Liquid drainage

Drainage can be troublesome when the bottom trays are flooded (i.e., covered with liquid), emd the rate of liquid withdrawal fiwm the column exceeds the rate of liquid downflow through the trays and downcomers (61,192,207,356). Under such conditions, a vapor gap may form below the bottom tray. The presence of this gap leaves the bottom tray with the task of supporting all the liquid above it, and may lead to failure of the bottom tray. Once the bottom tray fails and the liquid drains from it, the vapor gap moves to the space below the next higher tray. This tray may fail as well, and a cascade of collapsing trays may ascend the column (Fig. 12.8). 

Valve trays are most vulnerable to vapor gap problems, particularly if their downcomers and/or downcomer clearances are small. The 

The author is familieir with one incident where vapor gap formation is believed to have caused severe damage in a 10-ft vacuum column 

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Historically the most common gas disperser for cross-flow plates has been the bubble cap. This device has a built-in seal which prevents liquid drainage at low gas-flow rates. Typical bubble caps are shown in Fig. 14-20. Gas flows up through a center riser, reverses flow under the cap, passes downward through the annulus between riser and cap, and finally passes into the liquid through a series of openings, or slots, in the lower side of the cap. 

Liquid Drainage from Closed Relief System - Accumulation of liquid in closed rehef systems can impose appreciable back pressure and reduce relieving capacity. The following design features must be included to avoid these problems ... 

Ablauf fliissigkeitt /. discharge liquid, drainage liquid, -bahn, m. Ablasshahn. -dl, n. expressed oil run oil. -rohr, n., -rohre, /. outlet tube, waste pipe, -trichter, m. draining funnel, condensing funnel (inverted funnel with tubulated rim for carrying off condensed vapors). 

This Area Removed from Baffle to Allow for Liquid Drainage. Size Set to Suit Expected Flow. 

In some instances the baffle spacing must be rearranged to allow for a nozzle or coupling connection, ft is important that changes in baffle location be reviewed, as performance or pressure drop can be seriously affected. This is of extreme importance in vacuum units. Baffle orientation is sometimes misinterpreted by the fabricator, and this can cause serious problems where liquid drainage is concerned, or the revised vapor flow path can allow for bypassing the tube surface. 

Reversal (both selective and nonselective) of matter from the froth to the pulp through froth subduction, bubble coalescence and liquid drainage. 

To further improve the steam quality, the experienced design engineer places a dome, as shown in Fig. 15.4, on top of the kettle. The dome will have a sloped demister mesh pad. A demister acts to coalesce small droplets of water into larger droplets. The larger droplets easily settle out of the flowing steam. The demister is sloped, to promote liquid drainage from the surface of the pad. 

Any additives that can act to reduce the viscosity of foam films, and thereby increase the liquid drainage rate, will tend to reduce foam stability as a result. This includes any chemicals that can reduce surface viscosity and/or surface elasticity. Some alcohols can be use to produce these effects. 

Rosenberg, M., Wang, Z., Sulzer, G., Cole, P. 1995. Liquid drainage and firmness in full-fat, low-fat, and fat-free Cottage cheese../ Food Sci. 60, 698-702. 

Let us imagine two non-interacting rigid plates with zero thickness situated at the surfaces of tension at the film surfaces, i.e. at z = h/2. Besides, the pressure pc the plates are subjected to a variable external force 11/1 (A is the area of the plane-parallel film). This fdrce counterbalances the forces causing the thinning of a non-equilibrium film by liquid drainage from it into the adjacent liquid meniscus. Thus, the film thickness h can change reversibly at fixed values of the independent variables. 

Reynolds relation requires liquid drainage from the film to follow strictly the axial symmetry between parallel walls. Rigid surfaces ensure such drainage through their non-deformability, while non-equilibrium foam films are in fact never plane-parallel. This is determined by the balance between hydrodynamic and capillary pressure. Experimental studies have shown that only microscopic films of radii less than 0.1 mm retain their quasiparallel surfaces during thinning, which makes them particularly suitable for model... 

The rate of foam drainage is determined not only by the hydrodynamic characteristics of the foam (border shape and size, liquid phase viscosity, pressure gradient, mobility of the Iiquid/air interface, etc.) but also by the rate of internal foam (foam films and borders) collapse and the breakdown of the foam column. The decrease in the average foam dispersity (respectively the volume) leads the liberation of excess liquid which delays the establishment of hydrostatic equilibrium. However, liquid drainage causes an increase in the capillary and disjoining pressure, both of which accelerate further bubble coalescence and foam column breakdown. 

Applying the r(l) parabolic relation, there are two possible variants of change in border volume during liquid drainage [13] ... 

These equations can be transformed into expressions about both the rate of liquid drainage w and the volume of liquid AVi T released from the foam at the moment x... 

Foam stability is most commonly monitored by following its collapse and liquid drainage. Both are macroscopic properties and can easily be measured. These properties are not directly related to microscopic events such as drainage occurring from lamellae, but they allow a description of foam behaviour over time, and, in conjunction with the transient development of bubble size distributions (BSD), yield a complete description of foam destabilization mechanisms (Hailing 1981 Bisperink et al. 1992 Patino 1995 Lau and Dickinson 2005). The common parameter characterising foam stability is half-life time for liquid drainage or foam collapse (Deeth and Smith... 

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