Loschmidt diffusion cell

Loschmidt diffusion apparatus (a) curves showing schematically the quantity A7 = 75, - 7 (h) cell ... 

Equation 6.2.3 has exactly the same form as Eq. 5.1.3 for binary systems. This means that we may immediately write down the solution to a multicomponent diffusion problem if we know the solution to the corresponding binary diffusion problem simply by replacing the binary diffusivity by the effective diffusivity. We illustrate the use of the effective diffusivity by reexamining the three applications of the linearized theory from Chapter 5 diffusion in the two bulb diffusion cell, in the Loschmidt tube, and in the batch extraction cell. 

In an isobaric closed system like the diffusion cell or Loschmidt tube (see Chapter 5), any... 

McKay (1971) carried out experiments in a Loschmidt-type diffusion cell with a nine component system including nitrogen, carbon dioxide, and a number of paraffins. The experiments were carried out at the high temperatures and pressures typical of underground oil and gas reservoirs. McKay s paper includes a dimensioned drawing of the Loschmidt-type cell used for the experiment and the initial conditions for some of his experiments. Use the theory of Section 5.5 to predict the composition profiles in this system. State clearly any assumptions made in your calculations. You will need to estimate diffusion coefficients at high pressure. Consult Reid et al. (1987) for details of methods that can be used for this purpose. 

Modern gas-diffusion medium in low-temperature fuel cells is typically a highly porous carbon paper with porosity in the range of sgdl = 0.6-0.8 and with the mean pore radius in the order of 10 pm (10 cm). By the order of magnitude, the mean free path of molecules in atmospheric pressure air is = l/(A LO-fci ), where Nl = 2.686 10 cm is the Loschmidt number (number of molecules in a cubic centimetre of atmospheric pressure gas at standard temperature) and akin — 10 cm is the molecular cross-section for kinetic collisions. With this data we get 3 10 cm, or 3 10 pm. Obviously, mean pore radius in the GDL is nearly 3 orders of magnitude greater than I f and the physical mechanism of molecule transport is binary molecular diffusion. 

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