Introduction
Black liquor is a complex, hazardous and challenging liquid to analyse. Its highly concentrated nature makes measuring its make-up very difficult, and often requires the use of advanced laboratory techniques to identify its chemical constituents.
The need for laboratory analysis makes a detailed chemical analysis of strong black liquor out of reach for most pulp mills. Even making a simple residual effective alkali (REA) measurement requires titration - which is time consuming and expensive.
Spectrometers allow continuous and detailed measurement of chemical concentrations in real time. Unfortunately, the majority of process spectrometers are based on near infrared light, which is fundamentally less informative than mid infrared light. Conventional mid infrared spectrometers (which often use a Fourier transform are referred to as "FTIR spectrometers") have sensitive moving parts and fragile fiber probes – making them wholly unsuitable for production environments such as pulp mills.
The IRmadillo is a process analyzer built using FTIR spectroscopy, but with static optics: removing the need for moving mirrors or fiber probes, and dramatically improving stability, reliability, and ruggedness.
Example Use Case
This application note presents data collected at a single site in North America, where the IRmadillo is mounted directly into a strong black liquor line (~ 60 % solids). Originally designed purely to measure Na2S for environmental monitoring, the customer then expanded the calibration to become a universal chemical composition analyzer for black liquor. The IRmadillo was installed directly into an existing line with a 2" ANSI flange, no additional installation work was required.
Figure 1: An IRmadillo process analyser installed in strong black liquor line, with accompanying compressed air dryer and filtration unit.
After the IRmadillo was installed, the customer was able to make continuous measurements of composition - which they are intending to use to make process improvements, and to identify potential problems such as scale and corrosion early enough to fix them whilst they're small.
Results
Discrete calibrations were made for a large range of different chemicals, using laboratory analyses as reference data with a combination of partial least squares (PLS), support vector regression (SVR) and locally weighted regression (LWR) modelling techniques.
Average errors of measurement for the chemicals of interest are shown here.
- Total Dry Solids (TDS): 1 wt%
- Oxalate: 300 mg / kg
- Thiosulfate: 2800 mg / kg
- Sulfate: 2200 mg / kg
- REA: 0.29 %
- Sodium: 0.1 %
- Density: 0.006 g / mL
- Crude tall oil: 0.02 %
- Carbonate: 0.1 %
Figure 2: The measurement of 9 different chemicals and physical properties in a strong black liquor line performed simultaneously with a single IRmadillo installation over several months.
Conclusions
This work has shown that the IRmadillo spectrometer is easy to install directly into the strong black liquor line, and then operates as a single instrument measuring 9 different chemicals and properties simultaneously - updating to the DCS every two minutes.
The IRmadillo can measure both chemicals to high levels of accuracy (in many cases in the mg / kg, or ppm level) as well as physical properties such as density.
Additionally, the instrument is inherently stable with a single installation able to make measurements for several months with no maintenance or attention from the customer. A single dry air line source from a co-located compressed air dryer is all that is required to keep the instrument operational.
Keep In Mind
The IRmadillo is a static optics FTIR spectrometer that is then calibrated to measure the process of interest - changing it from a spectrometer outputting spectra, into an analyzer reporting concentrations and properties of interest.
It can be calibrated for almost any application in almost any process. Whether it's measuring reduction efficiency in green liquor or controlling the effective alkali in white liquor - the IRmadillo is a universal analyzer for pulp and paper making.
