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Principles of distillation
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DISTILASI
Disarikan dari Catatan Kuliah
Charles Musgrave
Chemical Engineering Department, Stanford University
Referensi
Seader dan Henley, Separation Process Principles
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
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
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
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
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)
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.
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.
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.
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
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.
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 .
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
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
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.
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
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
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.
Construction Lines for McCabe-Thiele Method
Equilibriumcurve
x=zFxB
y
yB
yN
xD
1
2
3
4
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.