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Figure 3: Filter mesh
Figure 2: Rangeability of TOC-analysers with different Oxidation Techniques without the use of diluting the sample with water
Catalytic combustion
This method oxidises organic compounds using a catalyst within a reactor that is maintained at a temperature of about 850°C. An aliquot of the sample is injected onto the catalyst. At this relatively low temperature, the catalyst (e.g. platinum pellets) is essential to be able to oxidise as many organic compounds as possible. For salty samples the reactor is maintained at a temperature lower than the melting point of NaCl (m.p. 800.4°C). The drawback is that the oxidation strength is not suffi cient enough to fully oxidise all organic species resulting in a reduced recovery rate, especially at higher TOC levels. Even when using a very small injection volume (e.g 50 micro litre), the maximum analyser range is approximately 2,000 ppm. Higher ranges are achieved by sample dilution. Moreover, the oxidation recovery is affected by ageing, coating and poisoning of the catalyst. Generally, these catalytic oxidation methods cannot handle organic particles larger than 50 microns and hence, measure mainly the DOC rather than TOC.
Non-catalytic combustion
The raw sample is injected into an inert ceramic reactor packed with ceramic beads. The reactor is placed in an electrically controlled oven maintained at a temperature of 1,200°C. This high temperature guarantees that all organics - regardless of their state (dissolved, particulate, volatile) - are oxidised completely. By-products such as salts and sulphur will sublimate keeping the reactor clean and unblocked. For high salt concentrations, the reactor is designed in such a way that the salts are spat out into a
special reactor foot, where they can be easily removed. The CO2 formed is measured by a NDIR detector. This True TOC analyser design can easily handle undiluted raw waste water samples with a level up to 50,000 ppm TOC.
Drawback of pre-dilution
Three of the above mentioned oxidation methods have a TOC range limitation of approx. 1,000 to 2,000 ppm, ranges above may be achieved by sample pre-dilution. Even without considering the technical consequences of adding a complex dilution system, we can conclude from an analytical perspective that the accuracy of the measurements is disputable. It is of the utmost importance that the sample remains representative in its composition of the original raw waste water despite having been diluted. Many types of water are diffi cult to dilute, or hard, even impossible to mix into a homogeneous solution. Furthermore, whilst diluting and mixing the sample with demineralised water, VOC could be lost into the ambient atmosphere resulting in a less exact TOC measurement.
Drawback of sample fi ltration
Three of the fi ve described oxidation methods have problems with raw waste water that contains particles bigger then 50 to 200 micrometer. These analysers need an additional conditioning system to fi lter out the particulate matter. Highly contaminated waste water normally contain a lot of particulates like slimes, algae and other suspended organic solids. If these particulate organics are fi ltered out, then the measured TOC results will be much too low. Instead of measuring the True TOC of the raw waste water, these analysers measure more or less only the DOC content.
Case study
At a huge petrochemical site, different factories are in operation, producing mainly liquid chemical base products for the production of specialised plastics. The discharged waste water of each factory fl ows into a single sewer system transferring the water to the main waste water treatment plant. When the processing operation is disrupted the waste water load may rapidly rise up to 10,000 and even 50,000 ppm TOC. However, under normal operational circumstances the TOC level is <500 ppm. Peak loads needed to be prevented, as costly products were being lost raising the waste water treatment costs on top. So, in the 1990ies, six TOC analysers were installed to continuously monitor the discharge loads of these streams. These analysers where based on batch injection and catalytic oxidation. The composition and consistency of the waste water were problematic since organic particles, slimes, algae and chlorinated hydrocarbons were present in high concentrations. As a consequence, the analysers’ wetted parts fouled and blocked, requiring almost daily maintenance. To achieve a better performance a special paper tape particle fi lter was installed and the particulate matter was fi ltered out. However, with not much effect.
It got to a point where the operators made the decision to select and install a new TOC analyser. Their goal was to really achieve improvements meaning less maintenance, a higher availability and better analytical performance. Finally, they selected the QuickTOCultra of LAR Process Analysers, whose principle is based on the non-catalytic combustion at 1,200°C. As they did not want to take any risks a 3 month trial period was agreed upon. To be able to compare the analysers performances, the new QuickTOCultra was installed at the most problematic waste water stream, side by side with the old analyser. The trial was a great success: Perfect analytical performance, no failures, no problems and naturally without needing the paper tape particle fi lter! LAR’s QuickTOCultra was directly measuring the TRUE TOC of the untreated raw waste water. The analyser proved not only to rapidly monitor peak levels of up to 50,000 ppm, it also showed that it has no memory effects meaning that it will return to low normal values quickly and easily. This was a break through for the site operators. There are currently six single and two-stream QuickTOCultra analysers installed on this petrochemical site.
Unique features
LAR’s QuickTOCultra uses a straightforward technique with only three wetted parts. Its oxidation technique is based on the non- catalytic thermal combustion at 1,200°C which proved to fully oxidise all types of organic carbon. Even the carbon compounds that are diffi cult to oxidise are completely cracked at this high temperature. The recovery is without a doubt, 100 percent. The range of this analyser is unique: Without using any dilution technique, it is able to measure from low ppm levels, all the way up to a level of 50,000 ppm TOC. Furthermore, the analyser uses an advanced robot operated injection system. Its simplicity is evident. The raw waste water sample fl ows through an overfl ow vessel where it is thoroughly mixed. A thick rugged needle sucks up an aliquot of the sample, including organic particles. It then injects a precise volume of that sample into the reactor. The organics in the waste water are fully combusted at the high temperature of 1,200°C. The amount of CO2
formed is directly proportional to the TRUE TOC concentration in the waste water.
Another feature is the explosion-proof housing used. Normally, water analysing equipment will be installed into analyser shelters with climate control. LAR offers stand-alone measuring equipment with special explosion-proof housings providing a easy-to-manage and easy-to-operate solution compared to analyser shelters. The protection housings are in accordance with the latest ATEX and IECEx standards. The protective housings are classifi ed as protection type Ex-p, with analyser housing under controlled overpressure. The analyser systems can be used in ATEX Zone 1 and Zone 2 (T3/T4) as well as in IECEx Zone 1 T4.
Summary
A well-designed TRUE TOC analyser is capable to handle waste water with a high load of suspended organic particles. Not only can it easily process waste water, it also completely oxidises all organics, including particles. An advanced robot driven injection technique, in combination with the non-catalytic 1,200°C thermal oxidation, make this possible. This method and analysis technique is known as the TRUE TOC. An excellent example of such a TRUE TOC measurement system, is the LAR QuickTOCultra analyser. Using the same approach, LAR offers also analysers for the determination of COD, TOD and TNb – combinations of these parameters are possible. These analysers are easy to maintain and measure the complete organic content of waste water and are characterised by exceptionally low operational costs.
Figure 4: Online TOC analyser in an ex-zone housing Author Details
Dr. Werner Arts, President, LAR Process Analysers AG Neukoellnische Allee 134, 12507 Berlin, Germany
• Tel: +49 30 2789580 • Email:
sales@lar.com • Web:
www.lar.com
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