Size of packing

The effective interfacial area depends on a number of factors, as discussed in a review by Charpentier [C/j m. Eng.J., 11, 161 (1976)]. Among these factors are (1) the shape and size of packing, (2) the packing material (for example, plastic generally gives smaller interfacial areas than either metal or ceramic), (3) the liquid mass velocity, and (4), for smaU-diameter towers, the column diameter. 

In a trayed absorber the amine falls from one tray to the one below in the same manner as the liquid in a condensate stabilizer (Chapter 6, Figure 6-4). It flows across the tray and over a weir before flowing into the next downcomer. The gas bubbles up through the liquid and creates a froth that must be separated from the gas before it reaches the underside of the next tray. For preliminary design, a tray spacing of 24 in. and a minimum diameter capable of separating 150 to 200 micron droplets (using the equations developed in Volume 1 for gas capacity of a vertical separator) can be assumed. The size of packed towers must be obtained from manufacturer s published literature. 

Because there are not sufficient data to serve completely for all types and sizes of packing, it may be necessary to estimate Kga values by ratioing packing surface areas and making the other appropriate correction for the problem conditions. 

The summary of HETP values of Vital [142] for various types and sizes of packings are believed to be referenced to typical industrial distributors for the liquid. This variation can influence the value of HETP in any tabulation the effect of distributor design is discussed in an earlier section of this chapter. Porter and Jenkins [143] developed a model to improve the earlier models of Bolles and Fair from about 25% deviation to about a 95% confidence using a 20% factor of safety [139]. 

In general, the largest size of packing that is suitable for the size of column should be used, up to 50 mm. Small sizes are appreciably more expensive than the larger sizes. Above 50 mm the lower cost per cubic metre does not normally compensate for the lower mass transfer efficiency. Use of too large a size in a small column can cause poor liquid distribution. 

Mass transfer in packed columns is a continuous, differential, process, so the transfer unit method should be used to determine the column height, as used in absorption see Section 11.14.2. However, it often convenient to treat them as staged processes and use the HETS for the packing employed. For random packings the HETS will, typically, range from 0.5 to 1.5 m, depending on the type and size of packing used. 

Band broadening processes and particle size of packing... 

In using Eq. (14-71), therefore, it should be understood that the numerical values of K a will be a complex function of pressure, temperature, the type and size of packing employed, the liquid and gas mass flow rates, and the system composition (e.g., the degree of conversion of the liquid-phase reactant). 

A corollary is that capacity increases with random packing size or with the space between structured packing layers. Comparing with the first objective, a tradeoff exists the ideal size of packing is a compromise between maximizing efficiency and maximizing capacity. 

Stage efficiencies for packed towers must be based on experimental tests with each type of packing. The efficiency varies, not only with the type and size of packing, but also with the fluid rates, the fluid properties, the column diameter, the operating pressure, and, in general, the extent of liquid dispersion over the available packing surface. 

For cncnrrfnr ga<-1igiikLHnwnflnuj over a packed bed, various flow regimes such as trickle-flow (gas continuous), pulsed flow, spray flow, and bubble flow (liquid continuous) can be obtained, depending upon the gas and liquid flow rates, the nature and size- of packing, and the nature and properties of the liquid. The flow-regime transition is usually defined as the condition at which a slight increase in gas or liquid flow rate causes a sharp increase in the root-mean-square wall-pressure fluctuations. 

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