Handling boiling acid fumes: Exhaust blowers vs. water aspirators/ejectors

By Bob Applequist, Senior Application Specialist

On June 12, 2014 in Articles

Back in the late 1800s, Danish chemist Johan Kjeldahl developed an analytical chemistry method that accurately measured the amount of nitrogen within organic samples. Brilliant! This method continues to be used today, and has dozens of variations. Depending on the specific equipment, small or large sample sizes can be boiled in pure sulfuric acid at extremely high temperatures.

The traditional Kjeldahl equipment (also called Macro) utilizes large 800 ml flasks where samples are boiled in sulfuric acid at 395° C. This creates enormous amounts of extremely corrosive acid fumes that must be removed from the laboratory environment, especially when a 6, 12 or 18-place digestion unit is fired up to complete the digestion procedure.

The first fume removal system, and still most frequently used with Macro Kjeldahl systems, was the blower exhaust system. Utilizing a highly corrosion-resistant PVC blower connected to the 6, 12, or 18-place fume removal manifold, acid fumes are drawn off the digestion flasks — each containing approximately 15-20 mls of boiling sulfuric acid. For decades these removal systems incorporated integral blowers, which held the heating burners. The acid fumes were suctioned out of the manifold, then pushed through the building’s ductwork to the outside. Although that design provides a mechanical convenience (no roof mounted blower required), it also presents a problem: Ductwork between the blower and outside exhaust is positively pressurized. Sounding much like a scene in a horror movie, any small leak in the ductwork would allow hot acid fumes to work through the leak and push into the building.

Alternatively, a far safer fume removal system incorporates a roof mounted blower. In this type of system, the entire ductwork system is kept under negative pressure created by the remote blower suction. By providing a safe ten-foot extension on the blower exhaust, the acid fumes are quickly diluted to safe concentrations by the prevailing winds outside the building. This preferred operation system is no different than traditional chemical fume hood setup using a remote blower.  

The second most common type of acid removal system incorporated into Kjeldahl Digestion systems has been the Water Aspirator/Ejector Removal system. This device relies on large volumes of pressured tap water flowing past to the acid fume collection manifold. A specifically sized tube constriction is employed in the flow pathway, causing increased water velocity, that in turn creates a vacuum due to the Venturi Effect.

This vacuum aspiration draws the acid fumes out of the manifold where they readily dissolve in the flowing water and are then discharged in a sanitary drain connection.

For optimal performance, the Water Aspirator/Ejector Removal system requires a water pressure of at least 30 psi. For large Aspirators such as those used on Macro Kjeldahl systems, a 6 to 8 gallon per minute flow rate is required to create a proper vacuum. Due to the rising cost of tap water and the environmental concerns directing the reduction of tap water use, Water Ejector/Aspirator systems have fallen out of favor.

Labconco has been making Macro Kjeldahl Systems since 1925. We offer Macro Kjeldahl products with a blower exhaust connection valve built-in to the 6, 12 and 18-place systems [Total number of 800 watt burners is 12, 24 or 32 respectively].