METHOD AND DEVICE FOR LIQUEFIING METHANE

Description

[0001] The invention relates to the field of liquefaction and filling of methane into a transport tank.

[0002] Natural gas and biomethane contain primarily methane. The former comes from the extractive industry, while the latter comes from the fermentation of waste.

[0003] In both cases, methane gas is liquefied to improve the energy density of the methane stream for transportation. Liquefaction of fossil natural gas is carried out in cryogenic units where the temperature of the methane is lowered to below -161°C at atmospheric pressure. The liquefied methane can be transported over long distances by ship at a realistic cost.

[0004] Thermal losses make it impossible to permanently maintain methane in a liquid state under its initial pressure. As a result, some of the methane vaporizes, and the pressure in the reservoir where the methane is stored increases accordingly. It is therefore necessary to manage the pressure increase in the reservoir. The solution then consists of extracting the evaporated methane from the reservoir. The extracted methane can be burned, re-liquefied, or used as fuel to power various turbines. This problem is the subject of extensive literature.

[0005] US Patent Application No. 2013 / 0192297 (JOHN MAK) presents a liquefaction unit. The gas stream arrives at a pressure of 31 bar, it is decontaminated then passes through a heat exchanger and is stored in a tank at a temperature of -141°C and a pressure of around 4.1 bar. In the tank, the gas fraction that evaporates is pre-cooled before joining the incoming gas stream. The liquefied gas in the tank is then injected into transport trucks at a pressure of 4.1 bar.

[0006] Australian Patent Application No. 2006241566 (SINGLE BUOY MOORINGS) describes a device for filling liquefied gas into a transport vessel commonly referred to as an LNG carrier. From an onshore station, the liquefied gas is delivered to the LNG carrier at a pipeline pressure of 1 bar. This document focuses particularly on the fraction of methane that evaporates in the pipelines, taking into account the length of these pipelines, pressure losses, and heat losses. This document describes, in particular, a device for returning the methane that has vaporized in the pipelines to the onshore unit.

[0007] These documents do not consider the energy consumption of these devices. Filling at these pressures requires that the methane be kept at a low temperature in the tanks.

[0008] The filling pressures commonly used in industry involve cooling the gas to very low temperatures. For example, at atmospheric pressure, methane is stored at a temperature below -161°C, and at 4.1 bar, it is maintained at a temperature below -141°C. However, the cooling systems that power the heat exchangers are extremely energy-intensive. One way to combat excessive energy consumption would be to liquefy the gas at a temperature higher than those mentioned above.

[0009] US2012 / 036888 describes the production of methane in a particular state (CCNG), at a very high pressure, greater than 45 bar, the pressure being greater than the critical point pressure, the temperature being lower than the critical point temperature. The process described in this prior art document uses a source of natural gas at a pressure of at least 60 psia (4.1 bar) and a temperature of at least 60°F (15°C), with a composition of approximately 95% methane, the remainder being nitrogen and carbon dioxide.

[0010] The objective is to propose a solution that allows methane to be liquefied and transported while optimizing the energy consumption of the installation.

[0011] For this purpose, a method for liquefying methane and filling a transport tank with the liquefied methane according to claim 1 is proposed firstly.

[0012] Secondly, a device for liquefying methane and filling a tank according to claim 2 is proposed, the device advantageously having the characteristics of claims 3 and 4.

[0013] Other objects and advantages of the invention will appear in the light of the description of an embodiment, given below with reference to the appended drawings in which: there figure 1 is a schematic representation of a methane liquefaction and filling device; figure 2 is a Pressure - Enthalpy diagram, with pressure represented on a logarithmic scale; figure 3a represents the variation in electrical power required in kW for 100 normal cubic meters of methane per hour at an initial temperature of approximately -100°C as a function of a given transport pressure and the liquefaction pressure. Each curve corresponds to a transport pressure while the ordinate and abscissa correspond respectively to the electrical power and the liquefaction pressure; figure 3b is similar to the figure 3a , except that the methane is initially at room temperature.

[0014] On the figure 1 A device 1 for liquefying methane and filling a transport tank 2 is schematically represented. This transport tank 2 can take different forms, namely a tanker truck, mobile silos or even a methane tanker.

[0015] In one embodiment shown in the figure 1 , the device 1 understand : a circuit 3 liquefaction of the initially gaseous methane, a circuit 4 filling liquefied methane into the tank 2 transport, a circuit 5 reinjection of a gaseous fraction of methane into the circuit 3 of liquefaction.

[0016] The circuit 3 liquefaction includes in the order of passage of the gas (represented on the figure 1 by arrows), a first interchange 6 thermal, a compressor 7, a second interchange 8 thermal, an exchanger 9 liquefaction and a pump 10. The circuit 3 liquefaction also includes a tank 11 buffer. The tank 11 buffer is intended to store liquefied methane and maintain a flow in the device 1 when the tank 2 transport is filled.

[0017] The circuit 4filling originates in the tank 11 buffer and conveys the liquefied methane to the tank 2 of transport.

[0018] The circuit 4 filling includes a first part 12 and a second part 13 connected to each other using a connector 14 injection. The first and second parts 12, 13 respectively comprise a first valve 15 stop and a second valve 16 stop. These valves 15, 16 stop allow, when the connection 14 injection valve is unlocked, to interrupt the flow of methane coming from one part of the reservoir 11 buffer and on the other hand the reservoir 2 of transport. The second part 13 further includes a regulator 17 allowing the pressure of liquefied methane leaving the tank to be lowered 11 buffer.

[0019] The circuit 5reinjection connects the reservoir 11 buffer and reservoir 2 transport at the entrance to the circuit 3 of liquefaction. Thus the fraction of methane vaporizing in the reservoir 11 buffer and in the tank 2 transport is recovered and sent to the circuit 3 liquefaction to be treated again. The tank 11 buffer includes a pressure gauge or sensor (not shown). This pressure gauge is connected to a computer unit (not shown). Thus when the pressure in the tank 11 buffer is, for one reason or another, too high the computer unit controls the pressure relief in the tank 11 buffer.

[0020] The circuit 5 reinjection connects the reservoir 11 buffer and reservoir 2 from transport to the circuit 3 of liquefaction. The circuit 5reinjection includes on the one hand a regulator 18 regulation located downstream of the reservoir 11 buffer and on the other hand a solenoid valve 19 reinjection, a valve 20 and a pressure gauge 21 (or pressure sensor) located downstream of the tank 2 transport. The solenoid valve 19 reinjection is manipulated by an actuator (not shown) which is also controlled by a computer unit (not shown). A connection 22 reinjection allows the reservoir to be separated 2 circuit transport 5 reinjection. The valves 19, 20 are arranged to block any flow of methane out of the reservoir 2 transport and out of the circuit 5 reinjection when the connection 22 reinjection is unlocked. The tank 11 buffer is permanently powered by the circuit 3liquefaction, the pressure varies slightly increasing due to the thermal inputs through the insulation of the tank 11 buffer. The regulator 18 regulation allows to regulate the pressure in the tank 11 buffer. This regulation is done by releasing the fraction of vaporized methane due to thermal losses in the reservoir 11 buffer, this gaseous methane is returned to the circuit 3 of liquefaction.

[0021] In the following, the liquefaction process will be described step by step. This process includes: a stage of liquefaction of the methane with compression of the latter and filling of the tank 11 buffer, a stage of pressure relief of the methane at the outlet of the tank 11 buffer to the tank 2 of transport.

[0022] During the liquefaction stage, gaseous methane enters the device 1at a temperature around -100°C and enters the first exchanger 6 thermal where it is heated to a temperature of around 20°C. The gaseous methane then enters a compressor 7 in order to be compressed to a pressure of 15 bar. The compressor 7 has a discharge temperature of around 80°C because it is cooled. The methane at the compressor outlet 7 is therefore at a pressure of 15 bar and a temperature of 80°C. The methane is cooled to a temperature of 20°C in a second exchanger 8 thermal. The methane then passes through the first exchanger again 6 thermal where its temperature is lowered to -90°C. The methane is still in a gaseous state at this stage. It is liquefied in the exchanger 9liquefaction, where it is cooled to a saturation temperature of around -114°C corresponding to the saturation pressure of 15 bar. The liquefied methane is then stored in the tank 11 buffer at a pressure of 15 bar and a temperature of - 114 °C.

[0023] The reservoir 2 transport is then filled with liquefied methane from the tank 11 buffer at a transport pressure. Typically, the transport pressure is between 1 and 8 bar, or the pressure in the tank 11 buffer is approximately 15 bar. Liquefied methane is fed from the tank 11 buffer to the tank 2 transport through the circuit 4 filling. In the circuit 4 filling, the pressure of the liquefied methane (15 bar) is lowered to the transport pressure (between 1 and 8 bar) thanks to the pressure reducer 17. So the tank 2transport is filled with liquefied methane at transport pressure.

[0024] The saturation pressure and temperature relationship is presented by the Pressure - Enthalpy diagram shown in the figure 2 At a pressure of 15 bar, the saturation temperature of methane is around -114°C.

[0025] It should be noted that the refrigerants are not identical in each of the heat exchangers. The first exchanger 6 thermal is cooled by the flow of gaseous methane entering the device 1. The methane being initially at a temperature close to -100°C, this temperature is used to cool the methane coming from the second exchanger 8 thermal. The second exchanger 8 heat exchanger has water as a refrigerant. The exchanger 9Liquefaction fluids use a hydrocarbon mixture as refrigerant. These fluids are particularly effective in the cryogenic industry, given their phase change temperatures.

[0026] In the tank 11 buffer, a fraction of methane vaporizes continuously taking into account thermal losses. This gaseous fraction is returned to the circuit 3 liquefaction through the circuit 5 reinjection thus maintaining a gas flow in the device 1.

[0027] At the same time, the reservoir 2 transport is filled from the tank 11 buffer at transport pressure below 8 bar. As the transport tank fills, the valve 19 regulates the pressure in the tank 2transport in order to maintain it at the desired pressure. This ensures that the passage of methane entering the tank is not obstructed. 2 transport while ensuring the safety of the installation. The tank 2 transport and tank 11 buffer includes a valve 23 safety valves triggered when the internal pressure in the tanks reaches a predetermined threshold deemed critical for the installation. The valves 23 are connected to a flare not shown.

[0028] When the tank 2 transport is filled, the valves 15, 16 injection are closed and the valves 19, 20 continue to regulate the pressure in the tank for a period of time if necessary 2 transport pressure. When this pressure is consistent with the desired transport pressure (indication given by the pressure gauge 21), the valves 19, 20are closed in turn and the connections 14, 22 are unlocked. The tank 2 transport can then be replaced by an empty tank.

[0029] Liquefying methane at pressures higher than those used for transport allows for substantial energy savings. The saturation temperature of methane at a pressure of 15 bar is higher than at a pressure of 4 bar, for example. Thus, to maintain methane in a liquid state, less energy is required because the liquefaction temperature is higher. It appears that increasing the pressure in the circuit 3 liquefaction is less energy-intensive than lowering the temperature in the same circuit.

[0030] This observation is validated by a series of calculations that we present below. The thermodynamic data of methane provided in the calculations are given by the Refprop 9 ®< software developed and marketed by the National Institute of Standards and Technology (NIST).

[0031] During liquefaction, the compression of methane gas and its cooling to a cryogenic temperature are the most energy-intensive operations.

[0032] We will therefore compare the liquefaction of methane brought to 15 bar and methane compressed to 4 bar.

[0033] The value 15 bar is not chosen arbitrarily. This value is the result of calculations carried out by the applicant. The curves of the figures 3a et 3b represent the variation in electrical power required for 100 normal cubic meters of methane per hour depending on the transport and liquefaction pressures. figure 3a presents this variation of power for a methane at a temperature of -100°C while the figure 3b applies to methane at room temperature. A study was conducted on methane at -100°C because at the outlet of the methane decontamination units and before entering the liquefaction units it is approximately at this temperature. The electrical power represented on the figures 3a et 3b combined for a unit of mass of methane to be liquefied: the electrical power of the compression of methane initially at 1 bar up to a liquefaction pressure for a compressor efficiency 7of 0.8, the electrical power required for liquefaction by taking a coefficient of performance (COP) of the refrigeration system of 30% to 40% compared to the ideal Carnot COP, the electrical power required for the reliquefaction of the evaporated fraction of methane at the moment when it is expanded to reach the transport pressure below 8 bar.

[0034] On the figures 3a, 3b the curve in line: continu corresponds to a transport pressure of 4 bar, pointillé corresponds to a transport pressure of 5 bar, mixte corresponds to a transport pressure of 6 bar, mixte long corresponds to a transport pressure of 7 bar, mixte et tiret corresponds to a transport pressure of 8 bar.

[0035] As can be seen on the figures 3a, 3b an optimum is visible at the abscissa 15 bar. The power consumed is therefore minimal at a liquefaction pressure of approximately 15 bar. This is regardless of the transport pressure for an initial methane temperature of - 100°C or at room temperature.

[0036] Methane compressed at 4 bar liquefies at a temperature of 131 K. To liquefy methane, the refrigeration system uses a refrigerant at a temperature obviously lower than the liquefaction temperature of methane. We take the example of a refrigerant at a temperature of 126 K. We consider the condensation temperature of the system at 20 °C or 293 K assuming normal climatic conditions of 15 °C. In the following calculations, the values ​​are rounded.

[0037] We calculate the coefficient of performance COP: COP = 126 293 − 126 = 0 , 75

[0038] Liquefaction is therefore ideally carried out with a refrigeration system having a COP equal to 0.75. By taking a COP of the refrigeration system of 30% compared to the ideal COP, we calculate a COP of 0.23.

[0039] In the device described, the methane being at -90°C or 163 K initially must fall below -142°C or 131 K. The energy required is 100 kJ / kg methane ( figure 2 ). To this value must be added the enthalpy of change of state of methane at a pressure of 4 bar, which is 468.4 kJ / kg methane ( figure 2 ). Liquefaction can therefore be done with a total enthalpy of 568.4 kJ / kg methane.

[0040] The electrical consumption is given to us by dividing the enthalpy by the COP, i.e.: 568 , 4 0 , 23 = 2511 kJ é lectrique / kg m é thane

[0041] Compression from 1 to 4 bar requires an energy of 183 kJ electric / kg methane. This makes a total electricity consumption of 2694 kJ electric / kg methane.

[0042] We now take the example of liquefaction at a pressure of 15 bar.

[0043] The liquefaction temperature of methane at 15 bar is -114°C or 159 K. Under the same climatic conditions as previously, the COP is given by the following relationship: 154 293 − 154 = 1 , 11

[0044] This gives us an actual COP of 0.33. Cooling from -90°C or 183 K to -114°C or 159 K requires an energy of 52 kJ / kg methane. The enthalpy of change of state is 378.8 kJ / kg methane, which makes a total cooling energy requirement of 430.8 kJ / kg methane. The required electricity consumption is: 430 , 8 0 , 33 = 1296 kJ é lectrique / kg m é thane

[0045] Compression from 1 to 15 bar requires an electrical energy of 526.5 kJ / kg methane. In total, an energy of 1822.5 kJ electric / kg methane is therefore required.

[0046] This liquefaction process therefore allows for an electrical energy saving of almost 48%. On an industrial scale, this process therefore allows for a considerable reduction in energy bills, reducing them by around half.

[0047] This process can also be applied to gases from natural deposits. In this case, it is possible to take advantage of the pressure of the deposit in order to avoid the compression step. Liquefaction can therefore be carried out at a pressure of 15 bar without first compressing the methane.

[0048] Advantageously, the relaxation of the pressure of the methane coming from the reservoir 11 buffer is done as close as possible to the tank 2 of transport. This provision applies in the case where the tank 2transport is an LNG carrier. It is common in port facilities for cryogenic units to be located far from the LNG carrier filling terminals. Thus, liquid methane travels through pipelines, sometimes for several kilometers, before being stored in the LNG carrier. The pipelines are generally underwater or buried and are not refrigerated. As a result, some of the transported methane vaporizes. The idea is to transport the methane at a pressure of around 15 bar and expand it just before entering the LNG carrier. Thus, the temperature difference between the methane and the external environment is not as high as if the methane were transported at lower pressures.

[0049] Another solution can however be considered. This involves carrying out an expansion stage. Indeed, for various reasons, it is possible that transporting methane at a pressure of 15 bar is not possible. Consequently, a first expansion can advantageously be carried out at the outlet of the buffer tank 11. The methane pressure is then lower than 15 bar but higher than the transport pressure. Thus, the methane is transported to the LNG carrier before being expanded again before entering it. This prevents too large a fraction of methane from vaporizing in the pipelines. Overall, the efficiency according to this other solution, which is not covered by the present invention, is improved since less methane vaporizes; this methane is then not sent back to the cryogenization unit to be liquefied again. The unit will therefore consume less energy.

Claims

1. A method for liquefying methane and filling a transport tank with liquefied methane, comprising: - a step of providing gaseous methane to a liquefaction device (1) comprising a liquefaction circuit, - a step of cooling the gaseous methane to its saturation temperature, in the liquefaction device (1), so as to obtain liquid methane, - a step of filling the transport tank (2) with the liquefied methane, comprising an expansion step during which in the transport tank (2), a fraction of the liquid methane transforms into gaseous methane, - the step of filling the transport tank (2) with liquefied methane at a transport pressure, is carried out from a buffer tank (11) included in the liquefaction circuit, - in the step of filling the transport tank (2), the liquid methane is introduced into the transport tank (2) by lowering its pressure to the transport pressure, this transport pressure being less than or equal to 8 bar, the fraction of liquid methane that has transformed into gaseous methane in the transport tank (2) and the fraction of methane vaporising in the buffer tank being returned to the liquefaction circuit, characterised in that - in the step of providing the gaseous methane to the liquefaction device, the gaseous methane comes into the liquefaction device (1) at a temperature of -100°C, - the step of cooling the gaseous methane to its saturation temperature is carried out at a pressure equal to 15 bar, - a step of filling the buffer tank (11) with liquefied methane is carried out, at a pressure of 15 bar and a temperature of -114°C, the gaseous methane entering the liquefaction circuit being heated to a temperature of about 20°C in a first heat exchanger (6), and then compressed to a pressure substantially equal to 15 bar by means of a compressor and to a temperature of 80°C, and then cooled to a temperature of 20°C in a second heat exchanger (8), and then cooled to a temperature of -90°C in the first heat exchanger (6), the gaseous methane obtained being then cooled and liquefied to a saturation temperature of approximately -114°C corresponding to the saturation pressure of 15 bar, and the gaseous methane from the transport tank and the buffer tank is returned upstream of the first heat exchanger (6).

2. A device (1) for liquefying methane and filling a tank (2) for transporting liquefied methane, enabling the methane liquefaction method according to the preceding claim to be implemented, the device (1) comprising: - an initially gaseous methane liquefaction circuit (3), - a circuit (4) for filling the liquefied methane in the transport tank (2), - a buffer tank (11) included in the liquefaction circuit (3), - a circuit for reinjecting the gaseous methane fraction sourced from the transport tank (2) and in the buffer tank (11) to return the gaseous methane to the methane liquefaction circuit, the filling circuit (4) sourced from the buffer tank (11) and conveying the liquefied methane to the transport tank (2), the liquefaction circuit (3) comprising at least one first heat exchanger (6) to heat the gaseous methane to a temperature of about 20°C, the liquefaction circuit (3) comprising a second heat exchanger (8), and a liquefaction exchanger (9), to liquefy the gaseous methane and means for relieving the pressure between the buffer tank (11) and the transport tank (2), this device (1) being characterised in that the liquefaction circuit (3) comprises a compressor (7), this compressor (7) being arranged to compress the gaseous methane exiting the first heat exchanger (6) at a pressure substantially equal to 15 bar and at a temperature of about 80°C, the second heat exchanger (8) cooling the methane exiting the compressor (7) to a temperature of 20°C, the methane exiting the second exchanger (8) passing through the first heat exchanger (6) to be cooled to a temperature of - 90°C, the gaseous methane exiting the second heat exchanger (8) being cooled and liquefied in the liquefaction exchanger (9) to a temperature of approximately -114°C corresponding to a saturation pressure of 15 bar, and the circuit for reinjecting the gaseous methane fraction conveys the gaseous methane upstream of the first heat exchanger (6).

3. The device according to claim 2, characterised in that it comprises a circuit (5) for reinjecting the gaseous methane fraction, this reinjection circuit (5) comprising means for measuring pressure in the transport tank (2) and in the buffer tank (11), these means being in the form of a pressure gauge (21) or a pressure sensor.

4. The device according to one of claims 2 or 3, characterised in that the device (1) comprises a computing unit programmed to execute the steps of the method according to claim 1.