Agitation time, effect, dispersant

Figure 15. Effect of segregation on polymerization of styrene in cyclohexane solution. Standard CSTR with h baffles and a 6-blade turbine, V = 670 cm, T = 75 °C. Dispersion Index DI vs. space time. Influence of agitation speed. Curves S (segregated flow) and M (well-micromixed flow) calculated from batch experiments. Initiator PERKAD0X l6, A = 0.033 mol L - -, kd = 5 x 10 5 s-1, f = 0.85 Mq = 6.65 mol L l, SQ = 2.22 mol IT1.

Starch is dispersed in the paper mill in large stainless steel tanks by injection of steam or by heat transfer from a steam-heated jacket. The tanks are stirred and equipped with baffles to prevent formation of a single vortex at the agitator shaft. A minimum heating time of 20 minutes at 95°C is normally required. Steam injection dilutes the starch paste by condensate, which must be considered for concentration control. Pastes that are prone to retrogradation are held at a temperature above 91°C or quickly cooled to 66°C to prevent amylose formation. Attention to storage temperature and water balance is an essential requirement for the effective use of starch in a paper mill. 

In this process, one starts with the phase that should become the dispersed phase. We call this phase A. One then slowly adds the other phase (B) to phase A while the system is agitated (or rapidly flowing, or homogenized in a suitable machine). Initially, droplets of phase B are formed, which are broken up into small droplets by the agitation. In time, more and more of these droplets are formed However, one ultimately wants to have an emulsion of in 5 therefore, the surfactant system is dissolved in phase B and not in phase A. Thus, at a certain time, the emulsion becomes so concentrated and the viscosity becomes so high, that the droplets of B are sheared apart droplets of B then start to coalesce. As soon as this coalescence sets in, all droplets start to coalesce, as a snowball effect. So, suddenly the droplets of B combine and start to form a continuous phase, taking up droplets of phase A, which starts to be the dispersed phase. 

---