【作者】 李清明；

【导师】 谭兴和；

【作者基本信息】 湖南农业大学 ， 园艺产品采后科学与技术， 2009， 博士

【摘要】 燃料乙醇作为一种可再生能源,可以用作汽车燃料的添加剂,并可以部分取代化石燃料,同时减少了环境污染,因而受到普遍关注。随着我国经济的高速发展,我国石油的需求量也越来越大,原油进口连年增长,2008年原油对外依存度已达49.8%。同时,化石燃料的燃烧排放了大量有害物质,严重威胁了我国的环境安全。为了确保能源安全、降低对进口石油的依赖和减少环境污染,我国也在积极发展燃料乙醇。能耗大和成本高是制约燃料乙醇发展的两个主要因素。燃料生产过程最大的能耗来自于乙醇和水的分离,由于乙醇-水混合物存在共沸现象,采用传统精馏的方法无法得到无水乙醇。而无水乙醇时燃料乙醇的重要基础成分。采用共沸精馏法提纯乙醇的能耗占到燃料乙醇工厂能耗的60%～80%。目前以节能为目的研发的乙醇的分离技术很多,其中吸附法由于能耗低而具有良好的应用前景。本文采用生物质吸附法,以马铃薯粉为吸附剂制备无水乙醇。马铃薯粉作为吸附剂具有原料易得、价格低廉、能耗低,使用失效后可作为发酵法生产乙醇的原料等优点。测定了马铃薯粉作为吸附剂的基本理化性质,并利用红外光谱、X射线衍射和扫描电镜等手段对吸附前后的马铃薯粉吸附剂的结构进行了表征;以马铃薯粉吸附剂为研究对象,设计了塔内径为25mm,有效填充高度为900mm的固定床恒温吸附柱用于研究乙醇-水体系在马铃薯粉上的吸附过程。测量了乙醇-水体系中水在马铃薯粉上的吸附等温线,测定了不同进料流量、床层温度、吸附剂粒度、进料浓度、不同床层高度和不同生物质吸附剂等操作条件下的透过曲线、透过时间、99.5%（v/v）生产能力和床层不同位置的温度变化曲线;对乙醇-水体系中水在马铃薯粉吸附剂上的吸附平衡进行了模型拟合、比较和分析;利用BET模型计算了马铃薯粉的吸附比表面积,通过反气相色谱法研究了马铃薯粉吸附剂对乙醇和水的吸附选择性;利用Klinkenberg模型和神经网络模型对马铃薯粉的吸附传质过程和透过曲线进行预测。试验结果如下:1马铃薯粉作为吸附剂的基本性质研究结果表明:马铃薯粉中淀粉含量为71.91%,淀粉中支链淀粉比例高达77%。40～60目的马铃薯粉床层空隙率达0.5,床层阻力小适合于作为淀粉吸附剂。利用红外光谱、X射线衍射分析和扫描电镜照片对马铃薯粉吸附剂表征,经吸附后马铃薯粉的结构未发生明显变化,说明吸附和再生操作对马铃薯粉的晶形和微观结构影响较小,马铃薯粉作为吸附剂具有良好的稳定性。马铃薯粉在90℃吸附15min后,在相同温度下经过30min即可完成解吸,说明马铃薯粉作为吸附剂易于再生,再生温度低,再生条件温和。经过1个月的反复吸附试验的马铃薯粉发酵后乙醇含量为9.6%,与对照相比较没有明显差异,马铃薯粉吸附剂在吸附活性下降后可以作为发酵原料进行利用。2吸附过程研究结果表明:在本试验条件下,透过时间和生产能力随床层温度升高、进料流量增大和粒径增大而变小,随床层高度增加而增大。吸附剂的粒径和床层高度的变化对透过时间和生产能力的影响最大,进料流量的影响次之,床层温度的高低对透过时间和生产能力的影响较小,但对吸附过程中的能耗影响较大。在吸附操作时,在共沸点附近浓度进行吸附是效率最高的,所以生产上可以采用传统的精馏方法使含水乙醇的浓度提高到共沸点附近,再进行吸附脱水制备无水乙醇。在操作温度为80℃、吸附剂粒径40-60目、原料乙醇进料浓度94.5%（v/v）和进料流量为3ml/min左右条件下,生产能力可达20.51gH₂O/100g吸附剂。在其他操作条件基本相同的情况下,床层高度与生产能力均和透过时间存在线性关系,其方程分别为t_b=0.085h-33.333和P=0.113h-49.445。3吸附平衡研究结果表明:乙醇-水体系中水在马铃薯粉中的吸附等温线为S型,属于Brunauer’s分类中的Ⅱ型吸附等温线。各模型拟合效果的优劣排序为:GAB模型、Henderson模型、Sircar模型、Oswin模型、Peleg模型、Langmuir模型、吸附势理论、Smith模型和Henry定律。根据BET模型,利用相对湿度下（0.05＜α_w＜0.35）的吸附平衡数据,计算出水在40-60目马铃薯粉的BET吸附比表面积为93.019 m²/g马铃薯粉。4吸附选择性研究结果表明:在70℃～140℃,乙醇和水在马铃薯粉中的吸附属于放热过程,分离因子和吸附自由能的变化规律均表明低温有利于吸附的进行,因此适当降低床层温度有利于乙醇-水的吸附分离。马铃薯粉对乙醇和水的吸附具有较强的选择性,其对水的吸附能力远大于乙醇,对于水分子能优先吸附,分离因子最大达203.65。粒径小的马铃薯粉由于表面积大,对乙醇和水的吸附作用强于粒径大的马铃薯粉,对乙醇和水的分离能力大于粒径大的马铃薯粉。70℃～140℃马铃薯粉对乙醇的吸附热ΔH_s在-12.55kJ/mol至-13.14kJ/mol之间,对水的吸附热在-24.88kJ/mol至-45.79kJ/mol之间,属于物理吸附的范围,水的吸附热显著大于乙醇吸附热,说明在马铃薯粉上水的吸附作用比乙醇强烈。5透过曲线预测研究结果表明:利用Klinkenberg模型求得80℃、84℃和88℃下马铃薯粉的总传质系数k分别为0.001413、0.001637和0.001936。并利用Klinkenberg模型和BP神经网络模型对不同温度的马铃薯粉透过曲线进行拟合与比较,利用本文给出的Klinkenberg模型和BP神经网络模型可以预测所需条件下的透过曲线,用于指导工业生产。更多还原

【Abstract】 Fuel ethanol is a kind of renewable fuel, which can be used as the additive of automobile fuel, and replace to the part of the mineral fuel, and reduce the pollution at the same time, and has been paid more attention widespread. With the high economic growth in china, petroleum consumption is rapidly increasing and imports of crude oil are also rising, 49.8% of domestic consumption has to rely on import. The increase in petroleum consumption is causing a serious air pollution problem. To deal with energy security and the air pollution problem, the Chinese government has strongly promoted the fuel ethanol.However, mail issues with fuel ethanol are high product cost and high energy consumption. The highest energy cost of ethanol is the separation of ethanol and water. The key point in ethanol dehydration is represented by the fact that ethanol forms with water an azeotropic mixture. This mixture is usually separated into its constituents by azeotropic distillation, a step which consumes 60%～80% of the overall energy required by the fermentative plant. Among the alternative techniques to dehydrate ethanol, adsorption dehydration system has good industrial application prospect in producing anhydrous alcohol.Adsorption with starchy materials was evaluated as an energy efficient technology for separation of the ethanol-water azeotrope. Potato powder was used as adsorbent for water removal in this study. The advantages of potato powder adsorbent as following, potato powder is cheap, the energy consumption is low, in addition, it can be used as the raw material of fermentation method to produce anhydrous alcohol.The physical and chemical properties of potato powder were investigated in this study, before and after adsorption, the adsorbents were examined with IR, X-RD and SEM. A bench test scale apparatus with a stainless steel column of 25mm inner diameter and of 900mm packed able height for fixed bed adsorption was designed to perform the experiments at constant temperature. The experiments were performed to measure water isotherms on potato powder, measure breakthrough curves with 99.5wt% ethanol as breakthrough point and temperature distribution curves for adsorption under various operation conditions of different levels of superficial velocity, bed temperature, bed depth, granularities adsorbent and inlet concentration. The analysis and the comparison of experimental data were also performed to attain series basic conclusions about the effect of operation conditions on adsorption capability of potato powder. Adsorption equilibrium models have been used to fit the experimental data for water adsorption isotherms on potato powder. BET model was used to calculate the specific surface area of water on potato powder. Inverse gas chromatography （IGC） was used to study the adsorption of water and ethanol on potato powder. Klinkenberg model and BP neural network model were used to analyze the breakthrough curves for different operation conditions.The main results of our research work are as follows: 1. Results from the adsorbent evaluate experiments indicated that starch content of potato powder is 71.91%, the ratio is approximately 23% amylose to 77% amylopectin in potato powder and bed voidage is 0.5 with small bed pressure drop,potato powder is suitable for adsorbent. adsorption did not create visible change, After adsorption, potato powder keep the granules structures, and conserve the size and shape.was shown by the IR spectrum, X-RD patterns and SEM micrographs. Under experiment conditions, potato powder is stable adsorbent. It operates and regeneration at relatively mild conditions, and is stable under cyclic use when regeneration is carried out with air at 90℃. In the event that capacity was lost over a period of time, the potato powder could still be used to make fermentation-derived ethanol.2. Results from the adsorption experiments indicated that the sensitive factors for the vapor adsorption process included the vapor superficial velocity, the temperature, granularities and bed depth. Under experiment conditions, the breakthrough time and production capacity of potato for alcohol-water systems decreases as temperture rise, vapor superficial velocity increases and the size of adsorbent increases, and increases as the bed depth increases. From 93% to 95% ethanol, potato powder is high performance. Distillation of fermentation strengh ethanol to a about 93% ethanol-water production below the azeotrope, followed by adsorption to remove the remaining water. the production capacity is about 21.51g H₂O/g adsorbent at 80℃and inlet ethanol concentration 94.5% （v/v）. When both the vapor superficial velocity and the feed composition are constant, the adsorption bed depth would be proportional to the breakthrough time and production capacity, the relation obtained was t_b = 0.085h-33.333 and P = 0.113h - 49.445, respectively.3. Results from the adsorption equilibrium experiments indicated that isotherm of potato powder exhibited Type II （according to the Brunauer’s classification）. The performances of the models examined in this work for experimental data fitting have the following order: GAB model, Handerson model, Sircar model, Oswin model, Peleg model, Langmuir model, adsorption potential theory of Polanyi, Smith model, and Henry law. GAB model included temperature parameters, and its precision was better than other mathematical models. Based on water-potato powder isotherm data in the range 0.05<α_w<0.35, the specific surface area was estimated for water adsorption as 93.019m²/g potato powder.4. Results from the IGC experiments indicated that water was adsorbed more strongly than ethanol on potato powder at 70～140℃. In addition, it was found that lower temperatures were more favourable for the adsorption of both solutes. In contrast, the separation capacities of potato powder with smaller particle size, i.e. 140 mesh and 100～120 mesh, were greater. The 140 mesh potato powder exhibited the highest separation at 70～140℃. Heats of adsorption were calculated from retention data are in the range from -12.55 to-13.1 kJ/mol for ethanol and -24.88 to -45.79 kJ/mol for water.5. Results from the predict of breakthrough curves indicated that by fitting the experimental results of breakthrough curves to Klinkenberg model, the overall mass-transfer coefficients were estimated as 0.001413, 0.001637 and 0.001936 at 80℃, 84℃and 88℃, respectively. Klinkenberg model and BP neural network model were successfully used for the prediction of breakthrough curves at different temperatures, the models can be used to instruct adsorption system design and operate.更多还原