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油页岩是我国储量丰富的非常规固体化石能源，通过干馏技术能够制取与原油性质接近的页岩油，有助于缓解我国原油进口压力。然而，目前通过固体热载体干馏工艺获得的页岩油中重质组分和粉尘含量较高，阻碍了页岩油的高值化利用。针对上述问题，本论文提出了分级冷凝技术方案，通过冷凝温度调控，促使热解油气中的粉尘和页岩油重质组分富集在高温冷凝单元，由此降低低温冷凝单元页岩油中的粉尘和重质组分含量，并实现页岩油在冷凝过程中的初步分离和提质。基于该技术方案，本论文通过实验和数值模拟相结合的方法，针对分级冷凝条件对热解油气除尘及组分分离的影响规律和作用机制进行了研究。主要研究内容及结果如下：1. 建立适用于高数浓度多分散颗粒的异相冷凝增长模型，通过该模型研究颗粒数浓度、初始颗粒粒径以及蒸气浓度对颗粒粒径分布的影响规律。模拟结果显示，低颗粒数浓度或高蒸气浓度能够增大颗粒平均粒径，同时使粒径分布变窄。初始粒径在（1-3 mm）所考察的范围内均长大到8 mm左右，该特性有利于高温油气中的粉尘脱除。由蒸气冷凝造成的蒸气消耗和热量释放会显著降低体系的过饱和度，抑制颗粒的增长。在本研究中，相对热量释放，蒸气消耗是限制颗粒进一步长大的主要因素。2. 建立油页岩流化床连续热解和三级冷凝器小试装置，在第二级和第三级冷凝温度分别为16 oC和-2 oC时，考察第一级冷凝温度（66-247 oC）对液体产物在三级冷凝器中的质量分布和组成变化的影响规律。结果显示，冷凝温度的降低同时提高页岩油和粉尘（甲苯不溶物）在第一级冷凝器的收率，证实了页岩油蒸气的冷凝对粉尘的捕集有促进作用，在本实验体系内最佳冷凝温度为158 oC。在此温度下，第一级油相的收率、重质组分含量和粉尘含量分别为18.03%、75.66%和6.99%，后两级油相收率之和为 81.97%，重质组分含量和粉尘含量分别为30.38%和1.83%。该结果证明了分级冷凝技术可以应用于热解油气除尘和组分初步分离。3. 随第一级冷凝温度的降低，管路内沉积颗粒物的聚集形态逐步由分散转变为较小聚团并进一步形成较大聚团，在本实验体系中特征转变温度为158 oC。蒸气的冷凝方式取决于蒸气在冷凝器入口与出口之间的温差：在低温差下，蒸气过饱和度低发生异相冷凝，在颗粒表面形成液膜，聚团是由颗粒之间相互碰撞形成；在高温差下，蒸气过饱和度高发生均相冷凝生成大量液滴，聚团是由液滴和颗粒相互碰撞形成。 4. 通过建立多组分蒸气冷凝模型，采用计算流体力学方法研究了热解油气在冷凝过程中的组分分布规律。模型分析表明，冷凝器内的温度分布对冷凝组分具有选择性，针对所研究的热解油气组成，常压瓦斯油馏分和汽油馏分开始冷凝的温度分别为157.3 oC和79.9 oC，与实验所得的冷凝温度基本一致，可以为分级冷凝工艺中冷凝温度的选择提供指导。5. 采用Aspen Plus对多级冷凝器串联工艺和油洗工艺进行模拟，结果表明，在产物收率接近的前提下，通过油洗工艺获得的产物纯度优于多级冷凝器串联工艺。在优化工艺参数下，可获得粗重油、粗常压瓦斯油和粗汽油三种产物，纯度分别为93.26%、93.43%和69.19%。;Oil shale is an unconventional solid fossil fuel with abundant resources in China. Shale oil obtained from pyrolysis of oil shale is similar to crude oil, which has become a significant supplementary oil resource. However, high contents of dust and heavy components in shale oil obtained from solid heat carrier technology impair its quality and are disadvantageous for its high-value utilization. To solve above problems, it is proposed to fractionate the oil into major fractions online by staged condensation, one of which contains the maximum content of dust and heavy components and the other contains the maximum light components with minimum dust. By the above method, the shale oil can be separated into several products preliminary in the condensation of pyrolysis vapor. On the basis of proposed technology, the impact of condensation temperature on the removal of dust and separation of components is studied by experiments and numerical simulation. The main findings are summarized as follows:1. A model was built to simulate the evolution of particle size distribution (PSD) at high number concentration by heterogeneous condensation. By the model, the effects of particle number concentration, initial diameter and initial vapor concentration were studied on the particle size distribution. It was found that a low particle number concentration or a high vapor concentration were beneficial to shift the PSD toward larger particles with a narrower distribution. The fluctuation of initial particle size (1-3 mm) had weak influence on the final particle size, which was around 8 mm. This feature was beneficial to the separation of dust from pyrolysis vapor at high temperature. Vapor depletion had greater effects than heat release on the final PSD at high particle number concentrations, which prevented the further increase of particle size. Based on the results, the condensation of vapor was preliminarily verified for the removal of dust.2. The staged condensation with three stages of condensers was applied in the pyrolysis of oil shale via a lab-scale fluidized bed. The temperature of the second stage and the third stage were 16 oC and -2 oC respectively. The effect of cooling temperature of the first stage (66-247 oC) on the distribution and characteristics of liquids was investigated. The results showed that the yield of shale oil and dust (defined as toluene insoluble matter) in the first stage increased with the decreasing temperature , which proved that the condensation of oil vapor had a positive influence on the removal of dust. The optimum temperature in this study was 158 oC. On the basis of above condensation temperatures, the yield, contents of heavy components and dust in the oil of the first stage were 18.03%, 75.66% and 6.99% respectively. The mixture of the oil in the latter stages accounted for 81.97% of the total oil, whose content of heavy components and dust were 30.38% and 1.83% respectively. The results preliminarily showed that it was feasible to apply staged condensation to the removal of dust and separation of components.3. When the temperature of the first stage was higher than 158 oC, the particles were dispersed uniformly, while the small aggregates were formed at 158 oC. As the temperature decreased, the aggregates became larger. The temperature gap defined as the difference vapor temperature between the inlet and the outlet of the condenser influenced the way of vapor condensation. When the gap was less than 242 oC, vapor condensation tended to occur on the particle’s surface called as heterogeneous condensation at low supersaturation, resulting in the agglomeration by the collision between particles. When the gap was larger than 242 oC, great amount of droplets could be formed by homogeneous condensation at high supersaturation and may collide with particles to induce agglomerates.4. A CFD model with the condensation of multicomponent was built to investigate the distribution of components in the condenser. The results showed that the distribution of temperature in the condenser had great influence on the condensation of each components. For the tested pyrolysis vapor, the dew point temperatures of the atmospheric gas oil and gasoline were 157.3 oC and 79.9 oC, which were in accordance with the experimental data and could be used to guide for the determination of the cooling temperature in the technology of staged condensation.5. The technologies of multistage condensers in series and oil wash were simulated by Aspen Plus respectively. When the yields of the obtained products were close to each other, the purities of products obtained from the technology of oil wash were higher than the other. With the optimum parameters, the pyrolysis vapor was condensed into crude heavy oil, crude atmospheric gas oil and crude gasoline, whose purities were 93.26%, 93.43% and 69.19% respectively.
|张华. 油页岩热解油气分级冷凝除尘及组分分离研究[D]. 中国科学院大学,2019.|
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