Predicting Contactor Performance

4 PREDICTING CONTACTOR PERFORMANCE 6.4-1 Physical Absorption in Packed Towers 

Icq = gas-phase ranss transfer coefficient (lb mole/s fl3 1 aun or kmoI/N s) kL - liquid phase mass transfer coefficient (ft/s or m/s) pL = density or liquid (lbm/ftJ or kg/m5) 

FIGURE 6.3-13 Diagram of curved equilibrium line showing inflection point and effective avemge slopes for both sections- (From Wilke nad von Stockar,10 copyright 1976. Used with permission of John Wiley Sons, New York.) 

In this model, a is assumed to be equal to aw% the welled area of the packing, and is calculated by die equation 

Belles and Fair evaluated a large data bese of published tests ou the performance of columns containing random packing and developed a mass transfer mode) that appeals to be superior to those previously 

Oc = a critical surface tension 61 dyn/cm for ceramic packing, 75 dyn/cm for steel packing, and 33 dyn/cm for polyethylene pacldng 

= dry outside surface area of packing a = wetted area of packing 

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Our objective here is to try to answer the following questions For a proposed type of gas-liquid contactor compatible with the properties and flow rates of the phases and with the reaction type, what are the likely values of the specific interfacial area and the gas and liquid mass-transfer coefficients by which the contact performance can be predicted And what is the expected accuracy of these values Table XVIII gives typical values of these parameters in typical contactors shown in Fig. 12 for fluids with properties not very different from those of air and water (especially, liquid viscosity under 5 cP where the liquid is nonfoaming). Because this review is especially concerned with the chemical method of determining these parameters, experimental data obtained by this method will be given in subsequent tables and figures. 

In the process industries, low viscosity Newtonian liquids predominate so this chapter concentrates on these. Even within this category, the performance of gas-liquid contactors can be influenced by poorly understood surface phenomena, which particularly affect bubble size and coalescence, so is not yet predictable from first principles. 

The characterisation of mass transfer is essential for the design of the micro-reactor. In liquid-liquid flows most studies have focused on the estimation of overall mass transfer coeflicients, while no model based on theory has been developed so far. The overall volumetric mass transfer coeflicient (kua) is a characteristic parameter of a system used to evaluate the performance of the contactors, and is a combination of the mass transfer coefficient (kp), which depends mainly on the difiusivity of solute, characteristic diffusion length and interfacial hydrodynamics, and of the specific interfacial area (a), which depends on the flow pattern. The prediction of the overall volumetric mass transfer coeflicient remains difficult due to secondary phenomena, tike interfacial instabilities. 

Mass transfer coefficients in micro-reactors are much higher than those obtained in conventional macro-contactors as it can be seen in Table 2.4. One common drawback of conventional contactors is the inability to predict precisely flow characteristics and interfacial area, because of the complexities of the governing hydrodynamics. This often leads to uncertainties in the design and causes limitations on the performance than can be achieved. Mass transfer rates in micro-reactors can be up to 2 orders of magnitude higher. In addition, consecutive reactions can be efficiently suppressed by strict control of residence time and its distribution (Kashid et al. 2011). 

They performed laboratoiy tests to measure the rate of COS absorption in MDEA and used a model based on the penetration theory to calculate the kinetic rate constant for equation 2-46. The results should be of value in rigorous design calculations to predict the fiac-tion of COS in a feed stream that will be absorbed in a commercial MDEA contactor. 

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