Counter-cocurrent Mode

An investigation into the applicability of numerical residence time distribution was carried out on a pilot-scale annular bubble column reactor. Validation of the results was determined experimentally with a good degree of correlation. The liquid phase showed to be heavily dependent on the liquid flow, as expected, but also with the direction of travel. Significantly larger man residence times were observed in the cocurrent flow mode, with the counter-current mode exhibiting more chaimeling within the system, which appears to be contributed to by the gas phase. 

As indicated by this schematic representation, different configurations vary with respect to the degree of coupling between the process streams. The strongest coupling occurs in the simultaneous mode, where chemical and thermal interaction occurs between the process streams. In contrast to this, the counter-cocurrent concept features only thermal interaction between the process streams localized in the reactor stage. 

This special case in Equation 3.28 is valid for any system, regardless of the phase equilibrium properties of the system. The significance of = — 1 is that this represents the point where the vapor flowrate in the CS tends to zero, and can be seen as the switching point from a counter-current mode of operation to a cocurrent operation mode. The only pinch point at these conditions is thus at Xa and profiles will resemble the mixing only process shown in Figure 3.12b. More importantly, though, this shows that any pinch point loci will always move through Xa, irrespective of phase equilibrium. 

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... 

Gallucd, F. and Basile, A. (2006) Cocurrent and counter-current modes for methanol steam reforming membrane reactor. Int. J. Hydrogen Energ., 31, 2243-2249. 

To establish the validity of the numerical scalar technique for RTD analysis, the normalized exit age distribution curve of both counter-current (Figure 1 (a-b)) and cocurrent (Figure 1 (c-d)) flow modes were compared. Table 1 shows that a good agreement was obtained between CFD simulation and experimental data. 

Tubular reactors are commonly used in laboratory, pilot plant, and commercial-scale operations. Because of their versatility, they are used for heterogeneous reactions as well as homogeneous reactions. They can be run with cocurrent or counter-current flow patterns. They can be run in isothermal or adiabatic modes and can be used alone, in series, or in parallel. Tubular reactors can be empty, packed with inert materials for mixing, or packed with catalyst for improved reactions. It is often the process that will dictate the design of the reactor, as discussed in this entry. 

An often used gas-liquid reactor is the bubble column. The gas is usually fed from the bottom through a sparger and the liquid flows either cocurrently or counter-currently. Counter-current operation is more efficient than co-current, but for certain types of parallel reactions, cocurrent operation can give better selectivity. Bubble columns are often operated in semi-batch mode the gas bubbles through the liquid. This mode of operation is attractive in the production of fine chemicals which are produced in small quantities - especially in the case of slow reactions. The flow patterns can vary a lot in a bubble column. Generally, as a rule of thumb, the liquid phase is more back-mixed than the gas phase. The plug flow model is suitable for the gas phase whereas the liquid phase can be modelled with the backmixed, dispersion, or plug flow model. 

A heat exchanger can be operated in co-, counter-, or cross-current mode. In practice the countercurrent mode is used almost exclusively since it is the most effective. Cocurrent heat exchangers are used only under special conditions (thermally labile... 

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