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Friday, July 6, 2007

LOWER HEATING VALUE

Is there any different between MASS lower heating value (kJ/kg) and lower heating value (gas)(VOLUMETRIC) (MJ/m3) in Aspen HYSYS ?
Yes...there are some differences...

MASS lower heating value (kJ/kg)
There are indeed two representations of Lower Heating Values in Aspen HYSYS. By default, HYSYS calculates the LHV on a molar basis as the sum of the heat of combustion (at 25C and 1 atm) multiplied by the mole fraction for each component in a mixture :

LHV = sum(x[i] * Heat_of_Combustion[i])

The heat of combustion in the above equation is a pure component property that comes directly from the HYSYS database. As a consequence, HYSYS will report if the Heat of Combustion is not specified for one of the components (i.e. hypothetical components that by default do not have a value specified).
In the events we do not know the Heat of combustion for hypothetical components, i may always suggest to represent the hypothetical compenent with nearest Molecular weight component in HYSYS standard library. From Science perspective, these figures are different but from engineering point of view, there are more or less similar...make equilavent between them is acceptable.
Lower heating value (gas)(VOLUMETRIC) (MJ/m3)
The second representation of the Lower Heating Value is calculated on a volumetric basis in much the same way as above, but with data from ISO 6976:1995(E). This calculation includes data for the following components (at 15C and 1 atm) :

Ammonia, Argon, CO, CO2, H2S, H2O, Hydrogen, Oxygen, Nitrogen, n-alkanes (C1 through n-C30, including i-C4 and i-C5), Ethylene, Propene, Cyclopentane, Cyclohexane, M-Cyclopentane, M-Cyclohexane, Benzene, Toluene, E-Benzene, 124-M-Benzene, Methanol, EGlycol, TEGlycol.
If the stream contains components other than these, then data for the hydrocarbon with the nearest molecular weight is used. If the molecular weight is greater than decane, then the data for decane is assumed.









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posted by Webworm, 1:00 AM

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