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An experimental investigation of salt-water separation in the vacuum flashing assisted with heat pipes and solid adsorption
Abstract
A novel single stage vacuum evaporator, as a salt-water separator, is developed to fully use ultra-low grade heat source as low as 50 °C. The novel design incorporates several technologies, including heat pipes (HPs), spray flashing and solid adsorption. Its performance is evaluated by separating water from NaCl solution with 3%concentration. The results of the experiment study show that HPs transfer heat to the droplets rapidly to assist a second evaporation and maintain the superheat degree for the salt-water separation process after the flash evaporation. It is also found that, when the adsorbent beds are applied, the evaporation pressure decreases by 0.4–1 kPa compared to the early experiment results, which results in an increase of the superheat degree. The introduction of solid adsorption in the salt-water separator provides a new way to utilize ultra-low grade heat source, although it brings a mixed success to the whole device while the separation ratio is slightly lower than expected due to the fact that there are both improved and impaired effects of heat transfer during the whole adsorption process. The lower cooling water temperature of the adsorption process, leads to better desorption effect, which improves the overall performance of the salt-water separator.
... Gao et al. [122] investigated an innovative single-stage vacuum evaporator to extract saltwater. The system was settled to utilize an ultra-low-grade heat source of 50 °C. ...
... This wide range illustrates the significant effect of ADS system design and operating conditions. Therefore, the next section summarizes Fig. 9 Schematic of the used experimental facility [122] the heat exchanger configurations and their effect on ADS performance. ...
Energy, freshwater, and the environment are interrelated features that infuse all human activities. Addressing this nexus in an integrated energy conversion system is a big challenge for the research community. Adsorption desalination system, which is a good alternative to traditional desalination systems, could solve this problem because it uses eco-friendly working fluids and can be powered by renewable energy. Many experimental prototypes for the adsorption desalination cycle were built and tested in the last decades. Also, different adsorbent materials were developed and characterized. Therefore, this paper reviews adsorbent materials with water vapor utilized in experimental adsorption desalination studies, which is considered the first step in constructing an efficient system. After that, the paper comprehensively reviews all previous experimental adsorption desalination studies. It focuses on the design of the experimental test rig, the mass of adsorbent material, and system performance, such as the specific daily water production, coefficient of performance, and specific cooling power. This work also discusses the properties of heat exchangers (i.e., adsorbent beds) employed in adsorption desalination systems.
... The schematic layout of the proposed LNG-assisted multi-generation system is illustrated in Fig. 1. The system comprises a topping singlestage ammonia-water APC sub-cycle driven by ultra low-grade heat (≤100°C [39,40]) representative of solar collector, geothermal heat, or industrial/power generation waste heat, and a bottoming LNG subcycle. In this instance, the system is assumed to be thermally activated by steam condensate having a temperature range of 70-100°C. ...
An ultra low-grade waste heat (≤100 °C) driven ammonia-water combined absorption power-cooling (APC) cycle-based multi-generation system with liquefied natural gas (LNG) cold exergy recovery is thermodynamically and exergoeconomically investigated for sustainable district cooling (DC) at two different temperature levels, and electricity generation in coastal hot climate regions serviced by LNG regasification terminals. At 70 °C heat source temperature, the system yields a specific net equivalent power output of 51.4 kWh/tonLNG at a cooling-to-power ratio of 7.3, and effective first-law and effective exergy efficiencies of 39% and 36%, respectively. Per million ton per annum (MTPA) LNG regasification capacity, the system would deliver 9.4 MW of space cooling, 6.5 MW of sub-zero refrigeration, and 2.2 MW of electricity for the district, while saving 8.4 ktons of natural gas and 19.3 kt of CO2-equivalent emissions per year, compared with conventional cooling, power and regasified LNG provision systems. The LNG refrigeration heat exchanger and APC absorber-evaporator are the most important components from exergoeconomic viewpoints. In addition to enhancing overall cooling and power generation capacity, the use of LNG as a cold source/sink reduces the minimum APC driving heat source temperature and rectification requirements, relative to an ambient water-cooled absorber sink. Additional benefits include the avoided use of conventional refrigerants, and reduced impact of LNG regasification on marine ecosystems.
Multi-generation concepts combining low-to-medium grade environmental or waste heat utilization, and waste cryogenic cold recovery, are investigated for (i) direct and indirect district cooling (DC) using a direct refrigeration unit and open Rankine cycle-driven compression chiller, respectively, (ii) district electricity generation using closed organic Rankine- or Brayton-based cycle configurations, and (iii) LNG regasification for local gas distribution. The Rankine-based system can yield equivalent energy savings of 41 kWh/tonLNG at effective first-law and effective exergy efficiencies of 24 % and 26.2 %, respectively, and the Brayton-based system, 135 kWh/tonLNG, at 43.1 % and 45.5 % efficiencies, respectively. Per MTPALNG regasification capacity, the Rankine- and Brayton-based systems could deliver 12.4 and 22.1 MW of DC, respectively, while generating 2.4 and 9.4 MW of net electrical power for the district, with 6.4 and 19.5 ktons of natural gas saved annually, respectively, and avoiding 16.9 and 51.5 kt of CO2-equivalent emissions annually, respectively. At a low-bound electricity tariff, corresponding net annual economic benefits of 5.3 and 8.3 million USD could be obtained for the Rankine- and Brayton-based multi-generation systems, with corresponding payback times of 2.6 and 3.9 years, respectively. Further environmental benefits would be achieved through reduced use of conventional refrigerants, and reduced impact on marine ecosystems.
Spray flash evaporation is an effective desalination method, which increases specific surface area of salty water by liquid atomization, thereby improving desalination performance and maximising low-grade heat source utilization. During evaporation, explosive boiling phenomenon occurs inside superheated droplets on a heated surface. In order to understand the mechanism of explosive boiling, spray flash evaporation of distilled water and 3.5 wt% salty water in a high vacuum vessel was observed visually. Meanwhile, a parametric study was carried out to scrutinize the impacts of the variation of ambient pressure, heat flux, and surface superheat degree. The experiment data indicates that nucleate site is located in the upper layer of a droplet due to internal superheated liquid and Marangoni convection. In different operating conditions, bubble fragmentation process or crown fragmentation process happens at nucleate site. The fragmentation time of pure water, which is mainly influenced by heat flux and surface superheat degree, shrinks with higher heat flux and higher surface superheat degree. The fragmentation time of 3.5 wt% salty water decreases with ambient pressure drops and superheat degree increments.
Radioactive fluoride wastes are generated during the operation of molten salt reactors (MSRs) and reprocessing of their spent fuel. Immobilization of these wastes in borosilicate glass is not feasible because of the very low solubility of fluorides in this host. Alternative candidates are thus an active topic of research including phosphate-based glasses, crystalline ceramics, and hybrid glass–ceramic systems. In this study, mixed fluorides were employed as simulated MSRs waste and incorporated into sodium aluminophosphate glass to obtain phosphate-based waste form. These waste forms were characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Leaching tests were performed in deionized water using the product consistency test A method. This study demonstrates that up to 20 mol% of simulated radioactive waste can be introduced into the NaAlP glass matrix, and the chemical durability is much better than that of borosilicate. The addition of Fe2O3 in the NaAlP glass matrix results in increases of the chemical durability at the expense of fluoride loading (to 6.4 mol%). Phosphate glass vitrification of radioactive waste containing fluorides is a potential method to treat and dispose of MSR wastes.
The applicability of the collocation spectral method (CSM) for solving nonlinear heat transfer problems is demonstrated in a convective-radiative fin with temperature-dependent properties. In this method, the fin temperature distribution is approximated by Lagrange interpolation polynomials at spectral collocation points. The differential form of the energy equation is transformed to a matrix form of algebraic equations. The computational convergence of the CSM approximately follows an exponential decaying law; and thus, it is a very simple and effective approach for a rapid assessment of nonlinear physical problems. The effects of temperature-dependent properties such as thermal conductivity, surface emissivity, heat transfer coefficient, convection-conduction parameter, and radiation-conduction parameter on the fin temperature distribution and efficiency are discussed. 2016
Based on the high heat transfer performance of heat pipe (HP) and large specific surface area of small diameter droplets, HP and spray technologies are combined in a novel single stage vacuum evaporator and artificial seawater with 3% saltness is studied experimentally. Parametric studies are carried out, such as cold source and heat source temperature, spray temperature, and spray flow. The results show that: the maximum heat flux density reaches 32 W cm− 2 on evaporator bland plate; HPs absorb energy effectively from low grade heat source (40–80 °C) then transfer the energy to the droplets already flashed, so as to maintain or even increase the superheat degree of droplets during evaporating process; This method can significantly improve the water separation rate (quality of condensed water / quality of dilute solution ∗ 100%); Changing the parameters can regulate water separation from 0% to 65%. Thus it has important significance for utilizing lower grade heat source effectively, gaining brine, and promoting the fresh water yield per unit volume and adaptability to variable load in desalination project.
Key management is one of the important issues for the ad hoc networks security. According to the characteristics of ad hoc networks, a threshold key management scheme without secure channel was proposed. In the proposed scheme, the role of the traditional certification authority (CA) is completed by a local register center and n distributed key generation centers, which can avoid the single point of failure. By using threshold cryptography, the ad hoc interior members collaborate to conduct the distributed generation the system private key. The mutual authentication between the user and distributed key generation centers was provided through the public cryptosystem based on the bilinear pairings. The user's private key is signed blindly to ensure that the private keys can be delivered securely in the public channel and cannot be eavesdropped. The analysis results show that the proposed scheme achieves the trust level III, has fault tolerance and is secure against the active and passive attacks. It is concluded that the proposed scheme satisfies the security requirements of ad hoc networks and reduces largely the cost of computation and storage.
This paper studies the water equilibrium uptake on two lype of adsorbents, namely silica gel RD powder and Zeolite FAM Z01. A novel adsorption isotherm model is proposed to regress the experimental data of the both adsorbents. For the silica gel-water pair, die proposed model generates more precise data regression than the classic DA and Tóth equations. In the case of the zeolite-water pair, the new model well fits its unique S-shape isotherms at various adsorption temperatures. The linear behavior at both low and high relative pressure regions, and the sharp rise in between the two regions arc accurately captured. The model produces R²value 0.997 and NRMSE error 3.15% for the former pair, and 0.997 R²and 6.05% error for the zeolite-water pair in the experiment range.
The rapid growth in technology nowadays has resulted in the densification of electronic components, which requires a highly reliable thermal management system. The heat pipes are the most promising heat transfer devices capable of transmitting high heat fluxes rejected by the modern electronic equipment. In the present study, the experimental analysis of thermal performance of a sintered-wick heat pipe is carried out at various inclinations with gravity assisted tilt. The results have shown that the performance of heat pipe varies with the geometrical orientation of heat pipe and it is observed the heat carrying capacity of heat pipe is well pronounced at 60⁰ gravity assisted tilt followed by 90⁰.
The performance of adsorption (AD) chillers or desalination cycles is dictated by the rates of heat and mass transfer of adsorbate in adsorbent-packed beds. Conventional granular-adsorbent, packed in fin-tube heat exchangers, suffered from poor heat transfer in heating (desorption) or cooling (adsorption) processes of the batch-operated cycles, with undesirable performance parameters such as higher footprint of plants, low coefficient of performance (COP) of AD cycles and higher capital cost of the machines. The motivation of present work is to mitigate the heat and mass "bottlenecks" of fin-tube heat exchangers by using a powdered-adsorbent cum binder coated onto the fin surfaces of exchangers. Suitable adsorbent-binder pairs have been identified for the silica gel adsorbent with pore surface areas up to 680 m2/g and pore diameters less than 6 nm. The parent silica gel remains largely unaffected despite being pulverized into fine particles of 100 μm, and yet maintaining its water uptake characteristics. The paper presents an experimental study on the selection and testing processes to achieve high efficacy of adsorbent-binder coated exchangers. The test results indicate 3.4-4.6 folds improvement in heat transfer rates over the conventional granular-packed method, resulting a faster rate of water uptake by 1.5-2 times on the suitable silica gel type.
The impact of the characteristics of the non-isothermal flashing droplet (on micron and millisecond scale), including the internal temperature gradient and heat convection, on vacuum flash-spray cooling was physically modeled, theoretically analyzed on the basis of the heat conduction equation in radial direction and diffusion-controlled evaporation, numerically simulated and experimentally measured. The dependence of the droplet temperature on the flash-spray conditions, such as the pressure, velocity and initial diameter of droplets, was evaluated. The simulated results show that the newly-modified non-isothermal model predicts the droplet's temperature more accurately than the conventional isothermal model. The surface evaporation of the droplet sharply decreases its own temperature but little affects its volume. Besides, we found that a faster flash evaporation originates from a lower pressure, a smaller size and a higher initial velocity of the droplet. The simulated results agree fairly well with the measured data.
The energy, water and environment nexus is a crucial factor when considering the future development of desalination plants or industry in the water-stressed economies. New generation of desalination processes or plants has to meet the stringent environment discharge requirements and yet the industry remains highly energy efficient and sustainable when producing good potable water. Water sources, either brackish or seawater, have become more contaminated as feed while the demand for desalination capacities increase around the world. One immediate solution for energy efficiency improvement comes from the hybridization of the proven desalination processes to the newer processes of desalination: For example, the integration of the available thermally-driven to adsorption desalination (AD) cycles where significant thermodynamic synergy can be attained when cycles are combined. For these hybrid cycles, a quantum improvement in energy efficiency as well as in increase in water production can be expected. The advent of MED with AD cycles, or simply called the MEDAD cycles, is one such example where seawater desalination can be pursued and operated in cogeneration with the electricity production plants: The hybrid desalination cycles utilize only the low exergy bled-steam at low temperatures, complemented with waste exhaust or renewable solar thermal heat at temperatures between 60 and 80 T. In this paper, the authors have reported their pioneered research on aspects of AD and related hybrid MEDAD cycles, both at theoretical models and experimental pilots. Using the cogeneration of electricity and desalination concept, the authors examined the cost apportionment of fuel cost by the quality or exergy of working steam for such cogeneration configurations.
Applying the process of superheated atomization (flash atomization) finely dispersed sprays with moderate droplet velocities can be obtained, without the necessity of high pressure or an additional gas phase. In this paper occurring evaporation phenomena inside a nozzle capillary and their impact on the generated spray are analyzed. Evaporation is enhanced for increasing superheating. An intensified evaporation induces a considerably extended spray and smaller droplet sizes are generated. Furthermore a reduced mass flux and decreased velocity of the droplets are to be expected. By increasing the amount of vapor spray pulsation, triggered by bubble bursting, is increased. This is shown in acoustic measurements, which also demonstrate a simple way to monitor, or even control the atomization of superheated liquids.
A new method of regenerating dehumidification solution is developed, which releases the dilute solution into vacuum to improve water evaporation instead of the traditional methods. Experiments on the temperature distribution in the lithium chloride solution droplets were carried out on the basis of the theoretical analysis. The experimental results indicate that both the surface temperature and center temperature declined sharply in the beginning stage of flash evaporation, then recover slowly, even though sometimes one is higher than the other in the whole process; pressure is the core factor of evaporation rate, the lower the pressure, the more intense flash; droplets with higher initial temperature only strengthen the intensity of the flash evaporation at the beginning. Radiant heat can significantly promote the strength of the flash evaporation which cannot be neglected especially for single droplet. The contrast between the experimental results and the numerical results prove that droplet flash evaporation model can be applied to the process without radiation, and the numerical results should be amended when used on the radiation condition.
Flash evaporation, as a new method to regenerate dehumidification solution, is investigated in this paper. Droplets of lithium chloride and/or calcium chloride solution were suspended on the junction of a thermocouple, whose characteristic change was captured by high-speed camera, infrared camera, and thermocouple, and analyzed by comparing the values of different conditions. The results indicate that the pressure determines the degree of superheat, and the absorbed heat affects the intensity of the evaporation. A stronger surface tension caused by an increase in concentration or in change of solute weakens the intensity of water evaporation; a higher initial droplet temperature improves the evaporation intensity in the beginning stage, having no influence on its final temperature. The smaller the droplet diameter, the lower the minimum temperature is; radiant heat plays a decisive role in the process of flash evaporation, which is almost the only heat source provided to the droplet for evaporation in the stable stage.
Desalination, other than the natural water cycle, is hailed as the panacea to alleviate the problems of fresh water shortage in many water stressed countries. However, the main drawback of conventional desalination methods is that they are energy intensive. In many instances, they consumed electricity, chemicals for pre- and post-treatment of water. For each kWh of energy consumed, there is an unavoidable emission of Carbon Dioxide (CO2) at the power stations as well as the discharge of chemically-laden brine into the environment. Thus, there is a motivation to find new direction or methods of desalination that consumed less chemicals, thermal energy and electricity.This paper describes an emerging and yet low cost method of desalination that employs only low-temperature waste heat, which is available in abundance from either the renewable energy sources or exhaust of industrial processes. With only one heat input, the Adsorption Desalination (AD) cycle produces two useful effects, i.e., high grade potable water and cooling. In this article, a brief literature review, the theoretical framework for adsorption thermodynamics, a lumped-parameter model and the experimental tests for a wide range of operational conditions on the basic and the hybrid AD cycles are discussed. Predictions from the model are validated with measured performances from two pilot plants, i.e., a basic AD and the advanced AD cycles. The energetic efficiency of AD cycles has been compared against the conventional desalination methods. Owing to the unique features of AD cycle, i.e., the simultaneous production of dual useful effects, it is proposed that the life cycle cost (LCC) of AD is evaluated against the LCC of combined machines that are needed to deliver the same quantities of useful effects using a unified unit of $/MWh. In closing, an ideal desalination system with zero emission of CO2 is presented where geo-thermal heat is employed for powering a temperature-cascaded cogeneration plant.
This paper reports a study on the effect of the condenser and evaporator resurfacing on overall performance of a 1m height closed two-phase thermosyphon. Water was used as working fluid with a fill ratio and operating pressure was 0.75 and 160mbar, respectively. The thermosyphon performances for plain and modified thermosyphon were studied at 44 power inputs from 43W to 668W. The results show that by making the evaporator more hydrophilic and the condenser more hydrophobic, it will be possible to increase the average thermal performance by15.27% and decrease the thermal resistance by 2.35 times compared with the plain one.
To complement the authors' previous experimental study of spray flash evaporation pursued at 60°C jet inlet temperature, an additional experimental study has been made of the effect of liquid temperature on the spray flash evaporation by changing jet inlet temperatures to 40 and 80°C.From the experimental results, a more general empirical equation suitable for predicting the variation of liquid temperature in the center of jet with residence time was deduced, and the recommendable operating conditions were proposed. Furthermore, an expedient method for prediction of the liquid temperature drop along the length of jet was devised for the recommendable operating conditions.It was realized that, even at lower liquid temperatures, the spray flash evaporation still has higher evaporation performance and extremely faster evaporation rate than the flash evaporation occurring in other systems.
An experimental study has been made of a spray flash evaporation occurring in a superheated water jet injected through a circular nozzle into a low-pressure vapour zone. The effects of nozzle diameter, flow rate and superheat on the rate of spray flash evaporation are shown. From the experimental results an empirical equation useful for predicting the variation of liquid temperature in the center of jet cross section with residence time is deduced. The rate of flash evaporation of water jet, that is, spray flash evaporation is considerably faster than that of a flowing liquid in a conventional multistage flash evaporator and that of pool water exposed to a sudden pressure drop in a container. (A)
A promising method of desalination suitable for low-populated islands and remote areas is experimentally investigated at a small desalination plant capable of producing 15.2 tons of fresh water per day based on the flash evaporation from superheated water jets. In this method, water at a temperature ranging from 24 to 40 °C is brought to superheat condition through the injection into a depressurized chamber maintained below the boiling pressure that corresponds to the injected water temperature. A part of the injected water quickly evaporates in order to regain equilibrium at the reduced pressure, and the generated steam is condensed on a surface condenser supplied with the cold water. Even at very low degrees of superheat, flash evaporation from the superheated jets has shown high efficiency in converting the latent heat of the liquid water into vapor within the jet residence time which does not exceed a fraction of a second. An exponentially decaying curve model that simulates the temperature decline curves at the centerline of the upward flowing jet is developed and found matching the experimental data considerably. Influence of flow conditions on the characteristics of the flashing jet and on the intensity of the flash evaporation is also clarified.
Experimental study on static flash evaporation of aqueous NaCl solution
- Zhang
Recent developments in thermally-driven seawater desalination: energy efficiency improvement by hybridization of the MED and AD cycles
- Ng
Current research status, difficulties and new strategy in utilization of low grade heat
- Li