Removal of Organic Acids during Mono-Ethylene Glycol Distillation

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Removal of Organic Acids during Mono-Ethylene Glycol Distillation and Reclamation to Minimize Long-Term Accumulation Adam Soames, Ahmed Barifcani, and Rolf Gubner Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00751 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Industrial & Engineering Chemistry Research

Removal of Organic Acids during Mono-Ethylene Glycol Distillation and Reclamation to Minimize Long-Term Accumulation Adam Soames1*, Ahmed Barifcani1, Rolf Gubner1 1.

WA School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Kent St, 6102, Perth W.A., Australia * Corresponding Author: [email protected] Abstract: Mono-Ethylene Glycol (MEG) is often used in offshore multiphase hydrocarbon transportation pipelines to prevent the formation of natural gas hydrates. Post hydrate inhibition, MEG is separated alongside the water phase and regenerated through a series of chemical and physical processes to remove excess water and process contaminants including mineral salts, production chemicals and organic acids. The level of organic acids within closed-loop MEG systems is often controlled via vacuum reclamation systems through the formation of non-volatile salt products allowing their separation from the evaporated MEG/water phase. However if a reclamation system is unavailable, or operated at low pH, organic acids will ultimately accumulate within the MEG loop. Likewise, removal of organic acids may be achieved during the distillation process by vaporization of the acids in their undissociated volatile form. Within both processes, the removal efficiency of an organic acid is ultimately dictated by the acids speciation behavior and system pH. The purpose of this study was to measure the removal efficiency of acetic acid during the regeneration and reclamation processes to identify key pH levels to minimize the long term accumulation of acetic acid. Two equations have been proposed to estimate the removal efficiency of acetic acid during distillation and reclamation individually with an average error compared to reported experimental data of 0.38% and 2.6% respectively. Combining the two proposed equations, the plant-wide removal of acetic acid can be predicted for a known rich glycol pH and pH rise across the regeneration system allowing practical application to industrial MEG regeneration systems.

1.0 Introduction Mono-ethylene glycol (MEG) has found widespread use as a thermodynamic hydrate inhibitor to prevent the formation of natural gas hydrates within offshore multiphase hydrocarbon transportation pipelines16 . In order to minimize the operational costs associated with MEG injection, post hydrate inhibition, the excess MEG is separated alongside the water phase to be regenerated and recycled for further use1-3,7,8. The MEG regeneration process entails a series of chemical and physical steps to remove a wide range of contaminants including excess water, mineral salts and process chemicals2,5,9,10. To produce a final lean MEG product suitable for reinjection at the wellhead, excess water is separated from rich MEG by distillation to regain a MEG concentration typically between 80-90% by weight1,2,5,11,12. To achieve the desired lean MEG concentration, the MEG regeneration unit (MRU) is typically operated between 120140°C at atmospheric pressure1,13,14. Alongside other contaminants, organic acids such as acetic, propanoic, butanoic and formic may be present within the regeneration system originating from the condensed water phase or following formation water breakthrough2,5,15. Furthermore, the thermal degradation of MEG at high temperature in the presence of oxygen may also lead to the formation of organic acids including glycolic, acetic and formic acid7,15-17. The presence of organic acids within industrial MEG regeneration systems can pose several operational issues including contributing to corrosion of downstream process equipment and pipe

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systems2. Organic acids have been found to increase the rate of corrosion of carbon and mild steel piping in natural gas and oil field systems18-21. In combination with carbon dioxide, acetic acid may also exacerbate the rate of Top-of-the-Line-Corrosion (TLC)15,21-26. Additionally, organic acids will directly reduce the pH of the liquid phase increasing the solubility of protective iron carbonate films, reducing corrosion protection and may also be directly reduced on the surface of metals enhancing the anodic reaction of the metal21,27,28. To minimize corrosion within natural gas pipelines two primary corrosion inhibition strategies can be utilized including pH stabilization and injection of film forming corrosion inhibitors (FFCI)2,3,5,9,19. The basis of pH stabilization entails artificially increasing system pH to reduce the availability of hydrogen ions for the cathodic corrosion reaction, whilst also promoting the formation of a protective FeCO 3 film on the surface of the pipeline. The pH within pipelines is typically increased through addition of hydroxides, carbonates or amine based compounds including methyldiethanolamine (MDEA)1,3,9,23. pH stabilization as a corrosion mitigation strategy is most effective when carbon dioxide is the main source of corrosion and is limited to systems where formation water breakthrough has not occurred due to the scaling risk at high pH5,23,29,30. Where scaling is a concern, FFCIs may instead be used due to their limited impact on system pH and hence scaling risk4,5,30,31. The removal of excess ionic species including salt components from closed-loop MEG can be performed using vacuum reclamation systems. MEG systems utilizing reclamation may operate under either fullstream reclamation whereby the rich glycol stream is flashed under vacuum to totally remove salts or using a slip-stream mode post distillation to remove primarily monovalent ions including sodium from a fraction of the produced lean glycol1,2,4,32-34. The type of reclamation is dictated by the expected salt production rate with systems operating at high salt loads requiring full-stream reclamation. However, for systems with low salt production rates, a lean glycol slip-stream reclamation system is often sufficient to control the salt content within closed loop MEG systems1,34,35. In a similar manner, the removal of organic acids from the MEG regeneration loop is typically achieved via vacuum reclamation systems, however, removal through distillation may also occur2. If a high pH is maintained within the reboiler during the distillation process, organic acids will exist in their ionic form and tend to remain within the lean MEG product resulting in their accumulation within the MEG loop2,23,29. As such, to facilitate their removal during distillation, it is essential to maintain a low pH (pH 60 hours). Organic acids formed through thermal degradation during reclamation will be subjected to removal as dictated by their speciation behavior. Reclamation systems operating at low pH will ultimately generate greater concentrations of acidic degradation products in the reclaimed solution. Figure 8 illustrates the levels of glycolic and formic acid produced in the reclaimed lean glycol solution following the reclamation process used within this study. Although the initial lean glycol solution was sparged with nitrogen to sub 20 ppb oxygen content, and the apparatus continuously purged with ultra-pure nitrogen, significant levels of glycolic acid were produced likely as the result of minor oxygen intrusion under vacuum. As such, unless a high pH is maintained, significant accumulation of glycolic and formic acids through MEG degradation can be expected in the reclaimed lean MEG solution. Furthermore, It can be suggested that the formation of acetic acid during the degradation process may account for the lower than simulated removal rates within the high pH regions (pH >9). Ultimately however, it was observed that minimal additional acetic acid was formed during the vacuum reclamation


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procedure through thermal degradation in this study. This suggests that the formation of acetic acid via MEG degradation primarily occurs after exposure to long term high temperatures as used by Odeigah, et al. 36. This however may still influence industrial reclamation systems where dissolved oxygen concentrations may be higher or oxygen intrusion via vacuum leakage more severe, potentially leading to lower than expected acetic acid removal rates compared those reported in this study. 3 HOCH2 CH2 OH + O2 → 2HCOOH + H2 O 2

(5)

HOCH2 CH2 OH + O2 → HOCH2 COOH + H2 O

(6)

300

10 9

250

8 7

200

6 150

5 4

100

3

Glycolic

2

Formic

50

Formic Acid (mg/L)

Glycolic Acid (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 0

0 4

5

6

7 8 Initial Lean Glycol pH

9

10

11

Figure 8. MEG organic acid degradation products generated during reclamation

3.5

Modelling of Plant Wide Acetic Acid Removal

Through a combination of removal through the distillation and subsequent reclamation system (if present), it may be possible to optimize the removal of organic acids over the entire regeneration system. A low targeted rich glycol pH into the distillation column in combination with a high pH rise across the reboiler may ultimately further facilitate a high rate of acetic acid removal during down-stream reclamation. Control of the lean glycol pH entering into the reclamation system can either be achieved through control of the initial rich glycol pH and dissolved CO2 concentration (Figure 4 and Figure 5) or through additional dosage of acids or bases following the regeneration process via in-line dosing points. To estimate the total plant wide acetic acid removal generated during distillation and reclamation or a combination of both, the experimental removal rates reported were fitted to the equations given by Equations (7) and (8) for distillation and reclamation respectively. The parameters A1/A2 and B1/B2 were regressed from the experimental data utilizing the objective function given by Equation (10). The equation parameters and average error of the individually simulated results (distillation and reclamation) compared to experimental data are given in Table 1, with visual compassion of calculated and experimental data given in Figure 9. From the individual distillation and reclamation Equations (7 and 8), the total plant wide acetic acid removal can be estimated using Equation (9).



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Figure 9. Comparison of modeled acetic acid removal rate during distillation and reclamation Distillation Removal, R D (%) = B1 −

B1 × 10−A1 10−pHdistillation + 10−A1

Reclamation Removal, R R (%) = 𝐹𝑅 × 100 − (B2 −

(7)

B2 × 10−A2 ) 10−pHreclamation + 10−A2

(8)

Where A and B are regressed parameters from experimental data and FR is the slip-stream reclamation fraction Total Removal, TR (%) = R D + R R (1 − OF =

RD ) 100

(9)

∑ |yexp − ycalc |

(10)

n

Table 1. Regressed model parameters and average model errors Parameter

Distillation Removal (1)

Reclamation Removal (2)

A1/A2

5.9161

7.7065

B1/B2

50.2688

99.5521

Average Error (%)

0.38

2.6

The model given by Equation (9) was then used to identify the optimum rich glycol pH that in combination with a known pH rise across the reboiler (Figure 4 and Figure 5), are required to achieve the optimal conditions necessary to maximize plant wide acetic acid removal. Figure 10 illustrates the total acetic acid removed by distillation at an initial rich glycol pH with incremental pH rises across the reboiler in


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combination with subsequent full-stream vacuum reclamation at the corresponding lean glycol pH. Systems with an initially low pH combined with low pH rise across the reboiler are ultimately constrained by the removal efficiency in the distillation system with minimal subsequent removal downstream in the reclamation system, reaching maximum acetic acid removal as per Figure 6. In contrast, systems generating a large pH increase across the reboiler, or utilizing in-line dosage of bases into the reclaimer feed achieved significant removal of acetic acid primarily due to the reclamation stage (Figure 7). As such, it is evident that the maximum acetic acid removal is primarily generated through the reclamation system particularly for systems operating using a combination of low initial rich glycol pH and large pH rise across the regeneration system. However, it is important to consider that MEG systems utilizing vacuum reclamation downstream of the regeneration unit typically operate using a slip-stream or partial reclamation mode to minimize operational costs and equipment size requirements1,2,5. Instead of full reclamation of the regenerated lean glycol product, only a fraction of the lean glycol is reclaimed with the fraction dependent on field requirements to maintain a specific level of salts within the closed-MEG loop. The study of Soames, et al. 2 utilized a slip-stream rate of 11% based on an Australian industrial MEG regeneration system processing up to 2500 m3 of MEG day. For the purposes of this study, a reclamation slip-stream rate of 20% was utilized to model the plant wide acetic acid removal rate illustrated by Figure 11. Due to the restricted reclamation rate, it is evident that the bulk of acetic acid removal is achieved when rich glycol with pH levels below six is fed into the distillation system. Ultimately, if a low reclamation rate is utilized in combination with high lean glycol pH, accumulation of acetic acid within the MEG loop will occur. In the study of Soames, et al. 2, a lean MEG slip-stream rate of 11% was sufficient to prevent the accumulation of organic acids within a closed loop MEG pilot plant where the produced water phase contained approximately 200 mg/L total organic acid content.

Figure 10. Simulated acetic acid removal rate full reclamation



Figure 11. Simulated acetic acid removal rate with 20% partial reclamation

4.0 Conclusion The level of organic acids within closed-loop MEG systems is often controlled via vacuum reclamation through the formation of non-volatile organic salt products allowing separation from the evaporated MEG/water phase. However if a reclamation system is unavailable, or operated at low pH, organic acids will ultimately accumulate within the closed-loop MEG system. To maximize the removal of organic acids during long term operation, the removal efficiency of organic acid during a combination of distillation and reclamation has been studied in conjunction with measured pH changes during the MEG distillation process. The experimental results generated during distillation and reclamation were compared to models developed using Aspen Plus® simulation software with good agreement found. Using the experimentally reported data, two equations have been proposed to estimate the removal of acetic acid during distillation and reclamation individually with an average model error of 0.38% and 2.6% respectively. The individual distillation and reclamation models can be utilized to estimate the total plant wide acetic acid removal within closed loop MEG systems. Overall, the results high light the importance of both operational pH during the individual distillation and reclamation processes and the expected pH rise across the regeneration system in determining total organic acid removal from closed loop MEG systems. Although complete removal of acetic acid during reclamation is achieved for reclamation systems with a pH above nine, closed loop MEG systems are ultimately constrained by the operating slip-stream rate and may face acetic acid accumulation if the designed slip-stream rate is insufficient2. For such systems, it may be advantageous to operate with a low rich glycol pH feed into the distillation system, as recommended by Soames, et al. 2, to allow simultaneous removal during the regeneration and reclamation processes given sufficient pH rise across the regeneration system. For MEG regeneration loops without reclamation facilities for salt handling, the removal of organic acids through the MRU may be the only option available to prevent long term accumulation besides costly replacement of MEG inventory. If left uncontrolled, accumulation of organic acids may lead to various operational issues including increased corrosion15,21-26, TLC15,21-26 and poor


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interaction with production chemicals such as oxygen scavengers6,51. However, if organic acids are to be removed alongside the produced water in the regeneration system to maximize plant wide removal, the potential corrosion risk to overhead systems due to low pH must also be considered 10.

5.0 Acknowledgments The authors would like to acknowledge the contribution of an Australian Government Research Training Program Scholarship in supporting this research.

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Removal of Organic Acids during Mono-Ethylene Glycol Distillation

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