Support, packing vapor distributing

A vapor-distributing support (Fig. 3.10) is a flat perforated plate containing perforated vapor risers. Liquid descends through the floor perforations, while vapor rises through the riser perforations. The bottom portion of the vapor risers is unperforated, so that vapor is injected above the liquid pool on the plate. Sumps are optional and recommended (289) where liquid inventory is to be reduced. The vapor-distributing support combines two internals (a vapor distributor and a support plate) into one. Compared to a vapor distributor, this saves both vertical space and internals costs. On the other hand, obstruction of perforations by pieces of packing and possible liquid overflow into risers may make its vapor distribution quality somewhat lower. 

In large-diameter packed colxunns where I-beams support the packing support plate, a larger space between the reboiler return nozzle and the packing support plate may be required. The prime consideration here is allowing sufficient open area between the bottom of the beam and the high liquid level for adequate vapor distribution. Detailed discussion is in Sec. 8.2. 

Structured packing. A layer of structured packing can sometimes be used to support a bed of random packings. This technique can add a fraction or even a full theoretical separation stage to the bed. The structured packing may also improve vapor distribution. 

Because a packing support plate usually is located immediately above the gas inlet in an absorber or the reboiler return in a distillation column, this plate could be used to control vapor distribution. Obviously, vapor maldistribution can reduce column efficiency in the same way as liquid maldistribution although due to the turbulence in the vapor phase, its rate of radial cross-mixing is at least three times that of the liquid phase. The potential for vapor maldistribution increases as column diameters or operating pressures increase. Fortunately, the vapor phase tends to maintain a uniform distribution once it has been established. Thus, usually only the packing support plate immediately above the vapor inlet needs to act as a vapor distributor. This support plate should be located at least one vapor-inlet diameter plus 12-in. above the center-line of the vapor inlet nozzle. 

One design of vapor-distributing support plate uses vapor orifices beneath perforated gas risers (see Figure 10-5). The liquid phase will pool on the horizontal deck to a depth equal to the pressure drop causing flow through the liquid orifices plus the gas-phase pressure drop through the plate. The gas phase will be injected into the packed bed through the perforations in the upper part of the gas risers. The lower part of the gas risers is left unperforated to accommodate the liquid pool. 

Support for packing is easily implemented by inserting flat circular discs above and below the packing that also served to ensure efficient distribution of the liquid and vapor streams. 

Packed Columns. These columns are usually constructed of stainless steel tubing with diameters of 1 /8 in. (3 mm) or 1 /4 in. (6 mm) and lengths of from 4 to 12 feet. The column is packed with a liquid or low-melting solid as the stationary phase distributed on a solid support material. The stationary phase must be relatively nonvolatile, that is, it should have a low vapor pressure and a high boiling point. Some typical stationary phases used with packed columns are listed in Table 22.1. Typical support materials are shown in Table 22.2. The most common support material consists of diatomaceous earth (Chromosorb). 

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