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Mass Transfer

The document discusses the calculation of the height of a dehumidification tower. Moist warm air at 85°C wet-bulb temperature and 45°C dry-bulb temperature enters the tower at a rate of 4,800 kg/m2h. Using a water flow rate of 1.4 times the minimum, the outlet air conditions are calculated to be 30°C wet-bulb temperature. The minimum water flow rate and outlet water temperature are determined. Integrating the enthalpy difference curve gives the number of transfer units for the tower. The height is then calculated as the product of the mass transfer coefficient and number of transfer units. The calculated height is 6.104 m.

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Pandia Rajan
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330 views4 pages

Mass Transfer

The document discusses the calculation of the height of a dehumidification tower. Moist warm air at 85°C wet-bulb temperature and 45°C dry-bulb temperature enters the tower at a rate of 4,800 kg/m2h. Using a water flow rate of 1.4 times the minimum, the outlet air conditions are calculated to be 30°C wet-bulb temperature. The minimum water flow rate and outlet water temperature are determined. Integrating the enthalpy difference curve gives the number of transfer units for the tower. The height is then calculated as the product of the mass transfer coefficient and number of transfer units. The calculated height is 6.104 m.

Uploaded by

Pandia Rajan
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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NPTEL Chemical Mass Transfer Operation 1

MODULE 6 HUMIDIFICATION AND AIR CONDITIONING


LECTURE NO. 9
EXAMPLE PROBLRMS ON DEHUMIDIFICATION Example Problem 6.4: Moist warm air is to be cooled and dehumidified to a wetbulb temperature of 30C using water of 25C in a dehumidification column. The air enters the tower with a dry-bulb temperature of 85C and wet-bulb temperature of 45C at 4800 kg/m2h rate. The overall gas phase mass transfer coefficient is estimated to be 2150 kg/ m3h. The flow rate of water is 1.4 times the minimum. Calculate height of the tower.

Solution 6.4:

Gs=4507.04 kg/m2.h Y2/ 0.027 / H2 98 kJ / kg

Ls=8439.27 kg/h.m2 TL2=25C

Dehumidification tower

Gs=4507.04 kg/m2.h TG1=85C Y1/ 0.065 H1/ 210 kJ / kg Given, TG1=85C; TW1=45C

Ls=8439.27 kg/h.m2 TL1=39.29C

Joint initiative of IITs and IISc Funded by MHRD

Page 1 of 4

NPTEL Chemical Mass Transfer Operation 1

From psychrometric chart, Y1/ 0.065 kg moisture/kg dry air and H1/ 210 kJ/kg dry air. or, Y/=0.047 and

H1/ (1.005 1.88 0.047) 85 2500 0.047kJ/kg


=210.4 kJ/kg Wet-bulb temperature of outlet air is 30C. Y2/ 0.027 kg moisture/kg dry air and
/ H2 98 kJ/kg.

We have, inlet water temperature, TL2=25C Locate top point Q(TL2, H 2/ ) (lower terminal of operating line) at Q(25, 98) on TLH/ plane. The enthalpy of feed moist air at the bottom point P (upper terminal) of the dehumidification tower is 210 kJ/kg. But the temperature of the exit water is unknown.

Calculation of minimum water rate: The equilibrium line is convexed downward. The pinch point is obtained by drawing the horizontal line through H1/ 210 kJ/kg to meet equilibrium line at P/. Points Q and P/ are joined. Line QP/ is the operating line for minimum water flow rate. The water temperature at P/ is TL1,max=45C (TW1).
Gs G1 (1 Y1/ )

Gs

4800 (1 0.065)

= 4507.04 kg/h.m2 Now, the minimum water rate Ls,min can be obtained from the following enthalpy balance equation:
/ Gs ( H1/ H 2 ) Ls ,min (TL1,max TL 2 )CwL

4507.04(210-98)=Ls,min(45-25)4.187 Ls,min=6028.05 kg/h.m2


Joint initiative of IITs and IISc Funded by MHRD Page 2 of 4

NPTEL Chemical Mass Transfer Operation 1

Actual water flow rate=1.4Gs,min=1.46028.05 kg/h.m2=8439.27 kg/h.m2 Outlet temperature of water TL1 is obtained from Equationas follows: 4507.04(210-98)=8439.27(TL1-25)4.187 TL1=39.29C Moisture removed from air=water condensed per hour

Gs(Y1/ Y2/ )
=4507.04(0.065-0.027) kg/h.m2. =171.27 kg/h.m2.

240 220 200 180 160

P(TL1,H1)

'

P (TL1, max,H1)

'

Equilibrium line Operating line

'

140 120 100 80 60 22.5 Tas 25.0 27.5


0 '

Q (TL2,H2)

30.0

32.5

35.0

37.5

40.0

42.5
0

45.0

TL2=25 C

TG, TL

TL1=39.29 C

TL 25 27.7 30

H/ 98 117 136.9

H */

1/[H/- H */ ] 0.043 0.031 0.026


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75 85 98

Joint initiative of IITs and IISc Funded by MHRD

NPTEL Chemical Mass Transfer Operation 1

32.5 35 37.5 39.29

155.4 176 195.3 210

110 130 145 156

0.022 0.022 0.020 0.019

0.05

0.04

0.03 1 [ H / H */ ] 0.02

0.01

0.00 25

26

27

28

29

30

31

32
0

33

34

35

36

37

38

39

TL ( C)

H toG

Gs 4507.04 m 2.096 m / 2150 KY a

N toG

dH / / ( H / H*/ ) (210 98) 0.026 2.912 H2

H1/

Height of dehumidification tower, Z=HtoGNtoG=2.0962.912 m=6.104 m. The Psychrometric chart can be obtained from the link: http://en.wikipedia.org/wiki/Psychrometrics

Joint initiative of IITs and IISc Funded by MHRD

Page 4 of 4

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