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Mass & Energy Balance Question Help

  1. Dec 12, 2009 #1
    I would appretiate help with the following 2 questions, I think I've got must of the first one right (Or went off t a tangent) but I don't now where to start on the second question

    1a) Calculate the flue gas composition if 100 kmol h-1 of a gas whose composition is given below are burnt with 10% excess air.
    b) Calculate the mass flowrate of air used

    2) Ethanol dehydration reaction is carried out in a reactor with the feed entering at 300oC
    C2H5OH --> CH3CHO + H2
    This is an endothermic reaction. The feed contain 90% mole % ethanol and the balance acetaldehyde. Heat is added to keep the temprature from dropping too rapidly. With a fraction of 30% and outlet temprature of 100oC.
    Calculate the heat addition rate per 253 mol of the feed gas.
    Below are the heat capacity data (KJ/mol-C)
    Cp [C2H5OH] = 0.11
    Cp [CH3CHO] = 0.08
    Cp [ H2] = 0.029
    Heat of reaction = 68.95 J

    Take Reference Temprature = 25oC

    Here my Working out for question 1:
    12.6CO2+6.3O2 -> 12.6CO2
    41.6H2+20.8O2 -> 41.6H2O
    26.6CH4+53.2O2 -> 26.6CO2+53.2H2O

    Total CO2= 12.6+2.6+26.6=41.8
    Total H2O=41.6+53.2=94.8
    Total O2=(6.3+20.8+53.2)X1.1+1.9=90.23
    Total N2=((79/21)X88.33)+14.7=346.99

    After that i worked out the %'s eg. %CO2(41.8/573.82)X100=7.3% (I'd put all of my % up but I'm not sure I'm right upto this point at best)

    1b) mrXmoles=mass
    O2, 32X88.33=2826.56
    N2, 28X332.3=9304.04
  2. jcsd
  3. Dec 14, 2009 #2
    I've sorted out question 1 but I still need help with question 2.
    Any help still welcome.
  4. Dec 16, 2009 #3
    Help with the language used would also be helpful, thank you
  5. Dec 17, 2009 #4
    Ethanol dehydration means that H2 is being removed from ethanol, which is stated in the reaction.

    So, do you know the difference between an endothermic and an exothermic reaction?
    One type of reaction "absorbs heat" and the other "gives off heat," which means that one type of reaction will feel cold and one will feel hot. Which one is which?

    Well from the question you can gather that an endothermic reaction will "absorb heat" and so it makes the ambient cool. Every material has some ability to store heat: this is the heat capacity. Here you have been given specific heat capacities (note the units) which is the amount of heat required to raise one mole of substance one degree C. The heat of reaction is the enthalpy change that occurs during the reaction (on a molar basis).

    I was planning on continuing, but I'm pretty tired.. hope this helps you.. I didn't check your work for #1 btw.
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