Tower/column diameter

The effective interfacial area depends on a number of factors, as discussed in a review by Charpentier [C/j m. Eng.J., 11, 161 (1976)]. Among these factors are (1) the shape and size of packing, (2) the packing material (for example, plastic generally gives smaller interfacial areas than either metal or ceramic), (3) the liquid mass velocity, and (4), for smaU-diameter towers, the column diameter. 

G = mass velocity of vapour (kg/m2 s of tower area), dc = column diameter (m),... 

Flooding is by far the most common upper capacity limit of a distillation tray. Column diameter is set to ensure the column can achieve the required throughput without flooding. Towers are usually designed to operate at 80 to 90 percent of the flood limit. 

Sieve trays are widely used in industry with column diameters up to 3.66 m (Ref. A6 p.21.74), this limit was imposed upon the testing procedure. Column diameters of less than 1.5 m would not prove economical under these conditions because of the very large tower height and number of trays required. Although a larger column diameter would substantially reduce the required number of trays,... 

Summary. Following the flooding checks, the column diameter remains at 6 ft. The tray layout remains- as specified at the beginning of this section. However, the tray spacing in the top section is extended from 18 to 21 in, so that the tower has 24-in tray spacing in the bottom section and 21-in tray spacing in the top section. 

The flood-point and the maximum pressure drop criteria gave comparable tower diameters. The more conservative of the two criteria gives diameters of 5.50 and 6.12 for the top and bottom section of the tower, respectively. As the diameters for the top and bottom sections are not much different, it is attractive to use uniform column diameter. The decision of whether to make the top and bottom section diameters the same is based on the same criterion as for tray columns (Sec. 6.5.3). The preliminary column diameter is the larger for the two column sections, i.e., 6.12 ft. This diameter is normally rounded to the next nearest half foot, but in this example it is rounded only to the next nearest quarter foot. A diameter of 6.12 is far closer to 6 ft than to 6.5 ft. Column diameter is relatively small, and three excessive inches substantially increase the costs. The column is operated at high pressure, and high-pressure shells are expensive. Therefore, the preliminary column diameter is 6 ft 3 in. 

Example 1 Determination of distillation-column diameter on basis of allowable vapor velocity. A sieve-tray distillation tower is to be operated under the following conditions ... 

Stage efficiencies for packed towers must be based on experimental tests with each type of packing. The efficiency varies, not only with the type and size of packing, but also with the fluid rates, the fluid properties, the column diameter, the operating pressure, and, in general, the extent of liquid dispersion over the available packing surface. 

When column diameters are less than 0.6 m (2.0 ft) packed towers can be considerably cheaper. However, if alloy metals are necessary, plate towers may result in less cost. Using ceramic or other similar resistant materials for packing and materials of construction, packed towers can serve to handle corrosive materials and acids. Because the gas flow in packed towers may offer less degree of agitation, packed tower operation may be better for liquids that tend to foam. When liquids are thermally sensitive, packed columns may offer less holdup and thus prevent changes taking place in the liquids due to thermal reaction. 

Choose materials of construction based on corrosion considerations. Column diameters are determined by specifying linear velocities for the two phases. Column heights are determined by estimating the actual number of stages based on the theoretical stage requirements and average stage efficiency. Internals in pulse columns are very similar to those in distillation towers, especially for sieve trays. Therefore, distillation correlations can be used to estimate FOB purchased and installed costs for continuous differential contactors, if they are assumed to be pulse columns. 

Because of the need for internal access to columns with trays, a packed column is generally used if the diameter calculated from equation (4-33) is less than 60 cm. Tray spacing must be specified to compute column diameter, as shown by equation (4-32). On the other hand, recommended values of tray spacing depend on column diameter, as summarized in Table 4.3. Therefore, calculation of the tower diameter involves an iterative procedure (a) an initial value of tray spacing is chosen (usually, t = 0.6 m) (b) the tower diameter is calculated based on this value of tray spacing (c) the value of t is modified as needed according to the recommendations of Table 4.3 and the current estimate of D (d) the procedure is repeated until conver-... 

Absorption and stripping are usually conducted in packed columns or in trayed towers. Packed columns are preferred when (1) the required column diameter is less than 60 cm (2) the pressure drop must be low, as for a vacuum service (3) corrosion considerations favor the use of ceramic or polymeric materials and/or (4) low liquid holdup is desirable. Trayed towers are preferred when (1) the liquid/gas ratio is very low, and (2) frequent cleaning is required. If there is no overriding consideration, cost is the major factor to be taken into account when choosing between packed columns and trayed towers for absorption or stripping. 

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