Design of a distillation column involves designing a theoretical column with several simplifying assumptions, and then applying a correction factor to compensate for actual conditions. Typically, the inlet flow to the distillation column is known, as well as mole percentages to feed plate, because these would be specified by plant conditions. This information may be given in terms of the light and heavy keys. For binary distillation, the light key is the more volatile component, and the heavy key is the less volatile component, or the component with the higher boiling point. The desired composition of the bottom and distillate products will be specified, and the engineer will need to design a distillation column to produce these results. When the problem is defined, design calculations can begin. For illustrative purposes, and for background information, tray-by-tray calculations will first be described before beginning the McCabe-Thiele Method.
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Key Components:
A Closer Look
The concept of "key components" is mentioned briefly for a binary system in the main text. These concepts can be used for binary or multicomponent systems, and have broader definitions then previously discussed.
- Light Key
- Heaviest of the light products
- Heavy Key
- Lightest of the heavy products
At first glance, these definitions are a little confusing. With a closer look, it can be seen that through these definitions, there is only one unique solution for any system. For a binary system, it is obvious that the more volatile components is the light key and the less volatile component is the heavy key. But, if a system existed with components A,B,C,and D, listed in the order of decreasing volatility, it is easier to see the need for this more extended definition. In this case, if A and B are desired in the distillate product, and C and D are desired in the bottom product, then B is the light key, (heaviest between A and B,) and C is the heavy key, (lightest between C and D.)
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The first step in tray-by-tray calculations is to determine the column pressure. This would involve a bubble or dew point calculation, depending on condenser conditions. The next step would be to specify a reflux ratio and perform heat and energy balances around the condenser in order to determine necessary heat transfer, and to determine the vapor composition leaving the top tray of the column. Assuming that the vapor leaving the top tray is in equilibrium with the descending liquid, the liquid composition can be obtain through dew-point calculations, as well as the temperature of that tray. With information known for the top stage of the column, and iterative method is applied to each stage in succession down the column until the feed plate is reached, using input from the last and the equilibrium assumption, until the liquid output of the last stage is finally determined. Before information on the stages in the stripping section would be able to be determined, heat and mass balances must be calculated for the reboiler. This is an iterative procedure where the vapor in equilibrium with the liquid leaving the final stage must be guessed. When the correct heat balance is obtained, a similar procedure is performed as described for the rectifying section; composition and flow rates are determined for successive stages, starting with the final stage, until the feed plate is reached.
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