Supercritical fluid extraction process operation

Figure 2, showing the solubility behavior of naphthalene in supercritical carbon dioxide, and Figures 3 and 4, which point out that the behavior is quite general, will be used to explain how a "generic" supercritical fluid extraction process operates. 

Supercritical fluid separation processes operate at pressures ranging from 1000 to 4000 lb/in.2, pressures that might be considered high, especially in the foods and essential oils industries. However, because of the factors just listed, supercritical fluid extraction has become eco-... 

For purposes of comparison, some supercritical fluid extraction processes have been calculated in which the extract is separated at the subcritical pressure p = 60 bar (Process 4). Such a process corresponds to that in Fig. 1 with the difference that a pump is employed to increase the pressure from state 1 to state 2, since the CO2 is cooled down to 17°C after separation, i.e. is present in the liquid state before the pressure is increased. Even for the most favourable variant with K = 0.062 DM/kg hop extract, the operating costs for this process are significantly higher than for processes employing supercritical separation. They can be reduced significantly by heat recovery with a heat pump as published by Sievers and Eggers 3. ... 

It is by now quite clear to the reader that the phenomenon of enhanced solubility in supercritical fluids has been known for more than a century. So why are there not scores of supercritical fluid extraction processes in operation today ... 

The forerunner of all the recent applications of SCF technology reported in the United States is the SCF regeneration of activated carbon first described at an American Chemical Society meeting in 1978 (Modell, de Filippi, and Krukonis, 1978). The phenomena in operation during adsorption of organics from wastewater and the desorption of organics from the activated carbon using supercritical carbon dioxide are similar to those active in other supercritical fluid extraction processes. Therefore, we examine the technical details here. 

For the case of a constant distribution coefficient, Treybal (1968) shows that the theoretical minimum solvent-to-feed ratio necessary to extract all of a component from a feed stream is equal to the inverse of the distribution coefficient. Recall, however, that the theoretical minimum value of the solvent-to-feed ratio requires an infinitely tall column. In actual practice a greater solvent-to-feed ratio, typically 1.3 to 1.5 times minimum, is employed. We present some elementary but informative graphical design procedures to illustrate the relationship between the distribution coefficient and the operation of the supercritical fluid extraction process for separating ethanol-water. 

Supercritical fluid extraction system - Hewlett Packard Model 7680A totally automated system with unlimited-capacity reciprocating pump, specially designed extraction chamber with safety interlocks, a variable restrictor nozzle and analyte collection trap. The operation of the extractor is controlled by a personal computer which is a Microsoft Windows-based system. An animated status screen provides real-time monitoring of the extraction process. Table II gives the SFE conditions for the HP extractor. 

Figure 5. Supercritical Fluid Extraction - a. Process Diagram b. Operating Paths.

This chapter discusses the principal aspects of the technique in its two modes, the devices typically employed by each and their amenability to coupling with subsequent operations of the analytical process. Also, the main analytical applications of both modes in analytical chemistry are described, and their advantages and disadvantages with respect to alternative techniques such as Soxhlet, ultrasound-assisted, microwave-assisted or supercritical fluid extraction, discussed. 

Supercritical Fluid Extraction This process generally involves the use of CO2 or light hydrocarbons to extract components from liquids or porous solids [Brunner, Gas Extraction An Introduction to Fundamentals of Supercritical Fluids and the Application to Separation Processes (Springer-Verlag, 1995) Brunner, ed.. Supercritical Fluids as Solvents and Reaction Media (Elsevier, 2004) and McHugh and Krukonis, Supercritical Fluid Extraction, 2d ed. (Butterworth-Heinemann, 1993)]. Supercritical fluid extraction differs from liquid-liquid or liquid-solid extraction in that the operation is carried out at high-pressure, supercritical (or near-supercritical) conditions where the extraction fluid exhibits... 

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. 

The Eastman process has operated in Texas (USA) with three continuous, stirred-tank reactors, a wiped-film evaporator, a distillation train and a continuous, counter-current, liquid-liquid extractor for recovery of the catalysts since 1996 (1400 metric tons per year capacity, e.g., semiworks facility), but full commercial capacity has not yet been reached. The main problem is still the formation of oligomers. Another answer to the problem of IL loss to the organic phase consists in the extraction of the products from the IL using a supercritical fluid [101], but operational costs are high. 

The examples in this chapter highlight many facets of the development of SCF processes and applications that need to be addressed, understood, and solved before scaleup is considered. The same procedure is necessary for any kind of separation process. Supercritical fluid extraction has the potential to reduce energy costs but that does not necessarily mean it will be less expensive overall. Frequently, capital and other operating costs can more than offset decreased energy costs. Subsequent chapters discuss developments in the application of supercritical fluid solvents to the solution of technically and economically difficult separation problems in which the improved performance of the processed materials can support the processing cost. 

This patent is concerned primarily with the polymerization of ethylene at conditions high above its critical temperature and pressure. The Krase and Lawrence patent covers polymerization but it also describes the separation of various oligomers by stagewise pressure reduction. The multistep sequence results in a lower energy recycle/separation process which produces discreet fractions of polyethylene of different molecular weight. A portion of the example and process operation is excerpted from the patent to point out once more that supercritical fluid extraction and separation have been known and understood for 40 to 50 years. 

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