Renewable Energy Global Innovations features: Thermodynamic analysis of siphon flash evaporation desalination system using ocean thermal energy

Significance Statement

Ocean thermal energy can be described as the thermal potential energy produced by the temperature difference between the warm surface and the cold deep seawaters. Reference to the large ocean area, ocean thermal energy reserves are huge. Ocean thermal energy is considered green in the sense that its production is without pollution. Nevertheless, this renewable energy sources suffers from low temperature difference between the deep seawater and the surface, which is generally in the range of 15-25K. The source also suffers the weakness of low specific heat that is approximately 4J/(gK), while seawater heat of vaporization is approximately 2400J/g.

To enhance the efficiency of the ocean thermal energy, it has been found profound to use the ocean thermal energy for seawater desalination directly. Using it directly can help skip the numerous conversion steps for converting ocean thermal energy to electricity and then converting the resulting electrical energy to chemical energy. Considering the scarcity of fresh water, it becomes paramount to produce fresh water using ocean thermal energy. However, in a previous system, energy consumption on seawater transportation was observed to be very high and this led to poor economic tradeoff of the systems.

In addition, the effect of a number of parameters on the performance of this system was not assessed. Above all, placing the evaporator and the condenser into a single unit caused the system to be very large, and the system’s exergy efficiency should be determined. Therefore, Zhejiang University researchers Zhi-jiang Jin, Hao Ye, Jin-yuan Qian and in collaboration with Hao Wang at Air Liquide Hangzhou Co., Ltd. And Hao Li at Nuclear Power Institute of China explained the working principle of siphon flash evaporation desalination system and analyzed the exergy efficiency of the entire system. They created a simulation model in ASPEN PLUS and analyzed the effects of a number of factors on the functioning of the system through the model. Their work is published in Energy Conversion and Management.

The vapor produced in the flash evaporator is normally absorbed into the condenser chamber reference to the pressure difference between the condenser and the evaporator. The vapor is condensed into freshwater by the cold deep ocean water. Owing to a particular degree of vacuum difference between the evaporator and the condenser, then the vapor can be absorbed into the condenser continuously.

However, the initial vacuum degree of the condenser shell side must be the same as the evaporator. There are two main functions of the ocean thermal energy; one is to generate and maintain the vacuum difference between the flash evaporator and the condenser. This will ensure that the surface water is vaporized continuously and absorbed into the condenser without extra energy consumption. The second function is that the cold deep seawater is used as a condensing agent for condensing the vaporized water into fresh water.

Under design conditions, the authors realized that the exergy efficiency of the entire system turned out very well at 7.81%. This value was higher than the typical utilization of the ocean thermal energy. The exergy efficiency of the flash evaporator was observed to reduce with a rise in the surface seawater temperature, but the condenser efficiency remained unchanged.

The flow rate of the deep seawater decreased with a rise of temperature change of the deep seawater. However, the flow rate of the surface water decreased with the increase in change in temperature of the surface water. The surface water flow rate also influenced the pressure difference between the condenser and the evaporator. Non-condensable gases in the water might have caused this. Therefore, taking into account the influence of non-condensable gases in the actual production is paramount.

About The Author

Zhi-jiang Jin, Ph.D., Professor

Institute of Process Equipment, Zhejiang University, China

Prof. Jin is the Deputy Director of Institute of Process Equipment, Zhejiang University, the Deputy Director of Energy Assessment Center, Zhejiang University. He is also a member of Pressure Vessel Branch Pipeline Committee, China Mechanical Engineering Society, and the Technical Committee of Chinese Safety and Pressure Relief Device Standardization. His research areas are focus on high efficient process equipment design and pressure pipeline safety technology.

About The Author

Jin-yuan Qian, Ph.D

Department of Energy Sciences, Lund University, Sweden

Dr. Qian received the B.Sc. and Ph.D. degrees both in Chemical Process Equipment from Zhejiang University, China in 2011 and 2016, respectively. He was a joint Ph.D. student at TU Bergakademie Freiberg, Germany, from 2013~2014. Currently, he is a postdoc fellow at Department of Energy Sciences, Lund University, Sweden. His research interests include Thermofluids, Micro/Nano Heat Transfer, Flow Control, Hydraulics, Computational Fluid Dynamics et al.

Reference

Zhi-jiang Jin, Hao Ye, Hao Wang, Hao Li, Jin-yuan Qian. Thermodynamic analysis of siphon flash evaporation desalination system using ocean thermal energy. Energy Conversion and Management, volume 136 (2017), pages 66–77.

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Renewable Energy Global Innovations features: Catalytic adsorptive desulfurization of model diesel fuel using TiO2/SBA-15 under mild conditions

Significance Statement

Removing trace amount of refractory sulfur compounds from diesel under mild conditions has been a great challenge in refinery. Recently, Xiao’s lab from South China University of Technology proposed an effective and economic desulfurization approach (catalytic adsorptive desulfurization, CADS) for ultra-clean fuel production under mild conditions.

A coupling oxidation-adsorption process was developed for selective desulfurization. Plausibly, such CADS approach shows superior desulfurization uptake at low sulfur concentration range, fast adsorption kinetics, excellent regenrability, operation at mild conditions, and facile and low-cost adsorbent synthesis. CADS may provide a promising path for ultra-deep desulfurization to achieve ultra-clean diesel. For more details, read X. Ren et al. / Fuel 174(2016) 118-125. 

About The Author

Prof. J. Xiao received her BS in applied chemistry and MS in Chemical Engineering from South China University of Technology, and her PhD majoring in fuel science from the Pennsylvania State University in 2012. Then, she joined the School of Chemistry and Chemical Engineering at South China University of Technology. Her research interests include adsorption for clean energy and environmental pollution control, i.e. fuel desulfurization and denitrogenation and CO₂ capture, new adsorbent materials, and environmental catalysis, etc. 

Journal Reference

Fuel, Volume 174,  2016, Pages 118–125.

Xiaoling Ren, Guang Miao, Zhiyong Xiao, Feiyan Ye, Zhong Li, Haihui Wang, Jing Xiao^, 

School of Chemistry and Chemical Engineering and Key Laboratory of Enhanced Heat Transfer and Energy Conservation, South China University of Technology, Guangzhou 510640, China.

Abstract

This study investigates catalytic adsorptive desulfurization (CADS) of model diesel fuel using TiO₂/SBA-15 under mild conditions. The TiO₂/SBA-15 was prepared by a facile incipient wetness impregnation method and characterized by N₂ adsorption and X-ray diffraction. The CADS referred to ADS performance were evaluated in a batch reactor.

High desulfurization uptake of 12.7 mg/g was achieved at low sulfur concentration of 15 ppmw-S by TiO₂/SBA-15 under CADS, which was two-magnitude higher than that under ADS without the in-situ catalytic oxidation of dibenzothiophene. Kinetic results suggested that the CADS equilibrium over TiO₂/SBA-15 was reached fast in 0.5 h. In the CADS–TiO₂/SBA-15 system, the TiO₂ loading, cumene hydroperoxide/dibenzothiophene ratio and CADS temperature were optimized to be 10 wt%, 2, and 35 °C, respectively.

Furthermore, desulfurization tests in 5 consecutive CADS-regeneration cycles suggested that the bi-functional TiO₂/SBA-15 can be regenerated by acetonitrile washing followed with oxidative air treatment. The CADS–TiO₂/SBA-15 mechanism went through the oxidation of DBT to oxidized DBTO₂ over TiO₂/SBA-15 by cumene hydroperoxide, which was followed by the adsorption of the oxidized DBTO₂ over TiO₂/SBA-15.

The superior desulfurization uptake at low sulfur concentration range, fast adsorption kinetics, excellent regenerability, operation at mild conditions, and facile and low-cost adsorbent synthesis make the CADS–TiO₂/SBA-15 system an effective and economic desulfurization approach for ultra-clean fuel production.

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