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

Title slide

We will learn how to

• Model a mixer and execute it
• Model a reactor
• Design 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 model the 3 components of this Flowsheet- mixer, reactor and separator and we will then assemble them to model the complete flow sheet.

Reactions

The equilibrium conditions are also given as follows

Here yi signifies mole-fraction

Flowsheet

In mixer, a fresh feed of ethane with n0 moles is mixed with a recycle stream of ethane with n6 moles

The outlet stream consists of 100 moles of ethane

Equations for Mixer

n0+n6 =100

I already have the code for this mixer in a text file.

Let me explain the code.

MODEL mixer;

I have named the model as mixer.

IS_A mole;

n_0, n_6 and n_tot.

Define the stream as a mole.

Output = Input

That is :

n_tot= n_0+n_6;

Note that label like 'equation 1' is not necessary to right, 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;

n_tot is assigned the value 100 mole

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

Under this method,

RUN specify and

RUN values

Now END the model mixture

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

Thus one can’t solve the model mixture separately in ASCEND

Flowsheet

As you can see there is one stream entering the reactor, and four streams leaving it.

Here ethane is dehydrogenated to ethylene and acetylene, so the out stream 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 stream as the mole.

The reaction extent ksi1 and ksi2 is also defined as mole

Equation 2-6 are defined as shown in the slide

Note that equation 7 and 8 are written as numerator on 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

Now under METHODS section, we define methods in a similar way as that for mixer.

Under values section, instead of giving a direct value, we initialize the value of the reaction extent ksi1 and ksi2 , we thus give a nominal value of ksi1 and ksi2

We also set upper bound for ksi1 and ksi2.

For assigning the nominal value, we write

ksi_1.nominal is equal to ksi_1

similarly for assigning the upper bound, we write

ksi_1.upper_bound is equal to n_tot into 2

Now, end the Method values

Define the method seqmod and END the reactor

Assignment: Equations for Separator

Using these model the separator on similar lines as that of mixture

Once we have modelled each component of the flowsheet

Let’s now wire them together. 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;

Note that the variable mixer, reactor and separator is used by ASCEND from the models defined before

Now let us state that output from mixture is same as input for the reactor and the output of the reactor is same as the input to the separator.

We equate the two by the help of the operator

ARE_THE_SAME

ARE_THE_SAME is a merging operator

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

Under METHODS section, we will again write the same methods as specify, values and seqmod.

Note that under method specify and method values, we run methods for each individual component under each method. 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 method specify for flowsheet

Now define the method seqmod and END the 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 under method section, click on seqmod, then click on RUN

Now click on the solve icon.

You will see the message converged in the panel below

Now click on m1 mixer, you will see ASCEND has calculated the values for the streams n_0 and n_6

It is also satisfying the equation

n_0+n_6 = n_tot

We encourage the learner to observe the values of remaining two components before and after solving the flowsheet for better understanding.

Summary

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

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Acknowledgements

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