Vapor cross flow

Vapor Cross-Flow Channeling on Sieve Trays... 

Figure 8-142. Vapor cross-flow channeling. Note entrainment near tray middle and outlet, and weep near tray inlet. Used by permission, Kister, H. Z., Larson, K. F. and Madsen, P. E., The American Institute of Chemical Engineers, Chem Eng. Prog. V. 88, No. 11 (1992), p. 86, all rights reserved.

Large fractional hole area, long flow path relative to tray spacing and high liquid flow rate are the key factors leading to the formation or intensification of vapor cross-flow channeling on sieve and valve trays. 

Conditions favor vapor cross flow These are the only conditions under which froth entrainment flooding is likely to be encountered when tray spacing exceeds 18 in. 

Froth entrainment flooding is only encountered at close tray spacing. It can be excluded from any capacity checks when tray spacing exceeds 18 in (except when conditions are conducive to vapor cross flow, Sec. 6.2.4). Note that at low tray spacing (< 18 in), either spray or froth entrainment flooding can restrict capacity. 

I-beam interference at the column base (Fig. 8.6a) can be prevented either by installing two feed inlets, one on either side of the I-beam, or by providing sufficient area under the I-beam for vapor cross flow. For a single feed inlet, it has been recommended (237) that the vertical distance between the bottom of the I-beam and the high liquid level be set such that the calculated pressure drop for half the feed vapor flowing under the I-beam is less than 10 percent of the anticipated pressure drop in the packed bed above. 

Techniques recommended for preventing I-beam interference at the column base also extend to chimney trays. It is best to provide sufficient open height for vapor cross flow between the top of the chimneys and the bottom of the I-beam (Sec. 4.10). If positioning the I-beams close to the chimneys cannot be avoided (e.g., in a revamp, where vertical space is limited), the chimneys should be designed to provide a... 

In small-diameter columns, D/Ha 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... 

The source term 5l in the momentum equation involves the drag force by the jetting vapor Tdrag and the resistance Fp created by the liquid-vapor cross-flow. 

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