ABSORBER

DESIGN

ABSORBER DESIGN

4.1. Absorber:

Gas absorbers are used extensively in industry for separation and purification of gas streams as

product recovery devices, and as pollution control devices. Gas absorbers are most widely used

to remove water soluble inorganic contaminants from air streams. Absorption is a process where

one or more soluble components of a gas mixture are dissolved in a liquid (i.e., a solvent).

Solute:

The component of gas that needs to be dissolved in a solvent

In our case the solute is ammonia that is dissolved in a lean solution of ammonia and water.

Solvent:

The substance that dissolved solute in it is called solvent.

Liquids commonly used as solvents include water, mineral oils, nonvolatile hydrocarbon oils,

and aqueous solutions. The solvent chosen should have a high solubility for the gas, low vapor

pressure, low viscosity, and should be relatively inexpensive.

Absorption, in chemistry, is a physical or chemical phenomenon or a process in

which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. This

is a different process from adsorption, since the molecules are taken up by the volume, not

by surface. In gas absorption, soluble vapors are more or less absorbed in the solvent from its

mixture with inert gas. The purpose of such gas scrubbing operations may be any of the

following;

For separation of component having the economic value.

As a stage in the preparation of some compound.

For removing of undesired component (pollution).

4.2. Types of Absorption:

Physical absorption,

Chemical Absorption

4.2.1. Physical Absorption:

In physical absorption mass transfer take place purely by diffusion and physical absorption is

governed by the physical equilibria. Physical absorption occurs when the absorbed compound

dissolves in the solvent. Physical absorption depends on properties of the gas stream and solvent,

such as density and viscosity, as well as specific characteristics of the pollutant in the gas and the

liquid stream. These properties are temperature dependent, and lower temperatures generally

favor absorption of gases by the solvent. Absorption is also enhanced by greater contacting

surface, higher liquid-gas ratios, and higher concentrations in the gas stream.

4.2.2. Chemical Absorption:

Chemical absorption occurs when the absorbed compound and the solvent react. When oxides of

nitrogen absorb in water the chemical reaction take place and nitric acid form this is common

example of chemical absorption.

4.3. Types of Absorber:

There are three major types of absorbers which are mainly used for absorption purposes:

Packed column

Plate column

4.3.1 Packed Tower:

Packed towers, which are the most commonly, used gas absorbers for pollution control. Packed

towers are columns filled with packing materials that provide a large surface area to facilitate

contact between the liquid and gas. Packed tower absorbers can achieve higher removal

efficiencies, handle higher liquid rates, and have relatively lower water consumption

requirements than other types of gas absorbers. However, packed towers may also have high

system pressure drops, high clogging and fouling potential and extensive maintenance costs due

to the presence of packing materials. Installation, operation, and wastewater disposal costs may

also be higher for packed bed absorbers than for other absorbers.

Packed column 1

4.3.2 Plate Tower:

Plate, or tray, towers are vertical cylinders in which the liquid and gas are contacted in stepwise

fashion on trays (plates). Liquid enters at the top of the column and flows across each plate and

through a downspout (down comer) to the plates below. Gas moves upwards through openings in

the plates, bubbles into the liquid, and passes to the plate above. Plate towers are easier to clean

and tend to handle large temperature fluctuations better than packed towers do. However, at high

gas flow rates, plate towers exhibit larger pressure drops and have larger liquid holdups.

4.3.3. Relative Merits of Plate and Packed Towers:

Plate column Packed column

Can handle wide range of liquid rates

without flooding

Flooding can occur due to fluctuation in

liquid rates

For large diameter column For small diameter column

Cannot be used for highly corrosive

liquids

Packed towers prove to be cheaper and

easier to construct if highly corrosive fluids

must be handled

Pressure drop more Pressure drop is low

Total weight of dry plate tower is less

than packed tower

Total weight of packed tower is high than

plate tower

Expensive Less expensive

The choice between use of a plate tower or a packed tower for a given mass-transfer operation

should, theoretically, be based on a detailed cost analysis for the two types of contactors. In

many cases, however, the decision can be made on the basis of a qualitative analysis of the

relative advantages and disadvantages.

The following general advantages and disadvantages of plate and packed towers should be

considered when a choice must be made between the two types of contactors:

4.4. Absorber System Configuration:

Gas and liquid flow through an absorber may be

Countercurrent

Crosscurrent

Co current.

4.4.1. Countercurrent:

The most commonly installed designs are countercurrent, in which the waste gas stream enters at

the bottom of the absorber column and exits at the top. Conversely, the solvent stream enters at

the top and exits at the bottom. Countercurrent designs provide the highest theoretical removal

efficiency because gas with the lowest pollutant concentration contacts liquid with the lowest

pollutant concentration. This serves to maximize the average driving force for absorption

throughout the column.

4.4.2. Crosscurrent:

In a crosscurrent tower, the waste gas flows horizontally across the column while the solvent

flows vertically down the column. As a rule, crosscurrent designs have lower pressure drops and

require lower liquid-to-gas ratios than both co current and countercurrent designs. They are

applicable when gases are highly soluble, since they offer less contact time for absorption.

4.4.4. Co current:

In co current towers, both the waste gas and solvent enter the column at the top of the tower and

exit at the bottom. Co current designs have lower pressure drops, are not subject to flooding

limitation. Co current designs are only efficient where large absorption driving forces are

available. Removal efficiency is limited since the gas-liquid system approaches equilibrium at

the bottom of the tower.

4.5. Packed Tower Internals:

4.5.1. Tower Shell:

The tower shell may be made of steel or plastic, or some combination which may require the

addition of liners or inner layers of rubber, plastic or brick. The mechanical problems of

attaching depending on the corrosiveness of the gas and liquid streams, and the process operating

conditions.

4.5.2. Mist Eliminator:

At high gas velocities, the gas exiting the top of the column may carry off droplets of liquid as a

mist. To prevent this, a mist eliminator in the form of corrugated sheets or a layer of mesh can be

installed at the top of the column to collect the liquid droplets, which coalesce and fall back into

the column.

4.5.4. Packing:

Packing materials provide a large wetted surface for the gas stream maximizing the area

available for mass transfer. Packing materials are available in a variety of forms, each having

specific characteristics with respect to surface area, pressure drop, weight, corrosion resistance,

and cost.

4.6. Packing Selection:

Packing materials are categorized as random or structured.

Dumped tower packing

Stacked tower packing

4.6.1. Dumped Tower Packing:

Random packing as the name implied, are dumped into a column during installation and allowed

to fall in random. Small packing’s poured randomly into a vessel is certainly the more popular

and commonly employed form of packed-tower design. However, in certain instances

where exceptionally low pressure drop and very high flow rates are involved, stacked or oriented

packing have also been used. Random packing’s are usually dumped into an absorption column

and allowed to settle.

4.6.2. Stacked Tower Packing:

Structured packing’s are considerably more expensive per unit volume than random packing’s.

They come with different sizes and are neatly stacked in the column. Structure packing usually

offer less pressure drop and have higher efficiency and capacity than random packing. Structured

packing may be random packing connected in an orderly arrangement,

4.7. Types of Packing:

4.7.1. Pall rings:

Pall ring that is improved on the basis of rashing rings. The design of pall rings provide higher

capacity and lower pressure drop than other packing the open cylindrical walls and inward bends

protrusions of pall rings allow greater capacity and lower pressure drop.

Lower pressure drop (less than half) than Raschig rings, also lower HTU (in some systems also

lower than Berl saddles), higher flooding limit. Good liquid distribution, high capacity.

Considerable side thrust on column wall. Available in metal, plastic and ceramic.

These are of two types:

4.7.2. Metal Pall Rings:

The rings are made up of metal.

4.7.3. Plastic Pall Rings:

The rings are made up of plastic material.

4.8. Distributor:

Distribution of the liquid onto the packed bed or structured packing is realsed by appropriate

liquid distributors. It is important to distribute the liquid flow equally across the column area in

order to secure an intensive mass transfer between the phases.

In addition to the task of regular liquid distribution the part has to meet following requirements:

pressure drop in the gas phase should be low

part should be resistant against dirt or solids in the liquid

high turn down ratio

low entrainment of droplets

prevention of irregular gas distribution

prevention of wall effect on liquid flow

Distributors are used for the good distribution of liquid over the packing so that the liquid come

in contact properly with incoming gas.

4.8.1. Types of The Distributors:

The following types of liquid distributors are available:

orifice distributor

trough distributor

rough-type distributor

ladder-type distributor

spray nozzle-type distributor

4.8.1.1. Trough Distributor:

Trough distributor provides good distribution under widely varying flow rates of gas and liquid.

the liquid may flow through simple V notched weirs, or it may flow through tubes that extend

from troughs to near the upper level of the packings.some deposition of solids can be

accommodated. Because of its large free area at is suitable for high gas rates

Orifice trough liquid distributor

4.8.1.2. Orifice Distributor:

Orifice distributor type which gives very fine distribution though it must be correctly sized for a

particular duty and should not be used where is disk of plugging. The orifice riser distributor is

designed to lay the liquid carefully onto the bed, with a minimum of contact with gas during the

process.

4.8.1.4. Perforated Pipe Distributor:

The perforated ring type of distributor for use with absorption columns where high gas rates and

relatively small liquid rates are ecounter.this is specially suitable where pressure loss must be

minimized, for the larger size of tower where installation through manholes is necessary, it may

be made up in flanged sections. The orifices are of 4 to 6 mm in diameter, and can be subject to

plugging if foreign material is present. The pipe must be carefully leveled for larger diameter

column.

4.8.2. Redistributors:

The liquid coming down through the packing and on the wall of the tower should be redistributed

after a bed depth of approximately 4 tower diameter for rashing rings and 5-10 tower diameters

for saddle packings.Collector/Redistributors, is very similar to the distributor in that it will

contain a deck and chimneys. The collector is used under a packed bed section to collect the

liquid to aid in mixing and redistribution. The difference is that the redistributors will contain

caps or hats to prevent the water falling from the packing from bypassing the collector.

4.9. Support Plates:

These are the simplest and least expensive type of packing supports. They also utilize the least

vertical space. They are designed for low to medium gas loading when used for dumped packing

and typically have 50 to 90% open areas depending on the material used.

The support grids are available in various materials such as plastic, FRP and metals. They can

also be used as bed limiters. Sometimes support beams are required for structural reasons

depending on the material and size of the support grate.

4.9.1. Gas Injection Support Plate:

It is a device used to hold the packing. It generally sits on a support ledge and can be supported

additionally by structural beams. There are two design criteria for the gas injection support plate.

It must hold the packing and liquid hold-up but also requires an open are greater than the cross

sectional area of the tower. The larger open area is accomplished by using slotted or perforated

plate that is corrugated or positioned in such a way to allow increased gas flow. Open area ranges

from 85% to greater than 100%.

4.9.2. Grid Type Support Plates (APS-GS):

Grid type packing supports are used for structured packing to provide a horizontal contact

surface and to prevent distortion of the packing. This design can also be considered for

random packing. A wide range of openings is available to prevent the packing from falling

through. The supports typically rest on support ledges. For larger towers with man ways,

sectional designs are standard.

4.10. Design of Absorber:

4.10.1. General Design Steps:

The designer is required to consider and determine

select suitable column type

select appropriate solvent

select type and size of packing

]material and energy balance

Calculate column diameter

Pressure drop calculation

Determine the number of transfer units

Determine height of transfer unit

Find the height of column

Select column internals

4.10.2. Input Data:

Operating temperature 263.5K

Operating pressure 61.22atm

6203571.4Pascal

Packing type Pall rings

Packing size 1.5 Inches

0.0381m

Packing factor, (Fp) 130/m

Surface area of packing (ap) 128 m2/m3

4.10.3. Gas Properties:

Gas flow rate 6357 kg/hr

587.52 kgmoles/hr

Gas pressure at entry 68.02 atm

Gas temperature at entry 249.82K

Gas mol weight (Mg) 10.82

Gas density (ρg) 35.88 kg/m3

2.24 lb/ft

4.10.4. Component to be Scrubbed:

Component Name Ammonia

Component flow rate 197 kg/hr

0.05472 kg/s

Molecular weight of comp (Mg) 17

4.10.5. Liquid Properties:

Liquid flow rate, L 80kgmoles

1439 kg/hr

Liquid Density, (ρl) 999.92kg/m3

Liquid Viscosity, (µl) 0.0014 Pa-s

1.4 Cp

Molecular wt of liquid (Ml) 17.99

4.10.6. Humidity Calculations:

Total pressure (Pt) = 6203.57 kpa

Vapor Pressure of water = 0.836 kpa

Molecular wt of exit gas (Mg2) = 10.69

H = V.P * Mg

(Pt-V.P) *Mg2

H = 0.000227 kg/kg dry gas

1.422 kg/hr

4.10.7. Material Balance:

Mol fraction of ammonia in entering gas = 5.5x10-4

Mol fraction of ammonia in exiting gas = 7.9x10-5

Mol fraction of ammonia in entering liquid = 0

Mol fraction of ammonia in exiting liquid = 4.5x10-3

4.10.8. Column Diameter Calculations:

Pressure drop for absorber = 15 to50 mmH2O/m of packing

Assume pressure drop = 42 mmH2O/m of packing

= 0.0569psi/m of packing

= 0.01734psi/ft of packing

4.10.9. Column Diameter:

D = 1.1283[G(kg/sec)/G(kg/secm2)]

Where G= gas mass flow rate

G = 200,000kg/h

L = liquid mass flow rate

L = 8500kg/h

X- Coordinate value:

X = 0.04368

Y- Coordinate value:

By using the assumed pressure drop = 42mmHg/m of packing

Y = 0.055 (unit operation by McCabe & smith edition 5 fig 4.1)

After putting the variables known, mass velocity of gas can be calculated as

G =1431 Lb/ ft2 h

Put in the equation;

Cs=0.184

µ=Cs

4.10.10. Area Calculation:

Ac = Π/4(D)2

Ac = 0.15 m2

4.10.11. Liquid Mass Velocity:

L = 2.665Kg/s m2

L = 0.5449 Lb/ ft2 sec

4.10.12. Pressure Drop Calculation:

Where:

∆P = 0.48 in H2O / ft of packing

∆P = Pressure drop in inches of water /ft of the packing height

G = Gas superficial mass velocity lb/s-ft2 tower cross section

L = liquid superficial mass velocity lb/s-ft2 tower cross section

ρg = Gas density ,lb/ft2

α & β are constant taken from (Applied process design for chemical & petrochemical plant by

Ernest E. Ludwig table 19-24)

4.10.13. Percentage Flooding:

K4’ at flooding from graph = 3.2

K4 = 1.991

Percentage flooding = (K4/ K4’) 0.5*100

= 78%

From graph 11.44: by using K4 = 1.77 and FLW =0.0429 the д P line come out to be 42mmHg/m

of packing and is same as was assumed

Δ Passumed = Δ Pcalculated

4.10.14. Calculation of Equilibrium Constant:

As our operating temperature and pressure is such that they are out of range of data so we

calculate equilibrium constant by using thermodynamic relationship which is as follow

Ke = yi/xi = γifiOL/PTфi

Where

γi = activity coefficient of component ammonia ‘i’=7.4

fiOL = (fugacity of pure liquid component ‘i’ ammonia N/m2)

fiOL = Pi фi [exp {(PT-Pi)Vi

L/RT]

Pi (vapor pressure of ammonia at 263.5 K) = 2.7 bar

PT = 62.03 bar

ViL = specific volume

After putting all values we get

f iOL = 1.7884 bar

фi = ( fugacity coefficient of pure liquid component ‘i’ ammonia unit less) calculated by

generalized correlation available in thermodynamics

ln фi = BoPr/Tr +ωB1Pr/Tr

For ammonia all values available in literature we get the value of фi = 0.57078

Putting all values in equation the value be:

Ki = 0.374

Which is the slope of operating line

Absorption factor = slope of O.L/ slope of E.L = L/mG

= 0.77/0.374

=2.1

As absorption factor is greater than 1 this indicate that more and more solute absorbed in liquid

cause the decrease in height of column and hence the cost.

Equilibrium curve plotted according to (ref McCabe and smith)

Slope of equilibrium line = 0.374

4.10.15. Number of Transfer Units Calculations:

NTU = A * (ln [(yb/ya)*(A-1) +1])

A-1 A

NTU = 9

4.10.16. Height of The Column:

As our packing size is 1.5 inch and column diameter less than 3 ft the HETP can be taken in the

range 0.4 to 0.75m so we select it

HETP = 0.7m

Dc = 0.438m

Ls = VL t / A

Where Ls =height of bottom section for liquid surge time ts = 10sec

VL = volumetric flow rate of liquid = 1439kg/hr

After putting values we get

Ls =1.598m

Zt = Ne * HETP + 3ft +0.25Dc + Ls

Zt = 8m

Height of packing:

Z = HOGNOG

HOG = height of overall gas phase transfer unit

NOG = number of overall gas phase transfer unit

As we know

HOG = HG + m ( G/L)HL

m = slope of equilibrium line

L/G= slope of operating line

To calculate HOG there are two methods:

Cornell’s method

Onda’s method

We use Cornell’s method:

According to it

4.10.17. Individual Height of Gas Phase Transfer Unit Calculations:

HG = 0.01ψh (Sc)v0.5(Dc/0.305)1.11(Z/3.05)0.23/ (L*f1*f 2*f3)

HG = height of gas phase transfer unit

ψh = at 58% flooding = 80 (from fig:4.2)

Lw = 2.67kg/m2sec

f1 = liquid viscosity correction factor

f1 = (µL/ µw)

f1= 1.089

f 2 =liquid density correction factor

f 2 = (ρw / ρL)1.25

f 2 = 1.0532

f3 = surface tension correction factor

f3 = (σw/ σL)

f3= 0.93

(Sc)v= gas phase Schmidt number= (µv/ ρvDv)

(Sc)v = 0.429

HG =0.699m

4.10.18. Individual Height of Liquid Phase Transfer Unit Calculations:

HL= 0.305фh (Sc)L0.5K3 (Z/3.05)0.15

HL = height of liquid phase transfer unit

K3 = at 58% flooding = 0.87 (from fig:4.3)

фh = 6.2 * 10-2 (from fig:4.4)

(Sc)L= liquid phase Schmidt number= (µl/ ρlDl)

(Sc)L = 976.47

HL = 0.6

4.10.19. Overall Height of Gas Phase Transfer Unit:

HOG = 1.03

Now

Y1/ Y2 = 0.02/0.0002021

Y1/ Y2 = 94.96

NOG = 6.2 (from fig: 4.5 by using mGm/L and Y1/ Y2)

Then

Z = HOGNOG

Z = 6.4m approximately same as calculated from the estimated value

4.10.20. Liquid Hold up Calculation:

Hlw =0.0004(L’/dp)

Hlw = water holdup (ft3 liquid/ ft3 vol of tower)

dp = equivalent spherical packing diameter (inches)

L= liquid rate (lb/ft2hr)

So

Hlw =0.0004(1961.64/1.5)0.6

Hlw = 0.03m3/m3 of tower (ref. applied process design for chemical & petrochemical

plant By Ernest E. Ludwig)

4.10.21. Minimum Wetting Rate:

Lmin = MWR*ap

Volumetric flow rate = V= L/ ρL

= 1.439m3/hr

Velocity= vol flow rate/ area of column

Velocity=9.54m/hr

MWR= v/ap

=0.00749m2/hr= 0.84 ft2/hr

(Ref. applied process design for chemical & petrochemical plant by Ernest E. Ludwig)

4.10.22. Check for Channeling:

=D/Dp

=17.5/1.5

=11.5

In the ratio 1:8 to minimize the channeling. (Ref. applied process design for chemical &

Petrochemical plant by Ernest E. Ludwig)

4.11. Mechanical Design:

4.11.1. Material Selection:

Low alloy steel 43XX (nickel 1.83%, chromium 0.80%, molybdenum 0.25%)

4.11.2. Thickness of The Column:

e = Pi Di/2f- Pi

Pi = internal pressure = 6.646N/mm2

f = design stress = 250N/mm2

e = 6.0597mm

e. = e + corrosion allowances

e. = 6.0597+ 2

e. = 8.059mm

4.11.3. Packing Support:

Simple grid and perforated plate supports

Function:

The function of support plate is to carry the weight of wet packing whilst allowing the passage of

gas & liquid.

4.11.4. Liquid Distributors:

Orifice type liquid distributor

Function:

Liquid distributors are needed to ensure the good distribution at all liquid flow rates.

4.11.5. Redistributors:

Wall wiper type redistributors

Function:

Redistributors can be equipped with wall wipers to collect the liquid clinging to the tower walls.

SPECIFICATION SHEET

Item Packed Absorption Column

No. required 01

Function To absorb ammonia in aqua

ammonia.

Operation Continous

Design Temperature 280 K

Design Pressure 6894 kPa

Height of packing section 5.22 m

Size and type of packing Plastic pall rings

Total height of column 8.27 m

Inside diameter 0.44m

Packing arrangement dumped