Abstract

This study was aimed at determining wastewater sewage sludge moisture sorption characteristics using a static gravimetric method at 303 K, 313 K and 323 K. Eleven mathematical models were adopted to simulate the experimental data, and the optimal model was analysed. The indexes included the net isosteric heat of adsorption (q_st), differential entropy (△S), spreading pressure (π), net integral enthalpy (q_in) and net integral entropy (△S_in) to evaluate the thermodynamic properties of the sludge. The isotherm curves exhibited type II behaviour according to the Brunauer--Emmett--Teller classification. The sorption isotherm fitting showed that the Gugghenheim--Anderson--de Boer model was able to reproduce the equilibrium moisture content evolution with water activity for a moisture range varying from 6% to 89% (0.991 < R² < 0.999). The net isosteric heat of sorption and differential entropy were evaluated through direct use of moisture isotherms via the Clausius–Clapeyron equation and used to investigate the enthalpy–entropy compensation theory. The net isosteric heat and differential entropy obviously decreased as the equilibrium moisture content increased, which satisfied the compensation theory. The spreading pressure of the sludge sorption process decreased as the temperature increased at a given water activity and increased as the water activity increased at a given temperature. When the spreading pressure was at a fixed level, the net integral enthalpy decreased as the equilibrium moisture content increased, whereas the net integral entropy decreased as the equilibrium moisture content increased to a minimum value of −93.17, −98.34 and −93.28 J/(mol.K) at 303 K, 313 K and 323 K, respectively, and then tended to increase.

中文翻译：

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污水污泥吸水等温线和热力学性质的数学建模

摘要

本研究旨在利用静态重量法在303 K、313 K和323 K下确定废水污泥吸湿特性。采用11个数学模型对实验数据进行模拟，并分析了最优模型。指标包括净等量吸附热（q _st）、微分熵（△S）、扩散压力（π）、净积分焓（q _in）和净积分熵（△S _in) 来评估污泥的热力学性质。根据 Brunauer--Emmett--Teller 分类，等温曲线表现出 II 型行为。吸附等温线拟合表明，Gugghenheim--Anderson--de Boer 模型能够再现平衡水分含量随水分活度的变化，水分范围从 6% 到 89% (0.991 <  R ² < 0.999)。通过克劳修斯-克拉佩龙方程直接使用水分等温线来评估净等量吸附热和微分熵，并用于研究焓-熵补偿理论。净等量热和微分熵随着平衡含水量的增加而明显减小，满足补偿理论。污泥吸附过程的扩散压力在给定的水分活度下随着温度的升高而降低，在给定的温度下随着水分活度的增加而增加。当摊铺压力处于固定水平时，净积分焓随着平衡水分含量的增加而降低，而净积分熵随着平衡水分含量增加至最小值-93.17、-98.34和-93.28 J/(摩尔。