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'Because of minimum wetting rate considerations, Ludwig (63) recommends against de- j signing for pressure drops lower than 0.1 in water/ft. Billet (56) also warns against designing j packed vacuum towers with too low a pressure drop.

'Reasons for the low pressure drop recommended for vacuum systems are prevention <d j mist and entrainment at the high vapor velocities experienced under vacuum (1), and minimizing bottom temperature, and therefore, material degradation (1,89). 1

"The maximum pressure drop is. low because of the high foaming tendency. This is equivalent to incorporating derating factors in tray column design (Sec. 6.2.10). Eckert (1) de- ; scribes an alternative and better means of derating packed tower designs for foaming systems. j

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