Desiccant Temperature Research Articles

Experimental investigation on compressed air drying performance using pressurized liquid desiccant

ABSTRACTAs a new compressed air-drying method, the compressed air dehumidification using a pressurized liquid desiccant was proposed in our previous study. The pressurized dehumidifier is a complex and core component of the drying system. The mass transfer performance between the compressed air and LiCl aqueous solution is experimentally studied in a counter-flow pressurized dehumidifier filled with structured packing. The humidity ratio of outlet compressed air, vapor removal of processed compressed air, moisture removal rate, and dehumidification efficiency were selected as the performance indices. The results show that the minimum humidity ratio of processed compressed air could reach 0.23 g/kg under 0.71 MPa. Compressed air-drying performance could be remarkably enhanced through increasing the air pressure and liquid desiccant inlet concentration while the influence of liquid desiccant temperature is negative. Furthermore, in order to ensure high compressed air-drying performance, reduce the power consumption of the air compressor and liquid desiccant pump, and the possibility of carryover, the optimum ratio of liquid to compressed air flow rate is recommended to be around 1.5 under pressure around 0.50 MPa. Meanwhile, the energy consumption for per-gram moisture removal of a liquid-desiccant-based compressed air-drying system can reach 1.42 kJ/g lower than cooling dehumidification under 0.3 MPa, which is 16.0% lower than a compressed air-cooling dehumidification system.

Numerical modeling of fluid flow and coupled heat and mass transfer in a counter-cross-flow parallel-plate liquid-to-air membrane energy exchanger

In this paper, a numerical model is developed and applied to investigate the fluid flow and heat and mass transfer in a counter-cross-flow liquid-to-air membrane energy exchanger (LAMEE). An iterative finite-difference method is used to solve two-dimensional steady-state flow field and the coupled heat and moisture transfer equations for the liquid desiccant and air streams. The model is validated with experimental data, and the agreement is within ±10%. The results of the numerical model show that the phase change energy plays a key role in the establishment of the temperature and moisture content distributions in the LAMEE. In the process of air cooling and dehumidification, the liquid desiccant temperature is increased not only by the sensible heat transfer, but also by the moisture transfer. Also, the capacity of the liquid desiccant to absorb moisture from the air is shown to be affected by the sensible energy transfer between the two fluids. Moreover, it is shown that the counter-cross-flow LAMEE effectiveness is between the effectiveness of pure counter-flow and pure cross-flow LAMEEs with the same membrane area. It is recommended for a practical design that the aspect ratio and entrance ratio should be less than 0.2 and 0.1 respectively, in order to achieve a high performance of the counter-cross-flow design that approaches within 2% the performance of a counter-flow design. Alternately, the membrane area of the practical design can be increased slightly (by 10%) so that the counter-cross-flow LAMEE has the same performance as a counter-flow LAMEE.

Experimental study on heat and mass transfer characteristics for a desiccant-coated fin-tube heat exchanger

In this study, we investigated the heat and mass transfer characteristics of a desiccant-coated fin-tube heat exchanger (DCHE), in which the desiccant was coated on the surface of a conventional fin-tube heat exchanger. It differs from desiccant wheels in which the desiccant is cooled or heated by the fluid inside the tube during the adsorption or desorption process, and from conventional heat exchangers, in which moisture transfer occurs and the latent heat transfer is caused by phase change of the adsorbed moisture. Firstly, the heat and mass-transfer model for the DCHE was presented in this paper, whereupon the influences of air velocity, desiccant temperature and moisture content on the overall mass-transfer coefficient were separately analyzed for desorption and adsorption processes. It emerged that the overall mass-transfer coefficient rose with increasing air velocity. The moisture content of the desiccant significantly impacted on the overall mass-transfer coefficient. Higher desiccant temperature resulted in lower overall mass-transfer coefficient. Finally, the heat transfer model given in this paper was applied to calculate the DCHE heat transfer rate and the values obtained by calculation were observed to correlate well with the experimental results.

ChemInform Abstract: Controlling the Catalytic Aerobic Oxidation of Phenols.

The oxidation of phenols is the subject of extensive investigation, but there are few catalytic aerobic examples that are chemo- and regioselective. Here we describe conditions for the ortho-oxygenation or oxidative coupling of phenols under copper (Cu)-catalyzed aerobic conditions that give rise to ortho-quinones, biphenols or benzoxepines. We demonstrate that each product class can be accessed selectively by the appropriate choice of Cu(I) salt, amine ligand, desiccant and reaction temperature. In addition, we evaluate the effects of substituents on the phenol and demonstrate their influence on selectivity between ortho-oxygenation and oxidative coupling pathways. These results create an important precedent of catalyst control in the catalytic aerobic oxidation of phenols and set the stage for future development of catalytic systems and mechanistic investigations.

Analytical Modeling and Performance Study of a Cross Flow Air Dehumidifier Using Liquid Desiccant

The dehumidifier is a key component in liquid desiccant air conditioning systems. Various mathematical models of heat and mass transfer processes inside the dehumidifier are introduced and numerically solved in the literature. Analytical solutions have more advantages than numerical solutions in studying the dehumidifier performance parameters. This paper presents the results from an analytical model for the performance of an adiabatic cross flow liquid desiccant air dehumidifier. Calcium chloride is used as desiccant material in this investigation. Both humidity and temperature effectiveness of the dehumidifier are used to predict the performance of the device under various operating conditions. Good accuracy has been found between analytical solution and reliable experimental results with a maximum deviation of +6.63% and -5.65% in the moisture removal rate. The method developed here can be used in the quick prediction of the dehumidifier performance. The exit parameters from the dehumidifier are evaluated under the effects of variables such as air temperature and humidity, desiccant temperature and concentration and air to desiccant flow rates. The results show that hot humid air and desiccant concentration have the greatest impact on the performance of the dehumidifier.

Optimization of Liquid Desiccant Dehumidifier Performance Using Taguchi Method

An attempt has been made to improve the performance of dehumidifier using liquid desiccant in this study. Experiments have been conducted based on L16 orthogonal array in order to experimentally investigate the influence of dehumidification process parameters like desiccant temperature, desiccant concentration, desiccant flow rate, air flow rate, and relative humidity on the effectiveness of dehumidifier. The optimum dehumidification condition has been found through Taguchi method. It has been confirmed that the optimum condition exhibited better effectiveness for the dehumidification process.

Statistical evaluation of a liquid desiccant dehumidification system using RSM and theoretical study based on the effectiveness NTU model

In the present study the performance of an air dehumidifier using lithium bromide (LiBr) as a desiccant was investigated. Response surface methodology (RSM) was used to assess individual and interactive effects of the six main factors (velocity, temperature and humidity of air, flow rate, temperature and concentration of desiccant) on dehumidification mass rate. A reduced quadratic statistical model was derived to predict dehumidification mass rate. The maximum dehumidification mass rate was obtained 0.154g/s under the optimal conditions of an air velocity of 4.1m/s, desiccant flow rate of 0.035kg/s, air humidity ratio of 0.0185kg/kg, desiccant concentration of 0.48kg/kg, air temperature of 29.5°C, and desiccant temperature of 21.8°C. The effectiveness number of transfer unit (NTU) model was employed to describe the coupled heat and mass transfer. The results of the model and the experimental data show good agreement. Dimensionless mass and heat transfer coefficients correlations are proposed; the average absolute differences between the predicted values and the experimental findings for Sh and Nu numbers were calculated as 2.14% and 5.27%, with the discrepancies mainly within ±9% and ±13%, respectively.

The Optimized Operation of a Solar Hybrid Desiccant/Displacement Ventilation Combined with a Personalized Evaporative Cooler

The study investigates by modeling and experimentation the performance of displacement ventilation (DV) aided with personalized evaporative cooler (PEC) system that operates in humid climate and uses a solid desiccant (SD) dehumidification system regenerated by parabolic solar concentrator thermal source. Predictive component models of the conditioned space, the SD, the solar concentrator system, and the PEC were developed and used for the prediction of associated operational energy consumption while utilizing an optimized control strategy. The control strategy seeks optimal values of supply air flow rate and temperature and the desiccant regeneration temperature for both cases with and without the aid of the PEC while meeting space load, indoor air quality, and thermal comfort requirements. Energy consumption was calculated for the optimized strategy using genetic algorithm optimizer and the integrated DV/SD-PEC models. The results agreed well with experimental data obtained from tests on a DV climatic chamber. In addition, votes of comfort recorded by participants in the experiment using PEC were very similar to predicted comfort. The optimized hybrid system performance was applied to a typical office space of area of 64 m2. The operation of the hybrid system with PECs resulted in a higher supply air temperature. The increment in supply air temperature is 1.1°C when PEC is used compared to case without PEC. This increase in temperature showed that an energy saving of 13.5% is achieved for the PEC aided hybrid system.

Performance Evaluation of a Desiccant Dehumidifier using Different Desiccants

ABSTRACTThis article reports an analysis of different empirical dehumidification effectiveness correlations for packed bed columns using two desiccant solutions viz. lithium chloride and calcium chloride. The analysis shows wide variations in effectiveness values ranging from 10% to 50% or more, with higher deviations occurring for lower ratios of liquid to gas flow rates. It is also reported that for same ratio of liquid to Gas flow rate, the dehumidification effectiveness is more for the desiccant system using LiCl. Design variables found to have the greatest impact on the performance of the dehumidifier, are desiccant concentration (slope = 2.7), desiccant temperature (slope = −1.4), air flow rate (slope = −0.9), and air humidity ratio (slope = 2.5).

A simple analytical method to estimate all exit parameters of a cross-flow air dehumidifier using liquid desiccant

The dehumidifier is a key component in liquid desiccant air-conditioning systems. Analytical solutions have more advantages than numerical solutions in studying the dehumidifier performance parameters. This paper presents the performance results of exit parameters from an analytical model of an adiabatic cross-flow liquid desiccant air dehumidifier. Calcium chloride is used as desiccant material in this investigation. A program performing the analytical solution is developed using the engineering equation solver software. Good accuracy has been found between analytical solution and reliable experimental results with a maximum deviation of +6.63% and −5.65% in the moisture removal rate. The method developed here can be used in the quick prediction of the dehumidifier performance. The exit parameters from the dehumidifier are evaluated under the effects of variables such as air temperature and humidity, desiccant temperature and concentration, and air to desiccant flow rates. The results show that hot humid air and desiccant concentration have the greatest impact on the performance of the dehumidifier. The moisture removal rate is decreased with increasing both air inlet temperature and desiccant temperature while increases with increasing air to solution mass ratio, inlet desiccant concentration, and inlet air humidity ratio.

Theoretical performance study of a liquid desiccant regenerator supported with scavenging air heat exchanger

This paper proposed a simplified theoretical model of the air and liquid desiccant parameters inside any packed bed liquid desiccant regenerator supported with scavenging air to air heat exchanger to reduce regeneration heat. The presented model depends mainly on the value of regenerator effectiveness regardless the packed bed structure type or the direction of flows inside the regenerator. Calcium chloride is used as desiccant material in this investigation. Both regenerator humidity and temperature effectiveness are applied for derivation of the numerical solution. The numerical solutions have showed good accuracy with a maximum deviation of 3.64% when compared with reliable experimental data available in the literature. The method developed here can be used in the quick prediction of the regenerator performance. The effects of variables such as air temperature and humidity, desiccant regeneration temperature, concentration, solution to air flow rate, and others on the regenerator performance have been investigated. The results reveal that operating variables such as desiccant concentration, desiccant regeneration temperature, and solution to air flow rate have the greatest impact on the performance of the regenerator. As a result of using scavenging air to air heat exchanger, the exit desiccant concentration increases by about 21.5% while the heat rate required for regeneration can be decreased by about 18% for the same exit concentration.

Mathematical modelling and experimental investigation of the hybrid desiccant cooling system

In this paper, an experimental study has been conducted on the hybrid desiccant cooling system by removing the latent heat and sensible heat of air separately by mixing it with the desiccant solution in a counter flow manner. This makes air totally dry and thus saves the energy to cool the air in the refrigeration system. The desiccant chosen here is the aqueous solution of calcium chloride. The packed bed inside the absorber as well as the regenerator consists of a polypropylene cascade ring for the efficient mixing of air and desiccant solution. The effects of various parameters such as desiccant inlet temperature, air inlet temperature, mass flow rate of air and desiccant solution have been studied to investigate the performance of the system. Comparing the results with previous studies, a fair agreement has been reported.

Thermodynamic analysis of a counter flow adiabatic dehumidifier with different liquid desiccant materials

Liquid desiccant systems have been proposed as energy saving alternatives to the conventional vapor compression systems for handling the latent load. This paper presents the results from a study of the performance of a counter flow liquid desiccant dehumidifier. A heat and mass transfer theoretical model of an adiabatic packed column has been developed, based on the Runge-Kutta fixed step method, to predict the performance of the device under various operating conditions. Good agreement was found between experimental tests and the theoretical model, with the maximum deviation being ±2.9% in air outlet temperature, ±15.9% in air outlet humidity ratio and ±2.8% in solution outlet temperature. Following the model validation, the rate and the efficiency of the dehumidification process were assessed under the effects of variables, such as air temperature and humidity, desiccant temperature and humidity and air and desiccant flow rates. The three most commonly used liquid desiccant solutions, namely LiCl, LiBr and CaCl2 were evaluated against each other. The results show that high absorber efficiency and system efficiency could be achieved under humid conditions, low air mass flow rates and LiCl as the desiccant solution.

Study of the Heat and Mass Transfer in a Dehumidification of Liquid Desiccant

Desiccant systems have been proposed as energy saving alternatives to vapor compression air conditioning for handling the latent load. Desiccants are classified as either liquid or solid. The main components for a liquid desiccant system are the dehumidification and regeneration towers. This paper presents the results from a study of the performance of a packed tower absorber for lithium chloride desiccant dehumidification system. A finite difference model was developed to determine the packing height of the dehumidification towers. The finite difference model was written in MATLAB language which is a suitable model to measure the optimum height of a tower. The paper also examines the effects of different design parameters on the height of a packed tower using a mathematical model. The effects of air and liquid flow rates, air humidity, desiccant temperature and concentration were reported on the packing height and humidity effectiveness of the column. In conclusion the results of the present study are compared with previous experimental studies.

Simplified analysis of heat and mass transfer model in liquid desiccant regeneration process

Current study reveals an analytical solution of coupled heat and mass transfer processes in a packed bed liquid desiccant regenerator. This solution showed good accuracy when compared with reliable experimental data available in the literature. In order to maintain the mass transfer gradient on the interface, a high liquid desiccant flow rate is often applied in the regenerator. Hence, the solution flow rate and concentration change very little in the whole processes. The experimental results confirmed the validation of this topic. Therefore, for narrow range of operating conditions in practical regeneration process, with acceptable error we can assume that equilibrium humidity ratio is constant on the interface. The assumption of constant humidity ratio was applied in this paper for derivation of analytical solution. The effects of variables such as air and liquid desiccant flow rate, air temperature and humidity, desiccant temperature and concentration were investigated on the evaporation rate. Furthermore, a comparison is made between the present study and analytical solution given in the literature. The results showed that the present analytical solution can give better results than other analytical solutions that presented in the literature.

An analytical solution for air dehumidification by liquid desiccant in a packed column

This paper presents a new analytical solution of heat and mass transfer processes in a packed bed liquid desiccant dehumidifier based on the equilibrium humidity on the interface is assumed to be constant. In order to maintain the partial pressure difference on the interface, a high liquid desiccant flow rate is often applied in the practical absorber. Therefore, for a narrow range of operating conditions for practical dehumidification process, we can assume that the equilibrium humidity ratio on the interface is constant. The assumption of constant humidity ratio is applied in this paper for derivation of the analytical solution. The model and the analytical solution predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. According to the Lewis definition in this present study, the Lewis number obtains 0.9. The effects of variables such as air and liquid desiccant flow rate, air temperature and humidity, desiccant temperature and concentration have been investigated on the condensation rate. The results present that design variables such as desiccant concentration, desiccant temperature, air flow rate, and air humidity ratio have the greatest impact on the performance of the dehumidifier. The liquid flow rate and the air temperature have not a significant effect. Furthermore, the effects of air and liquid desiccant flow rate have been reported on the humidity effectiveness of the column.

Performance analysis on the internally cooled dehumidifier using liquid desiccant

Dehumidifier is one of the most important components in liquid desiccant air-conditioning systems. Previous study shows that the internally cooled dehumidifier may have better mass transfer performance than the adiabatic unit. The effect of flow pattern, especially the flow direction of air to desiccant on the internally cooled dehumidifier performance is numerically analyzed in detail. The result shows that counter-flow configuration of air to desiccant has better dehumidification performance, and parallel-flow configuration performs the poorest with the same conditions, due to more uniform mass transfer driving force expressed in the counter-flow configuration. The decrease of the desiccant concentration is the main factor that influences the internally cooled dehumidifier's performance, while the increase of the desiccant temperature is the main performance restricting factor in adiabatic dehumidifier. Internally cooled dehumidifier has better mass transfer performance compared with adiabatic dehumidifier plus external heat exchanger.

Evaporation rate of a novel tilted solar liquid desiccant regeneration system

The regeneration system represents a vital part of any desiccant air conditioning system. The need of a solar assisted desiccant regeneration system is more important today. In this paper, an experimental study of a novel regeneration system modified from solar tilted still is carried out. A corrugated blackened surface is used to heat the desiccant and an air flow is used to regenerate calcium chloride solution. The effect of the liquid to air flow rate ratio; the desiccant temperature; the desiccant concentration and the inlet air humidity ratio on the evaporation rate has been studied experimentally. A wide range of liquid to air flow rate ratios are employed. The optimum value of the liquid to air flow rate ratio for higher evaporation rate is reported.

Experimental study on a new internally cooled/heated dehumidifier/regenerator of liquid desiccant systems

For providing good performance of dehumidifier and regenerator with certain dimensions, a new type of internally cooled/heated dehumidifier/regenerator based on the plate–fin heat exchanger (PFHE) was designed. To investigate the behavior of the new equipment, an experimental setup was established in an environment chamber with regulable temperature and humidity air. By the internally cooled dehumidification testing, effects of the cooling water temperature, the air flow rate and the desiccant temperature on the dehumidification performance and the cooling efficiency were presented. The behavior of internally cooled dehumidification process was compared with that of the adiabatic dehumidification process. The results suggested that the cooling efficiency decreased with the increasing of the cooling water temperature and desiccant with low temperature could bring more mass transfer coefficients. There is an optimal air flow rate to achieve the maximum absolute humidity decrease of the air. By the internally heated regeneration testing, effects of the air flow rate and the desiccant inlet temperature on the regeneration performance and air outlet parameters were discussed and also compared with those of the adiabatic regeneration process. It was concluded that the regeneration efficiency of internally heated regeneration was more than that of the adiabatic regeneration, and the internally heated regenerator could offer better thermal performance.

Application of Desiccant Technology for Concentrating Liquid Foods - Experimental Investigation

A single tube, stainless steel, liquid desiccant falling film evaporator was built and tested with the calcium chloride solution as the liquid desiccant to concentrate liquid foods. Experiments were conducted in the above setup with sugar solutions as the liquid food. To minimize water vapor mass transfer resistance, non-condensibles were removed from the system by a vacuum pump. No steam was used to drive this evaporator.The liquid food was heated to about 48ºC when the ambient temperature was 40ºC. The single tube double falling film evaporator was tested to determine the effect of desiccant concentration, temperature, and flow rate on the evaporation rate. The effect of liquid food concentration, temperature, and flow rate on evaporation rate was also studied. An uncertainty and error analysis for the experimental data was performed. It was demonstrated that liquid desiccants could provide enough energy, through heat of absorption, to evaporate a liquid food in a falling film evaporator with vapor transfer lines.