INDUSTRIAL AND ENGINEERING CHEMISTRY
1716
\Then the partial pressure of carbon dio3ide Tvas 1 atmosphere or more the surface had a marked tendency t,o pit. Addition of a small amount of acetic acid to such a 81-stem had little effect. n l t h acetic acid alone as the corrosive agent, the steel showed an even acid etch. Exposure of coupons to acetic acid and to carbon dioxide, ranging from 0.2 to 0.5 atmosphere resulted i n rapid initial attack with subsequent formation of a thick surface layer of ferrous carbonate, similar to that formed on the coupons from the intermediate type n-ell. A correlation bet!?-een the partial pressure of the carbon dioxide present in the wells studied here with the type of layers formed Tms made. The agreement is good for the first tn-o types of n-ells. Mlth the third type the metal was scarcelj- attaclced, even ivhen the partial pressure of carbori dioxide was as great p s that in highly corrosive Jvells, The third type of layer observed was previously described as being orgaiiic in nature ( 5 ) . The appearance of the films from the Tullos wells and the Carthage n.cll (Lower Pettit) n-we transparent initially arid thinner t h a n those formed in the intermediate n-ells. The values for the t h i c h c s s of the sui~l'acelayers after Iweekn' exposure are given in T a h l e I. 1 riietallogmphic examination of a cross section of the Tullos well S o . 2 surface layer on a coupoil exposed for 1 moiitii as made. It i ~ a fso u n d t o be about 5 , thick, n-liich is i n good agreemeill v i t h the value found for the noncorrosive Tullos ~vellS o . 1 iri this stud\-. The partial pressure of carbon dioxide in the noncorrosive Tullos !vel1 S o . 1 is as great as that found in the corrosive nells. From the appearance mid thiclaiess of the surface layers and t>he low rate of attack on the basis nieta.1 of the coupons cxposcd to this stream, it is reasonable t o conclude that a natural inhibitor is present, in the n-ell stream which forms a relatively impc~rvioui, coherent film. This is further borne out by the effect of organic substances added to the \Yell streanis as shon-n in the second exmade on the Hunt Stenart !vel1 and Grant well. The addition of naphthenic acid l o the Grant, \\-ell caused the coupons to increase in Tveight initially and the surface layers to become thicker. In the case of the H u n t Stewnrt v d l the film became thicker and the pitting n-hich had previously been observed ivas no longer found. d t one time during the exposure the rate curves assumed a slope approximating that' usually associated with a noncorrosive well. It, appears that the inhibitor decreased the rate of diffusion of t,he corroding const,itii-
Vol. 41, No. 8
ents. In other words, it seems to have plugged, to some extent a portion of the pores normally present in the surface layer. I t is believed that in every case the controlling step after the initial attack on the metal surface is the diffusion of t,he const'ituents through t8hefilm. This is shonm t o be t,he case with t h e first four vc-ells list,ed in Table I. For these the corrosion ratei.e.>the slope of the descaled x-eight loss curve thickness of the surface layers decreases. Coupons in the last four wells of Table iirrturally or by chromate, had thin, impervi the metal \%-asnot attacked to any degree. The one .ified i v d l listed iii the table apparently is of the noncorrosive rype according t o the data. ~
lCKYOWLEDGhlENT
The authors t,ake pleasure in acknoxledging the fiiiaiicial support of the Corrosion Research Project.; Committep of thc Natural Gasoline Association of .Imerica. They also wish t o express their apprec ion to the many field n-orkers IT-ho helyed in making the enupoil exposures. AIost of the n-orli referred t o may be found in the minutes of the Corrosion Research Project Comiiiittce ( S n t ~ u l a lGasoline Liesociation of -inicrica, 422 Kciinedy Building, Tulsa, Okla.). The information is du.e to m m y field vl.or.lters and inveAgat,nrs assnciated u-ith the committee iind much of the work is as yet unpublished. LITERATURE CITED
(1) Ani. 9 o c . Testing Materials, X-Ray Diffraction. Data C R ICIS,
Philadelphia, Pa. L-.Ii., "Metallic Corrosion. Paasivit;-. arid Protection,"
( 2 ) Evan,,
2nd ed., p . S I , London. EdwardA%rnoldand Co., 1946. ( 3 ) Hackelman, N . , and Shock, D. A , , IND.ENG. CHEM., 39, SCiS (1947).
(4) Shock, D. A , , N.G..k.l. Corrosion Research Projects Committee, M i n i i t e s , 3 , 373 ( 1 9 4 7 ) . (5) Shock, D . A , , and Hackerman, N., IKD.ENG.Cxmr., 39, 1283 (1947). (G) Uhlig, H. H., Chem. E?LQ. X e u s , 24, 3154 ( 1 9 4 8 ) . (7) Yale, IT. D.. Corrosion, 2, S5 ( 1 9 4 6 ) . RZCEIVED -4pril 8. 1918. Based in part o n a thepis submitted t o t h e faculty of he University of Texas i n partial fulfillment of t h e requirenicntb for t h e degree of doctor of Dliilosophy.
0 USING MEASUREMENTS OF FILM THICKNESS AND RIASS C. ICESNETH EIEERTS U . S . Bureau of Mines, Bartlesuille, Okla.
T
HE data published by Haclierman and Schniidt ( 1 ) suggest
t h a t the products of the corrosion they n-ere studying niay be partially identified by the density of the surface layers formed on the tcst c,oupons. The microscope procedure used by the authors ( 3 ) in determining the average thiclrness of the surface layers provided a nicasure of the volume of the corrosion product, in an area of 100 sq. cm. Det,ermining the n-eight loss of the coupons on exposure both before and after they were descaled provided a measure of the mass of the corrosion product jn the same area. Densities computed from the mass and the volume
of the corrosion product agree for the niost par0 xvith the densities giver1 in the literature for the compounds identified by x-ray analysis. Each set, of equat,ions given by Hackerman and Schmidt has been solved using the value of t (Jxeks of exposure) given in their Table I. The measured values of surface layer thickness, also given in Table I, and the difference in the coniput,ed values of the pairs of the equat,ioris have bccn used in the following equation to estimate the density of the corrosion products:
100 (10-%)d = y'
-y
(1)
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1949
1717
and the densities shown in column 4, Table I, of this comment TABLE I. IDENTIFICATION OF CORROSION PRODUCTS BY DENSITY were computed. The compounds identified as corrosion products MEASUREJIER’TS~
Average N a t u r e of Thickness y’ - zj, G. Density of Surface per 100 of Film. Film, X-Ray Test Layers, p Sq. Cm. G. per 1\11, Analysis H u n t Stewart 4.2 0.162 3.86 FeO, FeCOa 0.244 3.64 FeCOa Hunt Stexartb 6.7 Grant 7 6 0.294 3.87 FeO, FeCOa Grantb 9.9 0.378 3.82 FeCOa 6.2 0.234 3.77 PeCOa Cartilage Upper Pettit Carthag: Lower Pettit 3.3 0.126 3.82 FeO, FesOl Tullos h o . 1 4.6 0.174 3.78 FeCOs. FepOj .Jones No. 1 7 0 0 260 3 71 33-6 b 3 0 0 111 3 70 FeCOa, FeO, FesOa a Density.from literature: FeC03, 3.7 t o 3.9 g./ml.; FeO, 5.7 g./ml.; F?30i, 4.96 t o 6.40 &,’nil.: and FeS, 4.84 n./nil. b Inhibited.
where h d y
= thickness of the surface layer, p = density of corrosion products, grams per ml.
y’
=
=
weight loss of coupons as removed, grams per 100 sq. em. weight loss of coupons after descaling, grams PPI 100 sq. em.
Equation 1 was simplified as follows: d
=
100(,~’- y ) / h
Cracking of
by x-ray analysis are shown in column 5. Footnotes in Table I show the range of density values given in the literature for these products. Except for data obtained in the Carthage Lower Pettit tests, the data of Table I indicate close agreement between the measured values of corrosion product density and the literature values for products identified by x-ray analysis. Whether or not such close agreement can always be expected could best be determined by further testing. Assuming that film thickness nieasurements made by means of the microscope would not be more accurate than =tO.3,uL,the uncertainty reflected in most of the density computations would be 3 to 7 7 5 , The differences in thc densities of ferrous carbonatc, ferrous oxide, and ferrosoferric oxide range from 6 to 25%. Computed values of density would not, of course, show the respective amounts of each of three 01 more compounds present in a corrosion product but might distinguish between the oxides and ferrous carbonate. LITERATURE CITED
IKD. ENG.CHEM., 41,1712 (1949). (2) Hackerman, Norman, and Shock, D. A, Ibid., 39, 863-7 (1947). (1) Hackerman, Norman, and Schmidt, H. R.,
RECEIVED February 17, 1949.
igh Sulfur
USE OF STEAM WITH NATURAL CATALYST A. L. CONN AND C. W. BRACKIN, Standard
In
the catalytic cracking of high sulfur stocks with natural catalyst, the catalyst rapidly becomes poisoned, resulting i n excessively high coke and gas yields a t the expense of gasoline yield. Data are presented demonstrating the successful use of steam i n solving this probIsm. In one set of experiments, carried out i n a 2-barrelper-day fluid pilot plant, the introduction of steam to the regenerated catalyst standpipe serves to hydrate the catal3st prior to contact with oil, thus preventing loss of catalyst selectivity. In another set of experiments, including one full-scale plant test, the use of large quantities of stripping steam in addition to the hydration steam malies it possible to restore poisoned catalyst to normal selectivit,. I t is concluded that the rate of sulfur poisoning is dependent upon the extent to which the catalqst is hydrated at the time it contacts the oil, upon the concentration of sulfur in the feed, and upon the extent to which the poison is eliminated by replacement with steam as the cataljst passes through the stripper.
T
HE increase in demand for petroleum products has led to the
utilization of increasing amounts of high-sulfur crudes. Furthermore, in order to produce the required amounts of distillate fuels and a t the same time maximize gasoline production, it has been necessary to charge higher boiling gas oils to catalytic cracking units. Because the higher boiling fractions generally contain larger proportions of sulfur ( 1 ) both the crude source and the boiling range have combined to increase the amount of sulfur in catalytic crncking charge stocks. Under certain conditions, natural catalyst has an advantage over synthetic silica-alumina catalyst in the production of motor gasoline by catalytic cracking; hence, a number of catalytic crack-
Oil Company (Indiana), Whiting, Ind.
ing units have employed natural catalyst. I t has been found, however ( 2 , S ) , that the presence of large amounts of sulfur in tl e charge stock has a poisoning effect on natural catalyst. causing rapid deactivation and loss of selectivity. As an example of catalyst poisoning it was observed in one fluid cracking unit that, after a few weeks of operation, a progressive increase in coke and gas yields occurred at the expense of gasoline vield. Tests on the catalyst, using standard fixed-bed activity-testing methods ( 5 ) )indicated that the changes in yields weie the result of a change in the selectivity of the catalyst. For example, the carbon factor-the ratio of the carbon yield obtained with the catalyst tested to the carbon yield obtained with a standard catalyst at the same conversion of gas oil-was greater than 2.0. Similarly, the gas factor, which is derived in the same manner, was greater than 1.6. I n order to permit satisfactory operation with natural catalyst and high-sulfur stocks, tests were carried out t o determine means of preventing catalyst poisoning and methods LTere sought t o rejuvenate the catalyst after I t had become poisoned. I n small-scale test work reported bv Davidson (9), in which the operating conditions approximated those in catalytic cracking, the ability of natural catalyst to sorb moisture a t temperatures between 800 and 1060’ F. was studied. This moisture is believed to become associated with the cr)stal structure of the catalvst. Cracking tests, made in a small fixed-bcd unit with a high-sulfur stock ( g ) , demonstrated that hydration of the natural catalyst prior to contact with the oil feed (pichydration) greatlv reduced the rate of catalyst dctcrioration. In the current Tvork, preliminary results of which were reported bv Davidson, experiments confirming this observation were carried out in a small fluid catalytic pilot plant. In these experiments, the steam wae injectrd a t the base of the regenerated catalyst standpipe. Since that time, additional experiments have been carried out on the use of steam
