Vacuum Pumps for Degassing Liquids
CASE STUDY VACUUM PUMPS FOR DEGASSING LIQUIDS
THE DEGASSING PROCESS formation of the liquid frees up any trapped or dissolved gases. For normal degassing the pressure does not need to be lowered for liquid Similar to distillation, degassing is one of the many separation vaporization but only to what is required by Henry’s law to meet the processes such as distillation, drying, filtration, membrane specification. separation, adsorption, crystallization, etc. that rely upon the As with distillation, degassing can also be accomplished without differences in the physical properties of substances in a mixture. vacuum by only heating the liquid close to its boiling point to Degasification relies upon the differences in the solubility of a gas drive the dissolved gases out of it but in many cases the higher in a liquid with variations in temperature and pressure to provide a temperatures may cause decomposition of either the liquid or gases mechanism for separation. The solubility or miscibility of one fluid or a chemical reaction may occur. Vacuum degasification is the in another is normally considered a physical property if no chemical preferred method whether using a degassing vessel or in combination reaction occurs. The solubility of a gas in a liquid can be determined with membrane gas-liquid separation by passing the liquid through a from Henry’s law where tubular membrane which is evacuated on the outside and only allows Pi = xiHi and Pi is the partial pressure of the i component gas in the gas to pass through. Vacuum degassing provides a convenient and vapor phase directly above the liquid, xi is the mole fraction of the i efficient format for this separation at lower temperatures without component gas dissolved in the liquid, and Hi is Henry’s constant for harmful reactions with other gases such as Oxygen. the i component gas at a given equilibrium temperature. Air Solubility in Water vs. Total Pressure The most common degasification occurs with the removal of Oxygen, Carbon Dioxide, and Nitrogen from the most common liquid, water by for Various Water Temperatures reducing the pressure within a degasser tower as water runs through 0.03 it. The tower like a distillation column would be equipped with trays 5˚C or packing to provide much greater surface area and exposure to the 0.025 10˚C reduced pressure for better gas removal. This is done for the cooling Air by vol - 5˚C 0.02 Air by vol - 10˚C systems in power plants, chemical and pharmaceutical plants, 20˚C Air by vol - 20˚C semiconductor plants, etc. to eliminate the formation of gas bubbles Air by vol - 30˚C 30˚C 0.015 Air by vol - 40˚C Air by vol - 50˚C that can interfere with heat transfer or cause adverse reactions 40˚C Air by vol - 60˚C including corrosion. 50˚C 0.01 60˚C Air by vol - 70˚C 70˚C Air by vol - 80˚C
Similar to distillation pressure reduction and/or temperature 80˚C 0.005 elevation separate out the constituents. In fact distillation includes VOLUME ABSORBED AIR VOLUME TO WATER the degassing process by bringing the distillate to its bubble point 0 which insures that any dissolved gas is also removed as the vapor 0 100 200 300 400 500 600 700 800 TOTAL PRESSURE (mm Hg Abs)
MD-KINNEY • 4840 W. KEARNEY ST., SPRINGFIELD, MO 65803 USA P: 417-865-8715 OR 1-800-825-6937 • F: 417-865-2950
WWW.MD-KINNEY.COM K-ACS-AS-KLRC-SDV-KDP-VS-DG 1st Ed. 2/21 SELECTING VACUUM PUMPS Booster/Liquid Ring Vacuum System Similar considerations must be made in the choice of vacuum pumps for degassing as are made for distillation. Because both gases and liquids are involved in the process, the pump should be able to handle some liquid carryover in a process upset and be able to pump both the gases and entrained vapors from the liquids. Because various type liquids and gases could be handled, the pump should be able to deal with constituents that could be corrosive, flammable, or toxic and have compatible materials of construction including the elastomers, and proper electrical components including motors and safety devices. Liquid Ring Pumps meet these requirements. For lower pressure operation an Air Ejector/Liquid Ring Pump can be used and for both higher capacity at a lower pressure Booster/Air Ejector/Liquid Ring or Booster/Booster/ Liquid Ring systems can be used.
The Liquid Ring does not require internal lubrication and can run on most liquids such as water or low viscosity oil or many solvents that Air Ejector / Liquid Ring Pump are compatible with its materials and the process in terms of vapor pressure and viscosity. It can handle liquid slugs from process upsets or a continuous flow of liquid condensate from a pre-condenser. In some cases it can perform as both a vacuum pump for non- condensables and a direct contact condenser for vapors increasing its overall pumping capacity. It is one of the most reliable and durable mechanical pumps because of its simplistic design with one rotating shaft assemblage. It is also available in 316 stainless steel for greater corrosion resistance to process effluents.
The Rotary Screw Dry Pump also does not require internal lubrication and can handle some liquid carryover but as the name implies it is preferred to keep the pump dry for optimum performance. Knockout Liquid Ring FSR System Pots would normally be recommended to trap out liquid slugs. Since the Dry Pump contains no liquid within its pumping chamber it is not limited by the vapor pressure of the liquid and can achieve lower pressures without producing process contaminated waste products. The Dry Pump handles condensable vapors by keeping them in the vapor phase at an elevated temperature while traveling from suction to discharge so that they can be condensed out in an after- condenser. The Rotary Screw Dry Pump and Rotary Lobe Boosters are also available with optional protective coatings.
SDV Series Rotary Screw Dry Pump
MD-KINNEY • 4840 W. KEARNEY ST., SPRINGFIELD, MO 65803 USA P: 417-865-8715 OR 1-800-825-6937 • F: 417-865-2950
WWW.MD-KINNEY.COM K-ACS-AS-KLRC-SDV-KDP-VS-DG 1st Ed. 2/21