Phosphorus-free Scale Inhibitors：Polyaspartate Sodium (PASP) and PESA (Polyepoxysuccinic Acid sodium)

Introduction: This article focuses on organic polyphosphate scale inhibitors and water-soluble polymer scale inhibitors and dispersants, and briefly introduces several other types of scale inhibitors. The problems existing in phosphorus-based formulations are pointed out, and the development prospects of scale inhibitors are prospected. It is believed that scale inhibitors in the future will develop in a green and environmentally friendly direction.

The types and properties of commonly used scale inhibitors in industrial circulating cooling water are reviewed, focusing on organic polyphosphate scale inhibitors and water-soluble polymer scale inhibitors and dispersants, and briefly introducing several other types of scale inhibitors. The problems existing in phosphorus-based formulations are pointed out, and the development prospects of scale inhibitors are prospected. It is believed that scale inhibitors in the future will develop in a green and environmentally friendly direction.

When cooling water is used, it is continuously circulated and concentrated, and the mineral content in the water will continue to increase, causing scaling and corrosion of equipment pipes. The presence of scale will not only affect the heat transfer efficiency, but also cause under-scale corrosion. Therefore, scale prevention in circulating water is very important. At present, in the process of industrial circulating cooling water treatment, most of the methods of adding chemical agents are used to control the formation of scale. Chemical agents that can prevent the formation of scale and dirt or inhibit their deposition and growth are generally referred to as scale inhibitors. The commonly used forms of scale inhibitors in industry are mainly scale inhibitors and scale dispersants. Scale inhibitors and corrosion inhibitors are mainly of the following types: inorganic polyphosphates, organic polyphosphates, gluconic acid and tannic acid. At present, phosphorus-based formulas are mostly used in circulating water systems, among which organic polyphosphates are the most commonly used. Scale inhibitors and dispersants are mainly water-soluble polymers with medium and low relative molecular weights, including homopolymers and copolymers. Among them, homopolymers include polyacrylic acid and its sodium salt, hydrolyzed polymaleic anhydride, etc. There are many varieties of copolymers, mainly di- or terpolymers of acrylic acid and maleic acid.

 

1. Organic polyphosphate scale inhibitors and corrosion inhibitors

Organic phosphoric acid is a type of cathodic corrosion inhibitor. They are also a type of non-chemical equivalent scale inhibitor with obvious solubility limit effect (Threshold Effect). When they are used in combination with other water treatment agents, they can show an ideal synergistic effect. They have excellent chelating ability for many metal ions (such as calcium, magnesium, copper, zinc, etc.), and even have good deactivation effect on inorganic salts of these metals such as calcium sulfate, calcium carbonate, magnesium silicate, etc., so they are widely used in water treatment. At present, its varieties are still developing, so it is a relatively advanced and promising agent. However, the presence of phosphorus will have a certain impact on the environment.

1.1 Methylene phosphonic acid type

Methylene phosphonic acid type compounds are one of the earlier agents used in water treatment agents. Because it can form double five-membered ring chelates with metal ions such as Ca2+, Mg2+, Zn2+, Fe2+ in water, it has a good effect on calcium carbonate scale inhibition and corrosion inhibition performance for carbon steel. Common ones include aminotrimethylenephosphonic acid (ATMP), ethylenediaminetetramethylenephosphonic acid (EDTMP), diethylenetriaminepentamethylenephosphonic acid (DETPMP), hexamethylenediaminetetramethylenephosphonic acid (HDTMP), glycine dimethylenephosphonic acid (GDMP), and methylamine dimethylenephosphonic acid (MADMP).

1.2 Isocarbon diphosphonic acid type

Common isocarbon diphosphonic acid type compounds include 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and 1-aminoethylidene-1,1-12 diphosphonic acid (AEDP). Among them, HEDP is one of the agents developed earlier and used in water treatment. It can form a six-membered ring chelate with metal ions, and its antioxidant property is better than that of methylenephosphonic acid type compounds. In addition to calcium carbonate, it also has a good inhibitory effect on scale such as hydrated iron oxide and calcium phosphate. Hydroxypropylene diphosphonic acid (HPDP) developed by Bates Corporation of the United States is an excellent corrosion and scale inhibitor reported in foreign countries in the 1980s. Although it only has one more methylene group than HEDP in its molecular structure, its ability to control Ca2+ ions far exceeds that of HEDP, and it can effectively prevent the formation of calcium carbonate, calcium phosphate and calcium sulfate scale in cooling water, which is beneficial to increasing the concentration multiple of cooling water. Ai Shiyun et al. conducted a systematic study on HPDP, and also proved that HPDP is an effective water quality stabilizer with scale and corrosion inhibition performance that has a promising development and application.

1.3 Carboxylic acid phosphonic acid type

In some special occasions, such as high hardness water, high pH water, high heat exchange surface temperature and other harsh working conditions, or in cooling water media operating at high concentration multiples, the above-mentioned organic phosphoric acids are difficult to meet the process requirements. Domestic and foreign water treatment companies have successively developed more efficient carboxylic acid phosphonic acid corrosion and scale inhibitors, and their excellent corrosion and scale inhibition performance is suitable for operation under various harsh process conditions. The most representative products are 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA) and sodium 1,1'-diphosphonopropionylphosphonate (BPBP). PBTCA was first developed by H. Geffers et al. in Germany in the early 1970s and developed and applied in the late 1980s. There are three carboxyl groups and one phosphono group in the molecule, which has a strong chelating ability for metal ions and shows strong corrosion and scale inhibition capabilities. In recent years, East China University of Chemical Technology and Nanjing Institute of Chemical Technology have conducted systematic research on its synthesis and application. Experiments have shown that PBTCA is resistant to acid, alkali and oxidants, and does not hydrolyze at pH>14. It has good thermal stability. Under high hardness, high alkali and high temperature conditions, its scale inhibition performance is better than HEDP and ATMP, and it is an excellent cathodic corrosion inhibitor. Especially when used in high doses, it is a highly efficient corrosion inhibitor.

1.4 Macromolecular organic phosphoric acid

In order to increase the calcium ion tolerance of organic phosphoric acid scale inhibitors and improve their dispersing and stabilizing effects on oxides such as iron, zinc, and manganese and silicates, researchers have found that when organic phosphoric acid molecules exist in an appropriate structural form, as the relative molecular mass increases, the agent has obvious improvements in scale inhibition, corrosion inhibition, and compounding and synergistic performance. In the 1990s, macromolecular organic phosphoric acids with relative molecular masses in the range of hundreds were successfully developed. Typical products include polyamino polyether methylene phosphonic acid (PAPEMP), phosphonocarboxylic acid cotelomers (POCA) with relative molecular masses above 2000, and phosphono polyacrylic acid (PPCA). Since the relative molecular mass of such copolymers is relatively large, they not only maintain the scale inhibition and dispersibility of carboxylic acid polymers, but also improve their corrosion inhibition performance. They have multiple functions such as scale inhibition, dispersion, corrosion inhibition, and compounding and synergistic enhancement, becoming a truly multifunctional scale and corrosion inhibitor. He Huanjie once reviewed the research status and progress of macromolecular organic phosphoric acid.

2. Water-soluble polymer scale inhibitor dispersant

2.1 Water-soluble homopolymer

Water treatment agents with excellent scale inhibition performance, phosphorus-free, nitrogen-free and biodegradable have always been one of the directions of interest for researchers. At present, the homopolymers reported for industrial application mainly include polyepoxysuccinic acid (PESA) and polyaspartic acid (PASP). Polyepoxysuccinic acid (PESA) is a "green" water treatment agent variety that was successfully developed earlier. Betz Laboratory developed a phosphorus-free, nitrogen-free green scale inhibitor, polyepoxysuccinic acid (PESA), in the early 1990s. It has low toxicity, good biodegradability and is easily accepted by the environment. Xiong Rongchun also successfully prepared PESA. As a scale inhibitor, PESA is mainly used for cooling water treatment and is suitable for high alkali, high hardness and high temperature conditions. It has good compatibility with chlorine, and its scale inhibition performance is not affected by chlorine concentration. PESA has a good synergistic effect when used in combination with sodium phosphate, PBTCA and other agents. Under high hardness and high alkalinity water conditions, PESA has a better scale inhibition effect on calcium carbonate than ATMP and HEDP. Experiments show that PESA with a relative molecular mass of 400-800 has the best scale inhibition performance. Donlar produced and used polyaspartic acid (PASP) in the early 1990s, for which it won the 1996 "US Presidential Green Chemistry Challenge Award". Experiments have shown that polyaspartic acid has good corrosion and scale inhibition properties, can inhibit the formation of CaCO3, CaSO4 and BaSO4 scale, is resistant to high temperatures, has good thermal stability, and can be used in high temperature, high calcium water systems and reverse osmosis membrane treatment systems. Therefore, the research and development of this type of water treatment agent has become a hot topic abroad. Many companies in Germany, the United States, the United Kingdom, Japan, Russia, France, Poland and other countries have actively researched and developed PASP. Some studies have reported that PASP has the best calcium carbonate scale inhibition performance when the relative molecular mass is 2000-5000 and the mass concentration is 3-5 mg/L; the best calcium sulfate scale inhibition performance when the relative molecular mass is 1000-4000 and the mass concentration is 2-3 mg/L; the best barium sulfate scale inhibition performance when the relative molecular mass is 3000-4000 and the mass concentration is 4-5 mg/L.

2.2 Water-soluble copolymers

In recent years, copolymer scale inhibitors have been a hot topic in the research of circulating cooling water treatment agents. Since most copolymer structures contain not only carboxyl groups but also hydrophilic groups such as hydroxyl groups, ester groups, sulfonic acid groups or phosphonyl groups, the scale inhibition and dispersibility of copolymers are far superior to water-soluble homopolymers. Their characteristics are that they can not only inhibit calcium carbonate scale, but also have a good inhibitory and dispersing effect on calcium phosphate scale, calcium phosphonate scale, iron oxide, slime, etc. From early homopolymers, to binary copolymers, ternary copolymers, the variety of copolymer scale inhibitors and dispersants is increasing. Binary copolymers include maleic acid/acrylic acid copolymer (AA-MA), acrylic acid/hydroxypropyl acrylate copolymer (AA-HPA), acrylic acid/2-acrylamide-2-methylpropanesulfonic acid copolymer (AA-AMPS), maleic anhydride/styrenesulfonic acid copolymer, acrylic acid/organic sulfonic acid copolymer, etc. Terpolymers include maleic acid/acrylic acid/2-acrylamide-2-methylpropanesulfonic acid terpolymers, maleic acid/acrylic acid/acrylamide copolymers, acrylic acid/acrylamide/alkoxy polyethylene acrylate, etc.

3. Several other scale inhibitors

Oxidized polysaccharides were obtained by reacting for 90 minutes under the conditions of n(NaClO):n(C6H10O5)=3:4, 60°C, and pH 8.5. The relative molecular mass was 500-1200, the carboxyl mass fraction was 25%-42%, and it had good calcium complexing performance. The scale inhibition rate was as high as 97%. It can be used as a scale inhibitor for boiler water and circulating cooling water. It is a safe and non-toxic water treatment agent. Xu Liying and others used natural polymer starch as raw material to carry out chemical reactions to reduce the relative molecular mass, and introduced active groups such as carboxylic acid to obtain water-soluble oxidized starch (OS) that is not easy to rot. Experiments show that oxidized starch has good scale inhibition performance in the range of Ca2+ mass concentration of 50-300 mg/L, and the scale inhibition rate is 100%. Under the conditions of high temperature, high calcium, and high pH, ​​the compound use of oxidized starch and hydrolyzed polymaleic anhydride can achieve a good scale inhibition effect. In industrial circulating cooling water systems, oxidized starch compound agents are used. They are non-toxic and have no eutrophication problems. They are easily biodegradable when discharged into the environment, which can reduce environmental pollution. Therefore, this type of product has broad application prospects and good social benefits. Liu Guohua et al. first compounded tungstate with polyaspartic acid to screen out phosphorus-free tungsten-based water treatment agents. The resulting composite formula does not contain phosphorus and heavy metal ions, which meets the requirements of environmental friendliness. Experiments show that its scale inhibition rate is 91.67% and its corrosion inhibition rate is 98.71%.

4. Conclusion

Since phosphorus discharge will cause eutrophication of surrounding waters and promote the growth of bacteria and algae to form "red tides", developed countries in Europe and the United States have proposed phosphorus ban and phosphorus restriction measures, such as Germany's requirement that phosphorus discharge be ≤1mg/L. China's comprehensive sewage discharge standards also limit phosphorus discharge. From the long-term development trend, the production and application of phosphorus-based water treatment corrosion inhibitors will inevitably be restricted. In the future, scale inhibitors will develop in a green and environmentally friendly direction. We will strengthen the research on high-efficiency scale inhibitors that are phosphorus-free or low-phosphorus, especially the development of high-efficiency scale inhibitors that can still perform under harsh conditions. The development of products with dual functions of scale inhibition and corrosion inhibition has significant economic and social benefits.

Shandong Yuanlian Chemical Co.,ltd. is a leading manufacturer of Phosphorus-free Scale Inhibitors：polyaspartate sodium (PASP) and PESA (Polyepoxysuccinic Acid sodium),  any needs for MSDS, TDS, COA of MGDA, feel free to contact: [email protected]; Tel: +86 537 3739818