306

0-51; 50-100

^Signifies the approximate pH swing that valves will accommodate.

^Signifies the approximate pH swing that valves will accommodate.

REACTION TANK ATTENUATION TANK

FIG. 7.40.22 Two-sided feedback control of pH.

REACTION TANK 1 ATTENUATION TANK 2

FIG. 7.40.24 Cascade control of pH.

REACTION TANK ATTENUATION TANK

FIG. 7.40.22 Two-sided feedback control of pH.

because the materials are still reacting with each other after they have left the first tank. If the setpoint pHC-2 is too high, the pH of the final stream will be greater than desired. When flocculation is to be carried out downstream of the pH treatment facility, stable pH values can be extremely important.

A delicate balance must be struck in this type of system with respect to the size of the first vessel. A long residence time in the first tank ensures long contact time between reagents, thereby producing an effluent pH which is close to the desired value, but at the same time it may result in a sluggish control loop around this vessel. For efficient cascade control, response of the inner loop (control loop around the first tank) must be fast. The other control loop (pHC-2), sometimes referred to as the master, or primary, control loop, is usually tuned (control mode adjustments such as proportional band are set) so as to be less responsive than the inner loop. The tuning of pHC-1 will be a result of the dead time (a delay between a change

REACTION TANK 1 ATTENUATION TANK 2

FIG. 7.40.24 Cascade control of pH.

in reagent flow and the time when its effect is first felt), capacity, and process characteristics.

When this part of the process is dominated by dead time, the technique of sample data control may be useful in stabilizing the control system by a sample and hold device (Figure 7.40.24). This device may be a timer that automatically switches the controller between automatic and manual modes of operation. This can allow the controller to be in automatic for a fraction (x) of the cycle time (t)

and then can switch it to a fixed-output, manual condition for the rest of the cycle (1 - x)t.

In the second form of cascade (cascaded residences) (Figure 7.40.25), each vessel has its own feedback loop. This approach is recommended when the incoming material is very strongly acidic or basic (pH values less than 1 or greater than 13). The first stage controls the effluent at a pH of approximately 4 or 10, and the second stage brings the effluent to its final value, near 7. The choice of pH setpoint for the first stage depends on the characteristics of the material. For example, material having a titration characteristic like that of A in Figure 7.40.9 may have a setpoint of approximately 3.5. The purpose is to make the control problem as simple as possible by staying on as linear a portion of the titration curve as possible.

This approach is logical when one considers the process gain characteristic as well as the accuracy limitation of a reagent delivery system. The remainder of the neutralization control problem is then similar to that illustrated in part B of Figure 7.40.9. In this manner, the control system does not have to cope with the entire nonlinear characteristic of the process all at once. A sequenced pair of valves is shown in conjunction with the first stage in order to handle the pH load variations. A single valve would probably suffice for the second stage, because its influent is pH-controlled. Depending on the valve sizes and on the individual valve rangeabilities, this three-valve arrangement has a maximum possible reagent flow turndown of approximately 125,000:1 (50 X 50 X 50).

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