International Conference on Distillation & Absorption
30 September - 2 October 2002,
Baden - Baden, Germany
Separation of Ternary Heteroazeotropic Mixtures
in a Closed Multivessel Batch Distillation Column
Stathis Skouras and Sigurd Skogestad
NTNU
Presentation Outline
 Project objective
 The multivessel batch column
 The model
 Simulation results
• System 1: Serafimov’s topological class 1.0-2
• System 2: Serafimov’s topological class 1.0-1a
 Discussion
 Conclusions
 Future plans
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
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?
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
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.
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
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
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
System 1: Serafimov’s 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
NTNU
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
International Conference on Distillation & Absorption 2002, Baden-Baden
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)
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
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
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
System 2: Serafimov’s topological class 1.0-1a
DISTILLATION LINES MAP
 One binary heteroazeotrope
 No distillation boundary
 Final products in the vessels
depend on the feed
NTNU
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
International Conference on Distillation & Absorption 2002, Baden-Baden
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)
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
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
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
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’
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
Conclusions
 Separation of heteroazeotropic mixtures is feasible in a
closed multivessel batch distillation decanter hybrid
 Systems belonging to Serafimov’s topological classes 1.0-2
and 1.0-1a can be separated in the proposed novel process
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
Future Plans
 Which other azeotropic classes
can be separated in the proposed
column?
 System Water-Acetic Acid-Butyl
Acetate belongs to Serafimov’s
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?
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden
Serafimov’s classification
 Serafimov’s work back in the late 60’s 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/publications/thesis/2000/hilmen
NTNU
International Conference on Distillation & Absorption 2002, Baden-Baden