Process for purifying raw gas by membrane separation with control of the gas supply temperature to the membranes

Abstract

------ Process for purifying a raw gas by membrane separation with control of the gas supply temperature to the membranes The present invention relates to a process for purifying a raw gas by membrane separation, said process using n stages of membrane separation, n being greater than or equal to 1, said gas to be treated consisting of a mixture of at least two constituents A and B, characterized by the fact that it comprises the following steps: a nominal flow rate and / or a nominal composition of the incoming raw gas to be treated is established;For each separation stage i, at least one nominal characteristic operating parameter is determined, chosen from among the ratio RSCi0, the charge level of component A or B at stage i, denoted respectively TCAi0 and TCBi0, the overall charge level of component A or B at stage i, denoted respectively TCGAi0 and TCGBi0, the specific consumption CSP0. When the flow rate and / or composition of the incoming raw gas changes during the process, the permeability of at least one membrane in at least one stage i is adjusted by adjusting the feed temperature of the corresponding separation stage so that at any given time, and independently for each stage i, at least one of the following is measured and / or calculated in real time on each separation stage: the ratio RSCi, the charge level TCAi or TCBi, the overall charge level TCGAi or TCGBi, or the CSP, which approaches its corresponding reference value, RSCi0 or TCAi0. TCBi0 or TCGAi0 or TCGBi0 or CSP0.;

Description

Title of the invention: Process for purifying a raw gas by membrane separation with control of the gas supply temperature to the membranes

[0001] The present invention relates to a method for purifying a raw gas by membrane separation with control of the gas supply temperature of the membranes. As an example of gas to be purified, mention may be made of biogas from which it is desired to obtain a stream rich in carbon dioxide (CO2) and a stream rich in methane (CH4).

[0002] An example of a process for purifying a raw gas is described by European patent EP2588217B. In this process, three membrane separation stages are used, the raw gas being a mixture of at least two components A and B, A being the component with the highest permeance.

[0003] This process, which aims to separate the raw gas stream into a stream enriched in A and a stream enriched in B, is illustrated by the diagram in [Fig. 1].

[0004] As can be seen in this diagram: - the raw gas stream 1 is compressed in a compressor Cl to obtain a compressed gas stream 2 which is passed into a first membrane separation stage El to obtain: - a retentate stream 3; and - a permeate stream 4; - the retentate stream 3 is passed into a second membrane separation stage E2 to obtain: - a retentate stream 5, which constitutes the desired B-enriched stream; and

[0005] a permeate stream 6; - the permeate stream 4 is passed through a third membrane separation stage E3 to obtain: - a retentate stream 7; and - a permeate stream 8, which constitutes the desired A-enriched stream.

[0006] A pressure reducing valve VI is placed in the path of stream 7. The permeate stream 6 and the retentate stream 7 are recycled into stream 1 before introduction into the compressor CL.

[0007] When the operating conditions deviate from a nominal operating point defined from the operating conditions used for a precise flow rate and / or a precise composition of raw gas to be purified, the performance of the purification process will be affected by this deviation from the nominal operating point. It would therefore be desirable to have a simple means of controlling the process in order to be able to modify the operating conditions so as to find performances identical or close to those of the nominal operating point when the flow conditions and / or composition of the raw gas entering the system change.

[0008] To this end, the present invention relates to a method for purifying a raw gas by membrane separation, said method using n membrane separation stages, n being greater than or equal to 1, the or each membrane separation stage, denoted i with 1 < i < n, being constituted by: • at least one separation membrane whose permeability depends on temperature; • a gas supply inlet to said stage i through which: • for i=l, said raw gas to be treated is introduced, where appropriate in combination when n=l with a permeate recycling stream and / or a retentate recycling stream from the single stage and when n>l with a permeate recycling stream and / or a retentate recycling stream, said recycling streams coming from another stage; and • for i>l, the permeate or retentate from one of the other stages is introduced, • a retentate outlet; and • a permeate outlet,

[0009] said gas to be treated consisting of a mixture of at least two constituents A and B, the permeability of the membranes for constituent A being greater than that of constituent B,

[0010] characterized by the fact that it comprises the following steps: • a nominal flow rate and / or a nominal composition of the incoming raw gas to be treated is established, representing the nominal operating point; • for each separation stage i, at least one nominal characteristic operating quantity chosen from: • the RSC'o ratio, defined as the SCy / SCu'o or SCb'o / SCa'o ratio,

[0011] SCa'o and SCb^ each being defined as the ratio of the partial flow rate of the constituent A or B respectively in the permeate stream leaving stage i, noted respectively PcA'o and Pcb'o. to the partial flow rate of the same constituent in the feed of the same stage i, noted respectively Fa'o and FH'o; • the charge rate of constituent A or B at stage i, noted respectively TCA'o and TCb^, defined as the ratio Fa'o / Pa'o or Fb'o / Pb'o,

[0012] Fa'o and FBi0 being the partial flow rate of component A or B in the supply of stage i and PA'O or Pb'o the permeability of the membranes at the temperature of the nominal operating point for component A or B on stage i; • the overall loading rate of component A or B at stage i, noted respectively TCGa'o and TCGb'o, defined as the ratio of the flow rate of component A or B in the nominal flow rate to the permeability of the membranes at the temperature of the nominal operating point for component A or B respectively at stage i, i.e. PA'O or Pb'o; and • in the case where operation is carried out with several stages, where identical membranes are used on all stages i and the same feed temperature is established on all stages i for the nominal flow rate and / or the nominal composition of the incoming raw gas to be treated, the specific consumption CSP0, defined as the energy consumption of the process divided by the nominal flow rate of an incoming raw gas to be treated or of at least one stream leaving the process under the established nominal conditions; • when the flow rate and / or composition of the incoming raw gas, measured before any possible mixing with at least one recycle stream, changes during the course of the process, the permeability of at least one membrane in at least one stage i is adjusted and thereby the quantity of constituents A and B passing through the membrane(s) concerned, by adjusting the temperature of the feed to the corresponding separation stage so that at any time, and independently for each stage i, at least one of the RSO ratio or the loading rate TCA' or TCB' or the overall loading rate TCGA' or TCGB' or the CSP, measured and / or calculated in real time on each separation stage, approaches its corresponding reference value, RSOo or TCA i0 or TCb'o or TCGa'o or TCGb'o or CSPo.

[0013] The permeability or permeance of a membrane for a constituent is an intrinsic property of the material constituting the wall of the membrane to allow said constituent to pass through it. This permeability can be expressed in the form of a molar flow rate per unit of surface area and per unit of pressure, i.e. in mol / s / m2 / Pa. The dependence of the permeability of membranes on temperature can, for example, follow an Arrhenius-type relationship.

[0014] Selectivity is the ratio of the permeability of a first pure constituent to the permeability of a second pure constituent. Selectivity also varies with temperature.

[0015] In a particular embodiment according to which one operates with several stages, one uses identical membranes on all the stages i and one establishes the same supply temperature on all the stages i for the nominal flow rate and / or the composition nominal value of the incoming raw gas to be treated, the overall load rate TCGA'o or TCGb'o is chosen as the nominal characteristic quantity, and in the case of a change in the flow rate and / or the composition of the incoming raw gas, the feed temperature is adjusted in all stages using a single regulator associated with the overall load rate at any stage.

[0016] At any time and in real time and for each stage i, the permeability of the membranes for the constituent A or B, either PA'(t) or PeKO, can be determined by measuring and / or calculating the pressures, temperatures, flow rates and compositions of the incoming current and outgoing currents, and by numerical resolution.

[0017] Knowledge of the pressures, temperatures, flow rates and compositions of the incoming and outgoing streams makes it possible to establish material balances on a membrane. From physical models of these membranes and conventional numerical resolution methods, the person skilled in the art can access the permeability values.

[0018] For example, it is possible to use the physical models and numerical resolution methods described by Michael Binns et al. in Strategies for the simulation of multi-component hollow fibre multi-stage membrane gas separation Systems, Journal of Membrane Science, 497, (2016), 458-471.

[0019] The separation membranes may be polymer membranes chosen from polysulfone membranes and polyimide membranes.

[0020] The temperature of the feed to each stage can be adjusted using a heat exchanger. It is also possible to use any temperature adjustment means known to those skilled in the art.

[0021] In a particular embodiment, the constituent A of the raw gas is carbon dioxide, and the constituent B is methane. Description of Figures

[0022] - [Fig.l] is a schematic representation of a typical method serving as a basis for the method according to the invention; - [Fig.2] is a schematic representation of the process according to a first embodiment with 3 cascaded control loops, regulating the RSC ratios; - [Fig.3] is a schematic representation of the process according to a second embodiment with 3 cascaded control loops, regulating the TC load rates; - [Fig.4] is a schematic representation of the process according to a third embodiment with 3 cascaded control loops, regulating the overall TCG load rates; [Fig.5] is a schematic representation of the method according to a fourth embodiment with 1 cascade control loop, regulating the overall TCG load rate; and [Fig.6] is a schematic representation of the process according to a fifth embodiment with 1 cascade control loop, regulating the CSP consumption.

[0023] [Fig.2] is a resumption of [Fig.l], with the following implementation: • a temperature regulation device H1 on the current path 2 to modify the temperature of the power supply 2' of the first stage El; • a temperature regulation device H2 on the path of the retentate 3, to modify the temperature of the feed 3' of the second stage E2; and • a temperature regulation device H3 on the path of the permeate 4, to modify the temperature of the feed 4' of the third stage E3.

[0024] the same reference numbers and letters as in [Fig.l] having been retained to designate similar pipes and apparatus / devices.

[0025] In [Fig.2], the following regulatory organs appear: • T_F1, temperature sensor whose role is to measure the temperature of the 2' power supply of the first stage El; • T_F2, temperature sensor whose role is to measure the temperature of the 3' power supply of the second stage E2; • T_F3, temperature sensor whose role is to measure the temperature of the 4' power supply of the third stage E3; • PT, pressure sensor placed on stream 7 of retentate from E3, upstream of VI; and

[0026]

[0027]

[0028] • Regulatory bodies noted by blocks: • 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 • 0UT_MIN: Minimum output of the regulator and bearing the reference letters OR1 to OR8. The OR1 and OR2 components (2 cascade regulators) regulate the purification efficiency of the process. The purification efficiency is defined as the flow rate of component B recovered from the retentate 5 divided by the flow rate of component B entering the raw gas 1. The desired setpoint of the purification efficiency is entered as the value SP_INT of the OR1 organ. It also receives the measured value of the flow rate of constituent B in the retentate 5 divided by the flow rate of constituent B entering the raw gas 1 as the MEASUREMENT value.

[0029] The OR2 organ receives as MEASUREMENT value the pressure of the retentate 7 measured by the pressure sensor PT.

[0030] By comparing the desired setpoint value and the measured value of the purification efficiency, the OR1 member will send as output value OUT to the OR2 member the pressure value to be reached for the retentate 7, the OR2 member receiving it as value SP_EXT.

[0031] The OR2 organ will then control the pressure of the retentate 7 by sending the output value OUT to the control valve V1 in order to maintain the value of the purification efficiency.

[0032] The members OR3 and OR4 constitute a cascade regulation of two regulators whose function is to control the ratio RSC1 of the first membrane separation stage El by acting on the temperature of the feed 2' of the first stage by means of the temperature regulation device HL

[0033] The desired setpoint value of the RSC'o ratio under nominal conditions is entered as SP_INT value of the OR3 device and the OR3 device receives as MEASUREMENT value the value of the RSC1 ratio calculated in real time. The output of the OR3 device is a working temperature setpoint sent in SP_EXT to the OR4 device.

[0034] The device OR4 compares the setpoint SP_EXT coming from OR3 with the temperature MEASUREMENT value of the supply 2' of the first stage El measured by the temperature sensor T_F1 and acts on the temperature regulation device H1 so that it modifies the temperature of the stream 2 leaving the compressor Cl so that the supply 2' is at the right temperature so that the value RSC1 approaches the value RSC'o.

[0035] The OR5 and OR6 organs constitute a cascade regulation of two regulators whose function is to control the RSC2 ratio of the second membrane separation stage E2 by acting on the temperature of the supply 3' of the second stage by means of the temperature regulation device H2.

[0036] The desired setpoint value of the RSC20 ratio under nominal conditions is entered as SP_INT value of the OR5 device and the OR5 device receives as MEASUREMENT value the value of the RSC2 ratio calculated in real time. The output of the OR5 device is a working temperature setpoint sent in SP_EXT to the OR6 device.

[0037] The device OR6 compares the setpoint SP_EXT from OR5 with the temperature MEASUREMENT value of the supply 3' of the second stage E2 measured by the temperature sensor T_F2 and acts on the temperature regulation device H2 so that it modifies the temperature of the retentate 3 of the first stage E1 so that the 3' power supply is at the correct temperature so that the RSC2 value approaches the RSC20 value.

[0038] The members OR7 and OR8 constitute a cascade regulation of two regulators whose function is to control the RSC3 ratio of the third membrane separation stage E3 by acting on the temperature of the supply 4' of the third stage by means of the temperature regulation device H3.

[0039] The desired setpoint value of the RSC30 ratio under nominal conditions is entered as SP_INT value of the OR7 device and the OR7 device receives as MEASUREMENT value the value of the RSC3 ratio calculated in real time. The output of the OR5 device is a working temperature setpoint sent in SP_EXT to the OR8 device.

[0040] The device OR8 compares the setpoint SP_EXT coming from OR7 with the temperature MEASUREMENT value of the supply 4' of the third stage E3 measured by the temperature sensor T_F3 and acts on the temperature regulation device H3 so that it modifies the temperature of the permeate 4 of the first stage E1 so that the supply 4' is at the right temperature so that the value RSC3 approaches the value RSC30.

[0041] [Fig.3] is a replication of [Fig.2], with the installation of the regulating organs OR9 to OR 14 in place of the regulating organs OR3 to OR8, the organs OR1 and OR2 being identical.

[0042] The members OR9 and OR10 constitute a cascade regulation of two regulators whose function is to control the TCA* or TCB' ratio of the first membrane separation stage El by acting on the temperature of the feed 2' of the first stage by means of the temperature regulation device HL

[0043] The desired setpoint value of the TCa'o or TCb'o ratio under nominal conditions is entered as SP_INT value of the OR9 device and the OR9 device receives as MEASUREMENT value the value of the TCA* or TCB' ratio calculated in real time. The output of the OR7 device is a working temperature setpoint sent in SP_EXT to the OR10 device.

[0044] The OR 10 device compares the SP_EXT setpoint from OR9 with the temperature MEASUREMENT value of the supply 2' of the first stage El measured by the temperature sensor T_F1 and acts on the temperature regulation device H1 so that it modifies the temperature of the stream 2 leaving the compressor C1 so that the supply 2' is at the right temperature so that the TCA* or TCB' value approaches the corresponding TCA'o or TCB*o value.

[0045] The members OR11 and OR12 constitute a cascade regulation of two regulators whose function is to control the TCA2 or TCB2 ratio of the second membrane separation stage E2 by acting on the temperature of the supply 3' of the second stage by means of the temperature regulation device H2.

[0046] The desired setpoint value of the TCA2o or TCB2o ratio under nominal conditions is entered as SP_INT value of the OR11 device and the OR11 device receives as MEASUREMENT value the value of the TCA2 or TCB2 ratio calculated in real time. The output of the OR11 device is a working temperature setpoint sent in SP_EXT to the OR12 device.

[0047] The device OR12 compares the setpoint SP_EXT coming from OR11 with the temperature MEASUREMENT value of the feed 3' of the second stage E2 measured by the temperature sensor T_F2 and acts on the temperature regulation device H2 so that it modifies the temperature of the retentate 3 of the first stage E1 so that the feed 3' is at the right temperature so that the value TCA2 or TCB2 approaches the corresponding value TCA20 or TCB20.

[0048] The members OR13 and OR14 constitute a cascade regulation of two regulators whose function is to control the TCA3 or TCB3 ratio of the third membrane separation stage E3 by acting on the temperature of the supply 4' of the third stage by means of the temperature regulation device H3.

[0049] The desired setpoint value of the TCA30 or TCB30 ratio under nominal conditions is entered as SP_INT value of the OR13 device and the OR13 device receives as MEASUREMENT value the value of the TCA3 or TCB3 ratio calculated in real time. The output of the OR13 device is a working temperature setpoint sent in SP_EXT to the OR14 device.

[0050] The device OR14 compares the setpoint SP_EXT coming from OR13 with the temperature MEASUREMENT value of the supply 4' of the third stage E3 measured by the temperature sensor T_F3 and acts on the temperature regulation device H3 so that it modifies the temperature of the permeate 4 of the first stage E1 so that the supply 4' is at the right temperature so that the value TCA3 or TCB3 approaches the corresponding value TCA30 or TCB30.

[0051] [Fig.4] is a replication of [Fig.3], with the installation of the regulating organs OR 15 to OR20 in place of the regulatory bodies OR9 to OR 14, the bodies OR1 and OR2 being identical.

[0052] The members OR15 and OR16 constitute a cascade regulation of two regulators whose function is to control the ratio TCGA* or TCGB' of the first membrane separation stage El by acting on the temperature of the feed 2' of the first stage by means of the temperature regulation device HL

[0053] The desired setpoint value of the TCGA'o or TCGe'o ratio under nominal conditions is entered as SP_INT value of the OR15 device and the OR15 device receives as MEASUREMENT value the value of the TCGA* or TCGB' ratio calculated in real time. The output of the OR15 device is a working temperature setpoint sent in SP_EXT to the OR16 device.

[0054] The OR 16 organ compares the SP_EXT instruction coming from OR 15 to the value of MEASUREMENT of the temperature of the supply 2' of the first stage El measured by the temperature sensor T_F1 and acts on the temperature control device H1 so that it modifies the temperature of the stream 2 leaving the compressor Cl so that the supply 2' is at the right temperature so that the value TCGA* or TCGb1 approaches the corresponding value TCGa'o or TCGb'o.

[0055] The members OR17 and OR18 constitute a cascade regulation of two regulators whose function is to control the ratio TCGA2 or TCGB2 of the second membrane separation stage E2 by acting on the temperature of the supply 3' of the second stage by means of the temperature regulation device H2.

[0056] The desired setpoint value of the TCGA20 or TCGB2o ratio under nominal conditions is entered as SP_INT value of the OR17 device and the OR17 device receives as MEASUREMENT value the value of the TCGA2 or TCGB2 ratio calculated in real time. The output of the OR17 device is a working temperature setpoint sent in SP_EXT to the OR18 device.

[0057] The device OR18 compares the setpoint SP_EXT coming from OR17 with the temperature MEASUREMENT value of the feed 3' of the second stage E2 measured by the temperature sensor T_F2 and acts on the temperature regulation device H2 so that it modifies the temperature of the retentate 3 of the first stage E1 so that the feed 3' is at the right temperature so that the value TCGA2 or TCGB2 approaches the corresponding value TCGA20 or TCGB2o.

[0058] The members OR19 and OR20 constitute a cascade regulation of two regulators whose function is to control the ratio TCGA3 or TCGB3 of the third membrane separation stage E3 by acting on the temperature of the supply 4' of the third stage by means of the temperature regulation device H3.

[0059] The desired setpoint value of the TCGA30 or TCGB3o ratio under nominal conditions is entered as SP_INT value of the OR19 device and the OR19 device receives as MEASUREMENT value the value of the TCGA3 or TCGB3 ratio calculated in real time. The output of the OR19 device is a working temperature setpoint sent in SP_EXT to the OR20 device.

[0060] The device OR20 compares the setpoint SP_EXT coming from OR 19 with the temperature MEASUREMENT value of the supply 4' of the third stage E3 measured by the temperature sensor T_F3 and acts on the temperature regulation device H3 so that it modifies the temperature of the permeate 4 of the first stage E1 so that the supply 4' is at the right temperature so that the value TCGA3 or TCGB3 approaches the corresponding value TCGA30 or TCGB3o.

[0061] [Fig.5] is a repeat of [Fig.4], with the installation of the regulation organs OR21 to OR24 in place of the regulation organs OR 15 to OR20, the organs OR1 and OR2 being identical.

[0062] In this case, the membranes of the separation stages E1, E2 and E3 are identical. Under nominal conditions, the temperatures of the feeds 2', 3' and 4' are identical. The behavior of all stages is similar.

[0063] A single master regulator, regulator OR21, is used, in combination with slave regulators OR22 to OR24.

[0064] The member OR21 and the members OR22, OR23 and OR24 constitute a cascade regulation whose function is to control the ratio TCGA' or TCGB' of the first membrane separation stage E1 and to act on the temperatures of the feeds 2', 3' and 4' by means, respectively, of the temperature regulation devices H1, H2 and H3.

[0065] The desired setpoint value of the TCGA'o or TCGb'o ratio under nominal conditions is entered as SP_INT value of the OR21 member and the OR21 member receives as MEASUREMENT value the value of the TCGa1 or TCGb1 ratio calculated in real time. The output of the OR21 member is a working temperature setpoint sent in SP_EXT to each of the OR22, OR23 and OR24 members.

[0066] The members OR22, OR23 and OR24 compare the SP_EXT setpoint from OR21 with the temperature MEASUREMENT value respectively of the power supply 2' of the first stage E1 measured by the temperature sensor T_F1, of the power supply 3' of the second stage E2 measured by the temperature sensor T_F2 and of the power supply 4' of the third stage E3 measured by the temperature sensor T_F3. The members OR22, OR23 and OR24 then act on the temperature regulation devices H1, H2 and H3 respectively so that they modify the temperature of the streams 2, 3 and 4 respectively so that the power supplies 2', 3 / and 4' respectively are at the correct temperature so that the value TCGA' or TCGb1 approaches the corresponding value TCGa'o or TCG b'o.

[0067] [Fig.6] is a replication of [Fig.5], with the installation of the regulating organs OR25 and OR28 instead of the regulatory bodies OR21 to OR24.

[0068] In this case also, the membranes of the separation stages E1, E2 and E3 are identical. Under nominal conditions, the temperatures of the feeds 2', 3' and 4' are identical. The behavior of all stages is similar.

[0069] A single master regulator, regulator OR25, is used, in combination with slave regulators OR26 to OR28.

[0070] The OR25 device and the OR26, OR27 and OR28 devices constitute a cascade regulation whose function is to control the CSP consumption of the process and to act on the temperatures of the feeds 2', 3' and 4' by means, respectively, of the temperature regulation devices H1, H2 and H3.

[0071] The desired set value of the CSP0 consumption under the conditions nominal values ​​is entered as SP_INT value of the OR25 device and the OR25 device receives as MEASUREMENT value the value of the CSP consumption calculated in real time. The output of the OR25 device is a working temperature setpoint sent in SP_EXT to each of the OR26, OR27 and OR28 devices.

[0072] The members OR26, OR27 and OR28 compare the setpoint SP_EXT from OR25 with the temperature MEASUREMENT value respectively of the power supply 2' of the first stage E1 measured by the temperature sensor T_F1, of the power supply 3' of the second stage E2 measured by the temperature sensor T_F2 and of the power supply 4' of the third stage E3 measured by the temperature sensor T_F3. The members OR26, OR27 and OR28 then act on the temperature regulation devices H1, H2 and H3 respectively so that they modify the temperature respectively of the streams 2, 3 and 4 so that the power supplies respectively 2', 3 / and 4' are at the right temperature so that the consumption value CSP approaches the corresponding value CSP0.

[0073] The following Examples illustrate the present invention without, however, limiting its scope.

[0074] In these examples:

[0075] All percentages indicated are molar percentages.

[0076] The purification process used comprises three membrane stages and follows the architecture of the process illustrated in [Fig.l].

[0077] A raw gas composed, under nominal conditions, of 55% methane (component B), 44.3% carbon dioxide (component A), 0.2% oxygen and 0.5% nitrogen was separated. When the composition of the incoming raw gas is changed, the methane composition is varied and the carbon dioxide composition is adjusted to obtain 100%.

[0078] The raw gas flow rate is set at 400 NmVh under nominal conditions.

[0079] The temperature of the feed streams passing through the membranes is 25°C under nominal conditions. Pressure losses in the pipes and membranes are neglected.

[0080] The purification efficiency is defined as the molar flow rate of methane recovered from the retentate 5 divided by the molar flow rate of methane entering the raw gas 1. The value of the purification efficiency is set at 99.2%.

[0081] The supply pressure of stage El is set at 10 barg (1.0 MPa relative to the pressure gauge).

[0082] The pressure of the permeate of stage E2 is imposed at 0.1 barg (10000 Pa relative to the manometer).

[0083] The pressure of the permeate of stage E3 is imposed at 0.001 barg (100 Pa relative to the manometer).

[0084] The supply pressure of stage E3 is controlled by a control valve V1 located on the retentate of stage E3. This regulation aims to guarantee a purification efficiency of 99.2%.

[0085] Hollow fiber membranes marketed by UBE of the CO-810-FSC type with selectivity 72.7 at 25°C are used on the three separation stages, respectively 5, 4 and 5 membranes on stages E1, E2 and E3.

[0086] Specific consumption is defined as the quantity of electrical energy required by the process reduced to the quantity of raw gas treated. It is therefore expressed in kWhe / Nm3 of raw gas.

[0087] In the Examples, it is calculated using the following formula (1)

[0088] CSP—PucoMp+PliAL'^ (1)

[0089] Formula in which: • Q is the raw gas flow rate in Nm3 / h; • Pucomp is the power required (in kW) by the compressor Cl assuming an isentropic efficiency of 0.7; • PuCF is the power required (in kW) by the cooling unit located at the outlet of compressor Cl in order to cool the gas, assuming a coefficient of performance of 2.5; • PuAUX is the power required (in kW) by various auxiliaries necessary for the pretreatment of the raw gas before entering the purification process (not shown in the Figures).

[0090] Under nominal conditions, the following results are obtained: • a process recycling rate of 0.2871. (Definition of recycling rate = sum of molar flow rates 6 and 7 divided by molar flow rate 1); • a molar fraction of CH4 in retentate 5 of stage E2 of 97.34%; and • a calculated theoretical electricity consumption of 0.190 kWhe / Nm3 of gas raw.

[0091] These are the results that we seek to approach when the nominal flow rate of the raw gas or the nominal composition of the raw gas changes over time.

[0092] Example 1 (according to the invention) according to the diagram in [Fig.2]:

[0093] The process diagram described in [Fig.2] is used.

[0094] Table 1 shows the process results after changing the incoming raw gas flow rate to 90%, 75% and 50% of the nominal flow rate value.

[0095] [Tables 1] ÀÏSC rgguteiKXi on te RSC Without æ-gsteSiors on te 8SC ÀVgS' nsguteSste on .fe RSC .Without réguiatferc on te RSC Arec régussfeoïï on te RSC «sçuiasan on te RSC WWS) 92.34% 93% ? S % 53% 97.79.% 97.37% '35.37% SS. 32% §8.3? % CSP ter Landscape 3 SC 38?1 % e.isi 29.20 % as as 32 55 % 0 1% 39.85% 01% 42.79 % 2.198 M 75% e / ss 506% 1er T. F» S'Ci 25 55 45.73 23.05 19.65 25.33 3.92 25 3? RSC ■ 5 72 3 13 '7.2S 03 5.53 2é' èisgë T F2 C'C j 25 03 2W 25.30 25.35 3.72 25.83 S 55 8 .SS 7.44 8.53 8.55 4.0 T S3 CCÇ 25.03 WS 25 30 22^3 3OS 15.16 23 33 RSC 3 s 2.6? 42.87 <3.71 12^7 ®;74 12.37 14.0

[0096] As can be seen in Table 1, the regulation of the RSC values ​​of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under the nominal conditions while the values ​​obtained without regulation deviate much more. The electrical consumption increases in particular in the absence of regulation of the RSC' values ​​because of the increase in the recycling rate when the feed flow rate decreases.

[0097] Example 2 (according to the invention) according to the diagram in [Fig.3]:

[0098] The process diagram described in [Fig.3] is used.

[0099] Table 2 shows the results of the process after changing the gas flow rate. gross inflow at 50% and 150% of the nominal flow value.

[0100] [Tables2] With on ïë TC reoiffer. on the TC With regtilaSso ssr te TC regs.d3Éi«?i on the TC ■ % do norwîSTj %CK4_»2 toc % 87.34 % 57% SS.27 % 98.87% S8J?% 94.6S% CSP XS 'kWh <Wi ij 0 38-71 % 0.193 28,95-% 85.36 % Ci,18i 31 18% 0.188 23,10 % T_F1 i’Q 35.® 4,25 25..® 38,51 2S.SS TC_CH4_1 'SSWkQr 2 44E+0S 2.44E+89 1 4=3.+28 2,44E+-33 3.3SE+8S Se étegè T_F2 PC'S 35 00 3,98 25 22 38,30 25,® TC_CH4_2 2,®E+89 1,22E+® 2.2SE+® V7Ê+09- T F3ÇC1 25,® $> 57 25.® 35.® 25.88 ssCaiAg j 2.44E+D8 2.445+35 3.53E+S8 2..44E1-®

[0101] As can be seen in Table 2, the regulation of TC values ​​on methane of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate from them much more.

[0102] Example 3 (according to the invention) according to the schematic of [Fig.5]:

[0103] The process schematic described in [Fig.5] was used.

[0104] Table 3 shows the results of the procedure after varying the gas flow rate gross inflow at 75%, 50%, 125% and 150% of the rated flow value.

[0105] [Tables3] Ç ax Ci l 5¾ Q *>x SO rt^: O <^ ÇN' O a Ki c* O 1* w N bri' «N Cft O + LU Ù3 •05 cî' OV) CM R + LU & c-î R en £3 'ï f M CM cS s RT mJ iv te LUJ ÿ I*- * JS! £ SS. . •nw! & £ ü X» ■<C ; hjft es CM g O' 01 01 «N U'i CM iÎj Cl R O) LU c-f R wi CM ÙJ S> ** CM 'l1 t < Cl Cj ¢1 Cj + lu CM ¢9 S UJ h This is LU Without anything on the TCG O kf? >£.< cm •ÿ Page ¢1 O IC. ' M * UJ Û1 ■X1 is to UŸ CM O + AT HOME There> Cl <0 Q -b AT «> AS- [w « JS! « s^, 4'0 CM a) <s>cm o C' t- tj'.i<N + UJ T? k+ LU T-> s ¥ Ol g U « Æ $ S? LM U?! ..■S U5 K Cl CM CJ en rJ Q a sn cm 01' & .+ LU Ci £3 U1 Cm •0) + LU 05 *1 QO UM CJ Cft O itl ttJ SP « S1 < s § 97.81¾ 1 en C3 CM ry fl »3 id èr? O •E LU fe O? *0 kiS O i LU K. 15 iC i.Û \wm Q •h LU 8 MM%IO 05 tù CM U'S CM UJ rx> es Ml' CJ O + LU i-à çn. O IÏ5 «•> >• i 05 to î-UJ b--O? -, se ■B § ns fN ce s1 CM CSP vs <K-zrh / ^3; 4) g iv U. b" '5 k K "ê CJ y .J O î r* C CJ î, lC< U. 1" 01. 1 •w X <J £ o1 s h ® ter êîKgg Æi s* >>(p 3rd floor

[0106] As can be seen in Table 3, the regulation of the TCG values ​​on the methane of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate from them much more.

[0107] In this particular case where the membranes are identical on all stages and the controlled quantity is the TCG, the supply temperature is the same for the three stages.

[0108] Table 4 shows the results of the process after changing the composition of the incoming raw gas to a methane mole fraction of 50% and 60%.

[0109] [Tables4] tes sir te TCG Ssïïs J ?L ' * " ""V Gj: With on te TC S CsTscosam F0CK4 55 % 59% %CH4 R2 §7,34 % 97.27 % 87..51% 97,41 % 87.17 % CSP ve bÆgôE sm 0 5¾ Q Fit 0.193 8,189 28,71 % 23,3g % 31.85 % 25 21 % 1st floor 25.60 25. æ 27,87 25.05 1.97E+Q9 4 97E+Û8 1,79Efæ' 1.87E+89 249=+09 2nd floor T FO CCI .21,92. 25.88 37.87 35» TCG_CH4_2 1.S7E+O8 1.79E%% 1.57E+0S 245E+0® 3rd stage T Fsro .25..00 .21.92 25.80 27.87 .35,S» TCG_CH4_3 s WE+0S t .9^+60 1J^+S9 1.97E+SS

[0110] As can be seen in Table 4, the regulation of the TCG values ​​on the methane of the separation stages E1, E2 and E3 by adjusting the feed temperature of each stage allows the values ​​of the CH4 content in the retentate 5, the recycling rate and the electrical consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate from them much more. [YES] Example 4 (according to the invention) according to the diagram in [Fig.61:

[0112] The process diagram described in [Fig.6] is used.

[0113] Table 5 shows the process results after changing the incoming raw gas flow rate to 75%, 50%, 125% and 150% of the nominal flow rate value.

[0114] [T ableaux5 ] ~ to. 0 <y's sO a r o l 1 ûa Sï | 25.03 | ô KO CM "3 & sa CS a x< w • 1$ Kx- & C| ex ïï K SJX an <5 h* s’ w s» O ¢0 ev fi. à** ■M r 4 U'? O Q O •MM* co Ô’J ri o & O) £ i* tfi LÔ s »x> iM 'A ÀO CM yj CM CL 0 ,<■ -.nob .►V» •K'-' t-.^. J _LI CO ■li' SS? V- ■> M*' & 0 h 1 ecj ex r' LU •A 0 - .X ei Üv K» b-%. to rO ri ri «> ¢0 O ex UJS rtS CJ uv u7 Kft W $v <3 CO CM IM ri SGS 1.0 H 43 CI U 5,1 ■7 CM cc? A -j tï> £ 5<zr & si rz i OM ... ù. .b CA 0 s» $ « b> n*i OO ss ü» S* «S' CM CM' u< U'j ws» Ù7 dh 1 <M .- « t)’-' ù- O «5 s §§ g as & Y? à bx’ 1 t tr ir idî xr is. v.' !?ï i 1 •^- SJ? x <b ►•xi cu r c>0 <h 0 O & !■< yf‘ ÙM iïj CJ tA IfX <r>O CJ C 4 ■'4 T?. tN O -*- ... ... £ ç» F *7 0 ç? (j H X..' 1 '1

[0115] As can be seen in Table 5, the regulation of the CSP consumption by adjusting the supply temperature of each stage allows the values ​​of the CH4 content in retentate 5, recycling rate and electricity consumption to approach the values ​​obtained under nominal conditions while the values ​​obtained without regulation deviate much further.< / r> < / s>

Claims

Claims

1. - Method for purifying a raw gas (1) by membrane separation, said process using n membrane separation stages, n being greater than or equal to 1, the or each membrane separation stage, noted i with 1 < i < n, being constituted by: at least one separation membrane whose permeability depends on temperature; a gas supply inlet to said stage i through which: for i=l, said raw gas to be treated (1) is introduced, where appropriate in combination when n=l with a permeate recycling stream and / or a retentate recycling stream from the single stage and when n>l with a permeate recycling stream (6) and / or a retentate recycling stream (7), said recycling streams coming from another stage; and for i>l, the permeate or the retentate from one of the other stages is introduced, a retentate outlet; and a permeate outlet, said gas to be treated (1) consisting of a mixture of at least two constituents A and B, the permeability of the membranes for constituent A being greater than that of constituent B, characterized in that it comprises the following steps: a nominal flow rate and / or a nominal composition of the incoming raw gas to be treated is established (1), representing the nominal operating point; for each separation stage i, at least one nominal characteristic operating quantity chosen from: the RSC'o ratio, defined as the SCa'o / SCb'o or SCbVSC^o ratio, SCa'o and SCb'o being each defined as the ratio of the partial flow rate of the constituent A or B respectively in the permeate stream leaving stage i, noted respectively Pe^o and PcH'o, to the partial flow rate of the same constituting in the power supply of the same stage i, noted respectively FA' o and Fb'o; • the charge rate of constituent A or B at stage i, noted respectively TCa'q and TCb^, defined as the ratio Fa'q / Pa'o or Fb'q / Pb'o» Fa'o and FBi0 being the partial flow rate of component A or B in the supply of stage i and PA'o or PH'o the permeability of the membranes at the temperature of the nominal operating point for component A or B on stage i; • the overall loading rate of component A or B at stage i, noted respectively TCGA'o and TCGb'o, defined as the ratio of the flow rate of component A or B in the nominal flow rate to the permeability of the membranes at the temperature of the nominal operating point for component A or B respectively at stage i, i.e. PA'o or PH'o; and • in the case where operation is carried out with several stages, where identical membranes are used on all stages i and the same feed temperature is established on all stages i for the nominal flow rate and / or the nominal composition of the incoming raw gas to be treated, the specific consumption CSP0, defined as the energy consumption of the process divided by the nominal flow rate of an incoming raw gas to be treated (1) or of at least one stream leaving the process under the established nominal conditions; • when the flow rate and / or composition of the incoming raw gas (1), measured before any possible mixing with at least one recycling stream, changes during the course of the process, the permeability of at least one membrane in at least one stage i is adjusted and thereby the quantity of constituents A and B passing through the membrane(s) concerned, by adjusting the temperature of the feed to the corresponding separation stage so that at any time, and independently for each stage i, at least one of the RSO ratio or the TCA' or TCB' loading rate or the overall TCGA' loading rate or TCGB' or CSP, measured and / or calculated in real time on each separation stage, approaches its corresponding reference value, RSC'o or TCA'o or TCb'o or TCGa'o or TCGb^ or CSPq.

2. - Method according to claim 1, according to which one operates with several stages, characterized in that one uses identical membranes on all stages i and one establishes the same supply temperature on all stages i for the nominal flow rate and / or the nominal composition of the incoming raw gas to be treated, one chooses as nominal characteristic quantity the overall loading rate TCGA'o or TCGb'o, and in the case of a change in the flow rate and / or the composition of the incoming raw gas, one adjusts the supply temperature in all stages using a single regulator associated with the overall loading rate at any stage.

3. - Method according to one of claims 1 and 2, characterized in that at any time and in real time and for each stage i, the permeability of the membranes for the constituent A or B, i.e. PA'(t) or PBi(t) is determined by measurement and / or calculation of the pressures, temperatures, flow rates and compositions of the incoming current and outgoing currents, and by numerical resolution.

4. - Method according to one of claims 1 to 3, characterized in that the separation membranes are polymer membranes chosen from polysulfone membranes and polyimide membranes.

5. - Method according to one of claims 1 to 4, characterized in that the temperature of the supply of each stage is adjusted using a heat exchanger.

6. - Method according to one of claims 1 to 5, characterized in that the constituent A of the raw gas is carbon dioxide, and the constituent B is methane.

Images