51065880 industrial training report research packed column

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

1The Realization of a Packed-bed Tower

- Minimize the consumption of utilities

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.

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).


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

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

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

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.

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

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

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.).

‘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

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).

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

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.)

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.

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


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


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.

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

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

• 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 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.

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.

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

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)

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.

(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 kx 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 yi-y. So the design equation

corresponding to Eq. (5) becomes

(8)


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.

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

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.

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),

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 NtOG= 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

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]

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.

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

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


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.


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


http://www.separationprocesses.com/Absorption/GA_Chp02e.htm

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)


http://www.separationprocesses.com/Absorption/Fig134.htm

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


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/ft2

(1.25-5 m3/m2h); for structured packing it is 0.1-0.2 gpm/ft2 (0.12-0.25 m3/m2h).


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


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


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


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

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 Flv,

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 Cs,

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.

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)


(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


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 CO2

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


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

Random packing bed limiter (Non-interfering)


• Requires no vessel attachments• Minimizes interference of drip

point from high performance liquid distributors


Random packing bed limiter


• 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.


Table 8: The information of packing support

Support plates Diameter Specification Contour

Structured packing support grid


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


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)


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)


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)


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 EquipmentAbsorber (Packed Bed Scrubber) Heat ExchangerDesorber/Stripper/Regenerator CondenserPump ReboilerReflux Drum/Liquid Separator Solvent Cooler

Table 11: Optional Equipment of Carbon Dioxide Capture System

Optional EquipmentSemi Lean Flash DrumSemi Lean CoolerReclaimerFeed 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

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.

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 increaseCause Consequence Action

Possible fouling Flooding, poor

efficiency, off-spec

treated gas

Cleanout, identify root cause (eg.

Corrosion)

Problem: Differential pressure sudden increase, erratic actionCause 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

entrained hydrocarbon Problem: Low feed gas flow rate

Cause Consequence ActionUpstream 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 rateCause Consequence Action

Upstream process

change

Increase amine demand,

possible jet flooding

Increase amine flow

Problem: Low amine Flow rateCause Consequence Action

Change in controller

status or supply

Potential reduction in

acid gas recovery

Adjust conditions determining rate


pressureProblem: High amine Flow rate

Cause Consequence ActionChange 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 temperatureCause Consequence Action

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 balanceProblem: High lean amine temperature (90-130°F)

Cause Consequence ActionChange in upstream

process and/or

ambient conditions


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 temperatureCause Consequence Action

Change in upstream

process and/or

ambient conditions


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 ActionChange 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

Overcirculation Excessive utility

consumption

Reduce amine circulation rate

Problem: High rich amine loadingCause 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

drumProblem: Low pressure

Cause Consequence ActionSystem venting to

atmosphere or relief

system

May not get into the

regenerator

Find the source of leaking

Problem: Negative pressureCause Consequence Action

Relief stack draft causing a

vacuum on the system

Air may be drawn into

flash drum and

contaminate the amine or

Adjust relief system

pressure to hold positive

pressure on drum


cause an explosive mixtureProblem: High flash gas rate

Cause Consequence ActionHydrocarbon 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

drumProblem: High hydrocarbon level (normally 0-5 % level above amine)

Cause Consequence ActionInsufficient skimming Amine foaming, SRU

upset

Increase skim rate, check

gas and liquid absorber

operation hydrocarbon

carryunderProblem: High amine level

Cause Consequence ActionWater 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 levelCause 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

Lean/rich exchangers


Table 15: Troubleshooting of lean/rich exchangers(Amine Basic Practices Guidelines

2007)

Problem: High rich amine temperatureCause Consequence Action

Low fuel rate Flashing and corrosion in

exchangers and

regenerator inlet

Check amine flows,

possibly bypass hot lean

amine flowsProblem: Low rich amine temperature

Cause Consequence ActionExchanger fouling Poor stripping in

regenerator and/or

increased reboiler steam

demand

Check all temperatures for

poor performance

(fouling), clean exchangers

Problem: High pressureCause 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 pressureCause Consequence Action

Failed pressure controller,

loss of reboiler heat

source, loss of feed, loss of

containment

Upset of or shutdown of

downstream sulfur unit,

release to atmosphere, unit

shutdown

Determine cause for loss

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 pressureCause 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 lineProblem: 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 rateCause Consequence Action

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

regeneratorProblem: Gradual decline in rich amine flow rate

Cause Consequence ActionDeclining rich amine flash

drum level resetting flow,

plugging from corrosion

products or salts, leaks or

open drains


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 ActionLoss of feed, loss of

reboiler heat input,

downstream unit

shutdown, plugged

overhead line, pressure

controller failure, loss of

containment, tower

internals malfunction


treating capability, relief,

unit shutdown

Restore feed. Restore

reboiler heating media.

Steam out of otherwise

heat overhead line; raise

reflux or pump around

return temperature

Problem: Sudden increase in acid gas product rateCause 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 filterProblem: Lean loading exceeds spec/treated gases and liquids fail to meet spec

Cause Consequence ActionInsufficient reboiler heat

input caused by:

insufficient heating media

Off-spec products,

excessive corrosion

Increase reboiler heat

input, decrease circulation,

increase amine strength,


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

remove heat stable salt

anions and sodium, clean

fouled exchangers, check

sodium level

Problem: Little or no tower pressure dropCause Consequence Action

Loss of feed, loss of

reboiler heating media,

tray blowout, tower


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

normalCause Consequence Action

Foaming, hydrocarbon

intrusion from flash drum,

hydrocarbon refluxed to

tower, reboiler heat input

flunctuations, tower


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 dropCause Consequence Action

• Buildup of

corrosion 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

Reduced throughput, high

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

• Discontinue

antifoam additions and

change carbon filters to

prevent antifoam

buildup and foaming

episodes

• Remove heat stable

salts anions and sodium

• Remove amine


(reclaimer)

• Shutdown and

chemical or water wash


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

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

systemCause Consequence Action

Insufficient heat input, too

cold reflux or pumparound

return temperatures, reflux

level or pumparound flow

control valve failure,

excessive rich loadings,

tower internals

malfunction

Increased lean loadings

leading to off-spec

products

Raise reboiler heat input.

Raise reflux or

pumparound return

temperatures.

Raise condenser cooling

media temperature

Problem: High top temperature for conventional condenser/drum overhead

systemCause 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

Reduce reboiler duty

Re-establish

condenser/pumparound

cooler cooling media

Reduce circulation


corrosion rates throughout

regenerator, accelerate

amine degradation, excess

energy costs

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

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 refluxCause 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

restrictedProblem: Reflux ratio too low

Cause Consequence ActionController 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 systemProblem: Loss of pumparound flow

Cause Consequence ActionLoss 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

coolerProblem: Increase in pumparound flow

Cause Consequence ActionImproper 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,

exchanger fouling, loss of

cooling water

• Reduced removal

efficiency

• Hydrocarbon

vaporization in

liquid/liquid contactors

Close cooler bypass,

calculate heat transfer

coefficients and clean

exchangers, check cooling

water supply, rapid loss of

cooling may be an

indication of mechanical

problemsProblem: Low temperature

Cause Consequence ActionCooler bypass closed, loss

of heat to regenerator

Condensation of

hydrocarbons in absorbers

• Open cooler bypass

• Check regenerator

bottoms temperature

for deviationProblem: High Pressure

Cause Consequence ActionFouling, 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 temperatureCause Consequence Action

Loss of reboiler heating

media, fouling of reboiler

or lean/rich exchangers

due to excessive corrosion

rates, failure of

reflux/pumparound control

leading to overcooling,

loss of contaminant

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

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


salts and anions

• Remove amine


(reclaim)Problem: High reboiler temperature

Cause Consequence ActionTower overhead or acid

gas product line plugged

or pressure control

problem, too hot heating

Increased corrosion rates

leading to filter plugging

and equipment fouling and

plugging, corrosion

• Clear overhead line

pressure restrictions

• Confirm

temperature and


media, too high heat input,

hydrocarbon incursion

causing higher pressure,

high heat stable salt anion

and sodium content,

improper circulation of

amine through reboiler,

improper design of

reboiler, high amine

degradation product levels

damage leading to reboiler

failure, decreased

throughput, acceleration of

amine degradation

pressure of heating

media, adjust as needed

• Skim rich flash

drum/reflux drum for

hydrocarbons

• Decrease reboiler

heat input


salt anions and sodium

• Remove amine


• Evaluate heat

transfer equipment for

coefficient and

hydraulics

• Clean reboiler

and/or lean/rich

exchangersProblem: Loss of heating media flow

Cause Consequence ActionController 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 rateCause 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

slightlyProblem: High steam rate

Cause Consequence ActionImproper setting, or


Bumping, carryover of

salts into regenerator,

inability to clean up MEA

Decrease steam flow until

bumping of violent boiling

is no longer heardProblem: High feed rate

Cause Consequence ActionImproper set point or


Carryover of salts into

regenerator, inability to

clean up MEA

Re-adjust level

Problem: Low feed rateCause Consequence Action

Improper set point or


Exposed tubes may cause

thermal degradation of

amine on hot tube surface

Re-adjust level

Problem: Low temperatureCause 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 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 hoursSemi-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


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


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.


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.


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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

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

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.

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.
