Selecting the right vacuum technology for chemical and pharmaceutical processing applications is often difficult, cautions Uli Merkle.
A system must deliver the required pumping speed at operating pressure and ensure the required pump-down time. It cannot be sensitive to process gases and must meet all requirements when it comes to CIP (clean-in-place) and gas recovery. Reliability and economic efficiency also play a significant role.
The three vacuum technologies most often used in chemical and pharmaceutical processing technologies are liquid ring, dry screw and oil-lubricated rotary vane vacuum pumps.
Liquid ring vacuum pumps
Liquid ring vacuum pumps (are rotating positive displacement pumps with an impeller eccentrically placed in a cylindrical housing .
Over many years, these vacuum pumps have proven themselves to be robust and reliable vacuum generators in chemical processes. The operating fluid in the compression chamber continuously dissipates the compression heat, so the vacuum pump operates nearly isothermally. This means that the process gas does not heat up to a notable degree and the vacuum pump operates at relatively low temperatures. This significantly reduces the risk of unwanted reactions or an explosion. Low operating temperatures also facilitate condensation of vapours and gases, which increases the nominal pumping speed of the vacuum pump.
The ultimate pressure of the vacuum pump depends on the vapour pressure and viscosity of the liquid. The viscosity of the operating fluid will impact the power consumption of the vacuum pump.
Liquid ring vacuum pumps are available on the market in different versions, materials and shaft seals.
Advantages of liquid ring vacuum pumps:
- Not sensitive at all to vapours or liquids entering the system
- The different material versions enable them to be tailored to the process gas
- Possible contamination of the operating fluid with condensate from the process gas, which makes it necessary to subsequently treat the operating fluid before its disposal
- High energy consumption
- Ultimate pressure depends on the vapour pressure of the operating fluid
Dry screw vacuum pumps
Dry screw technology is widely used in the chemical and pharmaceutical industries but relatively new compared to liquid ring technology.
In the 1990s, Busch launched the first dry screw vacuum pump on the market, the COBRA AC. The major difference compared to the liquid ring vacuum pump is that screw vacuum pumps (fig. 3) do not require operating fluid to compress the process gas. Hence the ‘dry’ screw label.
Screw vacuum pumps operate using water cooling, which ensures even temperature distribution throughout the pump body and thus thermal stability in the entire process.
Consult an expert and take into account all important parameters - process conditions, process gases and integration into process control, through to economic efficiency, safety and reliability of future vacuum generation
Generally, dry screw vacuum pumps operate at higher temperatures than liquid ring vacuum pumps. Condensation of process gas elements is thus largely eliminated. This enables the process gas to be conveyed through the vacuum pump without contamination or causing a reaction with an operating fluid. Cast iron is the standard material used for work pieces for all parts that come into contact with the pumped medium. It is either untreated or treated with a special coating to make it resistant to nearly all chemicals. After the end of the process, we recommend flushing the vacuum pump with a suitable cleaning fluid and to purge it with nitrogen to avoid corrosion and deposit forming during stand-still.
With different compression systems and various coatings, screw vacuum pumps from Busch can be configured to be compatible with any chemical.
Advantages of dry screw vacuum pumps:
- Dry compression, no contamination or reaction possible between process gas and operating fluid
- High vacuum level
- Energy efficient
- Can be designed for nearly all process gases thanks to material selection and temperature regulation
Disadvantages of dry screw vacuum pumps:
- Sensitive to particles entering the system
- Cannot be used with process gases that tend to be reactive at high temperatures
Once-through oil-lubricated rotary vane vacuum pumps
Successfully used in many fields for decades, these are amongst the most widely used mechanical vacuum pumps in the industry. Busch developed the Huckepack, a two-stage once-through oil-lubricated rotary vane vacuum pump in the 1960s, specially designed for chemical and pharmaceutical processing technology.
Busch ‘s Huckepack rotary vane vacuum pumps (fig. 5) have three significant distinguishing features when compared to other vacuum pumps that operate according to the rotary vane principle:
- Two compression stages are stacked and connected to each other makes it possible to achieve a lower ultimate pressure.
- Oil lubrication allows a defined amount of operating fluid, oil or other media-compatible fluid to injected into the compression chamber; other rotary vane vacuum pumps use oil circulating lubrication.
- Water cooling regulates operating temperature within a certain range.
Because the lubricant only flows through the vacuum pump once, nearly all liquids with a viscosity in the range of 150 centistokes (cSt) can be used. These constantly flush the vacuum pump during operation, protecting it from corrosion and deposits.
Advantages of once-through oil-lubricated rotary vane vacuum pumps:
- High vacuum level
- Extremely robust and reliable
- Easy servicing
- Perfectly suited for conveying acid vapours and monomers or products that lead to polymerization when other vacuum technologies are used
- Operating fluids must be treated or correctly disposed of
All of the vacuum generation technologies discussed here have advantages and disadvantages. There is no single ideal solution for all applications. It is therefore important to seek consultation from a vacuum expert and take into account all important parameters in the process, starting with process conditions, process gases and integration into process control, through to economic efficiency, safety and reliability of future vacuum generation. In most cases, consideration of these factors leads to a customized vacuum system that is directly tailored to the requirements.
Photo: liquid ring (top l), modern screw (bottom l) and rotary vane vaccum pumps (right)
Uli Merkle is head of marketing services, Busch Vacuum Pumps and Systems.