Water absorption by a solution of lithium bromide in wetted wall columns with tangential feed

INT. C O m . HEAT M A S S T R A N S F E R Vol. i0, pp. 49-58, 1983 ©Pergamon

WATER A B S O R P T I O N IN WETTED

BY A S O L U T I O N

WALL COLUMNS

0735-1933/83/010049-10503.00/0 Ltd. P r i n t e d in t h e U n i t e d States

Press

OF LITHIUM BROMIDE

WITH T A N G E N T I A L

FEED

s. GABSI R. BUGAREL du G4nie Chimique Chemin de la Loge 31078 TOULOUSE CEDEX

Institut

(C~l~t~ruicated

b y J. Gosse)

ABSTRACT One of the most effective methods of increasing the rate of heat trans fer is to carry it in form of thin film. The purpose of the research reported here is to investigate water absorption by a solution of lithium bromide under vaccum in w e t t e d wall columns with tangential feed in order to apply it in a cycle of a b s o r p t i o n h e a t pump. It is shown that film-wall heat transfer coefficient increases with pressure, vapor and liquid flow rate but it does not vary with the solution c o n c e n t r a t i o n in the range of our experiments.

Introduction Water-lithium

bromide

system

can be used in a a b s o r p t i o n

heat pump

/-1-~ 7. In fact, Lithium

certain

bromide

found to have high

substances,

solution

such as a salt will

is a hygroscopic

the best solubility

absorb water vapor.

salt solution

vapor pressure

w h i c h has been

relationship

to active

cycle efficiency. A limited

absorption water

of paper has reported

of pure vapor with

vapor

/~9-Ii

number

absorption

a solution.

in wetted

7. This work was realized

wall

experimental

Only

columns

in wetted

results

few authors /U7,SZ

for

reported

or in spray

wall columns

with

pure

columns

tangential

feed.

Description The

lithium

is corrosive

bromide

solution

in the presence

made of stainless

of apparatus generally

of air.

used in a b s o r p t i o n

For this reason,

steel. 49

the p i l o t

heat pumps plant

is

50

S. Gabsi a n d R. B u g a r e l

L i t h i u m chromate

inhibitor

been verified e x p e r i m e n t a l y on l i q u i d - v a p o u r

Vol.

is Used in a p r o p o r t i o n of

that the p r e s e n c e

I/I000.

I0, No.

It h a s

of the i n h i b i t o r has no e f f e c t

equilibrium.

The major components

of the s y s t e m are

:

- absorber -

generator

- evaporator -

condenser

- liquid-liquid

heat exchanger.

We will devote our work

Absorber

to the study of the absorber.

characteristic

Monotube-absorber It is a w e t t e d wall column w i t h t a n g e n t i a l was made of stainless

steel,

two coaxial

tubes

feed / 12 /. The column u s e d 40/49 mm and 61,5/70 mm w i t h

1,90 m length. A spiral holes

film is o b t a i n e d by i n t r o d u c t i o n

(diameter

i mm) d i a m e t r i c a l l y

film along the column wall,

opposed.

a stainless

of the s o l u t i o n

In order

to

from two

manage

the spiral

steel spiral was i n t r o d u c t e d

in

the inside p a r t of the tube. In the jacket of the absorber,

we also p l a c e d a spiral.

Multitube-absorber It is also a w e t t e d wall ~ree

tubes

(diameter

12/17 mm)

was a d m i t t e d by three holes In order

column w i t h t a n g e n t i a l

to insure

and

1,80 m length.

(diameter

feed c o n s t i t u t e d with

Solution

lithium bromide

I mm).

the a d i a b a c i t y

of the column,

it is i s o l a t e d w i t h

glass wool. The gas flows c o u n t e r - c u r r e n t l y .

Experimental The e x p e r i m e n t a l

study was r e a l i s e d

part,

we intend to d e t e r m i n e

tions

(0,63 < X

difference

ber was m a i n t a i n e d The solution about 5 m/s.

the m a x i m u m

< 0,68 w e i g h t LiBr)

P and 80°C which are r e l a t e d Temperature

condition in a looped circuit. capacity

In the first

for d i f f e r e n t

and e v a p o r a t o r

temperatures

concentraof 70,

75

to the a b s o r b e r pressure. between

entering

and leaving

liquid the a b s o r -

at 5°C. flow rate was chosen

such that the s o l u t i o n

velocity

is

1

Vol.

I0, No.

i

~%TERABSORPTION

Heat t r a n s f e r Absorber

fixed,

without

each value

flow rate

temperature

varied.

Knowing

and solution that,

flow

the system is

flow rate leads us to a new stable

that 4,17.10 -4 kg.s -I

: flow rate of solution

temperature

it i n c r e a s e s

TE

is

of vapor

flow rate lower

conditions

of e v a p o r a t o r

evaporator

power

difficulty.

For a v a p o u r rating

coefficient

initial

the e v a p o r a t o r

autostabilisant, regime

51

capacity

For concentration, rate

IN~~LLOOI/h~S

and concentration)

is not very significant

rapidly

(Fig.

(depending

while

on ope-

the v a r i a t i o n

for higher

vapor

I).

(°C)

84

0,

x

/

• ,22,0-, °63 • ,. ,0-' -

! !

00

82

•

/

/

/ I

/ /

/ /

•

80

I ~ I0 4 ( k g s - 1 ) J

I

I

I

3

4

5

6

Fig.

1 - Column

I

7

capacity

Heat b a l a n c e s with velocity

a maximum

capacity,

on the local h e a t

I - Overall

h e a t transfer

For the c a l c u l a t i o n s were

considered

transfer

the influence

of gas and liquid

coefficient.

coefficient of the h e a t exchange

rate,

two differents

ways

:

- first a balance

on the r e f r i g e r a n t

= Dr QABS i

. Cpr

- secondly

with

QABS2

. h(Xp.TEs)

= DL

we have studied

. AT

(i)

a heat balance + DV

on the system c a l o r i g e n - a b s o r b a n t

. Hv

(T E) -

(DL + DV) h

(XR°Tss)

(2)

52

S. Gabsi and R. Bugarel

Theoretically, found a maximum method

is m o r e

QABS2 With 5%) w h i c h with

the t w o b a l a n c e s

deviation precise

= U

. a

of

so t h i s

(AS)ml

relates

liquid

overall

and vapor

U = 1 8 5 . 1 0 -4

value

but we have

o f h e a t b y the s e c o n d

:

QABS2 U = a.(A@)ml

results, heat

the b e s t

transfer

flow rate

. DV 0'94

l e a d to the s a m e r e s u l t ,

calculated

is the o n e w e u s e d

÷

the e x p e r i m e n t a l

will

10%. T h e

Vol. i0, No. 1

is

(3)

correlation

coefficient

(maximum deviation

U refrigerant-solution

:

. DL 0'133

(4)

2 - L. o. c. a. l. . .h.e.a.t. .t.r.a.n.s.f.e.r. . .c .o .e .f .f .i .c .i .e .n t The overall

coefficient

wall-refrigerant 1 U

1

h +

was

related

and wall-resistance

e

to l o c a l

film-wall

coefficient

e D. D. + D 1 1 e h -l ~ i D 2 D e e 2-i L o c a l w a l l - r e f r i g e r a n t coefficient

A copper appears

simulated

transfer

Nu = 0,0737 h

e

.D

eq

interpreted

groups

/ 13 /

transfer.

It

t y p e o f flow. to o b t a i n

the w a l l - r e f r i g e r a n t

were

correlated

by

:

with

(6)

(7)

by comparing

the v a l u e s

the e x p e r i m e n t a l

predicted

values

b y the c o r r e l a t i o n

of

of SIEDER

: Re 0 ' 8 Pr I/3

It is f o u n d t h a t the p r e s e n c e

(~)0,14

the s p i r a l

data

(8)

increases

is m u l t i p l y e d

It c a n be e x p l a i n e d

by

by

the i n c r e a s e

heat

transfer

coefficient.

In

2,7. of

turbulence

produced

by

the

of a spiral.

2-2 L o c a l

film wall

Now having with

heat

4 . A P

Deq

NU = 0,027

presence

on this

in order

coefficients

1

dimensionless

fact,

the column

to increase

Re 0 ' 7 9 5 P r I/3

The runs were

and TATE

jacket

(5)

coefficient.

The experimental

Nu

s e t in the

that no study has been made

We h a v e heat

I ~ + e

spiral was

h., 1

I :

equation

he, w e

(5).

it is p r o p o s e d - solution

heat

transfer

can calculate

It is a c o m p l e x

to study

function

the i n f l u e n c e

flow rate

coefficient

h i relative

of

:

: hi

to l i t h i u m

of a large

bromide

number

solution

of parameters,

:

Vol. I0, No. i

WATER ABSORPTION I N ~ W A L L C O L t ~ S

-

vapor

-

evaporator

53

flow rate temperature

concentration. In order ficient

to show the interest

of falling

hi

film is calculed

1 6red

= 0,67

~2

of tangential

prl/3

Rel/9

by RAMM's

(~)

feed,

the exchange

relationship

coef-

/ 14 / :

I/3

(9)

1/3

~red = (--2----)

0

g

2-2-I In order coefficient centrations

Influence

to study

flow rate flow rate,

the variation

of the

h. with respect to the mass vapor flow rate for d i f f e r e n t con1 is shown in figure 2, while this v a r i a t i o n as a function of

inlet vapor velocity figure

of vapor

the e f f e c t of vapor

at d i f f e r e n t

evaporator

temperatures

is shown

in

3. h

10 - 2

(W m -2 °C -I)

Tangential

feed

i

DL(kg s-] ) TE(°C )

X

P • 0,63

122 ]0-4

<

80

• 0,647

:

0,664 0,679

film

J

i.

i DV.IO 4 A

(k(! s - 1 )

A

,1 Fig.

2 - hi as a function

Figures

2 and 3 reveal

characteristics

in a spiral

For a low flow rate,

too low to create For higher

vapor

200 per cent higher.

for various

some i m p o r t a n t

concentrations

aspects

of the heat

transfer

film.

the exchange

less than that of a falling were

of DV,

film.

coefficient

In this case,

the spiral

film.

flow rates,

local

for a tangential

the solution

heat transfer

feed is

flow rates

coefficient

about

54

S. Gabsi and R. B u g a r e l

h L 10-2

Vol.

i0, NO. i

(Wm -2 °c-l)

•l(-c) .L(kg s-*) xp / 122 10-4 0,664 /Tangential

/,

• 80

feed/

.,o " / Z . / .

/',.

I,_

,,/.

llingfilm v

(m s -t ) EG

i

. . . . . . . .

0,75

!

......

A

1

. . . . . . . .

L

1,25

,,,

1,5

Fig. 3 - hi as a function of VEG, for different evaporator temperatures This result brings together with b i b l i o g r a p h y analysis densation and evaporation,

heat transfer coefficients

: for the con-

increaSe with gas

velocity E I 5 - 2 0 j . In fact, h.l was related implicltely with h g and vapor flow rate. This result was compared with the ones of C A R P E N T I E R and C O L B U R N and TEPE and M U E L L E R / 21 / and is p r e s e n t e d on the figure 4. The p r e s e n t experiments

10 5

( w m -2 oC-I )

h 1

104

* our 0,63 <

results X

p

< 0,679

s

s~

T E " 80eC I

10 3

10 2

Data o~ C a c L ~ n t ~ e ~ - C o l b u ~ n andTepe-l~.tchardmon • water vapor • M e t . h a n o i vapor DV/A *

(kg s -I m -2)

!

06

10 7

Fig.

10 8

4 - Comparison of our results

"

10 9

Vol. i0, No. i

WATER ABSORPTION I N ~ ~ A L L C O L [ 9 ~ S

is the e x t r a p o l a t i o n

of CARPENTIER

a little modification At a constant

temperature.

domain

of experiments

the

flow rate with

and inlet

But,

it does

liquid velocity, not change with

h i increases

with eva-

the c o n c e n t r a t i o n

i n the

:

< Xp < 0,679.

2-2-2 The

for l o w e r v a p o r

o f the s l o p e .

vapor

porator

0,63

a n d all.

55

liquid

figure

Influence

5 where

has been plotted

the ratio between

versus

transfer

coefficient

velocity

in t h e c o l u m n

and evaporator

of liquid

flow rate

f l o w r a t e h a s an e f f e c t

the liquid

on the transfer

the transfer

Reynolds

area as shown on

area and the wall

number.

It also affects

area the heat

: the variations

o f h. w i t h r e s p e c t t o i n l e t l i q u i d l is s h o w n o n t h e f i g u r e 6 f o r d i f f e r e n t c o n c e n t r a t i o n s

temperatures.

a h

P

10 -2

( W m -2

°c -I)

i

Tangential

feed

7 ion 12 •

Fig.

Car

6

i

I

'

i _

l

2

3

4

x P • 0,679 0,664 • 0,664

5

a 5 - - - as a f u n c t i o n a

TE('C)

DV(kg s-I )

80

3,9.10-4

75 70

Falling film

!

I

5

6

VEL

! (ms-~

7

p

o f Re

L Fig.

As s h o w n o n

these

- the v a r i a t i o n the v e l o c i t y the

liquid

figures

The a n a l o g y

flow rate used

for the i n f l u e n c e as t h a t h a s b e e n

o f VEL

:

is l e s s a f f e c t e d

of vapor.

6 - h i as a f u n c t i o n

b y the v e l o c i t y

of figure

is i n the h i g h e s t

of concentration

5 and

paragraph.

than by

6 shows

z o n e o f the c u r v e .

and evaporator

f o u n d i n the p r e g o i n g

of liquid

figure

temperature

The

that, result

is t h e s a m e

56

S. Cx~bsi a n d R. B u g a r e l

2-2-3 C o r r e l a t i o n

of results

In a n a l o g y with the r e l a t i o n s h i p propose

Vol.

i0, No.

for spiral film of AKERS and all.

/ 10 /, we

:

h i = 0,67

(6r~d)

The o b t a i n e d

Pr I/3

(~)i/3

correlation

--Re L m + E Rev n --Z --/

valid

for the vapor

flow rate h i g h e r than

> 780 is in a good a g r e e m e n t w i t h the r e l a t i o n s h i p Re v d e v i a t i o n of 12%. Identification

m = 0,ii

result

leads

(IO)

with an average

:

(II)

(a) a

P

Re L

I) a__ = a P

i + B.C D(3-~

(12)

-

B = 2,9179 C = 0,0880729 D = 0,67933689 E = 6,8472

. 10 -26

n = 8,3132

a

was d e t e r m i n a t e d

- -

a

from CARRE e x p e r i m e n t a l

result / 22 /.

P

Conclusion Because ~e

absorber

of the limited in detail.

could e s t a b l i s h

a correlation

which can be used With change

for local

the c o m p a r i s o n

of S A L A Z A R ' s

area is one of the limiting

We

data,

film h e a t

we c o u l d ' n t

coefficients.

study But,

we

transfer coefficient

type of apparatus.

research,

factors.

we can show that the ex-

Also,

we have r e a l i z e d

a mul-

(see m u l t i t u b e - a b s o r b e r ) .

found that the c e n t r i f u g a l

the heat transfer

of a v a i l a b l e

for the d e s i g n of this

titubes-exchanger

coefficient

In conclusion, feed,

numbers

So we have used the o v e r a l l

the absorption,

so

is improved.

we e m p h a s i z e

of v a p o r and liquid

force i n c r e a s e s

the favorable

flow rate,

and of the i n c r e a s e d number of tubes

influence

of p r e s e n c e

of t a n g e n t i a l

of spiral in the jacket

in the absorber.

I

Vol.

I0, NO. I

W A T E R A B S O R P T I O N IN WI~i'l'~O W A L L COLU~qS

Nomenclature A a ap Cp D Deq DL Dr DV g H h L Nu Q P Pr Re T U X Q ~p

flow area (m 2) transfer area (m2/m 3) wall area (m2/m 3) specific heat (kJ/kg°C) diameter (m) e q u i v a l e n t diameter (m) flow rate solution (kg/s) flow rate refrigerant (kg/s) flow rate vapor (kg/s) gravitational constant (m/s 2) vapor enthalpy (kJ/kg) local heat transfer coefficient (W/m2°C) length (m) Nusselt number (dimensionless) heat transfer quantity (kJ/kg) exchanger perimeter (m) Prandtl number (dimensionless) Reynolds number (dimensionless) temperature (°C) overall heat transfer coefficient (W/m2°C) mass fraction (weight LiBr) heat conductivity (W/m°C) density (kg/m 3) dynamic viscosity (kg/ms) wall dynamic viscosity (kg/ms)

Subscripts ABS absorber E evaporator e wall-refrigerant ER inlet refrigerant ES inlet solution g gas i film-wall L liquid ml logarithmic mean p weak solution R rich solution SR outlet refrigerant SS outlet solution v vapor

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I

A. SALAZAR,

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S. GABSI, Thesis

3

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