Vapor flow bubbling

Uneven vapor flow bubbling-up through the tray deck will promote vapor-liquid channeling. This sort of channeling accounts for many trays that fail to fractionate up to expectations. To understand the cause of this channeling, we will have to quantify total tray pressure drop. 

For most trays, liquid flows across an active area of the tray and then into a downcomer to the next tray below, etc. Inlet and/or outlet weirs control the liquid distribution across the tray. Vapor flows up the tower and passes through the tray active area, bubbling up through (and thus contacting) the liquid flowing across the tray. The vapor distribution... 

Kister et al. [213] have concluded from examining reported cases of cross-flow channeling related to poor sieve tray column performance that under specific conditions the cross-flow channeling does occur. See Figure 8-142 [213] for diagram of the postulated vapor flow across a tray. It is known to occur for valve trays and bubble cap trays. This condition has not been studied very much in the open literature however, several investigators including myself have observed in industrial practice the... 

In a packed column, liquid and vapor flow counter-currently and separation between the liquid and vapor phases takes place continuously. In contrast, in a column with trays, separation occurs in stages. In a packed column, vapor does not bubble through the liquid as in the columns with trays. For this reason, and due to the absence of the vapor-flow orifices, packed columns operate at a much lower pressure drop. In addition, because liquid and vapor contact in a packed column is less agitated than in a trayed column, packed columns are less likely to foam. 

Fig. 2.32 Diabatic flow pattern map for vaporizing flow in uniformly heated micro-channel, R-134a, d = 0.5 mm, L = 70 mm, Tg = 30 °C, = 50 kW/m without subcooling at inlet. Flow patterns isolated bubble regime (IB), coalescing bubble regime (CB), annular (completely coalesced) regime (A), post-dryout regime (PD). Reprinted from Thome et al. (2006) with permission...

The increasing void fraction and acceleration of the flow also produce changes in the flow regime with downstream location. As shown in Figure 4.2, for vertical upward flow, bubbly flow at the onset location subsequently changes to slug, churn, and then annular flow. When there is a large difference in the liquid and vapor... 

As the pressure in the pipe decreases in the direction of the flow, the saturation temperature of the liquid decreases, promoting an increase in vaporization. More bubbles are formed these interact to form a continuous... 

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

A distillation tray works efficiently, when the vapor and liquid come into intimate contact on the tray deck. To this end, the liquid should flow evenly across the tray deck. The vapor should bubble up evenly through the perforations on the tray deck. The purpose of the outlet weir is to accomplish both these objectives, as follows ... 

On the other hand, bubble caps (or even the more ancient tunnel cap trays) are different, in that they do not depend on the vapor flow to retain the liquid level on the tray deck. More on this later. For now, just recall that we are dealing only with perforated tray decks. 

Bubble-cap trays may be operated over a far wider range of vapor flows, without loss of tray efficiency. It is the author s experience that bubble-cap trays fractionate better in commercial service than do perforated (valve, or sieve) trays. Why, then, are bubble-cap trays rarely used in a modern distillation ... 

Have about 15 percent less capacity because, when vapor escapes from the slots on the bubble cap, it is moving in a horizontal direction. The vapor flow must turn 90°. This change of direction promotes entrainment and, hence, jet flooding. 

The liquid on the tray deck was at its bubble, or boiling, point. A sudden decrease in the tower pressure caused the liquid to boil violently. The resulting surge in vapor flow promoted jet entrainment, or flooding. 

Bubbling area AB (also called active area) The total tower cross-sectional area minus the sum of downcomer top area Apr, downcomer seal area ADB, and any other nonperforated areas on the tray. The bubbling area represents the area available for vapor flow just above the tray floor. 

The assessment of the equipment vapor flow capacity should also take the cooling capacity of the condenser into account. This can be directly compared to the heat release rate. Further, the swelling of the reaction mass, due to the presence of bubbles, may also become critical for high degrees of filling (see Section 9.4.4). When both vapor and gas are released, obviously the sum of both velocities must be used in the assessment. 

At low and moderate gas rates, a major portion of the gas flow bubbles through the stagnant zones, contacting only a small fraction of the liquid flow. Froth formation in the central zone of the tray becomes relatively weak at low vapor rates (161), Solari etal. 

---