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Rigorous column simulation - SCDS

Problem & solution principle:

A rectification can be realistically displayed by performing a rigorous column simulation. It is

possible to simulate ideal and real mixtures. A detailed calculation of the rectification column

that provides more exact results is performed based on a tray-to-tray balance. Two rigorous

column types are available in CHEMCAD: TOWR and SCDS. In the following, we will introduce

the SCDS column.

In this tutorial, we will analyse a three-substance mixture consisting of benzene, acetone and

cyclohexane. This mixture is to be separated by means of rectification, and pure benzene of

approx. 99 mol% shall be obtained in the process. In addition, at least 99 mol% of the benzene

added in the feed are to be recovered. A mixture of

64 mol% acetone, 18 mol% benzene and cyclohexane is fed into the column. The simulation is

performed with an SCDS column in CHEMCAD.

Figure 1: Flow sheet SCDS column

http://www.chemstations.eu/en/tutoriallp


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Implementation of the SCDS simulation in CHEMCAD

The simulation is performed with CHEMCAD Steady State. Prior to the simulation, the

components and the thermodynamic model must be selected. At "Thermophysical: Select

Components", the components benzene (CAS no.: 71-43-2), acetone (CAS no.: 67-64-1) and

cyclohexane (CAS no.: 110-82-7) are selected. The subsequent "Thermodynamics Wizard"

suggests a suitable model after specification of the pressure and the temperature. For the given

example, CHEMCAD recommends the k-value model NRTL. For the enthalpy model, LATE

(latent heat) is suggested. This selection is a preselection made by the program, and should

always be verified by the user or synchronised with a decision diagram ([3], figure 8/9).

After closing the "Thermodynamics Wizard", the NRTL Parameter window opens (figure 2). It

lists the interaction parameters (BIPs: Binary Interaction Parameters) of each possible two-

substance mixture of the previously selected components. For the stated example, the two-

substance mixtures benzene/cyclohexane and acetone/cyclohexane are listed. Data for the

acetone/benzene mixture are missing. The missing NRTL data must be added; otherwise it is

assumed that the activity coefficient is one, followed by an ideal calculation of the respective

substance pair [6, chapter VLE]. The missing values can be calculated with UNIFAC and added.

There are three options to do this: "UNIFAC VLE", "UNIFAC LLE" and "modified UNIFAC".

Figure 2: "NRTL Parameter Set" window

For the stated example, the missing data is calculated using "UNIFAC VLE" and added. In case of

a miscibility gap, "UNIFAC LLE" must be selected. "Modified UNIFAC" should be selected for

high pressures and high temperatures.



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Prior to each simulation, the behaviour of the mixture must be examined in more detail to

determine possible rectification limitations (example: azeotrope, distillation limitations,

miscibility gaps). To detect miscibility gaps, the equilibrium diagram should be checked first for

each binary substance mixture at "Plot: TPXY". We can see that there is no miscibility gap. A

rough estimation concerning miscibility can also be made via the molecular structure. However,

if there is a miscibility gap, the vapour-liquid-liquid equilibrium must be selected at Global

Phase Option (Thermodynamic Settings, option: Vapor/Liquid/Liquid/Solid).

The residue curve is generated at "Plot: Residue Curves" (figure 3).

Figure 3: Residue curve of the three-substance mixture benzene, acetone and cyclohexane

The residue curve provides information about the present azeotropes and about their boiling

points and those of the pure components. Figure 3 shows that there are two azeotropes in the

three-substance mixture, and that possible distillation limitations can thus be identified.

In the examined example there is a distillation limitation between the binary azeotropes that

cannot be exceeded during a rectification. Hence two distillation regions develop in which

different bottom components can be obtained depending on the feed composition.

The given feed (red dot) is located in the right distillation region. The low boiler, the azeotrope

of cyclohexane and acetone, with a boiling point of 53.85°C is located in this region. The

azeotrope is expected as top component. Benzene is the high boiler (boiling temperature

80.09°C) and removed as bottom component.

With the residue curve, it is possible to estimate possible top and bottom components

beforehand and select expedient rectification parameters accordingly.



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Table 1: Data of relevance for the simulation

Units Components Thermo-

dynamics

Feed

streams

Unit operations

SI Benzene

Acetone

Cyclohexane

K: NRTL, H: LATE

1 SCDS column

1 feed stream

2 product

streams

The UnitOp (unit operation) of the SCDS column is entered in the flow sheet and allocated a

feed stream and two product streams. The feed stream is set to liquid boiling with the data

stated in table 1 (see figure 4).

Figure 4: Feed settings window



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Next, we will initialise the rectification column. The rectification parameters are required along

with the number of stages and the feed tray.

The number of stages and the feed tray are usually not known at the beginning and must be

estimated at this point. Later on, these can be optimised via a sensitivity study.

In this example, the indicated number of stages is 30 and the feed tray is to be located at half,

N=14 (figure 5). In CHEMCAD, the condenser and the evaporator each count as an individual

stage. Countdown is done from top to bottom.

Figure 5: Settings window SCDS column

By means of the residue curve, it has already been possible to discern the top or bottom

component to be expected. To verify this assumption, it is advisable to initially simulate the

column with infinitive reflux and assess the behaviour of the rectification column. In the

specifications, the reflux ratio is set to 1000 as top criterion. The mass flow is selected as

bottom criterion. Due to the infinite reflux ratio, 1000 kg/h will be discharged via the bottom.

The specification of the reflux ratio at the top and a discharged bottom mass flow are the

standard conditions with which the behaviour of a column should be examined. The settings

are illustrated in figure 6.



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Figure 6: Settings of the rectification column with infinitive reflux

The SCDS column is supposed to converge for the selected settings. At "Format: Add Stream

Box" it is possible to output a value chart with the properties of the streams (figure 7).

Figure 7: Value chart of the streams with infinite reflux ratio

We can see that - as expected - the azeotrope develops on the top (distillate) of the column.

The benzene fraction in the top is so slight that it is therefore regarded as zero. Due to the fact

that no component is extracted at the top, the composition of the bottom is the same as the

composition of the feed.

The expected results, which were already evident in the residue curve, are confirmed.



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The simulation is performed in the next step with the desired rectification parameters. The

objective is to obtain almost pure benzene with 99 mol%, and that with a recovery rate of 99 %.

Figure 8: Setting of the rectification column for the desired criteria

The required mole fraction of the benzene (bottom component mole fraction) is stated for the

bottom criterion. 99 % of the benzene is to be recovered. At the top, the recovery rate

(distillate component fraction recovery) of benzene is set at 1 % (figure 8).

The simulation is restarted, and the results are displayed in the Streambox and in the

UnitOpBox (figure 9). We can see that the desired purity is achieved in the bottom, and that

more than 99 % of the applied benzene is extracted. The azeotropic mixture is extracted at the

top.

A reflux ratio of 3.26 and an evaporator output of 1595.77 MJ/h are calculated with the stated

rectification parameters.



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Figure 9: Properties of the streams and column after the simulation

The number of stages and the feed tray were estimated at the start. Now the optimum number

of stages and the optimum feed tray can be determined with a sensitivity study.

To determine the optimum number of stages, the evaporator output is applied across the

number of stages and examined for a minimum.

The number of stages is varied from 5 to 50 and the evaporator output calculated for each

stage in the process (figure 10).



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Figure 10: Settings of the sensitivity study for the optimum number of stages

Once the sensitivity study has been performed, the data can be output in a diagram. In figure

11, the evaporator output is entered across the number of stages. Convergence problems occur

between stages 5 and 16. As of stage 24, the evaporator output takes on a constant minimum.

Figure 11: Influence of the number of stages on the evaporator output (sensitivity study 1)



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In the column properties, the number of stages is changed to 24 and the simulation is restarted.

Next, the optimum feed tray is defined. A second sensitivity study is performed for this

purpose. The feed tray is varied across the column height, and in doing so the influence on the

reflux ratio is analysed. The feed tray that is varied from stage 4 to stage 20 is used as variable.

The reflux ratio calculated for each stage is set as dependent variable. The correlation is

illustrated in figure 12.

Figure 12: Influence of the feed position on the reflux ratio

We can see that the reflux ratio takes on a minimum around stage number 10. The lower the

reflux ratio, the lower also the energy consumption of the column. For this reason, the feed is

supplied at stage 10.

The column settings are adapted again, and the simulation is started.

It is advisable to check to what extent the feed position influences the equilibrium position

within the column. The temperature profile across the stages can be generated at "Plot: UnitOp

Plots: Column Profiles" (figure 13).



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Figure 13: Temperature profile within the column

At the 10th stage, we can see that the position of the feed tray together with the feed

composition has a limited influence on the temperature profile within the column. The feed

position is therefore deemed optimal.



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The procedure for initialising a rigorous rectification column (SCDS column) is summarised once

again in table 2.

Table 2: Summary "Simulation of an SCDS column"

Procedures Benefits/information

- Select components & thermodynamic

model

[Thermophysical [Select Components] &

[Thermodynamics Wizard]

- Setting of the calculation bases

- Selection of the thermodynamic model has a

pronounced influence on the calculation

- Plotting of the residue curve

[Plot] [Residue Curve]

- Identification of possible azeotropes and

distillation limitations

- Definition of the top and bottom component

to be expected

- Creating the flow sheet

- Assumption: Number of stages and feed tray

- Setting of an infinite

reflux ratio and complete extraction via

the bottom

- Analysis of the column behaviour

- Determination of the top concentration

to be expected

- Setting of the rectification parameters

- If the rectification parameter is

e.g. an azeotrope, successive

approximation is expedient

- At Convergence in the settings window

of the column, the option Reload

Column Profile can be used to facilitate

successive approximation

- Optimising the number of stages by means

of a

sensitivity study

[Run] [Sensitivity]

- Optimising the number of stages by

determining the correlation between

evaporator output and number of stages

- Economically optimal number of stages with

minimum evaporator output

- Determination of the feed tray by means of

a

sensitivity study

[Run] [Sensitivity]

- Determination of the feed tray position

by determining the correlation

between reflux ratio and feed tray

- Economically optimal feed tray with

minimum reflux ratio



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Rating

The properties of the streams and the column are shown at "Format: Add Stream Box und Add

UnitOp Box“ (figure 14).

We can see that the azeotrope consisting of cyclohexane and acetone is separated at the top of

the column. Benzene is only extracted in very slight quantities. Almost pure benzene is

obtained at the bottom.

The results show that it has been possible to reduce the evaporator output to 1355.03 MJ/h. It

has also been possible to reduce the reflux ratio to 2.58.

Figure 14: Results after simulation of the rigorous column



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Fundamental principles

This tutorial investigates the simulation of the rigorous column SCDS. No simplifications are

made in the calculation of rigorous columns, which is otherwise the case with the shortcut

method. Each tray is balanced individually, which results in a complex equation system that

must be solved by means of numerical algorithms. The rigorous column simulation is

mathematically more extensive in comparison to the shortcut method, but provides much

more exact and realistic results.

Ideal mixtures can be quickly displayed as rough estimates with the shortcut column. The

problem is, however, that it cannot be used for non-ideal mixtures, like for example azeotropic

mixtures, because it no longer reflects realistic conditions due to the pronounced

simplifications in the calculation. For this reason, rigorous column simulation is used for non-

ideal mixtures.

The SCDS column is one of the rigorous columns that can be used in CHEMCAD. SCDS stands for

"Simultaneous Correction Distillation System". It is a very versatile column model that is suited

for all rectification processes.

When calculating the rigorous SCDS column, a stationary state between liquid-vapour phase or

liquid-liquid phase is assumed for each tray. The following assumptions are made:

1) Each tray is defined as a thermodynamic system in which the phase equilibrium is

reached.

2) No chemical reactions occur.

3) The uptake of liquid drops in the gas phase and the inclusion of gas bubbles in the liquid

phase are not considered.

The thermodynamic system of a stage is illustrated in figure 15.

Feed

with : Vapour stream

: Liquid stream

Figure 15: Simplified thermodynamic system of a stage within the column

Stage j



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For this thermodynamic system, the required equilibrium equations of relevance for the design

are summarised using the MESH method. MESH stands for Material balance, Equilibrium,

Summation condition and Heat balance.

This way, a complex equation system results for each tray. The mathematical calculation is very

extensive and convergence algorithms are required to solve it. The respective literature states a

large number of iterative solution approaches for solving these non-linear algebraic equation

systems.

General solution algorithms that do not have any restrictions and are applicable for all cases are

the simultaneous correction method and the inside-out method. They can be used for all

column types and all feed compositions. Both algorithms are used in CHEMCAD.

With the simultaneous correction method (SC), all MESH equations as well as their

combinations are solved simultaneously with the help of the iterative Newton-Raphson

method.

Further application options of SCDS are:

Column simulation with packing

Column simulation with special trays

Adsorption and/or absorption processes



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The above simulation was generated in CHEMCAD 6.4.0.

Are you interested in further tutorials, seminars or other solutions with CHEMCAD?

Then please visit our website.

www.chemstations.eu

Or please contact us.

Mail: [email protected]

Phone: +49 (0)30 20 200 600

www.chemstations.eu

Authors:

Lisa Weise

Sources:

[1] Kister, Henry Z.: Distillation design. McGraw-Hill, 1992

[2] Gmehling, Jürgen: Kolbe, Bärbel: Kleiber, Michael: Rarey, Jürgen: Chemical Thermodynamics

for Process Simulation. Wiley-VCH Verlag, 2012

[3] Edwards, John: Process Modeling Selection of Thermodynamic Methods

[4] Schmidt, Wolfgang: USER NRTL BIPS, 2011

[5] Sattler, Klaus: Thermische Trennverfahren: Grundlagen, Auslegung, Apparate. Wiley-VCH

Verlag, pp. 199-202

[6] CHEMCAD help

[7] Seader; Siirola; Barnicki: Perry's Chemical Engineers' Handbook, Section 13 Distillation, 7th

edition. McGraw-Hill, New York, (1997)

[8] Kontogeorgis, Folas: Thermodynamic Models for Industrial Applications, Wiley-VCH Verlag,

2010



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