동결농축법을 이용한 폐수처리의 동결거동 및 상용화 시스템 에너지 효율 향상에 관한 연구

Abstract

Water has a decisive effect to survival of all organisms including human being. But many countries have been experiencing a water shortage with more deterioration due to population growth, industrial development and improving the quality of human life. Another noticeable water problem which threatens the mankind is water pollution. Industrial development gave rise to waste-water containing harmful contaminants that caused severe environmental pollution. Water pollution inflicts a fatal effect on not only mankind but also an organism. Various techniques of waste-water management have developed continuously and applied to treat pollutants contained in waste-water. But most methods were suitable for a large quantity of waste-water.

Recently, waste-water treatment system is developed in small and middle size to get more economic advantage. Freeze concentration method based in the fact that pollutions can be separated and concentrated as non-frozen liquids when ice crystals are formed under the freezing point, is very useful to waste water treatment system of small and middle size. Freeze concentration system which can re-use purified water and cold energy obtained from ice has high thermodynamic efficiency and low energy consumption.

First, this study was progressed on the freezing behavior along the vertical tube in relation to freeze concentration method. The experiment was conducted on the freezing behavior of waste-water by cooling wall temperature, air-bubble flow method, ice-lining thickness in NaCl aqueous solution and the representative heavy metal aqueous solution containing Pb and Cr during the freeze concentration process.

The fundamental experiment was performed to investigate freezing behavior of NaCl aqueous solution along the vertical cooled circular tube for parameters such as cooling wall temperature and air-bubble injection method. In the experiment on heavy metal aqueous solution, Pb and Cr concentration of frozen layer and concentrated solution was compared at the effect of parameters such as cooling wall temperature, flow field effect and the ice-lining thickness. The following experimental factors were applied such as cooling wall temperature: -2 ℃, -7 ℃, -12 ℃ and -17 ℃, direct air-bubble injection method and ice-lining thickness: 1 ㎜ and 5 ㎜.

As the result of experiment, a decrease in the cooling wall temperature brought the high growth rate of ice front and more solute was involved in frozen layer. The method to inject directly air-bubble on ice-liquid interface through the ring shape nozzle gave higher purity of ice than indirect method. Ice-lining in 5 ㎜ thickness resulted in frozen layer with higher purity than 1 ㎜ thickness.

Also, the freeze concentration waste-water system which was designed as the small and medium sized capacity was analyzed about the rate of electric power consumption and the daily treatment capacity to suggest the direction of the system development. The virtual system possible to commercialize was composited and the design data was calculated to analyze energy efficiency and daily productivity with HYSYS simulation. At first, the power consumption and operation time of the system with fresh water precooler or without it was calculated by computer modeling and analysis. Subsequently, the change of design and operation treatment capacity was applied to the system with fresh water cooler.

The cost effectiveness of the freeze concentration system was analyzed with operation power consumption based on computer-aided analysis. Economical efficiency of the system for commercialization was studied at the point of energy recovery to utilize heat source of purified ice and condenser for air conditioning and to re-use water fusing ice.

As the result, the rate of electric power consumption was higher as 0.6 Wh/㎏ but daily treatment capacity increased in quantity as 19 % in the system with fresh water precooler. As design treatment capacity increased, the rate of electric power consumption was lower and daily treatment capacity was larger in quantity.

The operation cost of the freeze concentration system was lower up to 9.3 ~ 12.1 times in comparison with the evaporative concentration using electricity method and was lower up to 31.9 ~ 41.5 times in comparison with entrustment treatment.

As the energy recovery system, cooling efficiency was 1.12 and heating efficiency was 2.99 when ice thermal storage and condenser heat was fully utilized by air conditioning unit. At the comprehensive economic efficiency, cost saving was 32.5 times greater than operation cost in summer and 28.7 times in winter.