LOW TEMPERATURE SORBENTS FOR REMOVAL OF SULFUR COMPOUNDS FROM FLUID FEED STREAMS

A sorbent material is provided comprising a material reactive with sulfur, a binder unreactive with sulfur and an inert material, wherein the sorbent absorbs the sulfur at temperatures between 30 and 200° C. Sulfur absorption capacity as high as 22 weight percent has been observed with these materials.

DETAILED DESCRIPTION The present invention discloses compounds exhibiting high sulfur absorbing capacity for temperatures in the range of 30° C. to 200° C. The invented materials have shown very high sulfur capacity (15-20 weight percent) when contacted with sulfur-containing streams having sulfur ppm concentrations as high as 1.2 percent (12,000 parts per million). Specifically, the invented sorbents are capable of adsorbing sulfur compounds from a gaseous feed of about 5 to 22 weight percent based on the weight of the sorbent in the above-stated temperature range. The materials incorporated in the invented mixture are readily available and the method for preparing the pellets is very simple, therefore leading to cost-effective production. Also, the materials incorporated into the sorbent mixture are not hazardous and will not cause disposal problems. Since the sulfur capacity of the sorbent is very high, the amount of sorbent required in desulfurization processes is low; therefore the size of the reactor bed can be minimized. As noted supra, the invented sorbents can be utilized in fluidized/transport bed reactors or fixed bed reactors. The invented sorbent can be utilized in a myriad of forms. For the sake of simplicity, pellets were formed from the invented sorbent and utilized to provide the data contained herein. Chemically- and physically-stable sorbent pellets were utilized in sulfur removal processes in the temperature range of 30° C. to 200° C. Generally, the mixture comprises a material reactive with hydrogen sulfide, a diluent/support, and a binder unreactive with hydrogen sulfide. Component ranges of the invented sorbent are as follows: 1 Reactive material: 30 to 70 percent by weight; Inert diluent: 20 to 60 percent by weight; and Binder: 2 to 45 percent by weight. When sorbents are prepared utilizing impregnation of inert supports, reactive material concentration may vary from 5 to 60 wt %. Preparation Detail The sorbent material is being prepared by blending the reactive material (an exemplary material being copper hydroxide) with inert materials such as calcium sulfate or titanium dioxide, and a binder such as bentonite. The said mixture is mixed with water to produce a slurry and then either extruded or extruded/marumerized to make pellets with the desired shape. These materials can be spray dried or granulated to prepare sorbents suitable for fluidized bed/transport reactor applications. The sorbent pellets should be calcined to be converted to a usable form. The sorbents can also be prepared by impregnating inert materials with the said reactive materials. Other sorbent preparation methods, well known in the art, can be utilized in the preparation of sorbents with the reactive materials described in the patent. Pellets made up of the above components may be prepared by solid-state mixing and adding a sufficient amount of water to cause the pellets to agglomerate or adhere together. Mixer-pelletizer or compressing equipment and other methods of agglomeration known in the prior art may be used for this purpose. The agglomerated pellets are dried and calcined at an elevated temperature to convert them to durable form. Drying the pellets occurs in an oven at a temperature over 100° C. (212° F.) and preferably about 100° C. for approximately 7-10 hours. The dried pellets are then calcined at a temperature between 50° C. and 150° C. for less than nine (9) hours. At this calcination temperature and duration, the material reactive with hydrogen sulfide is unreactive with all other components of the mixture. The resulting pellets exhibit increased sulfur absorbing capacity in the temperature range of 30° C. to 200° C. compared to currently available commercial blends. The crush strength of the fresh pellets are in the range 3-4 lb per pellet and increases to 4-5 lb per pellet after sulfidation. Hydrogen Sulfide—Reactive Material Detail Reactive materials can be inorganic materials selected from the group consisting of copper hydroxide, copper (II) oxide, iron (III) hydroxide, potassium bicarbonate, rubidium hydroxide, zinc oxide, zinc oxide hydrate, lithium hydroxide, sodium peroxide, and mixtures thereof. Copper hydroxide is preferred for use over its effective temperature range of about 50° C. to 200° C. For operation between 30° C. and 50° C., rubidium hydroxide or lithium hydroxide may be employed. It should be noted that the reactive metal salts of the compounds, such as the acetates, formates, carbonates and nitrates can be used instead of the oxides inasmuch as the oxides can be derived from the salts. Generally, the reactive compound will contain a metal selected from the group consisting of copper, iron, potassium, rubidium, zinc, lithium, sodium, or combinations thereof. The reactive material reacts with the sulfur via the following reaction mechanism: MO&plus;H 2 S→MS&plus;H 2 O  Equation 2 wherein MO is a metal oxide, and MS is the salt formed with the sulfur. Inert Material Detail Inert material utilized in the invented composition can be homogenous in structure, or comprise a plurality of various grain sizes. In a preferred composition, the inert material is comprised of a first diluent portion and a second portion. The first inert portion (i.e. diluent) provides stability to the composition inasmuch as it does not enter into the reaction with hydrogen sulfide or otherwise alter during the reaction period. The diluent inert material may be selected from a group consisting of titanium oxide, titanium dioxide, calcium sulfate, calcium phosphate, calcium silicate, magnesium sulfate, zinc silicate, zinc aluminate, and alumino silicates. It is used at a concentration of 0 to 40 weight percent of the pellets and preferably 10 to 30 percent. In preparing pellets containing the diluent inert material, temperatures high enough to cause a reaction between this material and the reactive oxide are to be avoided to prevent loss of reactivity. Calcium sulfate and titania are preferred material for this component. The second portion of the inert material contains large particles so as to obtain necessary porosity in the pellets. This compares with the reactive component portion of the sorbent which have relatively smaller particle sizes for maximum reactivity, strength, and optimum formation of voids around the larger inert particles. Particle sizes of the second portion of the inert material may be varied, depending on the desired pellet sizes for different types of reactor systems. For fixed/moving bed reactors, spherical or cylindrical pellets over 1 millimeter (mm) in size, and typically 2 to 5 mm, are used. For pellets of this size, particle sizes of the second portion of the inert component with large particle size may be over 50 microns and preferably 75 to 700 microns (25 to 200 mesh). Fluidized bed/transport reactors employ pellets under 500 microns, and the second portion of the inert component with large particle size for this pellet size could be sized under 150 microns, preferably 0.5 to 5 microns. The second portion inert material containing larger particles for use in the pellets may be selected from the group consisting of silica gel, silica, alumina, alumina gel, titania gel, calcium sulfate, zinc silicate, zinc aluminate, and sand. Silica gel or calcium sulfate are preferred. As noted supra, the second portion of the inert material may incorporate material with varying particle sizes, but at least two (2) weight percent of the particles should be made up of particles approximately twice as large as the reactive material. Preferably between 2 and 30 weight percent of the total inert material (i.e., the first and second portions combined) should be comprised of particles twice as large as the reactive material. Up to 40 percent of the second portion of the inert material could be particles twice in size compared to the size of the particles comprising the reactive compound. The inert material may be provided in the pellets at a total concentration of 0 to 20 weight percent and more preferably at 2 to 10 weight percent. Other components of the pellets become loosely packed around the larger particles of this inert material, creating better porosity in the pellets. Upon being subjected to exposure at higher temperatures in preparation or operation, the large particle size inert material undergoes a decrease in surface area, but porosity of the pellets is increased due to creation of additional voids around the large particles. Binder Detail A binder is required in the pellets to keep them together. The binder may comprise inorganic or organic materials or a mixture thereof. For example, suitable inorganic materials include, but are not limited to, kaolinite, other alumino silicates, calcium sulfate, cement, or mixtures of these materials. Organic binders that can be used include substances selected from the group consisting of hydroxypropyl methyl cellulose, molasses, starch, polyvinyl acetate, cellulose, hydropropyl cellulose, lignin sulfonate, and mixtures thereof. Concentration of the binder in the pellets may range from 2 to 60 weight percent. It is noted that calcium sulfate is included within the listing of materials for both the first and second inert materials as well as for the binder. The binder facilitates shaping the sorbent material into a desired shape, such as pellets, spheres, rods, or other configuration to maximize sorbent contact with the sulfur laden fluid to be treated. The invention is illustrated by the following examples. TGA Data with Powdered Reactive Materials Extent of sulfur uptake by the powder was determined using a TA Instruments 951 Thermogravimetric Analyzer (TGA-2050 TA Instruments). Approximately 25-50 mg of sample was utilized for each test. Sulfur-containing gases were introduced at 90 cc.min at the desired sample temperature. Tests were conducted with sulfur gases in the presence of both reducing gases and non-reducing gases. The composition of the sulfur containing reducing gas mixture was 0.4% H 2 S, 51.9% H 2 , 22% CO 2 , 1.67% CH 4 and 24% N 2 , while the composition of the sulfur-containing non-reducing gas mixture was 1.28% H 2 S in nitrogen or argon. In the TGA experiments, weight gains of the pellet is measured after introduction of the gas. The amount of sulfur uptake by a solid material is usually calculated utilizing the weight gain. A typical TGA curve for copper (II) oxide is depicted in FIG. 1 . When secondary reactions do not occur during sulfur sorption, the weight gain in TGA is directly proportional to the sulfur uptake. However, when secondary reactions take place, the weight gain is not directly related to the weight gain and the solid is analyzed using a sulfur analyzer to determine the actual sulfur uptake after the TGA experiments. The analyzer is the SC-432DR™ model manufactured by LECO Corp. of St. Joseph, Mich. Sulfur uptake values (after exposure to H 2 S in reducing gas) determined by the TGA/LECO experiments are listed in Tables 1 and 2, below, for reducing and non-reducing gas, respectively. Compared to commercially-available materials, many of the invented sorbents showed exceptional sulfur uptake, 16-22 weight percent, in the temperature range of 50 ° C. to 200° C. Both rubidium and lithium hydroxide showed reasonable sulfur capacity, even at 30° C. These results are superior to those obtained from commercial sorbents, such as molecular sieves, carbon-containing industrial sorbents, and the commercial solvent methyl diethyl amine (MDEA). As such, all ten of the invented sorbents provide superior sulfur absorption compared to commercially-available compositions. The LECO/TGA sulfur uptake values after exposure to H 2 S in non-reducing gases (Argon or nitrogen) are listed in Table 2. 2 TABLE 1 Sulfur Loading Values Obtained from TGA/LECO Analysis with H 2 S in Reducing Gas Sulfur Uptake (Weight Percent) Compound 200° C. 150° C. 100° C. 50° C. 30° C. Copper Hydroxide 19.3 22.0 19.1 16.8 5.6 Copper II Oxide 17.6 12.1 3.3 0.3 0.2 Iron III Hydroxide 17.6 3.8 2.2 1.7 1.4 Potassium Bicarbonate 19.9 6.4 0.01 0.01 <0.01 Rubidium Hydroxide 6.1 4.7 6.6 5.1 7.5 Zinc Oxide 7.7 5.6 4.1 2.8 2.6 Zinc Oxide Hydrate 5.5 2.9 1.6 1.2 0.9 Lithium Hydroxide 3.7 0.3 0.5 8.4 6.9 Sodium Peroxide 4.9 6.2 9.2 3.3 4.2 Ferric Oxide 4.3 1.5 1.2 1.0 0.2 COMMERCIALLY- AVAILABLE SORBENTS: Activated Carbon 3.04% and 2.79% at 30° C. MDEA solvent 0.001-2.17 moles/mole (or 2.85 × 10 −4 to 0.62 wt %) at 40-65° C. Molecular Sieve 5A 0.03-0.21 weight percent at 30-200° C. The inventors found that most of the sorbents had a higher sulfur capacity in the presence of reducing gas but rubidium hydroxide seems to perform better in the presence of non-reducing gas. The powdered materials also were tested with 1 percent carbonyl sulfide in nitrogen at both 50 and 150° C. Copper hydroxide had a sulfur uptake of 5.5 weight percent and lithium hydroxide had a sulfur uptake of 1.7 weight percent at 150° C. but showed a lower sulfur uptake at 50° C. (copper hydroxide—0.9 wt % and lithium hydroxide at <0.01 wt %). This indicates that these two compounds are suitable for absorption of carbonyl sulfide at 150° C. 3 TABLE 2 Measured Sulfur-Uptake Values After Exposure to H 2 S in Non-Reducing Gas. Total Sulfur Uptake (Weight %) Compound 200° C. 150° C. 100° C. 50° C. 30° C. Copper Hydroxide 16.8 15.4 14.6 10.3 5.1 Rubidium Hydroxide 13.4 13.6 — 5.0 Iron Hydroxide 8.4 — 1.8 — — Copper Hydroxide 8.7 8.3 — — — When these materials were tested with tetrahydro thiophene (180 ppm) in nitrogen at both 50° C. and 150° C., the sulfur uptake values were very low with lithium hydroxide showing the highest absorption of 0.85 wt % at 50° C. Lithium hydroxide also had a sulfur uptake of 0.67 wt % and 0.79 wt % at 50° C. and 150° C. respectively when it was exposed to dimethyl sulfide (1500 ppm) in nitrogen. Copper hydroxide had a sulfur uptake of 0.22 and 0.60 wt % at 50° C. and 150° C. respectively when it was exposed to dimethyl sulfide. Test Results with Pelletized Sorbent A pelletized sorbent structure was constructed with the following general formulation: copper hydroxide present at between 60 and 65 weight percent; an inert material present at a weight percent of the material of approximately 7 to 12 percent; a binder material present at approximately 8 to 12 weight percent of the material; and a diluent material present at approximately 15 to 25 weight percent of the material. Specific sorbent pellets were prepared using the following composition: 4 Copper Hydroxide 550 grams Silica Gel 37.5 grams (35-60 mesh) Silica Gel 37.5 grams (100-200 mesh) Bentonite 90 grams Titanium dioxide 170 grams The powders were mixed with a sufficient amount of water, extruded and marumerized (spherical) to obtain pellets having an average diameter of 3 mm. The pellets were calcined at 100° C. for eight hours. These sorbent pellets were tested in the TGA at 150° C. with hydrogen sulfide in reducing gas. The measured (LECO) sulfur uptake of the solid was 15 weight percent. These results indicate that the sulfur capacity for the sorbent pellets was superior to typical sorbents. Typical sorbents have sulfur capacity less than 3 weight percent, as shown in Table 1. The sorbent pellets were also tested in an atmospheric fixed bed bench scale reactor. The reactor bed had a 6 inch bed height and 2 inch diameter. The inlet hydrogen sulfide concentration was 2000 ppm in nitrogen. The gas was introduced to the reactor at a space velocity of 1000 hr −1 . The temperature of the reactor bed was maintained at 150° C. The outlet hydrogen sulfide concentration measured as a function of time is shown in FIG. 2 . The outlet hydrogen sulfide concentration was near zero ppm during the first 40 hours of testing. At approximately 45 hours after testing began, sulfide concentration of the outlet gas begins to increase, thereby indicating saturation of the sorbent. This indicates that the sorbent has a very high efficiency and is capable of reducing the sulfur level from 2000 ppm to near zero ppm. While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims. For example, the sorbent can comprise a compound reactive to sulfur, wherein the compound is impregnated onto, into or otherwise reversibly adhered to inert porous substrates to form reactant sorbents. These porous substrates can be large granular materials selected from the group consisting of titania, silica, alumina, alumino silicate, zirconia, zeolites, carbon, or combinations thereof. These porous substrates can range in size from 100 microns to 3-4 millimeters.