Page 372 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Packed Towers 361

will release the gas, e.g., C02 from ethanolamine; howev-
er, in other cases chemical treatment must be used. In
order to fully use the absorbent liquid, recycle systems
1920/ (28.4 mol. wt of mixture) must be established, and the economics of absorbent
KGa = selection become of considerable importance, determin-
(0.95) (11.2)
ing the size of the system in many cases. Because the
absorption ability of the various combinations of gas and
= 6.35 mol/hr (ft3) (am)
liquid vary rather widely and depend upon the system con-
dition, it is often necessary to examine more than one
Converting this to ammonia-H2-N2 mixture:
absorbent for a given gas. The performance of several
important systems is gi~7en in summary or reference form:
KC;a= 6.35 - O.Og6 = 7.9 mol/hr (ft') (atm)
[ 0.063]
I. Carbon Dioxide or Sulfur Dioxide in Alkaline
Solutions
Then, substitilting,

( 1920/ ( 11.2mol wt H -N mixture) n (9 - 106)
H oG for this mixture = KGa = Z A Aplm f..
(7.9)(11.2atm)
= 1.875 ft
where @$ = gas coefficient; lb mol/(hr) (ft.$) (am)
n = C02 (or SOP) absorbed, Ib rnol/hr
A = tower cross-section area, ft2
Toruer Packing Height for Amm~zia-H~drogfmNiicrogen Mixture
Z = height of packed section in tower, ft
Z = (1 2375) (6.33) = 11.9 Ft packing Aplm = log mean partial pressure of gas in inlet and exit
gas streams, am
fa = fraction effective packing wetted
For 1-in. Berl saddles allow 2.0 ft for good liquid disai-
bution through the packing from the top.
Design Procedure for Alkali-Absorbers
Minimum packed bed depth = 11.9 + 2.0 = 13.9 ft
1. Calculate material balance, determining quantity of
Use: 15 ft (or perhaps 18 ft to allow for variations in per- gas to be absorbed, and alkali required.
formance) 2. Estimate a tower diameter and establish its operating
point from Figure 9-21. For initial trial set operating
Expected Pressure Drop point at 50-60% of flood point system based on vary-
ing gas mass velocity at variable tower diameters for a
Packing: (15) (0.5 in/ft) = 7.5 in. fixed
Support: (Grating type), approx. = 1.0 0.5
Total drop = 8.5 in. water

Inkt Liquid
3. Estimate the effective fraction wetted packing area
For this small diameter tower, bring water into center of by Figure 946 or 943. As a general rile, try not to
top of packing with a mned down 90" ell, placed 69 in. accept design if fraction wetted is less than 0.5. Ifit is
above packing. less, adjust tower conditions to raise value, bearing in
mind that this factor is based on scattered and very
Mass Transfer With Chemical Reaction incomplete data.
4. Determine Q based on inlet alkali normality and per-
Many absorption processes involve some chemical reac- cent conversion to carbonate (for carbon dioxide).
tion; however, it is fortunate that satisfactory correlation
can be made without delving into the complexities of the Kp Data and Corrections [42, 69, 7.21
reaction.
In many instances the gas being absorbed is to be recov- The necessary data required to properly design a C02
ered from the solution, and hence the effluent from the scrubber are separated into three sections, depending
absorp~on must be treated. Often the application of heat upon the CO2 concentration in the feed gas. Corrections