Cesium-Promoted Catalysts

The use of catalysts in which some of the potassium is replaced by cesium provided the more active catalyst anticipated from earlier development work. A striking temperature as low as 320°C was reported in a fiill-scale four-bed plant, and operation was possible at a stable bed-1 inlet temperatnre of 370°C. 

TABLE 2.8. Operation of Single Absorption and Double Absorption Sulfur-Burning 

Heat exchange cooling between beds 1-2, 2-3 and 3 with sulfur trioxide absorption after bed 4. 

Feed gas 9% sulfur dioxide and 10% oxygen. Catalyst loading 190 liters of catalyst per tonne of acid per day. 

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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-... 

However, the scmbber will not be required during steady running of the plant when a cesium promoted catalyst is used in adequate amounts in the last pass of the converter which is operated at 385-390 °C. 

A higher (10.0-10.5%) SO2 gas strength in the burner outlet gases will be possible due to use of a cesium promoted catalyst. This will also require lower volumes of gases to be handled, thus reducing power consumption. 

Using a DCDA process instead of SCSA and cesium promoted catalyst in the last pass of the converter. 

Use of cesium promoted catalysts in first and fourth/fifth passes. This would enable the start of conversion at about 385-390 °C earher than conventional catalysts having an ignition temperature of 410 °C, i.e., faster after any plant stoppage). This will improve the efficiency of conversion to as high as 99.9% instead of the 99.5% offered by DCDA (3 +1) systems. 

The fifth pass shall contain Cesium promoted catalyst which has a lower ignition temperature of 360 °C. As lower temperature favours higher overall conversion, it is obvious that a higher yield of the above reaction will be obtained as compared to the conventional (3 + 1) DCDA Process. 

Single contact acid plants oxidize 98-99% of their feed gas SO2, whereas double contact acid plants oxidize 99-99.7% with four catalyst beds and little or no cesium-promoted catalyst. The highest SO2 oxidation efficiencies, at around 99.95%, are... 

Table 29.2 Industrial percent SO2 oxidation in a four pass double absorption acid plant. The feed gas contains 10 volume% SO2, 11 volume% O2, 4 volume% CO2, and 75 volume% N2. The catalyst bed inlet temperatures are as follows bed 1 420 °C, bed 2 435 °C, bed 3 440 °C, and bed 4 410 °C. Cesium-promoted catalyst is installed in bed 4. ...

Cesium-promoted catalysts and the new catalysts LEAPS (Christensen and Polk, 2011) and GEAR (Felthouse et al., 2011) are used to maximize SO2 oxidatimi. They are more costly than conventional catalysts (potassium promoted), but give appreciably better SO2 oxidation performance. Table 29.3 provides industrial design data which show the benefits of using cesium-promoted catalyst in a four pass single coti-tact type metallurgical acid plant. 

Table 29.4 shows a comparison of an acid plant with and without MECS GEAR type catalysts. Additional cases are shown that also use cesium-promoted catalysts. 

The feed gas contams 10 volume% SO2, 14 volmne% O2, and 76 volume% N2. In this example, the use of cesium-promoted catalyst results in a 21 % decrease in SO2 emissions. Case 1 uses all conventional (potassium promoted) catalyst. Defined below Table 28.1. 

Hie feed gas contains 11.5 volume% SO2. GR are GEAR catalysts, SCX" are cesium-promoted catalysts, and XLP" are traditional potassium-promoted catalysts. 

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