Mass transfer in extraction

The mass transfer in extraction equipment using mixers requires careful study before scale-up. 

Mass transfer during formation of drops or bubbles at an orifice can be a very significant fraction of the total mass transfer in industrial extraction or absorption operations. Transfer tends to be particularly favorable because of the exposure of fresh surface and because of vigorous internal circulation during the formation period. In discussing mass transfer in extraction, it has become conventional (H12) to distinguish four steps (1) formation, (2) release, (3) free rise or fall, (4) coalescence. Free rise or fall has been treated in previous chapters. Steps 1 and 2 are considered here. 

Mass transfer in extraction from solid substrates in most cases depends heavily on the transport rate in the solid phase. The length of the transport path determines mass transport in the solid phase. In general the extraction rate increases with decreasing particle size. However, mass transfer into the fluid phase has to be achieved. If the smaller particles hinder the flow of the fluid in the fixed bed, then the mass transfer rate and the amount of extract decrease with smaller particles. 

Schiigerl, K., Blaschke,H.G., Brunke,U. and R. Streicher. "Interaction of fluid dynamics, interfacial phenomena and mass transfer in extraction processes". Recent Developments in Separation Science Vol. 3 (CRC Press, Cleveland, 1977). 

The contribution to the height of a transfer unit overall based on the raffinate-phase compositions is the sum of the contribution from the resistance to mass transfer in the raffinate phase plus the contribution from the resistance to mass transfer in the extract phase divided bythe extraction factor [Eq. (15-31)]. 

Prediction methods attempt to quantify the resistances to mass transfer in terms of the raffinate rate R and the extract rate E, per tower cross-sectional area Af, and the mass-transfer coefficient in the raffinate phase and the extract phase times the interfacial (droplet) mass-transfer area per volume of tower a [Eqs. (15-32) and (15-33)]. 

For liquid/liquid extraction, data on mass transfer rate of the system at typical operating conditions are required. Also required are an applicable liquid/liquid equilibrium curve and data on chemical reactions occurring after mass transfer in the mixer. 

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 ... 

Steps (i) and (ii) are controlled by molecular diffusion. Higher operating temperatures can improve the kinetics of mass transfer in all three steps. Vandenburg et al. [37] described the kinetics of PFE extraction using the hot-ball model [286] derived for SFE extractions. 

Following the assumption that the feed stream only dissolves extraction solvent in the feed stage, then the apparent feed concentration after the mass transfer in the feed stage can be calculated6 ... 

Leibson, I and BECKMANN, R. B. Chem. Eng. Prog. 49 (1953) 405. The effect of packing size and column diameter on mass transfer in liquid-liquid extraction. 

Mass transfer in polymeric solutions by molecular diffusion is a comparatively slow process, and in extraction equipment where mass transfer occurs through a thin wiped film, inordinately large equipment surface areas are often required in order to obtain substantial rates of mass transfer. In commercial practice when this situation occurs, the required surface areas may, instead, be obtained by either one of two methods, both of which involve reducing the pressure in the extraction zone. In one approach the extraction pressure is fixed at a value which is less than the equilibrium partial pressure of the monomer or solvent in the polymeric solution fed to the extraction zone. In these circumstances gas bubbles... 

The overall coefficients of liquid-liquid mass transfer are important in the calculations for extraction equipment, and can be defined in the same way as the overall coefficients of gas-liquid mass transfer. In liquid-liquid mass transfer, one component dissolved in one liquid phase (phase 1) will diffuse into another liquid phase (phase 2). We can define the film coefficients /C i (nr h" ) and k (m h ) for phases 1 and 2, respectively, and whichever of the overall coefficients A (m h ), defined with respect to phase 1, or Al2 (mh ) based on phase 2, is convenient can be used. Relationships between the two film coefficients and two overall coefficients are analogous to those for gas-liquid mass transfer that is,... 

The diffusion coefficient as defined by Fick s law, Eqn. (3.4-3), is a molecular parameter and is usually reported as an infinite-dilution, binary-diffusion coefficient. In mass-transfer work, it appears in the Schmidt- and in the Sherwood numbers. These two quantities, Sc and Sh, are strongly affected by pressure and whether the conditions are near the critical state of the solvent or not. As we saw before, the Schmidt and Prandtl numbers theoretically take large values as the critical point of the solvent is approached. Mass-transfer in high-pressure operations is done by extraction or leaching with a dense gas, neat or modified with an entrainer. In dense-gas extraction, the fluid of choice is carbon dioxide, hence many diffusional data relate to carbon dioxide at conditions above its critical point (73.8 bar, 31°C) In general, the order of magnitude of the diffusivity depends on the type of solvent in which diffusion occurs. Middleman [18] reports some of the following data for diffusion. 

The interest in mass transfer in high-pressure systems is related to the extraction of a valuable solute with a compressed gas. This is either a volatile liquid or solid deposited within a porous matrix. The compressed fluid is usually a high-pressure gas, often a supercritical fluid, that is, a gas above its critical state. In this condition the gas density approaches a liquid—like value, so the solubility of the solute in the fluid can be substantially enhanced over its value at low pressure. The retention mechanism of the solute in the solid matrix is only physical (that is, unbound, as with the free moisture), or strongly bound to the solid by some kind of link (as with the so-called bound moisture). Crushed vegetable seeds, for example, have a fraction of free, unbound oil that is readily extracted by the gas, while the rest of the oil is strongly bound to cell walls and structures. This bound solute requires a larger effort to be transferred to the solvent phase. 

The usual specific flow-rates for extraction are very small. In terms of space velocities, these are about 5 to 15 kg/h per litre of extractor volume, with superficial velocities in the range of 0.5 to 10 mm/s. With these small velocities, natural convection mass transfer is the favoured mechanism of transport. Gas densities are in the range of 500 to 800 kg/m3, and viscosities are about 5 x 10 7 kg/(m s), thus giving kinematic viscosities of about 10 9 m2/s, which is a very small value for a fluid. For example, the kinematic viscosity of water is 10"7 m2/s and that of ambient air is 2 x 10 5 m2/s. This makes free convection a principal mechanism for mass-transfer in high pressure gases. 

Brodkorb M, Slater MJ. Multicomponent and contamination effects on mass transfer in a liquid-liquid extraction rotating disk contactor. Trans IChemE 2001 79(part A) 335-346. 

Morters M, Bart HJ. Fluorescence-indicated mass transfer in reactive extraction. Chem Eng Technol 2000 23 1-7. 

Kubisova, L ., Sabolova, E., Schlosser, ., Martak, J. and Kertesz, R. (2004) Mass-transfer in membrane-based solvent extraction and stripping of 5-methyl-2-pyrazinecarboxylic acid and co-transport of sulphuric add in HF contactors. Desalination, 163, 27. 

Table 3.3 presents the approximate physical properties of gases, supercritical fluids, and liquids. It shows that the densities of supercritical fluids are close to that of a liquid, whereas their viscosities are gaslike. The diffusion coefficients are in between. Due to these unique properties, supercritical fluids have good solvating power (like liquid), high diffusivity (better than liquid), low viscosity, and minimal surface tension (like gas). With rapid mass transfer in the supercritical phase and with better ability to penetrate the pores in a matrix, extraction is fast in SFE, along with high extraction efficiency. 

The ASE system is fully automated. An autoseal actuator moves the cell from the carousel into the heating oven. The solvent is delivered from one or more solvent bottles into the extraction cell by a pump. The oven is heated, and the temperature and pressure in the cell rise. When the pressure reaches 200 psi above the preset value, the static valve opens to release the excessive pressure and then closes again. Then the pump delivers fresh solvent to the cell to bring the pressure back to the preset value. The addition of fresh solvent increases the concentration gradient and enhances both mass transfer and extraction efficiency. The extracts are collected in 40 or... 

The aim of extraction is to promote mass transfer or extraction reaction between two phases (a mutually insoluble liquid-liquid system) by dispersing one liquid phase in another. In principle, creating a larger interface area in this operation is advantageous. However, it is necessary to consider that interface phenomena depend on the type of system. Extractors are classified into three types ... 

Besides fluid mechanics, thermal processes also include mass transfer processes (e.g. absorption or desorption of a gas in a liquid, extraction between two liquid phases, dissolution of solids in liquids) and/or heat transfer processes (energy uptake, cooling, heating, drying). In the case of thermal separation processes, such as distillation, rectification, extraction, and so on, mass transfer between the respective phases is subject to thermodynamic laws (phase equilibria) which are obviously not scale dependent. Therefore, one should not be surprised if there are no scale-up rules for the pure rectification process, unless the hydrodynamics of the mass transfer in plate and packed columns are under consideration. If a separation operation (e.g. drying of hygroscopic materials, electrophoresis, etc.) involves simultaneous mass and heat transfer, both of which are scale-dependent, the scale-up is particularly difficult because these two processes obey different laws. 

Some advantages of the new system compared to extraction columns are the higher flow rates at low volumes because no flooding problems occur and a much higher specific surface which leads to a better mass transfer. In addition to this, the start-up of the system is fast, easy and allows a broad operation range. Another point is the low space requirement for the plant. The complete plant which has a flow capacity of 100 kg/h CO2 and up to 5 kg/h feed, has been built into a 3 x 3 x 3 m room. ... 

The mass transfer in solvent extraction takes place when a water immiscible organic phase is intimately mixed with an aqueous phase fron which one or more constitutents are transferred to the organic phase Stripping of the loaded organic phase with an aqueous solution concentrate the desired components, while at the same time regenerating the organii solvent for recycle. 

Mechanical expression of rice bran yields less oil, 10-12%, than solvent extraction, 16-18%. Rice bran is treated with steam and dried prior to pressure expression. Prepressing is usually carried out at 70 kg/cm followed by oil expulsion at 105-316 kg/cm (9). As a result of the low yield of oil from mechanical extraction, residual oil in the bran is recovered with hexane. Hexane extraction can be performed by batch or a continuous operation. Continuous operation uses countercurrent flow to improve mass transfer. Solvent extraction at high temperatures results in higher crude oil yield, but the crude oil is of lower quality. A new oil-extraction process, which involves premolding of rice bran at 14% moismre content and <40°C followed by hexane extraction at <15°C, was reported to yield a light-colored crude oil with no wax (9). 

Bocquet S, Torres A, Sanchez J, Rios GM, and Romero J. Modebng the mass transfer in solvent-extraction processes with hollow-fiber membranes. AIChEJ. 2005 51(4) 1067-1079. 

Bothun GD, Knutson BL, Strobel HJ, and Nokes SE. Mass transfer in hollow fiber membrane contactor extraction using compressed solvents. J. Membr. Sci. 2003 227(1-2) 183-196. 

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