Thermodynamic Simulation And Experimental Validation Of Spiral-wound LNG Heat Exchangers Applied In Land-based And Offshore LNG Plants

LNG spiral-wound heat exchangers have the advantages of high effectiveness,high operation pressure and large scale unit,and have been used as main cryogenic heat exchangers in large-scale land-based and offshore LNG plants.The LNG spiral-wound heat exchanger is the key equipment in liquefaction process,and its performance can directly determine the LNG production of the whole liquefaction process.And thus,an effective design method for LNG spiral-wound heat exchangers applied in land-based and offshore LNG plants is necessary.Simulation based design methods show great advantages of high effectiveness,less resource requirements and being capable of avoiding repeatedly manufacturing prototypes and doing experiments in heat exchanger design and optimization,especially for large-scale cryogenic heat exchangers,such as LNG spiral-wound heat exchangers.The basis of such a simulation based design method is usually an effective distributed-parameter model for heat exchangers due to high accuracy in predicting heat exchanger performance.Therefore,it is needed to develop an effective simulation approach based on distributed-parameter models for LNG spiralwound heat exchangers applied in land-based and offshore LNG plants.Although there are some researches on the models of LNG spiral-wound heat exchangers,some aspects including the fast calculation method for thermodynamic properties of natural gas mixtures,the distributed-parameter model for land-based LNG spiral-wound heat exchanger capable of reflecting the flow mal-distribution of parallel spiral tubes and the complex flow circuits arrangement of multi-streams,the distributed-parameter model for floating LNG spiral-wound heat exchanger capable of reflecting the shell-side maldistribution under rolling conditions,and the design and optimization method based on distributed-parameter models,are required to be further investigated.This thesis discusses the researches on the four aspects as follows.(1)A fast calculation method for thermodynamic properties of natural gas mixtures.To satisfy the requirements of fast speed,high accuracy,absolute stability and reversibility,a fast calculation method for thermodynamic properties of variablecomposition NG in supercritical pressure region is developed by extending the implicit curve-fitting models from the reference composition to variable composition.The implicit curve-fitting models for the reference composition are developed by dividing the supercritical pressure region into three subsections and regressing each subsection separately instead of a uniform equation for the entire range,which can increase the fitting accuracy of the subsection near the critical temperature where the thermodynamic properties change dramatically.The implicit curve-fitting models for variable composition are developed by regressing the precise influence of variable composition on thermodynamic properties based on the implicit curve-fitting models for the reference composition.The mean calculation speed of the present fast calculation method is about 104 times faster and the mean deviation is less than 0.5% compared with GERG-2008.(2)A distributed-parameter model for land-based LNG spiral-wound heat exchanger.The layer-by-layer control volume method has the ability to reduce the control volume number and to reflect the refrigerant distribution in shell side.The enthalpy of refrigerant confluence with different compositions iterated by the temperature of the mixed refrigerant will avoid the calculation error of directly using the mass flow rate weighted average method due to different enthalpy reference.The flexible flow circuits of SWHEs in different liquefaction processes can be described by a directed graph.The equivalent flow resistance is an effective way to solve the non-linear equations in calculating the mass flow rate distribution through flow circuits.The presented distributed-parameter model shows good accuracy and fast speed for predicting the performance of LNG SWHEs,especially for the cases with maldistribution in tube side.Compared with the experimental data of a small-scale spiral-wound heat exchanger,the maximum deviation of predicted heat exchange capacity of each stream is within 5%,and the mean deviation is 3.5%;the maximum deviation of predicted pressure drop of each stream is within 80 k Pa,and the mean deviation is 22 k Pa.(3)A distributed-parameter model for floating LNG spiral-wound heat exchanger under rolling conditions.A tube-by-tube model of floating LNG SWHEs under rolling conditions is developed.The mass flow rate distribution at arbitrary tilt angles is obtained by computing the three-dimensional mass transfer among adjacent control volumes to distinguish the gas and liquid rich regions,and then the heat transfer models for gas and liquid rich regions are developed,respectively.The deviation of the predicted heat exchange capacity from the experimental data of a small-scale spiral-wound heat exchanger applied in the sloshing platform is within 5.0%,and the deviation of shell-side outlet temperature is within 2 oC;the mean deviation of the predicted mass transfer rate distribution from the experimental data is within 16.3%.The effect of rolling amplitude on the heat transfer performance based on the proposed model is analyzed.The results show that the heat exchange capacity decreases with the increase of the rolling amplitude,and the decrease is from 2.2% to 6.7% under the rolling amplitude from 3° to 15° when the mass flow rate of shellside inlet is well distributed;the transient heat exchange capacity might increase when the mass flow rate of shell-side inlet is non-uniformly distributed.(4)A design and optimization method based on distributed-parameter models for LNG spiral-wound heat exchangers.Minimum heat transfer area is determined as the optimization objective.The idea to develop the design and optimization method for LNG spiral-wound heat exchanger is to obtain the preliminary optimal geometry by a segmented stream evolution model,to check the performance of preliminary optimal geometry by a distributed-parameter model and then to readjust the geometry.The algorithms for the geometry optimization process and the performance check and geometry readjustment process are developed respectively.Applying a segmented stream evolution model to optimize geometry can accelerate the optimization speed because the segmented stream evolution model is less complicated than the distributed-parameter model;applying a distributed-parameter model to check the heat exchanger performance can ensure the design accuracy;combining the above two models in the design and optimization method can simultaneously meet the requirements of fast optimization speed and high design accuracy.The present research can provide the design basis and method for LNG spiralwound heat exchangers applied in land-based and offshore LNG plants.