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IRmadillo – robust process analyser to measure reduction efficiency for real-time control of the recovery boiler

Abstract

This work presents the use of a static-optics FTIR spectrometer, Keit’s IRmadillo, to measure the concentrations of sodium sulphide (Na₂S) and sodium sulphate (Na₂SO₄) in the green-liquor line downstream of the smelt dissolving tank outlet at a pulp mill for real-time measurement and control of reduction efficiency in the recovery boiler.

Introduction

The recovery boiler is one of the most important parts of pulp production, being responsible for both the generation of energy recovery, and the reduction of sulphates and sulphites into sulphides for re-use. The performance of the boiler is commonly measured using reduction efficiency (RE):

RE = [Na₂S]/([Na₂S] + [Na₂SO₄])

Calculating the efficiency requires the ability to measure both the concentration of Na₂S and Na₂SO₄, and then calculate the RE value. Laboratory measurements of Na₂S and Na₂SO₄ are slow, can be prone to errors and require the handling of hot, caustic material by the operators.

Spectrometers allow continuous and detailed measurement of chemical concentrations in real time. Unfortunately, the majority of process spectrometers are based on near-infrared (NIR) light, which is fundamentally less informative then mid-infrared light. Conventional mid-infrared spectrometers (often using a Fourier Transform – referred to as FTIR spectrometers) have sensitive moving parts and fragile fibre probes – making them wholly unsuitable for production environments such as pulp mills.

Here we the present the use of a static-optics FTIR spectrometer – the IRmadilloDiamond – to measure RE in real time from a single installation downstream of the recovery boiler at a pulp mill.

“…we have found IRmadillo to be stable, robust, precise and well suited for industrial on-line applications”.

Senior Development Engineer, International Forest Products Company

Instrument Installation and Calibration

The IRmadillo instrument was installed directly into the outlet of the smelt dissolving tank at a pulp mill using a welded flanged connection (see Figure 1). The pipe is periodically flushed to prevent scale formation.
Spectra were acquired continuously with a two-minute averaging time to optimise signal to noise (SNR) ratio.

Calibration was performed using a range of partial least squares (PLS) and support vector regression (SVR) models, using a combination of the Unscrambler and Solo chemometrics packages. Reference data were provided by the mill using laboratory techniques.

IRmadillo Installed pulp mill

Figure 1: Photograph of the IRmadillo spectrometer installed in the outlet of the smelt dissolving tank

Results and Discussion

FTIR directly measures the fundamental chemical bond vibrations for the molecules being studied – for example, the S=O and S-O bonds in the sulphate ion.

The absorbance, A, at a given wavenumber is given by the Beer-Lambert law, A = εcl, where ε is the coefficient of absorbance of a given chemical at that wavenumber, c is the concentration of that chemical, and l is the path length through the sample. This law states that the relationship between absorbance and concentration is linear, which allows a regression model such as PLS to be built.

It is possible to build a calibration directly for reduction efficiency, but the best practice is to measure the fundamental chemical concentrations and then calculate reduction efficiency in real time from those values instead.

Figure 2 Performance of IRmadillo calibration model

Figure 2: Performance of calibration model for Na2SO4 using a PLS model

 

Discrete calibrations were made for both Na₂S and Na₂SO₄. The Na₂SO₄ calibration was built using a first order derivative – to account for temperature variations – and then an orthogonal scatter correction (OSC) to account for other spectral artefacts. The results of this calibration are shown in Figure 2. The Na₂S calibration was built using the same transforms as Na₂SO₄. The performance for both calibrations are:

Na₂SO₄ average error = 0.43 g / L
Na₂S average error = 2.52 g / L

These numbers are well within acceptable limits, and this allows the calculation of reduction efficiency from the reported values.

There is also a good linear agreement between the measured and predicted values, with R² > 0.9 for both chemical species of interest.

The aim of this work is for continuous, long-term measurement of RE, without the need for frequent re-calibration or background scan acquisition. Representative on-line measurements over a three-week period are shown in Figure 3, with the off-line reference data shown as red dots overlaid on the continuous measurement.

The data clearly show a very strong agreement between the reference data and the measurement, and even highlight that at least one data point is probably the result of human error (27th May sodium sulphide of 46 g / L is likely an error).

The calculated Reduction Efficiency is shown on the top row of the graph, with the target line of 95% efficiency highlighted. It can be seen that standard operating processes fall below the target value, but the on-line measurement will now allow optimisation and ultimately a closed-loop control.

Figure 3 sodium sulphide and sulphate

Figure 3: The measurement of sodium sulphide and sulphate in real time along with the calculated reduction efficiency

Conclusions

This work has shown that the IRmadillo spectrometer is easy to install directly into the smelt dissolving tank outlet, and that it has the required stability to make continuous, reliable measurements over an extended period of time (at the time of writing, the spectrometer has been installed for over six months with no additional background scan or probe cleaning).

The real-time and reliable measurement of Na₂S and Na₂SO₄ is possible, enabling continuous and real-time calculation of reduction efficiency. In turn, this enables the operators to understand how the recovery boiler is operating and make adjustments to better control it.

Availability of RE as a real-time measurement will enable closed-loop control of the recovery boiler to automate the improvements and move towards a more efficient reduction thereby decreasing operating costs and improving profitability for the mill.

Moreover, the installation in the green liquor line enables feed-forward control of the TTA (total titratable alkali) value for better control of the recausticising process, and – critically – to minimise and prevent over-liming. The next stage of this work will expand the calibration to Na₂CO3 and NaOH to enable real-time calculation of TTA

Pulp & Paper FAQs

How does the IRmadillo cope with build-up of scale (such as pirssonite) in green liquor?

Keit is currently developing an in-situ descaling system for installation sites that suffer from scaling, so you can install the instrument then walk away, knowing it will stop the build-up of scale. What’s more, the instrument checks every scan it takes for quality, and can be programmed to alert you if it detects a scale build-up for manual cleaning if something went very much out of specification on plant.

What sort of sample preparation or conditioning is required?

The IRmadillo was developed to install directly into your process, with no need for filters, sample conditioning systems or anything else. During installation scoping conversations, our engineers will discuss how best to seal the instrument into your process (such as an ANSI or DIN flange). If your process operates at a very high temperature then we may need to bring the temperature down to a manageable level.

Can you measure multiple chemicals at the same time?

The IRmadillo is based on FTIR technology meaning it continuously measures everything that’s present in the mixture – all the time, every time. For you, this means it can look for organic chemicals (such as cellulose and polymers), inorganic salts (such as sulphate) and the physical properties (such as effective alkali and total dissolved solids) all at the same time. It provides a reading every 2 minutes, giving you the tools you need to make changes in real time.

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Keep in Mind

The IRmadillo can be used to measure an enormous range of different chemicals, properties and process states. This example looks at RE, but other important properties such as TTA can also be measured.

Also, process states such as “in spec” and “out of spec” can be created using qualitative calibrations in addition to the quantitative calibrations shown here.

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Contact us

If you like the sound of this, please contact us for more information. We can arrange a trial installation on your plant for several months so you can see you yourself just what the IRmadillo can do.