Efficiencies of NaOH production methods in a Kraft pulp mill

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2009; 33:1341–1351Published online 20 April 2009 in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/er.1546

Efficiencies of NaOH production methods in a Kraft pulp mill

Tobias Richards1,�,y Carlos Pavletic1 and Johan Pettersson2

1Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 412 96 Goteborg, Sweden2Eka Chemicals AB, SE-445 80 Bohus, Sweden

SUMMARY

There are several processes in a Kraft pulp mill where there is a need for sodium hydroxide, e.g. in the digester and thebleaching plant. The objective of this study is to perform a preliminary evaluation, intended to select the best alternativefor producing sodium hydroxide on a Kraft pulp mill site. The first step of the evaluation consists of screening availableprocesses for producing sodium hydroxide needed in the mill. The first step of the evaluation shows that the two bestoptions for increasing the production of sodium hydroxide for internal use in a mill are the conventional lime cycleprocess or direct causticization with titanates. The second step of the evaluation compares the lime cycle and thetitanate process using first and second law analyses to determine the energy requirement and the exergy efficiencies ofboth processes. Such analyses show a higher energy requirement and a lower exergy efficiency in the titanate processthan in the lime cycle process without any heat integration. However, the titanate process shows better performance interms of energy requirement and exergy efficiency than the lime cycle, if heat is integrated into both processes. Thetitanate process requires, in the best case, only 80% of the energy required for a fully heat-integrated lime cycle process.Copyright r 2009 John Wiley & Sons, Ltd.

KEY WORDS: exergy analysis; energy analysis; pulp mill; direct causticization; lime cycle

1. INTRODUCTION

Sodium hydroxide is one of the most commonlyused chemicals in industry; it is used almosteverywhere where a strong base or source ofsodium ions is required. Sodium hydroxide, alsoknown as caustic soda because of its corrosiveaction on many substances, is normally used inindustry as a diluted solution, but for transporta-tion reasons, it is normally available as a white

crystalline substance in the shape of pellets or

flakes, or as a concentrated solution at 50%

weight. Sodium hydroxide does not occur in

nature but can be produced from readily available

materials such as sodium chloride or sodium

carbonate. In this study, the objective was to

evaluate methods for on-site production of sodium

hydroxide in the paper pulp industry, with a focus

on pulp bleaching where the added chemical

*Correspondence to: Tobias Richards, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Goteborg, Sweden.yE-mail: [email protected]

Received 12 June 2008

Revised 19 February 2009

Accepted 6 March 2009Copyright r 2009 John Wiley & Sons, Ltd.

cannot be recovered. This topic has acquiredimportance because of the trend in the pulpindustry to build large mills in remote locations,close to forests, which are easily renewed in areasonably short period of time. This trend isintended to reduce costs related to the transporta-tion of wood, but requires the transport of pulpingand bleaching chemicals to these distant locations,or the production of them on site or, at least, closeto the mill.

The aim of this investigation is to determinewhether or not there are any alternatives to thetraditional lime cycle process for producing NaOHon a Kraft pulp mill site. In addition, if so, howwell could such an alternative perform incomparison with the current process, i.e. whichprocess has the highest potential.

2. METHODOLOGY

The investigation started with a survey of differentavailable technologies for producing NaOH, anddetermined the advantages and disadvantages ofeach. The most promising alternatives were thenselected for further analyses. The analyses includeda theoretical study with respect to energy andenergy quality (exergy). The first and second lawsof thermodynamics were used to compare theenergy requirements, energy efficiency and possi-bilities for improvement of the processes. The firstlaw analysis was handled with an energy balance,and the second law analysis was handled with anexergy balance:

Energyin � Energyout ¼ Energyacc ð1Þ

In the energy balance, the terms were derivedfrom the heat of formation to include eventualreactions inside the system. It was assumedthat the kinetic and potential energy contri-butions could be neglected (it was potential thatwas investigated and not a specific operationalsite):

Exergyin � Exergydestruction � Exergyout

¼ Exergyacc ð2Þ

The concept of exergy combines both the secondlaw and the first law and gives information aboutthe usefulness of the energy contained in amaterial, heat or work stream. Exergy is definedas the maximum amount of work that can beproduced by a stream of matter, heat or workwhile achieving equilibrium with the referenceenvironment [1]. An exergy analysis requires theprecise definition of the environment that func-tions as a reference state for the analysis. Thismeans that temperature, pressure and mixture ofsubstances must be defined and given a zero exergyvalue so that all the exergy values for the differentstreams in the system will be equal to the distancefrom the defined reference state.

In this study, the exergy analyses performedused a reference state defined at temperatureT5 298K (251C) and pressure P5 1 atm. Thesereference substances can be selected as the mostvalueless substances found in abundance in nature[2]. The exergy calculations were based on theconcept with three modules as described by e.g.Kotas [3]. Figure 1 shows a simplified illustrationof this method.

Figure 1. Description of the exergy of a substance in a steady flow.

T. RICHARDS, C. PAVLETIC AND J. PETTERSSON1342

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Energy Res. 2009; 33:1341–1351

DOI: 10.1002/er

Module X stands for reversible physicalprocesses, i.e. this investigation omits bothpotential and kinetic contributions. Module Ystands for reversible reference reactions, andmodule Z stands for isothermal change in theconcentration. The maximum work for a fullyreversible physical process between the originalstate, 1, and the environment, state 0, is given bythe following equation (potential and kinetic workomitted):

½Wrev�01 ¼ h1 � h0 � T0 � ðs1 � s0Þ ð3Þ

It can thus be seen that it is necessary to calculatethe entropy, s, to determine the exergy balance. Inmodule Y, the substances undergo a fullyreversible chemical reaction. This reactionmodule modifies the original substances in thestream in such a way that they can be brought intoa state of chemical equilibrium with the substancesin the reference environment. The co-reactants andproducts are delivered and removed from themodule at environmental pressure andtemperature. The maximum work is equal to thedifference between the chemical potential of thereactants and the products, and equals the Gibbsfunction of the reactions (Dg0):

½Wrev�pr ¼ �Dg0 ð4Þ

It should be noted that both modules X and Yinclude a possible heat transfer with theenvironment, which must be taken intoconsideration in the complete calculation. Thefinal stage, module Z, uses an isothermalconcentration change in the species either to theconcentration needed in module Y or to theconcentration in the environment. This module isonly used when considering gaseous compoundsor compounds in a water solution. There is noreason to utilize the concentration difference whenconsidering solid material because e.g. thewithering process is so slow. A possible heatexchange can also be performed in module Z tomaintain constant temperature. In thisinvestigation, it is assumed that the productionmethod is continuous and in steady state, whichmeans that the accumulation terms can beneglected.

3. PRODUCTION METHODS

3.1. Electrolytical methods

Production of sodium hydroxide has traditionallybeen related to chlorine production since thepredominant methods for large-scale productionare based on the electrolytic decomposition ofsodium chloride in an aqueous solution (brine),which results in the generation of chlorine,hydrogen and a sodium hydroxide solutionaccording to the overall reaction

2Naþ ðaqÞ þ 2Cl� ðaqÞ þ 2H2O! 2Naþ ðaqÞ þ 2OH� ðaqÞ þ Cl2 ðgÞþH2 ðgÞ ð5Þ

There are three main methods for producingelectrolytic decomposition of brine: the mercurycell process, the diaphragm cell process and themembrane process. In all of these methods,chlorine is produced on the anode, while hydrogenand aqueous sodium hydroxide are produced onthe cathode [4].

In all electrolytic processes, the brine is usuallysaturated using solid salt and recycled brine fromthe electrolysis step or the concentration step.Then, the brine must be precipitated and filtratedto remove traces of metals that could decrease theefficiency of the process. This treatment isparticularly important in the mercury cellprocess, where dangerous operating conditionscould result, and in the membrane cell processwhere the membrane could be damaged bydivalent ions such as Ca21 and Mg21.

3.2. Methods using Na2CO3

The lime cycle is the conventional causticizationsystem in the Kraft process, but other technologiesfor sodium hydroxide production are also avail-able. These include direct causticization andautocausticization, which have shown good ex-perimental results, but have also shown somelimitations in dealing with chemicals present in theKraft process, such as sulfur. These limitations canbe avoided if the process is separated from thepulping process and used solely for the productionof sodium hydroxide using pure sodium carbonateas feedstock.

EFFICIENCIES OF NAOH PRODUCTION METHODS 1343


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3.2.1. Lime cycle. For many years, the lime cyclehas been the universally preferred method forproducing sodium hydroxide in the Kraft process.In a stand-alone process, the process is fed withdissolved sodium carbonate and solid calciumoxide, which are converted into sodium hydroxide,according to the following equations:

CaOþH2O! CaðOHÞ2 ð6Þ

Na2CO3 ðaqÞ þ CaðOHÞ2! 2NaOH ðaqÞ þ CaCO3 ð7Þ

Equation (7) is an equilibrium reaction where thesodium hydroxide concentration can, in practice,reach 120–140 g l�1 [5]. After caustici-zation, the calcium carbonate is separated fromthe formed sodium hydroxide in a clarifier. Theseparated lime mud is washed, filtered and thentransferred to the lime kiln where calciumcarbonate is converted to calcium oxide,according to Equation (8). Then calcium oxide isrecycled to the slaking unit (see Equation (6)):

CaCO3 ðheatÞ ! CaOþ CO2 ð8Þ

3.2.2. Direct causticization. Direct causticizationis a technology that uses a metal oxide (MxOy) as acausticizing agent. The process is carried out intwo basic steps: a reactor, e.g. a fluidized bed, anda leaching step. In the reactor, the metal oxidereacts with sodium carbonate, according to thegeneral equation [6]

aNa2CO3 þ bMxOy

! aNa2O � bMxOy þ aCO2 ð9Þ

The product is then hydrolyzed according to thefollowing equation:

aNa2O � bMxOy þ ðaþ zÞH2O! 2aNaOHþ bMxOy � zH2O ð10Þ

The metal oxide hydrate is insoluble in the sodiumhydroxide solution and can therefore be separatedand recirculated to the reactor. There is documen-tation of the successful use of titanium oxide(TiO2), iron oxide (Fe2O3) and ilmenite(FeO � TiO2) for direct causticization [7–10].

3.2.2.1. Titanium oxide. The method of producingsodium hydroxide by direct causticization using

titanium oxide is based on the formation ofsodium titanate through the reaction of sodiumcarbonate and titanium dioxide, according to thefollowing equation:

Na2CO3 þ TiO2 ðsÞ

! Na2O � TiO2 ðsÞ þ CO2 ðgÞ ð11Þ

Sodium titanate is decomposed when dissolvedin water, forming sodium hydroxide:

Na2O � TiO2 ðsÞ þH2O ðlÞ! 2NaOH ðaqÞ þ TiO2 ðsÞ ð12Þ

Titanium oxide is highly insoluble in alkalinesolutions and is easily separated from the sodiumhydroxide solution so that it can be recovered,dried and reused in the causticization process. Thischaracteristic makes titanium oxide very suitableas a causticizing agent because the resultingsolution contains very low concentrations oftitanium oxide and the amount of makeup is,therefore, very low.

The decomposition of sodium carbonate startsin solid state at 5001C, but at a very low rate [7].However, just above the melting point of sodiumcarbonate (8511C) the reaction proceeds quickly toform a compound where the molar ratio betweenNa2O and TiO2 is close to 4

5. This compound isdenoted as 4:5 sodium titanate (4Na2O � 5TiO2).Temperatures higher than 9501C accelerate thespontaneous decomposition of sodium carbonateinto carbon dioxide and sodium oxide (Na2O). Atsuch a high temperature, sodium oxide is volatile,which implies a material loss that increases rapidlyabove 11001C [7,8]. The formation of 4:5 sodiumtitanate occurs through several steps. A furtherreaction of the product can also be found, but onlyin the absence of carbon dioxide, which means thatthe main product leaving a reactor operatedbetween 850 and 9501C in the presence of CO2 is4:5 sodium titanate [11]. In the leaching step, theproduction of sodium hydroxide occurs accordingto Equation (13). Other products may occur, butthis is the main compound [12]:

3ð4Na2O � 5TiO2Þ ðsÞ þ 7H2O ðlÞ! 14NaOH ðaqÞ þ 5ðNa2O � 3TiO2Þ ðsÞ ð13Þ



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3.2.3. Autocausticization. The oxide formed inautocausticization is soluble and therefore cannotbe separated from NaOH. This method has beenused to increase the capacity of NaOH productionon site in pulp mills that have limited lime kilncapacity and in such cases boron is used. Althoughthe ability of sodium metaborate to producesodium hydroxide has been demonstrated, its usein a separate process, solely to produce a causticsoda solution, is not feasible because the causticiz-ing agent remains in the solution. This means thatthe quality of the product would be affected by theconcentration of sodium metaborate, and also thatthe economy of the process would be adverselyaffected by the permanent requirement of sodiummetaborate addition. These two effects are aserious drawback of autocausticization comparedwith the results obtained with direct causticization.

3.3. Selected technologies

Based on the review of the alternatives forproducing sodium hydroxide, it was only possibleto proceed with one technology for furtheranalysis and comparison with the conventionallime cycle process. First, technologies based onelectrolytic decomposition of brine were discardedbecause they all produce elemental chlorine forwhich there is limited use in new mills. Thesetechnologies are only economically viable in verylarge quantities. Several options are available fordirect causticization, however. Iron oxide, forexample, has a very low solubility but smallconcentrations of iron ions that might appear inthe resulting sodium hydroxide solution, andinterfere with chemical use in the bleachingprocess, particularly in stages employing hydrogenperoxide. This problem does not appear iftitanium oxide is used. Therefore, the use of ironoxide and ilmenite can be discarded, whiletitanium oxide appears to be the most promisingalternative.

4. PROCESS ANALYSIS

The two selected processes considered in theanalysis have slightly different inputs and outputs.Although both use sodium carbonate as a sodium

source, the lime cycle process requires a solution ofsodium carbonate, while the titanate processrequires solid sodium carbonate. In addition, theresulting caustic solutions differ in concentration.The typical concentration obtained from a limecycle process is 128 g NaOHkg�1 of solution,while a higher concentration, 190 g NaOHkg�1

solution, is expected from the titanate process.Today, Kraft pulp mills operate with concentra-tions consistent with the lime process, i.e. around128 g NaOHkg�1 solution. In order to comparethe two processes, the boundary of the analysiswas extended so that one theoretical unit wasadded for each process. A dissolving unit wasincluded in the lime cycle process to allow forstarting the analysis with solid sodium carbonate.A dilution unit was included in the titanate processto reduce the concentration of the caustic solutionfrom 190 to 128 kg NaOHkg�1 of solution.

Both the lime and the titanate processes wereassumed to have units that operate at 1223K andothers that operate at 373K. This can be justifiedby the fact that today’s pulp mills operate at about1223K in the lime kiln and at around 373K(boiling water at atmospheric pressure) for theslaking and causticization units. The hightemperature in the titanate process was assumedto be 1223K in order to attain as high aconversion as possible without severe materialloss. The leaching reaction, i.e. the production ofNaOH, occurs at 373K to maximize the reactionrate without evaporation of water. The heatrequired in both cases was assumed to beprovided by hot reservoirs at temperatures of1323 and 473K, both placed outside theboundaries of the system. In reality, the origin ofthese reservoirs differs but in this case the purposewas to compare the two different methods fromtheir potentials and not to get the exact exergy lossin a specific pulp mill configuration. Thetheoretical temperature difference of 100Kensured a fast and effective heat transfer. Theheat released by the system was absorbed by theenvironment, which was assumed to be a coldreservoir at the reference temperature, T0 5 298K.The assumption of hot and cold reservoirs outsidethe boundaries of the system meant that all theheat required or released by the process was



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considered as an interaction with the referenceenvironment. The change in temperature of anymaterial in the process was associated with achange in energy and in entropy. The energychange is given by

DS ¼Z 1223

373

CP

TdT ð14Þ

To define the composition of the reference state,some of the rules proposed by Morris and Szargut[13] were applied: A reference species must beadopted for each chemical element; the referencesubstance adopted must be a common componentin the environment; chemical exergy for thereference substances should be assumed as zero.Taking this into account, the reference compositionfor both processes was defined as a mix of H2O (l),CO2 (g), CaCO3 (s), Na2CO3 (s) and TiO2 (s). Thismix assured the existence of all elements containedin the different substances that participate in bothprocesses, and they were abundant enough in theenvironment to be assumed as zero chemicalexergy. The thermodynamic data used in thisstudy, enthalpy (including enthalpy of formation)and absolute entropy, were taken from the sourceslisted in Table I.

To determine the exergy of the materials in theprocesses, a number of reactions were used tovirtually transform the materials to the materialpresent in the chosen reference environment. Forthe lime cycle process, the following reactions wereused:

CaOþ CO2 ! CaCO3 ð15Þ

CaðOHÞ2 ! CaOþH2O ð16Þ

Na2CO3 ð163Þ ! H2OþNa2CO3 ðsÞ ð17Þ

2NaOH ð128Þ þ CO2

! Na2CO3 ðsÞ þH2O ð18Þ

Na2CO3 (163) means a water solution of Na2CO3

at a concentration of 163 g kg�1 solution. This willthen equal the amount necessary to produce aNaOH concentration of 128 g kg�1 solution in theproduct, denoted as NaOH (128):

Na2O � 3TiO2 þ CO2

! 3TiO2 ðsÞ þNa2CO3 ðsÞ ð19Þ

3ð4Na2O � 5TiO2Þ ðsÞ þ 7CO2

! 7Na2CO3 ðs; lÞ þ 5ðNa2O � 3TiO2Þ ðsÞ ð20Þ

2NaOH ð190Þ þ CO2 ! Na2CO3 ðsÞ þH2O ð21Þ

NaOH ð128Þ ! NaOH ð190Þ þH2O ð22Þ

NaOH (190) corresponds to the achievableconcentration in the titanate process (190 g kg�1

solution).The chemical exergy can then becalculated according to the following equation:

DE ¼ ðHproducts �HreactantsÞ � T0ðSproducts

� SreactantsÞ ð23Þ

4.1. The lime cycle

The slaker takes water from its surroundings andrecirculated calcium oxide from the lime kiln.These two compounds react to form calciumhydroxide, which is the causticizing agent for theprocess.

The equilibrium reaction that occurs in thecausticizer allows a maximum concentration of140 g l�1 of NaOH (at 1001C), which was used asthe reference result for this process. The productconcentration was approximately 128 gNaOHkg�1 of solution, which means that 2molof sodium hydroxide produced was dissolved in32.28mol of water, which entered the system in thedissolving unit. This amount of water dissolved1mol of sodium carbonate, and produced asolution with a concentration of 162.6 gNa2CO3 kg

�1 solution.The analysis assumed that the amount of water

in the sodium carbonate solution was the exactamount required to produce 128 g NaOHkg�1 of

Table I. Sources of thermodynamical data for differentsubstances.

Substance Source

CaO, Ca(OH)2, CO2,Na2CO3(s)

NIST [14]

CaCO3 Barin and Knacke [15]NaO � TiO2, NaO � 2TiO2,NaO � 3TiO2

Barin [16]

NaOH (aq), Na2CO3 (aq),H2O

Theliander [17]

Heat of reaction(reaction (13))

Richards and Theliander[18]



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solution. This is a simplification and in realitysodium hydroxide is formed through anequilibrium reaction (Equation (7)), and thussome Na2CO3 will remain in the solution [19]. Itwas also assumed that the calcium carbonate wasextracted dry from the causticizer and taken to thelime kiln where it was heated to 1223K, thetemperature at which it reacts to produce calciumoxide and carbon dioxide. The latter was used toheat the incoming calcium carbonate so that wastecarbon dioxide was released at 473K. Theregenerated calcium oxide was then cooled to373K and reused in the slaker. Figure 2 shows aflow sheet for the lime process.

4.2. The titanate process

Direct causticization with titanates appears to bethe only alternative to the lime process for on-siteproduction of caustic solutions. The titanateprocess is described in the simplified flow sheet(Figure 3).

The process consisted of a reactor where 4:5sodium titanate was produced by a reactionbetween sodium carbonate and 1:3 sodiumtitanate. This process also assumed fullconversion of Na2CO3. Sodium hydroxidesolution was produced in the leaching stepthrough a reaction between 4:5 sodium titanateand water. Since the solution obtained from theleaching unit can have a concentration as high as190 g NaOH per kg of solution, it was diluted to

128 g per kg of solution so that the final productwas equal to that obtained in the lime cycleprocess.

5. EVALUATION AND PROCESSIMPROVEMENTS

The enthalpy required or released in the processesis given in Tables II and III. It is clear that theoverall required heat for the lime cycle is lowerthan the heat needed for the titanate process:136.1 kJmol�1 NaOH and 157.4 kJmol�1, respec-tively. However, the energy analysis does not takeinto consideration the energy quality, and the heatof the reaction in the hot environment is quitedifferent between the two processes.

The exergy flow diagrams (Figures 4 and 5) forthe lime cycle and titanate processes were used as afirst step in comparing the two processes in moredetail.

The large inputs were heating lime and 1:3sodium titanate to the high temperature (1223K)and heating water to 373K. The lime cycle processshowed lower exergy consumption than thetitanate process prior to any heat integration inthe processes. The total exergy used in the limeprocess was 109.1 kJmol�1 NaOH and in thetitanate process it was 170.9 kJmol�1 NaOH.However, both the lime and the titanateprocesses had a large amount of exergy, whichwas consumed by cooling compounds from the

Figure 2. Flow sheet of the lime cycle. Figure 3. Flow sheet of the titanate cycle process.



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high-temperature level to the low-temperaturelevel, i.e. from 1223 down to 373K. Thesestreams can instead be used for heat integration,

and thus reduce the need for a high-temperaturereservoir. Therefore, we suggest that the solidmaterial exiting the hot reactor be allowed to pass

Table II. Enthalpy flow in the lime cycle process.

Source Required (kJmol�1 NaOH) Released (kJmol�1 NaOH) Temperature range

Lime kilnHeating of lime 49.4 373–1223Reaction 82.5 1223Cooling of CaO 22.0 1223–373Cooling of CO2 22.9 1223–298Slaker and dissolutionWater for final solution 85.8 298–373Dissolution of Na2CO3 11.3 373Water for slaking 2.8 298–373Slaking 33.8 373CausticizerReaction 2.0 373Total 222.5 86.4

Table III. Enthalpy flow in the titanate process.

Source Required (kJmol�1 NaOH) Released (kJmol�1 NaOH) Temperature range

ReactorHeating of 1:3 and Na2CO3 172.3 373–1223Reaction 32 1223Cooling of 4:5 120.6 1223–373Cooling of CO2 22.9 1223–298LeacherIncoming water 56.5 298–373Reaction 7.6 373DiluterIncoming water 32 298–373Dilution 0.5 373Total 300.9 143.5

Figure 4. Exergy flows in the lime cycle.



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through an indirect heat exchanger where the heatis then transferred to the material that is to bewarmed, i.e. reburned lime (CaO) in the limeprocess and 4:5 sodium titanate in the titanateprocess. If a counter-current indirect heatexchanger with a minimum temperature diff-erence of 10K is used where air comes in directcontact with the solid material leaving the hotreactor, a total of 22.0 and 120.6 kJ can betransferred as heat to the air from the limeprocess and the titanate process, respectively.Note that this is energy transfer and that theminimum temperature of the solids is defined as373K, which is more than the inlet temperature ofthe air, which is at the reference temperature of298K. To further increase efficiency (reduceexergy loss), the off-gases from the hot reactor,i.e. CO2, can be used in the heat exchanger. Theamount and temperature of produced CO2 are thesame for both processes and will transfer another22.9 kJ in each process. All energy from thisstream is transferred because it is assumed thatthe gases are mixed. The hot gas stream is thenused to preheat the incoming solids, and therebycomplete the energy transfer from one solid stream

to another. Again, assuming a minimumtemperature difference of 10K, the transferredenergy will be 40.8 kJ for the lime process and130.2 kJ for the titanate process. The outlet gaseshave a temperature of 383K after the heatexchanger. This decreases the need for heat fromthe hot reservoir, and the resulting heat demandsbecome 91.1 kJ for the lime process and 74.1 forthe titanate process. The corresponding exergyvalues are 70.6 and 57.4 kJ, respectively. Table IVshows the exergy input for the two investigatedprocesses for the two different levels ofintegration, i.e. no integration and partintegration.

It is clear that the integration between the twoprocesses is beneficial, especially for the titanateprocess where the exergy demand for the processafter integration is almost half of what it was beforeintegration. The energy need decreased in the hotreactor from 204.3 down to 74.1 kJ. Thisopportunity to substantially decrease the energydemand was also found in another study thatexamines the improvement of the chemical recoverycycle in a Kraft pulp mill [19], but in that study theincoming sodium carbonate was a smelt and would

Table IV. Exergy input and consumption for the lime process and titanate process (2 means after heat integration).

Exergy (kJmol�1 NaOH)

Lime process Lime process 2 Titanate process Titanate process 2

In 119.9 103.3 181.7 92.1Consumption 109.1 92.5 170.9 81.3

Figure 5. Exergy flow in the titanate process.



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therefore favor the titanate process. Heat is stillrequired to warm the incoming water in bothprocesses. However, this energy is the same in bothprocesses and, therefore, does not change theoverall comparison but has a large impact on theefficiency of each process.

6. CONCLUSIONS

Based on a screening of different methods forproducing NaOH on a Kraft pulp site, only directcausticization using titanates was found to be ableto compete with the conventional method basedon a lime cycle. All other methods produced eitherunwanted side products or intervened with pulpproduction.

The first and second law analyses show that inorder to compete with the conventional lime cycleprocess, the titanate process requires an effectiveheat integration system to recover part of theenergy used to heat the 1:3 sodium titanate thatenters the reactor. Good heat integration can alsoimprove the performance of the lime cycle.However, the potential for improvement is muchgreater in the titanate process, which at best mightrequire only 80% of the energy required by a heat-integrated lime cycle process. This is based on acomparison between the heat delivered at hightemperatures, i.e. 91.1 kJ for the lime process and74.1 kJ for the titanate process.

The dilution step considered in the titanateprocess analysis was created to achieve a productconcentration equal to that obtained in thelime cycle process for a consistent comparisonof energy performances. The capacity of thetitanate cycle to produce a more concentratedsolution, which is preferable for the bleachingprocess, should speak for the adoption of thistechnology, instead of the conventional lime cycleprocess.

The analysis does not take into account factorssuch as efficiencies of reaction, efficiencies offiltration and requirements for makeup. Furtheranalyses in these areas are expected to showfavorable results for the titanate process if theexperimental results reported in the literature,which show better reaction equilibrium, filtration

properties and particle breakup than the limecycle, can be applied industrially [19,20]. Onedrawback of the titanate process is the largeamount of solid material that must be handled.

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DOI: 10.1002/er

Efficiencies of NaOH production methods in a Kraft pulp mill

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