Rate Based Modeling of a Sulfite Reduction Bioreactor

Transcription

1 Rate Based Modeing of a Sufite Reduction Bioreactor S. Ebrahimi Dept. of Biotechnoogy, Deft University of Technoogy, Juianaaan 67, 2628 BC Deft, The Netherands, and Dept. of Chemica Engineering, Sahand University of Technoogy, Tabriz, Iran C. Picioreanu, R. Keerebezem, J. J. Heijnen, and M. C. M. van Loosdrecht Dept. of Biotechnoogy, Deft University of Technoogy, Juianaaan 67, 2628 BC Deft, The Netherands DOI /aic Pubished onine March 23, 2005 in Wiey InterScience ( A nove bioogica sufite/sufate reduction process has been evauated using a ratebased mathematica mode. The principe of this process is that sufate reducing bacteria reduce sufate or sufite to sufide using H 2 and CO 2 as energy and carbon source under anaerobic conditions. The H 2 S gas resuting from the bioogica sufate reduction is simutaneousy stripped off and, in a separate stage, absorbed and oxidized to eementa sufur. In this nove process, the bioogica sufate/sufite reduction reaction is accompanied by absorption of H 2 and CO 2, chemica reactions in the iquid phase and the subsequent desorption of the voatie reaction product, H 2 S. As a resut of the compicated nature of this process, accurate simuation of the system is essentia for the precise design, parameter estimation and contro of the process. The proposed mode adopts the fim theory and incudes both instantaneous equiibrium reactions and reactions with finite rates. Mass transfer resistance in the gas phase is aso taken into account. The rate based process mode was impemented in a packed-bed bioreactor mode to obtain reactor dimensions as a function of operationa variabes. A sensitivity anaysis was conducted on the mathematica mode to define the key parameters of the process design and performance, and to estabish the optima reactor operationa conditions American Institute of Chemica Engineers AIChE J, 51: , 2005 Keywords: sufite reducing bacteria, sufur dioxid, fue gas, mutiphase reactions, mathematica modeing Introduction Combustion of fossi fue resuts in the generation of fue gas containing SO 2 which is a major air poutant causing severa serious environmenta poution probems, such as photochemica smog and acid rain. Fue gas desufurization (FGD) is effective to prevent SO 2 emission. The most commerciay important fue gas desufurization technoogy at present is the use of soid, throwaway adsorbents, such as imestone and Correspondence concerning this artice shoud be addressed to S. Ebrahimi at s.ebrahimi@tnw.tudeft.n American Institute of Chemica Engineers doomite. This type of process resuts in the production of arge amounts of gypsum, that is, cacium sufate, which imposes a significant disposa probem, since ony part of the produced gypsum can be used as construction materia 1. As an aternative to the conventiona FGD process, a new regenerabe biotechnoogica process for fue gas desufurization has been deveoped by Paques Bio systems B.V. and Hoogovens Technica Services 2. This Bio-FGD process combines a conventiona caustic scrubber for the remova of SO 2 from the fue gas and two bioreactors for processing the iquid from the scrubber. In the scrubber, SO 2 is absorbed by a water phase buffered by carbonate. The bisufite/sufate is then transferred to the anaerobic bioreactor, where the sufite and sufate 1429

2 Figure 1. Combined chemica/bioogica process for conversion of SO 2 to H 2 S. The thick ine denotes the sufite reduction bioreactor modeed in this study. from the absorber iquid are reduced to sufide through sufatereducing bacteria. Finay, the sufide containing iquid is fed to an aerobic bioreactor where sufide is converted to eementa sufur. Eementa sufur is removed from the soution by a separator, and the regenerated bicarbonate soution is recircuated to the absorber. BIO-FGD offers many advantages compared with conventiona FGD processes. It resuts in consideraby reduced costs for waste gas desufurization, high SO 2 remova efficiency and ow reagent consumption. Moreover, the mass of the end product is reduced by more than 80% and the end product is reusabe (vauabe) eementa sufur. The main disadvantage of the Bio-FGD process is couping of the reactors by one arge iquid cyce. Due to hydrogen sufide inhibition in the anaerobic reactor, a arge cycic water fow is needed to keep a ow hydrogen sufide concentration. Since a of the reactors are connected by this iquid cyce, a arge iquid fow has to be circuated, and most reactors are hydrauicay imited. Consequenty the reactors become wider due to the imited superficia iquid veocities in combination with the arge cyce fow. To overcome the imit in the Bio-FGD process, we have deveoped an aternative biotechnoogica process for fue gas desufurization 3. In this process, the sufide produced in the sufite reducing bioreactor is stripped off to the gas phase and, hence, no need for the recircuation of a arge iquid fow. In this way, instead of one arge iquid cyce two sma iquid cyces and one gas cyce are appied which makes the process more fexibe. In the mean time, two iquid oops are working in a bioscrubber concept and there is no need to separate the biomass from the iquid phase. Aso, a suspended growth bioreactor can be used and the iquid containing suspended ces circuating between absorbers and bioreactors. The first iquid oop contains a sufur dioxide absorber, which converts SO 2 into HSO - 3, and a sufite reduction bioreactor where HSO 3 is converted by microorganisms into H 2 S - under anaerobic condition, using H 2 and CO 2 as energy and carbon source, respectivey (Figure 1). The H 2 S gas resuting from the sufite/sufate reduction is simutaneousy stripped off and transferred into a gas phase, which carries H 2 S to a second oop where H 2 S is absorbed and oxidized to eementa sufur. Two options are proposed for the second oop: (1) using akaophiic bacteria, in this case H 2 S gas is absorbed into an akaine soution and subsequenty bioogicay oxidized to eementa sufur, which is separated from the water phase, 4 and (2) using acidophiic bacteria, in this case an aqueous Fe 2 (SO 4 ) 3 soution is used as H 2 S absorbent. H 2 S is absorbed and chemicay oxidized to eementa sufur, whie Fe 3 is reduced to Fe 2 5. Eementa sufur is removed from the soution by a separator, and the reactant Fe 3 is regenerated from Fe 2 by bioogica oxidation in an aerated bioreactor, which is connected to the chemica H 2 S oxidizer with a iquid cyce. Bioogica sufite reduction is an important subprocess in this new integrated chemica-bioogica process for SO 2 remova, which is the focus of this study. For this purpose, the use of autotrophic sufate reducing bacteria in a suspended growth type bioreactor is proposed in which, hydrogen gas is used as an eectron donor. The use of suspended biomass is advantageous compared to the use of biofim because diffusion imitation is absent. By using a cosed iquid oop a suspended biomass cuture can be recircuated between the two reactors as ong as the HRT in the SO 2 scrubber can be negected, which is normay the case. An aqueous NaHCO 3 soution is used for SO 2 absorption, and subsequenty bicarbonate is regenerated via bioogica reduction of sufite in the bioreactor. The overa sufite conversion rate in H 2 utiizing bioreactors is often determined by the mass transfer capacity for H 2 6. The appication of suspended growth type bioreactors for this purpose offers severa advantages over biofim bioreactors. In the suspended growth bioreactors there is no need for space and expensive carrier materia. The ony mass transfer barrier is transfering compounds from the gas phase to the iquid phase due to the absence of the mass transfer resistances as a resut of the compounds transport to the biofim and diffusiona resistance within the biofim. In the suspended growth bioreactor, the SRT can be easiy seected by the amount of beed. In the sufite reduction bioreactor using hydrogen as the eectron donor, a strong competition exists for hydrogen between hydrogenotrophic sufite reducer and hydrogenotrophic methanogens (converting H 2 to undesired CH 4 ) occurs 6. The beed can be set in such a way to seect a residence time short enough to prevent growth of competing hydrogenotrophic methanogenic bacteria in the system. Sufate reducing bacteria (SRB) are obigate anaerobes, which convert sufate or sufite as eectron acceptor to sufide as end product. They are known to grow both heterotrophicay using sma organic moecues, and autotrophicay using H 2 as the eectron donor and CO 2 as the carbon source 7. Synthesis gas is considered to be highy attractive for the sufate remova process for the foowing reasons 8 : Low biomass yied is achieved using autotrophic sufate reducing bacteria. Synthesis gas is easiy produced on site from gasification of the coa. Low-grade coa, containing sufur deposits can safey be used for synthesis gas production, as sufur compounds woud be treated by the system. In this study the feasibiity and engineering aspect of the sufite reduction bioreactor were investigated. In this bioreactor 1430 May 2005 Vo. 51, No. 5 AIChE Journa

3 CO 2 0.2NH Y sx max 2.1HSO 3 1 Y sx Y sx max H 2 3 CH 1.8 O 0.5 N max 2.1HS 1 max 0.6H Y 2 O 0.2H (1) sx Figure 2. Compex reaction system. (a) Overa mass transfer frame in the countercurrent fow contactor; and (b) concentration profies in the two fim mode for a cross section of the coumn. Besides growth, bacteria aso use the energy created from H 2 and sufites for maintenance necessities. The simpified cataboic stoichiometric equation that appies to the bacteria maintenance reaction can be written as bioogica reaction is accompanied with severa chemica reactions in iquid phase, transport of H 2 and CO 2 from gas phase, and the subsequent desorption of a voatie reaction product, H 2 S. Due to the compicated nature of this process, an accurate simuation of the process is essentia for the precise design. In this regard, a rate based mode has aready been deveoped in the previous research 3 and modified to appy for the design and simuation of the sufite reduction bioreactor. Mathematica Mode Deveopment Process description The sufite reduction process is considered as a system where a sufite containing iquid is contacted with a gas fow containing H 2 as energy source and CO 2 as carbon source. The iquid is fed at the top of the bioreactor at a fow rate L, and it comes in countercurrent contact with a gas fow at voumetric rate G. H 2 and CO 2 from the gas phase must be transferred to the iquid phase, where the bioogica reaction occurs and sufite is converted to sufide. The produced sufide is transferred to the gas phase as H 2 S. In the iquid phase, in addition to the bioogica reaction a series of chemica reactions occur. The chemica reactions must be taken into account in the mode since they are reevant to transport and reaction rates cacuation as we as the ph. A representation of the system is depicted in Figure 2. In the proposed mode, the foowing assumptions have been made to simpify the anaysis of the bioreactor: Steady-state condition Mass transfer between the gas and iquid phase is described based on the two fim theory. No reaction takes pace in the gas phase. The contactor used is packed bed and the gas and iquid are in pug fow. The temperature differences can be negected. Mode Reactions Autotrophic sufate reducing bacteria use carbon dioxide as their soe carbon source to produce organic matter. A stoichiometric equation for bacteria growth on hydrogen can be derived from the biomass yied on H 2 (1/Y max sx is the moes of H 2 consumed per C-mo biomass produced), the eementa baances on, C, H, O, N and the charge baance and by assuming that the biomass composition is represented by CH 1.8 O 0.5 N H HSO HS H 2 O (2) The rate equation for the specific rate of hydrogen utiization with simutaneous inhibition by the product H 2 S can be described by the foowing equation 6 q H2 q H2 max C HSO3 C H2 C H2S 1 K HSO3 CHSO3 K H2 C H2 C H2S,max (3) Accordingy, the conversion rate of H 2 based on the reactor iquid voume r H2 (in mo of H 2 /m 3.h) is r H2 q H2 C x (4) By considering the biomass specific substrate consumption rate (q H2 ) and maintenance (m s ) as independent rates and using the Pirt equation 10, the foowing reation for specific growth rate can be obtained r x C x Y sx max q H2 m H2 C x (5) Consequenty, the reations between other conversion rates and r x can be obtained by the stoichiometric couping of the above growth and maintenance reactions (Eqs. 1 and 2) r C r x (6) r NH3 0.2r x (7) r SIV r SII max 2.1r Y x 1 sx 3 m sc x (8) The vaues of the kinetic parameters and maintenance coefficient are estimated using correations proposed by Heijnen et a. 9 and reported iterature data as shown in Tabe 1. The bioogica sufite reduction process using H 2 and CO 2 as respiration energy and carbon source aso invoves the exchange of SO 2, CO 2,H 2 and H 2 S between the iquid and gas phases. In addition a compex system of parae and consecutive ionic dissociation/association reactions in the iquid phase takes pace. The basic reactions considered are 1431

4 Tabe 1. Kinetic Parameters for Sufite Reducing Bacteria, with H 2 and CO 2 as Energy and Carbon Source at 55 C 9,12,13 Kinetic Parameter Vaue Units max q H K H moh 2 /C-mo h kmo/m 3 K HSO kmo/m 3 max Y H2 x 0.06 C-mo/moH 2 m H2 1.0 moh 2 /C-mo h max kmo/m 3 C H2 S Reactor mode SO 2 H 2 O ^ H HSO 3 HSO 3 ^ H SO 3 2 CO 2 H 2 O ^ H HCO 3 CO 2 OH ^ HCO 3 HCO 3 ^ H CO 3 2 (9) (10) (11) (12) (13) H 2 O ^ H OH (14) H 2 S ^ HS H (15) HS ^ S 2 H (16) The steady state mass baances for SO 2, CO 2,H 2 and H 2 Sin the gas phase assumed in pug fow through the reactor can be written as 1 RTS dgp j,b dz N g j,i a (17) where j represents the above gaseous species. The steady state mass baance for each species j in the buk iquid phase, aso assumed in pug fow, is L dc j,b S dz N j,b a r j,b where fuxes into or from buk iquid are (18) dc j N j,b D j (19) x Mass baances for individua chemica species can be combined to baances for tota sufite(siv), tota carbonate (C), tota sufide (SII) and hydrogen as foows L S L S L S dc SIV,b dc SO2 D dz SO2 D HSO3 x D SO3 2 dc SO3 2 dc HSO3 dc C,b dc CO2 dc D dz CO2 HCO3 D HCO3 x D CO3 2 dc 2 CO3 x dc SII,b dz D dc H2S dc H 2S HS D HS x L S dc S 2 D S 2 x SIV,b (20) x ar x r C,b (21) a x x a r SII,b (22) dc H2,b dc H2 D dz H2 a r H2,b (23) x Equations 17 and 20 to 23 contain the fuxes at the gas/iquid interface and from iquid fim/iquid buk. These fuxes are reated to the concentration gradients of the different components at the gas/iquid interface and buk iquid (see aso Eqs. 34 to 38). Therefore, the concentration profies of different components in the iquid fim are required. For this purpose, the mass baance equations in iquid fim for different species have to be integrated into the mass baances for the axia direction of the reactor. Note that, because of CO 2 hydroysis reaction in the iquid fim and changes in ph, concentrations of individua species change in the fim. However, because it is assumed that no bioogica reaction takes pace in the fim, the tota fuxes of SII, SIV and C are not affected. Consequenty, Eqs. 20 to 23 coud aso be formuated, based on the fuxes of H 2 S, SO 2,CO 2 and H 2 at the gas/iquid interface. Fim region The differentia equations describing the diffusion of each component j into or from the gas phase without reaction are dn j g 0 (24) where j is for SO 2,CO 2,H 2 S and H 2. The differentia equations describing the process of diffusion with simutaneous reaction of each species j in the iquid phase are dn j r j (25) Where Fick s aw is used to express the diffusive fux of each species: 1432 May 2005 Vo. 51, No. 5 AIChE Journa

5 N j D j dc j (26) After repacing the fuxes Eq. 26 in Eq.25, the mass baances for individua species can be combined to obtain the foowing baances for tota sufite, tota sufide, tota carbonate and tota sodium d 2 C SO2 D SO2 2 D HSO3 d 2 C HSO3 d 2 C 2 SO3 2 D 2 SO3 2 0 (27) interface (x 0) and boundary conditions in buk iquid (x ) is the thickness of mass transfer boundary ayer in the iquid phase which can be estimated using the mass-transfer coefficient k and the diffusivity for H 2 S as reference D H2 S/k H2 S. Boundary conditions at the gas iquid interface(x 0) The transfer rate of SO 2 is equa to the sum of the fuxes of the sufite species at the gas-iquid interface, which aso must be equa to the fux of SO 2 in the gas fim d 2 C CO2 D CO2 2 D HCO3 d 2 C HCO3 d 2 C 2 CO3 2 D 2 CO3 2 0 (28) g N SO2, j k g,so 2 RT p dc SO2 SO 2,b p SO2, j D SO2 x0 D H2S d 2 C H2S 2 D HS d 2 C HS d 2 C S 2 2 D S (29) dc HSO3 D HSO3 x0 D SO3 2 dc SO3 2 x0 (34) D H2 d 2 C H2 2 0 (30) D Na d 2 C Na 2 0 (31) By using Eq. 27 to 31, the bioogica reaction in the iquid fim is negected as can be justified by the sma voume of the iquid fim compared to the overa iquid voume. Because the reactions 9, 10 and 13 to 16 incudes proton transfers, instantaneous equiibrium is assumed for these reaction throughout the iquid fim; therefore, the chemica equiibrium reations of these reactions aso appy at a points in iquid phase. The hydroysis reaction of CO 2 is sow, therefore, instead of a chemica equiibrium equation a separate materia baance for CO 2 has to be incuded D CO2 d 2 C CO2 2 r CO2 (32) r CO2 represents the CO 2 hydroysis reaction rate and is a function of the reactive species invoved, and the reaction may proceed according to Eq. 11 or 12 depending on the ph 3. It has been shown that the impact of any eectric potentia gradient on the fux of ions may be disregarded under ow soute concentration conditions as ong as the mass fux equations are combined with a fux charge equation 11. Therefore, the ast equation needed to cose the system of baance equations in the iquid fim is the fux of charge Boundary conditions z j D j dc j 0 (33) The mode system consisting of mass and charge baances Eqs 27 to 33 and six chemica equiibrium reations for the reactions 9, 10 and 13 to 16 must be competed by the boundary conditions reevant to the fim mode at the gas iquid where and simiary for H 2 S and H 2 p SO2, j H SO2 C SO2,i (35) g N H2S,i k g,h 2S RT p dc H2S H 2S,b p H2S,i D H2S x0 D HS dc HS x0 D S 2 g N H2,i k g,h 2 RT p dc H2 H 2,b p H2,i D H2 x0 dc S 2 x0 (36) (37) Since the rate of the hydroysis reaction is sow, the fux of CO 2 in the gas fim is equa to the fux of dissoved CO 2 at the interface dc g CO2 N CO2,i k g,co2 /RT p CO2,b p CO2,i D CO2 x0 (38) - The fux of HCO 3 and CO 2-3 at the interface must be zero because the transport of carbon species is represented by CO 2 dc HCO3 D HCO3 x0 D CO3 2 The fux of Na is zero at the interface D Na dc 2 CO3 x0 0 (39) dc Na 0 (40) x0 and aso the net fux of charge must be zero at x

6 z j D j dc j 0 (41) x0 Since instantaneous equiibrium is assumed for the reactions 9, 10 and 13 to 16 at the interface, the equiibrium equations for these reactions appy aso at x 0. Boundary conditions in buk iquid (x ) Equiibrium is assumed for a the reactions in the buk iquid; therefore, besides the equiibrium equations for reactions 9, 10 and 13 to 16, aso the equiibrium equation can be written as foows C H C HCO3 C CO2 K 3 (42) together with other four equations obtained from mass baances for tota sufur species, tota carbon species, Na,H 2 and a charge baance C tot,siv C SO2,b C HSO3,b C SO3 2,b (43) C tot,sii C H2,S,b C HS,b C S 2,b (44) C tot,c C CO2,b C HCO3,b C CO3 2,b (45) C tot,h2 C H2,b (46) C tot,na C Na,b (47) Z j C j 0 (48) The system of seven Eqs 27 to 33 together with the six equiibrium equations for the reactions 9, 10 and 13 to 16 and the described boundary conditions are used to find the concentration profies of the 13 unknown species (H 2, H 2 S, HS, S 2, SO 2, HSO 3, SO 3 2, CO 2, HCO 3, CO 3 2, Na, H, OH )inthe iquid fim at each z position in the coumn height. These concentration profies aow the cacuation of the fuxes of SO 2,H 2 S, H 2 and CO 2. The fuxes are needed for integration of differentia mass baance equations in the buk gas and iquid aong the coumn in Eqs. 17 and 20 to 23. The iquid fim thickness and reactor height were both discretized in a spatiay uniform grid and second-order finite differencing was appied. The resuting system of noninear agebraic equations was soved numericay with a traditiona Newton-based method. Resuts and Discussion In a previous work we have described the mode based design of a scrubber for SO 2 remova from the fue gas in a 600 MW power pant using an aqueous NaHCO 3 soution 3. The fue gas fow rate considered was Nm 3 /h containing 1,000 ppm SO 2, and the partia pressure of CO 2 in the fue gas was 0.14 bars. In this study, the mode proposed wi be appied for the design of a sufite reduction bioreactor for the regeneration of the absorbent used in the SO 2 scrubber. The temperature and the tota pressure were assumed to be 55 C and 1.1 bar, respectivey, equivaent to the conditions in the first coumn. A packed bed reducing contactor is proposed. The packing materia is considered to be 35 mm pastic pa rings. The iquid phase with suspended ces enters the bioreactor at the top and fows countercurrenty with the gas phase. The bioreactor diameter is based on the 85% fooding condition. The height of the bioreactor is determined for fu conversion ( 99%) of sufite to sufide and 99% desorption of sufide produced to the gas phase. The correations used for estimation of the physica properties and mode mass transfer parameters are the same as described previousy 3. Bioreactor performance In the integrated process for SO 2 remova described in the introduction, a cosed iquid oop between the SO 2 scrubber and sufite reduction bioreactor is proposed. Hence, the absorbent iquid from the SO 2 scrubber is directy transferred to the bioreactor and the outgoing iquid from the bioreactor is recircuated to the SO 2 scrubber. Biomass is proposed to grow as a suspension in the bioreactor and the ces are transferred with the iquid phase between two reactors without separation stage. Hence, the objectives to be met in the sufite reduction bioreactor are: Transfer of CO 2 and poory soube H 2 from the gas phase to the iquid phase Bioogica sufite reduction. Transfer of sufide as H 2 S to the gas phase. To prevent subsequent desorption of H 2 S in the SO 2 scrubber, the outet iquid from the bioreactor shoud be free of H 2 S. Therefore, H 2 S resuting from sufite reduction shoud be either stripped off simutaneousy in the sufite reduction bioreactor or removed in a separate stage. Singe stage H 2 absorption and desorption of H 2 S in the sufite reduction bioreactor is preferred to keep the configuration of the integrated process for fue gas desufurization as simpe as possibe. Representative input conditions and design parameters and some mode resuts for the sufite reduction bioreactor are summarized in Tabe 2. These conditions are considered in the simuations. Regeneration of bicarbonate soution Typica resuts when the bioreactor is designed to regenerate the bicarbonate soution used as SO 2 absorbent are discussed. Composition of the inet iquid into the SO 2 reduction bioreactor is the same as the outet iquid from the SO 2 scrubber. Typica interfacia mass-transfer rate profies of the components aong the bioreactor height are shown in Figure 3. Positive and negative vaues of the component fux correspond to absorption and desorption, respectivey. The initia concentration of CO 2 in the gas phase, the concentration of the carbonate species in the iquid phase and the consumption rate of CO 2 determine the direction of the CO2 fux at the gas/iquid interface. In this case in the major part of the bioreactor the partia pressure of CO 2 in the gas phase, is higher than the partia pressure of CO 2 in the iquid phase and, hence, absorption of CO 2 occurs. And ony at the top of the bioreactor desorption of CO 2 occurs, some degree May 2005 Vo. 51, No. 5 AIChE Journa

7 Tabe 2. Typica Design Parameters for Sufite Reduction Bioreactor Bioreactor temperature, T(K) 328 Bioreactor pressure, P(bar) 1.1 Fow rates Inet iquid fow rate, L(m 3 /s) 1.11 Inet gas fow rate, G(m 3 /s) 11.1 Inet iquid composition C Na (kmo/m 3 ) 0.036, 0.114* C SV (kmo/m 3 ) C CIV (kmo/m 3 ) , * C P (kmo/m 3 ) 0.0, 0.072* C X (kg/m 3 ) 10 Inet gas phase composition H 2 (%) 70 CO 2 (%) 30 Maximum sufite conversion 99% Maximum sufide desorption 99% Interfacia area, a (m 2 /m 3 ) 125 % Fooding 85 Liquid hod up 10% Mass transfer coefficients k g,h2 s (m/s) k g,co2 (m/s) k g,h2 (m/s) k g,so2 (m/s) k,h2 s (m/s) k,co2 (m/s) k,h2 (m/s) k,so2 (m/s) iquid fim thickness, (mm) Cacuated coumn height (m) 30.4, 21.8* Cacuated coumn diameter (m) 6.13 In case of phosphate buffer. Figure 4. Conversion rate profie of various components aong the bioreactor, in case of bicarbonate buffer. H 2 S is produced in the iquid phase and subsequenty desorbed to the gas phase. At the top of the bioreactor the partia pressure of H 2 S is higher than the partia pressure of the H 2 Sin the fresh soution; therefore, H 2 S absorption occurs. However, overa H 2 S is transferred to the gas phase in the bioreactor. H 2 is transferred from the gas phase to the iquid phase where it is consumed by sufate reducing bacteria. In the ower part of the bioreactor, no substantia transfer of H 2 is observed, whereas H 2 S desorption occurs. Therefore, it can be concuded that in this region there is no bioogica sufite reduction reaction to consume H 2. In Figure 4, the reaction rate profies aong the bioreactor are shown, which ceary indicate that sufite conversion is accompished in the uppermost of the bioreactor. Therefore, it can be concuded when a bicarbonate buffer soution is used in the SO 2 scrubber, the bioreactor height becomes consideraby higher due to the need for H 2 S desorption. The function of the bottom part of the bioreactor is ony desorption of the remaining H 2 S in the iquid phase. The bioreactor height based on sufite conversion woud be about 60% of the tota bioreactor height needed for the H 2 S stripping. The desorption of sufite species, as SO 2 to the gas phase is negigibe, ess than 0.004% of tota the sufite species in the iquid phase transfers to the gas phase as SO 2 (simuation not shown). Stimuation of H 2 S desorption using phosphate buffer Simuation resuts show that H 2 S desorption is a physica mass transfer process without significant chemica enhancement. However, appropriate ph adjustment heps H 2 S desorption. The adjustment of ph impacts the H 2 S desorption by atering the reative amount of H 2 S, HS - and S 2 species. A decrease in the ph permits an increase in the reative ratio of H 2 S to the tota sufide concentration. The fraction of H 2 S concentration to the tota sufide concentration in the buk iquid using the equiibrium reations is given by C H2S/C tot,sii 11 K 1 /10 ph K 1 K 2 /10 2pH (49) Figure 3. Interface mass-transfer rate profie of various gases aong the bioreactor in case of bicarbonate buffer. where K 1 and K 2 are the first and second dissociation constants of H 2 S, respectivey. This equation indicates that the percentage of the H 2 S is 4.5%, 32 and 82% when the ph is adjusted at 8, 7 and 6, respectivey. Therefore, the H 2 S desorption can be stimuated utiizing an acidic buffer, but inhibition of the bioreaction imits the aowed ph. Buffer capacity, on the other hand, can be increased to reduce ph change of the soution aong the bioreactor. In the case of bicarbonate buffer, increasing bicarbonate concentration to achieve a better buffering is actuay unfavorabe to the process because it causes an increase in ph as we. 1435

8 Figure 5. Interface mass-transfer rate profie of various gases aong the bioreactor, in case of phosphate buffer. Figure 7. ph profie in the iquid buk aong the bioreactor. A combined bicarbonate-phosphate buffer soution was tested as an aternative for the pure bicarbonate buffer described previousy. The simuation resuts obtained when a phosphate buffer is used at the same inet iquid ph of the bicarbonate buffer (ph 6.7) are shown in Figure 5. The resuts indicate that by using a phosphate buffer neary simutaneous desorption of H 2 S in the reaction zone aong the bioreactor can be achieved. The bioreactor height substantiay decreases, about 30%, resuting in a more economica process. A simiar behavior is presented by reaction rate profies aong the bioreactor in Figure 6. The impact of phosphate buffer on ph of the iquid buk aong the bioreactor as compared to the bicarbonate soution is presented in Figure 7. Because the sufite species are converted to the sufide species and subsequenty are transferred to the gas phase as H 2 S, increase in ph of the iquid phase from the top to the bottom of the bioreactor is expected. However, the resuting changes in ph in the case of phosphate buffer due to higher buffer capacity are smaer than those occurring for the bicarbonate buffer. The ow ph of the iquid buk in the case of the phosphate buffer resuts increase of the reative ratio of H 2 S to the tota sufide concentration and, hence, the enhancement of H 2 S desorption. The partia pressure profies of H 2,CO 2 and H 2 S in the gas buk aong the bioreactor are shown in Figure 8. Changes in the partia pressures are the resuts of the gas/iquid interfacia mass transfer and change in the gas fow rate. As the gas phase moves up, H 2 and CO 2 are transferred to the iquid phase and consumed by sufate reducing bacteria. H 2 S produced desorbs to the gas phase, but based on the stoichiometry of the bioogica sufite reduction reaction H 2 S production is about one third of the H 2 consumed; therefore, the gas fow rate decreases aong the bioreactor height. The concentration profies of the tota sufite and tota sufide and H 2 S in the iquid buk aong the bioreactor are depicted in Figure 9. The concentration of the sufite species decreases from the top to the bottom of the bioreactor, due to the bioogica conversion to sufide. The concentration profie of the tota sufide species in the iquid buk is the resut of Figure 6. Conversion rate profie of various component aong the bioreactor, in case of phosphate buffer. Figure 8. Partia pressure profie of various gases aong the bioreactor, in case of phosphate buffer May 2005 Vo. 51, No. 5 AIChE Journa

9 Figure 9. Concentration profie of tota sufite, carbonate, sufide and hydrogen sufide aong the bioreactor, in case of phosphate buffer. absorption/desorption and production. The sufide species is produced and eventuay stripped off to the gas as H 2 S. It is cear that the H 2 S concentration in the iquid buk aong the bioreactor is far from the toxicity eve, kmo/m 312. Sensitivity anaysis A sensitivity anaysis is performed to quantify the effect of some of the mode input parameters on the process performance and to estabish the optima operating conditions. Beow the dependence of the reactor height on three independent variabes, the partia pressure of H 2, the suspended ces concentrations and the tota pressure are described. Effect of H 2 partia pressure on the bioreactor height The required bioreactor height to obtain fu conversion of sufite without considering 99% desorption of H 2 S, is shown in Figure 10. The bioreactor height decreases significanty by increasing the partia pressure of H 2 due to increase in the H 2 gas/iquid interfacia fux. By further increasing of H 2 partia pressure the sufite reduction process switches from mass transfer imited to kinetic imited regime, therefore, decrease in the bioreactor height for sufite conversion sows down. Figure 10 shows aso how increasing H 2 concentration in the inet gas to the bioreactor aters bioreactor height required for fu H 2 S desorption. The bioreactor height decreases by increasing H 2 partia pressure in the inet gas. However, further increase of H 2 partia pressure resuts in an increase of the bioreactor height. Since the inet gas fow and pressure are assumed to be constant high H 2 partia pressure is corresponded to the ow CO 2 partia pressure. And at the ow inet CO 2 partia pressure the ph at the bottom of the bioreactor increases resuting in poor H 2 S desorption and, therefore, the higher bioreactor height. Effect of biomass concentration on the bioreactor height In a cosed iquid oop bioscrubber concept for gas purification the poutant gas stream is uncouped from the absorbent Figure 10. Effect of inet gas H 2 concentration on the bioreactor height at constant tota gas pressure; the height required for H 2 S stripping equas the height for fu H 2 S remova minus the height required for sufite conversion. iquid containing biomass. Therefore, there is no need for biomass retention and a bioreactor with suspended biomass can be appied. In a cosed oop bioscrubber with in principe infinitey ong biomass retention times, much higher biomass concentration can be achieved. The maximum biomass concentration in the bioreactor is achieved when a substrate is used for the growth independent maintenance purposes. However, in practice some beed is required to a avoid toxicity effect resuting from accumuation of sats and haide ions (that is, F - and C ). Furthermore, the beed fow rate can be used to set the ce residence time (SRT) at a desired vaue to washout undesired sow growing microorganisms. The maximum beed corresponding to the ower imit of the SRT in the cosed oop bioscrubber, which is determined by the washout of the desired, suspended ces. The maintenance coefficient at 55 C is estimated to be 0.33 mosiv/c-mo.h, using the correation proposed by Heijnen et a. 9 The tota SIV, which shoud be removed, is about 85 kmo/h. By assuming a 1,000m 3 tota operating iquid voume incuding the scrubber, bioreactor and piping voume, the maximum biomass concentration can be cacuated to amount about 60 kg/m 3. A biomass concentration of 2 kg/m 3 corresponds to a SRT of 1.2h. In Figure 11, the bioreactor height is potted against the biomass concentration in the inet iquid. As biomass concentration in the inet iquid increases the bioreactor height decreases. However, by further increasing of the biomass concentration the sufite reduction process switches from the kinetic imited to the mass transfer imited regime, therefore, the cacuated bioreactor height remains constant. Effect of tota pressure on the bioreactor height The bioreactor height is controed by H 2 absorption rate and H 2 S desorption rate. Increasing the tota pressure at constant gas fow rate eads to a higher H 2 interfacia mass-transfer rate 1437

10 Figure 11. Effect of biomass concentration on the coumn height, and the SRT required. and using phosphate buffer aows simutaneous desorption of H 2 S. Therefore, the bioreactor height wi be decreased. Concusions A nove cosed oop bioscrubber concept for fue gas desufurization was evauated. This process is based on recycing of the absorbent soution between a scrubber and sufite reducing bioreactor without separation of the suspended bacteria ces. The main feature of the cosed oop bioscrubber concept is that much higher biomass concentration and high mass transfer capacity can be achieved. The integrated bioogica process seems a very viabe option for fue gas desufurization, and shoud be further deveoped for arge-scae appications. Feasibiity and engineering aspects of sufite reduction bioreactor using H 2 and CO 2 as energy and carbon source were studied. In this process the bioogica reaction is accompanied with severa chemica reactions in iquid phase, transport of H 2 and CO 2 from gas phase and the subsequent desorption of voatie reaction product H 2 S. To evauate the process a simpe, but reaistic mathematica mode was deveoped. The principe of the mode is, based on the two fim theory which governs the couping of mass transfer (absorption/desorption of the chemica components), specific features of eectroyte species, bioogica and chemica reactions. The proposed mode comprehensivey describes the compicated system of reaction and interface transport. In genera the mode can be used equay for other reactive separation processes. The mode resuts show that, simutaneous desorption of H 2 S produced during the bioogica sufite reduction can best be achieved in the same bioreactor by using phosphate buffer. Therefore, a separate stage for H 2 S desorption is not required. Acknowedgment The support of this project by STW, the Dutch Technoogy Foundation, and the support of the first author by Iranian Ministry of science, research and technoogy are gratefuy acknowedged. Notation a specific interfacia area, m 2 /m 3 C moar concentration in the iquid phase, kmo/m 3 C x biomass concentration, C-kmo/m 3 C j,b concentration of the component j in the iquid buk, kmo/m 3 D diffusion coefficient, m 2 /s G gas voume fow rate, m 3 /s He Henry s aw coefficient, atm.m 3 /kmo k reaction rate constant k g gas side mass-transfer coefficient, m/s k iquid side mass-transfer coefficient, m/s L iquid voume fow rate, m 3 /s ms maintenance coefficient of biomass on H 2, mo H 2 /C_mo. s N j fux of component j in the iquid fim per unit gas/iquid interfacia area, kmo/m 2 s N g j,i interfacia fux of component j per unit gas/iquid interfacia area, kmo/m 2 s P tota pressure, atm p j partia pressure of the component j, atm q H2 biomass specific H 2 consumption rate, mo H 2 /C-mo.s q H2,max maximum biomass specific H 2 consumption rate, mo H 2 /C-mo s r H2 H 2 consumption rate, kmo H 2 /m 3.s r x bacteria growth rate, C-kmo/m 3.s R gas constant, m 3.atm/kmo.K r j reaction rate of the component j, kmo/m 3.s S coumn cross section, m 2 T temperature, K x spatia coordinate in the iquid fim, m Y max sx The maximum yied coefficient of biomass on H 2, C-mo/mo H 2 z spatia coordinate on the coumn height, m z A eectric charge of species A Greek etters g gas fim thickness, m iquid fim thickness, m iquid hodup m 3 /m 3 specific growth rate, 1/s max maximum specific growth rate, 1/s Superscripts g in gas phase in iquid phase Subscripts b in the buk of the gas or iquid phase i at gas/iquid interface Literature Cited 1. Dasu BN, Subette KL: Microbia remova of sufur dioxide from a gas stream with net oxidation to sufate. App Biochem and Biotech. 1989,20: Ruitenberg R, Dijkman H, Buisman CJN: Bioogicay removing sufur from diute gas fows. Jom; the journa of the mineras, metas and materias. SOC : Ebrahimi S, Picioreanu C, Keerebezem R, Heijnen JJ, van Loosdrecht MCM: Rate-based modeing of SO2 absorption into aqueous NaHCO3/Na2CO3 soutions accompanied by the desorption of CO2. Chem Eng Sci. 2003, 58: Sorokin DY, Lysenko AM, Mityushina LL, Tourova TP, Jones BE, Rainey FA, Robertson LA, Kuenen GJ: Thioakaimicrobium aerophium gen. nov., sp. nov. and Thioakaimicrobium sibericum sp. nov., and Thioakaivibrio versutus gen. nov., sp. nov., Thioakaivibrio nitratis sp. nov. and Thioakaivibrio denitrificans sp. nov., nove obigatey akaiphiic and obigatey chemoithoautotrophic sufuroxidizing bacteria from soda akes. Int J of Systematic and Evoutionary Microbioogy. 2001,51: Ebrahimi S, Keerebezem R, van Loosdrecht MCM, Heijnen JJ: Kinetics of the reactive absorption of hydrogen sufide into aqueous ferric sufate soutions. Chem Eng Sci. 2003,58: van Houten RT, Yun SY, Lettinga G: Thermophiic suphate and 1438 May 2005 Vo. 51, No. 5 AIChE Journa

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