A PLANT FOR EXTRACTING SUBSTANCES IN GAS OR MIST FORM FROM A FLOW OF GAS

Abstract: The invention relates to a plant for extracting substances in a gas or mist form from a flow of gas. The plant includes in series a gas-flow collecting chamber (1), a compressor (5), a cooler (8) which functions to lower the gas temperature, and an expansion device (11) which functions to rapidly lower the pressure and the temperature of the gas flow and which includes a chamber (13) for separation of the condensation thus formed. According to the invention, the gas flow is recycled from the separation chamber (13) back to the collecting chamber (1), for repeated circulation of the gas flow between these chambers (1, 13).

Description: A PLANT FOR EXTRACTING SUBSTANCES IN GAS OR MIST FORM FROM A FLOW OF GAS The present invention relates to a plant of the kind defined in the preamble of the following Claim 1 for extracting substances in gas or mist form from a flow of gas. A number of more or less primitive plants of this kind are known to the art. These known plant, however, are unable to cleanse waste gases effectively and often leave solid residues which cannot be dumped or deposited without first being further processed or treated. The aversion of the general public to plants of this kind, such as waste incinerators or plants for the de¬ struction of environmentally harmful waste materials situated in the vicinity of built-up areas, can. be easily understood. Due to the risk of environmentally harmful material being spread over long distances from such plant, either by air or by water, it is difficult to obtain permission to construct such plants, and environmentally harmful material, such as mercury bat- teries and other scrapped articles containing heavy metals among other things, such as mercury, are at present stored in barrels in far from satisfactory conditions and forms. There is therefore a need to provide improved plants which are able to achieve desired separation, e.g. destruction, without contaminating the surroundings with environmentally harmful substances, as is the case at present. A step in the right direction has been taken in the case of a proposed plant in which environmentally harmful waste is charged to a furnace in which the waste is heated to a temperature of from 400°-600°C under vacuum. Mercury and organic materials are vaporized and led to an afterburner to which oxygen is supplied and in which the vaporized organic material is combusted at a temper¬ ature of about 850 °C, resulting in the formation of carbon dioxide and steam. The gases are then cooled and the majority of the mercury present is condensed in a condenser at a temperature of about 15°C. Finer extrac¬ tion of mercury from the waste gases is achieved by drawing the waste gases through an active carbon filter and a freeze chamber, temperature -20 " C, with the aid of a suction pump, whereafter the waste gases are dischar- ged through a chimney to atmosphere. According to calculations, these flue gases contain at most 0.13 mg 3 of mercury per m of gas, together with carbon dioxide among other substances. A single run of the plant takes from 12-15 hours to complete and involves the destruc- tion of one barrel filled with mercury batteries and other environmental harmful materials. The process includes the addition of nitrogen gas and oxygen and the control of temperature and subpressure, and is intended to be controlled fully automatically by means of a microprocessor, in dependence on the type of waste concerned. The scrap remaining in the barrel is intend¬ ed to contain at most 75 ppm mercury and shall be capa¬ ble of being dumped or deposited safely. The process is thus quite complicated and there is a risk of serious consequences to the surroundings in the event of a malf nction. The object of the present invention is to provide a novel plant of the kind defined in the introduction which is less liable to subject the surroundings to pollution. This object is achieved in accordance with the invention with a plant having the characteristic features set forth in the characterizing clause of the following Claim 1. The extraction or destruction process is thus carried out in an essentially fully enclosed system and comprises circulating a gas flow, alternately heating and contaminating the gas flow with, e.g. , said environ¬ mentally harmful waste, and thereafter cooling the gas pronouncedly to a temperature which is so low as to condense the environmentally harmful substances and separate these substances from the gas flow. The gas is circulated until the concentration of environmentally harmful substances has fallen to an acceptable level. The results of tests carried out on the inventive plant have shown the residual concentration of mercury for instance to be as low as 0.2 micrograms for each normal cubic meter of gas at a temperature of -60°C. The process can be continued for the length of time required to achieve the degree of extraction or separa¬ tion desired, since the temperature of the gas can be lowered readily to a temperature of -100°C for instance, in the expansion device, wherewith carbon dioxide and oxides of nitrogen are also condensed, therewith leaving solely a volume of inert gas upon completion of the process. The process time is from 0.2-0.5 h for a volume of about 200 litres (= 1 oil barrel). The invention will now be described in more detail with reference to exemplifying embodiments thereof and with reference to the accompanying drawings, in which Figures 1 and 2 illustrate schematically respective exemplifying embodiments of the inventive plant, comprising a col¬ lecting chamber in the form of a furnace; and Figure 3 illustrates schematically a corresponding plant compris¬ ing a collecting chamber which includes adsorption chambers and through which a solvent-containing gas flows. The plant illustrated in Figure 1 includes a furnace 1 having an openable bottom 2. The furnace 1 further includes inlet opening 3, which is connected to the outlet of a compressor 5 by means of a pipe 4, and an outlet opening 6, which is connected by a pipe 7 to a cooler 8 comprising a cooling coil 9 which is supplied with a suitable coolant. Cooling of the gas entering the cooler 8 may also be effected by spraying water into the gas. Connected to the cooler 8 is a pipe 10 which extends to the inlet of a screw expander 11, the outlet of which is connected by a pipe 12 to an inlet opening of a filter chamber 13. The chamber 13 has an outlet opening which is connected by a pipe 14 to the inlet of the compressor 5, and to a valve 22', which is normally closed, through which any surplus gas can be released through a pipe 22. The furnace 1, followed by the cooler 8, may also be connected in the pipe 14, between the outlet of the filter chamber 13 and the compressor inlet, as indicated by the broken single-line rectangle in Figure 1. The furnace 1 is connected in parallel with a control¬ lable bypass conduit 55, by means of a valve arrangement 54. This arrangement can have an important significance from the aspect of system control. For instance, the arrangement enables solely that amount of gas required to carry away the contaminants to be heated during plant operation. The compressor 5 and the expander 11 will constantly work with the same gas flow, while the flow of gas through the furnace 1 can be varied. The arrangement also provides the advantage of enabling the oxygen content of the furnace to be controlled, there¬ with enabling low-oxygen combustion to be achieved. The hydrocarbons produced are then unable to find a sufficient number of oxygen molecules and, as a conse¬ quence, form both carbon monoxide and free hydrogen. These components have a strongly reducing effect at high temperatures and therewith enable undesirable mercury compounds to be converted to purely metallic mercury, which is easier to handle. For the purpose of braking the rotors of the screw expander 11, an electric generator 15 is connected to the rotors which also functions to supply electricity to a heating coil 17 in the furnace 1, through an electric conductor 16. A motor 18 is connected to the com¬ pressor 5. The plant operates in the following manner: An oil barrel having a volume of about 200 litres and filled with environmentally harmful material, e.g. mercury batteries, is placed in the furnace l and heated to a temperature of 400-1800 β C by means of heating coils (not shown) or some other appropriate means. The compressor 5 is started-up by the motor 18, so as to pressurize the furnace 1 and the cooler 8. As a result of pressurizing the furnace and cooler, the screw expander 11 begins to rotate and is braked by the generator 15, which contributes towards heating of the furnace 1, through the electrical conductor ' 16 and the heating coil 17. A pronounced drop in pressure occurs over the screw expander 11, therewith pronounced cooling of the gas leaving the expander 11. When the tempera¬ ture of the gas is lowered in the cooler 8 by means of the cooling device 9 to a temperature below 40°C, the gas in the chamber 13 may be given a temperature of from -50° to -100°C, wherewith mercury, among others, con¬ denses and can be collected and removed from the chamber 13, while recycling the cold gas through the pipe 14 to the furnace 1, via the compressor 5, in a closed cir¬ cuit, wherewith mercury is progressively expelled from the material present in the furnace until essentially no mercury remains therein. Different substances carried by the gas flow can be extracted individually therefrom, by lowering the temperature of the gas in several stages with the aid of multi-stage expansions, optionally in combination with heat exchange. It is necessary to introduce fresh material into the plant and to discharge residues, slag and ash from said plant through gas-tight locks or valve mechanisms, so as to ensure that contaminated gas will not escape to atmosphere. When opening the furnace for the introduction of fresh material, the gas present in the compressor 5 is prefer- ably led immediately therefrom to the cooler 8 through the pipe 55, so as to maintain cooling of the chamber 13. The embodiment illustrated in Figure 2 is generally similar to the embodiment illustrated in Figure 1, with the exception of a further compressor 20 which is in¬ tended to supply air of combustion to the furnace 1 through a pipe 1 for the combustion of hydrocarbons, which are gasified in the furnace at higher tempera- tures. Although not shown, a pilot flame is kept burn¬ ing in the furnace during the process. The Figure 2 embodiment also includes a discharge pipe 22 to which there is fitted a valve 22' which functions to release surplus gas from the return pipe 14 forming part of the closed circuit. The flow through the compressor 20 is only a small part of the flow through the compressor 5. The invention can be applied within many different branches of industry where environmentally harmful solvents are used for painting and washing processes, and also in the manufacture of plastic articles, gluing processes, in printing plants, or in industries of other chemical activities. Such chemicals include thinners, toluene, styrene, trichloroethylene, petrochemicals, alcohols, acetone, benzene, etc. A common factor of all such chemicals is that they include different types of volatile hydrocarbons, although theoretically the inven¬ tion can also be applied with other volatile substances, provided that a suitable adsorbent can be found. It is earlier known that these substances can be adsorbed in filter plants with the aid of an adsorbent, e.g. an active carbon or so-called Zeolites. The carbon is then incinerated or, in the case of large plants, the car¬ bon/adsorbent is regener-ated. This regeneration is effected with the aid of heat or superheated steam. The steam is led into a condenser, where the solvent also condenses. The majority of hydrocarbons, although not alcohols, are not easily dissolved in water. The mix¬ ture can therefore be left to stand and the solvent fraction later removed and, normally, reused. Regeneration is an expensive process which places par¬ ticular demands on the filter plant and on the carbon used. Thus, in practice, the purifying efficiency of the system is lower than that which can be theoretically achieved with the adsorbent used. This process is simplified considerably by the inventive plant for cleaning crude gas illustrated in Figure 3 and with the process employed in this plant. " Smaller plants which provide a considerably improved degree of separa- tion or extraction can be constructed. The plant illustrated in Figure 3 is principly similar to the plant illustrated in Figure 2, although with the exception that a collecting chamber 1 which corresponds to the furnace l of the earlier embodiment is now locat- ed in the return pipe 14, and the generator 15 has been arranged to supply electrical energy to the motor 18. The collecting chamber 1 in Figure 3 is comprised of a tubular pressure vessel 30 which includes two mutually separate columns 31, 32 filled with an appropriate adsorption material, e.g. active carbon. The pressure vessel 1 is connected at one end thereof to a pipe 33 through"which solvent-containing crude gas is delivered to the vessel τ whereas the other end of the vessel is connected to a discharge pipe 34 for cleansed gas, these connections being achieved through the medium of two valves 35 and 36 which are intended to connect the columns 31, 32 alternately to the pipes 33, 34 so that when gas flows through one column, e.g. column 32, the other column, e.g. 31, is fully closed, with the excep¬ tion of a respective valve-fitted inlet and outlet 37, 38 connected to the pipe 14. A corresponding inlet and outlet in the column 32 are referenced 39, 40 respec¬ tively. Associated valves are identified by the same reference numerals to which a prime sign has been added, i.e. 37', 38', 39' and 40'. With the valves 35 and 36 are in the operational states illustrated in Figure 3, the adsorbent material in the column 31 has been saturated with a solvent from a passing crude-gas flow during a proceeding working phase. This solvent is now vaporized and is discharged by opening the valve 38' and placing the outlet 38 in communication with the suction side of the compressor 5. At the same time, gas flows through the column 32 from the pipe 33. By allowing a given volume of gas to flow. by opening the valve 37', it is possible to drive-off the last residues of solvent without needing to generate a high subpressure. The column 31 may also be heated slightly, in order to accelerate vaporization of the solvent. There is a risk that certain substances, e.g. styrene, will polymerize at excessively high tempera¬ tures, therewith destroying the adsorbent material. This can be prevented by passing cold, cleansed gas into the column, from the separation chamber 14 (not shown) . When the adsorbent in the column 32 becomes saturated with solvent, a condition which can be detected by means of a sensor 41 mounted at the outlet from respective columns 31, 32, the valves 35, 36 are reversed so that gas will flow from the pipe 33 into the column 31 and out through the pipe 34, wherewith solvent that accom¬ panies the gas will be re-adsorbed in this column. The valves 37' and 38' are closed at the same time, and the valves 39' and 40' opened, the valves 39' and 40' being set to a setting at which a desired subpressure is obtained in the column 32 with a suitable throughflow of gas, determined by the valve 39'. In this way, the solvents are collected alternately in the two columns 31, 32 of the pressure vessel 30, these columns, in turn, being connected alternately to the pipe 14. The pipe 14, with the aid of the valves 37' and 39' and a control valve 14' connected in the pipe 14, is maintained at a subpressure suitable for expel- ling the solvents in the columns 31, 32 during circula¬ tion of the gas in the circuit which includes the pipe 14 and the compressor 5, the cooler 8, the expander 11 and the separation chamber 13, therewith achieving continuous extraction of the solvents in the chamber 13, in the manner aforedescribe . The majority of solvents are explosive, which prohibits working at excessively high concentrations. A typical 3 concentration limit is 10 g/m n, which applies to tolu¬ ene, thinner, styrene and other similar substances. In order to ensure that these solvents are present in low concentrations, cleansed gas is recycled back to the column while the compressor applies suction at the other end at the same time. In the case of larger plants, the concentration limit may be raised from 3 to 5 times, by excluding oxygen from the flow of gas to the compressor. Extraction of oxygen from the gas flow can be achieved by filling the column with CO , N or some other inert gas, prior to compressor suction beginning. The return gas will also be oxygen-free. Subsequent to the gas having passed through the compres¬ sor 5, the gas is cooled in the cooler 8, which in the illustrated case is a pressurized heat exchanger. The pressure in the heat exchanger may be from 5-20. bars. The gas is further cooled by expanding the gas through a turbine or some other expansion machine 11, while gene¬ rating mechanical force at the same time. The cooling effect is very pronounced and temperatures in the range of -50° to -90°C can be achieved without difficulty. The solvent will then condense and can be subsequently readily separated in the chamber 13 and recovered. Only an insignificant amount of water is present and the residual content in the adsorbent is very low. The difference in the flow between the compressor 5 and the flow of crude gas in the chamber 13 may have almost any desired value, which is highly advantageous in practice. It may often be necessary to heat the gas departing from the separation chamber 13 to the collecting chamber 1. This can be achieved readily with the aid of a heating coil 52 in the pipe 14, hot water being passed through a pipe 53 from the cooling device 9. It will be understood that ' the invention is not restric¬ ted to the illustrated and described embodiments and that modifications can be made within the scope of the invention claimed. For example, the cooler 8 in which a number of substances condense may advantageously have the form of a scrubber 50 for intensified cooling of the gas and the device 51 for cleansing and removing conden- sation from the cooler 8. The plant may also, of course, include more columns than the two illustrated columns 31 and 32.

Claims: CLAIMS 1. A plant for separating or extracting substances in a gas or mist form from a gas flow, for example such substances as mercury and other environmentally harmful heavy metals when incinerating batteries or consumed solvents or other volatile substances released from chemical and metallurgical processes, said plant co - prising - a chamber (1) for temporarily collecting the gas flow containing the substances to be extracted; - a heat exchanger (8) which is connected to an outlet (6) of said chamber (1) and which functions to lower the temperature of the gas flow; - a compressor (5) which pressurizes the gas flow; and - an expansion device (11) , preferably a screw-rotor device, through which the gas flow passes while rapid- ly lowering the pressure and temperature of said gas so that at least a part of the substances to be ex¬ tracted condense and are separated in solid or fluid state in a separation chamber (13) which is connected to the outlet (12) of said expansion device and which is provided with an outlet (14) for the remaining gas flow, c h a r a c t e r i z e d in that the outlet (14) of the separation chamber (13) is connected to a throttle- able inlet (3) of the collecting chamber (1), either directly or via the compressor (5), such as to form a gas-circulation circuit, and is also connected to a throttleable residual gas outlet (22). 2. A plant according to Claim 1, c h a r a c ¬ t e r i z e d in that the collecting chamber (1) in- eludes a furnace for melting and/or incinerating batter¬ ies or other materials which contain environmental harmful substances; and in " that the furnace (1) is connected in parallel to a controllable bypass conduit (55). 3. A plant according to Claim 1 or 2, c h a r a c ¬ t e r i z e d in that the heat exchanger (8) is com¬ bined with a cleansing and cooling scrubber-type cooling device by means of which the gas temperature at the inlet (10) of the expansion device (11) is restricted to about 30°C. 4. A plant according to any one of Claims 1-3, c h a r a c t e r i z e d in that the connecting cham¬ ber (1) comprises a pressure vessel (30) which includes chambers (31, 32) provided with adsorbent material and valves (37', 38'; 39', 40') for guiding the gas alter¬ nately through the different chambers (31, 32) and connecting the chambers alternately to the suction side of the compressor (5) and to the outlet of the separa¬ tion chamber (13) and creating a subpressure in respec¬ tive chambers such as to vaporize the volatile substanc¬ es adsorbed on the adsorbent material. 5. A plant according to any one of Claims 1-4, c h a r a c t e r i z e d in that the heat exchanger (8) includes a scrubber (50) provided with means (51) for cleaning condensation removed from the heat exchanger (8).

Inventors: ALMLOEF GOERAN, LILLIEHOEOEK FOLKE, ALMLOEF, GOERAN, LILLIEHOEOEK, FOLKE
Applicants: BAL AB
Application year: 1992
Main classification: F25J3
IPC codes: B01D53, H01M6