Excellent advice, PaoloPemi. Step 1 is always the selection of the best thermo/VLE model for the problem.

Nicole, I have Aspen Plus V10 and I was researching your problem using the Methods Assistant. It seems to me that HYSGLYCO was tuned for Triethylene glycol/water pair at temperatures lower than your range in your pressure range and at pressures lower than your range in your temperature range:

"The Glycol property package should be applicable over the range of temperatures, pressures, and component concentration encountered in a typical TEG-water dehydration system: between 15°C to 50°C and between 10 atm to 100 atm for the gas dehydrator, and between 202°C to 206°C and 1.2 atmospheres for the glycol regenerator."

The Cubic-Plus-Association EOS may be a bit better and a more general purpose method if you have it in your version:

"The CPA method represents the Cubic-Plus-Association EOS modeldeveloped by Kontogeorgis and co-workers (Kontogeorgis, Voutsas, Yakoumis, Tassios, IECR 1996). The model combines the SRK cubic EOS with an association term similar to that of SAFT, as present in the PC-SAFT model. Mixing rules apply to the cubic, whereas combining rules are used for the association term. The model’s applicability covers the VLE and VLLE of mixtures containing hydrocarbons and polar/associating chemicals such as water, alcohols, glycols, esters, and organic acids."

The CPA method seems to have been developed to handle hydrogen bonding, which is exactly your problem's main non-ideality.

Using CPA in your problem definition, I got a saturation pressure of 226.56 psia at 202 C. Only 6 psi greater than your saturation pressure; 3% difference. Not much to worry about here, IMO. Paolo, can you double check that 270 psia saturation pressure?

Anyway, using my CPA properties, I used my company's HEM PSV software, similar to Breizh's article above, and got the following:

EMMA

Estimate of flow rate with 2 data states.

Areas and coefficients for nozzles.

Reservoir pressure = 427.20 psia

Surrounding pressure = 187.00 psia

Nozzle area = 0.503 in2

Nozzle coefficient = 0.4910

==================== PROPERTIES ====================

DATA STATE A B

========== ======= =======

Pressure, psia 226.60 187.00

Wt. fraction gas 0.00000 0.01993

Gas density, lb/ft3 0.51020 0.42360

Liquid density, lb/ft3 53.650 54.260

====================================================

REAL NOZZLE RESULTS:

Mass velocity = 122288.0 lb/hr/in2

Mass flow rate = 61511 lb/hr

Stored energy = 1287.18 Btu/ft3

NOZZLE THROAT:

Pressure = 226.57 psia

Thrust / area = 136.0 psi

Velocity = 91.3 ft/s

Wt. frac. gas = 0.00001

And the next API nozzle size down (0.307 in^{2} = 198 mm^{2}):

EMMA

Estimate of flow rate with 2 data states.

Areas and coefficients for nozzles.

Reservoir pressure = 427.20 psia

Surrounding pressure = 187.00 psia

Nozzle area = 0.307 in2

Nozzle coefficient = 0.4910

==================== PROPERTIES ====================

DATA STATE A B

========== ======= =======

Pressure, psia 226.60 187.00

Wt. fraction gas 0.00000 0.01993

Gas density, lb/ft3 0.51020 0.42360

Liquid density, lb/ft3 53.650 54.260

====================================================

REAL NOZZLE RESULTS:

Mass velocity = 122242.0 lb/hr/in2

Mass flow rate = 37528 lb/hr

Stored energy = 1287.18 Btu/ft3

NOZZLE THROAT:

Pressure = 226.57 psia

Thrust / area = 135.9 psi

Velocity = 91.3 ft/s

Wt. frac. gas = 0.00001

The next size down API nozzle size (0.196 in^{2} = 126 mm^{2}) was too small.

Notice I used the liquid coefficient, because in both cases my software predicts the pressure at the nozzle exit is essentially the saturation pressure with almost no vapor (Wt. frac. gas = 0.00001).

Now, I realise this mixes the methods I used with your methods and two-phase backpressure calculations, but it was the best I could do quickly. The real answer will be a convergence of the PSV and tailpipe calcs providing the backpressure on the PSV outlet using the same methods and property model. I hope this helps you.

**Edited by latexman, 15 January 2021 - 02:41 PM.**