Many chemical plants need colorimetric/photometric analysis of various compounds in their process and wastewater systems. Historically, processors relied upon lab-based bench top systems to perform this type of analysis. Today however, improved designs and digital technologies in field-based process analyzers can produce lab-quality results automatically, more frequently and with much less required labor.
Colorimetric/photometric analysis determines the concentration of a chemical element or compound in a solution by combining a reagent with a sample. This results in a specific sample color; its intensity is proportional to the concentration of the compound of interest.
The color intensity is measured photometrically based on the Beer-Lambert law. This law states that a linear relationship exists between a sample’s absorbance of light and the concentration of the absorbing species, in this case, color.
Lab-based systems rely upon hand-delivered and manually prepared samples for analysis. Field-based systems get samples via independent sample systems operating in concert with the analyzer.
Proper colorimetric/photometric analysis involves three elements: sample preparation and delivery; colorimetry (combining the sample and reagent to produce color); and photometry (measurement of the resulting color intensity). Improper or mediocre performance in any of these three areas will lead to measurement error. Therefore, it’s important to thoroughly understand the potential limitations of any given system.
Colorimetric/photometric analyzers can handle a range of compounds and suit a diverse range of applications. For instance, analyses of ammonium, phosphorus, iron and nitrite are crucial for water and wastewater. Ammonium measurement is critical in wastewater for monitoring ammonia reduction. Ortho-phosphate measurement is used in process control of phosphate removal, either biologically or through flocculation.
Measurements of total phosphorus now are vital to ensure effluents meet discharge permit levels, which are becoming increasingly stringent as state environmental agencies and the U.S. Environmental Protection Agency (EPA) clamp down on nutrient levels in effluent.
Drinking water plants need iron analysis, which falls under the non-mandatory standards with secondary maximum contamination levels (SMCLs); the EPA secondary standard for iron is 0.3 ppm. In industrial processes, iron can build up in tanks, storage vessels, water heaters and pipelines. Iron deposits can decrease capacity, reduce pressure and increase maintenance. They also can cause self-closing valves to stick.
Plant steam and cooling-water systems require monitoring and control of hardness to improve the quality of the water. Hardness isn’t a health hazard but the calcium, magnesium and other metals causing water hardness can lead to scaling in boilers, cooling towers and other equipment, reducing the efficiency of heat transfer. In steam systems, lower levels of impurities decrease corrosion rates in boilers and lower the frequency of blow downs, which cuts fuel costs
Benchtop spectrophotometers are a common tool for colorimetric/photometric analysis in industrial processes. These spectrometers use a small vial of sample, with colorimetric reagents added to achieve the desired color reaction. Because they operate over a range of light wavelengths, these devices can measure a range of colors — and thus can provide results for various compounds. It’s not uncommon to measure more than 35 different parameters with one spectrometer by using a variety of reagents. Newer benchtop systems incorporate barcode readers and RFID [radio frequency identification] technology to auto-identify the appropriate method and calibration adjustments.
However, benchtop systems are relegated to the lab, with testing performed by lab technicians. This necessitates sampling at the process and transferring those samples to the lab for analysis. These systems can produce an accurate result — but one only indicative of the sample at the time of analysis, not when it was taken.
Moreover, sample collection and transfer can require very specific criteria to ensure sample integrity prior to analysis. Samples may need filtering or treating and may demand analysis within a certain limited time. What may appear on the surface as a straightforward process—sample collection and preparation—can be time-consuming and prone to error.
For example, when analyzing for iron, dissolved iron requires a well-filtered sample while less filtering is necessary for total iron that includes undissolved iron. Turbidity, iron oxide, high iron content and other factors can interfere with the results and all must be taken into account.
In addition, the demand on lab technicians’ time severely limits the frequency at which they can perform the analyses. As with any analysis, cost also must be considered. Benchtop systems can run more than $4,000. To this you must add the cost of reagents and sample preparation, as well as labor to collect and deliver the samples to the lab.
Because lab results don’t represent what’s happening in the process in realtime, they at best enable manual adjustments to operations, with the impact of these adjustments not known until the next sample is analyzed.
A field-based colorimetric/photometric analyzer (Figure 1) eliminates or greatly reduces the need for manual sampling and lab analysis for many compounds. Today, in many processes, lab analysis serves to spot check and verify in-situ analyzers making measurements directly at the process and in nearly real time.
Often referred to as “cabinet analyzers,” these systems perform the same function as a benchtop spectrometer but generally use a photometer operating at a specific wavelength and so are limited to one parameter. This limitation rules these analyzers out for applications requiring simultaneous measurement of multiple compounds.
For many compounds, a straightforward reagent-based sample color reaction occurs that allows direct photometric measurement. However, total phosphorus analyzers need a digestion process ahead of the colorimetric analysis. There are a range of suppliers for these systems, with different approaches for how they perform the three basic functions of sampler preparation, chemical/sample management and photometry.
A high-quality sample preparation system that ensures a proper sample is provided to the analyzer is key to the success of a photometric/colorimetric analyzer installation. Such a system (Figure 2) at a minimum must incorporate filter media. Removal of particulates is crucial to protect the internal workings of the analyzer, such as the tubing and photometer, from plugging or coating. Analyzers are designed to handle small sample sizes, reducing the amount of reagent required for each analysis, thus keeping operating costs to a minimum. As a result, tubing, pumps, valves and other fluid components have narrow flow paths that a poorly filtered sample could easily block.
Some sample preparation systems require an external pump or process pressure to deliver the sample to the analyzer while others include a pump for sample delivery.
Installation of the sample system demands care. You must position the sample system filter in the process to ensure extraction of a representative sample. In addition, you must choose a filter porosity appropriate for the application. The filter porosity should match that used for any comparative lab tests. The filter should be readily accessible and easy to clean on a routine basis.
Ideally, a system should offer automatic cleaning with air backwash or even chemical cleaning. Chemical cleaning can be critical in an influent installation where greases or oils can be present, along with high biological activity that can lead to filter or sample line plugging.
For freezing weather environments, install tracing or another appropriate heating option on the sample system tubing.
Achieving the most economic and effective ongoing performance of a field-based analyzer requires paying adequate attention to three key factors: consumables, components and control system integration.
Consumables. Operating costs of colorimetric analyzers rise and fall with reagent consumption. The more reagent required for each analysis, the higher the operating costs will be. The key to keeping costs low is long reagent life and small consumption with each analysis.
Look for reagents that are easy to prepare and have a long shelf life prior to preparation and after installation in the analyzer. A longer shelf life allows you to purchase the materials well in advance, so product is on hand when reagent replacement is necessary.
Higher consumption rates mean reagents will need changing out more often, increasing overall maintenance costs. Systems that track reagent usage and indicate when it’s time to replace reagents assist in efficient analyzer maintenance. Some systems offer internal reagent cooling systems to help extend the life of reagents, reducing replacement frequency. Consider this approach to increase reagent lifetime.
Besides reagent consumption, each internal cleaning and calibration cycle consumes chemicals. In some analyzers, the rate of cleaner consumption will vary based on water hardness, which can add to the cost of operation.
Components. Any photometric/colorimetric analyzer will require routine maintenance that involves pump and tubing replacement, along with upkeep of valves and motors that may exist. Overall cost of ownership highly depends on the number of wear parts, their frequency of replacement, cost and impact on analyzer downtime. So, consider the inner workings of the analyzer and the wear components’ lifetimes stated by the manufacturer. Some analyzer designs require extensive part replacements that rely on an array of tools. Newer designs significantly reduce the number of parts requiring service, virtually eliminating the need for tools and dramatically decreasing the time required to perform routine maintenance.
Historically, peristaltic pumps handled reagent and sample dosing. New analyzers now rely on dosing syringe pumps that are more precise, consume less liquid, need replacement less frequently and are easy to change out.
Control system integration. Today’s photometric/colorimetric analyzers offer a range of output options from traditional 4-20-mA analog to fieldbus digital communications such as Profibus, Modbus or EtherNet/IP. Additionally, some systems provide the ability to integrate other sensor measurements within the analyzer, expanding its functionality and reducing capital costs for additional measurement values.
Take To The Field
Don’t relegate colorimetric/photometric analysis to the laboratory. Getting accurate and reliable measurement of various parameters in situ has become a more attractive option. The advent of newer analyzer designs dramatically reduces reagent consumption and eases maintenance. These newer systems, working in conjunction with improved sample preparation systems, can provide laboratory-quality results at a much lower cost of ownership than a lab-based system.
However, perhaps the biggest advantage of field-based analyzers is their generation of nearly real time data. This allows for tighter control of a process thanks to a higher frequency of accurate actual process data.
STEVEN SMITH is a Lafayette, Colo.-based senior product marketing manager – analytical for Endress+Hauser USA. Email him at email@example.com.