Patent Application
20100098618
The present invention relates generally to contaminant removal from gas streams. In another aspect, the present invention relates to a process for removing/recovering sulfur from a gas stream using a Claus-type reactor followed by contact with a regenerable sorbent.
Gas streams containing sulfur species originate from various sources. They are found in refinery off-gases as well as sulfur treatment units that are unable to convert all gaseous sulfur species to elemental sulfur. These gases contain SO2 and H2S at levels exceeding permissible emission limits which are currently set at 10 ppm H2S and 250 ppm SO2 in the United States. Gas compositions vary widely depending on the application. Often steam, syngas, and/or CO2 are found in these gases. Such gases are mostly free of O2 but often contain H2.
One way to treat such gases is by hydrotreating and amine scrubbing. Hydrotreating requires the whole gas stream to be heated to reaction temperature following a gas cool-down from 400° C. to near ambient temperatures prior to use. Inherent in this process is a significant energy penalty due to the heating and cooling steps required. The amine regeneration produces concentrated H2S which is returned to a Claus unit where it is converted to elemental sulfur.
Alternatively, the gas can be oxidized in a burner to form SO2 as the only sulfur species. This option also requires a cool-down phase and additional equipment to scrub the SO2 and to regenerate the scrubbing material. This is known as the CANSOLV® process (CANSOLV is a registered trademark of Cansolv Technologies, Inc.) and the regeneration produces concentrated SO2 which is recycled to a Claus unit.
Accordingly, a need exists for a process to remove sulfur from a gas stream that eliminates the heating-up and cooling-down steps from the alternative processes.
In one embodiment of the present invention, there is provided a process for the removal/recovery of sulfur, the process comprising, consisting of, or consisting essentially of:
a) contacting a mixture of: 1) a gas stream comprising H2S and 2) an SO2 gas stream comprising SO2 with a catalyst comprising alumina in a reaction zone to thereby form a reactor effluent gas stream comprising elemental sulfur, H2S and SO2;
b) cooling the reactor effluent gas stream to thereby form a liquid elemental sulfur stream comprising elemental sulfur and a tail gas stream comprising H2S and SO2;
c) contacting the tail gas stream with a sorbent in a sorption zone to produce a product gas stream and a sulfur-laden sorbent, wherein the sorbent comprises:
(i) zinc oxide;
(ii) expanded perlite;
(iii) alumina; and
(iv) a promoter metal,
wherein the promoter metal is present in an amount which will effect the removal of sulfur or sulfur compounds from the tail gas stream when contacted with same in this step c) and at least a portion of the promoter metal is present in a reduced valence state;
d) contacting at least a portion of the sulfur-laden sorbent with a regeneration gas stream comprising oxygen in a regeneration zone to produce a regenerated sorbent and an off-gas-stream comprising SO2; and
e) utilizing at least a portion of the off-gas stream as the SO2 gas stream in step a).
Referring to
In general, a gas stream comprising H2S and an SO2 gas stream comprising SO2 can be mixed and contacted, by lines 100 and 126, respectively, with a catalyst comprising alumina in reactor 12 thereby forming a reactor effluent gas stream comprising elemental sulfur, H2S and SO2. The reactor effluent gas stream exiting reactor 12 via line 110 is passed to cooler 13 for cooling to thereby form a liquid elemental sulfur stream comprising elemental sulfur and a tail gas stream comprising H2S and SO2.
The elemental sulfur stream is removed from reactor 12 via line 111. The tail gas stream exiting cooler 13, via line 112, can be contacted with a sorbent in sorption zone 14 to thereby remove one or more contaminants from the tail gas stream. The resulting, contaminant-depleted, product gas stream exiting sorption zone 14, via line 114, can be routed to product gas user 16, while at least a portion of the contaminant-laden sorbent, removed via line 116, can be dried in drying zone 18 prior to exiting drying zone 18 via line 120 and regenerated via contact with a regeneration gas in regeneration zone 20. The resulting off-gas stream comprising SO2 exiting regeneration zone 20 is routed to reactor 12 via line 126. At least a portion of the regenerated sorbent can then be returned to sorption zone 14 via conduit 124 for subsequent reuse. In one embodiment, at least one of the sorption, drying, and regeneration zones 14, 18, and 20 can be contained within the same process vessel. In another embodiment, at least one of the sorption, drying, and regeneration zones 14, 18, and 20 can be contained in two or more separate process vessels. Further, the sulfur removal/recovery system 10 depicted in
The gas stream charged to reactor 12 can be any gas stream comprising H2S. More particularly, the gas stream is a synthesis gas stream from a gasification process which comprises CO, H2 and H2S. Typical feeds to such a gasification process include, but are not limited to, liquid hydrocarbons, coal and coke. The gas stream from such a gasification process is preferably treated in a conditioning process prior to being charged to reactor 12 to remove tars, chlorine and other materials that would contaminate and possibly lead to failure of downstream equipment.
The gas stream can comprise in the range of from about 10 ppmv to about 60 volume %, from about 10 to about 25,000 ppmv, or from about 10 to about 6,000 ppmv of H2S. The alumina present in reactor 12 can be any alumina-containing catalyst useful for the Claus-type reaction of H2S with SO2 to form elemental sulfur and water. The reactor 12 is operated at a temperature from about 150 to about 375° C., about 175 to about 340° C., or about 200 to about 340° C.; and at a pressure from about −7 to about 3000 psig, about 0 to about 1000 psig, or about 0 to about 350 psig; and at a standard gas hourly space velocity (SGHSV) of about 100 to about 20,000 hr−1, about 1000 to about 20,000 hr−1, or about 1000 to about 10,000 hr−1. The reactor effluent gas stream is cooled in cooler 13 at a temperature from about 121 to about 155° C., about 121 to about 150° C., or about 121 to about 135° C.; and at a pressure from about −7.0 to about 3000 psig, about 0 to about 1000 psig, or about 0 to about 350 psig.
The tail gas stream from cooler 13 can comprise in the range of from about 1 ppmv to about 30 volume percent (1 vol. %=10,000 ppmv), from about 1 ppmv to about 10 volume percent, from about 1 ppmv to about 1 volume percent, or from 1 ppmv to 1000 ppmv of SO2. In one embodiment, the tail gas stream from cooler 13 can comprise in the range of from about 1 ppmv to about 60 volume percent, from about 1 ppmv to about 20 volume percent, from about 1 ppmv to about 5 volume percent, or from 1 ppmv to 5000 ppmv of H2S.
In one embodiment, the ratio of H2S to SO2 in the tail gas stream exiting cooler 13 can be about 100:1, 10:1, 2:1, or 1:1. The tail gas stream can further comprise compounds selected from the group consisting of steam, syngas (CO and H2), CO2, and combinations of any two or more thereof.
As depicted in
In general, the sorbent employed in sorption zone 14 can be any sufficiently regenerable zinc-oxide-based sorbent composition having sufficient contaminant removal ability. While described below in terms of its ability to remove sulfur contaminants from an incoming tail gas stream, it should be understood that the sorbent of the present invention can also have significant capacity to remove one or more other contaminants.
While not wishing to be bound by theory, it is believed that nickel subsulfide (NiS2) is formed by the reaction of nickel sulfide (NiS) and SO2 in the presence of hydrogen. Nickel sulfide can originate from the reaction of nickel oxide and H2S. The suspected reaction mechanism is as follows:
NiO+H2S→NiS+H2O+ΔH
NiS+SO2+2 H2→NiS2+2 H2O+ΔH
In one embodiment of the present invention, the sorbent employed in sorption zone 14 can comprise zinc and a promoter metal component. The promoter metal component can comprise one or more promoter metals selected from the group consisting of nickel, cobalt, iron, manganese, tungsten, silver, gold, copper, platinum, zinc, tin, ruthenium, molybdenum, antimony, vanadium, iridium, chromium, palladium, and mixtures thereof. In one embodiment, at least a portion of the promoter metal component is present in a reduced-valence state, such as a zero valence state. The valence reduction of the promoter metal component can be achieved by contacting the sorbent with a reducing agent within sorption zone 14 and/or prior to introduction into sorption zone 14. Any suitable reducing agent can be used, including, but not limited to hydrogen and carbon monoxide.
In one embodiment of the present invention, the reduced-valence promoter metal component can comprise, consist of, or consist essentially of, a substitutional solid metal solution characterized by the formula: MAZnB, wherein M is the promoter metal and A and B are each numerical values in the range of from about 0.01 to about 0.99. In the above formula for the substitutional solid metal solution, A can be in the range of from about 0.70 to about 0.98 or 0.85 to 0.95 and B can be in the range of from about 0.03 to about 0.30 or 0.05 to 0. 15. In one embodiment, A+B=1.
Substitutional solid solutions are a subset of alloys that are formed by the direct substitution of the solute metal for the solvent metal atoms in the crystal structure. For example, it is believed that the substitutional solid metal solution MAZnB is formed by the solute zinc metal atoms substituting for the solvent promoter metal atoms. Three basic criteria exist that favor the formation of substitutional solid metal solutions: (1) the atomic radii of the two elements are within 15 percent of each other; (2) the crystal structures of the two pure phases are the same; and (3) the electronegativities of the two components are similar. The promoter metal (as the elemental metal or metal oxide) and zinc (as the elemental metal or metal oxide) employed in the sorbent described herein typically meet at least two of the three criteria set forth above. For example, when the promoter metal is nickel, the first and third criteria, are met, but the second is not. The nickel and zinc metal atomic radii are within 10 percent of each other and the electronegativities are similar. However, nickel oxide (NiO) preferentially forms a cubic crystal structure, while zinc oxide (ZnO) prefers a hexagonal crystal structure. A nickel zinc solid solution retains the cubic structure of the nickel oxide. Forcing the zinc oxide to reside in the cubic structure increases the energy of the phase, which limits the amount of zinc that can be dissolved in the nickel oxide structure. This stoichiometry control manifests itself microscopically in an approximate 92:8 nickel zinc solid solution (Ni0.92 Zn0.08) that is formed during reduction and microscopically in the repeated regenerability of sorbent. In addition to zinc and the promoter metal, the sorbent employed in sorption zone 14 can further comprise a porosity enhancer (PE) and an aluminate. The aluminate can comprise a promoter metal-zinc aluminate substitutional solid solution characterized by the formula: MZZn(1-Z)Al2O4, wherein M is the promoter metal and Z is in the range of from 0.01 to 0.99. The porosity enhancer, when employed, can be any compound which ultimately increases the macroporosity of the sorbent. In one embodiment, the porosity enhancer can comprise perlite or expanded perlite. Examples of sorbents suitable for use in sorption zone 14 and methods of making these sorbents are described in detail in U.S. Pat. Nos. 6,429,170 and 7,241,929, the entire disclosures of which are incorporated herein by reference.
Preferably, the sorbent comprises:
(i) zinc oxide;
(ii) expanded perlite;
(iii) alumina; and
(iv) a promoter metal,
wherein the promoter metal is present in an amount which will effect the removal of sulfur or sulfur compounds from the tail gas stream when contacted with same and at least a portion of the promoter metal is present in a reduced valence state
Table 1, below, provides the composition of a sorbent employed in sorption zone 14 according to an embodiment of the present invention where reduction of the sorbent is carried out prior to introduction of the sorbent into sorption zone 14.
In an alternative embodiment where the sorbent is not reduced prior to introduction into sorption zone 14, the promoter metal component can comprise a substitutional solid metal oxide solution characterized by the formula MXZnYO, wherein M is the promoter metal and X and Y are in the range of from about 0.01 to about 0.99. In one embodiment, X can be in the range of from about 0.5 to about 0.9, about 0.6 to about 0.8, or 0.65 to 0.75 and Y can be in the range of from about 0.10 to about 0.5, about 0.2 to about 0.4, or 0.25 to 0.35. In general, X+Y=1.
Table 2, below, provides the composition of an unreduced sorbent employed in sorption zone 14 according to an embodiment where the sorbent is not reduced prior to introduction into sorption zone 14.
As mentioned above, when an unreduced sorbent composition is contacted with a hydrogen containing compound in sorption zone 14, reduction of the sorbent can take place in sorption zone 14. Therefore, when sorbent reduction takes place in sorption zone 14, the initial sorbent contacted with the raw gas stream in sorption zone 14 can be a mixture of reduced sorbent (Table 1) and unreduced sorbent (Table 2).
In general, the incoming tail gas stream can contact the initial sorbent in sorption zone 14 at a temperature in the range of from about 150° C. to about 1000° C., about 250° C. to about 700° C., or 350° C. to 550° C. and a pressure in the range of from about atmospheric pressure to about 5000 psig, about atmospheric pressure to about 1000 psig, or atmospheric pressure to 500 psig. At least a portion of sulfur-containing compounds (and/or other contaminants) in the tail gas stream can be sorbed by the sorbent, thereby creating a sulfur-depleted product gas stream and a sulfur-laden sorbent. In one embodiment, sulfur-removal efficiency of sorption zone 14 can be greater than about 85 percent, greater than about 90 percent, greater than about 95 percent, greater than about 98 percent, or greater than 99 percent.
As depicted in
As depicted in
Regeneration zone 20 can employ a regeneration process capable of removing at least a portion of the sulfur (or other sorbed contaminants) from the sulfur-laden sorbent via contact with a regeneration gas stream under sorbent regeneration conditions. In one embodiment, the regeneration gas stream entering regeneration zone 20 via conduit 122 can comprise an oxygen-containing gas stream, such as, for example, air (e.g., about 21 volume percent oxygen). In another embodiment, the regeneration gas stream in conduit 122 can be an oxygen-enriched gas stream comprising at least about 50, at least about 75, at least about 85, or at least 90 volume percent oxygen. In yet another embodiment, the regeneration gas stream can comprise a substantially pure oxygen stream.
According to one embodiment of the present invention, the regeneration process employed in regeneration zone 20 can be a step-wise regeneration process. In general, a step-wise regeneration process includes adjusting at least one regeneration variable from an initial value to a final value in two or more incremental adjustments (i.e., steps). Examples of adjustable regeneration variables can include, but are not limited to, temperature, pressure, and regeneration gas flow rate. In one embodiment, the temperature in regeneration zone 20 can be increased by a total amount that is at least about 75° C., at least about 100° C., or at least 150° C. above an initial temperature, which can be in the range of from about 250 to about 650° C., about 300 to about 600° C., or 350 to 550° C. In another embodiment, the regeneration gas flow rate can be adjusted so that the standard gas hourly space velocity (SGHSV) of the regeneration gas in contact with the sorbent can increase by a total amount that is at least about 1,000, at least about 2,500, at least about 5,000, or at least 10,000 volumes of gas per volume of sorbent per hour (v/v/h or h−1) above an initial SGHSV value, which can be in the range of from about 100 to about 100,000 h−1, about 1,000 to about 80,000 h−1, or 10,000 to 50,000 h−1.
In one embodiment, the size of the incremental adjustments (i.e., the incremental step size) can be in the range of from about 2 to about 50, about 5 to about 40, or 10 to 30 percent of magnitude of the desired overall change (i.e., the difference between the final and initial values). For example, if an overall temperature change of about 150° C. is desired, the incremental step size can be in the range of from about 3 to about 75° C., about 7.5 to about 60° C., or 15 to 45° C. In another embodiment, the magnitude of the incremental step size can be in the range of from about 2 to about 50, about 5 to about 40, or 10 to 30 percent of magnitude of the initial variable value. For example, if the initial regeneration temperature is 250° C., the incremental step size can be in the range of from about 5 to about 125° C., about 12.5 to about 100° C., or 25 to 75° C. In general, successive incremental steps can have the same incremental step sizes, or, alternatively, one or more incremental step sizes can be greater than or less than the incremental step size of the preceding or subsequent steps.
In one embodiment, subsequent adjustments to the regeneration variable(s) can be carried out at predetermined time intervals. For example, adjustments can be made after time intervals in the range of from about 1 minute to about 45 minutes, about 2 minutes to about 30 minutes, or 5 to 20 minutes. In another embodiment, the adjustments can be made based on the value(s) of one or more “indicator” variable(s). An indicator variable is a variable in the system monitored to determine the progress of the sorbent regeneration. Examples of indicator variables can include, but are not limited to, sorbent sulfur loading, regeneration sorbent bed temperature, regeneration zone temperature profile (i.e., exotherm), and off-gas stream composition. In one embodiment, the sulfur dioxide (SO2) concentration in the off-gas stream is monitored to determine when the flow rate of the regeneration gas and/or the regeneration temperature should be incrementally adjusted.
The regeneration process can be carried out in regeneration zone 20 until at least one regeneration end point is achieved. In one embodiment, the regeneration end point can be the achievement of a desired value for one or more of the adjusted regeneration variables. For example, the regeneration process can be carried out until the temperature achieves a final value in the range of from about 300 to about 800° C., about 350 to about 750° C., or 400 to 700° C. or the SGHSV reaches a final value in the range of from about 1,100 to about 110,000 h−1, about 5,000 to about 85,000 h−1, or 25,000 to 60,000 h−1. In another embodiment, the regeneration process can be finished after a predetermined number of variable adjustments. For example, the regeneration process can be carried out long enough for at least 1 or in the range of from about 2 to about 8 or 3 to 5 incremental adjustments to be made. In yet another embodiment, the regeneration process can be carried out until a final value of the selected indicator variable is achieved. For example, the regeneration process can be carried out until the concentration of SO2 in the off-gas exiting regeneration zone 20 declines to a value less than about 1 volume percent, less than about 0.5 volume percent, less than about 0.1 volume percent, or less than 500 ppmv. Regardless of the specific endpoint selected, the entire length of the regeneration process can be less than about 100 hours, or in the range of from about 30 minutes to about 48 hours, about 45 minutes to about 24 hours, or 1.5 to 12.5 hours.
In one embodiment, the above-described regeneration process can have a regeneration efficiency of at least about 75 percent, at least about 85 percent, at least about 90 percent, at least about 95 percent, at least about 98 percent, or at least 99 percent. The regenerated sorbent can have a sulfur loading that is less than about 10 weight percent, or in the range of from about 0.05 to about 6 weight percent, or 0.1 to 4 weight percent.
As illustrated in
Referring back to
The following examples are intended to be illustrative of the present invention and to teach one of ordinary skill in the art to make and use the invention. These examples are not intended to limit the invention in any way.
A sorbent was exposed to several simulated feeds representing various tail gas compositions. The feeds had a general H2S to SO2 ratio of about 2:1.
Sorbents containing nickel, zinc, alumina, and expanded perlite were crushed and sieved to obtain 100+/200− mesh size particles. The sorbents were then contacted with the simulated tail gas streams. For Example 2, the sorbent was reduced with hydrogen before being contacted with the feeds, and for Examples 3-5, the sorbents were reduced in-situ during contact with the feeds.
A 1:1 mixture of sorbent and alundum was used to prevent the reactor bed from plugging. This mixture was placed in a downflow fixed bed reactor and heated to 400° C. and slightly elevated pressure to warrant feed flow through the system. To prevent steam from condensing in the reactor, all sample lines, valves, and other sample system components were heat-traced to maintain a temperature above 150° C. both up-and downstream of the reactor. Before analyzing the downstream off-gases, the steam was condensed to protect the on-line analyzers. For Examples 3-5, where a pre-reduction step was carried out, the sorbent was exposed to a 20 volume percent H2/N2 gas mixture until water levels in the off-gas were back to approximately their initial levels.
This Example was conducted using an unreduced sorbent. The feed stream used contained N2 with 243 ppmv SO2 and 243 ppmv H2S.
In this Example, the sorbent was pre-reduced with H2. In this case, complete conversion and storage of both contaminants into the sorbent was achieved. This reaction continued as long as reduced active components were available. Even when these resources neared exhaustion (after 50+ minutes), H2S was still removed due to the excess availability of ZnO. This is shown in detail in
In this Example, a small amount of a reductant (H2) was added to the feed. This in-situ reduction forms active species capable of reducing SO2 and storing the resulting sulfur into the sorbent. This is shown in
A gas composition resembling refinery off-gases was simulated to show that both SO2 and H2S can be removed under these conditions. These gases tend to contain larger amounts of steam. The gas composition studied is shown in Table 3.
The results are shown in
A Claus unit tail gas simulation was also tested. This tail gas contains syngas, CO2, H2S and SO2, but very low moisture levels. Table 4 below shows the composition of the Claus unit tail gas tested.
