Entrainment trays

Entrainment occurs when spray or froth formed on one tray enters the gas passages in the tray above. In moderate amounts, entrainment will impair the countercurrent action and hence drastically decrease the efficiency. If it happens in excessive amounts, the condition is called priming and will eventually flood the downcomers. 

The model of theoretical equiHbrium trays with entrainment is readily treated by computer with methods analogous to those used for the design of fractionating columns. 

Entrainment Due to Gas Bubbling/Jetting through a Liquid Entrainment generally hmits the capacity of distiUation trays and is commonly a concern in vaporizers and evaporators. Fortunately, it is readily controllable bv simple inertial entrainment capture devices such as wire mesh pads in gravity separators. 

In distillation towers, entrainment lowers the tray efficiency, and 1 pound of entrainment per 10 pounds of liquid is sometimes taken as the hmit for acceptable performance. However, the impact of entrainment on distiUation efficiency depends on the relative volatility of the component being considered. Entrainment has a minor impact on close separations when the difference between vapor and liquid concentration is smaU, but this factor can be dominant for systems where the liquid concentration is much higher than the vapor in equilibrium with it (i.e., when a component of the liquid has a very lowvolatiUty, as in an absorber). 

At higher vapor loads, the kinetic energy of the vapor rather than the bubble burst supphes the thrust for jets and sheets of hquid that are thrown up as well as the energy from breakup into spray. This yields much higher levels of entrainment. In distillation trays it is the most common limit to capacity. 

Stichlmair uses the ratio of actual velocity to this maximum velocity together with a term that increases entrainment as the distance gets small between the hquid-vapor layer and the tray deck above. His correlation spans a 10 fold range in entrainment. He shows a sharp increase in entrainment at 60 percent of the maximum velocity and attributes the increase to a shift to the spray regime. 

The distance between trays Zt should be larger than h, sufficient so that (1) the streamers of dispersed hquid from the holes break up into drops before coalescing into the layer of liqmd on the next plate, (2) the linear velocity of continuous liquid is not greater than that in the down out to avoid excessive entrainment, and (3) the tower may be enterea through handholes or manholes in the sides for cleaning. 

The Souders-Brown correlation considers entrainment as the controlling factor. For high liquid loading situations and final design, complete tray hydraulic calculations are required. 

Sieve trays installed with holes behind false downcomer. Entrained liquid overhead. Limited lean oil flow. Vessel manufacturer error. 

Hats on chimney tray excessively restricted vapor resulting in liquid entrainment. Design error. 

Manways left off trays. Poor separation. Everything looked good. Scan interpretation did not identify problem until after the tower was opened. In fact, the trays with properly installed manways were thought to be entraining. Installation error. 

The VPS overhead consists of steam, inerts, condensable and non-condensable hydrocarbons. The condensables result from low boiling material present in the reduced crude feed and from entrainment of liquid from the VPS top tray. The noncondensables result from cracking at the high temperatures employed in the VPS. Inerts result from leakage of air into the evacuated system. Steam and condensable hydrocarbons are condensed using an overhead water-cooled condenser. The distillate drum serves to separate inerts and non-condensables from condensate, as well as liquid hydrocarbons from water. Vacuum is maintained in the VPS using steam jet ejectors. 

Another important consideration in tower design is tray downcomers size. At high ratios of liquid flow to vapor flow a proportionally greater area on the tray must be allotted to the downcomer channel opening. Downcomers are designed from basic hydraulic calculations. If the downcomer is inadequately sized and becomes filled with liquid, liquid level will build on the tray above. This unstable situation will propagate its way up to the tower and result in a flooded tower condition. Excessive entrainment can also lead to this same condition and, in fact, is usually the cause of flooding. 

A typical amine system is shown in Figure 7-4. The sour gas enters the system through an inlet separator to remove any entrained water or hydrocarbon liquids. Then the gas enters the bottom of the amine absorber and flows counter-current to the amine solution. The absorber can be either a trayed or packed tower. Conventional packing is usually used for 20-in. or smaller diameter towers, and trays or structured packing for larger towers. An optional outlet separator may be included to recover entrained amines from the sweet gas. 

Commonly, amine absorbers include an integral gas. scrubber section in the bottom of the tower. This scrubber would be the same diameter as required for the tower. The gas entering the tower would have to pass through a mist eliminator and then a chimney tray. The purpose of this scrubber is to remove entrained water and hydrocarbon liquids from the gas to protect the amine solution from contamination. 

Entrainment about three times that of perforated type plate or sieve tray. Jet-action accompanies bubbling. 

Entrainment Only about one-third that of bubble cap trays. 

Tray Spacing Can be closer than bubble cap due to improved entrainment. Fifteen inches is average, 9-in., 10-in. and 12-in. are acceptable, with 20- to 30-in. for vacuum. 

Flexitray Type A Figure 8-72 valve lift w/srp edge orifice, round Koch Higher than Sieve Tray. Lower entrain than Sieve Wide range Low Good... 

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

A bubble tray blows when the vapor rate is extremely high, regardless of the liquid rate, causing large vapor streams or continuous bubbles to be blown through the liquid. The efficiency and contact is low and entrainment is usually high. Here also low slot seals contribute to the sensitivity of the tray to such action. 

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