# Aspen v8 Tutorial - Flash Calculation - v8 Tutorial - Flash – Aspen’s’ﬂash’unitoperaon’with’an’appropriate’rigorous’thermodynamic’ model.’ 5. ... Aspen v8 Tutorial - Flash Calculation

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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.

2 [1]

• 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., 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).

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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 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

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) = 2.583 108

Component yi xi methanol 0.427 0.348

1-propanol 0.116 0.360

acetone 0.457 0.292

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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 Aspens 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 (Raoults 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). Perrys 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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