Molar flow binary diffusion

A thin plastic membrane separates hydrogen from air. The molar concentrations of hydrogen in Ihe membrane at the inner and outer surfaces are determined to be 0.045 and 0.002 kmol/m respectively. The binary diffusion coefficient of hydrogen in plastic at the operation temperature is 5.3 X lO mVs. Determine the mass flow rate of hydrogen by diffusion through the membrane under steady conditions if the thickness of the membrane is (a) 2 ram and (b) 0.5 mni,... 

Equimolar counter diffusion appears in the distillation of binary mixtures. In a distillation column the liquid falls downwards, and the vapour flows upwards, Fig. 1.43. As the liquid flowing down the column is colder than the vapour flowing upwards, chiefly the component with the higher boiling point, the so called least volatile component condenses, whilst the vapour from the boiling liquid mainly consists of the components with the lower boiling points, the more volatile components. The molar enthalpy of vaporization is, according to Trouton s rule, approximately constant for all components. If a certain amount of the least volatile component condenses out from the vapour, then the same number of moles of the more volatile substance will be evaporated out of the liquid. At the phase boundary between liquid and vapour we have cAwA = —cBwB. The reference velocity u is zero because cu = cAwA + cBwB. The molar flux transported to the phase boundary from (1.158) and (1.160) is... 

Eq. (1) is applicable to both pure diffusion and convective transfer in a laminar or turbulent flow. For a binary system, the total molar flux, which takes into account mass transfer by both molecular diffusion and convection because of bulk flow, can be expressed as ... 

Gas mixtures. Nomenclature to be used in describing the flow of a binary gas mixture through a diffusion barrier is shown in Fig. 14.4. The problem is to determine how the molar velocities of li t and heavy components, Gi and Gj, respectively, depend on upstream and downstream... 

Jj is the molar flux vector for species j with respect to the mass average velocity (kmol/m s). When the flow is laminar or perfectly ordered the term V Jj results from molecular diffusion only. It can be written more explicitly as an extension, already encountered in Chapter 3, of Pick s law for diffusion in binary systems, as... 

But this is not all, the same diffusion coefficient D may be used if we use molar quantities in formulation of Pick law for this binary concentration diffusion, cf. (4.562) below. Specifically, using the molar diffusion flow of constituent 1 defined as the corresponding (specific) diffusion flow given above divided by the molar mass Ml of the first constituent (molar quantities are denoted, in addition, by apostrophe e.g. the molar diffusion flow relatively to volume average velocity as... 

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