Cocurrent mode

Bubble columns and various modifications such as airlift reactors, impinging-jet-reactors, downflow bubble columns are frequently used in lab-scale ozonation experiments. Moderate /qa-values in the range of 0.005-0.01 s l can be achieved in simple bubble columns (Martin et al. 1994 Table 2-4 ). Due to the ease of operation they are mostly operated in a cocurrent mode. Countercurrent mode of operation, up-flow gas and down-flow liquid, has seldom been reported for lab-scale studies, but can easily be achieved by means of applying an internal recycle-flow of the liquid, pumping it from the bottom to the top of the reactor. The advantage is an increased level of the dissolved ozone concentration cL in the reactor (effluent), which is especially important in the case of low contaminant concentrations (c(M)) and/or low reaction rate constants, i. e. typical drinking water applications... 

Countercurrent bubble flow with liquid-supported solids, which can be affected by downward liquid fluidization of particles having a density lower than that of the liquid, has been referred to as inverse three-phase fluidization. The mass transfer potential of such a countercurrent operation is worthy of study, especially for cases in which dispersion of the gas rather than the liquid is called for and the required gas-liquid ratio and throughput can be effected without flooding. In contrast, the corresponding cocurrent mode has received more attention than all other cases and constitutes the majority of the literature on three-phase fluidization. 

From a theoretical point of view, the cocurrent mode of operation is in many cases not the optimal one, and countercurrent operation, in which gas and liquid flow in opposite directions over the fixed catalyst, should be preferable, as will be discussed below. Countercurrent flow of gas and liquid through a random packing of catalyst particles is difficult or hardly obtainable under industrial conditions, however. One possibility for obtaining the desired countercurrent flow is to use fixed catalysts with a well-defined specific geometry instead of a random packing. An example of a reactor with such a structured catalyst is the internally finned monolithic reactor (IFMR), to be discussed below. 

In most applications of trickle-flow reactors, the conversions generate heat that causes a temperature rise of the reactants, since the industrial reactors are generally operated adiabatically. In the cocurrent mode of operation, both the gas and the liquid rise in temperature as they accumulate heat, so there is a significant temperature profile in the axial direction, with the highest temperature at the exit end. When the total adiabatic temperature rise exceeds the allowable temperature span for the reaction, the total catalyst volume is generally split up between several adiabatic beds, with interbed cooling of the reactants. In the countercurrent mode of operation, heat is transported by gas and liquid in both directions, rather than in one direction only, and this may increase the possibility of obtaining a more desirable temperature profile over the reactor. 

Column contactors may be operated in cocurrent, countercurrent or fluidized bed modes of operation. The cocurrent mode means that the regenerant solution flows through the column in the same direction as the feed solution. The countercurrent mode has the regenerant flowing in the opposite direction as the feed solution. 

Another special case arises when R = 0 where the liquid flowrate tends to zero, and also represents a switch from counter-current to cocurrent mode. At these... 

Chamber dryers equipped with stirrers are commonly nsed for drying of flotation concentrates (Figure 51.4). The dryer operates in a cocurrent mode. Hot combustion gases supplied to the dryer by a duct (5) meet at the outlet the cold and moist coal supplied by another duct (7). The dryer consists of a chamber (1) fitted with baffles (9) and two shafts (2) rotating... 

On the left side is shown the development of temperatrrre and humidity in the apparatus for the cocurrent mode, whereas on the right side the countercrrrtent mode is... 

A mathematical formulation for the gas separation by hollow fibers operating in a cocurrent mode was developed by Pan [264], [265]. The assumptions made in his model were... 

Consider the demineralization of a SOOOppm NaCl solution to 400ppm. This is being implemented using a couple of ED membrane stacks in series. The diluate coming out of the ith membrane stack, (Aidu,i)i, becomes the feed brine to the next (i - - l)th membrane stack (Aidu,o)i+i- Each membrane stack is operating in cocurrent mode between the diluate and the concentrate streams. Suppose there are four membrane stacks in series and that the fractional demineralization / for the ith ED membrane stack is constant and equal for each stack (=/). Determine the value of/being achieved in any membrane stack. (Ans. 0.395.)... 

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