A concise review of relevant experimental observations and modeling of high-pressure trickle-bed reactors, based on recent studies, is presented. The following topics are considered:  flow regime transitions, pressure drop, liquid holdup, gas−liquid interfacial area and mass-transfer coefficient, catalyst wetting efficiency, catalyst dilution with inert fines, and evaluation of trickle-bed models for liquid-limited and gas-limited reactions. The effects of high-pressure operation, which is of industrial relevance, on the physicochemical and fluid dynamic parameters are discussed. Empirical and theoretical models developed to account for the effect of high pressure on the various parameters and phenomena pertinent to the topics discussed are briefly described.

https://crelonweb.eec.wustl.edu/ReactionEngineeringCourse/Day-8.pdf

High-Pressure Trickle-Bed Reactors: A Review

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 coc...

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

Data on design and operation of trickle beds at elevated pressures are scarce. In this study the influence of the gas density on the liquid holdup, the pressure drop, and the transition between trickle and pulse flow has been investigated in a tricklebed reactor operating up to 7.5 MPa and with nitrogen or helium as the gas phase. Gas-liquid interfacial areas have been determined up to 5.0 MPa by means of CO2 absorption from CO2/N2 gas mixtures into amine solutions. 
A comparison of the results from nitrogen as the gas phase to those of helium shows that at equal gas densities the hydrodynamic states are the same. The gas-liquid interfacial area increases when operating at higher gas densities. When the determined dimensionless interfacial areas agl/as are all within the range 0.25-0.8, the trickle-bed reactor is suggested to operate in the trickle-flow regime. The gas density has a strong influence on the liquid holdup. Due to the higher pressure gradients at elevated gas densities, the liquid holdup decreases noticeably. Besides, the boundary between the trickle-flow and pulse-flow regime shifts toward higher liquid throughputs: the region for trickle-flow operationg becomes larger. For the liquid holdup and the pressure gradient in the trickle-flow regime, correlations derived based on dimensionless numbers can be applied to high-prssure trickle beds.

/pdf/hydrodynamics-in-a-cocurrent-gas-liquid-trickle-bed-at-1yjg6sxrti.pdf

Hydrodynamics in a cocurrent gas-liquid trickle bed at elevated pressures

http://fs.teledos.gr:2206/%3ERESEARCH%20PUBLICATIONS/CHEMICAL%20ENGINEERING/Reactor/Three-phase%20catalytic%20reactors.pdf

Three-phase catalytic reactors

The paper presents experimental results concerning dynamic liquid holdup, wetting efficiency and local mass transfer coefficients between the liquid and solid surface in a fixed-bed three-phase reactor, in which both the gas and liquid flow cocurrently downwards. The experiments were conducted for two basic regimes of operation of trickle-bed reactors, namely the gas continuous flow regime and the pulsing flow regime under atmospheric pressure. The measurements of dynamic liquid holdup have been performed for a wide range of gas and liquid flow rates, three different packing diameters, and for two systems of working media of different physical properties. The common correlation developed determines with good accuracy the holdup values in both regimes as well as in the transition region between the two hydrodynamic modes. In experiments concerning the wetting efficiency a dynamic tracer method has been employed, for which an original mathematical model has been formulated. The results are presented in the form of diagrams and appropriate correlating formulae. The experiments concerning local solid—liquid mass transfer coefficients were carried out for two diameters of spherical particles, with the flow rates of both phases and physicochemical properties of the liquid varied over a wide range. The experimental results are correlated and compared with the appropriate literature data. A mathematical model describing the time-varying solid—liquid mass transfer process is formulated for the pulsed flow regime. The time-averaged mass transfer coefficients calculated on the basis of the model developed are compared with the experimental values. A good agreement between these two sets of values fully confirms the assumptions of the model elaborated.

Hydrodynamics and mass transfer in a three-phase fixed-bed reactor with cocurrent gas—liquid downflow

Flow regimes for nonfoaming and foaming two-phase downflow through catalyst beds were correlated in terms of the superficial volumetric gas-to-liquid ratio and a force ratio relating inertia plus gravity forces to viscous plus interphase forces. For nonfoaming systems, the resulting flow map identified five regimes: two continuous phases (trickling flow), gas continuous (spray or blurring flow), liquid continuous (dispersed bubble), pulsing, and bubbling/pulsing; for foaming systems, the regimes were two continuous phases, gas continuous, foaming, and pulsing with and without foaming. Several orders of magnitude variations in the Froude, Weber, and Reynolds numbers were covered by data obtained in a 29.2 cm dia bed for aqueous 0-46% glycerol solutions and literature data obtained during downflow in smaller diameter beds for nonfoaming systems, including air/diethylene glycol, natural gas/lubricating oil, carbon dioxide/lubricating oil, and Freon 12/silicone oil and for foaming systems, including air/silicone oil and natural gas/kerosine. In the pulsing flow regime, the frequency of pulses leaving large (29.2 cm) and small (6.58 cm) dia beds were correlated with the flow map coordinates. The results are applicable to existing or new pilot and commercial reactors. Diagram, graphs, and 17 references.

Two‐phase downflow through catalyst beds: Part I. Flow maps

Surveys of local and overall pressure drop in partially pulsing 29.2 cm diameter beds show the overall pressure drop steady but elevated, while local pressure drops across various sections of the bed oscillate in correspondence with the local pulse frequency. The increase in overall pressure drop due to pulsing varies between 0 and 100%, depending on the extent of pulsing and liquid holdup within the bed.

In foaming systems, the overall bed pressure drop can be higher than for pulsing alone, depending on liquid holdup and (apparently) the type of foam and its intensity. In some cases, pulsing appears to be the starting point for low frequency foaming pressure drop surges which are associated with cyclic foam formation and breakup within the bed.

E. Talmor

Papers

Two‐phase downflow through catalyst beds: Part I. Flow maps

Two-phase downflow through catalyst beds: Part II. Pulsing regime pressure drop