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The Study of Compensation Effect in Reverse Boudouard Reaction on Graphite in Presence of Activating Additives

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The Study of Compensation Effect in Reverse Boudouard Reaction on Graphite in Presence of Activating Additives id12648703 pdfMachine by Broadgun Software - a great PDF writer! - a great PDF creator! - http://www.pdfmachine.com http://www.broadgun.com ISSN(Print) : 2320 –1967 ISSN(Online) : 2320 –1975 Original Article ChemXpress 5(3), 078-088, (2014) The study of compensation effect in reverse Boudouard reaction on graphite in presence of activating additives Pavel Anatolyevich Nikolaychuk*, Aleksandr Vasilyevich Kolesnikov Department of Analytical and Physical Chemistry, Chelyabinsk State University, 454001, Bratyev Kashirinykh Street, 129, Chelyabinsk, (RUSSIAN FEDERATION) E-mail: [email protected]; [email protected] Received : 05th April, 2014 ; Revised : 03rd June, 2014 ; Accepted : 08th June, 2014 Abstract : In this study the results of calculation of vation energy for calcium, strontium and iron compounds rate constants for graphite gasification reaction (reverse were plotted. The isokinetic temperatures for various Boudouard reaction) taking into account heat and mass groups of catalysts were calculated. °C are transfer processes at temperatures of 900-1200 Global Scientific Inc. presented. The calculation of rate constants of C + CO2 reaction on graphite in presence of calcium, magnesium, Keywords : Graphite gasification; Reverse strontium, iron (II) and iron (III) oxides, strontium car- Boudouard reaction; Compensation effect; Isokinetic bonate and metallic iron was performed. The graphs of temperature. linear dependencies of pre-exponential factor on acti- INTRODUCTION ied very widely. Particularly, the studies concerning the influence of temperature on kinetics of the reverse The interaction of carbon dioxide with carbon plays Boudouard reaction reveal the presence of compensa- a major role in many pyrometallurgical high tempera- tion effect. ture processes, where coke, coal and other carbon ma- Compensation effect is the dependency between terials are used as solid reductants. Many studies are pre-exponential factor (A) and activation energy (EA) devoted to investigation of C + CO reaction in order 2 in Arrhenius equation ( ), which reveals to in crease yield of carbon monoxide, which is one of often, when studying heterogeneous catalytic pro- the most important raw materials for chemical produc- cesses[5-7]. In this case values of A and E increase or [1] [2-4] A tion . It was noted in papers , that basic solutions of decrease simultaneously, compensating each other, and the problem of carbon gasification reaction speeding the influence of temperature on rate constant becomes can be obtained from fundamentals of reaction kinet- less substantial. If the relationship between EA and A ics, stream dynamics, diffusion, heat and mass transfer. for multiple reactions strictly obeys linear nature (log A Therefore, the various aspects of this reaction were stud- á â = + EA), rate constants for all reactions become ChemXpress 5(3), 2014 Original Article79 equal at one certain temperature, which is called It is stated[3], that full amount of active centers per “isokinetic temperature” [7-9] è T . gram of carbon [C[t]] is the sum of amounts of free and The presence of compensation effect was noticed occupied active centers: at catalytic gasification of carbons, activated by various [C[t]] = [C[f]] + [C[o]] (5). [10] additives such as sodium lignosulphate , ferric ni- Assuming that rates of formation and vanishing of [10] [11,12] trate , sodium and potassium carbonates , orga- occupied active centers are equal and using steady- [13] nometallic compounds with V, Cr, Mn, Fe and Co , state approximation, the following kinetic equation can [14] [15] sodium vanadate and others . The influence of tem- be derived: perature on rate constants of C + CO2 reaction and compensation effect in presence of calcium and mag- nesium oxides, strontium oxide and carbonate, iron and (6). iron oxides is the subject of the present work. Expressing [C[f]] from equation (5) and substituting The reverse Boudouard reaction C + CO2 was it into equation (6) yields: [2-4,16-19] studied very widely , including studies with dif- k [CO ] [C ] = k [CO ] [C ] + k ferent coal types and at various carbon dioxide partial 1 2 [t] 1 2 [o] -1 [CO] [C[o]] + k2 [C[o]] (7). pressures. The mechanism of this reaction is postulated Solving for [C ] from equation (7) gives: to be the following[3,4,16,17,19]: [o] (8). (1), Dividing both numerator and denominator by k1 [CO2] gives: (2), (9). where C[f] is a free active center on carbon surface, k1 able to reaction, and C is an active center, occupied Defining the value of I and substituting it and [o] 2 k by oxygen atom, k , k–1 and k are rate constants of 2 1 2 the value of K from equation (3) into equation (9) yields: corresponding elementary stages. p Reaction (1) proceeds relatively fast and is charac- terized by following equilibrium constant: (10). Substituting equation (10) into equation (4) gives (3). the expression for overall carbon gasification rate: Here equilibrium concentrations of gases, [CO] and –3 [CO2], are expressed in mol cm and [C[o]] and [C[f]] (11), are the numbers of moles of occupied and free active The values of K and I do not involve any param- centers per one gram of carbon. p 2 Reaction (2), corresponding to carbon transfer from eter that is a function of physical properties of carbon, solid phase to gaseous one, which proceeds on occu- they also are expected to be independent of pressure. pied active centers, is much slower. The dependencies of these parameters on temperature [20] In this case, rate of gasification of carbon sample, sur- are taken from paper . rounded by gas of unified composition, can be ex- The aim of this study is:1) to investigate C + CO2 pressed in following way[3,4]: reaction on graphite in a flow reactor in presence of calcium, magnesium, strontium and iron compounds using (4), kinetic equation (11) and considering heat and mass dn transfer processes; 2) to calculate rate constants and where N is the rate of carbon transfer from solid c dt evaluate possible compensation effect between activa- –1 – mass of carbon, phase to the gaseous one, mol s , P tion energy and Arrhenius prefactor in presence of study- g. ing additives. 8O0 riginal Article ChemXpress 5(3), 2014 EXPERIMENTAL Analysis of experimental data The experimental results were presented in form of Performing of experiments dependencies of mass loss of graphite (F) on time (t): The commercially available graphite powder which (12), comes in the graphite pots was used in all experiments. Ð Ð where ê is current graphite mass, î is initial graphite The graphite was of spectral analysis quality and had – mass and P = P P is graphite mass loss. the particle size d 0.1 mm and impurities content of no o k -5 Kinetic curves were conditionally divided into three more than 2 10 weight percent. Activating additives – from start of experiment to time of sections: section I were of analytical quality with particle size d 0.1 mm – from third minute to the moment 3 minutes, section II and purity of 99%. Carbon dioxide from balloons, con- – 70 % graphite mass loss and section III – from of 65 taining no more than 0.1% CO and 0.01% H2 was used. – 70 to 100 % graphite mass loss. The example of The initial samples with the various concentrations 65 of the catalysts were prepared by the mechanical mix- the kinetic curves is shown at Figure 1. The rate con- ing of corresponding amounts of the graphite and the stants were calculated according to section II. In most additive in plastic cups during 10 minutes. A weight taken cases the experimental points in this section were de- from the sample mixture using analytical balance ADV- scribed by straight lines. The slopes of these lines ±0.0001 g was placed ÄF 200 with measurement error of ( Äô) were determined using less squares technique. into alundum bath that was preliminarily calcined at After this the average rates of graphite mass loss at the °C during 1 hour. The bath had been injected into 1200 second section were determined: porcelain tube and placed into thermobalance ATV-14 with carborundum heaters. Carbon dioxide flow was (13). passed through the tube with determined rate. The stud- ÄP ies were performed at temperature range of 900- After dividing the values of Äô by carbon molar °C –1 –1 1200 . Carbon dioxide consumption was 50 cm3 min mass (12.0107 g mol ) the rates of loss of graphite and gas flowing rate in reaction tube was 1.06 and 1.32 amount of moles (graphite consumption rates) were –1 °C respec- cm s at temperatures of 900 and 1200 calculated. tively. The alundum baths used in experiments were According to[23], the average reaction rate inflow weighted before and after the experiment and no mass dn system ( ) can be calculated independently by equa- loss was detected. dt Thermogravimetric analysis was applied to study tion: reaction kinetics. According to paper[21], TGA can be (14), freely used without needing to pay attention to chemi- where N is the average rate of interaction of carbon sorption dynamics if carbon active surface area does c –1 Õ Õ – the amounts of 2 dioxide with graphite, à and à not exceed 312 m g . However, active surface area ,i ,o ÑÎ 3 – [22] at reactor inlet and outlet respectively, cm , Fà of graphite without special processing is much lower , 2 –1 – reaction which allows using of TGA method freely. the rate of incoming substance, mol s , V 3 Thermogravimetric curves obtained from different space volume, cm . The values of N calculated in accordance with equa- weights taken from one sample mixture show good re- c producibility and compatibility with each other. tion (14) were found to be compatible with the values Since the heating area in thermobalance is small, of graphite consumption rate within their ranges of un- certainty.
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