Process for separating co2 from natural gas

Abstract

The present invention relates to a process for the separation of CO2 from natural gas by using a membrane with a selectivity for methane greater than for CO2. The reverse membrane generates a permeate with low CO2 content that will be used as fuel gas, thus reducing the energy demand of the CO2 reinjection system, since the stream with high CO2 content will remain at high pressure.

Description

Technical Field

[0001] The present invention relates to a method for separating CO2 from natural gas by using a membrane (hereinafter referred to as a reverse membrane) that is more selective for methane than for carbon dioxide.

[0002] This invention is applicable to the field of oil and associated gas production and is designed for operation in a full gas reinjection mode, i.e., no output, wherein the CO2 content is higher than 40 mol%. Background Technology

[0003] Currently, in the natural gas processing systems used on the platform, natural gas from the oil processing system is typically compressed, then passed through a dehydration system, and finally sent to the hydrocarbon dew point control and acid gas removal system to meet the requirements of the gas processing membrane.

[0004] Typically, when the gas content is approximately 10% to 75% CO2, the CO2 removal process in the total reinjection unit is performed by separation using a CO2-selective polymer membrane. The purpose of this process is to produce a fuel gas stream with a low CO2 content (relative to the generated content), and thus reduce greenhouse gas emissions primarily associated with turbine generator exhaust.

[0005] The CO2-selective membrane method operates at approximately 25 to 80 bar (2500 kPa to 8000 kPa). This separation method produces two streams: a permeate with low CO2 content (“CO-lean”) at high inlet pressure; and a permeate with high CO2 content (“CO-rich”) at low pressure. The permeate is used for fuel gas, while the permeate stream requires significant energy expenditure for recompression and reinjection into the reservoir.

[0006] As an example of existing technology Figure 1 A CO2 removal system consisting of a pre-membrane and a membrane is shown, typically with an inlet and an intermediate heater.

[0007] Because the membranes currently used are selective for CO2, CO2 is preferably permeated (CO2-rich and at low pressure), leaving the membrane system at a low pressure of approximately 4 bar (400 kPa) for subsequent compression and reinjection. Natural gas-rich streams with low CO2 content leave the membrane system, prepared for use as fuel gas consumption or for output and gas lift, such as... Figure 2 As shown.

[0008] Existing technologies disclose several documents using polymer membranes with superior selectivity for CO2 compared to methane, among which:

[0009] Document BR 112018011305-0 relates to the purification of natural gas using gas separation membranes. In this sense, the document discloses a separation unit comprising a gas separation membrane unit, which includes one or more membranes connected in parallel or series, wherein one unit has a selectivity for CO2 that is superior to that for methane. The membrane units in this document contain polymers or copolymers. However, the document does not describe the use of a reverse membrane with a selectivity for methane that is superior to that for CO2 to separate CO2 from natural gas.

[0010] Document US 6,565,631 describes an invention comprising a method for separating CO2 from natural gas, the method comprising: contacting a mixture of CO2 and natural gas with a first side of a carbon membrane such that a portion of the mixture passes through the carbon membrane to a permeate side. The resulting mixture on the permeate side becomes CO2-rich relative to the mixture on the first side. The carbon membrane comprises an asymmetric hollow filamentous carbon membrane containing partially carbonized products of symmetrical hollow filaments containing an aromatic imide polymer material. However, this invention does not describe the use of a reverse membrane, which has a higher selectivity for methane than for CO2, to separate CO2 from natural gas.

[0011] An embodiment of documents BR 112012011483-2 and BR 122020006337-8 relates to a method for reducing the concentration of acid gases in a natural gas stream, the method comprising providing a carbon membrane. Another aspect of the invention is a method for optimizing the CO2 / CH4 selectivity of the carbon membrane. It may be noted that these documents do not describe the method of the present invention using a reverse membrane to separate CO2 from natural gas.

[0012] Reference BR 112017013042-4 discloses membranes with different selectivity at different separation stages. However, it does not mention the preferential selectivity of CH4 over CO2.

[0013] Document WO 2018 / 236750 discloses a method for recycling compressed permeate to a distillation unit. However, this separation is intended to obtain CO2.

[0014] Document WO2016153958 discloses that permeate streams or other byproduct streams (e.g., NGL flash streams) can be recycled for fuel instead of being fed into a flare. However, it does not discuss the selectivity of the permeate for CH4.

[0015] Document WO 2017 / 069990 describes the advantages and disadvantages of cellulose acetate membranes in the field of gas separation of gas mixtures. This document discloses that cellulose acetate membranes still require improvement in many properties, including selectivity, performance durability, chemical stability, resistance to hydrocarbon contaminants, resistance to solvent swelling, and resistance to CO2 plasticization. However, it does not discuss the selectivity of permeate for CH4.

[0016] Given the prior art, those skilled in the art can confirm that no literature discloses a method for separating CO2 from natural gas using a reverse membrane, such as the present invention, which aims to reduce the equipment and energy costs associated with such gas processing methods that produce fuel in an SPU (Stationary Production Unit). Summary of the Invention

[0017] The objective of this invention is to separate CO2 from natural gas using a membrane that exhibits superior selectivity for CH4 compared to CO2. In this manner, the operation using a reverse membrane produces a "CO2-lean" permeate, which will be used as fuel gas, thus reducing the energy requirements of the CO2 reinjection system, as the CO2-rich stream will continue to operate under high pressure. Attached Figure Description

[0018] To make the invention more readily understood, the accompanying drawings numbered 1 to 3, which are attached to and form part of this specification, are provided by way of example but are not intended to limit the invention.

[0019] Figure 1 A simplified diagram of a prior art CO2 removal system is shown.

[0020] Figure 2 A simplified diagram of a prior art gas compression system following a membrane is shown.

[0021] Figure 3 A simplified diagram of the gas compression system following the reverse membrane method is shown. Detailed Implementation

[0022] This invention relates to a method for separating CO2 from natural gas using a reverse membrane. Reverse membranes are those produced such that methane permeability is greater than CO2 permeability (i.e., the selectivity for methane (CH4) is greater than 1 relative to carbon dioxide (CO2). In this manner, the treated gas stream will be permeate, unlike the situation in prior art membranes where the target gas is in the permeate, hence the name reverse membrane. The membrane comprises a polymer, inorganic, or mixed matrix material and can be in the form of hollow fibers or planar fibers.

[0023] An example of an inorganic membrane is the alumina membrane, which primarily exhibits a Knudsen-type transport mechanism, with its diffusion coefficient inversely proportional to the square root of the molecular weight. Therefore, for alumina membranes, methane, due to its lower molecular weight than CO2, has a higher diffusion coefficient, ensuring greater permeability across the membrane. Consequently, the CH4 / CO2 selectivity is greater than 1, making it suitable for reverse membrane methods used for CH4 separation.

[0024] In this manner, the present invention describes a method for separating CO2 from natural gas, the method comprising the following steps:

[0025] - A gas stream consisting of natural gas is fed into a gas separation unit comprising one or more membranes connected in series or parallel with selective layers, the selective layers being more selective for methane than for carbon dioxide; wherein the feed stream has a flow rate comprising 60% to 90% CO2 and a flow rate of 4 to 12 MMSCMD (million standard cubic meters per day), and operates at a feed pressure of 40 bar (4000 kPa) to 60 bar (6000 kPa);

[0026] - Remove the permeate stream enriched with methane relative to the feed gas at permeate pressures of 4 bar (400 kPa) to 16 bar (1600 kPa);

[0027] - Remove the permeate stream enriched with carbon dioxide relative to the feed gas at permeate pressures of 40 bar (4000 kPa) to 60 bar (6000 kPa).

[0028] One or more membranes in the separation unit are made of polymer, inorganic or mixed matrix materials.

[0029] As a result of using a reverse membrane in the method of the present invention, the permeate stream comprises a flow rate of 8% CO2, 90% CH4, and 0.73 MMSCMD, and the residual stream comprises a flow rate of 67% CO2, 23.9% CH4, and 5.27 MMSCMD. The permeate stream contains up to 20% CO2, which allows it to be combusted in the turbine.

[0030] In this way, unlike existing technologies, the operation of a reverse membrane produces a "CO2-lean" permeate, which is used as fuel gas, thereby reducing the energy demand of the CO2 reinjection system because the CO2-rich stream is maintained at high pressure. Therefore, the permeate gas stream is directed to the fuel gas system, and the residual gas stream is directed to the gas injection system.

[0031] The nominal capacity of separation and compression equipment is expressed by the gas flow rate supplied to it. In this way, reducing the gas flow rate means reducing the capacity and therefore the cost of the equipment. The reverse membrane method reduces the flow rate of the low-pressure gas stream generated in all membrane separation processes. This effect is amplified when the CO2 content of the gas to be treated exceeds 60 mol%. In this case, the permeate flow rate produced by a conventional membrane is 50% higher than that obtained using the reverse membrane method.

[0032] Using a reverse membrane allows for a flow rate reduction of approximately 30% compared to using a conventional membrane, which can be even greater depending on the CO2 concentration. This results in a 30% reduction in compression power consumption and a 20% reduction in the CAPEX of the compression and CO2 removal system.

[0033] Therefore, when compared with existing membranes, reverse membranes will potentially minimize the recompression flow and energy consumption of fuel gas generation systems in oil and gas production units.

[0034] Example

[0035] Those skilled in the art will value the knowledge provided herein and will be able to reproduce the invention in the embodiments and other variations provided, which are covered by the scope of the appended claims. In this manner, the following are examples describing the experimental procedures used.

[0036] To generate the results, a natural gas stream with a composition of approximately 60% CO2 and a flow rate of 6 MMSCMD was tested. From this stream, approximately 0.73 MMSCMD of fuel gas, with less than 10% CO2, would need to be produced.

[0037] Table 1 shows the flow rates of permeate (CO2-rich) and residual (CO2-lean) streams obtained using existing membranes under these fuel gas production conditions. Under these conditions, the fuel gas is a residual stream, and the permeate must be compressed and directed to the gas injection system.

[0038] For example, a reverse membrane was simulated, considering that the CO2 permeability of the reverse membrane was opposite to that of the CH4 permeability of the cellulose acetate membrane. To maintain consistency, the same relative permeability was maintained for other hydrocarbons.

[0039] Table 1 also provides the flow rates of permeate and residual streams obtained under the aforementioned fuel gas production conditions, but using a reverse membrane. Under these conditions, the fuel gas is a permeate stream, which must be compressed and directed to the fuel gas system.

[0040] Table 1 - Expected Traffic

[0041] Existing membranes Reverse membrane Feed flow rate [MMSCMD] 1.86 6.00 Permeate [MMSCMD] 1.13 0.73 Retentate [MMSCMD] 0.73 5.27 <![CDATA[Permeation area[m 2 >< 32,000 20,000

[0042] Use the following conditions:

[0043] -Feed pressure-50 bar (5000 kPa);

[0044] -Osmotic pressure -4 bar (400 kPa); and

[0045] -Residue pressure -48 bar (4800 kPa).

[0046] In addition to the results above, Table 2 provides the composition of the feed and output flows for existing and reverse membranes.

[0047] Table 2 - Flow Composition

[0048]

[0049]

[0050] Therefore, due to the lower permeate flow rate resulting from the reverse membrane, the present invention allows for: - a reduction in separation equipment;

[0051] -Reduce the number of compression devices;

[0052] - Lower energy consumption during service;

[0053] - Lower greenhouse gas emissions.

Claims

1. A method for separating CO2 from natural gas, characterized in that, The method includes the following steps: - A gas stream consisting of natural gas is fed into a gas separation unit comprising one or more membranes connected in series or parallel with selective layers in which the selectivity for methane is superior to that for carbon dioxide; wherein the feed stream has a flow rate comprising 60% to 90% CO2 and a flow rate of 4 to 12 MMSCMD, and operates at a feed pressure of 40 bar (4000 kPa) to 60 bar (6000 kPa); - Remove the permeate stream, which is rich in methane compared to the feed gas, at permeate pressures of 4 bar (400 kPa) to 16 bar (1600 kPa); - Remove the permeate stream, which is rich in carbon dioxide compared to the feed gas, at a permeate pressure of 40 bar (4000 kPa) to 60 bar (6000 kPa). The membrane of the separation unit is made of a polymer, inorganic or mixed matrix material.

2. The method according to claim 1, characterized in that, The permeate gas stream is directed to the fuel gas system, and the excess gas stream is directed to the gas injection system.

3. The method according to claim 1, characterized in that, The permeate stream is composed of up to 20% CO2, allowing it to burn inside the turbine.

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