Ethanol Dehydration To Ethylene Over Submicron ZSM-5 Zeolite: Study On Deactivation, Regeneration And Kinetics

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.