ert 318 unit operationsportal.unimap.edu.my/portal/page/portal30/lecturer notes... · ert 318 unit...

DISTILLATION

W. L. McCabe, J. C. Smith, P. Harriot, Unit Operations of Chemical Engineering, 7th edition, 2005.

ERT 318 UNIT OPERATIONS

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Outline: Batch distillation (pg. 724) Continuous distillation with reflux (pg. 666) Material balance in plate column (pg. 670) McCabe-Thiele Method (pg. 672)

Determination of the number of plates using enthalpy-composition diagram (pg. 694)

Introduction to multicomponent distillation Phase Equilibria in Multicomponent Distillation (Pg. 737) Flash Distillation of Multicomponent Mixtures (pg. 741) Fractionation of Multicomponent Mixtures (pg. 742) Azeotropic and Extractive Distillation (pg. 759)

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Batch Distillation

Fig. 21.35: Simple distillation in a batch still

• Often used in small plants to recover volatile products from liquid.• Vapor: - equilibrium with the liquid in the still

- Richer with volatile component- composition liquid & vapor are not constant

Vapor

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Batch Distillation

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Batch Distillation

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Batch Distillation

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Batch Distillation

Continuous Distillation with Reflux

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yn = vapor leaving platexn = Liquid leaving plateyn+1 = vapor entering platexn+1 = liquid entering plateV = vapor phasey = concentration of vaporL = Liquid phasex = concentration of liquid

Fig. 21.3: Material-balance diagram for plate n

Often used for large-scale production, far more common than batch distillation.

Tn-1 < Tn < Tn+1

Fig. 21.4: Boiling-point diagram showing rectification on ideal plates

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Continuous Distillation with RefluxCombination rectification and stripping

To purify by repeated distillation

Physical separation where more components are removed from a liquid stream by vapor stream

•Why feed is in the central?•What is feed plate?

Reflux provides the downflowing liquid in the rectifying section that is needed to act on the upflowing vapor.

No reflux no rectification

If no azeotropes,both overhead and bottom products obtained in any desired purity if enough plates and adequate reflux are provided.

Fig. 21.5: Continuous fractionating column with rectifying and stripping sections.

Continuous Distillation with RefluxMaterial Balances in Plate columns

Overall material balance for two-component systems

Fig. 21.6: Material-balance diagram for continuous fractionation column

Total material balance: F = D + B

Component A balance: FxF = DxD + BxB

Eliminating B: D/F = (xF-xB) / (xD-xB)

Eliminating D: B/F = (xD-xF) / (xD-xB)

(21.6)

(21.7)

(21.8)

(21.9)

Eq. 21.8 & 21.9 are true for all values of the flows of vapor and liquid within the column

Net flow rates:

D = Va - La

D = Vn+1 - Ln

DxD = Vaya – Laxa = Vn+1yn+1 - LnxnB = Lb – Vb = Lm – Vm+1BxB = Lbxb – Vbyb = Lmxm - Vm+1ym+1

(21.10)

(21.11)

(21.12)

(21.13)

(21.14)

Continuous Distillation with Reflux

111

n

aaaan

n

nn V

xLyVxVLy

111

n

Dn

n

nn V

DxxVLy

DLDxx

DLLy

n

Dn

n

nn

1

Operating lines : Because there are 2 sections in the column, there are also 2 operating lines; 1 rectifying section and 1 stripping section.

Component A balance: FxF = DxD + BxBFrom Eq. (21.7):

From Eq. (21.9):

(21.15)

(21.16)

(21.17)

The slope defined by Eq. (21.6) the ratio of liquid to the vapor stream: by eliminating Vn+1

Bmmmm BxxLyV 11

111

m

Bm

m

mm V

BxxVLy

Continuous Distillation with RefluxOperating lines:

Material balance over control surface II, below the feed plate:

(21.18)

In a different form, this becomes

(21.19)

This is the equation for the operating line in the stripping section.Slope =liquid flow / vapor flow. Eliminating Vm+1 from Eq (21.19) & (21.13) gives:

BLBxx

BLLy

m

Bm

m

mm

1 (21.20)

Continuous Distillation with RefluxNumber of ideal Plates; McCabe-Thiele Method

• ASPEN : Computer design program used to identify the number of plates required for a distillation problem.

• Mc Cabe-Thiele Method : A simplified graphical method for calculating the number of plates.

• When the operating lines used Eqs. (21.17) and (21.20) are plotted within the equilirium curve on the xy diagram, the McCabe-Thiele step-by-step construction can be used to compute the number of ideal plates

•Eqs. (21.17) and (21.20), show that unless Ln and Lm are constant, the operating lines are curved and can be plotted only if the change in these internal streams with concentration is known.

• Enthalpy balances are required in the general case to determine the position of a curved operating line.

• See Example 21.2.

• Tutorial 1: Problem 21.11

Example 21.2

Example 21.2: Solution

Example 21.2: Solution

Example 21.2: Solution

Example 21.2: Solution

Example 21.2: Solution

Example 21.2: Solution

Determination of the number of plates using enthalpy-

composition diagram

Enthalpy Balances

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• Variations in V and L streams depend on the enthalpies.

• Enthalpy data may be available from an enthalpy-concentration diagram (e.g., Fig. 21.2) or data bank from in computer program.

• Often distillation columns are designed using computers, the basic enthalpy balance equation are given.

• Refer to Example 21.5, which illustrate the small difference this makes in the McCabe-Thiele diagram for a typical ideal system.

• Benzene-toluene solutions are ideal.

• Temperature for x = 0.5 and y = 0.5 is not the same! (Refer to next slide)

• Slight curvature in Fig. 21.21 is due to the nonlinear change in the bubble-point and dew-point with mole fraction benzene.

Fig. 21.21: Enthalpy-concentration diagram for benzene-toluene at 1atm.

Tb, benzene = 80 oC

For:Liquid mixtures @ bubble point Vapor mixture @ dew point

Temp. range = 110.6 - 80 oCTb, toluene = 110.6 oC

Enthalpy Balances

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Figure 1: T-x-y diagram for Benzene-Toluene System @ 1atm

Enthalpy Balances

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• Consider an overall enthalpy balance for system shown in Fig. 21.6

Fig. 21.6: Material-balance diagram for continuous fractionation column

FHF + qr = DHD + BHB +qc

HF = enthalpy of feedHD = enthalpy of overhead productHB = enthalpy of bottom productqc = heat removed from condenser = qr

For given feed and product streams, only 1 of heat effects, qr or qc.

Normally,

qc is chosen in designing a column to correspond to the desired reflux ratio and moles of the overhead vapor. Then, qr can be calculated using Eq. (21.48).

In operating column, qr is often varied to change the vapor flow rate and reflux ratio, and changes in qc then follow.

(21.48)

(REPEAT)

Enthalpy Balances

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Enthalpy balances in rectifying and stripping sections

Enthalpy Balances

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Enthalpy balances in rectifying and stripping sections

Enthalpy Balances

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Enthalpy balances in rectifying and stripping sections

Example 21.5 A mixture of 50 mol percent benzene and toluene is to be separated by

distillation at atmospheric pressure into products of 98% purity using a reflux ratio 1.2 times the minimum value. The feed is liquid at the boiling point. Use enthalpy balances (Table 21.3) to calculate the flows of liquid and vapor at the top, middle, and bottom of the column, and compare these values with those based on molal overflow. Estimate the difference in the number of theoretical plates for the methods.

Table 21.3: Data for Example 21.5

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Component Enthalpy of vaporization,cal/g mol

Specific heat at constant pressure, cal/g mol. °C

Boiling point, °C

Liquid Vapor

BenzeneToluene

7,3607,960

3340

2333

80.1110.6

Tutorial 2 Problems: 21.1 21.4 21.5 21.11

Due Thursday, 08/10/2015.

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