Discontinuous extraction processes

Typical data for the high pressure solvent pumps used in the processes described above are given in Table 9.1. 

Application Discharge pressure (Pressure differential) Temperature Capacity 

There are a number of other applications of pumps and compressors in supercritical extraction plant. The exact nature of these depends on the particular process. Examples are  

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Modify the above program to model the case of a discontinuous five-stage extraction cascade, in which a continuous flow of aqueous phase L is passed through the cascade. The solvent is continuously recycled through the cascade and also through a solvent holding tank, of volume Vs, as shown below. This problem of a stagewise discontinuous extraction process has been solved analytically by Lelli (1966). 

Figure 8.12 shows the segmented ring quick closure system [12]. This special development of the Bredtschneider closure makes it possible to open or close the vessel in a very short time (3-4 minutes) by a computer-controlled hydraulic system. The technique has been operated successfully with discontinuous extraction processes for several years. Figure 8.13, for example, shows this type of closure on a pressure vessel used for the decaffeination of tea. The inner diameter of the vessel is t/i = 1100 mm. Closure and vessel were designed for a fluctuating load. 

The recovery and repressurisation of solvent gas following depressurisation of a section of plant, which has already been discussed (section 9.2.2) for discontinuous extraction processes, can be necessary for economic or antipollution reasons. As the necessary gas compressors do not usually form a very important part of the plant, the discussion below will be limited to some basic considerations. 

Ma.nufa.cture. Nickel carbonyl can be prepared by the direct combination of carbon monoxide and metallic nickel (77). The presence of sulfur, the surface area, and the surface activity of the nickel affect the formation of nickel carbonyl (78). The thermodynamics of formation and reaction are documented (79). Two commercial processes are used for large-scale production (80). An atmospheric method, whereby carbon monoxide is passed over nickel sulfide and freshly reduced nickel metal, is used in the United Kingdom to produce pure nickel carbonyl (81). The second method, used in Canada, involves high pressure CO in the formation of iron and nickel carbonyls the two are separated by distillation (81). Very high pressure CO is required for the formation of cobalt carbonyl and a method has been described where the mixed carbonyls are scmbbed with ammonia or an amine and the cobalt is extracted as the ammine carbonyl (82). A discontinued commercial process in the United States involved the reaction of carbon monoxide with nickel sulfate solution. 

Boliden (2) A lead extraction process in which a sulfide ore, mixed with coke, is smelted in an electric furnace, air jets forming vortices between the electrodes. Discontinued in 1988 in favor of the Kaldo process, using a rotating furnace. 

Of the various extraction processes the decaffeination with supercritical CO2 exhibits the most commercial advantages for bulk production. The process is a discontinuous one. Fig. 1.4-3 shows a number of serially arranged extractors (5) charged with the supercritical CO2 feed by the centrifugal circulation pump (1). 

This example concerns a discontinuous (batch) liquid-solid extraction process. Here, the quantity of extracted species (y, -< y >- = kgA/kg liq, A = type of species) depends on the following factors the ratio of mixing phases (mi/mg-associated to Zj -< mj/mg >-= kg liq /kg solid), the contact time (t associated to Zj, < T >-= min) the mixing rate (w = Tind -associated to Z3, -< w >-= m/s, n-rotation speed, d - mixer diameter) the mean concentration of one species carrier, which is placed in the liquid phase (Cg -associated to Z4, CgA >-= kg carrier/kg liq) the diameter of the solid particles (d-associated to Z5, d >-= m). The temperature can be another important factor in the process, but initially we can consider that it is constant. Nevertheless, it will be considered as an additional factor in a second step of this analysis. The experiments are carried out with a solid containing 0.08 kg A/kg solid. 

Of increasing interest are structural changes in or coatings of natural materials. Thus the volume increase of tobacco resulting from controlled pressure release is already used industrially. The discontinuous extraction of natural materials requires the plant to be depressurised after completion of the process, whereby the non-stationary behaviour of pressure and temperature in the extraction vessel is of great importance for the design of the pressure vessel. 

Note The inability to operate the process continuously (separations were made batch wise) and its rejection of the still useful enriched uranium caused the discontinuation of its application in favor of the solvent extraction processes, which overcame both of these limitations. 

Depending on the scale and nature of the extraction problem, it may be necessary to design the extraction process for continuous or discontinuous operation and for single or multistage extraction. Furthermore various different separation techniques may be used. 

Fig. 16 Experimental setup of the miniplant in a discontinuous mode of operation (a) as well as in continuous circulation of both phases (b), and an illustration of the processes occurring during the particle extraction process (c) (schematics were created by Leistner, 1)...

Decontamination of soils using supercritical fluids is an attractive process compared to extraction with liquid solvents because no toxic residue is left in the remediated soil and, in contrast to thermal desorption, the soils are not burned. In particular, typical industrial wastes such as PAHs, PCBs, and fuels can be removed easily [7 to 21]. The main applications are in preparation for analytical purposes, where supercritical fluid extraction acts as a concentration step which is much faster and cheaper than solvent-extraction. The main parameters for successful extraction are the water content of the soil, the type of soil, and the contaminating substances, the available particle-size distribution, and the content of plant material, which can act as adsorbent material and therefore prolong the extraction time. For industrial regeneration, further the amount of soil to be treated has to taken into account, because there exists, so far, no possibility of continuous input and output of solid material for high pressure extraction plants, so that the process has to be run discontinuously. 

Plate Theory. Envision the chromatographic system as a discontinuous process functioning the same as a distillation or extraction system, that, is comprised of a large number of equivalent plates. 

The high affinity shown by carboxylic acids for copper (II) compared with the remaining divalent metals of the first transition series appears to be due in part to the stabilization of the extracted complexes by the formation of the well-known dimeric structure (1) in which copper(II) carbox-ylates exist in the solid state and in non-donor solvents.54 The axial ligands, L, consist of undissociated carboxylic acid molecules55 or, in the absence of an excess amount of extractant, they may consist of water or other solvent molecules.56 Copper was successfully removed from nickel sulfate solutions on the base-metal plant at Matthey Rustenburg Refiners in South Africa by being extracted into Versatic 10 acid at a controlled pH value. The process is believed to have been discontinued only because improvements in the selective leaching of copper and nickel rendered it unnecessary. 

In this procedure it is assumed that musical signals are made up as additive combinations of tones (sinusoids) which represent the fundamental and harmonics of all the musical notes which are playing. Since this is certainly not the case for most non-musical signals, we might expect the method to fail for, say, speech extracts or acoustical noises. Fortunately, it is for musical extracts that pitch variation defects are most critical. The pitch variation process is modelled as a smoothly varying waveform with no sharp discontinuities, which is reasonable for most wow generation mechanisms. 

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