Regenerable system for the removal of sulfur compounds from a gas stream

Abstract

The disclosure relates to copper oxide-based sorbents, and processes for preparing and using them. The sorbents are preferably used to remove one or more sulfur species from gas streams. The sorbents comprise a porous silica support material impregnated with CuO nanoparticles. The nanoparticles are uniformly distributed throughout the porous silica support and sulfur compounds are adsorbed on the nanoparticles.

Claims

1. A sulfidized sorbent composition comprising a porous silica support material impregnated with CuO nanoparticles, wherein the nanoparticles are essentially uniformly distributed throughout the porous silica support and sulfur compounds are adsorbed on the nanoparticles, and wherein the nanoparticles have an average diameter of about 1 to about 8 nm.

2. The sulfidized sorbent composition of claim 1, wherein the silica support material has a median pore diameter of about 5 to about 15 nm.

3. The sulfidized sorbent composition of claim 1, wherein the CuO nanoparticles further comprise Zn.

4. The sulfidized sorbent composition of claim 3, wherein the Cu/Zn molar ratio is in the range of about 20:1 to about 1:1.

5. The sulfidized sorbent composition of claim 1, wherein the porous silica support has a pore volume in the range of about 0.3 to about 3.0 ml/g.

6. The sulfidized sorbent composition of claim 1, wherein an average interparticle distance between the nanoparticles is at the theoretical maximum distance.

7. The sulfidized sorbent composition of claim 1, wherein the silica support material comprises SBA silica, MCM silica, FDU silica, KIT silica or silica gel.

8. The sulfidized sorbent composition of claim 1, wherein the sulfur compounds comprise H.sub.2S.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of a desulfurization process of the invention.

(2) FIG. 2 depicts the results of a low angle XRD experiment showing patterns of SBA-15 and copper-zinc impregnated SBA-15. The inset in the figure shows wide angle patterns.

(3) FIG. 3 shows argon adsorption-desorption isotherms for SBA-15 and CuZnOSBA-15 measured at 87 K. Pore size distribution (inset) was obtained using the NLDFT equation (silica, cylindrical pores).

(4) FIG. 4 shows an HAADF STEM image of sulfidized CuZnOSBA-15.

(5) FIG. 5 shows breakthrough curves of cyclic adsorption/regeneration tests on CuZnOSBA-15.

(6) FIG. 6 shows breakthrough curves of cyclic adsorption/regeneration tests on CuZnOSBA-15.

(7) FIG. 7 shows breakthrough curves of cyclic adsorption/regeneration tests on CuZnO-silica gel.

(8) FIG. 8 XRD pattern of CuZnOAl.sub.2O.sub.3 adsorbent.

(9) FIG. 9 shows breakthrough curves of cyclic adsorption/regeneration (14 cycles) tests on CuZnOAl.sub.2O.sub.3 adsorbent.

DETAILED DESCRIPTION

(10) In the following detailed description, preferred embodiments of the invention are described to enable practice of the invention. Although specific terms are used to describe and illustrate the preferred embodiments, such terms are not intended as limitations on practice of the invention. Moreover, although the invention is described with reference to the preferred embodiments, numerous variations and modifications of the invention will be apparent to those of skill in the art upon consideration of the foregoing, together with the following detailed description. The parameters set forth herein are not defined by any one analytical technique. As is readily understood by those of ordinary skill, parameters may vary depending on instruments used to define certain parameters.

(11) Unless otherwise specified, the terms a, an, and the mean one or more.

(12) As used herein, about, approximately, and substantially will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which they are used. If there are uses of the term which are not clear to persons of ordinary skill in the art given the context, about or approximately will mean up to plus or minus 10% of the particular term and substantially will mean more than plus or minus 50% of the particular term.

(13) As indicated previously, the sorbent compositions of the invention can include a porous support material. The disclosed supports may include porous silicon dioxide materials (mesoporous silica-pore size range 2-50 nm) with ordered or disordered pore structure (e.g. SBA-15, SBA-16, MCM-41, MCM-48, KIT-6, FDU-12, and silica gel). As used herein, silicon dioxide refers to silica having the formula SiO.sub.2. Silicon dioxide may form a porous support, such as porous particles, which may be impregnated with sorbent material as disclosed herein. In some embodiments, the silica support material can have a median pore diameter of about 5 to about 50 nm, about 5 to about 15 nm, about 7 to about 8 nm, or about 8 nm. Median pore diameter can be measured using methods known to those of ordinary skill in the art, for example, by standard nitrogen or argon adsorption analysis.

(14) In some embodiments, the porous silica support has a pore volume in the range of about 0.3 to about 3.0 ml/g. In some embodiments, the silica support has a surface area of about 100 to about 1000 m.sup.2/g, or about 750 m.sup.2/g. Specific surface area can be calculated using conventional methods, including the Brunauer-Emmett-Teller (BET) theory.

(15) The sorbent compositions may be impregnated with copper material. Copper material may include copper metal, copper oxides, and copper salts (e.g., copper nitrate and copper acetate). After the disclosed sorbent compositions have been calcined, preferably the compositions comprise copper metal or copper oxide. The sorbent compositions may also be impregnated with zinc material. Zinc material may include zinc metal, zinc oxides, and zinc salts (e.g., zinc nitrate and zinc acetate). After the disclosed sorbent compositions have been calcined, preferably the compositions comprise zinc metal or zinc oxide.

(16) As used herein, impregnated refers to the introduction of a solution to a porous support material. In contrast to the term coating the term impregnated or impregnating means that the solution has permeated the support material or that the support material has become infused with the solution. Coating on the other hand only indicates that a layer of the solution has been deposited on the outer surface of the support material.

(17) In some embodiments, the silica support is impregnated with CuO nanoparticles. In some embodiments, the nanoparticles have an average diameter of about 1 to about 8 nm or about 3 to about 6 nm or about 6 nm or about 5 nm. Nanoparticle diameter can be measured using analytical methods known to those of ordinary skill in the art, for example, by transmission electron microscopy (TEM).

(18) In some embodiments, the silica support is impregnated with CuZnO nanoparticles. In some embodiments, the Cu/Zn molar ratio is in the range of about 20:1 to about 1:1. The sorbent can also be unsupported copper-zinc-aluminum oxides. Copper/zinc molar ratio is between 20:1 to 1:1 with a minimum Cu/Al ratio of 10.

(19) The nanoparticles are essentially uniformly distributed throughout the porous silica support. As used herein the term essentially uniformly distributed means that the nanoparticles are evenly distributed throughout the entire pore system. In some embodiments, the nanoparticles are spaced at equally sized and maximally spaced distances. Without being bound by any theory of the invention, it is believed that the uniform distribution of the nanoparticles on the support contribute to the high stability achieved in the sorbent compositions of the invention.

(20) In some embodiments, the sulfur compounds are adsorbed on the nanoparticles. As used herein, sulfur compounds may include, sulfur, hydrogen sulfide (H.sub.2S), carbonyl sulfide (COS) and other sulfur compounds such as SO.sub.2, CS.sub.2, and S.sub.2. Organosulfur compounds of the invention include compounds such as mercaptans or those thiophenic compounds found in cracked gasolines, which include, among others, thophene, benzothiophene, alkyl thophenes, alkyl benzothiophenes, and alkyldibenzothiophenes.

(21) The sorbents of the invention have a high sulfur capacity. One method of expressing sulfur capacity is miligrams sulfur per gram of sorbent (mgS/g). In some embodiments, the sorbents provide a sulfur removal capacity in the range of about 60 to about 300 mgS/g.

(22) In some embodiments, the sorbent compositions may be prepared by the following steps: impregnating the silica support material with an aqueous solution of a metal salt; drying the impregnated support material; calcinating the impregnated support material.

(23) In some embodiments, the silica support material is impregnated with an aqueous solution of zinc salt and copper salt having a molar ratio of Cu:Zn of about 2:1. The zinc salt can include zinc nitrate, zinc acetate, and a mixture thereof; and the copper salt can include copper nitrate, copper acetate, and a mixture thereof.

(24) The sorbent composition can be converted to a sulfidized sorbent composition by passing a gaseous stream of sulfur compounds through the impregnated support material. The step of impregnating can be performed by incipient wetness impregnating.

(25) The step of drying can be performed by vacuum drying. The vacuum drying can be performed at room temperature.

(26) The calcinating can be performed by heating the dried particles to a temperature range of about 200-600 C., or 500 C. The calcinating can be performed in an inert gas.

(27) The sorbent compositions may be utilized in methods for treating gaseous streams, liquid streams or both. In some embodiments, the sorbent composition is utilized to treat a hydrocarbon stream (e.g., a fuel stream). In a preferred embodiment, the sorbent compositions are utilized for treating gaseous streams containing sulfur compounds in Claus process tail gas or remote small natural gas processing units, syngas (H.sub.2/CO) clean-up, and potential automobile exhausts.

(28) In some embodiments, sulfur compounds are removed from a gas stream, by passing the gas stream through an effective amount of a sorbent composition for an effective amount of time to produce a sulfidized sorbent composition and to reduce sulfur compounds in the gas stream to a level of less than about 1 ppm (or about 1 to 3 ppm, or about 1 to 5 ppm).

(29) The gas steam is heated to a temperature of less than about 400 C. (or about 150 to 250 C.) before being passed through the sorbent composition.

(30) In some embodiments, the gas stream (e.g., Claus tail gas) is treated with a hydrogenating agent before being passed through the sorbent composition. This ensures that a significant portion of the gas is converted to H.sub.2S. Typically, reducing gases present in the tail gas (e.g. H.sub.2, CO, H.sub.2O) are sufficient to convert all of the sulfur compounds to hydrogen sulfide via hydrogenation/hydrolysis reactions. In some embodiments, the hydrogenation agent is hydrogen gas. In some embodiments, the sulfur compounds in the gas stream are primarily H.sub.2S. In some embodiments, the H.sub.2S concentration in the gas stream is lower than 1%.

(31) The sorbent compositions are highly regenerable. As used herein, regnerability relates to the ability of the same sorbent to be used for multiple cycles of adsorption after stripping the adsorbed species (e.g., H.sub.2S) and then to be used for subsequent cycles of adsorption. In some embodiments, the sorbent compositions do not exhibit substantially reduced sulfur capacity after regeneration in comparison to a sorbent composition that has not been regenerated. In some embodiments, the regenerated sorbent has a sulfur capacity that is at least about 60% of that of the sorbent composition that has not been previously used, or preferably about 70%, or 80%, or 90% or 95% of the sorbent composition that has not been previously used.

(32) In some embodiments, the sorbent composition is regenerated (i.e., the sulfur compounds adsorbed to the sorbent composition are removed) by heating the sulfidized sorbent composition to a temperature between about 100-700 C. (or about 400-550 C.) and passing an oxidizing agent over the sorbent composition. The oxidizing agent can include air, pure oxygen, diluted oxygen, ozone, and hydrogen peroxide, or a combination thereof. In some embodiments, the oxidizing agent is diluted oxygen. In some embodiments, the diluted oxygen is 1-5 mol % oxygen in an inert carrier gas.

(33) The regeneration step can involve several oxidation-reduction cycles. For example, in some embodiments, a reducing agent is passed over the sorbent composition after the oxidation step. In some embodiments, the reducing agent includes hydrogen, methane, and carbon monoxide gas, or a mixture thereof. The hydrogen gas can contain 1-10 mol % hydrogen in an inert carrier gas. In some embodiments, an additional step of passing an oxidizing agent over the sorbent composition after the reduction step is included. The oxidation-reduction-oxidation cycle can repeated in tandem at least 2 times (or 2-10 times in tandem). In some embodiments, the adsorbent is flushed with an inert gas between each oxidation-reduction-oxidation cycle. In some embodiments, the inert gas is nitrogen gas. According to another embodiment, the regeneration procedure can be sequentially performed by exposing the bed to oxidizing/reducing/oxidizing gas mixtures with flushing the bed with inert gas or steam in between. In one example, oxidizing gas and reducing gas can be 1-10 mol % hydrogen and 1-5 mol % O.sub.2 in balance inert gases, respectively. In some embodiments, during the oxidation step converting copper sulfide to copper oxide, at least some copper sulfate is formed. Sulfates can react with H.sub.2S present in the gas phase and contaminate the stream with SO.sub.2 during adsorption cycle. In order to eliminate this negative effect, sulfates can be reduced using reducing gases after oxidation. In the final oxidation step, the metal copper can be converted to its oxide form which has a higher activity for sulfur removal.

(34) A schematic showing a desulfurization process of the invention is shown in FIG. 1. Ancillary equipment (e.g., compressors, valves, heaters) are not shown in the respective process flow diagram but are covered within the scope of the present invention. As shown in FIG. 1, a Claus tail gas 4 is passed through a catalytic hydro-treating reactor 1 and effluent containing hydrogen sulfide 5 is passed through the online adsorption bed 2. The resulting effluent 7 contains sub-ppm levels of hydrogen sulfide. For a continuous operation after hydrogen sulfide breakthrough (defined according to the regulations in place) in bed 1, hydro-treated flow 4 is diverted to bed 2. For regenerating bed 1, an oxidizing stream 8 comprising an oxygen-containing gas in the temperature range 100-700 C., preferably 400-550 C., is passed through the sulfided bed until complete regeneration (or any acceptable level) is achieved. Applying such low temperatures for regeneration is advantageous and permits construction of the sulfur removal unit with stainless steel, thereby lowering the capital cost of the unit. The effluent 6, containing sulfur dioxide, is returned to the Claus plant for sulfur capture.

(35) The method may be performed using a cyclic operation in a dual bed configuration with one active bed online until the breakthrough point is reached. The sorbent can then be regenerated in an oxidizing gas at temperatures up to 550 C. The sulfur dioxide-rich effluent can be returned to the Claus plant during the regeneration step. The hyro-treated stream can be directed to the second bed. In another embodiment, the method can include two beds filled with sorbents of the invention working in cyclic fashion using operational procedures apparent to the skilled person.

(36) The following examples illustrate preparation and use of currently preferred sorbents according to the present invention.

EXAMPLES

Example 1

Synthesis and H2S Capacity of CuZnOSBA-15 Adsorbent Material

(37) In this example, an adsorbent material composed of copper-zinc oxides on mesoporous silica (SBA-15) (CuZnOSBA-15) was prepared. CuZnOSBA 15 was prepared by incipient wetness impregnation of aqueous solution Cu(NO.sub.3).sub.2.3H.sub.2O and Zn(NO.sub.3).sub.2.6H.sub.2O (molar ratio Cu:Zn=2:1). SBA-15 was synthesized according to the procedure developed by Sayari et al. 1.0 mL of the above solution was added to 1 g SBA-15 in 0.2 mL batches. After impregnation, powders were vacuum dried at room temperature for 24 h and then calcined at 500 C. for 4 h under a nitrogen flow of 50 mL/min.

(38) As depicted in FIG. 2, XRD patterns of SBA-15 (calcined at 550 C. for 5 h) and CuZNOZnO-SBA-15 (molar ratio Cu:Zn:Si=2:1:12.5) confirmed formation of mesoporous silica and its structural stability after incorporation of copper and zinc oxides. After incorporation of oxide nanoparticles, characteristic reflections shifted slightly (0.2 nm) to higher angles indicating framework contraction. No peak associated with copper or zinc oxides was detected in the diffraction patterns of CuZnOSBA-15, suggesting a high dispersion of incorporated nanoparticles on the support. Argon adsorption isotherms (FIG. 3) follow IV type with H1 hysteresis; consistent with the mesostructure of SBA-15 with one-dimensional cylindrical channels. The cumulative pore volume obtained using NLDFT model on isotherms indicated that the cumulative pore volume decreased from 1.34 cc/g to 0.77 cc/g which can be attributed to the presence of nanoparticles in the pores. Embedding nanoparticles in SBA-15 resulted in a reduction in the median pore size from 7.9 nm to 7.6 nm. STEM-HAADF imaging of CuZnOSBA-15 after sulfidation revealed a very uniform distribution of copper-zinc sulfided nanoparticles (5 nm) in SBA-15 (FIG. 4).

(39) The CuZnOSBA-15 adsorbent was exposed to a flow H.sub.2S (102 ppmv in He) at 150 C. and 1 atm in a cyclic adsorption/regeneration fashion. Regeneration was conducted at 500 C. under a flow of 5 mol % oxygen in nitrogen. FIG. 5 shows the high performance of the adsorbent towards cyclic adsorption/regeneration.

(40) The adsorbent showed a high level of adsorption and regenerability performances towards a simulated gas (H.sub.2S 200 ppmv, CO 300 ppmv, C.sub.2H.sub.6 300 ppmv, CH.sub.4 0.25%, Ar 0.59%, CO.sub.2 19.88, N.sub.2 79.2) at 150 C. and 1 atm (FIG. 6). In this case, regeneration was also conducted at 500 C. under a flow of 5 mol % oxygen in nitrogen.

Example 2

Synthesis and H2S Capacity of CuZnO-Silica Gel Adsorbent Material

(41) CuZnO-silica gel was synthesized using the same procedure described in Example 1 with 0.75 ml of copper-zinc nitrate (4 M) solution.

(42) The CuZnO-silica gel adsorbent material was tested using the adsorption/regeneration procedure detailed in Example 1. As shown in FIG. 7, the CuZnO-silica gel adsorbent shows a high level of regenerability.

Example 3

Synthesis and H2S Capacity of CuZnOAl2O3 Adsorbent Material

(43) CuO/ZnO/Al.sub.2O.sub.3 precursors, zinc acetate (1.25 M), copper nitrate (1.25 M) and aluminum nitrate (1.25 M) (molar ratio Cu:Zn:Al=6:3:1), were co-precipitated by adding Na.sub.2CO.sub.3 solution (1.25 M) at a constant pH=7 in distilled water at 70 C. The resulting solution was aged for 1 h at 80 C. The precipitate was extensively washed with distilled water to afford pH=7 and was dried at 120 C. for 12 h. The powder was calcined at 550 C. for 5 h under flowing air. FIG. 8 shows the XRD pattern of CuZnOAl.sub.2O.sub.3.

(44) Exposing the CuZnOAl.sub.2O.sub.3 adsorbent to a flow of H.sub.2S (10.3 ppmv in He) at 150 C. and 1 atm in 14 adsorption/regeneration cycles revealed a loss in adsorption capacity for the first two cycles, thereafter the capacity stabilized at a high adsorption capacity of 98 mgS/g.sub.sorbent (FIG. 9). For this adsorbent some SO.sub.2 elution was seen for a short time right before H.sub.2S breakthrough.

(45) The invention has been described in considerable detail with reference to its preferred embodiments. However, it will be understood that numerous variations and modifications can be made without departure from the spirit and scope of the invention as set forth in the foregoing detailed disclosure and defined in the appended claims. All U.S. patents and published applications and other publications cited herein are hereby incorporated by reference in their entirety.

REFERENCES

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