Amine Thermal Degradation

1. Amine Thermal Degradation By: Jason Davis

2. Overview • Carbamate Polymerization of MEA • Background • Chemistry • Model • PZ and MEA/PZ Blends • Amine Screening

3. Amine Losses • Oxidative Degradation – A. Sexton • Thermal Degradation – degradation occuring at stripper and reclaimer conditions • Carbamate polymerization • Other thermal degradation • Volatility – M. Hilliard • Physical Losses

4. Amine Losses Thermal Degradation Oxidative Degradation Vapor Losses

5. Thermal Degradation • Industry standards currently limit MEA concentration at 30wt% (15wt% being the standard for natural gas treating) due to concern over increased corrosion and thermal degradation • Degradation can lead to ineffective CO2 capture, loss of expensive solvent, increased equipment corrosion, and an increased environmental impact

6. Chemistry + H+ + CO2 MEA MEA Carbamate + H2O + H+ 2-Oxazolidone Polderman Dillon and Steele (1955)

7. Chemistry - Continued + H2O + MEA 1-(2-hydroxyethyl)-2-imidazolidone (HEIA) + H2O + CO2 N-(2-hydroxyethyl)-ethylenediamine (HEEDA)

8. What Do We Know • MEA Carbamate Polymerization Factors • CO2 loading • Temperature • Amine concentration • Literature for MEA • No kinetic data available • Controlled when solutions held at 15 wt% in industrial applications

9. Sample Apparatus • Use high pressure sample containers made of 316L stainless steel tubing and endcaps • Forced convection oven to maintain constant temperature for a large number of samples • Maintains CO2 loading in solution at elevated temperature and pressure to accelerate thermal degradation • Simple experimental design and allows for a large number of solutions to be tested at one time

10. Analytical • GC • High temperatures can alter results • Separation of polar compounds difficult and cross contamination in sample port • HPLC • Amine detection difficult with standard detectors • Can identify and quantify nonionic species • Cation IC • Separates positively charged ions • Will not detect non-ionic species • Can measure amine disappearance and the formation of ionic species (highly polar)

11. MEA Experiments • Matrix of samples • MEA Concentration (15-40wt%) • CO2 Loading (0.2-0.5) • Temperature (100-150oC) • 100oC and 150oC experiments in 2ml sample containers • 120oC and 135oC experiments in 10ml containers

12. 11m MEA after 8 wks at 135oC MEA HEEDA

13. Emperical Data Regression where K is the temperature dependent rate constant given by: • MEAf = final MEA concentration (molality) • MEAo = initial MEA concentration (molality) • = Loading defined as moles CO2 per mole amine t = time (weeks) T = Temperature (K)

14. Effect of Loading (T=135C) a=0.2 a=0.4 a=0.5

15. Effect of Temperature (a=0.4) 100oC 120oC 135oC 150oC

16. Effect of Concentration (T=135oC a=0.4) 7m 3.5m 11m

17. HEEDA Formation 11m MEA at 135oC a=0.2 a=0.4 a=0.5

18. Thermal Degradation Costs • Approximately $2/ton CO2 allocated to solvent make-up in most cost models • Assumes 1.5kg MEA/ton CO2 and a cost of $1.32/kg MEA • 3.5m MEA, P=1atm, $0.10/ton CO2 • 11m MEA, P=2.5 atm, ~$1.60/ton CO2 • Does not include corrosion or reclaimer costs • Natural gas processing experience says reclaimer composes 50% of thermal degradation • Corrosion has been shown to increase in the presence of HEEDA

19. MEA Conclusions • Temperature has the greatest effect on thermal degradation in the stripper • Quadruples every 15oC • Double pressure = 15oC temp increase • Loading increases degradation slightly more than 1st order • Concentration has multiple effects • Slightly more than 1st order in concentration • In practice an increase in concentration yields increased stripper temperatures due to increased BP of solution (3.5m to 11m increases temperature by ~4oC and increases thermal degradation by 40%)

20. MEA/PZ Blended Systems • Made measurements of aqueous PZ and a 7m MEA/2m PZ blend at varying temperatures • PZ not expected to degrade since it does not have an alcohol group to form an oxazolidone intermediate • Unknown what the blended system would do

21. Aqueous PZ after 8 weeks at 150oC PZ These peaks are in the time 0 sample

22. Degraded MEA/PZ after 3 weeks at 135oC MEA Degradation Products PZ

23. Amine Losses after 2 Weeks *All systems have a loading of 0.4 and similar moles of alkilinity

24. PZ Blend Conclusions • PZ with a loading of 0.4 did not degrade at 150oC for over 8 weeks • The blended systems preferentially destroyed PZ, the more expensive solvent • PZ is a stronger nucleophile so it attacks the MEA oxazolidone structure more readily than MEA thereby increasing degradation

25. Other Amines • Set up several screening experiments on other amine systems including • EDA • DETA • MDEA • HEEDA • DGA • AMP • Only measured ionic degradation products

26. Amine Screening(T=135oC a=0.4 t=4wks)

27. MDEA after 4wks at 135oC MDEA

28. HEEDA after 4wks at 135oC HEEDA

29. Amine Screening Conclusions • HEEDA degrades very quickly compared to other amines studied • Industrially MDEA does not significantly degrade but this study shows it does shift to other amines • Arm shifting • Higher activation energy than other amines so increased temperature might effect more • Order from least to most degradation • PZ<DGA< MDEA< AMP<EDA< MEA< DETA< HEEDA

30. Future Work • Mechanistic model for MEA degradation • MEA with spikes of various degradation products to determine k values for reactions • Measure HEIA formation with HPLC for low temp samples to get a more accurate degradation rate • Thermal Degradation modeling in ASPEN • Various stripper configurations • Possible reclaiming simulations as well

31. Summary • Thermal degradation can be important in the overall cost of the MEA absorber/stripper system • Engineering controls can keep these costs reasonable • Further study of the reclaiming system is needed • PZ does not thermally degrade by itself, but does in the presence of alkanolamines • Many common amines do degrade under stripper conditions and this should be considered when choosing a solvent

32. QUESTIONS?