Four Catalyst Beds

Most industrial acid plants have 4 catalyst beds. The arrangements of these beds in order of decreasing industrial use are  

3 catalyst beds before intermediate H2SO4 making 1 catalyst bed after 

These results explain the widespread industrial use of the 3 -1 process. 

Beds before intermediate H2SO4 making - beds after 

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Fig. 20.1. Total S02 oxidation efficiencies of 4 four-catalyst-bed arrangements. The 3 - 1 bed arrangement is seen to be the most efficient.

In the conventional plant sulfur dioxide is converted to sulfur trioxide in a series of three or four catalyst beds with cooling between the beds to remove the heat of reaction. The overall conversion is limited by the equilibrium for the relative partial pressures of sulfur dioxide, sulfur trioxide, and oxygen and the temperature of the converter exit gas. This equilibrium is equivalent to about 98.5% conversion. 

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. 

In this last process (Figure 15), a mixture of liquid water and gaseous propylene is fed to the top of trickle-bed reactors. Each reactor has four catalyst beds. As the reaction is exothermal ( AH= "51 kJ/mol), each of the beds has its own quench system. Temperature is indeed a very important parameter, on which depend the efficiency of the catalyst, its resistance to hydrolysis and... 

UOP developed two varieties of zeolite catalyst designated as QZ-2000 and QZ-2001 (47,48). Its research indicated that the catalysts have Bronsted acidity values. As a result, oligomerization of propylene is essentially eliminated. Accelerated stability tests indicated that the percentage of the bed of catalyst employed for desired reactions increases with time of operation. QZ-2001 demonstrates improved stability and operation at benzene/propylene ratios of 2.0 in its liquid-phase process. Unreacted benzene and polyisopropylbenzenes are recovered and recycled so that cumene jdelds of about 99% are obtained. Four catalyst beds in series are employed. The catalysts need to be reactivated after about 2-5 years. UOP have licensed several units. 

As unlimited number of variations can be thought of when different percentages of feed distribution on each of the four catalyst beds are to be considered a change of feed to one bed may or may not be compensated for by variations of the flow rates on the other 3 beds. In the following two such cases studied by the simulation are described. 

A multibed reactor system that is used in the oxidation of SO2 to SO3, on a V2O5 catalyst, is illustrated in Figure 5.8. The reactors operate at atmospheric pressure, and the heat of the reaction is removed by external heat exchangers, which are also used to preheat the feed into the reactor. In modern sulfuric acid factories, at least four catalyst beds in series... 

Initially, the combined model was huge, containing more than 1.2 million non-zero terms in its matrix of variables. To allow the model to run in a reasonable amount of time on a Pentium III computer, we made some simplifications. In the reduced model, the four catalyst bed models are still fully rigorous. However, the hydrogen furnaces are represented with a heat-exchanger model, quench valves are modeled with mixers, a component splitter model is used for the wash-water system, and a group of component splitters is used for the fractionation section. These changes reduce the number of equations and non-zeros to 130,000 and 680,000 respectively. Despite these simplifications, the slimmed-down model remains, in our collective opinion, a useful tool for offline what-if studies and for economic comparisons of different process options. 

Figure 19.7 Four catalyst bed single contact acid plant. The third catalyst bed heatup path and intercept are the same as in Figs. 16.3 and 16.4. The fourth bed is new (Table S.4 in Appendix S). Note that (a) the fourth bed oxidizes less than half of the third bed s exit SO2 whereas (b) Fig. 19.6 s after-H2S04-making bed oxidizes 98.9% of the third bed exit SO2 (Section 19.6). This explains the greater efficiency of double contact acidmaking.

Calculate the equivalent SO2 oxidation efficiency with four catalyst beds but no intermediate H2SO4 making. Use the technique described in Appendix S with aU of Problem 19.1 s temperatures and pressures. 

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... 

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