| With the development of economic globalization,the air pollution from marine vesseles has been aggravated gradually,contaminating human health and ecological environment.The International Maritime Organization has proposed a series of restrictive measures to reduce pollution caused by the emission of sulphur dioxide from ships.Magnesium based-seawater desulfurization technology has a unique advantage and should be popularized as soon as possible.The key technical problems of magnesium based-seawater desulfurization system have been solved,but there are still some deficiencies in the design of flow field,prediction of desulfurization efficiency and treatment of magnesium sulfite.On the basis of fully investigating the ship's working condition,the desulfurization tower and internal structure are designed according to the experience of the predecessors.Using fluid simulation software,RFlow,and the subdomain technology,the physical mesh model of the tower was established.Via the finite difference method,the numerical results were calculated.The results of mesh quality test showed that the physical mesh model is reasonable and can be used for subsequent numerical simulation.In order to optimize the flow field in the tower,the velocity distribution of the exhaust gas in the tower was simulated based on the RANS turbulence model.The distribution of gas-liquid flow field in the desulfurization tower was simulated by coupling the multi fluid model with the dispersed k?s turbulence model.The influence of different exhaust gas inlet and spray layer design on the flow field in the tower was simulated and analyzed,the results show that the gas flow in the tangential direction and the reasonable design of the nozzle support arm can increase the uniformity of the gas-liquid flow field,reduce the pressure loss of the tower and optimize the design of the tower.Based on the simulation of the flow field in the tower,combined with the two-fielm theory,a prediction model for the desulfurization efficiency of the magnesium based-seawater desulfurization system is established,and the simulation results are verified by the real test on board.The results show that the model is reliable and can be used in the magnesium based-seawater marine exhaust gases desulfurization system.In order to explore the influence of different ship conditions on the desulfurization efficiency,this prediction model was used to simulate the desulfurization efficiency with different exhaust gas flow and spray flow,and the results show that with a reasonable gas-liquid ratio,the desulfurization tower can meet the requirements of the convention order.Due to the high desulphurization efficiency of the magnesium-seawater method,a large amount of magnesium sulfite is produced in the tower,which is harmful to the environment.It was found that magnesium sulfite will be oxidized to harmless magnesium sulfate,when contacting with oxygen in the exhaust gas.Therefore,this paper proposes a new method to improve the oxidation rate of magnesium sulfite,and studies the mechanism of the spraying oxidation of magnesium sulfite.The results showed that the oxidation rate of magnesium sulfite was 1.5-level response to its concentration under spraying condition,keeping ph at 7-8,and increasing the concentration of magnesium sulfate could keep a high oxidation rate.Comparative analysis of magnesium sulfite spray oxidation rate in fresh water system and seawater system,the results show that spray oxidation rates of magnesium sulfite are not very different under the two systems.The results of real ship test show that a reasonable renewal time of the circulating fluid and a proper concentration of magnesium sulfite can increase the rate of spraying oxidation and reduce the load of the subsequent waste water treatment system. |
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