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

Process Flowsheet Generation & Design Through a Group Contribution Approach 15

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?

Process Flowsheet Generation & Design Through a Group Contribution Approach 22

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

Process Flowsheet Generation & Design Through a Group Contribution Approach 23

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