Supercritical isobaric process

By rapid expansion of supercritical propane solution (RESS), and isobaric crystallisation (ICSS), isotactic polypropylene and ethylene-butylene copolymers were precipitated from the supercritical solution. The RESS process produced microfibres with a trace of microparticles, while the ICSS process produced microcellular products. Improvement in thermal stability was achieved by first synthesising a thermoplastic vulcanisate from polypropylene and ethylene-propylene-diene terpolymer from a supercritical propane solution, followed by RESS. 28 refs. 

Fractionation. Kerr-McGee developed the ROSE process for separating the heavy components of cmde oil, eg, asphaltenes, resins, and oils, in the 1950s. This process was commercialized in the late 1970s, when cmde oil and utility costs were no longer inexpensive. In the ROSE process (Fig. 11), residuum and pentane are mixed and the soluble resins and oils recovered in the supercritical phase. By stepwise isobaric temperature increases, which decrease solvent density, the resin and oil fractions are precipitated sequentially. 

For this work, a 9 1 volumetric flow ratio of liquid CO2 and methanol has been employed and this corresponds to a molar flow ratio of 8.5 1.5. From literatures [5], the critical parameters of such a mixture are about 50°C and 94 bar. From Figure 1, one can see that the two other isobars exhibit two maximas - one in the subcritical and the other in the supercritical region. It is a well-known fact that SFE processes are controlled either by solubility or mass transfer limitations [6]. As such, the shape of these isobars has to be explained in term of these two limitations too. 

Supercritical fluid extraction (SFE) is a suitable process for many separation problems. The regeneration of the supercritical fluid is as important as the extraction step itself Therefore this paper presents a method to do this in a more isobaric way than the customary pressure reduction regeneration. For the example of soil remediation we have investigated the activated carbon regeneration of supercritical carbon dioxide loaded with the low-volatile polycyclic aromatic hydrocarbon (PAH) pyrene. Characteristics of supercritical fluid extraction for soil remediation are elevated temperatures and pressures up to 370 K and 300 bar. For this reason adsorption isotherms of pyrene on activated carbon up to these conditions are measured first. Subsequently this method is used to regenerate carbon dioxide in a closed solvent cycle plant with a 4 1 extractor. An economic analysis using these results indicate that the soil remediation costs will decrease for about 20 - 30 % by means of an activated carbon adsorber. 

An excellent review or LLE applications in the food industry is given by Hamm,17 Since solvent toxicity is a major consideration, supercritical solvents, such as CO, which are nomoxic and LLE from an aqueous leachate are two popalar means of deriling with this problem. Figure 7.8-11 is a cooceptual supercritical extraction flowsheet. Because Tc is much lower than either 7 or rB, the solvent (e.g., C02) simply is flashed from the solute by throttling (an isenthalpic process). Then the vapors are compressed (an isentropic process) and cooled (an isobaric step) to complete the solveni recycle. Usually, costs are determined by the compressor requirearerUs. Several potential applications of supercritical or near-critical solvents are discussed in more datail else where,... 

Sincfle Pass System. Ihe single pass system for fractionation of fi oil operates at a given tertperature and utilizes a st wise adjustment of pressure in the extraction vessel and a lower pressure in the collector. A schematic of the process is own in Figure 2. A stepwise increase in the pressure of the extraction vessel enhances dissolution of the material to be extracted into the supercritical phase in the order of increasing boiling point or molecular wei t. Ihe dissolved material is then s >arated either isobarically or isothermally in the collector. 

TABLE 4. Energy consumption of a pump process. Throttling to the subcritical state. Isobaric heating after the pump. Supercritical solvent CO . Conditions of the extraction 40 MPa, 339 K Conditions of the regeneration 6 MPa, 301 K. 

FIGURE 12.3 Variations in the solubility of a low-volatility substance (liquid or soBd) in a subcritical (temperature < critical temperature, T ) or supercritical fluid (temperatnre> T ) as a function of process temperature, process pressure (segmented lines correspond to equal pressure or isobaric conditions), and solvent density (dotted lines correspond to equal density conditions). (Reproduced fmm Ref. [4] with permission of Elsevier.)... 

Now consider the rapid expansion of a supercritical fluid through the expansion nozzle depicted in Figure 3, which is in use in our laboratory and is similar to those typically reported in the literature (19,20). A tapered inlet (usually 120° angle) is followed by a cylindrical capillary section in which L/D typically ranges from 3 to 6000. Given the comparatively low viscosity of a supercritical fluid, the effects of acceleration and friction on the pressure are weak at the low flow speeds that exist upstream of the nozzle in the process tubing. Consequently, the pressure drop up to this point is small. However, when the fluid passes into the tapered inlet section of a typical RESS nozzle, the slow expansion gradually turns rapid. If, to simplify analysis, we subdivide the process into an isobaric part followed by a rapid expansion, the question of where the rapid expansion truly starts must be addressed. 

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]. 

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