Separation%20of%20Ternary%20Heteroazeotropic%20Mixtures%20in%20a%20Closed%20Multivessel%20Batch%20Distillation%20Column

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Separation of Ternary Heteroazeotropic Mixtures
in a Closed Multivessel Batch Distillation Column
International Conference on Distillation
Absorption 30 September - 2 October 2002, Baden
- Baden, Germany

• Stathis Skouras and Sigurd Skogestad

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Presentation Outline

• Project objective
• The multivessel batch column
• The model
• Simulation results
• System 1 Serafimovs topological class 1.0-2
• System 2 Serafimovs topological class 1.0-1a
• Discussion
• Conclusions
• Future plans

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Project Objective

• Is it feasible to separate heteroazeotropic
  mixtures in the closed multivessel batch column?
• What kind of heteroazeotropes can be separated in
  the novel column?

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The Multivessel Batch Column

• Characteristics
• Batch column
• 2 sections and 3 vessels
• Closed (total reflux) operation
• Vapor bypass configuration
• Indirect level control in the vessels with TCs
• Why multivessel column?
• Simple to operate. No off-cut fractions. Column
  runs by itself
• Ternary mixtures separated simultaneously in one
  closed operation. Final products accumulated in
  the 3 vessels
• Energy (time) savings due to the multieffect
  nature of the operation.

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The Model

• Staged column model, 25 stages in each section
• No vapor holdup - constant liquid holdup
• Constant vapor flows - liquid flows from the
  T-controllers
• Perfect mixing and equilibrium at all stages
• Ideal vapor phase
• VLE from UNIQUAC
• LLE from experimental data
• Atmospheric pressure P 1.013 bar
• Simulations performed in MATLAB

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System 1 Serafimovs topological class 1.0-2

• DISTILLATION LINES MAP
• One binary heteroazeotrope
• One distillation boundary (unstable separatrix)
• Two distillation regions
• Final products in the vessels depend on the feed

The problem Not all 3 original components can be
recovered by simple distillation The idea The
boundary is crossed by decantation and all 3
original components are recovered in a
distillation-decanter hybrid
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Simulations

• Step 1 Built up the composition profile
• Feed F in the left feed region
• Methanol in the top (unstable node)
• Heteroazeotrope in the middle (saddle)
• 1-Butanol in the bottom vessel (stable node)

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Simulations

• Step 2 Decant-Reflux the organic phase
• Decanter at the middle of the column (internal)
• Split the heteroazeotrope in the decanter
• Reflux the organic phase in the column
• Direct level control in the decanter

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System 2 Serafimovs topological class 1.0-1a

• DISTILLATION LINES MAP
• One binary heteroazeotrope
• No distillation boundary
• Final products in the vessels depend on the feed

The problem Separation stops because of the
heteroazotrope accumulation in the top The
idea The liquid-liquid split is used to overcome
the azeotropic composition, thus enhancing the
separation of the original mixture
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Simulations

• Step 1 Built-up the composition profile
• Feed F in the upper feed region
• Heteroazeotrope in the top (unstable node)
• Ethyl Acetate (EtAc) in the middle (saddle)
• Acetic Acid (AcAc) in the bottom (stable node)

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Simulations

• Step 2 Decant-Reflux the organic phase
• Decanter at the top of the column
• Split the heteroazeotrope in the decanter
• Reflux the organic phase in the column
• Direct level control in the decanter

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Discussion

• A combination of the closed multivessel batch
  column with a decanter was proposed for the
  separation of ternary heteroazeotropes
• Separation of two classes of heteroazeotropes was
  studied. Both of them can be separated in the
  novel column
• The decanter can be placed either in the middle
  (internal) or in the top of the column, according
  to the nature (class) of the mixture
• Part of the separation is accomplished by
  distillation, while the liquid-liquid split in
  the decanter is used for crossing the
  distillation boundary and overcoming the
  azeotropic composition
• Final products accumulated in the vessels at the
  end of the process
• The novel process is simple, there is no need for
  off-cut fractions and the column runs almost by
  itself

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Conclusions

• Separation of heteroazeotropic mixtures is
  feasible in a closed multivessel batch
  distillation decanter hybrid
• Systems belonging to Serafimovs topological
  classes 1.0-2 and 1.0-1a can be separated in the
  proposed novel process

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Future Plans

• Which other azeotropic classes can be separated
  in the proposed column?
• System Water-Acetic Acid-Butyl Acetate belongs to
  Serafimovs class 1.0-1b. Can it be separated?
• Experimental verification of our simulation
  results

• Energy (time) savings of the proposed process
  compared to conventional batch configurations
  e.g. batch rectifier?

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Serafimovs classification

• Serafimovs work back in the late 60s in USSR
• 26 classes of feasible topological structures of
  VLE diagrams
• Results presented again in 1996
• Serafimov, L.A., (1996). Thermodynamic and
  Topological Analysis of Liquid-Vapor Phase
  Equilibrium Diagram and Problems of Rectification
  of Multicomponent Mixtures, Book Mathematical
  Methods in Contemporary Chemistry, S.I. Kuchanov
  (Ed.) Gordon and breach Publishers, Amsterdam,
  557-605
• More material
• PhD thesis (2000) by E. K. Hilmen., Chapter 3
• Available at www.chembio.ntnu.no/users/skoge/publ
  ications/thesis/2000/hilmen