Distillation - Robert H. Perry.pdf - NAUKA - bogg

Source: Perry’s Chemical Engineers’ Handbook

Section 13

Distillation

J. D. Seader, Ph.D., Professor of Chemical Engineering, University of Utah, Salt Lake

City, Utah; Fellow, American Institute of Chemical Engineers; Member, American Chemical

Society; Member, American Society for Engineering Education. (Section Editor*)

Jeffrey J. Siirola, Ph.D., Research Fellow, Eastman Chemical Company; Member,

National Academy of Engineering; Fellow, American Institute of Chemical Engineers, American

Chemical Society, American Association for Artificial Intelligence, American Society for Engi-

neering Education. (Enhanced Distillation)

Scott D. Barnicki, Ph.D., Senior Research Chemical Engineer, Eastman Chemical Com-

pany. (Enhanced Distillation)

CONTINUOUS-DISTILLATION OPERATIONS

General Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13-4

McCabe-Thiele Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-27

Operating Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-27

Thermal Condition of the Feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-28

Equilibrium-Stage Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-29

Total-Column Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-29

Feed-Stage Location. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-32

Minimum Stages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-32

Minimum Reflux. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-32

Intermediate Reboilers and Condensers . . . . . . . . . . . . . . . . . . . . . . . 13-32

Optimum Reflux Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-32

Difficult Separations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-32

Stage Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-34

Miscellaneous Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-34

Complex Distillation Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13-4

Related Separation Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13-5

Synthesis of Multicomponent Separation Systems . . . . . . . . . . . . . . . . .

13-9

THERMODYNAMIC DATA

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-10

Phase Equilibrium Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-10

Graphical K- Value Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-10

Analytical K- Value Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-16

DEGREES OF FREEDOM AND DESIGN VARIABLES

Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-22

Analysis of Elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-22

Analysis of Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-23

Other Units and Complex Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-24

APPROXIMATE MULTICOMPONENT

DISTILLATION METHODS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-35

Fenske-Underwood-Gilliland (FUG) Shortcut Method. . . . . . . . . . . . . 13-35

Example 1: Calculation of FUG Method. . . . . . . . . . . . . . . . . . . . . . . 13-36

Kremser Group Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-37

Example 2: Calculation of Kremser Method . . . . . . . . . . . . . . . . . . . . 13-39

SINGLE-STAGE EQUILIBRIUM-FLASH CALCULATIONS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-25

Bubble Point and Dew Point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-25

Isothermal Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-25

Adiabatic Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-26

Other Flash Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-26

Three-Phase Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-26

Complex Mixtures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-26

RIGOROUS METHODS FOR MULTICOMPONENT

DISTILLATION-TYPE SEPARATIONS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-39

Thiele-Geddes Stage-by-Stage Method for Simple Distillation . . . . . . . 13-40

Example 3: Calculation of TG Method . . . . . . . . . . . . . . . . . . . . . . . . 13-40

Equation-Tearing Procedures Using the Tridiagonal-Matrix

Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-43

Tridiagonal-Matrix Algorithm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-44

BP Method for Distillation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-45

Example 4: Calculation of the BP Method . . . . . . . . . . . . . . . . . . . . . 13-46

GRAPHICAL METHODS FOR BINARY DISTILLATION

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-26

Phase Equilibrium Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-27

* Certain portions of this section draw heavily on the work of Buford D. Smith, editor of this section in the fifth edition.

13-1

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Equilibrium-Stage Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13-4

Distillation

13-2 DISTILLATION

SR Method for Absorption and Stripping . . . . . . . . . . . . . . . . . . . . . . 13-47

Example 5: Calculation of the SR Method . . . . . . . . . . . . . . . . . . . . . 13-47

Simultaneous-Correction Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-48

Naphtali-Sandholm SC Method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-48

Example 6: Calculation of Naphtali-Sandholm SC Method . . . . . . . . 13-49

Inside-Out Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-49

Example 7: Calculation of Inside-Out Method . . . . . . . . . . . . . . . . . . 13-51

Homotopy-Continuation Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-51

Stage Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-52

Rate-Based Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-52

Material Balances (2 C

Reactive Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-81

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-81

Simulation, Modeling, and Design Feasibility . . . . . . . . . . . . . . . . . . 13-81

Mechanical Design and Implementation Issues . . . . . . . . . . . . . . . . . 13-83

Process Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-83

2 Equations) . . . . . . . . . . . . . . . . . . . . . . . . . 13-53

Energy Balances (3 Equations) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-53

Mass-Transfer Rates (2 C

PETROLEUM AND COMPLEX-MIXTURE DISTILLATION

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-85

Characterization of Petroleum and Petroleum Fractions . . . . . . . . . . . . 13-86

Applications of Petroleum Distillation. . . . . . . . . . . . . . . . . . . . . . . . . . . 13-89

Design Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-89

Example 9: Simulation Calculation of an Atmospheric Tower . . . . . . 13-93

2 Equations) . . . . . . . . . . . . . . . . . . . . . . . 13-53

Summation of Mole Fractions (2 Equations) . . . . . . . . . . . . . . . . . . . 13-53

Hydraulic Equation for Stage Pressure Drop (1 Equation) . . . . . . . . 13-53

Interface Equilibrium ( C Equations) . . . . . . . . . . . . . . . . . . . . . . . . . 13-53

Example 8: Calculation of Rate-Based Distillation . . . . . . . . . . . . . . . 13-54

−

BATCH DISTILLATION

Simple Batch Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-96

Batch Distillation with Rectification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-96

Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-96

Approximate Calculation Procedures for Binary Mixtures . . . . . . . . . . . 13-97

Operating Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-98

Batch Rectification at Constant Reflux . . . . . . . . . . . . . . . . . . . . . . . . 13-98

Batch Rectification at Constant Overhead Composition . . . . . . . . . . 13-98

Other Operating Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-99

Effect of Column Holdup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-99

Shortcut Methods for Multicomponent Batch Rectification . . . . . . . . . 13-100

Calculational Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-100

Rigorous Computer-Based Calculation Procedures . . . . . . . . . . . . . . 13-100

Example 10: Calculation of Multicomponent Batch Distillation . . . . 13-102

Rapid Solution Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-103

ENHANCED DISTILLATION

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-54

Azeotropism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-54

Residue Curve Maps and Distillation Region Diagrams . . . . . . . . . . . . 13-56

Applications of RCM and DRD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-58

Extension to Batch Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-66

Azeotropic Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-68

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-68

Exploitation of Homogeneous Azeotropes . . . . . . . . . . . . . . . . . . . . . 13-69

Exploitation of Pressure Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-72

Exploitation of Boundary Curvature . . . . . . . . . . . . . . . . . . . . . . . . . . 13-73

Exploitation of Azeotropy and Liquid-Phase Immiscibility . . . . . . . . 13-73

Design and Operation of Azeotropic Distillation Columns . . . . . . . . 13-75

Extractive Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-75

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-75

Solvent Effects in Extractive Distillation. . . . . . . . . . . . . . . . . . . . . . . 13-76

Extractive Distillation Design and Optimization . . . . . . . . . . . . . . . . 13-77

Solvent Screening and Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-79

Extractive Distillation by Salt Effects . . . . . . . . . . . . . . . . . . . . . . . . . 13-81

DYNAMIC DISTILLATION

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-104

Ideal Binary Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-104

Multicomponent Distillation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13-105

PACKED COLUMNS

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Distillation

DISTILLATION 13-3

Nomenclature and Units

U.S.

Symbol

Definition

SI units

customary

units

Symbol

Definition

SI units

customary

units

Absorption factor

Vector of stage variables

Area

m 2

ft 2

Activity

Number of chemical species

Component flow rate

(kg

mol)/s

(lb

⋅

mol)/h

Distillate flow rate

(kg

mol)/s

(lb

⋅

mol)/h

in bottoms

Deviation from set point

Component flow rate

(kg

mol)/s

(lb

⋅

mol)/h

in distillate

Residual of heat-transfer

Btu/h

expression

Rate of heat transfer

Btu/h

Residual of phase

(kg

mol)/s

(lb

⋅

mol)/h

Fraction of feed

leaving in bottoms

equilibrium expression

Feed flow rate

(kg

mol)/s

(lb

⋅

mol)/h

Fugacity

psia

Vector of stage functions

Function in homotopy expression

Interlink flow rate

(kg

mol)/s

(lb

⋅

mol)/h

Function in homotopy expression

Volume holdup of liquid

m 3

ft 3

Residual of energy balance

Btu/h

Residual of energy balance

Btu/h

Height

Height of a transfer unit

Homotopy function

Enthalpy

J/(kg

mol)

Btu/(lb

mol)

Component flow rate

(kg

mol)/s

(lb

⋅

mol)/h

in liquid

Vapor-liquid equilibrium

ratio ( K value)

Pressure

kPa

psia

K C

Controller gain

Measure of thermal

condition of feed

K D

Chemical equilibrium

constant for dimerization

q c

Condenser duty

Btu/h

K d

Liquid-liquid

distribution ratio

q r

Reboiler duty

Btu/h

Sidestream ratio

Liquid flow rate

(kg

mol)/s

(lb

⋅

mol)/h

Liquid-sidestream ratio

Residual of component

(kg

mol)/s

(lb

⋅

mol)/h

Homotopy parameter

material balance

Time

Liquid holdup

⋅

mol

⋅

mol

Component flow rate in vapor

(kg

mol)/s

(lb

⋅

mol)/h

Number of transfer units

Weight fraction

Number of equilibrium stages

Mole fraction in liquid

N c

Number of relationships

Mole fraction in vapor

N i

Number of design variables

Mole fraction in feed

Greek symbols

N m

Minimum number of

equilibrium stages

N p

Number of phases

Relative volatility

N r

Number of repetition variables

Activity coefficient

N o

Number of variables

Convergence criterion

Rate of mass transfer

(kg

mol)/s

(lb

⋅

mol)/h

Scale factor

Pressure

psia

Murphree-stage efficiency

Residual of pressure-drop

psia

Time for distillation

expression

Parameter in Underwood

equations

P sat

Vapor pressure

psia

Heat-transfer rate

Btu/h

Holland theta factor

Q c

Condenser duty

Btu/h

Eigenvalue

Q r

Reboiler duty

Btu/h

Sum of squares of residuals

Residual of phase-equilibrium

expression

Feedback-reset time

Fugacity coefficient of

pure component

External-reflux ratio

R m

Minimum-reflux ratio

Entrainment or occlusion ratio

Residual of mole-fraction sum

Fugacity coefficient in

mixture

Sidestream flow rate

(kg

mol)/s

(lb

⋅

mol)/h

Stripping factor

Φ A

Fraction of a component in feed

vapor that is not absorbed

Vapor-sidestream ratio

Φ S

Fraction of a component in

entering liquid that is

not stripped

Temperature

Liquid-sidestream rate

(kg

mol)/s

(lb

⋅

mol)/h

Vapor flow rate

(kg

mol)/s

(lb

⋅

mol)/h

Factor in Gilliland

correlation

Vapor-sidestream rate

(kg

mol)/s

(lb

⋅

mol)/h

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⋅

Distillation

G ENERAL R EFERENCES : Billet, Distillation Engineering, Chemical Publish-

ing, New York, 1979. Fair and Bolles, “Modern Design of Distillation Columns,”

Chem. Eng., 75 (9), 156 (Apr. 22, 1968). Fredenslund, Gmehling, and Ras-

mussen, Vapor-Liquid Equilibria Using UNIFAC, a Group Contribution

Method, Elsevier, Amsterdam, 1977. Friday and Smith, “An Analysis of the

Equilibrium Stage Separation Problem—Formulation and Convergence,” Am.

Inst. Chem. Eng. J., 10, 698 (1964). Hengstebeck, Distillation—Principles and

Design Procedures, Reinhold, New York, 1961. Henley and Seader, Equilib-

rium-Stage Separation Operations in Chemical Engineering, Wiley, New York,

1981. Hoffman, Azeotropic and Extractive Distillation, Wiley, New York, 1964.

Holland, Fundamentals and Modeling of Separation Processes, Prentice-Hall,

Englewood Cliffs, N.J., 1975. Holland, Fundamentals of Multicomponent Distil-

lation, McGraw-Hill, New York, 1981. King, Separation Processes, 2d ed.,

McGraw-Hill, New York, 1980. Kister, Distillation Design, McGraw-Hill, New

York, 1992. Kister, Distillation Operation, McGraw-Hill, New York, 1990.

Robinson and Gilliland, Elements of Fractional Distillation, 4th ed., McGraw-

Hill, New York, 1950. Rousseau, ed., Handbook of Separation Process Technol-

ogy, Wiley-Interscience, New York, 1987. Seader, The B.C. (Before Computers)

and A.D. of Equilibrium-Stage Operations, Chem. Eng. Educ., Vol. 14 (2),

(Spring 1985). Seader, Chem. Eng. Progress, 85 (10), 41 (1989). Smith, Design of

Equilibrium Stage Processes, McGraw-Hill, New York, 1963. Treybal, Mass

Transfer Operations, 3d ed., McGraw-Hill, New York, 1980. Ullmann’s Encyclo-

pedia of Industrial Chemistry, Vol. B3, VCH, Weinheim, 1988. Van Winkle,

Distillation, McGraw-Hill, New York, 1967.

CONTINUOUS-DISTILLATION OPERATIONS

GENERAL PRINCIPLES

Separation operations achieve their objective by the creation of two

or more coexisting zones which differ in temperature, pressure, com-

position, and/or phase state. Each molecular species in the mixture to

be separated reacts in a unique way to differing environments offered

by these zones. Consequently, as the system moves toward equilib-

rium, each species establishes a different concentration in each zone,

and this results in a separation between the species.

The separation operation called distillation utilizes vapor and liquid

phases at essentially the same temperature and pressure for the coex-

isting zones. Various kinds of devices such as random or structured

packings and plates or trays are used to bring the two phases into inti-

mate contact. Trays are stacked one above the other and enclosed in a

cylindrical shell to form a column. Packings are also generally con-

tained in a cylindrical shell between hold-down and support plates. A

typical tray-type distillation column plus major external accessories is

shown schematically in Fig. 13-1.

The feed material, which is to be separated into fractions, is intro-

duced at one or more points along the column shell. Because of the

difference in gravity between vapor and liquid phases, liquid runs

down the column, cascading from tray to tray, while vapor flows up the

column, contacting liquid at each tray.

Liquid reaching the bottom of the column is partially vaporized in a

heated reboiler to provide boil-up, which is sent back up the column.

The remainder of the bottom liquid is withdrawn as bottoms, or bot-

tom product. Vapor reaching the top of the column is cooled and con-

densed to liquid in the overhead condenser. Part of this liquid is

returned to the column as reflux to provide liquid overflow. The

remainder of the overhead stream is withdrawn as distillate, or over-

head product. In some cases only part of the vapor is condensed so

that a vapor distillate can be withdrawn.

This overall flow pattern in a distillation column provides counter-

current contacting of vapor and liquid streams on all the trays through

the column. Vapor and liquid phases on a given tray approach thermal,

pressure, and composition equilibriums to an extent dependent upon

the efficiency of the contacting tray.

The lighter (lower-boiling) components tend to concentrate in the

vapor phase, while the heavier (higher-boiling) components tend

toward the liquid phase. The result is a vapor phase that becomes

richer in light components as it passes up the column and a liquid

phase that becomes richer in heavy components as it cascades down-

ward. The overall separation achieved between the distillate and the

bottoms depends primarily on the relative volatilities of the compo-

nents, the number of contacting trays, and the ratio of the liquid-

phase flow rate to the vapor-phase flow rate.

If the feed is introduced at one point along the column shell, the

column is divided into an upper section, which is often called the rec-

tifying section, and a lower section, which is often referred to as the

stripping section. These terms become rather indefinite in multiple-

feed columns and in columns from which a liquid or vapor sidestream

is withdrawn somewhere along the column length in addition to the

two end-product streams.

EQUILIBRIUM-STAGE CONCEPT

Until recently, energy and mass-transfer processes in an actual distil-

lation column were considered too complicated to be readily modeled

in any direct way. This difficulty was circumvented by the equilibrium-

stage model, developed by Sorel in 1893, in which vapor and liquid

streams leaving an equilibrium stage are in complete equilibrium with

each other and thermodynamic relations can be used to determine the

temperature of and relate the concentrations in the equilibrium

streams at a given pressure. A hypothetical column composed of equi-

librium stages (instead of actual contact trays) is designed to accom-

plish the separation specified for the actual column. The number of

hypothetical equilibrium stages required is then converted to a num-

ber of actual trays by means of tray efficiencies, which describe the

extent to which the performance of an actual contact tray duplicates

the performance of an equilibrium stage. Alternatively and preferably,

tray inefficiencies can be accounted for by using rate-based models

that are described below.

Use of the equilibrium-stage concept separates the design of a dis-

tillation column into three major steps: (1) Thermodynamic data and

methods needed to predict equilibrium-phase compositions are assem-

bled. (2) The number of equilibrium stages required to accomplish a

specified separation, or the separation that will be accomplished in a

given number of equilibrium stages, is calculated. (3) The number of

equilibrium stages is converted to an equivalent number of actual con-

tact trays or height of packing, and the column diameter is determined.

Much of the third step is eliminated if a rate-based model is used. This

section deals primarily with the second step. Section 14 covers the

third step. Sections 3 and 4 cover the first step, but a summary of meth-

ods and some useful data are included in this section.

COMPLEX DISTILLATION OPERATIONS

All separation operations require energy input in the form of heat or

work. In the conventional distillation operation, as typified in Fig.

13-1, energy required to separate the species is added in the form of

heat to the reboiler at the bottom of the column, where the tempera-

ture is highest. Also, heat is removed from a condenser at the top of

the column, where the temperature is lowest. This frequently results

13-4

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Distillation

CONTINUOUS-DISTILLATION OPERATIONS 13-5

FIG. 13-1 Schematic diagram and nomenclature for a simple distillation col-

umn with one feed, a total overhead condenser, and a partial reboiler.

column or split-tower arrangement of Fig. 13-3 a, which corresponds

somewhat to the energy-saving concept employed in multieffect evap-

oration, might be used. The feed is split more or less equally among

columns that operate in parallel, but at different pressures, in a cas-

cade that decreases from left to right. With proper selection of col-

umn-operating pressure, this permits the overhead vapor from the

higher-pressure column to be condensed in the reboiler of the lower-

pressure column. External heat-transfer media are needed only for

the reboiler of the first effect and the condenser of the last effect.

Thus, for N effects, utility requirements are of the order 1/ N of those

for a conventional single-effect column. Wankat [ Ind. Eng. Chem.

Res., 32, 894 (1993)] develops a large number of more complex mul-

tieffect schemes, some of which show significant reductions in energy

requirements.

In another alternative, shown in Fig. 13-3 b, the rectifying section

may be operated at a pressure sufficiently higher than that of the strip-

ping section such that heat can be transferred between any desired

pairs of stages of the two sections. This technique, described by Mah

et al. (op. cit.) and referred to as SRV (secondary reflux and vaporiza-

tion) distillation, can result in a significant reduction in utility require-

ments for the overhead condenser and bottoms reboiler.

When multicomponent mixtures are to be separated into three or

more products, sequences of simple distillation columns of the type

shown in Fig. 13-1 are commonly used. For example, if a ternary mix-

ture is to be separated into three relatively pure products, either of the

two sequences in Fig. 13-4 can be used. In the direct sequence, shown

in Fig. 13-4 a, all products but the heaviest are removed one by one as

distillates. The reverse is true for the indirect sequence, shown in Fig.

13-4 b. The number of possible sequences of simple distillation

columns increases rapidly with the number of products. Thus,

although only the 2 sequences shown in Fig. 13-4 are possible for a

mixture separated into 3 products, 14 different sequences, one of

which is shown in Fig. 13-5, can be synthesized when 5 products are

to be obtained.

As shown in a study by Tedder and Rudd [ Am. Inst. Chem. Eng. J.,

24, 303 (1978)], conventional sequences like those of Fig. 13-4 may

not always be the optimal choice, particularly when species of inter-

mediate volatility are present in large amounts in the feed or need not

be recovered at high purity. Of particular interest are thermally cou-

pled systems. For example, in Fig. 13-6 a, an impure-vapor sidestream

is withdrawn from the first column and purified in a side-cut rectifier,

the bottoms of which is returned to the first column. The thermally

coupled system in Fig. 13-6 b, discussed by Stupin and Lockhart

[ Chem. Eng. Prog., 68 (10), 71 (1972)] and referred to as Petlyuk tow-

ers, is particularly useful for reducing energy requirements when the

initial feed contains close-boiling species. Shown for a ternary feed,

the first column in Fig. 13-6 b is a prefractionator, which sends essen-

tially all of the light component and heavy component to the distillate

and bottoms respectively, but permits the component of intermediate

volatility to be split between the distillate and bottoms. Products from

the prefractionator are sent to appropriate feed trays in the second

column, where all three products are produced, the middle product

being taken off as a sidestream. Only the second column is provided

with condenser and reboiler; reflux and boil-up for the prefractionator

are obtained from the second column. This concept is readily ex-

tended to separations that produce more than three products. Proce-

dures for the optimal design of thermally coupled systems are

presented by Triantafyllou and Smith [ Trans. I. Chem. E., 70, Part A,

118 (1992)]. A scheme for combining thermal coupling with heat

pumps is developed by Agrawal and Yee [ Ind. Eng. Chem. Res., 33,

2717 (1994)].

in a large energy-input requirement and low overall thermodynamic

efficiency, when the heat removed in the condenser is wasted. Com-

plex distillation operations that offer higher thermodynamic efficiency

and lower energy-input requirements have been under intense inves-

tigation. In some cases, all or a portion of the energy input is as work.

Complex distillation operations may utilize single columns, as

shown in Fig. 13-2 and discussed by Petterson and Wells [ Chem. Eng.,

84 (20), 78 (Sept. 26, 1977)], Null [ Chem. Eng. Prog., 72 (7), 58

(1976)], and Brannon and Marple [ Am. Inst. Chem. Eng. Symp. Ser.

76, 192 , 10 (1980)], or two or more columns that are thermally linked

as shown in Figs. 13-3 and 13-6 and discussed by Petterson and Wells

(op. cit.) and Mah, Nicholas, and Wodnik [ Am. Inst. Chem. Eng. J.,

23, 651 (1977)].

In Fig. 13-2 a, which is particularly useful when a large temperature

difference exists between the ends of the column, interreboilers add

heat at lower temperatures and/or intercondensers remove heat at

higher temperatures. As shown in Fig. 13-2 b, these intermediate heat

exchangers may be coupled with a heat pump that takes energy from

the intercondenser and uses shaft work to elevate this energy to a tem-

perature high enough to transfer it to the interreboiler.

Particularly when the temperature difference between the ends of

the column is not large, any of the three heat-pump systems in Fig.

13-2 c, d, and e that involve thermal coupling of the overhead con-

denser and bottoms reboiler might be considered to eliminate exter-

nal heat transfer almost entirely, substituting shaft work as the prime

energy input for achieving the separation. More complex arrange-

ments are considered by Björn, Grén, and Ström [ Chem. Eng.

Process., 29, 185 (1991)]. Alternatively, the well-known multiple-

RELATED SEPARATION OPERATIONS

The simple and complex distillation operations just described all have

two things in common: (1) both rectifying and stripping sections are

provided so that a separation can be achieved between two compo-

nents that are adjacent in volatility; and (2) the separation is effected

only by the addition and removal of energy and not by the addition of

any mass separating agent (MSA) such as in liquid-liquid extraction.

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