ABSTRACT

Organic phosphonates are a family of compounds widely used as mineral scale and corrosion inhibitors in a variety of applications in industrial processes. The phenomena of scale and corrosion inhibition has been studied extensively. This study reports the tolerance of phosphonates with hardness ions under a variety of test conditions (i.e., temperature, type and concentration of hardness ions, total dissolved solids, etc.). The phosphonates tested include: hydroxyphosphono acetic acid, HPA; aminotris(methylene phosphonic acid), AMP; 1-hydroxyethylidine 1,1-diphosphonic acid, HEDP; 2-phosphono butane 1,2,4-tricarboxylic acid, PBTC; and polyamino polyether methylene phosphonic acid, PAPEMP. It has been found that tolerance of phosphonates with hardness ions strongly depends on phosphonates architecture. Results of this study also show that phosphonate tolerance to hardness ions can be extended by the addition of additives (polymeric and/or non-polymeric) to the aqueous system. Three types of polymers tested include: synthetic (homo-, co-, and terpolymers), biopolymers, and hybrid polymers. It has been found that terpolymer containing hydrophobic and bulkier groups perform better than the homo-, co-, and hybrid polymers.

INTRODUCTION

In many industrial systems including cooling, boiler, desalination, geothermal, oil and gas, etc., scale formation presents significant operational challenges. Problems are prominent when it deposits on equipment surfaces and causes various operational challenges including reduced heat transfer, and premature equipment replacement.1 Inorganic scales commonly encountered in industrial systems include carbonates, sulfates, and phosphates of alkaline earth metals. One of the main difficulties in anticipating scale formation is that various factors including water composition, pH, temperature hydrodynamics of the flow, dissolved and suspended impurities, heat exchanger metallurgy and surface roughness, presence of gas bubbles, etc., influence scale formation in the bulk solution and on equipment surface.

Phosphorous containing compounds such as polyphosphates and phosphonates are widely used in a variety of industrial applications such as water treatment, electroplating, paper and pulp slurries, scale removal, crude oil production and pigment dispersion. Phosphonates differ structurally from polyphosphates in that they have a P-C bond rather than a P-O bond. The structural differences account for their superior stability under stressed operating conditions i.e., high alkaline pH, high temperature. Phosphonates and polyphosphates prevent scale formation at “substoichiometric” dosages by adsorbing onto crystal growth sites of micro-crystallites thereby interfering with crystal growth and altering the crystal growth morphology. In addition, both phosphonates and polyphosphates have been shown to exhibit metal chelation and dispersancy activities. Phosphonates are also effective in controlling mild steel corrosion. As corrosion inhibitors, phosphonates may generally be described as cathodic inhibitors. They function by reacting with calcium and polyvalent metal ions, particularly those of the corrosion products. The phosphonate-cation products form a protective nonconductive layer on the metal surfaces. This barrier separates the metal from the bulk water and prevents diffusion of oxygen to the metal surface, thereby preventing corrosion. A drawback of certain phosphonates such as 1-hydroxyethylidene-1,1-diphosphonic acid, HEDP, and aminotris(methylene phosphonic acid), Amp, as well as other amino phosphonates is their sensitivity to oxidizing microbiocides, such as chlorine or bromine-based biocides necessary to control microbiological growth.2 Orthophosphate (PO4)3-, one of the degradation products, can cause calcium phosphate scaling in high hardness process waters.3

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