J. Baeyens

Gas fluidized beds operating at high velocities: a critical review of occurring regimes

Abstract Industrial processes often involve the interaction of a gas and a solid phase. Low gas velocity processes are characterised by limited carry-over and are performed in packed, fluidized or slugging beds. At higher gas velocities, the heterogeneous two-phase character of the fluidized bed is replaced by a condition of uniformity where large discrete bubbles or voids are on the whole absent. This so-called ‘turbulent’ fluidized bed regime has been described in many contradictory and non-decisive ways, as will be reviewed in the paper. When the gas velocity through the fluidized bed is increased above a critical velocity (transport velocity), entrainment becomes so important that the bed can no longer be maintained in the column unless entrained particles are collected and returned to the bed. Reactors with an external recycling system have a means to control the solids hold-up and are referred to as ‘Circulating Fluidized Beds’ (CFB). The CFB normally operates in the so-called ‘fast fluidization’ regime, which is situated between the dilute and dense pneumatic transport regimes. However, the terms ‘fast fluidization’ and ‘transport velocity’ are used in different and controversial ways. A critical literature assessment is included in the paper. Special attention is focussed on the axial density profile as being the key parameter of the riser. Experimental work was performed in a lab-scale CFB with a riser of 0.1 m i.d. The transport velocity is the minimum gas velocity required to develop a stable fast fluidization mode. Several techniques to measure this velocity are reviewed and a correlation, based on own experimental results and literature data is included. Comparison with other correlations is performed. Since there is a literature discrepancy about the fast fluidization regime, the transition between dilute and S-shaped profile will be determined as function of the solids circulation rate and the gas velocity, based on own experimental work. The new correlation is thereafter applied to literature data and good agreement is obtained.

Pneumatic drying: the use of large-scale experimental data in a design procedure

Abstract A pneumatic powder-dryer can be used for flash evaporation of surface moisture, where heat transfer dominates the drying phenomenon. Experimental data obtained in several industrial dryers were analyzed and fitted by the correlation Nu = 0.15 Re p . The design of pneumatic dryers was thereafter determined by following a strategy using basic powder/gas characteristics, drying thermodynamics and hydrodynamics. A stepwise calculation using the predicted heat transfer coefficient of the above equation predicts moisture and temperature profiles in the pneumatic dryer. The shape of the data fitting is very good. The calculation procedure moreover enables calculation of the required length of the drying path.

Macroscopic fluid flow conditions in spiral-wound membrane elements

Abstract Reverse osmosis (RO) finds increasing applications as separation technique in chemical and environmental engineering where desalination, selective separations in the agro-industrial processes or wastewater purification are well-established examples. To fully evaluate the potential of RO and facilitate scale-up procedures, the modelling of the process is an important tool and literature models analyze the separation efficiency in terms of mass transfer with material balances, pressure drop through the module and mass transfer coefficients as dominant parameters. Important underlying factors are the geometry of the module and the hydrodynamic flow regime since mass transfer and pressure drop are a function of these factors, as witnessed by several publications. Since the concentrate-side (which is also the feed side) of the membrane plays the key-role, measurements should focus on the concentrate-channel of the spiral-wound membrane element. The determination of the channel flow regime and hydrodynamics of spiral-wound RO-channels can be carried out through the measurement of the residence time distribution (RTD). The present paper describes our experimental investigations on RTD through the application of a step change in tracer concentrations and relates the RTD-response curves to the regime of flow through the concentrate channels. This tracer-technique is common in characterizing flow regimes in chemical reactors. Its application to RO flow channels is innovative and results obtained stress its applicability to this specific geometrical layout. Results indeed demonstrate that the experimental average residence time of the concentrate channel is smaller than the theoretically calculated residence time, with differences between both values gradually decreasing with increasing liquid flow rate. This observation corresponds with findings for traditional packed bed applications where the presence of dead volumes reduces the real average residence time. The presence of dead zones in the spiral-wound membrane element is therefore evident. Since in our experimental procedure, residence time distributions are measured both between the tracer injection point and respectively the inlet and outlet of the spiral-wound element, the nett RTD-contribution of the membrane element itself can only be determined by eliminating the influence of the inlet flow equalization zone. This is performed by applying the convolution principle. Experimental and calculated RTD-curves for values of the Peclet-number of approximately 20 are in very good agreement. In analogy with fixed bed applications, the interpretation of the findings corresponds to a laminar flow profile with a limited dispersion. The definition of this flow profile in the concentrate channel is important in the use of transport models to characterize the membrane performance, as will be shown in a further paper.

Modelling reverse osmosis by irreversible thermodynamics

Abstract Literature models for reverse osmosis are based either on irreversible thermodynamics (IT) or on transport mechanisms. Our research focuses on the evaluation of existing models through comparison of experimental and predicted data for the transport of solute and solvent through the membrane. The present paper reviews the fundamentals and design equations of the IT theory. Our experimental and literature data are thereafter used to check the validity of the IT approach. A further paper will similarly evaluate the transport models. Experimental data confirm that the osmotic pressure can be predicted with great confidence by the Pitzer equation for 2-1 and 2-2 electrolytes over a wide range of concentrations. Moreover, divalent ions and/or 2-2 electrolytes are rejected to a greater extent than, respectively, monovalent and/or 2-1 electrolytes. The results of the model analysis confirm the validity of the Kedem-Katchalsky predictions at high values of the volume flux and of the overall validity of the Spiegler-Kedem approach. The transformed results, however, illustrate that both cross coefficients are very small hence making it impossible to pass judgement on the applicability or otherwise of the Onsager reciprocal relations.

Modeling and scaleup of reverse osmosis separation

Over the past years reverse osmosis has become a commercially attractive microsolute separation process. This success is largely due to the development of new and more reliable polymeric membrane materials and of novel module configurations. Flat-sheet membrane units are preferred for laboratory experiments to determine the intrinsic membrane properties with respect to permeate flux and separation efficiency. These units are not expensive and allow different membrane sheets to be tested with the same unit because of the easy membrane replacement. The industrial use of these units is excluded due to the low membrane surface area to volume ratio. The separation characteristics of reverse osmosis also depend to a large extent on the geometry of the membrane elements and the associated hydrodynamic phenomena. Hence, residence time distribution experiments have been performed on industrial-scale spiral-wound modules to study the flow regime. The detailed results of these experiments are reported elsewhere, and...

Modeling osmotic pressures for aqueous solutions for 2-1 and 2-2 electrolytes

In an extensive research program, reverse osmosis has been investigated for the treatment of wastewater containing heavy metals such as electroplating effluents where more stringent pollution control standards impose the use of very efficient treatment techniques. The present paper relates to the prediction of osmotic pressures for 2-1 and 2-2 single or mixed electrolytes. The experimental determination of the osmotic pressure using a spiral wound asymmetric membrane and the use of predictive literature models was executed simultaneously. The experimental results correspond very well with predictions using Pitzers model equations, while the other model equations overrate the osmotic pressure for concentrated electrolyte solutions.

Cleaning of hot calciner exhaust gas by low-density ceramic filters

Abstract Efficient dust removal at high temperatures gains increasing interest and ceramic filters are suitable to achieve high collection efficiencies for (sub-)micron particles. The paper presents experimental results for their application in lime-kiln operations. Experimental results were transformed into design equations for (i) the baseline pressure drop and the effect of the cleaning cycles; (ii) the total pressure drop and the effect of the dust load; (iii) the prediction of the time between two cleaning cycles in function of the operating parameters.

Modelling and scale-up of reverse osmosis separation

Abstract Reverse osmosis has over the past years become a commercially attractive microsolute separation process. This success is largely due to the development of new and more reliable polymeric membrane materials and of novel module configurations. Flat sheet membrane units are preferred for laboratory experiments to determine the intrinsic membrane properties with respect to permeate flux and separation efficiency. These units are not expensive and allow different membrane sheets to be tested with the same unit because of the easy membrane replacement. The industrial use of these units is excluded due to the low membrane surface area to volume ratio. The separation characteristics of reverse osmosis also depend to a large extent on the geometry of the membrane elements and the associated hydrodynamic phenomena. Hence residence time distribution-experiments have been performed on industrial scale spiral wound modules to study the flow regime. The detailed results of these experiments are reported elsewhere and allow determination of suitable expressions to describe the pressure drop and concentration polarisation phenomena in the concentrate channel of the module. The modelling procedure that will be outlined in the paper, incorporates the underlying hydrodynamic and transport phenomena and is based on an integrated discretisation approach. The procedure is applicable for the simulation of a membrane plant with ‘Christmas tree’ configuration. The basic concept of the procedure is to stagewise determine the performance of one membrane element based on its inlet flow conditions. Eventually the outlet permeate flux and concentration are calculated by properly averaging the permeate fluxes of the individual membrane elements. Industrial experiments were carried out to evaluate the simulative accuracy of the modelling procedure for spiral wound modules. This plant was designed to treat wastewater issued from producing microelectronic chemicals. Finally, a sensitivity analysis is performed to determine the critical parameters for the design of reverse osmosis separation by spiral wound modules.