Continuous tray column

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

Continue doivn column using the composition calculated for tray above to substitute in Equation 8-176 to obtain the (xj/xh) for the tray below. 

In a packed column, liquid and vapor flow counter-currently and separation between the liquid and vapor phases takes place continuously. In contrast, in a column with trays, separation occurs in stages. In a packed column, vapor does not bubble through the liquid as in the columns with trays. For this reason, and due to the absence of the vapor-flow orifices, packed columns operate at a much lower pressure drop. In addition, because liquid and vapor contact in a packed column is less agitated than in a trayed column, packed columns are less likely to foam. 

Tray columns Continuous and well mixed on an individual tray Discontinuous but often assumed well mixed on an individual tray... 

Pulsing means that either the whole liquid content of a sieve tray column is continually pushed up and down by a piston that moves to and fro, or the whole plate package is moved up and down [3]. Figure 9.5 illustrates the two extractor constructions schematically. They show about the same efficiency... 

The very first continuous distillation column was the patent still used to produce Scotch whiskey in the 1830s. It had 12 bubble-cap trays with weirs, downcomers, tray decks, and bubble caps with internal risers. Current trayed towers are quite similar. As most distillation towers have always been trayed rather than packed, one would have to conclude that trayed towers must have some sort of inherent advantage over packed towers. And this is indeed true, in a practical sense even though, in theory, a packed tower has greater capacity and superior separation efficiency than a trayed column. 

As the liquid holdup increases, the effective orifice diameter may become so small that the liquid surface becomes continuous across the cross section of the column. Column instability occurs concomitantly with a rising continuous-phase liquid body in the column. Pressure drop shoots up with only a slight change in gas rate (condition C or C ). The phenomenon is called flooding and is analogous to entrainment flooding in a tray column. 

Davies B, Jeffreys GV. The continuous trans-esterification of ethyl alcohol and butyl acetate in a sieve-plate column. Part III trans-esterification in a six-plate sieve-tray column. Trans IChemE 1973 51 275-280. 

Equations (3) and (4) represent the component mass balances for continuous systems (packed columns). For discrete systems (tray columns), the differential terms transform to finite differences, and the balances are reduced to algebraic equations. 

In a Middle Vessel Batch Distillation Column (MVC) (Figure 2.2) the separation section is divided, as in the usual continuous distillation column, into rectifying and stripping sections, with a feed tray in the middle. The essential features of this type of column are ... 

Distillation is an ancient unit operation, and has been widely practiced for thousands of years. Early applicetions used crude vaporization and condensation equipment, often for concentrating the alcoholic content of beverages, The first vertical columnar continuous distillation still was developed by Cellier-Blumenthal in France in 1813. Perrier introduced an early version of the bubble-cep tray in England in 1822, Packings were used as early as 1820 by a technologist named Clement who used glass balls in an alcohol still. Coffey developed the first sieve tray column in 1830. The first book on fundamentals of distillation was La Rectification de I alcohol by Ernest Sorel in 1893. 

The separation operation called distillation utilizes vapor and liquid phases at essentially the same temperature and pressure for the coexisting zones. Various kinds of devices such as random or structured packings and plates or trays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cylindrical shell to form a column. Packings are also generally contained in a cylindrical shell between hold-down and support plates. The column may be operated continuously or in batch mode depending on a number of factors such as scale and flexibility of operations and solids content of feed. A typical tray-type continuous distillation column plus major external accessories is shown schematically in Fig. 13-1. 

FIG. 15-40 Sieve tray flooding data. System toluene (dispersed) + water (continuous). Tray spacing = 30.5 cm. Column diameter = 42.8 cm. [Taken from Seibert, Bravo, and Fair ISEC 02 Proc., 2, pp. 1328-1333 (2002), with permission. Copyright 2002 South African Institute of Mining and Metallurgy.]... 

The distillation of a mixture of ethanol(l)-Lbutyl alcohol(2)-water(3) in a sieve tray column was considered in Example 12.2.2. Here, we continue that problem by determining the point efficiencies for this system under the conditions specified in the prior example. For the record the composition of the vapor entering the tray is... 

High capacity in foaming system. Trayed columns use the continuous liquid phase to create a froth that is difficult to separate. Packed towers make the vapor phase continuous, and the liquid phase discontinuous. 

Depending on the tray geometry (diameter, weir height, downcomer design, etc.) in trayed columns or on the type of packing in packed columns, a certain amount of liquid is retained within the column as liquid holdup. An additional amount of liquid holdup is associated with the condenser assembly. The remaining space within the distillation system contains the vapor holdup. In continuous columns, holdup is usually not a factor since it does not affect the separation as long as the column operates at steady state. 

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