CO2 Cleaning In A Rotating Drum

The invention relates to a method for treating objects in dense phase CO2, wherein said objects are placed into a rotatable drum which is located in a pressure vessel, and wherein said pressure vessel is partly filled with dense phase CO2.

Dry-cleaning using liquid carbon dioxide is known as an environmentally friendly cleaning technique with favourable cleaning properties which can be used to remove contaminants from garments or textiles as well as from metal, machinery, workpieces or other parts.

Normally the objects to be cleaned are placed into a pressure vessel which is then filled with liquid carbon dioxide. In order to achieve the favourable dissolving properties of liquid carbon dioxide the contact between the objects to be cleaned and the liquid carbon dioxide should be as good as possible. Therefore, it is already known to use an impeller to vary the flow profile in the pressure vessel. It is further known to place the objects into a rotatable drum within the pressure vessel. However, in cleaning porous objects, for example textiles, the liquid carbon dioxide may not reach all dirt between thin fibres or within pores.

VVO 01/49920 discloses a method for cleaning laundry in a pressure chamber filled with liquid carbon dioxide wherein the pressure of the liquid carbon dioxide is intermittently and rapidly lowered in order to bring the liquid carbon dioxide into boiling. Thus steam bubbles are created which are considered as a micro-mechanical treatment of the laundry and help to loosen dirt particles from the laundry. A drawback of this method is that during said rapid pressure drop a certain amount of gas is withdrawn from the pressure vessel which has to be dealt with.

It is an object of the invention to provide a method for cleaning objects in dense phase carbon dioxide with improved cleaning performance, in particular for cleaning porous objects. This object is achieved by a method for treating objects in a dense phase gas, wherein said objects are placed into a rotatable drum which is located in a pressure vessel, and wherein said pressure vessel is partly filled with a dense phase gas, and wherein said drum is rotated at a high speed so that said dense phase gas is forced out of the centre of said drum by centrifugal forces.

As used herein, the term "dense phase gas" means any suitable gas in liquid or supercritical status. The preferred dense phase gas is liquid carbon dioxide.

As used herein, the term "treating" means any kind of effecting the object, in particular of effecting a surface of the object, for example cleaning, impregnating or coating. The preferred application of the inventive method is cleaning of objects, particularly of textiles, garments, pillows or matrasses.

As used herein, the term "high speed" means a rotation speed sufficiently high to force said dense phase gas out of the centre of said drum. During said phase of high speed rotation a kind of ring of dense phase gas is created at the inner wall of the pressure vessel.

According to the invention the objects to be treated, preferably to be cleaned, are placed into a rotatable drum which is located inside a pressure vessel. The pressure vessel is then partly filled with a dense phase gas, preferably liquid carbon dioxide. During the cleaning operation the drum is rotated at a high speed so that the objects as well as the dense phase gas are forced outwards in radial direction. The gaseous part will move to the centre of the drum due to its lower density. The dense phase gas will be forced into pores, holes or similar openings of the objects and partly pass through them. In addition, when treating flexible objects, for example textiles, the objects will be stretched to the wall of the drum and over baffles within the drum. The flow of dense phase gas will also force the fibres of the textiles to open-up which means that particles are less locked in the fibres and could be pushed away.

The invention is based on a different cleaning concept compared to the traditional dense phase cleaning methods. In the prior art the objects are only placed in the cleaning fluid or at the most tumbled within the cleaning fluid. The objects are brought into contact with the dense phase gas in order to dissolve any undesired particles or dirt in the dense phase gas which is then withdrawn from the pressure vessel. The cleaning performance essentially depends on the dissolving properties of the dense phase gas and possible additives.

The invention also makes use of the dissolving properties of the dense phase gas. But in addition, dirt and contaminants are physically washed out of the objects since the objects are accelerated and pressed to the inner wall of the rotatable drum. Further the dense phase gas is pressed through the objects by centrifugal forces whereby loosening dirt from the surface of the object and sweeping it away.

The inventive cleaning process is a combination of dissolving and pushing out dirt and contaminants from the objects, for example from the garment. Dirt very often also consists of particles which are not always soluble in the dense phase gas. The particles could be trapped in the object, for example in the fibres of the garment. The particles could have soluble dirt on the surface and by cleaning the surface could the particle came loose from the garment or at least easier be pushed out from the garment.

Preferably, said step of rotating said drum at high speed is carried out repeatedly with decreasing the rotation speed of said drum inbetween. This is in particular useful when treating flexible objects as garments. The objects are placed into the rotatable drum, the pressure vessel is filled with dense phase gas and then the rotation of the drum is started. The rotational speed is increased to sufficient high speed so that the objects are pressed to the wall of the drum by centrifugal forces. After a certain time the rotational speed will be decreased in order to alter the acceleration forces which effect the objects. These steps of first rotating the drum at high speed and then at a lower rotational speed may be repeated several times, preferably more than 10 times, more preferred at least 20 times.

Preferably, one sequence of rotating at high speed followed by decreasing the rotation speed will take 10 to 30 seconds, more preferred 15 to 20 seconds. During a total cleaning time of 10 to 15 minutes it is possible to repeat that sequence 30 to 40 times.

By altering the rotational speed during the cleaning operation the objects are alternately compressed and de-compressed. The decompressing / compressing steps force dense phase gas in and out of the object and thus the transport of material as dirt and particles out of the object increases. For example in cleaning textiles or garments the drum will first be rotated at a high speed so that the garments are hold and compressed to the inner wall of the drum. Then the rotational speed is reduced and the garments are de-compressed due to the lower centrifugal forces. The alternate application of compression and de-compression to the objects "pumps" dense phase gas in and out of the objects, thus washing out dirt and particles.

It has been found that the cleaning performance can be further improved if the direction of rotation is changed between two subsequent steps of rotating said drum at high speed. For example, the drum is first rotated at high speed in clockwise direction and then the rotational speed is decreased down to zero. The following rotation at high speed is carried out in a counter-clockwise direction. This procedure is advantageous since the position of the objects in the drum is changed and a good average cleaning of all objects and of all parts of the objects is achieved.

Preferably the drum is rotated at a rotational speed of more than 70 rpm, more preferred at a speed of more than 100 rpm and most preferred at a speed above 140 rpm during said step of high speed rotation. The rotation speed should be at least so high that said kind of ring of dense phase gas is created at the inner wall of the pressure vessel, which depends on the size and diameter of the drum. Between two of these steps of high speed rotation the rotational speed of the drum is preferably decreased to 20 to 60 rpm, preferably 30 to 50 rpm.

At a certain rotational speed the dense phase gas will create a ring around the inner wall of the pressure vessel with the centre of the pressure vessel filled with gas. Depending on the amount of dense phase gas, on the volume of the rotatable drum and on the volume of the pressure vessel, that ring of dense phase gas covers or does not cover the objects which are hold by the inner walls of the rotating drum.

According to a preferred embodiment of the invention such an amount of dense phase gas is filled into said pressure vessel that said objects are covered with dense phase gas during said step of rotating said drum at high speed. In this case the objects are soaked by the dense phase gas. In the traditional way of cleaning objects in liquid carbon dioxide, the liquid carbon dioxide is boiling to some degree so that there are always gas bubbles close to the objects. By performing the inventive acceleration, the .

gas phase and the dense or liquid phase are separated. Due to its lower density the gaseous phase collects near the centre of the drum whereas the dense phase and the objects are moved radially outwards. The objects are then only subjected to the dense phase. Since the solvent power of a dense phase gas is increased compared to a boiling liquid, the cleaning performance is essentially improved by using the inventive rotation.

Normally the rotatable drum is perforated in order to allow the dense phase gas pass through the walls of the drum, but to hold the objects within the interior of the drum. In this case it might be advantageous to fill the pressure vessel only to such a degree that the complete dense phase gas is forced out of the drum. The objects are then not soaked with dense phase gas during said high speed rotation step. But instead, the dense phase gas is first forced into holes and pores of the objects and then again out of the object. That passage of dense phase gas through the objects essentially increases the removal of particles and dirt.

It is further preferred to withdraw part of the dense phase gas during said step of high speed rotation, for example by a pump connected to a outlet in the bottom of the pressure vessel. This part of the dense phase gas is filtered or purified and then returned to the pressure vessel. The dense phase gas is circulated in order to achieve a multiple passage through the objects. Instead of using a pump to withdraw part of the dense phase gas from the pressure vessel, it is preferred to provide the upper part of the pressure vessel with an outlet. During the high speed rotation dense phase gas is forced into a radial direction and part of the dense phase gas leaves the pressure vessel through said outlet. A filter or another device will then purify the dense phase gas before re-entering the pressure vessel and the rotatable drum. Preferably the dense phase gas is introduced into the pressure vessel near the centre of the rotatable drum. It is also possible to spray the dense phase gas on the objects which are rotated at a high speed.

The inventive method has several benefits compared to the prior art. It is possible to soak all parts of the objects by the dense phase gas. For example in the preferred case when liquid carbon dioxide is used as dense phase gas, gaseous CO2 will be forced out of the objects. But also other non-condensed gases are driven out of the object due to the acceleration. This will improve the overall average cleaning performance on all surfaces of the objects. Further, the dense phase gas will be forced through the objects and will sweep out any undesired particles. For that reason the invention has particularly proven to be advantageous for the removal of mites from textiles, pillows, blankets and so on.

The invention may be combined with other methods to increase the cleaning efficiency. It might be advantageous to provide baffles in the rotating drum in order to avoid that the objects slip or slide along the inner surface of the rotating drum during the high speed rotation which would be counterproductive in view of the application of centrifugal forces. Several soft baffles could also be used to improve stretching of the objects, especially of garments.

The preferred dense phase gas is liquid carbon dioxide. Supercritical carbon dioxide may also be used, but in that case no phase separation will occur since there is only one phase. On the other hand, the garments will be stretched at the inner surface of the drum and by a good design of the drum, for example by small shovels or baffles, the supercritical CO2 will be forced by the rotation to move through the garment.