Deaeration of fiber suspensions using tailored dispersions formed in static mixers

Deaerators are gas removal chemicals, which improve the creaming velocity of dispersed gas bubbles by enhancing the coalescence rate. In paper production they are important process chemicals since air bubbles present in the paper processing liquids lead to diminished paper quality and reduced runnability of paper machines. Standard deaerators are supplied either as aqueous suspensions or liquid concentrates. Compared to suspensions, concentrates have the advantage of lower transport costs, longer shelf-life, and in general a less complex composition. On the other hand, suspensions ensure more uniform product quality since the particle size is adjusted in a well-suited dispersing process and is not determined by the flow field of the application process as it is the case with concentrates. By dispersing concentrates on-site just before their addition to the paper system the advantages of concentrates (low costs, long shelf life) and suspensions (uniform product quality) are combined. In addition, deaerators can be tailored this way regarding particle size for different application systems. For that purpose a mobile dispersing and dosing unit was developed, in which static mixing elements continuously form dispersions. The deaerator efficiency of dispersions formed with the new unit were investigated in a pilot plant fiber suspension flow loop as well as in the short circulation of an industrial paper machine. To evaluate the pilot plant results, a new efficiency parameter was introduced, which characterizes the speed of deaerating action. In a first step, the active deaerating material of a standard suspension (mixture of fatty alcohol, polyol ester, and tallow) was applied as concentrate. Aqueous dispersions of this concentrate formed with the dispersing unit succeeded in application on pilot plant as well as industrial scales. We even found that the on-site dispersed and therefore not stabilized dispersions exhibit higher deaerator efficiency than the standard suspensions containing stabilizers, which means that for a performance equivalent to the standard suspension the addition amount can be reduced. This phenomenon could be explained by the deaeration mechanism on a microstructural level. Additional experiments with different particle sizes revealed that in the particle size range of 5− 35 μm the deaerator activity linearly decreases with increasing particle size due to reduced number concentration. Since the active material of the standard deaerator is a multiphase system and only melts at temperatures of order of 70 °C, we investigated in a second step several deaerator concentrates with properties more appropriate regarding on-site application. We employed two liquid block copolymers of ethylene and propylene oxide as well as two solid systems, namely higher fatty alcohols and a newly developed chemical compound obtained by esterification of an alkoxylated fatty alcohol. The materials were analyzed using rheometry, tensiometry, light scattering, differential scanning calorimetry (DSC), nuclear magnetic resonance spectroscopy (NMR), and contact angle measurements. The results of deaerator efficiency tests were discussed according to the bridging-dewetting mechanism with its crucial parameters