THE SKY IS THE LIMIT Lost water can never be
retrieved...especially in
petrochemical industry where the costs of discharging can be sky high. Besides the expenses of waste water treatment
processing, energy supply, taxes and legal charges, valuable base chemicals or products can be lost. To monitor these effl uent streams a TOC analyser can be used that rapidly responds to any change in levels.
Normally, production installations have low waste water discharge levels, mostly within the range of 1,000 ppm TOC (total organic carbon). However, occasionally the installation can suffer failures or process disturbances take place. In this case, the range could very well reach a level of up to 50,000 ppm. If these go unnoticed, various consequences need to be considered:
• Loss of process fl uids and base chemicals • Risk of an infl uent shock loading in the WWTP
• Increased operating costs of the WWTP e.g. aeration, dosage of extra feed fl uids
• Increased effl uent discharge costs • Problems with authorities about exceeding effl uent discharge levels.
For safe and effi cient plant operation, the online TOC analysis is recommended, if not regulated. The operation of an TOC analyser is standardised (EN 1484:1997): Generally, using a reactor, a portion of the waste water sample is oxidised converting the hydrocarbon content to carbon dioxide (CO2
). A CO2 gas detector,
typically a Non Dispersive Infrared (NDIR) detector, measures the amount of CO2
formed. Various oxidation methods are available,
mainly differing in their oxidation techniques. At high levels of up to 50,000 ppm, the oxidation strength must be outstanding. However, many TOC analysers are not able to measure levels high enough and therefore need to pre-dilute samples. The drawback of dilution is that many types of waste water, for example with particulate, volatile, or oily contents, cannot be accurately diluted.
Oxidation methods
The heart of every TOC analyser is the reactor, where the oxidation to CO2
takes place. Many manufacturers claim that their design
offers the greatest performance. However, the needs and demands of industries are tough and the waste water compositions are diverse and challenging. Especially with high organic loads, where
organics may be present in different states and kinds (dissolved, volatile, mixed, solidifi ed, fats, free fl oating oils, particulates) many TOC analysers need a sample conditioning system that removes organic particles, algae, fats and/ or slime. Conversely, True TOC analysers determine the total of organic species irrespective of the sample composition and consistency.
UV oxidation
This wet-chemical technique oxidises the sample by use of UV light, a catalyst and a reactor made of quartz glass. It is well known and commonly used, but, however, not a very suitable method for samples containing particles. It is rather known as the DOC analysis method (Dissolved Organic Carbon). The recovery rate of this method differs depending on the type of organic component and on the analyser process parameters such as UV exposure time, UV light intensity, the oxidant concentration and the DOC level. In general, the UV exposure time is about 5 minutes resulting in a relatively slow response. The measuring range is limited to about 1,000 ppm DOC/TOC. Higher ranges are achieved by pre-diluting the sample. Salt, especially sodium chloride (NaCl) has a negative impact on measurements.
High pressure & temperature
This oxidation technique uses the physical phenomena that organic compounds in water are oxidised at a critical stage where both, the temperature and the pressure of the waste water sample are high. Firstly, the sample is mixed with a liquid catalyst and an aliquot of it is then drawn into a small reactor, basically a titanium tube. Secondly, two high pressure valves are activated and the sample is captured and sealed. An electrical heater is powered causing a rapid increase of the reactor temperature to >375°C. Subsequently, the pressure increases (P >215 bar). At this stage, the organics are oxidised to CO2
open and a carrier gas transfers the CO2
. The reactor cools down, the high pressure valves to a NDIR detector. This
oxidation technique can only handle fi ltrated samples (d < 200 micron) and as a consequence, mainly measures DOC. Particles may block the small reactor, possibly causing fouling of the high pressure valves. Any leak, even the smallest, will directly infl uence the correct reactor operation causing the oxidation recovery to drop. The oxygen needed for the oxidation of organic species comes from the liquid catalyst only (e.g. persulfate). Therefore, the maximum range is limited to about 1,000 ppm (increased by using pre-dilution). At higher TOC levels the recovery rate gets worse.
Hydroxyl radicals
With this oxidation technique samples are oxidised in a wet fl uid state. The heart of the analyser is a closed loop circuit consisting of a reactor, a circulation pump, a 4-port fi lling valve and tubing. A fi xed volume of reagent(s) and an aliquot of the sample are pumped into this loop. Furthermore, oxygen (O2
) is converted to
ozone use a high voltage corona reactor. By exposing the high pH reagents to a heavy concentration of ozone, hydroxyl radicals are created which oxidise the organics and produce CO2
.
The maximum range of this method depends on the hydroxyl radicals’ oxidation strengths, the concentration of the catalyst as well as the composition and consistency of the hydrocarbon organics in the sample. The less of the sample there is, the higher the range. Additionally, oils and fats can foul the complete circulation system causing absorption and adsorption effects and slowing down the response time.
Figure 1: Comparison of different oxidation methods.
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