Radio frequency energy offers significant technical and economic advantages for a wide range of chemical processing applications.
Radiofrequency (RF) energy is being used in a wide (and constantly growing) range of products and services. Its success is partly due to a better understanding of the physical aspects of RF application, such as heat generation and mass transport, and the use of numerical electromagnetic-field modeling methods. It is also related to the efforts of equipment manufacturers to provide electrical systems that are compatible with the industrial conditions and processes in which they operate.
Chemical process industries (CPI) applications of RF energy include: wood gluing and fiberboard manufacture; textile drying; paper making; polymer heating, melting, welding, and curing; oil recovery; and environmental remediation. RF systems for medical therapy, such as for electrosurgery, treatment of cardiac arrhythmias, and tissue removal, are being developed and are in use in clinical settings, but these are beyond the scope of this article.
The significant advantages of RF heating for industrial applications include: the potential for heating uniformity throughout the material volume; more rapid heating of the bulk material; selective heating; better and more rapid process control; achieving desirable physical and chemical effects; efficiency; and the low cost of the electrical energy required. RF equipment can be manufactured for compactness and mobility with high reliability and low maintenance requirements, and can be controlled remotely by computer, thereby eliminating the need for trained operating personnel.
However, existing RF technology has a well-known limitation: it is generally difficult to modify specialized equipment in order to do something different from what was intended. Therefore, for many new or nonstandard applications the initial investment cost may be high depending on the originality of the design.
The basics of RF heating
The portion of the electromagnetic spectrum between 30 MHz and 300 MHz, with wavelengths from 10 m to 1.0 m, respectively, defines the RF range. Additional specific frequencies approved by the Federal Communications Commission for industrial, scientific, and medical (ISM) applications, including 27.12 MHz and 13.56 MHz, are also referred to as RF frequencies.
Two kinds of physical processes describe how RF energy is absorbed at the molecular level in the material: (1) heating of the material by ionic conductivity, and (2) heating by dielectric polarization (where electric dipoles of molecules interact in an oscillating electric field). Both physical processes are defined by an effective electrical conductivity, (sigmaeff = (sigma + omega(epislon)". The dielectric loss factor, epsilon", describes the ability of dielectric materials to absorb RF energy. For example, at 10 MHz, melamine has a loss factor of 0.23, methyl methacrylate (such as Plexiglas) has a loss factor of 0.027, and a raw potato has a loss factor of 47.8.
The interaction of the electric field with a dielectric material provides the foundation for RF heating and has its origin in the response of electric charges to the applied electric field. The displacement of these charge particles from their equilibrium position creates induced dipoles, which respond to the applied field. Such induced polarization arises mainly from the displacement of electrons around the nuclei (electronic polarization) or is due to the relative displacement of atomic nuclei because of an unequal distribution of charge in molecule formation (atomic polarization). Also, some dielectric materials contain permanent dipoles due to the asymmetric charge distribution of molecules. A final source of polarization results from electric charge buildup at interfaces between two dielectric materials in a system of materials.
RF heating can be illustrated by a simple analysis of the energy absorption created by high-frequency currents applied to a dielectric material through direct-contact electrodes, as shown in Figure 1. Here we consider a slab of material of thickness d sandwiched between metal electrodes with surface area A. An alternating RF current source of voltage V and frequency f is applied to the electrodes. The resulting rms current in the dielectric is: where |E| is the electric field. If, instead, the dielectric material is heated by the radiation field from an antenna, then the electric field will attenuate as it propagates through the dielectric, giving up its energy in the form of heat. This method of heating is best described by the depth of penetration, Delta, given by Equation 6 below.
As can be seen from Eq. 5, the amount of absorbed power is linearly related to the electrical conductivity, rs, which increases with frequency, and the square of the electric field. Heat losses are ignored in this equation, despite the temperature dependence of epsilon and epsilon and the transfer of energy by conduction, convection, and radiation. Also, the electric field intensity, |E|, will vary as a function of both position and the temperaturedependent material properties.
However, this equation does show the fundamental relationships among the important variables - that is, the heating rate increases with increasing frequency, and the RF power required is directly proportional to the material volume. This relationship is very useful for estimating initial heating rates. Computer modeling is generally necessary to describe the distribution of heat within the volume taking into account heat losses by conduction, convection, and radiation, and the temperature-dependent dielectric properties.
Equation 6, describing depth penetration of RF energy, is a fundamental consideration for describing to what extent a given volume of dielectric material can be heated by RF energy when the energy coupling to the volume is derived from propagating electromagnetic waves.
RF dielectric process heating is similar to microwave dielectric process heating. The most significant differences between the two are power density and wavelength. Microwave frequencies are approximately two orders of magnitude greater than radiofrequencies. Since the power density within the material is proportional to frequency, power densities for microwave heating are usually higher than those for RF. Depth of penetration is directly proportional to wavelength, and since RF wavelengths are greater than microwaves, RF may be used to process thicker materials.
Technical considerations The feasibility of any RF heating application requires an understanding of the thermal and electrical behavior of the material to be treated and the changes in these properties with time during absorption by RF energy.
The electric field distribution must be known both spatially and temporally throughout the dielectric material to ensure uniform heating and avoidance of "hot spots" and electric discharges. A dielectric loss factor anywhere in the material that increases with temperature may give rise to a thermal runaway condition in which more RF power is absorbed and converted to heat. Certain ceramic materials have been known to have rapidly rising heat absorption above a critical temperature.
Another potential problem area with all high-temperature heating (greater than 300 deg C) is the surface radiation loss. Surface loss by infrared radiation may be excessive at high temperatures and actually result in cooling of the material. A possible solution is to provide an RF-transparent, but infrared-opaque, liner around the material to be heated.
Finally, the design of the structure that generates the electric field within the material is critical. A simple applicator is the parallel-plate electrode assembly shown in Figure 2. The electric field between the plates is essentially uniform throughout the dielectric material if the dielectric properties are homogeneous and the electric field near the edges of the electrodes is not employed for heating.
For example, in the design of an RF vacuum dryer for kiln drying of commercial softwood, the most important feature was the creation of a homogeneous distribution of electromagnetic energy throughout the material in order to avoid hot and cold spots, which would in turn affect drying uniformity and result in large variations in final moisture content. An applicator model was developed and different electrode shapes were analyzed for field uniformity during thermal treatment. The most significant problems solved by the modeling studies were related to the elimination of air gaps between the electrodes and wood and the minimizing of electric field fringing by bending the ends of the positive electrodes around the kiln wood load. Potential arcing problems were reduced by operating the kiln at reduced voltage (which the elimination of the air gaps allowed). This application resulted in significant reduction in drying time and improved wood quality (1).
A different applicator structure, which radiates electromagnetic energy into contaminated soils for environmental remediation or into tar sand deposits for the recovery of heavy oils, makes use of antenna radiation principles (2). Here, a halfwave dipole in a vertical borehole can be made to radiate and couple energy into the "earth" dielectric, with over 90% of the RF power transferred to it from the RF generator. A knowledge of the electrical and thermal properties of the soil as a function of temperature and frequency is necessary for successful implementation. To achieve uniformity of heating within the soil, several borehole applicators can be employed. By varying the electrical polarities and time sequencing of the RF voltages on each applicator, a heating pattern is created that will be nearly uniform.
Industrial processes
As mentioned earlier, RF processing applications are becoming more widely recognized as a result of newer methods available for understanding the complex interactions between the material and the electromagnetic field. These methods include dielectric measurement techniques for evaluating the dielectric properties, and advanced electromagnetic modeling techniques, such as finite-difference time-domain numerical programs for calculating the electric field distributions and specific absorption rate (SAR) within materials of arbitrary shape. Solid-state sources of RF power, coupled to computerized controls of output power, load regulation, and other system parameters, provide lower system cost, higher energy efficiencies, and compact equipment with lower maintenance and service costs.
Worldwide RF industrial applications over the past two decades include:
wood gluing and fiberboard manufacture; textile drying; dye fixation; baking operations; pasteurization and sterilization; paper making; and polymer processing, such as welding, preheating, melting, block heating, and curing.
The RF-heating wood gluing technique for the manufacture of furniture frames is based on the high dielectric loss of urea formaldehyde (UF) adhesive resin compared to the moisture content of the wood. Alignment of the RF electric field for optimum coupling to the glue layer is achieved by proper arrangement of the RF electrodes. Even though there is a move away from using UF for furniture making operations, RF-activated adhesives are preferred for preparing smaller sections that will be joined to make up larger sections due to the associated cost reductions.
Textile drying has benefited from the use of RF energy for drying because of the rapid but gentle heating produced by RF energy. Drying time in conventional equipment is many hours, and there is a tendency to overdry the outer fibers. causing loss of fiber strength and color. However, RF drying results in improved yields because the temperature of no part exceeds the wet bulb temperature.
European fiberboard manufacturers have widely adopted the use of RF heating of pre-compacted sheets of resin-coated fibers of lumber. The temperature in the middle of the sheets is raised by RF energy toward the cure temperature. Installations typically operate at 13.56 MHz and are in the several hundred kilowatt range, with electrode areas of many square meters.
In the paper industry, RF energy is being extensively applied to converting operations because of the increased use of water-based coatings and adhesives. Because of the substantial difference in evaporating temperatures and energy requirements, water-based products cannot be dealt with in the same way as solvent-based products. Conventional heating equipment cannot achieve the required speeds and still maintain end product quality, whereas RF heating can. RF has been applied successfully to laminating, varnishing, envelope gluing, and book binding (3).
Preheating of polymer injectionmolding powder is a long established application of RF heating. A satisfactory residual moisture content is necessary in the powder to avoid the creation of blow-holes. RF energy can be used to dry certain powders as they exit the feed hopper into the screw barrel with the additional advantage that, since the powder is also heated, ejection cycle times can be reduced (3).
Plastics welding using RF consists of heating the plastic under mechanical pressure. The RF electrodes, operating at 27.12 MHz, are made to form the required weld shape. The plastic is positioned either manually or mechanically. Then, the electrode/presshead assembly is advanced until it makes contact with the film, at which time the RF energy is cycled on. In a few seconds the plastic will melt and fuse. The RF power is then switched off, and the downward movement of the electrodes maintains pressure until the plastic joint is made (3).
RF cross-linking of composites with a polymeric matrix is expected to become a convenient curing process for composites because of its efficiency and the structural uniformity of the final product. The activation of the cross-linking reaction in various thermosetting resins, such as epoxy prepolymers, polyurethanes, and unsaturated polyesters, is based on the partial conversion of heat in the organic medium. The curing reaction starts when the thermal level of the reactants is sufficient. Preliminary results show RF heating at 27.12 MHz is efficient for the cross-linking of the resins and the epoxy-silica microsphere composites (4).
A recently introduced combination oven system, in which 5%-20% of the total heat is provided by RF energy and the balance by conventional fossil fuel burning, has been recently introduced. The basic concept is to heat the internal water content of the body and thereby drive the liquid water to the surface, where efficient evaporation of the surface water takes place (3). Installations in the textile and food industries have demonstrated substantial energy cost savings as well as reductions in dryer size. Even with just a very small percentage of the total energy being RF energy, processing time is reduced. For example, in bread baking, less than a 10% RF power addition halves the baking time. Nondrying applications are still in the early stages of development, but indications are that a very wide range of products, from meat pies to bread, snack foods to cakes, can all benefit substantially from this technique (3).
Environmental remediation
RF energy offers a unique method for in situ heat application, at high heating rates and with controlled and electronically steerable heating patterns throughout significant volumes of contaminated soil (5). RF technology can be used for rapid in situ heating of a variety of contaminants, thereby improving contaminant flow characteristics and facilitating subsequent separation and removal of the contaminants from subsurface soils.
Potential limitations relate to the reduced depth of penetration and increased costs when contaminants are removed from soils heavily saturated with water. The increased attenuation of the RF energy would require more closely spaced applicator boreholes, which add to drilling costs, and the additional energy required to supply the latent heat of vaporization if temperatures above 100 deg C are necessary for contaminant volatility and removal by soil vapor extraction (SVE).
In one common arrangement, a flexible coaxial transmission line and applicator (antenna) system is inserted into one or more vertical or horizontal boreholes in the area to be treated. RF generators supply energy through coaxial lines to electromagnetically coupled down-hole antennas, and the subsurface material between the antennas rises in temperature as it absorbs electromagnetic energy. Properly configured, the system provides a heating pattern that can be controlled spatially by varying the operating frequency, electrical phasing of antenna currents, and antenna length and position. Pumpable liquids or vapors released as a result of in situ heat absorption may be extracted through the same boreholes used to apply the electromagnetic energy.
Figure 3 shows such a single borehole system. The key elements for the application of subsurface RF energy are an applicator, an impedance matching network (which ensures maximum power transfer to the applicator at all times), and an RF generator.
RF heating was used to clean up gasoline-contaminated soil at the site of a former gasoline station in Minnesota (6). Since 1991, portions of the site have undergone active remediation via SVE, groundwater ventilation (GWV) or groundwater sparging, and pump-and-treat technologies. However, residual levels of contamination remained. RF was applied to heat the site, which caused the vapor pressure of gasoline compounds (primarily BTEX [benzene, toluene, ethylbenzene, and xylene]) in the soil, and the rate of contaminant recovery by both SVE and GWV, to increase markedly.
Oil recovery
The production of oil from heavy oil and tar sand deposits by thermal means has been routine for many years, and RF heating has been pursued for this purpose since the early 1970s. The advantage of RF heating technology over other conventional techniques, such as steam flooding or injection, is that it provides a well-defined and directed heating pattern to a subsurface oil deposit independent of soil permeability and fracture with minimum energy losses. Advances in solid-state generators and applicator technology have made RF heating economically viable.
Potential limitations relate to the water content of the reservoir rock or sands. The absorption of high-frequency energy in most oil reservoirs would involve mainly the water content of the formation, since dry rock of most varieties is generally considered to be a dielectric.
The absorption depends very strongly upon the electrical conductivity of the absorbing material, and the water content of the reservoir rock contributes significantly to its conductivity. All other factors being equal, the lowest absorption, or greatest heating range from the borehole, is expected with lower values of the product of porosity x water saturation x ionic content. Therefore, greater oil recovery per borehole of applied RF energy occurs when the volume of pore water is low with low ionic content.
An in situ borehole RF antenna heating system was installed at a depth of 620 ft at an oil recovery site in California (7). A pilot test was conducted to demonstrate the ability of the RF system to focus thermal energy at high efficiency into a particular subsurface deposit and thereby raise near-borehole temperatures to levels that would increase the rate of oil recovery. The test demonstrated a proof-of-concept for the controlled application of RF energy for thermal enhanced oil recovery (EOR) from a single borehole environment. The 18ft-long borehole applicator, which operated at 13.56 MHz at 25 kW, is shown in Figure 4.
Economic considerations
Most RF systems are custom-designed for a specific application, so costs can vary widely, ranging from $900 to $5,000 per kW of installed heating capacity. The time to heat a particular volume of material to a specific temperature must be known to estimate the total RF power requirements.
The power source usually represents 75% to 80% of the total system cost. For example, given the specific heat, density, and thermal and dielectric properties of the material to be treated, a simple heat balance provides a quick estimate of total power and energy requirement at a given frequency.
The remaining costs depend on the requirements for: applicator configuration; power transmission; instrumentation and controls; and installation.
The cost of the applicator is a function of the applicator type and electrode configuration. About 90% of all RF systems use a parallel-plate electrode system, although more recent applications in medicine and environmental remediation use either antennas or transmission lines. In parallel-plate electrode systems, the applicator represents 20% to 25% of the system cost, with engineering representing a significant part of that.
The cost of power transmission from the RF generator to the applicator is usually small since the distance between the applicator and the generator is small. Semi-rigid, rigid, or flexible coaxial cabling is used to convey the power, depending on power rating and system handling requirements. As frequency and power levels increase, size and ability to dissipate heat become more important, thereby increasing costs.
The cost of instrumentation and controls for RF systems is a function of system complexity and the amount of control desired for safe, efficient, and cost-effective operation. Typical instrumentation and control hardware may involve a laptop computer, modem, power measurement equipment, and temperature-sensing equipment connected to the computer for feedback control of the process.
Operating costs. RF system operating costs vary widely. For small medical systems, operating at a few tens of watts, operating costs may be negligible. For large industrial systems functioning at 50,000 W, operating costs will depend on the cost of electricity (related to energy conversion efficiency), replacement costs for consumable components, downtime, and routine maintenance requirements. The overall energy efficiency for RF systems may vary between 50%-60% for vacuum tube systems and 60%-80% for solidstate systems, depending on the conversion efficiency of 60 Hz power to RF, the transmission efficiency, and how well the material to be heated is coupled through the applicator to the generator/transmission-line system.
Consumable components include power tubes and vacuum capacitors for high-power RF systems. The typical lifetime of RF tubes is 5,000 to 10,000 hours. Tube cost varies with power level - $4,000 for 50 kW to $12,000 for 100 kW. By contrast, a small solid-state RF generator for medical applications may cost anywhere from $15,000 to $35,000, depending on complexity.
For further information on costs and economics, see (8). Future applications
The full potential of RF energy as a process tool for industry has yet to be realized. For example, the use of RF energy for diagnostics and/or sensing in the fields of environmental remediation, oil recovery, and medicine is in the early stages of development. The remote sensing of subsurface contaminants by high-resolution, ground-penetrating radar has yet to be fully developed for detecting subsurface liquid contaminants such as leaks from underground oil storage tanks. The detection of subsurface heavy oil deposits using RF propagation measurements has potential for both oil exploration and controlling the actual oil recovery process.
In every process application involving the use of RF to heat a dielectric material or provide diagnostic capability, the remote sensing of the changes in the physical and chemical properties of the material are fundamentally important to ensure RF process control and to achieve the desired economic and physical results. Changes in the dielectric properties with heat directly influence the intensity and phase relationships of the RF wave energy. Measurements of these two parameters during the process can be related to corresponding changes of the physical properties of the material being processed.
The challenges to overcome relate to a better understanding of: the thermodynamic aspects of RF heating in materials;
the electromagnetic field distributions and their control in materials during heating; and
material properties that interact with the RF fields to produce both thermal and possibly athermal effects.
RF (and microwave) heating process design has been and continues to be empirical and experienced-based. There is a need for improved understanding of the behavior of RF heating and mass-transfer systems. Because the thermodynamics of RF heating systems are generally considered to be no different than conventional heating systems, modifications of classical heat-transfer models are considered sufficient for describing the macroscopic behavior of RF heating and mass transfer. The lack of theory consistent with the features of RF heating and mass transfer has resulted in the lack of explanation and understanding for various published observations and results unique to the RF process (9). Analysis of the thermodynamics of microwave heating systems has revealed that, in addition to temperature and internal pressure gradients, the existence of electric field gradients within the material volume may reduce the activation energy for the microwave drying process and increase mass-transfer rates over those of conventional heating (9).
Activating effects induced by electromagnetic wave absorption remain a very controversial topic. Above all, the absorption of electromagnetic waves involves conversion to heat. However, athermal effects have been theoretically considered and experimentally observed in the curing of certain polymer systems and in sintering of ceramics. Under athermal effects, the electric field allows induced organization of the irradiated medium (10).
Basic questions remain as to whether or not electromagnetic energy at the molecular level is able to enhance or modify basic chemical reactions. Do chemical reactions proceed at the same rate with and without electromagnetic irradiation for the same bulk temperature? What are the thermodynamic effects of electric fields on chemical equilibrium?
The design of industrial RF systems based on detailed numerical and analytical models of the interaction of the electromagnetic wave energy with materials during the heating process and prediction of results at the molecular level will lead to a fuller realization of the use of RF energy for industrial processes. Further Reading
Majetich, G., and R. Hicks, 'he Use of Microwave Heating to Promote Organic Reactions," The Journal of Microwave Power and Electromagnetic Energy, 30 (1), pp. 27-45 (March 1995).
[Reference]
Literature Cited
[Reference]
1. Zwick, R. L., et al., "Design Considerations for Radio Frequency Vacuum Dryers," presented at the 30th International Microwave Power Symposium, Denver, CO, International Microwave Power Institute (IMPI), Manassas, VA (July 1995).
2. Kasevich, R. S., "Electromagnetic Apparatus and Method of In Situ Heating and Recovery of Organic and Inorganic Materials," U.S. Patent No. 5,065,819 (Nov. 19, 1991).
3. Jones, P. L., "Radiofrequency Processing in Europe," J. of Microwave Power and Electromagnetic Energy," 22 (3), pp. 143-153 (Sept., 1987). 4. Gourdenne, A., et al., "Industrial RF Applications," Proceedings of the 31st International Microwave Power Symposium, Boston, MA, IMPI, Manassas, VA. pp. 57-59 (July 1996).
5. Kasevich, R. S., et al., "Radiofrequency Heating for Soil Remediation," presented at the Air & Waste Management Association's 90th Annual Meeting and Exhibition, Toronto, Ont., Paper No. 97FA164.05 (June 1997).
[Reference]
6. Kasevich, R. S., et al., "Enhanced Remediation of Gasoline from the Capillary Fringe Utilizing Radio Frequency," presented at the 1 th Annual Conference on Contaminated Soils, Univ. of Massachusetts at Amherst, Amherst, MA (Oct. 1996).
7. Kasevich, R. S., et aL, "Pilot Testing of Radio Frequency Heating System for Enhanced Oil Recovery from Diatomaceous Earth," presented at the69th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, New Orleans, LA, SPE Paper No. 28619, SPE, Richardson, TX (Sept. 25-28, 1994).
8. Sutton, W. H., et. al., eds., "Microwave Processing of Materials," Symposium Proceedings, Vol. 124. Materials Research Society, Pittsburgh, PA, pp. 341-346 (1988).
9. Adu, B., et al., "Thermodynamics of Microwave (Polarized) Heating Systems," The Journal of Microwave Power and Electromagnetic Energy, 30 (2), pp. 90-96 (June 1995).
10. Stuerga, D. A. C., "Microwave Athermal Effects in Chemistry: A Myth's Autopsy, Parts I and II," The Journal of Microwave Power and Electromagnetic Energy, 31 (2), pp. 87-100 (Part I) and 101-113 (Part II) (June 1996).
[Author Affiliation]
Raymond S. Kasevich, KAI Technologies, Inc.
[Author Affiliation]
R. S. KASEVICH, PE., is president and founder of KAI Technologies, Inc., Portsmouth, NH (603/431-2266; Fax: 603/431-4920.) He has 30 years of corporate research and development experience in electromagnetic science and engineering applications covering a wide range of frequencies from full-scale RF oil recovery and environmental remediation systems to medical catheter systems for microwave hyperthermia. Also, he has 20 years of experience teaching electrical engineering. He holds 25 patents and has published numerous papers in professional journals. He holds an ME in electrical engineering from Yale Univ. and a BS in electrical engineering from the Univ. of Hartford (having done additional undergraduate studies at Case Western Reserve Univ.). He received a Ford Foundation grant for PhD studies at the Univ. of Michigan while working part-time at the Radiation Laboratory in Ann Arbor on network synthesis problems, and he continued PhD studies at Massachusetts Institute of Technology in the physics department as a special graduate student.
