VISUAL OBSERBATION OF HCFC141b GAS HYDRATE FORMATION/DECOMPOSITION PROCESS OUTSIDE OF A TUBE

In order to design a kind of heat exchanger suitable to the indirect-touched gas hydrate cool storage vessel, a visual observation of HCFC141b gas hydrate formation/decomposition process was presented through a self-designed small-scale visualization apparatus of gas hydrate cool storage. Based on the shooted photos and recorded temperatures, the formation/decomposition process of HCFC141b are described, some characteristics are concluded, and some suggestions of designing heat exchanger are indicated according to the specific characteristics of HCFC141b gas hydrate formation/decomposition process.

A developed transient gas-liquid-solid flow model with hydrate phase transition for solid fluidization exploitation of marine natural gas hydrate reservoirs

The multiphase flow characteristic is one of the most concerning problems during solid fluidization exploitation of marine natural gas hydrate reservoirs.In this research,a new transient gas-liquid-solid multiphase flow model with hydrate phase transition was developed.Meanwhile,this model considered the coupling relationship among convective heat transfer,hydrate dynamic decomposition,and multi-phase flow.The model can simulate the change of flow pattern from solid-liquid to gas-liquid-solid flow,and describe the distribution character of volume fraction of phase,wellbore temperature and pressure,and hydrate decomposition rate during transportation.The simulation results indicate that the hydrate decomposition region in the wellbore gradually expands,but the hydrate decomposition rate gradually decreases during the solid fluidization exploitation of hydrate.When mining time lasts for 4 h,and the bottom hole pressure decreases by about 0.4 MPa.Increasing NaCl concentration in seawater helps expand hydrate decomposition regions and improves the wellbore hydrate decomposition rate.When the Nacl mass fraction in seawater reaches 15%,it will raise the hydrate decomposition regions to the whole wellbore.In addition,the higher the wellhead backpressure,the lower the decomposition area and decomposition rate of hydrate in the wellbore.When wellhead backpressure reaches 2 MPa,the volume fraction of gas near the wellhead will reduce to about 12%.This work is expected to provide a theoretical basis for the development of marine hydrate reservoirs.

Texture,composition and properties of plugs formed by carbon dioxide hydrate and wax

Gas hydrates and wax are the major flow assurance problems for the transportation of produced hydrocarbons through pipelines.However,in most research works both these two problems are studied separately.Although simultaneous precipitation or deposition of these compounds in pipelines can lead to different mitigation/prevention strategies,the investi-gations in which both these problems are considered simultaneously appeared only recently.There is no information in the literature on the texture/composition and features of decomposition process of mixed wax/hydrate plugs.At the same time,this information could be useful to understand how to treat the problem of formation of these plugs.In this work,three wax/gas hydrate plugs were collected at quasi-static conditions from a water-in-oil emulsion to study their texture,composition and the features of decomposition process.Powder X-ray diffraction and IR(infrared spectroscopy)analyses showed that the plugs consisted of wax and gas hydrate.Thermovolumetric and DSC(Differential Scanning Calorimetry)experiments showed that the main part of gas hydrate in the plugs at the ambient pressure started to decompose at about 268 K.This temperature was higher than the equilibrium temperature of carbon dioxide hydrate at this pressure,indicating that the gas hydrate in the plugs could be effectively preserved at temperatures below the ice melting point(273.2 K).It was found through observation of the hydrate decomposition process in the plugs under the microscope that the gas in the samples released in small bubbles,while the hydrate particles were not visible at this magnification,indicating that the hydrate was indeed highly dispersed in the samples.A residual wax was jelly-like after decomposition of hydrate in all the cases.Rheological experiments showed that the plugs residues after decomposition of the hydrates had higher yield points and viscosities than the initial waxy crude oil origi-nally used for the experiments.

Composition and morphology of the thermal decomposition products of 3Mg（OH）2·MgCl2·8H2O nanowires

The thermal decomposition of 3Mg（OH）2·MgCl2·8H2O （318MHCH） nanowires synthesized from agglom- erated Mg（OH）2 microspheres was investigated. The influence of heating rate and temperature on the composition and morphology of the products was investigated. Thermogravimetric-differential scan- ning calorimetry, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction showed that increasing the heating rate from I to 20 ℃/min promoted the escape of crystalline water from the 318MHCH nanowires. 318MHCH nanowires were dehydrated stepwise to 310MHCH porous nanowires from room temperature to 320℃, and then to MgO cubic nanoparticles from 420 to 700 ℃. The nanowires retained their one-dimensional morphology, until the phase changed to MgO. The immediate collapse of the one-dimensional structure was attributed to the presence of Mg-O/Cl chains.