Distilasi (stanford).ppt

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DISTILASI Disarikan dari Catatan Kuliah Charles Musgrave Chemical Engineering Department, Stanford University Referensi Seader dan Henley, Separation Process Principles

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Principles of distillation

Transcript of Distilasi (stanford).ppt

Page 1: Distilasi (stanford).ppt

DISTILASI

Disarikan dari Catatan Kuliah

Charles Musgrave

Chemical Engineering Department, Stanford University

Referensi

Seader dan Henley, Separation Process Principles

Page 2: Distilasi (stanford).ppt

Metode McCabe-Thiele (1)

Total condenser

Feed

Overhead vapor

Boilup

N

2

1

Distillation

f

Reflux drum

Rectifying section stages

Stripping section stages

Feed Stage

Bottoms

Partial reboiler

Reflux

Distillate

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Metode McCabe-Thiele (2)Aliran berlawanan arah banyak tahap dalam kolom distilasi biner diperlihatkan sebagai berikut.(a) Kolom dengan N jumlah tahap teoritik(b) Kondensor total yang menghasilkan cairan refluks yang bertindak sebagai absorbent dan cairan

distilat(c) Reboiler parsial yang menghasilkan uap boilup yang bertindak sebagai stripping agent dan

produk bawah(d) Tahap umpan berada di tengah.

Konfigurasi ini menghasilkan pemisahan tajam, kecuali ada azeotrop dengan konsentrasi distilat dekat dengan titik azeotrop.

Tujuan distilasi adalah memperoleh distilat yang kaya komponen kunci ringan dan produk bawah yang kaya komponen kunci berat.

Total condenser

Feed

Overhead vapor

BoilupN

21

Distillation

f

Reflux drum

Rectifying section stages

Stripping section stages

Feed Stage

Bottoms

Partial reboiler

Reflux Distillate

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Metode McCabe-Thiele (3)

Komposisi umpan dinyatakan oleh fraksi mol komponen ringan (LK), ZF. Pada suhu dan tekanan umpan dapat berwujud cair, uap, atau campuran uap-cair. Komposisi distilat dinyatakan oleh fraksi mol komponen ringan (LK), XD, dan komposisi produk bawah XB.

Tingkat kesulitan dalam memperoleh pemisahan ditentukan oleh nilai relative volatility, antara LK (index-1) dan HK (index-2).

2

12,1 K

K

Jika dua komponen membentuk larutan ideal, sehingga Hukum Raoult dapat diterapkan:

P

PK

oi

i

Relative volatility adalah perbandingan tekanan uap murni:

o

o

P

P

2

12,1

Total condenser

Feed

Overhead vapor

Boilup

N

2

1

Distillation

f

Reflux drum

Rectifying section stages

Stripping section stages

Feed Stage

Bottoms

Partial reboiler

Reflux Distillate

Fraksi mol LK xD

Fraksi mol LK xB

Hanya tergantung T

Jika T naik, berkurang hingga mencapai satu yang berarti tidak terjadi pemisahan.

Fraksi mol LK zF

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Metode McCabe-Thiele (4)

Relative volatility dapat dituliskan:

11

11

11

11

22

11

2

12,1 1

1

1/1

/

/

/

yx

xy

xy

xy

xy

xy

K

K

Untuk suhu didih yang berdekatan, maka hampir konstan sepanjang kolom. Penyelesaian untuk fraksi mol ringan dalam uap menghasilkan:

Untuk komponen yang memiliki titik didih tidak berdekatan, akan berubah tergantung pada komposisinya.

y1 1,2x1

1 x1 1,2 1

y1

x1

KurvaKeseimbangan

Garis 45°

y1

x1

Garis 45°

Kenaikan relativevolatility

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Spesifikasi• Laju umpan total (F)• Fraksi mol umpan (zF)• Tekanan operasi kolom (P)• Kondisi fase umpan. • Kurva keseimbangan uap-cair• Jenis kondensor (total atau parsial)• Fraksi mol distilat (xD)• Fraksi mol produk bawah (xB)• Rasio reflux terhadap refluks minimum (R/Rmin)

Hasil• Laju distilat (D)• Laju produk bawah (B)• Jumlah tahap minimum tahap keseimbangan (Nmin)• Reflux ratio minimum, Rmin = Lmin/D• Reflux ratio, R = L/D• Boilup ratio, R’ = V/B• Jumlah tahap keseimbangan (N)• Lokasi umpan optimal• Komposisi uap dan cair tiap tahap

Metode McCabe-Thiele: Spesifikasi (5)

Page 7: Distilasi (stanford).ppt

Metode McCabe-Thiele: Neraca Massa (6)

FzF xDD xBB

Feed

BoilupN

2

1

f

Bottoms

Reflux

F, zF

D, xD

B, xB

Distillate

Neraca Massa (dalam komponen LK) :

Neraca massa total: F D BDiperoleh: D F

zF xB

xD xB

Jika D, F, zF diketahui, makaxD atau xB dapat dihitung.

Page 8: Distilasi (stanford).ppt

McCabe-Thiele Method: Rectifying Section (7)

Vn1yn1 Lnxn DxD

Yang dapat disusun menjadi:

Rectifying section dimulai dari tahap-1 hingga tahap di atas umpanJika dibuat neraca massa komponen ringan (light key)di sekitar tahap-n bagian rectifying section termasuk kondensor.

yn1 Ln

Vn1xn

DVn1

xDFeed (L/V)

Boilup

N

n

1

f

Bottoms

Reflux

ZF

L, xD= x0

xB

DistillatexD

n

1 Reflux

L0, xD= x0

DistillatexD

Lxn

Vyn+1

Jika L dan V tetap sepanjang kolom, maka persamaan di atas berupa garis lurus.

Page 9: Distilasi (stanford).ppt

McCabe-Thiele Method: Limpahan Molar Tetap (8)

Dn

nn

nn x

V

Dx

V

Ly

111

Jika L dan V konstan, maka persamaan di atas berupa garis lurus.Ini mensyaratkan:

(a) Dua komponen memiliki entalpi penguapan sama dan tetap.

(b) Perubahan kapasitas panas diabaikan dibanding panas penguapan.

(c) Kolom terisolasi sempurna hingga tidak ada kehilangan panas.

(d) Tekanan dalam kolom serbasama.

Kondisi ini disebut constant molar overflow.

Feed (L/V)

Boilup

N

n

1

f

Bottoms

Reflux

ZF

L, xD= x0

xB

Distillate

xD

Untuk kondisi ini jumlah uap yang dipindahkan ke aliran cair dalam tiap tahap sama dengan jumlah cairan yang dipindahkan ke aliran uap. Sehingga laju alir uap dan cairan konstan sepanjang bagian.

Page 10: Distilasi (stanford).ppt

McCabe-Thiele Method: Rectifying Section Operating Line

y LV

x DV

xD

The liquid entering stage one is the reflux L and its ratio to the distillate L/D is the reflux ratio R. If we have constant molar overflow, then R is a constant and

LV

L

L D

L / DL / D D / D

R

R1

DV

D

L D

1R1

and

We define this equation as the operating line of the rectifying section.

Feed (L/V)

BoilupN

n

1

f

Bottoms

Reflux

ZF

L, xD= x0

xB

Distillate

xD

In the case of constant molar overflow we can then drop the stage subscripts:

yn1 Ln

Vn1xn

DVn1

xD

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McCabe-Thiele Method: Operating Line

x

Equilibriumcurve

45° line

n

1

f

Reflux

xD= x0

Distillate

xD

L, xn V, yn+1

y LV

x DV

xDWe can then rewrite:

as y R

R1x

1R 1

xD

x0=xDx1

y

y1

y2

y 1

R1xD

Rectifying Section Operating lineSlope=L/V=R/(R+1)<1

If R and XD are specified then we can graph the line shown in the following plot.

Page 12: Distilasi (stanford).ppt

McCabe-Thiele Method: Stripping Section

Lxm Vym1 BxB

Which we can rearrange and use the constant molar overflow assumption to find:

The stripping section extends from the stage just below the feed stage to the bottom stage N. If we perform a material balance in the light key around the bottom stages of the rectifying sectionincluding the condenser we have:

y LV

x BV

xB

Feed (L/V)

BoilupN

n

1

f

Bottoms

Reflux

zF

L, xD= x0

xB

DistillatexD

y VB 1

VBx

1VB

x Band

Lxm

Vym+1

Boilup

NBottoms

B, xB

m+1

L, xN

V, yB

Since:

LV

V B

V

VB 1VB

L V B

ThenVB is called the boilup ratio.

VB VB

We define this equation as the operating line of the stripping section.

This is also the operating line of the stripping section .

Page 13: Distilasi (stanford).ppt

McCabe-Thiele Method: Stripping Section

x

Equilibriumcurve

45° line

xNx

B

y

yB

yN

Stripping Section Operating LineSlope=L/V=(VB+1)/VB

If VB and XB are specified then we can graph this as the line shown in the following plot.

y VB 1

VBx

1VB

x B

Lxm

Vym+1

Boilup

NBottoms

B, xB

m+1

L, xN

V, yB

xm

Ym+1

y VB 1

VBx

1VB

x B

Page 14: Distilasi (stanford).ppt

Feed Stage Considerations

In determining the operating lines for the rectifying and stripping sections we needed the bottoms anddistillate compositions and reflux and reboil ratios. The compositions can be independently specified, but R and VB are related to the vapor to liquid ratio in the feed.

FF

FFF

L

L

L

L

L

V

V V

V

VV

V

V VV VF V

V F V V F V

L F L L F L L L LF

L L L L

Subcooled Liquid Bubble Point Liquid Partially Vaporized

Dew Point Vapor Superheated Vapor

Page 15: Distilasi (stanford).ppt

Feed Conditions

So except in the cases where the feed is a supercooled liquid or superheated vapor the boilup is related to the reflux by the material balance:

V L D VF

VB V B

L D VF

B

Distillation operations can be specified by the reflux ratio or boilup ratio although the reflux ratio (or R/Rmin) is most often specified.

Dividing by B gives the boilup ratio:

L B V

V D L

VF LF D B

V V VF

L L LF

VF L L D B

VF L L D L V

V L D VF

Consider the cases where the feed is not a supercooled liquid or a superheated vapor:

Mass balance around the reboiler:

Mass balance around the condenser:

Mass balance around the column:

Vapor entering the rectifying section:

Liquid entering the stripping section:

Substitute this into the column balance:

Substitute in the reboiler balance:

In other words, the vaporentering the rectifying sectionis the vapor entering the condenserminus the feed vapor flow rate.

Page 16: Distilasi (stanford).ppt

The q-line

First, we define the parameter q by: q L L

F

yV Lx BxByV Lx DxD

Subtracting the two operating lines:

Gives: y V V L L x DxD BxB

Using a material balance in the LK: DxD BxB FzF

Using a material balance around the feed stage to elminate vapor flow rates:

F V L V L

Simplifying and using the definition of q results in the q-line:

y q

q 1

x

zF

q 1

x zF y zF

minus

y V V L L x FzF

V V F L Ly F L L L L x FzF

The q-line has slope q/(q-1)and intercepts the 45 degreeline at y=zF

Page 17: Distilasi (stanford).ppt

Construction Lines for McCabe-Thiele Method

Equilibriumcurve

45° line

x=zFxB

y

yB

yN

Stripping Section: Operating lineSlope=L/V=(VB+1) /VB

xD

Rectifying Section: Operating lineSlope=L/V=R/(R+1)<1

q-liney

LV

x DV

xD

y LV

x BV

xB

y q

q 1

x

zF

q 1

Page 18: Distilasi (stanford).ppt

Feed Stage Location Using McCabe-Thiele

Equilibriumcurve

x=zFxB

y

yB

yN

xD

Equilibriumcurve

x=zFxB

y

yB

yN

xD

1

2

3

4

1

2

3

4

5

Feed stage located one tray too low. Feed stage located one tray too high.

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Construction Lines for McCabe-Thiele Method

Equilibriumcurve

x=zFxB

y

yB

yN

xD

1

2

3

4

Page 20: Distilasi (stanford).ppt

Summary

This lecture:• We extended the analsis used for adsorption and stripping to binary distillation. • We described a typical binary distillation configuration. • We made definitions such as reflux ratio, constant molar overflow, etc.• We described operating lines.• We plotted the equilibrium curve.• We stepped through stages to show the change in composition as you go throughthe column.

Next lecture we’ll continue our discussion of binary distillation and the McCabe Thiele method.