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Surface Chemistry Interventions to Control Boiler Tube Fouling

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Surface Chemistry Interventions to Control Boiler Tube Fouling AECL SURFACE CHEMISTRY INTERVENTIONS TO CONTROL BOILER TUBE FOULING bY C.W. Turner, D.A. Guzonas and S.J. Klimas The work was co-funded by Atomic Energy of Canada Limited and the Electric Power Research Institute. Heat Exchanger Technology Branch Chalk River Laboratories Chalk River, Ontario, Canada KOJ 1JO 2000 June AECL- 12036 EACL UTILISATION DE LA CHIMIE DE SURFACE POUR LUTTER CONTRE L’ENCRASSEMENT DES TUBES DES GkNdATEURS DE VAPEUR Par C.W. Turner, D.A. Guzonas et S.J. Klimas RliSUMk L’adsorption d’ammoniac, de morpholine, d’ethanolamine et de dimethylamine sur la surface de magnetite et d’hematite colldidales a Cte mesuree a 25 “C. L’effet de l’adsorption sur le potentiel de surface a Cte quantifiC en mesurant le deplacement rt%ultant du point isotlectrique des produits de corrosion, et en mesurant directement la force d’interaction superficielle entre les produits de corrosion et l’Inconel600. Ces mesures ont permis d’appuyer l’hypothese selon laquelle l’adsorption d’amine influe sur la vitesse de depot de magnetite, en diminuant la force de repulsion entre la magnetite et la surface dIncone 600. La vitesse de depot d’hematite augmentait en meme temps que la concentration d’oxygene. On a determine un mecanisme pour prendre en compte l’accroissement des vitesses de depot avec des melanges de haute qualite (> 0,35) et on a demontre sa capacite a predire le comportement, conformement aux donnees experimentales et a celles de la centrale. Par suite de ces recherches, plusieurs criteres sont proposes afin de reduire l’importance des depots de produits de corrosion sur le faisceau de tubes. Un faible depot d’hematite est favorist par une faible concentration d’oxygene dissous, tandis qu’un faible depot de magnetite est favorise en choisissant une amine pour le controle du pH ayant peu tendance a s’adsorber sur la surface de magnetite. Afin de reduire au minimum l’adsorption, l’amine devrait posseder une force de base elevee et une grande <<empreinte>> sur la surface de magnetite. Dans le but d’empecher l’accroissement de depot avec des qt.&it& de melange ClevCes, on propose d’utiliser une amine modifiee pour reduire la tension superficielle, ou I’elasticite de l’interface eau-vapeur, ou les deux. Technologie des Cchangeurs thermiques Laboratoires de Chalk River Chalk River (Ontario) Canada KOJ 1 JO Juin 2000 AECL- 12036 AECL SURFACE CHEMISTRY INTERVENTIONS TO CONTROL BOILER TUBE FOULING bY C.W. Turner, D.A. Guzonas and S.J. Klimas ABSTRACT The adsorption of ammonia, morpholine, ethanolamine, and dimethylamine onto the surfaces of colloidal magnetite and hematite was measured at 25°C. The effect of the adsorption on the surface potential was quantified by measuring the resulting shift in the isoelectric point of the corrosion products and by the direct measurement of the surface interaction force between the corrosion products and Inconel600. These measurements have served to support the hypothesis that adsorption of amine affects the magnetite deposition rate by lowering the force of repulsion between magnetite and the surface of Inconel600. The deposition rate of hematite increased as the oxygen concentration increased. A mechanism to account for enhanced deposition rates at high mixture qualities (> 0.35) has been identified and shown to predict behaviour that is consistent with both experimental and plant data. As a result of this investigation, several criteria are proposed to reduce the extent of corrosion product deposition on the tube bundle. Low hematite deposition is favoured by a low concentration of dissolved oxygen, and low magnetite deposition is favoured by choosing an amine for pH control that has little tendency to adsorb onto the surface of magnetite. To minimize adsorption the amine should have a high base strength and a large “footprint” on the surface of magnetite. To prevent enhanced deposition at high mixture qualities, it is proposed that a modified amine be used that will reduce the surface tension or the elasticity of the steam-water interface or both. Heat Exchanger Technology Branch Chalk River Laboratories Chalk River, Ontario, Canada KOJ 1JO 2000 June AECL-12036 In a previous investigation into the effect of alternative amines on tube-bundle fouling that was jointly funded by Atomic Energy of Canada Limited (AECL) and Electric Power Research Institute (EPRI), it was shown that the surface chemistry of corrosion products has a significant influence on the particle deposition rate under flow-boiling conditions (Turner et al., 1997). For example, particles of hematite deposited at rates an order of magnitude greater than did particles of magnetite. In tests performed at constant ~HT, the deposition rate measured for magnetite depended upon the amine used for pH control. In addition, the amine with the highest base strength at 25°C (dimethylamine) resulted in the lowest deposition rate, whereas the weakest base (morpholine) was associated with the highest deposition rate. After correcting a problem associated with acidic impurities in the cover gas (most likely carbon dioxide) that resulted in excessive amounts of amine being required to achieve the desired pH, it was observed that the deposition rates for magnetite at fixed pHr were consistently lower when less amine was added. It was postulated that the difference between the deposition rates of magnetite and hematite was due to differences in the surface charge of these oxides under the test conditions, magnetite being negatively charged at ~HT = 6.2 and hematite, positively charged. Measurements of streaming potential at 25°C (P.V. Balakrishnan and C.W. Turner, Chalk River Laboratories, unpublished results) indicate that the surface of Inconel600 should also be negatively charged under the test conditions. Thus particles of magnetite will tend to be repelled from Inconel600, whereas particles of hematite will be attracted. To account for the dependence of the deposition rate of magnetite on the nature and concentration of amine used for pH control, it was postulated that the amine was adsorbing onto the surface of magnetite and changing its surface potential. The amine will bc present in solution as both the neutral and the protonated (positively charged) species. Thus adsorption of amine onto magnetite will tend to make its surface less negative. This adtiprption should reduce the force of repulsion between magnetite and Inconel600, and lead ICY ;\ correspondingly higher deposition rate. AECL ;md EPRI undertook a follow-up program to evaluate the hypothesis that the adsorption of amine affcds rtw Ptiicie deposition rate by altering the surface interaction potential between the particles and the wrfacc of Inconel600. The ultimate goal of this program is to identify those propenics of the amine that control the deposition behaviour and so be able to choose or design an amine that best controls tube-bundle deposition in the steam generator. The work scope proposed for the follow-up program includes measurements of the adsorption of amine onto the surface of corrosion products and measurements to determine the effect of adsorption on the surface potential. Laser Raman spectroscopy was chosen to measure the adsorption isotherms because this technique can be applied over a wide range of temperatures. Two techniques were chosen to measure the effect of adsorption on the surface potential of the corrosion products. Measurements of the electrophoretic mobility of a metal oxide particle versus pH provides a relatively simple way to detect the adsorption of a charged species. Every metal oxide has a characteristic pH at which the surface potential is normally zero, known as the - vii - isoelectric point (IEP). Evidence for the adsorption of the positively charged species is manifested by a shift in the IEP to a higher pH. The second method used to determine the effect of adsorption of a charged species on the surface potential is Atomic Force Microscopy (AFM). Measurements of the force exerted by the surface of Inconel600 on a particle as the two are brought together can be used to deduce the surface potential. The effect of the adsorption of amine on the surface potential can therefore be determined by performing these measurements as a function of amine concentration. By combining information on the adsorption of amines and the corresponding surface chemistry modifications with the high-temperature loop deposition data, we hope to identify properties of the amine that should be optimized to reduce deposition of corrosion products on the steam generator tube-bundle. Because the measurements performed in this investigation to determine the effect of adsorption on surface potential can only be made at ambient temperature, complementary experiments are being performed at the Oak Ridge National Laboratory in a parallel program to measure the effect of the adsorption of amine on the high-temperature point of zero charge (PZC) of magnetite. Additional loop deposition tests were also planned to complete the test matrix for amines examined in the previous investigation and to evaluate the effect of 3 additional amines- methoxypropylamine, pyrrolidine, and 4-aminobutanol-on the deposition rate of corrosion products. A detailed analysis of flow phenomena at high mixture qualities was also planned for the follow-up program to identify the cause of the high deposition rates observed in the previous investigation for mixture qualities in excess of about 0.35. Enhanced deposition at high mixture qualities was not observed in loop tests conducted in a separate
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