Table 1 - Synthetic water conditions Parameter Utility A Utility B Utility C Calcium ( mg / l as Ca 2 + ) 91.2 83.2 170.0 Magnesium ( mg / l as Mg 2 + ) 42.9 42.2 NR Bicarbonate ( mg / l as
HCO 3
2-
)
between a polymer additive and soluble ions , or surface interaction between polymer and forming crystal lattices , occurs , preventing precipitation . Typically , phosphonates do not have stabilisation properties .
Particulate dispersion is a suspension of particulates in an aqueous solution . It involves a mixture of finely divided particles , called the internal phase ( often of colloidal size ), being distributed in a continuous medium , called the external phase . These can be inorganic ( e . g .
CaCO 3
), organic ( e . g . biomass ) or a mixture of the two .
While phosphonates do not have true particulate dispersion functionalities , polymers can be quite effective . Polymer composition and molecular weight are key determinants in deriving functionality for effective particulate dispersion . The final mechanism discussed here in relation to scale control is
301.5 86.9 255.0
Carbonate ( mg / l as CO 3
2-
) ( total as CaCO 3 ) 100.0 ( total as CaCO 3
)
Sulfate ( mg / l as SO 4
2-
) 0.0 166.9 NR Chloride ( mg / l as Cl - ) NR 147.2 NR Phosphate ( mg / l as PO 4
3-
) 0.0 1.0 0.0 Final Solution pH 9.0-9.2 8.8-8.9 9.2-9.3 Temperature ( ̊C ) 50 ̊C 71 ̊C 50 ̊C Calcite Saturation 94.4X 43.5X 140.3X Langelier Saturation Index 2.5 2.0 2.7
Calcite Momentary Excess ( mg / l )
Treatments
46.1 19.2 35.4
Blank 500ppb PBTC 500ppb PEPMA / HPSP
Blank 500ppb HEDP 500ppb PEPMA / HPSP
Blank 500ppb HEDP 500ppb PEPMA / HPSP
crystal habit modification . A crystal habit is the normal size and shape of a precipitated substance in a given set of environmental conditions . The formation of crystals such as CaCO 3 and their subsequent deposition onto surfaces follow a process of nucleation , lattice formation and propagation , bulk precipitation and surface deposition .
Modification of crystal habit involves introducing a ‘ poison ’ or contaminating additive that disrupts normal lattice formation . This , in turn , yields crystals tending either to re-dissolve or to precipitate in abnormal forms that deviate from the substance ’ s untreated crystal habit . This effect tends to reduce cohesion of the crystals to each other ( dispersion ) and adhesion of crystals to system surfaces ( scaling ).
In some ways , crystal modification is the basis for mineral scale control using phosphonates and polymers .
Crystal modification impacts threshold inhibition in that it disrupts the growth and propagation of the crystal lattice such that crystalloids tend to redissolve . This frees up the polymer or phosphonate molecules to interact with other forming crystal lattices , which supports the substoichiometric relationship .
Additionally , distorted crystals tend to be more easily stabilised ( colloidal ) and dispersed as particulates . Finally , the modified crystals themselves are much less likely to form permanent attachments to surfaces . The comparative modification was demonstrated in an earlier publication . 2 It and can be readily shown in a comparison of untreated calcite versus a sample precipitated in the presence of PEMPA ( Figure 1
).
In the case of once-through cooling applications , the functionalities of polymers and phosphonates described do not necessarily all come in to play due to the short retention time in the condenser , high water flow velocities and ultra-low additive dosage . For example , at ppb dosage levels and high velocities , the polymers would not be expected to provide much functionality for dispersion or stabilisation .
Evalution methodology & results
In once-through cooling applications , threshold inhibition is primary mechanism of both polymers and phosphonates . These additives are typically used at ultra-low dosages in the ppb range , due to the short retention time of the water through the condensers . Therefore , the quantification of induction time versus additive dosage is a valuable comparison tool .
Parameter
Induction time ( seconds )
Frequency change ( delta Hz )
Induction time ( seconds )
Frequency change ( delta Hz )
Induction time ( seconds )
Blank < 5 > 250 < 5 190 30 140 500 ppb PBTC < 5 100 < 5 50 < 10 20
500 ppb PEPMA / HPSP
> 60 0 10 20-25 120 15
Frequency Change ( delta Hz )
Table 2- Comparative QCM data for the untreated blank , 500 ppb phosphonate treatment & 500 ppb PEPMA / HPSP blend
24 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981