Technical field

The present invention relates to pre-treatment of natural gas before it is converted to liquefied natural gas. More specifically, the present invention relates to removal of carbon dioxide (C0 ₂ ) from a natural gas stream. The present invention relates to a system and a method for polishing of the C0 ₂ content in the feed to a natural gas liquefaction system. In addition the present invention comprises use of the system and the method as such. Background art

A natural gas stream usually contains several species that have to be removed before the gas can be converted to liquefied natural gas (LNG). Facilities for LNG production have therefore a pre-treatment section which ensures that the composition of the feed gas to the natural gas liquefaction system meets prescribed specifications. The pre- treatment section in the LNG production facility is a combination of gas treatment systems, each having well defined tasks.

The carbon dioxide (C0 ₂ ) removal system is one of the larger systems within the pre- treatment section in a LNG production facility. Removing C0 ₂ down to a safe concentration is crucial. To avoid freeze-out of solids during liquefaction of the natural gas, removal of C0 ₂ down to a concentration of 50 ppmv is required. Thus, a malfunctioning C0 ₂ removal system can lead to reduced LNG production or in a worst case, blocked process equipment within the natural gas liquefaction system. Traditionally, C0 ₂ is removed from natural gas streams by absorption in a solvent consisting of water and one or more amines. The gas and the solvent are contacted counter-currently in a tall absorber column. The solvent flows downwards in the absorber column, driven by gravity. The gas flows upwards through the column. The C0 ₂ absorber column in a LNG production facility is a tall construction, often 30- 40 m high. Onboard a floating vessel, such as a Floating Production Storage and Offloading (FPSO) vessel, the column will be affected by movement and static tilt because the solvent is driven downwards by gravity. The result is maldistribution of the solvent in the absorber column. Hence, during tilt and motion, some of the C0 ₂ rich feed gas may pass through the absorber column with too little or no contact with the solvent. As a consequence, the treated gas exiting the absorber column may have a higher than acceptable C0 ₂ -concentration. Other technologies for removing C0 ₂ from natural gas include adsorption and membranes. Both these technologies are insensitive to motion, but they also have their own drawbacks. The adsorption method is not suited for bulk C0 ₂ removal. Membranes are not suited for removal of C0 ₂ down to the required low concentration (50 ppmv). C0 ₂ absorption technologies are therefore preferred in LNG production facilities. Absorption of C02 in other types of solvents, like chilled methanol, may be used to treat the gas to meet the very stringent C0 ₂ specification in a LNG production facility, but these technologies are affected by tilt and motion in the same way as conventional amine based absorption technologies. Several C0 ₂ absorption systems with two or more absorption stages have been proposed in literature. They use a conventional absorber column as the last absorption stage in the system. The system with a two stage amine process described in US 201 1/01 10833 A1 is one example of such a system. This system reduces the heat requirement for amine regeneration. The two stages include a semi-lean (partly regenerated) amine loop and a lean (fully regenerated) amine loop. A static mixer (first absorption stage) was used in the semi-lean amine loop before the partly treated feed gas entered the conventional absorber column (second absorption stage) in the lean amine loop. Although it is a two-stage process, because the last stage C0 ₂ absorption is done by the absorber column, it will not resolve the challenges with tilt and motion on a LNG FPSO. Furthermore, this system does not include methods or apparatus for polishing low C0 ₂ concentrations.

Hence, there is a need for a system and a method for removing carbon dioxide from a natural gas stream in a LNG production facility onboard a floating vessel down to a safe and acceptable concentration to avoid freeze-out of solids in the natural gas liquefaction system.

The present invention concerns C0 ₂ removal. The same concept could be used to remove the last traces of other species in the treated gas exiting a conventional absorber column. Absorption of water in glycol and absorption of H ₂ S in an absorption solvent (amine based or other) are two examples of technologies which traditionally are done in absorber columns, and which may benefit from the second absorption stage included in the present invention. SUMMARY OF THE INVENTION

The object of the present invention is to overcome the disadvantages of existing technologies and provide said method and system.

The present invention comprises system for C0 ₂ removal in a liquefied natural gas production facility comprising a first stage absorber column with a C0 ₂ rich gas feed inlet of said first stage absorber column, a C0 ₂ absorbent solvent inlet of said first stage absorber column and an absorbent solvent outlet of said first stage absorber column, wherein said system further comprises a gas outlet from said first stage absorber column in fluid communication with a downstream second C0 ₂ absorption stage, wherein said second absorption stage comprises at least one mixer and at least one separator. The second absorption stage of the present invention comprises at least one absorption step comprising at least one mixer and at least one separator. Further an outlet of a mixer of said second absorption stage is in fluid communication with an inlet of a separator of said second absorption stage. Furthermore at least one mixer is in fluid communication with a C0 ₂ absorbent solvent. In the present invention an absorbent solvent outlet-pipe from an absorbent solvent buffer tank is in fluid communication with at least one mixer of said second absorption stage. In addition an absorbent solvent outlet-pipe from an absorbent solvent buffer tank is in fluid communication with said absorbent solvent inlet of said first stage absorber column. According to the present invention absorbent solvent outlet-pipe from at least one separator of said second absorption stage is in fluid communication with an absorbent solvent inlet of the C0 ₂ absorber column of the first stage.

In one embodiment said second absorption stage comprises at least two absorption steps arranged in serial connection. Further at least one mixer is arranged as an inline mixing element located in the piping in a LNG production facility. In one embodiment at least one separator is arranged as an inline separator element located in the piping in a LNG production facility. Further at least a gas outlet from said first stage absorber column is in fluid communication with a water wash unit. The C0 ₂ reduced feed gas from a separator of said second C0 ₂ absorption stage is in fluid communication with a water wash unit.

In one embodiment said second C0 ₂ absorption stage is a retrofitted system in an existing liquefied natural gas production facility.

In one embodiment at least one remixer is preceding a separator within an absorption step.

Further, the present invention comprises a method for C0 ₂ removal in a liquefied natural gas production facility, in which a C0 ₂ rich gas feed is conveyed to a first stage absorber column, a C0 ₂ absorbent solvent is conveyed to said first stage absorber column, and an absorbent solvent is conveyed from said first stage absorber column, wherein said method further comprises conveying feed gas with reduced C0 ₂ concentration from said first stage absorber column to a downstream second C0 ₂ absorption stage. In the present invention an absorbent solvent is conveyed to at least one mixer in said second absorption stage. In addition an absorbent solvent is conveyed from a solvent buffer tank to at least one mixer of said second absorption stage. Further an absorbent solvent from a solvent buffer tank is conveyed to said first stage absorber column.

In one embodiment an absorbent solvent from at least one separator of the second absorption stage is conveyed to an inlet of the C0 ₂ absorber column of the first stage.

In one embodiment at least a C0 ₂ reduced feed gas from said second absorption stage is conveyed to a water wash system.

The present invention comprises use of a C02 removal system as described herein on an offshore facility located on a floating vessel. Further, use of a C02 removal method as described herein on an offshore facility located on a floating vessel is also comprised in the present invention. In one embodiment use of a C02 removal system as described herein on a Floating production Storage and offloading vessel is also comprised in the present invention.

In one embodiment use of a C02 removal method as described herein on a Floating production Storage and offloading vessel is also included in the present invention.

The present invention uses the superior properties of absorption technologies, with its tall absorber columns (first stage), combined with a second absorption stage which is unaffected by tilt and motion. Together, the first and the second stage will constitute the C0 ₂ removal system within the gas pre-treatment section in the LNG production facility. The second absorption stage utilise co-current flow of absorbent solvent and gas. It is the momentum of the gas that transports the solvent, dispersed as small droplets in the gas stream, through the second absorption stage. Tilt and motion do not affect the second absorption stage because gravity plays an insignificant role.

The C0 ₂ removal will be split between the two absorption stages in the following way: Almost all the C0 ₂ in the feed gas will be removed in the conventional absorber column within the first absorption stage. Whenever the treated gas exiting the absorber column does not meet the C0 ₂ specification, for instance due to the influence of tilt and motion on an offshore floating facility, the final removal down to 50 ppmv C0 ₂ in the gas will be done in the second absorption stage.

The second stage is thus a polishing stage because it only removes a small quantity of C0 ₂ , for instance from 6-700 ppmv down to 50 ppmv C0 ₂ in the gas. The wording polishing should be understood as the complete or partly removal of the last traces of a specie from a gas stream.

The present invention relates to a C0 ₂ removal system in the gas pre-treatment section of a liquefied natural gas production facility comprising a first stage absorber column with a C0 ₂ rich gas feed inlet of said first stage absorber column, a C0 ₂ absorbent solvent inlet of said first stage absorber column and a absorbent solvent outlet of said first stage absorber column, wherein said system further comprises a gas outlet from said first stage absorber column in fluid communication with a downstream second C0 ₂ absorption stage, wherein said second absorption stage comprises at least one mixer and at least one separator. Furthermore, the present invention relates to a method for polishing of the C0 ₂ content in the feed gas to a natural gas liquefaction system, in which a C0 ₂ rich gas feed is conveyed to a first stage absorber column, a C0 ₂ absorbent solvent is conveyed to said first stage absorber column, and an absorbent solvent is conveyed from said first stage absorber column, wherein said method further comprises conveying feed gas with reduced C0 ₂ concentration from said first stage absorber column to a downstream second C0 ₂ absorption stage.

In the said second absorption stage, the lean C0 ₂ absorbent solvent is first introduced in the step in which the gas has the lower C0 ₂ -concentration, wherein the now partly C0 ₂ -loaded solvent is further introduced into the mixer upstream said step.

The present invention comprises use of a system and method as described in the preceding in which said system is used offshore for C0 ₂ removal onboard a floating production, storage and offloading vessel.

The present invention also relates to a use of said system and method on an offshore facility located on a floating vessel as a Floating production Storage and offloading vessel.

In an embodiment, the invention relates to a system for removal of the C0 ₂ content in a natural gas stream. The system comprises the C0 ₂ absorber column with a gas-feed inlet at the bottom, a C0 ₂ absorbent solvent inlet at the top, and a gas outlet at the top. The gas outlet at the top of said C0 ₂ absorber column is in fluid communication with the second C0 ₂ absorption stage. The second C0 ₂ absorption stage comprises a mixer and a separator. In this embodiment the second C0 ₂ absorption stage comprises only one C0 ₂ absorption step including one mixer and one separator. Furthermore, the system comprises a C0 ₂ absorbent solvent outlet-pipe from the bottom of the separator in fluid communication with an inlet to the top of the C0 ₂ absorber column. Furthermore, the system comprises a C0 ₂ absorbent solvent outlet-pipe from a solvent buffer tank for the storage of lean C0 ₂ absorbent solvent in fluid communication with the inlet at the top of the C0 ₂ absorber column. Moreover, the system comprises a gas feed line from the second C0 ₂ absorption stage in fluid communication with a water wash unit. In an embodiment, the invention relates to a method for removal of the C02 content in the feed gas to a natural gas liquefaction system. The method comprises the steps of feeding a C0 ₂ -rich gas into the bottom of a C0 ₂ absorber column, followed by feeding a C0 ₂ absorbent solvent through an inlet at the top of said C0 ₂ absorber column. The method further comprises feeding the C0 ₂ -reduced gas through an outlet from the top of the C0 ₂ absorber column to a second C0 ₂ absorption stage. Furthermore, the method comprises feeding lean C0 ₂ absorbent solvent from a solvent buffer tank to a mixer in a C02 absorption step in said second C0 ₂ absorption stage and to the C0 ₂ absorber column in the first stage. Moreover, the method comprises feeding C0 ₂ absorbent solvent from a separator within said second C0 ₂ absorption stage to the C0 ₂ absorber column in the first stage or to other locations in the absorbent solvent circuit. Thereafter, the method comprises feeding the further C0 ₂ -reduced gas from said second C0 ₂ absorption stage to a water wash unit. In this embodiment the second C02 absorption stage comprises only one C0 ₂ absorption step including one mixer and one separator.

In an embodiment, the said second C0 ₂ absorption stage is a retrofitted system in an existing C0 ₂ removal system within the pre-treatment section in a LNG production facility.

In an embodiment, the method for polishing of the C0 ₂ content in a natural gas stream is used offshore for C0 ₂ -removal onboard an FPSO vessel.

The present invention can also be used for polishing of the water or H ₂ S content in a natural gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described in detail with reference to the attached figures. It is to be understood that the drawings are designed solely for the purpose of illustration and are not intended as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to schematically illustrate the structures and procedures described herein.

Figure 1 shows a schematic representation of a C0 ₂ -removal system according to the present invention. Figure 2 shows a schematic representation of a plurality of C0 ₂ absorption steps in the second C0 ₂ absorption stage.

Figure3 shows a schematic representation of two C0 ₂ absorption steps in the second C02 absorption step.

Figure4 shows a schematic representation of one C02 absorption step including use of a remixer.

DETAILED DESCRIPTION

Throughout the present description, the same reference numerals have been used to represent identical or corresponding parts.

Figure 1 shows a schematic representation of a C0 ₂ -removal system (50), in accordance with an embodiment of the present invention.

The C0 ₂ -removal system (50) includes a conventional C0 ₂ -removal stage (100) and an additional stage (200) which is downstream said conventional C02-removal stage (100). The conventional C0 ₂ -removal stage (100), which is also referred to as the first absorption stage, can operate independently as per the existing technologies. The downstream C0 ₂ absorption stage (200), which is also referred to as the second absorption stage, when operated in combination with the conventional C0 ₂ -removal stage (100), contributes to reduce or eliminate the disadvantages of existing technologies. The second absorption stage (200) contributes to clean out the remaining C02 in the natural gas stream. Such removal of remaining C0 ₂ is also referred to as polishing. It reduces the C0 ₂ concentration in the stream down to an acceptable level.

The C0 ₂ -removal system (50) to be placed in the pre-treatment section within a liquefied natural gas production facility comprises a first stage C0 ₂ absorber column (102). The C0 ₂ absorber column (102) has a C0 ₂ rich feed gas (101 ) inlet at the bottom. The C0 ₂ absorber column (102) also has an inlet at the top for connecting a pipe (169) carrying a lean C0 ₂ absorbent solvent. The C0 ₂ absorber column (102) further has an outlet for connecting a pipe (105) carrying a rich C0 ₂ absorbent solvent. Furthermore, the C0 ₂ absorber column (102) has an outlet at the top for connecting a pipe (104) carrying gas with a reduced C0 ₂ concentration. The pipe (104) is in fluid communication with a downstream second C0 ₂ absorption stage (200) and a bypass line (106). The gas flowing through the pipe (104) is either sent to water wash unit (172) through a bypass line (106) or sent to the second C0 ₂ absorption stage (200) through a pipe (108).

The C0 ₂ absorption stage (200) comprises at least one C0 ₂ absorption step (210). Fig. 1 illustrates one C0 ₂ absorption step (210). However, two or more than two steps can also be used. Each C0 ₂ absorption step (210) comprises a mixer (212) and a separator (216). The mixer (212) has an inlet for the pipe (108) and optional another inlet for a pipe (168). The pipe (229) which is not shown in Figure 1 carries the C0 ₂ absorbent solvent from an optional second separator in a second absorption step. The gas enters the separator (216) from mixer (212) through a pipe (214). The separator (216) has an outlet at the bottom for connecting a pipe (167) carrying the C0 ₂ absorbent solvent. The pipe (167) is connected to a pipe (166) and the pipe (169). The pipes (166, 169) carry the lean C0 ₂ absorbent solvent. The separator (216) has another outlet at the top for connecting a pipe (218) carrying the gas to a second mixer in an optional second absorption step (not shown in figure 1 ) or through a pipe (238) to a water wash unit (172).

The water wash system (172) has a provision for a water inlet (173) and a water outlet (176). In the water wash system (172) the entrained droplets and the dissolved C0 ₂ absorbent solvent is washed out of the natural gas. The water wash system (172) has another outlet for connecting a pipe (174) carrying the washed C0 ₂ -reduced gas to a dehydration system (not shown in figure 1 ).

The outlet pipe (105), of the C0 ₂ absorber column (102), carrying the rich C0 ₂ absorbent solvent is connected to a flash tank (1 12). The flash tank (1 12) has an outlet for connecting a pipe (1 14) carrying a first C0 ₂ stream. The flash tank (1 12) has another outlet for connecting a pipe (1 16) carrying the C0 ₂ absorbent solvent to a heat exchanger (122). The heated C0 ₂ absorbent solvent exiting the heat exchanger (122) is passed through a pipe (124) to a desorber column (132). A reboiler (152), connected to the desorber column (132), provides the necessary heat to heat the C0 ₂ absorbent solvent in the desorber column (132). The pipe (134) connects the overhead stream to an overhead condenser (142). The overhead condenser (142) has an outlet for connecting a pipe (144) carrying a second C0 ₂ stream. A pipe (136) connects the overhead condenser (142) to the desorber column (132). The desorber column (132) has an outlet for connecting a pipe (138) carrying the lean C0 ₂ absorbent solvent. The pipe (138) is connected to the heat exchanger (122) which cools the lean C0 ₂ absorbent solvent. The cooled lean C0 ₂ absorbent solvent exiting the heat exchanger (122) is passed through a pipe (126) to a solvent buffer tank (162). The heat exchanger (122) exchanges the heat between the C0 ₂ absorbent solvent stream entering the desorber column (132) and the lean C0 ₂ absorbent solvent stream leaving the desorber column (132). The solvent buffer tank (162) has an outlet for connecting a pipe (164) carrying the lean C0 ₂ absorbent solvent. The pipe (164) is connected to the pipes (166, 168). The lean C0 ₂ absorbent solvent in pipe (164) is split between the lean C0 ₂ absorbent solvent in the pipes (166, 168). Most of the lean C0 ₂ absorbent solvent from the solvent buffer tank (162) goes directly to the C0 ₂ absorber column (102) through pipe (166). Only a small fraction, typically 0-10%, of the C0 ₂ absorbent solvent flow rate goes in pipe (168) and further through pipe (218) to the separator in the second absorption stage (200). The C0 ₂ absorbent solvent from the separator (216) in pipe (167) is mixed with the remaining lean absorbent flow from pipe (166) and flows to the C0 ₂ absorber column in pipe (169). Therefore, the purity of the C0 ₂ absorbent solvent entering the C0 ₂ absorber column (102) is close to the purity out of the solvent buffer tank (162).

Pipe (167) can also be mixed with the main solvent flow at any other location within unit (100).

In an embodiment, the second absorption stage (200) comprises a plurality of sequential C0 ₂ absorption steps (210) arranged in serial connection as shown in Figure 2 and Figure 3.

In Figure 2 the mixer (222) has an inlet for the pipe (218) and another inlet for a pipe (239). The pipe (239) carries the C0 ₂ absorbent solvent from the separator (236). The feed gas enters the separator (226) from the mixer (222) through a pipe (224). The separator (226) has an outlet at the bottom for connecting the pipe (229) carrying the C0 ₂ absorbent solvent to the mixer (212). The separator (226) has another outlet at the top for connecting a pipe (228) which carries the feed gas to the mixer (232).

The mixer (232) has an inlet for the pipe (228) and another inlet for a pipe (168). The pipe (168) carries the lean C0 ₂ absorbent solvent. The C0 ₂ absorbent solvent used in the mixer (232) has the lower C02-loading, as this mixer treats the gas with the lower C0 ₂ -content. The feed gas enters the separator (236) from the mixer (232) through a pipe (234). The separator (236) has an outlet at the bottom for connecting the pipe (239) carrying the C0 ₂ absorbent solvent to the mixer (222). The partly loaded C0 ₂ absorbent solvent from the separator (236) is used in mixer (222) as the C0 ₂ - concentration in the gas is higher here. The same principle applies for mixer (212) as the C0 ₂ absorbent solvent here comes from the separator (226). This allows the C0 ₂ absorbent solvent to be gradually loaded with C0 ₂ .

Optionally, a remixer can be utilized in the second absorption step. The remixer (240) is placed between the mixer (212) and the separator (216) as illustrated in Figure 4. The remixer has an inlet for the pipe (214) and one outlet for the pipe (241 ). A remixer (240) can be utilized in the second absorption step to mix the gas (phase) (C0 ₂ ) and the liquid (phase) (solvent) from the inlet to the outlet. The remixer will increase the contact surface between the gas phase and the liquid phase. Several remixers can be placed in subsequent order between a mixer and a separator.

The second C0 ₂ absorption stage is insensitive to motion. Hence, the C0 ₂ -removal system (50) can be employed on LNG FPSOs.

In addition to polishing, the second C0 ₂ absorption stage (200) can also mitigate a very costly over-circulation of the C0 ₂ absorbent solvent. The C0 ₂ -removal system (50) comprises a bypass line (106). Using the bypass line (106), the second C0 ₂ absorption stage (200) can be bypassed and the C0 ₂ -removal system (50) can also function as a conventional C0 ₂ -removal unit (100).

In an embodiment, the C0 ₂ -removal system (50) comprises one or more mixers mixers (212, 222,232) as inline mixing elements located in the piping and the separators (216, 226, 236) as inline separators located in the piping. The inline mixing elements and inline separators are compact and will represent little extra weight or footprint for the C0 ₂ -removal system (50).

In another embodiment, said second C0 ₂ absorption stage is a retrofitted system in an existing C0 ₂ -removal system of the LNG production facility. The C0 ₂ -removal system (50) may be used offshore for C0 ₂ -removal onboard an FPSO vessel.

The absorbent solvent may be a solution of one or more amine in water.

The C0 ₂ -loadings of a rich amine solvent and semi-lean amine solvent will vary with application. The C0 ₂ -loading of the rich amine depends on the C0 ₂ -content of the feed gas. For the second absorption stage, the semi-lean amine has a low loading, usually lower than 0.1 mol C0 ₂ / mol amine when an activated MDEA, N-methyl-diethanol- amine, solvent is used (MDEA + piperazine or another activator).

It should be noted that in addition to the exemplary embodiments of the invention shown in the accompanying drawings, the invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough, and will fully convey the concept of the invention to those skilled in the art.