Trickle Flow Research Articles

The three-levels-of-porosity reactor. A novel reactor for countercurrent trickle-flow processes

A novel three-phase reactor concept featuring countercurrent trickle flow, the three-levels-of-porosity (TLP) Reactor, is introduced. A lab-scale TLP reactor is described. On the basis of visual observation and typical pressure drop and liquid holdup measurements using air and water, it can be concluded that, in the reactor packing, gas and liquid flow through separate pathways with the possibility of periodic contact. The reactor features very low pressure drop compared to randomly packed fixed-bed reactors. It can be operated countercurrently with gas and liquid flowrates well above those currently used in commercial cocurrent hydrotreaters and is therefore a promising alternative to the randomly packed fixed-bed trickle-flow reactor for hydroprocessing of heavy oils.

Comparison of hydrodynamic parameters for countercurrent and cocurrent flow through packed beds

AbstractHydrodynamic parameters have been determined in common equipment, i.e., same column and liquid and gas distributors, for cocurrent and countercurrent two‐phase flow through fixed beds. The piston/dispersion exchange model (PDE) with usual Danckwerts' boundary conditions (closed/closed system) has been used to describe the liquid flow. A new imperfect pulse method has been used to estimate the PDE model parameters directly from the experimentally nonideal input and output response. The transition between trickle flow and pulse flow, for two‐phase downflow, and the occurrence of flooding, for countercurrent flow, has been investigated using a macroscopic model for the two‐phase flow.

Trickle flow of dense particles through a fluidized bed of others

Dense (or large) particles fall through liquid fluidized beds of light (or small) particles. In this work, a new contactor is described in which continuous, countercurrent transport of dense particles in a stationary, liquid fluidized bed of light particles is exploited to obtain selective and continuous transport of the dense phase. The system is refered to as the ‘trickle flow fluidized-bed reactor’. This sytem is evaluated in its application as a countercurrent adsorptive reactor. By selecting a suitable adsorbent as the dense phase, and a catalyst as the light phase, simultaneous reaction and countercurrent product removal may be achieved within one contactor. For a systematic design and optimization of such an adsorptive trickle flow fluidized-bed reactor, a predictive hydrodynamic model is required. In this work, the relation between the volume fractions and the fluxes of the three phases is investigated. A laboratory scale trickle flow fluidized-bed reactor has been constructed for experimental studies on hold-up and flux. A multi-component transport model to predict the volume fractions, fluxes and operating conditions is developed. The model is validated with the experimental hold-up and flux data from the trickle flow fluidized-bed reactor. To apply this concept as a countercurrent adsorptive reactor, a high countercurrent adsorbent flow, a high hold-up of the dense adsorbent and light catalyst are required, together with a stable operation. Hence, the reactor should operate at a minimum liquid fraction of approximately 50 vol% and a free area fraction of the supporting plate between 0.3 and 0.5. It is demonstrated that the system is very sensitive to the bulk density of the multi-component suspension. At a high hold-up of the dense particles, the bulk density of the bidisperse suspension may increase to a level which causes a complete wash-out of the light particles. Therefore, the difference in density between the light catalyst and dense adsorbent particles should be as small as possible.

Miniaturization of hydroprocessing catalyst testing systems: Theory and practice

AbstractMain factors limiting downscaling of fixed‐bed integral reactors are discussed, which operate with a gas stream or with gas and liquid in trickle flow, as used in hydroprocessing of oil. Criteria developed for a sufficiently close approach to plug flow and for good contacting of the catalyst show that they can be met in very small reactors, both for processes with reactants in the gas phase and for trickle‐flow processes, provided that the catalyst bed is diluted with fine inert material in the latter case. Experimental tests show that microreactors with typical catalyst volumes of 5–10 mL can be used to obtain representative results that can very well match data from industrial reactors. It appears to be feasible to miniaturize catalytic test reactors further to “nanoscale” reactors with as little as 0.2–0.4 mL of catalyst while maintaining the results to be meaningful. Even though there are advantages as well as limitations, miniaturization can enhance safety and reduce manpower.

Residence time distribution of the liquid in two‐phase cocurrent downflow in packed beds: Air/newtonian and non‐newtonian liquid systems

AbstractNew data on the liquid residence time distribution for two‐phase downflow of air‐Newtonian and non‐Newtonian liquids through packed beds of porous and non‐porous particles are presented. The piston‐dispersion‐exchange model is used to describe the liquid flow. With porous particles the dynamic evolution of the tracer concentration in the particles is described in terms of diffusion phenomena. The axial dispersion is very important in the case of two‐phase downflow of air‐water (trickle flow regime) and air‐CMC systems through fixed beds with porous particles, and is negligible in the case of non‐porous particles. With the porous particles, a key value is the effective diffusion coefficient of the tracer in the pores of the particles.

On dynamic liquid holdup determination by the drainage method

A careful examination of the liquid drainage in packed-bed reactors using liquid superficial velocities characteristic of trickle flow regime and no gas velocity is both experimentally and theoretically carried out. True free-flowing or dynamic liquid holdups are determined in two different reactors packed with porous and nonporous particles employing several liquids. Dynamic liquid holdups are shown to be higher than those determined directly from total liquid drained volumes, which are usually reported when the drainage method is applied. It is found that a model of laminar liquid films properly addresses the influence of liquid superficial velocities and of liquid viscosity on the dynamic liquid holdup within the experimental conditions studied. A simple quantitative phenomenological model to describe the drainage kinetics is developed and is independently verified from experimental data. Satisfactory agreement is found for all the examined conditions.

Heat transfer in a packed bed reactor with cocurrent downflow of a gas and a liquid

The authors investigate hydrodynamics and radial heat transfer in packed column with cocurrent downflow of gas and liquid. The effect of flow regimes, of gas and liquid flow rates and of liquid properties are discussed. The heat transfer can be represented by the homogeneous model with one or two parameters: the bed effective radial thermal conductivity A r and if necessary a supplementary heat transfer resistance at the wall of the reactor 1 α w . The importance and complexity of the interaction between hydrodynamics and heat transfer are shown. The necessity of distinguishing two major hydrodynamic regimes (low and high interaction) to describe the heat transfer parameters is emphasized. Simple predictive relations are proposed for each regime, based on physical phenomena. Low interaction regime corresponds to trickling flow, whereas high interaction regime includes bubble flow and pulsed flow. The liquid holdup behaviour enables to interpret the heat transfer results in both cases. The results in cocurrent downflow are also compared to results in cocurrent upflow and to correlations of the literature.

A countercurrent adsorptive reactor for acidifying bioconversions

A countercurrent adsorptive reactor, based on the segregation tendency of dense sorbent particles through a liquid fluidized bed of light biocatalyst particles, is described for the acidifying production of the key pharmaceutical intermediate 6-Aminopenicillanic acid from Penicillin G. Mass transfer limited release of hydroxide from an ion exchange resin with simultaneous separation of the product by ion exchange, enabled axial control of the pH and product inhibition. The complexity of the countercurrent Trickle Flow Reactor requires a mathematical model, incorporating the effects of hydrodynamics, heterogeneous biocatalysis and multicomponent ion exchange. Such a model is presented and the parameters in the Three Phase-liquid, biocatalyst, sorbent-Model were determined from independent experiments. Model predictions gave an adequate description of experimental reactor performance. Using this model, the sensitivity of key parameters in the model, such as sorbent to liquid feed ratio and ion exchange selectivity and reactor optimization are readily studied.

Experimental and theoretical studies of pressure drop hysteresis in trickle bed reactors

Extensive experimental work was carried out with three different gas-liquid systems (air-water, air-water with surfactant C 12H 25SO 3Na, air-aqueous CMC solution) and three kinds of packings (Φ2.7, 4.0 and 8.0 mm glass beads) to examine the influence of various parameters on pressure drop hysteresis. Gas and liquid flow rates, physical properties of liquid and operation modes, influence the behavior of hysteresis in the packed reactor, and liquid flow rate is the most important factor. Hysteresis is not so pronounced for columns packed with large particles and it disappears in the pulsing flow regime. The mechanism responsible for hysteretic behavior resides in the variable uniformity of gas-liquid flow in the packed section. A parallel zone model for pressure drop in the trickling flow regime was established on the basis of experimental facts and analysis of flow structure. Theoretically predicted hysteresis loop of pressure drop is in satisfactory agreement with experimental data.

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 Holub et 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 constants E 1 and E 2, 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. Holub et 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.

High-Pressure Hydrogenation of Naphthalene Using a Reduced Iron Catalyst

Based upon the interest in iron as a potential dispersed phase catalyst for synfuels production, the high-pressure hydrogenation of naphthalene in mineral oil and cyclohexane solvents was studied in trickle flow and vapor-phase reactors, respectively, with an unsupported iron oxide powder as catalyst. Several forms of iron catalyst were examined including the original iron oxide, hydrogen prereduced, and in-situ sulfided forms. A commercial Ni-Mo/Al 2 O 3 catalyst was used for comparison. Iron oxide was not active for hydrogenation; however, in situ reduction by hydrogen yielded a remarkably active hydrogenation catalyst. The secondary hydrogenation reaction to form decalins from tetralin occurred only with the hydrogen reduced iron. Kinetic behavior was determined in the vapor phase at 6.9 MPa and at temperatures over a range of 160-300°C.

The gas-solid-solid packed contactor: Hydrodynamic behaviour of counter-current trickle flow of coarse and dense particles with a suspension of fine particles

The development of a new recovery system involving direct contact of a dilute suspension of fine particles with a rain of coarse and dense particles in a packed column is described. The experiments are complemented by an analysis of the hydrodynamic behaviour. The experimental results show that the behaviour of a dilute multiphase system may be described by two or more diphase systems.

Multicomponent mass and energy transport on different length scales in a packed reactive distillation column for heterogeneously catalysed fuel ether production

A reactive distillation column was operated which was packed with acid ion exchange rings catalysing the production of the octane enhancer methyl-tert.-butylether (MTBE) from methanol (MeOH) and isobutene (IB). Two types of catalytic rings were used: a block polymer (BP) with a capacity of 4.54 meq(H+)/g and a glass supported precipitated polymer (GPP) with a capacity of 0.9 meq(H+)/g. Fluid dynamic experiments with pure MeOH and pure MTBE show that the catalytic packings, which are located in the upper column section, work under trickle flow conditions. Nonreactive distillation experiments reveal sustained macroscale oscillations of fluxes, pressure and temperature that are caused by the distillation behaviour of the binary mixture MeOH/IB. These oscillations were also detected in the reactive distillation process with the highly stable, but less active GPP catalyst. The local compositions and temperatures from the nonoscillatory reactive distillation experiments with the BP catalyst are subjected to a detailed analysis of microscale solid-liquid and vapour-liquid transport resistances based on the Generalised Maxwell-Stefan approach. It turns out that the mass transport resistances inside the catalyst can vary significantly along the packing. The main mass transport resistance at the vapour-liquid interface is located on the liquid side.

Multicomponent mass and energy transport on different length scales in a packed reactive distillation column for heterogeneously catalysed fuel ether production

A reactive distillation column was operated which was packed with acid ion exchange rings catalysing the production of the octane enhancer methyl- tert.-butylether (MTBE) from methanol (MeOH) and isobutene (IB). Two types of catalytic rings were used: a block polymer (BP) with a capacity of 4.54 meq(H +)/g and a glass supported precipitated polymer (GPP) with a capacity of 0.9 meq(H +)/g. Fluid dynamic experiments with pure MeOH and pure MTBE show that the catalytic packings, which are located in the upper column section, work under trickle flow conditions. Nonreactive distillation experiments reveal sustained macroscale oscillations of fluxes, pressure and temperature that are caused by the distillation behaviour of the binary mixture MeOH/IB. These oscillations were also detected in the reactive distillation process with the highly stable, but less active GPP catalyst. The local compositions and temperatures from the nonoscillatory reactive distillation experiments with the BP catalyst are subjected to a detailed analysis of microscale solid-liquid and vapour-liquid transport resistances based on the Generalised Maxwell-Stefan approach. It turns out that the mass transport resistances inside the catalyst can vary significantly along the packing. The main mass transport resistance at the vapour-liquid interface is located on the liquid side.

Solid-liquid mass transfer to hollow pellets in a trickle bed

Local, instantaneous liquid-to-solid mass-transfer coefficients to both the internal (inside a hole) and external surfaces of hollow, nonporous pellets are measured in a trickle-bed reactor by an electrochemical technique for various gas (N 2 ) and liquid (water) flow rates and hole sizes in both the trickle and pulsing flow regimes. This information is needed for predicting the volume efficiency of hollow pellets. In the trickle-flow regime, partial wetting causes large variations in the mass-transfer rates among pellets with identical holes in the same packed bed. In the pulsing regime, the gas-flow rate has a negligible impact on the mass-transfer rate, while the product of the Sherwood number and the fractional wetting, ΦSh, increases with increasing liquid flow rate and hole diameter. The mass transfer is not uniform on the pellet surface, and the ratio of internal to external mass-transfer coefficients is approximately two-thirds. A correlation of the time-averaged mass transfer at both the pellet hole and external surfaces was developed. Mathematical simulations of the effectiveness and volume efficiency factor of pellets with several holes show that the nonuniformity in the mass transfer has a small impact unless the external mass-transfer resistance becomes the rate-limiting process

Pressure drop, liquid holdup, and flow regime transition in trickle flow

AbstractA phenomenological, pore‐scale, hydrodynamic model is developed for representation of the uniform, cocurrent, two‐phase flow in the low interaction regime in trickle bed reactors. Comparison of model predictions with numerous pressure drop and liquid holdup data reveals that phase interaction terms are negligible which results in a simplified model with no adjustable parameters. This model yields improved pressure drop and liquid holdup estimates for the low interaction regime. In addition, a criterion for the prediction of the trickle to pulsing flow regime transition is developed based on Kapitza's (1945) work on laminar film stability. This criterion compares favorably to data and to some other existing models for prediction of the trickle to pulsing flow regime transition.

Cerebral metabolism and circulatory arrest: Effects of duration and strategies for protection

Hypothermic total circulatory arrest (CA) is commonly used to facilitate repair of complex congenital heart defects. However, the “safe” period of CA remains to be defined. Extended periods of hypothermic total circulatory arrest may impair cerebral metabolism and cause ischemic brain injury. This study defines the relationship between increasing durations of CA at 18 °C and cerebral metabolism, and examines the protective value of topical cooling of the head or continuous “trickle” flow (5 to 10 mL · kg −1 · min −1). Thirty-three 1-week-old piglets were randomized to six experimental groups: control; 15, 30, or 60 minutes of CA; 60 minutes of CA with topical cooling of the head; and 60 minutes of trickle flow. Animals were placed on cardiopulmonary bypass (CPB) at 100 mL · kg −1 · min −1 and cooled to 18 °C. After the experimental period of CA or trickle flow (or 60 minutes of CPB at normal flow for the control group), animals were rewarmed to 37 °C and weaned from CPB. Data were obtained before and immediately after CPB at 37 °C, and before and immediately after the experimental period at 18 °C. Parameters measured included cerebral blood flow by xenon 133 clearance, arterial and sagittal sinus blood gases, and cerebral metabolism. Hypothermic total circulatory arrest caused an impairment of cerebral metabolism that was directly proportional to CA duration ( r 2 = 0.73; p = 0.0001), and recovery of metabolic function after 60 minutes of CA improved more than 50% if the head was packed in ice. If technically feasible, CPB flow of only 5 to 10 mL · kg −1 · min −1 during hypothermia is superior to CA with respect to cerebral protection. Future studies with this model can be used to develop optimal modes of cerebral protection during neonatal heart operations.

Mass transfer between gas and particles in a gas—solid trickle flow reactor

Gas-solids mass transfer was studied for counter-current flow of gas and millimetre-sized solid particles over an inert packing at dilute phase or trickle flow conditions. Experimental data were obtained from the adsorption of water vapour on 640 and 2200 μm diameter molecular sieve spheres at ambient conditions in a test column with a cross-sectional area of 0.06 x 0.06 m 2 and a packing height of 0.27 or 0.53 m. The packing consisted of a bank of regularly stacked, 0.01 m diameter bars made of stainless steel. Assuming the effective area for mass transfer to be equal to the external surface area of the spheres, the experimental values of the average gas—solids mass transfer coefficient were determined to amount to approximately 40–80% of the values calculated from the well-known Ranz—Marshall correlation for a single sphere in an undisturbed gas flow. These experimental values were identified as conservative estimates of the actual average gas—solids mass transfer coefficient because the pore diffusion resistance could not be eliminated completely. A comparison between experimental data on hydrodynamics (gas—solids momentum transfer) and gas—solids mass transfer indicated, furthermore, that the influence of particle shielding with interfering concentration boundary layers of different particles was small for the millimetre-sized particles and the experimental conditions investigated. Gas—solids mass transfer as well as gas—solids momentum transfer were mainly determined by single-particle flow behaviour.

Modelling of non-catalytic reactions in a gas-solid trickle flow reactor: dry, regenerative flue gas desulphurisation using a silica-supported copper oxide sorbent

A one-dimensional, two-phase dispersed plug flow model has been developed to describe the steady-state performance of a relatively new type of reactor, the gas-solid trickle flow reactor (GSTFR). In this reactor, an upward-flowing gas phase is contacted with as downward-flowing dilute solids phase over an inert packing. The model is derived from the separate mass heat balances for both the gas and (porous) solids phases for the case of a non-catalytic gas-solid reaction, which is first-order in the gaseous reactant. The reaction rate may also depend on the solid reactant concentration, but this concentration is assumed to be low and uniform throughout the solids volume. From the model, axial profiles can be calculated numerically for the four independent variables, viz. the gas-phase and solids-phase temperatures and the concentrations of the gaseous and solid reactant. Under isothermal conditions, the model equations can be solved analytically; the resulting expressions for the axial profiles of the gaseous and solid reactant are presented. The model is applied to predict the flue gas desulphurisation performance of a full-scale GSTF absorber in a dry, regenative process for the simultaneous removal of SO x and NO x from flue gases. In this process, to be operated at 350–400°C, the sorbent material consists of a porous silica support (spherical particles, 1.5mm diameter) with 7.5 wt% CuO deposited on this support by an ion-exchange technique. The model calculations are based on experimental findings from previous studies regarding reaction kinetics, hydrodynamics of the two-phase flow, gas-solids mass transfer and testing of the integrated process in a bench-scale plant. It appears that SO 2 removal efficiencies over 95% can be achieved in a GSTF absorber with a length of 15 m. Furthermore, the model predicts a large temperature peak for both phases in the absorber if the heat capacity ratio(defined as the ratio of mass flux times specific heat capacity for both phases) is close to one. This large temperature peak is due to the occurrence of the exothermic reaction of SO 2 with CuO in combination with efficient counter-current gas-solids heat exchange. Several parameters influencing the magnitude and axial position of the maximum gas-phase and solids-phase temperatures are discussed.

Macromixing versus hydrodynamics in trickle flow columns

The hydrodynamics in trickle flow columns is characterized by parameters such as the liquid flow maldistribution, the partial wetting, the liquid holdup, etc which are difficult to determine experimentally on large scale units. The usual way to avoid these hydrodynamic measurements consists in tracer experiments from which macromixing models may be developed. These models provide some information on the liquid flow pattern. Simple macromixing models such as the dispersed plug flow are often used in order to quantify the differences from ideal states of mixing. These models may be used to determine the effect of the hydrodynamics on the performance of the apparatus. Unfortunately, the representation of the mixing pattern remains too simple to give a realistic description of the liquid flow hydrodynamics. Much more elaborated macromixing models have been reported in the literature, claiming to provide an accurate analysis of the relationships between some characteristics of an RTD curve and the hydrodynamic mechanisms occuring in trickle flow columns. Actually, such analysis is often questionable because of the extraordinary complexity of the liquid flow and the relative simplicity of the macromixing models. For this reason, RTD and hydrodynamic models are often developed independently of each other. We try to reconciliate these two approaches by proposing a detailed analysis of macromixing and hydrodynamic mechanisms and by comparing the information inferred from both types of model