In the normal situation, where ethylbenzene quantities are small relative to toluene, integration is practically based on toluene kinetics. The xylenes enter the calculation in the dB term, since B = 1 - ( X Y 2)and may be readily calculated from the distribution relationships previously presented.
+ +
Conclusions
Grouping isomers and assuming that the activation energy for toluene hydrodealkylation is equal to that for xylene hydrodealkylation permitted establishing the kinetics of 5' xylene demethylation simply. The relative rate constants for the four individual Cg aromatic isomers to that for toluene define composition relationships exclusive of operating conditions. .4mathematical model suitable for computer programming for sizing reactors to charge any mixture of toluene and Cs aromatics is provided. In working with xylene demethylation, important differences from toluene demethylation must be considered. The temperature rise that occurs in an adiabatic system is higher per mole of benzene produced, since two methyl groups, rather than one, are removed from the aromatic nucleus. Ethane production and H1 consumption vary according to the quantities of ethylbenzene in the fresh feed. In characterizing xylene demethylation, xylene conversion is not a particularly useful variable, since this number will almost invariably be in the range of 90 to 99 mole yo. More useful is the concept of recycle to fresh feed ratio in providing a measure of reaction severity. Benzene production from a toluene demethylation unit will decrease when xylenes are added to the toluene feed, because consecutive reactions take place. However, the effect on reactor sizing is relatively small, since xylenes hydrodealkylate a t a significantly higher rate than does toluene.
Nomenclature = relative demethylation rate of toluene to grouped xylene
a
isomers Z, = o-xylene mole fraction (of total aromatics) Z, = p-xylene mole fraction (of total aromatics) 2, = m-xylene mole fraction (of total aromatics) Y = ethylbenzene mole fraction (of total aromatics) X = toluene mole fraction (of total aromatics) B = benzene mole fraction (of total aromatics) k = pseudo rate constant for grouped xylene isomers k , . k J j ,k,, k,. k , = rate constants for specific compounds .If = total moles of hydrogen and nonaromatic hydrocarbons per mole of aromatics (including recycle gas) = a constant in a n isochoric system R = gas constant 0 = empty reactor contact time T = absolute temperature x = total pressure c = concentration H = moles of hydrogen per mole of aromatics -v = H , - XI - YI - 2 ( 2 0 5 zm, ZDJ
+ +
SUBSCRIPT
i
= initial
literature Cited
Betts, W.D., Popper, F., J. AppZ. Chem. 8, 509 (1958). Betts, jV. D., Popper, F., Silsby, R. I., Ibid., 7, 497 (1957). Ind. Eng. Chem. 54, 28 (February 1962). Silsby, R. I., Sawyer, E. W., J.AppZ. Chem. 6 , 347 (1956). \Yeiss, A. H., Petrol. Rejner 41, 185 (1962). Weiss, A. H., Doelp, L. C., Logwinuk, -4.K., IND.ENG. CHEM..PROCESS DESIGN DEVELOP. 2, 169 (1963). (7) Weiss, A. H., Friedman, L., Ibid.,2, 163 (1963). (8) \Veiss, .4.H., Maerker. J. B., Newirth, R., Oil Gas J.60, N o . 4, (1) (2) (3) (4) (5) (6)
64 (1962).
RECEIVED for review March 27, 1963 ACCEPTED .4ugust 28, 1963
KINETICS OF THERMAL DEALKYLATION
OF ALKYLNAPHTHALENES JOHN C. B I X E L , LAVAUN S. M E R R I L L , JR., V. A L V I N L. BENHAM
DEAN ALLRED, AND
Denver Research Center, Marathon Oil Co., Littleton, Colo. The thermal dealkylation of 1 -methylnaphthalene, 2-methylnaphthalenef 1,6-dimethylnaphthalene, and 2,3,6-trimethylnaphthalene with hydrogen a t 61 1 O to 688" C., 300 to 900 p.s.i.a., and hydrogen-alkylnaphthalene feed mole ratios o f 2.2 to 7.9 indicates a first-order mechanism with respect to alkylnaphthalenes and half-order with respect to hydrogen. The reactions of 1 -methylnaphthalene and 2-methylnaphthalene to form naphthalene appear reversible, while the reactions of trimethylnaphthalene to form dimethylnaphthalene and dimethylnaphthalene to form monomethylnaphthalene appear to go to completion with no noticeable equilibrium, A possible reaction mechanism has been proposed, from which rate equations for the principal reactions involved have been derived. T
o ACHIEVE optimum design of a chemical process, it is desirable to understand the chemical reactions involved. With such a n understanding, mathematical equations can be obtained relating reaction rates and process variables. These equations are useful to engineers in chemical reactor design and process optimization. Processes for both the thermal and catalytic dealkylation of alkylnaphthalenes and alkylbenzenes in the presence of hydro78
I&EC PROCESS DESIGN A N D DEVELOPMENT
gen have been discussed in the recent literature (7, 3 ) . A possible mechanism for the dealkylation of toluene to benzene has been proposed by Sawyer and Silsby ( 4 ) and adapted to monomethylnaphthalenes by Betts and Popper (2). This paper presents data obtained in a study of the kinetic properties for the dealkylation of alkylnaphthalene constituents found in light catalytic cycle oil (LCCO) or reformate bottoms to form naphthalene. Dealkylation is accomplished by
removal of the methyl groups and substitution of hydrogen. The nature of the reaction is:
+
CH3 Trimethyl-
d3-
(CHI) Dimethyl-
Hz
+
+
(CH4) Monomethyl --
((334)
Composition of the off-gas was continuously monitored using a recording thermal conductivity unit. Analyses. Routine analysis of the liquid products \%as carried out on a gas-liquid chromatograph a t 175' C. The chromatographic column consisted of a 4-meter Type K packing (polyethylene glycol on inert packing). Gas analysis was carried out using a Fisher gas partitioner. Feed Materials. Methylnaphthalene feed materials used, and their purities are : 1 -Methylnaphthalene, 96-9970 purity by weight 2-Methylnaphthalene, 96-99Yo purity by weight 1,6-Dimethylnaphthalene,96-99yG purity by weight 2,3,6-Trimethylnaphthalene,9 9 5 purity by weight
The gaseous feed material was cylinder hydrogen. Operating Conditions. T h e range of conditions over which the data were obtained is shown in Table I.
Naphthalene
Preliminary investigation indicated that the rate-controlling reaction for the production of naphthalene is the conversion of monomethylnaphthalenes to naphthalene. Therefore, I-methylnaphthalene and 2-methylnaphthalene were studied to a greater extent than 1,6-dimethylnaphthalene or 2.3.6trimeth)lnaphthalene. 1,6-Dimethylnaphthalene and 2.3.6trimethylnaphthalene \cere taken as typical alkylnaphthalenes representing the dimethvl and trimethyl fractions. Experimental
Reactor System. T h e isothermal flow unit used is schematically illustrated in Figure 1. The heart of this equipment consisted of preheater, heater, and reactor units. T h e preheater was a 500-ml. stainless steel Hoke cylinder rate a t 1800 p.s.i., packed with stainless steel Raschig rings, and operated at about 750' to 800" F. Its purpose was to vaporize the feed and intimately niix it with the hydrogen gas stream. It was designed with a large volume to provide surge capacity and smooth out flow fluctuations. The heater unit consisted of a short length of stainless steel tubing, which served to bring the vaporized reactants u p to the reaction temperature. Small diameter tubing was used to achieve high vapor velocities. High vapor velocity \vas desirable to improve heat transfer and keep residence time to a minimum in the heater section. Conversion took place mostly in the reactor or soaker section of the unit. Two reactor configurations were used: 500-ml. Hoke cylinder. of fairly large diameter (11/'2-inch I.D.) and coils of 'ir-inch O.D.(0.18-inch I.D.) stainless steel tubing. All of the data used in the study were obtained using the Hoke cylinder. The ','4-inch O.D. tubing was used only to determine if backmixing was a problem. The Hoke cylinder reactors were packed with dense highfired alumina balls "* inch in diameter. These balls were of very low surface area and catalytically inert. They served to break u p the flow through the unit, limit backmixing, and provide a heat transfer medium. Residence time in these units was varied by changing the reactant feed rate. Temperature \vas measured b.,. a series of thermocouples a t 2-inch intervals in a thermowell through the center of the reactor, At the reactor outlet: for both reactor configurations, the product was cooled by a quench which consisted of 30 inches of ','r-inch O.D. stainless steel tubing coiled inside a water jacket, through which hot \cater was circulated. Following the quench was a 1';z-liter pressure vessel which served as a gasliquid separator and receiver. I t was maintained a t a temperature above the melting point of naphthalene. Hydrogen was metered into the system by displacement with \cater. The alkylnaphthalenes were metered from a graduated buret and combined with the hydrogen in a tee just above the inlet to the preheater. Liquid products were removed from the equipment after separation from the gas phase by accumulating in a high pressure receiver. The receiver was periodically removed and the amount of product determined by weight. The gaseous products were removed through a letdown valve and metered with a wet-test gas meter before being vented.
Table 1.
Temp., a
Min. Max.
c.
61 1 688
Operating Conditions
H2ILtquzd Feed Mole
Pressure, P.S.I.A. 300 900
Ratio 2.2:l 7,9:1
V,/F, L.4ec.I G. .Vole 20 172
For the monomethylnaphthalenes. conversions were obtained at 611'. 637O, 663O> and 688' C., Lvhile pressure and hydrogen-methylnaphthalene feed mole ratio were held constant at 600 p.s.i.a. and 5.5 to 1: respectively. Hydrogenmethylnaphthalene feed mole ratios of 2.2: 1, 3.9:1, 5 . 5 : l . and 7.9: 1 were studied at a constant 663' C. and a constant 600 p.s.i.a. In addition, conversion data were obtained a t 300 and 900 p.s.i.a. a t 663' C . and hydrogen-methylnaphthalene feed mole ratio of 5.5: 1. At least five values of r;.'F were obtained for each series of runs. holding temperature, pressure. and reactant ratio constant. Conversions of 1,6-dimethylnaphthalene were obtained for hydrogen ratios of 2.7: 1 and 6.4: 1 a t 663' C. and 600 p.s.i.a.; and 6.4: 1 hydrogen ratio at 637' C.: 600 p.s.i.a. Conversions of 2,1,6-trimethylnaphthalenewere obtained for temperatures of 637' and 663' C. at 600 p.s.i.a. and 6.0: 1 hydrogen ratio. Kinetic Evaluation
Several kinetic models \cere tested to determine the ability of the model to reproduce experimental conversion data. One of the first attempts at correlating the data was to use the 1.5-order kinetics of Betts and Popper and Silsby and Sawyer. Good agreement \cas found betxceen the experimental data and the calculated conversions a t low conversion levels, but at higher conversion levels the experimental conversions were lower than predicted. One possible reason for this deviation was that side reactions were becoming more evident a t the higher conversions. An analysis of the product, however, indicated that although the side reactions increased slightly, they could account for only a small percentage of the deviation. Another possibility \\-as that backmixing was influencing the data. This was checked by using a reactor of coiled l,la-inch O.D. tubing which Lvould eliminate the backmixing. The data obtained from this reactor did not deviate significantly from those obtained previously. The last possibility investigated \cas the reversibility of the naphthalene-forming reaction. This \vas checked by charging naphthalene to a small batch reactor, pressurizing it u p to 750 p.s.i.g. with methane, and heating it to 650' C . for 2 minutes. The products from this reaction contained both 1-methylnaphthalene and 2-methylnaphthalene along \vith some material of high molecular \ceight. This indicated that the reaction \cas reversible. VOL. 3
NO.
1
JANUARY
1964
79
PURGE GAS IN
Reaction Conditions Hydrogen Mol. Ratio = 5.5: I Pressure = 600 Ib./sq. in. abs - =Calculated Curves
METHYLNAPHTHALENE'~ FEED
HYDROGEN
080 0
E u)
% 0.60 E
Y
z
0.40
2
m
E 0.20 LIQUID P R o DUc RECEIVER
TY
Figure 1 .
-
> z
SCRUBBER
0 0
-0
1.00
t 0.80 Q,
0
e 0.60
60
80
Reaction Conditions Temperature = 6 6 3 ° C .oo - Pressure =600Ib./sq.in. abs. - =Calculated Curves Hydrogen Mol. Ratios:o
E
v
W
40
v r / F ( l i t e r . sec.) g. mole
0
-
\
v)
20
Figure 3. Effect of temperature on 2-methylnaphthalene conversion
Reaction Conditions Hydrogen Mol. Ratio ~ 5 . 5 ~ 1
YQ)
0
Unit for continuous flow dealkylation
0
-
E
Y
.60 .
z z > w
0201/
0 0
Figure 2. Effect of temperature on l-methylnaphthalene conversion
CioHiCH3 CloHr'
$
+ H'
+ H,
2"
(1)
$ CloH7' $
ClOH6
+ CH4
+ H'
I
I
I
I
20
40
60
80
v r / F ( l i t e r sec. g. mole
The model that gave the best fit to the data was then developed, utilizing the following reversible reaction mechanism : H1
OV
(2) (3)
This mechanism is similar to that used by Betts and Popper; the only difference is the reversibility of Reactions 2 and 3. The assumptions were made that Reaction 2 \vas rate-controlling while 1 and 3 were at equilibrium. With these assumptions the following rate equation \vas derived for the dealkylation of monomethylnaphthalenes :
)
I
Figure 4. Effect of hydrogen ratio on l-methylnaphthalene conversion
The same type of rate equation \vas assumed for the dealkylation of dimethylnaphthalenes and trimethvlnaphthalenes. except that the reverse reaction \vas assumed negligible. These rate equations have the form: ru
= -LiiaDair21'2
and l T = -KTa?aH21
Evaluation of Constants
Mole reacted per mole fed: Y, was plotted against reciprocar molar space velocity, V,lF, and slopes of the curves were taken for each data point on the curve to give reaction rates. 80
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
-0 Q)
u-
Q)
E 5
100
\ U
080
React ioR Conditions Temperature 663°C Pressure =600 Ib. /sq. in. abs. Hydrogen Mol. Ratios 0 = 7.9 : I 0=3.9:1
0
Reaction Conditions Temperature = 663°C Hydrogen Mol Ratio = 5 5 : I - = Calculated Curves
c-
-E Q)
1
0
e v)
060
E
Y
z
0 40
0 cn
cn
fi 020
0.20
> z
> z
0
0
0
I
I
I
I
20
40
( l i t e r 'sec.)
80
vr,
60
0
3
0
20
g. mole
Figure 5 . Effect of hydrogen ratio on 2-methylnaphthalene conversion
-0 Q)
c
-
Reaction Conditions Temperature = 6 6 3 " C . 100 - Hydrogen Mol Ratio =5.5: I - = Calculated Curves
80
60
Figure 7. Effect of pressure on 2-methylnaphthalene conversion
-0 Q)
y.
Q)
o
. sec. 1 ,, ( liter9. mole 40
Reaction Conditions Hydrogen Mol. Ratio = 6.4: I 1.00 - Pressure = 600 Ib./sq.in. abs.
/
zi
\ U
2 u
080 -
0 W
L
- 060 0
E
Y
z
0.40
1 /
s
cn
-= Calculated
E o >
Curves
z
0 0
vr,
( liter - sec. )
20
40
vr
(lirr
Analytical and reaction condition data were then used to calculate component activities. Activities were calculated as the ratio of fugacities of the component-Le., methane, HP, naphthalene. or methylnaphthalene-at the temperature and pressure of the system to its fugacity in the standard state. .The standard state \vas taken to be the pure component gas in its ideal state as I-atm. pressure and the temperature of the s)-stem. The fugacity of the component vias obtained from the fugacity of the pure component at the temperature and pressure of the s)-stern. assuming ideal solution behavior, In this manner reaction rate was determined as a function of component activities with temperature as parameter. Reaction rate constants and equilibrium constants \yere next
.set.
. mole
9. mole
Figure 6. Effect of pressure on 1 -methylnaphthalene conversion
60
80
)
I 10
Figure 8. Effect of temperature on 1,6-dimethylnaphthalene conversion
determined by the method of least squares in Lvhich the preceding rate equations were used. Finally, a digital computer program was written to aid in fitting the data. The constants obtained by the differential method were used as a first approximation to calculate x as a function of V,/F. L'alues of rate and equilibrium constants were then varied manually in the computer calculations to obtain the best calculated curves fitting the experimental data points. Results
The experimental data points are plotted in Figures 2 through 10. The curves \.\-ere calculated using the kinetic model. VOL. 3
NO.
1
JANUARY 1 9 6 4
81
Reaction Conditions Temperature.663'C.
c
.40r Reaction Conditions:
z E!
Hydrogen Mol. Ratio = 6.0 Pressure = 600 Ib./sq. in. absolute
.20
0
Figure 10. Effect of temperature naphthalene conversion Figure 9. Effect of hydrogen ratio on 1,6-dimethylno phtha lene conversion
Table II.
Tzmp., C.
688 663 637 611
Reaction Rate Constants and Equilibrium Constants
7-Methylnaphthalene EquilibReaction rate rium constant, k,, con.rtant, g. molell.-sec. K,
3.15 X lo-' 1.50 X 0.70 X 0.28 X
1 -Methylnaphthalene 2-Methylnaphthalene 1,6-Dirnethylnaphthalene 2,3,6-Trimethylnaphthalene
c.
637 663
2.4 0.9
0.85x 0.37 x 0.17 x 10-4
6.8 3.4 1.4 0.65
Activation Energy, Kcal./G. Mole
Arrhenius Constant, G. Mole/L.-Sec.
52 . O 54.5 44.6 53.5
2.49 X 108 8.90 X lo8 4.82 X l o 6 2.02 x 109
Reaction Rate Constant, k,, G. Mole/L.-Sec.
x 2.4 x 1.2
Reaction Rate Constants for naphthalene
Temp., O
c.
637 663
82
3
Reaction Rate Constants for 1,6-Dimethylnaphthalene
Temp.,
Table V.
1.9 X
15
Arrhenius Constant and Activation Energy
Table 111.
Table IV.
2-Methylnaphthalene EquilibReaction rate rium constant, k , I , constant, g . mole,Jl.-sec. K,'
10-4 10-4
2,3,6-Trimethyl-
Reaction Rate Constant, k t , G. ..Mole/ L.-Sec. 2.2 5
x x
10-4 10-4
l&EC PROCESS D E S I G N A N D D E V E L O P M E N T
on 2,3,6-trimethyl-
Monomethylnaphthalenes. The reaction rat€ and equilibrium constants giving the best fit for the 1-methylnaphthalene and 2-methylnaphthalene data points are shown in Table 11. Table I11 gives the Arrhenius constants and activation energies as calculated from the data. T h e reaction rate constants for I-methylnaphthalene and 2-methylnaphthalene thus follow these equations as a function of temperature: k , = 2.49 X 108 ,-~Z,OOOJRT k,' = 8.90 X 108 e-64,400/KT
1,6-Dimethylnaphthalene. No equilibrium appeared to be present in the disappearance of the dimethylnaphthalene; therefore, only the forward reaction rate constants were calculated (Table IV). The Arrhenius constant and activation energy were calculated from the data as plotted in Figure 11. These data gave values of 4.82 X 106 gram molejliter sec. and 44.6 kcal./'gram mole, respectivelv, and led to the equation : ku
= 4.82 X 106
e--:4,600fRT
for the temperature dependence of the reaction rate constanr. 2,3,6-Trimethylnaphthalene. Again no equilibrium was observed, so only the forward reaction rate constants as tabulated in Table V were calculated. These data gave an activation energy of 53.6 kcal. per gram mole and led to the equation: kT
2.02 X 109 p 3 , S O O RT
for the temperature dependence of the reaction rate constant. Discussion
Side reactions observed Ivere primarily of two typespolymer formation and destruction of the naphthalene ring. The polymers were probably formed by a condensation reaction to give compounds of three or more rings, some of which were tentatively identified as pyrene, chrysene, and naphthacene. The maximum amount of polymer formation observed was 3.1y0 of the total product, while the average of all runs was of the order of 1%. Destruction of the naphthalene ring was evidenced by the formation of lighter homologs, primarily benzene and alkyl benzenes. Again the maximum formation of such compounds amounted to 3%, while the average was about 1%.
The equilibrium constants, K , were determined in a manner to give the best fit to the experimental data. It appears that the equilibrium constant increases with increasing temperature, indicating that the reaction is endothermic with heats of reaction on the order of 55,000 cal. per gram mole (Figure 12). Theoretical calculations indicate that, although the reaction might be slightly endothermic a t the conditions studied, it should not be nearly so pronounced. This discrepancy indicates that the equilibrium constants presented are not real constants but rather are apparent values that give the best fit to the experimental data. This disagreement might also indicate that the mechanism chosen is not an accurate description of the reactions involved. -4fter development of the equations described, a dealkylation study was carried out on a n aromatic extract of light catalytic cycle oil of the following composition :
!O.O A
8 .O
0
6.0
2,3,6-Trimethylnaphthalenc I, 6-Dimethylnaphthalene I- Methylnaphthalene
4.0
2 .o
*
2
1.0
0.8 Y
0.6
w t . 70 Nonnaphthalene 2-Methylnaphthalene 1-Methylnaphthalene Dimethylnaphthalene Trimethylnaphthalene
0.4
0.2
0.1
1.04
1.06
- TxI
1.10
1.08
lo3
1.12
1.14-
1.16
OK-'
Figure 1 1. Relation of reaction rate constants to reciprocal temperature
n
1.02
1.04
44.9 37.3
The primary object of this study was to determine Ichether the equations developed \could accurately predict the yield of naphthalene from the abo1.e feed material. The LCCO extract \cas studied in the same equipment and in the same range of conditions as the pure methylnaphthalene studies. Figure 13 compares the predicted yields (solid lines) and the experimental data (points). The naphthalene yield is predicted accurately. The intermediate compounds, while not predicted as accurately, still give satisfactory comparisons, Analysis of the feedstock used in this run was uncertain with regard to the actual isomers of the naphthalene involved. A s a n example, a n ethylnaphthalene dealkylated directly to the naphthalene without first dealkylating to a monomethylnaphthalene, while the dimethylnaphthalenes first dealkylated to the monomethylnaphthalene. By necessity the ethylnaphthalenes and the dimethylnaphthalenes \cere lumped intoone group
-.-
0
12.7 3.0 2.1
50 -
Naphthalene
- - - - 2-Methylnaphthalene x--I - Methylnaphthalene 0-
Dimethylnaphthalene
-. .-.
I-Methylnaphthalene 2-Methylnaphthalene
106
1.08
1.10
1.12
I 14
CONTACT T I M E ( S e c . ) Figure 12. Relation of equilibrium constant to reciprocal temperature !, for 1 -methylnaphthalene and 2-methylnaphthalene
Figure 13. Calculated and experimental yields of naphthalenes from LCCO aromatic extract 663' C., 600 p d . , H*/LCCO mole ratio 7.8 to 1
VOL. 3
NO.
1
JANUARY
1964
83
in the analysis, and any alkylnaphthalenes that would be isomers of the trimethylnaphthalenes were also lumped into one group. Thus, satisfactory comparison would indicate primarily that the data points follow the same pattern as the predicted curve. The agreement shown in Figure 13 indicates that the equations obtained in this study are useful for reactor design and process optimization, even though the proposed mechanism may not be the actual mechanism involved. Nomenclature
= I-methylnaphthalene reaction
k,
=
K,,,
=
I;,,,
=
r
= = = =
T V, x
rate constant, gram mole/sec. liter 2-methylnaphthalene reaction rate constant. gram mole/sec. liter 1-methylnaphthalene equilibrium constant 2-methylnaphthalene equilibrium constant reaction rate, gram mole/sec. liter absolute temperature, " K. reactor volume, liters conversion, moles reacted per mole fed
literature Cited
= activity of methane = = = = = = =
k,
activity of dimethylnaphthalene activity of hydrogen activity of I-methylnaphthalene activity of naphthalene activity of trimethylnaphthalene 1-methylnaphthalene feed rate, gram mole/sec. dimethylnaphthalene reaction rate constant, gram mole/sec. liter
(1) .4sselin, G. F., Erickson, R. A., Chem. Eng. Progr. 5 8 , 47 (1962). (2) Betts, rV. D., Popper, F., J . Appl. Chem. 8 , 509 (1958). (3) Fowle, M. J., Pitts, P. M., Chem. Eng. Progr. 5 8 , 37 1962). (4) Sawyer, E. LV., Silsby, R. I., J . Appl. Chem. 6 , 347 il956). RECEIVED for review March 29, 1963 ACCEPTED September 3, 1963 Sational A.1.Ch.E. Meeting, New Orleans, La., March 1963.
FUNDAMENTAL STUDIES ON ANODIC PROTECT10 N A l l y 20 in Sulfuric Acid Z, A
, F0
R0
U
L I S , Esso Reseach and Engineerinz
Co., P.O. Box 209, .\fadison,
iV. J .
The anodic polarization behavior of Alloy 20 in sulfuric acid was investigated b y potentiostatic techniques, and the effect of acid concentration and temperature on the shape and relative position of the anodic polarization curve wiih reference to the current and potential coordinates was evaluated. Acid concentrations from 3 to 96% HzS04 were used in the temperature range 20" to 127" C. An increase in acid concentration up to 64% H2S04displaces the polarization curve toward more cathodic potentials and higher curreni densities. An inct ease in temperature results in higher current densities and slightly mote cathodic potentials. Particular emphasis was placed on the interpretation from an engineering viewpoint of the anodic polarizaiion parameters, such as critical current, passive current, etc., as affected b y changing acid concentration and temperature.
HE anodic passivation of metals and alloys has been known T f o r many years, mainly as a laboratory phenomenon (2, 4: 5, 9, 70, 74). However, with the introduction of new potentiostatic techniques, wherein potential can be controlled at a set value independent of the output current, this method appears to be in principle suitable for application in the petroleum and chemical industry to solve certain severe corrosion and contamination problems (7, 6-8, 77, 72, 75, 77) * To learn more about this technique, a program was initiated to study the anodic polarization characteristics of a commercial Alloy 20 stainless steel in various sulfuric acid concentrations (3 to 96y0H2S04) and examine the transition in behavior with changing acid concentration and temperature. In the simplest terms, anodic protection is the process of passivating a metal by impressing an external anodic current upon it. This technique is applicable to highly conductive
84
l&EC PROCESS DESIGN A N D DEVELOPMENT
solutions and in general to acids, the bases, and their salts. It should be attractive to processes handling acids such as H&SOd,HSPOd, " 0 3 , etc., or organic acids where product contamination and/or corrosion is a problem. Furthermore, it could allow the replacement of costly metals or alloys by cheaper structural materials for many severe corrosion applications. Experimental
Polarization measurements Lvere performed using an ~ ~ ~I~~~~~~~~~~ l ~ potentiostat, ~ i ~ ~h~ ~ polarization l cell and the electrode design were similar to the ones described bv Stern ( 7 3 ) . All potentials were measured relative to a s i t urated calomel electrode with a Luggin probe and a high impedance voltmeter. Commercially available .4lloy 20 was used to prepare the electrodey. The electrolyte was prepared from C.P. sulfuric acid using laboratory-distilled water. The temperature was controlled a t 1 0 . 5 " C. The over-all change D
