تجزیه و تحلیل ترمودینامیکی در مصرف انرژی نظری از نمک زدایی آب دریا

It's well known that there are various methods being used in seawater desalination, which lead to different energy consumptions. However, the theory of thermodynamics reveals that the theoretical energy consumption (TEC, equivalent to the minimum work Wmin) of seawater desalination is only related to the initial and final state. Thermodynamic analysis of the TEC can contribute to the energy-saving of seawater desalination. In this paper, a novel mathematical model is proposed to calculate the TEC of seawater desalination concerning the recovery ratio, by assuming seawater to be aqueous NaCl–MgCl2–MgSO4 solution rather than aqueous NaCl solution. The activity coefficients of salts and the osmotic coefficients of water are calculated by the Pitzer model. The effect of ionic strength, recovery ratio, the activity coefficients of salts and the osmotic coefficients of water, seawater substitution on the TEC are also discussed. It is shown that ionic strength, recovery ratio, activity coefficients of salts and osmotic coefficients of water, seawater substitution have their different effects.

Seawater desalination is an energy-consuming process. Therefore, the fundamental way of reducing its cost is to minimize the total energy consumption of seawater desalination process. This is one of the most challenging issues during the past decades of research and development on this topic [1] and [2]. The second law of thermodynamics, able to tell us a quantity of energy inefficiencies in each part of the process, is particularly useful in optimizing the design and operation of seawater desalination and promoting the plant performance [3]. The first step in such an analysis is the determination of the theoretical energy consumption (TEC). Dodge [4] considered all desalination techniques as a simple separation process and obtained a general minimum work for the separations. The minimum separation work given in the tables is calculated by extracting pure water from a 3.5% NaCl solution (mass fraction) at different recovery ratios. Gao [5] regarded seawater as an ideal solution of aqueous NaCl, and proposed a simplified model to calculate the TEC, whose results showed that the separation of 34,000 mg/L aqueous NaCl solution to obtain 500 mg/L potable and 136,000 mg/L concentrated seawater at 25 °C, the cost of which is 1.41 kW h t− 1. In the above studies, seawater is represented by aqueous NaCl solution. Cerci [6] considered the saline water as an ideal solution, obeying Rault's law. In his research, the solution was assumed as a dilute solution for the mole fraction of salt in the 3.5% NaCl solution (mass fraction) is about 0.011. The enthalpy of the solution was considered as the sum of the enthalpies of pure components in the solution. So the activity coefficients of salts and the osmotic coefficients of water were not taken into account. The other studies [7], [8], [9], [10], [11], [12], [13] and [14] using ideal mixture model of pure water and sodium chloride salt have the similar hypothesis.

This work proposed a novel method for calculating the TEC of seawater desalination based on the theory of thermodynamics. The results showed that the ionic strength, recovery ratio, the activity coefficients of salts, the osmotic coefficients of water and different seawater samples affect on the TEC. The TEC increases when recovery ratio or ionic strength of seawater sample becomes bigger. And the TEC almost increases nonlinearly with the increment of the ionic strength at the recovery ratio more than 40%, the reason is mainly attributed to the effect of the osmotic coefficients of water while not the activity coefficients of salts. However, the TEC increases linearly with the ionic strength at all recovery ratios without considering the activity coefficients of salts and the osmotic coefficients of water. Different deviations between the TEC of real seawater and that of aqueous NaCl solution reveal that the consideration of activity coefficients of salts and the osmotic coefficients of water is more important than seawater substitution at the ionic strength of the standard seawater, and at other ionic strengths the two factors are both important.