Theoretical Study Of Onboard Air Separation Unit And Inerting System

The mass transfer mechanism, process of mass transfer and the calculation method ofpermeability coefficient for the hollow fiber membrane was briefly introduced, and the influence ofthe temperature and the concentration on the permeability coefficient was analyzed. The mathematicalmodel of the hollow fiber membrane of OBIGGS in term of the differential method was set up, andsolved via Runge-Kutta method. The comparison with the experimental data published in theliterature reveals that the error is within10%. Several parameters including the flow of the feed gas ofunit membrane area, pressure ratio, the separation factor and separation ideal level are defined toanalyze the influence on the oxygen concentration and the separation efficiency of product gas.Through the establishment of the mathematical description on the pressure drop, the influence of themembrane length, membrane inner diameter, the operation pressure on the pressure drop in themembrane and the membrane inner diameter on separating performance are discussed. Consequently,the simulation of the ground-based inerting system and the on-board inerting system are calculate tocompare the ground-based oil prescribing system in reduced pressure, normal pressure andsupercharged pressure; The curve of the oxygen concentration ullage, pressure in tank and flow ofbleed air on the condition of the constant flow, varied concentration are obtained via the mathematicalsimulation.The result shows that the separation efficiency of the product rises with the increase of the flowof the feed gas of unit membrane area, but it also leads to the higher oxygen concentration. Therefore,the appropriate design process should be considered to overcome this disadvantage. The increase ofthe pressure ratio and the permeability coefficient of oxygen will make the decrease of the oxygenconcentration and the separation efficiency of the product. However, the increase of the penetrationratio has little effect on the separation efficiency of the product, so it is beneficial to the gas separationprocess. The pressure ratio makes greater effect on the separation ideal level, the deviation betweenthe actual separation process and the theoretical value increase due to the increasing of pressure ratio and the temperature affect less. The pressure in the fiber decrease as the increasing of the fiber length,and the speed increased gradually in addition. The increase of the membrane inner diameter canreduce the pressure drop in the fiber, and the increasing of the feed pressure also reduces the pressuredrop in the fiber. The membrane inner diameter has a great impact in the separation performance ofhollow fiber membrane, and the separation efficiency of product gas and the oxygen concentrationdecrease observably. The decompressed ground-based oil prescribing system can obviously decreasethe mass of oxygen dissolved in oil, and so is the most advantageous of all. The performance of thehollow fiber membrane used in the onboard inerting system varied during the flight and the oxygen inullage can reach a lower level than the ground inerting.The study in the paper could provide the theory base for the design of the OBIGGS and theground-based oil pre-scrubing system.