Desulfurization performance of zinc oxide and its application in industry Zinc oxide CAS 1314-13-2 With the development of the world economy, the position of coal and oil in the world's energy sources is becoming more and more important. However, in the process of using these two resources, a large amount of sulfur-containing substances will be produced. If these substances are used as raw materials in downstream chemical fields or power plants, environmental pollution can be reduced and resources can be rationally used. However, most of the catalysts in the chemical process are sensitive to sulfur, and the sulfur must be removed. Zinc oxide becomes the active composition of desulfurization catalyst with its characteristic of fine desulfurization. 1、The physical properties and application of zinc oxide Zinc oxide is mainly used in industries such as rubber, cosmetics, medicine, catalysts, and electronic materials. Studies in the field of catalysis have found that zinc oxide has a good desulfurization effect. If zinc oxide has an appropriately high specific surface area, its desulfurization effect will be more obvious. Therefore, the so-called active zinc oxide and nano-zinc oxide with specific functions have come out one after another, making The desulfurization effect of zinc oxide is more mature in industrial applications. When the particle size of activated zinc oxide is below 1um and the density is about 5.2g/cm³, it has good dispersibility, and is better than ordinary zinc oxide in application effect, and its consumption can be reduced by 15%. Its activity does not come from surface modification, but from particle size (specific surface area) and particle morphology. Its average particle size is 1/10 of that of ordinary zinc oxide, that is 0.035-0.05um, and its specific surface area is 45-50m²/10 times that of ordinary zinc oxide. The particle shape is basically spherical or chain-like, which is the same as ordinary zinc oxide. The column shape or the needle shape is obviously different, and the microfine size is small, and the surface disorder is high, which causes the atomic valence to be unsatisfactory, so it has great activity. The technical performance indicators of active zinc oxide in the industry standard of the Ministry of Chemical Industry are shown in Table 1. project First class Qualified product ZnO-content，% - 95~98 95~98 Moisture，% - ≤0.7 ≤0.7 Water soluble content，% - ≤0.5 ≤0.7 Fineness45um Sieve residue,% - ≤0.1 ≤0.4 Specific surface area/m²-g-1 - ≥45 ≥35 In recent decades, the use of zinc oxide has been on the rise, and domestic brands are quietly replacing imported brands. According to statistics, among all the catalysts used in synthetic ammonia plants, the consumption of zinc oxide desulfurizer has risen to the third place. The localization rate was still 81.3% before 1988 and reached 100% after 1988. 2、Desulfurization mechanism and reaction behavior of zinc oxide Among all oxides, zinc oxide has the most application to remove hydrogen oxide. The reaction mechanism and reaction behavior have been recognized. Zinc oxide also has the function of partial conversion and absorption, which can partially convert COS, CS2 and other organic sulfur into hydrogen sulfide for absorption and removal. Because the generated ZnS is difficult to dissociate and has high desulfurization accuracy (desulfurization to 0.05×10-6), it has been used in the fine desulfurization process. The reaction formula is as follows: ZnO+H2S=ZnS+H2O Zinc oxide is a typical n-type semiconductor. As a catalyst, its surface electronic properties play an important role. Since hydrogen sulfide is an acid gas, when it is removed with zinc oxide, it must first undergo a surface adsorption process, so the reaction rate is related to the surface alkalinity and the surface electron concentration. The surface electrons of zinc oxide increase, and alkalinity is strengthened, which is beneficial to the adsorption of hydrogen sulfide, which is beneficial to desulfurization, and vice versa. The adsorption of gas molecules on the surface of zinc oxide is often accompanied by the donation and acceptance of electrons. Therefore, some gases seem to have nothing to do with desulfurization. In fact, their adhesion to the surface of the solid reactant changes the surface electron concentration, thereby affecting the desulfurization process. Therefore, the atmosphere effect of zinc oxide desulfurization is very important. ZnO desulfurization is a gas-solid non-catalytic reaction. One of its major features is that the reaction not only proceeds on the solid surface, but also goes deep into the interior. When the desulfurization starts, ZnS is first generated on the surface of the ZnO grains, and then the sulfide ions on the surface layer migrate to the inner layer to replace the oxygen ions inside, which is the so-called solid diffusion. The product ZnS, then the surface layer of sulfide ions migrate to the inner layer, and the internal oxygen ions are replaced, which is called solid diffusion. The molar volume of the product ZnS is 1.64 times the molar volume of ZnO. Once ZnS is generated, the unreacted ZnO core will be densely wrapped, so that the solid diffusion resistance in the particles increases sharply with the progress of the reaction, which leads to Because the reaction slows down sharply or even stagnates in a state far from equilibrium, even after a long time of reaction, it is difficult for the outermost particles to react completely. Since the diffusion of H2S in the pores of the particles is easier than the diffusion of the particles, the H2S entering the particles will slide through the pores of the particles and diffuse in the direction of the inner core when the particles on the outer edges of the particles have not fully reacted, and are the same as ZnO particles The reaction is carried out so that the reaction zone extends along the diameter of the particle to near the core of the particle. The width of the reaction zone is wider at low temperatures and becomes narrower as the temperature increases. This is because the solid diffusion resistance is more sensitive to temperature than the pore diffusion resistance. The solid diffusion needs to overcome a certain energy barrier, which is quite large. Activation energy and pore diffusion resistance are not sensitive to temperature effects. Recently, other countries have begun to study the use of zinc oxide to directly remove organic sulfur from hydrocarbons, and even thiophene sulfur can be removed. No H2S is produced in the process, that is to say, the reaction mechanism is different from the reaction of H2S and ZnO. 3、Application of zinc oxide desulfurization 3.1 Purification of raw gas 3.1.1 IGCC process Integrated coal gasification combined cycle power generation technology (IGCC) is an advanced clean coal technology. In this technical process, the gas produced by the gasifier is used to drive the gas turbine to generate electricity, and the steam generated by the flue gas waste heat drives the steam engine to generate electricity. The outlet temperature of a typical gasifier is 877℃. If the gas can be purified at this high temperature and the total sulfur of the feed gas is reduced to less than 10×10-6, the thermal efficiency of the entire IGCC system will increase by 1% to 2%. At the same time, compared with low-temperature desulfurization, the production efficiency of desulfurization unit operations is significantly improved. In the early development of IGCC desulfurizer, a single oxide was mostly used as the desulfurizer, and later iron and titanium oxides were gradually introduced. From the perspective of efficient desulfurization, zinc oxide has a more efficient desulfurization effect than other metal oxides. The reason is that it is more thermodynamically beneficial, but the vulcanization kinetics of zinc oxide is slower. Therefore, composite zinc oxide is gradually being used. Grindley et al. mixed iron and zinc oxides to produce zinc ferrite (ZnFe2O4) and found that zinc ferrite has a higher desulfurization efficiency than zinc oxide. Due to its high acid capacity, zinc ferrite reacts quickly with H2S and has high desulfurization efficiency. In a reducing atmosphere, zinc ferrite is easily decomposed into ZnO and Fe3O4. In further research, Lew et al. found that zinc oxide with titanium oxide reduces to volatile zinc slower than pure zinc oxide. After conducting oxidation/regeneration cycle experiments on different Zn-Ti oxides, it was found that the desulfurization ability of zinc titanate in continuous cycle operation was relatively stable. 3.1.2 Chemical processes such as synthetic ammonia, hydrogen production, and methanol synthesis Because too many catalysts used in the chemical process are very sensitive to sulfur, refined desulfurization is required for chemical raw material gas {total sulfur <0.1mg/mm³ (standard condition)}. Traditional wet desulfurization has been restricted to use due to the large amount of sulfur waste liquid. In the dry desulfurization developed in recent years, the desulfurizer with zinc oxide as the active composition is the main research object. The raw material gas for synthetic ammonia has a wide range of sources, including naphtha, natural gas, dry refinery gas, coal and heavy oil. Due to the use of oxidative desulfurizers, the sulfur in the feed gas (oil) can be removed to less than 0.5~0.1ug/g, thus ensuring the downstream processes (steam reforming, low-variation, methanation, methanol, low pressure combined alcohol, carbonyl synthesis Etc.) Use catalysts containing nickel, copper, iron and precious metals to avoid sulfur poisoning. There are more than 10 types of zinc oxide desulfurizers currently used in this field. Among them, the T305 type desulfurizer adopts a special composite preparation process. Under the condition of ensuring sufficient ZnO content, the introduction of additives and auxiliary agents, the product has a good pore structure, and maximizes the utilization rate of the inner surface of the ZnO and H2S reaction. Performance indicators are higher than those of foreign products, and have the best performance in domestic industrial applications. Copper-based catalysts in the production of synthetic alcohols or combined alcohols are very sensitive to sulfur. The desulfurization process is generally used in series with zinc oxide desulfurizers and other desulfurizers. For example, gas plants use molecular sieve to remove organic sulfur and zinc oxide to remove residual sulfur, and chemical fertilizer plants 852 type hydrolysis agent in series with KT310 type zinc oxide desulfurizer, etc. In fact, KT310 itself is a good catalyst. It does not need to work with other desulfurizers, and can remove hydrogen sulfide and organic sulfur at room temperature of 10~40℃, and has a large sulfur capacity. It can be said that in the production of methanol or co-alcohol, the zinc oxide desulfurizer acts as a gatekeeper. The raw materials for hydrogen production are similar to synthetic ammonia, mainly natural gas, hydrogenated dry gas, reformed dry gas, coking dry gas and catalytic cracking dry gas and other gaseous hydrocarbons and straight-run naphtha, hydrogenated light naphtha and other liquid hydrocarbons. The hydrocarbon steam reforming process uses nickel-based catalysts, which are also very sensitive to sulfur. In order to protect the catalyst, after the hydrorefining, it is refined with zinc oxide before it can enter the reformer. In recent years, the raw materials of many hydrogen production plants have been changed from natural gas and light oil to coking dry gas and catalytic dry gas, which has further increased the loading of zinc oxide. If half of the equipment is successfully retrofitted, the annual demand for zinc oxide will increase by about 340t. That is to say, only the hydrogen plant, the demand for zinc oxide desulfurizer will reach about 2000t before 2005. Zinc oxide desulfurizer generally desulfurizes at medium temperature or high temperature (200~600℃). However, it is quite difficult to achieve dry desulfurization at room temperature (30~150℃) because of its low sulfur capacity at room temperature. The British company first developed the room temperature desulfurizer P2020 and completed its industrial application in the United States in 1987. The reaction temperature was below 65°C. QTS-01's room temperature fine desulfurizer is characterized by its strong adaptability to raw gas, and its effect is better in an atmosphere containing a proper amount of water vapor. T306 and T307 were successfully developed in the late 1980s. T306 is used for fine desulfurization of butanol synthesis gas, and T307 is used for liquid phase propylene desulfurization. Due to the addition of special additives, T306 type desulfurizer has a better removal effect on simple organic sulfur at a lower operating temperature (about 180°C); the development of T307 successfully achieved room temperature liquid phase desulfurization for the first time. KT-310 and TC-22 are new products developed in the 1990s. The desulfurization effect at room temperature is better than T307. At room temperature (around 30℃), the sulfur in liquid propylene and CO2 can also be removed to below 0.05×10-6. . 3.2 Oil refining 3.2.1 Jet fuel desulfurization The Catalyst Plant of the Chemical Industry Corporation and the Research Center of the Refining and Chemical Corporation jointly developed the NCT2000 jet fuel room temperature liquid phase fine desulfurizer, the main component of which is active zinc oxide. The catalyst is used in the refined desulfurization device of the refinery. The silver flake corrosion of the exported jet fuel is 0 grade, and the surface of the desulfurizer is active; there is no powdering phenomenon of the refined desulfurizer, indicating that the agent is adequately used strength. The catalyst has a refined adsorption effect on hydrogen sulfide and elemental sulfur with a content of less than 5×10-6 under normal temperature, normal pressure, and liquid phase conditions. 3.2.2 FCC gasoline desulfurization Petroleum companies have developed a new gasoline adsorption desulfurization process called S-Zorb, which can be used for FCC gasoline total sulfur desulfurization. The sulfur content of full-cut FCC gasoline can be reduced from 800×10-6 to <25×10-6, octane number Loss <1.0. The characteristic is that under hydrogen conditions, the adsorbent can selectively adsorb various sulfur in gasoline such as mercaptans, sulfides, disulfides and various thiophene sulfides, so that they can be stored on the adsorbent , And the hydrocarbon structure in the sulfide is released back into the process stream to achieve desulfurization. Unlike the previous zinc oxide, which can not be regenerated, the adsorbent used in this technology is oxidized with hot air, and the sulfur in the adsorbent generates sulfur dioxide, which is then regenerated in hydrogen.