Research On The Integration Of LNG Regasification And Air Separation Units

Liquefied Natural Gas(LNG)is a kind of clean energy,and releases a lot of cold energy in the downstream process of re-gasification to natural gas(NG)at normal condition.At present,China became the second largest importer of LNG.It will produce huge economic benefits by utilization of cold energy from those import LNG.In this dissertation,to make full use of LNG cold energy and solve the existing problem in this subject,the regasification of LNG integrated into the air separation unit with double rectification column and vapor recompression column are studied,and the heat transfer of supercritical fluid,the law of energy distribution in distillation column and the optimization of cold energy system is discussed in detail.It has an important theoretical significance and engineering application value for the efficient utilization of LNG cold energy and energy conservation of cryogenic air separation unit.The main contents of this dissertation are:(1)Due to strong nonlinear variation of supercritical LNG and nitrogen isobaric heat capacity with temperature and pressure,pinch point would occur in heat exchange process between LNG and nitrogen.The heat exchanger process is affected by the pinch point value,temperature difference at the cold end and pressure of nitrogen.The different value of pinch point have little effect on its location,and increasing the temperature difference at the cold end effectively increases the mass flow ratio and decreases the approach temperature difference,while rising the pressure of nitrogen leads to a valley value of the mass flow ratio,in contrast to a peak value in the approach temperature difference.(2)As the pinch point occur in heat exchange process,the LNG cold energy cannot be used efficiency and sufficiently in the air separation unit with double rectification column.Choosing the lower temperature difference at the cold end is good for the energy consumption and equivalent energy consumption per kilogram liquid production,but a large amount of LNG is needed and cryogenic energy could not be fully used;while the influence of pressure of nitrogen on the energy consumption presents different variation trend.The result shows the energy consumption per kilogram liquid production of proposed process is roughly 5-12 percent lower than that of convectional processes which the cryogenic energy is only used for cooling the nitrogen or the feed air.(3)Adopting the modification of MESH equation,the thermodynamic characteristics of three different distillation columns for air separation are analyzed.The results show the recovery ratio of product is decided by the total amount of heat duty and the scheme of heat distribution.Based on the above results,the first and second law is introduced to assess the thermodynamic characteristics of distillation columns.The results show the operating pressure of vapor recompression column is decided by the utilization of cryogenic energy,and it has the lowest demand of shaft work and minimum energy of separation compared with the double rectification column and internally heat-integrated distillation column.Therefore,a cryogenic air separation unit with the implementation of vapor recompression column and LNG cold energy is proposed.The optimization results show that the power consumption of proposed process is about 6.9%and 11.1%lower than it’s in the previous studies in producing vapor and liquid product respectively,and the minimum energy of separation is improved by use of LNG cryogenic energy.(4)The heat transfer characteristics of supercritical nitrogen and methane in horizontal tube is studied by numerical simulation.The results show the strong variation of thermo physical properties of supercritical methane and nitrogen would cause buoyance effect,and make the secondary flow generate in the cross-section.Therefore,the heat transfer near the pseudo-critical point would enhance.Adopting the existing semi-empirical heat transfer criterion correlations to calculate the heat transfer coefficient of supercritical nitrogen and methane in the micro-element section,the results show the Jackson heat transfer criterion correlations has the same predicted value with numerical result when the condition of supercritical nitrogen is near the pseudo-critical point,and the Gnielinski heat transfer criterion correlation is the appropriate correlation for the supercritical methane and supercritical nitrogen far away from the pseudo-critical point.