| Freshwater is necessary for fishing boats.It is supplied as the domestic water for the crew,and it ensures the normal operation of the boats,such as using as cooling water for cylinder jackets.Currently the main of fresh water is carried from ports,in this way,it not only takes up space but also makes no guarantee for long journeys.Nevertheless,this problem can be solved by desalination effectively.Compared with traditional seawater desalination technologies,membrane distillation,as a new separation method,which has the advantages of no vacuum or high pressure,and simple structure,has attracted much attention from industry.To solve the problem of high energy consumption in membrane distillation desalination process,in this paper,the first pilot-scale hollow fiber membrane seawater desalination system driven by diesel engine waste heat from fishing boats was built.This system was based on membrane humidification and dehumidification(MHDD)and the waste heat of the diesel engine was used as the heat source,which effectively solved the problem of high energy consumption.In addition,considering that the industrial-scale hollow fiber membrane modules are usually made in a random arrangement for fibers,and there are problems such as oversimplification and unreasonable boundary conditions in the modeling.In this paper,threedimensional computational fluid dynamics(CFD)models were established to analyze the effects of fiber arrangement,packing fractions,air Reynold numbers on the module performance.The details of heat and mass transfer in the membrane distillation processes were revealed.Theoretical guidance for the optimal design of the module was given.The specific work is consisted of the following parts:(1)The hydrophobic polyvinylidene fluoride(PVDF)hollow fiber membranes with the contact angle of 103.1° were prepared.4 membrane modules with staggered arrangement were designed by solving the heat and mass transfer equations.Every membrane module was comprised of 4 smaller modules,which reduced the core changing cost.The membrane area of every membrane module was 8.74 m2,and the packing fraction was 26%.The specific structural parameters of the condenser were designed based on the moisture transfer coefficient method.The total area of the condenser for heat transfer was 15 m2.The rest of the equipment in the system was designed and selected to build the whole hollow fiber membrane seawater desalination system driven by diesel engine waste heat.(2)Test experiments were conducted on fishing boats in Nantong,Jiangsu,to study the effects of different operating conditions(including flow rate,temperature of heated seawater,flow rates of inlet air and cooled seawater)on the performance of the hollow fiber membrane desalination system.From the experimental data,it could be seen that the conductivity of the produced water is only 16.57 μS·cm-1 and under the flow rate of heated seawater of 4.8 m3·h-1,inlet temperature of 82℃,the flow rate of inlet air of 720 m3·h-1,and the flow rate of cooled seawater of 6 m3·h-1,the freshwater production was 41 L·h-1 and the performance indices were satisfied.Under this condition,the specific humidification power on the basis of unit area of membrane was 1.2 kg·m-2·h-1,and the COP and COPE of the system were 0.646 and 16.746,respectively.(3)The CFD models with different arrangements were established,by using user defined functions(UDF)to simulate the transmembrane mass transfer process under coupled boundary conditions.Combining transport models for moisture and Realizable k-ε turbulence models for momentum,the models were solved.The effects of fiber arrangement on temperature polarization and heat and mass transfer performance under different air Reynold numbers and packing fractions were studied.The comprehensive performance of different components was evaluated with a newly proposed comprehensive performance indicator.The results showed that the overall performance of the staggered arrangement was the best under all operating conditions.The effects of packing fractions on the performance of a randomly arranged bundle were greater than that of a regularly arranged one.For the randomly arranged module generated by random number method,under high Reynold numbers or high packing fractions,the overall performance was even lower than an aligned arrangement.The optimal packing fraction was around 32%.For random modules with local compact characteristic,obvious deterioration of heat and mass transfer were observed because of flow channeling and stagnation.The performance would be improved by further increasing the packing fractions. |
