Patent Application
20080041227
The FIGURE represents a schematic block flow diagram of the process of the present invention.
The gas feed stream is first treated in a first adsorbent bed having a first section to remove water from the gas feed stream, such as a Na A zeolite. In the second section of the first adsorbent bed, preferred adsorbents are those which comprise constituents chemically reactive with mercury or mercury compounds. Various cationic forms of several zeolite species, including both naturally occurring and synthesized compositions, have been reported by Barrer et al. [J. CHEM. Soc. (1967) pp. 19-25] to exhibit appreciable capacities for mercury adsorption due to the chemisorption of metallic mercury at the cation sites. Some of these zeolitic adsorbents reversibly adsorb mercury and others exhibit less than full, but nevertheless significant, reversibility. An especially effective adsorbent for use in the present process is one of the zeolite-based compositions containing cationic or finely dispersed elemental forms of silver, gold, platinum or palladium. A particularly preferred adsorbent of this type is disclosed in U.S. Pat. No. 4,874,525 (Markovs) in which the silver is concentrated on the outermost portions of the zeolite crystallites. This adsorbent, as well as the other zeolite-based adsorbents containing ionic or elemental gold, platinum, or palladium, is capable of selectively adsorbing and sequestering organic mercury compounds as well as elemental mercury. Zeolite A containing elemental gold is disclosed as an adsorbent for mercury in U.S. Pat. No. 4,892,567 (Yan). The specific mention of these materials is not intended to be limiting, the composition actually selected being a matter deemed most advantageous by the practitioner give the particular circumstances to which the process in applied.
The temperature and pressure conditions for the filtration and the adsorption purification steps are not critical and depend to some degree upon the particular feedstock being purified and whether the adsorption step is to be carried out in the liquid or in the vapor phase. Temperatures typically range from about 16° to 60° C. in the beds during the adsorption-purification step. If the adsorption bed is to be regenerated the purge medium is heated to at least 100° C., and preferably at least 200° C., higher than the temperature of the feedstock being purified. Pressure conditions can range from about 140 kPa to about 17.5 Mpa (20 to 2500 psia) and are generally not critical, except during liquid phase operation where it is necessary to maintain sufficient pressure at the operating temperature to avoid vaporization of the feedstock.
In the present invention, it has been found that the in situ sulfidation of a copper oxide containing adsorbent provides very favorable results. The copper oxide adsorbent is an agglomeration which is preferably produced by using a transition-phase alumina; an oxysalt of a transition metal; an alkali metal compound (AM) and active water (AW).
The transition alumina usually consists of a mixture of poorly crystalline alumina phases such as “rho”, “chi” and “pseudo gamma” which are capable of quick rehydration and can retain substantial amounts of water in a reactive form. An aluminum hydroxide (Al(OH)3), such as Gibbsite, is the typical source for preparation of transition-phase alumina. The typical industrial process for production of transition-phase alumina includes milling Gibbsite to a particle size between 1-20 microns followed by flash calcination for a low contact time as described in U.S. Pat. No. 2,915,365. Amorphous aluminum hydroxide and other crystalline hydroxides, e.g. Bayerite and Nordstrandite or monoxides-hydroxides AlOOH such as Boehmite and Diaspore can also be used as a source of transition-phase alumina. In this invention we are using transition-phase alumina produced in the UOP plant in Baton Rouge, La. The BET surface area of this material is about 300 m2/g and the average pore diameter is about 30 Angstroms as determined by nitrogen adsorption.
A solid oxysalt of a transitional metal is used as a component of the composite. Oxysalt, by definition, refers to any salt of an oxyacid. Sometimes this definition is broadened to “a salt containing oxygen as well as a given anion”. FeOCl, for example, is regarded as an oxysalt according this definition. For the purpose of this work, we use basic copper carbonate (referred to as “BCC”) with a formula of Cu(OH)2CuCO3. This is a synthetic form of the mineral malachite, produced by Phibro-Tech, Ridgefield Park, N.J. The particle size of the BCC particles is approximately in the range of that of the transition alumina—1-20 microns. Another useful oxysalt would be Azurite with a formula of Cu3(CO3)2(OH)2. Generally, oxysalts of Cu, Ni, Fe, Mn, Co, Zn or mixture of elements can be successfully used
An alkali metal compound is another component of the composite or agglomerate. This compound can be a part of the transition alumina or added separately in the process of agglomerate preparation. Typically transition alumina contains about 0.3 mass-% sodium calculated as the oxide. Addition of NaOH in the agglomeration process is used in order to boost the Na2O content of the final composite to 0.6-0.7 mass-%. Thus, the pH of the liquid added in the course of the agglomeration process is between 13.1-13.7.
Finally, water is also a component used in making the reactive composite. The process of preparation of the reactive composites is a series of chemical reactions in which water plays a very important role. Typically, the amount of water added during the agglomeration process is about 50% of all other ingredients. In the course of the curing process, which can be performed at ambient temperature for at least 12 hours or at a slightly elevated temperature from 60° to 70° C., water participates in different processes which result in an attachment of water molecules to the other composite ingredients.
Various sulfur species are removed, including hydrogen sulfide, ethyl sulfide, methyl mercaptan, ethyl mercaptan, and other sulfur compounds. Carbonyl sulfide is a common contaminant that needs to be removed. The thermal treatment, which follows the curing step, leaves enough water in the material in order to carry out COS removal until the complete exhaustion of the scavenging element, which is the transition metal in this case. The final composite should contain excess water, beyond the water from the carbonate's hydroxyl groups, in order to convert all the Cu available to CuS through a reaction with COS.
Thus, the first step is preparation of a “hydrated” active component as described in the following equation, where “a”, “b” and “c” refer to gram moles. The “c” in the equation is at least equal to “a” and not higher than 10 times “a”.
(Cu(OH)2CuCO3)a.(Al2O3)b+cH2O=(Cu(OH)2CuCO3)a(Al2O3)b(H2O)c
The COS reacts then with the composite as shown below in this reaction:
(Cu(OH)2CuCO3)a.(Al2O3)b.(H2O)c+2aCOS=2aCuS+bAl2O3+3aCO2+(c+a)H2O
The alkali element (not shown for simplicity in the equations) provides for a higher rate of COS hydrolysis which is catalyzed by the alumina component. Since the alumina component plays not only the role of a COS hydrolysis catalyst, but is also the bearer of most of the reactive water, the ratio a/b is from 0.05 to about 1.2. The preferred ratio is in the 0.3-0.6 range. The alkali metal expressed as an oxide is usually not more than 5% of the mole fraction of the aluminum oxide—“b”. Finally the excess water is at least 15% of the mole fraction of the aluminum oxide—“b”
It should be noted that the ratios listed above are only an example for oxysalts similar to the basic copper carbonate. Other salts would require different ratios depending upon various factors including the content and valence of the transition element, the sulfur compound formed upon reaction with H2S and the hydroxyl content of the initial oxysalt.
The azurite Cu3(OH)2(CO3), for example, would require 2 moles of additional water available in order for the reaction of the Cu compound with COS to go to completion.
It is believed that agglomeration in a rotating pan followed by reactive curing and custom activation, either as a part of adsorbent manufacture or just before its use is a preferred way to practice the invention. The following example illustrates the production method for the adsorbent.
A four feet rotating pan device was used to continuously form beads by simultaneously adding transition alumina and basic copper carbonate (BCC) powders while spraying the powders with water. The pH of the water was adjusted to pH 13.5 by adding a NaOH solution. The transition alumina (TA) powder was produced by UOP LLC in Baton Rouge, La. The basic copper carbonate was obtained as “dense” powder from Phibro-Tech (Ridgefield Park, N.J.). The mass ratio of BCC: TA was 45:55, which corresponds to a mole ratio “a/b” of about 0.38. The water feeding rate was adjusted to provide for sufficient agglomeration and maximize the content of 8×14 mesh size fraction. The water feeding rate was approximately equal to the feeding rate of the BCC powder. The “green” agglomerates were collected after discharging from the rotating pan and subjected to “drum” curing at ambient temperature.
The product from the Example is then used to remove sulfur compounds, such as H2S, from a hydrocarbon stream. In removing the sulfur compounds, a large amount of CuS is formed in the adsorbent bed. We have found that accommodating large amount of the active component—CuS while maintaining high total surface area has a positive effect on the Hg removal capability of the final material.
A McBain-Baker adsorption apparatus was used to determine the H2S loading on different adsorbents. The following table shows the loading data at 5 torr H2S and 22° C. on an adsorbent made in accordance with the Example together with analytical data for S content as determined on the spent samples by the combustion method.
One can see from the data that there is a good correlation between the values obtained by the mass gain as measured in the McBain apparatus and the S loading. All samples achieve close to the theoretical limits of S pick-up determined by the following sulfidation reaction:
CuO+H2S=CuS+H2O
The data in the above table suggest that the samples can be easily sulfided at ambient conditions even at low partial pressure of H2S and static atmosphere. X-ray analysis of a spent sample confirmed that the CuS is the only copper containing crystalline phase present in the sample.
In conclusion, we have found through pilot plant testing conditions at which the adsorbent of the Example could be sulfided under the least favored conditions such as large excess of hydrogen in the gas mix.
A comparison between the present invention in column 1 and a current commercial product in column 2.
The material of the present invention contained more than twice the amount of sulfur, which may be attributed to a difference in the support material. The material of the Example is based on a transitional alumina support; while the commercial material contains gamma—theta type alumina as a support material. This explains the relatively low BET surface area of the commercial material.
The present invention provides a reactive copper component that converts easily to CuS upon sulfidation at mild conditions. Thus, a powerful mercury guard can be obtained by an in situ exposure of the adsorbent to sulfur contained in a hydrocarbon gas stream simultaneous to its use to remove mercury. The present invention removes at least 90% of the mercury present in a hydrocarbon gas stream, preferably at least 95% of the mercury and most preferably at least 99% of the mercury. Typically the hydrocarbon gas stream comprises at least 2.0 μg/nm3 of elemental mercury.
The FIGURE shows a simplified flow scheme. A gas feed stream, such as natural gas comes is shown as feed 1 that travels through adsorbent bed 2 containing an adsorbent for removal of at least water and mercury from the natural gas. A product stream that has been dried and purified of the mercury then leaves the adsorbent bed as purified feed 3. Normally in operation, there would be at least two adsorbent beds so that when a bed becomes saturated with impurities, it can be taken off line and regenerated leaving at least one adsorbent bed to continue removing impurities from the gas stream. In the FIGURE is shown an adsorbent bed 6 that is in regeneration mode, having a regeneration gas stream 4 that is first heated as shown by heat exchanger 5 before passing through adsorbent bed 6 to remove the water and mercury by using the heated regeneration gas. In some instances, the regeneration gas consists of a portion of product gas 3. Then the regeneration gas is sent through cooler 7 and then condenser 8 for removal of condensed water 10 and mercury 9. The cooled regeneration gas still contains an unacceptably high level of mercury and is sent to an adsorbent bed that contains a metal oxide adsorbent on an alumina support, preferably a copper oxide adsorbent on the alumina support. The regeneration gas further contains some sulfur compounds that react with the metal oxide to provide an effective adsorbent for removal of mercury. Spent regeneration gas 13 is then shown leaving adsorbent bed 12.
