A METHOD AND A DEVICE FOR HEATING A LIQUID

Abstract: A method and a device for heating a liquid in a continuous process, wherein the liquid is moved in downward direction in a free fall from a supply plate in a closed vessel, and carried out of said vessel via a discharge valve. Steam for heating the falling liquid is introduced into the vessel, which steam at least partially condenses in the liquid. Liquid is collected in a container at the bottom of the vessel, which liquid is stored in said container for some time before being discharged via the discharge valve. The level of the liquid in the container is measured, and the discharge valve is controlled in dependence on said liquid level, wherein the liquid level can be maintained at a constant predetermined value.

Description: A METHOD AND A DEVICE FOR HEATING A LIQUID The invention relates to a method for heating a liquid in a continuous process, wherein the liquid is moved in downward direction in a free fall from a supply plate in a closed vessel, and carried out of said vessel via a discharge valve, wherein steam for heating the falling liquid is introduced into the vessel, which steam at least partially condenses in the liquid. A method of this kind is known, for example from EP 0 650 332. By means of such a method, a liquid can be quickly heated to a predetermined temperature, and be briefly maintained at that temperature, for example for a fraction of a second or for a few seconds, after which the liquid can be discharged via the discharge valve, for example to an expansion vessel, where the temperature of the liquid is quickly reduced. The use of such a method makes it possible for example to extend the storage life of milk, because bacteria in the milk are killed by said heating, whilst the short duration of the heating process does not affect the taste of the milk. In order to achieve an optimum result, it is important that the temperature of the liquid during heating and the period of time during which the liquid is subjected to a high temperature are accurately controlled. The object of the invention is to provide a method wherein the various parameters of the process can be controlled in an efficient, reliable and accurate manner. In order to accomplish that objective, liquid is collected in a container at the bottom of the vessel, which liquid is stored in said container for some time before being discharged via the discharge valve. By collecting the liquid, which has been heated to the desired temperature while falling down from the supply plate, in a container and storing it therein for some time, the period of time during which the liquid is subjected to a high temperature can be set as desired. Said period includes at least part of the time it takes for the liquid to fall from the supply plate to the container, plus the storage time of the liquid in the container. Preferably the level of the liquid in the container is measured, and the discharge valve is controlled in dependence on the measured level, so that a desired liquid level in the container can be maintained by controlling the discharge valve. The liquid level can thereby be maintained at a constant predetermined value, which value, in combination with the volume of the liquid flow to the vessel, determines the residence time of the liquid in the vessel. Possibly it will be necessary thereby to take into account the fact that at least part of the steam being supplied to the vessel will condense, but the amount of liquid which is produced as a result of that is relatively small . It is preferred to measure the level of the liquid in the container by means of a differential pressure gauge, which measures the difference in pressure in the vessel between a location near the bottom side of the container and a location above the container, which difference in pressure is substantially caused by the liquid column in the container above said location near the bottom side of the container. In practice it has become apparent that this measurement makes it possible to determine the level of the liquid in the container in a reliable manner, in particular in a more reliable manner than is possible with a method wherein absolute values of a pressure are measured. Furthermore, this method for determining the liquid level in the container appears to be more accurate and reliable than measurements wherein the liquid level is measured directly, in a manner which is known per se for that matter. Preferably it can be determined by means of a sensor whether a specified location in the container is below or above the liquid level, so that means for determining the liquid level in the container can be calibrated by varying the liquid level during measuring, in such a manner that the liquid level will pass said specified location. In this way it is possible to compensate for any deviations of the pressure measurements, which may be caused by the fouling of the pressure sensor, for example as a result of deposition of solids thereon. The calibration of the differential pressure measurement may take place every day, for example, or each time the process is started, whereby the discharge valve reduces the amount of liquid being discharged until the liquid level passes the specified location of the aforesaid sensor, at which moment the differential pressure gauge is reset at the value associated therewith. This method of calibrating the level measurement in the present process for heating liquid may be considered to constitute a separate invention, which can be implemented independently of the other characteristic features. Preferably the residence time of the liquid in the vessel is regulated by measuring the liquid supply flow and setting a predetermined liquid level in dependence thereon, which is done by controlling the discharge valve. The measurement of the liquid supply flow can take place by means which are known, and if a specified residence time of the liquid in the vessel is desired, the associated liquid level in the container can be determined and subsequently be set . In practice it has become apparent that this makes it possible to maintain an accurate residence time of the liquid in the vessel during the continuous process. This method for controlling the residence time of the liquid in the vessel may be considered to constitute a separate invention . Particularly if the method is used in a continuous process, wherein the liquid being discharged from the vessel is packaged in a consumer packaging after having been subjected to any further treatment that may be required, it will be advantageous if the liquid flow through the vessel can be varied, without this having an effect on the treatment of the liquid in the vessel. Thus it is possible to avoid the use of buffer vessels for storing the discharged liquid temporarily before packaging it, which offers several advantages, among other things the fact that the possibility of bacteria getting into the liquid again is thus reduced. In the case of varying liquid flows through the vessel, it is possible to maintain a consistent treatment process of the liquid in the vessel, in particular as regards the residence time of the liquid in the vessel, by means of a corresponding control of the liquid level in the container. During the process, the liquid flow to the vessel is reduced, whilst the liquid level in the container is reduced in proportion to said reduction. In this manner a constant residence time of the liquid in the vessel can be achieved. Also this method may be considered to constitute a separate invention. In one preferred embodiment, a stirrer is present in the container, so that additional stirring of the liquid which is present in the container is possible. Especially in the case of less stable liquids, which contain fractions which tend to deposit on the wall of the container, thus fouling any sensors that may be present, for example, said additional stirring of the liquid in the container may offer advantages . Also the presence of a stirrer in the container may be considered to constitute a separate invention. In another preferred embodiment the liquid is moved in downward direction in a free fall from a supply plate in the closed vessel, and carried out of the vessel via a discharge valve, wherein steam for heating the falling liquid is introduced into the vessel, which steam at least partially condenses in the liquid, and wherein an excess of steam is supplied to the vessel, so that part of the steam being supplied exits the vessel together with the liquid via the discharge valve. The storage of liquid at the bottom of the vessel is prevented in this manner, so that the residence time of the liquid in the vessel corresponds with the time which the liquid requires for moving from the supply plate to the discharge valve. In a continuous process, wherein a constant residence time of the liquid in the vessel is required, this method is extremely effective, in particular because it makes it possible to vary the liquid flow through the vessel in a simple manner, without this having any influence on the treatment of the liquid in the vessel. Preferably it is detected thereby that an amount of steam passes the discharge valve together with the liquid, with the discharge being controlled in such a manner that a minimal yet detectable amount of steam will pass the discharge valve at all times. This is an effective method of preventing the accumulation of an amount of liquid at the bottom of the vessel, which might prolong the residence time of the liquid in the vessel, the pressure inside the vessel as well as the absence of any accumulated liquid can be maintained in an effective manner by adjusting the discharge valve so that a minimum amount of steam will pass the discharge valve at all times. The passage of steam can be detected by sound detection means, because sound is generated when steam passes through the discharge valve. In practice this appears to be a very effective method. The process wherein no storage of liquid takes place at the bottom of the vessel and the above manner of controlling said process may be considered to constitute separate inventions . In another preferred embodiment, the temperature of the falling liquid is measured by means of an infrared sensor, which is directed at the passing liquid jets. Such a sensor may be provided in the wall of the vessel, preferably near the bottom side of the vessel. The measurement of the temperature of the downwardly directed liquid jets is important in order to achieve an effective inspection and control and of the continuous process, and in practice it has become apparent, that an accurate and reliable result is obtained by measuring said temperature directly by means of an infrared sensor. Other sensors for measuring the temperature in the device in which the method is carried out appear to function less reliably in practice, because of fouling of the sensors in question. The measurement of the temperature of the liquid jets by means of an infrared sensor may be considered to constitute a separate invention. The increase in the temperature of the liquid during the time that said liquid is present in the vessel can be determined by means of a computer calculation, which calculation can be carried out continuously on the basis of a number of measured values, which measurements are carried out sufficiently accurately in practice for obtaining a reliable result of the calculation. A number of the following parameters, or all of them, can be continuously measured thereby. The parameters concerned are the volume of the liquid supply flow, the temperature thereof, the pressure thereof, the volume of the steam supply flow, the temperature thereof, the pressure thereof, the volume of the possible supply and discharge flow of cooling water, which for example functions to cool the wall of the vessel, the heat losses of the vessel to the ambient atmosphere, etc. The amount of heat absorbed by the liquid is determined by deducting the total outward heat flow from the inward heat flow, on the basis of which calculation the temperature increase of the liquid can be determined. The determination of the temperature increase of the liquid in the above manner can be considered to constitute a separate invention. The invention furthermore relates to a device for carrying out any one or more of the aforesaid methods. Further aspects of the invention are disclosed in the description of the figures and defined in the claims. Hereafter a few embodiments of a device for treating a liquid will be described by way of illustration with reference to the drawing. Figure 1 shows a device for treating a liquid, which device comprises a container for storing liquid; Figure 2 shows another embodiment of a container; and Figure 3 shows another device for treating a liquid. The various figures are merely diagrammatic representations, wherein corresponding parts are indicated by the same numerals. Figure 1 shows a vessel 1, in which a liquid can be subjected to a heat treatment. Said liquid is supplied via supply pipe 2, and the volume of the liquid flow to vessel 1 can be controlled by means of supply valve 3. The liquid is carried, via pipe 4, to supply plate 5, which is provided with a number of passages, so that the liquid being supplied falls down in a number of jets from supply plate 5. Said jets are diagrammatically indicated in the figures by means of dashed lines 6. The passages in supply plate 5 are not shown in the figures, they have a diameter of for example 3 mm. According to Figure 1, vessel 1 is provided on the bottom side with a container 7 in the form of an upwardly extending round tube, which has a diameter which is much smaller than the cross dimension of vessel 1. A closable opening 8 is present on the bottom side of container 7, the through- flow area of which can be adjusted and controlled by means of a valve member 9, which can be moved by means of a driving motor 10. Figure 1 furthermore shows a supply pipe 11 for supplying steam to vessel 1, whereby the volume of the supply flow is controlled by means of controllable supply valve 12. Inside the vessel 1, the liquid, which moves in downward direction in jets 6, is heated by the steam being supplied, whereby at least part of said steam condenses in the liquid. The fallen liquid is collected in container 7 and temporarily stored therein, because a predetermined liquid level 13 is maintained therein by means of valve member 9. The volume of the liquid supply flow and the liquid level 13 in container 7 determine the residence time of the liquid in vessel 1. After the liquid has passed discharge valve 8, 9, the liquid is carried to an expansion vessel, not shown, where the liquid is cooled. Arrow 14 indicates the direction of the liquid being discharged. In the device which is shown in Figure 1, the liquid level 13 is determined by means of a differential pressure gauge 15, which is connected, by means of line 16, to a pressure sensor 17 near the bottom side of container 7, and which is connected, via a line 18, to a pressure sensor present above container 7. The difference between the pressure measured by sensor 17 and the pressure measured by sensor 19 is a measure for liquid level 13. The device according to Figure 1 is furthermore provided with a sensor 20, which is connected to a differential pressure gauge 15 by means of line 21. Sensor 20 is a measuring device, which detects whether liquid level 13 is present above or under the location where sensor 20 is present. In fact, sensor 20 detects whether a gas or a liquid is present in the container at the location of said sensor. The determination of liquid level 13 by means of the differential pressure measurement by sensors 17, 19 can be calibrated by detecting from time to time that the liquid level 13 is located at the level of sensor 20, at which moment the differential pressure measurement is set to the associated value. In order to maintain a desired liquid level 13, driving motor 10 is controlled, via line 22, by means of differential pressure gauge 15. The supply of steam via pipe 11 can be adjusted by means of controllable valve 12, for example in dependence on the gas pressure that prevails in vessel 1. The amount of liquid being supplied can be changed during the process by means of supply valve 3, whilst the residence time of the liquid in vessel 1 can be maintained at a constant level by changing the liquid level 13 as well, , which is done by controlling driving motor 10. A specified liquid level 13 is used in conjunction with each volume of the liquid supply flow, so that a predetermined residence time can be obtained. The device according to Figure 1 is furthermore provided with an infrared sensor 23, which is connected to a control device 25 by means of line 24. The infrared sensor 23 is directed at the jets 6 of the liquid, so that the temperature of said liquid jets can be measured in a reliable manner. Control device 25 may also adjust other components of the device, for example steam supply valve 12. Figure 2 shows another embodiment of container 7, wherein discharge valve 8,9 is positioned beside the bottom side of container 7. The difference with the device according to Figure 1 is the presence of a stirrer 26, which stirrer is driven by means of a driving motor 28. Said stirrer is capable of preventing solids from depositing on the wall of container 7, especially when the liquid is not very stable. In this way it possible to prevent sensors 17, 20 from becoming fouled to such an extent that their detection accuracy is affected. Stirrer 26 furthermore provides an even temperature distribution within the stored liquid, thereby preventing prolonged contact between the liquid and the wall of container 7, so that the heat exchange that occurs thereby will not have any negative consequences for the liquid. Figure 3 shows a device for treating a liquid, wherein liquid is likewise supplied via supply pipe 2 and supply plate 5. This device does not comprise a container for storing liquid, however, on the contrary, liquid is prevented from collecting at the bottom side of vessel 1. This is done by supplying more steam via pipe 11 than is necessary for heating the jets 6, whereby said excess of steam is discharged along with the liquid via discharge valve 8, 9. As a result of this, the residence time of the liquid in vessel 1 will be constant at all times, that is, as long as the time which the liquid needs for falling from supply plate 5 to the bottom side of vessel 1. Vessel 1 may be designed to have a large height, in particular a height of more than 5 metres, preferably more than 7 metres, in order to achieve a sufficiently long residence time. In order to ensure that steam is constantly discharged along with the liquid from vessel 1, via valve 8, 9, a sound sensor 29 is provided, which is connected, via line 30, to a control element 31, which controls driving motor 10 via line 32. The position of valve member 9 may thereby be set to discharge a minimum amount of steam, which is just detectable, so that on the one hand it is ensured that no liquid is stored at the bottom of vessel 1, whilst on the other hand the amount of escaping steam is reduced to a minimum. One advantage of the fact that an amount of steam escapes, albeit only a small amount, is that an undesirable accumulation of gases in vessel 1 is prevented in this manner, which gases can be carried along with the steam in small amounts. A space 33 is depicted to be present round pipe 4 in figures 1 and 3, which space forms an insulation space between pipe 4 and the interior cf vessel 1. The fact is that the liquid must not be heated before it has left supply plate 5. To that end, plate 5 may be made of a material which does not conduct heat very well, so that there will be hardly any heating, if at all, of the liquid during the time that said liquid is present in the passages of supply plate 5. The illustrated embodiments are to be considered as mere examples, also other embodiments are possible.

Claims: CLAIMS 1. A method for heating a liquid in a continuous process, wherein the liquid is moved in downward direction in a free fall from a supply plate in a closed vessel, and carried out of said vessel via a discharge valve, wherein steam for heating the falling liquid is introduced into the vessel, which steam at least partially condenses in the liquid, characterized in that liquid is collected in a container at the bottom of the vessel, which liquid is stored in said container for some time before being discharged via the discharge valve. 2. A method according to claim 1, characterized in that the level of the liquid in the container is measured, and that the discharge valve is controlled in dependence on the measured liquid level. 3. A method according to claim 2, characterized in that the discharge valve is controlled in such a manner that the liquid level is maintained at a constant predetermined value. 4. A method according to claim 1 or 2 , characterized in that the level of the liquid is measured by means of a differential pressure gauge, which measures the difference in pressure in the vessel between a location near the bottom side of the container and a location above the container. 5. A method according to any one of the preceding claims, characterized in that it is determined by means of a sensor whether a specified location in the container is below or above the liquid level. 6. A method according to claim 5, characterized in that means for determining the liquid level in the container are calibrated by varying the liquid level during measuring, in such a manner that the liquid level will pass said specified location. 7. A method according to any one of the preceding claims, characterized in that the residence time of the liquid in the vessel is regulated by measuring the liquid supply flow and setting a predetermined liquid level in dependence thereon, which is done by controlling the discharge valve. 8. A method according to any one of the preceding claims, characterized in that the liquid flow to the vessel is reduced during the process, and that the liquid level in the container is reduced in proportion to said reduction thereby. 9. A method according to any one of the preceding claims, characterized in that the liquid which is stored in the container is additionally stirred by means of a stirrer. 10. A method for heating a liquid in a continuous process, wherein the liquid is moved in downward direction in a free fall from a supply plate in a closed vessel, and carried out of said vessel via a discharge valve, wherein steam for heating the falling liquid is introduced into the vessel, which steam at least partially condenses in the liquid, characterized in that an excess of steam is supplied to the vessel, so that part of the steam being supplied exits the vessel together with the liquid via the discharge valve. 11. A method according to claim 10, characterized in that it is detected that an amount of steam passes the discharge valve together with the liquid, wherein the discharge is controlled in such a manner that a minimal yet detectable amount of steam will pass the discharge valve at all times. 12. A method according to claim 11, characterized in that the passage of steam is detected by sound detection means . 13. A method for heating a liquid in a continuous process, wherein the liquid is moved in downward direction in a free fall from a supply plate in a closed vessel, and carried out of said vessel via a discharge valve, wherein steam for heating the falling liquid is introduced into the vessel, which steam at least partially condenses in the liquid, characterized in that the temperature of the liquid moving in downward direction is measured by means of an infrared sensor, which is directed at passing liquid jets. 14. A method according to claim 13, characterized in that the infrared sensor is mounted in the wall of the vessel, near the bottom side thereof. 15. A method for heating a liquid in a continuous process, wherein the liquid is moved in downward direction in a free fall from a supply plate in a closed vessel, and carried out of said vessel via a discharge valve, wherein steam for heating the falling liquid is introduced into the vessel, which steam at least partially condenses in the liquid, characterized in that the temperature of the liquid that exits the vessel ' is determined by a calculation on the basis of measurement of the amount of energy being supplied to the vessel. 16. A device for heating a liquid in a continuous process, which device comprises a closed vessel, wherein the liquid can be moved in downward direction in a free fall from a supply plate, and be carried out of said vessel via a discharge valve, and which furthermore comprises means for supplying steam to the vessel for heating the falling liquid, characterized in that a container is present at the bottom of the vessel, in which container fallen liquid can be collected and be stored for some time before being discharged via the discharge valve. 17. A device according to claim 16, characterized by means for measuring the level of the liquid in the container, and by means for controlling the discharge valve in dependence on the measured liquid level. 18. A device according to claim 17, characterized by a differential pressure gauge comprising a sensor which is present at a location near the bottom side of the container and a sensor which is present at a location above said container. 19. A device according to any one of the claims 16 - 18, characterized in that a sensor is mounted at a specified location in the container, which sensor is capable of detecting whether or not liquid is present at that location. 20. A device according to any one of the claims 16 - 19, characterized by means for regulating the residence time of the liquid in the vessel, which means control the discharge valve in dependence on a detected liquid level and on the volume of the liquid supply flow. 21. A device according to any one of the claims 16 - 20, characterized in that a drivable stirrer is present in said container. 22. A device for heating a liquid in a continuous process, which device comprises a closed vessel, wherein the liquid can be moved in downward direction in a free fall from a supply plate, and be carried out of said vessel via a discharge valve, and which furthermore comprises means for supplying steam to the vessel for heating the falling liquid, characterized by means for supplying an excess of steam to the vessel, so that part of the steam being supplied can exit the vessel together with the liquid via the discharge valve. 23. A device according to claim 22, characterized by means for detecting the amount of steam that passes the discharge valve, and by means for controlling the discharge in such a manner that a minimal yet detectable amount of steam will pass the discharge valve at all times. 24. A device according to claim 23, characterized in that sound detection means are present for detecting the presence of steam in the liquid which exits the vessel . 25. A device for heating a liquid in a continuous process, which device comprises a closed vessel, wherein the liquid can be moved in downward direction in a free fall from a supply plate, and be carried out of said vessel via a discharge valve, and which furthermore comprises means for supplying steam to the vessel for heating the falling liquid, characterized by an infrared sensor for measuring the temperature of the falling liquid. 26. A device according to claim 25, characterized in that the infrared sensor is mounted in the wall of the vessel, near the bottom side thereof. 27. A device for heating a liquid in a continuous process, which device comprises a closed vessel, wherein the liquid can be moved in downward direction in a free fall from a supply plate, and be carried out of said vessel via a discharge valve, and which furthermore comprises means for supplying steam to the vessel for heating the falling liquid, characterized by means for determining the amount of energy being supplied to the vessel, and by means for calculating the temperature of the liquid that exits the vessel on the basis of said determination.

Inventors: DEN HOLLANDER ADRIAAN GERRIT, DEN HOLLANDER, ADRIAAN, GERRIT
Applicants: DEN HOLLANDER ADRIAAN GERRIT, DEN HOLLANDER, ADRIAAN, GERRIT
Application year: 1998
Main classification: A23C3
IPC codes: A23C3, A23L3