The Experts below are selected from a list of 297 Experts worldwide ranked by ideXlab platform

Rodney O. Fox - One of the best experts on this subject based on the ideXlab platform.

  • pdf modeling of turbulent mixing effects on Initiator Efficiency in a tubular ldpe reactor
    Aiche Journal, 1996
    Co-Authors: Kuochen Tsai, Rodney O. Fox
    Abstract:

    The effect of turbulent mixing in the reaction zone of a tubular low-density polyethylene reactor was studied by combining a Lagrangian composition probability density function (LCPDF) code with a computational fluid-dynamics code. Because the LCPDF code can treat the chemical reaction terms in a turbulent flow without resorting to moment closures, it is used to describe the temperature and scalar fields of reactants including Initiator and monomer molar concentrations, and the moments of the molecular weight distribution. The chemical reaction terms are efficiently dealt with using a three-parameter chemical lookup table that contains the temperature and composition changes as functions of Initiator and monomer concentrations and temperature over a small time step. The reaction-rate constants from the study of Lee and Marano (1979) are functions of temperature and pressure. The flow fields are obtained using the k – e turbulence model. Because the temporal and spatial evolution of all fields in the reactor can be simulated, it is possible to study the effect of the Initiator injection location, flow rate and temperature of the monomer and Initiator feed streams on polymerization in considerable detail. Moreover, by observing the probability distribution of the composition fields, a better understanding of hot-spot formation is achieved, leading to improved reactor designs.

  • PDF modeling of turbulent‐mixing effects on Initiator Efficiency in a tubular LDPE reactor
    AIChE Journal, 1996
    Co-Authors: Kuochen Tsai, Rodney O. Fox
    Abstract:

    The effect of turbulent mixing in the reaction zone of a tubular low-density polyethylene reactor was studied by combining a Lagrangian composition probability density function (LCPDF) code with a computational fluid-dynamics code. Because the LCPDF code can treat the chemical reaction terms in a turbulent flow without resorting to moment closures, it is used to describe the temperature and scalar fields of reactants including Initiator and monomer molar concentrations, and the moments of the molecular weight distribution. The chemical reaction terms are efficiently dealt with using a three-parameter chemical lookup table that contains the temperature and composition changes as functions of Initiator and monomer concentrations and temperature over a small time step. The reaction-rate constants from the study of Lee and Marano (1979) are functions of temperature and pressure. The flow fields are obtained using the k – e turbulence model. Because the temporal and spatial evolution of all fields in the reactor can be simulated, it is possible to study the effect of the Initiator injection location, flow rate and temperature of the monomer and Initiator feed streams on polymerization in considerable detail. Moreover, by observing the probability distribution of the composition fields, a better understanding of hot-spot formation is achieved, leading to improved reactor designs.

Zheng Zhang - One of the best experts on this subject based on the ideXlab platform.

  • modeling of the bulk free radical polymerization up to high conversion three stage polymerization model ii number average molecular weight and apparent Initiator Efficiency
    Polymer, 2002
    Co-Authors: Zheng Zhang
    Abstract:

    Abstract The equations for predicting the number-average molecular weight are derived on the basis of the three stage polymerization model (TSPM) in this paper. By applying the equations, a plotting approach is proposed to determine the apparent Initiator Efficiency defined as f [( α td +1)/2] and the constant of chain transfer to monomer, where f is the Initiator Efficiency and α td denotes the fraction of the termination rate constants by disproportionation. Using the approach to plot the experimental data in the literature, it is found that the chain transfer to monomer can be neglected for both methylmethacrylate (MMA) and styrene (St) polymerizations, but it can exert a significant effect on ethylmethacrylate (EMA) polymerization. In addition, the apparent Initiator Efficiency is found to be independent of reaction temperature and Initiator concentration at each stage. The values of f [( α td +1)/2] at gel effect stage are slightly reduced as compared with that at low conversion stage for MMA and EMA polymerizations. However, it decreases significantly at gel effect stage for St polymerization. Using the equations derived and the apparent Initiator efficiencies obtained from TSPM plots, the number-average molecular weights at different conversions can be predicted. Comparisons show that the agreement between predictions and experimental data is satisfactory.

  • Modeling of the bulk free radical polymerization up to high conversion—three stage polymerization model. II. Number-average molecular weight and apparent Initiator Efficiency
    Polymer, 2002
    Co-Authors: Jiguang Qin, Wenping Guo, Zheng Zhang
    Abstract:

    Abstract The equations for predicting the number-average molecular weight are derived on the basis of the three stage polymerization model (TSPM) in this paper. By applying the equations, a plotting approach is proposed to determine the apparent Initiator Efficiency defined as f [( α td +1)/2] and the constant of chain transfer to monomer, where f is the Initiator Efficiency and α td denotes the fraction of the termination rate constants by disproportionation. Using the approach to plot the experimental data in the literature, it is found that the chain transfer to monomer can be neglected for both methylmethacrylate (MMA) and styrene (St) polymerizations, but it can exert a significant effect on ethylmethacrylate (EMA) polymerization. In addition, the apparent Initiator Efficiency is found to be independent of reaction temperature and Initiator concentration at each stage. The values of f [( α td +1)/2] at gel effect stage are slightly reduced as compared with that at low conversion stage for MMA and EMA polymerizations. However, it decreases significantly at gel effect stage for St polymerization. Using the equations derived and the apparent Initiator efficiencies obtained from TSPM plots, the number-average molecular weights at different conversions can be predicted. Comparisons show that the agreement between predictions and experimental data is satisfactory.

Kuochen Tsai - One of the best experts on this subject based on the ideXlab platform.

  • pdf modeling of turbulent mixing effects on Initiator Efficiency in a tubular ldpe reactor
    Aiche Journal, 1996
    Co-Authors: Kuochen Tsai, Rodney O. Fox
    Abstract:

    The effect of turbulent mixing in the reaction zone of a tubular low-density polyethylene reactor was studied by combining a Lagrangian composition probability density function (LCPDF) code with a computational fluid-dynamics code. Because the LCPDF code can treat the chemical reaction terms in a turbulent flow without resorting to moment closures, it is used to describe the temperature and scalar fields of reactants including Initiator and monomer molar concentrations, and the moments of the molecular weight distribution. The chemical reaction terms are efficiently dealt with using a three-parameter chemical lookup table that contains the temperature and composition changes as functions of Initiator and monomer concentrations and temperature over a small time step. The reaction-rate constants from the study of Lee and Marano (1979) are functions of temperature and pressure. The flow fields are obtained using the k – e turbulence model. Because the temporal and spatial evolution of all fields in the reactor can be simulated, it is possible to study the effect of the Initiator injection location, flow rate and temperature of the monomer and Initiator feed streams on polymerization in considerable detail. Moreover, by observing the probability distribution of the composition fields, a better understanding of hot-spot formation is achieved, leading to improved reactor designs.

  • PDF modeling of turbulent‐mixing effects on Initiator Efficiency in a tubular LDPE reactor
    AIChE Journal, 1996
    Co-Authors: Kuochen Tsai, Rodney O. Fox
    Abstract:

    The effect of turbulent mixing in the reaction zone of a tubular low-density polyethylene reactor was studied by combining a Lagrangian composition probability density function (LCPDF) code with a computational fluid-dynamics code. Because the LCPDF code can treat the chemical reaction terms in a turbulent flow without resorting to moment closures, it is used to describe the temperature and scalar fields of reactants including Initiator and monomer molar concentrations, and the moments of the molecular weight distribution. The chemical reaction terms are efficiently dealt with using a three-parameter chemical lookup table that contains the temperature and composition changes as functions of Initiator and monomer concentrations and temperature over a small time step. The reaction-rate constants from the study of Lee and Marano (1979) are functions of temperature and pressure. The flow fields are obtained using the k – e turbulence model. Because the temporal and spatial evolution of all fields in the reactor can be simulated, it is possible to study the effect of the Initiator injection location, flow rate and temperature of the monomer and Initiator feed streams on polymerization in considerable detail. Moreover, by observing the probability distribution of the composition fields, a better understanding of hot-spot formation is achieved, leading to improved reactor designs.

Guy B. Marin - One of the best experts on this subject based on the ideXlab platform.

  • Initiator Efficiency modeling for vinyl chloride suspension polymerization
    Chemical Engineering Journal, 2009
    Co-Authors: Joris Wieme, Marie-françoise Reyniers, Guy B. Marin
    Abstract:

    Abstract A unified methodology is developed to calculate the fraction of Initiator derived radicals that initiates the polymerization, i.e. the Initiator Efficiency, as a function of polymerization time and polymerization conditions. In the presented approach the most important reactions that Initiator derived radicals undergo are taken into account. All involved parameters have a clear physical and fundamental meaning and no adjustment to experimental data is required. The presented methodology is applied to diacyl peroxide Initiators that are commonly used in industrial vinyl chloride suspension polymerization processes: dodecanoyl peroxide and benzoyl peroxide. Validation is performed by comparing calculated and experimental data for the monomer conversion and averages of the molar mass distribution within a polymerization temperature range of 323–333 K and an Initiator concentration range of 0.26–4.2 wt% based on the monomer.

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

  • 110th anniversary nonideal mixing phenomena in high pressure low density polyethylene autoclaves prediction of variable Initiator Efficiency and ethylene decomposition
    Industrial & Engineering Chemistry Research, 2019
    Co-Authors: Prokopis Pladis, C. Kiparissides
    Abstract:

    A segregation–backmixing model is developed to simulate the dynamic operation of multifeed high-pressure low-density polyethylene (LDPE) autoclaves, calculate the specific Initiator consumption (grams of Initiator per kilogram of LDPE), and assess the risk of ethylene decomposition under different nonideal mixing and operating scenarios. To describe the nonideal macro- and micromixing phenomena in a LDPE autoclave, a user-specified multizone model representation of the actual reactor is established. One of the key features of the present model is that the continuously fed Initiator into a reaction zone can exhibit two distinct states, namely, a “segregated” or a “molecular” one. Simulation results are presented, showing the effects of macro- and micromixing model parameters on the spatial temperature distribution and specific Initiator consumption with respect to the Initiator feed concentration to a two-compartment reaction zone. Finally, the operational conditions and process faults that can lead to eth...

  • 110th Anniversary: Nonideal Mixing Phenomena in High-Pressure Low-Density Polyethylene Autoclaves: Prediction of Variable Initiator Efficiency and Ethylene Decomposition
    Industrial & Engineering Chemistry Research, 2019
    Co-Authors: Prokopis Pladis, C. Kiparissides
    Abstract:

    A segregation–backmixing model is developed to simulate the dynamic operation of multifeed high-pressure low-density polyethylene (LDPE) autoclaves, calculate the specific Initiator consumption (gr...

  • development of a general mathematical framework for modeling diffusion controlled free radical polymerization reactions
    Macromolecules, 1992
    Co-Authors: Dimitris S Achilias, C. Kiparissides
    Abstract:

    A new theoretical framework is proposed for modeling diffusion-controlled free-radical po- lymerization reactions. Termination and propagation rate constants as well as Initiator Efficiency are expressed in terms of a reaction-limited term and a diffusion-limited one. The latter is shown to depend on the diffusion coefficient of the corresponding species (i.e., polymer, monomer, primary radicals) and an effective reaction radius. All parameters appearing in the diffusion-limited part of the kinetic rate constants have a clear physical meaning and can be evaluated in terms of the physical and transport properties of the reacting species. It is shown that the proposed approach for modeling diffusion-controlled reactions does not require the introduction of critical break points to mark the onset of various diffusional effects (Le., gel effect, glass effect). The ability of the present model to elucidate the mechanism of diffusion-controlled reactions is demonstrated by analyzing the free-radical polymerizations of styrene and methyl methacrylate initiated by the thermal decomposition of AIBN, AIBME, AVN, and LPO chemical Initiators. It is shown that, at high conversions, Initiator Efficiency strongly depends on the size of Initiator molecules. The present model predictions are in excellent agreement with experimental data on monomer conversion, total radical concentration, and average molecular weights measured in different laboratories by ODriscoll and Huang42 and Zhu et al.l0