E.-U. Schlünder

Fluidized bed spray granulation: Investigation of the coating process on a single sphere

Abstract The coating and granulation of solid particles in a fluidized bed is a process which converts pumpable and atomizable liquids (solutions, slurries, melts) into granular solids in one step by means of drying. The solution to be processed is sprayed onto a fluidized bed. Particle growth can take place either via surface layering or agglomeration. In the case of surface layering the atomized droplets deposit a thin layer of liquid onto the seed particles. The solvent is then evaporated by the hot fluidizing, leaving behind the dissolved material on the surface. Although fluidized bed spray granulation and film coating have been applied in industry for several years, there is still a lack of understanding of the physical fundamentals and the mechanisms by which spherical granules are formed. Hence a new method was developed which allows the direct observation of the subsequent particle-forming mechanisms such as droplet deposition, spreading, wetting and drying. The authors present a laboratory scale apparatus in which a single freely suspended particle can be coated under well defined and constant coating and drying conditions. With this device, particle-growth-rate and the development of particle morphology were measured and investigated under various experimental conditions.

Drying of porous material during the constant and the falling rate period : A critical review of existing hypotheses

Abstract During drying of porous material a so-called “falling rate” period is observed, where the drying rate decreases as the moisture content decreases. This behavior is usually described by the well known “shrinking core model.” This model, however, contradicts experimental findings and violates basic laws of multiphase mass transfer in porous media as well. A new model, named “wet surface model,” is suggested which eliminates those discrepancies. #Dedicated to Professor J. King on the occasion of his 70th birthday.

Flow regimes of free jets and falling films at high ambient pressure

At atmospheric pressure the following flow regimes have already been reported for liquid free jets and falling films, that are “draining”, “dripping”, “Rayleigh disintegration”, “wavy disintegration” and “spray” for jet disintegration as well as “smooth film”, “waves”, “troughs” and “crests” for falling films, respectively. In a high-ambient-pressure environment there exists an additional falling film flow regime, i.e. “breakup into droplets”. At elevated pressures the viscosity of the liquid phase and the interfacial tension of the substances which are used decrease significantly due to dissolved carbon dioxide. In addition, the influence of the ambient gas-phase increases, so that the respective regimes occur at considerably lower liquid flow rates. In this work the conditions of the flow regimes have been studied and new own correlations have been developed. The results may be used for process development and reactor design in order to reduce the size of the apparatus or to generate surface area for mass transfer not by means of special contacting devices but by using atomization devices. Also, the formation of undesired aerosols can be avoided, which are almost impossible to remove and might be responsible for backmixing. As a sign for the strong interdependence of mass transfer and fluid dynamics it has been discovered that the flow patterns of saturated liquid free jets and falling films are clearly different from those of the corresponding undersaturated flows. This fact is especially important for scale-up of high-pressure reactors: exactly the same flow conditions have to be set in pilot and production column, e.g. to provide the required separation efficiency.

Fluidized bed granulation—the importance of a drying zone for the particle growth mechanism

In fluidized bed granulation, either surface layering or agglomeration may lead to granule growth. Agglomeration will occur if a wet particle collides with another particle and is bound by a liquid bridge which becomes solidified during the subsequent drying period. In the case of a two-fluid-nozzle spraying upwards into the fluidized bed, wetted particles may be transported pneumatically by the nozzle air above the bed plenum and dry before they fall down in the outer regions of the bed. Hence, a drying zone above the fluidized bed is formed, leading to significantly less agglomeration.

Fluidized bed granulation: gas flow, particle motion and moisture distribution

Abstract In fluidized bed granulation the liquid is often distributed by means of a two-fluid-nozzle that is positioned inside the bed. Then the wetting process takes place in the jet that is formed by the atomizing air. A model has been derived for a free jet to calculate the particle and gas velocities, the void fraction and the cross-sectional area of the jet. The results are compared with experimental data for the particle circulation time in the fluidized bed. Based on the particle and gas motion the liquid deposition is calculated, resulting in particle moisture distributions. The influence of different parameters on the moisture distribution and possible consequences on the granulation process are discussed.

Sorption isotherms for methanol, benzene and ethanol on poly(vinyl acetate) (PVAc)

Abstract The sorption isotherms for three organic solvents on PVAc have been measured via the isopiestic method using a simple inexpensive device with a spring inside the measuring chamber as the force sensor. A robust steel spring was used successfully instead of the sensitive quartz spring described in the literature. The sample was a thin aluminium foil coated with a thin polymer layer to yield a short diffusion path length and sufficient equilibrium load at an acceptable tare mass. This enabled equilibrium to be attained within 1–2 h. The experimental data may be well described by the Flory—Huggins theory. The interaction parameter was fitted as a linear function of the volume fraction. Temperature was of no significant influence within the range 20–50 °C or at 60 °C, respectively. The sorption isotherms can also be predicted by the UNIFAP (UNIFAC for polymers) method without the use of experimental data. PVAc exhibits highly sorptive behaviour, e.g. 10 wt.% methanol at 80% activity. The measuring device employed may be useful for the study of adsorbents that can be coated as a film onto aluminium foil or exist as a thin film themselves (e.g. membranes).

Horizontal flow boiling experiments of saturated pure components and mixtures of R846-R12 at high pressures

Abstract In the present discussion about the decomposition of the ozone layer due to the effect of chlorofluorocarbons, non-azeotropic mixtures are suggested as mid-term alternatives. Thermodynamic data and calculation methods are available, but there is a lack of information on heat transfer of such mixtures at horizontal flow boiling conditions. An experimental study of nucleate boiling within a horizontal tube is reported here. Local (point) heat transfer coefficients at flow boiling of saturated pure components (R846(SF 6 ), R12) and binary mixtures at moderate and high pressures have been measured in a wide range of mass velocities (80 to 400 kg m −2 −1 ) and flow qualities (0.05 to 0.9). A model of E.-U. Schlunder has been used to calculate the local heat transfer coefficients at flow conditions. The circumferential averaged heat transfer coefficients were based on the local values for the conditions of constant heat flux or wall superheat. Calculations were in satisfactory agreement with experimental data.

Measurement and simulation of the vacuum contact drying of pastes in a LIST-type kneader drier

Abstract The drying of paste-like products is a common industrial task. Usually a batch vacuum contact drying process is used and the process is carried out in a kneader dryer with a low stirrer speed due to very high torques appearing during the drying process. The process can be divided into two parts, the paste and the particulate regime. Both regimes are separated by a sharp maximum in the torque curve. The position of the maximum depends on the type of product. In the paste regime the drying rate decreases with decreasing moisture content of the product, while torque increases and the product temperature remains nearly constant. In the particulate regime the drying rate decreases further with decreasing product moisture content. At the same time product comminution takes place, the torque decreases and the product temperature rises. Drying rate curves are measured for aqueous suspensions of china clay using a batch kneader drier at pilot plant scale. The influence of the main drying parameters on the drying process is investigated experimentally and theoretically. The measured drying rate curves are compared to calculations based on the penetration model for contact drying. Neglecting mass transfer effects and assuming that the particle size distribution of the product remains constant, the penetration model is able to describe both the paste regime and the particulate regime of the drying process over the whole moisture range. Measured and calculated drying rate curves are in good agreement for a variety of drying parameters.

Thermal conductivity of packed beds consisting of porous particles wetted with binary mixtures

Abstract The thermal conductivity of particles wetted with a binary mixture and packed beds of such particles has been investigated both theoretically and experimentally. The model of Zehner and Bauer has been used to combine the conductivites of the (wetted) pores and the solid matrix to provide an effective thermal conductivity for the particle as well as to predict the effective conductivity of the packed bed. The latter is obtained both from the conductivity of the single particles and from the conductivity of the moist gas in the cavities, the additional transport of energy via the evaporation and recondensation of the vapours of both components being taken into account through the introduction of an equivalent thermal conductivity. Two different techniques have been employed in the experimental investigations. The thermal conductivities of single particles were obtained by means of a transient method, in contrast to the conductivities of fixed beds which were measured in a steady-state apparatus. By fitting two unknown parameters to the results for the single particles, a good prediction of the thermal conductivities of packed beds was obtained showing a strong temperature dependence in the case of wet particles.

Adsorption equilibrium of solvent mixtures on silica gel and silica gel coated ceramics

Abstract This paper presents the sorption isotherms of the mixtures 2-propanol/water and n-heptane/methylcyclohexane as well as the isotherms of the pure solvents on the adsorbent silica gel and silica gel coated ceramics. The equilibrium data were obtained gravimetrically via an experimental set-up based on the through flowing method. The single component isotherms of all solvents are type IV of Brunauer classification, indicating strong interactions typically with a mesoporous solid. Due to small differences in molecule size, physical properties and interaction forces to silica gel, the mixtures of n-heptane and methylcyclohexane show a nonselective adsorption behavior over the complete composition range. An unusual phenomena has been observed for the adsorption equilibrium of the 2-propanol–water mixtures: at low relative pressures and low alcohol contents, 2-propanol is preferentially bounded so that the selectivity behavior according to the vapor liquid equilibrium of the fluid mixture is entirely inverted.