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Tunisia, like many other countries in the world, suffers from a shortage of conventional energy resources. Further, it exhibits large and increasing volumes of organic wastes that cause environmental and health problems. In this study, the considered wastes consist of the organic fraction of municipal solid wastes (OFMSW), the olive mill wastewater (OMWW), and the chicken wastes (CW). These bio-wastes can therefore be valorized by a biological process such as the anaerobic digestion to satisfy on the one hand the energy demands and on the other hand, to reduce the environmental problems. Accordingly, the first aim of this work is to investigate the biomethane potential of the mentioned bio-wastes and to assess the energetic content of the obtained biomethane production. Secondly, a theoretical analysis is conducted by using the Buswell equation to predict the biomethane generation and to determine the biodegradability of the studied substrates mixture. Finally, the experimental findings are used to carry out a kinetic study, by using four kinetic models to assess their suitability to fit the present anaerobic digestion process. To do this, the biomethane potential estimation of these biomasses, under mesophilic conditions, is carried out at a laboratory scale by using the biochemical methane potential (BMP) tests. These testes are performed under an optimal substrate-to-inoculums ratio and a controlled temperature during the testing period at a laboratory scale. The experimental results show that the net yield of the biomethane from the studied bio-wastes mixture is equal to 0.384 Nm3/kg VS, which is equivalent to 130.29 Nm3 of biomethane, extracted from one ton of these bio-wastes mixture. The theoretical and the experimental results of the net produced biomethane show that the substrates mixture biodegradability is found equal to 66.89%. The studied kinetic models show good agreements with the experimental results, with a correlation coefficient R2 ≥ 95. It is found that the Gompertz model is the best one for predicting the anaerobic digestion process, due to the lowest root mean square prediction estimate (RMSPE) coefficient, as compared with the other studied kinetic models.

The motion of the humid air inside a solar still is due to double-diffusive natural convection, where convective heat and mass transfer phenomena are encountered. The humid air, an intermediate medium between the evaporating and the condensing surfaces of the solar still, is regarded as a heat engine that absorbs heat from the free surface of the warmed brackish or saline water and converts it into work to carry up the water vapor molecules to the inner surface of the glass cover. Like all heat engines, the humid air work is accompanied by a degraded energy due to the presence of irreversibilities. The aim of the present study is to give answers about the internal irreversibility origins and the associated work fluxes of the humid air inside the solar still due to double-diffusive natural convection. It is for the first time that the internal entropy generation and the work fluxes formulations are developed and investigated for the humid air inside a solar still. It is found that the irreversibility rate due to fluid friction is the major contributor of the total entropy generation for N > 0.3. The increase in the temperature and the concentration gradients enhances the augmentation of all irreversibility rates. The second law efficiency is an important parameter that should be evaluated to assess the system performance. The mean value of the irreversibility percentage is found equal to 6.83%. The reversible and the lost work fluxes increase with the increase in the humid air temperature.

Tunisia is a country that suffers from energy demand problems and environmental matters. Thus, Tunisian authorities desire to encourage the development of renewable energy sources, especially from biological processes, like anaerobic digestion. Therefore, this study is focused on the evaluation of biogas and bio-methane yield from the co-digestion of three available and abundant bio-wastes in the southern regions of Tunisia. The three different raw materials are an organic fraction of municipal solid waste, chicken manure, and olive mill wastewater. In this context, experimental work to evaluate the potential of biogas and bio-methane production was carried out at mesophilic temperature 35 &deg, C and batch mode. The present work highlights the possibility of generating biogas from these organic wastes and reducing the amounts of the wastes to dispose of in landfills. The experimental study of the co-digestion process under specific conditions of carbon to nitrogen ratio (C/N), T, pH, and inoculums to substrate ratio ISR provided a high yield of net methane and net biogas, in comparison with other research works. Results showed a higher specific net methane production per kg of volatile solids, which is equal to 0.338 Nm3 methane/kg VS and 0.430 Nm3 methane/kg VS for two studied cases. The obtained volatile solids reduction was found to be 91% of the initial content, for a hydraulic retention time (HRT) of 40 days.

Driving into renewable energy production from organic resources and wastes becomes the most effective solution to cover the energy demand and to reduce the environment issues. Recent studies report that anaerobic digestion, especially the co-digestion, is an efficient alternative technology that combines bio-energy production with sustainable waste management. Tunisia is one of the countries which suffer from energy demand problems and environment issues. Thus, Tunisian government turns on the development of renewable energy. Therefore, this study focuses on the evaluation of biogas and bio-methane yield from the co-digestion of three available and abundant bio-wastes in the south regions of Tunisia. In this context, bio-methanisation potential test (BMP) was carried out to this evaluation. As a result, the biogas and bio-methane cumulative were respectively equal to 0.263 Nm3 biogas/ Kg VS, 0.166 Nm3CH4/Kg VS for bottle 1 and 0.134 Nm3biogas/ Kg VS, 0.069 Nm3CH4/Kg VS for the blank. This test gave volatile solids reduction equal to 98.99% and 96.81 %.

The paper reports a comprehensive study of heat transfer, mass transfer, and entropy rate of humid air for a single solar still, concerning a typical summer day in a south east region of Tunisia. Transient humid air characteristics such as: partial pressures of components, temperature, humidity ratio, molecular diffusivity, and thermal diffusivity are determined and investigated. Entropy rate equation is derived and studied. Influences of humid air temperature and pressure on heat and mass transfer and on entropy rate are studied. It was found that the increase of water vapor partial pressure and humid air temperature are the best conditions for improving solar still productivity. Humid air can be regarded as a heat engine that generates kinetic energy by transporting energy from heated water surface to inner glass cover surface. Entropy rate of humid air largely depends on dry air entropy rate. The latter presents negative sign, meaning dissipation of convection heat transfer between evaporating and condensing surfaces during circulation. Inversely, entropy rate of water vapor has a positive sign, meaning that it receives energy during phase change by evaporation and transport by humid air from saline water surface towards the inner glass cover surface.

In this paper, entropy generation of double-diffusive natural convection in a 2D dimensional enclosure with magnetic and Dufour effects has been numerically performed. Dirichlet boundary conditions for temperature and solute concentration are applied to the two vertical walls of the enclosure; wheras the two horizontal walls are adiabatic and insulated. The governing equations of continuity, momentum, energy and concentration are numerically solved by using a Control Volume Fined Elements Method, CVFEM of Patankar. The governing parameters of the problem are the thermal Grashof number (GrT), the buoyancy ratio (N), the Hartmann number (Ha), the Dufour parameter (Du) and the Prandtl number (Pr). The obtained results were presented graphically via the velocity field components, temperature and concentration distributions, entropy genertion rate behaviour and by isotherms, streamlines and isentropic lines maps. The average Nusselt and Sherwood numbers are also derived and discussed numerically. The investigated results showed that the flow field and then entropy generation are notably influenced by the considering parameters.

The pre-incubation of digestate and recycling of microbes inside a continuously stirred tank reactor (CSTR) are effective ways to optimize the anaerobic digestion process and improve the performance of biogas and methane production, also in existing biogas plants. In this study, a digestate incubation system using a nutrient mix to boost the activity of microbes was coupled to a CSTR to boost biogas and methane production. This system has been tested both on a lab scale and on an industrial scale. On a pilot scale, the system achieved an increase of +16.47 v% in biogas production with respect to the conventional anaerobic digestion process, and an increase of +2 v% in methane content (from 65.94 v% to 67.84 v%). On an industrial scale, the use of this incubation reactor with a capacity of 1 m3 has led to an increase in methane yield of 12 v%. This system allows to maintain the syntrophic relationship between acid-producing bacteria and methanogens and contemporary push the development of methanogens. Moreover, it is an economic system to be integrated into an existing biogas plant given the small volume and the simplicity of the incubation reactor.

In this paper, an analytical study about the effect of a falling gas–liquid absorption film temperature on entropy generation is carried out. Entropy generation formulations due to viscous effects and mass transfer are derived. Results in terms of viscous, mass transfer and total irreversibilities are graphically presented and discussed.

The performance of a single solar still is numerically studied by solving the energy balance equations, concerning Gabes region. Effects of the glass cover thickness and the saline water depth on water-to-inner glass cover temperature difference and on daily yield for June 21st are determined and graphically presented. It was found that optimum values of glass cover thickness and water depth giving highest productivity are 3 mm, and 1.5 cm, respectively.

The article reports a numerical study of entropy generation in double-diffusive convection through a square porous cavity saturated with a binary perfect gas mixture and submitted to horizontal thermal and concentration gradients. The analysis is performed using Darcy–Brinkman formulation with the Boussinesq approximation. The set of coupled equations of mass, momentum, energy and species conservation are solved using the control volume finite-element method. Effects of the Darcy number, the porosity and the thermal porous Rayleigh number on entropy generation are studied. It was found that entropy generation considerably depends on the Darcy number. Porosity induces the increase of entropy generation, especially at higher values of thermal porous Rayleigh number.

Evaluation of inner glass, outer glass water and basin temperatures as well as yield of a simple solar distiller (SSD) still is performed. Numerical computation based on the Runge-Kutta method is used. Experimental results and those calculated are then compared in terms of the above-mentioned parameters. Results concern the climatic conditions of Gabes (south east region of Tunisia). Effects of different parameters, namely the orientation and the use of single or double glass cover of the SSD still are studied. It is important to notice the significant effects on daily yield due to the studied operating parameters. It was found that the double glass cover use provides higher values of temperatures as well as the still productivity.

The study of mass transfer into falling films constitutes a significant aspect for numerous applications in the chemical technology and is considered the subject of many theoretical and experimental researches. Evaluating the second law of thermodynamics is one of the contemporarily used methods to determine the performances of an industrial process and to study various sources of irreversibility. Expressions of the liquid velocity, the gas concentration, the entropy generation rate as well as the main sources of irreversibility in the case of gas absorption (carbon dioxide) into a laminar falling viscous incompressible liquid film (water) without chemical reaction, are analytically derived and graphically presented and discussed.

The present study is based on the use of three correlation groups which are: the Lewis number, the Dunkle and the ‘Kumar and Tiwari’ correlations for evaluating mass transfer in two types of solar stills. Theoretical results are compared with those obtained experimentally for a Simple Solar Distiller and a Hybrid Simple Solar Distiller/Heat pump stills. Experimental results and those calculated by Lewis number correlation show good agreements. Theoretical results obtained using Dunkle and ‘Kumar and Tiwari’ correlations are not satisfactory with the experimental ones.

The influence of an external oriented magnetic field on entropy generation in natural convection for air and liquid gallium is numerically studied in steady-unsteady states by solving the mass, the momentum and the energy conservation equations. Entropy generation depends on five parameters which are: the Prandtl number, the irreversibility coefficients, the inclination angle of the magnetic field, the thermal Grashof and the Hartmann numbers. Effects of these parameters on total and local irreversibilities as well as on heat transfer and fluid flow are studied. It was found that the magnetic field tends to decrease the convection currents, the heat transfer and entropy generation inside the enclosure. Influence of inclination angle of the magnetic field on local irreversibility is then studied.

This paper numerically investigates the effect of an externally evanescent magnetic field on total entropy generation in a fluid enclosed in a square cavity by using a control volume finite element method to solve the conservation equations at Prandtl number of 0·71. The values of relaxation time of the magnetic field are chosen, so that the Lorentz force acts only in the transient state of entropy generation in natural convection. The total entropy generation was calculated for, fixed value of irreversibility distribution ratio, different relaxation time varying from 0 to 1/5 and Grashof number varying from 104 to 105. The effects of the Hartman number and the magnetic field inclination angle on the evolution of total entropy generation throughout the transient regime were investigated. Results show that the application of evanescent magnetic field not only suppresses the fluctuation of the total entropy generation in the transient state, but also reduces the gap for magnetic field relaxation time less than 1/10.

This paper proposes a numerical analysis of entropy generation during mixed convection inside a porous Poiseuille–Benard channel flow, where the Darcy–Brinkman model is used. Irreversibilities due to heat transfer and viscous dissipation have been derived, and then calculated by numerically solving mass, momentum, and energy conservation equations, by using a control volume finite element method (CVFEM). For a fixed value of the thermal Rayleigh (Ra = 104) and the modified Brinkman (Br* = 10−3) numbers, transient entropy generation exhibits a periodic behavior for the medium porosity ε ≥ 0.2, which is described by the onset of thermoconvective cells inside the porous channel. Highest irreversibility is obtained at ε = 0.5. More details about the effects of the Darcy, the Rayleigh, and the modified Brinkman numbers on entropy generation and heat transfer are discussed and graphically presented.

This paper reports the numerical determination of the entropy generation due to heat transfer, mass transfer and fluid friction in steady state for laminar double diffusive convection, in an inclined enclosure with heat and mass diffusive walls, by solving numerically the mass, momentum, species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, the thermal Grashof number and the buoyancy ratio on total entropy generation were investigated. The irreversibilities localization due to heat transfer, mass transfer and fluid friction is discussed for three inclination angles at a fixed thermal Grashof number.

The present paper reports a study about entropy generation analysis for the case of chemical absorption of a gas into laminar falling liquid film. The CO2 absorption into monoethanolamine (MEA) aqueous solutions has been considered. Temperature and concentration expressions are determined by using Laplace transform and used for the entropy generation calculation. The effects of irreversibilities due to heat transfer, mass transfer, viscous effects, coupling effects between heat and mass transfer, and chemical reaction on the total entropy generation of the considered system are derived. The obtained results show that entropy generation is mainly due to chemical reaction irreversibility at the gas–liquid interface. Between this interface and the reaction film thickness (where the reaction take place), entropy generation is due to both chemical reaction and mass transfer irreversibilities. More details concerning the contribution of each kind of irreversibility to entropy generation through the falling film are graphically presented and discussed.

The paper provides a numerical investigation of the entropy generation analysis due to natural convection in an inclined square porous cavity. The coupled equations of mass, momentum, energy and species conservation are solved using the Control Volume Finite-Element Method. Effect of medium permeability and inclination angle on entropy generation is analysed. It was found that according to the Darcy number and the porous thermal Raleigh number values, the entropy generation could be mainly due to heat transfer or to fluid friction irreversibility and that entropy generation reaches extremum values for specific inclination angles., {"references":["A. K. Sen , Natural convection in a shallow porous cavity-the Brinkman\nmodel, int. J. Heat Mess Tran., 5. pp. 855-868. May1987.","Lauriat, G., Prasad, V., Natural convection in a vertical porous cavity: a\nnumerical study for Brinkman-extended Darcy formulation. J. Heat.\nTrans. 109, pp 688-696 Aug. (1987).","P. Alam, S. R. Ansari, and R. Kumar, Numerical study on natural\nconvection in a porous cavity due to patial heating and cooling at\nvertical walls, Int. J. of Appl. Math. and Mech. Vol. 7(18): pp1-21, feb.\n2011.","M. S. Selamat, I. Hashim, M. K. Hasan, transient Natural Convection in\nPorous Square Cavity Heated and Cooled on Adjacent Walls, Math.\nProb. in Eng.Vol 2012, jan. 2012.","B.C. Nader , B.B. Brahim , L. Taieb, Influence of thermal boundary\nconditions on natural convection in a square enclosure partially heated\nfrom below. Int. Comm. in Heat and Mass Trans., vol. 34, pp. 369-379,\nMar.2007.","Shiina, Y., Hishida, M.: Critical Rayleigh number of natural convection\nin high porosity anisotropic horizontal porous layers, Int. J. of Heat and\nMass Trans., Vol 53, pp 1507-1513 Mar. 2010.","A. C. Baytas, optimisation in an inclined enclosure for minimum entropy\ngeneration in natural convection. j. Non-Equilib. Thermodyn. Vol 22, pp\n145-155, Oct.1997.","A. C.Baytas, Entropy generation for natural convection in an inclined\nporous cavity. Int. J. Heat and mass Trans. Vol 43, pp 2089-2099,\nJun.2000.","Famouri, M., Hooman, K.,Entropy generation for natural convection by\nheated partitions in a cavity. Int. Com. in Heat and Mass Trans. Vol 35,\npp 492-502, Apr. 2008.\n[10] N.Hidouri, H. abbassi, M. Magherbi, A. Ben Brahim, Entropy generation\nin double diffusive convection in presence of the Soret effect, progress\nin computational fluid dynamics, Vol. 7, 5 , pp 237 - 246, Jun.2007.\n[11] Hidouri, N., Ben Brahim, A. Influence of Soret and Dufour effects on\nentropy generation in transient double diffusive convection, Int. J.\nExergy, 7, 4, 454-481, Jun. 2010.\n[12] A. Mukhopadhyay, Analysis of entropy generation due to natural\nconvection in square enclosures with multiple discrete heat sources, Int.\nComm. in Heat and Mass Transfer vol.37, pp867-872, Aug. 2010.\n[13] I. Zahmatkesh, on the importance of thermal boundary conditions in heat\ntransfer and entropy generation for natural convection inside a porous\nenclosure, Int. J. of Ther. Sci., 47, pp 339-346, Mar. 2008.\n[14] K. Hooman, F. Hooman, S. R. Mohebpour, Entropy generation for\nforced convection in a porous channel with isoflux or isothermal walls.\nInt. J. Exergy. Vol.5, pp 78-96, Jan. 2008.\n[15] H. J. Saabas, B.R. Baliga, Co-located equal-order control-volume finiteelement\nmethod for multidimensional incompressible fluid flow. Part I:\nFormulation. Numer. Heat Transfer Pt. B-Fund.vol 26, 381-407, Jan.\n1994.\n[16] R. Younsi, A. Harkati, D. Kalache, Numerical simulation of double\ndiffusive natural convection in a porous cavity: opposing flow. The\nArabian J. for Sci. and Eng. Vol 27, pp181-194, Dec. 2002.\n[17] P. Nithiarasu, K.N Seetharamu, T. Sundararajan, Double-diffusive\nnatural convection in an enclosure filled with fluid- saturated porous\nmedium: a generalized non-Darcy approach. Num. Heat Trans. Vol 30,\npp 413- 426, Mar.1996."]}

This paper reports an analytical study of the second law in the case of gas absorption into a laminar falling viscous incompressible liquid film. Velocity, temperature, and concentration profiles are determined and used for the entropy generation calculation. Irreversibilities due to heat transfer, fluid friction, and coupling effects between heat and mass transfer are derived. The obtained results show that entropy generation is mainly due to coupling effects between heat and mass transfer near the gas-liquid interface. Total irreversibility is minimum at the diffusion film thickness. On approaching the liquid film thickness, entropy generation is mainly due to viscous irreversibility.

Thermosolutal convection in a square cavity filled with air and submitted to an inclined magnetic field is investigated numerically. The cavity is heated and cooled along the active walls with a mass gradient whereas the two other walls of the cavity are adiabatic and insulated. Entropy generation due to heat and mass transfer, fluid friction and magnetic effect has been determined in transient state for laminar flow by solving numerically the continuity, momentum energy and mass balance equations, using a Control Volume Finite—Element Method. The structure of the studied flows depends on four dimensionless parameters which are the Grashof number, the buoyancy ratio, the Hartman number and the inclination angle. The results show that the magnetic field parameter has a retarding effect on the flow in the cavity and this lead to a decrease of entropy generation, Temperature and concentration decrease with increasing value of the magnetic field parameter.

The influence of Soret and Dufour effects on entropy generation in transient double diffusive convection of a binary gas mixture for the special case of opposing buoyancy forces with equal intensity is numerically studied. It is an interesting case since it corresponds to minimum entropy generation. For moderate thermal Grashof number (GrT = 104), Soret and Dufour parameters induce a slight increase of entropy generation, for relatively higher thermal Grashof number (GrT = 5 × 104), oscillatory behaviour of entropy generation is obtained. Contributions of Soret and Dufour effects on entropy generation, on heat and mass transfer are discussed.

Influence of the Soret effect on entropy generation in a square cavity filled with a binary gas mixture in double diffusive convection is numerically studied. It was found that entropy generation increases with the increase of thermal diffusion and buoyancy ratios for fixed thermal Grashof number. Total irreversibility takes a minimum value at a special buoyancy ratio ranging between 1 and 0. Soret effect causes the increase of total irreversibility by more than 50% at higher values of thermal Grashof number and buoyancy ratio. More details of the irreversibilities localisation are discussed.

The influence of Dufour effect on entropy generation in double diffusive convection is numerically investigated in this paper. The dimensionless total entropy generation is evaluated as a function of: the buoyancy ratio N, a parameter characterising the solutal buoyancy forces by report to the thermal ones; the Dufour parameter KF, a parameter characterising the influence of the diffusion-thermo effect; the thermal Grashof number. More details of the investigation is focused for the special case when N = −1.