CN115728348A - Method for measuring activation energy in thermal reaction process - Google Patents
Method for measuring activation energy in thermal reaction process Download PDFInfo
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Abstract
A method for measuring the activation energy of a thermal reaction process is provided, wherein the activation energy of the reaction is obtained by fitting calculation by measuring the conversion rate of the reaction process represented by mass flow, heat flow and/or mass spectrum signals of a specific product or reactant in a normal section of the thermal reaction process and the true temperature and true temperature change rate of a sample. The method does not need to estimate the mechanism function or the mode function of the reaction in advance, and the analysis result is established on an objective test rather than relying on subjective prejudgment, so the reliability of the result is high.
Description
Technical Field
The invention relates to the technical field of thermal analysis, in particular to a method for analyzing the kinetic characteristic parameter, namely activation energy of a thermal reaction process by utilizing a thermal analysis technology and a mass spectrum combined test means, wherein the kinetic characteristic parameter comprises the activation energy of analyzing elementary reaction processes, physical processes (such as phase change and crystal form transformation), volatilization, desorption and adsorption.
Background
The classical kinetic studies of chemical reactions basically use the following formula:
dx/dt＝f(x)k(T) (1)
where dx/dt represents the rate of change of the variable x, and f (x) and k (T) are functions of the variable x and the temperature T, respectively.
The most commonly used temperature function is the Arrhenius formula:
k(T)＝A exp(-E/RT) (2)
where A is called the pre-exponential factor, E is the activation energy, and R is the gas constant.
At present, the measurement and analysis of the kinetic characteristic parameter of the thermal reaction process, namely activation energy, are mainly based on a conventional kinetic equation. For an isothermal process its form is [1]:
for non-isothermal processes then:
in the two formulas, E is activation energy, A is a pre-factor, beta is a heating rate, alpha is a conversion rate in the reaction process, R is a gas constant, T is time, T is temperature, and f (alpha) is a reaction kinetic mode function.
The conversion relation between the conversion rate alpha, temperature difference, heat flow and time or temperature in the reaction process can be measured by various thermal analysis means, such as thermogravimetry, differential thermal analysis, differential scanning calorimetry (hereinafter abbreviated as TG, DTA, DSC) and the like, and the characteristic parameter E1 of the reaction kinetics is obtained by the two formulas through very complicated calculation analysis.
For example, the Kissinger method, which is commonly used in thermal analysis, first considers that the mode function of the reaction process is an nth-order function, n is the reaction order,
f(α)＝(1-α) n (5)
the equation is substituted into equation (4) and is converted into a linear equation with specific parameters, the activation energy is calculated by fitting the test results under different heating rates for many times, and the equation expression of linear fitting is shown as equation 6.
T pi Is composed of
At extreme valueTemperature of time [2 ]]。
The method is taken as a traditional method, the main source of the method is that the previous analytical equation of the homogeneous reaction power is borrowed, and the method is converted from the previous analytical equation simply and directly without real physical meaning and explanation. The conversion relationship is as follows:
wherein f (C) is a reaction mechanism function, and k (T) is a reaction rate function [3,4].
Regardless of the form of the kinetic equations, homogeneous or heterogeneous reaction systems, and isothermal or non-isothermal processes, there are many international discussions of how to test and analyze characteristic parameters of the reaction kinetics, which are mainly summarized as follows:
1. the reaction mechanism function f (C) or the reaction kinetic pattern function f (α) must be determined in advance, and although efforts by numerous scholars have given dozens (or even more) of functional relationships, these relationships are merely guesses and estimates of the reaction mechanism [5,6,7]. Because different types of reaction systems are complex, and the form of the function is more difficult to determine for various unknown reactions, the reliability of measurement and analysis is seriously influenced, and many analysis processes are difficult to carry out reverse-extrapolation verification and extrapolation calculation. Also, even if the above estimated functional relationship is able to calculate the dynamics of the reaction process, there may be a lack of rationality [8]. In summary, the reaction mechanism function (or mode function) itself has limited the application of this type of test method.
2. The conversion rate α in the analytical relation is substantially different from the concentration C because the introduction of the concentration concept defaults that the kind of the analysis object is definite, while the conversion rate α in the reaction process is actually unknown for the kind of the analysis object, which means that the conversion rate α represents the comprehensive progress of all the substances in the reaction process, i.e. the total package reaction. The change of the data is very complex and the connotation is very rich, and the data is formed by the cross integration of a plurality of elementary reaction steps [3], so that no clear theoretical basis exists for the data to serve as the characteristic parameters of the analysis reaction.
3. Since the kinetic mechanism of the thermal reaction process changes with the change of temperature, a complicated mathematical problem, namely temperature integration, needs to be introduced in the calculation process of the method, and the thermal reaction process has large endothermic or exothermic characteristics, so that the actual temperature of the sample changes nonlinearly and even has a very strong self-heating or cooling phenomenon. However, the existing analytic methods often adopt simplified fitting to the test process, which affects the reliability of the test analysis results [3].
4. The method can not analyze elementary reaction in complex process from the principle of kinetic equation, and even does not know the conversion rate of each elementary reaction for the test sample or process with unknown or unknown composition, unless the reaction process is a single elementary reaction process. For the intermediate processes of various types of reaction processes and the influence of elementary reactions on each other, it is even more impossible to test and analyze [4].
5. The characteristic parameters of the reaction kinetics obtained by the test and analysis according to the method are difficult to objectively verify, namely, the real change of the reaction process is difficult to calculate, and meanwhile, the condition change working condition is difficult to extrapolate, so the reliability and the applicability of characteristic parameter estimation have great problems [1].
In short, the traditional test analysis method is based on the existing reaction kinetic process, has large limitation and cannot correctly understand the reaction process, and the reaction mechanism models need to be estimated in advance, and the models are mathematical corrections for ensuring the effective calculation of a formula and cannot be verified and proved in a physical sense, namely, the selection of the model cannot clearly determine why the model is selected but not the other model, and the determination of the model parameters cannot give a reasonable explanation. Therefore, the method for measuring the reaction characteristic parameters based on the estimation model has the essential problem, so that the traditional test analysis method is difficult to obtain the correct result.
List of references:
1. hu Rongzu, high victory, et al. Thermal analysis kinetics. Beijing: scientific Press, 2008,2-18
2. Liu Changwei, xi Tonggeng, thermal mass spectrometry, shanghai: shanghai science and technology literature Press, 2002, 104-106
3.Vyazovkin S，Wight C A.Int.Reviews in Physical Chemistry，1998，17(3)：407-433
4.Vyazovkin S.Int.Reviews in Physical Chemistry，2000，19(1)：45-60
5.Koga N，Tanaka H J.Thermal.Anal.，1994，41(2-3)：455-469
6.Sestak J.J.Thermal.Anal.，1990，36(6)：1997-2007
7.Galwey AK，Brown B E.Thermal Decomposition of Ionic Solid，Elsevier：Amsterdam，1999
8.Vyazovkin S.Thermochim.Acta，2000，357～358：133-140
Disclosure of Invention
The invention provides a novel method for measuring activation energy in a thermal reaction process, which obtains the activation energy of the reaction by measuring the conversion rate (which can be represented by mass flow, heat flow, mass spectrum signals and the like of a specific product or a reactant) of the specific product or the reactant in the thermal reaction process and the real temperature change rate of a sample through fitting calculation.
According to an aspect of the present invention, there is provided a method of measuring activation energy of a thermal reaction process, comprising:
(1) Placing the sample in a thermal analysis device;
(2) Thermally reacting the sample by a change in temperature;
(3) In the normal phase of the thermal reaction process, i.e. the conversion rate of the reaction process
(conversion rate in the reaction process)
Represented by mass flow, heat flow and/or mass spectral signals of a particular product or reactant characterizing the thermal reaction process) reaches a maximum,
measuring the real temperature T and the real temperature change rate beta of the sample, namely the time differential of the temperature T; and is
Measuring the conversion of said specific product or reactant
Calculating its marginal conversion
Wherein the content of the first and second substances,
marginal conversion for time differential of conversion in the course of the reaction
Reflecting the specific product or reactant conversion per unit of reaction time
A variation amount of;
(4) Performing parameter fitting according to the following formula to obtain the activation energy E of the thermal reaction process:
wherein, the first and the second end of the pipe are connected with each other,
is an external impulse, i.e., an external impulse generated by the change of the external temperature to the reaction process, E is the reaction activation energy, and R is the gas constant.
According to some embodiments, the reaction process conversion rate
Derived from at least one of thermogravimetric data (TG or DTG) and mass spectral data (MS) of the sample, or from at least one of thermal flow Data (DSC), differential thermal Data (DTA) and mass spectral data (MS).
According to some embodiments, the thermal reaction process is a process involving endotherms or exotherms, including chemical reactions, phase changes, crystal form transformations, volatilization, desorption, and adsorption.
According to some embodiments, the thermal reaction process is a homogeneous system reaction, or a heterogeneous system reaction in which at least one reactant and at least one product are in different phases, including a pyrolysis process or a gas-solid reaction process.
According to some embodiments, the temperature variation mode includes a temperature-increasing mode and a temperature-decreasing mode, wherein the temperature-increasing mode includes an isothermal process, a linear temperature increase, a modulated temperature increase, and a random temperature increase.
According to some embodiments, a modulated or random temperature increase is selected that approximates the actual temperature of the sample.
According to some embodiments, the activation energies obtained under different temperature variation patterns are the same.
According to some embodiments, the thermal reaction process is carried out in an inert atmosphere, or in a reactive gas atmosphere.
According to some embodiments, the fitting employs a least squares method.
According to some embodiments, the method further comprises: by changing the conditions of the reaction process, the change characteristics of the reaction process are calculated by using the activation energy obtained by the test for verification, wherein the change characteristics comprise mass flow, heat flow and mass spectrum data.
The method does not need to estimate the mechanism function or the mode function of the reaction in advance, and the analysis result is established on an objective test rather than relying on subjective prejudgment, so the reliability of the result is high.
Drawings
FIG. 1 is a schematic flow diagram of an activation energy test analysis of a thermal reaction process according to the present invention;
fig. 2 is a schematic diagram of various temperature measuring points of a typical thermal analysis device, wherein a: a schematic view of a vertically arranged furnace body; b: a schematic view of a horizontally arranged heating furnace body;
FIG. 3 is a schematic diagram of four temperature ramp processes;
FIG. 4 shows CaCO in example 1 3 Temperature and rate of temperature rise during pyrolysis;
FIG. 5 shows CaCO in example 1 3 Comparison of calculated mass flow of evolved gas with test weight loss rate (DTG) in pyrolysis tests (in the figure, the designation Cai is CO calculated according to kinetic characteristic parameters 2 Mass flow rate, DTG is the sample weight loss rate directly measured by the test equipment);
FIG. 6 shows CaCO in example 1 3 Activation energy test analysis during thermal decomposition (M in the figure) f To escape gas CO 2 Marginal conversion of (2), M e As the reaction external impulse);
FIG. 7 is a measured temperature versus temperature increase rate curve for the moderate temperature process + modulated temperature increase procedure of example 2;
FIG. 8 shows 30mg CaCO in example 2 3 A mass spectrum chart of the thermal decomposition process (the side surface in the graph is a schematic curve of weight loss rate DTG and weight loss TG);
FIG. 9 shows 30mg CaCO in example 2 3 CO in thermal decomposition 2 A mass flow curve;
FIGS. 10a to 10c show CaCO in example 2 3 Marginal conversion of reaction during thermal decomposition with external impulse (M in the figure) f To escape gas CO 2 Marginal conversion of (2), M e As the reaction external impulse);
FIG. 11 shows CaCO in example 3 3 Weight loss Rate (DTG) and CO of isothermal pyrolysis test 2 Calculating the mass flow rate;
FIG. 12 shows CaCO in example 3 3 Marginal conversion and mass flow data for evolved gases in isothermal pyrolysis tests (M in the figure) f For evolution of gas CO 2 Marginal conversion of (D), M e As the reaction external impulse);
FIG. 13 shows CaCO in example 4 3 A weight loss rate (DTG) and temperature rise rate curve of a random temperature rise pyrolysis process;
FIG. 14 shows CaCO in example 4 3 Marginal conversion and external impulse of evolved gases in random temperature rise pyrolysis test (M in the figure) f To escape gas CO 2 Marginal conversion of (D), M e As the reaction external impulse);
FIG. 15 shows CaCO in example 4 3 Comparing the calculated mass flow of the escaping gas in the random heating pyrolysis test with the test weight loss rate (DTG) data;
FIG. 16 is CaCO of example 4 3 Comparing the calculated weight loss with the test weight loss (TG) data in the random heating pyrolysis test;
FIG. 17 shows DTG and TG data on the thermal decomposition of silicone rubber in example 5;
FIG. 18 shows a mass spectrum of thermal decomposition of silicone rubber in example 5;
FIG. 19 shows PI mass spectra at the maximum weight loss temperature point during the thermal decomposition of silicone rubber in example 5;
FIG. 20 shows the major ion shift curves (numbers marked in the figure correspond to the mass-to-nuclear ratio m/z) for PI during thermal decomposition of silicone rubber at elevated temperature in example 5;
FIGS. 21a-21d show the marginal conversion to external impulse (M in the graph) of the main evolved gas during thermal decomposition of silicone rubber at modulated elevated temperature in example 5 f To escape gas CO 2 Marginal conversion of (D), M e As the reaction external impulse);
FIG. 22 shows a three-dimensional graph of the rate of weight loss (DTG) measured during carbon dioxide ambient gasification of carbon dust in example 6;
FIG. 23 shows the weight loss (TG), weight loss rate (DTG) and temperature rise rate curves of the carbon powder carbon dioxide atmosphere gasification process in example 6;
FIG. 24 shows the marginal conversion rate and external impulse (M in the graph) of carbon powder carbon dioxide ambient gasification in example 6 f To escape gas CO 2 Marginal conversion of (D), M e To reflect external impulse).
Detailed Description
The invention reconstructs a general reaction kinetic equation based on the actual characteristics of material conversion in the reaction process, is suitable for various thermal reaction processes, particularly the reaction of a heterogeneous system, and more importantly, can determine the kinetic characteristics of elementary reaction by utilizing mass spectrum data. The test result faces to a real reaction process and is not influenced by external parameter changes, such as different sample amounts and random temperature rise processes, the reliability and repeatability of the test analysis result are very good, and the data result can be used for extrapolation analysis.
Brief introduction to testing principles
The kinetic equation adopted by the invention is constructed by taking time as a variable according to the mass change of a specific product or reactant in the thermal reaction process and combining the characteristic of positive and negative reaction interaction influence, and the specific form is as follows:
first item of the above formula
Is the marginal conversion of a particular substance during the reaction, of
For conversion of a particular substance (a particular product or a particular reactant) with respect to mass to be able to characterize a reaction process, e.g. as CaCO 3 During the thermal decomposition, with CO 2 For a particular substance that characterizes a thermal decomposition process, its mass flow rate can reflect the actual conversion rate of the process;
is the time differential of the mass flow; marginal conversion
By (a) is meant the specific substance per conversion during the reaction
The growth amounts below represent the trend of the reaction process.
Second item of the above formula
Is the external impulse of the reaction process, where E is the activation energy of the reaction, R is the gas constant, T is the true measured temperature of the sample, β is the true rate of temperature rise of the sample, i.e., the time derivative of the temperature T, and thus,
representing the external impulse generated by the external temperature change to the reaction process.
Item three of the above formula
Is the internal inertia of the reaction process, wherein A is a pre-exponential factor,
a multiple reaction factor, representing the degree of interaction between reactions in a multiple reaction process, is 1 for an independent reaction process (i.e., the reaction process under study during which the reactants and products do not have any relationship to other reactions) and;
the whole term represents the internal inertia of the reaction process itself.
Formula (I) can be understood simply as: the amount of change in a particular species (first term, marginal conversion) is determined by the difference between the external effect (second term, external impulse) that pushes the reaction forward and the resistance that prevents the reaction forward or the effect (third term, internal inertia) that pushes the reaction backward.
The above reaction kinetics relationship has the following characteristics:
because the marginal conversion rate is determined by the mass flow rate of a specific substance, which can be directly measured by mass spectrometry, thermogravimetry, calorimetry and other technical means, the measurement value does not need to determine the initial mass of the reaction sample in advance, which is very important for researching unknown mixture, elementary reaction and intermediate reaction, because the initial reaction mass of the reaction process is unknown or even can not be measured.
The external impulse of the reaction is determined by the true temperature and the activation energy of the sample, which can be determined entirely by the true temperature and the rate of temperature rise of the sample if the activation energy is considered as an assumed predictive value; secondly, the rate of temperature rise has a very large effect on the external impulse of the reaction compared to the true temperature of the sample.
The internal inertia term of the reaction is generally resistance in the temperature rise process, the influence in the normal section of the reaction is very weak, and the influence is far lower than the external impulse, so that the normal section (front section process) of the reaction is mainly considered in the process of measuring the activation energy, and the first two terms in the formula (I) are used for parameter fitting comparison, so that the real value of the activation energy can be obtained.
For processes with multiple reaction effects, if the characteristics of elementary reactions, intermediate reactions, etc. need to be determined, the interaction between the reactions can be reduced by using a variable random temperature rise process, a change in interferents, and a change in test atmosphere.
The specific steps of the test
One process of the activation energy testing process is shown in fig. 1, the process is suitable for measurement of various reaction processes, the key point is selection of testing conditions, and equipment for implementing the test comprises thermogravimetry, a thermal analyzer, a mass spectrum and the like, wherein when the mass spectrum test is adopted, the equipment is not necessarily combined with the thermal analyzer, the key point is to accurately test the real temperature of a sample, but not the reference point temperature or the furnace body temperature, and the relationship of the three is shown in fig. 2.
The specific implementation steps of the reaction process activation energy test can comprise:
1. selection of a sample: according to the characteristics of the test sample, a small amount of sample is selected for testing, under the condition of ensuring the validity of the test signal, different sample amounts can be adopted for testing, and the sample amount is selected according to the principle that the signal-to-noise ratio S/N of the peak value of the test signal is at least larger than 10. For an unknown substance or a test sample containing an interfering substance, the uniformity of sampling needs to be ensured, the particle size of the sample is reduced as much as possible, and the interfering substance test needs to ensure that the sample is fully mixed.
2. Setting temperature rising conditions: the temperature raising program comprises an isothermal process, linear temperature raising, modulated temperature raising, random temperature raising and the like. Since the reaction is carried out in the process of reaction, the phenomena of heat release and heat absorption can occur, and the self-heating and self-cooling of the sample are caused, so the temperature setting program is only a preliminary set value, and the actual temperature rise process is based on the actual test temperature at the sample. The temperature raising program is selected mainly according to the characteristics of a tested sample, linear temperature raising can be selected for a reaction system with known characteristics and relatively single characteristics, and modulation temperature raising or random temperature raising close to the actual condition is selected for a complex reaction process or a verification link. The modulation temperature rise refers to adding a periodic temperature rise process to the temperature rise process on the basis of normal linear temperature rise or isothermal temperature rise, wherein the periodic temperature rise can be a sine alternating signal or a periodic temperature rise process of pulse, triangle or step. The random temperature rise is executed according to any temperature rise process without periodic signal characteristics. The four temperature raising processes are schematically shown in fig. 3. It is noted that in addition to the temperature increase, other measures may be taken, such as temperature reduction, pressure adjustment, addition of interferents, change of test atmosphere, etc.
3. Determination of atmospheric conditions: the selection of the atmospheric conditions depends not only on the reaction conditions required for the reaction process, such as the reactants of the reaction need a specific gas atmosphere themselves, but also on how effectively the reaction process under study is highlighted in a complex reaction system. For simple and definite reaction processes, the selection of atmosphere is definite; for complex reaction processes, the choice of atmosphere must be combined with the characteristics of the actual process itself. In general, an inert gas atmosphere may be selected, or a specific reactive gas atmosphere may be selected according to a specific reaction.
4. After the sample, the temperature program and the atmosphere condition are determined, the testing process is executed on equipment such as a thermogravimetry instrument, a thermal analyzer and a mass spectrometer according to the settings.
5. In the testing process, the characteristics of testing equipment are combined, the variation curves of the temperature of a sample, the variation of the sample amount (thermogravimetric data, TG or DTG), the heat flow (heat flow data, DSC), the differential heat (differential thermal data, DTA) and the mass spectrum (mass spectrum data, MS) in the testing time range are respectively tested, and the thermogravimetric or mass spectrum data is used for giving out the mass flow of a specific substance
A variation curve.
6. Calculation of temperature and rate of change of temperature using the samples tested
Calculating marginal conversion rate of reaction process by using mass flow data of specific substance
7. Marginal conversion rate
And with
And comparing to obtain an activation energy E predicted value, continuously adjusting the predicted value to enable the external impulse to be subjected to parameter fitting with the marginal conversion rate, and obtaining an optimized and calculated value (for example, the mean square error is less than 1%) which is the activation energy E in the reaction process.
8. The reaction process conditions such as sample amount and temperature rise program are changed, and the change characteristics such as flow, heat flow, mass spectrum and the like in the reaction process are calculated by using the activation energy obtained by testing for verification.
Technical effects
Compared with the traditional reaction kinetics characteristic parameter testing and analyzing method, the method has at least one of the following characteristics:
1. the method for testing and analyzing is based on a new reaction kinetic equation, and the method is based on a reasonable physical meaning and is not simply borrowed from common knowledge in other fields.
2. The test analysis has no mechanism function or mode function of pre-estimated reaction, so the analysis result is established on an objective test instead of subjective prejudgment, and the result has good credibility.
3. Since it is not necessary to determine the amount of sample that can participate in the reaction in advance, it is suitable for measuring both the elementary reaction and the intermediate reaction, and also for measuring the course of the reaction in unknown mixtures.
4. The actual temperature of the sample is used in the test process, but not the furnace body temperature and the reference point temperature, so that the influence of heat absorption or heat release in the reaction process is effectively avoided.
5. The test signal may be overall data such as DTG, DSC, and DTA, or may be mass spectrum data. When mass spectrum data is used for analysis, specific mass-to-nuclear ratio signals can be selected for specific different reactions for analysis, only sum analysis by relying on signals such as DTG, DSC and DTA is avoided, and elementary reactions and interaction thereof in a complex reaction process can be explored.
6. Introducing a modulation mode of the heating rate, namely the heating rate adopts a nonlinear periodic mode, and even introducing a cooling process of the modulation mode; the kinetic parameters of the complex reaction process can be more clearly and accurately tested and analyzed by utilizing the nonlinear temperature rise rate. For a more complex reaction system, a random temperature rising mode can be introduced, and the temperature rising rate can be randomly changed according to requirements and is not limited to a periodic temperature rising mode.
7. Because the temperature change used by the method can be any random temperature-rising program, the requirement on the hardware of the testing equipment is reduced, and the accuracy, stability and reliability of the activation energy testing analysis are greatly improved.
8. By utilizing the temperature rise program with multiple changes, on one hand, the precision of test analysis can be improved, on the other hand, the verification of test analysis data can be synchronously realized, and the single test analysis can be realized, so that the test analysis precision is improved, the verification of an analysis result can be realized, the test analysis is simplified, and the efficiency is improved.
9. The reaction kinetic characteristic parameters obtained by the test analysis can be directly verified and extrapolated for application, the initial and termination temperatures of the reaction do not need to be set in the actual verification and extrapolation, and the reaction mechanism can be more effectively and objectively reflected in the full-time process.
10. The activation energy of different elementary reactions in a complex reaction system can be effectively analyzed by combining random or modulated temperature rise programs and utilizing a mass spectrometry combined analysis technology, and the mutual relation among the reactions can be further analyzed.
11. When the characteristics of the reaction process are analyzed by using the mass spectrum, the reaction process and the mass spectrum ionization process can be effectively identified and distinguished through test analysis results under different mass-to-nuclear ratios, so that the mechanism of the real reaction process is found out.
Examples
The following examples are provided to aid in the understanding of the present invention, but they should not be construed as limiting the scope of the present invention.
Example 1: caCO 3 Activation energy testing of pyrolysis linear temperature ramp process
The test selects the super grade pure CaCO of the national drug group 3 As test samples, the specific conditions of the test were: the device is Thermo Mass Photo of Japan science company, the sample amount is about 10mg, the error is controlled to be +/-5%, the sample holding crucible is a Pt crucible, the temperature rise rate of the test setting program is respectively selected to be 5, 10, 15 and 20K/min, the carrier gas is He, the carrier gas flow is 300ml/min, the change of the sample weight in the temperature rise process is measured by a thermal balance, and the signal of the escaping gas is collected by Mass spectrum.
The actual temperature and temperature rate changes during the reaction are shown in fig. 4, and the weight loss rate (DTG) data during the reaction are shown in fig. 5. It is clear from fig. 4 that the actual temperature and the temperature rise rate of the sample change, although the set temperature rises linearly, the actual temperature rise rate changes very complexly and does not show a single stable value, especially in the time period when the reaction occurs most intensely, the temperature rise rate fluctuates greatly up and down, and the fluctuation is larger when the temperature rise rate is larger. Therefore, the actual heating rate of the reaction is used in the analysis and calculation, and the actual heating rate can be replaced by the mean value of the actual heating rates at different times in the whole reaction process.
FIG. 6 shows CaCO 3 The thermal decomposition process takes the analysis result of the temperature rise rate test of 10 and 15K/min, and for the convenience of calculation, the value obtained by dividing the activation energy E by the gas constant R is directly taken as the test target in the embodiment, and M in the figure e For external impulse
And M f To marginal conversion
The two are very consistent at 550-720 ℃, the test value of E/R is 22860 (unit is K), and the mean square error is less than 17. By comparing the marginal conversion with the external impulse, it can also be determined that the reaction start temperature is about 540 ℃.
In order to verify the reliability of the characteristic parameters of the kinetics of the reaction, the measured value of the activation energy is applied to formula (I) to calculate the CO of the actual reaction 2 The gas evolution flow rate, which is compared with the experimentally measured DTG data, is very good in agreement with the latter, as shown in figure 5, where the 5, 20K/min heating rate is the extrapolation calculation. It should be noted that the verification process does not require the start and end temperature points of the reaction process, and excludes the start of the artificial design reaction; meanwhile, the mechanism function of the reaction is not estimated in advance in the calculation and analysis process, and even the dynamic change of the reaction process can still be expressed in the later stage of the reaction peak value.
Example 2: caCO in the temperature rise process is prepared by the same amount of different samples 3 Pyrolysis activation energy test
High-grade pure CaCO of national drug group selected by testing under different sample quantities 3 As test samples, the specific conditions of the test were: the test equipment was Thermo Mass Photo by Japan science, the sample amounts were selected to be 6, 10, 30mg, respectively, and the sample holding crucibles were Pt crucibles. In order to confirm the reliability and comparative data of the experiment, the temperature rise process of the test adopts the same modulation temperature rise, the temperature rise process is shown as figure 7, the temperature and the temperature rise rate in the figure are the measured values of the sample points, the setting curve of the temperature rise program is not, he is selected as the carrier gas, the flow rate of the carrier gas is 300ml/min, and the signal of the escaping gas is collected by mass spectrometry.
FIG. 8 shows 30mg CaCO 3 The mass spectrum of the thermal decomposition process can clearly see CO 2 The characteristic peak change trend of the method is the same as that of the DTG curve, and CaCO is calculated through the transformation of mass spectrum signals 3 CO in the pyrolysis process 2 Mass flow rate [ a specific algorithm may beSee Hongde Xia, kai Wei. Equivalent dielectric spectra analysis in TG-MS system, thermochimica Acta,2015, 602:15-21]From this signal, the marginal conversion M of the reaction process is calculated, as shown in FIG. 9 f And calculating the reaction external impulse M by using the measured temperature, the heating rate and the predicted activation energy parameter e As shown in fig. 10. Marginal conversion by least squares
Linear fit, E/R =22850K for such reaction course was determined, and results of the same treatment for the test of 6, 10mg sample amount are shown in fig. 10.
In addition, the following points are required to be emphasized: in the process of modulating the temperature rise, because a large amount of samples which can participate in the reaction are consumed in the normal stage reaction, the amount of the remaining samples which can participate in the reaction is actually unknown, and the reaction in the later stage cannot be calculated and analyzed by using a relative parameter, namely the degree of reaction conversion. This is a problem in that the conventional test method does not mechanically provide an effective solution at all.
It is clear from FIG. 10 that although the samples tested were very different, the starting time points of the reactions were identical and the external pulses of the reactions were identical
And marginal conversion
Completely overlapped in the reaction front section. For different sample size tests, only the length of time that the marginal conversion coincides with the external impulse is different. This also indicates that the marginal conversion of the reaction process reflects the characteristics of the process and is also an important parameter for the similarity.
Example 3: caCO 3 Validation testing of isothermal pyrolysis process
Because the traditional reaction kinetic parameter test uses an isothermal pyrolysis mode, compared with the analysis test method, the CaCO with the national drug group of superior purity is still selected 3 As a test sample,the specific conditions of the test are as follows: the testing equipment is Thermo Mass Photo of Japan science company, the sample amount is about 10mg, the sample holding crucible is a Pt crucible, the temperature rising rate of the test is 10K/min, the temperature rising rate is 0 at 650 ℃, namely, the temperature is kept at 650 ℃, he is selected as carrier gas, the flow of the carrier gas is 300ml/min, and the signal of the escaping gas is collected by Mass spectrometry.
The change in the ramp rate in the actual test is shown in fig. 11, where the temperature stabilized to 650 ℃ over time due to the slow response of the heat transfer, as shown more clearly by the change in the ramp rate, which in fig. 11 was changed to 0 by a large continuous change. CO in FIG. 11 2 The flow is calculated by dynamic parameters obtained by test analysis, and because the temperature rise process in the test process is typical nonlinear temperature rise and the temperature rise rate is greatly reduced, the temperature rise rate used in the calculation is the real temperature rise rate calculated according to the temperature change. From the figure, CO can be seen 2 The mass flow calculation result is very consistent with the tested DTG, which shows that the calculation of the kinetic parameters is very accurate.
The marginal conversion was calculated from the true gas evolution curve for this test, as shown in FIG. 12, in order to obtain activation energy utilization
The term fits the front end of the reaction process and it can be seen from figure 12 that the marginal conversion is very consistent with the external impulse in the normal phase of the reaction. Thus, E/R =22850 can be obtained. The characteristics of the whole process can be verified according to the parameters, and as shown in the calculation result of fig. 11, the coincidence of the rear segments is very good. More importantly, the starting time point and the ending time point of the reaction process are calculated and are not manually set.
Example 4: caCO 3 Random temperature rise pyrolysis process test analysis
In order to prove the applicability of the test analysis method, the test analysis is carried out by utilizing the process of random temperature rise, and the test still selects the pure CaCO of the national drug group 3 As test samples, the specific conditions of the test were: the test equipment is Japan scienceThe sample amount of Thermo Mass Photo of the company is about 10mg, the sample holding crucible is a Pt crucible, the temperature rise rate is 20K/min before the reaction in the test process, when the temperature rise rate is close to the reaction starting point, the temperature rise rate vibrates randomly in a large range, the actual situation is as shown in figure 13, he is selected as carrier gas, the carrier gas flow is 300ml/min, the signal of the escaping gas is collected by using a Mass spectrum, the test result is as shown in figure 13, and the gas flow can be seen to have large vibration.
Also in this example, the marginal conversion was calculated from the true gas evolution curve, as shown in FIG. 14, in order to obtain the activation energy utilization
The equation fits the front end of the reaction process and it can be seen from figure 14 that the marginal conversion is very consistent with the external impulse at the front end of the reaction. Thus, E/R =22860 can be obtained. Due to the adoption of the random temperature rising mode, the activation energy can be directly calculated by utilizing the amplitude in the fitting process, so that the influence of baseline drift can be directly eliminated by utilizing the analysis method for analyzing test signals such as DSC and DTA.
The characteristics of the whole process can be verified according to the parameters, and as shown in the calculation results of fig. 15 and 16, the coincidence of the whole process is very good no matter the weight loss rate DTG curve or the weight loss TG curve is obtained.
Example 5: test analysis of activation energy of elementary reaction in silicone rubber pyrolysis process
The embodiment is a typical application of the test analysis of the activation energy of the elementary reaction in the complex thermal decomposition process. The test sample is silicon rubber provided by Beijing aerospace new material and process research, the test process adopts linear temperature rise firstly, and completes two tests in EI and PI ionization modes to determine the main characteristics of silicon rubber decomposers, then selects the mass-nuclear ratio (m/z) to be accurately tested for mass spectrum test, and analyzes the characteristics of elementary reaction by using PI ionization mode and modulation temperature rise process. The reaction conditions were: the testing equipment is Thermo Mass Photo of Japan science company, the sample amount is about 20mg, and the crucible for holding the sample is Al 2 O 3 The crucible and the carrier gas are He gas, and the flow rate is 300ml/min.
Fig. 17 shows the weight loss rate DTG and weight loss TG data of silicone rubber under linear temperature rise, fig. 18 shows the mass spectrum three-dimensional maps of the two, respectively, in the EI map in fig. 18a, the broken peaks of the mass spectrum signals overlap more, and the features of the escaping gas and the related reactions thereof are difficult to distinguish, while in the PI map in fig. 18b, the release process of the feature substances can be clearly observed, better explaining the characteristics of the test method, and the mass-to-nuclear ratios m/z are respectively 15, 78, 207, and 222 as the research objects, because these mass-to-nuclear ratios are the main substances at the maximum weight loss point in the pyrolysis process, as shown in fig. 19.
After selecting a representative nucleus ratio, a modulation temperature-rise program is adopted to study the pyrolysis process, and the temperature-rise program, the weight loss TG curve and the variation curves of different nucleus ratios in the pyrolysis process are shown in figure 20. Because the marginal conversion rate of elementary reaction can be directly calculated by using the ion current intensity of different proton-nuclear ratios, and the activation energy of different reaction processes is different, various elementary reaction characteristics can be given, as shown in fig. 21. The activation energy of different reactions can be solved by using a fitting method, the starting time point of the elementary reaction is definitely given at the starting point of coincidence of the marginal conversion rate curve of different elementary reactions and the external impulse in the graph 21, the sequence among the starting time points shows the reaction relationship among the starting time points, and the internal relationship of a complex reaction system can be effectively analyzed and judged, for example, m/z =15 represents methyl, which represents the reaction process of methyl removal; m/z =78 represents a benzene ring, the release reaction of which is different from that of a methyl group; in addition, m/z =207, 222 is the product of the same reaction process because the activation energy is the same. The E/R for the different elementary reactions is shown in Table 1.
TABLE 1 activation energy of different elementary reactions of thermal decomposition of silicone rubber
| m/z | E/R(K) |
| 15 | 7340 |
| 78 | 12180 |
| 207 | 32210 |
| 222 | 32210 |
When the gas characteristics are analyzed by mass spectrometry, an ionization means (ionization means) is needed to determine the characteristics of the gas, so that the phenomenon that the characteristics of the reaction process and the ionization rearrangement process coexist in the actual reaction process is analyzed by mass spectrometry, but the fragmentation peak after the gas is ionized is synchronous with the main peak, so that the change trend is the same, so that the reaction process and the ionization rearrangement process can be distinguished by using the activation energies of different mass-to-nuclear ratios, as shown in fig. 21, the activation energies of m/z =15, 78 and 207 are different and all represent the characteristics of the reaction process, and the activation energies of m/z =207 and 222 are the same, which indicates that the reaction process and the ionization rearrangement process are the same, wherein m/z =207 is the fragmentation peak of m/z =222, and the two are different by one methyl group (m/z = 15).
Example 6: activation energy test analysis of carbon powder gasification process
This example is a process of gasifying carbon powder in a carbon dioxide atmosphere, in which CO is present 2 As a typical reactive gas, participate in the reaction process. The solid sample for reaction is graphite powder with purity of 99.95% provided by Aladdin company, the particle size is 100 meshes, and the sample amount is about 10 mg. The specific conditions of the test are as follows: the test equipment is Thermo Mass Photo of Japan science, the sample holding crucible is Pt crucible, the temperature rise process is modulation temperature rise program, the modulation program is approximate triangular wave, the temperature rise rate is different in different temperature rise periods, the actual situation is as three-dimensional DTG curve shown in figure 22,the horizontal plane projection is a temperature rise program, he is selected as carrier gas, the flow rate of the carrier gas is 300ml/min, a DTG signal of carbon powder is obtained by utilizing thermogravimetry, a test result is shown in figure 23, the fact that the DTG signal of the weight loss rate has large vibration can be seen, and the weight loss of the carbon powder can directly represent the characteristics of the reaction process, so that the marginal conversion rate of the reaction process is calculated by using DTG data of the weight loss rate.
The marginal conversion rate of the reaction process calculated by using the DTG signal in fig. 23, and the external impulse calculated from the temperature signal and the predicted activation energy of the reaction in fig. 23, as shown in fig. 24, were calculated, and thus E/R =40932K was estimated, and it was found that the two substantially coincide with each other.
Because the temperature change in the example is a complex temperature rise program combining multiple temperature rise rates, the precision of test analysis can be improved under multiple changing conditions, the verification process can be synchronously realized in the same test, the analysis precision can be improved, the verification of an analysis result can be realized through a single test analysis reaction characteristic, the simplification of test analysis is facilitated, and the efficiency is improved.
Claims (10)
1. A method of measuring activation energy of a thermal reaction process, comprising:
(1) Placing the sample in a thermal analysis device;
(2) Thermally reacting the sample by a change in temperature;
(3) In the normal phase of the thermal reaction process, i.e. the conversion rate of the reaction process
(conversion rate in the reaction process)
Represented by mass flow, heat flow and/or mass spectral signals of a particular product or reactant characterizing the thermal reaction process) reaches a maximum,
measuring the real temperature f and the real temperature change rate beta of the sample, namely the time differential of the temperature T; and is provided with
MeasuringConversion of the particular product or reactant
Calculating its marginal conversion
Wherein the content of the first and second substances,
marginal conversion for time differential of conversion in the course of the reaction
Reflecting the specific product or reactant conversion per unit of reaction time
A variation amount of;
(4) Performing parameter fitting according to the following formula to obtain the activation energy E of the thermal reaction process:
wherein the content of the first and second substances,
is an external impulse, i.e., an external impulse generated by the change of the external temperature to the reaction process, E is the reaction activation energy, and R is the gas constant.
2. The method of claim 1, wherein the reaction process conversion rate
Derived from at least one of thermogravimetric data (TG or DTG) and mass spectral data (MS) of the sample, or from at least one of thermal flow Data (DSC), differential thermal Data (DTA) and mass spectral data (MS)And (6) obtaining.
3. The method of claim 1, wherein the thermal reaction process is a process involving endotherms or exotherms, including chemical reactions, phase changes, crystal transformations, volatilization, desorption, and adsorption.
4. The method of claim 3, wherein the thermal reaction process is a homogeneous system reaction or a heterogeneous system reaction in which at least one reactant and at least one product are in different phases, including a pyrolysis process or a gas-solid reaction process.
5. The method of claim 1, wherein the temperature change mode comprises a temperature ramp up mode and a temperature ramp down mode, wherein the temperature ramp up mode comprises an isothermal process, a linear ramp up, a modulated ramp up, and a random ramp up.
6. The method according to claim 5, wherein the modulated temperature rise or the random temperature rise close to the actual temperature is selected according to the change of the actual temperature of the sample.
7. The method of claim 5, wherein the activation energies obtained for different temperature profiles are the same.
8. The method of claim 1, wherein the thermal reaction process is carried out in an inert atmosphere, or in a reactive gas atmosphere.
9. The method of claim 1, wherein the fitting employs a least squares method.
10. The method of claim 1, further comprising: by changing the conditions of the reaction process, the change characteristics of the reaction process are calculated by using the activation energy obtained by the test for verification, wherein the change characteristics comprise mass flow, heat flow and mass spectrum data.
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