Solvent cycle

During the investigation of pollution in coastal seawaters, Werner and Waldichuk [24] pointed out the need for concentrating and isolating trace amounts of certain substances with a continuous solvent extractor. They constructed a modified Scheibel apparatus by changing the organic solvent cycle system. 

After sufficient extraction the dissolved substances have to be separated from the fluid in a following step. Decreasing the pressure at constant temperature reduces the fluid-density, and therefore the solvent-power of the fluid. The extracted substances are collected at the bottom of the separator, as shown in Fig. 6.6-2. To close the solvent cycle, the fluid has to be recompressed to extraction pressure. 

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. 

Figure 2.8 Cyclic voltammograms and QCM results for the cycling of PVF in LiC104 electrolyte in both aqueous (3117 A film) and acetonitrile (4775 A film) solvent. Cycling was performed at a scan rate of 5mV/s from +0.1 to +0.6 V vs SCE.

The necessary operations for changing conditions of state and composition in the solvent circuit can be carried out in different ways which depend on the nature of the substances involved, the scale of the process unit, and the operating conditions of the processing unit. The main difference in solvent cycles is whether the solvent is cycled in the supercritical (gaseous) or subcritical (liquid) state. In both cycles the solvent can be driven by a pump or by a compressor. 

Solvent cycle with heat recovery Conditions of the extraction 40 MPa, 323 K ... 

Fiinure 31. Comparison of compressor and pump solvent cycle. Extraction temperature 313 K [I, 22]. 

Separation of extract and solvent by an absorbens or adsorbens makes feasible a solvent cycle at nearly constant pressure. Considering energy consumption, this is the... 

In multi Cycle processes, conditioning vessels are usually required between each solvent cycle, and an evaporator may also sometimes be introduced in order to keep the feed concentration to the second cycle at a low value and therefore economize in the size of the second-cycle extraction plant. 

The camotite ore of the Colorado Plateau area contains a workable proportion of vanadium in addition to uranium, and processes have been devised for the simultaneous recovery of both elements by solvent extraction methods. The Shiprock plant in New Mexico, for example, extracts first the uranium from a sulphate leach liquor, by means of a solvent containing 10 per cent D2EHPA with 2-5 per cent TBP, in kerosene. A second solvent cycle, with different proportions of the two phosphates, then extracts vanadium from the first cycle raffinate. Sodium carbonate is then used for backwashing the uranium and 10 per cent sulphuric acid for the vanadium. 

Separation of dissolved substances from the fluid is performed by precipitation. Most applications use an isenthalpic decrease of pressure and temperature to reduce the fluid density and, consequently, the solvent power of the fluid. After pressure reduction, the extraction dense gas is heated so that it reaches the gaseous state. In this phase, no solvent power is present for any substances and therefore nearly complete separation of the extracted substances takes place. The dissolved substances precipitate and can be discharged at the bottom of the separator. The solvent cycle is closed by recycling the recompressed CO2, either cooled in case of a compressor process or condensed by using the pump process. 

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. 

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