# Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters
## 3 Results and Discussion
### Characteristics of Hydrate Formation
Therefore, the process of hydrate formation in the solution with crystal nuclei may be very slow and required a long induction time. In addition, previous studies have also showed that homologous polyols such as pentaerythriol and 2,2-dimethyl-1,3-propanediol cannot form hydrates, which indicates that the hydrophobic methyl groups are crucial for the formation of TME hydrates [27]. This could be one reason why TME can nucleate and grow in a short induction time. To decrease the induction time of the hydrates, a 1 cm\({}^{3}\) ice cube was added to the pure water system. Under the same temperature conditions, ice had a positive effect on the hydrate crystallization. Upon addition of the ice cube, the induction time decreased markedly, and significantly high formation rates were finally obtained. R141b started to form a hydrate slurry within approximately 10 min. The water conversion rate continued to increase, and finally a dense and relatively uniform mass was obtained, as shown in Figure 2(b2). TME also began to form in large quantities at approximately 10 min, but there was still some remaining water at the end, as shown in Figure 2(b4). TBAB/TBAC began to form in large quantities at approximately 5 min, and formed a relatively dense hydrate, as shown in Figure 2(b6). The rate of hydrate formation mainly depended on the energy supply rate (when the hydrate grew to the surface) and the rate of surface heat removal (when the hydrate was formed) [38]. Hydrate formation was faster upon the addition of ice under the same conditions because ice positively affected the crystallization of the hydrates. This may be because ice can better accelerate the initiation of hydrate formation by providing additional nuclei for heterogeneous nucleation [39]. Meanwhile, after the ice melted, the water could retain its structure in the liquid phase, and this structure served as a template for the formation of hydrate nuclei [40]. In addition, melting ice absorbed the large amount of energy generated during the formation of the hydrates, thereby increasing the rate of hydrate formation [40]. Previous studies have reported the same experimental phenomenon. [PERSON] et al. [40] investigated the crystallization and growth of methane hydrate in static melting ice and showed that nucleation only occurred at the methane-water interface. They concluded that nucleation ultimately affects the rate of hydration formation, and the melting ice on the ice surface provides a framework for the nucleation of hydrates, which increases the formation of hydrates. The effect of magnetic stirring, and of magnetic stirring combined with the addition of 0.1 g graphene, on hydrate formation by TBAB/TBAC was investigated (Figure 2). Hydrate formation increased only when magnetic stirring was used (500 rpm), but at a significantly lower rate than achieved with the addition of the ice cube. The final result was a hydrate slurry with a light yellow color, as shown in Figure 2(b7). For the combined method, there was a moderate increase in the induction time, whereas the formation rate was significantly increased, and a white hydrate slurry was obtained, as shown in Figure 2(b8). Stirring is commonly used as an auxiliary method for hydrate formation. Stirring promotes the mixing of incompatible gas and liquid, and it rapidly renews the reaction interface, thereby enhancing the transfer of heat and mass. Continuous stirring simultaneously increases material diffusion, accelerates material dissolution in the liquid phase, shortens the induction period, and increases the rate of hydrate formation [41; 42]. [PERSON] et al. [43] suggested that hydrate formation is limited by insufficient gas-liquid mixing. In addition, the induction time mainly depended on the stirring speed. Therefore, although the induction time can be shortened by increasing the stirring speed, this must be controlled because high speeds can greatly inhibit hydrate growth [44].