Distillation column design features

Fair, J. R. and W. L. Bolles, Monsanto Co., A Feature Report on Modern Design of Distillation Columns, Chemical Engineering, April 22, 1968. 

The developments in this subsection have revealed that high-purity distillation columns exhibit a dynamic behavior with three time scales. Thus, according to the results in Sections 7.4 and 3.4, the design of a control system involves the synthesis of a tiered structure featuring three levels of control action. 

An extra feature of the equipment shown in Figure 21.3 is the design of the condensation train. It is known that under certain conditions, the pyrolytic products when condensed, form a mist or aerosol, similar to the smoke produced by cigarettes or a barbecue. These aerosols, may accumulate as a wax on the walls of condensers, but often they do not settle inside collection vessels. This problem was solved by making the gases flow upwards through a vertical condenser, effectively creating a reflux-like effect, similar to a distillation column. Furthermore, this condenser was operated at a temperature that cooled the products to a point at which they condensed, but did not solidify, and allowed the liquid droplets to coalesce and hence be collected. The optimal operation of the condenser was with water at temperatures between 50 and 60°C [86]. 

Qualification of equipment—The qualification of BPC process equipment including reaction vessels, receivers, crystallizers, centrifuges, dryers, filters, distillation columns, solvent distribution systems, etc. is a well-defined activity. While this equipment is somewhat different in design and operating features, than the dosage form equipment that has been the subject of the majority of papers on the subject, the same general principles apply. Reaction vessels, receivers, and crystallizers differ only minimally from formulation and water for injection tanks. Some BPC dryers are identical to those utilized in tablet departments. Solvent distribution systems are piping systems and may resemble WFI distribution systems. Some pieces of equipment such as distillation columns... 

There is no claim that the material is all new. The steady-state methods are discussed in most design textbooks. Most of the dynamic material is scattered around in a number of papers and books. What is claimed is that this book pulls this material together in a coordinated easily accessible way. Another unique feature is the combination of design and control of distillation columns in a single book. 

Membranes can also be used to purify a mixmre and attain composition beyond the azeotropic composition. The pervaporation process features a liquid feed, a liquid retentate, and a vapor permeate. While gas-phase membrane processes are essentially isothermal, the phase change in the pervaporation process produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. An important application is the removal of water from the ethanol-water azeotrope. Chapter 14 will discuss the details of design and control of such processes. 

Well designed distillation columns are close to electronic filters and while complex their performance can be well predicted from thermodynamic data. The complexity of chemical reactors varies from case to case. The performance of a well designed methanol or ammonia reactor can be predicted as well as that of a distillation column, whereas in more complex systems the risks of predictions is greater. In some sense identification of chemical reaction models has some features of a purely statistical correlation. Even in a simple well defined system such as isomerization of xylene where we can measure all reaction... 

This chapter covers the design of separating columns. Though the emphasis is on distillation processes, the basic construction features, and many of the design methods, also apply to other multistage processes such as stripping, absorption and extraction. 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

The design of RD is currently based on expensive and time-consuming sequences of laboratory and pilot-plant experiments, since there is no commercially available software adequately describing all relevant features of reactions (catalyst, kinetics, holdup) and distillation (VLE, thermodynamics, plate and packing behavior) as well as their combination in RD. There is also a need to improve catalysts and column internals for RD applications (1,51). Figures 8 and 9 show some examples of catalytic internals, applied for reactive distillation. 

Another approach intensifies distillation by combining two columns into one, a so-called dividing wall column (Figure 3.8). This arrangement obviates a second separate column and its evaporator and condenser. The unit features a vertical wall in the middle part of the column, creating a feed and draw-off section in this part of the column. The dividing wall, which is designed to be gas- and liquid-sealed, permits... 

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