pH Modification Pilot Plant Trials

Abstract

This work was undertaken for the purpose of designing, testing and optimising a small scale pilot plant process control system for acid dosing of geothermal fluid to mitigate silica deposition in fluid oversaturated with respect to amorphous silica solubility. The pilot plant was based on an approach used for automated laboratory titration. Three acid injection points were included, each upstream of a static mixer. Automated pH measurement was provided at ambient temperature with rapid sampling and cooling to provide pH feedback to the process control.

In the pilot plant the first acid injection was 98% sulfuric acid in sufficient volume to shift the pH to a “set point” near the shoulder of the titration curve. This injection rate was determined by fluid flow rate and a pH trim and controlled through a PLC. The successive second and third acid injections downstream were 20% sulfuric and controlled to a pH set point only (no flow component).

The acid dosing methodology was shown to be highly effective in maintaining the target pH with excellent pH stability over days of operation as well as reasonably quick recovery to target pH conditions after large changes in flow rate.