Ponchon savarit.pdf

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SIDESTREAMS AND MULTIPLEFEEDs USING McCabe THIELE

GRAPHICAL METHOD.


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How many theoretical stages are needed for the

distillation separation shown below? Where is the

feed and the take-off stages? Use the McCabe-Thiele graphical method. At 1 atm (14.696 psia),

methanol boils at 64.7oC and ethanol boils at 78.4oC.

The vapour pressure of ethanol at 64.7oC = 8.2 psia.

The vapour pressure of methanol at 78.4oC = 25

psia. Consider the mixture to be ideal. For the two

components assume that the relative volatility is

1.745. Assume that the pinch occurs at the feed

stage.


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GRAPHICAL MULTISTAGE CALCULATIONS BY THE

PONCHON-SAVARIT METHOD

Brief overview of the Method

• The McCabe-Thiele method assumes that molar liquid and vapourflows are constant in each section of the column. This assumption

eliminates the need to make an energy balance for each stage. A

graphical method that includes energy balances as well as material

balances is the Ponchon-Savarit method.

• The Ponchon-Savarit method uses Enthalpy-concentration diagrams

such as the one shown below. This diagram includes curves for the

enthalpies of saturated liquid and vapour mixtures. Terminal points of

tie lines connecting these two curves represent the equilibrium

vapour and liquid compositions.

• For the purpose of this section we are always going to assume that

Murphree’s plate efficiency is 100% which implies that liquid and

vapour streams leaving each plate are in perfect equilibrium with

each other in terms of their compositions.


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Consider a single stage, n-1, where vapour from stage n-2 below is mixed

adiabatically with liquid from stage n above to give an overall mixture,

denoted by the mole fraction z , and then brought to equilibrium


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Figure 10.4


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Feed Stage Location

In the diagram above, P’ is the difference point for

stages above the feed stage and P’’ for stages below the

feed stage, it follows that the transition from onedifference point to the other occurs at the feed stage.

Therefore the tie line that crosses the line P’P” will be

ideal feed stage.

Hence the feed stage is stage 5.


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Limiting Reflux Ratios

Total Reflux At total reflux, the minimum number of stages is required and F=0, B = 0, D

= 0 , therefore the points (hD – qD, x D) and (hB – qB, x B) lie at infinity.

The operating lines for each section of the column thus become straightvertical lines and the minimum number of stages in calculated as shown in

the diagram below.

Minimum Reflux Ratio


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Minimum Reflux Ratio

• The first step in determining the minimum reflux ratio is to use the

feed condition. In the diagram below the difference point P’ and P”

are located by extending the equilibrium tie line through z F to itsintersections with the two difference point.

• However in the Figure below, the extension of the feed point tie line

places the difference points at P’1 and P’’1.

• The pinch point however manifests itself at tie line T. This suggests

that the minimum reflux ratio in the enriching section is set by the

highest intersection made by the steepest tie line in the section with

the line x = x D.

• Use of difference point P’ in stepping off stages gives an infinitenumber of stages with the pinch point occurring at x T

• x = x B. If there is a tie line in the stripping section that gives a lower

P”, this must be used; otherwise, a pinch region will develop in the

stripping section.


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The Partial Condenser The Figure below shows the construction of operating line corresponding to

an enriching section with a partial condenser. Here V n and LR are on the

same operating line, and LR is in equilibrium with D.


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