Method for obtaining liquid co2

Abstract

The invention relates to a method for obtaining liquid CO2, the method comprising carrying out Joule-Thomson expansion of the starting gas mixture in order to obtain liquid CO2 from a gaseous mixture of CO2, CH4, and impurities, after it has been compressed (in C2) and then cooled, and distilling the mixture resulting from the Joule-Thomson expansion under temperature and pressure conditions that allow liquid CO2 to be obtained at the outlet at the bottom of the distillation column (CD). According to the invention, cooling the compressed mixture (in C2) is carried out (in A2); cooling of the cooled mixture (in A2) is carried out (in H3); cooling of the cooled mixture (in H3) is carried out by heat transfer to the reboiler associated with the distillation column (CD), wherein the cooled (in H4) mixture is then sent for Joule-Thomson expansion.

Description

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 the fermentation of organic matter and composed mainly of methane and carbon dioxide. Although biogas can be burned directly to produce heat and electricity (cogeneration), it is also possible to process it to recover its two main components, CH4 and CO2, separated and purified.

[0003] Document FR3114516 discloses a biogas purification process enabling liquid carbon dioxide to be obtained.

[0004] Generally speaking, 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 purification process of 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 of the cooled mixture (in A2) is carried out (in H3) using an external cold source, a portion of said cooled mixture (in A2) being, 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, a portion of the cooled mixture (in H3) being able to be sent directly to the Joule-Thomson expansion without passing through cooling (in H4).

[0008] The gaseous mixture of CO2, CH4 and unavoidable impurities can be obtained 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 preliminary compression (in C1) of the mixture; by cooling (in A1), if necessary followed by a heat exchange (in H1) with a cold group; and by drying to eliminate the water present in the starting gas mixture.

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

[0012] The temperature of the external cold source can be between -30°C and 15°C, being in particular 0°C.

[0013] Liquid CO2 leaving the distillation column can be stored at a pressure of 10 - 25 barA, and the CO2 that vaporizes during this storage can be recycled into the starting gas stream before compression in C2. In particular, CO2 that vaporizes during storage can be recycled into the stream before drying, or after cooling in A1.

[0014] Condensation of the vapors leaving the top of the distillation column can be carried out by heat exchange with a cooling unit, with a view to returning them to the distillation column, the non-condensable vapors being evacuated.

[0015] 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 liquid CO2.

[0016] 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 stream 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 stream 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 towards the heat transfer to the boiler in H4 is set at 50 - 100%; the setpoint 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 opening of the second flow separation device towards the heat transfer to the boiler in H4 is measured; 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 towards the heat transfer to the boiler in H2 is reduced; the purity of liquid CO2 at the outlet at the bottom of the distillation column is measured; 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.;

[0017] 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 stream 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 stream 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 towards the heat transfer to the boiler in H4 is set at 50 – 100%; the setpoint 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;we measure the temperature T and the pressure P of the stream at the outlet at the bottom of the distillation column; 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 T; bullewhich 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 in H4 is measured; 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 towards the heat transfer to the boiler in H2 is reduced; the temperature T of the liquid CO2 at the outlet at the bottom of the distillation column is measured; the opening of the second flow separation device towards the heat transfer to the boiler in H4 is increased 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 bulle determined.

[0018] 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.

[0019] This is a schematic representation of a facility for obtaining liquid CO2 according to the prior art described in FR3114516.

[0020] This is a view similar to that of a schematic representation of a plant for obtaining liquid CO2 according to the invention.

[0021] This is a schematic representation of the installation on which other measuring elements at the bottom of the distillation column outlet and the various regulation diagrams have been shown.

[0022] If we refer to the, we can see that the installation comprises arranged in succession: a compressor C1 capable of receiving, via an inlet pipe 1, a gaseous stream enriched in CO2 which is a permeate resulting from a membrane separation of biogas and for which the aim is to further purify it in CO2 and obtain liquid CO2; an air cooler A1 whose inlet is connected via a pipe 2 to the outlet of the compressor C1; a heat exchanger H1 whose inlet is connected via a pipe 3 to the outlet of the air cooler A1, and which ensures the cooling of the introduced stream using a circuit 4 including a refrigeration 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 discharging 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 stream leaving the dryer S; an air cooler A2 whose inlet is connected via a pipe 7 to the outlet of the compressor C2; a heat exchanger H2 whose inlet is connected, via 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 expansion valve JT which receives, via a pipe 9, the cooled stream leaving the exchanger H2 and from which exits via a pipe 10 an expanded stream introduced as a charge at the top of the column CD.;

[0023] The steam leaving the top of column CD is sent to a condenser CO via a line 11, providing condensate which is returned via line 12 to column CD and non-condensables evacuated from the system via a line 13. Condenser CO is cooled by the GF2 refrigeration unit using circuit 14 maintained for example at -30°C.

[0024] At the bottom of column CD, the liquid CO2 is drawn off via a pipe 15, to be sent to a valve Vt1 for regulating the level of liquid CO2 at the bottom of column CD, then is sent, at the outlet of valve Vt1 via a pipe 16, to a storage tank R (for example at a pressure of 19 barA).

[0025] From the tank R, the liquid CO2 can be evacuated to a transport via a pipe 17. During this storage, part of the liquid CO2 passes into the gaseous phase, in particular due to the expansion carried out when passing through the valve Vt1 (the “boil-off” phenomenon), this fraction then being returned in a pipe 18 to an expansion valve Vt2 to reduce the pressure before being redirected via the pipe 19 into the pipe 3 for recycling.

[0026] The installation according to the invention shown differs from that which has just been described with reference to the by the presence of: a first three-way valve V1 arranged on the path of the current passing in 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 arranged on the outlet of the heat exchanger H3; and yet another heat exchanger H4 which serves as a boiler for the column CD and which returns the cooled current to the Joule-Thomson expansion valve JT.

[0027] 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 the and for which the same reference numbers are used.

[0028] Valve V1 allows part of the current coming from line 8 to be sent: via line 20, to exchanger H2; and the remaining part via line 21 to heat exchanger H3, the latter also receiving via line 22 the cooled current coming from H2, the combined currents arriving at H3 via line 23.

[0029] The heat exchanger H3 is cooled by the stream 24 on which a cold source SF is placed, being for example at a temperature of 0°C. This cold source can be a geothermal source or a cold group.

[0030] Valve V2 allows part of the current coming from line 25 at the outlet of exchanger H3 to be sent via line 26 to heat exchanger H4, and via line 27 to the Joule-Thomson expansion valve JT, the latter also receiving via line 28 the cooled current coming from H4, the combined currents arriving at valve JT via line 29.

[0031] On the route of pipe 15, an AT analyzer was placed, the role of which is indicated below.

[0032] 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 allows the liquid to be introduced into the distillation column after the Joule-Thomson expansion at a low temperature allowing the distillation to be carried out at a lower pressure than in the case of the installation of the.

[0033] Carrying out the distillation at a lower pressure also has the advantage of requiring less stress on the GF2 cooling unit for the condensation of vapors at the top of the CD column.

[0034] Another advantage is a lower pressure difference between distillation and storage of liquid CO2, which reduces the boil-off phenomenon and thus the recycling of vaporized CO2.

[0035] We will now describe the process control strategies with reference to Figures 2 and 3.

[0036] On the, we have represented two regulation organs OR1 and OR2 for which we have used the following notations:SP_INT: Internal adjustment point (setpoint) of the master regulatorSP_EXT: External adjustment point (setpoint) of the slave regulator corresponding to the output of the master regulatorSP_MAX: Maximum adjustment pointMESURE: Incoming measurement in the regulatorOUT: Output of the regulatorOUT_MAX: Maximum output of the regulatorOUT_MIN: Minimum output of the regulator

[0037] The OR1 and OR2 components 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 GF2 refrigeration unit. The maximum flow must therefore be sent to H4, which implies an opening setpoint for 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 setpoint 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 V1 in order to supply the necessary energy while guaranteeing the maximum opening of V2.

[0038] The CO2 purity value at the bottom of the distillation column CD is entered into the control unit OR1 as the SP_INT value. The setpoint value for opening the second three-way valve V2 is entered into the control unit OR2 as the SP_INT value.

[0039] 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 setpoint value, this means that more flow can be passed through V2 to the exchanger H4.

[0040] The OR2 regulation organ will therefore send as an OUT signal a signal to reduce the opening of the first valve V1 to the H2 exchanger.

[0041] The amount 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 control unit OR1 as a MEASUREMENT signal. In response, the control unit OR1 will send an order to open valve V2 to the heat exchanger H4 as an OUT signal until the purity value of the CO2 at the outlet at the bottom of the distillation column CD corresponds to the set value.

[0042] The installation according to the invention shown differs from that which has been described with reference to the one in 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.

[0043] The installation also has two regulation organs OR3 and OR4 replacing regulation organ OR1 and whose role will be indicated below.

[0044] The objective of the regulation of the installation presented in is identical to that of. 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.

[0045] In a first embodiment called predictive mode, the regulation member OR3 is inactive. In this embodiment, the purity of the CO2 is not measured directly by the analyzer AT but is determined using the pressure and temperature of the liquid CO2 stream at the outlet at the bottom of the distillation column CD.

[0046] 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 allows us to go back to the temperature value which corresponds by reading on the bubble curve to the purity considered. We then obtain the temperature T bulle which corresponds to the outlet pressure at the bottom of the CD distillation column and the desired CO2 purity.

[0047] 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 T is determined. bulle corresponding to the desired CO2 purity.

[0048] This temperature T bulle is entered into the OR4 regulator as SP_INT value.

[0049] In the same way as in the case illustrated in, the opening setpoint value of the second three-way valve V2 is entered into the control unit OR2 as the SP_INT value. The opening value of the second three-way valve V2 is measured and sent to the controller OR2 as the MEASUREMENT value. The control unit OR2 will therefore send a signal to reduce the opening of the first valve V1 to the exchanger H2 as the OUT signal.

[0050] The TT_CO2 analyzer measures the outlet temperature at the bottom of the distillation column CD and sends this temperature to the control unit OR4 as a MEASUREMENT signal. In response, the control unit OR1 will send an order to open valve V2 to the heat exchanger H4 as an OUT signal until the CO2 outlet temperature at the bottom of the distillation column CD corresponds to the setpoint value T bulle .

[0051] The value of T bulle can be calculated by a calculator called T_bulle calculator, which allows the system to operate in a so-called automatic mode. This calculator 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.

[0052] The value of T bullecan also be entered directly by the operator, the system then operating in manual mode, without control of the CO2 purity at the column outlet.

[0053] In a second embodiment called analysis mode, the OR3 regulation organ is active.

[0054] The OR3 control unit receives as setpoint value SP_INT the purity value of the CO2 at the bottom of the distillation column.

[0055] In this regulation mode, the regulation organ OR3 determines the temperature T bulle and sends it as an OUT signal to the OR4 control unit which receives it as a SP_EXT signal.

[0056] Then, the regulation is carried out as in the predictive mode by measuring the outlet temperature at the bottom of the distillation column CD and acting on the valves V1 and V2 until the measured CO2 purity corresponds to the CO2 purity setpoint.

[0057] The following examples illustrate the present invention without, however, limiting its scope.

[0058] Reference Example 1

[0059] The installation described in the is used. The ambient temperature considered is 10°C and the energy optimum corresponds to compression at 35 barA by compressor C2.

[0060] Table 1 shows the pressure and temperature conditions as well as the molar compositions of each of the streams in the pipes considered.

[0061] Pipeline123 (before recycling)3 (after recycling)56Pressure (barA)1.1132510.710.610.610.510Temperature (°C)201602013.078512.961212.9612Flow rate (mol / h)4495.034495.034495.035185.945185.945163.47Molar fraction CH40.0150.0150.0150.01300220.01300220.0130588Molar fraction CO20.9750.9750.9750.978330.978330.982588Molar fraction O20.0010.0010.0010.0008667760.0008667760.000870549Molar fraction N20.0040.0040.0040.003467090.003467090.00348218Molar fraction H2O0.0050.0050.0050.004333860.004333860Conduit789101315Pressure (barA)3534.934.9313131.026Temperature (°C)16020-3.24255-7.32554-29.1937-4.58889Flow rate (mol / h)5163.475163.475163.475163.47195.8494967.62Molar fraction CH40.01305880.01305880.01305880.01305880.3442639.94E-07Molar fraction CO20.9825880.9825880.9825880.9825880.5409790.999999Molar fraction O20.0008705490.0008705490.0008705490.0008705490.02295156.03E-09Molar fraction N20,003482180,003482180,003482180,003482180,0918065,71E-11Molar fraction H2O000000Conduit16171819Pressure (barA)19191910.7Temperature (°C)-21.2978-21.2978-21.2978-35.8436Flow rate (mol / h)4967.624276.71690.911690.911Molar fraction CH49.94E-074.33E-074.47E-064.47E-06Molar fraction CO20.99999910.9999960.999996Molar fraction O26.03E-092.23E-092.95E-082.95E-08Molar fraction N25.71E-111.38E-113.25E-103.25E-10Molar fraction H2O0000,

[0062] Example 2 of the invention

[0063] The installation described in the is used. The ambient temperature considered is 10°C and the energy optimum corresponds to compression at 44 barA by compressor C2.

[0064] Table 2 shows the pressure and temperature conditions as well as the molar compositions of each of the streams in the pipes considered.

[0065] Pipeline123 (before recycling)3 (after recycling)56Pressure (barA)1.1132510.710.610.610.510Temperature (°C)201602019.388519.275319.2753Flow rate (mol / h)4495.034495.034495.034549.74549.74527.23Molar fraction CH40.0150.0150.0150.01481990.01481990.0148934Molar fraction CO20.9750.9750.9750.97530.97530.980142Molar fraction O20.0010.0010.0010.0009879840.0009879840.000992889Molar fraction N20.0040.0040.0040.003951930.003951930.00397155Molar fraction H2O0.0050.0050.0050.004939920.004939920Conduit7823252910Pressure (barA)43.643643.543643.543643.443643.443620Temperature (°C)160207.073412-19.1656-25.622Flow rate (mol / h)4527.234527.234527.234527.234527.234527.23Molar fraction CH40.01489340.01489340.01489340.01489340.01489340.0148934Molar fraction CO20.9801420.9801420.9801420.9801420.9801420.980142Molar fraction O20.0009928890.0009928890.0009928890.0009928890.0009928890.000992889Molar fraction N20,003971550,003971550,003971550,003971550,003971550,00397155Molar fraction H2O000000Conduit131516171819Pressure (barA)2020.02619191910.7Temperature (°C)-29.319-19.6091-21.298-21.298-21.298-35.8437Flow rate (mol / h)384.1024143.134143.134088.4554.672454.6724Molar fraction CH40.1755319.94E-079.94E-078.85E-079.13E-069.13E-06Molar fraction CO20.7659560.9999990.9999990.9999990.9999910.999991Molar fraction O20.01170265.60E-095.60E-094.82E-096.40E-086.40E-08Molar fraction N20.04681081.76E-111.76E-111.36E-113.21E-103.21E-10Molar fraction H2O000000,

[0066] Example 3 Calculation of Specific Electricity Consumption

[0067] The specific electricity consumption, i.e. the amount of electricity consumed to obtain 1 kg of liquid CO2 at the desired purity, is calculated for each of Examples 1 and 2.

[0068] The following assumptions were used: pressure drop in dryer S: 0.5 bar consumption of dryer S: 3.5 kWe isentropic efficiency of compressor C1: 0.62 isentropic efficiency of compressor C2: 0.67 consumption of air coolers A1 and A2: 1 kWe each for an air flow rate less than or equal to 6000 Nm 3 / hconsumption of auxiliaries (solenoid valves, regulators, instrumentation): 2 kWecoefficient of performance of the cooling unit 0 / 4°C GF1: COP_GF1 = -0.07214*T_ambient+24.9615coefficient of performance of the cooling unit -30°C GF2: COP_GF2 = -0.0292*T_ambient+10.20498

[0069] The formula for calculating the theoretical specific consumption of Example 1 CSP1 is:

[0070] CSP1 = (PuC1 + PuC2 + PuA1 + PuA2 + PuS + PuV + PuH1 / COP_GF1 + PuCO / COP_GF2) / mCO2

[0071] The formula for calculating the theoretical specific consumption of Example 2 CSP2 is:

[0072] CSP2 = (PuC1 + PuC2 + PuA1 + PuA2 + PuS + PuV + (PuH1 + PuH3) / COP_GF1 + PuCO / COP_GF2) / mCO2

[0073] Formulas in which:PuC1 is the electrical power consumed by compressor C1PuC2 is the electrical power consumed by compressor C2PuA1 is the electrical power consumed by dry cooler A1PuA2 is the electrical power consumed by dry cooler A2PuS is the electrical power consumed by dryer SPuV is the electrical power consumed by auxiliaries (solenoid valves, regulators, instrumentation)PuH1 is the thermal power exchanged by exchanger H1PuH3 is the thermal power exchanged by exchanger H3PuCO is the thermal power exchanged by the distillation column condensermCO2is the mass flow rate of CO2 in line 17 (CO2 actually recovered)

[0074] Table 3 below shows the values ​​of the different elements.

[0075] Example 1Example 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)35.0043.64Column working pressure (barA)3120Incoming gas flow rate (Nm3 / h)100.00100.00Liquid CO2 storage pressure (barA)1919Liquefied CO2 flow rate (kg / h)188.22179.93Boil-off recycling rate14.98%0.81%Non-condensable recycling rate2.42 %6.7%C1Electrical energy consumption (kWe)14.2014.20C2Electrical energy consumption (kWe)6.947.53A1High temperature (°C)160.0160.0Low temperature (°C)20.020.0Thermal energy consumption (kWh)7.397.39Electrical energy consumption (kWe)1.001.00A2High temperature (°C)160.0160.0Low temperature (°C)20.020.0Thermal energy consumption (kWh)9.398.73Electrical energy consumption (kWe)1.001.000H1High temperature (°C)13.119.4Low temperature (°C)13.019.3Thermal energy consumption (kWh)0.000.00H2High temperature (°C)20.020.0Low temperature (°C)-4.167.1Thermal energy consumption (kWh)14.382.60H3High temperature (°C)-7.1Low temperature (°C)-2.0Thermal energy consumption (kWh)0.0010.32H4High temperature (°C)-2.0Low temperature (°C)--19.19Thermal energy consumption (kWh)0.0003.02CO (condenser)High temperature (°C)-29.0-29.0Low temperature (°C)-29.0-29.0Thermal energy consumption (kWh)16,054,19BoilerHigh temperature (°C)-4,16-19,19Low temperature (°C)-4,16-19,19Thermal energy consumption (kWh)-14,38-5,62Auxiliaries (valves, regulators, instrumentation)Electrical energy consumption (kWe)2,002,00Dryer SElectrical energy consumption3,503,50Ambient temperature (°C)10,0010,00COP chiller -30°C GF21,941,94COP chiller 0 / 4°C GF14,534,53Total electrical energy consumption (kWhe)36,933,7Specific consumption (kWhe per Nm3 of incoming gas)0,3690,337Specific consumption (kWhe per kg of liquefied CO2)0.1960.187,

[0076] 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%.

[0077] Table 4 compares the specific consumption values ​​as a function of ambient temperature.

[0078] Specific electricity consumption (kWhe per kg of liquid CO2) Ambient temperature (°C) Example 1 Example 2 Deviation (%) 100,1960,1875% 200,2080,1947% 400,2470,21712%

[0079] As can be seen in Table 4, the higher the ambient temperature, the greater the consumption gap in favor of the process using the installation according to the present invention.

Claims

– 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 by the fact that: in A2 a cooling, called "first cooling", of the compressed mixture is carried out; in H3 a cooling, called "third cooling", using an external cold source, of the mixture cooled in A2, a portion of said mixture cooled in A2 being, 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.; – 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. – Method according to one of claims 1 and 2, characterized in that the working pressure of the compressor is between 20 and 60 barA. – 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 “pre-cooling”, in A1, where appropriate followed by a heat exchange, called “pre-heat exchange”, in H1 with a cold group (GF1); and a drying to eliminate the water present in the starting gas mixture. – 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. – Method according to one of claims 1 to 5, characterized in that the temperature of the external cold source is between -30°C and 15°C, being in particular 0°C. – Method according to one of claims 1 to 6, 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 into the starting gas stream before compression. – Method according to claim 8 taken in combination with claim 4, characterized in that the CO2 which vaporizes during storage is recycled in the stream before drying, or after prior cooling in A1. – Method according to one of claims 1 to 8, characterized in that a condensation of the vapors leaving 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 non-condensable vapors being evacuated. – Method according to one of claims 1 to 9, characterized in that the temperature at the bottom of the distillation column (CD) is adjusted between -40°C and -12°C. – Method according to one of claims 1 to 10, characterized in that: a first flow separation device (V1) 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) towards the heat transfer to the boiler in H4 is set at 50 - 100%; the setpoint 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 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 its set value then: the opening of the first flow separation device (V1) towards the heat transfer to the boiler in H2 is reduced; the purity of liquid CO2 at the outlet at the bottom of the distillation column (CD) is measured; the opening of the second flow separation device (V2) 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 (CD) corresponds to its set value.; – Method according to one of claims 1 to 10, characterized in that: a first flow separation device (V1) 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) towards the heat transfer to the boiler in H4 is set at 50 – 100%; the setpoint 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 stream at the outlet 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 T; bullewhich 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 (V1) towards the heat transfer to the boiler in H2 is reduced; the temperature T of the liquid CO2 at the outlet at the bottom of the distillation column (CD) is measured; the opening of the second flow separation device (V2) towards the heat transfer to the boiler in H4 is increased 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 T bulle determined.

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