Corrosion of Steel in Sulfur-Producing e FRASCH PROCESS 1). A.
SHOCK AND NOKMAS HACBEKMAN University of Texas, Austin, Tex.
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EVERE corrosion of Coupons were exposed in a container placed in the flow The coupons which had equipment has long line from a sulfur well. Rates of corrosion were found been contacting sulfur a t the time of removal had a n adbeen accepted as a part of to vary with variation in production conditions. Specifiherent layer of sulfur on the cally, the rate was negligible i n the presence of sulfur alone sur.ace, hi^ sulfur film normal operation in the Frasch process of mining sulbut increased tremendously during water contacting helped protect the coupon fur. The presence of sulfur, periods i n the presence of sulfur. The rate and nature surface layers from altering of attack were reproduced in the laboratory by a simple aPPreciablJ7 before examinasulfur compounds, water, dissolved salts, and air, as well as bomb reactor. The conclusions given on the basis of tion. because high temperatures> subject field experiments were corroborated by laboratory work. of this, none of the corrosion products on the surface were the production equipment to lost. However, i t was necesvery severe corrosive condisary to remove the sulfur film in the laboratory in order to study the coupons. This was accomtions. The large number of corrosive agents present makes it difficult, if not impossible, to specify corrosion-resistant materials plished by hanging the metal piece in a reflux still containing carbon disulfide so that the condensing liquid ran back over the on the basis of existing data. West (3)has reported on rates of coupon. After this treatment there was still some undissolved attack of sulfur on various nonferrous metals and steel alloys a t amorphous sulfur on the coupon. This was removed by heating for 1 hour a t 90 c. and then repeating the refluxing operation. mining and processing temperatures. The presence of water was The coupon was then examined microscopically, n'eighed, descaled, sholr-n to have a marked accelerating effecton the rate. Fanelli and weighed again. (1) found that the introduction of the halogens, especially RESULTS chlorine, increased the rate of attack on aluminum and alloy I n general, there are three types of conditions t o which the steels.. Severe corrosion is found especially in the sulfur-prometal in the producing equipment might be subjected. These ducing tubing and exterior of the tubing which supplies the air to conditions are found in normal operation of a sulfur well. I n lift the sulfur. I n these sections where the accelerating agents, one case water is introduced into the well in order to start the water and dissolved salts, are present in normal operation, such flow of sulfur. This is known as a boost or wash. I n another, corrosive effects might be anticiuated. Little is known. however. about the rate andmechanism bf attack taking place during the 20 FT. various phases of well operation. k 1 This study was undertaken t o evaluate the corrosion rates k 8 1 N . - 8 1 N . - B ING 8 I N . 8 I N . - 8 IN.' experienced during production and to gain a better understanding of the mechanisins by which the corrosion takes place.
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FIELD APPARATUS
A coupon container similar to t h a t used in condensate well studies ( 2 ) was designed t o fit into the flow line from a sulfur well (Figure 1). Collars were welded into a 20-foot section of steam-jacketed 3-inch line, set a t approximately 8-inch centers. Thus, there was space for exposure of 30 coupons at one time. The coupons, 3/4 x 3 x 1/16 inch strips, were bolted into a slit rod which was welded to the center of a collar plug. This assembly was screwed into the collar to a snug fit and turned until the main face of the coupon was parallel with the flow. A bypass section with shut-off valves was placed at the ends of the container so that the coupons could be removed at any time. The coupons to be evaluated were shipped to and from the field in containers made of 4- or %inch sections of 1.25-inch pipe capped a t either end. A small bag of silica gel was placed a t one end to keep the air dry in the container. A good seal was ensured by pulling the caps down tight on the pipe sections.
SIDE VIEW
PROCEDURE
The coupons used t o evaluate the behavior of steel were cut from SAE 1020 sheet. The surface was abraded on a belt sander so t h a t all mill scale and surface markings were removed. Care was taken not to let the temperature of the specimen exceed 70" C. during this operation. Coupons were numbered and weighed in the laboratory, placed in shipping desiccators, and installed in the field container on arrival. The coupons were removed by backing out the plugs, placing a suitably sized envelope over the sample, and unbolting it from the plug. They were then placed in the shipping desiccators and sent back to the laboratory for evaluation.
S E C T I O N THROUGH COLLAR Figure 1.
1974
Field Coupon Container
1975
INDUSTRIAL AND ENGINEERING CHEMISTRY
September 1949
DLSCRLED VALUES 0WITH SCALE - S R E f l O V E D O--.
N O R M A L PRODUCTION
111 ~
3 6 9 T I M E IN DAYS Figure 3.
4
a
a
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Weight Change Curves during Sulfur Production
I t
TIME IN DAYS Figure 2.
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Weight Loss Curves during Production Cycle
during normal sulfur flow, only sulfur and air contact the metal. I n the third case, sulfur and water from the formation come up the tubing. This is due to the level of the molten sulfur pool d r o p ping below the perforations far enough to allow the overlying water t o come up the tubing. This high pressure water (approximately 750 pounds per square inch and 300' F. at formation) flashes on reaching a lower pressure (approximately 50 pounds per square inch and 250 O F.) at the surface, so t h a t some liquid sulfur, some water, and some steam and air are in the tubing during this time. This effect is known as a blow. These periods are not altogether clear cut, nor do they necessarily occur in sequence, because water can be produced in small amounts during sulfur-pumping periods and a well can be p u t back into production after the sulfur pool drops by boosting, thus avoiding a blow. However, the cycle of boost, sulfur pumping, and blow is considered normal well behavior. Figure 2 shows the effect of such a cycle on corrosion rate. Thirty coupons were inserted in the container and two were removed each day for a 15-day period. The well pumped sulfur normally the first 5 days. On the fifth day the well stopped pumping sulfur, but a boost of 2.5 hours restored the well t o normal production until the ninth day. On the tenth day the well was boosted again; it produced normally on the eleventh day and blew on the twelfth day, was sealed on the thirteenth day, and then continued to produce normally for the remaining time given. Plotting the loss in weight against time in Figure 2 shows t h a t during sulfur-pumping periods the penetration was relatively low, but that when water contacted the coupons very large rates of attack were obtained. Straight lines, drawn t o give the average slopes, show comparative rates of 0.588 inch per year during the first boost, 0.084 inch per year during the intermittent period, and 3,175 inch per year during blow and boost period. This showed that the rates of corrosion were considerably different for the various operating conditions. Results obtained on an uninterrupted sulfur flow are shown in Figure 3. The coupons were weighed with and without the surface reaction products-i.e., descaled and with scale but with sulfur removed. The relatively flat curves show t h a t after the first day, during which a surface layer-is formed, very little change took place. The total penetration at the end of 14 days
was 0.0001 inch per year of uniform metal loss. Microscopic examination of the coupon showed a relatively thin uniform coating of iron sulfide. There was no evidence of pitting or other kinds of nonuniform attack. Coupons which had been subjected to a period of severe corrosive attack and then exposed for 3 months to continuous sulfur flow showed a uniform penetration of 0,0009 inch per year for this period. The corrosive conditions during the wash or boost period were studied by exposing a set of coupons both to normal sulfur flow and t o a boosting operation. The coupons were removed during the normal pumping period as well as during the boost itself. Figure 4 shows the results of such a run. The rate of attack during the period of sulfur pumping was again negligible. Coupons were removed at 9:45 A.M. before the boost started, at 11:50 A.M. when the boost was about completed, and at 12:45 P.M. when the well had sealed and was pumping intermittent
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NORMAL
PRODUCTION
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v,
z
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0.0 DESCALEO WITH S C A L E
0 Figure 4.
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8 T I M E IN DAYS
IG
Weight LOSSCurves during Wash or Boost Period
19'16
INDUSTRIAL AND ENGINEERING CHEMISTRY 2
IN.
P Y R E X GLASS
OR WATER
Laboratory Test Bomb
slugs of sulfur and water. Kormal pumping was restored by the time coupons were removed the nest day. The upper curve indicates the descaled weight losses and the lower the M eight loss with the scale still on the coupon. It is evident that the corrosion rate was high only during the times in which water contacted the metal and decreased when the water was no longer present. The divergence of thc two curves indicates the thickening of the scale which v a s evident from the examination (of the coupons. hlicroscopic examination showed a gencral Toughening of the surfaces but no marked pits. Calculation of penetration rate for the few hours during the wash gives a value of 1.18inches per year. Study of the blow period shoned that a corrosion rate even higher than that during the boost period is t o be expected. Evidence of rapid removal of corrosion products, erosion, and pitting were found on examining the coupons microscopically. R a t e of corrosion during the blow shown in Figure 2 was 3.175 inches per year. IS
The field experiments indicated t h a t the high rate of corrosion taking place in the wells was not due t o the presence of molten sulfur and air alone. Most of the attack took place in the presence of water when the sulfur was in close contact with the st,eel. >In approach to these conditions in laboratory experiments was made in the apparatus shown in Figure 5 , Test bombs were made of a 2-inch heavy duty pressure nipple and cap. Pyrex liners were made to fit' the bombs and the coupon and liquid were placed in the glass liner. The cap was sealed and the bomb heated in an oil bath to bhe required temperahre, usually 130" C. (266" F.). This temperature was chosen for oil bath operation in order to bring the bombs from room temperature to the temperature of molten sulfur and still not be too high to be much above the 121 O C. (250 F.) temperature of well head conditions. The constant temperature oil bath was constructed to accommodate a number of the bombs. The temperature of the bombs was taken by inserting a dial type industrial thermometer into the bomb cap and letting the thermometer casing dip into the liquid in the bomb.
SULFUR
Figure 5 ,
L.A€tORATORY IYVESTIGATION
CAP
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Vol. 41, No. 9
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Because the coupons and exposure medium inside the bomb would take some time t o come to equilibrium, and because the corrosion rate is a function of temperature, it was desirable to determine the time it took for the bomb temperature to reach a desired value. Figure 6 shows both the bomb temperature and the weight loss of sulfur-covered coupons as a function of time. The reaction is essentially complete within an hour; the rate thereafter is rather lox, as demonstrat'ed by the nearly horizontal portion of the curve. Inspection showed that attack began near t.he point where the sulfur became molten. A black surface developed between the sulfur and the steel Tvith the short exposure times, first a t a few places, then with the longer exposure times finally covering the entire surface of the coupon. This black coating increased in thickness also with the increasc in time. Not all of the sulfur reacted; some of it ran off the coupon and collected in a button in the bottom of the glass liner. These results indicated that the rate of attack for molten sulfur in good contact with steel and with water present must be considerably higher than the rates resulting from attack in water
U"n
1
I
1
I
TOTAL R R E A O F C O U P O N S U L F U R COVERED-
0.12
I
MONLOF COUPOU A R E A 5 U L F U R COVCRE
0
0
I
1
IO
20
A30
SULFUR COVERED AREA, CM.' Figure 6. Weight Loss of Coupons and Bomb Temperature as Function of Time
Figure 7 . Relationship of Weight Loss and Area 0% Coupon Covered b y Sulfur Two-IIour Exposure a t 130' C.
September 1949
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1977
alone. Also, the rate is not dependent on the amount of sulfur present and not contacting the steel, as in the case of the excess sulfur i n the experiment first discussed. To show this more definitely, the following experiments were carried out:
corrosion found in the field was the same as those found in these laboratory experiments,
Coupons were exposed to sulfur alone and water alone, and sulfur-covered coupons to water. I n the first case, a 3-hour exposure caused only a slight discoloration. Coupons exposed in water alone showed some attack (slight rusting). Weight losses were about 0.2 mg., just within the limit of experimental error. The relationship between the extent of attack and sulfur-covered areas of the coupon with constant weight of sulfur in each case is shown in Figure 7. I n this experiment, coupons were immersed t o different levels in liquid sulfur for 2 hours, so that adherent coatings covering a variety of areas on the coupons were obtained. Approximately 1.75 grams of sulfur were found to adhere t o the totally covered coupons. I n order t o maintain a constant quantity of sulfur in each bomb, sulfur was added to the amount needed to bring the weight present as a coating t o 1.75 rams. The results shown in Figure 7 indicate that the total wei Et loss after 2 hours at 130" C. bears a linear relationship to t i e area covered by the sulfur. The rate of attack with no sulfur coating was considerably higher than the rate in water alone with no sulfur present. These losses were 3.0 mg. (shown in Figure 7 a s the value at 0 area) for water and sulfur and 0.2 mg. for water alone.
ments, t h a t much of the corrosion experienced in the producing tubing and air line could be cut appreciably by decreasing t h e water contact time. This means decreasing the periods of boost and blow. The highest corrosion rates are apparently experienced when liquid sulfur as well as liquid water and air is in good contack with steel. The rate a t which steel is attacked by sulfur and air under normal pumping conditions is negligible compared t n the rates during the boost and blow periods. The use of a multiple coupon container in the field was effective in studying the variation in corrosion during the sulfur production cycle. Evaluation of alloys in these recognized corrosive conditions can be made in the field by the same means. A laboratory method has been described which appears useful in studying and evaluating the highly corrosive conditions found during certain periods of sulfur well operation.
These data suggest t h a t the severely corrosive condition in sulfur-producing wells is due to the effect of water and sulfur together when the latter is in intimate contact with the steel. Confirmatory evidence is offered by an experiment in which the lower end of a n uncoated steel coupon, which was exposed to water under the usual conditions, touched a 1.75-gram sulfur button. The extent of attack was again low, b u t most of t h a t which took place was in the area of contact with the sulfur. Furthermore, the visible effects were clearly decreased as the distance along the coupon increased. The corrosion product and type of attack of the totally covered coupon resembled t h a t on the coupons from the field taken during the boosting period. The order of magnitude of the types of
SCMRZARY
It is evident, on the basis of the field and laboratory experi-
ACKNOWLEDGMENT
The authors wish to acknowledge the financial support of the Freeport Sulphur Company in carrying on this work. They also wish to thank the field and production personnel for their cooperation. LITERATURE CITED
(1) Fanelli, Rocoo, IND.ENG.CHEW,38,39-43 (1946).
(2) Hackermsn, Norman, and Shock, D. A.,Zbid., 39,863-7 (1947). (3) West, James R., Chem. Eng.,53,No. 10,225-34 (1946). RECEIVED November 4, 1948. Presented before the Division of Industrial and Engineering Chemistry at the 114th Meeting of the AMERIUAN CEEMICAL St. Louis, Mo. SOCIETY,
Effective Thermal Conductivities in Gas-Solid Systems J
D.G.BUNNELL, €3. B. IRVIN, R. W. OLSON, AND J. M. SMITH, b
*
Purdue University, Lafayette, Ind.
Point to point temperatures have been measured for both the solid and gas in a %inch reactor, packed with 0.125-inch alumina cylinders, and through which hot air was passed at mass velocities from 150 to 500 lb./(hr.) (sq. ft.). The entering air temperature was maintained a t $00' C. and the reactor wall held at about 100" C. by a boiling water jacket. Measurements were made at seven points across the diameter of the reactor and at packed bed depths of 0, 2, 4, 6, and 8 inches. While significant temperature gradients were observed, even near the center of the tube, the solid and gas temperatures were identical
within the accuracy of the measurements. This combined with the fact that the temperature varied considerably with mass velocity indicated that the radial heat transfer in a packed bed, under the conditions of this work, depended primarily on the characteristics of the gas rather than the solid pellets. Effective thermal conductivities computed from the temperature measurements ranged from about 0.1 to 0.4 B.t.u./(hr.)(ft.)(' F.). The increase in effective thermal conductivities over the value at static conditions was directly proportional to the mass velocity of gas flow.
T
This paper reports experimentally measured temperature gradients, both in the direction of gas flow (longitudinally) and perpendicular t o i t (radially), in a 2-inch inside diameter tube packed with 0.125-inch cylindrical alumina pellets. From these data effective thermal conductivities, K values, were computed assuming t h a t a heat transfer mechanism based upon conduction could be used to interpret the temperature data. Actually, the heat transfer rates within the gas-solid bed are
HE design of equipment for gas-solid catalytic reactions is complicated by the fact that the reaction rate may change rapidly with temperature and gas composition. This is especially important when the reactor is jacketed for heating or cooling because of the existence of radial temperature gradients within the reactor bed. Therefore, a knowledge of temperature from point t o point in the gas-solid system is necessary in order t o understand what takes place in a reactor.
