# A Review of Reactor Designs for Hydrogen Storage in Clathrate Hydrates
## 2 Hydrate-Formation Technologies
### Bubble Forming  
The bubble-tower reactor is a known reactors developed for gas/liquid reactions in industrial application and found to be suitable for gas hydrate formation. [PERSON] et al. [26] patented a microbubble technology for hydrate formation. This technology is developed based on the idea behind the ejector-type loop reactor (ELR). In ELR the kinetic energy of a high-velocity liquid jet is being used to the form microbubbles in the bulk fluid which will result to a full mixture of the gas and the liquid [27]. The advantages of using an ELR system over a stirred-tank reactor can be listed as [25]:  
1. An enhanced gas and liquid mixing characteristics and mass transfer. 2. The possibility of using an external heat exchanger for controlling the liquid's temperature to avoid the disadvantages of using internal cooling coil installations. 3. Gas circulation provides the optimum use of the gas and eliminate the need for any additional gas compression step. 4. Easier operation and less sealing problems at high pressure due to the absence of moving parts in the design. Figure 4a presents the ELR assisted hydrate-formation system. The details of the ejector are illustrated in Figure 4b. In the water-circulation system, the cooled-down liquid will be mixed with gas inside the ejector and introduced again to the bottom of the reactor through the diffuser. Limited heat transfer rates during the hydrate formation and difficulty in the separation of formed hydrate at the gas/liquid interface constitute the main problems with this type of design, which can result in an overall lowering of efficiency of hydrate formation. In the water-recirculation pathway, and during the heat-exchange process through the surface of smooth tube, the temperature near the inner wall was found to be lower than that in the centre of the tube. This will be followed by heterogeneous nucleation of hydrate on the surface of the internal wall. The adhesion of hydrate to the wall surface will decrease the heat-transfer efficiency and inhibit hydrate formation in the reactor. Figure 3: Standard stirred tank hydrate formation reactor. Therefore, [PERSON] et al. [28] proposed a new hydrate-preparation system under an endogenous force-field. In this design, instead of a smooth tube, a tube with an internal spiral groove was used (Figure 5) to generate a secondary flow and centrifugal force (endogenous force field) [29; 30]. This force-field and the existing density difference between water, gas and hydrate, will cause the removal of the formed hydrate attached to the wall. On the other hand, an enhanced degree of heat-transfer of the fluid inside the tube was observed due to the production of secondary flow phenomena that also serves to increase the extent of synergism between the velocity and temperature fields [31; 32; 33]. Finally, in this family of reactors, the use of gas nano-bubbles (including those of hydrogen) may also be an excellent way to increase surface-to-volume ratios of gas in water to facilitate the growth of hydrogen hydrate, and intimate gas-liquid contact [34]. As a result of surface design, the complex secondary flow and centrifugal force phenomenon, known as endogenous force-field, would be generated [29; 30]. In the endogenous force-field, because of the density difference amongst the three phases (water/gas/hydrate), the gases near the wall of the tube and the hydrate attached to the wall could be removed. As a result of the rapid renewal of the gas-liquid and hydrate-wall interfaces, mass-transfer processes of hydrate-reaction systems were improved. Meanwhile, according to the field-synergy principle [31; 32; 33], the production of the secondary flow phenomenon increases the  
Figure 4: (**a**) Schematic plot of the hydrate formation system with ELR. (**b**) Scheme of the ejector. Figure 5: Schematic diagram of the internal spiral-grooved tube. level of synergism between the velocity and temperature fields, as mentioned previously, and improves greatly the heat-transfer efficiency of the fluid inside the tube.