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Using carbon dioxide for enhanced coagulation

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Process control
CO2 Feed System
CO2 feed system with motive stream booster pumping, feed control panel and dissolution skid.

Control for the CO2 feed system can be manual or automated. Manual process control may be limited to a manual flow control valve, rotameter, and a solenoid valve that is energized when flow is observed in the process stream. Automated process control may be performed based on CO2 feed rate or pH set point control. For dosage based control, a programmable logic controller (PLC), process stream and gas flow meters with analog outputs, and gas modulating flow control valve are required.

Based on a calculation programmed into the PLC and operator input for CO2 dosage rate (mg/L), the flow control valve modulates to set the gas feed rate. The operator can make adjustments to the dosage rate, based on the observed pH measurement taken from a point downstream of injection.

Process control can be further automated by adjusting the CO2 gas feed rate to meet a target pH level. In this case, the PLC utilizes feedback from a pH analyzer downstream of the CO2 injection point to control the position of the modulating gas flow control valve. The dosage rate is automatically adjusted until the pH reaches the target set point.

Conclusions

Carbon dioxide can be a viable alternative to acid for pH suppression in the enhanced coagulation process. A CO2 feed system can provide high-level process control while using a product that is safer and easier to handle. Many challenges posed by using mineral acids for pH suppression can be mitigated. There are negligible differences in overall capital and operational costs.

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Andrew Kleisinger, P.Eng., and M. Jason Stusick, P.Eng., are with MPE Engineering. This article appears in ES&E Magazine’s February 2016 issue.

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