생물필터 공정에서 톨루엔 제거의 정상상태 모사 및 그 해석

노창현, 윤명수*, 최호석

충남대학교 공과대학 화학공학과, (주) 한탈*

Steady-State Modeling and Analysis

on the Removal of Toluene in Biofilter

Chang-Hyun Rho, Myung-Su Yoon, Ho Suk Choi

Department of Chemical Engineering, Chungnam National University

Taejon, 305-764, Korea

Hantal Co., Ltd.

Introduction

Biofiltration is an air pollution control technology in which microorganisms immobilized on organic/inorganic packing materials at an off-gas containing biodegradable volatile organic or inorganic compounds and is often a highly efficiency and low-cost alternative to other, more conventional, air pollution control technologies such as thermal oxidation, catalytic incineration, refrigerated condensation, carbon adsorption by chemicals and water scrubbers. Recently, in Europe and Japan, it is the most commonly employed odor control technique.(1) In this study, through the steady-state simulation of the removal of toluene in a known biofilter system, the performance of a biofilter under given conditions and the relationship between parameters such as media surface area, biological activity, biofilm thickness, etc and toluene removal are investigated.

Model Development for Biofilter process

A general model for the biofilter process contains three phases such as gas phase, biofilm phase and solid phase. For steady-state two mass balances in the biofilm are

[eqn: { PARTIAL s SUB t } over { partial t }~=~f(X sub v ) D sub tw { PARTIAL sup 2 s SUB t } over { partial x sup 2 }~-~ X SUB v over Y sub t mu sub t (s sub t , s sub o )]                               (1)

[eqn: { PARTIAL s SUB t } over { partial t }~=~f(X sub v ) D sub ow { PARTIAL sup 2 s SUB o } over { partial x sup 2 }~-~ X SUB v over Y sub t mu sub t (s sub t , s sub o )]                               (2)

Mass balances in the gas phase are :

[eqn: □ {partial c sub t } over {partial t }~=~-u sub g { partial c sub t } over { partial h } ~+~D SUB tw f (X sub v ) alpha A SUB s sup * ({partial s sub t } over {partial x }) sub x=0 ~-~k sub a (1-alpha )A sub s sup * ( c sub t - c sub t sup * )]	(3)

[eqn: □ {partial c sub 0 } over {partial t }~=~-u sub g { partial c sub 0 } over { partial h } ~+~D SUB ow f (X sub v ) alpha A SUB s sup * ({partial s sub o } over {partial x }) sub x=0]	(4)

Mass balance in the solid phase (particles) is

[eqn: (1-□)□ sub p { partial c sub tp } over { partial t } ~=~k sub a (1- alpha)A sub s sup * (c sub t - c sub t sup * )]                               (5)

By setting the left-hand side of eqs(1),(2),(3),(4),and (5) equal to zero one gets the steady-state version of the model. In such a case, since [eqn: c_t`=`{c}`_{t } ^{* }], eq.(13) becomes a tautology and can be neglected. Furthermore, the last term on the right-hand side of eq.(5) also drops, and the resulting four equations are identical with the model proposed earlier by Shareefdeen et al.(2)

The biodegradation kinetics for toluene is

[eqn: mu sub t ( s sub t , s sub o ) ={ {mu sub t sup * s sub t } over { K sub t + s sub t + {s sub t sup 2 } over {K sub {I t} }}} { { s sub o } over {K sub ot + s sub o } }]                                    (6)

The following Freundlich adsorption isotherm is used to predict the amount of adsorption.

[eqn: c sub tp ~=~k sub d ( c sub t sup * ) sup n]                                                         (7)

Two effectiveness factors are defined as:

[eqn: e sub t = { f (X SUB v ) D SUB tw ( {partial s sub t }over{ partial x} ) sub x=0 } over { delta ( {X SUB v }over { Y sub t } ) [ mu sub t ( s sub t, s sub o )] sub x=0 }]                                             (8)

[eqn: e sub o = { f (X SUB v ) D SUB ow ( {partial s sub o }over{ partial x} ) sub x=0 } over { delta ( {X SUB v }over { Y sub ot } ) [ mu sub t ( s sub t, s sub o )] sub x=0 }]                                             (9)

Results

Fig. 1 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. 1, 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. 2 represents toluene removal rate which is defined as [eqn: (`c_Ti`-c_Te`)/ tau] and calculated from the predicted value for [eqn: c_Te]. This value was obained by solving the steady-state versions of eqs (1), (2), (3) and (4), using a computer code of shooting method. As the space time increases, the toluene removal rate decreases. The  toluene removal rate increases with increasing the inlet toluene concentration. Table 1 represents the outlet concentration of toluene at different inlet concentrations of toluene and different inlet gas velocities. As shown in the table, the outlet concentration of toluene gradually increases with increasing inlet concentration of toluene at a fixed space time and decreases with increasing the space time at a fixed inlet concentration of toluene. However, the toluene removal rate becomes higher at large inlet concentration and small space time. This is because the biofilter still has more capability to remove toluene at large inlet concentration and high inlet velocity than at poor operation conditions. The limitation of exit VOC is 50ppm. Therefore, as shown in Table 1 one has to operate the biofilter at the operating condition near solid front line in the table.

Conclusion

We solved a steady state problem of the biofilter process and then obtained a functional form of the effectiveness factor which depends on both gas phase concentration and inlet concentration of toluene. The removal rate is enhanced with increasing both the inlet concentration and the inlet velocity of toluene. Since, however, the environmental allowance of exit VOC is limited under 50ppm, one can decide optimum operating condition of the biofilter process from this simulation.

Fig 1. The effect of inlet conc.   Fig 2. The change of toluene                   of toluene on the effectiveness              removal rate with space time

factor	and inlet toluene conc.

Table 1. Model predicted outlet toluene concentration

Q×105 (m3/s)	CTI(ppm) □(min)	401		377		352	328			304
		CTe
1.63	5.72	105	97		88			80	73
1.55	6.02	97	89		81			74	67
1.47	6.35	89	81		74			67	61
1.38	6.73	80	73		67			61	55
1.30	7.15	71	65		60			54	49
1.21	7.7	61	56		51			46	42
1.14	8.17	54	49		45			40	37
1.06	8.79	45	41		37			34	30
0.98	9.53	36	33		30			27	25
0.89	10.4	28	26		24			21	19
0.81	11.43	21	19		17			16	14
0.73	12.7	15	13		12			11	10

References

1. Shareefdeen,Z., Baltzis,B.C., 1994, Biofiltration of toluene vapor under

steady-state and transient conditions. Chem. Eng. Sci 49 4347-4360

2. Shareefdeen,Z., Baltzis,B.C., Oh,Y.-S. and Bartha,R., 1993, Biofiltration of

methanol vapor. Biotechnol. Bioengng 41, 512