Sulfur oxides reformate

Emissions from catalytic reforming (Figure 4.14) include fugitive emissions of volatile constituents in the feed and emissions from process heaters and boilers. As with all process heaters in the refinery, combustion of fossil fuels produces emissions of sulfur oxides, nitrogen oxides, carbon monoxide, particulate matter, and volatile hydrocarbons. 

The reformer off-gas is directed to a hypochlorite caustic scrubber. A continuous emission monitor (CEM) is used to monitor scrubber off-gas. The composition of this gas is expected to be primarily carbon dioxide (CO2) and oxygen (O2) with small amounts of water, nitrogen oxides (NOJ, sulfur oxides (SO, ), hydrogen (H2), and carbon monoxide (CO). 

Sulfur dioxide is a primary pollutant leading to the atmospheric corrosion of zinc. It controls the corrosion rate when the relative humidity is in the area of 70% or above. Sulfur oxides and other air pollutants are deposited on zinc surfaces either by dry or wet deposition. Regardless of the method of deposition, the sulfur dioxide deposited on the zinc surface forms sulfur-ous or other strong acids, which react with the protective zinc oxide, hydroxide, or basic carbonate film to form zinc sulfate. The film of protective corrosion products is destroyed by the acids, which reforms from the underlying metal, causing continuous corrosion by an amount equivalent to the film dissolved, hence to the amount of sulfur dioxide absorbed. Corrosion rates increase even further when the relative humidity exceeds 85%. 

Common impurities include those formed in reformate fuel streams (carbon oxides (COx), methane (CH4), hydrogen sulfide (H2S), and ammonia (NH3)), pollutants found in air (sulfur oxides (SOJ, nitrogen oxides (NOJ, ammonia (NH3), and organic compounds (propane, benzene, toluene)), catalyst fines carryover, rust from piping, salts, and dust. Impurities may adsorb onto the Pt surface (CO, H2S, SO, and CP), carbon support (H2S and SOJ, or gas diffusion layers (salts, dust, and organic compounds), and adsorb into the ionomer (silica, cations (M+), NH4" ), or simply plug up the flow passages. 

The sulfur oxides that are formed are directly related to the amount of sulfur found in the fuel. These emissions are typically low when PSA offgas is burned with natural gas. The PSA offgas is sulfiir-free due to feedstock pretreatment to protect catalyst beds within the hydrogen plant. Natural gas also typically contains very low levels of sulfur. However, if refinery fuel gases are used as a makeup fuel to the reformer, then the sulfur emissions can increase dramatically as these streams often eontain large amounts of sulfur. 

Sulfur oxides and other corrosive species react with zinc coating in two ways dry deposition and wet deposition. Sulfur dioxide can deposit on a dry surface of galvanized steel panels until a monolayer of SO2 is formed. In either case, the sulfur dioxide that deposits on the surface of the zinc forms a sulfurous or other strong acid that reacts with the film of zinc oxide, hydroxide, or basic carbonate to form zinc sulfate. The conversion of sulfur dioxide to sulfur-based adds maybe catalyzed by nitrogen compoimds in the air (NO t compoimds). This factor may affect corrosion rates in practice. The acids partially destroy the film of corrosion products that will then reform from the underl)dng metal, thereby causing continuous corrosion by an amount equivalent to the film dissolved, hence the amount of SO2 absorbed. 

Impurities can be removed by formation of a gaseous compound, as in the fire-refining of copper (qv). Sulfur is removed from the molten metal by oxidation with air and evolution of sulfur dioxide. Oxygen is then removed by reduction with C, CO, in the form of natural gas, reformed... 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

Steam Reforming Processes. In the steam reforming process, light hydrocarbon feedstocks (qv), such as natural gas, Hquefied petroleum gas, and naphtha, or in some cases heavier distillate oils are purified of sulfur compounds (see Sulfurremoval and recovery). These then react with steam in the presence of a nickel-containing catalyst to produce a mixture of hydrogen, methane, and carbon oxides. Essentially total decomposition of compounds containing more than one carbon atom per molecule is obtained (see Ammonia Hydrogen Petroleum). 

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

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. 

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