Acordis uses the other eight million gallons for cooling purposes, which is returned to the Lune after it is used, a little warmer but otherwise unchanged, via four separate streams into a beck which feeds the river. So sensitive is the plant¹s setting, that Acordis has invested over £0.5 million in a scheme to protect the river from even the slightest eventuality of accidental contamination.

The acetate process involves the manufacture of a derivative of raw cellulose from acetic anhydride using a small amount of sulphuric acid as a catalyst. Condenser pipes carry acetic acid and methylene chloride mixtures. While separate from the cooling system there is the faintest possibility of cross contamination should minor corrosion, perhaps of pinprick proportions, occur in a condenser tube.

Organic contamination could affect the oxygen demand load to the river. The Environment Agency (EA) consent allows Acordis to return the waters to the river with a slight increase of 5ppm BOD over the intake level. To ensure that organic/ acid contamination does not occur by infiltration into the cooling waters, Acordis has for many years monitored the quality of the waters for pH and temperature rises, on a daily basis. In this way any increase in, for example, acetic acid levels can be picked up well before a breach of BOD levels.

In 1995, the company completed a new computer-controlled system, its River Lune Protection Scheme (RLPS) to automatically detect any rise in cooling water contaminant levels and if necessary to divert the individual stream of water to safety to a 70,000 gallon underground collection tank, stationed beneath the company¹s main car park.

In designing the RLPS, the company considered the use of on-line parameters to automate the measurement of organics in the cooling water drains. Monitoring would need to be very sensitive as the background levels in the River Lune are themselves very low, around 3ppm, and at times fall to 0.8ppm.

Any instrumentation would need to be highly sensitive yet sufficiently robust for on-line and continuous monitoring. Total organic carbon (TOC) offers such sensitivity and initially was considered in preference to on-line oxygen demand techniques such as total oxygen demand (TOD) which is well suited to dirtier waters to 100,000ppm and above. COD was considered too lengthy a process. UV persulphate TOC was considered but can prove slower and less effective in oxidising substances such as methylene oxide. Acordis therefore turned to high temperature oxidation instruments for TOC. Acordis chose the Ionics range of TOC analysers which use a high temperature catalyst at 900oC. However, prior to installation the approach altered from the use of TOC to TC (total carbon) as there is a risk of losing volatiles during TOC carbonate removal. TC is also a more rapid technique.

Ionics provided the monitoring instrumentation in two analyser houses. Sited on opposite sides of the plant, each house monitors two drains. Each system sequentially computes TC, pH and temperature of the return water in the two drains versus intake water from the river. In the event of a higher than expected value to be recorded then the system automatically diverts the drain to the holding tank.

Typical TC values of river water intake and return are 25ppm. The system being sensitive to the quite low ppms, a sequence of measurements was specially designed by Acordis to cross-check the readings for each drain before effecting diversion. This prevents accidental diversion due to unrepresentative readings from algae or river debris, twigs and the like.

Improved filtration and modified cleaning procedures were also adopted to remove Œspikes¹ from TC readings. Given the nature of the process, in the normal course of events, erratic variations from the 25ppm level are not anticipated. Acordis is more interested in trends data over, for example, 24 hours or one week, as a gradual leak is more likely than a major peak. Data analysis procedures were therefore designed to pay most attention to gradual rises in ppm levels and to alert the process supervisor to any changes.

The computerised RLPS presents a Œlayered' system of information on each part of the plant. On screen, a schematic of the entire system indicates, via colour-coded symbols, the status of different elements. The operator can call up an individual page for each element which gives them Œrealtime' TC and pH values, whether diversion to the holding tank has been necessary and the direction of each control valve.

The intake water readings not only act as comparisons with the discharged waters, they also indicate the realtime quality of the water in the river. This has two further benefits ­ it ensures that the river water is of sufficient purity for the plastic process and it provides the EA with information on the Œhealth' of the river upstream of Acordis.


This article first appeared in Water & Wastewater Treatment May 1999 page 40


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