OHMIC HEATING APPARATUS AND METHOD FIELD OF INVENTION

The invention relates to improvements in ohmic heating.

BACKGROUND

Ohmic heating (also referred to as electrical resistance heating, direct resistance heating, and joule's heating) is a heat treatment method in which an electric current is passed through a process fluid and heat is generated within the fluid. It may be used in food processing to achieve pasteurization or sterilization of liquid food products such as milk, for example. Ohmic heating offers the potential for thermal processing of products without relying on conduction of heat from a heated surface into the fluid.

However ohmic heating suffers from a disadvantage of deposition or "cooking" of the product onto the electrode surfaces. When this occurs it rapidly reduces current flow through the product, and therefore heating of the product being thermally treated via ohmic heating. In addition, regular production down time for cleaning the electrode surfaces is uneconomic. Also, ohmic heating at mains frequency can cause damage to the electrodes via electrolysis, so that the electrode surfaces become pitted which further increases the rate at which electrode fouling occurs.

It is an object of the invention to ameliorate this fouling of the electrode surfaces, and to preferably also reduce electrolysis damage to the electrode surfaces, in ohmic heating.

SUMMARY OF INVENTION

In broad terms in one aspect the invention comprises apparatus for ohmic heating a product which includes an electrode clearing device arranged to contact or move over a surface of an electrode or electrodes of the apparatus to clear product from the electrode(s) during operation of the apparatus.

In a preferred form the electrode clearing device or in particular the operative scraping edge thereof (herein referred to as a "scraper") is arranged to move about an axis of rotation for the scraper so that the scraper moves continuously over a cylindrical surface or surfaces of the electrode(s) during ohmic heating.

In broad terms in a further aspect the invention comprises apparatus for ohmic heating a product, comprising concentric electrodes separated by a product space for containing or passage therethrough of the product during ohmic heating of the product and a scraper device arranged to rotate about a common axis with the electrodes between the electrodes over a cylindrical surface of at least one of the electrodes.

Tlie apparatus comprises at least one scraper which contacts the electrode surface(s) but may comprise a multiple number of scrapers which move simultaneously over the electrode surface (s).

Most preferably the one or more scrapers will simultaneously contact and move over the exposed surface of each electrode to clear the surfaces of both electrodes as the scraper(s) move(s). The scrapers are formed of an electrically non-conductive or insulating material, so that a short circuit is not formed between the electrodes. In a preferred form the scrapers are formed of metal coated with a coating of a non-conductive material such as a hard plastics material, but the scrapers may be of alternative constructions as is referred to subsequently. Typically the ohmic heating apparatus will thermally process a product as it moves continuously or semi-continuously past or between the electrodes, but in an alternative form the apparatus may be arranged to batch process and ohmic heat a substance (in batches).

The product may be a flowable product such as a liquid or pasty product, in a food processing application for example, of the ohmic heating apparatus. The product may be a low viscosity liquid which will flow or may be pumped relatively freely through the ohmic heating apparatus, or a higher viscosity liquid or pasty product which is pumped under higher pressure.

The ohmic heating apparatus and method of the invention may be used for heat treating non-food products and general industrial applications, such as oils, industrial chemicals such as detergents, polymeric substances and similar, where it is desired to heat treat the product for any reason. The term "product" in the specification should be understood accordingly.

Preferably the apparatus also includes a power supply or frequency converter arranged to provide an alternating voltage to the electrodes at a frequency sufficiently above mains frequency

(50Hz) sufficient to also avoid damage to the electrodes by electrolysis. Preferably the frequency is a frequency above 500 Hz, more preferably above UdHz, and most preferably above 5kHz. We have achieved particularly good results with frequencies of about lOkHz.

In broad terms in a further aspect the invention comprises a method for ohmic heating comprising applying an alternating voltage across a substance between electrodes while causing an electrode clearing device which contacts one or more electrodes to move over the electrode(s) during ohmic heating to reduce fouling of the electrodes. Preferably the method also includes applying between the electrodes an alternating voltage at a frequency sufficiently above mains frequency (50Hz) to also avoid corrosion of the electrodes by electrolysis.

The term 'comprising' as used in this specification and claims means 'consisting at least in part of, that is to say when interrupting independent claims including that term, the features prefaced by that term in each claim will need to be present but other features can also be present.

BRIEF DESCRIPTION OF THE FIGURES

The invention is further described with reference to the accompanying figures in which:

Figure 1 is a schematic cross-section of one form in the direction of product flow, of ohmic heating apparatus of the invention,

Figure 2 is a schematic transverse cross-section of the apparatus of Figure 1 along line I-I of Figure 1,

Figure 3 is a schematic cross-section similar to Figure 1 but of another form of ohmic heating apparatus of the invention, Figure 4A shows an ohmic heating arrangement used in trials in the subsequently described experimental work, and Figure 4B schematically shows the surfaces of concentric electrodes of the ohmic heating apparatus used in the experimental work,

Figures 5-7 are plots of current versus time from trial 1 subsequently described, showing in Figure 5 a typical skim milk (5 weight percent) fouling curve in the ohmic heater used in the trial, in Figure 6 the effect of milk inlet temperature on electrode fouling, and in Figure 7 the effect of milk flow rate on electrode fouling,

Figures 8 and 9 are plots of current versus time from subsequently described trial 2, showing in Figure 9 the effect of scraping and a high frequency power supply in reducing electrode fouling and in Figure 9 the relative impacts of scraping and a high frequency power supply in reducing electrode fouling, and

Figures lOa-c are photographs of electrode surfaces which are also further referred to in the subsequent description of experimental work.

DETAILED DESCRIPTION OF EMBODIMENTS The embodiment of the ohmic heating apparatus schematically shown in cross-section in

Figure 1 comprises a cylindrical outer electrode 1 and a concentric cylindrical inner electrode 2, with a product space between the two electrodes. End caps 3 and 4 close the product space at the ends of the electrodes. The arrangement shown is schematic and ohmic heating apparatus of the invention comprising spaced concentric electrodes may be configured in various forms suitable for different industrial applications.

The electrodes and typically other elements of the ohmic heating apparatus may be formed from stainless steel for example.

A product to be thermally processed enters the product space between the electrodes 1 and 2 through inlet port 5, and exits via outlet port 6. Any suitable arrangement of one or more each inlet and outlet ports may be provided. In use the product completely fills the product space

between the concentric electrodes, and the rate of product flow through the apparatus is such as to provide the desired residence time for the product within the product space, for example sufficient residence time to pasteurise or sterilise milk or another food product.

In some applications it may be desirable to also cool or heat one or both electrodes. An external heating or cooling jacket (not shown) may be provided around the exterior of outer electrode 2. A secondary passage 7 may be provided through the centre of the inner electrode 2 with an inlet 8 and outlet 9 schematically shown, to allow circulation of cooling or heating fluid within the centre electrode 2.

Scraper blades, 9a-9d (see particularly Figure 2) operate within the product space between the electrodes 1 and 2 and are arranged to move over the surfaces of the electrodes 1 and 2 contacted by the product within the product space during operation of the apparatus. The scraper blades span between and contact the outer surface of the inner electrode 2 and inner surface of the outer electrode 1, over substantially the full height of the electrodes as shown.

The scraper blades are formed of an electrically non-conductive or insulating material so they do not form a short circuit between the electrodes. The scraper blades may be formed of a rigid plastics material for example. Alternatively the scraper blades may be formed of metal coated with a non-conductive or insulating material such as a coating of a hard plastics material such as Nylon ^Tm for example, or any other suitable hard polymeric non-conducting material. Alternatively again the scraper blades may be formed of a non-conductive ceramics material for example, which again may be coated with a polymer material if desired.

The individual scraper blades are in the embodiment shown connected by an arm 10 which is in turn coupled to a drive shaft 11 journalled for rotation at 12 as shown. The drive shaft is connected to an electric or hydraulic motor (not shown) so that in operation of the ohmic heating apparatus the scraper blades are caused to rotate in unison about the common axis of the concentric electrodes 1 and 2, with the scraper blades simultaneously moving over the inner and outer electrode surfaces exposed to the product within the product space, to clear the electrodes of fouling.

Four scraper blades may be provided, which may be equi-angularly spaced about their axis of rotation. Alternatively but less preferably only a single scraper blade may be provided, or three or alternatively again any number of multiple scraper blades may be provided.

The scraper blades 9a-9d are straight or linear over their height but in alternative form may be curved so as to have a helical form for example, which may possibly assist both in removing product from the electrode surfaces and in moving the product through the ohmic heating apparatus.

The scraper blades move over the surface(s) of the electrodes duting operation of the ohmic heater, to reduce fouling. The rotational speed at which the scraper blades rotate in an ohmic heater configuration as shown in Figures 1 and 2 will depend on the diameter of the ohmic heater and its electrodes, but the linear tip speed of the scrapers over the electrode surfaces is preferably such that a scrapet passes over any point on the electrode surfaces at least once per second, and more preferably at least twice per second (once every half second) or higher. The rate of product flow through the heater will also impact on the optimally effective scraper speed. The number of scraper blades provided, and the rotational speed of the scrapers should take into account the product flow rate and the desired scraper blade linear speed over the electrode surfaces, to ensure that electrode fouling is minimised or substantially eliminated.

Figure 3 schematically shows in cross-section another embodiment of the ohmic heating apparatus of the invention. In this embodiment product flows through the apparatus in the direction indicated (liquid in — liquid out), and as product passes between the electrodes 15 and 16 which are spaced in the direction of the product flow it is subjected to an alternating voltage which generates heat within the product. Rotating scraper blades 17 are provided mounted about a central shaft 18 which is suitably connected, external to the product flow path, to a drive motor (not shown).

Operation of the electrode clearing system comprising one or more scrapers during the operation of the ohmic heating apparatus as product passes between the electrodes reduces fouling of the electrodes. Further, it is preferred that the frequency of the alternating voltage applied between the electrodes is higher than mains frequency, sufficiently to minimise damage to the electrodes due to electrolysis which can also otherwise occur.

The effectiveness of combined electrode clearing system and high frequency power source is illustrated by the subsequently described experimental work:

Trial 1 - Without Scraper And With Low Frequency Voltage

We investigated electrode surface fouling occurring during pasteurisation or sterilisation of reconstituted skim milk solution in an ohmic heater.

The milk solution was prepared by dissolving 5 wt % skim milk powder in tap water. The skim milk solution was pumped by a pump from a tank through the ohmic heater. The ohmic heater consisted of two concentric stainless steel cylinders as the two electrodes with a product flow through space between them. A schematic diagram of the experimental set up is shown in Figure 4a. The dimensions of the electrodes of the heater are given in Table 1.

Table 1: Outside diameter of the inner electrode 5.575 cm

Inside diameter of the outer electrode 6.975 cm

Length of electrodes 11.50 cm

Annulus 0.7 cm

Cross-sectional area 9.5994 cm ²

Volume 110.393 cm ³

A set voltage difference was applied across the two electrodes, causing a current to pass through and heat the milk flowing through the annulus. The current density at the start of the heating process was around 1250 A/m ² , with the voltage and current values being around 10 V and 28 A respectively.

The milk solution was pumped through the ohmic heater using a peristaltic pump. The hot milk leaving the ohmic heater was returned to a holding tank which was kept at a constant temperature with the help of a temperature controlled electric heater and a stirrer. A computer controlled data acquisition system was used to record the experimental data. The inlet and outlet temperatures of the milk were measured and recorded continuously throughout the experiment using K-type thermocouples. The current and voltage values were recorded using a current transducer and a voltage transducer respectively. Fouling was monitored by observing the drop in the current passing through the ohmic heater, caused by the formation of the fouling deposits on the electrode surfaces. A series of experiments was performed over a range of operating conditions and a typical fouling curve is shown in Figure 5. Fouling reduces over time the overall current passing through the ohmic heater. The milk flow rate was 15.5 kg/hr, corresponding to an average velocity of 0.45 cm/s (laminar flow, Re ~ 80). The mill?; temperature in the tank was set at 78°C and the corresponding temperature at the inlet of the ohmic heater was slightly lower i.e. 75.6°C. At the start, the current passing through the milk was 27.8 A (average current density = 1225 A/m ² ) for a voltage difference of 10.15 V, resulting in an initial power input of around 280 W (mains power, electric frequency 50 Hz). Under these conditions, the outlet temperature of milk was around 88.3°C. After 4 hours, the power input into the ohmic heater was around 190 W and the outlet temperature decreased to around 84.5 ⁰ C. During the course of this fouling experiment, the voltage increased by around 12%.

The effect of milk temperature at the inlet of the ohmic heater is illustrated in Figure 6. A reduction in the inlet temperature of around 7 ⁰ C, corresponding to the tank temperature of 7O ⁰ C, was expected to result in lower fouling. In contrast, the rate and amount of fouling were observed

to be higher. The initial power input for this trial was around 290 W (28.1 A, 10.3 V). The operating conditions after 4 hours of operation were: power - 185 W, current 16 A, voltage — 11.65 V.

The effect of milk flow rate on fouling is shown in Figure 7. Over 100% increase in the flow rate (from 15.5 kg/hr to 31.5 kg/hr) had little effect on the fouling rate. The power input at the higher flow rate was observed to be similar, around 275 W (28.5 A, 9.6 V), since the flow rate should have had a minimal effect on the electrical resistance of the milk solution. Increasing the . flow rate resulted in a lower temperature gain (outlet temperature = 81.4°C) and allowed faster convection of the proteins to the electrode surfaces. These effects may have been counteracted by the increasing fluid hydrodynamic forces at the electrode surfaces that would allow lower settling rates.

Significant pit corrosion of both electrode surfaces was also observed. The use of mains power supply with frequency of 50 Hz meant that apparently each cycle was long enough (1/50 s) to promote electrolysis of the stainless steel surfaces. In general, the corrosion reactions not only affect the local pH but also degrade the electrode surfaces resulting in the availability of sheltered places for the fouling deposits. The corresponding changes in the hydraulic and thermal conditions promote fouling. It is likely that the corrosion of the stainless steel electrodes had an effect on the localised fouling rates.

Trial 2 - With Scraper and High Frequency Voltage

Experimental work as described above was repeated using the same ohmic heater but with the following changes:

— The damaged electrode surfaces of the heater used above were machined smooth. The new dimensions of the ohmic heater are detailed in Table 2. — A variable, high frequency power supply was used.

- A stainless steel scraper with four blades made of high molecular weight polyethylene, in a configuration shown in Figure 1 and 2, was added to the ohmic heater, to scrape the surfaces of the electrodes during operation of the heater. The blades were arranged so that two blades, positioned 180° apart, scraped the inner electrode while the other two blades scraped die outer electrode.

Table 2: Outside diameter of the inner electrode 5.520 cm

Inside diameter of the outer electrode 7.090 cm

Length of electrodes 11.50 cm

Anmαlus 0.785 cm

Cross-sectional area 11.3491 cm ²

Volume 130.515 cm ³

A number of trials were carried out using this modified ohmic heater. Figure 8 illustrates the current profiles obtained in three different trials. The corresponding operating conditions are given in Table 3. The frequency of the high voltage power supply was 10.1 IcHz. The milk flow rate was increased slightly in order to account for the increased cross-sectional area but the flow velocity was similar to that in earlier trial. The rotation speed of the scraper was between 30 and 47 rpm.

Table 3:

Inlet Voltage Frequency

^X111CL Scraper speed

Run Temperature

(rpm)

(V) (kHz)

RUNOIl 65.5 14.5 10.1 30

RUN012 66.4 14.5 10.1 32-33

RUN013 68.2 15.7 10.1 46-47

In all three runs the current value did not decrease with time, which indicates that no fouling took place on the electrode surfaces. Similar observations were made when the ohmic heater was opened and the electrode surfaces were observed visually after every trial. The surfaces were also found to have no corrosion.

The relative impact of high frequency power supply and scraping of the electrode surfaces on fouling can be determined by analysing the results illustrated in Figure 9. Three current profiles are shown corresponding to three different trials performed at i) normal mains frequency and without electrode scraping, ii) high frequency and without electrode scraping, and iii) high frequency and with electrode scraping. The corresponding operating conditions are given in Table 4. Table 4:

Inlet Voltage Frequency ^ιτneτ Scraper speed

Run Temperature

(° ^C ) (V) (kHz) ^(IP

RUN009 68.3 10.33 0.05 0

RUNOl 6 68.4 15.90 10.1 0

RUN013 68.2 15.70 10.1 46-47

The operation of the scrapers on the electrode surfaces substantially reduced electrode fouling. The use of a high frequency power supply also contributed to a significant reduction in fouling since it simultaneously reduced the electrolysis damage to the electrodes. The impact of the high frequency power supply and scrapers was also investigated by visually analysing the fouled electrode surfaces after each trial. Figures 1 Oa-I Oc are photographs of the inside electrode after each of the three different trials mentioned above (Figure 9 and Table 4). Figure 10a shows the electrode after the trial with a mains frequency power source and without electrode scraping, and significant electrode damage is apparent. Figure 10b shows the electrode after the trial with a high frequency power source and without electrode scraping and it can be seen that electrode damage is reduced. Figure 10c shows the condition of the electrode after the trial with scraping and with a high frequency power source and can be seen that electrode damage and fouling are minimal. In summary, scraping during ohmic heating substantially reduces electrode fouling and use of a high frequency power supply also reduces electrolysis damage to the electrodes and thus further reduces electrode fouling.

The foregoing describes the invention including specific embodiments thereof and trials relating thereto, by way of example. Alterations and modifications as would be obvious to those skilled in the art are intended to be incorporated in the scope thereof as defined in the accompanying drawings.