Greener Industry - Supercritical Carbon Dioxide

Supercritical Cleaning
Liquid carbon dioxide is already used by some dry-cleaning systems in place of tetrachloroethene (commonly referred to as "perc"), which is a very volatile organic compound and suspected carcinogen. Even mild exposure can have unpleasant effects, and dry cleaners must dispose of spent material as hazardous waste. Other solvents, including chlorinated fluorocarbons (CFCs), are used for degreasing machine parts and other industrial cleaning.

Photoresist Removal

A fully automated process using supercritical carbon dioxide has been developed for cleaning photoresist and etching residues from semiconductor wafers. Manufacturers normally use sulphuric acid, hydrogen peroxide and organic solvents, and may typically generate 18 million litres of wastewater per day.

The cleaning is done using supercritical carbon dioxide with a small amount of propylene carbonate, an environmentally benign material. This acts a bit like a detergent, aiding the dissolving of the residues. A final "rinse" with pure scCO₂ ensures the wafers are spotlessly clean and dry, and both the carbon dioxide and propylene carbonate can be re-used.

Machine Component Degreasing

image: loading point of supercritical degreaser

Equipment maintenance often requires very thorough cleaning of metal components. This has traditionally been done using organic solvents, but the treatment of used solvent is an additional expense. Very thorough cleaning can be achieved using scCO₂, and equipment is now available for this purpose.

Fine Powders
In many applications materials are required in the form of a very fine powder. This can be particularly important in the manufacture of pharmaceutical products. The normal approach is to re-crystallise the material from a solvent and then grind it (known as "milling").

A more even result can be produced by dissolving the material in scCO₂, and then releasing the pressure rapidly from a nozzle. Very fine powders can be made in this way.

Much research has gone into finding alternatives to the many volatile organic solvents in use in products such as paints. One of the most common solvents, however, is water. It is used in very large quantities by some industries, but cannot be discharged unless it is free from contamination. There are several areas where scCO₂ may be introduced as an alternative.

Spray painting	It is not possible to "capture" the solvent vapour released when spray-painting large objects like ships and aircraft. A system using scCO₂ as a solvent is now available, and has been of particular interest for military use. It greatly reduces the quantity of VOCs released into the atmosphere.
Yarn sizing for woven materials	High-speed fabric weaving requires smooth yarn, and so it is usually coated (or "sized") to achieve this, and the size is removed afterwards. The process uses a lot of water and heat energy, but research has shown that the size could be applied using scCO₂ instead of water, and heat is no longer needed for drying the yarn. Find out more
Cleaning	"Dry" cleaning and industrial de-greasing can be carried out with scCO₂ and small quantities of environmentally friendly additives. The cleaning of integrated circuit wafers for computer use is one area where using scCO₂ can reduce the amount of contaminated wastewater needing treatment. Find out more
Powder production	The production of a fine powder with even-sized particles is not as easy as it sounds. It is, however, often vital for the effective absorption of drugs into the body. The rapid de-pressurisation of scCO₂ containing the dissolved material is one way of achieving this. Follow this link for more information
Dyes and preservatives	Effective penetration of dyes and preservatives into materials is often slow and incomplete using liquid solvents. If scCO₂ is used instead, it will penetrate as effectively as a gas, but carry the quantities of dye or preservative normally associated with a liquid solvent, giving a better result.

Woven Fibres
Yarn used in weaving is "sized" before weaving. The process, known as "slashing", involves coating the yarn with a material such as starch or poly(ethanol) (polyvinyl alcohol, PVA) to glue down loose fibres and allow it to slide more easily during weaving. The size is washed off afterwards.
image: yarn before and after sizing
Both sizing and washing require very large quantities of water, which must be treated to remove excess size before it can be re-used or discharged. A water-based size also needs time, space and energy to dry before the yarn can be used, and consequently the machinery occupies a lot of space.

A method has been developed using supercritical carbon dioxide (scCO₂) as a solvent for the size. This has the potential to:
· Reduce the energy used in the slashing process, and the space occupied
· Produce a more even layer of size, which can therefore be thinner
· Drastically reduce the amount of water used for sizing and washing
Key to this development is the ability to coat yarn in a rapid, continuous process at the high pressures necessary. This has been achieved on a laboratory scale using a specially constructed "venturi" tube, shaped to create the pressure drop needed to precipitate the size. A series of baffles reduce pressure towards the ends of the tube - this is to prevent size and carbon dioxide being squirted out the ends!
diagram: coating yarn with size using supercritical carbon dioxide

The system is not yet used commercially, but several companies have shown interest.