snuffy wrote -
This talk of hypothetical but non-existent vacuums in systems open to the atmosphere is misleading and decidedly Aristotelian. There are positive and negative pressure differentials.
i'm sorry if i implied that a vacuum exist in a compound refractionating column. i was merely trying to show/explain that
if a vacuum existed it would be eliminated through revaporization of condensate.
I agree that the vapor being produced in the boiler displaces the atmosphere. However, this displacement does not, ipso facto, produce a positive pressure inside the column. The density of the vapor is crucial. If the vapor is lighter than air, there will be a negative pressure where the column connects to the outside atmosphere.
the vapor is constantly being produced and expanding away from the boiler, condensate returns to the boiler, not vapor. what do you call the force exerted by one substance against another when the substances are vapors/gases? i've been taught that this is called pressure.
think of this: under standard conditions you are given a container with an open top, a closed bottom and an inlet at the bottom. attach a hose to the inlet and fill the container with helium. it won't work because the helium being lighter than air, pushes past the air in the container and out the open top. retry by turning the container over, it works because the helium is trapped by the now closed top of the container and the air is pushed out the now open bottom. if, in the first part of this experiment, carbon dioxide had been used, then the CO2 being heavier than air would have pushed the air out the open top of the container.
Thought experiment: a refluxing column has a LM head mounted directly over the VM head. Both takeoff heads have their valves adjusted to a tiny trickle, so as to minimize the interaction. If VM works as I described above, when the VM cutoff point approaches, the VM head will cease producing as the density of the vapor falls and the LM head will continue. The boiler runs with constant heat input throughout and its rate of vapor production has no effect on the observed results.
if the vapor management head is properly working, there will not be any vapor getting by its reflux condensor to reach the liquid management head.
rad14701 wrote -
It seems that if the condenser is too efficient then it would push the boundary layer down below the take-off, thus causing take-off to cease... Vapor rising and colder air pushing it back down... That's more or less how the process works in layman's terms, aside from the normal concept of having the vapor condense on the condenser and drop as liquid
nixon and mccaw discuss this in detial in their book, "the compleat distiller". basically it's a self canceling/regulating process, if there is no vapor reaching the reflux condensor there is no condensate to drip back and cool the top of the column. heat flows to cold. cold
does not radiate away from the condensor. one of the basic laws of thermodynamics.
it's not
colder air that is pushing down, the molecules in the vapor have absorbed heat and have phase jumped from liquid to vapor, the molecules in the vapor are carrying the heat, the molecules in the vapor release the heat energy to the reflux condensor when they run into it. when the heat/additional energy that allowed them to phase jump is gone, they revert back to liquid form.
edit for spelling and grammar