improving window of operations of olefin plant using aspen plus · pdf file ·...

© 2012 Aspen Technology, Inc. All rights reserved

Improving Window of Operations of Olefin Plant Using Aspen Plus An example of application of E/O modeling

Phil Edwards, Senior Process Engineer, SABIC UK

Rob Hockley,

Senior Business Consultant, AspenTech

Hosted by:

Ron Beck,

Product Marketing, AspenTech

Optimizing Operations Webinar Series

September 20, 2012

© 2012 Aspen Technology, Inc. All rights reserved |

2

Agenda

1. Introduction to equation-oriented (E/O) modeling in Aspen Plus (Rob Hockley)

2. Olefin Plant “window of operation” modeled and improved with Aspen Plus by SABIC UK (Phil Edwards)

3. Q & A with Rob and Phil


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Ongoing Series of Technical Webinars Engineering Webinars for education and best practices

UPCOMING WEBINARS OF INTEREST:

Oct 24, 2012: Modeling Conversion of Carbon to Biomass in Algal Systems (Dr. Eric Dunlop, Pan Pacific)

Nov 14, 2012: Integrated Economics: Take it to the Next Level

Dec 12, 2012: Solid Phase Modeling in Specialty Chemicals (Ajay Lakshmanan, AspenTech)

OTHER RECENT WEBINARS :

Recent webinars on many engineering topics can be viewed on demand on www.aspentech.com including:

Sept 5, 2012: Getting Started with Aspen HYSYS Dynamics

Sept 13, 2012: Improved Heat Exchanger Design with aspenONE EDR Integrated with Process Modeling


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Aspen Plus Sequential (SM) cf. Equation-Oriented (EO)

Sequential Modular • Easy-to-use

• Reliable initialization

• Slow when lots of recycles, optimization

S1 S2 S3

S6

S4

S7

S5 MIXER

B1

MIXER

B2

FSPLIT

B3

FSPLIT

B4

S1 S2 S3

S6

S4

S7

S5

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

0= y - f1 (u,o,x)

Equation-Oriented

• Each block modelled by equations

• All variables visible to solver

• Initialization from SM

• Fast convergence

• Simulation, Optimization, Reconciliation


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Model Equations

Knowns:

Fc, Fh, Cpc, Cp

h, Tc1, Th1, UA

Unknowns:

Q, Tc2, Th2

• Convergence scheme must be programmed

• Different set of known variables requires a different solution algorithm

Fc, Cpc

Fh, Cph

Tc1 Tc2

Th2 Th1

Aspen Plus Sequential Modular (Closed Form) Model

Q = Fc * Cpc * (Tc2- Tc1)

Q = Fh * Cph * (Th1- Th2)

Q = UA * ((Th1 - T

c2) - (Th2 - T

c1))/

ln((Th1 - T

c2)/(Th2 - T

c1))


6

Model Equations

Model Variables:

Fc, Fh, Cpc, Cp

h, Tc1, Th1, UA, Q, Tc2, Th2

• Non-linear solver drives residual functions to zero

• Knowns and unknowns interchangeable

Tc2 Tc1

Fc, Cpc

Fh, Cph

Th2 Th1

Aspen Plus Equation Oriented (Open Form) Model

f(1) = Q - Fc * Cpc * (Tc2 - Tc1)

f(2) = Q - Fh * Cph * (Th1 - Th2)

f(3) = Q - UA * ((Th1 - Tc2) - (Th2 - Tc1))/

ln ((Th1 - Tc2) / (Th2 - Tc1))


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Equation Oriented Solver Where should you consider using it?

Large models that take a long time to run

– e.g. with many recycles & design specs

– e.g. recycles including multiple distillation columns

– e.g. where recycles strongly affect design specs and vice versa

– EO can run up to 50 times faster in Aspen Plus compared to SM

Multi-variable optimization

– Optimizing plant operations

– Optimizing designs

– Data reconciliation

– EO Optimizer requires an additional license; it is not part of standard Aspen Plus


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Aspen Plus Equation Oriented Workflow

First run the flowsheet in the normal sequential modular (SM) mode

More advanced options: – Leave out some design specs from the SM calculation

and only use them in the EO

– Don’t bother fully converging the SM simulation i.e. initialize EO with an unconverged SM result

– Break open some recycle streams in SM and “reconnect” them in EO

S1 S2 S3

S6

S4

S7

S5 MIXER

B1

MIXER

B2

FSPLIT

B3

FSPLIT

B4


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Click on “More” in the Control Panel


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In the expanded Control Panel, you have the option to set the Solution Mode – Sequential Modular

– Equation Oriented

– Mixed Mode

When you select Equation Oriented it will start EO Synchronization – Builds the EO

flowsheet

– Initializes the EO variables with the current SM results


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Set up additional EO calculations

– Spec Groups (roughly equivalent to Design Specs)

– Optimizer Objective Fun.

Click on the Run button

View the EO results

– Click on

– Or, view EO Configuration > EO Variables in the Data Browser


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EO Variables Table for Each Block / Feed Stream

In the Data Browser, each block, or feed stream, also has its own EO Variables folder


13

Aspen Plus Conventional Results Forms

The normal stream results forms will show the results from the EO solution

The normal block results will also match the EO solution

But …

The normal “SM” input forms will not necessarily match the EO solution


14

Aspen Plus Existing Models

In my experience, switching an existing, LARGE, model from SM to EO may fail

You may use “tricks” in large SM models to aid convergence e.g. writing values into recycle streams. This is invalid in EO mode and causes it to fail.

EO is a more rigorous solution e.g. trace components in a recycle may solve in SM but if they cannot leave the loop in EO it will not solve

If this happens:

– Build up the model again in sections and try out in EO mode after adding each new section

– Leave some design specs out and only add them in EO mode


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Unsupported in EO

These features are completely unsupported and will not run in EO mode:

Non-conventional solids

Solid Particle Size Distributions

Solids-handling unit operation models relying on PSDs

Balance Blocks

Data-Fit (could use Reconciliation mode in EO)

SM Optimization/Constraint blocks (use EO Optimization run mode instead)

Excel Calculator blocks

Sensitivity Blocks (EO sensitivity gives derivatives)


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Perturbation Layer

Not all Aspen Plus unit operations are supported in EO mode

However, some unsupported blocks will still run using the “Perturbation Layer”

EO solver then sees the block as a “black box” - similar to one equation with a number of inputs and outputs

The Perturbation Layer numerically changes (perturbs) the inputs to see the effect on each output

You can choose to expose any of the internal block variables to the EO solver e.g. for use in Spec Groups

– In the Block Options folder, EO Var/Vec tab

If the model has many Perturbed blocks it will slow down the EO simulation


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Unsupported Blocks that Work with Perturbation Layer

Rigorous Exchanger models inside HeatX blocks e.g. Aspen Shell&Tube (very slow however)

Distl / DSTWU / SCFrac (shortcut distillation models)

MCompr (use multiple Compr blocks instead)

MHeatX

MultiFrac

Pipe & Pipeline

RBatch

RGibbs

User/User2 blocks (ACM User models work ok)


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Unsupported Blocks that Work with Perturbation Layer

Not all of the options within the supported blocks will work in EO mode

Use of these unsupported options will also trigger the Perturbation Layer for these blocks

Examples:

– Rigorous option in HeatX blocks

– Phase-specific heat transfer coefficients in HeatX

– Multiple performance curves at different speeds in Compr

See Aspen Plus Reference section in the online Help; look in EO Usage Notes for each type of block


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Equation Oriented Aspen Plus

Benefits

– Up to 50 times faster when converging big models

– Can more easily converge highly integrated flowsheets (e.g. with lots of interacting recycles and Design Specs)

– Much more powerful optimization features

Extra Challenges

– Users have to learn the extra concepts for EO

– Users have to learn how to drive the extra User Interface forms for EO

– If convergence failure occurs, harder to debug

If the benefits outweigh the challenges then it is worth using Equation Oriented!

IMPROVING WINDOW OF OPERATION OF OLEFINS PLANT USING ASPEN PLUS

Phil Edwards 20th Sept 2012

No. 21

Presentation contents

Context of C3 splitter retray

What will be achieved by increasing the window of operation

Olefins 6 optimizer and how it is used in this project

C3 splitter retray

Performance of new trays

Concluding comments

No. 22

Context of C3 Splitter retray: an olefins plant

naphtha

propane

butane/LPG Olefins Plant

hydrogen

other C4s gasoline fuel oil

ethylene propylene butadiene

ethane & propane

C5s

steam

fuel (methane)

No. 23

feed

17 furnaces

840oC

Steam

raising

-165oC

H2 methane

ethylene

ethane cold fractionation

propylene

propane

C4s C5+

8 psig

590 psig

primary

fractionator gasoline

heat

recovery fuel oil

refrig

CW

Context of C3 Splitter retray: an olefins plant

warm fractionation

No. 24

Context of C3 Splitter retray: original column

C3-

C4+

C2-

propylene

propane

93 trays per column

460 mm tray spacing

4-pass valve tray

C3

No. 25

feed

17 furnaces

Steam

raising

H2 methane

ethylene

ethane cold fractionation

propylene

propane

C4s C5+

primary

fractionator gasoline

heat

recovery fuel oil

refrig

CW

Feed flexibility project – increasing window of operation

Constraints:

Furnaces

Cooling water

Cold fractionation

Warm fractionation

refrig

warm fractionation warm fractionation

cold fractionation

No. 26

Feed flexibility project – increasing window of operation

The project is a package of modifications to extend the operating

envelope of Olefins 6 and improve its energy efficiency.

It enables the production of high value co-products to be increased

for a fixed ethylene production rate.

Average propylene / ethylene production envelope

Pro

pyle

ne p

roduct

ion

Ethylene production

original project

not (0,0)

No. 27

Olefins 6 optimizer

Aspen Plus Model

Rigorous

Equation-oriented

Connected to:

Plant measurements

APC constraints

Price set

Steady-state

(incorporating SPYRO for the furnaces)

No. 28

Olefins 6 optimizer

Steady state detection

Data input and validation

Model configuration

1st run - Parameterization

2nd run - Optimization

Set point implementation

Implementation delay

Optimization cycle

H&MB + state of equipment + constraints + objective function

optimum operation + associated information

rigorous model + plant measurements

heat and material balance (H&MB) + state of equipment

1st run : parameterization

2nd run : optimization

No. 29

Use of Olefins 6 optimizer for feed flexibility project

The optimizer was run offline to create heat and mass balances for many possible cases:

taking account of the many recycle streams and the heat integration

changing the design specs as the basis for the project was refined 4 cases selected to define new wider operating window:

high and low proportion of LPGs in feed slate

(different feeds produce different product mix)

high and low severity (severity relates to furnace conditions)

(different reaction conditions produce different product mix)

A key part of this was to uprate the C3 splitter. The project philosophy was to maximize throughput for the existing column shell

‘get in as many trays as possible’!

(but within shutdown window and without welding to column shell)

No. 30

EO - solved together

Olefins 6 Optimizer; used as basis of design for feed flexibility project

heat integration recycles 4 cases

feeds from optimizer

SM - solved separately

Uprate design cases

selected equipment only

A

A

B

B

C

C

more detail: heat exchangers (S &T)

column DPs

No. 31

Fixed top pressure

Feed calculated from

upstream unit

Fixed slippages

propane

propylene For initial operation DP variable is

calculated from known values

(measured DP, calculated loads,

tray design)

For modified operation

DP is calculated from DP variable

DP

The advanced process controller provides a DP limit (set from experience to avoid

operational problems).

Olefins 6 Optimizer; C3 splitter retray

-Optimized operation

-Flexibility project cases

For plant optimization runs set point moves have to result in a DP within this limit

For feed flexibility project it was clear that not all cases could be achieved

Enables required reflux

rate to be calculated

No. 32

C3 splitter retray; modelling remarks

The tray vendor wanted a full set of plant readings around the C3 splitter - raw plant data had mass balance inconsistencies - but optimizer provides balance that is always consistent - choice of period important. Need to be steady so that steady-state model represents true situation (large inventories can mask imbalances).

We discovered a discrepancy between our model and that of tray vendor - difference in theoretical stages - arose from difference in propane-propylene binary interaction parameter - AspenTech said that the default parameters reflect a wide range of operating conditions

- recommended refitting parameter just for our operating conditions - tailored parameter was between AspenTech’s old and new values (default was changed between two versions of AES)

- optimizer model updated, even though previous efficiency/bip worked fine together

No. 33

Reflux flo

w

Performance of new trays

No. 34

Performance of new trays - successful high rate operation

Sustained operation with reflux rate at 800 te/hr.

Propylene in base of C3 splitter reduced:

less unnecessary recycle of product (efficiency and throughput benefits)

Propane in top well controlled

Pressure drop steady at <25 psi

Get good match on reflux rate with average efficiency >90%

No. 35

Concluding comments

We found this to be a good approach: Equation oriented whole plant model:

• Included all important recycles and heat integration

• Easy to make changes to model specifications

• Run time just a few minutes

• Ensured consistency between SM models

Connection to plant data:

• Made it possible to assess current condition of equipment

• Allowed some constraints to be identified during EO modelling

Sequential modular modelling of smaller sections of plant:

• Accessible to contract engineers unfamiliar with EO modelling

• Allowed more complex models (eg rigorous exchanger options) to be used.


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Want to see similar results?

http://training.aspentech.com

Consider a training class from AspenTech

http://training.aspentech.com/


37

Online Training Easy to Access Training Content

Convenient, on-demand access from inside the product – Aspen Plus, Aspen HYSYS

– Aspen EDR, Aspen Economic Evaluation

– Aspen Basic Engineering

Superior user experience – Getting Started

– What’s New

– Multiproduct Integration

– Best Practices

Library of rich training content

Links to additional support and training resources

All from within the product!


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OPTIMIZE 2013 6 – 8 May 2013 The Westin Waterfront Hotel Boston, MA USA For more information, visit www.aspentech.com/agc

http://www.aspentech.com/agc


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Questions?


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Contact Information

Phil Edwards, Senior Process Engineer, SABIC UK Petrochemicals Email: [email protected] Rob Hockley, Senior Business Consultant, AspenTech Email: [email protected] Ron Beck, Senior Product Marketing Manager, AspenTech Email: [email protected] For any Aspen Plus product family communication:

Email: [email protected]

For more information on Aspen Plus and related products:

http://www.aspentech.com/core/aspen-plus.aspx

mailto:[email protected]










IMPROVING WINDOW OF OPERATION OF OLEFINS PLANT USING ASPEN PLUS C3 Splitter re-tray at the Olefins 6 facility

Phil Edwards 20th Sept 2012

improving window of operations of olefin plant using aspen plus · pdf file ·...

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