Silvia Azucena Nebra

The use of biomass residues in the Brazilian soluble coffee industry

Abstract The objective of this paper is to discuss the use of coffee grounds in the Brazilian soluble coffee industry. This residue is used as a fuel in the boilers of the same industry; so, data about their utilization are presented and analysed, discussing the actual technology and the advantages of improving the drying of the biomass with the exhaust combustion gases. After that, an experimental study is reported on the characteristics of this material, which are important for the combustion process, including the transport, storage and drying, the mean diameter of the particles, talus angle, apparent and real density, sphericity, surface area, terminal velocity, spontaneous ignition temperature and heat of combustion.

Simultaneous moisture transport and shrinkage during drying of solids with ellipsoidal configuration

This work presents a two-dimensional diffusional model to predict the simultaneous mass transfer and shrinkage during drying of solids with prolate spheroidal shape, considering that the changes in volume of solid are equal to the volume of evaporated water. The resulting equations are numerically solved, using the finite-volume method. This model was used to study numerically the effect of the air-drying conditions and shrinkage on the drying kinetic of banana peel for six experiments, considering the natural shape of this fruit. Here, it was treated as an ellipsoid of revolution. Several results are shown and analyzed such as the comparison between numerical and experimental data; the dimensionless shrinkage parameters; the relationships of length, superficial area and volume; the moisture content distribution and finally the mass transfer and diffusion coefficients.

Cost attribution methodologies in cogeneration systems

Abstract The design and operation analysis and optimization of complex thermal plants, in terms of both thermal and economical variables, is the subject of Thermoeconomics. Several different thermoeconomical methodologies are presented in the literature, each one based upon different principles and therefore presenting somewhat different results. In this article, four methodologies developed by diverse authors are applied to a simple gas turbine cogeneration system. The results are compared: the importance of the division of exergy into mechanical and thermal components and the allocation of the cost of external irreversibility are discussed.

Thermoeconomic evaluation of a gas turbine cogeneration system

The main purpose of this work is to apply thermoeconomics concepts to a projected steam injected gas turbine cogeneration system, which aims at providing the thermal and electrical demands of an industrial district sited in Cabo (Pernambuco, Brazil). The power plant is evaluated on the basis of the First and Second Laws of Thermodynamics. A thermoeconomic analysis, using the Theory of Exergetic Cost, was performed in order to determine the production costs of electricity and steam. Two hypothetical operational conditions, as regards the level of electric power generation, were considered.

THEORETICAL ANALYSIS OF THE DIFFUSION PROCESS INSIDE PROLATE SPHEROIDAL SOLIDS

ABSTRACT A numerical solution of the diffusion equation to describe solute transport inside ellipsoids, assuming a constant difiusion coefficient and a corrective boundary condition, is presented. The difiusion equation in a prolate spheroidal coordinate system was used for a hidimensional case. The finite volume method was employed to discretize the basic equation, utilizing a uniform grid size. The equation was solved irteratively using the Gauss-Seidel method. The effect of Biot number and the aspect ratio of the body on diffusion rate and concentration during the process is presented. Plots are presented for Biot numbers from 0.05 to infinity and aspect ratios of 1.1, 2.0 and 5.0. To investigate the effect of the aspect ratio, different results changing the aspect ratio for Bi= 1.0 are shown. The results show that the model is consistent and it may be used to solve other cases such as those that include cylinder and sphere geometry, and/ or those with variable diffusion coefficients with small modifications.

Ethanol Production from Sugar Cane: Assessing the Possibilities of Improving Energy Efficiency through Exergetic Cost Analysis

The sugar and ethanol production is one of the most important economical activities in Brazil, mainly due its high efficiency and competitiveness. Ethanol production is done by a series of steps: juice extraction, treatment, fermentation, and distillation. The juice extraction and treatment is a common operation of both the sugar and ethanol industries. The process begins with the sugar cane juice extraction, usually done by mills, where the cane is compressed between large cylinders for the separation of the juice from the bagasse. Recently, a juice extraction system, called a diffuser, was introduced in some sugar and ethanol plants. In diffusers, after the sugar cane preparation stage was completed with knives and shredders, the cane passes through a bed where the juice is separated from bagasse by the addition of imbibition water and steam, resulting in a leaching process. The present study evaluates different possibilities of decreasing the thermal energy consumption through exergetic cost analysis. The base case is a traditional ethanol production plant, for which the unitary exergetic cost of ethanol and electrical energy are determined. In the following cases, two proposals were assessed: the use of the diffuser as an extraction system and the use of pinch technology to perform an energetic integration between distillation and extraction (diffuser) systems. The results of exergetic efficiency, irreversibility generation, and unitary exergetic cost of products of the three cases are analyzed and compared. The results show the feasibility of using diffusers and heat recovery to decrease thermal energy consumption in ethanol production plants.

DRYING IN CYCLONES—A REVIEW

ABSTRACT This paper presents an overview of the flow, heat and mass transfer characteristics of vortex (or cyclone) dryers. The focus is on the potential of the cyclone configuration for drying of particulates. A selective review is made of the literature pertains to single phase and gas-particle flow in cyclone geometries. Recent data on drying of particulates in cyclone dryers are summarized.

NUMERICAL SIMULATION OF DRYING IN A CYCLONE

ABSTRACT This work presents a complete improved mathematical model of drying in cyclone. The slip condition of the particles on the wall, the heat transfer wall-panicle and the shrinkage of the panicles during the drying process were considered. The mathematical model considers a two-dimensional turbulent gas-particle flow where the panicle phase is treated as a continuum. The momentum equations of both particle and gas phases were written in cylindrical coordinates. The discretized equations were solved by the SIMPLE algorithm. Considering the slip condition to the panicle phase and the shrinkage of the material during the drying process it was revealed a better fitness between numerical and experimental results than the previous model.

THEORETICAL STUDY OF INTERMITTENT DRYING (TEMPERING) IN PROLATE SPHEROIDAL BODIES

The process of intermittent drying in prolate spheroidal bodies was simulated assuming liquid diffusion to be the sole mass transport process, a constant diffusion coefficient and equilibrium conditions at the surface. The mass diffusion equation in a prolate spheroidal coordinate system was used for a two-dimensional case. Due to the use of a dimensionless coordinate system, a Fourier number for tempering is defined, in order to determine the dimensionless time required to achieve a flat moisture content profile. Assuming that the drying process is stopped only once at a fixed point, the tempering Fourier number was determined for aspect ratios of 1.1, 2.0 and 5.0. Many cases were studied, changing the tempering Fourier and aspect ratio of the body. Focusing on the drying rate, the effect of one, two and four drying passes was studied for different interruption points in the process and aspect ratios of 1.1, 2.0 and 5.0, in all cases using the continuous process as a comparison. From the numerical results it was found that during the tempering process, the drying rate and the final mean moisture content are affected by the tempering Fourier number, multipass drying and geometrical dimensions of the body.

Bagasse Dryer Role in the Energy Recovery of Water Tube Boilers

The sugar and ethanol industry, which uses sugarcane as raw material, utilizes the by-product, bagasse, as fuel. The bagasse is fed with a moisture content of 50% (w.b). Dryers are not being used in the way it is proposed here. This article proposes the introduction of bagasse dryers connected to the energy recovery system of the boilers operating with the exhaust gases. The combination of the dryer with other thermal energy recovery devices, such as an air heater and economizer, is considered. The performance improvement is evaluated. Costs are also taken into account. Finally, an optimized solution for the boiler energy recovery system is proposed.