BRUSSELS, Aug 1, 2014 (PPI Magazine) - The chemical recovery process continues to be a critical component of pulp mills, infusing chemical, steam, and power back into the system. The economic viability of mills rests in large part from efficient and optimized chemical recovery operations. At the heart of the process is the recovery boiler and the dissolving tank.
Fig. 1 - Fourier-transform near infrared analyzer: Platform technology providing whole mill measurements of liquor chemistry
Aside from consumed air control and density control of the raw green liquor (RGL), little has been done to further advance automation and control of the recovery boiler and dissolving tank. The common practice in a recovery boiler is to manipulate the percent solids of black liquor (BL), firing temperature, airflow, and gun pressure. However, changes in the BL chemistry are often unknown. As such, swings in strong BL chemistry result in problematic fluctuations in boiler performance and smelt compositions.
At the smelt dissolving tank, molten smelt from the recovery boiler is mixed with weak wash such that the density of the resultant RGL is within a desirable limit. The green liquor (GL) is then clarified and slaked with lime to form white liquor (WL) through the causticizing process. Common practice at the dissolver is to control density using either hand tests, such as Baumé, or the use of bubble pipes, refractometers or nuclear density meters in conjunction with manual titrations for total titratable alkali (TTA). Downstream, clarified GL control is accomplished by trim control using nuclear density while at the recausticizing unit operation, manual titrations paired with differential temperature and conductivity have been applied to slaker and causticizing efficiency (CE) control.
The use of these traditional techniques (density, temperature, conductivity) all lack chemical specificity and are often prone to drift due to scaling. Process supervisor control using these traditional techniques have failed and stifled advancement in developing supervisory control of the chemical recovery operations. With rising energy, chemical, raw material, and labor costs, there is renewed emphasis on optimizing the chemical recovery operation.
New technology brings opportunities for automation
Fourier-transform near infrared spectroscopy (FT-NIR) has been fully developed by FPInnovations and its Alliance Partners as proven process analyzers for online measurements of various streams in the chemical recovery processes, including: dissolving tank GL, clarified GL, causticizer WL samples from the first causticizer (1C) and fourth causticizer (4C), final WL for complete compositions including reduction efficiency (RE). For weak black liquor measurements provide residual effective alkali (REA), lignin, organics, inorganics, and total solids content.
FT-NIR is based on the principle of vibrational spectroscopy. Infrared light illuminates a sample and the resulting absorbance spectrum with its characteristic feature bands can provide quantitative determination of the liquor compositions including: effective alkali (EA), active alkali (AA), TTA, sulfide, carbonate, sulfate, lignin, organic solids, inorganic solids, etcetera. As an optical method of measurement, analysis time is in the order of minutes and does not require reagents or reaction vessels. Water is used to clean the cell and the sample lines.
Figure 1 illustrates the platform technology measurement capability. Figure 2 illustrates the implementation of an FT-NIR analyzer at a mill's recovery/recaust area with the analyzer station installed above the causticizing tanks.
Applications for recovery boiler smelt dissolving tank control
Control of the dissolving tank has traditionally been accomplished through continuous measurements of the RGL density. While density is correlated to TTA, it is more advantageous to control TTA since it defines the liquor strength. A higher and more stable RGL TTA is desirable as it reduces volumetric flow. This decrease in flow increases residence time in the GL clarifiers and allows for further trim control of the CGL, improving the stability of the liquor feeding into the slake. However, density measurements drift from scaling and are not sufficiently reliable.
Fig. 2 - FT-NIR liquor analyzer sampling station showing sampling tree and transmission cell
Combining the measurement capability of the FT-NIR technology with the smelt dissolving tank TTA supervisory control that incorporates a soft-sensor, mills have access to a continuous interpolation of TTA from the relationship between density and TTA. The drift in the density is corrected by the measurements of TTA from the analyzer at a frequency of every 5 to 10 minutes. The TTA is used as input to a controller that compares the value to the set point and cascades to a flow controller, which in turn manipulates the weak wash valve to achieve the desired dissolver TTA, Fig. 3.
Fig. 3 - Smelt dissolving tank TTA control strategy
The FT-NIR analyzer provides complete liquor compositions, including reduction efficiency (RE), making it possible to calculate the saturation limit of the liquor. Moreover, constraints can be put in place to avoid pirssonite deposition due to liquor properties excursions. Using this control strategy, TTA variability can be significantly reduced, by as much as 70%, Fig. 4. By stabilizing the dissolver TTA, a higher TTA can be targeted to take further advantage of higher strength liquor, leading to debottlenecking of recovery and recausticizing processes at a mill.
Fig. 4 - Comparison between traditional density control and TTA control using FT-NIR
With the ability to measure sulfate, RE can be calculated, providing valuable information on the reduction process in the boiler. Mill process data has shown that RE correlates extremely well with changes in strong BL solids and primary airflow. A contour map was generated by monitoring the RE of the dissolver GL during a period which saw a rise in the strong BL solids content from 69% to 78% and was accompanied by changes in the primary flow. The contour map shows the possible combinations of solids content and primary air that would result in the best RE, Fig. 5.
Fig. 5 - Contour plot relating primary air flow with black liquor solids content to green liquor reduction efficiency. Red indicates higher RE
New technology brings benefits and cost savings
The progress of FT-NIR technology for true liquor composition measurements have led to new advancements in control strategies that take advantage of the multitude of liquor chemical compositions supplied by the FT-NIR analyzers. Advanced control strategies for smelt dissolving tank TTA are now in practice at many mills. Trim control of the clarified green liquor as well as feed-forward slaker control and final CE control have also been applied at mills. The integration of process measurements and advanced control strategies have resulted in significant reductions in process variability, typically from 40% to 60%, allowing for target shifts to higher TTAs and higher CEs.
Mills have also reported sizeable reductions in purchased lime usage and all mills have claimed reductions in pressure filter maintenance, reduced plugging in evaporators and in the recovery boiler. In one case, an increase in WL strength and debottlenecking recovery and recausticizing area substantially increased digester production. All of this translates into an ROI ranging from $500,000 to $3 million. This is just the tip of the iceberg. With a successful implementation of FT-NIR analyzers, pulp mills can realize further potential cost savings in chemical recovery optimization.
Thanh Trung, FITNIR Analyzers Inc., Canada
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