Method of Removing Iron Oxide Deposits From Heat Transfer Surfaces

Most industrial heat exchangers are composed of bundles of ferrous metal tubes. In some instances, non-ferrous metals such as admiralty metal are used. These heat exchange systems are water-cooled, with the heat absorbed by the water being removed atmospherically by cooling towers. These industrial cooling systems rapidly form iron oxide deposits which reduce their heat transfer efficiency. It is common to mechanically clean these systems when the iron oxide deposits become excessive. Mechanical cleaning, while effective in many cases, is time-consuming and expensive.

The heat exchangers thus described should be distinguished from the heat transfer surfaces of boilers. The distinction is that the scale in boilers is most often composed of calcium or magnesium salts and is relatively low in iron oxide. Industrial heat exchangers of the type described normally contain deposits which are predominantly composed of the oxides of iron. Therefore, the specification and claims, when referring to heat transfer surfaces and heat exchangers, means industrial heat exchangers and not boilers.

A method for removing iron oxide deposits from heat transfer surfaces which comprises the sequential steps:

(a) contacting such surfaces with an aqueous solution which contains at least 25 parts per million of a hydrolyzable tanning extract and has a pH of not more than 8.5 for a period of time sufficient to modify a substantial portion of the iron oxide deposits; and then,

(b) removing the modified deposits formed in step (a) with an aqueous solution having a pH not greater than 4 which contains at least 1000 parts per million of citric acid.

This group of tanning extracts represents a distinct species of tannins over the so-called condensed tanning extracts. The hydrolyzable tanning extracts most advantageously employed in the practice of the invention are sumach, valonea, or chestnut tannin, with the latter being preferred. For a more detailed discussion of tannins, see the Encyclopedia of Chemical Technology, Second Edition, Volume 12, Interscience, 1972, page 321 et. subs.

The hydrolyzable tanning extracts are most preferably employed at ranges between 50-1000 ppm with solution concentrations of 100-300 ppm appearing to be optimal. The pH of these solutions should not exceed 8.5 and is preferably within the range of 3.0-7.5. While the hydrolyzable tanning extracts are effective when used alone, it is oftentimes beneficial that they be used in conjunction with a water-dispersible surfactant, preferably a nonionic surfactant. Surfactants of this type are described in McCutcheon's Detergents & Emulsifiers, 1974 North American Edition, Published by McCutcheon's Division, Allured Publishing Corporation. A preferred surfactant is nonyl phenol reacted with 9 moles of ethylene oxide. The amount of time necessary for the hydrolyzable tanning extract to act upon and complex with the iron oxide deposits varies depending upon a number of conditions. A general rule is that the minimum time required is at least 12 hours with time periods ranging from between 12 hours to as long as several days sometimes being required to adequately complex with the iron oxide deposits. Such variables as the temperature of the system during the treatment with the hydrolyzable tannin extract, the nature and quantity of the deposit, the pH of the system, and the like will govern the time of the treatment which cannot be expressed with exactitude. The optimum conditioning parameters for chestnut tannin were found to be a 100-500 ppm solution circulated for 2-3 days with the pH being about 3-7.

The citric acid treatment which follows after the hydrolyzable tanning extract treatment should employ citric acid solution which contains at least 1000 ppm with the pH not being in excess of 4. In most instances, the pH of the citric acid solution should be about 3.0-3.4. A pH of 2.8-3.8 should be maintained to obtain maximum advantage of citric acid. A preferred dosage range of the citric acid is within 2000-4000 ppm.

The time required for the citric acid to remove the hydrolyzable tannin extract iron deposits will vary between a few hours up to as long as a day or more depending upon the environment of the system, e.g. pH, tannin extract employed, quantity of suspended or tannated iron oxide in the system, temperature and the like. In most cases, a time of about 18 hours using optimum citric concentration and pHs will give good cleanup.

Rather than continuing the citric acid treatment for a fixed period of time, it is possible to monitor the soluble iron levels during the citric acid treatment. The treatment can be discontinued when the iron levels are above about 500-600 ppm.

The treatment with the hydrolyzable tannin extract and the citric acid may be conducted over a wide temperature range but below the boiling point of the treating solutions used to practice the invention. While ambient temperatures may be used, it is preferred that the temperatures in excess of 100.degree. F. be used with a preferred temperature range being 100.degree.-150.degree. F.

A typical cleaning procedure for an iron fouled heat exchanger would be as follows:

1. Discontinue the corrosion inhibition program, if used.

2. Add 200-300 ppm tannin and 5-10 ppm of Comp. N.sup.1 to the system and circulate. Maintain the pH at 6-7 and a temperature of 110.degree.-130.degree. F. As the tannin concentration is reduced to less than 50 ppm by consumption, add more tannin to increase the dosage to 200-300 ppm.

3. Discontinue tannation after 2-3 days depending upon the severity of the fouling.

4. Dump the system or blow-down heavily.

5. Refill with clean water and add citric acid at 2,000-4,000 ppm, pH 2.7-3.2, and a temperature of 110.degree.-130.degree. F.

6. Monitor soluble iron levels and when soluble iron reaches 500 ppm, blow-down heavily and add more citric acid.

7. Repeat step 6 for three or four times. (Blow-down may contain fragments of iron tubercles at this stage.)

8. Blow-down system and return to the normal corrosion inhibition program, if used. If possible, the system should be monitored for leaks throughout the program and discontinue treatment if leaks develop.

Unusually thick iron oxide deposits or deposits containing large amounts of silica are extremely difficult to remove using the above chemical treatment. In such cases, the mass of deposit should be removed by mechanical means prior to chemical treatment.

Iron complexes of gallotannic acid, Quebracho tannin, wattle tannin, and chestnut tannin were prepared in the following manner. Ten grams of FeCl.sub.3 dissolved in a minimum of water was added to five grams of the appropriate tannin or tannic acid dissolved in water. The dark purple to black precipitate that formed was filtered, washed, and dried. In the case of chestnut tannin, its iron complex was extremely finely divided and probably colloidal. A water suspension of this complex had to be evaporated to dryness for the solubility tests.

To determine the solubility of each of the iron complexes in citric acid (and hence its ease of removal from a tannated iron substrate), the following scheme was used. A 100 mg. sample of each iron complex was placed in a separate 100 ml portion of citric acid ranging in concentration from 500 to 5,000 mg/l. After two hours of intermittent stirring, the suspensions were filtered and dried. The amount of dissolution was determined by weight differences before and after citric acid treatment. Data from these experiments, conducted at 72.degree. and 120.degree. F. are shown in FIG. I.

Clearly, the solubility of the iron-Quebracho complex is far too low to be considered for practical usage. Indeed, Quebracho might lead to fouling in iron laden systems that would not be recovered in the subsequent citric acid step. Based solely on solubility considerations, the chestnut tannin is preferred since its chance of dissolution approaches 100 percent in heated systems. The iron-gallotannic acid complex is adequately soluble in citric acid; however, the high cost of the acid could preclude its usage.

Heat transfer unit experiments were run to determine the effects of tannins and citric acid on mild steel heat transfer tubes. In most cases, the heat flux was 10,000 BTU/ft.sup.2 /hr. and the flow rate was 2.8-3.6 ft/second. Bulk water temperature was 125.degree. F. Three types of water were used ranging in hardness from 100 to 1200 ppm Ca, but no significant differences were evident.

Twelve of the most significant runs are outlined in Table I. All were conducted in three cycle Chicago tap water. After each test listed, the significance of the findings of that test is given. Many of the findings of the solubility testing were verified during this phase of the work. The appropriate tannin type, concentration, time of tannation, and the relative unimportance of pH during deposit conditioning were determined. Optimum conditioning parameters were found to be chestnut tannin, 100-500 ppm, 2-3 days, and pH 3-7.

Citric acid must be applied at a minimum dosage of 1000 ppm and a pH of 3.0-3.4. Lower concentrations and higher pH values are not effective in deposit removal. However, higher concentrations and lower pHs improve the rate of deposit removal at the expense of increased corrosion rates. Surfactants and dispersants have some utility in the process, primarily for systems with oil or silt present.

Eleven pilot cooling towers were used to verify all the conditions found for optimum iron oxide cleanup during previous testing. Significant differences between PCT and HTU tests are lower temperatures for the PCTs (100.degree. F. vs. 125.degree. F.) and slightly lower flow rates (0.1-2.5 ft/sec. vs. 2.8-3.6 ft/sec). The PCTs also incorporate the possibility of using mixed metallurgies with the inherent possibility of fouling from corrosion of other tubes in the system.

Two of the pilot cooling towers, A and B, used 7-tube shell side heat exchangers. These towers as well as Towers D and E used heat exchanger tubes equipped with thermocouples to follow fouling and defouling during all phases of the procedure.

The PCT experiments are outlined in Table II with a summary of each run given at the end of the test. For Towers A, B, D, and E, FIGS. 2-5 show graphically the results of each phase of the program.

Many of the parameters and conditions discovered in HTU work were confirmed and new facts were uncovered. For instance, chestnut tannin is preferable to wattle tannin; concentrations of 50-200 ppm are adequate, thick, aged deposits are difficult to penetrate and remove; oil and silt should present no unsolvable problems; low pH conditions are absolutely necessary for citric acid to adequately remove tannated deposits; repeated "shocks" up to 3,000 ppm of citric acid are preferable to constant feeding, long-term treatments; high concentrations of ferric ion cause increased corrosion and should be removed as soon as possible; and it is possible to passivate a cleaned system with Comp. F.sup.1 and an appropriate corrosion inhibitor.

The Heat Transfer Unit tests (HTU) as well as the Pilot Cooling Tower tests (PCT) are described in detail in the article, "Small-Scale Short-Term Methods of Evaluating Cooling Water Treatments. . . Are They Worthwhile?" by D. T. Reed and R. Nass, Nalco Chemical Company, presented at the 36th Annual Meeting of the International Water Conference, Pittsburgh, PA, Nov. 4-6, 1975, which is incorporated herein by reference. Various lettered materials used in Tables I & II are set forth in the Glossary.

B--Benzotriazole

D--A glassy polyphosphate

E--A low molecular weight sodium polyacrylate

F--A film forming passivator for metal systems containing sodium pyrophosphate, sodium acid pyrophosphate, nonyl phenol R.sub.x 8 moles ethylene oxide (surfactant), and benzotriazole.

G--Corrosion inhibitor containing chromate and zinc in a 7 to 1 ratio.

H--A scale dispersant containing hydroxyethylidene diphosphonic acid and sodium polyacrylate.

I--A biocide whose active agents include methylene bis thiocyanate and 2,4,5-trichlorophenol.

J--A corrosion inhibitor containing sodium lignosulfonate, zinc chloride, and polyolester (see U.S. Pat. No. 3,502,587).

L--Deposit from a commercial cooling tower basin, Chicago area. Contains 28% Si, 21% Ca, 17% Fe, 7% Al, 4% Mg, 4% S, 2% Zn, 13% carbonate, and 5% CHCl.sub.3 extractables.

M--Modified polyethoxylated straight chain alcohol (nonionic).

N--Octyl phenoxy polyethoxyethanol (surfactant).

0--A corrosion inhibitor containing a glassy polyphosphate and polyolester (see U.S. Pat. No. 3,502,587)

P--A surfactant-dispersant combination containing:

(a) octyl phenoxy polyethoxyethanol;

(b) polyethoxylate;

(c) a low molecular weight sodium polyacrylate.