This paper presents a numerical model to analyze the thermal and fluid dynamic behavior of a mechanical vapor compression MVC desalination system. The MVC desalination is a method to obtain distilled water using the evaporation and condensation processes at the same time, both occur at low pressure (values lower than atmospheric pressure). This method requires a compression work to increase the saturation temperature of the vapor mass flow obtained in the evaporator, which is used to feed the co...
This paper presents a numerical model to analyze the thermal and fluid dynamic behavior of a mechanical vapor compression MVC desalination system. The MVC desalination is a method to obtain distilled water using the evaporation and condensation processes at the same time, both occur at low pressure (values lower than atmospheric pressure). This method requires a compression work to increase the saturation temperature of the vapor mass flow obtained in the evaporator, which is used to feed the condenser. Then, the compressed vapor is condensed, and its latent heat is transferred to the feed seawater.
The MVC desalination is used at low and medium scale in comparison with other techniques such as multistage flash desalination (MSF) or reverses osmosis (RO). The MVC desalination unit studied in this paper uses renewable energy to supply the electricity required by the different devices that make up the unit. The reason to use renewable energy is that the MVC desalination system has been though to work in remote places, where an electric grid is not available. Consequently, the system is obliged to work with a variable power supply. This variability should be well defined to avoid damage and establish secure partial working operation of the MVC desalination system. The transient and steady-state behaviors of the desalination system are evaluated taking into account the variability of the renewable energy sources, where both solar and wind energy are feasible.
The MVC desalination system is divided into three different subsystems, following the strategy proposed by Mazini (2014). The first subsystem is the evaporator and condenser, in which the evaporation and distillation processes are performed. The second is the vacuum and deaeration subsystem, where the low pressure is achieved, and non-condensed gas is stripped. The last subsystem is the mechanical compressor, which is modeled to know its energetic requirement in function of the desalination performance and the climatic conditions. The mathematical formulation of the evaporator/condenser and vacuum subsystem is based on mass, energy and salt balance conservation equations. Compressor model is based on the root blower laws, in which the volumetric flow, velocity, power and the displacement by revolution values are related.
The group of equations is solved by means of the in-house object-oriented tool called NEST, which is capable of linking and solving different elements that making up a system (Damle, et. al., 2011). The MVC desalination system presented in this paper has different components: an evaporator/condenser, a compressor, a deaerator, two heat exchangers and a group of pumps. Although in this numerical platform each component is an object, the whole system resolution is carried out iteratively for solving all its components and transferring the appropriated information between them.
The numerical model has been validated using experimental data obtained from the technical literature (Namine et al. 2014). After that, a virtual test has been carried out, in which the relation between the variability of the renewable energy sources and the capacity of the desalination system (distilled water production) has been evaluated.
Morales Ruiz, S., Castro, J., Rigola, J., Oliet, C., Oliva, A. Dynamic Performance of a Mechanical Vapor Compression (MVC) Desalination System. A: International Refrigeration and Air Conditioning Conference at Purdue. "17th International Refrigeration and Air Conditioning Conference at Purdue : 2018 conference proceedings". Purdue Mall, West Lafayette, Indiana: Purdue University, 2018, p. 1-9.