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Transfer Masa antara Dua Film

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Transfer massa
antar fase dua film
OTK 2
kelas IV C
TEKNIK KIMIA FT UNS
Sperisa distantina
1
(Review) Peristiwa transfer massa:
1. Difusi molekuler  transfer massa di
fase diam atau laminar.
2. Difusi turbulen  transfer massa di
fase turbulen = transfer massa satu
film
3. Transfer massa antar fase  transfer
massa 2 film
2
Difusi molekuler
Difusi
Turbulen
(satu film)
Transfer massa
antar fase dua film
3
Transfer massa
antar fase
1. transfer massa dari badan utama fase I (gas) ke permukaan antar
fase.
NA = k g (PAG − PAi )
2. di batas antar fase, tidak ada tahanan, sehingga terjadi
keseimbangan.
PAi = f (XAi ; keseimbangan)
3. transfer massa antar fase dari antar fase ke badan utama fase II
(cairan):
NA = k x (XAi − XAL )
4
 Perpindahan massa akan
berlangsung selama ada
perbedaan konsentrasi
di lapisan film.
 Jika konsentrasi di batas
fase sudah sama dengan
konsentrasi di badan
utama, maka keadaan
jenuh atau
keseimbangan telah
tercapai.
 Dari uraian di atas, tampak bahwa hubungan
keseimbangan menjadi sangat penting untuk diketahui.
Hal ini dapat digunakan untuk menentukan profil
konsentrasi di kedua fase.
5
 Hubungan keseimbangan gas-cair (di interface):
data kelarutan, hukum Henry.
pA = H . XA
pA = tekanan parsial A di fase gas.
XA = fraksi mol A di fase cair.
H = konstanta keseimbangan Henry.
Bentuk persamaan keseimbangan ditentukan
satuan H.
 Sering dijumpai di operasi alat-alat transfer
massa yang berdasarkan perbedaan kelarutan
(absorber, stripper).
6
Contoh data keseimbangan
Welty
Data keseimbangan disajikan dalam bentuk data
atau kurva.
7
(Welty)
Berdasarkan satuan H, maka pers. keseimbangan:
pA = H . XA
pA = tekanan parsial A (bar)
XA= fraksi mol A =
mol A
mol air
8
pA = H . CA
pA = tekanan parsial A (kPa)
CA= konsentrasi A (mol A/m3)
9
Geankoplis
Contoh:
Henry O2 di air pada 20OC:
H x 10-4 = 4,01
H= 2,01 x 104
atm
Satuan H  fraksi mol
104
atm
fraksi mol
H = 2,01 x
pA = H .XA
10
Contoh aplikasi keseimbangan antar fase
NH3 is selectively removed from an NH3
mixture by absorption into water. In this
steady state process, NH3 (species A) is
transferred by molecular diffusion through a
stagnant gas layer 2 cm thick, and then
through a stagnant water layer 1 cm thick as
shown in the figure.
The concentration of NH3 at the upper boundary of the gas layer is 3.42 mol%, and
the concentration at the lower boundary of the water layer is essentially zero. The
temperature of the system is 15oC and the total pressure on the system is 1 atm.
The concentration of NH3 at the interface between the gas and the liquid phase is
given by the following equilibrium data:
PA = 0.7736 CA
Determine the flux of NH3 across both the gas and liquid films. At 15oC, the
diffusivity of NH3 in air is 0.215 cm2/s, and the diffusivity of ammonia in liquid
water is 1.77x 10-5 cm2/s. Assume that the concentration of NH3 in the liquid water
11
is very small (dilute solution).
Dicari: banyaknya A yang melewati lapisan gas dan cairan.
Penyelesaian:
Difusi A di fase gas:
Difusi A melalui udara diam = NA1
BC: 1). Z=0  YA0 = 0,0342
𝐩𝐀𝐢
2). Z= -2 cm  YAi =
𝐏
Batas fase: hubungan keseimbangan:
pA = H. CA
pAi = H . CAi
Difusi A di fase cair:
Difusi A melalui air diam= NA2
BC: 1). Z= -2  CAi =
𝐩𝐀𝐢
𝐇
2). Z = -3  CA=0
Neraca massa A di gas dan cairan:
NA1 = NA2= NA
12
Hasil=?
NA=?
13
Konsentrasi di antar fase sangat sulit diukur, maka
perlu dicari bentuk persamaan lain.
Dibayangkan: dua film diwakili oleh satu film
khayal, dan film khayal itu dapat dianggap di fase I
ataupun di fase II.
14
FILM GABUNGAN DI
FASE CAIR
Gas
Cairan
FILM GABUNGAN DI
FASE GAS
Gas
Cairan
PAG
PAG
xAL
NA =
∗
K x (XA
∗
PA = H X A
PA*
xA*
xAL
− X AL )
NA = K g (PAG − PA∗ )
PA∗ = H XAL
Hubungan kx, Kx, kg, Kg?
15
Hubungan kx, Kx, kg, Kg?
Film gabungan di fase cair
∗
∗
(X A
− X A ) = (X A
−XAi ) + (XAi − X AL )
=
PAG
(
H
PAi
− ) + (XAi
H
Tr. massa Individual di film gas
−X AL )
Individual di film cairan
Dibagi NA dan dimanipulasi menjadi:
1
KX
=
1
H kg
+
1
kX
16
Film gabungan di fase gas
Film individual
(PAG − PA∗ ) = (PAG − PAi ) + (PAi − PA∗ )
= (PAG − PAi ) + H(XAi − XAL )
Dibagi NA dan dimanipulasi menjadi:
1
Kg
=
1
kg
+
H
kX
17
18
19
1
KX
1
Kg
=
=
1
kX
H
kX
+
+
1
H kg
1
kg
gaya pendorong
kecepatan perpindahan 
tahanan
∗
NA = K x (XA
− XAL )
Driving force=gaya pendorong
𝑁𝐴 =
X∗A −XAL
tahanan
1
Kx
20
Latihan 1
Chlorine water for pulp bleaching is being prepared by
absorption of chlorine in water in a packed tower
operating at 293 K and 1.013 x 105 Pa pressure.
At one point in the tower the chlorine pressure in the gas is
5 x 104 Pa and the concentration in the liquid is 1 kg/m3.
Data on the solubility of chlorine in water at 293 K are as
follows:
Cl2 pressure in Pa, x 10-4
0.674
1.21 2.63
4.93
4.96
9.77
Solubility in mol Cl2/m3, x 10-1
1.58
2.25 3.69
5.84
5.94
10.21
If 75% of the resistance to mass transfer lies in the liquid
phase, what are the interfacial compositions?
21
algoritma
1. Data diketahui: skema dilengkapi data dan
simbol variabel.
2. Data yang dicari?
3. Susun persamaan hubungan koefisien
transfer massa gabungan dan individual.
4. Penyelesaian perhitungan data yang
dicari.
22
Latihan 2
Kg= 2,74 10-9 kmol/m2/s/Pa.
Diketahui:
H=1,358 103 Pa/kmol/m3.
Cairan
Gas
PAG
CAL
Di gas: mol A =nA=8 mol
Di cairan: CA=0,064 kmol/m3
Kondisi operasi: 293 K; 1,013 105 Pa.
PA*
Tahanan di fase gas=85% dari tahanan total.
NH3=A
Dicari: kg dan kc, dan
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komposisi di batas fase?
Latihan 3
Welty, 2008
24
TUGAS ke-6 (in a group, @ 3 students)
Welty, 2008
25
26
Welty, 2008
Welty, 2008
27
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