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# How To Interpret Hetp From Rate-Based Simulation?

hetp packed tower

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### #1 victornassau

victornassau

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Posted 17 July 2018 - 01:49 AM

Hello,

I want to understand better the physical meaning of the height equivalent to a theoretical plate(HETP) calculated from rate-based distillation. I'm using Aspen Plus to evaluate different packings for a glycol separation. My goal is to get the most efficient packing to minimize the height, as I need 80 theoretical stages. As input I give the packed height, say 4m per section and I'm using 8 sections. As a result of the simulation I have a chart showing the HETP per stage, for example the HETP plot attached. In this example, the median value is around 0,54m. Does that mean the total packed height I need is 80 x 0,54 = 43,2m instead of the 32m in the input?

I got confused when I was studying with the book "Separation process principles : chemical and biochemical operations / J. D. Seader, Ernest J. Henley, D. Keith, Roper.—3rd ed.". In example 12.2, page 466, they simulate a trayed tower, with 20 trays, 0,5m tray spacing, total 10m, and calculate a Murphree efficiency between 0,73 and 0,79, depending on the component, and state that " the 20 trays are equivalent to approximately 15 equilibrium stages". Fine. But then in example 12.3 they do the same separation using a packed tower. With 3 sections, one of 13m and 2 of 6,5m, and calculate a HETP around 0,5m, depending on the component. But that gives an equivalent of 13 theoretical plates for the same separation. I don't understand this difference. Since the trayed tower has 10m and the packed, 26m, the choice is for the trayed tower, correct?

Thank you,