Periodic switching

Of course, moving the Hquid feed and withdrawal positions continuously is impractical. However, approximately the same effect can be produced by providing multiple Hquid-access lines to the bed and periodically switching each stream to the adjacent line. Functionally, the adsorbent bed has no top or bottom and is equivalent to an aimular bed. Therefore, the four Hquid-access positions can be moved around the bed continually, always maintaining the same distance between the various streams. 

Since usually the reference electrode is not equipped with a capillary probe (see Fig. 2-3), there is an error in the potential measurement given by Eq. (2-34) in this connection see the data in Section 3.3.1 on IR-free potential measurement. The switching method described there can also be applied in a modified form to potential-controlled protection current devices. Interrupter potentiostats are used that periodically switch off the protection current for short intervals [5]. The switch-off phase is for a few tens of microseconds and the switch-on phase lasts several hundred microseconds. 

The modeling of SMB can be addressed by simulating the system directly, taking into account its intermittent behavior, or by representing its operation in terms of a true countercurrent system. The first model represents the real SMB and considers the periodic switch of the injection and collection points. The second is developed assuming the equivalence with the TMB, where solid and fluid flow in opposite directions. 

If the catalyst degrades (e.g. as a result of coke formation on the surface), then a fixed-bed device will have to be taken off-line to regenerate the catalyst. This can either mean shutting down the plant or using a standby reactor. If a standby reactor is to be used, two reactors are periodically switched, keeping one online while the other is taken offline to regenerate the catalyst. Several reactors might be used in this way to maintain an overall operation that is... 

FIG. 16-48 General scheme of a simulated moving bed (SMB) adsorption system. Bed rotation is simulated by periodic switching of ports in the direction of fluid flow. A is less strongly retained than B. 

In a simulated moving bed reactor, as its name already indicates, the movement of the solid phase is simulated. This is achieved by using a set of fixed-bed reactors (columns) connected in series and periodically switching the feed and withdrawal points from one column to the other. A schematic presentation of a simulated moving bed chromatographic reactor is shown in Fig. 8. 

The countercurrent movement of a stationary phase is cumbersome in practice but it can be circumvented by an array of short columns connected by multi-position valves connected with eluent, feed, extract and raffinate, a method referred to as simulated moving bed chromatography (SMB) (Schulte and Strube, 2001). In SMB chromatography, the continuous countercurrent flow of the fluid and of the solid adsorbent is simulated by periodically switching the different inlets and outlets in the multi-column unit. Enantioselective SMB-LC has first been demonstrated for racemic 1-phenylethanol resolved on the polysaccharide CSP Chiralcel OD. In this pioneering work the principle of the method and the set-up has been depicted in a lucid educational fashion (cf. Figures 22 23) (Negawa and Shoji, 1992). 

Dielectrophoresis is the translational motion of neutral matter owing to polarization effects in a non-uniform electric field. Depending on matter or electric parameters, different particle populations can exhibit different behavior, e.g. following attractive or repulsive forces. DEP can be used for mixing of charged or polarizable particles by electrokinetic forces [48], In particular, dielectric particles are mixed by dielectrophoretic forces induced by AC electric fields, which are periodically switched on and off. 

Periodic switching of helical flow in curved channels - chaotic advection... 

The flow rate of 8.5 mm s 1 corresponds to the maximum flow rate induced by period switching of the flow [26],... 

Sample and hold controllers are periodically switched from manual to automatic and back again. They are used mostly on dead-time processes. 

The practical implementation of the TCC reactor imposes several technical problems (as discussed earlier), and consequently the simulated moving-bed (SMB) technology emerged. Movement of the solid is simulated by periodically switching the inlet and outlet ports in the direction of the liquid flow, as shown in Fig. 6.5. 

Figure 1. (a) Step-wise function of inlet parameters change, (b) Fixed-bed reactor operated at periodic switching between two dilTerent feeds that provide for step-wise control. 

The rotary valve is used to periodically switch the position of the liquid feed and withdrawal points in the adsorbent chamber. The process operates in a continuous mode at low temperatures in a liquid phase. 

Description The process simulates a moving bed of adsorbent with continuous counter-current flow of liquid feed over a solid bed of adsorbent. Feed and products enter and leave the adsorbent bed continuously, at nearly constant compositions. A rotary valve is used to periodically switch the positions of the feed-entry and product-withdrawal points as the composition profile moves down the adsorbent bed. 

The hot coke in an endothermic reaction produces mainly carbon monoxide and hydrogen, and cools down. The process is periodically switched to combustion air to regain the high coke temperatures necessary for the water splitting reaction. Since carbon monoxide is relatively difficult to separate from hydrogen, and also because it is possible to obtain a further mole of hydrogen from each mole of carbon monoxide, the whole water gas stream is normally put through a shift conversion step (Eq. 11.17). 

The True Moving Bed (TMB) approach, assumes that the SMB process is equivalent to the true counter-current moving bed (TMB). This TMB model neglects the dynamics associated with the periodic switching of the columns and gives the mean concentration profiles over a switching period. 

In this approach, an equivalent countercurrent movement of solid is assumed instead of the SMB process. This equivalent true moving bed (TMB) neglects the d3mamics associated with periodic switching and produces mean concentration profiles over a switching period. 

ToF SIMS separated ion beams are used for etching and analysis purposes. A depth profile is obtained by periodically switching between etching and analysis. In addition, the pulse mode operation in ToF SIMS further slows down the process. ToF SIMS, however, can be used to obtain more accurate depth profiles if time is not a concern. The next section introduces depth profiling based on the dynamic SIMS technique, which is very popular in analyzing electronic materials. 

Even though in the example of a 3M0 star of Langer et al. (1999) the star evolves close to rigid rotation on the main sequence, the extreme expansion of the hydrogen-rich envelope and the compactness of the C/O-core on the AGB create an enormous shear at the core-envelope interface. While this remains without drastic consequences as long as active nuclear shell sources separate both regions well in terms of entropy, this is different during the thermal pulses where both nuclear shell sources are periodically switched off. 

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