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NIR Liquor Analyzer in the Pulping and Recovery area

BACKGROUND
Surprisingly, the NIR technology has been around for more than 30 years, mainly utilized in the agricultural industries. Laboratory Spectroscopy has been used for analysis of organic and inorganic compounds and in-depth studies for online sensors measuring liquors in the Kraft processes have been used since the mid-1980s. However, low maintenance, cost effective instrument technology to continuously monitor liquor composition at key locations in Kraft processes has not been readily available.

The most practical work in NIR Spectroscopy development has been conducted at the Auburn University Pulp and Paper Research and Educational Center in Auburn, Alabama, USA.

NIR Spectroscopy
The NIR spectral region is defined as the band of wavelengths from 750 to 2500 nm. The quantitative analysis begins with acquiring a spectrum of a sample using a number of wavelengths within this range. Figure on the right shows a typical Transmission Cell arrangement for analyzing liquid samples such as white, green and/or black liquors. This part of the analysis takes 15 seconds. It is critical that the construction materials meet the harsh process environment that is present; therefore all metal parts are made of titanium and the lenses are sapphire.

The spectrum is linearized using the Beer-Lambert relation. Extraction of data is done utilizing Partial Least Square Regression (PLSR) technique. Using spectral data measured from samples with known properties (e.g. EA, AA, TTA etc.) a regression model is built. This regression model is then compared to spectra from unknown samples to measure the chemical/physical properties of the unknown sample.

Recent Advances
Even though the NIR technology had shown much promise at several industrial installations, no commercially viable, cost effective analyzer has been available until recently. Some of the critical issues include: the ability to collect samples that are under process pressure and/or atmospheric pressure, keeping the sampling lines from either plugging with solid particles (mud, chips, fibers) or scaling up due to liquors. At the analyzer: sampling valves must meet the requirements of being leak-proof against slurry and/or high pressure while optics are kept clean by periodic and automatic acid cleaning followed by a self-calibration cycle.

These sampling related problems along with spectrometer robustness issues have been resolved with a physically small, semi portable, environmentally self contained and fully automated analyzer package. A commercial version is now available for white, green and black liquor analysis at key measurement locations in the Kraft process.

Chemical composition of process liquors is determined through the use of near-infrared (NIR) spectroscopy. This spectroscopic technique is a subset of a larger class of analytical techniques that fall in the category of optical spectroscopy techniques. The following figure shows the electromagnetic spectrum with the portion relating to optical spectroscopy expanded.

Optical spectroscopy techniques have been used quite successfully for decades in the laboratory setting to analyze liquids, solids and gases composed of a multitude of chemical species. In the past twenty five years the development of miniaturized and durable electronic and optical components has allowed many of these techniques to be moved from the laboratory setting to the process environment.
Development of new computational techniques along with the microcomputers to implement them have further advanced the use of spectroscopic techniques for both qualitative and quantitative analysis at the process.

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Applications
NIR based analyzers can be utilized in several Kraft pulp mill processes.

Some of these applications have already been in service for several years while some are just now being implemented

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Utility Requirements

  • Electrical: 110-120 VAC/60 HZ, 30Amp
  • Air: Instrument air 70-120 psi
  • 3/8” stainless steel tubing
  • Sample Lines: ½” Stainless tubing
  • Drain: 1” pipe back to process
  • Water: Mill water 40-80 psi 3/4“ tubing
  • I/O: 4-20mA for each component  or MODBUS/TCP

Maintenance RequirementsDuralizer Liquor Analyzer

  • Light Source: Bulb replacement once per year
  • Lens cleaning Acid: Every 6 months
  • Maintenance Kit: Available

Desktop NIR-Liquor Analyzer

The lightweight desktop analyzer is capable of providing any test results previously discussed, and many more, in just 25 seconds. Operator bias is eliminated due to sample volume and endpoint variations. The NIR analyzer does not need pH indicators, pH probes or titration chemicals or any of the other peripheral support materials required for titration based testing. Excess lab equipment is eliminated and test repeatability is enhanced.

Manufacturer: R.E. Hodges; Auburn, Alabama

Liquor Analysis:

The NIR spectroscopy has shown to be able to measure e.g. the following Kraft mill liquor properties with an accuracy of 0.99 or better correlation coefficient with the lab:

  • TTA – Total titratable alkali
  • AA – Active Alkali
  • EA – Effective alkali
  • TDS – Total Dissolved solids
  • TDD – Total Dissolved dead load
  • RE – Reduction efficiency
  • REA – Residual effective alkali
  • RAA – Residual active alkali
  • TDO– Total Dissolved Organics

Duralizer Liquor Analyzer

The transmission cell provides a means for NIR radiation to interact with the process sample while isolating the light source, fiber optic cable and spectrometer from the process. A typical transmission cell is composed of a body with appropriate sample inlet and outlet connections and a pair of optical couplers to deliver light to the sample and collect light after interaction with the sample.

The optical couplers house a set of lenses to focus the radiation onto the tip of the fiber optic cable. The ends of the couplers in contact with the process sample have windows, usually sapphire, to provide a transparent optical path for the entering and exiting light as well as providing isolation from the process sample. Sapphire is usually the material of choice for the coupler windows due to its combination of hardness, chemical and heat resistance and transparency over a broad range of wavelengths.

Duralyzer Installations

Spectroscopic based measurement solutions hold the key to solving most if not all of the difficult measurement applications in the process industries. R. E. Hodges, LLC was formed to develop these measurement and control solutions so traditional lab based testing for quality control can be replaced by real time online measurements coupled with advanced control methods. Duralyzer, our NIR Liquor Analyzer is unique compared to traditional spectrometer manufacturers in the fact that it is a turnkey measurement and control solution, based on spectroscopic methods, that has been tailored to suit the application.

When an analyzer leaves our manufacturing facility it has been optimized for the application and is guaranteed to have minimal installation requirements and minimal continuing maintenance requirements. We are so confident in our products that we include a complete money back guarantee if the customer is not totally satisfied with the performance of the product.