Reheating furnace is a furnace that using fuel combustion energy to heat steel products before hot rolling. Materials need to reach the temperature around 1400K uniformly after heating in reheating furnaces. Steel oxidizes during the reheating process. Oxidize scale layer on the surface will changed the heat transfer properties of surface and increase the inner stress of material, reducing the quality of the steel. In this study, a model of scale formation under reheating furnace working condition is developed. The model can be coupled into computational fluid dynamics (CFD). The commercial software, ANSYS FLUENT®, was utilized to give a prediction of furnace atmosphere and calculate the formed scale. A calculator is also developed to predict the scale formation of a single point during the reheating process using measurable flow field data. Furthermore, a series of parametric studies has been investigated to study the influence of operating conditions.

Scale formation is one of the persistent problems in mineral processing and related industries. One of the main components of the scale is frequently gypsum or calcium sulphate dihydrate (= CaS04.2H20). Gypsum is formed through the process of crystallisation, and it is well known that crystallisation process is significantly affected by the presence of admixtures. Industrially, scale formation occurs in an environment which is very rarely free from the presence of admixtures. In a typical mineral processing industry, certain types of admixtures are present, which may include metallic ions (e.g. originated from corrosion products) and certain types of the flotation agents used. The effect of admixtures on crystallisation kinetics and cyclical morphology can be very significant, even if they are present in trace amounts. It is important to emphasise that the effects are generally specific, that there is no unified theory that applies to all and every situation. The present study has investigated the effect of certain admixtures on gypsum crystallisation, and was accomplished in three phases of experiments: (1) seeded batch crystallisation; (2) seeded continuous crystallisation, and (3) once through flow system under isothermal condition. The three phases of the work used equimolar solutions of CaC12 and Na2SO4 to produce CaS04 which is the precipitating species. The seeded batch crystallisation experiment explored the effect of two flotation agents commonly used in mineral processing plants: (1) sodium isopropyl xanthate (= SIPX) and, (2) isopropyl thionocarbamate. The experiments were performed at 25, 35, and 45°C, respectively. The initial concentration of the crystallising solution was 2,000 ppm of Ca 21 and it reached the equilibrium concentration values of between 1,000 and 8,00 ppm of Ca 2+ in 90 minutes. / The effect of the two selected admixtures on crystallisation was measured by continuous monitoring of the desupersaturation of the crystallising solution with time, which subsequently resulted in the determination of the crystallisation rate constant. The results arc as follows. Firstly, the admixtures selected (either individually or in combination) were able to retard the growth rate of gypsum. In the absence of any admixture, the second order rate constant was between 1,405 x 10-6 and 1,561 x 10-6 ppm-1 min-1. Addition of SIPX at a typical plant dosing level 0.200 g/L) reduced the rate constant to 475 x 10-6 PPM-1 min', while isopropyl thionocarbamate at a typical plant dosing level (= 0.070 g/L) decreased the rate constant to 254 x 10-6 ppm-1 min-'. However, addition of a combination of the two admixtures, each at a typical plant concentration level, reduced the rate constant to 244 x 10-6 ppm-1 min-1, which was only slightly below that in the presence of isopropyl thionocarbamate. Thus, in these batch crystallisation studies, isopropyl thionocarbamate seemed to be dominant over SIPX. Secondly, the batch crystallisation system in the current work did not show any induction time. It was concluded that the seeds added into the batch system could be capable of eliminating the induction time. Thirdly, the reduced growth rate of the gypsum crystals as affected by the admixtures was probably caused by the adsorption of admixtures onto the crystal surface. The second phase of the project involved a seeded continuous (MSMPR) crystalliser. Some parameters used in this experiment (mean residence time, agitation speed and type of one admixture) were taken from the batch experiment carried out in the first phase of the project. / Three admixtures were chosen for the seeded continuous crystallisation: (1) SIPX, (2) Fe3+, (3) Zn2-, and they were used either individually on in combination with each other. SIPX was chosen as it is one of the most common flotation agents used in mineral processing. Metallic ions: Fe3+ and Zn2+ were selected, since they were found in substantial amounts in both scale samples and process water in certain minerals processing industries. In general, the admixtures tested were found to be able to inhibit the crystal growth rates, but to enhance the nucleation rates. In addition, the growth rate was found to be dependent on crystal size, and hence, a correlation between these two parameters and the admixture concentration was formulated. For a fixed level of concentration (f 700 ppm of Ca z+ at steady state) and crystal surface area, it was proved that for each crystallisation temperature: 25 and 40°C, the correlation function can be represented as G = k Lα (1 +C)β where: G = linear growth rate, micron/hour; k, α, and β = dimensionless constants; L = (sphere equivalent) crystal size, micron; C = concentration of the admixtures used, ppm. For both the crystallisation temperatures used, the correlation function shows that the growth rate is significantly dependent on crystal size, but a weak function of admixture concentrations. The mechanism of crystal growth inhibition was assumed to be that of adsorption of admixtures onto the active growth sites, thereby decreasing or stopping the growth. Similar to the first phase of the present study, this seeded continuous crystallisation also showed no induction time. The third phase of the project investigated the gypsum scale formation in a oncethrough pipe flow system under isothermal condition and in the presence of admixtures. / Four types of pipe materials were tested: PVC, brass, copper and stainless steel. Two admixtures were selected: SIPX and Fe3+. The behaviour of the gypsum scale formation was measured as the mass of the gypsum scale deposited on the substrate per unit area of the pipe surface. Within the range of the experimental conditions applied in this scale formation study, the following results were obtained. Firstly, the mass of the gypsum scale increased with concentration (in the range: 2,000 to 6,000 ppm of Ca t+) and that the correlation between the mass and the concentration can be represented by quadratic functions. Secondly, the mass of the gypsum scale decreased with increasing concentration of the admixtures used. Thirdly, the flow rate of the scaling solutions (in the range: 0.4 to 1.3 cm/sec) did not significantly affect the mass of the gypsum scale. PVC produced the highest mass of gypsum scale, followed by brass, copper, and stainless steel, respectively. Fourthly, the presence of admixtures caused the surface of the scale deposit to become rougher than was the case in a pure system, and longer scaling experimental times resulted in denser scale deposits. In this scale formation project, the induction time was investigated. In contrast with the first and the second phase of the projects, the induction time in the scale gypsum formation experiment was significant. At a concentration of 2,000 ppm of Ca 2+' pure gypsum solutions had induction times of about 105 minutes at 18.3°C and 97 minutes at 20.3°C. In the presence of 10 ppm of SIPX, the scaling solution at 2,000 ppm of Ca2+ and 19.2°C had an induction time of 1,400 minutes. The present study produced three important findings. / Firstly, the presence of Fe 3+ or sodium isopropyl xanthate (SIPX) reduced the growth rate of gypsum crystallised either in a vessel (= a continuous crystalliser) or in a pipe flow system. Secondly, the rate of growth of gypsum crystals was found to be consistently higher in the vessel than in the pipe flow system. The rate of growth of the pure gypsum in the crystalliser at 25°C was 0.0389 kg/ m2 hour while those in the pipe flow system were between 0.0289 and 0.0202 kg/m2 hour, depending on the pipe material and the scaling solution flow rate. Thirdly, with respect to gypsum scaling, PVC was the least favourable material, followed by brass and copper, while the most favourable was stainless steel. It is believed that the present study has significantly contributed to the understanding of the effect of admixtures on crystallisation of gypsum, especially in relation to the scale formation.

Yes / Fouling on heat transfer surfaces due to scale formation is the most concerned item in thermal desalination industry. Here, a dynamic fouling model is developed and incorporated into the MSF dynamic process model to predict fouling at high temperature and high velocity. The proposed dynamic model considers the attachment and removal mechanisms in the fouling phenomena with more relaxation of the assumptions such as the density of the fouling layer and salinity of the recycle brine. While calcium sulphate might precipitate at very high temperature, only the crystallization of calcium carbonate and magnesium hydroxide are considered in this work. Though the model is applied in a 24 stages brine recycle MSF plant, only the heat recovery section (21 stages) is considered under this study. The effect of flow velocity and surface temperature are investigated. By including both diffusion and reaction mechanism in the fouling model, the results of the fouling prediction model are in good agreement with most recent studies in the literature. The deposition of magnesium hydroxide increases with the increase in surface temperature and flow velocity while calcium carbonate deposition increases with the increase in the surface temperature and decreases with the increase in the flow velocity.

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La production de poly(chlorure de vinyle) (PVC) sous forme de suspension chez INEOS ChlorVinyls est réalisée en réacteur fermé agité, dont les parois sont en émail ou en acier inoxydable. Il se forme en cours de polymérisation un dépôt de PVC (croûte) sur les parois du réacteur qui génère de nombreux inconvénients. Afin de limiter cet encroûtement, INEOS ChlorVinyls et l’ensemble des producteurs de PVC appliquent à chaque batch (par exemple pour INEOS ChlorVinyls 50 fois par jour pour ses 22 réacteurs) un revêtement organique. L’application systématique du revêtement et la formation de croûtes ont des conséquences économiques non négligeables (arrêts de production, coût de main d’œuvre et matière, coût de traitement des déchets, qualité du PVC contaminé par le revêtement…).La compréhension du phénomène d’encroûtement en vue de développer un revêtement permanent devient donc nécessaire pour améliorer la qualité des produits, diminuer les coûts et dégager un avantage concurrentiel favorable à INEOS vis-à-vis de ses concurrents. Le sujet de thèse a été divisé en deux parties bien distinctes avec premièrement l’étude du mécanisme d’encroûtement et la mise en place d’un scénario permettant d’expliquer de manière physique et chimique la formation de la croûte sur les parois du réacteur. Une seconde partie a été dédiée au développement d’un revêtement polymère avec la sélection d’un système résistant au milieu réactionnel de polymérisation en suspension du chlorure de vinyle puis à l’optimisation de l’adhésion du revêtement polymère sur acier inoxydable afin d’obtenir des performances maximales et durables. / The suspension synthesis of PolyVinyl Chloride (S-PVC) at the INEOS ChlorVinyls facility in Mazingarbe (FRANCE) is realized thanks to a closed-reactor technology with reactor walls made of stainless steel or enamel. One of the major problems during the production of PVC by suspension polymerization is the formation of a deposit (called crust or scale) on the reactor walls. The formation of scale leads to numerous sorts of drawbacks like a decrease of the reactors’ productivity, the need to clean the reactors after each batch, the exposure of the operators to VinylChloride Monomer (VCM) which is classified CMR, some quality issues… At the moment, a coating is applied before each batch (50 times per day for the 22 reactors at Mazingarbe) in order to lower the amount of scale formed during the S-PVC batch. The application of the coating added to the formation of scale leads to important extra costs. The comprehension of the scale formation mechanism with the aim of then developing a durable protective coating becomes a priority in order to increase the final product quality, lower the costs and gain a competitive advantage for INEOS ChlorVinyls. The Ph.D. subject was divided into two parts with the first year dedicated to the comprehension of the scale formation mechanism and the establishment of a complete scenario explaining the formation of scale from a chemical and physical point of view. The second part of this project was dedicated to the development of a polymer coating with the selection of an adapted polymer candidate and then the optimization of its adhesion onto stainless steel in order to obtain the optimal performances and the durability of the coating.