Using Super Cesium Catalyst to Increase Sulfuric Acid Production

Transcription

1 Using Super Cesium Catalyst to Increase Sulfuric Acid Production David D. Clark P.E., Atis Vavere, Ph.D. and John R. Horne MECS, Inc. St. Louis, MO Presented at AIChE Convention Clearwater, Florida June 2007

2 Abstract Super cesium sulfuric acid catalyst has been used to increase sulfuric acid production at the same or lower sulfur dioxide emissions. Super cesium catalyst is much more active than conventional potassium-promoted vanadium catalyst and can operate at temperatures as low as 735 o F. By taking advantage of the low temperature properties of this catalyst in the final converter pass, the converter gas strength and/or air flow rate can be increased to make more sulfuric acid production. This paper will describe applications of this catalyst in several sulfuric acid plants. Introduction A current focus within the sulfuric acid industry is to reduce sulfur dioxide emissions to the atmosphere while maintaining or increasing acid production. This trend is supported by an industry-wide consolidation of production capabilities and increased emphasis on improving air quality. The use of conventional potassium-promoted sulfuric acid catalyst has nearly reached the practical limit regarding minimization of emissions with increasing acid production rates. The development of an exceptional low temperature catalyst promoted by cesium (Cs) compounds has expanded the useful operation range of sulfuric acid plants, allowing for increased acid production rates (with higher gas strengths and greater volumetric gas flow) while maintaining or reducing SO 2 emissions. The MECS Super Cesium catalyst (SCX-2000) has revolutionized this growing industrial effort and helps to generate improved plant operations in an economical fashion. This paper presents the capabilities of this novel catalyst and discusses several successful applications of this technology. Catalyst Development and Applications In the contact sulfuric acid process, there is often an interest in reducing the inlet temperatures to various adiabatic catalyst beds in order to provide more favorable equilibrium conditions. The addition of cesium (Cs) salts to the conventional alkalivanadium sulfuric acid catalyst formulations has long been known to enhance the low temperature properties of the catalyst (1). The cesium promoter stabilizes the vanadium +5 oxidation state (V 5+ ) at temperatures below 790 o F (420 o C) and keeps the active vanadium species solubilized in the molten salt. In the conventional K-V sulfuric acid catalyst, the various vanadium compounds begin precipitating from the molten salt at low bed temperatures, leading to catalyst deactivation (2, 3). At relatively high operating temperatures (> 806 o F/430 o C), the reaction rate is approximately the same for both the conventional material and the cesium-promoted catalyst. However, if the temperature is below 780 o F (415 o C), the conventional catalyst begins to deactivate due to the precipitation while the stabilized cesium catalyst continues to perform well. Therefore, the useable temperature range for effective conversion is greatly expanded, providing versatility to the overall operation. 2

3 MECS has been focused on advancements in Cesium catalyst for many years. A patent (U.S. Patent No. 4,193,894) was issued in 1980, recognizing the specific low temperature activity of the Cs-K mixture of vanadium sulfates (4). After continuing developments, the MECS Cs-110 catalyst was introduced in The newest MECS cesium-promoted formulation was developed to maximize the low temperature catalytic activity while maintaining ring strength and long term stability, and was introduced to the market in The SCX-2000 catalyst incorporates a high surface area support for optimum catalyst utilization and an enhanced cesium/potassium/vanadium catalyst component which maximizes the catalyst activity. The six lobed-star shape provides optimum dust management leading to extended operating periods and minimum bed pressure drop. The combination of all of these properties leads to an ideal lower bed catalyst. The conventional cesium-promoted product (XCs-120) is designed for the harsh environment in the upper beds while providing the versatility of a cesium-promoted formulation. Therefore, for increasing acid production while maintaining or reducing SO 2 emissions, the use of the MECS SCX-2000 ring catalyst is the ideal material for use in fourth/fifth beds after Interpass absorption. SCX-2000 Catalyst Applications A classic example of the application of the SCX-2000 Super Cesium catalyst is provided by a 3x1 sulfur burning plant where increasing acid production was desired while maintaining SO 2 emissions at or below 3.5 lbs./stpd. In this case, a full fourth bed of SCX-2000 was installed and the bed inlet temperature was optimized at 750 o F (399 o C). In this situation, an increase in acid production of approximately 19% was possible, relative to circumstance with a full fourth bed of the standard LP-110 catalyst: Table 1 SCX-2000 versus LP-110 MAXIMUM POSSIBLE ACID PRODUCTION SULFUR BURNING PLANT Optimum 4 th Bed Maximum Acid SO 2 4 TH Bed Catalyst Inlet Temp. Production Rate Emissions SCX-2000 RINGS 750 O F/399 O C 2675 STPD 3.5 lbs./stpd LP-110 RINGS 775 O F/413 O C 2250 STPD 3.5 lbs./stpd 3

4 In the case of this sulfur burning plant, one can examine the other advantages of SCX-2000 catalyst at a given production rate relative to operating the plant with a full bed of standard LP-110. The following chart shows results which demonstrate the emissions reduction capabilities of the Super Cesium catalyst: EFFECT OF SCX-2000 CATALYST SO 2 Emissions (lbs./stpd) SCX-2000 (750oF) LP-110 (750oF) LP-110 (780oF) 4 TH Bed Catalyst (Bed Inlet Temperature) Acid Production Rate = 2550 STPD Fourth Bed Loading = 64.5 L/STPD As can be seen in this illustration, the SCX-2000 catalyst allows the plant to achieve very low emissions which would not be possible using the LP-110 catalyst under the same conditions. Another example of the application of the unique properties of the SCX-2000 Super Cesium catalyst is in a fourth bed installation in a sulfur burning 3x1 plant. In this case, the attainable 4 th bed inlet temperature was relatively low (768 o F/409 o C), the maximum catalyst loading was about 54 L/STPD, and the low atmospheric pressure at this site contributed to the conversion challenge. A full bed of the SCX-2000 catalyst rings provided the required low temperature activity to achieve the required conversion and increase the acid production rate by over 100 STPD. With conventional K-V catalyst in the fourth bed (at the low inlet temperature), the SO 2 emissions would have been nearly double relative to the SCX-2000 case: 4

5 4 th BED CATALYST SO 2 EMISSIONS ACID PRODUCTION SCX-2000 SUPER CESIUM 395 ppm 1520 STPD K-V CATALYST RINGS 883 ppm 1405 STPD (Bed Inlet Temperature = 768 O F/409 O C) 1600 Benefits of Super Cesium Catalyst: Production and Emissions ACID PRODUCTION (STPD or EMISSIONS (ppm) PRODUCTION STANDARD CATALYST SCX-2000 EMISSIONS Again, the operation flexibility afforded by the SCX-2000 Super Cesium catalyst allows the plant operators to select the conditions which provide the needed acid production with acceptable sulfur dioxide emissions. A third example of the application of the SCX-2000 catalyst is a spent acid plant with a relatively low final bed inlet temperature. The focus of this SCX-2000 application was to reduce SO 2 emissions while maintaining acid production. There were some temperature limitations to Bed # 4 which also leads to the use of the Super Cesium catalyst. In this application, a loading of about 77 L/STPD of the SCX-2000 catalyst was installed in Bed # 4 and the bed inlet temperature was set to 732 o F (389 o C). Under these conditions, SO 2 emissions were reduced to less than 1.0 lb./stpd while maintaining the desired acid 5

6 production rate. The use of conventional K-V catalyst under these conditions would have resulted in extremely low conversion through Bed # 4, resulting in emissions exceeding 1000 ppm SO 2 from this double absorption plant. Conclusion There are numerous applications of the newly developed SCX-2000 Super Cesium sulfuric acid catalyst throughout the world. The use of this catalyst in the final bed after Interpass absorption allows the plant operators to increase acid production while maintaining emissions or alternatively, significantly reducing SO 2 emissions with the desired acid production rate. References (1) Tandy, G. H., J. Appl. Chem. 6, 68 (1956) and the references therein. (2) Villadsen, J., and Livbjerg, H., Catal. Rev. Sci. Eng., 17, 203 (1978) (3) Boghosian, S., Fehrmann, R., Bjerrum, N. J., and Papatheodorou, G. N., J. Catalysis 119, 121 (1989) (4) Villadsen, J., Catalyst for Oxidation of Sulfur Dioxide, U.S. Patent Office, Patent No. 4,193,894 (3/18/1980) 6