Patent Application
20130081328
The present disclosure relates to a process for treating a gaseous mixture such as a refinery fuel gas to provide a low-carbon fuel gas. The disclosure further relates to the use of such low-carbon fuel gas as fuel for refinery equipment.
Within operations for refining petroleum products, refinery equipment such as heaters and boilers are a significant source of carbon dioxide emissions. Such equipment is typically fueled by refinery fuel gas (RFG) used throughout the refinery and containing hydrogen, methane as well as other hydrocarbon components. The carbon dioxide emissions can be controlled using a known amine solvent-based post-combustion carbon capture process. However, this requires a large amount of area which is not always available in a refinery.
It has been proposed that refinery equipment be fueled by hydrogen fuel produced by reforming RFG in a steam methane reformer (SMR) or autothermal reformer (ATR) fitted with a carbon dioxide capture technology. Since the RFG can contain as much as 30 mol % hydrogen, it is advantageous to separate a majority of the hydrogen from the RFG first. Known hydrogen separation membranes can be used for this purpose. The pressure differential across the membrane determines the flux, i.e. the flow rate of hydrogen per unit area per unit time across the membrane. In order to achieve high recovery of hydrogen, the size in terms of area of the membrane can be increased, the feed can be compressed to increase the pressure on the feed side and/or the pressure on the permeate side of the membrane can be reduced. Each of these has disadvantages in terms of cost, space requirements and energy usage for compression of the feed or recompression of the permeate.
It would be desirable to provide an integrated process that would produce a low-carbon fuel gas for use in refinery equipment in a way that is energy efficient and reduces carbon dioxide emissions from such refinery equipment.
According to one embodiment, the present disclosure relates to a process for providing a fuel gas for refining operations, in which a first gaseous mixture containing hydrogen and methane is passed across one side of a first hydrogen separation membrane to form a first hydrogen-enriched stream and a first hydrogen-depleted retentate stream containing methane. A sweep gas is passed across the other side of the membrane to enhance hydrogen flux across the membrane. The first hydrogen-depleted retentate stream is subjected to a reforming operation to form a second gaseous mixture containing hydrogen, carbon monoxide, carbon dioxide and water. The second gaseous mixture is passed through a water gas shift reactor to form a third gaseous mixture containing hydrogen and carbon dioxide. The third gaseous mixture is separated into a second hydrogen-enriched stream and a carbon dioxide stream. The first and second hydrogen-enriched streams are combined to form a low-carbon fuel gas stream containing at least about 50 mol % hydrogen.
A process and system are disclosed to reduce CO2 emissions from combustion equipment found in refineries such as heaters and boilers, herein also referred to as “refinery equipment.” Conventionally, the refinery equipment is fueled by refinery fuel gas (RFG) and/or natural gas.
As a result of the presence of carbon components in the fuel, such equipment is a significant source of CO2 emissions in a refinery. A low-carbon fuel gas stream containing at least about 50 mol % hydrogen, also referred to interchangeably herein as “H2 fuel,” can be used as a fuel for refinery equipment to reduce or eliminate CO2 emissions.
The H2 fuel can be produced by reforming RFG in a reformer, such as a steam methane reformer (SMR) or an autothermal reformer (ATR). By “reforming” is meant the reaction of a hydrocarbon mixture with an oxidizing agent such as oxygen and/or steam, usually in the presence of a catalyst, to form hydrogen, carbon monoxide, carbon dioxide and water. The RFG may contain up to about 30 mol % H2, so it is advantageous to separate a majority of H2 from the RFG before sending it to the reformer. The separated H2 is mixed with the H2 formed in the reformer and used as a fuel in the refinery equipment. Removal of H2 from the RFG has a number of potential advantages, including a decrease in the required size of the reformer, increase in hydrogen formation in the reformer, and decrease in the amount of O2 required in the ATR (where applicable).
The H2 can be separated from the RFG by use of a hydrogen membrane, also referred to herein as the “first hydrogen membrane.” Several membrane types are suitable for use in the present process, including polymeric, metallic, and porous inorganic membranes including carbon molecular sieve (CMS) and ceramic. The RFG is sent to the feed or retentate side of the membrane and H2 permeates through the membrane to the permeate side in the first hydrogen-enriched stream. The remaining RFG, also referred to as the hydrogen-depleted retentate stream, now substantially devoid of H2, leaves the membrane from the retentate side. The difference in the partial pressure of the H2 between the two membrane sides forms the driving force for H2 separation, and the hydrogen flux across the membrane is directly proportional to this partial pressure difference. By “hydrogen flux” is meant the volumetric flow rate of hydrogen permeate across the membrane per unit of membrane area over time, which can be expressed as cm3/cm2/min. Thus, in order to increase the hydrogen flux, to increase the H2 recovery and/or to reduce membrane area, the feed pressure is increased or the retentate pressure is reduced, or both. Since the feed RFG contains significant amounts of components other than H2, its compression to higher pressures requires a significant amount of energy. Therefore, it is advantageous instead to reduce the partial pressure of the H2 in the retentate stream by sending a sweep gas stream to the permeate side of the membrane, which increases the hydrogen flux across the membrane. According to one exemplary embodiment, the sweep gas stream utilizes N2 taken from the exhaust gas of refinery equipment and/or from an air separation unit (ASU). The former source of the sweep gas is advantageous when there is no ASU located nearby, for example, when the H2 is produced from RFG in a SMR instead of an ATR.
In one embodiment, N2 taken from the exhaust gas from refinery equipment is used as a sweep gas. The refinery equipment is fired with the H2 fuel thus obtained in the presence of air. The exhaust gas contains primarily N2, H2O and a small amount of O2. The exhaust gas has a pressure of near-atmospheric and a temperature generally in the range of about 100° C.-about 300° C. In order to utilize a portion of the exhaust gas as the sweep gas, the exhaust gas is first optionally cooled in a heat exchanger to a temperature in the range about 30° C.-about 50° C. to condense out most of the water which is next optionally removed in a separator column. A direct contact cooler may also be used to lower the flue gas temperature. The cooled sweep gas stream is next compressed to a pressure of about 3 to about 6 bar in a compressor. The O2 in the sweep gas stream is optionally removed in a catalytic de-oxo unit, which is supplied with a small amount of H2, which can be taken from the hydrogen-enriched permeate stream of the H2 membrane. The removal of O2 upstream of the membrane unit can be advantageous to limit the temperature rise in the membrane from the reaction between H2 and O2 in the sweep stream, particularly when the membrane is operated at temperatures in excess of about 150° C. The use of the compressor and the catalytic de-oxo unit can increase the temperature of the sweep gas stream, as known to those skilled in the art. This can be advantageous to approximate the temperature of the membrane unit, if high-temperature membranes are employed. The H2-enriched permeate stream is mixed with the H2 produced in the reforming process and can be supplied to refinery equipment as a high hydrogen content and low carbon content fuel.
In an alternative embodiment, the membrane sweep gas stream may be obtained from a reforming process. In this embodiment, a reforming process converts hydrocarbons into a mixture of hydrogen and carbon oxides. Any known reforming process can be used, including steam methane reforming (SMR) in which hydrocarbons (HCs) react with steam in an endothermic process, partial oxidization (PDX) in which HCs react with oxygen in an exothermic process, and a combination of these two referred to as autothermal reforming (ATR) in which HCs react with both steam and oxygen to produce H2 and carbon oxides. In the reforming process, a pre-reformer may be employed to convert higher HCs (C2+) into methane, and thus avoid carbon formation in the main reformer. The gas exiting the reformer, also referred to as syngas, consists of H2, CO, CO2, H2O, and unconverted CH4. The syngas is sent to a heat recovery steam generator (HRSG) to cool the gas stream and simultaneously generate high pressure steam. The cooled syngas is sent to a high temperature shift reactor (HTS) and low temperature shift (LTS) reactor to convert most of the carbon monoxide (CO) and water vapor into hydrogen (H2) and carbon dioxide (CO2) by employing the water gas shift (WGS) reaction.
Separation of H2 from the shifted syngas can be accomplished by sending the shifted syngas stream to a H2 selective membrane, also referred to as the “second hydrogen membrane,” where a majority-H2 stream, also referred to as the second hydrogen-enriched stream, is separated from the syngas stream. More than a single stage of membranes may be used and the shift reactor may be integrated with the membrane process to increase the CO conversion into H2 and simultaneously increase H2 recovery across the membrane. Suitable membranes for this application include metallic membranes, and porous inorganic membranes such as carbon molecular sieves (CMS) and ceramic membranes. These membranes are suitable for use in the temperature range about 150° C.-about 500° C. The hydrogen-depleted retentate stream from the membrane consists primarily of CO2 with minor amounts of H2, CO, and CH4. The CO2 from the hydrogen-depleted retentate stream can be separated from the rest of the components in a cryogenic CO2 purification unit (CPU), such as those described in U.S. Patent Application Nos. U.S. 2010/0024476, U.S. 2008/0176174, and U.S. 2008/0173585, incorporated herein in their entirety by reference. The carbon dioxide-depleted stream from the CPU containing primarily CH4, H2, and CO with minor amounts of CO2, which can be advantageously used as a sweep gas for use with the H2 membranes (the first and/or second hydrogen membranes), in combination with the RFG stream to recover additional H2 in the first membrane, and/or as a recycle stream to the reformer after any pre-reformer to recycle the CH4. A purge on the recycle stream may be conducted to avoid accumulation of inert gases in the process.
Referring to
Within membrane module 30, first gaseous mixture 1 passes across one side, herein referred to as the retentate side 35a, of a hydrogen separation membrane 35. The other side of the membrane is referred to as the permeate side 35b. A sweep gas 2 is passed across the permeate side 35b. A hydrogen-depleted retentate stream 7 containing methane is removed from the membrane module 30 from the retentate side 35a of the membrane 35. A permeate stream containing hydrogen, also referred to as a first hydrogen-enriched stream 3, is also removed from the membrane module 30, from the permeate side 35b of the membrane 35. The first hydrogen-enriched stream 3 contains at least about 10 mol % hydrogen.
Suitable hydrogen separation membranes for use in the present process include metallic membranes (e.g., palladium alloy membranes), porous inorganic membranes (e.g., silica, zeolite, alumina and carbon molecular sieve membranes), and nonporous polymeric membranes.
The hydrogen-depleted retentate stream 7 is reformed to form a second gaseous mixture 8 containing hydrogen, carbon monoxide, carbon dioxide and water in a reformer 50. Second gaseous mixture 8 can also include unreacted methane. The retentate stream 7 may be compressed to a pressure in the range about 20 to about 30 bar using a compressor (not shown). In one embodiment, reformer 50 is an autothermal reformer used in conjunction with an air separation unit 55. Air separation unit 55 receives air 14 which is separated into nitrogen-containing gas stream 15 and oxygen stream 16, which in turn feeds reformer 50.
According to one embodiment, a portion of the nitrogen-containing gas stream 15 removed from air separation unit 55 is used as the sweep gas 2 across the permeate side of the membrane 35.
The second gaseous mixture 8 is next sent to a water gas shift reactor 60 in which the carbon monoxide and water vapor of the gas mixture 8 are converted in the presence of a suitable catalyst to a third gaseous mixture 9 primarily containing hydrogen and carbon dioxide, as known to those skilled in the art.
The third gaseous mixture 9 is next sent to a separation unit 70 which separates the gaseous mixture 9 into a second hydrogen-enriched stream 12 and a carbon dioxide-containing stream 11. The second hydrogen-enriched stream 12 can contain small amounts of methane, carbon monoxide and carbon dioxide. In one embodiment, the second hydrogen-enriched stream 12 contains at least about 70 mol % hydrogen. The carbon dioxide-containing stream 11 can contain other components as well.
In one embodiment, the separation unit 70 is a carbon dioxide absorption unit in which carbon dioxide is absorbed into a solvent. Suitable solvents can be selected from, for example, methyl diethanolamine, dimethyl ether of polyethylene glycol, refrigerated methanol and the like.
In an alternative embodiment, the separation unit 70 is a membrane module utilizing a hydrogen separation membrane. Passing the third gaseous mixture 9 over the membrane results in second hydrogen-enriched stream 12 as the permeate stream and carbon dioxide stream 11 as the hydrogen-depleted retentate stream.
In yet another alternative embodiment, shown in
The second hydrogen-enriched stream 12 is then combined with the first hydrogen-enriched stream 3 to form the low-carbon fuel gas 4. The low-carbon fuel gas 4 contains at least about 50 mol % hydrogen. By way of example and not limitation, the fuel gas can have a lower heating value (LHV) between about 280 and about 310 BTUs per standard cubic foot. This low-carbon fuel gas 4 can be used within refining operations as fuel for refinery equipment, such as, for instance, heaters and boilers. Carbon dioxide emissions from such heaters and boilers using the low-carbon fuel gas 4 are at least about 50% lower than carbon dioxide emissions when conventional refinery fuel gas or natural gas is used as fuel. In one embodiment, the fuel gas 4 can be mixed with conventional refinery fuel gas or with natural gas prior to using in refinery equipment, e.g. heaters and boilers, to reduce the carbon dioxide emissions as compared with refinery fuel gas or natural gas alone. In another embodiment, steam or water can be injected in the combustion zone of the refinery equipment in order to absorb heat and moderate the combustion temperature.
The pressure of gas stream 25 can be maintained at between about 3 and about 6 bar depending on the fuel gas pressure required at the refinery equipment feed point, in order to avoid recompression of the low-carbon fuel gas 4 prior to its use. The temperature of the gas stream 25 may be adjusted suitably in a heat exchanger (not shown) depending on the membrane temperature requirement. The flow rate of the gas stream 25 is adjusted to obtain the required H2 purity in the combined fuel gas stream as described above as would be apparent to one skilled in the art.
A de-oxo unit 95 may optionally be used to remove oxygen from nitrogen-containing gas stream 25, resulting in stream 27 which can be used in the same ways as nitrogen-containing gas stream 25, as previously described. In this case, the O2 in the nitrogen-containing gas stream 25 is removed in a de-oxo unit in which oxygen and hydrogen react in the presence of a catalyst to form water. The de-oxo unit 95 is supplied with a small amount of H2, which can be taken from a hydrogen-enriched stream, i.e., at least one of hydrogen-enriched streams 3, 12 and 74. For instance, a portion of hydrogen-enriched permeate stream 3 can be fed to de-oxo unit 95 as hydrogen stream 3a. Similarly, a portion of hydrogen-enriched stream 12 can be fed to de-oxo unit 95 as hydrogen stream 12a. Alternatively, a portion of hydrogen-enriched permeate stream 74 can be fed to de-oxo unit 95 as hydrogen stream 74a. The removal of O2 upstream of the membrane unit can be advantageous to limit the temperature rise in the membrane from the reaction between H2 and O2 in the sweep stream, i.e., at least one of sweep streams 2 and 78, particularly when the membrane is operated at temperatures in excess of 150° C. The compressor 90 and the de-oxo unit 95, in addition to an optional heat exchanger (not shown), can be used to increase the temperature of the gas stream 27. This can be advantageous when gas stream 27 is used as sweep gas stream 2 and/or 78 in that the temperature of stream 27 can be adjusted to approximate that of the membrane unit, if high-temperature membranes are employed.
In one embodiment, the carbon dioxide-containing stream 11 may further be compressed to a suitable pressure, e.g., about 100 bar to about 200 bar, and injected into a subterranean formation for enhanced oil recovery or storage in suitable geological formations.
Where permitted, all publications, patents and patent applications cited in this application are herein incorporated by reference in their entirety, to the extent such disclosure is not inconsistent with the present invention.
Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “comprise,” “include” and its variants, are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions, methods and systems of this invention.
From the above description, those skilled in the art will perceive improvements, changes and modifications, which are intended to be covered by the appended claims.
