A waste gas stream contains 50 mol % CH4, 10 % CO2, 20% water and the rest nitro

Question

A waste gas stream contains 50 mol % CH4, 10 % CO2, 20% water and the rest nitrogen. This stream is to be burned with pure oxygen.   For a basis of 100 moles of the waste gas stream, determine:

- The stoichiometric flow of oxygen required

- The flowrate and composition (mol%) of the exit stream if the waste gas is burned with 25% excess oxygen.

A waste gas stream contains 50 mol % CH4, 10 % C02, 20% water and the rest nitrogen. This stream is to be burned with pure oxygen. For a basis of 100 moles of the waste gas stream, determine: The stoichiometric flow of oxygen required The flowrate and composition (mol%) of the exit stream if the waste gas is burned with 25% excess oxygen.

Explanation / Answer

Feed = 100 moles of waste gas

Gas contans

CH4= 50moles and CO2=10 moles Water= 20 moles and Nitrogen= 100-(50+10+20)= 20 moles

The reaction is CH4+2O2--à CO2 +2H2O

1 mole of CH4 required 2 moles of oxygen

50 moles require 50*2= 100 moles of oxygen

Since pure oxygen is supplied ( not air), oxygen actually supplied =100*1.25= 125 moles

Product

CO2 formed = 50 moles ( 1mole of CH4 produces 1 mole of CO2)

CO2 initially present = 10 moles, total moles of CO2= 50+10= 60 moles

Moles of water formed= 2*50 =100 moles of water

Water initially present= 20 moles, total moles of water = 20+100=120 moles

Oxygen remaining = 25 moles, nitrogen= 20 moles

Products :CO2= 60 moles H2O= 120 , N2= 20 and O2= 25

Total = 60+120+20+25= 225 moles

Product composition : CO2= 100*60/225=26.67% H2O= 100*120/225=53.33% O2= 100*25/225=11.11% and N2= 100*20/225=8.89%

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