modeling of equilibrium and dynamic multi-component adsorption in a two-layered fixed bed for purification of hydrogen from methane reforming products

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1 modelng of equlbrum and dynamc mult-component adsorpton n a two-layered fxed bed for purfcaton of hydrogen from methane reformng products Mohammad A. Ebrahm, Mahmood R. G. Arsalan, Shohreh Fatem * Laboratory of Adsorpton Processes, Department of Chemcal Engneerng, Faculty of Engneerng, Unversty of Tehran. * P.O.Box-11155/4563, Enghelab Ave. Tehran, Iran, Fax: ( ) , Emal: shfatem@ut.ac.r Adsorpton s one of the most common ndustral processes for Hydrogen purfcaton. Layered bed columns wth dfferent adsorbents consstng actvated carbon and zeolte materals are used for effcent adsorpton of lght gases such as, CO 2, CO, N 2 and CH 4, n methane reformng gas product and purfcaton of hydrogen wth hgh qualty. A dynamc mult-component adsorpton model was developed n a fxed bed of ndustral scale contanng two layer of adsorbent. Lumped knetc model wth lnear drvng force between sold phase concentraton and ts equlbrum condton was assumed for mass transfer between the gas and sold phase. Plug flow was consdered wth axal dsperson and varyng velocty along the bed. Extended Langmur model was chosen for the equlbrum adsorpton of each component n the gas mxture and ther parameters were predcted by sngle gas adsorpton experments. The breakthrough curves of the gases where determned at adsorpton and desorpton modes. The senstvty analyss of the model was carred out to ndcate the effect of feed pressure and flow rate on purty, and breakthrough curves of hydrogen. Keywords: adsorpton; Hydrogen purfcaton; double layered; dynamc modelng. 1. Introducton Hydrogen purfcaton usng layered bed pressure swng adsorpton (PSA) unts s one of the most common ndustral processes to produce Hydrogen wth purty for fuel and other ndustral applcatons. (Grande et al., 2007, Papadas et al., 2009 and Lopes et al., 2010). For the proper desgn and smulaton of the PSA unts, the adsorbent characterstcs and the effectve physcal parameters should be frstly dentfed. The equlbrum adsorpton model and ts parameters are the key components n characterzaton of the adsorbents propertes and they control the mass transfer zone and breakthrough curves n dynamc mode of adsorpton column. Therefore, dentfcaton of these parameters s the target of the present work. The sotherm and physcal parameters derved from the present work would be used n a further research for smulaton of the PSA process of hydrogen purfcaton. In the present work, frstly the expermental sotherms of H 2, N 2, CO, CO 2 and CH 4 are obtaned for the two adsorbents; actvated carbon and zeolte 5A at two temperatures. Thereafter, they are best ftted on Langmur equaton. The Langmur equaton and ts parameters are used n the dynamc model of a two- layered packed bed

2 column, and the senstvty analyss of the breakthrough curves and mass transfer zone s nvestgated wth operatonal condtons for the gaseous mxture. 2. Equlbrum adsorpton experments Frstly, the Zeolte5A adsorbents were condtoned for a perod of 3 hours by a 100 ml/mn flow of Helum at 573 K for pre-treatment. Before each test, both adsorbents loaded n adsorpton cell, were mantaned n vacuum (-0.8 barg) at 353 K for a perod of 3 hours to completely desorb all the adsorbed gases. In ths work, the measurements of pure gas equlbrum sotherms were conducted usng a statc volumetrc setup (Gholamhosseny M., et al., 2008 ). The experments were started wth a known ntal pressure and t was recorded to approach to a constant equlbrum condton. These tests were performed for all gases (H 2, CO, CO 2 and CH 4 ) on both actvated carbon and zeolte 5A n K and K at dfferent pressures up to 21 barg. The sotherm data were best ftted on Langmur model and the parameters of ths model were determned by fttng the model on the expermental results. 3. Dynamc adsorpton modelng The mass balance for each component of the mxture and the total mass balance n the element of fgure 1 were wrtten as follows: Fgure 1: Schematc dagram of adsorpton cell used for modelng Component mass balance: 2 N Y Y Y Y P RT (1 εb ) q q = Dz u + ρ 2 P Y ρp t z z P t P ε (1) b t = 1 t Total mass balance: N C ( uc ) (1 ε ) ρ q = t t t t b P z ε b = 1 Mass transfer rates are expressed as lnear drvng force (LDF) model. q * = k( q q ) (3) t The mult-component adsorpton equlbrum model s represented by the extended Langmur sotherm, In whch the constants b and q s depend on the temperature as equaton (5),(6) followng way: (2)

3 q * sbpy q = (4) () n 1 + bpy j = 1 b =b exp (b / R T ) 0 1 (5) q s =a +a /,1,2 T (6) Ergun s equaton was appled to the pressure drop along the bed. 2 2 dp 150 µ (1 - ε ) V 1.75 ρ(1 - ε ) V = + (7) dz ( φ d p ) ε φ d pε Boundary condton: PY P u RT z = 0: = RT [ Y Y f ], u = u F, P = P z D z F (8) z PY RT = L : = 0 z (9) Intal condton: Y H2 (z, 0) =1, Y (z, 0) =0, P (z, 0) =P f (10) To solve the above system of partal dfferental equatons, the spatal dervatves are dvded usng a backward dfference scheme, and the resultng ordnary dfferental equatons are solved wth the Runge-Kutta method n Matlab programmng software. 4. Result and Dscusson 4.1 Equlbrum models Sngle gas equlbrum sotherm tests were performed for the gases (H 2, CO, N 2, CO 2 and CH 4 ) on actvated carbon and zeolte 5A adsorbents n 298K and 308K. The results are shown graphcally n fgure 1. The result of equlbrum adsorpton experments was well ftted usng Langmur sotherm models. The parameters of ths model are derved and shown n table 1. As shown n fgure 1 equlbrum adsorpton of Hydrogen on both adsorbents obeys Henry's rule, therefore sotherm data of Hydrogen were ftted on Henry's model (eq. 11). The parameters for ths model are shown n table Dynamc adsorpton model The model was solved numercally for the adsorpton bed consstng of a 5.38 meter Actvated Carbon layer at the bottom of the bed and a 1.87 meter Zeolte 5A layer at the top loaded n a column of meter dameter. Feed to the column conssts of a

4 H 2 :CO 2 : CO: N2:CH4 wth molar composton of 0.757:0.18:0.007:0.024:0.032 and flow rate of 0.6 m 3 /s and pressure equal to 21 barg. Fgure 2(a) llustrates the breakthrough curve of the layered bed. The breakthrough curve s shown n fgure 2(b). At tme of 180 seconds the purty of Hydrogen approaches to In mass transfer zone of CO 2, the stronger adsorpton of CO 2 causes desorpton of other mpurtes as shown n the form of rollups n the breakthrough curves of other gases. Table1: The calculated parameters of Langmur model AC Zeolte a 1, a 2, b 0, b 1, CO E CO E N E CH E CO E CO E N E CH E Fgure 1: Result of sothermal equlbrum adsorpton experments ((a) Actvated Carbon, 308 K (b) Zeolte 5A, 308 K (c) Actvated Carbon, 298 K (d) Zeolte 5A, 298 K) Fgure 3(a) presents the concentraton profle of CO 2 along the bed length n varous tmes. The mass transfer zone moves along the bed length n the same drecton wth the

5 gas moton and gets to the end of the bed. In fgure 3(b) the model results of Hydrogen mole fracton s llustrated at varous tmes durng the desorpton step by the purge flow of pure hydrogen equal to the feed flow rate n adsorpton step. The amount of hydrogen n column s ncreased by ncreasng the tme because of more desorpton of mpurtes. * Table2: Parameters of Henry's model for adsorpton of Hydrogen ( q = IP P) Adsorbent Actvated Carbon Zeolte IP 1 (mol.bar -1.gr -1 ) Fgure 2: Breakthrough curve of all components (a) at the end of the column, (b) at the end of the AC layer Fgure 3: Mass transfer zone of CO 2 a) at adsorpton mode, b) at desorpton mode Fgure 4(a) llustrates the breakthrough curve of Hydrogen at dfferent feed pressures. Wth ncreasng the feed pressure, the bed s saturated faster resultng n lower breakthrough tme. Fgure 4(b) demonstrates the breakthrough curve of Hydrogen at dfferent feed flow rates. Wth ncreasng the feed flow rate, the rate of adsorpton ncreases resultng n a shorter breakthrough tme.

6 Curves n a layered bed of Actvated Carbon and 5A Zeolte s studed n the ndustral scale column. The adsorpton sotherm models and ther parameters were determned by 5. Conclusons In the present work, dynamc adsorpton of methane steam reformng products; mxture of H 2, CO 2, CO, N 2 and CH 4, s studed by mathematcal modelng of breakthrough the statc experments carred out for each type of adsorbent; AC and 5A zeolte and they were well ftted on the Extended Langmur equaton for the mxture adsorpton. Fgure 4: Effect of feed pressure (a) and feed flow rate (b) on breakthrough of Hydrogen The dynamc adsorpton-desorpton model was developed for the gas speces wth pressure drop and varaton of the gas velocty along the bed. Breakthrough curves for both adsorpton and desorpton modes were determned and reported. It was concluded that after a perod of 180 seconds workng at adsorpton mode, the purty of Hydrogen would be 99.9%. References Gholamhosseny M., Fatem S. and Rasoolzadeh M., 2008, Hydrogen adsorpton and equlbrum models on multwalled carbon nanotubes at moderate temperatures and pressures, Internatonal Journal of Chemcal Reactor Engneerng 6, art. No. A80. Grande C.A., Lopes F, Rbero A., Lourero J., and Rodrgues A., 2008, Adsorpton of Off-Gases from Steam Methane Reformng (H2, CO2, CH4, CO and N2) on Actvated Carbon, Separaton Scence and Technology, 43: Lopes, F.V.S., Grande C.A., Rodrgues A.E., 2010, Actvated carbon for hydrogen purfcaton by pressure swng adsorpton: Multcomponent breakthrough curves and PSA performance. Chemcal Engneerng Scence, do: /j.ces Papadas D., Ahmed SH., Kumar R. and Joseck F., 2009, Hydrogen Qualty for Fuel Cell Vehcles A Modelng Study of The Senstvty of Impurty Content n Hydrogen to The Process Varables n The SMR PSA Pathway, nternatonal journal of hydrogen energy 34,