Synthesis of
Reactive Distillation Processes
Matthias Groemping
Supervisor: Dr Megan Jobson
1-1
XVII PIRC Annual Research Meeting 2000
Abstract:
The reaction and separation systems of processes are usually designed independently. However,
they influence each other due to recycles and should thus be designed simultaneously. Reactive
distillation is a special case, which integrates reaction and separation in one unit. For some
processes this is a cost-effective option.
In the process synthesis, we need to determine which process structures can achieve our
requirements and how the selected process should be operated. We should consider all options
and their operational parameters, such as reactor cascades with a separate separation system,
reactive distillation columns, pre-reactors, recycles and novel designs.
This presentation will introduce two tools for the synthesis of reactive distillation processes. The
first tool determines whether reactive distillation is a favourable process for the given system and
which other flowsheet structures are equally promising. Stochastic network optimisation is used
for this purpose.
The second tool presented is a conceptual design methodology for quick and effective screening
of feasible design parameters for reactive distillation columns. The methodology is based on the
concepts developed for non-reactive multicomponent nonideal distillation and allows the designer
to initialise the design of single feed multicomponent reactive distillation columns with both
reactive and inert sections.
The methods are illustrated using examples of industrial relevance.
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1-2
Synthesis of Reactive Distillation Processes
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1-3
Synthesis of Reactive Distillation Processes
Why use reactive distillation?
R1 + R2
P1 + P2
Conventional flowsheet
Reactive distillation
Recycle R2
P2
R2, P2
R1
I
R2
I
R1
II
II
III
P2
R2
R2,
P1,P2
P1
•
•
•
•
1-4
III
P1
Lower investment cost
Avoid side reactions
Avoid / overcome azeotropes
Direct heat integration
Synthesis of Reactive Distillation Processes
The conventional flowsheet on the left consists of separate reaction and separation sequences. In a
reactor cascade the reactant R1 is converted almost completely so that the excess of reactant R2,
the by-product P2 and the product P1 enter the separation sequence. In the first column the
product is withdrawn at the bottom and in the second column the unconverted reactant is
separated from the by-product to be recycled.
This is a conventional process. But what is reactive distillation? We take the reactors 1, 2, 3 and
stack them on top of each other into the first column. We obtain a reactive distillation column
with a reactive section in the middle, and two 'inert' sections. In the reactive section separation
and reaction take place simultaneously and in the inert section a classical distillation is performed.
The most obvious advantage of reactive distillation is that we can reduce investment costs for
shells and external recycling. Due to the continuous removal of components from the reactive
zone it might be possible to suppress side reactions and thus increase selectivity. Where there are
azeotropes in the system, their behaviour is changed in a reactive system. The reaction can thus be
used to deliberately avoid or overcome azeotropes. In exothermic reactions the heat of reaction
can be used to evaporate the liquid and thus reduce the reboiler duty. This direct heat integration
is usually to be preferred over indirect heat integration, which requires matching temperature
levels and always incurs thermodynamic losses.
However, a reactive distillation column can only work efficiently for certain mixtures. For
example the operating conditions for reaction and separation have to be similar and the relative
volatilities of the components should favour the countercurrent arrangement within a reactive
distillation column, i.e. the reactants should be middle boiling components. The investigation of
these aspects is part of the process synthesis.
Process synthesis
Reactants
Specifications
? Process ?
Conventional
process
Novel
design
Reactive distillation
Reactive
zones
1-5
Maximum selectivity
Maximum yield
Minimum cost
Feed
location
Holdup
Additional reactor,
Downstream separation
Synthesis of Reactive Distillation Processes
During process synthesis an engineer has to decide on a flowsheet structure. One
wants to select the structure, which for given reactants and specifications promises
the highest selectivity or the lowest cost. If a reactive distillation scheme is selected,
the placement of reactive and inert zones has to be addressed. Other questions to be
answered in the synthesis and design process include: How should one introduce
reactants so that they are distributed favourably? How large should the holdup be?
Is a pre-reactor beneficial? Is downstream separation required?
At the same time, reactive distillation competes with other flowsheet structures,
such as a conventional process with separate reaction and separation sequences, and
completely different, novel designs.
During process synthesis we thus have to determine which process is the most
promising (with respect to an appropriate objective function) and how to implement
it.
Current methods for synthesis of
reactive distillation processes (1)
• Trial and error
Simulation using commercial packages is possible
Iterative and time consuming
Few guidelines exist for hybrid processes
Suitable once initial process synthesis decisions have
been taken
Reference: Fair, Chem. Eng., Oct., 158-162 (1998)
1-6
Synthesis of Reactive Distillation Processes
The most common method to answer some of these questions is probably the "trial
and error" method. It is possible to model a reactive distillation in a commercial
simulation package. However due to the complexity of the problem this approach is
very time consuming, especially as there are few heuristics available for hybrid
processes such as reactive distillation. It is therefore more useful once the main
flowsheet and design characteristics are fixed, i.e. after the processes is synthesised.
Reference:
Fair, J.R.: Design aspects for reactive distillation. Chem. Eng., Oct., 158-162 (1998)
Current methods for synthesis of
reactive distillation processes (2)
• Distillation lines
Visualisation of basic underlying phenomena (boundaries)
Graphical interpretation
Limited degrees of freedom
#
#
Not applicable to multicomponent mixtures
Not applicable to columns with both reactive and inert zones
Distillation
boundary
A
Chemical
equilibrium
C
B
References: e.g. Barbosa and Doherty, Chem. Eng. Sci., 43, 1523-1537 (1988)
Espinosa et al., Chem. Eng. Sci., 50, 469-484 (1995)
Bessling et al., IECR, 36, 3032-3042 (1997)
1-7
Synthesis of Reactive Distillation Processes
For non-reactive ternary azeotropic systems composition profiles are displayed in
triangular diagrams to analyse the physical boundaries of a system. Following this
approach, reactive composition profiles were developed, to visualise the boundaries
for separation processes with superimposed reactions. This concept allows an initial
assessment of opportunities and limits of reactive distillation. However, the number
of components is limited for this approach, to facilitate the visual interpretation.
Also it is not possible to address columns with both reactive and inert zones.
References:
Barbosa, D.; Doherty, M.F.: Design and minimum reflux calculations for singlefeed multicomponent reactive distillation columns. Chem. Eng. Sci., 43, 1523-1537,
(1988)
Espinosa, J.; Aguirre, P.A.; Perez, G.A.: Some aspects on the design of
multicomponent reactive distillation columns including non-reactive species. Chem.
Eng. Sci., 50, 469-484, (1995)
Bessling, B.; Schembecker, G.; Simmrock, K.H.: Design of processes with reactive
distillation line diagrams. Ind. Eng. Chem. Res., 36, 3032-3042, (1997)
Current methods for synthesis of
reactive distillation processes (3)
• MINLP (Mixed Integer Non-Linear Programming)
Optimal solution, but:
#
#
Good initialisation required
Global optimum not guaranteed
Highly non-linear equation system aggravates convergence
Limited structural varieties
r
n,Vol
Optimise:
Feeds, n, r, s, Volumes
s
Reference: Ciric and Gu, AIChE J., 40, 1479-1487 (1994)
1-8
Synthesis of Reactive Distillation Processes
Using the mathematical tool MINLP (mixed integer non-linear programming) both
continuous variables, such as the reflux ratio, and discrete variables, such as those
representing the existence of a specific stage or feed point, can be optimised
simultaneously. MINLP is a gradient-based optimisation tool, which for a given
initial point finds an optimal solution. However due to the complexity of the
reaction kinetics and thermodynamic models the variables have to be bounded,
scaled and initialised very carefully.
Reference:
Ciric, A.R.; Gu, D.: Synthesis of nonequilibrium reactive distillation processes by
MINLP optimization. AIChE J. 40 (1994) 1479-1487
Current methods for synthesis of
reactive distillation processes (4)
• Stochastic network optimisation
Was applied in recent years to reactor networks (Mehta, PIRC 1998)
and reactor-separator networks (Linke, PIRC 1999)
Is applicable to highly non-linear equation systems
Large number of structural varieties can be analysed
In this presentation we will show the use of stochastic
optimisation in the context of reactive distillation process
synthesis.
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Synthesis of Reactive Distillation Processes
At the Department of Process Integration stochastic network optimisation was
successfully used in process synthesis applications: namely in multiphase reactor
networks and reactor-separator networks. In contrast to the three previously
outlined methods for the synthesis of reactive distillation processes, both a large
number of structural varieties and -as it will be shown- thermodynamically complex
systems can be handled by stochastic network optimisation techniques.
In this presentation we will introduce the use of stochastic network optimisation in
the context of reactive distillation processes.
Ref.:Mehta V., PIRC presentation, (1995-1997)
Linke P., PIRC presentation, (1999)
Areas of application for different approaches
Trial &
error
Stochastic
network
optimisation
NLP
MINLP
Conceptual
approaches
Variety of flowsheet configurations
1 - 10
Synthesis of Reactive Distillation Processes
A number of approaches can be used for the synthesis of reactive distillation
processes. Each of them has its own merits and limits. Some can handle a great
depth of modelling, some can address a large number and high complexity of
flowsheet structures simultaneously, some are fast and some allow strong user
interaction. In this diagram we ranked the approaches to two important aspects in
the synthesis of reactive distillation processes: the variety of flowsheet
configurations that they can address simultaneously -displayed on the horizontal
axis- and the thermodynamic complexity that they can handle -displayed on the
vertical axis. The shaded areas indicate, where the application of a technique is
most appropriate. They usually can be applied to a wider range. Stochastic network
optimisation, for example, can be applied over the entire range, but there are
reasons not to do so, such as required time or limited direct user interaction.
In reactive distillation we face highly complex systems, as both non-linear VLEmodels such as the Wilson or NRTL equation and complex reaction kinetics have to
be incorporated. For process synthesis purposes we want to analyse a large variety
of structures at the same time. Stochastic optimisation therefore seems to be a
promising tool for the synthesis of reactive distillation processes. On the other end
of the scale, a conceptual approach is desirable, once we have decided to use a
specific flowsheet structure. Such an approach would avoid trial-and-error
simulations by screening design options to understand sensitivities and trade-offs
and initialise rigorous simulations.
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1 - 11
Synthesis of Reactive Distillation Processes
Gradient-based and stochastic “optimum”
TARGET and design options
“Optimal solution”
possibly local optimum
single design obtained
close to global optimum
several alternative designs obtained
Design Parameter
1 - 12
Synthesis of Reactive Distillation Processes
Non-linear equation systems feature many local optima. We can, for example,
optimise the design parameters of a spray-column, so that it is the best spraycolumn for this system. However we could lose sight of other arrangements that are
much more promising. Stochastic optimisation performs a global search over all
design options (displayed on the horizontal axis) and reveals the potential or
"target" of the process for an objective function (displayed on the vertical axis). It is
worth finding alternative flowsheet structures that approach the target. These
different solutions can then be analysed regarding other criteria such as
practicability or safety, and be further optimised using a gradient-based method,
simulations or experiments.
How can we combine all these design options
in a generic way?
1 - 13
Synthesis of Reactive Distillation Processes
But how can we combine all these design options in a generic way? We make use
of a so-called superstructure, which encompasses all options and combinations
systematically.
Single-phase reactor network superstructure
Phase 1
Feed
Product
Splitter:
- single phase
Reactor:
- well mixed / multistage (plugflow)
Can be extended to any number of phases
Mehta, PIRC presentation (1995-1997)
1 - 14
Synthesis of Reactive Distillation Processes
This is the superstructure of a single-phase reactor network, consisting of two units,
a feed and a product. Every source is connected to every sink and each reaction unit
-or compartment- can represent either a well-mixed reactor (CSTR) or a plug flow
reactor (PFR), which is modelled as a cascade of CSTRs. The selection between the
options is controlled using binary variables, as is the existence of each connection.
By adding more phases the superstructure for a multiphase reaction system is
generated. Each compartment is then allowed to exchange mass with its equivalent
compartment in the other phase. Such a superstructure was developed by Mehta for
the synthesis of reactor networks.
Ref.:Mehta V., PIRC presentation, (1995-1997)
Reactor-separator superstructure
Reactor / mass
exchanger
Separator
Phase p
Phase change
units
Phase q
Splitter
Intraphase stream
Interphase stream
Linke, PIRC presentation (1999)
1 - 15
Synthesis of Reactive Distillation Processes
Last year, Linke presented a superstructure for reaction-separation synthesis. Here
we see the two-phase superstructure consisting of the reactor compartments that act
as reactors and/or mass exchangers. Also there are separator units in each phase,
which perform sharp splits between key components or "black-box" component
separations, and there are phase change units to shift mass between phases or
pressure levels (total reboiler/condenser, pump/throttle). This superstructure
therefore contains reaction and separation units to address reaction–separation
process synthesis.
Ref.:Linke P., PIRC presentation, (1999)
Optimisation method
Stochastic search using “Simulated Annealing”
Annealing = Controlled cooling of a melted metal into a
perfectly ordered crystal state
• Guarantees optimality, regardless of initialisation.
• Does not get trapped in local optima.
• Quality of results depends on time spent searching
= on the rate of cooling the metal.
• A different solution is generated every time,
because it is a random search.
• If standard deviation of objective between different
solutions is smaller than 5%, we have confidence in
the quality of results.
1 - 16
Synthesis of Reactive Distillation Processes
The superstructure allows us to represent complex structures in a systematic way. A stochastic
optimisation tool called "Simulated Annealing" is employed to reduce this superstructure to those
structures that promise optimal results.
In metallurgy, annealing is the controlled cooling of a melted metal into a perfectly ordered
crystal state. Simulated Annealing, by analogy, reduces the superstructure into a state with an
optimal value of the objective function, by iterative and random modifications of any structural or
operational parameter. If a modification is accepted, it is used as the base for the next
modification. At the beginning of the optimisation -i.e. when the metal is hot- all rearrangements
are accepted, whereas in the end only those modifications that improve the objective are
approved.
Since all modifications are accepted at the beginning, the optimisation is independent of the
initialisation. We do not need to supply a good guess beforehand. Also the technique circumvents
local optima, as even moves that do not improve the process performance can be accepted.
The cooling rate of the metal has a critical effect on the quality of the final solution. If it is too
high, i.e. when we quench, we cannot achieve a good result, whereas a low cooling rate
guarantees an optimal solution. We therefore face a trade-off between the quality of the solution
and the time required for simulations during the annealing.
As every modification is selected randomly, each stochastic experiment, i.e. each optimisation,
generates a different answer. The standard deviation between several stochastic experiments is a
measure for the confidence in the results. Is it lower than 5%, we can assume that the solutions
are close to the maximum potential of the process. Thus we can determine the stochastic
optimum, the target of the process, without the risk of getting trapped in local optima.
Framework for stochastic optimisation
Input data
• Superstructure, initial structure
• Physical properties, ...
Simulation of a specific structure
• Material balances
• Energy balances
• Objective function (e.g. cost, yield, ...)
Perturbation Moves
• Network changes
• Operational changes
Stochastic Optimiser
• Accept or reject perturbation move
• Simulated Annealing
Optimised structure
• Network structure
• Operational parameters
• Objective close to target
1 - 17
Synthesis of Reactive Distillation Processes
The optimisation search is based on the modification of states and their assessment,
using the objective function. Starting from an initial structure, we modify, simulate
and either accept or reject the modification. At the end we obtain a structure that
performs close to the target, as well as the associated operating conditions.
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1 - 18
Synthesis of Reactive Distillation Processes
Superstructure for reactive distillation networks
Phase 1 (vapour)
Feed
Product
Unit
1
Unit
2
Condensation
Evaporation
Mass transport
Product
Feed
Phase 2 (liquid)
Single phase reactor
(CSTR / PFR)
Reactionseparation
unit
Inert distillation section
(rigorous / short-cut)
Single phase splitter /
reboiler / condenser
1 - 19
Reactive distillation section
(counter- / cocurrent)
Synthesis of Reactive Distillation Processes
The superstructure accommodates the liquid and the vapour phases independently.
The two phases are coupled by condensation, evaporation and mass transport within
a reaction-separation unit.
Within each reaction-separation unit, the options for mass-transfer, reaction,
number of stages, flow direction and depth of modelling effort can be controlled, so
that each unit either represents a well mixed reactor (CSTR), a cascade of CSTRs, a
plug-flow reactor (PFR), a reactive distillation section in co- or countercurrent
mode, an inert distillation section or a short-cut distillation column. The splitters
can function as a partial reboiler or a partial/total condenser, as well as simply
facilitating connections between the reactions separation units of a given phase.
Example of structures contained within the
superstructure
Conventional process:
- reactor cascade
- distillation column
- recycle
Reactive distillation column:
- reactive rectifier
- inert stripper
- distributed vapour feed
Novel designs are also included in superstructure
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Synthesis of Reactive Distillation Processes
These are two examples of structures that the superstructure encompasses. On the
left-hand side we see a conventional process consisting of a reactor cascade, a nonreactive distillation column and a recycle. On the right hand side is a reactive
distillation column with a reactive top section, an inert stripper and distributed
vapour feed. Apart from these standard designs, completely different, novel designs
are included as well.
Key features of new reaction-separation model
• Simultaneous reaction and mass transfer
Realistic VLE-models (e.g. Wilson, NRTL) used to compute transfer
between phases
• Energy balance included
Consistent approach to relate thermal and phase equilibrium
• Detailed separation models included
Short-cut distillation models when appropriate
Rigorous models for non-ideal VLE behaviour
These features are required to describe reactive distillation processes
1 - 21
Synthesis of Reactive Distillation Processes
The key features of the new reaction-separation model are related to the depth of
modelling required for reactive distillation. We need realistic VLE models, such as
the Wilson or NRTL equations, because many reactive distillation columns are used
to avoid or overcome azeotropes that aggravate downstream separation. The energy
balances allow a consistent approach to the modelling of thermal and phase
equilibrium.
For separation units, short-cut methods based on Fenske-Underwood are
appropriate for ideal VLE behaviour. For non-ideal behaviour, rigorous tray-to-tray
calculations have to be performed.
Assessing feasibility of a design
Convergence of a simulation is
a) hard to achieve for feasible designs (highly non-linear models)
b) not achievable for infeasible designs (e.g. sections might dry up)
• Initialise simulation with results of previous structure
• If simulation does not converge
Use a continuation method to guide simulator
• If continuation method does not converge
Conclude that structure is infeasible for the given operating
conditions
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Synthesis of Reactive Distillation Processes
The complexity of the modelling required for reactive distillation processes makes
it difficult to assess the feasibility of a specific flowsheet structure with a set of
operating parameters. Because novel structures are generated during the
optimisation, we cannot use short-cut methods to assess feasibility, but have to rely
on simulation.
However due to the non-linearities, a converged simulation is hard to achieve for
feasible designs. When a simulation does not converge, it is not clear whether this is
due to inadequate simulation techniques or infeasibility of the design.
During Simulated Annealing we change the structures in step-wise fashion. We can
thus initialise each simulation with the result of the previous structure. If the
simulation fails, we use a continuation method, where we break down the change of
the structure into incremental steps and thus guide the simulation. If this
continuation method also fails, we conclude that the underlying design is infeasible.
Continuation method
Without continuation method
New
Old
New
Move 2
Move 1a
Move 1
Old
Move 1b
Move 1c
• New design does not
converge
/ Infeasible ?
/Reject move
• Try a different move
/ If no feasible new design is
found, optimisation ends at
sub-optimal old design
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With continuation method
• Move in increments
• Use previous step for initialisation
• New design may converge
/ Approximately 50% of apparently infeasible
designs converge with continuation method
/Not trapped in designs from which it is
difficult but feasible to move away from
• Do the next move
/More feasible designs obtained
/ Better optimisation results achieved
/ Less time wasted
Synthesis of Reactive Distillation Processes
What are the advantages of using the continuation method? Let's start from an old
design and modify it (move 1). If we do not use a continuation method and the
simulation does not converge, we have to conclude that the new design is infeasible,
reject the move and try different moves. We waste time trying unsuccessfully to
simulate structures and if we cannot converge any new design, the optimisation
ends at an sub-optimal design.
On the other hand, we can break the move from the old to the new design into
incremental steps, for example by reducing the flows into a unit instead of removing
the unit immediately. Each incremental step serves as initialisation for the next one.
If we apply this continuation method, about 50% of previously failed simulations
converge. We thus do not get trapped in sub-optimal solutions from which it is hard,
but feasible to move away, but can move towards new designs.
The continuation method therefore allows a more efficient optimisation search, as a
wider range of network modifications is analysed. Also, the final results are better
and achieved faster, even though more simulations are performed using incremental
steps, as less time is wasted with unsuccessful simulations. A continuation method
is necessary for the non-linear modelling requirements in reactive distillation.
Summary: Stochastic network optimisation
• All operational and structural varieties considered in a
generic superstructure
- The superstructure consists of:
• Homogenous CSTR / PFR
• Reactive distillation zones
• Short-cut / rigorous distillation
• Reboiler / condenser
• Mixer / splitter
- The superstructure encompasses:
• Reactive distillation processes
• Conventional processes
• Combined pocesses
• Novel designs
• Superstructure is optimised using Simulated Annealing
and a continuation method
1 - 24
Synthesis of Reactive Distillation Processes
We described how to consider all structural and operational varieties in a generic
superstructure, which consists of homogeneous CSTR and PFR, reactive distillation
zones, rigorous and short-cut distillation, reboiler, condenser, mixer and splitter.
The superstructure therefore encompasses reactive distillation processes, as well as
conventional processes and totally different, novel designs.
A robust stochastic optimisation technique in the form of Simulated Annealing is
used together with a continuation method to aid the simulation. From the stochastic
experiments we obtain the target for the system performance and a set of nearoptimal designs that help us to understand trade-offs and design trends.
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1 - 25
Synthesis of Reactive Distillation Processes
Examples for stochastic network optimisation
1. Ethylene Glycol:
- Comparison with MINLP optimisation.
2. MTBE:
- Application to a thermodynamically complex system.
3. Industrial case study:
- Which flowsheets are promising?
- How to separate a contaminant?
- How to design a reactive distillation column?
1 - 26
Synthesis of Reactive Distillation Processes
We will now present three examples to illustrate different issues. The first example
compares stochastic network optimisation with an MINLP optimisation from the
literature for ethylene glycol production. Then we will look at a highly non-linear
system. MTBE is known for its complexity, modelling difficulties and
multiplicities. Finally we will present an industrial case study, where we will
investigate promising flowsheets, establish how to separate contaminants and give
guidelines about how to set up a reactive distillation column.
Example I: Ethylene glycol
• System parameters:
#
#
#
#
Four components
Autocatalytic reactions: EO + H2O Ú EG (product)
EO + EG Ú DEG (by-product)
Heat of reaction = -90 kJ/mol
VLE using Antoine type of equation (EO,DEG>H2O,DEG>EG,DEG>1)
• Objective:
#
#
1 - 27
Maximise the economic potential (ratio of yield to total annualised cost)
Penalty for contamination of product stream with water
Synthesis of Reactive Distillation Processes
There are four components in the ethylene glycol system: Ethylene oxide reacts
with water to ethylene glycol. Ethylene oxide reacts with ethylene glycol to produce
the unwanted by-product diethylene glycol. The second reaction can be suppressed
using a large excess of water, so that in a conventional process more than 40 moles
of water per mole ethylene glycol have to be recycled to achieve a high selectivity.
Reactive distillation is an attractive alternative, as the products are the highest
boiling components and thus can be withdrawn at the bottom of a reactive
distillation column. As the reaction is exothermic, the heat of reaction can be used
directly in the column to reduce the reboiler duty. During stochastic network
optimisation, the economic potential, which is the ratio of yield to total annualised
cost, was maximised.
Optimal design from literature
H2O
Obj = 13 mol/$
Q = 6.7 MW
X = 95.0%
EO
(Objective)
(Reboiler duty)
(Yield)
EG, DEG
MINLP optimisation:
• Reactive rectifying section and inert stripper
• Total reflux
• Water feed at top
• Ethylene oxide feed distributed over reactive section
Reference: Ciric and Gu, AIChE J., 40, 1479-1487 (1994)
1 - 28
Synthesis of Reactive Distillation Processes
In the literature this example was optimised using the gradient based MINLPmethod. A value of 13 mol/$ is obtained. Unfortunately the values given in the
paper are neither complete nor consistent, so that it is not possible to quote a more
precise value for the economic potential. The feed injection points, number of
stages and their holdup were optimised along with the operating parameters (reflux
and reboil ratio). It was found that a reactive rectifying section should be coupled
with an inert stripping section. To obtain water-rich streams in the reactive section,
the top stage vapour is refluxed completely and water is fed at the top. The ethylene
oxide is distributed over the length of the rectifying section to achieve an optimal
distribution of the reactants.
Reference:
Ciric, A.R.; Gu, D.: Synthesis of nonequilibrium reactive distillation processes by
MINLP optimization. AIChE J. 40 (1994) 1479-1487
Near-optimal structures from stochastic optimisation
Obj = 13.07 mol/$
Q = 5.2 MW
X = 94.5%
Obj = 13.07 mol/$
Q = 5.2 MW
X = 94.5%
Obj = 13.14 mol/$
Q = 4.6 MW
X = 94.5 %
Obj = 13.16 mol/$
Q = 5.1 MW
X = 95.1 %
H2O
H2O
H2O
EO
H2O
EO
EO
EG, DEG
EG, DEG
• Reactive top sections
• Inert or reactive stripper
• High or total reflux
• Water feed at top
1 - 29
EG, DEG
EO
EG, DEG
• Ethylene oxide as stripping agent
reduces reboiler duty
• Counter- or cocurrent sections (similar
performance due to high water content)
Synthesis of Reactive Distillation Processes
The best structures found from stochastic network optimisation also all feature a
reactive section with superimposed mass transfer, indicated by the cross in the
representation. The water feed is located at the top and using internal or external
recycles, the water content is maintained at a high level. The structures usually have
a stripping section, where the light boiling ethylene oxide is used as stripping agent.
It reduces the vapour pressure of water and encourages water separation. Because
the water content is high in the reactive section, the difference between co- and
countercurrent operation is marginal. Both structures are found in final design, but
as a countercurrent arrangement is easier to implement, an engineer would probably
consider a reactive distillation column with ethylene oxide as stripping agent.
Summary: Ethylene glycol
• Optimised superstructure containing four units:
Target: 13.2 mol/$
Average: 12.87 mol/$
Standard deviation: 2.5%
• Confidence that target is close to global optimum,
because of small standard deviation.
• Results similar to those from MINLP optimisation - but not
restricted to reactive distillation column only.
• Results suggest using ethylene oxide as stripping agent.
Feature not identified in literature
1 - 30
Synthesis of Reactive Distillation Processes
For the given objective function, a target of 13.2 mol/$ was found. The small
standard deviation of 2.5% indicates that this target is repeatedly approached in a
series of stochastic optimisations and we can thus have confidence in the quality of
the results.
We find similar results as in the MINLP optimisation, but do not restrict the
structure to reactive distillation only. For example the feature to feed ethylene oxide
as stripping agent in the vapour phase was not detected in the MINLP optimisation,
maybe because the feed was restricted to the liquid phase. This example therefore
highlights the suitability of stochastic network optimisation.
Example II: MTBE-Synthesis
• System parameters:
#
#
#
#
#
#
#
#
#
Four components
Equilibrium reaction: iso-Butene + MeOH Û MTBE
Inert component n-Butane present
Heterogeneous catalyst (e.g. Amberlyst 15)
Heat of reaction = -37.7 kJ/mol
Kinetic expression based on activities
VLE using Wilson equation
Three minimum boiling azeotropes in non-reactive system, two reactive
azeotropes
Multiplicities
• Objective:
#
#
1 - 31
Maximise the economic potential (ratio of yield to operating cost)
Revenue from separation of n-Butane to vapour product
Synthesis of Reactive Distillation Processes
The second example is the production of MTBE. Iso-butene reacts with methanol to
produce methyl-tert-butyl-ether in a heterogeneously catalysed reaction. N-butane is
introduced to the process together with the iso-butene and acts as an inert. The VLE
is modelled using the Wilson equation. Due to the three azeotropes in the nonreactive system, the separation of MTBE and the inert requires a series of columns
in a conventional reaction-separation scheme. The behaviour of the azeotropes
however is changed in the reactive system This change in behaviour can be
exploited in a reactive distillation column. Due to the non-idealities in the liquid
phase the reaction kinetics are modelled using activities (Thiel et al., 1997).
Simulation however is not only aggravated by these non-linearities, but also induces
multiplicities, where, for an identical column set up, different stable operating
points are obtained.
During the stochastic network optimisation, the ratio of yield to operating cost, i.e.
the economic potential, was maximised.
Reference:
Thiel C., Sundmacher K., Hoffmann U.: Residue curve maps for catalysed reactive
distillation of fuel ethers MTBE and TAME. Chem. Eng. Sci., 52, 993-1005, (1997)
Alternative structures for MTBE
nC4
nC4
MeOH
MeOH
obj=64.46 mol/$
Yield=98.5%
Q=1.6 MW
IB/nC4
IB/nC4
IB/nC4
IB/nC4
MTBE
obj=64.26 mol/$
Yield=97.7%
Q=1.1 MW
MTBE
Features: • Reactive distillation columns and similar flowsheets
• Reactive rectifying section and inert stripper
• Methanol feed above iso-Butene/n-Butane feed
• Results compare with those for superstructure restricted
to reactive distillation options
1 - 32
Synthesis of Reactive Distillation Processes
The structures obtained from stochastic network optimisation are generally reactive
distillation columns -as on the right- and similar arrangements -as on the left-, with
a reactive rectifying section and an inert stripper. The heavier boiling methanol is
fed above the C4-feed to ensure optimal distribution of the reactants in the reactive
zones. Due to the continuous removal of MTBE and n-Butane from the reactive
zone, yields of 98% can be achieved.
Summary: MTBE
• Optimised superstructure containing four units:
Target: 64.6 mol/$
Average objective: 58.5 mol/$
Standard deviation: 5.0 %
• Confidence in quality of solution, because of acceptable
standard deviation.
• System with very challenging thermodynamics can be handled.
• Standard flowsheets are also obtained.
1 - 33
Synthesis of Reactive Distillation Processes
For the given objective function, a target of 64.6 mol/$ was found. The acceptable
standard deviation of 5% indicates that we have confidence into the value of the
target and the quality of the result. The stochastic network optimisation thus proved
to be applicable even to systems with highly challenging non-linear
thermodynamics. Also, standard flowsheets such as a reactive distillation column
are part of the set of near-optimal designs obtained.
Example III: Industrial case study
• System parameters:
Six components
Equilibrium reaction: A + B Û C
# Homogenous catalyst
# Water and non-condensable H contaminate reactants
2
# Exothermic reaction
# VLE using NRTL
#
#
• Objective:
#
#
1 - 34
Maximise the economic potential (ratio of yield to total annualised cost)
Penalty for water in product stream => water separation necessary
Synthesis of Reactive Distillation Processes
The third and final example for stochastic network optimisation is an industrial case
study, where in a homogeneously catalysed ether-type reaction, C is produced from
A and B. The reaction kinetics are expressed in terms of activities and the Wilson
equation is used to calculate liquid-phase non-idealities. One of the reactants is
contaminated with water and hydrogen and the objective function includes a penalty
term relating to water in the product stream, so that apart from a high yield, an
efficient separation scheme has to be found by the optimisation routine. A simple
investment cost model is included, to avoid large volumetric flows and
unreasonable column diameters.
Case study: Process synthesis and design issues
(A) Which flowsheets are promising?
(B) How to deal with contaminants and catalyst?
Separate contaminants before or after the reaction?
Boiling order: Hydrogen, A, B, Catalyst, Water, C
(C) Identify structural and operational parameters for an
reactive distillation column.
1 - 35
Synthesis of Reactive Distillation Processes
There are several questions to be answered in the case study. Which flowsheets are
promising? How do we deal with the contaminants and the homogeneous catalyst?
Shall we separate the contaminants before or after the reaction? And finally, if a
reactive distillation scheme is appropriate, which structural and operation
parameters should be used?
Alternative structures for industrial case study
H2O separation before reaction
Obj=20.96, Q=2.3 MW, X=99.2%
0.88 H2
0.22 B
H2O separation before reaction
Obj=21.43, Q=1.0 MW, X=99.9%
A, H2O,
0.88 H2
(H2 recycle)
H2
B
C
0.78 B,
Cat
H2O
A, Cat,
H2O,H2
H2O separation after reaction
Obj=20.97, Q=2.4 MW, X=98.6%
H2, H2O
(B/H2O) split
(H2 recycle)
H2O
B
Cat
(B/C) split
C
1 - 36
A,
H2O,H2
Synthesis of Reactive Distillation Processes
C
Some near-optimal structures are represented here. The reboiler duty Q is an
indicator of the operating cost, which together with the yield X forms the objective
function. Water contamination of product C is negligible in all final designs. The
design on the left for example separates the water in a distillation column. The
distillate is used as stripping agent in a reactive distillation type of arrangement. The
bottom product of the reactive section is fed to another distillation column, where
the unconverted reactant is recovered and the product C is separated at the bottom.
Another design, where the water is separated before the reaction is shown on the top
right. Here, part of reactant B is used as absorbent and water is withdrawn at the
bottom. The reactants present in the overheads are condensed and fed to a series of
CSTRs. The uncondensable hydrogen is used as a stripping agent in the rectifier,
which separates the product C at the bottom. This design is similar to a
conventional reaction-separation arrangement.
Alternatively the water can be withdrawn after the reaction, as in the third design. It
is a combination of a reactive distillation section with a series of CSTRs and an
inert stripping section to separate the product C from the water. The high boiling
reactant B is fed above the low boiling A to the reactive distillation section and part
of the hydrogen is recycled from the top of this section to the bottom of both the
reactive and the inert section.
Confidence in optimisation results
• Optimised superstructure containing five units:
Target: 21.4 mol/$
Average: 20.7 mol/$
Standard deviation: 2.5%
• Confidence in quality of solution, because of small
standard deviation.
1 - 37
Synthesis of Reactive Distillation Processes
Again, the confidence in the optimisation results is high, as the standard deviation
of 2.5% is low. Obviously the target is repeatedly approached during the
optimisation experiments.
Restricted superstructures
Reactive distillation
1-ξ
r
ξ
Reactive absorption
H2
H2 , A, B, H2O
C
H2O
B
B
Cat
Cat
A, H2, H2O
H2O
A, H2, H2O
H2O, A, B
s
s
C
C
Reactive distillation
(without H2 recycle)
Obj=21.2, Q=2.6-2.9 MW, X=99.3-99.7%
Reactive absorber with reboiler
(with C and H2O as washing liquid)
Q=0.5 MW, X=45%
Reactive distillation
(with H2 recycle)
Obj=21.3, Q=1.5-2.3 MW, X=99.4-99.7%
Reactive absorber without reboiler
(with C and H2O as washing liquid)
Q=0 MW, X=30%
Reactive distillation more attractive
because of significantly higher yields
1 - 38
Synthesis of Reactive Distillation Processes
If we restrict the superstructures to a reactive distillation column followed by nonreactive distillation to separate the water, similar objectives, duties and yields as in
the unrestricted optimisation are achieved. The case study revealed that hydrogen,
which contaminates one of the reactants, is beneficial to the reactive distillation
process, as it can be used as a stripping agent to reduce the reboiler duty by up to
50%.
An absorber with water and/or product C as washing liquids was also investigated.
A different objective function was employed, so that only reboiler duties and yields
can be compared with those obtained for the reactive distillation column. However,
because the overhead losses of reactants are large, the yields of an absorber are
significantly less than those obtained in a reactive distillation column.
Insights from optimisation
Promising flowsheets:
• Both conventional and reactive distillation flowsheets are promising.
Contaminants and catalyst:
• Use hydrogen in distillation columns as stripping agent to reduce
reboiler duty and to force reactants into liquid phase.
• Separate water either before or after the reaction.
• Catalyst can be kept within or recycled to reactive zones.
Reactive distillation column design:
• Recycle hydrogen.
• Feed lighter boiling reactant below higher boiling reactant.
• Use reactive rectifying section and inert stripping section.
• Use both condenser and reboiler.
• Condense as much as possible.
• Absorbtion using washing liquids is not appropriate.
These insights can give guidance for further process
synthesis and design.
1 - 39
Synthesis of Reactive Distillation Processes
We can gain valuable insights from the optimisation results. Both conventional and
reactive distillation designs seem to be promising. We can use the hydrogen in
distillation columns as a stripping agent to reduce the reboiler duty and to force the
reactants into the liquid phase. Water can be separated either before or after the
reaction and the catalyst can be kept within the reactive zones or recycled back to
them.
In a reactive distillation column, hydrogen should be recycled and the lower boiling
reactant should be fed below the higher boiling reactant to obtain efficient
distribution of the reactants in the liquid phase. A stripping zone with a low
residence time is required to suppress the reaction whilst separating the product C.
An absorption using product C or water as washing liquids is not appropriate, as the
overhead losses cannot be suppressed. We should rather condense as much as
possible. These insights can give guidance in further process design and
development.
Conclusions: Stochastic network optimisation
• Step 1: Determine the target
• Step 2: Impose limitations and restrictions
• Step 3: Analyse results and compare with target
Can be applied to highly non-linear systems when using a
continuation method to support simulation.
Screen wide range of structural and operational parameters.
Obtain set of near-optimal structures for further analysis.
Obtain promising solutions without a good initial guess.
Initialisation for further simulations, optimisations or experiments.
A systematic approach to gain insights and develop novel designs.
1 - 40
Synthesis of Reactive Distillation Processes
Process synthesis using stochastic network optimisation is divided into three main
steps. Firstly, we determine the target for the system performance. In a second step
we can impose restrictions to the superstructure, to reflect practical considerations.
The resulting structures are then compared with the target and are further analysed.
The methodology can be applied to highly non-linear systems, when a continuation
method is used to guide simulations. It allows the engineer to screen a wide range of
structural and operational varieties and identify a set of close-to-target solutions
rather than a single -potentially local- optimum without a good initial guess. The
results can be used to initialise further investigations using simulations,
optimisations or experiments. It is a systematic approach, from which we gain
insights and ideas to develop not only conventional, but also novel designs.
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1 - 41
Synthesis of Reactive Distillation Processes
Stochastic network optimisation gives us the flowsheet structure and
operating conditions.
BUT WE NEED
Conceptual design method for a fixed reactive distillation structure to:
•
•
•
•
1 - 42
screen column design options
understand sensitivities
understand trade-offs
initialise rigorous simulation
Synthesis of Reactive Distillation Processes
Why do we need another tool? Stochastic network optimisation reveals promising
network structures and operating conditions for a given system with any number of
components and reactions and great depth of modelling. Also the superstructure can
be restricted to analyse only reactive distillation columns.
However, once we fix the structure, conceptual design methods are also appropriate
to quickly screen column design options and understand sensitivities and trade-offs
ahead of rigorous simulation.
Synthesis of reactive distillation columns
• Stochastic network optimisation:
Time consuming
• Short-cut methods (such as Fenske): Not available
• Distillation lines (Ternary diagrams):
Not applicable to multicomponent mixtures
• Calculation of profiles in a column:
Applicable and conceptual
Make use of concepts developed for
non-reactive, multicomponent and non-ideal
(e.g. azeotropic) distillation columns.
1 - 43
Synthesis of Reactive Distillation Processes
For the synthesis of a single reactive distillation column, stochastic network
optimisation is applicable but can be time consuming and is more appropriate for
considering a large variety of structures. We lack short-cut methods, such as
Fenske, for reactive systems and the concept of composition profiles and distillation
lines using visualisation in a ternary diagram is not applicable to multicomponent
systems or columns with both reactive and inert sections. However, we can
calculate composition profiles in a column and can make use of conceptual design
procedures developed for non-reactive, multicomponent and non-ideal distillation
columns.
Column modelling
r
r
xD
xD
Rectifying section
n
n
xF
m
m
s
Stripping section
xB
s
xB
Calculations initiated from product compositions
1 - 44
Synthesis of Reactive Distillation Processes
In the design and analysis of distillation columns separating azeotropic mixtures,
the columns are divided into two sections: the rectifying and the stripping section.
Both sections are modelled independently and are initiated from the product
compositions.
Isobutanol, 107.9°C
Column modelling: Non-reactive ternary distillation
1
0.8
xB
0.6
0.4
Composition profiles are
calculated from a fixed
product composition.
s=2
xB
0.2
s=4
s=10
0
0
0.2
N-butanol, 117.7°C
0.4
0.6
0.8
s=50
1
2-butanol, 99.7°C
e.g.: Stichlmair et al., CEP, 1, 63 (1989)
Wahnschafft et al., IECR, 31, 2345 (1992)
1 - 45
Synthesis of Reactive Distillation Processes
For non-reactive ternary distillation, a bottom composition xB can be specified,
from which composition profiles at a specific boil-up ratio s for successive number
of stages can be calculated. The same applies to a rectifying section.
References:
Stichlmair et al., CEP, 1, 63 (1989)
Wahnschafft et al., Ind. Eng. Chem. Res., 31, 2345 (1992)
Chloroform, 61.2°C
Stage composition lines
m=1 m=2
0.4
m=3
Composition profiles (dashed):
Same reboil ratio (s)
Different stages (m)
m=4
0.3
xB s=1
0.2
m=5
Stage composition lines (solid):
Same stage number (m)
Different reboil ratios (s)
s=2
m=6
s=3
0.1
m=7
s=4
s=6
s=10
s=20
0
0
Benzene, 80.2°C
0.2
0.4
0.6
0.8
1
s=100
Acetone, 56.1°C
Castillo, PhD Thesis (1997), PIRC Presentations (1994 - 1996)
1 - 46
Synthesis of Reactive Distillation Processes
The composition profiles of this stripping section start from the bottom product
composition along the length of a column for a specific reboil ratio s. By
rearranging the points of a set of composition profiles, we can also display stage
composition lines, which show the composition on a specific stage m for various
reboil ratios s. The stage composition lines are just a different representation of the
composition profiles, but they offer advantages in column design procedures, as
pointed out by Castillo and Thong.
References:
Castillo, F., PhD thesis (1997), PIRC presentations (1994 - 1996)
Thong, D., PhD thesis (2000); PIRC presentation (1999)
Chloroform, 61.2°C
Each intersection between rectifying and stripping
stage composition lines indicates a feasible column design
xD
1
r=5.9
0.8
n=20
Azeotrope
64.5°C
m=15
0.6
s=2.7
xB
0.4
xB
xD
0.2
xD
0
0
0.2
0.4
0.6
= rectifying stages
= stripping stages
= reflux ratio
= boil-up ratio
1
Acetone, 56.1°C
Benzene, 80.2°C
xB
1 - 47
0.8
n
m
r
s
Castillo, PhD Thesis (1997)
Synthesis of Reactive Distillation Processes
Specifying both top and bottom product allows one to plot stage composition lines
for both the rectifying and stripping section. The intersection of two of these lines
indicates a feasible column design. The two product compositions xD and xB for
example are feasible and a continuous composition profile connecting them can be
found for values for a reflux ratio (r) of 5.9, a reboil ratio (s) of 2.7, when the
rectifying section contains 20 stages (n) and the stripping section 15 stages (m).
Column design for non-reactive ternary systems
Each intersection between rectifying and stripping stage
composition lines indicates a feasible column design.
But:
• What about multi-component systems?
• What about reactive systems?
1 - 48
Synthesis of Reactive Distillation Processes
For ternary non-reactive systems we can carry out column design by intersecting
rectifying and stripping stage composition lines. But, can we do the same in
multicomponent systems and reactive systems?
Column profiles can also be calculated for multicomponent mixtures
e.g., 4-component mixture
Isobutanol
107.9°C
• Two lines do not usually intersect
in 3-D and higher space
• Position of lines is very sensitive
to product compositions
1
0.8
xB
1
0.8
0.6
0.4
n-butanol
117.8°C
0.2
0.6
0
2-butanol
99.6°C
0
0.4
0.2
0.2
0.4
xD
0
tert-butanol
82.4°C
0
0.6
0.2
0.8
0.4
0.6
Column profiles are not convenient when C>3
0.8
Thong, PhD Thesis (2000)
1 - 49
Synthesis of Reactive Distillation Processes
Composition profiles can be calculated for mixtures with any number of
components. It is however not practical to use composition profiles or stage
composition lines for multicomponent mixtures, as two lines usually do not
intersect in 3-D and higher dimensional spaces. Also, because both product
compositions have to be specified completely, the location of the profiles is very
sensitive to changes in the amounts of trace components in the products.
Ref.:Thong, D., PhD thesis (2000); PIRC presentation (1999
Manifolds for multicomponent mixtures
Instead of an exact product, specify the required purity (e.g. 99% tert-butanol)
n=5
r=2
Isobutanol
107.9°C
Each manifold indicates the
set of feasible compositions
on tray n for reflux ratio r to
obtain required product
purity.
n=5
r=1
1-butanol
117.8°C
Product region for
99% tert-butanol
2-butanol
99.6°C
tert-butanol
82.4°C
n = rectifying trays
r = reflux ratio
n=5
r=0.5
n=5
r=5
n=5
r=οο
Thong, PhD Thesis (2000)
1 - 50
Synthesis of Reactive Distillation Processes
Instead of specifying a product completely, we define a product purity. All
compositions satisfying this purity form the product region. Instead of using
composition profiles, which are all points leading to an exact product composition,
Thong (PhD Thesis, 2000) suggested the use of composition manifolds, which are
the set of all compositions leading to a product region. Manifolds at different values
of stage number and reflux ratio can be constructed.
Ref.:Thong, D., PhD thesis (2000); PIRC presentation (1999
Specify an exact co-product composition and calculate the
stage composition lines
99% tert-butanol
Isobutanol
107.9°C
1
0.8
xB
0.6
1-butanol
117.8°C
0.4
1
r=5
0.8
n=5
0.6
m = 12
0.4
s=2
0.2
xB
0.2
0
0tert-butanol
0 82.4°C
0.2
2-butanol
99.6°C 0
0.2
0.4
0.4
0.6
n=5
0.8r=5
0.6
0.8
n
m
r
s
= rectifying stages
= stripping stages
= reflux ratio
= boil-up ratio
Every intersection between a rectifying manifold and
stripping stage composition line indicates feasible parameters
Thong, PhD Thesis (2000)
1 - 51
Synthesis of Reactive Distillation Processes
The co-product composition from the column is then specified explicitly to
calculate stage composition lines (or composition profiles, which are equivalent).
Each intersection between a stage composition line and a manifold corresponds to a
unique combination of feasible parameters. We can automate this procedure and
determine all feasible designs for a range of reflux and reboil ratios and a range of
stripping and rectifying stages.
Instead of specifying an exact co-product composition, we can also relax the
specifications and define a product purity to calculate manifolds for the stripping
section. Each intersection between the rectifying and stripping manifolds would
then indicate feasible parameters.
Ref.:Thong, D., PhD thesis (2000); PIRC presentation (1999
Column design for non-reactive
multi-component systems
Each intersection between rectifying and stripping
manifold indicates a feasible column design for
multicomponent systems.
But:
• What about reactive systems?
Assume reactions reach equilibrium
1 - 52
Synthesis of Reactive Distillation Processes
For multicomponent non-reactive systems we can carry out column design by
specifying required product purities and intersecting rectifying and stripping
manifolds. Can we do the same in reactive systems, when we assume that the
reactions reach chemical equilibrium?
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1 - 53
Synthesis of Reactive Distillation Processes
Chemical equilibrium in reactive mixtures
A + B C with inert D
ideal system
K = 10
D
50.5°C
Everything is bent!
A
65.3°C
C
100°C
B
81.6°C
Every reaction reduces dimension of composition space
e.g: 4 components, 1 reaction => 2 dimensional equilibium surface
1 - 54
Synthesis of Reactive Distillation Processes
Not all compositions in a reactive mixture are stable. The components will react
until the system reaches equilibrium. Here we have an example for a reaction of A
and B to C with D being an inert component. Pure C for example is not a stable
solution, as it will always decompose to A and B. Every reaction therefore reduces
the dimension of the composition space and all compositions have to lie on the
equilibrium surface. The result of this is that everything is bent.
Composition profiles in reactive distillation column
xD
D
50.5°C
D
50.5°C
xD
xD
n=oo
r=oo
(Same plot from
different angles)
A
65.3°C
r =1
r =2
r =5
r =10
C
100°C
n=6
A
65.3°C
n=7
n=8
n=9
C
100°C
B
81.6°C
B
81.6°C
We can calculate and plot composition profiles,
as in non-reactive distillation.
References: e.g. Barbosa and Doherty, Chem. Eng. Sci., 43, 1523-1537 (1988)
Espinosa et al., Chem. Eng. Sci., 50, 469-484 (1995)
Bessling et al., IECR, 36, 3032-3042 (1997)
1 - 55
Synthesis of Reactive Distillation Processes
All composition profiles in a reactive section of a distillation column are located on
the equilibrium surface. Using this additional information, we can calculate the
composition profiles within this section. Here for example we see the profiles for a
reactive rectifying section from two different angles.
References:
Barbosa, D.; Doherty, M.F.: Design and minimum reflux calculations for singlefeed multicomponent reactive distillation columns. Chem. Eng. Sci., 43, 1523-1537,
(1988)
Espinosa, J.; Aguirre, P.A.; Perez, G.A.: Some aspects on the design of
multicomponent reactive distillation columns including non-reactive species. Chem.
Eng. Sci., 50, 469-484, (1995)
Bessling, B.; Schembecker, G.; Simmrock, K.H.: Design of processes with reactive
distillation line diagrams. Ind. Eng. Chem. Res., 36, 3032-3042, (1997)
Column design for quaternary systems
with single reaction
Reaction reduces composition space.
• If reaction takes place in all column sections, all profiles
are located on a bent 2D-surface.
• Project this surface onto a flat plane (concept of
transformed variables).
• Find intersections between rectifying and stripping
profiles, as in non-reactive ternary systems.
But:
• What about multi-component reactive systems?
• What about inert sections in the column?
1 - 56
Synthesis of Reactive Distillation Processes
As long as we can plot the profiles in 2-D, we can use the same concepts as in nonreactive distillation. For quaternary systems with a single reaction, the equilibrium
surface is two-dimensional. If reaction takes place in all column section, all profiles
are located on this bent 2-D surface. We can project the surface onto a flat plane,
using the concept of transformed variables, and find all intersection between
rectifying and stripping profiles.
But, it was shown earlier that this approach is not appropriate for multicomponent
mixtures, as two lines usually do not intersect in 3-D and higher dimensional
spaces. Using an inert section does not help to reduce the dimension, as the inert
section operates in the full composition space.
So, what do we do in multicomponent reactive systems? And what about when we
need an inert section to separate a component?
Use manifolds as for multi-component
non-reactive systems
Instead of an exact product, specify the required purity
Product region for
99% tert-butanol
1-butanol
117.8°C
tert-butanol
82.4°C
D
50.5°C
Isobutanol
107.9°C
7
2-butanol
99.6°C
Product region for
99% inert D
A
65.3°C
?
C
100°C
B
81.6°C
Non-reactive case:
Linear approximation of manifolds
Linearisation is OK!
1 - 57
Reactive case:
How to approximate manifolds?
Everything is bent!
Synthesis of Reactive Distillation Processes
We can use manifolds as in non-reactive systems. Instead of an exact product
composition, we specify a product purity. In non-reactive systems, Thong showed
that linear approximations of the manifolds can be used. We calculate composition
profiles from the corners of the product region and assume that all compositions
leading to the product region are located on flat plane between the points on these
composition profiles.
In reactive systems however everything is bent, so that we cannot use this linear
approximation for the manifolds.
Use patches
We suggest the following method for reactive systems:
• Use more starting points on edge of product region.
• Approximate the bent manifolds by piecewise
linearised patches.
The method is also applicable to increase quality of approximation
for non-reactive manifolds.
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Synthesis of Reactive Distillation Processes
In this work, we suggest to use more starting points on the edge of the product
region and approximate the bent manifolds by piecewise linearised patches. This
procedure is can also be used to increase the quality of approximations in nonreactive sections.
Patching procedure
How can we approximate the manifold on the equilibrium surface?
x1 (n,r)
x1 (n,r)
x3 (n,r)
x2 (n,r)
Start from corners of
product region only
As in non-reactive
approximation
No good at all for
reactive systems
;
1 - 59
x5 (n,r)
x4 (n,r)
Obtain location of equilibrium
Start a profile from one
surface at intermediate points
additional point
Select point on equilibrium Approximates shape of
equilibrium surface
surface between corners
Approximates shape of
The more points the
composition manifold
more precise
7
Synthesis of Reactive Distillation Processes
How does the patching procedure to approximate the manifolds on the equilibrium surface work?
On the left, we see the top section of the composition space with the equilibrium surface for an
A+B=C type of reaction with the inert D, which must be separated. When we start two
composition profiles from the corners of the reactive product region, the approximation of the
manifold is a straight line between two points on these composition profiles. This is the linear
approximation, as it was used in non-reactive systems. In reactive systems however, this line does
not approximate the real location of the manifold on the equilibrium surface at all.
When we add another starting point, located on the equilibrium surface, on the edge of the
product region between the two corners, we can approximate the manifold using two linear
patches. This already approximates the shape of the composition manifold.
If we locate the equilibrium surface for intermediate points on the two linear patches, we further
improve the approximation. This kind of patching is important when the distance between two
points (e.g. x1 and x3) is large, which will commonly occur close to pinch points.
Adding more additional starting points for the calculation of composition profiles and projecting
more intermediate points onto the equilibrium surface obviously increases the quality of the
approximation. On the other hand there is a trade off in the accuracy of the representation and
time required to calculate the composition profiles. The generation of the patches and the test for
intersection is not time-consuming at all, as only a set of linear equations and some data handling
is involved, which can be automated.
Patches for: A + B C with inert D
(Same plot from
different angles)
n=2
n=4 r=4
r=4
D
50.5°C
n=8
r=4
n=8
r=4
n=4
r=4
n=2
r=4
A
65.3°C
C
100°C
A
65.3°C
D
50.5°C
B
81.6°C
C
100°C
B
81.6°C
Patched linear manifolds give good approximation.
1 - 60
Synthesis of Reactive Distillation Processes
These are the linear patched approximations for the ideal example introduced
earlier. We used three additional starting points, apart from the two on the corner of
the product region. This generates five points on each composition manifold. Each
of the four resulting line segments is further improved by projecting the
intermediate point of the segment onto the equilibrium surface. These projected
points do not necessarily represent an exact point on the composition manifold, but
they improve the shape of the approximated composition manifolds with respect to
the equilibrium surface.
The patched manifolds give a good approximation. Actually, the use of only the
central additional point with one intermediate projection of the two resulting
segments would have been sufficient in this case, as the other points hardly improve
the quality of the approximation.
Design procedure
• Specify both products and their purities and the quality of the feed
(either sat. liquid or sat. vapour feed)
• Specify which sections are reactive and which are inert
• Supply additional starting points to calculate manifold patches in
both column sections
• Specify the reflux and reboil ratios (r and s) and the number of
stages in each column section (n and m)
• Calculate feed concentration and product flowrates.
• Calculate patched manifolds for rectifying and stripping section
(both reactive and non-reactive options)
• Identify intersection of manifolds
Calculate total annualised cost to rank feasible design options
• Repeat calculation for new set of r, s, n and m
1 - 61
Synthesis of Reactive Distillation Processes
In the design procedure for multicomponent, single feed reactive distillation
columns with both inert and reactive sections, we first specify both products and the
required purity in addition to the quality of the feed. The feed has to be either a
saturated liquid or a saturated vapour. We also supply any required additional
starting points to calculate composition manifolds in the column sections and
specify the reflux and reboil ratios (r and s) and the number of stages in both
rectifying and stripping section (n and m). The sections can be either reactive or
inert. From the reflux and reboil ratios r and s, the feed quality q and the product
compositions we can calculate the product flowrates and the feed composition.
Then we calculate the patched manifolds and test for intersection. Every
intersection indicates a feasible design and we can determine the total annualised
cost to rank the results. Finally, we repeat the procedure for a different set of r, n
and m to rapidly screen different design options.
Instead of investigating the influence of the feed concentration on feasible designs,
one could also specify one feed concentration or one product flowrate ahead of the
design procedure. This removes one degree of freedom. so that reboil and reflux
ratio are not independent anymore. Either the reflux or the reboil ratio has to be
calculated instead of the feed concentration. This speeds up the analysis, as fewer
composition profiles are determined.
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
1 - 62
Synthesis of Reactive Distillation Processes
Case study: MTBE
i-Butene + MeOH MTBE and inert n-Butane
Equilibrium surface
nB/MeOH
Azeotrope
n-Butane
69.4°C
Non-reactive
distillation boundary
azeotrope
nB/MeOH
67.0°C
r =?
n =?
q=1
m= ?
s =?
MTBE
i-Butene
61.4°C
MTBE
136.2°C
azeotrope
iB/MeOH
59.6°C
MeOH
128.0°C
1 - 63
azeotrope
MTBE/MeOH
119.9°C
Specs: i-B MeOH nB
Top
*
0.096 0.904
Feed 0.223 0.277 0.500
Bottom *
*
*
Product purities: 99.9%
* = traces
Liquid feed: 180 kmol/h
MTBE
*
0.999
Synthesis of Reactive Distillation Processes
The design procedure is applied to the production of MTBE in a reactive distillation
column. On the left-hand side, we see the bent equilibrium surface (dark
shading/red) and the non-reactive distillation boundary (light shading /blue) located
between the three minimum boiling non-reactive azeotropes. Two of these
azeotropes are eliminated by the reaction. The n-butane/methanol azeotrope
survives and an additional reactive saddle azeotrope very close to pure n-butane is
generated. We can therefore produce the n-butane/methanol azeotrope at the top of
a reactive rectifying section and pure MTBE from an inert stripping section.
We specify the feed such that it contains 50% n-butane and no MTBE. The molefractions of iso-butene and methanol in the feed are calculated by mass balance over
the column. High product purities of 99.9% are specified.
Results of design procedure
• Apply column design procedure using one additional starting point for
both top and bottom product to generate patches
• Determine sets of n, m, r, s for feasible column designs
e.g.:
r=1
r=2
r=3
r=4
n=9
m=8 m=9 m=10 m=11
9.9
9.9
- 12.4
12.4
12.4
Matrix entries
represent
reboil ratios s
• Select one feasible column design, e.g.: n=9, m=10, r=3, s=9.9
• Cost of this design: TAC = 312,000 £/yr (101% of optimal design)
• Use values of n, m, r, s as input to rigorous simulation
1 - 64
Synthesis of Reactive Distillation Processes
The design procedure is applied and some of the resulting sets of feasible
parameters are displayed. We select one of the sets, e.g. n=9, m=10, r=3, s=9.9. The
cost of this design is 312,000 £/yr. The design procedure also allows the optimal
design to be identified. In this case, the cost of the optimal design with a saturated
liquid feed is about 1% less than that of the design selected.
We use the values of n, m, r and s as input to a rigorous simulation.
Rigorous simulation results
Liquid mole fraction
100%
80%
60%
40%
20%
0%
0
5
10
15
20
25
Stages
x Isobutene
x Methanol
x MTBE
x n-Butane
• Results of conceptual design procedure used to set up simulation
• Increasing reflux ratio from r=3 to r=4.03 meets product specification
• Multiplicities occur: Different profiles for idential column specification
1 - 65
Synthesis of Reactive Distillation Processes
Using the results of the conceptual design procedure to set up the rigorous
simulation generates a set of composition profiles, which do not meet the product
specifications. By increasing the reflux ratio from 3 to 4.03, the desired purity and
100% iso-butene conversion can be obtained, as shown in the diagram.
The profile generated by the original set of parameters is a low conversion profile,
where MTBE is produced on the top stages and then decomposed towards the feed.
In the bottom section the profile pinches close to the MTBE/methanol azeotrope.
By increasing the reflux to 4.03, the reactants are distributed more efficiently and
MTBE is produced on all rectifying trays. The stripping section efficiently separates
methanol and MTBE.
Note that multiplicities occur. Both a low conversion profile and the high
conversion profile (as shown above) are stable solutions for the same column set-up
and the same design parameters. This obviously severely aggravates trial-and-error
design approaches: if a design does not meet the product specifications, one cannot
say whether this is because they are not achievable or because the initial guess led
the simulation to converge on a low conversion profile. The occurrence of these
multiplicities for the MTBE system is well reported in the literature and was
experimentally verified.
MTBE case study: Summary
Screening and feasible column design achieved with little effort
• For a highly complex system
• For a column with a reactive and an inert section
• Preliminary costing procedure included
• Conceptual method provides good input to rigorous simulation
• Multiplicities and non-linearities make trial-and-error simulations a tedious task
Conceptual methodology in good agreement with rigorous simulation
• Discrepancies arise:
Vmin 310
assumption of constant molar overflow is not satisfied: =
= 62%
Vmax 500
patched manifolds are linear approximations
• Conceptual methodology could be improved by:
including energy balances to remove assumption of constant molar overflow
using more patches for better approximation of manifolds
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Synthesis of Reactive Distillation Processes
Screening and feasible column design is achieved for the highly complex MTBE
system with little effort. We can design a column with both an inert and a reactive
section, perform a preliminary costing procedure to rank feasible designs and
provide a good input to rigorous simulations. Multiplicities encountered for this
system would make trial and error design approaches a very tedious task.
The design procedure is in good agreement with the rigorous simulations.
Discrepancies arise because the assumption of constant molar overflow, used in the
design procedure, is not satisfied. The rigorous simulation reveals that the vapour
flow through the rectifying section varies by almost 40%. Also, even though one
additional starting point was used to generate patches, the patches are still a linear
approximation of the real location of the manifolds.
The design procedure could be improved by introducing energy balances to
eliminate the constant molar overflow assumption and by using more patches to
represent the manifolds more precisely.
Conclusions: Conceptual methodology
• We can now carry out column design
for single-feed columns separating reactive
multicomponent mixtures, including azeotropic mixtures
assuming reaction equilibrium is reached on each stage
for columns with both reactive and inert sections
for any type of split
without having to specify exact product compositions
• Conceptual method identifies wide range of
feasible operating points
• Preliminary costing procedure can be used to
screen for most promising designs
• Results of methodology are in good agreement
with those of rigorous simulation
1 - 67
Synthesis of Reactive Distillation Processes
The new conceptual methodology allows us to carry out column design for singlefeed columns separating reactive multicomponent mixtures, including azeotropic
mixtures, for columns with both reactive and inert sections, for any type of split,
without having to specify exact product compositions.
The conceptual method identifies a wide range of feasible operating points, which
can be ranked for screening using a costing procedure. The methodology is in good
agreement with rigorous simulation.
Outline
1. Motivation and current approaches
2. Synthesis using stochastic network optimisation
- Previous work
- Extension to reactive distillation
- Applications
3. Synthesis using conceptual methodology
- Previous work
- Extension to reactive distillation
- Application
4. Outlook
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Synthesis of Reactive Distillation Processes
Future Work
• Conceptual methodology
Calculation of composition profiles with energy balances to eliminate
assumption of constant molar overflow
Extension to multiple zones (inert rectifier, reactive middle section, inert stripper)
Extension to two-feed columns and complex columns
Sequencing using reactive and non-reactive columns
• Stochastic network optimisation
Application of superstructure approach to other process units, e.g. reactive
extraction, crystallisation, ...
• Combination of conceptual methodology and stochastic optimisation
Optimisation of complex columns (including reactive sections)
Optimisation of column sequences with recycles (including reactive columns)
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Synthesis of Reactive Distillation Processes
Ideas for future work to improve the conceptual design methodology include the
calculation of composition profiles using energy balances to remove the constant
molar overflow assumption. The methodology should be extended to include
multiple sections, such as an inert rectifier, a reacting middle section and an inert
stripper. Two-feed columns are often required for reactive distillation and complex
column arrangements with reactive sections are also worth analysing. As reactive
distillation is a potential way of simplifying column sequences, for example when
azeotropes can be avoided or overcome, the sequencing procedure developed by
Thong (PhD Thesis, 2000) should be extended to accommodate reactive distillation
columns.
On the stochastic network optimisation front, the superstructure approach could be
applied other process units, such as reactive extraction or crystallisation.
A combination of the conceptual methodology and the stochastic optimisation
technique seems to be a promising way to approach the optimisation issues faced in
complex column design and column sequencing including recycles.
Acknowledgements
Gratitude is expressed to Degussa-Hüls AG,
Germany, for sponsoring this research.
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Synthesis of Reactive Distillation Processes