Michael L . Standish of Radical Polymers describes a newly patented alternative to chelating agents such as EDTA and HEDP
Use of polymers for deposit control in once-through utility systems
Michael L . Standish of Radical Polymers describes a newly patented alternative to chelating agents such as EDTA and HEDP
Mineral scale inhibitors have been used in once-through utility systems since at least the 1970s . In recent years , the industry has predominantly employed phosphonates such as hydroxyethylidene diphosphonic acid ( HEDP ) for calcium carbonate ( CaCO 3
) control , along with secondary levels of sulfonated copolymers to help manage transition metals such as iron and manganese .
Recently , supply chain issues , notably COVID , labour shortages , regulations and restrictions , transportation bottlenecks , port congestion and energy costs , have limited the availability of HEDP and raised costs to two to three times historic levels . Users have had to search for alternatives . This paper describes the viability of a combination of a PEPMA patented enhanced polymaleic acid ( PEMPA ) 1 and a high purity sulfonated polymer ( HPSP ) as an alternate for HEDP .
Function of polymers & phosphonates
In once-through and recirculating cooling applications , mineral scale control is mainly achieved with phosphonates , polymers or , more commonly , a combination . This is due to the increased functionality of polymers and phosphonates and the ability to apply these materials at sub-stoichiometric levels with exceptional results .
Traditional chelating agents such as ethylenediaminetetraacetic acid ( EDTA ) and other discrete or nonpolymeric small molecules require
Figure 1 - Untreated calcite ( left ) & calcite treated with PEPMA ( right )
dosages based on a stoichiometric relationship with the metal ions present in the treated water . As such , chelating agents are typically only employed in applications like boiler water treatment , where hardness levels are typically low and make-up water volume is minimised .
Polymers and phosphonates provide mineral scale control using the primary functionalities of threshold inhibition , sequestration , chelation , stabilisation , dispersion and crystal modification . Threshold inhibition involves extending the solubility of an otherwise insoluble salt beyond normal saturation limits by using an additive which functions at sub-stoichiometric levels to delay the onset of precipitation .
In a once-through system , untreated or uninhibited water passing through a condenser may take less than five seconds to begin precipitating mineral scale in a given set of operating conditions . Alternatively , effectively treated water containing a few hundred ppb of phosphonate or polymeric inhibitor may prevent the onset of precipitation by 30 seconds
or more . This difference is referred to as induction time .
In a once-through cooling application , the residence time of water passing through the condenser is typically less than ten seconds . Thus , treating a system with ppb levels of an effective polymer and / or phosphonate that extends the induction time to over ten seconds would mean the difference between scaling and not scaling the condenser . By comparison , in a recirculating system with a holding time of 24 hours or more , the level of polymer or phosphonate needed to inhibit formation of scale can range between 3 and 10 ppm on an active basis .
The extent and duration of threshold inhibition may be related to a variety of factors or conditions . These include : the driving forces for precipitation ( pH , temperature , concentrations of scale-forming ions , etc .); the particular efficacy of the threshold inhibitor ; other water impurities ( dissolved gas and / or suspended solids ); the rate of water concentration or evaporation ; and the frequency of additive dosage .
22 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981