Milorad P. Dudukovic

Multiphase catalytic reactors: a perspective on current knowledge and future trends

ABSTRACT Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas–liquid downflow, trickle-beds with countercurrent gas–liquid flow, and packed-bubble columns where gas and liquid are contacted in cocurrent upflow. The advantages of cyclic operation are also outlined. This is followed by a discussion on conventional reactors with mobile catalysts, such as slurry bubble columns, ebullated beds, and agitated reactors. Several unconventional reactor types are reviewed also, such as monoliths for two-phase flow processing, membrane reactors, reactors with circulating solids, rotating packed beds, catalytic distillation, and moving-bed chromatographic reactors. Numerous references are cited throughout the review, and the state-of-the-art is also summarized. Measurements and experimental characterization methods for multiphase systems as well as the role of computational fluid dynamics are not covered in a comprehensive manner due to other recent reviews in these areas. While it is evident that numerous studies have been conducted to elucidate the behavior of multiphase reactors, a key conclusion is that the current level of understanding can be improved further by the increased use of fundamentals.

Flow mapping in bubble columns using CARPT

A noninvasive Computer Automated Radioactive Particle Tracking (CARPT) Facility is used for the investigation of liquid recirculation and turbulence in a bubble column. The motion of a single neutrally buoyant radioactive particle is monitored by an array of scintillation detectors and analyzed by an on-line computer to map the flow field. Results for the mean flow patterns and turbulence in a 12″ diameter column are reported for the air-water system. This communication marks the first application of CARPT for tracing the liquid flow in bubble columns.

Numerical simulation of gas}liquid dynamics in cylindrical bubble column reactors

In this paper, we have attempted to validate a transient, two-dimensional axisymmetric simulation of a laboratory-scale cylindrical bubble column, run under bubbly flow and churn turbulent conditions. The experimental data was obtained via gamma-radiation based non-invasive flow monitoring methods, viz., computer automated radioactive particle tracking (CARPT) provided the data on liquid velocity and turbulence, and computed tomography (CT) determined the gas holdup profiles. The numerical simulation was done using the FLUENT software and compares the results from the algebraic slip mixture model, and the two-fluid Euler–Euler model. Reasonably, good quantitative agreement was obtained between the experimental data and simulations for the time-averaged gas holdup and axial liquid velocity profiles, as well as for the kinetic energy profiles. The favorable results suggest that the simple two-dimensional axisymmetric simulation can be used for reasonable engineering calculations of the overall flow pattern and gas holdup distributions.

A phenomenological model for pressure drop, liquid holdup, and flow regime transition in gas-liquid trickle flow

Abstract A phenomenological, pore-scale, hydrodynamic model is developed for representation of the uniform, two-phase, gas-liquid cocurrent flow in the low interaction regime in trickle bed reactors. The model provides improved predictions for both the pressure drop and liquid holdup using the parameters obtained exclusively from single phase flow data. In addition, a new criterion for prediction of trickle to pulsing flow regime transition is developed based on Kapitzas (1948) work on laminar film stability. Agreement with available data is good.

A γ-ray tomographic scanner for imaging voidage distribution in two-phase flow systems

Abstract A computed tomographic scanner using γ-rays has been implemented for the measurement of void fraction and its distribution in two-phase flow systems such as fluidized beds and bubble columns. The automated scanner is capable of imaging flows in test sections between 2.5 cm and 45.0 cm in diameter. The developed system hardware, the adopted algorithm for image reconstruction and possible sources of error in measurement are discussed. Typical results for void fraction distribution in bubble columns are presented.

Catalyst wetting efficiency in trickle-bed reactors at high pressure

Abstract Trickle-bed reactors are widely used in industrial high pressure operations (up to 30 MPa). The knowledge of catalyst wetting efficiency as a function of operating conditions is needed for relating laboratory and pilot scale reactor data to large scale reactor operation. The available predictions of the wetting efficiency rest on data collected at atmospheric pressure. In this study a phenomenological analysis has been developed to relate the wetting efficiency with operating conditions such as reactor pressure, gas and liquid flow rate. Experimental data for the wetting efficiency at high pressure have been obtained via a tracer technique. The data support the developed model which can be expressed by the following correlation: ƞ CE = 1.104 RE L 1/3 1 + [( ΔP Z )/p L g] Ga L 1/9 This correlation is also in good agreement with the data correlated previously at atmospheric pressure and provides the means for assessing liquid-catalyst contacting at all operating pressures.

Pressure drop and liquid holdup in high pressure trickle-bed reactors

The vast majority of commercial trickle-bed reactors, fixed bed of catalyst particles contacted by cocurrent downflow of gas and liquid, operate at high pressure. In this study Holubet al.s (1992, 1993) phenomenological model has been extended to describe the effect of high pressure (i.e. increased gas density) on pressure drop and liquid holdup in the trickle flow regime. This model, based on annular two-phase flow in a slit, ties pressure drop and liquid holdup but was previously verified only at atmospheric pressure. The advantage of this model is that the Ergun constantsE1 andE2, required by the model, are determined from single phase (gas) flow through the packing of interest and no two-phase flow data is needed. Experiments were conducted at high pressure over a range of gas and liquid velocities and different bed characteristics. The developed phenomenological analysis, describing the effect of high pressure and gas flow rate in terms of five limiting cases, matches well the experimental observations. The high pressure data collected in this study was used as a basis for comparing the prediction for pressure drop and liquid holdup of the model and of the available high pressure correlations. Holubet al.s (1992) model matches experimental observations better than available correlations. It also predicts all trends in pressure drop and holdup correctly for all changes in operating variables such as pressure, liquid and gas superficial mass velocity and with physical properties of the gas and liquid.

Void growth and resin transport during processing of thermosetting — Matrix composites

The fabrication of composite laminates having a thermosetting resin matrix is a complex process. It involves simultaneous heat, mass, and momentum transfer along with chemical reaction in a multiphase system with time-dependent material properties and boundary conditions. Two critical problems, which arise during production of thick structural laminates, are the occurrence of severely detrimental voids and gradients in resin concentration. In order to efficiently manufacture quality parts, on-line control and process optimization are necessary, which in turn require a realistic model of the entire process. In this article we review current progress toward developing accurate void and resin flow portions of this overall process model.

Dynamic simulation of bubbly flow in bubble columns

Gas–liquid bubbly flow in two-dimensional bubble columns is studied by numerical simulation. An Eulerian–Eulerian two-fluid model is used to describe the time-dependent motion of the liquid driven by small, spherical gas bubbles injected at the bottom of the columns. The simulations are able to capture the large scale structures as observed experimentally in the laboratory. The numerical results, which include the characteristics of large scale structures such as wave length and frequency, mean velocities, turbulence intensities and turbulence shear stress, are analized and compared with the experimental data of Lin et al. (1996) [A.I.Ch.E. J., 42, 301–318] and Mudde et al. (1997) [A.I.Ch.E. J., 43, 913–926] and show good agreement.

Frontiers in Reactor Engineering

The key challenge for reactor engineering is to establish the scientifically based sustainable technologies necessary for meeting the future energy, environmental, and materials needs of the world. This goal requires advancing our scientific understanding of multiscale kinetic transport interactions to enable better reactor choice and to ensure higher reactor and process efficiencies.