WO1983002607A1

WO1983002607A1 - Process for inhibiting scale

Info

Publication number: WO1983002607A1
Application number: PCT/US1982/000121
Authority: WO
Inventors: Chemical Company Dearborn
Original assignee: Kuhn, Vincent, R.; Ventura, Ederlyna, M.
Priority date: 1982-01-29
Filing date: 1982-01-29
Publication date: 1983-08-04
Also published as: GB2125025A; EP0099883A4; GB2125025B; SE8301135D0; EP0099883A1; ES519364A0; GB8325799D0; ES8406566A1; DE3249177T1; SE8301135L

Abstract

A process for inhibiting scale formation on metal surfaces in contact with aquaeous systems by adding a threshold quantity of copolymers of 2-acrylamido-2-methylpropane sulfonic acid and acrylic or methacrylic acid or water soluble salts thereof.

Description

PROCESS FOR INHIBITING SCALE

Background of the Invention The present invention relates to the treatment of aqueous systems, and, more particularly, to inhibiting or preventing the deposition of scale onto metal surfaces in contact with the aqueous systems.

Typical apparatus in which the metal parts are fouled by scale deposition include evaporators, single and multi-pass cooling towers and associated equipment and the like. As the aqueous system is passed through or over the apparatus a portion of the aqueous system evaporates causing a concentration of the dissolved salts either naturally or synthetically contained in the system. These dissolved materials approach and reach a concentration at which they deposit out of the system (in certain instances they have solubility- in inverse relation to temperature) and form scale on the metal surfaces. This is especially true when the aqueous system is formed from nearby natural resources which contain various amounts of dissolved solids such as of calcium, magnesium and phosphates. Such deposits typically reduce heat transfer rates, and may impede the flow of the aqueous systems.

Various materials have been used to prevent scale deposition. These include the treatment of the aqueous system with sulfuric acid. Such an acidic agent poses storage problems and is corrosive to many metals. U.S. Patent 2,782,162 to Lidell discloses the use of polyphosphate-lignosulfonates as scale preventatives. However, these materials are unstable at the elevated temperatures normally encountered in operation and the selve form a sludge capable of depositing and, therefore, have limited application.

The use of a variety of polycarboxylates including acrylate polymers, as dispersants in aqueous systems has

OMPI been known for many years. It is well recognized, however, that these materials suffer a considerable decrease in effectiveness when used in hard water, for example, water containing more than 300 ppm calcium hardness. This is obviously a serious deficiency in view of the fact that the majority of processes use recirculating aqueous systems where substantial concentration is developed.

A method of inhibiting the formation of scale, particularly calcium phosphate and the like, depositions on metal surfaces of apparatus in contact with circulating aqueous systems is highly desired. It is especially desired to have a method which uses an agent which is stable, non-corrosive, and easily handled, which is effective at very low dosages and which is capable of inhibiting the formation of scale on metal surfaces of apparatus in contact with aqueous systems which tend to concentrate.

Summary of the Invention The present invention is directed to a method of inhibiting sc le formation on metal surfaces which are in contact with aqueous systems by incorporating into the aqu ous system a threshold quantity of a copoly er of 2-acrylamido-2-methylpropane sulfonic acid and acrylic or methacrylic acid or water soluble salts of said copolymers.

Detailed Description

The present invention is directed to a method which is more effective in inhibiting and preventing scale deposition, especially deposition of calcium phosphate found in hard water, to a greater degree than is currently available.

According to the present invention, it has been surprisingly found that the inhibition of scale deposition, particularly of calcium phosphate, on metal

<& surfaces can be achieved to a very high degree by the -^ inclusion of threshold quantities of a specific copolymer of 2-acrylamido-2-methylpropane sulfonic acid and acrylic or methacrylic acid.

The subject copolymer is formed from 2-acrylamido-2-methylpropane sulfonic acid with either acrylic acid or methacrylic acid. The molar ratio of the two types of monomeric units in the copolymer should be from 5:1 to 1:5, and more preferably from 2:1 to 1:2. The copolymer should be of low molecular weight with a weight average molecular weight of from 1,000 to 10,000, preferably from 4,000 to 6,000 and most preferably of about 5,000. Although small amounts of high molecular weight copolymer can be tolerated in the system, such high molecular weight copolymers tend to behave as flocculating agents and, therefore, their presence should be avoided.

The copolymer is formed from carboxylic acid and sulfonic acid containing monomeric units. These acid units may be in the form of the free acid or preferably at least partially neutralized to a water soluble salt i.e. salts of alkali metals, such as sodium or potassium or ammonium salt with sodium being most preferred.

The copolymers required for use in the subject invention may contain minor amounts of up to about 5 mole percent of other monomeric units which are substantially inert with respect to the subject process such as lower (C,-C₅) esters of acrylic or methacrylic acid, «t» acrylonitrile and the like.

The copolymers required for use in the subject *• invention are surprisingly effective in inhibiting scale formation, particularly of calcium phosphate, when present in the aqueous system in threshold amounts which will depend, to some extent, on the nature of the aqueous

% JΕEX

OMPI system. Normally dosages of from 0.1 to 100 parts per million and more preferably from 1 to 25 parts per million provide effective inhibition. The copolymer may be added to the aqueous system coming in contact with the metal surfaces of an apparatus by any convenient mode, such as by first forming a concentrated solution of the copolymer with water and then feeding the concentrated solution to the aσueous system at some convenient point in the operation. Alternately, the subject copolymer can be added directly to the aqueous system.

The copolymer found useful in performing the subject process can be formed by conventional vinyl polymerization techniques. The monomers of 2-acrylamido-2-methylporpane sulfonic acid, methacrylic acid and acrylic acid (as appropriate) are each commercially available. The monomers are mixed in appropriate molar ratios to form the desired product and are polymerized using conventional redox or free radical initiators. Formation of low molecular weight copolymers may require the presence of chain terminators such as alcohols and the like in manners known in the art.

The subject copolymers have been found surprisingly effective in inhibiting scale formation especially with respect to inhibiting calcium phosphate deposition onto metal surfaces. Calcium ions are normally present in hard water such as obtained from natural sources as, for example, seawater, lakes, rivers, etc. Phosphates are normally present in aqueous systems either naturally or through treatment of the water for various purposes. The calcium and the phosphate form a precipitate which can be generally represented as the normal phosphate C «(PO ) although it has also been proposed to be a calcium phosphate known as hydroxyapatite which has the formula Ca (PC>₄) OH or 3Ca₃( 0₄) ^• Ca(OH) .

O P

^" A consideration of the solubility products of the various calcium salts show CaCO_ as 4.8 x 10 -9;

Ca₃(P0₄)₂ as 1.3 x lθ^"32 and Ca^PO^OH as 3 x lθ^"58 and thereby indicates that the basic calcium phosphate salts are a prime source of deposition. Inhibiting the deposition of calcium phosphate has been found to be readily accomplished using the specific copolymeric material described hereinabove.

The following examples are given for illustrative purposes only and are not meant to be a limitation on the description in the present application except as defined by the claims appended hereto. All parts and percentages are by weight unless otherwise indicated.

Example I

A copolymer of 2-acrylamido-2-methylpropane sulfonic acid and methacrylic acid (mole ratio 1:2) as its sodium salt and having a molecular weight of about 5,000 was evaluated as a calcium phosphate scale inhibitor in a dynamic test apparatus.

The apparatus comprises a vertical glass cylinder having an eight kilowatt cylindrical stainless steel cartridge heater inside. The cylinder was closed with a ballcock at the top and was equipped with temperature measurement probes at the inlet and outlet ports. An open plastic vessel having a five liter capacity was used as the open reservoir. This reservoir had an outlet tube connected to a centrifical pump which fed (via a rotometer) into the bottom inlet port of the cylinder.

-* The outlet port is connected to the reservoir to return the aqueous fluid thereto. The reservoir also contains a cooling coil to maintain the reservoir water at 123° F. and a water make-up actuated by a diaphragm pump. The apparatus was maintained at a circulation rate of one

OMPI gallon per minute with an inlet temperature of 123° F. , an outlet temperature of 130° F. and a pH of 8.2 + 0.1 using a. Precision pH probe.

The aqueous system which circulates through the dynamic test apparatus was a water solution containing 750 ppm potassium chloride, 374 ppm calcium sulfate hemihydrate, 136 ppm calcium chloride, 341 ppm sodium chloride, 518 ppm magnesium sulfate heptahydrate, 380 ppm calcium as calcium carbonate and 15 ppm phosphate as PO (phosphoric acid) to simulate a hard natural water source. Total hardness of the aqueous system (as Ca and Mg) was 591 ppm.

A concentrate of 1,000 ppm of the copolymer was formed and an aliquot portion was taken and mixed into the reservoir to provide a 20 ppm total dosage of copolymer in the aqueous system. The aqueous system being tested was circulated through the apparatus for a 22 hour period. The amount of calcium phosphate on the metal surface of the heater was determined by dissolving the scale formed with hydrochloric acid and conducting standard colorimetric analysis for phosphate. As a standard, a duplicate test was performed in which the aqueous system did not contain the copolymer agent. The analysis showed that 102.6 mg. of phosphate scale formed in the standard while only 43.0 mg. formed when the subject copolymer was present in the aqueous system. This shows a 58.1 % inhibition to calcium phosphate scale formation.

Example II A test was performed in the same manner as described in Example I above except that the copolymer used therein was replaced by a copolymer of

2-acrylamido-2-methylpropane sulfonic acid and methacrylic acid (mole ratio 1:1) as the sodium salt which contains approximately 1-2 percent of butyl acrylate and acrylonitrile.

The test results showed that only 35.5 mg. of phosphate scale formed during the test which was a 65.4 % inhibition to phosphate scale formation.

Example III

A test is performed in the same manner as described in Example I above except that the copolymer is in the form of the free acid. The results are substantially the same as attained in Example I with the copolymer salt.

While the invention has been described herein in connection with certain preferred embodiments, it is not intended to limit the invention to the particular form set forth, but, on the contrary, it is intended to cover such alternatives, modifications and equivalents as defined by the appended claims.

*s-

Claims

WHAT IS CLAIMED IS

1. A process for inhibiting scale formation on metal surfaces in contact with aqueous systems comprising incorporating into the aqueous system capable of forming scale on the metal surface an effective amount of a low molecular weight copolymer formed from the monomeric units of (A) 2-acrylamido-2-methylpropane sulfonic acid and of (B) a vinyl carboxylic acid selected from methacrylic acid or acrylic acid; said sulfonic acid and carboxylic acid groups contained in said copolymer are present in the form of the free acid or as a water soluble salt; and the mole ratio of (A) to (B) is 5:1 to 1:5.

2. The method of Claim 1 wherein the aqueous system is capable of depositing calcium phosphate salts on the metal surfaces and said copolymer is present in said^" aqueous system in from 0.1 to 100 ppm.

3. The method of Claim 2 wherein the low molecular weight copolymer has a weight average molecular weight of from 1,000 to 10,000 and is present in said aqueous system in from 1 to 25 ppm.

4. The method of Claim 2 wherein the copolymer has a weight average molecular weight of from 4,000 to 6,000 and contains monomeric units (A) and (B) in molar ratio of from 2:1 to 1:2.

5. The method of Claim 2 wherein the sulfonic acid and carboxylic acid groups of said copolymer are at least partially in the form of an alkali metal salt or ammonium salt.

6. The method of Claim 2 wherein the aqueous system passes over metal surfaces of an apparatus which causes a concentration of the dissolved salts therein.

7. The method of Claims 1, 2, 3, 4 or 5 wherein the copolymer contains up to about 5 mole percent of other monomeric units which are substantially inert with respect to the subject process.^'