All French Creek Software programs combine an intuitive and easy to learn interface with a rigorous, consistent calculation engine capable of modeling waters of high TDS, temperature and pressure.
DownHole SAT models water over a range of parameters rather than just a single point, displaying results in an array of color coded graphs and descriptive tables.
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We all have favorite, or at least familiar indices which assist in the assessment of the scale potential of a water, which provide some insight into the impact of concentrating, injecting or heating a water, or of mixing waters in various ratios.
Commonly used traditional indices provide an excellent starting point for evaluating a water chemistry and scale potential. Prior to the advent of cost effective personal computers, sophisticated indices were beyond the reach of most water chemists.
Traditional indices and calculation methods that are practical for manual use are simplifications, including numerous assumptions. These cause misleading interpretations, and limit the applicability of the indices to a single brine, or similar waters. French Creek brings main frame calculation power for water treatment to the level of the PC, eliminating (or at least greatly minimizing) the need for simplification and sometimes misleading assumptions.
The interface takes modeling chemistry a step further, allowing you to model numerous scales and ranges of parameters in the same amount of time it would take to work a spreadsheet or slide rule.
Simple indices include the Langelier Saturation Index, Ryznar Stability Index, Stiff-Davis Saturation Index, and Oddo-Tomson Index for calcium carbonate. Indices have also been developed for other common scales such as calcium sulfate and calcium phosphate.
The simple indices provide an indicator of scale potential, but lack accuracy due to their use of total analytical values for reactants. They ignore the reduced availability of ions such as calcium which occurs due to association with sulfate and other ions. The simple indices assume that all ions in a water analysis are free and available as a reactant for scale forming equilibria.
For example, the simple indices assume that all calcium is free. Even in low ionic strength waters, a portion of the analytical value for calcium will be associated with ions such as sulfate, bicarbonate, and carbonate (if present). This leads to over-prediction of the scaling tendency of a water in high ionic strength waters. The impact of these "common ion" effects can be negligible in low ionic strength waters. They can lead to errors an order of magnitude high in high ionic strength brines. Table 1 outlines some of the ion associations which might be encountered in natural waters.
A majority of the indices used routinely by water treatment chemists are derived from the basic concept of saturation. A water is said to be saturated with a compound (e.g. calcium carbonate) if it will not precipitate the compound and it will not dissolve any of the solid phase of the compound when left undisturbed, under the same conditions, for an infinite period of time.
A water which will not precipitate or dissolve a compound is at equilibrium for the particular compound. By definition, the amount of a chemical compound which can be dissolved in a water and remain in solution for this infinite period of time is described by the solubility product (Ksp). In the case of calcium carbonate, solubility is defined by the relationship:
(Ca)(CO3) = Ksp
where
(Ca) is the calcium activity (CO3) is the carbonate activity Ksp is the solubility product for calcium carbonate at the temperature under study.
In a more generalized sense, the term (Ca)(CO3) can be called the Ion Activity Product (IAP) and the equilibrium condition described by the relationship:
IAP = Ksp
The degree of saturation of a water is described by the relationship of the ion activity product (IAP) to the solubility product (Ksp) for the compound as follows: If a water is undersaturated with a compound:
If a water is at equilibrium with a compound:
If a water is supersaturated with a compound:
The index called Saturation Level, Degree of Supersaturation, or Saturation Index, describes the relative degree of saturation as a ratio of the ion activity product (IAP) to the solubility product (Ksp):
Saturation Level= IAP/Ksp
In actual practice, the saturation levels calculated by the various computer programs available differ in the method they use for estimating the activity coefficients used in the IAP; they differ in the choice of solubility products and their variation with temperature; and they differ in the dissociation constants used to estimate the concentration of reactants (e.g. CO3 from analytical values for alkalinity, PO4 from analytical orthophosphate).
Table 2 defines the saturation level for common scale forming species encountered in industrial applications. These formulas provide the basis for discussion of these scales in this paper.
The Saturation Index discussed can be calculated based upon total analytical values for the reactants. Ions in water, however, do not tend to exist totally as free ions.6,7,8 Calcium, for example, may be paired with sulfate, bicarbonate, carbonate, phosphate and other species. Bound ions are not readily available for scale formation. The computer program calculates saturation levels based upon the free concentrations of ions in a water rather than the total analytical value which includes those which are bound.
Early indices such as the Langelier Saturation Index (LSI) for calcium carbonate scale, are based upon total analytical values rather than free species primarily due to the intense calculation requirements for determining the distribution of species in a water. Speciation of a water is time prohibitive without the use of a computer for the number crunching required. The process is iterative and involves:
The use of ion pairing to estimate the free concentrations of reactants overcomes several of the major shortcomings of traditional indices. Indices such as the LSI correct activity coefficients for ionic strength based upon the total dissolved solids. They do not account for "common ion" effects.
Common ion effects increase the apparent solubility of a compound by reducing the concentration of reactants available. A common example is sulfate reducing the available calcium in a water and increasing the apparent solubility of calcium carbonate. The use of indices which do not account for ion pairing can be misleading when comparing waters where the TDS is composed of ions which pair with the reactants versus ions which have less interaction with them.
Pitzer Coefficient Estimation Of Ion Pairing Impact The ion association model provides a rigorous calculation of the free ion concentrations based upon the solution of the simultaneous non-linear equations generated by the relevant equilibria. A simplified method for estimating the effect of ion interaction and ion pairing is sometimes used instead of the more rigorous and direct solution of the equilibria.
Pitzer coefficients estimate the impact of ion association upon free ion concentrations using an empirical force fit of laboratory data. This method has the advantage of providing a much less calculation intensive direct solution. It has the disadvantage of being based upon typical water compositions and ion ratios, and of unpredictability when extrapolated beyond the range of the original data.
The use of Pitzer coefficients is not recommended when a full ion association model is available.
Saturation level is the ratio of the Ion Activity Product to the Solubility Product for the scale forming specie.
For calcium carbonate: SL = (Ca)(CO3)/Ksp'
For barium sulfate: SL = (Ba)(SO4)/Ksp'
For calcium sulfate: SL = (Ca)(SO4)/Ksp'
Saturation Levels should be calculated based upon free ion activities using the solubility product for the form typical of the conditions studied (e.g. calcite for low temperature calcium carbonate, aragonite at higher temperatures.)
Saturation levels can be interpreted as follows:
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