Flooding and Allowable Loads

The correlation of Eckert (Fig. 13.37) combines a pressure drop relation and safe flow rates insofar as staying away from the flooding point is concerned. A flooding line corresponds to pressure drops in excess of 2 in. water/ft. In use, a pressure drop is selected, and the correlation is applied to find the corresponding mass velocity G from which the tower diameter then is calculated. Another correlation recommended by a manufacturer of packings appears in Rgure 13.40. Example 13.16 compares these correlations for a specific case they do not compare any more closely than could be expected from the scatter of flooding data. 

Liquid distributor/ redistributor Hold down grid 

System Die. ID Packing Type Size, ID Bed Depth m HETP, HTU, m System press., kPa 

Fatty acid Benzene/mono- 0 76 Pall rings 0 03 8 12 19 085 4 94 

DBOC bateh CrtjCi/ CH3CIj/ CHClj/ 0.46 Pall rings 0 03 8 9 14 0 49 267 

General information Type Knitted multifilament Knitted multifilament Corrugated Rolled screen Multiple unsealed 

Approximate number of units 12 in and larger 610 90 woven-wire fabric 500 with spacers 43 downcomer trays on close tray spacing 120 

Process and system considerations Minimum head pressure, torr l 1 0.5 0.5 5 

Sensitivity to uneven initial liquid Moderate Moderate Fairly low High Moderate 

Pressure drop, mmHg per foot of packed height See Fig. 3 See Fig. 2 See Fig . 4 

---

Packed Towers 433 Kinds of Packings 433 Flooding and Allowable Loads 433 Liquid Distribution 439... 

Calculate minimum allowable superficial vapor velocity (flooding) for equal vapor and liquid loads, for the X-100 and S-100 packings. 

Packed lowers, 687-708 allowable velocity in, 697-700 cost of components of, 708-713 flooding point in, 697-702 liquid distribution in, 691-692 loading point in, 692-694 pressure drop in, 692-697 relative merits of plate and, 706-708 Packings ... 

Using the modified GPD curve (Fig. 7), the flow parameter (X), and the flood pressure drop, find the allowable packing load parameter, Lp. Fig. 7 shows pressure drop isobars at flood for different flow parameters. Determine the Souders-Brown coefficient at flood ... 

For vacuum distillation, a preliminary test for leakage is necessary. The apparatus is evacuated when the required pressure has been reached the vacuum pump is stopped and the rate of increase of pressure is observed (section 5.4.1). A further test for tightness is performed when the column is in operation and the thermal load has been established. The still pot heater is switched on only when the desired pressure is attained, and the column jacket heater as soon as vapour evolution starts. The cooling water should be tiumed on before the heaters are put into operation during the initial period its rate of flow should be checked at intervals since it usually tends to vary somewhat at the start with the expansion of tap washers. When the vapour has reached the colunm head the load is adjusted to the required value (section 4.10.7) unless the column is to be flooded (section 4.10.8) to wet the packing. The apparatus is then allowed to run for at least half an hour at total reflux, so as to allow the initial boiling point to be accurately determined, and only then is it adjusted to the required reflux ratio (section 4.10.4). 

Equation (5.7-12) is the basis for most correlations used to predict maximum allowable vapor velocity. The correlations are based on observed flood conditions in operating columns as evidenced by a sharp drop in efficiency (Fig. 5.7-5), a sharp rise in pressure drop, or a liquid loading condition giving difficulty in maintaining operating stability. A discussion of entrainment flo< measurement has been given by Silvey and Keller. ... 

---