FIELD OF THE DISCLOSURE

The presently disclosed subject matter relates to systems and methods for production of liquid carbon dioxide. Particularly, the liquid carbon dioxide can be a low pressure stream of carbon dioxide formed from carbon dioxide produced in a power production system and method, specifically a system and method using carbon dioxide as a working fluid.

BACKGROUND

Carbon capture and sequestration (CCS) is a key consideration of any system or method that produces carbon dioxide (C0 ₂ ). This is particularly relevant to power production through combustion of a fossil fuel or other hydrocarbon-containing material. Several power production methods have been suggested where CCS is achievable. One publication in the field of high efficiency power generation with CCS, U.S. Pat. No. 8,596,075 to Allam et al., provides for desirable efficiencies in closed cycle oxy-fuel combustion systems utilizing a recycle C0 ₂ stream. In such system, the C0 ₂ is captured as a relatively pure stream at high pressure.

Current proposals for C0 ₂ disposal often require transportation in high pressure pipelines as a high density, supercritical fluid at pressures of 100 bar (10 MPa) to 250 bar (25 MPa). Such pipelines require high capital expenditures. Piped C0 ₂ is either sequestered in an underground geologic formation, such as a deep saline aquifer, or may be used to economic advantage, such as for enhanced oil recovery (EOR).

The use of C0 ₂ for EOR necessitates its availability over a wide area of an oil rich region. This would require the extensive use of a network of pipelines extending over the region. This becomes prohibitively expensive in many uses, particularly off-shore oil fields. It thus would be useful to provide bulk quantities of C0 ₂ (such as produced from a power production system and method) in liquid form that would be easier for delivery to off-shore oil production platforms. Other beneficial uses of C0 ₂ gathered from power production facilities can be envisioned if the C0 ₂ could be provided in a liquefied form.

SUMMARY OF THE DISCLOSURE

The present disclosure provides systems and methods useful in the production of liquid C0 ₂ . The disclosed systems and methods can make use of C0 ₂ from any source. The systems and methods, however, may be particularly beneficial in connection with a system and method that produces a high pressure C0 ₂ stream, particularly a high pressure C0 ₂ stream at near ambient temperature. The present systems and methods are further beneficial in that liquid C0 ₂ can be produced with substantially high purity, particularly with low levels of oxygen, nitrogen, and noble gases (e.g., Argon).

In certain embodiments, a C0 ₂ source that can be used for producing liquid C0 ₂ can be a power production system, particularly oxyfuel combustion systems and methods, and more particularly combustion methods utilizing a C0 ₂ working fluid. Systems and methods for power production wherefrom a C0 ₂ stream may be obtained are described in U.S. Pat. No. 8,596,075, U.S. Pat. No. 8,776,532, U.S. Pat. No. 8,959,887, U.S. Pat. No. 8,986,002, U.S. Pat. No. 9,068,743, U.S. Pub. No. 2010/0300063, U.S. Pub. No. 2012/0067054, U.S. Pub. No. 2012/0237881 , and U.S. Pub. No. 2013/0213049, the disclosures of which are incorporated herein by reference in their entireties.

In some embodiments, the present disclosure relates to methods for production of a low pressure liquid carbon dioxide (C0 ₂ ) stream. Such methods may compriseproviding a high pressure C0 ₂ stream at a pressure of about 60 bar (6 MPa) or greater, about 100 bar (10 MPa) or greater, or in a pressure range as otherwise disclosed herein. The methods further may comprise dividing out a portion of the high pressure C0 ₂ stream and expanding the portion to form a cooling stream that may be useful as a refrigerant. For example, the cooling stream may be at a

temperature of about -20 °C or less or in a temperature range as otherwise disclosed herein. The methods further may comprise cooling the high pressure C0 ₂ stream to a temperature of about 5 °C or less (preferably about -10 °C or less) by passing the a high pressure C0 ₂ stream through a heat exchanger in a heat exchange relationship with the cooling stream. The methods further may comprise expanding the high pressure C0 ₂ stream so as to form a low pressure C0 ₂ stream at a pressure of down to about 6 bar (0.6 MPa). The methods further may comprise passing the low pressure C0 ₂ stream through a separator effective to separate a vapor stream therefrom and provide the low pressure liquid C0 ₂ stream.

In further embodiments, the present disclosure relates to systems useful in the production of a low pressure liquid carbon dioxide (C0 ₂ ) stream. In some embodiments, such systems may comprise one or more components adapted for providing a high pressure C0 ₂ stream, one or more heat exchangers, one or more expanders (e.g., valves), one or more separators, and one or more distillers. In a non-limiting example, a system according to the present disclosure may comprise: piping adapted for passage of a high pressure C0 ₂ stream; a divider adapted for dividing the high pressure C0 ₂ stream into a cooling fraction and a bulk stream; an expander adapted for expanding and cooling the cooling fraction of the high pressure C0 ₂ stream; a heat exchanger adapted for cooling the bulk high pressure C0 ₂ stream against the warming expanded and cooled cooling fraction of the high pressure C0 ₂ stream; an expander adapted for expanding and cooling the bulk high pressure C0 ₂ stream to form a two phase, low pressure C0 ₂ stream; a separator adapted for removing a vapor fraction from the two phase, low pressure C0 ₂ stream; and a distiller adapted for removing at least a portion of non-C0 ₂ components and providing a low pressure, liquid C0 ₂ stream.

In still other embodiments, the present disclosure relates to methods for production of a low pressure liquid carbon dioxide (C0 ₂ ) stream from a high pressure C0 ₂ stream from a power production process. In some embodiments, such method can comprise combusting a carbonaceous or hydrocarbon fuel in a combustor in the presence of oxygen and a recycle C0 ₂ stream at a pressure of about 100 bar (10 MPa) or greater and a temperature of about 400 °C or greater to form a combustor exit stream comprising C0 ₂ . The combustor exit stream in particular may be at a pressure of about 200 bar (20 MPa) to about 400 bar (40 MPa). The combustor exit stream in particular may be at a temperature of about 800 °C to about 1,600 °C. The method further may comprise expanding the combustor exit stream in a turbine to generate power and form a turbine exit stream comprising C0 ₂ at a pressure of about 50 bar (5 MPa) or less. The turbine exit stream particularly may be at a pressure of about 20 bar (2 MPa) to about 40 bar (4 MPa). The method further may comprise cooling the turbine exit stream in a heat exchanger with heat transferred to the heating recycle C0 ₂ stream. Cooling may be to a temperature of about 80 °C or less, such as to near ambient temperature. The method also may comprise further cooling the turbine exhaust stream against ambient cooling means and separating condensed water in a separator. The method further may comprise pumping C0 ₂ from the turbine exit pressure to a pressure of about 100 bar (10 MPa) or greater to form a high pressure C0 ₂ stream. In particular, the high pressure C0 ₂ stream may be at a pressure of about 100 bar (10 MPa) to about 500 bar (50 MPa) or about 200 bar (20 MPa) to about 400 bar (40 MPa). The C0 ₂ from the cooled turbine exit stream may be compressed to a first pressure, cooled to increase the density thereof, and then pumped to the second, greater pressure in the range noted above. A portion of the high pressure C0 ₂ stream can be passed back through the heat exchanger to be heated against the cooling turbine exit stream prior to passage back into the combustor. Further heating also may be applied to the stream after compression and before passage into the combustor, such further heating coming from a source other than the turbine exit stream. A portion of the high pressure C0 ₂ stream (which portion can comprise any net C0 ₂ produced in combustion) can be cooled to a temperature of about 5 °C or less, such as, for example, in a heat exchanger using a refrigerant. The refrigerant may comprise a portion of the high pressure C0 ₂ stream that can be utilized as a cooling fraction by expanding the portion to a pressure that is about 30 bar (3 MPa) or less but is above the triple point pressure of C0 ₂ . The cooling fraction can be a temperature of about 0 °C or less or about -20 °C or less. In particular embodiments, the cooling fraction of the high pressure C0 ₂ stream may be cooled to a temperature of about -55 °C to about 0 °C. The portion of the high pressure C0 ₂ stream that is cooled in the heat exchanger against the C0 ₂ cooling fraction can be expanded to a pressure of down to about 6 bar (0.6 MPa) (preferably always maintaining a pressure above the triple point pressure of C0 ₂ ) so as to form the low pressure liquid C0 ₂ stream. In particular, the cooled portion of the high pressure C0 ₂ stream may be expanded to a pressure that is about 30 bar (3 MPa) or less but is above the triple point pressure of C0 ₂ .

Methods as described above further may comprise further elements. For example, the cooling of the turbine exit stream particularly may be to a temperature of about 70 °C or less or about 60 °C or less. One heat exchanger or a plurality of heat exchangers may be used. For example, an economizer heat exchanger may be used followed by a cold water heat exchanger. After cooling, the methods also may comprise passing the turbine exit stream comprising C0 ₂ through one or more separators to remove at least water therefrom. Further prior to said pumping step, the methods may comprise compressing the turbine exit stream comprising C0 ₂ to a pressure of up to about 80 bar (8 MPa) (e.g., a pressure of about 60 bar (6 MPa) to about 80 bar (8 MPa)). Still further, the methods may comprise increasing the density of the turbine exit stream comprising C0 ₂ , such as by cooling the stream in a cold water heat exchanger. The density, for example, may be increased to about 600 kg/m or greater, about 700 kg/m or greater, or about 800 kg/m or greater. The turbine exit stream may be compressed prior to increasing the density of the stream.

The methods may further comprise, after said cooling of the bulk high pressure C0 ₂ stream in a heat exchanger and prior to said expanding, passing the bulk high pressure C0 ₂ stream through a re-boiler. The re-boiler may particularly be combined with a distiller (e.g., a stripping column). As such, the re-boiler may provide heating to the distiller.

The methods may comprise further processing of the bulk low pressure liquid C0 ₂ stream. For example, the low pressure liquid C0 ₂ stream may be a two phase material including the liquid phase and a vapor phase. Thus, the methods may comprise passing the low pressure liquid C0 ₂ stream through a separator effective to separate a vapor stream therefrom. In some embodiments, the vapor stream may comprise up to about 8% (particularly up to about 4% or up to about 6%) by mass of the low pressure liquid C0 ₂ stream passed through the separator. In some embodiments, the vapor stream may comprise about 1% to about 75% by mass C0 ₂ . In some embodiments, the vapor stream may comprise about 25% to about 99% by mass of a combination of N ₂ , 0 ₂ , and argon (or further inert gases). The methods also may comprise passing the remaining low pressure liquid C0 ₂ stream (e.g., after withdrawing the vapor phase therefrom) through a distiller, such as a stripping column (which may include the re-boiler, as discussed above).

After the distillation step, the liquid C0 ₂ may be provided to a pump to increase its pressure to a desired value. The cold discharge stream from the pump may be supplied to a heat exchanger upstream of the reboiler to supplement the cooling duty of the high pressure C0 ₂ that is expanded to create a refrigerant. The warmed refrigerant C0 ₂ and/or the overhead stream from a stripping distillation column may be provided to a compressor that discharges the flow at a pressure compatible with the system where the high pressure C0 ₂ stream was sourced. The vapor phase stream from the separator may also be provided to a system performing additional separation processes. Alternatively, the vapor phase stream may be vented.

The low pressure liquid C0 ₂ stream provided according to the present disclosure in particular may have only a very low oxygen concentration. In some embodiments, the low pressure liquid C0 ₂ stream may have an oxygen content of no more than about 25 ppm, particularly no more than about 10 ppm. The low pressure liquid C0 ₂ stream also may have a similarly low

concentration of inert gases, such as nitrogen and argon.

As non-limiting examples, the present disclosure can relate to the following embodiments. Such embodiments are intended to be illustrative of the broader nature of the disclosure as a whole.

In some embodiments, the present disclosure can provide methods for production of a low pressure liquid C0 ₂ stream. For example, such method can comprise: combusting a carbonaceous or hydrocarbon fuel with oxygen in a combustor in the presence of a recycle C0 ₂ stream at a pressure of about 100 bar (100 MPa) or greater and a temperature of about 400 °C or greater to form a combustor exit stream comprising C0 ₂ ; expanding the combustor exit stream in a turbine to generate power and form a turbine exit stream comprising C0 ₂ at a pressure of about 50 bar (5 MPa) or less; cooling the turbine exit stream in a first heat exchanger to form a cooled turbine exit stream; pumping C0 ₂ from the cooled turbine exit stream to a pressure of about 100 bar (10 MPa) or greater to form a high pressure C0 ₂ stream; dividing the high pressure C0 ₂ stream into a bulk portion and a cooling portion; expanding the cooling portion of the high pressure C0 ₂ stream to reduce the temperature thereof to about -20 °C or less; cooling the bulk portion of the high pressure C0 ₂ stream to a temperature of about 5 °C or less by passing the bulk portion of the high pressure C0 ₂ stream through a second heat exchanger against the expanded cooling portion of the high pressure C0 ₂ stream; and expanding the cooled, bulk portion of the high pressure C0 ₂ stream to a pressure that is about 30 bar (3 MPa) or less but is greater than the triple point pressure of C0 ₂ so as to form the low pressure liquid C0 ₂ stream. In further embodiments, such method may include one or more of the following statements, which statements may be combined in any number and any combination. Moreover, such method may include any further elements as otherwise described herein.

The combustor exit stream can be at a pressure of about 200 bar (20 MPa) to about 400 bar (40 MPa).

The combustor exit stream can be at a temperature of about 800 °C to about 1,600 °C.

The turbine exit stream comprising C0 ₂ can be at a pressure of about 20 bar (2 MPa) to about 40 bar (4 MPa).

The turbine exit stream can be cooled in the heat exchanger to a temperature of about 80 °C or less.

The method further can comprise passing the cooled turbine exit stream comprising C0 ₂ through one or more separators to remove at least water therefrom.

The method further can comprise heating one or both of the oxygen and the recycle C0 ₂ stream in the heat exchanger against the turbine exit stream.

The high pressure C0 ₂ stream can be at a pressure of about 200 bar (20 MPa) to about 400 bar (40 MPa).

The bulk portion of the high pressure C0 ₂ stream can be cooled to a temperature of about - 55 °C to about 0 °C.

The method further can comprise, after cooling of the bulk portion of the high pressure C0 ₂ stream and prior to expanding of the bulk portion of the high pressure C0 ₂ stream, passing the bulk portion of the high pressure C0 ₂ stream through a re-boiler.

The re-boiler can be in a stripping column.

The method further can comprise passing the low pressure liquid C0 ₂ stream through a separator effective to separate a vapor stream therefrom.

The vapor stream can comprise up to about 8% by mass of the low pressure liquid C0 ₂ stream passed through the separator.

The vapor stream can comprise about 1% to about 75% by mass C0 ₂ and about 25% to about 99% by mass of one or more of N ₂ , 0 ₂ , and Argon.

The method further can comprise passing the remaining low pressure liquid C0 ₂ stream into a stripping column.

The low pressure liquid C0 ₂ stream exiting the stripping column can have an oxygen content of no more than about 25 ppm.

The method can comprise pumping the low pressure liquid C0 ₂ stream to a pressure of at least about 100 bar (10 MPa).

The method can comprise delivering the pumped liquid C0 ₂ stream to a C0 ₂ pipeline. The method further can comprise mixing an overhead vapor from the stripping column with the cooling portion of the high pressure C0 ₂ stream exiting the second heat exchanger.

The method further can comprise adding the mixture of the overhead vapor from the stripping column and the cooling portion of the high pressure C0 ₂ stream exiting the second heat exchanger to the cooled turbine exit stream.

In further exemplary embodiments, the present disclosure can provide systems configured for production of a low pressure liquid C0 ₂ stream. For example, a system can comprise: a splitter configured for dividing a high pressure C0 ₂ stream into a first portion and a second portion; a first expander configured for expanding and cooling the first portion of the high pressure C0 ₂ stream; a heat exchanger for cooling the second portion of the high pressure C0 ₂ stream against the cooled first portion of the high pressure C0 ₂ stream exiting the expander; and a second expander configured for expanding the cooled second portion of the high pressure C0 ₂ stream so as to form the low pressure liquid C0 ₂ stream. In further embodiments, such system may include one or more of the following statements, which statements may be combined in any number and any

combination. Moreover, such system may include any further elements as otherwise described herein.

The first expander can be configured for cooling the first portion of the high pressure C0 ₂ stream to a temperature of about -20 °C or less.

The heat exchanger can be configured for cooling the second portion of the high pressure C0 ₂ stream to a temperature of about 5 °C or less.

The second expander can be configured to expand the cooled second portion of the high pressure C0 ₂ stream to a pressure that is about 30 bar (3 MPa) or less but is greater than the triple point pressure of C0 ₂ .

The system further can comprise a combined stripping column and re-boiler.

The stripping column can be in line downstream from the second expander, and the re- boiler can be in line downstream from the heat exchanger and upstream from the second expander.

The system further can comprise a liquid/vapor separator positioned downstream from the second expander and upstream from the stripping column.

The system further can comprise a compressor configured for receiving the first portion of the high pressure C0 ₂ stream from the heat exchanger.

The system further can comprise: a combustor configured for combusting a carbonaceous or hydrocarbon fuel with oxygen in a combustor in the presence of a recycle C0 ₂ stream at a pressure of about 100 bar (10 MPa) or greater and a temperature of about 400 °C or greater to form a combustor exit stream comprising C0 ₂ ; a turbine configured for expanding the combustor exit stream to generate power and form a turbine exit stream comprising C0 ₂ ; a further heat exchanger configured for cooling the turbine exit stream; and a pump configured for pumping C0 ₂ from the cooled turbine exit stream to form the high pressure C0 ₂ stream.

The invention includes, without limitation, the following embodiments.

Embodiment 1 : A method for production of a low pressure liquid carbon dioxide (C0 ₂ ) stream, the method comprising: combusting a carbonaceous or hydrocarbon fuel with oxygen in a combustor in the presence of a recycle C0 ₂ stream at a pressure of about 100 bar (10 MPa) or greater and a temperature of about 400 °C or greater to form a combustor exit stream comprising C0 ₂ ; expanding the combustor exit stream in a turbine to generate power and form a turbine exit stream comprising C0 ₂ at a pressure of about 50 bar (5 MPa) or less; cooling the turbine exit stream in a first heat exchanger to form a cooled turbine exit stream; pumping C0 ₂ from the cooled turbine exit stream to a pressure of about 100 bar (10 MPa) or greater to form a high pressure C0 ₂ stream; dividing the high pressure C0 ₂ stream into a bulk portion and a cooling portion; expanding the cooling portion of the high pressure C0 ₂ stream to reduce the temperature thereof to about -20 °C or less; cooling the bulk portion of the high pressure C0 ₂ stream to a temperature of about 5 °C or less by passing the bulk portion of the high pressure C0 ₂ stream through a second heat exchanger against the expanded cooling portion of the high pressure C0 ₂ stream; and expanding the cooled, bulk portion of the high pressure C0 ₂ stream to a pressure that is about 30 bar (3 MPa) or less but is greater than the triple point pressure of C0 ₂ so as to form the low pressure liquid C0 ₂ stream.

Embodiment 2: The method of any previous or subsequent embodiment, wherein the combustor exit stream is at a pressure of about 200 bar (20 MPa) to about 400 bar (40 MPa).

Embodiment 3 : The method of any previous or subsequent embodiment, wherein the combustor exit stream is at a temperature of about 800 °C to about 1,600 °C.

Embodiment 4: The method of any previous or subsequent embodiment, wherein the turbine exit stream comprising C0 ₂ is at a pressure of about 20 bar (2 MPa) to about 40 bar (4 MPa).

Embodiment 5: The method of any previous or subsequent embodiment, wherein the turbine exit stream is cooled in the heat exchanger to a temperature of about 80 °C or less.

Embodiment 6: The method of any previous or subsequent embodiment, further comprising passing the cooled turbine exit stream comprising C0 ₂ through one or more separators to remove at least water therefrom.

Embodiment 7: The method of any previous or subsequent embodiment, further comprising heating one or both of the oxygen and the recycle C0 ₂ stream in the heat exchanger against the turbine exit stream. Embodiment 8: The method of any previous or subsequent embodiment, wherein the high pressure C0 ₂ stream is at a pressure of about 200 bar (20 MPa) to about 400 bar (40 MPa).

Embodiment 9: The method of any previous or subsequent embodiment, wherein the bulk portion of the high pressure C0 ₂ stream is cooled to a temperature of about -55 °C to about 0 °C.

Embodiment 10: The method of any previous or subsequent embodiment, further comprising, after said cooling of the bulk portion of the high pressure C0 ₂ stream and prior to said expanding of the bulk portion of the high pressure C0 ₂ stream, passing the bulk portion of the high pressure C0 ₂ stream through a re-boiler.

Embodiment 11 : The method of any previous or subsequent embodiment, wherein the re- boiler is in a stripping column.

Embodiment 12: The method of any previous or subsequent embodiment, further comprising passing the low pressure liquid C0 ₂ stream through a separator effective to separate a vapor stream therefrom.

Embodiment 13: The method of any previous or subsequent embodiment, wherein the vapor stream comprises up to about 8% by mass of the low pressure liquid C0 ₂ stream passed through the separator.

Embodiment 14: The method of any previous or subsequent embodiment, wherein the vapor stream comprises about 1% to about 75% by mass C0 ₂ and about 25% to about 99% by mass of one or more of N ₂ , 0 ₂ , and Argon.

Embodiment 15: The method of any previous or subsequent embodiment, further comprising passing the remaining low pressure liquid C0 ₂ stream into a stripping column.

Embodiment 16: The method of any previous or subsequent embodiment, wherein the low pressure liquid C0 ₂ stream exiting the stripping column has an oxygen content of no more than about 25 ppm.

Embodiment 17: The method of any previous or subsequent embodiment, comprising pumping the low pressure liquid C0 ₂ stream to a pressure of at least about 100 bar (10 MPa).

Embodiment 18: The method of any previous or subsequent embodiment, comprising delivering the pumped liquid C0 ₂ stream to a C0 ₂ pipeline.

Embodiment 19: The method of any previous or subsequent embodiment, further comprising mixing an overhead vapor from the stripping column with the cooling portion of the high pressure C0 ₂ stream exiting the second heat exchanger.

Embodiment 20: The method of any previous embodiment, further comprising adding the mixture to the cooled turbine exit stream. Embodiment 21 : A system configured for production of a low pressure liquid carbon dioxide (C0 ₂ ) stream, the system comprising: a splitter configured for dividing a high pressure C0 ₂ stream into a first portion and a second portion; a first expander configured for expanding and cooling the first portion of the high pressure C0 ₂ stream; a heat exchanger for cooling the second portion of the high pressure C0 ₂ stream against the cooled first portion of the high pressure C0 ₂ stream exiting the expander; and a second expander configured for expanding the cooled second portion of the high pressure C0 ₂ stream so as to form the low pressure liquid C0 ₂ stream.

Embodiment 22: The system of any previous or subsequent embodiment, wherein the first expander is configured for cooling the first portion of the high pressure C0 ₂ stream to a

temperature of about -20 °C or less.

Embodiment 23: The system of any previous or subsequent embodiment, wherein the heat exchanger is configured for cooling the second portion of the high pressure C0 ₂ stream to a temperature of about 5 °C or less.

Embodiment 24: The system of any previous or subsequent embodiment, wherein the second expander is configured to expand the cooled second portion of the high pressure C0 ₂ stream to a pressure that is about 30 bar (3 MPa) or less but is greater than the triple point pressure of C0 ₂ .

Embodiment 25: The system of any previous or subsequent embodiment, further comprising a combined stripping column and re-boiler.

Embodiment 26: The system of any previous or subsequent embodiment, wherein the stripping column is in line downstream from the second expander and wherein the re -boiler is in line downstream from the heat exchanger and upstream from the second expander.

Embodiment 27: The system of any previous or subsequent embodiment, further comprising a liquid/vapor separator positioned downstream from the second expander and upstream from the stripping column.

Embodiment 28: The system of any previous or subsequent embodiment, further comprising a compressor configured for receiving the first portion of the high pressure C0 ₂ stream from the heat exchanger.

Embodiment 29: The system of any previous embodiment, further comprising: a combustor configured for combusting a carbonaceous or hydrocarbon fuel with oxygen in a combustor in the presence of a recycle C0 ₂ stream at a pressure of about 100 bar (10 MPa) or greater and a temperature of about 400 °C or greater to form a combustor exit stream comprising C0 ₂ ; a turbine configured for expanding the combustor exit stream to generate power and form a turbine exit stream comprising C0 ₂ ; a further heat exchanger configured for cooling the turbine exit stream; and a pump configured for pumping C0 ₂ from the cooled turbine exit stream to form the high pressure C0 ₂ stream.

These and other features, aspects, and advantages of the disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below. The invention includes any combination of two, three, four, or more of the above-noted embodiments as well as combinations of any two, three, four, or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined in a specific embodiment description herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosed invention, in any of its various aspects and embodiments, should be viewed as intended to be combinable unless the context clearly dictates otherwise.

BRIEF DESCRIPTION OF THE FIGURES

Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 shows a flow diagram of a system according to embodiments of the present disclosure for formation of a low pressure liquid C0 ₂ stream; and

FIG. 2 shows a flow diagram of a system according to embodiments of the present disclosure for formation of a low pressure liquid C0 ₂ stream utilizing a portion of a high pressure C0 ₂ stream drawn from a power production process.

DETAILED DESCRIPTION

The present subject matter will now be described more fully hereinafter with reference to exemplary embodiments thereof. These exemplary embodiments are described so that this disclosure will be thorough and complete, and will fully convey the scope of the subject matter to those skilled in the art. Indeed, the subject matter can be embodied in many 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 satisfy applicable legal requirements. As used in the specification, and in the appended claims, the singular forms "a", "an", "the", include plural referents unless the context clearly dictates otherwise.

The present disclosure relates to systems and methods adapted for production of low pressure liquid carbon dioxide (C0 ₂ ). The systems and methods particularly may be adapted to intake a stream comprising non-liquid C0 ₂ (e.g., gaseous C0 ₂ or supercritical C0 ₂ ) and convert at least a portion of the non-liquid C0 ₂ to liquid C0 ₂ . The intake stream may comprise a fraction of liquid C0 ₂ ; however, the intake stream preferably comprises no more than about 25%, no more than about 10%, no more than about 5%, or no more than about 2% by weight liquid C0 ₂ .

Liquid C0 ₂ produced according to the present disclosure can be produced at a low pressure in that the pressure of the produced liquid C0 ₂ is less than 50 bar (5 MPa) but greater than the triple point pressure of C0 ₂ so as to preferably avoid substantial formation of solid C0 ₂ . In some embodiments, the produced liquid C0 ₂ can be at a pressure of down to about 6 bar (0.6 MPa), in particular about 30 bar (3 MPa )to about 6 bar 0.6 MPa), about 25 bar 2.5 MPa) to about 6 bar (0.6 MPa), or about 15 bar (1.5 MPa) to about 6 bar (0.6 MPa). The temperature of the produced liquid C0 ₂ preferably is in the range of the saturation temperature at the given pressure. For example, the temperature can be in the range of about 5 °C to about -55 °C, about -5 °C to about -55 °C, or about -15 °C to about -55 °C.

Methods of producing liquid C0 ₂ according to embodiments of the present disclosure generally can comprise cooling and expanding the C0 ₂ from the intake stream. Depending upon the source of the intake stream, the methods may comprise one or more compression steps. In preferred embodiments, the intake C0 ₂ can be at a pressure of about 60 bar (6 MPa) or greater, about 100 bar (10 MPa) or greater, or about 200 bar (20 MPa) or greater. In other embodiments, the pressure of the intake C0 ₂ can be in the range of about 60 bar (6 MPa) to about 400 bar (40 MPa). The temperature of the intake C0 ₂ may be greater than 10 °C or may be in the range of about 10 °C to about 40 °C, about 12 °C to about 35 °C, or about 15 °C to about 30 °C. In some embodiments, the intake C0 ₂ can be at about ambient temperature.

An embodiment of a system and method according to the present disclosure useful in the production of liquid C0 ₂ is shown in FIG. 1. As seen therein, a high pressure C0 ₂ stream 24 may be cooled by passage through a water cooler 50 (which may be optional depending upon the actual temperature of the high pressure C0 ₂ stream). The high pressure C0 ₂ stream 24 is then divided into a first portion and a second portion using a splitter 68 (or other suitable system element configured for dividing a stream) to provide a high pressure C0 ₂ side stream 57 that can be expanded, such as through a valve 58 or other suitable device, to form a cooling C0 ₂ stream 56. The remaining high pressure C0 ₂ stream 62 passes through a heat exchanger 10 where it is cooled by the cooling C0 ₂ stream 56, which exits as C0 ₂ stream 33. The cooled, high pressure C0 ₂ stream 51 exiting the cold end of the heat exchanger 10 can be at a temperature of about 5 °C or less, about 0 °C or less, about -10 °C or less, or about -20 °C or less (for example, about 5 °C to about -40 °C or about 0 °C to about -35 °C). The cooled, high pressure C0 ₂ stream 51 can be expanded to form the liquid C0 ₂ stream. As illustrated in FIG. 1, the cooled, high pressure C0 ₂ stream 51 first passes through a re-boiler 52, which is part of a stripping column 53 in FIG. 1, and thus supplies heating for the distillation therein, which is further described below. Passage through the re-boiler thus may be optional. The high pressure C0 ₂ stream 55 leaving the re-boiler 52 is expanded to form the low pressure liquid C0 ₂ stream 35 at a temperature and pressure in the ranges described above. In FIG. 1, stream 55 is expanded through a valve 48, but any device useful for expanding a compressed C0 ₂ stream may be used. For example, the expansion device can be a work producing system, such as a turbine, which lowers the enthalpy of the C0 ₂ between the inlet and the outlet and further lowers outlet temperature.

The expansion of the high pressure C0 ₂ stream (e.g., from the range of about 60 bar (6 MPa) to about 400 bar (40 MPa)) to form the low pressure C0 ₂ stream (e.g., at a pressure of about 30 bar (3 MPa) or less but greater than the triple point pressure of C0 ₂ ) can result in a two phase product stream formed of a gas and liquid mixture having the same total enthalpy as the C0 ₂ stream input to the valve (or other expansion device). The temperature of the two phase mixture leaving the valve (or a turbine per the exemplary, alternative embodiment noted above) particularly can be at the saturation temperature of the liquid at the reduced pressure. In FIG. 1, stream 56 exiting valve 58 and stream 35 exiting valve 48 may both be two phase streams. The two phase, low pressure C0 ₂ stream 35 exiting valve 48 may be passed through a separator 9 to provide the C0 ₂ vapor fraction stream 49 and the C0 ₂ liquid fraction stream 36.

In embodiments wherein the input high pressure C0 ₂ stream is from an oxy-combustion power production system, the vapor fraction that can be separated from the low pressure liquid C0 ₂ stream will contain the bulk of the inert gases (e.g., nitrogen, excess 0 ₂ , and noble gases, such as argon) that are present in the oxygen source and the fuel source (e.g., natural gas). As a non- limiting example, an oxy-combustion power production process may be carried out with a 1% excess oxygen stream flow into a combustor, the oxygen stream being formed of approximately 99.5% oxygen and 0.5% argon. The resulting net C0 ₂ product can include 0 ₂ at a 2%

concentration and argon at a 1 % concentration.

According to the present disclosure, cooling of a C0 ₂ product from a power system as exemplified above by indirect cooling means to a temperature which, on expansion through a valve to a pressure of, for example, 10 bar (1 MPa), results in a flash vapor fraction of approximately 4%. In various embodiments, the vapor fraction may be up to about 6%, up to about 5%, or up to about 4% by mass of the total liquid C0 ₂ stream (e.g., stream 35 in FIG. 1). The vapor stream (e.g., stream 49 in FIG. 1) can comprise about 1% to about 75% by mass C0 ₂ and about 25% to about 99% by mass of a combination of N ₂ , 0 ₂ , and argon (or other inert gases). In further embodiments, the vapor stream can comprise about 60% or greater, about 65% or greater, or about 70% or greater by mass of the combination of N ₂ , 0 ₂ , and argon (or other inert gases). The flash vapor fraction (e.g., stream 49 leaving the separator 9 in FIG. 1) may be vented to the atmosphere or captured. Production of the flash vapor stream is beneficial in embodiments where the input C0 ₂ stream is derived from an oxy-combustion process as removal of the vapor fraction will prevent a build-up of inert argon and/or nitrogen (which may be present in natural gas and/or coal derived fuel gas that is combusted and which may be present in an oxygen stream derived from a cryogenic air separation plant). To form the flash vapor fraction, it can be useful to cool the high pressure C0 ₂ stream (e.g., stream 62 in FIG. 1) to a temperature of about -30 °C or less or about -33 °C or less prior to expansion. In embodiments where the input high pressure C0 ₂ stream is from a source that may be substantially or completely devoid of inert gases (and optionally oxygen), it may not be necessary to form the flash vapor fraction. In embodiments using natural gas fuel having a significant fraction of N ₂ in the oxy-fuel power production process, it can be useful to adjust the temperature to which the stream 51 is cooled so as to ensure the removal of the bulk of the N ₂ with the 0 ₂ and argon in stream 49 together with a minimum loss of C0 ₂ in stream 49.

Preferably, the majority of the concentration of 0 ₂ and argon (and other inert gases) from the input C0 ₂ stream is removed in the flash vapor fraction such that the C0 ₂ liquid fraction stream (e.g., stream 36 in FIG. 1) has only a minor concentration of N ₂ , 0 ₂ , and argon - e.g., about 1% or less, about 0.5% or less, or about 0.2% or less by mass. This minor concentration of N ₂ , 0 ₂ , and argon can be stripped from the C0 ₂ liquid fraction stream, such as by using a distillation apparatus (e.g., the stripping column 53 in FIG. 1). Alternatively to the illustration of FIG. 1, a stripping section may be fitted in the lower part of the flash separator. In embodiments utilizing the stripping column, a re-boiler (component 52 in FIG. 1 as discussed above) can be included to withdraw remaining available heat from part or all of the high pressure C0 ₂ stream (e.g., stream 51 in FIG. 1 ). Such heating can be varied to provide the necessary liquid to vapor ratio to reduce the oxygen concentration in the net liquid C0 ₂ product (stream 54 in FIG. 1). The oxygen concentration in the net liquid C0 ₂ stream can be no more than about 25 ppm, no more than about 20 ppm, or no more than about 10 ppm.

In further embodiments, the product liquid C0 ₂ stream 54 can be pumped to a high pressure and heated in heat exchanger 10 (or in a further heat exchanger or by further means) for delivery into a CO-2 pipeline. The product liquid C0 ₂ stream particularly may be pumped to a pressure of about 100 bar (10 MPa) to about 250 bar (25 MPa).

Returning to FIG. 1 , the top product 63 leaving the stripping column 53 may be further reduced in pressure if desired, such as in valve 64 and then combined with C0 ₂ stream 33. The combined streams may be compressed in compressor 34 to provide a return high pressure C0 ₂ stream 21, which may be, for example, combined with the input high pressure C0 ₂ stream 24 or added to a further C0 ₂ containing stream (see FIG. 2).

The foregoing embodiments for forming a low pressure liquid C0 ₂ stream can be economically desirable in that about 95% or greater, about 96% or greater, or about 97% or greater by mass of the C0 ₂ in the net low pressure C0 ₂ stream (e.g., stream 35 in FIG. 1) can be removed as the low pressure liquid C0 ₂ stream. In the embodiments described above, about 1.5% to about 2.5% by mass of the net C0 ₂ product may be vented to the atmosphere with the combined N ₂ , 0 ₂ , and argon stream (e.g., stream 49 in FIG. 1), thus providing a C0 ₂ removal efficiency of about 97.5% to about 98.5%. In embodiments wherein the above-described method is carried out in connection with a closed cycle power system using C0 ₂ as the working fluid, the stream 49 preferably is vented to the atmosphere because removal of the inert components is desirable to keep their partial pressure and concentration as low as possible. Optionally, the stream 59, following pressure reduction in valve 60, can be routed through a set of passages in the heat exchanger 10 to provide extra refrigeration for cooling the stream 62 before the stream 59 is vented.

The utilization of an input high pressure C0 ₂ stream 24 provides a unique ability to provide indirect cooling to the high pressure C0 ₂ stream. As described in relation to the embodiments above, the indirect cooling can be provided by dividing out a portion of the high pressure C0 ₂ stream at near ambient temperature and then expanding this divided portion of the high pressure C0 ₂ stream to a temperature of about -20 °C or less, about -30 °C or less, or about -40 °C or less (e.g., approximately -40 °C to about -55 °C). This can be achieved by reducing the pressure of the high pressure C0 ₂ stream 24 down to less than about 20 bar (2 MPa), less than about 10 bar (1 MPa), or less than about 8 bar (0.8 MPa) (e.g., about 20 bar (2 MPa) to about 5 bar (0.5 MPa) or about 12 bar (1.2 MPa) to about 5 bar (0.5 MPa), particularly about 5.55 bar (0.555 MPa)). The resulting liquid plus vapor stream (e.g., stream 56 in FIG. 1) in then used to cool the bulk high pressure C0 ₂ stream indirectly in a heat exchanger.

The systems and methods of the present disclosure are particularly beneficial when used in combination with a power production method utilizing a C0 ₂ working fluid, such as the systems disclosed in U.S. Pat. No. 8,596,075, the disclosure of which is incorporated herein by reference in its entirety. In particular, such process can use a high pressure/low pressure ratio turbine that expands a mixture of a high pressure recycle C0 ₂ stream and combustion products arising from combustion of the fuel. Any fossil fuel, particularly carbonaceous fuels, may be used. Preferably, the fuel is a gaseous fuel; however, non-gaseous fuels are not necessarily excluded. Non-limiting examples include natural gas, compressed gases, fuel gases (e.g., comprising one or ore of H ₂ , CO, CH ₄ , H ₂ S, and NH ₃ ) and like combustible gases. Solid fuels - e.g., coal, lignite, petroleum coke, bitumen, and the like, may be used as well with incorporation of necessary system elements (such as with the use of a partial oxidation combustor or a gasifier to convert the solid or heavy liquid fuels to a gaseous form). Liquid hydrocarbon fuels may also be used. Pure oxygen can be used as the oxidant in the combustion process. The hot turbine exhaust is used to partially preheat the high pressure recycle C0 ₂ stream. The recycle C0 ₂ stream is also heated using heat derived from the compression energy of a C0 ₂ compressor, as further discussed herein. All fuel and combustion derived impurities such as sulfur compounds, NO, N0 ₂ , C0 ₂ , H ₂ 0, Hg and the like can be separated for disposal with no emissions to the atmosphere. A C0 ₂ compression train is included and comprises high efficiency units that ensure minimum incremental power consumption. The C0 ₂ compression train can particularly provide a recycle C0 ₂ fuel compressor flow that can be recycled in part to the combustor and directed in part to the liquid C0 ₂ production components as the input high pressure C0 ₂ stream.

Figure 2, for example illustrates a power production system combined with elements as described herein to produce the net C0 ₂ product derived from carbon in the primary fuel in the form of a low pressure liquid with an oxygen content in a minimal range as described herein. An embodiment of such system is described in the Example below in connection to FIG. 2.

The magnitude of the total C0 ₂ net product flow can be vary depending upon the nature of the fuel used. In embodiments utilizing a natural gas fuel, the total C0 ₂ net product flow can be about 2.5% to about 4.5% (e.g., about 3.5%) of the total recycle C0 ₂ fuel compressor flow. In embodiments utilizing a typical bituminous coal (e.g., Illinois No. 6), the total C0 ₂ net product flow can be about 5% to about 7% (e.g., about 6%) of the total recycle C0 ₂ fuel compressor flow. The quantity of recycled C0 ₂ used for refrigeration can be in the range of about 15% to about 35% or about 20% to about 30% (e.g., about 25%) by mass of the net C0 ₂ product flow.

In some embodiments, liquid natural gas (LNG) can be used as a refrigeration source in a manner such as described in U.S. Pat. Pub. No. 2013/0104525, the disclosure of which is incorporated herein by reference in its entirety. In particular embodiments, the LNG can be heated to a temperature approach to the condensing temperature of the C0 ₂ turbine exhaust (e.g., at a pressure of about 20 bar (2 MPa) to about 40 bar (4 MPa)). The turbine exhaust flow leaving the water separator can be dried in a desiccant drier to a dew point below about -50 °C before being liquefied using refrigeration derived from the high pressure LNG, which is in turn heated. The liquid C0 ₂ can now be pumped to a pressure of about 200 bar (20 MPa) to about 400 bar (40 MPa) using a multi-stage centrifugal pump. The high pressure natural gas will be at a temperature typically in the range of about -23 °C (for turbine exhaust leaving the economizer heat exchanger at about 20 bar (2 MPa)) to about 0 °C (for turbine exhaust leaving the economizer heat exchanger at about 40 bar (4 MPa)) using a 5 °C approach to the saturation temperature of C0 ₂ at these pressures. This cold, high pressure natural gas can be used to pre-cool the high pressure C0 ₂ at about 60 bar (6 MPa) to about 400 bar (40 MPa) prior to expansion to produce liquid C0 ₂ in the pressure range of about 6 bar (0.6 MPa) to about 30 bar (3 MPa). This refrigeration can be supplemented by additional refrigeration derived from expansion of high pressure C0 ₂ as described above to give a temperature of the cooled net CO ₂ product which on expansion to the required pressure of the liquid C0 ₂ product results in a gas fraction containing about 50% to about 80% by mass of (0 ₂ + N ₂ + Ar). The effect is to significantly reduce the quantity of additional C0 ₂ which must be recycled for refrigeration.

EXAMPLE

Embodiments of the present disclosure are further illustrated by the following example, which is set forth to illustrate the presently disclosed subject matter and is not to be construed as limiting. The following describes an embodiment of a combined power production system and method and system and method for production of low pressure liquid C0 ₂ , as illustrated in FIG. 2.

As seen in FIG. 2, a natural gas fuel stream 42 (which in this Example is pure methane) at about 40 bar (4 MPa) is compressed to about 320 bar (32 MPa) in a compressor 44 to provide a compressed natural gas fuel stream 43, which in turn enters a combustion chamber 1 where it combusts in a preheated oxidant stream 38, which comprises about 23% by mass of oxygen mixed with about 77% by mass of diluent C0 ₂ . In the illustrated embodiment, the total oxygen quantity contains approximately 1% by mass more oxygen than is required for stoichiometric combustion. The combustion products are diluted in the combustor 1 by a heated recycle C0 ₂ stream 37 at about 304 bar (30.4 MPa) and about 707 °C. A combustor exit stream 39 at a temperature of about 1153 °C is passed to a turbine 2 inlet, the turbine being coupled to an electric generator 3 and a main C0 ₂ recycle compressor 4.

The combustor exit stream 39 is expanded in the turbine 2 to provide a turbine exit stream 45 at about 30 bar (3 MPa) and about 747 °C, which in turn is passed through an economizer heat exchanger 15 and is cooled to about 56 °C leaving as cooled turbine exit stream 16. The cooled turbine exit stream 16 is further cooled against cooling water in a water cooler 7 to near ambient temperature (stream 17 in FIG. 2). The cooled turbine exit stream 17 is passed through a separator 6 where a liquid water stream 18 is separated from a gaseous C0 ₂ overhead stream 19, which itself is divided into separate flows (streams 22 and 20 in FIG. 2).

The gaseous C0 ₂ overhead bulk stream 22 enters the C0 ₂ recycle compressor 4, which operates with an intercooler 5 and compresses the ambient temperature gaseous C0 ₂ overhead bulk stream 22 (derived from the turbine exit stream 45) from a pressure of about 28.2 bar (2.82 MPa) to about 63.5 bar (6.35 MPa) - i.e., compressed C0 ₂ stream 23.

The gaseous C0 ₂ overhead fraction stream 20 is used to dilute the 99.5% 0 ₂ stream 28 (which is at a pressure of about 28 bar (2.8 MPa)) that is produced by the cryogenic air separation plant 14. Combined streams 20 and 28 form the low pressure oxidant stream 26, which is compressed to about 320 bar (32 MPa) (stream 27) in a compressor 11 with inter-coolers 12. The high pressure oxidant stream 27 is heated in the economizer heat exchanger leaving as the preheated oxidant stream 38 at about 304 bar (30.4 MPa) about 707 °C.

A first side-stream 32 at about 1 10 °C is taken from the heating high pressure recycle C0 ₂ flow and heated to about 154 °C (stream 31 in FIG. 2) in side heat exchanger 13 against a heat transfer fluid (entering the side heat exchanger as stream 30 and exiting as stream 29) which removes heat of compression from the air compressors in the cryogenic air separation plant 14. The ASU has an atmospheric air feed 40 and a waste nitrogen exit stream 41 which is vented to the atmosphere.

A second side-stream 61 at a temperature of about 400 °C is taken from the heating high pressure recycle C0 ₂ stream and used in the turbine 2 for internal cooling.

The compressed C0 ₂ stream 23 at about 63.5 bar (6.35 MPa) and about 51 °C is cooled in a heat exchanger 46 against cooling water to provide stream 47 at about 17.5 °C with a density of about 820 kg/m ³ , which is pumped in a multi-stage centrifugal pump 8 to a pressure of about 305 bar (30.5 MPa). The pump discharge flow is divided into two parts.

High pressure recycle C0 ₂ stream 25 from the pump discharge flow is passed through the economizer heat exchanger 15 and functions as the flow from which the first side-stream and the second side-stream are taken (as discussed above).

The stream 24 from the pump discharge flow comprises the net C0 ₂ product stream derived from carbon in the natural gas. Stream 24 preferably can include an additional content of C0 ₂ for use in refrigeration. The additional C0 ₂ content can be up to about 50% by mass, up to about 40% by mass, or up to about 30% by mass of the recycle C0 ₂ . In some embodiments, the additional C0 ₂ content can be about 5% to about 45% by mass, about 10% to about 40% by mass, or about 15% to about 35% by mass of the recycle C0 ₂ .

The high pressure C0 ₂ stream 24 is cooled to near ambient temperature in a water cooler 50 and divided into two parts. High pressure C0 ₂ fraction stream 57 is reduced in pressure to about 8.2 bar (0.82 MPa) in valve 58 to form a cooling C0 ₂ stream 56, which is a two phase mixture at a temperature of about -45 °C. The cooling C0 ₂ stream 56 is passed through heat exchanger 10 where it evaporates and heats to near ambient temperature leaving as C0 ₂ stream 33. High pressure net C0 ₂ product stream 62 is passed directly into the heat exchanger 10 where it is cooled against the cooling C0 ₂ stream 56 to a temperature of about -38 °C leaving as cooled high pressure net C0 ₂ product stream 51. This stream is then passed through a small re- boiler 52 in the base of a stripping column 53 leaving as stream 55. This stream is reduced in pressure to about 10 bar (1 MP a) in valve 48 to form a two phase net C0 ₂ product stream 35, which is then passed through a separator 9.

The overhead vapor stream 49 exiting the top of the separator 9 encompasses about 4% by mass of the flow of two phase net C0 ₂ product stream 35 and is formed of about 30% by mass C0 ₂ and about 70% by mass of a combination of 0 ₂ and argon. The overhead vapor stream 49 is reduced in pressure in valve 60 and then vented to the atmosphere (stream 59 in FIG. 2).

Optionally, stream 59 can be heated in heat exchanger 10 to near ambient temperature providing extra refrigeration and then further heated to above ambient temperature to make the vent stream buoyant.

The liquid C0 ₂ stream 36 exiting the separator 9 at a pressure of about 10 bar (1 MPa) comprises about 96% by mass of the flow of two phase net C0 ₂ product stream 35. Stream 36 is fed to the top of the stripping column 53.

Exiting the bottom of the stripping column 53 is the low pressure liquid C0 ₂ product stream 54, which comprises the net C0 ₂ produced from carbon in the primary fuel feed to the power system. In the illustrated embodiment, stream 54 has an oxygen content below 10 ppm.

The top product stream 63 exiting the stripping column 53 is reduced in pressure to about 8 bar (0.8 MPa) in valve 64 and added to C0 ₂ stream 33. Combined streams 33 and 63 are compressed in compressor 34 to about 28.5 bar (2.85 MPa). The discharge stream 21 compressed in the C0 ₂ compressor 34 is mixed with gaseous C0 ₂ overhead bulk stream 22 and compressed back up to about 305 bar (30.5 MPa) in the C0 ₂ compressor 4 and the pump 8.

In the above example, specific values (e.g., temperature, pressure, and relative ratios) are provided to illustrate working conditions of an exemplary embodiment of the present disclosure. Such values are not meant to be limiting of the disclosure, and it is understood that such values may be varied within the ranges as otherwise disclosed herein to arrive at further working embodiments in light of the overall description provided herein.

Many modifications and other embodiments of the presently disclosed subject matter will come to mind to one skilled in the art to which this subject matter pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the present disclosure is not to be limited to the specific embodiments described herein and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.