Humidification-dehumidification System Research Articles

Solar-powered adsorption desalination utilizing composite silica gel with a humidification-dehumidification desalination system

This work comes in light of addressing crises of freshwater scarcity, energy demand, and climate change to investigate innovative configurations of hybrid adsorption desalination integrated with a humidification dehumidification system powered by solar energy or waste heat in four operation modes. The investigation includes economic cost analysis and system performance using raw silica gel and silica gel/CaCl2 adsorbents under the weather data of Assiut-Egypt. System performance has been evaluated regarding daily water production, gain output ratio, and cooling effect using TRNSYS and MATLAB software. The combined configuration with heat recovery (the fourth mode) using silica gel/CaCl2 gives a relatively high freshwater production (69 m3/ton.day in June) compared with the basic adsorption desalination system using raw silica gel (7.1 m3/ton.day in June). Silica gel/CaCl2 in the first mode presented the highest cooling effect (555 W/kg) in June compared with silica gel in the primary system (197 W/kg). The third configuration's solar-powered silica gel/CaCl2 system shows a clear superiority of 1.8 in gain output ratio, compared with the basic system with and without heat recovery (0.54 and 0.4, respectively). The third configuration also provided the cheapest freshwater production among all configurations at 2 $/m3 in June.

Multi-stage humidification–dehumidification system integrated with a crystallizer for zero liquid discharge of desalination brine

Desalination plants discharge a massive volume of brine every day, which negatively affects the ecosystem, particularly the subsurface ecology and marine life. To mitigate the brines' detrimental effects on the environment and facilitate the recovery of salt crystals and freshwater, the Zero Liquid Discharge (ZLD) approach was proposed. This study proposes four different configurations of multistage humidification dehumidification systems integrated with a crystallizer, exploring their energy and economic implications. Key findings from the study include the higher Gained Output Ratio (GOR) in series configurations due to lower temperature differences, resulting in lower heater energy consumption. The crystallizer significantly enhances water productivity, but it is a major consumer of both electricity (85%) and thermal energy (50.45–57.7%) in the system. Increasing the heat source temperature reduces the Specific Electrical Energy Consumption (SEEC) and the specific area while slightly increasing the Specific Thermal Energy Consumption (STEC). Elevated cooling water temperature raises SEEC and specific area but has negligible effects on STEC. Higher feed water flow rates increase SEEC but lower both STEC and specific area. Feed salinity elevation decreases productivity, SEEC, STEC, and specific area. The crystallizer constitutes 15.43–26.44% of the total capital cost, with series configurations incurring higher costs. Unit freshwater production costs, considering salt selling, range from 1.733 to 4.821 $.m−3. In a waste heat scenario, parallel configurations become profitable, where freshwater can be distributed at zero cost, while the produced salt crystals can be sold at an even lower price of $58.09 per tonne and $57.04 per tonne, respectively.

Evaluation of two-stage humidification-dehumidification desalination systems operated by a modified heat pump

The humidification-dehumidification desalination system operated by a heat pump is ideal for the decentralized small-scale desalination plant, due to its advantages of low investment and stable freshwater provision. To overcome the heat source limitation of multi-stage humidification-dehumidification system, a modified heat pump with a subcooler is used to operate the two-stage humidification-dehumidification system. With the difference of heated object, two layouts of the heat pump operated two-stage humidification-dehumidification systems are proposed, which are twice water-heated system and twice air-heated system. A parametric study is conducted to analyze the effect of feed seawater temperature and water/air mass flow ratio on the thermodynamic and economic performances of the proposed systems. A performance comparison is also carried out to analyze the difference between water heating and air heating, together with advantages over the reported systems. According to the results, it can be concluded that optimal feed seawater temperature and water/air mass flow ratio exist for the proposed systems. The twice water-heated system outperforms the twice air-heated system with the maximum freshwater productivity and gain output ratio of 34.69 kg/h and 7.69, while the lowest specific electricity consumption and freshwater production cost of 89.18 kWh/m3 and 14.52 $/m3. These results are also superior to those of previously reported HP-HDH systems, fully demonstrating the value of research and development for this novel system and its promising market application prospects.

Design of a solar air heater for a direct-contact packed-bed humidification–dehumidification desalination system

The performance of solar desalination systems based on a humidification–dehumidification (HDH) approach is significantly enhanced by preheating the air entering the evaporator. To estimate this potential benefit, we present an analysis of integrating a novel solar air heater (SAH) with an HDH system. The combination of both units leads to intriguing applications, such as incorporating these systems into buildings. A model of the solar thermal system is developed, and its predictions are validated with experimental data. The SAH model is combined with a previously validated model of an HDH. This allows us to estimate the performance of a combined SAH/HDH system under different environmental conditions and to evaluate the annual water treatment capacity at various sites around the world. Simulations suggest that a system with a 3.5 m2 solar collector in arid regions (Middle-East, north of Africa, south-west of the USA, East of South America), has the potential to treat 30 tons of water and reduce the CO2 emission by 150 kg annually.

RETRACTED: Enhancing desalination efficiency using waste heat from household air conditioning: A heat pipe-assisted HDH system performance analysis

This study presents an experimental exploration of a desalination system that produces freshwater from saltwater using a novel heat pipe-assisted humidification-dehumidification (HDH) process. The system utilizes waste heat recovered from residential air conditioners that use vapour compression refrigeration systems. The study focuses on the mass and heat transfer within the system's components, with performance evaluations conducted at three different air conditioning temperatures (19 °C, 20 °C, and 21 °C). The experiments highlight the freshwater production of the R32 heat pipe over the R134a and R600a heat pipes. The study estimates the system's cost-effectiveness at $0.0085/L, emphasizing its economic viability. Preheating resulted in significant improvements in freshwater yield, ranging from 43.33 % to 51.5 %, at temperatures of 21 °C and 20 °C, respectively. The Gain Output Ratio (GOR) doubled from 0.75 to 1.52, indicating a more energy-efficient process with significant improvements in the effectiveness of humidification and dehumidification. The pH levels improved from 8.32 to 7.07, representing a 15 % reduction and falling within the WHO-recommended range of 6.5–8.5. Electrical Conductivity (EC) shows a notable reduction from 62,720 to 826.1 micromhos/cm, representing a 98.68 % reduction, and well below the permissible limit of 1500 micromhos/cm. Total Dissolved Solids (TDS) also experienced a drop from 31,777 to 413.1 ppm, representing a 98.7 % reduction, and falls under the WHO guideline of <600 ppm for drinking water. The experiments prove that the use of waste heat from household air conditioners for sustainable freshwater production through the HDH desalination method, with the added efficiency benefits of heat pipe.

Performance analysis of a novel two-bed adsorption humidification-dehumidification desalination system using Metal-organic framework 801

Adsorption desalination techniques have been increasingly studied over the last century as a sustainable option for meeting the rapidly growing demand for fresh water. One of the attractive factors for using adsorption desalination systems is the use of low-grade thermal energy normally wasted in the atmosphere. The current study introduces a novel adsorption desalination system integrated with a humidification dehumidification system for producing potable water. Four system configurations are presented. The main distinction between them is the source of hot water used for the humidifier. Scheme #1 utilizes the heat of condensation, Scheme #2 applies the heat of condensation plus the heat of adsorption, while Scheme # 3 adds the outlet of heating water from the desorber as a source of heat to the humidifier. Scheme #4 is a standalone adsorption desalination system that applies chilled water to cool the condenser. The novelty of this system is the application of the chilled water from the evaporator to the dehumidifier to create the highest possible temperature difference necessary for the humidification and dehumidification process. This advantage allows larger water productivity, especially in hot humid regions. In this case, it is possible to extract water vapour in the ambient humid air, water vapour gained from the humidification process and water vapour condensed in the condenser of the adsorption desalination system. Moreover, we have used a novel material Metal Organic Frameworks 801 as an adsorbent for a highly efficient process. Based on the results, Scheme #3 produced the maximum amount of fresh water, 210 kg/h while Scheme #2 obtained the maximum gained output ratio, 4.4 with a minimum cost of 0.0079 $/L. A standalone adsorption system produced the minimum amount of distillate water and the lowest gained output ratio. It is not advisable to use a standalone adsorption system to produce distillate water; as the full desalination potential is not achieved.

Study on the Heat and Mass Transfer Characteristics of Humidifiers in Humidification–Dehumidification Desalination Systems

The humidifier plays a key role in a humidification–dehumidification (HDH) desalination system; it directly affects both the freshwater production efficiency and energy consumption ratio of the system. In this study, for a humidifier in an HDH system, a heat–mass coupled differential equation model of spray water and air on the surface of the packing material was established, and the effects of parameters such as the spray water temperature (tw), mass flow rate of spray water (mw), air temperature (ta), and air mass flow rate (ma) on the humidification performance of humidifiers composed of eight different types of packing materials were investigated. The results show the following: (1) Under the same inlet spray water and air conditions, the humidification performance of different packing materials from good to bad is as follows: cellulose paper, polypropylene, hackettes, saddles, snowflakes, wooden slats, polyvinyl chloride, gunny bag cloth. (2) Increasing the tw can significantly improve the humidification performance. To achieve higher humidification energy efficiency, it is recommended to increase the tw to above 80 °C. (3) With the increase in the mw, although the humidification efficiency (εhum) decreases slightly, the humidification rate (mhum) increases, and the specific humidification energy ratio (ηhum) decreases accordingly. To maintain a high mhum and a low ηhum, it is advisable to control the mw at not less than 0.5 kg/s. (4) Increasing the humidifier inlet ta can improve the mhum, εhum, and ηhum, although not as effectively as increasing tw. (5) Increasing the ma can improve mhum and εhum. However, it simultaneously increases the ηhum. The results of this study can provide theoretical guidance for the selection of efficient packing materials and the optimization of humidifier operating conditions in HDH desalination systems.

Thermodynamic comparison of the operation of the HDH system with water heating or air heating in closed water circulation and semi-open-air circulation

The present study focuses on doing a thermodynamic analysis of a desalination system that utilizes the humidification–dehumidification (HDH) process with semi-open-air (SOA) circulation. The HDH desalination system is a notable example of a developed system utilized for freshwater production. This system has the advantage of being able to derive its energy from solar sources. The categorization of desalination units is determined by the configuration of their components, the direction of water and air movement, and the kind of heating employed for each stream. The present study noted that the utilization of this particular air circulation strategy, while maintaining a constant maximum system temperature, leads to enhanced system efficiency when combined with water heating. However, no substantial impact was detected when this method was employed in conjunction with air heating (AH). Each type of HDH desalination system has a different ratio of mass flow rates (MFRs) for the streams. This means that the efficiency of the system stays the same, no matter how the air moves through the system. The examination of temperature and relative humidity characteristics in the environment has indicated that this particular desalination unit is applicable in places characterized by both arid and humid conditions. Furthermore, in both air and water heating modes, it has been observed that the system employing SOA circulation has superior efficiency when the environment temperature is elevated. In the context of water and/or air heating, when a specific value is assigned to the MFR ratio of the two streams, modifying the mode of air circulation does not have an impact on the overall efficiency of the system. It is shown in this article for HDH with air heater the highest GOR is attained at either FCR = 1 or FCR = 0, depending on the operational temperature of the system. Also The system clearly works at its most efficient (GOR = 3.4) when it's in a Semi-open-air mode (FRC = 0.77) and the temperature is 50 °C. On the other hand, the system efficiency is 2.94 when the air exchange system is closed (FRC = 1). In the same way, the overall efficiency is 2.88 if the air flow is open (FRC = 0).

Experimental analysis on hybrid adsorption and open air cycle humidification dehumidification (AD–OHD) desalination system

In the current status, an Adsorption Desalination System (ADS) has been found to have lower water production capacity than conventional desalination technologies. This study proposes a hybrid desalination system integrating the Adsorption (AD) and Open air cycle Humidification-Dehumidification (OHD) process to overcome the challenge of lower water production capacity in ADS. This hybrid system known as AD-OHD system comprises of a single bed ADS and a Humidification-Dehumidification (HDH) unit. The main objective of this work is to analyse the performance of the hybrid AD-OHD system based on parameters such as Gain Output Ratio (GOR) and amount of desalinated water production. The performance of the hybrid AD-OHD system is studied under three different operating conditions of switching (tswt) and sorption time (tspt), namely, tswt > tspt, tswt = tspt, and tswt < tspt. The results indicate that the optimal working condition is achieved when the hybrid AD-OHD system operates at condition tswt < tspt. The amount of desalinated water and GOR under the optimal condition is found to be 1.12 l/h and 0.78, respectively. The study also conducts experimental investigations to evaluate the GOR of the hybrid AD-OHD system under different operating conditions, including the temperature of the water entering the evaporator and the temperature of the hot and cooling water input to the adsorbent bed. Experimental data indicates a positive correlation between GOR and hot water temperatures, while increased cooling water temperature reduce GOR due to reduced adsorption behaviour. Although the evaporator inlet water temperature initially boosts GOR, it declines as the adsorption capacity diminishes. The study highlights the superior performance of the hybrid AD-OHD system compared to both the standalone single-bed ADS and open cycle HDH desalination systems. The hybrid AD-OHD desalination system exhibits an 80 % improvement over the single-bed ADS system and a 50 % improvement over the open cycle HDH system in desalinated water production. Additionally, a theoretical model of the adsorption desalination cycle is developed and validated, demonstrating the viability of the AD-OHD system for desalination applications.

Experimental study on optimum performance of two-stage air-heated bubble-column humidification–dehumidification system

An experimental investigation of a small-scale air-heated humidification–dehumidification (HDH) desalination system with bubble-column humidification and dehumidification units was conducted. The study addressed the performance of the multistage air-heated bubble-column HDH system, which has limited coverage in the literature, by operating two bubble-column humidifiers in series for the air humidification process with air reheating. The effect of operating parameters such as airflow rate, air temperature, and saline water levels in both humidifiers on the performance metrics of the system were investigated. The product distillate rate, energy consumption, gain output ratio (GOR), and specific energy consumption (SEC) are the main indicators of performance for the proposed desalination system. Response surface methodology (RSM) was applied to the current system using the design of experiment (DoE) for the prediction of variables that greatlyaffect productivity and energy input. The airflow rate, air temperature, and water level of the second humidifier have a favourable effect on the distillate rate and GOR of the system. In contrast, the effect of the water level inside the first humidifier is insignificant. Furthermore, the RSM optimization approach was used to obtain the optimum distillate productivity. An optimized distillate rate of 0.45 L/h and a GOR of 0.4 are achieved at 1.5 SCFM (standard cubic feet per minute) of airflow rate, and 6.5 cm of water level in the second humidifier with 140°C air inlet temperature. The numerical optimization reveals the optimal operating parameters, that correspond to maximum distillate production of 0.3 L/h with minimum input energy of 0.71 kW, to be 139°C air temperature, 1.13 SCFM of airflow rate, 6.5 cm and 3 cm water levels of second and first humidifier, respectively.

Energy and exergo-economic analysis of a parallel feed multi-effect system integrated with humidification–dehumidification system for brine recovery

Desalination technologies reject large amounts of brine with significant value back to sea. The concept of hybridization of different desalination technologies has proven that it can be effective in terms of reducing rejected brine and increasing the freshwater production rate as well as reducing the freshwater cost. In this work, brine recovery to improve water production through a simple modified configuration of integrating a multi-effect desalination (MED) system with humidification–dehumidification system (HDH). The rejected brine of the MED system is used as the feed for the HDH system without the need for preheating the rejected brine since it leaves the MED at a suitable temperature for HDH application. The study focuses on investigating the effect of different operating conditions on the increase in system freshwater production rate and recovery ratio as well as the exergetic efficiency. Parameters investigated include steam temperature, feed salinity, number of brine streams, cooling water flow rate, and ambient temperature. An exergo-economic analysis has also been conducted using the cost flow method to evaluate the freshwater production cost for the modified system. Results indicate that the integration of HDH can increase the water production rate by a maximum of 7.82% and produce fresh water at 2.08 $/m3 compared to 2.094 $/m3 when using the standalone system under the same conditions.

Performance of different HDH desalination units powered by diesel engine generator waste heat

Diesel generators used for off-grid power generation and other applications waste a significant amount of input power as heat (about 65%), contributing to global warming. To mitigate this, waste heat recovery and utilization systems can be employed. The current study, focuses on utilizing waste heat from a 2.5 MW diesel generator for freshwater production through integrated humidification dehumidification (HDH) systems. Four different HDH system layouts are investigated, considering factors such as dehumidifier and humidifier effectiveness, temperature differentials, mass flowrate ratio, and ambient temperature. The aim is to analyze and optimize the system's pure water productivity, recovery ratio, specific entropy generation, and gained output ratio. The results show that the layout featuring an air heated – air heated cycle (AH-AH) performs the best, with the highest productivity and gained output ratio of 234.48 m3/day and 3.01, respectively. On the other hand, the water heated – water heated cycle (WH-WH) layout exhibits lower performance. The study demonstrates the potential to recover 46–70% of the waste heat, significantly improving the diesel engine's utilization factor from 36% to 68%. The proposed system also proves cost-effective, with freshwater production costing as low as 0.62 $/m3 when utilizing waste heat, compared to 4.11 $/m3 and 13.96 $/m3 when using steam or electric heating elements. Furthermore, the integrated system helps reduce the environmental impact by minimizing waste heat discharge into the environment and thus contributes to mitigating global warming.

Thermodynamic and thermoeconomic modeling of humidification-dehumidification desalination systems with bubble column dehumidifier

Humidification-dehumidification (HDH) desalination systems are popular among researchers due to their simple structure, ability to operate with low-grade and renewable energy sources, and the capacity to work with high-concentration water. This paper presents a thermodynamic and thermoeconomic analysis of an HDH system with a multi-stage bubble column dehumidifier. There was a lack of economic studies on bubble column dehumidifier systems in previous literature; also, no research had examined the impact of different geometric and temperature parameters on the entire system's performance. The fixed surface method was used to model the humidifier and dehumidifier, and the effect of mass flow rate ratio, top and bottom temperature, pipe length within each dehumidification stage, and the number of dehumidifier stages on the system's performance was studied. System performance was compared under various conditions, including gain output ratio (GOR), water production, and cost of produced freshwater. The highest GOR is obtained in the highest pipe length and the highest number of dehumidifier stages. Increasing the length of the pipe and the number of dehumidification stages increases the cost, so achieving the lowest cost of produced water is a compromise between higher cost and higher efficiency. The system achieved a GOR of 3.03, the lowest cost price of produced water is 18.29 $m3, and the highest acheived exergy efficiency is 22.4 %.

Design and performance a novel hybrid membrane distillation/humidification–dehumidification system

Water shortage is a burden on governments with the significant increase in population and industrial activities. There is an urgent need to devise innovative systems for providing potable water with low production costs. The availability of seawater around the world with the presence of desalination technologies can offer various solutions. A novel air gap membrane/humidification-dehumidification (AGMD/HDH) hybrid system was designed and implemented to produce distilled water. The influence of operational parameters on the hybrid process was studied in detail. Freshwater productivity gained output ratio (GOR), energy efficiency, specific thermal energy consumption (STEC), and feasibility study was investigated to optimize the process. Air flowrate, seawater feed temperature, and water flow rate were ranged from 100 kg/h to 250 kg/h, 40 °C, 65 °C, and 240 L/h to 600 L/h, respectively. The hybrid system produced freshwater of 24.4 kg/h at optimum conditions. The leading performance indicators were 6.8 for GOR, energy efficiency (80%), and 240 kWh/m3 for STEC. Humidification-dehumidification (HDH) system was mainly powered by low-grade energy power, so the proposed hybrid system was able to recover heat loss from the operation of the air gap membrane desalination (AGMD) unit. The cost estimation of distilled water is 9.34 US $/m3 with an annual production rate of 210.62 m3/year. The proposed hybrid air gap membrane distillation/humidification–dehumidification units showed remarkable improvement in total water productivity at a low cost.

Process design and technoeconomic analysis for zero liquid discharge desalination via LiBr absorption chiller integrated HDH-MEE-MVR system

A novel multipurpose seawater desalination process is presented that integrates a Lithium Bromide (LiBr) absorption chiller, humidification-dehumidification (HDH), multi-effect evaporator, and mechanical vapor recompression (MVR) to improve the energy effectiveness of conventional desalination processes. The proposed process uses the LiBr absorption chiller cycle to preheat the seawater prior to the steam generation in the multi-effect evaporators while the MVR system activates the LiBr cycle. The integration of HDH further allows to recover the excess water in the process stream and concentrates the brine effluent for zero liquid discharge processing. A stand-alone HDH desalination is comparatively studied with the integrated system to quantify their respective performances over several metrics. Technoeconomic analysis is also performed to assess the economic feasibility and a sensitivity analysis is carried out to observe the effects of changing process variables on the overall performance of these systems. The results show that the proposed process design achieves several times higher gain-output ratio compared to a conventional HDH system of equal capacity, with a levelized water production cost of $8.4/m3 and 6 kW of cooling capacity that can be used for air conditioning.

On using artificial neural network models for a thermodynamically-balanced humidification-dehumidification system: Design and rating analysis

This research focuses on rating and sizing a thermodynamically-balanced humidification-dehumidification (HDH) system with zero, single, and double extractions of humid air from the humidifier to the dehumidifier. In order to implement thermodynamic balancing appropriately, optimal thermal design is essential to be estimated via a reliable and straightforward model. Thus, this study suggests an artificial neural network to predict the system performance and size parameters. The artificial neural network is trained, tested, and validated using a rich data set created to cover possible operating ranges. The considered performance parameters are gained output ratio, water-to-air mass flow rate ratio, recovery ratio, seawater and air temperatures, energy effectiveness, and critical enthalpy pinch, and the design parameters involve humidifier volume and dehumidifier area. Four inputs are used: seawater inlet temperature, heating temperature, enthalpy pinch, and extractions count. The developed model works best for inlet temperature, heating temperature, enthalpy pinch, and extractions count within 20 – 40 °C, 60 – 80 °C, 1 – critical enthalpy pinch in kJ kgd-1, and 0 – 2 extractions, respectively. The results indicate that the developed artificial neural network has high accuracy in predicting the performance and design parameters, demonstrating minimum accuracy of 98.3%, 96.4%, and 98.9% for zero, single, and double extractions, respectively. Furthermore, the generated neural network is validated against numerical and experimental data from the literature, showing a maximum discrepancy percentage of less than 4%. Thus, the neural network provided by this study is helpful for HDH desalination engineers, designers, researchers, and scientists.

Modeling of a direct-contact humidification-dehumidification desalination unit in a 256 MW steam power plant using effluent streams: Case study

Considering the waste of fresh water in a 256 MW steam power plant located in the southeast of Iran, and to compensate a part of its water consumption, a humidification-dehumidification (HD) desalination system is proposed. Because of sedimentation problems in indirect-contact HD systems, in this research, the application of direct-contact HD method to recover the effluent stream in a steam power plant is presented. Considering unit cross-sectional area for HD towers and based on primary results of the modeling, up to 119 lit/hr can be produced when the height of both humidifier and dehumidifier is 1.8 m and the flow rate of air is 0.7 kg/s. Three different scenarios based on (I) full capacity of available steam from the blowdown of boilers, (II) available chemical effluent (salt water), and (III) available cooling water, are examined and compared in this research. As, required salt water or/and cooling water are much more than the available amounts in the power plant, scenario I and II are rejected. Based on obtained resulted from the scenario III, the cost of production is 0.8 $/m3 of fresh water, and the GOR of proposed HD desalination unit is 1.03. With consumption of less than 5% and 10% of the available steam and salt water in the power plant, respectively, more than 7.5 m3/day of fresh water can be produced by the proposed direct-contact HD desalination system.

A critical experimental analysis of Humidification-Dehumidification desalination system with mass extraction

The present work reports an experimental investigation of a zero- and single-extraction humidification-dehumidification (HDH) desalination system. While rich thermodynamic modeling of mass extraction/injection-based HDH systems exists in the literature, studies providing design guidelines are lacking. This work aims to elucidate practical design aspects experimentally and report a simple tool for component and system sizing based on the understanding gained from experiments. The effect of the operating parameters such as airflow rates, water flow rates, extraction airflow ratio, top cycle temperature, and salinity on the energy efficiency and yield of the HDH desalination system with and without mass extraction/injection was experimentally examined. It was observed that there is an optimal airflow rate and water flow rate for a zero extraction HDH desalination system at a given top-and-bottom-cycle temperature. The design of mass extraction/injection in advanced HDH cycles involves identifying two key elements of design: (a) location of extraction and (b) amount of airflow extraction/injection. The effect of the airflow extraction location and ratio on the thermal energy efficiency of the single extraction HDH system is reported for different air flow rate conditions. The maximum gained-output ratio for zero- and single-extraction HDH systems was 1.74 and 1.86, respectively. A simple effectiveness-based mathematical model for a single-extraction HDH system is reported that enables component and system sizing.

Performance evaluation of bubble column humidifier using various air distributors in a humidification-dehumidification desalination plant

A humidification-dehumidification (HDH) system using a bubble column humidifier was optimized using response surface methodology (RSM). The humidifier was integrated with four types of air distributors entitled the horizontal cuts (H-cuts) and vertical cuts (V-cuts) spargers with the same total hole area, as well as the multiple orifice nozzles (M-orifice) and perforated plate with equal hole diameters. The experimental investigation consisted of a parametric study, optimization using RSM, and bubble characteristics using image analysis. The effects of a qualitative parameter (i.e., the type of air distributors) and quantitative parameters (i.e., the inlet water temperature, superficial air velocity into the humidifier as well as the water height inside the humidifier) on the absolute humidity of the air leaving the humidifier were studied. The H-cuts and M-orifice spargers were found as the best air distributors in their subclasses due to higher gas holdup, specific interfacial area, and lower Sauter mean diameter. The humidifier performance was optimized in the lowest superficial air velocity (0.195 m/s), the highest water height inside the humidifier (50 cm) and the inlet water temperature to the humidifier (90 °C). It was concluded that the H-cuts and M-orifice spargers at the optimum values of the quantitative parameters achieved the maximum absolute humidity of about 0.18 and 0.143 kg water vapor/kg dry air, respectively.

Effect of thermal characteristics on the chemical quality of real-brine treatment through hydrophilic fiber-based low-grade heat-powered humidification-dehumidification process

Considering the increasing demand for desalination plants and their byproduct brine, this study investigated a humidification-dehumidification (HDH) system for treating membrane distillation-generated real high-salinity brine using low-grade heat (45–70 ℃) to explore its feasibility for sustainable energy-efficient minimal liquid discharge. A novel super-hydrophilic fabric was adopted for accelerated humidification, and its impact on brine droplet miscarriage characteristics was evaluated. The influence of the operating fluid thermal properties (cycle 1: air preheating; cycle 2: air and brine dual-fluid preheating; and cycle 3: air post-heating after humidification) on the brine treatment efficiency, energy consumption, and chemical quality of freshwater produced was analyzed in detail to establish their characteristic nexus. It was identified that, during humidification, increasing the brine temperature (up to 55 ℃) influenced its ionic mobility, thereby promoting efficient separation of the salts/minerals and contributing to achieving better freshwater quality. Furthermore, although cycle 3 exhibited improved system thermal efficiency (gained output ratio equal to 1.77), its non-preheated air contributed to a negative effect of the reduced humidity ratio (∼17 g/kg), leading to a lower freshwater productivity of 67% than that of cycle 2 (29 g/kg and 70%). The present study also illustrates a novel effect of evaporative deposition occurring due to air-water interaction on the fabric humidifier surface, with an exploration of its effect on reducing freshwater chemical quality. The freshwater generated from optimum thermal cycle 2 exhibited reduced pH (by ∼63%), sodium (99.9%), chloride (99.9%), toxic boron (99.7%), and other chemical contaminants, thereby satisfying the major international water reuse standards.