The issue of negative enthalpy values in Aspen
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I have also been troubled by this issue. Recently, while studying theories related to combustion (I major in thermal energy), when I encountered the concepts of absolute enthalpy and standard enthalpy of formation, I felt that the enthalpy values in Aspen must refer to absolute enthalpy, as shown in the formula. Absolute enthalpy = formation enthalpy + apparent enthalpy. The formation enthalpy is negative, while the apparent enthalpy depends on temperature: it is positive when the temperature is higher than the standard condition temperature, and negative otherwise. Therefore, it is not surprising that the enthalpy values calculated in Aspen can be either positive or negative. Another example: the standard enthalpy of formation of water vapor is -241,845 kJ/mol, and the change in enthalpy is at most a few thousand units; therefore, the absolute enthalpy of water remains negative. This is why the enthalpy values of water calculated in Aspen are all very large negative numbers.
You are absolutely right; the enthalpy value in Aspen is calculated based on the definition of absolute enthalpy. Absolute enthalpy consists of formation enthalpy and apparent enthalpy; the formation enthalpy is generally negative, while the apparent enthalpy varies with temperature and can be either positive or negative. When the temperature is below the standard condition, the apparent enthalpy can be negative, which may result in a negative overall enthalpy value. Taking water vapor as an example, its standard enthalpy of formation is indeed low, which results in the absolute enthalpy value of water in Aspen usually being a large negative number. This is normal in thermodynamic calculations, especially for certain compounds. .
It’s all about the enthalpy difference; only the reference point is different.