ASPEN软件模拟吸收

Design Procedure for an Absorption Unit on

the AspenPlus Software

Author:

Brigitte McNames

This manual presents all steps necessary to design an absorber using the AspenPlus

simulation software.

The manual also includes useful tips, recommendations, and

explanations throughout the design procedure.

The following example will be used:

Example 1

Problem Statement:

Absorption of Acetone in a Packed Tower

Acetone is being absorbed by water in a packed tower having a diameter of m at

293 K and kPa (1 atm).

The inlet air contains mol% acetone and outlet

mol% acetone.

The total gas inlet flow rate is kmol/h.

The pure water inlet flow

is kmol/h.

(This example is

EXAMPLE

taken from reference 1).

Schematic:

Gas Outlet

x

acetone

=

P

ure Water Inlet

F = kmol/h

Absorber

T = 293 K

P = 1 atm

Gas Inlet

x

air

=

x

acetone

=

F = kmol/h

L

iquid Outlet

x

acetone

=

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Procedure

Logon to the AspenPlus system and start a blank simulation.

The flowsheet area should

appear.

(Refer to

“AspenPlus

Setup for a Flow

Simulation”

if you need help.)

Shown above is the Columns subdirectory.

Choose the RateFrac block from the

subdirectory by clicking on it.

If you click on the down arrow next to the RateFrac block,

a set of icons will pop up.

These icons represent the same calculation procedure and are

for different schematical purposes only.

Choose the block that best represents the process

that you are designing.

For our example we will use the

RATEFRAC

rectangular block

at the top left corner.

RateFrac is a rate-based nonequilibrium model for simulating all types of multistage

vapor-liquid operations such as absorption, stripping, and distillation.

RateFrac simulates

actual tray and packed columns, rather than the idealized representation of equilibrium

stages.

A column consists of segments (see schematic for a packed

column at right).

Segments refer to a portion of packing in

a packed column or one or more trays in a tray column.

RateFrac performs an initialization calculation where all

segments are modeled as equilibrium stages.

The results

from the initialization step are used to perform the rate-

based nonequilibrium calculations.

To learn more about

RateFrac and its applications refer to the

“RateFrac”

help

pages.

Packed segment n-1

Packed segment n

Packed segment n+1

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First, create a schematic similar to the one above using the RateFrac block.

Refer to

“AspenPlus

Setup for a Flow

Simulation”

if you need help.

Attach the liquid inlet and

gas inlet streams to the feed port.

Attach the gas outlet to the vapor distillate port and the

liquid outlet to the bottoms port.

Once the flow sheet is complete, click the

“Next”

button (

?

) and the title screen should appear (see below). Give the example a title and

change the units from English to Metric on the same screen.

Click the

?

button.

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The components screen should appear next.

Enter in the species used in the example (see

above).

The

“Find”

button on the bottom of the screen enables you to quickly search for

components in the databanks by formula, name, CAS registry number, molecular weight,

and normal boiling point.

The

“Elec

Wizard”

button can be used to generate electrolyte

components and reactions for electrolyte applications from components you entered.

A

custom component that is not found in the databanks can be created using the

“User

Defined”

button.

The

“Reorder”

button will simply reorder the components that are

already defined on the selection sheet.

When all components have been entered, click the

?

button.

On the next screen, choose a

Property Method from the list by

pressing on the down

button to the right of the

box.

If you need help

refer to

“AspenPlus

Setup

for a Flow

Simulation.”

This example will use

NRTL.

Then click the

?

button.

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Shown at the left is the

input page for the air/acetone inlet

gas.

Enter all the data from

the problem statement

for temperature, pressure, flow

rate,

and composition.

Make sure that units

and definitions correspond to the

values that you are

entering.

The input sheet for the gas inlet stream should appear (see above).

Enter the values from

the problem statement.

If values are unknown, leave the respective boxes blank.

When

finished, click the

?

button.

The input sheet for the liquid inlet stream should appear.

Once again, enter the respective values from the problem statement, and click the

?

button.

The input sheet for the absorber block will appear next.

A column consists of segments

that are used to evaluate mass and heat transfer rates between contacting phases.

A

segment refers to a portion of packing in a packed column or a series of trays in a tray

column.

Enter the number of segments.

As a rule of thumb, there should be one segment

per foot of column height.

However, more segments could be used to increase the

accuracy.

The height of the segment should not be less than the average size of the

packing used.

This example will use ten.

Also on this screen, you can select the

condenser and reboiler type.

Since we are modeling an absorber, select

“none”

for

condenser and reboiler.

Click the

?

button.

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The view selector that

appears on the next

screen allows you to

select the type of pressure

specification

that you want to enter.

Choose Top/Bottom

and enter 1 atm pressure from the

problem statement for

segment 1.

Segment

1 will refer to the first

segment at the top of

the tower.

Click the

?

button.

The button will automatically bring you to the tray specification sheet.

Since our column

consists of packing rather than trays, choose

“PackSpecs”

from the data browser at the

left.

You will be brought to the packing specification sheet.

Choose

“New”

to create

your packing specification for the tower.

Start

“pack

segment

number”

at 1, which is the

top packed section in the column.

The screen shown below will appear for entering

packing specifications.

Enter a value for the ending segment.

For our example, enter ten

since it is the last segment of packing in our column.

For this example, I have arbitrarily

chosen ceramic raschig rings as packing since the packing type was not specified

in the problem statement.

Guess a packing height that may give us the separation we

need to get our final gas and liquid concentrations.

Since I have already tackled this

problem, I know that the required height of packing necessary to achieve the separation

we need is m.

Click the

?

button to continue.