Scaling of Calcium Carbonate on Heated Surfaces - Crystallization or Particulate Fouling?

Abstract

Abstract One of the main challenges with mono ethylene glycol (MEG) loops is to achieve controlled removal of produced salts and at the same time avoid uncontrolled precipitation and scaling on heat exchanger surfaces. Scaling is described in the literature as occurring both by particulate and crystallization fouling which is of particular interest in MEG regeneration processes with heat exchanger surfaces being exposed to solutions with significant quenatities of particles in the liquid bulk. In order to control the process it is essential to understand to which extent scale formation is driven by crystal growth on the surface as compared to particulate fouling of particles originally precipitated in the liquid bulk. Surface scaling was studied in a laboratory setup where a heated stainless steel tube is inserted in a well-mixed continuously stirred tank reactor allowing for control of the steady state supersaturation and the bulk temperature. To test if particulate fouling is contributing to the scale formation premade particles of a different calcium carbonate polymorph was seeded to the system. By running the system with pre-scaled tubes of calcite and at the same time seeding with vaterite particles, it was shown that particulate fouling is not an important growth mechanism. For aragonite scales produced on a tube heated at 90 °C, we found that vaterite particles were incorporated to a certain extent into the porous scale structure. However, the simultaneous consumption of supersaturation by the vaterite particles in solution as well as by the scale layer, atually lowered the scale thickness. This reduced scale layer growth was confirmed by heat exchanger data. We found that particulate fouling is not an important mechanism for build-up of calcium carbonate scales. Presence of bulk particles can even be advantageous by reducing the available supersaturation for surface scaling.