Process and apparatus for liquefying a gas rich in carbon dioxide

Abstract

Title of the invention: Method and apparatus for liquefying a carbon dioxide-rich gas. An apparatus for liquefying a carbon dioxide-rich gas comprises a first heat exchanger (E1), a second heat exchanger (EC101), an organic Rankine cycle comprising a turbine (T1) and a pump (P1), means for sending a flow of liquefied natural gas (LNG) to the first heat exchanger where it is heated to form a heated flow, means for sending the heated flow to be reheated in the second heat exchanger to form a natural gas (NG) flow, means for sending a carbon dioxide-rich gas flow (1) to the second heat exchanger to be at least partially liquefied by indirect heat exchange with the heated flow, the organic Rankine cycle being connected to the first heat exchanger to supply heat to it. Abstract figure: Figure 2

Description

Title of the invention: Method and apparatus for liquefying a gas rich in carbon dioxide

[0001] The present invention relates to a method and apparatus for liquefying a gas rich in carbon dioxide.

[0002] A carbon dioxide-rich gas contains at least 90 mol% carbon dioxide, preferably at least 95 mol% carbon dioxide, or even at least 99 mol% carbon dioxide.

[0003] In liquefied natural gas (LNG) terminals, LNG is stored in cryogenic conditions at low pressure. In order to use it in gaseous form and transport it over long distances by pipeline, the LNG must be pumped and vaporized. At most LNG terminals installed worldwide, this vaporization is achieved by using either seawater as a heating medium or flue gases produced by burning some of the natural gas. In both cases, the cold is simply lost to the environment, which is inefficient.

[0004] Today, the use of organic Rankine cycles is being studied and demonstration plants are beginning to be installed worldwide. This process is based on the use of pure or mixed components as a heating medium.

[0005] While the Rankine cycle can operate with water as the working fluid for applications such as geothermal heat recovery, the use of organic fluids evaporating at low temperatures makes it possible to exploit low-temperature cold sources. This is known as the organic Rankine cycle or ORC cycle (for "Organic Rankine Cycle" in English).

[0006] ORC cycles are conventionally industrialized using LNG as a cold source and seawater as a hot source, but they have relatively low energy yields, of the order of 20 kWh per ton of vaporized LNG, i.e. 0.015 kWh / Nm3.

[0007] According to an object of the invention, there is provided a method for liquefying a gas rich in carbon dioxide in which: i. A flow of liquefied natural gas or natural gas, or supercritical natural gas, the flow being at most -100°C, is sent to a first heat exchanger where it heats up forming a heated flow ii. The heated flow is heated in a second heat exchanger forming a natural gas flow at a temperature above 0°C iii. A carbon dioxide-rich gas stream at a pressure of at least 20 barg is sent to the second heat exchanger to at least partially liquefy by indirect heat exchange with the reheated stream and an at least partially liquefied stream exits the second heat exchanger iv. An organic Rankine cycle provides heat to the first heat exchanger by sending an expanded gas into a turbine in the cycle, the expanded gas liquefies in the first heat exchanger forming a liquid, the liquid is pressurized by a pump, the pressurized liquid heats up in the first exchanger and then in the second exchanger before being sent to the turbine.

[0008] According to other optional aspects: • liquefied natural gas vaporizes in the first heat exchanger. • the pressurized liquid vaporizes at least partially in the first heat exchanger. • the pressurized liquid or the pressurized liquid at least partially vaporizes enters the second exchanger at between -10°C and -50°C. • a second flow rich in CO2 at a pressure of at least 40 bars g, liquefies in the second heat exchanger forming a second liquefied flow. • the partially liquefied stream leaving the second exchanger is separated by partial condensation and / or by distillation to form a liquid product rich in CO2 and a gas containing CO2 which is returned to the stream of gas rich in carbon dioxide at a pressure of less than 20 bar g, being compressed with the stream of gas rich in carbon dioxide in a compressor. • the partially liquefied flow leaving the second exchanger is separated by partial condensation and / or by distillation to form a liquid product rich in CO2 and a gas containing CO2, at least part of the gas heating up in the second heat exchanger. • electricity is generated using a generator connected to the turbine. • a liquid formed by partial condensation and / or distillation is expanded to a first pressure, possibly lower than the storage pressure, sent to a phase separator operating at this first pressure and the liquid from the separator is pressurized to a pressure higher than the first pressure.

[0009] According to another object of the invention, there is provided an apparatus for liquefying a gas rich in carbon dioxide comprising a first heat exchanger, a second heat exchanger, an organic Rankine cycle comprising a turbine and a pump, means for sending a flow of liquefied natural gas or natural gas or supercritical natural gas, the flow being at most -100°C, to the first heat exchanger where it heats up forming a heated flow, means for sending the heated flow to heat up in the second heat exchanger forming a flow of natural gas at a temperature above 0°C, means for sending a flow of gas rich in carbon dioxide at a pressure of at least 20 barg to the second heat exchanger to liquefy at least partially by indirect heat exchange with the heated flow, means for outputting an at least partially liquefied flow from the second heat exchanger, the organic Rankine cycle being connected to the first heat exchanger to provide heat by sending an expanded gas into the turbine of the cycle, means for sending the liquefied expanded gas into the first heat exchanger forming a liquid at the pump,means for sending the pressurized liquid from the pump to the first heat exchanger to be heated and vaporized there, means for sending the vaporized liquid in the first heat exchanger to the second heat exchanger and means for sending the vaporized liquid heated in the second heat exchanger to the turbine.

[0010] According to other aspects of the invention: • the apparatus comprises means for liquefying a second stream rich in carbon dioxide at a pressure greater than 20 barg, or even at least equal to 40 barg at least partially in the first heat exchanger.

[0011] The first and second heat exchangers may constitute a single exchange line, so that the liquefied natural gas (or not) passes from the first to the second heat exchanger without leaving the exchange line. In this case, the first and second exchangers may form part of the same stack of plates forming passages between them.

[0012] Otherwise the first and second exchangers can be separated from each other.

[0013] [Fig-1] illustrates that in this process, the liquefied natural gas LNG is introduced into a brazed aluminum heat exchanger El at high pressure, typically between 40 and 120 bar g, in the supercritical state. The liquefied natural gas is at a temperature of at most 100°C and can be replaced by natural gas or supercritical natural gas. Under these conditions, the densities decrease as the temperature increases and the natural gas NG leaves the exchanger El between -10 and -50°C. In order to reach ambient temperature, the cold NG is introduced into an exchanger E3 supplied with seawater which allows it to be heated between 0 and 15°C depending on the temperature of the seawater.

[0014] On the heat transfer fluid side, the fluid in liquid form is stored in the tank VI, pumped by the pump PI and partially or totally vaporized in the exchanger EL. At the outlet of the exchanger El, the cycle fluid HM is still cold (between -10°C and -50°C) and possibly not completely vaporized and must be reheated to temperature ambient by the E2 exchanger supplied with seawater before being expanded in a T1 turbine. The higher the temperature at the turbine inlet, the better the energy recovery. A V2 tank protects the turbine from liquid overflows in the event of a malfunction. The energy recovered by the turbine is converted into electricity by the generator. This allows a recovery of 35 kWh / t LNG but some of the cold is still lost in the seawater.

[0015] As the effects of global warming become more noticeable every day, the capture and liquefaction of CO2 in industrialized regions is being studied, with this CO2 then being transported by ship to very distant underground storage sites.

[0016] Each industrial site produces gases containing CO2 with different compositions. For this reason, CO2 purification processes are often far removed from liquefaction processes that collect CO2 from several sources.

[0017] It is known from CN105545390 to use the frigories of a flow of liquefied natural gas to liquefy CO2 and to produce electricity with two organic Rankine cycles. However, the present invention uses a single organic Rankine cycle, the cycle fluid heating up against the CO2-rich gas which liquefies at least partially. The advantage lies in greater ease of operation.

[0018] The present invention proposes to use the frigories from the vaporization of liquefied natural gas or from the heating of natural gas to provide cold for an organic Rankine cycle and then to liquefy a CO2-rich flow.

[0019] The invention will be described in more detail with reference to the figures where:

[0020] [Fig.2] represents a method according to the invention.

[0021] [Fig.3] represents a method according to the invention.

[0022] In [Fig.2], a flow of liquefied natural gas LNG heats up in a heat exchanger E1 and vaporizes in a heat exchanger EC101 to produce natural gas NG at the hot end of the exchanger EC101. The liquefied natural gas is at a temperature of at most 100°C and can be replaced by natural gas or supercritical natural gas.

[0023] A gas flow 1 containing at least 90 mol% of carbon dioxide, preferably at least 95 mol% of carbon dioxide, or even at least 99 mol% of carbon dioxide is compressed in a first stage K101-1 of a centrifugal compressor forming a compressed gas 3 and is then cooled forming a cooled gas 5. The flow 1 contains at least one other component lighter than carbon dioxide, for example nitrogen, oxygen, argon, carbon monoxide, hydrogen, methane. The gas 5 is compressed to at least 20 bar g, for example 22 bar g in a second stage K101-2 of the compressor. After cooling the gas 7 compressed in the second stage is sent to the hot end of the heat exchanger. heat EC101 where it cools by passing through the entire exchanger and partially condensing. The partially condensed flow is separated in a phase separator VI02 whose gas 11 feeds a washing column C in the bottom. The liquid 13 from the phase separator VI02 is expanded in a valve and mixed with a bottom liquid 35 from column C. The two flows 13 and 14 are expanded to a pressure of approximately 5500mbarg and sent to a phase separator V103.

[0024] The gas 23 formed in the separator V103 is enriched in at least one lighter component and is sent to mix with the compressed and cooled gas 5.

[0025] The liquid from separator V103 is divided into two, a portion 29 being pressurized by a pump P3. The liquid pumped into P3 is divided into two, a portion 31 being returned to separator VI03 and the remainder 33 serving as liquid product at 7 barg.

[0026] The storage (not shown) for which the liquid 33 is intended is at low pressure and often not right next to the liquefaction unit of [Fig.2]. The pressure is expanded from 22 to 5.5 barg at the liquefaction unit and the gas formed is recycled into the inlet gas 1. By expanding the liquid 13, 14 to a low pressure, this makes it possible to inject a sub-cooled liquid which will compensate for the thermal inputs.

[0027] Thus the liquid 33 is pressurized by the pump P3 downstream of the separator V103 to reach the storage pressure by passing through a pipeline, which involves overcoming the pressure losses.

[0028] The remainder 25 of the liquid is pressurized by the pump P2. The liquid 25 pumped into P2 is divided into two, a portion 27 being returned to the separator V103 and the remainder 26 serving as washing liquid for column C.

[0029] The overhead gas 34 of column C contains carbon dioxide as well as at least one light impurity present in gas 1 such as oxygen, nitrogen, argon, carbon monoxide, etc. This gas 34 is sent to the atmosphere. Otherwise, at least a portion 17 of the gas heats up in the heat exchanger EC101.

[0030] A cold production cycle connects the two heat exchangers EC101, EL A gas 4 is expanded in a turbine T1 forming the expanded flow 43 which cools in the exchanger El against the LNG flow. The expanded gas partially condenses in the exchanger El and arrives in a phase separator VL The liquid formed 45 is pressurized by a pump PI and returned in part 47 to the separator VL The rest of the pressurized liquid vaporizes in the exchanger El forming the flow 49 which heats up in the exchanger EC101 to be sent to a separator V2 and then as flow 4 to the turbine T1, forming a closed cycle.

[0031] Electricity is generated by means of a generator connected to the turbine TL

[0032] This scheme allows 1.7 tonnes of CO2 to be liquefied for 1 tonne of liquefied natural gas. vaporized.

[0033] In a simpler variant, the partially liquefied flow(s) are separated by partial condensation or by distillation. The presence of column C is not essential.

[0034] [Fig.3] differs from the previous figure in that it includes a compressor additional KB. A portion 8 of the CO2-rich flow 7 at 22 bars is boosted in the KB compressor to at least 40 bars g, for example 45 bars g, liquefied in the EC101 exchanger and expanded to a pressure of approximately 5500mbarg to be sent to the VI03 separator.

[0035] This scheme allows liquefying 2 tons of CO2 for 1 ton of vaporized liquefied natural gas. If the process is a little less efficient than the previous one in terms of energy consumption, this scheme on the other hand minimizes the approach of temperatures in the EC101 exchanger, which allows the use of a brazed aluminum plate and fin exchanger less expensive than the technology necessary for [Fig.l] which requires the use of a brazed stainless steel exchanger or a printed circuit heat exchanger or a diffusion bonded heat exchanger.

[0036] Stream 8 is not necessarily formed by overpressurizing a portion of flow 7 but may be an independent CO2-rich stream available at a higher pressure, such that compressor KB is not required.

Claims

Claims

1. A method for liquefying at most two gases, the single liquefied gas or both liquefied gases being carbon dioxide-rich, wherein: i) A flow of liquefied natural gas (LNG) or natural gas, or supercritical natural gas, the flow being at most -100°C, is sent to a first heat exchanger (El) where it heats up forming a heated flow ii) The heated flow heats up in a second heat exchanger (EC101) forming a flow of natural gas (NG) at a temperature above 0°C iii) A carbon dioxide-rich gas stream (1) at a pressure of at least 20 barg is sent to the second heat exchanger to at least partially liquefy by indirect heat exchange with the heated flow and an at least partially liquefied stream exits the second heat exchanger, no part of the carbon dioxide-rich gas stream being sent to the first heat exchanger, iv) An organic Rankine cycle (4,43,45, 47, Tl,PI) provides heat to the first heat exchanger by sending an expanded gas (43) into a turbine of the cycle (Tl), the expanded gas liquefies in the first heat exchanger forming a liquid (45), the liquid is pressurized by a pump (PI), the pressurized liquid heats up in the first exchanger and then in the second exchanger before being sent to the turbine.,

2. A method according to claim 1 wherein the liquefied natural gas (LNG) vaporizes in the first heat exchanger (El).

3. A method according to claim 1 or 2 wherein the pressurized liquid vaporizes at least partially in the first heat exchanger (El).

4. Method according to one of the preceding claims in which the pressurized liquid or the at least partially vaporized pressurized liquid (49) enters the second exchanger (EC101) at between -10°C and -50°C.

5. Method according to one of the preceding claims in which a second CO2-rich flow (8) at a pressure of at least 40 bar g, liquefies in the second heat exchanger (EC101) forming a second liquefied stream, with no part of the second CO2-rich stream being sent to the first heat exchanger.

6. Method according to one of the preceding claims in which the partially liquefied stream (13) leaving the second exchanger is separated by partial condensation and / or by distillation (C) to form a CO2-rich liquid product (33) and a CO2-containing gas (23) which is returned to the carbon dioxide-rich gas stream at a pressure of less than 20 bar g, being compressed with the carbon dioxide-rich gas stream in a compressor (K101-2).

7. Method according to one of the preceding claims in which the partially liquefied flow leaving the second exchanger (EC101) is separated by partial condensation and / or by distillation (C) to form a liquid product rich in CO2 (14) and a gas containing CO2 (34), at least a portion (17) of the gas heating up in the second heat exchanger.

8. Method according to one of the preceding claims in which electricity is generated by means of a generator connected to the turbine (T1).

9. Apparatus for liquefying at most two carbon dioxide-rich gases comprising a first heat exchanger (El), a second heat exchanger (EC101), an organic Rankine cycle comprising a turbine (Tl) and a pump (PI), means for sending a flow of liquefied natural gas (LNG) or natural gas or supercritical natural gas, the flow being at most -100°C, to the first heat exchanger where it heats up forming a reheated flow, means for sending the reheated flow to heat up in the second heat exchanger forming a flow of natural gas (NG) at a temperature above 0°C, means for sending a flow of carbon dioxide-rich gas (1) at a pressure of at least 20 barg to the second heat exchanger to at least partially liquefy by indirect heat exchange with the reheated flow, means for outputting an at least partially liquefied flow from the second heat exchanger,the organic Rankine cycle being connected to the first heat exchanger to provide heat by sending a gas (43) expanded in the turbine of the cycle, means for sending the liquefied expanded gas (45) into the first heat exchanger forming a liquid at the pump, means for sending the pressurized liquid, from the pump to the first heat exchanger to be heated and vaporized there, means for sending the vaporized liquid in the first heat exchanger to the second heat exchanger and means for sending the vaporized liquid (4, 49) heated in the second heat exchanger to the turbine, the apparatus not comprising any means for sending a gas rich in carbon dioxide to the first heat exchanger,

10. Apparatus according to claim 9 comprising means for liquefying a second stream (8) rich in carbon dioxide at a pressure greater than 20 barg, or even at least equal to 40 barg at least partially in the first heat exchanger.

11. Apparatus according to claim 9 or 10 in which the first and second heat exchangers (El, EC101) constitute a single exchange line.

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