Oscillation bottom flow

When the bottoms stream provides the bulk of the column preheat, bottom flow swings may cause fluctuations in feed enthalpy. Unless the feed temperature controller can suppress these rapidly and effectively, the disturbances will reenter the column and interact with the composition controller. In one column controlled with scheme 16.4a, this resulted in severe oscillations of the composition controller. 

Fig. 3a indicates that the bubble-rise velocity measured based on the displacement of the top surface of the bubble ( C/bt) quickly increases and approaches the terminal bubble rise velocity in 0.02 s. The small fluctuation of Ubt is caused by numerical instability. The bubble-rise velocity measured based on the displacement of the bottom surface of the bubble (Ubb) fluctuates significantly with time initially and converges to Ubt after 0.25 s. The overshooting of Ubb can reach 45-50 cm/s in Fig. 3a. The fluctuation of Ubb reflects the unsteady oscillation of the bubble due to the wake flow and shedding at the base of the bubble. Although the relative deviation between the simulation results of the 40 X 40 x 80 mesh and 100 x 100 x 200 mesh is notable, the deviation is insignificant between the results of the 80 x 80 x 160 mesh and those of the 100 X 100 x 200 mesh. The agreement with experiments at all resolutions is generally reasonable, although the simulated terminal bubble rise velocities ( 20 cm/s) are slightly lower than the experimental results (21 25 cm/s). A lower bubble-rise velocity obtained from the simulation is expected due to the no-slip condition imposed at the gas-liquid interface, and the finite thickness for the gas-liquid interface employed in the computational scheme.

Houcine et al. (64) used a non-intrusive laser-induced fluorescence method to study the mechanisms of mixing in a 20 dm CSTR with removable baffles, a conical bottom, a mechanical stirrer, and two incoming liquid jet streams. Under certain conditions, they observed an interaction between the flow induced by the stirrer and the incoming jets, which led to oscillations of the jet stream with a period of several seconds and corresponding switching of the recirculation flow between several metastable macroscopic patterns. These jet feedstream oscillations or intermittencies could strongly influence the kinetics of fast reactions, such as precipitation. The authors used dimensional analysis to demonstrate that the intermittence phenomenon would be less problematic in larger CSTRs. 

Simulated Moving Bed An implementation of preparative chromatography in which a series of identical columns is used. The continuous injection of a feed stream and of the mobile phase stream, the continuous collection of two fraction streams are made in positions which are periodically moved by one column length. The result is a semisteady concentration profile for each component, which oscillate slowly, and permits the collection of two streams of constant composition. The process is equivalent to moving the stationary phase down the column while the mobile phase flows upward, the faster moving component eluting at the top, with the mobile phase, the slower moving one at the bottom, writh the stationary phase. 

Reboiler oscillations can often be dampened by increasing flow resistance in the reboiler inlet lines or reducing flow resistance in the reboiler outlet lines. This shifts flow resistance from the outlet to the inlet lines. This is analogous to dampening U-tube oscillations by increasing friction in the bottom of the tube while easing the restriction at the open end of the tube. Design practices for reboiler inlet and outlet lines were discussed in Sec. 15.2. 

PE is measured using a particle charge detector (PCD-02, Miiteck). This consists of a cylindrical polytetratiuoroethylene (PTFE) bath, equipped with two silver electrodes, located at the top and bottom and linked to an amplifier. A PTFE piston mounted in the bath oscillates vertically at a constant frequency, making the liquid flow along the sides of the bath (Figure 10.6). This apparatus is connected to an automatic titrator used to add polyelectrolyte. 

The oscillatory deep-channel rheometer described by Nagarajan and Wasan (227) can be used to examine the rheological behavior of liquid/liquid interfaces. The method is based on monitoring the motion of tracer particles at an interface contained in a channel formed by two concentric rings, which is subjected to a well-defined flow field. The middle liquid/liquid interface and upper gas/liquid interface are both plane horizon tal layers sandwiched between the adjacent bulk phase. The walls are stationary while the base moves. In the instrument described for dynamic studies of viscoelastic interfaces the base oscillates sinusoidally. This move ment induces shear stresses in the bottom liquid that are transmitted to the interface. The interfaces are viewed from above through a microscope attached to a rotary micrometer stage which is coaxial to the cylinders. 

Streaks and chatter downweb lines air flow at knife causes oscillations in the upstream bead/distance between the bottom of the air knife and the coating pan edge is too large/inadequate contact angle between the liquid and the web. 

FIG. 5. Change of the rate oscillations with variation of the gas phase heat conductivity by partial substitution of He feed gas by Ar Methanol to ojygen ratio 2.4 1 (flow), sample polycristalline copper foil, T = 688 K. Top Feed gas 87 vol-% He, bottom feed gas mixture of 47 vol-%He and 40 vol-% Ar. 

Wu and Cheng [1] conducted experiments using 8 parallel silicon microchannels heated from the bottom. They observed water flows with large-amplitude or long-period oscillating boiling modes as a function of heat flux and mass flux depending on whether the water outlet is at saturation temperature or superheated. 

The equilibrium between these two effects causes mixed convection and stratified flow. The stratification interface oscillates and may induce severe damage to the neighbouring structures. Furthermore, the support structures at the bottom of the hot pool have to be protected from hot sodium. The behaviour of such a region has been studied, namely for SPXl [8.27] and EFR [8.19]. Studies were mainly conducted through scale model tests, because computations are not yet able to predict the fluctuation characteristics for such complex situations. As the main physical phenomenon of interest is the interaction between buoyancy forces (natural convection) and inertia forces (forced convection from the main pool... 

Figure 2.3 The two most common scale-down simulators. The graphics below each picture depict the substrate concentration that a cell circulating in the simulator would experience, (a) Two interconnected stirred-tank reactors (STR-STR) with independent environmental parameter control. Cells are circulated between them, experiencing a square oscillation as depicted in the graphic at the bottom. The liquid volumes and flow rate determine the average to be evaluated, (b) A plug-flow reactor connected to a stirred-tank reactor (STR-PFR). Environmental perturbations...

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