市政污泥吸附等温线模型和热力学性质

Mathematical modelling of water sorption isotherms and thermodynamic properties of municipal sewage sludge

采用静态重量法测定了市政污泥在30℃、40℃、50℃下的吸附等温线,选用11个常见的数学模型对实验数据进行了拟合并对最佳模型进行了解析,通过净等量吸附热qst、微分熵ΔS、扩散压力π、净积分焓qin和净积分熵ΔSin等指标评价污泥的热力学性质。试验结果表明,在温度恒定时,等温曲线属于Ⅱ型,GAB模型拟合效果最佳,能较好地反映平衡含水量随水分活度的变化。应用Clausius-Clapeyron方程,利用等温线模型计算净等量吸附热和微分熵,随着平衡含水率的增加,净等量吸附热和微分熵明显降低,调和平均温度Thm与等速温度Tl不等,焓-熵补偿理论成立。在一定的水活度下,扩散压力随温度的升高而减小,在温度恒定的情况下,扩张压力随水分活度增大而升高。净积分焓随平衡含水率的增加而减小,而净积分熵在低平衡含水率时随平衡含水率的增加而减小,在30℃、40℃和50℃时分别达到最小值-75.698J/(K∙mol)、-78.987J/(K∙mol)和-82.687J/(K∙mol),然后呈上升趋势。

The adsorption isotherms of municipal sludge at 30℃,40℃,and 50℃were determined by static gravimetric method.Eleven common mathematical models were selected to fit the experimental data,and the best model was analyzed.The thermodynamic properties of sludge were evaluated by net isosteric adsorption heat,differential entropy,diffusion pressure,net integral enthalpy and net integral entropy.The results showed that the isothermal curve belongs to typeⅡat constant temperature,the GAB model fitted well the isotherms data of sludge and was considered as the best model for predicting equilibrium moisture changes with water activity.Additionally,sorption isotherms data were used to determine the thermodynamic properties such as isosteric sorption heat,sorption entropy,spreading pressure,net integral enthalpy and entropy.Net isosteric heat of sorption and differential entropy were evaluated through direct use of moisture isotherms by applying the Clausius-Clapeyron equation and used to investigate the enthalpy-entropy compensation theory.The net isosteric adsorption heat and differential entropy decrease obviously with the increase of equilibrium water content.The harmonic mean temperature Thm was not equal to the constant velocity temperature Tl,so the enthalpy-entropy compensation theory was established.The spreading pressure increased with the increase of water activity at a given temperature,and decreased with the increase of temperature at a given water activity.The net integral enthalpy decreased as the equilibrium moisture content increase.However,the net integral entropy decreased with the increase of equilibrium water content at low equilibrium water content,and reached the minimum values of-75.698J/(K∙mol),-78.987 J/(K∙mol)and-82.687 J/(K∙mol)at 30℃,40℃and 50℃,respectively,and then increased.