Process Puzzler: Fix a Fluid Flow Flaw

Sept. 15, 2009
Readers suggest solutions for dryer difficulties.

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?

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

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

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.

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.

Is the dehumidifier still performing to specs? The chilled water lines could be partially plugged and not providing the humidity removal. Is it possible to obtain a humidity probe that could be inserted into the process air stream to monitor the dewpoint or relative humidity percentage? If it works better in the winter than in the summer, this may be a clue.

The filter shaker mechanism undergoes violent forces by design. By nature, it can be a maintenance headache. Is a rebuild in order?

Is the air stream flow monitored? Are there differential pressure (dP) transmitters measuring across the HEPA filter, the product bowl and the exhaust filter? Are any of these dPs showing unusual behavior that would indicate a loss of air flow during normal (non-filter-shake) conditions? If there is an air flow transmitter, is it acting erratically? Installation of an ammeter on the blower is a cheap and consistent way of monitoring air flow across the dryer since most blower curves are almost linear for motor load versus airflow.

Opening the dryer bowl gate to fluidize the bed makes me wonder if the pressure drop of the abandoned steam coils is compromising the available blower static pressure. Is there a payback to increasing production by eliminating the pressure drop of the abandoned coils? Can this dP be measured?
Steve Duckro, associate director process engineering
Eurand, Inc., Vandalia, Ohio

One of our welders got hurt cutting into a carbon steel header for a chilled brine line to install another chiller in the closed-loop system. He purged the piping but not the room. An invisible gas ignited from his torch. Our plant environment contains chlorine and other corrosives. What was source of the gas? How can we prevent future accidents and what do we need to change in the system to avoid such safety problems?

Send us your comments, suggestions or solutions for this question by November 6, 2009. We’ll include as many of them as possible in the December 2009 issue and all on Send visuals — a sketch is fine. E-mail us at [email protected] or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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