QEEFoam: A Quasi-Eulerian-Eulerian model for polydisperse turbulent gas-liquid flows. Implementation in OpenFOAM, verification and validation

In this paper we present a new multiphase computational model for polydisperse turbulent gas-liquid flows. In this model the gas phase is transported by a single convection equation and the effect of turbulent dispersion is addressed by including a diffusion term. In order to close the system of equations, the gas phase velocity is calculated by employing the slip velocity concept or by solving an ODE. This procedure shares similarity with the Euler-Lagrange (E-L) method, in which the gas velocity is updated by bubble Lagrangian tracking, and with the Euler-Euler (E-E) method, and for this reason it is called the Quasi-Eulerian-Eulerian (Q-E-E) method. In order to account for polydispersity one single transport equation is added to describe the effects of bubble breakage and coalescence on the mean bubble size. The novel Q-E-E method was implemented in the open-source code OpenFOAM-7 and was used to simulate turbulent gas-liquid flows with three different geometries operating under different conditions. The predictions for the dynamical vortex structures, local phase fraction, global gas holdup, mean bubble size and vertical/horizontal liquid velocities were verified against the solution provided by the E-L solver or against published experimental data. Good agreement was found and with extremely small computational costs

Computational fluid dynamic modeling of fluidized bed polymerization reactors

Polyethylene is one of the most widely used plastics, and over 60 million tons are produced worldwide every year. Polyethylene is obtained by the catalytic polymerization of ethylene in gas and liquid phase reactors. The gas phase processes are more advantageous, and use fluidized bed reactors for production of polyethylene. Since they operate so close to the melting point of the polymer, agglomeration is an operational concern in all slurry and gas polymerization processes. Electrostatics and hot spot formation are the main factors that contribute to agglomeration in gas-phase processes. Electrostatic charges in gas phase polymerization fluidized bed reactors are known to influence the bed hydrodynamics, particle elutriation, bubble size, bubble shape etc. Accumulation of electrostatic charges in the fluidized-bed can lead to operational issues. In this work a first-principles electrostatic model is developed and coupled with a multifluid computational fluid dynamic (CFD) model to understand the effect of electrostatics on the dynamics of a fluidized-bed. The multifluid CFD model for gas-particle flow is based on the kinetic theory of granular flow closures. The electrostatic model is developed based on a fixed, size-dependent charge for each type of particle (catalyst, polymer, polymer fines) phase. The combined CFD model is first verified using simple test cases, validated with experiments and applied to a pilot-scale polymerization fluidized bed reactor. The CFD model reproduced qualitative trends in particle segregation and entrainment due to electrostatic charges observed in experiments. For the scale up of fluidized bed reactor, filtered models are developed and implemented on pilot scale reactor

Development, Verification, and Validation of Multiphase Models for Polydisperse Flows

This report describes in detail the technical findings of the DOE Award entitled 'Development, Verification, and Validation of Multiphase Models for Polydisperse Flows.' The focus was on high-velocity, gas-solid flows with a range of particle sizes. A complete mathematical model was developed based on first principles and incorporated into MFIX. The solid-phase description took two forms: the Kinetic Theory of Granular Flows (KTGF) and Discrete Quadrature Method of Moments (DQMOM). The gas-solid drag law for polydisperse flows was developed over a range of flow conditions using Discrete Numerical Simulations (DNS). These models were verified via examination of a range of limiting cases and comparison with Discrete Element Method (DEM) data. Validation took the form of comparison with both DEM and experimental data. Experiments were conducted in three separate circulating fluidized beds (CFB's), with emphasis on the riser section. Measurements included bulk quantities like pressure drop and elutriation, as well as axial and radial measurements of bubble characteristics, cluster characteristics, solids flux, and differential pressure drops (axial only). Monodisperse systems were compared to their binary and continuous particle size distribution (PSD) counterparts. The continuous distributions examined included Gaussian, lognormal, and NETL-provided data for a coal gasifier

Computational analyses for modeling fluidized bed gasification processes

In-situ adaptive tabulation (ISAT) method, which can treat complex chemistry efficiently, has been implemented into a multiphase computational fluid dynamics (CFD) code. The numerical algorithm is improved to solve the chemical source term. Two-dimensional and three-dimensional non-isothermal detailed silane pyrolysis cases are used to test the features of ISAT, such as scale factors and multiple tables on multiple processors. ISAT can reduce the computational time for chemistry drastically. For example, a speed-up of 50 times is achieved with the non-isothermal three-dimensional case. Coal gasification with detailed chemical reactions is simulated to test the performance of ISAT with complex gas-solid reaction mechanisms. The speed-up of chemistry is around 16 times with 10% reduction in total CPU time for simulations. The elutriation phenomenon in fluidized beds is also studied in order to increase the efficiency of biomass reactors. In the simulations, fine and coarse particles are solved as two individual phases. Effects of gas velocity, and fine and coarse particle diameters are investigated for elutriation rate constants. The elutriation rate constants are proportional to approximately the 5.6th power of the gas velocity and decrease with increasing fine diameters. The diameters of coarse particles have little influence on elutriation rate constants. The simulation results are compared with experiments from the literature and performed at Iowa State University. Apart from the mainstream of this research, the effects on the use of coordinate systems and configurations to model fluidized bed reactors are tested. Three different fluidization regimes: bubbling, slugging and turbulent regimes are investigated. The results indicate that a two-dimensional Cartesian system can be used to successfully simulate and predict a bubbling regime. Caution must be exercised when using two-dimensional simulations for other fluidized regimes

Kinetic modelling simulation and optimal operation of fluid catalytic cracking of crude oil: Hydrodynamic investigation of riser gas phase compressibility factor, kinetic parameter estimation strategy and optimal yields of propylene, diesel and gasoline in fluid catalytic cracking unit

The Fluidized Catalytic Cracking (FCC) is known for its ability to convert refinery wastes into useful fuels such as gasoline, diesel and some lighter products such as ethylene and propylene, which are major building blocks for the polyethylene and polypropylene production. It is the most important unit of the refinery. However, changes in quality, nature of crude oil blends feedstock, environmental changes and the desire to obtain higher profitability, lead to many alternative operating conditions of the FCC riser. There are two major reactors in the FCC unit: the riser and the regenerator. The production objective of the riser is the maximisation of gasoline and diesel, but it can also be used to maximise products like propylene, butylene etc. For the regenerator, it is for regeneration of spent or deactivated catalyst. To realise these objectives, mathematical models of the riser, disengage-stripping section, cyclones and regenerator were adopted from the literature and modified, and then used on the gPROMS model builder platform to make a virtual form of the FCC unit. A new parameter estimation technique was developed in this research and used to estimate new kinetic parameters for a new six lumps kinetic model based on an industrial unit. Research outputs have resulted in the following major products’ yields: gasoline (plant; 47.31 wt% and simulation; 48.63 wt%) and diesel (plant; 18.57 wt% and simulation; 18.42 wt%) and this readily validates the new estimation methodology as well as the kinetic parameters estimated. The same methodology was used to estimate kinetic parameters for a new kinetic reaction scheme that considered propylene as a single lump. The yield of propylene was found to be 4.59 wt%, which is consistent with published data. For the first time, a Z-factor correlation analysis was used in the riser simulation to improve the hydrodynamics. It was found that different Z factor correlations predicted different riser operating pressures (90 – 279 kPa) and temperatures as well as the riser products. The Z factor correlation of Heidaryan et al. (2010a) was found to represent the condition of the riser, and depending on the catalyst-to-oil ratio, this ranges from 1.06 at the inlet of the riser to 0.92 at the exit. Optimisation was carried out to maximise gasoline, propylene in the riser and minimise CO2 in the regenerator. An increase of 4.51% gasoline, 8.93 wt.% increase in propylene as a single lump and 5.24 % reduction of carbon dioxide emission were achieved. Finally, varying the riser diameter was found to have very little effect on the yields of the riser products

Finite-Volume Filtering in Large-Eddy Simulations Using a Minimum-Dissipation Model

Large-eddy simulation (LES) seeks to predict the dynamics of the larger eddies in turbulent flow by applying a spatial filter to the Navier-Stokes equations and by modeling the unclosed terms resulting from the convective non-linearity. Thus the (explicit) calculation of all small-scale turbulence can be avoided. This paper is about LES-models that truncate the small scales of motion for which numerical resolution is not available by making sure that they do not get energy from the larger, resolved, eddies. To identify the resolved eddies, we apply Schumann’s filter to the (incompressible) Navier-Stokes equations, that is the turbulent velocity field is filtered as in a finite-volume method. The spatial discretization effectively act as a filter; hence we define the resolved eddies for a finite-volume discretization. The interpolation rule for approximating the convective flux through the faces of the finite volumes determines the smallest resolved length scale δ. The resolved length δ is twice as large as the grid spacing h for an usual interpolation rule. Thus, the resolved scales are defined with the help of box filter having diameter δ= 2 h. The closure model is to be chosen such that the solution of the resulting LES-equations is confined to length scales that have at least the size δ. This condition is worked out with the help of Poincarés inequality to determine the amount of dissipation that is to be generated by the closure model in order to counterbalance the nonlinear production of too small, unresolved scales. The procedure is applied to an eddy-viscosity model using a uniform mesh

Multiphase flow dynamics and mass transfer in different multiphase reactor for shear controllable synthesis process

In short, this thesis gives insight in detailed particle synthesis process modelling in two main multiphase reactors (IJR and SVFR) and experiments of synthesis of FePO4 and SiO2 aggregated particles are performed to analysis effect of hydrodynamics on particle properties. The attempt of combination of fast-mixer and external ultrasound field in micro/nano-particle synthesis has successfully intensified turbulence and such combination has positive effects on aspects of chemical reaction, mixing performance and mass transfer. Except for experimental work, the simulation work has been carried out to analyse flow patterns, turbulent intensity, chemical reaction as well as particle fluid interaction in particle synthesis process in multiphase reactors. Such investigation makes it possible to build correlations on hydrodynamic parameters and particle characteristics and helps to predict the behaviour and properties of particles. This is meaningful for design, upgrade and scale-up of multiphase reactor in synthesis process

Multiphase flow dynamics and mass transfer in different multiphase reactor for shear controllable synthesis process

In short, this thesis gives insight in detailed particle synthesis process modelling in two main multiphase reactors (IJR and SVFR) and experiments of synthesis of FePO4 and SiO2 aggregated particles are performed to analysis effect of hydrodynamics on particle properties. The attempt of combination of fast-mixer and external ultrasound field in micro/nano-particle synthesis has successfully intensified turbulence and such combination has positive effects on aspects of chemical reaction, mixing performance and mass transfer. Except for experimental work, the simulation work has been carried out to analyse flow patterns, turbulent intensity, chemical reaction as well as particle fluid interaction in particle synthesis process in multiphase reactors. Such investigation makes it possible to build correlations on hydrodynamic parameters and particle characteristics and helps to predict the behaviour and properties of particles. This is meaningful for design, upgrade and scale-up of multiphase reactor in synthesis process