Pressure drop in fixed-bed adsorbers

Abstract

The effect of pressure drop on the dynamic behavior of a fixed-bed adsorber during adsorption and desorption steps is studied for two operating modes (constant volume and molar flow rate at the bed inlet). For saturation with a constant volume flow rate, neglecting pressure drop gives rise to a late breakthrough for the concentration wave compared to the case with Δ P, in other words, overestimates the breakthrough time. This is essentially due to the increase in molar flow at the bed entrance inherent to the appearance of an axial pressure gradient. On the other hand, when a molar flow rate is maintained constant at the bed inlet, it is shown that neglecting pressure drop leads to an underestimation of the breakthrough time. For the desorption step using a constant volume flow rate, it is seen that pressure drop engenders a shortening of regeneration. This apparent result is misleading. In fact, when reasoning in terms of gas quantity needed to regenerate the bed, it appears that pressure drop leads to an overconsumption of desorbing gas. This is confirmed when working with a constant molar flow rate. Thus, as intuitively expected, pressure drop is unfavorable to regeneration.