3 replies to this topic

[ankur2061]

Dear all,

Most of us nowadays depend on simulators (HYSYS, Pro II, Chemcad & others) for obtaining flash vapor & liquid compositions by modelling a separator with an inlet stream of a given composition and pressure and temperature. A vapor stream & liquid stream from the separator will give the vapor and liquid compositions respectively. However, the actual mechanism & equations for isothermal flash in simulators is not known to many. The attached spreadsheet is a humble attempt to provide flash calculations using a set of algebraic equations. The spreadsheet is basically provides the solved example given in the reference book from which the spreadsheet is developed.

Comments are most welcome.

Regards,
Ankur.

Attached Files

•  Flash_composition.xls   212KB   985 downloads

[jaffrey]

I'm a second year chemical engineer and we're learning all the flash calculations by hand at the moment, whilst learning Hysis alongside. What I find most challenging are adiabatic flash calculations which requires the iterative method for solving problems involving enthalpies. Is there any further help regarding these sort of calculations. Thanks for the information above.

[mbeychok]

ankur2061:

Your spreadsheet performs the flash calculation for a multicomponent liquid by using the Rachford Rich equation which is well and good.

However, the flash calculation for a single component liquid (i.e., propane, chlorine, etc.) is much simpler.

Read this article in the online Citizendium encyclopedia which covers both single component and multicomponent flash calculations:

Flash evaporation

[PaoloPemi]

actually many simulators do use Newton solvers or minimizers, these are more reliable in critical region, for these solvers you need the derivatives (fugacity, activity etc.) you can calculate the derivatives by numerical differentiation, with methods as Broyden or analytically.
Unfortunatelly Excel VBA language is not the best to code a good solver but you can access a external library which provides the derivatives or the complete solver for two phases or multiphase, there are free tools which do that, see for example Properties at "www.prode.com" for details.