Countercurrent Fixed-Bed

The conditions in countercurrent fixed beds have been investigated for many years in order to improve the understanding of two-phase flow and to develop reliable design methods. However the proposed correlations available for fluid dynamics and mass transfer are practically all based on experimental data obtained at atmospheric pressure. Extrapolation of the results to high pressure is questionable and not recommended. Moreover the results of systematic investigations in the high-pressure range are scarce in the open literature. 

A major conclusion from these relatively few studies is, that it is necessary to promote the investigation of the hydrodynamics of countercurrent fixed beds and the physical properties of... 

The lack of gas-side mass-transfer data at elevated pressure for countercurrent fixed beds packed with large and small solid particles should be underlined. 

The countercurrent fixed-bed reactor is obviously self-adaptive in that it establishes the necessary temperature level for the respective reaction, whereas in the standard type the temperature level is directly proportional to the slope of the temperature front in the heat exchanger section, dT/dz, which in turn is proportional to the adiabatic temperature rise Taj. 

It is interesting to note that in the limit of rapid flow reversal the reverse flow reactor and the countercurrent fixed-bed reactor show completely similar temperature and conversion profiles [44]. This can be understood with the help of Fig. 25 With rapid flow reversal the catalyst temperature will remain constant due to the large heat capacity of the packing while the gas temperature will be below the catalyst temperature in the respective feed section and above in the exit section (Fig. 25B). This behavior is completely similar to that of a countercurrent fixed-bed reactor, where the catalyst is placed at the separating walls between the up and down flowing gas (Fig. 25C). It only has to be considered that instead of pushing the reacting gas for a short period in one and for another period in the other direction, now half of the mass flow will go per-... 

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

Continuous Countercurrent Systems Most adsorption systems use fixed-bed adsorbers. However, if the fluid to be separated and that used for desorption can be countercurrently contacted by a moving bed of the adsorbent, there are significant efficiencies to be realized. Because the adsorbent leaves the adsorption section essentially in equilibrium with the feed composition, the inefficiency of the... 

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

Focusing discussions on carbon adsorption processes, in a pulsed-bed adsorber, the carbon moves countercurrent to the liquid. The effeet is of a number of staeked, fixed-bed eolumns operating in series. Spent earbon is removed from the bottom of... 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

In the SMB operation, the countercurrent motion of fluid and solid is simulated with a discrete jump of injection and collection points in the same direction of the fluid phase. The SMB system is then a set of identical fixed-bed columns, connected in series. The transient SMB model equations are summarized below, with initial and boundary conditions, and the necessary mass balances at the nodes between each column. 

In fact, it is extremely difficult to operate a TMB because it involves circulation of a solid adsorbent. Thus, the concept must be implemented in a different way where the benefit of a true countercurrent operation can be achieved by using several fixed-bed columns in series with an appropriate shift of the injection and collection points between the columns. This is the SMB implementation as presented in Fig. 10.2. 

In the first class, the particles form a fixed bed, and the fluid phases may be in either cocurrent or countercurrent flow. Two different flow patterns are of interest, trickle flow and bubble flow. In trickle-flow reactors, the liquid flows as a film over the particle surface, and the gas forms a continuous phase. In bubble-flow reactors, the liquid holdup is higher, and the gas forms a discontinuous, bubbling phase. 

Laboratory reactor for studying three-phase processes can be divided in reactors with mobile and immobile catalyst particles. Bubble (suspension) column reactors, mechanically stirred tank reactors, ebullated-bed reactors and gas-lift reactors belong the class of reactors with mobile catalyst particles. Fixed-bed reactors with cocurrent (trickle-bed reactor and bubble columns, see Figs. 5.4-7 and 5.4-8 in Section 5.4.1) or countercurrent (packed column, see Fig. 5.4-8) flow of phases are reactors with immobile catalyst particles. A mobile catalyst is usually of the form of finely powdered particles, while coarser catalysts are studied when placing them in a fixed place (possibly moving as in mechanically agitated basket-type reactors). 

Fixed-bed reactors are used for testing commercial catalysts of larger particle sizes and to collect data for scale-up (validation of mathematical models, studying the influence of transport processes on overall reactor performance, etc.). Catalyst particles with a size ranging from 1 to 10 mm are tested using reactors of 20 to 100 mm ID. The reactor diameter can be decreased if the catalyst is diluted by fine inert particles the ratio of the reactor diameter to the size of catalyst particles then can be decreased to 3 1 (instead of the 10 to 20 recommended for fixed-bed catalytic reactors). This leads to a lower consumption of reactants. Very important for proper operation of fixed-bed reactors, both in cocurrent and countercurrent mode, is a uniform distribution of both phases over the entire cross-section of the reactor. If this is not the case, reactor performance will be significantly falsified by flow maldistribution. 

Fixed-bed systems are the most common, but some countercurrent fluidized beds are in use. Flow diagrams are given in reference 47. The superficial velocities of gases in fixed beds should be about 1 ft/sec (0.3 m/sec) and those for liquids about 1 ft/min (0.3 m/min).48 See references 48 and 49 for more design information. 

With fixed-bed updraft gasifiers, the air or oxygen passes upward through a hot reactive zone near the bottom of the gasifier in a direction countercurrent to the flow of solid material. Exothermic reactions between air/oxygen and the... 

Activated carbon adsorption may be accomplished by batch, column, or fluid-ized-bed operations. The usual contacting systems are fixed-bed or countercurrent moving beds, as shown in Figure 8.2. The fixed beds may employ downflow or upflow of water. The countercurrent moving beds employ upflow of the water and downflow of the carbon, since the carbon can be moved by the force of gravity. Both fixed beds and moving beds may use gravity or pressure flow. 

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