| Air separation system is a typical type of high energy-consumption system. The increasing scale of the air separation unit in recent years contribute much to the total energy consumption, the design difficulty of key machine and the subsystems. The theoretical obstcles of large-scale air separation units require deep research. It is a chanlenge to deal with all these issues. In the present paper, we conduct the research toward the modeling of the outer-elevated type air separation unit with the oxygen production of 120,000Nm3/h and and the methods to reduce its energy consumption, and aim to clarify and solve some obstcles of large-scale air separation units, and to reduce the energy consumption of the large-scale air separation system.Based on the analysis of the flowsheet parameters of conventional outer-elevated bype air separation unit with the oxygen production of 120,000Nm3/h, the exergy analysis of the air separation system is conducted. It is found from the exergy analysis that the exergy loss distributes rather ununiformly in six sub-systems. In detail, compression and cooling system, distillation system, adsorption and purification system and the main heat exchange system are the key sources of system exergy loss, and therefore are the most important research directions. The expansion system and subcooling system only contribute to 1.7% of total system exergy loss, and it is difficult to obtain obvious efficiency increase by improving these two subsystems. On this basis, a scheme is investigated to recovery the waste heat of air compressor to heat the waste nitrogen used in the adsorber desorption. It turns out that by employing the heat recovery scheme, the annual electricity consumption is reduced from 27.153million kW·h to 12.094million kW·h; the exergy efficiency of air compression and cooling system increases from to 54.3% to 55.8%; the total system exergy loss is reduced by 5.64%.A novel adsorber with large air processing capacity is studied which is characterized by parallel airflow passages, and the adsorption model is established. The isotherms of activated alumina and 13X zeolite is validated by experiment data. The research indicates that the adsorber volumes of two-passage and three-passage adsorbers are reduced by 27.8% and 31.7%, respectively; and the pressure drop is reduced by 10.8% and 56.4%, respectively. Moreover, the breakthrough time of two-passage adsorber is increased by 23.1%, increasing the utilization of the adsorption capacity.A passage arrangement scheme is proposed in the present paper based on minimizing the cumulative heat load. The distributed-parameter heat transfer model is employed to obtain the heat transfer performance. On this basis, a futher passage arrangement optimizatin scheme is propsed to improve the heat exchange efficiency. The comparable study with literature indicate that the cumulative heat load obtained by the present work is reduced by 2.9%. The outlet temperature ununiformity of the air stream, the waste nitrogen stream, the nitrogen stream are reduced by 7.8%,4.1% and 82.0%, respectively, contributing to the increase of heat exchange efficienty. Moreover, to reduce the complexity of modeling, the Modelcia modeling is applied to the air distillation process and is compared with conventional Boiling-point algorithm. The oxygen and nitrogen concentration profiles of both methods are also obtained to validate the accuracy.By conducting the research on large-scale air separation system in terms of modeling and low energy consumption, the performance of the air compressin and cooling system, the molecular sieve prepurification system, the multi-stream plate-fin heat exchange system are improved. The efficiency of the air separation system is increased and the energy consumption is reduced. The obtained results is of significance to the research on large-scale air separaton unit in the future. |
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