AU1428488A

AU1428488A - Method for recovering volatile compounds from atmosphere

Info

Publication number: AU1428488A
Application number: AU14284/88A
Authority: AU
Inventors: Karl-Erik Ungerholm
Original assignee: UTILEX INGF
Current assignee: INGENJORSFIRMA UTILEX
Priority date: 1987-02-27
Filing date: 1988-02-24
Publication date: 1988-09-26
Also published as: EP0346379A1; SE8700832L; WO1988006481A1; FI893999A0; KR890700388A; FI893999A; SE8700832D0; SE459158B; JPH02502614A

Description

liethod for Recovering Volatile Compounds rom Atmosphere

Technical Field

The present invention relates to a method for recovering volatile compounds other than water from the atmosphere evacuated from spaces, in which materials containing volatile compounds are manufactured, handled, stored or used. These volatile compounds have attracted much atten¬ tion as an inconvenience to the surroundings and as a menace to the environment. Besides there is an interest of recovering and reusing these compounds. The invention is particularly related to solvents and dispersion vehicles for paint, printer's inc, adhesive cement, plastics and to cleaning and extracting liquids. Those compounds usually consist of hydrocarbons or halogenated hydrocarbons, but the invention also covers other volatile compounds for instance carbondisulfid.

Evaporation of solvents occurs inter alia at the manu¬ facturing, handling, storing and using cf paint, tluch attention has been given to the emission of solvent vapours when painting and lacquering. Such lacquering is performed in spray boths or chambers which are being passed by big amounts of ventilation air, which removes the main part of the solvents from the paint. Since the lacquering is performed at room temperature the evacuated air must be replaced by a corresponding volume of fresh air, which during the colder part of the year must be warmed to room temperature. The need of preheating may be reduced by recirculating part of the air, but thereby the content of solvents in the air is increased and there is no reduction of the total emission. Background Art

The evacuated atmosphere usually contains comparatively small amounts of volatile compounds in a big amount of air and no effective and reliable method of cleaning such effluents has been found. Trials have been made with combustion of the volatile compounds in sand beds or absorption in organic and inorganic absorption materials. It has also been proposed to condense the volatile compounds but with bad results as the humidity in the atmosphere diεturbes the process. In European patent

No 124 997 it has been proposed to recover solvent vapour in the atmosphere evacuated from a plastic curing oven. The atmosphere which mostly consists of nitrogen is cooled by liquid nitrogen and after reheating returned to the oven. In this way introduction of water vapour, which otherwise will freeze and tends to clog piping or other conduits is almost avoided.

In European patent No 71 907 the atmosphere evacuated from dry-cleaning plants is cooled by a heat pump to a temperature at which both solvent and water can be removed as liquids. To free the atmosphere from volatile compounds to an acceptable degree it is, however, necessary to cool the atmosphere to a temperature at which the normal humidity of air will form ice that clogs the equipment.

Disclosure of Invention

According to the invention an atmosphere containing volatile compounds is first relieved from water vapour and then cooled to condensation of the volatile compounds other than water. The condensate is recovered. The present invention makes it possible to purify evacuated atmosphere of volatile compounds to any degree wanted in every special case. As the invention is based on cooling and condensation of the volatile compounds the residue of such compounds in the finally discharged atmosphere depends on the vapour pressure of the compounds in question at the end temperature that has been chosen.

The invention preferably comprises a first step in which the atmosphere is relieved from water vapour by adsorp¬ tion on a medium with a specific capability of adsorbing water and very small ability to adsorb other compounds. Such adsorbents are well known in the art and is e.g. described in the US patent 3 078 635, which describes a molecular sieve consisting of a specially composed zeolite. This first step is performed at room tempera- ture, which considerably reduces the risk that the adsorbing medium will be fouled by the volatile com¬ pounds. The adsorbent is then regenerated by hot air and reused. In a second step the atmosphere is cooled by a heat pump preferably of the type in which a gaseous medium is compressed and cooled to condensation, where¬ after the condensed medium by evaporation generates cold. Such heat pumps makes it possible to cool the atmosphere to a temperature of -30 to -40 without using extreme pressures. The volatile compounds condensed in this step are removed. -

In a third step the atmosphere is further cooled until the rest of the volatile compounds are condensed. The condensate is removed. The cooling in the third step may - be effected by compressing dry gas, for example a small part of the atmosphere from the first step, cooling the compressed gas and then let it expand, preferably into the atmosphere in the third step. This atmosphere is before that cooled by heat exchanging it with the atmosphere leaving the third step. The volatile compounds _τ that have condensed in the second and third steps are separated and e.g. paint solvents may be recovered so pure that they can be reduced. The invention gives a great freedom to chose very low temperatures. The invention also makes it possible to recover the heat content of the evacuated atmosphere and to use it for heating purposes. The use of a second step with a conventional heat pump working with a condensing medium and a third step using a small cooling machinery has proved to be a very advantageous combination in regard of the equipment needed and its reliability.

Brief Description of Drawing

The figure is a schematic sketch of a preferred method for performing the invention. Evacuated atmosphere containing volatile compounds in a first step passes means 4,5 for relieving it from water vapour, in a second step 12 it is cooled by a heat pump and in a third step 24 it is further cooled to condensation of the rest of the volatile compounds. The condensates are removed 18,21.

Mode for Carrying Out the Invention

A mode for carrying out the invention will be described with reference to the figure.

Atmosphere, evacuated from a space where volatile co - pounds are vaporised and which is of room temperature e.g. 18-20 C, is first conducted -to a filter 2 for removing solid and liquid contaminations e.g. paint residues and dust. The atmosphere then is blown 3 through means performinσ the different steps of the invention. The first step has the purpose to remove as much water vapor from the atmosphere so that no difficulties owing to formation of ice may occur in the later steps. The water vapor is suitably removed by adsorption in beds , 5 charged with an adsorbent that preferably adsorbs water vapor. Artificial zeolites often called molecular sieves may suitably be used for this purpose. These sieves can be made with selective ability to adsorb certain matters for instance water vapor. In this case it may eventually be of interest also to adsorb carbon dioxide if it is wanted to cool to a very low temperatures in the third step. The adsorption of volatile compounds can be kept very low with the right choise of adsorbent material.

The beds are connected alternately by valves 6, 7, 8, 9 and are regenerated in between with hot air 25 which is connected by valves 26, 27, 28, 29. The atmosphere relieved from water vapor is conducted through a pipe 10 to a first cooling apparatus 12 in which the second step according to the method of the invention is accomplished. In the cooling apparatus the atmosphere is cooled by a heat pump 11, 13, 14, 16. In this a suitable refrigerant is compressed in compressor 13, is cooled and condensates 14 and after pressure reduction 16 is vaporized 11 by the incoming atmosphere containing vaporized solvents which thus is cooled. As refrigerant in heat pumps at present fluorhydrocarbons are usually used. With for instance CHCLF„ (CFC22, Freon22) the atmosphere can be cooled to -30 to -40 C. A part of the volatile compounds is condense and separated 17 and tapped 18. For condensation 14 of the refrigerant it is cooled 15 with water or possibly air which thus is being heated and can be used for heating purpose. For instance the incoming fresh air for the ventilation can be heated bv the produced hot water. In order to accomplish the 3:rd step according to the method of the invention the cooled atmosphere is conduc¬ ted from the evaporator 11 and separator 17 to the final cooling 19 where the temperature is further lowered in order to obtain a satisfactory condensation of the rest of the volatile compounds. These liquifies on the inner surfaces of the cooler 19 and are tapped 21. The outgoing atmosphere from the final cooler 19 is heat exchanged 22 with the ingoing atmosphere.

Further cooling 20 is accomplished with a suitable method. According to one method dry gas e.g. a part 30 of the waterfree atmosphere from the first step is compressed, cooled by heat exchange with the outgoing atmosphere from the 3:rd step, and after expansion introduced in to the cooling 19.

Another method of cooling is to use an external cooling agent e.g. liquid nitrogen, if it is available to a reasonable cost.

The whole cooling apparatus 24 of step 3 is suitably insulated from the surroundings. In this way the cooling machinery in the third step may be dimensioned only for the condensation of the volatile compounds and for removing the heat that passes thr insulation.

The outgoing atmosphere 23 can at summertime for instance be used for cooling of fresh ventilation air. Even if the use of volatile compounds in the actual space is stopped, the equipment for performing the invention can totally or partly be used for heat recovery from outgoing air or from fresh ambient air and -thus produce heat for useful purposes. Industrial Applicability

As an example of the industrial applicability of the method according to the invention the spray painting or lacquering of automotive car bodies is chosen.

The air flov; evacuated from a typical car body spray both

3 amounts to about 200 000 m /h and has a content of

3 solvents emanating from the paint of about 0,25 g/m . At a temperature of 20 C and a relative humidity of 60 % the

3 water content is about 10 g/m . The air after having passed a filter 1 is blown through one of the water adsorbing beds 4 , 5. As soon as traces of water appear in the air leaving the bed, the air is blown through the other bed and the first bed is regenerated.

The air then passes step 2 in which it is cooled to -40 C. Only small amounts of the solvents will condense.

In the third step the air is cooled to -70 C. In this step most of the solvents will condense and are with¬ drawn.

By circulating most of the air evacuated from the spray both the amount of air may be reduced. For instance if the air is circulated 10 times the air flow to the equipment, may be reduced to 20 000 m /h and its content

3 of solvents increased to 25 g/m . In this case most of the solvent will condense in the second step and the rest in the third step.

From a typical spray both 50 kg solvents in this way may be collected. For instance at 6 000 hour operation per year 300 000 kg solvents are recovered from the ventila¬ tion air.

Claims

1. Method for recovering volatile compounds from atmosphere by condensing c h a r a c t e r i z e d in that an atmos¬ phere (1) containing volatile compounds is first relieved (4,5) from water vapour and then cooled (U2,24) to condensation of the volatile compounds other than water, whereafter the condensed compounds are removed (18,21) from the atmosphere.

2. Method according to claim 1. c h a r a c t e r i z e d in that it comprises a first step (4,5) in which the atmosphere is relieved from water vapour by adsorption, a second step (12) in which it is cooled by a heat pump (Ul,13,14) to a temperature of -30 to -40°C, and a third step (23) in which the atmosphere is further cooled and the volatile compounds are ultimately condensed.

3. Method according to claim 2. c h a r a c t e r i z e d in that the atmosphere in the first step is relieved from water by adsorption in molecular sieves (4,5) .

4. Methods according to claim 2. c h a r a c t e r i z e in that the atmosphere in the third step is cooled by expanding a compressed cooled dry gas into said atmosphere.

5. Method according to claim 2. c h a r a c t e r i z e d in that the atmosphere in the third step is cooled by a from the outside supplied cooling agent.