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Precision Problems In Aspen

rgibbs aspen precision equilibrium

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#1 joshkt

joshkt

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Posted 04 February 2015 - 10:40 PM

Hi there,

 

I am attempting multiphase equilibrium calculations using the RGIBBS reactor in Aspen Plus. Specifically, I am interested in calculating the difference in the Gibbs energy of the system between the initial and equilibrium states. Currently, I am doing this by getting Aspen to calculate the Gibbs energy of the mixture (i.e. the physical property GMX) in the inflow and outflow material streams, and then differencing these two numbers to obtain the Gibbs energy change. However, because I am differencing two very large, similar numbers, I've discovered that Aspen's precision is insufficient to calculate the difference accurately. The Gibbs energy of the inflow and outflow streams are only given to 4 or 5 significant figures. Of course, I can easily change the number of significant figures displayed in the stream output, but this isn't helpful since the additional figures are all zeros. 

 

I was wondering if there is any way of getting Aspen to report more precise Gibbs energies. Presumably this is doable since the RGIBBS reactor works by minimizing Gibbs energy, and so Aspen must calculate these quantities more precisely than it is displaying.

 

Many thanks,

Josh



#2 Pilesar

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Posted 05 February 2015 - 01:56 PM

Have you tried using a Calculator block to let Aspen calculate the difference directly? The software likely carries more significant figures internally.  



#3 joshkt

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Posted 05 February 2015 - 11:50 PM

The calculator block works; thanks very much!






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