Counterflow trays

Counterflow Trays Dualflow Multiple downcomer Baffle trays (splash decks) ... 

The character of the two-phase mixtnre above the tray floor is different from that of a conventional crossflow tray becanse of the connterflow of the phases, and this makes it possible for a counterflow tray to achieve more than one theoretical stage. A paper by Garcia and Fair provides approaches to modeling pressure drop and efficiency of dualflow trays, based on the FRI database. 

As in the case of counterflow trays, baffle trays are often used for fouling services, e.g., handling slurries that would plug conventional crossflow trays. Their simplistic design does not lead to mass transfer efficiencies as high as those for counterflow trays, but their pressure drop and capacity can have advantages over the conventional devices. 

Counterflow trays Round openings ( Dualflow ) Rectangular openings ( Tkubogiid )... 

Trays, counterflow—Trays on which the vapor and liquid flow counter-currently either through the same or separate openings. 

Downcomer Choke Flooding This is also called downcomer entrance flood or downcomer velocity flood. A downcomer must be sufficiently large to transport all the liquid downflow. Excessive friction losses in the downcomer entrance, and/or excessive flow rate of gas venting from the downcomer in counterflow, will impede liquid downflow, initiating liquid accumulation (termed downcomer choke flooding) on the tray above. The prime design parameter is the downcomer top area. Further down the downcomer, gas disengages from the liquid and the volumes of aerated liquid downflow and vented gas... 

The flow parameter is more suitable for packed columns (12), where vapor-liquid counterflow exists, but has also been used in tray columns, For tray columns the author prefers QL [Eq. (6.6)] to the flow parameter. 

A variation of the normal sieve tray is the Turbogrid tray developed by the Shell Development Company. The Turbogrid tray uses slots instead of circular holes and operates as a counterflow plate unit with no downcomers. A typical tray consists of a flat grid of parallel slots. The slots can be stamped perforations in a flat metal sheet or the open spaces between parallel bars. 

For counterflow plates, the curves of Fig. 14-25 may be used for open areas of 20 percent or greater. Plates with 15 percent open areas have about 85 percent of the curve values, and open areas of less than 15 percent are not recommended. For counterflow-plate columns of the segmental-baffle type, 50 percent cut, allowable C y values are about 15 percent greater than those shown in Fig. 14-25, when vertical spacings of the baffles are equal to the tray spacings shown. 

Extraction involves the transfer of components between two liquid phases, much as absorption or stripping involves the transfer of components from liquid to vapor phase or vice versa. As in vapor-liquid multistage separation processes, the device employed to carry out liquid-liquid extraction is usually a counterflow column that performs the function of a number of equilibrium stages interconnected in counterflow configuration. In each stage, two inlet liquid streams mix, reach equilibrium, and separate into two outlet liquid streams. As in vapor-liquid columns, the lack of complete equilibrium in liquid-liquid extractors is accounted for by some form of tray efficiency. Liquid-liquid extraction may also be carried out in a cascade of mixing vessels connected in series in counterflow. 

Devices for effecting contact of vapor and liquid may be classified as shown in Table 12.6. Recently, there has been a trend away from the conventional crossflow devices, in the direction of counterflow devices, mostly packings. Still, an enormous number of older columns are operating with trays, and thus the technology of tray design and analysis is important. 

Pressure drops of counterflow perforated trays are difficult to predict, because the open area nsed by the vapor varies with loading, and one must predict the fraction of opening for vapor at a given load point. Forms of the simple two-resistance eqnation are used ... 

A model for predicting the efficiency of dualflow-type trays has been reported and is mechanistic in character. A large efficiency database, provided by Fractionation Research, Inc. (FRl), was used in constracting the model. Efficiency data for counterflow baffle tray columns are sparse, and the only model available is based largely on a heat transfer database, using the appropriate analogies. 

Sometimes the holes in the sieve tray are covered with caps or checkvalves to help prevent the liquid from coming down through the hole. If the vapor flow remains strong, however, it will prevent counterflow itself. 

Even though tray columns are normally crossflow devices (as opposed to counterflow devices such as packed columns), certain aspects of coumerflow theory are used with nays. This involves the use or transfer units as measures of the difficulty of the separation. For a simple binaty system, and for distillation, the number of transfer units mny be obtained from... 

For packed columns, which are classified as counterflow columns (most trays are crossflow devices), mass transfer is related to transfer tassa in connterflow vapor-liquid cou(acting Hie most-used criterion of efficiency is the height equivalent to a theoretical plate (HETP) thet was first introduced by Peters 1... 

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