Session: 11-42-01: Heat and Mass Transfer in Heating, Cooling, and Power Systems

Paper Number: 113754

113754 - Performance of a Forward Feed Multi-Effect (MED-FF) Thermal Desalination System With Feed Preheating 

Extended Abstract

Water scarcity is one of the major concerns facing mankind due to limited fresh water resources and continuous increase in population, industrialization and urbanization. The demand on water rises steadily such that more than 75 countries are facing water scarcity. United Nations sources indicate that by 2025, about 1.8 billion people will suffer from water shortage. This number is expected to increase month after another despite the fact that 70% of the earth is covered with water, out of which 2.5 is fresh water. Only a limited fraction of this freshwater is available to consume directly, the rest is hidden in Glaciers, swamps or subsurface moisture. The direct solution to this problem is through industrial water desalination of seawater and brackish water.

Several systems are used for desalinating salty water in more than 20000 desalination plants worldwide with almost half of this capacity is produces in Middle East and North Africa (MENA region). Large scale Desalination technologies are utilized since 1950’s. Desalination technology can be classified into thermal desalination systems such as MSF, MED, HDH and mechanical desalination systems like RO and FO systems. There are other emerging systems that do not produce large quantities of fresh water. Thermal desalination systems were dominating earlier, then significant improvements in RO systems resulted in making it the most utilized technology. Each category has its pros and cons. Energy consumption is the major key factor through which preference is made for selecting the appropriate technology.

Accordingly, enhancements in energy consumption (reduction) are investigated continuously to desalinate water effectively and economically. Multi effect desalination systems (MED) are typical thermal systems that can produce high purity water in large quantities. They are powered by steam extracted from a power plant. The current study includes a mathematical model developed to reduce energy consumption and accordingly enhance the performance of MED-FF system through energy recovery from hot rejected brine.

The layout of the system includes feed seawater that flows to the first affect after passing by a single of multiple heaters where the feed temperature increases through heat transfer from the rejected brine. The sprayed feed would be sensibly heated in the first effect to saturation temperature before part of it evaporated to form water vapor that can condense in the tubes of the next effect into fresh water releasing its latent heat that is used to heat and evaporate feed of the next effect. The process is repeated from one effect to the following one that operates at lower pressure and temperature.

Typically feed reheating was achieved through the use of some of the vapor [1] (or all of it [2]) leaving the effect and condensing in the preheater to transfer heat the feed. This results in energy loss of steam that could otherwise condense in the tube bundle of the following effect to generate more vapor. Instead, part of the rejected brine flows in to the feed preheater to heat the feed. The performance of the MED-FF plant with no feed preheater is compared with that of feed preheater with a portion of the vapor generated within the effect. Both are compared with the proposed layout with single or multiple preheaters. Performance has significantly improved through the energy recovery reflected in the performance ratio. The effect on the total heat transfer surface area and specific flow rate of cooling water is also taken into consideration.

 

Presenting Author: Mohamed Antar King Fahd University of Petroleum and Minerals

Presenting Author Biography: Dr. Antar is a Professor in the Mechanical Engineering Department at King Fahd University of Petroleum and Minerals. His research interests include heat transfer and thermodynamic analysis applied to thermal desalination systems, membrane based desalination systems. He has developed related desalination based courses in addition to a long list of publications and patents in this area.

Authors:

Azeez Qudah KFUPM
Abdulsalam Hasan KFUPM
Mohamed A. Antar KFUPM