Plate contactors

Cross-flow plates are the most common type of plate contactor used in distillation and absorption columns. In a cross-flow plate the liquid flows across the plate and the vapour 

Other types of plate are used which have no downcomers (non-cross-flow plates), the liquid showering down the column through large openings in the plates (sometimes called shower plates). These, and, other proprietary non-cross-flow plates, are used for special purposes, particularly when a low-pressure drop is required. 

Three principal types of cross-flow tray are used, classified according to the method used to contact the vapour and liquid. 

This is the simplest type of cross-flow plate. The vapour passes up through perforations in the plate and the liquid is retained on the plate by the vapour flow. There is no positive vapour liquid seal, and at low flow-rates liquid will weep through the holes, reducing the plate efficiency. The perforations are usually small holes, but larger holes and slots are used. 

In which the vapour passes up through short pipes, called risers, covered by a cap with a serrated edge, or slots. The bubble-cap plate is the traditional, oldest, type of cross-flow plate, and many different designs have been developed. Standard cap designs would now be specified for most applications. 

The most significant feature of the bubble-cap plate is that the use of risers ensures that a level of liquid is maintained on the tray at all vapour flow-rates. 

A larger spacing will be needed between certain plates to accommodate feed and side stream arrangements and for manways. 

The principal factor that determines the column diameter is the vapor flow rate. The vapor velocity must be below that which would cause excessive liquid entrainment or a high-pressure drop. The following equation, which is based on the well-known Souders and Brown (1934) equation (see also Lowenstein, 1961), can be used to estimate the maximum allowable superficial vapor velocity and hence the column area and diameter  

= maximum allowable vapor velocity, based on the gross (total) column cross-sectional area, m/s  

This approximate estimate of the diameter would be revised when the detailed plate design is undertaken. 

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The performance of real stages is related to an equilibrium stage by the concept of plate efficiencies for plate contactors, and height of an equivalent theoretical plate for packed columns. 

Cross flow plates are the most common type of plate contactors used in tray distillation column. In a cross-flow plate, the liquid flows across and down from the plate and the vapour flows up through the plate. The flowing liquid is transferred from plate to plate through vertical channels called downcomers. A pool of liquid is retained on the plate by an outlet... 

The transfer of mass from one phase to another is involved in the operations of distillation, absorption, extraction, humidification, adsorption, drying, and crystallization. The principal function of the equipment used for these operations is to permit efficient contact between the phases. Many special types of equipment have been developed that are particularly applicable for use with a given operation, but finite-stage contactors and continuous contactors are the types most commonly encountered. A major part of this chapter, therefore, is devoted to the design aspects and costs of stagewise plate contactors and continuous packed contactors. 

Enriched steam for the synthesis-gas generator (A) is produced in the series of sieve-plate contactors (E), (F), and (G). In (E) deuterium is transferred from methylamine liquid to methylamine vapor, reducing the deuterium content of the liquid from 10.916A to 1.06A while increasing that of the vapor from 1.02N to lOA. In (F) deuterium is transferred from methylamine vapor to water, increasing the deuterium content of the latter from N to 8A. This two-step transfer of deuterium from liquid methylamine leaving (C) to water leaving (F) is necessary to prevent chemical reaction between water and the catalyst dissolved in liquid methylamine. 

Reciprocating plate contactor, 516-522 Recovery fraction, 461 Rectift ing section, 293 Redlich-Kwong equation, 148, 599 Reflux, 12 drum, 514 total, 323, 435 Refluxed stripper, 9, 13 Reflux ratio cost, 451 external, 310 internal, 310 minimum, 323, 443... 

Plate contactors such as valve trays are evaluated in terms of a plate efficiency, which is inversely proportional to how closely the composition of the streams leaving a stage approach the predicted compositions at thermodynamic equilibrium. To obtain the number of stages required for any given separation, it is necessary to divide the computed number of theoretical stages by an average empirical fractional plate efficiency. 

The HTU is then calculated for membrane module 272/6.61 = 41.3 cm. A comparison of contactors with reciprocal plate contactor are presented in Table 4.11. 

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