m

H-I

SUPER-DEHYDRATION WITH GLYCOLS

by Steve Wor ley

B lack , S iva l l s ~ Bryson , Incorporated Ok lahoma C i ty , Ok lahoma

Deve lopment of Dehydrat ion w i th G lyco ls

The f i r s t g lyco l type dehydrators fo r natura l gas were custom des igned and nonpor tab le un i t s w i th fa i r l y e laborate rebo i l ing and re f lux ing equ ipment . Most un i t s were des igned in a manner s imi la r to the ambient - temperature lean o i l gaso l ine p lants in use in about 1945. D ie thy lene g lyco l was genera l ly used a l though some used e thy lene g lyco l . Most un i t s were l im i ted to dew po in t depress ions of about 40OF - 50OF.

Unt i l about 1947-1948, when t r ie thy lene g lyco l (TEG) un i t s were in t roduced on the market , d ry des iccant dehydrat ion was necessary when dew po in t depress ions greater than about 50OF were requ i red . Cont inued progress w i th TEG un i ts now a l lows dew po in t depress ions as great as 160OF fo r f ie ld un i t s . The pat tern o f p rogress has been about as ind icated in Tab le I be low. The spread o f temperatures l i s ted ind icates the e f fec t o f contact temperatures in the range of 70F to 160OF. P ressure has no measurab le e f fec t except as i t a f fec ts the amount o f water to be removed.

TABLE I

MAXIMUM DEW POINT DEPRESSION RANGE

YEAR OF

1947 50 1948 60 1950 65 1953 60 - 75 1957 75 - 90 1958 90 - 110 1959 100 - 140 1964 110 - 150 1966 120 - 160

Res idua l water contents as low as 0 .15 lb . per MMSCF and water dew po in ts as low as -57F have been a t ta ined w i th TEG in f ie ld un i t s . At the same t ime there has been l i t t le inc rease in g lyco l dehydrat ion cos t due to re f inements that have a l lowed increased capac i ty and decreased ut i l i ty cos ts .

The most s ign i f i cant deve lopment in TEG dehydrat ion began in 1946. At that t ime, P ro fessor Laurance S. Re id , Un ivers i ty of Ok lahoma, began work on samples o f t r ie thy lene g lyco l w i th s t rong suppor t f rom Mr. Les Po lderman of Un ion Carb ide Chemica ls Company

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Dur ing the next f i ve o r s ix years cons iderab le e f fo r t was devoted to tak ing laboratory data fo r the sys tem t r ie thy lene g lyco l -water - natura l gas . P ro fessor Re id d i rec ted severa l g raduate s tudents in numerous invest igat ions , some of wh ich were sponsored by fe l lowsh ips . P ro fessor Re id and J . A. Por ter repor ted some o f the ear ly f ind ings in 1949 (5 ) P ro fessor Re id and .Pro lessor Mark Townsend made add i - t iona l repor ts in 1951 and 1953 ~6, 7 , 8 ) .

Ma jor emphas is was p laced on the use of t r ie thy lene g lyco l in 1948 and 1949 w i th the in t roduct ion on the market o f g reat ly s imp l i - f i ed and packaged we l lhead g lyco l dehydrat ion un i ts . One of the f i r s t fo rmal reoor ts o f such ins ta l la t ions was that o f Mr. L. H. Peah l in 1959 (3 ) F igure 1 i s a p ic ture of one of the 1949 mode l un i t s d i scussed by Mr. Peah l .

Fur ther p rogress w i th g lyco l dehydrat ion un i ts was s low unt i l 1957 when both B lack , S iva l l s and Bryson , Inc . and Parkersburg deve loped the s t r ipp ing gas method of super - reconcent ra t ion of g lyco ls . Fur ther p rogress was de layed unt i l more accurate equ i l ib r ium data were deve loped.

Des ign Var iab les

Unt i l about 1957, glycol dehydrat ion was cons idered in te rms o f 65OF dew po in t depress ions , 3 ga l lons /pound water c i rcu la t ion , and a 4 - t ray contactor tower . Today spec i f i c un i t s a re des igned fo r dew po in t depress ions f rom 40OF to 160F , fo r g lyco l c i rcu la t ion ra tes of f rom 1 ga l . / lb , o f water to as much as 8 ga l . / lb , o f water and fo r t rays f rom 2 to as many as 16. Even more t rays w i l l be used in spec ia l cases in the fu ture .

Tab le I I be low ind icates the usua l range of des ign var iab les encountered in 1950 and in 1967.

TABLE II

USUAL RANGE OF DESIGN VARIABLES

Year Number t rays Dew po in t depress ion - OF Rebo i le r temperatures - OF G lyco l ra tes - ga l . / lb , water Contact temperatures - OF G lyco l concent ra t ions - % Pressure - ps ig

1950 1967

4 2 - 12

40 - 60 40 - 140

350 375 - 400

3 i - 8

50 - I00 40 - 160

96 - 98 99 - 99 .98

500 - 1200 25 - 2500

In i t ia l l y , regenerat ion techn iques prov ided fo r lean g lyco l so lu t ions of 95% to 96% g lyco l . In rea l i ty , concent ra t ions as h igh as about 98 .5% were probab ly in i t ia l l y a t ta ined but ex is t ing ana ly t i ca l and f ie ld sampl ing techn iques d id not a l low detect ion of th i s fac t unt i l about 1953. The deve lopment of more soph i t i ca ted regenerat ion techn iques and more accurate ana ly t i ca l methods have a l lowed regenerat ion of t r ie thy lene g lyco l up to as h igh as 99 .98% g lyco l

H-3

Consistent regenerat ion to concentrat ions as high as 99.98% TEG has been attained in f ield units from standard designed high concen- tration regenerators.

A brief visual compar ison of the first TEG dehydrators with those fabr icated today wil l not indicate a great change For instance, compare Figure 2, a picture of a recent s imilar sized unit~ with that of Figure I. Compar isons of the process f low diagrams for the two units would also indicate little change. In spite o:f the small out- ward appearance changes, dehydrator performances have been greatly improved. For example, $9~sider Figure 3. The curve indicated was first publ ished in 1952 x~'. At that time the curve indicated the maximum absorber contact temperature at which 7 lb. per MMSCF water content could be attained at various absorber pressures. The basis for the curve was a 65F water dew point depression, the greatest that could be rel ied upon at that time. Today the same curve almost exact ly represents the maximum absorber contact temperature at which 1/4 Ibo per MMSCF water content can be obtained.

Improved Data fo r Sys tem Gas-Water -TEG

An important step in the ref inement of glycol dehydrator perfor- mance has been in more accurately establ ish ing equi l ibr ium water dew points in the TEG-WATER-GAS sMstem. Figure 4 indicates two sets of data for the system in 1966 for one contact temperature.

It is s igni f icant to note that nothing has changed regarding the true equi l ibr ium between water and TEG. The only thing that has changed has been the accuracy of our knowledge of the system.

Curve number B of Figure 4 represents predicted per fo rm~e based on exper imental dat~_~aken by Professor Mark Townsend

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exhaust ive s tudy of dew po in t depress ions obta inab le w i th t r ie thy lene g lyco l . The tes ts were conducted w i th fu l l - sca le s i ze equ ipment . The tes t fac i l i t i es inc luded prov is ions fo r c i rcu la t ing up to l0 MMSCF/D of gas a t p ressures up to 1 ,000 ps i w i th as much as 100 ps i p ressure drop in the c losed gas sys tem. Add i t iona l s tud ies a re in p rogress w i th up to 20 contactor t rays and a t a w ide range o f operat ing cond i - t ions .

Re fer r ing to F igure 6 the tes t gas leaves the rec i rcu la t ion com- pressor , passes through a heater where i t i s p reheated , and then i s contacted w i th water in a mul t i - t ray vesse l . The water enters the saturat ion vesse l a t 180OF or h igher . The preheated gas and water p rov ides fo r the heat o f vapor i za t ion fo r the water that i s vapor i zed in to the dry -gas s t ream. The wet , hot gas s t ream then i s coo led down to the p lanned operat ing temperature . Care i s taken to make sure that water cont inuous ly accumulates in the separator jus t ahead o f the dehydrat ion contactor in o rder to assure saturat ion of the inlet gas s t ream.

G lyco l , hav ing cons is tent concent ra t ions in the range f rom 98 to 99 .98 per cent , was obta ined f rom the regenerat ion sys tem. The des iccant was c i rcu la ted over vary ing numbers of t rays in the con- tac t ing tower . Water dew po in ts were taken between t rays and down- stream from the unit with both an e lectr ica l dew point analyzer and the U. S. Bureau of Mines dew point tester.

It is s igni f icant to note that the glycol ut i l ized was typical of that in field use rather than new, clean glycol. Some of the data was taken w i th g lyco l that had been exposed to temperatures as h igh as 450OF fo r extended per iods o f t ime in o rder to reproduce typ ica l f ie ld cond i t ions .

Over 1 ,000 dew po in ts were taken in the in i t ia l ser ies of runs . G lyco l concent ra t ions were var ied f rom 98 to 99 .97 per cent . P res - sures were var ied f rom 200 to 900 ps i . Temperatures were var ied f rom 600 to 130OF. G lyco l c i rcu la t ion ra tes were var ied f rom 1 ga l . to 15 ga ls . o f g lyco l / lb , o f water in the in le t gas s t ream.

The dew po in t data were p lo t ted and cor re la ted so that the e f fec ts o f g lyco l concent ra t ions , g lyco l c i rcu la t ion ra tes , contact temperatures , contact p ressures and number of contact t rays cou ld be pred ic ted . Smal l re f inements have been e f fec ted as a resu l t o f care fu l s tud ies made on f ie ld operat ing un i ts dur ing the past 8 years .

A l l g lyco l samples , whether f rom f ie ld un i t s o r f rom the laboratory f ie ld s i ze un i t , a re ana lyzed fo r water content by the Kar l F i scher t i t ra t ion method. Compar i son o f many dup l i ca te samples ind icates reproduc ib i l i ty as fo l lows :

TEG G lyco l Concent ra t ion Range

99 .95 - 100 .00% 99 .50 - 99 .95% 99 .00 - 99 .50% 98 .00 - 99 .00%

Di f fe rence Between Dup l i ca tes

0.005% o.o1% 0.02% 0.04%

m

H-5

New Regenerat ion Method

The most ef fect ive ref inement in the performance of t r iethylene glycol dehydrat ion units has been in the method of regenerat ion as indicated in U. S. Patent No. 3,105,748 and Canadian Patent No. 733,820.

In summary, the new regenerat ion pr inciple involves the removal of water from glycol by convent ional regenerat ion techniques up to about 99.1% glycol and the further pur i f icat ion by gas stripping. The gas str ipping technique consists of preheat ing a very small quant i ty of natural gas and contact ing it countercurrently with 99.1% tr iethylene glycol as it leaves the reboi ler and f lows to the storage tank. The small amount of countercurrent ly f lowing natural gas reduces the partial pressure of water vapor in contact with the part ia l ly regenerated glycol to a very low quantity. This forces a shaft of the equi l ibr ium so that addit ional water vapor leaves the glycol and enters the vapor phase at essent ia l ly the same temperature resul t ing in very low residual water contents. This technique is part icu lar ly effect ive when accompl ished with countercurrent contact- ing techniques.

F igure 7 represents the expected g lyco l concent ra t ions under var ious cond i t ions of regenerat ion . As noted , a 400F rebo i le r temperature a t 1200 feet e levat ion w i l l resu l t in about 99.1% TEG wi th no s t r ipp ing gas be ing in jec ted . The e f fec t of ins ta l la t ion and operat ion a t h igher e levat ions i s to a l low h igher concent ra t ions than ind icated a t any s t r ipp ing gas ra te . The g lyco l concent ra t ion resu l t ing f rom 400OF rebo i le r temperature and zero s t r ipp ing gas a t sea leve l i s about 99.0% TEG.

Similar curves to those indicated in F igure 7 can be drawn for any regenerator temperature or any elevation. The effect of lower reboi ler temperatures is paral le l curves at somewhat lower concentrat ions.

If str ipping gas is injected direct ly into the reboi ler the results are as indicated by the lower curve. The upper curve represents the glycol concentrat ion result ing from counter -current ly in ject ing str ipping gas after the major amount of water vapor has been removed. The dramatic d i f ference in the glycol concentrat ions produced from the two methods results from two major principles. One is the pr inciple of mass transfer by countercurrent contact of the liquid and gas phases as compared to a single cocur rent contact of the phases. The second pr inciple is the greater dr iv ing force resul t ing from inject ion of str ipping gas after most of the water is previously removed by the appl icat ion of heat. This is compared to the lower dr iv ing force result ing from injection of str ipping gas in the same area where the bulk of the water is released by boil ing.

The pr inciple of in ject ion of str ipping gas into the reboi ler was a standard feature on all Standard regenerators furnished in

H-6

1950 by Black, Sival ls & Bryson, Inc. The pract ice was d iscont inued in early 1951 because of lack of evidence of ef fect ive results that would just i fy the quant i ty of str ipping gas required. The later development of stabi l ized Karl F ischer reagent and the e lectronic detect ion of the analyt ical end points al lows accurate measurements of small changes. In addition, the development of e lectronic water analyzers for gas streams al lows accurate detect ion of a few degrees change in dew points.

Super -Dehydrat ion App l i ca t ions

Effect ive ut i l izat ion of glycol dehydrat ion for dew point depress ions in excess of 100OF offers new potent ia ls for savings in equipment costs and operat ing costs. For example, it is now more often pract ical than in the past to dehydrate at high pipel ine f lowing temperatures rather than cool the gas stream and separate any condensed l iquids prior to dehydrat ion with solid or liquid desiccants.

This s ituat ion has come about as the result of careful studies of the effect of glycol rates and trays upon dew point depress ions as well as from a better knowledge of the true equ i l ibr ium and the avai labi l i ty of higher TEG concentrat ions.

The opt imum appl icat ion of super-dehydrat ion necess i tates careful cons iderat ion of many variables. For example, consider the dehydra- tion of a high temperature and low pressure gas stream. The gas wil l contain several hundred pounds of water per MMSCFD. There is a need to minimize the amount of glycol c i rculated to reduce the fuel and pumping power requirements. Exhaust ive studies have indicated it is pract ical to ut i l ize as l itt le as 1 gal. TEG/lb. of water removed in some cases. This is practical, however, only with addit ional trays and higher TEG concentrat ions. In addition, accurate knowledge of the thermodynamics of the system must be known and cons iderat ions must be given to the type of gas being treated.

For example, consider F igure 8. The temperature data were taken from a unit where several hundred pounds of water per MMSCFD were being removed. Heat evolved by the absorpt ion of water vapor by the glycol resulted in a cons iderable increase in the contact temperature.

Note the fo l low ing :

(a) Gas out le t temperature 17F h igher than the in le t temperature

(b) G lyco l out le t temperature 15F h igher than the in le t temperature

(c) Maximum t ray temperature 7OF h igher than the out le t gas temperature .

It is also s igni f icant to note that the above temperature d i f fe rences" change with the specif ic gravity of the gas being dehydrated when all

H-7

o ther var iab les remain unchanged. Th is fac t resu l t s f rom a change in the heat absorb ing capac i ty o f the gas due to the changed mass o f gas and , to a smal le r degree , the changed spec i f i c heat o f the gas .

When temperature bu lges of the type ind icated in F igure 7 occur, care fu l eva luat ion must be made of both the temperature and equ i l ib r ium bu lges that occur w i th in the contactor tower.

I n sp i te o f the apparent compl ica t ions d i scussed above , F igure 7 represents an ac tua l app l i ca t ion where dehydrat ion a t 140OF in le t temperature to 22OF dew po in t was jus t i f ied ins tead o f coo l ing pr io r to dehydrat ion .

Most dehydrat ion app l i ca t ions have less than I00 Ib . /MMSCFD o f water to be removed and there fore resu l t in on ly a few degrees increase in temperature in the contactor .

Perhaps , the greates t use fo r super -dehydrat ion w i th g lyco ls w i l l be in the area o f gas cond i t ion ing ahead o f re f r igerated lean o i l absorpt ion p lants . In many cases g lyco l dehydrat ion w i l l app ly where out le t dew po in ts in the range o f -20OF to -50OF are requ i red . Th is method prevents many potent ia l p rob lems o f g lyco l d i s t r ibut ion and g lyco l - condensate separat ion that occur w i th g lyco l in jec t ion sys tems.

F igure 9 ind icates the re la t ionsh ip o f g lyco l concent ra t ion changes fo r normal app l i ca t ion w i th a la rger number o f t rays . In genera l , s t ra ight l ines resu l t but they are somet imes a f fec ted when la rge quant i t ies o f water a re removed w i th re la t ive ly low g lyco l ra tes .

Use of D i rec t F i red Rebo i le rs

Super dehydrat ion can be a t ta ined a t rebo i le r temperatures o f 400F or lower . The lower rebo i le r temperatures requ i re more s t r ip - p ing gas or more e laborate regenerat ion equ ipment to a t ta in the same TEG concent ra t ion .

Up to ten years ' exper ience o f operat ion o f severa l hundred un i ts a t 400OF rebo i le r temperatures fa i l s to ind icate any ev idence o f measurab le losses by degradat ion .

F igure I0 ind icates typ ica l f lue gas and meta l sk in temperatures fo r a d i rec t f i red g lyco l rebo i le r f i re tube w i th 400OF bu lk tempera- tu re . The temperatures a re representat ive o f an average f lux ra te o f about 8 ,000 BTU/hr . - f t . 2. Whi le look ing a t F igure l0 i t i s easy to v i sua l i ze what e f fec ts w i l l resu l t by chang ing the length o f the f i re tube so that the average f lux ra te var ies f rom 6 ,000 to i0 ,000 BTU/hr . - f t . 2. In th i s manner , i t i s eas i ly seen that the maximum tube wa l l temperature in contact w i th the bo i l ing g lyco l so lu t ion i s not changed by chang ing the average des ign f lux ra te .

Maximum f lux ra tes in d i rect f i red rebo i le r f i re tubes is a funct ion of the forced convect ion coef f i c ient on the ins ide and the mean beam length of the rad ia t ion component . D i rec t and hard impinge- ment of the f lame aga ins t the f i re tube should be avoided, however .

H-8

F igure I i ind icates the approx imate temperature drops that occur across the f i re tube wa l l ind icated in F igure i0 a t the po in t of maximum f lux ra te .

By referr ing to F igure I0 and Figure ii, it is easy to v isual ize that the use of high pressure steam for reboi ler heat ing media can result in much higher skin temperatures than when ut i l iz ing a direct fired reboiler. For example, consider the use of 500OF steam. The condensing steam heat transfer coeff ic ient is about equal to the boi l ing glycol coeff ic ient. In that event the metal skin tempera- ture in contact with the boi l ing glycol would be about 450F. The use of high velocity, high temperature heat transfer oil can also result in metal skin temperatures that are in excess of those for direct fired reboi lers if not proper ly designed.

Summary

Super -dehydrat ion w i th g lyco ls i s an e f fec t ive method o f reduc ing cap i ta l equ ipment cos ts and operat ing cos ts in most app l i ca t ions where h igh dew po in t depress ions are requ i red . I t can be e f fec t ive ly app l ied to a lmost any gas , a t a lmost any pressure and a t a w ide range of contact temperatures .

H-9

Bibliography

. "Some Critical Aspects of Designing for High Dew Point Depression with Glycols," J. A. Loomer and J. W. Welch, Gas Conditioning Conference, University of Oklahoma, March, 1961

. Scauz i l l o , F rank R . , "Equ i l ib r ium Rat ios o f Water in the Water - T r ie thy lene G lyco l -Natura l Gas System," Journa l o f Pet ro leum Techno logy , Ju ly , 1961.

. Peah l , L. H . , " Ins ta l la t ion , Operat ion and Per fo rmance o f a Sk id Mounted Gas Dehydrat ion P lant , " O i l and Gas Journa l , Ju ly 13, 1950, pp. 92-96

. John M. Campbe l l and Lawton L. Laurence , "G lyco l Dehydrat ion , " Par t I I I o f "Dehydrat ion o f Natura l Gas and Hydrocarbon L iqu ids , " Pet ro leum Ref iner , Vo l . 31, No. 11, pp. 109 (1952)

5. Por ter , J . A. and Re id , L. S . , p resented a t A . I .M .E . meet ing , San Anton io , Texas , October , 1949.

6. Po l i t z iner , I . , Townsend, F. M . , and Re id , L. S . , S t . Lou is meet ing A . I .M .E . , Pet ro leum Branch (1951)

7. Townsend, F. M. "Vapor-Liquid Equil ibrium Data for DEG and TEG- Water-Natural Gas System," Gas Conditioning Conference, May, 1953.

. Townsend, F. M., "Equ i l ib r ium Water Contents of Natura l Gas Dehydrated by Aqueous D ie thy lene and Tr ie thy lene G lyco l So lu t ions a t Var ious Temperatures and Pressures , " Ph .D. d i sser ta t ion , The Un ivers i ty of Ok lahoma (1955)

9. Wise, H. , Puck, T. T. and Failey, C. F., Journal of Physical Chemis t ry , (1950) 54, 734

i 0 . U . S . Patent No. 3 ,105 ,748 ass igned to B lack , S iva l l s & Bryson , Inc.

l l . Canad ian Patent No. 733 ,820 ass igned to B lack , S iva l l s & Bryson , Inc .

I I I I I I l i

Fig. 1 Early Day Packaged TEG Dehydrator (9)

F ig . 2 H igh Concent ra t ion Dehydrator

1 t 1 1 I i ! I 1 1 I 1 I I 1 ! 1 I l

120 ' '

II0

/ /

/ /

6O

0 ~

I00

I - - - r , . )

~- 8Q

p~ 7o ' 4 (

Fig. IS

f . /

/

I i f

m ~ ALL, O I t .E C.tS

~ l t l tE TO

~/m IX 1952 fq)

801} 1200 1600 2900 2WOO

~ P'~.

Dehydrator Performance in tqS2 and in 196~

10-0 -- 8--

6~ TEG- iT lm sYslTn EI~IILImlUN MATIEII OM POINTS IOO"F cmtl'lcr ~ I tTLqf~ ,.f

, , , , , i v / .G

.q t /

-" .3 , ' /~"

- _,4, ~ , , - m //

.I /

.DO /

" / .o3 /

.01 -60

f /

/

-qo

I~G.q

A- - F IELD DATA

B - - $CIUZILLO {2)

-20 0 "20 "~ DE;; POINT " r

EQUILI]~ILIN DEll POINTS FOR IO0"F CONTk"T PtJBLISXED IN 1966

"60 +80

~._ ~ . . . . . , ,

4o .o .o .Oco~c T ,~To~to_ ,4~ ,.o ,.o ,To

FIG. 5 EQUILIBRIUM WATER DEW POINTS WITH VARIOUS CONCENTRATIONS TEG.

I 1 ] I I I I I ! I I ! I I i l i I l

m I i i

f v t m IUrTI~ W m N - ~ m t98 *'

-

I IZO0 rl~ m

G]m~Nrllm~. m ~NBtXI'I~

4 6 8 I0 12 14

99.2

99.0 0 Z

! I I l t i

z o

| -

o a~

~z

120 124 128 132 136 140 144 148 152 156 I~ 164 168 IT~ TEMPE RATURE - F

FIG, 8 GAS AND GLYCOL TEMPERATURE I:~ORLES FOR FIE~WY WATER LOA OS. FIG g GLYC-OI__ C,I::::I, ICZNTR, eQ'K3N FT:I3FLE FOR NORMAL ~e~TER LOADS.

( J

~_ 404F ur,

900 F

I 2 f::~)Oo F

430 F MAX

o

407 F

,,,

410F /

II O0F ~ 1400F

_ - - 2oO0F

/o 425 F 420 F

I

415 F

GLYCOL BULK TEMP = 400F

FIG. I0 TYPICAL DIRECT FIRED GLYCOL REBOILER TEMPERATURE PROFILES OF FLUE GAS AND FIRE TUBE.

~ L O W RESISTANT BOILING HEAT TRANSFER COEFF,C,ENT

0 Q ~ .rTH,s ~RF, CE ,N CONTACT

| , ~ ( I WITH BOILIN6 GLYCOL

SECTION OF FIRE ~/ / / / / / / / / / / / / / / / / / fe~Iw- - - - T U B E WALL

, _ / - HIGHLY RESISTANLTO HEAT RADIATING FLUE GASES TRANSFER ('H I ., 30)

430F MAX METAL SKIN TEMP ON GLYCOL SIDE

445"F METAL KIN TEMP GAS 51DE

- " - - -POINT FLUE GAS TEMP. = 2600 F

~ ULK TEMP. = 400 F , , , . . . . I AT GLYCOL = 30F

,AT METAL = #5F

~T FLUE GAS = 2155F

_ FIG. If TYPICAL FIRETUBE TEMPERATURE DROPS FOR DIRECT FIRED GLYCOL REBOILERS

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