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 centimetres, 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.
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.
Roots pumping stations for thin-film and short-path distillation.
Maintaining the exact vacuum pressure required in the vaporizer is vitally important for the quality of the separation pro- cess and imposes big demands on the vacuum control system and the quality of the vacuum pumps used.
Pfeiffer Vacuum therefore offers Roots pumping stations with liquid ring pumps as proven solution. Depending on the number of Roots piston stages, a pressure of 10-3 hPa can be reached without great effort. It is also possible to operate the liquid ring pump with the substance that needs to be distilled. One example is the processing of rolling oil. The oil contaminated by the rolling operation is reprocessed through distillation. For this, three-stage Roots pumping stations consisting of two Roots pumps and a liquid ring pump are used. The rolling oil to be distilled serves as the operating liquid for the liquid ring pump. At a pressure of approximately 5 hPa, the rolling oil evaporates and is condensed in the downstream condenser. The possibility of leakage air saturated with oil vapors being sucked into the vacuum pump system, where the oil vapors can then condense again in the liquid ring pump, cannot be completely ruled out, however. But if the rolling oil is used as the operating liquid, this will not negatively affect the through- put of the pump. The level of the liquid in the circulatory container of the liquid ring pump rises slowly. When the maximum permitted level is reached, the operating liquid is automatically discharged and fed to a suitable treatment process. Depending on the application, dry backing pumps can also be used instead of the liquid ring pump.
Vacuum solutions for vacuum distillation
Pfeiffer Vacuum offers customized solutions to create the vacuum conditions required for the various applications. Especially with regard to applications in the chemical industry that require a pressure of less than 33 hPa, the Roots pumping stations of Pfeiffer Vacuum’s OktaLine have established as solutions. Depending on the required pumping speed and ultimate pressure, different pumping stages can be built in. Roots pumps with a pumping capacity of 145 m³/h to 8,000 m³/h are available as standard. In special cases, Roots pumps with a pumping speed of up to 25,000 m³/h can be manufactured.
The gas circulation cooled version of the Okta allows compression to ambient pressure and is used predominantly in critical processes in the chemical industry. The standard pumps are made of spheroidal graphite cast iron (GGG40), which ensures the high-pressure shock resistance (16 bar) of the pump housing. This is particularly important for ATEX applications. For particularly corrosive applications, the Okta can be made of stainless steel. It is also possible to apply a plasma-polymer coating to the parts of the suction chamber that come into contact with the product. More demanding requirements regarding the tightness of the pump are achieved with the aid of magnetically coupled drives with leakage rates of less than 1E-6 Pa m³/s.
Rotary vane, screw and gas circulation cooled Roots pumps from Pfeiffer Vacuum are available as backing pumps. For ATEX applications including Zone 1 indoor/outdoor, suit- ably certified pumps are available.