Mass plate column

In this equation, represents the rate of energy dissipation per unit mass of fluid. In pulsed and reciprocating plate columns the dimensionless proportionahty constant K in equation 38 is on the order of 0.3. In stirred tanks, the proportionaUty constant has been reported as 0.024(1 + 2.5 h) in the holdup range 0 to 0.35 (67). The increase of drop si2e with holdup is attributed to the increasing tendency for coalescence between drops as the concentration of drops increases. A detailed survey of drop si2e correlations is given by the Hterature (65). 

The pulsed-plate column is typically fitted with hori2ontal perforated plates or sieve plates which occupy the entire cross section of the column. The total free area of the plate is about 20—25%. The columns ate generally operated at frequencies of 1.5 to 4 H2 with ampHtudes 0.63 to 2.5 cm. The energy dissipated by the pulsations increases both the turbulence and the interfacial areas and greatly improves the mass-transfer efficiency compared to that of an unpulsed column. Pulsed-plate columns in diameters of up to 1.0 m or mote ate widely used in the nuclear industry (139,140). 

A small perforated-plate column of conventional design was pulsed by Goldberger and Benenati [Jnd. Eng. Chem., 51, 641 (1959)] with marked improvement in mass-transfer rates. 

Stewart, W. S., Heat and Mass Transfer Coefficients for the System Air-Water in a Perforated Plate Column, thesis presented in partial fulfillment of Master s Degree in chemical engineering, Louisiana State University (1958). 

Distillation may be carried out in plate columns in which each plate constitutes a single stage, or in packed columns where mass transfer is between a vapour and liquid in continuous countercurrent flow. Plate columns are now considered, and packed columns are discussed in Section 11.11. 

Earlier studies in mass transfer between the gas-liquid phase reported the volumetric mass-transfer coefficient kLa. Since kLa is the combination of two experimental parameters, mass-transfer coefficient and mterfacial area, it is difficult to identify which parameter is responsible for the change of kLa when we change the operating condition of a fermenter. Calderbank and Moo-Young (1961) separated kta by measuring interfacial area and correlated mass-transfer coefficients in gas-liquid dispersions in mixing vessels, and sieve and sintered plate column, as follows ... 

Spray, packed, and sieve-plate columns give poor mass-transfer rates for consequently require greater height. The mass transfer in such columns can be significantly improved by providing mechanical agitation. Remen (1951) and Oldshue and Rushton (1952) introduced the rotating-disk contactor (see Fig. 26b) and the mixed column (see Fig. 26c). 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

The gas flow, leaving the downdraft pyrolysis reactor, is quenched in a water scrubber. The resulting aerosol is then forced through a glass fibre filter. This gas cleaning system has later been optimised to release as little metals as possible the aerosol at the outlet of the water scrubber is forced through a plate-column, a liquid-gas separation vessel, an extra ice-water condenser and finally through a tube filled with cotton wool that acts as a filter. A metal mass balance was calculated over this system for the three metals (Cu, Cr and As). This downdraft pyrolysis system is described in more detail by Helsen et al. [ 11, 12, 14]. 

Plate columns are used for operations requiring a large number of transfer units, high pressure, high gas flow rates and low liquid flow rates, when it is necessary to supply or to remove heat, when solids are present in the liquid (or gas), and when the diameter is greater than 70 cm. They have the ability to handle large variations in gas and liquid flow rates. Mass-transfer data will be presented here for the most common designs— bubble-cap plates and sieve plates. 

FIG. 15-38 Mass-transfer data for sieve plate and modified bubble plate columns. System benzoic acid + water + toluene, except where noted. To convert feet to meters, multiply by 0.3048 to convert inches to centimeters, multiply by 2.54. [Data taken from AUerton, Strmn, and Treybal,Tram. AIChE,39jp. 361 (1943) Row, Koffolt, and Withrow, Trans. AIChE, 37, p. 559 (1941) and Treyhal and Dumoulin, Ind. Eng. Chem.,34,p. 709 (1942).]... 

This type of calculation does not have to be carried out for a plate column because the two phases are well mixed on each plate. This means that on each individual plate a state of equilibrium can be presumed. Therefore a volume element is identical to an equilibrium stage, and the height of the column can be obtained from the number of equilibrium stages required for a particular separation. This is a thermodynamic rather then mass transfer problem. This explains why a mass transfer device, such as a distillation column can be sized without any knowledge of the laws of mass transfer. 

MPa, respectively. Suchak and Joshi (1994) and Pradhan et al. (1997) have examined the effects of mass transfer characteristic and residence time on the designs of packed and plate column. 

In designing an absorber or stripper, one mnst ascertain some index of difficnlty of separation. As in the case of distillation separations, this index is either the required number of theoretical stages or the required number of transfer units. These parameters are interchangeable, with stages often nsed for plate columns and transfer nnits for packed columns. Backgronnd on the parameters is provided in Chapter 12. For convenience in handling the associated hydranlics and mass transfer calculations, the remainder of this chapter is divided into three parts Plate Columns, Packed Columns, and Special Devices. ... 

Steam stripping is often performed in plate columns since, in the past, few studies of mass transfer in packed columns have been made. A detailed study of stripping toluene from water in a... 

Plate columns (not shown in the figure), which can be used for the same purposes as packed columns, have many horizontal plates that are either perforated or equipped with so-called bubble caps. The liquid supplied to the top of the column flows down the column, in horizontal fashion, over each successive plate. The upwards-moving gas or vapor bubbles pass through the liquid on each plate, such that a gas-liquid mass transfer takes place at the surface of the bubbles. 

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