UNSTEADY AND STEADY-STATE GAS PERMEATION THROUGH ACTIVE POROUS WALLS
Advancing the chemical engineering fundamentals
Membranes and Membrane Science (T2-8P)
Keywords: active scavenger, gas permeation rate, steady-state, unsteady-state
There are various ways to improve the barrier properties of plastic packaging walls, which can generally be classified as providing either passive or active protection. Passive protection consists in simply supplementing the original plastic material with a further component with superior barrier properties, so that the overall performance is improved. Examples are the addition of polyamide or ethylene vinyl alcohol compounds to the polymer.
Active protection, on the other hand, implies the addition of a scavenging compound to the plastic matrix; this reacts with, and thus consumes, the permeating gas, thereby greatly reducing, for example, contact of a sensitive food product with oxygen. Examples of oxygen scavengers in current employment are iron, ascorbic acid, photosensitive dyes, enzymes, unsaturated fatty acids and immobilised yeasts.
In cases where the use of an active scavenger is contemplated, it is clearly important to be able to quantify its effectiveness with regard to the amount of oxygen, or other contaminant, which permeates the packaging wall to reach the packaged contents.
Steady and unsteady-state gas permeation rates through packaging walls containing active (scavenger) materials are determined as functions of the system's physical parameters and the scavenger load. With the simplifying assumptions of constant scavenger concentration and first order reaction kinetics, steady-state analysis shows that there is a minimum quantity of scavenger that must be added to the packaging wall if any reduction of gas permeation is to be achieved. Unsteady-state studies have established the dependence of the time needed to reach stationary behaviour on the system's physical parameters.
We have been able to quantify (analytically for the steady-state case and numerically for the unsteady-state case) gas transmission rates in active packaging walls. This information allows for the quantitative design of the wall, enabling a desired performance criterion to be realized. Analysis of the more complete system, not subject to the simplifying assumptions employed above, is currently in progress and will be presented in due course.
Presented Tuesday 18, 13:30 to 15:00, in session Membranes and Membrane Science (T2-8P).
