Vapor distributor

Support plate used as a vapor distributor with restricted open area. Reduced capacity in order to make separation. Design error. 

Structured packing used as a vapor distributor for random packed bed dislodged and blown sideways. High pressure drop. Hold down not provided. Design eiTor. 

Liquid distribution in a packed bed is a function of the internal vapoi/liquid traffic, the type of packing employed, and the quality of the liquid distributors mounted above the packed bed. Vapor distribution is controlled by the internal vapor/liquid traffic, by the type of packing employed, and by the quality of the vapor distributors located below the packed beds. 

Figure 7.4 shows a modern, narrow-trough, liquid collector-vapor distributor chimney tray. While the initial vapor distribution through a packed bed is not quite so critical or difficult as the liquid distribution, it is still important. 

A properly designed valve or sieve tray will act as a vapor redistributor. Thus, poor initial vapor distribution will only lessen the efficiency of the bottom tray. But if a packed-bed vapor distributor does not work properly, vapor channeling will be promoted through the entire bed. 

The narrow-trough vapor distributor shown in Fig. 7.4 is intended to disperse the vapor evenly across the bottom of the packed bed. The width of the chimney does not exceed 6 in. The older-style chimney trays, which may have had a few large round or square chimneys, reduced the separation efficiency of the packing. To work properly, the vapor distributor has to have a reasonable pressure drop, in comparison to the pressure drop of the packed bed. For example, if the expected pressure drop of a 12-ft packed bed is 10 in of liquid, the pressure drop of the vapor distributor ought to be about 3 to 4 in of liquid. 

A packed tower can successfully fractionate with a very small pressure drop, as compared to a tray. For a modern trayed tower, to produce one single theoretical tray worth of separation (that s like a single, 100 percent efficient tray), a pressure drop of about 6 in of liquid is needed. A bed of structured packing can do the same job, with one inch of liquid pressure drop, even when allowing for the vapor distributor. In low-pressure fractionators, especially vacuum towers used to make lubricating oils and waxes, this can be of critical importance. 

The need for a proper vapor distributor, to achieve proper initial vapor distribution... 

Flashing Feed and Vapor Distributors When the feed or reflux is a flashing feed, the vapor must be separated out of the liquid before the liquid enters a liquid distributor. At low velocities (only), this can be achieved by a bare nozzle (Fig. 14-70a). A V baffle (Fig. 14-70h) is sometimes installed as a primitive flashing feed or vapor distributor. 

FIG. 14-70 Flashing feed and vapor distributors, (a) Bare nozzle, (b) Rounded V baffle, (c) Peripheral flash box—the box extends right around the tower wall, with the collected liquid descending via downpipes to a liquid distributor below, (d) Gallery distributor—the feed enters the gallery area (upper plate). (Parts a-c, courtesy of Sulzer Chemtech part d, courtesy of Koch-Glitsch LP.)... 

FIG. 14-72 Chimney tray vapor distributor. (Reprinted courtesy of Koch-Glitsch LP.)... 

Vapor distribution can be troublesome, especially in large-diameter columns. This type of maldistribution is best tackled at the source by paying attention to the vapor inlet arrangements. Common commercial vapor distributor designs are discussed elsewhere (23,40,152). 

Low-velocity vapor-only feeds often enter via bare nozzles or V baffles (above). At higher velocities, perforated vapor spargers are used. At high velocities, vapor horns and Schoepentoeters are often preferred. Alternatively or additionally, a vapor distributor may be mounted above the feed. The vapor distributor is a chimney tray (Fig. 14-72) where liquid is collected on the deck and flows via downcom-... 

Liquid collectors provide for withdrawal of products, mixing of new feeds with the internal liquid traffic, and remixing to avoid composition gradients after a certain height of packed bed has been traversed. Liquid collectors are often designed as one item to be integral with vapor distributors and liquid redistributors. 

Vapor distributors ensure a smooth vapor distribution before the vapor enters the packed bed. They fall into two classes. The first class consists of those that distribute the vapor fiow profile by imposing a pressure drop on the vapor steam. The second class consists of devices that mechanically redistribute the vapor flow by means of vanes. 

Vane vapor distributors use directional vanes to ensure vapor distribution of a vapor stream. Vane-type distributors are commonly used for distribution of high-velocity, mixed-phase feeds. Various proprietary and patented designs are in use. Both radial entry and tangential entry designs are available. Tangential entry designs are the better choice for two-phase flows. However, radial entry designs will work acceptably for most applications. 

The main consideration for introducing reflux or intermediate feed into a packed tower is adequately distributing the incoming stream to the packing. Unlike most tray columns, packed towers are sensitive to distribution. Maldistribution is detrimental to packing efficiency and turndown. The main devices that set the quality of distribution in a packed column are the top (or reflux) distributor, the intermediate feed distributor, the redistributor, and sometimes the vapor distributor. Adequate hydraulics in the inlet area is also important failure to achieve this can affect distributor performance and can also cause premature flooding. 

The common types of flashing feed and vapor distributors are the baffle type (Fig. 3.9a), the vapor-liquid separator type (Fig. 3.96), the gallery type (Fig. 3.9c), and the tangential entrance tjrpe (Fig. 2.2j). Some of the vapor distributors discussed in Sec. 3.12 are also sometimes used for flashing feeds, especially when liquid distribution to the section below is not critical (e.g., when it contains trays). 

Most vapor distributors are essentially chimney trays (Chap. 4) specifically designed to promote vapor distribution. A typical vapor distributor contains a large number of uniformly spaced vapor risers and... 

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