Reverse flow regeneration

Konstandopoulos, A. G., and Kostoglou, M. Periodically reversed flow regeneration of diesel particulate traps. SAE Technical Paper No. 1999-01-0469 (1999b). 

In order to sustain the benefits from fixed bed counterflow regeneration it is imperative that the resin bed remains consolidated and undisturbed at all times during loading and reverse flow regeneration. Some ways this is achieved are discussed later in Chapter 10, but it follows that conventional backwashing of a counterflow designed column is undesirable since the inevitable bed disturbance causes exhausted resin to mix with the lightly loaded resin zone at... 

When forward-flow regeneration is employed it is essential that the adsorption cycle should not be terminated prematurely otherwise desorption will be very inefficient since the sorbate must first be pushed through the unsaturated region of the bed before it can be removed from the outlet. The volume of purge required to desorb the partially saturated bed is therefore essentially the same as for a fully saturated bed. By contrast, with reverse-flow regeneration premature termination of the adsorption cycle means only that the full capacity of the adsorbent is not utilized and the purge requirement/mole desorbed is not increased by operating with an incompletely saturated bed. 

Theoretical simulations of a two-bed adsorption system with a single adsorbable component have been carried out by Tan and Spinner for linear systems and by Bunke and Gelbin and Chao for nonlinear systems. In Gelbin s analysis the advantages of reverse-flow regeneration are clearly shown but the quantitative conclusions are of limited practical value since the analysis is restricted to systems in which both temperature and flow rate are maintained constant throughout the entire cycle. For the reasons already discussed it is impractical to operate an adsorption system in that way except when the adsorption isotherm is linear. 

Part of the time during the exhaust period, gas actually flows simultaneously toward both ends of the bed (curve 3) from a pressure maximum. As the exhaust period continues, the pressure maximum both declines and moves toward the product end, so that near the end of the exhaust period the maximum is essentially at the product end. Thus, all parts of the bed ultimately are subjected to the reverse flow necessary to purge adsorbed nitrogen. The fact that the purge gas for regenerating the adsorbent comes from the bed itself constitutes a major difference between this process and PSA, in which purge gas comes primarily from another bed. 

Figure 1.8 illustrates the operating behavior of the reverse-flow CATOFIN process in the limit of equal heat capacities during reaction and regeneration cycle (h = 1). The inlet temperature of the regeneration gas is set approximately ATai above the inlet temperature of the endothermic reaction feed. In the periodic steady state, only two narrow zones close to both reactor ends contribute considerably to the conversion, while the major part of the fixed bed cools down to a temperature level well... 

Fig. 1.8. Reverse-flow CATOFIN process at equal heat capacity fluxes during production and regeneration cycle periodic temperature profiles (top) and conversion profiles (bottom) at the end of the endothermic semicycle (t = tcyc/2) and the regeneration...

Fig. 1.9. Coupling of dehydrogenation of ethylbenzene to styrene and hydrogen combustion in a catalytic fixed-bed reverse flow reactor [9]. (a, b) Fixed-bed temperature profiles during production and regeneration cycle.

Fig. 1.13. Schematic picture of the evolution of temperature profiles in a reverse-flow reactor with distributed side feed during production (left) and regeneration cycles (right) [25].

A simple tandem reverse-flow reactor scheme has been proposed for this purpose [11] (Fig. 7.5). By condensing the sulfur formed in the reactor outlet and reheating the residual anhydrous inert gas stream, one obtains a thermally efficient integration of the elutive adsorbent regeneration into the reactor operation. The arrangement depicted represents an adsorptive equivalent to the reverse-flow reactor with removal of a hot side-stream [6]. 

As outlined above, supramolecular binding offers new possibilities in this regard. Solids functionalized with a single acceptor motif can be used in more than one application, and the effective cost of the synthesis of the support is reduced. After (partial) catalyst decomposition, the catalyst can be removed easily, and the support can be reused and the catalyst regenerated. Leaching of immobilized catalysts remains the key problem, even without decomposition the leached catalyst can be handled by applying reverse-flow techniques in an "oversized bed. However, no applications of this approach have been reported, but it can be improved. 

A full discussion of these factors has been given by Cartei who has also presented a numerical simulation of the regeneration of an adiabatic air drier showing clearly the advantage of reverse flow. In Carter s simulations the initial moisture profile through the bed was taken as the profile at the end of the adsorption cycle, which was terminated just prior to breakthrough. The... 

FIGURE U.4. Simulated desorption curves for regeneration of a partially saturated bed (HjO-AI2O3) in forward flow (--) and with reverse flow (—). (a) Effluent concentration, (b) effluent temperature, and (c) rale of removal of moisture. (After Carter. )... 

Since FCC catalyst is kept above minimum fluidization conditions everywhere in this catalyst circulation loop, the fluidized catalyst is free to flow from one place to another. Thus the catalyst circulation is driven by the overall pressure balance of the unit, and the circulation rate is regulated by the two slide valves, i.e., the stripper slide valve and the regenerator slide valve. A minimum pressure drop across each slide valve is set in the control system to guard against flow reversal, which is a very serious safety issue. For instance, a reverse flow of hydrocarbon vapor from the reactor to the oxygen-rich regenerator can lead to a sudden increase in combustion reaction and regenerator temperature. In the extreme case, a catastrophic explosion could occur. The overall pressure balance of the unit determines the pressure drops available for the slide valve control and hence the maximum catalyst circulation rate. 

In addition to entering the RPE from the circulation, all-lrany-retinol passes into the RPE cells through their apical surfaces during bleaching of ihodopsin in the ROS. This phase of the visual cycle is better characterized than the reverse step, namely the reverse flow of retinoid that culminates in visual pigment regeneration. 

The design should be such that it is possible to carry out by remote control reverse flow flushing, washing, regeneration and change of resins. 

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