1
A. J. BRANDEL California Research Corp., El Segundo, Calif,
Recirculation of Cooling Water in Petroleum Refining Problems arising from use of recirculated cooling water may be greatly reduced b y proper water controls and treatments, but treatment effective in one system may be of little value in another
WATER
is of major importance in the refining of petroleum. I t is used chiefly for cooling, but is also used extensively for steam production and for treating operations. An average of 770 gallons of water is used in the refining of one barrel of crude petroleum (7). A large refinery would, a t this rate, require from 50,000,000 to 10C,000,000 gallons of water a day. Conservation measures usually are necessary for refineries, not only because of the cost of water but because the available supply may be inadequate. Where refinery locations permit, sea water may be used for cooling in unlimited quantities. However, salt water causes high corrosion and fouling rates. Use of fresh water is frequently economical, even where sea water is available. I n Southern California, as in other areas of water shortage, the limited supply of water necessitates conservation in its use. Because the major use in petroleum refining is for cooling purposes, the greatest reduction in water usage may be realized by recirculation over cooling towers. This practice, however, introduces many problems. This paper discusses these problems and some remedial measures which have been practiced a t the El Segundo Refinery of Standard Oil Co. of California.
Common Problems in Use of Recirculated Cooling Water Difficulties encountered in use of recirculated cooling water include various types of corrosion, deposition of scale on cooling surfaces, plugging of heattransfer equipment with biological growths, and deterioration of cooling towers. These problems vary in severity in different areas and systems because of different water qualities, heat-transfer equipment, and operating conditions. Such difficulties are restricted by limiting dissolved solids concentrations in the cooling water by blowdown, control of water alkalinity, and use of corrosion inhibitors, polyphosphate suspending agents to control scale deposition, and toxic agents to limit the growth of algae and bacteria. Protective coatings and cathodic protection
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are in limited use but are not considered here. The problems of corrosion, scale deposition, and other difficulties encountered in recirculating cooling water are interrelated, and the corrective measures that may be taken should not $e considered individually. For example, reducing the alkalinity of cooling water to eliminate scale deposition may increase, decrease, or change the type of corrosion encountered in a cooling system. I t may also affect the growth of algae and bacteria or the effectiveness of treatments for their control, as well as alter the rate of cooling tower deterioration. I n the following discussion, the various factors in cooling water treatment are treated separately, but their interrelation should always be kept in mind. Materials present in the make-up water to recirculated cooling systems may contribute to corrosion, scale, and cooling tower deterioration. Chlorides in high concentrations accelerate corrosion; calcium and magnesium salts deposit as scale on heat-exchange surfaces; and many salts, such as carbonates, accelerate delignification of cooling tower lumber. Blowdown, or continuous removal of part of the recirculated water from the system, is required to limit the concentrations of these materials. External softening may reduce the amount of blowdown required. T h e sca5ng tendencies of recirculated cooling waters are commonly minimized by a combination of blowdown to control solids build-up, alkalinity control to reduce precipitation of solids, and use of chemical additives to prevent deposition of precipitated solids as scale. Where the water used has low scaling tendencies, many installations operate satisfactorily with only blowdown. Others use a combination of blowdown and alkalinity control. Polyphosphates are used in most cooling water systems, in addition to blowdown and alkalinity control, to prevent scale deposition. Such treatment permits increased concentration of the recirculated water, with resultant savings in water consumption and costs of chemical treating. Low concentrations of polyphosphates, in the range of
INDUSTRIAL AND ENGINEERING CHEMISTRY
1 to 5 p.p.m. in cooling water, will usually prevent scale deposition in heatexchange equipment, if utilized with proper control of solids concentrations and alkalinity. Alkalinity, or pH control, should be uniform and reliable and is provided most effectively by automatic acid-feeding equipment. The concentration of salts in recirculated cooling water normally does not affect corrosion rates greatly. Corrosion of equipment by cooling water is commonly minimized by chemical corrosion inhibitors, together with alkalinity control. Alkalinity control is needed for most effective corrosion control, even where not needed to limit scale deposition. Some of the corrosion inhibitors in common use are chromates (Z), polyphosphates ( 3 ) , combinations of chromates with relatively high concentrations of polyphosphates, commonly known as “dianodic” treatment (5), and polar organic compounds ( I ) . Many other corrosion inhibitors (4, 6 ) are also in use? though to a lesser extent. Chromates and polyphosphates are used most frequently for corrosion control. Examples of commonly used compounds are sodium dichromate and sodium hexametaphosphate. The concentrations of chromates used for corrosion control usually range from 50 to 200 p.p.m., although they are sometimes effective in much lower concentrations. The p H of recirculated cooling water is usually controlled a t some level between 6.5 and 8.0, and more commonly between 7.0 and 7.5 when chromate inhibitors are used. Chromate inhibitors accelerate or localize corrosion when present in very low concentrations. They are also subject to reduction to chromic oxide by sulfides, mercaptans, and other reducing materials encountered in petroleum refining. Polyphosphate concentrations of 25 to 50 p.p.m. (as POr) in cooling water are used for controlling corrosion. Phosphate inhibitors are most effective with a water pH between 5.5 and 6.5, which is somewhat lower than for chromates. Polyphosphates are not generally so effective as chromates for reducing corrosion but they do not accelerate corrosion when present in low concentra-
RE-USE OF W A T E R B Y I N D U S T R Y
Figure 1. Equipment located beside cooling tower for adding water -conditioning chemicals
tions in cooling water and are not affected by reducing materials. Polyphosphates, however, deposit as phosphate scale if high concentrations are used. Combination chromate-phosphate treatments frequently provide better control of corrosion than either chemical used separately at comparable concentrations. A water p H of 6.0 to 7.0 is commonly maintained when this method of treatment is used. Good control of water alkalinity is nearly as important as the chemical corrosion inhibitor itself in reducing corrosion. The problem of biological growths in recirculated cooling water systems varies widely with location and operating conditions. Algae grow on exposed portions of cooling towers and, if detached, can cause serious plugging of heat exchange equipment. Algae growth is normally less in mechanical draft towers as compared with natural draft towers because of the reduced wetted area exposed to light, which is needed for algae growth. Bacteria growth within equipment can reduce water circulation and heat transfer greatly and some types can cause serious pitting corrosion. Chemicals used for controlling biological growths include chlorine, bromine, chlorinated phenols, copper and mercury salts, and quaternary amine compounds. A difficulty frequently encountered is the development of resistance or tolerance by the growths to the toxic chemicals used for their control.
Figure 2. General view of cooling tower and waterchlorination facilities
treatment. The system serves a variety of tubular heat-exchange equipment with stock temperatures up to 300" F. and water temperatures to 130" F. and circulates about 13,000 gallons of cooling water a minute. Typical analyses of the makeup and circulating waters are shown in Table I. Normal operation conditions in this system are shown in Table 11. The water treatment used in this system consists of blowdown to limit the solids content, p H control by means of a p H controller-recorder which operates a sulfuric acid injection pump, and addition of a combination chromate-phosphate (dianodic) mixture for control of corrosion and scale deposition. Biological growths are controlled by alternating shock chlorination and treatment with a quaternary amine compound (alkyl benzyl trimethyl ammonium chloride).
Table 1.
Typical Water Analyses
PH Alkalinity (as CaCOa), p.p.m. Total dissolved solids, p.p.m. Total hardness, p.p.m. Chlorides (Cl), p.p.m. Sulfate (Sod), p.p.m. Phosphate (Pod), p.p.m. Chromate (CrOl), p.p.m.
Table II.
Make-Up Water 8.6
Circulated Water 7.0
110
30
650 125 80 275
4400
... ...
...
550 2500 20
35
Operating Conditions
Circulation rate, gal./min. Make-up rate, gal./min. Hot water temperature, O F. Cold water temperature, F. Dianodic treatment used, lb./day Acid used (HzSOI),lb./day
13,000 475 105 75 60 500
Experience with Cooling Water Treatments
A recirculated cooling water system at the El Segundo Refinery of Standard Oil Co. of California provides an example of one type of cooling water
Figure 3. Compact installation for adding treating chemicals to recirculated cooling water VOL. 48, NO. 12
DECEMBER 1956
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With the treatment described, no scale deposits have been observed in this refinery recirculated cooling water system during several years of operation. Corrosion has caused no significant metal loss, but corrosion products building u p in sections of low water velocity, such as channel sections of heat exchangers, have at times caused some loss of cooling capacity by blocking the ends of heat exchanger tubes. The occasional occurrence of such fouling has been attributable to failure to maintain the prescribed treatment, generally to improper alkalinity control. It has been observed from operations of several recirculated cooling water systems over a period of years that good p H control is essential in avoiding both corrosion and scale deposition. Failure of p H control normally has more effect than occasional lapses in maintaining the desired concentrations of corrosion inhibitors and suspending agents. No biological growths have been found in this system, except for one occasion when failure of a chlorinator prevented adequate treatment and bacteria (sulfate-reducing type) were detected in some heat exchangers. T h e equipment used for adding waterconditioning chemicals to this system is shown in Figures 1 and 2. T h e equipment includes a tank for dissolving corrosion and scale control chemicals from which the chemical solution is added to the cooling water by means of an eductor. The p H controller-recorder shown adjusts the rate of an acid-injection pump to maintain the desired p H of cooling water. A chlorinator located in the small shelter in the pictures adds chlorine to the circulating water from the chlorine storage drums shown in Figure 2. A storage shelter for treating chemicals is included. Figure 3 shows similar facilities in another cooling water system. The El Segundo Refinery has nine recirculating cooling water systems with a total circulation rate of about 85,000 gallons a minute. Currently, most of the water treatments used are similar to the treatment discussed above. The combination of blowdown, alkalinity control, and chromate-phosphate (dianodic) treatment has been used in several systems for up to 7 years with generally good results. Considerable difficulty was encountered in one system, where reducing agents entered the cooling water a t times and reduced the chromate corrosion inhibitor to chromic oxide. Metal loss was not serious even in this case, but accumulation of iron corrosion products and precipitated chromic oxide fouled heat exchangers so that frequent cleaning was required. The problem subsided when leaks of reducing Faterial into the cooling water were controlled to prevent chromate reduction. In other systems using the same treatment,
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heat exchangers have been used for u p t o 5 years without cleaning. Another effective chemical treatment for recirculated cooling water consists of 50 p.p.m. of chromate (CrOd) to control corrosion, together with a “threshold” polyphosphate treatment of 2 to 4 p.p.m. (as Pod). Solids are limited by blowdown, as in the treatment described above, and p H is controlled to 7.5. This treatment is not truly dianodic, as the concentration of polyphosphate is too low to be effective as a corrosion inhibitor. This combination is not quite so effective as the dianodic treatment in controlling build-up of corrosion products in areas of low water velocities, but is otherwise equivalent for limiting corrosion and scale deposition. Such treating agents are readily prepared from open market chemicals and the proportions may be varied to suit different conditions. A variation of this treatment has been used in a cooling system serving mostly open box condensers where high water losses and high local temperatures make it difficult to maintain desired concentrations of treating chemicals. Chromate concentration averages about 20 p.p.m. in this system and polyphosphate about 1 p.p.m. Water p H is maintained a t 7.5. This seemingly inadequate treatment has reduced corrosion and plugging of tubular heat exchangers with corrosion products, so that periods between cleanings have been extended from about 6 weeks to over a year. Equipment life has been extended by a large but, as yet, unknown amount. The p H control and polyphosphate treatment also caused previously deposited scale to detach from heat-exchange surfaces and prevented redeposition of scale. Other cooling water treatments have been tried a t the El Segundo Refinery, including an organic chromium compound for limiting corrosion and a high polyphosphate treatment for controlling corrosion and scale. These treatments were not so effective as methods now used, although they have been effective in other plants. Several chemical treatments for preventing growth of algae and bacteria in recirculated cooling water systems have been tried a t the El Segundo Refinery during the past 15 years. Chemicals used a t various tinies include chlorine, bromine, chlorinated phenols, organic copper compounds, and a quaternary amine. All of these treatments controlled biological growths for varying lengths of time, but resistant strains of algae and bacteria have been observed after any one of these chemicals was used for 6 months to 2 years. Growth of slime-forming bacteria has a t times been so rapid that heat exchangers had to be cleaned after only 6 weeks of operation. Algae growth from cooling towers has
INDUSTRIAL AND ENGINEERING CHEMISTRY
entered the circulating water with similar results. The treatment now uscd for control of biological growths-shock chlorination alternated with a quaternary amine addition-has been used up to 5 years with generally good results. Experience with chlorine used alone, both as uniform and shock treatment, has shown that gradually increasing amounts of chlorine are required to control fouling. Similar results were observed with the other chemicals tested. Bromine was found to be as effective as chlorine and can be used with less expensive injection equipment for treating small systems. The copper compound tested (a commercial organic copper algicide) effectively controlled algae growth and had a residual effect because of absorption in the cooling tower lumber; however, it was ineffective against bacteria after 6 months’ use. A combination treatment with the copper compound alternated with shock chlorination provided good control of both algae and bacteria in one system for several years. The quaternary amine compound used prevented algae growth for u p to 5 years. but it was decreasingly effective against bacteria after 1 to 2 years. The quaternary amine treatment alternated with shock chlorination currently provides control of algae and bacteria in the refinery recirculated cooling water systems. Summary
The methods for minimizing difficulties presented by use of recirculated cooling water a t the El Segundo Refinery were developed by experimentation over about 15 years. A treating procedure which is effective in one system or one area may be of little value in systems using different water, operating under different conditions, or located in a different area. However, the problems arising from use of recirculated cooling water may be greatly reduced by proper water controls and treatments: possibly similar to those discussed. The best treatment in a given situation should be determined by making extensive trials of possible treating procedures. Literature Cited N., IKD. ENG.CHEM.44,
,.(1952). Ibid., (Symposium), 37,702-59 (1945). Kahler, H. L., Gaughan, P. J., Ibid., 44, 1770-4 (1952). (6) Kahler, H: L., ’George, C., Petroleum ReJner 34, 90.7, 144-8 (1955). ( 7 ) Maher, J. C., Zbid., 30, No. 11, 101-6 (1951).
RECEIVED for review April 10, 1956 ACCEPTED October 2, 1956
