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NEQ Cell Model

An issue that is not adequately addressed by most models (EQ and NEQ) is that of vapor and liquid flow patterns on distillation trays or maldistribution in packed columns. Since reaction rates and chemical equilibrium constants are dependent on the local concentrations and temperature, they may vary along the flow path of liquid on a tray, or from side to side of a packed column. For such systems the residence time distribution could be very important, as well as a proper description of mass transfer. On distillation trays, vapor will rise more or less in plug flow through a layer of froth. The liquid will flow along the tray more or less in plug flow, with some axial dispersion caused by the vapor jets and bubbles. In packed sections, maldistribution of internal vapor and liquid flows over the cross-sectional area of the column can lead to loss of interfacial area. [Pg.227]

To deal with this shortcoming of earlier non-equilibrium models, both steady state and dynamic NEQ cell models have been developed [12-15]. The distinguishing feature of this model is that stages are divided into a number of contacting cells, as shown in Fig. 9.6. These cells describe just a small section of the tray or packing. [Pg.227]

The unit cell for homogeneous systems (and for heterogeneous systems modeled as though they were homogeneous) is depicted in Fig. 9.8. The equations for each cell are given in Table 9.1. [Pg.228]

A schematic diagram of the unit cell for a vapor-Uquid-porous catalyst system is shown in Fig. 9.9. Each cell is modeled essentially using the NEQ model for heterogeneous systems described above. The bulk fluid phases are assumed to be completely mixed. Mass-transfer resistances are located in films near the vapor-liquid and liquid-solid interfaces, and the Maxwell-Stefan equations are used for calculation of the mass-transfer rates through each film. Thermodynamic equilibrium is assumed only at the vapor-liquid interface. Mass transfer inside the porous catalyst may be described with the dusty fluid model described above. [Pg.228]

Equation type and number Liquid phase Vapor phase [Pg.230]


Table 9.1 Equations describing dynamic NEQ cell model ... Table 9.1 Equations describing dynamic NEQ cell model ...
Column hardware choice can have a significant influence on the conversion and selectivity such aspects can be properly described only by the NEQ cell model, or by a still more sophisticated model based on computational fluid mechanics (such models have yet to be developed). It is insufficiently realized in the literature that, say, for tray RD columns, the tray design can be deliberately chosen to improve conversion and selectivity. Even less appreciated is the fact that the design methodology for RD tray columns is fundamentally different from that of conventional trays. Liquid residence time and residence time distributions are more important in RD. The froth regime is to be preferred to the spray regime for RD applications this is opposite to the design wisdom normally adopted for conventional distillation. For relatively fast reactions, it is essential to properly model intra-particle dif... [Pg.237]


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