Neste trabalho é apresentada a avaliação da inversão da sacarose em um sistema constituído de um Evaporador de filme descendente com promotor de película. Foi feita a modelagem da inversão considerando-se os balanços de massa e de entalpia e a cinética de inversão no tubo de evaporação. Os principais parâmetros considerados foram: constante cinética (k), taxa de evaporação(mv), concentração de açúcares totais (C), vazão do líquido no tubo de evaporação (q), pH, temperatura (T) e tempo de residência no tubo de evaporação (T). Usaram-se dois métodos para a resolução do modelo, a primeira através da integração numérica do modelo (Runge Kutta de 4ta ordem) e a outra através de equacionamento simplificado a partir valores médios das propriedades, pH, e calculando as constantes cinéticas nas temperaturas efetivas em cada zona do tubo de evaporação, Tefa (zona de aquecimento) e Tefe (zona de evaporação), possibilitando assim, a integração analítica do modelo. Foram calculados os valores do volume do líquido (V) através dos dois métodos. Os resultados foram expressos em função da vazão do líquido (q). Verificou-se que os resultados calculados pelos dois métodos foram muito próximos. Os modelos desenvolvidos podem ser aplicados para a otimização do processo de evaporação visando à minimização da inversão da sacarose. O método desenvolvido possibilita a determinação do tempo de residência no tubo de evaporação. / In this work the evaluation of sucrose inversion in a system formed by falling film Evaporator with Promoter of film is presented. The modeling was made considering: mass and enthalpy balances and the kinetic of inversion in the evaporation tube. The mean parameters investigated were: kinetic constant (k), evaporation rate (mv), concentration of total sugars (C), volumetric velocity of the liquid in the evaporation tube (q), pH, temperature (T) and residence time in the evaporation tube (T). Two methods was used to solve the model, the first by numeric integration (Runge Kutta 4th Order) and the other by analytic integration of simplified model considering mean values of properties and pH, and the kinetic constant calculates at effective temperatures in each zone of the evaporation tube, Tefa and Tefe, allowed the analytic integration of the model. The volume of liquid in the evaporation tube (V) was calculated by the two methods (numeric integration and simplified methods). The results were expressed as a function of liquid flow rate (q). It was verified that the values obtained by the two methods were very closed. These models can be used for the optimization of the evaporation process emphasizing the minimization of the sucrose inversion. The methodology could be used for the determination of the residence time in the evaporation tube.

Evaporation is used in the dairy industry to reduce the production costs of powder production (including milk powder) as it is more energy efficient to remove water by evaporation than by drying. There are significant economic reasons why gaining a greater understanding of the complex interactions occurring between the liquid and vapour phases in evaporators is advantageous. The multiphase flows in industrial dairy falling film evaporators were studied. Several computational fluid dynamic (CFD) models were created using Ansys CFX 10. Two case studies were chosen. The first case involved modelling the dispersed droplets that require separation from the water vapour evaporated from the feed of the evaporator. The CFD results were able to show that fouling was not caused by a lack of separation. The predicted separation agreed with experimental measurements. The atomisation process was found to be critical in the prediction of the separation. The atomisation process is not well understood and introduced the greatest error to the model. A plug flow assumption is currently used as a basis for the design the separators. The CFD solutions found no validity to this assumption. The second case study aimed to model and solve the distribution of the feed into the heat transfer tubes at the top of the falling film evaporators. The goal of this study was to be able to accurately predict wetting of the tubes. The volume of fluid (VOF) method using the continuum surface force method (CSF) to account for surface tension was chosen to model the system. The poor curvature estimate of the CSF method was found to produce parasitic currents that limited the stability of the solutions. Small VOF timesteps prevented the solver from diverging and the parasitic currents would oscillate the interface around the correct location. The small timesteps required significantly more computational power than was available and the model for the distribution process could not be solved. The CSF VOF method showed considerable promise, particularly because it can predict free surface topography without user input. There are still questions about numerical creeping of films, but the method was able to correctly predict several different surface tension and contact angle dominated film flows expected to be needed to accurately model the distribution of the falling film evaporator. Validated solutions of jet, meniscus, sessile, "overfall" and 3-D weir models were obtained and these agreed with published results in literature. A 2-D weir solution showed qualitative agreement with the expected form of the film. A 2-D hydraulic jump model without surface tension was created and agreed with experimental work in the literature to within 22%. The 3-D hydraulic jump solution only showed partial agreement with published experimental, the solutions were not mesh independent and not well converged so few conclusions can be drawn. The solutions of a rivulet model showed qualitative similarities with experimental work. The predicted wetting rate did not agree with values in the published literature because the spatial domain modelled was believed to be too narrow. An extended model of rivulet flow agreed with the idealised rivulet profile in literature and the predicted wetting rate agreed with some of the published literature. Again the solutions were not mesh independent so few conclusions can be confirmed.

Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: This work focussed on a local multiple chamber falling film evaporator (FFE). The FFE is currently under operator control and experiencing large amounts of lost production time due to excessive fouling. Furthermore, the product milk dry mass fraction (WP) is constantly off specification, negatively influencing product quality, while the first effect temperature (TE1) runs higher than the recommended 70°C (this is a main cause of fouling). A two month period of historical data were received with the aim to develop a controller that could outperform the operators by keeping both control variables, WP and TE1, at desired set points while also increasing throughput and maintaining product quality. Access to the local plant was not possible and as such available process data were cleaned and used to identify two data based models, transfer function and autoregressive with exogenous inputs (ARX) models, as well as a semi-empirical-model. The ARX model proved inadequate to predict TE1 trends, with an average TE1 correlation to historical data of 0.36, compared to 0.59 and 0.74 for the transfer function and semi-empirical-models respectively. Product dry mass correlations were similar between the models with the average correlations of 0.47, 0.53 and 0.51 for the semi-empirical, transfer function and ARX models respectively. Although the semi-empirical showed the lowest WP correlation, it was offset by the TE1 prediction advantage. Therefore, the semi-empirical model was selected for controller development and comparisons. The success of the semi-empirical model was in accordance with previous research [1] [2] [3], yet other studies have concluded that ARX modelling was more suited to FFE modelling [4]. Three controllers were developed, namely: a proportional and integral (PI) controller as base case, a linear quadratic regulator (LQR) as an optimal state space alternative and finally, to make full use of process knowledge, a predictive fuzzy logic controller (PFC). The PI controller was able to offer zero offset set point tracking, but could not adequately reject a feed dry mass (WF) disturbance (as proposed and reported by Winchester [5]). The LQR was combined with a Kalman estimator and used pre-delay states. In order to offer increased disturbance rejection, the feedback gains of the disturbance states were tuned individually. The altered LQR and PFC solutions proved to adequately reject all modelled disturbances and outperform a cascade controller designed by Bakker [6]. The maximum deviation in WP was a fractional increase of 0.007 for LQR and 0.005 for FPC, compared to 0.012 for PI and 0.0075 for the cascade controller [6] (WF disturbance fractional increase of 0.01). All the designed controllers managed to reduce the standard deviation of operator controlled WP and TE1 by at least 700% and 450%, respectively. The same level of reduction was seen for maximum control variable deviations (370%), the integral of the absolute error (300%) and the mean squared error (900%). All these performance metrics point to the controllers performing better than the operator based control. In order to prevent manipulated variable saturation and optimise the feed flow rate (F1), a fuzzy feed optimiser (FFO) was developed. The FFO focussed on maximising the available evaporative capacity of the FFE by optimising the motive steam pressure (PS), which supplied heat to the effects. By using the FFO for each controller the average feed flow rate was increased by 4.8% (±500kg/h) compared to the operator control. In addition to flow rate gain, the controllers kept TE1 below 70°C and WP on specification. As such, the overall product quality also increased as well as decreasing the down time due to less fouling. / AFRIKAANSE OPSOMMING: Hierdie projek het op ‘n vallende film verdamper (VFV) gefokus. Die VFV word tans beheer deur operateurs en ondervind groot hoeveelhede verlore produksie tyd a.g.v oormatige aangroeisels. Die vorming van aangroeisels is grootliks te danke aan die eerste effek temperatuur (TE1) wat gereeld 70°C oorskrei. Die produk droë massa fraksie (WP) is ook telkens nie op die gewenste vlak nie, wat produk kwaliteit negatief beinvloed. Data, wat oor ‘n twee maand periode strek, was verkry met die doelstelling om ‘n beheerder te ontwerp wat beter sou vaar as die operateurs, deur beide WP en TE2 om ‘n nou stelpunt te beheer. Ter selfde tyd moet die ontwerpte beheerder die produksie tempo en produk kwaliteit verhoog. Geen toegang tot die plaaslikke VFV was moontlik nie, dus was die data skoongemaak en gebruik om twee data gebasseerde modelle te identifiseer, nl. oordragsfunksie en outoregressiwe met eksogene insette (ORX) modelle, asook ‘n semi-empiriese model. Die ORX model kon nie TE1 goed voorspel nie, met ‘n korrelasie faktor (tot die historiese data) van 0.36, vergeleke met die 0.59 en 0.74 van die oordragsfunksie en semi-empiriese modelle onderskeidelik. WP korrelasie faktore was meer konstant tussen die modelle, met waardes van 0.47, 0.53 en 0.51 vir die semi-empiriese, oordragsfunskie en ORX modelle onderskeidelik. Alhoewel die semi-empiriese model die laagste WP korrelasie vertoon het, was die tekortkoming vergoed deur die beter TE1 voorspelling. Gevolglik was die semi-empiriese model gebruik vir beheerder ontwerp en vergelyking. Die sukses van die semiempiriese model stem ooreen met vorige studies [1] [2] [3], tog het ander studies al bevind dat die ORX model beter gepas is vir die VFV proses [4]. Drie beheerders was ontwikkel, nl. ‘n proporsionele en integreerder (PI) beheerder as basis geval, ‘n liniêre kwadratiese reguleerder (LKR) as optimale toestands beheer alternatief en laastens ‘n voorspellende wasige logika beheerder (VWB) om volle gebruik van proseskennis te maak. Die PI beheerder kon foutlose volging van die stelpunte lewer, maar kon nie ‘n inset voer droë massa fraksie (WF) versteuring (soos voorgestel en weergegee deur Winchester [5]) na wense verwerp nie. Die LKR was saamgevoeg met ‘n Kalman afskatter en het gebruik gemaak van onvertraagde toestande. Die versteuringstoestande was individueel verstel om beter versteurings verweping te weeg te bring. Die aangepaste LKR en VWB kon beide die WF versteuring verwerp en het beter gevaar as ‘n kaskade beheer oplossing wat deur Bakker [6] ontwerp was. Die WP afwyking is beperk tot ‘n fraksie droë masse verandering van 0.007 vir LKR en 0.005 vir VWB, vergeleke met die afwykings van 0.012 vir die PI beheerder asook die 0.0075 van die kaskade beheerder [6]. Die ontwerpte beheerder kon ook die standaard afwyking van beide WP en TE1 met ten minste 700% en 450% onderskeidelik verminder. Soortgelyke verbeterings was gesien vir die maksimum beheer veranderlikke afwyking (370%), die integraal van die absolute fout (300%) en die gemiddelde fout (900%). Dus het die ontwerpte beheerders wesenlik verbeter op die operateur beheer. Ten einde om gemanipuleerde veranderlikke versadiging te voorkom, asook die voer vloei (V1) te optimiseer, was ‘n wasige logika optimiseerder (WVO) ontwerp. Die WVO het die beskikbare verdampingskapasiteit ten volle benut deur te sorg dat die stoom druk (PS), wat energie verskaf vir verdamping, ge-optimiseerd bly. ‘n Gemiddelde V1 stygging van 4.8% (±500kg/uur), vergeleke met operateur beheer, is waargeneem. Al die beheerders kon steeds die WP en TE1 stelpunte volg en dus TE1 onder 70°C hou (wat verminderde vormasie van aangroeisels tot gevolg gehad het). Daarom het die produk kwailiteit verhoog en die verlore produksie tyd verminder.

Neste trabalho é apresentada a avaliação da inversão da sacarose em um sistema constituído de um Evaporador de filme descendente com promotor de película. Foi feita a modelagem da inversão considerando-se os balanços de massa e de entalpia e a cinética de inversão no tubo de evaporação. Os principais parâmetros considerados foram: constante cinética (k), taxa de evaporação(mv), concentração de açúcares totais (C), vazão do líquido no tubo de evaporação (q), pH, temperatura (T) e tempo de residência no tubo de evaporação (T). Usaram-se dois métodos para a resolução do modelo, a primeira através da integração numérica do modelo (Runge Kutta de 4ta ordem) e a outra através de equacionamento simplificado a partir valores médios das propriedades, pH, e calculando as constantes cinéticas nas temperaturas efetivas em cada zona do tubo de evaporação, Tefa (zona de aquecimento) e Tefe (zona de evaporação), possibilitando assim, a integração analítica do modelo. Foram calculados os valores do volume do líquido (V) através dos dois métodos. Os resultados foram expressos em função da vazão do líquido (q). Verificou-se que os resultados calculados pelos dois métodos foram muito próximos. Os modelos desenvolvidos podem ser aplicados para a otimização do processo de evaporação visando à minimização da inversão da sacarose. O método desenvolvido possibilita a determinação do tempo de residência no tubo de evaporação. / In this work the evaluation of sucrose inversion in a system formed by falling film Evaporator with Promoter of film is presented. The modeling was made considering: mass and enthalpy balances and the kinetic of inversion in the evaporation tube. The mean parameters investigated were: kinetic constant (k), evaporation rate (mv), concentration of total sugars (C), volumetric velocity of the liquid in the evaporation tube (q), pH, temperature (T) and residence time in the evaporation tube (T). Two methods was used to solve the model, the first by numeric integration (Runge Kutta 4th Order) and the other by analytic integration of simplified model considering mean values of properties and pH, and the kinetic constant calculates at effective temperatures in each zone of the evaporation tube, Tefa and Tefe, allowed the analytic integration of the model. The volume of liquid in the evaporation tube (V) was calculated by the two methods (numeric integration and simplified methods). The results were expressed as a function of liquid flow rate (q). It was verified that the values obtained by the two methods were very closed. These models can be used for the optimization of the evaporation process emphasizing the minimization of the sucrose inversion. The methodology could be used for the determination of the residence time in the evaporation tube.

Les poudres laitières sont obtenues par concentration du produit liquide (membranes, évaporation sous vide) puis séchage du concentré. Contrairement au séchage par atomisation, peu de travaux ont été menés sur la concentration par évaporation sous vide. Pourtant, la maîtrise du schéma global de production des poudres gagnerait en une meilleure compréhension de cette opération réalisée dans des évaporateurs à flot tombant. Le présent travail vise à étudier comment améliorer les performances des évaporateurs ; il s’appuie sur deux facteurs limitants de l’opération : l’encrassement des surfaces d’échange et la viscosité maximale en sortie des évaporateurs. La contribution des différents constituants des produits à l’encrassement a été étudiée grâce à une méthodologie développée sur un évaporateur pilote.Les résultats obtenus ont permis d’identifier les éléments minéraux qui contribuent à l’encrassement lors de la concentration de microfiltrat et d’ultrafiltrat de lait, de lactosérums acides chlorhydrique et lactique. Afin de dépasser l’extrait sec maximal en sortie des évaporateurs à flot tombant, deux évaporateurs rotatifs à couche mince, de conceptions différentes, ont été testés à l’échelle pilote. Les études de faisabilité ont été menées jusqu’à la poudre. Le potentiel des équipements a été évalué suivant les propriétés fonctionnelles des poudres et le coût énergétique d’élimination de l’eau. Dans l’un des cas, le remplacement de la tour de séchage par deux évaporateurs rotatifs en série constitue une rupture technologique pour la production de poudres de perméat et dimi / Dairy powders are produced using a two-step process: concentration of the liquid product (membrane, concentration by vacuum evaporation) and drying of the concentrate. Contrary to spray-drying, vacuum concentration has been poorly investigated. However, a better understanding of this unit operation carried out in falling-film evaporators would lead to an improved control of the global process scheme for the production of powders. This PhD project aims to highlight how to enhance the performances of falling-film evaporators; it focuses on two limiting factors of the vacuum concentration: the fouling of the heating surfaces and the maximal viscosity at the outlet of the falling-film evaporators. The contribution of the different product components to the fouling was investigated using a methodology developed at pilot-scale.Our results showed which mineral elements favored the fouling of falling-film evaporators during the concentration of milk microlfiltrate and ultrafiltrate as well as hydrochloric and lactic acid wheys. Besides, two thin-film rotative evaporators of different designs were tested at pilot-scale in order to exceed the maximum dry matter achieved in falling-film evaporators. The feasibility studies were conducted up to the production of powders. The capabilities of the equipment were evaluated according to the functional properties of powders and the specific energy costs relative to the removal of water. In one case, the combination of two rotative evaporators replacing the spray drier constitutes a technological breakthrough in the production of permeate p

[pt] Um sistema térmico de armazenamento da energia com pasta de cristais de gelo foi desenvolvido para aplicações de condicionamento de ar e resfriamento de processos. O sistema usa um evaporador orbital de haste, um trocador de calor vertical do tipo tubo e carcaça com intensificação mecânica de transferência de calor. A pasta de gelo é produzida continuamente sem acumulação no evaporador e é compatível com unidades condensadoras convencionais, tanques de armazenamento e bombas. Soluções aquosas diluídas ou soluções inorgânicas de salmoura promovem a formação de cristais de gelo, e o gelo líquido resultante pode ser bombeado ou por gravidade alimentar um tanque de armazenamento. O circuito hidráulico de refrigeração (carga térmica) pode ser desacoplado da produção do gelo utilizando-se o tanque de armazenamento. O armazenamento de gelo líquido fornece temperaturas consistentemente baixas à medida que se derrete o gelo, que por sua forma pode ser derretido também muito rapidamente. Com suas altas temperaturas características de evaporação e elevados fluxos do calor, os sistemas de geração de gelo líquido apresentam potencial para reduzir significativamente os custos de capital inicial e operação, quando comparados com tecnologias de sistemas estáticos de gelo ou ice harvesting. / [en] New ice crystal slurry thermal energy storage (TES) system has been developed for both HVAC and process cooling applications. The system uses an orbital rod evaporator (ORE), a vertical shell-and-tube heat exchanger with mechanical heat transfer augmentation, as a dynamic ice maker to generate liquid ice. Ice forms continuously without accumulation in the ORE and is compatible with conventional condensing units, storage tanks, and pumps. Dilute glycol or inorganic brine solutions promote formation of ice crystals, and the resulting liquid ice may be pumped or gravity fed to a storage tank. The cooling load circuit can be hydraulically decoupled from ice production at the storage tank. Stored liquid ice provides consistently low solution supply temperatures over significant portions of the ice melt period and may be melted very rapidly. With its characteristic high evaporator temperatures and high heat fluxes, ORE TES systems have the potential for significantly lower capital and operating costs than static ice or ice harvesting technologies.