【作者】 王飞；

【导师】 肖文德；

【作者基本信息】 华东理工大学 ， 化学工程， 2011， 博士

【摘要】 生物乙烯是一条绿色、可再生和可持续发展的石油替代路线。乙醇脱水制乙烯是该路线中的关键。本文研究了亚微米ZSM-5分子筛催化乙醇脱水制乙烯过程中的催化剂失活、再生和反应动力学,并进行了工厂中间试验。主要研究内容如下：第一、催化剂改进。目前可以用于乙醇脱水的催化剂主要有活性氧化铝、金属或过渡金属氧化物、杂多酸和分子筛。活性氧化铝催化反应温度较高而且空速较低,在重时空速为0.19 h-1时当温度升高到360℃以上乙醇达到完全转化,乙烯选择性在312℃时达到最高,为92.6%。分子筛催化剂在重时空速为1.6h-1,温度25℃左右即可达到98%以上的乙醇转化率和98%以上的乙烯选择性,其活性比氧化铝高得多。影响分子筛催化活性的因素有：硅铝比、原粉粒径和改性方法等。低硅分子筛的酸量较高而酸强度较低,适合乙醇脱水反应；水蒸气处理和酸处理等改性可调节催化剂的酸量,提高催化剂的活性。亚微米(100～1000 nm)晶粒分子筛能有效提高反应物和反应产物在催化剂中的扩散性能,与传统的分子筛,比如NKC-3分子筛催化剂相比,亚微米催化剂的微孔孔容可增加一半以上,其活性比NKC-3也明显提高。亚微米ZSM-5分子筛在乙醇浓度80 wt%、重时空速1.6 h-1和温度230℃下,乙烯选择性达到99%,乙醇转化率达到99%,表现出良好的低温活性。1000 h的长周期实验的升温幅度仅24℃,器外再生的活性几乎完全恢复,再生催化剂的在线1000 h升温幅度与新鲜催化剂相同。第二、催化剂的失活特性。乙醇脱水反应中,积炭是催化剂失活的主要原因。积炭可能堵塞分子筛孔道,降低催化剂表面酸性,因而降低反应物分子扩散性能及可用活性中心数量,导致催化剂失活。反应温度越高,积炭速率越大,且导致积炭的不饱和度增加,加速了积炭的演变。重时空速升高时,积炭速率降低,但乙醇转化率和乙烯选择性也降低；当重时空速过低时,积炭速率很高且催化剂活性下降很快。水能够降低积炭速率,并且降低积炭的不饱和度,有利于保持催化剂稳定性。较大的催化剂颗粒由于扩散路径较长其积炭速率也增加,而催化剂颗粒过小时也会导致积炭速率升高,催化剂粒径应在一个合适的范围内。本文开发了一种预积炭方法,能够有效地降低积炭速率,提高催化剂的活性和稳定性。该法将催化剂在较低温度进行一定时间的预积炭,将部分酸性过强的酸性中心进行积炭失活,从而降低催化剂正常操作时的积炭速率,延长了催化剂的单程使用寿命。第三、失活催化剂的再生特性。随再生温度升高,积炭烧除速率增加,但温度高于550℃时对催化剂损坏较大。采用程序升温方法的再生效果优于恒温,且程序升温终点温度为500℃时再生效果最好。再生时加入水蒸汽会导致再生催化剂的活性降低。器内再生的最佳条件为：升温至400℃,保持2 h,然后升温至500℃,保持约4 h,空气空速约为600 h-1,总再生时间约为8 h。催化剂在再生后微孔恢复89%以上,总酸量恢复约72%,再生后催化剂的活性与新鲜催化剂相近。第四、反应动力学和反应器模拟。在固定床积分反应器中研究了亚微米催化剂上乙醇脱水的动力学,结果表明乙醇脱水属于串联反应,符合L-H机理模型,表面反应为速率控制步骤,模型经过统计检验,高度适合。亚微米催化剂上乙醇脱水的动力学方程如下：根据动力学模型对乙醇脱水反应的管式反应器进行模拟,结果很好地反映了各操作参数的影响规律,与实验结果基本一致。换热介质温度对反应结果影响较大,换热介质温度应控制在280℃左右；重时空速提高虽然可以提高乙烯的时空产率,但是降低乙烯的选择性,增加副产物乙醚的选择性,因此应当选择合适的重时空速；乙醇浓度对反应结果的影响较小,但应采用含水乙醇以提高乙烯选择性；在管径为0.032 m时反应器长度应选择3米为宜。第五、中试研究。中试在处理乙醇量为10吨/年的固定床管式反应器中进行。催化剂装填量约为1.5 kg,原料为95%(v/v)乙醇,重时空速约为1.25 h-1。新鲜催化剂在中试装置上的乙醇转化率达到98%以上,乙烯选择性接近100%,单程使用时间大于2000 h。反应温度约为240-350℃。失活催化剂的原位再生约35 h。采用逐渐升高温度和提高含氧量的再生方法,积炭逐渐烧除,催化剂床层内未出现飞温现象。少部分积炭需要在500℃以上才能烧除。再生催化剂上乙醇转化率在96%以上,乙烯选择性在99%左右,在线使用2000 h时温升仅45℃。反应器中不同位置的催化剂失活情况不同。由进口向出口方向,反应物料中乙烯浓度越来越高。入口处催化剂积炭较轻,出口处催化剂积炭最严重,剩余孔容最小,晶系变化最大,脱铝情况最严重,剩余酸量也最小更多还原

【Abstract】 The bio-ethylene route is a green, renewable and sustainable alternative route. Ethanol dehydration to ethylene is a key step in this route. Catalyst deactivation, regeneration and kinetics in ethanol dehydration to ethylene over submicron ZSM-5 zeolite were studied in this work. A pilot-plant study was also carried out.The main research contents are as follows:1. Catalyst improvement. Currently, reported catalysts for ethanol dehydration include activated alumina, metal or transition metal oxide, heteropolyacid and zeolites.The activated alumina needed a higher reaction temperature and a lower space velocity. Ethanol completely converted at temperatures higher than 360℃with a weight hourly space velocity (WHSV) of 0.19 h-1. The selectivity of ethylene reached a maximum of 92.6% at 312℃. Zeolite catalysts showed an ethanol conversion of 98% and an ethylene selectivity of 98% at around 250℃with a WHSV of 1.6 h-1. It can be seen clearly that zeolite catalysts possess much higher activity than activated alumina.The influential factors of the zeolite activity include Si/Al ratio, crystalline size and modification methods. The zeolite with lower Si/Al ratio was more favorable to ethanol dehydration with a higher acid content and a lower acid intensity. Hydrothermal dealumination and acid treatment can improve the activity of zeolite by adjusting the catalyst acid content. The submicron (100-1000 nm) ZSM-5 is effective to increase the diffusion ability of reactants and products. In comparison with the conventional zeolite such as NKC-3, the micropore volume of the submicron zeolite catalyst increased more than 50%, thus the submicron catalyst has much higher activity.The submicron ZSM-5 catalyst exhibited good low-temperature activity. Under an ethanol concentration of 80wt.%, a WHSV of 1.6 h-1 and a temperature of 230℃, both ethanol conversion and ethylene selectivity reached 99%. The temperature rise in 1000 h long duration experiments over fresh catalyst was only 24℃. The activity was almost recovered completely by ex-situ regeneration and the temperature rise in 1000 h on line was equal to that of fresh catalyst.2. Catalyst deactivation behavior. During ethanol dehydration, the deposition of coke is the main cause of catalyst deactivation. The coke deposits may block the zeolite channel, decrease the surface acidity of the catalyst, reduce the diffusion ability and available active sites, and thus cause the catalyst deactivation.As the reaction temperature increasing, the coke deposition rate and the unsaturated degree of coke increased, and the evolution of coke was facilitated. As the WHSV increasing, the coke deposition rate decreased while ethanol conversion and ethylene selectivity decreased. At a very low WHSV, the coke deposition rate became relatively high and the activity of catalyst declined quickly. Water in feed could reduce the coke deposition rate, reduce the unsaturated degree, and thus improve the catalyst stability. Large size catalyst extended the diffusion length and caused an increase in coke deposition rate, while too small size catalyst also increased the coke deposition rate. There is a proper particle size.A precoking method was proposed in this work. This method could effectively reduce coke deposition and improve the active stability. The precoking method deactivated the strong acid sites by coking at a lower temperature, and thus reduced the coke deposition rate at normal operation and prolonged catalyst life.3. Regeneration behavior of deactivated catalyst. As the regeneration temperature increasing, the burning rate of coke deposits increased. The catalyst would suffer damage under temperatures higher than 550℃. Temperature programming regeneration would obtain a favorable effect rather than constant temperature regeneration. The best result was obtained at a final temperature of 500℃. The presence of water during regeneration could decrease the activity of regenerated catalyst.The optimal regeneration conditions are as follows:the temperature was raised to 400℃and maintained for 2 h, and then it was raised to 500℃and maintained for 4 h; the space velocity of air was about 600 h-1; the total regeneration time was about 8 h. The catalyst recovered more than 89% of micropore volume and 72% of surface acidity. The activity of regenerated catalyst was very close to that of fresh catalyst.4. Kinetics and reactor simulation. The kinetics of ethanol dehydration was studied in a integral fixed-bed reactor. The results showed that ethanol dehydration to ethylene and diethyl ether was a parallel reaction, and surface reaction was rate-determining step in this L-H mechanism model. The statistical tests showed that this model was of high reliability and validity.The kinetics of ethanol dehydration on the submicron zeolite catalyst is as follows: Based on the above kinetics, the simulation of a fixed-bed tubular reactor for ethanol dehydration was carried out. The simulation results exhibited the influences of operational parameters on ethanol dehydration, which were in accordance with experimental results. The temperature of heat-exchange medium illustrated major effects on the reaction results and it should be controlled around 280℃. An increase in WHSV could increase the space time yield of ethylene at the cost of the decrease of ethylene selectivity and the increase of diethyl ether selectivity. Consequently, an appropriate WHSV should be chosen. Although the concentration of ethanol has a little effect on the reaction results, hydrous ethanol should be employed to achieve higher ethylene selectivity. A length of 3 m should be chosen for aФ0.032 m reactor.5. Pilot plant study. The pilot plant study was carried out in a fixed-bed tubular reactor with an ethanol consumption of 10 t/a. The catalyst loaded was about 1.5 kg. The feed was 95% (v/v) ethanol and the WHSV was 1.25 h-1. Ethanol conversion was more than 98% and ethylene selectivity was close to 100% over fresh catalyst in the pilot plant. The fresh catalyst could be used more than 2000 h in the temperature range of 240-350℃in a single run.An in situ regeneration of deactivated catalyst took about 35 h. The coke was burnt off under stepwise rising temperature and oxygen content. No temperature runaway was observed during the regeneration process. A little part of coke needed to be burnt at temperatures higher than 500℃. Ethanol conversion and ethylene selectivity were more than 96% and about 99% over regenerated catalyst, respectively. The temperature rise over regenerated catalyst was only 45℃in 2000 h.The catalysts suffered varying degrees of deactivation along the reactor. Ethylene concentration in the product mixture increased along the tubular reactor. The catalyst at the forepart of the reactor suffered less coke deposition. The catalyst at the end of the reactor suffered severest deposition of coke, and it has the lowest pore volume, biggest syngony transformation, severest dealumination, and lowest acidity.更多还原