Conventional Thermal Processes

Abstract

Thermal desalination processes account for about 50% of the entire desalination market. The remaining market share is dominated by the reverse osmosis (RO) process. The main thermal desalination processes include multi-stage flash desalination (MSF), multiple-effect distillation (MED), and mechanical vapor compression (MVC). Other thermal desalination processes, e.g., solar stills, humidification dehumidification, freezing, etc., are only found on a pilot or experimental scale. Thermal desalination processes consume a larger amount of energy than RO; approximately the equivalent of 10–15 kWh/m3 for thermal processes versus 5 kWh/m3for RO. Irrespective of this, the reliability and massive field experience in thermal desalination keeps its production cost competitive compared to the RO process. Also, the large scale production capacity for a single MSF unit, approximately 75,000 m3/day, is sufficient to provide potable water for 300,000 inhabitants. An increase in production capacity for the MED system has been realised recently, with unit production capacities of up to 30,000 m3/day. This chapter covers various aspects of thermal desalination processes. It includes a review of design, operating, and performance parameters. The analysis for each process includes a brief review of some of the recent literature studies, process descriptions, process models, and an illustration of system design and performance analysis. The chapter is divided into two parts, the first is on evaporation processes, which includes MED and MVC, and the second is on flashing processes, which include MSF. Each section starts with a description and analysis of the individual stage, for either evaporation or flashing. This is to simplify the explanation of the main processes that takes place during evaporation or flashing. Each division gives a complete description of each desalination process, together with the main modelling equations. Performance charts are presented for each system and explained in terms of main design and operating conditions.