Falling film absorption

INTRODUCTION

Efficient gas absorption depends on the following

This is achieved in a Falling Film Absorber which is essentially a Shell & Tube heat exchanger in which both gas to be absorbed and absorbing liquid flow co-currently downward with extraction of heat by circulation of coolant in the shell. The Absorbing liquid is circulated through a tank till desired concentration is achieved. The liquid flows at such a rate that the tubes do not flow full of the liquid but instead, descends by gravity along the inner walls of the tubes as a thin film. Obviously, this produces a much greater liner velocity for a given rate flow than could be obtained if the tube flowed full.

The equipment works as a number of water cooled wetted-wall columns in parallel and each tube is provided with distribution system on top to effect uniform distribution of both liquid and gas and also formation of a thin liquid film on the inner surface of the tube.

Approximately 5% of the gas remains unabsorbed in the falling film unit. A tail tower is required if this remaining quantity of gas is to be absorbed. In this case the unabsorbed gases enter at the bottom of tail absorber and process water enters at the top. The water absorbs the hydrogen chloride gas to form dilute acid which flows by gravity to the top of main absorber.
 

SALIENT FEATURE

LIMITATIONS

Sr.
No.

Normal
Size
(mm)

Absorber Area
(M²)

No. of Tubes/
Tube OD
(mm)

Max. Gas
Absorption Rate
(Pure HCL)* Kg/hr

Max. Acid Prod.
Rate ( As 30 % HCl)
(Kg/hr)

Height
(m)

1

80

1.00

4/20

30

100

4400

2

100

1.76

7/20

60

200

4500

3

150

4.80

19/20

150

500

4600

4

225

7.80

31/20

250

833

4920

5

300

15.30

61/20

500

1667

5050

6

400

36.00

143/20

1175

3917

5300

7

450

47.00

187/20

1500

5000

5700

8

600

84.00

333/20

2700

9000

5800