Formal Lab Report-packed Beds

7/24/2019 Formal Lab Report-packed Beds

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DELA CRUZ, Patricia Bianca B. Date Performed: March 3, 20154 ChE B Date Submitted:April 25, 2015

Problem B1

Pressure Drop ad !loodi" i a Pa#$ed Colum

%& %trodu#tio

Packed towers occur in almost all chemical plants for separation processes such

as gas asorption, sol!ent e"traction, distillation or chemical reactions. #he packed

column in figure 1 consists of a gas and li$uid inlet and outlet, a distriuting space at the

top and ottom, and importantl%, the packings. #he entering gas flows from the

distriuting space elow the packed section to the packing interstices where it contacts

the descending li$uid. &t also operates in a wa% where two different fluid phases,

particularl% gas and li$uid, were allowed to flow countercurrentl% enaling a chemical

component, known as solute, to e transferred from one phase to the other phase.

'igure 1 Packed (olumn

Meanwhile, the packings pro!ide the large surface area needed for intimate contact

etween the li$uid and the gas phase. As shown in figure 2, the most commonl% used

commercial packings are raschig rings, lessing rings, erl saddles, and pall rings )1*.


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'igure 2 Most commonl% used packings+ a- raschig rings, - lessing rings, c- erl

saddles, and d- pall rings

#his e"periment mostl% deals with the gas asorption separation process

in!ol!ing the airwater s%stem. /ne of the oecti!es of this e"periment is the

determination of !oid fractions of the packed eds. &n gasli$uid flow s%stems, !oid

fraction is defined as the fraction of the channel crosssectional area that is occupied %

the gas phase )2*. &t is one of the most important parameters used to characterie two

phase flows and ha!e a fundamental importance in models predicting the pressure drop)3*.

/ther oecti!es for this e"periment are the determination of the effects of li$uid

holdups on the pressure drop of the packed column and determination of packing factor

e"perimentall% with the use of flooding !elocit% calculations. 'rom a fluid mechanical

perspecti!e, the most important issue is that of the pressure drop re$uired for the li$uid

or the gas to flow through the column at a specified flow rate. rgun e$uation is one of

the man% e$uations to sol!e for the pressure drop across a packed ed length ut with

the limitation of onl% ha!ing an a!erage of 0. !oid fraction )*.

P

Z =

150 vo(1 )2

2Dp

2 +

1.75g vo2 (1 )

3Dp

Ergun Equation

4here !osuperficial gas !elocit%, p is the particle diameter, 6 is gas !iscosit% and 7

represents the !oid fraction. Also, the rgun e$uation descries flow for oth laminar

and turulent. 8owe!er, one e$uation that was onl% applicale for a laminar flow was %

Blake9oen% which is actuall% the first term of the right side of the rgun e$uation.

Another separate e$uation % BurkePlummer was the second term of the rgun

e$uation applicale onl% for turulent flows. Meanwhile, 'ahien and :chri!er ga!e a


3/18

modified rgun e$uation for computing pressure drop as function of porosit% as shown

elow )1*.

L= 136

(1)0.38

;aminar 'low

T= 29

(1)1.45 2+1.87

0 /75N , p

(1)0.26 #urulent 'low

I=q L+(1q)T &ntermediate


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Sump Tank Water Pump

Air Flow Meter

Water-Dye Manometer

Air Pump

Air and Water Knobs

Equipment On-O Swit!"

Air Flow #al$eWater Flow #al$e

Dis!"ar%e Pipe #al$e

&olumn

Pa!ked 'eds

Water Flow Meter

Fpd=6(1)

3Dp

%%& 'ethodolo"(

'or this e"periment, the Armfield @as;i$uid Asorption (olumn apparatus was

used as shown on the figure elow.

'igure 1 Armfield @as;i$uid Asorption (olumn

Before the e"periment was conducted, length of the packed eds and the

diameter of the gas column were first measured. All remaining water in the e$uipment

was also drained and the sump tank was cleaned. Afterwhich, the sump tank was filled

again with water up to >5 of its capacit%. 'urthermore, the onoff switch and knos

were turned off as depicted % the figure elow.


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'igure 2 #urned off+ e$uipment switch left-, air and water knos right-

#he air and water flow !al!e together with the drainage !al!e found at the ottom of the

sump tank was also closed. /n the other hand, the discharge pipe !al!e and all

pressure taps were opened.

'or startup, the switch was turned on to run the air pump where the flow rate

was set to 150 ;min for 15 minutes for the remo!al of an% water in the column. #he

threewa% glass cocks were also adusted such that all the gas flowing were directed to

the manometer alread% containing water and a redorange d%e.

/n the e"periment proper, the air control !al!e was throttled ack to C0 ;min.

#he differential pressure in mm82/ was measured and recorded accordingl%.

Afterwards, the gas flow rate was increased with an increment of ten 10- up to the 150

;min flow rate accompanied % the measurement of differential pressure for each

inter!al. #he procedure was repeated ut with different water flow rates from 1 ;min upto > ;min with an increment of one e"cept that pressure was also recorded for the

water flow rate of C.5 ;min.

'or a proper shutdown of the e$uipment, all water was drained with the gas rate

set to 150 ;min for 15 minutes. 'inall%, the pump and the switch were turned off

properl%.

%%%& Results ad Dis#ussio

'rom the raw e"perimental data of pressure difference ased on the manometer

fluid height, pressure drop was computed as follows with a specific gra!it% of 1.0.

Cg

hgP

=


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where densit% D- is for water at 25o(.

#ale 1. "perimental Pressure rop

L%)U%D!L*+ RA,E

-L.mi/

0.0 1.0 2.0 3.0 .0 5.0 C.0 C.5 >.0

A%R !L*+RA,E -L.mi/

PRESSURE DR*P -lb.ft0/

20 0. 0.? 0.? 1.C 1.C 2.0 2.0 2.0 2.0

30 0. 0.? 1.2 2.0 2. 2. .1 .5 C.1

0 0. 0.? 1.2 2. 2.= .5 C.= =.0 10.C

50 0.? 0.? 2.0 3.> .1 ?.2 10.C 1C.3 1C.>

C0 1.C 1.C 2. 5.3 5.> 13.= 21.2 23.2 flood

>0 2.0 2.0 3.> >.3 >.> 1=.C 32.2 3=.5

?0 2.= 3.3 5.> 10.C 11. 2=.? flood flood

=0 3.3 .5 >.3 15.= 1C.3 3?.3100 3.> 5.3 11. 1>.5 22.? flood

110 .5 C.5 11.? 20.0 33.

120 5.3 >.> 13. 22.0 1.2

130 5.> ?.2 1.> 2.= 51.

10 C.1 =. 15.5 32.C flood

150 C.5 =.? 1C.3 33.

'rom the computed data, pressure drop increases as the air and li$uid water flow

rate increases. #he highest pressure drop reading was 51. lft2where li$uid and air

flow rates were ;min and 130 ;min respecti!el%. 4ith a gas flow rate of 10 ;min

and same flow rate for water, li$uid accumulation at the top of the packings was

oser!ed signaling flooding. ue to this oser!ation, flooding is therefore defined as the

condition where a large pressure drop occurs with a small change in gas !elocit%.

Additionall%, the lowest air flow rate that produces flooding was C0 ;min with the

corresponding ma"imum allowale li$uid flow rate of > ;min. #hus, flooding could e

also oser!ed with lower air flow rate when li$uid flow rate was high.

Eoid fractions were computed using the formula of 'ahien and :chri!er

e"pressing pressure drop as function of porosit% which was the modified e$uation of

rgun. 'or this e"periment, the following were the !oid fractions otained.

#ale 2. Eoid 'ractions 7- for ifferent Air 'low


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L%)U%D!L*+ RA,E

-L.mi/0.0 1.0 2.0 3.0 .0 5.0 C.0 C.5 >.0

A%R !L*+RA,E

-L.mi/

*%D !RAC,%*2S -3/

200.?C

50.3?

0.3?

0.2?=

?0.2?=

?0.2>2

50.2>2

50.2>2

50.2>2

5

300.5C

00.3?C

20.3?

0.30

50.2?=

?0.2?=

?0.251

0.2

>0.22

0

00.1

0.1

0.3>5

00.313

20.300

50.2C5

0.23

>0.21>

=0.20>

5

500.3>

10.3>

10.3?

50.2=?

20.2?=

?0.23?

?0.221

0.1=5

30.1=3

=

C00.3?C

20.3?C

20.3?

50.2?3

50.2>>

?0.21C

00.1=0

>0.1?5

C flood

>00.3>=

C0.3>=

C0.32C

30.2>0

0.2CC

30.20

30.1>C

30.1C5

?

?00.3C0

C0.3?

50.300

50.253

20.2?

00.1?>

? flood flood

=00.35=

20.330

>0.2?=

?0.233

0.231

>0.1?0

1000.35?

00.325

0.2C

00.233

=0.21C

> flood

1100.3?

0.315

?0.2C?

50.231

50.1==

120

0.31

3

0.30?

>

0.2C5

0.230

=

0.1=2

1300.31

?0.311

10.2C

=0.22?

10.1?

5

100.32

30.305

C0.2CC

30.215

5 flood

1500.32

>0.30>

?0.2C>

>0.21?

3

Ealues of computed !oid fractions range from 0.?C5 to 0.1C5?. #he lowest air

and li$uid flow rates showed the highest !oid fraction. 4hile, an air flow rate of >0 ;min

with C.5 ;min li$uid water flow rate otained the lowest !alue of !oid fraction. Also, !oidfraction !alues decrease with increasing flow rates for oth air and water.

Meanwhile,


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( )

=

1

0

Re

vDN

P

where 6 and D is for air.

'or dr% packings where water flow rate was e$ui!alent to 0 ;min, pressure drop !ersus

the


9/18

*)))) *0)))

-,*))))

-)*0)))

)*))))

)*0)))

,*))))

,*0)))

*))))

12)

12,

1212(

12.

120

12/

12/*0

123

Log (G)

Log (DP/Z)

'igure . Plot of ;og PH- !s ;og @- for ifferent ;i$uid 'low


10/18

'igure 5. Plot of Pressure rop P- !s @as ;oading 'actors @f-

;oading one is the enhancement of mass transfer ut as rates were increased

further, flooding occurs. #his results when gas !elocit% also ecomes the function of the

li$uid holdup instead of ust a function of li$uid rate. &t is where the pressure drop

increases at an accelerated rate that e!entuall% leads to flooding. #hus, in figure 5, the

loading one was descried % the shaded region.

Packing factors calculated for different !olumetric flow rates of water where

shown on tale 3. A!erage packing factors was also otained from the calculated

porosit% !oid fraction- !alues in each li$uid flow rate. 'rom the tale, it was oser!ed

that packing factors increases with also an increasing li$uid flow rate.

#ale 2. Packing 'actors of ifferent ;i$uid 'low 3=.10

3 132C.C0

1?50.?>

5 1?>>?.?C

C 20??5.C0

C.5 25?2.5

> 20?==.5=

'or dr% packings, pressure drop calculated from the rgun and


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#ale 3. Pressure rop ased on "perimental ata, rgun $uation and


12/18

tale 3, oth the rgun and


13/18

and gas phase since it pro!ides large contact area etween the two 2- phases.

#he fresh li$uid entering from the top of the tower flows countercurrentl% with the

gas phase and asored the solute present in the richgas thus, lean gas lea!es

the top. #he soluteenriched li$uid flows down where concentrated li$uid lea!es

the ottom of the tower through the li$uid outlet.

3. ifferentiate etween static and d%namic or operating holdup. 8ow does this

affect the pressure drop through a packed columnK

:tatic li$uid holdup is defined as the !olume fraction of li$uid that remains in

the ed after complete draining while the d%namic or operating li$uid holdup is a

freedraining li$uid not contained in the particles of the packed ed and collects

at the ottom of the column after a sudden shutoff of the li$uid feed )>*.

;i$uid holdup is a function of the li$uid rate onl% up to the loading region.

4hen loading region is entered, it also ecomes a function of the gas !elocit%.

#he holdup uilds up as the gas flow rate is increased, there%, resulting in the

reduction of free space. &n conse$uence, the pressure drop also increases at an

accelerated rate and e!entuall% leads to flooding )C*.

. efine loading and channelingK @i!e the rele!ance of these two factors inpacked column operation.

;oading is characteried % a mild li$uid uildup on the packing where

packed column operation is fre$uentl% most economical in this loading region.

#his also gi!es reasonal% high capacit% coefficient since the packing is fairl%

well wetted and pressure drops are still comparati!el% low )C*.

/n the other hand, channeling occurs when the fluid flowing through the

packed ed finds a preferred pathN through the ed. #his effect happens when

li$uid films grow thicker in some places of the packing surface while thinner in

others, thus, the li$uid collects into small ri!ulets and flows along localied paths

through the packing. &n low li$uid rates, the packing surface is most likel% dr% or

co!ered % a stagnant film of li$uid resulting in the poor performance of large

packed towers especiall% when filled with stacked packings )?*.


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5. 8ow does the packing factor otained from the flooding !elocit% differ from the

one estimated empiricall% with the use of the correlation of ;oo et alK

Packing factor otained from the flooding !elocit% considers the point

where flooding occurs. 8owe!er, packing factor estimated empiricall% from the

correlation of ;oo et al is onl% dependent on the ed porosit% and does not

consider flooding. #hus, packing factor !alue from the ;oo et al correlation is

different from the !alue otained from flooding !elocit%.

& Co#lusio

#he !oid fractions in packed eds, pressure drops and packing factor were

successfull% determined in the e"periment. A!erage !oid fraction for dr% packings was

calculated as 0.3?2. /n the other hand, porosit% near all flooding point showed a

smaller !alue. #hus, further decrease in porosit% results ecause li$uid holdups take up

space inside the packings which then e!entuall% leads to flooding. (onse$uentl%,

pressure drop was also large when li$uid holdup was oser!ed ecause gas flow could

not pass through without disturance of the li$uid holdup. Meanwhile, C mm ceramic

raschig rings used in calculation ha!e an effecti!e diameter of 0.22 inch with C2 !oid

fraction and a dr% packing factor of 5350m. Packing factor otained for dr% packingpackings e"perimentall% was 3C=C.>5ft or 1212?.m.

;astl%, it was also concluded that flooding is an important matter in packed tower

applications and the appropriate t%pe of packing material was also of importance to

calculate the pressure drop and flooding.


15/18

%& Refere#es

1. @eankoplis, (. O. 1==5-. :tage and (ontinuous @as;i$uid :eparation

Processes. &n (. O. @eankoplis, Transport Processes and Unit Operations3rd

ed., pp. 5?C32-. :ingapore+ Prentice 8all &nternational.

2. 8ewitt, @. '. 2011-. Eoid 'raction. Thermopedia.

doi+10.1C15AtoH.!.!oidfraction

3. n.d.-. Eoid 'ractions in #woPhase 'lows. &n Engineering Data Book IIIpp. 1>1

1>33-.

. 'ahien,


16/18

5. Perr%, -. Perrys !hemical Engineers "and#ook>th ed.-.

Gew Qork+ Mc@raw8ill Book(o.

C. ;e!a, M. 1=53-. To$er Packings and Packed To$er Design./hio+ #he Jnited

:tates :tone 4are (ompan%. C2=S!iewI1upSse$I>

>. de 9lerk, A. 2003-. ;i$uid 8oldup in Packed Beds at ;ow Mass 'lu".%I!hE

&ournal' ()C-, 15=>2000.

?. Mc(ae, 4. ;., :mith, O. (., R 8arriott, P. 200C-. @as Asorption. &n 4. ;.

Mc(ae, O. (. :mith, R P. 8arriott, Unit Operations of !hemical Engineeringpp.

5C5C12-. Gew Qork+ Mc@raw8ill.

%%& Appedi#es

Appedi A Ra6 Data of Pressure Differe#e i #etimeter

L%)U%D!L*+ RA,E

-L.mi/0.0 1.0 2.0 3.0 .0 5.0 C.0 C.5 >.0

A%R !L*+

RA,E -L.mi/ PRESSURE D%!!ERE2CE -#m/

20 0.2 0. 0. 0.? 0.? 1.0 1.0 1.0 1.0

30 0.2 0. 0.C 1.0 1.2 1.2 2.0 2.2 3.0

0 0.2 0. 0.C 1.2 1. 2.2 3. . 5.2

50 0. 0. 1.0 1.? 2.0 .0 5.2 ?.0 ?.2

C0 0.? 0.? 1.2 2.C 2.? C.? 10. 11. flood


17/18

>0 1.0 1.0 1.? 3.C 3.? =.C 15.? 1=.

?0 1. 1.C 2.? 5.2 5.C 1.C flood flood

=0 1.C 2.2 3.C >.? ?.0 1?.?

100 1.? 2.C 5.C ?.C 11.2 flood

110 2.2 3.2 5.? =.? 1C.120 2.C 3.? C.C 10.? 20.2

130 2.? .0 >.2 12.2 25.2

10 3.0 .C >.C 1C.0 flood

150 3.2 .? ?.0 1C.

Appedi B Properties Used i Cal#ulatiosDensitywater

50& 612/*,,)

,3 lb78t(

9ra$ity % 2 (* 8t7s

9ra$ity&onstant %&2 (*

:lbm*8t;7:lb8*s;

Diameter!olumn D 2 34*0 mm

Densityair 5 0&6%2

)*)3(/43

lbm78t(

Diameterparti!le Dp2 / mm#is!osityair 5

0&


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