Distillation columns limiting behavior

There are two limits at which we can examine the behavior of a distillation column. The first is at total reflux (i.e., with an infinite reflux ratio, which is often called infinite reflux conditions). The other extreme is to operate at minimum reflux. In this section we shall limit our discussion to the total reflux case in later sections we shall look at operating columns at finite reflux (ratio) conditions. Intuitively, we tend to expect that a column will give its maximum separation when run at infinite reflux. While this is true for ideally behaving species, it does not have to be true when separating nonideally behaving species. Thus, we need to look carefully at running colunons all the way from minimum to total reflux conditions. 

We shall determine the limiting behavior for simple distillation columns by discovering all the products of such a column, regardless of the number of stages it has or the reflux ratio used in operating it. We present and extend here the ideas advanced in Wahnschafft (1992) and Wahnschafft et al. (1992). 

To determine the behavior of a distillation column over wide ranges of operating conditions, calculational procedures are needed for treating certain limiting conditions which arise such as D = 0 and D — F. Procedures are presented and discussed below. 

It has been shown (Popken et al, 2001 BeRling et al, 1998) that the first model usually provides the highest deviations between calculation results and the behavior of real reactive distillation columns. The assumption that chemical equilibrium is reached is not adequate for most of the chemical reactions of commercial interest. Changes in composition and heat are taken into account by using the equilibrium constant K and the heat of reaction. The second model caimot be recommended because chemical reactions are slower than the time needed to reach VLE. Therefore, it makes no sense to assume kinetic limitations for the distillation part but to neglect the reaction kinetics. 

It should be mentioned that the majority of the work presented here is graphically based simply because it is easier to grasp column into-actions and behavior when approached from this point of view. However, this need not be a limitation for the methods. The authors would also like to stress that it is not necessarily the specific material and problems presented in the book that are important, but more the tools that the reader should be equipped with. The concepts we present simply put tools into the designer s hand for him/her to create a unique column or separation structure that may solve his/her particular separation problem. For instance, both distributed feed and reactive distillation columns are discussed independently, although it is of course entirely possible to conceive of a reactive distributed feed system, which is not discussed. The tools in this book will thus first allow the reader understand, analyze, and design well-known and frequently encountered distillation problems. Second, the tools can be used to synthesize and develop new systems that peihaps have not even been thought of yet. This principle applies to virtually all the work in this book and the reader is urged to explore such concepts. 

In the first selection step, the non-minimum phase behavior of the various combinations was analyzed. For this distillation column, the closed-loop bandwidth is mainly limited by the RHP zeros. Therefore, all combinations with RHP zeros smaller than 0.01 rad/s were rejected and the number of control structures was reduced to seven. 

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