Difference between revisions of "Scaling Tendencies"

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(Scaling Tendency and Prescaling Tendency in OLI Software)
(Scaling Tendency and Prescaling Tendency in OLI Software)
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Let’s consider a 1.0 molal NaHCO<sub>3</sub> solution:
Let’s consider a 1.0 molal NaHCO<sub>3</sub> solution:
IAP = [gamma.Na<sup>+</sup>]*[m.Na<sup>+</sup>]*[gamma.HCO<sub>3</sub><sup>-</sup>]*[m.HCO<sub>3</sub><sup>-</sup>]
IAP = [gamma<sub>Na<sup>+</sup></sub>]*[m.Na<sup>+</sup>]*[gamma.HCO<sub>3</sub><sup>-</sup>]*[m.HCO<sub>3</sub><sup>-</sup>]

Revision as of 19:29, 29 January 2019

What is Scaling?

Scaling is the deposition of a mineral salt on processing equipment. Scaling is a result of supersaturation of mineral ions in the process fluid. Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, leading to a precipitation of solids (usually crystals).

For example, changes in temperature can cause that calcium bicarbonate precipitates as calcium bicarbonate and releases CO2 into the solution, as is shown in the following reaction:

Ca(HCO3)2(aq) ---> CaCO3(s)+CO2(g)+H2O

The saturation level of a salt in water is a good indicator of the potential for scaling.

What is a Scaling Tendency?

Scaling Tendency is the ratio of the ratio of the concentration of the ions of the salt of interest divided by the equilibrium constant of the same salt (Ksp). For example for the following reaction

CaCO3(s) = Ca+2 + CO3-2

The scaling tendency (ST) is:

ST = [Ca+2][CO3-2]/Ksp

The three possible ST scenarios are the following:

    Scaling Tendency      Result
    < 1.0	           The solid is under-saturated and is not expected to form
    = 1.0	           The solid is saturated and is expected to form
    > 1.0	           The solid is over-saturated and may or may not form

Scaling Tendency and Prescaling Tendency in OLI Software

Scaling tendencies are essentially saturation ratios. When the scaling tendency for a species is 1.0, it indicates that this species is in equilibrium with water. In other words, the species is in the solid phase. A scaling tendency below 1.0 indicates sub-saturation and that the solid phase will not form.

Scaling Tendency is the saturation ratio after all potential solids come to equilibrium. This is the true equilibrium condition (time=∞).

Prescaling Tendency represents the condition before any solids are allowed to form. This is a non-equilibrium condition and can be viewed as the condition where time=0.

Below there is an example of how the software calculates the Scaling Tendency. Consider the dissolution of sodium bicarbonate:

NaHCO3(s) = Na+ + HCO3-

Scaling Tendency is defined as ST = IAP/KSP

IAP is the Ion Activity Product, and is defined as the product of specific ions (in this case the ions resulting from the dissociation of a particular solid); and the Solubility Product (Ksp) is the thermodynamic limit of ion availability.

Let’s consider a 1.0 molal NaHCO3 solution:

IAP = [gammaNa+]*[m.Na+]*[gamma.HCO3-]*[m.HCO3-]

Assuming Ideal Solution Activities

gamma.Na+ = 1.0

gamma.HCO3- = 1.0

m.Na+ = 1.0

m.HCO3- = 1.0

IAP = (1.0)(1.0)(1.0)(1.0) IAP = 1.0

Defining the Ksp

Ksp = gamma.Na*m.Na*gamma.HCO3*m.HCO3

KSP = 0.403780

The Scaling Tendency is then the ratio of available ions to the thermodynamic limit.

ST = IAP/KSP ST = 1.0/0.403780 ST = 2.48

Was assuming ideal conditions valid??

The actual species concentration and activity coefficients are:

gamma.Na = 0.598

gamma.HCO3 = 0.596

m.na = 0.894

m.HCO3 = 0.866

This results in a different IAP

IAP= (0.598)(0.894)(0.596)(0.866)


The new Scaling Tendency is therefore:

ST = IAP/Ksp

ST = 0.276/0.40378

ST = 0.683

Why are the concentrations not equal to 1.0? Speciation and chemical equilibria tend to form complexes which provide a “Sink” for carbonate species. In this example:

CO2o = 0.016 molal

NaHCO3o = 0.101 molal

CO32- = 0.012 molal

NaCO3- = 0.004 molal

What does the Scaling Tendency Mean?

If ST < 1, then the solid is under-saturated If ST > 1, then the solid is super-saturated If ST = 1, then the solid is at saturation Scaling Index = Log (ST)

The post-scaling tendency can go over 1.0 in special cases. If you have disabled a solid from forming (via the Menu Item: Chemistry: Model Options: Phases Tab) then it is possible to have a scaling tendency greater than 1.0. Also if the temperature is above a solid’s Temperature Range (TRANGE) then it can also be above 1.0. Please refer to the TRANGE support tip.

This was OLITips39.