Cascade compositions

We want to compare tray temperature control with two types of composition control. In both, the composition of the distillate propane product is measured directly and controlled at 2 mol% isobutane impurity. The first type is direct composition control in which a single PI controller is used with reboiler heat input manipulated. The second type uses a cascade composition-to-temperature control structure. 

Runs are made to compare the dynamic and steady-state performance of the two alternative control structures (temperature control and cascade composition/temperature control) with the R/F and QR/F ratio installed. The column is subjected to disturbances in feed flow rate and then feed composition. 

Of course, this problem of steady-state shift in product purity for feed-composition changes could be solved by using a cascade composition/temperature-control structure. Keep in mind, however, that the RR would have to be fixed at the highest value needed to handle the range of feed compositions. 

The two-column system provides stable operation but does not maintain tight control of product composition for some feed composition disturbances. A cascade composition/ temperature structure may be required. 

More than 7.5 MW could be added from a hot utility to the first interval, but the objective is to find the minimum hot and cold utility. Thus from Fig. 6.186, QHmin = 7.5MW and Qcmm = 10MW. This corresponds with the values obtained from the composite curves in Fig. 6.5a. One further important piece of information can be deduced from the cascade in Fig. 6.186. The point where the heat flow goes to zero at T = 145°C corresponds to the pinch. Thus the actual hot and cold stream pinch temperatures are 150 and 140°C. Again, this agrees with the result from the composite curves in Fig. 6.5a. 

The initial setting for the heat cascade in Fig. 6.18a corresponds to the shifted composite curve setting in Fig. 6.15a where there is an overlap. The setting of the heat cascade for zero or positive heat flows in Fig. 6.186 corresponds to the shifted composite curve setting in Fig. 6.156. 

The grand composite curve is obtained by plotting the problem table cascade. A typical grand composite curve is shown in Fig. 6.24. It shows the heat flow through the process against temperature. It should be noted that the temperature plotted here is shifted temperature T and not actual temperature. Hot streams are represented ATn,in/2 colder and cold streams AT iJ2 hotter than they are in practice. Thus an allowance for ATj in is built into the construction. 

Example 6.3 The problem table cascade for the process in Fig. 6.2 is given in Fig. 6.18. Using the grand composite curve ... 

Figure 6.30 shows the grand composite curve plotted from the problem table cascade in Fig. 6.186. The starting point for the flue gas is an actual temperature of 1800 C, which corresponds to a shifl ed temperature of (1800 — 25) = mS C on the grand composite curve. The flue gas profile is not restricted above the pinch and can be cooled to pinch temperature corresponding to a shifted temperature of 145 C before venting to the atmosphere. The actual stack temperature is thus 145 + 25= 170°C. This is just above the acid dew point of 160 C. Now calculate the fuel consumption ...

Figure 16.19 shows the grand composite curve plotted from the problem table cascade. The two levels of steam generation are shown. 

Both control schemes react in a similar manner to disturbances in process fluid feed rate, feed temperature, feed composition, fuel gas heating value, etc. In fact, if the secondary controller is not properly tuned, the cascade control strategy can actually worsen control performance. Therefore, the key to an effective cascade control strategy is the proper selection of the secondary controlled variable considering the source and impact of particular disturbances and the associated process dynamics. 

FIG. 8-36 Block diagram of the cascade co composition disturbance, while L2 would be a change in the cooling water temperature. 

Figure 3.37. Flaw and composition inputs to stage n of the cascade.

Comparing the composite curve, Figure 3.22, with Figure 3.237 shows that the heat introduced to the cascade is the minimum hot utility requirement and the heat removed at the bottom is the minimum cold utility required. The pinch occurs in Figure 3.23b where the heat flow in the cascade is zero. This is as would be expected from the rule that for minimum utility requirements no heat flows across the pinch. In Figure 3.23b the pinch temperatures are 80 and 90°C, as was found using the composite stream curves. 

Figure 5.22. (e) Batch distillation, reflux flow cascaded with temperature to maintain constant top composition... 

---