Analysis of Continuous Distillation

We focus on binary distillation - i.e. 2 components: A (MVC) and B (LVC)

Refer to the Figure below which showed the nomenclature adopted:

Feed stream F with mole fraction xF Distillate D with mole fraction xD Bottoms B with mole fraction xB The Reflux Liquid is L. For a total condenser, it has the same concentration as the distillate.

The feed usually enters the column somewhere near the middle of the column. Note that there are 2 sections in the column: the Rectifying Section (Enriching Section) located above feed entry; and the Stripping Section (Exhausting Section) located below feed entry.

Separation of the feed is achieved via:

Distillate D which is richer in the more volatile component of mole fraction xD

Bottoms B which is richer in the less volatile component, where the mole fraction of the more volatile component is xB

In other words, we want the distillate to be as pure in MVC as possible (maximum xD = 1.00), and the bottoms to be as pure in LVC as possible (minimum xB = 0.00).

Our analysis of continuous distillation shall focus on trayed columns. Trays can be one of 3 major types: sieve, valve or bubble caps. In addition, packed columns are also commonly used. Various types of packings had been used. For more information, visit the PLANT OPERATION & MONITORING Section.

There are 2 graphical methods for the anaylsis of continuous distillation. Click here for more details.

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The Reflux Ratio, R

An important parameter in the analysis of continuous distillation is the Reflux Ratio, defined as the quantity of liquid returned to the distillation column over the quantity of liquid withdrawn as product from the column, i.e. R = L / D.

The reflux ratio R is important because the concentration of the more volatile component in the distillate ( in mole fraction xD ) can be changed by changing the value of R.

For the design of a new column, a specified value of R is used to carry out the design (for example, using the McCabe-Thiele Method), i.e. to determine the number of trays or height of packing required to achieve a desired separation.

Note: As will be shown in later section with more discussion on Reflux Ratio, there is a "trade-off" between the reflux ratio and number of trays to be used. In other words, the same separation can be achieved by using different combination of reflux ratio and number of trays. Recall that trays facilitate mass transfer between the vapour and liquid for better separation. Using a lower reflux ratio for the design will result in a higher number of trays required for the separation and vice versa. This is viable only if the tower is yet to be built (i.e. in the design stage).

For an existing tower, it is no longer possible to add trays or packings. Hence, to increase the distillate MVC mole fraction, the reflux ratio must be increased. Higher reflux ratio implied more re-contacting of the liquid with the rising vapour, thus leading to more separation, and better distillate purity.

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