مقاله انگلیسی رایگان در مورد تشکیل هیدرات گازی در آب نمک دارای NaCl و MgCl2 – الزویر 2019

Hydrate-based desalination (HBD) has been developed to obtain fresh water from seawater in an economic and environmental sustainable manner. As a low-pressure hydrate former for the HBD process, chlorodifluoromethane (R22) hydrates were formed in the pressure range of 1–6 bar under brine environments, aqueous NaCl and MgCl2 solutions. Synchrotron X-ray diffraction and Raman spectroscopy measurements revealed that the R22 hydrates formed with NaCl and MgCl2 show structure I and enclathration of R22 molecules into the large cages. These results also confirmed that the salt ions cannot be encaged in the gas hydrate framework, reflecting the ion exclusion behavior for HBD above the eutectic of water and hydrated salts. The formation kinetics of R22 clathrate hydrate in the presence of salts show that both the initial growth rate and pressure drop of the R22 hydrates heavily depend on the salinity of aqueous solutions. A new theoretical approach adopting the transient time-dependent apparent rate constant of hydrate formation with salts was proposed to predict the formation kinetics of R22 hydrates under brine environments, which was in good agreements with the experimental results. These results provide good information for separating ionic compounds from aqueous solutions by hydrate-based separation processes.

Introduction

Over the past several decades, demand for fresh water has continuously increased throughout the world as a result of sustained population growth and improved living standards [1–4]. At the same time, the availability of good quality water is central to sustainable development and further improvements in quality of life. Given the limited availability of fresh water resources, and unreliable water quality for industrial and human needs, desalination technologies involving processes to recover pure water from sea water, have attracted considerable attention [5–10]. Fundamentally, desalination technology removes salts from sea water to provide fresh water, and various approaches have been proposed over the last several decades to accomplish this, with a range of efficiencies, performance and cost. Gas hydrates are ice-like inclusion compounds comprised of a ‘host’ water-framework and small ‘guest’ molecules such as methane, ethane, carbon dioxide, etc. Hydrate structures are typically categorized based on differences in the size and shape of the hydrate cages. Huge amounts of gaseous molecules can be selectively stored in the hydrate cages [11–23]. For this reason, gas hydrates are currently considered one of the promising functional materials in the energy and environmental fields because of their possible applications for gas storage and transportation [24–27], carbon capture and sequestration [28–32], the selective separation of gas species [33–38], and desalination [39–41]. Among the various desalination approaches, hydrate-based desalination (HBD) technology is a promising method for recovering fresh water from sea water. The key mechanism of the HBD technology is a simple temperature- and pressure-dependent phase transition from liquid to solid. HBD technology is considered a novel method for reducing the energy costs of desalination, and increasing the level of freshwater recovery. In addition, HBD technology is also regarded as an environmental-friendly method because hydrates can be directly formed from seawater under low-temperature and high-pressure conditions [39–41]. However, achieving the appropriate high-pressure condition for gas hydrate formation is a major obstacle to improving hydrate formation efficiency, and accordingly, the development of an efficient way to form gas hydrates under more moderate conditions is one of the key objectives of current research