Dynamics of pressurization and depressurization during pressure swing adsorption

Abstract

Experiments with a single fully instrumented 3.0 m long column packed with either non-adsorbent or adsorbent particles revealed that times required for pressurization and depressurization differed substantially. For all the non-adsorbent packings except sand, the column could be filled to pressures of up to 7 bar absolute with air, Ar, O 2 or N 2 within periods of 1 s. When the column was packed with small particles of sand, pressurization with gas took up to 10 s. The length of time to pressurize with gas was proportional to gas viscosity and the square of the length of column packing and inversely proportional to bed permeability and particle size. When the column was packed with a 5A molecular sieve and filled with N 2 or O 2 the time required for the column to reach the final pressure was correspondingly longer than for the non-adsorbent packings except sand. Results obtained when the column was depressurized showed that blowdown of the column always took a longer time than the corresponding pressurization time. The results are interpreted in terms of two quite distinct models. One model consists of a conventional unsteady-state bed conservation equation coupled with either a modified Ergun or Darcy equation to take account of axial pressure gradients. Results show that application fo the steady-state Ergun equation to the unsteady-state processes of pressurization and depressurization is unsatisfactory. Step boundary conditions are also shown to be inadequate for non-adsorbing gas—solid systems. For the first model it was therefore necessary to impose experimentally determined time-dependent boundary conditions for each system and to assume, additionally, instantaneous equilibrium between N 2 and also O 2 and the 5A zeolite. Simulations then agreed reasonably well with experimental results, the greatest discrepancies occuring near the gas inlet and at the lowest terminal absolute pressures (3 and 4 bar). The second model considered is an electrical analogue representation of the system. The difficulties is handling boundary conditions (inherent in the conventional approach), and the constraints of instantaneous gas—solid equilibrium, are circumvented by adopting the latter model. It is also shown that determining Ergun coefficients by curve fitting pressure as a function of time and distance is a more satisfactory way of proceeding thant utilizing coefficients measured from steady-state experiments. Micropore diffusion control as a rate determining event is a more satisfactory model for representing the pressurizing and depressurizing of the column with N 2 or O 2 in the presence of the 5A zeolite packing than a model based on equilibrium control.