Purity versus recovery in FricDiff separations for feed-side and sweep-side products

Frictional Diffusion (FricDiff) is a separation technology based on differences of diffusivities in a mass-separating sweep gas. Previously, Frictional Diffusion has been applied to the separation of azeotropic mixtures and gas mixtures where the product is on feed side. In this work, the application range is extended by studying hydrogen–hydrocarbon separations. In this case, the target product is obtained on the sweep-side. In contrast to the feed-side product case, it is shown that the purity of a sweep-side product is inherently limited by the ratio of the diffusion coefficients. This limitation can, however, be overcome with a small pressure difference which induces a convective flow superimposing diffusion. Thereby, the purity of product increases from 65% to more than 95%. On the other hand increasing pressure difference decreases the recovery to lower than 40% for the sweep-side product. The positive effect of pressure difference is confirmed by using a single-tube FricDiff unit. A good agreement between experimental and modeling data is observed which suggests the potential of FricDiff for higher purity separations for both feed-side and sweep-side products.

Research highlights▶ comparison of FricDiff performance for feed and sweep-side products by varying three different design parameters: sweep ratio, feed in flow rate and pressure difference. ▶ Sweep and a feed side product have a limitation of the purity of the sweep-side product if only diffusion is used for the separation. ▶ This limit is given by the ratio of the feed mixture components’ diffusion coefficients. ▶ The limitation in enrichment for the sweep-side product can be overcome by increasing the pressure on the sweep side. ▶ Increased friction due to the induced convection improves the separation. ▶ A successful correspondence between experimental and simulation data.