汇总distillation

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AspenPlusHelpSessionDistillation

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Aspen Plus Help Session: Distillation

*Note: The following instructions assume certain knowledge of Aspen. If any of the instructions

are vague, or the instructor is moving too quickly, please let him/her know. They will be happy to

review or clarify any information.

In the following tutorial, SLC=Single left-click; SRC=Single right-click.

1. Select Aspen Plus User Interface under Program/Applications. When Aspen Plus

window pops up, select “Blank Simulation”. At the Connect to Engine Window,

under Server Type select ‘Unix Host’ and in the User Info, under Node Name,

type ‘Sunblast’.

2.

Enter username and password. Disregard Working Directory and enter. You

will be informed when the connection has been established.

Distillation Options:

The three different methods available in Aspen Plus are

1.

a) DSTWU

b) DISTL

RADFRAC

DSTWU

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This approach uses Winn-Underwood-Gilliland shortcut design calculations. It is

designed for a single feed and 2 product distillation column. Remember, for additional

information regarding this method, select the DSTWU icon and press F1.

In order to use this option you must specify the recovery of the light and heavy keys.

Winn - estimates minimum # of stages

Underwood - estimates minimum Reflux Ratio (RR)

Gilliland - relates actual number of stages and RR

DSTWU calculates the minimum reflux ratio and minimum number of theoretical plates

for the specified recovery. It then calculates the actual reflux ratio for the specified

number of stages, or

the actual number of stages for the specified reflux ratio, depending on which is entered.

It also determines the optimal feed location and reboiler and condenser duties. The

model assumes constant molar overflow and constant relative volatilities.

DSTWU Example

One hundred lbmol/hr of an equimolar mixture of ethane/ethylene is available at 25

C and 1 atm. This stream needs to be separated in a distillation column capable of

recovering at least 99.6% of the light key (ethylene) and 99.9% of the heavy key (ethane,

entered as .001 recovery in distillate). The column has a total of 40 stages. Calculate

the minimum reflux ratio, actual reflux ratio, minimum stages, and feed location for this

column. Use RK Soave model. The whole column operates at 300 psi (Both Reboiler

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and Condenser are at 300 psi).

Answer: Min RR:

Now suppose that the reflux ratio is specified at 20% above the minimum reflux.

Calculate the

minimum and actual stages, minimum and actual reflux ratio, and feed location.

Answer: Min RR:

Note that the minimum RR and minimum stages are identical. This should be expected

since these are only a function of the specified recovery for the light and heavy key.

Finally, we wish to see how the RR ratio changes as a function of actual stages. From

TPII class you should be aware that the RR and the number of stages are inversely related.

We want to monitor the RR for a range of 40 - 100 stages. From the Data Browser,

choose “Blocks” and then the appropriate block name. Select Input and then the

“Calculations Options” tab. Choose to “Generate table of reflux ratio vs number of

theoretical stages.” Enter 40 as the initial number of stages and 100 as the final number

of stages. Select “Increment size for number of stages” and enter 2. Press F4 or the

‘N’ buttons on the toolbar and re-run the simulation.

Note: your values must be above the minimum number of plates!

5.937

Actual RR: 7.12

Actual # Stage : 63.38

5.937

Actual RR: 19.96

Feed location: plate 24.22 Min stages: 34.09

Min stage: 34.09

Feed plate location: 37.80

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To see a plot, display the results data browser and select Blocks/”Block

Name”/Results. Choose the Reflux Ratio Profile tab. Designate the independent and

dependent variables. SLC on the independent variable column heading and press

Ctrl+Alt+X. SLC on the dependent variable column heading and press Ctrl+Alt+Y.

You can also define the dependent and independent variables through the “Plot” option

on the upper pull down menu.

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DISTL

This approach is also for a single feed and 2 product distillation, but uses the

Edmister method to calculate product composition. You must use a column icon under

“Distl” rather than DSTWU. If you are modifying the previous example, you must

delete the DSTWU block and replace it with a Distl block, and then reconnect the streams.

You’ll find this option by selecting a stream with the left mouse (SLC), and then SRH

(single right hold) with the right mouse. . To use this method, the following must be

specified:

The results are the feed stage temperature, bottom stage temperature, top stage

temperature, and feed quality as well as the product composition. It also assumes

constant molar overflow and constant relative volatilities.

Basically, this method can be used when everything in the column is specified and you

need to verify the product results.

DISTL Example

Replace the DSTW block with the DISTL The same feed as the previous example

Number of stages

Feed location

RR

Pressure profile

D:F (distillate to feed Ratio)

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enters a column of 12 stages (including condenser and Reboiler), with the feed located at

the 4th stage (from the top, including the condenser). The column operates at 300 psi

with a RR of 7 and a D:F ratio of 0.8. Calculate the product composition.

Suppose that you would like to monitor the ethane composition at the bottom as a

function of the

RR. In other words, we would increase or decrease the RR and see the effect it has on the

molar composition of the bottom stream. To do so we must perform a sensitivity

analysis where we will vary the RR from 7 to 60.

Go to the Data browser window and select the Model Analysis Tools folder, then the

Sensitivity folder, and then click “NEW” to create a new sensitivity analysis. Call it any

ID you want. The next screen has three tabs: define, vary, and tabulate.

In the define tab you create a new variable; click on “NEW” and call your variable

XETHAN. The variable definition window will pop-up. Fill the reference area as

follows:

Type: mol-frac

BOTTOMS

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Distillate: ethane mole fraction = 0.384

Bottoms: ethane mole fraction = 0.965

Stream :

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Component: ethane

Leave Category as “All”, and Substream as (Mixed)

Press F4 or the ‘N’ button twice or go the vary tab. Here we will tell aspen how to

vary the RR. Start by choosing clicking on the pull down menu and selecting “new “.

Aspen will automatically assign a variable number. Fill this form as follows:

Select the Range Option

For your Report labels you may choose

Line 1: RR

Lower Limit:

Upper Limit:60

Incr: 2

7

Type:

Block:

Variable:

Block-Var (Block variable)

name of your column

RR

Then Press F4 or the ‘N’ button to go to the Tabulate tab and do the following:

Col: 1

XETHAN Tabulate:

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You can bring up a list of variables by SRC on the Tabulated variable option, and

selecting variable list from the pop-up menu; then SLC. You can also click on the

table format button to add labels to your table. Press F4 or the ‘N’ button.

Run the simulation and notice how Aspen solves each point. To see a plot, display the

results data browser and select Blocks/”Block Name”/Results. Choose the Reflux Ratio

Profile tab. Designate the independent and dependent variables. SLC on the

independent variable column heading and press Ctrl+Alt+X. SLC on the dependent

variable column heading and press Ctrl+Alt+Y. You can also define the dependent and

independent variables through the “Plot” option on the menu. Choose Plot from the menu

and select Display Plot.

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RADFRAC

RADFRAC is the ASPEN’s rigorous distillation method. No assumptions are made!

Its can do: absorption, stripping, extractive distillation, azeotropic distillation, and

ordinary distillation. It is capable of handling any number of feeds and side product

streams. You’re probably better off starting a new simulation since a lot of things will

be different from the previous examples.

RADFRAC Example

Four hundred lbmol/hr of methylcyclohexane (MCH) and toluene (equimolar) is

available at 220F and 20 psia. Since these two components have narrow boiling

points, it is been proposed to use phenol as an extractive agent to aid in the separation

(phenol likes toluene more than MCH). Two hundred lbmol/hr of phenol feed is

available at 220 F and 20 psia. We intend to optimize the phenol flow rate in such a

way that MCH can be recovered with 97% purity in the distillate. The column has a

total of 22 stages with the phenol being fed at the 7th stage and the feed mixture at the

14th stage. It is also desired to have a total distillate flow rate of 200 lbmol/hr.

The column operates at a pressure of 16 psi at the top and 20.2 at the bottom. The

reflux ratio was estimated at 8. Use the UNIFAC model. Connect the streams and your

screen should look like the following figure. Use the F4 key or the ‘N’ button to

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input your components, property method and the feed and phenol streams as in the

previous examples (remember we use UNIFAC for this example).

Pressing the F4 key or the ‘N’ should take you to the distillation block. You

should the Setup options and Operating Specifications sections as follows:

Take a second to go over the options and see all the variables that you can choose (read

the description on the bottom). Press F4 or the ‘N’ button, or go directly onto the

Streams tab. The Streams tab requires that you specify the feed locations which you

would set as follows:

Also, note that two options are available, above stage and on stage.

Above stage: Feed between two adjacent stages where the liquid flows to stage n and the vapor

flows to stage n - 1

Number of Stages:

Condenser:

22

total (Vapor/liquid = 0)

(leave the rest as they are)

Distillate: 200 (lbmol/hr)

Reflux ratio: 8

Feed: 14 above stage

above stage Phenol: 7

On stage: Both, vapor and liquid flow to the same stage.

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Press F4 or the ‘N’ button, or go directly to the Pressure Tab. First select the Pressure

profile option from the view pull-down menu. Then enter the profile as follows:

1.

1.

Run the simulation to see how close we are to the process demand (97% MCH).

Note the iteration method used by the simulation engine. It is important to see if your

calculations are converging otherwise you can be there for a long time and get junk out.

It took 6 iterations for our simulation to converge. We obtained the following results:

A very useful feature of RADFRAC is that it keeps track of the behavior of all the

species in all the stages. The temperature, flows, pressures, enthalpies, vapor and liquid

mole fractions, and distribution coefficient profiles are all stored in a menu. From the

Radfrac folded on the setup window, click on the Profiles option. If you want to plot the

profiles, you would first designate the independent and dependent variables. SLC on the

independent variable column heading (like stage#), and press Ctrl+Alt+X (You may

Distillate: x_MCH = 0.88 x_tol = 0.12

Stage Pressure

16

20.2

Bottoms: x_MCH = 0.06 x_tol = 0.44

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skip this step since stage # is the default independent variable for this case). SLC on the

dependent variable column heading (say liq flow) and press Ctrl+Alt+Y. You can also

define the dependent and independent variables through the “Plot” option on the upper

pull down menu. Another way of doing this is through the plot wizard option on the

“Plot “ option on the upper pull down menu. This option will let you customize your

plots and it is worth to spend sometime looking at it.

We still need to change the phenol flow rate to meet the process demand (97%).

Note that we can do a sensitivity analysis that will give us an idea what the flow rate

should be approximately, but since our intention is to calculate the actual flow rate of

phenol we will perform a Design-Specification analysis.

You may perform your design spec as shown in the previous tutorials or directly

at the Radfrac block. From the data browser click on BlocksRadfracDesign Spec and

create a new ID at the object manager. This ID has to be a number and we chose to call

it “1” . The Specifications tab will appear on which you should fill the following

information:

On Design Specifications:

Type: Mole purity

On Specification:

Target: .97

Stream type as product should be selected

Move onto the next tab ( or Press F4 or the ‘N’ button) and specify MCH as the

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component to be used in the design. You do this on either the Base Components or the

Available components menu, and then clicking on the single right arrow, which moves the

name of the component to the selected components list.

Now we are ready to provide information on the manipulated variable, i.e. the

amount of Phenol. Press F4 or the ‘N’ button or go directly to the Vary subfolder from

your distillation block. At the Object manager, create a new variable number just as

above ( we called it “2”). The specifications tab will appear and you would fill the

information as follows:

Type: feed rate

phenol

200

5000

Stream Name:

Lower Bound:

Upper Bound:

We have just specified that in order to reach our target value we will manipulate

the phenol molar flow rate between 200 and 5000 lbmol/hr hoping that the answer in this

range (if not we can always change it).

Run the simulation and note the number of iterations. If an error results

re-initialize all blocks and rerun it before you make any changes. For us it took 22

iterations to converge to a solution.

Answer:

Phenol flow rate: 1119 lbmol/hr

MCH mole frac: .9700

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To view and print a table with your results, go to the results folder and select any of the

result stream subfolders. You should see a bottom named Stream Tables. Click and a

table will show on the Process Flow Sheet window. You may also copy the figure and

table on the clipboard by selecting them with the mouse, using FileCopy or Ctrl+C, and

the pasting them into other windows Applications (Work, Word Perfect Power Point)

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