A method for measuring the liquid level in a pressure vessel

Abstract

Summary of the invention: A method for measuring the liquid level (L) in a pressure chamber (2), the present method comprising: providing a thermometric wall (10) inside the pressure chamber, heating the inner surface of the thermometric wall that is not in contact with the liquid, detecting the temperature of at least one detection point of said inner surface; estimating the position of the liquid level based on the difference between the reference temperature and the temperature thus detected.

Description

A method for measuring the liquid level in a pressure chamber. Description Field of invention The present invention relates to a method and system for measuring the liquid level of a liquid confined in a pressure chamber. Prior knowledge Liquid level measurement in a pressure vessel is a common task in most equipment found in chemical plants, including reactors (converters), strippers, condensers, and the like. Typically, a pressure vessel contains a liquid in equilibrium with a gaseous phase above the liquid, and a liquid level measurement is required in order to properly control the process taking place in the vessel. The task of measuring liquid level may be challenging under certain conditions, for example when either the pressure or temperature of the liquid is above a critical value, in the presence of a boiling liquid, when a chemical reaction occurs in the liquid and / or in the gas phase, in the presence of a corrosive environment. A notable example of a pressure vessel where liquid level measurement may be difficult is a reactor for the synthesis of urea from carbon dioxide and ammonia. As disclosed in the prior art (see, for example, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag), most modern urea processes employ a high-pressure synthesis loop consisting of a reactor, stripper, and condenser operating at high pressures of approximately 130 to 160 bar. The urea synthesis reactor operates under high temperature and pressure, possibly in a near-critical or supercritical condition, and contains a corrosive liquid, especially due to the presence of ammonium carbamate. Other equipment in urea plants operating under severe conditions includes the high-pressure stripper and the high-pressure condenser. Another notable example is the reactor for converting urea to melamine. Urea synthesis and melamine synthesis may be carried out in the same facility, known as a urea-melamine facility. The known method for liquid level detection, especially in the case of the above-mentioned pressure vessels operating under harsh conditions, includes the following: Membrane-based transmitters are subject to corrosion and hydrogen penetration. The technique of measuring radiation levels, for example with gamma rays, clearly poses serious safety risks. Radar-based measurement, the accuracy of which is affected by the characteristics of the environment above the liquid surface. The radar technique is essentially based on the gas phase and the liquid phase has a different response to the radar signal. Therefore, the accuracy of this technique is affected when the gas phase and the liquid phase have the same characteristics, which may occur under certain conditions such as the temperature and / or pressure being close to the critical point or the liquid being in a supercritical state. Therefore, this method may be affected by the evaporation of small liquid droplets or by the formation of foam above the liquid surface. Despite the above disadvantages, the radar technique has been investigated for application in the urea field, especially for urea reactors. An example of a radar-based technique is disclosed in WO 2013 / 036108. Therefore, it has been proposed to solve the disadvantages of radar-based measurements by means of a hollow tube. However, this does not completely eliminate the influence of steam or foam on the liquid. Furthermore, the installation of a relatively large hollow tube in a urea reactor is an additional cost and may adversely affect the operation of the reactor itself. Summary of the invention The aim of this invention is to provide a new method for detecting the level of a liquid in a pressure vessel, which is accurate and suitable for operation in aggressive environments and / or under severe operating conditions, especially in the presence of liquid in near-critical or supercritical conditions. This object is achieved by the method according to claim 1, some preferred embodiments of which are disclosed in the dependent claims. The method of the present invention comprises detecting the temperature of one or more detection points on a heated inner surface of a thermometric wall that is partially immersed in a liquid. The method further comprises comparing at least one reference temperature and a temperature that is precisely detected. Based on said comparison, the liquid level in the pressure chamber is estimated. The present invention is based on the findings that the heat transfer coefficient of the liquid surrounding the thermometric, at the liquid-gas interface, varies significantly. The gas phase has a much lower heat transfer coefficient than the liquid phase. If surrounded by the liquid phase, the inner surface of the wall tends to approach the liquid temperature, due to good heat transfer between the inside and outside of the wall. If the inner surface is surrounded by the gas phase, on the contrary, the heat transfer is generally poor and affects the temperature of the inner surface. At least one reference temperature, used to estimate the liquid level, may include one or more reference values. The reference temperature(s) may include or be expressed by one or more of the following: An expected temperature of at least one detection point; one or more previous values ​​of previously detected temperature at at least one detection point; One or more temperature values ​​of the liquid contained in the pressure chamber; One or more temperatures on the outer surface of the thermometric wall. Therefore, the difference between the reference temperature(s) and the actual temperature is a function of the heat transfer through the side wall of the thermometric wall, and said heat transfer depends on the coefficient of the surrounding liquid. Based on this relationship, this method can estimate whether the surrounding liquid at the temperature detection point is liquid or gas, and, thus, the point (height) of the liquid surface can be indirectly detected. Preferably, the inner surface of the thermometric wall is heated to a target temperature that is higher than the temperature of the liquid surrounding the thermometric wall. Thus, the temperature of the inner surface of the thermometric wall, at the point where the wall is surrounded by the gas phase, will be close to the target temperature due to low heat exchange. At the point where the wall is surrounded by the liquid phase, on the contrary, the temperature will be lower and closer to the liquid temperature. Based on this detection, the position of the liquid surface can be estimated, for example, by using the expected temperature of the inner surface as a reference temperature. To increase the accuracy of the method, the temperature at several points on the inner surface of the wall can be detected. For example, a sudden change in temperature between two points indicates that the liquid level is located between the two points. The liquid contained in the pressure vessel may be at a temperature and / or pressure close to or above the critical value. The liquid may also be a supercritical fluid (SCF), which indicates that both the temperature and pressure are above the critical value. A desired application of the invention is liquid level measurement in pressure vessels of urea plants, melamine plants, and urea-melamine plants. Brief explanation of the problem Figure 1 is a schematic of a pressure vessel, e.g., a urea reactor, including the temperature measurement system of the present invention. Figure 2 is a schematic of the thermometric wall of Figure 1. Detailed description The present invention, in its broader form, can be carried out by the following methods: Providing a thermometric wall inside the pressure chamber; Heating the inner surface of the thermometric wall; detecting the temperature of at least one detection point of said internal surface; Estimate the position of the liquid surface based on the difference between at least one reference temperature and the temperature detected in the previous step. The thermometric wall extends vertically into the pressure chamber, preferably to the extent that the wall is within the expected range of the measured liquid level. The outer surface of the thermometric wall is exposed to the liquid contained in the pressure chamber. The inside of the thermometric wall is properly sealed. Therefore, the inner surface is not in direct contact with the liquid contained in the pressure chamber. The outer surface and the inner surface are separated by the side wall of the thermometric wall. The step of heating the inner surface of the thermometric wall is preferably carried out by heating the inner surface to a target temperature higher than the ambient temperature contained in the pressure vessel. In this way, heat tends to transfer from the inside to the outside of the pressure wall. The at least one reference temperature, which is used to determine the position of the liquid surface, may comprise an expected temperature of the detection point. Said expected temperature may be a function of the heating process of the inner surface, i.e., it may be equal to a target temperature set for said heating process, or it may be a function of the power transferred in the heating process itself. At least one reference temperature may also comprise previous temperature values ​​of said inner surface. Using only the expected and / or previous temperature(s) of the inner surface as the reference temperature may be preferable, as this method does not require measuring the temperature outside the thermometric wall where the temperature sensor is exposed to the liquid. However, the at least one reference temperature may also include one or more values ​​of the temperature of the liquid contained in the pressure chamber or the temperature of the outer surface of the thermometric wall, preferably at a point having the same height as the inner surface detection point. The temperature of the medium contained in the pressure vessel is preferably detected at a point close to the thermometric wall and close to the detection point of the inner surface. The temperature of the medium may include the temperature of the liquid phase and / or the temperature of the gas phase. The comparison between the detected temperature and the reference temperature(s) is used to determine the actual heat transfer between the inner and outer surfaces and, therefore, the heat transfer coefficient of the fluid at the detection point height. The internal surface temperature may be detected at multiple detection points, at different heights, to increase the accuracy of the system. The temperature may be detected at multiple discrete points or also continuously by a fiber optic distributed temperature sensing (DTS) system. The discrete detection points may be equally spaced, along the vertical direction, in a preferred embodiment. Based on the detected temperatures at several detection points, the thermal profile of the inner surface of the thermometric wall can be determined. Starting from the individual measurements, the temperature profile may be appropriately interpolated. The position of the liquid surface can be estimated based on said thermal profile. The above tasks can be performed by a suitable processing unit, for example, a suitable hardware configured to implement the method of the present invention. For example, in one example, the method may detect a pair of consecutive detection points at different heights (i.e., one above the other) on the inner surface of the thermometric wall, where the temperature difference between the detection points is greater than the temperature difference between the other pairs of consecutive detection points. This pair is called a singular pair because it exhibits a singularity (i.e., a sharp change) in the delta T between the consecutive points, compared to the other pairs. The method assumes that the singularity is due to the change in heat transfer between the two detection points resulting from the different aggregation state (liquid versus vapor) of the surrounding medium. Therefore, the method assumes that the liquid level between the detection points of the pair is singular. Based on the detected values, the method may also calculate the liquid position with very high accuracy within the range defined by the pair. If a temperature profile is obtained or calculated, the position of the liquid surface may be identified by a region where the temperature changes dramatically over a small vertical distance, due to the boundary between the liquid and vapor. In a simplified embodiment of the invention, a single temperature detection point may be used, in which case the detection process is repeated over time, and when a temperature change greater than a threshold difference is detected, the liquid level is assumed to have passed the detection point. Hence, this method provides a level switch. Heating the inner surface of the thermometric wall can be accomplished by an electrical resistor mounted in the thermometric wall or other heating means. An aspect of the invention is also a system for measuring the liquid level of a liquid in a pressure vessel, in particular in a chemical reactor, comprising a vertically extending thermometric wall, wherein the thermometric wall has an inner surface not in contact with said liquid, which is sealedly separated from the interior of the pressure vessel containing the liquid, wherein the thermometric wall comprises at least one heater arranged to heat said inner surface to a target temperature, and the system comprises hardware configured to detect the liquid level around the thermometric wall by a method according to one of the examples described above. Said hardware may comprise one or more temperature sensors for collecting the temperature at selected points of the surface inside the thermometric wall, and an electronic board, PLC or equivalent suitably adapted for carrying out the programmed method, for example for comparing the detected temperatures and the expected position of the liquid surface obtained. The temperature sensors are of known type and may comprise, for example, thermocouples. The advantages of the invention are as follows: The liquid level may be measured indirectly, without a level gauge in direct contact with the liquid. The level may also be detected indirectly from within the thermometric wall, without any sensing means in direct contact with the liquid, which is particularly desirable when the liquid has harsh conditions such as temperature, pressure, corrosion. Thermometric walls that can withstand such conditions are available at a reasonable cost, while sensors designed to operate under the same conditions are usually very expensive. The invention has no safety concerns: the present invention is insensitive to problems with visual / radar systems, especially in the presence of vapor or foam on the liquid surface or the liquid in a critical or supercritical condition. The accuracy of the inventive method can be determined by a suitable number of temperature detection points. Referring now to a preferred embodiment, shown in the figures, a chemical reactor 1 has a vertical pressure vessel 2 with a vertical axis A and, very schematically, has an inlet 4 of reactants and an outlet 5 of reaction products. Inside the reactor 1, there is a liquid level L. Above the liquid level, the reactor 1 contains a vapor phase which is in equilibrium with the liquid phase. As will be apparent to the skilled person, reactor 1 may include other inlets and / or outlets. For example, reactants may be introduced via a separate stream and an overhead gas stream may be drawn from above. The reactor 1 has a liquid level detection system operating in the thermometric wall 10. The thermometric wall 10 extends vertically into the pressure chamber 2. As shown in Figure 2, the inner surface 13 of the thermometric wall 10 is heated to a target temperature T* by a suitable heater 12. The temperature of the inner surface 13 is detected at selected points, which include, for example, points P1, P2, P3. The temperature is detected by suitable sensors. The graph of Figure 2 shows the detected temperatures T1, T2, T3 at points P1, P2, P3 respectively along the vertical y coordinate. The graph also shows the target temperature T*, which is the point of heater 12. The target temperature T* is greater than the temperature Tf of the liquid contained in the pressure chamber 2. In the example, the temperatures T1 and T2 detected at points P1 and P2 are close to the target temperature T* due to poor heat transfer and therefore diffusion of the heat input of the heater 12, through the side wall 11. The poor heat transfer is due to the presence of a gas phase around the wall 10 at the height (y-coordinates) of points P1 and P2. However, at point P3 where wall 10 is surrounded by liquid, the detected temperature T3 is lower than T1 and T2 and closer to temperature Tf. This is due to the much higher heat transfer coefficient of the liquid (compared to vapor) and therefore the heat transferred to the liquid through the side wall 11. So the system assumes that the liquid level L is between points P2 and P3. The system may assume that the difference (T2-T3) is greater than a certain threshold. In an example (not shown) only one detection point, for example P1, is provided. In this case, the system may use previous temperature values ​​as a reference. The method may therefore detect a sudden change in temperature at point P1 and assume that the liquid level has passed the height P1. In another example not shown, the ambient temperature Tf is used as a reference to determine the position of the liquid surface. For example, the system may include a primary thermocouple located to sense the temperature of the interior surface 13 and a second thermocouple located below the expected liquid surface L to sense the liquid temperature. The system may also include another thermocouple located to sense the gas phase temperature above the surface L.

Claims

CLAIMS 1) A method for measuring a liquid level (L) of a fluid contained in a pressure vessel (2) the method comprising: i) providing a thermometric well (10) inside the pressure vessel, wherein the thermometric well extends vertically in the pressure vessel; ii) wherein said thermometric well has an inner surface (13) which is not in contact with the fluid and is separated from the fluid by a side wall (11) of the thermometric well; iii) heating said inner surface of the thermometric well; iv) detecting the temperature of at least one detection point of said inner surface; v) estimating a position of the liquid level based on the difference between at least one reference temperature and the actual temperature detected at step iv); wherein the pressure vessel is a piece of equipment of a plant for the synthesis of urea or of a plant for the synthesis of melamine.2) Method according to claim 1, wherein the step iii) of heating the inner surface of the thermometric well includes heating the inner surface to a target temperature greater than the temperature of the medium contained in the pressure vessel.3) Method according to claim 1 or 2, where, in the step v), the at least one reference temperature includes an expected temperature of the at least one detection point.4) Method according to any of the previous claims wherein, in the step v), the at least one reference temperature includes a temperature previously detected at said at least one detection point.5) Method according to any of the previous claims, where, in the step v), the at least one reference temperature includes a temperature of the fluid contained in the pressure vessel.6) Method according to any of claims 1 to 5, wherein the step iv) includes detecting the temperature of at least two detection points of said inner surface at different elevation.7) Method according to claim 6, wherein the step v) includes: determining a thermal profile of the inner surface of the thermometric well, based on the temperature detected at said detection points, and the position of the liquid level is estimated on the basis of said thermal profile.8) Method according to claim 6 or 7, where step v) includes: measuring the temperature at several detection points on the inner surface, the detection points being at different elevations; detecting a singular pair of consecutive detection points wherein the difference temperature between said detection points is greater than the difference of temperature of other pairs of consecutive detection points; assuming that the liquid level is located between the detection points of said singular pair.9) Method according to any of claims 1 to 5, wherein the step iv) is performed by detecting the temperature at only one detection point and is repeated over time, and the step v) includes that the liquid level is assumed to have crossed the detection point when a change of temperature greater than a reference threshold is detected.10) Method according to any of the previous claims, wherein the step iii) is performed by means of an electric resistance installed in the thermometric well.11) A system for measuring the liquid level of a fluid in a pressure vessel, particularly in a chemical reactor, comprising a thermometric well (10) which extends vertically in the pressure vessel, wherein the thermometric well has an inner surface (13) which is not in contact with said fluid, being sealedly separated from the inside of the pressure vessel where the fluid is contained, wherein the thermometric well includes at least one heater (12) arranged to heat said inner surface (13) to a target temperature , the vessel being a piece of equipment of a plant for the synthesis of urea or of a plant for the synthesis of melamine and the system includes a hardware configured to detect the liquid level around the thermometric well with a method according to any of claims 1 to 10 .

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