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Effect of Water Losses by Evaporation and Chemical Reaction in an Industrial Slaker Reactor

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Effect of Water Losses by Evaporation and Chemical Reaction in an Industrial Slaker Reactor 339 Vol.50, n. 2 : pp.339-347, March 2007 ISSN 1516-8913 Printed in Brazil BRAZILIAN ARCHIVES OF BIOLOGY AND TECHNOLOGY AN INTERNATIONAL JOURNAL Effect of Water Losses by Evaporation and Chemical Reaction in an Industrial Slaker Reactor Ricardo Andreola 1, Osvaldo Vieira 2, Onélia Aparecida Andreo dos Santos 1 and Luiz Mario de Matos Jorge 1* 1Departamento de Engenharia Química; Universidade Estadual de Maringá; Av. Colombo, 5790 - Bloco D90; [email protected], 87020-900; Maringá - PR - Brasil. 2Klabin Paraná Papéis; Fazenda Monte Alegre; Bairro Harmonia; 84275-000; Telêmaco Borba - PR - Brasil ABSTRACT A dynamic model of the slaker reactor was developed and validated for Klabin Paraná Papéis causticizing system, responsable for white liquor generation used by the plant. The model considered water losses by evaporation and chemical reaction. The model showed a good agreement with the industrial plant measures of active alkali, total titratable alkali and temperature, without the need of adjustment of any parameter. The simulated results showed that the water consumption by the slaking reaction and evaporation exerted significant influence on the volumetric flow rate of limed liquor, which imposed a decrease of 4.6% in the amount of water in reactor outlet. Key words: Slaker reactor, modeling and simulation, white liquor INTRODUCTION Ca(OH) 2(s) . In turn, green liquor is a liquid solution with a large quantity of sodium carbonate that The causticizing reactor system from Klabin reacts with calcium hydroxide producing sodium Paraná Papéis produce white liquor used in hydroxide and calcium carbonate, according to the cellulose digesters. This system is composed by causticizing reaction: Na 2CO 3(aq) + Ca(OH) 2(s) → 2 one slaker reactor followed by nine causticizing NaOH (aq) + CaCO 3(s) . This reaction is slightly reactors in sequence. According to Fig. 1, green exothermic. liquor and lime are continuously introduced in the At the outlet of the slaker reactor, there are two stirred slaker reactor. The lime from lime kiln is distinct streams: limed liquor and grits. The limed mainly composed of a large quantity of calcium liquor is basically formed by a sodium hydroxide oxide and a small amount of inert solids. solution with calcium carbonate, calcium The calcium oxide instantaneously reacts with hydroxide, inert solids and eventually unreacted water from green liquor producing calcium calcium oxide in suspension. hydroxide, according to a highly exothermic reaction, called slaking reaction: CaO (s) + H 2O(l) * Author for correspondence Brazilian Archives of Biology and Technology 340 Andreola, R. et al. Lime Green liquor Grits Stirrer Limed liquor Figure 1 - Scheme of slaker reactor In the sequence, the flow is fed in causticizing dm m q PM PM reactors by gravity, while the grits are removed e = w − e −V e r +V e r dt e 0, V PM 1 PM 2 from the reactor by a screw. Na 2O d The main objective of this article was evaluate the (5) effect of water losses by evaporation and chemical dm m q PM reaction in Klabin Paraná Papéis slaking reactor. f = w − f +V f r (6) dt f 0, V PM 1 Na 2O dm m q g = − g MATERIAL AND METHODS wg 0, (7) dt V Mathematical Modeling Where: a = Na CO , b = NaOH, c = Na S, d = The mathematical model was developed by 2 3 2 CaO, e = Ca(OH) , f = CaCO , g = inert solids. Andreola (2001) from mass balances on each 2 3 component, Equations (1) to (7), and from energy The model predicted the actual concentration of balance, Equation (9), over the slaker reactor, considering perfect mixture and constant volume. each soluble component (Ci*, i= a, b, c); the mass The hypothesis of perfect mixture was based on of insoluble solids inside the reactor (m j, j= d, e, f, the research developed by Hypponen and Luuko g) and the reactor’s temperature (T). Volumetric (1984) that showed the same value of residence flow rate (q) and mass flow rates (w j) must be time distribution for solids and liquid in a known. For model purposes, Causticising (r 2) and causticizing reactor system. The assumption of Slaking (r 1) kinetics were corrected by molecular constant volume (V) was based on the fact that the weights (PM) ratios. volume of slaker reactor was constant. The salker The actual concentration of a generic component i reactor has an outlet for the liquor at the top of (C i) was determined from its apparent reactor, according to Fig. 1, that keeps the level of concentration (C i*), using Equation (8). This liquid constant. procedure was necessary because the apparent * concentration represented the mass of a specific dC 1 ∗ PM a = ()q C − qC − r a (1) component dissolved per unit volume of reactor. dt V 0 a 0, a 1 PM Na 2O However, part of this volume was occupied by ∗ insoluble solids: CaCO 3, CaO, Ca(OH) 2 and inert dC 1 ∗ PM b = ()q C − qC + 2r b (2) solids. Each volume of the solid compounds 0 b 0, b 1 dt V PM Na O = ρ 2 (V j m j / j , j = d, e, f, g ) must be subtracted ∗ dC 1 ∗ from the total volume (V) to obtain the effective c = ()q C − qC (3) dt V 0 c 0, c volume (Vr) and the real concentration of each dm m q component, Equation 8. d = w − d −Vr (4) dt d 0, V 2 = ∗ V Ci Ci . (8) Vr Brazilian Archives of Biology and Technology Effect of Water Losses by Evaporation and Chemical Reaction in an Industrial Slaker Reactor 341 m m m m While the reactor temperature (T) was lower than Where: Vr = V − f + e + d + g ρ ρ ρ ρ the boiling temperature of the reacting mixture f e d g (T eb ), water evaporation did not occur, therefore, qevap = 0. However, when the reactor temperature The operational reactor temperature was very close reached T eb, water evaporation occured at constant to the boiling temperature of the reacting mixture. temperature and the rate of evaporation could be Both reactions were exothermic and the reacting instantaneously evaluated from the solution of the mixture temperature increased until it reached the following equation: boiling point. Consequently, there was energy loss C1 - q evap C2 = 0. by evaporation. To take this energy loss into As mentioned previously, the limed liquor was account, this loss was included in Equation (9), composed of an aqueous solution of sodium admitting that the enthalpy and boiling hydroxide, calcium carbonate, calcium hydroxide, temperature (T eb ) of the solution were identical to inert solids and, eventually, calcium oxide that of pure water ( ∆Hevap ) at the same pressure, unreacted. Consequently, the volumetric flow rate because the reacting mixture was composed of a of limed liquor (q) was composed of five distinct diluted solution. Insoluble solids were dispersed in terms: water (q H2O ); calcium carbonate (q CaCO3 ); solution and jointly form a slurry that had distinct calcium hydroxide (q Ca(OH)2 ); inert solids (q inerts ) density ( ρslurry ) and heat capacity (Cp slurry ). The and calcium oxide (q CaO ), according to Equation reactor’s liquid phase was called liquor. Liquid (10). (∆Hliquor ) and solid ( ∆Hsolids ) enthalpy changes were evaluated from heat capacities of solids (Cp ) q = q + q + q + q + q (10) j H 2O CaCO 3 Ca (OH 2) CaO inerts and dissolved compounds (Cp j). Where: dT = − C q C (9) m f m dt 1 evap 2 q = q ; q = q e ; CaCO 3 ρ Ca (OH 2) ρ Where: V f V e (∆ + ∆ + + − ) H liquor H solids Q1 Q2 Q3 m = md g C1 q = q ; q = q . Vρ Cp CaO ρ inerts ρ slurry slurry V d V g = −∆ = −∆ = − Q1 ( H r1)Vr 1; Q2 ( H r 2 )Vr 2 ; Q3 UA (T Tamb ) ∆ = * + * + * ∆ Hliquor Cp [( Ca 0, Cb 0, Cb 0, )q 1 The volumetric water flow rate (q H2O ) could be GL 0 ; obtained from water mass balance, admitting − (C* + C* + C*)q∆ ]; a b c 2 unsteady-state behavior, where the consumption of md 0, q m q water was given by the slaking reaction (q r1 ) and ∆ = 0 ∆ − d ∆ + H solids Cp d 1 2 by evaporation (q evap ), according to Equation (11). V V q = q − q − q (11) m q m q H 2O 0 r2 evap Cp e 0, 0 ∆ − e ∆ + e 1 2 PM V V r V H 2O Where: q = 2 r2 ρ PM m q m q H 2O Na 2O f 0, 0 ∆ − f ∆ + Cp f 1 2 V V The volumetric flow rate of limed liquor, q, could be obtained from Equations (10) and (11), mg 0, q0 mg q ∆ − ∆ ; Cp g 1 2 according to Equation (12). V V ∆ = − ∆ = − ; V 1 (Tin Tref ); 2 (T Tref ) = ( − − ) q q qr2 q (12) ρ ∆H Vr 0 evap C = H 2O evap . 2 Vρ Cp slurry slurry The reaction rate equations for the slaking reaction (r 2), CaO (s) + H 2O(l) → Ca(OH) 2(s) , and causticizing reaction (r 1), Na 2CO 3(aq) + Ca(OH) 2(s) 2NaOH (aq) + Brazilian Archives of Biology and Technology 342 Andreola, R. et al. CaCO , employed in this work were those 3(s) 1 1 ew 1 employed by Swanda (1994), given by Equations = + + (15) UA h A A k h A (13) and (14). Both reactions were exotheric and it ext lm w int took account in the model by heats of slaking Wall thermal conductivity (k w) was taken from the (∆Hr2 ) and causticizing ( ∆Hr1 ) reactions. literature, wall thickness (e w) and logarithmic m averaged area (A lm ) from design specifications.
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