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Predictions of moisture removal efficiencies for packed-bed dehumidification systems
Abstract
Liquid desiccant-based dehumidification systems have been widely used to remove water vapour from air in a packed column using different liquid desiccants. The liquid desiccants are usually grouped into two categories: aqueous solutions of inorganic salts and aqueous solutions of organic compounds. In order to design such a desiccant—dehumidification system, correlations of the column performance parameters are necessary. A correlation of column efficiency for different packings and desiccant solutions was developed in this study using lithium chloride (LiCl) as the inorganic salt and triethylene glycol (TEG) as the organic compound. This correlation involves the air and liquid flow rates, air and liquid inlet temperatures, column and packing dimensions, and the equilibrium properties of the desiccant solutions. The correlation was tested for polypropylene Flexi rings, ceramic Berl saddles, glass Raschig rings and polypropylene Pall rings. The average value of the errors between predicted values and experimental data was about 7%.
... Since the evaporating cooler water temperature, T w , and the ambient air temperature, T aa , have not changed much in our present experiments, we have evaluated another correlation based on popular Chung's correlation [40] which is used for the counter Transactions of the ASME flow packed-bed dehumidifier. It is given by ...
... The important depending parameters, which influence these performance parameters, are mass flowrate of solution, solution temperature, inlet air temperature, and relative humidity. The moisture and enthalpy effectiveness are evaluated and compared with those of Cheng et al. [35] and the new correlation for moisture effectiveness is developed based on the depending parameters and also another correlation is developed based on Chung's correlation [40]. In order to study the effects of different parameters on system performance, a number of experiments have been carried out and the important findings are as follows: ...
The present paper deals with the experimental study of the liquid desiccant air conditioning system using the single storage solution tank. The novelty of the system is that the dehumidification and regeneration are carried out in a single compact unit. The regeneration of solution is done using the marquise-shaped solar collector. The liquid desiccant solution used here is calcium chloride and water. The moisture removal rate (MRR), moisture, and enthalpy effectiveness are used as the performance parameters. The effects of mass flowrate of solution, solution temperature, inlet air temperature, and relative humidity (RH) on the performance are investigated. The experimental outcome shows that when the solution flowrate is increased from 0.263 to 0.437 kg/s, the MRR is improved from 5.08 to 7.82 g/kg and when the RH is increased from 70% to 92%, the MRR is enhanced from 5.56 to 12.45 g/kg. The new correlation for moisture effectiveness is developed based on the experimental values and depending parameters, and also another correlation is developed based on Chung’s correlation (Chung, T.-W., 1994, “Predictions of Moisture Removal Efficiencies for Packed-Bed Dehumidification Systems,” Gas Sep. Purif., 8(4), pp. 265–268).
... The analysis of the dehumidifier is based on the model developed by Gandhidasan [37] and Chung [38] which is based on the following assumptions. ...
... The effectiveness of the dehumidifier is expressed as follows [38]. ...
In this paper, we provide thermal analysis and design methodology of a liquid desiccant assisted dew point indirect evaporative cooling system. The purpose of the system is to serve as an alternative for conventional vapour-compression based building air conditioning systems in providing satisfactory human thermal comfort conditions in the hot and humid climatic regions. The main features of the study are the following: i) novel incorporation of a forced parallel flow direct solar regenerator and a dew point indirect evaporative cooler within the same air conditioning unit; ii) detailed thermal modelling of each of the system components with lesser simplifying assumptions with respect to earlier works; iii) large cooling capacity (∼18.8 TR) under harsh climate; and, iv) a comprehensive year-around case study for system operation in a hot and humid location (Kolkata, India). Our thermal model is validated with a reference model study. The maximum room air temperature predicted by the current system for yearlong analysis is 26.7 °C. The thermal COP of the system for diurnal operation in the most humid month of a calendar year (July) varied between 0.40-0.96. The cooling system can prevent overheating of the conditioned space, as specified by ASHRAE Standard 55-2017, throughout the year.
... These parameters affected the overall mass transfer coefficient with a similar trend though less sensitivity was shown towards the desiccant flow rate. In a related study, Chung et al. [49] presented different correlations for various packing materials and mixtures of a couple of desiccants for predicting moisture removal efficiencies. ...
... Moon et al. [80] through experimental study, provided mass transfer data of a CaCl 2 cross-flow dehumidifier with structured packing. An attempt to compare the results with those of counter-flow in [49] and cross-flow in [75] failed to show good agreement. Given this development, a novel empirical correlation was formulated with regards to dehumidifier effectiveness whose outcomes fitted well within the range of 0.4-0.8 at ±10% discrepancy with the experimental values. ...
In this paper, a comprehensive technical review of liquid desiccant (LD) dehumidification and regeneration techniques is presented. The operational features, processes and performance indices of various flow configurations of adiabatic dehumidifier and regenerator are extensively covered. The heat and mass transfer assessment is presented in terms of past experimental and modelling evaluations and procedures. The existing adiabatic dehumidifier/regenerator heat and mass transfer models are categorized into finite difference, effectiveness-number of transfer units and simple empirical correlation models. The respective performance prediction models are critically analysed in details and compared in terms of assumptions, iterative procedures, solution methods, accuracy, computation time, output variables and applications. The solar regenerator models are also highlighted with a focus on the collector module. The ideal settings, formulation procedures, current state-of-the-art and opportunities for improvements are outlined. The review provides meaningful insight into the research status and available opportunities in the LD adiabatic dehumidifier and regenerator modelling and optimization as well as conceptualization of the applicable models. Finally, some very impactful suggestions for improvement and further research are outlined.
... The coupled heat and mass transfer process between the solution and the air in the packing tower is a rudimentary issue that needs to be considered both in the dehumidifier and in the regenerator. Simulation models and experimental results for this process have been presented and optimized with numerous influencing factors, including the packing structure, such as the packing dimension [3] and packing thickness [4], solution type [3], flow path (mainly counter flow, cross flow, or parallel flow), inlet states of the air and solution [5], etc. The effects of the inlet parameters of the air and solution on the process are of great importance [6], and usually include the flow rate, temperature, concentration of the solution, humidity ratio of the air, etc. Moon et al. [7] has investigated the effects of the above influencing factors on the dehumidifier performance, which have also been investigated in other studies [8,9]. ...
... The coupled heat and mass transfer process between the solution and the air in the packing tower is a rudimentary issue that needs to be considered both in the dehumidifier and in the regenerator. Simulation models and experimental results for this process have been presented and optimized with numerous influencing factors, including the packing structure, such as the packing dimension [3] and packing thickness [4], solution type [3], flow path (mainly counter flow, cross flow, or parallel flow), inlet states of the air and solution [5], etc. The effects of the inlet parameters of the air and solution on the process are of great importance [6], and usually include the flow rate, temperature, concentration of the solution, humidity ratio of the air, etc. Moon et al. [7] has investigated the effects of the above influencing factors on the dehumidifier performance, which have also been investigated in other studies [8,9]. ...
Direct contact between humid air and water/liquid desiccant (LD) solution is common in air-conditioning systems, where transfer (heat/mass) and conversion (evaporation/condensation) processes occur. This work analyzes these processes using exergy theory and a two-film model. The exergy flow and exergy destruction of heat transfer and mass transfer can be expressed using a unified exergetic expression and depicted by a psychrometric chart. Results show that the equivalent air film of the water/solution is a medium for exergy flow and is thus used to determine the exergy change of the water/solution. Direct evaporative cooling and condensation dehumidification are two representative cases between humid air and water. In the former, the thermal exergy and humid exergy both flow from the humid air to water, where exergy destruction and conversion of the humid exergy into thermal exergy are inevitable. In the latter, thermal and humid exergy flows are all reversed. Dehumidification and regeneration are two crucial and reversed processes between the humid air and solution. The concentration exergy of the solution is involved in the thermal and humid exergy flows. The solution exports its concentration exergy to complete the exergy flows during dehumidification, and it obtains the concentration exergy from exergy flows during regeneration.
... Experimental performance on adsorber in previous investigations and earlier researches has been tabulated in (Table 1). (Table 2) The observations proposed by [45][46][47] were primarily proposed and evaluated packing, but it can be referred by many investigators afterwards for structured packing's [48]. Moreover, standard relations are used mainly in design for desiccant dehumidifiers only, correlation by [44,49] can be applied in case of packed bed type desiccant regenerator also. ...
... The experimental error observed case in effectiveness evaluation was found as ± 0.08 for dehumidifier and ± 0.17 for regenerator. The relations proposed by [44,46,50] found to be more accurate as they are in nondimensional form and account not only for the working existing room air and liquid conditions but also for the packing size. (Table 2) show empirical formulation for the dehumidification efficiency and regenerator performances respectively. ...
Potential option to replace the traditionally used vapor compression system with desiccant based dehumidification and cooling to overcome the problems in VCR use like as substantial consumption of high grade electrical energy and to eliminate the use of the CFC based refrigerants which are responsible for the depletion of ozone layer. The desiccant cooling can be proved to be an efficient in highly moist atmosphere to handles the latent cooling load of the conditioned space. The present overview explains about the detailed ideas for making use of various chemicals as the desiccant solution for their optimum cost and characteristics. The desiccant cooling can handle both humidity and temperature separately and effectively to produce necessary thermal comfort within the conditioned space. The desiccant cooling can find optimum use of renewable solar energy in air conditioning by applying them for desiccant regeneration to lower the consumption of electricity which produced mostly by fossil fuel based power plants which leads to problem of pollution subsequently. The present review provides the direction for effective use of the desiccant based cooling for separable control over temperature and humidity in case of both residential and industrial use to ameliorate the dual-energy and cost saving.
... The predicted values from both of their correlations agreed with the experimental results within ±18%. Moon et al. [8] calculated the humidity effectiveness using the equations of Liu et al. [9] and Chung [10] and compared the calculated values to their experimental results, but they found that the calculated values from both equations do not fit well with their experimental data. Hence, they fitted their experimental data to the general form of the dehumidifier effectiveness from Refs. ...
... Hence, they fitted their experimental data to the general form of the dehumidifier effectiveness from Refs. [9,10] and compared the predicted values from their equations with their experimental data, which resulted to within ±10% deviations. Liu et al. [11,12] utilized analytical solution to derive equations for the enthalpy and moisture effectiveness. ...
This paper presents general types of correlation for the heat and mass transfer coefficients inside an air-solution contactor as expressions of Reynolds-Prandtl numbers and Reynolds-Schmidt numbers, respectively. These general equations summarize the physical and thermophysical properties of the air, the solution, and the contactor, which make them capable to be used for parametric studies provided they are fitted in a wide range of experimental data that include all the properties involved. In this work, a liquid desiccant system with an adiabatic structured packed bed as contactor and an aqueous lithium chloride as solution was constructed. The experimental data taken at various air superficial velocities and solution flow rates were fitted to the general correlations, and comparisons between the predicted and experimental results for both coefficients are within ±10%, for both dehumidification and regeneration processes. In addition, the calculated values of the outlet air humidity ratio and temperature agree well with the experimental data for both processes. The particular equations for the heat and mass transfer coefficients can be used to perform parametric studies at different air superficial velocities and solution flow rates with very good accuracy. Results from this study can help improve the system design and operation methods of air-solution contactors.
... The performance of this type of packing is compared with other two types of structured packing available in literature. The experimental data are compared with the predicted values using correlations reported by Chung (1994) and Moon et al. (2009). New empirical correlations of the moisture and enthalpy effectiveness are developed and validated in a wider experiment range. ...
... Empirical correlations of moisture effectiveness and enthalpy effectiveness As mentioned above, the dehumidification capacity of the dehumidifier (i.e. the moisture removal rate) can be predicted by Eq. (9) with the moisture effectiveness correlation. Moon et al. (2009) andChung (1994) developed the moisture effectiveness correlation for a cross flow and a counter flow dehumidifier with structured packing, respectively. The comparison between the predicted moisture removal rates from their correlations and the present experiment results are shown in Fig. 5. ...
This paper presents an experimental study on the dehumidification performance of a counter flow liquid desiccant dehumidifier using structured packing with a high specific surface area (650 m² m⁻³). New empirical equations correlating the moisture effectiveness and the enthalpy effectiveness with critical inlet parameters are developed, which can be used to conveniently predict the performance of a similar dehumidifier. The empirical correlations are validated using the experimental data of this study, and compared with the experimental data reported by another researcher. The deviations are within ±10% for the former and within ±15% for the latter. The performance of the present type of packing is also compared with other two types of structured packing available in literature. The influences of the inlet conditions of the air and the desiccant as well as the packing height on the dehumidification performance are also investigated and compared with the results reported in previous studies.
... The dehumidification effectiveness correlations presented by past researchers Moon et al. [29] Packed bed adiabatic Fig. 12 Representation of temperature profile of three fluids in internally cooled dehumidifier [35,36] are summarized in Table 4. It is evident that unlike packed bed towers [9,[26][27][28][29][30][31], the number of effectiveness correlations in falling film towers is limited [16,32,33]. The existing correlations in falling film towers were developed by considering experimental/numerical observation mainly for metallic surfaces, hence on validation, these correlations performed poorly against current experimental readings. ...
Plastics as air-liquid desiccant contacting surfaces are being explored as a promising alternative to metallic surfaces in falling film towers. Nevertheless, poor wetness of plastic has severely restricted its extensive practical applicability. In the current experimental study, surface modification technique is employed to enhance the dehumidification performance of single polypropylene plastic plate surface. Dehumidification ability of three kind of plastic plate surface: Plain PP plate, Modified PP plate A and Modified PP plate B were investigated and compared in an adiabatic vertical falling film tower. It was found that the best Modified PP plate offered 48.1 - 127.0% improvement in dehumidification effectiveness as compared to the Plain PP plate with an average improvement of 83.3%. Furthermore, a new correlation is developed to predict the dehumidification effectiveness of plastic and metallic surfaces of adiabatic/non-adiabatic falling film tower by incorporating the wetting factor and sensible cooling information in the correlation equation. The new approach takes into the account the sensible cooling (non-adiabatic absorber) of the desiccant solution in the upper half of the absorption column. The proposed correlation demonstrates excellent accord with eleven experimental datasets, with an average error of 12.7%.
... In order to eliminate the effect of experimental errors, 72 sets of experimental data available from the present and previous work [25,26,28] were used to obtain an empirical correlation of the latent effectiveness and inlet parameters as given by Eq. (30). The form is based on Chung's correlation for adiabatic absorption process [43], but further takes into account the effects of the cooling water flow rate and cooling wa- (30) to achieve a trade-off for the given conditions. As shown in Fig. 12, the predicted results agree with the experimental data with 84.7% of the data within the relative discrepancy of ±20%. ...
Liquid desiccant dehumidification provides a pathway to high-flow air pretreatment of air compressors for energy savings. However, high air-to-solution flow ratios (i.e., over 4.0) may result in an unacceptable decrease in dehumidification effectiveness, and few studies have managed to overcome this challenge. This study aims to experimentally demonstrate that the multi-stage internally-cooled liquid desiccant dehumidifier (MILDD) is capable of improving the effectiveness at extremely high air-to-solution flow ratios over 10.0. A laboratory bench of the MILDD is designed and tested in various operational conditions. Based on the finite difference model, the experimental results of dehumidification effectiveness are analyzed in terms of the heat and mass transfer process such as irreversible loss and driving forces. The specific cooling capacity associated with the energy efficiency is further studied by considering different desiccant regeneration efficiency. In addition, the experimental latent effectiveness from present and previous work is compared and correlated. Results show that the measured latent effectiveness of the MILDD exceeds 0.42 and goes even up to 1.02 at high air-to-solution flow ratios, i.e., 8.6–20.1, while existing liquid desiccant dehumidifiers maintain a comparable effectiveness only at much lower flow ratios, i.e., below 4.0. The proposed model and correlation also have been validated with a considerable accuracy for predicting the performance of internally-cooled dehumidifiers. This work has experimentally demonstrated the ability of the multi-stage internally-cooled liquid desiccant dehumidifier to overcome the low effectiveness at high gas–liquid flow ratios, which advances the potential application of liquid desiccant dehumidification in the air compression process.
... When the u decreased from the higher figure to the initial figure, some new paths that formed under the former higher u condition were still reserved [40]. The liquid hysteresis was inferred considering that the response of the liquid drainage process lagged behind the irrigation amelioration [47]. The amount of immobile liquid could not totally flow out of the heap and was still retained in intra-particle pores ( Figure 5C). ...
Liquid is a crucial medium to contain soluble oxygen, valuable metal ions, and bacteria in unsaturated heap leaching. Liquid retention behavior is the first critical issue to be considered to efficiently extract low-grade minerals or wastes. In this study, the residual liquid holdup of an unsaturated packed bed was quantitatively discussed by liquid holdup (θ), residual liquid holdup (θresidual), relative liquid holdup (θ′), and relative porosity (n*) using the designed measuring device. The detailed liquid holdup and the hysteresis behavior under stepwise irrigation are indicated and discussed herein. The results show that relative porosity of the packed bed was negatively related to particle size, and intra-particle porosity was more developed in the −4.0 + 2.0 mm packed bed. The higher liquid retention of the unsaturated packed bed could be obtained by using stepwise irrigation (incrementally improved from 0.001 to 0.1 mm/s) instead of uniform irrigation (0.1 mm/s). It could be explained in that some of the immobile liquid could not flow out of the unsaturated packed bed, and this historical irrigation could have accelerated formation of flow paths. The θ was sensitive to superficial flow rate (or irrigation rate) in that it obviously increased if a higher superficial flow rate (u) was introduced, however, the θresidual was commonly affected by n* and θ′. Moreover, the liquid hysteresis easily performed under stepwise irrigation condition, where θ and θresidual were larger at u of the decreasing flow rate stage (DFRS) instead of u of the increasing flow rate stage (IFRS). These findings effectively quantify the liquid retention and the hysteresis behavior of ore heap, and the stepwise irrigation provides potential possibility to adjust liquid retention conditions.
... As mentioned above, the moisture absorption capability of ILs consisting of small non-protic imidazolium cations decreases as the cation alkyl chain length grows, according to Ficke and Brennecke. 18 This trend is generally obtained also for monocationic quaternary salts studied here, that is, [N 1 2 showed the best performance in the dehumidification rate (entry 8) among 16 different quaternary ammonium salts, as listed in Table 1; for example, its rate per mole is 5.6-fold faster than [HMC2][DMPO 4 ] 2 . However, we found it very difficult to dehydrate aqueous solutions of the ether-oxygen-containing dicationic quaternary ammonium salts due to their strong deliquescence nature. ...
... The performance analysis was based on the development of correlations to include in the energy and mass balance equations of the liquid desiccant process [7]. Based on the analysis of experimental results [8][9][10][11][12][13][14] and correlations available in the literature [13][14][15][16][17][18][19][20], Eq.(1) was developed for the prediction of the moisture effectiveness, εω, and enthalpy effectiveness, εh, of dehumidifiers and regenerators [21] with cellulose structured packing: (1) ...
Automotive manufacturers are continuously striving to reduce the energy consumption of their production process due to economic, environmental, and policy compliance reasons. This study investigated the integration of the liquid desiccant technology into the conventional air management system for painting operation. The technology was appealing for its ability to control humidity, work independently as a dehumidifier and a regenerator, and store energy in a thermo-chemical form. In addition, waste heat sources available in the paint shop can be used for the regeneration of the liquid desiccant solution. Techno-economic assessment analysis of this novel process was conducted in the study for two different outdoor air conditions. Economic savings in terms of electricity and fuel consumption were achieved, resulting in a payback period of approximately 6 years and important benefits over the technology lifetime.
(Published in the "Proceedings International Sorption Heat Pump Conference 2021 - Part II Call 2021" under the
creative commons license CC-BY-NC 4.0.
https://creativecommons.org/licenses/by-nc/4.0/deed.en
http://dx.doi.org/10.14279/depositonce-10430.2)
... A comparative study was done for HVAC systems with three commonly used desiccants namely, lithium chloride, calcium chloride and ethylene glycol. Similarly, Chung [19] had conducted a study for predicting the efficiency of moisture removal in packed bed dehumidification systems. The study focused on lithium chloride and TEG desiccants. ...
Air water generators that harvest water from air humidity have the potential to counter the ever-rising problem of drinking water scarcity. There are many different types of air water generation systems that work on various different principles. Desiccant based air water generation systems work on the principle of moisture absorption, consisting of a packed bed dehumidifier that absorbs the moisture from air. This reduces the energy requirement of the system. To discuss the efficiency of the system, it is crucial to understand the working of the packed bed column. In this paper, a mathematical model has been developed for a packed bed dehumidification system using aqueous CaCl2 as the liquid desiccant. This model has been developed using water saturation pressure and equilibrium relative humidity models. The packed bed model has been used to study the effect of various input parameters like air and desiccant flow rate, packing material, relative humidity and desiccant concentration, on the capacity of the desiccant to absorb water from air. The results so obtained can be used to predict the water that can be absorbed by the desiccant in the packed bed column for given inlet conditions.
... Reviews of the early works on the development of liquid desiccant systems have also been provided by several researchers [7][8][9][10]. Packed columns are the conventional devices to evaluate these systems [11][12][13][14][15][16]. However, separation in Packed columns can lead to some problems such as foaming, channeling, flooding and liquid hold up. ...
In this study, tri-ethylene glycol (TEG) based nanofluids of carbon nanotubes (CNT), silica (SiO2) and aluminum oxide (Al2O3) in different weight percents of 0.01 and 0.05 wt% were employed to improve the properties of TEG for water vapor absorption in a hollow fiber membrane contactor (HFMC). To do so, the impact of various operational parameters including nanofluid type, nanoparticles (NPs) concentration, liquid and gas flow rate on water vapor separation were elucidated. According to obtained results, CNT nanofluid was more capable on water vapor absorption toward SiO2 and Al2O3, so that the injection of CNT nanofluid led to water vapor absorption enhancement up to 10.8 % as compared to TEG solution alone. The results also indicated that the gas flow rate has an inverse relationship with the amount of water vapor absorption efficiency, whereas increasing the liquid flow rate has no tangible effect on absorption efficiency, which is because of the very high solubility of water vapor in TEG. Ultimately, dynamic light scattering (DLS) analysis was carried out to evaluate the stability of nanofluids and size distribution of nanostructures in the base fluid.
... The solution must be heated before entering the REG because the humidification process is an endothermic reaction. The LD system is continuously operated by repeating the dehumidification and regeneration processes [3,14,15]. The solution is cooled and heated by the heat sources, and a solution HX is used to reduce the heating and cooling load for the solution. ...
The objective of this study was to modify an existing liquid desiccant and indirect/direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) for humidification operation in winter. The energy benefit of the liquid desiccant-assisted humidification approach during the operation of LD-IDECOAS over the conventional method with a steam humidifier was evaluated through a detailed energy simulation. The humidification and enthalpy effectiveness values of the liquid desiccant humidification measured from laboratory tests were 0.41 and 0.49, respectively, which were applied to the energy simulation for the modified system. Both systems with the proposed and conventional humidification approaches were simulated using an engineering equation solver combined with a TRNSYS 18 energy simulation program. The results demonstrated that the modified LD-IDECOAS consumes less energy for humidification compared to the existing system with a steam humidifier. The proposed system also exhibited considerable heating energy-saving potential. Consequently, modified LD-IDECOAS consumed 42% less primary energy during winter operation after being modified for liquid desiccant-assisted humidification.
... This design provides a large contact area between liquid desiccants and air. It provides direct contact, which increases heat and mass transfer rate [146,147]. Packing materials can be either random or structured packing, which is shown in Fig. 6, Random packings include raschig rings and intalox saddles; and structured ones are wood grids and double spiral rings [24,148]. Random packing needs LD's flow rate for wetting, and it usually has a high-pressure drop, but they establish good contact between air and liquid desiccant [91]. ...
Air conditioners (ACs) use an enormous amount of energy as they are widely used to provide people with favorable indoor conditions. In recent years, liquid desiccants have received considerable attention as they can reduce the energy consumption of ACs in particular. This paper examines the different liquid desiccant materials, properties, operating conditions, desiccant dehumidifiers, performance improvements with different regeneration configurations. The paper also highlights the effect of the liquid desiccant system on indoor air quality and, more specifically, on volatile organic carbon, particulate matter, and the removal of bacteria. Besides, key performance parameters are introduced in order to understand the performance of liquid desiccant better. The operation of different materials or mixtures of liquid desiccants can be effectively compared with these parameters. Finally, various methods of liquid desiccant regeneration, including the most advanced method of photovoltaic/electrodialysis regeneration, are elucidated. The review could provide a suitable guidance for design and operation of a liquid desiccant-based building air conditioning system.
... Table 4 presents the summary of the effectiveness correlations proposed by the previous researchers. It is clear that most of the previous humidity effectiveness correlations had been developed for the packed bed absorber/regenerator [33,[36][37][38][39][40][41][42]. Very few effectiveness correlations are developed for the falling film tower. ...
Recent interest in falling film tower put them ahead of the contemporaries for the development of low flow hybrid liquid desiccant air-conditioning system targeting residential applications. Air-liquid desiccant contacting surface in case of metal suffer from corrosion problem thus require frequent replacement. Non-metallic surface such as plastic suffers from poor wettability. In the current experimental study, regeneration of aqueous lithium chloride solution over adiabatic single polypropylene (PP) plate vertical falling film tower has been carried out. Two types of PP plates: Plain and Modified are prepared and tested. Regeneration rate of the Modified PP plate is around 54.6% superior to the Plain PP plate for the tested conditions. Also, new correlation for the estimation of air humidity effectiveness of the falling film regenerator has been developed using wetting factor information. Developed correlation predicts accurately the performance of the metallic as well as non-metallic surfaces for both adiabatic and internally heated conditions. The mean effective error for the current correlation is 18.1% for the eight datasets.
... Summary of packing materials and operating conditions as well as the resulted performance in direct-contact DRs[49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][65][66][67]. ...
Liquid desiccant cooling is being considered as an alternative to vapour compression air conditioning systems and has been extensively investigated in recent decades. The dehumidifiers and regenerators are the key components of liquid desiccant cooling systems and their heat and mass transfer performance significantly influences the performance of liquid desiccant cooling systems. This paper provides an overview of heat and mass transfer improvement techniques used to enhance the performance of direct-contact and indirect-contact dehumidifiers and regenerators used in liquid desiccant cooling systems. A number of techniques such as using a third heat transfer fluid, selection of packing materials, and tube arrangement, were reviewed and the performance of the dehumidifiers and regenerators using such techniques was summarised. The results showed that a large number of heat and mass transfer improvement techniques have been developed and used to enhance the performance of dehumidifiers and regenerators while further investigations on additives in liquid desiccants and surface modifications for direct-contact dehumidifiers and regenerators, and membrane deflection of indirect-contact dehumidifiers and regenerators might be still needed. In the meanwhile, design optimisation of packing materials (e.g. geometrical parameters) and fins with complex geometries for direct-contact dehumidifiers and regenerators, and inserts for indirect-contact dehumidifiers and regenerators may further improve their heat and mass transfer performance.
... With the fast developing of desiccant material, component performance, and system configurations of LDS, the capability of convenient and flexible simulation of LDS is highly desired to facilitate research and development. Various models describing the heat and mass transfer in LDS components were developed including dynamic model [21], steady-state finite-difference models from the solution side [22] or the air side [23,24], analytical models [25,26] avoiding iterative solution of governing differential equations, as well as curve-fitted empirical correlations connecting the performance of specific components to a range of operating conditions [27]. ...
Liquid desiccant systems (LDS) have recently seen an increase in research interest as they can utilize low-grade heat resources and separate the sensible and latent cooling loads by efficiently removing moisture in the air without cooling it to the dew point. However, simulation and analysis of LDS had remained complex and demanding due to the limited resources of LDS simulation tools. This work presents the new LDS module developed in the Sorption system Simulation program (SorpSim), which is an open-source and flexible platform for steady-state simulation and analysis of various sorption systems. First, the new LDS module containing a finite-difference model and an effectiveness-NTU model for the heat and mass transfer in LDS dehumidifier/regenerator component was introduced. Then the simulation results of the new module were verified using data from the literature. Finally, a case study was carried out in SorpSim where an LDS cycle was built and simulated using the new module. The impacts of design and operating parameters on the simulated LDS performance were investigated. The parametric study revealed that a high source temperature improved moisture removal rate (MRR) but reduced the system coefficient of performance (COP); the COP increased monotonically with the desiccant solution recirculation ratio, while the MRR peaked at a ratio of 85%; and an internal solution heat exchanger with UA of 800 W/K was found to be sufficient for optimal performance under high recirculation ratios. The case study demonstrated the LDS module’s capability to facilitate the analysis of LDS design and operation. The LDS module can be further coupled with other component models in SorpSim to simulate and analyze various liquid-desiccant-based systems.
... The DEH is modeled based on the works of Gandhidasan [6] and Chung [7]. The governing equations are given in equations (1) through (5). ...
In this paper, exergy analysis of a novel solar powered liquid desiccant assisted air
conditioning system is presented and simulated. The system aims to provide suitable thermal
comfort conditions inside large office buildings with high internal loads situated in the hot and
humid tropical/subtropical countries of the world. The system consists of process and
regenerating air streams, a liquid desiccant solution loop and a cooling water loop. The primary
objective of this study is to present the exergy of cooling capacity along with the overall
exergy efficiency of the proposed system. The study helps to quantify the optimum operating
and design parameters for system operation based on the second law of thermodynamics. For
the base case, which is representative of a hot and humid climatic condition, the proposed
system is able to maintain the room air conditions within the moderate thermal acceptability
criterion. The exergy of cooling capacity and exergy efficiency for the base case is about 2900
W and 2 % respectively. Parametric analyses show that the system performs the best under
conditions of high ambient insolation and temperature, low ambient humidity and a process air
to desiccant solution mass flow rate of about 3 in the dehumidifier.
... To regenerate the desiccant solution, an open packed bed system is used. This type of vapor-liquid contact devices is known for its high mass transfer efficiency in a liquid desiccant system [30]. In this study, a hot desiccant with a counter-flow configuration was considered due to its higher performance compared to other configurations [31]. ...
Displacement ventilation (DV) combined with liquid desiccant membrane cooled ceiling (LDMC-C) is an efficient Heating, ventilation and Air conditioning (HVAC) system. However, the LDMC-C/DV has limited performance in office spaces located in hot and humid climates or characterized by high internal latent loads due to the inability to control the humidity. This study proposes a cascaded liquid desiccant system in which the liquid desiccant flow exiting the LDMC-C is directed into a liquid desiccant heat and mass exchanger in the DV supply duct to control the humidity in the space and conserve energy. To achieve this goal, an integrated model for different components of the cascaded liquid desiccant system is developed to predict the comfort level and the air quality inside the space as well as the energy consumption of the system. The model was validated experimentally in a climatic chamber. The validated integrated system model was applied to a case study to assess the effectiveness of the system in hot and humid climate. Two configurations of the LDMC-C and mass exchanger were simulated and found to provide acceptable humidity for comfort in the occupied zone of the space. Moreover, the cascaded system cooling and heating energy were found lower by 21.25% and 16.23% respectively in August when compared to a LDMC-C/DV, and to DV membrane exchanger system with different configuration. The cooling and heating energies were also lower in the cascaded system compared to a LDMC-C/DV while using conventional supply air dehumidification method.
... The applied aqueous solutions of salts used for absorbing water are lithium chloride, calcium chloride and lithium bromide. The different types of glycols are ethylene glycol, di-ethylene glycol, tri-ethylene glycol, and tetra-ethylene glycol [22]. Some drawbacks to the aqueous solutions of salts are corrosion and salt crystallization; therefore, glycols are usually preferred in industrial applications [7]. ...
... The flow directions of process air and desiccant solution (i.e., counter-flow, parallel-flow, and cross-flow) are one of the main concerns for the design of the liquid desiccant dehumidifier configuration [14][15][16]. As for the counter-flow configuration, Chung [17] experimentally investigated the dehumidification performance of the counter-flow packed-tower dehumidifier with different packing materials and desiccant solutions, and then developed empirical correlations predicting the dehumidification effectiveness. Koronaki et al. [18] proposed a theoretical model, which was validated by experimental data, of an adiabatic counter-flow packed-tower type dehumidifier with three different desiccant solutions. ...
In a liquid desiccant dehumidifier, the air-to-solution flow direction plays an important role in the dehumidification performance and the physical size of the dehumidifier tower. In this research, the dehumidification performance of counter-flow and cross-flow liquid desiccant dehumidifiers were compared via a series of experiments carried out in the environmental test chamber. To predict the dehumidification performance of both flow type dehumidifiers in various operating conditions, empirical models returning the dehumidification effectiveness and the enthalpy effectiveness, which were adopted as performance indices, were also derived based on the experimental data. From the sensitivity analysis conducted by using the proposed empirical correlations, it was observed that when the inlet air humidity ratio increased from 10.1 g/kg to 22.7 g/kg, the dehumidification effectiveness and enthalpy effectiveness of the counter-flow liquid desiccant dehumidifier increased from 51.2% to 82.4% and 40.2% to 72.5%, respectively. Meanwhile, both effectiveness values of the cross-flow dehumidifier increased from 62.3% to 63.4% and 46.3% to 55.0%, respectively. Likewise, when inlet solution temperature varied from 15.2 to 31.1 °C, the dehumidification effectiveness and enthalpy effectiveness of the counter-flow type dehumidifier decreased from 71.7 to 45.4% and 64.4 to 47.3%, respectively, while those of a cross-flow type dehumidifier decreased from 65.3 to 54.8% and 55.8 to 45.2%. Consequently, one may conclude that the cross-flow liquid desiccant dehumidifier would provide relatively stable dehumidification performance regardless of the changes in operating parameter, but the cross-flow dehumidifier may have limited dehumidification performance, especially at the lower desiccant solution temperature and in highly humid process air conditions.
... The dehumidification effectiveness of the LD was evaluated using an existing model (Equations (10) and (11)) found in the literature [27,28]. The model is an empirical model applicable to an LD unit with a lithium chloride (LiCl) solution. ...
The aim of this study is to analyze the operating energy savings of a liquid desiccant and an indirect and direct evaporative cooling-assisted 100% outdoor air system (LD-IDECOAS) for marine applications. The LD-IDECOAS comprises a liquid desiccant (LD) unit and indirect and direct evaporative coolers (IEC and DEC) to meet the target supply air (SA) conditions. In this study, seawater was used as the cooling source and the waste heat reclaimed from the engine was used as the heating source in the proposed system. The operating energy of the LD-IDECOAS was determined based on detailed energy simulations conducted on two cabins with an area of 16.5 m², which was compared to a conventional system. The thermal loads on the cabins were estimated using design weather data under various oceanic climate conditions (normal, extremely hot, and extremely cold climates), by adhering to the ISO-7547 standard. The operating energy consumption of the LD-IDECOAS was calculated by modeling the proposed system with a commercial equation solver program (i.e., EES). The results were then compared to those of a conventional constant air volume (CAV) system. The operating energy consumption of the proposed system was reduced by 57–70% in cooling operations and 39% in heating operations under normal climate conditions. In the extremely hot climate regions, the energy consumed by the proposed system was reduced by 56–63% for cooling applications and 39% for heating applications.
... The authors[81,82]have also compared their results with Martin et al.[83]and found good agreement. Further, the moisture removal rate has been compared with Chung et al.[84]as shown inFig. 16. ...
In a global era, continuous increment in energy requisition with its associated cost and relevant climate problems is causing accentuation in exploring more efficient ways to provide air conditioning in enclosed space without degradation of the environment. In the hot and humid area, major part of conventionally produced electrical energy is consumed by air conditioning. Also in the rapidly growing world scarcity of clean water is dilemma as equal as greenhouse and ozone layer depletion. Thus, there is emergence of dehumidified air for human comfort conditions with having potable clean water same time. In summer with largely available solar radiation, use of hybrid solar system is viable option to overcome problems of dehumidification of liquid desiccant as well as of potable water. Up till now the conventional dehumidification with desiccant solution has been improved or replaced by less costly new systems. This paper provides an extensive literature review on development of liquid desiccant regeneration using solar energy. The paper also includes the recent findings of hybrid solar system in which either two sources of heat is used for regeneration of liquid desiccant or solar energy is used for regeneration of liquid desiccant along with other application.
... Better performance of regeneration with random packing could be obtained when the mass flow rate ratio of desiccant to air ranged from 2.5 to 3.0 ( (Huang et al. 2010). Additionally, one correlation of column efficiency was developed by Chung (1996) with LiCl and triethylene glycol aqueous solutions considering desiccant/air flow rate and temperature, packing column structure, and desiccant property. However, as most of the packings used above were self-made, performance studies of commercial packings, especially the comparison study of different packing types, are seldom conducted, and a full study is expected for helping the design of LDAC systems. ...
Packed liquid desiccant dehumidification systems are regarded as energy-saving and environmentally friendly technologies compared with conventional dehumidification technologies. The packing type plays a significant role in the system performance, but comparative studies of the different types are limited. First, this article investigates the dehumidification performance of liquid desiccant dehumidification with three packing types: corrugated structured, S-shaped polyvinyl chloride, and globular-shaped polypropylene. Then, the operation performance of solar-assisted liquid desiccant air-conditioning systems with different packings is simulated. A commercial building in Hong Kong was chosen as the case study. Results show that each specific packing surface area and structure determines the dehumidification performance by significantly influencing the liquid/air contact area and time. The enthalpy and moisture effectiveness of the corrugated structured packing were found to be 1.69 and 1.60 times those of S-shaped polyvinyl chloride packing, and 1.97 and 1.87 times those of globular-shaped polypropylene packing. The main reason is that the corrugated structured packing has a denser structure with a smaller volume of individual modules, which could provide a large specific surface area, but the dehumidification performance did not increase proportionally with the specific surface area. When the specific surface area increased from 81.8 to 537.3 m²/m³, the enthalpy and moisture effectiveness increased from 0.35 to 0.6 and from 0.38 to 0.61, respectively. Furthermore, the simulation results showed that for the commercial building, the maximum electricity savings percentage for globular-shaped polypropylene packing liquid desiccant air-conditioning systems, S-shaped polyvinyl chloride packing liquid desiccant air-conditioning systems, and corrugated structured packing liquid desiccant air-conditioning systems was 11, 21, and 42%, respectively. For globular-shaped polypropylene packing liquid desiccant air-conditioning systems, a minimum solar collector area of 219m² was required to avoid consuming more electricity. The authors also found that the dehumidification performance could be enhanced with an increase of desiccant/air mass flow rate ratio and inlet air temperature. This article will help researchers and engineers optimize the design of liquid desiccant air-conditioning systems with different packing types.
... Chung developed correlations for column efficiency of different types of packings and desiccant solutions (40% LiCl as well as 95% tri-ethylene glycol, TEG). He modified a previous study of Gandhisadan et al. and replaced the concentration parameter with the fraction of vapor pressure depression to vapor pressure of pure water [21]. He gave a more direct description of the relationship between the desiccant solution and column efficiency. ...
Liquid desiccant air conditioning systems (LDACs) are energy efficient and environment-friendly means of providing cooling as they are powered by low-grade thermal sources. They have favorable features like high density energy storage, ease of transfer of cooling media, possible higher COPs and better indoor air quality. The performance of liquid desiccant systems relies heavily on heat and mass transfer characteristics of two critical components: the dehumidifier and the regenerator. The popular choices for dehumidifier and the regenerator are packed bed tower and falling film columns. The spray tower design does not give high effectiveness and is not very popular. The purpose of this paper is to provide a review of heat and mass transfer studies done on both the packed bed towers and falling film type liquid desiccant-air contacting devices for absorption and desorption processes in dehumidifier and regenerator respectively. The review would help developing an insight into the subject to the new researchers in this area.
... In an ideal case the moisture absorbed by the desiccant solution must be equal to the moisture desorbed by the desiccant solution. But in actual case they may not be equal, as the air moisture removal effectiveness [11,[20][21][22][23][24], the enthalpy effectiveness [3,[25][26][27], and the vapour pressure difference ratio [27] suitable for dehumidification process will differ from the moisture absorption effectiveness [9,15], the vapour pressure difference ratio [9] and the enthalpy effectiveness [9] of the regeneration process. Therefore, a new co-relation has been developed to know the overall performance of the liquid desiccant dehumidification system by relating the moisture removal rate (condensation rate) of the dehumidifier with the moisture absorption rate (evaporation rate) of the regenerator. ...
Liquid desiccant dehumidification system is one of the emerging technology which can be used as a promising alternative to remove the excess moisture content present in the air before its being cooled. This paper describes a relatively simple mathematical model for the preliminary analysis of a regeneration process occurring in a counter flow adiabatic regenerator through a dimensionless water vapour difference ratio. To derive all the exit parameters of a liquid desiccant regenerator aforementioned ratio is used in terms of known inlet parameters. The model predictions are compared with the reliable set of experimental data available in the literature and a good agreement is found between them. The effects of condensation rate and humidification effectiveness on the performance of the regenerator are also studied and the results are presented in this paper.
... The study of dehumidifier is carried out by various researchers [6][7][8][9][10][11][12][13][14][15][16][17][18]. Chung and Wu [6] have designed packed dehumidifier with an inverse U-shaped tunnel and tested for the dehumidification of air. ...
Dehumidifier is the main component of liquid desiccant dehumidification system. Effect of the inlet parameters on various outlet parameters of the dehumidifier is studied in the present paper with structured pads as packing material and calcium chloride as liquid desiccant to process the air. The outlet parameters are change in specific humidity, mass transfer coefficient, moisture removal rate, air temperature, solution temperature, effectiveness and the corresponding inlet process parameters; mass flow rate of air, temperature of air, temperature and flow rate of desiccant solution. It is observed that mass transfer coefficient and moisture removal rate increase with increasing mass flow rate of the air and desiccant while these parameters decrease with increasing temperature of air and desiccant solution. Dehumidifier effectiveness gets increased with increasing solution flow rate. The present investigations are compared with the results of the researchers in the past.
... Thus, humidity effectiveness is necessary to indicate the tower efficiency in terms of humidity reduction compared to the theoretical limitation. The humidity effectiveness is expressed by the following equation as: To compare the results of present study with previous study, [12] empirical correlation to predict humidity effectiveness for packed bed tower was used and given by: ...
... Then, the humidity effectiveness shows almost a constant trend. This phenomenon is also observed from [6,7] who used a cross flow configuration for dehumidification process. Figure 6 shows the effect of inlet air temperature on the moisture removal rate and humidity effectiveness. ...
... (6)) was selected among the existing models for predicting ε ab in this research. Chung's model [27] was modified to this model, which showed approximately 7% deviation between the predicted values and experimental data. Martin and Goswami's [28] (Eq. ...
With the recent development of the independent control of temperature and humidity, the demand for dehumidification has significantly increased, and liquid desiccant dehumidification systems have attracted considerable attention because of a high dehumidification efficiency. This study presents a new design for a liquid desiccant dehumidification system that applies a membrane-assisted dual sump for maintenance the solution concentration with a low solution cooling and heating loads. A detailed heat and mass transfer analysis was conducted to evaluate the feasibility of the membrane for the maintenance the solution concentration according to the mass transfer resistance of the membranes. Moreover, the variation in the solution temperature and concentration was predicted by performing detailed simulations. The simulation results indicated that the low-mass-transfer-resistance membrane requires a large amount of solution load and the high-mass-transfer-resistance membrane cannot be used to maintain the solution concentration. Thus, the mid-mass-transfer-resistance (i.e., 15,000 s/m) membrane is suitable for the liquid desiccant dehumidification system. Compared with the conventional liquid desiccant dehumidification system based on a solution exchange, the membrane-assisted liquid desiccant dehumidification system could save about 19% load for solution cooling and heating owing to the lower heat and mass transfer rate on the absorber and regenerator sump.
The paper developed correlations for predicting moisture and enthalpy effectiveness of the liquid desiccant dehumidification and regeneration process. Experimental dehumidification and regeneration data available from the literature with different system dimensions, liquid desiccant solutions (LiBr and LiCl), geometry and size of the packing and flow configuration (counter- and cross-flow) were gathered for correlations development. The developed correlations involved the mass flow rates of air and desiccant solution, the inlet temperature of the air and the desiccant solution, the moisture content and enthalpy of the inlet air, the moisture content and enthalpy of air at an equilibrium state with the inlet desiccant solution, the geometry, dimensions and wetting of the packing and the contact time between air and desiccant solution. The comparison between the calculated and experimental effectiveness showed a good match, which had errors ranging 4.37–7.2%, and performed better when compared to other correlations available in the literature. These newly developed correlations will be useful for quick system design and performance analysis and to establish cost-effective solutions for liquid desiccant technology.
The corrosion problem of liquid desiccants hampers the development of liquid desiccant-based air-conditioning systems. Plastic surfaces can be a promising alternative because of their anti-corrosive property. However, plastic has the inherent problem of low wettability. The present experimental study investigated the regeneration performance of a liquid desiccant over multi-inline vertical polypropylene circular cylinder surfaces in an adiabatic mode, to find a substitute of vertical plate surfaces for developing low flow falling film towers. The mechanical surface modification method was explored to elevate the performance level of the circular cylinder surface. A comparative performance analysis was conducted between the circular cylinder and plate surface. It was found that the regeneration rate of the Conventional circular cylinder surface was superior to the Conventional plate surface by 50.5% for the evaluated conditions. The regeneration rate of the circular cylinder surface can reach up to 0.684 g/s at a 2.5 mass flow rate ratio of the solution to air. A new generalized empirical correlation was proposed to estimate the effectiveness of falling film regenerators by incorporating the driving potential of heat and mass transfer process and wetness parameters. The prediction accuracy of the present correlation was 16.5% (i.e. mean effective percentage error) against nine datasets. The outcome of the present study would be beneficial for developing low-flow solar-assisted plastic falling film liquid desiccant regenerators.
Researches on packed liquid desiccant dehumidifiers are abundant. However, only a few works concentrate on modeling the dynamic dehumidification process as it requires detailed heat and mass transfer coefficients along the dehumidifier, which are impacted by many factors such as flow pattern of liquid desiccant and hard to be obtained, hindering the development of dynamic models. Data driven modeling method could eliminate the requirement for detailed heat and mass transfer coefficient along the dehumidifier. A time delay neural network (TDNN) model is a general way to describe the dynamic process without the requirement of much knowledge on physical mechanism, yet the number of time steps for each inlet parameter is hard to be determined. In this paper, an improved time delay neural network (iTDNN) model was proposed. Through analyzing characteristics of dehumidification process, model inputs were simplified, which is the core of improvement of TDNN model. Besides, a new combined algorithm including back propagation (BP) algorithm and improved genetic algorithm (GA) was proposed to acquire the optimal weight values globally. The predicted results agree well with the experimental data for validation part. The method presented in the paper provides a reference for modeling multi-variables and large delay nonlinear dynamic systems.
This study investigated methyldiethanolamine (MDEA) effects on gas dehydration using tri-ethylene glycol (TEG) solution in a packed tower. The gas phase has entered the bottom of the contact tower, and it contacted the TEG to separate the moisture after passing through a humidifying tower. The key parameters were the concentration of MDEA in TEG solution, the concentration of water in the solution, and the gas and liquid flow rates. The results revealed that existence of MDEA in the dehydration process did not decrease the absorption efficiency and it even slightly increased the absorption performance of water vapor at some concentrations. The results also showed that increasing the liquid flow rate has a little effect on the absorption efficiency which can be due to the very high solubility of water vapor in TEG. However, the gas flow rate increase reduced the absorption efficiency, which showed that the main mass transfer resistance was in the gas phase. Also, the presence of 5 vol% water in the absorbent solution reduces the water vapor absorption efficiency by approximately 20%. Finally, the effect of MDEA concentration on foaming of solution was investigated, and it was shown that the presence of MDEA had no significant effect on foaming in the experimental condition.
This paper provides a bibliography of the 1994 journal literature for adsorptive and membrane-type separations. The references are taken from the 45 most important chemical engineering journals. This paper provides an update to the literature as provide in previous bibliographic papers (Ray 1990a, 1991, 1994, 1995). A bibliography of the chemical engineering journal literature from 1967–88 has been published by the author (Ray 1990b), and can provide access to a wider range of topics. A complete bibliographic listing of the chemical engineering journal literature from 1989 to 1995 (with subsequent six-monthly updates) is available on a CD-ROM database and full details can be obtained from the author.
The papers included here have been divided into the following subject groups: theory; design data; adsorbents; PSA and cyclic systems, and applications; liquid-phase adsorption; ion exchange, chromatography, etc.; membranes; and membrane-type separations.
The liquid desiccant air-conditioning system allows reducing energy consumption compared to the conventional compressor-type air conditioners. In order to develop desiccant materials for air conditioners, we have investigated the dehumidification capability of quaternary ammonium Ionic Liquids (ILs) and the equilibrium water vapor pressure of aqueous solutions of these ammonium salts. Among the seven tested types of ILs, 2-hydroxy-N,N,N-trimethylethan-1-aminium dimethylphosphate ([Ch][DMPO4]) displayed the best dehumidification capability and the lowest equilibrium water vapor pressure. Furthermore, the 80% aqueous solution of [Ch][DMPO4] exhibited a less corrosive effect on four types of metals, i.e., steel (hot dip zinc-aluminum alloy plated steel), copper (C1100P), aluminum (A5052), and stainless steel (SUS: SUS304). It should be noted that this [Ch][DMPO4] is not only non-toxic but also exhibits a stable nature; the aqueous solution produced no odor after storing for over 1 year under ambient conditions.
Liquid desiccant cooling systems are considered a promising technology for accurate humidity control and high energy efficiency. The dehumidifier and the regenerator are the two main components in the system, and their performance directly determines the system performance. This paper is a comprehensive review of the empirical correlations for the determination of mass transfer coefficient and moisture effectiveness in both adiabatic and internal cooling/heating dehumidifier/regenerators, and it further discusses approaches to enhance their mass transfer performance. These methods include structural improvements, such as structural modification, ultrasound atomisation and membrane-based modules, and modification of liquid desiccants, such as the addition of surfactants and nanoparticles. Finally, a brief summary and some suggestions for future work are outlined and addressed.
Suitable control of the humidity can contribute to electric energy savings. However, the present dehumidification system has many weak points. The liquid desiccant air-conditioning system has recently gained growing interest from the stand point of reducing energy consumption during dehumidification. In order to find the appropriate ionic liquids (ILs) as a desiccant for the liquid desiccant air-conditioner system, we conducted a systematic evaluation of the humidification capability of 16 types of ILs. Among the tested ILs, tributyl(methyl)phosphonium dimethyl phosphate ([P4441][DMPO4]) exhibited the best dehumidification capacity and had a less corrosive effect on four types of metals as possible piping materials. It should be noted that this [P4441][DMPO4] has a very stable nature and produced no odor while conducting the experiment and storing for over 1 year at room temperature under ambient conditions. Furthermore, it was revealed that a 77% (w/w) aqueous solution of [P4441][DMPO4] worked as an efficient desiccant liquid for the liquid desiccant air-conditioner system. © 2019 Institute of Process Engineering, Chinese Academy of Sciences
The purpose of air-conditioning systems is to provide a suitable indoor environment with respect to temperature, humidity and fresh air. Liquid desiccant dehumidification constitutes an effective method for extracting moisture from humid air with less energy consumption. Current studies mainly focus on dehumidification system under certain heat and mass transfer ability (number of mass transfer units NTUm). This paper will investigate NTUm of counter-flow packing tower and its influence on system performance. An experiment including a counter-flow packing tower was conducted. When the height of tower was 0.7 m, the volumetric mass transfer coefficient was between 1 to 4 kg/(m³·s) and NTUm presented a range between 0.9 to 1.4, when air flow rate changes from 1 kg/(m²·s) to 2.5 kg/(m²·s). When the height changed to 2.1 m, the NTUm of the tower presented a variation from 3.3 to 4.3. Furthermore, a simulation model is developed, which is validated by experimental data and past research results. The influence of NTUm on total circulation flow path is mainly caused by heat and cold offset and an NTUm value approximately of 4 constitutes a suitable value of increasing heat and mass transfer ability. Comparing three typical flow paths, when NTUm is lower than 3.5, inter-stage circulation presents improved performance in system COP. When NTUm is large, total circulation is efficient. The system COP cross points of flow paths are caused by heat and cold offset and concentration difference of the solution circulation between the dehumidifier and the regenerator, which can be quantified by loss coefficient ε and χ respectively. The heat and mass transfer ability of the system should be considered in the designing process of flow path configuration to obtain an efficient performance for different range of NTUm.
The structured packing has been widely used in the liquid desiccant systems due to low air pressure drop and large heat and mass transfer area. However, the structured packing is usually expensive and poor in mass transfer against the random packing. In order to provide the cost-effective structured packing with excellent mass transfer performance, Z-type packing made of gauze is proposed and designed in this article. The dehumidification and regeneration performance of the proposed packing are tested experimentally under summer conditions. It is found that the Z-type packing has excellent dehumidification performance with the mass transfer coefficient ranging from 6.5 to 14.2 g/m²s, about 70% higher than the corrugated cellulose packing and the plant fiber packing on average. Although the novel packing is lower in the moisture removal rate and the dehumidification efficiency, it could be made more tightly by shortening the distance between gauze sheets to overcome the shortcoming. The regeneration performance of the Z-type packing is also comparable with the plant fiber packing. The economic analysis is further conducted and it verifies the low cost of the Z-type packing under the same moisture removal/evaporation rate. The proposed packing is expected to be an alternative to the existing packings.
The main purpose of this research was to propose a practical correlation for predicting the dehumidification effectiveness of a liquid desiccant (LD) system. The experimental data were collected during the operation of a typical liquid desiccant dehumidification unit. A lithium chloride (LiCl) solution was used as the liquid desiccant solution. The 2000-m3/h LD unit was installed in a real building, and the variation of the dehumidification effectiveness values was monitored under various operating conditions. The data were statistically analyzed by the response surface methodology (RSM), and the impact of each system operation parameter on the dehumidification effectiveness was estimated quantitatively. Six operating parameters − the mass flow rate of process air, the mass flow rate of the desiccant solution, the dry-bulb temperature and the humidity ratio of the inlet air, and the inlet temperature and concentration of the desiccant solution − significantly affected the dehumidification performance of the LD unit. Consequently, a simplified linear equation model was derived as a function of the operation parameters that significantly impacted the dehumidification effectiveness. The proposed model was verified by comparison with existing models.
In this study, hollow fiber membrane contactors have been applied for gas dehumidification using tri-ethylene glycol as absorbent. The effects of gas and liquid flow rates, inlet concentration of tri-ethylene glycol and flow direction on dehumidification efficiency have been investigated. The results reveal that membrane contactors can remove water vapor very effectively using tri-ethylene glycol, and very low water dew point as low as −50 °C can be achieved. It also shows that liquid phase is not the limiting resistance in this system, as a result of very high solubility of water in tri-ethylene glycol.
Liquid desiccant dehumidification system, which presents great potential for energy saving in HVAC industry, has drawn much attention in recent years. Various dehumidifiers have been proposed for achieving the better performance and are used to be evaluated by the indicator: dehumidification effectiveness. However, this widely-used indicator is found varying significantly with the operational conditions and cannot distinguish the inherent mass transfer capability of dehumidifiers from the influence of the properties of airstream and desiccant. In view of this, this paper presents a novel concept named dehumidification perfectness, based on the conversion laws of mass and energy, for realizing the impartial evaluation of the inherent mass transfer capability of dehumidifiers, getting rid of the influence from the airstream and desiccant solution. Experimental data from the open literatures was employed to validate the concept and its affecting factors were then discussed. It was found that higher degree of dehumidification perfectness was obtained with bigger/longer effective liquid-gas contact area/time provided by the dehumidifier. Furthermore, with different properties of various dehumidifiers, such as the surface area densities of packing, their dehumidification perfectness was varying significantly. The concept developed here demonstrates promising potentials for comparing, predicting and improving the performance of various dehumidifiers.
Part 2 presents two numerical examples of application of air dehumidification by means of lithium chloride hydroscopic solutions to specific cases of air conditionings and drying. Appropriate design calculations and diagrams are offered. A brief economic analysis of energy costs is given. It is shown that this process is energy wise competitive with conventional air conditioning systems and with air-air heat pumps.
Packed towers can be used for solar regeneration of aqueous solutions and dehumidification of air using aqueous solutions. These processes involve simultaneous heat and mass transfer with heat effects. In order to design a packed tower for aqueous solution-air contacting operations, heat and mass transfer coefficients for each phase are required. For the present study, aqueous calcium chloride solution is used; ceramic Raschig rings and Berl saddles are used as the packing materials. In this paper air phase transfer coefficients are correlated with flow rates of air and liquid and the temperature of air, whereas liquid phase coefficients are correlated with rates of air and liquid flow, and the temperature and concentration of the liquid.
In the open cycle desiccant cooling system, the room air or the room air mixed with ambient air is drawn through the desiccant dehumidifier where water vapor is absorbed. Then the dry air is cooled by a sensible heat exchanger by passing the air through an evaporative cooler. The desiccant may be either solid or liquid. Liquid desiccant systems are not as well developed as solid desiccant systems. Any attempt to improve the performance of a liquid desiccant should include a performance analysis of the dehumidifier. One of the efficient dehumidifiers for the liquid desiccant system is the packed bed dehumidifier and shown schematically. Because of the abundance of variables involved with the packed bed dehumidifier, the analysis becomes increasingly complex and is achieved at very high computational cost. Hence there is a need to define the performance of a packed bed quantitatively. A definition of air moisture removal effectiveness is presented based on the rigorous heat and mass transfer calculations for an adiabatic counterflow packed bed dehumidifier. This note also studies the influence of relevant parameters on tower performance quantified by the moisture removal effectiveness.
Mass Transfer Fundamental and Applications Prentice-Hall
- A L Hines
- R N Maddox
- J L Bravo
- J A Rocha
- Fair
Hines, A.L. and Maddox, R.N. Mass Transfer Fundamental and Applications Prentice-Hall, NJ (1985) Bravo, J.L., Rocha, J.A. and Fair, J. R. Hydrocarbon Process (January 1985) 91
Handbook of Aqueous Electrolyte Solutions: Physical Properties, Estimation, and Correlation Methods Halsted Press
- A L Horvath
Horvath, A.L. Handbook of Aqueous Electrolyte Solutions: Physical Properties, Estimation, and Correlation Methods Halsted Press, New York (1985)