Supercritical carbon dioxide initiators

Kinetic profile of extraction of tin residues after ring-opening polymerisation in supercritical carbon dioxide initiated by dibutyltin dimethoxide... 

Cyclohexane oxidation has been studied in supercritical carbon dioxide medium for homogenizing the initial reaction mixture to produce cyclohexanone and cyclohex-anol as the chief reaction products. Conversions obtained are low compared to the liquid phase oxidation because of dilute concentrations of the reactants. Cyclohexanone is more selectively formed and favored by both pressure and temperature. Results suggest that the reaction in supercritical CO2 medium can be gready manipulated (Srinivas and Mukhopadhyay, 1994). 

A cylindrical extractor, 1-m long, is filled with crushed-vegetable-oil seeds. The oil is to be extracted with pumping supercritical carbon dioxide at a density of 500 kg/m3 through the packed bed. The estimated solubility of the oil in the dense gas at this density is 3.425 kg/m3. The superficial velocity of the carbon dioxide in the bed will be 1 mra/s. This fluid velocity is sufficiently small for the fluid to become saturated with oil. We are required to estimate the minimum time of operation for complete extraction of the oil from the bed. The initial oil fraction is 12% (wt/wt) based on wet seeds, the void fraction of the bed is 40%, and the density of the particles is 900 kg/m3. 

It has been shown that, in supercritical carbon dioxide, increases in water concentration result in increases in enzyme activity. The amount of added water needed for this increase varies and can depend on many factors, such as reaction type, enzyme utilized, and initial water content of the system. This is true until an optimal level is reached. For hydrolysis reactions, activity will either continue to increase or maintain its value. For esterification or transesterification reactions, once the optimal level of hydration has been reached, additional water will promote only side reactions such as hydrolysis. Dumont et al. (1992) suggests that additional water beyond the optimal level needed for enzyme hydration may also act as a barrier between the enzyme and the reaction medium and thereby reduce enzyme activity. Mensah et al. (1998) also observed that water above a concentration of 0.5 mmol/g enzyme led to lower catalytic activity and that the correlation between water content of the enzyme and reaction rate was independent of the substrate concentrations. 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

The experimental solubility data for solid naphthalene in supercritical carbon dioxide, given as moles naphthalene dissolved per liter, are shown in Figure 6. Qualitatively the three pressure-composition isotherms show characteristic behavior for a solid-supercritical fluid system. Each isotherm initially shows a large increase in solubility with increasing pressure, and then a limiting value is reached at higher pressures. 

Although supercritical carbon dioxide is a poor solvent for many highly polar substances, and the reactivity of carbon dioxide will always limit its use as a reaction solvent, university-based champions of the use of supercritical carbon dioxide, working with adventurous industrialists, initially in the area of a range of hydrogenation reactions, will help to pave the way for other applications. 

The present work reports results and observations on the enzymatic synthesis of oleyl oleate (which is a synthetic analogue of jojoba oil) in supercritical carbon dioxide. Special stress was laid on the comparison between batch and continuous systems for the above mentioned synthesis. Influence of different reaction parameters on the reaction yield and initial reaction rates was studied. 

Lipases are the most common enzymes used in non conventional media like organic solvents and supercritical carbon dioxide. Lipases usually hydrolyse fats into fatty acids and glycerol. The special property of lipases is their ability to act at the interface between water and oil. In these experiments lipase (EC 3.1.1.34) from Rhizopus arrhizus (Boehringer Mannheim) was used to investigate the effects of lipase under hydrostatic pressure. The analysed reaction was the hydrolysis of p-Nitrophenyllaureate at different concentrations at 35 °C. The dependance of the kinetic constants between 1 bar and 3000 bar is presented in table 2. Like the thermophilic GDH at 1000 bar lipase is activated under pressure as well. The initial reaction rate increases by a factor of 1.5 at 1000 bar compared to the initial reaction rate at ambient... 

As a comparison and reproduction to recent works done by DeSimone and coworkers [2], attempts were made in the free radical polymerization of acrylic acid (AA) in supercritical carbon dioxide (SCCO2) with Azoisobutyronitrile (AIBN) as initiator. 

The following discussion introduces the considerations applicable to initial design and subsequent scaleup of any supercritical or near-critical fluids parts cleaning vessel, then centers on dimensionless variables and principles for applying these variables to actual scaleup of a parts cleaning vessel. The last part of the discussion focuses on the application of these concepts to an agitated, supercritical carbon dioxide, parts cleaning vessel. 

The initial equipment costs and operating costs for the more conventional processes are higher than for supercritical carbon dioxide. The higher equipment costs are due to the additional parts needed for environmental treatments (i.e., scrubbers, vapor incinerators, etc.) The consumables costs are lower for the supercritical fluid processes due to the closed-loop recycle design of the supercritical fluid system, elimination of water for rinsing and the reduced electricity costs associated with not having to dry the parts. 

A detailed description of the experimental apparatus and procedure used for the aqueous study are given elsewhere (Roop and Akgerman, Ind. Eng. Chem. R., in review) Static equilibrium extractions were carried out in a high pressure equilibrium cell (300 mL Autoclave). After the vessel is initially charged with 150 mL of water containing 6.8 wt.% phenol and supercritical carbon dioxide (and a small amount of entrainer, if desired), the contents were mixed for one hour followed by a two hour period for phase separation. Samples from both the aqueous phase and the supercritical phase were taken for analysis and the distribution coefficient for phenol calculated. 

In addition to advantages like higher initial reaction rate and higher conversion, supercritical fluids provide an easy separation of products and unreacted substrates. This ecologically safe recovery of the products is a unique advantage provided by supercritical carbon dioxide. ... 

The environmentally benign nature of carbon dioxide comes from its very stable molecular bonds, which in turn do not provide high polarity. In fact, a carbon dioxide molecule has only a weak quadrupole moment, due to minor charge separation on oxygen and carbon atoms. Hence, the molecular interaction with most polar and heavy substances of interest is minor, providing only a weak solvent power. If needed, a small amount of cosolvent (also termed as entrainer or modifier) is added to enhance polarity and affinity with solutes. In many applications, however, the design limitation is the solubility of the substance in supercritical carbon dioxide. Therefore, the solubility data are essential both for the initial feasibihty study and final process design. 

Supercritical-fluid chromatography (SFC) was extensively evaluated in the late 1980s, in both the capillary and the packed-column formats, for a variety of compounds, including pharmaceuticals—primarily with negative results. Subsequent evaluation of SFC has shown that the primary source of this disillusionment is that supercritical carbon dioxide is substantially less polar than initially predicted. In its simplest form, supercritical carbon dioxide can be thought of as providing approximately the same polarity as hexane. As a... 

Fig. 8.4 Initial reaction rate (bars) and selectivity (points) exhibited by free Candida antarctica lipase B for butyl propionate synthesis in supercritical carbon dioxide and in four different ionic liquids/supercritical carbon dioxide systems. The reaction conditions were r=50°C, vinyl propionate 150 mM and 1-butanol 100 Mm [38]...

As evident from Fig. 8.4, an increase in the selectivity has been observed in IL/ scCOj biphasic systems media (>99.5%) with respect to scCO assayed alone (95%). These results could be explained by the use of water-immiscible ILs which have a specific ability to reduce water activity in the enzyme microenvironment. The synthetic activity of the immobilized lipase in IL/scCO biphasic systems is lower than that in scCO assayed alone. Similar results were found by Mori et al. [40] in IL/ hexane biphasic systems. These authors reported that the enzymatic membranes prepared by simple adsorption of CaLB onto the surface were more reactive than membranes prepared with ILs. As can be observed in Fig. 8.4, the initial reaction rate in the assayed IL/scCO biphasic systems increased in the following sequence [bdimim ][PF ]<[bmim ][PFg ]<[bmim ][NTfj ]<[omim ] [PF ], which was practically in agreement with flie activity sequence reported by these authors using free Candida antarctica lipase B in homogeneous ionic liquid systems ([bmim ] [PF ]<[bdmim+][PFg ]<[bmim+][NTfj ]<[omim ][PF ]), with the exception of [bmim [PF ] and [bdimim+][PFg ]. These results were explained taking into account that biotransformation occurs within the ionic liquid phase, so substrates have to be transported from scCOj to the ionic liquid phase. The mechanism of substrate transport between the ionic liquid and the supercritical carbon dioxide could be by three consecutive steps diffusion of the substrates through the diffusion... 

The shape of the curves indicate that the initial stage of the extraction is limited by solubility. Note the initial rise was greatest at 40 C (0.95 g/mL) at those conditions the recovery reached 82 % when only 60 mL of supercritical carbon dioxide had passed through the extraction thimble. For the same volume of the supercritical fluid (60 mL), the recovery was only 78 % for 80°C(0.81 g/mL), and 58 % for 120 C (0.67 g/mL). However, as the analyte flux begins to diminish (the easy-to-extract solutes are removed, and the more difficult solutes deeper within the matrix are now being removed), the extraction mechanism is no longer limited by saturation solubility. Rather, the extraction mechanism in the later stages of SFE is diffusion limited for... 

The term supercritical fluid is used to describe any substance above its critical temperature and pressure. The discovery of the supercritical phase is attributed to Baron Cagniard de la Tour in 1822 [3], He observed that the boundary between a gas and a liquid disappeared for certain substances when the temperature was increased in a sealed glass container. While some further work was carried out on supercritical fluids, the subject remained essentially dormant until 1964 when a patent was filed for using supercritical carbon dioxide to decaffeinate coffee. Subsequent major developments by food manufacturers have led to the commercialization of this approach in coffee production. The use of supercritical fluids in the laboratory was initially focused on their use in chromatography, particularly capillary supercritical fluid chromatography (SFC). However, it was not until the mid-1980s that the use of SFE for extraction was commercialized. 

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