Catalyst Bed Acid Plant

Chapters 10 through 17 do separate 1 , 2 and 3 catalyst bed calculations. This chapter joins these calculations. Its objectives are to  

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Figure 29.1 shows an example of industrial SO2 oxidation for a three catalyst bed acid plant. It shows heatup paths that do not intersect the equilibrium curve as in Fig. 18.2. Slightly higher SO2 emissions from the acid plant occur because equilibrium SO2 oxidation is not reached in each catalyst bed.

Figure 29.1 Nonequilibrium SO2 oxidation in a three catalyst bed acid plant. The feed gas conditions and catalyst bed pressures are identical to those in Fig. 7.8. The heatup paths do not intersect the equilibrium curve. This results in slightly less SO2 oxidation and slightly higher SO2 emissions to the atmosphere.

Oxidation. Naphthalene may be oxidized direcdy to 1-naphthalenol (1-naphthol [90-15-3]) and 1,4-naphthoquinone, but yields are not good. Further oxidation beyond 1,4-naphthoquinone [130-15-4] results in the formation of ortho- h. h5 ic acid [88-99-3], which can be dehydrated to form phthaUc anhydride [85-44-9]. The vapor-phase reaction of naphthalene over a catalyst based on vanadium pentoxide is the commercial route used throughout the world. In the United States, the one phthaUc anhydride plant currently operating on naphthalene feedstock utilizes a fixed catalyst bed. The fiuid-bed process plants have all been shut down, and the preferred route used in the world is the fixed-bed process. 

Incieased catalyst-bed piessuie diop caused by dust fouling reduces production of acid and significantly increases energy consumption by the plant s blower. To avoid these problems, first converter-pass catalyst pellets are screened at every significant turnaround, typically every 12—24 months. 

Downstream from the 3rd bed, the gas is cooled and passed to an intermediate absorption tower, in which the S03 formed is absorbed in recirculating sulphuric acid. The cold and practically S03-free process gas is reheated to 380-440°C and returned to the converter, where the remaining SO2 is converted to S03 in a 4th catalyst bed. The rest of the S03 is subsequently recovered in a final absorption tower before the process gas, containing a small fraction of unconverted S02, is emitted through the stack. The combustion air is dried with the 98 wt% product acid in order to avoid corrosion and acid mist problems in the plant. The sulphuric acid process normally operates close to atmospheric pressure with the combustion air blower dimensioned just for compensation of the pressure drop through the plant. 

The desire to save energy calls for low pressure drop over the catalyst layers because they account for a significant part of the total pressure drop through the sulphuric acid plant. According to simple correlations such as the Ergun equation [12], the pressure drop over a catalyst bed per bed length at a given flow rate and properties of the gas only depends on the bed void fraction e and a characteristic pellet diameter... 

Adequate control of the chemistry in the front end furnace can significantly effect the lifetime and efficiency of the downstream catalyst beds in a sulfur plant. Inadequate removal of Ce+ hydrocarbons from the acid gas feed can result in catalyst fouling by polymeric materials formed under furnace conditions. Toluenes, ethylbenzenes and xylenes have been shown to be particularly troublesome in this regard. Oxygen breakthrough into the catalyst beds can also shorten the effective lifetime of the Alumina catalyst by sulfation i.e. 

The single absorption contact process for sulfuric acid is characterized by four main process steps gas drying, catalytic conversion of S02 to S03, absorption of S03, and acid cooling. The maximum S02 conversion for a single absorption plant is about 97.5-98 percent. By adding a second S03 absorber with one or two catalyst beds between absorbers, the S02 conversion can be increased to 99.5-99.8 percent or even as high as 99.9 percent with a cesium-promoted catalyst, resulting in lower S02 emis-... 

Fig. 25.11. Sankey energy flow diagram for a 1000ton/day sulfur-burning double absorption sulfuric acid plant (feed gas 10% S02). A Blower B Sulphur furnace C Waste heat boiler D Catalyst bed 1 E Steam superheater F Catalyst bed 2 G Boiler H Catalyst bed 3 J Intermediate heat exchangers K Intermediate absorber L Converter bed 4 M Economizer N Final absorber O Air dryer P Acid coolers. (Courtsey Lurgi GmbH, Frankfurt, Germany.)...

Sulfur is quite versatile it can be used as an agricultural insecticide or as a raw material for making sulfuric acid, as shown in Figure 2.12. To make sulfur, acid gas (hydrogen sulfide, sulfur dioxide, and carbon dioxide) from the various refinery amine units is collected and fed to a sulfur plant. In a typical sulfur plant, the acid gas is fed to a reaction furnace. The hydrogen sulfide is first partially burned at 2,500° F (1,370° C) and 15 psia (103 kPa) in the reaction furnace to form sulfur dioxide next, it is passed through a waste heat boiler and then passed over catalyst beds at 500°F (260° C) and 15 psia (103 kPa) in the converters. Sulfur is condensed from the effluent of successive converters and solidified in pits. 

Fig. 7.7. Photograph of catalyst bed converter, courtesy Outokumpu OYJ www.outokumpu.com Gas inlet and outlet flues are shown. Others are hidden behind. Fig. 7.6 s gas coolers are also hidden behind. Converter walls and roofs are designed to be strong enough to withstand their acid plant s main blower shutoff pressure without damage (Friedman and Friedman, 2004). Catalyst tray supports are also strong enough to withstand the downward force exerted by the descending feed gas (at the converter s operating temperature).

Catalyst deactivates when it is cooled below its solidification temperature. This happens when a catalyst bed is fed with cold gas or when the acid plant is shut down. 

The calculations of this chapter are all based on feeding 1 kg-mole of dry gas into the acid plant s first catalyst bed. The kg-mole of each component (e.g. S02) in this feed gas are calculated by equations like ... 

Industrial catalyst bed gas pressure varies slightly between acid plants depending on altitude. It also tends to increase slightly over time as catalyst beds become clogged with dust and catalyst fragments. 

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