Column internals random packing

Figure 8.7 provides fractionation column internal trays that are to be added to the overall pressure vessel cost. A packed-column internal (random packing) cost is also given as cost per cubic foot of column-packed volume. 

It is assumed that the mass tiansfer occurs in the layers on both sides of the phase boundary. According to the model, the mass transfer is caused by molecular diffusion. Therefore, a linear concentration profile is built up in the boundary layers. At the phase boundary itself, thermodynamic equilibrium is reached. More or less empirical correlations for the mass transfer coefficients /I and for the effective phase interface area are usually provided by the simulation program for different column internals (random packings, structured packings, and trays). The necessary diffusion coefficients can again be calculated according to Section 3.3.6. 

As indicated above, packed column internals include hqiiid distributors, packing support plates, redistributors (as needed), and holddown plates (to prevent movement of packing under flow conditions). Costs of these internals for columns with random packing are given in Fig. 14-80, based on early 1976 prices, and a Marshall and Swift cost index of 460. 

FIG. 14-81 Cost of internal devices for columns containing random packings, (a) Holddown plates and support plates, (b) Redistributors. [Pikulik and Diaz, Chem. Eng., 84(21), 106 (Oct, 10, 1977).]... 

Furthermore, a CFD based description of single-phase and multi-phase flows in column internals is given in (Yin et ah, 2000 Yin et ah, 2002) for random packings, in (Petre et ah, 2003 Larachi et ah, 2003) for structured packings and in (Trubac et ah, 2001) for a structured catalytic packing. 

The hardware issues regarding RD columns focus on the column internals such as the packings (random or structured packing, Fig. 4), distillation trays, and downcomers. Typical hardware design information is given in the standard sources as for conventional distillation. Some of the important issues on hardware design aspects are discussed below.l ... 

Fig. 4.6 Diagrammatic view of continuous column. AH the internal elements of both structured and random packing are included though it would be very unusual for both to be present in a single column.

If the investment costs of column internals are to be compared, the solution of a separation problem is first analyzed. Suitable internals have to be specified with criteria for comparison purposes, such as the specific column volume v, as the column volume per vapor or gas load, and separation efficiency for the optimum operating condition. For random and regular packing. 

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