# Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters
## 3 Results and Discussion
### Stability of TBAB/TBAC Hydrate in Cyclic Formation  
The practical use of a mixed-hydrate working substance will facilitate significant future developments in the cold storage industry. At present, there are few studies on this topic, and a suitable mixed-hydrate working substance has not yet been fully developed. The rapid and stable formation of hydrates is of great importance for the application of cold-storage technology. A shorter induction time could decrease the time and energy costs, thereby increasing the efficiency of the cold-storage system. From the previous analysis,  
Figure 4: Crystal morphologies of (**a**) R141b; (**b**) TME, and (**c**) TBAB/TBAC hydrates at 5 \({}^{\circ}\)C and 0.1 MPa, for a sequence of repeated experiments (Case 1–3). the TBAB/TBAC hydrate was found to have a shorter induction time and faster formation rate than the other hydrates. The formation-dissociation experiments were conducted using different promotion methods such as adding a 1 cm\({}^{3}\) ice cube, magnetic stirring at 500 rpm, and combining 500 rpm magnetic stirring with the addition of 0.1 g graphene, each of which ran for 10 cycles. In this study, the hydrate dissociation time was considered to be the time from which the reaction vessel reached ambient temperature to the time when all the hydrate crystals dissociated. The experimental results show that the induction time gradually decreased with increasing numbers of formation cycle, and finally stabilized, as shown in Table 4 and Figure 5. After hydrates were formed at the first cycle, there was more and more water with a memory effect in the following formation cycles, so the induction time had a tendency to decrease. Eventually, almost all of the water in the system had formed hydrate and the induction time tends to be stabilized. In addition, as more water in the system had a memory effect, it may provide more nucleation sites within the system and more dispersed hydrate particles would be formed when the hydrate was formed again. The dispersed particles may have a larger heat exchange area which may reduce the dissociation time in the following dissociation cycle. When the formation process stabilized, the hydrate dissociation process also stabilized. In general, with the use of ice, the formation process and dissociation processes require less time than required with the two other promotion methods. While, in the ice-added system, the induction time in the second formation was longer than that for the first formation step. This may be due to the fact that no more ice was added after the first cycle. After dissociation of the hydrates, no ice provided additional nuclei for heterogeneous nucleation and molten ice was no longer available to provide a template for forming the hydrate nuclei, which led to a longer time for the secondary formation of hydrates. The overall trends for magnetic stirring alone and magnetic stirring combined with 0.1 g graphene were the same. However, the single method was more effective than the combined method for the first five formation-dissociation cycles. Subsequently, the hydrate induction time for the combined method gradually decreased, affording a shorter induction time than that of the single promotion method. The time required for the formation and dissociation of the TBAB/TBAC hydrate after multiple cycles was stabilized at about 0.75 and 1.25 h, respectively, with good long-term stability. Other studies have also shown hydrate slurries with similar properties [60; 61]. This indicates that the TBAB/TBAC hydrate shows potential for use as a cold-storage working substance.