Michel Crine

Local measurements of void fraction and liquid holdup in packed columns using X-ray computed tomography

Abstract A ‘tailor made’ computed X-ray tomographic scanner has been developed as a tool for the analysis of the distribution of gas, liquid and solid phases in packed columns. The very good spatial resolution of the scanner has first been assessed by the imaging of objects of known shape and size, called ‘physical’ phantoms. Images have then been realized on a 0.6 m diameter and 2 m height column packed with Cascade Mini-Ring 1A packing elements, which is a random polypropylene packing designed to be used in absorption columns. The solid phase distribution, leading to the void fraction distribution, has been analyzed on cross-section images of the dry packed column (without any liquid flowrate). The measured value of the bed void fraction is equal to the value provided by the manufacturer. The analysis of the axial profile of void fraction shows that the ‘end effect’ may be neglected, whereas the analysis of the radial profile evidences the existence of a non negligible ‘wall effect’. Images have then been carried out on the packed column irrigated by a liquid flowrate ranging between 0 and 6000 l h−1 (0–6.10−3 m s−1). Hold-up values have been measured in different cross-sections of the column and averaged in order to obtain the total hold-up value in the bed. The computed values are in very good agreement with those reported in the literature for similar packing. The dependence of the liquid hold-up on the liquid superficial velocity can be expressed in terms of a power law. The fitted value of the exponent, equal to 0.65, is in the range of exponent values found in correlations of the literature.

Use of X-ray microtomography to follow the convective heat drying of wastewater sludges

ABSTRACT X-ray microtomography is proposed as a new tool to investigate the evolution of size, shape and texture of soft materials during a drying operation. This study is focused on the drying of mechanically dewatered sludges from a secondary wastewater treatment. The shrinkage phenomenon is shown to play a crucial role in the control of the drying process. The shrinkage curves are determined by analysing the shape and size of cross sectional microtomographic images of sludge extrudates at different levels of drying. The observation of drying and shrinkage curves allows us to determine 3 critical water content values, which define different drying zones where extragranular, intragranular or mixed limitations prevail. When drying is externally controlled, the decrease of the drying rate observed during experiments can be related to the reduction of the external area of the sample, i.e., to shrinkage. When drying is internally controlled, resistances inside the solid govern the process. Between these two extreme situations, the drying rate reduction is the result of both the external area decrease and the development of internal resistances limiting drying. A multizone model is proposed to describe quantitatively these observations. The analysis of the internal texture of the sludge extrudates reveals crack formation at the end of the drying process. The onset of crack formation is clearly related to the appearance of internal transfer limitations, i.e., humidity and temperature gradients inside the material.

Convective Drying of Wastewater Sludges: Influence of Air Temperature, Superficial Velocity, and Humidity on the Kinetics

Abstract The influence of air temperature, velocity, and humidity during convective drying of two different sludges (A and B) is investigated through a 33 factorial design. For sludge A, a constant drying flux period is observed, while sludge B is characterized by a long decreasing drying flux phase. A sensitivity analysis shows that temperature is the main operating parameter affecting the drying kinetics. Mass and heat transfer coefficients as well as water evaporation capacities are calculated from drying curves. Transfer coefficients are related to operating conditions through dimensionless relations. For both sludges, a linear relation is found between water evaporation capacity and the maximal measured drying flux.

Imaging of liquid distribution in reactive distillation packings with a new high-energy x-ray tomograph

We describe a new, high-energy (420 kV), large-scale (0.45 m in diameter, 4 m in height) x-ray tomograph developed to investigate gas and liquid flow through fixed bed like absorption, distillation and reactive distillation columns. The first results obtained with this set-up on test objects (physical phantoms), such as a cylindrical container filled with water or a large diameter structured metallic packing, validate the technique as a quantitative tool for geometrical measurements. Very detailed two-dimensional (2D) and three-dimensional (3D) images of a 0.09 m diameter KATAPAK-SP 12, a reactive distillation packing, are presented. Quantitative information relative to liquid hold-up distribution may be obtained from tomographic imaging performed on an irrigated column packed with this element.

Measurement of Shrinkage and Cracks Associated to Convective Drying of Soft Materials by X-ray Microtomography

Abstract Traditionally, the measurement of shrinkage occurring during drying is performed by destructive or poorly accurate techniques such as volume displacement methods. Cracks detection and quantification are realised either by destructive techniques or sophisticated but expensive nondestructive ones (NMR imaging). X-ray microtomography in combination with image analysis provides an accurate, nondestructive and easy to use technique to determine simultaneously shrinkage and crack extent. Results reported in this article concern drying of wastewater sludges whose management will become a real challenge in the years to come. These results show a clear relation between drying kinetics and crack development. This could be related to the development of internal diffusional limitations inducing moisture gradients and mechanical stresses leading to cracks formation.

Influence of back mixing on the convective drying of residual sludges in a fixed bed

The influence of a backmixing operation on the convective belt drying of two wastewater sludges was studied. The expansion of the sludge extrudates bed due to increasing additions of dry product was quantified by using X-ray tomography. This non-invasive technique was used to determine the bed porosity and the total exchange area available for heat and mass transfers, for increasing levels of backmixing. For a same drying flux, the expansion of the drying bed leads to higher drying rates, allowing a reduction of the total drying time. In this context, rheological properties of the sludges are key properties.

Investigation of Liquid Maldistribution in Packed Columns by X-Ray Tomography

Ideas which have developed during this century about liquid maldistribution in packed beds are summarized. A statistical model of the liquid flow is used to analyse new experimental results obtained by X-ray computed tomography.

Analysis of electrical resistance tomography measurements obtained on a bubble column

Measurements of gas hold-up obtained by using pressure transducers and an optical probe are compared with those obtained by an electrical resistance tomography method. The use of a neural network, the input of which is the electrical measurements, can lead to quantitative results for the gas hold-up distribution. An interesting qualitative diagnosis of the sparger functioning is observed from reconstructed images.

Image analysis of X‐ray microtomograms of soft materials during convective drying

X‐ray microtomography is used to explore the textural evolution that soft materials undergo during a drying treatment. An original image processing algorithm is applied to vertical projections and reconstructed cross‐section images in order to quantify the texture at different stages of drying. Measurements are performed both on grey‐level and on binary images. It is shown that X‐ray microtomography is a very promising tool in the field of drying investigations. It can be used to determine internal moisture profiles, and to follow crack development and shrinkage in an accurate and non‐destructive way. This information is crucial to validate drying models. Waste‐water sludges are used as test materials to assess the validity of the proposed methodology. The management of these sludges, often including a drying stage, will become a challenge in the forthcoming years in accordance with environmental regulations. Samples collected in two waste‐water treatment plants are investigated. Their analysis by X‐ray microtomography brings to the fore two different drying behaviours, illustrating that sludge drying is a complex unit operation very sensitive to the way the material is produced.

Convective Drying of Wastewater Sludge: Introduction of Shrinkage Effect in Mathematical Modeling

Drying of two kinds of wastewater sludge was studied. The first part was an experimental work done in a discontinuous cross-flow convective dryer using 1 kg of wet material extruded in 12-mm-diameter cylinders. The results show the influence of drying air temperature for both sludges. The second part consisted of developing a drying model in order to identify the internal diffusion coefficient and the convective mass transfer coefficient from the experimental data. A comparison between fitted drying curves, well represented by Newtons model, and the analytical solutions of the equation of diffusion, applied to a finite cylinder, was made. Variations in the physical parameters, such as the mass, density, and volume of the dried product, were calculated. This allowed us to confirm that shrinkage, which is an important parameter during wastewater sludge drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by the air temperature and the origin of the sludge. The values of the diffusion coefficient changed from 42.35 × 10−9 m2 · s−1 at 160°C to 32.49 × 10−9 m2 · s−1 at 122°C for sludge A and from 33.40 × 10−9 m2 · s−1 at 140°C to 28.45 × 10−9 m2 · s−1 at 120°C for sludge B. The convective mass transfer coefficient changed from 4.52 × 10−7 m · s−1 at 158°C to 3.33 × 10−7 m · s−1 at 122°C for sludge A and from 3.44 × 10−7 m · s−1 at 140°C to 2.84 × 10−7 m2 · s−1 at 120°C for sludge B. The temperature dependency of the two coefficients was expressed using an Arrhenius-type equation and related parameters were deduced. Finally, the study showed that neglecting shrinkage phenomena resulted in an overestimation that can attain and exceed 30% for the two coefficients.