Still monitoring after all these years
Continuous monitoring of industrial effluent, to ensure compliance with consent levels, can be prone to long periods of downtime when wastewaters are difficult to handle and their monitoring faces other factors. Through a careful maintenance regime at its Fife ethylene processing plant, ExxonMobil Chemicals has achieved 98 per cent uptime on an instrument installed more than 15 years ago to constantly provide continuous oxygen demand (COD) measurements every few minutes.
The ethane is treated and preheated, before being mixed with steam and cracked in the site's seven furnaces at 800°C. The mix is then cooled and compressed and the cracked gases and liquids separated. Unreacted ethane is recycled and by-product hydrogen is used as fuel and hydrocarbons C5+ sold on as intermediates.
Following cracking and several stages of cooling, the ethylene is passed up through a quenching tower against a downward flow of quenching water which cools the ethylene further and removes tars and oils left over from the cracking process. After quenching, the water - which will contain some liquid long-chain hydrocarbons plus sulphur and salts, a by-product of caustic scrubbing - is passed to the site's main wastewater drain. The drain, which also takes rainwater run-off, is discharged into the River Forth under consent from the Scottish Environment Protection Agency (SEPA).
Since the opening of the plant more than 16 years ago, ExxonMobil has needed to monitor wastewaters every few minutes for any organic contaminants to ensure that levels fall below their internal threshold of 100ppm - well within the SEPA consent levels of 250ppm COD. Monitoring also has to overcome the difficulties of handling water samples that included salts, liquid long-chain hydrocarbons and algal growth in the tubing to the analyser.
For COD measurements, ExxonMobil chose the on-line parameter TOD (Total Oxygen Demand), which correlated closely to COD. TOD is capable of completely breaking down difficult-to-oxidise, long-chain molecules because the technique uses high temperature catalytic oxidation. Representative samples are introduced via a ceramic sliding plate valve and injection tube onto a ceramic catalyst at 900°C.
Further advantages of TOD instruments are that they are based on a mechanically simple technique, can handle physically difficult waters including those containing high salts and oily samples. They generally need relatively low maintenance - TOD systems are often used for remote locations where they can be left unattended for days on end.
Every few minutes
From the outset, ExxonMobil monitored TOD using an Ionics M82 on-line TOD system to infer the COD value. The M82 acted as part of a wider process control system, giving a measurement every five minutes, to alert operators and automatically control the discharge pumping of the wastewaters to the river.
The Mossmorran TOD's longevity is mainly due to a highly effective maintenance regime adopted by ExxonMobil involving an extra set of key spares including the catalyst and injection tube. Twice a week the system is tested with benchmark calibration solutions. With any fall in performance, rather than go through a lengthy fault-finding exercise, Exxon-Mobil's analyser technicians merely replace key parts of the instrument with the substitute spares, cleaned since the last service using acid washing of the catalyst and ultrasonic cleaning of other elements. In this way routine maintenance and instrument downtime have been reduced to around one hour every six weeks.
To overcome the problem of algal growth, which blocked pipework and potentially the tubing in the instrument, ExxonMobil introduced a fast-flow feed of wastewater to the analyser house through copper tubing; the oxides of the latter to help as a biocide to control the growth. Samples from the fast-loop stream are gathered by a special cup system within the analyser, designed to give a representative sample at all times.
In 2000, however, ExxonMobil faced a new monitoring challenge. While the TOD's mechanical elements were working almost as well as when new, electronics and processing had come a long way in the past decade and a half. ExxonMobil decided to 'pension off' its reliable servant in favour of more modern and supportable technology - a 7800 On-line TOD analyser - which also provided enhanced data processing and slightly faster analysis times (three minutes). Before discarding the Model M82, however, the company ran the two alongside each other for several weeks - just to make sure.