Countercurrent flow reactors advantages

Hasselt BWv, Lebens PJM, Calis HP, Kapteijn F, Sie ST, Moulijn JA, Bleek CMvd. A numerical comparison of alternative three-phase reactors with a conventional trickle-bed reactor. The advantages of countercurrent flow for hydrodesulfurization. Chem Eng Sci 1999 54 4791-4799. 

Falling-film reactors have a liquid reactant flowing down the walls of a tube with a gaseous reactant flowing up or down (usually countercurrent). This reactor is particularly advantageous when the heat of reaction is high. The reaction surface area is minimal, and the total reaction heat generated can be controlled. 

The second issue for cooled tubular reactors is how to introduce the coolant. One option is to provide a large flowrate of nearly constant temperature, as in a recirculation loop for a jacketed CSTR. Another option is to use a moderate coolant flowrate in countercurrent operation as in a regular heat exchanger. A third choice is to introduce the coolant cocurrently with the reacting fluids (Borio et al., 1989). This option has some definite benefits for control as shown by Bucala et al. (1992). One of the reasons cocurrent flow is advantageous is that it does not introduce thermal feedback through the coolant. It is always good to avoid positive feedback since it creates nonmonotonic exit temperature responses and the possibility for open-loop unstable steady states. 

New reactor types for three-phase operation are still being developed. An example is the application of structured reactors, which may have certain advantages in three-phase operation, and can be operated with co-, cross- as well as with countercurrent flow. A very recent development is the use of monolith reactors (Fig. 8.9) in three-phase operations. Their advantages are the small pressure drop, the good external mass transfer, the short diffusion distance, and the low adiabatic temperature rise. Disadvantages are the higher catalyst costs, importance of liquid distribution, and moderate catalyst load. 

A third method of operation is to use counterflow of coolant and reactant, which results in a profile of the type shown in Figure 5.19c. By adjusting the coolant rate, the peak reactor temperature can be kept about the same, but the final reactor temperature is close to the coolant inlet temperature. The low concentration and temperature make the final reaction rate quite low, and a larger reactor would be needed to reach the same conversion. Unlike normal fluid-fluid heat exchange, there is no inherent advantage, and perhaps a penalty for countercurrent operation. Flowever, for practical reasons, countercurrent flow is often used with upflow in the shell and downflow in the tubes to prevent fluidization. 

The intrinsic complexity of three phase systems creates some difficulties in the scale-up and in the prediction of performances of three phase reactors. But this complexity is also often a serious advantage, as the simultaneous occurrence of three phases offers such a large number of design possibilities that almost all technical and chemical problems (heat removal, temperature control, selectivity of the catalyst, deactivation, reactants ratio etc..,) can be solved by a proper choice of the equipment and of the operating conditions. For example, countercurrent flow of gas and liquid can be used to overcome thermodynamic limitations and solvent effects can be used to improve selectivity and resistance to poisoning of the catalyst. 

Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

In the design of optimal catalytic gas-Hquid reactors, hydrodynamics deserves special attention. Different flow regimes have been observed in co- and countercurrent operation. Segmented flow (often referred to as Taylor flow) with the gas bubbles having a diameter close to the tube diameter appeared to be the most advantageous as far as mass transfer and residence time distribution (RTD) is concerned. Many reviews on three-phase monolithic processes have been pubhshed [37-40]. 

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... 

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