# Conventions¶

## Units of Measurement and Reference States¶

Due to the flexibility provided by the IDAES Integrated Platform, there is no standard set of units of measurement or standard reference state that should be used in models. Units of measurement are defined by the modeler for the 7 base quantities (time, length, mass, amount, temperature, current and luminous intensity) in each property package, and the platform makes use of this and Pyomo’s Units container to automatically determine the units of all variables and expressions within a model. Thus, all components within a model using a given property package must use units based on the units chosen for the base quantities (to ensure consistency of units). However, flowsheets may contain property packages which use different sets of base units, however users should be careful to ensure units are converted correctly where property packages interact. For more detail on defining units of measurement see Defining Units of Measurement.

Pyomo also provides convenient tools for converting between different units of measurement and checking for unit consistency, of which a few are highlighted below:

• units.convert(var, to_units=units) - returns a Pyomo expression including the variable var and the necessary conversion factors to convert it to the desired set of units (units). This method will return an Exception if the units of var are not consistent with those requested by the user.

• units.assert_units_consistent(object) - checks for consistency of units in object and raises an AssertionError if they are not. object may be a Block, Constraint or Expression.

The IDAES developers have generally used SI units without prefixes (i.e. Pa, not kPa) within models developed by the institute, with a default thermodynamic reference state of 298.15 K and 101325 Pa. Supercritical fluids have been consider to be part of the liquid phase, as they will be handled via pumps rather than compressors.

## Standard Variable Names¶

In order for different models to communicate information effectively, it is necessary to have a standard naming convention for any variable that may need to be shared between different models. Within IDAES, this occurs most frequently when information regarding the state and properties of the material, which is calculated in specialized PropertyBlocks, is used in others parts of the model.

### Standard Naming Format¶

There are a wide range of different variables which may be of interest to modelers, and a number of different ways in which these quantities can be expressed. In order to facilitate communication between different parts of models, a naming convention has been established to standardize the naming of variables across models. Variable names within IDAES follow to the format below:

{property_name}_{basis}_{state}_{condition}


Here, property_name is the name of the quantity in question, and should be drawn from the list of standard variable names given later in this document. If a particular quantity is not included in the list of standard names, users are encouraged to contact the IDAES developers so that it can be included in a future release. This is followed by a number of qualifiers which further indicate the specific conditions under which the quantity is being calculated. These qualifiers are described below, and some examples are given at the end of this document.

#### Basis Qualifier¶

Many properties of interest to modelers are most conveniently represented on an intensive basis, that is quantity per unit amount of material. There are a number of different bases that can be used when expressing intensive quantities, and a list of standard basis qualifiers are given below.

Basis

Standard Name

Mass Basis

mass

Molar Basis

mol

Volume Basis

vol

#### State Qualifier¶

Many quantities can be calculated either for the whole or a part of a mixture. In these cases, a qualifier is added to the quantity to indicate which part of the mixture the quantity applies to. In these cases, quantities may also be indexed by a Pyomo Set.

Basis

Standard Name

Component

comp

Indexed by component list

Phase

phase

Indexed by phase list

Phase & Component

phase_comp

Indexed by phase and component list

Total Mixture

No state qualifier

Phase

Standard Name

Supercritical Fluid

liq

Ionic Species

ion

Liquid Phase

liq

Solid Phase

sol

Vapor Phase

vap

Multiple Phases

e.g. liq1

#### Condition Qualifier¶

There are also cases where a modeler may want to calculate a quantity at some state other than the actual state of the system (e.g. at the critical point, or at equilibrium).

Basis

Standard Name

Critical Point

crit

Equilibrium State

equil

Ideal Gas

ideal

Reduced Properties

red

Reference State

ref

### Constants¶

IDAES contains a library of common physical constants of use in process systems engineering models, which can be imported from idaes.core.util.constants. Below is a list of these constants with their standard names and values (SI units).

Note

It is important to note that these constants are represented as Pyomo expressions in order to include units of measurement. As such, they can be directly included in other expressions within a model. However, if the user desires to use their value directly (e.g. to initialize a variable), the value() method must be used to extract the value of the constant from the expression.

Constant

Standard Name

Value

Units

Acceleration due to Gravity

acceleration_gravity

9.80665

$$m⋅s^{-2}$$

6.02214076e23

$$mol^{-1}$$

Boltzmann Constant

boltzmann_constant

1.38064900e-23

$$J⋅K^{-1}$$

Elementary Charge

elementary_charge

1.602176634e-19

$$C$$

96485.33212

$$C⋅mol^{-1}$$

Gas Constant

gas_constant

8.314462618

$$J⋅mol^{-1}⋅K^{-1}$$

Newtonian Constant of Gravitation

gravitational_constant

6.67430e-11

$$m^3⋅kg^{-1}⋅s^{-2}$$

Mass of an Electron

mass_electron

9.1093837015e-31

$$kg$$

Pi (Archimedes’ Constant)

pi

3.141592 [1]

Planck Constant

planck_constant

6.62607015e-34

$$J⋅s$$

Stefan-Boltzmann Constant

stefan_constant

5.67037442e-8

$$W⋅m^{-2}⋅K^{-4}$$

Speed of Light in a Vacuum

speed_light

299792458

$$m⋅s^{-1}$$

[1] pi imported from the Python math library and is available to machine precision.

Values for fundamental constants and derived constants are drawn from the definitions of SI units (https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf) and are generally defined to 9 significant figures.

Acceleration due to gravity, gravitational constant and electron mass are sourced from NIST (https://physics.nist.gov) and used the significant figures reported there.

### Thermophysical and Transport Properties¶

Below is a list of all the thermophysical properties which have standardized names.

Variable

Standard Name

Activity

act

Activity Coefficient

act_coeff

Bubble Pressure

pressure_bubble

Bubble Temperature

temperature_bubble

Compressibility Factor

compress_fact

Concentration

conc

Density

dens

Dew Pressure

pressure_dew

Dew Temperature

temperature_dew

Diffusivity

diffus

Diffusion Coefficient (binary)

diffus_binary

Enthalpy

enth

Entropy

entr

Fugacity

fug

Fugacity Coefficient

fug_coeff

Gibbs Energy

energy_gibbs

Heat Capacity (const. P)

cp

Heat Capacity (const. V)

cv

Heat Capacity Ratio

heat_capacity_ratio

Helmholtz Energy

energy_helmholtz

Henry’s Constant

henry

Internal Energy

energy_internal

Mass Fraction

mass_frac

Material Flow

flow

Molality

molality

Molecular Weight

mw

Mole Fraction

mole_frac

Osmotic Pressure

pressure_osm

pH

pH

Pressure

pressure

Speed of Sound

speed_sound

Surface Tension

surf_tens

Temperature

temperature

Thermal Conductivity

therm_cond

Vapor Pressure

pressure_sat

Viscosity (dynamic)

visc_d

Viscosity (kinematic)

visc_k

Vapor Fraction

vap_frac

Volume Fraction

vol_frac

### Reaction Properties¶

Below is a list of all the reaction properties which have standardized names.

Variable

Standard Name

Activation Energy

energy_activation

Arrhenius Coefficient

arrhenius

Heat of Reaction

dh_rxn

Entropy of Reaction

ds_rxn

Equilibrium Constant

k_eq

Reaction Rate

reaction_rate

Rate constant

k_rxn

Solubility Constant

k_sol

### Solid Properties¶

Below is a list of all the properties of solid materials which have standardized names.

Variable

Standard Name

Min. Fluidization Velocity

velocity_mf

Min. Fluidization Voidage

voidage_mf

Particle Size

particle_dia

Pore Size

pore_dia

Porosity

particle_porosity

Specific Surface Area

area_{basis}

Sphericity

sphericity

Tortuosity

tort

Voidage

bulk_voidage

### Naming Examples¶

Below are some examples of the IDAES naming convention in use.

Variable Name

Meaning

enth

Specific enthalpy of the entire mixture (across all phases)

flow_comp[“H2O”]

Total flow of H2O (across all phases)

entr_phase[“liq”]

Specific entropy of the liquid phase mixture

conc_phase_comp[“liq”, “H2O”]

Concentration of H2O in the liquid phase

temperature_red

Reduced temperature

pressure_crit

Critical pressure