Calibration method for a heat flow calorimeter, especially in the cement industry

Abstract

The present invention relates to a method for the automatic calibration of a heat flow calorimeter in order to enable an immediate start of measurement with reproducible and meaningful measured values, even within the first hour.

Description

[0001] The invention relates to a method for the automatic calibration of a heat flow calorimeter in order to enable an immediate start of measurements with reproducible and meaningful measured values, even within the first hour.

[0002] Isothermal heat flow calorimetry is used in cement research to model the reaction behavior of binders over time. The isothermal heat flow calorimeter has at least one precise measurement range within approximately... 2100 until 11000 The calorimeter chamber is temperature-controlled. This calorimeter chamber contains one or more calorimeter cells, usually eight. The temperature of the calorimeter cells is stabilized by the calorimeter chamber. A heat flux sensor in each calorimeter cell measures the heat flowing to a heat sink, thus determining the current temperature difference between the sample and the heat sink. The amount of heat released depends on the composition of the binder, its fineness, and any other additives. The cumulative specific heat of the sample is a material constant, and the reaction kinetics can only be altered by changing physical parameters such as the fineness or composition, or by adding concrete admixtures such as water reducers, accelerators, retarders, and the like.Typical experiments in cement chemistry investigate the influence of external parameters, such as fineness, composition, and additives, on the reaction process. The heat flow or cumulative heat over time is plotted, resulting in a representation of the intensity of cement hydration over time.

[0003] Sample preparation is typically performed manually. A binder sample is first weighed into an ampoule and then mixed with a defined quantity of water to achieve a selected water / binder ratio (water / cement ratio). After adding the water, the powder and water are mixed to a homogeneous paste, either manually with a spatula or mechanically by shaking or vibration, either before filling the ampoule or directly within the ampoule. Once homogenized, the ampoule containing the homogeneous paste is inserted into one of the calorimeter cells in the calorimeter chamber of a commercially available calorimeter using a tool such as a hook or suction cup. The measurement is then initiated using software provided by the calorimeter manufacturer to continuously record the amount of heat released.

[0004] In this widely used method, the first 30-60 minutes of the measurement are typically disregarded because any temperature differences between the temperature inside the isothermal calorimeter and the sample prepared outside the calorimeter are captured along with the measurement signal, leading to a significant measurement error. After a waiting period of 30-60 minutes, the stabilization of the baseline indicates that the additional heat generated by the temperature difference, not released during hydration, has been completely dissipated, and the heat subsequently released can be attributed solely to the sample.

[0005] This effect can be clearly demonstrated with an inert (sand) sample. This sample does not react upon the addition of water or an excitation liquid and releases no energy from a chemical reaction. Therefore, in an ideally temperature-controlled system, the sample should not release any measurable heat when introduced into a calorimeter cell. If the sample is not at the same temperature as the calorimeter's interior, a deviation from the zero line is observed even for the inert sample: negative values ​​if the sample is colder than the calorimeter's interior temperature, and positive values ​​if the sample is warmer. These observations are technically explained by a heat flow from the heat sink to the sample and vice versa. The isothermal calorimeter is thus an ideal instrument for determining absolute temperature differences in inert samples.

[0006] In a real binder sample, however, unlike in an inert sample, the heat generated by sample hydration and the heat generated by a temperature difference between the sample and the calorimeter thermostat cannot be distinguished. This leads to a systematic measurement error when measurements are taken during a period in which this error is still significant. However, it is known that within 30-60 minutes, the heat generated by the sample temperature is completely or largely dissipated via the heat sink. Therefore, reliable measurements are only possible after a waiting period of 30-60 minutes.

[0007] In practice, two methods have been proposed to determine the initial heat release within the first 30–60 minutes of the reaction. In the in-situ measurement, powder and water are introduced separately into the calorimeter cell in a specialized ampoule. The binder and mixing water are temperature-stabilized in separate containers within the calorimeter chamber. After temperature stabilization, usually for several hours (typically 2 to 20 hours), the water container is opened externally via a mechanism, allowing the water to flow into the powder. Finally, a paste of water and binder is mixed within the calorimeter cell, either manually or mechanically using a stirring device. The measurement begins upon the addition of the water. This method is not used in routine measurements and cannot be automated.Reasons for this include the long waiting time for temperature stabilization of the individually introduced paste components in the calorimeter and thus only a low sample throughput, the mixing quality which cannot be verified due to lack of visual contact, and the specialized ampoules and stirrers which are only partially reusable or difficult to clean.

[0008] WO 2016 / 041717A1, WO 2017 / 162639A1, and US 10352885B2 employ a different approach: performing the entire sample preparation and optional intermediate storage of the sample in a sample preparation chamber with direct access to the calorimeter. The temperature equality of the sample preparation and calorimeter chambers minimizes the temperature difference between the samples upon introduction into the calorimeter chamber, and the isothermal calorimetric measurement can begin immediately after sample introduction without delay and without waiting for baseline stabilization.

[0009] In practice, a sample preparation chamber is maintained at a constant, precisely controlled temperature. Optionally, a sample storage area can also be provided, as proposed, for example, in WO 2023 / 094419. The sample storage area is stabilized with a second temperature controller to more quickly compensate for any sample temperature difference between the ambient temperature of the temperature-controlled sample preparation chamber and the sample preparation chamber itself. The isothermal calorimeter connected to the sample preparation chamber has its own temperature controller for monitoring and regulating the temperature within the calorimeter chamber containing the calorimeter cells.

[0010] Temperature measurement with thermocouples is very well suited for measuring relative temperature differences when the ambient temperature changes. Measuring absolute temperatures precisely with thermocouples is more difficult. This is only possible if the thermocouples have been precisely calibrated and the typically occurring variable conductivity characteristics and manufacturing-related deviations are compensated for. For this reason, the absolute offset of thermocouples is often determined, for example, in ice water (0 °C) or boiling water (100 °C), and then the temperature measurement is corrected with a specific correction value.

[0011] Another difficulty, particularly with larger volumes, is that the temperature at the thermocouple's installation location may differ from the setpoint on the controller, which, due to vertical temperature stratification, reflects the temperature introduced into the volume, including the thermocouple's display error. Finally, dynamic air currents within a volume can also create temperature differences, even in a closed cell.

[0012] The procedure according to WO 2016 / 041 717 A1, WO 2017 / 162 639 A1, or US 10 352 885 B2 requires, during initial commissioning and, if necessary, during ongoing operation, an adjustment of the temperatures in the sample preparation chamber with the internal temperature of the calorimeter (calorimeter chamber) to ensure consistently reliable measurements based on temperature equilibrium and to prevent errors due to temperature differences between the sample and the isothermal calorimeter. Direct calibration of the thermocouples relative to each other is not possible due to the installation situation in the instruments. For this reason, the temperature of the sample preparation chamber and the temperature in the calorimeter are adjusted using an inert sample to determine temperature differences between the sample preparation chamber and the calorimeter interior. The controller settings are then adjusted so that the temperature differences approach zero.

[0013] For this purpose, one can, for example, introduce inert samples into the calorimeter at variable temperatures and establish a correlation between the measurement signal and the sample temperature. From this correlation, the necessary adjustment to the temperature controllers can then be estimated and iteratively optimized based on the observed temperature difference. In practice, the temperature of the calorimeter's thermostat is used as a fixed point, and in repeated measurements, the controller setting for the heating / cooling unit of the sample preparation chamber is initially adjusted so that, upon reintroduction of the inert sample, the measurement signal shifts towards the zero line. The endpoint is reached when no measurement signal or a negligible measurement signal can be detected for the inert sample using the isothermal calorimeter.

[0014] The process becomes more complex—particularly time-consuming due to waiting times—when a second or more calorimeter chambers are connected to a shared sample preparation setup. In this case, the internal temperature of the calorimeter chambers typically differs initially due to multiple thermocouples, as does the temperature of the sample preparation chamber. Therefore, adjustment of all subsequent calorimeter chambers with the first calorimeter chamber is necessary to ensure that, ultimately, an inert sample, regardless of its location in the first or any of the subsequent calorimeter chambers, produces a correct measurement signal close to zero. It is evident that the adjustment of the setpoints becomes more complex with an increasing number of calorimeter chambers.Temperature equalization of the calorimeter chambers is essential, as otherwise, due to the altered reaction kinetics caused by the unequal internal temperatures, comparable measurements cannot be carried out in different calorimeter chambers.

[0015] The object of the invention is to create an automated control system that detects real temperature differences at different components in an automated calorimeter and regulates the controllers of the cooling / heating units in an automated procedure despite differing temperature readings, so that one or more calorimeters and the sample preparation chamber can be operated permanently at the same or a sufficiently similar temperature without measurement errors and the effort required for temperature adjustment is minimized.

[0016] This problem is solved by the method with the features specified in claim 1. Advantageous further developments are described in the dependent claims, the following description, and the drawings.

[0017] The method according to the invention serves to calibrate a heat flow calorimeter with at least one sample preparation chamber and at least one first calorimeter chamber. The calorimeter chamber is where the actual measurement of the heat flow is performed. Industrially used calorimeters often have eight or more calorimeter cells that can be operated in parallel within a single calorimeter chamber, all of which can be equivalent. The selection of the first calorimeter chamber from the total can then be defined arbitrarily. Simultaneously, the calorimeter chamber can contain two or more calorimeter cells, each capable of performing an independent measurement as a calorimeter, but all calorimeter cells within the calorimeter chamber are jointly temperature-controlled by a single, common temperature control system. Therefore, for the method according to the invention, the calorimeter cells are indistinguishable and equivalent.The sample preparation chamber has a preparation chamber temperature control system and a preparation chamber thermocouple. The first calorimeter chamber has a first calorimeter temperature control system and a first calorimeter thermocouple. The thermocouple serves to measure the temperature, which is then used to regulate the temperature control. However, since absolute temperatures are difficult to measure with a thermocouple, as already explained, a difference arises between the different thermocouples even at the same measured temperature. This means that the different temperature control systems are regulated to slightly different temperatures, which in turn leads to a sample being transported from one area to another having a different temperature than the area to which it is being transported. This, in turn, distorts the measurement of the heat flow in the calorimeter. The heat flow calorimeter has a control unit.The control unit is connected to the preparation chamber thermocouple and the first calorimeter thermocouple for transmitting the measured temperatures to the control unit. Furthermore, the control unit is connected to the preparation chamber temperature control and the first calorimeter temperature control for control purposes. The process comprises the following steps: a) Introducing an inert sample and an excitation liquid into the sample preparation chamber, b) Tempering the inert sample and the excitation liquid in the sample preparation chamber, c) Introducing a mixture of the inert sample and the excitation liquid into the calorimeter chamber, d) Determining the heat flow into or out of the mixture of the inert sample and the excitation liquid, e) Evaluating the heat flux recorded in step d) and distinguishing in case I) a positive heat flux out of the mixture of the inert sample and the excitation liquid and in case II) a negative heat flux into the mixture of the inert sample and the excitation liquid, f) In case I) lowering the temperature of the preparation chamber temperature control and / or raising the temperature of the first calorimeter temperature control, and in case II) raising the temperature of the preparation chamber temperature control and / or lowering the temperature of the first calorimeter temperature control, g) Repeat until in step e) no heat flow is detected within the limits of measurement accuracy and thus the preparation chamber temperature and the first calorimeter temperature are set to the same absolute temperature.

[0018] The introduction in step a) can be done from the outside, as would normally be the case for samples. However, since this is only an internal calibration with an inert sample, the same sample, already as a mixture, can be reused, as explained below. Therefore, the introduction in step a) can also be done from the calorimeter chamber. Depending on the introduction method, the subsequent tempering step b) can vary in length. For example, if a sample is reused and introduced from the calorimeter chamber into the sample preparation chamber, the temperature difference will be smaller than for a sample introduced from the outside, so a shorter tempering time is sufficient.

[0019] Between tempering in step b) and introduction in step c), mixing takes place, provided the inert sample and the excitation liquid are separate in step b). This is advantageous because mechanical work is also performed during mixing, thus introducing heat. However, if the sample is "reused," it is already mixed, so remixing is unnecessary. Nevertheless, the process can be carried out to retain the energetic aspect of mixing.

[0020] The data acquisition in step d) takes place within a time window of up to 60 minutes. After this time, all thermal effects have subsided. Therefore, according to current best practices, the measurement is also started after this time window.

[0021] The evaluation in step e) is preferably carried out by integration over the time course, i.e., by determining the total amount of heat released or absorbed, as is otherwise common practice in calorimetry.

[0022] The amount of heat transferred determines whether the sample preparation chamber is warmer or colder than the first calorimeter chamber. Accordingly, in step f), an adjustment is made until no difference can be detected and the sample preparation chamber and the first calorimeter chamber have the same absolute temperature.

[0023] The advantage of this method is that it can be carried out automatically to enable the absolute calibration of different thermocouples to each other and thus allow an immediate start of measurements in the calorimeter, which in turn allows for the evaluation of the initial peak, which is necessary to obtain rapid feedback on the reactivity in the cement area.

[0024] In a further embodiment of the invention, the heat flow calorimeter has a second calorimeter chamber. This second calorimeter chamber can also have two or more calorimeter cells. As already mentioned, a larger number, for example eight calorimeter chambers, is common. The second calorimeter chamber has a second calorimeter temperature control unit with a second calorimeter thermocouple. First, the procedure for the first calorimeter chamber is carried out up to step g). This involves first aligning the sample preparation chamber with the first calorimeter chamber. This ensures that the setting for the sample preparation chamber can no longer be changed. Therefore, the procedure for the second calorimeter chamber is then carried out, wherein in step f) the temperature of the second calorimeter temperature control unit is lowered in case I) and the temperature of the second calorimeter temperature control unit is raised in case II).Only the second calorimeter chamber is adjusted, bringing it to the same temperature level as the first calorimeter chamber and the sample preparation chamber. This process can then be repeated for additional calorimeter chambers. This ensures that all calorimeter chambers are successively brought to precisely the same temperature, allowing each sample to be measured in every calorimeter chamber and yielding reproducible results.

[0025] In a further embodiment of the invention, the heat flow calorimeter has a sample storage chamber. The sample storage chamber is arranged upstream of the sample preparation chamber in the normal sample flow and can be used to introduce and pre-temper a larger number of samples. The sample storage chamber includes a sample storage temperature control unit and a sample storage thermocouple. First, the procedure for the first calorimeter chamber is carried out until step g) is reached, thus setting the sample preparation chamber to the final temperature. Subsequently, the inert sample and the excitation fluid are introduced into the sample storage chamber, and then, in step a), the sample is introduced from the sample storage chamber into the sample preparation chamber. Then, in step f), in case I), the temperature of the sample storage temperature control unit is lowered, and in case II), the temperature of the sample storage temperature control unit is raised.This ensures that the sample storage chamber is set to a temperature that does not affect the measurement. Due to the sample processing in the sample preparation chamber, it can be assumed that the absolute temperature of the sample storage chamber only needs to be within a certain temperature range.

[0026] In a further embodiment of the invention, quartz, corundum, titanium dioxide, or a mixture thereof is selected as the inert sample. These exhibit the best comparability to clinker or cement, for example with regard to heat capacity and other properties, but show no reaction when mixed with water or an aqueous solution, for example, even a weak aqueous alkali.

[0027] In a further embodiment of the invention, water or an aqueous solution, in particular a basic aqueous solution, is used as the excitation liquid.

[0028] In a further embodiment of the invention, the evaluation in step e) is quantitative, and the adjustment in step f) is proportional to the quantitative deviation. Thus, the larger the deviation, the faster the adjustment will be. This has the advantage of reducing the number of iteration steps.

[0029] In a further embodiment of the invention, the adjustment is carried out in predetermined steps. The heat flow is determined for each adjustment. The setpoint for the zero crossing, and thus the optimal temperature setting, is determined by means of linear regression.

[0030] In a further embodiment of the invention, a waiting period is observed before repeating the process steps according to step f). The waiting period is selected depending on the degree of adjustment.

[0031] In a further embodiment of the invention, the mixture of the inert sample and the excitation liquid is reused. Since this mixture is already at the correct temperature, this significantly speeds up the execution of the process.

[0032] In a further embodiment of the invention, the detection in step d) takes place over a period of 2 min to 40 min, each time starting with the introduction in step c).

[0033] The method according to the invention is explained in more detail below with reference to an embodiment shown in the drawings. Fig. 1 Heat flow calorimeter Fig. 2. Temperature dependence

[0034] In Fig. Figure 1 shows an exemplary, schematic heat flow calorimeter. The heat flow calorimeter consists of a pre-existing sample storage chamber 50, a central sample preparation chamber, and three calorimeter chambers 21, 22, and 23. The sample storage chamber 50 is temperature-controlled by the sample storage temperature control unit 51, the sample preparation chamber 10 is temperature-controlled by the preparation chamber temperature control unit 11, the first calorimeter chamber 21 is temperature-controlled by the first calorimeter temperature control unit 31, the second calorimeter chamber 22 is temperature-controlled by the second calorimeter temperature control unit 32, and the third calorimeter chamber 23 is temperature-controlled by the third calorimeter temperature control unit 33. Thermocouples 12, 41, 42, 43, and 52 are provided for temperature control of the calorimeter chambers 11, 31, 32, 33, and 51. The relative calibration of these, which has already been carried out, is particularly relevant.The sample storage chamber 50 has a sample storage thermocouple 52, the sample preparation chamber 10 has a preparation chamber thermocouple 12, the first calorimeter chamber 21 has a first calorimeter thermocouple 41, the second calorimeter chamber 22 has a second calorimeter thermocouple 42, and the third calorimeter chamber 23 has a third calorimeter thermocouple 43. All temperature control units 11, 31, 32, 33, 51 and all thermocouples 12, 41, 42, 43, 52 are connected to a control unit 60, which monitors the temperatures and controls the temperature control units 11, 31, 32, 33, 51 accordingly.

[0035] Fig.Figure 2 shows the relative calibration of the thermocouples between two areas, for example, between sample preparation chamber 10 and the first calorimeter chamber 21. In this case, the temperature T in sample preparation chamber 10 is varied, for example, in steps of 0.1 K. The heat flux E is determined for each of these points. The zero crossing, i.e., the setting at which the two areas, for example, sample preparation chamber 10 and the first calorimeter chamber 21, have exactly the same temperature, can easily be found in the graph. This then allows for immediate measurement to begin after the sample is introduced and thus enables the evaluation of the initial peak. Reference sign 10 Sample preparation chamber 11 Preparation chamber temperature control 12 Preparation chamber thermocouple 21 first calorimeter chamber 22 second calorimeter chamber 23 third calorimeter chamber 31 First calorimeter temperature control 32 second calorimeter temperature control 33 third calorimeter temperature control 41 first calorimeter thermocouple 42 second calorimeter thermocouple 43 third calorimeter thermocouple 50 Sample storage chamber 51 Sample storage temperature control 52 Sample storage thermocouple 60 Control unit QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2016 / 041 717 A1 [0008, 0012] WO 2017 / 162 639 A1 [0008, 0012] US 10 352 885 B2 [0008, 0012] WO 2023 / 094 419

[0009]

Claims

[1] Method for calibrating a heat flow calorimeter with at least one sample preparation chamber (10) and at least one first calorimeter chamber (21), wherein the sample preparation chamber (10) comprises a preparation chamber temperature control (11) and a preparation chamber thermocouple (12), wherein the first calorimeter chamber (21) comprises a first calorimeter temperature control (31) and a first calorimeter thermocouple (41), wherein the heat flow calorimeter comprises a control unit (60), wherein the control unit (60) is connected to the preparation chamber thermocouple (12) and the first calorimeter thermocouple (41) for transmitting the measured temperatures to the control unit (60), wherein the control unit (60) is connected to the preparation chamber temperature control (11) and the first calorimeter temperature control (31) for control, wherein the method comprises the following steps: a) Introducing an inert sample and an excitation liquid into the sample preparation chamber (10), b) Tempering the inert sample and the excitation liquid in the sample preparation chamber (10), c) Introducing a mixture of the inert sample and the excitation liquid into the calorimeter chamber (21), d) Determining the heat flow into or out of the mixture of the inert sample and the excitation liquid, e) Evaluating the heat flux recorded in step d) and distinguishing in case I) a positive heat flux out of the mixture of the inert sample and the excitation liquid and in case II) a negative heat flux into the mixture of the inert sample and the excitation liquid, f) In case I) lowering the temperature of the preparation chamber temperature control (11) and / or raising the temperature of the first calorimeter temperature control (31) and in case II) raising the temperature of the preparation chamber temperature control (11) and / or lowering the temperature of the first calorimeter temperature control (31), g) Repeat until in step e) no heat flow is detected within the limits of measurement accuracy and thus preparation chamber temperature control (11) and first calorimeter temperature control (31) are set to the same absolute temperature. [2] Method according to claim 1, characterized by, that the heat flow calorimeter has a second calorimeter chamber (22), wherein the second calorimeter chamber (22) has a second calorimeter temperature control (32) and a second calorimeter thermocouple (42), wherein the procedure for the first calorimeter chamber (21) is carried out first until step g) is reached, wherein the procedure for the second calorimeter chamber (22) is then carried out, wherein in step f) in case I) the temperature of the second calorimeter temperature control (32) is lowered and in case II) the temperature of the second calorimeter temperature control (32) is raised. [3] Method according to any of the preceding claims, characterized by, that the heat flow calorimeter has a sample storage chamber (50), wherein the sample storage chamber (50) has a sample storage temperature control (51) and a sample storage thermocouple (52), wherein the procedure for the first calorimeter chamber (21) is carried out first until step g) is reached, wherein the inert sample and the excitation liquid are then introduced into the sample storage chamber (50) and then the sample preparation chamber (10) is introduced from the sample storage chamber (50) in step a), wherein in step f) in case I) the temperature of the sample storage temperature control (51) is lowered and in case II) the temperature of the sample storage temperature control (51) is raised. [4] Method according to any of the preceding claims, characterized by that quartz, corundum, titanium dioxide or a mixture thereof is chosen as the inert sample. [5] Method according to any of the foregoing claims, characterized bythat water or an aqueous solution, in particular a basic aqueous solution, is used as the excitation liquid. [6] Method according to any of the foregoing claims, characterized by , that the evaluation in step e) is quantitative and that the adjustment in step f) is proportional to the quantitative deviation. [7] Method according to any of the foregoing claims, characterized by , that a waiting period is observed before repeating the procedural steps according to step f), the waiting period being chosen depending on the level of the adjustment. [8] Method according to any of the foregoing claims, characterized by that the mixture of the inert sample and the excitation liquid is reused. [9] Method according to any of the foregoing claims, characterized by , that the collection in step d) takes place over a period of 2 min to 40 min after the introduction in step c).

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