Mass transfer efficiency, structured packing

Figure 9-6MM. Nutter BHS structured packing with expanded metal texture, which maintains high mass transfer efficiency across all operating conditions. Nutter Engineering designs and manufactures BSH in North America under exclusive license from Julius Montz GmbH of Germany. Used by permission of Nutter Engineering, Harsco Corp.

Surface features. The surfaces of a few structured packings are smooth (e.g., Fig. 8.10a,g). Most structured packings have a roughened or enhanced surface that assists the lateral spread of liquid, promotes film turbulence, and enhances the area available for mass transfer. Laboratory measurements of absorption rates showed that both mass transfer efficiency and wetted area are enhanced by texturing metal surfaces (28). The extent to which mass transfer was improved varied with the type of texturing used. Texturing employed by... 

Structured packings have replaced trays and random packings as their cost has decreased and more is known of their performance behavior. Initially thought to be appropriate only for high vacuum distillations, they are now used for absorbers, strippers, and pressure distillations. Because of their open structure (over 90% voids) and large specific surface areas, their mass transfer efficiency is high when proper distribution of liquid and gas over the cross section can be maintained. Table 13.15 shows a comparison of features of several commercial makes of structured packings. 

Ultimately, the choice of packing is based on pressure drop, mass transfer efficiency, and cost. Structured packings made from sheet metal now cost less than 100 per cubic foot. The hydraulics of packed beds will be discussed later in this section. 

Structured Packings. A number of models for predicting mass transfer efficiency of a given structured packing have been... 

Example 13.18 is included to show the predicted mass transfer efficiency of an acetone-water column in which structured packing is used. Of particular interest are the results from Eqs. 13.255, 13.259, 13.261 and 13.262 are used to determine transfer units. The value of HETP is calculated using Eq. 13.172. Plug flow of the phases is usually assumed, although extreme ratios of the flowing phases can cause some back mixing. 

Static extraction columns Spray column Baffle column Packed column (random and structured packing) Sieve tray column Deliver low to medium mass-transfer efficiency, simple construction (no internal moving parts), low capital cost, low operating and maintenance costs, best suited to systems with low to moderate interfacial tension, can handle high production rates Petrochemical Chemical Food... 

Structured packing is nowadays largely used in industry because of lower pressure drop and higher mass transfer efficiency. There are numerous types. Following Kister (1992), stmctured packing has a definite advantage over modem random packing only at low liquid loads (< 20 gpm/ft2). 

Fair, J. R. and Bravo, J. L., Prediction of Mass Transfer Efficiencies and Pressure Drop for Structured Tower Packings in Vapor/Liquid Service, University of Texas at Austin, 1987. 

It has been well established that the efficiency of random packings is significandy affected by the quality of liquid distribution onto the packed bed. As was true when saddle-type packings first were developed, initially structured packings were assumed to be able to rectify the initial inferior liquid distribution quality. More recent work has shown that liquid distribution quality also is critical to achieving high mass transfer efficiency with structured packings [9]. 

J. R. Fair, J. L. Bravo, Prediction of mass transfer efficiency and pressure drop of structured tower packings in vapour/liquid service", Inst Chem. Eng. Symp. Ser. 104, (1987) A183. 

This is the first reactor reported where the aim was to form micro-channel-like conduits not by employing microfabrication, but rather using the void space of structured packing from smart, precise-sized conventional materials such as filaments (Figure 3.25). In this way, a structured catalytic packing was made from filaments of 3-10 pm size [8]. The inner diameter of the void space between such filaments lies in the range of typical micro channels, so ensuring laminar flow, a narrow residence time distribution and efficient mass transfer. 

The large porous particles are the oldest of these materials, and are no longer used in analytical hplc, although because of their high sample capacity they are still useful in preparative work. Columns packed with the large particles have relatively low efficiencies because of the long time it takes for solute species to diffuse into and out of the porous structure (slow mass transfer). 

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