Packed columns random packings

It is most convenient to make connections with standard taper joints. The checkers used with equal satisfaction a 50-cm. column randomly packed with short sections of glass tubing. 

Chen8,9 studied the gas holdup of a 7-cm i.d. 244-cm long column randomly packed with open-end screen cylinders of various sizes (1.27 cm x 1.27 cm and 1.9 cm x 1.9 cm) and screen meshes (8-14 mesh). The results with an air-water system were obtained in the bubble-flow regime. The screen cylinders were found to reduce the gas holdup. The results showed that for t/0g < 4 cm s, the gas holdup was a linear function of gas velocity, a result similar to the one obtained in an unpacked bubble-column bul not in a column packed with Raschig rings or other conventional packings. He also showed that for low gas velocity, l/0G < 3.64 cm s 1 the parameter (hG - 1ig)//ig was a unique linear function of liquid velocity (independent of gas velocity). Here, /iG is the gas holdup at zero liquid velocity. He also obtained a relationship between the gas holdup and the slip velocity between gas and liquid. All the data were graphically illustrated, however, no analytical correlation was presented. 

Shestopalov et al.47 measured the liquid-phase axial dispersion in a 246-mm-diameter and 1.5-m-high glass column. Randomly stacked Raschig rings with dimensions of 10 mm x 10 mm, 15 mm x 15 mm, and 25 mm x 25 mm were used as packing. Methylene blue dye solution was used as a tracer, and its steady-state... 

Fig. 6. Computed lateral voidage profile for a three-dimensional column randomly packed with uniform spheres. After Zimmerman and Ng 4).

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... 

Tray Columns Packed Columns (Random) Spray Contactors... 

It is preferable if the reactions in RD columns are catalyzed by heterogeneous catalysts because they are easily kept in the reactor and they allow hybrid RD columns combining reactive and non-reactive sections. Fig. 5.25 illustrates a column randomly packed with supported ion exchangers in rings. Such a heterogeneous cat-... 

In many applications packed columns appear to be the wave of the future. Columns packed with structured packings have a combination of low pressure drop and high efficiency that often makes them less expensive than either randomly packed columns or staged columns. Random packings can be made in a wide variety of materials so that practically any chemicals can be processed. Many of the flow distribution problems that limited the size of packed columns appear to have been solved. 

If three equilibrium stages are required, use a mixer-setder, a sieve tray column, a packed column (random or structured), or a membrane contactor. 

Table 2-7. Data relating to the experimental flooding point values, diagrammed in Fig. 2-17b, in columns randomly filled with plastic (PP, PVDF) packing elements. No. of test system acc. to Table 2-2...

Fig. 2. Packing materials for packed columns, (a)—(f) Typical packing elements generally used for random packing (g) example of stmctured packing.

The situation is very much poorer for stmctured rather than random packings, in that hardly any data on Hq and have been pubHshed. Based on a mechanistic model for mass transfer, a way to estimate HETP values for stmctured packings in distillation columns has been proposed (91), yet there is a clear need for more experimental data in this area. 

Fig. 21. Random packing elements for distillation columns (a), Raschig ring (metal) (b). Bed saddle (ceramic) (c), Intalox saddle (ceramic) (d), PaH ring...

Typical experimental values of HETP for a random packing such as 50-mm PaH rings, and a stmctured packing, such as Intalox 2T of Norton Co., under the same system conditions, are shown in Figure 25. Many designers of packed columns prefer the use of HETP instead of but the latter is more fundamental and discrirninates between Hquid- and vapor-phase resistances. It should be noted that terms such as H and N are based on... 

Introduction Packed columns for gas-liquid contacting are used extensively for absorption, stripping, and distillation operations. Usually the columns are filled with a randomly oriented packing material, but for an increasing number of applications the packing is very care-... 

Distillation Applications Packings are now routinely considered for distillation columns with diameters up to 10 m or more. The pressure drop advantages of the modern, through-flow random pack-... 

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. 

Once packing heights are determined in other sections from HETP (distillation) or Koa (absorption), the height allowances for the internals (from Figure 1) can be added to determine the overall column height. Column diameter is determined in sections on capacity and pressure drop for the selected packing (random dumped or structured). 

Bed limiters commonly are used with metal or plastic tower packings. The primary function of these devices is to prevent expansion of the packed bed, as well as to maintain the bed top surface level. In large diameter columns, the packed bed will not fluidize over the entire surface. Vapor surges fluidize random spots on the top of the bed so that after return to normal operation the bed top surface is quite irregular. Thus the liquid distribution can be effected by such an occurrence. 

Equation (1) can be used in a general way to determine the variance resulting from the different dispersion processes that occur in an LC column. However, although the application of equation (1) to physical chemical processes may be simple, there is often a problem in identifying the average step and, sometimes, the total number of steps associated with the particular process being considered. To illustrate the use of the Random Walk model, equation (1) will be first applied to the problem of radial dispersion that occurs when a sample is placed on a packed LC column in the manner of Horne et al. [3]. 

The dispersion of a solute band in a packed column was originally treated comprehensively by Van Deemter et al. [4] who postulated that there were four first-order effect, spreading processes that were responsible for peak dispersion. These the authors designated as multi-path dispersion, longitudinal diffusion, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase. Van Deemter derived an expression for the variance contribution of each dispersion process to the overall variance per unit length of the column. Consequently, as the individual dispersion processes can be assumed to be random and non-interacting, the total variance per unit length of the column was obtained from a sum of the individual variance contributions. 

In a packed column the individual solute molecules will describe a tortuous path through the interstices between the particles and some will randomly travel shorter routes than the average and some longer. The multi-path effect is diagramatically depicted in Figure 5. 

Driven by the concentration gradient, solutes naturally diffuse when contained in a fluid. Thus, a discrete solute band will diffuse in a gas or liquid and, because the diffusion process is random in nature, will produce a concentration curve that is Gaussian in form. This diffusion effect occurs in the mobile phase of both packed GC and LC columns. The diffusion process is depicted in Figure 6. 

The term porosity refers to the fraction of the medium that contains the voids. When a fluid is passed over the medium, the fraction of the medium (i.e., the pores) that contributes to the flow is referred to as the effective porosity of the media. In a general sense, porous media are classified as either unconsolidated and consolidated and/or as ordered and random. Examples of unconsolidated media are sand, glass beads, catalyst pellets, column packing materials, soil, gravel and packing such as charcoal. 

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