November Process Puzzler: Deter dryer difficulties

Readers suggest how to improve reliability and avoid danger, in this month's Process Puzzler.

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Dr. Rudy Lisa, sr. research associate
BASF Corp., Mt. Olive, NJ

 

Form a game plan

Clearly, this dryer is an accident waiting to happen. Consider these ideas to improve the safety and the reliability of the dryer:

  1. a new spray nozzle material;
  2. reinstall the existing knockers and add others later after inspection;
  3. create a temperature monitoring alarm system;
  4. monitor blower bearings and consider alternatives;
  5. install gas sniffers at top of dryer;
  6. consider replacing pneumatic burner control valve with electric actuator;
  7. evaluate alternatives for eliminating unreliable positive displacement pumps;
  8. evaluate pressure losses in system to improve blower performance;
  9. consider elimination of unreliable shaker arms in baghouse;
  10. inspect fire control systems; and
  11. conduct time study on shut-downs and clean-outs.

Apparently, the spray nozzle material is eroded by the slurry. This isn’t surprising. In drying a slurry, the droplets must be very fine. To quote Williams-Gardner’s Industrial Drying (Leonard Hill, London, 1971), p. 56: "A pump-able slurry, paste, or sludge will require the maximum possible immediate moisture evaporation on its introduction into the dryer in order that it may break down into manageable particles of semi-dried material and not build up on the internal surfaces of the dryer." Without sufficient stress, the slurry will cake the inside of the dryer, becoming a fire hazard as fine dust break free from the cake and forms into tinder material. I would recommend using nickel or a nickel-alloy for the spray nozzles.

Replace or repair the existing bin activators or knockers. Add knockers to the lower-middle cone of the dryer itself. Don’t add knockers near the base of the dryer — this could promote flaking in the cake at the bottom of the dryer. This cake will begin to form regardless of how clean the operation is. There’s no point in kicking a sleeping dog!

Initiate a "check-weight" program. Every time the dryer is shut down to collect the product, weigh the material collected where it’s not supposed to be: the duct, the bottom of the baghouse, cyclones, and dryer bottom. You’re making progress when the bag weight decreases. Add additional knockers as needed based on this study.

Operations is flying blind. The most likely spot for a fire — the bottom of the dryer — is neglected in the present layout. Add two thermocouples 180° apart at the bottom of the cone above the 180° elbow. Add a thermocouple near the top of the dryer to detect fluctuations in the burner. Add another thermocouple downstream of the 180° elbow to detect a rise in heat as the product catches fire in the bottom of the dryer: heat follows flow.

Gas in a confined space is a recognized danger. Dryers tend to be installed in ceiling areas where dry air removes moisture from product leaking from high-pressure line. This is a serious potential hazard. Install multiple gas sniffers, check the sprinkler system and look at pipe expansion at the burner inlet: a common source of leaks.

Electric actuators can outperform pneumatics in fine controls. This is worth looking into for the burner temperature control valve.

Perhaps there’s an alternative to high pressure. Maybe a portion of the combustion air could be used to atomize the slurry. Gas/liquid atomizers often operate at a lower pressure than high pressure liquid. This idea only works if high pressure compressors are not involved, otherwise you risk poor reliability.

Building up product in ducts is mainly a problem to flow. Backpressure will cause a centrifugal compressor to back down its curve, producing less fluidizing gas: solids fall out of suspension. Investigating the entire system for pressure losses and dead areas will eliminate this potential problem.

Shaker arms are often replaced by more reliable pulse-jet systems. This may not work here because the blower capacity may be limited but it’s worth investigating.

If it hasn’t been done recently, the fire control system should be inspected.

Clean-out is inevitable with this type of batch dryer. There should be ways to reduce downtime. I would do several detailed time-studies on this operation to identify ways to make the operator’s job easier and thereby increase the run time for the dryer.

Dirk Willard, senior process engineer
Ambitech Engineering, Hammond, Ind.

 

 

 

January's Puzzler

A hydrolysis catalyst bed is designed for removing COS, H2S and HCN at 430°F from 40,000 lb./hr. of H2 gas at 400 psig. The diameter of the packed bed is 5 ft. The maximum allowable heat-up rate for the alumina catalyst is 90°F/hour. At the extreme minimum ambient temperature, 20°F, it will take almost five hours to heat the bed safely. One idea proposed for heating the bed is to recirculate gas through a mineral oil heater and the bed until the catalyst can safely reach reaction temperature. An estimate of the heat-up time for the catalyst was made using Schumann’s curve and the method described on pp. 668–670 of Kern’s Process Heat Transfer. Unfortunately, an estimate of the rise in the bed by recirculating the gas indicates that it will take only 53 minutes, which is a gradient of 464°F/hour. Besides this problem, the largest practical blower that could circulate low-pressure gas would need over 1,000 hp. and could only operate up to 15,000 lb./hr. at 3 psig. Further work with the Schumann curve showed that it could take over 16 hours to achieve temperature with such a blower. After discussion, another start-up problem emerged: condensation of water on the catalyst. What can you recommend to ameliorate the condensation problem? Because a blower is not practical, what other ideas can you suggest?

Send us your comments, suggestions or solutions for this question by December 10, 2007. We’ll include as many of them as possible in the January 2008 issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net 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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