Heat capacity measurements of amine-water solutions used for Post ...

Heat capacity measurements of amine-water solutions used for PostCombustion Carbon dioxide applicationsJohanna KuuselaDepartment of Chemical Engineering at Lund University, Department of Chemical and BiochemicalEngineering at Technical University of Denmark (2010)AbstractGrowing consumption of fossil fuel leads to increased emissions of the greenhouse gas carbon dioxide,which contributes in a major way to the global warming. It is believed that fossil fuels will continue to bethe main important source of energy for the near future. Therefore ways are investigated for removingCO 2 from the flue gases from fossil fuel power plants. Natural gas- and coal fired power plants stands for40 % of world’s anthropogenic CO 2 emissions and consequently efforts are placed on finding ways ofreducing emissions from these plants.One method of cleaning the flue gases is post combustion carbon dioxide capture (PCC) where the gas isscrubbed with a water based solution of amine or other solvent which can react with CO 2 . To correctlybe able to dimension heat exchangers in the PCC process thermodynamic data such as heat capacitiesfor the amine-water solutions are needed. Heat capacity data for several amines are scarce or there canbe a lack of data in literature. The aim of this study was to experimentally investigate heat capacities foramine-water solutions with different amounts of carbon dioxide. The solutions investigated were 10 wt% MEA-, 30 wt % MEA-, 10 wt % AMP- and 30 wt % AMP-water solutions respectively. Differentconcentrations of carbon dioxide gas were injected to each sample to investigate its effect on the heatcapacity of the solutions.IntroductionTo reduce the carbon dioxide emissions to theenvironment from fossil fuel power plants thereare three different methods of capturing carbondioxide: Oxy Fuel combustion capture, precombustion capture and post combustioncapture (1). In the PCC method differentabsorbents, mostly amines, in water solutionare used to scrub carbon dioxide from the fluegas after the combustion process (1). The postcombustion carbon dioxide capture processsimply described consists of an absorptioncolumn, a desorption column and a heatexchanger between the columns. Where CO 2rich solvent from the absorber is heatexchanged against the lean and warmer solventfrom the desorber. The amine solution reactswith CO 2 mainly through two different reactionpaths (2):Reaction path for primary and secondaryamines:→←1

→←→←Reaction path for tertiary amines and stericallyhindered primary and secondary amines:→←→←In the first reaction path two moles of amineare consumed per mole of CO 2 , making it haveless capacity than the second reaction pathwhere only one mole amine is consumed permole of CO 2 . The reaction rate is faster for thefirst reaction path while tertiary amines moreeasily desorb the CO 2 .Amines chosen for this studyFor this study the amines MEA and AMP werechosen to make the heat capacitymeasurements. Heat capacity data for MEAwater solutions with different concentrations ofCO 2 are available in literature (3). MEA wastherefore chosen for the experiments to be ableto compare experimental heat capacity valueswith literature values and by that way evaluatethe reliability of the experimental method used.Investigations have previously been made onthe ability of AMP to react with CO 2 in theabsorption reaction (4). These experimentsshowed a good ability for AMP to be used inPCC applications. Heat capacity data for AMPwatersolutions however, have not been foundin literature and for these two reasons AMP waschosen for the heat capacity experiments madein this study.ChemicalsThe chemicals used in this study are:99% pure Monoethanol amine (MEA)from Sigma-Aldrich99% pure 2-amono -2-methyl-1-propanol (AMP) from Sigma-AldrichBoiled distilled waterExperimental methodTo determine the heat capacities for the aminewatersolutions with different amounts of CO 2 ,a method with micro calorimetry has beenused. The principle of the method is to measureheat flow from the sample to the environment.The micro calorimeter used for theseexperiments is the Bio Activity Monitor 2277BAM (5).A sample from the amine solution(approximately 1.5 g) was put in to a smallstainless steel vessel and lowered in to themicro calorimeter.The carbon dioxide gas was allowed to flow into the sample vessel through a small tube forwhich a port was opened. The tube wasconnected from the small carbon dioxide flaskto the sample vessel inside the calorimeter.A constant heat supply (300 µW) was applied tothe sample inside the calorimeter until constanttemperature was reached in the sample. Thiswas performed at least two times to get somereplicate measurements.An experimental data set of heat flow from thesample as a function of time was obtained.Calculations on the experimental data weremade in Excel. A program with the extendedUNIQUAC (6) model was used to calculate theheat capacity data for the solutions. The results2

given by UNIQUIAC were compared with thecalculated results from the experimental data.Theory for the measurementsWhen a constant heat supply is applied to thesample, some of the heat is stored by thesample while the rest is transferred from thesample to the environment via two thermoelements where the heat flow is detected. Theheat equation can be described according toequation 1.Equation 1Where P(t) supplied is the constant heat flowsupplied to the sample cup, P(t) transferred is theheat not absorbed by the sample and thereforeflowing to the heat sink and P(t) absorbed is theabsorbed heat by the sample.Since and the equationabove can also be expressed according toequation 2 below.Equation 2Equation 3( )Equation 4 below is the solution to equation 3.Equation 4In equation 4 is the current temperatureof the sample, is the temperature of thesample at a stage where a constant uppertemperature during the heat supply has beenreached, is the constant temperature of thesample when no heat is supplied and t is thetime during the experiment. The parameter τ isa curve constant obtained from the plot of heatflowing from the sample to the environmentagainst the time during the experiment.Both sides of equation 4 are subtracted with T 0and multiplied with B transferred to get amathematical function for the powertransferred from the sample to theenvironment.Equation 5Where m and C p are constants in the form ofmass and specific heat capacity of the sample,B transferred is a constant that has to do with thethermal properties of the apparatus and has theunit W/°C. In equation 2, m*C p is set to theconstant C 1 , the constant B transferred is set to C 2and P(t) supplied is set to the constant C 3 .According to this, equation 3 below is obtainedwhich can be solved as a differential equation.In equation 5is the heat flow fromthe sample to the environment and is thehighest value for this heat flow obtained, whichis a constant value when the sample hasreached its maximum heat absorption capacity.In equation 5 above and are theactual values. But measurement is alwaysconnected with a measurement error which hasto be taken in to account. In this case the erroris that the baseline at the lower steady state ofthe power curve should be zero when no poweris flowing to the sample but this was not thecase during the measurements. Instead thelower steady state of the power curve had a3

P(t) µWvalue somewhat above zero. The power at thelower steady state of the power curve and thusthe deviation from zero will be called .Therefore the measured value of the powertransferred to the heat sink and the power atthe upper steady state of the power curve canbe written according to equations 6 and 7below.15001000500073000-50078000 83000 88000Equation 6-1000time (s)Figure 1. Experimental data of heat flow from the sampleas a function of timeEquation 7Equations 6 and 7 expresses that the measuredvalue is equal to the actual value plus ameasurement error.In equation 7,is a mathematicalfunction for the measured heat flow from thesample to the environment and representsthe measured constant heat flow from thesample when the maximum heat absorptioncapacity is reached.Equation 7 was used in Excel to make acalculated curve for the heat flow to becompared with the experimental data for theheat flow.An example of a power curve where theexperimental data of heat flow is plotted as afunction of time can be seen in figure 1 below.In figure 1 to replicate measurements wasmade.Calculations on the experimentaldataThe heat capacity for the sample is obtainedfrom the measurements by recognizing that theτ value for each sample is proportional to theproduct of its mass and heat capacity.To obtain a τ value for the sample, a leastsquares fitting of the experimental data to themathematical function was performed. A sumof squares error was calculated according toequation 8.Equation 8∑By minimizing the sum of squares errorbetween experimental and calculated heatflows, a τ value for equation 7 above wasobtained in Excel.Each specific sample has its own τ value. A heatflow measurement was also made for an emptyvessel to be able to eliminate the vessel’scontribution to the heat capacity. A4

Cp (kJ/kgK)765432100 0,5 1 1,5extendedUNIQUACmodel heatcapacity for30wt% MEAsolution(kJ/kgK)experimentalheat capacityvalues for30wt% MEAsolution(kJ/kgK)Figure 4. Comparison between extended UNIQUAC modelCp values and Cp values from this study for 30 wt% MEAsolutionThe extended UNIQUAC model shows an upperdip in the heat capacity values for 30 wt% MEAsolution as can be seen in figure 4 above, thisoccurs at a loading in the region of 0.1.Moreover the heat capacity values calculatedwith the model are larger than the experimentalvalues. The odd replicate heat capacity value of2.789 at the loading 0.21 for the experimental30 % MEA solution has been left out in figure 4.Table 3. Heat Capacity (kJ/kg K) at 25 °C for CO 2 loaded10wt% and 30 wt % AMP-water solutions from presentstudyLoading 10wt% AMP 30wt% AMP(mol/mol)0 4.440, 3.753, 3.057, 3.142.9030.11 4.021 3.070.14 3.7030.27 3.4600.32 3.0290.42 4.4890.46 3.5280.52 3.9920.53 3.275The three heat capacity values for 10 wt% AMPat the loading 0 in table 3 are measurementsmade for different samples from the samesolution.The results from the present study for 10 and30 wt% AMP solutions are plotted in figures 5and 6 below.54,543,5Table 3 below is a list of the heat capacitymeasurement results for 10 and 30 wt % AMPat different CO 2 -loadings.32,520 0,2 0,4 0,6loading (mol CO2/mol AMP)Figure 5. Experimental Cp-values for 10% AMP solutionfrom present study7

Cp (kJ/kgK)3,83,63,43,232,82,62,42,220 0,1 0,2 0,3 0,4 0,5loading (mol CO2/mol AMP)Figure 6. Experimental Cp-values for 30% AMP solutionfrom present studyAs can be seen in figures 5 and 6 above, bothAMP solutions have a minimum for heatcapacity roughly at the loading of 0.3. In the 10wt% solution the minimum value is situatedsomewhat lower and in the 30 wt% solution it isat a slightly higher loading than 0.3. Figure 5also shows a large variation in the threereplicates at the loading 0. The replicate valuesin figure 5 span over the whole plotted area inthe graph.The expected result for all the measurementsmade in this study was that the heat capacitywould decrease with increasing carbon dioxideloading. A simple reason for the expected resultis that the mass of the sample gets larger withincreasing CO 2 concentration.Why decreasing heat capacity with increasingCO 2 concentration is not the observed result inall the measurements could have many reasons.In principal the fluctuating measurement resultscould depend on the apparatus used or that thesolutions in fact differed in composition.A replicate measurement was made for 30 %MEA solution at a CO 2 loading of 0.21 a dayafter the first measurement on the exact samesample (table 2 and figure 2).If it is assumed that the bio activity monitor hadthe same performance, this could mean thatequilibrium in the solution was in fact notestablished before the first measurement. Thecarbon dioxide might have needed a longertime to get absorbed by the solutionIn the measurements which involved carbondioxide injection in to the sample an errorsource could also be the accuracy with whichthe CO 2 content in the sample was measured(CO 2 gas flask was weighed before and after theinjection).For the 10 % AMP solution three measurementswere made with zero CO 2 in the sample (table 3figure 5). These measurements were made fordifferent samples taken from the sameprepared solution. Since the amine is soluble inwater it is more questionable if these differingresults depends on not reaching equilibrium.Furthermore each measurement result for heatcapacity shown above is a mean valuecalculated from at least two replicatemeasurements. The replicate measurementswere made for the same sample approximatelyone hour apart.From all the replicate measurements made itcould be seen that some of the measurementsdeviated a great amount from each otherdespite the short time difference.These results could in fact question theperformance of the apparatus used.The bioactivity monitor was originally designedto measure heat flow from living systems likecells and living organisms. This makes it8

extremely sensitive (it can detect heat flowsdown to 0.1 µW).The sensitivity of the bio activity monitor BAM2277 could also be a reason for differingmeasurement results if it was for exampleexposed to external vibrations or other kind ofdisturbance during the measurement.ConclusionsFor many reasons which were discussed in thesection above, more experiments would havebeen needed to be able to evaluate thereliability of the measurement results of heatcapacity for the different amine solutions.The method used could be improved by moreaccurately measuring the concentrations ofdifferent components in the sample solution.For example the concentration of CO 2 could bedetermined by titrating instead of weighing.It is necessary to be certain that the solutionhas reached equilibrium before starting themeasurement.In summary, further investigation of the aminesolution heat capacities would be interestingand necessary in the future.AcknowledgmentsThank you Ingemar Odenbrand at the department ofChemical Engineering at Lund University for helpfulsupervision and encouragement during this work.Thank you Kaj Thomsen , Philip Loldrup Fosbøl andErling H. Stenby at the department of Chemical andBiochemical Engineering at Technical University ofDenmark for help and guidance during this study.Thank you Zacarias Tecle for technical instructionsconcerning the Bio Activity Monitor and guidance inthe laboratoryThank you Karin Petersen for safety instructions inthe laboratory.Thank you Willy wan Well at Dong Energy forcontacting me with the people at the department ofChemical and Biochemical Engineering at TechnicalUniversity of Denmark.Finally I thank all you people at the department ofChemical and Biochemical Engineering at TechnicalUniversity of Denmark for making me feel welcome.References(1) Jochen Oexmann, Christian Hensel, AlfonsKather, 2008, "Post combustion CO2 capture fromcoal fired power plants: Preliminary evaluation of anintegrated chemical absorption process withpiperazine- promoted potassium carbonate" inInternational journal of greenhouse gas control, 2,541-544(2) Graeme Puxty, Robert Rowland, 2009, “CarbonDioxide Postcombustion Capture: A Novel ScreeningStudy of the Carbon Dioxide AbsorptionPerformance of 76 Amines”, CSIRO EnergyTechnology, CSIRO Molecular and healthtechnologies, The University of Newcastle, 43, 6427-6433(3) Ralph h. Weiland, John C. Dingman, 1997, “HeatCapacity of Aqueous Monoethanolamine,Diethanolamine, N-Methyldiethanolamine, and N-Methyldiethanolamine Based Blends with CarbonDioxide”, Chem. Eng. Data, 42, 1004-1006(4) Mohammad R.M. Abu-Zahra, Paul H.M. Feron,2009, “New process concepts for CO 2 postcombustioncapture process integrated with coproductionof hydrogen”, International journal ofhydrogen energy, 34, 3992-4004(5) Instruction manual for Bio Activity MonitorBAM 2277(6) Leila Faramarzi, Georgios M. Kontogeorgis, KajThomsen, Erling H. Stenby, 2009, “ExtendedUNIQUAC model for thermodynamic modeling ofCO 2 absorption in aqueous alkanolamine solutions”,Fluid Phase Equilibria, 282, 121-1329