Dosage Of Chemicals Using Proportional Feed Pumps

The slopes of the ORP curves shown in Figures 3 and 4 are steepest at the equivalence point. This creates the potential of overdosing the amount of reactant, which is alleviated by the use of a proportional addition system. For this purpose a millivolt-to-frequency converter is added to the ORP controller. This converter is interfaced with the chemical feed pump and controls the pump stroke frequency linearly between low and high set points as indicated in Figure 5. In this fashion, the chemical feed rate increases linearly with an increase in the ORP deviation from the ORP set point. The feed rate (pump stroke frequency) slows down as the ORP approaches the desired ORP set point. The dosage of the chemicals with proportional feed pumps is especially useful if the pollutant concentrations vary in the feed streams. The pH adjustments are achieved in the same way. In addition, ORP and pH controllers can activate relays at low and high alarm set points. In this fashion, visual or acoustical alarms can be activated and feed pumps can be shut off.

Electrode Potential [mV]

Equivalence Point

400-

300-

100-

-100

Equivalence Point

Electrode Potential [mV]

400-

300-

100-

-100

8 10 12 14

Hypochlorite Solution [ml]

Figure 3 Oxidation-reduction potential (ORP) of a KCN solution. 100 ppm of CN titrated with hypochlorite reagent. Electrodes: Pt/saturated calomel.

8 10 12 14

Hypochlorite Solution [ml]

Figure 3 Oxidation-reduction potential (ORP) of a KCN solution. 100 ppm of CN titrated with hypochlorite reagent. Electrodes: Pt/saturated calomel.

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