Chemical vacuum drying and degassing remove moisture, solvents or dissolved gas under process-specific pressure and temperature conditions.
Reducing process pressure lowers solvent boiling temperature, increases the driving force for volatile removal and allows heat-sensitive products to be processed under less severe thermal conditions. The resulting gas and vapour load must then be removed by a properly engineered vacuum system.
A roots vacuum booster can increase pumping speed within a defined pressure range when it is combined with a suitable backing pump. The booster does not normally replace the backing pump and should not be treated as an independent atmospheric-discharge vacuum source. Its effectiveness depends on the complete system, including condensers, filters, separators, piping, controls and product-containment equipment.
Distinguishing the Three Processes
Although drying, evaporation and degassing all use vacuum, they have different process objectives.
Vacuum Drying
Vacuum drying removes moisture or solvent from a solid, filter cake, powder, granule, paste or slurry. Heat is transferred to the material while reduced pressure lowers the volatile component’s boiling temperature.
Vacuum Evaporation
Vacuum evaporation removes solvent from a liquid solution to produce a more concentrated liquid, slurry or solid residue. Unlike fractional distillation, the primary objective is usually concentration rather than separation of several volatile components into purified fractions.
Vacuum Degassing
Vacuum degassing removes dissolved, entrained or chemically released gases from liquids, oils, resins, polymers and other process materials. It may also remove residual solvent or monomer from viscous products.
The vacuum system must be sized for the specific gas-release mechanism. A vessel pump-down calculation alone is not sufficient.
Roots Vacuum Boosters in Chemical Drying
Why Vacuum Reduces Drying Temperature
A liquid boils when its saturation pressure becomes equal to the surrounding absolute pressure:
System Pressure
- Psystem = absolute process pressure
- Psaturation = saturation pressure of the liquid
- Tboiling = boiling temperature
Reducing process pressure lowers the boiling temperature of water or solvent. This can protect products that would degrade, oxidize, polymerize or lose quality at higher temperatures.
Vacuum drying can also improve solvent recovery because the removed vapour is directed into an enclosed condensation and collection system.
Chemical Vacuum-Drying Equipment
Roots vacuum boosters may be integrated with:
- Vacuum tray dryers
- Rotary cone dryers
- Vacuum paddle dryers
- Agitated filter dryers
- Nutsche filter dryers
- Vacuum shelf dryers
- Vacuum belt dryers
- Reactor-dryer systems
- Vacuum ovens
- Batch powder dryers
- Continuous vacuum dryers
Pharmaceutical freeze-drying is also a valid mechanical-booster application, but it should be treated separately under pharmaceutical processing because its condenser temperatures, sterility requirements and pressure profile are substantially different from conventional chemical drying.
Drying Stages
A vacuum-drying cycle commonly includes several stages.
Initial Evacuation
The backing pump removes air from the vessel and connected piping. The Roots booster may start after the suction and discharge pressures enter its permissible operating range.
Product Heating
The material receives heat from a jacket, internal agitator, heated shelves or another heat-transfer surface. At low pressure, convective heat transfer through gas is reduced, so conduction and product mixing can become particularly important.
Constant-Rate Drying
When sufficient free liquid is available at the product surface, evaporation may be limited mainly by heat transfer and vacuum-system capacity. Vapour load can be high during this stage.
Falling-Rate Drying
As surface liquid is depleted, moisture or solvent must migrate from within the particles or product structure. Internal diffusion and desorption increasingly control the drying rate.
A large roots booster may produce limited benefit during the final drying stage if volatile release from the product, rather than vacuum-pump capacity, is controlling the process.
Moisture Content
Moisture or solvent content on a dry basis can be expressed as:
Liquid Content (Dry-Solids Basis)
- X = liquid content on a dry-solids basis
- mliquid = mass of water or solvent
- mdry_solid = mass of dry solids
The average evaporation rate required for a batch can be estimated as:
Average Evaporation Rate
- ṁevap = average evaporation rate
- Xinitial = initial liquid content on a dry basis
- Xfinal = final liquid content on a dry basis
- tdrying = drying time
The peak evaporation rate may be considerably higher than the batch average. Condenser and vacuum-pump sizing should be based on the credible peak load.
Drying Heat Requirement
The approximate thermal requirement includes heating the product, heating the retained liquid, evaporating the removed liquid and compensating for equipment heat losses.
The latent heat component is:
Latent Heat Duty
- Qevaporation = latent heat duty
- ṁevap = evaporation rate
- λ = latent heat of vaporization at the operating condition
A broader heat balance is:
Total Heat Load
The roots booster does not supply the evaporation energy. It maintains the pressure required for volatile removal, while the dryer’s heating system supplies the necessary thermal energy.
Roots Vacuum Boosters in Chemical Evaporation
Purpose of Vacuum Evaporation
Chemical evaporation concentrates a solution by removing water or another solvent. Operating under vacuum can:
- Lower the required boiling temperature
- Protect heat-sensitive compounds
- Reduce product discoloration
- Limit unwanted chemical reactions
- Permit use of lower-temperature heating media
- Improve recovery of volatile solvent
- Reduce atmospheric emissions
The process may use single-effect or multiple-effect evaporators, falling-film evaporators, forced-circulation units, thin-film equipment or batch evaporators.
The roots booster handles the residual gas and vapour load after the process condenser, subject to the selected system arrangement.
Boiling-Point Elevation
A solution does not necessarily boil at the same temperature as its pure solvent. Dissolved solids and non-volatile components can increase boiling temperature.
The approximate relationship is:
Solution Boiling Temperature
- Tboiling,solution = actual solution boiling temperature
- Tsaturation,solvent = pure-solvent saturation temperature at process pressure
- ΔTBPE = boiling-point elevation
Boiling-point elevation can reduce the effective temperature difference available for heat transfer. It should be considered when defining evaporator pressure and heating duty.
Evaporation Mass Balance
For a non-volatile dissolved solid:
Overall Mass Balance
The dissolved-solids balance is:
Solids Mass Balance
- ṁfeed = feed mass flow rate
- ṁconcentrate = concentrated product mass flow rate
- ṁvapour = evaporated solvent flow rate; not used in this solids mass balance
- xfeed = solids mass fraction in the feed, dimensionless
- xconcentrate = solids mass fraction in the concentrated product, dimensionless
Use the same mass-flow units for all mass-flow terms.
The vapour load obtained from this balance is used to size the condenser. Only the uncondensed vapour and non-condensable gas should normally continue to the vacuum booster.
Glossary
Condensate receiver: Vessel that collects liquid formed upstream of vacuum equipment.
Degassing: Removal of dissolved or entrained gas from a liquid or material.
Desorption: Release of molecules from a material surface or internal structure.
Residual moisture: Water remaining in the product after the specified drying step.
Vacuum drying: Removal of moisture or solvent from a material under reduced pressure.
Vacuum evaporation: Concentration of a liquid by vaporizing solvent under reduced pressure.
Vacuum hold pressure: Absolute pressure maintained during the specified drying or degassing period.
Vapor load: Rate at which process vapor enters the vacuum system.