Pressure Drop and Flooding in a Packed Column

Baual, Ann Christne Evariso, Evariso, Carla Rae Gregorio, Justn Edrik Vinluan, Justn Timohy

Pressure Drop and Flooding in a Packed Column

I.

Introduction Chemical processes usually occur as mixture of different components in the gas, liquid

or solid phase. Separation processes such as absorption, distillation, liquid-liquid extraction, leaching, membrane processing, crystalliation, adsorption and ion exchange can be used to separate or remo!e one or more components components from its original mixture by contacting this mixture "ith another phase. phase. For the solute#s to diffuse diffuse from one phase to the other, other, the t"o phases "hich are some"hat miscible to each other are brought into more or less intimate contact. $he components of the original mixture reallocate bet"een t"o phases during contact and these phases are separated by simple physical methods. %&eankoplis, '(()* &as absorption is a unit operation in "hich soluble components of a gas mixture are dissol!ed in a liquid. $o make an intimate contact bet"een the gas and liquid "hich usually flo"s flo"s counter counter currently currently,, packing packing elements elements are placed placed in the !ertical !ertical,, cylindric cylindrical al columns columns or  to"ers. to"ers.

$hese $hese packing packing elements elements also ser!e to pro!ide de!elop de!elopment ment of interfac interfacial ial surface

through "hich mass transfer takes place. %Perry, '((+*   packed column are mainly use for counter-current counter-current gas-liquid flo" for heat and mass transfer. n a packed column, liquid, dri!en by gra!ity flo"s do"n through the random structured packing packing and is charact characterie eried d by lo" pressure pressure drop and high operati operating ng range. range. %acko"i %acko"iak, ak, '(/(* ost commonly kno"n packing materials are raschig rings, berl saddle, pall ring, intalox metal metal %$P* %$P* and 0aeger metal metal tri-pack. tri-pack. %&eanko %&eankoplis plis,, '(()* '(()*  packing packing has three three importan importantt geometrical characteristics1 surface area, sie and !oid fraction %free !olume*. $he !olume of  the free space of the packing is packing !oid fraction "hich is related to /m ) of its !olume. 2hile the area related to /m ) of the !olume the packing is its specific surface area. %3ole!, '((4* 5ne of the important hydrodynamic parameter of the packed bed is the packing pressure drop "hich is equal the difference bet"een the pressures at outlet and inlet of the packing. Pressure drop is usually presented as a function of the gas superficial !elocity %3ole!, %3ole!, '((4*

"hich starts to rise at a faster gas rate. Superficial gas !elocity is the gas !elocity through a pipe assuming that there is no obstruction present in the system. &as flo" rate is directly proportional to the liquid hold-up or accumulation "hich causes flooding. $he liquid can no longer flo" do"n through the packing and is blo"n out "ith air at the flooding point. $he to"er cannot operate abo!e the flooding !elocity. nd the optimum economic gas !elocity is about one-half or more of the flooding !elocity. %&eankoplis, '(()*

Ergun Type Equation $he 6rgun equation is deri!ed by Chemical 6ngineer Sabri 6rgun in /78'. t is an estimation of the pressure drop through a packed bed due to rate of fluid flo", fluid properties, density of packing and physical properties of the packing material %Sandidge, Shin, 9egaFuentes, : 2illiams, '((8*. $herefor 6rgun modified ;eynolds , density ?, !oid fraction @ and particle diameter D p. %Auan Bia : la!ka, '((7*

%/*

6rgun also defined the friction factor f p as a function of pressure drop P, the length of the packed bed E, particle diameter, !oid fraction, superficial !elocity and density gi!en by equation %'*.

%'*

 fter se!eral experiments "ith different packing material "ith different flo" rates, 6rgun "as able to formulate the general form of the equation1

%)*

$o calculate for the pressure drop, the 6rgun equation can be defined as1

%*

Leva-type Equation ax Ee!a in /787 created a semi-empirical equation for the prediction of minimum fluidiation !elocity u mf   for gas fluidiation as a function of particle diameter D p, density ? and !iscosity > as sho"n belo"1

 %8*

n order for the Ee!a equation to be used in liquid phase, it is modified using experimental data for liquid phase and making an equation fitting the data %ohammed, '((7*. $he modified Ee!a equation can be "ritten as follo"s1

%4*

$he pressure drop prediction may be estimated by the correlation gi!en by1

2here modified friction factor f m is a function of of particle diameter D p, density ?, shape factor G, exponent as a function of ;eH n, !oid fraction @, pressure drop p, length of bed E and fluid superficial mass !elocity &.

Robbins Equation ;obbins correlation for pressure drop in a random particle packed to"ers are based on a dry packing factor unlike the most manufacturerHs published !alue "hich is based on packing factor from "et data %Eud"ig, /77I*. ;obbins generalied equation of pressure drop for random to"er packings is gi!en by1

2here P is the total pressure drop, & f  is gas loading factor, E f  is liquid loading factor, F pd is a dry packing factor, ? is density and > stands for !iscosity. $here are se!eral methods in using ;obbins depending on different parameters and requires careful attention to dimensions. o"e!er the use of the equation has been simplified through the introduction of Fig. /. %Perry, '((+*

Figure 1: The Robbins generalized pressure-drop correlation. Fro! L. Robbins "he!. Eng. #rogr. $ay 1%%1. p.&'(

$his experiment aims to determine the !oid fractions of the packed beds, to determine the effects of liquid holdups on the pressure drop of the packed column and to determine the packing factor experimentally "ith the use of flooding !elocity calculations.

II.

Methodology

Preliminary $he dimensions of the packed to"er and packings "ere recorded. $he sump tank containing the "ater "as cleaned and refilled to I8J of its capacity. 5n-off s"itch, knobs, flo" meter and drainage !al!es "ere closed#turned off "hile the !al!e if the return line and all pressure taps "ere opened. ll liquid in the tubes connected to the pressure taps "ere also drained.

Start-Up $he equipment "as turned on as "ell as the compressor and pump. $he air flo" rate "as set at maximum for /8 minutes to dry the remaining "ater in the to"er.

Pressure Drop of dry and wet packings Starting from '( E#min, the air flo" rate "as increased by /( E#min until it reached the maximum flo" rate of /( E#min. $he "ater flo" rate "as set to ero. fter each ad0ustment, the pressure drop in the manometer, containing colored "ater, "as measured and recorded. Knlike the pre!ious process, "ater flo" rate "as no" increased periodically for all the gi!en air flo" rates. $he pressure drop of the manometer "as measured and recorded. $he air flo" rate increment "as stopped "hene!er flooding occurs in the packed column.

Shutdown $he packed to"er "as drained of all liquid and the pumped "as turned off. Similar to the start-up process, the air flo" rate "as set at maximum for /8 mins to allo" the packed to"er to dry. $he compressor and on-off s"itch "ere turned off.

III.

Treatment and Discussion of Results

I.

!nswers to "uestions

These are some of he characeristcs ha a acking should have for i o !e emloyed in mass ransfer oeraton" #a$ he shae of he ackings should avoid sagnan ools of li%uid, ra gas !u!!les and violen changes in he directon of he gas& #!$ ' should imrove (e)ng and li%uid disri!uton& #c$ ' should rovide more oen area for vaor rise& #d$ The rato of o(er diameer o random acking should !e greaer han en& #e$ ' should have highly corrosive service& #f$ *ue o he ossi!iliy of deformaton, lastc acking should !e limied o an unsuored deh of +-+. / #0-1 m$ (hile meal acking can

(ihsand 2-2. / #3-4&3 m$& #g$ 5acking facors& The caaciy, e6ciency and ressure dro characeristcs vary (ih acking si7e and ye& The higher surface area, he more e6ciency acking& 5acking facors are indicaors of caaciy& The lo(er acking facors, he higher caaciy& 8oe ha ceramic acking (ill !e he 9rs choice for corrosive li%uids, !u ceramics are unsuia!le for use (ih srong alkalis& 5lastc acking are a:acked !y some organic solvens and can only !e used u o moderae emeraures, so are unsuia!le for distllaton columns& ;here he column oeraton is likely o !e unsa!le, meal rings should !e seci9ed, as ceramic acking is easily !roken& A acked !ed column conains a suor lae, a li%uid disri!uor, and a mis eliminaor& The li%uid sream