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 defined as 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 may or may not form > 1.0 The solid is over-saturated is expected to form
Scaling Tendency and Pre-scaling Tendency in OLI Software
Scaling Tendency is the saturation ratio after all potential solids come to equilibrium with water. This is the true equilibrium condition (time=∞). In OLI Software the Scaling Tendencies are reported under the name of Post-Scaling.
Pre-scaling 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.
Scaling Tendency (Reported in the software as Post-Scale)
Scaling tendencies are essentially saturation ratios. When the scaling tendency for a species is 1.0, it indicates that its solid form is in equilibrium with water. A scaling tendency below 1.0 indicates sub-saturation and that the solid phase will not form.
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
Where 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 Ksp is the Solubility Product Constant which gives the thermodynamic limit of ion availability.
Let’s consider a 1.0 molal NaHCO3 solution. The IAP is calculated as follows:
IAP = [gammaNa+]*[mNa+]*[gammaHCO3-]*[mHCO3-]
Assuming Ideal Solution Activities:
gammaNa+ = 1.0
gammaHCO3- = 1.0
mNa+ = 1.0
mHCO3- = 1.0
IAP = (1.0)(1.0)(1.0)(1.0)
IAP = 1.0
Defining the Ksp
Ksp = [gammaNa+]*[mNa+]*[gammaHCO3-]*[mHCO3-]
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 (calculated by the OLI software) are:
gammaNa+ = 0.598
gammaHCO3- = 0.596
mNa+ = 0.894
mHCO3- = 0.866
This results in a different IAP:
The new Scaling Tendency is the following:
ST = IAP/Ksp
ST = 0.276/0.40378
ST = 0.683
Scale Index (Reported in the software as SI, Index)
Scale Index (SI) is the log value of the Scaling Tendency (ST):
SI = Log (ST)
There are three scenarios for the Scale Index (also referred to as the Saturation Index)
- SI = 0 IAP = Ksp → saturated (in equilibrium)
- SI < 0 IAP < Ksp → undersaturated
- SI > 0 IAP > Ksp → supersaturated
Pre-Scaling Tendency (Reported in the software as Pre-Scale)
OLI software defines a Pre-Scaling tendency as the scaling tendency before solids precipitate. The Pre-scaling tendency is calculated the same as the Post-scaling tendency:
ST = IAP/Ksp
Pre-Scaling Index (Reported in the software as SI, Index)
Pre-Scale Index (SI) is the log value of the Pre-Scaling Tendency (PST):
PSTI = Log (PST)
Note: Many industries, notably the up-stream oil & gas industry, use the pre-scaling tendency to make design decisions about adding anti-scaling and anti-fouling agents or if the asset is as risk.
Difference between Post-scale and Pre-scale
Pre-Scale: The saturation ratio before solids precipitate. Post-Scale: The saturation ratio AFTER solids precipitate (if solids are selected). S, ST – Saturation, Scale Tendency: The ratio of the concentration (activity) to its solubility (S=1). SI – Scale Index: Log(S).