Brine concentration with cooling tower evaporation for a zero liquid discharge system. TRNSYS modeling and experimental results

Resumen

The treatment for elimination and/or storage of the rejection flow, brine, from the inverse osmosis desalination requires large space and high cost. This elimination process through natural evaporation is not practical neither technically feasible. In addition , most of international policies and initiatives provide a sustainable economy and energy framework, promoting solutions based on minimal (or zero) liquid discharge specially for environmental protected areas. The Mar Menor saltwater lagoon, located in SouthEast of Spain, is a clear example of such areas. Moreover, this lagoon has been on the brink of an ecological collapse since 2015, as a direct consequence of years of brine spills and drainages of agricultural origin. Under this scenario, this paper describes the TRNSYS©modeling and experimental results of a evaporation process of brine by means of a cooling tower solution. This system is part of a renewable prototype system under the framework of the Life European Project Desirows. This project proposes an industrial solution for brackish denitri-fication and desalination purposes based on local renewable resources in a zero liquid discharge framework. This paper thus aims to propose theoretical operating points capable of predicting the cooling tower performance. The main component of the installation, and a biomass boiler performance are modeled as well, arranged to provide thermal energy to the brine and evaporation capacity. In order to estimate the cooling tower performance , a theoretical model is proposed and subsequently validated using the TRNSYS©software. The different components are validated, and different versions of the system-prototype are analyzed as well. The system is modeled under different operating point conditions to determine the energy consumption efficiency derived from the brine evaporation. The simulation is carried out for a year, and the operation of the installation is analysed in terms of critical parameters. Limitations are studied and the energy cost derived from its operation is calculated. On other hand, experimental results show the relevance and influence of environmental variables, the brine evaporation increases with low temperatures and low relative humidity. From the experimental results, the inlet temperature of brine to the system is the most critical variable, determined as reduction of energy cost per evaporation brine volume (kWh/m 3) of 100 kW/m 3 with an increment of 10 o C of inlet temperature of brine in the range of 20-30 o C. Extensive results and detailed system descriptions are also included in the paper.