Method and apparatus for non-thermal pasteurization of living-mammal-instillable liquids

A non-thermal plasma reactor is provided for treating a liquid with non-thermal plasma species. The reactor includes a liquid inlet, a liquid outlet, a reaction volume between the liquid inlet and the liquid outlet and at least one non-thermal plasma electrode adjacent to the reaction volume. The non-thermal plasma electrode is isolated physically and electrically from the flow path by a dielectric barrier.

BACKGROUND OF THE INVENTION

The present invention relates to non-thermal pasteurization, sterilization or disinfection of a living-mammal-instillable liquid to destroy live pathogens living in the liquid.

Various methods of pasteurizing liquids such as liquid foods, fermentation broth, biological fluids, blood products, medicines, vaccines, etc., have been used for destroying live pathogens, including bacteria, viruses and fungi, living in the liquids. However, these methods typically generate heat during the pasteurization process to kill live pathogens. This heat may introduce impurities depending on the process and can also easily damage active components, ingredients or other desirable characteristics of the liquid, such as food nutrients and sensory attributes, including flavors, aromas and colors. If these products are thermally processed, they will become unacceptable or their commercial values will be greatly reduced. In the case of biological fluids, living cells may be altered or damaged. Therefore, a number of minimal thermal processes have been developed for some of these applications, including ultra-filtration, ozonation, pulsed ultraviolet light, irradiation, high hydrostatic pressure (HHP) and pulsed electric field (PEF) discharge.

Of these methods, PEF discharge has been shown to be very effective for killing bacteria within liquids. PEF discharge is considered to be one of the premier new technologies with a great potential of replacing thermal, chemical and other pasteurization and sterilization technologies for the treatment of liquid foods and pharmaceuticals. However, there are a number of drawbacks of the PEF discharge technology. For example, ohmic heating occurs during the PEF discharge, which causes the temperature of the liquid being treated to rise. Hence, a cooling system must be used in order to maintain the liquid at a low temperature. A significant amount of energy is wasted with unwanted heating and cooling of the liquid. Also, the requirement of a cooling system adversely increases the time required to treat the liquid. In addition, the PEF electrodes are immersed directly in the liquid. Since the electrodes contact the liquid, they are regarded as a major contamination source to the liquid due to oxidation of the electrodes during discharge. The electrodes must therefore be replaced regularly, which increases maintenance time and costs.

Improved methods of non-thermal pasteurization are desired for pasteurizing liquids without degrading the natural characteristics of the liquids.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a non-thermal plasma (NTP) reactor. The reactor includes a reactor inlet, a reactor outlet, first and second electrodes, and a reaction volume between the first and second electrodes. The reaction volume includes a discharge initiation region and a treatment region. The discharge initiation region is positioned between the first electrode and the treatment region, and the treatment region is positioned between the discharge initiation region and the second electrode. The treatment region is coupled to the reactor inlet and the reactor outlet. A dielectric barrier separates the discharge initiation region from the treatment region.

Another embodiment of the present invention is directed to a non-thermal plasma (NTP) reactor. The reactor includes a liquid inlet for receiving a liquid to be treated, a liquid outlet, first and second electrodes, and a reaction volume positioned between the first and second electrodes and coupled to the liquid inlet and the liquid outlet. A dielectric barrier is positioned between the first and second electrodes. The first and second electrodes and the reaction volume are oriented generally vertically such that the liquid entering the reaction volume from the liquid inlet passes through the reaction volume toward the liquid outlet by the force of gravity.

Another embodiment of the present invention is directed to a non-thermal plasma reactor for treating a liquid with non-thermal plasma species. The reactor includes a treatment flow path for passing the liquid to be treated, a gas injector and a non-thermal reactor cell. The gas injector is coupled in the treatment flow path and has a liquid inlet, a gas inlet and a gas-liquid outlet. The reactor cell is coupled in the treatment flow path and includes an inlet coupled to the gas-liquid outlet, an outlet, a reaction volume between the inlet and the outlet of the cell and a first non-thermal plasma electrode adjacent to the reaction volume. The first non-thermal plasma electrode is isolated physically and electrically from the flow path by a first dielectric barrier. The first dielectric barrier has an upper surface along the reaction volume, which has a plurality of recessed channels extending along the treatment flow path.

Another embodiment of the present invention is directed to a method of at least partially sterilizing a liquid comprising living pathogens. The method includes: (a) passing the liquid with a gas in the form a gas-liquid mixture through a reaction volume between first and second electrodes while maintining a gap in the reaction volume between the gas-liquid mixture and at least one of the first and second electrodes; and (b) electrically exciting the first and second electrodes to generate a non-thermal plasma within the reaction volume and thereby kill at least a portion of the pathogens within the liquid of the liquid-gas mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a diagrammatic view of a “silent type”, volume discharge non-thermal plasma reactor100, which can be used for pasteurizing and/or at least partially sterilizing living-mammal-instillable liquids to kill live pathogens living in the liquids. Non-thermal plasma reactor100includes a liquid inlet102, a liquid outlet104, a reaction volume106between liquid inlet102and liquid outlet104, electrodes108and110, and dielectric barriers112and114. Flow path116indicates the liquid flow path from inlet102to outlet104, through reaction volume106. Each of the electrodes108and110is physically and electrically isolated from the liquid in flow path112by a respective one of the dielectric barriers112and114.

Dielectric barriers112and114are separated from one another by a gap, which defines the effective width of reaction volume106. Dielectric barriers112and114can include Teflon, tempered or regular glass, ceramic, quartz or epoxy resin, for example. Other insulating materials can also be used. In one embodiment, each electrode108and110is embedded within an epoxy resin. In one embodiment, the thickness of dielectric barriers112and114can range from 0.01 millimeters to 3 millimeters, for example. Thicker or thinner barriers can also be used. The discharge gap between electrodes108and110can be sized to suit a particular application. For example, electrodes108and110can be separated by a distance of zero to 5 centimeters, or up to 30 centimeters. A larger gap can be used if voltage and insulation conditions permit. In one particular embodiment, electrodes108and110are separated by 10 millimeters, with an effective gap between dielectric layers112and114of about 7 millimeters. Both single and multi-layer NTP reactors can be used.

Electrodes108and100can have a variety of configurations. For example in the embodiment shown inFIG. 1, electrodes108and110are each formed of a thin, planar sheet of conductive metal, such as a copper foil. Other conductive structures can also be used such as a conductive mesh, wire or strip. The combination of electrodes108and110can have a variety of different types, such as plate-to-plate, mesh-to-mesh, plate-to-wire, wire-to-wire, plate-to-mesh and wire-to-mesh, for example. The shapes of electrodes108and110can also be varied. For example, electrodes108and110can be arranged coaxially with one another, wherein the outer electrode is tubular and the inner electrode is either tubular or a wire. Other arrangements can also be used. However, in each arrangement, both electrodes108and110are physically and electrically isolated from the liquid in the reaction volume by a dielectric barrier in order to prevent an electrical conduction path through the liquid and contamination of the liquid due to contact with the electrodes.

High voltage power supply124supplies power to electrodes108and110. Electrode108is electrically coupled to a first terminal120of power supply124, and electrode110is electrically coupled to a second terminal122of power supply124. One of the electrodes108and110serves a ground electrode, such as electrode110, and the other, such as electrode108, serves as a high voltage electrode. Power supply124can include a direct-current (DC) or an alternating-current (AC) power supply that is capable of producing a voltage across electrodes108and110so as to form an electric discharge path, shown by arrows126, across reaction volume106. In one embodiment, the voltage potential generated between electrodes108and110is a substantially constant AC or DC voltage, such as a continuous AC voltage in the range of 5 kV-35 kV, with a frequency of 1 Hz to 1000 Hz. Other voltage ranges can also be used, such as voltage ranges between 1 kV and 500 kV. Power supply124can be operated at either low or high frequencies and can produce pulses with a single polarity or can produce bipolar pulses.

With electrodes108and110having opposite polarity, electrodes108and110generate a strong electrical field across reaction volume106. The strong electrical field is applied to gas in the liquid, which generates non-thermal plasma species, including electrically neutral gas molecules, charged particles in the form of positive ions, negative ions, free radicals and electrons, and quanta of electromagnetic radiation (photons). These non-thermal plasma species are highly reactive and are effective in destroying live pathogens, such as bacteria, viruses and fungi, living in the liquid being treated. Because of the non-thermal nature of reactor100, reactor100preserves the quality and other heat-sensitive attributes of the liquids being pasteurized.

Examples of liquids that can be treated include any liquid that is instillable in a living mammal, such as a human, dog, horse, cat, etc. The term “instillable” includes all liquids that are non-toxic to a living mammal when introduced into the mammal by methods such as oral ingestion, inhaling, transdermal absorption, rectal (as with enema or other such solutions), direct insertion into arterial vessels, venal vessels (IV), lymphatic vessels, the spinal canal, and body cavities such as the abdomen, the lungs or the liver, intramuscular injection, and subcutaneous injection.

One example of such a liquid is a liquid that is capable of being consumed and assimilated by a living mammal as nourishment. Such liquids include water, juices (such as fruit juices), milk, carbonated and non-carbonated soft drinks, flavored non-carbonated beverages, soups and other dilute and pumpable liquid foods (including liquids with food particles in suspension). Other treatable liquids may include fermentation broth, medications and vaccines of all types, total parenteral nutrition (TPN) liquids, including sugars and lipids, etc., intravenous (IV) fluids such as Lactated Ringers or D5, etc., renal dialyzing fluids (which are instilled and drawn back off), biological fluids, human and animal fluid products, and bodily fluids that must be returned to the body without damage to viable components such as platelets and leukocytes. Such bodily fluids include blood, blood products and cerebrospinal fluid (CSF).

It has been found that the reduction in pathogens living in the liquid being treated is greatly enhanced if fine gas bubbles are introduced into the liquid being treated by the plasma or if the liquid has a large surface area that is exposed to a gas. The interaction of gas or gas bubbles with the plasma has been found to enhance the sterilization effectiveness. The resulting liquid-gas mixture can include a gas dispersed in a liquid or a liquid dispersed in a gas. The gas can be mixed with the liquid in a variety of ways, such as by diffusion or injection. Various gas injection devices can be used, such as a Venturi tube gas injector made by Mazzei Injector Corporation. Alternatively, the liquid can be sprayed through the reaction chamber to form droplets of liquid separated by gas. In one embodiment, the liquid-gas mixture has a thickness along flow path116of 0.1 millimeters to 30 millimeters, for example. Other thicknesses can also be used. Reactor100can be constructed in various arrangements to expose the liquid-gas mixture to the plasma discharge for a time between 0.1 second to 10 minutes, for example. Other treatment times can also be used.

Introducing fine gas bubbles into the liquid greatly enhances the generation of plasma in reactor100for killing pathogens living in the liquid being treated. As the gas-liquid mixture is passed through NTP reactor208, the gas bubbles in the liquid become excited by the applied electric field, generating non-thermal plasma. The non-thermal plasma species then interact with and kill pathogens living in the liquid. Parameters associated with gas injection include composition of the gas, amount and distribution of the gas in the liquid, the size of the gas bubbles, velocity of the liquid relative to the physical motion of the gas, and the gas injector orifice size. Experiments have shown in liquid containing gas bubbles, especially with a gas containing 90% oxygen, bacteria kill is increased substantially as compared to the bacteria kill in liquid containing no gas bubbles.

Various factors that may affect the killing power of the reactive NTP species within reaction volume106include the ratio of gas to liquid (from very low to very high), size of gas bubbles, degree of mixing of gas and liquid, and compositions of the gas and liquid. Preferably, the system is adapted to obtain a 5 log to 10 log reduction in pathogens living in the liquid. A high gas-to-liquid ratio can be obtained by injecting the liquid into a gas phase. For example, it was observed that the killing power of the NTP species was greater with smaller gas bubbles than with larger gas bubbles. Also, it has been found that the more evenly the gas bubbles are distributed in the liquid, the more effective the plasma generation and pathogen reduction. In one embodiment, the ratio of gas volume to liquid volume (Gas Volume/Liquid Volume) is preferably 0.1 to 20, more preferably 0.3 to 5, and most preferably 0.5 to 1. However, other ratios outside these ranges can also be used. A variety of gas compositions can be used, such as air, oxygen, ozone and nitrogen, or a mixture of these or other gases. One type of gas may be more effective than the other in a particular application, depending on the type of liquid and the types of pathogens being killed. For example, the gas bubbles can consist of 100% by volume oxygen (e.g., O2) or 100% by volume nitrogen.

FIG. 2is a diagram which schematically illustrates a non-thermal plasma liquid pasteurization system200, which introduces gas bubbles into the liquid according to one embodiment of the present invention. System200includes liquid source tank202, pump204, gas mixing device206, non-thermal plasma reactor208, high voltage power supply210and liquid receiving tank212. Source tank202, pump204, gas mixing device206, non-thermal plasma reactor208and receiving tank212are coupled in series with one another within a treatment flow path214, which can be formed of a series of tubes or other liquid channels for passing the liquid to be treated from one element in path214to the next.

Tank202contains the liquid to be treated. Pump204pumps liquid from tank202to tank212, through treatment flow path214. Additional pumps can be placed at various locations along treatment flow path214in alternative embodiments. Also, pump204can be eliminated in embodiments in which another mechanism, such as gravity, is used for moving the liquid along treatment flow path214. The output of pump204is coupled to the input of gas mixing device206. The flow rate of the pump is set based on factors such as the desired treatment time, the applied voltage, the dimensions/structures of reactor208, and the size of gas mixing device206. Gas mixing device206can include any device that is capable of introducing gas bubbles into the liquid flowing through treatment flow path214. Various mixing devices can be used, such as a gas diffuser or a gas injector. In one embodiment, gas mixing device206includes a Venturi tube injector. Other types of gas mixers can also be used. Gas mixing device206has a gas inlet216for receiving the gas to be mixed into the liquid.

The gas-liquid mixture is then provided to liquid inlet220of non-thermal plasma reactor208. Reactor208can include reactor100shown inFIG. 1, for example. High voltage power supply210is electrically coupled to the electrodes within reactor208. As the gas-liquid mixture passes through reactor208, from liquid inlet220to liquid outlet222, the non-thermal plasma generated in reactor208pasteurizes the liquid by destroying at least a portion of the live pathogens living in the liquid. The treated liquid then exits through liquid outlet222and is collected in receiving tank212.

In one embodiment, the liquid being treated within reactor208is kept under a pressure that is greater than an ambient pressure surrounding the reactor so as to maintain the gas bubbles substantially uniformly distributed in the liquid and of a small size. The pressure can be increased by providing liquid outlet222with a cross-sectional area that is less than the cross-sectional area of liquid inlet222. Also, the internal reactor flow path can be designed to provide a back pressure in the liquid and to provide turbulent flow.

FIG. 3is a diagram illustrating a Venturi tube injector300, which can be used for the gas mixing device204shown in FIG.2. Injector300has a main flow path302between an inlet304and an outlet306and has a flow constriction308. A gas inlet310is coupled to the main flow path302at the flow constriction308. As liquid flows along main flow path302a pressure difference between inlet304and outlet306creates a vacuum inside the injector body, which draws gas into the injector through gas inlet310and results in a mixture of gas and liquid at outlet306. A Venturi tube injector is a high efficiency, differential pressure injector. It has been found that this type of injector mixes gases with liquids very well. As a result, bubbles in the gas-liquid mixture produced at the output of injector300are extremely fine and uniformly distributed.

FIG. 4is a diagram which schematically illustrates a cross-sectional view of a non-thermal plasma reactor which has a winding, serpentine flow path and can be used for reactor208(shown inFIG. 2) according to one embodiment of the present invention. Reactor400includes a liquid-gas inlet401, a treated liquid-gas outlet402and a plurality of oppositely polarized non-thermal plasma electrodes404and406which are arranged to form a serpentine liquid flow path indicated by arrows408. As described above, each electrode404and406is physically and electrically isolated from the liquid flow path by a respective dielectric barrier. In one embodiment, electrodes404and406are each formed as a planar electrode panel that is parallel to and separated from the other electrode panels. Each electrode panel404and406has a polarity that is opposite to the polarity of the next adjacent electrode panel. This creates a plurality of reaction volumes, which are coupled together in series to form flow path408. Each reaction volume is defined by the gap between a respective pair of electrodes404and406. The serpentine flow path can be used to increase the liquid residence time within reactor400and to increase the turbulence of the liquid flow, which may assist in keeping the gas bubbles more evenly distributed and of a small size in the liquid. Any number of reaction volumes can be used in alternative embodiments. For example, reactor400can include a single reaction volume such as shown inFIG. 1, two reaction volumes that form a U-shaped flow path, or a plurality of reaction volumes as shown in FIG.4. In an alternative embodiment, the individual reaction volumes extend parallel to one another from inlet401to outlet402.

FIG. 5is a cross-sectional view of a tubular non-thermal plasma reactor500according to an alternative embodiment of the present invention. Reactor500has a tubular structure, with flow going into or out of the page in FIG.5. Reactor500includes a tubular ground electrode502and a wire high voltage electrode504, which is coaxial with electrode502. In an alternative embodiment, electrode502is a high voltage electrode and electrode504is a ground electrode. Electrodes502and504are separated by a gap which defines a reaction volume506. Electrodes502and504are physically and electrically isolated from reaction volume506by respective dielectric barriers508and510. Dielectric barriers508and510prevent electrodes502and504from contaminating the liquid being treated and provide electrical isolation that prevents the liquid within reaction volume506from shorting electrode502to electrode504.

FIG. 6is a perspective, schematic view of a non-thermal plasma reactor600having narrow strip electrodes602and604. Electrodes602are biased at one polarity, and electrodes604are biased at an opposite polarity. Electrode strips602and604are arranged perpendicular to one another and are spaced about a reaction volume. Each individual electrode602and604is insulated by a dielectric barrier. For example, all of the electrodes602can be embedded within one sheet of dielectric material, and all of the electrodes604can be embedded within another sheet of dielectric material. With this type of electrode structure, the local electric fields around electrodes602and604are greatly enhanced, which ensures discharge takes place easily and effectively in the gas bubbles.

FIG. 7Ais a side cross-sectional view of a non-thermal plasma reactor700according to another alternative embodiment of the present invention. Reactor700includes a housing702and at least one “surface” discharge electrode704. Housing702has a liquid inlet706, a liquid outlet708and a pair of flow paths710extending on either side of surface discharge electrode704. Surface discharge electrode704includes a plurality of adjacent conductors712and714having opposite polarity. Conductors712and714are electrically insulated from flow paths710by a dielectric material715. In one embodiment, conductors712and714are each individually coated with a dielectric material that forms an electrically insulating sheath. In an alternative embodiment, conductors712and714are embedded in a dielectric material to form an electrode sheet. Conductors712and714can have diameters of about 0.1 to about 3.0 millimeters, for example, and are each separated by a gap in the range of 0 to 6 millimeters, for example.

Excitation of conductors712and714generates micro-current electric field discharge paths716along the surfaces of electrode704. Electric field discharge through discharge paths716generate non-thermal surface plasma species within the liquid being treated, along the surface of electrode704. These non-thermal surface plasma species are highly reactive and destroy pathogens living in the liquid, similar to the embodiments discussed above. Electrode704can have a variety of shapes, such as planar or tubular.FIG. 7Bis a plan view of electrode704in planar form, which illustrates one possible arrangement of conductors712and714.

FIG. 8is a side view of a non-thermal plasma reactor800according to another alternative embodiment of the present invention. Reactor800includes fluid inlet801, fluid outlet802, electrodes804and806and dielectric barriers808and810. Electrodes804and806are separated from one another by a gap, which defines a reaction volume between dielectric barriers808and810. Reactor800further includes a sprayer812, which is coupled to fluid inlet801for receiving the liquid to be treated. Sprayer812spays the liquid through the reaction volume, between dielectric barriers808and810to form a fine mist within the reaction volume. The treated liquid then exits through liquid outlet802. Sprayer812assists in generating a gas-liquid mixture within the reaction volume, which helps the plasma in destroying pathogens living in the liquid.

FIG. 9illustrates an NTP reactor900having a set of barriers used to increase the back pressure within the liquid being treated. Briefly referring back toFIG. 2, the stream of the gas-liquid mixture from gas mixing device206to reactor208is of high speed and high pressure. To some extent, the distribution of gas bubbles in the liquid depends on the back pressure of the mixture. The higher the back pressure, the higher the solubility of the gas in the liquid. In one embodiment, a large tank202can be used to increase the back pressure in the system.

In the embodiment shown inFIG. 9, the arrangement of electrode panels is used to increase the back pressure. As liquid is pumped through tube901, gas injector902draws gas into gas inlet903and produces a gas-liquid mixture at the outlet of the injector. Tube904delivers the gas-liquid mixture from gas injector902to inlet908of NTP reactor900. NTP reactor900has a plurality of electrode plates905and906, which are arranged to form a serpentine flow path from inlet908to outlet909and are arranged perpendicular to inlet908. With this arrangement, electrode plates905and906form barriers to the liquid stream entering from inlet908and being passed from one portion of the flow path to the next. These barriers further increase back pressure within the gas-liquid mixture.

Experimental Results

Several experiments were performed to demonstrate the effectiveness of non-thermal plasma in reducing pathogens living in a liquid. These experiments are described below.

The first experiment was performed to test the effect of air injection conditions and applied electric field on the viability of Salmonella in a liquid carrier (i.e., distilled water).

In a first test a “static” reactor was used, which had stripped electrodes similar to the electrodes shown in FIG.6. In the static reactor, the liquid to be treated was placed into the reactor with no flow. The gaps between individual electrode strips were 10 mm, and the effective reaction volume had a gap of 7 mm. A liquid containing Salmonella and no gas bubbles was placed in the reaction volume. The liquid was then treated by operating the electrodes at 25 kV. Next, a liquid containing Salmonella was placed in the reaction volume and bubbled with air at 1-2 CFH to introduce air bubbles into the liquid. The electrodes were again operated at 25 kV. Finally, a liquid containing Salmonella was placed into the reaction volume and bubbled with oxygen at 1-2 CFH. The electrodes were again operated at 25 kV.

Table 1 shows that the reduction in bacteria is minimal when there are no gas bubbles in the liquid and is increased substantially with the presence of air bubbles and especially with the presence of oxygen bubbles, in the liquid.

The reductions in bacterial load were evaluated using standard approaches involving serial dilutions of a solution, which were plated onto culture plates. Following incubation, colonies were counted to evaluate the number of organisms in the diluted solutions. Using the dilution values, estimates were obtained of the original bioload.

Next, Salmonella reduction was tested with a “static” NTP reactor having oppositely polarized plate electrodes, which were operated at 15 kV and were separated by dielectric barriers. The gap between the electrodes was 10 mm, and the effective reaction volume between the dielectric barriers had a gap of 7 mm. Liquid containing Salmonella was placed in the reaction volume, bubbled with air and treated. The resulting bacteria reduction as a function of time is shown in Table 2.

The smaller applied voltage, as compared to the voltage used to produce the results in Table 1, resulted in a smaller log reduction of Salmonella bacteria in the liquid.

Next, Salmonella reduction was tested by placing a liquid containing Salmonella into the reaction volume, bubbling the liquid with oxygen and then treating the liquid-oxygen mixture by operating the electrodes at 15 kV. Again, the gap between the electrodes was 10 mm, with an effective reaction volume gap of 7 mm. The results of this test are shown in Table 3.

Looking at Tables 1-3, the use of non-thermal plasma to treat a liquid having injected gas bubbles is effective in achieving at least a five log reduction in Salmonella. Comparing Tables 2 and 3, the use of oxygen bubbles as compared to air bubbles increased the amount of Salmonella reduction per unit of treatment time.

In the second experiment, the use of non-thermal plasma was tested for effectiveness in killingE. Colibacteria within a liquid. The test apparatus used in the second experiment was similar to that shown in FIG.2. The gas mixing device included a Venturi tube injector, which introduced air and oxygen at 1-2 CFH, and the NTP reactor had a serpentine flow path such as that shown inFIG. 4with two individual reaction volumes. The gaps between the electrodes in the reactor was 10 mm, and the effective reaction volume between the dielectric barriers had a gap of 7 mm. An untreated liquid inoculated with five logs ofE. Coliwas placed in tank202and passed through NTP reactor208to tank212. Samples were then taken from the untreated liquid in tank202and the treated liquid in tank212and cultured in a similar fashion as described above with reference to Table 1. Bacterial colonies were found in the cultured untreated samples, while no bacterial colonies were observed in the cultured treated samples. Based on these observations, it was concluded that the pasteurization system shown inFIG. 2was effective in producing a five log reduction inE. Coli.

In the third experiment, the NTP pasteurization system shown inFIG. 10was built and tested. System1000included five NTP reactors1001connected together in series with each NTP reactor1001having its own source tank1002, pump1003and gas injector1004. The outlet of each NTP reactor1001was coupled to the source tank1002of the next reactor1001in the series. The plurality of gas injectors1004ensured that the gas-liquid mixture contained sufficiently fine bubbles throughout the flow. Air was injected through each injector1004at 2 cubic feet per hour (CFH). Pumps1003pumped the liquid through system1000at 10 gallons per hour. The electrical connections to the NTP reactors1001were coupled together in parallel with one another and were excited at 20 kV. The number of NTP reactors1001in system1000was varied so that the effect of the number of reactors on Salmonella bacterial reduction could be examined.

FIG. 11is a graph illustrating the log Salmonella bacterial reduction in the liquid as a function of the number of NTP reactors1001in FIG.10. With five NTP reactors1001, a five log bacterial reduction was be obtained with the system shown in FIG.10. However, this five log bacterial reduction was not observed when only one gas injector was used prior to the first NTP reactor in the system. This suggests the importance of gas bubbles in the liquid. Looking atFIG. 11the log bacterial reduction increased with the number of NTP reactors. This increase can be attributed to both the increased energy input and the increased amount of air bubbles in the liquid.

In the fourth experiment, the log reduction of Salmonella bacteria was tested as a function of applied voltage. The same system was used in Experiment 4 as was used in Experiment 3, with five NTP reactors connected together in series. Experiment 4 was conducted at 30 gallons per hour, and with 2 CFH air injection in each injector1004.FIG. 12shows the log reduction in Salmonella bacteria as a function of the voltage applied to each NTP reactor1001. As can be seen fromFIG. 12, log reduction in bacteria increases with increasing applied voltage. More than three logs of bacterial reduction is achieved at 30 kV.

In the fifth experiment, the pasteurization system shown in FIG.10and described above in Experiment 3 was used under three conditions: (1) without air injection; (2) with air injection; and (3) with oxygen injection. Otherwise, the same operating conditions were used as were used in Experiment 3, with five NTP reactors1001connected together in series. If oxygen can be replaced with clean air, the equipment and running costs of the system can be reduced. The results of Experiment 4 are shown in the graph of FIG.13.FIG. 13is a graph illustrating the log reduction of Salmonella bacteria for each of the test conditions. As shown inFIG. 13, without any air or gas input into the system, the system was only partially effective in killing Salmonella. With air injection, a two log reduction of bacteria was achieved. With oxygen injection, a five log reduction of bacteria was achieved. This suggests that air is a possible gas media in the NTP pasteurization system, but modifications of the system shown inFIG. 10may be needed to achieve a five log reduction with air injection. For example, the resident time of the treated liquid within NTP reactors1001can be increased.

The above-experiments show that non-thermal plasma is effective in reducing viable bacteria in a liquid sample. Non-thermal plasma can therefore be used for at least partially sterilizing liquid food such as juices and milk. Since there is substantially no ohmic heating, energy consumption during non-thermal plasma sterilization is small, and there is no need to cool the liquid being treated. This allows the system to be easily scaled-up accommodate a very large treatment volume. The desired treatment time can be obtained by passing the liquid through multiple NTP reactors connected together in series with one another or by cycling the liquid through the same reactor multiple times. Also, the number of series-connected reaction volumes in the same reactor can be increased or decreased. Because of the non-thermal nature of the system, the system preserves the quality and other heat-sensitive attributes of the liquid, such as taste and vitamin content. Other possible applications include pasteurization/sterilization of fermentation broth, biological fluids, blood products, medicines and vaccines. Also, since each electrode is physically and electrically isolated from the liquid being treated, the electrodes do not act as a source of contaminants to the liquids. The following figures illustrate further embodiments of the present invention.FIG. 14is a simplified, perspective view of two mesh-type non-thermal plasma electrodes1020and1022that can be used for pasteurizing liquids. Electrodes1020and1022are each formed of a conductive wire mesh, which has been coated with a dielectric material such that the wire mesh is electrically insulated from the liquid being treated. The dielectric coating is formed so that the area between each conductive segment in the mesh is open to fluid flow. Any coating technique can be used, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD).

The liquid to be treated is passed through electrodes1020and1022in the direction of arrow1024, substantially perpendicular to the planes formed by electrodes1020and1022. As the liquid passes through meshes1020and1022, electrodes1020and1022are electrically coupled to opposite voltage potentials, which creates a plasma within gap1026for treating the liquid present within the gap. If the openings in electrodes1020and1022are sufficiently small, the openings can further assist in breaking-up larger gas bubbles and maintaining the gas bubbles in the liquid at a sufficiently small size. Other arrangements can also be used, and meshes1020and1022can be non-planar. Also, a series of electrode pairs1020and1022can be used, wherein the liquid flows sequentially through each electrode pair for treatment. In an alternative embodiment, a gas injector or diffuser is not used to mix the gas and liquid. Rather, the gas is supplied through a tube into the reactor and is then broken into small bubbles as the gas and liquid are forced through the small openings in the mesh electrodes.

FIG. 15is a diagram, which schematically illustrates a multiple-plate non-thermal plasma reactor1500according to another embodiment of the present invention. Reactor1500has a liquid source tank1502and a liquid outlet tank1504. Pump1506draws liquid1508from tank1502through tube1510and supplies the liquid to gas injector1512through tube1514. As liquid1508is pumped through gas injector1512, gas injector1512draws gas into gas inlet1513and produces a gas-liquid mixture at the outlet of the injector. Tube1516delivers the gas-liquid mixture to valves1518, which control flow to a plurality of parallel NTP reactor cells1520. The term “tube” as used in the specification and claims can include any conduit or passage formed of any suitable material and having any suitable cross-sectional shape.

Each cell1520has a reaction volume1522and a pair of oppositely polarized electrodes1524, which are electrically and physically isolated from the reaction volume by dielectric barriers1526. Tubes1528deliver the gas-liquid mixture to reaction volumes1522for treatment. Dashed lines1530represent the upper surfaces of the gas-liquid mixtures in each reaction volume. Spacers1527define the height of reaction volumes1522, between opposing surfaces of dielectric barriers1526.

High voltage power supply1540delivers electrical excitation energy to electrodes1524through conductors1541for generating non-thermal plasma within reaction volumes1522. In one embodiment, power supply1540delivers an AC voltage of 5 kV to 30 kV at a frequency of 1 Hz to 1000 Hz, for example. Other voltages and frequencies can also be used. The treated gas-liquid mixture1530is then returned to tank1504through tubes1542. Electrodes1524and dielectric barriers1526can have any structure and gap size, such as those disclosed in the present application. Any number of parallel NTP reactor cells1520can be used in alternative embodiments of the present invention.

FIG. 16is a diagram that schematically illustrates a two-dielectric barrier NTP reactor1600having a discharge initiation region according to another alternative embodiment of the present invention. The same reference numerals are used inFIG. 16as were used inFIG. 15for the same or similar elements. In this embodiment, a film or plate1602divides reaction volume1522into a treatment region1604and a discharge initiation region1606. Film1602is suspended in the space between dielectric plates1526by spacers1608, for example. Tube1516delivers the gas-liquid mixture1530into treatment region1604, and tube1542returns the treated gas-liquid mixture to tank1504. Film1602contains gas-liquid mixture1530in treatment region1604and prevents the gas-liquid mixture from entering into discharge initiation region1606. Discharge initiation region1606can be filled with various gases, such as air, another gas or a gas mixture. Discharge initiation region1606can also be substantially void of any gas and held under a vacuum at below-normal atmospheric pressure. In this embodiment, electrodes1524are parallel plates, and discharge initiation region1606and treatment region1604are rectangular volumes.

In one embodiment, film1602is formed of a dielectric material, such as a transparent membrane of polytetrafluoroethylene from E.I. du Pont de Nemours and Company. In alternative embodiments, film1602can be formed of a transparent epoxy resin or other types of film or sheet materials. Film1602has good dielectric properties and allows one or more of the non-thermal plasma species to pass from discharge initiation region1606to treatment region1604. However, film1602should not allow the gas-liquid mixture1530to pass into discharge initiation region1606. Film1602can also be non-dielectric, as long as there is at least one other dielectric barrier between electrodes1524. Film1602can also include an ion-selective membrane. In one embodiment, film1602is made as thin as possible and transparent so as to limit absorption or reflection of the non-thermal plasma species passing through to treatment region1604. For example, film1602can have a thickness between 0.02 millimeters to 1 millimeter. Smaller or larger thicknesses can also be used. The surfaces of film1602can be hydrophilic or hydrophobic.

During operation electrodes1524are energized. The resulting electrical field between the electrodes generates non-thermal plasma species within regions1604and1606. Non-thermal plasma species within region1606are easily generated, and the discharge across region1606is fairly uniform. This assists in generating more consistent and uniform plasma species within treatment region1604. Without discharge initiation region1606, it has been found that the discharge within the gas-liquid mixture1530can be inconsistent or non-uniform, depending on the particular apparatus. The NTP species generated within initiation region1606that pass into treatment region1604react with the gas-liquid mixture to kill more evenly and consistently pathogens living in the liquid. Film1602also protects the upper electrode1524and the upper dielectric barrier1526from contamination or staining by gas-liquid mixture1530.

In addition, the discharge initiation region1606can be used to limit the generation of ozone more easily in applications where ozone is not desired. This region can be filled with a gas other than air, such as nitrogen, carbon dioxide or another gas, and still provide an effective treatment of any live pathogens in the liquid. In these embodiments, gas injector1512can be used to inject a gas other than air to further limit the generation of ozone. However, air can also be used if desired. Discharge initiation region1606can also be held under a small vacuum to further limit the amount of gas in the region and therefore the amount of ozone that is generated.

In an alternative embodiment, NTP reactor1600further includes a gas source1620, which supplies gas to discharge initiation region1606through tube1622. In addition, a tube1624can by coupled between discharge initiation region1606and gas inlet1513of gas injector1512. During operation, gas injector1512draws gas containing the non-thermal plasma species from initiation region1606into gas inlet1513to further enhance the mixture of non-thermal plasma species in the liquid being treated. Gas source1620replaces the gas drawn out of discharge initiation region1606. In another embodiment the NTP species generated in region1606is mixed with the gas-liquid mixture1530at the outlet of NTP cell1520. Mixing can be accomplished through a gas injector similar to injector1512, a diffuser or any other apparatus or method that forces or assists in the NTP species passing through or contacting the treated liquid.

In a further embodiment (not shown in FIG.16), a second dielectric film1602is positioned on the other side of treatment region1604, between treatment region1604and bottom dielectric barrier1526. The second dielectric film can be spaced from the bottom dielectric barrier1526by a further discharge initiation region1606, such that both sides of treatment regions1604have a discharge initiation region1606.

One or more of the dielectric barriers1526and1602can be eliminated as long as there is at least one dielectric barrier between electrodes1524. For example, both dielectric barriers1526can be eliminated such that dielectric film1602serves to separate regions1604and1606and as the sole dielectric material between electrodes1524. In yet a further embodiment, dielectric film1602is eliminated and one or both of the dielectric barriers1526are spaced from their respective electrodes1524. In this embodiment, the liquid being treated will still have no direct contact with electrodes1530, and the spaces between dielectric barriers1526and their respective electrodes1524can be used as discharge initiation regions similar to region1606.

FIG. 17is a diagram, which illustrates an NTP reactor1700according to another alternative embodiment of the present invention. Again, the same reference numerals that are used inFIG. 17as were used inFIGS. 15-16for the same or similar elements. In this embodiment NTP cell1520has a dielectric film1602, which separates gas-liquid mixture1530from discharge initiation region1606and a bare metal electrode1702. The upper dielectric barrier1526(shown inFIG. 16) adjacent the upper electrode1524has been removed. In another embodiment, the lower dielectric barrier1526can also be removed such that dielectric film1602serves as the main dielectric barrier between electrodes1524.

FIG. 18is a diagram, which schematically illustrates an NTP reactor1800according to another embodiment of the present invention. NTP reactor1800is similar to NTP reactor1600shown inFIG. 16, but has no dielectric film1602. Reaction volume1522has a height1802that exceeds the height1804of the gas-liquid mixture1530flowing through reaction volume1522to create a gap1806between the upper surface of mixture1530and the bottom surface of the upper dielectric barrier1526. As long as the gap1806is maintained during operation, the gap can serve as a discharge initiation region. The gap can be maintained by controlling or otherwise setting the volume flow of gas-liquid mixture1530through the inlet and outlet of reaction volume1522such that the gas-liquid mixture remains confined to the treatment region. Gap1806can be filled with air or any other suitable gas.

FIGS. 19-21show the electrode structure of one of the NTP cells1520shown inFIGS. 15-18, according to one embodiment of the present invention.FIG. 19is a top plan view of the NTP cell1520in which upper electrode1524and upper dielectric barrier1526are partially cut-away to expose a portion of bottom dielectric barrier1526.FIG. 20is a cross-sectional view of NTP cell1520, taken along lines20—20of FIG.19.FIG. 21is a cross-sectional view of NTP cell1520taken along lines21—21of FIG.19.

InFIGS. 19-21, dielectric film1602is removed for clarity. A pair of opposing end spacers1608and1609and opposing sidewall spacers1906define the reaction volume between the upper and lower dielectric barriers1526and contain the gas-liquid mixture being treated. End spacer1608has a plurality of passages1902(shown in dashed lines inFIG. 19) for passing the gas-liquid mixture from tube1516(shown inFIGS. 15-18) to the reaction volume. End spacer1609(FIG. 21) has similar passages1902for passing the treated gas-liquid mixture to tubes1542(shown in FIGS.15-18).

Within reaction volume1522, upper surface of the lower dielectric barrier1526can include a plurality of raised ridges or separating walls1910that maintain a dispersed flow of the gas-liquid mixture through reaction volume1522. Separating walls1910define a plurality of recessed channels1912along which the gas-liquid mixture flows. Separating walls1910can have heights that are equal to the height of reaction volume1522or less than the height of reaction volume1522. Spacers1608,1904, and1906and separating walls1910can be formed of the same material as dielectric barrier1526or from different material.

FIG. 22is a diagram, which schematically illustrates an NTP reactor2200according to another alternative embodiment of the present invention. Again, the same reference numerals are used inFIG. 22as were used inFIGS. 15-21for the same or similar elements. NTP reactor2200has a cylindrical NTP cell2202having a central axis2204, which is oriented normally (i.e., vertically) with respect to the floor on which reactor2200is supported and therefore parallel to the gravitational forces of the earth. NTP cell2202has a lower end2216, an upper end2218, a cylindrical inner stainless steel ground (or alternatively high voltage) electrode2206, a cylindrical inner dielectric barrier2208and a cylindrical outer high voltage (or alternatively ground) electrode2210. Cell2202has an inlet2212and an outlet2214located at the bottom end2216of cell2202. The space between the outer diameter of dielectric barrier2208and the inner diameter of high voltage electrode2210forms a reaction volume2222within which gas-liquid mixture1530is treated.

Tube1516is coupled between valve1518and inlet2212. The interior of cylindrical ground electrode2202and dielectric barrier2208serves as a passageway2220for delivering gas-liquid mixture1530(shown in dashed lines) to top end2218of NTP cell2202. As gas-liquid mixture1530exits the top of passageway2220, the gas-liquid mixture falls through reaction volume2222due to the force of gravity. The treated gas-liquid mixture1530then exits outlet2214and returns to tank1504through tube1542. The falling gas-liquid mixture1530maintains the mixture of gas and liquid and increases the surface area of the liquid that is exposed to the NTP species. This can further increase the effectiveness of the NTP treatment. Alternatively, inlet2212can be positioned at upper end2218.

NTP cell2202further includes a cylindrical dielectric film2230, which separates reaction volume2222into a treatment region2232and a discharge initiation region2234. Discharge initiation region2234can be filled with a gas or a vacuum, as discussed above, and is physically isolated from the gas-liquid mixture being treated in region2232. In an alternative embodiment, initiation region2234is positioned between treatment region2232and electrode2210. Additional discharge initiation regions can also be used, as discussed above.

FIG. 23is a cross-sectional view of a cylindrical NTP cell2300according to an alternative of the present invention.FIG. 24is a cross-sectional view of NTP cell2300taken along lines24—24of FIG.23. The same reference numerals are used inFIGS. 23 and 24as were used inFIG. 22for the same or similar elements. NTP cell2300is similar to NTP cell2202, but further includes an outer cylindrical dielectric barrier2302positioned between reaction volume2222and the inner diameter of outer electrode2210.

FIG. 25is a diagram, which schematically illustrates an NTP reactor2500in which NTP cell2300(shown inFIGS. 23 and 24) can be used. Again, the same reference numerals are used inFIG. 25as were used inFIG. 22for the same or similar elements.

FIG. 26is a diagram, which illustrates a conical NTP reactor2600according to another alternative embodiment of the present invention. Reactor2600includes a conical NTP cell2600having a conical inner electrode2604, a conical inner dielectric barrier2606, a conical outer dielectric barrier2608and a conical outer electrode2610. The space between the outer diameter of dielectric barrier2606and the inner diameter of dielectric barrier of2608defines a reaction volume2612through which gas-liquid mixture1530passes for treatment. NTP cell2602has a central axis2614, which is aligned vertically similar to the NTP cells shown inFIGS. 22-25. Inlet2212is positioned at the base of cell2602, and includes a passage2614, which extends through the interior of conical electrode2604to the top of reaction volume2612. In an alternative embodiment, inlet2212is positioned at the top of NTP cell2602. Dielectric barriers2606and2608isolate electrodes2604and2610from the gas-liquid mixture1530within reaction volume2612.

In alternative embodiments, the cylindrical or conical NTP cells shown inFIGS. 22-26can further include one or more dielectric films and discharge initiation regions similar to those shown or described with reference toFIGS. 16 and 17. Also, the cylindrical or conical dielectric barriers can be spaced from their respective electrodes to provide one or more discharge initiation regions between the electrodes and dielectric barriers.

FIG. 27illustrates a non-thermal plasma reactor2700in which the liquid is sprayed into the reaction volume, according to another alternative embodiment of the present invention. Reactor2700has an NTP cell2702, which includes vertically aligned electrode plates2704and2706, dielectric barriers2708and2710and reaction volume2712. A spraying nozzle2714is positioned at a top end2716of reaction volume2712as is coupled to valve1518through tube1528. Spraying nozzle2714sprays the liquid1508through reaction volume2712, between dielectric barriers2708and2710to form a fine mist2718within the reaction volume. Gravity pulls the liquid droplets in mist2718downward toward outlet2720at which the liquid droplets are returned to tank1504.

Any of the reactor cell structures discussed in the present application can be used in the NTP reactor2700in alternative embodiments of the present invention. NTP cell2702can have parallel plate electrodes or concentric cylindrical electrodes, for example, and can have one or more discharge initiator regions as discussed above.

FIG. 28is a diagram, which illustrates an NTP reactor2800according to another alternative embodiment of the present invention. The same reference numerals are used inFIG. 28as were used inFIG. 27for the same or similar elements. Similar to the embodiment shown inFIG. 27, NTP reactor2800includes an NTP cell2802having vertically aligned electrode plates2704and2706, dielectric barriers2708and2710and reaction volume2712. In addition, NTP cell2802includes a pair of dielectric films2804and2806, which separate reaction volume2712from dielectric barriers2708and2710, respectively. The space between dielectric film2804and dielectric barrier2708forms a discharge initiation region2808. Similarly, the space between dielectric film2806and dielectric barrier2710forms a discharge initiation region2810.

NTP cell2802further includes a thin curtain-forming tube2812, which is coupled to tube1528at the top end2816of cell2802. As tube1528delivers liquid1508to curtain-forming tube2812, the liquid falling from tube2812forms a “curtain”2820of liquid through reaction volume2712. The curtain of liquid2820significantly increases the surface area of the liquid that is exposed to the NTP species and encourages mixing of the liquid with the surrounding gas in reaction volume2712. The treated liquid returns to tank1504. Curtain forming tube2812can include a horizontal tube with holes in the bottom or with overflow openings along the sides of the tube to form the curtain of liquid. Other structures can also be used to form a continuous or intermittent liquid “curtain”.