INVESTIGATION OF THE CONVECTION COMBINED RADIATION EMITTED DURING THE SYNTHESIS OF FERRITES BY CARBON COMBUSTION IN OXYGEN Текст научной статьи по специальности «Физика»

ТЕХНИЧЕСКИЕ НАУКИ

A.A. Markov1, LA. Filimonov2, Mkhitar Hobosyan3 and Karen Martirosyan3.

1 Institute for Problems in Mechanics, Russian Academy of Sciences, Moscow, 119526, Russia 2 Institute of Structural Macrokinetics and Materials Science (ISMAN), Russian Academy of Sciences

Chernogolovka, Moscow, 142432, Russia 3 Department of Physics, University of Texas Rio Grande Valley, Brownsville, TX 78520, USA

INVESTIGATION OF THE CONVECTION COMBINED RADIATION EMITTED DURING THE SYNTHESIS OF FERRITES BY CARBON COMBUSTION

IN OXYGEN

Abstract The paper presented investigates the synthesis of ferrites by means of CCSO[1,2]. The authors have successfully established the main aspects of combustion in such systems based on the experimental data obtained and the numerical simulations of the Ni-Zn and Mg-Zn ferrite synthesis as the examples.

Introduction The CCSO synthesis of the

Nio.35Zno.65Fe2O4 ferrite was carried [1,2] out via the reaction:

0.35NiO+0.65ZnO+Fe2O3+r(C+O2) Nio 35Zn:, 65Fe204 +ICO2Î (1)- while

the gross scheme of magnesium-zinc ferrite formation is as follows [1,2]:

0.25MgC03 (s) + 0.75Zn0(s) + Fefi, (s)+a (C(s)+O2 (g ) ) ^ (a + 0.25) CO (g ) + Mg025Zn015Fe2OA (s)

where 2<L<14.

The main difference between reactions (1) and (2) is that at the same content of carbon in the green mixture (at the same a) the greater amount of gaseous carbon dioxide is formed during the synthesis of magnesium-zinc ferrite (compare eq.(1) and eq. (2)). As one can see from (1,2) the magnesium-zinc ferrite synthesis

Table 1. Experimental data obtained

(2)

is more promising than the synthesis of nickel-zinc ferrite in terms of gaseous carbon oxide generation since it produces 1 + 0.25/a moles of carbon oxide per each mole of solid carbon contained initially in the green charge. While during the synthesis of nickel-zinc ferrite

just about one mole of CO only is released per each mole of the carbon contained initially in the green mixture.

System Tign. •c Eact, kJ/mol Eheat kJ/g Porosity, vol. % Tc.max •c Twall •c 02 flow rate. Lmin' 1-2 cm Combustion wave velocity, cm/s Density, expérimental

Carbon in oxygen 405 114±3.4 7645 88 1108 570 1.95 0.02 3.25 g/cm3

Mixture witha=10 in oxygen 366 73±Z2 5770 88 780 520 1.95 0.04 3.6 g/cm3

With the help of data from table1 one can estimate first the characteristics of pure carbon combustion in oxygen:

i) longitudinal velocity of oxygen moving into the pure carbon combustion front:

PcUC

uo2 =

Po,

0.25 x 0.02 „ , cm

-r = 7.14 — (3);

0.7 x 10 s

ii) longitudinal velocity of carbon oxide moving out of the pure carbon combustion front:

prUr 0.25 x 0.02 „ cm uCO ^t^^ =- = 4-(4), i.e.,

CO2 Po 1.25 x 10-3 5

oxygen moves into the front faster than carbon oxide moves out of it. Therefore, the pure carbon combustion front consumes gas as a whole. The ratio of the oxygen to carbon oxide velocity (or ratio of estimate (3) to that of (4)) achieves the value: -1.7 well corresponding to that estimated for CCSO previously ( -1.8[3]). As shown above the situations with propagation of the CCSO front of magnesium-zinc or nickel-zinc ferrite synthesis differ strongly from that with the pure carbon combustion in oxygen. They both release the large

conductivity( 100K) :

the carbon heat

¿parallel ^ (m • K),

AperpendIc ^ 316W(mK),

We consider that the temperature gradient is about equal to that in CCSO (1000K/cm), i.e.:

amounts of carbon oxide and this peculiarity should be certainly taken into account at the numerical simulation of both magnesium-zinc and nickel-zinc ferrite synthesis.

Let's estimate the thermal conduction flux for pure carbon combustion in oxygen, q£ . With the help of the values known [4]:

qt(C = \-AmeanyT\ = (4.14 + 3.16)/2 x 103 = 3.65 x103 W/cm2 = 3.65kW/cm2 (5)

and the thermal flux generated by combustion of pure carbon in oxygen appears to be certainly higher than that of CCSO (compare with [3]). This fact can be explained by higher gas flows and the greater relative impact of convection during CCSO then those generated during reaction of pure carbon in oxygen. The inverse relative relationship between them is true for the

; 0.25 x 0.02 x 0.69 x (1108 - 20) = 3.7536k J (cm2 • 5) (6).

convection combined radiation heat flow during CCSO and combustion of pure carbon in oxygen. Using the data of table 1 one can also estimate the integral heat flux developed by combustion of pure carbon in oxygen:

int egral qC

The ratio of value (5) to value (6) predicts that more than 97% of the integral heat flux is provided by heat conduction while the impact of convection combined radiation appears to be less than 3% and can be neglected within the accuracy of calculations not exceeding 3% under simulation of pure carbon combustion of in oxygen.

Mathematical Formulation of the Problem Let's introduce the non-dimensional variables of Frank-Kamenetsky [5] for which the non-dimensional equations of heat and mass conservation contain

•s)

as the main combustion parameters, and the parametric values of

i

exp

л

to =

RT

0 у

lo =

V

Л/о

CV Pi

0

(8)

ß = RTo

E

7 =

cPT0ß

serve as the characteristic scales of time and length.

Then the following variables for the Cartesian non-dimensional coordinates marked by tilde can be considered:

Q

(7)

Xi,

— xh HQ,t — t/tQ.

uk = u}

/щ,к = 1,2,3; uQ =l0/t0

The non-dimensional temperature, T , can be de-

(T-T0)E

T =

termined [5] as follows: and the following

Retoc =

ûl0p,

0 Mg

RT(;

relations

(9), are

valid:

PoCv _ Л

L

= f, T = T0(\ + ßf)

Re. = Re,

Jloc

l0 l0 (10), while the following local Re and Pe can be introduced:

Pe^„ = P e

üh uoPo

upg

'Tloc

uoPo

(11)

where 14 is the gas velocity longitudinal component. The heat transfer coefficient is applied in the form [5]

0.3 r> „ 0.3'

K = K

0

1 + Re P e

1 ^ Reloc P e'

'Tloc

в

macro g,rad

A

R

P

The non-dimensional radiation heat flux can be expressed as follows [6] :

[(\+ßfw)A ~(\+ßfg)A\

(12),

T

A t 3

R л 0

л

0 1

V '1

where r1,s are the pore scale and emissivity coef- pore solid wall and Tg is that of gas in the pore. The ficient; T is the non-dimensional temperature of a equation of heat balance for gas phase reads:

XPgC

g g

\

д T

—- + u-VT

д t 8

л

с

= V-

У

л

\

Ре

V PeT

VT

-к(т8 - Ts ) + Q^ + Qm

(13),

i /~\macro , s^macro ,, , , ™

where Qg rad and Qg is the heat flux provided correspondingly by radiation and chemistry (due to gases released or consumed by the reactions) in the gas phase.

J

The equation of heat balance for solid phase is written as:

(1 -x)TPjSc дT

Г

'jS

д t

V■

л

Pe

\ PeT

VT

+ K(Tg - Ts ) + QST -Q

\macro sg ,rad '

(14),

Ömacro

gS

QJgs

Eqs. (13), (14) include the heat transfer coefficient K [5,6] that describes the heat exchange between gas and solid. These equations are the basic energy balance equations. The equations are coupled thermally both through the heat fluxes contained in the right hand sides of them as well as with the set of equations [3], that are the equations of momentum and mass conservation for gas species, particle number density, particle size variation. Due to the cylindrical symmetry of the reacting sample as well as that of the boundary and the initial conditions imposed correspondingly at the sample exit , entrance and lateral surface, it is assumed that the V operator applied numerically has a cylindrical symmetry

Simulation results and discussion

The results of simulation refer to the following parameters (7):

P* 0.24,^« 0.288, x = 0.5, ^ = 0.25-0.5 (15),

where X, tinit are the porosity and ignition time

respectively. The initial values of densities are as follows

„0 C „0 c 0

Ps — 5 Pg — 5.2,

Pq02 — 5.2, PCOO2 = 0 PC — 2.5 ^

0 0

Where ps, p denote the total initial densities

^,0 _ _0 . 0 of solid and gas phase, p — Pq2 Pcoi.

Fig. 1 Synthesis of the magnesium-zinc ferrite. Distribution of the MgCO3 (s) reagent (left) and that of the Mg025Zn075Fe2O4 (s) product (right) just before the complete conversion into the ferrite.

The CCSO synthesis of the magnesium-zinc ferrite has been illustrated on fig. 1. The sample is ignited from the bottom. Therefore, the combustion front propagates upward along the sample and the front of the ferrite synthesis is accompanied by the MgCO3 (s) thermal decomposition producing the excessive amount (0.25 moles ) of carbon oxide with respect to the synthesis of nickel-zinc ferrite(see fig.2). As the result, the excessively porous product with large pores and solid particles (up to -10^22% of the sample diameter) is

formed in the bottom part of the sample. While a more uniform layer of the magnesium-zinc ferrite is growing on the sample lateral surface. On the one hand the hot front promotes thermal decomposition of MgCO3 (s)

releasing carbon oxide, and on the other hand it causes simultaneously the growth of the ferrite large particles. In this relation, the sample lateral surface is much colder and is preferred to a greater extent for the ferrite synthesis of a finer dispersion.

Fig.2 Synthesis of the nickel-zinc ferrite. Distribution of the nickel oxide (leftward) and the nickel-zinc ferrite (at the right) just before the complete conversion into the ferrite.

Simulations of the nickel-zinc ferrite synthesis are presented on fig.2. In contrast to the magnesium-zinc ferrite synthesis (see fig.1) there is only one source of gaseous carbon oxide generation (pure carbon combustion). There are no other sources of CO similar to

MgCO3 (s) in this case. Therefore, more or less uniform layer of the nickel-zinc ferrite of the same porosity forms in the sample bottom part and on the lateral surface. The simulation results presented on figs. 1,2 complement sufficiently and enrich the results obtained earlier for the sulfide systems [7,8].

Conclusions

The simulation results we obtained lead us to the

in course of the combustion synthesis of ferrites considered. Due to the MgCO3 (s) contained initially in

the magnesium-zinc green charge and the decomposition of this reagent accompanying the magnesium-zinc ferrite synthesis the intensive flow of gaseous carbon oxide is generated in the product zone of the combustion wave which prevents formation of an uniform and fine dispersed magnesium-zinc ferrite. The structurally unstable magnesium-zinc ferrite with large pores and grains is formed in this case. Vice versa, an absence of any additional sources of gas generation except for the pure carbon combustion guarantees that the uniform fine dispersed and structurally stable ferrite is produced during the nickel-zinc ferrite synthesis.

conclusion about the decisive role of chemical kinetics

Literature

1. Martirosyan K.S., and Luss D. Carbon combustion synthesis of oxides: process demonstration and features, AIChE Journal, 51, 10, 2801-2810, 2005.

2. Martirosyan K.S., and Luss D. Carbon Combustion Synthesis of Ferrites: Synthesis and Characterization. Ind. Eng. Chem. Res. 46, 1492-1499, 2007.

3. Markov A.A., Filimonov I.A., and Martirosyan K.S. Simulation of front motion in a reacting condensed two phase mixture, J. Comput. Phys. 231, 20, 6714-6724, 2012.

4. Physical Magnitudes. Handbook, Moscow, Energoatomizdat, 1991, eds. I.S. Grigoriev, E.Z. Meilikhov, 1232 p. (in Russian)

5. Frank-Kamenetskii, D.A. Diffusion and Heat Transfer in Chemical Kinetics. Moscow Nauka, 3rd edition revised and enlarged 1987, 501 p. (in Russian)

6. Markov, A.A. Heat- and mass- Transfer in the pores of a submicron diameter with the movement of the heat front in a channel Preprint № 1108 of the Institute for Problems in Mechanics Moscow2015, 45p. (in Russian)

7. Markov A.A., Filimonov I.A., and Poletaev A.V. Two Temperature model of the sulfides synthesis. .XX

International Conference on Computational Mechanics and Current Software Systems, Alushta 2015, (in Russian)

u

Wschodnioeuropejskie Czasopismo Naukowe (East European Scientific Journal) #9(13)/2016 HSkl 8. Markov A.A., Barinov, V.Yu., Umarov, L.M., Filimonov I.A., Thermal and Visible radiation during the synthesis of zinc sulfide NPNJ'2016 pp. 97-99 , Alushta 2016 (in Russian)

Anderson A. Y.

Postgraduate student, Odessa National Academy of Food Technologies

Kologrivov M.M.

Candidate of engineering sciences, senior researcher, Odessa National Academy of Food Technologies

THERMAL DESIGN OF GEOTHERMAL CIRCULATION SYSTEM

Abstract: The paper studied the thermal processes in a geothermal circulation system for heating oil in accordance with the proposed model. The effect of heat leakage in the injection well on the heat exchange in the "underground boiler" and the working period of its operation are taken into account for the first time. The numerical simulation of the "underground boiler" for the long-term period with and without heat exchange and energy dissipation of the flow in the wells is done.

Keywords: injection well, underground boiler, energy dissipation.

1. Introduction

The development of geothermal energy due to its enormous resource potential and the ability to produce clean energy cheaper than using fuel. Under the geothermal circulation system means the totality of engineering structures, technical equipment and processes of heating, processing and delivery to the consumer under the hot coolant of the geothermal source. This system includes a natural or synthetic natural reservoir, operating (a mining) and the injection wells and ground technological complex [1,2]. The condition for the efficient extraction of hot rock energy heat transfer agent is the presence in the "underground boiler" developed heat exchange surface.

At the hearth of reliable methods for calculation of technological parameters of such systems are the study of processes of hydrodynamic and heat exchange assuming non-isothermal filtration of heat transfer agent in the «underground boiler».

On the other hand the main method of transporting highly viscous oil and petroleum products is their "hot swapping". The main aim of heating - reduction viscosity of the product in order to reduce flow resistance and energy consumption through the pipeline.

40

2. Formulation of the problem

Geothermal heating of heavy oil, which is pumped through the main pipeline and heating of petroleum in terminals has features. Heat transfer agent temperature (circulation water) at the outlet of the «underground boiler» varies in the range 60°C - 100°C. When the heat transfer agent flows in wells up to a depth of three kilometers there is a significant change in the water temperature due to heat exchange with the surrounding array. With increasing of operating time there is a noticeable impact on the water temperature which arises from the increasing of flow energy dissipation on the rough surface of the pipe.

In the paper [3] has shown that the greatest change in temperature of the water in the wells takes place in a total period of 12 years of operation. The amount of heat leakage from the energy dissipation in the total heat transfer varies with time to the injection well from 8% to 200%, and for the production from 0.7% to 32%. The error in the estimation of the flow temperature at the outlet of the wells excluding dissipation - up to 2°C.

Ф <j ■С о

■S «

2 5

(0 ^ i- с

8 3 E2

I?

2 с

С D

39,5 39 38,5 38 37,5 37 36,5 36 35,5 35

Temperature

excluding

dissipation

2 „.3 4 . 5. ,6. 7. ,.8 9 10 11 12

Operation period of circulation system, years

Fig. 1. Dependence of the inlet temperature in the underground boiler on the operation period of circulation system