Packings volumetric mass transfer

Traditional Design Method The traditionally employed conventional procedure for designing packed-tower gas-absorption systems involving chemical reactions makes use of overall volumetric mass-transfer coefficients as defined by the equation... 

With regards to handling data on industrial apparatus for gas-liquid mass transfer (such as packed columns, bubble columns, and stirred tanks), it is more practical to use volumetric mass transfer coefficients, such as KqU and K a, because the interfacial area a cannot be well defined and will vary with operating conditions. As noted in Section 6.7.2, the volumetric mass transfer coefficients for packed columns are defined with respect to the packed volume - that is, the sum of the volumes of gas, liquid, and packings. In contrast, volumetric mass transfer coefficients, which involve the specific gas-liquid interfacial area a (L L 5), for liquid-gas bubble systems (such as gassed stirred tanks and bubble columns) are defined with respect to the unit volume of gas-liquid mixture or of clear liquid volume, excluding the gas bubbles. In this book, we shall use a for the specific interfacial area with respect to the clear liquid volume, and a for the specific interfacial area with respect to the total volume of gas-liquid mixture. 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

When comparing film flow monolithic reactors with conventional catalytic packed reactors, one can conclude that the critical hydrod)mamic characteristics (hydraulic capacity, pressure drop, and volumetric mass transfer rates) are similar, but monoliths have distinct advantages greater flexibility, easier scale-up, the susceptibility of fhe surface to coating procedures, and advances in control of flooding—all allowing the use of very small channels and therefore efficienf cafalysf ufilizafion. 

Overall volumetric mass-transfer coefficient more than five times larger than that of the packed bed device [19]... 

Eq. (22) defines the volumetric mass transfer coefficients KyU and Kxa, which have the typical unit moles/(hrm mole fraction). Note also that the term Adz represents the differential packed tower volume. Therefore, Eq. (22) can serve as a model to determine packing height in a packed absorption tower. 

The interfacial surface area per unit volume, a, in many types of packing materials used in industrial towers is virtually impossible to measure. Both a and the mass-transfer coefficient depend on the physical geometry of the equipment and on the flow rates of the two contacting, immiscible streams. Accordingly, they are normally correlated together as the volumetric mass-transfer coefficient, kca. 

Estimate volumetric mass-transfer coefficients in packed towers. 

Air at 294 K and 1 atm enters a fixed-bed adsorber at a flow rate of 0.146 m3/s with a benzene vapor concentration of 29 g/m3. The cylindrical adsorber is 0.61 m in inside diameter and is packed to a height of 1.83 m with 331 kg of silica gel particles having an effective diameter of 2.6 mm and an external porosity of 50%. The adsorption isotherm for benzene has been determined experimentally and found to be linear over the concentration range of interest, given by q = kc, where q is in kg benzene/kg gel, c is in kg benzene/m3 of gas, and k = 4.127 m3 of gas/kg of gel. It has been estimated that the overall volumetric mass-transfer coefficient for the conditions prevailing in the bed is Kc.a = 8.79 s-1. Assuming isothermal and isobaric operation, calculate ... 

This program calculates the diameter of a packed column to satisfy a given pressure drop criterium,and estimates the volumetric mass-transfer coefficients. Data presented are from Example 4.4. 

Packing volumetric heat transfer coefficient, Uy = 3 kW/m °C superficial gas velocity = 1 m/s mass loading liquid/gas = 1.5-3/1. 

Figure 3-11 shows the product of the volumetric mass transfer coefficient fig and effective volume specific surface for mass transfer, as a function of gas and liquid loads for different types of random packings, found by empirical means. 

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