Laminar dispersion and the monitoring of flocculation processes

Abstract

The development of a simple technique for the monitoring of aggregates in flowing suspensions has led to several possible test procedures for the laboratory evaluation of flocculants and dispersants. While a batch method is fairly easy to implement and can give very useful information, there are some advantages in a continuous technique, especially for on-line applications. A very convenient means of causing flocculation is by laminar tube flow, especially since the degree of aggregation can be monitored directly in the tube. However, there are some relevant aspects of tube flow which need to be examined. Because particles close to the wall of a straight tube move very slowly, they take a long time to emerge from the tube and cause a response of the monitor. Since most flocculation occurs in regions of high shear, close to the wall, the full effect of a change in flocculant concentration may be reached very slowly. This is especially apparent with anin situ monitor. Coiling the tube should give a narrower range of axial velocities and hence a more rapid response of the monitor, but, even so, a steady-state reading may be achieved only slowly. Instead of a continuous flow of suspension, it is possible to inject a small sample pulse and to cause this to flow to the monitor. This gives a response which peaks soon after the first appearance of suspension at the detector, giving the possibility of more rapid tests. Experiments with latex suspensions flowing through straight and coiled tubes are reported, within situ monitoring of the transmitted light intensity. Both the average value and the fluctuating component of the intensity are monitored, the former giving the extinction caused by the flowing particles and the latter a sensitive indication of their state of aggregation. Although some of the results are in broad agreement with theoretical predictions, there are difficulties in interpreting some of the response curves obtained, especially with pulse input of a destabilized sample. Nevertheless, there are very clear differences between the responses of aggregated and single particles and a simple empirical test method could be developed.