Liquid-flow interruption

Fig. 19. Time-average rate of S02 oxidation under periodic liquid flow interruption as a function of cycle period, cycle split, and the time-average superficial liquid velocity at 26°C, 1 bar using a BPL activated carbon. (Figure from Haure et al., 1990, with permission, 1990 Elsevier Science Publishers.)...

Filters must be provided for all gas buffers in places where the gas will pass through the gas seal. The gas should be filtered to a nominal value of 2 microns. On most critical systems, dual filters will be required to permit servicing the filters without having to shut down the compressor. This requires the use of a transfer valve that can be switched without causing a flow interruption. If liquids are anticipated, heat tracing or an alternate gas source should be the first consideration. Should this not prove to be practi-... 

From a mass spectrometry perspective, the injector is of little concern other than the fact that any bubbles introduced into the injector may interrupt the liquid flow, so resulting in an unstable response from the mass spectrometer. 

For their experimental investigation of flow interruption, Haure et al. (1989) chose the catalytic oxidation of S02 over a high-surface-area activated carbon catalyst. Several research groups have studied this catalytic reaction and kinetics are available. It proceeds rapidly at 25°C and is controlled, at least partially, by 02 transport through the liquid phase. 

Conversion and the mean rate of reaction were obtained by titrating the liquid recovered from the bed for total acidity before and after dissolved S02 and sulfurous acid were oxidized to sulfuric acid. The difference between these measurements allowed calculation of the S02 conversion. An extension of the Haure study (Metzinger et al., 1992) added gas phase analysis and a check of the results through a sulfur balance. Computer control of flow interruption and acquisition of the S02 analyzer readings were also added. 

Haure et al. (1989) also undertook experiments in which the liquid flow rate was periodically reduced rather than interrupted. Switching between time-average liquid velocities of 4.0 and 1.2 mm/s at s = 0.5 resulted in about a 10% increase in the time-average rate of SO2 oxidation over steady state. The rate improvement was independent of r over the 2 to 60 min range explored. This is considerably less than the increase when flow interruption is utilized. 

Fig. 20. Schematic diagram showing the estimation of the time-average rate of S02 oxidation under periodic flow interruption or reduction employing steady-state oxidation rate vs liquid loading data (Figure from Haure etal., 1989, with permission, 1989, American Institute of Chemical Engineers.)...

The success of periodic flow interruption is due to the liquid static holdup within the porous catalyst pellets and the interstices of the catalyst bed. 

Liquid flowing slowly over the surface of a solid sphere is in essentially laminar flow, except that there must be a radial component due to the increase of the amount of surface at the equator relative to that at the poles. This system, useful as a model of packing in an absorption column, has been studied mathematically (33, 34). Further, if a vertical string of touching spheres is studied, one finds that this laminar flow is interrupted at each meniscus between the spheres complete... 

Periodic interruption of liquid flow in a trickle-bed reactor... 

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