Dense gas extraction

The diffusion coefficient as defined by Fick s law, Eqn. (3.4-3), is a molecular parameter and is usually reported as an infinite-dilution, binary-diffusion coefficient. In mass-transfer work, it appears in the Schmidt- and in the Sherwood numbers. These two quantities, Sc and Sh, are strongly affected by pressure and whether the conditions are near the critical state of the solvent or not. As we saw before, the Schmidt and Prandtl numbers theoretically take large values as the critical point of the solvent is approached. Mass-transfer in high-pressure operations is done by extraction or leaching with a dense gas, neat or modified with an entrainer. In dense-gas extraction, the fluid of choice is carbon dioxide, hence many diffusional data relate to carbon dioxide at conditions above its critical point (73.8 bar, 31°C) In general, the order of magnitude of the diffusivity depends on the type of solvent in which diffusion occurs. Middleman [18] reports some of the following data for diffusion. 

It should be pointed out that for a low pressure gas the radial- and axial diffusion coefficients are about the same at low Reynolds numbers (Rediffusion effects may be important at velocities where the dispersion effects are controlled by molecular diffusion. For Re = 1 to 20, however, the axial diffusivity becomes about five times larger than the radial diffusivity [31]. Therefore, the radial diffusion flux could be neglected relative to the longitudinal flux. If these phenomena were also present in a high-pressure gas, it would be true that radial diffusion could be neglected. In dense- gas extraction, packed beds are operated at Re > 10, so it will be supposed that the Peclet number for axial dispersion only is important (Peax Per). 

In this Chapter, fundamentals of design criteria in relation to processes and equipment are reviewed for dense-gas-extraction from solid matrices. Although, as mentioned in previous chapters, numerous dense gases can be used as solvents. In the following discussion we concentrate on the most extensively used gas-carbon dioxide. The reason for this is its nontoxic, non-flammable and inert nature, the possibility of gentle treatment of thermally sensitive materials, and the fact that it is inexpensive and an environmentally acceptable material. 

For dense gas extraction plants the building design shall consider, that ... 

The problems connected-with separation processes, units, and equipment are treated in the Sixth Chapter, focusing the reader s attention on high-pressure distillation and on dense-gas extraction from solids and liquids. 

Relevant safety issues arising in the design and operation of high-pressure plants are addressed in Chapter Seven. After a general section where testing procedures, safe plant operation, and inspection are summarized, two examples are dealt with in detail dense-gas extraction units and polymerization reactors. 

Chapter Eight is concerned with a major question connected with the development of high pressure technologies in the process and chemical industry, i.e., the economic evaluation of production carried out at high pressures. In this case, also, the matter is discussed in relation to three important examples dense gas extraction, polymerization and supercritical antisolvent precipitation processes. 

Gabelman A, Hwang S-T, and Krantz WB. Dense gas extraction using a hollow fiber membrane contactor Experimental results versus model predictions. J. Membr. Sci. 2005 257(1-2) 11-36. 

Membrane contactor offers potential solution in a wide range of gas/liquid and liquid/liquid applications gas adsorption and stripping, liquid/liquid extraction, dense gas extraction, fermentation and enzymatic transformation, pharmaceutical applications, protein extraction, wastewater treatment, chiral separations, semiconductor manufacturing, carbonation of beverages, metal ion extraction, protein extraction, and VOCs removal from waste gas [55]. 

Supercritical fluid extraction - also referred to as dense gas extraction or near critical solvent extraction - means that the operational temperature of the process is in the vicinity of the critical temperature of the solvent. Since the extraction of herbal raw materials requires non-drastic gentle process temperatures the choice of suitable near critical solvents is limited to pure or partly halogenated C,-Cj hydrocarbons, dinitrogen monoxide and carbon dioxide. All these solvents, especially carbon dioxide, exhibit favourable properties in view of the afore-mentioned aspects. 

Fig, 2,25 Diagram of a dense gas extraction plant, letters A-G refer to Figure 2.26... 

Stahl, E., and K. W. Quirin. 1983. Dense gas extraction on a laboratory scale A survey of some recent results. J. Fluid Phase Equilib. 10 269. 

The extraction of metals based on a membrane contactor system with conventional solvents is a process widely studied using different configurations, extractants, and extraction solvents. One of the upcoming applications of membrane contactors is supercritical extraction. This process is called porocritical extraction. Porocritical process or porocritical extraction is a commercial supercritical fluid extraction (SFE) technique that utilizes an hollow fiber membrane contactor (HFMC) to contact two phases for the purpose of separation. As an improvement, the extraction of Cu + from aqueous solutions by means of dense gas extraction was achieved by using a hollow fiber membrane contactor device [7]. The authors... 

Serpil, T., Aeskenazi, O.N., Akman, U. and Horta9SU, 0. (1997) Application of serially-interconnected perfectly mixed tanks model to dense-gas extraction of plants, Proc. 4th Int. Symp. Supercritical Fluids, 299-302. 

The economic question is of great importance to the commercialisation of any dense gas extraction process. The high pressures required raise the capital investment in processing plant very substantially above those for conventional solvent extraction processes. It should be noted, however, that the use of a non-flammable dense gas such as carbon dioxide instead of a flammable organic solvent will produce some compensating economies. 

The energy requirements for dense gas extraction will depend on the method employed for solvent regeneration. With careful design they should not exceed those for a conventional solvent recovery process and they may be considerably lower. Energy consumption for processes where solvent recovery involves a throttling expansion are discussed in chapter 9. 

The dense gas extractions currently in use for solid materials utilise continuous multi-batch processes and batch processes are notoriously uneconomic. 

Unfortunately the continuous extraction of solids at high pressures presents many very difficult engineering problems which have not so far been fully overcome. The design of solid product entry and exit valves for high pressure vessels has been described by Eggers et al. [2] and is also discussed in chapter 8, which contains an account of a pilot plant for the continuous extraction of oil seeds. However, the use of true continuous dense gas extraction processes does not appear to be a commercial practicality at present. 

Use of carbon dioxide for dense gas extractions rule of thumb solubility rules... 

The first symposium exclusively on dense gas extraction was held in Essen in 1978 and further conferences have been held subsequently on an almost annual basis. A large proportion of the studies carried out on dense gases have originated from Germany. 

Reviews on the subject have been published by Schneider et aL [8], Irani and Funk [9], Randall [10], Brunner [11], Blood [12], Paulitis etaL [13], Rizvi et al. [14] and more recently Stahl et aL [15]. Most of these reviews carry details of both the principles and the techniques used in dense gas extractions. 

Many patents have been granted for dense gas extraction applications though generally, for commercial reasons, they contain little specific data referring to the optimum conditions for extraction. Some of the literature on work in commercially important areas is summarised in Tables 2.8-2.11. Existing and proposed extractions can be generally classified into the following main application areas ... 

Table 2.8 Suggested applications of dense gas extraction with CO2 in the production and fractionation of edible oils, fats and waxes...

Contrary to conventional extraction processes, dense gas extraction enables more freedom in designing the optimum solvent cycle by means of the T,s diagram. Depending on solubilities and corresponding selected thermodynamic conditions, the CO2 circulation system can be driven by means of either a pump or a compressor, whichever needs less consumption of energy. Eggers [24] and Lack [18] compared both processes by means of a T,s diagram. 

H. Kallio and K. Kerrola, Dense gas extraction as a preparation method in food analysis, Curr. Status Future Trends Anal. Food Chem., Proc. Eur. Conf. Food Chem., 8th, Volume 1 (G. Sontag and W. Pfannhauser, eds.), Austrian Chemical Society, Vienna, 1995, p. 30. 

Gableman, A., and Hwang, S.-T. (1999). Hollow fiber membrane contactors. J. Membr. Sci. 159, 61. Gableman, A., Hwang, S.-T., and Krantz, W. B. (2005). Dense gas extraction using a hoUow fiber membrane contactor Experimental results versus model predictions. J. Membr. Sci. 257, 11. Hestekin, J. A., Bachas, L. G., and Bhattacharyya, D. (2001). Poly(amino acid) functionalized cellulosic membranes Metal sorption mechanisms and results. Ind. Eng. Chem. Res. 40, 2668. Imai, M., Furusaki, S., and Miyauchi, T. (1982). Separation of volatile materials by gas membranes. Ind. Eng. Chem. Process Des. Dev. 21, 421. 

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