absorption columns

ABSORPTION COLUMNS

Presented by

ANSHIKA JAIN-70700007

GEETINDER PUNIA-707000017

ASHIMA KANGO-70700011

BHUMIKA NATH-70700013

ABSORPTION COLUMN IN INDUSTRY

CONTENTS

INTRODUCTIONGENERAL CONSTRUCTION PACKING MATERIALSDESIGNWORKING APPLICATIONS

ABSORPTION

The removal of one or more selected components from a mixture of gases is called absorption.

In any mixture of gases, the degree to which each gas is absorbed is determined by its partial pressure. Example a slightly soluble gas like oxygen requires a much higher partial pressure of the gas in contact with the liquid to give a solution of a given concentration.

Also solubility of a gas is inversely proportional to the temperature of the gas. Temperature should be sufficiently low for adequate degree of absorption.

GAS-LIQUID ABSORPTION

• An example of the liquid gas system is an absorption process where a soluble gas is scrubbed from a mixture of gases by means of a liquid.

Liquid in

Liquid out

Gas in

Gas out

Packed bed

Packing support

Distributor hold down plate

ABSORPTION COLUMNS

Absorption columns or towers are commonly used for gas absorption. A soluble vapor is absorbed from its mixture with an inert gas by means of a liquid in which the solute gas is more or less soluble.

An overview

Cylindrical column with a gas inlet and distributing space at the bottom.

A liquid inlet and distributor at the top. Gas and liquid outlets at the top and bottom respectively. Column packing to ensure intimate contact between the liquid

and the gas. Packing support to give strength .

Basic Design

GENERAL CONSTRUCTION

• The shell of the column may be constructed from metals,ceramics,glass or plastics materials, or from metals with a corrosion-resistant lining.

• The column should be mounted truly vertically to help uniform liquid distribution.

• The bed of packing rests on a support plate which should be designed to have at least 75% free area for the passage of the gas so as to offer as low a resistance as possible.

• The simplest support is a grid with relatively widely spaced bars on which a few layers of large raschig or partition rings are stacked.

The gas injection plate

• The gas injection plate is designed to provide separate passageways for gas and liquid so that they need not compete for passage through the same opening. this is achieved by providing the gas inlets to the bed at a point above the level at which liquid leaves the bed.

The gas injection plate

THE GAS INJECTION PLATE

THE GAS INJECTION PLATE

LIQUID GAS

GAS INJECTION PLATE

GAS IS DISTRUBUTED DIRECTLY INTO PACKED BED

Distributor

• At the top of a packed bed a liquid distributor of suitable design provides for the uniform irrigation of the packing which is necessary for satisfactory operation.

SIMPLE ORIFICE TYPE

NOTCHED TYPE

PERFORATED RING

NOTCHED TROUGH

DISTRIBUTORS

Simple orifice type

• A simple orifice type which gives fine distribution through it must be correctly sized for a particular duty and should not be used where there is any risk of the holes plugging.

Simple orifice type

Perforated ring type

• The perforated ring type of distributor for use with absorption columns where high gas rates and relatively small liquid rates are encountered. this type is specially designed where pressure losses is minimun.For the larger size of tower, where installation through manholes is necessary,it may be made up in flanged sections.

Notched chimney type

Notched chimney type

• The notched chimney type of distributor,which has a good range of flexibility for the medium and upper flowrates,and is not prone to blockage.

Notched trough distributor

The notched trough distributor which is specially suitable for large sizes of tower, and, because of its large free area, it is also suitable for the higher gas rates.

Requirements of packing

Provide a large surface area-for better contact Have an open structure- for low resistance to

gas flow Promote uniform liquid distribution on the

packing material Promote uniform vapor gas flow across the

column cross section

TYPES OF PACKING

DUMPEDSTACKED

STRUCTURED

RANDOM PACKING

STACKED DUMPED

Those which are stacked by hand

Those which are dumped at random into the tower

RANDOM PACKING

Random packing – rings, saddles and proprietary shapes, which are dumped into the column

Metal saddles

are durable

Ceramic saddles

Resistance to heat

Resistance to abrasion

High mechanical strength

Berl saddles

perform better as compared to Raschig rings in the aspects of even fluid distribution and low resistance

Plastic saddles

used in acidic as well as alkaline media

Pall rings

are improved Raschig rings

Provide higher capacity

Provide lower pressure drop

Ceramic honeycombs

low thermal expansion high heat shock resistance anti oxidation large specific area corrosion resistance

Plastic pall rings Raschig rings

Reduce pressure drop

Maintain economy and efficiency

Ceramic ball

Adequate mechanical strength good thermal and chemical stability larger surface area low resistance

Structured Packing

•Made up from wire mesh or perforated metal sheets•Material is folded and arranged with a regular geometry•Have a high surface area•Have high void fraction

lower pressure drop

used for difficult separations

higher efficiency

ADVANTAGES OVER RANDOM PACKING

WORKING PRINCIPLE

Air pass through openings on trays from the lower part of the tray at the certain velocity and enter up comer, liquid also enter up comer through space between trays and up comer to become membrane. Gas and liquid co-current ascend, at the same time they contact on the surface of liquid membrane.

Then they enter packing and liquid is cracked on the surface of packing to contact air furthermore. Vapor which comes from packing enter upper packing, but liquid enter lower tray to continue contacting and separating.

Working

The inlet liquid, which maybe a pure solvent or a dilute solution of solute in the solvent and which is called the weak liquor, is distributed over the packings uniformly by the distributors.

The solute containing gas, or rich gas enters the distributing space below the packing and flows upwards through the spaces in the packing in the counter current to the flow of the liquid.

CONTD………

The packing provides a large area of contact between the liquid and gas.

The solute in the rich gas is absorbed by the fresh liquid entering the tower, and dilute or lean, gas leaves the top.

The liquid is enriched in solute as it flows down the tower, and concentrated liquid, called the strong liquor, leaves the bottom of the tower through the liquid outlet.

SIMPLE FLOW CHART OF WORKING OF ABSORPTION COLUMN

• LIQUID SOLVENT ENTERS FROM TOP

• GAS MIXTURE ENTERS FROM BOTTOM

LIQUID DESCENSDES AND

GAS ASCENDS

IN THE COLUMN

• LIQUID GETS DISTRIBUTD THROUGH DISTRIBUTION PLATE ON TOP

• GAS IS INJECTED THROUGH GAS INJECTION PLATE

BOTH START TRICKLING

IN PACKED BED

• INTERACTION BETWEEN LIQUID AND GAS IN COLUMN

IMPURITY GETS

ABSORBED IN

LIQUID

VARIOUS FACTORS AFFECTING DESIGN

& WORKING OF ABSORPTION COLUMNS

MATERIAL BALANCE

• One of the very important factors governing design of absorption column design and working is the gas-liquid ratio.

The L/G ratio is important in the economics of absorption in a counter current column

Increasing the ratio increases the driving force but it gives a diluted product thus increasing the cost of stripping or recovery of solvent

SOME BASIC TERMS

LIQUID HOLD-UP:-

When a liquid flows through a bed, a part of the open space is occupied by the liquid,this is called liquid hold-up.

LOADING POINT:-

When the gas rate is increased at a constant velocity up to a certain point liquid hold up increase up to point called loading point.

FLOODING POINT:-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 called flooding

Effect of Pressure

Absorption columns are often operated under pressure to give increased capacity and higher rates of mass transfer.

Gas flow is turbulent, and up to a certain point it varies with volumetric flow rate as uG1.8

1

23

4

y

xx

y

Gas velocity

Pressure drop

• Curve one represents the dry gas flow rate• Curve two represents the wet gas flow

rate• Curve three represents low liquid rate• Curve four represents high liquid rate

• But as the liquid hold up starts increasing,up to a certain point X called the loading point pressure drop rises more steeply as uG2.5 up to a certain point Y called flooding point after which the curve rises vertically. Absorption columns generally operate in range X to Y.

EFFECT OF TEMPRATURE

• Generally absorbers deal with dilute gas mixtures and liquids, and in those cases it’s frequently satisfactory to assume operation isothermal.

• But actually absorption operations are usually exothermic, and when large quantities of solute gas are absorbed to form concentrated solutions, the temperature effects can’t be ignored

• If by absorption the temperature of the liquid is raised to a considerable effect, the equilibrium solubility of the solute will be appreciably reduced and the capacity of the absorber decreased or else much large flow rate of liquid is required.

• If the heat evolved is excessive, cooling coils can be installed in the absorber or the liquid can be removed at intervals, cooled and returned to the absorber.

Contd……

ABSORPTION WITH CHEMICAL REACTION

• Reaction of the absorbed solute and a reagent leads to destruction of absorbed solute as it forms a compound reducing the equilibrium partial pressure of the solute, consequently increasing the concentration difference between the bulk gas and the interface; and the absorption rate is also increased.

CHOICE OF SOLVENT FOR ABSORPTION

CONSIDERATIONS:-a)PURPOSE OF ABSORPTION:- If the principal purpose of absorption is to

produce a specific solution then the nature of solvent is determined by the nature of product or if the principal purpose is to remove some constituent from the gas, many choices are possible

b)GAS SOLUBILITY:- The gas solubility should be high, thus

increasing the rate of absorption and decreasing the quantity of solvent required. Generally solvents of chemical nature similar to that of solute to be absorbed will provide good stability.

c) VOLATILITY:- The solvent should have low vapor pressure

since the gas leaving an absorption operation is ordinarily saturated with the solvent and much may thereby be lost.

d) VISCOSITY:- Low viscosity is preferred for reasons of rapid

absorption rates, improved flooding characteristics in absorption towers.

e) MISCELLANEOUS:- The solvent if possible should be non-toxic, non-

flammable, chemically stable, non- corrosive and economical.

APPLICATIONS

• The absorption process is most commonly applied to post-combustion capture.

• In post-combustion systems, the flue gas needs to be cooled and impurities removed so that the solvent can efficiently be reused.

• The flue gases containing CO2 and N2 are then fed into the absorber tower. The flue gas comes into the bottom of the tower while the solvent is fed into the top of the tower. The flue gas flows up through the packing in the tower, making contact with the solvent as it falls down. The CO2 is absorbed by the solvent as the flue gas rises so that the gas that comes out of the top of the tower contains very little CO2.

• The solvent, with dissolved CO2, is then removed from the chamber. The N2 is released as it is not absorbed in the solvent. The recovery of CO2 from the solvent is called desorption. The usual parameter for recovery of CO2 from the solvent is temperature change. Other parameters include pressure changes and the use of membranes with solvents.

• For recovery, where absorption is most important criteria; packed bed provides maximum surface area.

• In immobilized bio reactor catalyst is being immobilized by packed bed, substrate is converted into product when substrate is in contact with surface area, after sometime we get steady state, constant product is formed continuously.

REFRENCES• Coulson & Richardson’s CHEMICAL

ENGINEERING

VOLUME 6 by R K Sinnott

Article 11.14• UNIT OPERATIONS OF CHEMICAL

ENGINEERING by Warren L.McCabe,Julian C.Smith,Peter Harriott

Chapter 18 Gas Absorption

CONTD…….

• www.google.co.in• www.chereresources.com• www.yahoo.com• http://www.itoms.com/

packed_beds_flow_measurement_bubble_columns

• www.co2crc.co.au• MASS-TRANSFER OPERATIONS by Robert

E.treybal chapter 8

Contd……..

• Coulson & Richardson’s CHEMICAL ENGINEERING VOLUME 2

by J F Richardson & J R Backhurst

chapter 4 and chapter 12

THANK-YOU