Pumps, supercritical fluid extraction

The instrumental requirements for supercritical fluid extraction are quite simple. A pump is essential to generate the extraction pressure in a themostated extraction vessel. The soluble sample components are then swept from the vessel through a flow restrictor into a collection device that is normally at ambient pressure. The fluid used for supercritical fluid... 

Figure 28-14a shows how a supercritical fluid extraction can be carried out. Pressurized fluid is pumped through a heated extraction vessel. Fluid can be left in contact with the sample for some time or it can be pumped through continuously. At the outlet of the extraction vessel, the fluid flows through a capillary tube to release pressure. Exiting C02 evaporates, leaving extracted analyte in the collection vessel. Alternatively, the C02 can be bubbled through a solvent in the collection vessel to leave a solution of analyte. 

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. 

Of all the materials available for use as a supercritical fluid, CO2 has become the material of choice because of its chemical properties. Instruments have been developed to utilize the principles described to effect extractions of compounds from a variety of sample matrices including asphalt, plant material, and soils (Figure 25.1). The supercritical fluid is pumped through the sample, through a filter or column to a trap where the fluid vaporizes and solvent is added to transfer the analyses to a vial for analysis. More recent instruments combine the supercritical fluid extraction system with a variety of columns and detectors to acquire data from complex samples. 

The single-stage supercritical fluid extraction process for solid natural materials is shown schematically in Figure I. The solvent is conveyed from the low pressure to the high pressure by a pump or compressor V. Extraction is at pressure p and temperature t in extractor E, where the soluble substances are transferred from the natural material to the solvent. Normally, the extractor consists of several autoclaves connected in series in the solvent flow. In throttle valve D the solvent loaded with extract is relaxed to the lower pressure. The extract is separated from the solvent in separator A at separation pressure p and temperature t. Heat exchangers WI, W2 and W3 are installed to achieve the desired temperatures. 

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. ... 

Pure CO2 has been the preferred solvent due to its favorable properties. The CO2 is used in siphoned cylinders to assist the transference of the solvent to the pump. CO2 passes through a cooling system to increase its density before being inserted in the heating system. When required, a vessel containing a modifier can be added to the system in a way similar to that already well known in supercritical fluid extraction (SFE). 

Fig. 2 Schematic for a typical laboratory-scale supercritical fluid extraction system (A) fluid supply (B) pump/delivery device (C) extraction vessel (D) collection vessel (E) heated zone (P) pressure measurement device (T) temperature controller. View this art in color at www.dekker.com.)...

Truly continuous is the extraction of liquids if the extractor is replaced by a column. The liquid is pumped continuously onto the head of the pressurised column and flows down by gravity. Supercritical fluid extraction is normally operated in the so-called droplet regime and not in the film regime. This means the liquid in contact with the... 

Carbon dioxide is pumped from the bottom of the liquid tank and compressed, heated, and then the samples are extracted (Fig. 9.3). The analytes are trapped in an organic solvent as the carbon dioxide is vented from the extractor. Supercritical fluid extraction (SFE) involves some experimentation with the proper pressure to achieve the extraction of the analyte of interest. Several vendors sell the automated supercritical fluid extractors. Both the accelerated solvent extractor and the supercritical fluid extraction are expensive methods, in the price range of 50,000 each. Several general reviews of SFE include Gere and Derrico (1994), Smith (1995), and a general sample preparation review by Majors (1995). 

Supercritical fluid extraction (SFE) can be coupled to SFC using a series of switching valves and either a loop or an accumulator trap interface [79,172,174,187-192]. The loop interface is used with a closed-loop static extractor, sometimes equipped with a recirculating pump. The fluid from the extraction cell passes continuously through the injection valve loop and back to the extraction cell. Injection of an aliquot of the extract onto a packed column is made by periodically switching the loop so that it is in-line with the flow of mobile phase to the separation column. This approach is used to determine fundamental parameters of the extraction process more so than for analysis. 

Supercritical Fluid Extraction. The properties of cryogenic fluids under supercritical conditions give them considerable potential for regenerating spent adsorbents. The solution characteristics of the fluid should be compatible with the adsorbed components and, when the fluid is pumped through the adsorbent bed, it will dissolve the adsorbed spiU components. The supercritical fluid can be evaporated easily due to its volatility, and in some cases both the solvent and the contaminant from the spill can be recovered. The high solvation character of the supercritical fluids is due to low intermolecular distances between the solvent molecules. This novel process is expensive at present and therefore the spilled material should be high value added and recoverable, such as pharmaceutical products. 

Supercritical fluid extraction (SEE) is a selective technique of sample preparation that enables the preparation of matrices by varying several physical parameters. Nowadays, it is considered to be the best replacement for many extraction technologies, such as accelerated solvent, Soxhlet solvent, microwave assisted extraction and so on. It was originally marketed as a universal extraction tool in 1988 by Isco Inc. (Lincoln, Nebraska, USA), Lee Scientific (Salt Lake City, Utah, USA) and Suprex Corp. (Pittsburgh, Pennsylvania, USA). The basic components of the SFE instmment are a carbon dioxide reservoir, a pump, an extraction vessel, an oven, a restrictor... 

The procedure is described as follows supercritical fluid extiactions were performed with an automated ISCO SFX m 3560 instrument using 6 mL extraction vessels. The extraction vessels were filled with 100 mg of dried plant samples mixed with anhydrous sodium sulfate. The extracted analytes were collected into 10 mL of methanol. The internal standard was added to the collection vials immediately after extraction. The collection temperature was 5 °C. The SFE instrument was equipped with a 260 mL syringe pump for the addition of carbon dioxide at a flow rate of 1.5 mL/min and a manually controlled Jasco PU-980 HPLC pump for addition of the modifier (methanol) at flow rates of 0.04-0.1 mL/min (2.6-6.6 %). The restrictor temperature was set at 60 °C in all extractions [11]. In this paper, the extracts from SFE were found to be much cleaner in comparismi with those obtained by solid-liquid extractions or Soxhlet extractions. The results showed that supercritical fluid extraction is a valuable alternative technique to traditional extraction methods of Catharanthus alkaloids from dried leaves. 

Figure 3. Flow-sheet diagram of the supercritical fluid extraction semi-pilot plant at the University of Burgos. CO2 storage (1), Purge valves (PI, P2, P3), Shutoff valves (SI, S2,..., S8), Check valve (Cl), Coohng bath (2), Temperature controllers (TC), Total mass flow meter (3), Pump (4), Rupture disks (5, 8), Heating bath (6), Extractor (7), Temperature transducers (TT), Pressure transducers (PT), Expansion valve (El), Separator (9).

Supercritical fluid extraction (SFE) is an extraction procedure which utilizes solvents in their supercritical state as extractive agents. These supercritical fluids have similar densities to liquids, but lower viscosities and so higher solvation power. Supercritical fluids constitute an acceptable alternative to conventional liquid solvents in the analytical extraction of enviromnental samples [137]. Supercritical fluids are obtained in commercially available instruments, applying high temperatures and pressures to the solvent to ensure conditions above the critical point. The sample is located in an inert extraction cell, where the supercritical fluid is pumped. 

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