Patent Application
20130011323
The present invention relates to an energy efficient process for recovering hydrogen along with high pressure and high temperature carbon dioxide utilizing a reformer unit, a water gas shift reactor, one or more fixed sorbent beds containing a magnesium based sorbent and a hydrogen pressure swing adsorption unit.
In the typical prior art steps for carbon dioxide removal and regeneration, a hydrocarbon feed stream is treated in a steam hydrocarbon reformer unit, a water gas shift reactor and a pressure swing adsorption unit in order to obtain a pressure swing adsorption unit tail gas which must then be further treated in an additional process to remove the carbon dioxide present in the tail gas stream. In this schematic, the carbon dioxide is removed after the stream is treated in the hydrogen pressure swing adsorption unit.
A number of different products have been proposed for use in prior art methods for the removal of the carbon dioxide. However, most of the products used have to be regenerated at low pressure thereby resulting in the production of a carbon dioxide stream that is at low pressure. For example, U.S. Pat. No. 6,322,612 describes a pressure swing adsorption process for carbon dioxide removal. However, carbon dioxide is produced at low atmospheric or sub-atmospheric pressure. Solvent scrubbing processes such as the amine scrubbing process requires gas cooling below 40° C. thereby resulting in a loss of thermal efficiency. Sorbents such as zeolites have their capacities lowered at temperatures above about 200° C., and are strongly affected by the presence of moisture. In addition, sorbents such as calcium based sorbents and lithium based sorbents have been shown to adsorb carbon dioxide within the 200° C. to 400° C. temperature range but must be regenerated at low pressure and much higher temperatures (from 700° C. or greater) thereby requiring a large amount of regeneration energy.
New sorbents have been proposed for the removal of carbon dioxide. The publication “Novel Regenerable Magnesium Hydroxide Sorbent for CO2 Capture at Warm Gas Temperatures” by Rajani V Siriwardane and R. W Stevens of NETL (hereinafter “Novel Regenerable Magnesium Hydroxide Sorbent for CO2 Capture”) describes a sorbent based on Mg(OH)2 that can capture carbon dioxide at temperatures from 200° C. to 315° C. and can regenerate carbon dioxide at 20 bar and from 375° C. to 400° C. The noted article indicates that this sorbent may be used in applications such as coal gasification systems. U.S. Pat. No. 7,314,847 sets forth a process for preparing this sorbent. These sorbents produce CO2 streams at elevated pressure and temperature, however the CO2 stream needs further treatment to remove contaminants.
Accordingly, while there are a variety of different sorbents and different processes for removing carbon dioxide, there still exists a need to provide for a process that allows for the economical recovery of hydrogen as well as carbon dioxide where it is possible to remove the carbon dioxide at high pressure.
The present invention relates to a process for recovering hydrogen along with high temperature high pressure carbon dioxide from one or more hydrocarbon gas streams by incorporating a carbon dioxide recovery unit which utilizes a magnesium based sorbent into a process that includes a reformer unit, a water gas shift reactor and a hydrogen pressure swing adsorption unit. By incorporating such a carbon dioxide recovery unit into such a process, it makes it possible to provide a more economical recovery of carbon dioxide, thereby improving the overall economics of hydrogen and carbon dioxide production. The high temperature/high pressure carbon dioxide stream obtained can be used as carbon dioxide product (stored until used) or can be used directly. In addition, by using the process of the present invention, a high temperature/high pressure purge effluent gas stream is produced during the purge phase of the carbon dioxide removal step of the present process that can be recycled to the reformer unit to enhance the thermal efficiency of the overall process and improve the purity of the carbon dioxide product produced.
The process of the present invention provides for the incorporation of a sorbent based carbon dioxide removal unit into a process for the production of hydrogen and carbon dioxide, in order to improve the overall efficiency of the process through the phases of sorption, purge, carbon dioxide release and rehydroxylation. The process also includes a reformer unit, a water gas shift reactor and a pressure swing adsorption unit. By incorporating this sorbent based carbon dioxide removal unit between the water gas shift reactor and the hydrogen pressure swing adsorption unit, it is possible to effectively remove the carbon dioxide present in the water gas shift effluent, and produce a concentrated carbon dioxide product at high pressure. In addition to producing a concentrated carbon dioxide product at high pressure, during the purge of the sorbent beds, the sorbent beds are purged with high pressure steam to remove the hydrogen, carbon monoxide and methane trapped in the void spaces of the sorbent, with the pressure of the steam used being high enough to enable this hydrogen, carbon monoxide and methane containing stream (hereinafter “purge stream”) to be recycled to the reformer unit where the purge stream can be used as a supplemental feed for the reformer unit. The amount of steam used for purging the bed correspondingly reduces the amount of steam added to the reformer. This presents the further advantage of no net steam utilized for purging. The recycle of hydrogen, carbon monoxide, and methane at high temperature improves efficiency of the reformer. The purge phase of the carbon dioxide removal step improves the purity of the carbon dioxide stream, which is important if part of the carbon dioxide stream is used elsewhere as a product. Note that the pressure during the purge phase is higher than the pressure during the sorption phase. Following the purge phase, the sorbent bed is depressurized to the desired level and further heated to desorb the pure carbon dioxide at the desired pressure.
The resulting carbon dioxide depleted stream obtained as a part of these process steps is passed along to a pressure swing adsorption unit for producing a high purity stream of hydrogen. These process steps in turn maximize the use of energy contained in streams produced during the sorption phase of the carbon dioxide removal step while minimizing the additional treatment often necessary for use of the various streams produced according to conventional processes.
The process of the present invention involves recovering high purity hydrogen and high purity carbon dioxide from one or more hydrocarbon feed streams utilizing a reformer unit in combination with a water gas shift reactor, a carbon dioxide removal unit comprising one or more sorbent beds and a pressure swing adsorption unit. As used herein, the phrase “high purity carbon dioxide” refers to a carbon dioxide stream that contains greater than 90% carbon dioxide, preferably greater than 95% carbon dioxide and even more preferably, greater than 99% carbon dioxide. Furthermore, as used herein, the phrase “high purity hydrogen” refers to a hydrogen stream that contains greater than 90% hydrogen, preferably greater than 95% hydrogen and even more preferably, greater than 99% hydrogen. More specifically, the process involves introducing one or more hydrocarbon feed streams into a reformer unit to generate a hydrogen rich effluent, treating the hydrogen rich effluent in a water gas shift reactor to obtain a water gas shift effluent, subjecting the water gas shift effluent to treatment in a carbon dioxide removal unit containing one or more sorbent beds to produce a carbon dioxide depleted stream, a purge effluent gas and a carbon dioxide rich stream, introducing the carbon dioxide depleted stream into a hydrogen pressure swing adsorption unit to allow for the recovery of high purity hydrogen, recycling the purge effluent gas to the hydrocarbon feed stream as a supplemental feed, and withdrawing the high purity carbon dioxide product.
Those of ordinary skill in the art will recognize that the carbon dioxide depleted stream and the purge effluent gas may also contain residual amounts of carbon dioxide as well as the other components that may be present in the original gas stream treated. As used herein, the phrase “residual amounts” when referring to the amounts of other components that may be present in the carbon dioxide depleted stream refers collectively to an amount that is less than about 5.0%, preferably less than about 3.0% and even more preferably less than about 1.0%.
The process will be further described in more detail with reference to the single FIGURE contained herein (
In the second step of the process of the present invention, the hydrogen rich effluent obtained from the reformer unit 3 is treated in a water gas shift reactor 6 to further enrich the hydrogen content of the hydrogen rich effluent and to also increase the carbon dioxide content in the hydrogen rich effluent by oxidizing a portion of the carbon monoxide present in the effluent to carbon dioxide thereby obtaining a water gas shift effluent. In this embodiment, the hydrogen rich effluent stream is introduced via line 5 into the water gas shift reactor 6 (which can contain a variety of stages or one stage; stages not shown) to form additional hydrogen and carbon dioxide. Note that additional steam may also be added (not shown) upstream of the water gas shift reactor 6 along line 5. The result is a water gas shift effluent that is also at high temperature/high pressure. The conditions under which the water gas shift reaction is carried out are well known to those skilled in the art. Accordingly, the present process is not meant to be limited with regard to a specific water gas shift reactor 6 or the process for carrying out the reaction in the water gas shift reactor 6. Accordingly, any water gas shift reactor 6 known in the art may be used in the process of the present invention.
In the third step of the present process, the water gas shift effluent that is obtained from the water gas shift reactor 6 is subjected to treatment in a carbon dioxide removal unit 8 that contains at least one fixed sorbent bed 14 (only one bed depicted in
Accordingly, as used herein with regard to the present process, the term “fixed sorbent bed” or the plural thereof refers to any device that is designed to hold a fixed sorbent 15 while allowing for the injection and flow through of a water gas shift effluent from one side of the fixed sorbent bed 14 (functioning as an entrance) to the other side of the fixed sorbent bed 14 (functioning as an exit). The sorbent 15 is positioned within the confines of the fixed sorbent bed 14. With regard to the fixed sorbent beds 14, each fixed sorbent bed 14 allows for the injection of a product from line 7 (in this case the water gas shift effluent) into the fixed sorbent bed 14 and the exit of a stream that is essentially carbon dioxide free via line 10. As used herein, the phrase “essentially carbon dioxide free” refers to a stream that contains less than about 1.0% carbon dioxide, preferably less than about 0.5% carbon dioxide and even more preferably, less than about 0.1% carbon dioxide. However, as noted before, those skilled in the art will recognize that these streams often contain residual amounts of other components that may be present in the original syngas stream to be treated as well. Accordingly, as noted hereinbefore, the amount of the components will typically be present in the residual amount of less than 5.0%, preferably less than 3.0% and even more preferably less than 1.0%.
The sorbent 15 that is utilized in the one or more fixed sorbent beds 14 of the process of the present invention is highly selective for carbon dioxide and is selected from magnesium based sorbents, more particularly, magnesium hydroxide sorbents. As used herein, the sorbent 15 is in the form of a bed that contains beads, granules, crumbs or pellets of the sorbent 15. Of these sorbents 15, the most preferred with regard to the present process are the magnesium hydroxide sorbents such as those disclosed in U.S. Pat. No. 7,314,847 and Novel Regenerable Magnesium Hydroxide Sorbent for CO2 Capture, the full contents of each incorporated herein.
The fixed sorbent bed 14 retains the carbon dioxide because of the chemical reaction (adsorption) of the carbon dioxide with the sorbent 15. In addition, because of the manner in which the sorbent 15 is placed within each bed 14, there becomes spaces or voids due to the positioning of the sorbent particles 15 (the spaces created when the sorbent particles are in proximity to one another). Typically components such as hydrogen, carbon monoxide and methane will non-specifically fill or become trapped within the void spaces of the sorbent 15. These non-specifically filled or trapped components are then removed during the purge phase via line 13 as shown in
The actual treatment of the water gas shift effluent in the one or more fixed sorbent beds 14 involves four phases: the sorption phase, the purge phase, the carbon dioxide release phase and rehydroxylation phase. The first of these phases, the sorption phase, involves introducing the water gas shift effluent into one or more fixed sorbent beds 14 in the carbon dioxide removal unit 8 thereby allowing for the carbon dioxide in the water gas shift effluent to selectively react with the sorbent 15 as the water gas shift effluent passes through the fixed sorbent bed 14. Note that the temperature at which the water gas shift effluent in introduced into the one or more sorbent beds 14 will depend upon the specific sorbent 15 utilized as well as the conditions under which the water gas shift reaction are carried out. Typically, the water gas shift effluent will be introduced into the one or more fixed sorbent beds 14 at a temperature from about 100° C. to about 315° C. and at a pressure from about 20 bar to about 40 bar. Preferably, with regard to the present process, the water gas shift effluent will be introduced into the one or more sorbent beds 14 at a temperature that ranges from about 100° C. to about 250° C. and at a pressure from about 20 bar to about 40 bar. With regard to this reaction, the sorbent 15 reacts with the carbon dioxide in the water gas shift effluent to produce a carbonate and water. For example, in the case of magnesium hydroxide the reaction is:
Mg(OH)2+CO2→MgCO3+H2O
The magnesium hydroxide reacts with the carbon dioxide to yield magnesium carbonate and water. While a majority of the carbon dioxide present in the water gas shift effluent will react with the magnesium hydroxide sorbent to form a carbonate, a small amount of the carbon dioxide will remain unreacted. Generally greater than 90% of the carbon dioxide in the water gas shift effluent will be removed from the water gas shift effluent stream by the sorbent 15, preferably greater than 95% and even more preferably greater than 99%.
As noted previously, void spaces are created in the fixed sorbent bed 14 due to sorbent particle size and shapes (either as beads, granules, crumbs or pellets). These void spaces cause the non-specific “trapping” of components of the water gas shift effluent. The remaining components of the water gas shift effluent that are not trapped in the void spaces of the sorbent 15 are discharged from the fixed sorbent bed 14 via line 10 as a carbon dioxide depleted stream which can be further treated to obtain a hydrogen rich stream as described hereinbelow. As used herein with regard to the sorption phase, the phrase “remaining components” refers to hydrogen, carbon monoxide, methane, water vapor and other components as defined hereinbefore. In addition, the remaining components may also include a small amount of carbon dioxide that does not react with the sorbent 15 and becomes trapped in the void spaces. This carbon dioxide depleted stream is then passed to the hydrogen pressure swing adsorption unit 11 in order to remove the hydrogen present as a high purity hydrogen product stream via line 16.
Note that the period of time that the water gas effluent stream passes through the fixed sorbent bed 14 will depend upon the particular sorbent 15 utilized. As used herein, with regard to the sorption phase, the term “capacity” and phrase “high capacity” each refer to the amount of carbon dioxide that the sorbent 15 will remove from the water gas shift effluent stream. More specifically, the term “capacity” and phrase “high capacity” each refer to the amount of reactive sites (hydroxyl sites) of the sorbent 15 that react with carbon dioxide.
The next phase in the treatment of the water gas shift effluent in the carbon dioxide removal unit 8 is the purging of the fixed sorbent bed 14. As the sorbent 15 becomes saturated due to the carbon dioxide capacity of the sorbent 15 being reached (or almost being reach), the introduction of the water gas shift effluent stream into the sorbent bed 14 is stopped and high pressure superheated steam is injected into the fixed sorbent bed 14 through line 9. Note that at this point the pressure of the superheated steam used to purge the fixed sorbent bed 14 will be such that the purge stream created is at pressure higher than the pressure at the inlet of the reformer unit 3. This will allow the purge stream obtained to flow along line 12 to the reformer unit 3 without any additional compression. Note that a purge surge drum 17 may also optionally be included along line 12 to allow for the mixing of the purge stream for a more consistent stream. The superheated steam pressure utilized generally ranges from about 20 bar to about 60 bar.
Those skilled in the art will recognize that the flow through sorbent beds 14 can be controlled through strategically placed valves and lines. Furthermore, those skilled in the art will recognize that this embodiment may be carried out with regard to any number of fixed sorbent beds 14. In the preferred embodiment of the present process, the schematic configuration utilized with regard to the carbon dioxide removal unit 8 is a configuration that contains two or more fixed sorbent beds 14. More specifically, this embodiment can be carried out with as many as from four to eight fixed sorbent beds 14. Accordingly, in such a configuration rather than terminating the introduction of the water gas shift effluent into the fixed sorbent bed 14, the stream is simply diverted to another fixed sorbent bed 14 which is in the sorption phase of the four phases of the treatment in the carbon dioxide removal unit 8. Therefore, in such configurations, it is possible to use multiple fixed sorbent beds 14 which are staggered with regard to one another in terms of these four phases. By way of example, the configuration may include eight total fixed sorbent beds 14 with two fixed sorbent beds 14 running parallel to one another and being in the sorption phase at the same time, two fixed sorbent beds 14 being in the purge phase at the same time, two fixed sorbent beds 14 being in the release phase at the same time and two fixed sorbent beds 14 being in the rehydroxylation phase at the same time. Those skilled in the art can see that cycle sequence and step time can be tailored in many different ways to satisfy transfer of heat as well as transfer of carbon dioxide molecules. By using a configuration which is the same or similar in nature to this, it is possible to constantly run the process without the need to interrupt the process. In other words, it is possible to run the process online continuously. Note that the valves and their placement are not meant to be limiting to the present process and configuration.
During the purge phase, the superheated steam injected into the fixed sorbent bed 14 serves to dislodge a large portion of the remaining components that are trapped or lodged in the void spaces of the sorbent 15, thereby producing a purge effluent gas (also referred to as a purge stream) which contains these dislodged components. This purge effluent gas is withdrawn from the fixed sorbent bed 14 for example through a reversible flow conduit (not shown) and recycled via line 12 along with the superheated steam used to dislodge these components to the hydrocarbon feed stream 2 that is to be introduced into the reformer unit 3. This purge effluent gas which contains hydrogen, carbon monoxide and methane is used as a supplemental feed to maximize the production of hydrogen and carbon dioxide. This superheated steam is introduced into the fixed sorbent bed 14 and allowed to pass through the fixed sorbent bed 14 (for example, from one side to the opposing side of the sorbent bed 14). It is important to control this step as excess steaming will heat up the fixed sorbent bed 14 and start the release of the carbon dioxide from the sorbent 15 due to the decomposition of the carbonate. For example, MgCO3 starts decomposition at from about 350° C. to about 400° C. depending upon the pressure of the sorption bed 14. Those skilled in the art will recognize that higher pressures need higher temperatures for decomposition to start.
The composition of the purge gas will vary during the purge phase, being rich in hydrogen, carbon monoxide, methane in the beginning of the purge, and being lean in these components towards the end of the phase. A purge gas drum 17 is provided to smooth out the composition and flow of recycle stream to the reformer.
The next phase of the treatment in the carbon dioxide removal unit 8 is the carbon dioxide release phase which provides a high purity carbon dioxide stream that is also at high pressure and high temperature. This is accomplished by first increasing the temperature of the fixed sorbent bed 14. This increase in temperature of the fixed sorbent bed 14 may be accomplished in one of two manners. The temperature of the superheated steam stream provided via line 9 can be increased or additional heating means such as an indirect heat exchanger (not shown) may be used to increase the temperature of the sorbent 15 from about 180° C. to about 315° C. to from about 350° C. to about 420° C. In each of these cases, the temperature is increased to allow for the release of carbon dioxide from the sorbent 15 thereby producing a carbon dioxide stream that is considered not only hot but also wet. This high pressure high temperature carbon dioxide rich stream is withdrawn from the fixed sorbent bed 14 via line 13. The pressure in the fixed sorbent bed 14 during this phase is maintained at the desired pressure of the high purity and high pressure carbon dioxide product to be obtained—preferably at least in the range of from about 10 bar to about 30 bar. It is not until the carbon dioxide is released that the next phase, the rehydroxylation of the sorbent 15, takes place. More specifically, with regard to the sorbent 15, once the carbonate is formed in the sorption phase, the carbon dioxide can be released and the rehydroxylation can take place. In line with the previous example, this is demonstrated by the reactions as follows:
MgCO3→MgO+CO2
MgO+H2O→Mg(OH)2
As shown in this example, during the release phase, the magnesium carbonate is subjected to the noted temperatures (from about 350° C. to about 420° C.) to yield magnesium oxide and carbon dioxide. The temperature is maintained at this level for a period of time that is sufficient to allow for the release of the carbon dioxide. Once the carbon dioxide is released, for the rehydroxylation phase the temperature in the fixed sorbent bed 14 is then reduced using heat transfer media to a temperature from about 200° C. to about 300° C. in order to allow for the rehydroxylation of the sorbent 15. This phase occurs while the sorbent 15 in the fixed sorbent bed 14 is being contacted with steam or any other moisture containing stream that is being supplied. During the rehydroxylation phase, the magnesium oxide then reacts with water present (from the steam or other moisture containing stream) to yield magnesium hydroxide (a regenerated sorbent 15).
The next step of the present process as shown in
Finally, the purge effluent gas obtained from the carbon dioxide removal unit 8 is recycled via line 12 along with the superheated steam used to purge the sorbent beds 14 to the hydrocarbon feed stream 1 that is to be introduced into the steam hydrocarbon reformer unit 3. Therefore, the purge effluent gas, which is also at high temperature/high pressure will generally not require further processing to be utilized as feed for the steam hydrocarbon reformer unit 3. Accordingly, this purge effluent gas does not have to be cooled to be compressed, and the energy that is often lost from such streams is utilized in the steam hydrocarbon reforming step. As the hydrocarbon feed stream from the steam hydrocarbon reformer unit 3 typically requires the addition of steam to react in the steam hydrocarbon reformer unit 3, a portion of the steam can be from the sorbent bed 14 being purged with the high pressure steam.
A still further embodiment of the present invention involves modifying the carbon dioxide removal unit 8 to allow for the recovery of the heat of sorption and the heat of rehydroxylation in the fixed sorbent beds 14 in order to either generate high pressure steam that can be utilized within the carbon dioxide removal unit 8 or that can be utilized in the reformer unit 3 or to utilize the heated liquid as a source of heat in the reforming process. The modified carbon dioxide removal unit 8 would therefore comprise at least one fixed sorbent bed 14 that contains sorbent 15 and a heat transfer surface (not shown). The heat transfer surfaces would have a heat transfer media running therethrough so that when the fixed sorbent bed enters either the sorption phase or the rehydroxylation phase, this media could be run through the surfaces and allowed to adsorb the heat of sorption or the heat of rehydroxylation. More specifically, the heated transfer media could be used to generate high pressure steam for the carbon dioxide removal unit 8 or the reformer unit 3 or as a source of heat for the reforming process. A variety of different types of heat transfer media are available to be utilized in this manner. Examples of such heat transfer media include, but are not limited to, a molten carbonate salt mixture or any inorganic or organic compound with a boiling point that ranges from about 250° C. to about 350° C.
