Here is something you probably have never heard of, but maybe you want to. Electron beams. They are not radiation, but they do irradiate and thereby disinfect. We use them all the time (about 20,000) the them to irradiate and disinfect food, post office packages, and more. But we don’t use them for water or wastewater treatment. So would they work? This is a four-part blog to outline electron beams and how we might use them to solve some recalcitrant water treatment problems. But first, what is an electron beam? There are two case studies that will be discussed in Parts 2 &3, which the more recent research outlined in part 4. Sludge was the first area of investigation, but emerging contaminants, pathogens and the like are all subject to destruction by electron beams.
The basic concept of the technology is to accelerate electrons in a vacuum and focus those electrons using a magnetic field to create a concentrated, high-energy beam that can be directed at a target. In 1913, William Coolidge developed a high vacuum, thermionic cathode that he used to produce an prototype of the modern electron beam accelerator at General Electric (Coolidge 1916; Coolidge 1917). In 1925, Coolidge placed a thin foil window at one end of a high vacuum tube and studied the effects of the electron beam on a variety of materials using a 200 keV tube (Coolidge 1926; Coolidge and Moore 1926; Coolidge 1933). In the early 1930s, John Cockcroft and Ernest Walton developed circuitry for increasing the voltage for the particle discharge, which was the basis for many high-current, mid-energy electron accelerators (Berejka and Cleland, 2011). This design was improved by Willem Westendorp, who developed one of the first industrial electron beam accelerators at GE which were the first industrial electron beam processing (Westendorp 1940). In 1937, William Hansen and Sigurd Varian developed the klystron amplifier, which increases the amount of available power levels of microwave linear accelerators (linac), which with one or two milliamps of average beam current at 10 MeV are widely used for medical device sterilization and food treatment, accounting for most of the current industrial applications (Berejka and Cleland 2011). By 1941, technological innovations brought forth the commercialization of the industrial computerized tomography accelerator (Berejka and Cleland 2011).
Since the mid-20th century, electron beam technology has provided the basis for a variety of applications. Among the companies that became active with accelerators were High Voltage Engineering Company, Vivirad-High Voltage, Cryovac division of the Sealed Air Corporation, Nissin-High Voltage (NHV) and Wasik Associates. Arno Brasch and Wolfgang Huber developed a pulsed accelerator, based on capacitor banks being charged in parallel and discharged in series, made commercially available through the Electronized Chemicals Corporation (Berejka and Cleland 2011). With their pulsed accelerator, they showed that short pulses of high voltage, high current electron beams could effectively sterilize and preserve food with minimum damage.
Marshall Cleland and Kennard Morganstern founded Radiation Dynamics, Inc. (RDI) in 1958 to sell their Dynamitron which could attain the combination of higher electron energy and higher beam currents that other accelerators (many of which remain in operation -Berejka and Cleland 2011). The Dynamitron can operate at up to 5.0 MeV with total beam power up to 300 kW, thereby forming the basis for the electron transformer-rectifier (ELV) electron beam accelerators produced by the Budker Institute of Nuclear Physics in Novosibirsk, Russia (Berejka and Cleland 2011). Berejka and Cleland (2011) and Nayak et al. (2016) report that the Budker Institute has accelerators that operate between 400 keV and 2.5 MeV with a maximum beam power of 400 kW at 1.0 MeV. In addition, high current pulsed beams, radiofrequency accelerators which operate between 700 keV and 5.0 MeV with a high current version have been developed, and researchers are working on a 10 MeV at 100 kW accelerator.
The Efremov Research Institute of Electrophysical Apparatus in Saint Petersburg also produces a variety of industrial electron accelerators ranging between 0.5 and 2.5 MeV with electron beam power ratings up to 100 kW (Berejka and Cleland 2011). Ford Motor Company initially used energy electron beams (400 keV or less) to cure coatings, which generated a host of companies including Radiation Polymer Company (now Broadbeam Equipment part of PCT Engineered Systems), Energy Sciences Incorporated (ESI), Applied Advanced Technologies (now known as Advanced Electron Beams – AEB), and Ion Beam Applications SA (IBA). The IBA design has become better known as the Rhodotron™. The US Postal Service uses a Rhodotron to sanitize critical US Federal government mail (Berejka and Cleland 2011). The major competition for particle accelerators is gamma irradiators. The gamma irradiators have issues with radiation. As a result, there are 8 times as many particle accelerators in use as gamma irradiators.
New accelerators include changes to the older designs to improve efficiency and reduce operating costs. The basic unit of acceleration in particle accelerators is the RF cavity. Conventional accelerators are made from copper cavities and referred to as warm accelerating technology. More recently, superconducting materials like niobium, referred to as cold accelerating technology because of the need to operate cryogenic temperatures, have gained favor because of their ability to operate more efficiently. Bulk materials processing applications require multi-MeV energy for penetration and thousands of kW (or even MW) of beam power. Inherent losses in copper accelerators limit their efficiency (heat vs beam power). Heat removal limits duty factor, gradient and average power. Superconducting radio frequency (SRF)-based accelerators, found typically only in big science, are huge with complex cryogenic refrigerators, cryomodules, etc. High wall plug power efficiency of these SRF accelerators (e.g. ~75%) allows a large fraction of the input power to go into the beam and ultimately the target.
Recent efforts at institutions like the DOE’s Fermilab have incorporated several new technologies into superconducting RF accelerators to remove the need for liquid cryogens thus greatly reducing the size and complexity of the accelerator. This is in part made possible using cryocoolers to remove heat conductively. Since less heat removal is possible with conduction and the cryocoolers than convection and liquid helium, a bulk of the other technology advances like Nb3Sn thin films, low loss power coupling and accelerator operating parameters are made to reduce heat load which ultimately improves efficiency. The advantages of such an accelerator includes energy efficiency (lower operating cost), smaller foot print (portable and fits into existing operations more easily), less complexity and therefore more robust and higher power allowing for treatment of more mass per unit time.
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