Sulfur removal with zinc oxide

G. A. White Final removal of sulfur, of course, is effected conventionally by zinc oxide which will work at atmospheric pressures as well as at any other pressure. The bulk sulfur removal can work equally well at atmospheric pressure in our system. That is, in a commercial plant we have removed sulfur to less than 1 ppm through the combination of amine and caustic. So, if we want to follow that with zinc oxide, obviously we can meet the requirements for any methanation system. 

The zinc oxide process was developed by Johnstone and Singh (1940) at the University of Illinois. Although development was utility sponsored, the process has not been used commercially. However, a considerable amount of pilot-plant work was conducted, and features of the process design were worked out in considerable detail. The process is illustrated in Figure 7-17. The flue gas is contacted with a solution of sodium sulfite and bisulfite and sulfur dioxide is absorbed, thus causing an increase in bisulfite content. The solution is next passed into a clarifier, in which particulate matter removed from the gas stream is separated, and finally into a mixer in which it is treated with zinc oxide. At this point, the original ratio of sulfite to bisulfite is restored, and zinc sulfite is precipitated in accordance with the following reactions ... 

Zinc and Zinc Alloys. Zinc metal is highly reactive in acid solutions such as sulfuric, hydrochloric, and nitric dissolving rapidly at acid concentrations normally used to pickle steel and aluminum. Dilute (1—4%) solutions of these acids can be used with caution to remove zinc oxides. 

Like M( F(7s, S()F(7s can integrate fuel reforming within the fuel cell stack, A prereformer converts a substantial amount of the natural gas using waste heat from the fuel cell, (iornpoiinds containing sulfur (e,g, thiophene, which is cornrnonlv added to natural gas as an odorant) must be removed before the reformer. Typically, a hvdrodesiilfii-rizer combined with a zinc oxide absorber is used. 

N2. Sulfur containing odorants (mercaptans, disulfides, or commercial odorants) are added for leak detection. Because neither fuel cells nor commercial reformer catalysts are sulfur tolerant, the sulfur must be removed. This is usually accomplished with a zinc oxide sulfur polisher and the possible use of a hydrodesulfurizer, if required. The zinc oxide polisher is able to remove the mercaptans and disulfides. However, some commercial odorants, such as Pennwalf s Pennodorant 1013 or 1063, contain THT (tetrahydrothiophene), more commonly known as thiophane, and require the addition of a hydrodesulfurizer before the zinc oxide catalyst bed. 

There are high temperature and low temperature methods to remove sulfur from a fuel reformate stream. Low temperature cleanup, such as hydrodesulfurizing (limited to fuels with boiling end points below 205°C), is less difficult and lower in cost so should be used where possible, certainly with low temperature cells. Sulfur species in the fuel are converted to H2S, if necessary, then the H2S is trapped on zinc oxide. As previously mentioned, a minimum bed volume of the zinc oxide reactor is achieved at temperatures of 350 to 400°C. Simple... 

Zinc also may be produced by electrolysis of zinc sulfate solution. The zinc oxide in the roasted concentrate is leached with sulfuric acid. The oxide is converted to soluble zinc sulfate. Impurity metals, such as iron, copper, cadmium, arsenic, tin, and cobalt are removed by precipitation, floe formation, and other methods. The purified zinc sulfate solution is electrolyzed using aluminum cathodes and lead anodes. Zinc is deposited on the cathode. 

Another difference between Co and Fe is their sensitivity towards impurities in the gas feed, such as H2S. In this respect, Fe-based catalysts have been shown to be more sulfur-resistance than their Co-based counterparts. This is also the reason why for Co F-T catalysts it is recommended to use a sulphur-free gas feed. For this purpose, a zinc oxide bed is included prior to the fixed bed reactor in the Shell plant in Malaysia to guarantee effective sulphur removal. Co and Fe F-T catalysts also differ in their stability. For instance, Co-based F-T systems are known to be more resistant towards oxidation and more stable against deactivation by water, an important by-product of the FTS reaction (reaction (1)). Nevertheless, the oxidation of cobalt with the product water has been postulated to be a major cause for deactivation of supported cobalt catalysts. Although, the oxidation of bulk metallic cobalt is (under realistic F-T conditions) not feasible, small cobalt nanoparticles could be prone to such reoxidation processes. 

The absorption of sulfur dioxide in alkaline (even weakly alkaline) aqueous solutions affords sulfites, bisulfites, and metabisulfites. The chemistry of the interaction of sulfur dioxide with alkaline substances, either in solution, slurry, or solid form, is also of great technological importance in connection with air pollution control and sulfur recovery (25,227,235—241). Even weak bases such as zinc oxide absorb sulfur dioxide. A slurry of zinc oxide in a smelter can be used to remove sulfur dioxide and the resultant product can be recycled to the roaster (242). 

Natural gas feedstock enters the fuel processing subsystem at about 63 psig (4,5 atm). The fuel is first processed in hydrodesulfurizing unit (HDS) and zinc-oxide (ZnO) beds lo remove any sulfur compounds. The desulfurized fuel is mixed with process steam and preheated to about 850°F (454cC) before entering the reformer, which consists of reactor tubes containing a... 

When only potassium carbonate or organic solvents are used, the effects are less important. Potassium carbonate blocks the catalyst pores, and can be removed by washing with water to restore normal performance. Methanation catalysts can be protected from poisons by installing a guard bed of zinc oxide absorbent. This will remove traces of sulfur and droplets of liquid from the carbon dioxide removal system70. 

After inactivation treatment, the catalyst is removed, and phenolic antioxydants and other stabilizers are added. Cross-linking curing is realized on unsaturated pendant groups. Peroxides are avoided because they cause chain scission and therefore systems with sulfur as cross-linker and zinc oxide, 2-mercaptobenzothiazole and tetramethylthiuram monosulfide as accelerators are used. 

In the steam-reforming process, any sulfur compounds present in the hydrocarbon feedstock have to be removed because the nickel-containing catalysts are sensitive to poisons. This is either achieved by hydrodesulfurization (see Hydrodesulfurization Hydrodenitrogenation), generally with a combination of cobalt-molybdenum and zinc oxide... 

A typical layout of the steam-reforming section of a syngas plant with hydrocarbon feedstock is illustrated in Fig. 6. The first step is purification of the feedstock to remove sulfur so as to avoid poisoning of the downstream reformer catalysts. This is typically accomplished in a two-step process. In the first step, organic sulfur compoimds are converted into hydrogen sulfide by a hydrogenation catalyst. In the second step, H2S is absorbed by zinc oxide by the following reaction ... 

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