Species Table 9.3 - Specific Heat (Cp)

From SysCAD Documentation
Jump to: navigation, search

Navigation: User Guide -> Species Table 9.3 -> Species Table 9.3 - Specific Heat (Cp)

Species Table Heat of Formation
and Entropy
Density Specific Heat (Cp) Phase Change
(solubility)
BPE &
Acid/Base Ka/b
Vapour Properties Viscosity

Related Links: Editing User Species Database 9.3, Species Properties ($SDB)

This page is valid for SysCAD 9.3. For SysCAD 9.2, please see Species Table 9.2 - Thermodynamic Data.


Introduction

The Cp 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

Notes:

  1. The unit for Heat Capacity is J/mol.K.
  2. All Cp equations must use temperature in degrees Kelvin (K). If you have a Cp relation expressed in terms of degrees Centigrade or Fahrenheit it will need to be converted to a function of degrees Kelvin.
  3. 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.
    [math] \Delta H = \int\limits_{T_1}^{T_2}Cp dT\, [/math]
    Where T1 is Initial temperature and T2 Final temperature
  4. 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:
    [math] \Delta H _{T_1 \to T_{LowerLimit}}= Cp_{T_{LowerLimit}}*\left ( T_{LowerLimit} - T_1 \right)\, [/math]
    • Similarly, if T2 > TUpper Limit then:
    [math] \Delta H _{T_{UpperLimit} \to T_2}= Cp_{T_{UpperLimit}}*\left ( T_2 - T_{UpperLimit} \right)\, [/math]


The user may choose from a number of different functions for the Cp value, as shown in the image below:

Species Editing Cp 11.png

Each of these will be described in the sections below:

Constant Cp value

The first 2 functions represent constant Cp values:

  • Constant Value; and
  • Constant Value with Temperature Range.

With these 2 options Cp does not vary as a function of temperature. However, with the option that includes a Range, the user may define a specific temperature range for the value.


Laliberte´ Cp Function

The user may choose to use the Laliberte´ method for calculating the specific heat of an aqueous species. This method uses the constants as calculated by Laliberte´ based on data obtained from many references.

The liquid specific heat is calculated using the water specific heat, Cpw and the solutes heat capacity using following equation:

[math]\mathbf{\mathit{Cp_m = m_wCp_w + \sum{m_iCp_i}}}[/math]


The heat capacity of each solute in solution is calculated from:

[math]\mathbf{\mathit{Cp_i= a_1e^{\alpha}+a_5(1-m_w)^{a_6}}}[/math]

Where

[math]\mathbf{\mathit{\alpha= a_2*T +a_3e^{0.01T}+a_4(1-m_w)}}[/math]


mw = mass fraction of water
mi = mass fraction of solute species i
Cpw = Heat capacity of water (at stream temperature and pressure), kJ/kg.K
Cpi = Heat capacity of solute i, kJ/kg.K
Cpm = solution Heat capacity, kJ/kg.K
T = Temperature in °C
a1 to a6 = dimensionless empirical constants for each solute species.

Notes:

  1. The constants for most aqueous species are valid for temperatures between approximately 5 and 1200C.
  2. If the unit temperature is outside of the species temperature range, then SysCAD will use the values at the temperature limit.
  3. Water heat capacity is calculated in SysCAD as described here: Water and Steam Properties.
  4. The user MUST define all 6 constants and also the valid range for the function.
  5. To remove the temperature dependence, set a2 = 0 AND a3 = 0 (or a1 = 0).
  6. Due to the method of solving, a2 = 0 is not allowed unless a3 = 0 or a1 = 0.
  7. This method calculates the Cp of a solution based on ALL of the aqueous species dissolved in the solution. Therefore, if the user wishes to use this method, then it is recommended that all aqueous species in a project use the Laliberte´ method for the results to be consistent.

Reference:

Laliberte´ M. A Model for Calculating the Heat Capacity of Aqueous Solutions, with Updated Density and Viscosity Data. J.Chem Eng. Data 2009.


Cp as a Function of Temperature - Table of Values

Select this option if Cp varies as a function of Temperature and the user has data points for this.

See the example below:

Species Editing Cp Table.png

Cp Equation Format (Single or Multi segment)

The user may define a single temperature range for the Cp value, SingleSegment, OR multiple temperature ranges, MutilSegment. In both cases, the Cp equations that are used have the same form, which are described further on in this section.


Single Segment

If the user selects 'SingleSegment' then they need only enter a single Cp equation and the range for that equation. The form of the equation can be any of those shown below.

Species Editing Cp3.png

The forms of the equations are explained in the sections below.

Multi Segment

If the user selects 'MultiSegment':

  1. They may enter a number of Cp equations covering different temperature ranges;
  2. Each Cp equation must be specified with the temperature range for which it is valid;
  3. Temperature ranges MUST NOT overlap;
  4. Temperature ranges MUST be contiguous, i.e. there must not be any gaps in the temperature ranges.
  5. Different forms of the Cp equation can be used for different temperature ranges.

See the example below:

Species Editing Cp 4.png

To insert a new equation before the first equation, select the first equation and press the Insert button.

To insert a new equation after the last equation, press the Append button.

To insert a new equation inbetween current equations, select the nearest equation and press the Insert button. An additional equation with the same parameters will be added after the selected equation. The previous temperature range for the selected equation will be split equally over the selected and new equation.

Cp Equation Format

The format for the Cp equation and corresponding Temperature Range is the same if the user has chosen single or multi segment. The format is as follows:

Cp Equation Name(a,b,...):Range(C or K, TL, TH)
Where:
  1. The each 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.

The valid Cp equation formats are given below.

NOTES

  1. All temperatures in the above formulae are in Kelvin. It does not matter if you give the range in units of C, K or F, the correlation, must use K.
  2. If you want to use a correlation for Cp that is given as a function of C or F, set up a spreadsheet to generate the data for the range you want and then curve fit the data in degrees K and enter that into SysCAD (NB you should be able to reproduce an exact fit of your original correlation).

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.

CRC Equation

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

CRC Equation (alternative format 1)

CRC1_Cp(a,b,c,d)
[math]C_p = a + b.10^{-3} T + c.10^{-6} T^2 + \frac{d.10^5}{T^2}\,[/math]
where T - Temperature in K

CRC Equation (alternative format 2)

CRC2_Cp(a,b,c)
[math]C_p = MolecularWeight * \Bigg( a + b.T - \frac{c}{T^2} \Bigg)\,[/math]
where T - Temperature in K

HTE Equation Format

HTE_Cp(a,b,c,d)
[math]C_p = 4.186 * \Bigg(b + 2*c.10^{-3} T - \frac{d.10^5}{T^2}\Bigg)\,[/math]
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)
[math]C_p = a + b.10^{-3} T + \frac{c.10^5}{T^2} + d.10^{-6} T^2\,[/math]
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,e)
[math]C_p = a + bT + cT^2 + dT^3 + eT^4\,[/math]
where T - Temperature in K

NOTE: The Poly_Cp will support upto 5 terms, user can ignore the later constants if the polynomial has lesser terms.

For example:

For Cp = 28.5 + 0.04T + 0.01T^2,
use Poly_Cp(28.5,0.04,0.01)

General Polynomial Equation format

GenPoly_Cp(c1,p1,c2,p2,c3,p3,c4,p4,c5,p5,...)
[math]C_p = c_1 T^{p_1} + c_2 T^{p_2} + c_3 T^{p_3} + c_4 T^{p_4} + c_5 T^{p_5} ...\,[/math]
where T - Temperature in K
Notes:
  1. This equation can have any number of terms, so the parameters are optional. You only need to enter as many parameters as are needed.
  2. For Polynomials with 5 terms or less, we recommend using the Polynomial Equation format (Poly_Cp) format.

Shomate Equation

Shomate_Cp(a,b,c,d,e)
[math]C_p = a + b.10^{-3} T + c.10^{-6} T^2 + d.10^{-9} T^3 + \frac{e.10^6}{T^2}\,[/math]
where T - Temperature in K
The Shomate Equation is used for many applications. An example is to fit data from the NIST web site for gas components with higher temperature ranges. eg Oxygen

Examples of Cp Equations

Species
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

Important Points about Cp

  1. Heat capacity cannot be zero (except for special case of H+(aq), see next point). 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: Species: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. H+(aq) should have a heat capacity of zero. In Build 137 or later, projects with H+(aq) which use a non-zero value will not load.
  3. 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".