Vacuum evaporation is the process of causing thepressure in a liquid-filled container to be reduced below thevapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal. Although the process can be applied to any type of liquid at any vapor pressure, it is generally used to describe the boiling ofwater by lowering the container's internal pressure below standardatmospheric pressure and causing the water to boil atroom temperature.[1]
The vacuum evaporation treatment process consists of reducing the interior pressure of the evaporation chamber below atmospheric pressure. This reduces the boiling point of the liquid to be evaporated, thereby reducing or eliminating the need for heat in both the boiling and condensation processes. There are other advantages, such as the ability to distill liquids with high boiling points and avoiding decomposition of substances that are heat sensitive.[2]
When the process is applied tofood and the water isevaporated and removed, the food can be stored for long periods without spoiling. It is also used when boiling a substance at normal temperatures would chemically change the consistency of the product, such asegg whitescoagulating when attempting todehydrate thealbumen into a powder.
This process was invented byHenri Nestlé in 1866, ofNestlé Chocolate fame,[citation needed] although theShakers were already using avacuum pan before that (seecondensed milk).
This process is used industrially to make such food products asevaporated milk formilk chocolate andtomato paste forketchup.
In the sugar industry vacuum evaporation is used in thecrystallization ofsucrose solutions. Traditionally this process was performed in batch mode, but nowadays continuous vacuum pans are available.[3]
Vacuum evaporators are used in a wide range of industrial sectors to treatindustrial wastewater.[4] It represents a clean, safe and very versatile technology with low management costs, which in most cases serves as a zero-discharge treatment system.
Vacuum evaporation is also a form ofphysical vapor deposition used in thesemiconductor,microelectronics, andoptical industries. In this context it is used to depositthin films of material onto surfaces. Such a technique consists of pumping avacuum chamber to low pressures (<10−5torr) and heating a material to producevapor to deposit the material onto a cold surface. The material to be vaporized is typically heated until itsvapor pressure is high enough to produce a flux of severalAngstroms per second by using anelectricallyresistive heater or bombardment by a highvoltage beam.
Thermal evaporation has been investigated for the production of organic light-emitting diodes (OLEDs) and organic photovoltaic cells.[5][6] In organic photovoltaic cells, the purity of the organicsemiconductor layers influences the device's energy conversion efficiency and stability.[7]