# A Review of Reactor Designs for Hydrogen Storage in Clathrate Hydrates
## 3 Conclusions  
Bearing all available designs in mind, it can be concluded that interdisciplinary approach will play a main role in developing gas hydrate in gas storage application. A broad knowledge of materials science, surface engineering, fluid dynamics and process design is required to address the challenges allied with large-scale hydrate formation. The most common and potentially fruitful collaboration is between chemists and chemical engineers where the knowledge on gas hydrate behavior, accelerated formation, and extended stability will meet the engineering aspects of reactor design, hydrate formation and storage. Future studies in this area can be categorised in one of the following main subjects:  
* _Laboratory work from micro to macro scales_: On a molecular and micro scale, it would be highly desirable to determine the influence of additives on H\({}_{2}\)-hydrate formation kinetics, in addition to thermodynamic properties, structure, composition, and cage occupancy of the resulting hydrate phase; the molecular-migration behaviour of the H \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reactor** & & **Features** \\ \hline \multirow{3}{*}{**Water dispersion**} & \multirow{3}{*}{+} & Forming small hydrate droplets = Fast hydrogen charge/discharge \\ \hline \multirow{3}{*}{Water-Spraying Reactor} & \multirow{3}{*}{+} & Fast formation rate \\  & & + & Suitable for continuous generation \\  & & \(-\) & Difficult heat dispersion \\ \hline \multirow{3}{*}{**Gas dispersion**} & \multirow{3}{*}{+} & Forming bulky hydrate = Slow hydrogen charge/discharge \\ \hline \multirow{3}{*}{Stirred-Tank Reactor} & \multirow{3}{*}{+} & Common in research reactors \\  & & + & Suitable for water treatment applications \\  & & + & Suitable for dewatering (solid/water separation) \\  & & \(-\) & Slow formation rate \\ \hline \multirow{3}{*}{Bubbling Reactor} & \multirow{3}{*}{+} & Gas injection from bottom \\  & & + & Fast formation rate \\ \hline \multirow{3}{*}{Water-in-Oil Emulsion} & \multirow{3}{*}{+} & Suitable for two component hydrates \\ \hline \multirow{3}{*}{Fixed-Bed Reactor} & \multirow{3}{*}{+} & Extra solid additive for hydrate \\  & & nucleation \\ \hline \multirow{3}{*}{Unidirectional Growth} & \multirow{3}{*}{+} & Suitable for desalination, \\  & & + & Suitable for heavily polluted wastewater \\ \cline{1-1}  & & \(-\) & Slow formation rate \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hydrate formation reactors, advantages and disadvantages. molecules in the hydrate phase requires detailed study. Also, the interaction between additives and the hydrate phase needs to be investigated. Various analytical methods, such as Raman spectroscopy, differential-scanning calorimetry, and X-ray diffraction need to be considered. On a medium to large scale, the efficiency of H\({}_{2}\)-hydrate formation with respect to H\({}_{2}\)-storage capacity, hydrate-formation kinetics, as well as phase equilibrium, require investigation using high-pressure autoclaves. This background experimental data could, and, indeed, should, be used for important mechanistic and parametric modelling, to increase our level of insight into fundamentals of hydrate formation in the context of scaled-up hydrate reactors. * _Molecular-dynamics simulation_: Based on experimental data, molecular-dynamics (MD) simulations need to be performed to ascertain and elucidate crystallisation mechanisms of hydrogen hydrates. Indeed, insights from experiments can be used to inform and parameterise force-fields for MD, so as to allow MD to replicate as closely as possible experimental findings. Based on such insights into molecular mechanisms, it will be possible to assess free-energy profiles for transitions to the hydrate state, as well as to restructuring of water in the hydration layers around promotors and additives. Conceptualization, M.R.G. and N.J.E. ; writing--original draft preparation, M.R.G., J.M.S. ; writing--review and editing, M.R.G., J.M.S. and N.J.E. ; visualization, M.R.G. ; supervision, N.J.E.