Chemical Processing - September 2007 - (Page 62)

>> PROCESS PUZZLER >> Vent A ticking bomb? FIT PI TCV Combustion blower PI Product Product TE Product Product Natural gas Cartridge ﬁlter Slurry Reciprocating pumps Hopper TE Product Exhaust blower Filter Plug Spray nozzle TE PSH PSL PI Product Super sack Figure 1. Spray dryer for bioproduct feed slurry suffers from poor reliability and poses operating dangers. >> NOVEMBER’S PUZZLER A bulk spray dryer operates between 2,800–3,200 psig with a bioproduct feed slurry (Figure 1). The dried solid auto-ignites at 430°C, and this has occurred twice in four years. The reliability is poor. The on-stream factor is about 75% for numerous reasons: erosion of the single spray nozzle, which lasts only about three days before pressure drops below 2,800 psig; build-up of solids — knockers were deactivated due to poor maintenance; shaker failure in the baghouse; repeated bearing failure because of dust and poor location of blowers; frequent gas leaks, especially during start-ups; and failure of the oil seals on the high pressure pumps. Safety issues include operating without an inlet filter element, removing the cartridge filters on the pump to achieve a higher atomization pressure, and infrequent tune-ups of the burner. Any suggestions on how we can improve safety and reduce breakdowns? Send us your comments, suggestions or solutions for this question by October 12, 2007. We’ll include as many of them as possible in the November issue and all on CP.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net 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. need to have secondary seals that don’t swell in hot water or steam. For example, Viton is unsuitable. Possible materials are EPDM, food-grade silicone, Aflas or Kalrez. The contacting seal materials also need to be compatible with steam. Steam-grade carbon binders can be used. The stationary seat can be ceramic, tungsten carbide 62 • September 2007 or silicon carbide. If the agitator is to be run during sterilization, its seal should probably be of balanced construction unless it can be chilled well below the steam saturation temperature, to avoid “flashing” of steam. Greg Benz, president Benz Technology International, Inc., Clarksville, Ohio Consider a lower temperature It is highly likely that both the agitator seals and mechanical seals will be detrimentally affected by the steam. Replacing the agitator seals will pose the biggest expense. Fortunately, the mechanical seal manufacturers have kept up with the technology and seals can be replaced even in very old agitators. There is an alternative, though, that should be considered. Just because steam is available at 230°F does not mean you have to operate there. Steam sterilization is based on principles established for autoclaves. The relationship between sterilization time and temperature follows the Arhenius law, i.e., it is exponential. As a rule of thumb, a temperature of 225°F requires approximately 10 seconds of exposure time, if memory serves. This was the time we used for steam dwell tubes used to sterilize mash, or molasses, used in fermentation. Following the same principle, the temperature can be reduced to one safe for the existing mechanical seals. However, it is best to keep the temperature above boiling, or 212°F, assuming atmospheric operation, which may, or may not be true for your fermenter. If the fermenter runs at a higher pressure it may be necessary to consider venting and sterilize at barometric pressure. Since the mechanical seals will be thrown away anyway, why not calculate a lower autoclave temperature — below the safe temperature for the seals. Then, run a sterilization cycle at the temperature. Collect samples for lab testing for bacteria and spores. If these results are negative then your problem is solved. CP Dirk Willard, senior process engineer Armitech Engineering, Downers Grove, Ill. www.chemicalprocessing.com http://CP.com http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - September 2007

Contents From the Editor Field Notes In Process Energy Saver Compliance Advisor Succeed at Simulation Rethink Your Approach to Process Safety Avoid Blending Blunders Get the Right Cartridge or Bag Filter Wireless Proponents Take HART Membrane Boasts Material Benefits Process Puzzler Plant InSites Chem Show Product Preview ISA Product Preview Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - September 2007

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