Page 129 - Applied Process Design For Chemical And Petrochemical Plants Volume II

118 Applied Process Design for Chemical and Petrochemical Plants

accomplishing the desired absorption. As the number of Feed Gas
trays in the absorber is increased, the amount of heavier Mol or Volume %
material (larger A values) absorbed increases greater than 18.5
the lighter components (lower A values). More heavier 22.3
materials are also absorbed as the temperature of absorp- 20.3
tion is lowered. Thus, a cold lean oil has more capacity per 0.5
gallon than a warm oil. This requires a study of the entire 22.0
process and not just the one unit. Heat economy, oil flows, 0.7
and tower costs all enter into a full evaluation of the 2.5
absorber as it fits into the plant system. 13.0
Many designs are set up by assuming the number of the- 100.0
oretical trays, using best available information for tray effi-
ciencies and then calculating the expected performance.
A series of such studies might be made. Determine the oil rate and the number of theoretical and
actual trays required. (Note: this example illustrates that
unreasonable results must be examined, notjust accepted.)
Absorption and Stripping Efficiency
16,000,000
1. Inlet rich gas rate =
Unfortunately, the efficiencies for tray and overall col-
umn operation are incomplete and nullify to a certain (24)
extent some very high quality theoretical performance
design. Tray efficiencies may be estimated by Figure 8-29 = 1,756 mols/hr
or Table 8-1 1. Mols ethylene in = (20.5/100) (1,756) = 360 mols/hr
Mols ethylene in outlet gas = (100 - 98/100) (360)
= 7.20 mols/hr
Table 8-11 Mols ethylene absorbed in oil = 360 - 7.2 = 352.8 mols/hr
AbsorptionStripping Approximate Tray Efficiencies** 2. Specified ethylene separation = 0.98 = E~
- - Mols in - Mols out
pressure
Range
me Range, psig Temp., "F Efficiency % Mols in
Absorption 3. Minimum L/V for ethylene:
Hydrocarbon Oils &
Vapors 04300 30-1 30 35-50
Propme-key 100-2,100 - 30-37*-38
Butane-key 100-2,100 - 28-33*-36
StriPP* Average tower conditions for K:
Hydrocarbon Oils
with Steam 0-130 300-350 30-80 Temperature = (100 + 80)/2 = 90°F
Unsaturates in Oil Pressure: allow 20 psi pressure drop, then top pressure
with closed reboiler 0-50 - 25-35
would be 70 - 20 = 50 psig
*Average value
**Hull, R J. and K Raymond, Oil and Gas Journul, Nov. 9, 1953-March Average: (50 + 70)/2 = 60 psig
15, 1984 [32], used by permission. K (ethylene) at 60 psig and 90°F = 11.5 (from equilibrium
charts).

Example 8-34: Determine Number of Trays for Specified (%Imin = (11.5) (0.98) = 11.28
Product Absorption
A gas stream is to have the ethylene removed by absorp 4. Operating (L/VN + ll0 = (1.25) (11.28) = 14.1
tion in a lean oil of molecular weight 160, sp. gr. 0.825.
The inlet gas is at 70 psig and 100°F and the oil is at 80°F. 5. OperatingAio = (;+l),(+-)=g=1.227
-
The gas rate is 16,000,000 scf/day (60°F). Examine the
tower performance for 98% ethylene recovery at 1.25
times the minimum L,/VN. 6. Theoretical trays at operating (Lo/V~ + 1):