PISONICS
PS7020 Series
Ultrasonic Concentration Meter
Sound velocity · inline spool · no moving parts
Chloride-Process Titanium Dioxide — Tail-Gas Scrubber Liquor Monitoring
—— The same chemistry as a chlor-alkali tail-gas tower, in a different industry ——
【Key measurements: residual NaOH wt% / available chlorine】
The Stream
Chlorination and purification in chloride-process TiO₂ production vent chlorine-bearing tail gas, which is scrubbed with caustic. The reaction in the tower is the one a chlor-alkali emergency-chlorine tower runs: Cl₂ + 2 NaOH → NaOCl + NaCl + H₂O. The circulating liquor therefore carries the same three solutes: unreacted caustic, hypochlorite, by-product chloride.
This measuring point is not our invention. Pangang Group Titanium's granted patent CN111678951B, on calculating the caustic concentration of chloride-process TiO₂ tail-gas scrubbing liquor online, states the range of this stream plainly — NaOH 0–15 %, ClO⁻ 0–100 g/L — and back-calculates the two concentrations by regression on conductivity, oxidation-reduction potential and circulation-tank level.
That patent also hands us a useful cross-check. Taking ClO⁻ at 100 g/L and converting to available chlorine as Cl₂ gives about 138 g/L; at the density of this liquor that is roughly 11–12 wt% — which lands on the switch-out point computed from the reaction equation on our chlor-alkali tail-gas page (5 % residual alkali ↔ 11.3 % available chlorine). Two independent sources arriving at the same figure is good evidence that loops like this really do run down one reaction path.
So the Method Carries Over
Same chemistry, same path, so the selection logic transfers whole rather than needing to be argued again:
- Stoichiometry pins the three solutes to one dimension, so a single sound-velocity reading locates how far the reaction has gone;
- residual alkali and available chlorine are one progress number under two names — report either, the conversion is fixed;
- density barely moves along this path, so Coriolis and tuning-fork instruments cannot see the loop change;
- water loss, CO₂ and HCl in the gas all bias the reading high, so the alarm carries margin and the shift titration stays.
Each of those is derived, with magnitudes, on the chlor-alkali tail-gas page and is not repeated here. What follows is only what differs on the TiO₂ side.
Three Things That Differ Here
| Difference | Why it matters | What to do |
|---|---|---|
| The gas is not chlorine alone | Chlorination tail gas commonly carries HCl as well. HCl also consumes caustic but makes no hypochlorite, running a path of different slope, so a scale built on pure chlorine reads high | Calibrate on the real gas mixture; where the ratio swings, add another step of alarm margin |
| Solids may be present | Dust and hydrolysis products carried into the scrubber leave traces of solids in the liquid. High solids is on the sound-velocity exclusion list — scattering costs the echo its lock | Take a sample and look at the solids. Traces are fine; visibly turbid liquor needs assessment, or the PS7000 acoustic-impedance route instead |
| Corrosion is more complicated | Besides hypochlorite and chloride there may be acidity and metal ions, so wetted materials cannot simply copy a chlor-alkali selection | Confirm materials against a full stream analysis. 316L is out in oxidising, chloride-bearing media |
Where It Goes
- Full-bore vertical run on the circulating pump discharge. Not the tower return line and not the pump suction — aerated duty is on the PS7020's own exclusion list and a scrubber return is exactly where entrained gas lives.
- Upstream of the make-up tee. The point is the loop's own margin; blended with fresh caustic it no longer reports it.
- Sample valve on the same spool, temperature element close to the sound path.
Before We Quote
- Rough Cl₂ to HCl ratio in the tail gas, and whether it is stable.
- Solids content of the circulating liquor — one settled sample is enough.
- Make-up caustic strength and source; any separate water addition.
- Temperature range, working pressure, loop bore and material.
- Hazardous-area classification and required Ex rating.
- Whether switch-out is called on residual alkali or available chlorine, and at what threshold.
- Any set of “sample + laboratory result (free alkali / available chlorine) + temperature at the time”.
What this page is: a method and selection note, not a case study. There are no unit counts, saving percentages or payback periods here — we do not invent those. Where the page says what the industry does, it cites something you can check for yourself (published vendor literature, granted patents) rather than implying we have installed base there. Send us the process conditions and you get a selection opinion you can check.
