The article below is strictly for internal use of the peoples / groups associated with
Maxwell Additives Pvt. Ltd. And it is meant for sharing knowledge only. It is a work of great
scientists and organizations working in the field for the conservation of water and nature.
It has no professional / commercial motto involved
Introduction
In virtually all processes where untreated water is heated, fouling of equipment surfaces is the single most serious problem encountered. Affected application areas include cooling, boiler, geothermal, power generation, and many other production processes. The problems associated with fouling include:
- Reduced heat transfer by formation of an insulated layer of mineral scales
- Corrosion
- Flow restrictions including blocked pipes, heat exchangers, and nozzles
For example, the fouling of reverse osmosis membranes adversely degrades product quality, reduces product quantity, increases energy consumption, increases membrane cleaning frequency and cost, and reduces membrane life. Over the years, environmental restrictions and water conservation measures have combined to make these problems increasingly challenging to solve. Accordingly, deposit control treatment has become an essential to the operation of industrial water systems.
Commonly encountered foulants in industrial water systems include corrosion products, particulate matter, microbiological mass, and sparingly soluble salts of alkaline earth metals. Various approaches have been developed to control these problems. However, the control of iron-based foulants (e.g., Fe2O3, Fe3O4, Fe(OH)3, FePO4) is generally considered to be one of the most challenging problems. This article focuses on the control of iron foulants, particularly iron oxide.
Deposit prevention and removal of iron foulants can be achieved by either of two methods as follows:
- Mechanical - Control of cycles of concentration, reduced recovery, side stream filtering; or
- Chemical - Use of corrosion inhibitors or dispersants
However, mechanical approaches to control iron-based foulants are typically not economical and are ineffective if not used in combination with chemical treatment. Treating industrial water systems involves using a wide variety of chemicals to prevent the build-up of deposits on equipment surfaces. Water treatment programs typically include the following components:
- Scale inhibition
- Scale control agents - Polyphosphates, phosphonates, poly(acrylic acid), poly(maleic acid)
- Corrosion inhibitors - Molybdate, orthophosphate, polyphosphate, phosphonate, benzotriazole, tolyltriazole;
- Dispersants - Acrylic - maleic- sulphonic acid-based polymers
- Biocides - Oxidizing and nonoxidizing chemicals
- Environmental acceptability
- Compatibility with biocide
- Hydrolytic stability
| Product | Composition |
|---|---|
| POLYMAN®-1000 | Homopolymer |
| POLYMAN®-2000 | Homopolymer |
| POLYMAN®-1240-A | Terpolymer |
| POLYMAN®-4905 | Copolymer |
| Poly-E | Competitive terpolymer |
| Poly-F | Poly(diallyl dimethyl ammonium chloride) |
| Poly-G | Sodium lignosulfonate |
| HEDP PHOSPHOMAN®-111C | 1-Hydroxyethylidine-1,1 diphosphonic acid |
| SHMP | Sodium hexametaphosphate |
| ATMP PHOSPHOMAN®-222C | Amino tris(methylene phosphonic acid) |
| PBTC PHOSPHOMAN®-333C | 2-phosphonobutane-1,2,4-tricarboxylic acid |
| Surfactant | Block polymer of ethylene oxide and propylene oxide |
Historically, many deposit control chemicals were natural products (e.g., modified corn starches, tannins, lignins, alginates). However, natural dispersants are not used today because they provide marginal performance, temperature stability problems, and nutrients for biological growth (especially in the case of starch).
Since the development of synthetic polymers in the 1950s, essentially all effective water treatment formulations have incorporated deposit control polymers (DCPs) to improve system efficiency and reduce operating costs. The DCPs in these formulations are used as scale control agents and/or dispersants. DCPs inhibit the precipitation of scale forming salts such as calcium carbonate, calcium sulfate, and calcium phosphate. DCPs are also invaluable in industrial water systems for dispersing or suspending a wide variety of solids that would otherwise settle out to form scale or deposits.
Environmental concerns and rising operational costs are among the driving forces for industries to investigate continuously new approaches to conserve (use and reuse) water. Water conservation efforts have led operators to minimize cooling tower blowdown resulting in increases in the cycles of concentration. This increases the concentration of dissolved ions (e.g., calcium, magnesium, iron, sulfate, suspended matter), thus increasing fouling potential.
The selection of DCP(s) for a particular application can be a very challenging and time-consuming process. Water treatment formulators must consider a myriad of factors and have a great variety of DCPs available. DCPs are characterized in many ways including composition, molecular weight, ionic charge, charge density, and product form (liquid or solid). Criteria for selecting DCPs should include:
- Dispersion of suspended matter (e.g., iron oxide, clay, and calcium phosphate)
- Retention of activity in the presence of high calcium and magnesium hardness
- Retention of activity in the presence of iron(III)
- Tolerance to cationic flocculant
- Scale inhibition
- Compatibility with biocide
- Hydrolytic stability
- Environmental acceptability
It has been shown that the performance of water treatment formulations is affected by various factors including pH, temperature, suspended solids, and flocculating agents. This article furthers the investigation of the role of process water variables on the performance of DCPs. The goal of this paper is to expand and improve the criteria used by water treatment technologists to select dispersants that ensure optimum system performance. The study presented herein addresses the effect of system variable including dispersant dosage, water hardness, pH, total dissolved solids, and flocculating agents on the performance of commercially available dispersants. Table 1 lists the dispersants evaluated in this study. The dispersancy power of the polymers was evaluated on the bases of generally accepted laboratory test methods (Bottle tests).
Effect of Dispersant Dosage
Dispersancy as a function of active product dosage was evaluated using standard test conditions. Conducting Iron Oxide Dispersion as Function of Product Dosage: it is observed that low polymer dosages have a marked effect.
- An increase in dispersant concentration results in increased dispersancy.
- POLYMAN®-4905 (co-polymer) exhibits better performance than Poly-E (competitive terpolymer) and superior performance compared to POLYMAN®-1000 (Homopolymer) and POLYMAN®-2000 (Homopolymer).
- PBTC (a well-known calcium carbonate inhibitor) shows the poorest performance in terms of dispersing iron oxide in aqueous solution.
- The presence of additional carboxyl groups in a homopolymer (i.e., POLYMAN®-2000 vs. POLYMAN®-1000) does not significantly improve the dispersancy power of the dispersant.
Although PBTC and POLYMAN®-4905 are both strong acids, the performance of the POLYMAN-4905 is far superior to PBTC. This suggests that the ionic charge, size, and adsorption power of the functional group play important roles in imparting more negative charge on iron oxide particles. It is well known that the higher the negative charge imparted on the particles, the stronger the repulsive forces between the charged particles, hence better dispersancy.
The dispersancy data also presents some interesting findings like increasing the dispersant concentration from 0.25 to 0.50 mg/L results in approximately 50% DISPERSANCY increase for the POLYMAN®-4905 compared to approximately 35% increases in the case of POLYMAN®-1000 and POLYMAN®-2000. The findings further revealed that increasing the dispersant concentration from 0.5 to 1.0 mg/L only shows marginal improvement. Therefore, a thorough evaluation is necessary in selecting a dispersant for developing a formulated product to achieve optimum performance. From a practical point of view, the ability of a dispersant such as POLYMAN®-4905 to provide superior performance at low dosage is a desirable characteristic.
Effect of Divalent Cations
Calcium and Magnesium Ions
Water hardness affects iron oxide dispersancy by decreasing the amount of negative charge from the polymer on the agglomerating particles, hence decreasing charge repulsion. Additionally, the hardness ions shield the initial charge on the particles, allowing increased agglomeration. The influence of water hardness (calcium and magnesium) on the performance of two dispersants namely POLYMAN®-1000 (Homopolymer) and POLYMAN®-1240A (Terpolymer) was evaluated by conducting a series of dispersancy experiments.
Iron oxide dispersancy data for POLYMAN®-1000 (Homopolymer) and POLYMAN®-1240A (Terpolymer) in the presence of varying concentrations of calcium ions clearly shows that calcium ions have a marked influence on the performance of dispersants. In zero hardness water (i.e., distilled water) both POLYMAN®-1000 (Homopolymer) and POLYMAN®-1240A (Terpolymer) show excellent dispersancy activity (i.e., >95%). However, increasing the calcium ion concentration to 50 mg/L results in a dispersancy reductions (approximately 50% and 5% decreases, respectively) for both POLYMAN®-1000 (Homopolymer) and POLYMAN®-1240A (Terpolymer). These dispersancy data indicate that POLYMAN®-1240A (Terpolymer) retains its dispersancy activity far better than POLYMAN®-1000 (Homopolymer) in the presence of high calcium hardness (especially at 400 mg/L).
The dispersancy of iron oxide by various above cited polymers can, thus, be illustrated as: POLYMAN®-1240A >POLYMAN®-4905 = Poly-E >POLYMAN®-2000 =POLYMAN®-1000
POLYMAN®-4905(copolymer) displays better performance than either homopolymers (i.e., POLYMAN®-1000, POLYMAN®-2000). The performance increase observed for POLYMAN®-4905 vs. POLYMAN®-1000 may be attributed to the presence of additional strong acidic group.
POLYMAN®-1240A (terpolymer) is the best polymer in terms of dispersant effectiveness. The competitive terpolymer (Poly-E) under these stressed high hardness experimental conditions shows significantly less iron oxide dispersancy. Overall, the dispersant performance trend observed is terpolymer > copolymer > homopolymer and this is also true for inhibiting the precipitation of calcium phosphate and calcium phosphonates.
Iron (II), Manganese, and Zinc Ions
The influence of divalent cations (e.g., Fe(II), Mn(II), Zn(II)) on the performance of various dispersants was also tested. The presence of Fe(II) and Mn(II) at low concentrations does not significantly affect the dispersancy power of the polymers. However, zinc ions appear to have an antagonistic effect on the performance of dispersants. The observed decrease in dispersancy activity may be due to the precipitation of zinc hydroxide thus providing additional surface area for the adsorption of dispersant molecules. The adsorption of dispersant on freshly precipitated zinc hydroxide decreases the effective concentration of dispersant in solution thereby resulting in overall poor performance.
Influence of Trivalent Cation
We tested two terpolymers (POLYMAN®-1240A and Poly-E) for dispersancy performance in the presence of iron (III) which clearly shows that approximately 16% and 35% additional amounts of Carboxylic-Sulfonate Ter-Polymer POLYMAN®-1240A and Poly-E, respectively, are needed to achieve performance similar to that obtained in the absence of iron (III). In cooling water applications where low concentrations of iron (III) are encountered, the incorporation of a DCP in the formulation that exhibits more tolerance to iron (III) and other metal ions (e.g., Ca, Mg, Zn) is beneficial.
It is interesting to note that sodium lignosulfonate (Poly-G), which is a natural dispersant containing sulfonate groups, shows better dispersancy power in the absence of iron (III) than either homopolymer (POLYMAN®-1000 or Maleic acid homopolymer POLYMAN®-2000). However, the performance of Poly-G in the presence of 1 mg/L iron (III) is markedly reduced.
It also shows that approximately 400 times more divalent cations (i.e., Ca, Mg) are required to obtain a similar decrease in dispersant performance as is obtained from 1 mg/L of a trivalent metal ion. The observed change in dispersant performance caused by divalent and trivalent cations may be attributed to the difference in charge density between calcium and iron (III) ions.
Effect of Solution pH
Based on the studies, it is observed that between pH 5 to 9 the performance of the phosphonate increases with increasing pH of the solution. This fact is true for polymeric inhibitors (polymers / dispersants) also.
Because the evaluated polymers contain acidic groups, it is expected that the ionization of these groups will increase with the increasing pH, thus increasing the potential for adsorption of polymers onto iron oxide particles and improving dispersancy.
Effect of Cationic Polymer
Waters containing suspended matter are typically treated with coagulating or flocculating agents like DADMAC (that is cationic) before entering the cooling water system. DADMAC (Poly-F) has been known to "carryover" and could potentially interfere with the performance of anionic polymers used in treating industrial water systems.
The presence of as low as 0.1 mg/L concentrations of Poly-F has a marked antagonistic effect on the performance of various dispersants.
Effect of Polyphosphates, Phosphonates, and Surfactants
Polyphosphates and phosphonates are a family of compounds widely used in industrial water treatment programs to control mild steel corrosion and calcium carbonate scale formation.
The experiments shows that phosphorous-containing compounds (Polyphosphates and phosphonates) that are effective in preventing the precipitation of calcium carbonate, barium sulfate, calcium sulfate, etc., exhibit poor iron oxide dispersancy power compared to POLYMAN®-1240A (terpolymer). It is also observed that the surfactant does not show any significant activity in terms of dispersing iron oxide.
Miscellaneous
Many water treatment programs frequently incorporate chlorination for controlling microbiological fouling. Our testing on POLYMAN®-1240A and Carboxylic AMPS Co-polymer POLYMAN®-4905 show that under simulated field conditions these polymers do not lose any activity in the presence of chlorine. In addition, testing of both POLYMAN®-1240A and POLYMAN®-4905 after storage for 2 years at room temperature shows no significant decrease in both the performance for iron oxide dispersancy and calcium phosphate inhibition.
Conclusions
In summary, it has been shown that iron oxide dispersant performance can be adversely impacted by a variety of conditions commonly encountered in industrial water systems. The results presented in this study suggest that the following factors be considered when selecting a dispersant for developing a water treatment program to ensure optimum performance.
- Polymers are the most effective class of chemicals for dispersing particulate iron oxide.Polyphosphates and phosphonates exhibit poor dispersancy power.
- Polymer dosage is critical to dispersant performance. The ability of a dispersant to perform at low dosages is an important selection criterion.
- Polymer performance as a dispersant strongly depends upon ionic charge, monomer type, functional group, and molecular weight. Based on the dispersancy data, the ranking of the dispersant is: Terpolymer > Copolymer > Homopolymer
- Cation charge and concentration are the two most important water quality factors that impact the performance of a dispersant.
- Cationic polymer exhibits adverse influence on dispersant performance.
References:
The paper is prepared referring many international papers / journals followed by extensive performance tests / trials at our in-house laboratory.
Please, always make a practice to prepare a small batch at lab level and check the same as required before going to bulk production.