Laboratory vacuum generation

John Morris Scientific Pty Ltd
Wednesday, 08 October, 2003



Vacuum technology for the chemical laboratory mainly focuses on vacuum systems in the rough and fine vacuum range (700-10-3 torr). In the past, the majority of pumps used in the chemical laboratory were water-jet pumps and oil-sealed rotary vane pumps. About 10 years ago, these systems were nearly completely replaced by systems using diaphragm pumps.

Diaphragm pumps are oil-free, highly reliable and flexible pumping systems. In combination with modern vacuum controllers and accessories like catchpots and exhaust-waste vapour condensers many processes are optimised and solvent recovery is possible.

Vacuum pumps for the rough vacuum range

In many chemical applications, vacuum conditions are essential for the separation or processing of reaction products in a controlled and energy-saving way. For example, distillations are typically run under vacuum to lower the boiling temperature. Usually, in chemical applications large quantities of substances are evaporated, generating huge amounts of vapours, which are often corrosive or hazardous. Therefore, special care has to be taken to protect both pumping systems and the environment.

Water-jet pumps and diaphragm pumps are commonly used when rough vacuum is needed. Waterjet pumps are robust and cheap but cannot prevent the emission of pumped vapours into the air or the drainage of trapped gases into the wastewater. Hence, they are more often replaced by oil-free diaphragm pumps. Diaphragm pumps are available in models using chemical resistant materials and are designed for an easy and effective solvent recovery.

Economical and ecological arguments rule out water-jet pumps. They are cheap to buy, but expensive to operate. The water consumption is typically 160 L/h and amounts to 1000 tonnes of water per year.

Also from an ecological point of view water-jet pumps are very disadvantageous. The pumped vapours are either dissolved into the wastewater or are dissipated into the air. Containment, re-use or disposal of solvents is not possible with water-jet pumps.

Diaphragm vacuum pumps

Modern diaphragm pumps are available with pumping speeds up to 220 L/min and an ultimate vacuum down to about 1 torr. They are completely oil-free and dry, and due to the design there are no restrictions on the mounting position. Chemistry diaphragm pumps - in which all wetted parts are made of PTFE-compound materials - also provide high resistance against chemical attack. Designs that exploit planar diaphragms require minimum maintenance and are easy to service.

The compression ratio of a single stage diaphragm pump is typically greater than 10, providing an ultimate vacuum of around 60 torr. To reach lower vacuum levels several stages can be connected in series, until the ultimate vacuum is limited by the gas dynamics and valves operation. Depending on the dynamic response of the valves - which is correlated with its mass, geometry, modulus of elasticity and the driving gas flow - this lower limit is around 1 torr. That means that in series connections of only up to four stages are of practical use.

To understand how these multistage systems operate, look at a two-stage diaphragm pump (Figure 3). At the beginning of the cycle the diaphragm in the first stage moves down and gas flows into the pumping chamber. The diaphragm in the first stage then moves upward, pushing the gas into the second stage, and from here the gas is exhausted to the atmosphere. With multi-stage designs, a large variety of pumping speeds and ultimate vacuum levels can be achieved (see Figure 4).

Chemistry diaphragm pumps, made of PTFE and even better corrosion resistant fluorinated plastics, offer an optimum of resistance against corrosion and chemical attack. A well defined gas ballast can be turned on to prevent condensation inside the pump. The pumped vapours can easily be condensed at the exhaust side. Thermodynamically, condensation is more effective at atmospheric pressure than under vacuum. With the help of an exhaust-waste vapour condenser, solvent recovery of close to 100% is reached. In comparison to common vacuum pumps, like water-jet pumps and oil-sealed rotary-vane pumps, diaphragm pumps provide advantages for vacuum generation in the chemical laboratory. Diaphragm pumps do not consume any water, nor do they produce any wastewater. Due to the complete absence of oil, no reactions of the pumped gases or vapours with a lubricant or sealing oil can occur. Protective accessories such as cold traps are not necessary. In addition, the diaphragm pump is very insensitive to condensation inside the pump. The emission of volatile and harmful chemicals into the environment is reduced by an efficient solvent recovery system.

Vacuum pumps in the fine vacuum range

Several applications in the chemical laboratory like freeze drying, short path and molecular distillation require vacuum in the fine vacuum range (1 torr to 10-3 torr). Chemical resistant diaphragm vacuum pumps which are the preferred choice in the chemical laboratory for applications in the rough vacuum range are limited to an ultimate pressure of about 1 torr. Thus, whenever vacuum below 1 torr is needed other pump types like oil-sealed rotary vane pumps and roots pumps have to be used. These pumps are neither completely dry nor corrosion resistant like the diaphragm pump. For low-pressure applications (< 1 torr), the Chemistry-Hybrid-pump, a combination of a rotary vane and a diaphragm pump, offers the vacuum performance of a rotary vane pump, but avoids its disadvantages with regard to oil deterioration and corrosion.

Oil-sealed rotary-vane pumps

Oil-sealed rotary-vane pumps under clean conditions are known as reliable and robust mechanical vacuum pumps. The pumping range extends from atmospheric pressure down to 10-3 torr. Sizes from 20 to 25,000 L/min in one- or two-stage designs are available. In the chemical laboratory only small pumps with pumping speeds between 30 and 150 L/min typically are in use (see Figure 1). In chemical applications the pumped substances are mainly solvent vapours. If the dew-point is reached during the compression cycle of the pump, condensation will occur. Condensation inside the pump leads to deterioration of the vacuum performance and to a reduction of the lubrication and sealing properties of the oil. Eventually, corrosion of the pump components will occur. To reduce this problem, accessories (dry-ice, a liquid-nitrogen trap or a mechanical refrigerated condenser) to trap the vapours prior to its entry into the pump are used and inconvenient, frequent oil changes are required. All these measures are expensive and often they do not prevent the generation of contaminated waste oil and corrosion of the pump.

Most modern pumps are fitted with a gas ballast, which enables the pump to cope with a certain amount of vapour. A controlled quantity of air is admitted to the pump during the compression phase and increases the portion of non-condensable gases. In most cases, then the gas mixture can be compressed and expelled without condensation. Due to the extra work needed to compress the additional air admitted to the pump, the pump temperature rises. This temperature rise also helps to prevent vapour condensation inside the pump.

The performance and corrosion resistance under chemical load can be improved by a suitable selection of the pump fluids. In order to achieve a low vapour pressure, high service life and low level of back migration only distilled mineral oils are used in the vacuum industry. Mineral oils with a pH value above 7 are used for pumping acid vapours. Technical white (TW) oil is a hydrocarbon fluid which is distilled and extensively processed to remove naturally occurring impurities. White oils offer higher stability against oxidising media and are especially advantageous for pumping CO2 lasers.

These fluids have a two-to-three-times longer service life than regular distilled oils in comparable applications. They are therefore recommended for applications where exposure to reactive or corrosive gases is prevalent. Mineral oils can react explosively with oxygen. Therefore, inert fluids such as perfluoropolyether PFPE (eg, Fomblin) are used because of their high oxygen tolerance and their stability against reactive or corrosive gases.

Chemistry-Hybrid-pumps

Since its development more than 80 years ago, the oil-sealed rotary-vane pump has become the standard device for generating vacuum down to the 10-3 torr level. Its vacuum performance is excellent, but for a number of applications it has one severe disadvantage: it requires oil. Whenever condensable vapours and corrosive gases are pumped, these vapours deteriorate the properties of the oil, often totally spoiling the pump performance. In addition, there may occur strong corrosion within the pump. In the past, many attempts have been made to improve the capability of handling condensable and corrosive vapours. Besides the introduction of a gas ballast and the use of special pump fluids, expensive external oil filter systems and coating of the internal pump materials were used. A much better performance is achieved by the evacuation of the oil reservoir of the rotary vane pump. This has been put into effect in the commercially available chemistry-Hybrid-pump (see Figure 3).

The chemistry-Hybrid-pump is a combination of a rotary-vane and a chemistry diaphragm pump. Both pumps are directly coupled to a dual-shaft motor. The two-stage rotary-vane pump has a direct transfer line between the stages and is surrounded by an oil reservoir. It expels the pumped gases into the space above the oil level. From here the gases are evacuated by the diaphragm pump via an oil mist filter. The permanent evacuation of the oil reservoir through the diaphragm pump improves the working conditions in the rotary vane pump due to several effects:

  • The condensation is avoided in the oil-sealed section, but will occur in the diaphragm pump having a much higher condensate tolerance.
  • The oil in the reservoir at a mean temperature of about 50 to 60°C is permanently outgassed. This results in much larger oil change intervals, reducing both maintenance and amount of waste oil. Furthermore, the partial pressure of oxygen and other corrosive gases in the oil reservoir is lowered (by a factor of 50). Therefore, the reaction rates are lowered and the corrosion is drastically reduced.

Therefore, in the majority of applications the chemistry-Hybrid-pump does not need a cold trap to protect the rotary-vane pump. The price of a chemistry-Hybrid-pump is similar to a rotary-vane pump with its necessary accessories. Similar to a chemistry diaphragm pump, vapours can be pumped through the chemistry-Hybrid-pump and a highly efficient but effortless exhaust condenser can be utilised. Solvents can be easily collected and recycled or properly disposed. Furthermore, the expensive waste disposal of contaminated oil is reduced. In summary, chemistry-Hybrid-pumps offer significant advantages over common oil-sealed rotary vane pumps.

Roots pumps

Roots pumps are booster pumps, with two 8-shaped rotors rotating in opposite directions. Both rotors, moving past each other in the pumping chamber with small clearances, are synchronised by external gearing and push the gas from the inlet side to the outlet side. They provide only small compression ratios and must be backed by a pump which can deliver to atmospheric pressure. The gears and rotor bearings are oil-lubricated and placed outside the pumping chamber. Shaft seals are used to keep the oil outside the pumping chamber. The pumping chamber and rotors do not need any oil or lubricants for sealing. Therefore, roots pumps are considered to be technical dry or oil-free vacuum pumps.

Combinations of roots pumps with chemistry diaphragm pumps have large pumping speeds between 100 and 0.1 torr (see Figure 5) and show clear advantages in a growing number of applications where high pumping speed below 40 torr is required and where environmental considerations rule out other vacuum pumps. Due to an ultimate vacuum below 10-1 torr these pumping units can be used in applications where a controlled thermal treatment of substances is required, ie, distillation and drying.

Summary

For chemical applications in the fine vacuum range oil-sealed rotary-vane pumps and roots pumps are typically used. Oil-sealed rotary-vane pumps require frequent oil changes, thus producing waste oil. Furthermore, they are sensitive to corrosion. The combination of diaphragm pumps and oil-sealed rotary-vane pumps or oil-free Roots pumps results in both cases in an improvement with regard to applications where corrosive gases or condensable vapours have to be pumped. The diaphragm pump, which discharges against atmospheric pressure, allows the recovery of condensable vapours at the exhaust side.

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