Page 326 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Packed Towers 31 5

Example 9-3: Alternate Evaluation of Tower Condition 1. Examine packing characteristics.
and Pressure Drop Size a/Es Surface area, ft*/fts
1 In. Rashig Rings 158 38
Using Example 92 as reference, the tower will be exam- 1 In. Intalox 124 78
ined for critical liquor rate, L,, using Figure 9-35. 1 In. Berl Saddles 125 76
1 In. Pall Ring* 45 66.3
1% In. Intalox 69 59.5
centipoise 2
Centistokes = for caustic sol’n. = - 1.64 1% In. Berl Saddle 79 44
=
1.22
*Metal, all others ceramic.
2. Percent of flooding for various packings, holding
reading: L, = 16,000 lb/(hr) (ft2)
tower flow rates (including reflux) constant
Refer to Flooding, Loading and Pressure Drop
Because actual L’ = 2250 is less than 16,000 this tower Chart, Figure 9-21D.
operates in the gas continuous region.
L 0.5 remains constant for same separation at
Pressure drop at flooding from Table 9-33 and Figure (PG/PL) increased production rate
9-34D
G2 $2 p0.2 increases as G2 at increased production
APf - 4 in. water/ft (not exact figure, because table is for rate for a fixed a/ 2.
ceramic rings) PG PLgc

Percent of Flooding at 2,000,000 Ib/mo.
Correction = 0.94
rate referenced to flooding at 1 in.
Packing ceramic Raschig rings
Then actual expected pressure drop at flooding: 1 In. R R = 100%

APf =. (0.94) (4) = 3.76 in. water/ft
1 In. Intalox (A)* (s) 96.9%
(100)
=
For 45 ft of packing:
1 In. Pall Rings
(A) (g) (100) = 35.2%
APf total = (45) (3.76) = 169 in. water (metal)
1
Comparison with Figure 9-21C gives 3 in. water/ft (para- 1- In. Intalox (g (3 (100)=53.9%
2
meter) or a total of (3) (45) = 135 in. water. Neither of
these values represents a condition (flooding) that should
be considered for tower operation, except under known 1- 1 In. Berl Saddles (A)* (g) (100) = 61-82
2
experience studies. Distillation operations sometimes
operate above flooding, but other types of contacting nor- The flooding of the packing is a direct function of
mally require operations in the loading region (or below) the therefore it is valid at constant separation to
for stable performance. examine the performance as shown. The metal Pall
rings appear to allow for a considerable increase in
Example 94 Change of Performance with Change in capacity. In fact the condition at 35.2% of flood might
Packing in Erristing Tower not be good from a contact efficiency standpoint.
3. Selection
The 1!4-in. Intalox or Berl ceramic saddles would
A tower is packed with 1-in. ceramic Raschig rings. It
presently floods while drying water from a product at a be the preferred choice because: (1) the flooding
point is sufficiently low and yet probably not too far
production feed rate of 1,800,000 lbs/month with 0.25 from the load point (only flood data available, but
mol% being water. Flooding does not start at the bottom, would estimate 70445% of load); (2) the surface area
but at some intermediate point up the tower. What can be per cubic foot is essentially the same as for the exist-
done to eliminate the flooding? Is it possible to increase ing 1-inch Raschig rings. By reference to the effective
production rate to 2,000,000 Ibs/month?
interfacial area graphs, and by using the Berl saddle
data instead of Intalox as an estimate because it is not