Mass transfer area coefficient

Drugs are cleared from the blood during peritoneal dialysis in an analogous fashion to the clearance of creatinine or other plasma constituents. Transport between the blood and the peritoneal cavity is symmetric in the sense that transport rates are equivalent in each direction, and therefore if the PA (mass transfer-area coefficient) for a substance of equivalent molecular weight is available, the transfer rate can be estimated from... 

Another term used to characterize the transport properties of dialysis membranes is the so-called mass transfer area coefficient (MTAC), which is the product of the mass transfer coefficient (Ko) times the membrane surface area (A), or KoA. Usually, the terms MTAC and KoA reported are those for urea. While Ko should equal the maximum clearance obtained at high blood and dialysate flow rates, reports in the dialysis Hterature (Leypoldt et al., 1997) discuss the variation of KoA with dialysate flow rate. Such reports reflect the manufacturers or others inappropriate extrapolation of KoA from data obtained at typically clinically relevant flow rates, which arc not high enough to minimize boundary layer resistance. While the measurement of in vivo rather than in vitro characteristics of dialyzers is meant to provide more accurate or realistic information, it can be misleading in this context. 

Leypoldt, J. K., Cheung, A. K., Agodoa, L. Y., Daugitdas, J. T., Greene, T., and Keshaviah, R R. (1997). Hemodialyzer mass transfer-area coefficients for urea increase at high dialysate flow rates. The Hemodialysis (HEMO) Study. Kidney Int. 51, 2013. 

Keshaviah P, Emerson PF, Vonesh EF, Brandes JC. 1994. Relationship between body size, fill volume, and mass transfer area coefficient in peritoneal dialysis. / Am Soc Nephrol 4(10) 1820-1826. 

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)]. 

The mass-transfer coefficients depend on complex functions of diffii-sivity, viscosity, density, interfacial tension, and turbulence. Similarly, the mass-transfer area of the droplets depends on complex functions of viscosity, interfacial tension, density difference, extractor geometry, agitation intensity, agitator design, flow rates, and interfacial rag deposits. Only limited success has been achieved in correlating extractor performance with these basic principles. The lumped parameter deals directly with the ultimate design criterion, which is the height of an extraction tower. 

Ogl gas-liquid mass transfer area to reactor volume relation c concentration D diffusion coefficient... 

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

Data Effective interfacial area for 25 mm packing = 280 m2/m3 Mass transfer film coefficients ... 

Since the mass transfer coefficient, k, and the specific interfacial area, a, vary in a similar manner, dependent upon the hydrodynamic conditions and system physical properties, they are frequently combined and referred to as a ka value or more properly as a mass transfer capacity coefficient. 

Let us try to describe some of these phenomena quantitatively. For simphe-ity, we will assume isothermal, constant-holdup, constant-pressure, and constant density conditions and a perfectly mixed liquid phase. The gas feed bubbles are assumed to be pure component A, which gives a constant equihhrium concentration of A at the gas-liquid interface of CX (which would change if pressure and temperature were not constant). The total mass-transfer area of the bubbles is Aj j- and could depend on the gas feed rate f constant-mass-transfer coefficient (with units of length per time) is used to give the flux of A into the liquid through the liquid film as a flinction of the driving force. 

In surface aeration, the absorption rate is also measured with 02 electrodes in the liquid volume. By this method, the liquid-side overall mass transfer coefficient, kLa, is determined (a - volume-related mass transfer area = surface of all gas bubbles in the liquid volume). Due to the fact that the mass transfer in surface aeration occurs almost solely in the liquid surface, A, and by no means in the liquid volume, V, the measured kLa has to be multiplied by V to obtain the target quantity kLA = kLa V. 

A common feature of all models for the upper part of circulating fluidized beds is the description of the mass exchange between dense phase and dilute phase. Analogously to low-velocity fluidized beds, the product of the local specific mass-transfer area a and the mass-transfer coefficient k may be used for this purpose. Many different methods for determining values for these important variables have been reported, such as tracer gas backmixing experiments [112], non-steady-state tracer gas experiments [117], model reactions [115], and theoretical calculations [114],... 

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