absorption characteristics of ammonia-water system in the cylindrical tube absorber

7/29/2019 Absorption Characteristics of Ammonia-water System in the Cylindrical Tube Absorber

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_Corea~ J. Ch in . E ~ , 19~ 1 h :-:7-9203021

Absorp tion Characteristics of Ammonia-W ater Systemin the Cylindrical Tube AbsorberK i - B o n g L e e , B y u n g - H e e C h u n , J a e - C h e ol L e e , C h a n ~ I i n P a r k * a n d S u n g - H y u n K i m t

Depatlm ent of Chemical Engineering, Kore a Univers ity , Seoul 136-701, Kor ea*Department of Environmental Engineering, Junior College o f Inchon, Inchon 402-749, Ko rea

(Rec~zved 5 _~S~rch 2c ~ l 9 ~ cc~ p t ~ d 2 7 J z dy 2001 )

Ab stra ot-E xpe rim enta l analysis was per fom led in a cylindiicsi tube absorber which is considered to be suitablefor the lml:hle mode. Char acteri~ cs such as concentration, temperature, and pressure were m eam red and they reflectedthe cond ition of absorber wall. The variation o f characteristics was co nspicuo ns near the inlet region o f the amm oniagas. The ammonia gas and the solution flowed cocurrenfly and countercurrenfly and the results were compared.Ke y words: A bsorption System. Ammor~a-water, Cylinc~ical Tube Absorber , Mass Transfer, Ga s Holdup

I N T R O D U C T I O NDue to the ozone depletion problem associated with the use of

CFC a nd HCF C refiigemnts, absorption heat pu mp s and refiigera-tion systems have gained increasing interest in recent years Moreand more, they are regarded not only as environmen|ally fi'iendlyaltmaatives to CFC based systems, but also as energy efficient heat-hag and cooling tech nolo gy [Herald et at, 1996].

Th e absorber, uahieh is a major com ponen t in absorption refiig-eration systems, greatly affects the overall system perfonn ance. B ub-ble mode has been recom mended to enhance heat and mass Wans-fer perfon nance in anmo nia-w.ater absorption systems [Chtistensenet al,, 1996]. Falling film modes provide relatively high heat trans-fer coefficients and are stable during operation. However, fallingfilm modes have wettability problems and need good liquid dis-tributors at the inlet of the liquid flow. Bub ble mo des provide notonly high hea t transfer coefficients but also go od wetmbility andmixi ng bm,eeen the liquid and the vapor. However, the bubb le mode srequire vapor dislribution. Generally, vapor distribution is e~sierthan liquid dg~lmt ion. Recently, bubble mode s were recomm endedstrongly for ammonia-water absorption systems becmse the lowwettability in the falling film mode s is critical to the perfonn anceo f the sy stem [Ch ristensen et al., 1 996: Han et al., 1985].

Over the last ten year~ ammonia-water bubble mode has beenextensively investigated both numerically and analytically [Herbineand Perez-Blanco, 19 95: Kang et al., 1997: "& et al., 2001: Sung ,et al., 2000: Tsutsumi et al., 1999: Jang et al., 1999: Hwang et al.,1999: Lira et al., 1999: Noda et al., 2000]. However, few papershave been found concerning the heat and mass lransfer analysis ofthe bubble absorber. Merrill et al, tested three co mpa ct bubble ab-sorbers developed for generator-absorber hen exchange absorptioncycles (GAX ) [Mewill et al, 1998]. Their resalls show that enhance-ment techniques are effective in reducing absorber length and in-

TTo wh om correspondence should be addressedE-mall: [email protected]*Presented atthe Inf/Syrup. on Chem. Eng. (Cheju, Feb. 8-10, 2001)dedicated to Pros I-I. S. Chun on the occasion ofh ls retirement fromKor ea Ur~versity.

creased tube diameters may increase absorber performance.In the present study, the characteristics of the bubble mo de in

the cylinckical tube absorber were studied by experknent. Th e resultscan be used as fimdumental datato design the optimal cylindricaltube absorber.

E X P E R I M E N TThe schematic diagram o f the cylindrical absorber for the bub-

ble mod e studies is given in Fig. 1. The cylinckical co han n is 1 min height and 0.03 m in diameter. Five sample ports are installed at

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8. Cartridge heater9. Data acquisition system10. Heating tank11. Solution pum p12. Ho w meter13. Neutralization tank

sorbic.1. Absorb er2. Manometer3. N H 3 bomb4. Sampling port5. Thermocouple6. Valve7. Mass flo w controller

Hg . 1. Experhnental abserpfion system for c~Md rical tube ab-


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88 K.-B. Lee et al.0.2 rn intm,als axially and twe lve therm ocouples are set to measurethe behavior cs Iransfer and the thermal state of re.her experi-mental column. Also, seven mana neters are equipped to measure thepresmre profile The cylindrical column made o f a ~ fi c resin is Irans-parent, so the state of bubble flow can be observed In the experi-ments, fl ow rate of @aamoniagas was 1-9 L/rain nd the inlet liquidconcenlration was selected as 20-30 weight% Amm onia gas flow edupward, utfile a nm onia solution flowed b ah upward and downwarc[

A N A L Y S I SThe absorption rate is expressed as the following equations by

using the overall mass Iranffer coetfidents, K [McCabe et at, 1993:Kim et aL 2001-" Cho k 2001: Tsutsumi et al., 1999: Park, 1999].

n ~ = K p A ~ s A x ~ , a (i )[x;g-x,j -[x;~-xo~,lAx~,s - (2)ln[(x~ -x,~),"(x~ -xo.O]

]'he physical Wop~ies needed to determine the eoetfidents werereferred to literature [Reid et al., 1986: K DB ].

Gas holdup, "which is defined as the vo lum e ratio of gas phase

in the binary mixture, is an imporlant paranete r for char act~izingthe absorption performance o f the absorber. The g as holdup influ-ences the interracial area between the liquid and the gas and thusdetelmines the mass transfer rates during the absorption process[C I~ et at, 1978: Fukuji, 1999: Yarnashita et al., 1999: Choi et al.,1992: Mok et at, 1990: Ide et al., 1999: Kim et al., 1990: Park etat, 2000]. The g as holdup can be obtained in(~'ectly from the pres-sure variation:

Po- Pl=plg~=p~gZ1 (3 )Po- e~ ~ g ~ =e,p, Ze. (4)

whe re subscript 0 is standard position, 1 is initial x~Jue wh en onl ysolution flow ed, and 2 is final value when the gas flowe d ~ thesolution.

Dividing Eq. (3) by Eq. (4) and arranging yieldsAP

e g -p l _ : Po (5 )

R E S U L T S A N D D I S C U S S I O N1, Effect of Gas Flo w Ra te on Concentrat ion Prof i le

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Absorption Characteristics of Ammonia-Water Sys tem in the Cylindrical Tube Absorber 89Fig. 2 shows the effect of the gas flow rate on the concentration

profile vvhen the gas and the solution flows are cocutrent At lowgas fl ow rate, the concentration of solution is aknost sam e above012 ml At high gas f lo w rate, the concenlration of solution increasesw~h increasing height of absorber Most of the amm oni aga s wa sabsorbed imm ediately after injection at low gas flo w rate. How -ever. as gas fl ow rme increased, the remaining gas which vcas notabsorbed increased

Fig. 3 shows the effect of the gas flow rate on the concentrationprofile W en the gas md the solution flows are counte~cmlvnt Con-trmy to cocurrent flow , the concentration of solution increases withdecreasing height of ~ ~ l Similar to cocurrent flow, the con-centration of am monia is aknost same above 0.2 m at low gas f lowrate. At high gas fl ow rate, compared to low gas fl ow rate, the un-absorbed gas increa sed so that the length of absorber wh ich wasneeded to absorb gas was increased It is becanse as the amount ofinput gas increased, the capacity of solution that can absorb the gasdecreased. The un-absorbed gas was gradually absorbed as it flowedupward, A s can be seen in Fig. 3, the unlabsorbed gas was more inthe 3~o amm onia solution than in the 2~.o a mm onia solution.2. Ef fect of Gas Flow Rm e on Tem perature lh 'o l i le

Fig. 4 shows the effect of the gas flow rate on the temperm~

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.... ~~": : , : ,~ :T.~ , ' . , : , : , : ' , : , : .7 .~

9 ', " -, ,; :: :,zl _--?,- 'L , '

Fig. 6. The effect of ga~ flow rate o n ga~ h~ du p profile (cocurrent).

gas holdup. The slope is steep in the lower part o f the abs&o er andit is gentle in the higher pat. It is because the ammonia gas re-mained in the lower pa t and the pressure varied sharply as the gaswas absorbed. After the gas was absorbed, the presa~re variationbecame fiat. Pressure variation means the difference between thepressure when only the solution flowed and the pressure when thegas flow ed together. The region where the amm onia gas was mainly

, " ( , FL f : t ~ , ,) :) ~,,~ i'r t~ ': ~ r ;,4~, ',3 ~ [r,~]

Fig. 7. The effect r gas flow rate on gas holdup profile (counter-oarr~t ) .

ab~ogoed c ~ l d be l ~o wn by the t:ressure variation orthe gas holdulxOptimum height of absorber can be obtained by analyzing con-

cenlration, temperature and gas holdup. It is approximately 0.4 mand 0.6 m, when the gas flo w rate is 9 L/ra in md the concentrationof input solution is 20?,5 and 3~,o, respectively.4 . Co m p a r i s o n o f M a s s Tr ~ a s f er Co e f f i ci e n t s

Table 1. Results of mass traasfer coeff icientMass transfer co efl id ent at 209'0 input solu tion [m3/min]

C~s flo w rate [IJmin] I 5 9Plate type absorber Falli ng fil m mod e 1 . 1 8 5 9 10 - 3 1 . 94 2 . 10 - '~ 3 . 81 5 - 10 - '~

Bubble mode 7 . 1 3 1 9 1 0 - 3 2 . 8 8 5 9 10 - '~ 5.7 60 .10 --~Cylinda:ical tube absorber Cocurrent fl ow 1 . 62 7 . 10 --~ 3 . 01 8 . 10 - '~ 8 . 72 8 . 10 --~

Countercurrent flow 1 . 56 4 . 1 0 --~ 3 . 27 5 9 10 - -~ 5.9 10 - 10 --~Mass transfer coeffid ent at 30~ input solution [ m ~ / m i n ]

G-as fl ow rate [L/min] 1 5 9Plate type absorber Falli ng fil m mod e 9 . 2 5 7 . 1 0 - 3 4 . 7 1 2 . 1 0 - '~ 9 . 3 3 4 - 1 0 -'~

Bubble mode 4 . 11 0 . 10 - '~ 8 . 66 4 . 10 - '~ 1 . 65 5 - 10 - 1Cylinda:ical tube absorber Cocurrent fl ow 1 . 8 3 9 . 1 0 - 1 2 . 2 3 4 . 1 0 - 1 2 . 5 8 1 - 1 0 - 1

Countercurrent flow 7 . 4 6 8 . 1 0 --~ 1 . 5 4 6 . 1 0 - 1 2 . 2 2 8 - 1 0 - 1Januaxy, 2002


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Absorpt ion Charact eri st ics & Am mo nia-W ater Sys t em in the C yl indr i ca l Tube A bsorberTable 1 s twnmari zes t he r esul t s of m ass t r ans f er coef f i c i ent wh en

t i l e concenb-a ti c~l of i np ut so lu t ion was 2(~.0 and 3(~ .0 . The r esul t so f p l a t e t y p e a b s o r b e r a r e f r o m t h e f o r m e r s t u d y [ L e e e t a l ., 2 1 )) 0] .T h e m o d e o f c y l i n d ri c a l t u b e a b s o r b e r w a s t h e b u b b l e m o d e b o t hi n t h e c o c m r e n t a n d i n th e c o u n t e r c t a r e n t fl ow . T h i s t a b l e sh o w st h a t th e m a s s t r a n s f e r c o e f f ic i e n t s i n th e b u b b l e m o d e w e r e g r e a t e rf fl sz l t hose in t he f a l l i ng f i lm mod e. I n par t i cu lm, t he m ass b- ans ferp e r f o r m a n c e i n t h e c y l i n d r i c a l t u b e a b s o r b e r w a s e x c e l l e n t T h e r e -s u lt s b o t h i n t h e c o , n - r e n t a n d i n t h e c o u n t e m u r r e n t f l o w w e r e m u c ht h e s a m e c o m p a r e d w i t h th e r e s u l ts i n t h e p l a te t y p e a b s o r b e l :

C O N C L U S I O N SF o r t h e f i ar th e r u n d e r s t a n d i n g o f b u b b l e m o d e , t h e c h a r a c t e ri s -

t i c s o f b u b b l e m o d e i n th e c y l i n & i c a l t u b e w e r e s t u d i ed . T h e a b -s o r p t i o n a n d c o m p l e x f l o w b e h a v i o r i n m u l t i p h a s e f l o w s y s t e m sw e r e i n t e rp r e t e d b y a n a l y z i n g s t a te v a r i a b l e s s u c h a s c o n c e n t r a t i o n ,t erape~atu~e, and gas holdup . T he fo l lowing conc lus ions were dr aw nf r o m t h e p r e s e n t e x p e r i m e n t a l s tu d i e s :

1 . A s t h e g a s f l o w r a t e i n c r e a s e d , t h e c o n c e n t r a t i o n , t e m p e r a t u r ea n d g a s h o l d u p o f s o l u ti o n i n c r e a s e d

2 . W h e n t h e g a s a n d t h e s o l u t i o n w e r e c o c u r r e i ~ t h e c o n c e n t r a -t i o n a n d t e m p e r a t u r e o f s o l u t i o n i n c r e a s e d w i t h i n c r e a si n g h e i g h tof absorber . However , when counter c twrent~ the concent r a t ion andterape~atu~e o f so lut ion increo~sedw i t h d e c r e a si n g h e i g h t o f a b s o r b e l:

3 . F o r t h e g a s h o l d u p , i n t h e l o w e r p a r t o f th e a b s o r b e r t h e s l o p ew a s s t e e p a n d i n t h e h i g h e r p a r t t h e s l o p e w a s g e n t le .

4 . O p t i m u m h e i g h t o f a b s o r b e r c a n b e o b t a i n e d t o a n a l y z e c o n -c e nt ra ti or l~ t e m p e r a t u r e a n d g a s h o l d u p i n t h e a b s o r b e r

A C K N O W L E D G E M E N TT h i s s t u d y w a s s u p p o r t e d b y r e s e a r c h g r a n ts f r o m t h e K o r e a S c i -

e n c e a n d E n g i n e e r i n g F o u n d a t i o n ( K O S E F ) t h r o u g h t h e A p p l i e dRh eolog y Center (AR C) , a n of f i c i a l ERC , a t Kore a Uif ivers it y , Seoul ,K o r e a .

N O M E N C L A T U R EA : are a [ ra~ ']g : g r a v i t a t i o n a l a c c e l e r a t i o n [ m / s 2 ]K : o v e r a l l m a s s t r a n s f e r c o e f f i c ie n t [ m / m i n ]r a : m a s s f l o w r a t e [ k g / r a i n ]P : p r e s s u r e [ m m H g ]x : c o n c e n t r a t i o n [ w e i g h t % ]Z : h e i g h t [ m ]G r e e k L e t t e r sp : d e n s i t y [ k g / m ~]e : h o l d u pS u p e r s c r i p te q : e q u i l i b r i u m

S u b s c r i p t sa b s : a b s o r p t i o n

91g : g a si n : i n p u tl : l i q u i dl m : l o g m e a no u t : o u t p u t

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January, 2002

absorption characteristics of ammonia-water system in the cylindrical tube absorber

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