That's a good question. It turns out to be easy to give an answer to
someone who's studied a little Statistical Mechanics, but I'll try to
give an answer that doesn't assume that sort of background.
"Sam Clayton" wrote in message
...
| When they make Maple syrup they boil off the water in order to
concentrate the
| sugars in the sap. The question is: Could you save money by setting
up a vacuum
| to lower the boiling point?
Maybe, but the added complexity of the system probly negate any
advantage. The normal process uses multiple vats, heated to
progressivly higher temperatures to deal with the higher bioling point
of the syrup. Why? Sugar can easily dissolve in liquid water. However,
taking the solute (sugar) out of the water and putting it in the gas
phase (air) requires a lot of energy. At temperatures around the water
boiling point, these solutes stay in the liquid.
Now the total pressure in the syrup and the air at the boundary are
the same, otherwise one would push the other into a smaller space.
Part of the pressure in the syrup comes from the solutes, not the
water. So the pressure due to the water alone is reduced compared to
that of pure water at the same temperature. The vapor pressure, that
is, the pressure of water vapor that would stay in equilibrium with
the syrup, is reduced by the same amount because of the solutes.
Syrup boils when the vapor pressure of the syrup gets to be as big as
the pressure of the atmosphere. At that point, vapor bubbles in the
syrup can grow. You have to heat the syrup with solutes up more to get
the vapor pressure in it to equal the atmospheric pressure, so it has
a higher boiling point.

Well of course you say that's my whole point. And sure enough it is.
But syrup production isn't a batch process, it's a continuous process,
controled by temperature alone, no other instrumentation is required.
Even the reverse osmosis used today to pre-consentrate the sap before
it enters the vats requires little instrumentation.

However, it you seal the process, as would be required to produce a
vacumm, you will now need instrumentation and process control
equipment which will increase your cast of production signiticantly.

| vacuum the steam away. Would it work and would it be worth doing?
|