The Role of Process Integration in Process Synthesis

Jeffrey J. Siirola Purdue University / Carnegie Mellon University International Process Integration Jubilee Conference 18 March 2013

Process Innovation Sequence Need Identification Vendor Specifications Product Design Component Acquisition Basic Chemistry Construction Plan/Schedule Detailed Chemistry Plant Construction Task Identification Operating Procedures Equipment Design Commissioning/Start-up Plant Engineering Production Plan/Schedule Detailed Engineering Operation

Each step involves generating (synthesizing) alternatives Each step repeated several times at differing level of detail

Process Synthesis

♦The generation of process flowsheet alternatives

♦ Identification of... • Phenomena to exploit • Equipment to implement phenomena • Interconnections among equipment

Process Synthesis Approaches ♦ Evolutionary Modification

• Traditional Method • Alter Existing Design

♦ Superstructure Optimization

• Simplify Redundant Design • Generalized Disjunctive Programming

♦ Systematic Generation

• Build Up From Basic Components • Means-Ends Analysis • Role of Integration is Explicit

Means-Ends Analysis Systematic Generation Approach ♦ Define initial and goal states in terms of physical

properties ♦ Detect physical property differences ♦ Identify technological tasks to reduce differences ♦ Add unit operations to create flowsheet

• Retrosynthetically to goal state and specify inputs • Synthetically to present state and determine outputs • Strategically awaiting resolution of preconditions

♦ Recursively meet any technology preconditions

Hierarchical Systematic Generation Process Synthesis Paradigm ♦ Reaction Path Synthesis

• Chemistry

♦ Species Allocation • Input-output and recycle structure

♦ Task Identification • Composition, phase, temperature, pressure

♦Task Integration ♦ Utility Infrastructure ♦ Equipment Design

Onion Model, AIDES, BALTAZAR, PIP, PROSYN

Task Coordination and Integration I ♦ Integrate complementary (especially

enthalpy-changing) tasks for utility efficiency • Heat exchanger networks • Power integration • Pinch Technology

Resulting Network may Use more than the Minimum Utility Requirement

Heat was transferred across the pinch (for convenience or simplification)

Integration was not selected because utilities were desired to enable start-up

Integration was not selected because utilities were used to be able to reject disturbances

Integration was not selected because network was too complex (or insufficiently robust)

Integration was not selected because it divided streams into too many small segments

Complementary Task Integration Status

♦Wide use of heat exchanger networks ♦Power integration with expander-

compressors and some use of heat pumps

♦Techniques: Heuristic stream matching; mathematical programming; Pinch Technology

♦Overcome disturbance rejection problems with computer model-based control

♦Typical savings: 10% NPV

Task Coordination and Integration II ♦Coordinate tasks at different levels of

the process synthesis hierarchy to maximize efficiency-improving integration opportunities

• Heat-integrated distillation sequences • Multiple-effect distillation • Heat-pumped distillation

Multicomponent Distillation Sequences

A B C

D

A B

D C

A

B

C

D

D

A B

C D C B

A

Conventional Distillation Sequence

A B C

D Each sequence involves 13 enthalpy-changing tasks

1.0 Atm 1.0 Atm 1.0 Atm

Distillation Sequencing Heuristics for Constant Relative Volatility Ordering

(Seader & Westerberg, 1977; Nath & Motard, 1981; Nadgir & Liu, 1983)

1. If any product is multicomponent, favor sequences that do not split these components

2. All other things equal, use ordinary distillation

3. Avoid refrigeration (sufficient column pressure to condense overhead for reflux with water)

4. Avoid vacuum (unless reboiler temperature results in bottoms decomposition or requires expensive heating utility)

5. If on extremes of volatility ordering list (i.e., lowest boiler or highest boiler), remove unstable, reactive, or corrosive component next

6. Delay the most difficult separation (smallest relative volatility between keys) until last (i.e., in the absence of non-key components)

7. Delay recovery of the component required in highest purity until last.

8. If on extremes of volatility ordering list, remove dominant component (>50% of feed) next

9 If a separation is difficult (small relative volatility <1.05), consider manipulation of activity coefficients by azeotropic or extractive distillation with introduced mass separating agent entrainers or solvents

10. If mass separation agents have been introduced, recover as soon as possible

11. Do the easiest separation (largest relative volatility between keys) next

12. Remove the most volatile species as distillate next

Pressure-Optimized Distillation Sequence

D

A B

C

0.3 Atm 2.4 Atm 1.1 Atm

Typical 10-15% Net Present Cost Savings

Pressure-Optimized Distillation Sequence Followed by Heat Integration

D

A B

C

Typical 15-20% Net Present Cost Savings

0.3 Atm 2.4 Atm 1.1 Atm

Simultaneous Distillation Sequence Pressure Optimization

Including Heat Integration

D C

A B

Typical 35-40% Net Present Cost Savings

0.3 Atm 8.7 Atm 5.6 Atm

Pressure Optimization Allowing Steam Generation as a Cooling Utility

D

A B C

Low Pressure Steam Generator

Low Pressure Steam Generator

High Pressure Steam

Medium Pressure Steam

Medium Pressure Steam

Typical 35-40% Net Present Cost Savings

11 Atm 2.7 Atm 3.3 Atm

ABC

ABC ABC

ABC

ABC

A B

C

A

B(A)

C

ABC

A B

C

C

A

B

A

B(C)

C

A

B C

Direct Sequence

Indirect Sequence Sidedraw above Feed

Sidedraw below Feed Sidestream Rectifier

Sidestream Stripper

Column and Exchanger Capital

ABC ABC ABC

A A A

C C C

B B B

Prefractionator Petyluk System Divided Wall

A(B) A(B) A(B)

C(B) C(B) C(B)

Thermally-Coupled Columns

ABC ABC

C(B)

ABC

A A A

C C C

B B

A(B) A(B)

A(B)

C(B) C(B)

B

Petyluk System Shifted Bottom Section Shifted Top Section

Lo P

Lo P

Hi P

Hi P

Overcoming Operational Issues in Thermally-Coupled Columns

Multi-effect Distillation Five basic configurations

Split feed (both columns designed for sharp splits)

First column with sloppy distillate followed by second sharp split column

Either first column or second column is high pressure column

First column with sloppy bottoms followed by second sharp split column

Either first column or second column is high pressure column

L

H

L L L L

H H H H

B

A A A AB A

B AB B B

A A A A AB

B B B B AB

Typical 40-50% Energy Savings

Heat Pump Configurations Use work to transfer heat from a condenser to

a reboiler (same column or different columns)

Heat pump cycle with separate working fluid Use of distillate as working fluid (vapor

recompression) Use of bottoms as working fluid (reboiler flashing)

Generally advantageous to pump heat across the pinch

Benefit of pumping heat on same side of pinch depends on efficiency of generating work and coefficient of performance of heat pump

Heat Pump Vapor Recompression Reboiler Flashing

W W W

Heat Pumped Distillation

Hierarchical Task Coordination Status

♦Heat-integrated distillation sequences ♦Multiple-effect distillation ♦Heat-pumped distillation

♦Techniques: Heat integration network

synthesis within column pressure optimization; exhaustive discontinuity-based search; parametric studies

♦Typical savings (optimized heat-integrated distillation sequences): 35% NPV

Task Coordination and Integration III ♦ Integrate multiple technologies for enhanced

performance or synergism • Using LLE to overcome VLE constraint

+Decanter to split heterogeneous azeotrope

• Azeotropic and extractive distillation + Mass separation agents to alter solution activity

coefficients and improve relative volatility

• Reactive separation + Simultaneous separation to enhance reaction + Reaction to eliminate the need for separation

Feed Alternate Feed

A B

Breaking Heterogeneous Minimum Boiling Azeotrope

Use of a Liquid-Liquid Decanter to Overcome Vapor-Liquid Distillation Boundary

Azeotropic Distillation Deliberate Formation of a Heterogeneous Azeotrope

For systems that form homogeneous minimum boiling azeotropes it may be possible to introduce a mass separation agent (entrainer) which increases activity coefficients to form a heterogeneous ternary azeotropic distillate whose two phases are then decanted and processed in a way analogous to the processing of heterogeneous minimum boiling azeotropes

This approach can also be used enhance the separation of a nonazeotropic but pinched systems producing a heterogeneous binary azeotropic distillate

Feed

A B

Use of Entrainer to Ease Distillation Pinch

Entrainer Feed

B

Use of Entrainer to Convert Homogeneous Binary Azeotrope to Heterogeneous Ternary

Azeotrope

Entrainer

A

Azeotropic Distillation

Extractive Distillation For systems that form minimum boiling azeotropes

or pinched distillates, it may be possible to introduce into the top of the column a mass separation agent (solvent) which decreases activity coefficient of one of the components so that the azeotrope does not form or the distillate becomes less pinched

The solvent may be chosen to interact (associate) with any of the feed species, but is usually selected to extract the species with the highest boiling point

The solvent exits with the bottoms and is usually recovered from the bottoms in a subsequent column

Feed

A B

Use Solvent to Break Homogeneous Azeotrope

Solvent

Extractive Distillation

Synergistic Technology Integration Reactive Separation

Use of Separation to enhance Reaction Remove product to shift equilibrium Remove catalyst inhibitor Improve reaction selectivity (consecutive reactions)

Use of Reaction to enhance Separation

Combine reaction equilibria and phase equilibria to alter azeotrope formation React to completion to eliminate a subsequent separation task

Multiple Technology Integration Status

♦Azeotrope-breaking schemes ♦Azeotropic and extractive distillation ♦Reactive separation ♦Hyphenated separators

♦Techniques: Reactive and non-reactive

residue curves; boundary-crossing heuristics; attainable region theory; process simulation

♦Typical results: Achieve performance not otherwise possible

Task Coordination and Integration IV

♦ Integrate consecutive tasks for process simplification

• Design equipment to accomplish multiple tasks

♦Equipment design for process intensification

• Manipulate areas, gradients, and fields for faster transport and kinetics and smaller equipment

Acetic Acid

Original Methyl Acetate Flowsheet

Azeo

Methanol Catalyst

Methyl Acetate

Solvent

Solvent/Entrainer

Water

Heavies Water

Water

Synergistic Task Integration Routing Task Coproduct Streams

Catalyst Methanol

Methyl Acetate

Acetic Acid Water

Acetic Acid

Water

Methyl Acetate Methanol

Acetic Acid Equilibrium Reaction

Task A

Distillative Separation

Task G

Solvent-Enhanced Distillative Separation

Task F

Distillative Separation

Task C

Distillative Separation

Task D

Separative Reaction

Task E

Separative Reaction

Task B

Consecutive Task Integration for Process Intensification

Single Column Methyl Acetate Process

Extractive Distillation

Task F

Distillation Task G

Reactive Distillation

Task E Reaction Task A

Reactive Distillation

Task B

Distillation Tasks

C and D

Acetic Acid

Catalyst

Methanol

Methyl Acetate

Water

Consecutive Task Integration Status

Design equipment to accomplish multiple tasks Techniques: Ad-hoc empiricism; equipment and process simulation; experimental verification Capital (and energy) reductions can be dramatic: 80% NPV

Process Intensification ♦ Fundamental process technology research

• Change driving force • Change area • Change gradient • Change coefficient • Combine multiple phenomena • Integrate functionality

Equipment Intensification Status

♦Manipulate areas, driving forces, and turbulence for faster transport and kinetics and smaller equipment

♦Techniques: Ad-hoc empiricism; equipment and process simulation; experimental verification

♦Typical savings more related to improved reaction selectivity than to down-sized equipment cost reduction

Summary Process synthesis is the generation of flowsheet alternatives Economically superior process synthesis will incorporate a number of elements of process integration

Selective integration of reaction steps Component recycle within the flowsheet Integration of complementary tasks to minimize utilities Coordination of task operating conditions to optimize the opportunity for such complementary task integration Integration of multiple phenomena to accomplish tasks Integration of multiple tasks into single equipment Adjustment of design elements, geometries, and conditions to enhance or better balance reaction and transport kinetics

All of these process intensification techniques are in routine use in industrial chemical process synthesis