ENERGETIC OPTIMIZATION OF WET AIR OXIDATION PROCESS BASED ON THE PRELIMINARY STUDY ON LIQUID-VAPOR EQUILIBRIUMS. Sébasten Lefèvre 1,2, Jean-Henr Ferrasse 1, Rémy Faucherand 2, Alan Vand 2, Olver Boutn 1* 1 - Ax Marselle Unverstés, Laboratore M2P2 UMR CNRS 6181, Europôle de l Arbos, BP80 13545 Ax en Provence Cedex 4, France 2 - A3 Parc de la Chocolatere 26290 Donzère *Phone number: 33 442 90 85 12 Fax number: 33 442 90 85 15 E-mal address: olver.boutn@unv-cezanne.fr ABSTRACT Wet ar oxdaton process s used to treat wastewater wth hgh chemcal oxygen demand. Temperatures up to 350 C and pressures up to 30 MPa, are operated to develop a noncatalytc process. In the lterature, there s some data about the phase equlbrum of water-ar system but on larger domans and not accurate enough n our range. To study the process, t s necessary to complete the data wth adapted pressures, temperatures and compostons. An expermental set-up has been developed to realze equlbrum measurements as well as trals on waste degradaton. It conssts of a strred reactor of 150 ml equpped wth two sapphre wndows (dameter 4.1 cm) allowng a total vsualzaton of the reactve chamber. A motorzed pump assures the precson of the njectons of components (lqud and gas). A hot/cold regulaton system s also mplemented. The results of experments gve the volume occuped by the lqud and the gas phases, for a combnaton of temperature, pressure and global composton. Measurements are realzed to establsh dew curves for water-ar, waterntrogen and water-waste-ntrogen systems. Ntrogen has the same behavor as Oxygen and can replace t n mxtures for the phases equlbrum studes wth the waste to avod any reacton. The presence of the waste, n the concentratons studed, has no sgnfcant nfluence on the equlbrum ponts. Those data serve to model the lqud-vapor equlbrum n the approprate doman frst wth an deal model and then wth equaton of state Soave-Redlch- Kwong and approprate mxng rules. Those equlbrum models are then ncluded n commercal engneerng software to smulate WAO processes. Wth the objectve to develop a process energetcally optmzed, the second part of ths work s dedcated to perform exergy balances for each component and for the whole process. INTRODUCTION In a world context of legslatons ntensfcaton on the envronmental protecton, t s necessary to mprove the processes of urban water treatment and ndustral aqueous effluents. Domestc waste water comes from varous domestc uses of water. They are essentally carrers of organc polluton (cleaners, fats, solvents, organc molecules). Industral waters have characterstcs whch vary from an ndustry to the other one (refneres, dstlleres, pharmacy, mcroelectroncs ). Besdes organc, ntrogenous or phosphorous matters, they can also contan toxc products, solvents, heavy metals, organc mcropollutants and hydrocarbons. The Wet Ar Oxdaton (WAO) can be adapted to these problems of treatment. It conssts n oxdsng the organc fracton of an effluent contanng a strong concentraton of 1
organc matter not bodegradable (Chemcal Oxygen Demand COD lasts from 10 to 150 g.l - 1 ). For ths process, an oxdzer (ar, pure oxygen, peroxde of hydrogen) s put n contact wth the lqud effluent. WAO processes work n sub-crtcal condtons: pressure between 1 and 20 MPa and temperature between 100 and 320 C. The strong pressures allow mantanng the oxdaton reactons n the lqud phase. The frst stage of ths process s thus the transfer of gaseous oxygen towards the lqud phase. Durng the concepton of a WAO process, t s necessary to consder f oxygen quckly spreads towards the lqud phase. A basc work to develop ths process s the thermodynamc equlbrum that can be reached between the consttuents of the phase gas and the lqud phase. In partcular the knowledge of phase equlbra water - ar s necessary. In order to recover a maxmum of energy, the choce of the ndustral equpments wll depend n partcular on respectve quanttes of gas and on lqud gong out of the reactor. The knowledge of the balances of phase water - ar at frst, but also n the presence of a 3rd compound representng the waste (acetc acd, phenol, etc.), wll allow to choose the most adapted condtons of pressure and temperature. In ths last case, n order to have no chemcal reactons durng the measurement, t s necessary to work wth water / ntrogen system and to valdate that they have an dentcal behavour wth water / ar system. For the exstng expermental data, we can menton Hedemann [1] who measured the quantty of water n ntrogen and n the mxtures CO2 / N2, accordng to the temperature and the pressure. Before that, Hmmelblau [2] studed the solublty of nert gases n water. Closer to our works, studes carred out by Japas and Franck [3] [4] concern the systems water - oxygen and water-ntrogen. Ther expermental data are often used to estmate varous parameters of equatons of state. They were made by trals conducted n a cylndrcal autoclave (volume: 95 ml) equpped wth two sapphre wndows, between 150 and 300 C, and 20 to 270 MPa. These studes showed a smlarty n the behavour of two mxtures ntrogen / water and oxygen / ntrogen. However, ther range of pressure beng hgher than that of WAO (untl 35 MPa), ther curves are thus dffcult to explotable n our operatng doman. In s necessary to develop an accurate method n our expermental doman. MATERIALS AND METHODS Materals The expermental desgn used s presented on Fgure 1. It conssts manly of a vew cell wth a volume of 150 ml. Ths system can reach a maxmum pressure of 300 MPa and maxmum temperature of 350 C. The system s regulated for the pressure wth a hgh pressure syrnge pump and for temperature wth a coolng jacket and an electrc heater. A strrng system allows a very good mxng of the medum. Sapphre wndows allow vsualsng the entre volume of the cell, recorded wth a CCD camera. The camera s used to detect the transton between a bphasc state gas/lqud wth a monophasc lqud state. 2
Fgure 1 Expermental set-up for the measurement of phase equlbra Expermental procedure The expermental procedure s the followng. In the case of the study of a water - ntrogen system for example, t s necessary to fx at frst the quantty of ntal water to be ntroduced and the molar water fracton n the mxture. The ntal ntrogen pressure s then calculated to reach the desred zone of temperature and pressure. The ntrogen ntroduced by the pump s at 20 C. By usng a table of the molar volumes molar of ntrogen at 20 C accordng to the pressure (calculaton made by the equaton of state of Soave-Redlch-Kwong and compared wth the NIST database), we obtan the ntal pressure of ntrogen n the cell. The cell s frst pressurzed to approxmately 2 MPa, to stck wndows sapphre and avod any problem of leaks durng the heatng of the system. All the crcut around the pump (arrval ppes water tank, 50 cm 3 pump cylnder and ppe towards the optcal cell) s then flled wth water and pressurzed, and the quantty of chosen water s njected n the cell. Once the cell solated (zone 1-4 on Fgure 1), the pump crcut s purged wth ntrogen, then ntrogen pressure n the cell s ncreased untl t reach the calculated pressure. Thermodynamc calculatons As mentoned by J [5], attenton has been pad to the modellng of the Henry constant and thanks to the equaton of state relable fttng was provded. Nevertheless testng those equatons for dew pont gves mportant gaps wth data. In ths work two models have been used n order to represent the expermental data obtaned. From a general pont of vew, dew pont can be calculated by consderng the general equlbrum between phases. For a gas - lqud equlbrum the smple relaton to solve s the fugacty equalty: x l f f (1) 3
Ideal model The so-called deal model s based on the Raoult law stpulatng that the partal pressure n the gas phase s proportonal to the molar fracton n the lqud phase. Actvty coeffcents beng chosen equal to unty, a general defnton s gven by the followng equaton: P (2) σ P x The equaton of state for the gas phase s agan the deal gas law. Below the water crtcal pont, for a gven temperature, the dew pont beng well defned, the calculaton can be drectly realsed. The ntrogen beng supercrtcal n the studed doman, t acts more as a permanent gas. Model usng Equaton of State Usng equaton of states consst by defnng from the equaton of state of the pure component an equaton of state for the mxture and then by solvng ths equaton of state to calculate the fugacty coeffcents for both components and both phases. The equlbrum defnton s then gven by: l g x y (3) The equaton of state used s the Soave Redlch Kwong one. P = RT a V - b V(V b) (4) The mxng rule s the MHV2 coupled wth the UNIFAC group nteracton parameters. It conssts n calculatng the parameter a of the equaton of state by dervng them from an actvty model coeffcent. Ths mxng rule s strongly recommended for calculaton close to crtcal pont and for mxture ncludng polar and non polar molecules. The complete mxng rules descrpton can be found n Mchelsen [6]. EXPERIMETAL RESULTS AND MODELLING The expermental results obtaned wth the vew cell are presented on Table 1. In ths Table are presented the dfferent operatng condtons: ntal volume of water placed n the vew cell, the ntal ntrogen pressure, the resultng water molar content. The expermental result ndcates the pressure and the temperature correspondng to the transton between a bphasc and a monophasc state. The condtons tested n partcular for pressures between 12 and 30 MPa, whch are the most employed condtons for Wet Ar Oxdatons processes. The thermodynamc modellng results are shown on Fgure 2 for a water molar fracton of 0.62 and on Fgure 3 for water molar fractons of 0.8 and 0.85. 4
Operatng condtons Transton condtons Water (ml) Intal ntrogen pressure (MPa) Water molar fracton P (MPa) T ( C) 9 1.51 0.85 12.61 325 11 1.88 0.85 15.27 341.9 12 2.11 0.85 15.95 343.7 9 2.11 0.8 16.62 348 8 1.88 0.8 14.08 330.7 7 1.67 0.8 11.63 317.3 10 2.4 0.8 17.75 351.3 11 6.61 0.62 28 348 10 6 0.62 26.15 345.8 9 5.4 0.62 22.74 336.8 8 4.8 0.62 20.55 335.9 8 4.8 0.62 19.67 336.9 9 5.4 0.62 22.38 337.8 Table 1 Expermental results Even f the deal model seems smple and out of scope, n the studed doman of the WAO process, the results gven by the deal model are the closest that can be obtaned for the dew PTx data. Comparson of both models can be seen on Fgure 2 and Fgure 3. The deal model s always closer to expermental data. For a gven pressure, temperature gaps reach 10 C for the deal model but can ncrease to 30 C for the EOS model. Errors due to molar fracton estmaton can be vew on Fgure 2 where results wth a 0.02 absolute error are presented and on Fgure 3 wth a 0.05 absolute error. The dfference s not so hgh to explan the dfference due to the model and for a same model, the dfference wth expermental data. When water molar fracton ncreases to unty both models tend to superpose (see curve for x=0.9 on Fgure 3), and of course to jon the saturated propertes of water. Fgure 2: Comparson of models and expermental ponts for x water = 0.62 ncludng molar fracton error 5
Fgure 3: Comparson of models for x water = 0.8 and 0.85 ncludng molar fracton error and pure water saturaton curve. CONCLUSION WAO processes are operated by pressure and temperature. One of the leadng ponts for the success of oxdaton s to avod that the flud phase dsappeared. Oxdaton leadng to gas formaton, the fracton of gas ncreases durng reacton, meanng that for a gven temperature, pressure would have to be ncreased. In order to manage the process functonng, t s mportant to mplement a thermodynamc model n a process smulator. Takng nto account our expermental obtaned n the range of the process operaton, dfferent thermodynamc models have been tested. The condtons of pressure and temperature beng not very far from the crtcal pont of water, t s dffcult to obtan an accurate modellng. Fnally, the best result has been obtaned wth an deal model. REFERENCES [1] HEIDEMANN, R.A., PRAUSNITZ, J.M., Industral and Engneerng Chemstry, Process Desgn and Development, Vol. 16, 1977, p. 375. [2] HIMMELBLAU, D.M., J. Chem. Eng. Data, Vol. 5, 1960, p. 10. [3] JAPAS, M.L., FRANCK, E.U., Ber. Bunsenges. Phys. Chem., Vol. 89, 1985, p. 1268. [4] JAPAS, M.L., FRANCK, E.U., Ber. Bunsenges. Phys. Chem., Vol. 89, 1985, p. 793. [5] JI, X., LU, X., YAN, J., Flud Phase Equlbra, Vol. 9, 1990, p. 21. [6] MICHELSEN, M.L., Flud Phase Equlbra, Vol. 222, 2004, p. 39. 6