INDUSTRIAL AND ENGINEERING CHEMISTRY
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difficulty or not a t all. I n the second place, i t makes possible the recovery from the solvent extract of the lighter fractions of high-viscosity oil t h a t the solvent would normally carry with it. Unfortunately, the amount of propane required to perform these two separate functions does not in all cases coincide. A certain amount of propane may be required to do effective deasphalting, and more or less propane may be required to wash back from the extracts the lighter fractions which would normally be carried with it. If the amount of washing propane is increased beyond that required to recover the proper percentage from the extract, there will be washed back from the extract low-quality oils which must be subsequently removed by the other solvent. This procedure would necessarily require larger amounts of propane and larger amounts of solvents than that system 11-hich performed the deasphalting and washing steps independently. Figure 20 shows diagrammatically a system which utilizes propane for these tn-o functions independently. I n the first five extraction stages propane is acting as a deasphalting and auxiliary solvent medium. I n the last two washing stages propane is being used t o recover from the extract the lighter fractions that the solvent n-ould carry d h it. I n the solvent extraction results previously reported, the amount of propane used in these latter two stages was less than half of that used in the extraction stages. When an effort was made t o feed all of the propane into the two washing stages. and thence into the extraction stages, the yield and quality of the resulting raffinate decreased, unless the amount of selective solvent was correspondingly increased. ACID-TREATING.The Standard Oil Company (Indiana) has just recently completed the installation of a combined deasphalting and acid-treating plant a t its Whiting refinery. The general flow is shown in Figure 21. This plant is designed t o handle 1100 barrels a day of Midcontinent residuum and to produce a n acid-treated cylinder stock n-hich for some prod-
VOL. 28, NO. 9
ucts is percolated through clay for finishing. The period of operation has been so short that no average data from the plant are as yet available, but in the first few weeks of operation, when charging a residuum with a Saybolt Universal viscosity of 400 seconds a t 210' F., and when deasphalting with a propane ratio of 4:1,the yield was 75 per cent of deasphalted oil with a viscosity of 203 Saybolt seconds a t 210' F. The asphalt produced had a melting point of 130 O F. During this test the acid-treating portion of the plant was not in use. Subsequently the acid-treating plant was p u t into operation on a stock with a viscosity of 375 Saybolt seconds a t 210" F. T h e n deasphalting with a propane ratio of 4:l and treating with 0.75 pound of acid per gallon of charge, the yield m-as approximately 70 per cent. While the laboratory work indicated that the color on acid-treating would be '/z to 3/4 Tag-Robinson dilute, the actual color produced froni the plant has been 13j4to l 3 / 8 Tag-Robinson dilute.
Acknowledgment The writers desire to acknowledge the valuable assistance of W. H. Bahlke and H. 0. Forrest in assembling many of the data for this paper.
Literature Cited (1) Bahlke, IT. H., et al., PTOC. Am. Petroleum InJt., May, 1935. (2) Bray, U. B., Swift, C. E., and Cam, Donald, Re.her Natural Gasoline Mfr., 13,333 (Sept., 1.934). (3) Godlewics, hlarjan, and Pilat, Stanislaw, Oil Gas J . , 34, 76 (Dec. 26, 1935). (4) Sage, Schaafsma, and Lacey, ISD. ESG.CHEM., 26, 1218 (1934). (5) Tuttle, hf. H., PTOC. Am. Petrolewp Inst., 16,Pt.111, 112 (1935). (6) Wlson, R.E., and Keith, P. C., Jr., Oil Gas J . , 33, 14 (July 19. 1934). RECEIVED July 28, 1936. This paper, presented at the Chemical Engineering Congress of the World Power Conference, London, June, 1936, is reproduced by permission.
CORROSION OF METALS By Water and Carbon Dioxide under Pressure LTHOUGH experience has shown t h a t many metals are rapidly attacked by solutions of carbon dioxide under pressure, we have only incomplete and unsystematic informaGon as to t h e rate of corrosion of metals by moderately concentrated aqueous solutions of carbonic acid. Such information should be of some practical importance in connection with the selection of materials for use in the construction of equipment for t h e manufacture of solid carbon dioxide, t h e purification of hydrogen produced from water gas, and various other processes involving t h e handling of carbon dioxide under pressure and in t h e presence of water. ~
~~
Experimental Procedure A steel autoclave 24 cm. (9.4 inches) high and 15 cm. (5.9 inches) in internal diameter was connected with a steel cylinder containing liquid carbon dioxide. An empty cylinder was also connected into the system to act as a balancing reservoir. The line to the autoclave was provided with a calibrated pressure gage and with a release valve. The samples of the metal of which the rate of corrosion was to be determined were suspended, by glass hooks, in water in large test tubes held in a rack within
F. H. RHODES
AND JOHN M. CLARK, JR. Cornel1 University, Ithaca, N. Y.
the autoclave. The rack held four such tubes, each containing 150 cc. of water. The samples of metal were filed smooth on all faces and polished successively with coarse emery, No. 1, No. 0, and No. 00 BehrManning metallographic paper. During and after the final polishing the samples were not touched by the hands. The finished pieces were weighed, measured, and placed in the test tubes. When the samples were placed in the tubes in the autoclave, 150 cc. of distilled water were placed in each tube, the cover of the autoclave was fixed in position, and the cylinder of carbon dioxide was attached. The system was evacuated, the exhaust valve was closed, and carbon dioxide was admitted until a pressure of 150 pounds per square inch was attained. This pressure was maintained for 2 minutes, then the evacuation was repeated. After four successive evacuations and refillings, carbon dioxide was admitted and the pressure was raised to the value to be maintained during the test. During the test period the pressure was maintained constant to within 2 pounds of the desired value. The experiments were made in a room in which the temperature was constant t o 22.5' * 0.5' C. (72.5' * 0.9" F.). At the end of the test the exhaust valve was opened very gradually. The samples were removed, cleaned by rubbing with a cloth under a stream of water, wiped dry, and stored in a desiccator until weighed. This method of cleaning was satisfactory,
SEPTEMBER, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
The rate of corrosion of carbon steel by water and carbon dioxide increases rapidly as the pressure is raised to about 300 pounds per square inch. Further increase i n pressure of the carbon dioxide to 450 pounds per square inch has little effect on the rate of corrosion. Most nonferrous metals are corroded less rapidly than steel. Stainless steel is practically unattacked. These results were obtained with practically pure carbon dioxide and quiescent solutions. In any attempt to predict from these data the amount of corrosion under conditions other than those of the experiment, the effects of the agitation and of the presence of foreign gases must be considered.
since in no case was adherent scale formed on the surface of the test piece. On the samples of steel a loose black coating was present. This coating was easily removed by wiping with cloth. The final cleaned and dried samples were weighed. From the lose in weight and the original superficial area of the piece, the rate of corrosion, in inches penetration per month, was computed.
Preliminary Experiments Preliminary experiments showed t h a t in the corrosion of metals by solutions of carbon dioxide the initial rate of attack is often much higher (sometimes almost thirty times higher) than the final rate attained after the test piece has been in contact with the solution for some time. That initial rates of corrosion are often abnormally high has been observed by other investigators (1, 2) and has commonly been attributed to the presence on the polished specimen of a thin layer of very reactive worked or “flowed” metal. Results obtained here indicated that,. qualitatively a t least, the ratio of the initial to the final equilibrium rate of corrosion is an inverse function of the hardness of the metal. With relatively soft aluminum and electrolytic iron, very high ratios were found; with a harder steel of 0.2 carbon content the ratio was only 1.1. I n some cases, and particularly with the steels, the rate of corrosion decreased somewhat with the duration of the test period. This was true even with samples t h a t had previously been corroded long enough to remove the film of worked metal. The concentration of ferrous bicarbonate in the solution in which a steel (0.2 per cent carbon) had been exposed for 3 days was 0.03 molar; the solution was distinctly green. The retardation of the corrosion in long-continued periods of exposure and its reacceleration when the solution of ferrous bicarbonate was replaced by fresh water may have been due to the effects of the salt in decreasing the solubility of the carbon dioxide or in diminishing the dissociation of the carbonic acid, This retardation was negligible in test periods lasting not over 3 days.
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Effect of Carbon Dioxide Pressure The effect of the pressure of the carbon dioxide on the rate of corrosion of carbon steel is shown in Table I. The steels used were as follows: Component
Sample A
Sample B
% C
%
0.18 0.39 0.019 0.036 0.18
Mn S
P Si
0.22 0.66 0.033 0.021 0.14
Duplicate experiments were made, covering 2 4 a y periods of exposure after the samples had been exposed to a preliminary corrosion by carbon dioxide and water under pressure to remove the original f?lm of worked metal. The rate of corrosion increases rapidly as the pressure is increased to about 300 pounds per square inch; further increase in pressure has little effect.
Corrosion Rates of Various Metals The results of corrosion tests made with various nonferrous metals, with carbon dioxide under a gage pressure of 450 pounds per square inch, are shown in Table 11. Of these metals, the only one t h a t showed any pitting waa zinc. A series of tests made with stainless steels of various compositions indicated that these alloys are practically unattacked by carbon dioxide and water under a pressure of 450 pounds per square inch (Table 111). TABLE I. EFFECTO F PRESSURE O F CARBON DIOXIDE Gage Pressure of CO? Lb./sq. in. 450 300 150 80
-Rate Sample A
of Penetration-Sample B Inch per month 5.07 X IO-$ .4.97x 10-3 4.96 x 10-3 4.84 x 10-3 4.16 x 10-3 :3.96X 10-2 2.56 X 10-3 4.43 >: 10-3
-______
TABLE 11.
CORROSIOX
RATESOF SONFERROUS ~IETALS
Metal Brass (Cu til, Zn 3970) Monel metal ( S i 68.7 C u 28.6 Fe 0 1 M n l.OyG) Nickel, malleable (Ni’99.5.Cu’0.1, F e ’ O . l , L l n 0.1%) Copper, electrolytic Duralumin (Cu 4.0,h l n 0.5, M g 0.5%) Aluminum (99+ %) Lead, chemical Zinc
Penetration R a t e Inch/rnonth 2.7 x 10-5 1.4 X 1 0 - 6 1 . 4 X 10-6 e 0 x 10-r 1 . 5 X 10-4 2.5 x 10-4 1.4 x 10-4 5 . 2 x 10-4
TABLE111. CORROSION RATESOF STAINLESS STEELS Sample
Cr
Si
Si
%
%
%
Penetration R a t e Inch/month
Literature Cited (1) Calcott and Whetzel, Trans. Am. Inst. Chem. Engrs., 15, 1 (1923). ( 2 ) Pratt and Parsons, ISD.ENQ.CHEM.,17, 376 (1925). RECEIVEDJuly 10, 1936
