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It has been observed that this promising method preferred a single phase between liquid substrate and carbon dioxide‐hydrogen system. The ascendancy of the supercritical carbon dioxide medium is established in comparison with the conventional organic solvent and solvent‐less conditions.
Table of contents

• See a Problem?
• Green chemistry using liquid and supercritical carbon dioxide
• Supercritical fluids
• Supercritical fluids: the superheroes of green chemistry

Introduction, J. Young, J. DeSimone, and W. Phase behavior and its effects on reactions in liquid and supercritical CO2, L. Blanchard et al. Advances in homogeneous, heterogeneous and biphasic metal catalyzed reactions in dense phase carbon dioxide, T. Ikariya et al. CO 2 as a reactant and solvent in catalysis, T. Ikariya and R. Noyori 4. Free radical chemistry in supercritical CO 2 , J. Tanko 5. Fluorous phases and compressed carbon dioxide as alternative solvents for chemical synthesis: a comparison, W.

Leitner 6. Enzyme chemistry in carbon dioxide, R. Rodney and A. Solubility of polymers in CO 2 , M. McHugh 8. Interfacial phenomena with CO 2 -soluble surfactants, K.

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However, larger numbers of fit parameters tend to decrease the predictive power of the model. A discussion of the benefits and drawbacks of various models can be found in several reviews Brennecke and Eckert, ; Johnston et ah, We have chosen the Peng-Robinson equation since it is known to give reasonably good representations of solubilities in sc C0 2 Brennecke and Eckert, ; Johnston et ah, , because it is relatively simple to use, and because it is readily available for use in industry.

The Peng-Robinson equation requires inputs of critical temperatures and pressures, acentric factors w and binary interaction parameters, ky. The pure component properties can be taken from the literature or estimated Reid et ah, ; Stradi et al. The completely reliable computational technique that we have developed is based on interval analysis.

Since the phase equilibrium problem i. However, thus far the method described here is the only general purpose method that can be applied to any equation-of- state model, i. Experimental Techniques A discussion of a variety of techniques to measure high-pressure phase equilibria can be found in a review by Dohrn and Brunner Here, we use two different apparatuses. It consists of a high-pressure glass tube, into which a liquid sample can be loaded.

The saturated solution is passed through a restrictor and the precipitated solute collected, usually in a liquid collection solvent that is analyzed by UV-visible spectrometry. Thus, these two apparatuses are used in a complementary fashion to obtain full information about the high-pressure phase behavior. The C0 2 was Coleman Instrument Grade with a minimum purity of The key to obtaining good representation of experimental data is fitting a single binary interaction parameter, ky, to each set of binary data.

Computational Techniques The phase equilibrium problem consists of two parts: the phase stability calculation and the phase split calculation. Unfortunately, this problem—which can be formulated as an optimization problem or the equivalent set of nonlinear equations — frequently has multiple minima and maxima. As a result, conventional phase equilibrium algorithms may fail to converge or may converge to the wrong solution.

It does not require any initial guesses and is guaranteed, both mathematically and computationally, to converge to the correct solution. The interval analysis method and its application to phase equilibria using equation-of-state Phase Behavior and Reactions in Liquid and Supercritical C0 2 9 models has been described elsewhere Hua et al. It is a general-purpose technique that can be applied to any equation-of-state or excess Gibbs free energy model, and it guarantees correct solution to the phase equilibrium problem.

Supercritical fluids

In addition to the interval method developed, we also used standard modeling tools from Aspen Plus Aspen Technology, Inc. At just a slightly higher pressure At a feed composition of 0. Based on the binary measurements and modeling, we estimated the multi- component high-pressure phase behavior for the reaction mixture, extending the entire way from all reactants to full conversion Stradi et al. However, if the reaction were run at bar the reactant mixture would be two phase, and at 50 bar the system would be two phase all the way from reactants to full conversion.

Thus, by maintaining the pressure above just bar, one would expect the reaction mixture to remain single phase throughout the reaction.

This pressure is significantly 10 Catalysis and Chemical Synthesis in Carbon Dioxide below the bar used originally for this reaction. Thus, by using modeling tools to interactively guide experimental work, an improved design was achieved that uses a much lower pressure than originally proposed.

Tumas and co-workers W. For this demonstration, we have not included the catalyst, A1C1 3 ; however, its impact on the mixture phase behavior will be discussed later. Naphthalene is a solid at room temperature [melting point of A detailed description of the modeling of this particular system with the Peng-Robinson equation can be found in Xu et ah Also shown in the figure is the modeling done with the Peng-Robinson equation of state. Using binary interaction parameters of 0.

Also shown on the graph are the predicted values of the vapor-phase compositions that would be in equilibrium with the liquid phases measured. The symbols indicate experimental measurements and the lines indicate the results of modeling with the Peng-Robinson equation of state.

Figure 1. However, for this system, we also measured the solubility of the l'-acetonaphthone in the C0 2 -rich vapor phase using the dynamic extraction apparatus at pressures up to bar.

Also shown in the figure is the Peng-Robinson equation-of-state modeling using a binary interaction parameter of 0. These values gave the best fit to the liquid-phase composition data; that is, we did not use the vapor-phase measurements in obtaining this value. Yet, the Peng- Robinson equation, with a ky determined from liquid-phase compositions, gives remarkably good estimates of the vapor-phase compositions, as shown in the figure. Moreover, equimolar mixtures of 1'- and 2'-acet- onaphthone are liquid even at room temperature. This was easily achieved using both the static and dynamic apparatuses.

For instance, 2'-acetonaphthone was melted to introduce it into the glass cell of the static apparatus.

Also shown are the solubilities of 2'-acetonaphthone in the C0 2 -rich vapor-phase. Based on these binary measurements and the ky values determined from them , we have performed phase equilibrium computations for the multi- component mixtures that would result in the course of this reaction system. The critical temperatures, pressures, and acentric factors for all of the components, as well as their binary interaction parameters with C0 2 , that were used in these calculations, are shown in Table 1.

A value of 0. Since both extremely high-pressure and very large moderate-pressure vessels would require significant capital investment, these options are not likely to be economically attractive. It should be noted that in the real reaction system, two-phase operation would probably be unavoidable. In this example, we have neglected to include the A1C1 3 catalyst. The A1C1 3 actually forms a complex with the acetyl chloride and is required to be present in greater than stoichiometric amounts. Preliminary investigations revealed that the complex is highly insoluble in C0 2 and, when a 1.

Conclusions Phase behavior is an extremely important issue in designing and evaluating processes that use C0 2 as a replacement solvent. Evaluation of systems that might exhibit complex high-pressure phase behavior is further frustrated by the inability of conventional flash algorithms and process modeling tools to reliably compute the phase behavior given a particular model. Here, we have presented a methodology to model and compute complex high-pressure phase behavior for reaction systems, taking a limited amount of binary experimental data.

Conversely, it appears that the Friedel-Crafts reaction would have to be performed under multiphase conditions, which is likely to make it a poor candidate for solvent substitution with C0 2. References Brennecke, J.