Chemical manufacturers, petrochemical plants, and electric utilities all require large volumes of clean steam to produce products and services. Feedwater polishing and steam generation are very expensive and fairly uniform processes, so many chemical/petrochemical plants have joined forces with electric utilities to share the steam generated by one boiler system.
These cogeneration arrangements reduce operation cost for both facilities. Several relationships between the generator and the partner facility may exist, but the key concern is producing clean steam for both. Specifically, the amount of total organic carbon (TOC) in the stream is most important because it can have a major negative impact on turbines, manufacturing equipment, and water polishing systems. Most chemical processes have the potential to increase TOC levels.
A large increase may indicate a leak in the heat exchangers in the process stream. To safeguard each cogeneration partner, on-line TOC analysers are used to continuously monitor the steam after use in one facility and before it reaches turbines/equipment of the partner facility. If excess organics are present, the TOC analyser makes the re-use/waste of the steam an automatic, objective decision. When the TOC levels exceed the agreed limit, the partner responsible for a significant increase in TOC may face financial consequences from the other. It is in everyone's best interest to monitor outgoing and return steam to prevent costly equipment problems.
Two typical applications for on-line TOC analysers are:
1. Because each business unit in a large organisation is judged on its own profitability, process water from each business unit (and sometimes each individual stream) is monitored for TOC. This assures that the business unit is fairly charged for the clean up. The analyser is installed in the water line between the business unit and the waste stream. Without a TOC measurement at each unit outfall, a combined outfall TOC measurement is taken and all the business units bear the cost of clean up evenly. Then the combined outfall is sent to the wastewater plant for treatment.
2. Chemical and petrochemical plants are surrounded by stormwater ditches. Because of the nature of chemical and petrochemical processes, the stormwater may have a high content of organic contaminants. When the TOC measurement is higher than the permitted level, it is diverted into a retention (holding) tank to be slowly released into the wastewater treatment plant. When the TOC measurement is below a certain lower limit, it is discharged untreated.
The Hach astroTOC UV Turbo Analyser is a process analyser designed to continuously monitor organic carbon in the liquid sample stream in the range of 0 to 2, up to 0 to 50 mg/litre. In the analysis process, carbon is converted to carbon dioxide, which is measured with an IR detector. Ideal standards can be prepared with reagent-grade urea, ethylene glycol, or KHP to automatically calibrate the analyser. The analyser has two 4 to 20 mA analog recorder outputs selectable for sample concentration, analyser system warning, auto range indication, or the rate of change for early trend detection. There are five programmable alarms selectable for sample concentration, analyser system warning, and analyser system shutdown.
Petrochemical and chemical plants have many applications for TOC monitoring. After 25 years of TOC analysis experience in industry, the following Hach instruments are available from PREI Instrumentation:
astroTOC HT
This is designed for samples with salt content (anything above 0,1%) and hard to oxidise carbons such as humic substances, amines, zylene and benzene.
astroTOC UV
Designed for sample with high concentration of solids.
astroTOC UV TURBO
Designed to provide a fast response (T90 <5 minutes, T20 <3 minutes)
For more information contact Steve Herbst, PREI Instrumentation, 011 448 2172, [email protected], www.eprei.co.za
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