Mass transfer efficiency random packings

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. 

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). 

The amount of inteifacial area that is available in the packed column is of vital importance in determining the mass transfer efficiency of the packed column. However, it is rarely known with any degree of accuracy. It bears some relation to the specif area of the packing, that is, the amount of dry packing surface provide by the packing elements. But even this parameter is not really known with accuracy in the case Of the random packings, since an exact count of elements contained in the column is not feasible for commercial scale columns, and even for controlled pilot-scale units the elements may nest or otherwise block out area that can then serve no purpose for mass transfer. 

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]. 

Wagner I, Stichlmair J, Fair JR (1997) Mass transfer in beds of modem, high-efficiency random packings. Ind Eng Chem Res 36 227-237... 

The efficiency curves described above are somewhat idealized. In practice, efficiency curves generally follow the above principles, but may deviate from the patterns shown in Figs. 8.18a to c. Kunesh (51) states that point E in Fig. 8.16a is not always observed, and that in many cases the curve between points B and F is flat. Figure 6.16d is an example of experimentally measured curves for a random packing that deviate from Figs. 8.16a and b. The hydraulics and mass transfer processes taking place in a packed bed are extremely complex and are not well-understood. 

Like their random-packing efficiency model (above), the Bravo, Fair et al. structured-packing model is based on the two-film theory. The HTU is calculated from the mass transfer coefficients and interfacial areas using Eqs. (9.23) and (9.24). The HETP can be calculated from the HTU using Eqs. (9.12) and (9.13). The mass transfer coefficients are evaluated from... 

Figure 13.45. Flooding correction factor for liquid phase mass transfer Bolles/Fair correlation for random packing efficiency.

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