Supercritical phase equilibria involving solids

Fluid Phase Equikbria,

10 (1983) 159-172 Publishing Company, Amsterdam

Elsevier Scientific

PHASE EQUILIBRIA

SUPERCRITICAL R. KONINGSVELD

159 -Printed

in The Netherlands

INVOLVING SOLIDS

and G.A.M. DIEPEN

DSM, Research & Patents, Geleen, The Netherlands

ABSTRACT The solubility of solids in supercritical solvents is reviewed in a phenomenological discussion of binary and ternary systems containing one highly volatile component. Solubility and selectivity are greatly determined by the course of the binarv critical curves. the ternarv critical end-point curves, and the locations of the triple points of the solids. The mean-field lattice-gas model is used to review some important molecular parameters.

INTRODUCTION 'Supercritical'

behaviour

(mutual) solubility

usually relates to unexpected

upon small variations

Two classes of such supercritical

changes in

in pressure and/or temperature.

phenomena exist, viz. those involving

fluid phases only (ref.11 and those where fluids are in equilibrium

with

one or more solid phases, the second class forming the subject of this paper. The general phenomenological explained

in a consideration

is the supercritical A remarkable particular

principles

of fluid/solid

equilibrium

of binary systems in which the first component

fluid and the second a solid compound of low volatility.

feature of supercritical

dissolution

is the selectivity

solids it often shows. We discuss this phenomenon

ternary systems consisting components.

are

Concurrent

of a supercritical

partial miscibility

to

examining

fluid and two solid

phenomena

in the fluid (or

liquid) phase will be left out of consideration. Hannay and Hogarth probably were the first to observe unexpected in the solubility

of salts in alcohol close to the latter's critical

(ref.2). Diethyletherlanthraquinone, to be treated quantitatively solubility milestone

and Van Zon's work on the

water may be seen as another

in the field (ref.3).

037%3812/83/$03.00

point

studied by Smits, was the first system

(ref.2). Van Nieuwenburg

of quartz in supercritical

changes

0 1983 Elsevier Science Publishers

B.V.

160

In recent years chemical technology

has found useful applications

such

as the selective removal of caffeine from coffee beans, nicotine from tobacco, or the winning of valuable compounds like hops, mostly substances whose thermal stability call for mild dissolution conditions most cases the solubility

is so small that figures drawn to scale would not

clearly bring out the principles. Therefore, qualitative

(ref.5). In

all figures presented here are

only.

BINARY SYSTEMS Supercritical equilibria

behaviour in binary systems arises when three-phase

come to interfere with the critical curve

connects the two nonvariant pure components equilibrium

vapour-liquid

(L = VI. The latter

critical points Ca and Cb of the

and describes how the binary critical vapour-liquid

depends on pressure

(~1, temperature

(Tl and the composition

of

the mixture

(Xb, mole fraction of component -b). The three-phase equilibrium refers to a vapour phase and two condensed phases, in this paper a liquid

and a solid. In

figure la we recognize the familiar p(T) diagram for single

components, solid-liquid

e.g. b, with the three monovariant (SLlb, liquid-vapour

curves intersect in the triple point (0) three-phase

equilibrium

two-phase equilibria

(LVlb and solid-vapour

(SVlb. The three

which represents the nonvariant

SLV. The liquid-vapour

curve ends in the critical

point Cb where, upon an increase in p and T along the curve, both phases become identical in density and other properties. On each of the two sides of the three-dimensional

diagram we have similar situations for pure

-a(xb = 0) and pure b(xb = I). Some isothermal p(xb) sections diagrams) are indicated. The nonvariant single-component becomes monovariant three-phase

three-phase

equilibrium

(binary phase

SbLV (A)

upon addition of component a and there will be a

curve SbLV springing from the triple point of b_. At a

given p and T, the three coexisting phases are represented

by points

V, L and Sb on a straight line parallel to the xb axis and all three obviously project onto the p(T,

Xb

=

1)

plane in one and the same

point. Sets of these points for various p and T form the three-phase curve SbLV. The eutectic three-phase

equilibrium SaSbL also gives rise to a similar,

161

b

Fig. la. p[T, xb) Bakhuis-Rooteboom diagram for a binary system rithout 'supercritical' phenomena. The solubility of component b in liquid 5 is smail

S

S

c'b

S'a L

I.

1

3-

Fig. lb.

p(T) and T(xbj projections

L-V

for fig. la

162 but much

steeper

curve

in

the p(T, xb = 1) projection. The intersection of

SaSbL and SbLV represents the nonvariant quadruplepoint Q (0) phases

COeXi

St

where four

(SasbLv)‘.

Figure la shows that portion of the binary Bakhuis-Roozeboom

p(T,xb)

diagram (ref.6) which is relevant for the present discussion. The use of three-dimensional

figures is usually avoided and p(T) and T(xb) projections

are considered. Such projections, going with fig. la, are shown in fig. lb. Figs. la,b show a situation where the critical point of i (Ca) is located at a temperature comparable to that of the triple point of b. It is also assumed that the solubility of b in the liquid phase is relatively small, which involves the SbLV curve to be located at temperatures only slightly below those of the boiling-point curve of a_, (LV),, and to have a maximum. Also, there will be a relatively large p-T range in which the V and L branches of the SbLV equilibrium curve are found in the d-rich regions of the diagram (small xb).

An increase of the disparity in volatility, again combined with a small solubility of b_, may lead to the necessary condition for supercritical dissolution, viz. interference between the V and L branches of the SbLV equilibrium and the critical curve CaCb beyond C, (Figures '2a,b). Vapour and liquid become identical and the three-phase curve SbLV ends in the nonvariant first critical end point (L = V)lSb; C,), where a critical fluid is in equilibrium with a solid phase. An increase of T brings two-phase equilibria which, at low p, have the character of sublimation (SV). Raising the pressure gradually densifies the vapour and, at high p, the fluid assumes a density representative for liquids (SbL)* From the triple point of b_, another branch of the SbLV curve must originate as in fig. lb, which implies the existence of a second critical end point ((L = V)zSb; Cz) where the V and L branches intersect the critical curve starting from Cb. It can be seen that the shape of the p(xb) phase diagram may change considerably with temperature. The region important for the dissolution of solid __ b in supercritical a is indicated on the T-axis. Fig. 3 illustrates how the shape of the

p(Xb)

diagram may vary with

temperature, the system represented (in a qualitative sense) is diethyletherlanthraquinone

(ref. 3). At low pressures no appreciable

dissolution will take place at any temperature. Close to the second critical end point C2, however, the solubility will suddenly increase if the pressure is raised to that of Cz and above. This occurs because at C2 the

163

b

Fig. 2a. p(T, Xb) Bakhuis-Roozeboom-diagram for a binary system with two critical end points (L = VI1 and (L = VI2 [Cl and C2 in fig. 2b.I.

'b

T

V -''......._ .._,

... .._____,

'b b

I h

Fig. 2b.

, T1

TC*

p(T) and T(xb) projections for fig. 2a.

165

a

-.._ X Scale

enlarged

L

‘....,

.‘...

‘. ..._.

--. ,, “L.__,

...,

Fig. 4. top: p(T) pro'ection for a ternary system with two ternary critical end points SbS,(L = V3 (Type C). The solubilities of b and c in a are small (x-scale enlarged in the binary T(xb) and T(xc) proje?tionsT bot%om).

166 It would seem that type C represents the most probable situation if the solubilities of both b and 2 are small. Fig. 4 shows a p(T) projection and also indicates possible shapes for the T(xb) and T(xcl projections of the binaries -a/b and -a/c. At the low temperature end the x-scales have been enlarged in the indicated area because the concentrations of b and c in d are usually extremely small in that range. We can now construct p(x) isotherms for a number of temperatures and from them isobaric ternary diagrams. Examples are given in figs. 5 -'8. Drawn curves refer to singleor two-component

systems, dashed curves to ternary equilibria. Dotted cur-

ves indicate metastable states. It is seen that jumps in solubility similar to those at binary critical end points may arise at ternary critical end points which represent ternary S(L = V) equilibria for given temperatures. At TI for instance, we have a three-phase region S,LV until the pressure is raised to that of the ternary critical end point curve Sc(L = VI where L and V become identical. It may then happen that tne ScL binodal at that pressure assumes an S-shape like sketched, thereby giving rise to considerable enlargements of the ternary solubility range. At T2 such a ternary critical phenomenon occurs twice (fig. 6). Such relatively wide ternary ranges of solubility may again develop upon small changes in pressure and/or temperature. This effect may be favourable or not, depending on the objective of the separation, and must be reckoned with. Beyond the temperature of the first ternary critical end point we enter the ternary intercritical end-point range where the pressure may be expected to influence ternary solubility markedly without critical phenomena occurring

(fig. 71. At T4, beyond the ternary second critical end

point (fig. 81, phase relationships show a similar pattern to those at T2 (fig. 6). If the locations of the binary T(x) projections are as chosen here (fig. 41, the ternary solubility ranges will now be found in the b-rich part of the ternary diagram (fig. 8). Type B differs from C in that there are no ternary critical end points and hence, no ternary intercritical end point range. Type A differs from B on two points, firstly there is only one ternary critical end point curve and, secondly, the solubility of b is appreciable. The LV ranges may therefore be expected to be larger than with types B and C. These figures are qualitative examples only, actual phase relationships depend on the properties of the system on hand. Therefore

it iS difficult

to state general rules for selective dissolution because the necessary procedure is determined by the details of such ternary diagrams. Also, the objective may vary a great deal. For instance, if winning of a valuable

167 Fig. 5-8. Binary isothermal p(Xb) and p(xc) phase diagrams showing the location of the ternary three- and four-phase equilibria (dashed) and some ternary isobaric sections derived from them (see fig. 4). At T1 and T2 we only show the a-rich part of the diagrams (see T1, PsbLv, fig. 5). C

,

c

a

XC

a

Fig. 5.

3

!

'b

b

b

168

FI

S,FI

-4c-----SbFl

_

C

_=

----------.------~

a

a

a

Fig.

6.

SbS,LV

b

169

T3

I

p2

SbFl

:.o-

I

I(

)l b

C

a

b

Fig. 7. admixture b from an excess of c is aimed at, it is not probable that two or three - phase regions SbL(SbV) or SbLV can be employed. ScL or ScLV regions more naturally offer themselves and can be used under conditions for maximum solubility of b in the fluid phase and, preferably, minimum solubility of c. This qualitative recipe is also valid if purification contaminated

of a

solid, e.g. c_, is wanted. Separation of a 50/50 mixture of b

and c will depend totally on details of the phase relationships

in the

a-rich corner of the ternary diagram where the location of the ternary eutectic plays a determinative

role. In any case, the three-phase region

SbScL is to be avoided, as well as LV separation ranges which give only relatively small enrichments

in the components.

170

FI

I

SbFl

s,

(L-V)

S&=V) iSbScLV

a

b

Fig. 8.

CONCLUSIONS It follows that the main features determining selective solubility are: 1. the course of the binary critical curves 21 the course of the ternary critical end-point curves

3. the location of the triple points of the solids 4. the location of the ternary eutectic

172 bring about sufficient

selectivity.

Finally,

the disparity between the cri-

tical points has an evident influence. Within the model this implies at least an indirect influence of the relative size and shape of the molecules (ref. 7).

REFERENCES G.M. Schneider, Proceedings Faraday Supercritical Fluids Meeting, Cambridse, 13-15th September 1982; Fluid Phase Ea.. this volume J.B. Hannay and J. Hogarth, Proc.Roy.Soc. London,'&, 324 (1879) A. Smits, Z. Elektrochem., 2, 663 (19031; Z.phys.Chem., 5l_, 193; 52, 587 (1905) C.J. v. Nieuwenburg and P.M. v. Zon, Rec. Trav. Chim., 54_, 129 (1935) See e.g. Angew. Chem., Intern. Ed., 17, 701 (1978) R. Bakhuis-Roozeboom. Die heterogenen Gleichgewichte, Vieweg, Braunschweig, 1901-1918 L.A. Kleintjens, Proceedings Faraday Supercritical Fluids Meeting, Cambridge, 13-15th September 1982; Fluid Phase Eq., this volume