Mass Tr. Process Simulation

8/3/2019 Mass Tr. Process Simulation

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Colin S. ‘Chip’ Howat Ph.D., P.E.

John E. and Winifred E. Sharp Professor

Kurata Thermodynamics Laboratory

Department of Chemical & Petroleum Engineering

University of Kansas

Process Simulat ion

Mass

Transfer



This is a tool, albeit broken!

Chemical Engineering Practice





Chemical Engineering Practice

The trick to using chemical engineering tools effectively is to use the proper tool for the task at hand ~ AVOID OVERKILL!



Process Simulation

Chemcad

(As I prepared lecture last night, Chemcad 5 authorization failed. These slides are from CC4.)

This is the opening screen for

Chemcad 4. I know that youhave used Chemcad 5. I will

be, too.

Unfortunately, one of the

frustrating aspects of Chemcad

or any licensed software is

the authorization. On 2/11/01

my authorization of 5 failed.

We will use Chemcad 5.

Conceptually, though, the

process is the same no matter

the package being used.



Process Simulation

Chemcad

There are generally a series of six steps that you will follow when setting up a flowsheet forsimulation. Remember that the simulation should be a confirmation of what you suspect is true.

If the simulation is taking inordinate time, consider using another tool.

Directory DesignationJob Designation

Component Identification

Thermodynamics Selection

Flowsheet Development

Stream InputEquipment Specification

Calculation

Recording

Evaluation

Re-specification/Resimulation, as necessary

Any time that you approach

a simulation problem, you

should approach it in the

same way that you would approach a laboratory

experiment! Do fall to the

Song of the Lorelei!



Process Simulation

Directory Designation

The directory should be

unique to you. You cansave it to a ZIP drive, your

user-directory or the default.

You must keep track of where

you keep your jobs.

The ‘JOB’ is the study, at

hand. It can consist of a

variety of ‘CASES’ which

may use the same properties

and options but have special

aspects that require separate

study.

When beginning, you start

with a NEWJOB.



Process Simulation

Job Designation

This is the defaultscreen. If you are

using a common

directory, you may

have a variety of jobs

shown. In this case, Iam using a new directory

for this class.

The next slide shows

a different directory.



Process Simulation

Job Designation

This is my defaultdirectory that I use for

a variety of jobs.

However, for this class

I am setting up a specialdirectory for all

simulations.

If you use the default

directory, everyone willhave access to your work,

to use and modify as

they see fit.



Process Simulation


The components arestored as part of the

library. This is accessed

under the Chemcad 4

component option and

the equivalent in Chemcad5.

There is no benefit to using

more components than

you need.

Beware! You do not know

where the properties came

from. Therefore, use the

library with caution.



Process Simulation


The components canbe listed by entry

number, as in the right

hand slide. They can

also be listed by

formula and synonym.

You can have as many

components as you want.

Let’s look at problem4.19 which is the home-

work for Wednesday.

I need benzene (a

carcinogen) and toluene.



Process Simulation


These easiest way to listthem when the components

are relatively simple, is

by synonym. Then, you

can type in the name, or

part thereof, to search forthe chemical of interest.

When it is found, add it

to the list. Then, search

for another.



Process Simulation


I have found benzeneand toluene. This

completes the list.

The most difficult part

of the process simulationcomes next. This is the

selection of the

thermodynamics. As I

have said in class,

THEMODYNAMICS RULES!

Failure to heed this will

result in inaccurate

simulations.



Process Simulation

Thermodynamic Selection

I would avoid using the‘expert’ option. This is

a cop-out of your

responsibilities as a

chemical engineer. You

do not know the selectionprocess.

I strongly recommend that

you make the decision as

to what you think is theapplicable option.

For most systems, you can

use latent heat for enthalpy

or SRK if the material isnoncondensable.

Let’s focus on K-values!



Process Simulation


Look at all of the

options from which

you can choose!

Which one applies?

Answering that is

part of the engineering!



Process Simulation


I have chosen UNIFACbecause this is a

conventional hydrocarbon

system which should

be properly represented

by the structural groupsincluded in UNIFAC.

Regular solution theory

could also have been

chosen.

The best is to have data

but that is overkill for this

homework problem.



Process Simulation


I suspect that there is amajor difference between

4 and 5 here.

The next step is to select

the unit operations thatyou will be using for the

problem. In 4, you move

to the draw flowsheet

screen by selecting the

T-square icon.

All streams begin and end

at a unit operation. For a

flash, I need a feed, two

products and a flash.



Process Simulation


Once I have selected myunit operations, I need to

select the streams.

I believe that the flowsheets

should be drawn neatly.However, when setting up

initially, draw them

quickly and edit later.

Focusing on the neatestlayout before you have

developed your entire

flowsheet will cost you too

much time.



Process Simulation


I have completed theflowsheet and am now

ready to simulate the

flash. Note how the

feed is connected from

a feed ‘unit operation’to the flash. The products

leave the flash and connect

to the product ‘unit

operation’.

I try not to clutter my

flowsheets with unit

operation names and

numbers. My formal

flowsheet is hand or CADdrawn. This flowsheet is

merely a tool.



Process Simulation

Stream Input

These are my defaultunits. When you set up

your directory, you can

set units before building

a flowsheet. You can

also change them withina flowsheet to the values

you wish.

There is also an option

to set units for streamediting allowing you

to enter mole fractions,

weight fractions etc.



Process Simulation

Stream Input

The stream of interest ischosen by merely clicking

on it. I input the conditions

that I wanted. Note that

no temperature is given

in the problem statement.I arbitrarily selected one

and a corresponding

pressure.

I then selected ‘FLASH’ tosee if the input was clean,

i.e. gave the conditions

that I wanted. When it did,

I had entered the feed

properly.



Process Simulation

Equipment Specification

Each piece of equipmenthas a set of specifications

that you can choose from.

Many are seductive.

The purpose of this problemis to determine the recovery

of benzene in the vapor

when 90% of the toluene

is recovered in the liquid.

This problem is necessarily

iterative. I will iterate on

the V/F ratio. I will keep a

run log in order to record

my progress and to minimizthe number of simulations.



Process Simulation


Since the calculation isto be done at atmospheric

pressure, I have selected

14.696 psia.

It should be no surprise thatthere are two specifications

to be made on the flash.

You have 3C+10 variables

and 2C+6 equations. Once

C+2 feed specifications aremade, you have 2 specs left.

I am merely guessing V/F of

0.5 for this calculation.



Process Simulation

Calculation

Once you have set upthe flowsheet, you can

run it. This is a simple

flowsheet with one operation.

So, running all is OK.

However, for complex

flowsheets, you can save

time by focusing only on

those operations that you

are interested in. In that caseyou select specific unit

operations. This also helps

when setting up a flowsheet

that has multiple recycles.



Process Simulation

Calculation

The recovery of tolueneis only 58.9%. Therefore,

I will need to lower the V/F

in order to recover more

in the liquid.

You MUST keep a run log!

There are no exceptions. You

are to approach process

simulation as if it were a

laboratory experiment. Youwrite each of your

observations down and state

what you will adjust and why.

This must accompany any

homework or project work.



Process Simulation

Recording

The run log is a recording of all simulations that you make. It contains the target values for the variables,

the independent specifications that you are varying and the results. In addition, after each simulation,

it includes the observations and the reason for the next adjustment.

Specifications Dependent Variables

V/F ... ... %Tol ... ...

Target 90%

0.5 58.9

Too little toluene. I need more liquid.

Decrease V/F

0.2 85.6

Decrease V/F to .151 by linear interpolation.

0.151 89.4

OK. Close enough for these purposes.

Benzene recovery in the vapor is:21.9% (Note that this is with UNIFAC. The

results would be different with another solution model. Scary, huh?)



Process Simulation

Evaluation

This is a straightfoward problem. Consequently, the results are fairly easy to evaluate. However, if we

had a more complex flowsheet, we would need to evaluate each stream against each desired specification.

These may include compositions, recoveries, temperatures, percentage conversions etc. We would also

need to evaluate the sensitivity of our results against the uncertainties in the underlying data base.

For example, if I choose Regular Solution with the previous V/F, the toluene and benzene recoveries are

89.5% and 22.0% in the liquid and vapor, respectively.

Ideal solution gives 89.6 and 22.2%, respectively.

There isn’t much difference here, but suppose that this were a nonideal system? I also leave it to you to

see if the computed temperatures have changed.



Process Simulation

Re-specification/

Re-simulation

Often, flowsheet

modifications are required

as you learn more about

the process. This will

require editing, re-

specification and re-

simulation.

For example, if one stage

does not give the benzene

recovery that you want,

perhaps two stages would

be better?



Process Simulation

Reporting

Whenever you report simulation data, be selective. Hundreds of pages of output unless a final detailed

design run, are useless. Always include the cover sheet which gives the specifications. Always include

the equipment specification sheet. Include the streams of interest in the units of interest.

Heavily annotate by hand the output to direct the readers’ attention to what you want them to know!

Undocumented output is useless and unacceptable in professional practice, i.e. in this class and

elsewhere.

Good Luck and Good Simulating!



Process Simulation


This is Chemcad 5

component selection. The

screen is similar to that

used for Chemcad 4. The

concept is the same.





Process Simulation


In Chemcad 5, the

thermodynamic

selection and the

component selection

are in the same pull down

menu.

Other than that, it is the

same.




Process Simulation

In Chemcad 5, you will

note that the options

appear to be the same

with only slightly

different formatting.



Process Simulation


In Chemcad 5, the flow-

sheet building is accessed

by pressing the S/G

button.

Unlike CC4, you will seea pallet on the screen. This

contains the unit operations

of interest.

As before, I will use aflash.



Process Simulation


Building the flowsheet

is essentially the same

with the exception that

instead of two separate

menus for streams and

unit operations, they

all come up on one screen.

The unit is selected and

placed on the flowsheet

with a left click.

Streams are added. When

finished, right click.

Once finished, press S/Gto return to the simulation.



Process Simulation

Stream Input

To reach the stream of

interest, you need to

press the Specifications

button. You can then

select the stream of

of interest.

While the screen looks

different, the input is the

same.



Process Simulation


You will see that the

input to the flash is

equivalent, as well.

Do no be fooled by the

changes in format. Know

what you need to do and

you will find that the

simulation is easily

accomplished.



Process Simulation

Calculation

Is this the same result?

Good Luck and

Good Simulating!

Documents

Mass Tr. Process Simulation