Tall columns

If the tall column is a stripper, the separation will approach that of... 

The formation of micelles in the foam breaker does not affect the limiting equations because of the theoretically unhmited opportunity in a sufficiently tall column for their transfer from the reflux to the ascending stream [Lemhch, Principles of Foam Fractionation, in Perry (ed.), Progress in Separation and Purification, vol. 1, Interscience, New York, 1968, chap. 1]. 

The slowly rising foam in a tall column can Be employed as the sorbent for continuous chromatographic separations [Talman and Rubin, Sep. Sci., 11, 509 (1976)]. Low gas rates are also employed in short columns to produce the scumlike froth of batch-operated ion flotation, microflotation, and precipitate flotation. 

Wind loading will only be important on tall columns installed in the open. Columns and chimney-stacks are usually free standing, mounted on skirt supports, and not attached to structural steel work. Under these conditions the vessel under wind loading acts as a cantilever beam, Figure 13.19. For a uniformly loaded cantilever the bending moment at any plane is given by ... 

An allowance of 0.4 m should be added for a caged ladder. The calculation of the wind load on a tall column, and the induced bending stresses, is illustrated in Example 13.3. Further examples of the design of tall columns are given by Brownell (1963), Henry (1973), Bednar (1990), Escoe (1994) and Jawad and Farr (1989). 

Tall columns sway in the wind. The allowable deflection will normally be specified as less than 150 mm per 30 metres of height (6 in. per 100 ft). 

Helical strakes (strips) are fitted to the tops of tall smooth chimneys to change the pattern of vortex shedding and so prevent resonant oscillation. The same effect will be achieved on a tall column by distributing any attachments (ladders, pipes and platforms) around the column. 

The term (ae/g) is called the seismic constant Ce, and is a function of the natural period of vibration of the vessel and the severity of the earthquake. Values of the seismic constant have been determined empirically from studies of the damage caused by earthquakes, and are available for those geographical locations which are subject to earthquake activity. Values for sites in the United States, and procedures for determining the stresses induced in tall columns are given by Megyesy (2001), Escoe (1994) and Moss (2003). 

Skirt supports are recommended for vertical vessels as they do not impose concentrated loads on the vessel shell they are particularly suitable for use with tall columns subject to wind loading. 

Distillation. The distillation method of separating butene-1 is difficult. It requires a column with more than 100 trays operating under a reflux ratio of about 150 1 (Chemical engineer jargon—it means a very tall column, with lots of recycle—very energy intensive.)... 

Asphaltic materials may be separated from the residue by adsorption on clay. The oil is percolated through a relatively tall column of comparatively coarse clay or mixed with a selected clay of fine mesh at relatively high temperatures, and the spent clay is removed by filtration. 

The process called fractional distillation uses a method of continuous redistillarion to separate mixtures of liquids the mixture is heated and the rising vapor passes up through a tall column packed with material having a high surface area, such as glass beads (Fig. 8.42). The vapor begins to condense on the beads near the bottom of the column. 

It is more economical to install two towers in series than to erect one column taller than 200 ft. Two 100-ft-tall columns are therefore used in this application for the IC5 fractionator and two 100-ft-tall columns each for the NC5 fractionator. Thus four columns, each 100 ft tall, are required to make the IC5 and NC5 overhead product streams. A bottoms transfer pump between each column pair is of course required to transfer the bottoms liquid to the downstream top column tray. The vapors flow in a countercurrent direction from the bottom tower top to the bottoms section of the upper column. The transfer pumps placed between these two columns transfer 1000 gpm of liquid from the bottom tray of one to the top tray of the second column, which is the bottom column of the two. 

The height of the column section is determined mainly by the number of stages or plates required which in turn depends on how easy or difficult the mixture is to separate, i.e. the extent of separation. If the boiling points of the components to be separated are close, then a large number of stages are required leading to a very tall column. The column height is limited, however, by factors such a wind conditions, etc. 

In the second method a part of the condensate is returned (known as reflux) so that it comes in contact with the rising vapor on its way to the condenser. This technique is known as rectification. The efficiency of separation depends on the ratio of condensate returned to the still to the liquid product removed (which is known as the reflux ratio). The vapor and the reflux are made to come in intimate contact in a tall column located on top of the still. In the column the low boiler (liquid with a lower boiling point) spontaneously diffuses from the liquid reflux to the vapor, while the high boiler (the liquid with the higher boiling point) diffuses from vapor to liquid. So as the vapor rises in the column, it becomes enriched in low boiler and, as the liquid descends the column, its content of high boiler increases. A reboiler is provided at the bottom to vaporize the liquid and feed it up the column (Fig. 3.5). 

Since liquid maldistribution may become a problem unless the flow is periodically redistributed in a tall column, the total packing height must be broken up into a number of individual beds ... 

Woodburn55 measured the gas-phase RTD for extremely high ratios of liquid rate/gas rate in a 29.2-cm i.d. and 97-cm tall column packed with 2.5-cm stoneware Raschig rings (e = 0.714). The data were obtained in the range... 

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