Field of the Invention The invention relates to the processing of LNG from a production field. In particular, the invention relates to the removal of adverse gases including CO2, ¾S and N2 from a field originating feed stream

Background In order to manage the low quality of natural gas for a field, which is laden with impurities such as high CO2, ¾S and N2, typically requires a complex cryogenic separation process the separation to meet product specifications. Cryogenic separation is normally associated with tall column and heavy weight to meet the separation target of sales gas/LNG specs.

Alternatively, solvent-based removal processes for pre-treatment of nitrogen involve high energy consumption.

Both such processes require high CAPEX, OPEX, weight and footprint when applied to offshore conditions.

Summary of Invention

In a first aspect, the invention provides a system for processing an LNG feed, the system comprising: a bulk removal stage arranged to remove and release CO2 liquid from the inflow feed, said bulk removal stage including a first HGMT device, and; a polishing stage arranged to receive a lean CO2 feed from the first HGMT device, said polishing stage arranged to remove and release residual CO2, the polishing stage including a second HGMT device; wherein the polishing stage is arranged to release an outflow of CO2 stripped LNG. In a second aspect, the invention provides a method for processing an LNG feed, the method comprising the steps of: separating and releasing CO ₂ liquid from the inflow feed using a first HGMT device; receiving a lean CO ₂ feed from the first HGMT device at a second HGMT device; polishing said lean CO ₂ feed and releasing residual CO ₂ using the second HGMT device, and; releasing an outflow of CO ₂ stripped LNG.

In a third aspect, the invention provides a system for processing an LNG feed, the system comprising: an N ₂ removal stage arranged to remove and releasing N ₂ liquid from the inflow feed, said N ₂ removal stage including an N ₂ separation HGMT device, and; wherein the N ₂ separation HGMT device is arranged to release an outflow of N ₂ stripped LNG.

The development of both CO ₂ and N ₂ separation technologies will enable the monetisation of undeveloped gas fields having a high level of impurities. However, relying upon high cost, and large footprint, processes such as column distillation lessen the economic and technical viability of such fields.

High gravity mass transfer (HGMT) devices solve that part of the process, but the incorporation of HGMT devise so as to optimise the removal of such impurities remains key. HGMT devices eliminate the need for solvent based processes for conventional acid gas removal, as well as the necessity for excessive pre-cooling prior to entering the cryogenic process, as well as the requirement for additional dehydration units for pre treatment, particularly for N ₂ removal. Brief Description of Drawings

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. Figure 1 is a schematic view of a CO2 removal system according to one embodiment of the present invention;

Figure 2 is a schematic view of an N2 removal system according to a further embodiment of the present invention;

Figure 3 is a schematic view of a combined CO2 and N2 removal system according to a further embodiment of the present invention; and

Figure 4 is a schematic view of a generalised high gravity mass transfer device according to a further embodiment of the present invention.

Detailed Description

Figure 1 shows a schematic view of a CO2 removal system 5 according to one embodiment of the present invention. The system 5 receives an inflow of a feed stream From natural gas stream, which may, for instance have more than 40% CO2 content, less than 15% N2 and including a portion of FhS. . This is delivered to a high gravity mass transfer device 10 which in this case forms the basis for a bulk removal and polishing stage. The intention of the first HGMT 10 is to remove the majority of CO2 and FhS from the feed stream 20, to release 32 liquid CO2 from an outlet 30 in the HGMT 10. This first bulk removal may represent from 90 to 95% of the CO2 present in the feed stream. Meanwhile, stream 35 returned to HGMT 10 to further refine the bulk removal stage. The feed stream having lean CO2 is then directed 25 to a polishing stage having a second HGMT 15. The feed 25 may release a maximum of 14% CO2, and a proportionally increased N2 component. The feed 25 is cooled by a heat exchanger 42 and passed to a vessel 40 in which the liquid CO2IS returned 45 to HGMT 10. A portion 48 of the liquid is cooled and expanded in a heat exchanger 44 to a lower pressure prior to passing to an upper section of second HGMT 15. Meanwhile, the lean-C0 ₂ vapour 50 will heat up in a further heat exchanger 46 and expand before entering lower section of HGMT 15. HGMT 15 must always operate at lower pressure than HGMT 10 however at maximum operating pressure of 40 bar. Residual CO2 is removed from the feed stream and returned 65 to the first HGMT 10 via pump for the hydrocarbon recovery from HGMT 15. Stream 65 rich in C02 introduces to the HGMT 10 will avoid solid CO2 solid region. The polished feed stream having the substantial CO2 component removed therefrom is directed and cooled 60 to a vessel 70. The liquid 75 will be pump back to HGMT 15 to further enhance the separation. The CO2 lean stream 80 contains minimal CO2 content of at least 50 ppm with a proportionally increased N2 content in hydrocarbon rich stream. In this further embodiment, the polishing stage receives the feed stream under cryogenic conditions, although cryogenic conditions used in this embodiment is slightly different than conventional definition cryogenic conditions. To this end, a refrigeration unit 55 which may use for instance liquid nitrogen is directed to the heat exchangers leading into the HGMT 15 to cool down the process streams to meet the cryogenic operating conditions.

Figure 2 shows a further aspect of the present invention whereby an N2 removal stage 85 includes an HGMT device 90 for receiving a feed stream 95 that is nitrogen laden. The nitrogen laden gas passes through a preconditioning stage 100, to cool down the stream 105 temperature to the cryogenic temperature of -60°C to -120°C, into the HGMT device 90. The HGMT device 90 according to this embodiment, has been modified, in that internal housing elements, rotating parts, apparatus to avoid flow maldistribution, etc., have been modified from conventional HGMT devices, so as to meet the low temperature condition and produce on-spec LPNG product.

A separator vessel 125 is separates a vapour component 130 and liquid component 127. A nitrogen rich stream containing at least 97 % nitrogen is drawn off from overhead the vessel 125. The liquid component 127 is passed to HGMT device 90 via pump to further enhance the liquid outflow 110 which contains mostly liquid hydrocarbon with very minimal CO2 and N2 content. This component passes through a heat exchanger and is drawn off 85 as the LNG product. A return 120 stream is directed back to the HGMT device 90 for further separation.

Figure 3 shows a system 140 of an integrated HGMT process for CO2 and N2 removal from natural gas to LNG production. The integrated system 140 combines a multi stage separation process whereby a feed stream 160 is passed into the HGMT 145 for bulk removal of CO2 and subsequent disposal of liquid CO2 rich streaml80 with minimal hydrocarbon content. The lean CO2 stream from the HGMT 145 is directed 165 to the second polishing stage of the second HGMT 150 to meet the utmost CO2 content of 50 ppm in the 170 stream. A proportionally increased of N2 content in stream 170 is directed to a third HGMT 155 for the nitrogen removal to produce LNG product of stream 185 (contains less than 1 % of nitrogen). Furthermore, the nitrogen rich gas stream 190 is drawn off at the overhead section of HGMT 155.

This invention can be further employed to meet the nitrogen content in the fuel gas stream for the self-consumption with minimal impact to LNG production.

Figure 4 shows a generalized view of an HGMT device 200 comprising a high gravity mass transfer device housing which includes a rotational part 207, an internal cavity 211 for an arranged elements to enhance contact area for gas and liquid phases and one or more quantity apparatus can be installed in the housing to minimise the flow maldistribution inside the HMGT device.

Two phase feed stream 215 is introduce into a chamber 205 having a rotating elements 207. The arranged elements experienced the rotational movement resulting in high centrifugal force producing smaller liquid droplets amplified the mass transfer and heat transfer with higher overall separation efficiency and shorter residence time. Under this condition, most of the gas will be diverted to centre of the arranged elements due to the difference in velocity and drawn off at the overhead section 222 of HGMT device. Meanwhile, liquid product stream 224 is produced at the bottom section of HGMT device. A liquid stream 220 from an overhead vessel is diverted back to HGMT device to assist the separation process.