Efficiency of Irreversible Processes

This chapter establishes a direct relation between lost work and the fluxes and driving forces of a process. The Carnot cycle is revisited to investigate how the Carnot efficiency is affected by the irreversibilities in the process. We show to what extent the constraints of finite size and finite time reduce the efficiency of the process, but we also show that these constraints still allow a most favorable operation mode, the thermodynamic optimum, where the entropy generation and thus the lost work are at a minimum. Attention is given to the equipartitioning principle, which seems to be a universal characteristic of optimal operation in both animate and inanimate dynamic systems. 

The mechanism of the enone-aikene [2+2] photocycloaddition presumably follows the scheme below. Upon irradiation 1) a triplet exciplex is irreversibly formed from the triplet enone and ground state alkene 2) the triplet exciplex collapses to one or more 1,4-biradicals. 3) the biradicals either cyclize to the cyclobutane or revert to starting materials and 4) the biradical reversion decreases the overall efficiency of the process. 

Throttling maybe viewed as expansion in a turbine that is so irreversible that no work is produced. Why should one waste the potential of a pressurized gas by throttling it rather than using a turbine to expand it Because using a valve is much cheaper to install and maintain (no moving parts) than a turbine. Replacing a throttling valve with a turbine will increase the efficiency of a process, but the decision to replace it would have to be made on a case-by-case basis, based on cost-benefit analysis. 

The Use of Linear Thermodynamics of Irreversible Processes (LTIP) for Calculation of Parameters Related to Conservative Mechanisms in the Process of Light-to-Chemical Energy Conversion P/2e Calculation and Analysis Thermodynamic Efficiency and Energetic Coupling Analysis Experimental Validation of the Proposed Model for Different Simple Geometric Structures of a Photobioreactor... 

The power output of an electro-chemical cell (or a battery) is the product of the applied voltage times the current. A reversible electro-chemical cell has zero current and hence zero power output, and zero entropy production. Power output requires an irreversible process run in finite time therefore power output is concurrent with entropy production (dissipation). One use of the term efficiency of a process, such as a reaction, is in reference to energy transduction, for example what part of AG of a reaction is used to produce work there are other uses of this term. 

Irreversible Processes. Irreversible processes are among the most expensive continuous processes. These are used only in special situations, such as when the separation factors of more efficient processes (that is, processes that are theoretically more efficient from an energy point of view) are found to be uneconomicaHy small. Except for pressure diffusion, the diffusion methods discussed herein are essentially irreversible processes. Thus,... 

Real irreversible processes can be subjected to thermodynamic analysis. The goal is to calciilate the efficiency of energy use or production and to show how energy loss is apportioned among the steps of a process. The treatment here is limited to steady-state, steady-flow processes, because of their predominance in chemical technology. 

There is an apparent paradox here that as the cooled cycle contains an irreversible process (constant pressure mixing), its efficiency might be expected to be lower than the original uncooled cycle. The answer to this paradox follows from consideration of all the irreversibilities in the cycle and we refer back to the analysis of Section 3.2.1.1, for the rational efficiency of the [CHT]ru cycle. The irreversibility associated with the heat supply is unchanged, as given in Eq. (3.3), but the irreversibility associated with the heat rejection between temperatures T(, and T) = Ta becomes... 

It must be emphasised that the heat q which appears in the definition of entropy (equation 20.137) is always that absorbed (or evolved) when the process is conducted reversibly. If the process is conducted irreversibly and the heat absorbed is q, then q will be less than q, and q/T will be less than AS the entropy change (equation 20.137). It follows that if an irreversible process takes place between the temperatures Tj and 7 , and has the same heat intake q at the higher temperature 7 2 as the corresponding reversible process, the efficiency of the former must be less than that of the latter, i.e. 

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