Distillation is a physical process that can separate mixtures containing at least one liquid. It is most commonly encountered as the natural process wherein water from the earth’s surface is evaporated by the sun’s energy and passes into the atmosphere as water vapor. Once condensed, the water vapor forms clouds and eventually returns to the earth's surface in the form of precipitation.
Distillation is capable of separating mixtures because each substance in the mixture has its own unique boiling point. Therefore, when a mixture is heated the temperature of the mixture rises until it reaches the temperature of the lowest boiling substance in the mixture. The lowest boiling substance boils away quantitatively before the liquid temperature rises to the boiling point of the next higher boiling substance which then begins to vaporize. This process continues sequentially until each substance has vaporized.
Because of its ability to separate substances, distillation is used in a wide variety of processes, including concentration of alcohol following fermentation in the production of spirits and to separate crude oil into a myriad of petroleum products such as diesel and gasoline.
In water treatment, the advantages of distillation include its ability to separate water from virtually all dissolved and suspended contaminants as well as the effect of high temperature to kill viruses, bacteria and spores. Since volatile organic compounds (VOC’s) present in the water will vaporize, provision must be made in the design of distillers to remove these. Design for removal is based on the lower boiling point of these compounds.
Water distillation requires provision of heat sufficient to convert the water into steam and a means to capture and cool the steam to produce water. There are advantages to pressurizing the steam over the water so that water boils at a temperature higher than 100° C (212° F) (e.g., higher assurance of destruction of viruses, bacteria and bacterial spores). Since distillation is extremely energy intensive, considerable still design involves energy recovery from the condensation step to preheat raw water. Vapor compression stills are a common design which use complex engineering to enhance energy recovery.
Distillation has historically been the technology of choice for producing water meeting US Pharmacopoeia requirements for USP and water for injection (WFI). Still design for these applications must take into account all aspects of sanitary design including sloped piping to drain, minimization of dead legs, low point drains and sanitary pipe unions. Means must be provided to vent VOC’s and to prevent carryover of liquid into the steam.
Because distillation leaves all dissolved and suspended substances in the boiler as water vaporizes, scaling by hardness, silica and strontium and barium sulfates are distinct possibilities depending on their concentration in the raw water. Manufacturers of sophisticated, highly-engineered distillers have stringent pre-treatment requirements. The WaterProfessionals® can help you meet these requirements with demineralizers, reverse osmosis or softeners.