Vacuum Distillation

Separating sensitive substances with vacuum distillation

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.

Application requirements

Maintaining the exact vacuum pressure
Reliable vacuum control system
High quality of the vacuum pumps
Long maintenance intervals – reducing operating costs

How does it work?

Vacuum distillation is a process used to separate substance from liquid mixtures into their pure starting substances. This can be achieved by simple heating at atmospheric pressure, as is done with alcohol distillation. But not all substances can withstand high temperatures. With some mixtures, it can lead to chemical reactions or to disintegration of one or more individual substances. For all these applications, distillation under vacuum is vital. Unlike distillation at atmospheric pressure, vacuum distillation occurs under lowered pressure. This allows for evaporation already at lower temperatures. The basis for this is the vapor pressure curve of the respective substance. Water, which begins to boil at 100 °C under atmospheric pressure, can already boil at lower temperatures in a vacuum, for example at 90 °C and approximately 800 mbar. This makes it possible to carefully separate heat-sensitive materials from each other or those who would begin to break down at high temperatures. Vacuum distillation, for example, is used for separating oils, fatty acids, esters or monoglycerides.

Vacuum requirements & process examples

Maintaining the exact vacuum pressure required in the vaporizer is vitally important for the quality of the separation process and imposes big demands on the vacuum control system and the quality of the vacuum pumps used. If the vapor pressures of the substances to be separated are very close to each other, vacuum rectification columns are used. In these counter-current distillation systems, the vapor flows through a vertically aligned column to the condensed liquid. Installations such as structured packings ensure good mixing of the two phases so that a phase equilibrium can be achieved. Dwell times and pressure levels are higher than with thin film vaporizers, however. The two processes are often combined.

Vacuum distillation is enormously important for the polyisocyanate production. Following the actual production of the isocyanate, the highest possible concentration is ensured by means of a multi-stage distillation process. It requires fine vacuum conditions, i.e., absolute pressures in the range of 0.05 hPa.

The oil contaminated by the rolling operation is reprocessed through distillation. For this, three-stage Roots pumping stations, consisting of two Roots pumps and one liquid ring vacuum pump, are used. As the operating fluid for the liquid ring pump, the rolling oil to be distilled is used. At a pressure of around 1 mbar, only the rolling oil evaporates and can be condensed out again in the liquid ring pump. In this way, contaminants are removed and the rolling oil can be used again.

Product portfolio

For short path and molecular distillation, Pfeiffer Vacuum offers a large selection of suitable vacuum equipment. Depending on the required pumping speed and ultimate pressure, different pumping stages can be built in. They are available in gas-cooled or standard air-cooled versions as well as in different materials, for example, spheroidal graphite cast iron or stainless steel. Specific coatings and coupling types are also available and can be combined for individual requirements. Liquid ring, rotary vane, screw, and gas-cooled Roots pumps can be used as backing pumps. Furthermore, ATEX certified Roots pumps are also available for use in potentially explosive environments. From conception to implementation, the experts at Pfeiffer Vacuum develop individual solutions together with customers from all different areas of the chemical industry. These solutions are exactly designed for the requirements of the individual application.