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Robert A Taylor - One of the best experts on this subject based on the ideXlab platform.

  • off design simulation and performance of molten salt cavity receivers in solar tower plants under realistic Operational modes and control strategies
    Applied Energy, 2016
    Co-Authors: Saeed Mostafavi S Tehrani, Robert A Taylor
    Abstract:

    Abstract Solar irradiation is intermittent, but concentrated solar thermal (CST) plants are typically designed and analyzed solely based on their steady design point. Unlike coal power plants, however, CST plants frequently experience thermal loads well above and below their rated design point, leading to off-design operation for much of the Operational year. Importantly, if a latent heat thermal energy storage (LHTES) system is employed, the receiver inlet temperature can vary under these conditions. To date, there is a clear lack of knowledge for how to handle off-design conditions in terms of developing appropriate control strategies to maximize the receiver thermal output and its Operational Region. In this study, a thermal model was developed and validated that is suitable for design/off-design performance analyses of molten salt cavity receivers in both steady state and transient conditions. The study investigated two control strategies – a fixed receiver flow rate (FF) and fixed receiver outlet temperature (FT) – for their off-design performance in each of two off-design Operational modes (storage and non-storage). Solar field utilization (SFU) is variable in non-storage mode, but in the storage mode, it is whether variable or fixed at design point (SFU = 1). The feasible operating Region in this study refers to the zone restricted by maximum allowable Operational parameters defined based on design point analysis, mainly maximum receiver outlet temperature, maximum flow rate, and maximum receiver surface temperature. Through this analysis, it was found that receiver inlet temperatures above the design point (560 K) degrade the receiver performance in both control strategies and under all Operational modes. The results also revealed that the maximum allowable receiver inlet temperature that maintains the receiver operation inside the feasible Region could not go beyond ∼700 K or 600 K with the FF and FT strategies (in the storage mode with variable or fixed SFU), respectively. These values also indicate the charging cut-off temperature for the fluid flowing out in LHTES systems. In the non-storage mode, the receiver inlet temperature is remained constant at design point by varying the SFU over the time. While the design point direct normal irradiation (DNI) was 900 W m−2, the maximum allowable DNI is 700 W m−2 and 500 W m−2 with the FF and FT strategies, respectively. These results motivate a hybrid control strategy that switches between the FF and FT strategies to maximize the performance and the number of Operational hours of a CST plant during the day. As a final aspect of this study, off-design receiver efficiency correlations are developed that can be used in any simulation environment to accurately predict receiver performance.

  • off design simulation and performance of molten salt cavity receivers in solar tower plants under realistic Operational modes and control strategies
    Applied Energy, 2016
    Co-Authors: Saeed Mostafavi S Tehrani, Robert A Taylor
    Abstract:

    Solar irradiation is intermittent, but concentrated solar thermal (CST) plants are typically designed and analyzed solely based on their steady design point. Unlike coal power plants, however, CST plants frequently experience thermal loads well above and below their rated design point, leading to off-design operation for much of the Operational year. Importantly, if a latent heat thermal energy storage (LHTES) system is employed, the receiver inlet temperature can vary under these conditions. To date, there is a clear lack of knowledge for how to handle off-design conditions in terms of developing appropriate control strategies to maximize the receiver thermal output and its Operational Region. In this study, a thermal model was developed and validated that is suitable for design/off-design performance analyses of molten salt cavity receivers in both steady state and transient conditions. The study investigated two control strategies – a fixed receiver flow rate (FF) and fixed receiver outlet temperature (FT) – for their off-design performance in each of two off-design Operational modes (storage and non-storage). Solar field utilization (SFU) is variable in non-storage mode, but in the storage mode, it is whether variable or fixed at design point (SFU=1). The feasible operating Region in this study refers to the zone restricted by maximum allowable Operational parameters defined based on design point analysis, mainly maximum receiver outlet temperature, maximum flow rate, and maximum receiver surface temperature.

Saeed Mostafavi S Tehrani - One of the best experts on this subject based on the ideXlab platform.

  • off design simulation and performance of molten salt cavity receivers in solar tower plants under realistic Operational modes and control strategies
    Applied Energy, 2016
    Co-Authors: Saeed Mostafavi S Tehrani, Robert A Taylor
    Abstract:

    Abstract Solar irradiation is intermittent, but concentrated solar thermal (CST) plants are typically designed and analyzed solely based on their steady design point. Unlike coal power plants, however, CST plants frequently experience thermal loads well above and below their rated design point, leading to off-design operation for much of the Operational year. Importantly, if a latent heat thermal energy storage (LHTES) system is employed, the receiver inlet temperature can vary under these conditions. To date, there is a clear lack of knowledge for how to handle off-design conditions in terms of developing appropriate control strategies to maximize the receiver thermal output and its Operational Region. In this study, a thermal model was developed and validated that is suitable for design/off-design performance analyses of molten salt cavity receivers in both steady state and transient conditions. The study investigated two control strategies – a fixed receiver flow rate (FF) and fixed receiver outlet temperature (FT) – for their off-design performance in each of two off-design Operational modes (storage and non-storage). Solar field utilization (SFU) is variable in non-storage mode, but in the storage mode, it is whether variable or fixed at design point (SFU = 1). The feasible operating Region in this study refers to the zone restricted by maximum allowable Operational parameters defined based on design point analysis, mainly maximum receiver outlet temperature, maximum flow rate, and maximum receiver surface temperature. Through this analysis, it was found that receiver inlet temperatures above the design point (560 K) degrade the receiver performance in both control strategies and under all Operational modes. The results also revealed that the maximum allowable receiver inlet temperature that maintains the receiver operation inside the feasible Region could not go beyond ∼700 K or 600 K with the FF and FT strategies (in the storage mode with variable or fixed SFU), respectively. These values also indicate the charging cut-off temperature for the fluid flowing out in LHTES systems. In the non-storage mode, the receiver inlet temperature is remained constant at design point by varying the SFU over the time. While the design point direct normal irradiation (DNI) was 900 W m−2, the maximum allowable DNI is 700 W m−2 and 500 W m−2 with the FF and FT strategies, respectively. These results motivate a hybrid control strategy that switches between the FF and FT strategies to maximize the performance and the number of Operational hours of a CST plant during the day. As a final aspect of this study, off-design receiver efficiency correlations are developed that can be used in any simulation environment to accurately predict receiver performance.

  • off design simulation and performance of molten salt cavity receivers in solar tower plants under realistic Operational modes and control strategies
    Applied Energy, 2016
    Co-Authors: Saeed Mostafavi S Tehrani, Robert A Taylor
    Abstract:

    Solar irradiation is intermittent, but concentrated solar thermal (CST) plants are typically designed and analyzed solely based on their steady design point. Unlike coal power plants, however, CST plants frequently experience thermal loads well above and below their rated design point, leading to off-design operation for much of the Operational year. Importantly, if a latent heat thermal energy storage (LHTES) system is employed, the receiver inlet temperature can vary under these conditions. To date, there is a clear lack of knowledge for how to handle off-design conditions in terms of developing appropriate control strategies to maximize the receiver thermal output and its Operational Region. In this study, a thermal model was developed and validated that is suitable for design/off-design performance analyses of molten salt cavity receivers in both steady state and transient conditions. The study investigated two control strategies – a fixed receiver flow rate (FF) and fixed receiver outlet temperature (FT) – for their off-design performance in each of two off-design Operational modes (storage and non-storage). Solar field utilization (SFU) is variable in non-storage mode, but in the storage mode, it is whether variable or fixed at design point (SFU=1). The feasible operating Region in this study refers to the zone restricted by maximum allowable Operational parameters defined based on design point analysis, mainly maximum receiver outlet temperature, maximum flow rate, and maximum receiver surface temperature.

Andrea Lopes De Oliveira Ferreira - One of the best experts on this subject based on the ideXlab platform.

  • Síntese enzimática de ampicilina em reator integrado.
    Universidade Federal de São Carlos, 2004
    Co-Authors: Andrea Lopes De Oliveira Ferreira
    Abstract:

    A rota enzimática para produção de antibióticos atua em condições suaves de temperatura, pressão e pH, em meio aquoso, além de ter bem menor impacto ambiental. A síntese enzimática de ampicilina a partir de éster metílico de fenilglicina (EMFG) e ácido 6-aminopenicilânico (6-APA) foi estudada como alternativa à rota de produção química. O uso de enzimas imobilizadas é comum na literatura devido aos benefícios que estas apresentam em relação às enzimas solúveis: separação do meio reacional e reutilização do biocatalisador. A enzima penicilina G acilase [EC 3.5.1.11] foi imobilizada em dois diferentes suportes (sílica e agarose). O primeiro passo estudado para produção do antibiótico foi a determinação das condições ótimas de operação. Assim, estudouse a influência das seguintes variáveis: pH, temperatura, concentração inicial de 6- APA, concentração iônica do meio reacional e presença de solvente orgânico (metanol). As variáveis-resposta utilizadas foram produtividade, seletividade e rendimento. Os ensaios foram conduzidos segundo um planejamento fatorial 25. As variáveis que mais influenciaram nas respostas foram: temperatura, pH e concentração inicial de 6-APA. O uso de EMFG para acilação do núcleo β- lactâmico, 6-APA, resulta num sistema reativo série-paralelo, cujas reações indesejadas de hidrólise (do EMFG e do antibiótico) ocorrem simultaneamente à de formação do antibiótico. Assim, a concentração máxima atingida de antibiótico depende das velocidades relativas das três reações. Observou-se que o maior rendimento de síntese foi obtido na temperatura de 4C, pH 6,5, sem adição de solvente orgânico e em meio reacional de baixa concentração iônica. Os resultados também mostraram que é possível operar o sistema com alta produtividade, e ainda se manterem os rendimento elevados a 25C, concentração mais elevada de reagentes, ausência de tampão fosfato e pH 6,5. Após a seleção das condições reacionais (25C, pH 6,5), ensaios, utilizando-se a PGA imobilizada em sílica, foram realizados. O derivado enzima-sílica sofre desgaste por cisalhamento em reator convencional, o que levou à utilização de reator de leito fixo. Parâmetros de resistência à transferência de massa externa e interna no suporte sobre a velocidade global de reação foram estimados a partir de dados experimentais, com auxílio de modelo matemático heterogêneo do leito catalítico com recirculação total, operando em regime transiente. O outro suporte empregado na imobilização de PGA foi agarose intercruzada, que tem maior resistência ao cisalhamento. Experimentos de síntese com concentrações iniciais de reagentes variando de 50 a 250mM, utilizando-se o derivado enzima-agarose, formaram o domínio para treinamento de redes neurais que pudessem prever as velocidades de reação, pois um modelo mecanístico das três reações, levando em conta efeitos inibitórios e alostéricos de reagentes e/ou produtos, mostrou-se muito complexo, com número intratável de parâmetros cinéticos. Simplificações desse modelo, diminuindo-se o número de parâmetros ajustáveis, não conseguiram representar os dados experimentais em toda a faixa de interesse. Conjuntos de dados para treinamento supervisionado foram alimentados a redes feedforward com uma camada oculta. Essa abordagem permitiu uma boa representação da síntese de ampicilina na faixa operacional estudada e também direcionou os ensaios para região onde se maximizassem produtividade e seletividade. As velocidades de reação previstas pelas redes neurais foram incluídas nas equações fundamentais de balanço de massa, compondo um modelo híbrido do processo. As simulações com o modelo híbridoneural direcionaram os ensaios de síntese para região onde se maximizaram a produtividade e seletividade da reação. Observou-se que aspecto importante para a melhoria da seletividade na produção de ampicilina era a operação do reator em uma região onde ocorria precipitação do antibiótico, pois quando este cristalizava no próprio reator, diminuía-se sua perda por hidrólise. Um biocatalisador adequado, que suportasse a precipitação da ampicilina no meio reacional, foi desenvolvido, patenteado e utilizado em reator de escoamento em vórtices de Taylor, onde as tensões de cisalhamento são menores. Sínteses utilizando concentrações altas de reagentes foram realizadas nesse reator, de modo a precipitar o antibiótico, incrementando-se rendimento, seletividade e produtividade do sistema.Enzymatic route to produce antibiotic acts in mild reaction conditions (aqueous medium, neutral pH and moderate temperatures). Furthermore it reduces the number of reaction steps and decreases the amount and toxicity of waste products per kilogram of antibiotic. The enzymatic synthesis of ampicillin from phenylglycine methyl ester (PGME) and 6-aminopenicillanic acid (6-APA) can be an attractive alternative, replacing the chemical route. The use of an immobilized enzyme to catalyze the synthesis is very important to reduce costs. Penicillin G acylase (PGA) [EC 3.5.1.11] from E. coli was immobilized on two supports (agarose gel and silica). This work undertakes an optimization study of the enzymatic synthesis of ampicillin, to find out optimized process conditions. Therefore, it was studied the influence of the following variables: pH, temperature, 6-APA initial concentration, buffer concentration and the presence of methanol. Response variables were productivity, selectivity and yield (based on 6- APA initial concentration). The assays were carried on accordind to factorial design 25. Temperature, pH, and 6-APA initial concentration influenced At synthesis of ampicillin from PGME and 6-APA, two other reactions compete with the synthesis reaction: hydrolysis of PGME (a parallel reaction) and hydrolysis of ampicillin (in series with the synthesis). The yield of the synthesis of ampicillin depend on the rates of three different reactions. The highest yield was achieved at 4C, pH 6.5, without methanol, and with low buffer concentration. The results also indicate that it is possible to work with this system at high productivities, and it still keeps high yields at 25C, without buffer, and pH 6.5. After the selection of reaction conditions (25C, pH 6.5), assays with PGA immobilized on silica carrier were realized. Convensional reactors may cause shearing on derivative enzyme-silica, which led using fixed-bed reactor. Mass transport parameters were estimated by fitting heterogeneous mathematical model to experimental data of catalytic bed with recirculation, running on transient state. Another used support on immobilized enzyme was agarose gel. Domain of experimental assays used in the neural network training and validation were initial substrate concentrations ranged from 50 to 250mM. A mechanistic model to represent the synthesis of ampicillin from PGME and 6-APA is a set of seriesparallel reactions (ampicillin and PGME hydrolysis are undesirable side reactions) would be too complex, with an intractable number of kinetic parameters. Simplified models could not represent all the experimental data, and a hybrid model was used. Neural networks were trained to predict reaction rates and used in the mass balance equations. A feedforward neural network, with one hidden layer was used. Results of the simulation were promising. The Operational Region that of high productivity and selectivity of antibiotic could be successfully mapped. An important aspect to improve the selectivity of ampicillin synthesis is to precipitate the antibiotic because the hydrolysis reaction would be decreasing. An approprieted biocatalyst which preventing the precipitation was developed, and used in a Taylor vortex reactor where shears are smaller. Synthesis assays using high substrate concentrations were performed in this reactor, occurring precipitate of antibiotic during reaction, to improve yield, selectivity, and productivity of this system

  • distribution of suspended particles in a taylor poiseuille vortex flow reactor
    Chemical Engineering Science, 2001
    Co-Authors: Miriam Maria De Resende, Raquel L C Giordano, Paulo Waldir Tardioli, V M Fernandez, Andrea Lopes De Oliveira Ferreira, Roberto C Giordano
    Abstract:

    Vortex flow reactors (VFRs) are specially suited to work with shear-sensitive particles, due to the gentle and efficient stirring characteristics of Taylor vortices. For heterogeneous VFRs, the distribution of the solid phase must be accounted for in detail. This work presents residence-time distributions (RTDs) of agarose gel particles (Φ av =214 μm), suspended in different liquid solutions. Reactor porosity was 90%. The mass transfer coefficients of a lumped-parameter model of the reactor are estimated from residence time distributions of a tracer. The VFR has radius ratio η=0.664 and aspect ratio Γ=14.9. Axial flow rates were selected to provide a mean residence time of 1850 s, adequate for many applications. Minimum rotation rates of the inner cylinder were imposed by suspended particles homogeneity criteria. The VFR Operational Region was: 0.0645vortices were stationary, and circumvented by a by-pass stream. The results indicate that the gel particles are unevenly distributed in the vortex core and by-pass Regions. Mean residence times for the particles are substantially greater than for the liquid. The mathematical model presented in this paper can accommodate this behavior, using a partition coefficient for the solid phase.

Angeliki Alexiou - One of the best experts on this subject based on the ideXlab platform.

  • Operational Region of overlay d2d communications
    International Conference on Communications, 2015
    Co-Authors: Stelios Stefanatos, Antonis G Gotsis, Angeliki Alexiou
    Abstract:

    An important enabler towards the successful introduction of any new element/feature to the cellular network, such as the ability to allow for direct, device-to-device (D2D) communications, is the investigation and characterization of the Operational conditions, e.g., density of users, where its deployment will enhance performance. This paper attempts to provide a mathematically rigorous characterization of the set (Region) of Operational conditions, for which overlay D2D communications enhance performance in terms of average user rate. Using tools from stochastic geometry, and for the practically interesting case of a heavy loaded network, the Operational Region is identified in closed form as a function of a variety of parameters reflecting a wide range of Operational conditions. It is shown that, under the appropriate deployment scheme, D2D communications can be beneficial not only in the typically assumed regime of “proximal communications” but to a wide range of Operational conditions that include D2D link distances comparable to the distance from the cellular access point(s) as well as significantly large user densities.

  • Operational Region of d2d communications for enhancing cellular network performance
    arXiv: Information Theory, 2015
    Co-Authors: Stelios Stefanatos, Antonis G Gotsis, Angeliki Alexiou
    Abstract:

    An important enabler towards the successful deployment of any new element/feature to the cellular network is the investigation and characterization of the Operational conditions where its introduction will enhance performance. Even though there has been significant research activity on the potential of device-to-device (D2D) communications, there are currently no clear indications of whether D2D communications are actually able to provide benefits for a wide range of Operational conditions, thus justifying their introduction to the system. This paper attempts to fill this gap by taking a stochastic geometry approach on characterizing the set (Region) of Operational conditions for which D2D communications enhance performance in terms of average user rate. For the practically interesting case of a heavy loaded network, the Operational Region is provided in closed form as a function of a variety of parameters such as maximum D2D link distances and user densities, reflecting a wide range of Operational conditions (points). It is shown that under the appropriate deployment scheme, D2D communications can indeed be beneficial not only for the usually considered regime of "proximal communications" but to a wide range of Operational conditions that include D2D link distances comparable to the distance to the cellular access point and considerably large user densities.

  • Operational Region of D2D Communications for Enhancing Cellular Network Performance
    IEEE Transactions on Wireless Communications, 2015
    Co-Authors: Stelios Stefanatos, Antonis G Gotsis, Angeliki Alexiou
    Abstract:

    An important enabler towards the successful deployment of any new element/feature to the cellular network is the investigation and characterization of the Operational conditions where its introduction will enhance performance. Although there has been significant research activity on the potential of device-to-device (D2D) communications, there are currently no clear indications of whether D2D communications are actually able to provide benefits for a wide range of Operational conditions, thus justifying their introduction to the system. This paper attempts to fill this gap by taking a stochastic geometry approach on characterizing the set (Region) of Operational conditions for which D2D communications enhance performance in terms of average user rate. For the practically interesting case of a heavy-loaded network, the Operational Region is provided in closed form as a function of a variety of parameters such as maximum D2D link distances and user densities, reflecting a wide range of Operational conditions (points). It is shown that, under the appropriate deployment scheme, D2D communications can indeed be beneficial not only for the usually considered regime of “proximal communications” but to a wide range of Operational conditions that include D2D link distances comparable to the distance to the cellular access point and considerably large user densities.

Roberto C Giordano - One of the best experts on this subject based on the ideXlab platform.

  • distribution of suspended particles in a taylor poiseuille vortex flow reactor
    Chemical Engineering Science, 2001
    Co-Authors: Miriam Maria De Resende, Raquel L C Giordano, Paulo Waldir Tardioli, V M Fernandez, Andrea Lopes De Oliveira Ferreira, Roberto C Giordano
    Abstract:

    Vortex flow reactors (VFRs) are specially suited to work with shear-sensitive particles, due to the gentle and efficient stirring characteristics of Taylor vortices. For heterogeneous VFRs, the distribution of the solid phase must be accounted for in detail. This work presents residence-time distributions (RTDs) of agarose gel particles (Φ av =214 μm), suspended in different liquid solutions. Reactor porosity was 90%. The mass transfer coefficients of a lumped-parameter model of the reactor are estimated from residence time distributions of a tracer. The VFR has radius ratio η=0.664 and aspect ratio Γ=14.9. Axial flow rates were selected to provide a mean residence time of 1850 s, adequate for many applications. Minimum rotation rates of the inner cylinder were imposed by suspended particles homogeneity criteria. The VFR Operational Region was: 0.0645vortices were stationary, and circumvented by a by-pass stream. The results indicate that the gel particles are unevenly distributed in the vortex core and by-pass Regions. Mean residence times for the particles are substantially greater than for the liquid. The mathematical model presented in this paper can accommodate this behavior, using a partition coefficient for the solid phase.

  • analysis of a taylor poiseuille vortex flow reactor i flow patterns and mass transfer characteristics
    Chemical Engineering Science, 1998
    Co-Authors: Roberto C Giordano, Raquel L C Giordano, D M F Prazeres, Charles L Cooney
    Abstract:

    This paper shows the results of flow visualization and residence time distribution experiments in a Taylor-Poiseuille vortex flow apparatus. It is the first of a series that starts with the identification of flow patterns inside the device and goes up to the assessment of its performance as an enzymatic reactor. The approach is to study in depth one single geometric configuration (radius ratio η=0.677 and aspect ratio Γ=18.30), adequate for use as a heterogeneous reactor and or adsorption system in bio-processes, rather than spanning a range of geometries and proposing empirical expressions for mass transport coefficients. The range of rotations and axial flow rates used in this article correspond to low moderate rotational Reynolds numbers (Re θ from 130 to 615, with 1.6Operational Region: the vortex drift velocities are less than one, and decrease continuously with increasing rotations, until a full stop. Except for Re θ close to the critical value, the downstream displacement of vortices is slower than the mean axial velocity. The implications of this fact on the reactor performance are discussed.