Patent Application
20140366446
The present disclosure generally relates to methods and systems for gas separation. More particularly, the present disclosure relates to methods and systems for separating carbon dioxide from natural gas or synthesis gas streams.
The separation and removal of carbon dioxide (CO2) from hydrogen or hydrocarbon-containing impure gas streams is desired, among other reasons, to improve the heating value of the gas product and to meet applicable environmental guidelines regarding CO2 capture. Differences in a number of properties between CO2 and hydrogen or light hydrocarbons (i.e., C1-C3 hydrocarbons) serve as potential bases for gas separations. These differences include solubility, acidity in aqueous solution, and molecular size and structure. Possible separations therefore rely on physical or chemical absorption into liquid solvents or pressure swing absorption with solid absorbents, for example.
Liquid solvent absorption (i.e., “wet”) systems, for example, are commonly used for gas separation to remove minor amounts of CO2. This contaminant is preferentially absorbed in physical solvents such as dimethyl ethers of polyethylene glycol or chemical solvents such as alkanolamines or alkali metal salts. The resulting CO2-rich (i.e., “loaded”) solvent is subsequently regenerated by pressure-based separation methods to recover both CO2 and a regenerated solvent that may be recycled for further use in absorption. Solvent regeneration is normally conducted at a reduced pressure relative to the upstream absorption pressure, to promote vaporization of absorbed CO2 from the solvent. Solvent absorption and solvent regeneration are usually carried out in different columns containing packing, bubble plates, or other vapor-liquid contacting devices to improve the efficiency of mass transfer between phases. The CO2 may be recovered in more than one stream, for example in the vapor fractions of multiple pressure-based separators.
Chemical solvents, and particularly amines and other basic compounds, react with contaminant CO2, an acid gas, to form a contaminant-solvent chemical bond. Considerable energy release is associated with this bond formation during the thermodynamically-favored, acid-base reaction. Due to the substantial heat input required to break the bonds of the heat-stable salts formed as chemical reaction products, chemical solvents are not economically regenerated. Physical solvents, on the other hand, do not react chemically with gas contaminants, but instead promote physical absorption based on a higher contaminant equilibrium solubility at its partial pressure in the impure gas (i.e., a higher Henry's law constant).
Physical solvents that remain chemically non-reactive with the impure gas stream are therefore desirable in gas separation systems due to the ease of solvent regeneration. However, one of the drawbacks of physical solvents is their tendency to co-absorb small amounts of hydrocarbons along with the CO2. When the solvent is subsequently regenerated, the hydrocarbons are liberated from the solvent, along with the CO2, resulting in an impure CO2 product. Thus, prior art systems have required recycling of the impure CO2 back to the absorption column. This impure CO2 recycling necessitates an increase in size of the absorption column, an increase in solvent circulation rate, an increase in system cooling requirements, and an increase in solvent inventory, all of which increase the operating costs of the system in terms of utilities and materials used.
Accordingly, it is desirable to provide systems and methods for separating CO2 from hydrogen and hydrocarbon-containing gas streams that reduce or eliminate the need for impure CO2 recycling. It is further desirable to provide such systems and methods that reduce input costs, such as utility costs and material costs. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.
Systems and methods for gas separation are disclosed. In one exemplary embodiment, a method for gas separation includes the steps of contacting a feed gas stream that includes a product gas and an impurity gas with a liquid-phase absorption solvent and absorbing the impurity gas and a portion of the product gas of the feed gas stream into the liquid-phase absorption solvent. The exemplary method further includes the steps of subjecting the liquid-phase absorption solvent to a first reduced pressure environment to remove the portion of the product gas and a portion of the impurity gas from the liquid-phase absorption solvent and separating the portion of the product gas from the portion of the impurity gas.
In another exemplary embodiment, a system for gas separation includes an absorptive separation unit configured to contact a feed gas stream that includes a product gas and an impurity gas with a liquid-phase absorption solvent so as to absorb a portion of the product gas and the impurity gas into the liquid-phase absorption solvent. The system further includes a first pressure-based separation unit configured to subject the liquid-phase absorption solvent to a first reduced pressure environment so as to remove a portion of the impurity gas and the portion of the product gas from the liquid-phase absorption solvent. Still further, the system includes a membrane separation unit configured to separate the portion of the product gas from the portion of the impurity gas.
The gas separation systems and associated methods will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:
The following detailed description is merely exemplary in nature and is not intended to limit the disclosure or the application and uses of the disclosed embodiments. All of the embodiments and implementations of the gas separation systems and associated methods described herein are exemplary embodiments provided to enable persons skilled in the art to make or use the same and not to limit their scope, which is defined by the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary, or the following detailed description.
Embodiments of the present disclosure are generally directed to gas separation methods in which a contaminant, present as a minor component of an impure feed gas, is selectively absorbed into a solvent. The methods advantageously recover significant portions of the impure feed gas components, including the contaminant, in purified product gas streams. Representative impure gas streams include those that contain hydrogen (H2) and/or light hydrocarbons (e.g., C1-C3 hydrocarbons such as methane, ethane, and propane), and non-hydrocarbon gas contaminants, such as carbon dioxide (CO2). Examples of such gas streams include synthesis gas, which is typically derived from the gasification or steam reforming of carbonaceous materials, and natural gas, which is typically derived from terrestrial sources. Natural gas and synthesis gas streams generally include CO2 at contaminant levels, that is, in an amount of about 10% or less by volume, such as an amount from about 1% to about 10% by volume (the remaining about 90% or greater by volume being occupied by the hydrogen and/or hydrocarbon gasses noted above), or about 5% or less by volume. For simplicity, the illustrative embodiments are described hereinafter with respect to such hydrogen and/or hydrocarbon and CO2 systems with the latter component being present at contaminant levels, although it will be appreciated that the disclosure is broadly applicable to the separation of impurity gasses from impure gas feeds in which the impurity gas, present in a minor amount, is preferentially absorbed into a liquid solvent, and particularly a physical solvent.
The embodiments disclosed herein further employ a membrane separation system to separate hydrocarbons from CO2 in the impure CO2 product that is produced from the regeneration of the physical solvent used to separate the CO2 from the hydrogen and/or hydrocarbon-containing gas streams. By separating the hydrocarbon impurities from the CO2 product after liberation from the solvent, it is not necessary to recycle any impure CO2 product back to the absorption column. This, in turn, reduces the required size of the absorption column and further reduces the utility and material costs required to operate the system. The described embodiments find particular application in synthesis gas and natural gas processing and purification applications, although other implementations are possible.
Reference is now made to
Subsequent to contacting with the impure feed gas, the solvent from the CO2 absorption column 152, which collects in the bottom of the tower, is partially or fully “rich” or “loaded” (i.e., absorbed with) with CO2. A CO2-rich solvent stream 104 exits at the bottom of column 152 and is routed to a first pressure-based separation system, for example a first flash separation drum 154. In the first flash separation drum 154, the CO2-rich solvent is subjected to a first reduced pressure environment to cause some of the dissolved CO2 and any dissolved hydrocarbons to be transferred to the gas phase. The first flash separation drum 154 generally operates at a pressure of less than or equal to about 27 barg (400 psig) (e.g., from about 2 barg (30 psig) to about 27 barg (400 psig)). In one embodiment, flash separation drum 154 operates at a pressure from about 21 barg (300 psig) to about 27 barg (400 psig).
A separated overhead gas stream 106 containing primarily CO2 and some hydrocarbon impurities exits through an end, such as the top of first flash separation drum 154 and is routed to a compressor 156, wherein the pressure of the gas is increased to a range of about 21 barg (300 psig) to about 100 barg (1500 psig). Thereafter, a pressurized gas stream 108 (again, containing primarily CO2 and some hydrocarbon impurities) is routed to a cooling system 157, wherein excess heat generated by compression is eliminated. The cooling system 157 reduces the temperature of the gas to a range of about 30° C. (90° F.) to about 50° C. (120° F.). Stream 110 is optionally sent to a compressor “knock-out” drum (not shown in the figures) or other suitable device to separate any liquid from the vapor phase; the gas outlet of the separator may be equipped with a mesh blanket or other suitable device to remove entrained liquids so that the membrane is not exposed to liquids.
A cooled gas stream 110 from the cooling system 157 is then routed to a membrane separation system 158. As noted above, the embodiments disclosed herein employ membrane separation system 158 to separate any hydrocarbon impurities from the CO2 in the hydrocarbon-contaminated CO2 gas stream 106/108/110 that results from flash-separating the dissolved gasses from the loaded solvent. By separating the hydrocarbon impurities from the CO2 product after liberation from the solvent using the membrane separation system 158, the required size of the absorption column 152 and the utility and material costs required to operate the system 100 are reduced. The structure and operation of membrane separation system 158 is described in greater detail below in connection with
Membrane separation systems for gas separation processes are generally based on the relative permeabilities of the various components of the gas mixture, resulting from a gradient of driving forces, such as pressure, partial pressure, concentration, and/or temperature. Such selective permeation results in the separation of the gas mixture into portions commonly referred to as “residual” or “retentate”, e.g., generally including the components of the mixture that permeate more slowly and “permeate”, e.g., generally including the components of the mixture that permeate more quickly.
Membranes for gas separation processes typically operate in a continuous manner, wherein a feed gas stream is introduced to the membrane separation module on a non-permeate side of a membrane. The feed gas is introduced at separation conditions that include a separation pressure and temperature that retains the components of the feed gas stream in the vapor phase, well above the dew point of the gas stream, or the temperature and pressure condition at which condensation of one of the components might occur.
Separation membranes are commonly manufactured in a variety of forms, including flat-sheet arrangements and hollow-fiber arrangements, among others. In an exemplary embodiment of the present disclosure, a flat-sheet separation membrane is novelly employed in membrane separation system 158. In a flat-sheet arrangement, the sheets are typically combined into a spiral wound element. An exemplary flat-sheet, spiral-wound membrane element 200, as depicted in
In one embodiment the permeate CO2 gas, which exits the membrane separator 158 via stream 114, is available at, for example, about 5 barg (about 80 psig) to about 10 barg (about 150 psig). The residue hydrocarbon gas, which exits the membrane separator 158 via stream 112, is available at about 14 barg (about 200 psig).
In an exemplary embodiment, whether the spiral-wound membrane 200 or the hollow fiber membrane 300 is employed, the membrane may be constructed of a glassy polymer material. In one example, the glassy polymer material includes cellulose acetate. In another embodiment, the glassy polymer material includes a polyimide/per-fluoro polymer-based material.
Returning to
Returning now to the description of first flash separation drum 154, a first flashed solvent stream 116 (i.e., the solvent stream having been exposed to the reduced pressure environment of the first flash separation drum 154) exits an end, such as the bottom of the drum 154 and is routed to a second pressure-based separation system, such as a second flash separation drum 160 that operates at a lower pressure than the first flash separation drum 154. In the second flash separation drum 160, the solvent is subjected to a second reduced pressure environment to cause an additional amount of the dissolved CO2 to be transferred to the gas phase. The second flash separation drum 160 generally operates at a pressure of less than or equal to about 21 barg (400 psig) (e.g., from about 2 barg (30 psig) to about 21 barg (300 psig)). In one embodiment, flash separation drum 160 operates at a pressure from about 14 barg (200 psig) to about 21 barg (300 psig). The second separation flash drum 160 is operated at a pressure that is less than the first flash separation drum 154 such that additional dissolved CO2 in the solvent is caused to transfer to the gas phase. The absorbed hydrocarbons having been substantially eliminated from the solvent by the operation of the first flash separation drum 154, the second flash separation drum produces a gas phase CO2 product stream 118 that is substantially pure.
A second flashed solvent stream 120, which exits the bottom of the drum 160, is directed to two further pressure-based separation systems, e.g. two further flash separation drums: a third, low-pressure flash separation drum 162 and a fourth, vacuum-pressure flash separation drum 164. The third flash separation drum 162 operates at a pressure that is lower than the second flash separation drum 160, and the fourth flash separation drum 164 operates at a pressure that is lower than the third flash separation drum 162. For example, the third flash separation drum 162 generally operates at a pressure of less than or equal to about 14 barg (200 psig) (e.g., from about 2 barg (30 psig) to about 14 barg (200 psig)). In one embodiment, flash separation drum 162 operates at a pressure from about 7 barg (200 psig) to about 14 barg (200 psig). Further, the fourth flash separation drum 164 generally operates at a pressure of less than or equal to about 7 barg (100 psig) (e.g., from about 2 barg (30 psig) to about 7 barg (100 psig)). A second gas phase, substantially pure CO2 product stream 122 exits from the top of the third, low-pressure flash separation drum 162. A third flashed solvent stream 124 exits the bottom of the drum 162 and is routed to the fourth flash separation drum 164. A third gas phase, substantially pure CO2 product stream 126 exits from the top of the fourth, vacuum-pressure flash separation drum 164. Due to the very flow pressure of the stream 126, a compressor 168 is typically provided to increase the pressure of CO2 product stream 126. Thus, a compressed CO2 product stream 128 is produced by compressor 168. As alluded to above, the CO2-rich permeate stream 114 is joined with the compressed CO2 product stream 128 generated by the fourth, vacuum-pressure flash drum 164.
Although a series of four pressure-based separation systems, e.g. a series of four flash separation drums 154, 160, 162, and 164 are illustrated in system 100, it will be appreciated by those having ordinary skill in the art that more or fewer pressure-based separation systems may be provided in an embodiment. For example, the system 100 may alternatively be provided with one, two, three, or more than four pressure-based separation systems, as may be desired for a given system implementation. It will be appreciated that the more pressure-based separation systems that are provided, the more complete the regeneration of the absorption solvent (i.e., the more complete the removal of absorbed CO2 gas therefrom) will be.
A fourth flashed solvent stream 130 exits the bottom of the drum 164 and is routed to a pumping system 170 that delivers the semi-lean (i.e., having at least a portion of the absorbed CO2 removed therefrom) solvent to a cooling system 172. The cooling system 172 reduces the temperature of the stream 130 and produces a cooled, semi-lean solvent stream 134 that is recycled back to the absorption column 152 for use in further gas separations. Cooled, semi-lean solvent stream 134 may be combined with an optional make-up solvent stream (not shown) to provide a solvent stream that is introduced into absorption column 152 as described above. The optional make-up solvent stream replaces the total solvent losses throughout the gas separation system 100.
As such, the present disclosure provides various exemplary embodiments of methods and systems for gas separation that employ a membrane separation system to reduce or eliminate the need for impurity-containing product stream recycling. The described embodiments allow for a reduction in size of the required gas separation column, a reduction in solvent circulation rate, a reduction in system cooling requirements, and a reduction in solvent inventory, all of which reduce the operating costs of the system.
While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes may be made in the processes without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of this disclosure.
