Supercritical systems carbon dioxide

The same types of catalyst have been employed in 1-octene hydroformylation, but with the substrates and products being transported to and from the reaction zone dissolved in a supercritical fluid (carbon dioxide) [9], The activity of the catalyst is increased compared with liquid phase operation, probably because of the better mass transport properties of scC02 than of the liquid. This type of approach may well reduce heavies formation because of the low concentration of aldehyde in the system, but the heavies that do form are likely to be insoluble in scC02, so may precipitate on and foul the catalyst. The main problem with this process, however, is likely to be the use of high pressure, which is common to all processes where supercritical fluids are used (see Section 9.8). 

Experimental results are presented for high pressure phase equilibria in the binary systems carbon dioxide - acetone and carbon dioxide - ethanol and the ternary system carbon dioxide - acetone - water at 313 and 333 K and pressures between 20 and 150 bar. A high pressure optical cell with external recirculation and sampling of all phases was used for the experimental measurements. The ternary system exhibits an extensive three-phase equilibrium region with an upper and lower critical solution pressure at both temperatures. A modified cubic equation of a state with a non-quadratic mixing rule was successfully used to model the experimental data. The phase equilibrium behavior of the system is favorable for extraction of acetone from dilute aqueous solutions using supercritical carbon dioxide. 

The system carbon dioxide - acetone - water was investigated at 313 and 333 K. The system demonstrates several of the general characteristics of phase equilibrium behavior for ternary aqueous systems with a supercritical fluid. These include an extensive LLV region that appears at relatively low pressures. Carbon dioxide exhibits a high selectivity for acetone over water and can be used to extract acetone from dilute aqueous solutions. 

A pilot plant was built to study the influence of different process parameters on the particle size produced by RESS-process (Rapid Expansion of Supercritical Solutions). Particles smaller than 4 pm were obtained for the system carbon dioxide-anthracene. 

Supercritical fluids such as carbon dioxide can be used as solvents to extract organic compounds from aqueous solutions. In order to achieve recoveries of these products often in low concentration, cosolvents as methanol or other alcohols have been added to improve the solubility and the selectivity of the primary fluid. To optimize the extract recovery, the knowledge of phase equilibria of the ternary system carbon dioxide-methanol-water is required at different temperatures and pressures. 

Shariati A, Peters CJ. Measurement and modeling of the phase behavior of ternary systems of interest for the GAS Process I. The system carbon dioxide + 1-propanal + salicylic acid. J Supercrit Fluids 2002 25 195. 

The mechanics of increasing the pressure on the fluid can he done in a couple of ways. One would he to increase the temperature surrounding the hquid carbon dioxide [14]. As noted above, the pressme exerted on the hquid carbon dioxide in the cyhnder is a function of its ambient temperature. Only at higher ambient temperatures would the resultant system pressures be enough to reach the nearly hquid-hke densities of 0.1 to 1.0 g/mL that are required to use supercritical fluid carbon dioxide as an extraction solvent for extraction temperatures ranging from 40 C to 150°C. This approach is not widely used at this time. Liquid pumps (such as those used in liquid chromatographs) are more commonly used. 

Using mainly water non-misdble solvents several approaches are possible. In most cases, the organic solvent has to be saturated with water in order not to remove the boundary water surrounding the enzyme, which otherwise results in deactivation. In such microaqueous systems the pH of this tiny amount of water should be carefully chosen for optimal enzyme activity. The control of water activity can be achieved by addition of salts or utilization of saturated salt solutions I81, 821. The simplest way of using an enzyme in organic solvents is to suspend the insoluble enzyme in the required solvent. This technique was first reported in 1900 [83] and has been extended over the last few years to encompass many systems (mainly proteases and lipases) [75, 84, 85L Organic solvents may be replaced by supercritical liquid carbon dioxide, which exhibits similar properties to hexane[86, 146]. 

Phase Equilibria. From recent research (Schneider and Peters) it became apparent that in the near-critical region of certain ternary carbon dioxide mixtures, due to co-solvency effects of the two solutes relative to each other, the fluid multiphase behavior can be quite complex. Phenomena like immiscibility windows and holes are not unusual, which have their consequences for separations in near-critical processing. Peters stressed that for many applications in supercritical technology carbon dioxide is not an appropriate choice since for many solutes it is a poor solvent that would require the use of a cosolvents. If safety and environmental constraints permit, it is certainly worthwhile to consider alternatives for carbon dioxide. Gulari, Schneider and Peters emphasized the importance of studying representative model systems in order to obtain insight into the systematic variations of the complex phase behavior that may occur in near-critical multicomponent mixtures. Debenedetti stressed the importance of focusing on complex fluids like emulsions. 

Oil-in-oil emulsion systems display a relatively strong ER effect. Examples of such ER active emulsions are chlorinated paraffin/polydimethylsiloxane [11], castor oil/polydimethylsiloxane [13], urethane-modified polypropylene glycol/dimethylsiloxane [12] etc.. The ER effect in emulsions is attributed to the stretched droplets that Ibrm fibrillation chains along the direction of the electric field. This is a typical feature for any emulsion system in which the two liquids have a quite different dielectric constant and conductivity. Figure 17 shows the water droplet chains formed in a supercritical fluid carbon dioxide medium under a 60 Hz ac field of a very low field strength, Emax=IO V/mm [115]. A synergetic effect is observed in an system composing of polyanilines dispersed in a chlorinated paraffin/silicone oil emulsion [107],... 

Supercritical fluids Carbon dioxide forms a supercritical phase under relatively low temperatures and pressures and it therefore has been explored as a solvent for tbe Heck reaction. Following the reaction, the gas can be collected and recycled, allowing the design of true waste-free technology. The dielectric constant of supercritical CO2 is similar to pentane and therefore modified catalytic systems are often required to aid solubility.Unusually, one of the best catalytic precursors in supercritical CO2 is Pd(OCOF3)2, a strong electrophile and oxidant. Heck reactions in supercritical water have also been investigated. ... 

Phase Behavior. One of the pioneering works detailing the phase behavior of ternary systems of carbon dioxide was presented ia the early 1950s (12) and consists of a compendium of the solubiHties of over 260 compounds ia Hquid (21—26°C) carbon dioxide. This work contains 268 phase diagrams for ternary systems. Although the data reported are for Hquid CO2 at its vapor pressure, they yield a first approximation to solubiHties that may be encountered ia the supercritical region. Various additional sources of data are also available (1,4,7,13). 

The fugacity coefficient of thesolid solute dissolved in the fluid phase (0 ) has been obtained using cubic equations of state (52) and statistical mechanical perturbation theory (53). The enhancement factor, E, shown as the quantity ia brackets ia equation 2, is defined as the real solubiUty divided by the solubihty ia an ideal gas. The solubiUty ia an ideal gas is simply the vapor pressure of the sohd over the pressure. Enhancement factors of 10 are common for supercritical systems. Notable exceptions such as the squalane—carbon dioxide system may have enhancement factors greater than 10. Solubihty data can be reduced to a simple form by plotting the logarithm of the enhancement factor vs density, resulting ia a fairly linear relationship (52). 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

In principle, the sample transfer from the Supercritical state is relatively easily adaptable to other systems, due to the high volatility of the fluid at atmospheric pressure, particularly for carbon dioxide which is the most frequently used fluid. 

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

Details are given of the enzymatic transformation of polycaprolactone into repolymerisable oligomers in supercritical carbon dioxide. The object was to establish a sustainable chemical recycling system for polycaprolactone. 14 refs. 

Methylene chloride is probably the most generally used solvent for decaffeination processes, but others, some of which are already found in small amounts in coffee beans, are coming into use. For example, ethyl acetate,8 formaldehyde-dimethylacetal, ethanol, methanol, acetone,9 propane,10 benzyl alcohol,11 carbon dioxide,12 and supercritical carbon dioxide with an acid13 are used. Generally the pressure and temperature of the system are adjusted to keep the solvent in the liquid state. Coffee oil itself is even described for this use in one patent.14... 

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