Observation and analysis of the temperature curve of the molecular sieve purifier

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Currently, molecular sieve purifiers are widely used in air separation equipment to absorb moisture and carbon dioxide from the air, with the majority of these systems employing the \"temperature swing adsorption\" (TSA) process. During operation, a molecular sieve purifier usually requires monitoring of its inlet and outlet temperatures. During the adsorption process, the two curves formed by the temperature changes of air entering and leaving the purifier are known as “adsorption temperature curves”” ； During the regeneration process, the two curves formed by the temperature changes of the contaminated nitrogen gas as it enters and exits the purifier are referred to as “regeneration temperature curves”. The performance of a molecular sieve purifier is reflected in its temperature curve. Therefore, it is of great practical significance to carefully examine and analyze the temperature curve during the operation of the molecular sieve purifier. 1 Adsorption temperature curve: Air inlet temperature, Air outlet temperature. Figure 1 shows the adsorption temperature curve. Time, Temperature, °C. The typical adsorption temperature curve is as shown in Figure 1. Under normal circumstances, as long as the air pre-cooling system is functioning properly, the temperature of the air entering the purifier remains constant; therefore, the temperature curve is a horizontal line. The temperature of the air exiting the purifier remains high only for a short period at the beginning, and thereafter changes very little, thus it also approximates a straight line. 　　The temperature of the air increases after passing through the purifier. This is because the moisture and carbon dioxide in the air are adsorbed by the molecular sieve, and adsorption is an exothermic process. For air separation units with a fully low-pressure process, the pressure of air entering the purifier is around 0.6 Mpa (G). If the air pre-cooling system is equipped with a chiller, the temperature of the air entering the purifier is around 10°C. In this case, the temperature difference between the air entering and leaving the purifier is about 4°C. 　　If the temperature of the air entering the purifier increases, the temperature difference will also increase accordingly, as a higher air temperature leads to an increased water content in the air. If, during the operation of the purifier (except for the initial period after it is put into use), the temperature of the air exiting the purifier rises suddenly, while the temperature and pressure of the air entering the purifier remain relatively stable, this situation usually indicates that air has carried water from the air-cooled tower into the molecular sieve purifier. 　　During the initial period after a molecular sieve purifier is switched from regeneration mode to operational mode and adsorption begins, the temperature of the air exiting the purifier is relatively high; at this time, the outlet temperature is nearly 20°C higher than the inlet temperature. In most cases, this phenomenon is not caused by inadequate cold blowing during the regeneration process, but rather by the pressurization process carried out on the purifier before it is put into use. 　　The 13X molecular sieve used in air separation equipment to adsorb moisture, carbon dioxide, and other substances has an adsorption capacity not only for polar molecules such as water and carbon dioxide, but also to a certain extent for non-polar gases like nitrogen and oxygen. The pressurization process is one in which pressure increases; as the pressure rises, the static adsorption capacity of the molecular sieve increases, allowing more nitrogen and oxygen to be adsorbed by it. This process is also an exothermic one, and this heat release causes the temperature of the molecular sieve bed to rise. When the pressurized purifier is put into use, air carries away the heat from the molecular sieve bed, thereby causing an increase in the outlet temperature. 　　Since this phenomenon is not caused by incomplete cold blowing, it cannot be resolved by extending the cold blowing time. In some air separation units, adding a “mixing” step can reduce the adverse effects of such temperature fluctuations on the main heat exchanger. “The “mixing” step refers to the period during which, after a newly regenerated purifier is put into use, the original purifier that was performing the adsorption function continues to be used, operating in parallel for a while. In this way, since the temperature of the air coming out of the previously used purifier is lower, the temperature of the air mixed together will not be as high as it would be if only one purifier were used. 2 Regeneration temperature curve: Inlet temperature of polluted nitrogen gas, Outlet temperature of polluted nitrogen gas. Figure 2 Regeneration temperature curve: A B      C                   D      E Time, °C Temperature. Compared to the relatively simple adsorption temperature curve, the regeneration temperature curve is more complex. The typical regeneration temperature curve is shown in Figure 2. 2.1 Pressure relief stage (A–B) The molecular sieve purifier carries out the adsorption process at higher operating pressures (above 0.5 Mpa), while desorption and regeneration take place at lower pressures (around 10 Kpa). When the purifier switches from adsorption to regeneration, the pressure inside the purifier is first reduced. When the pressure decreases, the static adsorption capacity of the molecular sieve decreases, and some of the gas molecules or water molecules that were previously adsorbed will desorb from the molecular sieve. 　　Corresponding to the exothermic effect of the adsorption process, the desorption and regeneration process is one that requires heat absorption. During the depressurization phase, the heat required for desorption can only come from the molecular sieve bed itself, which causes the bed temperature to drop. As a result, the temperatures at the air inlet (dirty nitrogen outlet) and the air outlet (dirty nitrogen inlet) have begun to drop. 2.2 Heating stage (B–C) After the heating stage begins, although the inlet temperature of the contaminated nitrogen gas rises rapidly, the outlet temperature continues to drop, reaching around –10°C before it starts to increase gradually. The high-temperature contaminated nitrogen gas, heated by the regeneration heater, causes the molecular sieve in the upper part of the bed to heat up as it passes through the molecular sieve bed from top to bottom, thereby regenerating the molecular sieve in that upper section. During this process, the heat from the contaminated nitrogen gas is transferred, on one hand, to the molecular sieve located above; on the other hand, the moisture from the carbon dioxide that is desorbed carries away some of this heat. As a result, the temperature of the contaminated nitrogen gas drops rapidly, and by the time it reaches the bottom, its temperature is already very low. Therefore, the outlet temperature of the contaminated nitrogen gas does not rise quickly. 　　During the heating phase, the main parameter that needs to be monitored is the inlet temperature of the contaminated nitrogen gas; together with the flow rate of this gas and the heating time, it determines the amount of heat introduced into the purifier. The inlet temperature of the contaminated nitrogen gas is primarily determined by factors such as the pressure and temperature of the heating steam, as well as the flow rate of the contaminated nitrogen gas. Sometimes, the drain valve behind the heater malfunctions (this problem does not occur with electric heaters), and the condensate cannot be removed in time; as a result, the effective area inside the heater decreases, and the temperature of the contaminated nitrogen gas entering the purifier drops. 　　Generally, during the heating phase, desorption mainly occurs in the upper and middle parts of the molecular sieve bed, with heat being stored within that bed. 2.3 Cold blowing stage (C–D) During the cold blowing stage, on the one hand, the heat stored in the molecular sieve bed from the heating stage is used to continue desorbing molecules from the sieve; on the other hand, the heat present in the bed is removed, thereby preparing it for reuse. After cold blowing begins, the inlet temperature of the contaminated nitrogen gas drops rapidly, but the outlet temperature continues to rise; it only starts to decline after reaching a certain maximum value. 　　The temperature variation curve of the dirty nitrogen gas at the cold blowing stage (hereinafter referred to as the cold blowing curve) is particularly important. The highest temperature point on the cold blow curve is called the “cold blow peak,” which is a key indicator of whether the regeneration process is complete. During the regeneration process, the temperature of the molecular sieve in the bed decreases from top to bottom; therefore, the molecular sieve at the bottom is always regenerated the least thoroughly. If the cold-blow peak reaches 160°C, it indicates that all the molecular sieves in the purifier have been regenerated above this temperature (except for the areas near the edge of the cylinder due to heat dissipation issues). 　　The factors affecting the cold blow peak mainly include the low temperature of the contaminated nitrogen gas entering the purifier during the heating stage, the flow rate, and the duration of the heating time. Furthermore, if the molecular sieve absorbed more moisture and carbon dioxide during the previous usage cycle, the cold blow peak will decrease. If water enters the molecular sieve, the cold blow peak will decrease significantly. 　　Sometimes, two peaks appear on the cold blow curve; we refer to this as the “double peak” phenomenon, as shown in Figure 3. Based on our experience using molecular sieve purifiers for over a decade, this “secondary peak” phenomenon is a typical characteristic of an uneven molecular sieve bed. Figure 3 Temperature curve during cold blowing. Time, Temperature, °C. In a well-performing molecular sieve bed, the temperature gradient at any horizontal cross-section should be small; in such a bed, during regeneration, the temperature throughout the molecular sieve in the lowest layer remains more or less constant, and the temperature variation curve is also the same. What the instrument records is the temperature variation curve resulting from the mixture of gases coming from various sources; it can be considered as a curve formed by the combination of a series of waveform curves. Since all these waveforms are identical and have no phase difference, the shape of the resulting composite curve remains unchanged. 　　In another case, when the thickness of the molecular sieve bed is uneven, there is less molecular sieve in the thinner sections and more gas flows through them, resulting in a faster temperature change of the molecular sieve; the opposite is true in the thicker sections. In this way, when the conditions at various lowest levels reach their peaks simultaneously, it is possible for two or even three peaks to appear in the resulting waveform curve. Generally, when the molecular sieve bed is uneven, the on/off points of the cold blow curve also become somewhat \"shorter\" and \"rounder\". 　　The temperature of the contaminated nitrogen gas exiting the purifier at the end of the cold blow is another parameter that needs to be controlled; it is primarily determined by factors such as the cold blow time, the flow rate of the regeneration gas, and the amount of heat introduced during the heating process. Generally, when the molecular sieve bed is uneven, it takes longer to cool down to the specified temperature. 2.4 Pressurization Stage (D–E) During the pressurization stage, the pressure inside the purifier increases. As mentioned earlier, this is a process in which impurities in the air are adsorbed by the molecular sieve, while the temperature of the bed rises. Due to the bed temperature and the residual heat in the insulation layer, the inlet and outlet temperatures of the polluted nitrogen gas will increase.