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ID:19880567
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时间:2018-10-07
《第8章气体吸收 §8.1概述》由会员上传分享,免费在线阅读,更多相关内容在教育资源-天天文库。
1、第17章相际扩散与传质原理概述扩散是一种在物理驱动下单组分通过混合物的运动。扩散最通常的原因是扩散组分浓度梯度的存在。Diffusionisthemovement,undertheinfluenceofaphysicalstimulus,ofanindividualcomponentthroughamixture.Themostcommoncauseofdiffusionisaconcentrationgradientofthediffusingcomponent.Aconcentrationgradienttendstomovethecomponentinsuchadirectiona
2、stoequalizeconcentrationsanddestroythegradient.浓度梯度驱使扩散组分朝着使浓度均匀化的方向扩散,从而使浓度梯度趋于消失。Whenthegradientismaintainedbyconstantlysupplyingthediffusingcomponenttothehigh-concentrationendofthegradientandremovingitatthelowconcentrationend,thereisasteady-statefluxofthediffusingcomponent.Thisischaracteristic
3、ofmanymass-transferoperations.当不断地向高浓度一端提供扩散组分,从低浓度一端除去扩散组分时,扩散组分便具有稳定的传质通量。这就是许多传质操作的特点。例如,当在填料塔中用水吸收气体中的NH3时,在塔内的任意一点上,在气相浓度梯度的驱使下,NH3由气相主体扩散至气~液界面,在界面上溶解,然后,在液相浓度梯度驱使下,扩散至液相主体。Forexample,whenammoniaisremovedfromagasbyabsorptioninwaterinapackedcolumn,ateachpointinthecolumnaconcentrationgradient
4、inthegasphasecausesammoniatodiffusetothegas-liquidinterface,whereitdissolves,andagradientintheliquidphasecausesittodiffuseintothebulkliquid.Instrippingasolutefromaliquidthegradientsarereversed;herediffusionbringssolutefromthebulkliquidtotheinterfaceandfromthereintothegasphase.当从吸收液中解吸溶质时,浓度梯度与吸收过
5、程相反。在此,扩散作用使溶质从液相主体移至界面,再从界面移至气相。Insomeothermass-transferoperationssuchasleachingandadsorption,unsteady-statediffusiontakesplace,andthegradientsandfluxesdecreasewithtimeasequilibriumisapproached.在其它一些诸如浸取和吸咐操作中发生的是不稳定扩散,随着过程趋于平衡,扩散组分的浓度梯度和通量随时间而降低。Althoughtheusualcauseofdiffusionisaconcentrationg
6、radient,diffusioncanalsobecausedbyanactivitygradient,asinreverseosmosis,byapressuregradient,byatemperaturegradient,orbytheapplicationofanexternalforcefield,asinacentrifuge.虽然扩散的一般原因是浓度梯度,但能动梯度也能引起扩散如反渗透过程,压力梯度、温度梯度和外场力都可引起扩散,如离心分离。Moleculardiffusioninducedbytemperatureisthermaldiffusion,andthatfr
7、omanexternalfieldisforceddiffusion.Bothareuncommoninchemicalengineering.Onlydiffusionunderaconcentrationgradientisconsideredinthischapter.由温度引起的分子扩散称为热扩散,由外场力引起的扩散称为强制扩散。这两种扩散方式在化学工程中并不常见。本章仅讨论由浓度梯度引起的质量扩散。Diffusionisn
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