Specific heat capacity of water

[avinashresearch1]

Hi. Thanks for the excellent and extremely useful package!

I am trying to get a better understanding of querying for properties using Unitful but I am uncertain about the units of the results (it says J/K-1 but I am unable to get the expected specific cp even converting from moles:

using Unitful
import Unitful: bar, °C
water_model = SAFTVRMie(["water"]) # Also tried a few others e.g `PR` and `IAPWS95`.
cp = isobaric_heat_capacity(water_model, 1bar, 25°C)

[pw0908]

Hi! Thank you for the nice words!

For heat capacities, the output is using extensive units. You can see this by changing the molar amount:

julia> using Clapeyron, Unitful
julia> import Unitful: bar, °C, mol
julia> model = SAFTVRMie(["water"]);
julia> isobaric_heat_capacity(model,1bar,25°C,[1.0mol])
75.3275793331559 J K⁻¹
julia> isobaric_heat_capacity(model,1bar,25°C,[2.0mol])
150.65515866633206 J K⁻¹

Some functions which do provide intensive properties are molar_density and mass_density:

julia> molar_density(model,1bar,25°C)
55344.55768394381 mol m⁻³

julia> mass_density(model,1bar,25°C)
997.047039017707 kg m⁻³

We certainly should try to support the intensive versions of a few of the properties provided in Clapeyron...

[avinashresearch1]

Thanks for the quick reply! I was able to get it to work with IAPWS95, but quite surprising the variation in results. Good reminder to pick good defaults :)

julia> using Clapeyron
julia> using Unitful
julia> import Unitful: bar, °C, mol, kg
julia> p_test = 1bar
1 bar
julia> T_test = 25°C
25 °C
julia> model = IAPWS95()
IAPWS95 with 1 component:
 "water"
julia> mw = model.consts.Mw
18.015268

julia> isobaric_heat_capacity(model, p_test, T_test,[1.0mol])/mw
4.181318830958147 J K^-1

julia> model_2 = SAFTVRMie(["water"])
SAFTVRMie{BasicIdeal} with 1 component:
 "water"
Contains parameters: Mw, segment, sigma, lambda_a, lambda_r, epsilon, epsilon_assoc, bondvol

julia> isobaric_heat_capacity(model_2, p_test, T_test,[1.0mol])/mw
2.948594793493912 J K^-1

julia> model_3 = GERG2008(["water"])
GERG2008 with 1 component:
 "water"

julia> isobaric_heat_capacity(model_3, p_test, T_test,[1.0mol])/mw
4.404685005385272 J K^-1

julia> model_4 = PCSAFT(["water"])
PCSAFT{BasicIdeal} with 1 component:
 "water"
Contains parameters: Mw, segment, sigma, epsilon, epsilon_assoc, bondvol

julia> isobaric_heat_capacity(model_4, p_test, T_test,[1.0mol])/mw
2.363334753223272 J K^-1

[pw0908]

Almost forgot! In the case for non-empirical equations of state (I.e. SAFTs or cubics), the ideal contribution doesn't include vibrations and rotations, which leads to significant underestimates of the heat capacity. See below:

julia> model = SAFTVRMie(["water"]);�������
julia> isobaric_heat_capacity(model,1e5,298.15)/model.params.Mw[1]
2.948638658240215

julia> model = SAFTVRMie(["water"];idealmodel=ReidIdeal);

julia> isobaric_heat_capacity(model,1e5,298.15)/model.params.Mw[1]
3.663218112417885