Introduction to Process Modeling

Introduction to Process Modeling

Aspen Plus: Process Modeling Aspen Plus is based on techniques for solving flowsheets that were employed by chemical engineers many years ago. Computer programs were just beginning to be used, were of the stand-alone variety, and were typically used for designing single units. The solution of even the simplest flowsheet without recycle required an engineer to design each unit one at a time and, manually, introduce the solution values of a previously designed unit into the input of the next unit in the flowsheet. When it became necessary to deal with a recycle, the calculations began with a guess of the recycle values, and calculations ended when the values produced by the last unit in the loop matched the guesses. This involved much repetitive work and convergence was not guaranteed. This procedure evolved through the construction of rating models of units, as opposed to designing models, which could be tied together by software in a way that emulated the procedure above and employed robust mathematical methods to converge the recycle elements of the process. This type of system is termed a sequential modular simulator. An excellent example of such software was Monsanto Corporation Flowtran (1974), which eventually became the kernel upon which Aspen Plus was built.

Subsequently, Aspen Plus, although still basically a sequential modular simulator, has grown considerably and has many advanced functionalities, such as links to a variety of specialized software, such as detailed heat exchanger design, dynamic simulation, batch process modeling, and many additional functions. It also has a facility for using an equation-based approach in some of its models, which permits convenient use of design specifications in process modeling.

The User Interface

From the Start button go to: All Programmes/AspenTech/Process Modeling/Aspen Plus/Aspen Plus

To learn more about ASPEN PLUS see:

Teach Yourself the Basics of Aspen Plus Ralph Schefflan Wiley, 2011 ISBN 978-0-470-56795-1

Exercise: Benzene Flowsheet Workshop Objective: Begin building a Benzene process flowsheet

VAP1

Cooler Feed Feed T = 1000 F P = 550 psia Hydrogen = 405 lbmol/h Methane = 95 lbmol/h Benzene = 95 lbmol/h Toluene = 5 lbmol/h

T = 100 F P = 500 psia Cool

FL1

VAP2

T = 200 F Pressure drop = 0

LIQ1

FL2

P = 1 atm Q=0

LIQ2

Workshop 1 Benzene process properties Double-click the Aspen Plus icon.

Select the New option.

Select General with English Units, click Create.

In the first Component ID field, type HYDROGEN, and then press Enter.

In the next blank Component ID field, type METHANE.

Repeat for BENZENE and TOLUENE. Click the NEXT button to go to the next required data entry field.

Click the arrow next to the Method Name field, Scroll down the alphabetical list, and select PENG-ROB.

The parameters folder in the Navigation Pane shows the required input incomplete symbol. Click the NEXT button.

View available binary interaction parameters.

Click the NEXT button to go to the next step.

Click OK. From the FILE menu, select Save As then ASPEN PLUS BACKUP.

Fill in the FILE name line with BENZENE PROPERTIES.BKP. Select No to save in bkp format only.

Workshop 2 Benzene process simulation Open an existing simulation, BENZENE PROPERIES.BKP To turn the automatic naming feature off, select FILE/OPTION, then on the FLOWSHEET window, uncheck boxes 1 (block) and 3 (streams) under Stream and unit operation labels.

Click on Simulation

Click Exchanges,

Move the cursor into the Process Flowsheet Window and click where the heater should be located.

Enter the name COOLER.

Click Flash2, move the cursor into the Process Flowsheet Window and click where the unit should be located. Enter the name FL1.

Repeat the step above and call the unit FL2.

Click Material Streams and move the cursor into the Process Flowsheet Winodw.

Note the inlet and outlet ports of the blocks are highlighted. Move the cursor onto the Feed port of block COOLER; click. Move the cursor to the left, click where the feed icon should be placed; name the stream FEED.

Move the cursor onto the Product port of the COOLER unit, click move the cursor to the FEED port of FL1 and click. Name the stream COOL. Now stream 2 from COOL to FL1 is connected.

Draw a stream named LQ1 from the Liquid port of FL1 to the Feed port of FL2.

Draw a stream named VAP1 from the vapour port of FL1 to Blank.

Draw a stream named LQ2 from the Liquid port of FL2 to Blank.

Draw a stream named VAP1 from the vapour port of FL1 to Blank.

Click the Select Mode button to leave the stream connection mode.

Place the cursor in the blank field below Temperature in the State variable area.

Enter 1000, the unit F is fine.

Similarly, enter 550 for pressure.

In the Composition Area, enter the mole flows of the individual components.

Click NEXT to go the next required data entry field.

Specify a temperature of 200 F, and a pressure of 0 psia.

Click NEXT. Specify a temperature of 100 F and a pressure of 500 psia.

Click NEXT. Click the arrow next to the field showing Temperature, select Duty (heat) and specify a value of 0.

Enter a value of 1 for pressure and choose unit atm.

Click NEXT.

Click OK and run the simulation.

Check the results.

See the results by click on STREAMS.

Save As BENZENE.BKP