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Rate of Extraction

In extraction, the driving force is the difference between the concentration of the component being transferred, the solute, at the solid interface and in the bulk of the solvent stream. For liquid-hquid extraction, a double film must be considered, at the interface and in the bulk of the other stream. [Pg.100]

The rate of change in quantity of solution, dw/dt is given by the following equation [Pg.100]

In the simple case of batch extraction from a solid in a contact stage, a mass balance on the solute gives the equation  [Pg.100]

Example 7.2 In a pilot scale test using a vessel Inn in volume, a solute was leached from an inert solid axid the water was 75 percent saturated in 10 seconds. [Pg.100]

in a full-scale unit, 1000 kg of the inert solid containing, as before, 25 percent by mass of the water-soluble component, is agitated with 100m of water, how long will it take for all the solute to dissolve, assuming conditions are equivalent to those in the pilot scale vessel Water is saturated with the solute at a concentration of 3.0 kgjvn .  [Pg.101]


Interfacial Mass-Transfer Coefficients. Whereas equiHbrium relationships are important in determining the ultimate degree of extraction attainable, in practice the rate of extraction is of equal importance. EquiHbrium is approached asymptotically with increasing contact time in a batch extraction. In continuous extractors the approach to equiHbrium is determined primarily by the residence time, defined as the volume of the phase contact region divided by the volume flow rate of the phases. [Pg.62]

If the solute is uniformly distributed through the soHd phase the material near the surface dissolves first to leave a porous stmcture in the soHd residue. In order to reach further solute the solvent has to penetrate this outer porous region the process becomes progressively more difficult and the rate of extraction decreases. If the solute forms a large proportion of the volume of the original particle, its removal can destroy the stmcture of the particle which may cmmble away, and further solute maybe easily accessed by solvent. In such cases the extraction rate does not fall as rapidly. [Pg.87]

The overall extraction process is sometimes subdivided into two general categories according to the main mechanisms responsible for the dissolution stage (/) those operations that occur because of the solubiHty of the solute in or its miscibility with the solvent, eg, oilseed extraction, and (2) extractions where the solvent must react with a constituent of the soHd material in order to produce a compound soluble in the solvent, eg, the extraction of metals from metalliferous ores. In the former case the rate of extraction is most likely to be controUed by diffusion phenomena, but in the latter the kinetics of the reaction producing the solute may play a dominant role. [Pg.87]

Diffusion and Mass Transfer During Leaching. Rates of extraction from individual particles are difficult to assess because it is impossible to define the shapes of the pores or channels through which mass transfer (qv) has to take place. However, the nature of the diffusional process in a porous soHd could be illustrated by considering the diffusion of solute through a pore. This is described mathematically by the diffusion equation, the solutions of which indicate that the concentration in the pore would be expected to decrease according to an exponential decay function. [Pg.87]

In case A the solvents are immiscible, so the rate of feed solvent alone in the feed stream F is the same as the rate of feed solvent alone in the raffinate stream R. In like manner, the rate of extraction solvent alone is the same in the stream entering S as in the extract stream leaving E (Fig. 15-12). The ratio of extraction-solvent to feed-solvent flow rates is therefore S /F = E /R. A material balance can be written around the feed end of the extrac tor down to any stage n (see Fig. 15-12) and then rearranged to a McCabe-Thiele type of operating line with a slope of F /S [Eq. (15-11)]. [Pg.1461]

It cannot be overemphasized that knowledge of the characteristics of such eqidpment is surprisingly underdeveloped. The number of quantities that influence the rate of extraction is veiy large, and many of them are not well understood. Most of the available data were taken... [Pg.1473]

The number of variables that are known to influence the rate of extraction is exceedingly large, and includes at least the following Size, shape, and material of packing Tower diameter Packing depth... [Pg.1477]

The (I)-(III)-samples sorption ability investigation for cationic dyes microamounts has shown that for DG the maximum rate of extraction is within 70-90 % at pH 3. The isotherm of S-type proves the physical character of solution process and a seeming ionic exchange. Maximal rate of F extraction for all samples was 40-60 % at pH 8 due to electrostatic forces. The anionic dyes have more significant affinity to surface researching Al Oj-samples comparatively with cationic. The forms of obtained soi ption isotherms atpH have mixed character of H,F-type chemosorption mechanism of fonuation of a primary monolayer with the further bilayers formation due to H-bonds and hydrophobic interactions. The different values of pH p for sorbents and dyes confirm their multifunctional character and distinctions in the acid-base properties of adsoi ption centers. [Pg.266]

The rates of extract involved in industrial ventilation are by nature of a high volume. It is of interest to consider the energy required to heat one cubic meter of air from, say, an outdoor temperature of -5 °C to be discharged into the space at 20 °C. [Pg.711]

The terms oil production and gas production refer to rates of extraction of liquid and gaseous hydrocarbon materials from natural underground deposits. Reserves and resources, on the other hand, refer to amounts of oil and gas that are present in the deposits, the difference between reserves and resources being whether or not the amounts can be economically recovered under current conditions. Supply refers to the amount of a product that becomes available for... [Pg.923]

The rate of extraction depends on the mass transport coefficient (f), the phase contact area (F) and the difference between the equilibrium concentration and the initial concentration of the dissolved component, which is usually expressed as the driving force of the process (a). The rate of extraction (V) can be calculated as shown in Equation (135) ... [Pg.267]

It is not surprising that for most solvent-water combinations and for many anions, the rate of extraction increases with the number of C atoms of the catalyst cation. To a first... [Pg.118]

Mass conservation at the rhizoplane means that the diffusive flux towards the root, Eq. (4), must equal the rate of extraction by the root, Eq. (9), leading to the boundary condition... [Pg.336]

Contimious liquid extraction techniques are used when the sample volume is large, the distribution constant is small, or the rate of extraction is slow. The efficiency of extraction depends on many factors including the viscosity of the phases, the magnitude of the distribution constant, the relative phase volumes, the interfacial surface area, and the relative velocity of the phases. Numerous continuous extractors using llghter-than-water and heavier-than-water solvents vee been described [3,2 7,42,73,74]. Generally, either the ligi Pr or heavier density... [Pg.385]

Supercritical fluid extraction can be performed in a static system with the attainment of a steady-state equilibrium or in a continuous leaching mode (dynamic mode) for which equilibrium is unlikely to be obtained (257,260). In most instances the dynamic approach has been preferred, although the selection of the method probably depends just as much on the properties of the matrix as those of the analyte. The potential for saturation of a component with limited solubility in a static solvent pool may hinder complete recovery of the analyte. In a dynamic system, the analyte is continuously exposed to a fresh stream of solvent, increasing the rate of extraction from the matrix. In a static systea... [Pg.409]

Various models of SFE have been published, which aim at understanding the kinetics of the processes. For many dynamic extractions of compounds from solid matrices, e.g. for additives in polymers, the analytes are present in small amounts in the matrix and during extraction their concentration in the SCF is well below the solubility limit. The rate of extraction is then not determined principally by solubility, but by the rate of mass transfer out of the matrix. Supercritical gas extraction usually falls very clearly into the class of purely diffusional operations. Gere et al. [285] have reported the physico-chemical principles that are the foundation of theory and practice of SCF analytical techniques. The authors stress in particular the use of intrinsic solubility parameters (such as the Hildebrand solubility parameter 5), in relation to the solubility of analytes in SCFs and optimisation of SFE conditions. [Pg.85]

The efficiency of extraction is mainly dependent on temperature as it influences physical properties of the sample and its interaction with the liquid phase. The extraction is influenced by the surface tension of the solvent and its penetration into the sample (i.e. its viscosity) and by the diffusion rate and solubility of the analytes all parameters that are normally improved by a temperature increase. High temperature increases the rate of extraction. Lou et al. [122] studied the kinetics of mass transfer in PFE of polymeric samples considering that the extraction process in PFE consists of three steps ... [Pg.118]

The flow rate of extracted air can be determined by considering the air velocity, as determined by Darcy s Law (Equation 14.8), and the radial distribution of pressure (Equation 14.11). The solution for air velocity as a function of the radial distance is given in Equation 14.13 ... [Pg.529]

Tests have been carried out on the rate of extraction of benzoic acid from a dilute solution in benzene to water, in which the benzene phase was bubbled into the base of a 25 mm diameter column and the water fed to the top of the column. The rate of mass transfer was measured during the formation of the bubbles in the water phase and during the rise of the bubbles up the column. For conditions where the drop volume was 0.12 cm3 and the velocity of rise 12.5 cm/s, the value of Kw for the period of drop formation was 0.000075 kmol/s m2 (kmol/m3), and for the period of rise 0.000046 kmol/s m2 (kmol/s m3). [Pg.189]

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance... [Pg.95]

Solvent. The liquid chosen should be a good selective solvent and its viscosity should be sufficiently low for it to circulate freely. Generally, a relatively pure solvent will be used initially, although as the extraction proceeds the concentration of solute will increase and the rate of extraction will progressively decrease, first because the concentration gradient will be reduced, and secondly because the solution will generally become more viscous. [Pg.503]

Temperature. In most cases, the solubility of the material which is being extracted will increase with temperature to give a higher rate of extraction. Further, the diffusion coefficient will be expected to increase with rise in temperature and this will also improve the rate of extraction. In some cases, the upper limit of temperature is determined by secondary considerations, such as, for example, the necessity to avoid enzyme action during the extraction of sugar. [Pg.503]

In most cases the interfacial area will tend to increase during the extraction and, when the soluble material forms a very high proportion of the total solid, complete disintegration of the particles may occur. Although this results in an increase in the interfacial area, the rate of extraction will probably be reduced because the free flow of the solvent will be impeded and the effective value of b will be increased. [Pg.504]

As already pointed out, the rate of extraction will, in general, be a function of the relative velocity between the liquid and the solid. In some plants the solid is stationary and the liquid flows through the bed of particles, whilst in some continuous plants the solid and liquid move countercurrently. [Pg.507]


See other pages where Rate of Extraction is mentioned: [Pg.141]    [Pg.75]    [Pg.388]    [Pg.1461]    [Pg.1467]    [Pg.1480]    [Pg.1488]    [Pg.1015]    [Pg.1016]    [Pg.55]    [Pg.268]    [Pg.282]    [Pg.25]    [Pg.243]    [Pg.199]    [Pg.916]    [Pg.512]    [Pg.95]    [Pg.532]    [Pg.376]    [Pg.168]    [Pg.168]    [Pg.170]    [Pg.72]    [Pg.502]    [Pg.223]   


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