Besides well-established applications in the food, pharma and cosmetic sectors there is great potential for CO2 as a process medium. Its unique properties, especially in the supercritical stage, make it an ideal tool. CO2 is often used because of its mobility in even difficult material structures, while still acting as an efficient transportation agent. Additionally, it disappears in ambient conditions without any residue. Such qualities are used in processes where it removes components from a matrix distributing a soluble substance uniformly.
Wide spectrum of applications
- Cleaning of cork from TCA
- Impregnation of wood
- Tanning of leather
- Dyeing of fabrics and wood products
- Drying of Aerogels
- Cleaning of subsea tubing
- Cleaning of different fabrics
- Cleaning of polymers
- Removal of organic solvents
- Production of sulphur-free cellulose
- Recycling of batteries and other materials
Aerogels are materials with a high micro-porous structure, sometimes a nano-porous structure, both with a very low bulk density. These aerogels can have very interesting properties depending on the composition material. Silica aerogels can be transparent with a high thermal resistance therefore very appropriate to be used for insulation. Aerogels made from metal oxides have a high surface and can be used in catalytic systems. Lately, aerogels made from renewable resources are also being studied, especially the ones made from cellulose.
Production of aerogels has been studied by us since 2001 - Aerocell project (6th EC Framework).
The preparation of an aerogel involves 3 steps
- Preparation of the precursor gel (4 routes – example: Cellulose aerogels - cellulose-NMMO route , cellulose-NaOH route, cellulose ester route and cellulose carbamate path);
- Solvent change (mutual solubilities of water and CO2 are very limited and SC-drying of water wet aerogels creates capillary forces causing collapsing of micro-porous structure; to overcome this problem is slowly replacing the water by a water mixable organic solvent);
- Supercritical drying, as a pre-condition the water mixable organic solvent must also be completely mixable with CO2 or any other dense gas. Under the EC project the process of choice was with supercritical CO2. The design of this process requires accurate knowledge of phase equilibria between solvent and supercritical fluid - the solvent is replaced by CO2 in a single phase in order to avoid the appearance of capillary forces.
Drying tests in different sized plants were performed by us, determining best operating conditions and a SC drying unit was developed. These tests were used to study the economics of the SC drying process. A comparison between a compressor and pump driven process was made resulting that the compressor process requires up to about 50% less energy.
The final samples of the cellulose aerogels were tested for different applications such as for packaging, cosmetics (powdered delivery system), and carbon aerogels (samples were pyrolised and tested as “Super Capacitors”, as electrodes in fuel cells and in lithium ion batteries.
SC impregnation was a major topic in development of high pressure processes. The research covers a wide range of applications and scales: processing of high value products in small plants (cross-linked UHMW-PE with -tocopherol - medical purposes); impregnation of low value products such as timber at industrial scale (Superwood® process); treatment in medium sized plants (metal complexes into polymer fibres followed by electroless plating). Impregnation of cellulose fibres with -tocopherol and D-panthenol - special medical skin protection and for cellulose towels with disinfection effect. Impregnation of nuts with antioxidants in the food industry – a promising process.
Impregnation of wood, the most successful application of SC impregnation at industrial scale since 2002 in Hampen, Denmark. The Superwood® process use SC CO2 to dissolve a mixture of fungicides (tebuconazol, propiconazol and IPBC) for impregnation.
Impregnation of oil containing fruits and seeds, antioxidants (carnosic acid, carnosol) into nuts and coffee beans - stabilizing and prolonging shelf life.
Impregnation of polymer fibres, new applications: electroless plating of aramid fibres (metal organic compounds impregnation in aramid fibres and later a metal film was deposited on the surface by conventional electroless plating - obtaining conductive fibres); surface modification of PET fibres with natural polymers (sericin, collagen, chitosan); impregnation of PET fibres with PEG to enhance moisture absorption; impregnation of PET fibres with pyrroles to become conductive; surface modification of nylon fibres with organic metal complexes to improve the UV resistance.
Impregnation of cellulose fibres and aerogels, with vitamins and D-panthenol - wound healing - slow release of active substances.
Impregnation of polyethylene components for medical purposes, SC CO2 impregnation of UHMW-PE pieces with -Tocopherol - used in prosthetics production.