Thermal Drying in Controlled Atmospheres
Certain materials require protection from oxygen or other atmosphere contaminants during or after drying.
Many processed materials need to be thermally dried under controlled conditions. Most drying is done in open atmosphere or oxidizing ovens in batch, continuous belt, or tunnel feed styles. However, certain materials require protection from oxygen or other atmospheric contaminants during or after drying. This can be accomplished in a controlled-atmosphere and/or vacuum furnace system.
In addition to the removal of water, water vapor and other impurities being thermally driven off, the temperature could be above the boiling temperature of water (100°C/212°F). Higher temperatures speed the drying process, as long as it does not harm the material being processed. When adding solvents and/or binders, the temperature will need to be adjusted accordingly. The reasoning for the controlled atmosphere is that the material being dried could reabsorb moisture from the atmosphere or be oxidized at elevated temperatures.
Thermal drying can be done in vacuum, inert gas, nitrogen gas and reducing gas such as hydrogen. Proprietary gas mixes may also be used, depending on the needs of the process. Clean gasses are required for best results. Two techniques may be used depending on the desired outcome: dry room and glovebox integration.
The first option is to do the thermal drying in an oxygen-free environment, either in a vacuum or in inert gas, cooling the material below its oxidation level while it is still in the controlled atmosphere. The furnace can have temperature interlocks programmed to prevent operator error by having the material exposed to the surrounding atmosphere too quickly. These furnaces are sometimes operated in dry and/or clean rooms to prevent moisture absorption after the drying. Materials can then be sealed during packaging for protection.
Furnaces that are operated in a dry room typically cannot use cooling water. A water leak in a dry room may be dangerous or, at the very least, undesirable. Cold-wall furnaces are liquid cooled, most often with water. Therefore, dry room furnaces need to be cooled with a non-water industrial coolant (several brands are available). The coolant is recirculated in a closed-circuit heat exchanger. A liquid-to-air heat exchanger, which is usually required, eliminates the possibility of water contamination in the dry room.
The second option is to use a controlled atmosphere furnace integrated with a glovebox. This is used for materials that are sensitive before and/or after drying. Gloveboxes are also used when materials are mixed in the glovebox and then fired at high temperatures. Since the end product is sensitive to atmosphere contamination, it is sealed and packaged in the glovebox for protection during storage and shipping.
Glovebox furnaces come in many sizes, shapes and system access. The furnace can be mounted inside the glovebox or integrated to the glovebox. For low-temperature drying, ante-chambers manufactured with elements to heat the material up to 600°C are common and economical.
Furnaces mounted on the top, bottom or back wall of the glovebox are all possible. These furnaces take up less space within the glovebox and can reach temperatures up to 2,800°C. Consideration needs to be made as far as operator access to the furnace for loading and unloading. The door and cover of these furnaces are usually automated to open and close. The top-mounted glovebox furnace is a bottom-loading style. To load the furnace, the operator lowers the bottom cover and places the work on the work support fixture, and then closes the furnace and runs the process. A rear-mounted furnace may have a sliding door that can be either manually or automatically operated. A loading device is usually needed to place the work inside the furnace.
Furnaces mounted in the floor of the glovebox will have a manual door with either hinged or swivel access. The furnace will be loaded by lowering the material onto the work support at the bottom of the heat zone. If the work is heavy, an overhead crane from the roof of the glovebox will be employed.
One concern with glovebox users is the increase of gas temperature in the glovebox with a furnace running at an elevated temperature. However, because the maximum exterior temperature of the cold-wall designed furnace is 120°F, very little heat is released into the glovebox.
Controlled atmosphere thermal drying systems are often custom built to meet the requirements of a given process; many different configurations are possible. A custom vacuum/controlled atmosphere furnace manufacturer can help with specifying a furnace that will meet the process needs. Many require a non-disclosure agreement between the end user and the manufacturer of the thermal equipment.
On occasion, ultra-pure gas is required, and gettered argon or nitrogen gas is the solution. Standard bottled gas may have as much as 10 parts per million (ppm) of oxygen, which is the equivalent of a rough vacuum (> 10-2 Torr). Gettered gas will be able to clean the gas to less than 10-6 ppm, equivalent to a 10-8 Torr vacuum level. At this level, very few materials would experience any oxidation. For precise control of thermal drying, a vacuum, controlled and/or atmosphere system should be considered for ultimate results.
Considerations for Controlled Atmosphere Thermal Drying/Protective Processing
What materials are being dried and outgassed (e.g., H2O, solvents or binders)?
Does the processed material need protection from reabsorbing moisture?
Will the finished product be susceptible to oxidation?
- At room temperature?
- At higher temperature?
What will the process temperature be?
What gases are compatible with the product?
- Vacuum (at what level?)
- Inert gas such as argon or helium
- Nitrogen gas that can be reactive at higher temperatures
- Hydrogen gas, for a reducing atmosphere
- Proprietary blends for specific reaction to the process material
How will the material be handled before and after processing?
Will a dry room be needed to protect the material after processing?
Would a glovebox be preferable to protect the material before and/or after the processing?