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Fig. 16-10. Entrainment in bubble-cap plate columns.

Fig. 16-10. Entrainment in bubble-cap plate columns.

that there be sufficient liquid head in the down pipes. Figure 16-11 shows schematically the liquid head in the down pipes and its causes. The plate spacing must be great enough to allow for the sum of these heads plus some extra height to handle short periods of excess flow. Values of hp normally range from 2 to 4 in. of fluid; the various liquid heads on the plate (hw + hcr + hQ) amount to 3 to 4 in. A plate spacing of 6 in. leaves little margin of safety and requires the use of low liquid heads on the plate and low vapor velocities to give low pressure drop and reasonable entrainment. A plate spacing of 12 to 24 in. would appear to be more feasible.

In addition to the items included in Fig. 16-11, there are the factors of foaming and cross flow of the vapor. Foaming of the liquid may be

hu> 03 height of exit weir her = height of crest on weir ha « liquid gradient ho « roversal loss in down pipo h/ = friction in down pipe he ** pressure drop through riser and inside cap hs — pressure drop through slot hi » liquid head above slot hp =» pressure drop through plate

Fig. 16-11. Schematic drawing of the liquid heads in a bubble-plate column.

hu> 03 height of exit weir her = height of crest on weir ha « liquid gradient ho « roversal loss in down pipo h/ = friction in down pipe he ** pressure drop through riser and inside cap hs — pressure drop through slot hi » liquid head above slot hp =» pressure drop through plate

Fig. 16-11. Schematic drawing of the liquid heads in a bubble-plate column.

so great that it extends from one plate to the next and results in high pressure drop and entrainment, but it is usually not that serious. However, a relatively small amount of foam may cause difficulties by filling the upper part of the down pipe and hindering the liquid flow. In many cases, foam blocking the down pipes is the limiting factor in column capacity. The foam is produced largely by the vapor on the plate, and its plugging effect in the down pipes can be reduced by including a short calming section before the down pipes or by baffles that will hold the foam back on the plate and prevent it from flowing into the down pipes. If relatively stable foams are produced, it may be necessary to add some foam-breaking agent to the column. Experimental data indicate that the average depsity of the liquid and entrained vapor mixture is about one-half that of the liquid itself. For design purposes, it is therefore desirable to have a downspout height equal to approximately twice the value of the calculated liquid head.

Vapor flow across the column in the vapor space results from the nonuniform vapor distribution and from the usual reverse direction of liquid flow on succeeding plates. On a given plate % to % of the total vapor may flow up the downstream half of the plate. This necessitates from to H of the vapor flowing across the center of the plate to enter the other half of the plate above. Using the factor of for purposes of illustration,

VcfHsD (16-37)

where Vcf = cross-flow velocity Vc = superficial velocity IIs = free clearance between plates D = column diameter

In small-diameter columns, D/Hs is usually so small that the effect of cross flow is negligible, but in large columns it may become so great that the kinetic head equivalent to Vcf may be significant in terms of liquid head. Under these conditions, the pressure in the vapor space is not constant across the cross section, and its variation is such that it forces an increase in the hydraulic gradient. In order to make Hs as large as possible, any beams or projection on the bottom of a plate should be positioned to aid the vapor cross flow. Good vapor distribution on the plate will eliminate the effect of vapor cross flow.

An effect equivalent to the action of vapor cross flow is frequently obtained when the vapor is introduced through the side of the fractionating column. For example, the vapor from the reboiler is normally introduced into the side of the column near the bottom. If this vapor pipe terminates at the column wall, the kinetic energy of the vapor may be so great that it will cause a high impact pressure on the opposite side of the column. This type of action can result in very poor vapor distribution for plates up the column, and some type of distributor should be employed. These frequently take the form of baffles which serve to direct the vapor over the whole cross section.

Allowable Vapor Velocity. A factor closely related to plate spacing is the allowable superficial vapor velocity that can be employed. The limiting factor can be either the liquid-handling capacity of the down pipes or the loss of rectification efficiency due to entrainment. The limiting capacity in the first case is calculated on the basis of the pres-

20 30 Plate spacing, Inches Fig. 16-12. Allowable superficial velocity.

sure drops and the liquid heads as outlined in the preceding sections. For the capacity limited by entrainment, correlations have been presented by Souders and Brown (Ref. 33) and Peavy and Baker (Ref. 26). Their results plus other data have been used to construct Fig. 16-12. The superficial vapor velocity, Ve, in feet per second multiplied by the square root of the density ratio, is plotted as a function of the plate spacing, taken as the distance from the liquid surface to the plate above. Lines for different values of liquid seals above the top of the slots are given. High liquid levels above the slots give increased splashing and entrainment. The values given by this figure are reasonable design factors for most cases.

Plate Layout. A large number of types of bubble caps have been employed, but in most cases circular caps 4 to 6 in. in diameter are used. The slots are usually rectangular, although triangular and trapezoidal shapes are also employed. In some cases, the slots have been cut through the caps shell tangentially, but in the majority of cases they are cut radially. Tunnel caps are employed in some cases, but long caps of this type are sensitive to hydraulic gradient because the corrective action of the pressure drop through the risers is less effective. Hexagonal caps have been used but do not appear to have any advantage over circular ones.

Caps are usually arranged on triangular centers, and they are most commonly placed so that the liquid flow is at right angles to the rows of caps. This requires the liquid to follow a staggered path across the plate, and it has been assumed that this gave good contact of the liquid with the vapor.

Small-diameter caps can give more slot area and more free space for liquid flow than large caps. Caps larger than 6 in. in diameter are seldom employed and, except in small laboratory columns, caps less than 3 in. are not used because of the mechanical problem of handling the large number of them needed even for plates of moderate diameter. With the usual cap design and plate layout, the ratio of total slot area to superficial area generally falls between 0.1 to 0.2.

The caps should not be placed too near the weirs or walls. A clearance of 2 to 3 in. between the caps and the weirs and 1 to 2 in. at the walls is usually adequate. Clearances of 1 to 2 in. between caps are employed; i.e., 4-in.-diameter caps might be placed with centers at the corners of equilateral triangles with sides to 6 in.

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