Natural gas market has seen a phenomenal growth in the past decade due to shale gas boom and also the fact that natural gas is the cleanest burning fossil fuel. Currently, natural gas is pre-dominantly transported in the form of Liquefied Natural Gas (LNG) from source (producing countries) to distant end users (receiving countries like Singapore, Korea, Japan etc). LNG is then converted back to natural gas by removing cold energy for the usage of natural gas for power generation (primary use in Singapore). LNG cold energy needs to be harvested; it is now wasted worldwide including Singapore. Sea water is presently used for re-gasifying LNG by heating it from -162 ˚C to 20 ˚C at LNG terminals. Developing innovative technologies to make use of the cold energy is a key area in research and development.
A team from the NUS Centre for Energy Research and Technology (CERT) under my leadership is developing a prototype to produce purified water from seawater by harnessing the cold energy of LNG via the gas hydrate based desalination (HBD) technology. This research project is funded in part under the Energy Innovation Research Programme (ERIP), which is administrated by the Energy Market Authority (EMA) and funded by the National Research Foundation (NRF) of Singapore and Industry Partner, Royal Dutch Shell.
So, what is gas hydrate? Gas hydrate is an ice-like compound that is made up cages made by water molecules with gas/liquid molecules (known as guests) trapped inside them at high pressure and low temperature conditions. Gas/liquid molecules such as carbon dioxide methane, propane, cyclopentane etc are some molecules that can get trapped inside the hydrate cages. However, salts and other impurities are naturally rejected during hydrate crystal formation. These crystals can be separated from left over brine (when seawater is used as feed) and dissociated to get back gas hydrate guest and purified water.
Although HBD was proposed several decades ago as a potential technology for desalination and progress has been made since then, successful commercialisation of this process has not happened due to major challenges - the slow kinetics of hydrate formation, crystal separation from concentrated brine solution and cold energy required for the process.
The NUS team is developing a new technique that employs fixed porous bed made up of sand and water. When propane is employed as one of the hydrate formers in a gas mixture, hydrate crystals form above the silica sand bed. This results in both enhanced kinetics and a natural separation of crystals from the concentrated residual brine solution. The schematic of the concept is illustrated in Figure 1.
High energy requirement is another pressing issue that needs to be addressed for industrial scale application of the HBD process. In order to solve this issue, the NUS team is the first to propose an innovative approach by integrating the HBD process with LNG cold energy of LNG re-gasification terminals. By using this cold energy for cooling the HBD process it would be possible to drastically reduce the energy requirement and make it economically feasible. In order to prove its economic and technical feasibility, NUS team is now working to build/demonstrate a prototype of the HBD technology. If successful this could be the beginning of a new chapter in innovative desalination technology and gas hydrate based applications.
Figure 1: Schematic of HBD concept [Chem Eng Sci (2014), 117, 342-351]
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