SEPARATION OF CARBON DIOXIDE FROM OTHER GASES

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/625,202, filed November 5, 2004, which is incorporated herein by reference in its entirety.

FIELD

The present disclosure relates to processes for the separation of carbon dioxide from other gases, and more particularly to processes for the separation of carbon dioxide from methane-containing gas mixtures.

BACKGROUND

Adsorptive separation systems are known for use in separation of gas streams comprising multiple gas components. Pressure swing, temperature swing, and partial pressure swing or displacement purge adsorption are exemplary processes known in the art for performing such separations of multi-component gas mixtures. In the exemplary case of separation of methane-containing gas mixtures, existing adsorption based separation processes do not provide for desirably efficient separation of methane from other gas components. Existing adsorptive methane separation processes typically include sequential pressure or temperature swing adsorption systems requiring multiple adsorption steps and corresponding multiple adsorption systems or subsystems, and typically result in the delivery of product methane gas streams at relatively low pressure, commonly as a component of desorption exhaust. Existing processes may also result in delivery of methane product gas at relatively low purity and/or recovery due to the lack of efficiency in adsorptively separating methane from other common gas mixture components. Such other common gas mixture components may for example comprise carbon dioxide, nitrogen and oxygen in impure methane-containing feed gas streams. Examples of

such impure methane-containing gas streams may include biogas, landfill gas, impure natural gas or other methane-containing commercial gas streams. In applications where such impure methane streams are desired to be stored and used as compressed or liquified gas fuels, the relatively low purity of the methane product streams typically produced by some existing adsorptive separation processes may result in inefficiencies and/or impracticalities in the compression and/or liquifaction processes used to store such fuel gases.

SUMMARY

Disclosed herein are inventive adsorptive gas separation processes that addresses some shortcomings of the prior art. In one embodiment, an adsorptive gas separation process is provided capable of producing a purified methane product gas as a light non-adsorbed product gas (as opposed to a heavy desorbed exhaust gas component) from a feed gas mixture comprising at least methane, and carbon dioxide. In an exemplary embodiment, the feed gas mixture may comprise at least about 10% carbon dioxide, and the purified methane product gas may be desirably purified to contain less than about 5000 ppm carbon dioxide, hi another exemplary embodiment, the feed gas mixture may comprise at least about 20% carbon dioxide, and the purified methane product gas may be desirably purified to contain less than about 5000 ppm carbon dioxide, hi yet a further exemplary embodiment, the feed gas mixture may comprise at least about 50% carbon dioxide, and the purified methane product gas may be desirably purified to contain less than about 5000 ppm carbon dioxide. In any of the preceding exemplary embodiments, the purified methane product gas may be more desirably purified to contain less than about 1000 ppm carbon dioxide, and most desirably less than 100 ppm carbon dioxide.

In another embodiment, an adsorptive gas separation process is provided capable of producing a purified methane product gas at a recovery of at least about 50% in a single adsorption step with a single adsorption device, from a feed gas mixture comprising at least methane and carbon dioxide, m an exemplary embodiment, the feed gas mixture may comprise at least about 10% carbon dioxide, ^v and the purified methane product gas may be desirably purified to contain less than

about 5000 ppm carbon dioxide. In another exemplary embodiment, the feed gas mixture may comprise at least about 20% carbon dioxide, and in yet another exemplary embodiment, may comprise at least about 50% carbon dioxide. In any of the preceding exemplary embodiments, the purified methane product gas may be more desirably purified to contain less than about 1000 ppm carbon dioxide, and most desirably less than 100 ppm carbon dioxide.

In a further embodiment, an adsorptive gas separation process is provided capable of producing purified methane product gas from a feed gas mixture comprising at least methane and carbon dioxide, by adsorption of the feed gas mixture in an adsorber in contact with at least a first adsorbent material suited for adsorbing at least a majority of the carbon dioxide in the feed gas stream, and a second adsorbent material suited for substantially removing remaining amounts of carbon dioxide while substantially excluding methane from adsorption, hi an exemplary embodiment, the feed gas mixture may comprise at least about 10% carbon dioxide, and the purified methane product gas may be desirably purified to contain less than about 5000 ppm carbon dioxide, hi a further exemplary embodiment, the feed gas mixture may comprise at least about 20% carbon dioxide, and in yet another exemplary embodiment, may comprise at least about 50% carbon dioxide. In any of the preceding exemplary embodiments, the purified methane product gas may be more desirably purified to contain less than about 1000 ppm carbon dioxide, and most desirably less than 100 ppm carbon dioxide.

In a further embodiment, an adsorptive gas separation process is provided capable of providing enriched methane product gas from a feed gas mixture comprising at least methane and carbon dioxide, by adsorption of at least a majority of the carbon dioxide on an alumina-based adsorbent material to produce the enriched methane product gas. In an exemplary embodiment, the feed gas stream may comprise at least about 15% carbon dioxide, and the enriched methane product gas stream may desirably comprise less than about 5% carbon dioxide. In a further exemplary embodiment, the feed gas stream may comprise at least about 25% carbon dioxide, and the enriched methane product gas stream may desirably comprise less than about 5% carbon dioxide. In yet a further exemplary embodiment, the feed gas stream may comprise at least about 50% carbon dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a purely exemplary 6 bed pressure swing adsorption (PSA) cycle comprising a single common adsorber pressure profile (i.e., relative pressure vs. phase angle) suitable for implementing an embodiment of the presently disclosed adsorption processes using PSA techniques. As is known in the art, the "phase angle" of a PSA cycle is employed to divide the progress of a one complete cycle into 360° to be able to determine the implementation of each individual step during the cycle.

DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS

In an above-mentioned embodiment, the inventive process may be used to produce a relatively purified methane product gas stream by adsorption from an impure methane containing feed gas additionally comprising at least about 10% carbon dioxide, where the purified methane product is produced as the light, non- adsorbed product gas, and delivered at the pressure of the adsorption process, hi another exemplary embodiment, the feed gas may comprise at least about 20% carbon dioxide. The purified methane product gas may desirably be purified to comprise less than about 5000 ppm of carbon dioxide, more desirably to comprise less than about 1000 ppm carbon dioxide, and in particular may be purified to comprise less than about 100 ppm carbon dioxide, such as is desirable for use in some commercial applications such as the liquifaction of methane gas for storage as a liquified gas fuel, hi an exemplary embodiment, the inventive adsorption process may comprise a pressure swing adsorption (PSA) process wherein the feed gas mixture may be passed over an adsorbent material to produce a non-adsorbed product gas delivered at a higher pressure, and a desorbed exhaust gas delivered at a lower pressure of the process, wherein the purified methane product gas (comprising desirably less than about 5000 ppm carbon dioxide, or more desirably less than about lOOOppm carbon dioxide, or particularly less than about 100 ppm carbon dioxide) may be delivered as the non-adsorbed product gas.

Common impure methane feed gas mixtures may additionally comprise gas components other than methane and carbon dioxide, which may include for example, and without limitation, nitrogen and oxygen. In addition to the removal of carbon dioxide from the purified methane product gas, the present inventive process may also be used to desirably purify the methane product gas by substantially reducing the concentration of oxygen in the methane product gas relative to the feed gas mixture, such as is desirable to reduce oxygen concentration in the purified methane product gas in applications where oxygen levels may become a safety concern in downstream uses of the methane product gas. This may be the case in liquifaction applications where oxygen in the methane product may become concentrated during the liquifaction process to produce a liquified methane fuel.

In a further embodiment, as summarized above, the inventive process may be used to produce a purified methane product gas from a feed gas mixture comprising at least methane and carbon dioxide by adsorption of at least a portion of the carbon dioxide component in a single pass through the adsorption process, in a single adsorption device, wherein the methane product gas is purified to contain less than about 5000 ppm carbon dioxide while recovering at least about 50% of the methane in the feed gas. In one example, a purified methane product gas may be obtained with a gas flow path through a single adsorption process unit represented by a single adsorber element (which may or may not include a single adsorbent bed provided the multiple beds are contiguous with each other along the gas flow path). In other words, a purified methane product gas may be obtained without having to introduce a feed gas into a first adsorbent element to obtain an intermediate product that is subsequently introduced into a second adsorber element In an exemplary such embodiment, the feed gas mixture may comprise at least about 10% carbon dioxide, and the purified methane product gas may desirably be further purified to contain less than about 1000 ppm carbon dioxide, and particularly less than about 100 ppm carbon dioxide, such as is desirable for liquifaction of the purified methane product gas for storage as a liquified methane fuel gas. In a further exemplary embodiment, the feed gas mixture may comprise at least about 20% carbon dioxide, and in a yet further exemplary embodiment, the feed gas mixture may comprise at least about 50% carbon dioxide. Additionally, the present inventive adsorptive separation

process may be used to desirably produce a purified methane product gas containing less than about 5000 ppm carbon dioxide at a recovery of at least about 65% and particularly at a recovery of at least 75% of the methane in the feed gas mixture.

In a further embodiment of the present inventive adsorptive gas separation processes, multiple adsorbent materials in a single adsorber unit or element may be used to produce the desired purified methane product gas from the feed gas mixture comprising at least methane and carbon dioxide by means of adsorptive separation. In an exemplary embodiment, a first adsorbent material may be used to adsorb at least a majority of the carbon dioxide from the feed gas mixture. Such first adsorbent material may be selected from suitable adsorbent materials either known in the art, or developed in the future, such suitable adsorbent materials having both desirably high adsorptive capacities for adsorption of carbon dioxide, and relatively low adsorptive capacity for methane, resulting in a distinct selectivity for adsorption of carbon dioxide in preference to methane. Such suitable adsorbent materials may comprise for example, and without limitation, aluminas, silicas, hydrophobic zeolites (such as zeolite Y or ZSM-5 for example), activated carbon, and combinations thereof. In particular, activated alumina and silica gel, or a combination thereof may desirably be applied to the inventive process as a suitable first adsorbent material, as they both demonstrate desirably high adsorptive carbon dioxide capacity and relatively low adsorptive methane capacity, and are therefore selective for the adsorption of carbon dioxide over methane. In a further exemplary embodiment, silica gel followed in the adsorption path by activated alumina may be used as a first adsorbent material in the inventive processes. A second adsorbent material may be used in the present inventive adsorptive gas separation processes to adsorb any remaining portion of carbon dioxide in the process gas stream following adsorption of at least a majority of the carbon dioxide from the feed gas mixture by the first adsorbent material. In order to substantially adsorb the remaining portion of carbon dioxide, the second adsorbent material may be selected from any suitable adsorbent materials known in the art, or subsequently developed, such suitable adsorbent materials having a desirably high adsorptive selectivity for carbon dioxide relative to methane. In particular, adsorbent materials with very high adsorptive selectivities for carbon dioxide over methane to the point of substantially excluding

methane from adsorption on the adsorbent material may be preferably selected for use as the second adsorbent material in the inventive processes. Such suitable second adsorbent materials may comprise for example, and without limitation, zeolite 3 A, carbon molecular sieves, titanosilicate molecular sieve materials (also known as mixed tetrahedral/octahedral molecular sieve materials) and combinations thereof. More desirably, such suitable second adsorbent materials may comprise carbon molecular sieves, titanosilicate molecular sieves materials (such as ETS-4 for example), and combinations thereof, wherein such more desirable second adsorbent materials may be prepared to substantially exclude methane from adsorption while actively adsorbing carbon dioxide, hi all cases, suitable adsorbent materials for application in any of the embodiments may be formed into any suitable shape or form for adsorption of gas components in a suitable adsorption apparatus, such forms which may comprise for example, and without limitation, powders, pellets, beads, sheets and particularly thin sheets, cloths, fabrics or combinations thereof, such as are known in the art or hereafter developed for use in adsorption processes.

In embodiments of the present inventive gas separation processes adapted to pressure swing adsorption (PSA) techniques, a maximum, or higher pressure for the PSA process may be selected that desirably provides for both bulk adsorption of at least a majority of the carbon monoxide on the first adsorbent material in the PSA adsorbers, and adsorption of substantially any remaining amounts of carbon dioxide on the second adsorbent material in the PSA adsorbers. In an exemplary embodiment of the inventive processes adapted for PSA, such suitable maximum PSA pressure maybe desirably chosen to be between about 100-250 psig and more desirably between about 130-230 psig. hi further embodiments adapted for PSA, a minimum, or lower PSA pressure may be chosen that desirably provides for the desorption of adsorbed gas components from the adsorbent material(s) during regeneration of the PSA adsorbers between adsorption cycles. Such minimum PSA pressure may be chosen as ambient pressure, or in an exemplary embodiment comprising vacuum assisted desorption of the PSA adsorbers, such minimum pressure may be desirably chosen to be below ambient pressure.

An exemplary, and non-limiting PSA adsorber pressure profile suitable for adaptation of the present inventive adsorptive separation processes to a 6-adsorber

PSA device for the purification of a methane product gas by PSA from a feed gas mixture comprising at least methane and carbon dioxide components is illustrated in Figure 1. The relative pressure levels of various stages of the exemplary PSA cycle for a single PSA adsorber vessel are shown against the progressive phase of the cycle, wherein the progressive phase of the 6-adsorber PSA cycle are represented as 12 half-steps. The disclosed pressure cycle comprises three depressurization steps, and three re-pressurization steps, and both a supply purge stream and a product purge stream in order to desirably increase the recovery of methane in the purified methane product gas, relative to some simpler PSA cycles, such as are known in the art, which may employ product purge alone to regenerate the adsorbent materials. In view of the many possible embodiments to which the principles of this disclosure may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the invention.