Role of Metal Surface Catalysis in the Thermolysis ... - ACS Publications

Nov 23, 2014 - D. H. Moed , S. Weerakul , D. H. Lister , N. Leaukosol , L. C. Rietveld , and A. R. D. Verliefde. Industrial & Engineering Chemistry Re...
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Role of Metal Surface Catalysis in the Thermolysis of Morpholine and Ethanolamine under Superheater Conditions David H. Moed,*,† Arne R. D. Verliefde,‡ and Luuk C. Rietveld† †

Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands Particle and Interfacial Technology Group, Ghent University, Coupure Links 653, 9000 Gent, Belgium

‡

S Supporting Information *

ABSTRACT: Amines like ethanolamine and morpholine have great potential for protecting steam−water cycles against corrosion, but their thermal stability is limited and acidic decomposition products are a concern because of increased corrosion risk. In this study, metal catalysis of amine thermolysis caused by oxides on the inner surface of superheater tubes has been investigated by using a flow reactor and metal tubes of varying sizes and elemental composition. The kinetics of morpholine and ethanolamine thermolysis decreased as the tube size increased. The relationship between the S:V ratio and the degradation rate constant k was linear. Heterogeneous thermolysis accounted for 82−92% of the value of the degradation rate constant k at an S:V ratio of 4.65 mm−1. This decreased to only 6−17% at an S:V ratio of 0.4 mm−1. Although the results varied between the two applied materials, there is no consistent trend that can link thermolysis kinetics to the tube wall composition. Organic acid anion production was weakly related to the amine structure, temperature, and tube diameter but strongly related to the metal oxide composition, with formate and acetate altering as the dominant organic acid anion. A distinction between homogeneous and heterogeneous thermolysis has been made, but the results indicate that a laboratory study with a large enough tube diameter will lead to more reliable predictions.

1. INTRODUCTION Industrial size boilers heat water to high temperatures to produce steam to drive high-, intermediate-, and low-pressure turbines. The demineralized water used in these systems is typically chemically conditioned to raise the pH of the water and reduce failures due to corrosion. One of the commonly applied chemical treatment methods is all-volatile treatment (AVT) which uses ammonia to raise the pH of both the boiler (feed) water and steam or condensate. Although AVT is commonly applied for steam−water cycles (SWCs), it requires high water purity1 and corrodes any copper that might be present in an SWC. Also, because ammonia is more volatile than water, it offers limited protection to corrosion in areas where flashing occurs or the first droplets of condensate are formed.2,3 Alkalizing amines like morpholine (MOR) and ethanolamine (ETA) offer a less volatile alternative to ammonia,4 making the SWC more resilient to corrosion.5−7 Amines have been applied in nuclear power plants for decades, and some guidelines have been made available by research institutes.3 Their use in fossil-fired plants is under debate, due to the higher operating temperatures and pressures, because amines are known to have limited thermal stability and may decompose to form organic acid anions, which can lower the pH and therefore increase corrosion risk.2,8 Some experimental data for this thermal degradation of amines are available, but most data are only focusing on conditions found in nuclear plants, and the data in fossil-fired plants are more limited.9−12 Mori et al.13 tried to simulate the thermolysis of six amines at fossil-fired plant conditions at higher temperatures with a boiler−superheater test loop. They considered the influence of the superheater on thermolysis by varying the superheater exhaust temperature. They concluded © 2014 American Chemical Society

that MOR breakdown in the boiler was undetectable and started only from 500 °C onward in the steam phase, while ETA started degrading already at a steam exhaust temperature of 450 °C. The retention time was not mentioned, making any calculation of the kinetics impossible. The importance of the superheater in the applicability of alkalizing amines in fossilfired plants was also pointed out by Bull14 in a report detailing the results from a fossil-fired plant survey on the use of amines. In another study, MOR was dosed (9 ppm) to investigate the influence of the pressure, temperature, and retention time on the amine degradation kinetics in a flow reactor, mimicking a superheater.15 MOR degradation kinetics were found to be first-order, increasing with the temperature and decreasing with the pressure. The tubes used in the flow reactor, however, were only 1.5 mm in diameter, making the volume-to-surface (S:V) ratio much bigger than that in a full-scale fossil-fired plant, of which the tube diameter is at least 10 times larger. Degradation studies in a similar flow reactor revealed the importance of the S:V ratio during ammonia oxidation.16 This study assumes a linear relationship between the S:V ratio and k but bases this finding on measurements with just two tube diameters. Except for the S:V ratio, the composition of the tube could have an influence as well, as is shown in metal oxide catalysis studies.17 In this study, a more thorough investigation of amine degradation under fossil-fired plant conditions was carried out. MOR and ETA were selected to investigate the influence of the temperature, amine structure, S:V ratio, and metal surface Received: Revised: Accepted: Published: 19392

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Veolia WS, Ede, The Netherlands) and sparged with argon until oxygen was below 20 ppb O2, after which 9 ppm of the amine of interest was added. Then carbohydrazide (SigmaAldrich, 99% trace metal basis) was dosed at 0.1 ppm to scavenge any remaining oxygen, after which the solution was again briefly sparged with argon. During the remainder of the experiment, the solution was kept under a nitrogen blanket, to prevent oxygen intrusion. When the temperature was stable, the experiment was started and a 4 mL sample was taken at the desired contact time. This was repeated for all of the conditions listed in Table 1 and executed in duplicate (64 experiments in total, with 4 retention times tested during each experiment).

composition on the kinetics of the thermolysis of amines at superheater conditions. This was done by equipping a flow reactor with four different tube diameters in two different materials (so eight combinations in total). The influence of the temperature on MOR and ETA degradation was tested at 500 and 530 °C at a constant 13.5 MPa pressure to ensure that the pressure would not influence the results, as was observed by Moed et al.15 The condensed steam was sampled and analyzed for amine and organic acid anion concentrations, with the latter being measured because of its importance in practical situations. The resulting data were used to find a relationship between the S:V ratio and amine degradation kinetics, thereby showing the importance of the metal surface and its potential catalytic effects when degradation reactions occur. Parameters that can introduce uncertainty such as oxygen and heating/ cooling issues were minimized.

Table 1. Conditions Applied during the Experiments for Both MOR and ETAa

2. MATERIALS AND METHODS 2.1. Flow Reactor. The flow reactor (Figure 1) used in this study is an adaptation of the flow reactor used by Moed et al.18

material

S:V (mm−1)

T (°C)

P (MPa)

tretention (s)

SS 316 C276

4.65 2.32 0.89 0.4

500 530

13.5

0−5 5−10 10−20 20−30

a T = temperature, P = pressure, tretention = retention time of the steam, material = tube composition, and S:V ratio = inner surface divided by the volume of the tube.

The outer diameters (o.d.’s) of the tubes were 1.59, 3.18, 6.35, and 12.7 mm, with a wall thickness of 0.36, 0.71, 0.89, and 1.27 mm, respectively. The inside of the tubes had to have a metal oxide layer before the experiments started to ensure that this did not change during the run. To create a metal oxide layer on a new tube before the experiments were started, the tubing was pretreated by flushing for 24 h with a 17 ppm (1 M) NH3 solution at 300 °C and 10.0 MPa while maintaining 10− 20 ppb O2 in the influent. Table 2 shows the elemental composition range for stainless steel (SS) 316 and Hastelloy C276 (C276), in percentages, before the materials were oxidized. 2.3. Analyses. Analysis of organic acid anions was performed using a Metrohm 881(Schiedam, The Netherlands) ion chromatography system. A Metrohm A Supp 16 4.0/250 anion column was operated at 67 °C with an eluent containing 7.5 mM Na2CO3 + 0.75 mM NaOH in ultrapure water. The suppressor was regenerated with 50 mM H2SO4, and the limit of detection was 1 ppb. For cation analysis, a Metrohm C5 cation column was used, with 3 mM HNO3 as an eluent, for which the limit of detection of MOR and ETA was 5 ppb. The surface composition of the larger tubing was analyzed with X-ray diffraction (XRD). The samples were fixed with plasticine on the holder. The instrument used was a Bruker D8 Advance diffractometer, with Bragg−Brentano geometry, a graphite monochromator, and a Vantec position-sensitive detector. Co Kα radiation was applied, with a divergence slit of 1 mm, 45 kV/35 mA, and no sample spinning. The scatter screen position was lowered on the wall of the tube, preventing the X-ray beam on the wall from reaching the detector. Data were evaluated with Bruker software Dif f rac.EVA, version 3.1. The tubing with o.d.’s of 3.18 and 1.59 mm was too small for successful XRD analysis, and therefore scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were employed to obtain the chemical composition of the inner surfaces exposed. The micrographs were obtained from a JEOL JSM 6500F instrument using an electron beam with an energy of 15 keV and a beam current of approximately 200 pA. The micrographs obtained were acquired in secondary electron

Figure 1. Schematic of the flow reactor used during the experiments, with, from left to right, 200 bar of argon, the bottle containing the solution of interest, an HPLC pump, a fluidized sand bed, a cooling spiral, and a back-pressure regulator.

A glass vessel containing the solution of interest was equipped with a pH and oxygen meter to monitor the influent quality. The vessel also contained a tube for argon sparging to remove oxygen and an high-performance liquid chromatography (HPLC) pump inlet equipped with a filter. This HPLC pump provided the pressure to send the water through stainless steel tubing inside a fluidized sand bath at a fixed flow rate to rapidly heat the feedwater. The fluidized sand bath itself was an Omega FSB-3 (Stamford, CT) bath containing fine aluminum oxide for heat transfer. The fluidized sand bath temperature was regulated by a Parr 4838 (Frankfurt am Main, Germany) temperature controller. Temperature feedback was provided by a thermal couple from the fluidized sand bath to the controller. Upon leaving the sand bath, this water was immediately cooled in a cooling spiral that was submerged in a beaker with water, which was kept at 20 (±2)°C by letting tap water run through it. The back-pressure was controlled by an adjustable relief valve (Swagelok, Waddinxveen, The Netherlands). The volume of the tubes inside the reactor differed per tube material and diameter. Because water and steam increases in volume at higher temperatures, the retention time in the tubing is a function of both the flow velocity and fluidized sand bath temperature. 2.2. Procedure and Conditions. At the beginning of each experiment, the relief valve was first set to the desired pressure and the temperature controller at the right temperature. The glass vessel was filled with ultrapure water (Elga Purelab Ultra, 19393

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Table 2. Elemental Composition Range of the SS 316 and C276 Alloy in Percentagesa SS C276 a

Fe

Cr

Ni

Mo

62−68 4−7

16−18 14.5−18.5

11−14 51.5−60

2−3 15−17

W 3−4.5

Co*

Mn*

Si*

C*

S*

2.5

2 1

0.75 0.08

0.035 0.01

0.03 0.03

For elements with asterisks, the maximum value is given.

Figure 2, which represent the experimental degradation curves for MOR thermolysis at 530 °C and 13.5 MPa in SS tubes of

image and back-scattered electron image modes. EDS was performed with an Thermo-Fisher “Ultradry” solid-state drift detector and processed with Noran System 7 software. The compositions of the point measurements were determined semiquantitatively using computer-stored reference spectra of the pure elements with an acquisition time of 30 s. 2.4. Temperature and S:V Relations. First-order isobaric degradation kinetics as a function of the absolute temperature were modeled according to the following equations:19 r = k(T )[C]

(1)

k(T ) = A e−Ea / RT

(2)

where r is the degradation rate, k(T) is the degradation rate constant as a function of the temperature, [C] is the concentration of the degrading compound, A is the preexponential factor, Ea is the activation energy, T is the temperature, and R is the universal gas constant. The latter equation can be written in logarithmic form as ln(k) = ln(A) −

Ea 1 RT

Figure 2. C/C0 (on a logarithmic scale) versus retention time for MOR degradation at 530 °C and 13.5 MPa on SS for four tube sizes. R2 = 0.990, 0.959, 0.993, and 0.988 for 0.4, 0.89, 0.32, and 4.65 mm−1 respectively.

four different diameters, with C/C0 plotted on a logarithmic scale. Because the different curves are represented quite well by first-order kinetic models (eq 4), the value of the kinetic rate constant k for each experiment could be derived from the exponential regression line through the data points and t = 0, C/C0 = 1. The resulting k values from different experiments with different amines at different temperatures were combined for different metals and were used to determine the slope and y intercept of the S:V ratio plotted against k, which is shown in Figure 3. A linear relationship between k and the S:V ratio was found for both ETA and MOR for both materials, with k increasing as the S:V ratio increased, although some variation in the k value for the same experiment can be observed. MOR was more stable than ETA at both tested temperatures. The metal oxide catalysis effect of the tube wall surface appeared to be bigger for thermolysis of ETA than that of MOR (because the slope of k as a function of the S:V ratio is steeper for ETA). As the S:V ratio approaches 0 (i.e., the tube diameter becomes infinitely large), only homogeneous thermolysis is occurring and the degradation kinetics (k) are not influenced by metal oxide catalysis any more. No large differences between the experiments with SS and C276 were seen, although it can be seen from the graphs that the homogeneous part of k, derived from the y intercept, was different for the tubes made of SS and C276 for ETA at 500 °C. This should not be possible, so the error margin is too large in these results. The stronger effect of metal surface catalysis on ETA has an influence on the reproducibility. Although no consistent differences in the kinetic rate constant between the two materials were found, XRD analysis of the surface composition of the tubes revealed large differences in the oxide composition for each metal tube [see the Supporting Information (SI), Table 1]. The SEM/EDS results obtained for the smaller diameters reveal that, by weight, there were more oxides formed on the SS tubes than on the C276 tubes (see SI,

(3)

Therefore, in an Arrhenius plot of ln(k) versus 1/T, the slope of the regression line through the data points gives the experimental value of Ea/R, while the y intercept corresponds to ln(A). In order to find the experimental values of k, an integrated first-order rate law is fitted to the experimentally determined decrease in the concentration of the amines upon degradation. First order is assumed because this was found to be the case for amine degradation in most previous studies: C = C0e−kt

(4)

The reaction rate constant k can be found by plotting C/C0 against time and calculating the exponential regression function y = e−kx. Subsequently, A and Ea can be found from eq 3 as described above. By investigating the degradation at different S:V ratios, both homogeneous and heterogeneous degradation kinetics of the amines can be calculated. Segond et al.16 provided evidence for a linear relationship between the S:V ratio and k according to k(T ) = AHe−Ea;H / RT + AW e−Ea:W / RT

S V

(5)

where AH and Ea;H describe the homogeneous degradation part of the kinetic equation and AW and Ea;W relate to the heterogeneous (wall effect and catalysis) part of the equation. Results in this study show whether assuming such a linear can be justified. It will also show to what extent the surface area of the reaction is important.

3. RESULTS AND DISCUSSION 3.1. S:V-Ratio-Dependent Kinetics. During all experiments, the degradation behavior was investigated at several retention times, and each experiment led to plots as shown in 19394

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Figure 3. Calculated values of k plotted against the S:V ratio for C276 (left) and SS (right).

Table 2). However, the XRD did not detect much chromium or nickel oxide in the C276 tubes, while SEM/EDS suggests that this should have been the case. The surface compositions were distinctly different, both visually (see SI, Figure 1) and chemically, and vary even for the same material, but there was no consistent difference noticeable in the k values for firstorder thermolysis that can be attributed to the metal oxide composition. To illustrate this, the relative standard deviations (RSDs) for the k values found for SS and C276 at the same conditions (in duplicate) have been plotted in Figure 4. Each S:V ratio was used in four experiments for each combination of amine and temperature.

Table 3. Calculated Homogeneous and Heterogeneous Arrhenius Constants for ETA and MOR Thermolysis at 13.5 MPa

ETA on C276 ETA on SS MOR on C276 MOR on SS

Ea;H (kJ/mol)

ln(AH)(kJ/mol)

Ea;W (kJ/mol)

ln(AW) (kJ/mol)

215.3 389.3 324.0

29.11 55.18 44.45

150.7 131.1 179.5

19.37 16.51 22.32

343.1

47.49

137.0

15.66

the homogeneous part of the equation. The combined errors cause the linear regression lines of ETA degradation at 500 °C to cross the y axis at 0.0124 and 0.062 s−1 for C276 and SS, respectively. This 50% difference causes a 45% difference between the homogeneous activation energies found for both materials and is also reflected in a 47% difference between the natural logarithms of the homogeneous prefactors AH. The differences in the degradation kinetics for MOR were smaller, which is again reflected in the Arrhenius constants. The large effect of errors over the whole set of experiments on the homogeneous Arrhenius constants for ETA indicates that trying to account for wall effects during modeling of amine thermolysis in superheaters introduces more uncertainty. The data show that, at the lowest investigated S:V ratio, catalytic wall effects accounted for only 6−17% of the total value of k, while at the highest S:V ratio, this went up to 82−92%. Therefore, thermolysis of amines at superheater conditions should be investigated in a laboratory setting, using a tube large enough to represent an actual superheater. From the above results, it can be concluded that the results of previous studies15,20 with smaller tube diameters have given an overestimation of the degradation kinetics during amine thermolysis at superheater conditions. To make predictions of amine longevity in fossil-fired power plants and industrial SWCs possible, a data set with a larger tube diameter (1.25 mm or more) has to be created for all amines that are currently being applied, covering a large temperature and pressure range. Extrapolation to only homogeneous catalysis based on different S:V ratios is not recommended because this introduces large errors. Another factor of uncertainty that will be introduced when experimental data are translated to full-scale SWCs is that heating will not be as instantaneous as that in the laboratory. There is a temperature profile over the length of the superheater tube, and retention times at a certain temperature can be uncertain. On top of that, the tube wall will be hotter

Figure 4. RSDs for k values found for the same experiments with SS compared to C276, executed in duplicate.

If the surface composition had a strong influence on the degradation kinetics, the RSDs found would have decreased along with the S:V ratio. The highest differences in the k values between SS and C276 were noticeable for ETA, and it is not certain that this can be attributed to differences in the surface composition because this was not observed during MOR thermolysis. It is more likely that, because the ETA degradation kinetics are faster, there is also more room for error. On the basis of the results for both materials at both low and high S:V ratios, the composition of the tube wall should have little effect on the degradation kinetics in an actual superheater. The homogeneous and heterogeneous Arrhenius constants, as determined from the experimental data using eq 5, are shown in Table 3. The value of 0.044 mm−1 calculated from these constants for k at 510 °C and 13.5 MPa is on the same order of magnitude as the 0.051 mm−1 found in a previous study.15 The previously discussed errors found between the results for metal surface catalysis of SS and C276 during ETA thermolysis are reflected in the Arrhenius constants found for 19395

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than the steam and the exhaust temperature is regulated. Therefore, wall effects could start to play an increasing role again. 3.2. Organic Acid Anion Formation. As stated in the Introduction, potential organic acid anion formation during amine thermolysis is currently preventing the wider acceptance of amines for application in SWCs. Thermolysis of ETA and MOR in this study led to the formation of mostly acetate and formate, although some traces (up to 20 ppb) of glycolate were found for both amines and traces of propionate were found after MOR degradation. Figure 5 shows the relationship between the highest achieved degradation percentage and the formate and acetate production during each experimental run for MOR and ETA. Figure 6. Maximum degradation during an experiment versus formate and acetate production on SS and C276 for all temperatures, tube sizes, and amines.

Figure 5. Maximum degradation versus formate and acetate production from 9 ppm MOR and ETA for all temperatures, tube sizes, and materials.

MOR thermolysis in a superheater will, in practice, produce more organic acid anions than ETA, based on the concentrations at which they are applied. The higher stability of MOR does give it a slight advantage when it comes to organic acid anion production, but ETA’s better basicity at high temperature and lower concentration to maintain sufficient pH and the resulting lower organic acid anion concentrations will make ETA more attractive for use as an alkalizing amine in SWCs than MOR. There will be a maximum temperature for both amines to be applied though, for which more research is necessary.

The highest concentrations of organic acids were measured for ETA, but this was also the amine that reached the highest degradation percentages. The spread was high, and there were some severe outliers for both amines for both acetate and formate. There does not seem to be a clear relationship between the organic acid anion production, amine structure, and degradation percentage. When the same data for organic acid anion formation is sorted by the tube o.d. it can be seen that experiments performed with larger tube diameters produced more organic acids (plot not shown). Higher temperatures on average produce more acetate and less formate (plot not shown), which was also previously observed by Moed et al.18 A more obvious relationship was observed when the same data were plotted for formate and acetate sorted by the tube material used (Figure 6). Experiments performed with tubes made of SS produced more acetate than formate, while the exact opposite was observed for tubes made of C276. Although no large influence of the surface composition was found for larger diameters, when it comes to the degradation kinetics, catalytic effects of the wall did have an influence on the type of organic acids produced for all tube sizes. The material composition of the wall surface had more influence on the degradation pathway than the temperature, tube diameter, or amine composition. The concentrations for ETA used in this paper are exaggerated because much less is needed to maintain sufficiently high pH in a SWC (1.51 ppm for a pH of 9.2 at 25 °C). MOR concentrations, in practice, are around 10 ppm to maintain the same pH in the condensate because of the low pKa of MOR (8.94 ppm for a pH of 9.2 at 25 °C). Therefore,

4. CONCLUSIONS This study has led to the following conclusions: (1) There was an influence of the metal oxides on the inner tube wall on the amine thermolysis kinetics, with the degradation rate decreasing as the tube size increased. (2) The relationship between the S:V ratio and the degradation rate constant k was linear. (3) Metal surface catalysis during thermolysis of MOR and ETA accounted for 82−92% of the value of the degradation rate constant k at an S:V ratio of 4.65 mm−1. This decreased to only 6−17% at an S:V ratio of 0.4 mm−1. (4) Although the results varied between the two applied materials, there is no consistent trend that can link the thermolysis kinetics to the tube wall composition. (5) A distinction between homogeneous and heterogeneous thermolysis has been made, but the results indicate that a laboratory study with a large enough tube diameter will lead to more reliable predictions. (6) Organic acid anion production was weakly related to the amine structure, temperature, and tube diameter but strongly related to the metal oxide composition, with formate and acetate altering as dominant organic acid anions. (7) The higher stability of MOR does give it an advantage when it comes to organic acid anion production, but ETA’s better basicity at high temperature and lower concentration to maintain sufficient pH and the resulting lower organic acid anion concentrations will make ETA more attractive for use as an alkalizing amine in SWCs than MOR. (8) There is a maximum combination of the temperature and retention time for both amines to be applied, for which more research is necessary. 19396

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(15) Moed, D. H.; Verliefde, A. R. D.; Heijman, S. G. J.; Rietveld, L. C. Thermolysis of Morpholine in Water and Superheated Steam. Ind. Eng. Chem. Res. 2014, 53 (19), 8012−8017. (16) Segond, N.; Matsumare, Y.; Yamamoto, K. Determination of Ammonia Oxidation Rate in Sub- and Supercritical Water. Ind. Eng. Chem. Res. 2002, 41, 6020−6027. (17) Jackson, S. D.; Hargreaves, J. S. J. Met. Oxide Catal. 2009, 1−2, 1−866. (18) Moed, D. H.; Verliefde, A. R. D.; Rietveld, L. C.; Heijman, S. G. J. Organic acid formation in steam-water cycles: influence of temperature, retention time, heating rate and O2. Appl. Therm. Eng. 2014, 65 (1−2), 194−200. (19) Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper Collins Publishers, Inc.: New York, 1987. (20) Moed, D. H.; Verliefde, A. R. D.; Heijman, S. G. J.; Rietveld, L. C. Degradation kinetics of six alkalizing amines. 16th International Conference on the Properties of Water and Steam, London, Sept 1−5, 2013; Institution of Mechanical Engineers: London, 2013.

ASSOCIATED CONTENT

S Supporting Information *

Results of XRD and SEM/EDS analysis to support the findings described in this paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +31 (0) 152786588. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank EPRI and Evides Industriewater for financial and technical support. Ruud Hendrikx and Kees Kwakernaak from the Materials Science Department of Delft University of Technology are acknowledged for XRD and SEM/EDS analysis, respectively. Specific gratitude goes out to Michael Carravagio and James Mathews from EPRI for useful input and discussions.

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REFERENCES

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