This document discusses the development of a new framework for modeling multi-component multi-phase mixtures in Modelica. The goals are to support both native Modelica media implementations and connections to external thermophysical property databases, address limitations of the existing Modelica.Media interface, and enable new applications involving multi-phase processes. A model-based interface structure is proposed to overcome restrictions of using pure functions. An example demonstrates the approach for simulating air separation processes using both a simple native Modelica air media and a connection to Refprop. Further testing and development is still needed to fully realize the new framework.

1. MULTI-PHASE MIXTURE MEDIA
Johan Windahl1, Katrin Prölss1, Maarten Bosmans2,
Hubertus Tummescheit1, Eli van Es2, Awin Sewgobind2
1Modelon, 2Vortech

2. • MODELICAPROP - CleanSky SGO project
 In collaboration with Modelon and Vortech
• Objectives
 Add support for mixtures with multiple phases
 Analyze Modelica.Media structure
 Interface external property databases
BACKGROUND

3. • Meeting in Delft - 17 people
 Academia, Industry, Modelica design group, CO-Lan
• Conclusion
 Performance important (framework should support caching)
 Challenge: Not possible to share interface
BACKGROUND
model Example
replaceable package Medium=MyMedium;
Medium.MyExternalObject eo;
Modelica.SIunits.Density d;
equation
d=Medium.density_pT(1e5,300,eo);
end Example;

4. • Thermal separation
 Distillation processes
MOTIVATION – NEW APPLICATIONS
• Thermodynamic cycle
– Temperature glide
• Transport of flows
– Compositional pipe network
– CPU expensive

5. • Modelica.Media interface
• Limitations
 Single substance medium with two phases (liquid and vapor)
 Designed for power/energy applications (mass-based)
 Different ways of usage – confusing
 Relies on common subexpression elimination
• Find a new structure!
MODELICA MEDIA

6. 1. Structure and user-friendliness
A. Share interface
- Native Modelica media and external C-code based media
B. Easy to use, understand and implement.
- Details should be hidden from the user
2. Multi-component multi-phase (new functionality)
 Phase equilibrium
 Properties for a specified present phase
Identification of phases and compounds
 Mole and mass based properties
Chemical industry works in mole while energy in mass
GENERAL REQUIREMENTS

7. Function based Model based
INTERFACE STRUCTURE
Record based

8. INTERFACE: FUNCTION VS MODEL
• Advantages model based interface
• Share interface between external and native Modelica media
• Hide implementation details (external object)
• Equation based implementation
• Tool solve flash calculations
• User friendly - work graphically
• Minimalistic interface
• Avoid new functions for new input combinations
• Avoid dependency on special trick as common-sub expression
elimination
• Drawbacks
• No usage inside functions
• User unfriendly to calculate parameter
• Not calculate property on demand

9. MULTI-PHASE MIXTURE
• Package with models and helper functions
• ThermoProperties - similar to
BaseProperties in MSL
• parameters: inputs.pTX, init, optimization
• variables: p,T, d, … (mass and mole based)
• MultiPhaseProperties
• Overall (_overall) and single phase (_1ph)[nP]
properties
• phaseComposition, phaseFraction

10. EXAMPLE – THERMO-PROPERTIES

11. • C/C++ Modelica property interface
 Backends to CAPE-OPEN, RefProp, FluidProp
EXTERNAL MULTI-PHASE MIXTURE MEDIA

12. • A few similar functions (calcThermoProperties_XXX)
 Combines flash and property calculation
C-INTERFACE

13. • External property code
 Not designed for dynamic simulation
 Error when calling outside validity area
 Non-converging region
 Limited support for partial derivatives
 No support for providing iteration start values
• General no support for overall derivatives
 One-phase region: analytical calculation
 When multiple phases exist: calculated numerically (Modelica side)
EXTERNAL INTERFACE - CHALLENGES

14. • Modelica tool
 Not possible to calculate iteration start values from a property model
• Modelica specification
 Inconvenient to use a model/block based structure to calculate
parameters
LIMITATIONS

15. • Case 1: Native Modelica Air media
 3 component simple media, incompressible liquid phase and ideal gas
assumptions
 VLE, fugacities polyniominal adapted to data- solved by tool
 Simulate fast (Initialization is a challenge)
TEST CASE 1 - AIR SEPARATION UNIT
0 4000 8000
0
4
8
Simulation time (s)
CPUtime

16. • Case 2: Refprop Air media
 3 component air media, mixture of multi-parameter eos
 Different state selections and media inputs tested
 Solver getting stuck or failure when calling outside validity region
 Needs to be analyzed further
TEST CASE 2 - AIR SEPARATION UNIT

17. • New framework for multi-component multi-phase
 Open up new areas where the Modelica technology can be used
 New model based media
Support both native Modelica and external media
 Finding a structure more challenging than expected
Due to Modelica restrictions, combination of pure and differentiable
functions
CONCLUSION

18. • More testing and evaluation of concept
 New use cases
• Infrastructure for native Modelica implementations
 Equation of state, mixing rules
 Initialization part (homotopy?)
• Extend the C-interface backend
 More property packages: e.g. MultiFlash
 New functionality
• We encourage you to take part in continuing the
development!
 Open-source GitHub Repository
• Thanks EU for the financial support!
FUTURE WORK