Entrainment plate design

From these results, we conclude that the plate design is appropriate from the pont of view of pressure drop, entrainment, weeping, and plate efficiency. The next step in the design is to determine the number of real stages required. Since the plate efficiency does not change much from one end of the tower to the other, we will assume a constant value taken as the arithmetic average of the efficiency values at the tower ends Emce = (0.77 + 0.73)/2 = 0.75. 

Plate-Column Capacity The maximum allowable capacity of a plate for handling gas and liquid flow is of primaiy importance because it fixes the minimum possible diameter of the column. For a constant hquid rate, increasing the gas rate results eventually in excessive entrainment and flooding. At the flood point it is difficult to obtain net downward flow of hquid, and any liquid fed to the column is carried out with the overheaa gas. Furthermore, the column inven-toiy of hquid increases, pressure drop across the column becomes quite large, and control becomes difficult. Rational design caUs for operation at a safe margin below this maximum aUowable condition. 

These two types of flooding are usuaUy considered separately when a plate column is being rated for capacity. For identification purposes they are caUed entrainment flooding (or priming ) and downflow flooding. When counterflow action is destroyed by either type, transfer efficiency is lost and reasonable design hmits have been exceeded. 

There is essentially no published work on specific tests with these trays as relates to entrainment, etc. However, the very close similarity betw een a perforated plate rvith-out downcomers and one wdth downcomers is sufficient to justify using some data for one in the design of the second. 

Since much of the vented material will be liquid, separators such as knockout pots or tangential entry separators can provide disengagement and possible recovery. Figure 5 is a typical vapor-liquid separator design found to be effective for these applications. Inlet design superficial vapor velocity is about 100 ft/sec, with sufficient volume provided to accumulate the entire reactor liquid contents. The lip on the outlet vapor line and the horizontal plate to separate the accumulated liquid are important features to prevent re-entrainment. 

Inlet gas uniformity as measured by the inlet root mean square (RMS) should be kept less than 20%. Nonuniform gas flow or maldistribution will result in poor collection, excessive reentraimnent, or hopper sweepage. The precipitator is designed to have very little flow at the bottom of the collection plate to allow the particles to drop freely into the hopper without entrainment back into the gas stream. 

Figure 183. Drums with coalescers for assisting in the separation of small amounts of entrained liquid, (a) A liquid-liquid separating drum equipped with a coalescer for the removal of small amounts of dispersed phase. In water-hydrocarbon systems, the pot may be designed for 0.5 ft/sec (Facet Enterprises, Industrial Division), (b) An oil-water separator with corrugated plate coalescers (General Electric Co.).

Intermediate liquid outlets. Liquid may be withdrawn using a chimney tray or from a downcomer. A chimney tray is a flat, unperforated plate with vapor risers. It permits total withdrawal of liquid a downcomer drawoff permits only partial withdrawal because some weeping occurs through the tray. A downcomer drawoff may contain some entrained gas, which must be separated downstream or allowed for in downstream equipment design. 

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