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해수 담수화와 열역학

No
27
작성자
수질연구
조회수
938
첨부

지금의 우리 시대는 저탄소 녹색성장이 모든 산업공정에서 메가트렌드로 자리잡고 있다.
이는 지속가능한 발전을 위해 현 싯점의 필요 불가결한 주문이기도 하다.
열역학 제2법칙은 소금물을 염과 물로 분리하는데 필요한 최소한도의 일(work)을 제시해온 것은 주지의 사실이다.
마침 흥미있는 연구결과가 있어 소개하려고 게시판에 올립니다.
해수의 담수화 방법에 있어서 역삼투, 증류, 냉동방법들은 최소한도의 일(에너지)은 장치와 공정에 무관하게 계산되기 때문에 모등 공법에서 꼭 같이 적용기 때문에 논할 필요가 없다. 다만 연구자료인 첨부물에서는 RO(역삼투), NF(나노필터), EDR(역전기투석)장치와 MSF(다단증류)를 운전하는 단계별 시나리오별로 열역학 제1법칙과 제2법칙에 입각하여 공학적 관점에서 수준높게 분석한 자료입니다. 펌프와 전동기 단위의 에너지 손실은 MSF(78%)>EDR>RO>NF(23.6%)순이었고, 2법칙의 효율(카르노 효율)이 낮은 순서는 MSF(4%)>EDR>RO>NF(6.3%) 로 연구되었다.
중요한 결론은 효율이 낮은 장치는 에너지 소비를 줄임으로써 열역학적 성능을 향상시키기 위해 중요한 기회임을 의미한다는 점이다.
다음은 연구 요약문이며, 첨부는 전문을 게재합니다
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Reverse osmosis, distillation, and freeze desalination processes were analyzed using the first and second-laws of thermodynamics with particular attention to the minimum separation work requirement and the flow exergy. The minimum work for complete separation was investigated by first considering reversible processes for which entropy
generation and exergy destruction are zero. Minimum work relations for complete separation of mixtures were obtained and presented in various convenient forms. These relations were later employed to develop the minimum separation work for incomplete separation of saline water solution encountered in desalination plants. The minimum work input was determined for various salinities of incoming saline water and outgoing brine and product water, and the results were tabulated and plotted. The minimum work values show that a lower and an upper limit for the minimum work exist at corresponding recovery ratios of 0 percent and 100 percent. The plots of the minimum work versus recovery ratio at various salinities of the incoming saline water also show that there is an optimum value of the recovery ratio which decreases with increasing salinity. Using reverse osmosis, distillation, and freeze desalination processes, it is shown that the minimum separation work is independent of any hardware or process and thus the same for all processes.
Next, the exergy analysis of typical ideal and actual desalination processes was conducted together with the discussion of the minimum separation work requirement. The exergy changes of major components were calculated and illustrated using exergy flow diagrams for four desalination systems using actual plant operation data. Three systems were part of a brackish water desalination plant in California that incorporates RO (reverse osmosis), NF (nano-filtration), and EDR (electrodialysis reversal) units. Each unit produces about one million gallons of fresh water per day. The fourth plant is located near the city of Al-Jubail at the Arabian Gulf coast. This MSF (multi-stage flash) plant consists of 40 distillation units, and each unit consists of 22 flashing stages. The plant is capable of producing distilled water at a rate of 230 million gallons per day. Exergy flow rates were evaluated throughout the plant, and the exergy flow diagrams were prepared. The rates of exergy destruction and their percentage were indicated on the diagram so that the locations of highest exergy destruction could easily be identified. The analysis showed that most exergy destruction occurs in the pump/motor and the separation units. The fraction of exergy destruction in the pump/motor units is 39.7 percent for the RO unit, 23.6 percent for the NF unit, 54.1 percent for the EDR unit and 78 percent for the MSF unit. Therefore, using high-efficiency pumps and motors equipped with VFD drives can reduce the cost of desalination significantly. The second-law efficiencies for these systems were: 8.0 percent for the RO unit, 9.7 percent for the NF unit, 6.3 percent for the EDR unit, and 4 percent for the MSF unit. These very low efficiencies indicate that there are major opportunities in the plant to improve thermodynamic performance by reducing exergy destruction and thus the amount of energy supplied, making the operation of the plant more cost effective.


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2011-08-19 17:35:49
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