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Measurement and Control of Optical Brighteners Using Inline Sensor Technology

  Optical brightening agents are substances which adsorb nonvisible UV light (with a wave length of 300-400 nm) and reflect this as visible blue light with a wavelength between 400 and 450 nm. OBAs are generally anionic, water-soluble and colorless. The brighteners used for papermaking all have the same basis molecule based on Diaminostilbene-disulfonic acid. OBAs can be categorized according to the number of sulphonic acid groups (Di-, tetra- or hexa-sulphonated). The smaller the number of groups, the less affinity the brightener has for cellulose fibers. Di-sulphonated groups are generally dosed directly to the stock and hexa-sulphonated OBAs are dosed in the size press or to coating color. Tetra-sulphonated types are allrounders. In grades with a high brightness demand OBAs are added both internally and to the surface. The aim of this thesis was to examine the optical brightener dosage and its effect through inline process measurement. It should be checked if a control loop of the brightening agent leads to a higher paper quality and lower production costs. The analysis was carried out on PM6 in the Sappi Biberist mill in Switzerland. Currently the dosage of the OBA, blue and violet dyes at PM 6 is adjusted manually according to the values of the pope paper scanner. If the value delivered is too high the operator slows the rotation frequency of the pump, thus adding less brightening agent to the system. Is the value measured too low, the frequency of the pump is increased. Due to the late measurement of the pope scanner the operator only sees the result of his actions after a delay. Additionally, the operator has no value which to control the dosage by if there is a web break. A simultaneous measurement of the values and recording of the dosage of OBA should be achieved by the installation of the BT-5400 UV in the scope of the thesis. The instrument can measure fluorescence as well as the color (L*a*b*) of the pulp suspens[...]

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