GB2607630A

GB2607630A - A method and system for monitoring drying of matter

Info

Publication number: GB2607630A
Application number: GB2108330.8A
Authority: GB
Inventors: Hutchinson John; Monroe Charles
Original assignee: Mechatech Systems Ltd
Current assignee: Mechatech Systems Ltd
Priority date: 2021-06-10
Filing date: 2021-06-10
Publication date: 2022-12-14
Also published as: GB202108330D0

Abstract

A method for monitoring drying of matter, for example nuclear waste, contained in a vessel 2 at sub-atmospheric pressures. The vessel has an inlet conduit 12 and an outlet conduit 3. The method comprises the steps of: a) controlling the pressure of the vessel, such that the matter is under partial vacuum; b) introducing an incondensable gas having a first predetermined flow rate into the vessel via the inlet conduit; c) taking a first dew point temperature measurement of the vapour in the outlet conduit; and d) determining a residual moisture content of the matter using the dew point temperature measurement. The method may further comprise the steps of: e) introducing further incondensable gas having a second predetermined flow rate; f) taking a second dew point temperature measurement of the vapour in the outlet conduit; and g) determining the residual moisture content of the matter using the dew point temperature measurements. Steps c) to g) may be repeated until a water evaporation rate is equal to a threshold value, or drying is complete. The incondensable gas may be nitrogen, argon, dry air, helium, neon, krypton or xenon. The pressure may be in the range of 1 to 20 kPa.

Description

A Method and System For Monitoring Drying of Matter

Field of disclosure

[0001] The present invention relates to a method and system for monitoring drying of matter. For example drying of waste at sub-atmospheric pressures.

Background

[0002] Management of nuclear waste involves careful storage and disposal of the waste. Typically, storage and disposal methods are regulated by government agencies in order to protect human health and the environment. Nuclear waste may be temporarily stored, for example in ductile cast iron containers or vessels, until a long term storage procedure is developed.

[0003] Nuclear waste is often treated to adjust its characteristics in order to improve its safety. For example, the waste may be compacted to reduce its volume, filtered and/or ion exchanged to remove radionuclide content. The waste may be precipitated to induce chemical changes in the composition. Subsequently, the waste must be transformed into a form which will neither react nor degrade for an extended period.

[0004] Typically, the waste is dried at high temperatures so that it adopts a solid form (and is thus resistant to leaching) which allows the waste to be stored long term, either above or below sea level. The time taken for the waste to dry will vary depending on the specific composition of the waste.

[0005] Several ways of monitoring drying of waste, in particular nuclear waste, are known.

[0006] It is known to take a sample of waste from a vessel during the drying process, in order to estimate a residual moisture content of the waste. If the waste is contaminated with nuclear matter, it may be dangerous to directly handle the waste. Taking a sample of waste requires breaking the vacuum and thus stopping the drying process. Further, removal of a lid closing an opening of the vessel is required, followed by removal of the sample. The lid is required to be replaced, the vacuum reinstituted and drying restarted. This interrupts the drying process, thus increasing the time and thus the costs required to dry the waste.

[0007] It is also known to use a gas flow meter in an outlet conduit of the container or vessel in which the waste is contained to monitor the flow of vapour exiting the vessel and to thus determine a degree of drying of the waste. Known gas flow meters do not provide satisfactory function at low pressure. There is also a risk of contamination of the gas flow meter due to contaminants which may be present in the gas produced by the waste (the off-gas). In addition, some types of gas flow meters pose an explosion risk.

[0008] A pressure drop should be minimised when drying waste at low pressures in order to reduce the evaporation temperature. The presence of a gas flow meter in the outlet conduit means that it cannot be guaranteed that the gas flow meter readings are representative of the actual level of dryness of the waste, which could lead to waste being prepared for long-term storage containing an unsafe level of moisture. In addition, the presence of a gas flow meter in a nuclear waste drying system may provide an ignition source where a hydrogen/air mixture is present. Further, if the gas composition is unknown, a gas flow meter will not assist with providing a water mass flow rate. Thus, a gas flow meter may provide inaccurate readings and an unsafe drying system. It is a challenge to measure very small quantities of residual water (down to a few grams) in a large mass of material (many kilograms).

[0009] It is an object of the present invention to mitigate problems such as those described above.

Means for solving the problem 100101 According to a first aspect of the invention there is provided a method for monitoring drying of matter contained in a vessel. The vessel has an outlet conduit for removal of vapour produced by the matter arid an inlet conduit for introducing a gas into the vessel. The method comprises the steps of a) controlling the pressure of the vessel, such that the matter is under partial vacuum; b) introducing an incondensable gas having a first predetermined flow rate into the vessel via the inlet conduit; c) taking a first dew point temperature measurement of the vapour in the outlet conduit; and d) determining a residual moisture content of the matter using the dew point temperature measurement.

[0011] Advantageously, the method enables the residual moisture content of the matter contained in the vessel to be quantified, and therefore the level of dryness. Thus, the method overcomes the problem of needing to prolong drying times in order to be confident that the matter is dry. The residual moisture content of the matter can also be determined without requiring a gas flow meter in the outlet conduit. The method also does not require matter to be sampled from inside of the vessel. Thus, the present method of monitoring of drying of the matter can be performed without interrupting the drying process. Accordingly, the time taken for matter to dry is reduced, which means energy required is reduced, thus providing a cost saving.

100121 The method in its broadest definition is applicable in a waste drying system where there is no leakage or outgassing from the waste. The incondensable gas has a known and constant flow rate, which allows the evaluation of the evaporated water mass flow rate and ultimately the residual moisture content of the waste. Other aspects apply where these assumptions cannot be relied upon.

[0013] A significant advantage of the present method is a saving on drying time and as a consequence, increased throughput of waste.

[0014] Under-drying waste is also problematic, because the waste will contain unsuitable levels of moisture. Thus, the waste will be in an unsuitable form for long term storage, which is of particular concern where the waste is nuclear waster.

100151 Advantageously, dew point temperature measurements of the vapour are quick, easy and safe to obtain and can be taken on-line (i.e. in real time). Dew point temperature measurements can be converted to water vapour partial pressures and subsequently water vapour mass flow rates, which allows for the residual moisture content of the waste to be determined. The dew point temperature measurement taken may be a single dew point temperature measurement. The preferred method allows the operator to determine the point at which the waste has reached a required level of dryness in order to prevent wasteful over-drying or inefficient under-drying. This provides a cost saving. The operator can determine the minimum period of time over which heat energy may be required for drying the waste. The time period required for heating the vessel and the waste and the quantity of incondensable gas can be minimised.

[0016] A dew point temperature measurement can be converted to partial pressure. The incondensable mass flow rate is known. These two process variables and the gas pressure can be converted to water vapour mass flow rate. The analysis of evaporated water mass flow rate with time allows for the residual moisture content remaining in the waste to be quantified, e.g. at point A, as shown on figure 3. (It also allows for the initial moisture content to be quantified). Measuring the water vapour flow rate as a function of time, integrating the water vapour flow rate with time to determine the mass of water removed up to point A and plotting this against the water vapour mass flow rate indicates the total mass of water to be removed from point A to the vertical axis (where water evaporation rate is zero). The total mass of water to be removed less the mass of water actually removed at point A gives the residual moisture content in the waste contained in the vessel at that point.

100171 In particular for nuclear waste, it is important that specific levels of moisture content have been removed from the waste so that the waste can be stored safely, long term. Thus, the monitoring of residual moisture in the waste is highly important for ensuring safe storage and disposal of nuclear waste.

100181 Preferably, the method further comprises the steps of: e) introducing further incondensable gas having a second predetermined flow rate; 0 taking a second dew point temperature measurement of the vapour in the outlet conduit; and g) determining the residual moisture content of the matter using the dew point temperature measurements.

100191 When there is leakage in the system and/or outgassing from the waste, it is advantageous to measure the dew point temperature of the vapour in the outlet conduit at two predetermined incondensable gas flow rates. Thus, the overall flow rate after performing steps e) to g) is different to the flow rate after performing steps a) to d). A reason for taking a second dew point temperature measurement is to quantify gas leakage and outgassing. The two predetermined incondensable gas flow rates may be different, or they may be the same. Steps e) to g) allow the water mass flow rate and the combined leakage plus outgassing gas flow rate to be calculated. The further incondensable gas having a predetermined flow rate may be introduced directly into the vessel. Alternatively, the further incondensable gas may be introduced into the outlet conduit [0020] Preferably, the method further comprises repeating steps c) to g) until it is determined that i) a water evaporation rate is equal to a threshold value, or ii) drying is complete [0021] Advantageously, the present method can be implemented either until the waste has been dried to a predetermined level, or is completely dry. Thus, the waste may be dried until the water evaporation rates indicate that the waste is dried to a particular extent.

[0022] It may not be required to implement the method until the matter is completely dry (i.e. until the moisture content of the waste is zero). Normally, a residual mass of water remaining in the waste is acceptable for storage purposes.

[0023] The incondensable gas is incondensable at the operating temperatures of the system and is therefore incondensable relative to water vapour. Preferably, the incondensable gas is selected from the group comprising nitrogen, argon, dry air, helium, neon, krypton or xenon [0024] Advantageously, these gases are inert and are thus safe for use in a method and system for monitoring drying of waste. Accordingly, these gases do not disrupt the composition of the waste. Some of the aforementioned gases are also naturally abundant and are easy to obtain in large quantities and are relatively inexpensive.

[0025] Preferably, the pressure is controlled to achieve a value in a range of from 1 to 20 kPa. More preferably, the pressure is controlled to achieve a value in a range of from 3 to 7 kPa.

[0026] The present method provides particularly advantageous effects when the pressure applied to the vessel and thus the waste is lower than atmospheric pressure, e.g. a value in a range of from 1 to 20 kPa or 3 to 7 kPa. Within these pressure ranges, water will advantageously evaporate at lower temperatures than it would at atmospheric pressure. This means that the temperature applied to the vessel can be kept relatively low, which ensures minimal impact on the composition of the waste.

[0027] In combination with the relatively low pressure maintained in the vessel, the evaporation temperature of the water from the waste is also relatively low. Preferably, the constant temperature is a value in a range of from 10 °C to 60 °C.

100281 More preferably, the constant temperature is a value in a range of from 25 °C to 40 °C.

[0029] Use of a low temperature in either of the above ranges minimises the impact on the composition of the waste. For example, operation at approximately 33 °C has been observed to produce effective results.

[0030] Preferably, the first and second predetermined flow rates of the incondensable gas are between 0.01 Ncmh (normal cubic metres per hour) to 3 Ncmh. Introducing an incondensable gas into the outlet conduit having a predetermined flow rate in this range has been observed to provide particularly effective results.

[0031] It is useful to be able to monitor the water content in different types of matter, such as nuclear waste (but also biocides, cyanides, metal phosphides and other hazardous waste), in order for it to be determined whether the waste is suitable for safe storage. The ideas and processes described herein can be used for drying other materials where a high degree of dryness is required, such as pharmaceuticals and desiccants. It is both useful and energy saving to not use excessive heat to dry such materials.

100321 The extent to which the waste requires drying will depend on its specific composition. Sufficient drying of waste (especially nuclear waste) prevents deleterious effects during long term storage, including gas generation which can cause pressurisation of the vessel in which the waste is contained. By monitoring the dryness of waste, issues which may occur during long term storage of the waste, such as corrosion of the vessel, microbial activity and radiolysis.

100331 Preferably, determining a residual moisture content of the waste comprises the step of integrating a water vapour mass flow rate with time.

Additionally or alternatively, determining an original moisture content of the waste comprises the step of integrating a water vapour mass flow rate with time.

[0034] According to a second aspect of the invention there is provided a system for monitoring drying of matter contained in a vessel comprising: a vessel for containing the matter, wherein the vessel has an outlet conduit for removal of a vapour produced by the matter and an inlet conduit for introducing an incondensable gas into the vessel; means for controlling the pressure of the vessel, such that in use, the waste is under partial vacuum; means for measuring the dew point temperature of the vapour in the outlet conduit; a source of incondensable gas; and a controller. The controller is configured to perform the steps of: 1) taking and receiving a pressure measurement of the gas in the outlet conduit; 2) introducing the incondensable gas into the system, wherein the incondensable gas has a first predetermined flow rate; and 3) taking and receiving a first dew point temperature measurement of the vapour in the outlet conduit.

[0035] Advantageously, the system allows for matter to be dried and the drying process to be monitored using, for example, the method described in the first aspect of the invention.

[0036] Preferably, the controller is further configured to perform the steps of: 4) introducing further incondensable gas into the system, wherein the further incondensable gas has a second predetermined flow rate; 5) taking and receiving a pressure measurement of the gas in the outlet conduit; and 6) taking and receiving a second dew point temperature measurement of the vapour in the outlet conduit.

[0037] Advantageously, the controller is configured to take dew point temperature measurements at two incondensable gas flow rates. This allows the drying of matter, preferably waste, to be monitored where there is outgassing from the waste or a leakage in the system.

[0038] Preferably, the controller is further configured to repeat steps 1) to 6) until it is determined that i) a water evaporation rate is equal to a threshold value, or ii) drying is complete [0039] Advantageously, the present method can be implemented either until the matter has been dried to a predetermined level, or is completely dry. Thus, the matter may be dried until the water evaporation rates indicate that the waste is dried to a particular extent.

[0040] Preferably, the incondensable gas is selected from the group comprising nitrogen, argon, dry air, helium, neon, krypton or xenon. Advantageously, these gases are inert and are thus safe for use in a method and system for monitoring drying of waste. Accordingly, these gases do not disrupt the composition of the waste. Some of the aforementioned gases (particularly nitrogen) are also naturally abundant and are easy to obtain in large quantities and are relatively inexpensive.

[0041] Various embodiments and aspects of the present invention are described without limitation below, with reference to the accompanying figures.

Brief description of the drawings

[0042] Figure 1 shows a schematic representation of a system for monitoring drying of matter.

[0043] Figure 2 shows a plot of time vs. water mass flow rate for nuclear waste contained in a vessel undergoing drying [0044] Figure 3 shows a plot of water evaporation rate vs. water collected.

[0045] Figure 4 shows a plot of the relationship between the remaining mass of the vessel (containing the waste) over the drying process, compared with the relationship between the mass of water collected over the drying process

Detailed description of a preferred embodiment

[0046] Referring to figure 1, there is illustrated a system 1 for monitoring drying of matter. The matter may be waste. In a preferred embodiment, the waste may be nuclear waste. The drying of nuclear waste is a requirement for safe disposability and long term storage of the waste. The description herein is of a preferred embodiment wherein the matter is waste.

[0047] The system 1 includes a vessel 2 for containing the matter, which has an outlet conduit 3 for removal of vapour produced by the matter. The system I further comprises suction/evacuation means 11 (e.g. a pump) for removing incondensable gases (e.g. nitrogen and air) and water vapour from the vessel 2. Pump 11 may decrease the pressure of the system to 1 kPa. Pressure control valve 16 is operable to control the pressure in vessel 2 such that the waste or other matter contained in vessel 2 is under partial vacuum. Pressure meter 13 indicates the pressure of the system 1. The system 1 includes optional heating means 5 for heating the vessel 2 and a means 6 for measuring the dew point temperature of the vapour in the outlet conduit 3.

100481 The system includes a source of incondensable gas 7. A conduit 12 is connected to the source of incondensable gas 7. Conduit 12 allows the incondensable gas from source 7 to be fed into vessel 2 via inlet conduit 4. Inlet conduit 4 includes a valve 4' for controlling entry of incondensable gas into vessel 2. Conduit 8 branches off from conduit 12 and allows for the incondensable from source 7 to be introduced into outlet conduit 3. Valve 12' is positioned along conduit 12 at a position upstream (in the direction of fluid flow) of conduit 8. Valve 12' is a pressure regulating valve, which controls fluid flow along conduit 12 and controls supply pressure from source 7. Thus, valves 4' will pass a constant flow rate. Conduit 8 includes a valve 9 for controlling entry of incondensable gas into outlet conduit 3. Valve 9 will also pass a constant flow rate. In order to determine the incondensable gas flow rates, flow meters 24 and 29 are positioned upstream of valve 4' and valve 9, respectively, in the direction of gas flow.

[0049] The system 1 also includes a controller 14 connected to the means 6 for measuring the dew point temperature and to the valve 9 (and generally connected to other sensors and items under control). The controller 14 is also connected to pressure meter 13 positioned in the outlet conduit 3 and to pressure control valve 16 which is regulated by the controller 14 in view of the pressure value on pressure meter 13 in order to control the pressure in the vessel 2.

[0050] The system further includes a condenser 10 for cooling the vapour in outlet conduit 3 and a drain 15 for collection of liquid from the system 1 [0051] In use, air is evacuated from the system using means 11 (e.g. a pump).

Pump 11 moves the water vapour plus leakage and outgassing Of any), plus the incondensable gas which is fed from source 7. Pump 11 maintains low pressure in the vessel 2, regulated by pressure control valve 16, as a continuous process. Thus, the system 1 is under partial vacuum and the pressure is controlled by pressure control valve 16 and controller 14. The vessel 2 (containing the matter, which is preferably waste) is preferably heated using heating means 5, preferably controlled by controller 14. In a preferred embodiment, the vessel is heated to a temperature in the range of from 25 °C to 40 °C. A constant pressure of system 1 is maintained in a preferred embodiment in the range of from 3 to 7 kPa.

[0052] In a preferred embodiment, pressure regulating valves 12' and 4' are always open to allow incondensable gas from source 7 to be introduced into vessel 2 via conduits 12 and 4. The incondensable gas has a predetermined, constant flow rate as determined by valves 12' and 4'. In a preferred embodiment, the flow rate is 0.03 Ncmh. In a preferred embodiment, the incondensable gas is nitrogen. Introduction of incondensable gas into vessel 2 when valve 4' is opened provides an inert atmosphere. Subsequently, means 6 measures the dew point temperature of the vapour in outlet conduit 3 and may feed this information to controller 14. A residual moisture content of the matter can be determined by calculating the water evaporation mass flow rate by use of the dew point temperature measurement in combination with the pressure reading obtained from pressure meter 13, and the incondensable gas flow rate(s).

100531 If there is outgassing from the waste contained in vessel 2, or a leakage in system 1, incondensable gas having a predetermined flow rate may be introduced into outlet conduit 3 via conduit 8, by opening solenoid valve 17. Gas flow is controlled by valve 9. Alternatively, further incondensable gas can be introduced directly into vessel 2, e.g. by suitable control of valve 4'. Thus, conduit 8, valve 9, solenoid valve 17 and flow meter 29 are optional features of system 1. The two incondensable gas flow rates (which are measured and known) and the two dew point temperature measurements in combination with pressure measurements allow the total unknown incondensable flow rate plus evaporated water mass flow rate to be determined. The gas flow in conduits 4 and 8, the water evaporation mass flow rate, and any system leakage and waste outgassing generate a gas flow rate which is in equilibrium with the flow capacity of pump 11 and the pressure control valve 16, which regulates the pressure of system 1 to achieve a desired value.

[0054] The further incondensable gas (which is either introduced directly into the vessel 2, e.g. via inlet conduit 4, or into outlet conduit 3 via conduit 8) typically has the same composition as the incondensable gas initially introduced into vessel 2.

100551 In use, a first dew point measurement of the vapour in the outlet conduit 3 is taken by the controller 14. The first dew point temperature measurement is taken with valve 4' open and before further incondensable gas is introduced into either the vessel 2 via conduit 4 or the outlet conduit 3 via conduit 8.

100561 After the first dew point temperature measurement has been taken, further incondensable gas is introduced into vessel 2, e.g. via inlet conduit 4, or valve 9 is opened to allow a small amount of incondensable gas to flow from conduit 12, into conduit 8 and subsequently into outlet conduit 3. The controller 14 is configured to take and receive a second dew point temperature measurement of the vapour in the outlet conduit 3, with both valves 4' and 9 open.

100571 In an alternative embodiment, three dew point temperature measurements may be taken. A first dew point temperature measurement may be taken with valve 4' open and valve 9 closed; a second dew point temperature measurement may be taken where valve 4' is closed and valve 9 is open; and a third dew point temperature measurement may be taken where both valve 4 and valve 9 are open.

100581 In a preferred embodiment, further increments of incondensable gas are introduced into vessel 2, e.g. via inlet conduit 4, or into outlet conduit 3 via conduit 8. After introduction of each increment of incondensable gas, the controller 14 is configured to take and receive a dew point temperature measurement of the vapour in the outlet conduit 3. The two (or more) dew point temperature measurements at predetermined incondensable flow rates allow the unknown flow rate of outgassing and/or leakage to be determined, as well as the water evaporation mass flow rate.

[0059] In the situation where there is outgassing from the waste, the vapour produced by the waste will inevitably be a mixture of water vapour and incondensable gas. The specific proportions depend on the composition of the waste and its degree of saturation. The water vapour mass flow rate and the mass flow rate of the incondensable gas produced by the waste are both unknown. The method performed by system I involves introducing a known quantity (i.e. a predetermined flow rate) of incondensable gas (e.g. nitrogen) into the vessel 2 via the inlet conduit 4 and taking a first dew point temperature measurement of vapour in outlet conduit 3, and subsequently introducing further incondensable gas into the system 1 (such as directly into the vessel 2 or into the outlet conduit 3 via conduit 8) and taking a second dew point temperature measurement of vapour in outlet conduit 3. By measuring the first and second dew point temperatures, the water vapour mass flow rate of the water vapour produced by the waste, and thus a degree of drying of the waste (i.e. the residual moisture content of the waste), can be determined. Advantageously, it is easier to accurately measure the flow rate of nitrogen (or another incondensable gas) before it enters the drying system, rather than measuring the flow rate of water vapour flow rate in the outlet conduit 3 (i.e. the vapour extract line).

[0060] Measuring the dew point temperature of firstly the vapour produced by the waste after introducing an incondensable gas such as nitrogen, and secondly of the mixture of the water vapour and further incondensable (e.g. nitrogen) gas can be easily and quickly performed in real time. Furthermore, dew point temperature measurements can easily be converted to water vapour partial pressures (for example, by using Sonntag's or Antoine's equation) which can in turn be used to estimate the water vapour mass flow rate throughout the drying process. The water vapour mass flow rate is deduced from the dew point temperature measurement(s), the incondensable flow rate(s) and the pressure reading(s).

100611 Advantageously, the system 1 and therefore the method for monitoring drying of waste described herein do not require a gas flow meter in the vacuum extract line. Accordingly, disadvantages associated with using a gas flow meter in known systems and methods are circumvented. Such disadvantages include a pressure drop across the measurement point, and an ignition source where a hydrogen/air mixture is present.

100621 In a preferred embodiment, the steps of introducing additional incondensable gas having a predetermined flow rate into the outlet conduit 3 or directly into vessel 2 and taking a dew point temperature measurement of the vapour in outlet conduit 3 are repeated until it determined that drying is complete, or that a water evaporation rate is equal to a threshold value. In order to determine the residual moisture content of the waste, analysis of the water mass flow rate with time is performed.

[0063] The determination of water vapour flow rate allows a drying curve to be plotted. This data can be used to identify when the water vapour mass flow rate (and thus the drying rate) approaches zero. This allows the operator to ascertain when to stop the drying process. Costs incurred in a waste drying facility are determined by the asset cost of the facility, the operator time and the need to complete decommissioning and clean-up as quickly and cost effectively as possible. The methods and system described above provide a cost saving because the operator is able to determine the residual moisture in the waste. Unnecessary drying of waste is avoided and a higher waste throughput is achieved. Figure 4 shows the relationship between the remaining mass of the vessel (containing the waste) over the drying process, compared with the relationship between the mass of water collected over the drying process. As can be seen from figure 4, as the mass of water collected increases over time, the mass of the vessel (containing the waste) decreases. These two relationships can be compared to ascertain the degree of drying of the waste.

[0064] Figure 2 shows a theoretical plot of time vs. water mass flow rate for a vessel containing nuclear waste, for an ideal drying process. The theoretical studies applied a drying temperature of 50 °C and a pressure of 5 kPa.

[0065] As can be seen in Figure 2, between Oh and approximately 4.5h the water mass flow rate (i.e. the mass flow rate of the vapour in outlet conduit 3) is at a constant high value. During this time period, water is free to evaporate according to the normal rules of evaporation. The water molecules are 'free' in that they are not bound to the waste by intermolecular forces. During this initial drying phase, the water mass flow rate is governed by the amount of heat available for evaporation of the liquid from the waste, as the movement of moisture within the waste (i.e. the rate at which moisture reaches the surface of the waste) is fast enough to maintain a saturated surface of the waste.

[0066] Referring to the plot shown in figure 2, from 4.5h onwards, the drying process shows the evaporation of bound water as an exponential decay of the water evaporation mass flow rate with time. Water may be bound to the waste by intermolecular forces, and evaporation of bound water takes longer than evaporation of free water. At this stage the drying rate is limited by the internal moisture movement to the surface of the waste. It may be difficult to measure a threshold in the evaporated water mass flow rate, as it tends to an equilibrium (in this case zero), which is the nature of exponential decay. If this evaporated water mass flow rate is integrated over time, the resulting variable is the accumulated water collected. The accumulated water collected versus water evaporation mass flow rate can be plotted against each other (as is shown in figure 3 and discussed below). In order to move from the plot shown in figure 2 to the plot shown in figure 3, the water mass flow rate is integrated with time.

[0067] Nuclear waste processing facilities may require that it is determined whether the residual mass of moisture present in contained waste is equal to a threshold value. The method for monitoring drying of waste as described herein can be used in determining a residual mass of moisture present in the contained waste. As shown in Figure 3, the water vapour evaporation rate (i.e. the water vapour mass flow rate) decreases as the mass of water collected from the waste increases. In other words, the water evaporation rate decreases as the waste becomes drier. The water vapour mass flow rate can be determined using a single dew point temperature measurement if there is no outgassing or leakage in system 1. If there is outgassing and leaking in system 1, then first and second dew point temperature measurements are used to determine the water vapour mass flow rate.

[0068] Figure 3 shows a plot of water evaporation rate vs. water collected. Figure 3 shows real data obtained using the method of the present invention in the final stage of evaporation of the waste (the bound water stage). The drying process and the method for monitoring drying of the waste may be stopped when the evaporation rate reaches a predetermined rate or a threshold rate, wherein this predetermined rate is approaching zero. Extrapolation of the water evaporation rate (as shown by the dashed line in Figure 3), intercepts the vertical axis to show a theoretical amount of water to be collected when the water evaporation mass flow rate is zero. By the nature of an exponential decay, it would take an infinite amount of time to reach zero. The difference between the amount of water collected, by way of integration of the water evaporation rate and time, and the theoretical amount of water collected (determined by extrapolation), gives the residual amount of water remaining in the waste contained in the vessel 2. Thus, the amount of residual water remaining in the waste can be predicted by external and relatively low cost measurements.

10069] Waste may contain components which undergo crystallisation, for example, brine. Crystallisation of salt particles may occur during the drying process. Crystallisation absorbs energy and causes the evaporation water flow rate to decrease. This occurs due to heat energy being absorbed in the crystallisation process, rather than in the evaporation process. Once the crystallisation process has proceeded to an extent, heat energy becomes available for the evaporation process. Thus, instead of the evaporation water flow rate carrying on decreasing towards zero, the water vapour mass flow rate increases once heat energy is available for evaporation.

100701 There is thus a possibility that a false positive is obtained, due to the decrease in evaporation water flow rate during crystallisation, which appears to indicate that the waste is on its way to complete drying. In order to avoid obtaining a false positive, in a preferred embodiment, the method may include repeating steps c) and d), or steps e) to g) until it is determined that an evaporation rate of the mixture is equal to a threshold value The threshold value may be chosen to be a lower value than the lowest evaporation rate obtained during the crystallisation process. Monitoring of drying can be stopped at this point and extrapolation to zero can occur to calculate the residual content of water remaining in the waste contained in the vessel 2 100711 The method and system described herein may also be used in other applications where it is useful to measure the end point of drying of matter, for example, freeze drying or desiccant drying. Freeze drying will require lower temperatures and pressures than those discussed herein.

Claims

Claims 1 A method for monitoring drying of matter contained in a vessel having an outlet conduit for removal of vapour produced by the matter and an inlet conduit for introducing a gas into the vessel, the method comprising the steps of a) controlling the pressure of the vessel, such that the matter is under partial vacuum; b) introducing an incondensable gas having a first predetermined flow rate into the vessel via the inlet conduit; c) taking a first dew point temperature measurement of the vapour in the outlet conduit; and d) determining a residual moisture content of the matter using the dew point temperature measurement 2. A method according to claim 1, further comprising the steps of: e) introducing further incondensable gas having a second predetermined flow rate, 0 taking a second dew point temperature measurement of the vapour in the outlet conduit; g) determining the residual moisture content of the matter using the dew point temperature measurements 3. A method according to claim 2, further comprising repeating steps c) to g) until it is determined that i) a water evaporation rate is equal to a threshold value, or ii) drying is complete.4 A method according to any preceding claim, wherein the incondensable gas is selected from the group comprising nitrogen, argon, dry air, helium, neon, krypton or xenon.5. A method according to any preceding claim, wherein the pressure is controlled to achieve a value in a range of from 1 to 20 kPa.6. A method according to any preceding claim, wherein the pressure is controlled to achieve a value in a range of from 3 to 7 kPa.7. A method according to any preceding claim, wherein the vessel is heated to a temperature in a range of from 10°C to 60°C, 8. A method according to any preceding claim, wherein the vessel is heated to a temperature in a range of from 25 °C to 40 °C.9. A method according to any preceding claim, wherein the first and second predetermined flow rates of the incondensable gas are between 0.01 Ncmh to 3 Ncmh.10. A method according to any preceding claim, wherein the matter is nuclear matter.11 A method according to any preceding claim, wherein determining a residual moisture content of the matter comprises the step of integrating a water vapour mass flow rate with time.12. A system for monitoring drying of matter contained in a vessel comprising: a vessel for containing the matter, wherein the vessel has an outlet conduit for removal of a vapour produced by the matter and an inlet conduit for introducing an incondensable gas into the vessel; means for controlling the pressure of the vessel, such that in use, the matter is under partial vacuum; means for measuring the dew point temperature of the vapour in the outlet conduit; a source of incondensable gas; a controller configured to perform the steps of: 1) taking and receiving a pressure measurement of the gas in the outlet conduit; 2) introducing the incondensable gas into the system, wherein the incondensable gas has a first predetermined flow rate, 3) taking and receiving a first dew point temperature measurement of the vapour in the outlet conduit.13. A system according to claim 12, wherein the controller is further configured to perform the steps of: 4) introducing further incondensable gas into the system, wherein the further incondensable gas has a second predetermined flow rate; 5) taking and receiving a pressure measurement of the gas in the outlet conduit; 6) taking and receiving a second dew point temperature measurement of the vapour in the outlet conduit.14. A system according to claim 13, wherein the controller is further configured to repeat steps 1) to 6) until it is determined that i) a water evaporation rate is equal to a threshold value, or ii) drying is complete.A system according to any of claims 12 to 14, wherein the incondensable gas is selected from the group comprising nitrogen, argon, dry air, helium, neon, krypton or xenon.