Sulfuric minimum operating temperature

In the test method, a weighed test portion is mixed with a promoting agent (that aids in the quantitative release of all sulfur present in the test portion as sulfur dioxide) and ignited in a tube furnace at a minimum operating temperature of 1350°C (2460°F) in a stream of oxygen. Some promoters may contain sulfur, and the sulfur content should be determined by analyzing the promoter as a sample and appropriate corrections made based on the mass of the promoter and its sulfur content. 

Ammonium sulfates deposit and accumulate onto the catalyst, if the temperature is not high enough, leading to deactivation of the catalyst. The minimum working temperature (ie, the salt formation) depends on the SO2 and NH3 concentrations typically a catalyst temperature higher than 300°C is required for stable operation (9). The deposition of ammonium salts on the catalyst surface can be reversed upon heating. Besides ammonium sulfates and sulfuric acid deposit and accumulate onto the cold process units downstream of the catalytic reactor, primarily onto the air pre heater (APH). In spite of the fact that slip NH3 and SO3 are present both at ppm levels, still this causes severe corrosion and pressure drop problems due to the huge flow rates of gas treated, in the order of several hundreds of thousands Nm /h. 

All air preheater heat exchangers are subject to corrosive attack caused by the condensation of sulfur trioxide. Refineries must burn fuel gas with less than 160 ppm of sulfur. Regardless of the calculated dew point of the sulfur trioxide, operating experience has shown that a minimum temperature of 300 F-350°F in the outlet flue gas is required to minimize corrosion and fouling of the heat exchanger. Uneven cooling of the flue gas results in the need to keep the flue gas 50°F-100°F hotter than its calculated dew point. 

Up to the 1960s sulfur trioxide was recovered by a single absorption stage at the outlet of the fourth bed. A typical plant design limited the catalyst volume used in the first bed by the minimum inlet temperature of up to 420°C, at which the catalyst was active, and the approach to the equilibrium conversion of sulfur dioxide to sulfur trioxide at about 600°C. By coincidence, the maximum reasonable operating temperature to avoid deactivation in the first bed of catalyst was about 600°-650°C. Catalyst volumes in the final three beds were also designed to maximize conversion within the same limits of inlet temperature and equilibrium. This made it difficult to achieve more than 98.5% conversion consistently in four beds, even with large volumes of catalyst, as shown in Table 2.8B. 

Relatively low temperatures are required in alkylation reactors using sulfuric acid catalyst. They are necessary in order to slow down polymerization reactions and reduce the formation of undesirable acid soluble and hydrocarbon soluble by-products. Typically, most commercial units operate with reaction temperatures in the range of 35 F. to 65 F. Design temperatures are usually set at 50 F. For minimum acid make-up the reactor section should be operated as cold as possible. This means operating... 

Temperature and Pressure Specification. Since equilibrium conversion of sulfur dioxide to sulfur vapor increases with decreasing temperature, the reduction process was designed so that the primary reduction stage operates at the minimum temperature consistent with reaction kinetics which avoids sulfur condensation on the catalyst. Taking these factors into account, the primary catalytic reactor is operated at 350 °C (623°K). 

In certain high-temperature caustic applications where sulfur is present, alloy 600 is substituted for alloy 201 because of its improved resistance. Inconel, is however, subject to stress corrosion cracking in high-temperature, high-concentration alkalies. For this reason, the alloy should be stress-relieved prior to use and the operating stresses should be kept to a minimum. Alloy 600 is almost entirely resistant to attack by solutions of ammonia over the complete range of temperatures and concentrations. 

All air preheaters are subject to corrosive attack caused by condensation of sulfur trioxide. At a 150-ppm sulfur in the fuel gas, operating experience shows that a minimum temperature of 350 to 400°F minimizes corrosive attack. Uneven coohng in the preheater results in the need to keep the flue-gas outlet temperature 50 to 100°F above the calculated SO dew point. The flue-gas outlet temperature can be... 

The two principal problems of this process are oxidation of sulfur dioxide to form ammonium sulfate and loss of ammonia by vaporization. The oxidation problem can be alleviated by acidifying a portion of the circulating absorbent to release sulfur dioxide and produce anunonium sulfate solution or by carefully controlling the ammonium sulfate concentration in the circulating stream so that the required amount can be removed by a crystallization step. At the Trail plant, the first operation was used because ammonia acidification units were available elsewhere in the plant. Control of the ammonia-vapor-loss problem requires maintenance of minimum temperatures in the absorbers and careful adjustment of solution concentrations. 

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