Dist 001H C2Splitter

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    Distillation of Close Boiling Components with Aspen HYSYS V8.0

    1. Lesson Objectives Distil lation column modeling Column profiles Custom stream results Material balance across distillation column

    2. Prerequisites Aspen HYSYS V8.0 Experience inserting blocks and connecting streams in HYSYS Introduction to vapor liquid equilibrium

    3. BackgroundEthylene is an important monomer, and is made from ethane. The conversion of the reaction is not perfect, so

    the ethylene must be separated from the system. Ethane and ethylene are molecularly similar, and so are

    difficult to separate. The difficulty of the separation is compounded by the fact that polymer production

    requires extremely pure feedstocks.

    The examples presented are solely intended to illustrate specific concepts and principles. They may not

    reflect an industrial application or real situation.

    4. Problem Statement and Aspen HYSYS SolutionProblem: A stream containing 68.5wt% ethylene with a total flowrate of 7.3 million lb/day is fed into a

    distillation column consisting of 125 stages. It is desired to produce a distillate product stream containing a

    minimum of 99.96 wt% ethylene with a total flowrate of 5 million lb/day. It is also desired that the bottoms

    product contains no more than 0.10wt% ethylene. Determine if this separation is feasible.

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    Assumptions:

    - 100% tray efficiency- Total condenser- 300 psig column operating pressure- A refrigerant utility stream capable of condensing the ethylene mixture (not included in model)- Feed mixture is at 350 psig and is a vapor- 125 stages- Feed enters the column at stage 90- Peng-Robinson equation of state

    Aspen HYSYS Solution:

    4.01. Start Aspen HYSYS V8. Select New to start a new simulation.4.02. Create a Component List. In the Properties environment, select the Component Lists folder in the

    navigation pane and click Add to add a new HYSYS component list. In Component List 1 add Ethane

    and Ethylene to the selected components list. You may need to use the search function by typing in

    ethene and pressing enter to find the component ethylene.

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    4.03. Create a Fluid Package. Click on the Fluid Packages folder in the navigation pane and select Add to add anew HYSYS fluid package. Select Peng-Robinson as the property package. The Peng-Robinson equation

    of state is typically used to model systems containing hydrocarbons at high pressures.

    4.04. You are now ready to enter the simulation environment and construct the flowsheet. Go to thesimulation environment by clicking the Simulation button at the bottom left of the screen. On the main

    flowsheet insert a Distillation Column Sub-Flowsheet from the Model Palette under the Columns tab.

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    4.05. Double click on the Distillation unit on the main flowsheet, or go to UnitOps | T-100 in the navigationpane. The Distillation Column Input Expert window will appear. Here we will create the feed and

    product streams, as well as define the operating conditions of the column. On the first page, enter 125

    for # Stages and define the feed and product streams as shown below.

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    4.06. Click Next and page 2 will appear. This page allows you to configure the reboiler. Select Regular Hysysreboiler for Reboiler Type Selection, and select the Once-through radio button. Click Next.

    4.07. This next page allows you to input the pressure and pressure drop in the condenser and reboiler. Entera Condenser Pressure and Reboiler Pressure of 300 psig (2170 kPa). This indicates that there is no

    pressure drop through the column, which is acceptable for this simplified example.

    4.08. Click Next. This page (page 4 of 5) allows you to enter optional temperature estimates for thecondenser, top stage, and reboiler. In this case we will leave these fields blank. Click Next. The final

    page allows you enter a liquid distillate rate or a reflux ratio. From our problem statement, we know

    that distillate rate will be 5 million lb/day of ethylene. Select Mass for Flow Basis and enter 5000000

    lb/day (94498.4 kg/h) for Liquid Rate. Select Done to enter the column property window.

    4.09. We must first define the feed stream going into the column. In the Column property window, go to theWorksheet tab. On the Conditions form, enter a pressure of350 psig (2514 kPa), a Vapour fraction of

    1, and a Mass Flow of 7.3e+06 lb/day (1.380e+005 kg/h) for the Feed@COL1 stream. Go to the

    Compositions sheet in the Worksheet tab. Here we will define the composition of the feed stream.

    Type a number into the Feed composition grid for Ethane to open the Input Composition for Stream

    window. Select the Mass Fractions radio button, enter 0.315 for Ethane and 0.685 for Ethylene. Click

    OK. The feed stream on the main flowsheet should turn blue, indicating that all required input has been

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    entered and that it has solved for all properties. If you open the Feed stream property window you will

    see a green status bar saying OK at the bottom.

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    4.10. Now we must enter design specs to reach the desired product compositions as provided in the problemstatement. In the Column property window, go to Design | Specs. Our two design specifications for this

    simulation will be distillate rate and mass fraction of ethylene in the bottoms stream. In the column

    specifications form there should already be a Distillate Rate spec that was created from the column

    input expert. Make sure that the distillate specification value is correct (9.450e+004 kg/h) and that the

    spec is checked as active.

    4.11. In the Column Specifications form, click Add and select a Column Component Fraction specification. Inthe Comp Frac Spec window select the Stream radio button for Target Type. Enter Ethane@COL1 for

    Draw, Mass Fraction for Basis, and 9.0e-004 for Spec Value. Select Ethylene for Components. Close

    this form when all information is entered.

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    4.12. Make sure that this spec (Comp Fraction) is checked as Active in the Specification Details area. You willnotice that after activating the Comp Fraction spec, the Degrees of Freedom changes to -1. This means

    that the problem is over specified. To fix this, simply deactivate one of the column specifications, in this

    case the Reflux Ratio. Once the reflux ratio design spec is deactivated, the column should solve. You will

    notice that the status bar at the bottom of the sheet wil l turn green and sayConverged.

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    4.13. Check results. In the Column property window, go to the Performance tab. On the Summary sheet youcan see the flowrates and compositions of the feed and product streams. You can see that both

    specifications described in the problem statement are met.

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    4.14. In the Column Profiles sheet you can view the calculated reflux ratio, boilup ratio, as well as materialand energy profiles through the column.

    4.15. On the Plots sheet you can create plots such as temperature and composition along the column, asshown below.

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    4.16. Finally, on the Cond./Reboiler sheet you can view the calculated operating conditions for both thecondenser and reboiler.

    5. ConclusionsThe 125 stage column was able to exceed the specification of 99.96wt% ethylene at 5 million lb/day, as well as

    the bottoms having less than 0.10wt% ethylene. It could then be concluded that this column is capable of

    completing the desired separation. Aspen HYSYS allows engineers to model existing equipment and see if it is

    possible to repurpose it or, otherwise, design new equipment that would meet very specific criteria.

    6. CopyrightCopyright 2012 by Aspen Technology, Inc. (AspenTech). All rights reserved. This work may not be

    reproduced or distributed in any form or by any means without the prior written consent of

    AspenTech. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESSED OR IMPLIED, WITH

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