Influence of Injected Projectiles on the Induction Period of Crystallization Fouling

Projectiles of various shapes and hardness are increasingly used in process industries to mitigate fouling in tubular heat exchangers. It is a common practice to inject the projectiles at the early stage of fouling, though laboratory results are quite scarce in the open literature to assess whether this is an appropriate operating procedure. The present work aims at investigating the influence of injected projectiles on the induction period of CaSO₄ crystallization fouling. Fouling experiments have been performed in a plain heated tube. The projectiles were of spherical shape with diameter of 20.2 mm, that is, 1% bigger than the inner diameter of the heated tube, and were injected at various intervals. It has been observed that overall the attempted projectile reduced the induction period and thus expedited the fouling process. The asymptotic behavior of crystallization fouling is also approached more quickly but much less so than that of no injection. The induction period increased linearly with the flow velocity in case of no injection, while it was independent of the flow velocity when the projectile was injected as long as the injection rate was kept constant. Increasing the injection rate decreased the induction period and started the fouling process earlier. This is because the propulsion of projectiles induces air bubbles into the heat exchanger tube, which would in turn promote fouling to occur more quickly, and thus shorter induction periods are expected. Therefore, it is highly recommended to inject projectiles only after the induction period, to make use of the fouling-free operation during the induction period.