Concentration of bleached-chemi-thermo-mechanical pulp effluent by propane hydrate formation

During the past several years there is increasing public pressure for stringent environmental control for pulp mill water discharge to the receiving environment. The industry responded by developing zero liquid discharge technology that applies to mechanical pulp mills. The goal of these mills is to recover water suitable for reuse. That technology is based on evaporation. Another method of recovering water from a solution with impurities is through the formation of clathrate hydrate crystals. However, this method is not industrially proven. Only evaporation operates at an industrial scale. Clathrate hydrate crystals are inclusion compounds that can exist below or above the normal freezing point of water under suitable pressure and temperature. Effluent concentration through clathrate hydrate formation is based on the fact that impurities present in the effluent are not contained in the clathrate structure. The motivation for using clathrate hydrates for effluent concentration is the fact that the process can operate at low temperatures (1 to 5 °C for propane hydrate) and hence, the potential for scaling and corrosion is reduced. In this work, we chose to use vapour and liquid propane to form hydrate crystals for the purpose of concentrating BCTMP mill effluent, and recovering clean water. A new experimental apparatus was designed and built. The main characteristic of this apparatus is the fact that we are able to form the hydrate crystals, drain the effluent concentrate, wash and melt the hydrate crystals in one vessel. All unit operations were performed in a batch wise manner. We grew crystals at different conditions by varying the quantity of hydrate former, the impeller geometry, and varying the driving force. Experimental runs were performed with and without washing. The crystals were washed with either liquid propane or distilled water. Because BCTMP effluent is a dark colour solution that makes it difficult to observe the crystallization and separation process we worked first with a 2.5 wt % NaCl and then with the BCTMP effluent. The effectiveness of separation for both the brine solution and the effluent was determined by analyzing the amount of impurities in the recovered water, the spent wash solutions, and the concentrate. We found that crystals grew the fastest with a large driving force while using liquid propane mixed in a rigorous manner during the crystallization process. The quality of the recovered water depended upon the amount of hydrate former used during the crystal growth, and the extent of drainage. Improved drainage was achieved by displacing concentrate from the crystal using liquid propane. Washing the crystal with water did not perform well because there was poor contacting between water and the crystals. It was also found that the rate of crystal growth did not affect the quality of recovered water. The average reductions in the conductivity of the brine solutions were 31.5%, 66.1%, 32.1% for the no washing, washing with water and displacement with propane experiments respectively. In the effluent experiments the corresponding reductions in conductivity and TOC (total organic carbon) were, 22.5%, 22.5%; 23.1%, 11.4%; 45.1%, 27.3% respectively.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat