Choosing a gas scrubber: matching the machine to the molecule
A selection guide for laboratory and industrial exhaust treatment. How to decide between wet, dry and thermal scrubbing, size the unit to your real gas load, and prove it is still working a year after handover.
A gas scrubber is usually the last line priced in a laboratory fit-out and the first one questioned when the budget tightens. That is understandable, because it is the one item on the schedule that produces nothing. It exists so that what leaves the building is not a problem for the people downwind, for the plant on the neighbouring roof, or for a Department of Environment inspector. Choosing one badly is expensive in a quiet way: the unit sits in the plant room, consumes water and reagent, removes far less than the paperwork claims, and nobody finds out until somebody finally measures the stack.
Start with the gas, not with the catalogue
The most common error in scrubber procurement is asking for a scrubber by airflow. Airflow decides the size. The chemistry decides the type, and if the type is wrong then no amount of size will fix it. Before you ask anyone for a quotation, write down four things: which gases can enter the duct, at what worst-case concentration, at what temperature, and whether anything in the stream is flammable, pyrophoric or particulate.
Worst case is not the average. A packed tower sized on the average acid load from six teaching hoods will be swamped by one broken winchester of hydrochloric acid. Ask what the credible release actually is: a bottle dropped in a hood, a cylinder valve left open overnight, a chamber clean cycle, a reactor vent during an exotherm. That figure, not the daily average, is what the unit has to survive without breaking through.
Matching the mechanism to the molecule
What is in the exhaust
Treatment mechanism that suits it
Semian family to ask about
Acid gases and mists: HCl, HF, nitric and sulphuric fume
Wet absorption into a circulating alkaline liquor
SSW 200-E, SSW 200-K, SENTINEL series
Alkaline gases and ammonia
Wet absorption into a circulating acidic liquor
SSW wet scrubbers, SENTINEL series
Low or intermittent loads where water and drainage are awkward
Dry chemisorption or adsorption onto a solid medium
SSD 100-S chemisorption, CYRUS dry adsorption
Flammable and pyrophoric process gases
Thermal oxidation followed by a wet or dry polishing stage
Perfluorinated compounds from plasma etch and chamber clean
High-temperature abatement designed for PFC destruction
TRINITY PFC abatement
Single fume hood or one small process tool
Compact packaged scrubber sized for hood-scale duty
MIDAS-I, MIDAS-II, SBW 200-C, SBW 200-CE
Residual risk at a gas cabinet or cylinder change point
Passive treatment, no power and no services
ALTAIR passive gas treatment device
Semian splits these mechanisms into separate product families rather than offering them as options on one machine, which is useful at specification stage: the family name tells you whether you are buying absorption, adsorption or oxidation. Vvon Technologies supplies the range in Bangladesh and will size a unit against your gas list, but the gas list has to come from you or from your process engineer. No supplier can invent it, and one that offers to is guessing.
Wet, dry and thermal, and what each one costs you to own
Wet absorption passes the gas counter-current to a liquid that neutralises it. It copes with high and variable loads, tolerates spikes, and is the default for acid and alkali fume. In exchange you take on a water supply, a reagent supply, a recirculation pump, a pH loop and an effluent that has to be neutralised before it reaches drain. In a building with no effluent treatment, that last point is what stops the project.
Dry chemisorption and adsorption pass the gas through a solid medium chosen for the target gas. No water, no pump, no effluent, and the unit can go where a wet tower cannot. In exchange the medium has a finite capacity, the running cost is the medium, and when it is exhausted it stops working with no outward sign at all. A dry bed without a breakthrough check is a bed that will be found exhausted after the event rather than before it.
Thermal oxidation, burn-dry and burn-wet destroy the molecule instead of capturing it, which is what flammable, pyrophoric and perfluorinated streams need. These are the most involved units to own: fuel or electrical heating, combustion air, flame supervision, and a downstream stage to capture the acid products of combustion. They belong on the process tool that generates the stream, not on a general laboratory hood.
Passive devices treat small residual releases with no power and no services. They do not replace a scrubber on a live process, but they close off one specific risk at a gas cabinet or a cylinder change point for a fraction of the cost and with almost no operating burden.
Sizing: flow first, then contact time
Add up the exhaust volume of every hood, cabinet and tool the scrubber will serve, taken at the design sash or slot opening rather than at the closed position. Add an allowance for duct leakage and for the hood you will install in three years.
Fix the duct velocity. Industrial ventilation practice keeps gas and vapour ducts in the region of 5 to 10 m/s, and higher where fume or dust can settle out. Too slow and condensable material drops out inside the duct; too fast and fan power and noise become the problem.
State the required outlet condition. There are two limits and they are not the same: the discharge limit the Department of Environment applies, and the concentration that is acceptable where the plume can re-enter the building through a fresh air intake or an open window. On a tight Dhaka plot with neighbouring roofs at the same level, the second is often the harder of the two.
Let the supplier size the internals against your gas list and your outlet requirement, and ask for that calculation in writing: packing depth or media volume, gas residence time, liquid to gas ratio for a wet unit, and the expected removal efficiency for each named gas. A quotation stating a removal efficiency without naming the gas and the inlet concentration is not a design.
Select the fan after the scrubber, never before. The scrubber pressure drop is part of the system resistance and it rises as the packing fouls. A fan chosen on clean-bed pressure drop will be at the end of its curve within a year.
Check the plant room before you sign: floor loading, drainage, water supply, safe storage for reagent drums, and enough clearance around the unit to withdraw the packing. That last one is routinely forgotten and turns a two-hour job into a two-day job.
Media, reagent and the maintenance you have signed up for
Every scrubber type carries a consumable and a failure mode invisible from the outside. Price both at tender stage.
Wet units: reagent strength drifts, nozzles scale, packing fouls and the mist eliminator blinds. The pH probe is the most neglected component on the whole plant. It drifts, and once it has drifted the dosing system will faithfully hold the wrong pH. Re-calibrate it on a written interval, not when somebody remembers.
Water quality matters more than most specifications admit. Hard make-up water scales spray nozzles and packing. If the site supply is hard, budget for softening or for more frequent packing cleaning.
Dry units: the medium is the running cost, and its life depends on the load it actually sees rather than on the datasheet. Log throughput so a change can be planned instead of reacted to, and keep a spare charge on site. Imported media does not clear customs quickly, and a bed you cannot recharge is a process you cannot run.
Every type: a pre-filter that keeps particulate off the packing or the media pays for itself. In the dry season Dhaka dust will blind a bed that the chemistry alone would have left untouched for a year.
Corrosion in the humid months: check fixings, panel gaskets and enclosure seals on outdoor units before and after monsoon. Water in the control cabinet is the most common reason a scrubber will not start.
Proving it still works
A scrubber is the one item in the extract system whose failure is completely silent. The fan still runs, the hood still pulls, the operator sees nothing different. Build the instrumentation in at purchase, because adding it later means cutting into a live duct.
Differential pressure across the bed, trended rather than merely displayed. A rising trend means fouling; a falling trend means channelling or a collapsed bed, which is worse, because on the gauge it looks like an improvement.
pH and conductivity of the liquor, alarmed, with reagent consumption recorded alongside. A sudden fall in reagent use almost never means the process got cleaner. It means the dosing pump has stopped.
Liquid level and pump run status, interlocked to the process. This is the interlock that matters most in Bangladesh. When the supply drops and the changeover takes a few seconds, the recirculation pump stops and does not always restart, while the process tool and the extract fan carry on regardless. Wire the tool so it cannot run without a proven scrubber, and put the scrubber controls on the same essential supply as the extract fan.
An independent gas detector on the discharge is the only monitor that tells you what is actually leaving the stack. A fixed head alarming to the same panel as the rest of the laboratory turns an invisible failure into a visible one.
A periodic stack measurement by a third party, filed with the commissioning report. Environmental compliance and laboratory accreditation both ask for it, and the first test also gives you the baseline against which every later reading is judged.
Where procurement goes wrong
Buying on airflow alone. Two units with identical rated airflow can be built for completely different chemistry, and the cheaper one is usually the one that does not suit yours.
Splitting scrubber, ductwork and fan into separate tender packages. Nobody then owns the system resistance, and commissioning becomes an argument between three contractors while the laboratory stays shut.
Leaving the effluent out of scope. Neutralisation, holding and discharge are part of the installation, not a problem to hand to the maintenance department in month two.
No spares in the contract. Ask for a first-year spares kit priced inside the original order: pH probe, dosing tube, nozzles, gaskets, one packing or media charge. Buying them afterwards means a full import cycle for parts worth a small fraction of the unit.
No commissioning test named in the bill of quantities. If the tender document does not say the unit will be tested on site against a named gas at a named inlet concentration, you will be handed a delivery note and a manual, and the first honest measurement will happen years later.