Scott B. McCray

Reverse osmosis cellulose acetate membranes II. Dependence of transport properties on acetyl content

Abstract The effects of acetyl content on the transport behavior of asymmetric cellulose acetate membranes were determined. Acetyl content (AC) was varied by hydrolyzing commercial reverse osmosis cellulose acetate membranes. Transport behavior was characterized by the Kedem-Katchalsky (K-K) and solution-diffusion (S-D) membrane models. Except for AC>38.5% for one of the commercial membranes, the salt and water permeabilities increased as acetyl groups were removed by hydrolysis. A comparison of the K-K and S-D models indicates that both adequately describe the transport behavior of the membranes at high salt rejection, but in the K-K model the salt-water coupling plays an ever increasing role in determining the salt flux through the membrane as acetyl groups are removed. The observed dependence of the transport behavior on acetyl content is attributed to the type of water-polymer interaction and amount of water in the membrane.

Synergistic, membrane-based hybrid separation systems

Abstract A design concept is described for a synergistic hybrid process for separating process streams into two component streams: a solute-rich stream and a solvent-rich stream. This hybrid design concept, which is particularly well-suited for membrane-based unit operations, combines a solute-removal unit operation and a solvent-removal unit operation through use of a recycle stream. By combining the two unit operations in a hybrid system, efficiency is maximized, because each of the unit operations can be carried out under optimal operating conditions. Flexibility is maximized, because system operating conditions can be easily adjusted to accommodate changes in the feed stream or, in the case of membrane-based unit operations, to compensate for changes that occur in the membrane over time. Examples are presented that show that hybrid processes may lower industrial separation costs significantly and result in more-complete separations than are possible using conventional processes.

Reverse osmosis cellulose acetate membranes.I. Rate of hydrolysis

Abstract Measurements of hydrolysis rates of homogeneous porous and dense cellulose acetate membranes were analyzed in terms of external and internal mass transfer, and intrinsic chemical reaction. These results indicate that the external mass transfer controls the overall rate for porous membranes, while internal mass transfer and/or chemical reaction control the rate for dense membranes. By extending this analysis to asymmetric cellulose acetate membranes, the hydrolysis rate in the dense layer is shown to be much slower than the rate in porous layer.

A Novel Membrane Device for the Removal of Water Vapor and Water Droplets from Air

One of the key challenges facing NASA engineers is the development of systems for separating liquids and gases in microgravity environments. In this paper, a novel membrane-based phase separator is described. This device, known as a water recovery heat exchanger (WRHEX), overcomes the inherent deficiencies of current phase-separation technology. Specifically, the WRHEX cools and removes water vapor or water droplets from feed-air streams without the use of a vacuum or centrifugal force. As is shown in this paper, only a low-power air blower and a small stream of recirculated cool water is required for WRHEX operation. This paper presents the results of tests using this novel membrane device over a wide range of operating conditions. The data show that the WRHEX produces a dry air stream containing no entrained or liquid water - even when the feed air contains water droplets or mist. An analysis of the operation of the WRHEX is presented.

Investigation of humidity control via membrane separation for advanced Extravehicular Mobility Unit (EMU) application

This paper describes the development of a membrane-based process for dehumidifying the Extravehicular Mobility Unit (EMU). The membrane process promises to be smaller, lighter, and more energy efficient than the other technologies for dehumidification. The dehydration membranes were tested for 90 days at conditions expected to be present in the EMU. The results of these tests indicate that membrane-based technology can effectively control humidity in the EMU.

Operation of a breadboard liquid-sorbent/membrane-contactor system for removing carbon dioxide and water vapor from air

Processes to remove and recover carbon dioxide (CO2) and water vapor from air are essential for successful long-duration space missions. This paper presents results of a developmental program focused on the use of a liquid-sorbent/membrane-contactor (LSMC) system for removal of CO2 and water vapor from air. In this system, air from the spacecraft cabin atmosphere is circulated through one side of a hollow-fiber membrane contactor. On the other side of the membrane contactor is flowed a liquid sorbent, which absorbs the CO2 and water vapor from the feed air. The liquid sorbent is then heated to desorb the CO2 and water vapor. The CO2 is subsequently removed from the system as a concentrated gas stream, whereas the water vapor is condensed, producing a water stream. A breadboard system based on this technology was designed and constructed. Tests showed that the LSMC breadboard system can produce a CO2 stream and a liquid-water stream. Details are presented on the operation of the system, as well as the effects on performance of variations in feed conditions.

The Use of Membranes in Life Support Systems for Long-Duration Space Missions

The use of membrane processes in a long-duration manned missions regenerative environmental control and life-support system is presently discussed, in the cases of treatment for hygiene water, urine, humidity condensate, and phase-change distillate, as well as of water-vapor and CO2 removal from spacecraft air. Attention is given to the design of a tube-side-feed hollow-fiber module for membrane support and fluids-feed, as well as to the schematics for a membrane-based urine processor, an air recirculator, a potable-water producer, and a two-stage urine treater.

Water vapor recovery from plant growth chambers

NASA is investigating the use of plant growth chambers (PGCs) for space missions and for bases on the moon and Mars. Key to successful development of PGCs is a system to recover and reuse the water vapor that is transpired from the leaves of the plants. A design is presented for a simple, reliable, membrane-based system that allows the recovery, purification, and reuse of the transpired water vapor through control of temperature and humidity levels in PGCs. The system is based on two membrane technologies: (1) dehumidification membrane modules to remove water vapor from the air, and (2) membrane contactors to return water vapor to the PGC (and, in doing so, to control the humidity and temperature within the PGC). The membrane-based system promises to provide an ideal, stable growth environment for a variety of plants, through a design that minimizes energy usage, volume, and mass, while maximizing simplicity and reliability.

Preliminary analysis of a membrane-based atmosphere-control subsystem

Controlled ecological life supprot systems will require subsystems for maintaining the consentrations of atmospheric gases within acceptable ranges in human habitat chambers and plant growth chambers. The goal of this work was to develop a membrane-based atmosphere comntrol (MBAC) subsystem that allows the controlled exchange of atmospheric componets (e.g., oxygen, carbon dioxide, and water vapor) between these chambers. The MBAC subsystem promises to offer a simple, nonenergy intensive method to separate, store and exchange atmospheric components, producing optimal concentrations of components in each chamber. In this paper, the results of a preliminary analysis of the MBAC subsystem for control of oxygen and nitrogen are presented. Additionally, the MBAC subsystem and its operation are described.