The figure shows components of a distillation column intended for use as part of a system that produces high-purity liquid oxygen (LOX) from air by distillation. (The column could be easily modified to produce high-purity liquid nitrogen.) Whereas typical industrial distillation columns for producing high-purity liquid oxygen and/or nitrogen are hundreds of feet tall, this distillation column is less than 3 ft (less than about 0.9 m) tall. This column was developed to trickle-charge a LOX-based emergency oxygen system (EOS) for a large commercial aircraft.
A description of the industrial production of liquid oxygen and liquid nitrogen by distillation is prerequisite to a meaningful description of the present miniaturized distillation column. Typically, such industrial production takes place in a chemical processing plant in which large quantities of high-pressure air are expanded in a turboexpander to (1) recover a portion of the electrical power required to compress the air and (2) partially liquefy the air. The resulting two-phase flow of air is sent to the middle of a distillation column. The liquid phase is oxygen-rich, and its oxygen purity increases as it flows down the column. The vapor phase is nitrogen-rich and its nitrogen purity increases as it flows up the column. A heater or heat exchanger, commonly denoted a reboiler, is at the bottom of the column. The reboiler is so named because its role is to reboil some of the liquid oxygen collected at the bottom of the column to provide a flow of oxygen-rich vapor. As the oxygen-rich vapor flows up the column, it absorbs the nitrogen in the downflowing liquid by mass transfer. Once the vapor leaves the lower portion of the column, it interacts with down-flowing nitrogen liquid that has been condensed in a heat exchanger, commonly denoted a condenser, at the top of the column. Liquid oxygen and liquid nitrogen products are obtained by draining some of the purified product at the bottom and top of the column, respectively.
Because distillation is a mass-transfer process, the purity of the product(s) can be increased by increasing the effectiveness of the mass-transfer process (increasing the mass-transfer coefficient) and/or by increasing the available surface area for mass transfer through increased column height. The diameter of a distillation column is fixed by pressure-drop and mass-flow requirements. The approach taken in designing the present distillation column to be short yet capable of yielding a product of acceptably high purity was to pay careful attention to design details that affect mass-transfer processes.
The key components in this column are the structured packing and the distributor. The structured packing is highly compact. Each section of packing is about 1 in. (about 2.5 cm) in diameter and 3 in. (about 7.6 cm) long. The column contains a total of seven sections of packing, so the total length of packing in the column is 21 in. (about 53 cm). The packing promotes transfer of mass between the up-flowing vapor and the down-flowing liquid. The liquid distributor, as its name suggests, helps to distribute the liquid as nearly evenly as possible throughout the cross section of the column so as to utilize the packing to the fullest extent possible and thereby maximize the mass-transfer effectiveness of the column.
In operation, saturated air at a pressure of 70 psia (absolute pressure of 0.48 MPa) enters the reboiler and partially condenses. The air is then fully condensed by an external refrigeration source, such as a small cryocooler. The air then goes through a pressure drop of about 50 psi (about 0.34 MPa) in a throttling valve and thereby becomes partially vaporized. This pressure drop sets the column pressure at about 20 psia (about 0.14 MPa). This column pressure is required to obtain a significant temperature difference in the reboiler. The two-phase flow then enters a separator, where the vapor is vented, and the liquid is sent to the distributor. Once operation has reached a steady state, mass transfer between the down-flowing liquid and the up-flowing vapor results in the collection of 99-percent-pure LOX in the reboiler. The nitrogen-rich vapor is vented as waste at the top of the column. The structured packing enables the column operation to be insensitive to tilt angles of up to 20°, with respect to the local gravity vector. We are currently working to further miniaturize the distillation technology to provide a portable, lightweight, and low-power source of high-purity nitrogen and oxygen for other applications.
This work was done by Jay C. Rozzi of Creare, Inc., for Glenn Research Center. Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4–8
21000 Brookpark Road
Cleveland, Ohio 44135.
Refer to LEW-17593-1.