- Email: [email protected]
Bioscrubbing of air contaminated with high concentrations of hydrocarbons
Biotechniquesfor Air Pollution Abatement and Odour Control Policies A J . Dragt and J . van Ham (Editors) 0 1992 Elsevier Science Publishers B.V. All righrs reserved.
77
Bioscrubbing of air contaminated with high concentrations of hydrocarbons H.J.G. Kok, Institute of Environmental and Energy Technology TNO P.O.Box 342,7300 AH Apeldoorn, The Netherlands Abstract Bioscrubbing is a cleaning technique for waste air contaminated with biodegradable compounds. In the scrubber part of the bioscrubbing system, the contaminating compounds are absorbed in a water phase. The contaminated water is transported to the bioreactor, where the compounds are biodegraded by aerobic microorganisms (mainly to carbon dioxide, water, and biomass). The application range of bioscrubbing systems is mainly found in the biodegradation of odorous compounds in such sectors of industry as rendering plants, livestock farming, food industry, and foundries. The concentration of biodegradable compounds in the waste air streams is less than 100 - 500 mg/m3of air. In these cases, the bioreactor is an activated-sludge tank in which the microorganisms are suspended in water. Within the framework of the Dutch “Hydrocarbon 2000” Programme, many sectors of industry, with hydrocarbon concentrations in the waste gases often ranging from 0.5 t o 5 g/m3, will have to reduce their hydrocarbon emissions stringently (e.g. chemicals industry, coating and painting industry, fibre production, chemical cleaning business). Most water-soluble hydrocarbons are biodegradable. Bioscrubbing may be promising in this field when bioreactor techniques can be used with high degradation capacities and a capability of accommodating fluctuating concentrations and longer periods of production stops. In these branches, bioscrubbing systems with active-carbon pellets having a buffer capacity for hydrocarbons, and acting as a support material for microorganisms, have better chances than conventional bioscrubbing systems with activated-sludge tanks. 1. INTRODUCTION
Within the framework of the Dutch “Hydrocarbon2000” Programme, an average 65%reduction in hydrocarbon emissions, as compared to 1981emissions, must be realised by the year 2000. Where this emission reduction cannot be reached
78
through process-integrated measures (such as solvent-poor lacquers, recovery and reuse, other process routes, etc.), various cleaning techniques can be applied. For removing high concentrations of hydrocarbons in waste gases (5 - 10 g/m3),thermal incineration, for one, is a good, reliable cleaning technique.For removing low concentrations of hydrocarbons in waste gases (< 0.5 g/m3),biological waste gas purification is a relatively reliable cleaning technique for biologically easily degradable hydrocarbons (such as many aromatic compounds). In fact, a cheap and reliable cleaning technique for the intermediate concentration range is not yet available. Commissioned by the Dutch Ministry of Housing, Physical Planning and the Environment, a feasibility study has been conducted, in cooperation with Dutch industry, into the technical and economic aspects of applying biological cleaning techniques to waste gases with hydrocarbon concentrations of over 0.5 g/m3 [ll. In biofiltration, a contaminated waste gas stream is led through a humid, biologically active filter material (compost, peat, etc.), in which the contaminants are degraded by the microorganismspresent, after absorption (and adsorption). In conventional bioscrubbing, the contaminants are first removed from the waste gas stream in a wash column and subsequently biologically degraded (usually in a separate activated-sludge reactor) (see Fig. 1). 4
clean gar
parked column
-
w a s t e gas
-
C3 m
hioreactor
LT-4 Figure 1 Constructional principles of a bio-scrubber
r a n t a m m a t e d fresh water water
2. BIOLOGICAL CLEANING TECHNIQUES
Biological cleaning techniques for waste gas streams can be divided into two techniques: biofiltration and (conventional)bioscrubbing. Both techniques are the same in that they degrade the hydrocarbon compounds into mainly carbon dioxide, water, and biomass by means of aerobic microorganisms that are present in the water phase (as a result of the growth of microorganisms). When applying biological cleaning techniques, attention should be paid to the physical, (bio)chemical and technical hindrances and limitations. In essence,
79
bioscrubbers have better applicabilities for higher concentrations of hydrocarbons than biofilters, as the mass transfer and the degradation occur at different places; in other words, the wash column and the bioreactor can be optimised separately. Some other advantages of conventional bioscrubbing systems over biofilters are: - A better control of the nutrient supply and the degree of acidity - A more compact installation - A slower occurrence of toxic concentrations in the water. Conventional bioscrubbers (wash column + bioreactor with suspended sludge) have some disadvantages over biofilters: - With reasonably to badly soluble compounds (distribution coefficient H > 0.01), high wash columns and a large water flow are required (e.g. for ethers, aromatics, and chlorinated hydrocarbons). (H, in this respect, is the ratio between the concentration of hydrocarbons in the gas phase [g/m31 and the concentration of hydrocarbons in the liquid phase [g/m31in equilibrium). - The chance of the slowest-growingmicroorganisms being washed out. - Stagnation periods of some days are not permitted. - Disposal of the sludge (surplus sludge). - A more complicated start-up procedure. - The required addition of extra oxygen at high degradation capacities. - Generally, higher operational costs. Fig. 2 sketches the application range of biological cleaning techniques for reasonably to easily biodegradable hydrocarbon compounds. It shows that with respect t o badly soluble hydrocarbons (H > 0.1) a good, conventional biological cleaning technique is not available for the concentration range between 0.5 and 5 g/m3of waste gas. Furthermore, problems will occur in all systems mentioned in Fig. 2, if there are strong fluctuations in the supply of compounds to be degraded. Therefore, solutions to these problems should be sought. roncentration [ g / m 3 air]
t I
lo
bubble column
t
\
I
packed column1
tricklebed ifinel
tricklebed itoarsel
10-
Figure 2 Application area of diverse biological cleaning systems
-
10-5
10-L
lo-' 1 distribution-coefflclent H
10'
80
3. SPECIAL CONSTRUCTIONS FOR BIOSCRUBBING SYSTEMS Literature mentions some special constructions for bioscrubbing systems (plate column bioscrubber [21, membrane bioreactor [31, and watedsolvent emulsion in the bioscrubber [4]). Specifically with watedsolvent emulsions, good results have been obtained on a laboratory scale for compounds badly soluble in water, such as aromatic compounds. However, this causes a waste stream to originate (water/ solvent/surplus sludge) which is difficult to clean biologically. The first four disadvantages of conventional bioscrubbing systems mentioned above can possibly be met by applying activated carbon in the bioscrubbing system. In this respect, three properties of activated carbon can be used: - Improvement of the mass transfer in the scrubber [5] - Buffer capacity for intercepting fluctuations in the supply of contaminants - Immobilisation of the microorganisms on the activated carbon as a carrier material. The limitations regarding mass transfer in the scrubber and the possibilities for improving this situation by adding activated carbon to the wash water can be summarised as follows: - Through scrubbing, hydrocarbons with a distribution coefficient of H < 0.01 can be removed reasonably easily from a waste gas stream (many alcohols, ketones, aldehydes, and esters). - For reasonably soluble compounds (0.01 c H c O . l ) , the volume of the wash column can be restricted by adding a maximum of 15 wt.% of activated carbon in the form of powdered carbon andlor fine granular activated carbon. For a good utilisation of the buffer capacity, it is necessary that clean carbon should be supplied t o the scrubber and that the residence time in the scrubber should be sufficiently long. The application of a coarser grain will, accordingly, increase the necessary residence time. - Scrubbingcan scarcely remove hydrocarbons with a distribution coefficient of H > 0.1 from a waste gas stream by scrubbing, as this would require a large quantity of water to be pumped around (this is true for most aromatic compounds, chlorinated hydrocarbons, and aliphatics). The possibilities of improved degradation in the bioreactor for reasonably soluble hydrocarbons (compared to a conventional activated-sludge reactor) are summarised below. The emphasis here is put on the use of activated carbon as a carrier for the microorganisms and as a buffer capacity for intercepting fluctuations in the supply of contaminants. - In bioreactors, a specific surface area of 2,000 to 3,000 m2/m3of reactor can be realised by sludge on a granular carrier. Thus, a higher degradation capacity by a factor of 5 t o 10, compared t o activated-sludge, biofilter, or trickle-bed filter installations, is possible. - In the bioreactor, granular activated carbon has in many cases advantages over powdered carbon. With granular activated carbon, a larger buffer capacity can
81
be realised for bridging periods without contaminant supply. Also, higher degradation capacities can be realised with sludge on a granular carrier than with activated-sludge systems with powdered carbon (PACT) [6]. With granular carriers, advanced bioreactors must be used (e.g. air-lift reactors [7]). - Due to the porous structure of active carbon grains, it is also possible to degrade those hydrocarbons that can be degraded only by way of a combined anaerobic/ aerobic route (e.g. chlorinated hydrocarbons). For reasonably soluble to rather insoluble hydrocarbons, the quantity of liquid to be pumped around can be limited by circulating slurries of activated carbon between the scrubber and the bioreactor. Important aspects in this respect that need further investigation are: - The buffer capacity of the circulating carbon can probably be utilized only in a limited way, as the residence times in the scrubber and bioreactor are relatively short. - In applying circulating slurries of activated carbon, the adsorption of contaminants is somewhat delayed due to the presence of a biolayer, while desorption is accelerated by it (bioregeneration). - In strongly discontinuous waste gas streams and/or strongly fluctuating waste gas compositions, it is better to apply a separate buffer tank with activated carbon (in the gas phase or in the liquid phase). 4. COSTS OF BIOSCRUBBER SYSTEMS
Fig. 3 gives an estimate of the running costs (in Dutch guilders) per kilogramme of removed hydrocarbon, as a function of the hydrocarbon concentration in the waste gas to be cleaned. This estimate is based on cost data from literature and on some examples calculated in cooperation with the industry [8,9].
BF : Biofiltration BS, : Bioscrubbing (good soluble/ degradable, continuous) BS, : Bioscrubbing (good soluble/ degradable, discontinuous) BS, : Bioscrubbing (badly soluble and/or badly degradable) A, : Afterburning (continuous) A, : Afterburning (discontinuous)
6
I
2
0
0
1 __t
2
3
ront
Iq
1
5
6
H C I m l air]
Figure 3 Workin expenses of biofiltration, bioscru%bingand afterburning (estimation)
82
For waste gas streams with higher hydrocarbon concentrations (> 0.5 g hydrocarbons/m3), it is true that: - the investment costs of bioscrubbing systems are largely determined by the bioreactor construction and, to a lesser extent, by the scrubbing section; - the running costs of bioscrubbing systems vary between Dfl. 2.00 and over Dfl. 5.00 per kilogramme of removed hydrocarbon compound.
5. CONCLUSIONS Compared to thermal incineration systems, bioscrubbing systems have lower running costs, if the hydrocarbon concentrations in the waste gas t o be cleaned are less than about 3 g hydrocarbon/m3 waste gas, and the compounds are relatively easily soluble and biologically easily degradable. This holds both for continuous and discontinuous supply of contaminants. Compared to thermal incineration, the running costs can be lower for relatively insoluble and/or recalcitrant compounds, specifically in the case of discontinuous waste gas streams and concentrations around 1 g hydrocarbon/m3 waste gas. Bioscrubbing systems with circulating slurries of activated carbon may offer prospects for this application range. Further investigation into the fields mentioned above is therefore desirable. 6. REFERENCES
1 H.J.G. Kok, Biowassysteem voor de behandeling van koolwaterstofhoudende afgassen (fase 1: Haalbaarheidsstudie). TNO-report 91-151, April 1991. 2 F. Wolff, Biologische Abluftreinigung mit einem neuen Biowascherkonzept. VDI-Berichte Nr. 735, 1989. 3 K. Fischer, Biologische Elimination von schlecht wasserloslichen Abluftinhaltstoffen mit Hilfe eines Membranverfahrens. VDI-Berichte Nr. 735, 1989. 4 E. Schippert, Das Biosolv-Verfahren von Keramchemie zur Absorption von schwer wasserloslichen Losemitteln. VDI-Berichte Nr. 735, 1989. 5 J.T. Tinge, Selective sorption of gases in slurries of activated carbon in water. Thesis at Groningen University, 1987. 6 RIZA, Biologische zuivering van industrieel afvalwater met poederkooldosering. November 1986. 7 E. Buren, Bio-catalytische Abgasreinigung in einer Kernmacherei. VDIBerichte Nr. 735, 1989. 8 H. Kohler, Biologische Abluftaufbereitung. WLB Wasser, Luft und Betrieb 1/2, 1983. 9 U. Penzeel, Abluftreinigung zur Entsorgung organischer Emissionen. Fachveranstaltung: Abluftreinigung und Ruckgewinnung von organischen Losungsmitteln. Haus der Technik, Essen, 1990.
77
Bioscrubbing of air contaminated with high concentrations of hydrocarbons H.J.G. Kok, Institute of Environmental and Energy Technology TNO P.O.Box 342,7300 AH Apeldoorn, The Netherlands Abstract Bioscrubbing is a cleaning technique for waste air contaminated with biodegradable compounds. In the scrubber part of the bioscrubbing system, the contaminating compounds are absorbed in a water phase. The contaminated water is transported to the bioreactor, where the compounds are biodegraded by aerobic microorganisms (mainly to carbon dioxide, water, and biomass). The application range of bioscrubbing systems is mainly found in the biodegradation of odorous compounds in such sectors of industry as rendering plants, livestock farming, food industry, and foundries. The concentration of biodegradable compounds in the waste air streams is less than 100 - 500 mg/m3of air. In these cases, the bioreactor is an activated-sludge tank in which the microorganisms are suspended in water. Within the framework of the Dutch “Hydrocarbon 2000” Programme, many sectors of industry, with hydrocarbon concentrations in the waste gases often ranging from 0.5 t o 5 g/m3, will have to reduce their hydrocarbon emissions stringently (e.g. chemicals industry, coating and painting industry, fibre production, chemical cleaning business). Most water-soluble hydrocarbons are biodegradable. Bioscrubbing may be promising in this field when bioreactor techniques can be used with high degradation capacities and a capability of accommodating fluctuating concentrations and longer periods of production stops. In these branches, bioscrubbing systems with active-carbon pellets having a buffer capacity for hydrocarbons, and acting as a support material for microorganisms, have better chances than conventional bioscrubbing systems with activated-sludge tanks. 1. INTRODUCTION
Within the framework of the Dutch “Hydrocarbon2000” Programme, an average 65%reduction in hydrocarbon emissions, as compared to 1981emissions, must be realised by the year 2000. Where this emission reduction cannot be reached
78
through process-integrated measures (such as solvent-poor lacquers, recovery and reuse, other process routes, etc.), various cleaning techniques can be applied. For removing high concentrations of hydrocarbons in waste gases (5 - 10 g/m3),thermal incineration, for one, is a good, reliable cleaning technique.For removing low concentrations of hydrocarbons in waste gases (< 0.5 g/m3),biological waste gas purification is a relatively reliable cleaning technique for biologically easily degradable hydrocarbons (such as many aromatic compounds). In fact, a cheap and reliable cleaning technique for the intermediate concentration range is not yet available. Commissioned by the Dutch Ministry of Housing, Physical Planning and the Environment, a feasibility study has been conducted, in cooperation with Dutch industry, into the technical and economic aspects of applying biological cleaning techniques to waste gases with hydrocarbon concentrations of over 0.5 g/m3 [ll. In biofiltration, a contaminated waste gas stream is led through a humid, biologically active filter material (compost, peat, etc.), in which the contaminants are degraded by the microorganismspresent, after absorption (and adsorption). In conventional bioscrubbing, the contaminants are first removed from the waste gas stream in a wash column and subsequently biologically degraded (usually in a separate activated-sludge reactor) (see Fig. 1). 4
clean gar
parked column
-
w a s t e gas
-
C3 m
hioreactor
LT-4 Figure 1 Constructional principles of a bio-scrubber
r a n t a m m a t e d fresh water water
2. BIOLOGICAL CLEANING TECHNIQUES
Biological cleaning techniques for waste gas streams can be divided into two techniques: biofiltration and (conventional)bioscrubbing. Both techniques are the same in that they degrade the hydrocarbon compounds into mainly carbon dioxide, water, and biomass by means of aerobic microorganisms that are present in the water phase (as a result of the growth of microorganisms). When applying biological cleaning techniques, attention should be paid to the physical, (bio)chemical and technical hindrances and limitations. In essence,
79
bioscrubbers have better applicabilities for higher concentrations of hydrocarbons than biofilters, as the mass transfer and the degradation occur at different places; in other words, the wash column and the bioreactor can be optimised separately. Some other advantages of conventional bioscrubbing systems over biofilters are: - A better control of the nutrient supply and the degree of acidity - A more compact installation - A slower occurrence of toxic concentrations in the water. Conventional bioscrubbers (wash column + bioreactor with suspended sludge) have some disadvantages over biofilters: - With reasonably to badly soluble compounds (distribution coefficient H > 0.01), high wash columns and a large water flow are required (e.g. for ethers, aromatics, and chlorinated hydrocarbons). (H, in this respect, is the ratio between the concentration of hydrocarbons in the gas phase [g/m31 and the concentration of hydrocarbons in the liquid phase [g/m31in equilibrium). - The chance of the slowest-growingmicroorganisms being washed out. - Stagnation periods of some days are not permitted. - Disposal of the sludge (surplus sludge). - A more complicated start-up procedure. - The required addition of extra oxygen at high degradation capacities. - Generally, higher operational costs. Fig. 2 sketches the application range of biological cleaning techniques for reasonably to easily biodegradable hydrocarbon compounds. It shows that with respect t o badly soluble hydrocarbons (H > 0.1) a good, conventional biological cleaning technique is not available for the concentration range between 0.5 and 5 g/m3of waste gas. Furthermore, problems will occur in all systems mentioned in Fig. 2, if there are strong fluctuations in the supply of compounds to be degraded. Therefore, solutions to these problems should be sought. roncentration [ g / m 3 air]
t I
lo
bubble column
t
\
I
packed column1
tricklebed ifinel
tricklebed itoarsel
10-
Figure 2 Application area of diverse biological cleaning systems
-
10-5
10-L
lo-' 1 distribution-coefflclent H
10'
80
3. SPECIAL CONSTRUCTIONS FOR BIOSCRUBBING SYSTEMS Literature mentions some special constructions for bioscrubbing systems (plate column bioscrubber [21, membrane bioreactor [31, and watedsolvent emulsion in the bioscrubber [4]). Specifically with watedsolvent emulsions, good results have been obtained on a laboratory scale for compounds badly soluble in water, such as aromatic compounds. However, this causes a waste stream to originate (water/ solvent/surplus sludge) which is difficult to clean biologically. The first four disadvantages of conventional bioscrubbing systems mentioned above can possibly be met by applying activated carbon in the bioscrubbing system. In this respect, three properties of activated carbon can be used: - Improvement of the mass transfer in the scrubber [5] - Buffer capacity for intercepting fluctuations in the supply of contaminants - Immobilisation of the microorganisms on the activated carbon as a carrier material. The limitations regarding mass transfer in the scrubber and the possibilities for improving this situation by adding activated carbon to the wash water can be summarised as follows: - Through scrubbing, hydrocarbons with a distribution coefficient of H < 0.01 can be removed reasonably easily from a waste gas stream (many alcohols, ketones, aldehydes, and esters). - For reasonably soluble compounds (0.01 c H c O . l ) , the volume of the wash column can be restricted by adding a maximum of 15 wt.% of activated carbon in the form of powdered carbon andlor fine granular activated carbon. For a good utilisation of the buffer capacity, it is necessary that clean carbon should be supplied t o the scrubber and that the residence time in the scrubber should be sufficiently long. The application of a coarser grain will, accordingly, increase the necessary residence time. - Scrubbingcan scarcely remove hydrocarbons with a distribution coefficient of H > 0.1 from a waste gas stream by scrubbing, as this would require a large quantity of water to be pumped around (this is true for most aromatic compounds, chlorinated hydrocarbons, and aliphatics). The possibilities of improved degradation in the bioreactor for reasonably soluble hydrocarbons (compared to a conventional activated-sludge reactor) are summarised below. The emphasis here is put on the use of activated carbon as a carrier for the microorganisms and as a buffer capacity for intercepting fluctuations in the supply of contaminants. - In bioreactors, a specific surface area of 2,000 to 3,000 m2/m3of reactor can be realised by sludge on a granular carrier. Thus, a higher degradation capacity by a factor of 5 t o 10, compared t o activated-sludge, biofilter, or trickle-bed filter installations, is possible. - In the bioreactor, granular activated carbon has in many cases advantages over powdered carbon. With granular activated carbon, a larger buffer capacity can
81
be realised for bridging periods without contaminant supply. Also, higher degradation capacities can be realised with sludge on a granular carrier than with activated-sludge systems with powdered carbon (PACT) [6]. With granular carriers, advanced bioreactors must be used (e.g. air-lift reactors [7]). - Due to the porous structure of active carbon grains, it is also possible to degrade those hydrocarbons that can be degraded only by way of a combined anaerobic/ aerobic route (e.g. chlorinated hydrocarbons). For reasonably soluble to rather insoluble hydrocarbons, the quantity of liquid to be pumped around can be limited by circulating slurries of activated carbon between the scrubber and the bioreactor. Important aspects in this respect that need further investigation are: - The buffer capacity of the circulating carbon can probably be utilized only in a limited way, as the residence times in the scrubber and bioreactor are relatively short. - In applying circulating slurries of activated carbon, the adsorption of contaminants is somewhat delayed due to the presence of a biolayer, while desorption is accelerated by it (bioregeneration). - In strongly discontinuous waste gas streams and/or strongly fluctuating waste gas compositions, it is better to apply a separate buffer tank with activated carbon (in the gas phase or in the liquid phase). 4. COSTS OF BIOSCRUBBER SYSTEMS
Fig. 3 gives an estimate of the running costs (in Dutch guilders) per kilogramme of removed hydrocarbon, as a function of the hydrocarbon concentration in the waste gas to be cleaned. This estimate is based on cost data from literature and on some examples calculated in cooperation with the industry [8,9].
BF : Biofiltration BS, : Bioscrubbing (good soluble/ degradable, continuous) BS, : Bioscrubbing (good soluble/ degradable, discontinuous) BS, : Bioscrubbing (badly soluble and/or badly degradable) A, : Afterburning (continuous) A, : Afterburning (discontinuous)
6
I
2
0
0
1 __t
2
3
ront
Iq
1
5
6
H C I m l air]
Figure 3 Workin expenses of biofiltration, bioscru%bingand afterburning (estimation)
82
For waste gas streams with higher hydrocarbon concentrations (> 0.5 g hydrocarbons/m3), it is true that: - the investment costs of bioscrubbing systems are largely determined by the bioreactor construction and, to a lesser extent, by the scrubbing section; - the running costs of bioscrubbing systems vary between Dfl. 2.00 and over Dfl. 5.00 per kilogramme of removed hydrocarbon compound.
5. CONCLUSIONS Compared to thermal incineration systems, bioscrubbing systems have lower running costs, if the hydrocarbon concentrations in the waste gas t o be cleaned are less than about 3 g hydrocarbon/m3 waste gas, and the compounds are relatively easily soluble and biologically easily degradable. This holds both for continuous and discontinuous supply of contaminants. Compared to thermal incineration, the running costs can be lower for relatively insoluble and/or recalcitrant compounds, specifically in the case of discontinuous waste gas streams and concentrations around 1 g hydrocarbon/m3 waste gas. Bioscrubbing systems with circulating slurries of activated carbon may offer prospects for this application range. Further investigation into the fields mentioned above is therefore desirable. 6. REFERENCES
1 H.J.G. Kok, Biowassysteem voor de behandeling van koolwaterstofhoudende afgassen (fase 1: Haalbaarheidsstudie). TNO-report 91-151, April 1991. 2 F. Wolff, Biologische Abluftreinigung mit einem neuen Biowascherkonzept. VDI-Berichte Nr. 735, 1989. 3 K. Fischer, Biologische Elimination von schlecht wasserloslichen Abluftinhaltstoffen mit Hilfe eines Membranverfahrens. VDI-Berichte Nr. 735, 1989. 4 E. Schippert, Das Biosolv-Verfahren von Keramchemie zur Absorption von schwer wasserloslichen Losemitteln. VDI-Berichte Nr. 735, 1989. 5 J.T. Tinge, Selective sorption of gases in slurries of activated carbon in water. Thesis at Groningen University, 1987. 6 RIZA, Biologische zuivering van industrieel afvalwater met poederkooldosering. November 1986. 7 E. Buren, Bio-catalytische Abgasreinigung in einer Kernmacherei. VDIBerichte Nr. 735, 1989. 8 H. Kohler, Biologische Abluftaufbereitung. WLB Wasser, Luft und Betrieb 1/2, 1983. 9 U. Penzeel, Abluftreinigung zur Entsorgung organischer Emissionen. Fachveranstaltung: Abluftreinigung und Ruckgewinnung von organischen Losungsmitteln. Haus der Technik, Essen, 1990.