Electrolyte Chemistry Modelling: The Game-Changing Tool For Sour Water Operations In Downstream Oil And Gas

In the Downstream Oil and Gas sector, productivity and reliability are foundational to success. More than ever, companies are striving to adopt the latest operating methods and technologies to enhance refinery operations, enabling them to sustain production, boost yield, and increase profits while driving down operational expenses.

However, companies industry-wide often overlook water management and severely lack the tools to optimise and efficiently operate water-containing process units – a significant threat to production and bottom lines.

Corrosion, scaling, and inefficient operations plague water-containing systems as they are often poorly understood and neglected. Over time, these operational issues lead to losses in processing capacity, increased maintenance costs, unplanned shutdowns, and even catastrophic failure.

Corrosion is especially rampant as water readily dissolves solids, salts, and other harsh impurities in process units that use water washes like crude distillation and fluid catalytic cracking. Problems continue downstream as contaminants are removed in the sour water stripper where hydrogen sulphide, ammonia, and other volatile species are stripped for the purposes of water reuse or further processing in the wastewater treatment plant.  

As pressures are mounting to maximise water reuse, the sour water stripper has become arguably the most critical water-based unit in oil and gas processing. These units can be a significant bottleneck to refinery operations, yet many companies lack the deep chemistry understanding required to optimise sour water strippers, maximise water processing, eliminate corrosion, and reduce scale deposition.  

Advanced Electrolyte Modelling Tools

Water chemistry plays a major role in the efficient and reliable operation of sour water strippers. To fully understand the impact electrolytes have on thermodynamic properties (i.e. pH) and phase equilibrium (i.e. vapour stripping, scale deposition), an advanced electrolyte modelling tool is required.  

Advanced electrolyte modelling tools use speciation reactions as the basis for modelling electrolyte behaviour, these reactions are necessary when wastewater is complex or high in dissolved salts.  Speciation variations have a significant effect on phase equilibria such as salt solubility and vapour formation.  

Speciation chemistry is the inherent reason for non-ideal properties of an electrolyte solution and therefore needs to be accounted for in any calculation.  An example of speciation is shown in the following sour water system: water, ammonia, carbon dioxide and hydrogen sulphide.  While only having 4 components in the system, H₂O, NH₃, CO₂ H₂S; when you consider speciation, the number of components increases to 12: H₂O, H⁺, OH^–, CO₂⁰, CO₃^-2, HCO₃^–, NH₃⁰, NH₄⁺, NH₂CO₂^–, H₂S⁰, HS^–, S^-2. This expansion is further explained when studying the speciation reactions listed below.  

H₂O = H⁺ + OH^–

CO₂⁰ + H₂O = H⁺ +HCO₃^–

HCO₃^– = H⁺ +CO₃^-2

NH₃⁰ + H₂O = NH₄⁺ + OH^–

NH₂CO₂^– + H₂O = NH₄⁺ + CO₃^-2

H₂S⁰ = H⁺ +HS^–

HS^– = H⁺ + S^-2 

All 12 components will exist in some concentration in the sour water system and accounting for all speciation becomes critical in accurate, electrolyte-based modelling. Figure 1 and Figure 2 describe how considering all speciation is essential for accurate process models.

The parity plots shown below depict calculated and experimental solubilities for the same sour water system: CO₂, H₂S, and NH₃ at 20 and 60 C.  When speciation is not considered in predicting dissolved gas concentrations (Figure 1) the error is up to 10000x compared to measured values.  

Figure 1 – Parity plot for CO2, H2S, and NH3 vapour pressures when only the Henry's constant is used to predict solubility (no aqueous speciation).

When speciation reactions are considered, the error decreases significantly (Figure 2).

Figure 2 – Parity plot for CO2 and H2S solubility in water when a full speciation model is used to predict solubility

Electrolyte models like these are used in every industry to predict fluid properties, not only for predicting volatile gas stripping from sour water (sour water stripping) but in other common processes like combustion gas scrubbing to remove harmful components like mercury, SOx/NOx, and HCl, and most recently, as the first step in carbon capture.

In refining, predicting when a free acid phase will condense from a gas stream – a particularly pernicious example of this is at the top of the atmospheric crude distillation unit in an oil refinery where improper operations can lead to highly corrosive acidic conditions.  

Electrolyte Modelling for Scaling and Corrosion Risk

Electrolyte models provide insights to corrosion and fouling risks. The sour water stripper is highly susceptible to fouling, as the operating pH has the unintended consequence of solids precipitation in the complex water streams.  

Thermodynamic analysis (electrolyte modelling) of these streams at varying water qualities and operating conditions (temperature, pressure, pH, and composition) can be used to evaluate scaling (fouling) potential.  These insights allow the engineer to adjust operating conditions to avoid potential costly tower and heat exchanger fouling.

Similarly, sulphur-containing sour water comes with an increased corrosion risk.  With changing vapour/liquid composition and temperatures throughout the stripper, corrosion risk will also change.  Individual streams can be analysed at various operating conditions, to aid in the selection of materials for tower internals, heat exchangers, and overhead systems.  This can further optimise the tower design and establish operating envelopes that avoid corrosive conditions extending the life and reliability of the units.

Electrolyte chemistry modelling is critical in understanding the behaviour of complex water and wastewater in downstream oil and gas process units. This is particularly true in the sour water stripper that is highly susceptible to upsets due to small changes in water quality and pH. An electrolyte model can provide deep insight into these complex systems, informing the engineer on operating efficiencies, and corrosion and scaling risks.  

For more information on scaling, corrosion and electrolyte modelling, or to learn how modelling and simulation software can enhance the reliability, efficiency and performance of your O&G operations, contact OLI Systems today or email sales@olisystems.com.