In this screencast I am going to describe the operation of a triple effect evaporator
where we have backward feed. There is a separate screencast that describes a triple-effect
evaporator with the feed in the same direction that the steam is fed. And we will see exactly
what that means for this backward feed system we are going to feed into the third effect,
so we will call this the 3rd, the 2nd and the 1st. And we are going to feed a dilute
solution. For example, a solid in water. And then, our objective is to create a concentrated
solution out of the first effect. And the idea of using a triple-effect evaporator is
that we use significantly less energy than a single effect. We can essentially, say for
water, evaporate 3 kilograms of water for feeding in 1 kilogram of steam. So the way the evaporation
is going to work in the various stages, what we are going to do is feed into the heat exchanger
coil, steam, and the outlet of this is going to be condensate, namely the steam that is
condensed liquid water. We use the energy from the steam to evaporate out this first
effect. What we are going to do with this vapor is use it now as the heating source
for the second evaporator. So this means we are going to feed in, now, this steam again
to the heat exchanger coil. Again, coming out it is going to be liquid, as we condense
the steam. And that steam of course is going to evaporate. So we are going to have vapor,
call that vapor 2. Well that vapor is then going to be fed into the heating coils for
the third effect and coming out of the third effect again is liquid water. So that is part
of the steps of how we are getting evaporation and of course in the third step means, we
are again forming vapor which is the solvent. Well we fed into this third effect a dilute
solution. Because we are evaporating some of the liquid of the solvent we have made
a more concentrated solution so that means we have more concentrated solution leaving
the third effect. So we want to feed that now to the second effect. However, one consequence
of running this system in the direction we are running it is that temperature 2 for this
system, namely that is the temperature here, that is the temperature of the more concentrated
material that is leaving here. So T2 is going to be higher than T3. So if we are going to
feed the liquid coming out of the third effect into the second effect And it becomes the
feed for the 2nd effect, we actually have to raise the pressure because P3 is less than
P2 because we are talking about saturation temperature and of course bonding pressure.
So we can't feed it in without raising the pressure. So that means we take out this more
concentrated liquid, we have to use a pump to raise the pressure and then we can feed
the concentrated liquid into the second effect. And then the same thing, the outlet from the
second effect, because, again, temperature 1 is greater than T2. Which means P1 is greater
than P2. So the liquid leaving the second effect also needs a pump to raise its pressure
so it can be fed into the first effect where it is concentrated to form our final concentrated
solution. And so this describes a triple-effect evaporator. A couple more things to point
out. The mole fraction now in the liquid of our solute, x1, is greater than x2 is greater
than x3. x3 is what is leaving here, x2, here, x1, here, and this would be x of the feed
and feed mole fraction is even lower. So this describes the operation where we take advantage
of a vapor that we form in one effect as the heat source, and that is why T1 must be greater
than T2, in order for it to be able to evaporate the liquid in the second effect. Likewise
the T2 must be greater than T3 to have a temperature difference such that we can evaporate the
vapor 3. There is a couple reasons why we might run the triple effect evaporator with
this backward feed. One is if our fresh feed that is coming in here, this is our dilute
solution. If our fresh feed is cold. The reason being now, it will take a smaller amount to
heat it to our lowest temperature rather than heating it to our highest temperature. The
second would be that as we concentrate the solute the liquid becomes more viscous. So
the viscosity increases as x increases, the mole fraction of the solute. And therefore
we want the hottest temperature to be when we have the highest concentration of the solute
because the heat transfer is going to be higher when the material is less viscous and at a
higher temperature it is going to be less viscous. Other aspects of this are the same
as the forward feed, and an important aspect is of course that we can get a lot more evaporation
for the same amount of feed of the steam. So this is the big cost in the process separate
from the capital costs is this steam being provided to the system. And the more we can
evaporate the steam the more economical the process is.