PISONICS
PS7020 / PS7110
Ultrasonic Concentration Meter · Inline Refractometer
Sound velocity / critical-angle refractometry
Hydrogen Chloride Absorption — Acid Strength and Scrubber Liquor Monitoring
—— The easiest measurement on this list and the hardest materials problem ——
【Key measurements: HCl wt% / residual NaOH wt%】
Two Routes, Two Different Measurements
| Route | What it does | Liquid system | What the number is for |
|---|---|---|---|
| Water absorption falling-film absorber | HCl dissolved in water to make commercial acid — recovery and product at once | HCl + water, binary | Holds the outlet strength (commercial acid is commonly delivered around 31 %) and decides when to switch tanks; also shows when absorber efficiency is falling off |
| Caustic absorption tail-gas scrubber | Residual HCl neutralised before discharge — treatment, not recovery | NaOH → NaCl + H₂O, one-dimensional path | How much neutralising capacity is left in the loop, hence make-up and switch-out |
The Measurement Half: Neither Is Hard
On the water-absorption side this is a textbook binary. Sound velocity in hydrochloric acid varies monotonically with concentration over a wide span, so a real-liquid calibration is stable; refractive index is monotonic too, which makes the PS7110 route equally valid. Which one you pick turns on materials and whether a hygienic build is needed — not on whether the measurement works.
The caustic side is structurally the same as a chlorine scrubber: HCl + NaOH → NaCl + H₂O consumes caustic and makes chloride, stoichiometry pins the components to one path, and a single sound-velocity reading locates it. One thing to watch — the slope is not the same as the chlorine path: for the same one point of caustic consumed, the pure-HCl path moves sound velocity about 82 % as far as the pure-chlorine path does. A scale built for chlorine scrubbing cannot simply be carried over.
Mixed tail gas is where this goes wrong. Plenty of chlorination processes vent Cl₂ and HCl together, so both paths run in the same tower and the real slope sits between them, moving with the gas ratio. Such a tower is either calibrated on the actual mixture or given margin against the least favourable slope. Calibrating on either pure path and stopping there is not an option.
The Materials Half: All of the Difficulty
Commercial 31 % hydrochloric acid is severely aggressive to metals, and 316L is simply unusable on that line — there is no argument about it. The viable routes are these, each with its own boundary:
| Route | Where it fits | Watch for |
|---|---|---|
| Non-metallic lining | PTFE / PFA lined spool — the standard answer for concentrated acid | The joint between lining and sensor is the design problem; the lining sets the temperature ceiling |
| Tantalum | Outstanding in hydrochloric acid; the choice at higher temperature or strength | Expensive, and sensitive to fluoride-bearing media |
| Hastelloy C-276 | Good in reducing acids; usable at moderate strength and temperature | Margin falls away quickly as strength and temperature rise — confirm against the real duty rather than trusting the grade name |
| Titanium | Good in oxidising chloride media | Not for plain reducing hydrochloric acid — the opposite of the conclusion on the hypochlorite side; do not carry that experience across |
The caustic side is the easy one. Its liquor is NaOH and NaCl — alkaline, no strong oxidiser — and 316L is usually adequate. That differs from a chlorine scrubber, where hypochlorite is present and 316L is excluded. Same words on the P&ID, opposite materials conclusion, because the gas is different.
Where It Goes
- Water-absorption side: in the full-bore outlet run from the falling-film absorber, or on the product tank inlet. Not in the absorption section itself, which is two-phase by design.
- Caustic side: full-bore vertical run on the circulating pump discharge, upstream of the make-up tee.
- Both: temperature element close to the sound path, sample valve on the same spool.
Before We Quote
- Which point — the acid product, the scrubber loop, or both.
- HCl to Cl₂ ratio in the tail gas, and whether it is stable.
- Concentration and temperature range on the acid side; together those decide the material.
- Bore, pressure, hazardous-area classification.
- What the tank-switch and make-up decisions rest on today.
- Any laboratory results from field samples (strength + 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.
