In case you wouldn't know: The retrograde dew point of a mixture is the pressure where for a given temperature a decrease in pressure causes the vapour to condense. This is in contrast with the normal dew point which is the pressure where for a given temperature an increase in pressure causes the vapour to condense. This is best seen when looking at the phase envelope of a mixture.
A similar behaviour can be seen for a fixed pressure. in the envelope below, at 100 barg there is a dew point at around -30 C and another one at around 30 C.
The question is, how do you get the retrograde dew point pressure without having to read it from a plot? In a HYSYS stream, when you specify a temperature and a vapour fraction of 1, the result will be the dew point pressure. To get the retrograde dew point pressure, replace the vapour fraction value with -1. It's that simple. For a retrograde dew point temperature, specify the pressure and set the vapour fraction to -1.
Of course, physically a vapour fraction of of minus one doesn't make sense, it is just a trick to tell HYSYS you want the retrograde instead of the normal dew point.
A similar behaviour can be seen for a fixed pressure. in the envelope below, at 100 barg there is a dew point at around -30 C and another one at around 30 C.
The question is, how do you get the retrograde dew point pressure without having to read it from a plot? In a HYSYS stream, when you specify a temperature and a vapour fraction of 1, the result will be the dew point pressure. To get the retrograde dew point pressure, replace the vapour fraction value with -1. It's that simple. For a retrograde dew point temperature, specify the pressure and set the vapour fraction to -1.
Of course, physically a vapour fraction of of minus one doesn't make sense, it is just a trick to tell HYSYS you want the retrograde instead of the normal dew point.
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