Evaluation of a Novel Reactorâ Biofilter System

752

Ind. Eng. Chem. Res. 2003, 42, 752-763

Evaluation of a Novel Reactor-Biofilter System Li Xiaobing, S. Farooq,* and Shekar Viswanathan† Department of Chemical and Environmental Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

A two-stage biofiltration system with a rusty iron reactor followed by a conventional biofilter to remove H2S and methyl ethyl ketone (MEK) from an air stream under ambient temperature and pressure was developed. Experimental results indicated that rusty iron efficiently catalyzed the oxidation of H2S by O2 in air. A properly designed reactor removed nearly all of H2S, and the biofilter was protected from acidification, which was expected if H2S was allowed to enter the biofilter. A model was developed to simulate the removal of H2S in a rusty iron reactor and to provide a basis for design and prediction. A comparative experiment showed that, in treating air containing both H2S and MEK, the performance of a two-stage biofiltration system was stable during the experimental period, while a single biofilter had declining efficiencies for H2S and MEK removal in the same period. Introduction Biofiltration is a simple and reliable waste gas cleaning technique. It is a method of employing microorganisms that have the ability to biodegrade volatile organic substances present in air streams. The microorganisms used are immobilized on a porous support, on which liquid biolayers are formed. The support material is placed in a packed-bed configuration. Contaminated air streams are passed through the bed, known as the biofilter, and the pollutants are transferred to the biolayers where they undergo biological destruction. While biofiltration has emerged as an attractive technique in the treatment of volatile organic carbon (VOC)-containing gas streams, it is not completely free of problems. One problem is that H2S is present in many VOC-containing discharge streams and is also an important target compound for removal. When the emitted gas streams contain H2S, a biofilter is often acidified and loses its function. Although biofilter has been originally applied to control H2S emission, a number of studies have reported H2S-associated problems.1-5 Second, the oxidation of H2S in the biofilter bed produces sulfuric acid and, sometimes, elemental sulfur. The optimal pH for most aerobic biological processes is usually in the range of 7-8.6 However, the presence of sulfuric acid results in declining pH, which in turn reduces the solubility of H2S in the aqueous phase, slows down the mass transfer of H2S in liquid, and inhibits the activity of microorganisms that degrade compounds other than hydrogen sulfide. Third, the accumulation of elemental sulfur may also result in clogging of the biofilter bed. Fourth, the life span of compost, which is the most widely used biofilter medium, is significantly shortened because of the presence of H2S in the gas stream.8 This is also a problem due to compaction, shortcircuiting, and increased back-pressure across the bed. Most published studies to date on biofiltration focus on either the removal of H2S or the removal of VOCs. A few deal with emissions containing both H2S and VOCs. * Corresponding author. Tel: (65) 6874 6545. Fax: (65) 6779 1936. E-mail: [email protected]. † Present address: National University, 11255 North Torrey Pines Road, La Jolla, CA 92037.

A single biofilter hardly works efficiently in treating both. To seek the solution for the above problem, Chitwood et al.1 evaluated a two-stage biofilter used for treating waste air polluted with H2S and VOCs. In their biofiltration system, two biofilters were connected in series. The first one, called acid gas biofilter (AGB), was packed with lava rock as support to overcome the problem of decay from acidification. It eliminated most H2S using Thiobacillus sp. that flourished under a lowpH environment and was adapted to sulfide oxidation as the energy source. The second one was a conventional wood chip biofilter that was responsible for removing VOCs and the rest of H2S. Their results indicated that the biofilter was effectively protected from acidification. Under steady state, the AGB and the total system had H2S removal efficiencies of 95.4% and over 99%, respectively. However, fluctuating input H2S concentrations in their experiments led to a poor daily average removal efficiency in the beginning until the microorganisms became acclimatized to the conditions. Fluctuations in the flow rate and pollutant concentration are common for gaseous emissions and waste effluent streams from many industries, in particular, emissions from wastewater treatment operations. Boon and Boon8 reported that H2S could be quickly oxidized if O2 were present in humid air with the aid of oxidized iron, under ambient temperature and pressure. The overall percentage removal of H2S was found to be more than 95%, regardless of the highly variable H2S feed concentrations ranging from 34 to 500 ppm. The reaction was instantaneous. They estimated that the life span of such a fixed-bed reactor would be around 10 years or even longer. This covers completely the entire life of a conventional biofilter. To our knowledge, there is no published study that combines an iron filter with a traditional biofilter for treating both H2S and VOCs present together in gaseous emissions. The present study was, therefore, undertaken to investigate the feasibility of using a novel biofiltration system in which an oxidized iron reactor is installed to remove H2S from an air stream containing VOC before the VOC is metabolized in a conventional biofilter. Methyl ethyl ketone (MEK) was used as a model VOC in a recently concluded biofilter study in this labora-

10.1021/ie020284r CCC: $25.00 © 2003 American Chemical Society Published on Web 01/14/2003

Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 753

Figure 1. Schematic diagram of the setup for the comparative biofiltration experiment.

tory.9,10 In that study, activated carbon and compost were separately used as support media for the growth of microorganisms. In this study, MEK was retained as a model VOC and compost was packed in the biofilter. This study attained the following objectives: (i) Reaction kinetics for the oxidation of H2S was established, and a model was developed to simulate the behavior of a rusty iron reactor used for the removal of H2S from air. (ii) The performance of the proposed iron reactorbiofilter combination was compared to that of a conventional biofilter performing without the protection of an iron reactor but otherwise operating under identical conditions. Materials and Methods Experimental Setup. A schematic of the experimental system is shown in Figure 1. The air stream Q1 was directly taken from a cylinder. This air stream had a very high H2S concentration. H2S-free compressed air was split into two streams. One stream was passed through two connected humidifiers (sparging containers filled with water). The flow rate of the stream leaving the humidifier was Q2. Another air stream was passed through a 500 mL sparging bottle containing 99.5% liquid MEK. The flow rate of the stream leaving the sparger containing MEK was Q3. The three streams were mixed together to form a total flow of QT, which was then evenly divided into two streams. One stream was introduced into a conventional biofilter (refer to the single biofiltration system in the Introduction). The other stream was introduced into the catalytic iron reactor-biofilter system connected in series, as shown in the diagram. The balance of flow between the two

systems was achieved by adjusting the two two-way valves, cv1 and cv2. Biofilters 1 and 2 were identical in size. The treated gas streams were finally bubbled into a chemical solution before they were released into the fume hood. Samples for the measurement of the concentration were drawn from several ports detailed below and shown in Figure 1: (i) For the H2S feed concentration, a sample was taken at port 3 by switching the three-way valve before the start of the experiment. (ii) For the H2S concentration at the exit of the rusty iron reactor, a sample was taken at port A. (iii) For the MEK feed concentration, a sample was taken at port B. (iv) Samples taken at ports 1 and 2 were for the measurement of the H2S and MEK concentrations in a treated air stream coming out of biofilters 1 and 2, respectively. Note that the rusty iron reactor was placed before biofilter 1. Three mass flow controllers were used to control the flows of Q1 (Brooks Instrument, 5850E, 0-200 SCCM), Q2 (Alicat Scientific, 0-25 SLM), and Q3 (Brooks Instrument, 5850E, 0-20 SCCM). The flow controllers were calibrated using a digital bubble flowmeter. During calibration, the gas flow was diverted to the vent using the appropriate three-way valve. Rusty Iron Reactor. Two round columns made of Plexiglas were mainly used in the rusty iron reactor study. (A reactor with a rectangular cross section, which will be discussed in a later section, was also used in some experiments.) Named as reactors A and B, the internal diameters of the round column reactors were 3 and 1.9 cm, respectively. Mild steel sheets were cut

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Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003

into small chips and then twisted in order to create enough void space when packed in the bed. The twisted steel chips used were from two batches of different sizes. Batch 1 chips were approximately 15 mm × 4 mm × 0.5 mm, while batch 2 chips were approximately 20 mm × 6 mm × 0.5 mm. The bed voidage varied from 0.84 to 0.86. The packing height was flexible in reactors A and B (i.e., the height varied depending on the amount of the added catalyst). The chips were fully oxidized by heavy outdoor exposure to air and rain for 5-10 days. The following equations were used to calculate the iron catalyst volume and available surface area for any given experimental run:

catalyst volume (cm3) )

weight of catalyst used (g) 3

density of iron ()7.8 g/cm )

surface area (cm2) ) catalyst volume (cm3) 2 thickness of a chip ()0.05 cm) The surface area calculated from the above equation neglected the contribution from the thickness of the chips (0.05 cm). The contribution was about 13.7% and 9.8%, respectively, of the surface areas calculated from the geometric dimensions of the chips from batches 1 and 2. A conservative estimate was considered to be more reasonable to account for the loss of available area due to contact between the twisted chips. Biofilter. The Plexiglas biofilter columns had an internal diameter of 5 cm and a height of 50 cm. The support medium was compost (Tholander biomass Nr.BM4), obtained from Tholander Biofilter, Gmbh, Germany. The average wet density of the material was 620 kg/m3. The voidage of the packing was calculated to be 0.42. A mixed culture was used. Some amount of a dried mixed culture for MEK degradation supplied with the compost was subjected to the following enrichment procedure for revival and multiplication: 50 cm3 of a dried bioculture was mixed with 1 L of water, 10 g of powder sugar, and 10 g of peptone. This mixture was then fermented for 24 h with continuous aeration. The pH value of the resulting solution was finally adjusted to 7 before it was evenly distributed over the surface of the biomass. No additional mineral nutrient source was added after this preparation stage. Gases. Premixed air and H2S with H2S concentrations of 997, 3075, and 4132 ppm, respectively, were obtained from Soxal Pte Ltd., Singapore. Compressed air, which was free of H2S and MEK, was humidified by conducting it through two water-filled sparging containers that were connected in series. It was then mixed with two air streams containing high concentrations of H2S and MEK, respectively. The mixed air flow was then split into two equal streams, one of which was sent to the stand-alone biofilter and the other to the iron reactor-biofilter systems. Humidity of the Air Stream. The detail pertinent to humidification is discussed in a later section (transient model). It is known that a sufficiently humid air stream is essential for rapid simultaneous regeneration of the spent rusty iron catalyst.8 In this study, 95-97% relative humidity was achieved in the experiments by connecting two humidifiers in series. Analytical Methods. A portable gas chromatograph (GC; 10S Plus Photovac) equipped with a photoioniza-

tion detector (PID) and a packed column was used to measure the H2S concentration. The oven temperature was set at 35 °C. Ultrahigh-purity-grade air at 12 cm3/ min was used as the carrier gas. A Hewlett-Packard 4860 GC equipped with a flame ionization detector (FID) and a siloxane capillary (HP-PLOT Q) was used for measuring the MEK concentration. The oven temperature was maintained at 200 °C. The gas flow rates were 400, 40, and 40 cm3/min for air, hydrogen, and helium, respectively. Modeling H2S Reaction in the Rusty Iron Reactor The removal of H2S in the iron reactor is assumed to be a combination of the following two steps: (i) mass transfer from the bulk stream to the surface of the catalyst; (ii) reaction at the catalyst surface. Overall Reaction Rate Model. The following assumptions were made to arrive at the intrinsic reaction rate model: (i) A first-order irreversible reaction is used. (ii) Catalytic sites lose their function after oxidation of H2S but resume after regeneration. When some of the catalytic sites become inactive, the concentration of the active sites becomes less. For a fresh catalyst, active sites are evenly distributed on the surface of the rusty iron. The distribution of catalytic active sites along the fixed bed is assumed to remain unchanged for a short period from time zero. (iii) The reaction takes place only on the surface of the catalyst. When some catalytic sites become inactive, the oxidation of H2S takes place on the available fraction of the total surface. This is called active surface, which is assumed to be directly proportional to the concentration of the active sites. For a fresh catalyst, the total surface is taken as active. (iv) The oxidation of mild steel does not change its surface area. (v) Oxidation of steel surface does not change the overall density of mild steel. (vi) The area at the contact points among catalyst particles packed in the reactor bed is taken as negligible. (vii) There is no concentration gradient in the radial direction or along the width of the reactor. (viii) The catalyst in the early period of operation is in a pseudo steady state in which the active gas-solid contact area is the same as that at time zero. On the basis of the experimental results, it was determined that the removal rate was very fast, yet the rate of change of the active surface was much slower compared to the residence time. This provides the basis for assumption viii and is fully supported by the experimental result shown in Figure 2. The experimental details are given in the figure caption. The first three points in Figure 2 are (60, 8.607), (150, 8.723), and (300, 8.868). The maximum relative difference among the three H2S exit concentrations is about 3%. Thus, a safe quantification of the early period mentioned in assumption viii was taken to be 50 times the residence time from the start when H2S exit concentrations measured had less than 3% relative difference. The two-step process to represent the removal of H2S, proposed earlier, is schematically shown in Figure 3. The concentration in the bulk gas phase is Cg. All subprocesses are sequentially described as follows.


rH2S ) r2. Hence,

KCg ) kg(Cg - mC′s) ) krC′s

(5a)

When C′s is eliminated, the relation among K, kg, and kr is obtained:

1 1 m 1 1 ) + ) + K kg kr kg k′r

(5b)

Hence, the overall reaction is given by Figure 2. H2S concentration at the reactor exit during the early part of breakthrough (reactor B; chips from batch 1; Vb0 ) 13.4 cm3; H2S inlet concentration ) 19.4 ppm; τ0 ) 0.54 s; A0 ) 5.53 cm2/cm3).

-r ) KCg where K is related to the gas film mass-transfer coefficient and intrinsic reaction rate constant according to eq 5b. The mass-transfer coefficient can be calculated from the available correlation. The following mass-transfer correlation11 was used for the calculation of the masstransfer coefficient for the twisted chips packed in the reactor:

( ) ()

d eu 0 ) 1.17 ν u0 kg

Figure 3. Schematic of the assumed steps of the H2S removal process.

(i) From the bulk of the gas phase to the interface on the gas side, the mass-transfer rate is given by

-r1 ) kg(Cg - C′g)

(1)

where kg is the mass-transfer coefficient, Cg is the concentration of H2S in bulk gas phase, and C′g is the concentration of H2S at the gas-solid interface. (ii) At the interface, the concentration, C′g, on the gas side is in equilibrium with the interfacial concentration, C′s, on the solid side:

C′g ) mC′s

(2)

where m is the distribution coefficient. (iii) On the surface of oxidized iron, H2S is immediately oxidized by a first-order process:

-rH2S ) krC′s

(3)

where kr is the intrinsic reaction constant for H2S oxidation. For the overall process:

-r2 ) K(Cg - C/g)

(4a)

In the above equation, K is the overall rate constant, C/g ) mCs, where Cs is the concentration of H2S in the rusty iron phase. The experimental result will be presented in a later section to show that rusty iron did not seem to adsorb H2S. This means that Cs ) C/g ) 0. Hence, eq 4a becomes

-r2 ) KCg

(4b)

Because there is no accumulation at the interface, r1 )

-0.42

-0.67

ν D

(6)

In the above equation, u0 is the superficial velocity, ν is the kinematic viscosity, D is the molecular diffusivity, and de is the equivalent diameter of a sphere that has the same surface area as that of the actual particle. The equivalent diameter is given by

de ) xa/π

(7)

where a is the surface area of the actual particle. For the twisted chips, which were made from twisted pieces, a was calculated from the geometric dimensions given in an earlier section. Diffusivity was calculated using the ChapmanEnskog equation.12 The value of ν under ambient temperature (21-24 °C) was obtained from the literature.13 Pseudo-Steady-State Model. Having estimated the overall reaction model, the next step is to develop the reactor model for establishing the value of k′r. A pseudo-steady-state approach is proposed in which the reaction is analyzed based on data obtained within a very short time from the start. In this short period, the active catalyst area is assumed to remain constant at the value of the fresh catalyst. Mass balance over a differential volume segment of the reactor dVb gives

KCgA0 dVb ) Q[Cg - (Cg + dCg)]

(8)

where A0 represents the active surface area per unit empty bed volume at time zero, Vb is the reactor volume variable, and Q is the volumetric gas flow rate. Equation 8, upon rearrangement and integration from the reactor inlet to the exit assuming A0 to be constant, yields

Cge ) Cgi exp(-KA0τ0)

(9)

In the above equation, Cgi and Cge are H2S concentrations at inlet and outlet, respectively, τ0 ()Vb0/Q) is the nominal residence time, and Vb0 is the total volume of the reactor. Recall that the assumption of constant A0 is valid for a short time from time zero.

756


Transient Model. The pseudo-steady-state model developed in the previous section is applicable for a very short time from the start. A transient model is necessary to simulate the complete behavior from the beginning until the final steady state is reached. The following overall reaction represents iron-catalyzed oxidation of H2S: Fe2O3‚H2O

2H2S + O2 98 2H2O + 2S

(r1)

H2S-contaminated air that is sufficiently humid guarantees the following sequence of simultaneous consumption and regeneration of iron oxide, which at steady state gives the above overall reaction.8

2Fe2O3‚H2O + 6H2S f 2Fe2S3 + 8H2O

(r2)

2Fe2S3 + 3O2 + 2H2O f 2Fe2O3‚H2O + 6S (r3) Experimentally, it was observed that there was a long unsteady period before the reactor reached a steady state. This implied that initially the consumption of active iron sites [reaction (r2)] dominated over the regeneration of consumed sites [reaction (r3)]. With an increase in the Fe2S3 concentration, the rate of regeneration increased. The rate of regeneration ultimately became equal to the rate of consumption when the reactor reached its steady state and the H2S concentration became constant. The following assumptions are made for developing a transient model for the reactor: (i) There is no concentration gradient in the radial direction in the round column reactor. (ii) The H2S concentration changes only with time and the height of the reactor. (iii) It is assumed that H2S does not penetrate rusty iron, and these two reactions take place only on the surface. Only the catalytic sites on the surface are affected. Therefore, the surface of rusty iron is very important. When the removal of H2S has proceeded to some extent, part of the total surface is occupied by Fe2S3 and becomes inactive. Eventually, a balance between the active and inactive surfaces is achieved. In reality, there may be a gradient of active area along the column height. (iv) Because the concentration of O2 is high when compared with that of H2S, it can be assumed that O2 is always in excess and that the rusty iron consumption rate is directly related to the amount of H2S oxidized. The more H2S is oxidized, the more Fe2O3‚H2O is consumed. (v) The regeneration rate is affected only by the amount of Fe2S3 formed, which in turn is directly proportional to the inactive surface. Subject to the above assumptions, the following equations constitute the transient model. H2S mass balance in the reactor is given by

∂Cg ∂Cg + KAtCg ) 0 +Q ∂t ∂Vb

(10)

subject to the following initial and boundary conditions:

at t e 0, Cg ) 0 (everywhere in the reactor) Vb ) 0, Cg ) Cgi

Figure 4. Response to the rusty reactor exit after the withdrawal of H2S from the reactor feed gas (reactor B; chips from batch 1 Vb0 ) 59.25 cm3; H2S inlet concentration ) 36.5 ppm; τ0 ) 2.96 s, A0 ) 6.17 cm2/cm3).

The rate of change of the active surface area at any location in the reactor is given by

dAt ) k2(A0 - At)n2 - k1(KAtCg)n1 dt

(11)

at t e 0, At ) A0 (everywhere in the reactor) KAtCg denotes the volume of H2S oxidized per unit time per unit volume of the reactor at a given location. Although it is possible that the rate of active surface area consumption is first order with respect to KAtCg, such an assumption is not obvious from reaction (r2), which governs the reaction leading to the consumption of active surface area. Similarly, it is not obvious from reaction (r3) that the regeneration is first order with respect to the inactive area (A0 - At). Hence, the exponents, n1 and n2, are used. In the above equations, k1, k2, n1, and n2 are the unknown constants, At is the local active surface area per unit empty reactor volume and any time, and t is the time variable. The other variables have already been introduced. The theoretically predicted Cg, as a function of t (from 0 to ∞) and Vb (from 0 to Vb0), can be found by simultaneously solving eqs 10 and 11. Results and Discussion Adsorption of H2S by Rusty Iron. To ascertain if the rusty iron had any adsorption capacity for H2S, a desorption experiment was carried out and the result is shown in Figure 4. The experimental details are given in the figure caption. A sample was taken at the reactor exit 2 min after H2S was withdrawn from the feed stream. In these 2 min (∼40τ0), the H2S concentration dropped from 2.86 to 0.25 ppm and within another 2 min fell below the measurable limit. If the rusty iron had any reversible adsorption capacity, then the H2S concentration at the exit would have gradually dropped to zero after its withdrawal from the feed. The very rapid drop to practically negligible concentration is, therefore, a clear indication that the rusty iron did not have any appreciable reversible adsorption capacity for H2S. Hence, in this study, H2S adsorption on rusty iron was taken as negligible. Verification of the Role of Oxygen in Regenerating Reduced Rusty Iron. An experiment was carried out to verify the contribution of oxygen in air to the regeneration of the rusty iron catalyst. In this experiment, a mixture of H2S and humidified air was first passed through reactor B packed with twisted chips from batch 1. This was continued until some H2S

Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 757 Table 1. Experimental Conditions and Results for the Determination of the H2S Oxidation Rate Constant with Twisted Catalyst from Batch 1a wt of catalyst (g)

empty reactor volumeb (cm3)

A0 (cm2/cm3)

flow rate (cm3/min)

τ0 (s)

H2S inlet (ppm)

H2S exit (ppm)

Kc (cm/s)

kgd (cm/s)

k′r e (cm/s)

average k′r (cm/s)

16.2 (average of all of the runs)


6.2


6.2

0.95 0.96 1.11 1.15 1.19 1.47 1.53 1.86 2.76 3.20 1.37 1.52 1.66 1.77 1.96 1.99 2.25 2.59 2.71 3.32

21.79 22.23 25.66 22.03 27.41 28.30 29.41 26.96 40.07 61.51 35.58 47.51 65.22 55.31 66.65 67.47 76.33 203.91 213.73 200.30

4.76 4.86 4.59 3.84 4.46 3.34 3.15 1.96 0.91 0.98 4.11 4.68 5.37 3.81 3.27 3.62 2.98 5.13 4.10 2.01

0.262 0.257 0.252 0.248 0.248 0.236 0.237 0.230 0.223 0.210 0.257 0.248 0.244 0.246 0.25 0.239 0.234 0.231 0.237 0.225

1.665 1.646 1.514 1.489 1.457 1.287 1.259 1.126 0.895 0.820 1.701 1.600 1.518 1.465 1.378 1.368 1.274 1.174 1.143 1.016

0.311 0.304 0.302 0.298 0.300 0.290 0.293 0.289 0.298 0.283 0.303 0.294 0.291 0.296 0.305 0.290 0.288 0.289 0.300 0.290

0.296


850 833 722 701 676 545 525 433 291 251 882 794 725 682 613 606 566 466 444 363

a The catalyst amount used each time was kept fixed as much as practically possible. b Empty reactor volume ) cross-sectional area of reactor B × packed height. c From eq 9. d From eq 6. e From eq 5b.

Figure 5. Verification of spent rusty iron regeneration with humidified air (reactor B; chips from batch 1; Vb0 ) 50.3 cm3; H2S inlet concentration ) 100 ppm; τ0 ) 1.72 s, A0 ) 6.27 cm2/cm3).

breakthrough was monitored at the exit. In the second step, H2S was withdrawn from the feed and only humid air was flushed through the bed for 3 days. Because this experiment was done before establishing the regeneration rate, the flushing time was kept arbitrarily long to ensure complete reoxidation of the reduced rusty iron. In the third step (i.e., after flushing for 3 days), H2S was reintroduced in the feed stream at the same concentration level and breakthrough was monitored at the exit. Steps two and three were repeated, but humidified nitrogen was used in place of humidified air for flushing the bed. The experimental results are shown in Figure 5. The experimental conditions are detailed in the figure caption. It is clear that the catalytic function was regained after flushing the bed with humidified air. It is also clear that nitrogen flushing had no regeneration effect because the first H2S exit concentration after this step was close to the final ones recorded in the first and third steps. Validation of the Pseudo-Steady-State Reactor Model. A set of experiments was carried out to validate the proposed pseudo-steady-state model for the removal of H2S over a short contact time, during which A0 was assumed to remain constant. Reactor A and twisted iron chips from batch 1 were used. In these experimental runs, the H2S feed concentration was varied but the flow rate was kept unchanged so that residence time τ0 would

Figure 6. Experimental results showing linearity of outlet vs inlet H2S concentration according to eq 9 (reactor A; chips from batch 1; Vb0 ) 13.4 cm3; τ0 ) 0.54 s, A0 ) 5.53 cm2/cm3.)

remain constant. For every run, the same amount of fresh rusty iron (14.45-14.47 g) was used and the H2S exit concentration was measured within a short time from time zero. The use of fresh catalyst in every run guaranteed that A0 would be the same in all of the runs. The plot of exit vs inlet H2S concentration according to eq 9 is shown in Figure 6. The common experimental conditions are included in the figure caption. The linearity of the plot is consistent with the proposed reactor model. This result is in agreement with those of other published studies.8,14 Determination of the Intrinsic Reaction Rate Constant. According to the proposed model, the intrinsic reaction rate constant, k′r, is related to the overall rate constant, K, by eq 5b. k′r is the fundamental reaction parameter and is independent of flow and catalyst shape. In addition to the twisted rusty iron chips (detailed in the section on the rusty iron reactor), flat 2 cm (width) × 15 cm (length) × 0.05 cm (thickness) rusty iron strips were also used as the catalyst to vary the shape and hence the contributions of mass-transfer resistance across the external gas film. These strips were placed in a Plexiglas column that was 15 cm high and had a 2 cm × 4.75 cm rectangular cross section. Spacers were used to maintain equal spacing between the strips, the magnitude of which depended on the number of strips used. The surface area available for the reaction was

758


Table 2. Experimental Conditions and Results for the Determination of the H2S Oxidation Rate Constant with Flat Iron Plates no. of steel sheets

empty reactor volumea (cm3)

A0b (cm2/cm3)

flow rate (cm3/min)

τ0 (s)

H2S inlet (ppm)

H2S exit (ppm)

Kc (cm/s)

kgd (cm/s)

k′r e (cm/s)

average k′r (cm/s)

8

142.5

3.368

142.5

1.684

2.80 3.41 4.17 5.34 6.95 9.19 11.25 13.13 15.18 4.34

20.38 18.83 29.53 50.53 102.13 208.70 392.80 678.63 20.38 9.40

4.67 3.31 2.94 2.94 2.84 3.9 4.07 4.53 4.68 3.36

0.156 0.151 0.149 0.128 0.121 0.116 0.105 0.103 0.098 0.141

0.340 0.308 0.278 0.246 0.216 0.187 0.169 0.157 0.146 0.273

0.288 0.298 0.319 0.268 0.278 0.302 0.276 0.302 0.297 0.291

0.292

4

3050 2510 2050 1600 1230 930 760 651 563 1970

0.291

Empty bed volume ) rectangular cross-sectional area × length of the flat plate ) 2 cm × 4.75 cm × 15 cm ) 142.5 The available surface area was calculated from actual dimensions, but the contribution from thickness was neglected because these faces were attached to the reactor walls and were not exposed to gas flow. c From eq 9. d From eq 12. e From eq 5b. a

cm3. b

directly calculated from the known dimensions of the strips (see the footnote in Table 2 for details). In every experiment, the fresh catalyst was used so that the available surface area (per unit volume of bed) could be taken as equal to the active area at time zero, A0. The flow rate and inlet H2S concentration were varied, and the corresponding H2S concentration at the outlet was measured within 50 times of the residence time, as discussed in a previous section. The mass-transfer coefficients were calculated from the available correlation. Equation 6 was used for the twisted iron chips. The following equation11 was used for the groups of flat plates:

kgL Lu 1/2 ν 1/3 ) 1.328 D ν D

( ) ()

(12)

where L is the length of flat plate and u is the average linear velocity. Other symbols have the same meaning as those in eq 6. The above equation was found to give satisfactory results for experiments in which different numbers of plates were used. K for each run was calculated from eq 9. k′r for each run was then calculated from eq 5. The operating conditions and results are summarized in Tables 1 and 2. Determination of the Parameters of the Transient-State Model. To obtain the unknown parameters k1, k2, n1, and n2 of the transient model presented earlier, the coupled differential equations (10) and (11) were solved by the method of orthogonal collocation. Equations 10 and 11 were made dimensionless by introducing the following dimensionless variables:

C)

Cg Vb Vb0 At t , At ) , Vb ) , τ ) , τ0 ) Cgi Vb0 τ0 Q A0

The following dimensionless form of eq 10 was obtained:

∂Cg ∂Cg )- (KA0τ0)CgAt ∂τ ∂V

(13)

b

KA0τ0 is a dimensionless group of constants. The dimensionless boundary condition is Cg ) 1 at Vb ) 0. The dimensionless initial condition is Cg ) 0 at τ ) 0 (everywhere in the reactor).

Table 3. Experimental Details of the H2S Oxidation Runs Conducted To Extract Transient Model Parameters for the Rusty Iron Reactora run no. 1 catalyst weight (g) packed height (cm) empty reactor volume, Vb0 (cm3) bed porosity A0 (cm2/cm3) flow rate (cm3/min) τ0 (s) H2S inlet concentration (ppm)

2

3

4

59.19 8.0 56.55

59.6 7.5 53.02

53.38 7.3 51.60

59.97 7.8 55.13

0.87 5.3 1805 1.88 40.29

0.86 5.7 940 3.38 50.25

0.87 5.23 1200 2.58 44.68

0.86 5.51 1070 3.09 59.35

a K values used in the simulations were estimated from eq 5b using k′r from Table 1 and kg values calculated used eq 6.

In the dimensionless form, eq 11 becomes

dAt k2τ0 k1τ0 ) [A0(1 - At)]n2 (A C )n1(KAtCg)n1 dτ Vb0A0 Vb0A0 0 gi (14) where k2τ0/Vb0A0 and (k1τ0/Vb0A0)(A0Cgi)n1 are dimensionless groups of constants. The dimensionless initial condition is At ) 0 at τ ) 0 (everywhere in the reactor). Four experiments were carried out to collect the H2S exit concentration versus time data. Reactor A and twisted iron chips from batch 2 were used. The experimental conditions are detailed in Table 3. Equations 13 and 14 were collocated in space (i.e., reactor volume dimension), which gave rise to a system of ordinary differential equations (ODEs) in the time domain (Finlayson16). The collocated equations are given in the appendix. Fifteen internal collocation points were used. The system of ODEs was integrated using Gear’s integration method to obtain Cg and At at the collocation points along the bed as a function of time. The four model parameters k1, k2, n1, and n2 were obtained by minimizing the sum of the squares of residuals between the experimental and theoretical transient exit concentrations of H2S. Four sets of experimental runs were simultaneously fitted to the model in order to increase the reliability of the extracted parameters. The subroutine DBCONF provided in the IMSL17 was used for the optimization. The subroutine uses the quasi-Newton method and a finite difference gradient. This analysis indicated that there was no distinct global minimum for the problem. Depending on


Figure 8. Profiles of (a) dimensionless active surface area (the lines represent time in minutes in the following order: 10, 30, 50, 70, 100, 150, and ∞) and (b) dimensionless H2S concentration (the lines represent time in minutes in the following order: 0, 30, 50, 70, 100, 150, 300, and ∞) in an iron reactor. The experimental conditions correspond to those of run 1 in Table 3.

by fixing n1 ) n2 ) 1. The optimum values of k1 and k2 thus obtained are shown in eq 16. The corresponding residual is 944.

dAt ) 0.1251 × 10-3(A0 - At) - 0.467 × 10-4(KAtCg) dt (16)

Figure 7. Comparison between experimental and model results for transient response leading to steady state (see the text for the definitions of models 1 and 2).

the initial guesses of the four parameters, the optimization program converged at several local minima. As a result, a manual search was conducted by systematically varying all of the four unknown parameters over a wide range by making small increments. The combination of parameters that gave a minimum residual from the systematic manual search was used as the initial guess of the optimization subroutine (DBCONF) to obtain the final parameter values shown in eq 15.

dAt ) 0.1236 × 10-3(A0 - At)0.99738 dt 0.5442 × 10-4(KAtCg)0.93632 (15) The residual corresponding to the above parameters is 911. Because the values of n1 and n2 naturally came out to be close to 1, a second optimization was carried out

The model predictions are compared with the experimental results in Figure 7. Equations 10 and 15 constitute model 1 while eqs 10 and 16 constitute model 2. Profiles of the Active Surface Area and Concentration. Both the active surface area and concentration are functions of the axial position and time. Simultaneous integration of the coupled system of ODEs resulting from the collocated forms of eqs 10 and 14 gave a dimensionless active surface area (At) and a dimensionless H2S concentration (Cg) at the collocation points along the reactor length as a function of time. Representative results corresponding to the experimental conditions of run 1 are shown in Figure 8. Comparison between Model Prediction and Experimental Results at Steady State. Eight experiments were carried out to test the accuracy of the steady-state prediction by the transient model. The experimental conditions, H2S exit concentrations, and predictions are detailed in Table 4. The percentage deviation of the model prediction is relatively large when it is used to predict H2S exit concentrations lower than 1 ppm. Above 1 ppm, the predictions are within 10% on the average. Predictions using model 1 are mathematically better in all cases compared to those from model 2. The model can be effectively used for the design of a new reactor to meet treatment requirements or perfor-

760


Table 4. Experimental Details of the H2S Oxidation Runs Conducted To Evaluate the Steady-State Performance Prediction by the Transient Modela run no. catalyst weight (g) empty reactor volume, Vb0 (cm3) bed porosity A0 (cm2/cm3) flow rate (cm3/m) τ0 (s) H2S inlet concentration (ppm) experimental exit concentration (ppm) prediction by model 1 (ppm) percentage deviation from model 1 prediction by model 2 (ppm) percentage deviation from model a

1

2

3

4

5

6

7

8

137.93 129.36 0.86 5.41 945 8.21 50.01 0.45 0.181 -59.7 0.154 -65.7

137.93 129.36 0.86 5.41 1335 5.81 49.70 2.51 2.264 -9.9 2.019 -19.6

67.00 60.79 0.86 5.59 940 3.88 33.44 3.96 3.481 -12.1 3.235 -18.3

72.20 65.03 0.86 5.55 1340 2.91 40.01 11.80 11.575 -1.9 11.219 -4.9

59.60 53.02 0.86 5.69 940 3.38 50.25 13.68 14.321 +4.7 13.934 -1.9

53.38 51.60 0.87 5.23 1200 2.58 44.68 20.31 19.007 -6.4 18.623 -8.3

59.19 56.55 0.87 5.30 1805 1.88 40.29 24.16 21.120 -12.6 20.834 -13.8

59.97 55.13 0.86 5.51 1070 3.09 59.35 25.36 24.601 3.0 24.209 -4.5

K values used in simulating these runs were estimated from eq 5b using k′r from Table 1 and kg values calculated used eq 6.

Figure 9. Breakthrough of H2S and MEK in the catalytic rusty iron reactor.

mance evaluation of an existing reactor under the given operating conditions. In deducing the transient model equations, we did not take into account the effect of the deposition of elemental sulfur produced by the reduction of H2S on the removal efficiency of the catalytic iron reactor. Any observable effect of sulfur was not evident during the several months of the experimental period. Preparatory Experiments for the Biofiltration System. One experiment was conducted to examine the removal of MEK and hydrogen sulfide and their mutual influence when humidified air stream containing both H2S and MEK was passed through the rusty iron reactor. In the experiment, the H2S inlet concentration was ∼59 ppm. After the exit concentration reached steady state, 121 ppm of MEK was introduced. Both H2S and MEK exit concentrations were measured. The experimental results are shown in Figure 9. Clearly, the catalytic iron reactor did not remove MEK because the exit concentration was equal to that in the feed. Because the exit H2S concentration was not affected by the addition of MEK, it is also clear that MEK did not influence the oxidation of H2S in the rusty iron reactor. Another experiment was conducted to confirm that both biofilters that were prepared had biodegradability consistent with the results from other published studies and the two had the same performance. The two biofilters were connected in parallel. Only MEK was used in this experiment. The dimensionless exit MEK concentrations approaching steady state are shown in Figure 10. The removal efficiency was ∼47% in the two biofilters. In a previous study9 conducted in this laboratory, the

Figure 10. MEK exit concentrations from two identical biofilters connected in parallel showing the approach to steady state (internal diameter of the biofilter column ) 5 cm; packed height ) 50 cm; packed weight of the wet bed media ) 0.35 kg; gas flow rate to each biofilter ) 1080 cm3/min; concentration of MEK in the feed )120 ppm).

removal efficiency under similar conditions was in the range of 46-47%. The performance in the present study was indeed close to that observed previously. The results also indicated that the two biofilters had approximately the same biodegradability. Their back pressures were also found to be close. In the experiments, a stratification phenomenon was clearly observed. Ten to fifteen days after start-up of the biofilters, the water content was stratified along the biofilters. The flow was upward and the bed media close to the bottom appeared dry while significant water condensed on the top of the biofilters and in the tubes connected to the outlets of the biofilters. No condensed water was found at the influent ports. It was observed that the water content gradually increased along the flow direction. A possible explanation for this phenomenon is given as follows: (i) Temperature at the bottom of the bed might have been slightly higher than in that the other parts because the reaction is exothermic. Therefore, the relative humidity close to the inlet was comparatively low. The biodegradation follows the reaction18 given as follows: microorganisms

MEK + O2 98 CO2 + H2O + heat (ii) The population density of bacteria in the influent region, where food (MEK) was most plentiful, was highest. Therefore, the reaction rate was the highest, and a heat was produced there. (iii) Along the flow direction, more and more water was produced and was brought upward. Condensation


Figure 11. Dimensionless H2S and MEK exit concentrations from the two biofilters during the comparative study (details of the biofilters are the same as those in the caption of Figure 10; flow rate to each biofilter ) 550 cm3/min; H2S feed concentration ) 65.5 ppm; MEK feed concentration ) 102 ppm; rusty iron reactor A was used; Vb0 ) 119.5 cm3; τ0 ) 13 s; A0 ) 5.37 cm2/cm3).

started to form once the moisture content of the gas stream reached the saturation level. Comparative Experiment. In the comparative study, the two biofilters were operated continuously for 40 days. The flow rate to each biofilter was 550 cm3/min. The MEK feed concentration was 102 ppm. Before the introduction of an air stream containing H2S, the MEK exit concentrations were found to be 9.7 ppm from biofilter 1 and 1.8 ppm from biofilter 2. In the region that gave very high removal efficiency, it was not possible to operate the two beds to give identical performance. Although the same amount of compost taken from the same batch was packed in both columns, it was certainly not possible to ensure the same packing density in both. This minor difference in packing was significant because it noticeably influenced the performance when operated at high removal efficiency. A rusty iron reactor, which was later connected to biofilter 1 for removing H2S, was prepared. H2S was directly introduced in biofilter 2. Before it was subjected to H2S load, the MEK concentration was somewhat lower at the outlet of biofilter 2. Humidified air containing H2S and MEK was fed at the same time at equal rate to the two-stage reactorbiofilter system and to a single biofilter. As previously mentioned, the two biofilters were identical in size. The comparative experiment was run for 40 days. The H2S and MEK feed concentrations in the feed were 65.5 and 102 ppm, respectively. Dimensionless concentrations of H2S and MEK at the exits of the rusty iron reactor, biofilter 1, and biofilter 2 are plotted against time in parts a and b of Figure 11, respectively. No H2S was detected at the exit of the biofilter 1 during the entire period of the experiment.

(i) H2S Removed by the Rusty Iron Reactor. According to Figure 11a, the H2S exit concentrations from the rusty iron reactor were quite stable. This was expected because it had been shown earlier that the iron reactor reached a steady state. The exit concentrations varied between 0.06 and 0.07 ppm. The model developed above suggested a value of 0.01 ppm, but it was discussed that the percentage deviation of the model prediction was relatively higher when the exit concentration was less than 1 ppm. In the very low concentration level, it is perhaps more meaningful to judge the model by absolute deviation. From this perspective, the prediction of the model is quite satisfactory. (ii) H2S Removed by Biofilter 2. Although biofilter 2 was designed to remove MEK, it was still capable of removing H2S. The H2S concentration measured at the exit of biofilter 2 reached 14.6 ppm within 2.5 h after the introduction of H2S but then slowly decrease to a very low value before beginning to gradually rise again after about 400 h. (This observation is based on the actual coordinate values in the concerned region. It is not clear from Figure 11a because of the very long time scale of the complete experiment.) A possible explanation for this profile may be that some H2S degrading bacterial strain might have been present in the mixed culture, which took time to get acclimatized and start biodegrading. The second rise in the H2S concentration at the exit of biofilter 2 can perhaps be attributed to the acidification of the bed media. It is known from the literature18 that the mass transfer of H2S slows down because of the decrease of the pH in the liquid film. The acidification was confirmed by measuring the pH in the two biofilters using pH paper. Material samples were drawn from the side tabs located at 26, 38.5, and 50 cm from the inlet. While the material in biofilter 1 did not show any change from the starting pH of 7, the pH of the material in biofilter 2 dropped to ∼1.5 (1.3 to 1.7 from inlet to exit). (iii) MEK Removed by the Biofilters. The following discussion is based on Figure 11b. Because the two biofilters were fed with a higher MEK concentration (360 ppm) in the preparatory experiments, the population of bacteria biodegrading MEK was higher than that necessary for a feed containing ∼100 ppm MEK. When MEK with a lower concentration was introduced, some part of the bacterial population was affected because of insufficient food supply. However, the impact on the population may not have resulted in a quick and sudden destruction. The population might have decreased slowly. Hence, the MEK concentration at the exit of biofilter 1 slowly reached its steady state in about 500 h. It is more important to note that the exit concentration was stabilized at a level that was consistent with the result of a previous study.9 It appears that 0.06-0.07 ppm H2S that entered biofilter 1 did not affect the performance of the biofilter for MEK biodegradation in the experiment period of 40 days (nearly 1000 h). On the other hand, without the protection of an iron reactor, the performance of biofilter 2 in biodegrading MEK continuously declined. The H2S exit concentration went up from ∼2 to ∼13 ppm immediately after introduction of H2S and then returned to the previous steadystate value (∼2 ppm) in about 72 h. It appears that the MEK-biodegrading bacterial population was disturbed by the introduction of H2S but regained the previous level of activity in a few days. Biofilter 2 performed

762


steadily at that level for nearly 150 h, after which the MEK exit concentration rose again and reached 95% of the inlet concentration when the experiment was stopped after 40 days of operation. H2S was also simultaneously biodegraded in biofilter 2 during this period. It has already been discussed that, with the progress of the biodegradation of H2S, more and more sulfuric acid was produced and the bed gradually became acidic. The acidic environment is unfavorable for MEK-biodegrading bacteria. The colony of MEK-biodegrading bacteria was pushed away in the direction of flow by the progressing acid front. This is equivalent to the effect of gradually shortening the biofilter height, which was responsible for the gradual rise in the MEK concentration at the exit, ultimately becoming almost equal to the feed in about 1000 h. Conclusions The presence of H2S adversely affecting the performance of a biofilter-treating VOC-containing waste gas stream has been addressed in this study. It has been demonstrated that catalytic oxidation of H2S in a rusty iron reactor by the oxygen present in air prior to biofiltration of the polluted air stream can solve the problem. Mass-transfer and reaction characterization experiments were carried out. A transient model has been proposed and validated for the design of the rusty iron reactor for the oxidation of H2S. The present study has led to the following conclusions: (i) The oxidation of H2S by oxygen at the rusty iron surface followed an irreversible first-order reaction. 5(ii) The transient model established should be able to effectively predict the behavior of a rusty iron reactor for given operating conditions. Alternatively, the model may also be used for designing a reactor for a specified performance. (iii) It is possible for a properly designed reactor to remove nearly 100% H2S and give a stable performance for a long time. (iv) The presence of MEK did not affect the performance of a rusty iron reactor for the oxidation of H2S. (v) The effect of H2S on the biodegradation of MEK was experimentally demonstrated. It was demonstrated that the proposed two-stage reactor-biofilter system could effectively treat an air stream containing ∼65 ppm H2S and ∼100 ppm MEK. The performance remained stable over a period of 40 days. Appendix: Collocated Forms of the Transient Model Equations When eqs 13 and 14 were written in collocation form based on a Legendre-type polynomial to represent the trial function, the following sets of ODEs were obtained: M+2 dCg A(j,i) Cg(i) - KA0τ0Cg(j) At(j) (A.1) (j) ) dτ i)1

∑

where j ) 2, ..., M + 2; τ e 0, Cg(j) ) 0 where j ) 1, 2, ..., M + 2; Cg(1) ) 1

k2τ0 dAt (j) ) {A [1 - At(j)]}n2 dτ Vb0A0 0 k1τ0 (A C )n1[KAt(j) Cg(j)]n1 (A.2) Vb0A0 0 gi

where j ) 1, 2, ..., M + 2; τ ) 0, At(j) ) 1 where j ) 1, 2, ..., M + 2. In the above equations, M denotes the number of internal collocation points. Ax(j,i) denotes the collocation coefficient for the gradient. In this study 15 internal points were used. Solutions were also checked for M ) 13 and 17 to confirm that 15 points were adequate. Solutions for both M ) 13 and 17 were practically identical. Notation a ) total surface area of a particle, cm2 A0 ) active surface area per unit bed volume at time zero, cm2/cm3 At ) active surface area per unit bed volume at any location at time t, cm2/cm3 At ) dimensionless active surface area, At/A0 Cg ) H2S concentration in the gas phase, ppm or mol/m3 Cge ) H2S exit concentration, ppm or mol/m3 Cgi ) H2S inlet concentration, ppm or mol/m3 C′g ) H2S concentration at the interface on the gas side, mol/m3 C′s ) H2S concentration at the interface on the solid side, mol/m3 Cg ) dimensionless H2S concentration, Cg/Cgi D ) binary molecular diffusivity (H2S diffusivity in air in this study), cm2/s de ) equivalent particle diameter, cm K ) overall rate constant, cm/s kg ) mass-transfer coefficient, cm/s kr ) reaction rate constant in eq 3, cm/s k′r ) modified reaction rate constant in eq 5b, kr/m, cm/s k1 ) rate constant for iron catalyst consumption, cm-1 sn1-1 k2 ) rate constant for iron catalyst regeneration, cmn2-1 s-1 L ) length of flat plat, cm m ) distribution coefficient n1 ) exponent in the equation defining the rate of catalyst consumption n2 ) exponent in the equation defining the rate of catalyst regeneration Q (Q1, Q2, Q3, QT) ) volumetric flow rate (at various locations), cm3/min or cm3/s -rH2S ) H2S oxidation rate by a catalytic iron reactor, mol/ m2‚s t ) time, min or s u (u0) ) average (superficial) linear velocity, cm/s Vb (Vb0) ) reactor volume variable (total reactor volume), cm3 Vb ) dimensionless reactor volume variable, Vb/Vb0 Superscripts * ) equilibrium ′ ) at the interface Greek Letters ν ) kinematic viscosity, cm2/s τ0 ) nominal residence time, s

Literature Cited (1) Chitwood, D. E.; Devinny, J. S.; Reynolds, F. E. Evaluation of a Two-stage Biofilter for Treatment of POTW Waste Air. Environ. Prog. 1999, 18 (3), 212. (2) Cho, K. S.; Hirai, M.; Shoda, M. Degradation Characteristics of Hydrogen Sulfide, Methanethiol, Dimethyl Sulfide and Dimethyl Disulfide by Thiobacillus thioparus DW44 Isolated from Peat Biofilter. J. Ferment. Bioeng. 1991, 71 (6), 384.

Ind. Eng. Chem. Res., Vol. 42, No. 4, 2003 763 (3) Chung, Y. C.; Huang, C. P.; Tseng, C. P. Biodegradation of Hydrogen Sulfide by a Laboratory-scale Immobilized Pseudomonas Putida CH11 Biofilter. Biotechnol. Prog. 1996, 12 (6), 773. (4) Degorce-Dumas, J. R.; Kowal, S.; LeCloirec, P. Microbiological Oxidation of Hydrogen Sulphide in a Biofilter. Can. J. Microbiol. 1997, 43 (3), 264. (5) Yang, Y. H.; Allen, E. R. Biofiltration Control of HydrogenSulfide. 1. Design and Operational Parameters. J. Air Waste Manage. Assoc. 1994, 44 (7), 863. (6) Leson, G.; Winer, A. M. Biofiltrationsan Innovative Airpollution Control Technology for VOC Emissions. J. Air Waste Manage. Assoc. 1991, 41 (8), 1045. (7) Wani, A. H.; Branion, R. M. R.; Lau, A. K. Degradation Kinetics of Biofilter Media Treating Reduced Sulfur Odors and VOCs. J. Air Waste Manage. Assoc. 1998, 48, 1183. (8) Boon, A. G.; Boon, K. Catalytic-iron Filters for Effective and Low-cost Treatment of Odorous Air. Water Environ. Manage. 1999, 13 (3), 189. (9) Amanullah, Md. Dynamics of Biofilters. Master’s Thesis, National University of Singapore, Singapore, Singapore, 1999. (10) Amanullah, Md.; Farooq, S.; Viswanathan, S. Modeling and Simulation of a Biofilter. Ind. Eng. Chem. Res. 1999, 38 (7), 2765. (11) Sherwood, T. K.; Pigford, R. L.; Wilke, C. R. Mass Transfer; McGraw-Hill: New York, 1975.

(12) Bird, B. R.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; John Willey & Sons: New York, 1960. (13) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987. (14) Toogood, S. J. Odor Control. British Patent Application No. 8714191, June 17, 1987. (15) Deshusses, M. A. Biodegradation of Mixture of Ketone Vapors in Biofilters for the Treatment of Waste Air. Ph.D. Thesis, Swiss Federal Institute of Technology, Zurich, Switzerland, 1994. (16) Finlayson, B. A. Method of Weighted Residuals and Variational Principles; Aademic Press: New York, 1972. (17) IMSL. Math/Library User’s Manual; Visual Numeric, Inc.: Houston, 1994. (18) Swanson, W. J.; Loehr, R. C. Biofiltration: Fundamentals, Design and Operations Principles, and Applications. J. Environ. Eng. 1997, 123 (6), 538.

Received for review April 17, 2002 Revised manuscript received December 3, 2002 Accepted December 3, 2002 IE020284R

Evaluation of a Novel Reactorâ Biofilter System

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