The present invention generally relates to gas adsorption and, more particularly, to an apparatus and method of selectively adsorbing nitrogen over oxygen from a gas mixture.
The separation of gases, including nitrogen from oxygen, can be useful in a wide variety of environments. As an example, separating nitrogen from air for use in a cockpit of an aircraft continues to be problematic. Of course, an inefficient means of oxygen supply can impact the performance of the pilot. It can also translate to higher maintenance costs for the separation system. Many attempts to design gas separation systems have been made.
For example, a process for separating mixtures of oxygen and nitrogen by the use of an adsorbent is shown in U.S. Pat. No. 5,672,195. Therein, it is explained that zeolites have been used selectively to separate nitrogen from oxygen based on the strong interactions between the quadrupole moment of the nitrogen molecule and the cations of the zeolite. It was further noted that varying the temperature and pressure has also been used to optimize the nitrogen adsorption and desorption efficiency. The invention employed zeolites that were agglomerated preferably with an inorganic binder and formed into spheres. The porosity of the adsorbent was optimized to accelerate the adsorption kinetics. Some drawbacks to this design include loss of capacity due to dilution by the binder and, particularly, the generation of dust particles due to attrition of the binder and movement of adsorbent particles.
In U.S. Pat. No. 4,687,573, a sorbing apparatus for removing one or more components from a gas or liquid fluid is disclosed. The sorbing apparatus includes a chamber having a bed of sorbent particles bound together by a polymeric binding agent and/or bound to the chamber to prevent movement of the particles relative to one another. The chamber includes ports to allow an inflow and outflow of a fluid having the components to be removed, as well as a fluid used to purge the components. The process of making the bed of sorbent particles includes preheating inorganic sorbent particles and mixing the heated particles with a powdered polymeric binding agent. The mixture of particles and binding agent is placed under pressure at a solid-liquid transition temperature and then cooled. Optionally, the mixing and application of pressure may be done in a mold to provide a desired shape. Some disadvantages, however, to the above include the fact that the process is difficult to control.
Latex polymer bonded crystalline molecular sieves in an aqueous medium are shown in U.S. Pat. No. 4,822,492. It is pointed out therein that zeolites bonded to inorganic oxides deteriorate in aqueous media. However, the use of organic polymer binders in lieu of inorganic polymer binders has alleviated such problems. Yet, the use of an organic solvent at a high level has decreased the adsorptive capacity. Thus, the invention used latex in place of the organic polymer binder. The latex was generally a suspension of polymer particles in water. A crystalline inorganic compound, such as a zeolite, is mixed into the latex. An optional inorganic oxide binder was used. The resulting mixture was dried to remove water and then ground. Disadvantages with this disclosure include dust formation.
Currently Honeywell Normalair-Garrett Limited of England markets a generic Onboard Oxygen Generation System (OBOGS) based on the selective adsorption of nitrogen from air. This system consists of a pressure swing adsorption unit in which air is fed into the first of three cartridges at an elevated pressure. The first cartridge contains an inorganic adsorbent that is selective for nitrogen over oxygen. The adsorbent can be clay bound activated zeolite 13X (HP-13X). The nitrogen is thereby removed from the air, providing oxygen-enriched air for use in an aircraft. Periodically, the cartridge in use as a nitrogen adsorbent is changed, the pressure on the first cartridge is reduced, thereby allowing the adsorbed nitrogen to be desorbed. However, the capacity of the adsorbent mass within the cartridge is highly variable and the performance of each cartridge also varies.
As can be seen, there is a need for an improved apparatus for and method of separating nitrogen from air. Also needed is an apparatus and method that has a high capacity and reliability for nitrogen adsorption. A further need is for a method of making a nitrogen adsorbent that is simple and low in cost. A nitrogen adsorbent apparatus is also needed and that can be utilized in the form of a cartridge that allows easy use and replacement, while minimizing shifting of the adsorbent material within the cartridge due to vibration or other shocks. The cartridge should also provide good mass transfer kinetics and withstand heating up to about 200xc2x0 C.
In one aspect of the present invention, a method of making a gas adsorbent comprises dissolving a polymer in at least one first solvent to form a polymer solution; mixing an inorganic adsorbent with the polymer solution to form a precursor; molding the precursor; and leaching the at least one first solvent from the precursor.
In another aspect of the present invention, a gas adsorbent system comprises a housing; and a molded gas adsorbent within and substantially completely filling the housing such that the gas adsorbent is prevented from shifting within the housing in the absence of a component that adheres the gas adsorbent to the housing, with the gas adsorbent having been formed from a polymer solution and an inorganic adsorbent.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.