Process Puzzler: Fix a Fluid Flow Flaw

Readers suggest solutions for dryer difficulties.

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THIS MONTH’S PUZZLER
Our plant relies on an old batch fluid bed dryer. We can’t afford buying a new dryer, let alone its floor space. The dryer is difficult to operate because of loss of fluidization during the early part of the drying process. The baghouse employs a timer-controlled shaker that’s mechanically unreliable. When the shaker operates, the discharge valve to the blower momentarily closes. This is all that’s needed for solid to accumulate in the bowl above the screen. Operators often beat the bowl to keep the solid fluid. They have another trick: To fluidize the solid early in the batch, they open the door of the bowl — when the discharge valve opens air is drawn into the bowl, creating a vacuum to lift the wet solid. When it was installed, the dryer ran without so much operator attention. Since then two steam heaters have corroded and have been abandoned in-place (the air flows through them). Can you suggest some ways to avoid dryer problems and make the dryer easier to operate?

TAKE A SHOTGUN APPROACH
The problem seems to be mainly due to insufficient air heating during the early part of the batch drying. The following actions can help to avoid your existing dryer problems and make the dryer easier to operate: 1) replace the damaged steam heaters; 2) ensure that the filter is clean before the batch drying — monitoring the differential pressure is recommended; 3) improve the effectiveness of the mechanical shaker/vibrator or increase shaking frequency during initial part of the batch drying; and 4) check the blower for performance deterioration and the blower suction pipe/duct for obstructions.
C. C. S. REDDY, lead process design engineer
Singapore Refining Company Pte. Ltd., Singapore


INCREASE THE VELOCITY
The problem described appears to be as a result of alteration in the fluidization arrangement of the dryer. The unit was not properly designed. I suggest using a cyclone separator instead of a bag house. The corrosion of the two steam heaters is as a result of humid air condensing at a lower temperature on the heaters. Try looking at increasing the fluidization velocity without increasing the size of the unit.
Omenka Dennis, chemical engineer
Guinness Nig. plc, Lagos


CHANGE THE BAGHOUSE
The problem described seems straightforward. Change the baghouse from an old shaker type to pulse jet type. A pulse jet type baghouse doesn’t need to shut off the exhaust during the cleaning cycle. Another choice is to separate the mechanical shaker type baghouse into at least two compartments. Alternate the shaking and only shut off airflow to the one compartment when the dryer is in cleaning mode. The old steam heaters should be removed — they are causing unwanted pressure drop and costing you energy.
Frank Hwang, asset management group
Ashland Inc., Wilmington, Del.


ELIMINATE THE PRESSURE DROPS
The batch dryer sounds familiar. Are the condensate and steam connections to the abandoned steam coils blinded off? Condensate or steam leakage could be re-introducing humidity. If this is a problem, consider introducing slip blanks or breaking the connections altogether.

Corrosion could be a contributing factor. The dryer bowl plate supporting the screen may have suffered corrosion as well. If some of the plate holes have been occluded (with rust) or expanded (with metal removal), the air dispersion may have been altered, causing the product to be poorly fluidized in one sector of the bowl, especially after the filter shake sequence is activated.

If the dryer is sequenced through a temperature profile (typical to prevent case hardening), can the timing for the filter shake be altered so that the delay between shakes is longer during the colder (and wetter) steps? When the product cake is wet, the bed lift usually is lower so that the exhaust filters are not as taxed as they will be in the later (and hotter) steps when the bed is lighter and more likely to blind the filters.

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