Page 26 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: LDK Revised Pages
Encyclopedia of Physical Science and Technology EN001H-01 May 7, 2001 16:18

Absorption (Chemical Engineering) 23

−1
Downcomers layout. Usually, segmental downcom- F p Packing factor, ft −1 (m )
2
ers are used, in which the downcomer area extends from G Gas flow rate (Fig. 16 only), lb/(s ft )
2
the weir to the column wall (Fig. 17), but other designs (kg/(s m ))
are not uncommon. The design must consider down- G Gas flow rate, lb/h (kg/h)
2
comer hydraulics as well as mechanical and structural g c Conversion factor, 32.2 (lb ft)/(lbf s )
factors. (1.0(kg m)/(N s ))
2
The need for positively sealing the downcomer is deter- G M Molar gas-phase mass velocity,
2
2
mined in this phase. This could be achieved by installing lb mol/(h ft ) [kmol/(s m )]
an inlet weir, which is a weir installed at the tray inlet to G Molar gas-phase mass velocity of rich gas,
M
2
2
keep the downcomer outlet immersed in liquid. A similar lb mol/(h ft ) [kmol/(s m )]
device, which extends below the tray floor, is a seal pan H Enthalpy, Btu/lb mole (kJ/kmol) (Fig. 12
(Fig. 17). Both devices provide positive assurance against and Eq. (33) only)
vapor rising up the downcomer, but they may also trap H Henry’s Law constant, atm (kPa)
solids and dirt and cause blockage. A seal pan must al- h Height parameter for packed towers, ft (m)
ways be used in the downcomer from the bottom tray; H a Hatta number, defined by Eq. (12),
otherwise there is nothing to prevent vapor from rising up dimensionless
the bottom downcomer. H G Heightofatransferunitbasedongas-phase
resistance, ft (m)
Height of a transfer unit based on liquid-
H L
NOMENCLATURE phase resistance, ft (m)
Height of an overall gas-phase mass-
H OG
A Component A transfer unit, ft (m)
A Absorption factor, L M /(mG M ), h T Contactor height, ft (m)
dimensionless k 2 Second order reaction rate constant,
3
3
a Effective interfacial mass transfer area per ft /(h lb mol) [m /(s kmol)]
3
2
2
3
unit volume, ft /ft (m /m ) k G Gas-phase mass-transfer coefficient for
2
A Modified absorption factor, given by dilute systems, lb mol/(h ft mole fraction
2
Eq. (31b) solute) (kmol/(s m mole fraction solute))
A e Effective absorption factor, given by k G Gas-phase mass-transfer coefficient for
Eq. (31a) concentrated systems, same units as k G
B Component B k Gas-phase mass transfer coefficient for
G
b Number of moles of component B reacting multicomponent systems, same units as k G
with 1 mole of component A k L Liquid-phase mass-transfer coefficient for
C Component C dilute systems, same units as k G
c Number of moles of component C k Liquid-phase mass-transfer coefficient for
L
produced when 1 mole of component A concentrated systems, same units as k G
reacts with b moles of component B k Liquid-phase mass transfer coefficient for
L
Concentration of reactant A in the liquid,
C A multicomponent systems, same units
3
3
lb mole/ft (kg mole/m ) as k G
Concentration of reactant B in the liquid, o
C B k L Liquid-phase mass-transfer coefficient for
3
3
lb mole/ft (kg mole/m ) pure physical absorption (no reaction),
Concentration of reactant B in the bulk
C B 0 same units as k G
3
3
liquid, lb mole/ft (kg mole/m ) K OG Overall gas-phase mass-transfer
C SB Flooding capacity parameter, given in coefficient for dilute systems, same units
Fig. 20, ft/s (m/s) as k G

D A Diffusion coefficient of component A in K OG Overall gas-phase mass-transfer
2
2
the liquid phase, ft /h (m /s) coefficient for concentrated systems, same
D B DiffusioncoefficientofcomponentBinthe units as k G
2
2
liquid phase, ft /h (m /s) K Overall gas-phase mass transfer coefficient
OG
E Energy transfer rate across interface, Btu/h for multicomponent systems, same as units
(kJ/s) as k G
√
2
F iv Flow parameter, (L/G) ρ G /ρ L , L Liquid flow rate (Fig. 16 only), lb/(s ft )
dimensionless (kg/(s m ))
2