Calcium Carbonate Scale Adsorption and
Desorption Studies on Uncoated and
Coated Stainless Steel Surfaces
Adsorptions- och desorptionsstudier av kalciumkarbonat på obelagda och belagda ytor
av rostfritt stål
Emelly Todor
Faculty of Health, Science and Technology
Degree Project for Master of Science in Engineering, Chemical Engineering 30 ECTS
Supervisor: Dr. Olga Santos (Alfa Laval, Lund) and Dr. Gunilla Carlsson Kvarnlöf (Karlstad University) Examiner: Prof. Magnus Lestelius
I
Abstract
The calcium carbonate scale is one of the most common inorganic scales deposited on the surface of oil and water production facilities. This can cause major problems that can become expensive to repair, if not handled properly. The formation of calcium carbonate scale can slow down the production by, for example, blocking the flow of liquid in a heat exchanger.
This study was done to gain a better understanding of the mechanisms involved, in the early stages of scaling (induction, nucleation and crystal growth) on stainless steel (uncoated and coated) and efficiency of detergents for removing the scaling. Ellipsometry, Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy were chosen as the main methods in the study, as they are well-established tools for monitoring the scaling process. The ellipsometer was chosen to study adsorption and desorption, in situ, whilst varying experimental conditions (temperature, turbulence, water hardness) are possible. Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy were chosen to study structure and composition of calcium carbonate scale. This was complemented with quartz crystal microbalance measurements, as problem with interpreting ellipsometer data was encountered under some experimental conditions.
It was found that there are many factors that affect the formation of calcium carbonate scale, such as temperature, time, nucleation and induction. It was established that the adsorption rate was almost three times higher at 60 °C than at 25 °C, for stainless steel. Of the coated surfaces that were examined, it was found that a coating with sol gel (coating 1) is slightly better than the others to resist calcium carbonate scaling. An assumption was made that heterogeneous nucleation is the one that occurs most and that the roughness of the surface has an impact on the scaling.
II
Sammanfattning
Kalciumkarbonatpåväxt är en av de vanligaste oorganiska beläggningarna som deponeras på ytan av olje- och vattenproduktionsanläggningar. Detta kan orsaka stora problem som kan bli dyra att reparera om de inte hanteras ordentligt. Bildningen av kalciumkarbonatpåväxt kan sakta ner produktionen genom att till exempel blockera vätskeflödet i en värmeväxlare och att minska värmeöverföringen.
Denna studie gjordes för att få en bättre förståelse av de involverade mekanismerna, i de tidiga stadierna av påväxt (induktion, kärnbildning och kristalltillväxt) på rostfritt stål (obelagd och belagd) och tvättlösningens effektivitet för att avlägsna påväxten. Ellipsometer och svepelektronmikroskopi och energi-spridande röntgenspektroskopi valdes som huvudmetoder i studien, eftersom de är väletablerade verktyg för att övervaka påväxtprocessen. Ellipsometern valdes för att studera adsorption och desorption, in situ, under det att varierande experimentella förhållanden (temperatur, turbulens, vattenhårdhet) är möjliga. Svepelektronmikroskopi och energi-spridande röntgenspektroskopi valdes för att studera strukturen och sammansättningen av kalciumkarbonatpåväxt. Detta kompletterades med kvartskristallmikrobalansmätningar, eftersom problem med tolkning av ellipsometerdata påträffades under vissa experimentella förhållanden.
Det visade sig att det finns många faktorer som påverkar bildningen av kalciumkarbonat, såsom temperatur, tid, kärnbildning och induktion. Det konstaterades att adsorptionshastigheten var nästan tre gånger högre vid 60 °C än vid 25 °C, för rostfritt stål. Av de belagda ytorna som undersöktes, fann man att en beläggning med sol gel (beläggning 1) är något bättre än de andra för att motstå kalciumkarbonat. Ett antagande gjordes att heterogen kärnbildning är den som förekommer mest och att grovheten på ytan påverkar bildandet av kalciumkarbonat.
III
Acknowledgement
I would like to send my gratitude to my supervisor Gunilla Carlsson Kvarnlöf from Karlstad University for all the support and guidance during this master thesis.
Special thanks to my supervisors Olga Santos from Alfa Laval for all the guidance, encouragement and expertise during my thesis. Without your knowledge and interest, the outcome of this project would not have been the same. Also a big thank you to my examiner Magnus Lestelius, Tommy Nylander (professor at Lund University), Polina Naidjonoka (doctoral student at Lund University) and Ben Humphreys (Postdoc at Lund University) for their advice to overcome challenges during the project.
Finally, I want to thank my family and friends for all the support, love and encouragement during my years of study. None of this would have been possible without you; my gratitude to you is indescribable.
“Our greatest weakness lies in giving up. The most certain way to succeed is always to try just one more time”
VIII
Contents
Abstract ... I
Sammanfattning ... II
Acknowledgement ... III
List of Figures ... IV
List of Tables ... VI
Nomenclature ... VII
1 Introduction ... 1
1.1 Objectives of research ... 22 Background ... 3
2.1 The fundamental of scaling ... 3
2.2 Types of crystallization scales ... 3
2.2.1 Calcite ... 4
2.2.2 Aragonite ... 4
2.2.3 Vaterite ... 5
2.3 The formation of calcium carbonate scale ... 5
2.4 Chemical background of scaling ... 6
2.4.1 Solubility product ... 6
2.5 Adhesion ... 7
2.5.1 The mechanisms of adhesion ... 7
2.5.2 Adhesion forces and surface energy ... 8
2.6 The scaling process ... 9
2.6.1 Chemical potential ... 9 2.6.2 Supersaturation ... 10 2.6.3 Induction time ... 10 2.6.4 Nucleation ... 11 2.6.5 Primary nucleation ... 11 2.6.5.1 Homogeneous nucleation ... 12 2.6.5.2 Heterogeneous nucleation ... 13 2.6.6 Secondary nucleation ... 14 2.6.7 Crystal growth ... 14
2.6.7.1 Adsorption layer theory ... 15
2.6.7.2 Diffusion Theory ... 15
2.6.7.3 Surface energy theory ... 16
2.7 Factors that impact scaling ... 17
2.7.1 Impact of temperature ... 17
2.7.2 Impact of pH ... 17
2.7.3 Impact of supersaturation ... 18
2.7.4 Impact of pressure ... 18
2.7.5 Impact of flow rate ... 19
2.7.6 Impact of impurities ... 19
2.7.7 Impact of solution chemistry ... 20
2.7.8 Impact of surface roughness ... 20
2.8 Desorption ... 20
IX
2.9 Plate Heat Exchanger ... 21
2.10 Uncoated and Coated Stainless Steel ... 21
2.11 Ellipsometry ... 22
2.12 Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDX) ... 24
2.13 Refractometer ... 26
2.14 Quartz Crystal Microbalance with Dissipation monitoring ... 27
3 Methodology ... 28
3.1 Experimental materials ... 28
3.2 Surface Preparation ... 28
3.3 Methods ... 29
3.3.1 Ellipsometry ... 29
3.3.2 Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDX) ... 31
3.3.3 Refractometer ... 32
3.3.4 Quartz Crystal Microbalance with Dissipation monitoring ... 32
4 Results and Discussion ... 34
IV
List of Figures
Figure 2-1: Coloured SEM image of calcite scale crystals, image retrieved from (10). ... 4
Figure 2-2: Coloured SEM image of aragonite scale crystals (needles) from limescale, image retrieved from (14). ... 4
Figure 2-3: Coloured SEM image of vaterite scale crystals (rosettes), image retrieved from (16). ... 5
Figure 2-4: The main parts of the plate heat exchanger, image retrieved and modified from (62). ... 21
Figure 2-5: (a) Schematic diagram of an ellipsometer, where Φ is the angle of incidence, image retrieved and modified from (68). (b) Upon reflection at an interface, the light interacts with the material. This, in turn, changes the state of polarization and gives sensitivity to film thickness and optical properties if there is a film present. The incoming wave ki, is partly reflected, kr, and partly transmitted, kt, into the medium. The s and p vectors indicate the direction of the polarized light. ... 22
Figure 2-6: (a) Schematic diagram of a SEM, image retrieved and modified from (71). (b) Interaction volume, by Freundchen, 2015 (72). ... 24
Figure 2-7: (a) Principle of EDX, by Muso, 2007 (74). (b) Spectrum from EDX, by Ziel Rainer, 2008 (75). ... 25
Figure 2-8: (a) Spectral lines at different wavelengths, (b) Abbe Utility Software and (c) Actual photo of the refractometer. ... 26
Figure 2-9: Schematic diagram of an QCM, where CE is counter electrode, RE is reference electrode and WE is working electrode, by Zhao Ruoha, 2020 (79). ... 27
Figure 3-1: Stainless steel surface that stands vertically in the beaker. ... 29
Figure 3-2: Photo of the Rudolph ellipsometer. ... 30
Figure 3-3: Photo of the SEM-EDX. ... 31
Figure 4-1: Adsorbed amount of CaCO3 on stainless steel at 25 °C. ... 36
Figure 4-2: Adsorbed amount of CaCO3 on stainless steel at 25 °C (test 1). ... 37
Figure 4-3: Thickness of the adsorbed layer of CaCO3 on stainless steel at 25 °C (test 1). ... 37
Figure 4-4: Adsorbed amount of CaCO3 on stainless steel at 25 °C (test 2). ... 38
Figure 4-5: Thickness of the adsorbed layer of CaCO3 on stainless steel at 25 °C (test 2). ... 38
Figure 4-6: Adsorbed amount of CaCO3 on stainless steel at 60 °C (test 1). ... 40
Figure 4-7: Thickness of the adsorbed layer of CaCO3 on stainless steel at 60 °C (test 1). ... 40
Figure 4-8: Adsorbed amount of CaCO3 on stainless steel at 60 °C (test 2). ... 41
Figure 4-9: Thickness of the adsorbed layer of CaCO3 on stainless steel at 60 °C (test 2). ... 41
V Stainless steel at 30 s, (b) coating 1 at 30 s, (c) stainless steel at 1 min, (d) coating 1 at 1 min, (e) stainless steel at 5 min and (f) coating 1 at 5 min. ... 44 Figure 4-12: SEM images of CaCO3 crystals at temperature 60 °C, at different time and on different surfaces. (a)
Stainless steel at 30 s, (b) coating 1 at 30 s, (c) stainless steel at 1 min, (d) coating 1 at 1 min, (e) stainless steel at 5 min and (f) coating 1 at 5 min. ... 45 Figure 4-13: SEM images of CaCO3 crystals at temperature 60 °C, at different time and on different surfaces. (a)
Coating 2 at 30 s, (b) coating 3 at 30 s, (c) coating 2 at 1 min, (d) coating 3 at 1 min, (e) coating 2 at 5 min and (f) coating 3 at 5 min. ... 46 Figure 4-14: SEM images of CaCO3 crystals at air-water interface on different surfaces, at 60 °C, 5 minutes. (a)
Stainless steel, (b) coating 1, (c) coating 2 and (d) coating 3. ... 49 Figure 4-15: (a) SEM images of stainless steel rinsed with Alfa Phos (from ellipsometer) and (b) the elements
analysed from SEM-EDX. ... 51 Figure 4-16: QCM-D of the adsorbed layer of CaCO3 on stainless steel at 25 °C. ... 52
VI
List of Tables
Table 4-1: The effective complex refractive index of the substrate. ... 35
Table 4-2: Calculation of refractive index of hard water at 25 °C. ... 35
Table 4-3: Calculation of refractive index of Alfa Phos at 25 °C. ... 35
Table 4-4: Calculation of refractive index of hard water at 60 °C. ... 35
Table 4-5: Calculation of refractive index of Alfa Phos at 60 °C. ... 35
Table 4-6: The average amount of adsorption of CaCO3 (stainless steel, at 25 °C). ... 39
Table 4-7: The average amount of desorption of CaCO3 (stainless steel, at 25 °C). ... 39
Table 4-8: The average rate of adsorption of CaCO3 (stainless steel, at 25 °C). ... 39
Table 4-9: The average rate of desorption of CaCO3 (stainless steel, 25 °C). ... 39
Table 4-10: The average amount of adsorption of CaCO3 (stainless steel, at 60 °C). ... 42
Table 4-11: The average amount of desorption of CaCO3 (stainless steel, at 60 °C). ... 42
Table 4-12: The average rate of adsorption of CaCO3 (stainless steel, at 60 °C). ... 42
Table 4-13: The average rate of desorption of CaCO3 (stainless steel, at 60 °C). ... 42
Table 4-14: The size of the largest crystal or area on various surfaces and temperature. ... 48
Table 4-15: The percentage by weight and errors of Ca from EDX at 22 °C. ... 49
Table 4-16: The percentage by weight and errors of Ca from EDX at 60 °C. ... 50
VII
Nomenclature
Δ: relative phase shift (degree)
Ψ: relative change in amplitude (degree) Γ: adsorbed amount (mg/m2)
R: molar gas constant (JK-1mol-1) T: absolute temperature (K) tind: induction time (min) tr: “relaxation time” (min)
tn: time for system to reach steady state (min) tg: time before the nuclei can be detected (min) 𝛾: interfacial tension (mJ/m2)
J: nucleation rate (cm-3s-1) k: Boltzmann constant (JK-1) Vm: molecular volume (cm3mol-1) 𝜌: density (Kg/m3)
NA: Avogadro number (mol-1) km: mass transfer coefficient (m/s)
rc: minimum size at which a nucleus is stable (cm) ΔGcrit: Gibbs free energy (Jmol-1)
A: surface area of the crystal (m2)
c: dissolved concentration in the solution (mol/L) c*: equilibrium saturation concentration (mol/L) kd: mass transfer coefficient by diffusion (m/s) kr: rate constant for the surface reaction process (molL-1s-1)
ci: dissolved concentration (mol/L) WA: adhesion work (Nm-1)
θ: contact angle (degrees) R: desorption rate (mol/gs) r: rate constant for desorption (s-1)
A: frequency or pre-exponential factor (s-1) Ea: activation energy of desorption (J/mol) λ: wavelength (nm)
1
1 Introduction
Gustaf de Laval, was born in 1845 in Dalarna in Sweden. He founded Alfa Laval in 1883. Alfa Laval is a global supplier of products in separation, flow management and heat transfer. Their products are found in many industries, but mainly in food, environment, energy and the marine industry. Products that are sold are heat exchangers, separators, pumps, valves, etc. (1).
Today's energy comes with certain production challenges. Heat exchangers mostly contain liquids, which means that they can easily be exposed to inorganic and organic scales. Scaling is described as the unwanted crystallized layer at the solid surface in contact with a flowing medium. It is a hard salt that is formed due to changes in environmental conditions such as temperature, pH, water hardness, pressure, etc. (2,3). Scaling is a major problem that can lead to reduced heat transfer, reduced fluid flow and increased pressure drop. This can be very expensive to repair, so scale prevention is an important part of ensuring optimal production (4).
This study focuses on the adsorption and desorption of calcium carbonate (CaCO3), which is an inorganic scale. CaCO3 is one of the most common inorganic scales in oil and water production. CaCO3 scaling leads to an increased pressure drop and a reduced heat transfer due to the hard insulating layer that builds up at the metal surface (4).
2
1.1 Objectives of research
The aim of the research is to develop an understanding of scale deposition on the heat exchanger surfaces by focusing on surface growth and crystal structure. Another aim is to follow the scaling process in situ, on both uncoated and coated stainless steel surfaces. The specific objectives are:
• To study the adsorption of calcium carbonate on the various surfaces in terms of adsorption rate and adsorbed amounts.
• To study how fast and how much calcium carbonate that can be desorbed on the various surfaces, with Type 1 water and with a detergent (Alfa Phos).
• To determine the effect of temperature and static flow.
• To follow the calcium carbonate crystal structure on various surfaces at different time periods and temperatures.
3
2 Background
This section provides the basic understanding of scaling, factors that affect scaling and scaling mechanisms. It also contains a description of the methods used in this study.
2.1 The fundamental of scaling
Scaling is described as the unwanted crystallized layer at the solid surface in contact with a flowing medium. It is a hard salt, which is deposited from solution and adheres to the wall of the process equipment due to
supersaturation (2,3). Scaling is formed due to the change in environmental conditions such as temperature, pH, water hardness, pressure, etc. This can result in reduced heat transfer, reduced fluid flow and increased pressure drop. Depending on how much scaling has been deposited, this can lead to major problems and can be very expensive to repair (4). Calcium carbonate (CaCO3) and calcium sulfate (CaSO4) are the main types of scaling. Formation of carbonate and sulfate scale can slow down production by, for example, blocking the fluid flow in the heat exchanger. Therefore, it is important to determine the amount of calcium carbonate and calcium sulfate scale that will precipitate during oil and water production. Crystallization is formed due to supersaturation. There will be an excess of salts from the solution, which precipitates on the solid surface. The amount of precipitation depends, for example, on the solution chemistry and degree of saturation. For calcium carbonate and calcium sulfate, the amount of scaling increases with increasing temperature and pressure (3,4).
2.2 Types of crystallization scales
There are different categories within which a crystal structure is formed. The 14 Bravais lattices is used to describe these structures, explained in more detail in (5–7).
Calcium carbonate appears mostly in three anhydrous crystalline polymorphs. These are calcite, aragonite and vaterite. Calcite belongs to the rhombohedral crystal structure, aragonite to the orthorhombic structure and vaterite to the hexagonal structure. In aqueous solutions, the stability of the polymorphs increases and its solubility
decreases in the order vaterite → aragonite → calcite (8). The stabilization and crystallization of these
4
2.2.1 Calcite
Calcite is the most stable crystal structure of calcium carbonate and the most common. It usually precipitates at lower temperatures. Throughout the precipitation process Ca2+ ions are consumed, which means that
supersaturation is reduced and more calcite is formed. With increasing temperature in water, calcite becomes less soluble, but can be dissolved with the help of an acid. Natural calcite most often occurs in sedimentary rocks, such as limestone (8,9). The crystal structure of calcite looks like a distorted cube.
Figure 2-1: Coloured SEM image of calcite scale crystals, image retrieved from (10).
2.2.2 Aragonite
Aragonite is one of the metastable crystal structures of calcium carbonate and is most often formed at high temperatures. Aragonite can change to calcite, depending on the temperature. If it is in contact with water, it can be transformed to calcite already at room temperature. Natural aragonite can be formed in living organisms and occurs in almost all mollusk shells. Aragonite is "needle-like", but can be changed to either "flower-like" or "flake-like" depending on the crystallization conditions (3,8,11–13).
5
2.2.3 Vaterite
Vaterite is also a metastable crystal structure of calcium carbonate, but is one of the most unstable and most difficult to detect. This means that it has a higher solubility than any of the other phases. Therefore, the vaterite can easily change to aragonite or calcite in an aqueous solution. The transformation from vaterite to calcite usually takes place at lower temperatures and the transformation from vaterite to aragonite takes place at higher temperatures (approx. 60 °C). Natural vaterite occurs, for example, in organic tissue and in mineral sources (8,12,15). Its structure is porous and usually consists of many hexagonal vaterite plates, which are arranged in "rosettes", even resembling small round "bubbles". They can also appear as only vaterite plates that are attached together.
Figure 2-3: Coloured SEM image of vaterite scale crystals (rosettes), image retrieved from (16).
2.3 The formation of calcium carbonate scale
Calcium carbonate scaling depends, for example, on the temperature, pH and ionic strength of the solution. In an aqueous solution, the chemical equilibrium can be described as carbon dioxide (CO2), which undergoes an
ionization reaction. The following equations (2-1 to 2-4) describe the ion pair formation and hydrolysis of calcium ions. When carbon dioxide comes in contact with water, carbon dioxide is formed (17,18).
𝐶𝑂!(𝑔) ⇄ 𝐶𝑂!(𝑎𝑞) 2-1 𝐶𝑂! 𝑔 + 𝐻!𝑂 ⇄ 𝐻!𝐶𝑂! 2-2
𝐻!𝐶𝑂!⇄ 𝐻𝐶𝑂!!+ 𝐻! 2-3
𝐻𝐶𝑂!!⇄ 𝐶𝑂!!+ 𝐻! 2-4
Calcium ions (Ca2+) react with bicarbonate ions (HCO3-) and forms calcium bicarbonate, which is shown in the equations below (2-5 to 2-7).
𝐶𝑎!!+ 𝐻𝐶𝑂
6 𝐶𝑎!!+ 𝐶𝑂 !!!⇄ 𝐶𝑎𝐶𝑂! 2-6 𝐶𝑎!!+ 𝑂𝐻!⇄ 𝐶𝑎𝑂𝐻! 2-7
The precipitation of calcium carbonate is given according to equation 2-8. 𝐶𝑎!!+ 𝐶𝑂
!!!→ 𝐶𝑎𝐶𝑂! 2-8
The equilibrium can be shifted to the right if, for example, the pressure decreases, the temperature increases or if dissolved CO2 compounds are lost. Causing deposition of calcium carbonate (17,18). The amount of carbon dioxide in water affects the solubility of calcium carbonate. If the pressure decreases, the CO2 partial pressure decreases, which can lead to reduced solubility (19).
2.4 Chemical background of scaling
Scale formation is a product that is due to an imbalance in the bulk solution or in the solution chemistry,
depending on the environment. The solubility is of great importance for the formation of scaling and is described as the concentration of a solute that is in a saturated solution. A saturated solution is a solution that is in
equilibrium. A solution that has a higher concentration of solute than the bulk solution itself is called
supersaturated. The deposition of calcium carbonate scale is due to the chemical equilibrium between CO2 ions in the water (20,21).
2.4.1 Solubility product
Solubility is a term used to describe the amount of solute that dissolves in a solvent (usually water), at a certain temperature. Salt is formed by the reaction between positive and negative ions. The product that is formed is called the solubility product (Ksp). The chemical equilibrium of the solubility product constant is defined according to equations 2-9 to 2-10 (22).
𝐴𝐵 ⇋ 𝐴!+ 𝐵! 2-9
𝐾!"= 𝐴! 𝐵! 2-10
7
effect on the solubility product of certain compounds. Calcium carbonate dissolves in water to produce Ca2+ and CO32-, as can be seen in the above equations. CO32- absorbs a proton from water, which then forms a basic solution (17,18,22).
2.5 Adhesion
Coating is due to adhesion and significantly affects the oil and water industry. Adhesion occurs when two surfaces meet and form an interface. It is the maximum energy required to separate the surfaces from each other. Through, for example, van der Waals forces, adhesion forces can be accomplished. What happens is that nucleation and crystal growth can form on the material or equipment wall, which can impair efficiency and performance. There is much that can affect adhesion, such as diffusion, electrostatic, chemical bonding and adsorption. Adhesion is very complex and there are many mechanisms and models for describing the phenomenon (23–25).
2.5.1 The mechanisms of adhesion
In order to be able to describe the mechanism for adhesion between surfaces, 4 theories are used (26):
• Physical adsorption. Occurs when the adhesive has a low surface energy that is sufficient to be able to wet the surface. Wetting is when a liquid can stay in contact with the solid surface, through intermolecular attractions. In order to be able to achieve good substrate wetting, the surface tension of the substrate (γs) should be higher than the surface tension of the coating material (γp), or equal. The liquid on the surface can spread out and have very good contact with the surface, if the surface has a sufficiently high free energy. In metallic surfaces, this free energy can be increased through flawless surface preparation, which means that chemical contaminants and adhered oxide layers are removed. This theory describes the bounding of adhesive to metallic surface very well.
• Chemical bonding. Here, stronger primary chemical bonds are formed, compared to physical adsorption. They can arise with the help of primer, which is applied flawlessly on the surface. Organosilane coupling agents can be used, which are monomeric silicone-based chemicals. The inorganic metal surfaces usually do not form primary chemical bonds with the organic polymers.
8
• Electrostatic attraction. When the surface is placed in a liquid, an electrostatic double layer is formed. This occurs when molecules that are divided into two layers settle at the interface with opposite charge to each other, due to the electrostatic charge at the surface. If adhesion has solidified on the surface, then this theory will not play such a big role when it comes to structural bonding.
2.5.2 Adhesion forces and surface energy
Between two materials, B and C, the adhesion work (WA), is the energy required to be able to separate the interfaces of the surfaces. According to the physical adsorption theory, this can be described by the following equation (2-11) (26).
𝑊!= 𝛾!+ 𝛾! − 𝛾!" 2-11 where γB is the free energy on the surface B, γC is the free energy on the surface C and γBC is the interface of free energy between the adhered surfaces.
The surface is stable if the energy is positive and if the energy is negative the surface is unstable, which can lead to debonding. γB and γC should be replaced with γBL and γCL if the debonding takes place in the vicinity of a liquid. It has also been pointed out that in the vicinity of water, the thermodynamic adhesion work is negative for epoxy and steel interfaces. This is because the bond is unstable and in the vicinity of moisture they can lead to
debonding. As mentioned above the free energy can be increased through flawless surface preparation and use of organosilane coupling agents, which can also improve the moist endurance of the bond interface (26).
The surface tensions play a major role in determining how a coating can get wet and adhere to a substrate. The contact angle also plays an important role for wetting. If the contact angle is low, it results in better wetting. The equilibrium contact angle is related to different surface tensions and can be described by Young's equation:
𝛾!"= 𝛾!"cos 𝜃 + 𝑦!" 2-12
cos 𝜃 =!!"!!!"
!!" 2-13
where γsa is the surface tension of the solid in air, γlv is the surface tension of the liquid that is in equilibrium with the vapor, γsl is the interfacial tension between that liquid and the solid and θ is the contact angle (23,27). There were some problems with Young's equation that needed to be changed. According to Harkins and
Livingston (27), γsl becomes smaller than γsa when the surface of the solid has an adsorbed film of steam (from the liquid) on it. Therefore, Harkins and Livingston proposed the term πe to demonstrate the reduction. Young's equation can be rewritten as:
𝛾!" = 𝛾!"cos 𝜃 + 𝑦!"+ 𝜋𝑒 2-14
9
By combining equations 2-11 and 2-14, you get:
𝑊! = 𝛾!" 1 + cos 𝜃 + 𝜋𝑒 2-15
πe is insignificant under certain conditions, so if that term can be ignored gives the Young-Dupré equation: 𝑊!= 𝛾!" 1 + cos 𝜃 2-16
where γlv and θ are determined experimentally (23,27).
2.6 The scaling process
The mechanism for scaling takes place in different stages. In the first step, cations (Ca2+) and anions (CO 32-) collide with each other to form ion pairs in solution. The ionic pairs then form micro aggregates, where some grow and become the actual nucleation site for the crystallization. The microcrystals agglomerate and grow larger, these then combine to form strongly bonded macro crystals. Through adsorption, these continue to grow together with the scaling ions from the solution and thereby forming a scale film on a surface (28)
.
2.6.1 Chemical potential
By using Gibbs free energy (∆G), the reactivity of chemical components in a solution can be measured. The chemical potential is determined based on how the free energy changes in the system, when molecules or the number of moles of the reactant change in a system at constant pressure and temperature (20,29). The relationship between Gibbs free energy and chemical activity is given by the following equation (2-17).
∆𝐺 = 𝑅𝑇𝑙𝑛 !"#
!!" 2-17
Where R is the gas constant (JK−1mol−1), T is the absolute temperature (K), IAP is the activity product of free ions (Ca2+ and CO32-) and Ksp is the solubility product (29).
10
2.6.2 Supersaturation
Supersaturation occurs when there is a change (increase) in the concentration of dissolved anions (CO32-) and cation (Ca2+), than in the bulk solution itself. This is the driving force for scale formation and is affected by, for example, pressure, temperature and pH (30).
The supersaturation ratio (S) of calcium carbonate is defined as:
𝑆 =(!!"!!!)(!!"!!!)
!" 2-18
where ai is the activity of a specific ion (in this case CO32- and Ca2+) (30).
Something that is also used to calculate the scaling tendency is the supersaturation index (SI), which is given as:
𝑆𝐼 = 𝑙𝑜𝑔 (!!"!!)(!!"!!!)
!!" 2-19
In order for scaling to take place from a solution, there are three possibilities, with the help of thermodynamics (30).
• S < 1, the solution is unsaturated and the disposition of the scale formation is not thermodynamically possible.
• S = 1, the solution is in equilibrium. The scale formation and the dissolution rate are the same, which means that no scale formation takes place in the solution.
• S > 1, the solution is supersaturated and there is a high tendency for scaling.
2.6.3 Induction time
The induction time (period) is affected by the supersaturation ratio in a solution. The induction time (tind) is a measure of the capability of a supersaturated system to remain in a metastable equilibrium. It is given by the following equation 2-20.
𝑡!"#= 𝑡!+ 𝑡!+ 𝑡! 2-20
Where tr is the “relaxation time”, tn is the time it takes for the development of stable nuclei and tg is the time before the nuclei can be detected. The induction time is of great importance regarding the kinetics of the nucleation process (31,32).
The induction time is inversely proportional to the nucleation rate (J), as given in equation 2-21.
𝑡!"#∝ 𝐽!! 2-21
11 𝑙𝑜𝑔𝑡!"# ∝ ! ! !!!"#$! 2-22
where γ is the interfacial tension. The equation says that for a certain temperature, logtind and (logS)-2 will give a straight line, where the slope can calculate a value of y. This can only be used for homogeneous nucleation (31,32).
The Arrhenius reaction rate equation can be rewritten in terms of induction time:
𝑡!"#= 𝐴𝑒𝑥𝑝 ∆!
!!! 2-23
where A is a constant and kB is the Boltzmann constant (1.3805 x 10-23 J/K). The Arrhenius reaction rate ratio is most often used for a thermally activated process to determine the rate (32).
2.6.4 Nucleation
In order for crystals to develop, there must be some form of solid body, such as nuclei. These act as a center for crystallization. Nucleation occurs at the surface that comes in contact with liquid or vapor-suspended particles. It can happen spontaneously or synthetically. Nuclear formation sites are most often formed in small cracks where a free gas-liquid surface is maintained or in spots on the surface that have a lower wetting property. The nucleus grows when atoms from the liquid attach to it (32,33).
Nucleation is divided into two sections, primary nucleation, where the formation of new crystals does not depend on the presence of existing crystals and secondary nucleation, where the formation of new crystals is driven by the presence of existing crystals in the bulk solution. Primary nucleations are also divided into two sections,
homogeneous (spontaneously) and heterogeneous (stimulated by external particles) (32).
2.6.5 Primary nucleation
12
2.6.5.1 Homogeneous nucleation
Homogeneous nucleation is the formation of nuclei that are not in the presence of any other external phase or molecular species. An example that is often used to describe a homogeneous nucleation is so-called condensation nuclei. These are microscopic droplets that are formed by supersaturated vapors, at the surface. Homogeneous nucleation occurs at very high concentration. The heterogeneous nucleation takes place at lower concentration, which means that it will have a maximum value before it transitions to a homogeneous nucleation. This means that the homogeneous nucleation will be essential for the nucleation mechanism (32–34).
If the nucleation mechanism is homogeneous, the rate of nucleation can be written as:
𝐽 = 𝐴𝑒𝑥𝑝 − !"!!!!!!
! !!!! !"#! 2-24
where vm denotes the molecular volume of crystals (calculated from vm = M/(ρNA), where M is the molecular mass, ρ is the particles density and NA is Avogadro number) and A is the constant of nucleation (31).
The induction time is inversely proportional to the nucleation rate and can therefore be describes as:
𝑡!"#= 𝐴𝑒𝑥𝑝 !"!!!!!!
! !!! !!"#! 2-25
In the case of a homogeneous nucleation, the interfacial tension can be calculated by the following equation 2-26 (31). 𝛾 = 𝑘 !"!!!! !! ! ! 2-26
Examining the kinetics of a homogeneous nucleation process can sometimes be difficult, as a system is usually not free of impurities. One way to describe the homogeneous nucleation process is through free energy (Gibbs free energy). The general change can then be described as the following equation 2-27 (32).
Δ𝐺 = ∆𝐺!+ ∆𝐺! 2-27
Where ∆G denotes the free energy between a small particle (with radius r) and the solute in the solution. ∆Gv denotes the free energy for the change between the solution and a large particle. ∆Gs denotes the energy that is connected to the newly created surface.
The general free formation energy (∆G) can reach a maximum value (∆G𝑐𝑟𝑖𝑡). This maximum value is the cause of the formation of stable particles, which represents the critical size of a nucleus (32). The critical radius (rc), relates to the minimum size at which a nucleus is stable and can be described as:
𝑟! =!!!
∆!! 2-28
13
The Gibbs free energy for heteronucleation (∆G𝑐𝑟𝑖𝑡) can be written as: ∆𝐺!"#$= !"!!
!
! ∆!!!=
!!"!!!
! 2-29
In a newly formed crystalline lattice structure, the nucleation properties of a supersaturated solution depend on its critical size. It can either grow or dissolve, depending on the following.
• If r < rc, then the particle will dissolve or evaporate. • If r > rc, then the particle will grow.
This means that there is a quadratic relationship between the characteristic size on the surface or as given by surface roughness and the Gibbs free energy for heteronucleation.
The Arrhenius reaction rate equation can also be rewritten in terms of nucleation rate.
𝐽 = 𝐴𝑒𝑥𝑝 !∆!!
!! 2-30
From equation 2-29 the critical size of a nucleus can now be rewritten as:
∆𝐺!"#$= !"!!!!!
! !!!"#$ ! 2-31
where γ denote the interfacial energy (mJ/m2), v the molecular volume (cm3/mol) and S the saturation ratio (32).
2.6.5.2 Heterogeneous nucleation
Heterogeneous nucleation is stimulated by external particles and impurities in the system. Heterogeneous nucleation occurs more often than homogeneous nucleation, because it requires less energy than homogeneous nucleation. It is formed by conversion between two phases of solid, liquid or gas. A system with a large volume has a greater chance of being contaminated by heteronuclei (active particles). The size of the heteronuclei is therefore of great importance (32).
It forms at pre-existing surfaces. Such surfaces have low surface energy and thereby decreasing the free energy needed for nucleation. The free energy required for heterogeneous nucleation is equal to the product of
homogeneous nucleation and a function of the contact angle (θ):
∆𝐺!"#$,!!"!#$%!&!$'( = 𝜃 ∙ ∆𝐺!"#$,!!"!#$%$!&' 2-32
where 𝜃 = !!!"#$ !!!"#$!
! .
14
𝛾!" = 𝛾!"+ 𝛾!"cos 𝜃 ⇒ cos 𝜃 =!!"!!!!"
!" 2-33
where γcl describes the interfacial energy between solid crystalline phase and liquid (c stands for the solid crystalline phase and l for the liquid), γsl describes the energy of the interfacial energy between solid external surface and the liquid (s stands for solid external surface), and γcs describes the interfacial energy between solid crystalline phase and solid external surface. θ is the contact angle between crystalline deposit and solid external surface, also called the wetting angle in liquid to solid system (32).
2.6.6 Secondary nucleation
Secondary nucleation is a type of nucleation, where the formation of new crystals is driven by the presence of existing crystals in a bulk solution. Secondary nucleation differs somewhat from primary nucleation, as it occurs by introducing new interfaces. This results in a lower energy, which makes it easier for the molecules to grow on the nuclei (35).
The crystal size is similar to nucleation and nuclear growth, which are also affected by supersaturation and temperature. It can be said that reduced crystal size results in increased nucleation rate of the secondary
nucleation. Solubility is also important for nucleation. Low temperatures lead to a high solubility, which leads to the crystallization process being dominated by the secondary nucleation. If the supersaturation is high, then the nucleation rate is high (for secondary) and if the growth rate is low, then the particle size is also low (35). Kashchiev and Firoozabadi developed a relationship for heterogeneous nucleation, which is based on the induction time and can be described as following (36):
𝑡!"#= 𝐾 𝑆 𝑆 − 1 !!
!
!!!! 𝑒𝑥𝑝 !
(!!!!)!"!! 2-34
where K is a kinetic parameter, m is a parameter that describes the relationship between the radius and time (t = rm) of the primary nuclei. The thermodynamic parameter B, are described below:
𝐵 = !!!!!!!
!" !!! ! 2-35
where c is a form factor (c3 = 36π for spherical nuclei) (33).
2.6.7 Crystal growth
15
grow to a visible crystal size. The crystal growth is related to its surface area and the free energy. If the surface energy is low, this will promote crystal growth stability (32).
Clusters that can be formed in a supersaturated solution can also form so-called seed crystals. These grow with the help of adsorbent ions in small cracks on the surface. This causes the free energy to decrease, which favors the growth of larger crystals. Larger crystals grow faster than smaller ones and can reduce the secondary nucleation rate (32).
The crystals formed on the surface by scaling are usually larger than those formed in the bulk solution. This means that the heterogeneous properties of the scaling process benefit the crystal growth (30). In crystal growth, there are three fundamental mechanisms, which are adsorption layer theory, diffusion theory and surface energy theory.
2.6.7.1 Adsorption layer theory
Adsorption means that atoms, ions or molecules from a dissolved, gas or liquid substance, sticks to a surface and forms a film. There are two types of adsorption, physical adsorption that occurs by London-van der Waals forces and chemisorption, which occurs by chemical bonding forces (37).
According to Max Volmer's adsorption layer theory (38), crystal growth is an interrupting process. It describes the adsorption on the surface where the mobility of adsorbed molecules is allowed, but no interaction is allowed between the adsorbed molecules. The adsorption layer is of great importance for crystal growth and for the secondary nucleation. What happens is that molecules or ions are connected to the lattice at the active center, where a layer is built up on the entire surface (under ideal condition). When the layer is complete, more layers can be built. This can be described using the Gibbs-Volmer two-dimensional nucleus (32).
By using the Arrhenius reaction rate equation, the rate of Gibbs-Volmer two-dimensional nucleation, can be described as:
𝐽 = 𝐵𝑒𝑥𝑝 !∆!!"#$
!!! 2-36
where J is the two-dimensional nucleation rate and B is the thermodynamic parameter defined by equation 2-34 (32).
2.6.7.2 Diffusion Theory
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According to Noyes and Whitney (40), a diffusion process takes place on the surface of the crystal, which is controlled by the difference in concentration at the solid surface and in bulk of the solution (32).
This can be defined by the following equation (2-37).
!"
!" = 𝑘!𝐴 𝑐 − 𝑐∗ 2-37
Where m is the mass (precipitated solute) at time t, km is the mass transfer coefficient, A is the surface area of the crystal, c is the dissolved concentration in the solution itself and c* is the equilibrium saturation concentration. Berthoud and Valeton altered the equation 2-37, by assuming that there was a diffusion process in the mass deposition. This process takes place in two steps; the dissolved molecules are transported from the bulk solution of the liquid phase to the solid surface, which is then followed by a surface reaction where the dissolved
molecules settle in the crystal lattice (32). These two steps are affected by different changes in the concentration and can be described as:
Diffusion: !"!" = 𝑘!𝐴 𝑐 − 𝑐! 2-38
Reaction: !"!" = 𝑘!𝐴 𝑐!− 𝑐∗ 2-39
where kd is the mass transfer coefficient by diffusion, kr is the rate constant for the surface reaction process, ci is the dissolved concentration in the solution at the crystal contact surface (32).
2.6.7.3 Surface energy theory
Gibbs claimed that a crystal at equilibrium should have a minimum surface energy, which means that the total surface is in equilibrium with the total free energy and should be at a minimum. This type of crystal form is called the equilibrium crystal form and is very rare at constant pressure and temperature (32,41). This equilibrium form is produced when the crystal is allowed to grow in a supersaturated medium, which means that the process of the various surfaces must have minimal total surface-free energy for specific volume (32).
Pierre Curie (42) then suggested that the growth rates of crystal faces are proportional to the free energies of the surface. After that George Wulff (43) came up with the suggestion that if the length of a vector is drawn normal to a crystal surface, it will be proportional to its surface energy, which can be written as:
!!
!!= 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 2-40
17
According to Wulff, the equilibrium shape of a crystal is correlated to the free energies of the faces. He meant that crystal faces would grow at velocities that are proportional to their respective surface energies. The growth rate would then be inversely proportional to the lattice density, which means that surfaces with high face index grow faster than low ones and is measured by the speed of movement that goes along the face direction (32).
2.7 Factors that impact scaling
As mentioned above, there are different factors that can affect scale formation, such as pH, temperature, pressure, flow rate, degree of supersaturation and the presence of interfering ions or other salts in the solution (4,45).
2.7.1 Impact of temperature
According to the study by Yong et al. (46), temperature is significant for the crystallization process and the structure they get. At lower temperature (at 25 °C) they could observe vaterite and calcite crystals and at higher temperature (at 60 °C) they could observe aragonite crystals. The change that occurs in polymorphs with respect to the temperature can be attributed to the thermal vibrations and that the temperature has an effect on the solubility and growth of calcium carbonate scale (46). As the temperature increases, the solubility of calcium carbonate decreases, therefore it is easier for crystallization on heated surfaces (47).
Another study showed that at room temperature, vaterite and calcite were the most prevalent, but that pH and calcium ion concentration also play a role in the structure and size (45).
If there is a high concentration of, for example, malic acid and a low temperature, it has been shown that there is a reduction in the amount of calcium carbonate scale. The activation energy increases at a higher temperature, which leads to a faster reaction rate (48).
2.7.2 Impact of pH
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In a study by Andritsos and Karabelas, it was found that an increase in the pH of the solution leads to a rapid deposition of calcium carbonate scale. The pH increases due to the loss of CO2 from the solution (49).
2.7.3 Impact of supersaturation
Supersaturation is one of the driving forces for crystallization, which has been described in the calcium carbonate system. Supersaturation affects crystal growth, crystal size and the amount of scaling (45).
According to Charkraborty and Bhatia (50), the ionic ratio between [Ca2+]/[CO32−] and the degree of
supersaturation in a solution is of great meaning for the particle size distribution and the crystalline form (46). In a study, "pure" precipitation scaling experiments were performed; it showed that if the critical supersaturation ratio were about 7-8, the initial deposition rate would increase greatly. If the flow rate and temperature were to increase, this would lead to an increase in the deposition rate. The induction time decreases with reduced supersaturation conditions (< 7). All this shows that the scale formation process is controlled by supersaturation conditions, if the conditions are larger than the critical, and at lower conditions it is controlled by a surface reaction mechanism (49).
In a study by Kitamura, it turns out that the crystal size of calcium carbonate decreases with concentration. At high supersaturation, nucleation rate increases and then a metastable vaterite has a tendency to form, which increases with increasing solution concentration. At low supersaturation, a stable calcite tends to form (51).
2.7.4 Impact of pressure
Scaling is usually due to the pressure decreasing in the formation water. A reduction in pressure can cause different materials to have a reduced solubility in the water (21). Calcium carbonate scaling is usually formed at high temperature and low pressure, which is the reason why deposits of calcium are formed on, for example, walls in production wells (19).
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2.7.5 Impact of flow rate
When it comes to deposition and removal, it has been shown that high velocities can reduce scale formation, but not when it comes to pure calcium carbonate precipitation. At high flow rates, more ions are formed which causes scaling to increase, so calcium carbonate scaling increases with increasing flow rate and results in a more compact scaling layer (47–49).
According to Pääkkönen et al., surface integration (is a term that describes the attachment of fouling species to the surface) and mass transfer controls the scaling process in the integration diffusion model. With increased flow rate, surface integration controls the scaling. The residence time of the liquid at the surface becomes shorter, which reduces crystals from adhering to the surface. If, on the other hand, the flow rate is low, the mass transfers controls scaling. With increased flow rate, the surface temperature is also reduced, which reduces the scaling. Rhombic calcite was formed at low flow rate (47).
It has been shown that at high flow rate or at low CaCl2 concentration (at start), that most spherical vaterite is formed. Although a low flow rate leads to a low supersaturation, because there is not enough CO2, which is important for the dissolution of the vaterite (46).
2.7.6 Impact of impurities
The presence of foreign ions or minerals has a strong influence on inorganic scaling. Foreign ions, such as Fe3+ and Cr3+, can affect nucleation and increase the induction time of inorganic salts in aqueous solutions. The ions can also have a strong inhibitory effect and the higher the charge on the cation, the stronger the effect. If impurities interfere with the primary nucleation, then secondary nucleations can be formed. However, if the impurities are adsorbed at, for example, cracks on the surface where crystals are present, this can improve
secondary nucleation and the crystals may break. If the solution structure or equilibrium solubility is changed, this can lead to different effects from the impurities. It is difficult to know exactly which are the most pronounced consequences impurities cause (32).
Foreign substances, in addition to the crystallization itself, are considered to be an impurity. There are various terms you can use to describe the foreign substances that are not the solvent, such as, inhibitors, additives and poisons. An impurity that you add (in large quantities), is called an additive. Impurities can both slow down and speed up the growth process. Those which slow down growth are called inhibitors and those which accelerate are called growth promoters. There are three parameters that are affected by impurity that is adsorbed on the crystal face, that is, the solubility of the crystal, the kinetic and thermodynamic terms that belong to the growth model (52).
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2.7.7 Impact of solution chemistry
Scale formation of calcium carbonate depends very much on the composition of the solution and various
environmental factors. Solution chemistry has a major impact on inorganic scaling. The ionic species present in a solution determine the degree of supersaturation. If the degree of ions in the solution increases, the solution becomes more supersaturated. This is also due to other factors such as pressure and temperature (54,55).
2.7.8 Impact of surface roughness
Surface roughness is included in the surface structure and depends on how large the deviations are in the direction of the normal vector on the surface. If they are small, it means that the surface is smooth and if they are large, the surface is rough (56). Surface roughness has a big impact because adsorption is very sensitive to change in the area and has an important role in heat transfer and fluid flow. Surface roughness affects the contact surface area; a rough surface increases the surface energy as opposite to a smooth surface. A rough surface has a stronger
adhesion, which can lead to a larger amount and higher strength of the scaling layer (4,57). If the surface roughness increases, this can lead to the formation of calcite (58).
Mottahed and Molki, concluded that the surface roughness drastically reduces the entrance area of circular tubes. This means that it can be considered that the mass and heat transfer coefficients are constant, even though the surface roughness increases (59).
The roughness of the pipes can also affect the deposition. Smooth pipe surfaces mean that the coefficient of friction increases and that the cooling water flow decreases, which means that deposition grows faster. On the other hand, smooth pipe surfaces can improve the performance of the heat exchanger and condenser, for example by reducing re-deposition on surfaces and thereby increasing the time between cleanings (4).
2.8 Desorption
Desorption is the release of a substance from another, either through or from the surface. Desorption occurs when there is a change in a system that is in an equilibrium between the adsorbed surface and the bulk solution (60). The desorption rate (R), can be described as:
𝑅 = 𝑟𝑁! 2-41
21
𝑟 = 𝐴𝑒!!!" 2-42
where A is the frequency or pre-exponential factor and Ea is the activation energy of desorption (60).
2.9 Plate Heat Exchanger
The plate heat exchanger is the heat exchanger that Alfa Laval produces the most of. The purpose of a plate heat exchanger is to transfer thermal energy between two mediums, without mixing them. It makes is possible to separate the hot medium from the cold. A major advantage of these heat exchangers is that the media can be spread over the plates, through flat channels that are next to each other. In every other channel one medium flows and in the other channel another. This increases the surface area of the liquids, which enables heat transfer and increases the rate of temperature change (61).
Figure 2-4: The main parts of the plate heat exchanger, image retrieved and modified from (62).
2.10 Uncoated and Coated Stainless Steel
Stainless steel is actually a general term that describes a variety of steel types and is mainly made of iron and carbon. There are different types of stainless steel and a three-digit number separates them. Austenitic stainless steels have a 200-series and a 300-series. In this study, stainless steel of type 316 was used. It contains, among other things, chromium, nickel and molybdenum, which makes it very resistant to corrosion. Type 316 is durable and easy to manufacture.
22
The gel is then applied to the substrate, either by dip coating or spin coating. Then dried to obtain a hard and shiny surface. This is an effective method for corrosion and oxidation protection (63).
Chemical Vapour Deposition, CVD is a process in which powders or films are formed by chemical reactions or dissociation of gaseous reactants on or near a heated surface. What happens is that the surface is exposed to one or more volatile precursors that react and/or decompose on the surface, leading to the formation of a coating. CVD coatings are found almost everywhere and are an effective method for corrosion resistance and scaling resistance (64).
2.11 Ellipsometry
Ellipsometry is a technique often used to analyze optical properties on a thin film. An ellipsometer consists of five main components: a light source, a polarization state generator (PSG), a sample surface, a polarization state detector (PSD) and a light detector (Figure 2-5, a) (65). The technique is based on measuring the reflection at a smooth sample surface. The light travels into the PSG where it is polarized and then reflected on the smooth surface (at a large angle of incidence). Upon reflection, the light is polarized in accordance with the Maxwell theory of electromagnetics and described by the Fresnel equation (66). Thereafter, the light travels into the
analyzer where the polarization state is measured and then reaches the detector. In the detector, the intensity of the light is measured, which can be calculated with the help of a computer and certain analysis software. What can be obtained in the measurement are, for example, the thickness, adsorption and refractive index of the thin film. The method is based on a change in the polarization state of the light at the reflected surface, at an interface. This change determines the measurement data. The change that occurs at the surface of the thin film is very sensitive to surface properties, but can be detected with high accuracy (65,66). At adsorption studies for proteins, surfactants and polymers, the measurements are made at the liquid/solid interface (67).
Figure 2-5: (a) Schematic diagram of an ellipsometer, where Φ is the angle of incidence, image retrieved and modified from (68). (b) Upon
reflection at an interface, the light interacts with the material. This, in turn, changes the state of polarization and gives sensitivity to film thickness and optical properties if there is a film present. The incoming wave ki, is partly reflected, kr, and partly transmitted, kt, into the
medium. The s and p vectors indicate the direction of the polarized light.
23
A null ellipsometer measures azimuth angle (where azimuth is a horizontal coordinate system) for polarizer, compensator and analyzer. The technique involves adjusting the azimuth angles so that the polarized light is reflected linearly from the sample surface, so that the light at the detector extinguishes. Here, PSG is a
polarization delay pair and PSD is a linear polarizer, also called an analyzer. The compensator and the polarizer work together as a polarization filter to be able to generate a light that is reflected linearly from the sample surface. The light source can either come from a laser or a xenon lamp (65,66). In this method, several zones are used to determine and correct the zero positions in the polarizer, compensator and analyzer. Here, the light travels into the polarizer and the compensator, where the angle setting is changed so that the light can be reflected from the sample surface linearly. Then the light travels to the analyzer and on to the detector to detect a minimum, hence its name null or nulling ellipsometer (65,66).
Most often, an optical model is used, which consists of homogeneous layers. The data that is obtained from the use of a null ellipsometer consists of two angles, which describes the change of the polarized light. They are represented as Δ, which is the relative phase shift, and as Ψ, which is the relative change in amplitude. These can be described according to the equations 2-43 to 2-44.
∆= 𝛿!!− 𝛿!! − 𝛿!! − 𝛿!! 2-43 tan Ψ = !!! !!! !!! !!! 2-44
where δ is the phase, A is the amplitude, r is reflected light, i is incident light, s is parallel to the plane of incidence and p is perpendicular to the plane of incidence (69).
The adsorbed amount, Γ, is calculated by using the de Feijter equation.
Γ = ! !!!!! !" !"
2-45
where d is the thickness of the adsorbed layer, n1 is the refractive index of the adsorbed layer, n0 is the is the refractive index of the bulk solution and dn/dc is the refractive index increment (69).
24
2.12 Scanning Electron Microscopy (SEM) and Energy-Dispersive X-Ray Spectroscopy
(EDX)
Scanning Electron Microscope is usually abbreviated SEM and is a very useful technique used in topological and crystal structure images of sample surfaces. The electron-optical column consists of an electron gun, an objective lens, two condenser lenses, an electron detection system and a set of deflectors (Figure 2-6, a). What happens in a SEM is that the sample is placed in a vacuum chamber and the electron gun emits an electron beam that reaches the sample, in energy ranges from 1 to 30 keV. The beam reaches a depth of about 1 µm and forms signal electrons which are then detected by a detector and give rise to an image. These signal electrons are secondary electrons (SE), backscattered electrons (BSE) and characteristic X-rays (Figure 2-6, b). The image is obtained with the help of a computer and specific software. The SEM detector can be selected in the software, depending on what the surface looks like and what kind of signal electrons it generates. SE is the most common type of electron used in SEM, due to their low energy (57,70). This means that detection of the sample surface can be done with just a few nanometers. SE can produce accurate images of the surface roughness and structure. If the SE does not reach all the way to the detector, you will see a dark contrast for the image. BSE is most often used for the detection of electrons with energy higher than 50 eV and can provide more information about the structure below the sample surface. In order for an image to be detected, the BSE must travel in a straight direction from the sample. The image generation for SE is dependent on the surface topography, while BSE depends mostly on the atomic number of elements present on the sample surface. SEM has no magnifying lenses, but the
magnification of the image is determined by the ratio between the length of the monitor and the length of the scan on the sample surface. Changing the current amplitude in the scanning coils can change the magnification. The electron gun consists of an electron source, either a tungsten filament or a semiconductor crystal. The most common is tungsten filament (cathode), which is surrounded by a Wehnelt cylinder (electrode). Together with an anode, the electron gun forms an electrostatic lens, which gathers the electrons to a crossover (57,70).
Figure 2-6: (a) Schematic diagram of a SEM, image retrieved and modified from (71). (b) Interaction volume, by Freundchen, 2015 (72).
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The advantages of SEM are that it is easy to use with the right training, works quickly and provides a very detailed picture. Also collects versatile information from the different detectors and can be combined with EDX, for a more detailed analysis of the elements on the solid surface. The disadvantages, however, are that they are quite large and cost a lot. SEM is limited to only small solid samples that can fit into the vacuum chamber and that can also withstand vacuum pressure (73).
There are other techniques that can be combined with SEM, to be able to get other images or information about the sample. Something that is most often used in combination with SEM is energy dispersive spectroscopy, which is abbreviated EDX or EDS and is a chemical microanalysis technique (Figure 2-7). It can be used for example, spot detection analysis (on the bypass with a diameter of up to 10 cm), identification and material evaluation of various elements on the sample (57,70). The technique involves detecting X-rays emitted from the sample during massive firing from the primary electron beam in the SEM to characterize the elemental composition of the analyzed volume. When the sample is fired at, an electron hole is formed at the nucleus of the atom, which is then filled with electrons from a higher state and an X-ray is emitted. The X-ray detector in EDX measures the emitted X-rays in comparison with their energy. When an X-ray reaches the detector, it creates a charging pulse that is proportional to the energy of the X-ray. The charging pulse is then converted to a voltage pulse and the signal is forwarded to an analyzer where the pulses are sorted by voltage. The energy of each X-ray is sent to a computer with specific software for display and additional data analysis. Here, the elemental composition of the volume on the sample surface is identified (57,70).
Figure 2-7: (a) Principle of EDX, by Muso, 2007 (74). (b) Spectrum from EDX, by Ziel Rainer, 2008 (75).
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2.13 Refractometer
A refractometer is a technique used to measure the refractive index (n) of a sample in the visible range (77). It consists of a light source, an upper and lower prism box, a measuring prism, and a field and scale telescope. The technique is based on placing a few drops of the sample in the middle of the measurement prism. The light will then enter the refractometer through the open front sliding shutter on the upper prism box and hit the interface of the sample, at different angles (77). When light is transmitted through the samples, different spectral lines will appear, where the colors represent different wavelengths (Figure 2-8, a). The contrast and colors of the spectral lines can be improved by adjusting the lens of the field telescope or by adjusting the opening of the incident light. In order to be able to obtain the refractive index, the cross that is visible in the field telescope must be placed at the borderline (Figure 2-8, a). Then the values for the different spectral lines can be obtained by looking in the scale telescope. By using a software (Abbe Utility Software) the RI values for the corresponding wavelengths can be calculated (Figure 2-8, b). By interpolating the calculated RI values (from the software), using the Cauchy's equation (described in equation 2-46), the refractive index at a specific wavelength can be determined.
𝑛 𝜆 = 𝐴 + !
!! 2-46
where λ is the wavelength, and A and B are tunable parameters. The refractometer has a measured accuracy of ± 10−4 (77).
Figure 2-8: (a) Spectral lines at different wavelengths, (b) Abbe Utility Software and (c) Actual photo of the refractometer.
a
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2.14 Quartz Crystal Microbalance with Dissipation monitoring
Quartz crystal microbalance (QCM) is a very sensitive technique used to measure mass balance, by measuring the frequency change of a quartz crystal sensor in nanograms. The QCM-D is used to measure mass changes in frequency and energy loss in processes occurring on the surfaces or in thin films adsorbed on a quartz sensor. The method is based on using a certain quartz sensor and by applying an electric field, a mechanical oscillation with a specific frequency can be formed on the crystal (78). When there is a change in the mass at the surface interface or in the thin film, the frequency shift is measured from the fundamental resonant frequency of the crystal (f0). In addition to measuring f0, changes in the energy loss factor are also measured, which has to do with the viscoelastic properties of the upper layer or of the adsorbed thin film to the sensors. When the generator is turned off, the crystal oscillation amplitude decreases exponentially and the energy loss factor can be measured by registering the amplitude of the oscillation as a function of time. The measurements are done using a software, which calculates the values of the frequency and the dissipation. The dissipation (D) is defined as:
𝐷 =!!=!!"##"$%&'!
!!!!"#$%& 2-47
where, Q is the quality factor, Edissipated is the energy lost during one oscillation cycle and Estored is the total energy stored in the oscillator (78).
Figure 2-9: Schematic diagram of an QCM, where CE is counter electrode, RE is reference electrode and WE is working electrode, by
Zhao Ruoha, 2020 (79).
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3 Methodology
This section describes all the methods used in this thesis, including the main techniques used for the analysis of adsorption and desorption of calcium carbonate scaling and also the materials that were used. Alfa Laval provided the material and the Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy. Lund University provided the Rudolph Ellipsometry, Refractometer and Quartz Crystal Microbalance with Dissipation monitoring.
3.1 Experimental materials
Four types of surfaces were selected as substrates: stainless steel surface (SS316) with a contact angel of 66 ± 6 °, two stainless steel surfaces coated with different sol gel (coating 1 and 2). The first sol gel has a contact angel of 105 ± 1 ° and a thickness of 5 ± 2 µm, and the second has a contact angel of 45 ± 1 ° and a thickness of 400 nm. One stainless steel surface coated with CVD (coating 3) with a contact angel of 75 ± 1 ° and thickness of 1.2 ± 0.6 µm. The selected coated surfaces are coatings that Alfa Laval uses for their plates in their heat exchangers. They were also considered to have the greatest chance to work in the ellipsometer measurements, but only two of the coated surfaces worked. That is why only coating 2 and 3 were used in the ellipsometer.
Two types of hard water were chosen as solutions, due to their hardness: tap water from Denmark (hard water 1) with a total hardness of 51 degrees of German hardness (° dH) and water from a well (hard water 2), from Marieholm (Sweden), with a total hardness of 26 ° dH. Why two different hard waters were chosen was because hard water 1 was good for short scaling periods, but was considered too hard for the ellipsometer.
Alfa Phos (Alfa Laval, Lund, Sweden) is a cleaning agent used for desorption of calcium carbonate. Alfa Phos contains, among other things, phosphoric acid, phosphates and anionic surfactants. It was diluted to 5 % (ml) with Type 1 water.
3.2 Surface Preparation
The four types of surfaces were prepared for the use of null ellipsometer and SEM-EDX.
For the use of null ellipsometer, the surfaces (stainless steel, coating 2 and 3) were cleaned with ethanol (99.5 %), Type 1 water and dried with nitrogen gas (N2). All surfaces had a size of approximately 13x37 mm.
