06-321 ChemE Thermodynamics
Bruno A. Calfa
Last Update: August 27th, 2014
Basics of Flash Calcula:ons Important applica:on of VLE. Liquid at a pressure equal to or greater than its bubble point
pressure flashes or par:ally evaporates when the pressure is reduced, producing a two-phase system of vapor and liquid in equilibrium.
Consider a P-T flash, i.e., both pressure and temperature are specified as well as the inlet (overall) composi:on.
Modeling (I/II) Total mole balance Component mole balance Phase-equilibrium ra:o In general, the K-values (Ki) depend on composi:on, T, and P.
Modeling (II/II) Combining equa:ons and given that yields:
This is a nonlinear algebraic equa:on. Note that only zi is specified. If the K-values are independent of
composi:ons (e.g., Raoults Law assump:on), then a simple itera:ve solu:on approach can be used to solve for the ra:o V/F. Root-finding methods, such as Newton-Raphson and bisec:on, can be employed.
A more convoluted itera:on procedure is needed if the K-values also depend on composi:ons (rigorous models).
Numerical Example Mixture: (1)methanol + (2)1-propanol + (3)acetone. P = 101.325 kPa, T = 343.15 K. z1 = 0.4, z2 = 0.2, z3 = 0.4. Approximate Method
Assume Raoults Law applies (use Antoine equa:on to calculate satura:on pressures).
Use MATLABs func:on fsolve.
Rigorous Method Aspens flash unit opera:on with an appropriate rigorous thermodynamic
See MATLAB script M-file FlashExampleApproximate.m. Solu:on
V/F = 0.656 f(V/F) = 2.583 108
Component yi xi methanol 0.427 0.348
1-propanol 0.116 0.360
acetone 0.457 0.292
Rigorous Method Add components.
Rigorous Method Select thermodynamic model. Use Method Assistant. NRTL seems to be
appropriate. Go to the subfolder Parameters under Methods to confirm that you will be using Aspens built-in binary interac:on parameters.
Rigorous Method Switch to Simula:on environment. Add flash unit opera:on (Flash2). You may rename the block (right click -> Rename Block).
Rigorous Method Click on Material. Red arrows are mandatory. Blue arrows are op:onal.
Rigorous Method Draw streams. You can rename streams (right click -> Rename Stream).
Rigorous Method Configure feed stream. Go to folder Streams, then 1 (feed stream). Use same
condi:on of feed for this example.
Rigorous Method Configure flash unit. Go to folder Blocks, then B1.
Rigorous Method Run simula:on. Click on blue play bukon. It converged! We are not
performing economic evalua:ons at this point, so click Close.
Rigorous Method Check results (Stream Summary).
Rigorous Method To see molar frac:ons, go to folder Setup -> Report Op:ons -> Streams.
Check Mole for Frac:on basis.
Rigorous Method Rerun simula:on. View Streams (Custom). Remember that stream 2 is the
vapor stream, and stream 3 is the liquid stream.
Summary: Approximate vs. Rigorous
Component yi xi Approx. Rigorous Approx. Rigorous
methanol 0.427 0.427 0.348 0.318
1-propanol 0.116 0.128 0.360 0.422
acetone 0.457 0.445 0.292 0.260
More considerable devia:ons in the liquid stream composi:on. Ideal solu:on (Raoults Law) may not be a good assump:on for this system. Need rigorous ac:vity coefficients models.
See file FlashExampleRigorous.apwz.
References  Green, D. W. (Editor). (2008). Perrys Chemical Engineers Handbook. McGraw-Hill Professional. New York, NY. USA.  Seader, J. D.; Henley, E. J.; & Roper, D. K. (2011). Separa:on Process Principles: Chemical and Biochemical Opera:ons. John Wiley & Sons, Inc. Hoboken, NJ. USA.