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Fayzullayev N. I., Samarkand State University Rakhmatov Sh.B., Bukhara medical institute E-mail: [email protected]
KINETICS AND MECHANISM OF THE REACTION OF THE CATALYTIC OXYCONDENSATION REACTION OF METHANE
Abstract. Internal diffusion retardation in oxycondensation process of methane was analyzed. Effect of diferent factors to the reaction rate of preparation ethylene from methane was studied. Based on experimental results the reaction mechanism for the reaction of ethylene formation from methane was proposed and kinetic equation characterizing the whole process was chosen and its adequacy was tested.
Keywords: methane, ethyele, oxycondensation, mechanism, volumetric rate, kinectic equation, diffusion.
Introduction
Detected reserve of the natural gas deposit of the Republic of Uzbekistan is almost 2 trillion m3, and the oil is 350 million tons. Currently 31-32% of oil deposits have been mined in the Republic. Only 0.5-2.0% of methane has been processed chemically worldwide. This puts the task to increase products based on methane from the natural gas and to broaden researches focused on researched synthesizing important materials for the national economy. Saving materials and heat-energetic resources is one of the important tasks of modern technique. Development of the energy saving technology is one of the main directions of chemical technology and scientific technical progress. In many chemical processes in industry energy loss is main part of the total extravagance. Moreover development of technology using heat resources (oil, coal, natural gas) are raw materials and energy sources in many chemical syntheses simultaneously is important
[1-3].
The only correct way of perspective processing the natural gas is oxycondensation reaction, and the process takes place in one step and at the normal atmospheric pressure.
More than 30 year passed since the oxycondensation reaction of methane was discovered, but stable catalyst with high activity and yield was not developed so far, thus the reaction has not been applied in industry. For this reason development of catalyst and optimization ofworking parameters of apparatus for the preparation planned products with maximal yield is important [4-10].
Experimental section
Based on the analysis of reactants and products, the following parameters of the process were determined:
1. Hydrocarbon conversion:
XYB -
Y'
■ n + c
YB
2. Oxygen conversion:
X =
(C
- CI, • K,
C 0
•100%
•100%
3. Selectivity relative to the reaction products:
CMaxc. nc
S; = =i-i--100%
1 ^ ^ CMaxc
4. Yield of the reaction:
CMaxc ■ n
n;
Y =
I
c
■ nc + CY
YB
■100%
where CyB - concentration of respective hydrocarbon from the reactor (mol.%); QMaxc - concentrate of the product I from the reactor (mol.%); nC -number of carbon atoms in the product molecule; CO - concentration of oxygen in initial mixture
N0
(mol%); K = —- - coefficient related to change
N N2
of net volume during the reaction, N2 - concentration of nitrogen input of the reactor; N'2 - concentration of nitrogen from the reactor.
The rate of the reaction according to individual components (reactants and products) in the mixture based on the their concentration in mixture was calculated suing the formula below.
W = v (C >0 - C )/v
v - catalyst volume; W.-rate of the depletion of reactant-i or formation of product-. (mol-s-1-cm-3); V - sum rate of stream of reaction mixture (normal conditions cm3/sec); Ci 0 and Ci - initial and final concentrations of components respectively.
Oxycondensation reaction of methane was studied in stream differential reactor in laboratory conditions. The reactor is made of quarts with the length 650 mm and inner diameter 8 mm. catalyst dimension is 0.25-0.5 mm. Methane: oxygen in 2^7-1volumetric ratio was sent to the reactor at the rate 1 ^ 15 l/hour. The temperature was changed from 700 to 850 °C. oxycondensation reaction of methane was carried out in stream differential reactor at the normal atmospheric pressure. Gas products of the reaction was analyzed using GC.
Catalyst with the composition Metan-ni oksikondensasiyalash reaksiyasi uchun (Mn2O3)x • (Na2MoO4) y. (ZrO2)z for the oxycon-densation reaction of methane was prepared using the sol-gel method. Phase content was carried out on DRON-4.0 (CuK a - beam) diffractometer, and particle size was determined on the scanning electron microscope ((JSM-6380 LV) and the transmission electron microscope (EMV-100BR).
Effect of internal diffusion retardation in the oxycondensation of methane was analyzed using the method proposed by Wagner:
(1)
Fs =
dj • z
4 • Dff •c
< 1
where F_ - Tile modulus; D „ - effective diffusion
S eff
coefficient of oxygen to the inner shell of catalyst, m2/s; dz - diameter of the catalyst particle, m; g - maximum rate of the reaction, mol/mol-s; c - re-actant concentration, mol/mol. The temperature difference in the interior and exterior of the catalyst granule effect on the rate of reaction was defined using the following equation;
dz - r-aH RT
—-<- (2)
4-T Ea W
where ah - reaction enthalpy, J/mol; \at - heat conductivity of the catalyst particle, Vt(mk); T - temperature, K; Ea - activation energy, J/mol; R - Universal gas constant, J/(mol-K). In order to evaluate effect of external diffusion retardation on the surface of the catalyst the criterion was applied, it resembles the rate of reaction on the surface of catalyst relation to diffusion rate of substances from gas or liquid phase. If the rate of reactants in the stream differs less than 5% from the concentration on the surface, the rate of reaction does not differ more than 5% from the kinetic rate, thus the non-equality fulfills;
„ j
< 0.15 yok (3)
r • d.
2 ß c dz • r -AH 2-a-T
< 0.15:
RT
(4)
where (3 - mass exchange coefficient, m/s. a - heat exchange coefficient, Vt/ (m2-K). Calculations show that FS = 0.35, the left side of equation (2) 1.6 • 10-3, the right side 0.045; the left side of nonequality (3) 0.03; the left side of nonequality (4) 2.6 • 10-3, the right side less than 6.7 • 10-3, thus oxycondensation reaction of methane in a kinetic range.
a
Results and discussion
In order to study the kinetic law of oxyconden-sation reaction of methane effect of partial pressure of methane and oxygen on the rate of formation of ethylene at the temperature 700 - 800 °C and volumetric rate 600 - 1200 hour-1 was studied.
Effect of partial pressures of reactants on transition the law the partial pressure of one gas was changed while the other gas partial pressure was kept
Table 1. - Effect the partial pressure rates and temperatures (Ptotal =
constants. In order to keep the linear rate required amount of argon gas was sent to the reactor. The volume of catalyst was tuned to the experiment conditions to keep the comparative rate constant.
Keeping the linear rate of gas stream at different values of the temperature and volumetric rate, effect of the partial pressure of methane on the oxyconden-sation process is given in (Table 1).
of methane on different volumetric 0.1 MPa, P = 0.014 MPa)
' oxygen '
Volumetric rate of methane, ml/ ml.cat.hour Partial pressure of methane, MPa Conversion degree of methane to ethylene,% Selectivity relative to ethylene S,%
T=700 °C T=750°C T=800°C T=700 °C T=750°C T=800°C
600 0.017 8.8 14.3 20.6 6.7 8.4 10.6
800 0.017 7.6 11.5 17.4 5.6 7.2 8.8
1000 0.017 6.7 9.1 13.5 5.2 6.4 7.6
1200 0.017 5.8 8.5 9.7 4.8 5.8 6.5
600 0.025 18.4 20.8 2405 7.2 9.5 12.8
800 0.025 15.2 17.6 21.0 6.4 8.2 11.3
1000 0.025 13.0 15.2 18.6 5.6 6.4 10.0
1200 0.025 10.6 12.8 14.4 4.8 5.0 8.1
600 0.033 25.8 27.4 28.6 23.2 34.6 41.8
800 0.033 24.6 30.2 35.8 41.9 54.5 57.8
1000 0.033 23.4 33.2 42.8 64.3 72.8 81.4
1200 0.033 20.8 28.5 39.2 48.8 62.5 75.9
As shown on the table, increase in the partial pressure of methane at different comparative volumetric rates and temperatures, decreases total conversion.
Effect of the partial pressure of oxygen on the kinetic law of the oxycondensation reaction at
the temperature 700 - 800 °C and volumetric rate 600 - 1200 hour-1 was studied. The partial pressure of oxygen changed from 0.014 MPa to 0.01 MPa, while the partial pressure of methane kept constant (0.033 MPa). Results are given in (Table 2).
Table 2.- Effect the partial pressure of oxygen on different volumetric rates and temperatures (Pttl = 0.1 MPa, P th = 0.033 MPa)
^ x total ' methane '
Volumetric rate of methane ml/ ml.cat.hour Partial pressure of oxygen, MPa Conversion degree of methane to ethylene,% Selectivity relative to ethylene S,%
T=700 °C T=750°C T=800 °C T=700 °C T=750°C T=800°C
1 2 3 4 5 6 7 8
600 0.010 17.2 20.8 24.6 9.8 12.3 14.5
800 0.010 14.8 18.4 21.2 8.2 10.0 11.3
1000 0.010 10.4 14.0 17.8 6.4 8.5 9.8
1200 0.010 7.7 11.2 15.5 4.9 6.0 7.5
1 2 3 4 5 6 7 8
600 0.012 20.5 23.8 27.2 15.0 20.4 24.8
800 0.012 18.2 20.0 24.5 13.2 17.5 19.9
1000 0.012 15.8 17.4 21.1 10.7 14.8 16.2
1200 0.012 13.9 16.2 18.7 8.5 11.2 13.8
600 0.014 25.8 27.4 28.6 23.2 34.6 41.8
800 0.014 24.6 30.2 35.8 41.9 54.5 57.8
1000 0.014 23.4 33.2 42.8 64.3 72.8 81.4
1200 0.014 20.8 28.5 39.2 48.8 62.5 75.9
As shown in the table, decrease in the partial pressure of oxygen also decreases the total conversion of methane, and yield of formation and selectivity as well.
Results of kinetic studies show that increase of contact time and elevated temperature the process parameters improve.
Increase in the ration of CH4 : O2 decreases the conversion of methane and oxygen. Input of ethane, ethylene, CO and CO2 also decreases the formation of products. nisbatining ortishi metan va kislorod konversiyasining pasayishiga olib keladi. Increase in the contact time increases conversion of oxygen and methane, and decreases the selectivity relative to ethylene. Elevation of temperature increases conversion of methane, and decreases the selectivity relative to ethane and ethylene. Increase in the methane: oxygen ratio increases selectivity to ethane and selectivity to ethylene stays unchanged.
Increase in the temperature increases the conversion of oxygen. At 700 °C conversion of oxygen reaches 95% for 0.9 sec, at 1000 °C contact time is 0.009 sec.
Increase in the conversion of oxygen decreases the selectivity of the process. Increase in temperature up to 700-800 °C, the selectivity of the process decreases, and later the selectivity increases at 850-9650 °C.
Oxycondensation reaction of methane takes place in homogeneous-heterogeneous mechanism. Activation of methane takes place on the surface of hard oxide catalyst. On the active site of the catalyst one hydrogen atom evolves from methane and methyl radical forms.
Ethylene formation from oxycondensation of methane is one step process and takes place at normal atmospheric pressure. The process can be described using the following (schemes 1):
Schemes 1. Methane oxycondensation reaction mechanism to get aimed products
0/ - Normal place of O in crystal lattice
Schemes 2. Reaction path of oxycondensation reaction of methane
4CH4 + * 2C2H6 + 2H2O W1 = 02
26
2 4
22
2CH4 + 02 * C2H4 + 2H20 W3 = k3 P 02 2C2H6 + 502 * 4C0 + 6H20 W4 = k4 P C2H6 * P O2
2CH + 70 * 4C0 + 6H 0 w = k • PC
CH + 20 * 2C0 + 2H 0 w = k • P
2 4 2 2 6 6
• P
c2h4 P o2
C2H4 + 302 * 2C02 + 2H20 W7 = k7 • P
• P
c2h4 P o2
Ethylene formation from ethane takes place the
temperature higher that 700 °C on the active site of
r b 2LH + O * 2LH, + 2HO w = k P CH • P,
.1.1 . r 2 6 2 7 4 7 7 2 '
the catalyst surface.
CH4 * [CH3] ads + 2[H]ads 2 [CHH3]ads* [C2H6]ads + ads
[qHJs + [o]s ^[C2H 5 ]s + [oh-]s
[C2H5]S + [O]S * [C2H4] + [OH"]S [C2H 5 ]S + [O2]S * [C2H4] + [OH']S [C2H6]S + [O2]S *[C2H4]S + 2[OH"]S
[c2h 5 ] + CH4 * c2h6 + ch 3
[C2H "]s + [oJs * [c2h4] + [oh']s
[C2H6]S + [O2]S *[C2H4]S + [H2O] [C2H6] ads * [C2H4] ads + H2
In presence of optimal catalyst with the content (Mn2O3) x • (Na2MoO4) y • (ZrO2) z the kinetic laws of oxycondensation process of methane were studied partial pressures of reactants and at different temperatures at the differential reactor conditions, and the kinetic model of the process was developed. Oxycondensation of methane can be described using the following equations:
P0
Kinetic studies and calculations give the following kinetic parameters:
lgk1 = 24,56-18020/T; lgk2 = 10,0-9997/T; lgk3 =38,72-34073/T; lgk4 = 13,48-2855/T; lgk5 = 18,21-6104/T; lgk6 = 13,01-10904/T; lgk7 = 12,87-10114/T; where Т - temperature, K.
Ethylene formation from oxycondensation of methane is one step process and takes place at normal atmospheric pressure. The process can be described using the following reactions:
2CH4 + 1/2O2 ■ C2H6 + H2O ДИ = -176.8 kJ/mol 2C2H6 + O2 ■ 2C2H4 + 2H2O ДИ = -105.5 kJ/mol 2CH + O ■ C H + 2HO ДH = -277.51 kJ/mol
4 2 2 4 2 J
C2H4 + O2 ■ 2CO + 2H2 ДH = -451.1 kJ/mol C2H4 + 3O2 ■ 2CO2 + 2H2O ДH = -1329.7 kJ/mol C2H6 + 7/2O2 ■ 2C02 + 3H2O ДH = -1490.2 kJ/mol CH4 + 2O2 ■ CO2 + 2H2O ДH = -803.9 kJ/mol CO2 + H2 ■ CO + H2O ДH = +412 kJ/mol
The process takes place with the formation of ethane and its dehydogenation forms ethylene. Taking into account all products, the following net reaction can be writtem:
400CH + 259O2 ■ 90C2H6 + 70CH + 64CO2 +
4 2 2 6 2 4 2
+ 374H2O +16H2 + 16CO
ЛЯ8оо»с =-514 kJ/mol. The catalytic oxycondensation reaction of methane takes place at high temperature and the condensation can be described using the kinetic equation Lang-muir-Hinshelwood mechanism. Taking into account abovementioned, the sorbtion process obeys to Lang-muir isotherm, the kinetic equation for the catalytic oxycondensation reaction of methane the following
Langmuir-Hinshelwood equations porposed:
k ■ K ■ P ■ K ■ P
W = сн4 сн4 ro2 (1)
W = ■
(1 + KCHi • PCH4 + K02 . P0
k ■ K ■ P ■ K ■ P
л ^ch4 ±CH4 ^vo2 ±O2
(1 + K
w =
P + K ■ P
CH 4 ±CHt1 vo2 ±o2
k ■ K ■ P ■ K ■ P
л ch4 ±ch4 ^vo2 -lO2
KCH. ■ PCH. + K02 ■ PO2
(2)
(3)
Thus, solutions of kinetic equations are determined using the rate constant k of the reaction, and adsorption coefficient of oxygen and methane (KOj, KCH^),
partial pressures of oxygen and methane (KOj, KCH^),
and experimental rate value (W). Checking the adequacy of equations were carried out based on the mean quadratic deviation (s) between the difference of experimental and theoretical results.
Based on given equations and experimental results, it was determined the paramaters of kinetic equation sum of quadratic deviation of experimental from the theoretically calculated value differs minimally. Basis of the adequacy of kinetic equation the fulfillment of the following condition was taken:
y(w r - w
/ j v amaia na
min.
i=1
So, adequate kinetic equation for the oxycondensation reaction of methane in presence of (Mn2O3)x •
• (Na2MoO4)y • (ZrO2^ (М^ОзХ- (KCl)y • (ZK^)
catalyst at differential reactor conditions were proposed and its adequacy was evaluated.
W = -
k ■ K ■ P
л CH . ±CH.
K02 ■ P02
(1 + K
p + k ■ P
CH. ±ch41^O2 ±O2
It is eesential to test the adequacy of chosen eqa-utions (1-3) of experimental kinetic laws taken of differential reactor conditions. Taking into account of given equations and results of experiments, parameters of kinetic equations sum of quadratic deviation of experimental results must be minimally differ from the theoretically calculated results. Because of nonlinearity of proposed equations, their solution could be unlimited in mathematic way. In kinetics, solutions to equations are determined testing the adequacy for the range of the change of kinetic parameters based on the results of experiments.
Based on constants of kinetic eqautions determined at different temperatures, the activation energy of the process (Е = 33.8 kJ/mol) was calculated.
During the derivation of the kinetic equation of the oxycondensation reaction of methane limiting step was the rate of adsorption of oxygen and methane on the catalyst surface. The adsorption process on the catalyst surface is monomolecular, since oxygen and methane are adsorbed individually at active sites.
Conclusions
1. The internal diffusion retardation at the oxycondensation process of methane was analyzed.
2. Effect of different factors on the raction of ethylene formation from methane was studied.
3. The mechanis of ethylene formation from methane was proposed based on obtained results.
4. Kinetic equation characterizing the whole process was chosen and its adequacy was tested.
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