The present invention relates to a method and installation for the removal of nitrogen from natural gas by means of cryocondensation.
Nitrogen is not wanted in natural gas because nitrogen does not contribute to a higher flame temperature, on the contrary, nitrogen is ballast; therefore nitrogen needs to be removed from natural gas.
Moreover, at high flame temperatures nitrogen is known to bind to oxygen, forming toxic NOx compounds.
These reasons also make it clear that for the combustion process nitrogen is a most undesirable gas.
It is generally known that oxyfuel is a much applied process, in which during the combustion process pure oxygen is used instead of air with the aim to improve the efficiency of ovens. It goes without saying that here also the nitrogen present in the air (about 79% by volume) is merely ballast which, apart from the disadvantages just mentioned, leads to heat loss because nitrogen is blown into the atmosphere as warmed up gas.
In US-A-5,036,671 a method is described for producing a methane-rich liquid stream from a stream of natural gas containing nitrogen. The natural gas is liquefied at a pressure above atmospheric pressure, after which it is expanded to a lower pressure, thereby concentrating the nitrogen in the vapor phase.
The object of the invention is the removal of nitrogen from natural gas and the use of pure oxygen, resulting in increased efficiency.
To this end the nitrogen present in the natural gas has to be removed before the natural gas enters the combustion process.
To this end the present invention provides a method according to claim 1 and an installation according to claim 4.
According to the prior art cryocondensation of hydrocarbons is carried out using cold nitrogen as cooling means.
According to the present invention the disadvantages mentioned above are effectively removed.
It has been shown that the method according to the invention is particularly suitable for the removal of nitrogen from natural gas having a nitrogen content of about 14% by volume.
Basically the general idea of the invention is that the liquid oxygen is boiled off in a cryocondensation unit, causing the hydrocarbons to condense. After heating, the condensed hydrocarbons, together with the gaseous oxygen that has developed, can be used in the combustion process.
The natural gas to be treated is preferably precooled in a heat exchanger to about -80°C before cooling with liquid oxygen to about -165°C.
In comparison to the prior art, the method according to the invention provides the following advantages:
Further, the present invention relates to an installation for the removal of nitrogen from natural gas, characterized in that the installation is provided with a storage tank for liquid oxygen, which tank is connected by means of a pipe with a first heat exchanger for the condensation of the natural gas which, via a pipe and the first heat exchanger is linked to a second heat exchanger, from which the nitrogen-free condensed natural gas via a pipe, after being heated in the second heat exchanger, arrives via a pipe in a burner, while liquid oxygen from the first heat exchanger enters via a pipe a third heat exchanger to be fed in the gaseous phase via a pipe to the burner and gaseous nitrogen is removed from the first heat exchanger via a pipe.
The invention will now be further elucidated with reference to the drawing which illustrates a preferred embodiment of the installation in accordance with the invention.
In the drawing the installation according to the invention is indicated by reference number (1). The installation (1) according to the invention is provided with a storage tank (2) for liquid oxygen.
The installation is further provided with first, second and third heat exchangers (3,4, and 11).
Natural gas, for instance from Groningen, having a nitrogen content of 14% by volume is fed via a pipe (7) to the first heat exchanger (4) in which the natural gas is cooled to - 80°C. Via valve (15) the thus cooled natural gas is led to the second heat exchanger (3) in order to be further cooled with the aid of liquid oxygen to -160°C to -165°C, which liquid oxygen is led from the storage tank (2) into the third heat exchanger via pipe (5) and tap (14). Via pipe (6) the hydrocarbons condensed in the second heat exchanger (3) are fed into the first heat exchanger (4), where the liquid hydrocarbons become gaseous and in that form are transported via a pipe (13) to a burner (12) of a melting furnace.
Further, oxygen coming from the second heat exchanger (3) in liquid form is led via a pipe (9) and a tap (16) to the the third heat exchanger (11) and then to the burner (12). In the third heat exchanger (11) cold gaseous oxygen is further heated to ambient temperature. Via a pipe (10) the gaseous oxygen is united with the hydrocarbons in the burner (12) of the melting furnace.