Hydrate Formation and Separation

Karsten Ofstad
Juli 1996


SUMMARY

The main object of this diploma thesis is to produce natural gas hydrates with a gas-content as high as possible. A method to achieve high values of gas- content in successive tests was attempted, and a maximum gas-content of 161 Sm3 gas per 1 m3 of hydrate was obtained in this study. The drying of the hydrate was done by nitrogen gas injection.

The influence the sub-cooling temperature had on the gas-content was studied. The results shows that the gas-content is varying for different sub-cooling temperatures, but no final conclusion could be drawn. The stirrer rate effect on the gas-content was also studied. With stirrer speed ranging from 100 rpm to 500 rpm, three series with constant formation pressure were carried out. No trend is visible. How the formation time affects the gas-content was also studied. The gas-content as a function of formation time shows a decrease in the gas-content for increasing formation time.

Three successive experiments proved that a high gas-content could be reached and that the experiments are repeatable. The work has also included reliability tests of the gas-content in the hydrates. For seven out of ten experiments the difference in the gas-content is in the range of 0.0 % to 7.0 %. An insulation box around the hydrate glass container gave higher or similiar gas-contents for the tests that were carried out, compared to previous tests of the same samples where no insulation box were used. Finally, a microtome-cryostat is an useful instrument for cutting very thin sections of hydrates, and a procedure for operating the instrument was carried out for further use.


INTRODUCTION

Due to the world's increasing demand for natural gas as a energy source, natural gas must be transported longer distances from smaller gas-fields to the consumer [1]. Today this transport is through large and expensive pipelines, or by costly LNG-transport (Liquified Natural Gas), also involving high risk. To reduce the cost and the risk when transporting the natural gas via pipelines or LNG-boats, the NGH (Natural Gas Hydrates) concept has been proposed. The basic idea is that if the gas hydrates are stored adiabatically, they will remain stable below the equilibrium area, i.e. at atmospheric pressure and at temperatures above the equilibrium temperature. Therefore, no pressurised tanks are needed for neither storage nor transport. This makes the hydrate chain simpler and cheaper than the LNG chain [2]. The NGH concept is patented by Professor Gudmundsson at the Norwegian University of Science and Technology (NTNU) in cooperation with Aker Engineering.

To convert natural gas into NGH gives small fields potentials for development, when the storage and transport of natural gas are made economical justifiable. The idea of storing gas as hydrates is also proposed by others, but not yet industrialised. Benesh (1942) proposed the use of hydrates to improve the load-factor of natural gas supply systems. The hydrates should be produced at 10 °C and 35 bar, and stored at -32 °C. Another possibility is to store the hydrates at high pressure and temperature. Chersky et al. (1975) proposed the production of hydrates at high pressure, and stored at pressures between 20 and 50 bar [3].

To achieve the highest possible gas-content, gas hydrates in earlier experiments were tried separated from the water by gravitational force, by Parlaktuna and Gudmundsson (1995) [4] and Nerland (1995) [5]. This could, however, not provide the necessary purification of the hydrates, and only in three experiments by Nerland was the gas-content measured to values higher than 130 Sm3/m3.

The main object of this diploma thesis is to produce natural gas hydrates with a gas-content as high as possible, preferably in successive tests. The drying of the hydrates will be done by nitrogen gas injection. By using nitrogen gas, water will evaporate into the gaseous state, and is thereby transported out of the reactor. Purified hydrate will then be left inside the reactor. The effect some of the other variables, e.g. the sub-cooling temperature, the stirrer rate and the formation time has on the gas-content will also be investigated. In addition, the diploma thesis includes a description of the microtome-cryostat. This instrument cuts thin sections of the hydrate in a cold environment. These sections are in turn put under a microscope, making magnified studying of hydrates possible.


CONCLUSIONS

- Using nitrogen gas to dry the hydrates in the reactor had small effect on the gas-content.

- The highest gas-contents were found on the stirrer area. The hydrates formed on the stirrer area contained less water than the hydrates formed on the wall.

- The gas-content of the hydrates can be affected by the formation pres- sure, the sub-cooling temperature, the stirrer rate and the formation time.

- The use of a microtome-cryostat for hydrate studies was promising.


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Last modified: Thu Feb 27 16:43:46 NFT 1997