In this screencast we are going to learn how to design a flow chart. When you are confronted with
a wordy problem that asks you to solve for one or more specific values the first thing
you should do is draw a flow chart. So what are the things that should be on this flow
chart? Well the first things you need are any of the process units that are involved.
This could be a distillation column, this could be a reactor, or crystallizer, but you
want to make sure that they are all drawn in there, and usually they are best drawn
as squares or rectangles. I save the rectangles for absorption columns or distillation columns.
In addition you need all of the streams that are flowing in and out. This is in and out of every one
of the units. Then you want to write down or label all the known quantities. So anything
that is given to you in the problem description, and finally you want to label the unknowns,
and usually what you want to do is have variable names that make sense to you, and one of the
things because there can be a number of unknowns and knowns in your problem. You want to
keep your variables to a minimum. So let's start with a relatively easy problem. So we
have an equal or equimolar, which means the same amount of moles of both methane
and oxygen, and they are fed to a furnace. So we know, first of all we have a furnace,
and we have a stream coming in that has methane and oxygen. Half of the methane is combusted
to CO2 and and H2O. So the CO2 and remaining oxygen exit the reactor. The unreacted methane
as well as the water are sent to a condenser where the water is condensed. Both the water
and the methane exit the condenser. What I have done is drawn the processes units and
the streams going in and out as I went through the problem. Now let's see what we know and
what we don't know. So we are told that 100 moles of methane are fed to the reactor. So
we will put in 100 moles of methane. We are also told that it is equimolar. So that means
there is 100 moles of O2 fed to the reactor. The CO2 and I will just call it number of
moles of CO2, and remaining oxygen, nO2, exit the reactor. The unreacted methane, so we
know that it is half. So that is 50 moles of methane, as well as the water are sent
to a condenser where the water is condensed. Both the water and the methane, and notice
nothing happens to the number of moles. It is just that the water is condensed exit the
condenser and what the problem is asking is to find the number of moles of water condensed.
Notice here I named each component, even though they exited in the same stream, and one of
the things that you will find when you are doing reaction problems that's a good way
to keep track of all of your different products. So now let's take a look at another example.
Here we have an aqueous solution of 25 weight percent of calcium chloride and it is sent
to a crystallizer. So here is our crystallizer and here is our stream going in. Enough water
is evaporated. So that means that we have water coming off of the crystallizer. So that
5 kg of crystals precipitate out. These crystals are suspended in an aqueous solution and then
they're sent to an evaporator where more water is evaporated, and exiting the evaporator are
10 kg of crystal and 5 weight percent aqueous solution, and how much water is evaporated from each
unit? So let's start here in the beginning. We have an aqueous solution that is 25 percent
or 0.25 calcium chloride, and because it is aqueous that means the rest of this is water. Enough
water is evaporated so that 5 kg of crystals precipitate out. We do not know how much
aqueous solution comes in and we also don't know how much water is evaporated. However
what we do know is that we have 5 kg of crystals and an aqueous solution that we don't know
the weight percent of. So we are going to call this x of the calcium and 1 minus x of
the calcium chloride, which is our water. The reason we can do that is that
if you sum the mass fractions of your components it is going to equal 1. So they're sent
to an evaporate where more water is evaporated and exiting the evaporator are 10 kg of crystals,
and 5 weight percent aqueous solution, and we don't know what the flow rate of this is,
and how much water is evaporated from each unit? What we are asking for is, what
is m2 and m4? So the things to be careful of in a problem like this is, first of all
if they say that water is evaporated, you need to make sure that you have this water
exiting the unit. The second thing is in here is an example of trying to minimize the number
of variables you have. Since we know that mass fractions equal 1. We can write the mass
fraction here of water, as 1 minus the mass fraction of calcium chloride. And finally here
when we are exiting the evaporator we have 10 kg of crystals, we know the weight or mass
percents here of the aqueous solution, but what we don't know is the mass flow rate
exiting here. So I hope that this gives you some idea of how to interpret these word problems
and turn them into flow charts.