The breakdown process from solid wood to paper involves quite a number of preparative steps. The main process which converts wood into pulp is named the Kraft process, which utilizes white liquor (a mixture of sodium hydroxide «NaOH» and sodium sulfide «Na2S») to break down the lignin and cellulose linkages.
In the Kraft pulping process, the wood chips are saturated with white liquor and cooked at high temperatures in pressurized digesters, forming a liquid stream consisting of pulp and black liquor. After a washing step, the resulting pulp is sieved, washed, and bleached to produce paper; and the now weak black liquor continues to the chemical recovery loop into the evaporators.
After passing through multiple evaporators, the now concentrated black liquor enters the recovery boiler, where sodium sulfate «Na2SO4» is reduced to Na2S. Then, the green liquor (containing mostly Na2S and sodium carbonate «Na2CO3»), is sent to the causticizing plant to react with lime «CaO» and regenerate white liquor for the pulping process. Here, the recovery cycle is completed.
In the Kraft recovery cycle, the constant monitoring of residual effective alkali «REA» of black liquor, total titratable alkali «TTA», active alkali «AA», and effective alkali «EA» of green and white liquor, are necessary for maintaining the optimal recovery process, improving reaction yield, and enhancing process optimization. However, these streams are very hot and corrosive (like white liquor), thus there are risks of accidents when sampling manually.
It is also important to consider that variations in one part of the cycle will disturb downstream unit operation. In turn, these disturbances can create further variations, causing a downward spiraling effect. Conventional laboratory analysis leads to long response times between the sampling and the analysis in case of process changes, undermining the recovery efficiency.
A great choice for online monitoring alkali, carbonate, hydroxide, and other such important parameters in pulping liquors is the 2060 Process Analyzer from Metrohm Process Analytics. Save time, avoid spillages, and increase efficiency without manually sampling process points.
The 2060 Process Analyzer is used to determine alkali (AA, EA, and TTA), carbonate, hydroxide, sulfide, and causticizing degree «CE%» online in pulping liquors. The analyzer is suitable to analyze white, green, black, and wash liquors with multiple sample lines, outputting results for closed loop control. This method conforms to the standard method SCAN-N 30:85, the pulp and paper industry standard.
0.1–3 mol/L CO32- and S2-, 0.1–6 mol/L OH-, alkali 0.1–8 mol/L. Higher and lower ranges are possible with the use of special preconditioning systems.
Additionally, the sulfate concentration can be measured online with thermometric titration in the same instrument. In combination with the ABC titration, this gives a perfect indication for the degree of reduction and information about the recovery boiler efficiency, which acts as a reactor. The thermometric titration gives a faster response and avoids the use of toxic chemicals. Other online applications in the pulp and paper industry are during the bleaching process.