Effect of NaCl on clathrate hydrate based desalination HyDesal process with fixed bed approach
Authors: Linga Praveen , Babu Ponnivalavan , Nambiar Abhishek 
Published: June 2019

Acute shortage of water has become one of the biggest problem facing many countries in the world. Safe and drinkable water is a basic necessity for survival. Hence, water resource management has become need of the hour. Since, seawater is the most abundant source of water available on the earth, desalination is one of the most promising technology to produce potable water. Desalination is the process of removal of salts and other impurities from seawater and brackish water. Thermal distillation and Reverse Osmosis (RO) are two of the most widely employed desalination technologies in the World. Thermal desalination uses energy inefficiently and also suffers from corrosion problems. RO on the other hand is used in more than 50% of the desalination plants in the World. In a typical RO desalination plant, water recovery from seawater is less than 55%. However, RO suffers from fouling problems which increase maintenance cost. Since, these technologies are energy intensive there is a need to address water-energy nexus by employing innovative energy efficient solutions. Clathrate hydrate based process which is based on freeze desalination approach has been proposed as a potential desalination technology to alleviate shortage of potable water in many parts of the World. Clathrate hydrate are non-stoichiometric crystalline solid compounds made up of water and gas. Similar to freeze desalination, clathrate hydrate excludes salts and other impurities present in seawater and brackish water from crystals. After separating formed hydrate crystals from left over brine, there is potential to get pure water and gas by decomposing hydrate crystals. Gas released during decomposition of hydrate can be recycled. In general, salts present in seawater act as thermodynamic inhibitor for gas hydrate formation. One of the major challenges faced by freeze desalination is ice crystal contamination due to crystal contact with left over brine. Similarly, hydrate contamination by left-over brine has prevented successful commercialization of HyDesal technology. Hence, efficient separation of hydrate crystals from brine and faster kinetics of hydrate formation play a major role in clathrate hydrate based desalination process. In order to strengthen energy-water nexus, waste cold energy from Liquefied Natural Gas (LNG) regasification terminals is being incorporated to mitigate energy requirement of this process. Previously, it has been reported that propane has shown ability to draw water out of fixed bed to form hydrates away from unreacted brine solution. Hence, there is a need to identify suitable co-guest gas with propane to maximize water recovery from HyDesal process. In the present work, an innovative approach has been employed by using Propane/ Nitrogen gas mixture in fixed bed made up of silica sand. Fixed bed approach in presence of propane enables natural separation of hydrate from left over brine which has been reported in an earlier work, which helps in minimizing hydrate contamination by salts present in left over brine. Faster kinetics has also been reported in fixed bed approach. In this work, water recovery from 1.5 wt% NaCl solution and 3 wt% NaCl solution has been studied and comparison has been made with kinetics of hydrate formation in deionized pure water. In this study, salt was found to act as kinetic inhibitor for hydrate formation.