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The membrane-based liquid desiccant dehumidification system is a newly developed method in the field of air dehumidification. In this study, double-layer nanofibrous membranes (DLNMs) with directional vapor transport and water repellency for liquid dehumidification were fabricated by a simple electrospinning process. Specifically, the combination of thermoplastic polyurethane nanofibrous membrane and polyvinylidene fluoride (PVDF) nanofibrous membrane forms a cone-like structure in DLNMs, resulting in directional vapor transportation. The nanoporous structure and rough surface of PVDF nanofibrous membrane provide waterproof performance for DLNMs. Compare with the commercial membranes, the proposed DLNMs have a significantly higher water vapor permeability coefficient, which is as high as 539.67 g· μ m m−2·24 h·Pa. This study not only provides a new route to construct a directional vapor transport and waterproof membrane, but also demonstrates the huge application prospect of the nanofibrous membrane formed by electrospinning in the field of solution dehumidification. [ABSTRACT FROM AUTHOR]

A new polymer-based composite desiccant, Polyvinyl alcohol-LiCl nanofibrous membrane (PVA-LiCl NFM), has been prepared by the electrospinning technique. The newly-developed desiccant membrane not only has large moisture adsorption capacity, fast moisture adsorption rate and favorable stability, but also is featured by its low regeneration temperature. In this work, adsorption and desorption kinetic models are explored for PVA-LiCl NFMs, which include the pseudo-first and the pseudo-second order model based on the rate law and a diffusion model based on the Fick's law. The results indicate that the moisture adsorption and desorption kinetics of the PVA-LiCl NFMs follow the pseudo-second-order law very well. As the key parameters in the diffusion model, the equilibrium adsorption capacity and the diffusion coefficient are focused on, and they can be well determined by the Brunauer–Emmett–Teller (BET) model and the radial-basis-functions artificial neural network (RBFANN) model, respectively. Finally, the adsorption and desorption activation energy of the PVA-LiCl NFM with a 0.15 mass ratio of LiCl to PVA are recognized as 24.36 and 26.52 kJ/mol, respectively. The work is of great importance to the better application of the newly-developed desiccant material (PVA-LiCl NFM). [ABSTRACT FROM AUTHOR]

Electrospun PVA (polyvinyl alcohol)-LiCl composite membranes were prepared as novel solid desiccants. Experimental results show that nanofibrous membranes (NFMs) exhibit notable advantages in sorption capacity, sorption rate and low-temperature desorption rate, as compared with the solution-cast PVA-LiCl membranes (SCMs). The PVA NFM with 15 wt% LiCl can sorb 1.04 g g −1 water at 25 °C and 90% relative humidity (RH), which is more than twice of the reported capacity of silica gel. Due to the nano-structure and small diffusion distance, the desiccant membranes have fast sorption and desorption rates. The desorption isobars show that about 90% of the sorbed moisture can be removed at temperatures between 40 °C and 60 °C, which makes it promising to utilize solar energy or exhaust heat for air dehumidification. The composite desiccant membranes can also be recycled without the degradation of sorption and desorption performance. [ABSTRACT FROM AUTHOR]

The physical mechanisms of heat and mass transfer enhancement by ultrasound have been identified by people. Basically, the effect of ‘cavitation’ induced by ultrasound is the main reason for the enhancement of heat and mass transfer in a liquid environment, and the acoustic streaming and vibration are the main reasons for that in a gaseous environment. The adsorbent regeneration and food drying/dehydration are typical heat and mass transfer process, and the intensification of the two processes by ultrasound is of complete feasibility. This paper makes an overview on recent studies regarding applications of power ultrasound to adsorbent regeneration and food drying/dehydration. The concerned adsorbents include desiccant materials (typically like silica gel) for air dehumidification and other ones (typically active carbon and polymeric resin) for water treatment. The applications of ultrasound in the regeneration of these adsorbents have been proved to be energy saving. The concerned foods are mostly fruits and vegetables. Although the ultrasonic treatment may cause food degradation or nutrient loss, it can greatly reduce the food processing time and decrease drying temperature. From the literature, it can be seen that the ultrasonic conditions (i.e., acoustic frequency and power levels) are always focused on during the study of ultrasonic applications. The increasing number of relevant studies argues that ultrasound is a very promising technology applied to the adsorbent regeneration and food drying/dehydration. [ABSTRACT FROM AUTHOR]

Applications of ultrasonic energy to enhance a wide variety of processes or to improve system efficiency have been explored since about 1960s. This work makes an overview of recent studies about the applications of ultrasound in the field of Heating, Ventilation and Air-Conditioning (HVAC), including air humidification/dehumidification, desiccant regeneration, air cleaning, heat enhancement and fouling reduction of heat exchanger, defrosting or frost suppression for evaporator of air-conditioner. The mechanisms for these ultrasonic applications can be illustrated by a series of ultrasonically induced effects such as acoustic cavitation, streaming and vibration. The literature reviews manifest that the parametric studies on the ultrasound-assisted enhancement are the main interest of people to help in the design, optimization, and scale-up of the ultrasound-assisted process. As a new technology applied to the HVAC, the possible major challenges are the design and development of efficient power ultrasonic systems capable of large scale successful operation specifically adapted to each individual process. [ABSTRACT FROM AUTHOR]

The surface vapor pressure of desiccant solutions is the key parameter for modeling the liquid desiccant system. Usually, the simple mixing rule (SMR) model and the nonrandom two-liquid (NRTL) model are employed by people to calculate the surface vapor pressure of mixture desiccant solutions during simulating the performance of a liquid desiccant system. However, there is little literature caring about the calculation accuracy of the two models for surface vapor pressure of mixture desiccant solution under different circumstances. In this paper, an experimental system is built to measure the surface vapor pressure of LiCl–CaCl 2 desiccant solutions under various conditions including temperatures ranging from 25 °C to 75 °C, total mass concentrations ranging from 5% to 40% and three mass mixing ratios of LiCl to CaCl 2 (e.g., 1:1, 1:2, 2:1). The two models for surface vapor pressure of mixture desiccant solution (i.e., the SMR and the NRTL) are compared with the test data. It is found that the SMR model has much better accuracy than the NRTL model in forecasting the surface vapor pressure of LiCl–CaCl 2 mixture desiccant solution when the solution mass concentration is lower than 10%, and it is the opposite for the case when the solution mass concentration is higher than 30%. Afterwards, the revised equations for the SMR and the NRTL model are proposed according to the test data within the conditions concerned in this study. The calculation accuracy of the two vapor pressure models is proved to be greatly improved after the revisions. [ABSTRACT FROM AUTHOR]

Abstract: The ultrasound is applied to the regeneration of a new honeycomb desiccant material to decrease its regeneration temperature. Different inlet air conditions, i.e., 40, 45 and 50 °C in temperature, 0.008 and 0.012 kg/(kg dry air) in humidity ratio, combined with 0.5, 0.7 and 0.9 m/s in airflow rate, are designed for the experimental study. The initial moisture content of desiccant material sample used in this study is 0.35 kg water/(kg dry sample), and the acoustic frequency and power of ultrasound are 23 kHz and 30 W, respectively. Three indexes, ERR (Enhancement Ratios of Regeneration), ERARR (Enhancement Ratios of Average Regeneration Rate) and AEE (Average Electricity utilization Efficiency), are used to evaluate the enhancement effect and energy-saving characteristic of using ultrasonic. The experimental results show that the enhancement effect of ultrasound on the regeneration is greatly influenced by material structural parameters and regeneration conditions. It increases with the airflow rate (increasing from 0.5 m/s to 0.9 m/s), and decreases with the drying air humidity (increasing from 0.008 to 0.012 kg/(kg dry air)). The drying air temperature has little effects on the ERARR. The AEE decreases with the regeneration air temperature, humidity ratio and velocity. Besides, applying ultrasound to the regeneration of M-type material is more effective. Furthermore, the contributions of ‘heating effect’ and ‘micro-vibration effect’ induced by power ultrasonic are discussed and the results show the contributions of the micro-vibration effect dominate. Meanwhile, a model is developed to predict the regeneration rate of the new desiccant material assisted by ultrasound. [Copyright &y& Elsevier]

Abstract: The energy-saving characteristic of silica gel regeneration with power ultrasonic was analyzed by introducing the conception of specific energy consumption. For the purpose, the experiments of silica gel regeneration with 21-kHz power ultrasound were performed under different drying air temperatures (i.e., 35, 45, 55 and 65°C) combined with different acoustic power levels (i.e., 0, 20, 40 and 60W). And the energy saving ratios of the ultrasonic-assisted regeneration were studied by the method of ANOVA (Analysis of Variance) and compared among different conditions. The influences of acoustic power and drying air temperature as well as the target moisture ratio (at which the regeneration process ended) on the total specific energy consumption (TSEC) and the excess specific energy consumption (ESEC) were also discussed. The results indicate that all the factors (drying temperature, ultrasonic power level and the interaction between the drying temperature and the power level) have a significant (P <0.05) influence on the energy saving ratio, among which the influence of drying temperature is the most significant (P <0.05). According to the analysis of specific energy consumption, the optimal drying conditions aiming at the minimum energy use can be obtained. For the present experimental conditions, the condition of 55°C (drying temperature) and 60W (acoustic power level) can achieve the lowest TSEC and ESEC. In addition, different thresholds of power level are required to achieve the energy-saving effect due to the application of ultrasonic in the regeneration. The method of specific energy consumption can be also used for the energy analysis of the new regeneration technology in the scale-up study. [Copyright &y& Elsevier]

Abstract: Regeneration of dehumidizers is the most important stage in the working cycle of desiccant system. The lower regeneration temperature will be favorable for the energy efficiency of the whole system. Ultrasonic technology may be a promising method of dehydration applied to the regeneration of desiccant. As a non-heating method, the power ultrasonic may help lower the regeneration temperature and bring about energy savings. In the present paper, the mechanism of ultrasonic regeneration is set forth based on the ultrasonic theory as well as the mass transfer model in solid–gas and liquid–gas system. The recent studies related to ultrasonic dehydration are extensively reviewed, which is of significant reference to the study of desiccant regeneration assisted by power ultrasound. In addition, this work gives the basic ideas of ultrasonic dehydrator for solid/liquid-desiccant regeneration, which will promote the development of relevant equipments. Finally, some unexplored issues on this topic are addressed, including insight into the effects of ultrasonic on the regeneration, drying kinetics model for ultrasonic regeneration and the challenges possibly faced for the ultrasonic transducer development. [Copyright &y& Elsevier]

Abstract: A new regeneration method using power ultrasound was put forward to overcome the limitations of silica gel in air-conditioning applications, such as high regeneration temperature and low regeneration efficiency. The technical feasibility of the new method was validated experimentally and demonstrated in detail from different sides. The experiments were performed under different regeneration temperatures, i.e. 45°C, 55°C, 65°C and 75°C. The power and frequency of ultrasound applied in this experimental study was set as 40W and 26kHz, respectively. The three indicators, including the regeneration degree (RD), enhanced rate of regeneration (ER) and energy-saving rate (ESR), were suggested to evaluate the effect of power ultrasound in the regeneration. The Crank’s diffusion model was used for the calculation of the moisture diffusivity in silica gel, and the Arrhenius equation for the determination of energy activation of moisture desorption on silica gel. The analysis results prove that the introduction of high-intensity ultrasound to the regeneration of silica gel can help to improve the regeneration efficiency and reduce regeneration energy. The benefits should owe to the special ‘heating effect’ and ‘micro-vibration effect’ caused by power ultrasound that can enhance the moisture diffusivity in silica gel and lower the activation energy of moisture desorption on silica gel. [Copyright &y& Elsevier]

It is well known that internally-cooled membrane-based liquid desiccant dehumidifier (IMLDD) has better dehumidification efficiency than adiabatic membrane-based liquid desiccant dehumidifier (AMLDD). The flow directions of air, solution, and cooling water in the IMLDD can dramatically affect dehumidification and cooling performance. In this paper, the mathematical models for the IMLDD of ten flow types were developed and validated experimentally. Based on the validated model, the differences of air temperature and humidity among various flow types of the IMLDD were investigated, and the effects of operating conditions on the performance of different flow types were also studied thoroughly. The main conclusions of this paper include the following: The flow types of IMLDD have different average temperature and humidity differences between air and solution. Under different operating conditions, the IMLDD of flow type b4 always has a maximum total cooling capacity. The moisture removal rate of flow type b4 is up to 8.2% greater than flow type a2 of worst performance, and for the sensible cooling capacity, the flow type b4 is up to 5 times flow type a2. This paper can help researchers and engineers to choose the appropriate form of the IMLDD according to inlet conditions. • Mathematical model for ten flow types of IMLDD was developed and validated. • Average temperature and air humidity differences for various flow types of IMLDD are different. • The cooling capacity and moisture removal rate of IMLDD are 155% and 64.6% than AMLDD. • Performances of different flow types under varying operation conditions are investigated. • Cross-flow between air and solution and counter-flow between air and cooling water is recommended. [ABSTRACT FROM AUTHOR]

• Mathematical model for IMLDD was developed and validated experimentally. • Dehumidification performances for IMLDD of different flow types were compared. • Exergy destruction and entransy disspation were adopted to analyze IMLDD. • Unmatched coefficients were used to represent the loss of mass transfer. • Flow type of IMLDD with the best performance has been found. Internally-cooled membrane-based liquid desiccant dehumidifier (IMLDD) can well alleviate the dehumidification deterioration due to the increase of desiccant solution temperature. In the IMLDD, air channels and desiccant solution channels are separated on both sides of semi-permeable membrane. Cooling media flows through tubes inside solution channels to reduce desiccant solution temperature. There exist many flow patterns of fluids (air, desiccant solution and cooling water), which have been proved to have significant effects on the performance of IMLDD. Therefore, to find the optimal flow type of IMLDD and operating conditions, the mathematical model for IMLDD was developed and validated experimentally, and the exergy and entransy theories were first adopted to analyze the effects of inlet parameters on the performance of IMLDD of various flow types. The results showed that counter flow arrangement between air and solution and parallel flow arrangement between solution and cooling water had the best dehumidification performance. The trend of dehumidification efficiency was opposite to exergy efficiency with the increase of inlet air humidity ratio and solution concentration. The increase of inlet cooling water temperature improved exergy efficiency but decreased dehumidification efficiency accordingly. The research of this paper is helpful for the design and optimization of IMLDD. [ABSTRACT FROM AUTHOR]

• A state-space model for a membrane-based liquid desiccant dehumidifier was developed. • The dynamic model was validated experimentally. • The transient responses to sudden changes of inlet parameters were investigated. • The state-space model is helpful for control design and optimization. In this paper, a state-space model for a membrane-based liquid desiccant dehumidifier with counter-flow configuration was developed and validated experimentally. Compared with experimental data, the maximum relative error of model results is less than 2%, and the mean relative error is about 1%, which indicates that the state-space model can well predict the transient characteristics of the membrane-based liquid desiccant dehumidifier. Further, the transient responses of the dehumidifier to the step changes of inlet parameters (air temperature and humidity, air flow rate, solution temperature and flow rate) were investigated in detail. This dynamic model can clearly reflect the transient relationships among input, state and output variables. It was found that the effect of perturbation of the inlet air temperature on the moisture transfer in the membrane-based dehumidifier was too small to be neglected, and the variation of the inlet air humidity had little effect on the heat transfer. The response time constant of the outlet solution temperature to the inlet perturbations was obviously larger than that of the outlet air temperature and humidity. The air and the solution flow rate can be taken as the controllable parameters for the outlet air humidity control of membrane-based liquid desiccant dehumidifier. With the advantages of short calculation time and simple solving process, the state-space model is helpful for the control design and the optimization of the membrane-based liquid desiccant dehumidifier. [ABSTRACT FROM AUTHOR]

The photo-crosslinked polyamide 6 (PA 6)-LiCl nanofibrous membranes (NFMs) with presence of benzophenone (BP) and triallyl isocyanurate (TAIC) have been successfully manufactured as novel desiccant materials. The FTIR spectra illustrated the crosslinking of the PA 6. The sorption isotherms suggested that the crosslinked PA 6-LiCl NFMs could sorb 0.7 g g−1 water vapor at 25 °C and 80% relative humidity, which was 3 times than that of the traditional porous desiccants. The novel desiccant membranes were also featured by fast sorption rate and low regeneration temperature such as 50 °C. The cycling tests showed that the sorption capacity of the crosslinked PA 6-LiCl NFMs decreased only about 2% after 100 cycles, which makes the new desiccant materials more applicable in real situations. • Photo-crosslinked PA6-LiCl nanofibrous membranes were successfully manufactured. • Sorption capacity of the novel desiccant membranes could reach more than 0.7 g g−1 at 25 °C/80% RH. • Novel desiccant membranes can be regenerated sufficiently under low temperatures between 40 °C and 60 °C. • The novel desiccant membranes are featured by fast sorption and desorption rate due to their nano-structure. • Photo-crosslinked PA 6-LiCl nanofibrous membranes have improved cycling stability. [ABSTRACT FROM AUTHOR]