Steam Utility Systems are not Business as Usual for Chemical Process Simulators

Transcription

1 Steam Utility Systems are not Business as Usual for Chemical Process Simulators Jonathan Currie PhD Candidate AUT University David Wilson Associate Professor AUT University Shahrul Zainal Abidin Custodian Engineer PETRONAS Nick Depree Research Fellow University of Auckland Lukman Karim Process Engineer PETRONAS An Industrial Experience Brent Young Professor University of Auckland AUT University University of Auckland

2 A Steam Utility System

3 Project Motivation PETRONAS recognised the potential savings of modelling & optimising their utility systems Utility costs can be the largest managed cost for a refinery PETRONAS owns an in-house chemical process simulator icon was jointly developed by PETRONAS Research & Virtual Materials Group (VMG) in 2003 The latest version includes dynamics, optimization & model regression. The project was proposed to build on their existing technology icon Utility Optimizer (iuo) was to be a add-in to a chemical process simulator Steady state only

4 Typical icon Flowsheet

5 Functionality Gained from icon Building on an existing package provides several advantages: Rigorous thermodynamic engine Advanced modular sheet solver Automatic mass & energy balance However there are questions: How to customize unit operations? Is the water & steam thermodynamic engine accurate enough for utility modelling? Will we be able to model utility models using icon how the client intended?

6 icon Utility Optimizer (iuo)

7 Unique Characteristics of Utility Systems Development revealed that a chemical process simulator did not mate naturally to utility models By far the most time was spent developing solving heuristics than the models themselves The major issue was: We can make the models in icon, but how do we get them to reliably solve? We had to uncover what were the fundamental (and philosophical) differences between chemical and utility models

8 Unique Characteristics Problems caused by Single Components & Zero Flows across Mixers: Single Component Problems Zero Flow Problems

9 Unique Characteristics Unexpectedly the steam & water thermodynamic engine turned out to be too accurate: By using a complex expression involving calculating Helmholtz energy the resulting calculation was too slow Model solving speed was severely reduced by the need to calculate so many thermodynamic properties using this expression However a general thermodynamic model (such as Peng-Robinson) was not accurate enough VMG was tasked to develop the later IAPWS Steam 1997 standard, now implemented

10 Unique Characteristics Multi-point Modelling: Specifications may be entered at a number of locations within a unit operation or within the model itself The utility model must be able to solve for any set of specifications as long as the degrees of freedom are met

11 Implementation in icon Having identified areas where special attention was required, one important question was raised: How do we ensure the user cannot create a model where one of these issues will affect its ability to solve? Given the flexibility of icon in allowing the user almost total control of the model, how can we make the utility add-in robust? We decided to give the user only High Level control and design the unit operations to counter the issues encountered

12 Implementation Models all contain multiple feedback paths:

13 Implementation Avoiding the Recycle Solver: A recycle solver will only propagate an estimate in one direction (direction of the stream arrow) In the example to the left (Steam Header) this meant the unit operation would not solve Use of controllers allowed estimations to propagate in both directions when the process variable and output are connected together

14 Implementation Identifying estimated variables via loops:

15 Implementation Deciding the estimated variable solve order required heuristics developed during this project: 1 st Variables which contain model wide loops (condensate return) 2 nd Variables which are specific to a unit operation, but affect other unit operations (header temperature) 3 rd Variables which are specific to a unit operation, but do not affect other unit operations (boiler air flow)

16 Industrial Application

17 Conclusion icon Utility Optimizer has been successfully implemented at PETRONAS Now in version 3.0 Virtual Materials Group acknowledged utility system specific problems icon version 6.0 included many updates to accommodate the add-in. The result is a full chemical process and steam utility modelling system in one package

18 Current Work JSteam Available via Excel & MATLAB:

19 Questions Thank you to the following companies for allowing us to present this work: PETRONAS Group Technical Solutions (Kuala Lumpur, Malaysia) Virtual Materials Group (Calgary, Canada) Further information on these packages are available from A/P David I Wilson: diwilson@aut.ac.nz