Ponchon–Savarit vs. McCabe–Thiele: What extra advantages does the enthalpy–composition method provide for binary distillation design?
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AIt is more accurate for determining the number of equilibrium stages.
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BIt explicitly accounts for enthalpy changes in the column.
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CIt allows direct estimation of reboiler and condenser heat loads from the same diagram.
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DAll (a), (b) and (c).
Answer
Correct Answer: All (a), (b) and (c).
Explanation
Introduction / Context:Two classic graphical techniques for binary distillation are McCabe–Thiele (x–y diagram) and Ponchon–Savarit (enthalpy–composition diagram). Understanding their differences is essential for accurate stage and duty estimates, especially when heats of mixing and sensible-heat effects are significant.
Given Data / Assumptions:
- Binary distillation under steady operation.
- Enthalpy changes may not be negligible.
- Ideal or non-ideal behavior may be present.
Concept / Approach:McCabe–Thiele uses an x–y equilibrium diagram and assumes constant molar overflow (CMO), which implies horizontal operating lines and often neglects enthalpy variations. Ponchon–Savarit uses an h–x (or H–x) chart with tie lines, capturing both composition and enthalpy simultaneously. This enables direct reading of stage-to-stage energy balances and utility duties.
Step-by-Step Solution:
Map feed, q-line, and operating conditions onto an enthalpy–composition diagram.Step off stages using equilibrium tie lines and overall/partial energy balances.Read condenser and reboiler duties directly from the enthalpy differences on the diagram.Verification / Alternative check:When heats of mixing are appreciable or when CMO fails, Ponchon–Savarit typically predicts a different number of stages and more accurate heat loads than McCabe–Thiele, consistent with rigorous simulations.
Why Other Options Are Wrong:
- Each statement (a), (b) and (c) is correct; therefore “All (a), (b) and (c)” is the best choice.
Common Pitfalls:Applying McCabe–Thiele blindly under strong non-idealities or large heat effects; ignoring energy balances can lead to significant duty errors.
Final Answer:All (a), (b) and (c).