EP0690660A1 - Method and apparatus for heating a flowing ionic fluid - Google Patents

Abstract

The liq. is circulated through an elongated enclosure (3) having identical electrodes (4,5) at its inlet (6) and outlet (7). An AC voltage of e.g. 5 kV is generated (8) and applied between two perforated discs (15) in which the number and size of the holes depends upon the viscosity and rate of flow. At its centre the enclosure is constricted to a narrow tube (9) whose cross section is calculated for the desired rate of flow and duration of passage. <IMAGE>

Description

The present invention relates to a device for heating a flowing ionic liquid comprising at least one elongated enclosure for circulation of the liquid, two identical electrodes disposed respectively at the inlet and at the outlet of the enclosure and means for generating an electric field between said electrodes arranged to heat the liquid by direct electrical conduction within said liquid.

Such heating by direct electrical conduction is also known by the name of "ohmic" heating.

The invention also relates to a method of ohmic heating and / or sterilization of a flowing ionic liquid.

It finds a particularly important although not exclusive application in the field of the food industry, for sterilizing or pasteurizing liquids.

It is known that to sterilize or pasteurize effectively, certain temperature conditions must be observed for specific times.

However, food products are particularly heat-sensitive and their vitamin, taste and nutritional characteristics are directly affected by the duration of treatment.

In order to best preserve the organoleptic qualities of food, we therefore seek to heat liquids, more or less pasty, to pasteurize or sterilize, for as short a time as possible.

The known sterilization techniques do not, however, allow very short heating times, in particular with viscous products.

Indeed, with a conventional exchanger, a high flow speed is accompanied by significant pressure drops which become unacceptable for the process. In addition, in the case where the temperatures of the walls directly in contact with the liquid are high, there is a risk of notable degradability of the heat-sensitive liquid.

The scraped surface heat exchangers, better suited to liquids with high viscosity, require a long treatment time.

Finally, steam injection systems cause dilution, or even pollution, of the fluid to be heated, which is an often major drawback.

There is also known an ohmic heating exchanger for the industrial sterilization of food products which may contain pieces of whole food. This has the known advantages of this type of heating, namely the absence of a hot wall and convective exchange, since the thermal energy is dissipated by the passage of an alternating electric current, directly within the liquid.

It thus allows uniform heating to the heart of the treated products.

But here again, such a sterilizer requires a stay of the product in a high temperature range for a fairly long time (several minutes).

The present invention aims to provide a heating device and method which responds better than those previously known to the requirements of practice, in particular in that it makes it possible to obtain very short heating times, of the order of a second, and this in a simple and easy to implement.

For this purpose, the invention essentially provides a device for heating a flowing ionic liquid comprising at least one elongated enclosure for circulation of the liquid, two identical electrodes disposed respectively at the inlet and at the outlet of the enclosure and means generating an electric field between said electrodes arranged to heat the liquid by direct electrical conduction within said liquid,
characterized in that
the enclosure comprises a central tubular zone connected to an inlet convergent and to an outlet diverging, and in that the electrodes are openwork surface elements suitable for being traversed by the liquid and respectively fixed to said inlet converger and to said audit diverging outlet, perpendicular to the direction of flow.

• les dimensions des éléments surfaciques correspondent respectivement aux sections internes du divergeant et du convergeant à l'emplacement de leur fixation sur ces derniers ;
• les éléments surfaciques sont respectivement fixés aux extrémités externes par rapport à la zone centrale, du divergeant et du convergeant ;
• la zone centrale tubulaire est cylindrique, et le convergeant d'entrée et le divergeant de sortie sont des portions de cônes identiques dont la section intérieure de la petite base est égale à la section intérieure du tube, et dont la section intérieure de la grande base correspond à la surface active des électrodes ;
• les électrodes sont des plaques métalliques percées de trous de passage du liquide ;
• le divergeant de sortie peut être associé à un système de refroidissement ;
• le dispositif comporte deux boîtiers de connexion avec le circuit d'alimentation et d'évacuation du liquide, respectivement disposés de part et d'autre de l'enceinte, à savoir un boîtier d'alimentation du liquide de section correspondant à la plus grande section du convergeant auquel il est relié via la première électrode, et un boîtier d'évacuation de section correspondant à la plus grande section du divergeant auquel ledit boîtier d'évacuation est relié via la seconde électrode ;
• la section de la zone centrale tubulaire est inférieure ou égale à de l'ordre de cinq fois la surface d'une électrode, par exemple inférieure ou égale à dix fois cette dernière ;
• le dispositif peut comporter trois enceintes en parallèle, dont les électrodes sont couplées pour permettre une alimentation en triphasée.

In advantageous embodiments, use is made of one and / or the other of the following arrangements:

• the dimensions of the surface elements correspond respectively to the internal sections of the diverging and of the converging at the location of their attachment to the latter;
• the surface elements are respectively fixed at the outer ends relative to the central zone, of the diverging and the converging;
• the central tubular zone is cylindrical, and the inlet convergent and the outlet diverging are portions of identical cones whose inner section of the small base is equal to the inner section of the tube, and whose inner section of the large base corresponds to the active surface of the electrodes;
• the electrodes are metal plates pierced with holes for the passage of the liquid;
• the outlet diverging may be associated with a cooling system;
• the device comprises two boxes for connection with the liquid supply and discharge circuit, respectively arranged on either side of the enclosure, namely a liquid supply box with a section corresponding to the largest section of the convergent to which it is connected via the first electrode, and a section evacuation box corresponding to the largest section of the diverging point to which said evacuation case is connected via the second electrode;
• the section of the central tubular zone is less than or equal to about five times the area of an electrode, for example less than or equal to ten times the latter;
• the device can include three speakers in parallel, the electrodes of which are coupled to allow a three-phase supply.

The invention also provides a method of heating an ionic liquid flowing in an enclosure between two electrodes between which an electric field is generated arranged to heat the liquid by direct electrical conduction within the liquid,
characterized in that a low current density j, for example 40 mA / cm, is generated on the surface of the electrodes.

A higher current density, for example up to 700 mA / cm, is however also possible in other embodiments of the invention and depending on the materials used.

The electric field is alternating.

Advantageously, the liquid is passed in a converging box through a first electrode, then in a tubular part, then in a diverging box before discharging it through a second electrode, the alternating voltage applied between the electrodes being between about a few hundred volts, for example 400 volts, and about 5000 volts.

In an advantageous embodiment, the liquid to be heated is passed through three chambers arranged in hydraulic parallel and supplied with three-phase.

The invention will be better understood on reading the following description of embodiments given by way of non-limiting example.

• La figure 1 est une vue en coupe de l'enceinte du dispositif selon le mode de réalisation de l'invention plus particulièrement décrit ici.
• La figure 2 est une vue schématique d'un dispositif selon l'invention comportant trois enceintes du type de celle décrite en référence à la figure 1, alimentées en triphasée.

It refers to the accompanying drawings in which:

• Figure 1 is a sectional view of the enclosure of the device according to the embodiment of the invention more particularly described here.
• Figure 2 is a schematic view of a device according to the invention comprising three speakers of the type described with reference to Figure 1, supplied with three-phase.

Figure 1 shows a device 1 for heating an ionic liquid 2 in flow.

The device comprises an enclosure 3 made of insulating material, for example PTFE, for circulating the liquid and two identical electrodes 4 and 5, in the form of a surface element, disposed respectively at the inlet 6 and at the outlet 7 of the enclosure . Means 8 for generating an alternating voltage between the electrodes 4 and 5 of between a few hundred volts, for example 400 volts, and 10,000 volts, for example 5000 volts, are also provided.

The enclosure 3 comprises a tubular central zone 9, the cross section of which is calculated as a function of the flow rate of the liquid and the desired residence time, of a converging cone 10 for entering the liquid and a diverging cone 11 for leaving the liquid , symmetrical of the cone 10.

The internal diameter of the small base 12 of the cones is equal to the internal diameter of the tube 9, the internal diameter of the large bases 13 of the cones being chosen as a function of the electrodes.

In the embodiment of FIG. 1, the cones 10 and 11 are identical, but their dimensions can also be different to take account in particular of the electrical conductivity of the fluids which varies as a function of the temperature.

Each cone has, on the side of its large base, an internal shoulder 14, on which the electrodes 4 and 5 are respectively pressed.

Each electrode is for example constituted by a metal disc of diameter equal to that of the large base increased on either side of a thickness corresponding to, or slightly less than, the width of the shoulder. The disc is made of a material which depends on the product to be treated. It can be stainless steel, graphite, titanium or platinum alloy, etc.

The discs are drilled with holes 15 the number, diameter and distribution of which depend on the flow rate and the viscosity of the liquid to be heated.

The invention is not limited to electrodes constituted by perforated plates, but also relates to other embodiments of openwork surface elements such as electrodes in the form of a grid or constituted by housings containing metallic particles.

The electrodes are connected to the voltage generator circuit 8, for example via a nut welded at their periphery, said nut receiving a threaded rod provided with a sealing gland (not shown).

The device 1 also includes cylindrical housings or flanges 16 and 17, for connection with the supply and discharge circuit of the fluid (not shown). These housings serve as sealing covers at each end and allow the electrodes to be held in place by compressing them on the shoulders. Each housing has a central connection orifice 18 for the inlet or outlet of the fluid 19.

Such a device makes it possible to obtain a very short heating time thanks to its geometry. Indeed, the electrical resistance represented by the fluid contained in a cone 10 or 11 is much lower than that of the fluid contained in the central tube 9.

The electrical power dissipated, and therefore the heating of the fluid, is therefore carried out almost entirely in the central tube, ie in less than a second depending on the diameter chosen.

The inlet cone 10 can also serve as a preheating zone for the product, while the outlet cone can be associated with a rapid cooling device (not shown).

The end cones also make it possible to implant electrodes of large diameter and therefore to reduce the current density thereon, which limits the electrochemical phenomena which take place at the metal interface produced.

CARACTERISTIQUES DU FLUIDE masse volumique (kg/m3) 1100 chaleur massique (J/kg.K) 4000 température d'entrée (°C) 80 conductivité d'entrée (S/m) 0,411 température de sortie (°C) 165 conductivité de sortie (S/m) 0,764 TABLEAU 2

By way of nonlimiting example, in table n ° 1, the characteristic parameters of a device of the type described with reference to FIG. 1 are given below for a fluid whose characteristics correspond to table n ° 2, the electrical operating parameters being given in table 3.

FLUID CHARACTERISTICS density (kg / m3) 1100 mass heat (J / kg.K) 4000 inlet temperature (° C) 80 input conductivity (S / m) 0.411 outlet temperature (° C) 165 output conductivity (S / m) 0.764 TABLE 2

FIG. 2 shows a device 20 comprising three identical enclosures 21, 22, 23, in parallel, hydraulic, for example of the type described above, the electrodes of which are supplied with three-phase. The electrode 26 of the enclosure 21 is in phase (1) (ph 1 in the drawing). The electrode 28 of the enclosure 22 is in phase (2) (ph 2), the electrode 25 of the enclosure 23 is in phase (3) (ph 3). The electrodes 24, 27 and 29 placed at the outlet of the enclosures 21, 22, 23 are connected to the ground and to the neutral of the system.

We will now describe the operation of the device according to the invention with reference to FIG. 1.

The viscous liquid to be sterilized at 160 ° C, for example a dairy product, is supplied with pressure (for example under 4 bars) and introduced into 6 into the enclosure at a temperature of the order of 80 ° C, after preheating.

It goes from 80 ° C to a temperature of 90 ° C at the inlet of the tube 9, with a residence time which is for example of the order of 8 s in the cone 10, then its temperature rises by 90 ° C at 160 ° C in a residence time of 0.3 s in the central tube 9, before leaving in the diverging 11 where its temperature is stabilized.

In the order of 10% of the total power has therefore been dissipated in the inlet cone. On the other hand, 85%, or even in some cases more than 95%, of the heat is dissipated in the tube and less than 5% in the outlet cone where additional cooling (not shown) can for example also be advantageously provided.

Claims (11)

Device (1,20) for heating an ionic liquid (2) in flow comprising at least one elongated enclosure (3,21,22,23) for circulation of the liquid, two electrodes (4,5; 26,24,28 , 27; 25,29) identical, disposed respectively at the inlet (6) and at the outlet (7) of the enclosure and means (8) for generating an electric field between said electrodes arranged to heat the liquid by direct electrical conduction within said liquid,
characterized in that
the enclosure (3,21,22,23) comprises a tubular central zone (9) connected to an inlet convergent (10) and to an outlet diverging (11), and in that the electrodes (4,5 ) are perforated surface elements suitable for being traversed by the liquid and respectively fixed to said inlet convergent and to said outlet diverging perpendicular to the direction of flow. Device according to claim 1,
characterized in that the dimensions of the surface elements correspond respectively to the internal sections of the diverging and of the converging at the location of their attachment to the latter. Device according to claim 2,
characterized in that the central tubular zone (9) is cylindrical, and in that the inlet convergent (10) and the outlet diverging (11) are portions of identical cones, the inner section of the small base (12 ) is equal to the inner section of the tube, and whose section inside of the large base (13) corresponds to the surface of the electrodes. Device according to any one of the preceding claims, characterized in that the electrodes (4,5) are metal plates pierced with holes for the passage of the liquid. Device according to any one of the preceding claims, characterized in that the outlet diverging part (11) is associated with a cooling system. Device according to any one of the preceding claims, characterized in that it comprises two boxes (16,17) for connection with the supply and discharge circuit of the liquid, respectively arranged on either side of the enclosure (1), namely a supply box (16), of section corresponding to the largest section of the convergent to which it is connected through the first electrode (4), and a discharge box (17), of section corresponding to the largest section of the diverging part to which said evacuation case (17) is connected through the second electrode (5). Device according to any one of the preceding claims, characterized in that the section of the tubular central zone (9) is less than or equal to about five times the area of an electrode. Device according to any one of the preceding claims, characterized in that it comprises three chambers (21,22,23) in parallel, whose electrodes (24,25; 26,27; 28,29) are coupled to allow a supply three-phase. Method for heating an ionic liquid (2) flowing in an enclosure (3) between two electrodes (4,5) between which an electric field is generated arranged to heat the liquid by direct electrical conduction within the liquid, characterized in what is generated between the electrodes (4,5) a current density j less than or equal to 40 mA / cm. Method according to claim 9,
characterized in that the liquid is passed through a converging box (10) through a first electrode (4), then through a tubular part (9), then through a diverging box (11) before discharging it to the through a second electrode (5), the alternating voltage applied between the electrodes (4,5) being between around a few hundred volts, for example 400 V, and around 5000 Volts. A method according to any one of claims 9 and 10, characterized in that the liquid to be heated is passed through three chambers (21,22,23) arranged in parallel and supplied in three phase.

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