Microwave drying can deliver several benefits. First and foremost, microwave drying can be carried out at a lower temperature and at a lower gross amount (in BTU/lb) of water removed than other heating methods. Second, microwaves can penetrate into products and directly cause molecules such as water to vibrate at the frequency of the microwaves. Heat therefore is generated within the product instead of being transferred from the outside surface of the product, as in conventional methods. The capability therefore exists for microwaves to heat products more quickly than conventional procedures, and without excess temperature in the product because conductive heat transfer is not used.
Defined as a wave that travels at the speed of light and consists of a combined electric and magnetic effect, all electromagnetic waves are essentially the same, whether they are carrying power to a home, or transmitting radio or television signals, radiant heat, or visible light. Microwaves are a portion of the electromagnetic spectrum and typically are defined as the 300 to 30,000 MHz range -- above radio frequency and below millimeter waves.
The frequency designation of hertz (Hz) for cycles per second is named after F.W. Hertz, who discovered and demonstrated electromagnetic waves around 1886. Table 1 shows a few common items and where they fall in the electromagnetic spectrum.
The frequencies allocated in the United States for microwave heating are 915 MHz and 2,450 MHz. The longer wavelengths penetrate deeper into any material; therefore; 915 MHz is preferred for industrial use. The waves at the microwave frequencies specified are non-ionizing and therefore cannot damage or change molecular structure; instead, they only heat by vibration. By contrast, ionizing waves take place at the ultraviolet region, which is 100,000 times higher in frequency. The higher the frequency, the shorter the wavelength, which is the measure of the distance between successive peaks of the traveling wave.
The microwave mechanism for generating heat in a product results from the microwave energy creating movement in the polar water molecules and ionic conduction for salt content. Motion of these particles imparts, through drag or impact, movement to the surrounding particles, even if they are electrically inert. The interior of the material therefore is directly heated by the microwave energy without the necessity to overheat the surface to create a flow of heat.
Because some of the microwave energy is absorbed on passing through the material, there is clearly a limit to the depth of material that can be effectively heated with microwaves. Materials are characterized by a power penetration depth (dp), which is the distance into an infinite slab at which 1/e of the incident power remains. In this equation, “e” is the Naperian logarithm of 2.718. For water, the penetration depth at 915 MHz is 1.6"; because in real-world products the initial water content will be well below 100 percent, a larger thickness of 2" of the material to be processed usually is suitable.
When drying a product, the elevation of the surface temperature increases the saturated vapor pressure at the surface and causes a flow of vapor between the surface and the air. The loss of water content in the region results in a capillary pressure gradient re-humidifying the surface from the interior. In a process where the heat is externally applied, the temperature gradient and humidity gradients oppose each other, which can cause a surface crust to form and hinder both heat getting in and moisture getting out.
- Start with external heat only.
- Complete finish drying with heat only (to effect crust formation).
- Utilize mainly microwave heating for finish drying (to achieve very low moisture levels, or for leveling the moisture level in a product).
When determining the heating method to use, it is important to remember that with microwave heating, the moistest area will preferentially heat.
For continuous processing, the product is conveyed into and through the microwave zone or oven on a belt. Although the oven operator can see the microwave heating sections into the oven through the oven’s opening, microwave oven-makers employ a special choke structure to prevent microwaves from traveling to the outside. Despite the large amount of power inside, microwave leakage from industrial process ovens is no more than from a home microwave.
A microwave system comprises:
Transmitters. They generate microwave power from 440 V, three-phase input.
Waveguide. These tubes transfer microwaves from the transmitters to the ovens.
- Oven. The oven applies the microwave to the product.
Two styles of ovens presently are in use. Multimode ovens are much larger than the product. They allow the power to “bounce around” inside the enclosure until it is absorbed in the product. By contrast, single-mode ovens are much smaller in height. In them, the microwave energy is beamed directly onto the product, similar to a flashlight. Any energy not absorbed on the first pass through the product is reflected back.
With this approach to drying, the BTU/lb of water removed is less than the theoretical value of BTU required to heat that amount of water from its starting temperature to the boiling temperature, plus supply its latent heat of vaporization.
The longest detailed history of an oven using circularly polarized waves for drying is a case study involving use of a multimode microwave oven for municipal sewage. In that case, data was collected over 24 days, involving 178.75 hours of operations, by Burch BioWave Inc., Fredericktown, Ohio. A central control room measured electrical power supplied to the system as well as the amount of natural gas used. During this time:
- 240.56 wet tons were process by the system.
- 355, 939 lb of water were removed.
- 78,074 kWh of electricity was used.
- 65,107 ft3 of natural gas was used.
Table 2 demonstrates the values further.
These values can be compared to that required to heat and vaporize by any other method. Theoretically, this is ideal if only heating and boiling were used at 100 percent efficiency (table 3).
Both oven types show a similar capability of drying at less than the theoretical BTU/lb of a pure heat process while keeping the product temperature at less than 190°F (88°C) for a variety of products.