Supercritical high-pressure column processes

Figure 8.17 High-pressure column processes with supercritical fluids.

Although a continuous and isobaric operated high-pressure column process seems to be beneficial, no absorption liquid with suitable distribution coefficients is often available. An incomplete purification of the supercritical fluid leads to strongly increased solvent to feed ratios. Thus, the extracted components are separated by pressure reduction or by adsorption like in supercritical processes for solid material. An example is given in Figure 8.18 for the deacidiflcation of vegetable oil [36]. 

Fractionation of liquid mixtures with supercritical carbon dioxide in counter-cur-rent columns can be operated continuously, because liquids can be easily pumped into and out of a column. This represents a big advantage over extrachon from solid materials, as it allows real process intensification - large quantities of feed can be processed with only a small volume under high pressure at any given time. Frac-tionahon, mostly of natural products or extracts, has been extensively studied at the laboratory and pilot-plant scale. The design principles of this type of column have been established, and scale-up procedures devised [1,6]. They can be operated with reflux, as in distillation, and frachonahon can therefore become an extremely se-lechve process. Difficult separahons can be effechvely carried out. 

In the air separation process (see Fig. 2.3A) liquid high pressure air (c), resulting from the internal compression of oxygen, is expanded via a throttle valve (22) into the pressure column (12). Alternatively this expansion can be performed in a so called dense fluid expander . This is a turbine for the expansion of a liquid or very dense supercritical cold fluid. A turbine expansion produces less exergy loss than a throttle expansion. Owing to this the use of a dense fluid expander reduces the work for gas separation or liquefaction. 

Table 8.2 visualizes a classification of supercritical processes regarding the solid or liquid state of the feed material to be treated. Solids are processed batchtvise in high-pressure vessels, tvhereas liquids are moved continuously via countercurrent columns either as a falling film or as a droplet spray. Most important is the formation of a phase boundary bettveen the feed material and the supercritical fiuid in order to... 

The processing of liquids with supercritical fluids mainly depends on their flow ability. Liquid feed materials of low viscosity like water-based solutions - for example, ethanol in water - are operated in high-pressure countercurrent columns that are... 

The hydrodynamic behavior of the countercurrent flow is of high importance because of the density difference between the two phases at high pressures. Thus, with increasing gas density, the risk of flooding the column reveals. In order to design a safe liquid dynamics of a supercritical countercurrent column process, Stockfleth [43] developed an equation nondependent on geometrical data that enabled the prediction of the flooding point. 

Liquid products can, of course, also be processed with this extraction technique in this case the liquid raw material is fed into a distillation column and slowly falls to the bottom in countercurrent against the rising supercritical gas. At the bottom of the column, the residue is discharged from the sump by a level regulation system, and the extracted components are once again precipitated in the separator. With this procedure, the extractor in Fig. 16 must be replaced by a countercurrent extraction column. This process can be run continuously because liquid can be metered into the high-pressure area without difficulty. 

Due to possible environmental problems with acetone, new technologies are being developed for the production of deoiled lecithins involving treatment of Hpid mixtures with supercritical gases or supercritical gas mixtures (10—12). In this process highly viscous cmde lecithin is fed into a separation column at several levels. The supercritical extraction solvent flows through the column upward at a pressure of 8 MPa (80 bar) and temperature between 40 and 55°C. The soy oil dissolves together with a small amount of lecithin. 

SFC is the application of a supercritical fluid, any substance at a temperature and pressure above its thermodynamic critical point (Figure 9.2) with both gas- and liquid-like abilities to diffuse through solids, and dissolve materials, respectively, as the mobile phase in the chromatographic process. The most widely used mobile phase for SFC is carbon dioxide because of its low critical pressure (73 atm), low critical temperature (31°C), inertness, low toxicity, and high purity at low cost [12,13], Historically there were two approaches in developing modern SFC the use of either the packed and microbore columns designed for HPLC application or the open-tubular capillary GC type columns [13,14], The conventional packed HPLC... 

The mobile phase plays different roles in GC, LC, and SFC. Ordinarily, in GC the mobile phase serves but one purpose — zone movement. As we have seen in Chapter 28. in LC the mobile phase provides not only transport of solute molecules but also interactions with solutes that influence selectivity factors (or values). When a molecule dissolves in a supercritical medium, the process resembles volatilization but at a much lower temperature than would normally be used in GC. Thus, at a given temperature, the vapor pressure for a large molecule in a supercritical fluid may be 10 ° times greater than in the absence of the fluid. Because of this, high-molecular-mass compounds, thermally unstable species, polymers, and large biological molecules can be eluted from a column at relatively low temperatures. Interactions between solute molecules and the molecules of a supercritical fluid must occur to account for their solubility in these media. The... 

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