Process Flowsheet Generation & Design Through a Group Contribution Approach
Loïc d’Anterroches
CAPEC
Friday Morning Seminar, Spring 2005
Process Flowsheet Generation & Design Through a Group Contribution Approach 2
Introduction
• Question: How to solve this problem?
Synthesis Problem
???Raw materials
Products
Find the best sequence of operations to convert
the raw materials into the desired products
Process Flowsheet Generation & Design Through a Group Contribution Approach 3
Introduction
• Industrial approach– Take a master student looking for a 6 month training session
Current Methods
TargetsDataData
(input, …)
6 months and some thousands simulations later, the problem is solved.
Process Flowsheet Generation & Design Through a Group Contribution Approach 4
Introduction
• More academic approaches– Knowledge based methods
• Database of cases and rules built with time from experience.
• Problem: Not a lot of alternatives, feasible but with no guaranteed optimality, need to solve the model at each decision step.
– Optimization based methods• Setup of a mathematical superstructure in which all the
alternatives are represented, setup the optimization to find the best.
• Problem: Optimality only within the mathematical superstructure, hard to setup, need to solve the model at each decision step.
Current Methods
Process Flowsheet Generation & Design Through a Group Contribution Approach 5
Introduction
• A new method need to address those issues while improving on the drawbacks of the current methods
General Issues
Select the needed unit operations
Setup each alternative and simulate it
Get the best alternative
Definition of the problemHow to avoid using too many different unit operations but
keeping the right ones?
How to do an exhaustive analysis of the alternatives with the correct design parameters
for each simulation?
How to be sure this is the best? Any metrics?
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Introduction
• Core Idea– Apply the group contribution approach for property
estimation to the synthesis, design and modelling of a process flowsheet, with process-groups representing units, bonds representing streams, rules for flowsheet feasibility and a function of group contributions predicting the performance of the flowsheet.
• Main Objectives– Efficient on a large range of problems– Ability to integrated and reuse knowledge over the time– Computer implementation for ease of use
Core Idea and Main Objectives
Process Flowsheet Generation & Design Through a Group Contribution Approach 7
Introduction
• Introduction & problem presentation• Molecular Group Contribution Approach• Flowsheet Group Contribution Approach
– Presentation – Property Model– Overview of the Framework– Synthesis Algorithm– Design of the Process
• Implementation• Conclusion & Future Work
Overview of the Presentation
Process Flowsheet Generation & Design Through a Group Contribution Approach 8
Molecular Group Contribution Approach
1. Given a molecule
Property Prediction
HOEthanol:
2. Represent it as a combination of groups
3. Evaluate the property as the sum of the contributions
HO-OH
-CH2-
-CH3
AWnp ii
Process Flowsheet Generation & Design Through a Group Contribution Approach 9
Molecular Group Contribution Approach
1. Given a set target property values
Computer Aided Molecular Design
2. Select the groups that will compose the molecules
3. Combine the groups to form molecules
-OH; -CH2-; -CH3
150K < Tm < 170K
4. Evaluate the property to find matching molecules
HO HO HO
HO : Ethanol, Tm = 159K
Process Flowsheet Generation & Design Through a Group Contribution Approach 10
Flowsheet Group Contribution Approach
• Same as for molecules but with flowsheets– A process-group (PG) represents a unit or a set of unit
operations
Presentation
Reaction PG Solvent based separation PG
– A process-group is mass-balance independent– Connectivity is component, P and T dependent
Process Flowsheet Generation & Design Through a Group Contribution Approach 11
Flowsheet Group Contribution Approach
• A flowsheet is represented as a combination of process-groups (PGs)
Presentation
Process Flowsheet Generation & Design Through a Group Contribution Approach 12
Flowsheet Group Contribution ApproachOverview of the Framework
Process Flowsheet Generation & Design Through a Group Contribution Approach 13
Flowsheet Group Contribution Approach
• Energy index for distillation column process-group
Property Model
• Energy index for solvent based separation
NGn
kkk
ij
kNGn
kkx ARa
Rd
pQE
11
)()(
1
NGn
kkk
ij
kNGn
kkx Aa
d
pQE
11
1
– pk : Topology factor– ak : Regressed contribution of PG k– dijk : Driving force between the 2 key components– A : Regressed constant
– R : Solvent reflux of solvent– dijk : Solvent free force between the 2 components
Process Flowsheet Generation & Design Through a Group Contribution Approach 14
Flowsheet Group Contribution Approach
Generation of new process alternatives1. Given a set of inlets and outlets, and, property targets2. Select the matching PGs (Knowledge based)3. Generate all the feasible alternatives starting from the inlets4. Evaluate the target properties5. Rank the alternatives
• No mass-balance is performed during the generation of the alternatives.
• Very fast (still depends on the computer implementation)
• Combinatorial explosion limited by the constraints
Synthesis Algorithm
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Flowsheet Group Contribution Approach
Retrofit of existing processes1. Represent the existing process as a combination of PGs2. Select the PGs with their connections that will be kept as
starting structure together with the inlets and outlets. (Several starting structures are possible)
3. Select the matching PGs (Knowledge based)4. Generate all the feasible alternatives from the starting
structures5. Evaluate the target properties6. Rank the alternatives
• Retrofit and synthesis of new alternatives are possible within the same framework.
Synthesis Algorithm
Process Flowsheet Generation & Design Through a Group Contribution Approach 16
Flowsheet Group Contribution Approach
• Reverse approach– From the process-groups to the process flowsheet.– Define the topology of the unit operations within the PG
(Optional).– Define the unit operation design parameters (all the
parameters to fully describe the process, i.e. being able to run a rigorous simulation).
Design of the Process
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Flowsheet Group Contribution Approach
• Reverse approach for a reaction group– Fully based on the Attainable Region concept
Design of the Process
Maximize the selectivity to C
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Flowsheet Group Contribution Approach
• Reverse approach for a solvent based separation
Design of the Process
Distillation 1 Distillation 2
Number of stages 35 30
Feed location 20 12
Top product specifications Acetone 0.965
Chloroform 0.965
Bottom product specifications Acetone 0.05 Chloroform 0.05
• A solvent fraction of 0.7 is selected to match the 0.25 driving force target
• Ex = 0.138
• Use of Bek-Pedersen reverse algorithm to get the full column description
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Flowsheet Group Contribution Approach
• Software implementation
Implementation
Process Flowsheet Generation & Design Through a Group Contribution Approach 20
Flowsheet Group Contribution Approach
• Software implementation
Implementation
Process Flowsheet Generation & Design Through a Group Contribution Approach 21
Conclusion & Future Work
• Efficient on a large range of problems– Current PGs:
• Simple distillation, pressure swing, solvent based, kinetic reaction, fixed conversion reaction
– Theoretically computationally efficient1
• Ability to integrated and reuse knowledge over the time– Yes, if an efficient sequence is found, a translation as a
PG is possible ease of use
• Computer implementation for ease of use– Going on.
1. in practice the current implementation of the underlining graph theory could be improved
What with Respect to the Objectives?
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Conclusion & Future Work
• Do not take into account interaction between PGs– Snowball effect, controllability of a reaction-separation
system– The only way is to create a PG including both reaction
and separation
• Steady State– Need to go batch
• Precision of a group contribution method• Database of PGs
– Need to be built with the time
Limitations
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Conclusion & Future Work
• Single stage separation– Flash, crystallization (Work already done)
• Try on a full scale complex process– Multiple reaction/separation blocks with complex units
and recycles between the reaction/separation blocks
Future Work