How it works
- why it is hard — pH is a logarithm of concentration. Between pH 2 and 3 you are neutralising ninety per cent of the acid; between 6 and 7 you are neutralising almost nothing. The same dose has wildly different effect depending on where you are on the curve.
- the vertical bit — Near neutrality the titration curve is close to vertical. A dose that is a few per cent too large takes you from pH 6 straight past 7 to pH 10. The control valve then reverses, and the plant oscillates around the setpoint forever.
- staging — The answer is to split the job. The first stage does the coarse work — say pH 2.5 to 5 — where the curve is flat and forgiving. The second trims 5 to 7 with a much smaller reagent valve. Each stage then operates on a manageable part of the curve.
- residence time — The probe cannot read what has not mixed yet. Give each stage at least five minutes and enough agitation to be well mixed, or the controller is reacting to a measurement that does not represent the tank.
- what governs it — Reagent quantity is simple stoichiometry and rarely the problem. Controllability is the problem — pull the stages down to one with a wide pH swing and watch the loop become impossible, no matter how good the valve is.
Design parameters
The panel opposite runs the same correlations as the T-1100 design sheet, so the animation and the calculator cannot disagree. Drag any of them and the picture responds.
- Flow — m³/h
- Influent pH — –
- Target pH — –
- Tank volume — m³
- Reagent strength — % NaOH
- Stages — off
Open the full T-1100 design sheet for the governing equations, the accepted design envelopes and the worked calculation.