Extraction, solvent applications, commercial

Extractions. SCFs are used as extraction solvents in commercial food, pharmaceutical, environmental, and petroleum applications (rl,r5,r6,rl6,r23). An excellent overview of the patent literature up to 1991 is available (1). 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene using lithium alkyls as catalyst. Other laboratory applications include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

The phase behavior of solids in supercritical fluids has practical significance as well as academic interest. Since the mid-1970 s, it has been recognized that supercritical fluids can be useful as solvents for commercial-scale extractions. While a variety of applications are documented in the literature (2,6), supercritical-fluid (SCF) extraction has been particularly useful in upgrading petroleum fractions (7), extracting volatile components from coal (8), and deashing oil shale (9) and coal liquids (10). The... 

A historical perspective on aqueous-organic extraction using membrane contactor technology is available in Refs. [1,6,83]. The mechanism of phase interface immobilization was first explored in Ref. [84], while application of membrane solvent extraction for a commercial process was first explored in Ref. [85]. Two aspects of liquid-liquid contact in membrane contactors that are different from typical gas-liquid contact are (1) the membrane used could be hydrophobic, hydrophdic, or a composite of both and (2) the membrane mass transfer resistance is not always negligible. Ensuring that the right fluid occupies the membrane pores vis-a-vis the affinity of the solute in the two phases can minimize membrane resistance. These aspects have been discussed in detail in Refs. [6,86,87]. 

A typical extraction sequence is similar to the traditional SPE, although 500 pL of the appropriate solvent is sufficient for disk conditioning and extraction of the analytes. Several types of disk extraction media are commercially available in different dimensions depending on the application and sample volume. The most prevalent are paper-based, membrane-based, glass fiber-based, and PTFE-based products. Commercially available products are Speediscs by Baker, Empore by 3M, Novo Clean by Alltech, and SPEC by Ansys Technol. 

Solute selectivity. In certain applications, it is important not only to recover a desired solute from the feed, but also to separate it from other solutes present in the feed and thereby achieve a degree of solute purification. The selectivity of a given solvent for solute i compared to solute j is characterized by the separation factor = K,/Kj. Values must be greater than Ofjj = 1.0 to achieve an increase in solute purity (on a solvent-free basis). When solvent blends are used in a commercial process, often it is because the blend provides higher selectivity, and often at the expense of a somewhat lower partition ratio. The degree of purification that can be achieved also depends on the extraction scheme chosen for the process, the amount of extraction solvent, and the number of stages employed. 

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... 

Commercial-scale application of solvents coming under the category of neutral reagents is largely found as applied to the nuclear industry materials, as in example, for the separation and refining of uranium, plutonium, thorium, zirconium, and niobium. A process flowsheet for extracting niobium and tantalum from various resources is shown in Figure 5.23. It will... 

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