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DIP 10.ppt

Separation of a mixture of methanol + water

• Property Method Selection • Model parameters• Experimental data correlation• Optimization • Columns specifications

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The problem...

Separate a 100 kmol/h stream mixture of methanol + water in an atmospheric column.

• Property Method: a collection of methods and models to evaluate physical properties and phase equilibrium.

• The first step is choosing the appropriate property method.

DIP_10.ppt p. 3

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Property Method Selection

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• Main characteristic: phase equilibria calculation. Classification:

• IDEAL• EOS (PR, RK, BWR,....)• Activity coefficient (NRTL, UNIFAC, ...)• Electrolites (ELECNRTL, PITZER)• Special methods (AMINES, STEAMNBS,...)

• Selection: Help

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1

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2

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3

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More about Property Methods ... APrSystem Help: /Data /Properties [F1]

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Separation of a mixture of methanol + water in an atmospheric column:

Open a new file (General with metric units) and define components (METHANOL, WATER) and Property Method (NRTL).

Polar ?

Electrolyte ?

Yes

P < 10 bar ?

No

ij ?

Yes

LL ?

Yes

Wilson, NRTL, UNIQAC

No

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How does the Property Method calculate a specific property?

The properties used in the simulation are classified into categories depending on how they are evaluated:

Some general concepts:

1. MODELS

2. METHODS

3. ROUTES

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How does the Property Method calculate a specific property?

The properties used in the simulation are classified into categories depending on how they are evaluated:

1. MODELS: models calculate properties by direct use of an empirical expression or a theoretical model. For example:

GAMMA: liquid mixture activity coefficients.Possible MODELS: GMRENON: NRTL original

GMUFAC: UNIFAC

GMUFDMD: UNIFAC, modif. Dormund

.... (up to 32 MODELS)

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2. METHODS: in this case the property is evaluated using a thermodynamic expression where required properties are evaluated with MODELS. Different thermodynamic expressions (METHODS) may be available to evaluate a specific property.

For example, four METHODS are available to evaluate the enthalpy of a liquid mixture (HLMX) :

METHOD 1: Empiric expressionHLMX = f(T, P, x_i,

parámetros)

METHOD 2: From the ideal liquid mixture enthalpy (Hi*L) y the excess

enthalpy (HEX).HLMX = Σxi Hi

*L + HEX

METHOD 3: From ideal gas mixture enthalpy (Hmig) and departure

enthalpy (Hdepart):HLMX = Hm

ig + Hdepart

METHOD 4: ElectrolytesHLMX = f(xTRUE)

How does the Property Method calculate a specific property?

The properties used in the simulation are classified into categories depending on how they are evaluated:

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3. ROUTES: The group of specific METHOD and MODELS selected to evaluate a property is named a ROUTE.

How does the Property Method calculate a specific property?

The properties used in the simulation are classified into categories depending on how they are evaluated:

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• /Properties / Property Methods /NRTL: Routes , Select HLMX, View

View a ROUTE description

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Types of properties depending on their use and how they are calculated:

MAJOR: Are those properties required for calculations by unit operation models. They ere evaluated according to ROUTES, and may depend on other MAJOR, SUBORDINATE and INTERMEDIATE properties .

Properties: PHI, H, G, S, V, MU, K, D, SIGMA (Pure and mixtures)

SUBORDINATE: these properties are also calculated using ROUTES, but are not directly required for unit operation model calculations. They are used to calculate MAJOR properties.

Ex.: HLXS (excess enthalpy), ...

INTERMEDIATE: calculated by MODELS, they are used to calculate MAJOR and SUBORDINATE properties.

Ex.: GAMMA (activity coefficients),...

How does the Property Method calculate a specific property?

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• /Properties / Property Methods /NRTL: Models , Select GMRENON

Check a MODEL

F1

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Model ParametersReid,Prausnitz y Pooling, The Properties of Gases & Liquids, 4th Ed, Cap. 8, p. 380-1.

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The recommended order of use of the parameters (the same one used by Aspen): 1) experimental values (parameters or data correlation) 2) bibliographic (literature or Aspen database) 3) Estimates (PCES)

Points 2 and 3 have already been presented in previous classes.

• Check parameters: /Properties /Parameters /Binary interaction /NRTL-1• Check residuals from correlation (press button [Reg. Info] after selecting the parameter)• Check parameters available from other Databanks.

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• Add a second model (/Properties /Specifications: Referenced -> WILSON) and compare residuals with NRTL model (F1).

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Experimental data correlation

Correlate equilibrium TXY values at 760 mmHg: Ramalho R.S. et al, Ind.Eng.Chem. 53, 89 (1961)

• Change Run Type in Setup form to Data Regression.• /Properties /Data: New

• ID: D-1• Select type: MIXTURE• Write literature citation in the comments form• Write data

T (ºC) xMEOH yMEOH95.2 0.0293 0.183189.2 0.0948 0.414277.6 0.3004 0.688274.0 0.4564 0.766865.7 0.9293 0.9771

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• /Properties /Regression: New

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Help on NRTL parameters (F1)

DIP_10.ppt p. 25

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• Modify the selection of parameters to be regressed: aij = 0bij regresscij = 0.30

Metanol+Agua_01.apw

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• Run: confirm parameters replacement when asked.

Metanol+Agua_02.apw

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NRTL parameters have been replaced in /Properties /Parameters /Binary Interaction /NRTL-1

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Plot using Plot Wizard from form: /Regression /R-1 /Results: Profiles

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Check other results in /Regression /R-1 /Results

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Selection criteria:

• Choose the model giving the lowest Residual Root Mean Square.

Non-trustable regressions:

• A standard deviation for a regressed parameter is 0.0, indicating the parameter is at a bound.

• A large residual root mean square error value. Normally, this value should be less than 10 for VLE data and less than 100 for LLE data.

• Your VLE data fail the thermodynamic consistency test.

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Optimization

• /Setup: Run Type -> Flowsheet.• Draw PFD, stream 1 and column.

It is intended to separate a 100 kmol/h stream containing a mixture of MeOH + water (60 mol% MeOH). The mixture is 1 bar and it is at its boiling point. Use first a shortcut (DSTWU) to design a tower with a recovery of 99% for further optimization.

Corriente 1

Metanol+Agua_03.apw

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Results...

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Operation will be optimized using an objective function to minimize total costs:

. . .. . B CN Q Q

F OLKR

2

0 012 8 3 4 6

, where N is the number of theoretical stages, QB the reboiler duty, QC the condenser duty and LKR is the light key recovery. Perform a sensitivity analysis to study the behavior of the tower prior to the optimization.

• /Model Analysis Tools /Sensitivity: New, S-1

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Results:

Metanol+Agua_04.apw

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• /Model Analysis Tools /Sensitivity: S-1, Hide• /Model Analysis Tools /Optimization: New, O-1

Optimization:

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/Convergence/Convergence/$OLVER01/Results

Metanol+Agua_05.apw

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NOTE: although the optimization block is hidden (Hide) or deleted (Delete) the value of the optimized variable is preserved, if not ‘Reinitialize' is performed. This is because the optimized value of the variable is retained in the results form.

/Blocks /D-110 /Input /Blocks /D-110 /Results

0.49601.093

0.4536mín

RR

RR

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• /Model Analysis Tools /Optimization: O-1 -> Hide• Write calculated design and operating variables: 31 stages, feed stage=20, D/F=0.598,

RR=0.4960• Change to RADFRAC• /Setup /Report Options /Stream: Fraction Basis – Mole: ON

Replace the shortcut (DSTWU) for a rigorous (RADFRAC) unit operation.

Resultados

Metanol+Agua_06.apw

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• /Blocks /D-110 /Design Specs: New = 1• Specify 99.2%mol MeOH in stream 2

Let's see now, forgetting the optimization, how purity may be specified in distillate and bottoms, allowing the software evaluate operating variables.

/Blocks /D-110 /Design Specs /1

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• /Blocks /D-110 /Vary: New = 1• Allow RR vary between 0.1 y 10

/Blocks /D-110 /Vary /1

• Run

/Blocks /D-110 /Design Spec /1 /Blocks /D-1101 /Vary /1

Metanol+Agua_07.apw

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• /Blocks /D-110 /Design Specs: New = 2• Specify 99%mol WATER in stream 3

Specify now 99%mol WATER in stream 3, varying another column specification

/Blocks /D-110 /Design Specs /2

/Blocks /D-110 /Vary /2

Metanol+Agua_08.apw

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/Blocks /D-110 /Vary /2

/Blocks /D-110 /Results /Blocks /D-110 /Stream Results