Pressure Ranges

Gentle thermal separation of liquid mixtures largely preserves the physical and chemical integrity of the relevant ingredients. It can be accomplished with a reduction of boiling temperatures. This, in turn, can conveniently be accomplished via a reduction of pressure.

A broad portfolio of vacuum pump types and sophisticated designs of pump sets are available to effectuate such vacuum conditions. Vacuum pressure ranges are usually subdivided into the intervals depicted in the adjacent diagram.

It is important to note the variation of typical particle densities (blue) and mean free path lengths (red) as a function of pressure. In Statistical Physics it is shown that p ~ λ^-1~ ρ, with p, λ, and ρ denoting pressure, mean free path length, and particle density, respectively.

Typical pressures applied for vacuum distillation are in the fine vacuum range, i.e., between 1 mbar and 10^-3 mbar. Corresponding mean free path lengths are in the centimeter range.

One might argue that a further reduction of the pressure into the range of high vacuum and below could even further increase the efficiency of vacuum distillation. This is in fact a remarkably popular misunderstanding which ignores the fact that thermal separation via vacuum distillation obviously also implies the flow of material. Particle densities in the high vacuum range and below make mass transportation very inefficient.

Another important phenomenon to consider is flow dynamics: at atmospheric pressure and in the rough vacuum range fluids show viscuous behaviour which implies that particles are more likely to collide with each other rather than with the boundaries (walls) of the equipment. The contrary is true in the range of high vacuum and below where mean free path lengths exceed typical dimensions of the equipment. The implied flow dynamics in this range is commonly labeled "molecular flow".

Under fine vacuum conditions mean free path length are comparable with typical dimensions of the equipment which implies so-called Knudsen flow. Model calculations and experience show that this fine vacuum pressure range is indeed best suited for vacuum distillation.

Pumps, Pump Sets

Vacuum pumps for the intended pressure range are gas transfer pumps of the "positive displacement" or the "momentum transfer" category. Characteristic parameters that determine the suitability of a pump type for a given task include:

suction capacity, speed
inlet and exit pressure
compression rate
operating fluids
working temperature
(chemical) resistance
maintainability

In plants from UIC you will preferably find the following pump types in varying sizes and performance categories:


Liquid Ring Pump	Rotary Vane Pump	Roots Blower	Steam Ejector Pump	Oil Diffusion Pump
postive displacement			momentum transfer

The design of an effective vacuum system for a plant requires a diligent and thorough analysis of the performance criteria including technical and commercial constraints. This includes the selection and combination of pump types and sizes as well as the piping of the vacuum system. Below you may find some examples of pump sets for different vacuum ranges.

Instrumentation, Automation

Besides temperatures and feed rates, pressure is one of the most important control parameters of each vacuum distillation plant. Adequate instrumentation requires pressure gauges in order to keep track of process performance and to enable automation.

Plants from UIC are typically equipped with membrane pressure gauges and Pirani-type vacuum gauges.

Leak Tightness

A key quality criterion for a vacuum distillation plant is its leak tightness.

Lab and pilot plants from UIC routinely achieve leak rates below 0.005 mbar·l·s^-1, whereas larger and more complex plants for industrial production achieve values below 0.05 mbar·l·s^-1.

Common tests for leak tightness include measurement of pressure increase and helium sniffing. Such tests may be ordered from UIC as a service.