【作者】 丁雪；

【导师】 杨朝合；

【作者基本信息】 中国石油大学 ， 化学工程与技术， 2010， 博士

【摘要】 催化裂化(FCC)是石油炼化的核心工艺过程,干气是该过程的副产物,其中含有大量的乙烯。随着我国炼油企业的不断发展,干气产量也在大幅度提高。目前我国催化裂化装置加工能力已达130Mt/a左右,干气产量为5.2Mt/a,其中含有乙烯约1.0Mt/a。然而由于分离困难,我国多数炼油企业通常将干气用作燃料烧掉,造成了乙烯资源的浪费;或者用干气中乙烯与苯进行合成制取乙苯,但也会受限于苯的供应。伴随着原油资源的日益紧缺,如果能通过简单有效的途径将干气资源加以利用,变废为宝,则可以弥补化工原料的不足,为催化裂化装置带来新的效益。目前,除了催化裂化干气生产乙苯以外,干气中乙烯的回收利用技术还有深冷分离技术、变压吸附技术、中冷油吸收技术、ARS回收技术等。由于我国炼厂规模普遍偏小,涉及到成本问题,这些技术的应用并不广泛。要实现催化裂化干气中乙烯的有效利用,迫切需要开发易于实现的粗乙烯应用技术。由此设想将催化裂化干气中乙烯通过低聚反应生成液化气(LPG,即C3~C4)成分,实现以廉价资源生产高附加值产品。本文在固定床微反和流化床微反装置上,进行了催化裂化脱硫后干气中乙烯的低聚研究。结果表明该反应路线可行,并且发现在干气中乙烯低聚的同时伴随着氢转移、裂化、异构化和芳构化反应,共同构成复杂的反应网络。选择合适的反应条件可以控制原料的转化程度,改善产物分布,从而满足不同的生产目的。与二聚催化剂、歧化催化剂的反应结果的对比表明,酸性分子筛催化剂的反应活性和实用性最好。催化剂要含有Br?nsted酸(即质子酸,简称为B酸),同时要具有合适的孔道结构,才能具有较好的活性和稳定性。HZSM-5分子筛是乙烯低聚反应合适的催化剂的活性组分,且质量含量以20~30%为宜,高岭土是适宜的载体。催化剂酸性越强,乙烯转化率越高,丙烯、丁烯收率越低,使用含30%HZSM-5的催化剂,当酸密度大于0.14mmolNH3/g时,丙烯、丁烯收率显著减少。调节催化剂的酸性可以改善产物分布,例如使用低温离子交换催化剂、提高催化剂Si/Al比、在催化剂制备过程中引入适量P、金属或者对其进行水热处理均可以降低催化剂酸量,提高烯烃产物的收率。低温离子交换催化剂的稳定性非常差。在不同Si/Al比催化剂上的反应表明,乙烯双分子反应不受分子筛酸位分布密度的影响,而丙烯和丁烯的氢转移反应受到影响;乙烯二聚存在Eley-Rideal机理反应过程,而丙烯和丁烯的氢转移反应遵循Langmuir-Hinshelwood机理。在多种含金属的催化剂中,MgZSM-5催化剂上的丙烯收率和烯烃收率(丙烯丁烯收率之和)最高,可达12.21%和17.28%。温度对干气中乙烯低聚的影响非常显著。若以提高乙烯转化率和LPG收率为目的时,应该选择活性较高的新鲜催化剂,在400oC温度下即可,但是由于烯烃发生二次反应,所得产物中烷烃较多。提高压力和降低空速都有利于低聚反应,但由于新鲜催化剂容易积碳,反应条件不宜太苛刻。在温度为400oC,压力为0.1MPa,干气流量100mL/min和催化剂装填量0.5g的条件下,乙烯转化率为82.28%,LPG收率可达34.46%。若以丙烯为目的产物,最好使用水热处理催化剂,选择适当的空速和较低压力,在C4+组分能够裂化的高温下进行反应,同时也有利于抑制氢转移反应,另外添加适量稀释气也是增加丙烯收率的有效方法。在温度为550oC,压力为0.3MPa,干气流量100mL/min,氮气/干气比为1.0,催化剂装填量1g的条件下,乙烯转化率为49.79%,LPG收率为27.33%,丙烯收率为14.47%。经计算得知,新鲜催化剂和水热处理催化剂上乙烯低聚反应的级数分别为2.50和2.62。由于催化剂积碳等问题,流化床反应器在工业应用中具有更好的可行性,为此,在自行设计的流化床反应器上进行实验研究,结果表明,在较低温度下就能得到与固定床反应相当的结果。使用水热处理催化剂,在温度为500oC,常压,催化剂装填量为10g,干气流量为400mL/min时,乙烯转化率为47.22%,LPG、烯烃和丙烯收率分别为30.80%、21.21%和14.32%。为了了解干气组分在反应时的吸附情况,使用巨正则蒙特卡罗方法(GCMC)模拟了干气组分在ZSM-5分子筛中的吸附行为。经计算得知,在室温下,乙烯比丙烯更容易吸附,而在反应温度下,乙烯吸附量降至比丙烯还低。相比干气中更大的分子,高温更不利于乙烯的吸附。当干气组分在ZSM-5分子筛上共同吸附时,小分子吸附位置不具有明显的选择性,大分子则趋于集中吸附在孔道交叉处。C4+组分吸附能力较强,会优先抢夺催化剂孔道空间和吸附位,影响乙烯的吸附和反应。综上所述,以催化裂化干气为原料,通过选择适宜的催化剂和操作条件,可以将催化裂化干气中乙烯直接转化为LPG成分,从而实现废气资源化,本研究为催化裂化干气有效利用提供了一条新途径。更多还原

【Abstract】 As the core of refining technology, Fluidized Catalytic Cracking (FCC) plays an important role in the deep conversion of crude oil; dry gas, which contains plenty of ethylene, is the byproduct of this process. Currently, the total FCCU productivity in China is 130Mt/a with dry gas of 5.2Mt/a containing ethylene of approximately 1.0Mt/a. With rapid expansion of refining capacity, the yield of FCC dry gas tends to increase.However, most dry gas is burnt out as fuel in China causing atmospheric pollution and wasting of ethylene resource or used to make ethylbenzene which is usually limited by short supply of benzene. From the viewpoint of energy conservation and emission reduction, it is meaningful to take full advantage of FCC dry gas resource to cover the shortage of chemical materials and make it a new economic growth point of FCCU. Therefore, how to deal with the ethylene in FCC dry gas scientifically will become a topic of high concern.Besides producting ethylbenzene, present recovery technologies of ethylene from dry gas are cryogenic separation, adsorption separation, ARS method and so on. These technologies have been used and made profits in some refineries. However, most refineries in China are in small scale, in consideration of production cost, these technologies have narrow application prospects or low economic benefits. Therefore, other route should be found to make effective use of ethylene in FCC dry gas. The direct oligomerization of ethylene in FCC dry gas to Liquefied Petrolem Gas (LPG) hydrocarbons would be a feasible route.In this work, the oligomerization of ethylene in FCC dry gas (after desulfuration) on fixed bed reactor and fluidized bed reactor was investigated. The results showed that this reaction was feasible; moreover, the oligomerization, hydrogen transfer, cracking, isomerization and aromatization existed togother. Through choosing modest catalysts or reaction condition, the improvement in ethylene conversion and product distribution could be obtained.Compared with dimerization catalysts and metathesis catalysts, proton-exchanged zeolite catalysts showed the best activity and practicality. Br?nsted acid sites (B acid sites for short) and modest pore property were necessary. The proper active component and supporter were HZSM-5 zeolite (20~30wt%) and kaolin, respectively.With the increase in catalyst acidity, the ethylene conversion could be increased, while propylene and butylene yields were decreased when acid density exceeded 0.14mmolNH3/g over 30%HZSM-5 catalyst. Using catalyst ion-exchanged at low temperature, increasing Si/Al ratio, adding phosphorus or metal in catalyst and using steam treated catalyst could decrease the catalyst acidity and enhance propylene and butylene yields. The dimerization of ethylene was not restrained by sparse distribution of acid sites, and it proceeded containing Eley-Rideal mechanism; while the hydrogen transfer reaction of propylene and butylene was suppressed when acid density was low, and it followed the Langmuir-Hinshelwood mechanism. Among all MZSM-5 catalysts investigated, MgZSM-5 gave the highest propylene yield and olefin yield (sum of propylene and butylene), 12.21% and 17.28% respectively.The effect of temperature on ethylene oligomerization was remarkable. When ethylene conversion and LPG yield was the goal, fresh catalyst should be selected, and reaction temperature should be set 400oC, but more paraffin in LPG was formed because of the secondary reaction of olefin. Increasing pressure or decreasing space velocity favored the oligomerization of ethylene, but reaction condition could not be severe concerning about carbon deposition. Ethylene conversion and LPG yield could reach 82.28% and 34.46% under the condition of 400oC, 0.1MPa, and space velocity of 18h-1. When propylene was the desired product, steam treated catalyst, modest space velocity and pressure should be selected, and temperature should be increased to when C4+ could be cracked. Adding dilute gas was another approach to enhance propylene yield. Ethylene conversion, LPG and propylene yield could reach 49.79%, 2.33% and 14.47% respectively under the condition of 550oC, 0.3MPa, space velocity of 18h-1 and nitrogen/dry gas (volume ratio) of 1.0. The reaction order of ethylene over fresh catalyst and steam treated catalyst are 2.50 and 2.62, respectively.Carbon deposition restricts practical application of fixed bed reactor and makes fluidized bed reactor more desirable. Similar product distribution could be obtained in fluidized bed reactor at relatively lower temperature compared with fixed bed reactor. Ethylene conversion could reach 47.22% over steam-treated catalyst under the condition of 500oC, one atmosphere, catalyst weight of 10g and flow rate of 400mL/min, the LPG, olefin and propylene yield were 30.80%, 21.21% and 14.32%, respectively.To understand the adsorption properties of FCC components over ZSM-5 zeolite, the simulation calculation was carried out using Grand Canonical Monte Carlo method. Ethylene adsorbed on ZSM-5 more easily than propylene at room temperature, while at reaction temperature, the adsorption of ethylene was inhibited dramatically. The adsorption site of small molecules has no selectivity, while C4+ molecules tend to adsorb at the crossing of channel and occupy pore space and adsorption sites, thereby affecting the adsorption and reaction of ethylene.To sum up, the ethylene in FCC dry gas could oligomerize to LPG component on proper catalyst under modest condition. It is anticipated that this research will pave the way for the future research in the use of ethylene in FCC dry gas.更多还原