Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering.pdf

Table of Contents

(Subject Area: Chemical Engineering)

Article

Authors

Pages in the

Encyclopedia

Absorption (Chemical

Engineering)

James R. Fair and Henry Z.

Kister

Pages 1-25

Adsorption (Chemical

Engineering)

Douglas M. Ruthven

Pages 251-271

Aerosols

G. M. Hidy

Pages 273-299

Batch Processing

Narses Barona

Pages 41-56

Catalysis, Industrial

Bruce E. Leach

Pages 491-500

Catalyst Characterization

Robert J. Farrauto and Melvin

C. Hobson

Pages 501-526

Chemical Process Design,

Simulation, Optimization,

B. Wayne Bequette and Louis

P. Russo

Pages 751-766

Coherent Control of

Chemical Reactions

Robert J. Gordon and Yuichi

Fujimura

Pages 207-231

Cryogenic Process

Engineering

Klaus D. Timmerhaus

Pages 13-36

Crystallization Processes Ronald W. Rousseau

Pages 91-119

Distillation

M. R. Resetarits and M. J.

Lockett

Pages 547-559

Electrochemical

Engineering

Geoffrey Prentice

Pages 143-159

Fluid Dynamics

(Chemical Engineering)

Richard W. Hanks

Pages 45-70

Fluid Mixing

J. Y. Oldshue

Pages 79-104

Heat Exchangers

Kenneth J. Bell

Pages 251-264

High-Pressure Synthesis

(Chemistry)

R. H. Wentorf, Jr. and R. C.

DeVries

Pages 365-379

Mass Transfer and

Diffusion

E. L. Cussler

Pages 171-180

Membranes, Synthetic,

Applications

Eric K. Lee and W. J. Koros

Pages 279-344

Metalorganic Chemical

Vapor Deposition

Russell D. Dupuis

Pages 495-511

Pollution Prevention from

Chemical Processes

Kenneth L. Mulholland and

Michael R. Overcash

Pages 593-609

Pulp and Paper

Raymond A. Young, Robert

Kundrot and David A. Tillman

Pages 249-265

Reactors in Process

Engineering

Gary L. Foutch and Arland H.

Johannes

Pages 23-43

Solvent Extraction

Teh C. Lo and M. H. I. Baird

Pages 341-362

Surfactants, Industrial

Applications

Tharwat F. Tadros

Pages 423-438

Synthetic Fuels

Ronald F. Probstein and R.

Edwin Hicks

Pages 467-480

Thermal Cracking

B. L. Crynes, Lyle F. Albright

and Loo-Fung Tan

Pages 613-626

Absorption (Chemical

Engineering)

James R. Fair

University of Texas at Austin

Henry Z. Kister

Fluor-Daniel Corp.

I. Absorption in Practice

II. Principles of Absorption

III. Models for Absorption Equipment

IV. Absorber Design

GLOSSARY

period of time so that equilibrium is obtained, and are

then separated.

Inerts Gas components that are not absorbed by the

liquid.

Interface Surface separating the liquid from the gas.

Equilibrium is assumed to exist at this surface.

LPG Liquiﬁed petroleum gas.

Lean gas Gas leaving the absorber, containing the inerts

and little or no solute.

Lean solvent Solvent entering the absorber, containing

little or no solute.

Mass transfer coefﬁcient Quantity describing the rate of

mass transfer per unit interfacial area per unit concen-

tration difference across the interface.

Number of transfer units Parameter that relates the

change in concentration to the average driving force.

It is a measure of the ease of separation by ab-

sorption.

Absorption factor Ratio of liquid to gas ﬂow rate divided

by the slope of the equilibrium curve.

Films Regions on the liquid and gas sides of the interface

in which ﬂuid motion is considered slow and through

which material is transported by molecular diffusion

alone.

Gas solubility Quantity of gas dissolved in a given quan-

tity of solvent at equilibrium conditions.

Hatta number Ratio of the maximum conversion of re-

acting components into products in the liquid ﬁlm to the

maximum diffusion transport through the liquid ﬁlm.

Height of a transfer unit Vertical height of a contactor

required to give a concentration change equivalent to

one transfer unit.

Ideal stage Hypothetical device in which gas and liquid

are perfectly mixed, are contacted for a sufﬁciently long

Absorption (Chemical Engineering)

Operating line Line on the y – x diagram that represents

the locus of all the points obeying the component

material balance.

Rich gas Gas entering the absorber, containing both the

inerts and solutes.

Rich solvent Solvent leaving the absorber, which con-

tains solute removed from the feed gas.

Slope of equilibrium curve Ratio of the change of the

solute concentration in the gas to a given change in so-

lute concentration in the liquid when the solvent and

solute are at equilibrium and when solute concentra-

tions are expressed as mole fractions.

Solute(s) Component(s) absorbed from the gas by the

liquid

Solvent Dissolving liquid used in an absorption process.

Stripping (or desorption) Process in which the absorbed

gas is removed from the solution.

y – x diagram Plot in which the solute mole fraction in

the gas is plotted against the solute mole fraction in the

liquid.

Some common commercial applications of absorption are

listed in Table I .

B. Choice of Solvent for Absorption

If the main purpose of absorption is to generate a speciﬁc

solution, as in the manufacture of hydrochloric acid, the

solvent is speciﬁed by the nature of the product. For all

other purposes, there is some choice in selecting the ab-

sorption liquid. The main solvent selection criteria are as

follows:

1. Gas solubility. Generally, the greater the solubility

of the solute in the solvent, the easier it is to absorb the

gas, reducing the quantity of solvent and the equipment

size needed for the separation. Often, a solvent that is

chemically similar to the solute or that reacts chemically

with the solute will provide high gas solubility.

2. Solvent selectivity. A high selectivity of the sol-

vent to the desired solutes compared with its selectivity

to other components of the gas mixture lowers the quan-

tity of undesirable components dissolved. Application of

a solvent of higher selectivity reduces the cost of down-

stream processing, which is often required to separate out

the undesirable components.

3. Volatility. The gas leaving the absorber is saturated

with the solvent. The more volatile the solvent is, the

greater are the solvent losses; alternatively, the more ex-

pensive are the down-stream solvent separation facilities

required to reduce the losses.

4. Effects on product and environment. For example,

toxic solvents are unsuitable for food processing; noxious

solvents are unsuitable when the gas leaving the absorber

is vented to the atmosphere.

5. Chemical stability. Unstable solvents may be difﬁ-

cult to regenerate or may lead to excessive losses due to

decomposition.

6. Cost and availability. The less expensive is the sol-

vent, the lower is the cost of solvent losses. Water is the

least expensive and most plentiful solvent.

7. Others. Noncorrosiveness, low viscosity, nonﬂamm-

ability, and low freezing point are often desirable

properties.

ABSORPTION is a unit operation in which a gas mixture

is contacted with a suitable liquid for the purpose of

preferentially dissolving one or more of the constituents

of the gas. These constituents are thus removed or par-

tially removed from the gas into the liquid. The dissolved

constituents may either form a physical solution with the

liquid or react chemically with the liquid. The dissolved

constituents are termed solutes , while the dissolving liquid

is termed the solvent . When the concentration of solute in

the feed gas is low, the process is often called scrubbing.

The inverse operation, called stripping, desorption, or

regeneration, is employed when it is desirable to remove

the solutes from the solvent in order to recover the solutes

or the solvent or both.

I. ABSORPTION IN PRACTICE

A. Commercial Application

Absorption is practiced for the following purposes:

1. Gas puriﬁcation, for example, removal of pollutants

from a gas stream.

2. Production of solutions, for example, absorption of

hydrogen chloride gas in water to form hydrochloric acid.

3. Product recovery, for example, absorption of liqui-

ﬁed petroleum gases (LPG) and gas olines from natural

gas.

4. Drying, for example, absorption of water vapor from

a natural gas mixture.

C. Absorption Processes

Absorption is usually carried out in a countercurrent tower,

through which liquid descends and gas ascends. The tower

may be ﬁtted with trays, ﬁlled with packing, or ﬁtted with

sprays or other internals. These internals provide the sur-

face area required for gas–liquid contact.

A schematic ﬂow diagram of the absorption–stripping

process is shown in Fig. 1 . Lean solvent enters at the top

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