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06-‐321 ChemE Thermodynamics
Flash Calcula:on
Bruno A. Calfa
Last Update: August 27th, 2014
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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.
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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.
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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., Raoult’s 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).
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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 Raoult’s Law applies (use Antoine equa:on to calculate satura:on pressures).
– Use MATLAB’s func:on fsolve.
• Rigorous Method – Aspen’s flash unit opera:on with an appropriate rigorous thermodynamic
model.
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Approximate Method
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• See MATLAB script M-‐file FlashExampleApproximate.m. • Solu:on
– V/F = 0.656 – f(V/F) = 2.583 × 10−8
Component yi xi methanol 0.427 0.348
1-‐propanol 0.116 0.360
acetone 0.457 0.292
Rigorous Method • Add components.
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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 Aspen’s built-‐in binary interac:on parameters.
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Rigorous Method • Switch to Simula:on environment. Add flash unit opera:on (Flash2). • You may rename the block (right click -‐> Rename Block).
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Rigorous Method • Click on Material. Red arrows are mandatory. Blue arrows are op:onal.
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Rigorous Method • Draw streams. You can rename streams (right click -‐> Rename Stream).
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Rigorous Method • Configure feed stream. Go to folder Streams, then 1 (feed stream). Use same
condi:on of feed for this example.
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Rigorous Method • Configure flash unit. Go to folder Blocks, then B1.
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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.
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Rigorous Method • Check results (Stream Summary).
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Rigorous Method • To see molar frac:ons, go to folder Setup -‐> Report Op:ons -‐> Streams.
Check Mole for Frac:on basis.
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Rigorous Method • Rerun simula:on. View Streams (Custom). Remember that stream 2 is the
vapor stream, and stream 3 is the liquid stream.
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Summary: Approximate vs. Rigorous
Approximate Rigorous
0.656 0.756
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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
V/F
• More considerable devia:ons in the liquid stream composi:on. Ideal solu:on (Raoult’s Law) may not be a good assump:on for this system. Need rigorous ac:vity coefficients models.
• See file FlashExampleRigorous.apwz.
References [1] Green, D. W. (Editor). (2008). Perry’s Chemical Engineers’ Handbook. McGraw-‐Hill Professional. New York, NY. USA. [2] 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.
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