Energy & Fuels 2005, 19, 285-291
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Study of Nitric Acid Formation in Direct-Contact Heat Exchangers Used for Desalination Processes Murat Tekelioglu* University of Nevada, Reno College of Engineering, Department of Mechanical Engineering, Reno, Nevada 89557
Tahsin Engin and Hakan S. Soyhan Sakarya University, Engineering Faculty, Department of Mechanical Engineering, Esentepe Campus, Sakarya, Turkey Received January 13, 2004. Revised Manuscript Received August 2, 2004
In desalination processes, it is important to ensure that the pure water, which is the main product, is free from any acidic components. After the process of direct-contact evaporation of seawater, the formation of acids such as carbonic and nitric acid must be considered, for the health of consumers. This paper investigates the possible formation of nitric acid in a directcontact heat exchanger (DCHE), based on the chemical equilibrium composition of the combustion products when the fuel is propane or hydrogen. Association and dissociation reaction mechanisms of the main products are written for both fuels. A detailed comparison of the combustion products is given for these fuels. The results obtained from the analysis indicate that there is low risk of nitric acid formation as a product in the evaporator products when propane or hydrogen is used as fuel.
1. Introduction The amount of potable water is diminishing throughout the world as the world population increases. This reduction is also accelerated by climatic effects, such as global warming. In an attempt to address the problem of potable water shortage, the desalination of seawater provides an attractive alternative. Almost three-quarters of the Earth’s surface area is covered with water, and, if desalinated, the seawater can be an excellent source of pure water. Most applications of desalination processes use direct-contact heat transfer, as opposed to processes that involve the use of indirect-contact heat transfer, to evaporate seawater, where presence of acids can contaminate the pure water, which is the main product of the process. Direct-contact heat exchange (DCHE) processes between two immiscible fluids have shown many advantages, because of higher effective heat-transfer coefficients, an absence of surface scaling, and a relatively simple design that provides a cost-effective performance.1 These heat exchangers can be used for preheating and evaporating a working fluid, using thermal energy at low temperatures (such as solar and geothermal energy), for use in power plants.2,3 It has been concluded that when a DCHE is used as a preheater/ evaporator in a power plant coupled to a solar pond, the * Author to whom correspondence should be addressed. Telephone: (775) 784-6735. Fax: (775) 784-1701. E-mail address:
[email protected]. (1) Siqueiros, J.; Bonilla, O. An Experimental Study of a ThreePhase, Direct-Contact Heat Exchanger. Appl. Therm. Eng. 1999, 19, 477-493.
capital cost is reduced by 25%.4 Also, these devices have been suggested for application in crystallization,5,6 energy recovery from industrial waste,7 thermal storage,6,8 and heating air in a bubble column9 for which the temperature gradient between the dispersed and continuous phases did not differ much (for example,
