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Homework Help: Solving a Large Steady-State System with ASPEN

  1. Mar 4, 2017 #1
    Hi Everyone,
    I'm currently working on solving a complex chemical process system in a program similar to Aspen (DWSIM). We aren't given any molar flow rates of feed streams, only constraints on various parts of the system (i.e. only 5 mol% of ethylene may enter the reactor at a time).

    1. The problem statement, all variables and given/known data

    E1 C2H4 + 0.5 O2 --> C2H4
    E2 C2H4 + 3O2 --> 2CO2+2H2O​
    Fresh ethylene and air at 20 C and 10 atmospheres are combined with a recycle stream containing unreacted ethylene, carbon dioxide, nitrogen and traces of water vapor. The combined product is passed through a heat exchanger, then a fired heater, and fed to a reactor at 225 C at a pressure of 10 atmospheres. The ethylene in the reactor feed must be maintained between 5 and 10 mole % for the catalyst to function properly. The single pass conversion may be adjusted between 20 and 95% to optimize performance. All of the oxygen in the feed reacts. The reactor effluent emerges at 275 C and is passed through the other side of the same heat exchanger and then another in which it is cooled (with cooling water) to 40 C. The cooled stream is fed through an absorber in which essentially all of the ethylene oxide is absorbed by water. The water-ethylene oxide stream is passed to a distillation column in which 98% of the ethylene oxide leaves in the top product as 99.9% pure EO, while the balance of the ethylene oxide and water is sent to a wastewater treatment facility. The gas stream leaving the absorber becomes the recycle stream previously mentioned. However, some of the recycle stream is purged so excess carbon dioxide and nitrogen do not build up in the system.
    You are limited by the selectivity data presented in Table 1 and by the operating requirement
    Percent Conversion Selectivity
    20 6.0
    40 5.8
    60 5.2
    80 3.6
    90 1.2
    2. Relevant equations

    3. The attempt at a solution
    I've never seen a problem like this before, but my best idea of where to start is at the reactor (considering we don't have molar flow rates for the stream). I drew the reactor in Aspen and am in the process of adding a reaction to the system. I have no problem with creating the reaction, but have no idea how to deal with excess nitrogen, argon or carbon dioxide. Also, any additional recommendations on how to solve this problem would be a great help because as I said, I have never seen a problem like this in my life.
  2. jcsd
  3. Mar 4, 2017 #2


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    Homework Helper
    2017 Award

    Not much of an attempt, this is :smile:
    E1 is a weird looking reaction equation ?!
    For your simulation you can simply begin with picking a flow, e.g. 100 kmol/h. Approximate composition follows from 5-10% C2H4, air such that enough O2 is present and the remainder is CO2 for a start. The recycle can be closed later on.
    Draw a flowsheet, annotate with your given info and improvise !
  4. Mar 6, 2017 #3
    I second BvU's recommendation to do nothing until you draw a flow sheet. Also, to solve this, you are going to need to select a basis for the calculations, such as 100 moles/hr pure feed.
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