Transport Phenomena In Particulate Systems

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The convective heat and mass transfers from a single particle, surrounded by a flowing fluid, have important practical applications in Chemical Engineering. Among them are process component cooling, adsorption, distillation, catalytic reactions, extraction, and drying in fixed and fluidized beds. This chapter is intended to provide a concise vision of the phenomena emerging during the drying of single particles. A case study is presented with the proposition of a mathematical model for the drying process of spherical gel systems considering the effect of fluid flow on mass transfer and shape evolution. The experimental results presented were evaluated under a laminar fluid flow with different particle diameters and fluid velocities. Shrinkage of the samples was observed through digital images, and it was used for the calculations of the shape factors and apparent specific mass. The mathematical model considers two-dimensional mass transfer inside the samples, variable effective diffusivity, linear shrinkage, and non-uniform mass transfer by convection. The results confirmed that the major factor for the mechanical alterations of the spherical gel particles was the non-uniform drying owing to the fact that the mass transfer rates over the forward surface of the sphere are greater than those at the rear hemisphere. The proposed mathematical model represented the two-dimensional moisture profiles inside the sample and their consequent shape evolutions. This chapter attests the need for the consideration of the fluid flow effect on the mathematical modeling and contributes to a better understanding and the technological development of the drying processes.

Heat transfer in packed beds is of interest for several industrial applications. This paper presents a historical view of both experimental and modeling developments and the future trends. Heat transmission in such beds has been modeled with two-phase models, which provide two balance equations, one for each phase (solid and fluid), and with one-phase models, which propose a single equation to represent an arbitrary representative elementary volume of the system (pseudo-homogeneous model). Two-phase models lack experimental confirmation, while one-phase models lack better detailing. Temperature measurement with little distortion within the bed is a difficult task. The inlet temperature profiles, as well as the downstream temperature profiles, have been measured by several researchers, but better experimental techniques are still required. Angular temperature measurements for a low tube-to-particle diameter ratio (D/dp) indicate that the models must include angular temperature variation. Coupling among bed structure, fluid flow profile, and temperature variation must be represented by new models, such as the ones solved by Computational Fluid Dynamics (CFD).

The food and pharmaceutical industries as well as power plants based on biomass deal with natural materials possessing a wide variety of characteristics. The variability of shapes, sizes, and morphological characteristics found in natural products brings additional difficulties to quantify the transfer processes in unit operations such as conveying, separation and drying, and even accurate description of their properties requires special precautions. The knowledge of how these materials behave as they are packed, conveyed, and fluidized is important to subsidize the design of equipment and processing plants. This chapter will be focused on the study of fluidization of natural materials of flat shapes. Although the fundaments of fluidization are well-known for powders and conventional particles, information is scarce in the literature for materials of sphericity under 0.5, which include a variety of particulate matter such as grains, seeds, stalks, and fresh leaves. The analysis will approach: an overview on the description of physical properties of particles; a review on the fundaments of fluidization and researches aimed at the study and investigation of fluidization applied to flat materials; and a case study with a description of the fluidization patterns obtained using different flat materials, and a discussion of the fluid dynamic curves obtained in the fluidization assays. The selected materials for this study include a few seeds and cereals (with sphericity around 0.5), some fresh leaves (with sphericity under 0.25) and manufactured particles. From this analysis one intend to draw information about the feasibility of fluidizing flat particles of different characteristics.

This chapter deals with CFD applied to particulate systems, especially the Eulerian modeling for granular flow in a spouted bed. Initially, a short overview about fluidynamic studied is presented. A general mathematical model will be presented using multiphase flow simulation, which will then be applied to the simulation of a spouted bed. A special topic is dedicated to a literature survey of current studies relevant to CFD simulation of spouted beds, covering its major points. In conclusion, granular multiphase flow, including a solid-fluid system in a spouted bed, is developed as a case study. We thus aim to present the principal steps of numerical simulation of particulate systems and the evaluation of the results.

This chapter deals with the main characteristics of the Eulerian granular multiphase model applied to simulation of the fluid dynamic behavior of spouted beds. General aspects of the Finite Volume Method, the generation of discretization grids, and the analysis of fluid dynamic profiles obtained for different geometries and configurations of the spouted bed are also covered in this chapter. The results presented showed that the Eulerian granular multiphase model adequately predicts the fluid dynamic behavior of several spouted bed configurations, characterizing it as a useful tool in the design and performance analysis of this equipment.

Spouted beds are largely used due to their high heat and mass transfer rates and good mixing of solids. Specially, spouted beds are applied to deal with large particles (dp>1 mm), where fluidized beds are not efficient. In the literature there are several experimental and simulation studies using batch spouted beds. However, little information can be found regarding continuous spouted beds. The continuous operation has some important advantages such as higher production capacity and lower operating costs. The objective of this chapter is to introduce some concepts in transport phenomena applied to continuous spouted beds, as well as, discuss different modeling and simulating approaches.

A short overview of the main caracteristics of a control system applied to drying processes is presented. This includes examining several considerations that must be taken into account during the design of a control system and the associated problems that must be resolved. Special attention is dedicated to analyze the control problems of spouted bed dryers. The performances of different control configurations such as a self-tuning PID, supervisory and optimizing controllers are discussed and their advances and drawbacks examine. In today’s competitive market, technology plays an important rule allowing the implementation of advance control structures, improving the quality of the dried products and simultaneously saving energy.

Pharmaceutical products comprise several types of materials including chemical and biological substances, such as enzymes and proteins, hormones, antibiotics, vaccines, proteins, vegetable extracts, as well as pharmaceutical granules and compression mixtures. On the other hand, some products are produced in small amounts, have high value and are tricky to dry, such as the biopharmaceuticals, including proteins, enzymes, hormones and monoclonal antibodies. The final product should achieve strict quality specifications, essential to ensure the quality, safety and efficacy of the finished product on the market. Several drying systems are used in the pharmaceutical sector, such as the spray drying, freeze-drying, fluidized and spouted bed dryer, and microwave drying. According to several pharmaceutical guidelines, these systems should follow very harsh specifications, as made of stainless steel with sanitary finishing. The selection of the drying system more adequate for a particular application is a challenging task, and should consider factors related to the cost of the product to be processed, physical and chemical properties, thermal sensitivity and the final use of the product, among others. In this chapter is presented an overview of the recent applications and concepts involved in field of pharmaceutical product drying. Some topics of a general nature will be approached, including fundamental aspects of drying, principal types of dryers, methods used for their selection, and regarding to the physicochemical properties of the materials. The drying of some pharmaceutical products will be presented, emphasizing extracts of medicinal plants, enzymes and proteins, microencapsulation and particle coating.

Hydrodynamics analysis of biological reactors for effluent treatment is an essential step towards reactor optimization. This is even more important for fluidized bed reactors, which have characteristic flow patterns. The present chapter provides some background on the hydrodynamics of biological fluidized bed reactors. The basic principles concerning the operation of this type of reactor are presented along with the definition of some important parameters, such as, minimum fluidization velocity and the transition between fluidized and fixed bed modes. At the end of the chapter, two case studies on the influence of adhered biomass over the reactor hydrodynamics are analyzed. It is noteworthy that fundamental studies are still required for a more comprehensive approach on this subject.

Aqueous effluents containing metal ions from a variety of industrial processes constitute an important source of environmental pollution, even when the concentrations of concentrations of metal ions are very low. In order to overcome this problem, electrochemical technology seems to be a technical and economic alternative to the conventional methods used to control and remove metal ions from industrial effluents. The use of electrochemical reactors provides an environmental friendly technology to substitute the conventional method that only transfer the problem from the liquid phase to the solid phase. The understanding of the charge and mass transfer phenomena and the controlling factors affecting the electrochemical kinetics is of major importance for the optimized design of electrochemical reactors and also to establish the best operational conditions to be applied to the treatment process. In this text, we present some important variables that must be considered in order to make the electrochemical technology an efficient and economically advantageous alternative to solve one of the major problems associated with water pollution: contamination by heavy metals. Finally, the use of an electrochemical reactor to reduce hexavalente chromium to its trivalent state is presented.