Design And Research Of Cold Energy Recovery Process With Using Phase Change Material(PCM) In LNG Receiving Terminal

During the LNG gasification process,a large amount of cold energy is released.Making full use of these cold energy can bring enormous benefit to LNG receiving terminals,cold energy users and society.However,the output load of LNG receiving terminals,which are affected by the downstream customers,fluctuate frequently.At the nocturnal output trough,the LNG gasification load is very small,even it can decrease to zero.Accordingly,LNG cold energy utilization devices may be forced to stop operating because of the interruption of cold energy,which reduce the project economy.If cold energy utilization device is designed based on the cold energy available from the minimum gasification load of LNG receiving terminal,it can ensure that the device operates steadily.But this kind of design will constrict device production capacity and waste the vast majority of cold energy from output load at peak time.In order to solve this problem,cold energy recovery process with using phase change material(PCM)in LNG receiving terminal is designed and studied in this paper.Firstly,the physical properties of composite PCM composed of alkane and alcohol binary mixture are studied by Aspen Plus simulation.Then eutectic points and melting enthalpy of different materials are obtained.Considering physical properties,cost and safety,some conclusions and selection principles of PCM used for LNG cold energy recovery are drawn after materials inter-comparison.It is advisable in this paper that different kinds of PCM can be used to help different LNG cold energy utilization devices operate steadily.Secondly,the asynchronous and synchronous cold energy supplying/storing process where PCM is used to store the cold energy released by LNG and to maintain the cold energy utilization device operating steadily are designed,respectively.Also the corresponding cold energy storage devices are projected.Besides,the advantages and disadvantages of these two processes have been analyzed.Thirdly,with using n-pentane and n-heptane respectively,each kind of cold energy storage devices has been simulated by Fluent in order to analyze the cold energy supplying and storing process.The simulation results reveal that when the asynchronous device uses n-C5 and n-C7 as the PCM,the ratio of time required for storing to the time required for supplying is 3 and 1.7,respectively.For the synchronous device,no matter cold energy is stored or supplied,the PCM outside the hot finned tube at the same position has a similar melting time in synchronous device.It indicates that the synchronous device can supply the cold energy steadily with refrigerant to the cold energy utilization device,regardless of the LNG flow rate.Finally,taking air separation unit as example,the adaptability between cold energy storage process and cold energy utilization device have been researched.The air separation unit will be kept operating steadily by using asynchronous process and 2 cold energy storage devices.The No.1 cold energy storage device with 288 finned tubes will use 672 t n-pentane as PCM and require 1374 t LNG to meet the demand of cold energy.The No.2 cold energy storage device with 294 finned tubes will use 697 t n-heptane as PCM and require 588 t LNG to meet the demand of cold energy.