FR2844337A1 - SYSTEM AND METHOD FOR TRANSPORTING COMPRESSED NATURAL GAS - Google Patents

Abstract

The present invention relates to a method for transporting pressurized natural gas, in which the following steps are carried out: a plurality of reservoirs comprising a lower orifice and an upper orifice are connected by connecting all the upper orifices to a first pipe L1 and to a second pipe L2 all the lower orifices, - the said reservoirs are filled with natural gas under pressure and temperature conditions determined so that the natural gas is in the form of two phases: liquid and vapor, - the vapor phase is transferred through the upper ports of the tanks, and the liquid phase is transferred through the lower ports of the tanks. The invention also relates to a transport system.

Description

The present invention relates to a maritime gas transport system

natural

  in compressed form. The system comprises a set of tanks, means for loading and unloading, or introduction and evacuation, of the compressed gas.

  The invention also relates to a method for transporting and loading and unloading compressed gas. All of the tanks can be assembled on a ship, a train, a truck.

  Currently, several means are known for transporting gas: by gas pipeline

  when the geographic location allows, in liquefied form using LNG tankers, in compressed form, for example according to document US-6339996.

  However, this system is only suitable for transporting compressed gas at high pressures, of the order of 20 MPa, which makes it relatively expensive industrially.

  Thus, the invention relates to a method of transporting natural gas under pressure, in which the following steps are carried out: - a plurality of reservoirs is connected comprising a lower orifice and an upper orifice by connecting all the upper orifices to a first pipe and at a second pipe all the lower orifices, - the natural gas tanks are filled under a pressure below the cricondenbar of the gas and at a temperature below that of the ambient medium, the pressures and temperature being determined so that the natural gas is under form of two phases: liquid and vapor, - the vapor phase transported by the upper orifices of the tanks is transferred,

  the liquid phase transported through the lower orifices of the reservoirs is transferred.

  According to the method, the gas can be stored under a pressure of between 5 and MPa. The temperature of the natural gas stored in said reservoirs is maintained between

+10 and -500C.

  You can empty the liquid tanks by replacing it with an inerting gas.

  The transfer of the vapor phase can be carried out substantially at constant pressure.

  The invention also relates to a system for transporting natural gas under pressure, which comprises: - a plurality of tanks comprising a lower orifice and an upper orifice connected to a first pipe by all the upper orifices and to a second pipe by all the lower orifices, means for filling and / or emptying the tanks with natural gas by the first line, means for filling and / or partially emptying the tanks with a liquid phase by the second line, means for maintenance of the gas pressure at a pressure below the gas cricondenbar, - means for maintaining the gas temperature between 100C and -50'C. 20 The tanks can be grouped into a sub-assembly connected to the first pipe

and the second line.

  Each sub-assembly can be placed in an external envelope.

  The envelope can be filled with a thermal insulating material.

  The envelope can be filled with an inerting gas, for example nitrogen or carbon dioxide, by inerting, it is necessary to understand a non-combustible gas with the gas

natural, and / or in the air.

  A methane detector can be placed on a leakage current of the inerting gas.

  The transport system according to the invention is particularly well suited to the transport of natural gas associated with the production of liquid hydrocarbons, containing a significant proportion of hydrocarbons heavier than methane. It in fact makes it possible to simultaneously transport methane, as well as the heavier fractions, which the associated gas generally contains, at pressures substantially lower than those which are considered for conventional transport systems. For these mixtures, such as associated gases, the maximum pressure at which two phases, liquid and gas, can coexist, is the cricondenbar of the mixture considered. This pressure of the cricondenbar is generally much higher than the critical pressure of methane (46 bars). The cricondenbar of a mixture, such as an associated gas, can be for example between 10 and 20 MPa. Transporting such a mixture in a single phase, over a fairly wide temperature range, requires high pressure, of the order of 20 MPa, or above. The possibility of significantly reducing the pressure makes it possible to use lighter storage enclosures

  and less expensive allowing an increase in transport capacity.

  The present invention will be better understood and its advantages will appear more

  clearly on reading the following description of exemplary embodiments, in no way

  limiting, illustrated by the appended figures below among which: - Figure 1 schematically shows a set of natural gas transport tanks according to the invention, - Figures 2a, 2b, 2c illustrate modes of arrangement of the tanks in subassemblies , s figure 3 gives the phase equilibrium curve for an example gas, - figure 4 illustrates the method of loading and unloading by displacement of the

gas by a liquid phase.

  The present invention relates to a natural gas transport system, treated or untreated, comprising in combination: - a set of cylindrical tanks connected in parallel to constitute a transport volume of natural gas in two-phase mixture; - Isolation and / or refrigeration means making it possible to maintain the temperature of the gas transported between + 100C and -50 'C; - a device for loading and unloading natural gas into the transport volume, based on the displacement of the liquid phase making it possible to maintain the gas at a constant, or almost constant, pressure during the filling and / or filling operations emptying the tanks while avoiding mixtures between natural gas and a substitute gas when the tanks are empty, for example an inerting gas; - a ship, or other means of transport: wagon, truck, ... which collects the tanks. FIG. 1 represents a set of tanks in parallel, which are connected at their upper ends to a first transfer line Li and at their lower ends to a second transfer line L2. Each reservoir may or may not communicate via each of its ends to the transfer lines Li and L2, depending on whether valves VH and VB

are open or closed.

  The storage conditions for the natural gas transported in the tanks can be chosen so as to: - obtain the best price / mass ratio of gas transported (or best weight ratio of gas transported / weight of tanks); - be at a pressure below the cricondenbar of the mixture, and preferably between 5 and 15 MPa;

  - be at a temperature between +10 and -500C.

  The pressurized gas tanks are advantageously grouped into subsets of tanks. Such an arrangement makes it possible in particular to simplify the network of supply and withdrawal lines and possibly reduce the number of valves.

of connection.

  Figures 2a and 2b illustrate how to group four cylindrical bottles. Figure 2a shows a side view and Figure 2b a top view. The four cylindrical bottles are connected to a single pipe at the head 1 and at the bottom 2. This arrangement makes it possible in particular to eliminate the eight valves VB and VH which would be necessary if all the bottles were connected to the supply and withdrawal pipes so as to 20 independent. This sub-assembly can be isolated by two valves, one at the head and one at the bottom. Each subset of tanks is advantageously placed in an enclosure CR which makes it possible to thermally isolate it, as well as to protect it and position it in the vessel of the ship. This enclosure CR can be filled with an insulating material, such as for example perlite, silicate, polyurethane foam, to promote the maintenance of the required temperature in the tanks. It is also possible to "inert" each of these enclosures (to place its interior space under an inert atmosphere which does not allow combustion or explosive reactions), by filling it with nitrogen, or with another inert gas. for natural gas, such as, for example, carbon dioxide. This also makes it possible to control the tightness of the tanks, by controlling the composition of the inerting gas and by

  monitoring for any trace of methane in this gas.

  It is also possible to produce sub-assemblies comprising a larger number of reservoirs. Figure 2c shows a plan view of a group of seven

  cylindrical bottles, placed in a hexagonal enclosure. The number of tanks can be even greater and include several tens of tanks.

  To make such assemblies, the most compact arrangements are preferred so as to significantly optimize the storage density. To achieve maximum compactness, it is also possible to combine bottles of different diameters and / or sections. The transport method according to the invention makes it possible to significantly improve the storage capacity for given pressure conditions, and thus to be able to significantly reduce this transport pressure, while keeping a high storage capacity. 20 This is illustrated by the following numerical example: Numerical example: The composition of an associated natural gas transported is as follows (in mole%): nitrogen: 0.10 methane: 76, 60 ethane: 2.75 propane: 7 , 85 butane: 2.25

C5 +: 0.45

  If this natural gas is transported under pressure in a gaseous monophasic state, at

  bars and 30'C, a storage density of 246 kg / m3 is obtained.

  Using the transport method according to the invention, the same gas can be transported in the form of two phases: liquid and vapor. Referring to FIG. 3 which represents the diagram of the phases as a function of the pressure and the temperature of the associated gas as an example, the cricondenbar is referenced in P. and the cricondentherm in T, the saturation points ("dew point ") at D and the critical point C. At a pressure of 70 bars and at -40 ° C., the molar fraction in the liquid phase is 0.58. The liquid phase has a specific mass of 443 kg / m3 and the gas phase a specific mass of 128 kg / m3. The mixture of the two phases allows a density of the

  storage of 236 kg / m3. The storage density is therefore close to the storage density obtained at 20 MPa, and this for a pressure approximately three times lower. This results in the possibility, for the same quantity of gas transported, of considerably reducing the weight and the cost of the tanks. Maintaining natural gas at a temperature of around 20 -40 ° C is resolved by thermal insulation of the tanks.

  The loading and unloading operations are based on the displacement of the liquid phase, making it possible to maintain the natural gas at a constant, or almost constant, pressure and also to avoid the mixing of large quantities of natural gas and the gas of filling when the tanks are empty, for example an inert gas such as

  nitrogen or a combustion gas containing almost no more oxygen.

  In the proposed system, the tanks can be loaded or unloaded either at the head or at the bottom. The gas fractions are transferred to the top and the liquid fractions are transferred to the bottom. The use of tanks in vertical position lends itself well to the proposed loading / unloading mode. Figure 4 shows the sequences

loading or unloading.

  Before loading natural gas, at the shipping terminal, the tanks contain gas at low pressure (possibly inerting gas) and the liquid phase 10 is at its low level NB. The liquid phase used may preferably be a phase

  gazoline type hydrocarbon (light oil) which can be extracted from the associated gas.

  In a first loading phase, illustrated by FIG. 4, of the liquid

  arriving via line L2 is pumped into the tanks (VH valves closed and VB valves open), so as to put the tanks back under pressure. At the end of this phase, the liquid level has reached the high NH level.

  In a second phase of loading, the natural gas arriving via line L1 is

  transferred at constant pressure (VH and VB valves open). The liquid is gradually evacuated to the intermediate level NI which corresponds to the transport conditions. Depending on the liquid fraction transported, the excess liquid used to pressurize the tanks is returned to a storage capacity.

  During transport the valves VB and VH are closed, the phase level

  liquid corresponding to the proportion of liquid phase transported.

  On arrival at the reception terminal, in a first unloading phase, the gas is evacuated by the line LI and transferred at constant pressure (valves VB and VH open), by pumping the liquid arriving by line L2 so as to compensate for the volume of gas evacuated. When the tanks have been emptied of gas, the liquid is evacuated, 5 VB valves open and VH valves closed. The pressure in the gas phase drops to a residual level of at least about 0.3 to 0.4 MPa, sufficient to evacuate the liquid phase. The drop in pressure can lead to the appearance of a liquid phase by retrograde condensation. This liquid phase is then also evacuated via line L2. In certain cases, taking into account the quantity of gas dissolved in the liquid phase, the residual pressure may remain too high, at the end of the operation and it may be necessary in this case to pressurize the tanks with a fraction stabilized liquid, i.e. a liquid fraction whose vapor pressure is reduced, to transfer the gaseous fraction remaining in the tanks and then evacuate the stabilized liquid phase used to move the gas into a storage capacity, so that it can use it in subsequent unloading operations. Nitrogen, C02, or a gas resulting from combustion can be sent if necessary to the tanks to inert the load

  and strengthen the safety conditions during the return journey.

  The two phases are thus charged and discharged separately, the gas phase

  being charged and discharged by line Li, while the liquid phase is charged and discharged by line L2.

  In a variant, the tanks may include an impermeable membrane,

  or a piston, between the gas and the fluid so as to physically separate them.

  The invention is not limited to elongated tanks arranged vertically. Indeed, the invention relates to all the tanks having a lower access for draining the liquid and an upper access for the gaseous phase. The shape of the tanks

can therefore be variable.

  We can use conventional metal tanks or envelopes,

  metallic or plastic, reinforced by shrinking.

Claims (8)

  1) Method for transporting natural gas under pressure, characterized in that the following steps are carried out: - a plurality of tanks is connected, comprising a lower orifice and an upper orifice, connecting all the upper orifices to a first pipe and to a second pipe all the lower orifices, - said natural gas tanks are filled under a pressure lower than 10 cricondenbar of said gas and at a temperature lower than that of the ambient medium, said pressure and temperature being determined so that said natural gas is in the form of two phases: liquid and vapor, - the vapor phase transported by the upper orifices of said reservoirs is transferred, - the liquid phase transported by the lower orifices of said reservoirs is transferred. 2) The method of claim 1, wherein said gas is stored under pressure between 5 and 15 MPa.   3) Method according to one of the preceding claims wherein the temperature of the natural gas stored in said tanks is maintained between +10 and -50 ° C.   4) Method according to one of the preceding claims wherein said vacuum is emptied   reservoirs of the liquid by replacing it with an inerting gas.   ) Method according to one of the preceding claims wherein the transfer of the phase   steam takes place at substantially constant pressure.   6) Transport system for pressurized natural gas, characterized in that it comprises: - a plurality of tanks comprising a lower orifice and an upper orifice 5 connected to a first pipe by all the upper orifices and to a second pipe by all the lower orifices, - means for filling and / or emptying the tanks with natural gas by the first pipe, means for filling and / or partially emptying the tanks with a liquid phase by the second pipe, - means for maintaining the pressure of said gas at a pressure below the gas cricondenbar,   - means for maintaining the gas temperature between 100C and -500C.   7) System according to claim 6, wherein said tanks are grouped in subassembly connected to said first pipe and said second pipe.   8) The system of claim 7, wherein each subset is placed in a outer envelope.   9) The system of claim 8, wherein said envelope is filled with a thermal insulating material.   ) System according to one of claims 8 or 9, wherein said envelope is filled with an inerting gas, for example nitrogen or carbon dioxide.   11) The system of claim 10, wherein a methane detector is placed on   a leakage current from the inerting gas.