Validate VLE / VLLE results with Phase Envelope Utility in PRODE

[hcostello]

recently I have been requested to verify a series of phase equilibria calc's in critical area for a mixture of 15 hydrocarbons + water, CO2, H2S

The mole fraction of water it's 0.01 and the water separates forming a second liquid phase.

The vapor-liquid-liquid phase envelope utility available in PRODE PROPERTIES allows to draw the whole phase diagram including the three phase line,

once the phase diagram has been generated it's possible to verify results from different flash operations available in PRODE PROPERTIES such as,

isothermal flash operations,
specified enthalpy and pressure or temperature
specified entropy and pressure or temperature
specified volume and pressure or temperature

the phase diagram, generated directly in Excel, includes the equilibrium points (t,p) on dew line, bubble line plus three phases lines plus cricondentherm, cricondenbar and critical points.

With these values one can solve a flash operation e verify if phase equilibria results are in accordance.
For this I have utilized Excel to create tables of values vs. several t, p starting from atmospheric pressure and up to cricondenbar.

PRODE PROPERTIES shows a reliable convergence also for points very close to critical (in my tests at distance of 0.05 K from critical point).

You may find additional information and download PRODE PROPERTIES in this thread
'http://www.egpet.net/vb/threads/27092-Excel-library-for-process-calc-s-including-distillation'

[carlo.stenali]

thanks for the information,
generally the phase envelope utility has reliable convergence in critical area being based on continuation method while other flash operations have local initializations...
I have utilized frequently the phase envelope utility in PRODE PROPERTIES to verify reliable convergence of isothermal flash (and other flash operations) in two (vapor-liquid) and three (vapor-liquid-liquid) phases area,
convergence of isothermal flash is robust however in some cases, in critical area, near some equilibrium line (dew or bubble) it detects a gas (or liquid) phase where there should be two,
this may happen at distances of a small fraction of degree from a equilibrium line,
but in some case the distances may be higher.
I can confirm (according my limited number of tests) that close to a critical point PRODE works well.
Another cause of problems could be in natural gas or petroleum streams the presence of high amounts of water,
PRODE shows (usually) a robust convergence,
but also in this case the phase envelope can be useful to detect problems.

[asanter]

thanks hcostello & carlo.stenali,
I have followed your procedure for a 13 components, natural gas mixture,
my tests were conducted in critical area at points near dew line and bubble line,
the calculated differences between the single phase condition (vapor or liquid) calculated by isothermal flash
and the points in dew or bubble line of VLLE phase diagram are about 0.1-0.2 K

[tinom]

thanks all for the contribute.
Engineers have the responsability to validate their calc's and the procedure described is useful in many cases.

[cbadia]

also you may consider the thermodynamic relations between different properties to validate the results,
as PRODE exports properties plus derivatives vs. P,T,W one can test congruence.

[abarre]

thank you very much for this information

[stawad]

thanks,
just a comment,
which are common applications where one wish to calculate the additional H-P, S-P V-P etc. diagrams ?
As I understand they are points on different lines of phase envelope, is this true ?

[carlo.stenali]

Stawad,
V,H,S are basic properties which you can utlize
to validate almost all flash operations,

PRODE PROPERTIES can solve many types of flash operations,
T,P specified temperature and pressure (isothermal flash)
Pf,P specified phase fraction and pressure
Pf,T specified phase fraction and temperature
H,P specified enthalpy and pressure
H,T specified enthalpy and temperature
S,P specified entropy and pressure
S,T specified entropy and temperature
V,P specified volume and pressure
V,T specified volume and temperature
V,H specified volume and enthalpy
V,S specified volume and entropy
H,S specified enthalpy and entropy

and includes true multiphase capability.