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F in a Compact Wet Scrubber
0957±5820/01/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol 79, Part B, March 2001
SEPARATION OF DUST, HALOGEN AND PCDD/F IN A COMPACT WET SCRUBBER M. LEHNER1 , F. MAYINGER2 and W. GEIPEL1 2
1 Rauschert Verfahrenstechnik GmbH, Steinwiesen, Germany Lehrstuhl A fuÈr Thermodynamik, Technische UniversitaÈt MuÈnchen, Garching, Germany
U
p-to-date exhaust gas cleaning systems of waste incineration plants are highly effective, yet expensive in terms of equipment and other expenses. Aiming at a simpler, but nevertheless effective system, a compact wet scrubber was developed that consists of several venturi scrubbers working in self-priming mode. The venturi scrubbers are located at the bottom of a multistage packed tower. This compact apparatus incorporates several of the steps of a conventional exhaust gas cleaning process, such as dust separation, quench, acid and basic wet cleaning. Therefore, investment and running costs for the gas cleaning process are reduced without any loss of separation ef®ciency. The design concept and important experimental results of the compact scrubber are reported. The scrubber has been employed at a hazardous waste incineration plant in order to test its applicability under real conditions. The experiments were used to identify bene®ts and drawbacks of the scrubber, and to improve the scrubber design concept. It is shown that the developed scrubber is highly effective in separating ®ne dust particles and inorganic-gaseous pollutants (HCl, HF, SO2). Additionally, the concentration of polychlorinated dioxins and furans (PCDD/F) is signi®cantly reduced in the off-gas of the scrubber. These compounds are adsorbed into plastic packing materials. Keywords: hazardous waste incineration; gas cleaning; wet scrubbers; dust separation; PCDD/F separation; venturi scrubbers.
INTRODUCTION
the high gas velocities in the throats of the venturi scrubbers, ®ne dust particles and even very small aerosols (dp < 1 mm) are separated4,5. However, the high gas velocities in the venturi scrubbers do not allow suf®cient residence time for effective gas absorption. Consequently, a second scrubber stage is required. A packed tower is the most suitable device for the removal of gaseous pollutants, since it provides a great interfacial area for mass transfer and a long residence time for the gas phase. Both scrubber stages, venturi and packed tower, which are usually kept separate, are incorporated within one compact apparatus. Figure 1 shows a scheme of the scrubber design. The compact wet scrubber can be divided into three sections according to their functions: the venturi scrubbers (3) and the two packed tower stages (4) (5) (Figure 1). Depending on the gas ¯ow rate, several venturi scrubbers connected in parallel are located at the bottom of the compact scrubber. They are submerged in the washing liquid and, therefore, work in a self-priming mode. The washing liquid is injected only by means of a pressure difference between the inside and the outside of the venturi. In the venturi scrubbers the hot exhaust gases are quenched whereas dust as well as aerosols are separated ef®ciently. The injected liquid carrying the separated dust particles is dragged with the gas ¯ow up to the top of the venturi scrubbers. About 80 to 90% of the injected liquid returns to the bottom of the column. The gas containing the remaining liquid droplets passes through the second scrubber stage, a packed tower (4) which serves as the acid wet cleaning
In the last two decades great efforts have been made by engineers and chemists to improve the exhaust gas cleaning of waste incineration plants due to the strict requirements for emission standards1. Therefore, on the one hand the emission of pollutants has been strongly reduced, but on the other hand the cleaning process has become more and more complex. Modern gas cleaning consists of several steps, such as dust separation, quench, acid and basic wet cleaning, deNOX, and an activated charcoal ®lter which is used for the removal of residual concentrations of mercury, dioxins and aerosols2,3. As a result of this, expenses for the exhaust gas cleaning process have risen continuously in the last few years. A further improvement of exhaust gas cleaning technology has to be achieved by simpler and more reasonable solutions which are still ef®cient in separating the pollutants. Therefore, the objective of the presented research project was the development of a new compact apparatus which incorporates several steps of the conventional exhaust gas cleaning process in order to reduce investment and operating costs, while maintaining the separation ef®ciency. SCRUBBER DESIGN In order to deposit dust particles and aerosols as well as inorganic gaseous pollutants like SO2 and HCl, a combined wet scrubber has been developed that consists of several venturi scrubbers and a packed tower (Figure 1). Owing to 109
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LEHNER et al. Table 1. Operating conditions and design data of the compact wet scrubber. Operating conditions
SI-unit
Total gas ¯ow F-factor Speci®c liquid load of packings Gas temperature, entrance Gas temperature, exit Pressure, entrance (g) Overall pressure drop
3500 2.3 15±20 160±180 55±65 ±30 100
scbm h ±1 Pa1/2 m3 m ±2 h ±1 8C 8C mbar mbar
Design data Diameter of column Number of packed beds Bed heights Packing material Speci®c surface area Void fraction Number of venturi scrubbers Venturi length
800 2 2.0 Rauschert Hi¯owÒ 50-6 90 94 10 1200
mm ± m ± m2 m ±3 % ± mm
Figure 1. Compact wet scrubber and its bypass integration as a pilot plant into the hazardous waste incineration at Schwabach, Germany.
into the crude gas pipeline. Detailed data of the experimental setup are given in Table 1.
stage. Additionally, it holds back the residual liquid droplets emerging from the venturi scrubbers. Finally, the third scrubber stage, the basic wet cleaning, is used to remove SO2 by means of caustic. At the bottom of the second packed tower (5) the trickling water is accumulated and drained off. The compact wet scrubber is supplied with two separated washing liquid circulations, an acid circulation (which incorporates the venturi scrubbers and the ®rst packed tower) and a caustic circulation (second packed tower). At the top of the compact wet scrubber a demister prevents liquid entrainment. By integrating the venturi scrubbers into the bottom of the packed tower a comparatively simple and compact apparatus has been achieved. Therefore, the investment costs as well as the operating costs of the gas cleaning process could be reduced. The space saving design of the compact wet scrubber enables an easy upgrading as well as a reconstruction of gas cleaning plants. The compact wet scrubber is suitable for a wide ®eld of applications, such as for gas cleaning behind hazardous waste incineration, decentralized residuals incineration in the chemical industry, as well as chemical reactors. After the experimental programme had been carried out successfully with a laboratory scrubber, the experiments were conducted at a hazardous waste incineration plant in order to prove the suitability of the compact wet scrubber for the severe working conditions that show up there. As shown in Figure 1, approximately 10% of the crude gas ¯ow rate (3,500 scbm/h) is drained off from the crude gas pipeline shortly after the electric precipitator. Passing through a gas/gas heat exchanger in which the gas cools down from 3008 C to 1708 C, the gas enters the compact wet scrubber from the bottom. In this case, ten self-priming venturi scrubbers, connected in parallel, quench the gas and separate the dust and aerosols. The following two packed towers are used to remove inorganic gaseous pollutants (HCl, HF, SO2). The fan pumps the cleaned gas through the gas/gas heat exchanger again where it reheats up to approximately 2008 C. The cleaned gas is resuspended
EXPERIMENTAL RESULTS The pilot plant compact wet scrubber has been in operation for approximately four months. The aim of this project was to conduct long term experiments, to examine the separation ef®ciency of selected pollutants, to draw out a balance of some selected elements of interest, and, last but not least, to prove the ef®ciency of the scrubber even under severe working conditions. Operational Behaviour Regarding the venturi scrubbers, two alternative operating modes are commonly encountered. Most industrial venturi scrubbers work in a forced feed mode. The liquid is introduced into the throat and/or in the converging part of the venturi scrubber by means of a pump, and the liquid ¯ow rate is adjusted independently from the gas ¯ow rate. However, the venturi scrubbers used in the present investigation work in a self-priming mode5. This operating mode makes use of the reduced static pressure of the ¯owing gas in the throat which results from the acceleration of the gas in the converging section of the venturi scrubber. The scrubbing liquid is supplied by a water reservoir surrounding the throat. It is drawn in due to a pressure difference between the outside and the inside of the venturi throat which results from the hydrostatic pressure of the liquid and the static pressure of the ¯owing gas. Figure 2 shows the operating behaviour of the compact wet scrubber which is signi®cantly in¯uenced by the ®lling level of the venturi water reservoir. According to measurements of the liquid ¯ow rate as a function of different ®lling levels, higher ®lling levels of the venturi water reservoir are accompanied by enhanced liquid loadings. But increasing the liquid loading of the venturi scrubbers results in a higher overall pressure loss. The pressure loss is highly dependent on the gas ¯ow rate which is expressed by the characteristic curve of the fan. Finally, the gas ¯ow rate conveyed by the fan decreases, and as a result of this, the overall pressure loss decreases also. Therefore, the Trans IChemE, Vol 79, Part B, March 2001
SEPARATION OF DUST, HALOGEN AND PCDD/F IN A COMPACT WET SCRUBBER
Figure 2. Operating characteristic of the pilot plant compact wet scrubber.
operating point of the compact wet scrubber depends on the ®lling level, the overall pressure loss of the venturi scrubbers and the characteristic curve of the fan. All operating points of the compact wet scrubber located inside the ®eld `working range’ in Figure 2 ensure a steady operation that is automatically controlled by the control equipment of the scrubber. Dust Separation in the Venturi Scrubbers Many investigations in the literature 4±7 deal with the efficiency and the optimization of venturi scrubbers. According to these reports, the essential mechanism for collecting the particles in a venturi scrubber is based on inertia effects. The aerosol collection ef®ciency as well as the overall pressure drop of a venturi scrubber are signi®cantly in¯uenced by · · · ·
the the the the
liquid loading gas velocity in the throat liquid disintegration design of the scrubber
The separation ef®ciency of a venturi scrubber is enhanced with the amount of liquid added per volume of gas and with increasing the gas velocity in the throat4,5. This fact is, on the one hand, due to the increase in the interfacial area for mass transfer at higher liquid charges. On the other hand, high gas velocities lead to large relative velocities between the gas and the washing liquid immediately after the injection. To simplify matters, it is often assumed in the literature 8 that the scrubbing liquid is dispersed into droplets immediately after its injection into the scrubber. The so-called droplet model was developed in order to enable the calculation of the separation ef®ciency and the required quantity of liquid. The dust particles colliding with the liquid droplet, assumed to be spherical, are considered to be collected. A comparison between the calculated separation ef®ciencies and the experimental data shows that this theory underestimates the cleaning ef®ciency of the scrubber by far, particularly for ®ne particles. Mayinger and Neumann9 , Roberts and Hill10, as well as some other authors11,12, investigated the liquid atomization in air streams, and particularly in the throat of venturi scrubbers, by using different optical techniques. They have shown that the Trans IChemE, Vol 79, Part B, March 2001
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atomization process is extremely complex. Mayinger and Neumann9 took high-speed photographs of the spray formation in the throat of a venturi scrubber. The liquid jet is fragmented primarily into tiny sheet-like particles and ligaments with a few parachute-shaped thin liquid ®lms as a result of the high sheer stress of the gas ¯ow. Subsequently, at the end of the throat, droplets begin to develop. However, it should be noted that the sheet-shaped particles have a life-time of only a few milliseconds. Then they revert to droplets of different sizes. Mayinger and Neumann have pointed out that these structures provideÐfor a short time after the injectionÐa large interfacial area for mass transfer and ensure a high separation ef®ciency even for very ®ne dust particles. Figure 3 shows the effect of a special venturi throat design6,11 on the separation ef®ciency that makes use of the complex liquid disintegration process described above. The used throat design consists of cylindrical ports of 4 mm into which the scrubbing liquid is introduced. These ports are located in one, three, or ®ve planes, respectively, which have a distance of only a few centimetres to one another. As shown in Figure 3, a signi®cant enhancement of the collection ef®ciency is noticed when using multistage water injection. This fact results from an interaction of several factors. As mentioned above, the liquid does not disintegrate directly into droplets but into a large number of sheet-like structures. The larger interfacial area of these lamellar-shaped sheets caused by a renewed formation after each injection improved the collection ef®ciency. Mayinger and Neumann9 performed measurements of the interfacial area in a venturi scrubber, which is pertinent for the collection process, by means of a fast chemical model reaction. They have shown that a distinct maximum of the interfacial area is observed a few centimetres downstream of the injection nozzle. By introducing the water in several planes at a distance of a few centimetres, these maxima are achieved several times, which enhances the separation ef®ciency. High relative velocities between the injected liquid and the ¯owing gas enhance the washing effect of the scrubber. The velocity gradient between the liquid jet and the gas stream is at maximum directly at the injection location. Initially, the scrubbing liquid is strongly accelerated at each injection plane. By using multistage injection, high relative velocities are achieved over a greater part of the throat length as opposed to the single-stage injection. Finally, by staggering the ports between each injection level, a far more uniform distribution of the liquid was attained all over the cross section of the throat, and liquid
Figure 3. Separation ef®ciency of single-, three- and ®ve-plane injection of the washing liquid.
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Table 2. Results of particle concentration measurements at the pilot plant compact wet scrubber. No. Crude gas particle concentration [mg m ±3] Cleaned gas particle concentration [mg m ±3] Overall separation ef®ciency [%]
1
2
3
4
5
28.4 0.4 98.6
24.2 0.3 98.8
19.1 0.5 97.4
21.2 0.8 95.8
20.2 <0.5 >97.5
free areas in the throat were reduced. Therefore, the probability of a collision between aerosol and scrubbing liquid could be increased. The fundamental measurements described above were carried out at the laboratory compact wet scrubber; the measurement technique used to obtain this data is described in the literature 5,11. In Table 2 some results of the aerosol separation ef®ciency are given which were obtained at the pilot plant compact wet scrubber by measuring the dust concentration in front of and behind the scrubber by using a plane ®lter according to VDI 2066, sheets 1, 2 and 7. The accuracy of the dust concentration measurements is: 6 10% of the measured value, minimum 6 1 mg m ±3. Unfortunately, it was not possible to connect the pilot wet scrubber to the waste incineration plant directly behind the waste-heat boiler due to limited space. Therefore, the connection was placed after the electrostatic precipitator, resulting in low crude gas particle concentrations, as can be seen in Table 2. Nevertheless it is dif®cult to separate this residual particle concentration, since only the ®ne dust fraction emerges from the electrostatic precipitator, while the ®ne dust can hardly be removed. It can be derived from Table 2 that the particle concentration of the cleaned gas is always below 1 mg m ±3 leading to overall separation ef®ciencies between 96 and 99%. Therefore, even ®ne dust and aerosols are almost completely separated with the venturi scrubbers working in self-priming mode. In particular, the ®ne dust fraction is highly contaminated with heavy metals and organic compounds. In order to meet the emission standards for these pollutants, a high-grade separation of the ®ne dust fraction is of major importance. Gas Absorption in the Packed Tower In addition to the separation of the dust and the aerosols, the main task of the compact wet scrubber is the removal
of the halogen-hydrogen compounds (e.g. HCl, HF) and of sulphur dioxide (SO 2). The absorption of these gaseous pollutants is predominantly carried out in the packed towers (Figure 1). In the course of the long-term experiments both continuous and discontinuous measurement techniques were applied to determine the separation ef®ciencies of these pollutants in the gas phase. Additionally, the concentrations of the ions Cl ±, Br ±, SO42±, I ± and F ± in the scrubbing liquid was determined by means of sample probes after they had been drained off discontinuously. Measurement techniques The discontinuous measurements in the gas phase were carried out by using a sample lance made of glass ®lled with quartz wool that is connected with two Erlenmeyer ¯asks in series followed by a silicagel dryer, a piston pump and a gas meter. The sampled gas volume ranged between 0.04 and 0.35 m3 gas. The gas samples were always made simultaneously at two different locations in the scrubber, for example at the scrubber entrance and exit. The Erlenmeyer ¯asks were ®lled each with 200 ml washing solution with different compositions. The composition of the different washing solutions is given in Table 3. Additionally, sample probes of the washing solution were taken from the ®rst and the second scrubber stage (results see Table 4). In the lower part of Table 3, the analysis methods are given that were used to determine the listed ions and liquid properties. These methods were used to determine these ®gures both in the Erlenmeyer ¯asks of the gas phase measurements and in the sampled washing solution of both scrubber stages. The measurement accuracy of the liquid analysis is always better than 5%. However, the total accuracy of the gas sampling device is lower. In the case of the gas measurements, it is not guaranteed that every gas component is completely deposited in the sample ¯asks. Additionally, no experiments were carried out to
Table 3. Measurement techniques used for the discontinuousanalysis of the gas and washing liquid composition. Compound to be detected Halogens (HCl, HBr, etc.) SO2 Hg
Composition of washing solution in the Erlenmeyer ¯ask 1 molar NaOH 1 molar NaOH 4 ml 30% H2O2 0.1 molar HNO3 KMnO3
Analysis methods used to determine the below listed ®gures in the washing solution: Parameter pH-value Conductivity Density F Cl , Br , SO24 , I Hg Ca
Measurement method according to
Sample preparation
DIN 38404 C5 DIN 38404 C8 DIN 38404 C9 DIN 38405 D4-2 ISE DIN 38405 D20 (IC) ( EN ISO 10304-1) DIN 38406 E12 (Hydrid-AAS) DIN 38406 E22 (ICP)
± ± ± ± Filtration 0.45 mm ± Filtration 0.45 mm
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SEPARATION OF DUST, HALOGEN AND PCDD/F IN A COMPACT WET SCRUBBER
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Table 4. Results of the scrubbing liquid sample probe analysis towards chloride, bromide, sulphate, iodide and ¯uoride (n.m.: not measured). Cl [mg l 1 ]
Br [mg l 1 ]
SO24 [mg l 1 ]
I [mg l 1 ]
F [mg l 1 ]
No.
Stage 1
Stage 2
Stage 1
Stage 2
Stage 1
Stage 2
Stage 1
Stage 2
Stage 1
Stage 2
1 2 3 4 5 6 7 8 9 10 11 12 13
78100 34300 20800 18700 21400 33900 8900 21100 22300 34400 32900 73300 85200
400 942 328 161 58 77 58 109 132 161 147 1180 3730
<25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25
2060 388 870 1560 1190 239 <25 537 803 <25 <25 838 963
450 395 453 374 697 4560 1410 1170 552 4660 1100 5890 778
60 60 54 50 306 530 3900 484 96 4440 1620 2030 931
186 386 331 204 25 363 108 349 492 358 339 2060 399
<10 <10 <10 <10 52 <10 62 <10 <10 <10 <10 <10 <10
<1 <1 1.05 3.38 <1 104 67.3 1.93 1.52 n.m. 2.41 1.24 1.03
11.9 14.7 6.91 6.34 3.78 4.39 44.8 18.7 14.1 n.m. 70.3 60 257
prove whether the location of the sample probes is representative. Experience gained with a series of measurements in the past show that an error in the range between 5 and 30 % has been taken into account with this kind of gas measurement device. Experimental results The ®rst absorption stage operates with pH-values in the range of approximately 0 in order to remove the halide compounds. By using caustic, the pH-value of the second stage is adjusted to a range of approximately 7. This is a good condition for the separation of SO2. In Figure 4, the HCl-concentrations of the crude gas as well as the cleaned gas column are shown for one operating day. The crude gas concentration ranges between 1 and 3 g m ±3. At times short peaks up to 10 g m ±3 can be observed which cannot be described in Figure 5 due to the time scale. However, it is important to point out that the emission standard according to 17. BImSchV (German federal decree of emission standards for waste incineration plants) can be met in any case, even if very high concentrations of HCl (up to 10,000 mg m ±3 for short periods of time) in the crude gas are present. The separation ef®ciency of HCl, for example, is approximately 99%.
Figure 4. Exemplary HCl-concentrations of the crude gas and the cleaned gas.
Trans IChemE, Vol 79, Part B, March 2001
Table 4 shows some results of the scrubbing liquid sample probe analysis. As can be derived from Table 4, Cl±-ions were found predominantly in the scrubbing liquid of stage 1 (®rst packed tower), and, therefore, HCl is more or less completely separated in the ®rst scrubber stage. Additionally, the fairly high concentrations of the Cl ±-ions con®rm the gas measurements shown in Figure 4. The ions Br ±, I ± and F ± are found to be less concentrated in the scrubbing liquid due to their small concentrations in the gas phase (see Table 4). Similarly to Cl±, for these halogen-ions the separating zone in the compact wet scrubber can be localized precisely. While Br ± and F ± are predominantly separated in the second scrubber stage, I ± is mainly found in the scrubbing liquid of the ®rst stage. A comparison of the concentrations of these ions shows that the exhaust gas of the hazardous waste incineration is contaminated with higher concentrations of bromine than with ¯uorine. Iodine is concentrated in merely small amounts. Polychlorinated Dioxins and Furans The concentrations of polychlorinated dioxins and furans (PCDD/F) were measured in the crude and cleaned gas of the compact wet scrubber. Additionally, their deposition at the packing material of the column was investigated. All measurements were performed with constant operating conditions of the compact wet scrubber in order to ensure their comparability. Measurement technique The analysis of PCDD/F in the gas was carried out according to EN 1948, part 1 to 3. The gas samples were taken isokinetically according to VDI 2066 using a one metre long lance made of glass ®lled with quartz wool that is connected with a cooler separating the condensable components and two Erlenmeyer ¯asks in series ®lled with diethylene glycol (C4H10O3). This set-up is followed by a droplet separator, a ®lter element for aerosol separation, a silicagel dryer, a regulated piston pump and two gas metres. The gas samples were always taken simultaneously at the scrubber entrance and exit. The determination of PCDD/F was carried out as described below: extraction of the ®lter materials, the condensate and the absorbent
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LEHNER et al.
Figure 5. Concentrations of PCDD/F in the crude and cleaned gas of the compact wet scrubber.
using toluene (Soxhlet-process), clean-up of the extract by column chromatography, determination with GC/MS. The following detection limits have to be taken into account: T4to H6CDD/CDF: 0.001 mg kg 1 ; H7CDD: 0.005 mg kg 1 ; H7CDF: 0.003 mg kg 1 ; OCDD/CDF: 0.010 mg kg 1 . The analysis of the plastic packings is more sophisticated. The preparation and the extraction of the sample probes is different to the process described above, whereas the clean-up and the analysis method is exactly the same. At ®rst, the plastic packing sample is crushed mechanically. In order to achieve a higher extraction ef®ciency compared to the standard toluene/Soxhlet-process, different extraction solvents were used for the different plastic materials. Dichlormethane (CH2Cl2) was used to extract PVC, benzene (C6H6) was used for PP. For lack of a suitable solvent for PVDF and PFA, the standard toluene/Soxhletprocess was performed for this material. The accuracy of these measurements is the same as that mentioned above. Experimental results It can be derived from Figure 5 that PCDD/F is markedly reduced while passing through the compact wet scrubber. The average mass ¯ow rate of PCDD/F amounts to 382 ng I-TE/h in the crude gas and 156 ng I-TE/h in the cleaned gas, respectively. This comparatively high separation ef®ciency of 59% can partially be ascribed to the fact that a part of these compounds are attached to the surface of the ®ne dust particles emerging from the waste incineration. Due to the highly ef®cient dust separation described above, these compounds are drained off together with the dust. Nevertheless, this mechanism is not a suf®cient explanation of the observed high separation ef®ciencies. Therefore, it is assumed that there is another sink for these compounds in the scrubber. During the operation of the compact wet scrubber,
samples of the packing material (polypropylene Hi¯ow rings) were taken after an operation of 7 weeks and 15 weeks from the ®rst and the second packed tower, and their PCDD/F content was analysed. Figure 6 gives a comparison of the PCDD/F contents of the packing material. In addition to the packing material of the compact wet scrubber, new packing material was also analysed which already shows up with very low content of PCDD/F. One should recognize that the axis of ordinates in Figure 6 is graded logarithmically in order to depict the very small concentration of the new material. A signi®cant enrichment of PCDD/F is noticed in the packing material used in the compact wet scrubber compared to the new material. The second stage has approximately 12 times greater contamination than the ®rst stage. The difference in the concentrations between both scrubber stages can only be explained by the different operating conditions, as the same packing material, polypropylene, has been used for both stages. Vogg et al.13 reported that polypropylene has a signi®cant temperature-dependent adsorption potential for PCDD/F. Therefore, the temperature difference of a few Kelvin between both scrubber stages is assumed to be the reason for the different adsorption levels. The measurement of the enrichment of the packing material after 15 weeks of operation shows that the PCDD/F-contents were increased by a factor of 3.3 (stage 1) and 2.4 (stage 2), respectively. Consequently, the second scrubber stage is still more contaminated than the ®rst one, but the average content is now only 8.6 times higher than that of the ®rst stage. This fact leads to the assumption that saturation of the packing material with these pollutants has begun. A further comparison of the relative PCDD/F-contents in the crude gas and in the packing material shows that the congener pro®le is quite similar. In any case, it can be stated Trans IChemE, Vol 79, Part B, March 2001
SEPARATION OF DUST, HALOGEN AND PCDD/F IN A COMPACT WET SCRUBBER
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Figure 6. PCDD/F-content of the packing material. Comparison of the ®rst and the second scrubber stage after 7 weeks of operation with new packing material.
that the packing material is signi®cantly contaminated with PCDD/F. Therefore, an additional sink for these pollutants has been found although its collecting capability highly depends on the temperature and is limited due to saturation effects. The contaminated material can easily be disposed of by burning in the waste incinerator, where organic compounds are completely destroyed by the high temperatures. CONCLUSIONS AND FUTURE PROSPECTS It can be derived from the long-term experiments performed that ®ne dust, inorganic halide compounds as well as polychlorinated dioxins and furans are separated very ef®ciently in the compact wet scrubber. The dust is almost completely separated by means of the venturi scrubbers, whereas the separation of the other mentioned pollutants takes place in the packed towers. During the experiments, PCDD/F was continuously enriched in the packing material. In order to use the packing material ef®ciently and speci®cally for the separation of PCDD/F, the mechanisms of the separation process, such as the in¯uence of the material and saturation effects, must be the subject of further investigations. Measurements of the aerosol generation by means of spontaneous phase transition were additionally carried out at the compact wet scrubber. They showed that aerosols could be generated in the wet scrubber under particular operating conditions. These particles are fractionated mainly in the range of 0.5 mm to 2 mm, and they are not retained by the demisters at the top of the scrubber. An idea to solve this problem may be to move the venturi Trans IChemE, Vol 79, Part B, March 2001
scrubber stage from the bottom to the top of the compact wet scrubber. Consequently, any possible generated aerosols emerging from the packings can now be separated ef®ciently. Additionally, the second packing can be used for a speci®c growth of the aerosols by cooling of the scrubbing liquid, for example. The planned further investigations also have to be directed towards the PCDD/F adsorption at the packing material in order to obtain more information about the fundamental mechanisms of the separation process. REFERENCES 1. 17. Verordnung zur DurchfuÈhrung des BImSchG (17. BImSchV) vom 23.11.1990. Bundesgesetzblatt 1 S. 2545. 2. Comfere, W., Reitz, P. and Wilop, A., 1994, Erfahrungen mit Rauchgasnachreinigungsanlagen unterschiedlicher Konzeptionen in Herten, Augsburg und Burgkirchen. GVC-VDI Gesellschaft Verfahrenstechnik und Chemieingenieurwesen (Ed.): Abfallwirtschaft Herausforderung und Chance (VDI-Verlag DuÈsseldorf), 233±250. 3. Nottrodt, A., 1994, Fortschrittliche Verfahren zur Abgasreinigung in Abfallverbrennungsanlagen. ThomeÂ-Kozmiensky, K. J. (Ed.), Thermische Abfallbehandlung (EF-Verlag, Berlin), 432±443. 4. Mayinger, F. and Lehner, M., 1995, Operating results and aerosol deposition of a venturi scrubber in self-priming operation, Chem Eng Proc, 34: 283±288. 5. Lehner, M., 1998, Aerosol separation ef®ciency of a venturi scrubber working in self-priming mode, Aerosol Science and Technology, 28: 389±402. 6. Tigges, K. D. and Mayinger, F., 1984, Experiments with highly ef®cient venturi scrubbers for aerosol separation from gases under multiplane water injection, Chem Eng Proc, 18: 171±179. 7. Leith, D. and Cooper, D. W., 1980, Venturi scrubber optimization, Atmo Env, 14: 657±664. 8. Barth, W., 1959, Grundlegende Untersuchung uÈber die Reinigungsleistung von Wassertropfen, Staub, 19: 175±180.
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9. Mayinger, F. and Neumann, M., 1978, Dust collection in venturi scrubbers, Ger Chem Eng, 1: 289±293. 10. Roberts, D. B. and Hill, J. C., 1981, Atomization in a venturi scrubber, Chem Eng Comm, 12: 33±68. 11. Lehner, M., 1996, Staubabscheidung im VenturiwaÈscher mit FluÈssigkeits-Selbstansaugung, PhD Thesis (Technische UniversitaÈt MuÈnchen, Munich, Germany). 12. Lane, W. R., 1951, Shatter of drops in streams of air, Ind Eng Chem, 43: 1312±1317. 13. Vogg, H., Kreisz, S. and Hunsinger, H., 1994, Wet scrubbersÐa potential PCDD/F source? Organohalogen Compounds, 20: 305±307.
ACKNOWLEDGEMENT The presented research project was part of the BayFORREST research
activities and was sponsored by the Bayerisches Staatsministerium fuÈr Landesentwicklung und Umweltfragen (Bavarian Ministry of Development and Environmental Protection) in co-operation with Rauschert Verfahrenstechnik, Steinwiesen. These ®nancial supports are gratefully acknowledged.
ADDRESS Correspondence concerning this paper should be addressed to Dr M. Lehner, Rauschert Verfahrenstechnik GmbH, Paul Rauschert Str 6, 96349 Steinwiesen, Germany. The manuscript was received 15 April 1999 and accepted for publication after revision 18 December 2000.
Trans IChemE, Vol 79, Part B, March 2001
SEPARATION OF DUST, HALOGEN AND PCDD/F IN A COMPACT WET SCRUBBER M. LEHNER1 , F. MAYINGER2 and W. GEIPEL1 2
1 Rauschert Verfahrenstechnik GmbH, Steinwiesen, Germany Lehrstuhl A fuÈr Thermodynamik, Technische UniversitaÈt MuÈnchen, Garching, Germany
U
p-to-date exhaust gas cleaning systems of waste incineration plants are highly effective, yet expensive in terms of equipment and other expenses. Aiming at a simpler, but nevertheless effective system, a compact wet scrubber was developed that consists of several venturi scrubbers working in self-priming mode. The venturi scrubbers are located at the bottom of a multistage packed tower. This compact apparatus incorporates several of the steps of a conventional exhaust gas cleaning process, such as dust separation, quench, acid and basic wet cleaning. Therefore, investment and running costs for the gas cleaning process are reduced without any loss of separation ef®ciency. The design concept and important experimental results of the compact scrubber are reported. The scrubber has been employed at a hazardous waste incineration plant in order to test its applicability under real conditions. The experiments were used to identify bene®ts and drawbacks of the scrubber, and to improve the scrubber design concept. It is shown that the developed scrubber is highly effective in separating ®ne dust particles and inorganic-gaseous pollutants (HCl, HF, SO2). Additionally, the concentration of polychlorinated dioxins and furans (PCDD/F) is signi®cantly reduced in the off-gas of the scrubber. These compounds are adsorbed into plastic packing materials. Keywords: hazardous waste incineration; gas cleaning; wet scrubbers; dust separation; PCDD/F separation; venturi scrubbers.
INTRODUCTION
the high gas velocities in the throats of the venturi scrubbers, ®ne dust particles and even very small aerosols (dp < 1 mm) are separated4,5. However, the high gas velocities in the venturi scrubbers do not allow suf®cient residence time for effective gas absorption. Consequently, a second scrubber stage is required. A packed tower is the most suitable device for the removal of gaseous pollutants, since it provides a great interfacial area for mass transfer and a long residence time for the gas phase. Both scrubber stages, venturi and packed tower, which are usually kept separate, are incorporated within one compact apparatus. Figure 1 shows a scheme of the scrubber design. The compact wet scrubber can be divided into three sections according to their functions: the venturi scrubbers (3) and the two packed tower stages (4) (5) (Figure 1). Depending on the gas ¯ow rate, several venturi scrubbers connected in parallel are located at the bottom of the compact scrubber. They are submerged in the washing liquid and, therefore, work in a self-priming mode. The washing liquid is injected only by means of a pressure difference between the inside and the outside of the venturi. In the venturi scrubbers the hot exhaust gases are quenched whereas dust as well as aerosols are separated ef®ciently. The injected liquid carrying the separated dust particles is dragged with the gas ¯ow up to the top of the venturi scrubbers. About 80 to 90% of the injected liquid returns to the bottom of the column. The gas containing the remaining liquid droplets passes through the second scrubber stage, a packed tower (4) which serves as the acid wet cleaning
In the last two decades great efforts have been made by engineers and chemists to improve the exhaust gas cleaning of waste incineration plants due to the strict requirements for emission standards1. Therefore, on the one hand the emission of pollutants has been strongly reduced, but on the other hand the cleaning process has become more and more complex. Modern gas cleaning consists of several steps, such as dust separation, quench, acid and basic wet cleaning, deNOX, and an activated charcoal ®lter which is used for the removal of residual concentrations of mercury, dioxins and aerosols2,3. As a result of this, expenses for the exhaust gas cleaning process have risen continuously in the last few years. A further improvement of exhaust gas cleaning technology has to be achieved by simpler and more reasonable solutions which are still ef®cient in separating the pollutants. Therefore, the objective of the presented research project was the development of a new compact apparatus which incorporates several steps of the conventional exhaust gas cleaning process in order to reduce investment and operating costs, while maintaining the separation ef®ciency. SCRUBBER DESIGN In order to deposit dust particles and aerosols as well as inorganic gaseous pollutants like SO2 and HCl, a combined wet scrubber has been developed that consists of several venturi scrubbers and a packed tower (Figure 1). Owing to 109
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LEHNER et al. Table 1. Operating conditions and design data of the compact wet scrubber. Operating conditions
SI-unit
Total gas ¯ow F-factor Speci®c liquid load of packings Gas temperature, entrance Gas temperature, exit Pressure, entrance (g) Overall pressure drop
3500 2.3 15±20 160±180 55±65 ±30 100
scbm h ±1 Pa1/2 m3 m ±2 h ±1 8C 8C mbar mbar
Design data Diameter of column Number of packed beds Bed heights Packing material Speci®c surface area Void fraction Number of venturi scrubbers Venturi length
800 2 2.0 Rauschert Hi¯owÒ 50-6 90 94 10 1200
mm ± m ± m2 m ±3 % ± mm
Figure 1. Compact wet scrubber and its bypass integration as a pilot plant into the hazardous waste incineration at Schwabach, Germany.
into the crude gas pipeline. Detailed data of the experimental setup are given in Table 1.
stage. Additionally, it holds back the residual liquid droplets emerging from the venturi scrubbers. Finally, the third scrubber stage, the basic wet cleaning, is used to remove SO2 by means of caustic. At the bottom of the second packed tower (5) the trickling water is accumulated and drained off. The compact wet scrubber is supplied with two separated washing liquid circulations, an acid circulation (which incorporates the venturi scrubbers and the ®rst packed tower) and a caustic circulation (second packed tower). At the top of the compact wet scrubber a demister prevents liquid entrainment. By integrating the venturi scrubbers into the bottom of the packed tower a comparatively simple and compact apparatus has been achieved. Therefore, the investment costs as well as the operating costs of the gas cleaning process could be reduced. The space saving design of the compact wet scrubber enables an easy upgrading as well as a reconstruction of gas cleaning plants. The compact wet scrubber is suitable for a wide ®eld of applications, such as for gas cleaning behind hazardous waste incineration, decentralized residuals incineration in the chemical industry, as well as chemical reactors. After the experimental programme had been carried out successfully with a laboratory scrubber, the experiments were conducted at a hazardous waste incineration plant in order to prove the suitability of the compact wet scrubber for the severe working conditions that show up there. As shown in Figure 1, approximately 10% of the crude gas ¯ow rate (3,500 scbm/h) is drained off from the crude gas pipeline shortly after the electric precipitator. Passing through a gas/gas heat exchanger in which the gas cools down from 3008 C to 1708 C, the gas enters the compact wet scrubber from the bottom. In this case, ten self-priming venturi scrubbers, connected in parallel, quench the gas and separate the dust and aerosols. The following two packed towers are used to remove inorganic gaseous pollutants (HCl, HF, SO2). The fan pumps the cleaned gas through the gas/gas heat exchanger again where it reheats up to approximately 2008 C. The cleaned gas is resuspended
EXPERIMENTAL RESULTS The pilot plant compact wet scrubber has been in operation for approximately four months. The aim of this project was to conduct long term experiments, to examine the separation ef®ciency of selected pollutants, to draw out a balance of some selected elements of interest, and, last but not least, to prove the ef®ciency of the scrubber even under severe working conditions. Operational Behaviour Regarding the venturi scrubbers, two alternative operating modes are commonly encountered. Most industrial venturi scrubbers work in a forced feed mode. The liquid is introduced into the throat and/or in the converging part of the venturi scrubber by means of a pump, and the liquid ¯ow rate is adjusted independently from the gas ¯ow rate. However, the venturi scrubbers used in the present investigation work in a self-priming mode5. This operating mode makes use of the reduced static pressure of the ¯owing gas in the throat which results from the acceleration of the gas in the converging section of the venturi scrubber. The scrubbing liquid is supplied by a water reservoir surrounding the throat. It is drawn in due to a pressure difference between the outside and the inside of the venturi throat which results from the hydrostatic pressure of the liquid and the static pressure of the ¯owing gas. Figure 2 shows the operating behaviour of the compact wet scrubber which is signi®cantly in¯uenced by the ®lling level of the venturi water reservoir. According to measurements of the liquid ¯ow rate as a function of different ®lling levels, higher ®lling levels of the venturi water reservoir are accompanied by enhanced liquid loadings. But increasing the liquid loading of the venturi scrubbers results in a higher overall pressure loss. The pressure loss is highly dependent on the gas ¯ow rate which is expressed by the characteristic curve of the fan. Finally, the gas ¯ow rate conveyed by the fan decreases, and as a result of this, the overall pressure loss decreases also. Therefore, the Trans IChemE, Vol 79, Part B, March 2001
SEPARATION OF DUST, HALOGEN AND PCDD/F IN A COMPACT WET SCRUBBER
Figure 2. Operating characteristic of the pilot plant compact wet scrubber.
operating point of the compact wet scrubber depends on the ®lling level, the overall pressure loss of the venturi scrubbers and the characteristic curve of the fan. All operating points of the compact wet scrubber located inside the ®eld `working range’ in Figure 2 ensure a steady operation that is automatically controlled by the control equipment of the scrubber. Dust Separation in the Venturi Scrubbers Many investigations in the literature 4±7 deal with the efficiency and the optimization of venturi scrubbers. According to these reports, the essential mechanism for collecting the particles in a venturi scrubber is based on inertia effects. The aerosol collection ef®ciency as well as the overall pressure drop of a venturi scrubber are signi®cantly in¯uenced by · · · ·
the the the the
liquid loading gas velocity in the throat liquid disintegration design of the scrubber
The separation ef®ciency of a venturi scrubber is enhanced with the amount of liquid added per volume of gas and with increasing the gas velocity in the throat4,5. This fact is, on the one hand, due to the increase in the interfacial area for mass transfer at higher liquid charges. On the other hand, high gas velocities lead to large relative velocities between the gas and the washing liquid immediately after the injection. To simplify matters, it is often assumed in the literature 8 that the scrubbing liquid is dispersed into droplets immediately after its injection into the scrubber. The so-called droplet model was developed in order to enable the calculation of the separation ef®ciency and the required quantity of liquid. The dust particles colliding with the liquid droplet, assumed to be spherical, are considered to be collected. A comparison between the calculated separation ef®ciencies and the experimental data shows that this theory underestimates the cleaning ef®ciency of the scrubber by far, particularly for ®ne particles. Mayinger and Neumann9 , Roberts and Hill10, as well as some other authors11,12, investigated the liquid atomization in air streams, and particularly in the throat of venturi scrubbers, by using different optical techniques. They have shown that the Trans IChemE, Vol 79, Part B, March 2001
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atomization process is extremely complex. Mayinger and Neumann9 took high-speed photographs of the spray formation in the throat of a venturi scrubber. The liquid jet is fragmented primarily into tiny sheet-like particles and ligaments with a few parachute-shaped thin liquid ®lms as a result of the high sheer stress of the gas ¯ow. Subsequently, at the end of the throat, droplets begin to develop. However, it should be noted that the sheet-shaped particles have a life-time of only a few milliseconds. Then they revert to droplets of different sizes. Mayinger and Neumann have pointed out that these structures provideÐfor a short time after the injectionÐa large interfacial area for mass transfer and ensure a high separation ef®ciency even for very ®ne dust particles. Figure 3 shows the effect of a special venturi throat design6,11 on the separation ef®ciency that makes use of the complex liquid disintegration process described above. The used throat design consists of cylindrical ports of 4 mm into which the scrubbing liquid is introduced. These ports are located in one, three, or ®ve planes, respectively, which have a distance of only a few centimetres to one another. As shown in Figure 3, a signi®cant enhancement of the collection ef®ciency is noticed when using multistage water injection. This fact results from an interaction of several factors. As mentioned above, the liquid does not disintegrate directly into droplets but into a large number of sheet-like structures. The larger interfacial area of these lamellar-shaped sheets caused by a renewed formation after each injection improved the collection ef®ciency. Mayinger and Neumann9 performed measurements of the interfacial area in a venturi scrubber, which is pertinent for the collection process, by means of a fast chemical model reaction. They have shown that a distinct maximum of the interfacial area is observed a few centimetres downstream of the injection nozzle. By introducing the water in several planes at a distance of a few centimetres, these maxima are achieved several times, which enhances the separation ef®ciency. High relative velocities between the injected liquid and the ¯owing gas enhance the washing effect of the scrubber. The velocity gradient between the liquid jet and the gas stream is at maximum directly at the injection location. Initially, the scrubbing liquid is strongly accelerated at each injection plane. By using multistage injection, high relative velocities are achieved over a greater part of the throat length as opposed to the single-stage injection. Finally, by staggering the ports between each injection level, a far more uniform distribution of the liquid was attained all over the cross section of the throat, and liquid
Figure 3. Separation ef®ciency of single-, three- and ®ve-plane injection of the washing liquid.
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Table 2. Results of particle concentration measurements at the pilot plant compact wet scrubber. No. Crude gas particle concentration [mg m ±3] Cleaned gas particle concentration [mg m ±3] Overall separation ef®ciency [%]
1
2
3
4
5
28.4 0.4 98.6
24.2 0.3 98.8
19.1 0.5 97.4
21.2 0.8 95.8
20.2 <0.5 >97.5
free areas in the throat were reduced. Therefore, the probability of a collision between aerosol and scrubbing liquid could be increased. The fundamental measurements described above were carried out at the laboratory compact wet scrubber; the measurement technique used to obtain this data is described in the literature 5,11. In Table 2 some results of the aerosol separation ef®ciency are given which were obtained at the pilot plant compact wet scrubber by measuring the dust concentration in front of and behind the scrubber by using a plane ®lter according to VDI 2066, sheets 1, 2 and 7. The accuracy of the dust concentration measurements is: 6 10% of the measured value, minimum 6 1 mg m ±3. Unfortunately, it was not possible to connect the pilot wet scrubber to the waste incineration plant directly behind the waste-heat boiler due to limited space. Therefore, the connection was placed after the electrostatic precipitator, resulting in low crude gas particle concentrations, as can be seen in Table 2. Nevertheless it is dif®cult to separate this residual particle concentration, since only the ®ne dust fraction emerges from the electrostatic precipitator, while the ®ne dust can hardly be removed. It can be derived from Table 2 that the particle concentration of the cleaned gas is always below 1 mg m ±3 leading to overall separation ef®ciencies between 96 and 99%. Therefore, even ®ne dust and aerosols are almost completely separated with the venturi scrubbers working in self-priming mode. In particular, the ®ne dust fraction is highly contaminated with heavy metals and organic compounds. In order to meet the emission standards for these pollutants, a high-grade separation of the ®ne dust fraction is of major importance. Gas Absorption in the Packed Tower In addition to the separation of the dust and the aerosols, the main task of the compact wet scrubber is the removal
of the halogen-hydrogen compounds (e.g. HCl, HF) and of sulphur dioxide (SO 2). The absorption of these gaseous pollutants is predominantly carried out in the packed towers (Figure 1). In the course of the long-term experiments both continuous and discontinuous measurement techniques were applied to determine the separation ef®ciencies of these pollutants in the gas phase. Additionally, the concentrations of the ions Cl ±, Br ±, SO42±, I ± and F ± in the scrubbing liquid was determined by means of sample probes after they had been drained off discontinuously. Measurement techniques The discontinuous measurements in the gas phase were carried out by using a sample lance made of glass ®lled with quartz wool that is connected with two Erlenmeyer ¯asks in series followed by a silicagel dryer, a piston pump and a gas meter. The sampled gas volume ranged between 0.04 and 0.35 m3 gas. The gas samples were always made simultaneously at two different locations in the scrubber, for example at the scrubber entrance and exit. The Erlenmeyer ¯asks were ®lled each with 200 ml washing solution with different compositions. The composition of the different washing solutions is given in Table 3. Additionally, sample probes of the washing solution were taken from the ®rst and the second scrubber stage (results see Table 4). In the lower part of Table 3, the analysis methods are given that were used to determine the listed ions and liquid properties. These methods were used to determine these ®gures both in the Erlenmeyer ¯asks of the gas phase measurements and in the sampled washing solution of both scrubber stages. The measurement accuracy of the liquid analysis is always better than 5%. However, the total accuracy of the gas sampling device is lower. In the case of the gas measurements, it is not guaranteed that every gas component is completely deposited in the sample ¯asks. Additionally, no experiments were carried out to
Table 3. Measurement techniques used for the discontinuousanalysis of the gas and washing liquid composition. Compound to be detected Halogens (HCl, HBr, etc.) SO2 Hg
Composition of washing solution in the Erlenmeyer ¯ask 1 molar NaOH 1 molar NaOH 4 ml 30% H2O2 0.1 molar HNO3 KMnO3
Analysis methods used to determine the below listed ®gures in the washing solution: Parameter pH-value Conductivity Density F Cl , Br , SO24 , I Hg Ca
Measurement method according to
Sample preparation
DIN 38404 C5 DIN 38404 C8 DIN 38404 C9 DIN 38405 D4-2 ISE DIN 38405 D20 (IC) ( EN ISO 10304-1) DIN 38406 E12 (Hydrid-AAS) DIN 38406 E22 (ICP)
± ± ± ± Filtration 0.45 mm ± Filtration 0.45 mm
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Table 4. Results of the scrubbing liquid sample probe analysis towards chloride, bromide, sulphate, iodide and ¯uoride (n.m.: not measured). Cl [mg l 1 ]
Br [mg l 1 ]
SO24 [mg l 1 ]
I [mg l 1 ]
F [mg l 1 ]
No.
Stage 1
Stage 2
Stage 1
Stage 2
Stage 1
Stage 2
Stage 1
Stage 2
Stage 1
Stage 2
1 2 3 4 5 6 7 8 9 10 11 12 13
78100 34300 20800 18700 21400 33900 8900 21100 22300 34400 32900 73300 85200
400 942 328 161 58 77 58 109 132 161 147 1180 3730
<25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25 <25
2060 388 870 1560 1190 239 <25 537 803 <25 <25 838 963
450 395 453 374 697 4560 1410 1170 552 4660 1100 5890 778
60 60 54 50 306 530 3900 484 96 4440 1620 2030 931
186 386 331 204 25 363 108 349 492 358 339 2060 399
<10 <10 <10 <10 52 <10 62 <10 <10 <10 <10 <10 <10
<1 <1 1.05 3.38 <1 104 67.3 1.93 1.52 n.m. 2.41 1.24 1.03
11.9 14.7 6.91 6.34 3.78 4.39 44.8 18.7 14.1 n.m. 70.3 60 257
prove whether the location of the sample probes is representative. Experience gained with a series of measurements in the past show that an error in the range between 5 and 30 % has been taken into account with this kind of gas measurement device. Experimental results The ®rst absorption stage operates with pH-values in the range of approximately 0 in order to remove the halide compounds. By using caustic, the pH-value of the second stage is adjusted to a range of approximately 7. This is a good condition for the separation of SO2. In Figure 4, the HCl-concentrations of the crude gas as well as the cleaned gas column are shown for one operating day. The crude gas concentration ranges between 1 and 3 g m ±3. At times short peaks up to 10 g m ±3 can be observed which cannot be described in Figure 5 due to the time scale. However, it is important to point out that the emission standard according to 17. BImSchV (German federal decree of emission standards for waste incineration plants) can be met in any case, even if very high concentrations of HCl (up to 10,000 mg m ±3 for short periods of time) in the crude gas are present. The separation ef®ciency of HCl, for example, is approximately 99%.
Figure 4. Exemplary HCl-concentrations of the crude gas and the cleaned gas.
Trans IChemE, Vol 79, Part B, March 2001
Table 4 shows some results of the scrubbing liquid sample probe analysis. As can be derived from Table 4, Cl±-ions were found predominantly in the scrubbing liquid of stage 1 (®rst packed tower), and, therefore, HCl is more or less completely separated in the ®rst scrubber stage. Additionally, the fairly high concentrations of the Cl ±-ions con®rm the gas measurements shown in Figure 4. The ions Br ±, I ± and F ± are found to be less concentrated in the scrubbing liquid due to their small concentrations in the gas phase (see Table 4). Similarly to Cl±, for these halogen-ions the separating zone in the compact wet scrubber can be localized precisely. While Br ± and F ± are predominantly separated in the second scrubber stage, I ± is mainly found in the scrubbing liquid of the ®rst stage. A comparison of the concentrations of these ions shows that the exhaust gas of the hazardous waste incineration is contaminated with higher concentrations of bromine than with ¯uorine. Iodine is concentrated in merely small amounts. Polychlorinated Dioxins and Furans The concentrations of polychlorinated dioxins and furans (PCDD/F) were measured in the crude and cleaned gas of the compact wet scrubber. Additionally, their deposition at the packing material of the column was investigated. All measurements were performed with constant operating conditions of the compact wet scrubber in order to ensure their comparability. Measurement technique The analysis of PCDD/F in the gas was carried out according to EN 1948, part 1 to 3. The gas samples were taken isokinetically according to VDI 2066 using a one metre long lance made of glass ®lled with quartz wool that is connected with a cooler separating the condensable components and two Erlenmeyer ¯asks in series ®lled with diethylene glycol (C4H10O3). This set-up is followed by a droplet separator, a ®lter element for aerosol separation, a silicagel dryer, a regulated piston pump and two gas metres. The gas samples were always taken simultaneously at the scrubber entrance and exit. The determination of PCDD/F was carried out as described below: extraction of the ®lter materials, the condensate and the absorbent
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Figure 5. Concentrations of PCDD/F in the crude and cleaned gas of the compact wet scrubber.
using toluene (Soxhlet-process), clean-up of the extract by column chromatography, determination with GC/MS. The following detection limits have to be taken into account: T4to H6CDD/CDF: 0.001 mg kg 1 ; H7CDD: 0.005 mg kg 1 ; H7CDF: 0.003 mg kg 1 ; OCDD/CDF: 0.010 mg kg 1 . The analysis of the plastic packings is more sophisticated. The preparation and the extraction of the sample probes is different to the process described above, whereas the clean-up and the analysis method is exactly the same. At ®rst, the plastic packing sample is crushed mechanically. In order to achieve a higher extraction ef®ciency compared to the standard toluene/Soxhlet-process, different extraction solvents were used for the different plastic materials. Dichlormethane (CH2Cl2) was used to extract PVC, benzene (C6H6) was used for PP. For lack of a suitable solvent for PVDF and PFA, the standard toluene/Soxhletprocess was performed for this material. The accuracy of these measurements is the same as that mentioned above. Experimental results It can be derived from Figure 5 that PCDD/F is markedly reduced while passing through the compact wet scrubber. The average mass ¯ow rate of PCDD/F amounts to 382 ng I-TE/h in the crude gas and 156 ng I-TE/h in the cleaned gas, respectively. This comparatively high separation ef®ciency of 59% can partially be ascribed to the fact that a part of these compounds are attached to the surface of the ®ne dust particles emerging from the waste incineration. Due to the highly ef®cient dust separation described above, these compounds are drained off together with the dust. Nevertheless, this mechanism is not a suf®cient explanation of the observed high separation ef®ciencies. Therefore, it is assumed that there is another sink for these compounds in the scrubber. During the operation of the compact wet scrubber,
samples of the packing material (polypropylene Hi¯ow rings) were taken after an operation of 7 weeks and 15 weeks from the ®rst and the second packed tower, and their PCDD/F content was analysed. Figure 6 gives a comparison of the PCDD/F contents of the packing material. In addition to the packing material of the compact wet scrubber, new packing material was also analysed which already shows up with very low content of PCDD/F. One should recognize that the axis of ordinates in Figure 6 is graded logarithmically in order to depict the very small concentration of the new material. A signi®cant enrichment of PCDD/F is noticed in the packing material used in the compact wet scrubber compared to the new material. The second stage has approximately 12 times greater contamination than the ®rst stage. The difference in the concentrations between both scrubber stages can only be explained by the different operating conditions, as the same packing material, polypropylene, has been used for both stages. Vogg et al.13 reported that polypropylene has a signi®cant temperature-dependent adsorption potential for PCDD/F. Therefore, the temperature difference of a few Kelvin between both scrubber stages is assumed to be the reason for the different adsorption levels. The measurement of the enrichment of the packing material after 15 weeks of operation shows that the PCDD/F-contents were increased by a factor of 3.3 (stage 1) and 2.4 (stage 2), respectively. Consequently, the second scrubber stage is still more contaminated than the ®rst one, but the average content is now only 8.6 times higher than that of the ®rst stage. This fact leads to the assumption that saturation of the packing material with these pollutants has begun. A further comparison of the relative PCDD/F-contents in the crude gas and in the packing material shows that the congener pro®le is quite similar. In any case, it can be stated Trans IChemE, Vol 79, Part B, March 2001
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Figure 6. PCDD/F-content of the packing material. Comparison of the ®rst and the second scrubber stage after 7 weeks of operation with new packing material.
that the packing material is signi®cantly contaminated with PCDD/F. Therefore, an additional sink for these pollutants has been found although its collecting capability highly depends on the temperature and is limited due to saturation effects. The contaminated material can easily be disposed of by burning in the waste incinerator, where organic compounds are completely destroyed by the high temperatures. CONCLUSIONS AND FUTURE PROSPECTS It can be derived from the long-term experiments performed that ®ne dust, inorganic halide compounds as well as polychlorinated dioxins and furans are separated very ef®ciently in the compact wet scrubber. The dust is almost completely separated by means of the venturi scrubbers, whereas the separation of the other mentioned pollutants takes place in the packed towers. During the experiments, PCDD/F was continuously enriched in the packing material. In order to use the packing material ef®ciently and speci®cally for the separation of PCDD/F, the mechanisms of the separation process, such as the in¯uence of the material and saturation effects, must be the subject of further investigations. Measurements of the aerosol generation by means of spontaneous phase transition were additionally carried out at the compact wet scrubber. They showed that aerosols could be generated in the wet scrubber under particular operating conditions. These particles are fractionated mainly in the range of 0.5 mm to 2 mm, and they are not retained by the demisters at the top of the scrubber. An idea to solve this problem may be to move the venturi Trans IChemE, Vol 79, Part B, March 2001
scrubber stage from the bottom to the top of the compact wet scrubber. Consequently, any possible generated aerosols emerging from the packings can now be separated ef®ciently. Additionally, the second packing can be used for a speci®c growth of the aerosols by cooling of the scrubbing liquid, for example. The planned further investigations also have to be directed towards the PCDD/F adsorption at the packing material in order to obtain more information about the fundamental mechanisms of the separation process. REFERENCES 1. 17. Verordnung zur DurchfuÈhrung des BImSchG (17. BImSchV) vom 23.11.1990. Bundesgesetzblatt 1 S. 2545. 2. Comfere, W., Reitz, P. and Wilop, A., 1994, Erfahrungen mit Rauchgasnachreinigungsanlagen unterschiedlicher Konzeptionen in Herten, Augsburg und Burgkirchen. GVC-VDI Gesellschaft Verfahrenstechnik und Chemieingenieurwesen (Ed.): Abfallwirtschaft Herausforderung und Chance (VDI-Verlag DuÈsseldorf), 233±250. 3. Nottrodt, A., 1994, Fortschrittliche Verfahren zur Abgasreinigung in Abfallverbrennungsanlagen. ThomeÂ-Kozmiensky, K. J. (Ed.), Thermische Abfallbehandlung (EF-Verlag, Berlin), 432±443. 4. Mayinger, F. and Lehner, M., 1995, Operating results and aerosol deposition of a venturi scrubber in self-priming operation, Chem Eng Proc, 34: 283±288. 5. Lehner, M., 1998, Aerosol separation ef®ciency of a venturi scrubber working in self-priming mode, Aerosol Science and Technology, 28: 389±402. 6. Tigges, K. D. and Mayinger, F., 1984, Experiments with highly ef®cient venturi scrubbers for aerosol separation from gases under multiplane water injection, Chem Eng Proc, 18: 171±179. 7. Leith, D. and Cooper, D. W., 1980, Venturi scrubber optimization, Atmo Env, 14: 657±664. 8. Barth, W., 1959, Grundlegende Untersuchung uÈber die Reinigungsleistung von Wassertropfen, Staub, 19: 175±180.
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9. Mayinger, F. and Neumann, M., 1978, Dust collection in venturi scrubbers, Ger Chem Eng, 1: 289±293. 10. Roberts, D. B. and Hill, J. C., 1981, Atomization in a venturi scrubber, Chem Eng Comm, 12: 33±68. 11. Lehner, M., 1996, Staubabscheidung im VenturiwaÈscher mit FluÈssigkeits-Selbstansaugung, PhD Thesis (Technische UniversitaÈt MuÈnchen, Munich, Germany). 12. Lane, W. R., 1951, Shatter of drops in streams of air, Ind Eng Chem, 43: 1312±1317. 13. Vogg, H., Kreisz, S. and Hunsinger, H., 1994, Wet scrubbersÐa potential PCDD/F source? Organohalogen Compounds, 20: 305±307.
ACKNOWLEDGEMENT The presented research project was part of the BayFORREST research
activities and was sponsored by the Bayerisches Staatsministerium fuÈr Landesentwicklung und Umweltfragen (Bavarian Ministry of Development and Environmental Protection) in co-operation with Rauschert Verfahrenstechnik, Steinwiesen. These ®nancial supports are gratefully acknowledged.
ADDRESS Correspondence concerning this paper should be addressed to Dr M. Lehner, Rauschert Verfahrenstechnik GmbH, Paul Rauschert Str 6, 96349 Steinwiesen, Germany. The manuscript was received 15 April 1999 and accepted for publication after revision 18 December 2000.
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