Periodic liquid flow

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.)...

Constant rate period. Liquid flows to the surface to replace that lost to evaporation. This is the stage where most of the shrinkage and warping occurs, as a result of the c pillaiy pressvire differentials within the film. 

The changing hydrodynamics is one important influence factor under periodical liquid flow rate in trickle-bed reactors. Because until now an exact mathematical description of the hydrodynamics in periodically operated trickle-bed reactors especially the liquid flow field and the liquid distribution within the catalyst bed, is impossible and detailed further experimental and theoretical studies of the hydrodynamics under unsteady-state conditions are required. Despite a broad basis of experimental data published in the literature no correlation with general applicability is yet available, if an accuracy of the liquid holdup level better than 30% is desired [37]. 

Figure 4.7 Simulation results of the time-averaged conversion (curve 3 Ua, s) and of the reduced concentrations (c ) for a periodic liquid flow rate (on-off modus, Rol = 0.23/0 at a constant cycle split of...

The periodic liquid flow changes the hydrodynamic regime and influences the external and internal catalyst wetting. Therefore the catalyst particle wetted area will be increased and decreased depending on the hquid duration. The degree of wetting will be crucially influenced. This has a strong effect on the intensity of the process, as it alters the conditions of transport of the reactants to the catalyst surface where adsorption, chemical reactions and desorption of the products take place. 

Stegasov, A. N., Kirillov, V. A., Silveston, P. L., Modeling of catalytic SO2 oxidation for continuous and periodic liquid flow through a trickle bed. Chemical Engineering Science, 1994, 49, 3699... 

The hquid flow rate is adjusted to prevent resia from passiag through the openings ia either directioa. Liquid flow is stopped for a short period to allow resia to drop through openings to the chamber below. Liquid flow resumes before resia can drop through more than one chamber. 

Coutor (U.S. Patent 2,036,924, 1936). See also Eisenlohr [Ind. Chem., 27, 271 (1951) Chem. Jng. Tech., 23, 12 (1951) Phorm. Ind., 17, 207 (1955) Trans. Indian Inst. Chem. Eng., 3, 7 (1949-1950)] and Husain et al. [Chim. Ind. ( 4ilan), 82, 435 (1959)]. This centrifuge revolves about a vertical axis and contains three actual stages. It operates at 3800 r/min and handles approximately 4.92 mVh (1300 gaPh) total liquid flow at 12-kW power requirement. Provision is made in the machine for the accumulation of solids separated from the hquids, for periodic removal. It is used, more extensively in Europe than in the United States, for the extraction of acetic acid, pharmaceuticals, and similar produces. 

The pressure drop remains essentially constant as long as the liquid flow on tray remains steady during the period point A to point B on the diagram (the open balance point) [201]. At point B all valves are completely open off their seats, but are on the verge of closing and may be oscillating from open to closed. At point B the vapor velocity through the holes, opened balance point is ... 

OS 68] [R 19] [P 50] A numbering-up of five mini mixers, tested at the pilot stage, was used [134]. Automation of the entire process was required liquid-flow splitting to the single reactors was ... by no means trivial... [134]. The capacity of one mini reactor was 30 ml s , i.e. 1081 h . The complete setup hence should be operated close to 5001 h . The micro-reactor plant was operated at intervals as the preceding step was carried out batchwise. The operation of the micro-reactor plant started in August 1998 after a period of only about 1.5 years for development. 

The reactor was first primed with a cleaning solution, then with the reacting solution, and fed by pumping for a longer period [72, 74]. Then, the liquid flow was set to 1 ml min The samples were analyzed typically after 48 h to ensure completion of dark follow-up reactions. 

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

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. 15 shows the detailed structure of the droplet from a viewing angle of 60°. Experimental images show that a hole is formed in the center of the droplet for a short time period (3.4 4.8 ms) and the center of the liquid droplet is a dry circular area. The simulation also shows this hole structure although a minor variation exists over the experimental images. As the temperature of the surface is above the Leidenfrost temperature of the liquid, the vapor layer between the droplet and the surface diminishes the liquid-solid contact and thus yields a low surface-friction effect on the outwardly spreading liquid flow. When the droplet periphery starts to retreat due to the surface-tension effect, the liquid in the droplet center still flows outward driven by the inertia, which leads to the formation of the hole structure. 

Another important highly selective and stable hydroformylation sol gel catalyst is made of silica-supported rhodium covalently bound to supported Xantphos family of ligands.36 By incorporating monoliths of the sol-gel doped material into the paddles of an autoclave stirrer, the catalyst (Rotacat) can be used in a continuous liquid flow process. A single sample of this catalyst was used for a variety of different hydroformylation reactions under widely varying conditions over a period of more than a year, still retaining its selective activity. 

In vibration atomization, each individual droplet is produced one at a time by means of a periodic disturbance. Therefore, the resultant droplet size is not greatly dependent on the liquid properties. For a given liquid, the droplet size is practically determined only by liquid flow rate and vibration frequency. 

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