| The air compression process is the most energy-consuming unit of cryogenic air separation units,accounting for 70-80% of the total power consumption;however,enormous compression heat is difficult to be recovered due to its low grade and large fluctuation features.With the large-scale and low-energy consumption development of air separation units,it is urgent to recover compression heat to improve system efficiency.Since high temperature and high humidity air will lead to problems such as increased compressor input power and water vapour condensation,the use of liquid desiccant dehumidification systems driven by compression heat to cool and dehumidify the compressor inlet air is a potential way to reduce the compressor energy consumption.However,the typical characteristics of the air compression process in air separation units are high airflow and large fluctuation in temperature and humidity throughout the year,so the liquid desiccant dehumidification technology featuring low effectiveness is difficult to meet the deep dehumidification requirement of the compressor inlet air under all working conditions.Based on process-level mechanism analysis,componentlevel improvement method and system-level experimental verification,this paper focuses on the distribution characteristics of mass transfer resistances in liquid desiccant dehumidification process,the improvement method of dehumidification performance under high gas-liquid flow ratios,and the liquid desiccant-based deep dehumidification technology for energy saving of the air compression process.Main contents and findings are listed as follows:(1)A conjugate model of flow,heat and mass transfer in membrane-based liquid desiccant dehumidifiers is established,and the distribution characteristics of mass transfer resistances in gas,solid and liquid domains during the dehumidification process are revealed.Since the underlying mechanism of dehumidification performance deterioration has not yet been clear,this paper proposes a three-dimensional conjugate heat and mass transfer model of membrane-based liquid desiccant dehumidifiers considering transmembrane permeation characteristics.Based on the theory of mass transfer resistance distribution and concentration polarization,it is found that local mass transfer resistances upstream mainly originate from the transmembrane permeation resistance,while it is mainly the air-side convection mass transfer resistance for downstream.The proportion of the solution-side convection mass transfer resistance is always less than 22%.In addition,the intrinsic relationship between average mass transfer resistances and concentration polarization is revealed in variable conditions.(2)A multi-stage internally-cooled structure is proposed,which can effectively improve the driving force distribution in the liquid desiccant dehumidification process and significantly improves the dehumidification effectiveness under high gas-liquid flow ratios.Since the driving force distribution is uneven and the effectiveness is low under high gas-liquid flow ratios for liquid desiccant dehumidifiers,this paper proposes a structure of multi-stage dehumidification and layered internally-cooling for improving the dehumidification effectiveness based on the uniform distribution theory of driving forces for heat and mass transfer.Compared with the single-stage adiabatic type,the distribution uniformity of heat and mass transfer driving forces of the multi-stage internally-cooled liquid desiccant dehumidifier is improved by 41% and 57%,respectively.In high gas-liquid flow ratios of 8.60~20.10,the experimental latent effectiveness reaches over 0.42,which is more than twice that of existing dehumidifiers.(3)A deep dehumidification system with integrated dew-point cooling and multistage internally-cooled liquid desiccant absorption is first designed and built for compression power saving of air separation units,which achieves deep dehumidification under common air conditions and proves its energy saving effect on the air compression process.Since liquid desiccant dehumidification systems are difficult to achieve deep dehumidification under the conditions of high air humidity and high gas-liquid flow ratios,this paper designs and builds a dew-point cooling and internally-cooled liquid desiccant integrated deep dehumidification system based on a cascade handling concept.Under the experimental air condition of 32 °C and 20 g/kg,even if the gas-liquid flow ratio is as high as 9.65,the cascade dehumidification system can still reduce the humidity ratio to 6.00 g/kg,thereby achieving the deep dehumidification at high air humidity and high gas-liquid flow ratios.For the traditional three-stage air compression process,the compression power saving rate is up to 8.33% after applying the deep dehumidification system. |
