Optimisation of Mono-Ethylene Glycol Regeneration Chemistry and Corrosion Inhibition Strategies

This research studies the optimisation of mono-ethylene glycol (MEG) regeneration chemistry and various corrosion inhibition topics related to natural gas pipelines. MEG, used for the inhibition of natural gas hydrate formation, is regenerated to facilitate its reinjection with operational issues often arising during the industrial regeneration process. Research was undertaken to solve several issues identified in industry including optimisation of separation processes, improved corrosion inhibition strategies and development of chemical/physical data relevant to MEG systems

Effect of pretreatment process on scale formation in the re-boiler section of monoethylene glycol regeneration plant

Monoethylene glycol (MEG) regeneration plants often use pretreatment vessels to precipitate divalent cations, such as Fe2+, Ca2+, and Mg2+, in order to avoid or reduce fouling in downstream reboilers and heat exchangers. This pretreatment process operates under alkaline conditions and moderate temperatures (~ 80 °C) to accelerate the formation of low-solubility divalent salts. The objective of the present research was to determine whether the pretreatment process could be minimized, without negatively impacts on the MEG regeneration process from to the formation of scale on the heater bundle in the presence of low concentrations of divalent cations in the rich MEG stream. Scale formation was analyzed under MEG regeneration process conditions using a dynamic scale loop (DSL) test and verification experiments were performed in a MEG regeneration and reclamation pilot plant, both with and without pretreatment conditions. The scaling tendencies of several rich MEG–brine mixtures were evaluated at different pH pretreatment levels and dissolved CO2 concentrations. An evaluation temperature of 180 °C was chosen to match the skin temperature of the reboiler heater bundle during the MEG regeneration process. The experiments of pH 7.24 showed high amounts of precipitation scale within the reboiler due to high remaining concentrations of mineral ions. In addition, small concentrations of calcium and magnesium ions led to the precipitation of calcite, dolomite, and magnesium hydroxide on the reboiler bundle and within associated filtered outputs even when a pretreatment vessel was present. These results were confirmed by the differential pressure build-up and Scanning Electron Microscopy analyses for each experimental condition. Another interesting finding is that pH increased within the reboiler due to CO2 gas boiling off at high operating temperatures, thus contributing to increased alkalinity levels, which in turn promoted scale formation. These results indicate that pretreatment should not be reduced, even with divalent ion concentrations as low as ~ 5 ppm TDS, due to the harsh conditions within the reboiler heater bundle

Measurement of mono ethylene glycol volume fraction at varying ionic strengths and temperatures

The estimation of Mono Ethylene Glycol (MEG) concentration is an essential criterion during the industrial regeneration of MEG to evaluate the efficiency of regeneration process and to control the concentration of MEG reinjected at the wellhead. Although many laboratory methods to determine MEG concentration exist, their application may be costly in terms of the time required to perform sampling and laboratory analysis. For this reason, an alternative method for determination of MEG concentrations has been proposed. This method can be performed on-site utilizing physical properties that can be readily measured using portable measurement devices including refractive index (n D ), electrical conductivity (EC) and total dissolved solids (TDS). The volume fraction (F vm ), n D , EC, and TDS of MEG solutions have been measured at (283.15, 298.15, and 323.15) K, (10–100) vol. %, and at (0, 0.125, 0.25, 0.5, 1.0) M NaCl total volume of solution) ionic strength (IS). The experimental results were then correlated to develop a simplistic model capable of estimating the volume fraction of MEG mixtures at varying ionic strengths. The proposed models will therefore allow a quick and convenient method for the determination of MEG concentrations in the field to quickly identify undesirable changes in produced lean MEG concentration

The influence of magnetic fields on calcium carbonate scale formation within monoethylene glycol solutions at regeneration conditions

One of the most discussed topics related to the effects of external magnetic fields (MF) on aqueous solutions is the influence on the scale formation of calcium carbonate (CaCO3). However, the extent of the effect of these forces on the scale formation in the non-aqueous solutions has not been investigated so far. So MFs will be applied to non-aqueous mixtures to find out the behavior of scale formation. This study presents the results of inorganic scale formation within MEG solutions containing Ca2+ and HCO3- ions, which has been investigated using both static and dynamic scale loop (DSL) evaluation techniques. Furthermore, the influence of MFs on scale formation using the dynamic technique has also been studied. Results were generated using brine/MEG solutions exposed to an external MF produced by a 0.65 T Neodymium magnet for 2.5 s. The degree of scale formation was examined by measuring the pressure build-up across a capillary coil as scale was developed. Moreover, differences in CaCO3 morphologies were evaluated for the exposed and blank trials via the DSL technique and compared with the results obtained from the static scale evaluation method. The results of this research have demonstrated that the short exposure (2.5 s) to a powerful MF can significantly reduce scale-formation in the rich MEG solutions within the capillary coil. This is due to the alteration of the proton spin inversion in the field of diamagnetic salts. Furthermore, a significant difference in CaCO3 morphology was observed for the scale formed during dynamic and static conditions. The generating results help to reduce the use of chemical scale inhibitors with MEG solution during the gas hydrate treatments, especially when the concentration of MEG in formation water is low and scale formation is more likely to occur

Influence of magnetic fields on calcium carbonate scaling in aqueous solutions at 150° C and 1 bar

The experiments performed as a part of this study were conducted to evaluate the effect of magnetic field treatment upon the scale forming tendency of brine solution composed primarily of calcium bicarbonate ions. The reported results were generated using a Dynamic Scale Loop system with the brine solution exposed to a magnetic field generated by a 6480 Gauss magnet of grade N45 SH in a diametrical orientation for 2.5 s. Following magnetic exposure, the brine solution was exposed to an elevated temperature 150 °C at 1 bar to promote the formation of scale within a capillary tube. The extent of scaling was measured by recording the differential pressure across the tube as scaling proceeded. Three important conclusions regarding the effect of magnetic field treatment upon scale formation in calcium bicarbonate solutions were reached. Firstly, the ratio of calcium to bicarbonate plays a key role in determining how magnetic fields influence scale formation, whether promoting or inhibiting it. Solutions containing high concentrations of the bicarbonate, or equal concentrations of the bicarbonate and calcium species showed inhibited scale formation following magnetic exposure. Secondly, the electrical conductivity of the calcium carbonate solution was noticeably impacted by the exposure to the magnetic field through manipulation of the ionic hydration shell and may also provide a measure of the extent of scale formation. Finally, the application of magnetic field treatment fo r scale inhibition may provide an alternative eco-friendly scale inhibition strategy in place of traditional chemical scale inhibitors

Acid Dissociation Constant (p<i>K</i>_a) of Common Monoethylene Glycol (MEG) Regeneration Organic Acids and Methyldiethanolamine at Varying MEG Concentration, Temperature, and Ionic Strength

The acid dissociation constants (p<i>K</i>_a) of four organic acids (formic, acetic, propanoic, and butanoic) commonly found in monoethylene glycol (MEG) regeneration systems and methyldiethanolamine (MDEA) were measured via potentiometric titration. Dissociation constants were measured within varying concentration of MEG solution (0, 30, 40, 50, 60, 70, and 80 wt %) and at varying temperature (25, 30, 40, 50, 60, 70, and 80 °C). Thermodynamic properties of the dissociation process including Gibbs free energy (Δ<i>G</i>° kJ mol^–1), standard enthalpy (Δ<i>H</i>° kJ mol^–1), and entropy (Δ<i>S</i>° kJ mol^–1 K^–1) were calculated at 25 °C using the van’t Hoff equation. Comparison of the reported experimental p<i>K</i>_a values and calculated thermodynamic properties in aqueous solution to the literature demonstrated good agreement. Two models have been proposed to calculate the p<i>K</i>_a of acetic acid and MDEA within MEG solutions of varying concentration, temperature, and ionic strength. The proposed models have an average error of 0.413% and 0.265% for acetic acid and MDEA, respectively

Operation of a MEG pilot regeneration system for organic acid and alkalinity removal during MDEA to FFCI switchover

The switch over from pH stabilisation using MDEA to film forming corrosion inhibitors (FFCI) may be beneficial following formation water breakthrough during hydrocarbon transportation and processing to prevent scaling at elevated pH and to extend the operational lifespan of a field. Where formation water is present, organic acids including acetic can be expected within MEG regeneration systems and can impose a corrosion risk together with carbon dioxide. A case study was performed to evaluate the potential of simultaneous removal of organic acids and MDEA/alkalinity during the switch over from pH stabilisation to film forming corrosion inhibitors (FFCI). Experimental testing was conducted using a MEG pilot regeneration plant operated by the Curtin Corrosion Engineering Industry Centre. Sufficient removal of organic acids was achieved to prevent accumulation within the MEG regeneration loop and subsequent corrosion issues through distillation by lowering the pH of the rich glycol feed to six to promote removal of organic acids with the water distillate. Simultaneously, removal of MDEA and reduction of lean glycol alkalinity was achieved through the reclamation system to facilitate FFCI switchover more rapidly than a comparative industrial operational methodology

Experimental Vapor–Liquid Equilibrium Data for Binary Mixtures of Methyldiethanolamine in Water and Ethylene Glycol under Vacuum

Methyldiethanolamine (MDEA) is a widely used chemical in the natural gas processing industry as a solvent for CO₂ and H₂S capture and as a basic compound for pH stabilization corrosion control. During pH stabilization corrosion control, the removal of MDEA during the (mono)­ethylene glycol (MEG) regeneration process may occur under vacuum conditions during reclamation in which the removal of salt cations is performed. Isobaric vapor–liquid equilibrium data for the binary MEG–MDEA system is presented at (20, 10 and 5) kPa and water–MDEA system at (40, 20, 10) kPa to simulate its behavior during MEG reclamation under vacuum. Vapor and liquid equilibrium concentrations of MDEA were measured using a combination of ion chromatography and refractive index. The generated experimental VLE data were correlated to the UNIQUAC, NRTL, and Wilson activity coefficient models, and the respective binary parameters were regressed