Trays flow patterns

Fig. 11. Two-truck tray dryer. A, air inlet duct B, air-exhaust duct with damper C, axial flow fan D, fan motor, 2—15 kW E, air heaters F, air-distribution plenum G, distribution slots and H, wheeled tmcks and trays. The arrows iadicate air and vapor flow pattern.

This overall flow pattern in a distillation column provides countercurrent contacting of vapor and hquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibriums to an extent dependent upon the efficiency of the contac ting tray. 

Figure 8-67B. For sieve tray layout arrangements typical one-, two-, and four-pass tray flow patterns with clarifying flow markings by this author. Used by permission, Glitsch, Inc., Bui. 4900-5th Ed.

Mechanism 2 of Figure 8-122B becomes apparent when the flow recirculation on the tray increases with increasing underflow clearance. The curvature of the column wall influences the movement of the liquid toward the center. High underflow clearance does not even out maldistribution due to backup where the irregular flow pattern enters into the tray below. This allows flow separation to occur on the downcomer floor, and leads to enhanced retrograde flow. 

Example 11.7 Carbon dioxide is sometimes removed from natural gas by reactive absorption in a tray column. The absorbent, typically an amine, is fed to the top of the column and gas is fed at the bottom. Liquid and gas flow patterns are similar to those in a distillation column with gas rising, liquid falling, and gas-liquid contacting occurring on the trays. Develop a model for a multitray CO2 scrubber assuming that individual trays behave as two-phase, stirred tank reactors. 

Figure 11.21. Liquid flow patterns on cross-flow trays, (a) Single pass (b) Reverse flow (c) Double pass...

A question to be resolved in predicting efficiency concerns the liquid-flow pattern. It is usual practice to assume that the vapour is fully mixed, but there is a diversity of treatments of the liquid phase. The two limiting cases are completely-mixed-liquid and plug-flow-liquid. Achieved efficiencies on well designed trays usually fall between these cases. The assumption of a well-mixed tray liquid is only valid for the smallest trays (pilot scale). 

Liquid Flow Patterns on Large Trays The most popular theoretical models (below) postulate that liquid crosses the tray in plug flow with superimposed backmixing, and that the vapor is perfectly mixed. Increasing tray diameter promotes liquid plug flow and suppresses backmixing. 

The rate-based models suggested up to now do not take liquid back-mixing into consideration. The only exception is the nonequilibrium-cell model for multicomponent reactive distillation in tray columns presented in Ref. 169. In this work a single distillation tray is treated by a series of cells along the vapor and liquid flow paths, whereas each cell is described by the two-film model (see Section 2.3). Using different numbers of cells in both flow paths allows one to describe various flow patterns. However, a consistent experimental determination of necessary model parameters (e.g., cell film thickness) appears difficult, whereas the complex iterative character of the calculation procedure in the dynamic case limits the applicability of the nonequilibrium cell model. 

Liquid flow patterns and maldistribution on large trays... 

Liquid flow patterns. Liquid entering a single-pass tray flows in a diverging channel until reaching the tray centerline, then in a converging channel as the outlet weir is approached. The liquid has little incentive to move sideways and follow the curved walls of the column. Instead, it seeks the shortest path from inlet to outlet, and channels through the tray center (Fig. 7.75). This leaves stagnant zones near the curved walls on the side. 

Figure 7.7 Flow patterns on distillation trays, (a) Plug flow (6) plug flow in center, stagnant regions near wall.

Tray tilt affects the liquid flow pattern and the boundaries between the regions (148). 

Case studies were reported (170,174) of large-diameter (> 15-ft) towers with sieve trays not reaching the expected efficiency. Maldistribution was cited as the culprit or at least one of the causes. Improving liquid flow patterns, often among other modifications, was the fix. The only other evidence that channeling adversely affects tray efficiency comes from the above-mentioned theoretical models. 

Improving liquid flow patterns. A number of special tray designs have been developed to improve liquid velocity distribution on large-diameter trays. Their main applications are vacuum distillation. In pressure distillation, liquid flows are usually high and multipass trays are used, so that stagnant zones are seldom a problem. Some means of improving the liquid flow patterns are... 

Using arc-shaped downcomers. Special tray designs (166,169) have arc-shaped downcomers, which direct liquid toward the column walls. These devices improve flow patterns, but their effect on efficiency has not been tested. 

Vapor maldistribution. Most popular theoretical models (such as the AIChE and the Chan and Fair models, Sec. 7.2.1) postulate perfectly mixed vapor flow. In larga-diameter columns, vapor is more likely to rise in plug flow. Modeling work showed (143,179,180) that in the absence of stagnant zones on the tray, vapor flow pattern has generally little effect on tray efficiency. When column efficiency exceeds 30 percent (143), or when stagnant liquid zones exist (171,173,180), vapor plug flow reduces tray efficiency. 

Liquid flow patterns for different types of finite-stage trays. 

The use of CFD models for gas-liquid bubble columns has also raised considerable interest only Euler-Euler and Euler-Lagrange frameworks have been employed for the description of the gas and liquid phase states [3.38-3.42]. Bubble trays, considered as particular kinds of bubble columns, have lately presented enormous interest for the flow description by CFD. The flow patterns on a sieve tray have been analyzed in the liquid phase, solving the time-averaged equations of continuity and momentum [3.43]. 

Either the entire amount of the liquid on a tray or part of it may be taken out and returned to the tray directly below it. The combined liquid flow (the pumparound and the liquid flowing directly to the tray below) would be the same as the liquid flow down to the tray below had there been no pumparound. From the standpoint of equilibrium stages, the column would perform exactly as though the pumparound did not exist although the tray efficiency may be affected due to the different flow pattern. If the liquid is returned several trays below the draw tray, the trays between the draw tray and the return tray are bypassed by the pumparound liquid. The amount of fractionation in that column section is therefore reduced. This pumparound thus tends to lower the overall number of effective trays in the column. 

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