Benefits of Vacuum Tumble Drying

Vacuum Tumble Drying is one of the most energy efficient, cost effective, environmentally sound and safest methods for drying all types of organic and inorganic solids from polymers and catalyst, to food ingredients and phamraucitcals. Capacities range from the smallest lab-scale dryer with 0.1 cubic foot capacity to large 400-500 cubic foot batch size.
 

The Benefits of Vacuum Tumble Drying

  • Safe for Friable Products - The low speed at which tumble dryer operate, usually just a few revolmiton per minutes, makes them ideal for delicate and friable solids such as crystals, macromolecules or agglomerated solids. Even the discharging from a Rota-Cone is low sear since no agitation is required – solid flow directly out of the smooth unobstructed 45 degree cone.
  • Complete Discharge – The simple 45 degree cone allows solids to flow out unobstructed and without assistance of agitation or.
  • Low Cross Contamination – Because there is only one moving part, the tumble dryer is an inherently simple and clean design, which reduced product hang-up, improved yield and reduced cross contamination. This also make tumble dryer easily Cleaned in Place (CIP) with the addition of a spray nozzle or two.
  • Full Inspection – All surface of the Rota-Cone Vacuum Dryer can be fully inspected from one point – the loading hatch/manway.
  • Complete Discharge – (less heel) – Unlike agitated dryers, Rota-Cone dryer have very little surface area to trap product. The simple 45 degree cone is smooth and has no obstructions to retain product.
  • Low Temperature Processing - Ideal for products that may be damaged by exposure to excessive heat.
  • Low Moisture Levels – Drying under deep vacuum will remove more bound moisture than any other drying method. External low elves of moisture, approaching 0% are often required for pharmaceutical, catalysts and polymers.
  • Enclosed Environment – Active pharmaceutical ingredient, drugs and toxic chemicals are easily isolated in a vacuum dryer. Loading and discharging connections can be automated, sealed and purged for worker protection. The closed system allows the containment and reuse of solvent after removal form the solids.
  • Reduced Energy – Since the evaporation of the liquid under vacuum occurs at a much reduce temperature, that is the temperature differential is increased (SEE SIDE BAR) , the entire system will operate at a lower temperature which will reduce heat losses and will be safer for operators.
  • Avoid Oxidation – The vacuum vessel is ideal for avoiding oxidation of chemical and with the addition of an inter gas such as nitrogen, an oxygen free environment can be maintained.
  • Single Vacuum Seal – A single seal on the non-rotating vacuum line reduced vacuum leakage, provides less area for contamination and reduces maintenance time and costs compared to the two seals normally found on a horizontal vacuum rotary dryer.

Vacuum Drying - Drying at reduced pressure lowers the vaporization temperature of water and solvents. This increases the temperature differential or thermal driving force needed to evaporate liquids and increases drying efficiency. For example, at 90 mmHG the vaporization temperature of water is reduced from 212°F to 120°F. Subsequently the temperature differential is increased by 92°F. he following dT graph explains this increases. There are many scales used to express reduced pressure or partial vacuum. These scales may be either expressed in gauge or absolute scales. The following vacuum conversion chart lists several for the more common vacuum scales.

The driving force in evaporation of liquid is the temperature differential between the dryer shell and the temperature at which the liquid evaporates. This temperature differential is known as the delta-T , dT or ΔT and will be important in understanding the basic drying equation Q=U∙A∙dt. There are two methods of increasing the dT.

  1. Increase the temperature of the heat source on the vessel jacket or heating medium available.
  2. Reduce the pressure on the liquid and allow the liquid to vaporize at a lower temperature.

Phases of Drying – Before drying occurs the first step is to heat the vessel, the solids and the liquids in the dryer. This first step is known as the Initial Heating Phase. During the initial heating phase, the temperature increases and the heat input is known as sensible heat because the changes in the temperature can be sensed. The next step is to start vaporizing the liquid at a given vacuum level. The heat put into the system to accomplish vaporization is known as latent (Latin for hidden) because there is not increase in temperature. Just as water holds steady at 212° when boiling, water or solvents will maintain a given temperature when evaporating under vacuum. This time of constant evaporation is known as the constant rate drying phase. The liquid that is free to evaporate is the unbound moisture or free moisture. Once this free moisture is removed, the remaining liquid or bound moisture will not come off as easily. Because the liquid is no longer providing evaporative cooling to the product as the same rate as the unbound moisture, the temperature of the product will increases until it is equilibrium with the heat input, vacuum level and rate of evaporation. The reduce phase of the drying is known as the falling rate phase. Just as a pot of building water never goes above 212°F, once the water is removed the temperature of the pot rapidly increases to the temperature of the heating medium.

true s removed ad certain percentage of moisture may or may not remain. Theis remaining level is known as the critical moisture level and reaching it make the transition to the falling rate drying phase, because the rate of evaporation is reduced and may continue to reduce for some time.

In practice the falling rate period can last for hours or even days.

Predictability and Control - Understating of the phases of drying and monitoring temperature and vacuum levels are the main indicators of what is happening inside the vacuum dryer and ultimately, predictable control of the system.

Dryer Design - Rota-Cone Vacuum Dryer design consist of a vacuum tight, jacketed, double-cone vessel with non-rotating vacuum line entering through one of the trunnions and hearing medium entering through the other. The vessel is supported by self-aligning, anti-friction roller bearing at either end and is driven by a low speed gear drive with secondary reduction. Product contact surfaces are typically 300 series stainless steel, but Hastelloy and other high nickel alloys are available as are fluropolymer coatings such as Halar.

Single Shaft Seal – The vacuum line is the only seal (unless an agitator is added). The vacuum line seal is normally a packing gland mounted externally with a dust ring inside. Mechanical seals are also available for deep vacuum applications.

Surface Finishes range from buff or glass bead blasting to highly polish for cGMP applications. Any finish as measured by RMS or other measures are available. External surfaces are often carbons steel with epoxy coating, or for sanitary or highly corrosive environments, they be fabricated from stainless steel.

Heating Jackets – Cod non-code all vacuum dryers have double wall heating jackets. Paul O Abbe has optimized the flow path to give the most consistent heating pater in the double cone vessel. Jacket is typically heated with hot water or hot oil which is easier to control precisely than steam. Above 120°F an insulation shroud is recommended to prevent operator from coming in direct contact with the hot jacket. At higher temperature insulation shroud help to conserve energy. Insulation shroud are available in epoxy coated carbon steel or stainless steel.

Depending the application, need for maintenance and sanitation connection are available in threaded, flanged to sanitary tri-clamp connection.

Loading and Unloading – Standard dryer are designed with a large swing hatch for top loading and a butterfly valve below for discharge. The butterfly valve may be manually or pneumatically actuated. Greater levels of automation are available for loading and unloading solids including:

  • Automatic Flange Docking – Out extension sleeve/docking flange allows for automatic connection of the dryer to flanged above for loading and below for discharging.
  • Drum Loading and Unloading – Our exclusive Automated Drum Loading and Unloading System fallows the use of fiber, plastic or steel drum of any size bins or Totes to be sealed, held in place and automatics load its content into the dry.
  • Integral pneumatic conveyor – traditional dilute phase pneumatic conveyors can be used with the Rota-Cone acting as the solids receiving vessel.


Process monitoring

  • Temperature Measurement - Standard on all dryers is an integral themowell running co-axially down the middle of the vacuum lien and protruding into the product. This gives accurate and continuous monitoring of the product temperature during al phases of the vacuum drying process.
  • Oxygen Monitoring – Oxygen sensors mounted in the vacuum line are often used for highly reactive product and to assure adequate inert gas purging is taking place.
  • Solids Samplers - These are simple devices but allow sampling of solids during drying without opening the vessel or allowing air to enter the vessel. Operating the sample is taken by stopping the vessel with the sampler point downwards, opening the main valve to allow solids to flow into the stainless steel or glass container. The valve is closed, a vent valve opened to break vacuum, and the sample jar is removed for analysis.

Liquid Addition – Liquids are often added before, during or after drying has been performed. Liquids can be added in several ways but typically a spray line is mounted through the vacuum line, terminating in a spray nozzle pointing down. This keep the spray nozzle our of the product and provide uniform application of liquid.

Agitators and Lump Breakers – During drying limps or agglomerates may form. These agglomerated can reduce drying efficiency by trapping moisture between the particles. A simple low speed pin agitator may be sufficient to break up these lump and improve drying performance. For harder agglomerated a high speed saw-tooth disintegrator can be used.

 

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Rota-Cone Vacuum Dryer

Vacuum dryers offer many benefits including: Predictable and Repeatable Results. Dry Temperature Sensitive Products - Because all liquids vaporize at lower temperature at reduced pressures, it is possible to vaporize the liquids at very (more...)

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