Packed Bed Dehumidifier Research Articles

Experimental investigation and performance analysis of a spray-type ionic liquid desiccant dehumidifier

Ionic liquid desiccants (ILDs) have vapor pressures similar to lithium chloride and no corrosion and crystallization problems, so they are considered a promising alternative to halide salt desiccant solutions. However, the cost of ILDs is more expensive than conventional halide salts, which limits their practical application. Thus, this study developed a spray-type dehumidifier using an ionic liquid desiccant to increase dehumidification performance and reduce the required amount of ILDs. The presented dehumidifier was tested at various operating conditions. Also, a theoretical model was established to analyze heat and mass transfer coefficients of the dehumidifier. Results indicate that the increase in the flow rate ratio of solution to air benefits absorption performance, and the mass flux of solution is suggested to exceed 170 mL s−1 to avoid the occurrence of air bypass. Moreover, compared with the packed bed type, the spray type can reduce the required solution flow rate by 35 % and save power consumption by 19 % under the same moisture removal rates. The dehumidification effectiveness of the presented dehumidifier was up to 0.64 and comparable to the conventional packed bed dehumidifier using lithium chloride solutions.

Water from Air: Desiccant System Design and Simulation

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.

Performance prediction of a liquid desiccant dehumidifier using artificial neural networks approach

This work aims to develop two main ideas: first, the use of the artificial neural network (ANN) approach to predict the moisture removal rate (MRR) and the dehumidifier effectiveness (ɛ) of a counter-flow liquid desiccant dehumidifier using calcium chloride as an absorption solution. Second, the Garson method is used to identify the most important working parameters influencing the performance of the packed-bed dehumidifier component. A network model was developed in a MATLAB environment based on a multilayer perceptron that included an input, a hidden and an output layer. The network input parameters were the dry and wet bulb temperatures, the air and liquid flow rates, and the liquid desiccant temperature and concentration. The network output included two variables; the MRR and the ɛ. The performances of the ANN predictions were tested using experimental data not employed in the training process. The predicted values were found to be in good agreement with the experimental values, with mean relative errors of less than 4.90% for the MRR and 3.85% for ɛ. In addition, the air wet bulb and dry bulb temperatures were the parameters with the most influence on the MRR and ɛ, with a relative importance of 35% and 25%, respectively.

Numerical investigation of performance trade-off characteristics of a packed bed dehumidifier using aqueous blends of lithium chloride and calcium chloride

Current research in dehumidification systems is mainly focused on thermally driven liquid desiccant systems. However, the selection of proper desiccant solution has a greater significance on the performance of a dehumidifier. In this perspective, current work is presented with governing equations of energy, mass and species balance to study the performance of a dehumidifier. The governing equations are solved using finite difference method coupled with Jacobi’s model for an air–desiccant contact system of a cross-flow packed bed adiabatic dehumidifier operating with different desiccant blends of lithium chloride (LiCl) and calcium chloride (CaCl2) solution. The effect of variation in blend proportion on outlet parameters of a packed bed dehumidifier is patterned under various desiccant to air (L/G) mass flow rate ratios. It is observed that the addition of CaCl2 desiccant solution into LiCl changes the nature of thermo-physical properties of blends‚ which are predicted using non-random two liquid (NRTL) equation. Further, a trade-off analysis among the performance parameters is presented. It demonstrates that dehumidifier performance paradox such as enthalpy effectiveness (ɛh), moisture removal rate (MRR) and moisture effectiveness (ɛm) have a tendency to be optimal for 35% LiCl with 5% CaCl2 blend (BL1) among all blends at L/G ratio of 2. The variation of thermodynamic properties (air temperature, humidity, desiccant temperature and desiccant concentration) inside the dehumidifier module is visualized in terms of contour plots. Furthermore, the irreversibility in heat and mass transfer operation for the optimal blend is investigated in terms of physical and chemical exergy destruction paradigm. The result of such investigation elucidated that the maximum chemical exergy destruction is 0.88 kW, whereas the physical exergy destruction is 0.06 kW.

Energetic, entropic and economic analysis of a heat pump coupled humidification dehumidification desalination system using a packed bed dehumidifier

In this paper, the humidification dehumidification (HD) desalination system, driven by a mechanical vapor compression heat pump (MVCHP), is proposed. The direct contact dehumidifier, with packings filled, is applied to condense the humid air for water producing. On the basis of the thermodynamic laws, the energetic and entropic analysis, with the modified effectiveness to describe the heat and mass transfer characteristics of humidification, are first achieved to assess the desalination performance of the combined system. The influence laws from the terminal temperature difference (TTD) of the evaporator, the pressure ratio (PR) during compression and pinch temperature difference (PTD) of the condenser on the desalination performance are also explored and presented. The research results presented that the top thermodynamic performance of the MVCHP coupled HD desalination system arises as 71.56 kg h−1 for the water production, 5.28 for the gained-output-ratio (GOR) and 1.73 kJ kg−1 K−1 for the specific entropy generation rate, while two fully coupled cases at 1.21 and 1.96 for the mass flow rate ratio will emerge. It is found that water production can be elevated effectively through increasing the pressure ratio, while the peak value of GOR keeps at the design conditions. Furthermore, the positive effects from lower values both for the TTDe and PTDcon on the performance of the MVCHP coupled HD desalination system are proved. After the economic analysis, it is calculated that the cost of fresh water is about 0.018 $ L−1 with a lifetime of 10 years. According to the performance comparison, the great potential of energy conservation for the MVCHP coupled HD desalination system are validated, although the water production will be reduced after the integration of the MVCHP.

Thermodynamic Analysis of A Novel Humidification Dehumidification Desalination System Driven by Heat Pump

In this paper, the humidification dehumidification (HDH) desalination system using a packed bed dehumidifier, with a mechanical vapor compression heat pump integrated, is proposed, and the condenser is coupled to raise the seawater temperature while the evaporator is applied to absorb the energy from the discharged brine as much as possible. For the proposed combined desalination system, the mass and energy balance equations are constituted, and then the corresponding thermodynamic performance at the design conditions, both for the desalination and heat pump unit, is obtained. The simulation results show that the peak values for the water production and gained-output-ratio (GOR) emerge as 76.40kgh-1 and 5.14 at the design conditions. Compared to the general HDH desalination system, the elevation magnitudes reach ΔGOR=1.69 and 2.86 at the same conditions.

Performance assessment of a solar-powered adsorption air conditioning system

Rotary desiccant air conditioning is a thermally driven and CFCs free technology which mainly uses low grade heat sources, such as solar energy and waste heat. The dehumidifier is the critical component of the desiccant air conditioning system and its performance affects in a decisive manner the effectiveness, size and manufacture cost of the whole system. This paper presents an adequate simulation tool to predict the behaviour of packed bed dehumidifier of rotary wheel type under appropriate working conditions to a solar-assisted configuration. The analysis is carried out through a one-dimensional gas-side resistance model of heat and mass transfer processes on air-solid desiccant interface and provides a large number of numerical results that can be used for design and operational optimization of the desiccant wheel.

Semi-analytical method for heat and moisture transfer in packed bed of silica gel

A semi-analytical model for the heat and mass transfer of adsorption and desorption processes of the vertical solid desiccant packed bed dehumidifier is presented on the basis of quasi-steady state assumption, and is solved using close form integration with the limits equivalent to bed and time increments, and numerically by Runge–Kutta Fehlberg and forward scheme finite difference techniques. The most important parameters during the dehumidifier operation, namely, (i) exit air temperature and humidity, (ii) axial temperature distribution in the bed and (iii) water content are evaluated. Stability of the semi-analytical method is investigated and found that the main parameters affecting the model stability are the bed and time increments size. A dimensionless parameter combining time and bed increments size and air velocity named velocity ratio is defined and investigated. It is found that when the velocity ratio equals the ratio of particle diameter to bed length, the method is stable, and as the velocity ratio is made smaller beyond the stable velocity ratio, the results remain unchanged. The results of semi-analytical and numerical models agree well with the experimental results for both desorption and adsorption processes. Using the proposed semi-analytical model, the minimum and maximum relative errors for exit air temperature are 2.24% and 11.78%, respectively and for exit air humidity the minimum and maximum errors are 3.79% and 27.17% respectively.

Improved utilization of desiccant material in packed bed dehumidifier using composite particles

Solid desiccant dehumidifiers are widely used in drying processes. In most of these dehumidifiers, the desiccant material is used as packed bed of granule or spherical particles. Investigations of intra-particle heat and mass transfer processes has shown that the entire portion of the particle is not participating effectively during adsorption as well as desorption processes [Pesaran AA, Mills F. Moisture transport in silica gel packed beds-I. Theoretical study. International Journal of Heat and Mass Transfer 1987; 30: 1037–49]. This is because the diffusion rate is very small compared to that of convection. In the present work, a new desiccant composite particle, in which the unutilized portion of the spherical desiccant particle is replaced with an inert particle, is proposed. By replacing the conventional particles with composite particles for the same mass of desiccant material, the available area for heat and mass transfer increases and more amount of desiccant material is effectively utilized. Further, in order to ascertain the improvement in the performance of the desiccant bed using the composite particles, various factors like thermo-physical properties of the inert material, composite particle thickness ratio, bed configuration, bed volume, the pressure drop and the increase in total adsorbed or desorbed mass have to be considered. In view of this, a theoretical investigation of the operation of vertical solid desiccant packed bed dehumidifier, using both conventional silica gel particles as well as the new proposed composite silica gel particles has been reported. A modified solid side resistance (MSSR) model is developed for the prediction of intra-particle temperature and water content profiles. Results of the present theoretical models, when applied to packed bed of conventional silica gel particles, agree well with the experimental results from the literature for both desorption and adsorption processes. From the theoretical results, more utilization for the desiccant material is obtained when ordinary silica gel particles are replaced by composite silica gel particles. For the same amount of desiccant material and same mass flow rate of air, using particles of 0.2 thickness ratio the pressure drop decreases by about 60% for the case investigated. In addition, an increase of about 11.07% and 20.46% in total mass adsorbed and desorbed respectively are obtained. At the time when adsorption process ends, an increase of 15.5% in the bed effectiveness has been obtained. In addition, the expected improvement in total mass adsorbed and desorbed is observed to be dependent on the inert material thermo-physical properties for thickness ratio less than 0.5. An optimization technique relating the composite particle design, resulting savings in pressure drop and bed volume increase is proposed.

Cosorption Processes of Triethylene Glycol in a Packed-Bed Liquid Desiccant Dehumidifier

The cosorption characteristics of water and toluene vapors in various concentrations of triethylene glycol (TEG) solution flowing through a packed-bed dehumidifier are investigated in this paper. A multi-component model was constructed using the reported equilibrium relationships of toluene and water vapors in TEG solutions together with the Krishna-Standart multi-component mass transfer correlation. The effects of liquid-to-air ratios, TEG inlet temperatures, air inlet temperatures were reported on the moisture and toluene removal rate as well as the moisture and toluene removal efficiency of the packed dehumidifier. Running the packed dehumidifier in a higher liquid-to-gas flow ratio generally increased the removal rates and efficiencies of both water vapor and toluene vapor from the airstream. Increasing inlet temperatures of the TEG solution led to a decrease in the removal rate of water vapor when running the packed dehumidifier at a high liquid-to-gas flow ratio. However, there was no significant change in the toluene vapor removal rate or toluene removal efficiency when the flow rate of the inlet TEG solution was increased.

Analytical solution of combined heat and mass transfer performance in a cross-flow packed bed liquid desiccant air dehumidifier

Heat and mass transfer between air and liquid desiccant in a cross-flow packed bed dehumidifier is investigated. Analytical solutions of air and desiccant parameters as well as enthalpy and moisture efficiencies are given in the present study, based on the analogy between the combined heat and mass transfer process in the cross-flow dehumidifier and the heat transfer process in the cross-flow heat exchanger. The results given by the analytical solution are compared with numerical solutions and experimental findings. Good agreement is shown between the analytical solutions and the numerical or experimental results. The analytical solutions can be used in the optimization of the cross-flow dehumidifier.

Prediction of pressure drop in a packed bed dehumidifier operating with liquid desiccant

In this paper a more rigorous model, which is valid for both structured and random packing columns, is used for predicting the irrigated pressure drop in a desiccant–air contact system. Calcium chloride solution is considered as the desiccant. Four different random packing materials and three different structured packing materials are considered in the present study. The effects of random packing shape and the type of structured packing on the hydraulic performance are studied. The model has been validated for a wide range of operating values available in the literature. It is found that the structured packing has the lower pressure drop and higher capacity compared with random packings. Among the random packing materials considered in the present study, Intalox saddles can provide the least irrigated pressure drop and among the structured packing materials the sheet-type Mellapak 250 Y has the lowest pressure drop.

Investigation of heat and mass transfer in a gauze-type structured packing liquid desiccant dehumidifier

In this experimental investigation, a packed bed column suitable for 5-ton hybrid cooling system has been designed to study the absorption of water vapour from moist air by contact with aqueous solutions of calcium chloride. The packing material used in the study was two elements of the BXPEP structured packing and the height of the each element was 17 cm. This packed bed dehumidifier handles desiccant flow rates from 10 to 321/min. This paper presents results from a detailed experimental investigation of the heat and mass transfer between a liquid desiccant (calcium chloride) and air in a gauze-type structured packing dehumidifier. The effects of different independent variables such as air inlet absolute humidity, desiccant inlet temperature, flow rate and its concentration on the performance of the dehumidifier have been investigated.

Regeneration of silica gel in multi-shelf regenerator

In the present paper an investigation on the regeneration of solid desiccant (silica gel) in a modified design of dehumidifier called “Multi-shelf Dehumidifier”, has been reported. A “Packed Bed Dehumidifier” was used as control. Both the dehumidifiers were fabricated and tested simultaneously. The effect of regeneration air temperature, bed-air velocity and number of shelves on regeneration of silica gel was investigated. The range of regeneration air temperature was 42–72°C, while bed-air velocity varied from 0.175-0.55 m/s and number of shelves from 2 to 4. The regeneration time decreased with the increase in regeneration air temperature, bed-air velocity as well as number of shelves. The values of regeneration air temperature and bed-air velocity for minimum energy input were found to be 52°C and 0.175 m/s irrespective of the number of shelves. The effect of the rest period was investigated for these optimum values of regeneration air temperature and air velocity for a number of shelves varying from 2 to 4. The rest periods used were 30, 60 and 180 mins. The regeneration time reduced with increase in rest period for all the values of number of shelves, but the reduction in regeneration time was not appreciable.

Experimental Investigations on the Performance of a Packed Bed Dehumidifier for Various Climatic Conditions

ABSTRACT To simulate space cooling or drying systems for various geographical regions (low, moderate, and high humidity regions) a liquid desiccant system consisting of a humidifier and dehumidifier has been designed and constructed. This system consists of two coupled humidifying and dehumidifying packed columns filled with plastic Intalox Snowflake packing material. Experimental measurements of the dehumidifier performance utilizing a new effective liquid desiccant mixture (CELD) total desiccant salt content 40%wt.to 45%wt (salt comprised of equal portions by weight of lithium chloride and calcium chloride ), have. been carried out, The effect of different independent variables such as packing height, air inlet temperature and humidity ratio, and liquid desiccant inlet temperature, flow rate and concentration on the performance of the dehumidifier has been investigated. Experimental results demonstrate that the CELD is a promising desiccant for cooling and drying operations.

Parametric study of a packed bed dehumidifier/regenerator using CaCl 2 liquid desiccant

The processes occurring in a packed bed dehumidifier, which is part of a liquid desiccant solar cooling system, are mathematically simulated. The air flows in a counter flow direction to the liquid desiccant (CaCl 2). The effect of varying the air, liquid flow rates and bed geometry are studied in addition to studying the effect due to varying the air and liquid desiccant inlet coditions. The inlet temperature of the liquid desiccant during the air dehumidification process has a strong effect on the other parameters, while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and while the air inlet temperature has a negligible effect. It is also noticed that higher temperatures of air and liquid desiccant enhance the liquid desiccant regeneration processes but by different ratios. The study showed that both the air and liquid desiccant flow rates have negligible effect on the bed exit humidity ratio of air whereas the liquid flow rate has a strong effect on the bed exit moisture content of the liquid. It is also observed that increasing the air flow rate enhances the liquid desiccant regeneration (air humidification) process. Regardless of the inlet moisture content of the liquid desiccant, it is found that as the product ( LA s ) gets larger, the exit air humidity gets less.

Effectiveness of Moisture Removal for an Adiabatic Counterflow Packed Tower Absorber Operating with CaCl2-Air Contact System

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.