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Optimal Operation of Distillation Columns - Course Summary

Learn about the optimal operation of distillation columns, including stabilization, regulatory control, primary variables, economic operation, cost minimization, active constraints, and maximizing product yield. Join the course in Berlin, Summer 2005 with Sigurd Skogestad to delve into the intricacies of distillation control.

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Optimal Operation of Distillation Columns - Course Summary

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  1. Distillation Course Berlin Summer 2005. Sigurd Skogestad. Part 3 Optimal operation of distillation columns and link to control

  2. Given feed (F). 5 dynamic control degrees of freedom (valves): L, V (QB), VT (QC), D, B

  3. Distillation control • First task: Stabilization and regulatory control (PID loops) • Use 3 degrees of freedom for: • Control condenser holdup (stabilization) • Control reboiler holdup (stabilization) • Control pressure • May want to add (does not remove any degrees of freedom!) • Flow controllers • Temperature controller (”stabilize profile”) • Here consider second task: Use of remaining 2 degrees of freedom to achieve optimal economic operation (steady-state) • Issue: Which ”primary” variables should we control?

  4. Primary controlled variables • 2 remaining degrees of freedom for control • What should we control? • Often composition in both ends (”two-point control”) but not always • Systematic approach: • Define optimal operation and find optimal point • To avhieve optimal operation in practice: • ”Control active constraints” • Control ”self-optimizing” for uncontrained degrees of freedom (if any)

  5. Optimal operation distillation column • Steady-state DOFs (given p and F): 2, for example L/D and V • Cost to be minimized (economcs) J = - P where P= pD D + pB B – pF F – pV V • Constraints Purity D: For example xD, impurity· max Purity B: For example, xB, impurity· max Flow constraints: min · D, B, L etc. · max Column capacity (flooding): V · Vmax, etc. Pressure: 1) p given, 2) p free: pmin· p · pmax Feed: 1) F given 2) F free: F · Fmax • Optimal operation: Minimize J with respect to steady-state DOFs cost energy (heating+ cooling) value products cost feed

  6. Solution to optimal operation distillation • Cost to be minimized J = - P where P= pD D + pB B – pF F – pV V • Optimal operation: Minimize J with respect to DOFs • General: Optimal solution with N DOFs: • N – Nu DOFs used to satisfy “active” constraints (· is =) • Nu remaining unconstrained variables Usually: Nu zero or small • Distillation at steady state with given p and F: • N=2 DOFs. Three cases: • Nu=0: Two active constraints (for example, xD, impurity = max. xB, impurity = max, “TWO-POINT” CONTROL) • Nu=1: One constraint active • Nu=2: No constraints active very unlikely unless there are no purity specifications (e.g. byproducts or recycle)

  7. Expected active constraints distillation • Cost to be minimized J = - P where P= pD D + pB B – pF F – pV V • Amount of valuable product (D or B) should be maximized • Implication for valuable product: Avoid quality give-away (overfractionation) )Product. spec. valuable product is always active (and should be controlled for optimal operation) • Methanol + water example: Keep xD, impurity = 0.5% (max.) • “Sell cheap product (water) as valuable product” • This also saves energy (because overfractionation requires larger reflux and more energy) methanol + water valuable product methanol + max. 0.5% water cheap product (byproduct) water + max. 0.1% methanol

  8. Expected active constraints distillation • Amount of valuable product (D or B) should be maximized • Implication for cheap product: We may reduce the loss of valuable product by over-fractionating the cheap end, but this costs more energy. Two cases: • Keep spec. (active constraint) if energy is expensive (Nu=0) • Overpurify if energy is cheap (a) Unconstrained optimum (Nu=1) : Optimal composition is determined by trade-off between energy costs and value of increased recovery, (b) Reach capacity constraint (Nu=0): Loss of valuable product is minimized by operating at V=Vmax. • Methanol + water example: Since methanol loss is anyhow low (0.1% of water), it may not be optimal to overpurify. With energy very cheap, it is probably optimal to operate at V=Vmax. methanol + water valuable product methanol + max. 0.5% water cheap product (byproduct) water + max. 0.1% methanol

  9. Expected active constraints distillation: Summary • Always control valuable product at purity spec. • Avoid quality give-away • Remaining degree of freedom. Most common cases: • Control cheap product at purity spec. (Nu=0) “TWO-POINT CONTROL” • If loss (of valuable product) in “cheap end” is small • Operate at max. load V=Vmax (Nu=0) “ONE-POINT CONTROL” • Maximize yield (of valuable product) if large difference in product values and energy is cheap • Unconstrained (Nu=1) Usually “TWO-POINT” but not always • Operate at optimal trade-off between energy costs and value of improved yield (of valuable product)

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