Co-current operation

Figure 1 Liquid Residence time distribution comparison - Experimental vs Numerical (a) counter-current operation Liquid flowrate l.SLmm (b) counter-current operation Liquid flowrate 3Lmin (c) co-current operation Liquid flowrate 1.5Lmm (d) co-current operation Liquid flowrate 3Lmm ...

Figure 2 Gas Residence time distribution comparison - Experimental vs Numerical (a) counter-current operation (b) co-current operation.

Mole balance expressions were developed for a general series reaction by Agarwalla and Lund [16], and the same procedures were used here to develop the species balance equations shown in Table I. Boundary conditions and parameter definitions are presented in Tables II and III. Note that the boundary conditions are given only for co-current flow of reactants and inert, which is the only configuration studied. Previous work [16], has shown that counter-current operation is less effective than co-current operation. 

Economic studies had indicated that the cost of pipeline gas from oil shale was promising however, there were two major disadvantages in co-current operation ... 

Co-current Operation of Combined Meso-scale Heat Exchangers and Reactors for Methanol Steam Reforming... 

The test results with the ultrasonic nozzle were obtained with an estimated steam to copper (S/Cu) ratio of 23 and the humidified Ar was injected co-currently with the CuCl2 solution. Several variables remain to be investigated, i.e. lower S/Cu ratios, counter-current instead of co-current operation, and subatmospheric pressures. LeChatelier s Principle predicts that reducing the pressure in the hydrolysis reactor should reduce the S/Cu ratio. The effect of a reduced pressure was quantified by the results of a sensitivity study using Aspen. Aspen predicts that a S/Cu ratio of 17 is needed for essentially complete conversion at 375°C and atmospheric pressure while a S/Cu ratio of 13 is required at 0.5 bar. The conceptual process design specifies that the hydrolysis reactor be run at 0.25 bar. The pressure drop in the reactor is achieved by adding a low temperature steam ejector after the condenser at the exit of the hydrolysis reactor in the conceptual design. 

First, the effects of gas and liquid flows, co-current versus countercurrent operation, pressure and temperature were checked. As expected, based on the influence of these parameters on the vapor pressure or the vapor-liquid equilibrium (VLE), the fraction of water stripped by the nitrogen increases with increasing gas flows, decreasing liquid flows, lower pressures and higher temperatures. Countercurrent operation is more efficient than co-current operation, because the liquid phase at the inlet was already enriched with the compound which was to be separated. 

Consider first the co-current operation. Let Vj be the superficial velocity of the flowing water relative to the stationary earth and V, be the velocity of the bed also relative to the stationary earth. Thus, the relative velocity of the flowing water V, relative to the bed is... 

Many workers (51-56) have considered the design of counter-current packed columns. The equipment can effectively be used in co-current operation since there is no disadvantage due to driving force as in the case of physical absorbers (57). Figure 7 shows schematically such a column and Table 3 gives the simplifying assumptions of Juvekar and Sharma (50). 

In Section 12.9 we noted that co-current operation of absorbers was often enployed when a single equilibrium stage was sufficient. Co-current operation has the advantage that flooding cannot occur. This means that high vapor and liquid flow rates can be used, which automatically leads to small-diameter columns. 

Hqq is related to the individual coefficients by Eq. (16-27a). Unfortunately, it is dangerous to use Eqs. (16-37) and (16-38) to determine the values for Hl and Hq for co-current columns because the correlations are based on data in countercurrent columns at lower gas rates than those used in co-current columns. Reiss (1967) reviews co-current contactor data and notes that the mass transfer coefficients can be considerably higher than in countercurrent systems. Gianetto et al. (1973) operated with a 15-fold velocity increase and observed a 40-fold increase in Icl when liquid-phase resistance controlled. They recommended co-current operation for absorption with chemical reaction. Harmen and Perona (1972) did an economic conparison of co-current and countercurrent columns. For the absorption of CO2 in carbonate solutions where the reaction is slow they concluded that countercurrent operation is more economical. For CO2 absorption in monoethanolamine (MEA), where the reaction is fast, they concluded that countercurrent is better at low liquid fluxes whereas co-current was preferable at high liquid fluxes. 

Summarising the calculations of Frauhammer et al., a counter-current concept of coupling steam reforming with combustion reactions leads to significant temperature gradients and consequenfly thermal stress within the reactor and is not a suitable solution. Rather, co-current operation would be preferred, as will be discussed below. 

Variable fluid tempoature T with co-current operation... 

Demonstrating the conservation equations we will focus on counter current flow since it is of greater importance compared to co-current flow. The derivation of co-current operation is obtained analogously. A differential description of the conservation relations is required to approximate the module performance. The streams necessary to derive the balances are illustrated in Figure 5.15. The list of symbols provides an overview over the abbreviations used in the following section. 

---