Degradation and hydrate phase equilibria measurement methods of monoethylene glycol

Graphical abstract


Specifications
Area Engineering More specific subject area: Hydrate Phase Equilibria Method name: Degradation and hydrate phase equilibria measurement methods of monoethylene glycol Name and reference of original method Resource availability

Method details
To meet energy demands, Natural gas has increasingly become a profitable alternative. However, a serious challenge is the formation of gas hydrates. The traditional technique to inhibit hydrate formation in pipelines is the injection of a thermodynamic hydrate inhibitor to shift the hydrate phase equilibrium boundary to lower temperatures, thus leaving the operating conditions of pipelines to be within a hydrate-safe region [1]. For the least, hydrates can cause blockages in pipelines, severely disrupting gas production, and also have the potential to cause explosions in pipelines. A common hydrate inhibitor that is utilized is Monoethylene glycol (MEG), it is mainly favorable due to its high recoverability. However, during the recoverability process MEG undergoes multiple phases of thermal exposure. This usually leads to thermal degradation in the MEG solution which results in an overall lower hydrate inhibitory performance [2].
In-order to understand how degradation occurs, its products, the impact on the equipment, and the hydrate inhibition performance of MEG, a method to degrade and test MEG is proposed in-detail. A study conducted by the authors that successfully utilized this method reported on the effect of regenerated MEG over multiple cycles [2]. The method essentially comprises of three stages; a) Degradation of MEG, b) Analysis of degraded MEG, and c) Hydrate testing of degraded MEG.

Degradation of MEG
The utilization of MEG as a continuous hydrate inhibitor necessitates ongoing regeneration to remove impurities such as produced water, reservoir fluids, salts, corrosion products and production/ drilling chemicals that have a tendency to accumulate within the MEG solution [3][4][5][6]. Reclamation is the process in which non-volatile chemicals and monovalent salts are removed from the MEG solution through processing a stream of re-concentrated MEG solution from the regeneration process. The process occurs in a flash separator operating in vacuum where the input solution (MEG-watercontaminants) are boiled off at a temperature greater than the boiling point of water and MEG. Both, the water and MEG will evaporate while leaving behind salts and other chemicals that can then be removed from the system [7]. Care needs to be taken to ensure temperatures do not rise beyond the thermal degradation temperature of MEG, even though degradation of MEG has been shown to be possible at reclaimer operating conditions which are considerably lower [2,8].
Two experimental apparatuses within the laboratory (reclamation unit and autoclave system) will be illustrated and their procedures to produce degraded MEG samples will be outlined. The reclamation process typically implemented in the field was reproduced by a rotary evaporator essentially a vacuum distillation unit (Fig. 1). Laboratory scale rotary evaporators are designed for different reclamation processes with vacuum control with slight modifications based on specific requirements. The rotary evaporator is utilized to carry out the separation of MEG from monovalent salts and insoluble contaminants where salt-laden MEG as an input solution is distilled by removing the salts as a crystalized residue, and pure lean-MEG is collected as condensate product. To achieve optimum operating conditions, a vacuum pump is utilized to avoid MEG degradation due to high temperatures while increasing the salt removal efficiency. The reclamation unit comprises of an overhead condenser, a vacuum flask partially submerged in an oil bath, a vacuum system, a liquid receiver and an integrated control box. Modifications have been made to allow for sparging with nitrogen (99.999 mol%) to ensure there is no oxygen contamination. To ensure operating temperatures remain within tolerable and desired levels, several K-type thermocouples ware retrofitted to measure the temperatures of the vapor and liquid-slurry phases, while being connected to the Programmable Logical Controller (PLC). A level sensor was utilized to control the flow of lean MEG into the evaporator flask based on the desired slip stream portion from the input (or from the regeneration unit in the case of field application). Other instruments were utilized to monitor the system in terms of pH, pressure, flowrates, electrical conductivity (EC), dissolved oxygen (DO).
Procedure for the preparation and degradation of test solution is as follows: 1 Preparation of initial solution (non-degraded salt-laden MEG solution) (a) Set-up the air-tight beaker system as shown in Fig. 2 1) and (2) to determine the required additional water (DM) to reach the desired MEG concentration for testing.
where M 1 and M 2 are the masses of the initial (undiluted solution) and final (diluted solution) in g respectively, C 1 and C 2 are the concentrations of the initial and final solutions respectively, and DM is the additional water required to reach the desired concentration (C 2 ) in g.
After the careful preparation of the test solution, it is ready for the degradation process as follows: 3 Degradation of prepared solution using the reclamation unit (Fig. 1).
(a) Transfer the initial solution to storage vessel 1 (SV1).  flask, and store it if required for future analysis (i.e. viscosity, SEM/ECM and particle analysis). (n) Extract the degraded MEG solution (contents of SV1 and SV2) for further analysis as outlined in step 2. 4 A slightly more simplified approach to attaining degraded MEG samples is the use of typical stainless steel high pressure/temperature autoclaves requiring no modifications ( Fig. 2(b)). The procedure for MEG degradation using an autoclave is as follows:

Hydrate testing of degraded MEG
To determine the hydrate phase equilibria of the degraded and non-degraded samples, a high pressure PVT Sapphire Cell can be utilized. The desired gas mixture can be introduced into the chamber according to the experimental design and the type of hydrate structure under study. Common methods of determining the hydrate phase equilibria can be employed such as the isochoric, isobaric and isothermal methods. A typical high-pressure PVTcell (Fig. 3) is made out of sapphire material so a complete visual of the internals of the chamber is available for detailed visual observations. The cell has been designed with an inner volume of 60 cm 3 to allow for sufficient gas and liquid to form hydrate. An automated magnetic stirrer fitted to the cell produced an agitation rate that helps in the complete transformation of the liquid water phase to hydrate, and encourages the renewal of the surface where there is a higher tendency for hydrate film to form. The recommended stirrer rate to be applied is 400-500 rpm. The cell is equipped with pressure and temperature sensors to capture PVT data for further analysis. the pressure to the desired pressure for the first point on the hydrate phase boundary. Close the gas input valve once desired pressure is achieved. (g) Enable the heating system to heat up the sample to 35 C to destroy any water memory profiles, then turn off the heater. (h) Enable data acquisition and ensure temperature, pressure and stirrer rate data are being recorded (at 5 s intervals). (i) Begin video recording using the camera and light beam focused on the sample within the cell. (j) Enable the cooling process to begin and set the cooling rate to 1 C/h via the control software. (k) Carefully note visual observations such as the growth, agglomeration and behavior of hydrate formation; the inter-phase conditions (i.e., clear, foaming, bubbling, grey or cloudy), film formation on the inner walls of the cell; the temperature at which the first hydrate particle is formed, the point at which the stirrer stops moving due to impeding hydrate solids, and the rate of reduction of the solution in the cell. (l) When all visible liquid has transformed into hydrate, continue the cooling process for a further 3 C but avoid going below 0 C (i.e., ice formation region). (m) Begin the slow step-wise heating process at a rate of 0.5 C/h with a maximum rate of 1 C/h so that a sufficient time is available for equilibrium to be achieved. The process can be ended when all visible hydrate solids are converted to liquid. (n) The PVT system can now be cleaned and shut-down. (o) From the acquired temperature and pressure data for the cooling and heating processes, the hydrate thermodynamic equilibrium point may then be determined from the intercept of the two curves. Use the computer script provided in the Supporting information for automated processing of data logs to determine the hydrate phase equilibria conditions. (p) Repeat the entire process (5) for at least another 3 more pressure points in order to plot the hydrate phase boundary.

Method validation
The degradation of MEG can be identified by the presence of by-products. Studies from literature that investigated degradation of MEG have found the by-products of MEG degradation to be formic, glycolic, acetic and oxalic [9][10][11][12][13][14]. Numerous studies have been conducted by our laboratory using our method which are outlined in Table 1 [2,15,16]. The results clearly show the presence of degradation products such as acetic acid between degraded and non-degraded samples. A study conducted by Psarrou et al., (2011) has reported that a sign of degradation in the reclamation process is the color of the solution where it changes to more of a yellow color [8]. The color changes have also been reported in Table 1, and it can be seen that the color has changed from clear to yellow to dark brown as the degradation amount increases amongst the MEG solution samples. Furthermore, the effect of MEG degradation on the hydrate phase boundary can be studied using this method. A pure MEG solution of 25 wt % was prepared and degraded for 100 h using this method. The changes in color, pH, EC and the shift in hydrate phase boundary have been reported in Table 1 and Fig. 4. It can be confirmed that degradation products and promotion of hydrate formation was found.
Experiments were conducted to determine the methane-water hydrate phase boundary using the set-up reported in this study. The phase equilibria data are plotted in Fig. 4. When compared to the widely available literature data [17][18][19], an absolute average relative error (AARE) of 0.98% was found, which confirms that our apparatus and procedure are highly accurate in determining hydrate phase equilibria (Fig. 4).

Conclusion
Flow assurance challenges such as gas hydrates and corrosion are a serious concern for the oil and gas industry. An array of chemicals (i.e., hydrate, corrosion, scale, wax inhibitors and oxygen scavengers) are injected into the hydrocarbon production and process pipelines to prevent, decrease and or mitigate these concerns. MEG is a conventional hydrate inhibitor that is commonly used in the industry due to its reusability. However, MEG may undergo degradation in the reboiler and reclamation units of a MEG regeneration plant. Thus, to study the effects of degradation of MEG especially in the presence of other chemical additives upon the adopted hydrate inhibition program becomes important. This study has outlined the necessary methods to mimic field-like degradation of MEG and analysis in terms of hydrate inhibition performance and degradation products.