THERMODYNAMIC ANALYSIS OF SINGLE- AND DOUBLE-EFFECT ABSORPTION REFRIGERATION SYSTEMS FOR COOLING ETHANOL FERMENTATION PROCESS

Abstract

Alcoholic fermentation is one of the most important stages in industrial ethanol production process, involving a biochemical and exothermic reaction. Sometimes cooling towers are not capable of supplying a cold utility with a temperature low enough to maintain the fermentative medium temperature in a desirable range. Absorption Refrigeration Systems (ARS) appears to be a good alternative to obtain the necessary refrigeration for the fermentation process. The aim of the present paper was to carry out a thermodynamic analysis of ARS, evaluating their performance through the First and Second Laws of Thermodynamics. ARS with different configurations were studied (single-effect and double-effect with series, parallel and reverse parallel flows), all of them operating with water/lithium bromide mixture as working pair, under different operating conditions in order to satisfy the cooling load required by an industrial alcoholic fermentation process. Another objective of this paper was to investigate the risk of LiBr crystallization, which can result in scaling formation, with the aid of the solid–liquid phase equilibrium curve of H 2 O / LiBr mixture. Among the double-effect configurations studied, it was observed that series flow presents the more significant crystallization risk, which represents a limit to improve its First and Second Law performances. It was verified that the Second Law performance for the single-and double-effect ARS analyzed are similar, but their First Law performance are considerably different. This is due to the amount and quality of the heat consumed in the first effect generators of these systems. For a base case studied, First Law performance measured by coefficient of performance (COP) of double-effect ARS is 72% greater than the one for single-effect, while for Second Law performance, measured by exergetic efficiency, an increase of 5% was observed.