Process for obtaining liquid CO2

Abstract

------ Process for obtaining liquid CO2 To obtain liquid CO2 from a gaseous mixture of CO2, CH4 and impurities, a Joule-Thomson expansion is carried out on the starting gaseous mixture after it has been compressed (in C2) and then cooled and the mixture resulting from the Joule-Thomson expansion is distilled under temperature and pressure conditions making it possible to obtain liquid CO2 at the outlet at the bottom of the distillation column (CD). According to the invention, the compressed mixture (in C2) is cooled (in A2); the cooled mixture (in A2) is cooled (in H3); the cooled mixture (in H3) is cooled (in H4) by heat transfer to the boiler associated with the distillation column (CD), the cooled mixture (in H4) then being sent to the Joule-Thomson expansion. Figure to be published with the abstract: Figure 2

Description

Title of the invention: Process for obtaining liquid CO2

[0001] The present invention relates to a process for obtaining liquid CO2 from a gaseous mixture of CO2, CH4 and unavoidable impurities, this gaseous mixture being for example derived from a biogas purification process, in particular by membrane separation.

[0002] Biogas is a gas produced by fermentation of organic matter and composed mainly of methane and carbon dioxide. Although biogas can be directly burned to produce heat and electricity (cogeneration), it is also possible to treat it in order to recover its two main components CH4 and CO2 separated and purified.

[0003] Document FR3114516 discloses a process for purifying biogas which makes it possible to obtain liquid carbon dioxide.

[0004] Generally, it is desirable to improve the energy performance of a process in order to reduce the energy consumption required to achieve a similar or better result.

[0005] The applicant company therefore sought to optimize the energy performance of the process for purifying a gaseous mixture of CO2, CH4 and unavoidable impurities in order to obtain pure liquid CO2 with consequently lower energy consumption for the entire process.

[0006] To this end, the present invention relates to a process for obtaining liquid CO2 from a gaseous mixture of CO2, CH4 and unavoidable impurities, a process according to which a Joule-Thomson expansion of the starting gaseous mixture is carried out after it has been compressed (in C2) then cooled and the mixture resulting from the Joule-Thomson expansion is distilled under temperature and pressure conditions making it possible to obtain liquid CO2 at the outlet at the bottom of the distillation column,

[0007] characterized by the fact that: • cooling of the compressed mixture (in C2) is carried out (in A2); • cooling is carried out (in H3), using an external cold source, of the cooled mixture (in A2), a part of said cooled mixture (in A2) may have been, before being cooled (in H3), subjected to cooling (in H2) by heat transfer to the boiler associated with the distillation column; • cooling of the cooled mixture (in H3) is carried out (in H4) by heat transfer to the boiler associated with the distillation column, the cooled mixture (in H4) then being sent to the Joule-Thomson expansion, part of the cooled mixture (in H3) being able to be directly sent to the Joule-Thomson expansion without passing through cooling (in H4).

[0008] The gaseous mixture of CO2, CH4 and unavoidable impurities can come from a biogas purification process, in particular by membrane separation.

[0009] The working pressure of the compressor in C2 can be between 20 and 60 barA.

[0010] The compression (in C2) of the starting gas mixture can be preceded by a prior compression (in Cl) of the mixture; cooling (in Al), if necessary followed by a heat exchange (in Hl) with a cold group; and drying to eliminate the water present in the initial gas mixture.

[0011] Cooling (in A1) and cooling (in A2) can in particular each be carried out using an air cooler.

[0012] A portion of the cooled mixture (in A2) may be subjected to cooling (in H2) by heat transfer to the boiler associated with the distillation column before being cooled (in H3).

[0013] The temperature of the external cold source can be between -30°C and 15°C, being in particular 0°C.

[0014] Liquid CO2 exiting the distillation column can be stored at a pressure of 10-25 barA, and CO2 that vaporizes during this storage can be recycled into the starting gas stream before compression to C2. In particular, CO2 that vaporizes during storage can be recycled into the stream before drying, or after cooling to AL

[0015] Condensation of the vapors leaving the top of the distillation column can be carried out by heat exchange with a cold group, with a view to returning them to the distillation column, the non-condensable vapors being evacuated.

[0016] The temperature at the bottom of the distillation column can be adjusted between -40°C and -12°C. This adjustment can depend in particular on the working pressure of the distillation column in order to obtain the desired purity of the liquid CO2.

[0017] According to a first embodiment of the regulation of the method according to the invention: • a first flow separation device is provided, such as a three-way valve, arranged on the path of the current leaving the cooling in A2 and comprising an outlet towards the heat transfer to the boiler in H2 and an outlet towards the cooling in H3; and a second flow separation device, such as a three-way valve, arranged on the path of the current leaving the cooling in H3 and comprising an outlet towards the heat transfer to the boiler in H4 and an outlet towards the Joule-Thomson expansion; • the opening setpoint value of the second flow separation device for heat transfer to the boiler in H4 is set at 50 - 100%; • the CO2 purity target value is set at 99 - 99.99% by volume liquid at the outlet at the bottom of the distillation column; • the opening of the second flow separation device towards the heat transfer to the boiler is measured in H4; • if the opening value of the second flow separation device towards the heat transfer to the boiler in H4 is lower than its set value then: • the opening of the first flow separation device is reduced towards the heat transfer to the H2 boiler; • the purity of liquid CO2 is measured at the outlet at the bottom of the distillation column; • the opening of the second flow separation device towards the heat transfer to the boiler in H4 is increased until the purity of liquid CO2 measured at the outlet at the bottom of the distillation column corresponds to its set value.

[0018] According to a second embodiment of the regulation of the method according to the invention: • a first flow separation device is provided, such as a three-way valve, arranged on the path of the current leaving the cooling in A2 and comprising an outlet towards the heat transfer to the boiler in H2 and an outlet towards the cooling in H3; and a second flow separation device, such as a three-way valve, arranged on the path of the current leaving the cooling in H3 and comprising an outlet towards the heat transfer to the boiler in H4 and an outlet towards the Joule-Thomson expansion; • the opening setpoint value of the second flow separation device for heat transfer to the boiler in H4 is set at 50 - 100%; • the set value for the purity of liquid CO2 at the outlet at the bottom of the distillation column is set at 99 - 99.99% by volume; • the temperature T and the pressure P of the outlet stream at the bottom of the distillation column are measured; • we choose from a set of binary CH4 / CO2 diagrams, each corresponding to a known pressure, the one which corresponds to the measured pressure P; • we calculate the temperature Tbune which corresponds to the measured pressure P and for the desired purity of liquid CO2; • the opening of the second flow separation device towards the heat transfer to the boiler is measured in H4; • if the opening value of the second flow separation device towards the heat transfer to the boiler in H4 is lower than the set value then: • the opening of the first flow separation device is reduced towards

[0019]

[0020]

[0021]

[0022]

[0023] heat transfer to the H2 boiler; • the temperature T of the liquid CO2 is measured at the outlet at the bottom of the distillation column; • the opening of the second flow separation device towards the heat transfer to the boiler is increased in H4 until the temperature value T of the liquid CO2 measured at the outlet at the bottom of the distillation column corresponds to the temperature value T buiie determined. In this second embodiment, a device for measuring the quality of the CO2 produced at the outlet at the bottom of the distillation column is not used. [Fig.l] is a schematic representation of an installation for obtaining liquid CO2 according to the prior art described in FR3114516. [Fig.2] is a view similar to [Fig.l] of a schematic representation of an installation for obtaining liquid CO2 according to the invention. [Fig.3] is a schematic representation of the installation of [Fig.2] on which other measuring elements at the outlet at the bottom of the distillation column have been shown, as well as the various regulation diagrams. If we refer to [Fig.l], we can see that the installation comprises the following arranged in succession: • a compressor Cl capable of receiving, via an inlet pipe 1, a gas stream enriched with CO2 which is a permeate resulting from a membrane separation of biogas and for which the aim is to achieve additional purification in CO2 and the obtaining of liquid CO2; • an air cooler Al whose inlet is connected by a pipe 2 to the outlet of the compressor Cl; • a heat exchanger H1 whose inlet is connected by a pipe 3 to the outlet of the air cooler Al, and which ensures the cooling of the introduced stream using a circuit 4 including a cold unit GF1 maintaining the circuit 4 for example at 0-4°C; the heat exchanger H1 is optionally equipped with a device for recovering and evacuating the condensates generated during the cooling of the stream entering it; • a dryer S receiving, via a pipe 5, the cooled stream leaving the heat exchanger H1; • a compressor C2 capable of receiving, via an inlet pipe 6, the current leaving the dryer S; • an air cooler A2 whose inlet is connected by a pipe 7 to the outlet of the compressor C2; • an H2 heat exchanger whose inlet is connected, by a pipe 8, to the outlet of the air cooler A2 and which will supply heat to a distillation column CD, in other words which serves as a boiler for said column CD; • a Joule-Thomson JT expansion valve which receives the cooled stream leaving the H2 exchanger via a pipe 9 and from which a relaxed stream introduced as a charge at the top of the column CD leaves via a pipe 10.

[0024] The steam leaving the top of the column CD is sent to a condenser CO via a pipe 11, providing a condensate which is returned via pipe 12 to the column CD and incondensables evacuated from the system via a pipe 13. The condenser CO is cooled by the refrigeration unit GF2 using the circuit 14 maintained for example at -30°C.

[0025] At the bottom of the column CD, the liquid CO2 is withdrawn via a pipe 15, to be sent to a valve Vtl for regulating the level of liquid CO2 at the bottom of the column CD, then is sent, at the outlet of the valve Vtl via a pipe 16, to a storage tank R (for example at a pressure of 19 barA).

[0026] From the reservoir R, the liquid CO2 can be evacuated to a transport via a pipe 17. During this storage, a part of the liquid CO2 passes into the gaseous phase in particular because of the expansion carried out when passing the valve Vtl (the “boil-off” phenomenon), this fraction then being returned in a pipe 18 to an expansion valve Vt2 to reduce the pressure before being readdressed via the pipe 19 into the pipe 3 for recycling.

[0027] The installation according to the invention shown in [Fig.2] differs from that which has just been described with reference to [Fig.l] by the presence: • a first three-way valve V1 arranged on the path of the current passing through the pipe leaving the air cooler A2; • another heat exchanger H3 which recovers the current leaving the exchanger H2 and the part of the current which comes from A2 and which has not been sent to the exchanger H2; • a second three-way valve V2 located on the outlet of the heat exchanger H3; and • yet another heat exchanger H4 which serves as a boiler for the CD column and which returns the cooled stream to the Joule-Thomson expansion valve JT.

[0028] We will now describe in more detail the path of the current to be purified between the air cooler A2 and the Joule-Thomson expansion valve JT. We will not describe further the part upstream of the air cooler A2 and the part downstream of the valve JT which are identical to those of [Fig.l] and for which the same reference numbers are used.

[0029] Valve V1 allows part of the current coming from pipe 8 to be addressed: • by a pipe 20, to the H2 exchanger; and • the remaining part via a pipe 21 to the heat exchanger H3, the latter also receiving via line 22 the cooled stream coming from H2, the combined streams arriving at H3 via line 23.

[0030] The heat exchanger H3 is cooled by the stream 24 on which a cold source SF is arranged, being for example at a temperature of 0°C. This cold source can be a geothermal source or a cold group.

[0031] Valve V2 makes it possible to address part of the current coming from pipe 25 at the outlet of exchanger H3, • via line 26, to heat exchanger H4, and • via line 27, to the Joule-Thomson JT expansion valve, the latter also receiving via line 28 the cooled stream from H4, the combined streams arriving at valve JT via line 29.

[0032] On the path of the pipe 15, an AT analyzer has been placed, the role of which is indicated below.

[0033] In the present invention, the mixture is condensed before the Joule-Thomson expansion valve using heat exchangers H2 and H3. The condensed liquid is then subcooled using exchanger H4, which makes it possible to introduce the liquid into the distillation column after the Joule-Thomson expansion at a low temperature making it possible to carry out the distillation at a lower pressure than in the case of the installation of [Fig.l].

[0034] Carrying out the distillation at a lower pressure also has the advantage of requiring less stress on the GF2 cold group for the condensation of the vapors at the top of the CD column.

[0035] Another advantage is a lower pressure difference between the distillation and storage of liquid CO2, which makes it possible to reduce the boil-off phenomenon and thus the recycling of the vaporized CO2.

[0036] The process control strategies will now be described with reference to Figures 2 and 3.

[0037] In [Fig.2], two regulatory organs OR1 and OR2 are shown for which the following notations are used: • SP_INT: Internal setpoint of the master regulator • SP_EXT: External adjustment point (setpoint) of the slave regulator corresponding to the output of the master regulator • SP_MAX: Maximum set point • MEASUREMENT: Measurement entering the regulator • OUT: Regulator output • 0UT_MAX: Maximum output of the regulator • OUT_MIN: Minimum output of the regulator

[0038] The members OR1 and OR2 aim to maximize the subcooling of the liquid upstream of the Joule-Thomson expansion valve JT in order to minimize the energy supplied to the condenser by the refrigeration unit GF2. The maximum of the flow must therefore be sent to H4 which implies an opening instruction for the valve V2 at the largest possible opening, for example 99%. The rest of the energy to be supplied to the boiler is exchanged by H2. The purity instruction of the liquid CO2 at the bottom of the distillation column CD determines the energy to be supplied to the boiler and therefore the opening of VI in order to supply the necessary energy while guaranteeing the maximum opening of V2.

[0039] The value of the purity of the CO2 at the outlet at the bottom of the distillation column CD is entered into the control unit OR1 as the value SP_INT. The set value for opening the second three-way valve V2 is entered into the control unit OR2 as the value SP_INT.

[0040] The opening value of the second three-way valve V2 is measured and sent to the regulator OR2 as a MEASUREMENT value. As long as the opening value of V2 is lower than its set value, this means that it is possible to pass more flow through V2 to the exchanger H4.

[0041] The regulation organ OR2 will therefore send as signal OUT a signal to reduce the opening of the first valve VI to the exchanger H2.

[0042] The quantity of heat exchanged by H2 and H4 at the bottom of the distillation column CD will therefore decrease and the purity of the CO2 at the outlet will also decrease. This purity value is measured by the analyzer AT. The analyzer AT sends this purity value to the regulating member OR1 as a MEASUREMENT signal. In response, the regulating member OR1 will send as an OUT signal an order to open the valve V2 to the heat exchanger H4 until the purity value of the CO2 at the outlet at the bottom of the distillation column CD corresponds to the set value.

[0043] The installation according to the invention shown in [Fig.3] differs from that which has been described with reference to [Fig.2] by the presence of a pressure sensor PT_CO2 and a temperature sensor TT_CO2 arranged on the path of the pipe 15 and the role of which is indicated below.

[0044] The installation also has two regulating organs OR3 and OR4 replacing the regulating organ OR1 and whose role will be indicated below.

[0045] The objective of the regulation of the installation presented in [Fig.3] is identical to that of [Fig.2]. The regulation differs only by the measurement which is made to determine the purity of the liquid CO2 at the outlet at the bottom of the CD distillation column.

[0046] In a first embodiment called predictive mode, the regulation member OR3 is inactive. In this embodiment, the purity of the CO2 is not measured di directly by the AT analyzer but is determined using the pressure and temperature of the liquid CO2 stream leaving the bottom of the CD distillation column.

[0047] At the outlet at the bottom of the distillation column, the fluid is in liquid form at the boiling temperature and is therefore on the bubble curve of the CO2 / CH4 binary diagram. For a pressure at the outlet at the bottom of the distillation column CD, there is a CO2 / CH4 binary diagram at this pressure. Knowing the CO2 purity at the outlet at the bottom of the distillation column makes it possible to go back to the temperature value which corresponds by reading on the bubble curve to the purity considered. We then obtain the temperature Tbune which corresponds to the pressure at the outlet at the bottom of the distillation column CD and to the desired CO2 purity.

[0048] The outlet pressure at the bottom of the distillation column is therefore measured by the PT_CO2 sensor. Based on this pressure, the corresponding CO2 / CH4 diagram is determined and the temperature value Tbune corresponding to the desired CO2 purity is determined.

[0049] This temperature Tbuue is entered into the regulation organ OR4 as value SP_INT.

[0050] In the same way as in the case illustrated in [Fig.2], the opening setpoint value of the second three-way valve V2 is entered into the control unit OR2 as the value SP_INT. The opening value of the second three-way valve V2 is measured and sent to the regulator OR2 as the MEASUREMENT value. The control unit OR2 will therefore send as the OUT signal a signal for reducing the opening of the first valve VI to the exchanger H2.

[0051] The analyzer TT_CO2 measures the temperature at the outlet at the bottom of the distillation column CD and sends this temperature to the control member OR4 as a MEASUREMENT signal. In response, the control member OR1 will send as an OUT signal an order to open the valve V2 to the heat exchanger H4 until the temperature of the CO2 at the outlet at the bottom of the distillation column CD corresponds to the set value Tbune.

[0052] The value of Tbune can be calculated by a computer called a T_bulle computer, which allows the system to operate in a so-called automatic mode. This computer allows continuous automation of the determination of the bubble temperature on the binary CO2 / CH4 diagram, knowing the pressure measurement at the outlet at the bottom of the column and the desired CO2 purity.

[0053] The value of Tbune can also be entered directly by the operator, the system then operating in manual mode, without control of the CO2 purity at the column outlet.

[0054] In a second embodiment called analysis mode, the regulation organ OR3 is active.

[0055] The regulation organ OR3 receives as setpoint value SP_INT the purity value of the CO2 at the outlet at the bottom of the distillation column.

[0056] In this regulation mode, the regulation member OR3 determines the temperature T buiie and sends it as signal OUT to the regulation member OR4 which receives it as signal SP_EXT.

[0057] Then, the regulation is carried out as in the predictive mode by measuring the temperature at the outlet at the bottom of the distillation column CD and acting on the valves VI and V2 until the measured CO2 purity corresponds to the CO2 purity setpoint.

[0058] The following examples illustrate the present invention without, however, limiting its scope.

[0059] Reference Example 1

[0060] The installation described in [Fig.l] is used. The ambient temperature considered is 10°C and the energy optimum corresponds to compression at 35 barA by compressor C2.

[0061] Table 1 indicates the pressure and temperature conditions as well as the molar compositions of each of the streams in the pipes considered.

[0062] [Tables 1] Line 1 2 3(before recycling) 3 (after recycling) 5 6 Pressure (barA) 1.11325 10.7 10.6 10.6 10.5 10 Temperature (°C) 20 160 20 13.0785 12.9612 12.9612 Flow rate (mol / h) 4495.03 4495.03 4495.03 5185.94 5185.94 5163.47 Mole fraction ch4 0.015 0.015 0.015 0.0130022 0.0130022 0.0130588 Mole fraction co2 0.975 0.975 0.975 0.97833 0.97833 0.982588 O2 mole fraction 0.001 0.001 0.001 0.0008667 76 0.0008667 76 0.0008705 49 N2 mole fraction 0.004 0.004 0.004 0.0034670 9 0.0034670 9 0.0034821 8 H2 mole fraction 0 0.005 0.005 0.005 0.0043338 6 0.0043338 6 0 Conduit 7 8 9 10 13 15 Pressure (barA) 35 34.9 34.9 31 31 31.026 Temperatu re (°C) 160 20 -3.24255 -7.32554 -29.1937 -4.58889 Flow rate (mol / h) 5163.47 5163.47 5163.47 5163.47 195.849 4967.62 Mole fraction ch4 0.0130588 0.0130588 0.0130588 0.0130588 0.344263 9.94E-07 Mole fraction 0.982588 0.982588 0.982588 0.982588 0.540979 0.999999 co2 Mole fraction O2 0.0008705 49 0.0008705 49 0.0008705 49 0.0008705 49 0.0229515 6.03E-09 Mole fraction N2 0.0034821 8 0.0034821 8 0.0034821 8 0.0034821 8 0.091806 5.71E-11 Mole fraction H2 O 0 0 0 0 0 0 Pipe 16 17 18 19 Pressure (barA) 19 19 19 10.7 Temperature (°C) -21.2978 -21.2978 -21.2978 -35.8436 Flow rate (mol / h) 4967.62 4276.71 690.911 690.911 Mole fraction ch4 9.94E-07 4.33E-07 4.47E-06 4.47E-06 Mole fraction co2 0.999999 1 0.999996 0.999996 Mole fraction O2 6.03E-09 2.23E-09 2.95E-08 2.95E-08 Mole fraction N2 5.71E-11 1.38E-11 3.25E-10 3.25E-10 Mole fraction H2 O 0 0 0 0

[0063] Example 2 of the invention

[0064] The installation described in [Fig.2] is used. The ambient temperature considered is 10°C and the energy optimum corresponds to compression at 44 barA by compressor C2.

[0065] Table 2 indicates the pressure and temperature conditions as well as the molar compositions of each of the streams in the pipes considered. 12

[0066] [Tables2] Line 1 2 3(before recycling) 3 (after recycling) 5 6 Pressure (barA) 1.11325 10.7 10.6 10.6 10.5 10 Temperature (°C) 20 160 20 19.3885 19.2753 19.2753 Flow rate (mol / h) 4495.03 4495.03 4495.03 4549.7 4549.7 4527.23 Mole fraction ch4 0.015 0.015 0.015 0.0148199 0.0148199 0.0148934 Mole fraction co2 0.975 0.975 0.975 0.9753 0.9753 0.980142 O2 mole fraction 0.001 0.001 0.001 0.0009879 84 0.0009879 84 0.0009928 89 N2 mole fraction 0.004 0.004 0.004 0.0039519 3 0.0039519 3 0.0039715 5 H2 mole fraction 0 0.005 0.005 0.005 0.0049399 2 0.0049399 2 0 Pipe 7 8 23 25 29 10 Pressure (barA) 43.6436 43.5436 43.5436 43.4436 43.4436 20 Temperature (°C) 160 20 7.07341 2 -19.1656 -25.622 Flow rate (mol / h) 4527.23 4527.23 4527.23 4527.23 4527.23 4527.23 Mole fraction ch4 0.0148934 0.0148934 0.0148934 0.0148934 0.0148934 0.0148934 Mole fraction 0.980142 0.980142 0.980142 0.980142 0.980142 0.980142 co2 Mole fraction O2 0.0009928 89 0.0009928 89 0.0009928 89 0.0009928 89 0.0009928 89 0.0009928 89 Mole fraction N2 0.0039715 5 0.0039715 5 0.0039715 5 0.0039715 5 0.0039715 5 Mole fraction H2 O 0 0 0 0 0 0 Pipe 13 15 16 17 18 19 Pressure (barA) 20 20.026 19 19 19 10.7 Temperature (°C) -29.319 -19.6091 -21.298 -21.298 -21.298 -35.8437 Flow rate (mol / h) 384.102 4143.13 4143.13 4088.45 54.6724 54.6724 Mole fraction ch4 0.175531 9.94E-07 9.94E-07 8.85E-07 9.13E-06 9.13E-06 Mole fraction co2 0.765956 0.999999 0.999999 0.999999 0.999991 0.999991 Mole fraction O2 0.0117026 5.60E-09 5.60E-09 4.82E-09 6.40E-08 6.40E-08 Mole fraction N2 0.0468108 1.76E-11 1.76E-11 1.36E-11 3.21E-10 3.21E-10 Mole fraction H2 O 0 0 0 0 0 0

[0067] Example 3 Calculation of Specific Electricity Consumption

[0068] The specific electricity consumption, i.e. the quantity of electricity consumed to obtain 1 kg of liquid CO2 at the desired purity, is calculated for each of Examples 1 and 2.

[0069] The following assumptions were used: • pressure drop in the dryer S: 0.5 bar • consumption of the S dryer: 3.5 kWe • isentropic compressor efficiency Cl: 0.62 • isentropic efficiency of compressor C2: 0.67 • consumption of air coolers Al and A2: 1 kWe each for an air flow rate less than or equal to 6000 Nm3 / h • consumption of auxiliaries (solenoid valves, regulators, instrumentation): 2kWe • coefficient of performance of the 0 / 4°C GF1 cooling unit: COP_GF1 = -0.07214*T_ambient+24.9615 • coefficient of performance of the cold group -30°C GF2: COP_GF2 = -0.0292*T_ambient+10.20498

[0070] The formula for calculating the theoretical specific consumption of Example 1 CSP1 is:

[0071] CSP1 = (PuCl + PuC2 + PuAl + PuA2 + PuS + PuV + PuHl / COP_GFl + PuCO / COP_GF2) / mC02

[0072] The formula for calculating the theoretical specific consumption of Example 2 CSP2 is:

[0073] CSP2 = (PuCl + PuC2 + PuAl + PuA2 + PuS + PuV + (PuHl + PuH3) / COP_GFl + PuCO / COP_GF2) / mC02

[0074] Formulas in which: • PuCl is the electrical power consumed by the compressor Cl • PuC2 is the electrical power consumed by compressor C2 • PuAl is the electrical power consumed by the Al air cooler • PuA2 is the electrical power consumed by the air cooler A2 • PuS is the electrical power consumed by the dryer S • PuV is the electrical power consumed by the auxiliaries (solenoid valves, regulators, instrumentation) • PuHl is the thermal power exchanged by the exchanger H1 • PuH3 is the thermal power exchanged by the H3 exchanger • PuCO is the thermal power exchanged by the condenser of the column of distillation • mC02 is the mass flow rate of CO2 in line 17 (CO2 actually recovered)

[0075] Table 3 below shows the values ​​of the different elements.

[0076] [Tables3] Example 1 Example 2 %CO2 in the incoming gas (% vol) 97.50% 97.50% %CH4 in the incoming gas (% vol) 1.50% 1.50% %O2 in the incoming gas (% vol) 0.10% 0.10% %N2 in the incoming gas (% vol) 0.40% 0.40% %H2O in the incoming gas (% vol) 0.50% 0.50% Compressor working pressure (barA) 35.00 43.64 Column working pressure (barA) 31 20 Incoming gas flow rate (Nm3 / h) 100.00 100.00 Liquid CO2 storage pressure (barA) 19 19 Liquefied CO2 flow rate (kg / h) 188.22 179.93 Boil-off recycling rate 14.98 % 0.81 % Recycling rate of non-condensables 2.42 % 6.7% Cl Electrical energy consumption (kWe) 14.20 14.20 C2 Electrical energy consumption (kWe) 6.94 7.53 Al High temperature (°C) 160.0 160.0 Low temperature (°C) 20.0 20.0 Thermal energy consumption (kWh) 7.39 7.39 Electrical energy consumption (kWe) 1.00 1.00 A2 High temperature (°C) 160.0 160.0 Low temperature (°C) 20.0 20.0 Thermal energy consumption (kWh) 9.39 8.73 Electrical energy consumption (kWe) 1.00 1.000 H1 High temperature (°C) 13.1 19.4 Low temperature (°C) 13.0 19.3 , Thermal energy consumption (kWh) 0.00 0.00 H2 High temperature (°C) 20.0 20.0 Low temperature (°C) -4.16 7.1 Thermal energy consumption (kWh) 14.38 2.60 H3 High temperature (°C) - 7.1 Low temperature (°C) - 2.0 Thermal energy consumption (kWh) 0.00 10.32 H4 High temperature (°C) - 2.0 Low temperature (°C) - -19.19 Thermal energy consumption (kWh) 0.000 3.02 CO (condenser) High temperature (°C) -29.0 -29.0 Low temperature (°C) -29.0 -29.0 Thermal energy consumption (kWh) 16.05 4.19 Boiler High temperature (°C) -4.16 -19.19 Low temperature (°C) -4.16 -19.19 Thermal energy consumption (kWh) -14.38 -5.62 Auxiliaries (valves, regulators, instrumentation) Electrical energy consumption (kWe) 2.00 2.00 Dryer S Electrical energy consumption 3.50 3.50 Ambient temperature (°C) 10.00 10.00 COP cooling unit -30°C GF2 1.94 1.94 COP cooling unit 0 / 4°C GF1 4.53 4.53 Total electrical energy consumption (kWhe) 36.9 33.7 Specific consumption (kWhe per Nm3 of incoming gas) 0.369 0.337 Specific consumption (kWhe per kg of liquefied CO2) 0.196 0.187

[0077] As can be seen in Table 3, the specific electricity consumption for Example 1 is 0.196 kWhe / kgCO2 while it is 0.187 kWhe / kgCO2 for Example 2, which corresponds to a difference of 5%.

[0078] Table 4 compares the specific consumption values ​​as a function of ambient temperature.

[0079] [Tables4] Specific electricity consumption (kWhe per kg of liquid CO2) Ambient temperature (°C) Example 1 Example 2 Difference (%) 10 0.196 0.187 5% 20 0.208 0.194 7% 40 0.247 0.217 12%

[0080] As can be seen in Table 4, the higher the ambient temperature, the more the consumption difference is in favor of the process using the installation according to the present invention.

Claims

Claims

1. - Process for obtaining liquid CO2 from a gaseous mixture of CO2, CH4 and unavoidable impurities, process according to which a Joule-Thomson expansion of the starting gaseous mixture is carried out after it has been compressed and then cooled and the mixture resulting from the Joule-Thomson expansion is distilled under temperature and pressure conditions making it possible to obtain liquid CO2 at the outlet at the bottom of a distillation column (CD), characterized in that: • a cooling, called "first cooling", of the compressed mixture is carried out in A2; • a cooling, called "third cooling", is carried out in H3 using an external cold source, of the mixture cooled in A2, a portion of said mixture cooled in A2 possibly having been, before being cooled in H3, subjected to a cooling, called "second cooling", in H2 by heat transfer to the boiler associated with the distillation column (CD);• in H4, a cooling, called “fourth cooling”, of the mixture cooled in H3 is carried out by heat transfer to the boiler associated with the distillation column (CD), the mixture cooled in H4 then being sent to the Joule-Thomson expansion, part of the mixture cooled in H3 being able to be sent directly to the Joule-Thomson expansion without going through the cooling in H4.;

2. - Method according to claim 1 characterized in that the gaseous mixture of CO2, CH4 and unavoidable impurities comes from a biogas purification process, in particular by membrane separation.

3. - Method according to one of claims 1 and 2, characterized in that the working pressure of the compressor is between 20 and 60 barA.

4. - Method according to one of claims 1 to 3, characterized in that the compression of the starting gas mixture is preceded by a preliminary compression of the mixture; by a cooling, called "preliminary cooling", in Al, where appropriate followed by a heat exchange, called "preliminary heat exchange", in H1 with a cold group (GF1); and drying to eliminate the water present in the initial gas mixture.

5. - Method according to claim 4, characterized in that the pre-cooling in A1 and the cooling in A2 are each carried out using an air cooler.

6. - Method according to one of claims 1 to 5, characterized in that a part of the mixture cooled in A2 is subjected to cooling in H2 by heat transfer to the boiler associated with the distillation column (CD) before being cooled in H3.

7. - Method according to one of claims 1 to 6, characterized in that the temperature of the external cold source is between -30°C and 15°C, being in particular 0°C.

8. - Method according to one of claims 1 to 7, characterized in that the liquid CO2 leaving the distillation column (CD) is stored at a pressure of 10 - 25 barA, and that the CO2 which vaporizes during this storage is recycled in the starting gas stream before compression.

9. - Method according to claim 8 taken in combination with claim 4, characterized in that the CO2 which vaporizes during storage is recycled into the stream before drying, or after prior cooling in Al.

10. - Method according to one of claims 1 to 9, characterized in that a condensation of the vapors leaving at the top of the distillation column (CD) is carried out by a heat exchange with a cold group (GF2), with a view to a return to the distillation column (CD), the incondensable vapors being evacuated.

11. - Method according to one of claims 1 to 10, characterized in that the temperature at the bottom of the distillation column (CD) is adjusted between -40°C and -12°C.

12. - Method according to one of claims 1 to 11, characterized in that: • a first flow separation device (VI) is provided, such as a three-way valve, arranged on the path of the flow leaving the cooling in A2 and comprising an outlet towards the heat transfer to the boiler in H2 and an outlet towards the cooling in H3; and a second flow separation device (V2), such as a three-way valve, arranged on the path from the outgoing cooling stream in H3 and comprising an outlet to the heat transfer to the boiler in H4 and an outlet to the Joule-Thomson expansion; • the opening setpoint value of the second flow separation device (V2) towards the heat transfer to the boiler in H4 is set at 50 - 100%; • the set value for the purity of liquid CO2 at the outlet at the bottom of the distillation column (CD) is set at 99 - 99.99% by volume; • the opening of the second flow separation device (V2) towards the heat transfer to the boiler is measured in H4; • if the opening value of the second flow separation device (V2) towards the heat transfer to the boiler in H4 is lower than its set value then: • the opening of the first flow separation device (VI) is reduced towards the heat transfer to the H2 boiler; • the purity of liquid CO2 is measured at the outlet at the bottom of the distillation column (CD); • the opening of the second flow separation device (V2) is increased towards the heat transfer to the boiler in H4 until the purity of liquid CO2 measured at the outlet at the bottom of the distillation column (CD) corresponds to its set value.

13. - Method according to one of claims 1 to 11, characterized in that that : • a first flow separation device (VI) is provided, such as a three-way valve, arranged on the path of the flow leaving the cooling in A2 and comprising an outlet towards the heat transfer to the boiler in H2 and an outlet towards the cooling in H3; and a second flow separation device (V2), such as a three-way valve, arranged on the path of the flow leaving the cooling in H3 and comprising an outlet towards the heat transfer to the boiler in H4 and an outlet towards the Joule-Thomson expansion; • the opening setpoint value of the second flow separation device (V2) for heat transfer to the boiler in H4; the set value for the purity of liquid CO2 at the outlet at the bottom of the distillation column (CD) is set at 99 - 99.99% by volume; we measure the temperature T and the pressure P of the outlet stream at the bottom of the distillation column (CD); we choose from a set of binary CH4 / CO2 diagrams, each corresponding to a known pressure, the one which corresponds to the measured pressure P; we calculate the temperature Tbune which corresponds to the measured pressure P and for the desired purity of liquid CO2; the opening of the second flow separation device (V2) towards the heat transfer to the boiler in H4 is measured; if the opening value of the second flow separation device (V2) towards the heat transfer to the boiler in H4 is lower than the set value then: • the opening of the first flow separation device (VI) is reduced towards the heat transfer to the H2 boiler; • the temperature T of the liquid CO2 is measured at the outlet at the bottom of the distillation column (CD); • the opening of the second flow separation device (V2) is increased towards the heat transfer to the boiler in H4 until the temperature value T of the liquid CO2 measured at the outlet at the bottom of the distillation column (CD) corresponds to the temperature value Tb determined.

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