耦合过热蒸汽干燥的MVR蒸发结晶系统热力性能分析

Performance analysis of a MVR evaporative crystallization system coupled with super-heated steam drying technology

机械蒸汽再压缩(MVR)作为目前最先进的蒸发结晶技术得到广泛关注和应用,本文结合过热蒸汽干燥技术的优势,提出一种耦合过热蒸汽干燥的MVR蒸发结晶干燥系统。基于EES软件建立数值模型,通过系统仿真的方法分析闪蒸压力pflash、进料浓度c0、循环倍率CR、换热温差ΔTmhex等对压缩机能耗Wcomp和系统能效系数COP、下降管结晶盐含水率的上限值ω和换热器UA值的影响情况。结果表明:耦合过热蒸汽干燥的MVR系统相比常规系统具有更高的COP、更低的Wcomp,并且能够获得干燥的结晶盐;循环倍率CR增大,压缩机压比π降低,有利于提高COP和降低Wcomp,但同时换热器UAmhex值会增大,设备成本增加;系统COP随pflash的变化关系与循环倍率CR有关,CR较低时,随Pflash的增大,系统COP增大,CR超过某一定值,随着pflash的增大,系统COP呈先升后降趋势,其最大值在pflash为40~50kPa;进入干燥器的下降管结晶盐含水率的上限值ω与进料浓度c0有关,随进料浓度c0的升高而增大;ΔTmhex对MVR系统性能有显著的影响,ΔTmhex每增大1K,COP平均降低5.6%、Wcomp平均升高5.8%、UAmhex平均减少14.4%,ω平均升高1.7%。

As the most advanced evaporative crystallization technology,mechanical vapor recompression(MVR)has been widely concerned and applied.In this paper,an advanced MVR evaporative crystallization drying system combined with the superheated steam drying technology was proposed.A mathematical model has been developed using EES software.The effects of the feed concentration(c0),flash pressure(pflash),circulation ratio(CR),heat exchanger temperature difference(ΔTmhex)on the compressor work(Wcomp),coefficient of performance(COP),the maximum value moisture content of salt crystals in downcomer(ω)and heat exchanger parameter(UA)were studied in detail by using system simulation.The results showed that,compared with the conventional system,the MVR system coupled with superheated steam drying technology obtained higher COP,lower Wcomp,and could get dry crystal salts.With the increase of circulation ratio CR,the compressor pressure ratioπdecreased,which is beneficial to improve COP and reduce Wcomp,but at the same time,the heat exchanger parameter UAmhex and the equipment cost will increase.The variation of COP with pflash was influenced by the CR.As the CR was under a lower value,the COP increased with the increase of pflash,and once CR exceeded a certain value,the COP increased first and then decreased with the rising of pflash.There existed a maximum value when the pflash was among 40—50kPa.The ω increased significantly with the increase of the c0.The ΔTmhex has obvious effect on the performance of MVR system,and the COP decreases by 5.6%,Wcomp increases by 5.8%,UAmhex decreases by 14.4%,andωincreases by 1.7%,respectively,per unit increase in ΔTmhex.