CS7-RR Structure

However, note that the product purities remain fairly close to the desired 95% level, even for the +5% change in vapor boilup. This suggests that the CS7-RR structure should perform better than the CS7-R stmcture if the dynamics can be improved. The next section demonstrates that this can be achieved by using a suboplimal steady-state design that has more reactive trays. 

Figure 10.11 shows the effectiveness of this modified CS7-RR structure. The disturbances are 10% increases and decreases in vapor boilup. The system is quite stable and achieves a new steady state in about 2 h. Product purities remain close to 95%. 

There are two reasons for the improvement in control. The first has already been discussed the elimination of inverse response. The second is equally important the two temperature controllers in the CS7-RR structure have the same action (direct). This means that when both control loops see a positive vapor boilup disturbance, which increases both tray temperatures, the two controllers will increase both fresh feeds. This helps to maintain the delicate stoichiometric balance between the reactants that is essential for neat operation of a reactive distillation column. Because a reactive distillation column acts like a pure integrator with respect to the reactants, this similar initial response is very important for CS7-RR, where feedstreams are used as manipulated variables. 

Thus far in this chapter, the holdup per tray is kept constant at 1000 mol when the number of reactive trays is increased in an attempt to improve the dynamics of the CS7-RR structure. This means that the suboptimal design (5/10/5) has a total of 10,000 mol of reactive holdup and the optimal design (5/7/5) has a total of 7000 mol. The question that arises is what if the total holdup of the optimal design is kept the same and just distributed over a larger number of trays ... 

There are two altcrrtalives for this control stracture a constant reflux flowrate and a constant reflux ratio. The left system in Figure 10.4 shows the control structure (CS7-R) where the reflux flowrate is fixed and the reflux drum level is controlled by manipulating the distillate flowrate. The right system in Figure 10.4 shows the alternative version (CS7-RR) where the reflux drum level is controlled by the reflux flowrate and the distillate flowrate is adjusted to give a constant RR. 

Looking at the Ui parameter in Figure 10.5, the SVD analysis suggests that tray 4 is the most sensitive temperature measurement for both control stractures. The temperature of this tray could be controlled by manipulating the fresh feedstream FoA > which is fed at the bottom of reactive zone. Looking at the U2 parameter in Figirre 10.5, different trays should be selected for the two control structures. For die CS7-R stmeture, tray 13 is the most sensitive and it is tray 10 for the CS7-RR sfructure. 

Because the dynamics of the 5/10/5 and 5/13/5 designs are similar, only the results of the case with 10 reactive trays will be discussed. We found that adding more reactive trays did not solve the problem of a drop in product purities for the CS7-R structure. Thus, we consider only the CS7-RR stmcture. 

The hrst type of disturbance studied is a range of plus or minus step changes in vapor boilup (the production rate handle). Results show that very large positive changes of up to +50% can be handled by the CS7-RR control structure. The purities of both products are maintained within 1% of the desired 95% specihcation. 

Figure 10.14 shows the operability regions for designs using control structure CS7-RR with 10 and 13 reactive trays. The borders shown in the figure are set by either system shutdowns or a minimum purity of either product that drops below 94%. The upper boundaries and the left-hand boundaries shown in Figure 10.14 are all set by shutdowns, and the lower boundaries are set by low product purities. 

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