2022 Volume 62 Issue 12 Pages 2536-2541
Desiccant humidity control systems have been garnering considerable attention in the attempt to achieve highly efficient utilization of low-temperature heat exhausted from various industries at temperatures less than 373 K. We have focused on thermosensitive polymers as new desiccants because a large amount of dehumidified water would be expected in the system because of their thermosensitivity. Our previous study focused on the water adsorption behavior of poly(N-isopropylacrylamide) (poly(NIPA)), which has a low critical solution temperature (LCST) of 306 K. In this study, poly(N-isopropylmethacrylamide) (poly(NIPMA)) and poly(2-(dimethylamino)ethyl methacrylate) (poly(DMAEMA)) cross-linked with N,N′-methylenebisacrylamide (MBAA) were investigated. These polymers are known to exhibit thermosensitivity in the temperature range of 313–319 K in water, which is a higher LCST than that of poly(NIPA). Poly(NIPMA) adsorbed water vapor linearly with increasing relative humidity. It was also observed that poly(NIPMA) prepared at MBAA concentrations of 200 mol/m3 exhibited a thermosensitivity in the temperature range of 303–313 K in water vapor adsorption. Meanwhile, poly(DMAEMA) adsorbed little water vapor up to a relative humidity (RH) of 40%; however, it exponentially adsorbed water at RH levels higher than 40%. From the estimation results of effective water adsorptivity, we found that poly(DMAEMA) is applicable in desiccant humidity control systems when the dehumidification process is performed at high RH.
Currently, there are growing concerns regarding global warming worldwide. With the enactment of the Paris Agreement, efforts to realize a decarbonized society are intensifying. To curb global warming and achieve the 1.5°C target, it is necessary to promote electrification using renewable energy and to ensure the effective use of energy.
The steel industry is a typical energy-intensive industry that consumes large amounts of fossil fuels. As a result, enormous amounts of carbon dioxide and waste heat at various temperature levels are released into the environment.1) One way to achieve carbon neutrality in the steel industry is to reduce fossil fuel consumption by effectively using waste heat. High-temperature waste heat at temperatures higher than 773 K has great potential to improve energy utilization efficiency owing to its high exergy; however, high heat loss and low heat exchange efficiency are major challenges in handling high-temperature waste heat. To overcome these challenges, various studies and developments have been conducted.2,3,4,5) For instance, Sakai et al. prepared latent heat storage pellets consisting of an Al‒Si phase change material (PCM) and Al2O3.4) They reported that the heat storage pellets could store high-temperature heat at approximately 850 K, which is the melting point of PCM, with a heat storage density of 73.5 kJ/kg. We have also developed a Cu‒Al PCM encapsulated in black alumina.5) The encapsulated PCM exhibited excellent durability even when exposed to air at 1373 K for 400 h. Meanwhile, low-temperature waste heat at temperatures less than 373 K is also exhausted enormously into the environment as hot water; for instance, it is estimated that 195 PJ of hot water is discharged annually in the steel industry in Japan.1) However, it is difficult to utilize it efficiently because of its low exergy. In addition, the temperature of waste heat is too low to be used directly in steel-making processes. As one of the applications of low-temperature waste heat, we considered it as a heat source to drive desiccant humidity control systems.
Desiccant humidity control systems generally use the adsorption and desorption characteristics of water vapor by desiccant materials, and they can efficiently control latent heat loads, independent of sensible heat load.6,7,8) During the dehumidification process, water vapor in humid air is adsorbed and removed by a desiccant material coated on a honeycomb rotor or heat exchanger surface, resulting in comfortable dehumidified air being supplied to living rooms, offices, and factories. During the regeneration process, the desiccant material is heated to desorb water vapor using hot air or hot water. In this process, heat energy less than 373 K exhausted from the steel industry can be used as the heat source.
Various candidate materials have been proposed for desiccant humidity control systems. A large effective water adsorptivity, which is the difference in the amount of adsorbed water between the dehumidification and regeneration processes, is required to achieve a high dehumidification performance. Silica gel and some zeolites are well-known porous adsorbents for these systems.9,10,11,12,13,14,15,16,17) Silica gel adsorbs water vapor almost linearly with relative humidity (RH) and can be operated over a wide range of RH.9,10,11) FAM-Z01 is a zeolite with a unique sigmoid water vapor adsorption isotherm.12,13) Organic sorbents, which are bridge complexes of sodium polyacrylate, have higher water adsorptivities than silica gel over the entire RH range. The sorbents are considered a suitable material for high RH conditions for desiccant humidity control systems.15,16) Poly(acrylic-acid-co-acrylamide) superporous hydrogel was prepared and its water vapor adsorption isotherms were measured at various temperatures. It exhibited a type III isotherm with a maximum water adsorptivity of 1.03 g-H2O/g-sample at 298 K.17)
Among the various polymers, we focused on thermosensitive polymers as new desiccants. Thermosensitive polymers have a unique feature that their hydrophilicity and hydrophobicity change drastically at temperatures lower and higher than the lower critical solution temperature (LCST). Poly(N-isopropylacrylamide) (poly(NIPA)) with a LCST of 306 K is a representative thermosensitive polymer.18,19) Poly(NIPA) absorbs water molecules at temperatures less than 306 K. Meanwhile, poly(NIPA) expels water molecules at temperatures higher than 306 K, indicating that poly(NIPA) can control the water absorption behavior in a small temperature change. Several studies have been conducted on the application of thermosensitive polymers to the separation and recovery of metal ions from aqueous solutions and drug delivery systems. However, there are few studies on the application of thermosensitive polymers to desiccant humidity control systems. Docker et al. recently performed a theoretical analysis of a thermosensitive polymer desiccant dehumidification cycle named “the LCST cycle”.20) According to their combined first and second law analysis, the new system can achieve a coefficient of performance of 5.1 when the polymer is regenerated at 368 K. Matsumoto et al. developed thermo-responsive adsorbents to recover water from air moisture, which is an interpenetrating polymer network gel comprising poly(NIPA) and sodium alginate chains.21) In addition, Zeng et al. estimated the dehumidification efficiency of a wheel system using thermo-sensitive adsorbents prepared by Matsumoto et al.22) In our previous study, we investigated the water vapor adsorption behavior of poly(NIPA).23) We observed that poly(NIPA) (homopolymer and cross-linked polymer with N,N’-methylenebisacrylamide (MBAA)) exhibited thermosensitivity in water vapor adsorption; however, the thermosensitivity under dry conditions was weaker than that in water. Thereafter, the applicability of poly(NIPA) as an adsorbent in desiccant humidity control systems was evaluated.
The LCST of poly(NIPA) is slightly low for application in desiccant humidity control systems. The ambient air temperature in Japan often exceeds 306 K in summer, meaning that poly(NIPA) cannot sufficiently adsorb water vapor in humid air. Therefore, the purpose of this study was to investigate the water vapor adsorption behavior of thermosensitive polymers with a higher LCST than that of poly(NIPA). Poly(N-isopropylmethacrylamide) (poly(NIPMA)) and poly (2-(dimethylamino)ethyl methacrylate) (poly(DMAEMA)) were used as desiccants. The LCST of poly(NIPMA) in aqueous solution was approximately 319 K24) and that of poly(DMAEMA) was approximately 313 K.25,26,27,28) Cross-linked polymer gels were synthesized via free-radical polymerization with MBAA. Water vapor adsorption isotherms were measured using a volumetric method. The applicability of thermosensitive polymers to desiccant humidity control systems was evaluated based on their effective water adsorptivity.
NIPMA and DMAEMA were used as monomers, whereas MBAA was used as a cross-linker. N,N,N’,N’-tetramethylethylenediamine (TEMED) and ammonium peroxodisulfate (APS) were used as the polymerization accelerator and initiator, respectively. Special grade DMAEMA, MBAA, TEMED, and APS were purchased from FUJIFILM Wako Pure Chemical Corp. NIPMA was purchased from Sigma‒Aldrich. All reagents were used without further purification. The molecular structures of NIPMA, DMAEMA, NIPA monomers and MBAA are shown in Fig. 1.
Molecular structures of monomers and MBAA.
First, ion-exchanged water (IEW) was bubbled with nitrogen gas for 60 min to remove dissolved oxygen, which inhibited polymerization. The monomer solution was prepared by dissolving DMAEMA, MBAA, and TEMED in IEW at predetermined concentrations of each chemical reagent. To prepare the NIPMA polymer gel, NIPMA was dissolved in ethanol because of its low solubility in IEW. An aqueous APS solution was also prepared. The monomer solution (9 mL) and 1 ml of the APS solution were heated to a polymerization temperature of 323 K with a thermostat and bubbled with nitrogen gas for 60 min. Thereafter, these two solutions were mixed to initiate radical polymerization. After 24 h of treatment at 323 K, the resulting polymer gel was washed with IEW to remove residual reagents and dried at 373 K for 24 h in a vacuum dryer. Finally, the obtained polymer gel was crushed and sieved to 300 μm. In this study, cross-linked polymer gels were prepared at a monomer concentration of 1000 mol/m3, MBAA conc. = 200–600 mol/m3, TEMED conc. = 20 mol/m3, APS conc. = 100, 200 mol/m3 of NIPMA, and 30 mol/m3 of DMAEMA. The concentrations of the monomer and MBAA are shown in the gels, which represent the concentrations of the monomer and MBAA solutions in the preparation of the gels.
2.3. Water Vapor Adsorption IsothermWater vapor adsorption isotherms were measured using a volumetric method (Belsorp 18, MicrotracBEL Corp.). Approximately 0.1 g of the polymer was first degassed at 373 K for 24 h in a vacuum. Thereafter, the amount of adsorbed water vapor was automatically measured in the temperature range of 303–333 K in the RH range of 0–95%.
The water vapor adsorption isotherms measured at 303 K are shown in Fig. 2 for poly(NIPMA) and poly(DMAEMA) with concentrations of monomer: MBAA = 1000:200 mol/m3. In this figure, the result for poly(NIPA) is also shown as a reference. For poly(NIPMA) and poly(NIPA), the amount of water adsorbed linearly increased with increasing RH up to RH = 70%. In addition, poly(NIPMA) exhibited almost a similar adsorption isotherm as poly(NIPA) within this RH range. As shown in Fig. 1, the NIPMA monomer is structurally similar to that of NIPA. Each monomer had one amide group, which is a well-known hydrophilic functional group. Therefore, it is considered that both polymers exhibited a similar water adsorption isotherm shape, especially at low and middle RH ranges. Meanwhile, poly(DMAEMA) exhibited little water adsorptivity at RH levels less than 40%. Subsequently, the amount of adsorbed water increased exponentially with increasing RH. Galvin et al. reported the swelling factor of poly(DMAEMA) at various relative humidities.29) The change in the swelling factor with RH exhibited a shape similar to that of the water adsorption isotherm in Fig. 2. According to the Brunauer, Deming, Deming, and Teller (BDDT) classification, this adsorption isotherm is classified as a typical type III isotherm, indicating that the polymer gel has a weak interaction with water vapor. Tertiary amines were present in the DMAEMA monomer. However, they have a weaker interaction with water molecules than the amide functional group, resulting in lower water adsorptivity at a lower RH range.
Water vapor adsorption isotherm at 303 K of poly(NIPMA), poly(DMAEMA), and poly(NIPA). Polymers were synthesized with the concentrations of monomer: MBAA = 1000:200 mol/m3 in the pre-gel solutions. (Online version in color.)
Figure 3 shows the effect of MBAA concentration on water adsorptivity of polymers at 303 K and at relative humidities of 50, 60, and 80%. The polymers were prepared using various concentrations of MBAA. The horizontal axis is the molar ratio of MBAA to the summation of the monomer and MBAA; for instance, [MBAA]/([NIPMA] + [MBAA]) is 0.17 for poly(NIPMA) with concentrations of 1000 mol/m3 NIPMA and 200 mol/m3 MBAA. For reference, the results for poly(NIPA) are also shown in Fig. 3.23) For each RH condition, the amount of adsorbed water of poly(NIPA) linearly increased up to a molar ratio of 0.20 with increasing amounts of MBAA additive.23) For poly(NIPMA), water adsorptivity at a MBAA ratio of 0.2 is similar to poly(NIPA), particularly at relative humidities of 50 and 60%. This was attributed to the similar shapes of the water vapor adsorption isotherms of poly(NIPMA) and poly(NIPA), as shown in Fig. 2. However, water adsorptivity did not significantly change and it became almost constant even if the MBAA ratio was increased from 0.17 (200 mol/m3) to 0.29 (400 mol/m3). Meanwhile, for poly(DMAEMA), the amount of adsorbed water increased slightly as the MBAA concentration increased from 200 to 600 mol/m3. This trend was observed for every RH condition.
Effect of MBAA concentrations in the pre-gel solutions on the amount of adsorbed water at 303 K and at relative humidities of 50, 60, and 80%. (Online version in color.)
Water vapor adsorption isotherms of poly(NIPMA) and poly(DMAEMA) with concentrations of monomer: MBAA = 1000:200 mol/m3 at varying adsorption temperatures are shown in Figs. 4(a) and 4(b). The adsorption isotherms of each polymer exhibited similar shapes at each adsorption temperature. Poly(NIPMA) and poly(DMAEMA) exhibited type II and III adsorption isotherms, respectively. The amount of adsorbed water decreased gradually as the adsorption temperature increased; however, this trend was not pronounced up to RH of 30% for poly(NPMA) and 40% for poly(DMAEMA). In the high relative pressure range, water adsorptivity decreased with increasing adsorption temperature. It can be observed that these thermosensitive polymers have a temperature dependence on water vapor adsorption.
Water vapor adsorption isotherm on poly(NIPMA) and poly(DMAEMA) at adsorption temperatures in the range of 303–333 K. Polymers were synthesized with the concentrations of monomer: MBAA = 1000:200 mol/m3 in the pre-gel solutions. (Online version in color.)
Figures 5(a) and 5(b) show the relationship between adsorption temperature and the amount of adsorbed water on poly(NIPMA) and poly(DMAEMA) with various concentrations of MBAA at RH of 40 and 85%, respectively.
Temperature dependency of the amount of adsorbed water on poly(NIPMA) and poly(DMAEMA) at relative humidities of 40 and 85%, respectively. Polymers were synthesized with 1000 mol/m3 monomer and various concentrations of MBAA in the pre-gel solutions. (Online version in color.)
For poly(NIPMA), water adsorptivity at RH = 40% is almost independent of adsorption temperature. In addition, water adsorptivity decreased with decreasing MBAA concentrations; however, the amount of adsorbed water is small and approximately 0.1 g-H2O/g-dry polymer even for poly(NIPMA) with 400 mol/m3 MBAA. In contrast, at RH = 85%, temperature dependence of water adsorptivity varied according to the MBAA concentration. When the MBAA concentration was set at 200 mol/m3, the amount of adsorbed water sharply decreased in the temperature range of 303‒313 K and thereafter gradually decreased with increasing adsorption temperature. Poly(NIPMA) seems to have weak thermosensitivity in the temperature range of 303‒313 K for water vapor adsorption although the LCST of poly(NIPMA) is said to be approximately 319 K in water. At 600 mol/m3 of MBAA, the water adsorptivity was maintained at 313 K and decreased linearly at temperatures higher than 313 K. In our previous study, we observed that the thermosensitivity of poly(NIPA) weakened as the amount of MBAA increased. A similar trend was observed for poly(NIPMA).23)
Poly(DMAEMA) exhibited a similar trend of water adsorptivity at RH = 40% to poly(NIPMA); that is, water adsorptivity was almost constantly independent of adsorption temperature. At a RH of 85%, the amount of adsorbed water in all DMAEMA polymer gels almost linearly decreased with increasing adsorption temperature. As mentioned above, poly(DMAEMA) is known to have a LCST of 313 K in water; however, a clear thermosensitivity could not be observed for water vapor adsorption.
3.4. Applicability of Polymers to Desiccant Humidity Control SystemsThe applicability of thermosensitive polymers to desiccant humidity control system was evaluated from the perspective of the expansion of the application of polymers. The effective water adsorptivity of polymer gels, which is defined as the difference in the amount of adsorbed water between the dehumidification and regeneration processes, was used as an evaluation index, as reported in our previous study. Figure 6 shows the effective water adsorptivity of poly(NIPMA) and poly(DMAEMA) under several operating conditions. The results for poly(NIPA) are shown in Fig. 6 as a reference. Silica gel is a typical conventional adsorbent used in desiccant humidity control systems. When regeneration is performed by increasing the temperature from 303 to 323 K while maintaining the same RH of 60% as for adsorption (Case 1), all adsorbents did not exhibit effective water adsorptivity (less than 0.05 g-H2O/g-dry polymer) for desiccant humidity control systems. Meanwhile, under the constant RH of 90% (Case 2), poly(NIPMA) and poly(DMAEMA) exhibited three to four times higher effective water adsorptivity than silica gel; however, their adsorptivities were low at approximately 0.1 g-H2O/g-dry polymer. In a more practical condition of Case 3, silica gel exhibited the highest adsorptivity of 0.25 g-H2O/g-dry polymer. However, effective water adsorptivities of poly(NIPMA) and poly(DMAEMA) were approximately 0.12 and 0.08 g-H2O/g-dry polymer because the amount of adsorbed water at RH = 60% of NIPMA and DMAEMA was only 0.17 and 0.09 g-H2O/g-dry polymer, respectively. In Case 4, where the dehumidification process was performed at RH = 90%, poly(NIPMA) exhibited almost similar high effective water adsorptivity of 0.28 g-H2O/g-dry polymer as silica gel. In particular, the effective water adsorptivity of poly(DMAEMA) attained 0.34 g-H2O/g-dry polymer and it is approximately 1.25-fold higher than silica gel. This was attributed to a significant increase in the amount of adsorbed water of poly(DMAEMA) in a high RH range. Based on these results, poly(DMAEMA) is expected to be a promising material for desiccant humidity control systems operated in high RH regions.
Effective water adsorptivity of poly(NIPMA), poly(DMAEMA), poly(NIPA), and silica gel under various operation conditions for desiccant humidity control system. Polymers were synthesized with the concentrations of monomer: MBAA = 1000:200 mol/m3 in the pre-gel solutions. (Online version in color.)
Poly(NIPMA) and poly(DMAEMA) were investigated as desiccant materials to develop a high-performance desiccant humidity control system because of their thermosensitivity in water and their LCST at 319 and 316 K, respectively. Poly(NIPMA) and poly(DMAEMA) were prepared by cross-linking them with MBAA. Water adsorption isotherms of the polymers were measured at adsorption temperatures in the range of 303‒333 K.
The water vapor adsorption isotherms of poly(NIPMA) and poly(DMAEMA) were classified as type II and III according to the BDDT classification, respectively. For poly(NIPMA), the water adsorptivity was almost independent of MBAA concentrations higher than 200 mol/m3. Meanwhile, the amount of water adsorbed by poly(DMAEMA) slightly increased with increasing MBAA concentration. Poly(NIPMA) is considered to have thermosensitivity in the temperature range of 303‒313 K for water vapor adsorption although the LCST of poly(NIPMA) is said to be approximately 319 K in water. However, when the MBAA concentration was higher than 200 mol/m3, the thermosensitivity of poly(NIPMA) was weakened. Meanwhile, poly(DMAEMA) did not exhibit clear thermosensitivity for water vapor adsorption. Finally, the applicability of the polymers in humidity control systems was evaluated based on effective water adsorptivity. When dehumidification was performed at RH of 90%, DMAEMA polymer gel achieved 1.25 times higher effective water adsorptivity than silica gel owing to a drastic increase in water adsorptivity in the high RH range. Consequently, the DMAEMA polymer gel is applicable in desiccant humidity control systems when the system is operated under high humidity conditions.
This research was partially supported by JSPS KAKENHI Grant Number 17K07028.
