December Process Puzzler: Debug a weigh feeder

Nov. 29, 2006
Readers offer ideas for improving reliability of a weigh feeder in Chemical Processing's monthly Process Puzzler feature.

Question from October's Chemical Processing

A weigh-feeder system measures a fine solid, a slurry, and a volatile organic into a mixer. The expensive volatile organic quickly evaporates. It is used in such small quantities that it must be carefully measured because it affects final product quality: too much and the concentrated organic could cause skin and eye irritations; too little and only the waxy smell of the solid will be present. The slurry tends to plug the feed nozzle. Sometimes, the fine-solid particle distribution ranges from dust to pellets; this variation sometimes blinds the dust collector downstream and, alternatively, plugs the pneumatic conveyor pick-up point because the solid particles are too large, or fine particles agglomerate, pick up moisture and seal up the pick-up point (Figure 1). How can we improve reliability of the system?

Figure 2. Block diagram of weigh-feeder system and troubles. (Click to enlarge.)

Install two cyclones

From your description, I found several problems that need to be solved to operate this process smoothly. I identified the following problems from the sketch and the description: 1.) variations in the particle size in the distribution of solids are causing pluggage of the bottom nozzle by two possible mechanism: with bigger particles blocking the nozzle or by bridge formation with fines; 2) the solid appears to be hygroscopic — it picks up moisture from the atmosphere and seals up the nozzle; 3) slurry is to plug the feed nozzle in its weigh tank; 4) and the volatile organic liquid is evaporating at a faster rate than expected by the system designers.

Based on these observations, I recommend the following improvements which I hope will improve the operational reliability: The present configuration for handling solids won’t work. To avoid choking the cyclone bottom nozzle, design two new cyclones, in series, for two different particle sizes. The bigger particles will be separated in first cyclone and fines in the second one. These sizes must be carefully estimated.

Without knowing what the distribution looks like, it will be necessary to define an “average” particle diameter for each cyclone that minimizes bridging. Also, the size should be for several days of running time so that production upsets are averaged into the analysis.

Managing the moisture problem will be a challenge. A small rotary feeder in the bottom nozzle of cyclone should help prevent uncontrolled contact of moisture and the particles. Pneumatic vibrators, thumpers or air blasters should be installed upstream of the rotary valves as a hedge against bridge formation with fines. The conveying air should be cooled sufficiently to avoid moisture content which can affect the free flowability of the material. You may want to go back to the air dryer and upgrade to a desiccant dryer. If possible, nitrogen gas might be considered for conveying. Since this is an expensive idea, perhaps, a small amount of nitrogen flow could be put introduced upstream of rotary feeders through rotometer. In this way, a trickle of nitrogen might reduce plugging problems in the air locks.

Because the second cyclone will collect the finer particles, you will want to verify the downstream filters on the pneumatic conveying system. This type of system, and variations in the particle size produced, may result in an unexpected carry-over.

The plugging of the slurry nozzle is a difficult problem. Perhaps a screw feeder would be an alternative. (Making it easy to change out the nozzle might be the only alternative.)

To reduce evaporation losses with the organic consider reducing the quantity to a minimum. Changing to a small plunger reciprocating pump with continuous flow could work. Chilling the organic to reduce its vapor pressure could work if chill water is available. Lowering the temperature should be checked to assure that viscosity or phase separation doesn’t become a problem. The vessel, where-in all three are mixed should be closed. Also a properly designed padding should be added to reduce vapor content over the mixing solid and liquid. Sealing the tank also will reduce any chance of contamination.

Kamal Parikh, general manager of technical services
Reliance Industries Ltd., Surat, India

Use an additive

Incorporate a vibrating device under the powder cyclone to avoid plugs. Reduce the evaporation of the volatile organic by adding a retardant. I suggest something like dipropylene glycol. To reduce the cost of the retardant, maybe add it using a dosing device. I would use an air atomizer, or something like it, to distribute the retardant evenly over the solid in the mixer below the weigh system.

Emilio Malaguti, technical manager
Chemtron, Hialeah, Fla.

February's Puzzler

A trucking company that supplies our fuming sulfuric acid has offered us a discount in price if we can double the pumping rate of our transfer pumps (Figure2). Management is excited about saving money but an inspection of the unloading station doesn’t inspire confidence. A new pump may be purchased but the suction line is limited because of the suction piping; spacing near the truck is very limited. The plug valves used throughout pose a significant pressure drop. Compressed air is already being used but is regulated down to 25 psig because of the danger of fatigue on the rupture discs protecting the tanker truck (35 psig is allowed by code; the setpoint for disc is 50 psig.). What can be done safely to increase the transfer rate? Can we double the rate? Are there any foreseeable problems if we do increase the rate? What improvements are needed in this old, grandfathered unloading station?

Figure 2. There is a discount in price if we can double the pumping rate of our transfer pumps. (Click to enlarge.)

Send us your comments, suggestions or solutions for this question by January 19, 2007. We’ll include as many of them as possible in the February 2007 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at [email protected] or mail to ProcessPuzzler, Chemical Processing, 555 W. Pierce Rd., 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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