TOWARDS SMART WATER ANALYSIS The road to predictive preventative maintenance in online TOC monitoring


Introduction Since last year, “new normal” is a buzzword re. digitalization in working environments. It covers not only industrial processes, but also new communication channels and digital access to information from virtually anywhere, often facilitating increased productivity while saving costs. Complex knowledge and the experience of experts are accessible from the comfort of the own home. In process automation, this trend has been gaining ground for years, a development that is gradually fi nding its way into


process analytics as well. Process analyzer technology


Today, many industrial processes are at least partially, if not fully, automated. There is little need remaining for human intervention in defi ned and unchanging environments. However, especially in processes where large quantities of water are treated, used, or transported, monitoring of analytical parameters is typically conducted.


While relatively simple sensors are used to determine physical parameters such as pressure, temperature and fl ow rate, the group of comparatively complex “process analyzers” is utilized for analytical measurement tasks. Examples are gas chromatographs, photometers, or probes. Such analyzers require some human interaction, mostly in form of maintenance, in order to ensure undisturbed operation and valid measurement results.


The increased effort and required knowledge are outweighed by the high benefi t of process analyzer technology (PAT), for it is classically used to determine the concentrations and compositions of substances. This in turn allows monitoring of both intentional and unintentional changes in substances and the fl ow of materials, making it an essential tool to continuously uphold process integrity – for the protection of equipment and the environment.


Many of the substances monitored in this way are organic compounds which occur both as natural ingredients as well as contaminants in water. For a thorough monitoring, careful analysis is necessary. This requires parameters that can be analyzed quickly, reliably, and can be automated to cover as many substances as possible.


While the determination of individual organic components is generally possible using various techniques, it simply would take too much time to provide a quick response to critical process changes. And although laboratory analysis can also deliver extremely accurate and even more fl exible measurement results, sample transport, evaluation, and reporting increase time expenditure. In many cases, only the sum of all organics is the parameter of interest for the process – and only when measured online.


Total Organic Carbon


One of the most meaningful parameters is TOC (Total Organic Carbon), which measures the sum of all organic carbon-containing molecules. The known quantity of distinct organic compounds is estimated to be about 40 million. This means that the largest group of chemical substances is captured in a single measured value.


Originally, TOC is a parameter determined by laboratories. However, since it is relatively easy to automate and provides results within minutes, it is also being determined “on-line” with the help of TOC process analyzers.


Shimadzu’s TOC-4200 is an example for a TOC analyzer specializing in online monitoring. Equipped with an automatic sampling station, it can automatically withdraw process water for analysis. It is important to adjust the sampling method to the respective water quality – e.g., if it contains particles, it has to be treated differently than saline, particle-free water.


Shimadzu offers various sampling modules allowing adaptation to various types of samples. Often, a single analyzer needs to monitor several measuring points at the same time. For that purpose, up to six different sample streams can be connected and measured by a single analyzer – automatically or on demand.


Important for TOC determination is the differentiation between organic and inorganic carbon. After all, carbonates and bicarbonates can be found in every natural water. The most used method for TOC determination is therefore the so-called NPOC (Non-Purgeable Organic Carbon) method. Utilizing its automated integrated sample pretreatment unit (ISP-Module), TOC-4200 acidifi es the sample to convert the carbonates and bicarbonates contained to CO2. Subsequently, the resulting carbon dioxide is expelled by gas purging.


Once the inorganic carbon is removed, an aliquot is injected automatically onto a 680 °C platinum catalyst where all existing organic compounds are oxidized to carbon dioxide. Water vapor is condensed, and combustion products that may be harmful to analyzer components, are removed from the gas fl ow by means of halogen scrubbers and fi lters. CO2 resulting from the combustion of organics is transported by a carrier gas fl ow to a highly sensitive CO2-selective NDIR detector and subsequently quantifi ed.


Based on an external calibration, the TOC concentration is then calculated. Meanwhile, the fl ow line is rinsed with clean water in order to prevent carry-over between individual analyses. Once rinsed, the next sample is already taken and prepared. Depending on the parameterization of the analyzer, a TOC value can be determined every three to four minutes. Utilizing an automatic dilution function, the measurement range can be extended in relation to the external calibration. This not only increases the wide measuring range, if necessary it can even be dynamically, but also enables the creation of calibrations from higher concentrated, longer shelf life standard solutions.


Maintenance requirements to online-TOC analyzers


TOC process analyzers could be roughly summarized as CO2 gas analyzers with automated liquid handling and oxidation module. They must be able to largely operate autonomously and unattended. Maintenance requirements should be as low as possible and service life as long as possible.


However, with all that in mind and even with constant improvement and advances in technology, every online-TOC analyzer ultimately requires maintenance. Filters for gas treatment and scrubbers for the protection of the detector have a maximum service life. The gas fl ow line has to be kept leak tight, combustion tubes for oxidation need to be cleaned or exchanged and chemicals require refi lls. In case of highly polluted samples, it is advised to clean the sampling line every now and then.


Ultimately, the nature of the sample, especially its matrix, can have a corresponding infl uence on how maintenance-intensively a device performs. Best practice is executing required actions on time; a distinction can be made here between “run-to- failure” and “preventative maintenance” – the latter is expressly recommended.


In case of processes that run in a very regular and defi ned manner, it is usually easy to set up maintenance schedules that, if followed, will result in little unscheduled maintenance. However, this is not always possible, as the composition of samples can change quickly and, in the worst case, unexpectedly due to accidents, environmental infl uences or simply the seasons. This can lead to the need for unplanned maintenance.


Unfortunately, such actions are often much more cost-intensive than planned activities, may require a higher level of experience and on-site support. Hence, one important goal in process analyzer technology is to identify a need for maintenance before it is required.


Remote diagnosis possibilities


In failure situations where trained personnel is not present on site, appropriate data transmission and diagnostic capabilities are crucial. Conventional data transfer in process analytics is realized for example, using the 4-20mA interface for the output of measured values, in combination with digital contacts for signaling


IET SEPTEMBER/OCTOBER 2021 WWW.ENVIROTECH-ONLINE.COM


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