Simulation of Adsorption Process in a Rotary Solid Desiccant Wheel

Specific humidity or moisture content is the actual mass of water vapor in 1 kg of dry air. Whereas relative humidity is the ratio of the actual mass of moisture in the air at a given temperature to the maximum amount of moisture that air can hold at the same temperature. Relative humidity and dry-bulb temperature are two parameters used to indicate the level of personal comfort. People commonly focus only on temperature but rarely on moisture; however, the high relative humidity of indoor air may have serious health implications for occupants. This is because, during the perspiration process, surrounding air that has a relative humidity close to 100% is unable to absorb the latent heat released by the human body. Sustained periods under these conditions can lead to people feeling thermally uncomfortable due to an increase in body temperature and can trigger dehydration and heatstroke. The effect citrus of this can result in respiratory and skin problems. Besides that, it can also create a humid environment conducive to the growth of bacteria (Satwikasari et al., 2018; Tharim et al., 2018). Therefore, it is necessary to control the humidity level in the air in order to ensure personal comfort in a confined space.

Malaysia is a tropical country, characterized by high daytime temperatures of 29–34°C and relative humidity of 70–90% throughout the year (Makaremi et al., 2012). The recommended temperature and relative humidity for the indoor environment are 23–26°C and 30–60%, respectively, as set out by ASHRAE Standard 55 (Yang & Zhang, 2008). In order to meet the ASHRAE requirement, air-conditioning systems are widely used in Malaysia. The number of air-conditioning systems in use has increased from 13,000 units in 1970 to more than 250,000 units in 1991, with the number expected to rise to around 1.5 million units by 2020 (Daou et al., 2006). However, the growing demand for air-conditioning has contributed to the massive consumption of electrical power. Other than sensible cooling, air-conditioning also performs the essential task of humidity control. In conventional air-conditioning units, the cooling process and air dehumidification are generally driven by a cooling coil (Nguyen & Aiello, 2013). The high humidity in Malaysia results in a significantly high air dehumidification load. The conventional method consumes a large amount of electricity as a result of the overcooling process to achieve lower humidity. Modern air-conditioning has recently included separate handling of the dehumidification load and sensible cooling capacity, which reduces its power requirement. This is usually integrated with the air-conditioning system to provide comfort inside buildings such as residential houses and offices, which require around 60–70% humidity, and hospital operating rooms, which need around 50–60% humidity (Sookchaiya et al., 2010).

A dehumidifier is a device that can be used to reduce the humidity of the air. Some industries, such as textile, foods, pharmaceutical, and battery production, are susceptible to moisture. These industries require an environment with low humidity within the range of 20–55% in order to maintain the quality of their products and machines (Kamar et al., 2016). Humid surrounding air will lead to the corrosion of metals, deteriorated characteristics of hygroscopic material, and increased harmful activity of micro-organisms in products (Moncmanová, 2007). The system has two different features, i.e., compressor-based (CBD) and desiccant-based dehumidifiers. CBD is a conventional method of removing water vapor by condensation based on the vapor compression refrigeration system (Rambhad et al., 2016). Humid air passes through a cooling coil where it is cooled below its dewpoint temperature in order for condensation to occur. However, the CBD system consumes large amounts of electrical energy during the cooling process. The desiccant dehumidification system, meanwhile, has received much recent attention as an alternative to the CBD type (Yamaguchi & Saito, 2013). Here, air is dehumidified without condensation, using only sorption from desiccant material instead. This can help reduce the electrical energy consumption of the CBD system.

Desiccant dehumidifiers can be characterized into two categories, i.e., liquid desiccant dehumidifier (LDD) and solid desiccant dehumidifier (SDD). The main components in an LDD system are the absorber and regenerator. Both parts are filled with constructed packing materials to enhance the contact area between the desiccant solution and process air. The absorber is concentrated with a desiccant solution to enable it to absorb water vapor from the process air. The process air, driven by a fan, flows in an upward direction within the liquid desiccant. Then, the dilute liquid desiccant flows out of the absorber and is pumped to the regenerator. In the regenerator, ambient air at a high air temperature flows in an upward direction within the diluted liquid desiccant. The ambient air absorbs the water vapor from the diluted liquid desiccant due to the difference in vapor pressure between the liquid desiccant and the air. The regenerator restores the ability of the liquid desiccant to absorb moisture for the next process cycle. The advantage of the liquid desiccant is that regeneration can be carried out at a lower temperature with high moisture removal capacity (MRC) (Misha et al., 2012). However, this can lead to corrosion of the dehumidifier components.

An SDD consists of a desiccant material constructed in the form of a wheel that rotates at a low speed, an air heater, and a drive motor. The wheel comprises a process air section and a regeneration air section. The humid process (ambient) air flows through the process air section, during which time the desiccant material adsorbs moisture from the air. The temperature of the air increases slightly since the adsorption process releases heat. As a result, the process air leaves the desiccant wheel with lower humidity and a somewhat higher temperature. On the other hand, hot regeneration air flows through the regeneration section of the wheel in the opposite direction to the process air. As this happens, the water vapor sitting on the desiccant material surface is desorbed by the hot air. This desorption process also involves heat transfer, and as a result, the regeneration air exits the wheel with higher humidity and a slightly lower temperature. The regeneration section of the desiccant wheel then becomes dry, ready for the new process air to flow through it. Since the desiccant wheel is rotating slowly, the above sequences of processes are continuous. This system consumes less electrical energy due to the lack of pump usage and moving parts (Wu & Wang, 2006). It is also more straightforward than liquid desiccants (Misha et al., 2012) because it has a low risk of crystallization and only a slight risk of damage due to high temperatures. Also, the solid desiccant material is environmentally friendly.

Two of the most critical components in the SDD system are the desiccant wheel and solid desiccant material itself, which is corrugated in the numerous channels inside the rotary wheel. Cheng et al. (2016) carried out a study on the influence of desiccant material properties on dehumidification effectiveness and showed that it is influenced by the thermal conductivity, specific heat, porosity, tortuosity, and thickness of desiccant materials. Jia et al. (2007) compared the effect of silica gel and composite materials on the coefficient of performance (COP) and MRC. It was found that the composite desiccant wheel adsorbed more moisture than the conventional one. Zhang et al. (2014) investigated the effects of ten types of desiccant materials on COP, specific dehumidification power (SDP), and dehumidification efficiency. The performance of the desiccant wheel is affected by several parameters, including wheel geometry, rotation wheel speed, inlet process air properties, inlet regeneration temperature, and velocity (Yamaguchi & Saito, 2013; Cheng et al. 2016). Jia et al. (2007) conducted a similar study using an experimental method. Others used a numerical method as an alternative to the experiment (Misha et al., 2012). This approach is less complicated, much cheaper, less time-consuming, and less laborious. Cheng et al. (2016) established a three-dimensional (3D) single-channel model representing desiccant material and found the model to be essential in obtaining accurate predictions. For simplification, many researchers have used a single-channel model to describe the airflow through the desiccant wheel. This may be for reasons of geometric similarity and in order to avoid prohibitive computation costs, and it is reasonable to use a single channel to represent the multiple channels in the desiccant wheel (Yadav et al., 2014).

Recent research and developments of SDD have focused on improving energy efficiency by using a low-grade heat source such as solar energy so that the regeneration temperature can be reduced. The regeneration temperature is determined by the properties of desiccant material, which should have high adsorption capacity and good regeneration ability. Though some novel materials have improved the performances of SDD systems, no material currently available can perfectly satisfy the entire demand for an energy-efficient, environmentally friendly, and affordable material. Therefore, more studies on the development of desiccant materials are needed in order to meet the requirements of industry. Traditionally, these have involved conducting experiments on the SDD system. However, a series of tests have needed to be performed as part of the experiments and clearly, due to the need to install a variety of desiccant wheel models, this practice is highly costly and time-consuming. In order to ensure efficiency in carrying out the parametric analysis, numerical modeling should be used where it can promote energy and cost-saving. There are still only limited studies in three-dimensional (3D) modeling representing solid desiccant material, where major past researchers have only developed simplified models in 1D or 2D. These models reduce the validity of simplified models of the SDD. Therefore, this study aims to examine the effects of air regeneration temperature and desiccant material on the performance criteria of solid desiccant material using numerical modeling. A 3D model of a single air channel was developed to represent a flow path of the process and regeneration air through the solid desiccant material. Flow simulations were carried out under a transient state to predict the average temperature and humidity of the process air at the channel exit. The model was validated by comparing the simulation results with experimental data obtained from the literature. The performance criteria considered are MRC, dehumidification effectiveness, and thermal effectiveness. This research produces an economical method for determining the performance criteria of solid desiccant materials. Thus, it could identify the most suitable materials that give the lowest possible humidity of process air at any given regeneration air temperature.