The present invention relates to a bound or compressed metal oxide particle for use in removing contaminants, including sulfur compounds, from fluids, and a method for making such compressed metal oxide composition. More preferably, the present invention relates to an extruded pelletized iron oxide composition, with the binder being a cellulose composition.
It is well known to use metal oxides, particularly iron oxide (FexOy) in a reactor bed to remove contaminants, typically sulfur compounds, especially hydrogen sulfide (H2S), from fluids, typically gas streams. Sulfur compounds are removed from fluids because they are known contaminants, which potentially make gas streams or other fluids unsalable. Gas that contains too much sulfur is known as sour gas. As such, in the gas industry, as well as related industries, it is considered necessary to remove sulfur compounds from fluids, including gas. Note that these fluids are typically devoid of oxygen. (It is known oxygen can increase reactivity between a metal oxide composition and contaminants.) For this reason, there is a need for products that remove sulfur compounds efficiently and cost effectively from fluids. It is further desired to have a method or composition that does not require the inclusion of activating agents, such as oxygen. Unfortunately, most commercially available iron oxide compositions (the most frequently used metal oxide material in removing sulfur) which operate at ambient conditions and are generally non-activated, hold an amount of sulfur equal to at most 20% by weight of the total iron oxide composition. More typically, iron oxide material (like that compressed in the present process) will hold on average 0.12 lbs of H2S/lb of iron oxide. (The percent held is dependent, in part, on the particular species of iron oxide used). Increased H2S absorption capacity for iron oxide compositions, above 20%, typically require the addition of a caustic or oxygen to the feed gas, which is dangerous and potentially difficult, especially at high pressures. This is problematic because approximately 80% of the total metal oxide product is unused. For this reason, frequent replacement of the metal oxide is required. Consequently, it is desired to increase the percent by weight of sulfur held by the total metal oxide product.
Sulfur removal on a level that treats up to millions of cubic feet of gas per day or on an industrial scale, typically requires the use of large reactor beds filled with the iron oxide media or product. Typically, this media is comprised of iron oxide and a carrier such as montmorillonite or wood chips. In order to eliminate frequent change-outs, that is the replacement of spent metal oxide media (media that no longer has suitable reactivity with sulfur) with new metal oxide media, large or numerous reactor beds are used. These reactor beds will either be very tall, 10 feet or higher, or multiple reactors will be lined up in succession so that a plurality of reactor beds will be used. If the reactor beds are too small or few, the metal oxide will be spent too fast. This is because when treating large volumes of gas or other fluids, the metal oxide found in the metal oxide media will be rapidly reacted. In order to have a sufficient bed life so that frequent changings of the metal or iron oxide media is not required, large amounts of metal oxide must be used. This is disadvantageous for a couple of reasons. First, the amount of sulfur held by the metal or iron oxide composition is low relative to the total weight of the product used. In order to increase efficiency, it is desired to have a product that holds a greater percentage of reacted sulfur per pound of total product. Secondly, the amount of area required to remove sulfur can increase costs. It is desired to have the option to decrease the total area required to remove H2S. In other words, it is desired to hold a greater amount of sulfur with a decreased amount of metal oxide composition.
One way to increase the amount of sulfur held in a reactor vessel is to pelletize or compress the metal oxide. The amount of sulfur held by the metal oxide composition is increased because there is more available metal oxide in the vessel. Normally, metal oxide is placed on a carrier, with the carrier comprising approximately 80% by weight of the metal oxide composition. Conversely, a pellet is typically comprised of an amount of binder equal to from about 1% to about 20% by weight of the pelletized mixture. As can be seen, the amount of metal oxide is significantly increased. The binders that have been used to form the pelletized iron oxide particles include cement, bentonite, and similar compositions, especially inorganic compositions. The pelletized particles made from these binders, however, have suffered from a problem in that it appears that the efficiencies have been lowered and that the reactivity of the metal oxides has been decreased. In particular, the amount of sulfur held is not significantly increased over the amount of sulfur held by the same species of metal oxide particle on a carrier. For this reason, prior attempts to pelletize metal oxide have been considered unsuccessful because of inadequate sulfur reactivity, in particular, holding capacity. Thus, it is necessary to find a binder that allows for sufficient binding of the metal or iron oxide particles without lowering the reactivity or efficiency with which the sulfur compounds are removed. More particularly, it is necessary to find a binder that permits the metal oxide to hold a greater amount of sulfur, in particular, H2S, without the presence of a caustic or the addition of oxygen in some form.
As stated, it has been known to pelletize metal oxides for use in removing sulfur compounds from fluids. In particular, U.S. Pat. No. 4,732,888, invented by Jha et al. discloses a zinc ferrite pellet for use in hot coal gas desulfurization. The patent discloses a composition comprised of zinc and iron oxide bound together with inorganic and organic binders, and a small amount of activator. Inorganic binders include bentonite, kaolin, and Portland Cement. The organic binders include starch, methylcellulose, and molasses. The pellets have a very specific product design because they are used in beds having temperatures of at least 650xc2x0 C. Because of the high temperatures, the organic binders dissipate leaving pellets that are fragmented and porous. Thus, the organic binders are included for the specific purpose of holding the pellets together, initially, and then dissipating so as to create greater porosity. While this design is outstanding for use in high temperature coal desulfurization processes, it does not provide for sufficient removal at ambient conditions. As implied, it has been observed that inorganic binders decrease the amount of sulfur removed by pelletized metal oxides. As a result, insufficient removal of sulfur will likely occur at ambient or near ambient conditions when inorganic binders are used to bind the pellets together. It should also be noted, that it has previously been believed that organic binders were unacceptable for forming pellets used at ambient conditions, because the organic binders generally do not provide for a pellet that has sufficient crush strength, or there is insufficient reactivity, or the use of the binders creates a pellet that is cost prohibitive.
The present invention relates to bound or compressed metal oxide particles used in the removal of contaminants, preferably sulfur compounds, from fluids and methods related thereto. The compressed metal oxide particle will be comprised of an amount of metal oxide equal to at least 80% by weight of the compressed metal oxide particle. Additionally, the compressed metal oxide particles have a crush strength equal to at least 1.0 kg and, more preferably, a crush strength equal to at least 3.5 kg. The compressed metal oxide will also retain an average amount of sulfur equal to at least 10% by weight of the compressed metal oxide particle and, more preferably, an amount of sulfur equal to at least 30% by weight of the compressed metal oxide particle. Importantly, the compressed metal oxide particle will hold a greater amount of sulfur than if the particular metal oxide species used to form the compressed metal oxide particle was used in association with a carrier. Generally, the compressed metal oxide particle will be able to hold an amount of hydrogen sulfide (H2S) equal to at least 0.27 per pound of metal oxide particle. The compressed metal oxide particle is further advantageous because it will sufficiently remove sulfur at temperatures of less than 150xc2x0 C. and, even more advantageously, at ambient conditions.
The compressed metal oxide particle will be comprised of an amount of metal oxide, preferably in powder form or having a small particle size, and a binder. The metal oxide will have a particle size ranging between about 0.1 microns and about 100 microns, which means that the metal oxide will be similar to dust, also known as fines. Any of a variety of metal oxides which are reactive with sulfur compounds may be used to form the bound metal oxide particles. Most preferably, the metal oxide will be of the formula MexOy, with Me selected from the group consisting of row 4, 5, 6, and 7 metals, with x equal to between 1 and 3, and y equal to between 1 and 4. It is more preferred if the metal oxide is an iron or zinc oxide composition, as these metal oxides have been known to readily react with sulfur compounds. In particular, iron oxide of the formula FeaOb will be preferred with a equal to between 1 and 3, and b equal to between 1 and 4. As such, iron oxide of the formula Fe3O4 is most preferred.
Any of a variety of organic binders may be used to hold the metal oxide particles together to thereby form the compressed or bound metal oxide particle. The binder selected must permit the metal oxide to be reactive with the sulfur compounds, and must also provide for a bound metal oxide particle having sufficient crush strength. Crush strength will be equal to at least 1.0 kg, as mentioned above, and is more preferably equal to at least 3.5 kg. As such, it has been determined that suitable binders, which provide for a bound metal oxide particle complying with the present invention are starch compositions and/or cellulose compositions. Substantially hydrophobic binders are preferred, with the cellulose composition being the most preferred. Additionally, lignin, bentonite, and lignosulfonate may also be used as binders. The binder can be added to the metal oxide in an amount equal to between 0.5% and 20% by weight, and more preferably in an amount equal to between 0.5% and 5% by weight
The method of the present invention involves combining the cellulose, which is the preferred binder, and the metal oxide particles and thoroughly mixing the two constituents. Once the two constituents are mixed, it is necessary to compress the composition so as to form the bound metal oxide particles. The techniques used to compress the constituents to form the bound metal oxide particles can be any of a variety of techniques or devices. Any compression device or method can be used as long as the bound metal oxide particles are suitably formed and have sufficient crush strength. It is most preferred, however, to pass the constituents through an extruder to form an extruded metal oxide composition. This has been found to produce pellets or particles which have sufficient crush strength and reactivity with contaminants, especially sulfur compounds. Additionally, once the material has been extruded, it is preferred to pelletize the material so as to form pelletized, extruded metal oxide particles. Any of a variety of extrusion devices may be used as long as the particles or pellets will have a diameter ranging between 3 mm and 20 mm, and a length ranging between 3 mm and 20 mm. More preferably, the pellets will have a diameter of approximately 3 mm to 6 mm, and a length of about 6 mm.
The present invention is advantageous for a number of reasons. In particular, the bound metal oxide particles allow for a product that can be used in a reactor bed, whereby the product reacts with a greater amount of sulfur so that a greater amount of sulfur is found in the reactor bed. This is desirable because a lesser amount of overall space can be used and fewer reactor vessel change-outs are required. The present invention is also advantageous because it demonstrates that a pelletized and extruded metal oxide particle can be formed that has sufficient reactivity with sulfur. This means that the particles are suitable for commercial use unlike many other known pelletized metal oxide compositions.