Difference between revisions of "Scaling Tendencies"

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==What is Scaling?==
  
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===What is Scaling?===
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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).
  
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Scaling is the deposition of a mineral salt on processing equipment. Scaling is a result
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To understand Scaling, the Scaling Tendency and Scaling Index definitions are used. These definitions depended on the Solubility Product Constant, K<sub>sp</sub> and Ion Activity Product, IAP definitions.
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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).
 
  
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For example, changes in temperature can cause that calcium bicarbonate precipitates as calcium bicarbonate and releases CO<sub>2</sub> into the solution, as is shown in the following reaction:
 
  
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Ca(HCO<sub>3</sub>)<sub>2(aq)</sub> ---> CaCO<sub>3(s)</sub>+CO<sub>2(g)</sub>+H<sub>2</sub>O
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'''Solubility Product Constant, K<sub>sp</sub>'''
  
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The saturation level of a salt in water is a good indicator of the potential for scaling.
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The solubility of ionic compounds of salts and minerals in water are governed by a solubility equilibrium expression and a solubility product constant known as K<sub>sp</sub>. It is important to note that the solubility product, K<sub>sp</sub> is a function of both temperature and pressure. Consider the general dissolution reaction below (in aqueous solutions):
  
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===What is a Scaling Tendency?===
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aA<sub>(s)</sub>⇌bB<sub>(aq)</sub>+dD<sub>(aq)</sub>
  
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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 (K<sub>sp</sub>). For example for the following reaction
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With equilibrium constant K<sub>sp</sub> defined as:
  
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CaCO<sub>3(s)</sub> = Ca<sup>+2</sup> + CO<sub>3</sub><sup>-2</sup>
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K<sub>sp</sub>=(''a''<sub>B</sub>)<sup>b</sup>∙(''a''<sub>D</sub>)<sup>d</sup>
  
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The scaling tendency (ST) is:
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Where, ''a''<sub>B</sub> and ''a''<sub>D</sub> are the activities of the aqueous species. The activity of any species i is defined as the product of its concentration in molality by its corresponding activity coefficient:  
  
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ST = [Ca<sup>+2</sup>][CO<sub>3</sub><sup>-2</sup>]/K<sub>sp</sub>
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''a''<sub>i</sub>=m<sub>i</sub> γ<sub>i</sub>
  
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The three possible ST scenarios are the following:
 
  
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    Scaling Tendency      Result
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'''Ion Activity Product, IAP'''
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    < 1.0           The solid is under-saturated and is not expected to form
 
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    = 1.0           The solid is saturated and is expected to form
 
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    > 1.0           The solid is over-saturated and may or may not form
 
  
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A real solution may not be in the state of equilibrium. This non-equilibrium state is described by the ion activity product (IAP). It has the same form as the equilibrium constant K<sub>sp</sub>, but involves the actual activities of the species in solution.
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IAP=(''a''<sub>B</sub>)<sup>b</sup><sub>actual</sub>∙(''a''<sub>D</sub>)<sup>d</sup><sub>actual</sub>
  
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===Scaling Tendency and Prescaling Tendency in OLI Software===
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==Scaling Tendency and Scaling Index==
  
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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.
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The Scaling Tendency (ST) is defined as the ratio of the Ion Activity Product (IAP) divided by the equilibrium constant (K<sub>sp</sub>).  
  
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'''Scaling Tendency''' is the saturation ratio after all potential solids come to equilibrium. This is the true equilibrium condition (time=∞).
 
  
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'''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.
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'''Equation (1)''' ST=IAP/K<sub>sp</sub>
  
  
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Scaling tendencies are essentially saturation ratios. Thus, if
  
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*ST < 1 Indicates sub-saturation, and the solid is not expected to form
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*ST = 1 Indicates saturation, and the solid is in equilibrium with water
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*ST > 1 Indicates supersaturation, and solids will form
  
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For Example, Consider this dissolution
 
  
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NaHCO3(s) = Na+ + HCO3-
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'''Note:''' The Scaling Tendency (ST) is reported in the software as Post-Scale.
  
  
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The Ion Activity Product (IAP) is defined as the product of specific  ions (in this case the ions resulting from the dissociation of a particular solid).
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The Scale Index (SI) (aka: Saturation Index in the literature), is given by the following relationship:
  
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Let’s consider a 1.0 molal NaHCO3 solution:
 
  
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IAP = gamma.Na*m.Na*gamma.HCO3*m.HCO3
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'''Equation (2)'''    SI =log<sub>10</sub>(IAP/K<sub>sp</sub>)
  
  
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[[File:Tip39 1.png|500px]]
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Thus, if
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*SI < 0 Indicates sub-saturation, and the solid is not expected to form
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*SI = 0 Indicates saturation, and the solid is in equilibrium with water
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*SI > 0 Indicates supersaturation, and solids will form
  
  
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Assuming Ideal Solution Activities
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'''Note:''' The Scaling Index (SI) is reported in the software as SI, Index
  
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gamma.Na = 1.0
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==Pre-Scaling Tendency and Scaling Index==
  
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gamma.HCO3 = 1.0
 
  
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m.na = 1.0
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Pre-Scaling tendency is defined as the scaling tendency before any solids are formed (this can be seen as all the species suspended in solution). The same formulas for ST and SI are applied (Equations 1 and 2).
  
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m.HCO3 = 1.0
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The Pre-Scaling tendency is reported in the software as Pre-Scale.
  
  
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IAP = (1.0)(1.0)(1.0)(1.0)
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'''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.
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IAP = 1.0
 
  
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The Solubility Product (KSP) is the thermodynamic limit of ion availability
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==Difference between Post-scale and Pre-scale==
  
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Ksp = gamma.Na*m.Na*gamma.HCO3*m.HCO3
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*Pre-Scale: The saturation ratio before solids precipitate.  
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*Post-Scale: The saturation ratio AFTER solids precipitate (if solids are selected).  
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*S, ST – Saturation, Scale Tendency: The ratio of the concentration (activity) to its solubility (S=1).  
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*SI – Scale Index: Log(ST).  
  
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KSP = 0.403780
 
  
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'''Scaling Tendency''' is the saturation ratio after all potential solids come to equilibrium with water. This is the true equilibrium condition (time=∞). I
  
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'''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.
  
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The Scaling Tendency is then the ratio of available ions to the thermodynamic limit.
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==Calculating Scaling Tendency: An Example==
  
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ST = IAP/KSP
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Below there is an example of how the software calculates the Scaling Tendency - using the AQ thermodynamic framework-
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ST = 1.0/0.403780
 
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ST = 2.48
 
  
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[[File:scaling tendency example.png]]
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Was assuming ideal conditions valid??
 
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The actual species concentration and activity coefficients are:
 
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gamma.Na = 0.598
 
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gamma.HCO3 = 0.596
 
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m.na = 0.894
 
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m.HCO3 = 0.866
 
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This results in a different IAP
 
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IAP= (0.598)(0.894)(0.596)(0.866)
 
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IAP=0.276
 
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The new Scaling Tendency is therefore:
 
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ST = IAP/Ksp
 
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ST = 0.276/0.40378
 
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ST = 0.683
 
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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:
 
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CO2o = 0.016 molal
 
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NaHCO3o = 0.101 molal
 
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CO32- = 0.012 molal
 
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NaCO3- = 0.004 molal
 
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What does the Scaling Tendency Mean?
 
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If ST < 1, then the solid is under-saturated
 
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If ST > 1, then the solid is super-saturated
 
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If ST = 1, then the solid is at saturation
 
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Scaling Index = Log (ST)
 
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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.
 
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This was OLITips39.
 
  
 
[[Category:Tips]]
 
[[Category:Tips]]

Latest revision as of 11:58, 6 March 2020

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).

To understand Scaling, the Scaling Tendency and Scaling Index definitions are used. These definitions depended on the Solubility Product Constant, Ksp and Ion Activity Product, IAP definitions.


Solubility Product Constant, Ksp

The solubility of ionic compounds of salts and minerals in water are governed by a solubility equilibrium expression and a solubility product constant known as Ksp. It is important to note that the solubility product, Ksp is a function of both temperature and pressure. Consider the general dissolution reaction below (in aqueous solutions):

aA(s)⇌bB(aq)+dD(aq)

With equilibrium constant Ksp defined as:

Ksp=(aB)b∙(aD)d

Where, aB and aD are the activities of the aqueous species. The activity of any species i is defined as the product of its concentration in molality by its corresponding activity coefficient:

ai=mi γi


Ion Activity Product, IAP

A real solution may not be in the state of equilibrium. This non-equilibrium state is described by the ion activity product (IAP). It has the same form as the equilibrium constant Ksp, but involves the actual activities of the species in solution.

IAP=(aB)bactual∙(aD)dactual

Scaling Tendency and Scaling Index

The Scaling Tendency (ST) is defined as the ratio of the Ion Activity Product (IAP) divided by the equilibrium constant (Ksp).


Equation (1) ST=IAP/Ksp


Scaling tendencies are essentially saturation ratios. Thus, if

  • ST < 1 Indicates sub-saturation, and the solid is not expected to form
  • ST = 1 Indicates saturation, and the solid is in equilibrium with water
  • ST > 1 Indicates supersaturation, and solids will form


Note: The Scaling Tendency (ST) is reported in the software as Post-Scale.


The Scale Index (SI) (aka: Saturation Index in the literature), is given by the following relationship:


Equation (2) SI =log10(IAP/Ksp)


Thus, if

  • SI < 0 Indicates sub-saturation, and the solid is not expected to form
  • SI = 0 Indicates saturation, and the solid is in equilibrium with water
  • SI > 0 Indicates supersaturation, and solids will form


Note: The Scaling Index (SI) is reported in the software as SI, Index

Pre-Scaling Tendency and Scaling Index

Pre-Scaling tendency is defined as the scaling tendency before any solids are formed (this can be seen as all the species suspended in solution). The same formulas for ST and SI are applied (Equations 1 and 2).

The Pre-Scaling tendency is reported in the software as Pre-Scale.


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(ST).


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

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.

Calculating Scaling Tendency: An Example

Below there is an example of how the software calculates the Scaling Tendency - using the AQ thermodynamic framework-

Scaling tendency example.png

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