MODELING AND SIMULATION OF REFORMER AUTO- THERMAL REACTOR IN AMMONIA UNIT

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1 Peet trool lleeuum & Cooaal ll IISSN Available online at Petroleum & Coal 9 (), 6-7, 7 MODELING AND SIMULATION OF REFORMER AUTO- THERMAL REACTOR IN AMMONIA UNIT Kayvan Khorsand *, Khadijeh Dehghan Research Institute of Petroleum Industry (RIPI),Tehran, Iran, P.O. Box: , khorsandk@rii.ir Received January 7, 7, acceted June, 7 Abstract In this aer, reformer auto-thermal reactor, which is the secondary reformer reactor in ammonia unit, has been modeled and simulated. The secondary reformer reactor is located after the rimary one and consists of a combustion chamber and a catalytic bed. For simulation of this reactor, combustion chamber (flame area) has been modeled assuming that the feed and excess inlet air to the reactor are fully mixed along the flame. Modeling of flame area yields flame temerature and concentration of reactor comonents and roduct, which have been used in modeling of catalytic bed. Finally, the results of this simulation have been comared with the industrial data taken from the existing ammonia unit in Khorasan Petrochemical Comlex, which show a relatively good comatibility. Key words: Auto-thermal, Reformer, Modeling, Simulation, Ammonia. Introduction Secondary reformer reactor lays an imortant role in ammonia roduction units. This reactor, which is laced after the rimary reformer, fulfils the rocess of methane to hydrogen conversion. In addition it will generate the required nitrogen for the rocess and adjusts the otimum nitrogen to hydrogen ratio required for the ammonia synthesis reaction. To achieve the otimum ratio, the synthesis gas to nitrogen ratio shall come close enough to the required value in the beginning of the rocess cycle, which is fulfilled by the secondary reformer. Excess air in accomany with fuel is entered to flame area to yield the desired amount for the outut of the reactor. The secondary reformer is imortant in the roduction of ammonia as it adjusts the N to H ratio and controls mixing of these two fundamental comonents in the reactor. In fact, oeration of the secondary reformer reactor deends uon incoming air mixture and gas feed which is the outut of the rimary reformer. Secondary reformer substantial reactions are as below: CH H O CO + 3H CO H O CO + H H. 5O H O CO O CO CH O CO + H + () + () + (3) + () + (5) Reactions () and () are for catalytic bed area and reactions (3-5) are related to combustion area. The substantial reactions conducted in combustion area of the secondary reformer reactor includes burning of hydrogen as the incoming gas from rimary reformer includes a lot of hydrogen and hence its burning kinetics has to be incororated to forecast the gas comonents concentrations and temeratures. The schematic diagram of the mentioned areas and the comonents abbreviations has been deicted in figure (). The following assumtions have been taken in to account for modeling of the reactor:

2 Kayvan Khorsand, Khadijeh Dehghan/Petroleum & Coal, 9() 6-7 (7) 65 a) The system is assumed to be unidirectional in the direction of fluid movement. b) As the fluid seed in the reactor is very high, the effects of mass enetration and temerature have been ignored. Also the fluid is not subject to radial diffusion. c) In studies related to the secifications of the gas mixture, relations of ideal fluid and in some cases ideal gas have been incororated. d) There is no oxygen after the combustion area. e) The reactor is adiabatic from thermal oint of view. f) It is assumed that the feed and the incoming excess air to the reactor are fully mixed along the flame. g) The reactions in combustion area are assumed to be under equilibrium. Modeling of flame area Flame area in secondary reformer has an imortant role in adjustment of N to H ratio. The amount of inlet air to the reactor should assure comlete combustion of O before its arrival to the catalytic bed and yield an otimum ratio for N and H +CO for synthesis reactor. In literature this otimum value is -3.5 [,,5]. By modeling of flame area it will be ossible to recisely estimate the temerature changes and gas comonents by changing the concentration of inlet feed and air to the reactor. Karim and Metwally (98) [] have investigated the kinetic behavior of steam-methane homogenous mixture in temerature range of 3 K, ressure of.5 atm. and steam to methane ratio of -5. They showed that in the mentioned oerating conditions and the amount of steam to methane ratio of more than, carbonization would not occur considerably and hence the nature of the reaction is homogenous. This result has been incororated in the assumtions of the current modeling. The imortant oint in reforming rocess is that comonents concentrations are less sensitive to rocess ressure. For instance, ressure imact on roduct concentration is very low in a wide range of residence times say - second. On the other hand, steam to carbon ratio has a high imact on the roducts concentrations such as CO, H and CO []. Fig. () Schematics of the main area of the secondary reformer reactor.. Heat and Mass balance in flame area Considering the assumtions made for modeling of the flame area, reactions have been considered to be in equilibrium [,5]. This assumtion seems reasonable, as the goal of the modeling is to derive flame area temerature, concentration of different comonents at the end of combustion and the otimum ratio of inut air to feed from rimary reformer to secondary one. Assuming l + m + n + + q = mol for inut feed to the secondary reformer and considering item d of the modeling assumtions, if x, x and x 3 are assumed as molar consumtions of oxygen in reactions (3), () and (5) resectively, then alying of molar balance equation of oxygen

3 Kayvan Khorsand, Khadijeh Dehghan/Petroleum & Coal, 9() 6-7 (7) 66 around the reactor yields x3 = z x x and considering item g of the modeling assumtions and equilibrium relations for all comonents, x and x can be derived. As illustrated in Fig (), the required relations for molar balance of the main comonents are as follow: N Balance: H Balance: O Balance: C Balance: c e = 79z (6) l + q + = d + v + w (7) m + l d + u z + n + = + c + f (8) m + n + q = u + w + (9) Simultaneous incororate of equilateral relations for reactions (3) and () yields: ( l + x )[ m x [ x + (z x x = 3 / + (z x x)]( n + x) )] ( K K ). 5 () According to relations given in [], equilibrium constants of reactions (3) and () are related to temerature based on the following equation:.5 Log (K3/K ) = /T c () Similarly for reactions () and (5), we have: DK [ x + (z x x )] [ m x + (z x x )] P = () [ q (.5 ( K 5 / K ) z x x)]( n + x)( + 63z x x) In which the equilibrium constants have the following relation with temerature:.5 Log (K /K ) /T c 5 = (3) x will have an integer value when DK=. After derivation of x, x, x3 and z, concentration of reaction comonents after combustion will be obtained by means of molar balance. For calculation of sensible heat changes the following relation will be used: T Tr HS = ( rc i i ) dt () T To r 5 T 5 ( ic i ) dt To i= i= Total heat of reactions (3), () and (5) is obtained by the following relation: HR 3,,5 = ( HF) r ( ), i HF T (5) c, i i Tc i i= i= Target function will be based on energy balance of the combustion chamber. If the temerature of the combustion area, T c, is calculated correctly, DHC will be zero: DHC T 3 c = HR3,, 5 HS (6) Tr

4 Kayvan Khorsand, Khadijeh Dehghan/Petroleum & Coal, 9() 6-7 (7) 67 Simultaneous solving of heat and mass balance equations, will yield the flame temerature and reactor comonents concentrations at the end of the flame area which will be considered as inut data to catalytic bed area. 3. Modeling of Catalytic Bed The mathematical model has been derived based on the models offered for the secondary reformer in [,3,,5]. The kinetics for the reforming reaction of methane with Alumina based nickel catalysts is according to those given in []. Considering the 5 imortant reactions conducted in combustion chamber and reforming catalytic bed and also the heat transfer restrictions stating that there is not any transfer of heat in the reactor walls and the heat transfer related to artial combustion and reforming will be only in combustion chamber, the secondary reformer reactor will have a one dimensional heterogeneous and adiabatic model [,3,]. It is of course imortant to consider the effect of local hot sot on activity and longevity of the catalyst. 3. Heat and Mass Balance in Catalytic Bed As the model used for the catalytic reactions of the secondary reformer reactor is one dimensional and heterogeneous, considering reactions () and (), mass balance equation for comonent i along the reactor will be as follows: ni = k= ξi, kηkrk ( ε ) A (7) Comonent i can be CO, H, CH, H O and CO. For nitrogen, argon and inactive comonents, the simle relation of ni = n T will be used. η k Is the catalyst effectiveness factor, r k is the rate of i= reaction k, A is the reactor cross section area, ξ i, k is the stoichiometry coefficient of comonent i in reaction k and ε is the orosity of catalytic bed. Considering that the resulted heat from heat transfer between gas and catalyst surface is equal to the reactions heat in steady state, the heat balance equation can be written as below: k = 6( ε ) h( Tg Ts ) ΔH kη k rk ( ε ) = (8) D s Heat transfer coefficient in catalyst surface will be derived using the below relation [5]: NU.6 3.Re r = (9) So the heat balance equation for the gas hase along axial direction will be as follows: Tg FgC ρ g = ΔH kηk rk ( ε ) A g k = () In which Fg is the molar flow rate of the gas hase. Using the mentioned equations it will be ossible to forecast the molar distribution of each comonent, gas temerature and catalyst surface temerature along the bed. As the simulation of the reactor is done in steady state, artial differentials will be converted to comlete differentials. 3.. Momentum Balance in Catalytic Bed Alying Ergun relation for catalytic bed, it will be ossible to calculate the ressure dro as be below [8] :

5 Kayvan Khorsand, Khadijeh Dehghan/Petroleum & Coal, 9() 6-7 (7) 68 dp d = G ρg D c ε 5 ) ε D ( 3 ( ε ) μ +.75G () μ - the average viscosity of gas mixture in the bed, D - the diameter of catalyst articles, ρ - the density of gas mixture inside bed and G - the mass flux of the gas inside bed. Relations of all the other hysical arameters with temerature, ressure and concentrations are according to reference [7].. Results and Discussion The oerating conditions as well as concentration of inut comonents to the reactor adoted from documents and PFD of Khorasan Petrochemical Comlex Ammonia Unit have been given in Tables () and (). The outut values of the reactor in both cases have been comared in Table (). Temerature distribution along the catalytic bed has been illustrated in Fig (). As the industrial data along the reactor are not available, only the inut temerature to the reactor, temerature of the combustion area and outut temerature have been comared with the industrial data. According to the simulation results, temerature of the combustion chamber is C, which is quite comatible with the real value of 7 C. Values of the comonents concentrations have been given in Table (). As there is no industrial data available in this area, they are not comarable. Considering the results obtained from modeling and the assumtions made for the combustion chamber, the main art of the model shows a good comatibility with the industrial unit. For better and recise forecast of this area behavior, the kinetic model of the combustion shall be also incororated. Fig (): Distribution of Temerature along the Catalytic Bed

6 Kayvan Khorsand, Khadijeh Dehghan/Petroleum & Coal, 9() 6-7 (7) 69 Fig (3): Distribution of Comonents Concentrations along the Catalytic Bed Distribution of comonents concentrations has been deicted in Fig (3). Concentration of the comonents in the outlet of the combustion chamber and inut to the reactor catalytic bed has been given in table (). Comarison has been only done in catalytic bed, as industrial data are not available for the other sections. Fig () illustrates the distribution of catalyst surface temerature along the catalytic bed. Fig (5) shows the ressure dro along the catalytic bed, which shows quite a good comatibility with the industrial reactor data. Fig (): Distribution of Catalyst Surface Temerature along the bed

7 Kayvan Khorsand, Khadijeh Dehghan/Petroleum & Coal, 9() 6-7 (7) 7 Fig (5): Pressure Dro Variation along the Catalytic Bed Parameters Temerature, o C Pressure, bar H N CH Air+ Inert CO CO O H O Table (): Inut Data of the Industrial Reactor [] Inut Feed to the Secondary Reformer (Outut of the Primary Reformer) Molar Flow Rates of the Comonents, kmol/hr Inut Air to the Secondary Reformer Table (): Outut Data of the Industrial Unit and Simulation Outut of the Outut of the Secondary Reformer Combustion Area and Parameters (Industrial Unit) Inut to Catalytic Bed (Simulation) Temerature, o C Pressure, bar H N CH Air+ Inert CO CO O H O Molar Flow Rates of the Comonents, kmol/hr Outut of the Secondary Reformer (Simulation)

8 Kayvan Khorsand, Khadijeh Dehghan/Petroleum & Coal, 9() 6-7 (7) 7 Acknowledgements We acknowledge the fully collaborations by Khorasan Petrochemical Comlex to disosal of industrial data within this investigation. References [] Karim, G.A. and M.M. Metwally, Int. J. Hydrogen Energy, Vol. 5, 98, [] Davies, J.; and Lihou, D. A.; Otimal Design of Methane Steam Reformer; Chemical and Process Engineering; Aril 97; : 7-8. [3] De Groote, Ann M., and Gilbert F. Froment, Alied Catalysis A: General, Volume 38, Issue, 996, Pages 5-6. [] Xu, J. & Froment G.F.; Methane Steam Reforming: II. Diffusional Limitations and Reactor Simulation; AIChE Journal; 989; Vol. 35; No.. [5] Yu. Y.H.; Simulation of Secondary Reformer in Industrial Ammonia lant; Chemical Engineering and Technology; Vol.5 (3); ; : [6] Froment, G.F.; and Bischoff, K.B.; Chemical Reactor Analysis and Design, 99, John Wiley & Sons, Ed. [7] Elashie S.S.E.H Elshishini; Modeling, Simulation and Otimization of Industrial Fixed Bed Catalytic Reactors; 993. [8] Witt, P. M. and L. D. Schmidt, Journal of Catalysis, Vol. 63, Issue, 996, Pages [9] Tsiouriari, V. A., Zhang, Z. and X. E. Verykios, Journal of Catalysis,Volume 79, Issue, 998, Pages [] Feasibility Study on Technology Transfer and Localization of Ammonia Production Unit, Research Institute of Petroleum Industries, 38. [] KHPC technical documents. [] htt://