RESUMO
Secondary hydrate formation or hydrate reformation poses a serious threat to the oil and gas transportation safety and natural gas hydrate exploitation efficiency. The hydrate reformation behaviors in porous media have been widely studied in large simulators due to their importance in traditional industries and new energy resources. However, it is difficult to understand the interfacial effects of hydrate reformation on the surface and in micropores of the porous media via a basic experimental apparatus. In this work, in situ X-ray computed tomography (X-CT) technology is used to detect the period, distribution, volume, and morphology characteristics of secondary hydrate formation during hydrate dissociation under depressurization, thermal stimulation, and the combined conditions. It is found that the secondary hydrate formation is inevitable under any conditions of hydrate dissociation. The secondary hydrate morphology varies among porous, grain-enveloping, grain-cementing, granular, and patchy structures, which are closely correlated to the hydrate reformation region and gas/water saturated conditions during hydrate dissociation. Accordingly, we revealed that the interfacial superheating phenomenon before hydrate dissociation could provide a supercooling condition for hydrate reformation. The gas flow along the interface of pores and inside the liquid water, as well as gas accumulation in noninterconnected pores, would exaggerate the hydrate reformation by increasing the local pore pressure. Meanwhile, the hydrate reformation aggravates the nonuniform distribution of gas hydrates in pores. In order to avoid hydrate reformation during dissociation, we further compared hydrate reformation and dissociation behaviors under three hydrate dissociation conditions. It is revealed that the combination of thermal stimulation and depressurization is an effective method for hydrate dissociation by retarding secondary hydrate formation. This study provides visual evidence and an interaction mechanism between interfacial heat and mass transfer, as well as secondary hydrate formation behaviors, which can be favorable for future quantitative research on secondary hydrate formation in different scales under various dissociation conditions.
RESUMO
Previous studies have suggested that L-cysteine regulates gut motility through hydrogen sulfide. However, the mechanisms involved in the L-cysteine-induced response have not been extensively studied. This study aimed to investigate the underlying mechanisms of action of L-cysteine on spontaneous contraction of rat colon. Longitudinal and circular muscle strips from rat middle colon were prepared to measure the spontaneous contractile activities of colon in an organ bath system. Whole-cell voltage-clamp techniques were applied to record the currents of L-type voltage-dependent Ca2+ channels (VDCCs) and voltage-gated K+ channels (Kv) in isolated smooth muscle cells (SMCs) from colon. L-cysteine inhibited the spontaneous contraction of longitudinal and circular muscle strips from the rat colon in a concentration-dependent manner. The inhibition induced by L-cysteine was significantly decreased by inhibitors of H2S synthesis (p < 0.05). Furthermore, the suppression induced by L-cysteine was partially attenuated by tetrodotoxin, L-NNA and glibenclamide (p < 0.05). Whole-cell voltage-clamp recordings showed that L-cysteine caused a remarkable reduction in the peak currents of VDCCs and significantly increased the membrane currents of Kv channels in isolated SMCs (p < 0.05). We concluded that L-cysteine inhibits the contractile activities of smooth muscle strips from the rat colon. The relaxation in response to L-cysteine may be in part mediated by a nitrergic pathway and by inhibiting the VDCCs in combination with a direct activation of the KV channels and KATP channels.
