2. Packed Columns
•Continuous contacting (differential contacting)
equipment.
•Basic characteristics: Simple construction; low
pressure drop; packings easily made of corrosion腐蚀
resistant materials.
•Applications: (1)Distillation with low pressure drop
and vacuum distillation; (2)Absorption and
distillation with falling liquid flow rate far greater
than upward vapor flow rate; (3)Corrosive
materials; (4)Small diameter column; (5)Easily
foaming materials; (6)Heat sensitive materials.
1
Illustrational
drawing of
packed column
Gas Gas
Gas
Liquid
Liquid
Gas
Liquid
Mist
catcher
Mist
catcher
Liquid
Mist
catcher
Clamp
plate
Clamp
plate
Mist catcher
Packing restrainer
Clamp plateColumnar
Columnar shell
shell shell
Columnar
Columnar
shell
Columnar
shell
Gas
Packings
Packings
Packings
Packings
Packings
Liquid Support
Support
plate Suppor
Packing
Packing
Support plate
plate
Support
Mist
catcher
Liquidredistribu
redistribu
tortor
Liquid
Liquid
redistrib
Liquid
redistribu
tor
Liquid redistribu
tor
Columnar
Columnar
shell
shell
Packing restrainer
Packings
Packings
Packing
Packing
Support
Suppor
Gas
Liquid
Liquid
redistribu
redistrib
tor
Liquid
Gas Mist catcher
Clamp plate
Liquid
2
Illustrational
drawing of
packed
column
3
3-2-1 Packings
1. Characteristics of packings
•Function of packings: Supplying space of mass
transfer.
(1)Surface area per unit volume of packings 比表面
积σ, m2 /( m3 packing layer)
σ = n σ0
σ0=Surface area of a packing, m2/a packing
n=Number of packings, n/m3
4
(2)Packing void fraction空隙率ε: m3 /(m3 packing layer)
V  NV0

 1  nV0
V
V0 = Volume of a packing, m3 /a packing
(3)Packing factor填料因子 : σ/ ε3 = Dry packing factor,
1/m
Φ=(σ/ ε3)wet= Wet packing factor
(4)Stacking density ρp, kg/m3
(Packing weight per unit volume of packings)
[Table 3-5][p.181]
5
•Basic requirement for packing: Large surface area
per unit volume of packing σ; Large packing void
fraction ε; Low Stacking density ρp; Good wetability;
Low fabricating cost; Enough mechanical strength.
2. Types of packing
•Classification according to the structure
(1)Solid packing: Rings/Saddles/Sheets
(2)Gauze packing: Net-like saddles / θ nets
/Corrugated nets
6
•Classification according to the manner of installation:
(1)Random packings ;
(2)Structured packings.
•Fluid mechanics and mass transfer performance
comparison among several kinds of common used
tower packings? (Rachig ring, Pall ring, Cascade ring
[pp.178-181]
7
鲍尔环
阶梯环
8
金属环矩鞍填料（Intalox Metal Tower Packing
球形填料
9
波纹型规整填料
格栅型填料
10
•Wall flow phenomenon壁流现象
and its decreasing.
Liquid redistributor of
packed column
11
•Main trends of new-type packing
development:
(1)Increasing flux;
(2)Improving the uniform distributions of fluid flow;
(3)Decreasing flow resistance.
12
•Two aspects of development:
1.Random packings:
Rings and saddles.
•Advantage of ring-type packings: Large flux;
Disadvantage of ring-type packings: Bad liquid
redistribution.
•Advantage of saddle-type packings: Good liquid
redistribution; Disadvantage of saddle-type packings:
Small flux.
•Combining the advantages of ring-type and saddletype packings: Intalox saddle; Conjugate ring.
13
2.Structured packings:
•Performance is much better than random packings
with very low pressure drop, good liquid
redistribution, large flux. But the fabricating cost is
expensive.
14
3-2-2 Fluid Mechanics of packed
columns填料塔流体力学性能
•Including:
(1)Pressure drop of gas flowing through
packed sections气体压降 ;
(2)Liquid flooding velocity液泛气速 ;
(3)Liquid and gas distributions气两相流体分布;
(4)Liquid holdup持液量 .
15
1.Pressure drop of gas flowing through
L
packing
depth (Relationship between ΔP/Z~u)
Superficial
gas velocity based on empty tower空塔气速u:
L
V
V
L L
u
L
2

DT
Z
L L
4
L
Ⅲ
R  P V V
V
Z Z
压降 P
Ⅱ
Z
Z
L
R
R
PPR  P
L
Ⅰ
L
V
L
V
u
 Dtower u
Gas velocity based on empty
Z
4
V
16
R  P
2
T
(1)L=0：Straight
line (Dry packing)
P
Z
u
Ⅲ
1.8~ 2.0
压降 P
Z
Ⅱ
(2)L  0: Curve
•Two turning points:
A(loading point载点 )
and B(flooding point泛
Ⅰ
点 ).
•Three zones:
Ⅰ.Constant liquid
V
holdup zone;
uon
 empty2 tower u
Gas
velocity
based
 DT
Ⅱ.Liquid loading zone;
Ⅲ.Flooding zone.
4
Logarithmic coordinates
17
•Under the same u,
the larger the L is,
the more left the
curve is, that is, 压降 P
Z
L（P/Z）
Question: Why
does the curve
move left when L？
Ⅲ
Ⅱ
Ⅰ
V
u
Gas velocity based on empty
DT2 tower u
4
•Under the same V
Logarithmic coordinates
and L  0，
Actual velocity uactual> Gas velocity of empty tower u
18
uactual> u, therefore,
under the same u,
Ⅲ
(P/Z) with L  0 is压降 P Z
greater than (P/Z)
with L=0.
Ⅱ
Ⅰ
u
V
Gas velocity based on
DT2empty tower u
4
Logarithmic
coordinates
19
•Before point A, when
u, the liquid holdup of
the column does not
change, this is constant
压降 P
Z
liquid holdup zoneⅠ.
• After point A, when
u, the liquid holdup of
the column increased,
this is liquid loading
zone Ⅱ.
Ⅲ
Ⅱ
Ⅰ
•At point B and
afterwards, flooding
V

happens, this is flooding Gas velocity based onu empty
tower
u
2
 D
zone Ⅲ.
4 T
Logarithmic coordinates
20
•Phenomena of
Ⅲ
flooding: The packed
sections fluctuate
violently. Pressure压降 P Z
Ⅱ
drop of packed
sections rises more
rapidly (P ).
Liquid rapidly
Ⅰ
accumulates, and the
entire column may fill
with liquid. Flooding
V

is detrimental
Gas velocity based onu empty
tower
u
2
 D
T
operation.
4
Logarithmic coordinates
21
2. Flooding
Normally, liquid is the scattered phase and gas the
continuous phase.
When flooding, liquid becomes the continuous phase
and gas the scattered phase.
•At flooding point B, the corresponding gas velocity
is uF 泛点气速
22
(1)Factors influencing (velocity at flooding)泛点气速
uF:
1) Characteristics of packings: 填料因子   uF 
(Flooding will not happen easily).
•For example, if void fraction  is large enough, the
permissible gas flux will be large enough too, and uF. 
is a comprehensive factorcombining the influences of , 
and packing structures on column performance.
2)Fluid physical properties:  V and    uF ； L 
 uF .
3)Ratio of liquid and gas flow rates液气比:
(wL/wV)  uF
23
(2)Determining the velocity at flooding uF (Eckert
generalized correlation: Figure 3-24, p.184)
wL
A
wV
 V

 L
1
 2
u 2 
 ，B 
g

A
A
B
B
 V  0.2

 
 L 
Flooding line
(u  u F(u)  u F )
泛点线
泛点线
P
P     
Z
Z
A
B
24
(3)Determining （P/Z）：
A
•After determining uF, take u =(0.5~0.85)u
B
F，
calculate A and B，then determine
from
(u  uP/Z
)
F
Eckert generalized correlation.
泛点线
P
A
Z
 
B
(u  u F )
泛点线
P
Z
 
A
B
25
3. Wettability
•Wetted surfaces of packings are effective mass
transfer area.
•Definition of spray density U [p.185]:
Liquid volume sprayed per unit time per unit cross
section of the column, [m3/m2.s]
•Minimum spray density Umin: the lowest value of U
that ensures the packings wetted.
26
U min  ( LW ) min  
(3-37)
 03/m.s]
.08 m
[m
3
当d 填料  75
m m，
( Lrate,
;
(LW)min——Minimum
wetting
W ) min
( m  h)
 ——填料的比表面积
3
m
当d 填料  75m m, ( LW ) min  0.012
(m  h) of
wetting rate: Liquid volume flow rate per unit length
packing periphery.
U min  ( LW ) min  
When d packing  75m m，
( LW ) min
3
m
 0.08
When d packing  75m m, ( LW ) min
3
m
 0.012
( m  h)
;
( m  h)
27
3-2-3 Calculations of Packed
Columns
1.Column diameter:
4VS
DT 
u
u  (0.5 ~ 0.85)u F
44VVSSflow rate, m3/s
VS=gas
volume
D
D 
1)U  U
TT
uu适宜
适宜
min
D
; 2) T
d 填料
 8(避免壁
•Attention: (1) DT should be rounded up;
uu适宜
 ((00..55 ~~ 00..85
85))uuFF
(2)Checking:
适宜 
D
D
TT
11))U
U U
Umin
;
;
2
2
)
)
min
 8(避免壁流现象
8 (Avoiding wall
) flow)
dd填料
packing
28
2.Column height
(1)Calculation from the number of transfer units:
Z=HOy*NOy; etc.
(2)Calculation from HETP: Z=HETP*NT, etc.
29
3-2-4 Accessory
Structures of
Packed columns
1.Packing supports
(a)Grid plate
30
•Function of packing support:
Supporting the packings and
the liquid holdup, ensure fluent
flow of gas and liquid.
•Basic requirement:
Enough mechanical strength,
and free cross sectional area.
(b)Riser升气管式
31
2.Liquid distributors
•Function: Uniformly distribute the liquid on the
surfaces of packings.
Shower nozzle type
Annular tubes with multi-holes
Overflow pipes
32
3.Liquid redistributor
Function: Reducing the non-uniform distribution
of liquid, and reducing the wall flow.
4.Mist catcher (Entrainment eliminator)
Function: Eliminating the entrained liquid drops in
the gas stream at the outlet.
5.Gas inlet
Function: Ensuring the uniform distribution of gas
flow on the cross sectional area of the column.
[Comparison between packed and plate towers?]
33