Stripping column diameter calculation

Determining the number of theoretical and actual trays in a distillation column is only part of the design necessary to ensure system performance. The interpretation of distillation, absorption, or stripping requirements into a mechanical vessel with internal components (trays or packing, see Chapter 9) to carry out the function requires use of theoretical and empirical data. The costs of this equipment are markedly influenced by the column diameter and the intricacies of the trays, such as caps, risers, weirs, downcomers, perforations, etc. Calculated tray efficiencies for determination of actual trays can be lost by any unbalanced and improperly designed tray. 

G2. [Note This problem is quite extensive.] Biorefineries producing ethanol by fermentation have several distillation columns to separate the ethanol from the water. The first column, the beer still, is a stripping column that takes the dilute liquid fermenter product containing up to 15% solids and produces a clean vapor product that is sent to the main distillation column. The main column produces a distillate product between about 65 mole % and the ethanol azeotrope, and a bottoms product with very litde ethanol. The calculated diameter of the main distillation column is much greater at the top than elsewhere. To reduce the size and hence the cost of the main column, one can use a two-enthalpy feed system split the vapor feed into two parts and condense one part, then feed both parts to the main column at their optimum feed locations. This method reduces the vapor velocity in the top of the column, which reduces the calculated diameter however, a few additional stages may be required to obtain the desired purity. 

Column diameter is calculated from the sizing relationships given in Chapter 3. A liquid height of 0.05 m is assumed on the stripping trays, giving a tray holdup of 650 mol on these trays. The holdup on the reactive trays is 1000 mol. The holdups in the column base and reflux dmm are sized to give a 5-min residence time when 50% frill, based on the total liquid entering. In the reflux drum this is the reflux flowrate, which is equal to the overhead... 

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). 

Note that the design should be checked at other locations in the column. Since Problem 10.D2 calculated a 12-foot diameter is needed in the stripping section, calculations need to be repeated for the stripping section (see Problem 1Q.D31. Problem 10.D3 shows that backup of liquid in the downcomers might be a problem in the bottom of the column even with a 12-foot diameter. This occurs because L = L + F = 3000 Ibmol/h, which is significandy greater than the liquid flow in the top of the column. This problem can be handled by an increase in the gap between the downcomer and the tray. 

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