Converter operations

The operation of a discontinuous-mode, flyback converter is quite different from that of a forward-mode converter, and likewise their control-to-output characteristics are very different. The topologies that fall into this category of control-to-output characteristics are the boost, buck/boost, and the flyback. The forward and flyback-mode converters operating under current-mode control also fall into this category. Only their dc value is determined differently. Their representative circuit diagram is given in Figure B-12. 

Catalytic reactions occur when the temperature exceeds 500 -6(X)°F (260°-3l6°C). Normal converter operating temperatures are 900°-1200°F (482°-649°C). 

Excessively rich A/F ratio causes converter operating temperatures to rise dramatically, thus causing converter meltdown. On the other hand, if the A/F ratio is too lean, the excess O2 will react with the CO, and the reduction of nitrogen with CO will not take place, Thus, catalytic converters cannot be used where there is excess air. 

Feed gases to most, if not all, methanation systems for substitute natural gas (SNG) production are theoretically capable of forming carbon. This potential also exists for feed gases to all first-stage shift converters operating in ammonia plants and in hydrogen production plants. However, it has been demonstrated commercially over a period of many years that carbon formation at inlet temperatures in shift converters is a relatively slow reaction and that, once shifted, the gas loses its potential for carbon formation. Carbon formation has not been a common problem at the inlet to shift converters. It has been no problem at all in our bench-scale work, and it is not expected to be a problem in our pilot plant operations. 

It may be perceived that a continuous, sulfur dioxide-rich stream can be produced if the smelting and converting operations of conventional technology can be combined in one continuous operation, and this has been attempted in the Noranda process. However, in the largest present-day installation subscribing to this process, the units are used primarily as smelters, with the conversion implemented in separate, conventional converters. 

A simplified version of the model in Table IX, neglecting accumulation of mass and heat as well as dispersion and conduction in the gas phase, predicts dynamic performance of a laboratory S02 converter operating under periodic reversal of flow direction quite well. This is shown by Fig. 13 taken from Wu et al. (1996). Data show the temperature profiles in a 2-m bed of the Chinese S101 catalyst once a stationary cycling state is attained. One set of curves shows the temperature distribution just after switching direction and the second shows the distribution after a further 60 min. Simulated and experimental profiles are close. The surprising result is that the experimental maximum temperatures equal or exceed the simu-... 

Fig. 14. Influence of inlet SO2 concentration on behavior and performance in adiabatic, packed-bed SOj converters operating under periodic flow reversal. Simulation results for t = 30 min, SV = 514 h 1, Ta = 25°C (a) effect of inlet SO2 vol% on the temperature profile in the catalyst bed, (b) influence of inlet S02 on converter performance and the velocity of the temperature front. (Figure adapted from Xiao and Yuan, 1996, with permission of the authors.)...

Fig. 17. Comparison of the variation of the time-average S02 conversion and the maximum bed temperature predicted for stationary cycling condition by an unsteady-state and a steady-state kinetic model for a packed-bed S02 converter operating with periodic flow reversal...

The studies described in the preceding two sections have identified several processes that affect the dynamic behavior of three-way catalysts. Further studies are required to identify all of the chemical and physical processes that influence the behavior of these catalysts under cycled air-fuel ratio conditions. The approaches used in future studies should include (1) direct measurement of dynamic responses, (2) mathematical analysis of experimental data, and (3) formulation and validation of mathematical models of dynamic converter operation. 

Cyanides have been detected in automobile exhaust. The average emission rate was 11-14 mg/mile for cars not equipped with catalytic converters and 1 mg/mile for cars with catalytic converters operating under optimum conditions. Cars with malfunctioning catalytic converters may emit as much or more hydrogen cyanide than cars without such equipment (Fiksel et al. 1981). 

The largest market for chemical reactors by far is the automotive catalytic converter (ACC), both in number of reactors in existence (many million sold per year) and in amount of reactants processed (mUhons of tons per year). There are >50 mtUion automotive catalytic converters operating (or at least existing) throughout the world, and everyone owns one if he or she has a car less than 10 years old. 

Figure 3.4 Ideal voltage and current waveforms for the buck converter operating in the continuous conduction mode.

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

Figure 17.22. Representative ammonia converters operating at various pressures and effluent concentrations (Vancini, 1971). (a) Original Uhde design operating at 125 atm typical dimensions, 1.4 x 7 m. (b) Haber-Bosch-Mittasch converter operating at 300 atm typical dimensions, 1.1 x 12.8 m. (c) Claude converter operating at 1000 atm typical dimensions 1.2 x 7 m. (d) Fauser-Montecatini (old style) converter operating at 300 atm with external heat exchange, showing axial profiles of temperature and ammonia concentration.

In the single-pressure and dual-pressure processes, the catalyst volatilizes at a rate determined by the converter exit-gas temperature. Experimental work indicates that the rate loss of catalyst (without a catalyst recovery system) is approximately three times more rapid at 973°C than at 866°C (Ref. PT18). From plant operation data, the loss from a dual-pressure converter (operating at 866°C) is estimated at about 0.10 g/tonne of 100% acid, and from a single-pressure converter (operating at 937°C) it is estimated at about 0.38 g/tonne of 100% acid. 

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