PHASE EQUILIBRIA MOLECULAR TRANSPORT THERMODYNAMICS Phase Equilibrium at Elevated Pressures in Ternary Systems of Ethylene and Water with Organic Liquids.
Salting Out with a Supercritical Gas JOSEPH C. ELGlN and JACQUES J. WEINSTOCK’ Department of Chemical Engineering, Princeton Univerrlty, Princeton, N. J.
u
nderlying most of t h e physical p h a s e change separation operations carried out by t h e chemical industry on homogeneous mixtures a r e t h e b a s i c p h a s e equilibrium conditions. T h e s e equilibria determine t h e natural (single-stage) limits of such separation p r o c e s s e s as distillation, extraction, and crystallization, and they usually must b e determined experimentally before reliable design calculations c a n b e made. Numerous chemical s y s t e m s a r e u s e d i n industry, but there a r e relatively few systems for which t h e p o s s i b l e p h a s e equilibria h a v e been defined or systematically studied experimentally over t h e operable range of conditions. In particular, d a t a for relatively few threecomponent (or ternary) s y s t e m s h a v e been reported over any appreciable range of temperature and pressure. T h i s article p r e s e n t s liquid-liquid and liquid-vapor p h a s e equilibrium d a t a a t 15OC. for p r e s s u r e s in t h e range from 0 to 1000 p.s.i.g. for a number of typical three-component s y s t e m s comprised of ethylene, water, and various organic liquids. It d e s c r i b e s t h e conditions and compositions existing i n several water-organic liquid mixtures which a r e normally either partially or completely miscible when they a r e subjected t o ethylene under pressure. It further pres e n t s a generalization of t h e p h a s e equilibrium behavior in three-phase patterns (typical isotherms) which cover a l l of t h e 26 s y s t e m s studied. T h e s e qualitative and quantitative r e s u l t s show that ethylene, when dissolved i n many waterorganic liquid mixtures a t elevated pressures, reduces t h e mutual solubilities of t h e s e liquid components. T h e result of t h i s effect is to introduce a region of immiscibility or t o i n c r e a s e t h e composition range covered by a n existing miscibility gap. T h i s p h a s e behavior provides a b a s i s for new separation methods for t h e dehydration of many t y p e s of organic compounds. EXPERIMENTAL METHOD
Apparatus. T h e investigation of t h e p h a s e equilibria of a three-component system in which one, two, or three p h a s e s may coexist requires a n equilibrium apparatus i n which t h e p h a s e s present may be agitated a t any desired temperature and pressure, and in which t h e material under examination Present address, Radiation Applications, Inc., 370 Lexington Ave., New Yo* 17, N. Y. VOL. 4, NO. 1, JANUARY 1959
may b e closely observed at a l l times. T h e apparatus must allow for the removal of samples of any p h a s e present i n t h e system without disturbing equilibrium conditions. T h e s e requirements were met by using t h e apparatus shown in Figure 1. T h e equilibrium chamber, or solubility “bomb,” was a standard transparent type Jerguson gage with a s t a i n l e s s s t e e l liquid chamber holding about 100 ml. T h e gage had
MERCURY
TEMPERATURE CONTROL
Figure 1.
UNIT
Pressure apparatus for studying phase equilibria
two thick borosilicate g l a s s windows. Adequate agitation w a s provided by a magnetically driven twisted strip of s t a i n l e s s . steel. Four t a p s were provided in the bomb for t h e admission of g a s or liquid and for t h e removal of samp l e s by displacement with mercury. T h e two sample cartridges shown in Figure 1 were arranged so that equilibrium samples of two p h a s e s could be removed from the bomb nearly simultaneously. T h e desired temperatures in the thermostat enclosing the equilibrium chamber were controlled by a Tenny thermos t a t i c circulated-air system to within about 0.1” and were measured with a calibrated thermometer. Desired press u r e s were applied from a compressed ethylene g a s cylinder or by means of compressed nitrogen g a s cylinder press u r e through a mercury seal. P r e s s u r e measurements were made with calibrated Bourdon pressure gages. All l i n e s
3
To verify t h e generalized p h a s e behaviors indicated by the qualitative study, quantitative p h a s e equilibrium diagrams for selected systems were then determined using t h e following procedure.
carrying solvent, gases, samples, or mercury were 1/8-inch s e a m l e s s s t e e l tubing. A second analytical apparatus w a s used for determining t h e volume of ethylene dissolved in each liquid sample. T h e s e measurements were made using g l a s s capillary tubing to connect three calibrated gas-measuring bulbs and two burets to the sample cartridge by way of a glass-tcmetal seal. T h e g a s volumes contained in t h e burets and t h e pressure of t h e gas-measuring system were varied by means of adjustable mercury reservoir bulbs connected to the bottoms of the burets by thick-walled Tygon tubing. For obtaining a qualitative general understanding of t h e p h a s e behavior existing in t h e systems under investigation, t h e 26 organic liquids listed in T a b l e I were studied by visual observation t o determine qualitatively t h e p h a s e s occurring in ternary compositions with water and ethylene. Each water-organic mixture w a s charged to t h e bomb and observed at a number of different equilibrium ethylene pressures over the range 0 t o 1000 p.s.i. T h e type, number, and relative volumes of t h e p h a s e s present were noted, and from t h i s information, generalized p h a s e equilibrium diagrams were developed.
Table
Ethylene-Water with Acetone Acetaldehyde Acetonitrile
VOl. 7 0 H,O in Liquid Binary
30 30
1.
Q u a l i t a t i v e R e s u l t s far E t h y l e n e - W a t e r J o l v e n t P h a s e E q u i l i b r i a
Temp., 0 C.
0
10 20 10 25
28
10
20
F o r e a c h system, several binary liquid compositions were mile the separately charged to the carefully cleaned bomb. desired equilibrium temperature w a s being established, agitation w a s begun and ethylepe w a s admitted t o the bomb. A s ethyle n e dissolved in the liquid, more w a s added, or the system pressure w a s maintained with nitrogen pressure through a mercury seal. T h e liquids to be sampled wore positioned for the appropriate sampling outlets. T h e cleaned, dried, and carefully weighed sample cartridges were put into position, and when the measured system temperature and pressure had remained, without adjustments, constant for 1 hour, the s a m p l e s were taken by slowly flashing the sample into the previously evacuated c a r tridge while simultaneously displacing the liquid being removed from the bomb with mercury so a s not t o change the equilibrium s y s t e m pressure. Using this technique, two p h a s e s could b e s a m p l e d a t the same time. After both s a m p l e s had been removed, the temperature-controlling unit w a s turned off, the bomb enclosure w a s opened, and the sample cartridges were removed. T h e e x c e s s liquid from the bomb-side tip of each cartridge w a s removed, the sample-containing cartridges were weighed, and the volume of ethylene contained in t h e sample waa then determined by allowing it to e s c a p e
Pressure, P. S.I.G.
3-Phase Region Observed
0-800
Yes Yes
0-1000 200, 400
No No
0-700
700, 1000
Acetic acid s e c - h t y l alcohol
tert-Butyl alcohol Ethyl alcohol
Ethyl acetate Ethylenediamine Ethylene glycol Formaldehyde Formic acid Glyoxal Methanol Methyl ethyl ketone
28 20 20
0 Two phases 25 5 5 50 Two phases 20
40 63 25 70
0 34 (two phases)
Methyl Cellosolve Methylal Morpholine Isopropyl alcohol wPropyl alcohol
Propionaldehyde Propionic acid Pyridine Triethylamine Triethanolamine Triethyl phosphate
20 20 20 (two phases) 25 25
1 1 50 15 25 Two phases 27 25 25 40 40
Light liquid p h a s e forms a t approx. 500 p.s.ig. Light liquid p h a s e forms a t approx. 450 p.s.i.g. Very low solubility of ethylene in liquid As temp. w a s lowered to 6OC. a t 1000 p.s.i. g a s p h a s e became liquid p h a s e
No
20 20 10.5 10 10
0 650 0-1000 0-1000 0-1000 0-890 0-900
30
900
Yesa
20 5 18 20
0-1000 0-1000 0-1000 0-1000
No
20 10 10.7 10.5 22
0-1000
No No
0-1000
Comments
Yes Yes
Three p h a s e s e x i s t a t all pressures above 200 p.s.i.
No Yes
NO Yesa
Light liquid p h a s e forms a t approx. 700 p.s.i.g. 890 p.s.i.g. is approx. critical pressure a t 10°C. Heterogeneous binary liquid a t 1 a t m ; additional separation of water occur8 up to 600 p.s.i.g. Separation almost quantitative; volume of water phage d o e s not increase a s temp. is lowered to 10eC. 3 p h a s e s e x i s t only a t or below critical temp. of ethylene
Yes
No Yes
0-900 0-1000 0-950
Yes Yes
A s temp. w a s lowered to 6OC. gas p h a s e became liquid Liquid binary components are practically completely immiscible Very low solubility of ethylene i n liquid Light liquid p h a s e e x i s t s between 700 and 730 p.s.ig. Light liquid e x i s t s between 700 and 715 p.s.ig.
NO
11 10 10 11
0-1000 0-1000
No No
0.800
Yes'
20 10 21
0-1000 0.1000 0-1000
No No
20 10 20 10 10 10 10
0-1000 0-1000 0-930 0-800
No No
0-1000 0- 1000
No Noa
1000
Yes'
11
0-800
Yes
10.5 10.4 10.5
0-1000 0-1000 0-1000
Yes Yes Yes
10.5 10.5
0-1000 0-1000
No
Yes
No No
Yesa
Gottschlich and Jaglom ( 5 ) found small 3-phase regions in binary and ternary systems, respectively Water is precipitated nearly quantitatively from satd. MEK a t ethylene p r e s s w e of 350 p.s.i.g. Very low solubility of ethylene in liquid Very low solubility of ethylene in liquid Ethylene d e c r e a s e s the solubility of methylal i n water Extremely low solubility of ethylene in liquid Extremely low solubility of ethylene in liquid 9 3 0 p.s.i.g. is critical pressure at 2OoC. 800 p.s.ig. is critical pressure a t 10°C. At 780 p.s.i.g. and 10°C., five p h a s e s coexist: solid C,H,-hydrate, water-rich liquid, propanol-rich liquid, ethylene-rich liquid, ethylene vapor Ethylene c a u s e s no additional separation above that occurring i n liquid binary Light liquid p h a s e forms a t 680 p.s.i.g. Light liquid e x i s t s between 700 and 790 p.s.ig. Heavy liquid p h a s e forms a t about 400 p.s.ig.; separation appears to be quantitative Extremely low solubility of ethylene i n liquid Light liquid p h a s e forms a t approx. 740 p.s.i.g.
.Solid ethylene hydrate forms in heavy or water-rich liquid phase.
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JOURNAL OF CHEMICAL AND ENGINEERING DATA
Figure 2.
S
.
Type 1 phase behavior in ethylenswater-solvent systems (Pattern 1)
Schematic isobaric isotherms and pressure-composition isotherm S A
S
S
A
S
slowly (at dry ice temperature) into the gas-measwing apparatus. Analysis of the remaining binary liquid w a s obtained by means of density or refractive index determinations.
RESULTS Q u a l i t a t i v e Observations. Qualitatively, t h e phase behavior observed for a l l systems studied can be classified into three generalized p h a s e behavior patterns. T h e s e a r e shown diagramatically by the triangular diagrams of Figures 2 , 3, and 4. In t h e s e diagrams, S represents a liquid organic compound, and pure ethylene i s in t h e g a s s t a t e (supercritical). F i s a fluid phase. At t h e low temperatures under study, 10' to 30°, the vapor pressures of both water and t h e organic liquids a r e so small that the composition of t h e g a s e o u s ethylene phase i s almost pure ethylene and c o i n c i d e s with t h e vortex representing pure gas. T h e classification into which the individual systems studied fall in relation to t h e s e patterns may be s e e n from T a b l e I together with t h e particular conditions of pressure, temperature, and composition under which they were 3bserved. T w o liquid p h a s e s apparently result froT two different sources: (1) development of mutual immiscibility between water and t h e organic liquid in the presence of dissolved ethylene, and (2) liquefaction of gaseous ethyle n e above i t s critical temperature because of nonideality induced in the vapor p h a s e by the presence of relatively small amounts of t h e vapor of t h e organic liquid. A liquid ethylene phase containing relatively small amounts of t h e liquid results. T h e three types of behavior differ in the degree of solubility exhibited i n the water-solvent binaries and in t h e ternary systems. Two or even a l l three f o r m s of the pressure-composition isotherms described may possibly occur in a single system a t different temperatures. At a given temperature (all quantitative data reported later are at 15.0' C.), the p h a s e behavior appears to depend primarily upon the hydrogen bond-forming tendency of t h e solvent. Pattern 1. Systems of ethylene and water with organic s o l v e n t s h a v i n g a strong tendency to form hydrogen bonds, exhibit i n the region of 10' to 15OC. t h e behavior illustrated in Figure 2. T h i s is designated a s T y p e 1. Representative compounds of t h i s c l a s s a r e ethylene glycol, triethanolamine, glyoxal, methyl Cellosolve, morpholine, ethylenediamine, and possibly acetaldehyde*, isopropyl alcohol*, and methanol*. (The experimentally indicated phase behavior classification for the starred compounds
VOL. 4, NO. 1, JANUARY 1959
Type 2 phase behavior in ethylene-water-solvent systems (Pottern 2)
Figure 3.
Schematic isobaric isotherms and pressure-composition isotherm
S
SOLTENT Figure 4.
Type 3 phase behavior in ethylenswater-solvent systems (Pattern 3)
Schematic isobaric isotherms and pressure-composition isotherm
5
here and below i s subject t 3 some uncertainty because of the limited number of observations made.) Other c l a s s e s of compounds may exhibit t h i s type of behavior a t higher temperatures-i. e., temperatures above the upper critical solution temperature for any liquid phase miscibility g a p s which e x i s t in the three-component system. Figure 2, A , illustrates the equilibrium which e x i s t s at P I , a moderately low pressure at which the solubility of ethylene in both water and t h e solvent i s low. T h e mutual solubilities of ethylene and water a r e very small, and they do not vary materially over t h e entire range of pressures used in this work (1 to 70 atm.) a t ordinary temperatures. At a given temperature, however, the solubility of ethylene in the organic solvent i n c r e a s e s markedly with pressure, i n contrast to i t s solubility in water, so that a t P, (an intermediate pressure level) the solvent-rich liquid phase in equilibrium with essentially pure ethylene contains about 50% of ethylene (Figure 2, B ) . A s the pressure i s raised above the critical pressure of the solvent-ethylene binary (Figure 2, C), t h i s binary becomes completely misc i b l e a s a s i n g l e fluid phase, F, and an isobaric c r o s s section of the pressure-composition isotherm a s s u m e s the form of t h e familiar ternary diagram with a single nonconsolute pair (Figure 2, 0 ) . In this phase behavior pattern, there e x i s t s a maximum of two p h a s e s (liquid in equilibrium with vapor or fluid) over t h e entire range of pressures. Pattern 2. Compounds which display t h i s behavior pattern (Type 2, illustrated in Figure 3) with ethylene and water a t temperatures of about 10' to 15OC. a r e liquids which have a l e s s e r tendency t o form hydrogen bonds, but have structures indicating both active hydrogen atoms and donor atoms. T h e s e include acetonitrile; n-propyl alcohol; tert-butyl alcohol; acetone; formic, acetic, and propionic a c i d s ; and triethyl phosphate*, triethylamine*, pyridine*, and formaldehyde*. T h e l a s t four contain no a c t i v e hydrogen atoms. Characteristic of t h i s behavior type i s the existence of a liquid phase miscibility gap within t h e pressure-composition prism which does not extend to t h e ethylen-solvent f a c e of t h e prism. .4t a low pressure PI t h e equilibrium i s identical with that of Pattern 1. A s the pressure i s increased to a point P;-however, the equilibrium between P I and P,-e.g., s h i f t s to t h e form represented by the Figure 3, 3. At this intermediate pressure, a second liquid phase forms at some intermediate water-solvent composition. T h i s c r e a t e s both a liquid-liquid area and a three-phase liquid-liquid-vapor region on an isobaric c r o s s section of the pressure-composition isotherm. A s t h e pressure i s further increased to t h e solubility of ethylene in the lighter liquid phase (the solvent-rich phase) increases, and the miscibility gap becomes larger a s i s shown in Figure 3, C. T h i s threephase region vanishes again at an upper pressure point a s t h e light liquid phase merges with the ethylene vapor phase (above the critical pressure of the ethylene-solvent binary). T h e result then, i s a fluid phase in equilibrium with t h e water-rich liquid phase (Figure 3, 0). Under t h e s e conditions, the isobaric composition triangle again becomes identical with that for similar pressures in Pattern 1. .Although phase equilibria for liquid-liquid-gas (supercritical) ternary systems of t h e type studied here do not appear to have been previously reported in the literature, liquid-liquid-solid systems exhibiting phase diagrams similar to that of Figure 3, C, are well known and several have been quantitatively measured. Typical among t h e s e i s t h e sodium chloride-ethanol-water system. In (10) t h e s e liquid-liquid-solid systems, t h e T-x isobars are similar in appearance to t h e P - x isotherms described above for liquid-liquid-gas systems. T h e phase equilibrium phenomenon exhibited by such systems in which an aqueous organic solution i s caused t o split into two p h a s e s by the addition of a solid solute, normally an inorganic s a l t , i s commonly called 6'salting out" and h a s long been employed
6
in both laboratory and plant a s a method of separating organic compounds from water. T h e solid used i s commonly called a salting out agent. Here by analogy, i s a salting out process in which a supercritical gas, C,H,, i s t h e salting out agent. T h e advantages of a salting out process employing a gaseous separating agent over a solid are obvious when t h e simplicity of separating and recovering the g a s from both phases, contrasted with removing a solid solute, i s considered. Pattern 3. When t h e miscibility gap in t h e three-component system i s large enough to intersect t h e water-solvent face of t h e pressure-composition prism, t h e p h a s e relations h i p s become as shown in Figure 4. T h e larger miscibility gap may occur either because of the chemical nature of t h e third (solvent) component or through a lowering of t h e temperature. Compounds which exhibit behavior of t h i s type a t about l o o t o 15'C. a r e t h o s e liquids whose structures p o s s e s s donor but no active hydrogen atoms. Specifically, they include methyl ethyl ketone, ethyl acetate, propionaldehyde, methylal, and sec-butyl alcohol. T h e constant-pressure c r o s s sections for t h i s c a s e differ from those of Pattern 2 a t the same pressures only in that a t all pressures considered here the solvent-water binary c o n s i s t s of two liquids over a wide range of binary compositions. T h e immiscibility i s enhanced by increasing t h e ethylene content a s i t is for Pattern 2. Hence, i t i s poss i b l e further t o dehydrate such solvents-e.g., methylethyl-ketone-by using supercritical ethylene a s a separating agent even though t h e solvents themselves a r e s t i l l partially miscible with water in the absence of ethylene. Quantitative Results. P h a s e equilibrium data for the systems quantitatively studied a r e presented graphically in the triangular p h a s e diagrams of Figures 5 t o 14, inclusive. Compositions in a l l c a s e s a r e expressed in weight per cent. T h e dashed l i n e s a r e experimentally determined t i e lines.
ACETONE
Figure 5.
Phase equilibrium diagram for sthylencwater-acetone at 415 p.r.i.a. and 1 5 O C .
T h e two samples determining each were taken nearly simultaneously, a s described in t h e procedure. Because t h e phase behavior type which appeared to hold most interest at the conclusion of the qualitative exploratory study w a s that in which a liquid p h a s e s p l i t OCcurred, a series of compounds found qualitatively to exhibit Behavior Pattern 2 or 3 w a s selected for quantitative investigation. T h e s e compounds a r e acetone, n-propyl alcohol, tert-butyl alcohol, a c e t i c acid, propionic acid, and acetonitrile, and methyl ethyl ketone (Pattern 3). T h e latter i s an inportant industrial solvent-e.g., in lubricating oil, dewaxing-and one which is somewhat difficult to JOURNAL OF CHEMICAL AND ENGINEERING DATA
dehydrate t o very low water contents. A single isobaric composition diagram w a s determined for each of t h e s e compounds (with ethylene and watet) a t 15OC. For t h e a c e t o n e system t h e diagrams were determined at three different pressures i n order to construct and examine t h e pressurecomposition isotherm. Acetone. F i g u r e s 5 , 6, and 7 show t h e isobaric c r o s s s e c t i o n s of t h e pressure-composition isotherm constructed from t h e experimentally determined p h a s e compositions for t h e acetone-water-ethylene system a t 415, 515, and 715 p.s.i.a., respectively, each a t 15.O"C. T h e complete isotherm i s given in the three-dimensional diagram of Figure 8. At 415 p.s.i.a., t h e difference between t h e solubilities of ethylene in water and in a c e t o n e is j u s t sufficient to s p l i t the liquid into two layers. Actually, t h e e x a c t press u r e a t which t h i s split commences w a s not determined experimentally, but extrapolation of t h e loci of t h e light liquid and heavy liquid compositions on t h e P - x isotherm i n d i c a t e s that it i s at about 350 p.s.i.a. At 415 p.s.i.a. t h e heavy liquid in t h e three-phase region contains 54% acetone by weight on an ethylene-free b a s i s ; t h e light liquid, 86%. At 515 p.s.i.a. t h e s e compositions a r e 41% acetone in t h e heavy liquid and 93% in t h e light liquid, respectively. At 715 p.s.i.a. they have become 21% and 95%, respectively. T h e
ACETONE ACETONE
Figure 8.
Ha0 Figure 6.
Phase equilibrium diaaram for ethylene-water-acetone a t 515 p.s.i.a. ond 15.0' C;
ethylene content of t h e s e liquid p h a s e s d e c r e a s e s in t h e heavy liquid from 5 to 1% of ethylene and i n c r e a s e s in t h e light liquid from 14 to 8440 a s t h e pressure i s increased from 415 to 715 p.s.i.a. In t h e a c e t o n e system, a s for other systems which exhibit p h a s e behavior of t h i s type, the second liquid p h a s e forms at an intermediate solvent-water composition, while t h e three-phase region vanishes at a point where t h e heavy liquid p h a s e a l s o h a s an intermediate composition. Methyl Ethyl Ketone. T h e next ketone above acetone in molecular weight, methyl ethyl ketone, exhibits P h a s e Behavior Pattern 3 with water and ethylene at 515 p.s.i.a. and 14.9OC. T h e diagram for t h i s system i s shown in Figure 9. Although the ketone-water binary still exhibits a rather wide miscibility gap, the mutual solubilities a r e somewhat greater than at atmospheric pressure. T h e i m -
METHYL ETHYL KETONE
ACETONE
Ha0 Figure 7.
A
c, H.
Ca H a Phase equilibrium diagram far ethylene-water-acetone at 715 p.s.i.0. and 15.0' C.
VOL. 4, NO. 1, JANUARY 1959
Pressurecomposition isotherm lor ethylenewater-acetone at 15.0a C.
Figure 9.
Phose equilibrium diagram far ethylene-waterqnethyl ethyl ketone at 515 p.s.i.0. and 14.9'C.
7
N-PROPYL ALCOHOL
miscibility rapidly increases, however, a s ethylene i s added t o t h e system. T h e two liquid p h a s e s which exist in the invariant three-phase region have t h e following ethylenefree compositions: 6.5% methyl ethyl ketone in t h e heavy liquid and 98% methyl ethyl ketone in the light liquid, respectively. T h i s split will be even wider a t higher pressure. T h e dehydration of methyl ethyl ketone t o water contents below 1%by weight by t h i s process should, therefore, b e relatively easy t o accomplish. n-Propyl Alcohol. n-Propyl alcohol with ethylene and T h e liquid water displays P h a s e Behavior Pattern 2. phase split, however, does not commence until t h e pressure exceeds 500 p.s.i.a. Consequently, t h e p h a s e diagram for t h i s system w a s determined at 715 p.s.i.a. (Figure 10). T h e ethylene-free compositions of the liquid p h a s e s i n t h e three-phase region a r e 21% n-propyl alcohol for t h e heavy liquid and 73% for t h e light liquid. T h e s e figures indicate t h e degree of separation which can be attained in a single contact at t h i s pressure ( m d temperature). At higher press u r e s and/or lower temperatures, the separation of propyl alcohol and water i s enhanced. tert-Butyl Alcohol. T h e ternary constant pressure and temperature diagram for t h e tert-butyl alcohol-water-ethyle n e system, presented in Figure 11, w a s determined at 515 p.s.i.a. I t resembles that for n-propyl alcohol a t 715 p.s.i.a. T h e ethylene-free compositions of t h e light and heavy p h a s e s in t h e invariant region a r e 69.5 and 25% tert-butyl alcohol, respectively. Again, a greater separation can be attained in t h e three-phase region at higher pressures and/or lower temperatures. Experiment shows that a wide split between water and alcohol occurs in t h i s system a t pressures of 915 p.s.i.a. even a t temperatures a s high a s 3OOC. Visually no noticeable change in t h e cont e n t s of t h e chamber occurred when t h e temperature w a s lowered to 10°C.-i.e., the relative volumes of t h e phases present in t h e bomb changed l i t t l e over t h i s temperature range. T h i s indicates that t h e liquid phase miscibility gap in t h i s system extends t o temperatures above 3OoC. and that the limits of t h e miscibility gap do not change appreciably over t h i s temperature range in t h e pressure region investigated. Acetic Acid. T h e p h a s e diagram for t h e a c e t i c acidwater-ethylene system shown in Figure 12 w a s determined a t a pressure, 783 p.s.i.a., slightly below t h e critical pressure for t h e ethylene-acetic acid binary. T h i s system and that for acetone a t 715 p.s.i.a. (Figure 7) represent t h e p h a s e equilibrium occurring in Behavior Pattern 2 at press u r e s between P, and P, (Figure 3). It i s evident that t h e concentration of solvest (acetic acid here) need not b e low in the heavy liquid phase a t pressures j u s t below the solvent-ethylene critical point. Under the conditions of Figure 12, t h e ethylene-free compositions of the light and heavy liquids existing in the invariant region are 99 and 78% acetic acid, respectively. Propionic Acid. Figure 13 i s the phase diagram found for t h e propionic acid-ethylene-water system. It differs from thpt for t h e acetic acid system at the same conditions of tem2erature and pressure. T h e isobaric c r o s s section of t h e P - x isotherm for t h e propionic acid system resembles more closely t h e behavior of the acetone system at 715 p.s.i.a. (Figure 7). T h e longer carbon chain apparently hinders the hydrogen-bonding tendencies of t h e acid sufficiently or t h e hydrophobic property of the hydrocarbon portion of t h i s compound i s sufficient t o produce a liquid-liquid equilibrium over a much greater range of water-acid compositions than for a c e t i c acid. T h e ethylene-free composition of the heavy liquid in the invariant region i s much lower in acid content, 31.5% propionic acid, and higher for the light liquid, nearly 100% propionic acid. Hence an ethylene extraction process for separating propionic acid and water should b e readily
H*O Figure 10. Phase equilibrium diagram far e t h y l e n r w a t e m . propyl alcohol at 715 p.s.i.a. and 15.OoC.
t e r t -BUTYL ALCOHOL
Figure 11.
Phare equilibrium diagram for ethylene-water-tortbutyl alcohol at 515 p.r.i.a. and 15.0°C.
ACETIC ACID
c
Figure 12.
8
Phase equilibrium diagram for ethylene-water-acetic acid at 783 p.si.a. and 15.0° C. JOURNAL OF CHEMICAL AND ENGINEERING DATA
f e a s i b l e a t or near t h e conditions employed. F o r t h i s s y s tem (see Figure 13) t h e solubility of ethylene in pure propionic acid w a s not measured. T h i s measurement w a s omitted in t h i s case, b e c a u s e a n a n a l y s i s of t h e light liquid p h a s e i n t h e invariant region revealed essentially n o water content. Acetonitrile. T h e separation of water present a s a n impurity, from acetonitrile under ethylene pressure, w a s first noted i n t h i s laboratory by S y k e s (9). Sykes' observations were made a t t h e considerably lower temperature of -15OC.
PROPI ON IC
ketone (Pattern 3). Wost of t h e small amount of water present in t h e saturated alcohol p h a s e i s precipitated into a second liquid p h a s e on t h e addition of ethylene a t 1 0 ° C . and 600 p.s.i.a. Methylal. In t h e binary methylal (dimeth0xyrnethane)water system, water i s very insoluble in t h e organic phase, while methylal i s appreciably soluble in water. At 21OC. and 1000 p.s.i.a. a portion of t h i s small quantity of methylal w a s found t o be extracted from t h e water by ethylene with t h e resultant formation of a second (lighter) liquid phase. Triethylamine. With ethylene ahd water, triethylamine follows Behavior Pattern 2 a t 10.5"C. 4 phase separation o c c u r s a t about 450 p.s.i.a. in which t h e removal of water from t h e amine a p p e a r s t o b e nearly quantitative. A t temperatures above 19'C., t h e lower critical solution t e m perature for t h e triethylamine binary, P - x isotherms according t o Pattern 3 may be predicted for t h i s system. I n t h e work subsequent t o that reported here, Meter ( 8 ) h a s studied water-organic amine-supercritical ethylene s y s t e m s in considerable detail. l l i s work verifies and ext e n d s t h e present results. DISCUSSION
Figure 13. P h a s e equilibrium diagram far ethylenewaterpropionic acid a t 783 p.s.i.a. and 15.0' C.
ACETONITRI LE
A
Effoct of M o l e c u l a r P r o p e r t i e s , Hydrogen Bonding. I t h a s been postulated above that t h e p h a s e behavior of a system containing water, ethylene, and an organic liquid under t h e present conditions depends primarily on t h e hydrogen bond-forming tendency of t h e organic Ilqulds. T h o s e which tend t o form strong hydrogen bonds a l s o tend t o be completely miscible with water. In t h i s category a r e t h e compounds which behave a s illustrated in Pattern 1. A s t h e hydrogen bond-forming tendency of t h e organic compound d e c r e a s e s i t s mutual solubility with water d e c r e a s e s , a n d t h e p h a s e behavior i n t h e ternary system s h i f t s next t o t h a t c l a s s i f i e d a s Pattern 2 and with further diminution of t h i s property t o Pattern 3. 4 useful c l a s s i f i c a t i o n of compounds according to their tendency to form hydrogen bonds h a s been proposed by Ewell, Ilarrison, and Berg ( 4 ) . Their c l a s s i f i c a t i o n is a s follows: C l a s s I. L i q u d s capable of forming three-dimensional networks of strong hydrogen bonds-e. g., water, glycol, glycerol, amino alcohols, hydroxylamine, hydroxyl a c i d s , polyphenols, amides, etc. Compounds such a s nitromethane and acetonitrile a l s o form three-dimensional networks of hydrogen bonds, but the bonds are much weaker than t h o s e involving OH and N H groups. Therefore, t h e s e types of compounds are placed in C l a s s 11. C l a s s 11. Other liquids composed o f molecules containing both a c t i v e hydrogen atoms and donor atoms (oxygen, nitrogen, and fluorine)-e.g., alcohols, acids, phenols, primary and secondary amines, oximes, nitro compounds with 3-hydrogen atoms, nitriles with a-hydrogen atoms, ammonia, hydrazine, hydrogen fluoride, hydrogen cyanide, etc. C l a s s III. L i q u d s composed of molecules containing donor atoms, but no a c t i v e hydrogen atoms--e.g., ethers, aldehydes, ketones, e s t e r s , tertiary amines (including pyridine type), nitro compounds and nitriles without
