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Novel Design of an Integrated Pulp Mill Biorefinery for the ...

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<strong>for</strong>ce. Washing is broken up into five individual stages: dilution, mixing, dewatering, diffusion,<br />

<strong>an</strong>d displacement. The water used <strong>for</strong> this process is ga<strong>the</strong>red from <strong>the</strong> drying <strong>an</strong>d evaporation<br />

stages.<br />

The first stage typically uses <strong>an</strong> acidic solution which binds to <strong>the</strong> lignin. In between each<br />

bleaching stage, <strong>the</strong> chemicals are drained from <strong>the</strong> pulp <strong>an</strong>d it is <strong>the</strong>n washed with <strong>the</strong><br />

a<strong>for</strong>ementioned water. Only in <strong>the</strong> last acid <strong>an</strong>d alkaline stages does <strong>the</strong> water have to be pure.<br />

Next, <strong>the</strong> lignin acid is removed in <strong>an</strong> E stage with sodium hydroxide. At <strong>the</strong> end <strong>of</strong> <strong>the</strong> process,<br />

<strong>the</strong> pulp is whitened by sodium hypochlorite, chlorine dioxide, or hydrogen peroxide.<br />

Because <strong>of</strong> restrictions placed by <strong>the</strong> United States government on effluent from<br />

bleaching pl<strong>an</strong>ts, elemental chorine free (ECF), total chlorine free (TCF) <strong>an</strong>d totally effluent free<br />

(TEF) are methods that have been put into practice in order to process paper environmentally<br />

sound. Yet all <strong>of</strong> <strong>the</strong>se processes require <strong>the</strong> earlier discussed oxygen delignification to be<br />

successful [26]. The best technology that exists <strong>for</strong> both TCF <strong>an</strong>d ECF bleaching if used on pulp<br />

with a kappa number <strong>of</strong> less th<strong>an</strong> 20 will result in <strong>the</strong> same effluent quality [27]. A comparison<br />

<strong>of</strong> chemicals used in each process is listed in <strong>the</strong> chart below:<br />

Table 1: Bleaching chemicals <strong>for</strong> ECF <strong>an</strong>d TCF bleaching processes[9].<br />

In comparison, taking two pulps <strong>of</strong> <strong>the</strong> same Kappa level number, <strong>the</strong> bleaching yield in<br />

<strong>the</strong> TCF pulp will be lower th<strong>an</strong> <strong>the</strong> one bleached with <strong>the</strong> ECF process [28]. In all <strong>the</strong> bleaching<br />

stages, bleaching chemicals consumption in <strong>the</strong> first-stage is directly proportional to <strong>the</strong><br />

incoming Kappa number [27]. Bleaching costs <strong>of</strong> TCF bleaching are on par with <strong>the</strong> cost <strong>of</strong> ECF<br />

bleaching at a Kappa number <strong>of</strong> around 20 [29]. However, total operating costs are signific<strong>an</strong>tly<br />

higher <strong>for</strong> TCF pulps. ECF adds $5-$10/ton <strong>of</strong> total production cost above chlorine bleaching,<br />

while TCF adds $40-60/ton, including capital expenditures (Pulliam, 1995). TCF pulps also are<br />

less bright <strong>an</strong>d not as strong as ECF pulps. Upgrading a pl<strong>an</strong>t from <strong>an</strong> elemental chlorine bleach<br />

pl<strong>an</strong>t to <strong>an</strong> ECF pl<strong>an</strong>t would require a 33% increase in energy usage <strong>for</strong> <strong>the</strong> bleach pl<strong>an</strong>t because<br />

<strong>of</strong> <strong>the</strong> chlorine dioxide creation [8]<br />

Recent ECF estimates approximate 2.3 GJ/ADt steam requirement <strong>an</strong>d 100 kWh/ADt <strong>for</strong><br />

bleaching [22].<br />

2.1.6 Causticizing <strong>an</strong>d Lime Kiln<br />

Green liquor, whose main components are sodium sulfide (Na2S) <strong>an</strong>d sodium carbonate<br />

(Na2CO3), is a byproduct <strong>of</strong> both <strong>the</strong> gasification <strong>of</strong> black liquor process <strong>an</strong>d <strong>the</strong> recovery boiler<br />

14

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