공기중의 톨루엔을 제거하는 바이오필터의 거동에 관한 연구:

I. 정상상태 실험 및 전산 모사

홍성도, 명성운, 최호석, 김인호, 박기돈*

충남대학교 공과대학 화학공학과

(주)한국 미생물기술*

Behavior of biofilters for the removal of toluene from air:

Steady-state experiment and simulation

Sung-Do Hong, Sung-Woon Myung, Ho-Suk Choi, In-Ho Kim, Kee-Don Park*

Department of Chemical Engineering, Chungnam National University

Korea Microbial Technology, Inc., Taejon, Korea*

Introduction

Biofiltration is an APC (Air Pollution Control) technology in which microorganisms are immobilized on organic/inorganic packing materials. The off-gas, containing biodegradable volatile organic or inorganic compounds is often a high-efficiency and low-cost alternative. The principle of biofiltration consists of a biologically active bed through which the contaminated off-gas is vented.

Biofiltration is very complex hybrid technology, which involve adsorption, absorption and biochemical reaction. Recently, numerous efforts have been made the process model. In early 1980s, Jennings et al.1 and Ottengraf2 first suggested biofilm models with assumptions of basic mass balance principles, simple kinetics, and a plug flow air stream. These models answered the value of overall order of reaction occurring in biofilms. Shareefdeen et al.3-4 proposed both steady-state and transient models in which the biodegradation was assumed to be first order with respect to both oxygen and the organic pollutant. The idea of their study was that some pollutants were adsorbed on the bare surface of support media while others were absorbed on the biofilm partially covering on the support media. Hodge and Devinny5 used an axially dispersed plug-flow model for gas phase and a linear driving force(LDF) model to approximate the interface mass transfer kinetics.

Although many efforts were made to accurately describe the biofiltration, either only simulation or only experiment did not allow us to fully understand the process. Thus, we tried to use both tools to understand the biofiltration. In this study, we first solved a steady state problem of the biofilter process based upon Shareefdeen et al.'s model6. To match simulation results with experimental results, we introduced a correction function for the reaction kinetics and determined some unknown parameters including specific surface area of the support media and biofilm thickness. The effect of inlet load on the removal efficiency or the elimination capacity has been investigated.

Experimental materials and methods

Figure 1 shows the schematic diagram of a biofilter system, which we used in the experiments. The main part of the unit was an acrylic column of 10.6cm diameter and 160cm height, equipped with a sampling port at each stage. The height of the biofilter bed was 120cm, leading to a value of bed volume equal to 10.59x10-3 m3. The packing material consisted of a mixture of peat and calstone(5:3 volume ratio before mixing). The microorganism used in the experiments was Pseudomonas.Putida type A and the strain of Pseudomonas.Putida is provided by Dr. Khoo, Korea Microbial Technology, Inc.  The culture medium used contained per liter: glucose, 1.1g; NH4Cl 0.1g; MgSO4 7H2O 0.05g; FeSO47H2O 0.005g; MnSO4H2O 0.005g ; CaCl2 0.00375g; Yeast extract 1g; 0.1M phosphate buffer solution 18ml. The microo+rganism cultured with the above medium in a shaking flask was well mixed with the packing materials and then packed to the column. The air stream was divided into two streams. The main one was passed through a water vessel for humidification. The second stream was passed through a closed toluene vessel. And then two streams were mixed before they entered the biofilter bed. The flow rates of two streams were controlled by two flow meters, respectively. The concentration of toluene was measured by a portable GAS-TEC(Flame Ionization Monitor Type 47674, Telegan Gas Monitoring Ltd., West Sussex, England) detector. The temperature of the biofilter bed was kept from 25℃ to 30℃ for maintaining high microbial activity. To avoid the pressure drop in the biofilter bed due to compaction, whole column was divided by 4 stages.

Figure 1. Schematic diagram of         Figure 2. Comparison of experimental data

experimental apparatus.             with steady state simulation results

Results and discussion

We used IMSL library(DIVPRK for gas phase and BVPFD for biofilm) included in Digital Visual Fortran(Digital Equipment Corporation, Maynard, Massachusetts) to obtain numerical solutions of both steady-state and transient models. We first obtained the steady state solutions at the air flow rate of 10 L/min and the toluene concentrations of 100, 200, 300 ppm, respectively. Figure 2 represents both simulation and experimental results. To fit the simulation results with experimental data, we had to introduce the following deactivating factor as a function of inlet toluene concentration since the deactivation observed with increasing the toluene concentration in the experiment,

(1)

where CTI is in g/m3. From the careful examination of the model equations, we found that possible parameters affecting the performance of our biofilter are , As*, □*, Xv. When we measured the number of microbes in the packing materials, it was gradually reduced with the average value of 108c.f.u./g-packing material. On the other hand, the sudden increase of inlet toluene concentration did not increase the absolute amount of toluene removed in our biofilter. Since we did not add any nutrients except carbon source from toluene, we decided that the nutrient limitation controlled our biofilter performance. Thus, we decided the correction of growth constant. However, we still have to investigate the effect of other important parameters such as biofilm thickness and kinetic constants

We also plotted the elimination capacity as a function of inlet loads in Figure 3. The elimination capacity is very important since it is related with the removal efficiency and inlet loads. The maximum elimination capacity is affected by the activity of microbes. The activity is a function of the environmental conditions such as temperature, humidity and inhibitory substances. Diks and Ottengraf6 reported various expressions of the elimination capacity but the definition we used in this study is

(2)

where Vbed is a volume of support media in biofilter and Q is a volumetric flow rate of air stream.

As shown in Figure 3, the elimination capacity increases with increasing the contaminated airflow rate since the amount of toluene transported from air stream to biofilm increases with increasing inlet loads from 0 to 20 g/m3hr. The elimination capacity, however, does not increase at the inlet loads of over 30 g/m3hr. From this result, we can know that the activity of microbes in the biofilm reaches a maximum and the maximum elimination capacity of our system is about 18 g/m3hr. In Figure 3, we also compared the experimental elimination capacities with simulation results obtained by changing either only inlet concentration of toluene or only the flow rate of air stream, respectively. Two simulation results show good agreements with experimental data. Although the analysis of biofiltration is very difficult due to numerous variables and unknown parameters, the introduction of simple correction function allows us to apply the simulation for the design or the optimization of biofilter system without conducting many experiments.

Figure 4 represents the change of removal efficiencies with respect to the inlet concentration of toluene. The experiment was conducted at the toluene concentrations of 0.411 to 1.233 g/m3 and the removal efficiency was obtained from dividing the difference between the inlet and the outlet toluene concentrations by the inlet toluene concentration. The removal efficiency of 65% at CTI=0.411 g/m3 decreases rapidly to about 35% at CTI=0.821 g/m3. At over CTI=0.821 g/m3, however, the removal efficiency decreases slowly.

Figure 3. Change of elimination capacity   Figure 4. Change of removal efficiency

with respect to inlet load.        with respect to the inlet concentration

This is because, at low inlet concentration of toluene, the diffusion of toluene through biofilm is a rate determining step(as also observed at Figure 2 with high curvature of 100ppm line) while, at high inlet concentration, the bio-decomposition reaction is a rate determining step.

Conclusions

We successfully solved steady state of the biofilter process by simply introducing a correcting function, which was obtained from the fitting of steady-state simulation results with experimental data. The removal efficiency decreased with increasing the inlet concentration of toluene since the nutrient limitation controlled the performance of our biofilter. Thus, the elimination capacity reached the maximum value of about 20g/m3hr at the inlet load of 30 g/m3hr. At the inlet toluene concentration of 100ppm, the diffusion of toluene into biofilm was obviously a rate determining step. Above 200ppm, however, biofilm already showed full activity. The steady-state simulation confirmed that the change of elimination capacity obtained by increasing only inlet toluene concentration was the same as that obtained by increasing only flow rate of contaminate air. The fact allows us to extend the simulation to the prediction of the elimination capacity at other operating conditions without doing additional experiment.

Reference

1. Jennings, P.A., Snoeyink, V. L., and Chian, E.S.K., Biotechnol.

Bioeng.,18,1249.(1976)

2. Ottengraf, S.P.P., Biotechnol. Bioengng.,19, 1411,(1977).

3. Shareefdeen, Z.;Baltzis, B.C.; Oh, Y.S.; Bartha, R. Biotech. Bioeng.,41,              512.(1993).

4. Shareefdeen, Z. and Baltzis, B.C., Chem. Eng. Sci.,49, 4347,(1994).

5. Hodge , D.S., Devinny, J. S., J. Environ. Eng. Div. (Am. Soc. Civ. Eng.), 121       (1), 21,(1995).

6. Diks, R.M.M., Ottengraf, S.P.P., Bioprocess Engng, 6, 93,(1991).