Design of irradiator
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The document outlines key design considerations for food irradiators that utilize radiation to extend shelf-life and reduce food-borne diseases. It discusses various types of irradiators, including cobalt-60 and electron beam types, emphasizing factors like dose uniformity, efficiency, cost, and safety systems. Additionally, the selection criteria for an irradiator depend on the product type, throughput requirements, and regulatory standards across involved sectors.
Design of irradiator
- 1. Design Considerations for Food Irradiators PREETI BIRWAL PH.D (NDRI) DEPT. OF DAIRY ENGINEERING
- 2. Introduction Food irradiation is a relatively new revolutionary process with essentially the same objectives as those of the traditional methods of food preservation i.e., extending shelf-life and reducing the incidence of food-borne disease. All food irradiators must expose the product to the radiation source effectively; the radiation may be either from 60Co or machine - generated. Many factors influence the design of a food irradiator and the choice of the radiation source. The major considerations include the uniformity of the distribution of the absorbed dose in the given product, efficient utilization of the radiation energy, and cost - effectiveness based on minimizing the combined capital and operating costs
- 3. Larger in size are pilot irradiators capable of conducting research, but they are mainly for irradiating large quantities of products for feeding tests, shipping studies, and commercial feasibility studies. Two examples of these are the Hawaii Development Irradiator built with the support of the then U.S. Atomic Energy Commission (AEC) in Honolulu in 1967, with an initial loading of 200 kCi of Co-60. Large quantities of papayas were irradiated, made into puree, and shipped to a U.S. laboratory in Illinois for animal feeding studies on safety. Another irradiator was the U.S. Marine Fisheries Irradiator, also built with AEC support in the mid-1960s at Gloucester, MA, for irradiation study of marine species. Two pilot electron accelerators were funded by the U.S. Department of Energy in the late 1980s under the program called Agriculture Demonstration Irradiators. One was installed at the Plant Industry Division of the Florida Department of agriculture and Consumer Services in Gainesville and another at Iowa State University in Ames. Both of these were made in France. The one in Ames, Iowa, can operate in both electron beam mode and x-ray mode. In 2001, Texas A & M University installed a pilot e-beam facility for research and development. Commercial Co-60 irradiators with initial loading of 500 kCi to several million curies have been built around the world, mainly for sterilizing disposable medical supplies and some cosmetics. Most electron accelerators have also been installed for industrial uses, such as the cable and plastic industry
- 4. IRRADIATOR SELECTION CRITERIA For the efficient operation of the irradiation facility, it is critical that the requirements of the intended applications are clearly understood before the facility is designed and built. When listing the requirements, it is essential that not only the present needs are considered but also the future ones (but realistic) are included. selection process is : – Type of product to be irradiated (size, density, homogeneity), Seasonality of product (some food products may be seasonal), Dose and dose uniformity requirements for the intended product(s) and process, Throughput requirement, product density can affect throughput. Is the irradiator part of a manufacturing or other process, or a service facility, and Is this a single or a multi purpose facility.
- 5. Besides these technical criteria, there are others that should also be considered during the selection process. These include: – Capital and operating cost of the total facility, Utility requirements such as electrical power and water supply, and Technical expertise available in the region, including human resources. Depending on the national regulations of the country, it would be necessary to obtain a license to construct and operate the facility. Several departments or ministries could be involved depending on the product to be processed, such as atomic energy authority, health ministry, food ministry, industry ministry, etc.
- 6. Types of Irradiators Panoramic dry source storage irradiators Underwater irradiators, in which both the source and the product being irradiated are under water. Panoramic wet source storage irradiators. Electron beam irradiation facilities, in which irradiation is performed in an area that is potentially accessible to personnel, but that is kept inaccessible during the irradiation process. X ray irradiation facilities, in which irradiation is performed in an area that is potentially accessible to personnel, but that is kept inaccessible during the irradiation process.
- 7. The irradiation treatment can be conducted in either a batch or continuous mode. The batch method is simpler to design, easier to operate and is more flexible. The continuous operation is more suitable for large volumes. It may involve a single-pass or a multiple pattern, the design of which allows a more uniform exposure of the food and a more efficient use of the source. Mobile irradiators are also available. These are compact units which are best suited for certain foods where season, location, transport and interval between harvest and processing, may be limiting factors, e.g., sea foods. The design should allow easy changing from one product to another without any product receiving doses outside the allowable limits.
- 8. Whatever type of irradiator is used, its operation must be reliable and must consistently provide a product that meets the design specifications. Controls and monitors must be provided that will stop the processing if the processing parameters are outside the design limits. Safety issues related to the product are a major concern when the process is designed to reduce the number of pathogenic organisms. All products must be processed within the required limits to ensure microbial safety. Safety systems to protect operating personnel from hazards caused by radiation must also be provided. These include appropriate shielding, radiation cell entry control and radiation monitoring systems. The design of the irradiator should also minimize the risk from other industrial hazards.
- 9. Design Requirement Wiring Source return Fire protection Access control Source rack Radiation monitors Water handling system Shielding Foundations
- 11. Cobalt Irradiators The 60 Co source elements are usually in the form of cylindrical pencils. A number of source pencils may be installed into individual modules to simplify the handling of the source. The modules are then distributed in a rack to form the desired source array. For panoramic irradiators with sources stored in a water - filled pool, the source handling is usually performed underwater. Source elements, source modules and a typical rectangular source rack. The size of the source rack should be sufficient to allow the addition of source pencils without requiring the return of source pencils until the end of their working life. Generally, a source rack is designed to allow the addition of new source pencils without removing sources for approximately 20 years.
- 12. Schematic illustration of a typical source rack composed of source modules, source pencils, inner capsules and cobalt slugs.
- 13. In many 60Co irradiator designs, the product is moved in parallel rows on both sides of a vertical rectangular source rack. The product may move at a controlled speed or may spend specified time increments in different static locations (shuffle dwell). The drive mechanism for moving the product may be pneumatic, hydraulic or electric. The product can be loaded into individual metal containers (totes) and transported on conveyors, or it can be loaded into hanging containers (carriers) for transport past the source.
- 14. Generally, the narrower the dimensions of the product in the direction perpendicular to the source rack, the more uniform the dose is, and the more rows of product, the better the efficiency. However, the distance from the source to the product and the amount and location of metal in the containers, conveyors and supporting structure also affect the throughput and the dose uniformity. For larger product volumes, such as entire pallet loads, it may be difficult to obtain the required dose uniformity, especially for higher density products. A different source and product geometry may be used to allow the product to be irradiated from four sides to improve dose uniformity. The product passes the source in four rows, two on each side of the source, and passes the source at two levels. The product extends past the ends of the source in both the vertical and the horizontal direction to minimize radiation loss and to obtain high radiation utilization.
- 15. As the source emits radiation in all directions and the rate of emission cannot be controlled, it is essential to control product movement past the source in the most efficient way possible, to make best use of the radiations. Another aim is to achieve the lowest possible dose uniformity ratio. For this reason, containerized products may follow a complex path around the source, often two or more product units in depth, the units being turned to effect two- sided irradiation. An example of product progress around a source in fig. The dose rates provided by isotope sources are generally low, so that irradiation may take around 1 h to complete. Therefore, product movement is often sequential with a finite time allowed in each position without movement, to allow absorption of sufficient radiation.
- 18. Electron Beam Irradiators The main designs of electron irradiator available are the Dynamitron, which will produce electron energies up to 4.5 MeV, or linear accelerators for higher energies. In either case, the resulting beam diameter is only a few millimeters or centimeters. To allow an even dose distribution in the product, it is necessary to scan the beam using a scanning magnet, which creates an alternating magnetic field (analogous to the horizontal scan of a television tube) which moves the beam back and forwards at 100–200 Hz. An even, fan-shaped field of emitted electrons is created, through which the food is conveyed. A simplified diagram of an electron beam machine. For electron beam irradiators, the product is usually transported at a controlled speed past a scanned beam of electrons. Because the electrons are stopped by a short depth of product, the irradiation zone is usually small and the dose is delivered in a very short time.
- 19. the low penetration of electrons requires that this technology is limited either to surface treatment of food or to foods of limited thickness (8 cm max., with two- sided irradiation). When electrons strike a target, they produce X-rays which can be utilized to give greater penetration depth, but which suffer from the disadvantage of a low conversion efficiency. The efficiency of conversion depends on the energy of the electrons and the atomic number of the target material. In practice, therefore, X-rays are produced by firing high-energy electrons at a heavy metal target plate, as shown schematically in Fig. 5.8. Even with 10 MeV electrons and a tungsten (atomic number 74) plate, the efficiency of conversion is only 32%, hence cooling water must be applied to the converter plate. Both electrons and X-rays deliver much higher dose rates than isotope sources, so that processing is complete in a matter of seconds. Also the beams may be directed, so that the complex transport of packages around the source is not required.
- 22. Special features: There may be specific requirements for some processes that could be incorporated in some of these designs. Irradiation of products in frozen or chilled condition: This is generally accomplished by the use of insulated containers. Incremental dose delivery: For a continuous mode of operation, this feature allows irradiating products with different dose requirements together. Products requiring less dose exit the irradiation room after less number of revolutions, while other products continue to go around the source for more dose. Low absorbed-dose applications: Because of mechanical speed limitations, various techniques may be used to reduce the absorbed-dose rates for such processes. These techniques include using only a portion of the source (e.g. raising one of several source racks to the irradiation position), using attenuators, and irradiating at greater distances from the source (which may be a separate loop)