In the previous screencast, I showed you how to take a wet gas and calculate the mole fractions
of the other components on a dry gas basis. Here we are given a dry basis composition.
We are also given a mole fraction of water, and we need to find the composition of the
stack gas on a wet basis. Here is the composition of our dry gas. In addition, the mole fraction of water
in the stack gas, not on the dry basis, is 0.06, and we want to know the complete composition on a wet basis.
So here we have the composition of our dry gas, but what we
want are the compositions of the overall gas. Let's figure out how to do it. So our first
step is figure out the number of moles of water in the dry gas, and what we know is that in
every one mole of wet gas we have 0.06 moles of H2O. We also know in every mole of wet
gas we have 0.94 moles of dry gas. So the next thing we will do is let's relate the two.
So we have again this 0.06 moles of H2O, and here is mole of wet gas, in every
1 mole of our wet gas there is 0.94 moles of dry gas. We cross-out these. What we end
up with is that there are 0.064 moles of H2O per every mole of dry gas. So the next thing
we have to do is assume a basis. So we are going to assume a basis of 100 moles of dry
gas, and if we do that we can find using the compositions we were given we can find the
moles of every component in the dry gas. So we have our 70 of N2, our 15 of CO2, 10 of
O2, and 5 of CO, but if you remember we have 0.064 moles of water for every 1 mole of dry
gas. So for every 100 moles of dry gas we are going to have 6.4 moles of water, and
now we find our compositions the way we normally would. With our total moles being 106.4 moles,
and if we go back to our picture. Here is the 100 moles that we assumed as a basis of
the dry gas, and here is that 106.4 moles that is our wet gas. So now we are going to
finally find the composition on a wet basis. So here is the composition of our N2, our
CO2, our O2, our CO, and finally our H2O. Note when we add this up this equals 1. Note
that we set a basis for both the wet and the dry gas analysis that we have done earlier.
We can do this because we are looking for a composition of the stream not a molar flow
rate. Try this yourself if using a different basis, such as 50 moles or 1 mole of dry gas,
and you will see that you get the exact same compositions.