摘要
变压吸附为目前处理回收挥发性有机化合物的主要方法之一。本文针对其中的真空变压吸附进行了研究,侧重于以往研究中常简化处理的难点——真空脱附阶段。研究内容主要有两部分:建立脱附传热传质的数学模型,分析其中传热传质的耦合作用,对丙酮活性炭上真空脱附过程中的传质过程进行数值模拟;真空变压吸附多组分有机气体的实验研究和数值模拟。
首先,为探讨脱附过程中传热传质耦合影响规律,采用线性驱动力和Langmiur吸附平衡模型,通过质量和能量守恒原理建立脱附的传热传质模型,从理论上分析脱附脱附过程中传热与传质之间的影响,发现在脱附过程中由于其温度变化较小,因此传热对传质的影响不大。脱附真空度越大,脱附初始吸附容量越大,但脱附速率相应变小;由于吸附剂微孔内的毛细管现象作用和脱附过程中的传热传质耦合影响,脱附浓度曲线表现出真空浓缩区、快速衰减区和缓慢衰减区三个阶段。由于温度的变化对传质系数影响不大,耦合影响的传质系数主要受吸附量相对于浓度变化率影响。通过对吸附等温线的分析发现吸附量对浓度的变化率在低浓度区和高浓度区虽然相差较大但分别近似线形。因此在模拟过程中分段求取传质系数和轴向扩散系数可以使数值计算结果很好的与实验数据吻合;对于同一初始吸附容量,脱附时丙酮分压越大其传质系数k越大;轴向扩散系数在真空脱附时不宜省略。
其次,实验研究了两床真空变压吸附甲苯和丙酮混合蒸气过程的传热和传质情况,讨论了平衡状态时浓度和温度随时间和床层方向的变化规律,实验发现在吸附床的后半部分浓度随时间的变化不是单调递减,有可能随着脱附时间的增加而有所维持甚至反弹。采用的传热传质耦合模型对吸附床的前半段模拟结果较好,模拟分析了稳定状态时浓度的变化规律及在床层中的分布。采用该模型对变压吸附的操作时间进行了优化:21.5℃下,丙酮和甲苯的吸附进气浓度分别为58.478g/m~3和18.014g/m~3,均压升和均压降时间各为5s,吹扫时间为10s时,当吸附时长为300s,最佳脱附时间为155s,此时丙酮的C_(脱附)/C_(吸附进气)为5.53,C_(净化)/C_(吸附进气)为0.10;甲苯的C_(脱附)/C_(吸附进气)为2.58;当脱附时间为60s,最佳吸附时间为165s,此时丙酮的C_(脱附)/C_(吸附进气)为4.93,C_(净化)/C_(吸附进气)为0.10;甲苯的C_(脱附)/C_(吸附进气)为2.98。
The pressure swing adsorption (PSA) is one of the primary methods of treating and recycling volatile organic compound (VOC). The paper researched on the mass and heat transfer of vacuum pressure swing adsorption (VPSA), focusing on the vacuum desorption process which is a difficult point and often simplified as the inverse process of adsorption in the previous documents. The study included two parts. One was building the model coupled with heat and mass transfer to analyze the interreaction of heat and mass transfer and simulating the mass transfer process of acetone desorption from activated carbon. The other one was the experimental research and simulation of VPSA process of multi-component VOC.
Firstly, the LDF model and Langmuir model were used to analyze the interreaction of heat and mass transfer coupled with heat and mass transfer and the principle of conservation of quantity and energy. It was found that heat transfer had little influence on mass transfer because of the little change of temperature. The higher desorption vacuum degree and the larger initial adsorption quantity of desorption led to the smaller desorption rate, which was attributed to the affect of capillarity of micropore in the adsorbent and the interreaction of the heat and mass transfer in the process of desorption. The changing process of the desorption concentration contained three phases: vacuum concentration, rapid attenuation and slow attenuation. Because of the little influence caused by the temperature changing, the mass transfer coefficient of LDF was mainly affected by the change rate of adsorption quantity through concentration. The adsorption isotherm showed that the change rate of adsorption quantity through concentration was much different in low concentration and high concentration, but it was nearly linear in low concentration and high concentration respectively. The influence of the coefficient of mass transfer on concentration was various in different phases of desorption, so it made the calculated value of simulation fit the experimental data well by selecting different coefficient of mass transfer or coefficient of axial diffusion. For the same initial adsorption quantity, the greater the partial pressure of acetone led to the larger the coefficient k of mass transfer, and the omission of the coefficient of axial diffusion of desorption was not suitable. Secondly, the dynamic and energetic characteristics of the organic gases of toluene and acetone were analyzed by experimental study and the concentration distribution of the equilibrium state was discussed. It was found that the concentration in the latter half of adsorption bed was not monotonely decreasing, and it was likely to maintain and even increase as desorption time increasing. The results of simulation fit the experimental data of the first half of adsoption bed better than which of the second half. Then the change of the concentration distribution in the bed in steady state was simulated and analyzed. The optimal operation timing of VPSA was obtained by the model. The fixed operating parameters were 21.5℃, the adsorption feed concentration of acetone(58.478g/m~3) and toluene (18.014 g/m~3), two 5s pressure equalizing time and 10s blowdown time. For 300s adsorption time, the optimal desorption time was 155s. And then for acetone, C_(desorption)/C_o was 5.53 and C_(purify)/C_o为0.10. For toluene, C_(desorption)/C_o was 2.58. For 60s desorption time, the optimal adsorption time was 165s. And then for acetone, C_(desorption)/C_o was 4.93 and C_(purify)/C_o为0.10. For toluene, C_(desorption)/C_o was 2.98.
引文
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