Abstract

If a glass electrode calibrated in aqueous pH standards is used to measure H+ concentration in aqueous glycol solutions the result will be in error due to the large change in solvent properties. Based on strong acid - strong base titrations in glycol solutions it is possible to calibrate glass electrodes. The titration does not only give the calibration constant for the electrode, but also the autoprotolysis constant for the solvent can be determined. The calibration method makes it possible to determine acid constants in glycol by potentiometric titration.

Introduction

Co-production of formation water and water carry-over into gas/condensate pipelines may result in scaling. The problem is closely associated with hydrate control. For long distance transportation, high dose hydrate inhibitors like methanol and glycol have been chosen rather than external heating or low dose kinetic inhibitors. High dose inhibitors provide means for corrosion control and are easy to regenerate. Ethylene glycol is preferred from a corrosion point of view. It reduces corrosion by itself and can easily carry corrosion inhibitors.

Carry-over of formation water may give scaling when glycol is used for hydrate and corrosion control. Ethylene glycol reduces the solubility of scale forming compounds as CaCO3, BaSO4 and SrSO4. Scale will rarely form when clean glycol is injected into a pipeline with limited amounts of formation water. The dilution of the formation water more than compensate for the reduction in solubility. However, depending on how the glycol is regenerated some formation brine will be recycled and accumulated until one or more compounds reach its solubility limit somewhere in the glycol loop. The scaling risk is enhanced further if the pH of the glycol is increased to reduce pipeline corrosion. Then solubility calculations and process control become critical to avoid precipitation of CaCO3.

To be able to predict scaling and set operational limits for glycol loops, it is at least necessary to know the solubility of the mineral salts and of the most water soluble gases CO2 and H2S. To fill in the lacking data for glycol solutions, IFE has run research programs to measure solubility and dissociation constants in glycol system. Most of these equilibria depend on pH and exact pH measurements are a requirement both in laboratory experiments and for process control. A simplified method for calibration of pH electrodes in glycol solutions is presented together with correction terms for changes in electrode response due to ethylene glycol.

Theory

pH measurements in solutions with high glycol concentrations are not straightforward. A glass electrode calibrated in aqueous pH standards will be in error due to the large change in solvent properties. Some pH reference value standards (RVS) are available for monoethylene glycol (MEG)1, but they are difficult to use in practice as they refer to solutions with zero ionic strength. The standards use KHftatalate that are difficult to use with glass electrodes as it has low buffer capacity and slow glass electrode response. They are also limited to maximum 70wt% MEG. The pH is defined by

  • pH = -log(aH) = -log(?HmH)(1)

The pH is the negative logarithm to the H+ activity and the pH scale depends on the chosen reference state. Mussini et al. chose the pure MEG-water solutions at zero ionic strength as reference state for their RVS1. For most practical use, the concentration of H+ is needed rather than the pH. The proton concentration can be obtained if a correction factor for glass electrodes can be established. The correction factor can be determined by performing a titration of strong base with a strong acid.2

Theory

pH measurements in solutions with high glycol concentrations are not straightforward. A glass electrode calibrated in aqueous pH standards will be in error due to the large change in solvent properties. Some pH reference value standards (RVS) are available for monoethylene glycol (MEG)1, but they are difficult to use in practice as they refer to solutions with zero ionic strength. The standards use KHftatalate that are difficult to use with glass electrodes as it has low buffer capacity and slow glass electrode response. They are also limited to maximum 70wt% MEG. The pH is defined by

  • pH = -log(aH) = -log(?HmH)(1)

The pH is the negative logarithm to the H+ activity and the pH scale depends on the chosen reference state. Mussini et al. chose the pure MEG-water solutions at zero ionic strength as reference state for their RVS1. For most practical use, the concentration of H+ is needed rather than the pH. The proton concentration can be obtained if a correction factor for glass electrodes can be established. The correction factor can be determined by performing a titration of strong base with a strong acid.2

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