Determine the number of stages required to achieve a separation at total reflux

McCabe-Thiele Analysis

In order to determine the number of stages necessary for a given separation (i.e., known feed composition, bottoms composition, and distillate composition), we could analytically solve stage-wise balance equations starting from the top (with the rectifying equations) and switching over to the stripping equations, once we reach the feed composition.

A simpler technique, however, involves a graphical approach. We already have two operating equations (lines), an equilibrium expression (line) using the relative volatility, and a feed expression (line).

EQUILIBRIUM LINE: $y=\frac{\alpha_{AB}x}{1+(\alpha_{AB}-1)x}$

RECTIFYING LINE: y = xL/V + xD(1-L/V)

STRIPPING LINE: y = xL'/V' - xB(L'/V'-1)

FEED LINE: y = xq/(q-1) - z/(q-1)

DEFINITION:

The McCabe-Thiele method is a graphical separations procedure that exploits the fact that a separation stage involves interaction between the mass balance and vapor-liquid equilibrium.

The simplest example of a McCabe-Thiele analysis involves the special case of no material being removed from the top or the bottom, so that the L=V=L'=V' and both the stripping and rectifying lines reduce to:

y = x

NOTE

This operating condition corresponds to a reflux ratio (L/D) and boil-up ratio (V/B) of infinity. The feed line is useless in this case.

This condition leads to a plot that involves the equilibrium curve and a 45 degree operating line

xy diagram

If we start at the desired distillate composition xD, and recalling that we need to use a total condensor if D=0, we note that the vapor composition yj coming off the top stage of the column is the same as xD (as is the liquid composition returing to the column x0). The 45 degree line, therefore, shows what the composition yj actually is. 

stepping on xy diagram 1

The composition of the liquid in equilibrium with that vapor (i.e., the one with composition xj) can be determined by looking vertically at the x value of the curve at that point yj

stepping on xy diagram 2

Our material balance (operating line) tells us the relationship between the  current  vapor composition and the composition of the next vapor stage. We, therefore, "perform" our material balance by first moving vertically (down) to the operating line (in this case the y=x line) then moving horizontally (left) to the equilibrium line, to tell us the y composition of the next lower stage. In order to solve the remainder of the problem, we continue with stage-wise stepping until we reach the bottoms composition:

stepping on xy diagram 3

NOTE

Each stair-step represents a stage within the column. This example, therefore, involved 3 stages.

OUTCOMES:

Determine the number of stages required to achieve a separation at total reflux using the McCabe-Thiele method