甲烷氧化偶联制乙烯反应器的数值模拟研究
发布时间：2018-07-16 11:13
【摘要】：甲烷氧化偶联制乙烯工艺是最有希望工业化的甲烷直接转化方法,但甲烷氧化偶联反应是强放热反应,催化剂床层容易形成热点,工业装置的温度难以控制。仅采用实验的方法实现反应器放大,会导致反应器开发成本高、周期长。计算流体力学是在流场计算的基础上,同时考虑流动、传热、化学反应等多物理化学现象间的耦合作用,可以准确预测反应器的特性,为反应器的放大提供工艺数据,降低开发成本、缩短开发周期。本文采用计算流体力学方法,利用修改参数后的Stansch反应动力学模型,首先建立了装填Na2WO4-Mn/SiO_2颗粒催化剂的OCM固定床反应器实验室装置的计算模型,模拟了原料气体流量为80ml/min常压操作下颗粒固定床反应器的反应特性。通过对比颗粒催化剂固定床反应器出口处CH_4转化率、CO_2等产物选择性的计算值与实验值,可以发现,所有参数的计算值与实验值误差范围为±2%,二者吻合较好,模型可靠；通过分析反应器的催化剂床层内组分浓度、反应温度、侧壁面热通量、压力、流体密度和流速等参数分布表明：在催化剂床层的进口附近,OCM反应较快,该区域的温度稍高于其它区域,包围该区域的反应器壁面放热较多,热通量最大值为17500W/m~2；颗粒催化剂床层内的OCM反应为变容积反应,并且生成物总摩尔数大于反应物总摩尔数,导致流体速度沿流动方向逐渐增加,密度逐渐减小。然后,模拟了装填Na_3PO_4-Mn/SiO_2/堇菁石整体式催化剂固定床反应器,采用与颗粒催化剂固定床反应器计算结果类似的处理办法,分析计算结果。研究表明,反应器出口处主要参数计算值与实验值的误差范围为±4%,模型可靠；由于边界层、化学反应和整体式催化剂结构的影响,导致催化剂床层内的流速沿内壁面法向方向先升高再降低。装填颗粒催化剂和整体式催化剂的两段式固定床反应器可以将这两种催化剂的优点互补。在上述计算的基础上,模拟研究了两段式固定床反应器的反应特性,并考察了反应温度和床层高度对反应器特性的影响。计算结果表明：颗粒催化剂床层内OCM反应比整体式催化剂床层内的剧烈；与颗粒催化剂床层相比,整体式催化剂床层在耗氧量较小的情况下,可以有效地提高C：产物选择性和收率；反应温度通过影响反应网络内各步反应的速率,进而影响各组分质量分数的分布,导致反应温度为800℃时,C_2产物选择性达到最优值66.7%；当整体式催化剂床层高度为50mm、位于上游的颗粒催化剂床层高度变化时,由于颗粒催化剂床层出口处各组分的质量分数存在差异,改变了下游的整体式催化剂床层的进口条件,最终导致C_2收率在颗粒催化剂床层高度为1 Omm达到最优值21.8%。由于两段式固定床反应器内整体式催化剂床层内氧气分压较低,降低了整体式催化剂的活性,导致整体式催化剂无法发挥其高C：选择性的优势。在两段式固定床反应器的两段床层之间补充氧气,可以有效提高C_2产物收率。本文在两段式固定床反应器模拟计算的基础上,对中间补氧的两段式固定床反应器进行模拟研究,并考察了补氧量和反应温度对反应器特性的影响。计算结果表明：对于不同补氧量下的P_(10)M_(50)固定床反应器,补充的氧气促进了整体式催化剂床层内的OCM反应,导致反应器出口处的C_2选择性和收率随补氧量的增加而逐渐升高；对于不同温度下补氧量为15%的P_(10)M_(50)固定床反应器,其整体式催化剂床层上的参数分布规律与无补氧的P_(10)M_(50)固定床反应器内整体式催化剂床层上的参数分布规律类似。在上述模拟研究的基础上,将两段式固定床反应器进行放大,设计了乙烯产量为300ton/a的两段式固定床反应器的中试装置,并采用与实验室小反应器模拟中相同的数学模型对其进行模拟,同时考察了冷却剂流量和冷却剂进口温度对反应器特性的影响。通过分析列管式固定床中试反应器内冷却剂流速、温度和反应气体的组分质量分数以及壁面热通量、对流换热系数等反应器的特性参数的分布,得到如下结论：颗粒催化剂床层温度是决定C_2H_6产量的主要因素,而整体式催化剂床层温度是决定C_2H_4产量的主要因素；当冷却剂流量为453ton/h,冷却剂进口温度为790℃,原料气体流量为594Nm~3/h,原料气体进口温度为775℃,烷氧比为3时,所设计的反应器满足设计要求。
[Abstract]:Based on the calculation of flow , heat transfer , chemical reaction and so on , it is possible to predict the characteristics of the reactor accurately and to reduce the development cost and shorten the development cycle .
By analyzing the concentration of components , the reaction temperature , the heat flux , pressure , fluid density and flow velocity in the catalyst bed of the reactor , it is shown that the OCM reaction is faster in the vicinity of the inlet of the catalyst bed , the temperature of the region is slightly higher than that of other regions , and the heat flux is more than 17500W / m ~ 2 .
The reaction of OCM in the bed of catalyst bed is variable volume reaction , and the total number of moles of product is greater than the total number of moles of reactants , resulting in a gradual increase in the velocity of the fluid along the flow direction and a gradual decrease in density . The results are analyzed by a similar approach to the calculation results of the fixed bed reactor of particulate catalyst . The results show that the error range of the main parameters at the outlet of the reactor and the experimental value is 卤 4 % and the model is reliable ;
Due to the influence of boundary layer , chemical reaction and monolithic catalyst structure , the flow velocity in the catalyst bed is increased and then decreased in the direction of the inner wall surface method . The two - stage fixed bed reactor filled with granular catalyst and monolithic catalyst can complement the advantages of the two catalysts . On the basis of the above calculation , the reaction characteristics of the two - stage fixed bed reactor are simulated and the influence of reaction temperature and bed height on the characteristics of the reactor is studied .
Compared with the granular catalyst bed layer , the monolithic catalyst bed layer can effectively improve the selectivity and the yield of the product under the condition that the oxygen consumption is small ;
By influencing the rate of reaction in the reaction network , the reaction temperature affects the distribution of the mass fraction of each component , which results in the selectivity of the C _ 2 product reaching the optimum value 66.7 % when the reaction temperature is 800 鈩，
本文编号：2126207
[Abstract]:Based on the calculation of flow , heat transfer , chemical reaction and so on , it is possible to predict the characteristics of the reactor accurately and to reduce the development cost and shorten the development cycle .
By analyzing the concentration of components , the reaction temperature , the heat flux , pressure , fluid density and flow velocity in the catalyst bed of the reactor , it is shown that the OCM reaction is faster in the vicinity of the inlet of the catalyst bed , the temperature of the region is slightly higher than that of other regions , and the heat flux is more than 17500W / m ~ 2 .
The reaction of OCM in the bed of catalyst bed is variable volume reaction , and the total number of moles of product is greater than the total number of moles of reactants , resulting in a gradual increase in the velocity of the fluid along the flow direction and a gradual decrease in density . The results are analyzed by a similar approach to the calculation results of the fixed bed reactor of particulate catalyst . The results show that the error range of the main parameters at the outlet of the reactor and the experimental value is 卤 4 % and the model is reliable ;
Due to the influence of boundary layer , chemical reaction and monolithic catalyst structure , the flow velocity in the catalyst bed is increased and then decreased in the direction of the inner wall surface method . The two - stage fixed bed reactor filled with granular catalyst and monolithic catalyst can complement the advantages of the two catalysts . On the basis of the above calculation , the reaction characteristics of the two - stage fixed bed reactor are simulated and the influence of reaction temperature and bed height on the characteristics of the reactor is studied .
Compared with the granular catalyst bed layer , the monolithic catalyst bed layer can effectively improve the selectivity and the yield of the product under the condition that the oxygen consumption is small ;
By influencing the rate of reaction in the reaction network , the reaction temperature affects the distribution of the mass fraction of each component , which results in the selectivity of the C _ 2 product reaching the optimum value 66.7 % when the reaction temperature is 800 鈩，
本文编号：2126207
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