Gas-to-liquid technology

#The present invention relates to a plant and a process for treating natural gas to produce a liquid product.

#It is well known that most oil wells also produce natural gas. At many oil wells natural gas is produced in relatively small quantities along with the oil. When the quantities of this associated gas are sufficiently large or the well is close to pre-existing gas transportation infrastructure, the gas can be transported to an off-site processing facility. When oil production takes place in more remote places it is difficult to introduce the associated gas into existing gas transportation infrastructure. In the absence of such infrastructure, the associated gas has typically been disposed of by flaring or re-injection. However, flaring the gas is no longer an environmentally acceptable approach, while re-injection can have a negative impact on the quality of the oil production from the field.

#Gas-to-liquids technology can be used to convert the natural gas into liquid hydrocarbons and may follow a two-stage approach to hydrocarbon liquid production comprising syngas generation, followed by Fischer-Tropsch synthesis. In general, syngas (a mixture of hydrogen and carbon monoxide) may be generated by one or more of partial oxidation, auto-thermal reforming, or steam methane reforming. Where steam methane reforming is used, the reaction is endothermic and so requires heat. The syngas is then subjected to Fischer-Tropsch synthesis. For performing Fischer-Tropsch synthesis the optimum ratio of hydrogen to carbon monoxide is about 2:1, and steam reforming has a benefit of providing more than sufficient hydrogen for this purpose.

#According to the present invention there is provided a gas-to-liquids plant for treating natural gas, wherein the natural gas is subjected to expansion through a flow restrictor so as to undergo cooling through the Joule Thomson effect, with separation of resulting liquids.

#The present invention also provides a process for treating natural gas in this way. The process of the invention relates to a chemical process for converting natural gas (primarily methane) to longer chain hydrocarbons.

#Ideally the expansion takes place without significant transfer of heat from the surroundings, the natural gas expanding into a lower pressure state. The flow restrictor may be a throttle valve, or alternatively may be an inlet nozzle of a vortex tube separator, or a turbo expander, or a Twister™ separator device. A vortex tube, or Ranque-Hilsch tube, splits the gas into a hot gas stream and a cold gas stream. The hot gas stream may be utilised elsewhere within the plant. However, as a result of the division into two streams, only a proportion of the gases are cooled. The expansion can cool the natural gas to below 0° C., more particularly below −10° C. for example below −15° C., with the consequence that longer-chain hydrocarbons condense from the vapour state into the liquid state, and can be separated from the remaining natural gas. The degree of cooling is selected to ensure that the output stream is at a pressure sufficient to drive gas through the process. Whilst the temperature can be dropped further in order to increase the recovery of higher hydrocarbons, this ceases to be advantageous when the cost of the increased compression requirements to re-pressurize the gas exceed the value of the additional longer chain hydrocarbons recovered. Preferably the natural gas stream is fed into the flow restrictor through a heat exchanger in which it is cooled by contact with at least one fluid that has been cooled by passage through the flow restrictor, so that the natural gas is below ambient temperature when it reaches the flow restrictor.