EFFECT OF NITRIC AND SULFURIC ACIDS ON NO X AND SO X ABSORPTION INTO OXIDO-ACIDIC SOLUTIONS

Distilltion Absorption 2010 A.B. de Hn, H. Kooijmn nd A. Górk (Editors) All rights reserved by uthors s per DA2010 copyright notice EFFECT OF NITRIC AND SULFURIC ACIDS ON NO X AND SO X ABSORPTION INTO OXIDO-ACIDIC SOLUTIONS Isbelle Liémns nd Dine Thoms Chemicl Engineering Deprtment, Fculty of Engineering, University of Mons, Rue de l Eprgne, 56, 7000 Mons, Belgium Emil: dine.thoms@umons.c.be Abstrct SO 2 nd NO x efficiencies were studied t 293 K nd 1 tm, in cble contctor, up to prtil pressures of 2000 ppm, in mixed queous solutions contining sulfuric nd nitric cids, s products of the wet oxidtive removl of SO x nd NO x, nd hydrogen peroxide, s oxidizing gent. The crrier gs ws essentilly crbon dioxide (90% vol). The CO 2 bsorption in oxido-cidic conditions ws found to be negligible. The SO 2 bsorption rte ws determined for vrious SO 2 prtil pressures nd different concentrtions of H 2 SO 4 nd HNO 3 (0-4M) nd hydrogen peroxide (0.05-1M) in the scrubbing liquid. It ws found tht the SO 2 bsorption rte increses shrply with the hydrogen peroxide concentrtion nd decreses s the H 2 SO 4 concentrtion increses. This influence of sulfuric cid is prepondernt nd, in presence of both nitric nd sulfuric cids, only slight influence of HNO 3 is noticed, modifying the properties of the scrubbing liquid but likely involving negligible effect on the kinetic of the rection between SO 2 nd H 2 O 2. Similr experiments hve been crried out with NO x s gs solutes, for different oxidtion rtios, showing tht n cid medium enhnces the NO x bsorption rtes. Keywords: bsorption, SO 2, NO x, hydrogen peroxide, mixed cids 1. Introduction In combustion processes, especilly with col s combustible nd ir s oxidnt, non negligible mounts of sulfur dioxide (SO 2 ) nd nitrogen oxides (NO x ) re produced, source of ir pollution involving cid rins nd corrosion problems in the industril instlltions. In desulfuriztion nd denitrifiction processes, most of the techniques re designed to remove SO x or NO x seprtely. Dry, semi-dry or wet lkline desulfuriztion processes 1,2 do not remove NO x in n efficient wy; SCR 3,4 or SNCR denitrifiction processes re quite inefficient for SO x btement, which cts moreover s poison in the system. In industril flue gses however these two compounds re very often present together, in vrible concentrtions up to 5000 ppm, depending on the combustible composition nd on combusting conditions. An efficient simultneous removl of SO x nd NO x is thus ttrctive. As lkline solutions produce liquid effluents contining nitrites, nitrtes, sulfites nd sulftes, downstrem wste tretment process is required. An bsorption technique tht hs been proposed 5,6 nd extensively studied 7,8 s well for desulfuriztion s for denitrifiction purposes uses hydrogen peroxide in the scrubbing liquid to oxidize SO 2 nd NO x in sulfuric nd nitric cids respectively, cids tht cn be reused or vlorized. Moreover, hydrogen peroxide is very clen compound from n environmentl point of view, dding only wter nd oxygen to the system nd thus generting no dditionl pollution problems. The im of this pper is to chrcterize the effect of HNO 3 nd H 2 SO 4 concentrtions in the scrubbing solution on the bsorption performnces of SO 2 nd NO x respectively. The following steps of this study will be focused on the comprehension of bsorption-rection mechnisms nd on the determintion of the best operting conditions of the industril process of simultneous bsorption nd, in finl step, to determine design prmeters. 2. Absorption mechnisms of SO 2 nd NO x in presence of H 2 O 2 Previous studies were chieved on seprte bsorption process of SO 2 into H 2 SO 4 +H 2 O 2 solutions 7 nd of NO x into HNO 3 +H 2 O 2 solutions 8. When SO 2 is bsorbed in solutions contining hydrogen peroxide, the bsorption-rection mechnism results in the formtion of sulfuric cid which concentrtes if the liquid solution is recycled in the process. The rection of SO 2 with H 2 O 2 occurs irreversibly, t finite speed, producing sulfuric cid ccording to: 325

I. Liémns nd D. Thoms. SSOO 2 + HH 2 OO 2 HH 2 SSOO 4 (1) Hydrogen peroxide is efficient even t low concentrtions, nd incresing its concentrtion in the liquid phse results in higher SO 2 bsorption rtes, enhncing the oxidtion rection. However the presence of H 2 SO 4 in the bsorbent (while recycling the solution for instnce) is unfvorble to the SO 2 removl. The nitrogen oxides of interest in liquid nd gs phses re NO, NO 2, N 2 O 3, N 2 O 4 nd, in presence of wter, HNO 2 nd HNO 3. The mounts of these species in the gs phse re not independent, following the next equtions (3)-(7). The presence of wter nd oxygen in the gs phse is tken into ccount in equtions (3),(6),(7): 2 NNNN + OO 2 2 NNOO 2 (2) NNNN + NNOO 2 NN 2 OO 3 (3) 2 NNOO 2 NN 2 OO 4 (4) NN 2 OO 3 (NNNN + NNOO 2 ) + HH 2 OO 2 HHHHOO 2 (5) The quite complex mechnism of bsorption of these nitrogen oxides into wter 9 is represented by equtions (6)-(8). The hydrolyses of NO 2, N 2 O 3 nd N 2 O 4 generte HNO 3 nd HNO 2 in the liquid phse: NN 2 OO 4 (2 NNOO 2 ) + HH 2 OO HHHHOO 2 + HHHHOO 3 (6) NN 2 OO 3 + HH 2 OO 2 HHHHOO 2 (7) A prt of the nitrous cid formed in liquid phse is decomposed with the undesirble relese of NO., s nitrogen monoxide hs very low solubility into wter: 3 HHHHOO 2 HHHHOO 3 + HH 2 OO + 2 NNNN (8) The presence of n oxidizing gent in the liquid phse ppers to be dul: on the one hnd to stbilize the dissolved NO x or HNO 2 formed in liquid phse by oxidtion in form of HNO 3, on the other hnd, to enhnce the mss trnsfer rte of HNO 2 from the gs phse. These two roles re dependent on the following rection: HH + HHHHOO 2 + HH 2 OO 2 HHHHOO 3 + HH 2 OO (9) Furthermore, it ws shown tht, when NO x re bsorbed in hydrogen peroxide solutions, the nitric cid formed enhnces the NO x bsorption for gseous mixtures contining trivlent NO x species (N 2 O 3 nd HNO 2 ) due to n utoctlysis of the oxidtion rection (9) 10,11. It ws previously proved tht the bsorption phenomenon of the species NO 2, N 2 O 3 nd N 2 O 4 is controlled by hydrolysis, nd there is no enhncement of the trnsfer rtes with H 2 O 2 for these compounds. 3. Experimentl set-up nd procedure The experimentl equipment is illustrted in Figure 1. It includes cbles-bundle lbortory scrubber which is well suited for kinetic studies s its specific surfce is quite insensitive to the liquid flow rte nd viscosity 12. The bsorption rector is ring-shped column mde of verticl glss tube in the xis of which stnds polypropylene rod which supports 6 twisted polypropylene cbles of 1.7 mm dimeter, constituting the pcking. Outside nd inside dimeters of the nnulus re equl to 0.045 nd 0.02 m, respectively nd the useful height of this micro-contctor is 0.54 m. The prepred scrubbing solution is fed to top distributing chmber with peristltic pump, the liquid being distributed round ech cble through individul holes drilled in the bottom of the chmber. A stedy flow hving the shpe of cylindricl films is set up round the yrns (Fig.2b). Gs nd liquid phses come into contct counter-currently in the scrubber. The liquid nd gs feed temperture is controlled by mens of thermosttic bth. The liquid phse nlysis is performed by clssicl titrtion of totl cidity with sodium hydroxide solution nd n iodometric method for H 2 O 2, using N 2 S 2 O 3 solution in presence of KI. In order to nlyze seprtely the two cids (HNO 3 nd H 2 SO 4 ), conductimetric titrtion method ws set up for the determintion of SO 4 2- ions concentrtion. 326

Effect of nitric nd sulfuric cids on NO x nd SO x bsorption into oxido-cidic solutions 13 3 2 4 12 wter 5 hood 11 6 1. crrier gs cylinder 2. humidifiction column 3. solute cylinder 4. flow indictor/control 5. het exchnger 6. cbles-bundle contctor 7. solution tnk 10 8. switch for solution recycling 9. liquid pump 10. het exchnger 11. liquid distributor 12. membrne dryers 9 13. gs nlysis Horizontl cross-section of the contctor 8 gseous flow upwrds 1 sewer sewer 7 liquid flow downwrds Figure 1.. Experimentl pprtus. b. Horizontl cross-section of the contctor The crrier gs, entering xilly t the bottom nd flowing out up to the top of the column, is humidified in sturtor, in which SO 2 (or NO x ) is dded to obtin the desired concentrtion. The gs flow rtes re metered by rotmeters. Smpling of gs simultneously t the input nd the output of the column is performed continuously through membrne dryers followed by two-chnnel U.V. nlyzer for SO 2, llowing to clculte the frction of SO 2 bsorbed A SO2 =( in out )/ in or chemiluminescence nlyzer for NO nd totl NO x, llowing the clcultion of the bsorption rte of NO x : A NOx =(y NOx in y NOx out )/y NOx in nd the oxidtion rtio O.R.:(y NOx in y NO in )/y NOx in, chrcterizing the inlet gs composition. All experiments reported here were crried out t tmospheric pressure nd temperture of 20±0.3 C with crrier gs contining 90%vol. of crbon dioxide nd nitrogen ±10%. The gs flow rte ws mintined t 0.85 m³/h resulting in superficil velocity of 0.2 m/s nd the liquid flow rte ws fixed t 185 ml/min (33.3 ml/min/cble which is the nominl vlue for n industril cble scrubber) (Tble 1). Tble 1. Operting conditions y in (ppm) O.R. (%) C H2O2 (M) C HNO3 (M) C H2SO4 (M) G (m³/s) L (m³/s) T (K) SO 2 600-2000 - 0-1 0-4 0-4 NO x 5000 15-98 0-1 0-3 0-2 2.45 10-4 3.13 10-6 293 4. Results nd discussion As preliminry, mss blnces were checked for the cse of SO 2 bsorption into queous solutions of H 2 O 2 by mesuring the quntities of bsorbed SO 2 (equivlent to H 2 SO 4 produced) nd consumed H 2 O 2. Moreover, s fr s the bsorption test results re concerned, since the bsorption rte depends on the contctor nd operting conditions, it cn be esily used for performnces comprisons of the different scrubbing solutions. Concentrtions of nitric cid, sulfuric cid nd hydrogen peroxide respectively represented in figures by x,y nd z in mol/l. In ll oxido-cidic conditions pplied in our bsorption tests it ws found tht CO 2 bsorption rtes cn be considered s negligible. 4.1. Effect of H 2 O 2, H 2 SO 4 nd HNO 3 on SO 2 bsorption Absorption tests of SO 2 into wter nd sulfuric-nitric cid solutions of different concentrtions contining hydrogen peroxide were chieved continuously for vrious SO 2 inlet concentrtions vrying from 600 to 2000 ppm (Tble 1). For the ske of comprison, SO 2 bsorption tests were lso performed with wter nd 0.5M sodium hydroxide solutions. 327

I. Liémns nd D. Thoms. As it cn be seen in Figure 2.., frctions removed with wter re reltively moderte. Indeed, the rection of SO 2 with wter, though instntneous, is limited by the equilibrium 13 : SSSS 2 + 2 HH 2 OO HH 3 OO + + HHHHOO 3 (10) Absorption of SO 2 into wter is not of gret interest for desulfuriztion process, except mybe becuse it is very esy nd chep to operte. If the liquid solution contins cids in ddition of wter 14, for instnce, to simulte process recycling the solution, these bsorption rtes drop drsticlly to very low vlues. When the oxidizing gent H 2 O 2 is dded to the cidic solution, quite higher SO 2 efficiencies re clerly observed. As shown in Figure 2.b., mximum bsorption rtes re observed for sodium hydroxide solutions. Actully the rection of SO 2 with NOH 2 : SSOO 2 + 2 NNNNNNNN NN 2 SSOO 3 + HH 2 OO (11) known s proton trnsfer rection is irreversible nd instntneous 1. The chrts here bove present reltive bsorption rtes, for which bsorption rtes into NOH solutions re used s reference (A=1). b.5,y=1,z=0.5,,z=0.5.5,y=1,z=0 z=1 z=0.5 x=1.5,y=3,z=0 Figure 2. Effect of hydrogen peroxide in cidic solutions on SO 2 bsorption performnces (), effect of H 2 O 2 concentrtion (b). It ppers quite clerly in Figure 2 tht in queous H 2 SO 4 -HNO 3 -H 2 O 2 mixtures the enhncement effect of the mss trnsfer, due to the chemicl rection of SO 2 with H 2 O 2, increses with the liquid phse content in peroxide, but remins lwys lower thn for sodium hydroxide solution, which implies tht the rte of rection with H 2 O 2 remins finite. The bsorption rte is independent of the prtil pressure of SO 2 in the inlet gs for NOH or sufficiently high H 2 O 2 content, ctully in presence of n excess of neutrlizing or oxidizing gent. b y=1 y=4 y=1 y=4 Figure 3. Effect of sulfuric cid concentrtion on SO 2 bsorption performnces in solutions contining hydrogen peroxide without () nd with (b) nitric cid. 328

Effect of nitric nd sulfuric cids on NO x nd SO x bsorption into oxido-cidic solutions The presence of sulfuric cid lwys leds to negtive effect on SO 2 bsorption rtes, s cn be seen on Figure 3 when incresing H 2 SO 4 for given H 2 O 2 content in the liquid phse. The presence of nitric cid does not seem to ffect pprecibly the result. On the other hnd, if rise in the nitric cid concentrtion decreses the SO 2 bsorption performnces, it is negligible compred to the effect of sulfuric cid, s shown on Figure 4 s down shift of the group of curves, for concentrtions of both cids up to 2M. At higher nitric cid concentrtions (4M on Figure 4.b), noticeble decrese of A SO2 cn be observed. The negtive effect of sulfuric cid on globl bsorption process is well known 7. But, concerning the nitric influence, since it hs only slight effect t low concentrtions (up to 2M), the decrese in bsorption rtes t higher concentrtions could probbly be explined by lower ph or lower SO 2 diffusivity or solubility in the liquid phse, mking the solute trnsfer more difficult. However, no kinetic effect cn clerly be pointed out. The presence of nitric cid in the bsorption solution, even if smll negtive effect hs been shown, is not rel disdvntge for the bsorption performnces since the negtive influence of H 2 SO 4 is predominnt nd its formtion, in SO 2 oxidtive bsorption process, is unvoidble. b.5 x=1.5 x=1 x=4 z=0.5 z=0.5 Figure 4. Effect of nitric cid concentrtion on SO 2 bsorption performnces in solutions contining hydrogen peroxide without () nd with (b) sulfuric cid. 4.2. Effect of H 2 O 2, HNO 3 nd H 2 SO 4 on NO x bsorption Compred to experimentl SO 2 bsorption study, fewer experiments were chieved for NO x study t this step of our study, leding however to interesting conclusions. Absorption of NO x into wter rises with the oxidtion rtio ((y NOx in y NO in )/y NOx in ). Actully, t low O.R. vlues, NO x re minly composed of NO, the fr lest soluble compound. The proportion of more soluble species rises with the oxidtion rtio, inducing better removl. The ddition of hydrogen peroxide into wter results in the sme bsorption rtes, but when nitric cid is present in the oxidizing solution, rise in the performnces cn be observed (Figure 5.) s the concentrtion of HNO 3 rises, especilly t intermedite O.R. (mximum NO x trivlent forms). b y NOx in = 5000 ppm reltive ANOx x=1 reltive ANOx.29.275 y NOx in = 5000 ppm O.R. (%) O.R. (%) Figure 5. Influence of nitric cid concentrtion () nd sulfuric cid concentrtion (in comprison with HNO 3 ) (b) on NO x bsorption performnces in solutions contining hydrogen peroxide 0.2M 329

I. Liémns nd D. Thoms. Figure 5.b illustrtes quite similr bsorption rtes obtined with HNO 3 solution (0.29M), nd H 2 SO 4 solution (0.275M) nd presenting equl H 3 O + concentrtion (M), nd both contining H 2 O 2 (0.2M). This clerly mens tht the enhncement is cused by the H 3 O + ions in the solution, whtever the H 3 O + source. It cn be concluded tht the presence of sulfuric cid in the bsorption solution together with nitric cid ppers therefore s n dvntge for the nitrogen oxides bsorption. 5. Conclusions Absorption performnces of dilute gseous SO 2 were studied in cble contctor t 293 K nd 1 tm, up to concentrtions of 2000 ppm, with mixed queous solutions contining sulfuric nd nitric cids nd hydrogen peroxide. The bsorption rte ws determined for vrious SO 2 prtil pressures nd different concentrtions of H 2 SO 4 nd HNO 3 (0-4M) nd hydrogen peroxide (0.05-1M) in the scrubbing liquid. It ws found tht the SO 2 bsorption rte increses with the hydrogen peroxide concentrtion nd decreses s the H 2 SO 4 concentrtion increses. While sulfuric cid concentrtion hs n unfvorble effect on SO 2 bsorption, HNO 3 does not seem to hve kinetic effect on the oxidtion rection (1). Its concentrtion somehow modifies the ph nd physicochemicl properties of the liquid solution, nd thus the trnsfer properties, ltering the bsorption performnces, but the influence of H 2 SO 4 remins predominnt. Similr experiments hve been crried out with NO x s gs solute, for different gs compositions nd 5000 ppm, confirming tht both sulfuric nd nitric cids enhnce the bsorption rtes for trivlent NO x species. Simultneous bsorption of SO 2 nd NO x into H 2 O 2 solutions, producing both sulfuric nd nitric cids, shouldn t pose problem in SO 2 nd NO x removl performnces. The next step of this study will be the determintion of the interction effects in simultneous SO 2 nd NO x bsorption in mixed solutions HNO 3 +H 2 SO 4 +H 2 O 2. It ims finlly t developing modeling of the bsorption process for simultneous reduction of SO 2 nd NO x with H 2 O 2 s oxidnt in liquid phse, nd formtion of nitric nd sulfuric cids s by-products. Acknowledgments The uthors re grteful to Air Liquide CRCD nd to the Fculté Polytechnique of UMONS for the finncil support of this study. References 1. H. Hikit, S. Asi, T. Tsuji, Am. Inst. Chem. Eng. J. 23 4(1977), 538 544 2. W. Vn den Broeke, J.B. Lefers, Chem. Eng. Tech. 56 10(1984), 792 793 3. L. Olsson, H. Sjovll, R.J. Blint, Appl. Ctl., B, 81 3-4(2008), 203-217 4. E. Tronconi, L. Lietti, P. Forztti, S. Mlloggi, Chem. Eng. Sci., 51 11(1996), 2965-2970 5. P.V. Deo, Chem. Prot. Environ. 34(1988), 275 292 6. Deguss, W. Von Wedel et.l., Europische Ptentnmeldung, 0 457 059 A1(1991) 7. D. Thoms, S. Colle, J. Vnderschuren, Chem. Eng. Process., 44(2005), 487-494 8. D. Thoms, S. Brohez, J. Vnderschuren, Trns. IChemE, 74B(1996), 52-57 9. H. Komiym, H. Inoue, Chem. Eng. Sci. 35(1980), 154-161. 10. E. Hlfpenny nd P.L. Robinson, J. Chem. Soc. 168(1952), 928-938 11. D. Thoms, J. Vnderschuren, M. Brignd, J. Chim. Phys. 95(1998), 523-535 12. S. Lefebvre, J. Vnderschuren, D. Verheve, F. Onyejekwe, Chem. Ing. Tech. 51 4(1979), 330-331 13. H. Hikit, S. Asi, H. Nose, Am. Inst. Chem. Eng. J. 24 1(1978), 147 149 14. Powerspn Corp., F. Alix et.l., Interntionl ptent, WO 01/87464 A1(2001) 330