Chemical Processing - January 2008 - (Page 41) >> PROCESS PUZZLER PROCESS PUZZLER < > THIS MONTH’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 safely heat the bed. 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 only take 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 more than 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 more than 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 isn’t practical, what other ideas can you suggest? Use a vacuum ejector Use a compressor and an eductor. The compressed gas can be dried to alleviate the condensation problem. Only compress the fraction of the gas necessary to get the flow rate you need. Shove the compressed gas through the motive port of the eductor to pull the remaining gas through. The question remains how much gas needs to be heated and where to best locate the compressor inlet and the eductor in relation to the gas/oil heat exchanger. Michael J. Eggers, senior process engineer Integrated Separation Solutions, Sharon, Wis. Try a desiccant We suggest installing a desiccant box at suction of blower feeding the catalyst bed. You must eliminate the water early in the process, at least during start-up. To avoid an increase in pressure drop and poor performance of the blower please provide several desiccant boxes in parallel. www.chemicalprocessing.com >> MARCH’S PUZZLER We use 5% caustic solution as a feedstock for a saponification reactor. To prevent freezing, the 1-in.-diameter pipe from the storage tank to the reactor pump is heat-traced and insulated. Nevertheless, this pipe frequently is plugged with caustic. What is causing this and what can we do to reduce maintenance on this line? Send us your comments, suggestions or solutions for this question by February 8, 2008. We’ll include as many of them as possible in the March 2008 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. January 2008 • 41 http://www.chemicalprocessing.com http://www.chemicalprocessing.com
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