Downcomer design

The downcomer area and plate spacing must be such that the level of the liquid and froth in the downcomer is well below the top of the outlet weir on the plate above. If the level rises above the outlet weir the column will flood. 

In terms of clear liquid the downcomer back-up is given by  

The main resistance to flow will be caused by the constriction at the downcomer outlet, and the head loss in the downcomer can be estimated using the equation given by Cicalese et al. (1947) 

Am = either the downcomer area Ad or the clearance area under the downcomer Aap, whichever is the smaller, m2. 

To predict the height of aerated liquid on the plate, and the height of froth in the downcomer, some means of estimating the froth density is required. The density of the aerated liquid will normally be between 0.4 to 0.7 times that of the clear liquid. A number of correlations have been proposed for estimating froth density as a function of the vapour flow-rate and the liquid physical properties see Chase (1967) however, none is particularly reliable, and for design purposes it is usually satisfactory to assume an average value of 0.5 of the liquid density. 

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Distillation trays constiircted of porous catalyticaUy active material and reinforcing resins Method described for removing or replacing catalyst on trays as a hquid slurry Catalyst bed placed in downcomer, designed to prevent vapor None specified Wang et al., Chinese Patent 1,060,228 (1992) Jones, U.S. Patent, 5,1.3.3,942 (1992)... 

Obtain downcomer design velocity, VD sg. Use the smallest value from these three equations... 

VDdsg = Downcomer design velocity, GPM/fT Vioad = Column vapor load factor WFP = Width of tray flow path, in. pL = Liquid density, Ibs/ft pv = Vapor density, Ibs/ft ... 

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

Figure 8-100. Segmental downcomer design chart. Used by permission, Belles, W. L. Pet. Processing Feb. thru May (1956).

Figure 11.24. Segment (chord) downcomer designs, (a) Vertical apron (b) Inclined apron (c) Inlet weir...

Downcomer liquid load For downcomer design, the liquid load is usually defined as the liquid velocity at the downcomer entrance (m/s or ft/s) ... 

Tray liquid load definitions. For tray (as distinct from downcomer) design the liquid load is usually defined as... 

Figure 8.30 Downcomer design chart, (o) Side segmental downcomer (b) center downcomer.

Downcomer check. Use the Koch correlation (Sec. 6.2.9), which is the more conservative correlation for downcomer design according to Lockett s analysis (12). 

Normally the pneumatically controlled pulse feeder operates with limited pressure difference between the hopper and the stop plate. When the pressure difference is in the direction opposite to that of the downward solids flow, the downcomer needs to be lengthened in order to seal the solids, or else other modes of downcomer design must be resorted to. 

Catalyst bed placed in downcomer, designed to prevent vapor Etherifications, alkylations ... 

Minimum theoretical stages at total reflux Heat transferred, Btu/hr Overall heat transfer coefficient, Btu/hrfP°F Vapor velocity, ft/sec Velocity under downcomer, ft/sec Downcomer design velocity, GPM/ft ... 

It is also required to size the downcomer for adequate phase separation. One method often employed for setting downcomer design velocity is laid out in the Glitsch design manual for valve trays. Here, the downcomer design velocity is correlated from the difference between liquid and vapor densities and the system factor as described above. For many low-pressure liquids, downcomer area at the top might allow 200-250 gpm/ft at 24 in. tray spacing. 

Adm Area of the downcomer, design minimum tm Valve thickness... 

Depending on the tray geometry (diameter, weir height, downcomer design, etc.) in trayed columns or on the type of packing in packed columns, a certain amount of liquid is retained within the column as liquid holdup. An additional amount of liquid holdup is associated with the condenser assembly. The remaining space within the distillation system contains the vapor holdup. In continuous columns, holdup is usually not a factor since it does not affect the separation as long as the column operates at steady state. 

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