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Solikhova Ozoda, assistant professor, Tashkent Chemical Technological Institute, Tashkent, Uzbekistan Organic chemistry and technology of basic organic synthesis department, Tashkent Chemical Technological Institute, E-mail: [email protected]
USAGE OF a-ALUMINUM OXIDE IN THE PROCESS OF ACETYLENE HYDROGENATION IN ETHANE-ETHYLENE FRACTION IN THE STUDY OF PALLADIUM CATALYST
Abstract: The research shows the purification processes of ethane-ethylene fraction of pyrolysis gas from acetylene hydrocarbons, which are based on the reaction of catalytic alkynes hydrogenation. The hydrogenation reaction happens after the usage of metallic or oxide catalysts. A contact containing 0.04 wt.% of palladium on a-aluminum oxide with specific surface area of 9 m2/g was used as a catalyst to research the kinetics of the acetylene hydrogenation processes. The results show that palladium content in supported hydrogenation catalyst affects the selectivity, the total catalytic activity and the duration of the catalyst operation, with the optimum palladium content of 0.2-0.3%. The introduction of alkaline and alkaline-earth elements into the composition of the catalyst does not have a noticeable effect on its properties. The addition of sulfur compounds to the catalyst composition leads to a decrease in its activity during the acetylene hydrogenation, without affecting the activity in the ethylene hydrogenation.
Keywords: industrial catalyst, acetylene hydrogenation, ethane-ethylene fraction, hydrogen conversion, acetylene conversion, pilot plant.
Light olefins (ethylene and propylene) are still Among them, ethylene and propylene have greatest
mainly produced with pyrolysis. Pyrolysis plants value due to being used for the production of poly-
today are capable of producing 113 million tons/ mer materials. Due to the fact that the presence of
year of ethylene that is 100% of world production alkynes can lead to the breakage of polymer chain
and 38.6 million tons/year of propylene that is a bit and decrease in the molecular weight of a polymer,
more than 65% of world production. The distribu- the production of polyethylene, polypropylene of
tion of feedstock produced by pyrolysis is as follows: various grades and ethylene oxides, requires the
ethane - 27.6% (wt.), liquefied gases - 14.0% (wt.), feedstock to have a high degree of purity (not more
straight-run gasoline (naphtha) - 53.1% (wt.), hy- than 10 ppm of acetyl). The ethane-ethylene frac-
drotreated kerosene-gasoil fractions - 5.3% (wt.). tion is subjected to catalytic hydrogenation to pu-
Separation of hydrocarbon stream into fractions: rify it from the acetylene impurity. Active palladium
methane-hydrogen, ethane-ethylene, propane- catalysts based on y-aluminum oxide, active carbon
propylene, etc. occurs at cooling at temperature (MA-15, PU-2, KhPU-1, G-58I) are the most com-
of 110-130 °C and pressure from 0.5 to 5.0 MPa. monly used for this process. Today the search for
USAGE OF a-ALUMINUM OXIDE IN THE PROCESS OF ACETYLENE HYDROGENATION IN ETHANE-ETHYLENE...
a catalyst comparable with known and widely used industrial catalysts in activity, but with a higher mechanical strength and selectivity, is important.
The ethane-ethylene fraction is subjected to catalytic hydrogenation to remove the acetylene impurity. The purification process of ethane-ethylene fraction (EEF) of pyrolysis gas from the admixture of acetylene hydrocarbons is based on the reaction of catalytic hydrogenation of alkynes. The hydrogenation reaction takes place in the presence of metallic or oxide catalysts. To research the kinetics of the acetylene hydrogenation process, a contact containing 0.04 wt.% of palladium on a-aluminum oxide with a specific surface area of 9 m2/g was used as a catalyst. Tests of the finished catalyst were carried out at temperatures of 40-60 °C and pressure of 2.3-2.5 MPa and a molar ratio of hydrogen and acetylene of 1.6-1.9. The selectivity of acetylene hydrogenation on this catalyst was 90%.
Experiment: an important stage in development and implementation of a new catalyst is the test of its main characteristics in conditions that are as close as possible to the industrial. This is necessary to predict the stability of its operational properties, such as activity, selectivity and performance life, which is of particular importance in the case of the use of expensive metal - palladium.
An estimation of activity and selectivity of test samples and their comparison with industrial catalysts during tests in four-level reactors (laboratory, pilot, experimental and industrial) lead to the completion of our task. Comparative tests of test sample and industrial catalyst G-58I on laboratory, pilot and experimental plants were carried out using a real industrial gas mixture - ethane-ethylene fraction of the composition: ethylene, approximately 55%, ethane 43%, acetylene 0.3%, hydrogen - 0.9%, the rest were methane, propylene, propane, carbon monoxide, etc. In the contact gas, the acetylene content should not exceed 10 ppm. We performed comparative tests on 32 test samples and industrial catalyst G-58I: at minimum temperatures of the acetylene hydrogena-
tion process, at which its complete destruction occurs (not higher than 1 ppm), hydrogen conversion and selectivity corresponding to these temperatures, the formation of a green oil as a result of the course of the ad hoc reactions of oligomerization of ethylene and acetylene at the acid sites of the inner surface of the carrier.
The results obtained on the experimental samples confirm the presence of two types of active centers on the inner surface of the given palladium catalyst, one of which is responsible for the acetylene hydrogenation and is present on a fresh catalyst, the second for the ethylene hydrogenation, its percentage on the surface increases during operation, which means that the selectivity of the hydrogenation process decreases. As a result of the treatment of G-58I catalyst with hydrogen sulphide, part of the active centers responsible for ethylene hydrogenation and acetylene does occur, the overall activity of the catalyst decreases. The rate of the hydrogenation reaction of ethylene increases with increasing content of palladium in the catalyst. As in the case of acetylene hydrogenation, an increase in the rate of ethylene hydrogenation with an increase in the palladium content from 0.03 to 0.1 wt.% can be observed. It should be noted that at the same value of the palladium content in the test sample and the industrial catalyst G-58I (0.5 wt.%), the hydrogenation rate of ethylene at the latter is greater, and the hydrogenation rate of acetylene is, on the contrary, less. Consequently, the selectivity of a sample with a palladium content of 0.5% during the acetylene hydrogenation in the ethane-ethylene stream is higher than the selectivity of G-58I.
Results and discussion: comparative tests of test samples and industrial catalyst G-58I on the laboratory, pilot and experimental plants were carried out using a real industrial gas mixture, where ethane-ethylene fraction of the composition is as follows: ethylene, approximately 55%, ethane - 43%, acetylene - 0.3%, hydrogen - 0.9%, the rest are methane, propylene, propane, carbon monoxide, etc. In the contact gas, the acetylene content should not exceed
10 ppm. Kinetic studies of the process of acetylene hydrogenation and ethylene were carried out in a laboratory plant. The temperature dependences of the rate constants of the hydrogenation reactions of acetylene and ethylene for test samples and industrial catalyst G-58I were presented. From the results of comparative tests on a pilot plant and kinetic studies on a laboratory installation, one can conclude that:
- reaction of acetylene hydrogenation is of the first order in acetylene, zero in hydrogen;
- reaction of ethylene hydrogenation - first order in hydrogen and zero in ethylene;
- the rate of hydrogenation reactions of acetylene and ethylene depends on the content of palladium in the catalyst; for test samples it was found that the optimal content is 0.2-0.3 wt.%, it has almost no effect on the reaction rate after 0.5 wt.%;
- additions of sodium sulfide and sodium formate reduces the speed of acetylene hydrogenation reaction, practically without affecting the rate of ethylene hydrogenation, which explains the decrease in the selectivity of the "seeded" test samples;
- activity of test samples in the acetylene hydrogenation prepared by the ethanolamine method is higher than in other samples and industrial catalyst G-58I;
- the best of the brands used for the preparation of test samples of the copper support, was a fresh cir-
cular one with a specific surface area up to 3.0 m2/g and a bulk density of790 kg/m3;
- no formation of green oil was noted on the experimental samples prepared by the ethanolamine method.
Conclusion: We can conclude that acetylene hydrogenation on palladium catalyst has the first order in acetylene and zero order for hydrogen; the order of the reaction of ethylene hydrogenation by hydrogen is equal to one, and by ethylene - to zero. The reaction takes place in the kinetic region at temperatures of 50-70 °C. The characteristics of the corundum carrier are determined by the performance life of the catalyst, its selectivity, the course of side processes (oligomerization of acetylene and ethylene with the formation of a green oil), its mechanical strength. The results show that palladium content in supported hydrogenation catalyst affects the selectivity, the total catalytic activity and the duration of the catalyst operation, with the optimum palladium content of 0.2-0.3%. The introduction of alkaline and alkaline-earth elements into the composition of the catalyst does not have a noticeable effect on its properties. The addition of sulfur compounds to the catalyst composition leads to a decrease in its activity during the acetylene hydrogenation, without affecting the activity in the ethylene hydrogenation.
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