ABSORPSI DENGAN KOLOM BAHAN ISIAN - Spada UNS

ABSORPSI DENGAN

KOLOM BAHAN ISIAN

Dalam praktek, kolom bahan isian sering dianalisis berdasar stage setimbang yang ekivalen menggunakan suatu nilai HETP (Height Equivalent to a

Theoretical Plate )

Kolom bahan isian adalah alat kontak kontinyu yang tidak mempunyai stage terpisah seperti pada kolom dengan tray.

Dengan mengetahui nilai HETP dan jumlah stage teoritis yang diperlukan tinggi kolom bahan isian dapat dihitung dengan mudah :

Konsep HETP, sayangnya tidak berdasar pada teori tertentu. Nilai HETP hanya dapat dihitung menggunakan data eksperimen laboratorium atau kolom

komersial

Pendekatan HETP

stages m

equilibriu equivalent

of number

height packed

HETP 

HETP n

H 

Dengan memperhatikan suatu elemen volum setebal dz pada suatu kolom bahan isian

S = luas tampang dari kolom

N

G G

L L

dz

Volume dari elemen volume = Sdz ��=����

Absorpsi: pendekatan transfer massa (Height of Transfer Unit (HTU), Number of Transfer Unit (NTU)

Fluks A � _�=�_�

(

^��,�− �_�,�

)

� _�=�_�

(

^��,�−�_�,�

)

�_�=�_�

(

^��,�−�^∗_�

)

�_�=�_�

(

^�∗�−�_�,�

)

dz z

z

  _



 





 ₃²

m a m column the

of volume

surface transfer

a mass

A terabsorpsi dari gas pembawa B

N

G G

L L

dz Mol A meninggalkan G = mol A masuk ke L

−�(�� )=�(��)

Fluks A

Dengan mengalikan suku kiri dengan dA dan suku kanan dengan aSdz Sdz

Laju transfer A −���=� � �=�_���

�_�=�_�

(

^��,�− �_�,�

)

^=��

(

^��,�−�_�,�

)

Jika perubahan laju alir molar diabaikan _�(��)=��� �(��)=� � �

dz z

z

Fase Gas

−���=�_��

(

^��,�−�_�,�

)

^{��� ���=�}��

(

^��,�−�_�,�

)

^���

−�_���

� ��= ��

(

^��,�−�_�,�

)

− �_���

�

∫

0

�

��=

∫

�

� ��

(

^�_�,�^−� _�,�

)

�_���

� �=

∫

�

� ��

(

^�_�,�^{− �}_�,�

)

�=�/�

�_��

∫

�

� ��

(

^� _�,�^{− �}_�,�

)

Fase Cair

�_�� �

� ��= � �

(

^��,�−��,�

)

�_�� �

�

∫

0

�

��=

∫

�

� ��

(

^��,�−�_�,�

)

�_�� �

� �=

∫

�

� ��

(

^��,�−�_�,�

)

�=�/�

�_��

∫

�

� ��

(

^�_�,�^−�_�,�

)

Menggunakan koefisien transfer massa keseluruhan

Fase Gas Fase Cair

�=�/�

�_��

∫

�

� ��

(

^��,�−�^∗_�

)

^�=

�/�

�_��

∫

�

� ��

(

^��

∗ −�_�,�

)

�=�/�

�_��

∫

�₂

�₁

��

(^� �,�− �_�,�)

�=�/�

�_��

∫

�₂

�₁

��

(^��,�−�_�,�)

�=�/�

� _��

∫

�₂

�1

��

(

^��,�− �^∗_�

)

�= �/�

� _��

∫

�₂

�1

��

(

^��

∗ −�_�,�

)

H_y

H_Ox H_Oy Hx

N_y

Nx

N_Oy

N_Ox Tahanan pada film gas

�_�=�_�

(

^��,�−�_�,�

)

� _�=�_�

(

^��,�−�_�,�

)

Tahanan overall pada film cair Tahanan overall pada film gas Tahanan pada film cair

� _�=�_�

(

^��,�− �^∗_�

)

�_�=�_�

(

^�∗�−�_�,�

)

H_y = height of transfer unit based on gas film H_x = height of transfer unit based on liquid film

H_Oy=height of transfer unit based on overall gas film H_Ox=height of transfer unit based on overall liquid film

N_y = number of transfer unit based on gas film N_x = number of transfer unit based on liquid film

N_Oy=number of transfer unit based on overall gas film N_Ox=number of transfer unit based on overall liquid film

�=�_� �_�=�_� �_�=�_{� �} �_{� �}=�_���_{� �}

�=�/�

�_��

∫

�

� ��

(

^��,�−�^∗_�

)

Evaluasi sisi kanan dari persamaan ini sering memerlukan integrasi numerik atau grafis.

Dengan fraksi mol bebas solute (Welty)

The mass balance for component A over the differential length dz is described by

The mass transfer of component A in the differential length dz is defined by

Gs = flux massa, mol/m².s = G/S

Contoh Welty (Example 6 Ch. 31)

Ammonia is to be absorbed from air at 293 K and 1.013105 Pa pressure in a countercurrent, packed tower, 0.5 m in diameter, using ammonia- free water as the absorbent. The inlet gas rate will be 0.2 m

³

/s and the inlet water rate will be 203 kg/s. Under these conditions, the

overallcapacity coefficient, K

_Y

, may be assumed to be 80 mole /(m

³

.s.∆Y

_A

) The ammonia concentration will be reduced from 0.0825 to 0.003 mol fraction. The tower will be cooled, the operation thus taking place

essentially at 293 K; the equilibrium data of example 3 may be used.

Determine the length of the mass exchanger.

• Equilibrium data for the system at 293 K and 1.013x10

⁵

Pa are as

follows: