Distillation sharp, feasibility

The previously discussed separation techniques for gas mixtures all involve a mass separating agent. Alternatively, thermal means is employed with partial condensation and cryogenic distillation. Bamicki and Fair (1992) recommend that partial condensation be considered for enrichment when the relative volatility between the key components is 7. For large-scale (>10-20 tons/day of product gas) enrichment and sharp separations, cryogenic distillation is feasible when the relative volatility between the key components is greater than 2. However, if the feed gas contains components, such as carbon dioxide and water that can freeze at the distillation temperatures, those components must be removed first. 

The process of sharp distillation is feasible only if each product point belongs to... 

In this chapter, we describe an algorithm for predicting feasible splits for continuous single-feed RD that is not limited by the number of reactions or components. The method described here uses minimal information to determine the feasibility of reactive columns phase equilibrium between the components in the mixture, a reaction rate model, and feed state specification. This is based on a bifurcation analysis of the fixed points for a co-current flash cascade model. Unstable nodes ( light species ) and stable nodes ( heavy species ) in the flash cascade model are candidate distillate and bottom products, respectively, from a RD column. Therefore, we focus our attention on those splits that are equivalent to the direct and indirect sharp splits in non-RD. One of the products in these sharp splits will be a pure component, an azeotrope, or a kinetic pinch point the other product will be in material balance with the first. 

Flash calculations and the application of the lever rule (overall mass balance relating the feed, distillate and bottoms product streams) to predict feasible sharp splits for a given feed condition. 

The piefened separation has the lowest energy demand of all sharp separations however, it does not provide pure products. Distillate as well as bottom fractions are binaiy fractions. In practice, however, either a pure low boiler or a pure high boiler has to be gained. Such separations are feasible but they generally have a higher energy demand. 

The important advantage of mixture separability analysis for each sharp split consists of the fact that this analysis, as is shown in this and three later chapters, can be realized with the help of simple formalistic rules without calculation of distillation. A spht is feasible if in the concentration space there is trajectory of distillation satisfying the distillation equations for each stage and if this trajectory connects product points. That is why to deduct conditions (rules) of separability it is necessary to study regularities of distillation trajectories location in concentration space. 

Feasible sharp reversible distillation split of ideal mixtures can be presented as follows 1, 2,... (n - 1) 2,3... n. Therefore, at the reversible distillation, components 2, 3,... (n - 1) are distributed among the top and the bottom products. At nonsharp and semisharp reversible distillation, both products contain all the components or one of the products does not contain the lightest or the heaviest component. At nonsharp reversible distillation, product points lie in the same straight line as at sharp distillation but at some distance from the hyperfaces of the concentration simplex. 

It follows from the aforesaid that sharp separation in a reversible distillation column is feasible only if the liquid-vapor tie-line of feeding is directed to the possible product composition region Reg at the boundary element C i of the concentration simplex and from region Reg at other boundary element... 

If the problem is stated in this way, it is necessary to determine what product compositions xd and xb of sharp separation are feasible at distillation of nonideal zeotropic and azeotropic mixtures. The theory of distillation trajectory tear-off from the boundary elements of concentration simplex answers this question. 

This allows us to actively influence the location of the pseudoproduct point of the intermediate section in order to maintain sharp separation (i.e., separation at which the intermediate section trajectory ends at some boundary element of the concentration simplex). This is feasible in the case when inside concentration simplex there is one trajectory of reversible distillation for pseudoproduct point x ) that ends at mentioned boundary element, and there is the second trajectory inside this boundary element. To maintain these conditions, pseudoproduct point x j) of the intermediate section should be located at the continuation of the mentioned boundary element, because only in this case can liquid-vapor tie-hues in points of reversible distillation trajectory located in this boundary element he at the lines passing through the pseudoproduct point x jy. We discuss these conditions in Chapter 4. It was shown that in reversible distillation trajectory tear-off point x[ev e from the boundary element the component absent in it should be intermediate at the value of the phase equUibrium coefficient between the components of the top product and of the entrainer rev,D > Kevj > Kev.s)- This condition is the structural condition of reversible distillation trajectory tear-off for the intermediate section. Mode condition of tear-off as for other kinds of sections consists of the fact that in tear-off point the value of the parameter (LfV) should be equal to the value of phase equilibrium coefficient of the component absent at the boundary element in tear-off point of reversible distillation trajectory ((L/V)m =... 

We have a considerable limitation of sharp extractive distillation process in the column with two feeds the process is feasible if the top product components number is equal to one or two. This Umitation arises because, in the boundary element formed by the components of the top product and the entrainer, there is only one point, namely, point iV+, that belongs to the trajectory bundle of the intermediate section. If Eq. (6.11) is valid, then the joining of the trajectories of the intermediate and top sections takes place as at direct split in two-section columns in the mode of minimum reflux. If Eq. (6.12) is valid then joining goes on as at split with one distributed component. 

Therefore, extractive distillation at three and more components in the top product is feasible in principle, but requires a search for an allowed composition of the pseudoproduct. If it is necessary to design a sharp extractive distillation column at ntr = 3, then it is not allowed to set the rate of the entrainer arbitrarily, but it is necessary to determine it from the conditions of joining of trajectories of the top and intermediate sections. The parameter E/D ensures an additional degree of freedom, which allows to increase by one the number of product components in the top product ntr. 

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