Difference between revisions of "ASCEND/C2/Simulating-flowsheet/English"

Title slide

• Model a mixer and execute it
• Model a reactor
• Connect different components in a single flowsheet
• Run the flowsheet

System Requirement

• Ubuntu Linux OS v. 12.04

• ASCEND v. 0.9.8

Pre-requisites

• Linux and
• ASCEND

To know more about ASCEND please visit ascend4.org

For relevant tutorials, please visit our website http://spoken-tutorial.org

Show the complete Flowsheet

This example is available on page 508 of the book.

We will then assemble them to model the complete flowsheet.

Reactions

The equilibrium conditions are also given as follows

Here yi signifies mole-fraction

Flowsheet

In mixer', a fresh feed of ethane with n0 (n knot) moles is mixed with a recycled stream of ethane with n6 'moles

The outlet stream consists of 100 moles of ethane.

Equations for Mixer

n0+n6 =100

Let me explain the code.

MODEL mixer;

I have named the model as mixer.

IS_A mole;

n_0 (n knot), n_6 and n_tot.

Define the streams as a mole.

n_0+n_6 = n_tot;

But this helps in debugging the code.

FIX n_tot ;

END specify;

Under METHOD specify, we fix the variable n_tot

n_tot := 100.0 {mole};

END values;

RUN clear;

RUN specify;

    RUN values;

END seqmod;

END mixer;

This signifies that given feed stream and unit operations specification, ASCEND will calculate the intermediate and outlet streams.

RUN specify and

RUN values

Note that model mixer is not square, as there is one equation and two unknowns.

Thus, one can’t solve the model mixer separately in ASCEND.

Flowsheet

Here ethane is dehydrogenated to ethylene and acetylene, so the outstream consists of ethane, ethylene, acetylene and hydrogen.

Equations for Reactor

To understand these equations, please refer the textbook.

Now let’s look at the code for reactor.

Define all the input and the output streams as a mole.

• numerator of the left hand side equal to
• right hand side into denominator of the right hand side

We don’t use division to avoid a poor initialization for solving the problem.

We thus give a nominal value of ksi1 and ksi2

For assigning the nominal value, we write

ksi_1.nominal is equal to ksi_1

ksi_1.upper_bound is equal to n_tot into 2

Define the METHOD seqmod and END the reactor.

Assignment: Equations for Separator

I have the code for modelling the flowsheet.

Let me explain the logic of connecting different components via code.

We define

m1 IS_A mixer;

r1 IS_A reactor; and

s1 IS_A separator;

• output from mixer is same as input for the reactor and
• the output of the reactor is same as the input to the separator.

ARE underscore THE underscore SAME

It merges different models or variables into one.

For example, we write

m1.n_tot,r1.n_tot ARE_THE_SAME;

This saves us from creating additional different equations.

For example, under METHOD specify we have written

RUN m1.specify and

RUN r1.specify

This command will execute the specify method for mixer under the METHODS specify for flowsheet

Now click on m1 mixer.

You will see ASCEND has assigned some default values to the streams n_0, n_6 and n_tot

Now click on the Solve icon.

You will see ASCEND has calculated the values for the streams n_0 (n knot) and n_6.

It is also satisfying the equation n_0 (n knot) +n_6 = n_tot

Summary

• Model a mixer
• Model a reactor
• Connect different components in a flowsheet
• Run the flowsheet

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Acknowledgements

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