Process Puzzler: Master a Musty Mess

Readers suggest how to deal with a contamination source.

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We feed grain via a screw conveyor from a silo to a mill. After grinding the material is conveyed by a pressurized pneumatic conveyor up six stories to cereal cookers. The pneumatic lines are bare pipe and run indoors. The mill room has fans but the air is humid in central Ohio in the summertime. Eventually, cereal is rolled into breakfast flakes. Unfortunately, the cereal often takes on a musty smell, which is worse in the winter. The cookers sometimes fail bacteria screening — the contamination has been traced back to the mill. Operators have seen water dripping from the conveyor ducts, although the moisture levels in the silo are only in the 8–9% range. What’s the source of the mysterious water and how can we eliminate or reduce the odor problem? How would you approach this problem?

The problem most likely arises from air being introduced into the process. The first question to ask is: what is the moisture content of the air streams introduced into the system? If air is being changed in the silos with fans then the moisture content measured there is not providing any useful data. The likely sources of water are:

1. Condensing water in either the mill or from the pneumatic conveyor due to Joules-Thompson effect; this tends to tie in with “it’s worse in winter.” Drying or reheating the air should address this cause. Renting an air dryer for a period is a first step.
2. Are the [blowers] water cooled? It is possible that saturated air is being carried forward.
3. Failing all these, look for places where any source of water may come in contact with the process: flushing lines, cleaning, cooling water, etc.
Ed Fish, senior consultant
Haztech Consultants, Winsford, U. K.

Dehumidify the pneumatic transport air by cooling below dew point before air enters the blower.
Eckehard Fuhrmann, engineer
SGL Technic, Valencia, Calif.

The problem is most likely caused by high dew point in the conveying air used. Assuming the user is making its own compressed air using compressors, I would suggest that the air dryers on the system be checked to ensure that the dew point is sufficiently low for the intended use to prevent condensation inside the equipment (-40°C should be sufficient). The condensation along with the grain is giving the bacteria all it needs to replicate and create the musty smell.
Chuck Cutler, maintenance engineer
Wyeth Vaccines, Sanford, N.C.

It sounds like moisture is getting in the system via the compressed air used in the pneumatic conveyor. Perhaps an air dryer between the air compressor and the pneumatic conveyor would benefit the process.
Robert Walker, laboratory technician
Mosaic, Mulberry, Fla.

Dripping of water from the duct indicates that the water source is the silos. Water vapor in the air is condensing in the silo during winter. High pressure and low ambient temperature will facilitate water condensation inside the silo. Proper insulation of silo and conveyor ducts will eliminate the problem. Another option is to maintain the silo under nitrogen pressure.
R. Gunasankar, senior process engineer
Chiyoda Malaysia SDN. BHD., Malaysia

Try drying out the compressed air to the air transport system.
George R. Strickland, consultant

Just such a mystery baffled Ralston Purina for years. I put together a test plan and it failed! I expanded my search; I looked at every source of water. The solution astonished me — but, as Sherlock Holmes said, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” That’s how it was for this puzzle. I measured the air humidity, poured over weather data, reviewed years of silo moisture tests, looked at maintenance records and ran calculations of the heat loss from the duct. The dripping water from the duct did not make sense but there it was. And then it dawned on me.
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