바이오필터의 모델링과 전산모사

최호석

충남대학교 공과대학 화학공학과

Modeling and simulation of a biofilter

Ho-Suk Choi

Department of Chemical Engineering, Chungnam National University

Introduction

Biofiltration is a very complex hybrid technology, which involves adsorption, absorption and biochemical reaction. Recently, numerous efforts have been made to model the process. 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. Deshusses et al.6,7 proposed a transient model for biofiltration of methyl ethyl ketone/ methyl isobutyl ketone mixtures in which kinetic interaction between the pollutants was taken into account in the degradation reaction model. Abumaizar et al.8 studied the removal of BTEX(benzene, toluene, ethyl benzene and xylene) and investigated the effect of adding GAC to the compost biofilter. Recently, Amanullah et al.9 developed a rigorous biofilter model to examine the effects of various operating variables and intrinsic properties on the transient response and conversion achieved at steady state.

Although many efforts were made to accurately describe the biofiltration, the biofilter models differed mainly in the following aspects:(1) model for fluid flow, (2) model for biodegradation reaction in the biofilm (3) details of interphase transport, and (4) role of the support media. In this study, we tried to anwser the following two questions. (1) What is the effect of various model parameters on biofilter performance? (2) How much details is required in the modeling of a biofilter?

Model development

The assumptions made in deriving the model are the same as Shareefdeen et al.'s6. The governing equations are as follows.

(Ⅰ) Mass balance in the biofilms are:

(1)

(2)

(Ⅱ) Mass balances in the gas phase are:

(3)

(4)

(Ⅲ) Mass balance in the solid phase(particles) is

(5)

For the biodegradation of pollutant j, the following interactive model was used

(6)

For the adsorption of pollutant j on solid media, the Freundlich isotherm was used

(7)

The transient model constitutes a set of partial differential equations in three dimensions(time, biolayer, and bed height). To simply solve the problem, a quasi-steady-state approximation was made for the biolayer, and the use of effectiveness factors was introduced by following the approach of Tong and Fan10.

(8)

(9)

The above system of equations was written in the dimensionless form and then discretized in space, using the method of orthogonal collocation. Ten points were used along the both bed length and biofilm thickness. The resulting set of ordinary differential equations was then integrated in the time domain using Gear's multistep method(DIVPAG) provided by IMSL library included in Digital Visual Fortran(Digital Equipment Corporation, Maynard, Massachusetts).

Results and discussion

The process was modeled with general mass balance equation which take into account reaction, mass transfer, and adsorption of the pollutant onto the packing material. In the model system, the height of the biofilter bed of the equipment used for the experiment was 68.6cm, and volume of the biofilter bed was 5.15 ×10-3m3. The packing material consisted of a mixture of peat and perlite (2: 3 volume ratio before mixing). The space time is □=7.7min and the inlet toluene concentration is 1.65g/m3. Fig. 1 represents the comparison of simulation with experimental results to fix the unknown parameters such as specific surface area of biofilter media. For the transient simulation, we used the expression of an effectiveness factor as a nonlinear function of inlet toluene concentration(Fig.2).

Fig. 1. Steady-state simulation results   Fig. 2. Effectiveness factors

Fig. 3. Dynamic response with respect to the step increase of inlet concentration.

Fig. 2 represents the effect of inlet toluene concentration on two effectiveness factors. Shareefdeen et al.(1) reported that two effectiveness factors were linear functions of only a gas phase toluene concentration. As shown in Fig. 2, however, the effectiveness factors are nonlinear functions which depend on the inlet concentration of toluene as well as a gas phase toluene concentration. Thus, we obtained several polynomial fitting functions with respect to inlet concentrations of toluene. Fig. 3 shows the result of the dynamic simulation under two step inputs of toluene concentrations. As shown in Fig. 1, the retention time of pollutants is about 2min. But we could know that it took long time to be steady-state as a result of careful observation of simulation results. This is because the adsorption of toluene is still active while the convection transport and the decomposition of toluene reaches steady-state.

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. Deshusses, M.A., Hamer, G., Dunn, I.J. Environ. Sci. Technol., 1995, 29 (4), 1048.

7. Deshusses, M.A., Dunn, I.J., In Proceedings of the 6th European Congress in Biotechnology, Alberghina, L., Frontali, L., and Sensi, P., Eds., Elsevier, The Netherlands, 1994, p.1191.

8. Abumaizar, R.J., Smith, E.H., Kocher, W., J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1997, 123 (6),606.

9. Amanullah, Md, Farooq, S. and Viswanathan, S., Ind. Eng. Chem. Res. 1999, 38, 2765-2774

14. Tong, C.C., Fan, L.-S., Biotechnol. Bioengng, 1988, 31, 24.