04 dehydration 1

8/11/2019 04 Dehydration 1

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Gas

Dehydration

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Gas Dehydration

Contents:

Principles of Gas Dehydration ..........................................................................3

Water Content of Gases......................................................................................4

Hydrate Formation...........................................................................................15

Factors Promoting Hydrate Formation............................................................17

Procedres and !esorces for Determining Hydrate"Formation

#emperatres of $%eet and $or Gas $treams ..............................................&'

#emperatre Control (ethods and )*ipment +sed #o ,nhi-it

Hydrate Formation in a atral Gas $tream....................................................&&

(ethanol ,n/ection !ate !e*ired #o ,nhi-it Hydrate

Formation in a atral Gas $tream.................................................................&0

Glycol Dehydration .........................................................................................3

Descri-ing the Glycol Dehydration Process ...................................................41

Glycol Dehydration $ystem Components .....................................................43

Process2Design aria-les ................................................................................4

ptimi6ing and #ro-leshooting Dehydrator perations................................51

$olid Desiccant Dehydration...........................................................................54

dsorption Calclations...................................................................................57

Process Flo% and the fnction of the ma/or components of $olid

Desiccant Dehydrators.....................................................................................01

Fnction of (a/or Components of $olid Desiccant Dehydrators....................00

,nstrmentation................................................................................................7'

ptimi6ing dsorption"#ype Dehydrators......................................................70

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PRINCIPLES OF GAS DEHYDRATION

Background

Liquid water and/or water vapor are reoved !ro natura" #a$ to%

8 Pre9ent formation of hydrates in transmission lines.

8 (eet a %ater de% point re*irement of a sales gas contract.

8 Pre9ent corrosion.

#echni*es for dehydrating natral gas inclde:

8 -sorption sing li*id desiccants.

8 dsorption sing solid desiccants.

#hrogh a-sorption; the %ater in a gas stream is dissol9ed in a relati9ely pre li*id

sol9ent stream. #he re9erse process; in %hich the %ater in the sol9ent is transferred

into the gas phase; is -ecase the sol9ent is sally reco9ered for rese

in the a-sorption step.

-sorption and stripping are fre*ently sed in gas processing and most gas

s%eetening operations; as %ell as in glycol dehydration.

#he second ma/or process -y %hich %ater 9apor is remo9ed from a gas stream is

called adsorption. dsorption is a physical phenomenon that occrs %hen molecles

of a gas are -roght into contact %ith a solid srface and some of them condense on

the srface.

Dehydration of a gas %ith a dry desiccant is an adsorption process in %hich %ater

molecles are preferentially held -y the desiccant and remo9ed from the stream.

&ater Content o! H'dro(ar)on

?ased on e@perimental data; Fig"& sho%s the sol-ility of %ater in s%eet

hydrocar-on li*ids. ,n sor hydrocar-on li*ids; %ater sol-ility can -e

s-stantially higher.

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Fig 2 - Solubility of Water in Liquid Hydrocarbons

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&ATER CONTENT OF GASES

#he satrated %ater content of a gas depends on pressre; temperatre; and

composition. #he effect of composition increases %ith pressre and is particlarly

important if the gas contains C& and2or H&$. For lean; s%eet natral gases

containing o9er 7'A methane and small amonts of hea9y hydrocar-ons;

generali6ed pressre"temperatre correlations are sita-le for many applications.

Fig."3is an e@ample of one sch correlation %hich has -een %idely sed for many

years in the design of Bs%eet natral gas dehydrators. #he gas gra9ity correlation

shold ne9er -e sed to accont for the presence of H&$ and C& and may not

al%ays -e ade*ate for certain hydrocar-on effects; especially for the prediction of

%ater content at pressres a-o9e 15'' psia. #he hydrate formation line is

appro@imate and shold not -e sed to predict hydrate formation conditions.

#he follo%ing e@amples are pro9ided to illstrate the se ofFig."3:

E*ap"e +,-. Determine the satrated %ater content for a s%eet leanhydrocar-on gas at 15'EF and 1;''' psia.

From Fig. "3;

W = &&' l-/((scf

For a &0 moleclar %eight gas;

Cg = '. =Fig. "3>

W = ('.)(&&') = &10 l-/((scf

For a gas in e*ili-rim %ith a 3A -rine;

Cs = '.3 =Fig. "3>

W = ('.3)(&&') = &'5 l-/((scf

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Fig- 3 : Water Content of Hydrocarbon Gas

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FG-#


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1'

FG-$


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cid gas %ater content is a 9ery comple@ s-/ect. #he data and methods

presented here shold not -e sed for final design. Fig. "4; "5; "0 and "7 are all

-ased on e@perimental data. crsory stdy of these figres re9eals the

comple@ities in9ol9ed. n accrate determination of %ater content re*ires a carefl

stdy of the e@isting literatre and a9aila-le e@perimental data. ,n most casesadditional e@perimental data is the -est %ay to 9erify predicted 9ales.

?elo% 4'A acid gas components; one method of estimating the %ater content ses

)* "1 and Fig. "3; "; and ".

W = yHC WHC + yC& WC& + yH&$WH&$ %q -&

11

FG-'


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%here:

W $atrated %ater content of gas stream; l- H&2(($CF

W@@ )ffecti9e satrated %ater content of each

component; l- H& 2(($CF

y@@ (ole fraction of component in gas stream

ote that Fig. " and "pro9ide 9ales for %hat is termed the Beffecti9e

%ater content of C& and H&$ in natral gas mi@tres for se only in )* "1. #hese

are not pre C&and H&$ %ater contents.

nother method for estimation of the satrated %ater content of acid gas

mi@tres p to 0''' psia ses Fig. "1' and"11.With gases containing C&; the C&

mst -e con9erted to an Be*i9alent H&$ concentration. For prposes of this

method; it is assmed the C& %ill contri-te 75A as mch %ater to the gas

mi@tre; on a molar -asis; as H&$.

1&

FG-(FG-)


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13

FG-&*


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E*ap"e +,-+ Determine the satrated %ater content of an'A C1; &'A C& mi@tre at 10' EF and &''' psia. #he e@perimentallydetermined %ater content %as 17& l-2((scf.

+ethod "ne

WHC = 107 l-/((scf =Fig. "3>

WC& = &4' l-/((scf =Fig. ">

W = ('.')(107) + ('.&')(&4')

= 1& l-/((scf

14

FG-&&


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+ethod ,o

First the composition mst -e con9erted for se %ith Fig. "1'.

yH&$ (psedo ) = ('.75)(yC&) = ('.75)('.&') = '.15

W = '.4 --l/((scf =Fig. "1'>

Density of %ater = 35' l-/--l

= ('.4)(35') = 17& l-/((scf

Fig. "1& compares the t%o methods presented for satrated %ater content

determination of high C&2H&$ gas mi@tres %ith some of the a9aila-le

e@perimental data. #he last for data points sho%n in Fig. "1& indicate the dangers

in9ol9ed %ith e@trapolation to higher C& or H&$ contents. ,n one case; the

estimated %ater content agrees %ithin 11A of the e@perimental 9ale. ,n another

case; the e@perimental 9ale is o9er 0 times the estimated %ater content.

Fig "1& Comparison of )@perimental 9s. Calclated Water Contents for cid Gases

(i@tre #; EF P; psigWater Content l-2((scf

)@perimental )* "1 Fig."1' "11

11A C&2A C1 1'' &''' 4'.0 4& 3.&

11A C&2A C1 10' 1''' &0 &77 &7

&'A C&2'A C1 1'' &''' 4'.0 43 44.1

&'A C&2'A C1 10' 1''' && &7 &7

A H&$2&A C1 13' 15'' 111 1'5 11&&7.5A H&$27&.5A C1 10' 1307 &47 &5 &73

17A H&$23A C1 10' 1''' && &7 &'

C12C&2H&$

3'A20'A21'A1'' 11'' 1 7&

C12C&2H&$

A21'A21A1'' 1'' 44& 7&

5.31A C124.0A C& 77 15'' 1'.& 3

5.31A C124.0A C& 1&& &''' 104.0 1'5

15


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H'drate Foration hydrate is a physical com-ination of %ater and other small molecles to prodce a

solid that has an Iice"li

Car-on dio@ide =C&>

)thane =C&>

Hydrogen slfide =H&$>

(ethane =C1>

itrogen =&>

Propane =C3>

Ji*id %ater has a mo-ile lattice strctre. #his lattice strctre has t%o 9acantlattice positions. When gas molecles fill these 9acancies; the lattice is immo-ili6ed;

and the gas and %ater form a solid strctre.

Co./osition

Hydrocar-ons %ith fi9e or more car-on atoms =C5K> do not fit into these lattice

9acancies; so they do not form hydrates. Pre n"-tane does not form hydrates -y

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itself; -t n"-tane does e@ist in hydrates %hen other smaller hydrate"forming

molecles are present. Hydrates are ' %tA %ater %ith specific gra9ities that range

from '.0 to '.. #herefore; hydrates float on %ater; -t sin< in hydrocar-on

li*ids.

Structure

Hydrates form one of t%o different crystalline strctres: $trctre , or $trctre ,,.

Figre"13 smmari6es the characteristics of hydrate crystalline strctres. Figre"14

sho%s $trctre , and $trctre ,, lattices.

$#!+C#+!) , $#!+C#+!) ,,

& small and large 9oids 10 small and 0 large 9oids

Generally formed -y CH4; C&H0; H&$;

C&

Generally formed -y C3H; i"C4H1';

CH&Cl&; CHCl35 324 %ater molecles per gas molecle

(L

17 %ater molecles per gas molecle

(L

F,G+!) 13: CH!C#)!,$#,C$ F HMD!#) C!M$#J $#!+C#+!)$

F,G+!) 14: HMD!#) $#!+C#+!)$

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Factors 0ro.oting Hydrate For.ation

0ri.ary

#he follo%ing are the primary conditions that promote hydrate formation: Free %ater =Gas is at or -elo% its de% point.>

High pressre

Jo% temperatre

Secondary

$econdary conditions that promote hydrate formation inclde the follo%ing:

High 9elocities.

Physical sites %here crystals might form sch as pipe el-o%s; orifices; or line

scale.

Pressre plsations.

$mall crystals of hydrates that may act as seed crystals.

#r-lence in gas streams =promotes crystal gro%th -y agitating spercooled

soltions>.

"ther Considerations

Gas composition greatly affects hydrate"formation temperatres. High H&$

concentrations promote hydrate formation. #he presence of ethane and propane also

promotes the formation of hydrates. Gases %ith higher specific gra9ities form

hydrates at lo%er pressres.

Hydrates ta


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H&$ and C& form sta-le $trctre", hydrates. H&$ in a gas stream strongly

promotes the formation of hydrates. -o9e 5EF H&$ does not form hydrates. #his

critical temperatre a-o9e %hich hydrates do not form is higher for H&$ than for

other components typically fond in natral gas streams. #herefore; in gas streams

of e*al densities; the gas stream %ith H&$ forms hydrates more readily.

1sing Gra/hical ,echniques to 0redict Hydrate-For.ation Conditions

Generally; hydrate formation temperatres shold -e e9alated anytime a gas stream

containing %ater and hydrate"forming components is cooled -elo% 'EF. ,t is not

necessary to e9alate hydrate formation temperatres -elo% 3&EF since pre %ater

free6es any%ay.

Figre 15 plots hydrate"formation temperatres of pre light gases. Hydrate

formation %ill occr in the region a-o9e and to the left of the cr9e for a gi9en

compond. #he discontinities in the lines correspond to changes in phase of thenonhydrate phases.

F,G+!) 15: HMD!#) F!(#, CD,#,$ F P+!) J,GH# G$)$

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)mpirical hydrate formation graphs may -e sed to determine the hydrate formation

conditions of a gas stream. #he gra9ity graphic method and the N"9ale method are

the t%o graphical methods sed to predict hydrate formation conditions.

Compter programs are more commonly sed; -t the gra9ity graphic method is still

sefl for roghly appro@imating hydrate formation conditions.

Graity Gra/hic +ethod

Figre 10 plots the hydrate"formation conditions of gases -ased on their specific

gra9ity relati9e to air =(W &>.

?ecase hydrocar-on mi@tres -elong to the same chemical family; the methodOs

se of specific gra9ity ma


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#he gra9ity graphic method roghly appro@imates hydrate formation conditions.

#his method shold not -e sed on gas streams %ith s-stantial concentrations of

non"hydrocar-ons =&; C&; and especially H&$>.

0rocedures and esources for Deter.ining the Hydrate-For.ation

,e./eratures of Seet and Sour Gas Strea.s 4Graity Gra/hic +ethod5

1. Calclate the %eight of component per mole of gas mi@tre -y mltiplying the

mole fraction of each component -y the moleclar %eight of each component.

!ecord the partial moleclar %eight of each component in the right colmn of the

ta-le pro9ided %ith the e@ercise.

&. Calclate the total moleclar %eight of the gas mi@tre. #o ma of the gas mi@tre.

sp. gr.=gas> =)*n. &>

%here: sp. gr.=gas> $pecific gra9ity of the gas stream

(Wgas (oleclar %eight of gas stream; l-2mole

(Wair (oleclar %eight of air

& l-2mole

4. +se Figre 10 to determine the hydrate"formation temperatre of the gas stream.

#he follo%ing sample pro-lem demonstrates the gra9ity graphic method

$ample Pro-lem: Determine the Hydrate"Formation #emperatres of $%eet and

$or Gas $treams =Gra9ity Graphic (ethod>

Calclate the appro@imate temperatre at %hich the gas stream entering a chill do%n

train at a Gas Plant forms hydrates. +se the gra9ity graphic method. #he left colmnof Figre 17 lists the composition of the gas stream and the right colmn is pro9ided

to help organi6e the calclations.

Gi9en:

Pressre 4&4 psig

&1

(Wgas

(Wair Q


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#emperatre from dehydrator 'EF

C(P)#(J)

F!C#,

(J)C+J!

W),GH#

l-2(ole F

(,L#+!)

& '.''00 &.' '.15

C& '.'''3 44.' '.'13&H&$ '.' 34.3 '.''

C1 '.0317 10.' 1'.1

C& '.&111 3'.1 0.35

C3 '.1' 44.1 4.'

i"C4 '.''7 5.1 '.453

n"C4 '.'&4& 5.1 1.41

i"C5 '.''31 7&.& '.&&4

n"C5 '.''4 7&.& '.340

n"C0 '.''14 0.& '.1&1

C7K '.'''& 1''.& '.'&'##J G$ $#!)( 1.'' "" &4.'

F,G+!) 17: #?J) F! CJC+J#,G #H) (J)C+J! W),GH# F

#H) G$ $#!)(

$oltion:

1. #he right colmn of Figre 17 sho%s the calclation of the %eight of each gas

component per mole of gas stream.&. #he %eight of each gas component per mole of gas stream is totaled at the -ottom

of the right colmn of the ta-le pro9ided in Figre 17.3. #he se of )*n. & to calclate the specific gra9ity =relati9e to air> of the gas

mi@tre reslts in the follo%ing:

sp. gr.

'.& =)*n.&>

4. From Figre 10; the hydrate"formation temperatre of the gas stream at 4&4 psig

=43 psia> is determined to -e 57EF.

ns%er:

Hydrates can form in this gas stream at appro@imately 57EF.

6lloable Gas %7/ansions

Graphical methods also predict permissi-le gas e@pansions. #hese graphs se the

gra9ity graphic method to determine the allo%a-le e@pansion =decrease in pressre>

&&

(Wgas

(Wair Q

&4.' l- 2 mole

&.' l- 2 mole Q


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at 9arios initial temperatres. s %ith the gra9ity graphic method; these graphs are

sefl for initial estimates; -t shold not -e sed for design.

Teperature Contro" 0etod$ and Equipent 2$ed To Ini)it H'drate

Foration in a Natura" Ga$ Strea

Heating a natral gas or depressri6ing it =ths cooling it> %hile it is nder hot

conditions can inhi-it hydrate formation. ,n a-o9e grond operations; the

temperatre drop cased -y depressri6ing =e@panding> a gas can reslt in the

temperatre of the gas stream dropping -elo% its hydrate"formation temperatre.

?ecase of the high temperatres ndergrond; a gas stream can -e e@panded

ndergrond %ithot the reslting temperatre dropping -elo% its hydrate"formation

temperatre. #herefore; e@panding a gas stream in a %ell -ore helps pre9ent hydrate"

formation in do%nstream processing.

#he t%o main pieces of e*ipment sed to control gas stream temperatre and

inhi-it hydrate formation are do%nhole reglators and indirect heaters. Do%nhole

reglators inhi-it hydrate formation -y e@panding gas streams %hile they are in the

%ell-ore. ,ndirect heaters inhi-it hydrate formation -oth at %ellheads =%ellhead

heaters> and along flo%lines =flo%line heaters>.

,ndirect heaters are often sed to inhi-it hydrate formation cased -y e@pansion or

to replace heat lost -y a flo%line to the srronding air and grond.

Donhole egulators

#he se of do%nhole reglators to inhi-it hydrate formation -y controlling gas

stream temperatres is generally feasi-le %hen the gas %ell has the follo%ing

conditions:

high reser9oir pressre that is not e@pected to decline rapidly

)@cess pressre

High capacity

#he temperatre and pressre of a gas stream as %ell as its composition determine

%hether hydrates %ill form %hen gas is e@panded into the flo%lines. Cooling occrs

as gas is e@panded across the cho


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throgh the %ell-ore t-ing. #he pressre drop across the reglator remains constant

and does not depend; %ithin a -road range; on the flo% rate of the %ell.

#he design of do%nhole reglators re*ires sing comple@ calclations that mst

accont for the follo%ing: Do%nhole pressres and temperatres

Well depth

Well-ore configration

ndirect Heaters

#%o types of indirect heaters are sed to inhi-it hydrate formation: %ellhead and

flo%line.#he e@pansion of gas streams at or near %ellheads often reslts in the formation of

hydrates.

Wellhead heaters %ithin a heater shell. #he

fire t-e is sally fired -y gas. #he coil contains the flid =the gas stream> to -e

heated and operates at fll gas pressre. #he heater shell operates at atmospheric

pressre. Figre 1 sho%s a typical indirect heater.

Floline Heaters" Flo%line heaters heat gas streams a-o9e their hydrate"forming

temperatres. ,n many cases; properly designed and placed %ellhead heaters pro9ide

sfficient heat to eliminate the need for flo%line heaters.

ndirect Heater Si8ing

#he determination of the si6e of a heater depends on the follo%ing conditions:

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monts of gas; %ater; oil; or condensate e@pected in the heater

,nlet temperatre and pressre

tlet temperatre and pressre =to a9oid hydrate"forming conditions>

#he si6e of heater coils to se depends on the 9olme of flid flo%ing throgh thecoil and the re*ired heat"transfer load.When heater coils are si6ed; it is important to consider operating conditions in

addition to normal; steady"state operating conditions. #ransient startp of a sht"in

%ell may re*ire e@tra heating capacity. #he temperatre and pressre conditions of

a sht"in %ell and the e@tra li*ids accmlated %hile the %ell %as sht in may

increase the heating load. ften; heaters are necessary only %hile %ells are -eing

started p. ,nstalling preheat coils ahead of cho


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#hey may not inhi-it hydrate formation dring startp. ,t may -e necessary to

inhi-it hydrate formation -y in/ecting either methanol or glycol ntil the gasflo% and temperatre sta-ili6e.

When %ell otpt falls -elo% normal prodction le9els; processors mst

remo9e and replace do%nhole reglators %ith another hydrate inhi-itionmethod.

When %or< is performed inside a %ell-ore; the %ell may -e permanently

damaged.

Indirect Heaters

#he ad9antages of sing indirect heaters to inhi-it the formation of hydrates inclde

the follo%ing:

(inimal maintenance or attention re*ired

ery lo% chemical re*irements

#he disad9antages of sing indirect heaters to inhi-it hydrates inclde the follo%ing:

Difficlty of spplying clean and relia-le fel to remote locations

Jarge operating =fel> costs if cheap fel is not a9aila-le

Potentially large capital costs

$ignificant plot space re*ired

$pecial safety e*ipment needed -ecase of fire ha6ard

Co./arison of ,e./erature Control +ethods

Figre 1 compares the se of do%nhole reglators and %ellhead heaters to inhi-it

hydrate formation. #he high capital costs of heaters generally limit their se to largehydrate inhi-ition installations. Do%nhole reglators %or< -est in large reser9oirs

%ith high gas pressres that are not e@pected to decline rapidly.

D)$,G FC#!$DWHJ)

!)G+J#!$

W)JJH)D

H)#)!$

,n9estment ery lo% ery high

Fel one ery high

perating (aintenance Jo% Jo%

Chemicals one ery lo%

Plot rea one ery high

Ha6ards High High

F,G+!) 1: C(P!,$ F #)(P)!#+!) C#!J ()#HD$

&0


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0etano" In3e(tion Rate Required To Ini)it H'drateForation in a Natura" Ga$ Strea

Chemical InjectionCrrently; methanol =(eH> and monoethylene glycol =()G> are the t%o

chemicals most commonly in/ected into gas streams to inhi-it hydrate formation.

Consider the se of chemical in/ection to inhi-it hydrate formation for the follo%ing:

Gas pipelines in %hich hydrates form at locali6ed points

Gas streams operating a fe% degrees a-o9e their hydrate formation

temperatre Gas"gathering systems in pressre"declining fields

$itations %here hydrate pro-lems are of short dration

Hydrate inhi-itors act similarly to antifree6e. dding a


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dri9en pmp in/ects the methanol into the gas stream pstream of the cho


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G"'(o" In3e(tion Rate Required To Ini)it H'drateForation in a Natura" Ga$ Strea

Ji

High initial cost

Possi-ility of glycol contamination

Jimited se =only non"cryogenic

applications>Cannot dissol9e hydrates already formed

(ethanol !elati9ely lo% initial cost$imple system

Does not generally need to -e

reco9ered

Jo% 9iscosity

When in/ected; distri-tes %ell

into gas streamsCan dissol9e hydrates already

formed

High operating costGenerally; se glycol in/ection if

methanol in/ection rate is o9er 3' gph

Jarge 9apor losses =high 9olatility>

F,G+!) &1: C(P!,$ F CH)(,CJ ,R)C#, ,H,?,#!$

Glycol does not e9aporate as easily as methanol. ,n some applications; glycol does

not dissol9e into li*id hydrocar-ons as easily as methanol. Glycol sol-ility in

hydrocar-on li*id increases %ith:

Glycol moleclar %eight

#emperatre increase

,ncrease in glycol concentration in %ater"glycol mi@tre

Glycol Concentration and Dilution:

,n addition to inhi-iting hydrate formation; yo also need to choose glycolconcentrations that do not free6e. Figre && sho%s the free6ing points of 9arios

a*eos glycol soltions

&


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F,G+!) &&: F!))S,G P,#$ F T+)+$ GJMCJ $J+#,$

ote that soltions %ith glycol concentrations -et%een a-ot 0' %t A and ' %t A

do not free6e. ?ecase of this; glycol soltions are generally


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Glycol n9ection and ecoery Syste. 1sing a ,hree-0hase Se/arator

Figre &5 sho%s a typical glycol in/ection and reco9ery system that ses a three"

phase separator. #he po%er"gas"dri9en pmp; the temperatre controller; and the

in/ection point sho%n in Figre 15 are similar to the methanol in/ection system

sho%n in Figre 5. gas dri9en pmp in/ects the glycol into the gas streampstream from the cho


33/81

F,G+!) &0: GJMCJ ,R)C#, D !)C)!M $M$#)(

LTSPC4-+,. D.

PC4-56+

E +,.A/7

G"'(o"

Rea( G"'(o"

LC4 +,8 D+LC4 569

E+,+A/7

F"are

LC4 +,8 D.LC4 5:.

SD456,SD45;.

SD4 5;5SD4 5:+

SD4 +,8SD4 5:8

PC4 566PC4 +,8D+

D-205

F"are

LeanGlycol

FromD-204

GasfromD-201

LP Sa"e$Ga$

F&'1

F&'1

?

D &1'

$


34/81

#he reco9ery side of the system sho%n in Figre &5 incldes a re-oiler and a three"

phase separator. #he glycol in/ection and reco9ery cycle is as follo%s:

#he in/ection no66le in/ects the lean glycol into the gas stream.

#he lean glycol a-sor-s the %ater and inhi-its hydrate formation in the cho


35/81

F,G+!) &7: GJMCJ $N,(()!

Inhibitor Pump

drm on top of a typical po%er"gas"dri9en pmp contains the inhi-itor: methanol

or glycol. #he drm connects directly to the pmp =generally; a positi9edisplacement pmp>. (ethods for monitoring the inhi-itor in/ection rate inclde

inserting a cali-rated dipstic< throgh the top of the drm or pmping the inhi-itor

into a measred 9essel. Drms are replaced %hen empty.

Glycol !osses

Glycol in/ection systems that in9ol9e -oth hydrocar-on li*ids and gases generally

lose glycol to the follo%ing:

$ol-ility =normally a-ot '.3 to 3 gallons of glycol per 1''' -arrels of

hydrocar-on li*id prodced>

Jea


36/81

;o88le Selection" o66le design is especially important in the design of glycol

in/ection systems for cold separation facilities. #he criteria for selecting a no66le

inclde the follo%ing:

Capacity

$pray angle $fficient pressre drop -et%een the no66le and the gas stream o9er the

e@pected range of operating conditions

ormally; a pressre differential of 1'' psi to 15' psi sfficiently atomi6es glycol.

lso; gas stream 9elocities a-o9e 1& ft2s help ensre atomi6ation.

;o88le 0lace.ent" ormally; no66les are located /st pstream of the heat

e@changer or chiller %here hydrates form. #he spray from a properly located no66leco9ers the entire t-e sheet of a heat e@changer.

,nade*ate atomi6ation cases the formation of glycol droplets that settle and floodthe -ottom of the heat e@changer. s a reslt; the glycol inhi-its hydrate formation in

the -ottom; -t not the top; of the heat e@changer. Flooding of the -ottom of the heat

e@changer also significantly decreases its effecti9eness.

,nade*ate co9erage can lea9e some t-es %ith a concentration of glycol that is too

lo%; %hich %ill reslt in the formation of hydrates. s sho%n in Figre 1; hydrates

plg the t-es; and there-y increase the differential pressre across the heat

e@changer.

F,G+!) &: ,C!)$) , P!)$$+!) D!P ?)C+$) F HMD!#) F!(#,

30


37/81

Figre & sho%s one no66le location -t three flo% rates. #oo lo% a no66le flo% rateprodces the same reslt as a no66le located too close to the t-e sheet. #oo high a

no66le flo% rate prodces the same reslt as a no66le located too far from the t-e

sheet.

F,G+!) &: SSJ) PJC)D # ) JC#,: #H!)) FJW !#)$

37


38/81

&ATER RE0O4AL PROCESSES

Liquid/Solid esiccants

,n those sitations %here inhi-ition is not feasi-le or practical; dehydration mst -e

sed. ?oth li*id and solid desiccants may -e sed; -t economics fa9or li*id

desiccant dehydration %hen it %ill meet the re*ired dehydration specification.

Ji*id desiccant dehydration e*ipment is simple to operate and maintain. ,t can

easily -e atomated for nattended operationU for e@ample; glycol dehydration at a

remote prodction %ell. Ji*id desiccants can -e sed for sor gases; -t additional

precations in the design are needed de to the sol-ility of the acid gases in the

desiccant soltion.

$olid desiccants are normally sed for e@tremely lo% de% point specifications as

re*ired to reco9er li*id hydrocar-ons.

Gl!col eh!dration

Background

#he more common li*ids in se for dehydrating natral gas are diethylene glycol

=D)G>; triethylene glycol =#)G>; and tetraethylene glycol =#!)G>. ,n general;

glycols are sed for applications %here de% point depressions of the order of 0'EF

to 1&'EF are re*ired.

D)G %as the first glycol to -e sed commercially in natral gas dehydration and can

pro9ide reasona-le de% point control. With the e@ception of #)G; D)G is the -est

li*id a9aila-le.

Ho%e9er; %ith normal field e*ipment; D)G can -e concentrated to only 5A

prity; %hereas #)G concentrations can reach to .5A %ithot special

e*ipment. lthogh -oth glycols perform sfficient dehydration in many sitations;

#)G is sed more commonly -ecase it re*ires lo%er circlation rates for a

compara-le de% point depression than D)G does and can reach lo%er de% points. ,t

is not ad9isa-le to se triethylene glycol for dehydration at lo% temperatres

=appro@imately 5'EF>; de to its high 9iscosity. #!)G is primarily sed %hen

dehydration conditions fall -et%een those encontered in normal #)G operations;and those in %hich gas stripping or 9acm distillation -ecomes necessary.

process flo% diagram of a glycol dehydration nit is sho%n in Figre 3'. Good

practice dictates installing an inlet gas scr--er; e9en if the dehydrator is near a

prodction separator.

#he inlet gas scr--er %ill pre9ent accidental dmping of large *antities of %ater;

hydrocar-ons; and2or salt %ater into the glycol contractor. )9en small *antities of

3


39/81

these materials can reslt in e@cessi9e glycol losses de to foaming; redced

efficiency; and increased maintenance.

FG1% 3*: 0"C%SS FL"W D6G6+ F" GL. #he glycol flo%s do%n throgh the contactor

contercrrent to the gas flo%. Water rich glycol is remo9ed from the -ottom of the

contactor; passes throgh the condenser coil; flashes off gas in a flash drm; and

flo%s throgh the glycol"glycol heat e@changer to the regenerator. ,n the regenerator;

a-sor-ed %ater is remo9ed from the glycol at atmospheric pressre -y heating. #he

regenerated glycol is cooled in the glycol heat e@changers and is recirclated to thecontactor -y the glycol pmp.

#)G %ill a-sor- a-ot 1 $CF of natral gas per gal at 1''' psig a-sor-er pressre.

#here %ill -e more a-sorption if aromatic hydrocar-ons are present. three to fi9eminte residence time in the flash drm is re*ired for degassing. )@cessi9e

hydrocar-ons in the glycol may case high glycol losses and foaming. #he o9erhead

9ent from the glycol regenerator may contain hydrocar-ons and shold -e piped to asafe location.

3


40/81


41/81

"SC#IBI$G %H" GL&COL "H'%IO$ (#OC"SS

,n general; the process of dehydrating natral gas streams %ith glycol is similar to

sing glycol in/ection to inhi-it hydrate formation. Ho%e9er; -ecase the glycol

mst not only a-sor-; -t also remo9e the %ater from the gas stream; dehydrationsystems a-sor- the %ater in contactors =also called a-sor-ers> instead of -y

in/ection. Glycol dehydration systems also re*ire higher and more precisely

reglated temperatres in their re-oilers.

FG1% 32: GL


42/81

glycol a-sor-s %ater from the gas stream. Dry otlet gas lea9es the top of the

contactor and rich glycol e@its the -ottom.

#he rich glycol enters the top of the stripping colmn and contercrrently contacts

steam rising from the re-oiler. #he rich glycol then enters the re-oiler; %hich -oilsthe %ater ot of the glycol. #he lean glycol lea9es the -ottom of the re-oiler and

enters the srge tan< for storage. #he pmp raises the glycol to system pressre;

preparing it for another dehydration cycle.

FG1% 33: GL


43/81

#his additional e*ipment impro9es the efficiency and effecti9eness of the

simplified system . Figre 34 sho%s another glycol dehydration system.

FG1% 3!: 0"C%SS FL"W "F 6 GL


44/81

,nade*ate scr--ing cases a-ot half of all glycol dehydration system pro-lems.

,nlet scr--ers remo9e free %ater and many contaminants from the inlet gas stream.

,n addition to free %ater; these contaminants inclde:

ils or hydrocar-ons )ntrained -rine

Do%nhole additi9es

$olids; sch as sand and corrosion prodcts

Figre 35 smmari6es the pro-lems cased -y these contaminants if not remo9ed -y

the inlet scr--er.

C#(,# P!?J)($

Free Water ,ncreases glycol recirclation; re-oiler heat dty; and fel

costs

,f the dehydration system -ecomes o9erloaded %ith %ater;glycol can carry o9er from the contactor and2or still .

$ales gas specification may not -e achie9ed

ils or

Hydrocar-ons

!edce the drying capacity of the glycol With %ater present;

can case foaming

+ndissol9ed oils can:

8 Plg a-sor-er trays

8 Fol heat transfer srfaces in the re-oiler

8 ,ncrease the 9iscosity of the glycol

8 Jight hydrocar-ons can flash in the stripping colmn

and case loss of glycol and2or damage to the pac


45/81

Contactor

Contactor to%ers contact the lean glycol %ith the %et gas stream. s Figre 5 sho%s;

contactor to%ers consist of three sections:

$cr--ing section in the -ottom

Drying =mass transfer> section in the middle

Glycol cooler and mist e@tractor in the top

ote: #he scr--er section in the glycol contactorOs -ase and the glycol cooling coil

are optional items =restricted to small field nits> .

FG1% 3$ : C";,6C," ,"W%

45


46/81

Scrubbing Section" #he gas stream enters the -ottom section of the contactor and

then enters a second scr--er integrated into the contactor and a %ire mesh mist

e@tractor. #hese remo9e any contaminants and entrained li*ids not remo9ed -y the

inlet scr--er. #his second stage of scr--ing frther minimi6es the contamination

of the glycol and helps pre9ent the free %ater from o9erloading the system.

Drying Section- ,n the middle section of the contactor; the gas stream flo%s p%ard

and thoroghly contacts the do%n%ard flo%ing lean glycol throgh 9al9e trays;

---le caps; or pac


47/81

potential increase to the foling rate of the rich side of the lean2rich glycol

e@changer.

Glycol 0u./

Glycol circlation pmps contain the only mo9ing parts in a glycol dehydration

system. #he three types of pmps sed in dehydration systems are:

)lectric"motor dri9en

High"pressre gas"operated

High"pressre li*id"operated

Jarger dehydration systems generally se electric"motor dri9en pmps. $maller

dehydration systems and those remotely located generally se high"pressre gas"

operated or high"pressre li*id"operated pmps.,nstalling a second =spare> pmp capa-le of pro9iding fll glycol circlation ensres

continos dehydration if the primary glycol circlation pmp fails.

Heat %7changers

Glycol dehydration systems often se three heat e@changers:

Jean gas2glycol

Glycol2glycol

!efl@ coil located in still colmn

Lean Gas@Glycol Heat %7changers se lean gas to frther cool the glycol to 5EF to

15EF a-o9e the e@it temperatre of the gas stream -efore the glycol enters the top of

the contactor.

Generally; glycol dehydration systems se do-le"pipe or shell"and"t-e heat

e@changers for lean gas2glycol heat e@changers.

Glycol@Glycol Heat %7changerspreheat the rich glycol lea9ing the -ottom of the

contactor -efore it enters the re-oiler and cools the lean glycol lea9ing the re-oiler

-efore it goes to the lean gas2glycol heat e@changer and the top of the contactor.

#he refl@ coil =sing cool; rich glycol> or the cooling fins on the still colmn

=%hiche9er is applica-le> maintains the temperatre at the top of the still colmn.#he glycol2glycol heat e@changers essentially increase the energy efficiency of the

system. #he large difference -et%een the e@it temperatres of the lean and rich

glycol from these heat e@changers re*ires conter crrent flo% to pre9ent

47


48/81

temperatre cross. #herefore; larger glycol dehydration systems generally se t%o

do-le"pipe or plate"and"frame heat e@changers in series.

Still Colu.n

eflu7 Coil- #he top of the still colmn contains a cooling coil that condenses

some of the steam rising from the re-oiler; pro9iding refl@ for the colmn. #his

cooling coil controls condensation and redces glycol losses.

,n addition to the re-oiler; the still colmn also reconcentrates glycol. $till colmns

contercrrently contact rich glycol %ith steam rising from the re-oiler. #his steam

strips %ater from the li*id glycol. $till colmns sally contain 4 to ft of ceramic

pac


49/81


50/81

temperatre in the top of the colmn is a-ot &&5EF. ,f the temperatre in the top of

the colmn drops too lo%; too mch %ater can -e condensed and %ashed -ac< into

the regenerator to flood the colmn and fill the re-oiler %ith e@cessi9e li*ids.

eboiler 0ressure!edcing the pressre in the re-oiler at a constant temperatre reslts in higher

glycol prity. #his pressre redction lo%ers the %ater partial pressre in the 9apor;

increasing the dri9ing force nder %hich %ater lea9es the glycol soltion.

Glycol Concentration

#he %ater content of the dehydrated gas depends primarily on the lean glycolconcentration. #he dry gas lea9es the contactor approaching e*ili-rim %ith the

lean glycol. #he leaner the glycol flo%ing to the a-sor-er; the more efficient the

dehydration. Figre 31 sho%s the effect of glycol concentration on gas de% point.

Glycol Circulation ate

When the nm-er of a-sor-er trays and glycol concentration are fi@ed; the de% point

depression of a satrated gas is a fnction of the glycol circlation rate. Whereas the

glycol concentration mainly affects the de% point of dry gas; the glycol rate controls

the total amont of %ater that can -e remo9ed. typical glycol circlation rate is

a-ot three gallons of glycol per pond of %ater remo9ed =se9en ma@imm>. #he

minimm circlation rate to assre good glycol"gas contacting is a-ot t%o gallons

of glycol for each pond of %ater remo9ed.

greater de% point depression is easier to achie9e -y increasing the glycol

concentration rather than -y increasing the glycol circlation rate =see Figre 4>. #o

se this plot; locate the glycol circlation rate; read p to the glycol concentration;

and then read across to find the de% point depression. n e@cessi9e circlation rate;

especially a-o9e the design capacity; o9erloads the re-oiler and pre9ents good

glycol regeneration. ,t also pre9ents ade*ate glycol"gas contacting in the a-sor-er;

increases pmp maintenance pro-lems; and can increase glycol losses.

5'

Figure 3(:

%ffect of ,%G circulation

rate and concentration onde /oint de/ression


51/81

Optii


52/81

decomposition prodcts; or acid gases pic


53/81

Foain#

Foaming can increase glycol losses and redce plant capacity. )ntrained glycol %ill

-e carried o9er the top of the a-sor-er %ith the sales gas %hen sta-le foam -ilds p

on the trays. Foaming also cases poor contacting -et%een the gas and glycol;

decreasing the drying efficiency.

$ome foam promoters are:

Hydrocar-on li*ids.

Field corrosion inhi-itors.

$alt.

Finely di9ided sspended solids.

)@cessi9e tr-lence and high li*id"to"9apor contacting 9elocities sally case

the glycol to foam. #his condition can -e cased -y mechanical or chemical

pro-lems.

#he -est %ay to pre9ent foaming is proper care of the glycol. #his in9ol9es effecti9e

gas cleaning ahead of the glycol system and good filtration of the circlating

soltion. #he se of defoamers does not sol9e the -asic pro-lem; and ser9es only as

a temporary control ntil the conditions generating foam can -e identified and

remo9ed.

'nal!sis and Control of Gl!col

nalysis of glycol is essential to good plant operation. (eaningfl analytical

information helps pinpoint high glycol losses; foaming; corrosion; and otheroperating pro-lems.

nalyses ena-le the operator to e9alate plant performance and ma


54/81

#he 9isal inspections shold ne@t -e spported -y chemical analysis. $amples of

the lean and rich glycol shold -e ta. #hese analyses

sally pro9ide sfficient information to determine the condition of the glycol.

Gl!col Loss (re,ention

Glycol losses can -e defined as li*id carryo9er from the contactor =normally '.1'gal2 ($CF %ith a standard mist eliminator> pls 9apori6ation from the contactor and

regenerator; and spillage. Glycol losses; e@clsi9e of spillage; range from '.'5

gal2($CF for high pressre; lo% temperatre gases to as mch as '.3' gal2($CF

for lo% pressre; high temperatre gases.

,here are seeral ays to reduce glycol losses

certain amont of glycol al%ays 9apori6es in the sales gas stream.de*ate cooling of the lean glycol -efore it enters the a-sor-er minimi6es

these losses.

ormally; most of the glycol entrainment is remo9ed -y a mist eliminator in

the top of the a-sor-er. )@cessi9e gas 9elocities and glycol foaming in the

a-sor-er sharply increase the glycol carryo9er. do%nstream gas scr--er

can pay for itself *ic


55/81

#hey shold -e a-le to operate p to pressre drops of &' to &5 psi. For -est reslts;

filters shold -e placed in the rich glycol line; -t the lean glycol can also -e filtered

to help


56/81

Solid esiccant eh!dration

7a(1#round

$ince solid desiccant nits cost more than glycol nits; their se is sally limited toapplications sch as 9ery sor gases; 9ery lo% %ater de% point re*irements;

simltaneos control of %ater and hydrocar-on de% points; and special cases sch

as o@ygen containing gases; etc. ,n cryogenic plants; solid desiccant dehydration

sally is preferred o9er methanol in/ection to pre9ent hydrate and ice formation.

$olid desiccants are also often sed for the drying and s%eetening of GJ li*ids.

Desiccants in common commercial se fall into one of three categories:

lmina " !egenera-le alminm o@ide -ase desiccant.

$ilica Gel " !egenera-le silicon o@ide adsor-ent.

(oleclar $ie9es " !egenera-le solid desiccants composed of crystalline

metal alminosilicates =6eolites>.

)ach desiccant category offers ad9antages in different ser9ices. #he -est choice is

not rotine.

cti9ated almina has a strong affinity for %ater and high internal adsorption area

de to the presence of pores or 9ery fine capillaries. lmina condenses and holdsthe %ater in the pores -y srface adsorption and capillary attraction. cti9ated

almina desiccant can -e sed for drying li*ids %hich do not contain nsatratessch as olefins or diolefins. ,t is less costly than moleclar sie9e desiccant -t its

capacity for a-sor-ing %ater also tends to -e lo%er; particlarly %hen attempting to

reach 9ery lo% %ater le9els; e.g. 5 %ppm in the prodct.

$ilica gel has a higher e*ili-rim adsorption capacity =see Figre 0> than almina

-ecase its a9aila-le srface is greater. De to silica gelVs higher price per pond;

almina is generally the economic choice. $ilica gel is not sed %here free %ater can

-e present; -ecase free %ater destroys silica gel. Free %ater o9er long"term

operation; either as droplets or slgs; %ill also damage moleclar sie9e and acti9ated

almina -y mechanical attrition and shold -e a9oided.

(oleclar sie9es ha9e the featre of niform pore si6e; %hich allo%s them to

e@clde molecles -ased on si6e. ?ecase different pore si6e moleclar sie9es are

prodced; selection of proper type of sie9e can alle9iate the pro-lem of ndesira-le

coadsorption.

50


57/81

(oleclarsie9es ha9e a higher design adsorption capacity than the other regenera-le

desiccants; -t this is often offset -y their considera-ly higher price per pond.

(oleclar sie9e dehydrators are commonly sed ahead of GJ reco9ery plants

%here e@tremely dry gas is re*ired. Cryogenic GJ plants designed to reco9erethane prodce 9ery cold temperatres and re*ire 9ery dry feed gas to pre9ent

formation of hydrates.

Dehydration to appro@imately 1 ppm% is possi-le %ith moleclar sie9es.

#%o types of moleclar sie9es; #ype 3 and #ype 4; are commonly sed for

drying hydrocar-on li*ids. #ype 4 sie9es are less costly than #ype 3 sie9es and

are sed for distillates %hich do not contain nsatrates. When nsatrates are

present in the feed; #ype 3 are sed to assre good regeneration.

$olid desiccants are sed in gas dehydrators containing t%o or more to%ers. Figre 7is a simple t%o"to%er system. ne to%er is onstream adsor-ing %ater from the gas;

%hile the other to%er is -eing regenerated and cooled. Figre sho%s a typical

moleclar sie9e dehydrator 9essel. Hot gas remo9es the adsor-ed %ater; after %hich

the to%er is cooled. #he to%ers are s%itched -efore the onstream to%er -ecomes

%ater satrated. Generally a -ed is designed to -e on line for to &4 hors. When

the -ed is ta


58/81

FG1% 3) : S"LD D%SCC6;, D%H


59/81

FG1% !*: ,


60/81


61/81

Dring the adsorption cycle; the -ed operates %ith three 6ones #he top 6one is called

the satration 6one. #he moleclar sie9e in this 6one is in e*ili-rim %ith the %etinlet gas. #he middle or mass transfer 6one =(#S> is %here the %ater content of the

gas is redced from satration to Y 1 ppm. ormally a system is designed so thatthere is a moistre analy6er to indicate %hen the mass transfer 6one is li


62/81

)9en thogh the (#S %ill contain some %ater; the satration 6one is calclated

assming it %ill contain all the %ater to -e remo9ed. #he length of the mass transfer

6one can -e calclated sing )*ation 7 from Wor< id 4. #he total -ed height is

the smmation of the satration 6one; mass transfer 6one; and gard -ed 6one

heights. ppro@imately si@ feet free space a-o9e and -elo% the -ed is needed.

FG1% !2 +"L% S%?% C606C,< C"%C,"; F"

1;S6,16,%D ;L%, G6S

FG1% !3 +"L% S%?% C606C,< C"%C,"; F"

,%+0%6,1%

0&


63/81

Pro(e$$ F"ow and te !un(tion o! te a3or(oponent$ o! So"id De$i((ant De'drator$+nli


64/81

s the flo% of gas contines; the (#SVs mo9e do%n%ard throgh the -ed and %ater

displaces all of the pre9iosly adsor-ed gas ntil; finally; the entire -ed is satrated

%ith %ater 9apor. When the -ed is completely satrated %ith %ater 9apor; the otlet

gas is /st as %et as the inlet gas. ?efore the desiccant -ed has -ecome completely

satrated; the to%ers mst -e s%itched from the adsorption cycle to the regenerationcycle =$ee Figre 44>.

ne regeneration"gas"spply scheme consists of ta of the entering %et"gas

stream throgh the regeneration system. ,n most plants; a flo% controller reglates

the 9olme of regeneration gas sed. #his gas is heated ntil it reaches 4''E to

0''EF; then it is piped to the to%er -eing regenerated. #he adsor-ed %ater -egins to

desor- at the start of the regeneration cycle if dry regeneration gas is sed. #he -l and the regeneration gas

in a t%o"to%er solid desiccant dehydrator.

FG1% !!: 0"C%SS FL"W "F ,W"-,"W% S"LD D%SCC6;, D%H


66/81


67/81

FG1% !#:0"C%SS FL"W "F ,H%%-,"W% S"LD D%SCC6;, D%H

6-&

D-&CD-&>D-&6

?-&

C

D

C

D

Dry GasTo

CoolingTrain

FeedGas

07


68/81

FG1% !$ 0"C%SS FL"W "F ,H%%-,"W% S"LD D%SCC6;, D%H


69/81

s %ith glycol dehydrators; inlet separators protect the dehydrator from imprities

sch as free %ater; salt; compressor oils; hydrocar-on li*ids; paraffins; corrosion

inhi-itors; glycol; amines; rst; iron slfide; iron o@ide; fractionation sands; drilling

md; pipeline scale; and slfr. #hese imprities impact the desiccant -ed and case

-rea canalso damage some adsor-ents.

on9olatile li*ids coat the desiccant and -loc< its pores. $olid imprities plg the

-ed increasing the pressre drop and crshing the desiccant. ll of these effects

shorten the operating life of the desiccant. ,f the dehydration nit is do%nstream of

an amine nit; glycol nit; or compressors; a filter"separator or li*id coalescer =for

li*id ser9ice> may -e needed.

6dsorber ,oer

#he adsor-er to%er holds the solid desiccant and contacts it %ith the process flid.

Figre 47 sho%s a typical adsor-er to%er.

0


70/81

FG1% !': 6DS">% ,"W%

#hree pro-lems that fre*ently case poor operation are insfficient distri-tion;

inade*ate inslation; and improper -ed spports.

Distribution" Poor gas distri-tion at the inlet and otlet of the desiccant -eds can

case gas channeling and desiccant damage. #he inlet gas distri-tor shold -e

16"Fillhole

Supportingscreen

1/8"ceramic

ball1/4"

ceramicball

1/16"mol. sieve(h!#

4"

4"

$

%.&.

7'


71/81


72/81

or %hole particles from plgging the screen openings. ormally; for 12"in.

desiccant; a &"3 inch layer of 12&"in. -alls is gently placed on the screen follo%ed -y

a smooth &"3 inch layer of 124"in. -alls. For 1210"in. desiccant =commonly sed for

li*id dehydration>; an additional layer of 12"in. -alls are placed on top of the 124"

in. -alls. #his complies %ith the &:1 ratio -et%een the layers as recommended -y thedesiccant 9endors.

egeneration Gas Heater

#he regeneration gas heater heats the regeneration gas to a-ot 5''EF. $olid

desiccant dehydrators se many types of heaters inclding salt -ath; direct fired; hot

oil; and steam.

$mall nits = ((?t2hr> generally se indirect"fired; salt -ath heaters for safety

reasons.

Jarger nits tend to se direct"fired heaters. ,n addition; other sorces of heat aresed inclding compressor"e@hast gases and %aste heat from tr-ines and other

heat sorces.

egeneration Gas Cooler

!egeneration gas coolers redce the temperatre of the regeneration gas to condensethe adsor-ed %ater and; sometimes; hydrocar-ons. Cooling the regeneration gas also

prepares it for frther processing.

Coolers are heat e@changers that se air; %ater; or natral gas to cool the

regeneration gas.

#ypically; they se am-ient air to cool the regeneration gas to %ithin 15EF to &'EF of

the air temperatre.

egeneration Gas Se/arator

!egeneration gas separators remo9e li*ids condensed -y the regeneration gas

cooler from the regeneration gas. ,f the li*id is primarily %ater; then a t%o"phase

separator; similar =e@cept smaller>; to the inlet separator is sed. ,f the li*id

contains s-stantial amonts of hydrocar-ons; then the dehydrator re*ires a three"phase separator to remo9e the li*id from the gas stream and separate the li*id into

%ater and hydrocar-ons.

Sitching ?ales

$%itching 9al9es direct the process flid and regeneration gas to the appropriatecomponent of the dehydrator. #%o"%ay 9al9es lea< less than three"%ay 9al9es.

$%itching 9al9es are in a harsh operating ser9ice as they mst operate %ith all

7&


73/81

com-inations of cold gas and hot gas on either side of the 9al9e. #his temperatre

cycling can case 9al9es to stic< and2or lea


74/81


75/81

egeneration 4Heating5 Cycle

ormally; regeneration gas flo%s in the direction opposite of the process flid in the

drying cycle. For drying hydrocar-on gas; this direction is p throgh the adsor-er

to%er. ,f the regeneration gas flo%s in the same direction as the process flid; thenregeneration gas mst displace the %ater and contaminants concentrated at the top of

the -ed do%n throgh the entire -ed. $ame direction flo% ris of the -ed strips contaminants from the desiccant. ,f the hot regeneration gas

sfficiently increases the partial pressre of the contaminants; the contaminants %ill

desor- off the desiccant. #his flo% direction also prodces e@tremely dry adsor-entat the -ottom of the adsor-er to%er.

Dring the dehydrating cycle; this dry adsor-ent remo9es the last amonts of %ater

from the process flid and prodces efflent %ith 9ery lo% %ater contents. ,f the

contaminants do not desor- off the desiccant; they %ill -ild p and potentially co is -eing dried; a mch

greater sefl capacity can -e e@pected for most desiccants than %hen partiallly

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force and to remo9e the increased mass of %ater in order to maintain the desired

efflent de% point. #he same mass flo% rate of incoming gas at a redced pressre

increases gas 9elocity and increases the -ed pressre drop. )@cessi9e pressre drop

cases dsting =adsor-ent -rea and damage to the desiccant. t pressres

a-o9e 1;3'' psia to 1;4'' psia; the coadsorption effects of hydrocar-ons sometimes-ecome significant.

Cycle ,i.e

#he drying cycle time is the rnlength of the drier -efore it re*ires regeneration.

#he design drying cycle sets the drier %ater loading and; hence; drier si6e. $ince

desiccant capacity decreases %ith age =nm-er of regenerations> initial cycle times

are considera-ly longer than design cycle times. Design cycle times are sed to

esta-lish %hen desiccant replacement is necessary. #he design cycle time is

appro@imately e*al to the regeneration time at the design flo% rate. #herefore;

%hen less time is re*ired to satrate a desiccant than to regenerate it; either thedessiccant mst -e replaced; or the flo% rate decreased.

#ypically; the adsorption cycle is operated on a fi@ed time. Fi@ed time cycles are

common as are dehydrator installations that s%itch -eds on %ater -rea: 1 hor

4. Heat to !egeneration #emperatre: aria-le5. Heat $oa< =not al%ays performed; discssed -elo%>: 1"& hors

0. Cool to #emperatre pproaching Drying #emperatre: aria-le

7. Fill =for li*id drying only>2Pressre: 1 hor

Z Where atomated regeneration facilities are pro9ided for small si6e e*ipment

considera-ly shorter times may -e practical.

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Gas ?elocities

nlet 40rocess5 Gas

Decreasing the gas 9elocity dring the drying cycle sally achie9es -oth lo%erefflent moistre contents and longer drying"cycle times. Figre 1 sho%s the

general effect of gas rate on the e@tent of dehydration. (inimm flo% rates tili6e

the desiccant flly. Ho%e9er; lo% linear 9elocities re*ire to%ers %ith large cross"

sectional areas. ,n selecting the linear flo% rate; a compromise mst -e made

-et%een the to%er diameter and the ma@imm tili6ation of the desiccant. high

linear flo% rate cases agitation of the granles; dsting and loss of capacity to

adsor-. ,n addition; flidi6ation can occr if the gas 9elocity =or li*id in the case of

li*id drying> p%ards throgh the -ed e@ceeds the flidi6ation 9elocity.

egeneration GasProdcing 9ery lo% efflent %ater contents =less than '.1 ppm> re*ires sfficiently

high regeneration gas 9elocities. Jo% gas 9elocities prodce channeling %hich

reslts in poor regeneration. Fre*ently; achie9ing 9ery lo% efflent %ater contents

re*ires regeneration gas 9elocities of at least 1' ft2min.

egeneration Gas Source

#he sorce of gas for heating and cooling desiccant -eds depends on plant

re*irements and; possi-ly; on the a9aila-ility of a sita-le gas stream. +sing dry

regeneration gas prodces efflent %ith lo% %ater contents. +sing %et feed gas

reslts in moderate efflent %ater contents. Graphs plotting isoteres =lines of

constant %ater loading> can -e sed to predict the regeneration gas conditions

re*ired to achie9e a gi9en efflent %ater content.

#he effecti9eness of reacti9ation can also play a ma/or role in retarding the decline

of a desiccant adsorpti9e capacity and in prolonging its sefl life. ot remo9ing all

of the %ater from the desiccant dring each regeneration sharply decreases its

seflness. For e@ample; if the dynamic adsorpti9e capacity of a thoroghly

reacti9ated desiccant is 1'A. 3A residal %ater remaining on the desiccant-ecase of insfficient regeneration; %old case its capacity to drop from 1'A to

7A.

lthogh gases rich in hea9ier hydrocar-ons may -e dried satisfactorily %ith

moleclar sie9es; the se of this same rich gas in a 5''E to 0''EF regeneration

ser9ice aggra9ates co


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ser9ice aggra9ates co


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Equipent Ite$

,n addition to the a-o9e process 9aria-les; engineers can optimi6e solid desiccant

dehydration e*ipment -y considering the follo%ing:

n accrate estimation of -ed si6es in order to realistically e9alatecompetiti9e -ids from desiccant 9endors.

ptimal design of adsor-er internals =inlet gas distri-tor; internal inslation

and -ed spports>; s%itching 9al9es; and control systems.

Proper design of regeneration gas systems.

$ince mole sie9e can prodce dst; filters are fre*ently installed

do%nstream to protect s-se*ent e*ipment.

04 dehydration 1

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