reactions in supercritical fluids - a review

Ind. Eng. Chem. Process Des. Dev. 1986, 2 5 , 1-12

REVIEW

1

Reactions in Supercritical Fluids-A Review

Bala Subramanlam' and Mark A. McHugh"

Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46556

The objective of this paper is to present a review of the fiekl of reactions in supercritical fluids (SCF's). The high pressure phase behavior of mixtures in their critical region has a direct bearing on the understanding and interpretation of kinetic rate data, and, therefore, a discussion of high pressure phase behavior is presented. Mention is made of transition-state analysis as applied to SCF reaction studies. Further, the unusual partial molar volume behavior of a heavy solute solubHized in an SCF solvent is described and related to the enhancement of the reaction rate near the critical point of the SCF. Several examples of experimental studies of reactions in SCF media are presented, and the advantages of an SCF reaction scheme as compared to a conventional reaction scheme are described. Finally, some observations are made based on our own experience as to the potential of this emerging technology.

1. In t roduct ion The potential use of supercritical fluid (SCF) solvents

in chemical separation processes has been of considerable research interest for the past decade. The fundamentals of SCF extraction technology and a number of potential applications for this technology are described in several recent review papers (McHugh, 1985; McHugh and Kru- konis, 1986; Paulaitis et al., 1983; Williams, 1981). One very interesting and, as yet, not fully tested offshoot of SCF extraction technology is the use of an SCF solvent as a reaction medium in which the SCF either actively participates in the reaction or functions solely as the solvent medium for the reactants, catalysts, and products. By exploiting the unique solvent properties of SCF's (e.g., wide variations in density and viscosity are possible with small changes in pressure and (or) temperature), it may be possible to enhance reaction rates while maintaining or improving selectivity. Also, separating products from reactants can be greatly facilitated by the ease with which the solvent power of the SCF solvent can be adjusted.

The objective of this paper is to present a review of the field of reactions in SCF's. The high pressure phase behavior of mixtures in their critical region has a direct bearing on the understanding and interpretation of kinetic

'Presently a t the Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, KS 66045.

* Author to whom correspondence should be submitted. Presently at the Department of Chemical Engineering, The Johns Hopkins University, Baltimore, MD 21218.

rate data, and, therefore, a discussion of high pressure phase behavior is presented. Mention is made of transition-state analysis as applied to SCF reaction studies. Further, the unusual partial molar volume behavior of a heavy solute solubilized in an SCF solvent is described and related to the enhancement of the reaction rate near the critical point of the SCF. Several examples of experimental studies of reactions in SCF media are presented, and the advantages of an SCF reaction scheme as compared to a conventional reaction scheme are described. Finally, some observations are made based on our own experience as to the potential of this emerging technology.

2. Phase Behavior at High Pressures The potential advantages of using a supercritical fluid

reaction medium are that it may be possible to increase the selectivity of a reaction while maintaining high con- versions, to dissolve reactants and catalyst in a single fluid phase so that the reaction occurs homogeneously, and to improve or greatly facilitate the separation of product species from reactants, catalyst, and unwanted byproducts by utilizing the phase behavior exhibited by the mixture in ita critical region. Also, reaction rates may be enhanced while operating in the mixture critical region due to a favorable pressure dependence of the reaction rate constant as well as the unusual volumetric behavior of heavy solutes solubilized in an SCF solvent.

To capitalize on the unique characteristics of an SCF reaction medium, it is necessary to be cognizant of the phase behavior which is exhibited by the reaction mixture a t high pressures. Ehrlich and Mortimer (Ehrlich and

0196-4305/86/1125-0001$01.50/0 0 1985 American Chemical Society

2 Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986

P I P

I p = I

( e )

Figure 1. Five types of schematic pressure-temperature diagrams for binary mixtures. The critical point of the more volatile component is located at C1, and the critical point of the less volatile component is located at Cz. The open triangles represent critical end points in which two phases become critically identical in the presence of a third, noncritical phase.

Mortimer, 1970) have argued that a large body of kinetic studies reported in the literature on the high-pressure polymerization of ethylene are of little value since the authors of those studies are unaware of the phase behavior involved with SCF mixtures. This situation will also be true when researchers attempt to analyze SCF reaction data without a knowledge of the number and types of phases present in their particular reaction system.

When operating a reaction in the mixture critical region, the location of phase border curves in pressure-temperature-composition (P-T-x) space is of paramount impor- tance. These phase border curves, which separate regions of differing states of matter, can be two-phase liquid-liquid (LL) or liquid-vapor (LV) boundaries, three-phase liquid-liquid-vapor (LLV) or solid-liquid-vapor (SLV) boundaries, and sometimes four-phase solid-solid-liquid- vapor (SSLV) or liquid-liquid-solid-vapor (LLSV) boundaries. For the purpose of describing the impact of phase behavior on reaction behavior in an SCF media, it is instructive to describe the schematic pressure-temperature (P-T) diagrams of binary mixtures since there are relatively few types of these diagrams, they are more easily comprehended than multicomponent phase diagrams, they represent a limiting case for the phase behavior of multicomponent mixtures, and they exhibit certain characteristics, such as three-phase LLV phenomena, which ap- pear in the same P-T regions for multicomponent mixtures. The simplest representation of binary-phase behavior is a P-T diagram since the regions of multiple phases are reduced in geometrical complexity because pressure and temperature are variables which are the same in each of the equilibrium phases (so-called "field" variables) (McHugh, 1985; Streett, 1983). Although three-di- mensional P-T-x diagrams reveal the full detail of the phase behavior of binary mixtures, a discussion of these

TEMPERATURE 'C

Figure 2. Phase behavior for the benzene-ethane system (Kay and Nevens, 1952).

, SUPERCRITICAL REGION -2 20001 I : w

In a HEXANE, c . p .

0,; ----*----- + ----- +--\d , U a 0 50 100 150 200 250

TEMPERATURE ("C ) Figure 3. Schematic representation of the pressure-temperature region used in the hydrocarbon isomerization process proposed by Leder et al. (1976).

phase diagrams is beyond the scope of this paper. The reader is referred elsewhere for this information (Streett, 1983; Rowlinson and Swinton, 1982; McHugh and Kru- konis, 1986).

Scott and van Konyenburg (Scott, 1972; Scott and Ko- nynenburg, 1970) have catagorized the types of binary- phase behavior by using five representative P-T diagrams as shown in Figure 1. The simplest possible binary P-T diagram (type I) is shown in Figure la. In this case, the liquids are miscible in all proportions, and a continuous critical mixture curve exists. In this and the other four diagrams in Figure 1, the overall mixture concentration varies along the critical mixture curve. An example of the variation of the mixture composition along the critical mixture curve is shown in Figure 2 for the benzene-hexane system where constant composition vapor-liquid loops have been superimposed on the P-T diagram.

In a patent describing a hydrocarbon isomerization process, Leder et al. (Leder et al., 1976) demonstrate that type I phase behavior is exhibited by a reacting mixture consisting of a hydrocarbon feed, such as n-hexane, a metal halide catalyst, a hydrogen halide solvent, such as HC1, and hydrogen. A schematic P-T diagram for this mixture is shown in Figure 3. These mixtures are reacted at P-T conditions within region I1 bounded by the points abcd shown in Figure 3. Although this reaction is not run in the supercritical region, it does provide an example in which the authors first determine the critical mixture behavior of the reactants (see Figure 4) and then proceed with the reaction studies in a well-defined region of the phase diagram.

The P-T diagram for the type I1 systems is shown in Figure lb. Notice that the liquids are no longer totally miscible at all temperatures and pressures. An LLV line ending at an upper critical end point (UCEP) (i.e., a point at which one of the liquid phases becomes critically identical with the gas phase in the presence of the other liquid

Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 1, 1986 3

(Alwani and Schneider, 1980). For type IV binary mixtures, the critical mixture curve may or may not exhibit such a pronounced pressure minimum as suggested in the schematic diagram of Figure Id.

Type V phase behavior shown in Figure l e is very similar to the previously described type I11 system. In this instance, however, the LLV region is bounded by the two branches of the critical mixture curve, and it does not reappear a t lower temperatures as shown in Figure IC.

If at a given set of conditions the reactant is soluble in the SCF phase while the product remains insolbule, then it may be possible to precipitate the product from the reaction mixture as the reaction proceeds. In this manner, the product can be immediately recovered from the reacting system and unwanted side reactions may be avoided. For instance, Paulaitis and Alexander describe the Diels-Alder reaction of isoprene with maleic anhydride in supercritical COB, wherein the product precipitates from the reaction mixture as the reaction proceeds (Alexander and Paulaitis, 1984). The reaction is run at fairly low concentrations of reactants in supercritical C02 near the critical point of pure C02. As the reaction proceeds, the product precipitates from the SCF phase as a solid and is easily recovered.

In cases where the product is split from solution by adjusting pressure or temperature, the SCF can be thought of as a phase-transfer agent which not only allows the reaction to occur homogeneously but also allows the product to be easily isolated from the reacting mixture and removed from the reactor.

The phase behavior shown in Figure 1 can be modeled by using a simple, cubic equation of state (EOS). It is not possible, however, to obtain better than qualitative to semiquantitative agreement between model-generated phase diagrams and experimental data unless multiple parameters are incorporated into the model. A certain amount of experimental phase behavior information is needed to fit the parameter(s) that is incorporated in the EOS. Nevertheless, a cubic equation of state with a fitted interaction parameter(s) can be used to identify regions in P-T space in which multiple phases can exist.

As mentioned earlier, it may be possible to enhance the reaction rate if the reaction is run in the mixture critical region. The rate enhancement is due to the pressure dependence of the reaction rate coefficient and due to the unusual partial molar behavior of a heavy solute solubilized in an SCF solvent. Transition-state analysis (Laidler, 1965; Eckert, 1972; Ehrlich, 1971) is used to explain the rate enhancement at high pressures. As described by Eckert (Eckert, 1972) for a bimolecular reaction, a chemical equilibrium is assumed between the reactants A and B and the transition state M.

(1)

The variation of the reaction rate constant k with

A + B F? M' - products

pressure is given by

700 1 5 00

50 70 90 110 130 150 170 190 210 230 250 270

T E M P E R A T U R E , O C

Figure 4. Experimentally determined critical mixture curves for various hydrocarbon-HC1 mixtures (Leder et al., 1976).

phase) is now evident a t temperatures lower than the critical point of either component. The UCEP appears at the intersection of the LL boundary curve and the LLV boundary curve as shown by the open triangle in this figure.

Type 111 phase behavior shown in Figure IC exhibits liquid immiscibility characteristics similar to that previously described for the type I1 system. However, the branch of the critical mixture curve starting at critical point C2 now intersects a region of liquid-liquid immiscibility at the lower critical solution temperature (LCST) (i.e., a t the LCST, the two liquid phases of the LLV line become critically identical in the presence of the gas phase) rather than ending at critical point C1. The other branch of the critical mixture curve which starts a t critical point C1 intersects the LLV line at an UCEP. This type of phase behavior is especially interesting since it may be possible to separate products from reactants a t conditions along the LLV line. If the resultant product species has a different substituent group as compared to the reactant, the reactant-product-SCF mixture may possibly exhibit a region of immiscibility near the critical point of the SCF. The types of substituent groups which affect the miscibility behavior of solute-nonpolar SCF mixtures are described by Dandge et al. and Stahl (Dandge et al., 1985; Stahl and Quirin, 1983). A reacting mixture will more likely exhibit multiphase LLV behavior as the difference between the molecular weights of the products, reactants, and SCF increases (Rowlinson and Swinton, 1982). In fact, for the extreme case of size disparity between solute and solvent-that is, polymer-solvent systems-LLV behavior near the critical point of the pure solvent has been known for more than 2 decades (Freeman and Rowlinson, 1960).

As shown in Figure Id, type IV phase behavior exhibits certain similarities to that of type I11 at conditions which are very near the critical point of the more volatile component, C1. However, the branch of the critical mixture curve which starts at the critical point of the less volatile component, C2, exhibits a minimum in pressure at a temperature near C1. This phase behavior suggests a number of interesting reaction/separation scenarios. For instance, if the reaction is run homogeneously at the temperature and pressure indicated by the asterisk in Figure Id, the product can be recovered by splitting the reaction mixture into two phases when the critical mixture curve is crossed by either isothermally reducing the pressure or by iso- barically increasing or decreasing the temperature. The phase behavior depicted in this diagram has been observed for the C02-squalane and the CO,-n-ClGH3, systems as well as other systems described by Alwani and Schneider

d In k aP

= -A V* / (R 7')

where AV*, the volume of activation, is the difference in the partial molar volumes of the activated complex and the reactants and is given by

If the volume of activation is positive, then the reaction will be hindered by pressure. However, if A F is a negative quantity, then the reaction rate will be enhanced by pressure.


0

- 5 8 7 atm 5 -400 - J u ~- 5 5 4 atin o u > - -600

1 , I I i I I I

01 0 2 0 3 0 4 05 06 07 0 8 09 I O M O L E F R A C T I O N V I N Y L C H L O R I D E

Figure 5. Calculated partial-molar volume behavior for the vinyl chloride-ethylene system near the critical point of pure ethylene (Ehrlich and Fariss, 1969).

Ehrlich (Ehrlich, 1971) uses transition-state analysis to interpret the unusual reaction behavior of ethylene polymerization in supercritical ethylene. He argues that vM, which has volumetric properties similar to the product polymer, can have a very large negative value in supercritical ethylene. An example of very large negative partial molar volumes of a heavy solute in a supercritical fluid is shown in Figure 5 for the vinyl chloride-supercritical ethylene system (Ehrlich and Fariss, 1969). Other inves- tigators have shown experimentally that the phenomenon of very large, negative partial-molar volumes exists for systems in which the solute is in dilute concentrations and the solvent is very close to its critical conditions (Eckert et al., 1983; Chappelear and Elgin, 1961; Ehrlich and Wu, 1973). Using a lattice-gas model, Wheeler (Wheeler, 1972) is able to theoretically explain this unusual partial molar volume behavior near the solvent critical point.

Simmons and Mason (Simmons and Mason, 1972) em- ploy transition-state theory with an equation of state (EOS) (i.e., Redlich Kwong and the virial equation) in an attempt to derive an expression for the pressure dependence of the dimerization rate constant of chlorotrifluoroethylene (T, = 378.8 K, P, = 40.1 atm). They fit the experimental rate data that they obtain at pressures ranging from 1 to 100 atm and temperatures from 393 to 423 K to an EOS to predict the volumetric properties of the reactants. With this approach, they obtain the ther- modynamic properties, such as the fugacity coefficient and the partial molar volume, of the activated complex. While the predicted fugacity coefficient of the activated complex has a pressure dependence characteristic of normal molecular species, the partial-molar volume of the activated complex, vM, decreases sharply near the critical point of the dilute reaction mixture (see Figure 6). This partial- molar volume behavior indicates that the volume of activation, AV*, and the reaction rate constant are strong functions of pressure near the critical point of the pure chlorotrifluoroethylene. However, Simmons and Mason report that the maximum pressure effect on the reaction rate constant is only 30% for pressures up to 100 atm. Unfortunately, they find that the Redlich-Kwong EOS does not correlate the experimental data within 30%. Hence, higher pressures are needed to increase the pressure enhancement and also to provide a more stringent test of the EOS.

For all practical purposes, the large rate enhancements which occur as a result of large solute partial molar volumes are limited to the dilute concentration region as shown in Figure 5 and implied in Figure 6. Although an enhancement of the reaction rate can occur as a result of

-3' I I I J 0 20 40 60 80 100

P R E S S U R E ( A t m )

Figure 6. Effect of pressure on the partial-molar volume of the activated complex formed during the dimerization of chlorotrifluoroethylene (Simmons and Mason, 1972).

LL--,,--L- 'P 1J 0 pc 200 400

P R E S S U R E ( a t m i

Figure 7. Physicochemical properties of a supercritical fluid.

hydrostatic pressure alone, excessively high pressures (i.e., greater than lo00 atm) are needed to have any appreciable effect (Eckert, 1972).

Finally, as noted in the Introduction, an SCF has unusual physicochemical properties which may have an effect on reaction behavior in the critical region. As shown in Figure 7, the diffusivity of an SCF is more gaslike than liquidlike at moderate to high pressures. Also, the viscosity of an SCF is more gas-like in the critical region. These properties should enhance the mass-transfer characteristics of an SCF reaction/separation process. Of course, if the reaction occurs in the single-phase, mixture-critical region, then interphase mass-transfer limitations on the reaction are absent.

3. Experimental Studies of Chemical Reactions in the Near-Critical and the Supercritical Regions

The unique solvent properties of near-critical and supercritical fluids can be exploited in chemical reaction schemes in a variety of ways. Each of these reaction schemes is briefly described.

(A) An SCF reaction medium can be utilized to lower the operating temperature of pyrolysis reactions. The carbon formation which occurs at the high temperatures normally encountered in pyrolysis reactions can therefore be minimized. Also, improved yield, selectivity, and product separation can be attained when operating in an SCF reaction medium as compared to conventional pyrolysis methods.

(B) For certain heterogeneous catalytic reactions characterized by catalyst deactivation caused by coking or fouling, the catalyst can be reactivated by adjusting the


Table I. Examples of SCF Reaction Schemes Reviewed in This Paper

temperature, improving yield, selectivity, and product separation

in situ catalyst reactivation of solid porous catalysts under mild SCF reaction conditions

study viscosity effects of solvent on chemical reactions using a single SCF solvent

reactions, producing enhanced reaction rates, selectivity, and efficient product separation

enhanced reaction rates and selectivities (e.g., free-radical reactions)

X

X

homogenous critical mixture phase X

reactants a t SCF conditions to achieve X

designation reaction scheme A B C D E reference(s)

SCF reaction conditions lower reaction x KOll and Metzger, 1978; KOll et al., 1983; KO11 and Metzger, 1978; Metzger et al., 1983

Tiltscher et al., 1981; Tiltscher et al., 1984

Squires et al., 1983

Kramer and Leder, 1975; Modell, 1982; Bhise, 1983; Randolph et al., 1985; Hammond et al., 1985

Patat, 1945; Blyumberg et al., 1965; Baumgarter, 1983; Ehrlich and Mortimer, 1970; Takahashi and Ehrlich, 1982; Ehrlich and Pitillo, 1960; Ehrlich and Kurpen, 1963; Ehrlich, 1965; Krase and Lawrence, 1966; Cottle, 1966

pressure and temperature so that the reacting medium is in the supercritical state. The low-volatile compounds which deactivate the catalyst can be stripped from the catalyst surface by the SCF phase. In this manner, the activity of the catalyst can be periodically regenerated by treatment with an SCF solvent or, in fact, the reaction can be run at supercritical conditions, thus maintaining high levels of catalytic activity for longer periods of time.

(C) An SCF medium can be used to study the effect of solvent viscosity on the chemical reaction. A wide range in solvent viscosity can be investigated with a single SCF by operating close to the solvent critical point.

(D) Supercritical fluid solvents can be employed as solvent media in chemically reacting mixtures especially where product separation by conventional techniques such as distillation are difficult to achieve or are prohibitively expensive. By adjusting the solvent power of the SCF (i.e., by varying the pressure and (or) temperature), it is possible to fractionate the reaction products. In certain cases, the use of SCF’s can also improve the product selectivity without adversely affecting total conversion.

(E) Certain chemical reactions, such as free-radical, vinyl-polymerization reactions, can exhibit enhanced reaction rates with different selectivities when the reaction occurs homogeneously in the mixture critical region as compared to heterogeneously in the subcritical gas-liquid region. Hence, it is possible to control the extent of reaction by isothermally adjusting the system pressure such that either the reaction proceeds rapidly when the reaction mixture exists as a single homogeneous supercritical mixture or the reaction slows considerably when the mixture exists as a heterogeneous gas-liquid mixture in the subcritical region.

Examples of each of these types of reaction schemes are described in the following section. Table I provides the reader with a summary of the various SCF schemes that have been reviewed in this paper.

3.1. Examples of Reaction Scheme A: High-Tem- perature SCF Reactions. KO11 and Metzger (KO11 and Metzger, 1978) use supercritical acetone as the reaction medium for the thermal degradation of cellulose and chitnin. Normally, the degradation temperatures used in the pyrolysis of these polysaccharides are so high that it is necessary to remove the primary products from the reaction zone as soon as they are formed to avoid secondary reactions which result in strong carbon formation. Both yield and uniform product distribution are adversely affected by the very high operating temperatures. Carrying out the pyrolysis under vacuum not only reduces the

Table 11. Products from Pyrrolysis of Cellulose a t 573 K (Koll and Metzger, 1978)

nitrogen -1 atm vacuum char 34.2(%) 17.8(%) tar 19.1 55.8 glucosan (1) 3.6 28.1 1,6-anhydroglucofuranose (2) 0.4 5.6

carbon formation but it also reduces the reaction rate due to the poor heat transfer to the reactants. A comparison of the reaction products of cellulose pyrolysis in a nitrogen atmosphere and under vacuum conditions at 573 K is given in Table 11.

KO11 and Metzger overcome the above problems by reacting the cellulose in the presence of supercritical acetone (T, = 408.9 K, P, = 47.0 atm) using a flow reactor. The pressure of the reactor is maintained at 250 atm and the temperature is increased slowly from 423 to 613 K. At SCF conditions, 98% extraction of the initial cellulose is achieved. The yield of glucosan, which is 38.8%, compares favorably with the 28.1% yield obtained from vacuum pyrolysis. Furthermore, there is negligible degradation of the cellulose by supercritical acetone. Even after 50% conversion, the cellulose residue has the same crystallinity as the starting cellulose, indicating the mild conditions of the reaction. Thus, the use of supercritical acetone as a reaction medium in the thermal degradation of cellulose results in an appreciable amount of extraction, less carbon formation, and better yield at temperatures lower than those used for conventional pyrolysis.

In a similar application, Koll and co-workers (KOll et al., 1983) degrade “Birch wood” with different organic solvents a t supercritical conditions (see Table 111) in a high-pressure/high-temperature flow reactor. The objective of this study is to disintegrate the wood into hemicellulose, cellulose, and lignin. All the organic solvents .shown in Table I11 dissolve 20-40% of the wood within 1 h. Degradation beyond 40% proceeds at a lower rate, with total degradation of the wood requiring temperatures higher than those in Table 111.

In this study, the carbohydrates and the lignin are de- graded at different rates depending on which SCF solvent is used (see Table I11 where Q is the ratio of the rate of lignin degradation to that of carbohydrate degradation). Alcohols seem to show higher selectivity for delignification, while esters, ethers, and alkanes preferentially attack the carbohydrates. The degradation rates of carbohydrates and lignin increase with temperature up to 553 K while maintaining the selectivity ratio Q approximately constant as shown in Figure 8. Beyond 553 K, there is an increase


Table 111. Degradation of Birch Wood with Supercritical Fluids (Sample Weight 3 g, Reaction Time 1.0 h, Pressure 98.6 atm, Solvent Feed Rate 1 mL/minL Q Gives the Ratio of Lignin-to-Carbohydrate Degradation (Koll e t al., 1983)

~~~

wt loss residual wt loss wt loss solvent T,, K Pc, atm temp, K wood, % lignin, % carbohydrate, % lignin, % Q

ester 467.6 35.5 523 20.77 28.02 29.99 0 n-pentane 470.2 33.0 523 24.29 34.26 38.90 0 2-propanol 508.0 53.0 523 24.62 17.20 23.39 30.06 1.3 acetone 508.0 47.0 523 22.77 19.55 23.72 18.57 0.8 methanol 513.0 78.7 523 25.30 13.04 20.25 47.47 2.3 ethyl acetate 523.0 37.8 543 36.76 26.55 42.98 9.44 0.2 2-butanol 538.0 48.0 543 31.00 16.52 29.29 38.52 1.3 1-propanol 536.7 49.9 543 32.45 11.88 26.92 56.72 2.1 2-methyl-1-propanol 553

0 -- 503 523 543 5 6 3

T ( K )

Figure 8. Effect of reaction temperature on birch wood degradation with 2-propanal (sample weight 3 g, solvent feed rate 3 mL/min, reaction time 1 h at -100 atm). Curve A lignin; B: carbohydrates; C: ratio (A/B) X 10 (Koll and Metzger, 1978).

in the pyrolitic decomposition of the carbohydrates, resulting in the formation of nonhydrolyzable condensation products which are also analyzed as lignin. This explains the drop in the degradation of lignin and the corresponding decrease in the selectivity ratio Q. Thus, SCF organic solvents are suitable for biomass degradation in a temperature range of 513 to 613 K.

During the extraction of biomass with supercritical acetone, Koll and Metzger (Koll and Metzger, 1978) also observe condensation products of acetone (diacetone alcohol and mesityl oxide) which are apparently formed by a thermally induced aldol condensation. The results of these studies prompted Metzger et al. (Metzger et al., 1983) to systematically investigate thermal intermolecular reactions at relatively high temperatures (-773 K) and high pressures ( - 500 atm) using a flow reactor with residence times of 1-10 min (KO11 and Metzger, 1978).

At these conditions, alkanes are added to alkenes (e.g., n-alkenes, acrylonitrile, methyl acrylate, methyl vinyl ketone), to 1,3-dienes (e.g., 1,3-cyclohexadiene), and to alkynes (acetylene). Thus, functional groups are added to hydrocarbons at SCF conditions. Typical alkanelalkene feed ratios range from 20 to 100. When cyclohexane (T, = 554 K, P, = 40.4 atm) is the representative alkane and methyl acrylate the representative alkene, the yield at 723 K based on the alkene increases significantly with pressure up to 100 atm and thereafter remains essentially constant. The increase in yield is explained by the increase of density of the reactants with pressure, thus enhancing the rate of the intermolecular reaction.

Other reactions which are studied by these authors in a similar manner include the thermal dimerization of methyl acrylate and the reaction of benzene with certain alkynes in a Diels-Alder reaction.

Supercritical extraction in coal processing may also be classified under this scheme. However, this subject is

39.61 12.66 35.26 58.78 1.7

r e o l i u o t i o n 5 2 3 2 K , 4 9 3 3 o lm

Y

5 10 deoc t i vo t i on 5232 K, 14 S otm 0 0 0

0 3 6 9 1 2 1 5 t ( h o u r s ) --

Figure 9. Conversion-time plot for the isomerization of 1-hexene in the presence of 2-chlorohexane (molar ratio 500:l) on A1203 catalyst. Catalyst deactivation is by side-reaction products; reactivation is done at supercritical (gas-phase) reaction conditions (Tiltscher et al., 1981).

reviewed adequately elsewhere (Williams, 1981) and, hence, is not included in this paper.

3.2. Examples of Reaction Scheme B: Heteroge- neous Catalytic Reactions at SCF Conditions. Tiltscher and co-workers (Tiltscher et al., 1981) report a novel approach for maintaining or reactivating the activity of a heterogeneous catalyst a t supercritical reaction conditions. They demonstrate their technique by using a high-pressure differential recycle reactor to study the catalytic isomerization of 1-hexene (T, = 504 K, P, = 30.7 atm) on 7-Al,03 catalyst with 2-chlorohexane as a co- catalyst. The isomers that are detected in the product solution are 1-hexene, cis-2-hexene, trans-2-hexene, and trans-3-hexene. The SCF reactivation method is demon- strated for three different modes of deactivation.

In one instance, the reaction is carried out in a gaseous reaction phase (T = 523.2 K, P = 14.8 atm). The resulting conversion vs. time curve is characteristic of the case where a deactivation process occurs in parallel with the reaction (Figure 9). The deactivation is caused by side reactions which produce low-volatile oligomeric compounds (Cl2-C30)s) which accumulate on the catalyst surface and eventually cause coking. These oligomers are desorbed by isothermally increasing the pressure to 493.3 atm, which is well above the critical pressure of the reactant mixture. Hence, the conversion level obtained before coking is restored. Tiltacher et d. (Tiltscher et d., 1984) observe that if high pressure is maintained, there is an approximately 2-fold increase in the overall conversion and about a 30% increase in the cisltrans-2-hexene ratio. The catalyst activity is maintained at precoking levels even after 12 h of reaction time as noted from the upper curve of Figure 9. The authors of this work do not clarify whether the 2-fold increase in the conversion is due to the rate enhancement that can occur as a result of operating in the mixture critical region.

It is interesting to note that while an isothermal increase in the pressure of a gaseous reaction phase usually favors a sorption process (and hinders desorption of low-volatile compounds) in heterogeneous catalytic reactions, in-


z 15 0

0 15 30 45 60 75

t [ h o u r s ) __c

Figure 10. Conversion-time plot for the isomerization of 1-hexene in the presence of 2-chlorohexane (molar ratio 500:l) on A1203 catalyst. Catalyst deactivated by fouling with MoSz under liquid-phase conditions; reactivation is done at supercritical conditions (Tilta6her et al., 1981).

5 0 I I I

. _. 4 . 1 '

0 0 6 12 18 24 30

t ( h o u r s ) - Figure 11. Conversion-time plot for the isomerization of 1-hexene in the presence of 2-chlorohexane (molar ratio 500:l) on A120a catalyst. Catalyst deactivated by poisoning of acidic sites with pyridine under liquid-phase conditions; reactivation is done at supercritical conditions (Tiltscher et al. 1981).

creasing the pressure to maintain the reaction phase in the mixture critical region results in the opposite effect, Based on their kinetic experiments, the authors also conclude that higher pressures are needed to enhance reactivation rates as the voltatility of the oligomeric coking compounds decrease. This is not a surprising result since it is well-known from polymer-SCF phase behavior studies, described in a later section of this paper, that as the molecular weight of the polymer increases, higher pressures are needed to solubilize the polymer. Finally, Tiltscher and co-workers note that reactivation can occur a t more modest pressures of 50-150 atm.

In another deactivation mode, a small amount of a finely dispersed catalyst fouling substance (MoS,) is introduced into the reactor under liquid-phase reaction conditions (T = 493.2 K, P = 493.3 atm). The conversion-time curve is once again characteristic of a reaction accompanied by catalyst deactivation (Figure 10). However, by isobaridy raising the temperature to 513.2 K, the reaction mixture is brought into the mixture critical region, and eventually the catalyst activity is restored. Although not stated ex- plicitly by the authors, presumably the trace amounts of MoSz are solubilized by the supercritical reaction mixture. Thus, catalyst fouling by low volatile impurities can be effectively controlled by operation in the single-phase mixture critical region.

In the final deactivation mode reported by Tiltscher et al. (Tiltscher et al., 1981), the active acidic sites of the catalyst are poisoned by continuous addition of a very dilute solution of pyridine in 1-hexene over a period of 12 h. The conversion decreases rapidly with time because of catalyst deactivation (Figure 11). The authors find that the catalyst can be reactivated by treatment a t supercritical conditions (T = 523.2 K, P = 493.3 atm). The catalyst poison is precipitated from the product solution as pyridinium chloride. Although nonpolar, hydrocarbon, supercritical fluid solvents are not expected normally to solubilize inorganic or organic salts, in this instance, it may

v)

0 o 3 0 -

'" - I I L1

I IO 100 1000 > 10

PRESSURE ( a t m 1 Figure 12. Variation of the viscosity of pure C02 as a function of pressure at various temperatures (Paulaitis et al., 1983).

I I 1 C I S

Omin 3 m i n 6 m i n

I R R A D I A T I O N T I M E

Figure 13. Transient behavior (prior to photostationary state) of the &/trans isomer ratio in stilbene radiation in supercritical COz (Squires et al., 1983).

be only necessary to solubilize parts per million of these salts to reactivate the catalyst. In our opinion, however, it is surprising that supercritical hexene can overcome the acid-base interactions that are occurring on the catalyst surface and, hence, remove the pyridinium chloride from the system.

The use of highly compressed supercritical reaction media for reactivating heterogeneous catalysts thus offers considerable advantages over conventional reactivation procedures that are currently in use. The SCF reactivation scheme obviates the need for a separate catalyst regen- eration unit since catalyst reactivation can be performed in situ a t relatively mild SCF conditions.

3.3. Example of Reaction Scheme C: Study of Viscosity Effects on Reaction Behavior Using an SCF Solvent Medium. Solvent viscosity can have an effect on the product distribution of certain reactions (Saltiel and Charlton, 1980). Squires and co-workers (Squires et al., 1983) describe an experimental approach to study the effect of solvent viscosity on the photochemistry of stilbene using supercritical COz as the solvent medium. Wide ranges in viscosities can be attained with small changes in pressure when operating very near the critical point of C02 (T, = 304.2 K, P, = 72.8 atm) (see Figure 12). The objective of their study is to identify the photostationary state (i.e., the cis/trans ratio of stilbene) achieved by irradiation of trans-stilbene in supercritical C02 at 313 K and 136.1 atm.

This reaction is performed in both the batch and flow modes. In the flow mode of operation, trans-stilbene is coated onto glass beads which are then packed into a feeder tube. Supercritical COz flows through the feeder tube and solubilizes some of the trans-stilbene. The COz-stilbene phase then flows through a quartz photo- reactor which is continuously irradiated with UV light. The rate of the photoisomerization reaction is followed by sampling the reactants a t regular intervals of time using an on-line gas chromatograph. At a given pressure and temperature (and hence a fixed solvent viscosity), the authors find that the photostationary state is attained in


less than 10 min. The change of isomer distribution with time during stilbene irradiation is shown in Figure 13. In a follow-up study, Aida and Squires (Aida and Squires, 1985) show how the &/trans ratio of stilbene changes dramatically as the pressure, and therefore viscosity, of the carbon dioxide is changed near its critical point.

In addition to being a suitable medium for studying solvent effects in organic chemical reactions, such as the photoisomerization of stilbene, an SCF can also be used as a solvent medium for subsequently separating the isomers formed during the reaction, as explained in the next section.

3.4. Examples of Reaction Scheme D: SCF Reac- tion/Separation Schemes. Kramer and Leder (Kramer and Leder, 1975) describe an SCF reaction scheme for isomerizing paraffinic hydrocarbons (4-12 carbon atoms). The isomerization reaction is catalyzed by using a Lewis acid catalyst (e.g., AlBr3, AlC13, and BFJ and is performed in an SCF medium selected from COP plus HBr or HC1 at temperatures above the critical temperature of the reaction mixture (up to 463 K) and at pressures such that the density of the reaction mixture is at least one-tenth of the density of the saturated solvent a t 293 K (P = 68.0-340.1 atm). The Lewis acid catalyst used in this instance is typically promoted with substances like water, HBr, or HC1.

There are several advantages with the SCF reaction scheme proposed by Kramer and Leder as compared to the conventional isomerization process a t subcritical conditions. The most striking advantage is that the catalyst can be dissolved in the supercritical fluid mixture and, therefore, the reaction can be carried out in a single homogeneous phase. Thus, better contact is promoted between the catalyst and the reactants. Further, small amounts of hydrogen can be easily dissolved into the SCF reaction phase as compared to a liquid reaction phase. This H2 addition facilitates isomerization as opposed to cracking reactions. It is interesting to note that a t SCF reaction conditions, the conversion level is essentially the same as the conversion level obtained when running the reaction in a heterogeneous liquid reactant-solid catalyst mixture. Another important advantage of the SCF reaction scheme is that the reaction products and the catalyst are subsequently separated from the SCF phase by carefully adjusting the pressure and/or temperature.

Modell (Modell, 1982) describes efficient processing methods for the oxidation of organic materials in supercritical water (Tc = 647.2 K, Pc = 217.6 atm). According to this patent, the reaction is performed in a single fluid phase at supercritical conditions. The reactor is well-in- sulated to minimize the loss of heat that is generated during the reaction. The heat of reaction is thereby efficiently transferred to the reactor effluent stream. The organics in the feed range from 2 to 25% based on the weight of the water. Oxygen is added in the form of pure oxygen or air in at least a stoichiometric amount required for total oxidation of the organic material.

An important advantage of the above SCF reaction scheme over conventional processing is that almost total oxidation of the organics can be realized with higher reaction rates since oxygen and nitrogen are completely miscible with supercritical water in all proportions and, hence, stoichiometric amounts of oxygen can be added for total oxidation of the organics. As compared to a two- phase liquid-vapor reaction system, a single, SCF-phase reaction scheme eliminates the need for mechanical mixing and thus simplifies the construction of the reactor although specialty materials of construction are probably needed.

H Z O t

Ethylene Oxide (EO)

CO2 +EO 1 Rich Phase

11 b

Y

HZO-Rich Phase

Ethylene Carbonate

+ Catalyst

T c a t a l y s t Carbonation 'T' Glycols. Catalyst,

Compressor

Recycle Catalyst

Polyethylene Glycols

Figure 14. Schematic diagram of an SCF reaction process for producing ethylene glycol (Bhise, 1983).

The above process also exploits the very low solubilities of inorganic salts in supercritical water (1 ppb to 100 ppm in the temperature range of 723-773 K). Therefore, if the reactor effluent stream is maintained between 723 and 773 K, inorganic salts in this stream can be easily precipitated and readily removed. Traces of sulfur in the feed can be oxidized to sulfate which precipitates from the reaction mixture. Also, the outlet water from the reactor is free of inorganic salts, thus eliminating the need for purifying feedwater from sources such as brine and seawater. Ef- ficient heat removal is another advantage of Modell's SCF oxidation scheme. The heat liberated during oxidation of the organics is directly recovered in the form of a super- heated, supercritical stream without the need for heat- transfer equipment.

A number of industrial applications, based on the SCF oxidation scheme, is cited by Modell. For example, the organic material can be a waste and/or toxic material of low calorific value which is merely oxidized and converted to an environmentally acceptable gas stream. On the other hand, the organic material can be useful as a fuel and can therefore be oxidized to recover useful energy for heating or to obtain a supercritical mixture of water and carbon dioxide for use as process water in power cycles. Another application involves a continuous desalination process where seawater or brine is used as the feedwater. The salt precipitates from the single fluid phase immediately after the reaction (i.e., after the temperature has reached 723-773 K), thus desalinating the feedwater.

Bhise (Bhise, 1983) describes a multistep process for the production of ethylene glycol from ethylene oxide in which near-critical or supercritical C02 is first used as a solvent and then used as a reactant. Ethylene oxide is normally produced by the vapor-phase oxidation of ethylene with molecular oxygen over a supported Ag catalyst. In conventional processing, the effluent containing the ethylene oxide is then scrubbed with water. The aqueous solution, which contains 10 mol % ethylene oxide, is further purified and the ethylene oxide recovered for the hydrolysis to ethylene glycol.

Shown in Figure 14 is a schematic diagram for an SCF reaction/separation process proposed by Bhise (Bhise, 1983). He shows that an ethylene oxide-rich COz phase


rates observed when operating in supercritical water are attributed to the solubility of very small amounts of inorganic salts in supercritical water. The phosphoric acid and salts of the phosphoric acid are dissolved in the supercritical steam and are split into ions. Patat lists several dissolution constants for primary ammonium phosphates in the supercritical steam. In this instance, the reaction performance is improved by operating the reaction homogeneously in the mixture critical region and, thus, promoting intimate contact between the reactants and the catalyst.

Blyumberg and co-workers (Blyumberg et al., 1965) describe the oxidation of n-butane a t temperatures and pressures close to the critical point of butane (T, = 425.1 K, P, = 37.5 atm) in a batch reactor. The oxidations are performed in both the SCF phase (T = 445.8 K, P = 146.3 atm) and the liquid phase (T = 420.8 K, P = 52.1 atm). This oxidation reaction involves two major steps. The first step is the formation of butane hydroperoxide via a free- radical chain mechanism. The second step is the break- down of the hydroperoxide into the products.

Sharp differences have been found in the rate of formation of the hydroperoxide and the product distribution in the SCF-phase and the liquid-phase oxidations. The liquid-phase oxidation is characterized by a long induction period for hydroperoxide formation, while the SCF-phase oxidation has much shorter induction times. The liquid- phase oxidation products are predominantly acetic acid and methyl ethyl ketone, while the SCF-phase oxidation products are formaldehyde, acetaldehyde, methyl, ethyl, and propyl alcohols, and formic acid. The reason for this difference in product spectrum has not been fully explained by the authors.

The increased reaction rates in the SCF phase may be associated with the more efficient production of free-radical pairs. The formation of free radicals is described in the following reaction in which molecule AB dissociates to form a geminate radical pair (A.B.) which may either diffuse apart to form a free-radical pair or may recombine before it can diffuse apart (so-called “cage-effect”) (Eckert, 1972).

AB FF (A-B-) - A- + B.

In the mixture critical region, it is expected that the resistance to diffusion will be lower than that in the liquid phase and, therefore, the (A.B.) radical pair should more readily diffuse apart in the critical region. Although increasing pressure favors the recombination of (A.B.) to form AB, it seems reasonable to assume that the rate of diffusion dominates the pressure effect as long as the system pressure is maintained below approximately lo00 atm. Hence, the formation of free radicals should be facilitated in the SCF phase as compared to the liquid phase and, therefore, shorter reaction times are to be expected.

The broader product spectrum obtained while operating in the SCF phase as compared to the liquid phase may be explained by the types of free radicals that are formed. In the SCF phase, the butane-derived free radicals have a higher probability of further decomposing into methyl radicals rather than terminating the reaction by recom- bining. The methyl radicals can undergo further oxidation, thus giving rise to a broad spectrum of products (Winkler and Hearne, 1961).

Baumgartner (Baumgartner, 1983) describes a process for enhancing tert-butyl hydroperoxide (TBHP) formation by reacting isobutane (T, = 415 K, P, = 37.0 atm) with oxygen in a dense-phase reaction mixture. In earlier studies, Winkler et al. (Winkler and Hearne, 1961; Winkler, 1958) show that the catalytic oxidation of isobutane in the

is obtained when the aqueous solution is contacted with near-critical or supercritical C02 at temperatures up to 373 K and pressures ranging to 300 atm. The ethylene oxide-C02 phase is then separated from the aqueous phase and subsequently contacted with a carbonation catalyst (e.g., organic quaternary ammonium halides) and reacted to form a catalyst-ethylene carbonateC02 stream (T = 293-363 K, P = 3 to 100 atm, catalyst/ethylene oxide = 0.014.15, residence time = 1-5 h). The catalyst-ethylene carbonateC02 stream is then delivered to another reactor and contacted with water to form ethylene glycol and COP (T = 363-573 K, P = 63 atm, water/ethylene carbonate = 1.1-2.5, and residence time = 0.5-4 h). The catalyst for the carbonation reaction also catalyzes the hydrolysis reaction. The C02 is flashed from the ethylene glycol stream and subsequently recycled. The ethylene glycol and the catalyst are then recovered.

The author claims that the ethylene carbonate is more efficiently hydrolyzed to mono(ethy1ene glycol) using the SCF reaction/separation scheme as compared to directly hydrolyzing ethylene oxide. Direct hydrolysis of the ethylene oxide tends to yield more of the higher glycols such as bis(ethy1ene glycol) as compared to the SCF reaction/separation scheme. Thus, in this case, the supercritical fluid medium not only acts as an extractant to remove the ethylene oxide from the aqueous stream but it also acts as a reactant which enhances selective ethylene oxide conversion to mono(ethy1ene glycol).

Biochemical reactions in supercritical fluids are an area which is not covered in depth in this review paper. The reason is that with a few noted exceptions, this area has yet to be pursued to any great extent. However, recently, two papers have appeared that describe enzymatic reactions in SCF’s (Randolph et al., 1985; Hammond et al., 1985). Randolph et al. describe a batch alkaline phos- phatase-catalyzed hydrolysis of p-nitrophenyl phosphate in supercritical carbon dioxide. They allow the reaction to procede for 4-24 h. In contrast, Hammond et al. describe a continuous-flow poly(pheno1 oxidase)-catalyzed oxidation of p-cresol in supercritical carbon dioxide and supercritical fluoroform. In this study, the reaction occurs in less than 30 min. Both of these SCF-reaction studies open new areas of research in biotechnology.

3.5. Examples of Reaction Scheme E: Enhanced Reaction Rates and Selectivities at SCF Reaction Conditions. The earliest reported study of using an SCF reaction scheme is attributed to Patat (Patat, 1945). Patat investigated the hydrolysis of aniline to phenol in a water-based acidic solution in near-critical and supercritical water (T, = 647.2 K, P, = 217.6 atm). Phosphoric acids and its salts are used as the catalyst for this reaction. The reaction proceeds extremely slowly under normal conditions and reaches equilibrium at low conversion levels. Raising the temperature of water and correspondingly increasing the pressure above the critical pressure of water increases the reaction rate by an order of magnitude. The conversion times, however, are still on the order of hours and the reaction rate is kinetic-controlled. For these reasons, the author chooses to study the reaction mechanism in the supercritical region to temperatures of 723 K and to pressures of 700 atm in a flow reactor. The hydrolysis reaction follows known, regular kinetics in the entire temperature and pressure space studied, and the activation energy of the hydrolysis (approximately 40 kcal/gmol) is found to be the same in the supercritical as well as in the subcritical region. The reaction is catalyzed by hydrogen ions formed from phosphoric acid dissolution in the supercritical steam. Hence, the enhanced reaction


vapor phase produces significant amounts of tert-butyl alcohol and minor amounts of other oxidation products such as acids, aldehydes, ketones, and other alcohols, in addition to the desired TBHP. Further, Winkler et al. demonstrate that reacting isobutane with molecular oxygen noncatalytically in the liquid phase of a two-phase liquid-vapor mixture at 373-423 K and at 28.2 atm produces reaction products consisting of TBHP and tertiary alcohol. However, this method suffers from very low reaction rates and a low selectivity for TBHP. The occurrence of a broader spectrum of produds in the vapor-phase oxidation of isobutane as compared to its liquid-phase oxidation is consistent with the observations of Blyumberg and co- workers in the case of the oxidation of n-butane (Blyum- berg et al., 1965).

In the work of Baumgartner, the isobutane oxidation is performed at a temperature and pressure which are significantly higher than the T, and P, of isobutane and also above the critical pressure of the reaction mixture ( T = 425 K, P = 68.0 atm). The reaction mixture is a single, dense, quasi-liquid phase. To attain enhanced TBHP selectivities, the authors show that reactor operating variables must be carefully optimized and controlled. Typical operating variables consist of a partial density of isobutane between 304.4 and 400.5 g/L, a feed molar ratio of isobutane to oxygen between 8 and 12, and a residence time between 15 and 80 min. The overall conversion levels at these operating variables vary between 5 and 20%, while the conversion level must not exceed 20% to maintain a high selectivity for TBHP. The authors also claim that in addition to enhanced selectivity, the rate of TBHP formation is significantly higher in the present case of homogeneous, dense-phase oxidation than the corresponding rate obtained in liquid-phase oxidation (Baum- gartner, 1983).

Within the last 2 decades, Ehrlich and co-workers have put together a comprehensive study on the free-radical polymerization of ethylene in supercritical fluid ethylene (T, = 9.7 OC, P, = 50.5 atm) (Ehrlich and Mortimer, 1970; Takahashi and Ehrlich, 1982; Ehrlich and Pittilo, 1960; Ehrlich and Kurpen, 1963; Ehrlich, 1965). This work is especially interesting because it combines the unique solubility characteristics of SCF solvents with the unusual solution properties of mixtures in the critical region to explain the industrially important high-pressure ethylene polymerization reaction.

Early chromatography work showed that polymers, especially polystyrene, can be fractionated when a supercritical fluid mobile phase is used (Jentoft and Gouw, 1970; Jentoft and GOUW, 1969). Ehrlich and Graham (Ehrlich and Graham, 1960) also found that polyethylene is soluble in supercritical propane when the pressure is increased above 500 atm. Knowledge of this solubility behavior is extremely important since it has a direct bearing on the interpretation of reaction rate data. Ehrlich has done a comprehensive study on the phase behavior of poly- ethyleneethylene mixtures a t high pressures. The results from his phase behavior studies are shown in Figure 15 as a schematic P-T-x diagram for the polyethylene-ethylene system. Notice that the two-phase liquid-vapor region for polymer-solvent systems persists to very high pressures. Ehrlich and Mortimer (Ehrlich and Mortimer, 1970) conclude that some of the kinetic studies reported in the literature on ethylene polymerization are often of little value since the authors of the studies are unaware of the phase behavior involved with SCF systems at moderate- to-high pressures. Many times, polyethylene polymerization studies reported in the literature are performed on

Figure 15. Schematic pressur&emperature-composition behavior for a polymer-solvent binary mixture (Ehrlich and Mortimer, 1970).

thermodynamically underdetermined systems due to the number of phases present and the number of constituents in the system.

The solution behavior of SCF mixtures is also used by Ehrlich to explain and interpret kinetic data. As mentioned earlier, very large, negative, partial-molar volumes of the polymer (and monomer) can occur at temperatures slighly above the critical temperature of the SCF solvent (see Figure 5). Abraham and Ehrlich (Abraham and Ehrlich, 1975) are able to calculate values for the partial-molar volume of a polymer in an SCF using the equation of state of Flory (Flory, 1970). Using transition-state analysis, Ehrlich shows how the effect of pressure on the polymerization rate constant is directly proportional to the negative of the partial-molar volume of the polymer dissolved in the SCF solvent (Ehrlich, 1971). Citing the unusual partial-molar volume behavior exhibited by heavy solutes dissolved in an SCF, he attempts to explain the large pressure effect on the polymerization rate as the pressure is dropped toward the mixture critical value, the small pressure effect on the polymerization rate when crystalline polymer is formed, and the very high anomalous activation energy for polymerization at a temperature of 373 to 403 K (the so-called critical polymerization boundary). I t should be noted, however, that the anomalous activation energy for ethylene polymerization in supercritical ethylene may be attributed to the presence of dissolved oxygen which can be both a free-radical in- hibitor in the subcritical liquid-gas region and a free- radical initiator in the supercritical region (Takahashi and Ehrlich, 1982; Ehrlich and Pittilo, 1960).

Krase and Lawrence (Krase and Lawrence, 1966) also describe an SCF reaction process for making ethylene polymers. They recommend reacting ethylene in the presence of a catalyst at temperatures between 4.0 and 400 OC and at pressures from 800 to 4000 atm. The polymer is then recovered by using a stepwise reduction in pressure with the objective of reducing compression costs. They note in this very early patent that appreciable quantities of the reaction products are still solubilized in the SCF phase at pressures as low as 150 atm. This solubility behavior suggests that the product polymer can be precipitated from solution essentially free of lower molecular weight oligomers, residual monomer, and perhaps some of the catalyst.

In another patent entitled “Supercritical Polymeriza- tion,’’ Cottle (Cottle, 1966) describes a process for reacting and separating polymers made from olefins. In particular, he proposes to react propylene to polypropylene using a catalyst and operating at conditions above the critical temperature and pressure for propylene (T, = 91.9 “C, P,


analysis of the different phases in the system. This ana- lytical problem is unique to each reaction system, depending upon the reactor configuration, the number of phases present, and the physical properties of the reactants and products. The research tools used to study reactions in gas and liquid phases should be applied to supercritical fluid-phase reactions. For example, Johnston and Kim (Johnston and Kim, 1985) and Hyatt (Hyatt, 1984) described how UV/visible absorption spectra of solvato- chromic dyes can be used to determine the polarity of an SCF solvent and, therefore, provide some measure of the solvent effect on reaction rates. Other such spectroscopic techniques may also provide useful information on the nature of SCF solvents and reaction media. Nomenclature D diffusion coefficient, cm k second-order rate constant P pressure, atm R gas constant, cal molm1 K-l T temperature, K u molar volume, cm3 mol-' Greek Symbols 4 viscosity, g cm-l s-l P density, g/cm3 Subscripts C critical point

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= 45.0 atm). After sufficient reaction time, the pressure is reduced to precipitate the crystalline polypropylene from solution while leaving the noncrystalline fraction solubilized in the propylene-rich phase. In this process, the SCF is used as both a reactant and a solvent similar to the ethylene polymerization processes previously described. Also, the example listed in this patent implies that the catalyst is soluble in the SCF phase, thus allowing for all the advantages of homogeneous reaction conditions.

Conclusion It is clear from the several examples cited in this review

that supercritical fluids can be advantageously used as reaction media. It may be possible to run a reaction in a single, homogeneous critical mixture phase, thus eliminating interphase mass-transfer limitations, and labile reaction products may be more readily isolated from the reaction mixture by adjusting the pressure and/or temperature to induce a phase split, thus avoding unwanted side reactions. To a lesser degree, reaction rates may be advantageously enhanced by running the reaction in the dilute mixture region at conditions close to the critical point of the pure SCF.

The field of reactions in SCF's has, as yet, not been fully investigated. A number of fundamental as well as practical questions need to be addressed: How does near-critical or supercritical conditions affect the equilibrium rates and paths of chemical reactions? What is the environment around a reactant species when dissolved in an SCF vis- a-vis a liquid solvent? How is the phase behavior of the initial reactants affected by product formation? Is it possible to exploit SCF reaction conditions to devise efficient reaction/separation schemes? How does critical and near-critical operation affect the adsorption, diffusion, reaction, and desorption steps involved in heterogeneous fluid-solid catalysis? What is the underlying mechanism of the catalyst reactivation phenomena that has been observed near the mixture critical region in the case of certain heterogeneous fluid-solid catalytic reactions? These questions pose considerable experimental and theoretical challenges.

Since reactions in supercritical fluid medium occur a t high pressures, it is very important to first determine the phase behavior of the reacting system. Many of the studies reviewed in this paper fail to include the phase behavior of the reacting mixture. Since multiple phases can occur in the mixture critical region, reaction studies need to be complemented with phase-behavior studies to gain an understanding of the fundamentals of the thermodynamics and kinetics of chemical reactions in solution. A simple, cubic equation of state (EOS) can be used to extend and complement the phase behavior studies. The location of phase border curves in P-T space can be determined by using an equation of state. However, the EOS approach should only be used as a guide to pinpoint P-T regions of interest. Also, an EOS can be used with transition-state theory to correlate the pressure dependence of the reaction rate constant when the prressure effect is large (Le., at relatively high pressures).

The experimental apparatus employed in the various high pressure phase equilibrium and high-pressure reaction studies cited in this review is primarily conventional, batch equipment. Tiltscher et al. (Tiltscher et al., 1979) describe a continuous differential recycle reactor suitable for in- vestigations of heterogeneous fluid-solid systems at kiigh pressures and temperatures. We believe that all experiments must have provision for visual observation of the number of phases present before, during, and after reaction. The experimental challenge of high pressure phase and reaction studies lies in the accurate sampling and

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Received for review May 6 , 1985 Revised manuscript received September 30, 1985

Accepted October 13, 1985

ARTICLES

Simultaneous Pyrolysis of Ethane-Propane Mixture in Pulsed Microreactor System

Zou Renjun”

Hebei Academy of Sciences, Sh#azhuang, China and Hebei Institute of Technomy, Tianjin, China

Lou Olangkun, Zhang Blngchang, CUI Hongwu, Guo Zhushan, and Song Xlaorul

Hebei Institute of Technomy, Tlanjin, China

This paper concerns the simultaneous pyrolysis of an ethane-propane mixture in a pulsed microreactor system designed and assembled personally at the fobwing reaction condition ranges: temperature, 759-925 OC; residence time, 0.038-1.1 s; fractional composition, 0-1 .O mole fraction. The ethylene peak yield showed a maximum, up to 57.39 mol % at 880 OC, 0.216 s, in the pyrolysis of a mixture composed of N2:C2H6:C3H8 = 0.4975:0.3077:0.1948. The synergistic effect of components of mixed feedstocks has been studied and the deviations of conversion and selectivity from the pure additivity behavior have been correlated and observed. The opposite views of different authors in the chemical literature are then discussed and explained. There are positive deviations of the overall selectivity of ethylene as well as both real and overall selectivities of propylene in simultaneous pyrolysis from these in individual pyrolysis. The results of this paper are of academic and economic significance to the utilization of oil field gas and natural gas resources as well as recycling ethane from ethylene plants using naphtha or AGO as feedstock.

Oil field gas and wet natural gas are composed of light hydrocarbons. These light hydrocarbons, especially ethane and propane, act as excellent feedstocks for olefin plants in petrochemical industry and are receiving worldwide interest.

For the sake of economizing on ethane and propane, enhancing ethylene and propylene, saving energy, and making greatest profit, several questions have to be re- solved: Which alternative do they prefer, simultaneous pyrolysis (the so called “co-cracking” appearing in some papers) of unseparated ethane-propane mixture or individual pyrolysis of separated ethane and propane? How do the variations in composition of ethane-propane mix-

* In accordance with the authors’ wishes, their family names are listed first.

0196-4305/86/1125-0012$01.50/0

ture and in operating condition of pyrolysis affect the process economics? What is the optimum operating condition for saving ethane and propane and enhancing ethylene and propylene?

These questions have to be answered not only at the beginning of an olefin plant project based on ethane and propane feedstock but also for existing ethylene plants using naphtha or AGO feedstock, because they have the problem concerning recyclic ethane and propane returning to ethane cracker and propane cracker. In present paper, the simultaneous pyrolysis of ethane-propane mixtures is studied and compared with the individual pyrolysis of ethane and propane, and there are naturally the answers to above-mentioned questions. Experimental Section

shown in Figure 1. The flow diagram of pulsed microreactor system is

I t includes the following units.

0 1985 American Chemical Society

reactions in supercritical fluids - a review

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