Amine Thermal Degradation By: Jason Davis

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

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

Amine Losses Vapor Losses Oxidative Degradation 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 CO 2 capture, loss of expensive solvent, increased equipment corrosion, and an increased environmental impact

Chemistry + CO 2 2-Oxazolidone + H 2 O MEA Carbamate MEA + H + Polderman Dillon and Steele (1955)

Chemistry - Continued + H 2 O 1-(2-hydroxyethyl)-2-imidazolidone(HEIA) + CO 2 N-(2-hydroxyethyl)-ethylenediamine(HEEDA) + MEA

What Do We Know MEA Carbamate Polymerization Factors CO 2 loading Temperature Amine concentration Literature for MEA No kinetic data available Controlled when solutions held at 15 wt% in industrial applications

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 CO 2 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

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)

MEA Experiments Matrix of samples MEA Concentration (15-40wt%) CO 2 Loading ( ) Temperature ( o C) 100 o C and 150 o C experiments in 2ml sample containers 120 o C and 135 o C experiments in 10ml containers

11m MEA after 8 wks at 135 o C MEA HEEDA

Emperical Data Regression where K is the temperature dependent rate constant given by: MEA f = final MEA concentration (molality) MEA o = initial MEA concentration (molality)  = Loading defined as moles CO 2 per mole amine t = time (weeks) T = Temperature (K)

Effect of Loading (T=135C)  =0.2  =0.4  =0.5

Effect of Temperature (  =0.4) 150 o C 135 o C 120 o C 100 o C

Effect of Concentration (T=135 o C  =0.4) 11m 7m3.5m

HEEDA Formation 11m MEA at 135 o C  =0.2  =0.4  =0.5

Thermal Degradation Costs Approximately $2/ton CO 2 allocated to solvent make-up in most cost models Assumes 1.5kg MEA/ton CO 2 and a cost of $1.32/kg MEA 3.5m MEA, P=1atm, $0.10/ton CO 2 11m MEA, P=2.5 atm, ~$1.60/ton CO 2 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

MEA Conclusions Temperature has the greatest effect on thermal degradation in the stripper Quadruples every 15 o C Double pressure = 15 o C temp increase Loading increases degradation slightly more than 1 st order Concentration has multiple effects Slightly more than 1 st 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 ~4 o C and increases thermal degradation by 40%)

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

Aqueous PZ after 8 weeks at 150 o C PZ These peaks are in the time 0 sample

Degraded MEA/PZ after 3 weeks at 135 o C MEA PZ Degradation Products

Amine Losses after 2 Weeks SolventTemp ( o C) MEA Loss (%) PZ Loss (%) Total Amine Loss (%) Pure PZ120-<2.0 Pure MEA MEA/PZ Blend Pure PZ135-<2.0 Pure MEA MEA/PZ Blend *All systems have a loading of 0.4 and similar moles of alkilinity

PZ Blend Conclusions PZ with a loading of 0.4 did not degrade at 150 o C 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

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

Amine Screening (T=135 o C  =0.4 t=4wks) AmineConcentration (molality) Remaining Amine Peak (%) Total Area Retention (%) PZ DGA79398 MDEA50 wt%7197 AMP39796 EDA MEA77680 DETA HEEDA3.5317

MDEA after 4wks at 135 o C MDEA

HEEDA after 4wks at 135 o C HEEDA

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

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

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

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