Species Table

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Navigation: User Guide -> Data Libraries -> Species Table

Related Links: Editing SysCAD Database, Project Configuration (cfg File), Species Data

NOTE: This page is valid for SysCAD 9.2, see Species Table in 9.1 for SysCAD 9.1.

Contents

Introduction

Species data can be added or edited in the database. The user can alter the data via the Edit Specie Database commands in SysCAD or the specie data can be edited directly in the MS Access database (care should be taken to get the formatting correct).

It is the responsibility of the user to review and check all the specie properties for the species used in your project.

The fields in the species table will be explained in the following sub-headings, but first, some important points to keep in mind:

HINT: If the user requires solid H2O (ice, or water trapped in a solid crystalline structure), then the Compound can be called 'H2O', the phase 'solid', but the Definition must NOT be H2O1, rather use O1H2.
  • SysCAD handles Sulfuric Acid (H2SO4) as follows:
The properties for H2SO4(l) and H2SO4(aq) are available in SysCAD. All Species Tables have H2SO4(l) and H2SO4(aq) as default species, with the default SysCAD data. (When a Species Table is upgraded from SysCAD 9.1 or earlier, the SysCAD default data for H2SO4(l) and H2SO4(aq) is inserted into the Species Table.) The user may override the default SysCAD values for these species, if they wish. Refer to Sulfuric Acid.
  • Each record in the Database must be unique. The uniqueness of the record is determined by fields [Compound] + [Phase]. These two fields may be set up as the primary key of the table. NOTE: There are no longer separate rows for different temperature ranges, there is now only one row per species.

For Example:

Compound PhaseTsTe Status
CaOH s 298.15 400 OK
CaOH l4003100 OK
CaOH l31006000 No longer allowed

General Species Data

This is general information that is required for ALL species.

Name

This field must be filled in.

This string describes the compound and is often the common or English name for the compound, e.g. Water, Lime, Nitric_Acid, etc. If the Display Tags option in the Access window is enabled, this will be used to display the compound name.

Compound

This field must be filled in.

It is used as a label to uniquely identify the compound.

The format for compound names is:

xxx.xx[xxx]x
Where x is any alphanumeric character
[ ] / are optional delimiters. Note "(" and ")" brackets are NOT permitted
Examples:
H2O
NaAl[OH]4
3CaO.Al2O3.1/2SiO2.5H2O

This field should be part of the primary key in the Access Database Table Design.


Definition

This field must be filled in.

This defines the elemental make up of the compound. SysCAD has a built in Periodic Table and will recognise the elements. Using the Periodic Table and the definition, SysCAD calculates the molecular weight of the compound. An elemental definition is required if the compound is a reactant or product of a chemical reaction. The format is:

AmBnCo

Where

A, B, C formula for element eg H for hydrogen, Au for gold, Pb for lead etc. This IS case sensitive, so Cl is NOT the same as CL; and Co is not the same as CO.

m, n, o are the number of moles of elements A, B and C in the compound, respectively. These can be expressed as integers, fractions or decimals.

Example:
For compound 3CaO.Al2O3.1/2SiO2.5H2O
The definition is Ca3O12Al2Si1/2H10 or Ca3O12Al2Si0.5H10

Notes:

  1. Each element must only be defined once. Therefore a definition of Ca3O1Al2O3Si1/2O1H10O5 where O is repeated is incorrect!
  2. If there is only one mole of an element the 1 must be specified. For example CO2 is incorrect, it should be C1O2.
  3. Element names are case sensitive.


If the elemental breakdown of a compound is unknown and not required, the user can insert a false element with it's own molecular weight.

Example:
Nn(123) (Nn will have a molecular weight of 123)
Ore(50) (Ore will have a molecular weight of 50)
Stuff1(37.3) (Stuff1 will have a molecular weight of 37.3)


Phase Occurrence

The user must select the appropriate phase for the species, either Gas, Liquid or Solid.

This is the actual phase in which the species occurs and it defines how the species is manipulated within SysCAD.

Individual Phase Label

This is the individual phase in which the species occurs and may be made up by any combination of alphabetical letters. This description is used for appearance and when splitting species based on Individual phases.

The user may select an Individual Phase from the drop down list, or if the required phase is not in the list, the user may type in a new Individual Phase.

Examples:
a) If the species is NaCl in the aqueous form, then the phase name could be aq, a or l. (recommended use is aq)
b) If the species is RNi, representing the organic nickel phase in a solvent extraction plant, then the phase name could be o, or og.

Note The following Individual Phase names are reserved for the specific Phase Occurrences as shown:

Individual Phase Occurrence
g Gas
l Liquid
aq Liquid
s Solid


This field is part of the primary key in the Access Database Table Design.

Density

This field is optional, but it should be filled in to ensure that the density calculations for any stream containing this compound are correct.

Note: SysCAD works with mass flows and uses the density to convert mass flows to volume flows. Thus in order to get accurate volume flows, the user must specify accurate density values for their species.

This is the density of the species in the defined phase. The unit for density is kg/m3.

Note: Generally a constant is expected, except in the case of gases, see special cases below.

If it is left blank SysCAD will assume the following:

  • A constant value of 2000 kg/m3 for solids,
  • A constant value of 1000 kg/m3 for liquids, and
  • Ideal gas density for gases.

The density provided here will also be used in volume calculations. The equation used is Volume = mass / density.

Refer to Stream Density for an example of how these individual densities are used to determine the density of a stream.

If the user enters a Density Correction function for an Aqueous Species (see Density Correction for more information), then the Density field will be greyed out.

Special Cases

  1. For gases the user may use one of the following three input formats (the formulation for Ideal Gas is also shown here):
    • Constant(value); or
    • LinearGasDensity(value) - the density value provided in brackets is expected to be at 0°C and Std. Pressure. Density @ T, P will be corrected based on:
      \mathbf {\mathrm{Density_{T,P} = Density_{0,StdP}*\frac{P}{StdP}*\frac{273(K)}{T(K)}}}; or
    • IdealGasDensity() - The density value will be calculated based on the Ideal gas law. Equations used are:
      (1) \mathbf {\mathrm{Density_{T,P} = \frac{m}{V}}} and (2) \mathbf {\mathrm{PV = nRT}} and (3) \mathbf {\mathrm{m = nM}}
      Rearranging the above equations will give:
      \mathbf {\mathrm{Density_{T,P} = \frac{PM}{RT}}}
      Where:
      m = mass of compound
      V = Volume of compound
      P = Partial Pressure of specie
      R = Universal Gas Constant = 8.314 472 J/mol.K (Reference: National Institute of Standards and Technology)
      T = Temperature in Kelvin
      n = number of moles of compound
      M = molecular weight of compound
  2. For Aqueous Solutions or ionic species where it's density changes according to the solution concentration, separate density correction functions can be used. This is described further down on this page, see Density Correction for more information.
  3. If the user has defined a species that will normally exist in the aqueous form, but the user does not have a density correction function, then it is recommended that the density of the species be set to the water density using the special density function LiqH2ORho(). This will ensure that the species has the same density as water and hence the species will not change the density of the solution. For further info on this equation see Water Density.

Thermodynamic Data

This is information is generally required for all species, unless the project is not using the Energy Balance Add-On and/or with Heat Calculations switched off.

Species that have been added to represent gangue material, such as Inerts, Ore, etc will generally not have any thermodynamic data.

Heat of Formation (H25)

This field is optional (but is usually defined).

This is the Heat of Formation at 25 °C. The unit is expressed in J/mol.

If this field is left blank, SysCAD will assume a value of 0 J/mol at 0°C.

This value is used to calculate heats of reaction for chemical reactions specified in SysCAD. If the compound is used in chemical reactions, this number will determine the accuracy of the energy balance. If this field is left blank due to lack of data, then the user should define the Heat of Reaction (HOR) manually in the reaction file. See Reaction Block (RB) for further information on reactions.

If the user enters values for the Heat of Dilution, for species such as acids, the heat of formation will be grayed out because SysCAD will use the Heat of Dilution values to determine the required heat of formation.

Refer to Stream Heat of Formation values (Hf) for an example of how these individual heats of formation are used to determine the heat of formation of a stream.

Note: All elements in their Standard State should have zero heat of formation at 25 °C. Examples of elements in their standard state: O2(g), N2(g), C(s) - where C is graphite.

Entropy (S­­25)

This field is optional.

This is the entropy value for the species. The unit for Entropy is J/mol.K.

Entropy values are normally only used in Free Energy Minimisation (FEM) calculations.

Heat Capacity (Cp)

  1. This field is optional, but if it is left blank it will have the following consequences:
    • SysCAD will assume a constant value:
      For solids and gases, SysCAD will assume a value of 1.0 kJ/kg.K,
      For liquids, SysCAD will assume a value of 2.0 kJ/kg.K.
      This assumption will be shown as a warning in the message window when loading a project.
    • If the species is used in a Reaction then the user will either:
      Have to define the Heat of Reaction (HOR) manually in the reaction file; or
      Receive a warning that Cp is not defined in the database.
      See Reaction Block (RB) for further information on reactions. (This is because Cp is used to calculate heats of reaction for chemical reactions specified in SysCAD
  2. The unit for Heat Capacity is J/mol.K.
  3. All Cp equations must use temperature in degrees Kelvin (K).
  4. Valid Cp equations must be supplied for all species used in the project to obtain a correct energy balance in SysCAD.
    • The Cp equations are integrated over a temperature range to obtain the change of enthalpy around a unit operation.
    \Delta H = \int\limits_{T_1}^{T_2}Cp dT\,
    Where T1 is Initial temperature and T2 Final temperature
  5. A species may have a number of Cp equations covering different temperature ranges. Each Cp equation must be specified with the temperature range for which it is valid.
    • The temperature ranges MUST NOT overlap for a single specie,
    • The temperature ranges MUST be contiguous, i.e. there must not be any gaps in the temperature ranges.
  6. The enthalpy calculation when the species is outside of the defined temperature ranges is calculated using the Cp at the limit of the defined temperature range:
    • If T1 < TLower Limit then:
    \Delta H _{T_1 \to T_{LowerLimit}}= Cp_{T_{LowerLimit}}*\left ( T_{LowerLimit} - T_1 \right)\,
    • Similarly, if T2 > TUpper Limit then:
    \Delta H _{T_{UpperLimit} \to T_2}= Cp_{T_{UpperLimit}}*\left ( T_2 - T_{UpperLimit} \right)\,


The format for the Cp equation and corresponding Temperature Range is as follows:

Cp Equation Name(a,b,...):Range(C or K, TL, TH)
Where:
  1. The Cp Equation is described below (Note: The equation always uses K); and
  2. The Range is the temperature range for which the Cp equation is valid -
    • The range can be given in degrees Celsius (C) or Kelvin (K) (Note, as above, while the range can be given in C or K, the actual Cp equation always uses K),
    • TL is the Lowest temperature at which the Cp equation is valid, and
    • TH is the Highest temperature at which the Cp equation is valid

Please see Examples of Cp Equations below for some examples of actual data.

SPECIFIC HEAT (Cp) EQUATIONS

The valid Cp equation formats are as follows:

  • Constant Cp
Const(a)
Cp does not vary as a function of temperature (it should still be defined for a specific temperature range)


  • CRC Equation
CRC_Cp(a,b,c,d)
C_p = a + b.10^{-3} T + \frac{c.10^5}{T^2} + d.10^{-6} T^2\,
where T - Temperature in K


  • CRC Equation (alternative format 1)
CRC1_Cp(a,b,c,d)
C_p = a + b.10^{-3} T + c.10^{-6} T^2 + \frac{d.10^5}{T^2}\,
where T - Temperature in K


  • CRC Equation (alternative format 2)
CRC2_Cp(a,b,c)
C_p = MolecularWeight * \Bigg( a + b.T - \frac{c}{T^2} \Bigg)\,
where T - Temperature in K


  • HTE Equation Format
HTE_Cp(a,b,c,d)
C_p = 4.186 * \Bigg(b + 2*c.10^{-3} T - \frac{d.10^5}{T^2}\Bigg)\,
where T - Temperature in K
Note: the first parameter a is required but is not used in the HTE equation and is ignored by SysCAD.


  • HSC Equation format
HSC_Cp(a,b,c,d)
C_p = a + b.10^{-3} T + \frac{c.10^5}{T^2} + d.10^{-6} T^2\,
where T - Temperature in K
Note: This equation is the same as the CRC format 1 equation.


  • Polynomial Equation format
Poly_Cp(a,b,c,d)
C_p = a + bT + cT^2 + dT^3\,
where T - Temperature in K


  • General Polynomial Equation format
GenPoly_Cp(c1,p1,c2,p2,c3,p3,c4,p4)


C_p = c_1 T^{p_1} + c_2 T^{p_2} + c_3 T^{p_3} + c_4 T^{p_4}\,
where T - Temperature in K


  • Shomate Equation (Gas Phase Heat Capacity)
Shomate_Cp(a,b,c,d,e)
C_p = a + b.10^{-3} T + c.10^{-6} T^2 + d.10^{-9} T^3 + \frac{e.10^6}{T^2}\,
where T - Temperature in K


NOTE: All temperatures in the above formulae are in degrees Kelvin.

Refer to Stream Specific Heat values (Cp) for an example of how these individual heat capacities are used to determine the heat capacity of a stream.


EXAMPLES OF Cp EQUATIONS

Specie
Cp
Total Temperature Range (C)
CaCO3(s) HTE_Cp(-9122, 23.8351, 3.2146, 5.1569):Range(C, 25, 927) 25 to 927
CO2(g) Shomate_Cp(19.7961, 0.07344, -5.60221e-005, 1.71541e-008):Range(K, 298.15, 1000) 25 to 726.85
Fe2(SO4)3(aq) Poly_Cp(-22429, 57.4101, 29.6643, 7.9582):Range(K, 298.15, 900) 25 to 626.85
2CaO*SiO2(s) HSC_Cp(145.896, 40.752, -26.192, 0.000):Range(C, 25, 848), HSC_Cp(134.557, 46.108, 0.000, 0.000):Range(C, 848, 1439), Const(205.016):Range(C, 1439, 2130) 25 to 2130


NOTES:

  1. Heat capacity cannot be zero. Therefore, if the value in the species database is zero for a species in the project, SysCAD will not initialise and the user will see the following error message: Specie:Bad CP: Value too small = 0J/mol at T 5 C (where species is the actual species with the zero Cp). The database Cp parameters will need to be corrected before you will be able to continue with a project.
  2. Heat capacity MUST increase with temperature with the exception of AQUEOUS compounds. Therefore, if as a result of the values in the species database for the specified temperature range, a negative increase for Cp with temperature is calculated, SysCAD will not initialise and the user will see the following error message: Bad Enthalpy: msH decreases with T for species (where species is the actual species with the negative Cp). The database Cp parameters will need to be corrected before you will be able to continue with a project. For an aqueous compound, the Cp data can effectively be used as a correction factor for that compound in solution. The Cp equation for an aqueous species may not be intended to be valid for the compound in it's pure state. To be recognised as an aqueous compound, the phase name must be "aq" or "a".
  3. The Shomate Equation is used to fit data from the NIST web site for gas components with higher temperature ranges. eg Oxygen


Solution Data

Solvent

  • This field is optional unless one or more of the following are true, in which case it is required:
  1. a Density Correction function is to be defined or;
  2. a Heat of Dilution function is to be defined or;
  3. a Phase Change Solubility function is to be defined or;
  4. a Phase Change at Mass Fraction function is to be defined.
  • All of the other fields in this section will remain grayed out until the user selects a solvent.
  • Only a single solvent may be defined for each species.
  • Once a solvent is specified the density correction function and/or heat of dilution function and/or phase change function may be entered.

Density Correction

This field is optional.

  • The density of a solution containing aqueous or ionic species changes according to the mass fraction of dissolved species in solution.
  • In these cases you may enter a density correction function instead of a value for density.
  • The Density Correction function describes the solution density as a function of solute mass fraction and has the following form:

Poly(a, b, c, d, e, f), Limit(Mf_max, MaxDensity, Warning)

This consists of 2 parts:

  1. The polynomial, which represents the density correction function (shown below)
    DensCorrFn(MF)_i = a + b.MF + c.MF^2 + d.MF^3 + e.MF^4 + f.MF^5 \,
    • Where MF is the mass fraction of the species in solution.
    • The user only needs to enter the required number of parameters. So, if the polynomial is 3rd order you need only enter a,b,c and d.
  2. The limiting values, which are described below:
    Mf_max is the maximum mass fraction of the solute for which the equation is valid.
    MaxDensity is the density of the pure solute in kg/m^3.
    Warning is either On or Off. If it is On (recommended), SysCAD will warn the user if the mass fraction of the solute is above the specified Mf_max.
    If the mass fraction of the solute in the solvent is above the Mf_max, then SysCAD calculates the density of the solution by linearly interpolating between the density at Mf_max and the density of the pure solute.

Example:

For FeSO4(aq): Poly(0.998,0.951,0.62), Limit(0.2, 2200, On)

Notes:

  1. If the user does not have a function relating the change in solvent density with solute concentration, then SysCAD will use the density in the species database in a pure mass weighted mean calculation.
  2. This will not usually produce the correct liquid density or volumetric flow rates.
  3. Please see Density Correction Calculations for the implementation method for density correction, a description of the 2 methods of calculating the specie Mass Fraction, MFi and Density Calculation Examples.
  4. The user may view the corrected density of a solution consisting on ONLY the solvent and the solute on the Species Data access window.

Heat of Dilution (Ht of Dilution)

This field is optional.

This field is normally used for aqueous acid or basic species. The data entered into the Heat of Dilution field will enable SysCAD to calculate the amount of energy released (exothermic) or absorbed (endothermic) when two streams with different concentrations of the relevant species are mixed.

The most well known example of this effect is mixing concentrated sulfuric acid with water - which is extremely exothermic.

The heat of formation is a function of the mass fraction of the specie in the solution (consisting of the species and the user specified solvent (usually water)). The user may enter the data in the form of a polynomial equation, or as TSpline data, where:

X = Mass Fraction of species, as a percentage, and
Y = Heat of Formation at 25°C, in J/mol.

Heat of Dilution data will override any value previously entered for Heat of Formation (H25).

Notes:

  1. If you use the Tspline to enter data, you must have data points for the mass fraction of the species at 0 and 100%.
  2. SysCAD has default data for Heat of Dilution for sulfuric acid. You may change this data if you wish. Please see Sulfuric Acid (H2SO4) in 9.2.

Phase Change

  • These fields are optional but once the Type is defined then the other two must be filled in.
  • All of the other fields in this section will remain grayed out until the user selects a Phase Change Type. Once a Type is chosen, the Paired and Function fields may be entered.
  • There are 3 types of Phase Change - Solubility, Temperature and Mass Fraction. This will be described below.
  • Only a single Phase Change Type may be defined for each species.

NOTE: If the user chooses one of the Solubility functions or the Mass Fraction function then they must select a Solvent in the solution data section.

Solubility

The solubility of the solute changes as a function of temperature in Kelvin. For example, the change of NaCl(s) to NaCl(aq) in H2O(l). One of the following Phase Change Types must be selected to enter data for solubility:

  1. Solubility (g/100g of solvent): The data is entered as g of Solute per 100g of Solvent as a function of Temperature. This is the most common way of specifying solubility. This option is saved as 'SolubleT-g/100g' in the database.
  2. Solubility (g/g of solvent): The data is entered as g of Solute per g of Solvent as a function of Temperature. This option is saved as 'SolubleT-g/g' in the database.

Paired

The paired species is the species that the selected species will change to if the specified conditions for solubility are met. The paired species must be the same component (i.e. contain the exactly the same number of each element). The paired species is chosen from the drop down list of available species.

For example, if defining the solubility of NaCl(aq) dissolved in water, then the paired species would be NaCl(s)

Function

The user may input a function describing the solubility of the solute as a function of temperature. If the user specifies a polynomial, then the function must be in Kelvin. If the user specifies the Tspline method, then the temperature is in Celsius.
Note: If the user wishes to use a CONSTANT solubility, i.e. independent of temperature, then select Poly(a,b,c,d..) and type in a single value, such as Poly(0.02).

The user selects the function describing the solubility from a drop down list. The following options are available:

  1. Poly(a, b, c, d, e, f)
    Example for NaCl(aq): Poly(0.5705,-0.0016,0.000003)
    For the polynomial (shown below), only the required number of parameters need to be entered.
    SolubilityFn(T)_i = a + b.T + c.T^2 + d.T^3 + e.T^4 + f.T^5 \,
    Where T is in degrees Kelvin.
  2. GenPoly(c1, p1, c2, cp2, c3, cp3...)
    Example for A2B3(aq): GenPoly(0.03425,0,-0.0020,1, 12.678, 2, 0.000003, 3)
    For the polynomial (shown below), only the required number of parameters need to be entered.
    SolubilityFn(T)_i = c_1.T^{p_1} + c_2.T^{p_2} + c_3.T^{p_3} + c_4.T^{p_4} \,
    Where T is in degrees Kelvin.
  3. Tspline(...)
    The data will be Solubility as a function of Temperature. The required data is as follows:
    • A table of data with X and Y values, where X = Temperature in C and Y = Solubility in g/g or g/100g (The user will define g/g or g/100g in Type)
    • Low Range - the lowest X value in the table. This is automatically picked up from the values inserted, the user does not have to type it in.
    • High Range - the highest X value in the table. This is automatically picked up from the values inserted, the user does not have to type it in.
    • Tension - this factor determines the shape of the curve between data points. If this factor is small, the curve approaches a cubic spline. As the tension increases the curve between points becomes more linear. The default value of 1 will normally be acceptable.
    See Entering Spline Data for further information on inputing this form of data.

The answer obtained is the maximum mass fraction (MFi) of the aqueous species, relative to the solvent, where

MF_i = \frac{Solute_i}{Solvent} \,

SysCAD will then calculate the maximum mass fraction of the aqueous species for each individual solute based on the stream temperature. The user may view the saturated values for species with solubility functions on the Species Data access window.

Notes:

  1. This solubility function is independent of pressure. Therefore, while the user may use this functionality to model the solubility of gas species in a solvent, this will not allow the user to include the pressure contribution.
  2. The associated enthalpy change with the change in the composition between the aqueous and other phases may result in a change in temperature of the stream.
  3. If the user chooses to switch on the solubility function in the model (see Plant Model - Species tab page), then SysCAD will make use of this calculated maximum mass fraction as follows throughout the whole project.
  4. If required, SysCAD will adjust the mass fractions of the solute in the aqueous and the other specified phase, either solid or gas, in order to ensure that the maximum mass fraction of the aqueous species is not exceeded.

For example, if the maximum solubility is a mass fraction of 0.35 (ie. 35 g per 100g of solvent):

  • If the specified mass fraction of the aqueous species is greater than 35% (relative to the solvent), SysCAD will adjust the mass fraction of the aqueous species down to 35% and transfer the excess to the other phase of the same species.
  • If the specified mass fraction of the species in the aqueous phase is less than 35% (relative to the solvent) and there is some mass in the other phase then SysCAD will adjust the mass fraction of the aqueous phase up to 35% and the excess will remain in the other phase.
  • The changes described above will happen automatically in a feeder (if solubility has been switched on) without adjusting the temperature of the stream.

For an overview of Solubility features, see Solubility. For an example of the use of the solubility function, see the Solubility Project.

PhaseChange at Temperature

The phase will change at a specified temperature. For example, change from one solid phase to another solid phase. For this option, the Phase Change Type selected must be PhaseChange at Temperature. This option is saved as 'Temperature' in the database.

Note: It is expected that the change in enthalpy at the specified temperature from the selected species to the paired species will be positive (i.e. endothermic). Any phase changes which are defined for exothermic phase changes will be disabled on loading of a project.

Paired

The paired species is the species that the selected species will change to at the specified temperature. The paired species must be the same component (i.e. contain the exactly the same number of each element). The paired species is chosen from the drop down list of available species.

For example, if defining the change of CuSO4(A) to CuSO4(B) at a specific temperature, then the paired species of CuSO4(A) is CuSO4(B).

Function

Specify the temperature (in Kelvin) at which the phase change occurs. This temperature will be the maximum temperature at which the specified species will exist. If enough energy is added to change some of the selected species to the paired species, but not enough energy to change all of it, then there will be a mixture of both species at the specified temperature. At any temperature higher than the one specified, all of the specified species will change to the paired species (i.e. change phase).

Note: If two species in the same project have the same temperatures specified for their phase change then SysCAD may change one species prior to the other at the specified temperature when a complete phase change of both species is not possible due to energy limitations.

PhaseChange at MassFraction

The phase changes at a defined mass fraction in the specified solvent. For example, the change of H2SO4(l) to H2SO4(aq) in H2O(l). For this option, the Phase Change Type selected must be PhaseChange at MassFraction. This option is saved as 'MassFraction' in the database.


Note that both paired species must be in the same phase, i.e. solid, liquid or gas, but have different Individual phases. Also only one of the phases will exist in a stream or unit, the 2 phases CANNOT both occur together. For example, if the user specifies a Mass Fraction change at 0.983 for H2SO4(aq), then SysCAD will convert ALL of the aqueous form to the liquid form at a mass fraction equal to or above 98.3%.

Paired

The paired species is the species that the selected species will change to at the specified mass fraction in the solvent. The paired species must be the same component (i.e. contain the exactly the same number of each element) in the same phase. The paired species is chosen from the drop down list of available species.

For example, if defining the change of H2SO4(aq) to H2SO4(l) at a mass fraction of 0.983, then the paired species for H2SO4(aq) is H2SO4(l). Note Both species in this case must be of the same phase, usually Liquid.

Function

Specify the mass fraction at which the phase change occurs. This will be the maximum mass fraction (relative to the specified solvent) at which the specified species will exist. At any mass fraction higher than the one specified, ALL of the specified species will change to the paired species (i.e. change phase).

Vapour Properties

These fields will only be writable for Vapour species, i.e. the user has chosen Gas for Phase Occurrence.


Vapour Pressure (Vp)

This field is optional.

If a component is to be used in Vapour Liquid Equilibrium (VLE) flash calculations, the Vapour Pressure must be provided for the vapour phase of the component. The Critical Temperature must also be defined. The other critical constants and the acentricity value are also normally defined but may not be required, depending on the form of the equation chosen.

The user does NOT provide this information for steam, as SysCAD will use in-built equations to calculate the vapour pressure of water. Please see Water and Steam Properties.

Where the VLE flash calculations will be used, the user should also check that both the liquid and vapour phase of the component are defined and selected for the project.

Note that in all cases, temperature T used in the equation is limited by the specified Critical Temperature (Tc). In other words if T>Tc, then Tc is used in the calculation.


General format

1) One representation of the Vapour Pressure equation is:

\mathbf {\mathrm{log_{10}(V_P) = \frac{a}{T} + b*log_{10}(T) + c*T + d}}

This vapour pressure function can be used in one of these formats:

  • Vp(a,b,c,d)
Where T is in Kelvin and Vp is in mmHg. SysCAD then converts the Vapour Pressure into kPa by multiplying 101.325/760 or 0.133322.
  • VpAtm(a,b,c,d)
Where T is in Kelvin and Vp is in Atmospheres. SysCAD then converts the Vapour Pressure into kPa by multiplying 101.325.
  • VpKPa(a,b,c,d)
Where T is in Kelvin and Vp is in KPa.


Antoine format

2) The Antoine Vapour Pressure equation is:

\mathbf {\mathrm {\ln V_p = A - \frac B{T+C}}}

The format is:

  • VpAnt(A,B,C)
where T is in Kelvin and Vp is in mmHg. SysCAD then converts the Vapour Pressure into kPa by multiplying 101.325/760 or 0.133322.


The equation may also be supplied with two additional parameters:

  • VpAnt(A,B,C, Tmin, Tmax)
where Tmin and Tmax are limits on the range of applicability of the equation specified in degrees Kelvin. If the temperature is outside this range, then the Lee-Kesler equation is used, which calculates the vapor pressure from the critical constants and accentric factor.

The Lee-Kesler equation takes the form V_p = P_c e^{L_0 + \omega_{ac} L_1} where

L_0 = 5.92714-\frac{6.09648}{T_r}-1.28862\ln(T_r)+.169347 \,T_r^6

L_1 = 15.2518-\frac{15.6875}{T_r}-13.4721\ln(T_r)+.43577 \,T_r^6

and Tr is the reduced temperature = T / Tc

Tc is the critical temperature

Pc is the critical pressure in kPa (to give a Vp in kPa)

ωac is the acentric factor in l/mol

Finally, if the equation is specified with *no* parameters:

  • VpAnt()

and the critical constants (Pc, Tc and Ac) are available, then the Lee-Kesler equation is used by default. SysCAD will report an error at load time if the critical constants (Pc, Tc and Ac) are not available.

Discussion

The Antoine equation is generally intended for low pressures, up to approximately 2-3 bar (absolute). It is based on experimental data obtained at these relatively low pressures and is useful (and very accurate) in this range. It should not be used above the range indicated as it does not extrapolate well.

The Antoine equation has a lower limit as well, but for temperatures below this limit the material is typically solid; there is no downside to using the equation below the minimum value.

The Lee-Kesler equation is very accurate for high pressures, and is exact for the critical temperature, since the terms in the equation sum to unity at that temperature. It is also exact for T_r = 0.7 T_c\, by the definition of the acentric factor:

w_{ac} = -\log \left(P_r^{sat}\right)_{T_r = 0.7} - 1\,

It is less accurate for lower temperatures, especially for polar molecules. For example, for water it predicts a vapor pressure of 90kPa at 100C compared to the true value of 1atm (101.3 kPa).

One concern is the transition between the two ranges. Discontinuities can lead to problems with the SysCAD solver, so we use interpolation in the range just above the Antoine maximum limit.

To find the overlap range, consider the possibilities: the discontinuity may be positive or negative, and the slopes may not match.

We calculate dT_a = \delta/f_{ant}'(t_{max})\, and dT_a = \delta/f_{mil}'(T_{max})\,, and (conservatively) set the overlap range limit to be

T_o = T_{max} + dT_a + dT_m\,

Within this range we linearly interpolate between the two equations, so that if \alpha = (T-T_{max})/(T_o-T_{max})\, we have

V_p = (1-\alpha)f_{ANT}(T) + \alpha f_{LK}(T)\,


Polynomial format

3) You can also use the polynomial equation for vapour pressure if required.

The format is:

  • Poly(a,b,c,x)
Where a, b, c etc are coefficients of the polynomial.
V_p = a + bT + cT^2 + dT^3 + ...\,


Related Topics:

Vapour Liquid Equilibrium (VLE).
Example for Vapour Pressure Data fitting

Critical Pressure (Pc)

This field is optional.

The critical pressure is the saturated pressure of the species at the critical temperature. Required in MPa.

This is required if the form of the vapour pressure equation chosen is the Antoine Equation with no parameters (ie. VpAnt()).

Critical Temperature (Tc)

This field is optional.

The critical temperature is defined as the temperature above which the species will not condense, no matter what the pressure. This temperature is required in Kelvin.

This is required for components involved in VLE flash calculations. This value is the maximum temperature that will be used in the vapour pressure equation. ie. if T > Tc, then Tc will be used in the vapour pressure equation.

It is also required if the form of the vapour pressure equation chosen is the Antoine Equation with no parameters (ie. VpAnt()).

Critical Volume (Vc)

This field is optional.

The critical volume is the specific volume of the species at its critical temperature and pressure. Required in l/mol.

Acentricity (Ac)

This field is optional.

This field is the Acentric Factor for the species.

This is required if the form of the vapour pressure equation chosen is the Antoine Equation with no parameters (ie. VpAnt()).

(Acid / Base) Dissociation (Ka/b)

This field is optional.

If the user requires a component to be included as an acid or base in the acidity, or pH, calculations, then this field must have the required acid or base dissociation constant. A number of 'standard' acids and bases are included in the default database with their associated dissociation constants. See Acidity Calculations for a full list of the included acids and bases.

The acid/base dissociation constant is the equilibrium constant for the dissociation reaction of the particular acid or base in water:

For the equation AB \rightleftharpoons A^+ + B^- , the dissociation constant, K (Ka for acid, Kb for bases), is calculated as:

K = \frac{[A ] [ B]}{[AB]}

where [AB] = concentration of the acid/base in mol/L in water

The form of the variable is:

Acids: Ka(Ka1, Ka2, Ka3)

where

Ka1 - required
Ka2 - optional (only relevant for diprotic and triprotic acids)
Ka3 - optional (only relevant for triprotic acids)

NOTE: For Ka ≥ 10, SysCAD assumes total dissociation.


Bases: Kb(Kb1)

where

Kb1 - required

NOTE: For Kb ≥ 1, SysCAD assumes total dissociation.


Examples: (with corresponding dissociation reactions)

Hydrofluoric Acid (HF) - Ka(6.8e-4) (a monoprotic acid) (HF \rightleftharpoons H^+ + F^- )

Phosphoric Acid (H3PO4) - Ka(7.2e-3, 6.3e-7, 4.2e-13) (a triprotic acid) (H_3PO_4 \rightleftharpoons H^+ + H2PO4^-\rightleftharpoons H^+ + HPO4^{2-}\rightleftharpoons H^+ + PO4^{3-})

NaOH - Kb(1) (NaOH \rightleftharpoons Na^+ + OH^- )

Boiling Point Elevation

This field is optional.

The Boiling Point Elevation (BPE) is calculated for aqueous solutions using the following equation1:

BPE = Kb*\sum(m*i) \,

where

Kb is the Molal Boiling point elevation constant, or sometimes called the Ebullioscopic Constant.
For water Kb = 0.512°C/m
m is the molality of each aqueous species - moles/kg water.
i is the van't Hoff constant for each aqueous species.

Notes:

  1. The user may enter a van't Hoff Constant for each aqueous species in the Species Database. SysCAD will then use these values, together with the above equation to calculate the BPE for each stream.
  2. The van't Hoff factor is a measure of the dissociation of the aqueous species in water. If the user cannot find a value for the van't Hoff factor in their references then the following approximations may be acceptable:
    • For relatively soluble species, then the van't Hoff Factor = number of ions in the specie, e.g. i(BeCl2) = 3, i(MnSO4) = 2
    • For relatively insoluble species, then the van't Hoff Factor = 1, e.g. i(BaSO3) = 1

See Boiling Point Elevation for a brief discussion on this topic.

The BPE value may then be used in vapour liquid equilibrium calculations, please see Vapour Liquid Equilibrium (VLE) for more information.

Example

For the following case:

Specie
Mass (kg)
Moles (g moles)
Molality
van't Hoff Factor (i)
Fe2[SO4]3(aq) 10 25.01 0.025 4.4
MgSO4(aq) 15 124.6 0.125 1.21
NaCl(aq) 5 85.55 0.086 1.68
H2O(l) 1000 55508 - -
BPE = 0.512 * (0.025 * 4.4 + 0.125 * 1.21 + 0.086 * 1.68)
BPE = 0.207°C

Reference

  1. Silderberg, M.S. “Chemistry - The Molecular Nature of Matter and Change”, 3rd Edition, 2003, pp508-509.

Reference Information

The following two fields, Checked and Reference are optional, but it is STRONGLY recommended that they are completed. This allows users to verify the data in the species table.

In addition to these two fields, SysCAD also places a user and date stamp here which allows users to see who last changed the data for each species in the database and when the change was made.

Checked

This field is Optional.

SysCAD uses the physical and thermodynamic qualities in this database to determine all of the stream properties in a project. Therefore, it is very important that the user verifies these values. This column allows the user to confirm that the values for the species have been checked and are correct. If the values are verified, then the user may type their initials, or any other reference, into this column. This allows other users to check the source of the data values.

When a project is loaded the following message will appear in the Message Window:

X Species not Checked, where X is the number of species without anything in the Checked column.

Reference

This field is optional.

It allows the reference of the data be entered. It is good practice to reference where the different data (eg Cp, Vp, density, etc) has been obtained from.

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