Transport Properties of Supercritical Carbon Dioxide

Frederic Lavanchy, Eric Fourcade, Evert de Koeijer, Johan Wijers, 

For many processes performed in supercritical fluids, the transport properties of the medium will play an important role. For polymerizations, this includes mass transfer for mixing reactants and to allow proper contact between monomer and catalyst. Polymerization reactions are usually highly exothermic, so that the heat of reaction needs to be absorbed and transported through the supercritical fluid. Virtually aU studies described in the literature have been performed on a relatively small scale, and scale-up aspects, for which mass and heat transfer are major issues, have generally been disregarded. This chapter will describe an experimental study of some aspects of mass and heat transfer in supercritical CO2 (SCCO2), and a comparison will be made with the behavior of standard liquid systems. 

Supercritical Carbon Dioxide in Polymer Reaction Engineerir  

Copyright 2005 WILEY-VCtt Verlag Gmbtt Co. KGaA, Weinheim 

The influence of the variation in transport parameters on the design of process equipment is not well studied when compared to the case of relatively incompressible liquids. Therefore, this Chapter will focus on experiments describing the hydrodynamic behavior and the heat transfer properties of scC02-Although this analysis is crucial for the study and promotion of chemical reactions in SCFs, it is also important for the application of SCFs in refrigeration and cooling. 

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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). 

To acquaint the reader with the key features of SCE, we display the supercritical region of interest (Eigure 6.13) and a map of diffusivities in liquids and supercritical carbon dioxide (Eigure 6.14), which clearly shows the superior transport properties of the latter. The increased extraction power of carbon dioxide with temperature and pressure becomes evident in Figure 6.15. We note in particular that doubling the pressure from 70 bar... 

Supercritical Extraction. The use of a supercritical fluid such as carbon dioxide as extractant is growing in industrial importance, particularly in the food-related industries. The advantages of supercritical fluids (qv) as extractants include favorable solubiHty and transport properties, and the abiHty to complete an extraction rapidly at moderate temperature. Whereas most of the supercritical extraction processes are soHd—Hquid extractions, some Hquid—Hquid extractions are of commercial interest also. For example, the removal of ethanol from dilute aqueous solutions using Hquid carbon dioxide... 

A way around this issue may have been found with the use of supercritical fluids. These materials, such as liquid carbon dioxide, have many interesting properties from the point of view of pharmacutical processing since they combine liquid-like solvent properties with gas-like transportation properties. Small changes in the applied pressure or temperature can result in large changes of the fluid density and, correspondingly, the solvent capacity and properties of the resultant particles. 

One promising way to proceed seems to be the extraction of the halogenated components by supercritical fluids. This technique is called supercritical fluid extraction (SFE). Because of their special properties supercritical fluids show solubilities like organic solvents and transport properties like gases. Especially carbon dioxide with it s low critical data (Tc = 31,3 °C, pc = 7,28 MPa) was found to be a good candidate for the extraction of organic substances. There are several applications of the SFE-technique with SC-CO2 for instance in the field of ... 

Supercritical fluids exhibit liquid-like solvent properties and gas-like transport properties. The combination of these properties makes supercritical fluids suitable for the various applications mentioned above. Carbon dioxide is the supercritical fluid of choice due to its mild critical temperature, nontoxicity, nonflammability, and low cost. Carbon dioxide becomes a supercritical fluid when it is heated above 31.1°C and simultaneously compressed above 73.8 bar. 

Many chemical reactions carried out in supercritical fluid media were discussed in the first edition, and those developments are included in total here after some recent work is described. In the epilogue (chapter 13) of the first edition we made reference to one of the author s work in enzyme catalyzed reactions in supercritical fluids that was (then) soon to appear in the literature. The paper (Hammond et al., 1985) was published while the first edition was in print, and as it turned out, there was a flurry of other activity in SCF-enzyme catalysis many articles describing work with a variety of enzymes, e.g., alkaline phosphatase, polyphenol oxidase, cholesterolase, lipase, etc., were published starting in mid 1985. Practical motivations were a potentially easier workup and purification of a product if the solvent is a gas (i.e., no liquid solvent residues to contend with), faster reaction rates of compounds because of gas-like transport properties, environmental advantages of carbon dioxide, and the like. 

The properties of fluids under supercritical conditions are considered ideal for extracting substances from exhausted activated carbons. Two supercritical fluids are of particular interest, carbon dioxide and water. Carbon dioxide has a low critical temperature of 304 K and a moderate critical pressure of 73 bar, while water has a critical temperature of 647 K and a critical pressure of 220 bar. The character of water at supercritical conditions changes from one that supports only ionic species at ambient conditions to one that dissolves paraffins, aromatics, gases and salts [65]. These supercritical fluids exhibit densities similar to those of liquids (high solvent strengths) and diffusion coefficients similar to those of gases (excellent transport characteristics), enabling them to effectively dissolve and/or desorb contaminants from the carbon surface and to easily enter/exit even the smallest pores and carry away any... 

The advantages of carbon dioxide as solvent have been well publicized it is, in fact, classified as GRAS - generally regarded as safe, it has low toxicity (threshold limit value - TLV = 5000 ppm), it is supercritical just above ambient temperature (critical temperature 31 C) and it is cheap. Also, like other supercritical fluids, it has advantageous gas-like transport properties, such as low viscosity and high diffusivity. 

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