Packed towers vapor distribution

Where pressure drop is critical, a sparger pipe with the bottom quadrant cut out (rather than perforated) is sometimes used, but at the penalty of inferior vapor distribution. Similarly, the sparger can be entirely eliminated and substituted by a dog house baffle parallel to the direction of fluid entry. This baffle is somewhat wider than the nozzle diameter and stretches from wall to wall parallel to the direction of the incoming fluid. The author is familiar with one experience where addition of a "dog house baffle eliminated a packed-tower vapor maldistribution problem. 

Ov Crall, the careful design of a distributor for liquid in the top of a packed tower, and for the redistribution of liquid flowing dow n multi-section packing in the tower, is essential to good consistent tower performance. However, the liquid flow is not alone, the uniformity of vapor distribution is likewise essential, because non-uniform vapor distribution can cause non-uniform liquid downflow. Then, there is the selection of the packing itself and its characteristics and requirements/sensitivity to the uniform distribution of the liquid and vapor. As earlier emphasized, the level of the distributor tray or trough can be critical to the consistent uniform liquid distribution. 

For commercial towers with good liquid/vapor distribution Norton [96] recommends standard designs use HETP ralues 13% above the system base HETP. If the system under consideration does not meet the physical properties limit, either use a conservative estimate or use actual plant or published data for the system. For comparison of HETP values for selected packings see Strigle and Rukovena [94], Figure 9-28. 

The problem we have just discussed—poor fractionation efficiency due to inadequate vapor and liquid initial distribution—is rather similar to tray deck dumping in trayed fractionators. And, just like trays, packed towers are also subject to flooding. 

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

Packed-tower efficiency and turndown are strongly dependent on the quality of initial liquid distribution. Uneven distribution may cause local variations in the liquid/gas ratio, localized pinch conditions, and reduced vapor-liquid contact. Figure 14 shows two common liquid distributor types, the ladder type (shown as the top distributor) and the orifice type (shown as the redistributor). The ladder type is a horizontal header of pipes, which are perforated on the underside. The orifice type is a flat perforated plate equipped with round or rectangular risers for gas passage. Other common types of distributors are a header equipped with spray nozzles (spray distributor) and a header of horizontal channels, with V notches cut in the vertical walls of the channels (notched-trough distributor). 

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 number of risers must be large enough to ensure good vapor distribution. This is essential in packed towers, especially when the chimney tray delivers vapor to a short or a low-pressure-drop bed. 

Packed-bed absorption towers are distributed systems and Figure 6.19 shows the molar flow rates across a small element A/ of one of these towers. We consider the absorption of component A from the gas (vapor) phase to the liquid phase. The rate of mass transfer from the vapor phase to the liquid phase is given by the relation... 

McMullan et al describe the modification of a large diameter tower from sieve trays to IMTP packing [10]. Due to the very low liquid flow rates, only three to four distribution points were used per sq ft in the liquid distributor in order to avoid plugging, which could occur with smaller orifices. Experience has demonstrated that uniform liquid and vapor distribution is required to obtain the large number of theoretical stages needed for this separation. This column now is providing a total of 63 theoretical stages in four packed beds. 

In the Coastal Refinery in Aruba, we used sieve trays with one-half-inch holes, which seemed to work fine. Avoid packed towers. They are subject to vapor-liquid channeling and poor fractionation efficiency due to sloppy installation, fouling, or poor liquid feed distribution. When calculating the required hole area for the trays, don t forget that the vapor loads and hence the required tray hole area will substantially diminish as the vapor flows up the column from the reboiler to the feed tray. This will normally require a reduction in the tray deck hole area in proportion to the reduced vapor flow rate. 

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. 

Since we do not rely on pumparounds to fractionate—but just to remove heat—good vapor-liquid distribution is not critical. A bed of 4 or 5 ft of structured packing is often, then, an excellent selection for the pumparound section of a tower. The capacity of such a bed potentially has a 30 to 40 percent advantage over trays. 

Figure 8-5 shows the configuration at the top of the tower. The reflux was distributed by an orifice plate chimney type, pan liquid distributor. Vapor flowed up through the chimneys reflux was supposed to be evenly distributed by the /2-in. holes drilled in the pan across the top of the packed bed. To determine if the reflux was truly being equally distributed across the structured bed, I proceeded as follows ... 

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