perpindahan massa-hk fick
TRANSCRIPT
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TK3201-01 Peristiwa Perpindahan – Kelas 01
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Perpindahan MassaObjek Bahasan : Campuran
• minimum 2 komponen :
komponen A dan B biner ⟨dibahas⟩
• 3 komponen/ lebih A, B, C, … :
multikomponen ⟨akan dibahasbila mungkin⟩
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Campuran KonsentrasiPengertian konsentrasi : jumlah i/volum
• konsentrasi massa i ( i =A atau B )
ρi : massa i per volum campuran
• konsentrasi molar i ( i =A atau B )
Ci : jumlah mol I per volum campuran
• Campuran biner: komponen A dan Bρ A dan ρB atau C A dan CB
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Dalam besaran massa:
A + B = 1
ρ A + ρB = ρ
Dalam besaran molar:
x A + xB = 1
C A + CB = C
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Konsentrasi fraksi ( atau % )Konsentrasi dinyatakan dalam fraksi
Sering digunakan, terutama untuk campuran biner
cukup perhitungan salah satu komponen Fraksi massa : A dan B
Fraksi mol : x A dan xB
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Hukum Fick Menggambarkan kuantifikasi laju perpindahan
komponen dalam campuran
Laju perpindahan komponenBerbanding lurus dengan gaya
penggeraknya/driving force bedakonsentrasi .
Berbanding terbalik dengan jarak
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Untuk komponen A (sama untuk komponen B):
Fluks dinyatakan dalam besaran massa
Fluks dinyatakan dalam besaran molar
Konstanta pembanding sifat milik campuran
D AB (koefisien difusivitas)
z j atau
z j A
AA
A
z
xC Jatau
z
CJ A
AA
A
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Hukum Fick
dz
dD j atau
dz
dD j A
ABAA
ABA
dz
dxCD Jatau
dz
dCDJ A
ABAA
ABA
Menggambarkan laju perpindahan komponen Adalam campuran AB secara molekular mediumdiam
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Pergerakan CampuranDalam campuran :
Komponen A bergerak dengan kecepatan
Komponen B bergerak dengan kecepatan Kecepatan A dan B
bisa tidak sama
bisa pada arahn yang berbeda
Kecepatan campuran (medium)?
kecepatan rata-rata dari kecepatan A dan B
Av
Bv
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• Kecepatan rata-rata:
• Mass average velocity
• Molar average velocity
Kecepatan Medium
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Ambil satu arah perpindahan (1 dimensi):
Atas dasar acuan diam:
• Kecepatan komponen : v A dan v B
• Kecepatan medium : v dan v*
• Kecepatan komponen i sebenarnya :
(v i – v) atau (v i – v*)
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Pendifinisian fluks komponen A
( satu arah/arah z ):Fluks molekular :
j A,z (fluks massa) atau J A,z (fluks molar)
Fluks atas dasar acuan diam:n A,z (fluks massa) atau N A,z (fluks molar)
Hukum dasar (Fick) merupakan fluks molekular
sedangkan aplikasi fluks atas dasar acuan diam?? hubungan j A atau J A dengan n A atau N A
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Fluks = konsentrasi x kecepatan
[ konsentrasi jumlahkomponen/volum]
j A
= A
. (v A
– v) atau j A
= A
. (v A
– v*)
J A = C A . (v A – v) atau J A = C A . (v A – v*)
n A = A . v A dan N A = C A . v A
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Hubungan antara j A dengan n A dan J A dengan N A
j A = n A - A .(n A + nB)
n A = j A + A .(n A + nB)
J A = N A - x A .(N A + NB)
N A = J A + x A .(N A + NB)
Penyelesaian :
harus mengetahui hubungan antara N A dan NB
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Dalam bentuk 3 dimensi (ruang)
AABA
AABA
xCDJ
D j
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Aplikasi mikroskopik Perpindahan massa komponen A adanya
distribusi konsentrasi A sepanjang ruang dimanakomponen A berpindah
Keadaan steady (tidak dipengaruhi oleh waktu) pengaruh ruang saja ( 3 dimensi )
Koordinat ruang:
Rektangular ( x, y, z) Silindris (r, , z)
Sperikal (r, , )
Unsteady 4 dimensi ( + dimensi waktu)
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Strategi Penyelesaian Melalui pembuatan neraca massa dalam suatu ruang
Masalah paling sederhana :
tidak dipengaruhi oleh waktu
steady state perpindahan satu arah banyak dalam praktek
medium mengalir laminer
Neraca dibuat dalam elemen ruang menghasilkan
persamaan diferensial
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Penyelesaian persamaan memerlukan kondisi batas
Kondisi batas yang umum:
Konsentrasi di permukaan diketahui,
Fluks dipermukaan diketahui,
Bila terkait dengan reaksi kimia laju direaksi
dipermukaan diketahui, Dipermukaan antar fasa konsentrasi di kedua
permukaan tidak sama
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Neraca Massa Keadaan steady
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Sublimasi padatan berbentuk bola
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Force Convection Mass Transfer
L
W
y
z
x
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Perpindahan Massa antar Fasa
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