Periodic reversal of flow direction

Periodic air blowing of the final stage of a multistage catalytic converter Periodic reversal of flow direction in a single-stage converter... 

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. 13. Comparison of simulated and experimental temperature profiles in a 2-m, near-adiabatic, packed-bed S02 reactor using a Chinese S101 catalyst and operating under periodic reversal of flow direction with r = 180 min, SV = 477 h"1, and inlet S02 = 3.89 vol% and T = 25°C. (Figure adapted from Wu et at., 1996, with permission of the authors.)...

Fig. 16. Variation in a stationary cycling state of catalyst temperature, S03, and complex concentrations in the melt phase and the concentration of gas phase species with time in a half cycle in the forward flow portion of a reactor operating under periodic reversal of flow direction with r = 40 min, SV = 900 h (Csodo = 6 vol%, (Co2)o = 15 vol%, Ta = 50°C. Curves 1, just after switching flow direction 2,1 min 3, 6.6 min 4, 13.3 min, and 5, 20 min after a switch in flow direction. (Figure adapted from Bunimovich et at., 1995, with permission, 1995 Elsevier Science Ltd.)...

With respect to periodic reversal of flow direction, this seems to be competitive for low levels of smelter gas S02, probably under 4 vol%, at ambient temperature. If S02 is too high, thermal runaway can occur. A second, important application is to reduce S02 in the off-gas from a conventional acid plant. Use of flow reversal avoids the problem of reheating the gas leaving the first-stage absorber. 

TABLE IX Model for a Packed-Bed Catalytic Reactor with Periodic Reversal of Flow Direction ... 

The strategies described so far are essentially chemistry based in the sense that different environments are created for a reaction to be initiated or enhanced. It is also possible to achieve enhancement by physically manipulating the same environment. One way of accomplishing this is by introducing controlled micromixing or optimizing the sequence of reactant additions. Another way is to operate under forced unsteady-state conditions by periodic reversals of the direction of flow. The main aspects of these strategies were considered in Chapter 13. 

This new method of SO2 oxidation is based on a periodic reversal of the direction of the reaction mixture flow over the catalyst bed. The process was developed by Dr. Matros at the Institute of Catalysis of the former USSR. Basically, a large bed of catalyst is used both as a reversing, regenerating heat exchanger and as a catalytic reactor for the SO2 oxidation reaction. 

III. S02 Converters Based on Periodic Reversal of the Flow Direction... 

The most common process used for this is depth filtration through a bed of sand or similar material charged in a vertical vessel. The incoming water flows from top to bottom. To improve the efficiency of such filters, two or more layers of media with various particle sizes are used. Coarse and less dense material such as anthracite is located at the top of the bed, whereas finer and denser particles of sand are placed at the bottom. Such multimedia filters can remove most particles larger than 10-20 pm. Periodically the filter bed is back-washed by reversing the flow direction (from bottom to top) and by increasing the flow rate. During backwash, the captured particles are removed and sent to drain, whereas heavier particles of the filter bed remain in the vessel and settle back at the end of the cycle. 

This technique uses periodic reversal of the flow direction of the reaction mixture through the fixed bed of catalyst (Fig. 3) which thereby serves both as an accelerator of chemical reactions and as a heat regenerator and/or accumulator [2], often flanked by layers of heat regenerative inert, typically ceramic, packing. Flow reversals induce continuous back-and-forth migration of the heat and reaction waves through the catalyst bed (Fig. 4). This allows for continuous autothermal operation without or with minimum external heat input. Theoretical basis of RFR operation, experimental results and commercial applications were described in a number of papers [2,35],... 

Unsteady-state operation of a catalytic reactor can be exploited to enhance the efficiency of a catalytic reaction. A simple way of achieving unsteady-state operation is to force it by periodic reversab of the direction of flow of the reactant mixture. An important parameter in creating this condition is the ratio ap so of the time between flow reversals to the time needed for transition to the steady state. For apuso < U unsteady operation results, and for ttpuso > U pseudosteady-state conditions prevail. 

Traditionally, the variation of evaporative rates across the stack has been counteracted by the installation of bidirectional fans and by periodically reversing the airflow direction through the stack. This policy has minimal effect on the drying rates in the center of the stack, but reduces the variation in behavior between the two end zones. If only moisture content variations are considered, many reversals are not needed to achieve this equalization (Pang et al., 1995 Nijdam and Keey, 1996 Wagner et al., 1996). However, if stress development in the surface layer with the likelihood of checking is taken into account, then the flow reversals for a timber such as Pinus sylvestris should be less than 2 h apart (Salin and Ohman, 1998). A period of 4 h is a common industrial practice for permeable softwoods such as P. radiata. 

More often the sediment contains carbonate and hydroxide. Therefore, the most common method to remove the sediment is concentrate acidification or current reversal [18]. Electrodialysis reversal (EDR) is an electrodialysis process with periodic reversal of the electric field. As the direction of electric current is switched desalination chambers become concentrating ones and vice versa. Colloidal and organic precipitate accumulates in the desaHnation chamber. As this chamber becomes a concentration one after switching the current direction, the organic or colloidal precipitate is washed away with concentrate flow. 

Since membrane fording could quickly render the system inefficient, very careful and thorough feedwater pretreatment similar to that described in the section on RO, is required. Some pretreatment needs, and operational problems of scaling are diminished in the electro dialysis reversal (EDR) process, in which the electric current flow direction is periodically (eg, 3—4 times/h) reversed, with simultaneous switching of the water-flow connections. This also reverses the salt concentration buildup at the membrane and electrode surfaces, and prevents concentrations that cause the precipitation of salts and scale deposition. A schematic and photograph of a typical ED plant ate shown in Eigure 16. 

Convection heat transfer is dependent largely on the relative velocity between the warm gas and the drying surface. Interest in pulse combustion heat sources anticipates that high frequency reversals of gas flow direction relative to wet material in dispersed-particle dryers can maintain higher gas velocities around the particles for longer periods than possible ia simple cocurrent dryers. This technique is thus expected to enhance heat- and mass-transfer performance. This is apart from the concept that mechanical stresses iaduced ia material by rapid directional reversals of gas flow promote particle deagglomeration, dispersion, and Hquid stream breakup iato fine droplets. Commercial appHcations are needed to confirm the economic value of pulse combustion for drying. 

A flow direction switch occurs at r/2 for a cycle period r set by the plant operator, or it may be initiated by the reactor or recuperator outlet temperature. Startup of the flow-reversal system requires a heating device to bring the catalyst bed or at least the frontal portion up to ignition temperature. This temperature is about 350°C for S02 oxidation using conventional catalysts. 

Several pilot plants have been built to test periodic flow direction reversal. Pilot-scale reactors with bed diameters from 1.6 to 2.8 m were operated with flow reversal for several years. The units, described by Bunimovich et al. (1984,1990) and Matros and Bunimovich (1996), handled 600 to 3000 m3/h and operated with cycle periods of 15 to 20 min. Table VIII shows the performance of these plants for different feeds and potassium oxide promoted vanadia catalysts. The SVD catalyst was granular the IK-1-4 was in the form of 5 (i.d.) x 10-mm cylinders, while the SYS catalyst was... 

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