Mixing feed point location

In circulating fluidized bed combustors, the volatile release and combustion are important with regard to pollutants emissions, heat release patterns and the subsequent reactivity of the char particles. For coals with high volatiles content, the release of volatiles is important in the selection of the feed point location, since the time constant for volatiles release and combustion is much shorter than the char burnout time. In addition, solid mixing and rate of devolatilization determine the region of volatile release. 

This means that the aggregate size distribution mirrors the turbulence spectrum of the reactor. Variables that determine mixing are reactor design, number and design of baffles, impeller design, power input, feed concentration, feed rate, location and number of inlet tubes, and so on. The position of the inlet tube(s) and the conditions near these feed points are also important and generally the solutions should be introduced close to the agitator [23, 24]. 

If two or more feed streams with difierent compositions are to be separated in the column, they should not be mixed and in at a single feed point. Instead they should be fed on separate feed trays at locations where tray compositions approximate feed compositions. 

Thus, the reactor will be perfectly mixed if and only if = at every spatial location in the reactor. As noted earlier, unless we conduct a DNS, we will not compute the instantaneous mixture fraction in the CFD simulation. Instead, if we use a RANS model, we will compute the ensemble- or Reynolds-average mixture fraction, denoted by ( ). Thus, the first state variable needed to describe macromixing in this system is ( ). If the system is perfectly macromixed, ( ) = < at every point in the reactor. The second state variable will be used to describe the degree of local micromixing, and is the mixture-fraction variance (maximum value of the variance at any point in the reactor is ( )(1 — ( )), and varies from zero in the feed streams to a maximum of 1/4 when ( ) = 1/2. 

This nomenclature is shown with Example 14.3. On the triangular diagram, the proportions of feed and solvent locate the mix point... 

Either the solvent feed ratio or the compositions Ej and RN serve to locate the mix point M. 

Both maximum and minimum limits exist of the solvent/feed ratio. The maximum is the value that locates the mix point M on the binodal curve near the solvent vertex, such as point Mmla on Figure 14.7(b). When an operating line coincides with a tieline, the number of stages will be infinite and will correspond to the minimum solvent/feed ratio. The pinch point is determined by the intersection of some tieline with line RnS- Depending on whether the slopes of the tielines are negative or positive, the intersection that is closest or farthest from the solvent vertex locates the operating point for minimum solvent. Figure 14.9 shows the two... 

The mix point is located by establishing the solvent/feed ratio. 

Figure 10-7 shows the concentrations of R and S and the product distribution Xs as a function of time for the feed location just above the impeller. The values are normalized with respect to the final values. R and S increase steadily with time. Xs increases at first, reaching a local maximum just before the species are mixed by the impeller. The improved quality of the mixture favors the first reaction and Xs decreases until it reaches a local minimum. At this point there is enough R present to allow the second reaction to occur even in relatively well mixed regions, and Xs increases again until it asymptotically reaches a final value. Figures 10-8a through 10-8h show the local concentrations of species A, R, and S as a function of time for the 600-1 tank at 100 rpm. 

Both maximum and minimum limits exist of the solvent/feed ratio. The maximum is the value that locates the mix point M on the binodal curve near the solvent vertex, such as point on Figure... 

---