Distillation tower design

Kister, H. Z., and K. F. Larson, Packed Distillation Tower Design, in Schweitzer, P.A. (Ed.), Handbook of Separation Techniques for Chemical Engineers, 3 ed., McGraw-Hill, New York, 1997, section 1.6. 

Why are distillation towers designed with controls that fix the tower pressure ... 

Solvent reeovery systems would also neeessitate the speeifieation of eondenser duties, distillation tower sizes, holding tanks, piping, and valves. It is important to note that the engineering design of an adsorption system should be based on pilot data for the partieular system. Information ean usually be obtained direetly from the adsorbent manufaeturer. The overall size of the unit is determined primarily by eeonomie eonsiderations, balaneing the operating eosts against the eapital eosts. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

Rukovena, F. and T. D. Koshy, Packed Distillation Tower Hydraulic Design Method and Mechanical Considerations, Ind. and Eng. Chem. Res., Vol. 32, No. 10 0993) p. 2400 (Used by permission. The American Chemical Society. All rights reserved.)... 

The kettle unit used in the reboiling service usually has an internal weir to maintain a fixed liquid level and tube coverage. The bottoms draw-off is from the weir section. The reboiling handled in horizontal thermosiphon units omits the disengaging space because the liquid-vapor mixture should enter the distillation tower where disengaging takes place. The chiller often keeps the kettle design but does not use the weir because no liquid bottoms draw off when a refrigerant is vaporized. 

Chemical process equipment is of two kinds custom designed and built, or proprietary off the shelf. For example, the sizes and performance of custom equipment such as distillation towers, drums, and heat exchangers are derived by the process engineer on the basis of established principles and data, although some mechanical details remain in accordance with safe practice codes and individual fabrication practices. 

Stable column operation is guaranteed by keeping the internal reflux of the distillation tower constant. Consequently, internal reflux controls are designed to compensate for changes in the temperature of the external reflux caused by ambient conditions. Figure 2.90a is controlled by a typical internal reflux control system (top) and the equations that need to be solved in calculating the required external reflux rate are shown at the bottom. This control system corrects for either an increase in overhead vapor temperature or a decrease in external reflux liquid temperature. 

Example 6 Determination of optimum reflux ratio. A sieve-plate distillation column is being designed to handle 700 lb mol (318 kg mol) of feed per hour. The unit is to operate continuously at a total pressure of 1 atm. The feed contains 45 mol% benzene and 55 mol% toluene, and the feed enters at its boiling temperature. The overhead product from the distillation tower must contain 92 mol% benzene, and the bottoms must contain 95 mol% toluene. Determine the following ... 

The designer usually wants to specify stream flow rates or parameters in the process, but these may not be directly accessible. For example, the desired separation may be known for a distillation tower, but the simulation program requires the specification of the number of trays. It is left up to the designer to choose the number of trays that lead to the desired separation. In the example of the purge stream/ reactor impurity, a controller module may be used to adjust the purge rate to achieve the desired reactor impurity. This further complicates the iteration process. 

Liebmann, K., Dhole, V. and Jobson, M., Integrated design of a conventional crude oil distillation tower using pinch analysis, Trans IChemE, 76, 1998. 

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