Corrosion bv and Deterioration of Glycol and GlycoLAmine Solutions J
DIETHY LENE GLYCOL SOLUTIONS W. G. LLOYD AND F. C. TAYLOR, JR. Technical Service and Development Laboratory, T h e Dou! Chemical Co., Bay C i t y , Mich.
IETHYLENE glycol 18 finding increasing use in industrial processes as a dehumidifying agent. For dehumidification of natural gas, the glycol typically comes in contact with an ascending stream of the moist gas in an absorption tower and, moving counterflow, is removed a t the base of the tower, pumped to a stripping still where the excess water is removed, and returned to the top of the absorption tower to complete the cvcle (6, 7 , 1 7 , 23). In this process the glycol, containing several per cent of water is dubjected to both intermittent exposure to elevated temperatures and a cumulative exposure to impurities in the feed stream -e.g., traces of light acids and oxygen. Consequently, in the absence of effective corrosion control, corrosion of steel equipment may develop to serious proportions over a period of time. Other worker8 (15, 81, SO) have recently described several specific instances of plant corrosion and several methods of corrosion control, The present n-ork is intended to contribute to the clarification of the chemical factors affecting corrosion by aqueous glycol solutions, and to indicate some means of controlling plant corroilion. Corrosion by glycol solutions has been determined by means of reflux tests employing a flask which is essentially a glass double boiler connected to two reflux condensers (Figure 1). This type of corrosion flask offers two advantages; the skin temperature of the inside flask containing the test solution may be regulated by the choice of a suitable heat-transfer fluid for the jacketing flaPk, and the overhead condenser for the inner flask is modified to make possible the attachment of a metal strip in such a manner that the strip is in continuous contact with the descending vapor condensate during reflux tests. The glycol solutions were made up to contain 7.5% by volume of diqtilled water; these solutions reflux a t 146" C. EXPERIMENTAL
All corrosion tests employed rectangular metal coupon3 with iurface areas of 2.5 sq. inches. Steel, where otherwise unspecified, is S.B.E. 1010 mild steel. Croloy is Croloy No. 5 steel (4-6 Cr 0.5 N o ) . Aluminum is 3s-H-14. The reflux corrosion teste employ flask assemblies as sho.icn in Figure 1, with Dowtherm E (boiling point, 177" C.) as the heat transler medium. Vapor-condensate coupons were suspended by small hooks from the reflux condenser drip loops In preliminary tests glass hooks were used, but they were found to be inconveniently fragile. Duplicate tests with glass and Chromel hooks showed no significant diff erences in corrosion rate, and accordingly Chromel hooks were employed. During the corrosion terts the gas atmosphere was maintained by a steady diffusion of the gas into the upper flask area a t a rate of 15 ml. per minute. Corrosion was measured glavimetrically, by weighing tared exposed coupons after scrubbing with Lava soap and a soft nylon bristle brush, rinsing twice in acetone, and drying in a desiccator. (Consequently, all corrosion penetration data reported are average, not maximum.) Data are reported in mils penetration per year (m.p.y,). The static immersion tests were carried out by immersing freshly scrubbed, weighed coupons in 100-ml. portions of test solutions, contained in 125-m1. Erlenmeyer flasks connected by tapered glass seals t o overhead reflux condensers and thermostated by a water bath a t 85" C. Corrosion of the metal coupons
after testing was measured in the same manner as in the reflux corrosion tests. Fractionation of the glycol in Table I was accomplished with a 10 X 3/? inch column packed with ceramic raschig rings. The glycol distilled a t 92-8" a t 1.5 mm. Tops and bottoms represent, respectively, the first and residual thirds of a 600-ml. charge. Hydroxyl exchange of the glycol r i t h Dowex 2 (OH) resin was carried out batchwise by bringing the glycol into contact with a very large excess of the resin (1 liter per 10 liters of glycol). The resin was dewatered prior to the exchange by allowing i t to stand 24 hours with occasional agitation under two successive portions of glycol (which were then discarded). The deionized sample of glycol mras further pretreated in a similar manner with Dowex 50 (H) resin. Glycol autoxidations were carried out in corrosGon flasks (Figure 1) but in the absence of metals, and with the aerator immersed in the glycol solutions. Carbon dioxide-free air was employed, and acids in exit gases were trapped with two standard aqueous caustic traps in series. For all tests the great preponderancy of acids formed remained in the glycol. Free acids were assayed by addition of excess 0. lAr aqueous sodium hydroxide, followed, after 3 to 4 hours' standing covered a t room temperature, by back-titration with 0.LV aqueous hydrochloric acid, End points were all close to p H 8.3, varying slightly with acid concentration. Conductance measurements were obtained from specific resistances measured with a Leeds and Northrup electrolytic conductivity bridge, with an L. and N. cell No. 4920. Thiosulfates were assayed by the iodometric method of Scott (E%), following pretreatment ~1ith excess freshly precipitated cadmium carbonate. Steam distillation of the plant-used glycol-amine solution was carried out after acidification with excess reagent phosphoric acid, followed by distillation to attrition; 1 to 3 volumes of water per volume of solution were maintained a t all times. Formic acid was isolated from the steam distillate as the sodium salt, exhibiting the folloming characteristics: reduced mercuric acetate; reduced alkaline permanganate; neutral equivalent (RCOOYa) = 68; p-bromophenacvl ester melting a t 141O , uncorrected (Fisher-Johns block). pormaldehyde and acetaldehyde, obtained from laboratory autoxidation, were identified by their dimedon derivatives, melting a t 191' and l53-9", respectively, and showing no depression when mixed with authentic derivatives. Viscosities were determined kinematically with a Fenskemodified Ostwald viscometer thermostated for the carbonate runs at 0" rt 0.1" C., and for the formate runs a t 54.8"& 0.2" 6. EFFECT OF GLYCOL ACIDITY
Four batches of diethylene glycol were employed in the tests. Ba,tches A and B were rendered slightly acidic by exposure to air a t elevated temperatures, while batches C and D were substantially neutral. The most acidic glycol sample, batch A, had a p H of 4.0. The results of reflux corrosion tests in nitrogen and oxygen atmospheres are shown in Table I. Four inferences may be drawn from these data. (1) Corrosion of the immersed metal strips vas negligible as compared with that of the suspended strips in contact with the vapor condensates. (While immersed strips have been included in all subsequent tests, all future data refer only to vapor-condensate corrosion.) (2) Samples which had been rendered acidic either by autoxidation or by addition of acetic acid were consistently more corrosive than comparatively neutral samples. ( 3 ) Neutral salts 2407
INDUSTRIAL AND ENGINEERING CHEMISTRY
2408
OF MILII STEEL BY DIETHYLESE GLI-~LTABLE I. CORROSION R A T E K 8OLr;TIOSS
l>EG
Treatment or ;idditire
Corrosion -
Immersed
Raies, __hI,I'.l-.~~ Yapor-phasc
Nitrogen Atnioy)licre, 100-1Iour Reflus
VoI. 46, No. 11
nieaiis of rendering the gl>-cwl noi~corrosivewhen thc gly1,oI isubjected to conditions proiiioting autosidatiun. Other workers have s h o ~ i ithc important effects of ositl;i\iou products upon corrosioii rate (if-6'), asid the beneficial eff(:c~tsof :~llralinebuffer additives (6, 11, 14, 15, 50). A further i~itli(?:~~i:tii of t'hc importance of p€I t,o corrosiori control may be inspection of the semilog p1c)t of glycol PIX (1 to 1 dilut distilled ii-t%ter)us. corrosion rat(: in nitrogen atmosphere ( f ? g i r t i 4 ).
B H
u
B
B
B B C C C
C C C C
C C C
None H O A c 0.2 wt. Naod'c, 0.2 \VI. /c XaCl, 0.2 wt. % NaOlI, 0 2 w b . 7' K?%TPOi.0.2 Wt. '6 IGHPOI and SaC1. 0 , 2 wt. yo each Sone TEPAb. 0 0.5 n-1. C+ h'a?I-IPO, 0 . 0 5 \i-t. 5
0.6 0.0 0.1 0.0 0.0 0.0 0.3
3 1. 220 30 II e 1
0.1 0 .o
10 0.3 0 0
Kone M E : A ~ , O . ~ ~%~ W ~ . T E P A c , 0.025 wt. % KpHPOn, 0 , 0 2 5 v t . % ' KaPO4, 0 . 0 2 5 wt. "0 N?H4, 0.20 wt. ?4 IIydrosyl-exchanged
0.1 0.1 0. J 0.1
.?:I
?,,
a Mils penetration per year. detns. c hlonoethanolamine. b Tetraethylenepentaniine
0 .2
0.4
0.0
0 1 1.0 2.3 2 ,4 0.1
0.1
0.0 0.1
hli data sveiaRos of two
0.2 01'
i n m e itvlicato
CONDENSER p1
data oxhibiting l,hr iiiiarly liiicar relationship iii 1Tiguu 4 obtained with a serics of mniples \Therein the pII i ~ a s; i t ] d by suitable additions of pot:tssium hydroxide and piiwphouic acid. The other &t:i aye talmi from a vari samples, iii vliich the p H \vas :iltoiwl variously thr tory :iutoxidatioris, miscclla t use, and addit>ioii of various organic and inorgrtnics The inference from this p H-corrosion relation sb ip is 1jH at, a minimum of ahout serious corrosion of mild steel. iesc
5OME ALKALINE ADDITIVES
'The desirability of neutralizing acidic glycol solutions i. i i i tlicatcd by the preceding dstn. The efficacy of a reprcsc?iititti\-c: ainiiie>2-aminoethanol (~iio~ioetha~iola~niiic, MEA), in chwking vapor-condensate corrociou by rendering the glycol solutions ialkuline, is illustrated by the data in Table 11. Although k i o t k glyool solutions arc corrosive in the absence of the amine, l.tcit,ii :],re reudcrcd noucorrosivo by the addition of about 0.03 75 Z-aininoct,tianoi. The use of 2-amiiioethanol a9 a coi,ronioi inhibitor has heen dcmonst~~itetl in concurrent work hy I Phenothiazine, 2% Doivanol R3B 10% Dowanol 33B: 10"$
1 i
80 100
1 1
20
30
feld and Biotim (26, 2 7 ) that uidinaq- z t w l is vulnerable to acid gas corrosion in this process.
(48-hour rrHux t e s t s .
Solution composition by volunic. 20 A 1 EA--75 D V Q JH~O) Corrosion Rates, B1.P.Y. -. AtlnosVaporAcid Dissociation Constant,s p1ier.c Immersed phase a t 25' C . (16) GO h-2 0.0 5 . 7 x 10 '-8 (1123 ; d11.3 -t f I ~) Hh 0.0 37 Hb', CI? 0.0 26 . 5 4 . 3 x 10-7 ( l I I C 0 6 ;=t € I + HCO,, )
coi
CO!.
c
I>
0.0 0.0 0 0
+
2
1
1.8X IO-: (HCOOH
€5'
+ 11COO
I
ALU.\IINUM COHKOSION
The corrosion of iiluminuni was initially surveyed by LI scrim of &hour reflux corrosion tests at vasious gas atmospheres. The immerjed aluminum 'coupons in all cases weye not attacked; however, the coupons exposed to returning vapor condensate showed corrosion rates ranging from 1 to 60 mils penetration per year, depending upon the gas atmosphere present. The results of these tests are shown in Table X. The observed variations in corrosion rate appear to correlate, qualitatively a t least, n.ith the acidity of the gas atmospheres (in the case of oxygen the gas atmosphere acidity is indicated in terms of a typical light acid). The correlation implied by these data in Table X, that vaporphase corrosion inhibition is a function of the acidity of the vapor phase, is of some utility in explaining plant corrosion experience. Riesenfeld and Blohm ( W ) , in rvorking with a stripping still containing aluminum t'ral-s and I>ubblecaps anti using a 20 niono-
TPoKi
Howcver, if the foregoing accounts for the corrosion supp~'cssion in region3 of high acid gas concentrations, it does not arcount for the corrosion suppression in the region just above t h ; ~ rehoilw in Figure 5. For a possible explanation of this seronci region of proteciion the effect of solvent polarity upon the rat(. of allcitline sttack upon aluminum may be considemi. SOLVENT POLARITY AND A L U M I N U M CORRQSlOX
;lluminuni is attacked severely by aqueous alkaline solutions. Experiments with aluminuni tube bundles and an :iqueous-aniiiic. gas-sweetening system resulted in tube failuw "after n f e w days" (26); tests wit,h 15% monoethanolamine solutions ~f varying proportions of glycol and water indicated that ii minimum of 40 volume diethylene glycol wis required to prcvcnt appreciable corrosion of immersed aluminum 25 (26). In the present ~ v w k:I series of 24-hour immersion tests n'as ciwried out a.t 85' C., xvitli
D I S S O L V ~ACID G A S E S
LEAN
-
; STRIPPING
STI-I
o
20 HL ACID G A S
P E R ML SCLUTIOY
APPRECIABLY CSSRODED SEt'ERELY PITTED
Figure 5 .
Corrosion of Aluniintcni 33 in JIunoethanolamine-Diethj lene G1j rol-Water Acid gas stripprnx rolninn
ethanolamine-40 diethylene glycol-40 water solution, encountered severe corrosion over a localized area of four or five trays roughly midw-ay between the feed tray and the reboiler. Figure 5 iridicat'es their corrosion experience whematically. Corroeion does not occur above the feed tray, nor does it occur significantly for a number of plates below the feed tray; then approximately midway down from the feed tray a rather well defined region of eorroaion is encountered. This region coincides with the region of sharp decrease in the acid gas concentration of the absorbent solution (and of the vapor phase). Furthermore, the corrosivc attack occurs chiefly on the bubble cap hold-down bolts and the
0.5
1 0
20
40
ah DEG Figute 6. Corrosion of Alumilium 3s by Monoethanolamine VOL
1,flw-t of diethylene glycol concentration on corrosion rate 24-honr immersion t e s t at 85' C.. 20 vol. 70M E A
November 1954
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
EFFECTSO F SOLVENT ADDITIVES ON CORROSIOK O F ALUMINUXB Y MONOETHANOLAMINE SOLUTIONS immersion tests a t 85' C. with 20 vol. yo MEA, 50 vol. 5% water)
TABLEXI, (24-houl
Solvent Additive (30 Vol %) Water (total, 80 vol. % water) Ethylene glycol (ethsnediol) Propanediol (1,2-) Butanediol (2,3-) Butanediol (mixed isomers) Hexanediol (mostly 12-1 Hexanediol (2-methylpentane-2,4-diol) Heptanediol (2,2-dieth~.lpropane-1,3-diol) Ethylene glycol Diethylene glycol Triethylene glycol Tetraethylene glycol (Polyglycol 13200) Sonaethylene glycol (Polyglycol E400) Trideoaethylene glycol (Polyglycol E600) Propylene glycol Dipropylene glycol TripIopylene glycol Heptapropylene glycol (Polyglycol P400)
Dowanol 33B (propylene glycol h l e ether) Dowsnol 5OB (dipropvlenc glycol M e ether) Dowanol 62B (triprop'ylene glycol M e ether) Methyl Cellosolve (ethylene glycol M e ether) Cellosolve (ethylene glycol Et ether) Butyl Cellosolve (ethylene glycol butyl ether) Methyl carbitol (diethylene glycol RIe ether) Carbitol (diethylene glycol ethyl ether) Butyl carbitol (diethylene glycol butyl ether) Other C-H-0 Compounds Glvcerol Acetone Dioxane (1,4-) Tetraethylene glycol dimethyl ether Amines Monoet.hanolamine (total, 50 vol % ' XEA) Diethanolpiperazine ( N , N ' - ) hi o r p holine
Corrosion Rate, 34.P.Y 140
180 170 37 12 7.5 8 4.5 3 6.5 37 6.5 3.5 2 7
ethanolamine-water); to be noncorrosive.
2413,
anhydrous monocthanolamine
appears
These inferences suggest aluminate ion solubility to be the determining factor for monoethanolamine-water-solvent systems. A semiquantitative check of this relationship was made by mixing equal volumes of each of five members of the ethylene glycol series with equal volumes of aqueous 5% sodium aluminate, allowing the solutions to stand for 100 hours a t 25' C., and gravimetrically determining the amount of aluminate precipitated from each solution. The resulting approximate data on aluminate solubility are compared in Figure 7 with the corresponding corrosion data from Table XI. The indications of positive correlation are strengthened by the parallel retrograde effects of nonaethylene glycol.
2
12
3.5 1
(immiscible a t 85') 9
4-
4 3.5 2 5.5
ETHYLENE GLYCOLS:
TETRA-
P
4.5
3.5 8 9 5 CORROSION RATE M P Y
220 5 0.5 0.5
A101 SOLV A S PPM A l p 0 ~
Figure 7 . Correlation between Aluminate Solubility in Glycol-Water and Aluminum Corrosion Rate in Monoethanolamine-Glycol-Water
16 4.5 1
aluminum 3s and 20% monoethanolamine solutions containing various balances of glycol and water. The results of these tests, shown in Figure 6, corroborate the findings of Riesenfeld and Blohni (65). In order to characterize the property of the glycol responsible for effecting this sharp inhibition of aluminum corrosion, a series of similar 24-hour immersion tests a t 85' C. was run employing solutions containing 20% monoethanolamine, 50% water, and 30% of each of a number of miscible organic solvents. The results of these tests are shown in Table XI. Several trends are indicated by these data.
For the several homologous series tested, increases in molecular weight -~ generally correlate with increases in efficacy of corrosion suppression. In comparing alkylated analogs of alcohols of the type RpCOH or glycols of the types RCHOH-RCHOH and RCHOHCH20R. where R eauals H or CH,, increased methylation consjstentli correlates \?-ith increased efficacy of corros!on suppression. The conspicuously highly polar solvent additives (water, glycerol, ethylene glycol) permit the highest corrosion rates, while the least polar additives (tetraethylene glycol dimethyl ether, dioxane) afford the lowest rates. Thus a rather consistent picture is obtained from these data in Table XI, in which polarity appears to be directly related to corrosion rate. One apparent anomaly is the reduction in corrosion rate in proceeding from 20 to 50% monoethanolamine in water. Rechecks show aqueous monoethanolamine solutions reach a maximum corrosion rate under these conditions a t about 20 volume % monoethanolamine. A minimum rate is obtained a t about 77 volume % monoethanolamine (1 to 1 molar mono-
Another correlation is suggestcd by t>he observation that polarity seems to be related to corrosion rate. The comparison between corrosion rates from Table X I with the dielectric constants of a dozen pure solvents for which reliable dielectric data are available (19) is shown graphically in Figure 8. When it is considered that this plot neglects all nonideal interactions of the solvent additives with the monoethanolamine-wnter-aluminum surface system, and that these additives show a considerable variation in molecular struct,ure and functional groups, the correlation in Figure 8 seems surprisingly good. Another and more accessible measure of solvent and solution characteristics is that of conductance. In order to minimize the chances of accidental or incidental correlations with other parameters (such as concent'rations us. corrosion rate or molecular weight us. corrosion rate) four series of solutions were prepared, all containing 20 volume % monoethanolamine. Scries A contained diethylene glycol in varying concentrations; series B contained homologous polyethylene glycols in 10 mole yo concentrations; series C contained the same series in 30 volume % concentrations; and series D cont'ained a group of alkyleiie monoglycols in 30 volume % concentrations. lleasurements of solution conductances a t 25" C. are plotted in Figure 9 against the 85 O C. immersion corrosion data reported above. On the basis of this relationship and assuming the corrosion test to be valid, it appears that the conditions for alkaline attack upon aluminum by monoethanolamine may be conveniently defined in terms of solution conductance; thus, solutions of conductances of 0.0004 mho per cm. or g r a t e r may be expected to be substant,ially corrosive, while solutions of Conductances of 0.0002 mho per cm. or lcss appear to be consistently noncorrosive toward immersed aluminum. The above relationships may somewhat quantify the factors in liquid-phase aluminum corrosion; however, they are not sufficient in themselves to predict the ext,ent of vapor condensata
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2414 300
---
I
I
P
I
GLYCERIN, I
TABLE XII. EFFECTS OF ADDITIVES os CORROSION OF ALUNINUN BY GLYCOL-AMINE SOLUTIONS (100-hour reflux corrosion tests in nitrogen, with solutions of composition 5 v d . 7, water, 20 vol. % MEA, additives as indicated, and balance DEG) C o r r o s i o e k e , h1.P.Y. Immersed Vapor condensate
Additive 10% Do~vanol38B 10% Dowanol 50B 10% butvlene alpcol (mixed isom&) 10% 2,3-butylene glycol Control. (no additive) 75% triethylene glycol 10% proi~l-leneglycol 10% ethylene glycol
>: d
i w
I-
Vol. 46, No. 11
0.0 0.0 0.0
0.0 0 0 0.0
0.0 0.0 0.0
2 40 50
2 2
70 1.50
PROPYLENE GLYC 10
K 2
0 0
a:
0 U
3.0
2-ME-2,
4-PENTANE010
I .o
0.3 0
20
40
DIELECTRIC CONSTANT OF PURE SOLVENT (25°C)
Figure 8. Correlation between Solvent Dielectric Constant and Aluminum Corrosion Rate in %TOLIQethanolamine- Solvent-Water
corrosion. Thus, the typical 20-1no1~oethanolair~i~1r 75-diethylene glycol-5-water solution is of low enough conductance (less than 0.00003 mho per cm.) to be noncorrosive t o immersed aluminum; yet the vapor condensate in equilibrium with this solution, consisting mostly of water and monoethanolamine, is extremely corrosive under certain conditione discussed above. Therefore, reduction of the vapor condensate corrosion by solution modification should be directed primarily nt modifying the composition of the vapor phwe in equilibrium with thr glycolamine solution. To determine the niodifying efYects uporr vapor c*ondrnaat~t. corrosion rate of other solvent component,s, a. series of experiniental solutions was prepared and subjected to 100-hour wflux oorrosion tests under nitrogen. The results of theve tests, shown iri Table X I , illustrate t x o requirements of t,he solvent additive which must be met: volatility and low polarity. Thus, ethylene and propylene glycols, t,hough more volatile than diethylene glycol, are ineffective because they are also more polar. Himilarly, triethylene glycol, although less polar, is ineffective becauw it is less volatile. On the other hand, the addition of eit'hei of the two glycol ethers, or of butylene glycol, effects substantial reduction of the vapor condenvate corrosion rate in this test, as these materials combine low polarity wit,h voldlities siilr~stant~ialIy in excess of that of diethylene glycol. The dat,a in Table XI1 suggest, a means of conipleting i i i i explanation of corrosion phenomena reported by Riewnfeld arid Blohm ( 2 7 ) and indicated schematically in Figure 5 , find also a means of removing this zone of high corrosion. The basis for. credit,ingthe high concentrations of acid gas with protection of t,he upper portions of the still column has been discussed abovt.. The protection of the lowest, plates in the still may be due to ii combination of factors. Higher temperatures reduce the effwtive base strength of the monoethanolamine, favor hydrolytic decomposition of aluminate salts to form protent,ive alumina, rid
inorcase the vapor pressure of t'he diethylene gl>-col. The corrosion then occurs only in the middle area, which is unprotected by any of these effects. Thia experience (which with a 40% water wlution, if not directly comparable, is analogous to that encountered with the more common 4 to 7% water solutions) suggests that a suitable solution modification along the lines indicated by the more successful solut'ions in Table XI1 may extend t,he zone of "neutral prot>ection." Such an overlap might be hoped to effect a rather coniplet'e eliminat,ion of aluminum corrosion in the stripping still. Unfortunately, the utilit'y of the two glycol ethers reported in Table SI1 is reduced by the fact that' they are difficult to separate from \later by distillat'ion; t,his could indicate a probability of uneconomic loss of the glycol et,hers with the overhead water in plant operations. mork ie now under way t'o investigate the utility of various formulationa, making use of this mechanibm to ( m t r o l aluminum corrosion in a model stripping column IWhile this particular utility has not been shown, the formulation< in Tahle XJI are covered by a patent held by th(1 Fl~ioi Corp I,td ( 2 ) 1
,
I
~ - - - - T - - - - ~ - - j - - - ~ ,
LIQUID PHASE
1
I
I
2
3
I
I
I
I
4 5 6
I
T
I
I
810
CONDUCTANCE, M H O I C M . x IO4 Figure 9. Correlation between Liquid-Phase Currosion of 4luminum 35 and Conductance of Solution SOLUTION I)ETEHIOKATION
The niaiii soul ced of gas-treating solution deterioration are the cumulative c-ollection of contaminants such as formic or acetic wids, (,arbon disulfide or carbonyl sulfide (10, 23, 2Q), formation of thiosulfate through reaction of hydrogen sulfide with trace amounts of oxygen (9, IO, S4), and direct autoxidation of com-
November 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
ponents of the absorbent solutiori by oxygen ($0,38, d6, 37, 80). Several observations made in the course of the present work may be relevant to this problem. Notwithstanding the failure of others (26) to detect thiosulfates in glycol-amine solutions exposed to hydrogen sulfide and oxygen at elevated temperatures, the present work indicates that autoxidation of hydrogen sulfide by oxygen does in fact occur; thus, after a 40-hour reflux of a solution of 20 monoethanolamine75 diethylene glycol-5 water in an atmosphere of 4 t o 1 hydrogen sulfide-oxygen, the solution was found t o contain 55 meq. per titer of thiosulfate. Similarly, a plant-used sample of similar composition was found to contain 41 meq. per liter of thiosulfate. Is thiosulfate is not removed by the cyclic stripping, its continued formation results in cumulative reduction in solution efficiency through reduction of total acid gas capacity and increase in viscosity. Autoxidation of laboratory solution8 a t reflux indicates that monoethanolamine, alone or in a glycol-amine solution, undergoee oxidative deamination, produces a steam-distillable acid mixture, itnd undergoes reactione reducing the titration-Kjeldahl assay ratio and Van Slyke-Kjeldahl assay ratio. Unstabilized diethylene glycol yields a mixture of aldehydic products, including formaldehyde and acetaldehyde, as well as the corresponding acids. I n glycol-amine solutions, free aldehydes would be likely to undergo Schiff-type condensations with the monoethanolamine present. Finally, analyses of a deteriorated glycol-amine solution from a gas-treating plant showed that very substantial portions of the original monoethanolamine were tied up. Only 48% of thr original amine (based on Kjeldahl assay = 100%) remained as primary amine and only 57% as free titratable amine. The solution contained 420 meq. per liter of formic acid and 41 meq. per liter of thiosulfate, these tying up the equivalent of about 15% of the remaining titratable amine. In addition, the presence of an organic material soluble in dilute acids and substantially insoluble in neutral or alkaline aqueous solutions suggests the presence of a polyamino compound, possibly related to the polyethylenirninee described by Jones and associates ( I S ) . All the contaminantP identified may be due to autoxidation. Extended tests by this laboratory as well as by others (84, 26, 28) indicate that in the absence of oxygen or of strongly acidic conditions, neither the glycol nor the amine is subject t b thermal degradation a t reflux temperatures. It is accordingly indicated that effective prevention of deterioration of the absorbent solution requires the moat rigorous pxclusion of oxygen from the system. SALT FORMATION AND SOLUTION VISCOSITY
In addition to the decrease in the free amine concentration, it11 important result of solution deterioration is the viscosity increase due to degradation products. Viscosity increase leads to a decrease in solution throughput, poorer heat-exchange properties, and poor absorption properties. A significant factor in this viscosity increase is the building up of nonregenerable salts. Ae was noted earlier, acid concentrations of the order of 0.4N may result from curnulat,ive oxidat’ion of a plant solution. The significance of such salt concentrations upon viscosity is illustrated by a series of determinations with fresh glycol-amine solutions cont,aining varying amounts of reagent formic acid. These data are shown in Table XIII. These data indicate an essentially linear relationship, in this concent,ration range, bekeen formic acid content and viscosity increase. The rate of increase, about 27% per IM. indicates t,hat the contribution by nonregenerable salts to viscosity increments in degraded solutions is significant. A similar series of viscosit,y determinations was made, at 0 ” C.! tjo determine the effect of small amounts of carbon dioxide upon viscosity. The results, shown in Table XTV, indicate a similar dependency. Thus a
2415
TABLE XIII. EFFECTO F FORNATE UPON VISCOSITY O F GLYCOLAMINESOLUTIONS (Solutions.
20 MEA-75 DEGTB HzO, with indicated amounts of formic acid added. Kinematic viscosities a t 54.8O i 0.2O C.)
Formic Acid Content Moles/liter Vol. %
Viscosity, Centistokes
l’iscosity Inorease, %
I\.Iolar Viscoulty Index0
-
e Molar viscosity index = cvo where Vc viscosity of solution containing C moles of HCOOH/liter Vo = viscosity of solution containing no HCOOH. and C = moles of HCObH/litcr
TABLEXIV. EFFECTOF DISSOLVEDCARBONDIOXIDEUPON VISCOSITY OB GLYCOL-ANINE SOLUTIONS (Solutions.
20 MEB-75 DEG-5 HzO, y i t h indicated amounts of diseolved COI. Kinematic viscosities a t 0’ i 0.lo C )
KOz Content in Solution Ml./ml. Moles/liK Nil Nil 0.11 0.0045 0.98 0.03?8 2.34 0 09 2.40 0:09;; 2.57 0.1044 25.0 I .Olfi
Visoosity, Centistokes
~ i ~ Increase, %
~ Molar ~ ~ i Viscosity Index
...
...
117.5
118.9 129.3 143.6 144.8 145.2 424.2
1.1 10.02
;;.;1 23: 5 ;
261
Av. S.D.
2.64 2.52 2.34 2.3 2.2; 2.57
2.4;
iz0.13
“lean” solution containing 1.0 ml. of carbon dioxide per ml. exhibits a viscosity a t 0 ” about 10% higher than the corresponding carbon dioxide-free solution. The data in Tables XI11 arid XIV indicate the significance of salt content with respect to the viscosity of these solutions. (The molar viscosity index for carbonic acid in these solutions i8 probably much greater than for formic acid; however, direct comparison of the indexes in Tables XIII and XIV is not justified, as these were obtained a t substantially different temperatures.) Distillation is reportedly being employed with success (9) t,o effect viscosity reduction and removal of other impurities in degraded solutions. It is not, a t present, known t o what extent a degraded glycol-amine solution may be (.leaned up by removal of anionic impurities. However, ion cxchange is currently undei iiivestigation in this laboratory ae a possible means of performing this function more efficiently and more economically. 4CKhOWLEDGMENT
The authors arr pleased to acktiowledge the assistance of H G Scholten, J. 1,. Arnold, and C. A. Lesinski in performing portionp of the experiments herein reported. They are also indebted to the El Paso Natural Gas Co for providing information o r the operation of plant equipment. LITERATLJRE CITED
( 1 ) Hlohm, C. L., Riesenfeld, F.C., and Fraaier, H. D., U. 8.P a t e n t
2,445,468(1948). (2) Ibid., 2,550,446(1981). (3) Bottoms, R. R., in Dunstan, A. E., and associates, “Science of Petroleum,” p. 1815, London, Oxford University P r e s s , 1938. (4) Brockschinidt, C. L., Gas. 18,No.4, 28 (1942). (5) Ruchan, R. C., Sullivan. H. R., Williams, M.,and S p a i n , H. IT., World Oil, 130, No. 6 , 148 (1980). (6) C a m p b e l l , J. M., Chrm. Eng. Progr., 48,440 (1952). (7) C a m p b e l l , J. M., and Laurence, L. L., Petroleum Refiner, 31, 109 (1952). (8) Chapin, W. F., Ibid.,26,537 (1947). (9) C o h n , A,. A., World Oil, 135,297 (1952).
t
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2416
Connors, J. S.,and I l i l l e r . .A .J. P c ~ w ~Processing, c ? ~ 5, 29 (1950). Gordon, .J. A.. Peiidei~ni LIW..2 5 , .i-74, T i , 80, 81, 83, 86
(ly. Hutchiiisoii. A. J . L.. U. S. I’atent 2,177,068 (1939). .Jones. G . 13.. Langs,ioeii, .I., Seuiiiann, AI. 31. C.. and Zomlefer, , (’hem., 9, 125 (19441, , a n d I3lolilrl. C’. I,..P P t i i E f b g . . 2 2 , No. 6, C-37
Kruger. 11. (I., and .\[u , .J. I-lon xirfacee by t 8 h c x sliding process. The initial and the constant find values of p s and the corresponding coriat,arit value of I*k are givcw in Table 1. If the slider is made to imverse the same pat,h a second tirw in the same direction without removal of deposits or abrasion oi the surfaces between traverscs, the valuei: of p8 :tnd j i k remain essentially constant throughout the run. The resulting values of and pk tire also given in Table I. Both viiliies are less than the
