ABSORBER DESIGN
ABSORBER DESIGN
4.1. Absorber: Gas absorbers are used extensively in industry for separation and purification of gas streams as product recovery devices, and as pollution p ollution control devices. Gas absorbers are most widely used to remove water soluble inorganic contaminants from air streams. Absorption is a process where one or more soluble components of o f a gas mixture are dissolved in a liquid (i.e., a solvent). Solute:
The component of gas that needs to be dissolved in a solvent n our case the solute is ammonia that is dissolved in a lean solution of ammonia and water. Solvent:
The substance that dissolved solute in it is called solvent. !iquids commonly used as solvents include water, mineral oils, nonvolatile hydrocarbon oils, and aqueous solutions. The solvent chosen should have a high solubility for the gas, low vapor pressure, low viscosity, viscosity, and should be relatively inexpensive. Absorption, in chemistr, is ! phsic!l or chemic!l phenomenon or ! process in "hich !toms, molecules, or ions enter some bul# ph!se $ %!s, li&ui' or soli' m!teri!l. (his is ! 'i))erent 'i))erent process )rom !'sorption, !'sorption, since the molecules molecules !re t!#en up b the volume, not b sur)!ce. n gas absorption, soluble vapors are more or less absorbed in the solvent from it s
mixture with inert gas. The purpose of such gas scrubbing operations may be any of the following" •
#or separation of component having the economic value.
•
As a stage in the preparation of some compound.
•
#or removing of undesired component (pollution).
4.*. (pes o) Absorption: • •
$hysical absorption, %hemical Absorption
4.*.1. +hsic!l Absorption: n physical absorption mass transfer ta&e place purely by diffusion and physical absorption is governed by the physical equilibria. $hysical absorption occurs when the absorbed compound dissolves in the solvent. $hysical absorption depends on properties of the gas stream and solvent, such as density and viscosity, as well as specific characteristics of the pollutant in the gas and the liquid stream. These properties are temperature dependent, and lower temperatures generally favor absorption of gases by the solvent. Absorption is also enhanced by greater contacting surface, higher liquid'gas ratios, and higher concentrations in the gas stream.
4.*.*. hemic!l Absorption: %hemical absorption occurs when the absorbed compound and the solvent react. hen oxides of nitrogen absorb in water the chemical reaction ta&e place and nitric acid form this is common example of chemical absorption.
4.-. (pes o) Absorber: There are three maor types of absorbers which are mainly used for absorption purposes* • •
$ac&ed column $late column
4.-.1 +!c#e' (o"er: $ac&ed towers, which are the most commonly, used gas absorbers for pollution control. $ac&ed towers are columns filled with pac&ing materials that provide a large surface area to facilitate contact between the liquid and gas. $ac&ed tower absorbers can achieve higher removal efficiencies, handle higher liquid rates, and have relatively lower water consumption requirements than other types of gas absorbers. +owever, pac&ed towers may also have high system pressure drops, high clogging and fouling potential and extensive maintenance costs due to the presence of pac&ing materials. nstallation, operation, and wastewater disposal costs may also be higher for pac&ed bed absorbers than for other absorbers.
+!c#e' column 1
4.-.* +l!te (o"er: $late, or tray, towers are vertical cylinders in which the liquid and gas are contacted in stepwise fashion on trays (plates). !iquid enters at the top of the column and flows across each plate and through a downspout (down comer) to the plates below. Gas moves upwards through openings in the plates, bubbles into the liquid, and passes to the plate above. $late towers are easier to clean and tend to handle large temperature fluctuations better than pac&ed towers do. +owever, at high gas flow rates, plate towers exhibit larger pressure drops and have larger liquid holdups.
4.-.-. !n'
Rel!tive +!c#e'
erits o) +l!te (o"ers:
+l!te column
+!c#e' column
!n h!n'le "i'e r!n%e o) li&ui' r!tes /loo'in% c!n occur 'ue to )luctu!tion in li&ui' "ithout )loo'in%
r!tes
/or l!r%e 'i!meter column
/or sm!ll 'i!meter column
!nnot be use' )or hi%hl corrosive +!c#e' to"ers prove to be che!per !n' e!sier li&ui's
to construct i) hi%hl corrosive )lui's must be h!n'le'
+ressure 'rop more
+ressure 'rop is lo"
(ot!l "ei%ht o) 'r pl!te to"er is less (ot!l "ei%ht o) p!c#e' to"er is hi%h th!n th!n p!c#e' to"er
pl!te to"er
E0pensive
ess e0pensive
The choice between use of a plate tower or a pac&ed tower for a given mass'transfer operation should, theoretically, be based on a detailed cost analysis for the two types of contactors. n many cases, however, the decision can be made on the basis of a qualitative analysis of the relative advantages and disadvantages. The following general advantages and disadvantages of plate and pac&ed towers should be considered when a choice must be made between the two types of contactors*
4.4. Absorber Sstem on)i%ur!tion: Gas and liquid flow through an absorber may be • • •
%ountercurrent %rosscurrent %o current.
4.4.1. ountercurrent: The most commonly installed designs are countercurrent, in which the waste gas stream enters at the bottom of the absorber column and exits at the top. %onversely, the solvent stream enters at the top and exits at the bottom. %ountercurrent designs provide the highest theoretical removal efficiency because gas with the lowest pollutant concentration contacts liquid with the lowest
pollutant concentration. This serves to maximie the average driving force for absorption throughout the column.
4.4.*. rosscurrent: n a crosscurrent tower, the waste gas flows horiontally across the column while the solvent flows vertically down the column. As a rule, crosscurrent designs have lower pressure drops and require lower liquid'to'gas ratios than both co current and countercurrent designs. They are applicable when gases are highly soluble, since they offer less contact time for absorption.
4.4.4. o current: n co current towers, both the waste gas and solvent enter the column at the top of the tower and exit at the bottom. %o current designs have lower pressure drops, are not subect to flooding limitation. %o current designs are only efficient where large absorption driving forces are available. -emoval efficiency is limited since the gas'liquid system approaches equilibrium at the bottom of the tower.
4.2. +!c#e' (o"er Intern!ls: 4.2.1. (o"er Shell: The tower shell may be made of steel or plastic, or some combination which may require the addition of liners or inner layers of rubber, plastic or bric&. The mechanical problems of attaching depending on the corrosiveness of the gas and liquid streams, and the process operating conditions.
4.2.*. ist Elimin!tor: At high gas velocities, the gas exiting the top of the column may carry off droplets of liquid as a mist. To prevent this, a mist eliminator in the form of corrugated sheets or a layer of mesh can be installed at the top of the column to collect the liquid droplets, which coalesce and fall bac& into the column.
4.2.4. +!c#in%: $ac&ing materials provide a large wetted surface for the gas stream maximiing the area available for mass transfer. $ac&ing materials are available in a variety of forms, each having specific characteristics with respect to surface area, pressure drop, weight, corrosion resistance, and cost.
4.3. +!c#in% Selection: $ac&ing materials are categoried as random or structured. • •
umped tower pac&ing /tac&ed tower pac&ing
4.3.1. Dumpe' (o"er +!c#in%: -andom pac&ing as the name implied, are dumped into a column during installation and allowed to fall in random. /mall pac&ing0s poured randomly into a vessel is certainly the more popular and commonly employed form of pac&ed'tower design. +owever, in certain instances where exceptionally low pressure drop and very high flow rates are involved, stac&ed or oriented pac&ing have also been used. -andom pac&ing0s are usually dumped into an absorption column and allowed to settle.
4.3.*.
St!c#e'
(o"er +!c#in%:
/tructured pac&ing0s are considerably more expensive per unit volume than random pac&ing0s. They come with different sies and are neatly stac&ed in the column. /tructure pac&ing usually offer less pressure drop and have higher efficiency and capacity than random pac&ing. /tructured pac&ing may be random pac&ing connected in an orderly arrangement,
4.. (pes o) +!c#in%: 4..1. +!ll rin%s: $all ring that is improved on the basis of rashing rings. The design of pall rings provide higher capacity and lower pressure drop than other pac&ing the open cylindrical walls and inward bends protrusions of pall rings allow greater capacity and lower pressure drop. !ower pressure drop (less than half) than -aschig rings, also lower +T1 (in some systems also lower than 2erl saddles), higher flooding limit. Good liquid distribution, high capacity. %onsiderable side thrust on column wall. Available in metal, plastic and ceramic. These are of two types*
4..*. et!l +!ll Rin%s: The rings are made up of metal.
4..-. +l!stic +!ll Rin%s:
The rings are made up of plastic material.
4.5. Distributor: istribution of the liquid onto the pac&ed bed or structured pac&ing is realsed by appropriate liquid distributors. t is important to distribute the liquid flow equally across the column area in order to secure an intensive mass transfer between the phases. n addition to the tas& of regular liquid distribution the part has to meet following requirements* •
pressure drop in the gas phase should be low
•
part should be resistant against dirt or solids in the liquid
•
high turn down ratio
•
low entrainment of droplets
•
prevention of irregular gas distribution
•
prevention of wall effect on liquid flow
istributors are used for the good distribution of liquid over the pac&ing so that the liquid come in contact properly with incoming gas.
4.5.1. (pes o) (he Distributors: The following types of liquid distributors are available* •
orifice distributor
•
trough distributor
•
rough'type distributor
•
ladder'type distributor
•
spray nole'type distributor
4.5.1.1. (rou%h Distributor: Trough distributor provides good distribution under widely varying flow rates of gas and liquid. the liquid may flow through simple 3 notched weirs, or it may flow through tubes that extend from troughs to near the upper level of the pac&ings.some deposition of solids can be accommodated. 2ecause of its large free area at is suitable for high gas rates
4rifice trough liquid distributor
4.5.1.*. Ori)ice Distributor: 4rifice distributor type which gives very fine distribution though it must be correctly sied for a particular duty and should not be used where is dis& of plugging. The orifice riser distributor is
designed to lay the liquid carefully onto the bed, with a minimum of contact with gas during the process.
4.5.1.4. +er)or!te' +ipe Distributor: The perforated ring type of distributor for use with absorption columns where high gas rates and relatively small liquid rates are ecounter.this is specially suitable where pressure loss must be minimied, for the larger sie of tower where installation through manholes is necessary, it may be made up in flanged sections. The orifices are of 5 to 6 mm in diameter, and can be subect to plugging if foreign material is present. The pipe must be carefully leveled for larger diameter column.
4.5.*. Re'istributors: The liquid coming down through the pac&ing and on the wall of the tower should be redistributed after a bed depth of approximately 5 tower diameter for rashing rings and 7'89 tower diameters for saddle pac&ings.%ollector:-edistributors, is very similar to the distributor in that it will
contain a dec& and chimneys. The collector is used under a pac&ed bed section to collect the liquid to aid in mixing and redistribution. The difference is that the redistributors will contain caps or hats to prevent the water falling from the pac&ing from bypassing the collector.
4.6. Support +l!tes: These are the simplest and least expensive type of pac&ing supports. They also utilie the least vertical space. They are designed for low to medium gas loading when used for dumped pac&ing and
typically
have
79
to
;9<
open
areas
depending
on
the
material
used.
The support grids are available in various materials such as plastic, #-$ and metals. They can also be used as bed limiters. /ometimes support beams are required for structural reasons depending on the material and sie of the support grate.
4.6.1. G!s In7ection Support +l!te: t is a device used to hold the pac&ing. t generally sits on a support ledge and can be supported additionally by structural beams. There are two design criteria for the gas inection support plate. t must hold the pac&ing and liquid hold'up but also requires an open are greater than the cross sectional area of the tower. The larger open area is accomplished by using slotted or perforated plate that is corrugated or positioned in such a way to allow increased gas flow. 4pen area ranges from =7< to greater than 899<.
4.6.*. Gri' (pe Support +l!tes 8A+S$GS9: Grid type pac&ing supports are used for structured pac& ing to provide a horiontal contact surface and to prevent distortion of the pac&ing. (his 'esi%n c!n !lso be consi'ere' )or r!n'om p!c#in% . A wide range of openings is available to prevent the pac&ing from falling
through. The supports typically rest on support ledges. #or larger towers with man ways, sectional designs are standard.
4.1. Desi%n o)
Absorber:
4.1.1. Gener!l
Desi%n Steps:
The designer is required to consider and determine • • • • • • • • • •
select suitable column type select appropriate solvent select type and sie of pac&ing >material and energy balance %alculate column diameter $ressure drop calculation etermine the number of transfer units etermine height of transfer unit #ind the height of column /elect column internals
4.1.*. Input D!t!: 4perating temperature
[email protected]
4perating pressure
68.??atm
[email protected]$ascal $ac&ing type
$all rings
$ac&ing sie
8.7 nches
9.9@=8m $ac&ing factor, (#p)
8@9:m
/urface area of pac&ing (a p)
8?= m?:m@
4.1.-. G!s +roperties: Gas flow rate
6@7B &g:hr 7=B.7? &gmoles:hr
Gas pressure at entry
6=.9? atm
Gas temperature at entry
?5;.=?
Gas mol weight (Cg)
89.=?
Gas density (Dg)
@7.== &g:m@ ?.?5 lb:ft
4.1.4. omponent to be Scrubbe': %omponent Eame
Ammonia
%omponent flow rate
8;B &g:hr 9.975B? &g:s
Colecular weight of comp (Cg)
8B
4.1.2. i&ui' +roperties: !iquid flow rate, !
=9&gmoles 85@; &g:hr
!iquid ensity, (Dl)
;;;.;?&g:m@
!iquid 3iscosity, (Fl)
9.9985 $a's 8.5 %p
Colecular wt of liquid (Cl)
8B.;;
4.1.3. ;umi'it !lcul!tions: Total pressure ($t)
[email protected] &pa
3apor $ressure of water
9.=@6 &pa
Colecular wt of exit gas (Cg?)
89.6;
+
3.$ H Cg
($t'3.$) HCg?
+
9.999??B &g:&g dry gas
8.5?? &g:hr
4.1.. !teri!l B!l!nce :
Col fraction of ammonia in entering gas
7.7x89'5
Col fraction of ammonia in exiting gas
B.;x89'7
Col fraction of ammonia in entering liquid
9
Col fraction of ammonia in exiting liquid
5.7x89'@
4.1.5. olumn Di!meter !lcul!tions: $ressure drop for absorber 87 to79 mm+?4:m of pac&ing Assume pressure drop
5? mm+?4:m of pac&ing 9.976;psi:m of pac&ing 9.98B@5psi:ft of pac&ing
4.1.6. olumn Di!meter: D < 1.1*5-=G8#%>sec9>G8#%>secm *9?
here
G gas mass flow rate G ?99,999&g:h ! liquid mass flow rate ! =799&g:h
@$ oor'in!te v!lue: I
= !:3( ρ g : ρ l − ρ g ) 9.7
I 9.95@6= $ oor'in!te v!lue:
2y using the assumed pressure drop 5?mm+g:m of pac&ing J
= G ? H # p µ l 9.8:( ρ g : ρ l − ρ g )9.7
J 9.977 (unit operation by Cc%abe K smith edition 7 fig 5.8) After putting the variables &nown, mass velocity of gas can be calculated as
G 85@8 !b: ft? h $ut in the equation"
%s9.8=5
F%s
I
= !:3( ρ g : ρ l − ρ g ) 9.7
4.1.1. Are! !lcul!tion: Ac L:5() ?
Ac
9.87 m?
4.1.11. i&ui' !ss elocit: ! ?.667g:s m? ! 9.755; !b: ft? sec
4.1.1*. +ressure Drop !lcul!tion:
here*
G ? ∆ P = α (89 β L ) ρ g
M$ 9.5= in +?4 : ft of pac&ing M$ $ressure drop in inches of water :ft of the pac&ing height G Gas superficial mass velocity lb:s'ft? tower cross section ! liquid superficial mass velocity lb:s'ft? tower cross section Dg Gas density ,lb:ft? N K O are constant ta&en from (Applied process design for chemical K petrochemical plant by Prnest P. !udwig table 8;'?5)
4.1.1-. +ercent!%e /loo'in%: 50 at flooding from graph @.?
A 5
=
?
[email protected] H # p ( µ l : ρ l ) 9.8 :( ρ v : ρ l
−
ρ v )
5 8.;;8 $ercentage flooding ( 5: 50) 9.7H899 < 5C
#rom graph 88.55* by using 5 8.BB and #! 9.95?; the Q $ line come out to be 5?mm+g:m of pac&ing and is same as was assumed R $assumed R $calculated
4.1.14. !lcul!tion o) E&uilibrium onst!nt: As our operating temperature and pressure is such that they are out of range of data so we calculate equilibrium constant by using thermodynamic relationship which is as follow e yi:xi Sif i4!:$Ti here Si activity coefficient of component ammonia Ui0B.5 f i4! (fugacity of pure liquid component Ui0 ammonia E:m?)
f i4! $i i Vexp W($T'$i)3i!:-T> $i (vapor pressure of ammonia at
[email protected] ) ?.B bar $T 6?.9@ bar 3i! specific volume After putting all values we get f i4! 8.B==5 bar
i ( fugacity coefficient of pure liquid component Ui0 ammonia unit less) calculated by generalied correlation available in thermodynamics ln i 2o$r :Tr XY28$r :Tr #or ammonia all values available in literature we g et the value of i 9.7B9B= $utting all values in equation the value be* i 9.@B5 hich is the slope of operating line Absorption factor slope of 4.!: slope of P.! !:mG 9.BB:9.@B5
?.8
As absorption factor is greater than 8 this indicate that more and more solute absorbed in liquid cause the decrease in height of column and hence the cost. Pquilibrium curve plotted according to (ref Cc%abe and smith) /lope of equilibrium line 9.@B5
4.1.12. Number o) (r!ns)er nits !lcul!tions: ET1
A
H (ln V(yb:ya)H(A'8) X8>)
A'8 ET1
A
;
4.1.13. ;ei%ht o) (he olumn: As our pac&ing sie is 8.7 inch and column diameter less than @ ft the +PT$ can be ta&en in the range 9.5 to 9.B7m so we select it +PT$ 9.Bm c 9.5@=m !s 3! t : A
here !s height of bottom section for liquid surge time ts 89sec 3! volumetric flow rate of liquid 85@;&g:hr After putting values we get !s 8.7;=m Zt Ee H +PT$ X @ft X9.?7c X !s Zt =m +eight of pac&ing* Z +4G E4G +4G height of overall gas phase transfer unit E4G number of overall gas phase transfer unit As we &now +4G +G X m ( G:!)+! m slope of equilibrium line !:G slope of operating line To calculate +4G there are two methods* ornells metho' On'!s metho'
e use %ornell0s method* According to it
4.1.1. In'ivi'u!l ;ei%ht o) G!s +h!se (r!ns)er nit !lcul!tions: +G 9.98[h (/c)v9.7(c:9.@97)8.88(Z:@.97)9.?@: (!Hf 8Hf ?Hf @) +G height of gas phase transfer unit [h at 7=< flooding =9 !w ?.6B&g:m?sec f 8 liquid viscosity correction factor f 8 (F!: Fw) f 8 8.9=; f ? liquid density correction factor f ? (Dw : D!)8.?7 f ? 8.97@? f @ surface tension correction factor
(from fig*5.?)
f @ (\w: \!) f @ 9.;@ (/c)v gas phase /chmidt number (Fv: Dvv)
(/c)v 9.5?; +G 9.6;;m
4.1.15. In'ivi'u!l ;ei%ht o) i&ui' +h!se (r!ns)er nit !lcul!tions: +! 9.@97h (/c)!9.7 @ (Z:@.97)9.87 +! height of liquid phase transfer unit @ at 7=< flooding 9.=B
(from fig*5.@)
h 6.? H 89'?
(from fig*5.5)
(/c)! liquid phase /chmidt number (Fl: Dll)
(/c)! ;B6.5B +! 9.6
4.1.16. Over!ll ;ei%ht o) G!s +h!se (r!ns)er nit: +4G 8.9@ Eow J8: J? 9.9?:9.999?9?8 J8: J? ;5.;6 E4G 6.? (from fig* 5.7 by using mGm:! and J8: J?) Then Z +4G E4G Z 6.5m approximately same as calculated from the estimated value
4.1.*. i&ui' ;ol' up !lcul!tion: +lw 9.9995(!0:d p) +lw water holdup (ft@ liquid: ft@ vol of tower) d p equivalent spherical pac&ing diameter (inches) ! liquid rate (lb:ft ?hr) /o +lw 9.9995(8;68.65:8.7)9.6
+lw 9.9@m@:m@ of tower (ref. applied process design for chemical K p etrochemical plant 2y Prnest P. !udwig)
4.1.*1. inimum Fettin% R!te: !min C-Ha p 3olumetric flow rate 3 !: D! 8.5@;m@:hr 3elocity vol flow rate: area of column
3elocity;.75m:hr C- v:a p <9.99B5;m?:hr 9.=5 ft?:hr
(-ef. applied process design for chemical K petrochemical plant by Prnest P. !udwig)
4.1.**. hec# )or h!nnelin%:
: p
8B.7:8.7
88.7
n the ratio 8*= to minimie the channeling. (-ef. applied process design for chemical K $etrochemical plant by Prnest P. !udwig)
4.11. ech!nic!l Desi%n: 4.11.1. !teri!l Selection: !ow alloy steel 5@II (nic&el 8.=@<, chromium 9.=9<, molybdenum 9.?7<)
4.11.*. (hic#ness o) (he olumn: e $i i:?f' $i $i internal pressure 6.656E:mm? f design stress ?79E:mm? e 6.97;Bmm e. e X corrosion allowances e. 6.97;BX ? e. =.97;mm
4.11.-. +!c#in% Support: /imple grid and perforated plate supports
/unction: The function of support plate is to carry the weight of wet pac&ing whilst allowing the passage of gas K liquid.
4.11.4. i&ui' Distributors: 4rifice type liquid distributor
/unction: !iquid distributors are needed to ensure the good distribution at all liquid flow rates.
4.11.2. Re'istributors: all wiper type redistributors
/unction: -edistributors can be equipped with wall wipers to collect the liquid clinging to the tower walls.
S+EI/IA(ION S;EE(
Item
+!c#e' Absorption olumn
No. re&uire'
1
/unction
(o !bsorb !mmoni! in !&u! !mmoni!.
Oper!tion
ontinous
Desi%n (emper!ture
*5
Desi%n +ressure
3564 #+!
;ei%ht o) p!c#in% section
2.** m
SiHe !n' tpe o) p!c#in%
+l!stic p!ll rin%s
(ot!l hei%ht o) column
5.* m
Insi'e 'i!meter
.44m
+!c#in% !rr!n%ement
'umpe'
