Multiple feeds and products, distillation

This subsection describes how to generate the feasible combinatorial possibilities of distillation column configurations for separation of mixtures that do not form azeotropes. Components are named A, B, C, D,. . . and they are listed in the order of decreasing volatility (or increasing boiling temperature). We limit our considerations to splits where the most volatile (lightest) component and the least volatile (heaviest) component do not distribute between the top and bottom product. For simplicity we consider only separations where final products are relatively pure components. Systems containing simultaneously simple and complex distillation columns are considered. Simple columns are the conventional columns with one feed stream and two product streams complex columns have multiple feeds and/or multiple product streams. 

While multi-product columns could be used to separate mixtures into products with distinct compositions, sharper separations usually require processing with multiple columns to achieve the desired splits and purities. A possible scheme would be to break down the separation process into a forward flowing series of single-feed, two-product distillation columns, each of which makes the separation between two key components. In a more complex multi-column approach, the process may include recycle streams, external separation-enhancing streams, stream splits, recombination of streams, energy integration, and so on, all aimed at achieving an economic optimum. 

The lower part of the column is covered by stepping off stages in a fashion similar to that in the upper part of the column, and the final conni of theoretical stages is then determined. The Ponchon-Savarit method may be used for many situations more complex lhan the simple one just described mixed vapor-liquid distillate product, side draw streams, multiple feeds, and so on. Standard unit operations textbooks should be consulted for more dentils on this methnd. As mentioned, it suffers from a need for enthalpy-concentration data, but even a crude approximation based on linear variation of enthalpy with concentration can be better than the McCabe-Thiele approach if there is a very large difference in the latent heats of vaporization of the iwo components being distillnd. 

Complex columns are in a broad sense all columns that are not simple columns, like the column shown in Figure 1.1. Complex columns may have multiple feeds, side product streams, stream transfers between two column units (thermally coupled columns), simultaneous chemical reaction(s) within the column body, hybrid membrane-distillation columns, and so on. Each of these columns present unique opportunities for cost saving. Typical complex columns are shown in Figure 1.3a d. 

Kister [94, 95] examines binary distillation systems with multiple feeds, one or more side products, one or more points of heat removal or addition, and various combinations. 

Multiple equilibrium stage processes simulated in this program are distillation, absorption, and stripping. Both simple and reboiled absorbers are included, and multiple feed plus side-stream products are possible from the fractionators. Matrix- and short-cut-type solution methods are provided in separate subroutines. 

Chemical engineers have been solving distillation problems by using the equilibrium-stage model since 1893 when Sorel outlined the concept to describe the distillation of alcohol. Since that time, it has been used to model a wide variety of distillation-like processes, including simple distillation (single-feed, two-product columns), complex distillation (multiple-feed, multiple-product columns), extractive and azeotropic distillation, petroleum distillation, absorption, liquid-liquid extraction, stripping, and supercritical extraction. 

If the distillation unit provides for side products and/or multiple feed locations, then the tray to tray calculations will use material balances for each tray that account for these additional streams. Furthermore, it is important to note that fraction cuts for crude oil fractionation are based more on boiling temperature ranges rather than on composition. 

Before we start searching for feasible designs and interpreting the effects of Xa s and 7 a s, we first have to define our column and break it into a series of CSs. Consider then a two product distillation column terminated at the top by a condenser and at the bottom by a reboiler, as shown in Figure 6.1. We shall define the total feed to the column as Ft, which is to be distributed at multiple points down the length of the column. For the general case where the feed is to be split into N — 1 substreams, at A 1 feed points, the result will be a column consisting of N CSs. 

At this input-output level the following assumptions are considered ( ) steady-state modeling (n) multiple feeds (nfeed ), side draw-off streams (notr Z+), heat streams (n, Z+), reactions rirx Z+) and components (uc Z+) and Hi) single bottom and distillate streams. Component and energy balances are performed over the structure. The component molar balances MC) include the distillate, bottom, feed and draw-off streams and the production (or consumption) of species due to chemical reaction. 

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