Development of an anti-clogging perforated plate atomizer for a zero liquid discharge humidification-dehumidification desalination system

Abstract

An anti-clogging perforated plate atomizer is designed for high temperature and salinity applications in a novel solar-powered zero liquid discharge humidification-dehumidification desalination system (US Patent Application US62882953). This paper presents a detailed discussion on its design, and operation. Experiments are performed on seven atomizers to study the effect of design, and operating parameters on spray cone angle, and average droplet diameter. Spray cone angle remains constant with change in air mass flux, and increases with increasing orifice diameter, and manifold diameter. It also increases initially with water mass flow rate, and manifold height, before attaining a constant value. Average droplet diameter increases with increasing water mass flow rate, and decreasing air mass flux, orifice diameter, and manifold diameter. Further, anti-clogging performance of the atomizer is tested with hypersaline water of 100,000 ppm total dissolved solid (NaCl) at elevated temperatures such as air at 175 °C and saline water at 45 °C. Results show no clogging for 2–13 h of operation. Near-complete suppression of atomizer clogging makes our new additively-manufactured perforated plate atomizer an ideal fit for high salinity, and zero liquid discharge humidification-dehumidification desalination systems. Additionally, its open-surface design allows additional surface modifications to further reduce clogging, and enhance self-cleaning characteristics.

Introduction

Demand for fresh water is increasing with the ever increasing population, and rapid industrialization: it doubles every fifteen years since 1950 [1]. Water desalination has been identified as a solution to accommodate this increasing demand of fresh water, since 97% of the water on earth is salt water [2]. This has led to a significant development in desalination technologies such as multi-stage flash (MSF), multi-effect evaporation (MEE), and reverse osmosis (RO) [3]. Total production rate of the desalination plants around the world (near 20,000) was estimated to be over 85 million tons per day in 2016 [4]. However, a large amount of waste brine is discharged from these desalination plants directly into municipal sewers, groundwater, coastal waters, and open land evaporation ponds [5]. Such disposal of waste brine has severe impacts on aquatic ecosystems, and land vegetation systems [6]. In this regards, zero liquid discharge (ZLD) desalination is seen as an efficient, and environmentally-friendly solution to the global water scarcity problem. It provides high water recovery, and zero waste generation coupled with valuable salt production. Accordingly, various efforts have been directed towards integrating ZLD systems with existing desalination technologies [7], [8]. For instance, Lu et al. [9] proposed a combination of freeze distillation, membrane distillation, and crystallization to achieve ZLD. They demonstrated a daily output of 2.5 kg of salt, and 69.5 kg of fresh water using a lab-scale hybrid ZLD desalination system.

Conventional desalination plants also have huge energy consumption relying on fossil fuels [10]. Accordingly, significant efforts have been concentrated on coupling desalination plants with a sustainable energy source, such as solar energy [11], [12]. However, high grade solar energy is not always available throughout the year [13]. Humidification-dehumidification (HDH) desalination suitable for low-grade heat sources, is therefore envisioned as a sustainable, and promising desalination technique. HDH involves evaporation of water from saline water into dry air at low temperatures (humidification). This water vapor is then condensed out from the air in a condenser to produce fresh water (dehumidification). HDH desalination is often complimented for low operating cost, lower operating pressure than RO, and easy installation [14]. It could also be used as a small-scale decentralized water production unit in rural areas to get fresh water for drinking, and other applications [15]. Accordingly, solar HDH is being investigated by many researchers.

A novel solar-powered desalination technology combining ZLD, and HDH for seawater to hypersaline water treatment has developed at the Oregon State University [16]. It atomizes saline water by thermally actuated air for higher evaporation rates during humidification. The saline water spray further evaporates to produce humid air, and salt particles in a particle-laden stream. The salt particles are separated from humid air in a cyclone separator. Vapor is condensed from the salt free humid air to produce fresh water. A two-phase separator is used to separate water from air. The technology is designed to be modular, portable, and powered by low grade heat to treat hypersaline water with 100,000–200,000 ppm total dissolved solids (TDS).

Fresh water production rate, and efficiency of a HDH desalination system could be increased by increasing the evaporation rate of the saline water or humidification [24]. Evaporation of water increases with mass flow rates, and temperatures of air, and water, and decreases with concentration of total dissolved solids in saline water [23]. Therefore, to accommodate high salinity as in our HDH system, we need to operate humidifier at high mass flow rates, and temperatures of air, and saline water to achieve higher evaporation rates. In the literature, two major methods are reported for humidification process: pad humidification, and spray humidification [17]. In a pad humidifier, saline water is sprinkled on an evaporator pad, and water evaporates in dry air passing though the pad. Literature has several experimental studies on pad humidifier using different material of construction. For instance, Hamed et al. [21] used a pad humidifier made of cellulous paper in a solar HDH desalination system. They used experimental results to validate their simulation model for predicting fresh water production under different operating conditions. They produced around 22 l/day of fresh water. Some other studies used a polypropylene packed bed [22], polyvinyl chloride (PVC) reaching rings pad [23], porous balls [20], and cellulous pad materials [24] in humidifier. However, pad humidifier doesn't fit the requirement of a ZLD system as all salt deposition would occur at the pad, and it would require frequent replacements.

In spray humidifier, saline water is atomized using pressure atomizers to generate a spray, and air is passed through it to facilitate water evaporation. For example, Hegazy et al. [18] experimentally studied the effect of flow directions of spray water (cross, counter and parallel), and air flow rate on humidification in a heat pump operated HDH desalination system. Cross water spray yielded the highest production of 3 l/h. El-Maghlany et al. [19] further improved the performance of HDH desalination system used by Hegazy et al. [18] using parallel flow spray in single, and two-stage dehumidification with 2 l/h, and 4 l/h fresh water production respectively. Some other studies utilizing spray humidifiers involve experimental study of a two-stage multi-effect HDH desalination system [25], a solar-assisted heat pump HDH desalination system [26], and a comparative study of pad, and spray humidifiers under different operating conditions [27]. However, in spray humidifiers atomizers rapidly clog at high temperature, and high salinity, which deteriorates the performance of the system. Clogging further results in increased maintenance cost, and reduced operating time of the plant, increasing the overall cost of fresh water [28]. This limits the system to lower operating temperatures, and salinities. Accordingly, various anti-clogging atomizers were patented to address the clogging of atomizer nozzles over the years. Most rely upon mechanical means, such as moveable pins, sleeves, or scrapers to either seal the nozzle from the atmosphere to prevent drying of un-dispensed product, or to mechanically clear dried product from the nozzle orifice [29], [30], [31], [32], [33], [34]. Such devices require frequently moving additional parts which adds to the design complexity, manufacturing cost, and poor aesthetic qualities. Further, incorporation of complex designs or methods also makes it difficult to maintain precision in atomizers which directly governs the quality of spray. Accordingly, it is be desirable to provide an atomizer which would exhibit a reduced tendency to clog with high temperature and salinity, yet be economical to produce and reliable in service with reduced complexities to make spray humidifier a viable option for a ZLD-HDH desalination system. The purpose of this paper is to develop an atomizer for hypersaline water spray humidifier to be incorporated in a ZLD-HDH desalination system with following characteristics: 1) stable operation; 2) fine spray for high evaporation rates; 3) high temperatures and high salinity: above 100,000 ppm TDS with suppressed atomizer clogging; 4) simple design and manufacturing.

In this regards, the novelty of this paper is carrying out an experimental study on the design, and characterization of a novel perforated plate atomizer for high temperature, and salinity applications in a ZLD-HDH desalination system. Airblast atomization is selected among commercially used atomization processes (mechanical atomization, pressure atomization, and airblast atomization), as it provides compact design, comparatively easy fabrication, and remarkable mixing of air, and liquid during primary, and secondary breakup for higher evaporation rates. The perforated plate atomizer is designed to harness the high evaporation rates of an airblast atomizer capable to accommodate high salinities, high mass flow rates, and high temperatures of air, and saline water, while suppressing any clogging.

This work presents a detailed description of the perforated plate atomizer design, and its operation. It also assesses the effect of atomizer design, water mass flow rate, and air mass flux on spray characteristics (spray cone angle, and average droplet diameter). In this regard, seven atomizer designs are tested at different operating conditions. A minimum average droplet diameter of 240 μm is achieved under a test condition having water mass flow rate of 2 g/s, and air mass flux of 45 g/cm² s. Further, anti-clogging performance of the perforated plate atomizer is tested with hypersaline water (100,000 ppm TDS of NaCl), and elevated temperatures for 2 h. No clogging of the atomizer is observed by the end of these experiments. Finally, the paper concludes with a 13 h anti-clogging performance test with hypersaline water (100,000 ppm TDS of NaCl) at 45 °C, and air at 175 °C to study the perforated plate atomizer performance over a longer span of time: no clogging of the atomizer is observed. Results conclude that the perforated plate atomizer provides a way to provide near-complete suppression of atomizer clogging while dealing with high temperatures, and salinities. These features make it an ideal fit for high salinity, and ZLD-HDH desalination systems, such as the one developed by our research team.