GB2333148A

GB2333148A - Liquifaction of gases

Info

Publication number: GB2333148A
Application number: GB9800250A
Authority: GB
Inventors: Michael John Pettman
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-01-08
Filing date: 1998-01-08
Publication date: 1999-07-14
Also published as: GB9800250D0

Abstract

A process for the production of a hydrocarbon gas involving the following steps: a) producing a stream of nitrogen-containing liquid at least partly by the utilisation of the refrigeration effect produced by the evaporation of LNG to form said hydrocarbon gas, b) transporting the so produced nitrogen-containing liquid in a cryogenic carrier to a location having a supply of gas from which LNG can be produced, c) producing a stream of LNG at least partly by the utilisation of the refrigeration effect produced by the evaporation of the nitrogen-containing liquid from step b), d) transporting the LNG produced in step c) in the same or a similar cryogenic carrier to the point where it may be utilised in step a).

Description

LIQUIFACTION OF GASES.

This invention relates mainly to the production and transportation of liquefied natural gas ("LNG" - hitherto predominantly methane). In particular it relates to a more efficient use of the refrigeration effect required to liquefy the LNG. In simple terms LNG is evaporated against a stream of nitrogen-containing gas which is itself thereby liquefied. This nitrogen-containing liquid is transported back to the hydrocarbon gas rich country where the LNG is made, and is there evaporated against the stream of gas which becomes the LNG. The invention having a concomitant benefit of making the LNG carrier ships cheaper in that the nitrogen containing liquid can be carried back in the same tanks as the LNG is carried to the land of its use - thereby greatly reducing the need for the LNG carriers to have additional ballasting tanks. Additional is the advantage that the location of the primary refrigeration equipment (i.e. a very significant part of the investment) may be located in the land where the LNG is to be used. This allows much smaller sources of hydrocarbon gases to be the source of the LNG because the local investment is much reduced compared with building at their location a full traditional LNG plant.

Presently, for LNG, the extremely expensive liquefaction plants have to be built in areas which are often remote lacking in infrastructure, and often climatically disadvantageous. By means of this invention the investment in plant in such areas is dramatically reduced.

This invention will now be described by reference to the production of liquid nitrogen as a cryogenic liquid, and natural gas as the LNG. However the inventor recognises that the only need is for a cryogenic liquid to be produced, and as such liquid air would be cheaper in that it does not have to be fractionated to separate the (inert) nitrogen from the oxygen. However, if safety precautions were not taken, air might become accidentally mixed with the natural gas and form an explosive mix. Naturally, any composition of nitrogen and oxygen containing mixture could be used as the cryogenic liquid, The inventor recognises that irl this invention, there is usually a need for additional refrigeration, and/or liquid nitrogen to be produced and added to each 'cycle'. This is because some of the cryogenic fluids evaporate during the transportation of the LNG and of the nitrogen containing liquid, and because of other normal thermodynamic inefficiencies in the invention e.g. when evaporating and condensing at both ends of the recycle.

This additional nitrogen-containing liquid may be separately produced by normal cryogenic means such as in a Linde cycle air separation unit (ASU) or by means of the use of molecular sieves, or its production may be integrated into either end of the cycle, but preferably into the LNG evaporation end i.e. in the land where the LNG is to be utilised, using cryogenic technology known to those skilled in that art. I.e. preferably utilising the refrigeration available from the evaporation of the LNG for the liquefaction of the nitrogen and the production of the additional nitrogen at the point(s) in the two cooling sections whereat it is most economically utilised.

In other words, this invention is: a process for the production of a hydrocarbon gas involving the following steps: a) producing a stream of nitrogen-containing liquid at least partly by the utilisation of the refrigeration effect produced by the evaporation of LNG to form said hydrocarbon gas, b) transporting the so produced nitrogencontaining liquid in a cryogenic carrier to a location having a supply of gas from which LNG can be produced, c) producing a stream of LNG at least partly by the utilisation of the refrigeration effect produced by the evaporation of the nitrogen-containing liquid from step b), d) transporting the LNG produced in step c) in the same or a similar cryogenic carrier to the point where it may be utilised in step a).

Preferably the nitrogen containing liquid is substantially pure nitrogen.

The transport of the LNG and /or the liquid cryogenic liquid can be in a floating carrier, a road car, a rail car, or a pipe.

Synthesis gas ( a gas mixture containing hydrogen and carbon monoxide) can be produced by means known to those skilled in that art, such as by partial oxidation means, utilising oxygen which is the by-product of the production of makeup nitrogen-containing liquid andnatural gas. In one utilisation of this invention, the hydrocarbon feedstock utilised for the production of synthesis gas comes from the LNG transported. This means of producing synthesis gas would be particularly suitable for the manufacture of iron by a direct reduction process.

This utilisation would be enhanced by the production and use of additional oxygen in a subsequent steel making process.

Another manner in which this invention could be utilised would be to greatly reduce the equipment needed to liquefy gas production from an off-shore platform.

Of course there is no pollution effect of evaporation and venting of the nitrogen. However it is an advantage of this invention that the nitrogen could be evaporated under preú.l-e and (with maybe additional compression) and fed into preferably adjacent o:- nearby wells so as to maintain well pressure or oil pressure in the reservoir. Additionally the nitrogen could be used on the platform or in any associated plant as blanketing gas.

Embodiment: General Description: Liquid nitrogen is produced at (normally elevated) pressure from a stream of gaseous nitrogen by the evaporation of incoming LNG. To this, makeup liquid nitrogen may be added and all this loaded onto the LNG carrier type of ship.

It is recognised that the incoming LNG may be transferred from the ship into a cryogenic storage tank so as to minimise demurrage on the expensive LNG carrier type of ship. For the same reason, the ship, having been emptied, may be refilled with liquid nitrogen from a storage tank.

The ship then travels to the location of the supply of the LNG - normally a region having a plentiful supply of natural gas or of oil-associated gas. Upon arrival, it discharges its cargo of liquefied nitrogen, preferably into storage (to again minimise demurrage) and takes on board its return cargo of LNG, preferably from storage for the same reason.

Because of the lower boiling point of liquefied nitrogen than methane, natural gas, which is largely methane, can be easily liquefied. Although by the use of different pressures in the liquid nitrogen evaporation section and the natural gas condensing section, the heat exchange area required could be optimised. Likewise the actual pressure utilised on either side can be optimised. It being usual, but not mandatory for this invention, to compress or utilise precompressed natural gas, andlor pressurised (e.g. by means of a pump) (initially) liquid nitrogen, on either side of the heat exchanger.

The inventor recognises that there is an economic optimum pressure to be used on either side of the exchanger. Given that the LNG is usually carried in a carrier ship at atmospheric pressure, one option being not to use any pressure at all thereby reducing or even eliminating the need to have significant rotary equipment at the LNG producing end of the cycle. In practice, it will probably be economic to have LNG pumps to load the ships, and to have recompression facilities so that the normal LNG boil-off caused by heat inleakage into the cryogenic storage tanks, can be recycled.

The inventor recognises that this invention might also be used to help reduce the capital and operating cost of local peak shaving units. In such units this invention would simply generate and store the cryogenic fluid locally. At times of high natural gas demand, the LNG would be evaporated, and the refrigeration thereby generated would be utilised to liquefy the e.g. nitrogen. During periods of low natural gas demand, (usually when the natural gas pipelines have surplus capacity, the e.g. nitrogen so liquefied would be used to at least assist (with respect to refrigeration effect) by its evaporation, with the liquefaction of more natural gas.

Embodiment: Detailed Description.

The description following serves to show the process condit: ons and equipment arrangement that may be employed to effect the invention. It is recognised that the precise arrangement and process conditions described are not the only ones which may be used, and indeed other more optimum conditions may be possible, depending on local costs for equipment, construction and energy and more importantly the integration of such a process in an existing complex at the same site.

The flow rates of fluids given in this description for the processing plant are those which have been calculated for a tanker operation of nominal capacity 135,000 cubic metres. The entire cycle time from unloading the LNG at the terminal wherein the LNG is vaporised and piped to users (the reception terminal) through to shipping the nitrogen to the terminal of the source of natural gas and wherein the natural gas is liquefied (the production terminal) and receipt again of the LNG at the reception terminal is assumed to be 25 days, allowing 24 days joumey time and one day for loading and unloading - 12 hours each. These assumptions may of course be changed in practice depending upon the size of carrier used, the voyage time and the time required for loading and unloading of the cryogenic cargoes. Other assumptions have been made for the amount of cargo loss due to evaporation. Anyone versed in the art of design of such facilities will be able to calculate the required flow rates for other carrier capacities, voyage, loading and unloading times and cargo loss.

The description commences at the reception terminal, recognising that the entire process is cyclic in operation.

Figure 1 shows the equipment necessary at the reception terminal. In this figure stream S1 of flow rate 4632 kgmol/h is essentially pure gaseous nitrogen manufactured from an air separation unit. The nitrogen to derive stream S1 may be separated from air in an air separation unit by means of a membrane, a pressure swing absorption system, or even cryogenically, using one of these well known and well practised technologies.

Stream S1 is mixed with stream S2 of essentially pure gaseous nitrogen and flow rate 5063 kgmol/ h and which is a recycle stream from the process. The combined stream S3 at slightly less than atmospheric pressure of about 100 kPa and temperature 251 K is compressed in a three stage compressor C4, C5 and C6 with after cooling exchangers E7, E8 and E9 respectively. The discharge pressure from the compression stage (of stream S10) is selected to be about 3600 kPa which is just above the critical pressure of nitrogen. The three stage compressor or compressors may be driven by steam turbines electric motors or gas turbines The total power of compression is estimated to be 31 million Watts taking into account inefficiencies of compression and other losses. Other compression arrangements and number of stages may be used, depending inter aiia on the cosi of energy and capital equipment. Stream SIG, at a temperature of about 308 K, exchanges heat with recycle stream S26 in exchanger E11 and is cooled to 262 K becoming stream S12.

LNG is pumped from the ship to atmospheric storage tank T13. For the purposes of description and calculation the following composition of LNG has been assumed, recognising that other compositions of LNG will also be possible.

Methane 95.30 mol h Nitrogen 0.02 mol % Ethane 4.11 mol % Propane 0.43 mol % Butane 0.04 mol % I-Butane 0.04 mol % Pentane 0.01 mol % Hexane + 0.05 mol % The LNG is pumped from atmospheric storage tank T13 by means of cryogenic pump P '4 at its bubble point temperature of 112 K to exchanger E15. The flow rate of liquid stream S16 is 5058 kgmovh, equivalent to194.3 cubic metres per hour, or 85.1 tonne/h. This flow rate is the hourly rate that requires processina over a 25 day cycle from a 120.000 cubic metre cargo of LNG allowing for a 2.5% evaporation loss of LNG on its 12 day journey.

The pressure of this LNG stream S16 at 180 kPa is sufficient to pass it through the exchanger E15. Higher pressures than this will reduce the temperature differences in exchanger E15. In exchanger E15 stream S12 is cooled from 262 OK to 125 OK at a pressure of 3500 kPa. Under these conditions, since the temperature is below the critical temperature of nitrogen, the nitrogen will condense to form a liquid nitrogen stream S17. The LNG in exchanger E15 will evaporate against the condensing nitrogen stream on the other side of the exchanger. The vaporised natural gas stream S18 leaving exchanger E15 at a temperature of 253 OK may be passed to a compressor C19 which may compress the natural gas to a suitable pressure for its subsequent use as pipeline gas or other use.

The liquid nitrogen stream S17 is further cooled in exchanger E20 to 121 "K against a recycle stream S25. The liquid nitrogen stream S21 from exchanger E20 passes through an expansion valve V22,the downstream pressure of which is slightly above atmospheric at 150 kPa. Under these conditions the stream S23 leaving the expansion valve V22 will partially vaporise and cool to 81 OK. Stream S23 is about 52% weight vapour. The vapour and liquid of stream S23 is separated in separator drum D24 from where the separated nitrogen vapour stream S25 is used as a recycle stream. Stream S25 is heated in exchanger E20 to 119 OK to form stream S26 and further heated to 212 OK in exchanger Ell to form stream S2, against the cooling nitrogen stream on the other side of both these two exchangers. The recycle stream S2 is mixed with stream S-l as described above.

The liquid nitrogen stream S27 from separator drum D24 is pumped by pump P29 to atmospheric storage tank T28. After the ship has discharged its cargo of LNG to tank T13, the ship is loaded with liquid nitrogen from tank T28 by means of pump P30. So that the ship does not remain at the terminal whilst the entire cargo of LNG is vaporised and the cargo of liquid nitrogen is produced, LNG storage tank T13 and liquid nitrogen tank T28 are provided for intermediate storage, both tanks being of suitable capacity for this purpose.

The exchangers Ell, E15, E20, the expansion valve V22 and the separator drum D24 may be contained in a cold box or cold boxes where suitable thermal insulation is used. The materials of construction and other details of design used for these process items, interconnecting pimping and associated instrumentation will be known to those versed in the art of cryogenics.

The amount of liquid nitrogen produced from the cargo of LNG is 164.5 cubic metres per hour or 130 tonne/h. This, due to thermodynamic inefficiencies and evaporation losses on the joumey, will be insufficient to liquefy a full cargo of LNG at the production terminal. In this embodiment, it is found that the amount of nitrogen produced at the reception terminal is 72% of the total amount required.

In this embodiment the remaining liquid nitrogen, amounting to 50.9 tonne/h, is produced by conventional means such as from an air separation plant, and loaded to the ship with the liquid nitrogen produced from the evaporation of LNG.

This total quantity loaded allows for an evaporation loss of 2.5% on the joumey to the production terminal. The total volume loaded is 135,000 cubic metres, representing the full capacity of the ship carrier used in this example.

Figure 2 shows the equipment necessary (and its simplicity) at the production terminal. Liquid nitrogen is off loaded from the ship and stored in atmospheric storage tank T31. Stream S32 of liquid nitrogen at a rate of 6295 kgmolJh or 176.3 tonnelh and a temperature of 77 OK is pumped by P33 at a pressure of 150 kPa to the exchanger E38. In this exchanger the nitrogen is vaporised by condensing natural gas on the other side of the exchanger. The vaporised nitrogen stream S34 leaves exchanger E38 at 272 K. The nitrogen may be discharged to atmosphere or used for some other purpose where nitrogen is required. Natural gas is pre-treated as necessary to remove e.g. sulphur containing compounds, water, mercury and carbon dioxide. Pre-treatment methods for removing these impurities which are detrimental to the liquefaction of natural gas and the equipment used therein are well known and widely practised.

Stream S35 is a pre-treated stream of natural gas of composition shown above. The flow rate of S35 is 5188 kg.mol/h and is sufficient to provide a cargo of LNG of 120,000 cubic metre during the cycle time given above. Stream S35 at a temperature of 293 OK and a pressure of 6900 kPa is reduced in pressure across valve V36 to a pressure of 3500 kPa to become stream S37 which is condensed in exchanger E38 by evaporating nitrogen on the other side of the exchanger. The exchanger E38 on the natural gas side is operated at the medium pressures quoted above as this will increase the temperature differences across the exchanger and increase the heat transfer coefficient thereby reducing the surface area requirement for this exchanger. The precise optimum pressure of operation of this exchanger on the natural gas side may be determined for the economic parameters of the location when known.

The exchanger E38 is designed to sub cool the condensed natural gas stream S39 below the atmospheric bubble point of the stream to prevent subsequent flashing of the gas in the atmospheric tank T42. The liquefied natural gas stream S39 leaves the exchanger at 110 OK and passes to the valve V40 wherein the pressure is reduced to atmospheric. Stream S41 at a temperature of 111 OK passes to the atmospheric LNG storage tank T42. LNG is pumped by P43 from tank T42 to the ship.

In the above description it has been assusned that additional nitrogen is added to the cargo from the reception terminal to satisfy the refrigeration requirement needed to liquefy the full cargo of natural gas at the production terminal. However it is possible to provide the additional refrigeration required in other ways. For example a much smaller conventional natural gas liquefaction plant, at or near the production terminal may be supplied to liquefy the natural gas for which the imported nitrogen is insufficient in quantity. Alternatively a smaller liquid nitrogen producing plant of conventional technology may also be supplied at or near the production terminal. Alternatively additional capacity in an existing conventional LNG plant at or near the production terminal could also be used. These three methods of supplying the additional refrigeration effort at the production terminal have the advantage that a smaller quantity of nitrogen is shipped as cargo, thereby reducing the cargo weight. This is an advantage since the weight of nitrogen to be shipped from the reception terminal of 108,530 tonne is greater than the weight of liquefied natural gas being shipped from the production terminal of 52,350 tonne in the balanced cycle. The volume of nitrogen shipped from the reception terminal is 135,000 cubic metres and the volume of liquefied natural gas shipped from the production terminal is 120,000 cubic metres. When the additional nitrogen is made up at the production terminal or additional LNG is manufactured in another plant at the production terminal the volume of liquid nitrogen required to be shipped becomes less and a better utilisation of the cryogenic ship is possible and larger quantities of LNG may be shipped from the production terminal up to the full capacity of the ship.

The calculation of instantaneous mass flows given above for the 25 day cycle have not allowed any non-operational time due to equipment failure and other operational difficulties. In practice the plants will be designed with additional capacity required to account for any downtime but this will not detract from the overall efficiency of utilisation of the refrigeration effect of the LNG described.

This invention may also be used to greatly increase the production of LNG from existing LNG producing locations by effectively converting those existing LNG-producing facilities to LNG towup facilities, whilst importing a nitrogencontaining liquid to carry out some of the refrigeration duty, thereby greatly increasing the total exporting capacity of the existing LNG facility.

Claims

Claims.:

1) a process for the production of a hydrocarbon gas involving the following steps: a) producing a stream of nitrogen-containing liquid at least partly by the utilisation of the refrigeration effect produced by the evaporation of LNG to form said hydrocarbon gas, b) transporting the so produced nitrogen-containing liquid in a cryogenic carrier to a location having a supply of gas from which LNG can be produced, c) producing a stream of LNG at least partly by the utilisation of the refrigeration effect produced by the evaporation of the nitrogen-containing liquid from step b), d) transporting the LNG produced in step c) in the same or a similar cryogenic carrier to the point where it may be utilised in step a).

2) A process as described in claim 1) wherein the nitrogen containing liquid is substantially pure nitrogen.

3) A process as described in claim 1) or 2) wherein the production of makeup nitrogen-containing liquid is integrated with the production of the nitrogencontaining liquid.

4) The production of synthesis gas utilising oxygen which is the by-product of the production of makeup nitrogen-containing liquid.

5) A process as described in claim 1) wherein some of the hydrocarbon feedstock utilised for the production of synthesis gas comes from the LNG transported.

6) Iron when produced by means of synthesis gas produced by means of claim 4 and/or 5 above.

7) A process as claimed in claim 1 wherein the transport is by means of a cryogenic carrier ship, a road car a rail car, or a pipe.