Three catalyst bed acid plant

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.

Figure 21.1 indicates how these requirements are achieved for a single contact sulfur burning acid plant with three catalyst beds. It shows that ... 

Fig. 20.5. 3 - 1 acid plant with one Cs catalyst bed (660 K gas input) and three K, Na catalyst beds (690 and 720 K). Maximum S02 oxidation is obtained with the Cs catalyst in bed 4, i.e. after H2S04 making. Bed 3 (just before H2S04 making) is nearly as good. The calculations are all based on Table 20.l s values - except for gas input temperature. 

As it turned out, ideas such as injecting catalyst into the beds via feed tubes were never realized because of a forced roast/acid shutdown in July 1999. Failure of the boiler water supply system resulted in a boiler tube rupture necessitating a total stoppage for three days. During this time, the No. 4 catalyst bed was opened. A sample of catalyst taken from the bed was analyzed and was foimd to have close to 100% activity. Some 2,000 liters of new catalyst were added and the plant was restarted. Little or no decrease in emissions was found and, consequently, a study to finally solve the problem was initiated. The options considered included ... 

To boost the conversion of SO2 to SO3 in the acid plant, additional catalyst was put into three of the four catalyst beds of the converter. In essence, all of the space in the catalyst beds has now been filled, and the acid plant has reached its limit for converting SO2 into sulfuric acid. Further improvements will require optimizing the operation of the converter, and eventually, rebuilding the acid plant. 

Minimise the escape of unreacted inputs from the plant by better process. A good example is the modified 3 + 2 DCDA process for the production of sulphuric acid wherein five catalyst beds are used instead of four. Three beds are used before the interpass absorption tower and two are after it. This results in the conversion of up to 99.85 % of SO2 to SO3. By providing separate acid circuit for the final absorption tower, the emission of SO2 in exit gases can also be brought down further as compared to the earlier design where aU acid towers had a common circulation tank. 

Figure 7.1 Bed of catalyst pieces for oxidizing SO2 to SO3. It is circular, 7-17 m diameter. Industrial SO2 oxidation is done in a converter of three to five such beds (Figs. 7.6 and 7.7). Downward gas flows are 25 Nm /min/m of top surface. Active catalyst consists of a molten V, K, Na, Cs, S, O phase supported on a solid porous diatomaceous earth substrate (Chapter 8). A top layer of silica rock or ceramic pieces holds the catalyst in place. A bottom layer prevents the catalyst from sticking to the stainless steel support grid. The rock and catalyst dimension are external diameters. Upward gas flow through catalyst beds has been used in a few acid plants.

Figure 21.1 Heat transfer flowsheet for single contact, sulfur burning sulfuric acid plant. It is simpler than industrial plants, which nearly always have fotu catalyst beds rather than three. The gaseous product is cool, SOs-rich gas, ready for H2SO4 making. The heat transfer product is superheated steam. All calculations in this chapter are based on this figure s feed gas composition and catalyst bed input gas temperatures. AU bed pressures are 1.2 bar. The catalyst bed output gas temperatures are the intercept temperatures calculated in Sections 12.2, 15.2, and 16.3.

Table 29.1 Industrial percent SO2 oxidation in a three pass single absorption acid plant. The feed gas contains 10 volume% SO2, 11 volume% O2, and 79 volume%N2. The catalyst bed inlet temperatures are as follows bed 1 415 °C, bed 2 425 °C, and bed 3 430 °C.

In the SCR process, NOX impurities are reduced with added ammonia in the presence of some residual oxygen from the furnace. The main NOX reduction reactions are shown in Table 11.5 together with some of the undesirable oxidation reactions, which can both produce sulfur trioxide and waste some of the added ammonia. Between 0.6-0.9 moles of ammonia per mole of NOX are added to limit the aimnonia shp to downstream equipment where it would deposit as sulfates. NOX conversion is therefore hmited to between 60-90%. At low NOX levels, there is little conversion to itrous oxide. Nitrous oxide formation is also inhibited by water. Gas leaving the boiler is usually at a temperature in the range 300-430°C and contains dust Dust is removed in an elee-trostatic precipitator with little heat loss before sulfur dioxide is removed as gypsum by reaction with lime. Alternatively, sulfur dioxide can also be eonvert-ed to sulfuric acid. The effluent is then vented to atmosphere. In the first power plants to be retrofitted with SCR units there were three possible loeations for the catalyst bed ... 

---