Vacuum Distillation: Vacuum technology in the chemical industry
The chemical industry is an indispensable supplier of raw materials to many economic sectors. The automotive industry, mechanical engineering, plastics, food, glass, or construction material industries, for example, all rely on substances that are produced by the chemical industry. These substances are employed in countless finished products that we use daily. A large proportion of these applications relies on vacuum technology. One of the most important applications is vacuum distillation.
Advantages and applications of vacuum distillation
The boiling point of many organic substances at atmospheric pressure lies within a temperature range from 200 to 400 °C. The separation of such mixtures therefore requires a high energy input at atmospheric pressure. Thermal decomposition of the mixtures can also occur in many cases at these temperatures.
Reducing the pressure into the low and medium vacuum range significantly reduces the boiling temperatures. In the case of monoglyceride, for example, which is used as an emulsifier, the boiling point is reduced from 300 °C at atmospheric pressure to about 220 °C at a pressure of < 0.1 hPa. Vacuum distillation is therefore widely used for processing temperature-sensitive substances such as those found in the food, pharmaceutical and petrochemical industries.
Another important factor that influences the preparation of temperature-sensitive substances is their dwell time in the vaporization device. In many cases, batch distillation in a reactor is unsuitable due to the dwell time, which can extend to hours, and due to the inadequate vacuum resulting from the fluid column in the vessel. A fill level of 10 cm, for example, means a density-dependent pressure of about 10 hPa. For this reason, thin film vaporizers and short-path evaporators are used in industrial applications. In these cylindrical systems, a very thin film (film thickness 1 to 3 mm) of the liquid to be vaporized is applied to the inside surface of the heated cylinder using rollers or wiper blades. Depending on the size of the system, the dwell time may be only a few seconds. Thin film vaporization works best within a pressure range from 1 to 100 hPa. Lower pressures are difficult to achieve due to pressure losses in the vaporous substances flowing from the device to the condenser.
However, for the separation of mono-, di- and triglycerides, for example, pressures in the range of 0.01 hPa are required. In this case, the so-called short-path distillation method is used. The condenser is located at the center of the cylindrical vaporizer and the distances between the hot wall and the water-cooled pipe coil are in the range of a few centimeters, depending on the size of the device. Pressure losses are minimized, since the material to be evaporated condenses directly on the cold surface. Since the mean free path of a molecule in a medium vacuum is in the range of the distance between the cylinder and the internal condenser, or is significantly greater, this method is also referred to as molecular distillation.
Read more about this state of the art technology in the attached application report.
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VTA vacuum distillation plant (Photo: VTA GmbH & Co. KG)