HYSYS Enthalpy Values
HYSYS Enthalpy Values
(OP)
I am curious as to why the values for enthalpy in HYSYS (Peng-Robinson with Lee Kesler Enthalpy Correction EOS) are negitive for typical natural gas. Does this mean that HYSYS compares the enthalpy to a refernce value because when I look up the enthalpy value in the API databook the value is always positive





RE: HYSYS Enthalpy Values
One thing that must be considered when using enthalpy differences as duties is that the differences have meaning only when there is no overall composition change, as there would be across reaction, unless all components involved in the reaction have zero reference enthalpy on the same basis. With the 1980 API TDB system, any reaction involving any of the ten light compounds having different zero basis automatically would give a problem. Regardless of zero enthalpy basis used, net heat of reaction can be calculated by getting enthalpy change to bring all reactants to the standard state for which heats of formation are defined, getting the heat of reaction by changes in heats of formation, adding those two heat terms (observing sign), and using the sum to heat the effluent composition from the standard state to effluent conditions. Either the shortfall is heat added or the excess is heat removed from the system. Either way, one can't always take difference between feed and effluent enthalpies as being net heat of reaction; that assumes too much about zero enthalpy basis for the compounds involved.
HTH, although it may have been more than you wanted.
Dick Russell
RE: HYSYS Enthalpy Values
I guess I find the absoulte values easier to understand cause I can see and reproduce the calculations that lead to them. As for HYSYS I just can't see any calculations.
RE: HYSYS Enthalpy Values
RE: HYSYS Enthalpy Values
Let this be a lesson to those who read this post that these simulators are not ment to spew forth thermodynamic properties but to simulate unit operations. Sometimes its just better to do it by hand