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We are designing a direct contact heat exchange system wherein there is a packed bed, hot water enters top of bed at 68 deg C and leaves the bed at 51 degC. Gas which is rich in methane & ethylene enters the bottom of the bed at 33 deg C and leaves the bed at 65 deg C. The intent is gas should get heated to 65 deg C. We are concerned about low approach temperature at the top of bed (68-65=3 deg C). Wanted to know anyone has any experienece with such system? Will that low approach is achievable? Please suggest any other similar system where we can check.
 
Answers
20/08/2015 A: S Banik, Centre for High Technology, sbdr@rediffmail.com
The system is like a cooling tower design. It is a case of gas heating with humidification. The gas would pick-up moisture from the water. Theoretically it is possible to design a column with 3 degC approach. However it would be a long tower preferably with structured packing. You would require to consider Lewis No which is a ratio of Schmidt no to Prandtl No.