Supercritical Fluid Science and Technology - American Chemical

Chapter 9

Four-Phase (Solid-Solid-Liquid-Gas) Equilibrium of Two Ternary Organic Systems with Carbon Dioxide

Downloaded by UNIV LAVAL on July 13, 2016 | http://pubs.acs.org Publication Date: August 29, 1989 | doi: 10.1021/bk-1989-0406.ch009

Gary L. White and Carl T. Lira Department of Chemical Engineering, Michigan State University, East Lansing, MI 48824-1226

Melting temperatures of organic solids may be depressed significantly when contacted with supercritical fluids. In this work, P-T traces are reported for the high temperature branch of the three phase (S-L-G) line for the naphthalene-CO and phenanthrene-CO systems and the four phase (S-S-L-G) lines beginning at the solid-solid eutectic for the naphthalene-phenanthrene-CO and naphthalene-biphenylCO systems. 2

2

2

2

Competent design o f chemical processes requires accurate knowledge of such process v a r i a b l e s as the temperature, pressure, composition and phase o f the process contents. Current p r e d i c t i v e models f o r phase e q u i l i b r i a involving s u p e r c r i t i c a l f l u i d s are l i m i t e d due to the s c a r c i t y o f data against which to t e s t them. Phase e q u i l i b r i a data f o r s o l i d s i n equilibrium with s u p e r c r i t i c a l solvents are p a r t i c u l a r l y sparse. The purpose o f t h i s work i s to expand the data base to f a c i l i t a t e the development of such models with emphasis on the melting point depressions encountered when s o l i d mixtures are contacted with s u p e r c r i t i c a l f l u i d s . Previous workers (1-7) have demonstrated that the melting points of s o l i d s i n binary systems with ethylene, ethane, or carbon dioxide at elevated pressures may be s i g n i f i c a n t l y reduced. This phenomenon i s due to the s o l u b i l i t y of the l i g h t component i n the l i q u i d phase. Temperature minimums have been observed along the SL-G three phase l i n e f o r some systems (4-7). For these systems, the three phase l i n e i n t e r s e c t s the c r i t i c a l locus f o r the mixture at the upper and lower c r i t i c a l end points (UCEP and LCEP) as shown i n Figure 1. A s i m i l a r phenomenon i s also observed f o r the e u t e c t i c melting point of c e r t a i n binary mixtures o f s o l i d s i n contact with gases at elevated pressures as i l l u s t r a t e d i n Figure 2. This could p o t e n t i a l l y lead to a s h i f t i n the r a t i o of the molten s o l i d s i n the e u t e c t i c l i q u i d . 0097-6156/89AM06-0111$06.00/0

o 1989 American Chemical Society

Johnston and Penninger; Supercritical Fluid Science and Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1989.


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Temperature Figure 1. P-T p r o j e c t i o n f o r a binary system with a discontinuous c r i t i c a l mixture l i n e .

0

1 Mole Fraction Component A (on a supercritical fluid free basis)

Figure 2. Depression of e u t e c t i c melting point by a s u p e r c r i t i c a l f l u i d i n an A-B-SCF system, where A,Β are immiscible s o l i d s and Έ < P < P . The upper l i n e s represent f i r s t f r e e z i n g and the lower l i n e s represent f i r s t melting (o - P Δ - P Q - P,). χ

i(

2

s

3>



9.

WHITE A N D LIRA

Four-Phase Equilibrium of Two Ternary Organic Systems 11

In work with a ternary mixture o f ethylene, naphthalene, and hexachloroethane, van Gunst e t . a l . (8) observed an analogous temperature depression o f the four phase (S-S-L-G) minimum melting s o l i d l i n e . As shown i n Figure 3, such system exhibited two ternary c r i t i c a l end points, designated as the "p" (lower temperature) and "q" (higher temperature) points. Ternary systems may display such an i n t e r r u p t i o n of the c r i t i c a l locus i f the b i n a r i e s mixtures o f the i n d i v i d u a l s o l i d s with the solvent gas also have interrupted c r i t i c a l l o c i . The existence o f the UCEP and LCEP f o r each o f the b i n a r i e s does not, however, mandate such behavior f o r the ternary c r i t i c a l locus. For the binary systems, only the P-T trace extending to the UCEP i s sought i n t h i s work. The LCEP i s t y p i c a l l y very near the c r i t i c a l temperature and pressure f o r the pure s u p e r c r i t i c a l f l u i d . S i m i l a r l y , f o r the ternary systems, only the q point branch of the four phase l i n e i s sought i n t h i s work. Systems studied here exhibit s o l i d - s o l i d immiscibility. Expérimental Apparatus A schematic o f the experimental apparatus i s shown i n Figure 4. Solvent gas from a supply cylinder i s compressed with an a i r operated gas booster (Haskell model AC-152) to a pressure above that neede i n the view c e l l . A High Pressure Equipment Company reactor v e s s e l i s used as a reservoir f o r the compressed gas. Flow of the solvent gas from the reservoir i s c o n t r o l l e d by a shutoff valve. The i n l e t and o u t l e t from the valve are to .03 inch I.D. tubing, which permits incremental pressure increases i n the c e l l of as l i t t l e as 2 p s i . Pressure i n the view c e l l i s measured with a Bourdon tube gauge (Heise model CMM-63457). The temperature within the c e l l i s measured with a c a l i b r a t e d thermistor (Omega Engineering, model THX-400-GP) which passes through a compression f i t t i n g into the c e l l . Temperature control i s achieved with an insulated water bath. A 600 watt copper tubing base heater and a Bayley Instruments model 123 temperature c o n t r o l l e r with a 1000 watt quartz bayonet makeup heater provide heat to the bath. Tap water flowing through c o l l e d copper r e f r i g e r a t i o n tubing provides cooling f o r the bath. Figure 5 shows a cross section o f the view c e l l . This c e l l i s f a b r i c a t e d from 316 s t a i n l e s s s t e e l . The i n t e r i o r of the c e l l i s illuminated through the window at the top o f the c e l l by a f i b e r o p t i c l i g h t . The states of the contents are observed through the side window by means of a closed c i r c u i t color camera connected to a borescope. Although view o f the upper region o f the c e l l i s r e s t r i c t e d , any a d d i t i o n a l phase formed i n t h i s region must have a mass density l e s s than the mass density of the s u p e r c r i t i c a l phase. Such behavior i s not anticipated with the systems studied here. The windows are 3/4 inch χ 3/4 inch quartz. A t r i a n g u l a r magnetic s t i r bar at the bottom o f the c e l l s t i r s the lower phase i n the c e l l . A rectangular s t a i n l e s s s t e e l wire mesh "flapper" on a shaft set into the magnetic s t i r bar s t i r s the upper phase. The s o l i d sample rests on a s t a i n l e s s s t e e l wire mesh platform at the l e v e l of the side window of the view c e l l .

F i r s t melting and f i r s t f r e e z i n g have been used to study melting point melting point depressions. Both methods may be used i n



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Temperature Figure 3. P-T p r o j e c t i o n f o r a ternary system with discontinuous four phase l i n e .

Compressor

Reservoir

Figure 4. Schematic of the experimental apparatus. (P - pressure gauge, Τ - thermistor, m - magnetic s t i r r e r f o r view c e l l ) .


WHITE AND LIRA

Four-Phase Equilibrium of Two Ternary Organic Systems 11


9.

Figure 5. Diagram of the view c e l l , (a - quartz windows, b magnetic s t i r bar, c - wire mesh flapper, d - sample platform, e - sampling p o r t s ) .



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binary systems, but only the f i r s t melting method i s suitable f o r ternary systems (see Figure 2). Sample preparation i s s i m i l a r f o r both the binary and the ternary systems. For binary system measurements, some of the s o l i d i s melted and drawn Into a section of c a p i l l a r y tubing. A 3/4 inch section of t h i s f i l l e d tubing Is placed on the wire platform and serves to hold the a portion of the sample being studied i n a constant p o s i t i o n and prevent i t from draining away when melting occurs. For ternary system measurements, the two s o l i d s to be studied are f i r s t mixed to provide intimate contact. The minimum melting point f o r the s o l i d - s o l i d binary at atmospheric conditions occurs at the eutectic composition. A s o l i d mixture of t h i s composition i s prepared i n the expectation that a s i m i l a r r a t i o of the components w i l l be present i n the l i q u i d phase at the f i r s t melting point i n the ternary mixture. The s o l i d mixture i s melted and s t i r r e d to obtain a homogenous l i q u i d . Some of the l i q u i d i s drawn into a c a p i l l a r y tube. The remaining l i q u i d i s poured out onto a clean sheet of aluminum f o i l . After the binary l i q u i d cools and s o l i d i f i e s , the t h i n sheet of s o l i d material i s broken up into "flakes" f o r loading into the view c e l l . Once the sample i s prepared, adequate s o l i d i s loaded into the view c e l l to ensure the presence of excess s o l i d at a l l conditions to be studied. The c a p i l l a r y tube i s placed on the wire mesh platform as close as possible to the side window and p a r a l l e l to i t . With the sample loaded, the c e l l i s sealed, placed i n the temperature control bath, and connected to the high pressure gas r e s e r v o i r . The c e l l i s purged with the solvent gas and then f i l l e d with enough of the solvent f l u i d to b r i n g i t to near the desired f i n a l pressure. The bath i s also brought up to near the desired temperature. Measurements are made by two methods. Method A i s used i n the i n i t i a l lower pressure region where the decrease In melting point with increasing pressure i s most rapid. The temperature of the bath i s held constant and the pressure i n the c e l l i s v a r i e d by adding and releasing small amounts of the gas or vapor phase. The pressure at which the f i r s t melting occurred within the c a p i l l a r y tube (near the ends i s recorded as the melting point. Accuracies f o r these measurements are ± 5 p s i a and ± .05 G. In the higher pressure region where the P-T curve becomes almost p a r a l l e l to the pressure axis, method Β Is used. The pressure of the c e l l Is r a i s e d to near the desired l e v e l and then the temperature i s raised slowly u n t i l the f i r s t melting i s observed. Method Β corresponds to the method used by McHugh (4-5) to determine s i m i l a r P-T traces. For t h i s method, both the pressure and the temperature are changing with time. Accuracies f o r these measurements are ± 1 p s i and ± .2 °C. e

Materials Chemicals used i n t h i s study are l i s t e d i n Table I. A l l chemicals were used without further p u r i f i c a t i o n . The p u r i t y of the naphthalene and phenanthrene were v e r i f i e d by measuring the melting point range of each at atmospheric pressure. Results The P-T data obtained In t h i s study are l i s t e d i n Table II and Table I I I . The P-T trace of the S-L-V l i n e of naphthalene-C0 was 2


9.

WHITE AND LIRA

Four-Phase Equilibrium of Two Ternary Organic Systems 11 Table I.

Purity of Materials Puritv 99+% 99% 98+% Bone dry grade

Supplier Aldrlch Aldrich Aldrich Linde Co.

Chemical Naphthalene Biphenyl Phenanthrene C0 2

determined and compared to l i t e r a t u r e data to v a l i d a t e the method. Data obtained i n t h i s study f o r the naphthalene-CO system are p l o t t e d i n Figure 6 with data from McHugh (4-5) and Cheong e t . a l . (6) f o r comparison. Figure 7 i l l u s t r a t e s the P-T trace of the ternary system napthalene-biphenyl-C0 . The P-T traces of the constituent binary systems are included f o r comparison. Biphenyl-C0 data shown i n Figure 7 f o r are those reported by McHugh (4-5) and Cheong et. a l . (6). The three phase P-T traces f o r both the naphthalene-CO and the biphenyl-C0 b i n a r i e s l i e above the c r i t i c a l temperature of carbon dioxide. For the ternary mixture, however, the four phase l i n e runs into the s u b c r i t i c a l region f o r carbon dioxide. The c h a r a c t e r i s t i c s of the phase t r a n s i t i o n change at s l i g h t l y above the l a s t reported data point (870 p s i a and 23.1 °C). In t h i s region, attempts to extend the S-L-V l i n e by increasing the pressure cause a C0 r i c h l i q u i d phase to begin forming i n the bottom of the c e l l . The c r y s t a l s on the platform appear to be unchanged during t h i s phase t r a n s i t i o n . Further experiments are i n progress to study the phase behavior of t h i s system. Figure 8 shows the data obtained f o r the naphthalene-C0 and phenanthrene-CO b i n a r i e s and the naphthalene-phenanthrene-CO ternary system i n t h i s study. In the ternary, the four phase l i n e f a l l s above the c r i t i c a l conditions f o r pure carbon dioxide but s i g n i f i c a n t l y below the three phase P-T l i n e f o r each of the single s o l i d - C 0 b i n a r i e s . Based on data taken, the ternary c r i t i c a l end point q point) f o r the f o r t h i s system l i e s between 32.0 and 34.2 °C and between 1335 and 1415 p s i a . Outside t h i s region, no melting of the s o l i d s i s observed with increasing temperature, but decreasing the pressure from 1415 p s i a while holding the temperature constant at 34.2 °C causes a l i q u i d to condense out which, when the pressure i s further decreased, completely


2

2

2

2

2

2

2

2

2

2

Table I I . P-T Data f o r the Naphthalene-Biphenyl-C0 Naphthalene/C0 PtDsia) 15 420 480 810 960 1090 1240 1480 1950 2470 2890

2

TCC) 80.2 74.1 73.2 68.8 67.0 65.6 63.8 61.65 58.5 58.6 59.5

2

System

Naphthalene/BIphenyl/C0 Pinsia) 15 210 250 390 560 650 735 870

2

Ti°C) 39.4 34.3 33.4 30.4 26.9 25.5 24.5 23.1


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4000

Δ - McHugh Ο - Cheong et al. 3000 Ο - This work

ΔΟ


2000

b

1000

ο

55

65

85

•75

Temperature fC) Figure 6. P-T p r o j e c t i o n f o r the naphthalene-C0 from Ref. 4-6 and t h i s work).

2

system. (Data

5000 Ο - Naphthalene/C0 I AO-Blphenyl/C0

4000 (· V -Ternary Ο - C 0 Critical Pt. 2

2

2

β

(0 Τα

3000

3

Φ

i-

3

Supercritical Fluid Science and Technology - American Chemical

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