NARRATOR: The Bypass procedure is similar to a recycle.
However, instead of components within the process
being sent back to the feed or another unit,
a stream splits from the feed.
Bypasses the unit,
and then mixes with the stream exiting the unit.
So let's look at the picture below.
Here we have a feed that's going into the system.
And right here, this feed splits into two streams.
One of them is what's known as the bypass stream,
and then the other right here goes into our process unit.
And if you know note, the bypass stream as it's called,
bypasses the entire unit.
So here, we have a stream that exits the process unit
and then here it combines with that stream,
and then the final stream is a product.
So we're gonna look at an example of how to use this.
So we're gonna start with a feed stream.
It's 10 kilograms per second.
And it is by mass 30%, A and 70% B.
So this stream splits such that 20% of it forms a bypass.
So here's our bypass and 20% of 10 kilograms per second
is two kilograms per second.
And what that means is here, the stream going into the unit
is eight kilograms per second.
Now this is very important.
Because there is a split right here,
all of the composition stay the same.
There's no reaction.
There's no removing of any of the components.
Therefore, this stream is gonna be 0.3A and 0.7B.
And this stream is gonna be 0.3A and 0.7B.
So the process unit removes enough B.
So let's say we have a stream here of B,
such that the final product is 50% A by mass.
So if this is 50% A, then it has to be 50% B as well.
So let's fill in the unknowns here.
So we don't know this mass flow rate
but we do know it's composition
because it says B is removed.
Here, we don't know the mass flow rate of this,
but we again know the composition.
And finally here, we don't know the mass flow rate.
When you have a bypass stream mixing with the stream
from the process unit, the compositions cannot be the same.
So this has some composition XA
and then XB is equal to one minus XA.
And what we wanna do here is
we wanna find all of these flow rates
as well as this unknown composition.
So we've labeled the system.
Where do you think we should do a balance?
Well, if we look at where we have the most information,
it makes sense to do an overall balance.
So note when we do an overall balance, we have two unknowns.
So we have M1 and M2, and we have two species A and B,
therefore we have zero degrees of freedom.
So let's write out the appropriate equations.
Coming in we have 10 kilograms per second,
and coming out, we have an M1 plus M2.
Now let's do an A balance.
And the reason to do an A rather than a B,
is it means we don't have to include stream M1
because it's 100% B.
So here we have 0.3 times 10 kilograms per second
and that's gonna be equal to 0.5 times M2.
And from those two equations,
we find that our M1 is equal to four kilograms per second.
And our M2 is therefore
equal to six kilograms per second.
So now that we have those, and let's put them in here.
So this is six kilograms per second,
and this is four kilograms per second.
Now we're going to have to do a balance
for our M3 and our XA.
So let's do our balance right here around our mixing point.
And again, remember because it is a mixing point
the compositions are not the same.
So we have here that M3 plus two kilograms per second
is equal to six kilograms per second.
Therefore, M3 is equal to four kilograms per second.
Now we'll do an A balance there.
So our 0.3, and this is coming from our bypass stream,
times two kilograms per second.
And we add that to our XA,
and this is times what we just calculated,
four kilograms per second.
And this is gonna have to equal
0.5 times six kilograms per second.
And from that, we find that our XA is equal to 0.6.
So now we can replace this right here.
That our M3 here is four kilograms per second.
And we know that it's 0.6A and 0.4B.
And let's see if that makes sense.
So if we have one stream here, that's 30% A,
one stream here that's 60% A,
the final composition should be somewhere in between there.
So you can see this is just a general material balance.
However the two important things to remember
about a bypass is,
A, the feed stream splits to form the bypass
and the stream into the unit.
And they are all the same composition.
And B, the bypass stream mixes with the stream from the unit
and that forms the product stream.
And these streams have different compositions.
As a final note, never assume that this feed stream up here
splits equally into the bypass stream
and the processing unit stream
unless you're specifically told that's the case.