MM Lecture 01

Unit Operations IENGG09038

Dr Mojtaba Mirzaeian

Room: D153Phone: 3567Email: [email protected]

Monday 11:00 -13:00Room: E335

SchoolofEngineering

Overview1. Distillation

General separation techniques and Gibbs Phase Rule & Degrees of Freedom Distillation Operation and Types of Distillation Operations Vapour Liquid Equilibrium (VLE) K-Value and Relative Volatility " Binary Vapour-Liquid Systems , T yA xA Diagram Dew/Bubble Points Calculation for Ideal and Non-Ideal Systems Flash Distillation and Batch Distillation Design Methods for Binary Systems (McCabe Thiele Method) Plate Efficiencies and Relationship Between Efficiencies Multi-Component Distillation Multi-Component Distillation Design Methods: Short-cut Methods Multi-Component Distillation Design Methods: Rigorous Methods

2. Mixing Agitation and Mixing Equipment for Agitation Flow Patterns in Agitated Vessels Power Consumption and Power Correlations Power Consumption in Non-Newtonian Liquids Mixing Times in Agitated Vessels Norwood and Metzner Correlation for Turbines Blending in Large Storage and Waste-Treatment Tanks Stratified Blending, Jet Mixing, static Mixers Suspension of Solid Particles Correlations for Suspension Dispersion Operations ( Liquid Liquid and Gas-Liquid Dispersions ) Mixer Selection and Scale-up

Overview

Lecture 1

In this lecture different separation processes and their importance in chemical engineering are introduced and discussed. Derivation of the equations for degrees of freedom, F, for vapour - liquid phase equilibria in a closed system and also for phase equilibrium in a flow system involving one feed steam, P product streams and C components are presented.

5 The separation of chemical mixtures into their constituents has been practiced, as an art, for millennia.

Early civilizations developed techniques to: Extract metals from ores Extract perfumes from flowers Extract dyes from plants Separate potash from the ashes of burnt plants Evaporate sea water to obtain salt Refine rock asphalt Distill liquors.

Human Body Kidney

Separation Processes

6Separation Processes Separations, including enrichment, concentration,

purification, refining, and isolation, are important tochemists and chemical engineers.

Chemists use analytical separation methods, such aschromatography, to determine compositions of complexmixtures quantitatively.

Chemists also use small-scale preparative separationtechniques, often similar to analytical separationmethods, to recover and purify chemicals.

7Separation Processes

Chemical engineers are more concerned with the manufacture of chemicals using economical, large-scale separation methods, which may differ considerably from laboratory techniques.

Chemists separate and analyze light-hydrocarbon mixtures by gas-liquid chromatography, while in a large manufacturing plant a chemical engineer uses distillation to separate the same hydrocarbon mixtures.

8Crude Distillation Column A typical separation process

is distillation used to separate crude into boiling-point fractions or cuts.

9General Separation Techniques The creation of a mixture of chemical species from the separatespecies is a spontaneous process that requires no energy input.

Separation of a chemical mixture into pure components, is not a spontaneous process and thus requires energy.

The feed and products may be vapor, liquid, or solid; one or more separation operations may be taking place; and the products differ in composition and may differ in phase.

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General Separation Techniques In each separation operation, the mixture components are induced

to move into different, separable spatial locations or phases by any one or more of the five basic separation methods:

The most common separation techniques:

1- Creation of a second phase, immiscible with the feed phase, byenergy (heat and/or shaft-work) transfer or by pressure reduction.Common operations of this type are distillation, which involves thetransfer of species between vapor and liquid phases, exploitingdifferences in volatility (e.g., vapor pressure or boiling point) amongthe species.

2- Addition of another fluid phase, which selectively absorbs, extracts,or strips certain species from the feed. The most common operations ofthis type are liquidliquid extraction, where the feed is liquid and asecond, immiscible liquid phase is added; and absorption, where thefeed is vapor, and a liquid of low volatility is added. In both cases,species solubilities are significantly different in the added phase.

General Separation Techniques

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3- The use of a barrier, usually a polymer membrane, which involves a gas or liquid feed and exploits differences in species permeabilitiesthrough the barrier.

4- Contacting a vapor or liquid feed with a solid agent. The solid agent consists of particles that are porous to achieve a high surface area, and differences in species adsorbability are exploited.

5- The use of external fields (centrifugal, thermal, electrical, flow, etc.), to liquid or gas feeds, with electrophoresis being especially useful for separating proteins by exploiting differences in electric charge and diffusivity.


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For all separation techniques, the size of the equipment is determined by rates of mass transfer of each species from one phase or location to another, relative to mass transfer of all species.

The driving force and direction of mass transfer is governed by the departure from thermodynamic equilibrium, which involves volatilities, solubilities, etc.

Applications of thermodynamics and mass-transfer theory to industrial separations are important.

Fluid mechanics and heat transfer also play important roles in separation operations.


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Choice/Synthesis of Separation Processes

Selection of a best separation process must be made from among a number of feasible candidates.

When the feed mixture is to be separated into more than two products, a combination of two or more operations may be best.

Even when only two products are to be produced, a hybrid process of two or more different types of operations may be most economical.

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Choice/Synthesis of Separation Processes

What to consider:

Separation methods ESAs (energy separating agents) and/or MSAs (mass

separating agents)

Separation equipment The optimal arrangement or sequencing of the equipment The optimal operating conditions of temperature and

pressure for the equipment

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Selection of Feasible Separations

Factors that influence the selection of feasible separation operations

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Cost of Recovery The cost of recovering and purifying a chemical depends strongly

on its concentration in the feed.

The more dilute the feed, the higher the product price.

When a very pure product is required,large differences in volatility or solubility or significant numbers of stages are needed for chemicals in commerce.

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Ease of Scale-Up

Operation Ease of Staging Need for Parallel Units

Distillation Easy No need

Absorption Easy No need

Extractive and Azeotropicdistillation

Easy No need

Liquid-Liquid Extraction Easy Sometimes

Membranes Re-pressurization required between stages

Almost always

Adsorption Easy Only for regeneration cycle

Crystallization Not easy Sometimes

Drying Not convenient Sometimes

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Ease of Scale-up of the Most Common Separation Operations

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Design of Separation Processes

Separation Processes problems can be solved using three sets of equations:

Equilibrium relationshipMass balanceEnergy balance

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Separation operations are subject to the conservation of mass.

Input + Generation = Output + Consumption + Accumulation

If no chemical reactions occur and the process operates in a continuous, steady-state fashion, then for each component, i, in a mixture of C components, the molar (or mass) flow rate in the feed, ni(F), is equal to the sum of the product molar (or mass) flow rates, for that component in the N product phases, P.

Mass Balance Equations

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Mass Balance Relationships

( ) ( ) (1) (2) ( 1) ( )

1

NF p N N

i i i i i ip

n n n n n n

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Gibbs Phase Rule & Degrees of Freedom The description of a single-stage system at physical

equilibrium involves intensive variables, which are independent of the size of the system, and extensive variables, which do depend on system size.

Intensive variables are temperature, pressure, and phase compositions (mole fractions, mass fractions, concentrations, etc.).

Extensive variables include mass or moles and energy for a batch system, and mass or molar flow rates and energy transfer rates for a flow system.

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Gibbs Phase Rule & Degrees of Freedom Phase-equilibrium equations, and mass and energy

balances, provide dependencies among the intensive and extensive variables.

When a certain number of the variables (called the independent variables) are specified, all other variables (called the dependent variables) become fixed.

The numbere of independent variables is called the variance, or the number of degrees of freedom. , F, for the system.

The phase rule of J. Willard Gibbs, which applies only to the intensive variables at equilibrium, is used to determine F.

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Gibbs Phase Rule & Degrees of Freedom

The rule states that

where C is the number of components and P is the number of phases at equilibrium.

The above equation is derived by counting the number of intensive variables and the number of independent equations that relate these variables .

F = V E

V = Number of intensive variablesE = Number of independent equations

2 PCF

The number of intensive variables, V, is:

where the 2 refers to the equilibrium temperature and pressure, while the term CP is the total number of composition variables (e.g., mole fractions) for components distributed among P equilibrium phases.

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2 CPV

The number of independent equations, E, relating the intensive variables is:

where the first term, P, refers to the requirement that mole or mass fractions sum to one for each phase and the second term, C(P-1), refers to the number of independent phase-equilibrium equations of the form:

where (1) and (2) refer to equilibrium phases.26


)1( PCPE

mole fraction of i in phase (1)mole fraction of i in phase (2)i

K

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For two phases, there are C independent expressions of this type; for three phases, 2C; for four phases, 3C; and so on.

For example, for three phases (V, L(1), L(2)), we can write 3C different K-value equations:

(1) (1)

(2) (2)

(1) (2)

/ 1toC/ 1toC/ 1toC

i i i

i i i

Di i i

K y x iK y x iK x x i

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However, only 2C of these equations are independent, because

Thus, the term for the number of independent K-value equations is C(P-1), not C.

(2) (1)/Di i iK K K

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The degrees of freedom is the number of intensive variables, V, less the number of equations, E.

F = C P + 2 When the number, F, of intensive variables is specified,

the remaining [P+C(P-1)] intensive variables are determined from the [P+C(P-1)] equations

2 1 22

F V E CP P C P C P

orF P C

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The number of degrees of freedom, F , is the minimumnumber of independent intensive variables(temperature, pressure, and concentrations) thatmust be fixed to define the equilibrium state of thesystem.

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Gibbs Phase Rule & Degrees of Freedom As an example, consider the vapor-liquid equilibrium

system shown in previous figure, where the equilibrium intensive variables are labeled on the sketch located above the list of independent equations relating these variables.

For 3 components and two phases the degrees of freedom, F, are 3 and the equilibrium intensive variables are T, P, x1, x2, x3, y1, y2 and y3.

If values are specified for T, P, and one of the mole fractions are known, the remaining five mole fractions are fixed and can be computed from the five independent equations listed in the previous figure.

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Irrational specifications lead to infeasible results.

For example, if the components are H2O, N2, and O2, and T = 100F and P = 15 psia are specified, a specification of xN2= 0.90 is not feasible because nitrogen is not nearly this soluble in water.

In using the Gibbs phase rule, it should be noted that the K-values are not variables, but are thermodynamic functions that depend on the intensive variables.

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The Gibbs phase rule is limited because it does not deal with feed streams sent to the equilibrium stage nor with extensive variables used when designing or analyzing separation operations.

However, the phase rule can be extended for process applications, by adding the feed stream and extensive variables, and additional independent equations relating feed variables, extensive variables, and the intensive variables already considered by the rule.


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Continuous single stage process with P=2

Additional Variables: F, (Zi, i = 1 to c), TF, PF, Q,

L , V C + 6 additional variables The additional independent

equations: C + 1 F = CP+2+C+6 [P+C(P-1)

+ C+1)] = C - P+7 = C + 5


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For a degrees-of-freedom analysis for phase equilibrium involving one feed phase, P product phases, and C components, phase rule can be extended by adding the above increments as:

If the C + 5 degrees of freedom are used to specify all ziand the five variables F, TF, PF, T, and P, the remaining variables are found.

5 CEVF

6)4()2( CCPPPCCPV1)1()]1([ CPPCPCPE

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MM Lecture 01