2. Packed Columns

baconossifiedMechanics

Oct 29, 2013 (3 years and 5 months ago)

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1

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.

2

Illustrational
drawing of
packed column



Illustrational
drawing of
packed column



3

Illustrational
drawing of
packed
column

4

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

σ,
m
2

/
( m
3

packing layer)


σ = n σ
0


σ
0
=Surface area of a packing, m
2
/a packing


n=Number of packings, n/m
3

5

(2)Packing void fraction
空隙率
ε:
m
3

/
(m
3

packing layer)

V0 = Volume of a packing, m
3

/a packing

(3)Packing factor
填料因子

:
σ/

ε
3
= Dry packing factor,
1/m


Φ=
(
σ/

ε
3
)
wet
= Wet packing factor

(4)Stacking density
ρ
p
, kg/m
3


(Packing weight
per unit volume of packings)


[
Table 3
-
5
][p.181]

6


Basic requirement for packing: Large surface area
per unit volume of packing
σ
; Large packing void
fraction
ε
; Low Stacking density
ρ
p
; 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

7


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]

8

鲍尔环



阶梯环


9

金属环矩鞍填料(
Intalox Metal Tower Packing


球形填料


10

波纹型规整填料


格栅型填料


11


Wall flow phenomenon
壁流现象
and its decreasing.

Liquid redistributor of
packed column

12


Main trends of new
-
type packing
development:

(1)Increasing flux;


(2)Improving the uniform distributions of fluid flow;

(3)Decreasing flow resistance.

13


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 saddle
-
type packings: Intalox saddle; Conjugate ring.

14

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.

15

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

.

16

1.Pressure drop of gas flowing through
packing depth (Relationship between
Δ
P/Z~u)

Superficial gas velocity based on empty tower
空塔气速






Gas velocity based on empty tower u



17

(1)L=0

Straight
line (Dry packing)

(2)L


〺⁃ur癥


Two turning points:
A(loading point
载点


and B(flooding point



).


Three zones:

.Constant liquid
holdup zone;


.Liquid loading zone;


.Flooding zone.

Logarithmic coordinates







Gas velocity based on empty tower u

18

Question: Why
does the curve
move left when L




Under the same u,
the
larger the L

is,
the
more left

the
curve is, that is,
L



/Z




Under the same V
and L


0


Actual velocity u
actual
> Gas velocity of empty tower u

Logarithmic coordinates







Gas velocity based on empty tower u

19

u
actual
> u, therefore,
under the same u,

(

/ZwithL


0is
greater than (

/Z
withL㴰.

Logarithmic
coordinates







Gas velocity based on empty tower u

20


Before point A, when
u

Ⱐthequ楤h潬u潦
the column does not
change, this is
constant
liquid holdup zone





After point A, when

u

Ⱐthequ楤h潬u潦
the column increased,
this is
liquid loading
zone




At point B and
afterwards, flooding
happens, this is
flooding
zone

.

Logarithmic coordinates







Gas velocity based on empty tower u

21

Logarithmic coordinates







Gas velocity based on empty tower u


Phenomena of
flooding
: The packed
sections fluctuate
violently
.

Pressure
drop of packed
sections rises more
rapidly
(







Liquidraidly
accumulates, and the
entire column may fill
with liquid. Flooding
is detrimental
operation.

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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 u
F
泛点气速

23

(1)Factors influencing (velocity at flooding)
泛点气速


u
F:

1)
Characteristics of packings:


填料因子





u
F



(Fldigwillthaeeasily.


For example, if
void fraction


islargeeugh,the
permissible gas flux will be large enough too, and u
F

.


isa
cmrehesiefactr
cmbiigtheifluecesf




adackigstructuresclumerfrmace.

2)Fluid physical properties:


V


and






u
F




L





u
F




3)Ratio of liquid and gas flow rates
液气比


(w
L
/w
V
)




u
F


24

(2)Determining the velocity at flooding u
F

(Eckert
generalized correlation: Figure 3
-
24, p.184)

25

(3)Determining


/Z
):



After determining u
F
, take u

=(0.5~0.85)u
F

calculate A and B

then determine

/Zfrm
Eckert generalized correlation.

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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, [m
3
/m
2
.s]


Minimum spray density U
min
: the lowest value of U
that ensures the packings wetted.

27

(L
W
)
min
——
Minimum wetting rate, [m
3
/m.s]



——
填料的比表面积

wetting rate: Liquid volume flow rate per unit length of
packing periphery.

(3
-
37)

28

3
-
2
-
3 Calculations of Packed
Columns

1.Column diameter:

V
S
=gas volume flow rate, m
3
/s


Attention: (1) D
T

should be rounded up;
(2)Checking:

(Avoiding wall flow)

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2.Column height

(1)Calculation from the number of transfer units:
Z=H
Oy
*N
Oy
; etc.

(2)Calculation from HETP: Z=HETP*N
T
, etc.

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3
-
2
-
4 Accessory
Structures of
Packed columns

1.Packing supports

(a)Grid plate

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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

32

2.Liquid distributors


Function: Uniformly distribute the liquid on the
surfaces of packings.

Shower nozzle type

Annular tubes with multi
-
holes

Overflow pipes

33

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?]