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1.0 Introduction
Fossil fuels supply more than 90% of the world’s energy needs. However, the
emission gas such as carbon dioxide will cause the greenhouse effect to the environment
due to the combustion process of fossil fuels. CO2 gas is produced during the reaction
contributes to additional absorption and emission of thermal infrared in the atmosphere
and eventually results in the global climate changes. As a result, the elimination of CO2
from combustion process is an important issue for now. There are a number of
technologies introduced for the CO2 capture such as absorption, membrane separation,
cryogenic and etc. Other than fossil fuels, the CO2 capture of remaining commercial
available fuel such as biofuel, coal biomass and natural gas is also our main concern. The
main challenge according to CO2 capture technology is to reduce the overall cost by
lowering both the energy and the capital cost requirements. Therefore, compromise
between cost and efficiency for an available CO2 capture technology is very important.
Commercial CO2 capture technology that exists today is very expensive and large energy
usage. Besides, they are a number of relatively low cost CO2 mitigation technologies
included improving energy supply and end-use efficiency by switching coal or oil to gas
where possible, forestation, and inexpensive renewable energy application. Definitely
they are sufficient for short term goals, but they will not be the final solution for long-
term.
1.1 Objective
Owing to the greenhouse gas emits mainly because of mankind activity, especially
combustion, which is always happen in industries. Therefore the major effect of this will
cause harm to human and also affect the climate change. Hence, the purposes of gas
scrubber system introduced to the industries are shown as below:
- Minimize the greenhouse effect
- Meet the requirement of Clean Air Act
- Minimize the losses of solvent
- Maximize the efficiency of plant and design integrity
- Minimize the consumption of utilities
1The Realization of a Packed-bed Tower
2.0 Packing
There is a variety of different packings in shape, size and performance are available and
they can be classified into three categories:
Random or dumped packings
Structured packings
Grid packings
Random packings are just dumped into the shell to give the packing pieces a random
orientation. Structure packings are stacked in the shell to take the shape of a packed bed.
Characteristics of tower packings
Besides low cost, the desirable characteristics of packings are described below (Kister,
1992).
a) A large surface area: Interfacial area of contact between the gas and the liquid is
created in a packed bed by spreading of the liquid on the surface of the packing.
Smaller packings offer a larger area per unit packed volume, but the pressure drop
per unit bed height becomes more.
b) Uniform flow of the gas and the liquid: The packed bed must have a uniform
voidage so that a uniform flow of the gas and of the liquid occurs. The shape of
the packing should be such that no stagnant pocket of liquid is created in the bed.
A stagnant liquid pool is not effective for mass transfer.
c) Void volume: A packed bed should have a high fractional voidage so as to keep
the pressure drop low.
d) Mechanical strength: The packing material should have sufficient mechanical
strength so that it does not break or deform during filling or during operation
under the weight of the bed.
e) Fouling resistance: Fouling or deposition of solid or sediment within the bed is
detrimental to good tower operation. Bigger packings are less susceptible to
fouling. Also, the packings should not trap fine solid particles that may be present
in the liquid.
2The Realization of a Packed-bed Tower
Types of tower packings
Tower packings are made of ceramics, metals or plastics. Kister (1992) and Larson
(1997) identified three generations of the evolutionary process of the random packings.
a) First generation random packings (1907 to mid-1950s): These included three
types of packings— Raschig rings, Lessing rings and other modifications of the
Raschig ring and Berl saddles. These are mostly packed randomly; ‘stacked’
packings are used in only a few cases.
i. Raschig ring: This is the oldest type of tower packing introduced by the
German chemist F. Raschig in 1907. It is a hollow cylinder having a
length equal to its outer diameter. The size of the Raschig ring ranges from
¼ inch to 4 inches. These rings are made of ceramic materials (unglazed
porcelain), metals or plastics (e.g. high-density polyethylene, HDPE).
Metal or plastic rings are made by cutting tubes of a suitable size. The
Raschig ring is probably the most rugged packing and can be used even
when a severe bumping or vibrating condition may occur. Other members
of the Raschig ring family are: (1) ‘Lessing ring’, which is similar to the
Raschig ring except that it has a partition along the axis of the ring. The
partition increases the surface area but the advantage is rather small in
3The Realization of a Packed-bed Tower
Random Structured Grid
First generation
Raschig ring, Lessing ring, Cross-partition ring, Berl saddle, Spiral ring.
Second generation
Pall ring (plastic and metal) and its modified versions (like Flexiring, Hy-pac, etc), Intalox saddle and its modifications, etc.
Third generation
IMPT (Norton), CMP (Norton, Glitsch), Nutter ring, Jaeger Tripac, Koch Flexisaddle, Fleximax, Norpac, Hiflow,etc
practice. This packing has not been quite popular. (2) The ‘cross-partition
ring’ that has two partitions instead of one in a Lessing ring. (3) The
ceramic ‘spiral ring’ that has an internal helix which creates internal whirl
of the gas and of the liquid and enhances the rate of mass transfer. The
latter two types are sometimes stacked in one or two layers on the support
grid of a randomly packed tower. Although Raschig rings are still in use,
the other variations of them are rarely used.
ii. Berl saddle: The berl saddle is the first modern packing developed in the
late 1930s. It is so called because it has the shape of a saddle. A packed
bed of Berl saddles has a larger specific surface area (i.e. surface area per
unit packed volume) and a smaller voidage than the Raschig ring.
Compared to the Raschig ring, the pressure drop is substanitially less
because of its ‘aerodynamic shape’. It has a rib on one surface that
prevents possible overlapping of the surfaces of two adjacent pieces. Berl
saddles offer higher capacity and a better performance than Raschig rings
but are more expensive.
Raschig ring Cross-partition ring
Lessing ring Berl saddle
Figure 1: Various types of first generation random packing ring
4The Realization of a Packed-bed Tower
b) Second generation random packings (mid 1950s to mid-1970s): The ‘Intalox
saddle’ may be considered to be the first member of the second generation
random packing developed by the Norton Chemical Products Corporation in the
early 1950s. It is an improved version of the Berl saddle and offers lesse ‘form
friction’ resistance to gas flow. Because of its particular shape two adjacent pieces
of the packing do not ‘nest’ and hence a stagnant pool of liquid is not created
between them. The area of the packing is almost fully utilized for effective
contact and mass transfer between the gas and the liquid phases. Similar to the
Berl saddle, it offers a larger specific interfacial area and a smaller pressure drop
compared to the Raschig ring. However, Intalox saddles are better packing than
the Berl saddles. Koch-Glitsch offers a similar ceramic packing under the trade
name ‘Flexisaddle’. The Intalox saddle and its modified varieties are of ceramic
or plastic make. The smooth edges of the Intalox saddle are scalloped and holes
inserted to make the super Intalox. This design promotes quick drainage of the
liquid, eliminates stagnant pockets and provides more open area, higher capacity
and efficiency. ‘Intalox snowflakes’, introduced by the Norton Corp. in 1987, is a
plastic packing of unique shape having a large number of liquid drip points,
causing continuous renewal of the liquid surface and superior mass transfer
performance.
Pall rings: The pall ring and its modifications evolved from Raschig ring, It is made by
cutting windows on the wall of a metal Raschig ring and bending the window tongues
inwards. While a bed of saddles offers reduced ‘form friction’ or drag because of the
aerodynamic shape, pall rings do so by allowing ‘through flow’ of the gas, because direct
passages on the wall are available. Since the interior surface is much more accessible to
gas and liquid flow, the capacity and efficiency of the bed are enhanced. Similar packings
are marketed by other companies under different trade names. The metal ‘Hy-Pak Tower
Packing’ of the Norton Corp., a slightly modified version of the Pall ring, has two bent
tongues in each window and is claimed to have better efficiency. Ceramic Pall rings,
which are Raschig rings with a few windows on the wall, have not been very popular.
5The Realization of a Packed-bed Tower
Intalox saddle Pall ring
Plastic Pall ring Metal ‘Flexiring’ (Koch Engg.)
Norton ‘Hy-Pak’ ring (metal)
Figure 2: Variouss type of second generation random packing ring
c) Third generationrandom packings (mid-1970s-): A pretty large number of metal
and plstic tower packings have been developed since mid-seventies that offer
improved performance in terms of lower pressure drop, less weight, larger
interfacial area and lesser liquid retention in the bed. Many of these packings
evolved from the intalox saddle. The ‘Intalox Metal Tower Packing’ (IMTP), a
6The Realization of a Packed-bed Tower
random packing developed by the Norton Corp., combines the high volume and
even distribution of surface area of a Pall ring and the aerodynamic shape of the
intalox saddle. The ‘Fleximax’ is an open saddle type packing from Koch-Glitsch.
‘Nutter rings’ have somewhat similar characteristics and are available in both
metal and plastic.
Several third generation random packings have been the offshoots of the Pall ring.
The Cascade Mini-Ring (CMP) is similar to the Pall ring but has a height-to-
diameter ratio (aspect ratio) of 1:3 compared to 1:1 of the latter. Because of low
height, such a packing element has a lower centre of gravity and therefore tends to
orient with the circular open end facing the vapour flow. This reduces friction and
enhances the mass transfer coefficient and effective surface area. The ‘Chempak’
or ‘Levapak’ ring is made by cutting the Pall ring in two halves, exposing the
tongues and promoting better performance. The Jaeger ‘Tri-Packs’ (metal or
plastic) resembles the Pall ring but has a spherical shape. This packing offers
more void volume and better distribution of surface area. It also prevents
interlocking of the pieces in the bed. HcKp (from Koch), NOR PAC (from Nutter
Engineering), LANPAC (from Lantec Products) are a few other third generation
random packings.
Intalox metal tower packing (IMTP) Nutter ring
Cascade Miniring
Figure 3: Various types of third generation random packing ring
7The Realization of a Packed-bed Tower
d) Structure packings: Structured packings have emerged as the formidable
competitor of random packings since the 1980s (Helling and DesJardin, 1994;
Bennett and Kovac, 2000). These are made from woven wire mesh or corrugated
metal or plastic sheet. Their major advantages are low gas pressure drop (because
of ‘through flow’ of the gas) and improved capacity and efficiency. The first
structured packing, called Panapak, made from thin metal strips to form a
honeycomb-like structure did not gain much popularity because if severe
maldistribution of liquid. Since the late 1970s and the early 1980s, Glitsch Inc.,
Sulzer and Nutter Engineering came up with acceptable high efficiency structured
packings made of corrugated metal sheets or wire mesh.
Intalox high performance corrugated structured packing (made from thin metal
sheets).
‘Flexeramic’ corrugated structured packing (Koch Engg.).
8The Realization of a Packed-bed Tower
‘Montz B1’ Structured packing Sulzer wire-gauge packing CY
(Nutter Engg. Corporation).
Figure 4: Various types of structure packing
i. Corrugated metal sheet structured packing: There are quite a few tower
packings of this category. These are fabricated from thin corrugated (or
crimped) metal sheets. The surface of a sheet is often made embossed,
textured or grooved to promote mixing and turbulence in the falling liquid
film and thereby to increase the mass transfer coefficient and efficiency.
A bunch of corrugated sheets are arranged parallelly, keeping a suitable
gap between the adjacent members to make a packing piece. A number of
such pieces are arranged and stacked one after another. A piece of packing
above is rotated at a certain angle relative to the piece immediately below
it. The height of a piece is typically 8 to 12 inches. The corrugation angle
of the sheets varies from 28o to 45o (Fair and Bravo, 1990; Olujic et al.,
2001). Perforations are sometimes made on the sheets to provide channels
of communication between the two surfaces of a corrugated sheet and to
improve wetting of the surfaces. A larger corrugation angle increases the
capacity in terms of the liquid load but reduces the mass transfer
efficiency.
ii. Wire mesh structured packings: Sulzer supply three types of such packing
marked AX, BX and CY. The packing elements are made of corrugated
layers of wire mesh. Sulzer packing type CY has a surface area of about
200ft2/ft3. Similar packings are marketed by Glitsch under the trade name
9The Realization of a Packed-bed Tower
Gempok, and by Norton Corp under the name ‘Intalox High-Performance
Wire Gauge Packing’. Glitsch also developed Goodloe for which the
knitted wire-mesh is used. A cylindrical tube made by knitting multi-
filament wires is flattened into a ribbon and then made in to a packing by
corrugation. It has a surface area above 550ft2/ft3. Montz A packing (Nutter
Engineering) is made from perforated wire mesh sheets with a specially
contoured corrugation. The surface area is about 150ft2/ft3. This packing is
similar to the Sulzer wire mesh packing.
iii.
Table 1: Characteristics of a few structured packings
Structured packing Material and surface Crimp angle Area
Mellapak Metals, plastics; grooved and
perforated
45o or 60o About
250m2/m3
Flexipak Similar to Mellapak -
Gempak Smooth or lanced 45o
Montz Metals, plastics; embossed Sinusoidal
MAX-PAC Metals; smooth; W-shaped
perforations
Sharp crimp
angle
Although developed in the late 1970s, the structured packings made visible inroads to
separation technology in the late 1980s. The first major application was in air separation
columns (Parkinson and Ondrey, 1997). The higher initial cost of such packings is amply
compensated by the lesser operating costs because of lesser pressure drop across the bed.
As a result, these packings have been very popular for use in vacumn distillation
columns. The packings have high efficiency (low ‘HETP’) as well. Also, the well-defined
geometric shape, particularly of those made from corrugated sheets, makes them
amenable to theoretical analysis, modeling and scale-up (Fair and Sticklemayer, 1998).
10The Realization of a Packed-bed Tower
Now, the structured packings are being used for near-atmosphere services as well (Bravo,
1997).
Another class of packings; so called ‘grid packings’, have been in use since long for high
gas/ vapour capacities at a low pressure drop.
Table 2: Common structured packings
Supplier Structured packing Metal grid packing
Sulzer Chemtech Mellapak series Mellagrid series
Koch Engineering Flexipak series Flexigrid series
Glitsch Inc. Gempak series C-Grid and EF-25 Grid series
Nutter Engineering Co. Montz series Snap-Grid series
Jaeger Products MaxPak series
Materials for tower packings
The common materials can be use for fabrication of tower packings are ceramic, metals,
and plastics. There are few factors to be considered for the selection of a material for
tower packings:
Ease of fabrication
Mechanical strength
Corrosion resistance
Wet ability
Ease of cleaning
Cost
Ceramic packings declined in popularity since the advent of plastic packings. They are
preferred for highly corrosive services; for example, the air-drying tower and SO3
11The Realization of a Packed-bed Tower
absorption tower in a H2SO4 plant as well as for the operation at elevated temperatures.
However, these have limited shapes (normally rings and saddles only), are prone to
breakage, and require more ‘downtime’ for filling, removal and cleaning. Metal random
packings offer higher capacity, efficiency and turndown ratio because of a smaller wall
thickness and more open area. Metal packings are unbreakable and have higher
compression resistance but have less wet ability than that of ceramic rings. For corrosive
services, a suitable type of stainless steel is used. Plastic packings are cheap, unbreakable,
light, and corrosion-resistance. Common materials are polyethylene, polypropylene,
PVC, and poly-yinylidine fluoride. Plastic packings may be made into a large number of
shapes. It is rather easy to fill them and clean them in situ by water or even stream, thus
reducing the downtime to a tenth of that for ceramic packings. The disadvantages of
plastics packings are: poor wet ability, brittleness at low temperature or on aging,
tendency to degrade in an oxidative environment or when exposed to UV. Plastic
packings are more expensive than the ceramic packings.
Capacity and Efficiency of Random and Structured Packing
Figure 5: Comparison of Structured packing and random packing (COLUMN INTERNALS n.d.)
12The Realization of a Packed-bed Tower
From the diagram above, it shows that the capacity of packing will increase when the
packing factor decrease while the efficiency will increase when specific surface area of packing
increase. Therefore, the result of capacity and efficiency of these two types of packing are shown
as below,
Table 3: Results of Capacity and Efficiency of Random and Structured Packing
Capacity Efficiency Type of packing used
High Low Structured and random packing can be used under the requirement
when the packing factor and specific surface area decrease.
Low High Structured and random packing can be used under the requirement
when the packing factor increase and specific area increase too.
However, from the figure above, structured packing is more
preferable to achieve this performance.
Low Low From the figure above, an increase in packing factor and decrease
in specific surface will achieve this performance. Thus, random
packing is more preferable under this performance.
High High In order to achieve this performance, an increase in specific
surface area and decrease in packing factor will do. Thus, from the
figure above, structured packing is more preferable under this
performance.
13The Realization of a Packed-bed Tower
3.0 Design and specification of a packed-bed tower
3.1 Packed-bed tower overview
Figure 6(a): Schematic diagram of a typical packed-bed tower
Figure 6(b): Diagram of a vapor distributor with packing support
14The Realization of a Packed-bed Tower
Figure 6(c): Diagram of a trough-type distributor
Figure 6(d): Diagram of a typical perforated pipe distributor
Figure 6(e): Diagram of a liquid redistributor
Figure 6(f): Diagram of a hold-down grib
Generally, packed towers are desirable whenever low pressure, whenever low holdup is
necessary, and whenever plastic or ceramic construction is required. In addition, some of
the newer structured (ordered) packings can provide more theoretical stages per unit of
tower height than other tower such as tray tower. Especially in large diameter towers,
liquid and vapor distribution are important considerations where others type of towers are
lacking of. In large towers, the cost of packing and other required internals, such as liquid
15The Realization of a Packed-bed Tower
distributors and redistributors can be the cheapest if compared to others tower; for
example, cross-flow tray tower.
Depth of plastic random packings may be limited by the deformability of the packing
elements to 10-15 ft. For metal random packings, this height can be 20-25 ft. For both
random and structured packings, the height between redistributors is limited to 20-25 ft.
because of the tendency of the phases to lose the function of distribution.
There are various kinds of internals of a generalized packed-bed tower are represented in
Figure XX, the individual parts of which are described one-by-one:
(a) is an example of a typical packed-bed column showing the inlet and outlet
connections and a variety of possible packings. Note that both random and
dumped packings are present as well as structured (ordered) packings.
(b) is a combination packing support and vapor distributor used for beds of
random packings. The serrated shape is used to increase the area for vapor
flow.
(c) is a trough-type distributor that is suitable for liquid rates in excess of
2gpm/sqft in towers 1ft in diameter and larger. In specialized forms, this type
of distributor can be used in very large towers where caution must be taken
not to ensure levelness of the device. Also, care must be taken not to starve
the far notches from their equal share of liquid flow. Trough distributors can
be fabricated form ceramic, plastic, or metal materials.
(d) is an example pf a perforated pipe distributor, which is available in a variety
of shapes. It is a very efficient type over a wide range of liquid rates but
suffers from its likelihood of plugging from even minute-size solids in the
liquid feed. In some large towers, spray nozzles may replace the perforations.
(e) is a device to redistribute liquid, which has a tendency to flow toward the
wall. It is sometimes called a wall wiper, and for structured packing elements,
it is an integral part of the element.
(f) is a hold-down grid to keep low density packings in place and to prevent
fragile random packings, such as those made of carbon; for instance, from
disintegrating because of mechanical disturbances at the top of the bed.
16The Realization of a Packed-bed Tower
There are two features that should be maximized in packed-bed towers are:
(a) Open area—the average percentage of the cross-sectional area of the tower
not blocked by the packing, and hence available for the flow of vapor and
liquid.
(b) Wetted surface area—the number of square feet of packing surface area
available for vapor-liquid contacting, per cubic foot of tower volume.
For the larger in the open area of packing, the will be greater the capacity of a tower. The
greater the wetted surface area of a packing, the higher the separation efficiency of the
tower. For instance, a packing consisting of empty space would have lots of capacity but
awful separation efficiency. In another way, a packing consisting of fine sand would have
great separation efficiency but very low capacity. Therefore, the selection of packing for
a column is a compromise between maximizing open area and maximizing the wetted
surface area.
Structured packing has about 50% more open area than random packings and two or three
times their wetted-surface area. Hence, structure packing has largely replaced packing in
the form of rings in many packed towers.
In a packed-bed tower, the entire packed volume is used for the vapor-liquid contacting.
This is comparable condition where the vapor-liquid contact occurs only on the 5 or 6 in.
above the tray deck in a tray tower and the majority of the tower’s volume is not used to
exchange heat or mass between vapor and liquid. Also, the entire cross-sectional area of a
packed-bed tower is available for vapor flow while in a tray tower; the area used for the
downcomer that feeds the liquid to a tray and the area used for fraining liquid from a tray
are unavailable for vapor flow.
Procedure of packing a tower
It is not advisable to pack a tower by dropping the packings into the tower from the top.
Ceramic packing may break if dumped from above. Also the packings may not get spread
uniformly; they may form a heap at the centre. A ring-type packing may roll down the
heap and get a preferential horizontal orientation. There are a few common techniques
(Figure XX) of installation of random packings. In the ‘wet packing technique’, popular
17The Realization of a Packed-bed Tower
with ceramic packings, the tower is filled with water or a suitable liquid and the packing
is dumped into it. Plastic packings cannot be filled in this way because they will float in
the liquid. ‘Dry packing’ may be done by lowering the packing in a wire bucket that is
led into the column through a manhole (Figure XX). The chute-and-sock method is also
used (this technique is very useful for loading a solid catalyst in a reactor).
Figure 47: Techniques of filling a tower with random packings: (a) wet-packing by filling
the tower with water, (b) dry-packing by lowering buckers filled with the packing, (c) the
chute-and-sock method of packing, and (d) packing through a chute only (Chen, 1984).
One rare occasion, a random packing like the Raschig ring is stacked in a column in
layers. The flow channels in such a bed are regular and the gas pressure drop becomes
less as a result. Structured packings are made in pieces to fit a column of given diameter
and are stacked in an appropriate way.
3.2 Design of a packed-bed tower
In order to choose a mechanically and commercially feasible scrubber system, the
following factors have to be taken into account.
Efficiency
Design variables
Sizing
Operation and maintenance
18The Realization of a Packed-bed Tower
Material used
Costs
Efficiency
The efficiency of an absorption process in part of the following:
The solubility of the pollutant(s) in the scrubbing solvent
Pollutant(s) concentration in the airstream being treated
Temperature and pressure of the system
Flow rates of gas and liquid (liquid/air ratio)
Gas-liquid contact surface area
Stripping efficiency of the solution and recycling of the solvent
In order to increase the higher absorption efficiency in a wet scrubber, the ability to
increase gas-liquid contact will always significant. The absorption efficiency will also be
improved in the scrubber if the temperature can be reduced meanwhile the liquid-to-air
ratio increased. In addition, the actual design of the tower (diameter, height, depth of
packed bed, etc.) will generally depend on the given vapor-liquid equilibrium for the
specific pollutant/ scrubbing solvents. The type of tower used as mentioned before will
affect the equilibrium as well.
However, such data are not always available for all pollutants encountered in industry
today. As if the data are available, empirical data will always be superior to theoretical
data for design purposes. Therefore, we can apply a similar type of pollutant having
available data to model another system if such empirical data are not available for the
corresponding pollutant with an added safety factor built into the design.
Design variables
Packed tower wet scrubber is commonly implemented in air pollution control
installations. The configuration used is somewhat simplified. For example, the tower is
packed with 2 in. ceramic Raschig Rings (note: 1 in. = 2.54cm) and the scrubbing liquor
(absorbent) used is water. The water is sprayed from top and the slurry is collected at the
bottom. The scrubbing liquor spray system is described as a once-through process with
no recirculation. It should be noted that in a field installation, this once-through method
19The Realization of a Packed-bed Tower
has the consequence of sending a large flow of water to a treatment facility. This example
is applicable for either organic or inorganic air pollutant control.
In any absorption process, possible removal efficiency is controlled by the concentration
gradient of the pollutant being treated between the gas and the liquid phases. As
previously defined, this concentration gradient is the driving force to mass transfer
between the phases. Therefore, the solubility of the given pollutant in the gas and liquid
phases will determine the equilibrium concentration of the pollutant in the given
example.
If a pollutant is readily soluble in the scrubbing liquor, the slope m of the equilibrium
curve is low. There is an inverse relationship between m and driving force; the smaller
the slope, the more readily the pollutant will dissolve into the scrubbing liquor. This
represents a high-driving-force system.
Theoretical models of flow through a packed tower
There have been a number of attempts to develop simplified models of two-phase (the
gas and the liquid) flow through a packed bed for a better understanding of the flow
phenomena as well as to theoretically determine the pressure drop and the flooding
capacity. Any such model visualizes a simplified picture of the bed and of flow through it
so as to make it amenable to theoretical analysis. There will be three models cited here.
a) The particle model: The packed bed is visualized as consisting of a number of
spheres of a size calculated on the basis of the void volume of the bed and the
surface area of the packing (Figure XX). When the voidage is large (i.e. >0.4), the
hypothetical spheres may not even touch each other (as if they remain
‘suspended’ but stationary). The pressure drop across the bed is a result of drag of
the following gas on the spheres. With increasing liquid flow, the void volume in
the bed decreases, the size of the hypothetical spheres increases and the pressure
drop also increases. In fact, an early version of the model was used by Ergan back
in 1952 to develop an equation for pressure drop across a packed-bed of solid.
Stichlmair et al. (1989) used this model to predict pressure drop and flooding for
both random and structured packings.
20The Realization of a Packed-bed Tower
b) The channel model: The packed bed is considered to act like a cylindrical block
with a number of uniformly distributed vertical channels in it. The hypothetical
channel diameter can be calculated from the voidage and specific area of the
packings. The liquid flows as a film along the walls of the cylinders and is subject
to shear force at the gas-liquid interface because if the upflowing gas. Billet
(1995) used this model to develop equations for gas-phase pressure drop and the
condition of beginning of loading of liquid in the bed. Loading starts when the
shear force at the interface is large enough to reduce the liquid and vapor
velocities at the interface to zero. The flooding conditions were also analytically
laid down as
and [uL=uLfl and uG=uGfl]
Here uL is the superficial liquid velocity and hL is the liquid holdup in the bed.
Billet (1995) and the German group used these criterion to determine the flooding
capacity of the bed. The model has been criticized by some people because it
visualizes the packing material to form a continuous medium. Nevertheless, it has
been used by other people too (e.g. Rocha et al., 1993) to predict packed-bed
pressure drop.
21The Realization of a Packed-bed Tower
c) Percolation model: This model (Hanley, 1994) assumes that a part of the liquid
flowing through the bed gets accumulated in certain locations of the bed causing
local blockage or ‘localized flooding’. This creates the enhanced pressure drop.
The number of flooded locations increases with the increasing liquid rate.
In a packed tower, on the other hand, the liquid flowing down through the packing
remains in contact with the up-flowing gas at every point of the packed section. Also, the
concentration of both phases change continuously. So, a packed column is called
‘continuous differential contact equipment’.
Sizing of a packed column basically includes the following steps: (i) selection of the
solvent; (ii) selection of packing; (iii) determination of the minimum and the actual
solvent rate; (iv) determination of the column diameter; (v) determination of the packed
height; and (vi) design of the liquid distributor and redistributor (if necessary), packing
support and the gas distributor, design of shell, nozzles, column support, etc. (including
selection of the materials to be used for the tower internals and to build the tower).
The following items and variables should be known or available for design purpose:
(a) Equilibrium data
(b) Flow rates and terminal concentrations of the gas and liquid phases
(c) Individual or overall volumetric mass transfer coefficients, sometimes called
‘capacity coeffecients’ (kyā, kxā, KGā, KLā, Kyā, etc.).
Design method based on the individual mass transfer coefficients
22The Realization of a Packed-bed Tower
Figure 48: The parameters on the sizing of the packed tower
Consider the packed-bed tower shown above; we use the mole fraction unit of the gas and
the liquid-phase concentrations. The flow rates (G’ and L’) are taken on the basis of the
unit cross-sectional area [i.e. mol/ (time) (area)] and the specific interfacial area of
contact between the gas and the liquid phases, ā, is taken on the basis of unit packed
volume and has the unit of m2/m3 or ft2/ft3. We make a steady state mass balance over a
small section of the column of thickness dh.
The rate of flow of the solute (with the carrier gas) = G’y mol/ (time) (area).
The change in the solute flow rate over the section= d(G ’y); this is intrinsically
negative in the case of absorption.
Let NA be the local flux and ky be the individual gas-phase mass transfer coefficient.
Then, the packed volume in the differential section for unit cross-sectional area of the
bed= (1) (dh)
Interfacial area of contact in the differential section= (ā) (1) (dh)
Rate of mass transfer of the solute= (ā) (dh) (NA)
A mass balance over the elementary section of the bed yields
(ā) (dh) (NA)= -d(G’y)= -G’dy- ydG’ (1)
23The Realization of a Packed-bed Tower
Since the carrier gas is not soluble, the change in the total gas flow rate is also equal to
the rate of mass transfer of the solute, i.e.
-dG’= (ā) (dh) (NA) (2)
Substituting Eq. (2) into Eq. (1), rearranging and putting NA= ky(y-yi),
(ā) (dh) NA (1-y)= -G’dy (3)
Thus, (4)
Integrating within the appropriate limits, we get
(5)
Evaluation of the integral above gives the height of the packing. The integration is not
straight-forward, since the interfacial concentration yi is not explicitly known as a
function of the variable y. The following steps should be followed in general (McNulty,
1994):
(a) Draw the equilibrium curve on the x-y plane for the particular gas-liquid system.
(b) Draw the operating line from the material balance equation.
(6)
If the liquid mass flow rate (i.e. the rate of flow per unit cross-sectional area) is
given, Ls is known. Otherwise, the minimum liquid rate on solute-free basis (Ls)min
is to be determined following the procedure detailed in previous section. The
actual liquid rate Ls is taken as a suitable multiple (commonly 1.2 to 2 times) of
the minimum rate. The outlet liquid concentration x1 is obtained from the overall
material balance.
24The Realization of a Packed-bed Tower
(c) Take any point (x,y) on the operating line, Figure XX. Using the known values of
kx and ky (or kxā and kyā), draw a line of slope –kx/ky from the point S(x,y) to meet
the equilibrium curve at R(xi, yi). So yi is known for the particular value of y. The
line SR is called a ‘tie’ line’.
(d) Repeat step (c) for a number of other points on the operating line. If k x and ky or
their ratio are constant, a set of lines parallel to the one drawn in step (c) may be
constructed. [Note that very often the mass transfer coefficients combined with
the specific interfacial area (i.e. kxā and kyā), rather than kx and ky, are given or
known.] Now we have a set of (y, yi) pair for y2≤ y ≤ y1.
(e) Calculate G= Gs (1+y) at each point. Note that Gs can be calculated from the
given feed gas flow rate.
(f) Calculate the value of the integrand fro a set of suitably spaced values of y.
Evaluate the integral in Eq. (5) graphically or numerically.
The height of the packing can also be determined using other types of individual mass
transfer coefficients (kx, kG, kL, Ky, Kx, etc.). The design equations given below can be
derived following the above procedure.
(7)
The height of the packing for a stripping column can be obtained in a similar way. But
here y2>y1 and the gas-phase driving force at any point is y i-y. So the design equation
corresponding to Eq. (5) becomes
(8)
25The Realization of a Packed-bed Tower
Figure 49: The flooding curve
Design method based on the overall mass transfer coefficient
If we express NA in terms of the overall mass transfer coefficient [NA= Ky(y-y*)], Eq. (4)
becomes
(9)
Here y* is the gas-phase concentration (in mole fraction) that is capable of remaining in
equilibrium with a liquid having a bulk concentration x. The required packed height is
obtained by integration of the equation between the two terminal concentrations.
(10)
Graphical or numerical integration of the right-hand side of the above equation is simpler
than that of Eq. (5). Plot the operating line, take any point (x,y) on the operating line,
draw a vertical line through it and extend up to equilibrium curve to reach the point y*. If
the values of the integrand for suitably spaced values of the variable y are calculated, the
integral can be evaluated graphically or numerically.
26The Realization of a Packed-bed Tower
Design equations similar to Eq. (7) can be obtained when the overall coefficient is given.
(11)
The locations of x, y, yi, y*, xi, x* are schematically shown in Figure XX. Design
equations based on the overall coefficients for a stripping operation can be easily derived
from above.
Design method based on height of a transfer unit
By using Eq. (5) and rewrite it in the following form
(12)
Where yiBM is the log mean value of yB [= (1-y)] defined as follows:
(13)
[Note that we are dealing with binary gas mixture in which B is the carrier gas (non-
diffusing), and yB= (1-y); the suffix M means ‘log mean’.]
The gas-phase mass transfer coefficient often varies as (G’)0.8. Also, the ‘Colburn-Drew
mass transfer coefficient’, ky’= kyyiBM, remains independent of the prevailing driving force
(but the coefficient ky depends upon the concentration through yiBM). As a result, the
quantity G’/kyā(1-y)iM remains fairly constant over the packed section of the bed although
the total gas mass flow rate, G’, varies. Chilton and Colburn (1935) called this quantity
‘height of a transfer unit’ based on the individual gas-phase coefficient or the ‘height of
27The Realization of a Packed-bed Tower
an individual gas-phase transfer unit’, denoted by HtG. Taking this quantity out of the
integral sign, we may rewrite Eq. (12) as
(14)
Where
and
The following table summarizes the expressions for the various forms of HTUs and
NTUs.
28The Realization of a Packed-bed Tower
Figure 7: The NTUs and HTUs
Packed-bed mass transfer data for gas-liquid systems are often reported in terms of the
height of a transfer unit. For a particular gas-liquid system, HTU depends upon the type
of packing and the gas and the liquid flow rates. The HTU data on typical systems maybe
obtained from the manufacturer of a particular packing.
Some qualitative physical significance can be attributed to the HTU and the NTU. The
HTU indicates inversely the relative ease with which a given packing can accomplish
separation for a particular system. For a ‘good packing’ (especially the one that provides
more specific interfacial area of contact), the value of HTU is less and the packed height
required for a specified degree of separation is smaller. The number of transfer units
(NTU), on the other hand, indicates the difficulty of separation. The greater the extent of
separation desired, the less will be the driving force available (particularly near the top of
the column in case of absorption and near the bottom of the column in case of stripping),
29The Realization of a Packed-bed Tower
and the larger will be the NTU. A quantitative significance can be attributed to NTU in
certain limiting cases. For example, in the case of absorption of a dilute gas [when (1-
y)*M/ (1-y)= 1], if the operating and the equilibrium lines are nearly straight and parallel,
(y-y*) is approximately constant. So
(15)
If we consider one overall gas-phase transfer unit, i.e. if we put N tOG= 1 in the above
equation, (y1-y2) (y-y*)av. Thus, a single transfer unit corresponds to the height of
packing over which the change in gas concentration is approximately equal to the average
driving force.
If the equilibrium relation is linear with slope m (i.e. y*= mx or y= mx*) the heights of the
individual and overall transfer units are related as follows (The derivation of these
equations is left as an exercise).
(16 a)
(16 b)
The relations may be considerably simplified if the solute concentrations are low. For
example, putting
and ,
We have
30The Realization of a Packed-bed Tower
Where Ā= L’/ mG’ (= L/mG) is the absorption factor, and = mG’/ L’ (= mG/ L) is the
stripping factor.
3.3 Column diameter of a packed-bed tower
Basically, the design of a packed-bed tower for a particular service involves a number of
things such as the selection of solvent, the selection of the type and size packing, the
determination of column diameter and height of packing, and the design of column
internals.. So far as column internals are concerned, there is no well-defined procedure. It
may be done by using the limited information available in the open literature and the
manufacturer’s catalogue. In this section, we discuss one very important item of design;
for example, the determination of the diameter of a packed column.
There are broadly two approaches. One of the approaches; based on the determination of
the flooding velocity by using the Eckert’s GPDC chart, proceeds as follows. (i) From the
total liquid and gas flow rates (either specified or calculated by material balance) the
abscissa (i.e. the flow parameter, Flv) is evaluated. (ii) The value of the ordinate is
obtained from the flooding curve and the mass flow rate of the gas at flooding is
calculated. (iii) The operating gas flow rate is normally taken as 70 to 80% of the
flooding velocity to guard against inherent errors in the flooding curve and also to keep
some flexibility in the design to take care of any sudden surge in the gas flow rate. Once
the design gas flow rate is fixed, the tower diameter and the pressure drop across the bed
may be estimated. The latter is obtained from the same chart. An algebraic correlation fro
the Eckert’s flooding curve (and a dozen similar equations) has been given by Piche et al.
(2001).
[G’ in lb/ft2.h; Fp in ft-1; ρ in lb/ft3; μ in cP; gc in ft.lbf/lbm.s2]
31The Realization of a Packed-bed Tower
The second approach does not use the flooding curve at all because of its limited
accuracy and applicability. The allowable pressure drop in the bed is taken as a basis of
design and the Strigle’s GPDC chart is used directly. The value of the flow parameter is
calculated and the capacity parameter corresponding to the allowable pressure drop is
obtained from the chart. The column diameter is now easy to determine. The pressure
drop at flooding for the particular packing can be calculated from the Kister and Gill
equation. The gas velocity at flooding can also be calculated from these results. A step-
by-step procedure is outlined in Ludwig (1997). A few practical values of the allowable
pressure drop, ΔP/L [(inch water)/ (ft packing)], (Ludwig,997) are: low to medium
pressure column operation: 0.4-0.6; absorption or similar systems: 0.25-0.4 for non-
foaming systems, 0.1-0.25 for foaming systems; atmospheric pressure distillation: 0.5 to
1.0 inch; vacuum distillation: 0.1-0.2. The recommended sizes of packing for different
column diameters are: Dc<1ft, dp<1 inch; Dc= 1-3 ft, dp= 1-3/1 inches; Dc>3 ft, dp= 2-3
inches. Normally, dp/Dc ranges between 1/20 and 1/10. Limited data and information on
pressure drop calculation for a bed of structured packings are available (Fair and Bravo,
1990; Strigle, 1994; Olujic et al., 2001).
For the first generation random packings, the flood point pressure drop is about 2-2.5
inch water per foot of packed bed; for Paul rings, it is 1.5 inch per foot. For most modern
packings, it is 0.5-1.5 inch per foot. Manufacturers of packings generally supply the
pressure drop and flooding characteristics of their products as plots of ΔP versus Fs
[=μsG(ρG)0.5]. It may be noted that the quantity Fs is also taken as a measure of the
‘capacity parameter’ or ‘factor’ for flow through a packed tower at low-to-moderate
pressure when ρG<<ρL. If enough data are available in the company’s catalogue, it is
desirable that the flooding point or pressure drop is determined by interpolation of the
available data.
In order to maintain proper vapor distribution through the bed, the operating bed pressure
drop should not be less than 0.1 inch water/ft. In a column operating near atmospheric
pressure, the superficial gas velocity normally remains below 1m/s; the liquid velocity
remains around 1cm/s. Common ranges of values of the more important packed-bed
parameters are given in Table 41.
32The Realization of a Packed-bed Tower
Table 4: Ranges of a few important packed-tower parameters
Random packing nominal size Dc/20 to Dc/10, Dc= column diameter
Bed voidage 70 to 90% (more for structured packings)
Open area of packing support
(for gas/ liquid flow)
70 to 85% or more
Re-distribution of liquid After 3 to 10 tower diameter (10 to 20 ft)
Gas pressure drop Less than 0.5 inch water per foot bed depth
Operating velocity 70 to 80% of flooding velocity
Minimum wetting rate 0.5 to 2gpm/ft2 for random packings; 0.1 to 0.2 gpm/ft2
for structured packings
3.4 Packing height of a packed-bed tower
The concept of the analysis of a packed column is mainly on the method of transfer units.
This method is more appropriate because the changes in compositions of the liquid and
vapor phases occur differentially in a packed column rather than in stepwise fashion as in
tray column.In this method, height of packing required can be evaluated either based on
the gas-phase or the liquid-phase. The packed height (z) is calculated using the following
formula:
z = N x H
Where,
N = number of transfer units (NTU) - dimensionless
H = height of transfer units (HTU) - dimension of length
33The Realization of a Packed-bed Tower
The number of transfer units (NTU) required is a measure of the difficulty of the
separation. A single transfer unit gives the change of composition of one of the phases
equal to the average driving force producing the change. The NTU is similar to the
number of theoretical trays required for tray column. Hence, a larger number of transfer
units will be required for a very high purity product.
The height of a transfer unit (HTU) is a measure of the separation effectiveness of the
particular packings for a particular separation process. As such, it incorporates the mass
transfer coefficient that we have seen earlier. Basically, the more efficient the mass
transfer (i.e. larger mass transfer coefficient) the smaller the value of HTU. The values of
HTU can be estimated from empirical correlations or pilot plant tests, but the applications
are rather restricted. ["Principles of Unit Operations" 2nd Ed., Foust et al, p.391]
Figure 8: The mass flow diagram of the packed tower
Determination of the packed height can be based on either the gas-phase or the liquid-
phase.
For the gas-phase, we have: z = NOG x HOG
34The Realization of a Packed-bed Tower
KY is the overall gas-phase mass transfer coefficient. "a" is the packing parameter that we
had seen earlier (recall the topic on column pressure drop, e.g. Table 6.3) that
characterize the wetting characteristics of the packing material (area/volume).
Normally, packing manufacturers report their data with both KY and "a" combined as
a single parameter. Since KY has a unit of mole/ (area.time.driving force), and "a" has a
unit of (area/volume), the combined parameter KY a will have the unit of mole/
(volume.time.driving force), such as kg-mole/ (m3.s.mole fraction). As seen earlier, other
than mole fraction, driving force can be expressed in partial pressure (kPa, psi, mm-Hg),
wt%, etc.
y1* is the mole fraction of solute in vapor that is in equilibrium with the liquid of mole
fraction x1 and y2* is mole fraction of solute in vapor that is in equilibrium with the liquid
of mole fraction x2.
35The Realization of a Packed-bed Tower
Figure 9: Mole fraction solute in vapor versus mole fraction solute in liquid
(y1 - y1*) is the concentration difference driving force for mass transfer in the gas phase at
point 1 (bottom of column) and (y2 - y2*) is the concentration difference driving force for
mass transfer in the gas phase at point 2 (top of column).
[Point P (x, y) as shown is any point in the column. The concentration difference driving
force for mass transfer in the gas phase at point P is (y - y*) as shown previously, this
time no subscripts are shown. ]
NOTE: Both equilibrium line and operating line are straight lines under dilute conditions.
Alternatively, equilibrium values y1* and y2* can also be calculated using Henry's Law (y
= m x, where m is the gradient) which is used to represents the equilibrium relationship at
dilute conditions.
Thus, we have: y1* = m x1; y2* = m x2
Similarly for the liquid-phase we have: z = NOL x HOL
36The Realization of a Packed-bed Tower
KX is the overall liquid-phase mass transfer coefficient and "a" is the packing parameter
seen earlier. Again, normally both KX and "a" combined as a single parameter.
Likewise, x1* is the mole fraction of solute in liquid that is in equilibrium with the vapor
of mole fraction y1 and x2* is mole fraction of solute in liquid that is in equilibrium with
the vapor of mole fraction y2. Refer to Figure 134 for finding values of x1* and x2* from
the equilibrium line.
Alternatively, x1* = y1 /m and x2* = y2 /m.
(x1* - x1) is the concentration difference driving force for mass transfer in the liquid phase
at point 1 (bottom of column) and (x2* - x2) is the concentration difference driving force
for mass transfer in the liquid phase at point 2 (top of column).
Table 5: Typical example of a packed-bed data-sheet (Basic design information and
parameters)
37The Realization of a Packed-bed Tower
3.5 Consideration
Packing factor
The Eckert chart contains a parameter Fp that characterizes the packing and is called
‘packing factor’ (another notation Cf can be used to denote the same quantity). The
packing factor introduced by Lobo in 1945 used to be taken as ap/ε3 (ap= surface area of
the packing per unit volume; ε= void fraction of the packed bed). The packing factor
could be calculated from these two properties of a packing. It was later found that the
pressure drop and flooding data could be better correlated if the packing factor was taken
as an empirical quantity. In fact, it is now taken to be so and is determined by
experimental measurement of pressure drop across a packed bed and using the
generalized pressure drop correlation discussed below. The values of Fp for different
packings are supplied by the manufacturers. The packing factor and a few other
characteristics of several random packings are given in the table below. The packing
factor inversely indicates the capacity of a packing; the specific surface area indicates its
mass transfer efficiency. It is intriguing that the values of the packing factor of the same
packing obtained from different soruces are found to vary.
Table 6: The information of particular packing
38The Realization of a Packed-bed Tower
Liquid holdup
In order to facilitate mass transfer on the packed-bed surface, there must be a reasonable
liquid holdup in the bed. However, excessive holdup increases pressure drop over the bed
and is also undesirable if the liquid is heat-sensitive. Generally, it ranges from a few
percent to about 15% of the bed volume. There are two types of liquid holdup (expressed
as volume of liquid per unit bed volume) have been defined.
Static holdup: It is the amount of liquid remaining in unit volume of the bed after the bed
is drained for a reasonable time. It is insignificant compared to the total holdup.
Operating holdup (hLo): It is the difference between the total holdup and the static holdup
when the bed is in operation. The is another term ‘dynamic holdup’ to denote the
scenario. Several correlations for estimation of the quantity are available (Kister,1992). A
recent correlation (Engel et al., 1997) for hLo (volume fraction of the bed) given below is
claimed to have an error within 16% for most systems.
Minimum wetting rate (MWR)
It is the liquid throughput below which the film on the packing surface breaks up
reducing the wetted area. A liquid rate below MWR is too small to wet all the packing
surface. The effective interfacial area of the gas-liquid contact decreases and the
efficiency of mass transfer decreases as a result. Among the many correlations available
for its prediction, the one due to Schmidt (1979) has been found to work very well.
Minimum liquid rate for random packings is reported to lie in the range 0.5-2 gpm/ft 2
(1.25-5 m3/m2h); for structured packing it is 0.1-0.2 gpm/ft2 (0.12-0.25 m3/m2h).
39The Realization of a Packed-bed Tower
Flooding in a packed tower
Knowledge of the hydrodynamic and mass transfer characteristics of a packed bed tower
such as the influence of the flow rate of the gas and of the liquid on pressure drop, liquid
holdup and the gas- and the liquid-phase mass transfer coefficients in the bed is essential
for the design of such a device.
Bed pressure drop and the phenomena of loading and flooding
The liquid distributed on the top of a packed bed trickles down by gravity. Flow of the
gas is pressure-driven and the pressure is generated by a blower or a compressor. The gas
undergoes pressure drop as it flows through the bed because (i) both skin friction and
form friction, (ii) frequent changes in the flow direction, and (iii) expansion and
contraction. When the packing is dry (there is no liquid throughput), the maximum area
for flow of the gas is available. However, when a liquid flows through the bed, a part of
the open space of the bed is occupied by the liquid (called ‘liquid holdup’ in the bed) and
the area available for gas flow decreases. This is the reason where the increasing liquid
throughput results in the increasing pressure drop of the gas. Typical gas flow rate vs.
pressure drop curves on the log-log scale for a dry bed (no liquid flow) and for two
constant liquid rates are qualitatively shown in the figure below.
Figure 10: Pressure drop curves on the log-log scale
The plot is linear for a dry bed. For an irrigated bed, such a curve is nearly linear with a
slope of about 2 in the lower region (i.e. ΔP varies nearly as the square of gas rate). The
40The Realization of a Packed-bed Tower
slope of the straight section, however, decreases slightly at higher liquid rates. If the gas
rate is increased at a constant liquid rate, the drag of the gas impedes the downward
liquid velocity. The liquid holdup in the bed increases as a result. This steady increase in
the pressure drop continues till the point B (Figure XX) is reached. At the point B and
beyond, the upflowing gas interferes strongly with the draining liquid. Over the region B-
C, accumulation or ‘loading’ of the liquid starts. The point C is called the point of
‘incipient flooding’.
If the gas flow rate is further increased, the liquid accumulation rate increases very
sharply. Liquid accumulates more in the upper region of the bed almost preventing the
flow of gas. This phenomenon is known as ‘flooding’. The bed becomes ‘flooded’ (point
D) when the voids in the bed become full of liquid and the liquid becomes the continuous
phase—a case of ‘phase inversion’. The definition of ‘flooding’, suggested by Bravo and
Fair (Kister, 1992) states that: It is a region of rapidly increasing pressure drop with
simultaneous loss of mass transfer efficiency. The visual and physical symptoms of
flooding are: (i) accumulation of a layer of liquid at the top of the bed, (ii) a sharp rise in
pressure drop, (iii) a sharp rise in liquid holdup in the bed, and (iv) a sharp fall in mass
transfer efficiency.
While the operation of the column becomes very unstable over the region CD and the
mass transfer efficiency drops significantly, some researchers have reported a reasonably
stable operation beyond the point D. This is because beyond the point D, the column
operates like a ‘bubble column with gas-liquid upflow’.
Prediction of pressure drop and flooding
In order to come out with a complete design of packed towers, the prediction of the
flooding point and pressure drop is essential. Charts, correlations and theoretical models
have been proposed for this purpose. Every packing has its own geometrical and surface
characteristics. Pressure drop per unit bed height as well as the flooding characteristics
are also different for individual packings even when all other parameters including
‘nominal packing size’ remain the same. However, it is not very realistic to work out
separate carrelations for pressure drop (and for mass transfer) for packings of different
41The Realization of a Packed-bed Tower
types and sizes. Instead efforts were made to develop a ‘generalized pressure drop
correlation’ (GPDC) that would be applicable to all kinds of random packings. The idea
of a GPDC was first introduced by Leva (1954). The major variables and parameters that
determine the pressure drop and flooding characteristics are: (i) the properties (density,
viscosity and surface tension) of the fluids and (ii) the packing type and its features (size,
voidage and surface area and surface properties). A number of charts and correlations
have been proposed by the US School (led by Leva, Eckert, Strigle, and Kister, to name a
few; see Kister, 1992) during the last fifty years. A second group of charts and
correlations have been proposed by the German School (led by Mersman, Stichlmair,
Billet and others; see Billet, 1995). Some of the correlations have a semi-theoretical basis
and include adjustable constants specific to a group of packing. Recently, Piche et al.
(2001) reviewed all important correlations proposed for the flooding point coming from
US and German Schools. These researchers also proposed a new correlation developed
by using artificial neural network (ANN) technique to 1019 data sets reported by
different workers.
Figure 11: Eckert’s curve
The GPDCs proposed by Eckert (1975 and before) of the erstwhile Norton Company
have been widely used for packed tower design. The 1970-version (Figure XX) gives a
42The Realization of a Packed-bed Tower
number of constant pressure drop curves and a flooding curve. It works well with most
first generation packings but not for several second generation packings and smaller
modern packings. Eckert’s 1975 version omitted the flooding curve because such a curve
always has a doubtful accuracy. For first generation packings, the ‘packing’ factor’ is
high (generally above 60ft-1) and the pressure drop is ΔP/L ≥ 2 inches of water per foot
packed height at ‘incipient’ flooding. Eckert’s chart wa). Eckert’s chart was further
refined by Strigle (1994) using a data bank of 4500 pressure drop measurements on beds
having different types and sizes of packings as well as using different liquids and gases.
The Strigle’s version (Figure XX) is now most popular for packed tower design (Larson
and Kister, 1997). The error in pressure drop prediction is claimed to be within ±11% for
normal ranges of operation. It has a ‘flow parameter’ as the abscissa and a ‘capacity
parameter’ as the ordinate.
Figure 12: Strigle’s curve
Flow parameter,
Capacity parameter, in ft/s
43The Realization of a Packed-bed Tower
Interestingly, the abscissa and the ordinate of the Strigle’s chart resemble the
corresponding quantities of Fair’s flooding chart for a tray tower. The flow parameter F lv,
represents the square root of the ratio of liquid and vapor kinetic energies. The ordinate
describes a balance between forces due to vapor flow (that acts to entrain swarms of
liquid droplets) and the gravity force that resists entrainment (Kister, 1992). Here Fp is a
characteristic parameter of the packing, called the ‘packing factor’. The quantity C s,
which is akin to the Souders-Brown constant, may be corrected for changes in interfacial
tension and viscosity, if necessary.
[σ in dyne/cm, μ in cP]
Strigle’s chart also excluded the curve for pressure drop at flooding. However, the curve
for ΔP/L= 1.5 inches water per foot is considered to represent the ‘incipient flooding’
condition. Kister and Gill (1991) proposed the following correlation for flood point
pressure drop in terms of the packing factor.
(inch water/ft; FP in ft-1)
Robbin (1991) proposed another set of correlations for pressure drop prediction over a
wide range of operating conditions. For dry bed pressure drop at nearly atmospheric
pressure, he suggested the following equation.
(inch water/ ft)
Here G’ is in lb/ft2.h, and ρG is in lb/ft3. The above equation has an important application.
The dry bed packing factor Fpd of any packing, which is now considered an empirical
quantity, can be calculated from the above equation by measuring the pressure drop
across an esperimental packed bed. However, the packing factors for dry and irrigated
beds are likely to be different. The dry bed pressure drop can also be calculated using the
Ergun’s equation.
44The Realization of a Packed-bed Tower
3.6 Mass transfer efficiency
A parametric study of carbon dioxide (CO2) absorption performance into an aqueous
solution of monoethanolamine (MEA) in the spray column was carried out
experimentally over wide ranges of process conditions. The performance of the spray was
interpreted in terms of the overall mass transfer coefficient, KGae and was found to vary
with process parameters, including gas flow rate, liquid flow rate, CO2 partial pressure,
MEA concentration, CO2 loading, and size of spray nozzle. The performance of the spray
column was compared to that of a packed column and showed a promise for CO2 capture
application.
The mass transfer performance was determined in terms of the volumetric overall mass
transfer coefficient by using the following equation
Where GI is inert gas flow rate in kmol/m2.hr, P is total pressure on the system in kPa, Z
is column height in m, yCO2,G and y*CO2 are mole fraction of CO2 in gas stream and
equilibrium mole fraction of CO2, and YCO2,G is mole ratio of CO2 in gas stream.
(a) CO2 partial pressure (kPa) (b) Gas flow rate (MEA concentration) (m3/m2.hr)
45The Realization of a Packed-bed Tower
(c) Gas flow rate (liquid flow rate) (d) CO2 loading (mole CO2/ mole MEA)
(m3/m2.hr)
(e) Liquid flow rate (CO2 loading) (f) liquid flow rate (CO2 loading)
(m3/m2.hr) (m3/m2.hr)
Figure 13: Effect of parameters on the performance
46The Realization of a Packed-bed Tower
Effect of parameters on the performance of the packed-bed tower
a) Effect of CO2 partial pressure. KGae decreases with CO2 partial pressure. By
considering mass flux of CO2 absorption (NCO2), an increase in CO2 partial
pressure leads to an increasing amount of CO2 transferred into liquid phase.
However, the increasing mass flux occurs in a lower extent compared to the
change in partial pressure, causing KGae to reduce as partial pressure increases.
This may be caused by the restricted diffusion and amount of reactive MEA in the
liquid phase. The mass transfer may be mainly controlled by CO2 reaction in the
liquid, thus resulting in only a small change in the amount of CO2 absorbed as the
partial pressure increases.
b) Effect of gas flow rate. KGae increases with gas flow rate to a certain point and
then remains constant. This suggests the gas-phase controlled mass transfer takes
place at low gas flow rates and the liquid-phase controlled mass transfer takes
over at high gas flow rate. In general, as the gas flow increases the amount of CO 2
molecules available for the absorption increases. This would lead to a higher mass
transfer flux. However, the overall rate of gas absorption is not only dependent
upon the gas flow rate, but also the liquid flow rate and availability of reactive
MEA in the liquid which as seen in this case controls the rate of mass transfer
after the gas flow rate reaches the point.
c) Effect of liquid flow rate. KGae increases with liquid flow rate. This is because
increasing the liquid flow increases effective interfacial area (ae), between liquid
and gas. Note that KGae increases more rapidly at low flow rates compared to at
high flow rates. The rapid increase was caused by (1) a reduction in size of spray
droplets from larger diameter to smaller diameter, thus resulting in an increase in
droplet surface area per unit volume of dispersed liquid and (2) an increase in
number of droplets produced by the nozzle and also the surface area available for
mass transfer. At the high liquid flow rate, the reduction in droplet size by the
increasing liquid flow is insignificant, leaving the increasing number of spray
47The Realization of a Packed-bed Tower
droplets to be the primary factor that defined the lower increase in mass transfer
performance.
d) Effect of MEA concentration. KGae increases with MEA concentration. This is
due to the fact that the increasing MEA concentration yields a higher amount of
the active MEA available to diffuse towards the gas-liquid interface and react
with CO2. This finding differs from the behavior observed in the packed column
in that the KGae of packed column decreases by 5% for every molarity of MEA
increasing. Such decrease in KGae is caused by an increase in solution viscosity.
This shows that the solution viscosity is more influential on the effective area in
the spray column than in the packed column.
e) Effect of CO2 loading. KGae decreases with CO2 loading. This is due to the fact
that as the CO2 loading increases the amount of active MEA decreases, causing
the KGae to decrease.
f) Effect of nozzle size. It was found that KGae of a larger nozzle is lower that of a
smaller nozzle at the low end of liquid flow rate. This is because the spray of the
lager nozzle is not fully developed, resulting in a lower effective area (ae). As the
liquid flow rate increases, the spray is more fully developed with the smaller
liquid droplets that offer higher ae, causing the KGae to increase accordingly.
3.7 Packed-bed tower internals
Bed limiter
Table 7: The information of bed limiter
Bed limiter Diameter Specification ContourStructured packing bed limiter (Non-interfering)
All column diameters
Minimizes interference of high performance liquid distributors
48The Realization of a Packed-bed Tower
Random packing bed limiter (Non-interfering)
All column diameters
Requires no vessel attachments Minimizes interference of drip
point from high performance liquid distributors
Random packing bed limiter
All column diameters
Fastens to vessel wall For use with traditional
distributors
Structured packing bed limiter/ Liquid distributor support
All column diameters with structured packing
Supports tubular, channel or trough type liquid distributors
Used for large diameter columns to reduce structural components
Provides limited uplift resistance
Support plates
Every packed-bed will need a support. However, there are some factors to be considered
in choosing the design of a packing support which is compatible to the corresponding
packed-bed.
It must physically retain and support the packed-bed under operating conditions in
the column including but not limited to packing type and size, design temperature,
bed depth, operating liquid holdup, material of construction, corrosion allowance,
and material buildup in the bed and surge conditions.
It must have a high percentage of free area to allow unrestricted counter-current
flow of down coming liquid and upward flowing vapor.
The specified flow rate at the time of order placement will not limit the capacity of the
packing they retain. Generally, a gas-injection type support is available for random
packings due to the separation passages for liquid and vapor flow so that the two phases
do not compete for the same opening. Packing elements are retained with specific slot
openings while the contour of the support provides a high percentage of open area. On
the other hand, structure packings allow itself to be supported by a simple open grid
structure due to the inherent construction of the packing.
49The Realization of a Packed-bed Tower
Table 8: The information of packing support
Support plates Diameter Specification Contour
Structured packing support grid
All column diameters
Supports all sheet metal or wire gauze packings
Random packing gas injection support plate
≥ 36 in. [900 mm] Gas injection design
Random packing gas injection support plate
12-48 in. [300-1200 mm]
Gas injection design
Light duty random packing support plate
4-36 in. [100-900 mm]
Low hydraulic loading, low support strength requirements
Table 9: The information of liquid collector
Liquid collector Diameter Specification ContourDeck style All diameters For total or partial
liquid draw-off Suitable to feed a
liquid distributor or trayed section below
Trough style ≥ 40 in. [1000 mm]
Permits thermal expansion
Total or partial liquid draw
25-40% open areaChevron vane ≥ 30 in. [760 mm] High vapor capacity
Low pressure drop Can be used for draw-
off or collection of liquid between packed-beds
50The Realization of a Packed-bed Tower
Distributor
Low flow rate (≤ 20gpm, 50m3/m2h)
Distributor Re-distributor availability
Turndown ratio
Flow rate Contour
Channel distributor with bottom orifices
Yes 2:1 2-16gpm/ft2 (5-40 m3/m2h)
Channel distributor with drip tubes
Yes 2:1 0.3-12gpm/ft2 (0.75-30 m3/m2h)
Tubular distributor No 2:1 (5:1 if sufficient column height)
0.3-8gpm/ft2 (0.75-20 m3/m2h)
Trough distributor with enhanced baffle plates
No - 0.3-8gpm/ft2 (0.75-20 m3/m2h)
Pan distributor with V-Notch risers
No High turndown ratio is available
1-8gpm/ft2 (2.5-20 m3/m2h)
Trough distributor with drip tubes
No 2:1 (10:1 when using multi-level orifices at each discharge conductor)
0.3-20gpm/ft2 (0.75-50 m3/m2h)
Pipe-arm distributor with orifices
No 2.5:1 1.5-10gpm/ft2 (4-25 m3/m2h)
51The Realization of a Packed-bed Tower
High flow rate (≥ 20gpm, 50m3/m2h)
Distributor Re-distributor availability
Turndown ratio
Flow rate Sample
Deck distributor Yes 2:1 4-80gpm/ft2 (10-200 m3/m2h)
Pan distributor Yes 2:1 ≥ 2gpm/ft2 (5 m3/m2h)
Pan distributor with bottom orifices
No 2.5:1 1-30gpm/ft2 (2.5-75 m3/m2h)
Deck distributor with bottom orifices
Yes 2.5:1 1-50gpm/ft2 (2.5-120 m3/m2h)
Trough distributor with bottom orifices
No 2:1 1-20gpm/ft2 (2.5-50 m3/m2h)
Trough distributor with weirs
No 2.5:1 2-40gpm/ft2 (5-100 m3/m2h)
Spray nozzle distributor
No 2:1 0.2-50gpm/ft2 (0.5-120 m3/m2h)
Enclosed channel distributor for offshore applications
No - 1-30gpm/ft2 (2.5-75 m3/m2h)
52The Realization of a Packed-bed Tower
4.0 Carbon Dioxide Capture System Equipments
Figure 14: Carbon Dioxide Capture System Split flow configuration (Vozniuk 2010)
Figure 15: Carbon Dioxide Capture System (Alstom and American Electric Power to
Bring CO2 Capture Technology to Commercial Scale by 2011 2007)
53The Realization of a Packed-bed Tower
By comparing both figures above, it could provide the information about the
essential equipment being used in the carbon dioxide capture system. Below are the list
of the equipments being used and their function,
Table 10: Essential Equipment of Carbon Dioxide Capture System
Essential Equipment
Absorber (Packed Bed Scrubber) Heat Exchanger
Desorber/Stripper/Regenerator Condenser
Pump Reboiler
Reflux Drum/Liquid Separator Solvent Cooler
Table 11: Optional Equipment of Carbon Dioxide Capture System
Optional Equipment
Semi Lean Flash Drum
Semi Lean Cooler
Reclaimer
Feed gas cooler
Table 12: Function of equipments
Equipment Function
Absorber It is function as the place to allow the feed stream components,
such as hydrogen, sulfur, carbon and others come in contact with
the solvent that absorb CO2
Solvent cooler It is function as a tools to cool down the solvent that recycle back
from desorber in order to allow more absorption of CO2
Heat Exchanger The type of heat exchanger being used is shell and tube. For the
hot stream, it is consist of the solvent that already absorb CO2 and
54The Realization of a Packed-bed Tower
leave the absorber. On the other hand, the cold stream consists of
solvent that required cooling down to allow more absorption of
CO2. Therefore, it is function as the tools to heat up the solvent
leave the absorber but before entering desorber and cool down the
solvent before reuse again to absorb CO2 in absorber at the same
time
Flue gas cooler It is the place allows the hot feed stream being cooled by cooling
water so as to achieve acceptable absorption efficiency.
Desorber In the desorber, the CO2 will be emerged and also allow removing
of water and traces of solvent. The CO2 being released will become
concentrated with water before proceed to transportation and
storage
Reboiler It is function as a tool to boil the water that being removed in the
desorber and transfer back to the desorber to drive the separation
process. At the same time, the solvent that being separated in the
desorber will recycle back to the absorber for absorption of CO2
Condenser It is a device to reduce a gas or vapour into liquid. Beside that, it is
operated by removing the heat from the gas or vapour, once the
heat being eliminated, the liquefaction will occur.
Reflux Drum It is a device to separate water from reflux when distilling
particular substance. Beside that, it also provides more time for the
operator to respond if they have exceeded the condenser’s capacity.
Furthermore, it also provides a place from which noncondensable
vapors may be vented.
Water wash It is a place to balance water in the system and to remove any
solvent droplets or solvent vapour carried over in order to prevent
excess emissions of solvent together with vent gas.
Reclaimer It is a device used to remove the solids and degradation product.
55The Realization of a Packed-bed Tower
Semi Lean
Flash Drum
It is a device used to recover solvent under split flow configuration
of carbon dioxide scrubbing process. The purpose of introducing it
can reduce the reboiler duty.
Semi Lean
Cooler
Its function same as solvent cooler.
4.1 Troubleshooting
Absorber
Table 13: Troubleshooting of Absorber(Amine Basic Practices Guidelines 2007)
Problem: Differential pressure consistently low (normally 0.1-0.2 psi/tray)
Cause Consequence Action
Possibly tray damage Poor efficiency, off-
spec treated gas
Mechanical repair
Problem: Differential pressure gradual increase
Cause Consequence Action
Possible fouling Flooding, poor
efficiency, off-spec
treated gas
Cleanout, identify root cause (eg.
Corrosion)
Problem: Differential pressure sudden increase, erratic action
Cause Consequence Action
Foaming, flooding Poor efficiency, liquid
carryover, general plant
upset
Add antifoam (discriminately),
reduce gas and/or liquid rates,
check relative gas/amine
temperatures to determine
likelihood of hydrocarbon
condensation, check feed gas for
56The Realization of a Packed-bed Tower
entrained hydrocarbon
Problem: Low feed gas flow rate
Cause Consequence Action
Upstream process
change
Reduced amine
demand, potential
reduction in mass
transfer due to weeping
Reduce amine flow, supplement
feed gas with recycle or clean gas
if warranted
Problem: High feed gas flow rate
Cause Consequence Action
Upstream process
change
Increase amine demand,
possible jet flooding
Increase amine flow
Problem: Low amine Flow rate
Cause Consequence Action
Change in controller
status or supply
pressure
Potential reduction in
acid gas recovery
Adjust conditions determining rate
Problem: High amine Flow rate
Cause Consequence Action
Change in controller
status of supply
pressure
Increased utility
consumption
Adjust conditions determining rate
Problem: Low feed gas temperature (normally 80-120 F)
Cause Consequence Action
Change in upstream
process and/or
ambient conditions
Reduced acid gas
recovery in extreme
cases
Increase temperature of feed gas
and/or amine
Problem: High feed gas temperature
Cause Consequence Action
57The Realization of a Packed-bed Tower
Change in upstream
process and/or
ambient conditions
Potentially reduced acid
gas recovery
Decrease feed gas temperature, or
increase amine flow rate to
improve heat balance
Problem: High lean amine temperature (90-130F)
Cause Consequence Action
Change in upstream
process and/or
ambient conditions
Potentially reduced acid
gas removal due to poor
equilibrium at high
absorber temperature.
Excessive moisture in
treated gas, with
potential downstream
condensation and
resultant
corrosion/fouling
Increase lean amine cooling
Problem: Low lean amine temperature
Cause Consequence Action
Change in upstream
process and/or
ambient conditions
Potentially reduced acid
gas removal from high
viscosity or low rate of
reaction
Reduce lean amine cooling or
supply heat
Problem: Low lean amine/feed gas temperature Differential (normally lean
amine at least 10 F hotter than feed gas)
Cause Consequence Action
Change in upstream
process and/or
ambient conditions
Condensations of
hydrocarbon,
potentially resulting in
foaming and/or
emulsification
Increase lean amine temperature
Problem: Low rich amine loading
Cause Consequence Action
58The Realization of a Packed-bed Tower
Overcirculation Excessive utility
consumption
Reduce amine circulation rate
Problem: High rich amine loading
Cause Consequence Action
Undercirculation Reduced acid gas
removal, corrosion
Increase amine circulation rate
Flash drum
Table 14: Troubleshooting of flash drum(Amine Basic Practices Guidelines 2007)
Problem: High pressure (45-65 psig for no rich amine pump while 0-25 psig with
rich amine pump)
Cause Consequence Action
Excessive hydrocarbon in
rich amine
Regenerator foaming, SRU
upset or fouling
Correct absorber
operation, clean up amine,
add antifoam, skim
hydrocarbon from flash
drum
Problem: Low pressure
Cause Consequence Action
System venting to
atmosphere or relief
system
May not get into the
regenerator
Find the source of leaking
Problem: Negative pressure
Cause Consequence Action
Relief stack draft causing a
vacuum on the system
Air may be drawn into
flash drum and
contaminate the amine or
cause an explosive mixture
Adjust relief system
pressure to hold positive
pressure on drum
Problem: High flash gas rate
59The Realization of a Packed-bed Tower
Cause Consequence Action
Hydrocarbon carryover
from absorber
Foaming and reduced acid
gas absorption (possible
violation), foaming in
regenerator (SRU upset)
Correct absorber
operation, clean up amine,
add antifoam, skim
hydrocarbon from flash
drum
Problem: High hydrocarbon level (normally 0-5 % level above amine)
Cause Consequence Action
Insufficient skimming Amine foaming, SRU
upset
Increase skim rate, check
gas and liquid absorber
operation hydrocarbon
carryunder
Problem: High amine level
Cause Consequence Action
Water leaking into system,
absorbers returning amine
inventory, absorber level
problem, imbalance in
amine flows
Amine carryover into gas
system, diluting amine
strength
Remove some amine from
plant. Check amine
strength
Problem: Low amine level
Cause Consequence Action
Dehydrating amine
system, absorber upset and
holding up or losing
amine, foaming in
absorbers or regenerators,
or system losses
Flash gas or liquid
hydrocarbon carryover in
rich amine to regenerator
due to low residence time
for gas or liquid
hydrocarbon separation
(SRU upset)
Add amine or condensate
to plant, find leak or loss
60The Realization of a Packed-bed Tower
Lean/rich exchangers
Table 15: Troubleshooting of lean/rich exchangers(Amine Basic Practices Guidelines
2007)
Problem: High rich amine temperature
Cause Consequence Action
Low fuel rate Flashing and corrosion in
exchangers and
regenerator inlet
Check amine flows,
possibly bypass hot lean
amine flows
Problem: Low rich amine temperature
Cause Consequence Action
Exchanger fouling Poor stripping in
regenerator and/or
increased reboiler steam
demand
Check all temperatures for
poor performance
(fouling), clean exchangers
Problem: High pressure
Cause Consequence Action
Fouling, equipment of
failure
Reduced circulation,
reduced heat transfer, high
lean amine temperature
and low regenerator feed
preheat
Locate the point of high
pressure drop
Regenerator/Stripper/Desorber
Table 16: Troubleshooting of Regenerator(Amine Basic Practices Guidelines 2007)
Problem: Low or decreasing reflux drum pressure
Cause Consequence Action
Failed pressure controller, Upset of or shutdown of Determine cause for loss
61The Realization of a Packed-bed Tower
loss of reboiler heat
source, loss of feed, loss of
containment
downstream sulfur unit,
release to atmosphere, unit
shutdown
of feed or heating medium,
install reflux purge and
clean water backup to
control corrosion.
Minimize velocities by
optimizing steam
consumption.
Problem: High or rising reflux drum pressure
Cause Consequence Action
Downstream unit
problems, blocked outlet
line, failed pressure
controller, flooded vessel,
excessive reboiler duty,
hydrocarbon
contamination
Relief, unit shutdown,
reduced heat input,
increased lean loadings,
decreased throughput,
downstream unit shutdown
or upset accelerate amine
degradation
Reduce feed, reduce
reboiler duty, drain
hydrocarbons from flash
drum, raise reflux
temperature, steam out
liquid gas product, drain
hydrocarbon liquid from
reflux drum, shutdown and
clean overhead line
Problem: High top pressure for conventional condenser/drum overhead systems
(normally 5-15 psig)
Cause Consequence Action
Condenser fouled and
plugged on the process
side, condenser fouled on
the cooling media side,
loss of cooling media
Upset of downstream
sulfur unit, foaming and
excessive entrainment,
relief, unit shutdown,
acceleration of amine
degradation, increased lean
loadings, reduce
throughput
Reduce feed, reduce
reboiler duty, drain
hydrocarbons from flash
drum, steam overhead line,
drain hydrocarbons from
reflux drum, shutdown and
clean overhead line
Problem: Sudden loss of rich amine feed rate
Cause Consequence Action
62The Realization of a Packed-bed Tower
Loss of flash drum or
contactor levels, flow
control failure, plugging
from corrosion products or
salts, high regenerator
pressure
Loss of throughput, loss of
treating capability, SRU
upset
Check rich flash drum
level control, check
contactor level control and
flows, check rich amine
feed circuit or plugging in
valves, orifices filters or
exchangers. Remove heat
stable salt anions and
sodium, remove
degradation products
(reclaim), check for
hydrocarbons to
regenerator
Problem: Gradual decline in rich amine flow rate
Cause Consequence Action
Declining rich amine flash
drum level resetting flow,
plugging from corrosion
products or salts, leaks or
open drains
Loss of throughput, loss of
treating capability
Balance flows in and out
of system. Check for
contactor foaming or
upset. Check for
maintenance activity (filter
changes, equipment
draining). Balance reflux
purges and makeup water
rates.
Problem: Sudden loss of acid gas product rate
Cause Consequence Action
Loss of feed, loss of
reboiler heat input,
downstream unit
shutdown, plugged
overhead line, pressure
Loss of throughput, loss of
treating capability, relief,
unit shutdown
Restore feed. Restore
reboiler heating media.
Steam out of otherwise
heat overhead line; raise
reflux or pump around
63The Realization of a Packed-bed Tower
controller failure, loss of
containment, tower
internals malfunction
return temperature
Problem: Sudden increase in acid gas product rate
Cause Consequence Action
Hydrocarbon intrusion into
regenerator, foaming,
tower internals
malfunction
Amine carryover,
upset/shutdown of
downstream sulfur unit
Skim rich amine
flash/reflux drums for
hydrocarbon, change
carbon filter
Problem: Lean loading exceeds spec/treated gases and liquids fail to meet spec
Cause Consequence Action
Insufficient reboiler heat
input caused by:
insufficient heating media
supply, over-circulation,
fouled lean/rich
exchangers, fouled
reboiler, loss of
regenerator level, leaking
lean/rich exchangers when
rich amine exceeds lean
amine pressure, caustic
contamination
Off-spec products,
excessive corrosion
Increase reboiler heat
input, decrease circulation,
increase amine strength,
remove heat stable salt
anions and sodium, clean
fouled exchangers, check
sodium level
Problem: Little or no tower pressure drop
Cause Consequence Action
Loss of feed, loss of
reboiler heating media,
tray blowout, tower
internals malfunction
High lean loadings to off-
spec products, unit
shutdown downstream
sulfur unit shutdown
Determine reason for loss
of feed, determine reason
for loss of heating media
Problem: occasional sudden rise in tower pressure drop then returning to
normal
64The Realization of a Packed-bed Tower
Cause Consequence Action
Foaming, hydrocarbon
intrusion from flash drum,
hydrocarbon refluxed to
tower, reboiler heat input
flunctuations, tower
internals malfunction
Amine and/or hydrocarbon
carryover into downstream
sulfur unit, high lead
loadings leading to off-
spec products, tray
blowout
Determine composition
of reflux to stop amine
loss, if high in amine
stop purge of reflux to
stop amine loss, if high
in hydrocarbon
increase purge
Test feed and bottoms
for foaming tendency,
add antifoam until
pressure drop is normal
Monitor carbon filter,
change if necessary
Skim rich amine flash
drum and reflux drum
for hydrocarbon
Ensure proper levels
are maintained in rich
flash and reflux drum,
check level instruments
Check reboiler heating
medium control for
flunctuating pressure,
temperature, or flow
Reduce feed rate
Reduce heat input
Problem: Gradual buildup or sudden permanent buildup of tower pressure drop
Cause Consequence Action
Buildup of corrosion Reduced throughput, high Discontinue antifoam
65The Realization of a Packed-bed Tower
products causing tray
plugging, damage
Excessive corrosion
rates caused by high
ammonia/amine
concentrations in
reflux, insufficient heat
input leading to
excessive lean loadings,
under-circulation
leading to excessive
rich loadings, high heat
stable salt anion content
Excessive particle
accumulation caused by
poor filtration,
increased filter pore
size to control filter
replacement cost, filter
replacement,
accumulation of
particles in rich amine
flash drum
lean loadings leading to
off-spec products,
increased energy
consumption, reduced
circulation leading to
higher rich loadings,
higher corrosion rates,
increase filtration costs
and increased plugging
additions and change
carbon filters to prevent
antifoam buildup and
foaming episodes
Remove heat stable
salts anions and sodium
Remove amine
degradation products
(reclaimer)
Shutdown and chemical
or water wash tower
Reduce filter pore size.
Clean rich amine flash
drum bottom
Add additional filtration
upstream of regenerator
Keep lean loadings
minimal, keep rich
loadings at or below
recommended levels
Problem: Low top temperature for conventional condenser/drum overhead
system
Cause Consequence Action
Insufficient heat input, too
cold reflux or pumparound
return temperatures, reflux
level or pumparound flow
control valve failure,
Increased lean loadings
leading to off-spec
products
Raise reboiler heat input.
Raise reflux or
pumparound return
temperatures.
Raise condenser cooling
66The Realization of a Packed-bed Tower
excessive rich loadings,
tower internals
malfunction
media temperature
Problem: High top temperature for conventional condenser/drum overhead
system
Cause Consequence Action
Too high heat input, loss
of reflux or pumparound
cooling media, too low
loadings
Overtax overhead system
leading to excess water
loss and poor downstream
sulfur unit feed, increase
amine in reflux water by
entrainment and/or
vaporization, increase
corrosion rates throughout
regenerator, accelerate
amine degradation, excess
energy costs
Reduce reboiler duty
Re-establish
condenser/pumparound
cooler cooling media
Reduce circulation
Reflux Drum
Table 17: Troubleshooting of Reflux Drum (Amine Basic Practices Guidelines 2007)
Problem: Acid gas product/Reflux temperature reads below 90 F (normally 90-
130 F)
Cause Consequence Action
67The Realization of a Packed-bed Tower
Too much cooling media,
too cold cooling media,
insufficient heat input,
reflux level control
problem, pumparound
temperature control
problem
Plugged overhead line
with hydrates of NH3
Confirm instrument
readings
Cut cooling media rate
or raise its temperature
Raise reboiler heat
input
Reflux Pump
Table 18: Troubleshooting of Reflux Pump(Amine Basic Practices Guidelines 2007)
Problem: Loss of reflux
Cause Consequence Action
Control failure, loss of
feed, loss of reboiler
heating media, leaks,
plugging, loss of cooling
media, fouled or plugged
overhead exchanger, pump
failure
Increased amine strength
leading to heat/mass
transfer problems or
pumping problems, loss of
levels reducing circulation
and throughput
Check controller, insure
correct valve trim and
metallurgy
Determine if feed lost,
correct upstream unit
Determine if heating
media lost, correct
supply problem
Determine if cooling
media lost or restricted
Problem: Reflux ratio too low
Cause Consequence Action
Controller problems,
insufficient reboiler heat
input, plugging, pump
problems
High lean loadings leading
to off-spec product
Check/repair controller
Raise reboiler heat
input
Clear plugging in reflux
system
68The Realization of a Packed-bed Tower
Problem: Loss of pumparound flow
Cause Consequence Action
Loss or decline in heat
input, increase in rich
loadings, loss of cooling
media, leaking draw tray,
insufficient makeup water,
too large purge rates,
coolant side of exchanger
fouled
Excess water to
downstream sulfur unit,
increase in pressure which
raise temperatures which
accelerates corrosion,
erratic amine strengths
Increase heat input to
reboiler
Increase lean
circulation
Restore cooling media
flow
Compute water balance
and adjust
purge/makeup
Clean pumparound
cooler
Problem: Increase in pumparound flow
Cause Consequence Action
Improper water balance,
too low tower top
temperature setting,
condenser leak
Erratic amine strengths,
plugged overhead lines
Compute water balance
and adjust
purge/makeup rates
Raise tower top
temperature
Analyze chloride level
of amine, shutdown
and repair condenser
Lean amine cooler
Table 19: Troubleshooting of lean amine cooler(Amine Basic Practices Guidelines 2007)
Problem: High temperature (Normally 90-130 F)
Cause Consequence Action
Cooler bypass open, Reduced removal Close cooler bypass,
69The Realization of a Packed-bed Tower
exchanger fouling, loss of
cooling water
efficiency
Hydrocarbon
vaporization in
liquid/liquid contactors
calculate heat transfer
coefficients and clean
exchangers, check cooling
water supply, rapid loss of
cooling may be an
indication of mechanical
problems
Problem: Low temperature
Cause Consequence Action
Cooler bypass closed, loss
of heat to regenerator
Condensation of
hydrocarbons in absorbers
Open cooler bypass
Check regenerator
bottoms temperature
for deviation
Problem: High Pressure
Cause Consequence Action
Fouling, equipment failure Reduced circulation or
high lean amine
temperature leading to off-
spec product, reduced heat
transfer
Locate the point of high
pressure drop
Reboiler
Table 20: Troubleshooting of reboiler(Amine Basic Practices Guidelines 2007)
Problem: Low reboiler temperature
Cause Consequence Action
Loss of reboiler heating
media, fouling of reboiler
or lean/rich exchangers
due to excessive corrosion
High lean loadings leading
to off-spec product,
emission of toxic gases to
atmosphere
Confirm initial and
final state and flows of
heating media and
calculate duty to
70The Realization of a Packed-bed Tower
rates, failure of
reflux/pumparound control
leading to overcooling,
loss of contaminant
exchanger
Compute tower heat
balance
Compute required heat
requirement and
compare to available
heat
Compute reboiler and
lean/rich exchanger
heat transfer
coefficients to
determine fouling.
Clean exchangers if
fouled.
Isolate leak
Remove heat stable
salts and anions
Remove amine
degradation products
(reclaim)
Problem: High reboiler temperature
Cause Consequence Action
Tower overhead or acid
gas product line plugged
or pressure control
problem, too hot heating
media, too high heat input,
hydrocarbon incursion
causing higher pressure,
high heat stable salt anion
and sodium content,
Increased corrosion rates
leading to filter plugging
and equipment fouling and
plugging, corrosion
damage leading to reboiler
failure, decreased
throughput, acceleration of
amine degradation
Clear overhead line
pressure restrictions
Confirm temperature
and pressure of heating
media, adjust as needed
Skim rich flash
drum/reflux drum for
hydrocarbons
Decrease reboiler heat
71The Realization of a Packed-bed Tower
improper circulation of
amine through reboiler,
improper design of
reboiler, high amine
degradation product levels
input
Remove heat stable salt
anions and sodium
Remove amine
degradation products
Evaluate heat transfer
equipment for
coefficient and
hydraulics
Clean reboiler and/or
lean/rich exchangers
Problem: Loss of heating media flow
Cause Consequence Action
Controller failure, loss of
supply, condensate system
bottleneck
Loss of treating capability,
off-spec product
Restore supply of
heating media
Lower condensate
system pressure
Reclaimer
Table 21: Troubleshooting of reclaimer(Amine Basic Practices Guidelines 2007)
Problem: Low steam rate
Cause Consequence Action
Improper setting, or
controller malfunction
Low boil up, extended
reclaiming run
Increase steam flow until
bumping or violent boiling
is heard, then reduce
slightly
Problem: High steam rate
Cause Consequence Action
Improper setting, or Bumping, carryover of Decrease steam flow until
72The Realization of a Packed-bed Tower
controller malfunction salts into regenerator,
inability to clean up MEA
bumping of violent boiling
is no longer heard
Problem: High feed rate
Cause Consequence Action
Improper set point or
controller malfunction
Carryover of salts into
regenerator, inability to
clean up MEA
Re-adjust level
Problem: Low feed rate
Cause Consequence Action
Improper set point or
controller malfunction
Exposed tubes may cause
thermal degradation of
amine on hot tube surface
Re-adjust level
Problem: Low temperature
Cause Consequence Action
High steam pressure
(above about 90 psig) or
salt concentration in
reclaimer too high
Thermal degradation of
MEA, possible corrosion
End the batch run, stop
steam flow, or, if steam,
pressure is problem reduce
it as appropriate
4.2 Maintenance plan for Carbon Dioxide Capture System
Table 22: Maintenance Plan of Carbon Dioxide Capture System(Example Packed Bed
Wet Scrubber Agency Operation & Maintenance Plan n.d.)
Daily
Check all the indicators, such as pressure drop indicator, temperature indicator and
others to ensure that they are not out of the normal operating range. If one of them
out of the normal operating range (to be specified by the facility), corrective action
is recommended to be taken within 8 hours to return the parameter to normal.
Conduct observation of the stack and areas adjacent to the stack to determine if
droplet reentrainment is occurring. The sign of droplet reentrainment may include
73The Realization of a Packed-bed Tower
fallout of solid-containing droplets, discoloration of the stack and adjacent
surfaces, or a mud lip around the stack. If the droplets reentrainment is occurring,
corrective action is recommended to be taken within 8 hours.
Weekly
Check liquid pressure gauges on supply headers to the scrubber to monitor for
problems such as nozzle pluggage, header pluggage and nozzle erosion. Pluggage
problem are indicated by higher than normal pressure and erosion problem
indicated by less than normal pressure. If the liquid pressure is out of normal
operating range (to be specified by the facility), corrective action is recommended
to be taken within 8 hours.
Quaterly (quarter of a year)
Conduct a walk around inspection of the entire system to search for leaks. If there
is leaking detected in the system, the appropriates action is recommended to be
taken within 8 hours
Semi-annually
Conduct an internal inspection of the system to search for signs of:
Corrosion and erosion
Solids deposits in the equipments
Plugged or eroded spray nozzles
If any of these condition exists, the appropriate action is recommended to be
taken within 8 hours
74The Realization of a Packed-bed Tower
5.0 Cost Estimation
Scrubbing Reagent
Below are the estimated specification and the market price of the scrubbing reagent.
Table 23: Estimated specification of Scrubbing Reagent
Specification Monoethanolamine (MEA)
Potassium Carbonate (K2CO3)
Operating Temperature (C)
40 40
Operating Pressure (bar) 1 1Inlet Solvent Flow Rate (kgmole/h)
148000 148000
MEA content in amine (mass %)
29 -
Total usage (kgmole/month)
28416000 28416000
Table 24: Market Price of Scrubbing Reagent
Scrubbing Reagent Primechem Malaysia Sdn Bhd
Best Chemical Co. (S) Pte Ltd
Monoethanolamine (MEA)
RM 6.50 / kg RM 6.80 / kg
Potassium Carbonate (K2CO3)
- RM 5.90 / kg
75The Realization of a Packed-bed Tower
6.0 Recommendation
From the technology available to capture carbon dioxide, there are still a lot of
improvements to go in order to enhance the performance of Carbon Dioxide Removal
Process. Therefore, there are some modifications can be made, such as produce a hybrid
system, which is the combination of advantages of two or more different technologies.
For example, combination of membrane technology and absorption technology result in
gas absorption membrane. Beside that, algae can remove carbon dioxide as well.
Therefore, plantation of algae near the source of CO2 can be one of the solutions for the
climate change issues. In addition, adsorption technology and cryogenics need to be
developed further in order to increase their performance before being commercialized
since it is cheaper compare with absorption technology.
On the other hand, it is the same goes to scrubbing reagent. So that, it may
produce better scrubbing reagent for carbon dioxide removal by increasing its capacity.
Furthermore, there are some research mention that room temperature ionic liquid can be
used as the scrubbing reagent as well but still under research phase. Therefore, it got
potential to classify as scrubbing reagent in the future.
76The Realization of a Packed-bed Tower
7.0 Conclusion
As a conclusion, packed bed scrubber is the type of scrubber that most often used
in the carbon dioxide capture system. Inside the packed bed scrubber, the internals
involved are spray nozzle, packing, packing support and mist eliminator. However, the
scrubbing reagent being used depends on the concentration of carbon dioxide in the feed
stream, which means using of physical solvent for higher concentration of carbon dioxide
in feed stream and chemical solvent for lower concentration of carbon dioxide.
Beside that, by conducting this project, it has met the objective of understanding
the purpose of gas scrubber system being used in the industry. However, it is still under
development stage to enhance its performance even there is a lot of CO2 capture already
exists in the world. This is because the carbon dioxide being produced more than the
amount that can be absorbed by the technology. Therefore, the best solution in preventing
the greenhouse gas effect effeciently is still an unknown.
77The Realization of a Packed-bed Tower
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85The Realization of a Packed-bed Tower
9.0 Appendices
Appendix A: Learning Outcome
Academic
From this industrial training, the learning outcome from the aspect of academic
are shown as below:
Learn about the process need to be go through in order to fabricate some of the
equipments, such as pressure vessel and heat exchanger
Learn about the types of machine involved to fabricate a particular product
Learn about the components that used together with fabricating a particular
product, such as flanges, gasket and others
Learn about the safety and precaution when working in the engineering factory
Learn more about welding process, such as welding tools, the way of welding
working, welding consumables parts and others
Learn about the side factors need to be considered in order to accomplish the
particular product
Learn one of the major problem faced by the industry, which is the effect of
greenhouse gas
Learn the way available to solve greenhouse gas effect of industry
Learn the flow of carbon dioxide scrubbing process and some different flow
configuration in order to achieve cost effective
Learn the transport and storage of carbon dioxide under carbon dioxide capture
and storage technology
Learn the consideration that involved in order to choose the best option for the
scrubbing process, such as scrubbing reagent, packed bed scrubber internals and
others
Learn the problem that may occur in the system and the action to fix it
86The Realization of a Packed-bed Tower
Non-academic
From this industrial training, the learning outcome from the aspect of non-academic
are shown as below:
Learn about the culture and principle of the company in working
Learn the way of communicate with supplier
Experience the working life as a research engineer
Experience the working environment of an engineering company, which is team
work environment
Learn better way to manage the workload given in order to be punctual
Experience the realistic side of a company during working
Expose to the strategy of a company in order to go through numerous challenges
and survive in this competitive era
Realize the important of action and planning in the way to achieve the target
Being shared with the success story of the company and realize the process in
order to develop the company from nothing become better
87The Realization of a Packed-bed Tower
Comment
During this industrial training, overall is good. This is because the tasks given
involved different attribute can be learned, such as experience on building up a pilot plant
and research on the renewable energy technology. Beside that, looking deep into the
equipments already fabricated and process already used in the factory in order to look for
further improvement, such as filter vessel and plasma arc welding. Then, it does help us
learn a lot from the aspect of academic and non-academic as an eye opener to the
industry.
However, there are some recommendations as well. Firstly, the company may
provide more practical work instead of research work. For example, arrangement of
intern student to experience about the welding process, which mean work on our own
hand under a qualified welder by using the waste material. The benefit of this practical
work is providing more knowledge for the way of welding, such as the angle of welding,
welding joint and others instead reading on the guideline only. Beside that, we can know
better about the proper consequence of welding from the aspect of quality. Furthermore,
we may work in this industry in the future, so that, we can perform as well instead of only
know the knowledge.
Hence, that’s all my comment and recommendation regarding to this industrial
training and I did appreciate this opportunity to join the training programme provided.
Thank you.
88The Realization of a Packed-bed Tower
Appendix B: Advanced Research and Development Pathways
Below are the future pathways of the research on carbon dioxide capture
technology in order to achieve improvements, such as reduce energy consumption for
solvent recovery stage, potential scrubbing reagent and others,
Solid Adsorbents
Metal-organic Frameworks
Functionalized Fibrous Matrices
Poly (ionic liquid)
Structured fluid absorbents
CO2 hydrates
Liquid crystals
Ionic liquid
Non-thermal regeneration method
Electrical Swing Adsorption
Electrochemical methods
Remarks:
Before proceeding to the advanced research topic, there are some researches still need to
be done, which are the controlled system needed for the carbon dioxide capture system,
the control loop, the complete cost estimation of carbon dioxide capture system.
89The Realization of a Packed-bed Tower
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