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16KINETIC FEATURES OF NICKEL OXIDE REDUCTION BY METHANE AT NON ISOTHERMAL CONDITIONS
S. Mamyan", H. A. Chatilyan*0, and S. L. Kharatyan"0
aYerevan State University, Yerevan, 0025 Armenia
bA.B. Nalbandyan Institute of Chemical Physics NAS RA, Yerevan, 0014 Armenia *e-mail: hakob@ichph.sci.am
DOI: 10.24411/9999-0014A-2019-10093
In the work the results on the kinetics of NiO reduction by methane at non isothermal conditions based on in situ measurements of reaction rate, as well as on periodic measurements of sample weight loss during reduction are presented.
The experiments were carried out on high-speed scanning electrotermography setup [1, 2] at linear heating regime by using Ni wires 100 ^m in diameter (NP-2 trademark, purity of 99.5%). The latters were preliminary oxidized in air (P = 1 atm., T = 1300oC, t = 150 s) with formation of NiO layer on metal surface with a thickness of ~ 6 ^m. Then the oxidized wires were reduced by methane at T = 800^1050°C and Pch4 = 5^300 torr; heating rates were varied between 1 to 1000°/s.
During the process the sample temperature, T, its electrical resistance, R(t), and total electrical power applied, W(t), for maintenance of linear heating regime with a given rate from T0 = 800°C up to reference temperature, 1050oC were continuously measured. The rate of heat release (in this particular case, heat absorption) due to NiO reduction by methane, Wr(t) was determined on the basis of non-stationary equation of thermal balance of metal wire with the environment as a difference of electrical powers, released on the wire at the first - reactive, W1(t), and the second - inert (after completion of the reduction reaction), Wi(t), heatings with the same temperature-time history: Wr(t) = W1(t) - W2(t). In Fig. 1 the reaction rate, Wr(t) (1), and conversion degree, a, vs. time (solid line (2)) at linear heating (3) with Vh = 500o/s, are presented, where
a(t ) = Q(' )
t t
J Wr (t)dt J[Wl(t) - W2(t)]dt
0
Q
tot
0
J Wr (t)dt \[Wi(t) - W2(t)]dt
0
0
The points indicate conversion degree determined by periodic weighing the samples. Of special interest is a comparison of the kinetics of nickel oxide reduction by methane and hydrogen. As it follows from Fig. 1, in contrast to hydrogen reduction at T > 800°C [3], the high-temperature reduction of NiO by methane is characterized by existence of a clearly marked induction period, followed by rapid acceleration of the process. As a result, the a(t) curves have S-shaped form both at constant and linear heating regimes, which is specific for the topochemical reactions proceeding via the stages of nucleation and growth, and are described by well known Avrami equation: a = 1-exp(-£tn) [4].
In Figs. 2a-2c there are dependencies of reaction rate, Wr(t) vs. time at linear heating the sample with heating rates, Vh = 50, 100, and 200o/s and methane pressure, Pch4 = 50 torr. As it follows from the data presented, increasing the heating rate moves the reaction from the heating stage (non-isothermal interaction) to the isothermal region. Thus, at Vh = 50°/s, the reduction
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reaction occurs and finished at heating stage. At Vh = 100°/s the reduction proceeds at transition from non-isothermal to isothermal period, while at Vh = 200°/s the reaction completely occurs at reference temperature.
Fig. 1. Dependencies of temperature, T, reaction rate, Wr(t) and conversion degree, a vs. time.
T, oC
1050
1000 -950 -900 850 H 800
a)
Vh=50 o/s
- 6 Wr,J/s ■■ 4
-- 2
0
2
Hh hnfcii nft i|lhii nil
4
6 t,s
T, oC
1050 ■
1000 ■ 950 900 850 800
b)
Vh=100 o/s
Wr, J/s
-- 7 -- 5 -- 3 -- 1
1
t, s
T, oC
1050
1000 950 900 850 800
Wr, J/s
7
■+ 5 3 ■+ 1
1
4 t, s 5
Fig. 2. Dependencies of temperature, T, and reaction rate, Wr(t), vs. time at reduction of NiO by methane. Linear heating with (a) Vh = 50, (b) 100, and (c) 200°/s. Pch4 = 50 torr.
0
0
1
2
3
4
0
2
3
It was shown that the increase of methane pressure significantly reduces the induction period and moves the whole reduction process to the shorter time region (Figs. 3a-3c). At that, in the case of linear heating up to certain reference temperature, depending of methane pressure and heating rate the reduction occurs either at heating stage or fully at reference temperature, i.e. at isothermal conditions.
Based on data obtained at various heating rates and methane pressure, a heating rate (Vh)-methane pressure (Pch4) diagram of reduction regimes was constructed (Fig. 4), according to the which at low heating rates and high pressures the reaction occurs mainly at heating stage. On the other hand, at high heating rates and low methane pressures, vice-versa, the process fully takes place at isothermal conditions.
Fig. 3. Dependencies of temperature, T, and reaction rate, Wr(t), vs. time at reduction of NiO by methane. Linear heating with Vh = 100o/s: (a) Pch4 = 10, (b) 50, and (c) 100 torr.
300 -250 -200 -150 -100 -50 -0
Pch4, torr
Non-isothermal
10
100
A Isothermal
V, o/s 1000
Fig. 4. PCH4-Fh diagram of NiO reducton regimes by methane.
In parallel to the kinetic measurements, SEM studies of the samples at different stages of the process were performed to reveal the causes of the specific shape of the kinetic curves. It has been established, that in the induction period, before intensive chemical interaction, nickel nucleus are formed on the surface of the nickel oxide, which subsequently extend and grow in the volume of oxide, forming highly porous nickel layer. The S-shaped kinetic curve appearance and the presence of the induction period are caused just by the nucleation and their further growth processes.
This work was supported by the Committee of Science MES of RA (Research grant 18T-1D051).
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4. J.E. House, Principles of Chemical Kinetics, 2nd ed., Elsevier, 2007.
