Method for removing clay deposits from cooling water systems

Abstract

Methods and compositions for removing deposits from surfaces in contact with the water of a cooling water system are disclosed. A lignosulfonate compound, alone or in combination with other materials, is effective in removing such deposits.

Description

FIELD OF THE INVENTION

The present invention is directed toward methods and compositions for removing deposits, such as clay deposits, from surfaces in contact with the water of a cooling water system.

BACKGROUND OF THE INVENTION

In cooling water systems, accumulations of clays are troublesome in that they reduce heat transfer efficiency of heat exchangers and the like. Also, accumulations of deposits on cooling tower fill reduce the evaporative efficiency of the cooling tower. In many instances, these deposits must be mechanically cleaned when they are present in excessive amounts. Hydrofluoric acid and ammonium bifluoride are known to dissolve silicas, silicates and clays. These compounds, however, would be unacceptable to remove deposits from surfaces in contact with cooling water systems, for several reasons.

Both compounds are extremely hazardous and toxic. The compounds are also highly corrosive to metals. The ammonia from the ammonium bifluoride is also known to be corrosive to metals made from copper. (Heat exchangers in cooling systems are often made from metals that contain copper.) These compounds also introduce the possibility of calcium fluoride scale formation which would reduce heat transfer. Cement is commonly used to construct cooling tower sumps and other structural parts. Since cement is a silicate, it would dissolve in the presence of the fluoride compounds.

The present invention is directed toward methods and compositions for chemically cleaning clay deposits from metal surfaces, particularly from heat exchange surfaces in cooling water systems, and non-metal surfaces such as plastic cooling tower fill.

PRIOR ART

Most currently practiced cooling system chemical cleaning procedures for removing deposits require depression of the system pH to an acidic range (e.g.,3-5) for several hours or several days. Such acid cleaning procedures are directed toward removing or dissolving acid-soluble solids (e.g. CaCO.sub.3, Ca.sub.3 (PO.sub.4).sub.2, iron oxide, etc.) Clays and silicas are not soluble in acid solutions.

In such cleaning approaches although deposit removal may be effected, high corrosion rates are experienced by reason of the maintenance of low pH during the process. For instance, in U.S. Pat. No. 4,190,463 (Kaplan), iron oxide deposits are removed from heat transfer systems by first contacting the deposits with an aqueous solution of a hydrolyzable tannin extract followed by subsequent use of dilute solutions of citric acid. The pH of the cooling system water should be maintained at about 2.8-3.0 during the citric acid addition step. At column 7, line 15 of the '463 patent, it is stated that use of citrates at higher pH's for long times are not effective for removing modified deposits.

In U.S. Pat. No. 3,297,580 (Pitzer), substantially neutral pH chemical cleaning compositions and methods are disclosed. These compositions and methods are directed toward removing oxide films from iron and ferrous alloys in inaccessible places. The cleaning compositions that are employed have a pH range of between 6.5 and 9.5 and involve utilization of buffered aqueous solutions of hydrazine and polycarboxylamine acid chelating agents. The preferred metal oxide is magnetite (Fe.sub.3 O.sub.4) and the preferred temperatures are 90.degree. to 100.degree. C. The presence of hematite (Fe.sub.2 O.sub.3) and other nonmetallic oxides in industrial systems and the normal system operating temperatures may have discouraged this approach.

In U.S. Pat. No. 4,789,406 (Holder) methods for removing iron oxide and other corrosion products from metal surfaces in contact with cooling water systems are disclosed. The aforementioned references do not disclose the use of specific chemical treatments to remove clay deposits from cooling water systems.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, a method of improving the efficiency of a cooling tower and removing deposits from surfaces in contact with the water of a cooling water system is disclosed. It has been discovered that a sulfonated lignin (lignosulfonate) is effective in removing deposits from cooling tower fill, either alone or in combination with a chelant. The chelant reduces the soluble hardness level (e.g., free or uncomplexed calcium and magnesium) of the water system to less than about 200 ppm as CaCO.sub.3. Examples of suitable chelants are ethylenediaminetetraacetic acid (EDTA) or citric acid. With the hardness level in the system complexed by the chelant, the clays (which may contain calcium) swell.

The pH of the system water is maintained within the range of from about 7 to about 11 throughout the entire program, thus minimizing the potential for corrosion which exists in many of the prior art treatment approaches utilizing low acid pH's for the cleaning methods. Bases such as Na.sub.2 CO.sub.3, NaHCO.sub.3, NaOH etc., may be used to adjust pH.

A sodium salt, such as Na.sub.2 SO.sub.4, NaCl, NaHCO.sub.3, Na.sub.2 CO.sub.3, etc., at from about a trace to about 10,000 ppm may optionally be added to improve the deposit removal. The sodium from the base, the chelant, or the naturally occurring sodium level in the cooling tower water may eliminate the requirement to add a sodium salt.

Subsequent to the addition of chelant to the aqueous system, a phosphonate or phosphate compound of from about 300-20,000 ppm may optionally be added. Exemplary phosphonate compounds include hydroxyphosphonoacetic acid, or preferably 1-hydroxyethylidene-1, 1-diphosphonic acid (HEDP). Exemplary phosphate compounds comprise tetrapotassium pyrophosphate (TKPP), sodium tripolyphosphate (STPP) or sodium hexametaphosphate (SHMP).

The treatment takes place over a period of from about 1 to 5 days until the deposits are removed from the affected surfaces. It is noted that, in order for the program to be effective, the pH of the cooling water system should be maintained within the range of from about 7 to 11. As will be shown, in certain instances, efficacy of deposit removal is enhanced at higher pH's in this range.

The methods and compositions of the invention are designed to penetrate and remove accumulated deposits from heat transfer surfaces and cooling tower fill surfaces. Unlike many of the conventional cleaning methods, the method is performed with the cooling system on-line (i.e., still operating) and at a pH from about 7 to 11. The unique ability to clean the system effectively without removing it from service can result in considerable savings in unit downtime. Many companies specializing in chemical cleaning do not offer effective on-line cleaning programs. As the cleaning method of the present invention removes clay deposits at a 7-11 pH range, copper alloy and mild steel equipment does not require isolation, and corrosion occurring during the cleanup is minimized. Copper corrosion rates could be further reduced by adding known copper corrosion inhibitors such as tolyltriazole. The present invention provides a distinct advantage over the traditional removal of silicas and silicates accomplished via the use of hydrofluoric acid and ammonium bifluoride solutions, which have the potential to cause serious damage to heat exchangers and associated valves and piping.

The components of the present invention perform their functions concurrently so there is no need for a pretreatment step. The chemicals are allowed to recirculate under normal operating temperatures (about 40.degree. F. to 160.degree. F. with about 100.degree. F. being most preferred) for cooling towers until the deposits have been removed (approximately 1 to 5 days). The cooling tower is then blown down and the normal cooling tower operation is resumed. Most of the removed solids will exit with the blowdown water.

Efficacy of the Present Invention

In order to demonstrate the efficacy of the present invention in the removal of deposits from surfaces in contact with the water of a cooling water system, the following experiments were performed. The experiments should be construed as being illustrative, and not as restricting the scope of the invention. All tests were conducted at room temperature unless otherwise noted.

The following beaker test procedure was used:

1. Cut fouled fill into coupons (approximately 2.5.times.0.5 inches). 2. Air dry tower fill coupons at room temperature. 3. Photograph coupons (unexposed). 4. Weigh coupons (initial weight). 5. Place coupons on holders (PVC rod with nylon screw). 6. Soak coupons in untreated test solution (control) for one-half hour with slow stirring. 7. Place 1.0 liter of test solutions in beakers and mix. 8. Add treatments and adjust pH if appropriate. 9. Immerse pre-soaked coupons in recirculating test water. 10. Remove coupons after specified time (about 3 days). 11. Weigh coupons when dry (air-dried at room temperature). 12. Photograph coupons (exposed). 13. Manually remove remaining deposits and weigh coupons when dry.

Example 1

Fouled Cooling Tower Fill from a Southeast U.S. Cooling System

Cooling towers use fill, or packing, to keep the water distributed evenly and to increase the water surface area for more efficient evaporation. Fill deposit in this example was analyzed to be primarily clay (kaolin) with a minor amount of calcium carbonate. Elemental analysis showed: 50% SiO.sub.2, 15% Al.sub.2 O.sub.3, 10% Fe.sub.2 O.sub.3, 15% LOI (loss on ignition), 5% MgO, 5% CaCO.sub.3. The test water had the following composition: 69 ppm Ca as CaCO.sub.3, 22.5 ppm Mg as CaCO.sub.3, 60 ppm M alkalinity.

______________________________________Test 1 - 3 days at 100 rpm with 0.5 hour presoak %Treatment Initial pH Deposit Removal______________________________________Untreated 7.8 63600 ppm HEDP, 0.002M EDTA 10 83600 ppm HEDP, 0.002M EDTA 10 961000 ppm sulfonated lignin______________________________________ ______________________________________Test 2 - 2 days with no presoak %Treatment Initial pH Deposit Removal______________________________________Untreated 7.8 74600 ppm HEDP, 0.002M EDTA 9 83600 ppm HEDP, 0.002M EDTA, 9 961000 ppm sulfonated lignin______________________________________ ______________________________________Test 3 - 2 days with no presoakTreatment Initial pH % Deposit Removal______________________________________Untreated 7.8 261000 ppm sulfonated lignin, 9.7 640.002M EDTA______________________________________ ______________________________________Test 4 - 3 days with no presoakTreatment Initial pH % Deposit Removal______________________________________Untreated 7.8 551000 ppm sulfonated lignin, 9.7 920.002M EDTA______________________________________ ______________________________________Test 5 - 1 day with no presoakTreatment Initial pH % Deposit Removal______________________________________Untreated 7.8 221000 ppm sulfonated lignin 7.8 581000 ppm sulfonated lignin 9.7 591000 ppm sulfonated lignin, 9.7 570.002M EDTA______________________________________ (The sulfonated lignin tested in the examples is available from Reed Lignin, Inc., as Maracarb .RTM. N1).

In this low hardness water, the lignin was able to remove deposits effectively with or without EDTA.

Example 2

Fouled Cooling Tower Fill from a Midwest plant

Fill deposit had the following composition: 34% LOI, 2% P.sub.2 O.sub.5, 2% SO.sub.2, 3% MgO, 2% Al.sub.2 O.sub.3, 24% SiO.sub.2, 9% CaO, 10% MnO.sub.2 and 14% Fe.sub.2 O.sub.3. The test water had the following composition: 776 ppm Ca as CaCO.sub.3, 361 ppm MO as CaCO.sub.3, 90 ppm SiO.sub.2.

______________________________________Test 1 - 1 day test duration with no presoakTreatment Initial pH % Deposit Removal______________________________________Untreated 7.5 131000 ppm sulfonated lignin, 9.0 370.012M EDTA______________________________________ ______________________________________Test 2 - 1 day test duration with no presoak;test conducted at 100.degree. F.Treatment Initial pH % Deposit Removal______________________________________Untreated 7.5 541000 ppm sulfonated lignin, 9.0 800.012M EDTA______________________________________ ______________________________________Test 3 - 1 day test duration with no presoakTreatment Initial pH % Deposit Removal______________________________________Untreated 7.5 71000 ppm sulfonated lignin 9.0 7______________________________________

Based on these tests, EDTA is needed for the lignosulfonate to be effective in a high hardness water.

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.

Claims

1. A method of removing clay deposits from surfaces in contact with the water of a cooling water system comprising the steps of:

(a) adding a lignosulfonate compound to said cooling water,

(b) adding a chelant selected from the group consisting of ethylenediaminetetraacetic acid and citric acid, and

(c) adding a compound selected from the group consisting of 1-hydroxyethylidene-1,1-diphosphonic acid, and hydroxyphosphonoacetic acid, and

(d) blowing down said cooling water to remove said clay deposits, the pH of said cooling water within the range of from about 7 to about 11 throughout the entire method.

2. The method as recited in claim 1 wherein said surfaces comprise plastic cooling tower fill

3. The method as recited in claim 1 wherein said chelant reduces the soluble hardness level of said cooling water system to less than about 200 ppm as calcium carbonate.

4. A method of removing clay deposits from surfaces in contact with the water of a cooling water system comprising the steps of:

(a) adding a lignosulfonate compound to said cooling water,

(b) adding a chelant selected from the group consisting of ethylenediaminetetraacetic acid and citric acid,

(c) adding a phosphonic acid, and

(d) blowing down said cooling water to remove said clay deposits,

the pH of said cooling water within the range of from about 7 to about 11 throughout the entire method.

5. The method as recited in claim 4 wherein said phosphonic acid is 1-hydroxyethylidene-1, 1-diphosphonic acid or hydroxyphosphonoacetic acid.