The present invention relates to a method of minimizing dischar- ge of and exposure to solvents and other volatile substances, and more specifically a method of preventing large volumes of air to be contaminated by e.g. processes based on solvents and to make possible the recovery of contaminants from evacuated process air.

When working with solvents in working spaces certain limit values of solvent concentrations must not be exceeded in the respiration air. Development points towards lower and lower limit values and in order to comply with the more restrictive limits the supply of fresh air is normally increased and thus there is a dilution with fresh air until the content of hazar¬ dous substances in the discharged air does not exceed the limit values. The reduction of a limit value from 25 ppm to 10 ppm would then result in an increase of the amount of fresh air with a factor of 2.5. In practice this method of diluting conta¬ minants requires the introduction of 250 to 375 times as much air as is needed for men working in an incontaminated working space in order to fall under the limit value with today's tech¬ nology, if other means do not exist for the protection of the people. Further, the concentration of a substance varies in the air depending on the localization in the room with respect to the source of the substance and the ventilation apparatus, re¬ spectively.

Because the supplied air must be heated, or, in a warm climate, cooled, the dilution of air for obtaining low limit values re¬ sults in appreciable energy costs for heating and cooling, re¬ spectively.

Besides toxicity, economical reasons for the recovery of vola¬ tile substances from process air may be of great importance.

Regarding the environment at large there are limit values for the nearby surroundings of process plants, while at the same time the capacity of the plants is limited by putting an upper limit to the purchase of a certain raw material. This demand can be said to be a serious threat against the ecology as well as the economy since it does not restrict the amount of the total discharge but only puts demand on the size of each individual plant. Thus it is allowed to increase a discharge provided that this is done in several small and thus unecomomic units.

The background to the above problem thus resides in the presump¬ tion that the inner environment at a working site only can be protected by contamination of large air volumes thus resulting in having such a low concentration of contaminants that it is impossible to remove these before discharging the air into the atmosphere.

The object of the invention is thus, motivated by toxic, econo¬ mical or other reasons, to provide a method of preventing the escape of volatile substances present in chemical processes into the working space or outside air and to make possible the recovery of said volatile substances from process evacuation air.

The above or other objectives are achieved with a defined method which according to the present invention is characterized by

- performing the process at negative pressure in a delimited chamber,

- compressing and cooling evacuation air from the process and separating off condensate formed thereby, and

- subsequently expanding the evacuation air and separating off possible further condensate.

By recirculation of condensed liquid in the process the process economy will also be improved. Further, the process according to the invention only prescribes evacuation of comparatively small amounts of air and thereby the great energy consumption for cooling/heating and for the transport of the large air volumes

required according to known technology will be eliminated. Despite it being less expensive per unit volume to transport air with the aid of fans compared with compressors or vacuum pumps, the process according to the invention still consumes much less energy. Further, the concentration of contaminants will be con¬ siderably higher with the process according to the invention, which also makes possible purification of the evacuation air.

The process according to the invention is useful for many diffe- rent types of processes where solvents or other volatile and/or poisonous substances exist, for examnple in the plastic manufac¬ turing and machining industry and lacquering industry.

According to one aspect of the invention the compression and cooling are performed in two or several steps, whereby the degree of purification might be increased to the desired level.

According to another aspect of the invention the expanded cooled evacuation air is used in the process for the cooling in the ne- gative-pressure chamber.

According to another aspect of the invention liquid-ring and liquid-jet pumps are used for evacuation, wherein as the pump liquid used is a volatile substance, the presence of which is to be avoided in the air.

The process according to the invention is especially well, but not exclusively, suited for use in plastic extrusion, whereby extrusion of polyester-styrene-resin onto glass fibre should be noted especially. Styrene is very volatile and toxic, and its high-temperature sensitivity also give rise to problems. Above about 50°C the styrene will polymerize to polystyrene, which means that air containing styrene cannot be compressed in the conventional way.

Further advantages, characteristics and objectives of the inven¬ tion will be evident from the following detailed description of

one example of the invention with reference to the accompanying drawing, in which

the figure shows a flow chart for performing the process accor- ding to the invention in connection with work with volatile sol¬ vents.

The figure schematically shows a working space, designated with the reference numeral 1, in which an apparatus for performing a solvent-based process is contained. In a chamber 2 a robot 3 is working. The robot is supplied with raw material in the form of material containing solvents from a tank 4 through the line 4 ' . A purification plant, which will be described mote in detail be¬ low, evacuates the robot chamber 2. The evacuated air is com- pressed and cooled whereby the solvent in the evacuation air is condensed. The solvent is recirculated to the tank 4 while at the same time the purification plant when necessary heats and cools, respectively, with the aid of a heat exchanger. Mechani¬ cal energy is stored in the embodiment shown and this is return- ed to the process when the compressed air after purification is expanded to atmospheric pressure.

The chamber 2 is evacuated through a line 2 ' to a compressing means 5, which compresses the air during cooling, whereby the solvent condenses and is returned through the line 6 to the tank 4. The cooling results in that heat is supplied to the space 1, which takes place through a heat-exchanger 7. The compressed air, which by cooling has been freed from the required part of the solvent, now proceeds in it's cooled condition to an expan- sion chamber in which the air during expansion emits mechanical energy to a pneumatic engine 8, contributing to the operation of the compression means, indicated on the drawing by the arrow 9. When the air expands in the pneumatic engine 8 the temperature is further lowered, whereby more solvent will condense and be recirculated through the line 12 to the tank 4. The cold air after the pneumatic engine 8 is then used, with the aid of the heat exchanger 10, to cool parts of the fabrication process. If, after the heat exchanger 7 but before the pneumatic engine 8, it

further cools the purification process, lower temperatures will be obtained but at the expence of the heat exchange in the heat exchanger 7. The air, the temperature of which now has increase during the passage through the heat exchanger 10, leaves the working space through the duct 11.

In order to avoid formation of ice in the process it might at certain pressure conditions be useful to dry the air with a known technology, for example with an adsorption drier, before the unit 7. Depending on the type of solvent and the prevailing pressure conditions in the specified process, it might on other occasions be convenient to let water vapor condense together with the solvent and, if necessary, subsequently separate water and solvent with known technology.

A plant working according to the invention has been generally described in connection with the figure. When for example styrene is a solvent in a process, a liquid-ring or liquid-jet pump is prefereably used in the compression plant, wherein styrene is the circulating liquid, continuously cooled by the above described air cycle or by cooling in a conventional way with a circulating cooling agent, such as freons . Hereby the temperature in the purification step may be kept well underneath the critical temperature at which styrene self-polymerizes, which is about 50°C.

The compression degree for the evacuation air may also be in¬ creased with the aid of e.g. a screw compressor, when the cooling is performed on a higher level. The cooling machinery can then be eliminated, since the screw compressor acts as a heat pump. Because of the high temperatures existing in a screw compressor, this is not suitable for e.g. styrene.

The pneumatic engine 8 does not constitute an essential part of the process and depending on the specific conditions in the individual case it may be used or may be left out.