. The temperature recorded on this thermometer when it reached stability was that of the boiling vapor phase. In order to prevent superheating in the bulb, a piece of platinum wire was sealed into it. Boiling temper- ' atures, however, were found to vary with the rate of heating, and Corveth suggested that the rate of heating be standardized for allsubsequent work. Internal Heaters A definite stride in the advance toward better equilibrium stills was the introduction of internal heaters to prevent superheating. Zawidski (150), in 1900, made a series of determinations on binary mixtures employing such a heater-in his still (Fig.4). A small thermometer was used and it was totally immersed in the flask to eliminate stem cor• rection. The condenser was equipped with a small receiver for collection of samples of the vapor phase. Although the apparatus had certain defects, such as no provision against entrainment of'.liquid in the vapor and nothing to prevent the changing composition of the liquid phase, Zawidski investig• ated about twenty-six liquid mixtures in all. FlG 4 -ZAW iDSKl'S APPARATUS (1900) FIG- fe" SWI ETOSLAWSKI'S BOILING POINT" APPARATUS 0925> FVG.T - ST»L-U OF 5AMELSHIMA INCORPORATING A VAPOR TRAP. Cottrell Pump There have been numerous modifications in various types of equilibrium stills of the air lift device first re• ported by Cottrell (22) in 1919 (Fig. 5). The original pumping apparatus consisted of a length of tubing flared out at the bottom and supported vertically by means of a platform A. The vapor passing up the tube forced liquid up with it and the effect of super-heating in the flask was;considered almost negligible. On the platform the vapor and liquid separated, the vapor passing upward into a condenser and the liquid returning to the boiler by way of the outside of the thermom• eter. Thus the temperature registered on the thermometer was that of the liquid in equilibrium with its vapor. Swietoslawski (122)(123) in 1925 and later was one of the first to adopt the Cottrell device for a very accurate apparatus for boiling temperature measurements. The apparatus (Fig.6) is quite simple and consists of a small bulb A fitted with three outlet tubes. Tube 1 is the entry tube for adding more liquid, tube 2 is the Cottrell pump and tube 3 is the liquid return. In operation the solvent is boiled in bulb A and tube 2 spurts the boiling liquid with its equilibrium vapor into trap B which contains a thermometer well. Ideal equilibrium conditions are considered to exist since the liquid and vapor are intimately mixed and hence the thermometer gives the true equilibrium temperature. The liquid is returned to bulb A by way of tube 3 and on condensation the vapor portion follows a similar path. FIG.5.-COTTRELL'S APPARATUS WITH AN AIR LIFT DEVICE (1919) The Vapor Trap The Japanese scientist, Sameshima (106), in 1918 developed an apparatus for equilibrium measurements incorpor• ating a vapor trap. This was an idea which was first struck by another Japanese, Yamoguchi (146), in 1913. By means of the vapor trap a relatively small amount of the liquid system could be used without the danger of a continuous change in composition. Sameshima's still was the forerunner of many of the modern equilibrium stills which incorporate similar devices. As shown in Figure 7, the vapors rise from the boiler A con• taining an internal heater and totally immersed thermometer, through a carefully logged and heated exit tube B into a condenser C. The condensate falls back into the vapor trap D and overflows into the downtake to vessel A. The condenser E was used to prevent vapor loss. Vapor and liquid samples could be removed with pipettes from A and D when required after equilibrium was reached as indicated by the stability of the temperature. 2. Modern Equilibrium Stills The Othmer Equilibrium Still Perhaps the one type of still which deserves a place by itself in a historical survey is the Othmer still. D.F. Othmer reported his first equilibrium still in 1928 (86). Since then there has been a steady stream of modifications by the same author as well as by others who have picked up the idea, but the main features of the apparatus have been retained. The original Othmer still is illustrated in Figure 8. The system is boiled in container A, and as vapors form and reflux on the cold walls of the container air is displaced through tap x. As the whole flask is warmed, vapors pass over into condenser B, the condensate fills trap C and over• flows back into vessel A. The equilibrium temperature is recorded on the thermometer protected by the tube, and samples are removed as simultaneously as possible from the boiler and vapor trap by means of stopcocks. Although it was claimed that the apparatus was easy to use and equilibrium was reached rapidly, some of its faults are obvious. Re- fluxing takes place on the upper walls of the boiler, flash• ing of the more volatile component occurs as the cold conden• sate returns to the hot liquid in A, superheating occurs,and the thermometer does not likely register the true temperature of the liquid in equilibrium with its vapor. A later modification of the Othmer Still (8?) in• corporates an internal heater which eliminates some of the superheating. Figure 9 shows how some of the general appear• ance has been altered but the fundamental principles remain unchanged. There were many papers published in subsequent years(88, 90, 91, 92, 93, 94)reporting determinations on Othmer stills with additional improvements. The latest modification of his still was reported by Othmer in 194# (#9). In this paper he reviewed some of the errors common to the various types of equilibrium stills and pointed out that these were essentially eliminated in his new, improved still. Figure 10 illustrates this apparatus and although a sincere effort has been made to eliminate all defects some of the sources of error inherent in this type of apparatus are still present. Carey and Lewis (14) took the essential features of the Othmer still and made a modification with an arrange• ment for the prevention of condensation in the upper regions of the boiler. Their still was made of copper sheet metal and tubing. The boiler walls were completely insulated as a shown in Figure 11 except for/small peep sight which was provided to view the end of the tube protecting the ther• mometer. Thus the cessation of condensation could be noted. The vapors pass over the thermometer and into chamber B. Any condensation which may take place here is caught in the chan• nel and passes out into the vapor trap. The remaining vapor condenses in the condenser and returns to the boiler by way of the trap. A simple type of equilibrium still which incor• porates some of the features of the Othmer still as well as a Cottrell pump is that of Rogers, Knight, and Choppin (102). This still (Fig. 12) was primarily designed for laboratory determinations where rapid attainment of equilibrium is required. Consequently, results obtained are certainly not of high accuracy. One of its main disadvantages is the use of one three way stopcock for removal of both boiler and condensate samples. It has provision, however, for accurate temperature measurement where a differential Beckmann thermometer can beused for precise readings. Hence it can be FiaiO.- NEW, IMPROVED OTHMER STILL. (1948') """ "•• —— FIQ.I3.- CHILTON'S STILU CI935") FIO. 12.- 5TIL. L OF ROGERS, KNIGHT AND CHOPPIN 0941) turned to molecular weight determination from boiling point elevation. Yft and Hickman (149) describe the use of the still for a laboratory determination of vapor liquid equilibrium in the system nitromethane-trichloroethene. Some of the other equilibrium stills which followed Othmer's design were those of Mizuta (78) in 1934 and Trimble and Potts (131) in 1935. Baker, Hubbard, Huguet, and Michal- orfski (4) effected modifications in the Trimble and Potts apparatus and reported it in a paper in 1939* However, the apparatus does not include any important development not already discussed. In 1942 Langdon and Keyes (70) modified Othmer1s still somewhat and incorporated a stirrer to mix the incoming condensate in the boiler. This, of course, would aid in achieving equilibrium conditions in the boiler for purposes of sampling and analysis. Chilton's Equilibrium Still Around the late twenties and early thirties equil• ibrium stills were becoming increasingly complicated in design and operation. An example of this is the highly elaborate arrangement of Nelson (83), (147) where two Cottrell tubes supply liquid from the boiler to an equilibrium chamber and a thermometer chamber. The Lansberger principle is employed where the equilibrium vapors are bubbled through the liquid to achieve equilibrium conditions. In an effort to simplify these units without sacrificing too much accuracy, Chilton (17) in 1935 reported the still shown in Fig. 13. It includes the Lansberger method of heating as well as the vapor trap. As the liquid in the boiler A is heated, the vapors formed bubble through the liquid in B past the thermometer and are finally condensed and collected in C. The excess condensate, of course, overflows back into the boiler. Liquid samples are siphoned out of B through a condenser to prevent loss of vapor, while vapor samples are removed by means of the stopcock at the bottom of C. Chilton claimed that his apparatus, as well as being simple to construct and operate, gave results whioh compared favorably with those of previous workers. Still of Scat chard, Raymond, and G-ilmann ...... The equilibrium still designed by Statchard, et al (110) about 1935 was a modification of the Swietoslowski appar• atus. They provided it with Chilton's device in the boiler along with some of the features of the Rosanoff, Lamb, and Briethut (104) apparatus. In operation, the liquid is placed in both vessels A and B (Fig.14) and that in A is heated. As vapors form they pass into the inner vessel by way of the inlet tube as shown in section E-E. Yapors and liquid from vessel B pass up the belled bottom Cottrell pump and spurt over the thermocouple well. The vapors rising from vessel B are considered the equilibrium vapors and they pass into the con• denser to reflux into the trap. By means of an overflow return tube the condensate returns to the boiler A. These researchers were probably the first in vapor liquid equilibrium studies who made a serious endeavour to measure the equilibrium temperature accurately. They employed Copper-manganin thermocouples which were standardized carefully ft h SECT»otH t-E. F1G.I4-APPARATUS of SCATCHARty RAYMOND and OILMANN. by comparison with a platinum resistance thermometer. Latest Developments in Equilibrium Stills. Elash Chamber One of the more important stills evolved recently is that of Jones, Schoenborn, and Colburn (64) reported in 1943. They introduced a flash chamber shown as B in Eig. lj> in which the returning condensate was vaporized and passed through the liquid in boiler A as in the Lansberger apparatus. Liquid is placed in boiler A and U-tube C. The boiler and the tube leading to the condenser are wound with resistance wire and are heated to a temperature just below equilibrium temperature. Chamber B is kept above equilibrium temperature to promote rapid vaporization. A thermometer or thermocouple placed into the well in A gives the equilibrium temperature. The by-pass and three way stopcock provided are for purposes of maintain• ing the vacuum when samples are being taken and preventing sucking back into the flash boiler. Although this still is of simple construction and appears relatively easy to operate, it actually requires con• stant attention during a run. This stems from the fact that superheating and distillation troubles in the apparatus are not easily avoided. The flash chamber heating coil must be adjusted so that a small drop of liquid just remains on the bottom of the tube. This indicates that superheating is not occurring. Gillespie Still with Vapor disengagement Chamber. One of the better equilibrium stills which seemed to eliminate many of the faults of previous stills was that put 54 forth hy Gillespie in 1946 (42). Some of the newer features of the apparatus (Fig.l6) were the inclusion of a vapor disengagement chamber, an internal platinum heater along with an external niohrome wire heater, and provision for the mixing of the vapor condensate with the liquid return before reaching the boiler. The Cottrell—tube from the boiler to the vapor disengagement chamber continually "pumps liquid and vapor over the thermometer bulb which accurately registers the equilibrium temperature. Liquid entrainment in the vapor is essentially eliminated as shown by Gillespie and later by Rieder, and Thompson (101). The internal heater prevents superheating as well as providing a means of pumping the liquid through the Cottrell tube. However, this still has certain disadvantages which introduce errors into any deter• minations. The liquid in the boiler is actually not the liquid in equilibrium with the vapor disengaged in the chamber. The liquid sample should be removed from a trap as close to the disengagement chamber as possible since it is here that equil• ibrium between the liquid and vapor actually exists. Secondly, condensation may take place in the Cottrell tube, and this necessitates good lagging. Finally, there is a pressure difference between A and B which may become serious at low pressures. Fowler Modification of the Gillespie Still. After making his review of equilibrium stills with a critical analysis of each (35)» Fowler designed and reported (36) a modification of the Gillespie apparatus. He made a FIG.IG.- GILLESPIE'S EQUILIBRIUM STILL (1946). complete evaluation of Gillespie1s still giving its advantages and disadvantages. His final apparatus, similar to that shown in Figure 29, includes a liquid trap D below the detachment chamber. The liquid from the chamber runs into a U-tube fitted with a tap at the bottom, overflows into a small bulb at the top of the U-tube and then mixes with the cold condensate on the way to the boiler. Liquid and vapor samples are removed from the appropriate taps. This apparatus eliminates the error due to the rectification which may take place in the Cottrell tube or due to the change in composition which may~result from the hydrostatic head between the boiler and the disengagement chamber. 4. Specialized Stills Stills for Partially Miscible Binary Mixtures. The first recorded equilibrium still used for deter• minations on partially miscible components was that of Stock- hardt and Hull (118) in 1931. It was used on the system butanol-water but was shown later (113) to produce vapors too rich in the more volatile component. A later still designed for such equilibrium measure• ments was that of Colburn, Schoenborn and Shilling (20) in 1943. An ordinary equilibrium still would not function properly in this case due to the separation of the components in the phases. These workers distilled the components separately, led them into a compartment with an approximate equilibrium mix• ture of the two components and allowed the liquid and vapor to reach equilibrium as indicated by the thermometer. The vapors 36 were condensed and analysed and the final liquid in the vessel was similarly analysed. The"inlet tube was surrounded by a baffle tube to give a circulating motion to the liquid and ensure intimate mixing. Results obtained on this apparatus were good. How• ever, skilled manipulation was required and the arrangement for separately distilling and then intimately mixing the com• ponents was rather clumsy. Recently, Smith and Bonner (113) in late 1949 pub• lished details of an equilibrium still which was used on the system 1-butanol-water. An ordinary condensate-recirculation type of unit would not be satisfactory for this system since the heavier layer would build up in the condensate trap. The design of the still (Fig.17) follows the general line of the Othmer-type (93) and is similar in principal to the unit of Baker, Hubbard, Huguet, and Michalowski. The charge is placed in the distillation flask A and the magnetic agitator is tur• ned on to prevent superheating. Heat is applied by means of the internal Nichrome wire resistance heater and all the air is vented out through the outlet C. When vapors begin to reflux on the neck walls, the vent is stoppered and the neck heater consisting of turns of resistance wire is turned on. The temperature of the neck is kept at about 3°C higher than that of the liquid to prevent condensation. Vapors are con• densed in the reflux condenser at B and the equilibrium vapor temperature is measured on the thermometer at D. During a run, the still is allowed to operate about O 3 FIG.I7 - OTHMER REFINED STILL MODIFIED BV SMITH I BONNER 09*9) an hour, the 5°C differential between liquid and vapor tem• peratures being maintained. Then the temperatures are recorded and a condensate sample is drawn off by means of the three way stop-cock and the auxiliary cooler F. By using a dye test, the authors found entrainment to be negligible. Equilibrium Stills for High Vacua C.A. Bishop (6), doing work on a Ph.D. thesis, was probably the first to design and construct an apparatus for the determination of vapor liquid equilibrium at low pressures. Details of his unit are unavailable at this writing. In 1947, at about the same time, two papers were published on vapor-liquid equilibrium at low pressures by different authors. Williams (143) described an apparatus which was a modification of Gthmer's (87) 1932 design. Refer• ring to Fig. 18, the liquid is put in the still body and stain• less steel helices are added to prevent bumping. The boiler A, as well as the removable oven body B, are insulated and heated by resistance wire. There is a condenser tube C mode of large diameter tubing to reduce the pressure difference between the vapor trap and boiler. The condensate oollects in the trap D and returns to the boiler. Williams reported good results at 0.1 mm.Hg. pressure on esters boiling 3*0° apart. Perry and Fuguitt (98) had a somewhat different design for their equilibrium Still (Fig. 19). The still proper was constructed entirely from Pyrex, while the stirrer shaft and top closure were fabricated of metal. The liquid is put into the boiler A, which is heated by an internal heater, and stirred by a paddle on a stem passing through a vacuum gland. FIG-20.-STILL OF GORDON AND BENSON FOR LOW TEMPERATURE EQUILIBRIA, 0^46> The upper portion of the boiler is wound with niohrome wire for heating to prevent condensation. By means of a rotating disc on the shaft, the rising vapors are deflected onto the walls of the vessel cooled by a water jacket. The condensate is caught in a trough and flows into a vapor trap B. From there it overflows back into the boiler. The whole apparatus is brought to a pressure of 0.1 mm of Hg and the authors report good results for components boiling 10°0 apart.. None of these units, however, have provisions for temperature measurement since this would increase the complex• ity of the apparatus considerably. Equilibrium Stills for Low Temperature Work Work with systems which are normally gaseous at room temperature involves added difficulty in apparatus design. Gordon and Benson (49) were probably pioneers in vapor liquid equilibria on substances with low boiling points. They carried out vapor liquid equilibria determinations on the system hydro• gen cyanide-cyanogen chloride at 15°C, the former component of the system having a boiling point of 26°C and the other 12.8GG. The apparatus they employed is illustrated very diagramatically in Figure 20. The part of the unit tothe right of the dotted line is enclosed in a constant temperature bath and the outlet F leads to the MoLeod gauge, vacuum pumps and mercury manometer. Bulbs C and D contained the two components which were distilled into the liquid phase bulb A by means of liquid air. Agitation was obtained with a small magnetic stirrer B. After temperature equilibrium was obtained the vapor pressure was measured on the mercury manometer. With gravimetrically determined samples of the components this gave the vapor pressure curve for the system. The equilibrium vapor-liquid composition was obtained by slow isothermal distillation of the system from bulb A into pycrometer G. From previous measurements the density-compos• ition curve was determined for the system and this was used for analysis of the phases. Almost within the last year, Stutzman and Brown (121) carried out vapor-liquid equilibrium determinations at consider• ably lower temperatures. Their measurements were made on natural gas and, of course, had to be made in a low temperature cryostat. B. Vapor Recirculation Type of Apparatus An Equilibrium apparatus belonging to this group is one where the vapor is continuously circulated through the system and is brought into repeated contact with the liquid. It is Considered by many authors to be far superior for accur• ate vapor liquid equilibrium measurements to those previously mentioned, Its advantages over the dynamic and continuous distillation types of units are, of course, quite obvious. Vapor recirculation in the apparatus under proper conditions should prevent superheating, entrainment of droplets of liquid in the vapor, and reflux in the lines. Furthermore, since there is no cold condensate to return to the hot boiler, flashing of the more volatile component is eliminated. However, deter• minations on such an apparatus must be carried out at constant temperature and with some mechanical means of circulating the vapor. Very often these involve certain complications which are not easily overcome. Consequently this type of apparatus is only rarely used and it has had only occasional mention in the literature. Early Forms One of the earliest types of apparatus employing the principle of vapor circulation was that of Rosanoff, Lamb, and Breithut (104) in 1909- They employed a modified Lans• berger device shown in Figure 21 and bubbled the vapor through the liquid to avoid superheating. The liquid system was placed in both containers A and B and that in A was heated by means of the internal heater. The vapor was led into vessel B by way of the inlet tube and was bubbled through the liquid. Vapors coming from B were passed through a disengagement device where any entrained liquid was removed and fell back into B. To prevent the steady loss of the more volatile component additional material was slowly supplied to the liquid in A, the rate being determined by the temperature reading in A. The vapor was then condensed and analysed, and a sample of liquid was removed from B by means of a pipette. Nelson ($3) in 1932 made an elaborate arrangement of a still incorporating two Cottrell pumps, a vapor trap, and the Lansberger principle of bubbling the vapor through the liquid. However, although this apparatus employs the scheme of vapor recirculation to a point, it cannot be classed en• tirely under this group. Constant .Temperature Type of Recirculati on Apparatus In 1929, Ferguson and Funnell (34) reported a novel apparatus for vapor liquid determinations. It was a rather FIG.2I." APPARATUS of ROSANOFF, LAMB and BRIETHUT 09OQ) . .. . • -• - 41 complicated unit for the recirculation of a vapor through its equilibrium liquid, and then for the analysis of the two phases. This was the first apparatus in which the vapors were recirculated through t he system by means of a specially designed, all-glass circulating apparatus (37). The vapor pressures of the liquid phase as well as that of the condensed vapor phase were measured on a mercury manometer. From a previously determined vapor pressure-composition curve the compositions of the two equilibrium phases were determined. A modification of the Ferguson-Funnell apparatus was that employed by Gordon and Hines (50) on the system ethanol-acetone. Referring to Figure 23, which illustrates a very similar apparatus, the liquid system was placed in the filling tube T from which it was distilled into the liquid phase bulb A and sealed off at x. The constant temperature baths, represented by solid lines for the water bath and broken lines for the air baths, were brought on temperature and circulating pump P was started. When pressure equilibrium was reached, as shown by the manometer at M, the pressure was read. Then the vapor and liquid phases were separated by sealing off the tubing at y and z, the vapor phase was con• densed into bulb B and sealed off from the rest of the appar• atus at W-. The baths were brought on -temperature again and the vapor pressure of the vapor phase measured. From a pres• sure-composition curve determined on the same apparatus the composition of each phase was obtained. The procedure will be more fully described in one of the- following sections since an apparatus closely related to that of Gordon and Hines was employed in this research. C. Other Methods of Vapor Liquid Equilibrium Determinations. The additional methods of vapor liquid equilibrium are not too widely used and will be dealt with only briefly in this review. The Bomb Method The apparatus consists of a closed evacuated vessel kept at constant temperature in a constant temperature bath. It has some provision for agitation such as a rocking device or shaker. The liquid sample is placed in the vessel, agi• tated until equilibrium is reached between the liquid and the vapor at constant temperature. By withdrawal of the vapor and liquid samples they can be analyzed. Some work with this method of determination was reported by Ferguson, Fried and Morris (-33). Certain difficulties are involved in such a deter• mination which may become serious. During sampling, pressure changes occur that are likely to induce evaporation or con• densation thus changing the equilibrium state. Also the sampling lines of small cross-section may fill up with liquid during the initial part of the operation- and this liquid may never come'to true equilibrium. Dynamic Flow Method Another procedure which has found a certain amount of use in vapor-liquid equilibrium studies is one in which a vapor is passed through a series of vessels containing liquids of suitable composition. A train of this type was.used in the work of Parks and Chaffee (95) in 1927 and, later in 1935 Washburn and Handorf (137) employed the same principle. If concentrations of all the vessels are made the same and a large number of vessels are used equilibrium will tend to be more nearly approached as the vapor passes through the unit. This can dispense with analysis of the liquid because if the concentrations in the vessels are made up accurately the con• centration in the last one will change only slightly and can be taken as the liquid composition. The apparatus has the advantage of being simple and involving a reasonably small amount of work. However, a fairly large amount of the components is required; an exact equilibrium cannot be attained due to the fact that a pres• sure drop is involved in passing the vapor through the system and there is a danger of entrainment. 44 CHAPTER IV APPARATUS A. The Vapor Recirculation Apparatus The apparatus first constructed was essentially that of Gordon and Hines (50) with a few small modifications for improved operation. Since the whole apparatus, from the high vacuum arrangement to the constant temperature cabinets, was built almost in its entirety in this research, it will be described here in some detail. High Vacuum System The apparatus for the production and measurement of high vacuum was constructed, assembled, and tested without too much difficulty. Figure 22 shows the arrangement of the manometer,' McLeod gauge, and diffusion pump. A reasonably high vacuum was obtained by introducing a narrow orifice or "jet" into the diffusion pump in its construction. Backed by a newj Welch "Duo Seal" mechanical forepump, it gave a pressure below 10~^mm. of mercury as measured on the McLeod gauge. The gauge was constructed roughly according to the specifications of Rosenberg (105) and was calibrated by the "squared scale" method (145)' Since the compression ratio of the gauge was calculated to be 2.48 x 10"^, the absolute pressure in the system could be measured accurately to a pressure at least corresponding to that figure in mm. of mercury. The mercury used, in this McLeod gauge was purified as described below and then distilled into the lower bulb to avoid contamination. Purification of Mercury A commercially pure form of mercury was further purified by a number of standard procedures (107). Firstly, dry, clean, filtered air was bubbled through the mercury covered by a dilute solution of approximately 1% nitric acid. This procedure oxidized any of the base metals such as copper, zinc and lead that may have been present. The oxidized metals appeared as a scum on the surface of the aqueous solution and were later removed by "pinholing" through a finely drawn down capillary. This oxidation procedure was carried on for about twenty four hours with frequent changes in the nitric acid solution. The mercury was then passed through a "scrubber" which consisted of a column some 150 cm. long terminating in a bent capillary in the form of a trap at the bottom. It was filled with about 5$ nitric acid solution. The mercury falls through the solution in the form of a spray which can be effected by passing the mercury through a funnel drawn to a fine jet. Thus any remaining alkali metals are oxidized and by a repetition of the procedure a number of times, the scum is removed at the funnel. Finally, the mercury was sprayed through distilled water twice and collected in a well cleaned and dried container. After removing any visible traces of water with filter paper, the mercury was transferred to. a distillation apparatus. Here it was twice distilled under vacuum to remove any traces of the noble metals such as gold or silver and tin. , - . , • 46 Main Equilibrium .Apparatus Referring to Fig.23, A is the liquid phase bulb of 1G0 ml. capacity, B is a bulb of 50 ml. for the conden• sation of the vapor phase, and C is a glass helix to allow the incoming vapor to reach temperature equilibrium. This part of the apparatus is enclosed in a constant temperature oil bath represented by the solid lines. The portion of the apparatus enclosed by the dotted lines, which represent the constant temperature air baths, consists of a 5 liter pressure equaliz• ing bulb D and Funnell-Hoover circulating apparatus P. L represents a leveling line on a small millimeter scale which can be viewed through a double glass window in the air ther• mostat^. The filling tube F is outside the constant temp• erature baths and contains the initial charge of the liquid system. K is a separate compartment made of galvanized sheet metal covered with asbestos paper and having a double-walled, insulated, sheet copper door. This allows access to the vapor lines joining the circulating apparatus with the bulbs in the oil bath without disturbing the equilibrium in the rest of the apparatus. A small Liebig condenser 0 was introduced into the line between the liquid phase bulb A and the vapor phase bulb B. This prevented condensation of the system into the bulb B on initial distillation from filling tube F. As a precaution against condensation in the horizontal line leading from A to F, nichrome resistance wire was wound about it for heating purposes. The manometer M permits measurement of the vapor pressure accurately with a cathetometer. By means of a levelling bulb J, mercury can be manipulated in the manometer for any desired height. All connecting glass lines on the circulatory part of the apparatus are made of 10 mm. outside diameter glass tubing to allow for rapid circulation. Circulating Pump This is the "heart" of the whole apparatus (Fig.25) and was perhaps the most difficult individual item to build. In the final construction of the glass parts, the specifications of Funnell and Hoover (37) were followed quite closely. How• ever, it was found that more efficient operation was obtained by grinding the valves into place with a very fine grinding compound consisting of 200 mesh carborundum well mixed with glycerine. Such treatment was also found helpful for the piston since the smooth surface of the glass was removed and thus helped to prevent binding. The solenoids were made 6.5 cm. long, but were finally wound with 1700 turns of No.22B&S cotton insulated copper wire. The extra 500 turns over the original solenoids gave a better field and the pump was found to operate for longer intervals without interruption. A suitable current for the coils is 1.1 amps using a 40 volt D.C. line. The resistance of each is 16 ohms and a 90 ohm rheostat in series gives sufficient control over the current input. As an added aid to prevent stalling of the piston, an electromagnet E was installed just above the centre of the cylinder of the pump. This was made by winding 1200 turns of cotton-insulated copper wire on a soft iron core 3 cm long which had iron flanges threaded on the ends. The flanges also "1\ PISTON ELECTROMA6N ET 3 L rr— I VALVES • ^ ^^^^ AIR CORI E S(>LENOID S 1 J7 FI6.25- CfRCULATING APPARATUS 4 tend to become highly magnetized and give the magnet a wider range of influence. Resistance of the electromagnet was found to be 11 ohms by means of a Simpson ohmmeter. With a 99 ohm rheostat in series the electromagnet, adjusted for a current of 1.3 amp from the 40 volt D.C. line, was found to have a considerable field of attraction. Commutator for Circulating Pump A modification has been made with regards to the commutator arrangement described in the B.A. thesis 033). In place of the automatic telephone commutator, a common record player motor and turn table have been employed. The motor operating at 78 revolutions per minute was arranged into a simple set up as shown in Fig. 24 to give the piston of the pump 78 cyles per minute. This speed produces about the maximum efficiency of the circulator as evidenced by tests made with the previous commutator which could be varied for speed. Although there have been electronic devices described in the literature (5) (125) for such timing operation, this commutator was found to function very satisfactorily without complications. Temperature Control The temperature range in which the apparatus can be employed is from room temperature up to 95°C. Temperatures in + the oil bath were to be maintained within - 0.002°C. Con• sequently, a serious problem was faced in designing baths which would completely enclose the apparatus and still permit certain manipulation on the vapor lines without excessively • v. : ' : 49 disturbing equilibrium during a run. A further complication is the .iu.se of fragile glass tubing and bulbs in connection with bulky and heavy thermostats on a metal rod frame which does not allow too. much firm support. Constant Temperature baths The air baths were fabricated of three-ply and were made with double walls, the space between filled with rock wool insulation. .One unit contains both compartments separated by a double wall. The walls, in most cases, are 1 l/k inches thick and were found to give efficient insulation. To offer complete freedom in working with the circulatory part of the apparatus in preparation for runs, the baths were made in two parts. The back portion is fastened firmly to the frame and contains the apparatus. A cover with access doors and vision windows slides over theback portion. Felt packing supplies further insulation in places where the back and cover come together and where glass tubing passes through the partition in the baths. The constant temperature oil bath is a large rec• tangular galvanized iron tank with a capacity of approximately 18 gallons. Around the outside of the tank is a plywood con• tainer. The space between sheet metal and plywood is packed loosely with rock wool insulation. A drain plug of 1" diameter at the bottom aids in the rapid drainage of the bath.-between runs and when condensation of the vapor phase is required. The bath is kept in place by a sturdy bench built especially for the purpose. Plate I shows the constant temperature baths as PLATE! I,- VAPOR RECIRCULATION EQUILIBRIUM APPARATUS. well as the remainder of the apparatus ready for operation. Stirring Equipment One drill rod steel shaft runs through the three constant temperature baths and bears the fans and propellers which supply the~circulation of air and oil. There are three brass bushings securely fastened to the frame of the constant temperature baths and serve as bearings for the shaft. The air circulators are five-inch diameter, four-bladed, aluminum fans firmly secured with keys and set screws sufficiently near the incandescent lamps and heaters to circulate the heat rapidly. The oil is circulated by two brass propellers three inches in overall length and carrying sufficient pitch in the two blades to throw the oil at a considerable rate. They are fastened to the shaft by means of set screws in a convenient position to give the most efficient stirring. The shaft is driven by a 1/8 h.p. motor and the belt is so arranged on the pulleys as to impart an r.p.m. of 1600 to the stirrers. It was made in two sections to facilitate dismantling. The lower portion of the shaft carrying the propellers for the stirring of the oil can be readily attached by means of a coupling and set screws just above the lower bushing. This was found convenient when only the air circulators were required to operate as in initial pressure measurement during a run. Figure 27 shows the constant temperature baths in sect ion with the stirring apparatus. ^3*0 THERMO REGULATOR EATER F\G.2 Heating. Temperature Measurement and Control. Air Baths The upper air both was heated by an "Edison-screw cone" heater of 660 watts controlled by a variac-type trans• former. A similar heater as well as an auxiliary incandescent lamp heater maintained the temperature in the lower air bath. The temperature in each compartment was individu• ally controlled with a Cenco, high capacity, DeKhatinsky thermoregulator connected to two-coil relays. Such a ther- moregulator consists essentially of a bi-metallic coil which on expansion or contraction with temperature makes contacts to activate either one or the other of the coils in the relay. The relay coils may open or close the circuit to the air bath heaters by their electromagnetic effect. To overcome the chatter of 110 volts A.C. in the relays, 40 volts D.C. were applied to the coils and 110 volts A.C. was left on the heaters. This type of thermostatic control was found to be effective for maintaining temperature to within - 0.5°C. Figure 26 gives a circuit diagram for such a unit. A long thermometer graduated from -10 to 110°C in divisions of 0.1°C and standardized against a platinum resis• tance thermometer with Bureau of Standards Certificate was used for temperature measurement in the air baths. Constant Temperature Oil Bath The heating in the oil bath consisted of four different heaters. Two blade heaters, one of 500 watts and the other of 250 watts, supplied the main heat. They were adjusted with rheostats in series and were kept on for a temperature just about a degree below that required of the bath. The vapor phase bulb B was heated separately by a nichrome wire heater in very close proximity. These three heaters could be controlled by a "metastatic" mercury thermo- regulator used in conjunction with a Cenco electronic relay control unit. This unit was found to give temperature control to within - 0.1°C. For the accurate temperature control an improvised electronic unit was employed. It included a triode for control of the activating current of the 3 milliamp relay. Figure 28(a) gives the circuit diagram. The finger-type thermo- regulator was simply constructed (Fig.2Sb) of pyrex glass tubing with a stopcock in one arm for adjustment of the mercury in the other arm. A needle fitted into a threaded housing of the capillary of the latter gave the finer adjust• ment. This thermoregulator works on the mercury expansion principle and the mercury was of the specially purified grade used for the McLeod gauge. The heater controlled by this unit was made of nichrome resistance wire (No.22 B8S gauge) loosely wound on a glass tubing framework around the inside of the bath. The whole heater had a resistance of 150 ohms and gave 80 watts on the full 110 volts of the A.C. circuit. For a greater wattage, however, the heater was tapped off at two points, the resistance between which was 80 ohms. This gave a total power of 150 watts and was found more suitable for the higher temperatures. A small Voriac was used for THERMO REGULATOR •O Q HEATER A.C. FIG. 28 CIRCUIT DIAGRAM OF ELECTRONIC CONTROL. UNIT FOR CONSTANT TEMPERATURE IN OIL BATH LEGEND FOR FIG. Z&d.. jtlCMOfitAC TT L,- 15 WATT TUN^STCN P«I.OT LAMP DO LJ. - 25 W*Tr TUNGSTEN UA>HP L» - 15 WATT TUNGSTEN »_AM*» 4 R, ~ I0 OHM CAffeOM RPStjrOR fc Rx- 2-2.*l° oHM CA*OOK* sesuroe Rj - 2-Z ll°4OHM CARBON RCSISTOK. Re - 3 m RCUA* T - VARIAO-TVPE. TRA*4SFORN*tR. 50 l_G TRIOOE. KJ FIG.26 6.- FINGER-TYPE THER M O REG U LATOR. adjustment of the heater. When all heaters in the oil bath were adjusted properly, the relay of the sensitive control unit would switch the current on and off to the nichrome wire at intervals of 5 seconds or even less. Under such conditions the temperature as noted on a Beckmann thermometer, did not vary more than - O.G02°C. The Beckmann was also calibrated with the Platinum resistance thermometer. B. The Liquid Recirculation Apparatus The apparatus was designed to be essentially the same as that of Fowler (36). Some of the small changes made were intended to be improvements and are listed as follows:- (i) A capillary stopcock was introduced for the removal of a sample from the liquid trap. This eliminates the larger quantities of liquid which stagnate in an ordinary 2 mm stopcock. (ii) The drop counter of the Gillespie still (42) was retained in the small bulb above-the. capillary leading to the boiler. This facilitates in adjusting for a proper rate of heating in the boiler. (iii) The vapor condensate trap was made smaller in order to achieve equilibrium more rapidly. This change was suggested as an improvement on the Gillespie still by Rieder and^Thompson (101). Since only refractive indices were ordinarily taken, there was sufficient condensate trapped for measurements. By means of a small pynometer density measurements could also be made. (iv) A type of condenser where dry ice and acetone or an ice-salt mixture can be used for the cooling material was introduced above the vapor condensate trap. Fowler suggested such an addition for equilibrium measurements under reduced pressure. (v) A thermometer well with a 19/38 Standard taper female joint was fitted into the upper unit to accommodate thermometers with ground glass joints used with the Cottrell- Choppin (102) equilibrium still. The still (Fig. 29) was made entirely of Pyrex glass by Mr. W. Pye, glassblower of the Chemistry Department. Tungsten leads were sealed into the standard taper stopper of the filling tube for electrical input to the internal heater. This heater consisted of 12 inches of 28 gauge platinum wire, wound into a small coil at the end and brazed to the tungsten leads with constantin as the flux. A current of 5 amp. con• trolled by a small variac was found suitable for proper pumping in the Cottrell tube. The boiler, Cottrell tube, disengage• ment chamber and thermometer well jacket were wound with a layer of asbestos cord for initial lagging. This was followed by a layer of asbestos cement. A small section of the Cottrell tube and two small openings at the junction of the tube to the disengagement chamber were left unlagged for observation of the pumping action. The external heater for the boiler was wound on the layer of asbestos cement and consisted of 28 turns of No. 22 B8S gauge nichrome resistance wire. It was wound right from the stopcock at the bottom of the boiler to the base of the filling tube, the last turns being kept in place by a few wrappings of thin asbestos cord. A potential of about 3$ volts giving 2.4 amp on a small variac was usually found sufficient for fairly low boiling mixtures. The external heater was covered by another layer of asbestos cement to give insulation to the wire. The whole apparatus was mounted on a small platform 24" by 16" with upright steel rods conveniently placed 14" apart. Plate II shows the apparatus without lagging C. Refractometer The refractometer employed in this research for analysis was a new instrument of the Spencer Abbe type. A description of the principles and working parts is given in most textbooks on Organic Analysis (112) and physical methods in organic chemistry. It was equipped with Amici compensating prisms so that a white light source could be used instead of monochromatic light. The scale of the refractometer is cal• ibrated directly in refractive index as measured with the D line of the sodium spectrum. It may be read accurately to the third decimal place and the fourth may be estimated with an accuracy of - 0.0002. The prisms were kept at the temperature c bath controlled within ± 0.05°C. The temperature on the re• fractomet er could be read directly to 1°C and estimated within - 0.2°C on a calibrated thermometer. This was about the limit of accuracy in reading the refractive index since a change in temperature of 0.2°C changed the refractive index of benzene by approximately .0001. Gibson and Kincaid (41) give a similar value for the experimental change in the refractive index of benzene with temperature. Kurtz, Amor and Sankin (67) have made a thorough review of the effeot of temperature on density and refractive index and summarized available data. equation They showed that the Eykman\represents experimental data for the effect of temperature on the density and refractive index of organic compounds quite accurately over a wide range of temperature. It was developed by J.F. Eykman (32) as an empir• ical formula n2 j- 1 x 1 - C (33) n + 0.4 d relating the refractive index n, the density d (both at the same temperature) and a constant C which was stated to be independent of temperature for the same compound. The refract- ometer was calibrated before and after any series of readings by means of a small piece of glass furnished with the instru• ment. This glass maintains a constant refractive index and is held in place on the refractometer prism by oapillary action with a very small quantity of mono-bromonaphthalene. As a further check doubly distilled water was used and it gave a refractive index of 1.3330 at 20°C as compared to 1.33299 given in the literature. D. Platinum Resistance Thermometer For accurate temperature measurement a platinum res• istance thermometer No. 169314 with a National Bureau of Standards Certificate dated August 17, 1937, was used. Where it was impractical to use this thermometer an accurate mercury thermometer was calibrated with it at short temperature inter• vals. The principle of the platinum resistance thermometer is based on the change in resistance of platinum with temper• ature. This change is positive with an increase in temperature and can be very accurately reproduced with pure platinum. The National Bureau of Standards which calibrates this type of thermometer issue a certificate giving the value of the resis• tance of the ice point, Ro; the fundamental interval (the difference in resistance between the boiling and freezing points of pure water), R1G0 - Ro = F; and , the Callendar constant, which-is obtained from the sulfur point. Calculation of the temperature is made by means of the Callendar Equation (13), t -fHt-Ro\ 1G0 +Sf{ t\2-/t\l Platinum resistance thermometers should be recalibrated at least every five years if they undergo a reasonable amount of use. The resistance of the platinum may change somewhat with time - and as a Consequence of handling. The ice point of the ther• mometer should be checked at least once a month and the value for Ro should not vary more than .002 ohms from the N.B.S. value (2). In the case where the deviation is too great for accurate laboratory resistance thermometry, the bridge should, be checked for calibration. If it appears to be all right, the resistance thermometer should be checked or sent into N.BiS. for recalibration. Measurements of resistance were made on a Leeds and Northrup 5-dial decade bridge. A commutator with mercury contacts was used to overcome the problem of lead resistances. The bridge and commutator are described in the Bulletin of the Bureau of Standards by E.J. Mueller (8l). The ice point of the thermometer was checked by the method given by Busse (11), and found to deviate by + .00045 ohms from that given by N.B.S. This error is quite insignificant for the higher temperature readings but becomes quite important in the lower temperature range from 0° to 10°C. Consequently, in the measurement of the freezing point of benzene such deviations were found to be quite significant. CHAPTER V MATERIALS A. Benzene A commercially pure grade of benzene as supplied by Merck & Co., Inc., was used. .It was thiophene free and eon- formed to A.C.S. specifications with a boiling range from ' 79.5°C to 8l.0°C. The freezing point minimum was given as 5.2°C Lot analyses for its maximum impurities were given as follows:- Non-volatile > • 0.001$ Acid or alkali ------passes test Substanoes darkened by H2SO4.-passes test Sulfur compounds (as S)- * 0,003% Thiophene 0.000$ Water - — ------passes test 1. Initial Purification The benzene was further purified according to the better features of purification reported in the papers of Gilmann and Gross (44), Glanville and Sage (46), Gornowski et al (51) and Tompa (130). About 1 liter of the CP. benzene was placed in a large (2 liter) separatory funnel and agitated for at least 10 minutes with CP* concentrated sulfuric acid. Only a slight turbidity appeared, the sulfuric acid was allow• ed to separate out for about 20 minutes and was then drained away through the stop-cock at the bottom of the funnel. The benzene was washed twice with 500 ml. portions of distilled water and in each case was allowed to stand for some time in order that oomplete separation take place before drainage. A solution of 0.1 N sodium hydroxide was made up of Baker's CP. NaOH in distilled water. The benzene was washed with two 500 ml. portions of this solution each for- about ten minutes followed by two washings with distilled water. About 150 ml. of mercury, highly purified as previously described, were then added to the benzene and thoroughly agitated. A slight scum of black, powdery material was left on the surface and may have been some combination of meroury and sulfur. The mercury was then removed and the benzene was finally washed three times with distilled water. On the final washing the benzene and water were allowed to separate completely and the water was drained. The benzene was poured out from the top of the fun• nel to prevent contamination with the water which had been drained at the lower outlet. Containers used from this point on were always thoroughly washed, rinsed with distilled water and oven dried. Baker and Adamson sodium metal of commercially pure quality was taken directly from the tin and ribbon was cut on the sodium press. The ribbon was pressed directly into the benzene in a distillation flask so as to prevent oxidation of sodium metal on standing in the atmosphere. The flask was then stoppered loosely with a ground glass stopper and the ben• zene was allowed to stand for one week. Thus any hydrogen formed from the reaction of sodium with water was freely evolved. 2. Distillation of Benzene The still employed for benzene distillation is illustrated in Fig. 30a. A is a one liter still pot with 19/4-2 standard taper joint and heated by a Glas-Col heating BENZENE. STILL. mantel controlled by a small variac type transformer. The column B is silvered, vacuum jacketed and packed with a mix• ture of glass beads, helices, and short lengths of tubing. The packed portion of the column is 122 cm (4 ft.) long and the internal diameter is 25 mm. C is a still head which is suit• able for both vacuum and atmospheric distillation. A ground glass joint reducer adapts the base of the still head to the top of the column. A total immersion thermometer graduated in divisions of 1.0°C ranging from -10° to 110°C was used to measure temperature of the distilling vapors. The reflux adjustment was controlled by a precision-ground stop-oock which could easily give reflux ratios as great as 100:1 E (Fig.30b), a new type of vacuum adapter as advertised by the Scientific Apparatus "Glass~~Co. and attributed.to E. Theimer (126)9was incorporated into the system. It was found extremely convenient in the elimination of stop-cocks and associated greases as found in the older conventional type of vacuum adapter (128). The principle of operation is merely the application of atmospheric pressure through a two-way top to a small valve ground to fit a constriction. This valve closes the actual distillation system and permits the receiver to be changed without disturbance of equilibrium within the column. The complete still was mounted into a sturdy frame in which it could be assembled or dismantled readily. Plate III is a photograph of the unit set up for vacuum distillation. The efficiency of the column was determined by the method described by Morton (80) and discussed in detail in a later section. At total reflux the column was found to have 14 PLATE: HL - BEN2fc~Nk 5TILL -theoretical plates with an H.E.T.P. of 8.56 cm. A vacuum distillation was attempted on the benzene. However, difficulties were encountered due to the high volatil ity of benzene and the heavy loss to the fore pump and atmos• phere. This was found to be the case even when the receiver was kept in a freezing mixture of dry ice and acetone and the trap in front of the forepumpwas similarly immersed. Since benzene is very stable and its boiling temperature at atmos• pheric pressure is quite low (80.1°C), it was decided that there is little advantage in distilling benzene under vacuum. Hence atmospheric distillation was carried out. Initially, the reflux regulator was adjusted for total reflux»and the benzene was refluxed over sodium ribbon for ten hours. The reflux ratio was then set for 30:1 and an initial fraction of about 100 ml was collected and discarded. The middle fraction which came over at a temperature of from 80.0°C to 80.1°C was collected and saved. 3. Fractional Reorystallization The distilled benzene was placed in a large 1-liter erlenmeyer flask and was slowly frozen in a dry-ice box. It was allowed to melt partially and the first 25 ml of liquid were discarded. The freezing process was repeated and the change in purity was followed by refractive index. However, since there was no definite change in refractive index with reorystallization, the material was considered to be pure and two recrystallizations sufficient. The purified benzene was then stored over freshly cut sodium in a ground glass-stopper• ed flask. 4. Check on Purity There were three separate determinations made on the purity of the benzene, - refractive index, freezing point and density. Each checked within experimental error with values given in the literature. (a) Refractive Index The Spencer Abbe-type refractometer was mainly used for most of the checks on purity with refractive index. Read• ings were taken at both 20GC and 25°C to compare with literature values at the two temperatures, and were found to be 1.5010 and 1.49 79, respectively. The values of other authors, are given in Table I. (b) Freezing Point The freezing point of benzene was determined on an apparatus similar to that of Mair, - Glasgow, and Rossini (74). Figure Jl is a diagrammatic illustration of the unit which was made suitable for freezing point determinations or both solids and liquids by the introduction of a heater. A is a silvered Dewar flask 18 cm high with an internal diameter of 6.5 cm»B is a small unsilvered Dewar (JO mm internal diameter) fitted with an outlet and stopcock for evacuation. The small Dewar was lagged with asbestos cord and wired with 27 turns of No. 22B S nichrome wire. The heater was found to have a resistance of 15 ohms and could be easily controlled with a variac. A thin layer of asbestos paste was added as insul• ation for the heater. As further insulation against the cool• ant a copper container was slipped over the heater and lagging 110 V. A.C LEGEND A- LARGE DE WAR B " UNSILVE RED DEWAR // i TH STOPCOCK C COPPER JACKET D - INSULATION AND HEATER WIRES EL - STIRRER F- PLATINUM RESISTANCE TH ERMOMETER FI G3I-F REEZING POINT APPARATUS and held A in place by means of a broad rim soldered on at the top. The rubber stopper in B accommodates the platinum resis• tance thermometer E and stirrer E. In operation, the cooling mixture is added to A and, in the case of benzene, a dry ice and acetone mixture is very effective. Purified benzene is placed in B to a height suf• ficient to cover the coiled portion of the platinum resistance thermometer. The temperature of the benzene should be around 20°C so that a reasonably long cooling curve can be obtained. The inner Dewar is evacuated by a vacuum pump and is then placed in the coolant. A resistance versus time curve is ob• tained by taking the resistance reading every minute for the first ten degrees of temperature and then every thirty seconds for the last few degrees. The cooling rate should be about 1°C or approx. 0.01 ohms in 1 to 3 min. in the range 3° to 10° above the freezing point. The heat transfer can be adjusted by introducing air into the jacket of the inner Dewar or evacuating down further. After recovery from undercooling is substantially complete, the resistance is recorded at intervals of one minute again and finally at 3 minute intervals. Read• ings are continued until difficulty is encountered in stirring. The cooling curve for benzene is shown in figure 32. A value of 3.A3A-°C was obtained for the freezing point and checks reasonably well with values given in the literature (Table I}. Since the platinum resistance thermometer employed has not been checked by the National Bureau of Standards since 1937, some of the constants may be out considerably. At low temperatures er• ror in the ice point calibration for Rft can be very significant. FOIUA C2 r"cH IPS b±;: ±i:P i u^-pr; '_._,, J ;j. 1::; .J u j.. the! " "*"' tfiJjJTLu.Li'...... ctpj iit,-'-; ;-TJ-:-. i- -Hi 3^ :*em:$ffi:a5^fe...u*i^ :.::iil.; "c 4^iJ:;_ ;a:'ni'ji_cLc. ; 1,1*11 nri j-Lu-u-i-u >+iu>-u*ii> .4».U«I,IIIIIIIHIIIII • •-arini MI I'liTtnV SB ^QlHj^fktt-i *fe-S^. 40 .50 60 70 (c) Density Density measurements were carried out in a vacuum- jacketed 25 ml. pycnometer bottle. The pycnometer was cal• ibrated with conductivity water at 2J?0C. For density deter• minations on benzene the bottle was thoroughly washed with soap and distilled water and dried with warm air. Absolute alcohol was used to remove any excess water and a rinse with ether eliminated the alcohol. The bottle was placed in a vacuum dessicator for at least one hour prior to weighing. It was weighed with calibrated weights accurately to 0.1 mg and then filled with benzene which was below 25°C. The bottle was then closed fairly tightly and placed in a constant "temperature water bath. The temperature of the bath was maintained at / 25°G - .02 according to a calibrated thermometer. After one hour, the bottle was removed from the bath, excess benzene was wiped off with tissue paper and the bottle and benzene were weighed accurately as before. The value obtained for the density of benzene at 25°C was 0.87368 gms fml. There are only a few values in the literature to compare with and they are listed in Table I. TABLE I PHYSICAL DATA FOR BENZENE FROM THE LITERATURE AUTHOR REFRACTIVE FREEZING- DENSITY INDEX POINT Renault (100) 4.45°C Lachowitz (69) 5.42 ± 0.02 Mangold (75) 5.5° Linebarger (73) 5.4 Smith and Manzies(114) 5.40 Washburn and Read (138) 5.48 Timmermans and Martin (127) 5.50 25 Wojciechowski(144) nD = 1.4981 5.51 O.87366 Gilman and Gross (44) 5.42-5.46 I.C.T. (58) •r- 1.5014 5.49 - 0.001 d|° = O.8788 Scatchard, Woodand Mochel (111) 5.53 Allen, Lingo, and Felsing (1) - 1.4980 5.5 0.8732 Eglof f ( 29) - 1.4977 5.49 25 n Gibbons et al(40) D - 1.4979 5.50 Glasgow, Murphy, Willingham, and Rossini (47) 5.53 ,25 Tompa (130) nD « 1.4981 5.49 Oliver, Eaton, and Huffman (85) 5.50 Handbook of Chemistry2Q and Physics (55) n. = 1.50142 d 5.51 4 - 0.8794 Glanville and Sage 25_ (46) nD = 1.5013 TABLE I (CONT'D) AUTHOR REFRACTIVE FREEZING 'DENSITY INDEX POINT Emerson, and on 25 Cundill (30) n£ =1.5011 d4= 0.8732 n^p = 1*4979 Natl. Bur. Stand- PO ards (82) n£ » 1.30II 25 nr/ « 1.4979 Pesce (97) n^5 = 1.49825 D^= 0.87362 Cohen and Buij(l9) D^= 0.87378 Smyth and Walls oc (116) n£ = 1.49815 Gibson and Kincaid 25 25 (41) nD - 1.4983 D4 = 0.87366 * 0.0002 B. Butanol The normal butyl alcohol used was of a commercially pure grade supplied by Fisher Scientific Company. The lot analysis data was given as follows: Boiling Range 116° - 117°C Free Acid(as Butyric) 0.01% Non-volatile matter 0.01% Further purification was carried out by first refluxing two liters of the butanol over freshly ignited calcium oxide for four hours. A three-necked flask was used and the condensers were held in place with ground glass joints. Calcium chloride drying tubes were attached to the ends of the condensers in order to protect the butanol from atmospheric moisture. The butanol was decanted from the lime and refluxed for another four hours with magnesium turnings. It was finally distilled in a vacuum-jacketed, silvered column similar to that used in the benzene distillation. The column was packed with glass helices for 3^ l/2 inches (93 cm) of its length. The distil• lation was carried on at a reduced pressure of approximately 500 mm as obtained by a water suction pump. The reflux ratio was set at 30:1 and the rate of distillation was adjusted just below flooding by means of a heating mantel controlled with a variac. The purity of the butanol was checked by Emerson and Cundill with respect to refractive index and density. At 20°C the refractive index was found to be 1.3994 and the den• sity was determined as O.8063 at 25°C A comparison with literature values is given in Table II. 69 TABLE H PHYSICAL DATA EOR BUTANOL FROM THE LITERATURE AUTHOR REFRACTIVE DENSITY B.P. INDEX Webb and Lindsley 20 (141) n£ = 1.5990 118°C Brunell, Crenshaw, and Tobin (9) n^5 = 1.5974 EE d4 0.8057 117.71 Smyth and Stoops 20 25 S3 4 0.8060 (115) nD - 1.39922 d S3 Jones and Christian (63) d4 0.8057 Washburn and Stand- shaw (140) ^ E3 O.8O649 Allen, Lingo, and 25 a23 Felsing (1) d 0.8057 nD - 1.3974 4 Handbook of 117.71 Chem. and Phys. 20 20 (55) 0.80978 Dp - 1.39931 V G. Biphenyl 1. Reorystallization The biphenyl used was of unknown purity supplied by Eastman Kodak Company. The initial purification consisted essentially of reorystallization from absolute alcohol. How• ever, there was a considerable amount of mechanical impurity such as dirt which could be only removed by filtration. About 250 grams of the crystalline biphenyl were introduced into a one-liter erlenmeyer flask. Absolute ethyl alcohol was added until it just covered the biphenyl. Then the mixture was heated just to boiling until all the biphenyl went into solution. The solution was quiokly filtered in a steam heated funnel with suction. The filtrate was allowed to cool slowly at room temperature and the biphenyl readily crystallized out without seeding. Filtration of the crystallized biphenyl was carried out in a large BQchner funnel (6.j> inch diameter) with suction. One layer of Whatman Filter Paper No. 1 was found sufficientvfor the filtration. The crystals were washed twice with cold absolute alcohol during each filtration. Reorystal• lization of the biphenyl was carried out three times and on the third filtration the mixture was kept somewhat warmer to eliminate the higher melting impurities. With each subsequent reorystallization the filtrate was found to be much clearer than previously. When water suction had removed all the al• cohol, the crystals were spread out between several large sheets of filter paper and were dried at atmospheric temper• ature and pressure for 24 hours. 2. Distillation (a) Determination of Conditions Since biphenyl has a melting point of around 69°C and a boiling point of approximately 254°C, it seemed quite imprac• tical to distil it at atmospheric pressure. Gn the other hand, if the pressure is reduced sufficiently there might be only sublimation with a resulting clogging up of the lines and other difficulties. The ideal distillation temperature appeared to be at about 125°C when the calculated vapor pressure is very nearly 14 mm. Fig. 33 gives the vapor pressure curve for bi• phenyl from Garrick (38)*nas calculated from the equation, log P (mm) - 7.0220 - 1723 - 245700 (57) T T^ which was fitted to the data of Chipman and Peltier (18). These researchers experimentally determined thempor pressure above l62°C and found the equation to fit within experimental error. There have been very few determinations made on the vapor pressure of biphenyl and those have usually been above 150°€ . (38, 791. The determination of an optimum in temperature and pressure was determined on a small semi-micro still (Fig.34) designed by Mr. A. Werner and built by Mr. W. Pye of the Chemistry department. In order to maintain a certain constant pressure above that given by a mechanical forepump, a pressure regulator, which will be described, had to be installed. Hot water was supplied to the condenser to prevent the solidific• ation of the biphenyl on its way to the receiver. About three grams of biphenyl were introduced into s 260 IZO I AO TEMPE.RATURE Fia.34 - SELMJ - MICRO 6TILL the still pot. All ground glass joints were sealed with a non• volatile, stiff stopcock grease (6enco. No. 15522-A). The still pot was heated in a glycerine bath and the column was carefully watched for the rise of biphenyl vapors. Temperature equilibr• ium was difficult to achieve with such a small quantity of material, but sufficient data was obtained for an approximate idea of conditions required. At 30 mm of mercury pressure the temperature recorded was 141°C. (b) Biphenyl Still Developed Although mention is made in the literature (130) of purification of such substances as biphenyl by vacuum distill• ation, there appears to be a general paucity of the details for a still for such a distillation. The literature was revie• wed from the earliest chemical abstracts to the present day. The only mention of such stills is usually found in the case of plant-scale production, almost invariably covered by patents and not described. Haehn (53) in 1907 described a small still for vacuum distillation of solid substances. It appeared rather impractical from the point of view of heating methods used to prevent solidification. Recently, a small still which could be used for solids was reported (25). However, this still was primarily designed for semi-micro quantities of a maximum of 20 ml. of material and would not be suited to our purpose. Consequently, it was decided to design and build a small still suitable for the distillation of liter quantities of biphenyl. The still described here can be built without too much difficulty by one who has had a reasonable amount of experience in glass blowing. There is nothing on the still 73 which, need be expensive, and in our particular case most of the still and accessories were made from scrap materials.1 It has been cbmpaotly assembled and is readily portable. The still itself is illustrated in Fig. 35. The still pot A is a 1-liter round-bottomed flask with a 29/42 standard taper female joint. Heat is supplied by a one liter size glas-Col heating mantle which is controlled by a small variac* The still pot was insulated from half way up the bulb to the top of the joint with asbestos cord and asbestos cement. A column was made essentially in two parts, the lower packed portion B which was vaouum-jacketed and silvered, and the upper portion for the thermometer. The packed portion is 25 cm. long, of 20 mm. tubing and the packing consists of single turn glass helices. A small platinum screen was placed into the takeoff from the column to prevent packing from passing over into the condenser and interfering with the vacuum adapter. The jack• eted portion of the column is 15 cm. long and the jacket is made of 37 mm. tubing. Silvering of the jacket was oarried out by the Rochelle salt process described in the Handbook;of Chem• istry and Physics (55) and in Strong's Uodern Physical Labor• atory Practicert. (119) Two unsilvered strips were left in the jacket for observation of the packing. The jacket was evacu• ated on a high vacuum system for eight hours and the pressure as measured on the McLeod gauge was less than a micron. The upper portion of the column which was to enclose the thermo• meter was also double-walled and equipped with an outlet for evacuation. This was unsilvered to give a completely '—"Si LEGEND A hLlTER STILLPOT B PACKED COLUMN G VACUUM-JACKETED STILLHEAD D THEIMER VACUUM ADAPTER E ELECTRICAL LY-H EATED RECEIVER F CONDENSATION TRAP G. ASBESTOS LAGGING H ELECTRICALLY-HEATED LAGGING TO ATMOSPHERE AND VACUUM \ *- F FIC.35.-BIPHENYL STILL. 74 unobstructed view of tne thermometer. The upper end was fit- ted with a 10/30. It carries a broken thermometer with a ground glass joint which was formed into a small hook just below the joint to hold a total immersion (0°-150°C) thermometer. The jacketed portion of the upper column was made 23 cm. long with 12 mm tubing inside and 21 mm tubing forming the outside jacket. The whole length was made such that the thermometer could hang low enough for the bulb to be in the direct stream of vapors during distillation. The jacket was evacuated in the high vacuum system and was finally degassed by a large bushy flame before being sealed off. In place of a regular type of condenser, a combinat• ion of vacuum adapter and condenser was employed. The adapter used.here is;, similar to; the one on the benzene still except that the outlet to the vacuum and atmosphere was made in the form of a bulb. It serves as a trap to condense escaping biphenyl vapors. Hot water for the condenser was produced by means of a copper coil enclosed in a large copper tube and heated with a Bunsen flame. The temperature was noted on a thermometer in a chamber between the heater and the condenser and was controlled by adjusting the size of flame or the flow of water from the tap. Those portions of tubing above and below the adapter condenser were lagged with asbestos cord and asbestos cement. They were also heated by 50 turns of No. 22 B&S nichrome wire embedded in the lagging. The temperature of the lower lagged portion was checked by means of a 0°-150° thermometer also embedded in the lagging. Figure 3& shows the lagged and heated portions of the still in dotted lines. The resistance of the heater as measured by a Simpson ohmmeter was found to be 16 ohms. It was connected in series with the nichrome wire heater of the receiver and the voltage input was controlled by a variac. The receivers were made of 30 mm tubing in 12 inch lengths with "5*24/40 female joints. They were constricted and thickened in the upper portion for sealing under vacuum. In order that the biphenyl fill the tube completely, the receivers were wound with a large number of turns of No.22 B&S nichrome wire. Pressure Control and Measurement The distillation of biphenyl was found to be very sensitive to pressure change, a variation of less than a mm Hg causing flooding in the column at lower pressures. Consequen• tly, a simple as well as easily adjustable pressure regulator was required. Various types of manostats as described by Morton (80) and Williams (143) were reviewed. Pressure control devices with a leak valve as reported by Jacobs (6l) were con• sidered. Emerson and Woodward (31) describe a pressure control apparatus where any desired pressure can be maintained by the use of a special mercury cut off between the exhausted appar• atus and the vacuum pump. There are the more elaborate devices for automatic pressure regulation in distillation as fully discussed by Coulson and Warne (23) in the Journal of Scientific Instruments* Theae regulators, in most cases, are of a too complicated nature for a simple laboratory distillation. A much, simpler device giving control within 0.5 mm was reported by Todd (129). Todd's apparatus is a refinement of the crude arrangements used by Newman (84) and Donahoe et al (27). New• man's regulator was merely a wash bottle so arranged that gas .being pumped out of an apparatus had to overcome a column of mercury. Todd, using the same principle, developed a regulator which could be readily adjusted for different pressures. Certain modifications in the Todd apparatus were found necessary to obviate the frequent loss of mercury due to violent turbulence. Pressure control was not too sensitive when mercury was used, because a certain pressure had to be built up within the still before it overcame the column of mercury and bubbled out. By introducing a trap with a small column of mercury between the regulator and the pump and reducing the height of the mercury in the regulator itself this problem was largely overcome. This modification also gives the regulator a wider range of usefulness since the head of mercury in the trap can be varied. The whole regulator was made longer with an extra annular ring on the inner tube and right angle bends in the arms leading to the pump and still to prevent any loss of mercury during turbulence. The final apparatus is diagrammat- ically illustrated in Fig. 36. The pressure in the still was measured on a differen• tial, mercury-filled manometer with a millimeter scale calib• rated against a standard meter bar with Bureau of Standards certificate. Atmospheric pressure was measured on a Central Scientific Co. mercury column type barometer, accurately to TILTING ROD FIG.36.- PRESSURE REGULATOR FOR VACUUM DISTILLATION. 77 + within - 0.1 mm. Pressure within the still could he regulated and measured to - 0.2 mm. of Hg. The still and all accessories, with the exception of the forepump were mounted on a solid wooden base covered with a sheet of transite. The base is. 23.5 by 12.5 inches and has heavy vertical rods 23 inches high conveniently spaced. A cross bar was put on for the attachment of other clamps. The manometer was fastened directly to the base and is in full view at very close proximity to the column. The whole apparatus is compact and is readily dismantled or assembled if necessary. Plate IV shows the apparatus assembled and ready for operation. Efficiency of the Column The theoretical plate efficiency of the column could not be checked too easily due to lack of a reflux adjustor. However, a fairly quantitative check was obtained by distilling a mixture of benzene and carbon tetrachloride. The vapor liquid equilibrium data for the system has been recently re• ported by Bushmakin and Voe^kova (10), and it has been shown to be without \ an azeotrope at atmospheric pressure, contrary to previous reports (135). It was decided to check the efficiency graphically by the method of McCabe and Thiele (77) as well as algebraically by the method of Dodge and Huffman ( 26). The materials used were of a commercially pure grade without further purificatidn; The carbon tetrachloride was a Baker and Adamson product and gave a refractive index of 1.4575 at 25°C. The benzene used was the C.P. material sup• plied by Nichols and gave a refractive index of 1.4980 at 25°C. PLATE EL- BIPHENYL STILL 78 The corresponding values given by I.G.T. ( 60) are n^= 1.45734 and,n^= 1.49794, respectively. The procedure employed for the efficiency check was that des• cribed by Morton (80). The values of Bushmakin and "Voeikova were used for the vapor-liquid equilibrium diagram and the refractive index vs. composition curve was taken from the anal• ytical data given for the system in the International Critical Tables (60). The y-intercept for the operating line was ob• tained with the expression given by Ward (135)» XCT. = y-intercept (58) PTTT The momenclature is explained in the following paragraph. A large scale graph was drawn for accurate plate determinations. As a check on the graphical method of efficiency determination, the equation of Dodge and Huffman for partial reflux was employed. (Log +l 2 n+1 - 2.303 2R + B 2Axcl + B -VB2-4AC . 2Axfi+B (B -4AC _2l|B2-4AC \ 2Axfx + B B^4AC 2Axcl+B+|B2-4ACJ 2 , + 1 log Axf, + Bxfl + C 1 AX<£ + BxG1 •+ C J (59) Where A = \R( 1- ) n = no. theoretical plates B • R( C = -xci Xf2 = mol fraction C^H^in Still xCl - mol fraction CCI4 in distillate xC2 = mol fraction C£H6 in distillate R = Reflux ratio (ratio of reflux to product) °^ - relative volatility For the value ofc^the relation of Bushmakin and Voeikova. was used, o( = 1.203 + 1.00203* (60) where x = mol percent, CCJ&4 in the liquid. These authors also pointed out that this relation fits the data of Rosanoff and Easley (103) although the latter only reported their results to 72 mol percent CC£4. In the x-y diagram the values of Rosanoff and Easley were above those of Bushmakin and Voeikova indicating a higher CC£4 content in the vapor phase. A reflux ratio: of 3»5 could be obtained in the column by proper adjustment of heat to the stillpot. Drops of the mixture refluxing into the stillpot as well as those going into the receiving tube could be counted. However, this was only approximate since there were no calibrated drop counters.; The two methods used cheeked quite closely, in each case giving about seven theoretical plates with an approximate H.E.T.P. of 3.5 cm. Improvements for the Still One of the major changes which could be made in the still is the installation of a device for reflux adjustment. There have been several types for a vacuum column described in the literature. One of the favourite designs is that incor• porating a solenoid-activated long-stemmed valve, described in simple arrangements by Collins and Lantz (21) and Diehl and Hart (24), and in a more complex form by Doty (28). Perhaps one type of reflux ratio: head which could be most easily adapted to the still described here is the one reported by Human and Mills (57). They claim that their stillhead is suit• able for substances which solidify above room temperature since the dividing mechanism is kept at the same temperature as the vapor* The reflux regulator has the advantage in this case of being simple with external manual control. Distillation Procedure The recrystallized biphenyl was placed in the stillpot with some boiling chips (marble chips). Pressure in the still was maintained at about 15 mm to give the most efficient oper• ation. Once distillation commenced the stillpot heater was adjusted so that there was a steady dripping of biphenyl at the rate: of about one drop every three seconds. The biphenyl was distilled twice, the middle fraction being saved in each case. On the final distillation the biphenyl was sealed in constricted receiving tubes so that it could be stored under vacuum. 3. Check on the Purity of the Biphenyl The freezing point of biphenyl was checked on the same apparatus as used for benzene. The cooling curve is given in Figure 37 which gives 69.191°C as the freezing point for biphenyl. This value agrees favourably with values given in the literature as seen in Table III which gives a chronol• ogical review of the freezing points found by different authors. ^-W-J-T-f-i-CP-U- ^ :klj'fi: m FOR BtRiA&NVL'p-i- 1 jjjjjjj ftyit'-bTrrht-hki;. lit Ul HE V7 ss "rP-i;;,-:t) ri4t.-trtfH Si. tf-H-Witi Hit Tf j-'-'H'-::- i Era ft HiHr Lrt minimit:j":}_' •• £>1 70 20 30 4-0 TIME- _\»-4 MINUTES TABLE III FREEZING POINT DATA FOR BIPHENYL FROM. THE LITERATURE AUTHOR P.P. Jaequerod and Wassamer (62) 69.0°C Washburn and Read (138)(139) 68.93 Garrick(38) 69.O Chipman and Peltier (18) 69.2 , Badger, Monrad, and Diamond (3) 136.6°F (69.2°C) Huffman, Parks, and Daniels (56) 69.I Montillon, Kohrbach, and Badger (79) 136.6°F ( 69.2PC) Spaght, Thomas, and Parks (117) 68.3 Gilman and Gross (44) 68.95 Eglof f ( 29) 70 Tompa (130) 69.I CHAPTER VI EXPERIMENTAL PROCEDURES A. Determination of Refractive Index-Composition Curve Ben zene-Butano1 The curve for the refractive index at 20°C versus mole fraction of benzene for the benzene-butanol system was determined by Emerson and Cundill (30) and will not be dis• cussed here. It is shown, however, in Figure 38. Benzene-Biphenyl Since biphenyl has a melting point of very nearly 70°C and the system is solid for a wide range of composition at 25°C, it was decided to determine the refractive index at 70°C. Certain difficulties were encountered, however, due to the nature of the system and the relatively high temperature. The benzene was found to evaporate very rapidly at this tem• perature and the biphenyl tended to solidify before it was placed on the prisms for refractive index readings. Consequen• tly, a technique had to be developed whereby errors due to these difficulties would be as nearly eliminated as possible. Weighings all had to be made repidly and refractive indices had to be taken immediately. It was found that with larger quan• tities of the system there was less danger of error due to the loss of the more volatile component. The procedure employed for the determinations was as follows. By calculation, the ratio by weight of benzene to biphenyl was evaluated for each .1 mole fraction benzene from 0 to 1.0. A set of calibrated weights was used for all weighings. Weighing bottles were kept quite warm on a hot plate before s the liquid, biphenyl was added. After the weighing bottle was carefully weighed alone, 2 to 1G grams of biphenyl were added. An accurate weighing was made of the biphenyl and then the weight of benzene required to satisfy a certain mol fraction was calculated. The benzene was added with an eyedropper until an approximate balance was achieved. The accurate weight of ben• zene and biphenyl was then obtained. If the concentration of biphenyl was high and the system solidified somewhat during the weighing,' it was warmed slightly on the hot plate with the cover of the bottle down tightly. Several drops of the solut• ion were placed on the lower prism of the refractometer and it was closed immediately. Temperature equilibrium was allowed . to be reached and at least three readings of refractive index were taken. It was often found that with a cold mixture a change in refractive index of ,0030 would take place from the time the solution was first clamped in place in the prisms to the time temperature equilibrium was attained. The change in refractive index was always negative which indicated that it was mainly due to the increase in temperature of the solution. There appeared to be no loss of benzene once the prisms were clamped in place since after about one minute when equilibrium was. reached there was no further change in the index. A check was made on the greatest possible error due to lack of equilibrium in refractive index determination. Of the three readings usually taken during determination there was seldom a greater difference than .0010 in refractive index be• tween the first and last reading. This corresponds to a maximum error of 1.5 mol percent in the most critical region of the curve, that is, at a mole fraction of between 0.4 and 0.6. However, since an average was. usually taken, this error would seldom exceed 1.0 mol percent. B. Vapor-Liquid Equilibrium Determination on the Gillespie- Eowler Still.•' The procedures used for the vapor-liquid equilibrium determinations in the two systems studied here varied somewhat due to the nature of the systems. However, since the initial runs made on the benzene-biphenyl system with a high mole frac• tion of benzene were similar in method to those of benzene- butanol only the former will be discussed. The first fifteen determinations up to a mole fract• ion of 0.5 in benzene were carried out at atmospheric temper• ature conditions. The apparatus was carefully cleaned, first, with -soap and water followed by 10% alcoholic-sodium hydroxide solution; It was then rinsed with clear water; .1 N hydro• chloric acid solution, clear water again and finally with three portions of distilled water. Before being used the still was dried in an oven at 120°C for twenty four hours. Stopcocks and standard taper joints were kept free of all greases or other lubricants. Initially, the boiler was charged with appr• oximately 200 ml of pure benzene-by way of the internal heater opening. The internal heater was set in place and the variac leads were attached. Cold water was run through the condenser to keep the benzene from vaporizing. The two heaters were turned on and a potential of about 35 volts was given the ex• ternal heater while the variac for the internal heater was kept at about 12 volts. When the charge in the boiler had warmed up and pumping in the Cottrell tube had commenced, the internal heater was adjusted so that smooth operation took place. It is difficult to place exactly the right quantity of liquid in the boiler at the outset so that when the liquid and vapor con• densate trap have filled adjustments may have to be made. Gn boiling, the liquid level should gently fluctuate in the small bulb above the capillary tube and in the small bulb at the top of the liquid trap. Attainment of temperature equilibrium usually took about an hour. It was found that some mercury in the ther• mometer well helped stabilize the temperature considerably. When temperature fluctuation was no greater than - ,05°C the boiling temperature was recorded and samples of the liquid and vapor condensate were removed as simultaneously as possible. The outlets from the two traps were always flushed out with some of the liquid before samples were taken in order to remove the stagnant liquid. Samples were put into weighing bottles with ground glass covers immediately fitted into place. Re• fractive indices were taken as rapidly as possible and the same precautions were taken as in the determination of the refractive index-composition curve. Atmospheric pressure readings were taken as nearly at the same time as the other readings as possible. A Cenco mercury column type of~barometer, located about 15 feet above the level of the vapor liquid equilibrium determinations in the early part of the runs at atmospheric temperature and at the same level in the final runs, was emplo• yed. In subsequent runs benzene was drawn from the vapor condensate trap and pure biphenyl was added to the required height in the boiler to effect changes in composition. This procedure gave points on the x-y curve quite evenly spaced at reasonably close intervals. When the concentration of biphenyl in the system became greater than 0.5 mol fraction solidification began to take place in. the liquid trap. Hence a hot air cabinet was required to make the remaining runs. This was found in a large o oven which could be controlled quite well around 70 C. Com• pressed air was run through the condenser and the stream of air was adjusted for whatever amount of cooling was required. It was found that at the temperature of the oven the benzene was volatilizing at a tremendous rate. To circumvent this diffi• culty the condenser abotfe the vapor condensate trap was con• stantly kept filled with ice. At the high temperature at which the system had to be boiled a serious problem was faced in the severe bumping which usually accompanies such temperatures with biphenyl. This was .largely overcome by maintaining the internal heater at a maximum potential and providing ample ebullition within the boiler. As a check on equilibrium values taken under the different external temperature conditions, at least one of the determinations made previously at a particular composition was repeated. The two points were found to fall very nearly on the same curve as shown at about a mole fraction of 0.5 in Fig. 45. C. Determinations on the Vapor-Recirculation Apparatus About 40 ml of the solution were placed in the filling tube J? which was attached to the main circulatory apparatus by means.of a ground glass joint. A very light film of stiff stopcock grease was put on the outside edge of the joint to prevent any air leakage. Tap 1 to the manometer and high vacuum system was kept closed and the sample was frozen with liquid air.. Then with mercury in the manometer just above tap J-and taps 1, 2, 4 and 5 open the system was evacuated. When a pressure below 10~^mm, as noted on the McLeod gauge, was reached, tap 1 was closed again and the sample was allowed to melt at room temperature. The freezing process was repeated at least twice in order to insure that the sample was thoroughly de• gassed. To distil the sample into bulb A it was first melted completely at atmospheric temperature. Cold water was kept running through the condenser N and bulb A was immersed in a cooling mixture of dry ice and acetone. A low potential of about 8 volts was applied to the heating tube and connecting line heaters by means of a variac. When the sample was com• pletely distilled over into A, the filling tube was sealed off carefully at x. Then with the sample frozen solid in A, the air baths Tg and Tj were brought on temperature, and mercury was raised in the manometer to the level L. Taps 3 and 4 were closed and the height of mercury was noted in the arm M by means of a cathetometer which read accurately to - 0.01 mm. The oathetometer was calibrated with a standard meter bar having a Bureau of Standards Certificate. The system in bulb A was then allowed to melt; constant temperature bath T-^ was filled with oil and brought on temperature. While temperature equilibrium was being reached, the mercury level was carefully watched at L so that it was not lowered out of the air bath by the incre• asing pressure within the system. Such a mishap would cause • vapors of the system to condense above the mercury and thus introduce errors into the determinations. Water to condenser 0 was turned off and the circulating pump B was started. Tem• perature in bath T,'was maintained within - 0.002°C of that required', while Tg and Tj were kept at 0.5°C and 2.5°C higher, respectively. Attainment of equilibrium as noted by the constancy of pressure in manometer M required about four hours. At this point, pump P was stopped, the equilibrium vapor pressure was measured on M with the mercury level still at L inside the bath. Circulation was then continued for another half hour and the pressure was again noted. When the variation in pressure be• tween successive readings did not exceed 0.1 mm of mercury, the vapor pressure was recorded as the equilibrium pressure. With the circulating pump stopped, the liquid phase bulb A was sealed off from the rest of the system at y and z. Bath T-j_ heaters were turned off and the oil was drained through the tap at the bottom. With T2 and T^ heaters still on, the vapor phase was condensed into bulb.B using a dry ice-acetone cooling mixture. Then B was sealed off from the remainder of the circulatory system at W. The mercury in the manometer was lowered just above tap 3 and any non-condensable vapors were pumped off. Bath Tg was brought on temperature, mercury was raised to the level L, tap 4 was closed, and the vacuum reading was again taken. The condensed vapor phase was 89 melted, oil "bath filled and brought on temperature and the pressure reading was recorded. To interpret the readings obtained for the pressure of the two phases in terms of composition, a vapor pressure- composition curve is required. Such a curve for benzene- biphenyl was previously obtained by Gilmann, and Gross (44-). Of course, the apparatus used here could be readily adapted for such readings by introducing a side arm to the manometer and checking the vapor pressures of gravimetrically determined samples. 90 CHAPTER VII RESULTS A. Benzene-Butanol The values obtained for refractive index at 20°C using different compositions of the benzene-butanol system were determined by Emerson and Cundill and are shown in Table 4. Errors due to volatilization of the more volatile compon• ent during weighing and refractive index determinations were calculated to be no greater than 0.065%. A large scale plot of refractive index versus composition was made for composition determinations during vapor liquid equilibrium operations. Figure 38 shows the curve on a reduced scale. TABLE 4 REFRACTIVE INDEX - COMPOSITION DATA FOR BENZENE-BUTANOL AT 20°C MOLE FRACTION REFRACTIVE INDEX OF BENZENE. AT 20°C e.0000 1.3994 0.1133 1.4093 0.2019 1.4172 0.3020 1.4263 0.4096 1.4368 0.4999 1.4452 0.5992 1.4558 0.6979 1.4662 0.8009 1.4772 0.9005 1.4884 1.0000 1.5011 The vapor liquid equilibrium values of refractive index, and corresponding composition as determined in this research on the Gillespie Fowler equilibrium still are tabulat• ed in Table 5« Values for equilibrium temperature and atmos• pheric pressure were also recorded and are shown. TABLE 5 EXPERIMENTAL VAPOR-LIQUID EQUILIBRIUM DATA FOR "MZEiNE. n-BU'TANOL Atf ATMOSPBSRIO PRESSURE • •' Run Duration Temp. Atmos. Liquid Phase Vapor Phase No. of Run PC press. R.I.n*0 . Mol.Fr. R.I. Mol.Fr, u mm. C6H6 C6H6 1 4.5 79.44 751.6 1.5010 1.0000 1.5010 1.000 2 2.25 79.72 751.6 1.4912 0.9025 1.4959 0.9175 3 3.5 81.40 750.7 1.4665 O.7080 1.4865 0.8720 4 2.75 87.50 750.7 1.4362 0.3935 1.4759 0.7910 5 4.0 9 2.20 752.4 1.425 3 0.2780 1.4679 0.7200 6 3.0 98.70 754.0 1.4137 0.1550 1.4531 0.5750 7 — 117.71 760.0 1.3996 0.0000 1.3996 0.00 A large scale plot was made of the composition of the vapor phase against the composition of the liquid phase with respect to the more volatile component. Emerson and Gundill (30) carried out extensive investigations into the system with both the Cottrell Ghoppin and the Gillespie Fowler units. Their equilibrium values, obtained on the latter still, were also plotted on the large graph as a check on the consistency of the apparatus. The two curves are shown'in reduced scale in Figure 39* The length of run was varied for each determination to note any change in equilibrium composition values. It was found that under ordinary conditions one hour of smooth oper• ation should be sufficient for accurate values. MOL-E FRACTION BENZE.NE. 1«M UlQUtD 92 The thermodynamic consistency of the system was checked by comparing experimental values of activity coefficients with those obtained theoretically. Experimental activity coeff• icients were evaluated from the expressions P yi and ^ - P 72 Pjx-^ •^>l^"2 discussed previously. Values of x± and Xg, mole fractions of respective components in the liquid phase, as well as y^ and yg, mole fractions of the components in the vapor phase, were taken from the experimental equilibrium data. The value of P was taken as the atmospheric pressure under which the deter• minations were carried out. Pressures of the pure components, o o P^ and Pg, at the particular boiling temperature were taken from vapor pressure curves. Stull (120) in his review of the vapor pressures of pure organic compounds gave the smoothed vapor pressure values for n-butanol as determined from the work of other researchers (12, 65, 54, 96). The vapor pressure curve is shown in Figure 40. In the case of benzene, the vapor pressure data for the range of temperature used for both the benzene-butanol system and the benzene-biphenyl system was obtained from various sources. Over the lower temperature region the vapor pressure values are given quite accurately by the equation logitfP (mm) = - 0.0j?223a + b (6l) where T is the absolute temperature, and a and b are constants having the following values; 0 to 42°C, a = 34,172 42 to 100°C, a •= 32,293 b - 7.9622 b = 7.6546 8 % THJIOlllJ ii Lttftl n 40 50 ! TEMPERATURE °C For the region of temperature from 100° to 130°C, the expert- mental values of Smith and Menzies (114) were drawn upon, while the higher temperature values were taken from the work of Gornowski et al {51}* TABLE 6 VAPOR PRESSURE DATA FOR BENZENE Temp. Pressure, mm Hg x 10~2 Source of Data 60 0.390 Calculated 2° .548 80 .754 « 90 1.022 tt 100 1.360 11 110 1.751 From Smith and Menzies (114) 120 2.S40 ti 130 2.825 From Gornowski et al (51) 140 3.518 it 130 4.331 it 160 5.277 it 170 6.368 tt 180 7.621 tt 190 9.050 ti 200 IO.669 it ao 12.495 it 220 14.546 11 230 16.851 11 240 19.425 n 230 22.306 " 260 25.520 tt TABLE 7 ACTIVITY COEFFICIENTS OF BENZENE AND n-BUTANOL FROM EXPERlME^AL VAPOR-LIQUID -EQUILIBRIA T°C P mm HgPi mm Hg Pp-mm Hg xi yi 79.44 751.6 750.0 160.0 1.000 1.000 1.000 6.30 79 . 7 2 751.6 755.0 1 62.0 0.90 25 0.9175 1.0L2 3.926 81.40 750.7 778.0 175.5 0.7080 0.8720 1.188 1.875 87.50 750.7 952.0 230.0 0.3935 0.7910 1.585 1.125 92.20 732.4 IO98.O 281.0 O.278O 0.7200 1.775 I.O38 98.70 754.0 1315.0 367.0 0.1550 0.5750 2.127 1.033 117.71 760.0. 2125.0 760.0 0.000 0.00 2.70 1.000 The activity coefficients for benzene, it, and for butanol, as evaluated from the experimental data are given in Table 7« A plot was made of log activity coefficient versus mole fraction benzene for both components in figure 42. The curves were extrapolated to Xj_ = 0 and x-^ = 1 for the end values of log^.and log^o., respectively. Constants A and B in the van Laar and Margules equations were taken as these terminal values, A =? log^, = 0.4J1 and B = log $A - 0 . 799. The theoretical values for activity coefficients were calcul• ated from the symmetrical forms of the van Laar equations: log = A and log = B 2 A-+-AX-JA2 /l + Bx2N k BxT 1 Sg/ As a check, the activity coefficients were also calculated with the Margules expressions rearranged into the two-term forms: log %f « ( 2B-A) x| + 2(A-B)X2 log 5z= (2A-B) xf + 2(B-A)xJ Table 8 gives the theoretical values of the activity coefficients as calculated with both the van Laar and the Mar• gules equations. The theoretical curves for the log activity coefficients versus mole fraction are shown along with the experimental curve in Figure 42. LOq ACTIVITY COEFFICIENT 23 WEOJ TABLE 8 THEORETICAL ACTIVITY COEFFICIENTS OF BENZENE AND n-BUTANOL Van Laar Margules 0.0 2.700 1.000 2.700 1.000 0.1 2.419 1.006 2.563 1.003 0.2 2.161 1.026 2.344 1.020 0.3 1.925 I.O67 2.086 1.061 0.4' 1.7H 1.137 1.824 1.141 0.5 1.520 1.253 1.584 1.282 0.6 1.354 1.445 1.379 1.519 0.7 1.215 1.770 1.217 1.920 0.8 1.105 2.56I I.098 2.613 0.9 1.029 3.543 1.025 3.869 1.0 1.000 6.300 1.000 6.300 The temperature-composition diagram for benzene-butanol is shown in Figure 43. B. Benzene-Biphenyl The refractive index-composition values for the system benzene-biphnyl at 70°C are given in Table 9. All the points obtained- are tabulated, but it was found that many of those taken initially showed a higher refractive index than those obtained as a check. Since technique was improved toward the last determinations, and a greater ;speed was achieved in weigh• ing and manipulation, the earlier readings can be suspected as being in concentration error due to evaporation of benzene. Consequently, the best values were taken and plotted on large mm. graph paper where 1 mm. on the abscissa was equivalent to .002 mole fraction and 1 mm. on the ordinate was equivalent to a change in refractive index of .0002. Fig. 44 gives a reduced scale curtfe'with all experimental points for re• fractive index against composition. Values of refractive index for even mole fractions were taken off the smoothed large KAO\_E- FRACTION ftELNZ-ELIME- R EL. F'R ACTIVE-"" mDE.^ scale plot and are given in Table 10. TABLE 9 REFRACTIVE INDEX-COMPOSITION DATA EOR BENZENE-BIPHENYL AT 70°C Run Moi Fraction Moi Fraction Refractive No. Benzene Biphenyl Index 1 0.00 1.00 1.5904 2 0.1090 0.8910 I.5869 3 0.0813 0.9187 1.5836 4 0.5287 0.4713 1.5618 0.5090 0.4910 1.5506 6 0.5052 0.4948 1.5493 7 0.6000 0.4000 - 1.5378 8 O.6967 0.3033 1.5246 9 0.7232 0.2768 1.5205 10 0.9075 0.0925 1.4903 11 1.0 0.0 1.4697 12 0.0 1.0 1.5904 13 1.0 0.0 I.4694 14 0.2870 0.7130 1.5733 15 0.4942 0.5058 1.5463 16 0.1732 0.8268 I.5800 X2 O.3092 0.6108 I.5605 18 0.0701 Q.,9299 I.5856 19 0.6119 0.3881 1.5380 k Values 12 to 19 were taken as a check on readings 1 to 11. TABLE 10 SMOOTHED REFRACTIVE INDEX COMPOSITION DATA FOR BENZENE-BIPHENYL . AT 70PC. . Moi Fraction Moi Fraction Refractive Index Benzene Biphenyl at 70°C 0.0 1.00 1.5904 0.05 0.95 1.5869 0.10 0.90 1.5835 0.15 O.85 1.5799 0.20 0.80 1.5762 0 . 25 0.75 1.5 7 24 0.30 0.70 I.5685 0.35 O.65 1.5642 0.40 0.60 1.5597 0.45 0.55 1.5549 0.50 0.50 1.5497 0.55 0.45 1.5440 0.60 0.40 1.5379 O.65 0.35 1.5314 0.70 O.30 1.5245 0 . 75 0 . 25 1.51 70 0.80 0.20 1.5091 O.85 0.15 1.5006 0.90 0.10 1.4916 0.95 0.05 1.4809 1.00 0.0 1.4697 In all, twenty six runs were made on the Gillespie- Fowler Still for the determination of vapor-liquid equilibrium of benzene-biphenyl. The first fifteen runs were taken at atmospheric temperature while the remainder were•obtained in an oven kept at 70°C. Table 11 gives all the experi;---yAe.nS data obtained with temperatures corrected for calibration with the platinum resistance thermometer. TABLE 11 EXPERIMENTAL VAPOR LIQUID EQUILIBRIUM FOR BENZENE- BIPHENYL AT ATMOSPHERIC PRESSURE. „ „ , ; m „ , ...... liquid Phase ' Vapor Phase Run Duration Temp. Atmos. — nn ."," •' "7Q '. " No of Run PC Press. R.I. n^ Moi Fr. R.I. n^ Moi Fr. Hours C£H6 C6H6 1 2 80.10 760.3 1.4700 1.000 1.4700 1.0 2 1.5 80.28 759.9 1.4716 0.991 1.4696 1.0 3 1.5 80.94 758.3 1.4772 .966 1.4700 1.0 4 2.0 8I.8I 759.2 1.4820 .945 1.4700 1.0 5 2.0 83.65 760.7 1.4912 .902 1.4700 1.0 6 2.5 84.54 760.5 1.4932 .891 1.4700 1.0 7 2.5 85.98 760.7 1.5025 .838 1.4700 1.0 8 3.0 87.IO 760.4 1.5100 .794 1.4700 1.0 9 2.0 88.80 76O.6 1.5170 .750 1.4700 1.0 10 5.0 91.85 755.0 1.5235 .706 1.4700 1.0 11 2.0 9 3.65 754.0 1.5265 .685 1.4700 1.0 12 2.5 95.57 754.3 1.5322 .644 1.4700 1.0 13 2.5 97.53 754.2 1.5376 .602 1.4700 1.0 14 1.5 98.98 754.4 1.5425 .563 1.4700 1.0 15 2.5 101.06 757.0 1.5500 .497 1.4700 1.0 16 1 102.26 757.2 I.5482 .504 1.4714 0;992 17 1 104.36 757.7 1.5569 . 430 1.4718 .990 18 1 126.2 757.7 1.5741 .228 1.4714 .992 A 19 1 132.6 757.2 1.5748 . 218 , 1.49 1 2 . 99 2 A 20 1 141.8 757.4 1.5790 .162 I.4787 .960 21 1.5 151.6 75 7.3 1.5 809 .135 1.4 7 29 .985 22 1 167.3 757.3 1.5845 .085 1.4870 .922 it 23 1 187.8 756.7 1.5873 .045 1.4877 .919 24 1.5 199 . 6 756.2 1.58 75 . 041 1.5076 .809 25 2 211.4 757.2 1.5893 .016 1.5493 .503 / 26 2 143.3 757.9 1.5802 .145 1.4782 .962 A Runs were definitely not at equilibrium when refractive indices' were taken. / Run was taken primarily as a rough check on the particular region of concentration. A large scale plot was made of all the experimental points with the exception of three which were obtained when the system was definitely not at equilibrium. Figure 45 shows the general form of the curve obtained. Smoothed values were taken from the large graph and are tabulated in Table 12 for even intervals of concentration in the liquid phase. TABLE 12 SMOOTHED VALUES FOR THE VAPOR-LIQUID EOJILIBRIUM OF BENZENE-BIPHENYL.: Moi.Fr. C6HD in Mol.Fr.C6H6in Liquid Phase Vapor Phase Temp.°C. 0.00 0.000 255.3°0 4 0.01 0.308 230.7""' 0.02 0.624 211.2 0.03 0.745 202.6 0.04 0.803 195.9 0.05 0.844 189.6 0.06 0.875 133.5 0.07 0.899 178.2 0.08 0.920 171.6 0.09 0.936 167.7 0.10 0.950 I62.4 0.12 0.971 151.5 0.14 0.9S4 140.6 0.16 0.96*9 133.3 0.18 0.990 129.8 0.20 0.991 I26.5 0.25 0.992 121.4 0.30 0.993 116.4 0.35 0.994 112.2 0.40 0.995 108.4 0.45 0.996 105.6 0.50 0.997 102.7 0.55 0.998 99.4 0.60 0.999 97.3 . O.65 1.000 95.0 0.70 1.000 92.9 0.75 1.000 90.7 0.80 1.000 88.6 0.85 1.000 86.5 0.90 1.000 34.3 0.95 1.000 82.2 1.00 1.000 80.1 A Temperature for the boiling point of pure biphenyl at 760 mm. taken from the literature (79) (18). For calculation of activity coefficients from the experimental data, pressures of pure benzene were taken from Figure 41 while those for biphenyl were obtained from Figure 22. The experimental activity coefficients for all vapor liquid equilibrium data are tabulated in Table 13. Only the starred values were plotted since they are the only ones which appear reasonable. Curves were drawn through these values and are shown in Figure 46. They were extrapolated to 3 0 and x-j_ = 1 and the values of A and B obtained were .415 and .393, respectively. The values of theoretical activity coefficients are tabulated in Table 14, and the curves are drawn in Figure 46. The temperature-composition data for benzene- biphenyl are plotted in figure 47. A smooth curve has been, drawn through the best points. LOG ACTIVITY COEFFICIENT 04 O 5 o 5 n YiOJFS O »+4^• " 0 3 b-4 O.5- O'fc 07 MOLE. FRACTION BENZENE. TABLE 13 ACTIVITY COEFFICIENTS OF BENZENE AND BIPHENYL FROM EXPERIMENTAL DATA T°C P mm Hg Pi mm Hg P2 mm Hg x-j_ v l do. 10 760.3 760 1.50 1.000 1.0 1.000 2.50 xx 80.28 759.9 763 1.60 0.991 £ 1.0 1.005 t» 80.94 753.3 777 1.75 .996 1.0 0.930 tt 81.81 759.2 300 1.90 .945 1.0 1.003 tt 33.65 760.7 345 2.00 .902 t 1.0 1.007 tt 34.54 760.5 365 2.05 .391 1.0 0.994 tt 35.93 760.7 905 2.15 .333 t 1.0 1.022 tt 37.10 760.4 936 2.25 .794 1.0 1.023 tt 33.30 760.6 990 2.40 .750 t 1.0 1.032 tt 91.35 755.0 1037 2.50 .706 1.0 0.934 n 93.65 754.0 . 1145 3.20 .685 1.0 0.961 tt 95.57 754.3 1203 3.60 .644 1.0 0.963 tt 97.53 754.2' 1273 4.00 .602 1.0 0.934 tt 93.93 754.4 1322 4.20 .563 1.0 1.013 tt 101.06 757.0 1393 4.50 .497 1.0 1.092 ' tt 102.26 757.2 1433 4.90 .504 A 0.992 1.039 2.491 IO4.36 757.7 1514 5.40 .430 A .990 1.151 2.46I 126.2 757.7 2535 14*70 .223 A .992 1.273 0.534 132.6 757.2 3000. 19.00 .213 .902 1.043 0.499 141.3 757.4 3664 26.70 .162 t .960 1.226 1.354 151.6 757.3 4492 37.20 .135 .935 1.232 0.353 I67.3 757.3 6020 65.30 .035 $ .922 1.363 0.939 137.3 756.7 3470 129.2 .045 .919 1.545 199.6 756.2 10270 134.2 .041 A .309 1.456 0.313^ 211.4 757.2 12610 257.0 .016 .503 1.337 1.490 143.3 757.9 3740 23.0 .145 ft .962 1.344 1.203 & Plotted values. xx Extrapolated values. TABLE 14 THEORETICAL ACTIVITY COEFFICIENTS OF BENZENE AND BIPHENYL Van Laar Margules xl i. V. a. 0.00 1.570 1.000 ~ "T:5W " 1.000 0.1 1.500 1.002 1.554 1.001 0.2 1.430 1.011 1.503 1.007 0.3 I.36O 1.023 1.429 1.024 0.4 1.292 1.053 1.345 1.059 0.5 1.225 1.105 1.257 1.119 0.6 1.161 1.130 1.175 1.216 0.7 1.103 1.299 1.104 1.367 0.3 1.052 1.496 1.049 1.596 0.9 1.015 1.340 1.013 1.943 1.0 1.00 2.500 1.000 2.500 102 CHAPTER VIII DISCUSSION OF RESULTS A. Benzene-Butanol From the plot of log of the experimental activity- coefficients versus mole fraction, the points obtained appear to fall on smooth curves. Although this in itself is not a criterion of thermodynamic consistency it does give a certain amount of indication in that direction (15). A check on the two curves' at x = 0.5 shows that the log ^ curve falls lower than that of log . This is further evidence of consistency since the log tf^curve has the higher end value, as pointed out in the theoretical discussion. A thermodynamic check on the consistency of the experimental curve shows a certain amount of deviation. The theoretical curves are completely dependent on the-accurate,- extrapolated end values of the experimental curves. It may be only fortuitous that the experimental terminal values are correct and have been accurately extrapolated. However, since the system does not form an azeotrope, these are about the only values upon which a thermodynamic check can be based. The van Laar and Margules curves do not coincide too closely. This can be expected since A, the ratio of the constants, B deviates considerably from unity. The temperature-composition curves in both this research and that of Emerson and Cundill conformed very well to the types of curves obtained for common non-azeotropic mixtures. The experimental points obtained deviated very 103 little from smooth curves, but in the high benzene concen• tration end of the diagram they showed considerable interest. It is here that the system appears to come very close to forming an azeotrope. B. B eh z en e- Bi ph en vl Evaluation of the activity coefficients from ex• perimental data for benzene and biphenyl gave only a few values falling above unity. When plotted, the points for benzene were within reason of a curve. However, those for biphenyl were off considerably, and only a very few had values above unity. This inconsistency may have been due to various reasons. In the experimental work, the temperature readings could have been taken too high resulting in a high vapor pressure for the pure components in calculations. Then in the expression •= Py2 the value of ^would be considerably too low. It appears 2.X2 that almost in all cases of biphenyl the ratio J2. was smaller than it should have been. This would indicate that the vapor phase was not exhibiting a large enough concentration of biphenyl. 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