Sponge iron production

Heat and Mass Transfer in Sponge Iron Reactor Reduction Zone

4th International Symposium on Advanced Fluid/Solid Science and Technology in Experimental Mechanics, 28-30 November. 2009, Niigata, Japan

Abstract

The aim of this research is to study heat and mass transfer in sponge iron reactor reduction zone through mathematical modeling arrangement and simulation. Kinetics equation of reduction of hematite to iron metal, methane reforming, and water gas shift reaction are taken into account in the model. After being validated with the reference data, the model was able to satisfactorily describe temperature and concentration profiles along reduction zone. The results also show an increase in reduction gas temperature inlet will increase the metallization degree.

Key words

Reduction zone, Heat transfer, mass transfer, Kinetics model, Sponge Iron reactor

1. Introduction

Sponge iron production rate in iron ore reduction plant can be increased by rising reduction gas temperature. However, the temperature rising can affect the product quality which is expressed by metallization degree. The investigation of the effect of reduction gas can be carried out by analyzing the performance of sponge iron reactor which in turn can predict the profitable optimum condition of reactor. However, the reactor analysis is difficult to be executed directly in the field plant; therefore it will need a simulator as support instrument. This simulator can be designed by arrangement of heat and mass transfer equation inside the reactor to reproduce reactor data from the field. By simulating heat and mass transfer equation, we can predict the effect of reduction gas on the performance of reactor.

Some iron ore reactor model had been proposed in few literatures. Iron ore reactor model proposed by Aguilar et al. (1995) was referred for fixed bed reactor. The model was derived based on unsteady state condition. Parisi et al. (2005) proposed a model for moving bed counter current reactor. The model was limited on reduction zone on iron ore reactor. Only reduction reactions were considered in the model. Plug flow were used as an approach for the model.

Another related model is formulated on direct reduction shaft furnace for producing sponge iron from iron ore as proposed by Takenaka et al. (1986). In his model, reduction rate equations were derived from the three-interface model, involves both mass and heat balances. Srinivasan (2002) showed a model for reduction iron oxides by carbon. This model was formulated on a circulating fluidized bed reactor. In their model, only the global direct reduction reactions were taken into account, methane reforming and water gas shift reaction are ignored.

The kinetic of direct reduction reaction plays an important role in sponge iron reduction zone mathematical model. Unreacted core model proposed by Levenspiel (1962) has been used widely for kinetics model of iron reduction. In this model, reaction was considered occurs first at the outer skin of the particle. The zone of reaction then moves into the solid, and may leave behind completely converted material and inert solid. Thus, at any time there exists an unreacted core of material which shrinks in size during reaction.

Mondal, et al (2003) proposed another kinetics model for iron reduction. The kinetic constants were derived based on Arrhenius equation. Cementite formation reaction was also taken into account in his model. This model is only applicable for iron reduction using CO as reducer. Iguchi (2004) also suggested kinetics model based on Arrhenius equation but cementite formation was ignored.

In this paper, heat and mass transfer in sponge iron reactor reduction zone are studied. Kinetics equation of reduction of hematite to iron metal, methane reforming, and water gas shift reaction are taken into account in the model. The model can be used to explore the performance of sponge iron reactor.

2. Reactor Modeling

Iron ore reactor is a moving bed reactor to produce sponge iron product from iron ore. Gas and solid phases are passed in counter current. Iron ore reactor is divided by 3 zone, those are reduction, isobaric, and cooling zone.

In the reduction zone, there are some reaction occurred between reduction gas and iron ore pellets. These reactions will extract iron metal from the ore to create sponge iron product. Hydrogen and carbon monoxide are used as reducer. The reduction reaction are shown as

1/3Fe2O3 + H2 Û 2/3Fe + H2O

1/3Fe2O3 + CO Û 2/3Fe + H2O

Methane reforming and water gas shift reaction are also possible to occur based on the composition of reduction gas and temperature through reaction

CH4 + H2O Û CO2 + H2

CO2 + H2 Û CO + H2O

Mathematical model of reduction zone in iron ore reactor is arranged on the gas and solid phase. Heat and mass transfer equations are formulated to perform the temperature and concentration of gas and solid phase respectively. The modeling is also carried out by applying kinetics model. Iron reduction, methane reforming, and water gas shift reaction are taken into account in the model.

The equations are formulated by assuming: (i) operating conditions have been steady-state, (ii) no heat loss over the wall of reactor reduction zone, (iii) the iron ore pellet consumption is governed by the unreacted shrinking core model, (iv) enthalpy is calculated based on temperature changes, and (v) plug flow is used to approach a model for gas and solid phase. By considering the above assumptions, heat and mass transfer equations can be stated as:

(1)

(2)

(3)

(4)

(5)

(6)

Equation (1)-(3) are mass transfer equation for gas phase, while Eq. (4) is for solid phase. Heat transfer equations are expressed by Eq. (5) and (6). Kinetics equations (r) are given by

(7)

(8)

(9)

(10)

(11)

These kinetic equations are adopted from literatures and rearrangement by inserting correction factor (Parisi, et al., 2004; Munster, et al., 1981; and Bustamante et al., 2004).

Heat capacities of gas and solid are calculated based on the temperature changes. On the gas phase, heat capacities are considered only for CO and H2 since the other substances have very small fraction. Heat capacities of gas are evaluated using:

(12)

(13)

Constant values of a, b, c, and d for CH4 and H2 respectively are obtained from Reklaitis (1983).

Standard reaction enthalpies (HR.(298K)) and reaction heat capacity (CpR) on the Eq. (5) are calculated for methane reforming, and water gas shift reaction respectively since they occur on the gas phase. On the contrary, HR.(298K) and CpR in the Eq. (6) is calculated based on iron reduction reaction. The values of HR.(298K) are obtained from Reklaitis (1983).

Simulation is carried out on sponge iron production capacity of 2500 tone/day. Reduction gas flow rate required is about 125000 m3/hour, while leaking cooling gas flow rate from isobaric zone is 4700 m3/hour. Some physical properties are obtained from Perry (1999).

2. Results and discussion

Calculation of the model is executed by using finite element method. The calculation is divided by many segments along height of reduction zone. Reaction constant rates in kinetics equation are adopted from the literatures. However, we have to adjust correction factor value in order to fit simulation results with reference data used in this research. Correction factor values used in the model are shown in Table 1.

Reaction constant rate value for methane reforming is not explained in literature. The value shown in Table 1 is obtained from the calculation after fitting with the reference data.

Table 1. Correction factor value

No.

Note

Methane Reforming

Water gas shift reaction

Iron reduction

by H2

by CO

1.

Correction factor for reaction constant rate

-

0.49

0.1

0.001

2

Reaction constant rate

*0.00199

-

-

-

3

Rf for gas phase

0.0006

4

Rf for solid phase

0.383

* Reaction constant rate (mol/m2.s.Pa0.94)

Table 2 resumes the comparison between reference data and simulation. From this table, one observed a good agreement between reference data and simulation. Gas composition data for flow number 1, is not available in the plant, however they can be calculated from mass balance resulted from mixing between reduction gas inlet (flow number 2) and leaking cooling gas (flow number 3).

Table 2. Comparison between reference data and simulation

* Flow

Note

Plant Data

Simulation results

2

1

3

Reduction gas inlet temperature (K)

Reduction gas temperature in the bottom of reduction zone (K)

CH4 (% vol.)

H2 (% vol.)

CO (% vol.)

CO2 (% vol.)

N2 (% vol.)

H2O (% vol.)

Leaking gas from isobaric zone (K)

1203

1193

1139

1193

2.9

42.1

9.58

2.26

3.72

1.36

8

REDUCTION GAS WASTES

Temperature (K)

CO2 (% vol.)

CH4 (% vol.)

CO (% vol.)

H2O (% vol.)

N2 (% vol.)

H2 (% vol.)

657.5

9.0

4.4

12.1

23.4

1.2

49.7

657.5

9.0

2.7

12.27

23.9

1.7

50.2

9

IRON ORE

Temperature (K)

Fe2+ (% mass)

Fe (% mass)

Fe2O3 (% mass)

Gangue (% mass)

400

0.5

0

93.2

6.3

400

0.5

0

93.19

6.3

10

REDUCED IRON

Temperature (K)

Fe2+ (% mass)

Fe metal (% mass)

Fe2O3 (% mass)

Gangue (% mass)

Metallization degree (%)

1169

0.5

87.69

5.5

6.3

94.22

Reduced iron data (flow number 10) at the outlet of reduction zone are also unavailable in the plant. However the simulation results from the calculation of reduction zone already have an agreement with simulation result produced from the calculation in isobaric zone (Alamsari, et al., 2009). Alamsari proposed model for isobaric and cooling zone of the same reactor. She found that metallization degree of the product has reduced around 1.72% after leave isobaric and cooling zone. As metallization degree of sponge iron product from the plant data is stated about 92.49%, it is implied that the product (reduced iron) before enter isobaric and cooling zone has metallization degree of 94.21%. This value has an agreement with simulation result produced from this research. Reduced iron shown in Table 2 produces metallization degree around 94.22%.

Alamsari et al., (2009) also showed solid temperature profile along isobaric and cooling zone. Reduced iron temperature was obtained about 1169 K. This value is close with simulation result shown in Table 3.

Temperature profiles along reduction zone for gas and solid phase are shown on 2a. Inlet and outlet temperature already have an agreement with the reference data. Temperature profiles along reduction zone show an increase in gas and solid temperature from the direction of solid flow.

Concentration profiles for gas phase are shown in 2b. Except nitrogen, other components show composition changes which are caused by reactions in the gas phase. Gas mole balance reactions for methane reforming, iron reduction, and water gas shift reaction already have an agreement.

Solid concentration profile on 2c resumes composition changes along reduction zone. At the beginning of reduction zone inlet, total Fe in the ore is dominated by Fe2O3. As reactions occur, Fe2O3 is slowly reduced by reduction gas to release oxygen. These reactions will change Fe2O3 to be Fe metal.

Conclusions

Reduction zone of iron reactor has been simulated. Kinetics equation of reduction of hematite to iron metal, methane reforming, and water gas shift reaction are taken into account in the model. Simulation results have an agreement with the reference data.

Temperature profiles along reduction zone show an increase in gas and solid temperature from the direction of solid flow. Gas concentration profiles show composition changes which are caused by reactions in the gas phase. Gas mole balance reaction for methane reforming, reduction, and water gas shift reaction already have an agreement.

At the beginning of reduction zone inlet, solid concentration profile shows total Fe in the ore is dominated by Fe2O3. As reactions occur, Fe2O3 is slowly reduced by reduction gas to release oxygen. Reduced iron from reduction zone produced metallization degree until 94.22%.

Nomenclature

Asp

pellets surface area per unit reactor volume, m2/m3

C

concentration, mole/m3

Cp

heat capacity, J/mole.K

D

molecular diffusivity of oxygen in gas

Dn

effective diffusivity of oxygen

ep

pellets porosity

EA

activation energy, J/mole

f

correction factor

M

solid molar flow rate, mole/sec.m2

Mw

molecular weight, kg/mole

h

convection heat transfer coefficient, J/s.m2.K

HR(298K)

standard reaction enthalpy, J/mole

k

pre-exponential constant mole C/sec.m3.Pa

L

Length of reactor cooling zone, m

p

pressure, Pa

r

reaction rate, mole/m3.sec.

R

gas constant, Pa. m3/mole.K

Rc

radius of unreacted core ,m

RP

pellets radius, m

Rf

relaxation factor

T

temperature, K

tp

pore structure tortuosity

U

global heat transfer coefficient, J/ m2.K.sec.

superficial velocity, m/s

z

space variable inside reactor, m

solid

solid

R

reaction
Superscripts

+

forward reaction

-

backward reaction
Subscripts

1

reduction reaction by H2

2

reduction reaction by CO

3

methane reforming reaction

4

water gas shift reaction

i

ith reaction

References

[1] Aguilar, J., Fuentes, R., and Viramontes, R., Simulation of iron ore reduction in a fixed bed, Modelling Simul. Mater. Sci. Eng. vol. 3, (1995), pp. 131-147.

[2] Alamsari, B., Torii, S., Trianto, A., and Bindar, Y., Numerical Simulation of Iron Ore Reactor Isobaric and Cooling zone to Investigate Total Carbon Formation in Sponge Iron, International Conference of Modeling and Simulation, Tokyo, (2009), pp. 88-92.

[3] Bustamante, F., Enick, R.,Rothenberg, K., Howard, B., Cugini, A., Cioco, M., and Morreale, B., High-Temperature Kinetics of the Homogeneous Reverse Wateras Shift Reaction, AICHE Journal, Vol. 50, No. 5, (2004), pp. 1028-1021.

[4] Iguchi, Y. and Yokomoto S, Kinetics of the reactions in carbon composite iron ore pellets under various pressures from vacuum to 0.1MPa, ISIJ International, Vol 44, (2004), pp.2008-2017.

[5] Levenspiel, O.,Chemical reaction engineering, John Wiley and Sons, New York, 2-nd ed., (1962), pp. 360-371.

[6] Mondal, K., Lorethova, H., Hippo, E., Wiltowski, T., and Lalvani, SB., Reduction of iron oxide in carbon monoxide atmosphere-reaction controlled kinetics, Fuel Processing Tecnology 86, (2004), pp. 33-47.

[7] Munster, P. and Grabke, HJ., Kinetics of the Steam Reforming of Methane with Iron, Nickel, and Iron-Nickel Alloys as Catalysts, Journal of Catalysts 72, (1981), pp. 279-287.

[8] Parisi, D.R. and Laborde, M.A., Modeling of counter current moving bed gas-solid reactor used in direct reduction of iron ore, Chemical Engineering Journal, vol. 104, (2004), pp. 35-43.

[9] Perry, Robert. H and Green, Don W., Perry's Chemical Engineer's Handbook, Mc. Graw-Hill Companies, Inc., 7nd. Edition, 1999.

[10] Reklaitis, G. V., Introduction to material and energy balances, John Wiley & Sons, 1st. edition, 1983.

[11] Srinivasan, N.S., Reduction of iron oxides by carbon in a circulating fluidized bed reactor, Powder Technology, vol. 124, (2002), pp. 28–39.

[12] Takenaka, Y., Kimura, Y., Narita, K., and Kaneko, D., Mathematical model of direct reduction shaft furnace and its application to actual operations of a model plant, Computers and Chemical Engineering, vol. 10, No. 1, (1986), pp. 67-75.

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