Downcomer pressure drop

The major factors governing the proper design far clearance under the downcomer (see Figure 8-63), and the distance between the bottom of the downcomer and the tray it is emptying onto are [190] (a) downcomer sealing, (b) downcomer pressure drop, and (c) fouling and/or corrosive nature of the fluids. TTie smaller the clearance, the more stable will be the tray start-up due to the greater restriction to vapor flow into and up the empty liquid downcomer. 

Downcomer Pressure Drop. When the downcomer is less than minimally fluidized, the pressure drop can be estimated with a modified Ergun equation substituting gas-solid slip velocities for gas velocities (Yoon and Kunii, 1970), as shown in Eq. (9). 

When the downcomer is fluidized, the downcomer pressure drop can be calculated as in an ordinary fluidized bed as ... 

When one of the three draft tube velocities was increased to simulate upset conditions, stable operations were still possible. These upset conditions could also be detected by pressure drop differences among various draft tubes and downcomers when differences in draft tube velocities were large. For severe upset conditions, where some of the draft tubes become downcomers, pressure drop measurement alone could not distinguish the solids flow pattern inside the draft tubes. 

The downcomer pressure drop is calculated using equation 14.55. 

In sieve tray columns, excessive orifice and/or downcomer pressure drop within the extractor causes formation of large coalesced layers that back up and overflow the trays. 

Three major factors govern the specification of clearance under the downcomer downcomer pressure drop, the fouling and corrosive nature of the system, and downcomer sealing. 

Downcomer pressure drop. If the clearance under the downcomer is too low, it may add substantially to the downcomer backup and consequently reduce downcomer capacity. Cases have been reported (61) where column capacity was increased by simply cutting 1 in off the bottom of the downcomer. Methods of estimating the backup caused by hydraulic losses through the opening under the downcomer are available in most distillation texts (48, 319, 371, 409). 

If a clearance greater than the weir height is desired, the author recommends that it be made as low as permitted by the downcomer pressure drop and fouling criteria that the clearance not exceed weir height by more than 1 in and that the clearance be made at least 2 in less than the clear liquid backup in the downcomer at minimum vapor and liquid loads. 

Capacity factor Figure 10-29 Crest height over weir Eq. M 0-261 Downcomer pressure drop Eq. (10=27)... 

Install recessed sumps under the downcomer outlets. In effect, this modification increases the open area under the downcomer and this reduces downcomer pressure drop. 

The downcomer pressure drop is determined by the differences between the liquid heads and static heads ... 

Pressure Drop and Downspout Backup. Pressure drop through the caps can be a little larger than the hydraulic gradient computed by the Davies equation [Eq. (16-4)] for tray stability. At the same time, pressure drop causes liquid to back up into the downcomer. Pressure drops, by the study of T. C. Dauphine for three sizes of caps, based on the Doctoral dissertation, Massachusetts Institute of Technology, 1939. 

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