Tower/column diameter calculations

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-... 

The support ring width should not be excessive, or it may restrict the column capacity. Strigle (386) gives the maximum recommended support ring width as a function of column diameter for columns between 4 and 12 ft in diameter. This width, W, in inches, can be calculated from W = D/4 + 0.5, where D is the tower diameter in feet. For small columns (< 4 ft in diameter), support rings sometimes restrict column capacity (386). 

Due to the change in vapor density with absolute pressure, the pressure drop should be calculated separately for each packed bed. Total column pressure drop is the summation of the pressure drops through the packed beds plus the pressure drops through the tower internals. Overall column pressure drop determined this way must be compared with the pressure drop originally assumed. This procedure should be repeated by varying column diameter or packing size until the assumed and calculated pressure drops agree. 

The net fractional area of vapor flow after deducting the downcomer area from the column area is often 0.8 the quantity (factor) is usually around 0.6. One calculates the value of d, for as well as Wfj, the larger value is used for the whole column. As indicated in Section 8.1.3.8, packed towers are used for towers whose diameter turn out to he less than 2 ft. 

Consideration of these two key factors is what we may call the process design stage of distillation column. On the other hand, the purpose of the mechanical design is to select the tower internals and column diameter and height. Some of the factors involved in design calculations include feed load size and properties and the type of distillation column utilized. 

Using and GPM (column liquid loading in gallons per minute), obtain approximate tower diameter for calculating flow path length. Use... 

ATM = Minimum column cross-sectional area, tV. Further detailed design calculations may result in a change in tower diameter. 

This is the boilup rate, which is approximately 3.3 ft vapor/sec. An approximately 1 ft 0 in. diameter column can handle this rate however, because it is in the usual size for a packed tower (or cartridge trays), the diameter must be checked using the packed tower calculations in Chapter 9 of this volume. 

When calculating the tower diameter, conditions at the bottom of the tower should be used because this corresponds to the maximum flow rates. At the bottom of the column ... 

It is informative to make some estimates about the size of the column used in this test. In normal liquid-liquid exuaction, Treyball discusses that a typical hole velocity, i.e., the velocity of the lighter liquid passing upward through the holes, is about 0.3 ft/sec, and that a typical hole area is about 10-20% of the plate area. If we use these values to estimate the diameter of the coffee decaffeination tower, we calculate that the total hole area is about 8 ft Assuming an average 15% hole area the cross sectional area of the column is 54 ft, or a diameter of 8 ft. Like the 495,000 Ib/hr flow rate related earlier, an 8 ft diameter column is quite a large column in which to carry out experimental decaffeination tests. Do the data that are reported in the example originate from the operation of a commercial facility somewhere or are the data made up. ... 

Gas absorption can be carried out in a column equipped with sieve trays or other types of plates normally used for distillation. A column with trays is sometimes chosen instead of a packed column to avoid the problem of liquid distribution in a large diameter tower and to decrease the uncertainty in scaleup. The number of theoretical stages is determined by stepping off plates on a y-x diagram, and the number of actual stages is then calculated using an average plate efficiency. The plate and local efficiencies are defined in the same way as for distillation [Eqs. 

A wastewater stream of 0.038 m3/s, containing 10 ppm (by weight) of benzene, is to be stripped with air in a packed column operating at 298 K and 2 atm to reduce the benzene concentration to 0.005 ppm. The packing specified is 50-mm plastic Pall rings. The airflow rate to be used is five times the minimum. Henry s law constant for benzene in water at this temperature is 0.6 kPa-m3/mol (Davis and Cornwell, 1998). Calculate the tower diameter if the gas-pressure drop is not to exceed 500 Pa/m of packed height. Estimate the corresponding mass-transfer coefficients. The diffusivity of benzene vapor in air at 298 K and 1 atm is 0.096 cm2/s the diffusivity of liquid benzene in water at infinite dilution at 298 K is 1.02 x 10 5 cm2/s (Cussler, 1997). 

The absorption tower will be filled with 50-mm ceramic Pall rings. Design for a gas-pressure drop not to exceed 400 Pa/m of packed depth. Assume that cooling coils will allow isothermal operation at 300 K. The gas will enter the column at the rate of 1.0 m3/s at 300 K and 1 atm. The partial pressure of methanol in the inlet gas is 200 mmHg (ScG = 0.783). The partial pressure of methanol in the outlet gas should not exceed 15 mm Hg. Pure water enters the tower at the rate of 0.50 kg/s at 300 K. Neglecting evaporation of water, calculate the diameter and packed depth of the absorber. 

Packed Tower Example. Equation (17-17) will be used to estimate the H.E.T.P. values for the atmospheric distillation of a benzene-toluene mixture in a packed tower 5 ft. in diameter. The liquid and vapor rates are 480 and 400 lb. mols per hr., respectively. The calculation will be made for the section near the bottom of the column where the liquid and vapor are essentially toluene. 

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