Simple Distillation: Recycle waste from biodiesel
production
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Boiling point composition of a mixture of
cyclohexane and toluene
Question: How far should you insert the thermometer into
the distillation head?
Answer: The thermometer should extend just below the
side arm connecting the condenser to the distillation
head.
• You are measuring the temperature of the vapor
that condenses in the condenser, you want the
thermometer bulb located just below the side
arm.
Water
• You always want the water to flow into the bottom
of the condenser. It is because water will flow up,
against gravity, and completely fill the
condenser’s outer jacket.
• If you were to add water at the top of the
condenser, the water would fall to the bottom of
the condenser and you would see an air pocket
in the cooling jacket.
To use a boiling point composition curve, you first need
to determine the mole percent of one of your compounds
in the two-component mixture.
Set-Up
Simple Distillation
• You are heating the mixture so the temperature
increases. The temperature will increase until it
hits the liquid composition line. This is the
temperature at which the mixture will begin to
boil.
Question: At what mole percent water should be in the
first drop collected?
Boiling Point Composition Curve
• A boiling point composition curve of a twocomponent mixture has temperature on the y-axis
and mole percent of one of the components on
the x-axis.
• The lowest curve on a boiling point composition
diagram is called the liquid composition line. The
highest curve is called the vapor composition line.
This line tells you the mole percent of the
compound when they are in the vapor phase at a
given temperature.
Answer: 100% mole percent water
The vapor consists of 100% water because water’s
boiling point (100 c) is so much lower than glycerol’s
boiling point (290 c).
Question: When you collect distillate in the collection
flask, what happens to the original mole percent of water
in the distillation flask?
Answer: Mole percent of water decreases.
The percentage of water, the lower boiling point
compound, is higher in the vapor phase.
Question: When you collected 50 ml of distillate in the
collection flask, the mole percent of water in the distillation
flask decreased to approximately 20 mole percent.
With your distillation set-up, what percentage of water will
be in the vapor phase at this position of the liquid?
Answer: 98% mole percent water
The mole percent water in the vapor is still very high
because there is such a big difference between the boiling
points of water and glycerol.
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When a compound boils, the temperature
remains constant. Since water boils at 100 c, you
can see water being distilled from the mixture as
a flat line at that temperature.
For a higher boiling point, the place where the
solution is being removed is the highest
temperature flat line on the temperature versus
volume plot.
Question: Where should you ideally change collection
flasks during the distillation process to get one flask with
pure water and one flask with pure glycerol?
You want to change during the transition between the two
boiling points.
3 mole methanol ≈ 2 moles water
Question: How does it differ from the glycerol-water
temperature vs volume plot that you observed in you first
experiment?
Answer: The boiling points of the two compounds are not
as transparent and there is no defined transition between
the two boiling points.
Question: At what temperature do you expect the 60 mole
percent methanol in water mixture to boil?
Answer: 72%
The liquid mixture in the flask should begin to boil at 72 c
if you are performing the experiment at sea level.
Remember, boiling point composition curves typically
assume an atmospheric pressure of 760 mm Hg.
Question: When you are performing this same distillation
at a higher elevation for example, at 300 meters above
sea level, how do you think the initial boiling point of the
mixture in the distillation flask would change?
Answer: The boiling point would decrease as compared
to the boiling point at sea level.
Question: If you collected the first drop from the
distillation, what would be the mole percent methanol in
that drop? Remember your, original distillation flask
contains 60 mole percent methanol.
Answer: 83 mole percent
You are getting some separation of water and methanol,
but the methanol is n0t 100% pure.
Question: If the mole percent of methanol in the liquid
phase in the distillation flask drops to 30%, what would be
the mole percent of the methanol collected in the
collection flask?
Answer: 67 percent mole percent methanol
As the mole percent of methanol decreases in the
distillation flask, the mole percent of methanol also
decreases in the collection flask.