Yaşar Demirel

Linear-nonequilibrium thermodynamics theory for coupled heat and mass transport

Abstract Linear-nonequilibrium thermodynamics (LNET) has been used to express the entropy generation and dissipation functions representing the true forces and flows for heat and mass transport in a multicomponent fluid. These forces and flows are introduced into the phenomenological equations to formulate the coupling phenomenon between heat and mass flows. The degree of the coupling is also discussed. In the literature such coupling has been formulated incompletely and sometimes in a confusing manner. The reason for this is the lack of a proper combination of LNET theory with the phenomenological theory. The LNET theory involves identifying the conjugated flows and forces that are related to each other with the phenomenological coefficients that obey the Onsager relations. In doing so, the theory utilizes the dissipation function or the entropy generation equation derived from the Gibbs relation. This derivation assumes that local thermodynamic equilibrium holds for processes not far away from the equilibrium. With this assumption we have used the phenomenological equations relating the conjugated flows and forces defined by the dissipation function of the irreversible transport and rate process. We have expressed the phenomenological equations with the resistance coefficients that are capable of reflecting the extent of the interactions between heat and mass flows. We call this the dissipation-phenomenological equation (DPE) approach, which leads to correct expression for coupled processes, and for the second law analysis.

Thermodynamic Analysis of Separation Systems

Abstract Separation systems mainly involve interfacial mass and heat transfer as well as mixing. Distillation is a major separation system by means of heat supplied from a higher temperature level at the reboiler and rejected in the condenser at a lower temperature level. Therefore, it resembles a heat engine producing a separation work with a rather low efficiency. Lost work (energy) in separation systems is due to irreversible processes of heat, mass transfer, and mixing, and is directly related to entropy production according to the Gouy‐Stodola principle. In many separation systems of absorption, desorption, extraction, and membrane separation, the major irreversibility is the mass transfer process. In the last 30 years or so, thermodynamic analysis had become popular in evaluating the efficiency of separation systems. Thermodynamic analysis emphasizes the use of the second law of thermodynamics beside the first law, and may be applied through (i) the pinch analysis, (ii) the exergy analysis, and (iii) the equipartition principle. The pinch analysis aims a better integration of a process with its utilities. It is one of the mostly accepted and utilized methods in reducing energy cost. Exergy analysis describes the maximum available work when a form of energy is converted reversibly to a reference system in equilibrium with the environmental conditions; hence, it can relate the impact of energy utilization to the environmental degradation. On the other hand, the equipartition principle states that a separation operation would be optimum for a specified set of fluxes and a given transfer area when the thermodynamic driving forces are uniformly distributed in space and time. Thermodynamic analysis aims at identifying, quantifying, and minimizing irreversibilities in a separation system. This study presents an overview of the conventional approaches and the thermodynamic analysis to reduce energy cost, thermodynamic cost, and ecological cost in separation systems with the main emphasis on distillation operations. Some case studies of cost reduction based on the thermodynamic analysis are also included.

Thermodynamics and bioenergetics

Bioenergetics is concerned with the energy conservation and conversion processes in a living cell, particularly in the inner membrane of the mitochondrion. This review summarizes the role of thermodynamics in understanding the coupling between the chemical reactions and the transport of substances in bioenergetics. Thermodynamics has the advantages of identifying possible pathways, providing a measure of the efficiency of energy conversion, and of the coupling between various processes without requiring a detailed knowledge of the underlying mechanisms. In the last five decades, various new approaches in thermodynamics, non-equilibrium thermodynamics and network thermodynamics have been developed to understand the transport and rate processes in physical and biological systems. For systems not far from equilibrium the theory of linear non-equilibrium thermodynamics is used, while extended non-equilibrium thermodynamics is used for systems far away from equilibrium. All these approaches are based on the irreversible character of flows and forces of an open system. Here, linear non-equilibrium thermodynamics is mostly discussed as it is the most advanced. We also review attempts to incorporate the mechanisms of a process into some formulations of non-equilibrium thermodynamics. The formulation of linear non-equilibrium thermodynamics for facilitated transport and active transport, which represent the key processes of coupled phenomena of transport and chemical reactions, is also presented. The purpose of this review is to present an overview of the application of non-equilibrium thermodynamics to bioenergetics, and introduce the basic methods and equations that are used. However, the reader will have to consult the literature reference to see the details of the specific applications.

Energetic and exergetic efficiency of latent heat storage system for greenhouse heating

In this research, solar energy has been stored using the paraffin with the latent heat technique for heating the plastic greenhouse of 180 m2. Energy and exergy analyses were applied for evaluation of the system efficiency. An average values of the rates of heat and thermal exergy stored into the HSU were 1 740 W and 60 W for the charging periods. It was determined that the average values of the net energy and exergy efficiencies of the system were 41.9% and 3.3%, respectively.

Thermodynamic analysis of convective heat transfer in an annular packed bed

A combination of the first and second law of thermodynamics has been utilized in analyzing the convective heat transfer in an annular packed bed. The bed was heated asymmetrically by constant heat fluxes. Introduction of the packing enhances wall to fluid heat transfer considerably, hence reduces the entropy generation due to heat transfer across a finite temperature diAerence. However, the entropy generation due to fluid-flow friction increases. The net entropy generations resulting from the above eAects provide a new criterion in analysing the system. Using the modified Ergun equation for pressure drop estimation and a heat transfer coeAcient correlation for an annular packed bed, an expression for the volumetric entropy generation rate has been derived and displayed graphically. In the packed annulus, the fully developed temperature profile and the plug flow conditions have been assumed and verified with experimental data. The volumetric entropy generation map shows the regions with excessive entropy generation due to operating conditions or design parameters for a required task, and leads to a better understanding of the behavior of the system. ” 2000 Elsevier Science Inc. All rights reserved.

Retrofit of distillation columns using thermodynamic analysis

Abstract Thermodynamic analysis provides the column grand composite curves and exergy loss profiles, which are becoming readily available for a converged distillation column simulation. For example, the Aspen Plus simulator performs the thermodynamic analysis through its Column–Targeting tool for rigorous column calculations. This study uses the column grand composite curves and the exergy loss profiles obtained from Aspen Plus to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a methanol plant. Effectiveness of the retrofits is also assessed by means of thermodynamics and economics. The methanol plant utilizes two distillation columns to purify the methanol in its separation Section. The first column operates with 51 stages, has a side heat stream to the last stage, a partial condenser at the top and a side condenser at stage 2, and no reboiler. The second column operates with 95 stages, has a side heat stream to stage 95, a total condenser, and high reflux ratio. Despite the heat integration of the columns with the other Sections and a side condenser in column 1, the assessment of converged base case simulations have indicated the need for more profitable operations, and the required retrofits are suggested. For the first column, the retrofits consisting of a feed preheating and a second side condenser at stage 4 have reduced the total exergy loss by 21.5%. For the second column, the retrofits of two side reboilers at stages 87 and 92 have reduced the total exergy loss by 41.3%. After the retrofits, the thermodynamic efficiency has increased to 55.4% from 50.6% for the first column, while it has increased to 6.7% from 4.0% for the second. The suggested retrofits have reduced the exergy losses and hence the cost of energy considerably, and proved to be more profitable despite the fixed capital costs of retrofits for the distillation columns of the methanol plant.

On the effective heat transfer parameters in a packed bed

In this study, wall-to-fluid heat transfer coefficient h w and effective radial thermal conductivity k er have been determined for a packed bed with 4.5<d e /d p <7.5 in the region of 200<Re<1450. Two values of Nu based on the bulk as well as the extrapolated fluid temperature at the wall have been determined and compared.

Entropy generation in a rectangular packed duct with wall heat flux

The entropy generation due to heat transfer and friction has been calculated for fully developed, forced convection flow in a large rectangular duct, packed with spherical particles, with constant heat fluxes applied to both the top (heated) and bottom (cooled) wall. An approximate analytical expression for the velocity profile developed for packed bed with H/dp > 5 has been used together with the energy equation of fully developed flow to calculate the non-isothermal temperature profiles along the flow passage. The velocity profile takes into account the increase in the velocity near the wall due to the higher voidage in this region of the bed. The effect of the asymmetric heating on the velocity profile is neglected under the thermal conditions considered. The volumetric entropy generation rate and the irreversibility distribution ratios have been calculated and displayed graphically for the values of H/dp = 5 and 20. It was found that the irreversibility distributions are not continuous through the wall and core regions, hence the optimality criterion of equipartition of entropy generations should be considered separately for these regions of the packed duct.

Thermal performance study of a solar air heater with packed flow passage

Abstract A solar air heater with the air flow channel underneath the absorber plate has been designed and tested. The performance tests have been carried out when the flow channel is empty (type I) and packed with Raschig rings made of hard plastic (type II). The packing causes a considerable increase in the efficiency. The theoretical analysis of the systems has been performed using a heat transfer model based on the quasi-steady state condition. The energy balance equations of the systems with one and two glass covers have been solved explicitly for various design and operating variables. The agreement between the measured and predicted performances is satisfactory for the type I, while it is relatively poor for the type II.

Optimization and economic evaluation of industrial gas production and combined heat and power generation from gasification of corn stover and distillers grains

Thermochemical gasification is one of the most promising technologies for converting biomass into power, fuels and chemicals. The objectives of this study were to maximize the net energy efficiency for biomass gasification, and to estimate the cost of producing industrial gas and combined heat and power (CHP) at a feedrate of 2000kg/h. Aspen Plus-based model for gasification was combined with a CHP generation model, and optimized using corn stover and dried distillers grains with solubles (DDGS) as the biomass feedstocks. The cold gas efficiencies for gas production were 57% and 52%, respectively, for corn stover and DDGS. The selling price of gas was estimated to be