INSTRUCTOR: I'm gonna describe
multiple-effect evaporation,
where the idea is we take a feed, that's in green here,
feed that is a five weight percent sugar solution
and we concentrate the sugar so that as we go through
the stages we end up with, in this case,
13.2% sugar solution by evaporating
some of the water from the solution.
Now we do the evaporation by having a heat exchanger
that we feed saturated steam, so the red,
saturated steam which then leaves here as saturated water.
And the advantage of these multiple effects
is that when we evaporate
some of the water from the sugar solution,
and that ends up being this blue stream,
that steam is used in the next effect of the evaporation.
So we move from left to right with the sugar solution,
the green, so we now feed a liquid into the second-effect,
we feed the liquid in that's higher concentration of sugar,
it goes in then to the third effect,
again at higher concentration,
until we end up with, in this case, 13.2%.
But the significant advantage is
instead of using additional outside steam
we use the evaporated water in the second stage.
In order for that to work and have a temperature difference
we run the second-effect,
the second stage, at a lower pressure
and therefore a lower boiling temperature.
And then the third stage, we do the same thing.
So the steam from the second stage
goes into the third stage, again,
at a lower pressure and a lower temperature, so again,
we have a lower temperature difference for heat transfer.
And the idea here, we calculate a steam economy
and that corresponds to the fact that instead of having
to feed fresh, saturated steam in each stage,
by using steam we created in the first stage
we can get more net water evaporated
from our sugar solution than steam that we feed in.
So for the conditions picked here,
we feed one kilogram of steam here and we evaporate
a total of 1.27 kilograms of water from our water solution.
The simulation allows us to vary
the feed pressure and the feed flow rate.
The other aspect of the simulation
is that I'm showing here the triple-effect.
Well we also can look at a double-effect
and a single-effect evaporator
to see the differences in efficiency.
And I should point out that this is a forward feed system
and that we're feeding the steam,
going in the same direction the evaporated water
and the more concentrated sugar solution.
It's also possible to do a reverse effect.
So let's look at the simulation
to understand a little better the behavior.
So here is the simulation, you notice it says
you can hover the cursor over each stream
and you get more information.
So if we look at this saturated steam
leaving the first-effect,
it gives us a flow rate and its temperature.
And then likewise, leaving the second-effect,
the saturated water from the heat exchanger
gives the flow rate.
Of course the same flow rate is coming in as steam.
We've concentrated the liquid in the first stage to 8.8%,
and then in the second stage to 11.3.
If I increase the feed pressure,
notice I increase the steam economy,
and I'm also of course increasing
the temperatures in the various effects.
If I increase the flow rate
so that we get more going through,
so let's go back and look at that,
notice the increase in the flow rate,
I'm lowering the sugar concentration in my concentrate
because I'm processing much more,
I'm increasing the steam economy so it's above two.
So, this demonstrates the significant increase
in energy efficiency by using a multiple-effect evaporator.
This simulation also has a double-effect evaporator.
In this case we can label the various streams
by clicking on show all streams,
and then the single-effect evaporator.
And you'll notice that we actually have
efficiency slightly greater than one
and this is just related to the fact
that heat of vaporization is a function
of temperature, or saturation pressure,
and so the feed stream at 154 degrees
has a different heat of vaporization
than the saturated steam that we feed in 200 degrees C.
So we use mass and energy balances to calculate
the differences for these multiple-effect evaporators
and it shows that we can get more efficiency,
utilize our energy better by having additional effects.