Amine Thermal Degradation

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Amine Thermal Degradation

• By Jason Davis

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Overview

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

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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

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Amine Losses
Thermal Degradation
Oxidative Degradation
Vapor Losses
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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

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Chemistry
H
CO2
MEA Carbamate
MEA
H2O
H
2-Oxazolidone
Polderman Dillon and Steele (1955)
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Chemistry - Continued
H2O
MEA
1-(2-hydroxyethyl)-2-imidazolidone (HEIA)
H2O
CO2
N-(2-hydroxyethyl)-ethylenediamine (HEEDA)
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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

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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

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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)

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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

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11m MEA after 8 wks at 135oC
MEA
HEEDA
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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)

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Effect of Loading (T135C)
a0.2
a0.4
a0.5
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Effect of Temperature (a0.4)
100oC
120oC
135oC
150oC
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Effect of Concentration (T135oC a0.4)
7m
3.5m
11m
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HEEDA Formation 11m MEA at 135oC
a0.2
a0.4
a0.5
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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, P1atm, 0.10/ton CO2
• 11m MEA, P2.5 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

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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)

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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

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Aqueous PZ after 8 weeks at 150oC
PZ
These peaks are in the time 0 sample
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Degraded MEA/PZ after 3 weeks at 135oC
MEA
Degradation Products
PZ
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Amine Losses after 2 Weeks
Solvent Temp (oC) MEA Loss () PZ Loss () Total Amine Loss ()
Pure PZ 120 - lt2.0 lt2.0
Pure MEA 120 4.0 - 4.0
MEA/PZ Blend 120 5.0 8.6 6.3
Pure PZ 135 - lt2.0 lt2.0
Pure MEA 135 18.1 - 18.1
MEA/PZ Blend 135 11.9 31.8 19.1
All systems have a loading of 0.4 and similar
moles of alkilinity
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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

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Other Amines

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

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Amine Screening(T135oC a0.4 t4wks)
Amine Concentration (molality) Remaining Amine Peak () Total Area Retention ()
PZ 3.5 100 100
DGA 7 93 98
MDEA 50 wt 71 97
AMP 3 97 96
EDA 3.5 64 91
MEA 7 76 80
DETA 2.3 9 71
HEEDA 3.5 3 17
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MDEA after 4wks at 135oC
MDEA
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HEEDA after 4wks at 135oC
HEEDA
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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
• PZltDGAlt MDEAlt AMPltEDAlt MEAlt DETAlt HEEDA

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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

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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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QUESTIONS?