process equipment update 13
agents such as chlorine and bisulphite in water. This is important as deionisation resins present in CEDI units can be damaged by oxidation. This method warns if the bisulphite solution is not properly removing chlorine as the presence of oxidising agents such as chlorine in the water flow raises the ORP level (positive values), whilst the existence of reducing agents such as bisulphite cause a drop in ORP (negative values) .
This method is simple but suffers from limitations in sensitivity, linearity and selectivity. ORP measurement detects the change caused by moving from extremely small parts per billion (ppb) concentration. However, the reliability of this system is questionable as there are other variables which could affect the results, such as pH, temperature and other oxidising or reducing agents that may be present in the water. Large concentration changes of sulphite are not always reflected in the same size change of ORP signal.
New technologies, such as sulphite monitoring systems, are currently being developed with the aim of eliminating many of the challenges associated with traditional approaches. Sulphite monitors are made up of three separate components: a chemistry module where the sample is pH adjusted for measurement, an inlet overflow assembly where raw sample is delivered to the system, and an electronic readout containing the sulphite concentration display, analogue output and alarm contacts.
as 0-200 ppb up to parts per million (ppm), keeping chemical consumption to a minimum. This is beneficial as controlling bisulphite excess significantly lowers operating costs. Early sulphite monitors used a direct measuring membraned sensor. These systems were beset with fouling problem caused by the presence of sulphur reducing bacteria.
R
Some innovative sulphite monitoring systems take a unique approach to the measurement of sulphite ion concentration.
First a small amount of sample water is mixed with acid, converting the sulphite ion into sulfur dioxide. The mixed sample then flows into a chamber where the sulphur dioxide is removed and a sensor located in the gas stream measures the released sulphur dioxide concentration, displaying the results in terms of equivalent sulphite ion concentration.
The main advantage of this process is that the sensor never comes into contact with the water sample. As a result, the system is able to continue functioning regardless of the quality of the water or the presence of sulphur reducing bacteria that can thrive in water containing excess sulphite.
Sulphite monitoring systems are more complex than ORP systems and similar in complexity to
ecent developments mean it is now possible, with the use of sulphite monitors, to continuously measure excess bisulphite over ranges of as low
chlorine monitors. These systems also require regular maintenance. For example, peristaltic pumps needed for sample and acid delivery require around ten minutes of maintenance to replace both tubes once every six months. In addition, the sulphite gas sensor requires occasional visual inspection to ensure no deposits have collected on the sensing membrane. This type of system also requires reagents and acid usage for pH adjustment in the chemistry module is around four litres per month.
Monitoring sulphites in solution using the gas stripping technique provides extremely high measurement sensitivity, and good accuracy at the cost of more maintenance and consumables. Calibration of sulphite monitors is similar to calibration of ORP and chlorine systems. A standard sulphite solution is prepared and introduced to the monitor, which is then adjusted. A typical calibration period is a monthly check of zero and span.
Conclusion
To properly protect CEDI modules and to greatly reduce the risk of irreversible damage, the need for a continual supply of deionised water in many manufacturing processes, the dechlorination of water before it reaches these electrodeionization system is essential. Methods of monitoring dechlorination, such as the use of chlorine monitors and ORP monitors, do not match the sensitivity, selectivity and overall accuracy of newly developed sulphite monitors. Sulphite monitors are slightly more complex and require more maintenance than ORP monitors and about the same as chlorine monitors; they are however an overall more cost-effective way of ensuring water is chemically safe to enter CEDI units. The fact they are able to maintain an appropriate, safe but not too excessive residual sulphite concentration, a problem associated with chlorine monitors, also reduces chemical consumption and keeps expense to a minimum.
The disadvantages of higher running costs and higher maintenance (compared to ORP) are offset by the advantages of direct, specific, sulphite only measurement.
Using this emerging technique, water samples can be kept down to low ppb levels of excess sulphite guaranteeing that no chlorine is present and keeping sulphite costs to a minimum.
Due to this method’s advantages over traditional techniques, sulphite monitoring is increasingly becoming the dechlorination method of choice among manufacturers utilising ultrapure water. Its flexibility and continuous development mean that sulphite monitoring can deal with challenging dechlorination processes in a range of applications.v
Dr Michael Strahand is General Manager Europe, Analytical Technology, Mossley, Ashton-U-Lyne, UK.
www.analyticaltechnology.com
www.engineerlive.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36