Multiple sample setup for testing the hydrothermal stability of adsorbents in thermal energy storage applications

The hydrothermal degradation of adsorbents depends on the temperature, the water vapour pressure and the duration of the exposure (see e.g. [12]). For testing the hydrothermal stability of a potential adsorbent, the discharging and charging modes of the application have to be experimentally emulated. The result is not readily transferable to other adsorption and desorption conditions, e.g. higher water vapour pressure during desorption. According to the application, up to several thousand cycles are necessary for characterizing a possible candidate as an adsorption material. Thus, testing the hydrothermal stability of adsorbents is still very time-consuming.

In order to reduce the time for detecting the right adsorbent for a defined application we propose the following method. The hydrothermal stability of an adsorbent is tested within a broad temperature and water vapour region and for defined times. Then, the obtained experimental data is used for setting up a model equation. This model can be applied for the prediction of the hydrothermal stability of the absorbent in different applications. As a consequence no further experimental time will be required.

For this proposed method, comprehensive and precise experimental data of the hydrothermal stability is necessary. For gathering the required data, a multiple sample setup was built, which shall allow studying the influences of the temperature, the water vapour pressure and the duration on the hydrothermal stability of an adsorbent. In contrast to the cycling tests mentioned above, the temperature and the water vapour pressure on the samples are maintained constant during the hydrothermal stress. After defined time spans, the degradation is measured by water uptake measurements. The constant conditions of temperature and pressure offer the possibility of modelling the stability.

The purpose of this paper is to present the functionality and the accuracy of the multiple sample setup. We first present the experimental setup and the measurement procedures. Second, we identify the reproducibility and measurement uncertainties of the multiple sample measurements. Third, the water uptake measurements of the experimental setup are compared with the adsorption isotherms determined using a volumetric measurement device.

The experimental setup allows varying the temperature, water vapour pressure and duration of the exposure independently. A sample material of an adsorbent is placed in 16 different cells. Groups of four cells are placed in one of four heated, nearly isothermal aluminium blocks with temperatures T1–T4. One sample cell of each isothermal block is connected to one of four temperate water reservoirs with the water vapour pressures P1–P4. Thus, 16 different data points can be recorded at once (figure 1).

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After a defined time, the water uptake of each sample is externally measured gravimetrically. The decrease in water uptake corresponds to the hydrothermal degradation of each sample. Figure 2 shows a sketch of the experimental setup with one of four temperate water reservoirs. A photograph of the experimental setup is given in figure 3.

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The temperature range of each block is between 30 °C and 350 °C  ±  0.7% of the reading. This measurement uncertainty results from the uncertainties of the temperature sensor calibration, the data acquisition system and potential temperature variations across an aluminium block. Influences on the temperature of one block to another could not be observed. The temperature can be set by three heating cartridges per block and is measured using two PT-100 RTD sensors. One sensor is below the sample cells, close to the bottom of the block and is also used for temperature control. The other sensor is close to the top of the block. The temperature difference of these sensors is less than 0.2% of the reading. The blocks are insulated with 25 mm-thick high-temperature thermal insulation.

Each sample cell is connected to one of four temperate water reservoirs via an automatic pneumatic valve. The water reservoirs stand in double-walled aluminium bins, which are insulated with 38 mm-thick thermal insulation to ensure a homogeneous temperature distribution. The temperature of each water reservoir can be set between 5 and 70 °C  ±  0.1 K using a peltier element. The temperature of the water reservoirs is measured using a PT-100 sensor that is located in the centre of the water reservoir. The temperature difference between the chamber and the bottom of the reservoir, where the peltier element is placed, leads to a small vertical temperature gradient inside of the water reservoir, which is less than 0.1 K.

The water vapour pressure of each water reservoir is measured using a capacitance manometer model 730 from Setra Systems. The pressure range is 100 kPa with an accuracy of  ±  0.25% of the reading. The pressure in the system is below the atmospheric pressure. Thus atmospheric gas can enter the system in case of leakages and prevent the water vapour transport from or to the samples. By comparing the temperature of the water reservoir and the water vapour pressure, it is possible to detect a pressure increase due to a leakage in the system.

The temperate blocks, the water reservoirs and the manometers are situated in a heated chamber with a temperature higher than that of the water reservoirs to prevent condensation. The system (samples or water reservoir) is evacuated with a rotary vane pump, which reaches a final pressure of 2  × 10^−  4 kPa. The evacuation of the samples can be done either via a needle valve or a stop valve. The needle valve is used at the beginning of the evacuation at pressures close to the atmosphere to prevent fluidization of the sample. At pressures below 0.1 kPa the stop valve is also opened to decrease evacuation time. Additionally, a metal sinter filter with a pore width of 2 µm is installed above the sample cell (see figure 4).

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Before a sample cell is disconnected using a quick connect from the experimental setup for weighing, the manual membrane valve is closed. This valve is installed above the sinter filter and below the quick connect. It prevents atmospheric gas from flowing into the cell, while the sample cell is disconnected. The water uptake of the samples is detected externally using a precision balance with a reproducibility of  ⩽  ±0.7 mg. The weight of the total arrangement of sample cell, filter, manual membrane valve and quick connect is about 550 g.

Each of the 16 sample cells are filled with about 5 g of adsorbent beads. The measurement procedure for testing the hydrothermal stability is illustrated in figure 5. By degassing the samples in the cells we obtain the dry mass m_d. During this time, the valves V2 and V3 are opened and V1 is closed (see figure 2). The degassing is performed in three steps to avoid the unintended ageing of the samples. At the beginning, the samples are degassed using the rotary vane pump until the pressure is less than 0.5 kPa. Then, an automated heating procedure is additionally used to desorb the final amount of water from the sample. For that purpose, the temperature is set to 150 °C with a holding time of 2 h, then 250 °C for 2 h and finally 350 °C for another 15 h. After cooling the sample cells to room temperature, they are disconnected from the system and their dry mass is measured using the precision balance.

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The next step is to prepare the water reservoirs for the water uptake measurements. De-ionised water in a flask is put in an ultrasonic bath for 30 min to degas the water before it is filled in the reservoirs. Then, each water reservoir is evacuated with the vacuum pump three times for 30 s (V1 and V3 is opened, V2 is closed). Thereby the amount of dissolved gas is further decreased and the space above the water is evacuated. As soon as the leakage rates of the empty water reservoir and the water reservoir with the degassed water are very close, the water is adequately degassed and no further degassing is required.

The water uptake of the samples X_s what is the quotient of the mass of the adsorbed water m_w and the mass of the dry adsorbent m_d is detected:

$$\begin{eqnarray}&&{{X}_{\text{s}}}=\frac{{{m}_{\text{w}}}}{{{m}_{\text{d}}}}\end{eqnarray} \tag{ 1 }$$

The temperatures of the four aluminium blocks with the sample cells are set at T  =  50 °C and the four water reservoir temperatures are set at T_w  =  36.7 °C corresponding to a relative water vapour pressure at the samples of:

$$\begin{eqnarray}&&\frac{p({{T}_{\text{w}}})}{p(T)}=50\%\end{eqnarray} \tag{ 2 }$$

One reason for choosing a relative water vapour pressure of 50% is that a change in the relative water vapour pressure has less influence on the water uptake than in regions of lower or higher relative water vapour pressures. Thus, the comparison of the water uptake of the 16 samples is more precise. The typical shape of an isotherm of a microporous adsorbent such as zeolite has a steep increase at low and high relative water vapour pressures and small changes at medium relative water vapour pressures (see e.g. figure 9). A further reason is that these conditions allow a comparison to the water vapour adsorption isotherms measured with the BELSORP apparatus.

The valves V1 and V2 are opened, the valve V3 is closed until the samples reach the adsorption equilibrium. This equilibrium state cannot be detected using the experimental setup. It is checked by increasing the time the samples could adsorb water and comparing afterwards the adsorbed mass weighted with the balance. As soon as no change in the adsorbed mass of the samples is detected, the equilibrium is reached. The difference between the mass of the samples after the water uptake measurement and the mass of the dry samples m_d is equal to the mass of the adsorbed water m_w of each sample.

Following this, the sample cells are placed in the aluminium blocks again. Then the degassing of the samples is performed again to prevent hydrothermal stress of the samples during the heating of the cells up to the desired temperature (see figure 5). The temperature of each block and each water reservoir is set to the conditions of the hydrothermal stress. As soon as the conditions are constant, the valves between the sample cells and the water reservoirs (V1 and V2) are opened for a defined time span, causing degradation according to the condition in each cell.

After this time span, the reduction of X_s is investigated by the water uptake measurement with the same conditions as before (50% relative water vapour pressure at 50 °C). The hydrothermal stability of the adsorbent Z corresponds to the ratio of remaining X_s,2 to the original water uptake X_s,1 (see figure 5).

For verifying the functionality of the experimental setup and the usability of the multiple sample measurements, a procedure with the same conditions for all 16 sample cells was performed. As a sample material zeolite NaMSX (sodium medium silicon zeolite X) from Chemiewerke Bad Köstritz GmbH (Germany) with a bead size over the range 2.5–5 mm was used. The zeolite beads are produced with about 18% of clay binder [21]. Corresponding to the measurement procedure shown in figure 5 the hydrothermal stability was tested at a temperature of 250 °C and a water vapour pressure of 19.93 kPa (equal to a water bath temperature of 60 °C). The duration of the exposure to the water vapour pressure and the temperature at the samples was set at 67 h. During that time, the water reservoirs were evacuated for a few seconds every 6 h. This evacuation prevents an increase in the atmospheric gas in the system. After 67 h the water uptake of the hydrothermally stressed samples (hereinafter called aged samples) was measured and compared to the new samples.

The water uptake of the new and aged samples measured using the experimental setup are compared by measuring the adsorption isotherms of the new and aged samples using the volumetric measurement device BELSORP Aqua 3 (BELSORP) from Bel Japan.

2.4.1. Sample preparation for the volumetric measurement.

2.4.2. Measurement settings for the volumetric measurement.

The adsorption equilibrium cannot directly be read during the water uptake measurement in the experimental setup. Thus, the time for reaching the equilibrium is determined in a kinetic experiment in advance. For defined time spans the samples are exposed to a temperature of 50 °C and a relative water vapour pressure of 50%. After each time span the water uptake of the samples is measured externally using the balance and compared to the adsorption isotherms measured by the BELSORP. After each weight measurement the experimental setup is evacuated and set to the same conditions of temperature and pressure, before the next time span starts.

Figure 6 presents the mean water uptake of the 16 sample cells filled with a new sample of zeolite NaMSX. Additionally, the water uptake determined from the adsorption isotherms measured by the BELSORP are illustrated. After 5.5 h, the mean water uptake measured using the experimental setup is equal to 0.263 kg kg^−  1 and close to the values that are measured by the BELSORP. Further measurements after a total time of 24 h and 25 h show that the water uptake increases slightly.

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In order to ensure that the adsorption equilibrium is attained, an increasing exponential decay function [22] is set up to estimate the final water uptake X_s,equil after infinite time:

$$\begin{eqnarray}&&{{X}_{\text{s}}}={{X}_{\text{s},\text{equil}}}{{(1-{{\text{e}}^{-bt}})}^{c}}\end{eqnarray} \tag{ 3 }$$

The measured values of the water uptake X_s are used as the independent variables in the non-linear regression analysis. The free parameters that have to be fitted in equation (3) are X_s,equil, b and c. The results are listed in table 1. The difference between the measured water uptake after 24 h and the calculated water uptake X_s,equil is minimal and within the error of the fit. The accuracy of the fit is good (R²  =  0.99). Consequently, we can set the time for reaching the adsorption equilibrium to 24 h.

Because of structural changes of the zeolite crystals after the hydrothermal stress, the adsorption equilibrium time may differ for new and aged zeolite. While on one hand the diffusion resistance of the aged zeolite should increase compared to the new zeolite [23], on the other, the water uptake of the aged zeolite is lower, which will decrease the integral heat of adsorption and possibly decrease the time for reaching the adsorption equilibrium.

Figure 7 illustrates the kinetic measurement of the aged zeolite NaMSX of the startup procedure (section 2.3). After 4 h, the mean water uptake, which was measured using the experimental setup, is close to the values measured by the BELSORP. However, a comparison of the water uptake measurements after 5.5 and 24 h shows that equilibrium was not attained. After 24 h the water uptake of the aged zeolite is equal to 0.206 kg kg^−  1 and about 0.5% above the value of the BELSORP with an equilibrium time of 1000 s. The final water uptake X_s,equil is calculated using equation (3) for the aged NaMSX, analogous to the new NaMSX. The results are presented in table 1 and show that no quantifiable increase in the water uptake is expected with increasing times. Thus, the time for attaining the equilibrium can be set to 24 h for the aged NaMSX also.

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The experimental setup is designed for setting different and accurate conditions (water vapour pressures and temperatures) in each of the 16 sample cells. This requires an excellent reproducibility and small measurement uncertainties. The reproducibility of detecting the dry mass and the water uptake of zeolite NaMSX in each sample cell was verified by repeating the degassing procedure as well as the water uptake measurement.

The maximum relative deviation of the dry sample mass between two measurements for all 16 samples is less than 0.05%. The maximum relative deviation of the adsorbed amount of water between two measurements for all 16 samples is less than 0.39%. The maximum expected measurement uncertainty of the water uptake ΔX_s/X_s is defined as the sum of the measurement uncertainties of the mass of adsorbed water Δm_w/m_w and of the mass of dry adsorbent Δm_d/m_d:

$$\begin{eqnarray}&&\frac{\Delta{{X}_{\text{s}}}}{{{X}_{\text{s}}}}=\frac{\Delta{{m}_{\text{w}}}}{{{m}_{\text{w}}}}+\frac{\Delta{{m}_{\text{d}}}}{{{m}_{\text{d}}}}=0.39\%+0.05\%=0.44\%\end{eqnarray} \tag{ 4 }$$

In literature, the uncertainty of detecting the water uptake using a Rubotherm thermobalance is reported to be 0.53% [24]. This value is calculated using a typical sample mass of 60 mg and a water uptake of 0.2 kg kg^−  1. The uncertainty of measuring the water uptake using a combination of a thermogravimetric apparatus and a humidity generator from Setaram is reported as 0.43%, assuming a sample mass of 80 mg and a water uptake of 0.2 kg kg^−  1 [24]. Thus, the measurement uncertainty of the multiple sample setup is comparable to these measurement uncertainties. However, the typical amount of sample that is needed for the characterization differs. The amount of sample that is placed in each of the 16 sample cells is about 5 g.

The hydrothermal stability Z is defined as the ratio of water uptake after the hydrothermal stress X_s,2 and water uptake before the hydrothermal stress X_s,1:

$$\begin{eqnarray}&&Z=\frac{{{X}_{\text{s},2}}}{{{X}_{\text{s},1}}}\end{eqnarray} \tag{ 5 }$$

Thus, the measurement uncertainty of the hydrothermal stability for each sample cell is equal to:

$$\begin{eqnarray}&&\frac{\Delta Z}{Z}=2\cdot \frac{\Delta{{X}_{\text{s}}}}{{{X}_{\text{s}}}}=0.88\%\end{eqnarray} \tag{ 6 }$$

Differences in the water uptake measurement between the sample cells will be discussed in the following section.

3.3.1. Results of the experimental setup.

Before the hydrothermal stability measurement was run, the water uptake of new zeolite NaMSX was measured at the reference water uptake conditions (50% relative water vapour pressure at a sample cell temperature of 50 °C) and an equilibrium time of 24 h. The measurement results of the 16 sample cells are plotted in figure 8. The error bars are calculated using the measurement uncertainty of equation (4). The standard deviation of all 16 samples is equal to 0.0018 kg kg^−  1 or 0.7%, relative to the mean water uptake of 0.275 kg kg^−  1. The standard deviation is on the same order of magnitude as the error bars. This confirms the hypothesis that all samples have the same water uptake under equal conditions.

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3.3.2. Validation of the water uptake measurement.

Three adsorption isotherms of the new adsorbent NaMSX at 50 °C were measured using the BELSORP, see figure 9. The equilibrium time for each measured value of the adsorption isotherms was set to 1000 s. At a relative water vapour pressure of 50%, the mean water uptake is equal to 0.267 kg kg^−  1 and the standard deviation is equal to 5.4  × 10^−  4 kg kg^−  1. The water uptake measured using the BELSORP at the relative water vapour pressure of 50% is 3% below the mean water uptake measured using the experimental setup.

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The difference in the water uptake can be explained by the different time spans that are set for reaching the thermodynamic adsorption equilibrium. The time until the water uptake was measured was set to 24 h using the experimental setup. The maximum time for reaching the equilibrium that can be set for each value of the adsorption isotherm using the BELSORP, is limited. The water uptake is calculated by measuring the pressure difference before and after a change of the relative water vapour pressure. As soon as the leakage rate of the system is of the same order of magnitude as the pressure difference, which would consequently falsify the result, the time for attaining the equilibrium cannot be increased.

3.4.1. Results of the experimental setup.

After the 16 samples were hydrothermally stressed with the conditions as mentioned in section 2.3, their stability is detected using the reference water uptake measurement. The results are plotted in figure 10. The mean value of the water uptake is equal to 0.206 kg kg^−  1 and the standard deviation is equal to 0.0017 kg kg^−  1 or 0.8% of the mean water uptake.

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The measured dry mass of the aged adsorbent is slightly above the dry mass of the new adsorbent. That is still the case after increasing the degassing time up to 33 h at 350 °C. The mean value of the increased mass for the 16 sample cells is equal to 0.4% relative to the new adsorbent, with a standard deviation of 0.046%. Because of the hydrothermal stress, the crystalline structure of the zeolite is partly decomposed. Some of the water molecules are possibly trapped because of blocked exit pores and the desorption of these molecules is prohibited. Further adsorption and desorption measurements show that part of the irreversibly adsorbed amount of water can be desorbed after adsorption took place. Nevertheless, the dry mass of the aged adsorbent is still above the dry mass of the new adsorbent.

The result of the hydrothermal stability is illustrated in figure 11. The hydrothermal stability is equal to a mean value of 0.749 with a standard deviation of 0.0047. Consequently, the hydrothermal stability of an adsorbent in each sample cell can be detected with an uncertainty of better than 2% (3 times the relative standard deviation of 0.6%). It is expected that this uncertainty is the same under different ageing conditions in each cell. Therefore, redundant measurements do not seem to be necessary and 16 different conditions can be measured simultaneously.

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3.4.2. Validation of the hydrothermal stability measurement.

The adsorption isotherms of the aged samples of three sample cells 2, 3 and 13 were measured using the BELSORP. Figure 12 illustrates the results of these measurements. At a relative water vapour pressure of 50% the mean water uptake is equal to 0.205 kg kg^−  1. The standard deviation is equal to 2.4  × 10^−  3 kg kg^−  1.

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The relative deviation between the water uptake measured using the experimental setup and the water uptake measured using the BELSORP is better than 0.5%.

The hydrothermal stability is tested by measuring the reduction of the water uptake at a relative water vapour pressure of 50%. This procedure implies that the water uptake is reduced by the same factor along the relative water vapour pressure. This assumption was verified by Storch [25] and can again be substantiated by comparing the relative reduction of the water uptake of the new and aged adsorbent.

Therefore, the data points of the adsorption isotherms of the new and aged NaMSX are interpolated and compared at 20 different points within the relative water vapour pressure of 0.05 and 0.9. Figure 13 shows the adsorption isotherms of sample 3 and the relative reduction of the water uptake. The arithmetic average of the relative reduction is equal to 0.24 with a standard deviation of 0.005.

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