Acid & Alkaline Gas Treatment Equipment

What is acid and alkaline gas treatment equipment?

An engineered package that removes acidic or basic pollutants from an industrial exhaust by bringing the gas into contact with a liquid that dissolves or neutralizes it: capture at the source, ducting and a fan, one or more absorption stages in a tower, a reagent and liquor circuit with dosing and bleed control, mist elimination, and the instrumentation that proves the outlet figure. Acid gases such as hydrogen chloride, chlorine, sulphur dioxide, hydrogen fluoride and hydrogen sulphide are washed with an alkaline liquor; alkaline gases such as ammonia and amines are washed with a dilute acid.

Why are acid exhaust and alkaline exhaust treated separately?

Because the two streams neutralize each other before either reaches a tower. Gathered into one duct they react in the ductwork, deposit ammonium chloride and similar salts on the wall, restrict the flow and corrode the steel, and the reagent bought to scrub the gas is spent on the other stream instead. Published practice in the industry that generates both streams divides the exhaust into an acid exhaust, an alkaline exhaust, an organic exhaust and a general exhaust, each with its own chemical scrubber, which makes the separation a ducting decision rather than a chemistry one.

Which tower configuration should be used for my duty?

The configuration follows from the solubility of the pollutant and the space available. A vertical countercurrent packed tower gives the highest theoretical removal because the cleanest gas meets the cleanest liquor and the average driving force across the column is maximised, and it usually needs a lower liquid-to-gas ratio, but it pays in pressure drop and must be kept clear of flooding. A horizontal crossflow tower has a lower pressure drop and a lower ratio but less contact time, so published guidance limits it to highly soluble gases where vertical space is the binding constraint. A cocurrent tower has the lowest pressure drop and the best fine-mist removal. A spray tower is the simplest and has the weakest mass transfer, which is why published guidance restricts it to particulate removal and to highly soluble gases such as sulphur dioxide and ammonia.

What is a liquid-to-gas ratio and why does it matter so much?

It is the volume of scrubbing liquor circulated per cubic metre of gas treated, and it decides whether the packing is wetted, how much contact surface is refreshed, and how much absorbed pollutant is carried away from the interface. Published design guidance gives 2.0 to 3.5 litres per cubic metre for hydrogen chloride, 3.0 to 5.0 for hydrogen fluoride and 4.0 to 6.0 for sulphur dioxide, and published experimental work recommends 2 to 3 for a packed tower removing sulphuric acid mist and vapour. An undersized ratio is the most common cause of a tower that fails to meet its guarantee, which is why the delivered ratio is verified at commissioning rather than taken from a pump curve.

What removal efficiency can be expected?

Published guidance for this class states that most gas absorbers exceed ninety per cent removal and that packed towers may reach as high as 99.9 per cent on some pollutant-solvent systems, with typical absorption duties at 250 to 10,000 parts per million by volume. Published equipment literature for acid gases quotes 95 to 99.5 per cent for common acid gases when the tower is properly sized with appropriate packing media and liquid rate, and 99 to 99.9 per cent and above for hydrogen chloride in a packed bed. The figure is only meaningful with the liquid-to-gas ratio, the inlet temperature, the stage count and the outlet allowance that produced it.

Why is a spray tower not enough on its own?

Because its mass transfer is the weakest of the arrangements in this class. Published guidance describes a spray tower as simple to operate and maintain with relatively low energy requirements, but as having the least effective mass transfer capability of the absorbers, restricted to particulate removal and to highly soluble gases such as sulphur dioxide and ammonia, with higher water recirculation rates needed to compensate. A spray stage earns its place as a pre-cooler ahead of a packed stage, or on a duty whose pollutant is genuinely highly soluble, but it is not the stage to specify for a poorly soluble pollutant or a tight outlet figure.

Why does an alkaline scrubber produce a salt, and where does it go?

Because the reaction that removes the pollutant is a neutralization reaction rather than a destruction reaction. Absorbed hydrogen chloride produces sodium chloride, sulphur dioxide produces sodium sulphate and hydrogen fluoride produces sodium fluoride in the circulating liquor, or the corresponding calcium salts where lime is used; the salt accumulates until it reaches the concentration the chemistry or the scaling behaviour will tolerate, and the bleed that holds it there is a stream with a volume, a concentration and a destination. Where a calcium reagent is used the sulphate and the fluoride are sparingly soluble and deposit on packing, nozzles and eliminator unless the concentration is held in its band.

How do I scrub ammonia or an amine?

With a dilute acid rather than with an alkali, because the pollutant is basic. Published practice for this duty holds the scrubbing liquor at a pH of two to four with sulphuric acid over a packed bed, and the reaction product is ammonium sulphate, recovered as a by-product or consigned for disposal. The design work is the acid dosing loop, the pH control band and the by-product route rather than the tower itself, and the liquor circuit has to be kept separate from any acid-side circuit, because an acid and an alkali in the same sump neutralize each other before either reaches a pollutant.

Why does hydrogen fluoride need special treatment?

For two reasons that are easy to miss. It demands a higher liquid-to-gas ratio than hydrogen chloride does, with published design guidance giving 3.0 to 5.0 litres per cubic metre against 2.0 to 3.5, which changes the circulating pump, the sump volume and the spray pressure. And it attacks glass, so glass-reinforced plastic and any glass-containing internals are excluded from the wetted parts rather than evaluated. The absorbed fluoride also precipitates as calcium fluoride wherever a lime reagent is used, so the circuit is specified with a salt concentration limit and a bleed rate rather than with a reagent alone.

Why is hydrogen sulphide difficult to scrub?

Because it is poorly soluble in water, so a plain water wash or an alkaline contact stage removes only a fraction of it. Published guidance for this class notes that the scrubbing medium should have a high solubility for the gas and that for low-solubility compounds such as hydrogen sulphide and hydrocarbons, amines or dimethylaniline are often used instead of water or alkali. A hydrogen sulphide duty is therefore specified with an oxidant, a chemisorbent or a two-stage arrangement, and its removal figure is quoted together with the reagent consumption it requires, which is materially higher than for a soluble acid gas.

Why is a redistributor fitted between packing sections?

Because liquid that has run down the wall of a single deep bed starves the middle of the column, and the transfer efficiency the tower’s height was bought for is lost in the dry zone. Published guidance states that redistributors are generally required for every eight to twenty feet of random packing depth, and that random packing is generally limited to an unsupported depth of twenty to twenty-five feet because the weight deforms the bed beyond that. A removal figure that slowly stops meeting its guarantee is very often a distribution problem rather than a chemistry problem.

Why is the tower run below its flooding velocity?

Because a packed tower has an upper limit to its gas flow above which the drag on the descending liquor is enough to hold it in the bed. Published guidance states that when the free volume in the packing fills with liquid and the liquid is carried back up the column the absorber is flooded, that the pressure drop rises and the packing stops mixing gas and liquor effectively, and that most packed towers therefore operate at sixty to seventy per cent of the flooding velocity. It also states the lower limit: a minimum liquid flow rate is needed to wet the packing sufficiently for effective mass transfer. A tower sized at the design flow with no flooding margin has no room for the day the flow rises or the bed loads up.

Which material should the tower be made of?

The material follows from the wetting liquid, the operating temperature and the halide content rather than from a preference. Published guidance states that packed columns are preferred to plate columns when acids are involved because the tower can then be built of fibreglass, polyvinyl chloride or another inexpensive corrosion-resistant material, that metal packings cannot be used for highly corrosive pollutants such as acid gas, and that plastic packings are not suitable for high-temperature applications. Published equipment literature quotes the envelope as polypropylene and polypropylene homopolymer for most acid and alkali gases below about eighty degrees Celsius, glass-reinforced plastic where a larger shell, a higher temperature or an outdoor installation is required, and stainless steel for oxidizing gases and sanitary duties. Where hydrogen fluoride is present, glass-containing materials are refused.

Why is the gas temperature important?

Because absorption falls as the temperature rises, and because hot gas brings two further problems with it. Published guidance states that the higher the gas temperature the lower the absorption rate and vice versa, that excessively high gas temperatures can lead to significant solvent loss through evaporation, and that precoolers such as spray chambers may therefore be needed to reduce the air temperature to an acceptable level. The operating temperature also decides which polymer may be used for the shell, the duct and the packing. Published guidance adds the compensation: where water is the solvent, the gas saturates adiabatically during absorption and cools the column substantially, so cooling coils are rarely required.

What causes a visible plume at the stack?

Almost always the mist eliminator rather than the chemistry. A scrubbing tower’s outlet carries droplets and mist, and the eliminator is the staged arrangement of corrugated sheets or mesh at the top of the column on which those droplets coalesce and drain back, with published equipment literature in this class claiming removal of droplets above roughly five micrometres. A blinded, shifted or bypassed eliminator lets that mist through, and the same fault raises the fan load and puts liquid into the duct. All three symptoms are maintenance findings, which is why the eliminator is specified with its own pressure drop, material, wash arrangement and access.

How is the bleed rate decided?

By the salt concentration the circuit will tolerate rather than by a level switch. The neutralization reaction converts the absorbed pollutant into a dissolved salt that accumulates, and the concentration is held where the chemistry, the scaling behaviour of the salt and the receiving route’s acceptance limits require it. With soluble sodium salts the constraint is usually the receiving route; with a calcium reagent it is the sparingly soluble calcium sulphate or calcium fluoride, whose concentration has to be held low enough to keep it off the packing and the eliminator. The bleed rate is delivered as a figure and verified at commissioning against the water balance, and it is one of the parameters most often reduced by an operator trying to save reagent, a saving paid for with a fouled bed.

Can one scrubber handle acid gas and nitrogen oxides together?

Partly, and the part it cannot handle is the part that matters. Nitrogen dioxide is soluble and is absorbed by an alkaline liquor, but nitric oxide is very nearly insoluble and passes through a water or alkaline wash almost untouched, and the absorption of nitrogen dioxide in caustic returns one nitrogen atom in three to the gas phase as nitric oxide through the reaction of nitrogen dioxide with sodium hydroxide to form sodium nitrate, nitric oxide and water. A nitrogen-oxide-bearing stream therefore needs an oxidation or reduction stage in series with the acid stage rather than a single acid scrubber, and that chemistry is worked on the nitrogen oxides treatment equipment page in the same range. The acid-gas and mist fraction of the same plume is the duty this page covers.

How do I know whether my acid and alkaline streams can share a duct?

By surveying the sources rather than by asking whether the gases are compatible in general. The test is the chemistry of each source: an acid-side source and an alkaline-side source cannot share a duct, because they neutralize each other and deposit salt in it; a reducing stream and an oxidant stream must not share a duct, because the result is a safety matter rather than a fouling matter; and a stream carrying a cyanide fraction must not be acidified before the cyanide is oxidized. The survey therefore records the assignment of every hood, tank, vessel and cell and states the physical consequence of every possible pairing before any duct is drawn or re-piped.

What is checked during commissioning of a scrubbing train?

Five measurements taken with the plant running its own material rather than a test gas. The two chemistries are checked as separated in the ducting as built. The capture is verified by holding the tank, vessel or cell boundary at a negative pressure at the peak of the duty, with the velocities recorded. The delivered liquid-to-gas ratio and the spray pattern over the bed are confirmed against the figures the removal was guaranteed at, and the pressure-drop baseline is recorded. The outlet is measured at the stack on the governing standard’s own basis and per species, and the pollutant balance is closed between the inlet load, the outlet gas, the bleed salt, the condensate and any recovered product. Each interlock is then proved individually, and the analyser is calibrated against its reference gas before the first production campaign begins.

What if the plant adds a tank or the emission limit tightens?

The source survey and the load accounting are re-run and the train is re-specified rather than pushed harder. Because the tower, the ducting, the reagent area and the control system were specified as modules, the usual answers are an additional packing section with its redistributor, a higher-capacity spray arrangement, a larger fan, an additional elimination stage, an additional reagent tank, a duct re-piped to separate two chemistries that were previously mixed, or an oxidation stage added where a poorly soluble pollutant was present all along and only became limiting when the limit tightened. Where the new load is higher than the measured one, the honest first step is to re-measure, to tighten the capture at the tank rather than enlarge the tower, and to spread the simultaneous sources apart in time.