Case study IV: Oxidative Scrubbers – How to, and When to Scrub VOC From Air.

• Methyl formiate from foundry waste air
• Formaldehyde from fumigation
• Acrolein from plastics manufacturer’s waste air
• Styrene from polyester coatings

which of these can be conveniently scrubbed from air?

„Forgotten“ waste gas purification technique

More than 50% of all waste gas purification plants are some type of combustion, be it regenerative, recuperative or catalytic oxidation. Another large market share (around 25%) is taken up by adsorption on activated carbon. Depending on the industry in question, the residual ca. 20% are either condensation, biofiltration or aqueous scrubbing.

What is the problem with aqueous scrubs? Why is this technique so rarely used, apart from dedusting? Some of the main issues with aqueous scrubs are:

1. The volatile organic compounds (VOC) have to have a sufficient solubility in water. The lower their solubility, the larger the scrubber has to be, and the higher the risk of breakthroughs will be. This makes the scrubber unsuitable for compounds such as xylene, many halogenated species or solvent naphtha, though traces of these compounds can often be treated successfully within the context of a VOC mixture.
2. The clean air leaving the scrubber is water saturated. Depending on the later destination of the air stream, it might have to be dried – an energy-consuming process.
3. The filter produces waste water. Since the vapor pressure of the circulating water determines the effectivity of the scrubbing process, the concentrations cannot be allowed to rise above a predetermined level.

On the other hand, aqueous scrubs can be a simple and effective method to treat certain waste gas streams:

1. If there is already a scrubber (to take care of dust and/or inorganic acids or bases), it can in many cases be retrofitted to treat the VOC as well.
2. In most cases, the aqueous scrubber is not sensitive to concentration peaks, provided there is enough capacity to adsorb the organic mass from peak flows.

How to reduce water consumption

In simple cases, such as with low concentrations of water-soluble, readily biodegradable molecules, water will simply be disposed of to the existing water treatment unit in a continuous side stream. Less soluble compounds can make this a costly proposition: The high organic vapor pressure of the scrubbing water will make it necessary to dispose of very large volumes of water.

In such cases, it is often possible to add an oxidation step to the scrub. This will not always oxidize the VOC into carbon dioxide and water, but it will break them down into compounds of increased water solubility, such as organic acids, which have a low vapor pressure in aqueous solution. This reduces the water consumption, often by one or two orders of magnitude.

Depending on the chemistry in question, oxidation can be effected by alkaline sodium hypochlorite solution (NaOCl/NaOH) or hydrogen peroxide. If these oxidants do not act fast enough on their own, a catalyst can be added which will increase turnover speed.

Should we even test an (oxidative) scrub?

There are several calculations you can run before you decide to start with actual tests:

1. You can calculate the water consumption for a simple scrubber based on chemical equilibrium. This yields a best-case scenario that will tell you how much water you need to dispose of per hour to comply with emission threshold levels, though it will not aid you in the design of the actual scrubber.
   If there is more than one VOC in the waste gas stream, and data for the mixture (which will almost inevitably be non-ideal) are not readily available from chemical literature, you can augment these calculations with some simple bench-scale tests.
2. You can calculate the consumption of oxidants (NaOCl, H₂O₂) for an oxidative scrub. For this, you will need information on the oxidation reaction: Reaction products, stoichiometry, reaction kinetics. Again, if these are not available from literature research they can be determined via bench-scale testing.

With these data, you can do a preliminary design of the scrubber which will help you decide whether it is even an option for your waste gas. The results will also give you a sound basis for a design of experiments (DoE) for the pilot plant scale.