Flow Distribution Analysis Of A Horizontal-type Molecular Sieve Adsorber Of Large Scale Air Separation Unit

Used for removing the impurities in feed gas, such as vapor, carbon dioxide and hydrocarbons, molecular sieve air purification system plays an important role for safe operation of an air separation system, hence, it is an indispensable part of air separation unit. The effectiveness of the molecular sieve purification system depends on the performance of its adsorber, whose internal flow field distribution, as an important performance index, heavily relates to the structure of adsorber. With the expanding scale of air separation units, the capacity of the adsorber in molecular sieve purification system should also be increased accordingly, which might induce the malfunction of the adsorption and regeneration. To achieve the goal of energy saving based on the stable operation of the molecular sieve purification system has become the focus of attention in the air separation industries and the research on the structure optimization has its realistic significance.Based on computational fluid dynamics(CFD), the flow distribution in a horizontal-type molecular sieve adsorber is analyzed with the commercial software FLUENT in this work, in order to offer a guide to the optimizating design of the adsorber in air separation unit. The main research contents include:(1) Upon introducing the important role of molecular sieve adsorber in the air separation unit, the shortcoming of traditional structure adsorber is analyzed for the case of an increased capacity of the molecular sieve adsorber. Then, several numerical simulation models are set up for the horizontal-type molecular sieve adsorbers with different structures. The flow distribution inside the molecular sieve adsorbers are computed and analyzed, in order to ameliorate the flow field by optimizing the distributor’s structure. In the simulation, the perforated-pipe, perforated-plate and adsorbent bed layer are all treated as porous media.(2) The traditional buffer-board flow distributor is replaced by the perforated-pipe structure of horizontal-type molecular sieve adsorber in a20000m3/h air separation unit, and the internal flow distribution with two simulation models are computed and analyzed. The results show the obvious uneven air distribution at the adsorbent bed layer and near the wall of horizontal-type molecular sieve adsorber in a20000m3/h air separation unit with traditional buffer-board flow distributor, while the perforated-pipe structure can overcome the shortcoming of the traditional adsorbers and can be beneficial for achieving better performance from the molecular sieve adsorber.(3) The one-perforated-pipe distributor is replaced by the multi-perforated-pipe structure of horizontal-type molecular sieve adsorber in a60000m3/h air separation unit, which can overcome the negative effects caused by the increased capacity. Meanwhile, compared with the structure of one-, two-and three-perforated-pipe, the flow distribution and the production cost of the molecular sieve adsorber with two-perforated-pipe distributor are obviously superior to the other two structures.(4) The accuracy of numerical simulation models are validated by the comparison between numerical results and actual operating parameters, and the independence of the simulation results on the mesh number is also analyzed. The results show that the inlet and outlet pressures and velocities are close to the actual operating parameters, and the error is in the controllable range. In addition, the accuracy of simulation results is also proved by the actual dosage of the adsorbent and the waste nitrogen.