MODELLING OF NEW REACTOR CONCEPT FOR HYDROTHERMAL OXIDATION

Transcription

1 MODELLING OF NEW REACTOR CONCEPT FOR HYDROTHERMAL OXIDATION S. Vielcazals*, J. Mercadier*, F. Marias*, P.Dutourié*, M. Bottreau**, F. Casell**. * Laboratoire de Thermique, Eergétique et Procédés EA 193 Rue J. Ferry, PAU, Frace Tel : , Fax : acques.mercadier@uiv-pau.fr ** Istitut de Chimie de la Matière Codesée de Bordeaux (ICMCB) 87, aveue du Dr Schweitzer, PESSAC Cédex, Frace Abstract: Hydrothermal oxidatio is a efficiet ad clea way for the trasformatio of waste-water cotaiig orgaic compouds. So-called hydrothermal oxidatio, these processes will be called subcritical oxidatio or supercritical operatio depedig upo pressure ad temperature coditios. The mai advatage of these processes is that byproducts are o toxic. Ideed, orgaic material (C,H,O) is exclusively coverted ito carbo dioxide ad water. These processes are particularly suitable for the treatmet of wastewater too cocetrated for biological treatmet or too wet for icieratio treatmet. The purpose of this work is to develop a mathematical model for ew reactor cocept for hydrothermal oxidatio, i order to improve its efficiecy. This reactor is horizotal ad tubular, ad is desiged with multi iectio poits of oxidat. Its diameter is very small with regard to its legth ad turbulece flow is fully developed i the reactor. The mathematical model is based o plug flow reactor behaviour assumptio. The goverig equatios are : mometum, mass, species ad eergy balaces. Thaks to this model, the temperature, cocetratio of wastes ad oxidat, ad velocity profiles are computed. Itroductio: Nowadays, idustrial societies geerate more ad more quatities of orgaic wastes. I Frace, sice 1 st July 00, oly ultimate passive wastes ca be put i tippig. So, importat research has bee maaged to develop ew techology i order to limit the toxic ed-products formatio, to reduce the waste volume ad to miimise the eergy cosumptio. Hydrothermal oxidatio process allow a cold combustio without eergy supply, due to the high exothermic reactio of oxidatio ad without toxic ed-products. Furthermore, CO obtaied as ed-product ca be easily collected ad used for further specific idustrial applicatio. I order to improve the waste treatmet by hydrothermal oxidatio from a techical ad ecoomic poit of view, we proposed a ew cocept of reactor schematically represeted o figure 1. It s a horizotal tubular reactor with a small diameter ad a great legth to obtai the desired residece time. This reactor operates uder turbulet flow coditios. Its particularity is that the oxidat is iected at 3 differet locatios. The mai advatages of this ew cocept are : (1) a best maagemet of the reactio s heat, () a better COD value i the iitial effluet (due to the low temperature ilet).

2 1 st iectio of oxidat (T i =5 C) d iectio of oxidat (T i =5 C) 3rd iectio of oxidat (T i =5 C) Waste : Q ilet = 1kg/h T ilet = 300 C P ilet =5MPa 1 st Area (9 m) d Area (9 m) 3 rd Area (30 m) Figure 1 : Schematic represetatio of a ew cocept of reactor with multi iectio poits. Numerical Procedure Model s Assumptios: Waste is cosidered like mixture of model orgaic compouds; Iected oxidat is supposed totally ad istataeously dissolved i the fluid phase; Fluid behaviour is cosidered as pure water for the calculatio of the thermodyamic properties: ideed, wastes are composed at least of 90% of water. We used the equatio of state IAPWS-IF97 (which is a ew formulatio of IFC-67) to calculate the properties of water (desity, viscosity ad ethalpy) accordig to temperature ad pressure. Reactor is assumed to behave as a plug flow reactor; Axial molecular diffusio ad thermal coductio are all egligible; For a total oxidatio, the stoichiometric equatio is : C H The reactio rate is assumed to be i the form of a global rate expressio: m ra = k exp A. Co R.T Where the activatio eergy (Ea) is expressed i kj mol -1 ; k is the pre-expoetial factor, C A ad C O are molar cocetratios i waste ad O i mol m -3 ; the rate is writte i mol A s -1 ; p ad q are the reactio orders with respect to orgaic compouds ad oxyge. Goverig Equatios: α β O γ β γ α.o 4 β.h O α. CO The field i the reactor is describes by the mass, mometum, species ad eergy coservatio [1], [], [3], [4]. O the iectors, these equatios are : ρ o = V u Mass coservatio

3 ρu u. V O P τ xx Mometum coservatio A k.exp R.T p A.Co q Orgaic compoud coservatio O O V M O = 1 α β 4 γ k.exp.c R.T p A.C q O O coservatio CO ρuh h O α k.exp.c 1 R.T O P 5 C = u. V = p A.C q O CO coservatio RT p q ( ) H.k.exp.C q& = 1 O the reactor, these equatios are : A O Eergy coservatio ρu Mass coservatio ρu P τ xx Mometum coservatio A k.exp R.T p A.Co q Orgaic compoud coservatio O = 1 α β 4 γ k.exp R.T p A.C q O O coservatio CO p q α k.exp.c A.C O x 1 R.T CO coservatio = ρuh P p q = u. H.k.exp.C A.C O q& x x 1 RT Eergy coservatio = Where x is the axis compoet, u the fluid velocity, ρ the fluid desity, P the static pressure, µ the viscosity, h the fluid ethalpy, h O (5 C) the oxyge ethalpy at 5 C (which is the iectio temperature of oxyge), T the fluid temperature, H the heat of reactio, O the V flow rate of oxyge per uit volume, M O the oxyge mass molar, τ xx the stress tesor, represets the th waste i the mixture ad the total umber of waste presets i the mixture (1 ). Furthermore, this program works with two thermal coditios: q & for a adiabatic reactor 4 or q& = h therm ( T T ) i the case of covective heat trasfer. With h therm the global d coefficiet of thermal exchage ad T the exterior temperature.

4 Resolutio: Fiite volume is used to discretize the system. The most attractive feature of the cotrol volume formulatio is that the resultig solutio would imply that the itegral equatios of coservatio are exactly satisfied over ay group of cotrol volumes ad, of course, over the whole calculatio domai [5], [6]. After this step of discretisatio, the system of algebraic equatios is solved thaks to the Newto-Raphso method. The use, i the Newto Raphso method, of a relaxatio coefficiet improves stability of the system especially whe the mass cotet i waste exceeds 3%w ad thus that the icrease i temperature becomes sigificat. The reactio of oxidatio takes place maily i the first meter after the oxidat iectio. Thus, i this part of the reactor the icrease i temperature is most sigificat. Therefore, the meshes will be more stregtheed ear the iectio poits. This grid takes the form of a geometrical cotiuatio of reaso q. x i x i1 x Figure : irregular grid for oe-dimesioal situatio. x i1 = q. x i x = q -1. x 1 Simulatio data: The simulatio of hydrothermal oxidatio process requires the kowledge of : 1) The chemical pathway; ) The reactio ethalpy ad thermal coditio [7]; 3) The thermodyamic properties of pure water (desity, viscosity, ethalpy) [8]; 4) The kietic parameters [9]; 5) Ethalpy of oxyge [10]; Moreover, may parameters are also eeded for the program: flow rate of solutio to be treated, flow rates of oxyge iected, iitial temperature, iitial pressure, waste cocetratio, adiabatic reactor or ot Process simulatio: For all followig simulatios, we suppose a adiabatic plug flow reactor. Simulatios were performed at 5 MPa with a ilet temperature of 300 C for a mass flow rate of 1 kg.h -1.

5 Usig a classical approach for the global chemical pathways, simulatios of temperature profiles, i this ew reactor cocept, are preseted i figure %w 490 6%w T( C) 440 5%w 390 Tc 4%w 340 3%w %w Leght of the reactor Figure 3 : Temperature profiles i the reactor for pheol oxidatio at differet ilet mass cotet. For each iectio, oe-third of the stoichiometric quatity of oxyge is iected. We observe o the figure 3 that, i the eighbourhoods of the critical poit, the icrease i temperature is less importat due to the highest value of the heat capacity as figure 4 demostrates it. 1%w

6 Cp kj/ (kg.k) 15 Cp kj/(kg.k) T C Figure 4: Heat capacity evolutio (Vs. Temperature) Thaks to figure 5, we ca determie the quatity of orgaic compouds ecessary to cross the critical poit. The compoud studied here is the pheol i a rage of mass cotet betwee 1%w ad 6%w. 550 T( C) Tc yc %w iitial Figure 5 : Evolutio of the outlet temperature for differet mass cotet i pheol. As figure 5 shows it, 1.65%w of pheol are ecessary to cross the critical poit for aour operatig coditios.

7 y%w y CO (%w) y O (%w) ywaste(%) yo(%) yco(%) 0 y phéol (%w) Legth of the reactor Figure 6: Example of mass cotets profiles i the reactor for pheol oxidatio (6%w). Sice we iect oe-third of the stoichiometric quatity i oxyge, for each iectio, there are oe-third of pheol oxidised ad oe-third of CO produced. Moreover, we observe that all the pheol is oxidised ito CO ad H O at the ed of the reactor. We have see that this program ca work with two thermal coditios (adiabatic or covectiv exchage with exterior). Figure 7 compares the temperature profiles for this two cases. I this simulatio h therm =W/(m².K) ad T =98,15K ; the others parameters are the sames tha the previous simulatios Xf=99,8% Xf=99,67% T( C) adiabatic covectiv heat trasfert Legth of the reactor Figure 7: Compariso of the temperature profiles accordig to the thermal coditio.

8 Coclusio The model preseted i this study allows to simulate the behaviour of a tubular reactor with multi iectio poits. It s a geeral program which leave the choice to the user to : 1)- the umber of orgaic model compouds preset i the mixture; )- the umber of oxidat iectios; 3)- the mass flow rates; 4)- the waste cocetratios; 5)- the iitial temperature ad pressio; 6)- the thermal coditios; 7)- the geometrical characteristics of the reactor. Here, we have studied the oxidatio of pheol. Temperature ad cocetratio profiles are preseted i this paper but the program is also able to computed, i particular, velocity, desity ad ethalpy profiles. Thaks to the temperature evolutio, it s possible to determiate the quatity of waste ecessary to cross the critical poit. Furthermore, thermal coditios are tested: adiabatic reactor ad covective trasfer. Refereces [1]- R. Byro, Warre E. Stewart, Edwi N. Lightfoot, Trasport Pheomea, Wiley Iteratioal Editio, []- R. Comolet, Mécaique expérimetale des fluides, Tome II, Dyamique des fluides réels, Masso Paris Mila Barceloe, [3]- W. Rohseow, H. Choi, Heat, Mass, ad Mometum Trasfert, Pretice-Hall, Eglewood Cliffs, New Jersey, [4]- I.E. Idel cik, Memeto des Pertes de Charge, Editios Eyrolles, [5]- C.T. Shaw, Usig Computatioal Fluid Dyamics, Pretice Hall Iteratioal, 199. [6]- V. Suhas Patakar, Numerical Heat Trasfert ad Fluid Flow, Taylor & Fracis, [7]- P. Dutourié, Thèse de Doctorat, Uiversité de Pau et des Pays de l Adour, 000. [8]- W. Wager, A. Kruse, The Idustrial Stadard IAPWS-IF97 for the Thermodyamic Properties ad Supplemetary Equatios for other Properties, Properties of Water ad Steam, Spriger, [9]- L. Li, P. Che, E.F. Gloya, Kietic Model for Wet Oxidatio of Orgaic Compouds i Subcritical ad Supercritical water, Supercrit. Fluid Eg. Sci., ch4, pp , [10]-Kacke, Kubaschewski, Hesselma, Thermochemical Properties of Iorgaic Subtaces, Spriger, Verlag, Secod Editio, 1991.