Ask Greg McMillan

Greg's Response:

For a reagent dose to be precise enough, the reagent’s logarithmic acid dissociation constant (pKa) should be close to the pH set point so that it tends to flatten the titration curve. The reagent viscosity should be low enough to ensure a fully turbulent flow which is important for consistent dosing. The reagent should be free from solids and slime because of the tiny control valve trims used. A fast reagent delivery requires that the reagent viscosity be low enough for the dose to start quickly and mix rapidly with the effluent. An extraordinary time delay has been observed for starting the flow of 98% sulfuric acid through an injection orifice because of its high viscosity. It has been compared to getting ketchup out of a bottle. Mixing an influent stream with a viscous reagent stream, such as 98% sulfuric acid or 50% sodium hydroxide that is about 40 times more viscous than the typical influent stream, is difficult and requires greater agitation intensity and velocity. A highly viscous reagent dose tends to travel as a glob through the mixture. Lastly, the reagent should be in the liquid phase.

The neutralization reaction of liquid components is essentially instantaneous once reagent streams are mixed with influent streams. Gas or solid reagents take seconds and minutes, respectively, to dissolve and get to actual liquid contact and mixing with the influent stream components. Reagent bubbles escape as a vapor flow when the bubble breakup time and gas dissolution time exceed the bubble rise time. Coating and plugging upstream control valves and downstream equipment from particles of unreacted reagent or precipitation of salts from reacted reagents can be so severe as to cause excessive equipment maintenance and downtime. Liquid reagents such as ammonia can choke the control valve from flashing in vena contracta or cause cavitation damage in the valve and the piping immediately downstream. Particles in the reagent can erode the valve trim. Some waste lime systems have rocks that can quickly tear up a valve seat and plug.

Dynamic simulations with representative physical properties and phases are needed to determine the best reagent.

For much more knowledge, see the ISA book Advanced pH Measurement and Control Fourth Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).