Thermal degradation of diethanolamine at stripper condition for CO2capture:Product types and reaction mechanisms☆

Idris Mohamed Saeed *,Brahim Si Ali,Badrul Mohamed Jan, *,Wan Jefrey BasirunShaukat Ali Mazari,Ibrahim Ali Obid Birima

1 Department of Chemistry,Faculty of Science,University of Malaya,Kuala Lumpur 50603,Malaysia

2 Department of Chemical Engineering,Faculty of Engineering,University of Malaya,Kuala Lumpur 50603,Malaysia

3 Department of Chemical Engineering,Faculty of Engineering,Dawood University of Engineering and Technology,MA Jinnah Road,Karachi 74800,Pakistan

Keywords:Diethanolamine CO2capture Degradation Mechanism

ABSTRACT Amine-based absorption/stripping is one of the promising technology for CO2capture from natural and industrial gas streams.During the process,amines and CO2undergo irreversible reactions to produce undesired compounds,which cause corrosion,foaming,increased viscosity and breakdown of equipment,ultimately contributing to the economic loss and environmental pollution.In this study,the thermal degradation of aqueous diethanolamine in the presence and absence of dissolved CO2was investigated.The experiments were performed in stainless steel cylinders.The results show that thermal degradation in the absence of CO2was a slow process;triethanolamine,and tris(2-aminoethyl)amine were only the degradation products identified in the mixture In addition,the rate of degradation was very low,only 3%degradation was observed after 4 weeks.But in the presence of CO2,sixteen degradation products were identified,nine of which were new degradation products reported for the first time in this study.The 3-(2-hydroxyethyl)-2-oxazolidinone,1,4-bis(2-hydroxyethyl)piperazine and triethanolamine were the most abundant degradation products.The remaining DEA concentration after 4 weeks was about 20%of the total amine concentration.The most probable degradation reactions and their mechanisms are also proposed.

1.Introduction

The rise in sea levels,melting of glaciers,warming of the ocean surface and natural disasters are the key indicators for global climate change.The fifth assessment report by the International Panel for Climate Change(IPCC)states that from 1880 to 2012,the average global temperature increase was 1.53°F(0.85°C)[1].Because of this,an effective strategy must be outlined to decrease the emission of greenhouse gases such as carbon dioxide(CO2)into the atmosphere.In addition,excess CO2in the earth's atmosphere must be captured and recycled into value added products.A wide range of technologies are available for the separation and capture of CO2from gas streams.Among these,pre-combustion,post-combustion and oxy-fuel combustion are the most established methods.The post-combustion method is based on an amine scrubbing process,which is widely used for the removal of acidic gases.This technique employs the absorption of acidic gases with different aqueous amine solvents[2-5].

Amines release irreversible degradation products during CO2absorption[6].The degradation of amines introduces difficulties which are associated with corrosion and environmental concerns,increasing the operation cost thus causing economic burden[7-10].In particular,amine degradation occurs at high temperature or through oxidative degradation.Thermal degradation occurs at high temperature,while oxidative degradation is induced in the presence of metal ions,CO2and oxygen in the system[10].The investigation of amine degradation products and their formation mechanisms are of a great importance for the correct selection and formulation of amines for the CO2capture process[11].

Diethanolamine(DEA)is one of the amines used in gas treatment process[11].DEA is suitable for the CO2capture process because it is less corrosive and more stable than most of the commercialized amines such as monoethanolamine(MEA)[12-14].The degradation of DEA has been studied since 1956[15],for both oxidative degradation[16-18]and thermal degradation[15,18-22].In early studies,Polderman and Steele[15]found that the loss in DEA concentration was negligible at 100 °C.However with the increase in temperature to 175 °C,a noticeable decrease in the DEA concentration of 97%was observed after 8 h.The 1,4-bis(2-hydroxyethyl)piperazine(BHEP)was identified as the degradation product,however no other degradation products were detected due to the lack of analytical instruments.Hakka et al.[22]investigated the thermal degradation of DEA and N,N,N′-tris(2-hydroxyethyl)ethylenediamine(THEED)was identified as the new degradation product.Kennard[19]studied the kinetics of DEA degradation in a 600 ml autoclave reactor.The initial concentration of DEA was varied between 5% and 100% while the temperature between 90°C and 250°C and pressure between 413.7 kPa and 6895 kPa.In the study,a number of degradation products were detected such as MEA,N-(2-hydroxyethyl)oxazolidin-2-one(HEOD),triethanolamine(TEA),N,N,N′,N′-tetrakis(2-hydroxyethyl)urea(TEHEU),N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine(TEHEED),BHEP and THEED.In another study,Hsu and Kim[23]investigated the thermal degradation of DEA using a stainless steel bomb at 140°C from 6 d to 16 d.A number of degradation products were identified and were in agreement with the literature such as BHEP,THEED and HEOD,while N-2-[bis(2-hydroxyethyl)-amino]ethyl-oxazolidin-2-one(HAO),N-2-[bis(2-hydroxyethyl)-amino]ethyl-N′-(2-hydroxyethyl)piperazine(HAP),N,N,N′,N′-tetrakis(2-hydroxyethyl)diethylenetriamine(THEDT)were reported as the new products.Lepaumier[18]investigated the thermal degradation of 4 mol·L-1DEA in the presence and absence of CO2at 140°C for 15 d.The results showed that the thermal degradation of DEA in the absence of CO2was relatively lower than its degradation in the presence of CO2.MEA,HEOD,BHEP,THEED were identified as the degradation products which is in agreement with the literature,while N-(2-hydroxyethyl)-N-(2-(2-hydroxyethylamino)ethyl)piperazine(HEAEHEP)and N-2-[bis(2-hydroxyethyl)-amino]ethylpiperazine(HEAEP)were the new degradation products.In a recent study,Islam et al.[24]investigated the degradation of DEA and identified the ionic degradation products in the absorber and striper conditions at 55 °C and 100 °C,using a 1.5 L double jacket reactor with concentrations between 2 and 4 mol·L-1.They also identified a number of ionic degradation products.

The chemistry and formation pathways of the degradation products are still unclear and require further investigation.It is important to understand the energy efficiency of the reaction in order to produce a viable absorption and stripping process.The energy penalty is burdened by the steam requirement and the inefficient recycling of the lean amine is due to the degradation of the amine.The decrease of amine degradation is possible by a clear understanding of the degradation mechanisms[25,26].In this paper,the thermal degradation of 30 wt%DEA was investigated in the presence and absence of CO2.The stability of the parent amine,the identification of degradation products and the reaction pathways are proposed.

2.Materials and method

2.1.Materials

Diethanolamine(DEA)(≥98% purity),3-(2-hydroxyethyl)-2-oxazolidinone(HEOD)(99%purity),1,4-bis(2-hydroxyethyl)piperazine(BHEP)(99% purity),triethanolamine(TEA)(≥99% purity),tris(2-aminoethyl)amine(TAA)(96% purity)were purchased from Merck(Malaysia).Barium chloride(BaCl2)and standard solutions of sodium hydroxide(NaOH),hydrochloric acid(HCl)and sulfuric acid(H2SO4)were also purchased from Merck(Malaysia).Carbon dioxide(CO2)gas(99.9%purity)and nitrogen(N2)gas(≥99.99%purity)were supplied by Linde(Malaysia Sdn.Bhd.).All chemicals were used as received without further purification.

2.2.Sample preparation and CO2loading experiments

DEA samples(wt%)were prepared in deionized water as the solvent.The CO2loading method used in these experiments was similar to our previous work[27,28].A magnetic stirrer and pH meter connected to a data acquisition system were used in the reactor.The variation of the solution pH with time was monitored by a pH probe from Metrohm(Malaysia).A water circulator was used to control the reaction temperature of the reactor by a double jacket cell(closed system).Fig.1 is a schematic diagram of the experimental setup.The reaction was initiated by introducing 100 ml 30 wt%of aqueous DEA into the double jacket reactor.The solution was purged with nitrogen gas for few minutes to displace the dissolved oxygen.After degassing the solution,CO2gas was introduced into the solution until it became saturated and the final pH was 7.Then the CO2saturated solution was poured into the cylinders and the CO2loading was determined by titration[29].In the CO2determination,two samples(0.5 g)were extracted from the reactor and transferred into a mixture of 50 ml NaOH(0.1 mol·L-1)and 25 ml BaCl2(0.5 mol·L-1)in a 250 ml Erlenmeyer flask.The CO2reaction in the BaCl2and NaOH mixture forms white precipitates of BaCO3.The samples were then heated,cooled and filtered through a 0.45 μm pore size silicon filter paper.The white precipitates were washed with 50 ml deionized water,followed by the addition of 0.1 mol·L-1HCl until the precipitates were completely dissolved.The acidified solutions were then titrated with 0.1 mol·L-1NaOH,which were performed using a 785 DMP Titrino auto-titrator installed with Tiamo 1.3-45.

Fig.1.Schematic diagram of the CO2loading experimental setup[28].

2.3.Thermal degradation experiments

The thermal degradation experiments were conducted using Swagelok 316 stainless steel cylinders(5 in.length and half inch diameter)equipped with end-caps.This method was introduced by Davis[25]who studied the thermal degradation of amines under stripper conditions.About 8 ml of CO2loaded/unloaded amine solutions was introduced into each cylinder.The cylinders were then placed in a Memmert 600 oven and heated to 135°C.Any leakage was checked by calculating the weight difference before and after the experiments.The cylinders were periodically removed from the oven;(once per week)for 4 weeks,cooled and transferred into glass vials for further analyses.The samples were kept in a refrigerator at 5°C to quench the reaction.

2.4.Analytical methods

2.4.1.GC-MS analysis

The Gas Chromatography-Mass Spectrometry(GC-MS)instrument was from Shimadzu(Japan).The analyses were performed on a Gas Chromatograph QP2010,coupled with Autosampler AOC 20I+S and mass spectrometer.The method used was similar to our previous work[28].Briefly,the separation of amine degradation products was performed on an RTX-5MS column and described in Table 1.The injector was on split mode to avoid contamination of the system and to increase the sensitivity.The identification of degradation products was performed by matching the mass spectra of the degradation products with the National Institute of Standards and Technology(NIST)database(1998).The samples were diluted in methanol at a 1:50 ratio to avoid column contamination and to improve the sensitivity.The samples were analyzed twice to check the reproducibility.Different commercial standards were obtained for the quantification of the initial amine and the main degradation products.The quantification was performed by comparing the calculated peak area of the analyte in the sample with the peak area of the standard of a known concentration.The concentration of minor degradation products was determined from the area under the curve in the GC-MS peaks,as standards were unavailable.

Table 1 Gas Chromatography-Mass Spectrometry operating conditions[28]

2.4.2.LC-QTOF-MS analysis

Liquid chromatography Quadruple Time of Flight-Mass Spectrometry(LC-QTOF-MS)was utilized to identify the degradation products in both techniques.The analysis of degradation products was performed using Agilent 1260 Infinity Liquid(LC)Chromatography coupled with a 6224 Time-of-Flight(TOF)Mass Spectrometer(MS).In LC-QTOF-MS,the molecules are converted into ions by the electrospray ionization(ESI)source.The column used for the component separation was Zorbax Eclipse Plus(2.1 μm×100 mm).The injection volume was 5 μl while the eluent was(0.10 wt%)formic acid in water(1)and methanol(2).A gradient profile of eluent concentration was set at a ratio of 49:1 for(1)and(2)for the first 6.0 min and changed to 1:4 for the subsequent 2 min.In addition,the ratio of(1)and(2)was set again to 49:1 from 8 to 14 min.The flow rate was set at 0.2 ml·min-1for the whole span of the analysis which was described by previous reports[28,30].

3.Results and Discussions

The loss of amine and formation of degradation products were investigated in the presence of CO2with 0.5 mol CO2per mole of alkalinity and in the absence of CO2at 135°C.The temperature in this study was close to the stripper temperature value because the thermal degradation of amine occurs at the stripper temperature.Another reason to use higher temperature was to shorten the degradation time,as amine degradation is a slow process at lower temperatures.

3.1.Identification of degradation products

In the thermal degradation of aqueous unloaded DEA,triethanolamine and tris-2-aminoethylamine were identified as the main degradation products.However,in the presence of CO2,a total of sixteen degradation products were identified.The main degradation products were 3-(2-hydroxyethyl)-2-oxazolidinone,1,4-bis(2-hydroxyethyl)-piperazine(BHEP),tri-ethanolamine(TEA)and N,N,N,N-tetrakis(2-hydroxyethyl)ethylene-diamine.As described in our previous work[31],volatile compounds such as ethylene oxide(EO)and ammonia could have been formed under ambient temperature,but these volatile products were released when the stainless steel cylinders were opened,it either evaporated or reacted with other compounds.In addition,monoethanolamine(MEA),N-(hydroxyethyl)ethyleneimine(HEM)which is one of the thermal degradation products identified in the literature was not found in this study[15,38].It was suspected that these degradation products were present in the solution.However,the identification of these products was not possible due to the limitations of our analytical techniques.A number of researchers also found that there are possible oxidative reactions which could generate heat stable amine salts(HSAS)such as formic acid and formaldehyde,which are considered as oxidative degradation products at high temperatures[32,33].This could be due to the presence of headspace oxygen[33]or from the reduction of CO2-containing molecule during high temperature degradation[32].Table 2 provides a list of the thermal degradation products identified in this study and compared with the literature results from other workers.

3.2.Thermal degradation of DEA-H2O system

The thermal degradation in the absence of CO2was performed to investigate the effect of high temperature at 135°C.The thermal degradation experiments were performed by heating 30 wt%aqueous DEA at 135°C over 4 weeks.GC-MS was used to quantify the percentage of amine loss and concentration of the degradation products.The results showed that DEA was very stable in the absence of dissolved CO2.In addition,the rate of degradation was very low,where only 3%degradation occurred after 4 weeks of heating.On the other hand,the quantity of triethanolamine(TEA)and tris(2-aminoethyl)amine(TAA)was also very low.Table 3 presents the percentage loss of DEA and the formation of DEA-H2O degradation products after four weeks.

3.3.Thermal degradation in DEA-H2O-CO2

The thermal degradation of 30 wt%DEA with CO2loading(0.5 mol CO2·mol-1amine)at 135°C was also investigated.The results showed that DEA was very unstable under these conditions.This means that the presence of dissolved CO2catalyzed the thermal degradation ofDEA.The concentration profiles suggest that the major degradation products in the mixture are i.e.3-(2-hydroxyethyl)-2-oxazolidinone(HEOD),1,4-bis(2-hydroxyethyl)piperazine(BHEP)and triethanolamine(TEA).The minor degradation products were identified as(TEHEED)N,N,N,N-tetrakis(2-hydroxyethyl)ethylenediamine,HEEP1-(2-(2-Hydroxyethoxy)ethyl)piperazine,MEAD,MPE and BHEAP as shown in Table 4.The concentration of HEOD increases initially before decreasing at longer reaction times.This suggests that HEOD is an intermediate compound which reacts further before forming the final product.Furthermore,it was noticed that the TEA concentration decreased during the experiment.This suggests that TEA was a key intermediate product which degraded further.The concentration of BHEP increased with time which indicated that it was a stable product and accumulated in the solution.Other minor degradation products such as TEHEED,HEEP,MEAD,MPE and BHEP did not contribute much to the loss of initial concentration of DEA.In addition,they have only a minor role in the formation of main degradation products and were absent in some samples due to the formation and deformation of products with time.

Table 2 Identified degradation products by GC-MS and LC-QTOF-MS in CO2loaded and unloaded DEA at 135°C

Table 3 Percentages of amine degradation and product formation in DEA-H2O at 135°C for four weeks

3.4.Possible pathways of formation of identified degradation products in DEA

An overall set of mechanisms is proposed,which could explain the formation of thermal degradation products.The objective was to uncover the types of reactions which took place during the thermal degradation and the formation of a particular degradation product.The possible reaction pathways for DEA degradation have been discussed by different researchers[18,19,23].The mechanisms proposed in this study are based on the identified degradation products and are in accordance with the general reaction pathways reported in the literature.

3.5.Triethanolamine

Triethanolamine is one of the main degradation products formed whether in the presence and absence of CO2,and it could be formed through various pathways.A possible reaction pathway was described in our previous work[31],which is between the reaction of DEA with ethylene oxide.Ethylene oxide is a highly volatile thermal degradation product of DEA,and has a higher selectivity towards TEA than MEA and DEA[34].At higher temperature in an aqueous alkaline environment,other reaction pathways could occur:DEA acts as a base and reacts with H2O to form a protonated DEAH+.Another DEA molecule attacks DEAH+to generate MEA and triethanolamine(TEA).In addition,at higher temperatures,the conversion of TEA occurs by hydrolysis to form tris(2-aminoethyl)amine in the presence of ammonia[35].Fig.2 presents the proposed mechanism of formation of TEA and TAA.

3.6.3-(2-Hydroxyethyl)-2-oxazolidinone(HEOD)

The reaction of secondary amines with CO2produces DEAcarbamates.This is followed by the protonation of DEA-carbamate to produce DEA-carbamic acid.The intermolecular cyclization of DEA-carbamic acid leads to the formation of HEOD.These reaction mechanisms were proposed by Kennard and Lepaumier[18,36].Fig.3 presents the reaction mechanism for the formation of HEOD.

Table 4 Percentages of amine degradation and product formation in DEA-CO2-H2O 135°C for four weeks

Fig.2.Proposed formation mechanism of TEA and TAA.

Fig.3.Reaction mechanisms for the formation of HEOD Kennard and Lepaumier[18,36].

Fig.4.Reaction mechanisms for the formation of THEED,TEHEED and BHEP Kennard and Meisen[36].

Fig.5.Proposed formation mechanisms of THEED and BHEP Kennard and Meisen[36].

Fig.6.Proposed formation mechanisms of TEHEED and BHEP Kennard and Meisen[36].

3.7.N,N,N′-Tris(2-hydroxyethyl)ethylenediamine(THEED)and N,N,N′,N″-tetrakis(2-hydroxyethyl)ethylenediamine(TEHEED)and 1,4-bis(2-hydroxyethyl)piperazine(BHEP)

Kennard[19]described the formation of THEED in three different ways.Firstly,the reaction of two DEA carbamates produces the carbamated-THEED,which further releases bicarbonate and proton to form THEED(Fig.4).Secondly,two molecules of DEA could also form THEED through the hydrolysis of water molecules when heated(Fig.5).The third possible pathway for the thermal degradation of DEA to form THEED is through the formation of N-(hydroxyethyl)ethyleneimine(HEM)from DEA by the release of a water molecule,(the formation pathway of HEM was reported previously)[31],which then forms THEED by the reaction with another DEA molecule(Fig.6).

Kennard and Chakma[36,37]postulated that BHEP could be formed by the dehydration of THEED to produce BHEP by intermolecular cyclization.The CO2initiated carbamation of THEED to produce BHEP is outlined in Fig.4.

Fig.7.Proposed formation mechanisms of THEED and TEHEED Kennard and Meisen[36].

Fig.8.Proposed formation mechanisms of THEED,TEHEED and BHEP.

In this work,we proposed an alternate pathway for the formation of TEHEED and THEED which is presented in Fig.7.The reaction begins by the dehydration of TEA-H+to release water molecules,followed by the attack of MEA to form THEED.The THEED further reacts with another MEA to form TEHEED molecule,which releases ammonia from MEA.

3.8.2-Morpholinoethanol

The proposed pathway for the formation of 2-morpholinoethanol(ME)is shown in Fig.8.The reaction of DEA with MEA occurs through the dehydration of water molecule to form 2-[2-(2-Amino-ethoxy)-ethylamino]-ethanol(AEEE),which further reacts to form ME through an internal cyclization.However,MEA was not identified as one of the degradation products in this study although it was reported in earlier works[18,38].

3.9.Cyclic piperazine derivatives

A number of piperazine ring derivatives were also identified.The formation of ring derivatives could be due to the internal cyclization of tris(2-aminoethyl)amine(TAA),releasing ammonia to form the aminoethylpiperazine(AEP)group.The substitution of ammonia in AEP by alcohol produces the(2-Hydroxyethyl)piperazine(HEP),which further reacts with ethylene oxide(EO)to produce 1-(2-(2-Hydroxyethyl)ethyl)piperazine(HEEP).The alkyl addition to piperazine occurs by the attack of formaldehyde and formic acid through the Eschweiler-Clarke reaction.The similarity to this reaction was described in our previous work[28].Briefly,the reaction begins by the methylation of amine with protonated formaldehyde to form an iminium ion intermediate.The formation of methylated ammonium ion occurs by a further reaction with formic acid.The deprotonation of the ammonium ion releases CO2as the by-product and affords the final methylated amine product.These reaction steps are repeated twice to produce the final tertiary amine 2-(4-Methyl-1-piperazinyl)ethanol(MPE).Fig.9 presents the proposed mechanism of formation of the cyclic piperazine derivatives.

Fig.9.Proposed formation mechanisms of MPE and AEP.

4.Conclusions

In this study,the thermal degradation of DEA was investigated at 135°C for four weeks in the presence and absence of dissolved CO2.In the absence of CO2,only two degradation products in small amounts were identified.However in the presence of CO2,numerous degradation products of DEA were formed in larger amounts.The TEA,BHEP and HEOD are the most abundant degradation products formed in the DEA reaction.BHEP was found as a stable degradation product,while HEOD and TEA were intermediates for the formation of other degradation products.Meanwhile CO2loading affected the loss of DEA as it catalyzed in the degradation reaction.It is anticipated that the greater loss of DEA is due to the formation of HEOD,which further reacts to form other degradation products.