Property Packages Guide¶

Choosing the right property package is one of the most important decisions in process simulation. The wrong choice can produce results that look reasonable but are quantitatively wrong by a wide margin. This guide helps you select the appropriate model for your system.

Decision Tree¶

graph TB
    Q1["Pure water or steam<br/>only?"]
    Q1 -->|Yes| ST["Steam Tables (IAPWS-IF97)"]
    Q1 -->|No| Q2["Hydrocarbon-only<br/>(CH4-C20+)?"]
    Q2 -->|Yes| Q2A["Pressure?"]
    Q2A -->|Low| LK["Lee-Kesler-Ploecker<br/>or SRK"]
    Q2A -->|Medium-High| PR["Peng-Robinson"]
    Q2 -->|No| Q3["Polar compounds<br/>(water, alcohols, acids)?"]
    Q3 -->|Yes| Q3A["VLE only<br/>or also LLE?"]
    Q3A -->|VLE| NRTL["NRTL or UNIQUAC"]
    Q3A -->|LLE| UNILL["UNIFAC-LL or NRTL"]
    Q3 -->|No| Q4["Electrolytes<br/>(salts, ions)?"]
    Q4 -->|Yes| EL["Sour Water,<br/>Electrolyte NRTL,<br/>or Seawater"]
    Q4 -->|No| GENERIC["Peng-Robinson (default)"]

Recommended Models by System¶

Water and Steam¶

System	Recommended	Notes
Pure water/steam	Steam Tables (IAPWS-IF97)	Reference accuracy across all conditions
Water + hydrocarbons (immiscible)	UNIFAC-LL, Sour Water	Captures liquid-liquid behavior

Hydrocarbons¶

System	Recommended	Notes
Light hydrocarbons (C1-C5)	Peng-Robinson	Industry standard for gas processing
Petroleum cuts	PR or SRK with pseudo-components	Generate pseudo-components from boiling curves
High pressure (> 100 bar)	PR or PRSV2	PRSV2 has better accuracy for polar species
Sour gas (with H2S, CO2)	PR with kij	Set binary interaction parameters

Polar Mixtures¶

System	Recommended	Notes
Alcohol/water	NRTL	Handles azeotropes correctly
Acid/water	NRTL or UNIQUAC	NRTL is the safer default
Aldehydes, ketones	NRTL	Verify with experimental data if critical
Wide-temperature alcohol/water	Modified UNIFAC (Dortmund)	Predictive, no DB needed

Predictive (when no data available)¶

System	Recommended	Notes
Mixed organics, no DB	UNIFAC	Group-contribution; works for many systems
Wide T range, polar	Modified UNIFAC (Dortmund)	More accurate at extreme T
Vapor-Liquid only	Standard UNIFAC
Liquid-Liquid	UNIFAC-LL

Electrolytes and Specialized¶

System	Recommended	Notes
Seawater	Seawater (IAPWS-08)	Reference for desalination
Sour water (NH3, H2S, CO2)	Sour Water	Specialized for gas treating
Aqueous electrolytes	Electrolyte NRTL	Plus / Patreon edition
Refrigerants	PR or REFPROP-style	PR works for most R-X compounds

Common Pitfalls¶

Using Raoult's Law for Non-Ideal Systems¶

Raoult's Law assumes ideal behavior in both phases. For non-ideal mixtures (water/ethanol, water/acetone), it gives wrong VLE predictions, especially near azeotropes. Always use NRTL or UNIQUAC for polar mixtures.

Using PR for Highly Polar Systems¶

Peng-Robinson is excellent for hydrocarbons and gases, but performs poorly for water/alcohol mixtures. The cubic EOS form does not capture the strong hydrogen bonding in polar systems. Use activity-coefficient models (NRTL) instead.

Mixing Property Package Conventions¶

Different packages may use different mole/mass fraction conventions or compound naming. When importing a flowsheet from another simulator, verify that all stream specifications are consistent with the new package.

Forgetting to Set Binary Interaction Parameters¶

For PR/SRK with strongly polar or quadrupolar compounds (CO2, H2S), the kij values matter. DWSIM's database has many published kij values, but always verify they are loaded.

Verification Strategy¶

Before trusting a simulation result:

Sanity check with hand calculations for simple cases (single component, ideal mixture)
Compare phases (vapor fraction, density) with NIST WebBook or experimental data
Check azeotropes appear at the right composition for known systems
Run with two different packages and compare; large discrepancies indicate the system is sensitive

Property Package List in DWSIM¶

The most common packages available in DWSIM (FluentAPI enum names):

Enum	Full Name
`SteamTables`	IAPWS-IF97
`PengRobinson`	Peng-Robinson (1976)
`PengRobinsonStryjekVera2`	PRSV2
`SoaveRedlichKwong`	SRK (1972)
`LeeKeslerPloecker`	LKP
`RaoultLaw`	Raoult's Law (ideal)
`NRTL`	NRTL
`UNIQUAC`	UNIQUAC
`Wilson`	Wilson
`UNIFAC`	Standard UNIFAC
`ModifiedUNIFAC_DDB`	Modified UNIFAC (Dortmund)
`UNIFAC_LL`	UNIFAC for LLE
`GraysonStreed`	Grayson-Streed
`ChaoSeader`	Chao-Seader
`SourWater`	Sour Water
`SeaWater`	IAPWS-08 Seawater
`Electrolyte_NRTL`	Electrolyte NRTL (Plus)

Refer to the FluentAPI Property Packages reference (DWSIM.FluentAPI/docs/api/property-packages.md) for the complete list.