Selective Paraffin Removal from Olefin/Paraffin Mixtures by Adsorption to Aluminum Methylphosphate-α: A Molecular Simulation Study

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1 Selectve Paraffn Removal from Olefn/Paraffn Mxtures by Adsorpton to Alumnum Methylphosphate-α: A Molecular Smulaton Study Maake C. Kroon 1,3, Carmelo Herdes and Lourdes F. Vega,3* 1 Process Equpment, Department of Process & Energy, Delft Unversty of Technology, Leeghwaterstraat 44, 68 CA Delft, The Netherlands Insttut de Cenca de Materals de Barcelona, Conseo Superor de Investgacones Centífcas, Campus de la UAB, 8193 Bellaterra, Barcelona, Span 3 MATGAS Research Center and Ar Products and Chemcals, Campus de la UAB, 8193 Bellaterra, Barcelona, Span E-mal: vegal@matgas.com; fax: In ths work the selectve paraffn removal from olefn/paraffn mxtures by adsorpton to the hybrd norganc-organc materal alumnum methylphosphonate polymorph alpha s nvestgated usng Grand Canoncal Monte Carlo smulatons. Ths materal selectvely adsorbs the paraffn from ethane/ethylene mxtures as well as ethyl chlorde/vnyl chlorde mxtures. Therefore, the key factor determnng the selectvty s found to be the molecular nteracton between the methyl group of ths materal and the methyl group of the paraffn. Introducton The olefn/paraffn separaton s the most energy ntensve separaton carred out n the petrochemcal ndustry. Currently, hgh-pressure dstllaton s used as purfcaton method, whch s energy-ntensve and expensve. A cheaper, more effcent, less energy-ntensve and cleaner method for separatng olefns from paraffns would be very desrable. One of these alternatve separaton methods s adsorpton, whch s stll n the research-phase. Commonly, t s the olefn that s preferentally adsorbed usng ths separaton method, because of the strong nteracton between the unsaturated bond of the olefn and a metal on on the surface of the adsorbent materal, formng a π-complex. Because the olefn s preferably adsorbed, t s the paraffn that s easly obtaned n ts pure form. Unfortunately, n ndustral processes the desred product s the olefn (raw materal for polymers) and not the paraffn. Therefore, adsorpton s not an opton to replace the current dstllaton columns (yet). However, when t s the paraffn that s selectvely adsorbed, the olefn can be easly obtaned n ts pure form, whch would make the adsorpton process a very attractve alternatve. Recently, for the frst tme, an adsorpton materal was found that was able to selectvely adsorb the paraffn nstead of the olefn. Ths research nvolved the selectve absorpton of ethyl chlorde compared to vnyl chlorde usng the hybrd norganc-organc materal alumnum methylphosphonate polymorph alpha (AlMePO-α) [1]. The study was performed by a combned expermental-molecular smulaton approach. However, t s stll unclear why AlMePO-α selectvely adsorbs the ethyl chlorde compared to the vnyl chlorde, and whether ths materal s also suted for other olefn/paraffn separatons. Here, we wll use molecular smulatons n order to understand the key factors determnng the selectve paraffn adsorpton by AlMePO-α (molecular nteractons, sterc effects, rgdty) and to nvestgate the sutablty of ths materal for other olefn/paraffn separatons. The molecular model used for the materal s the same developed n prevous work [], whle approprate force felds are 1

2 used for the dfferent olefns and paraffns under consderaton. As an example, the selectve adsorpton of ethane from ethane/ethylene mxtures by AlMePO-α s nvestgated. Methodology Molecular models: The AlMePO-α materal s modeled as consstng of sngle Lennard-Jones spheres for the ndvdual atoms, where the Lennard-Jones parameters of the Al, P and O atoms were taken from Schumacher et al. [3] and the methyl groups are represented as sngle Lennard-Jones stes located at the postons of the C-atom []. The parameters used n ths work are gven n table 1. The smulaton cell conssts of 96 alumnum atoms, 144 phosphorus atoms, 43 oxygen atoms, and 144 methyl groups. Table 1: Lennard-Jones parameters of AlMePO-α used n the smulatons [,3] Ste σ (nm) ε/k B (K) Al - - P - - O CH Both ethane and ethylene are modeled usng three dfferent force felds: two unted-atom molecular models.e. the OPLS-UA model [4] and the TraPPE model [5], and a two-center Lennard-Jones force feld wth pont quadrupole potental.e. the CLJQ model [6,7]. The frst two models wth fxed bond lengths (l ) between the unted-atom groups (CH 3 or CH ) only use the Lennard-Jones ntermolecular potental (φ LJ ) for the non-bonded nteractons, where ε/k B and σ are model parameters [4,5]: 1 6 σ = = σ φ LJ = 4ε (1) 1 1 r r The CLJQ model descrbes the total flud-flud potental as a sum of the Lennard-Jones nteractons (φ LJ ) and the quadrupolar nteractons (φ QQ ), where the quadrupole moment of the flud (Q) s an addtonal model parameter located at the geometrc center of the molecule: 3 Q φ = [ 1 5( c + c ) 15c c + ( c 5c c ) ] () QQ wth: 4 ( 4πε ) r 5 ab c = cosθ (3) c = cosθ (4) c = cos θ cosθ + snθ snθ cosφ (5)

3 where and refer to the LJ stes on the molecules, r s the dstance between the centers of molecules and, r ab s the center-to-center dstance amongst molecules, θ and θ are the polar angles of the molecular axs wth respect to a lne onng the molecular centers, φ s the dfference n azmuthal angles and ε s the vacuum permttvty. The LJ parameters for the CH 3 groups of ethane and the CH groups of ethylene for the three dfferent models and the quadrupole moment for the CLJQ model are shown n table. Table : Force felds and parameters for ethane and ethylene used n the smulatons [4-7] Component Force feld Parameters ε/k B (K) σ (Å) l (Å) Q (B) Ethane OPLS-UA TraPPE CLJQ Ethylene OPLS-UA TraPPE CLJQ Smulaton detals: Wth Grand Canoncal Monte Carlo (GCMC) smulatons, the number of molecules adsorbed versus the actvty s obtaned. In order to change the actvty nto pressure, the vral equaton of state n terms of actvty s used [8]: p p = ξξ + ( ρ ξ ) ( ρ ) + ξ ξ ξ (6) where ξ and ρ are the actvty and densty of the saturaton state pont of pure ethane or ethylene at the needed temperature. The state ponts were calculated from bulk smulatons and can be found n table 3. Table 3: State ponts of ethane and ethylene at dfferent temperatures Component T (K) P (kpa) ρ (mol/l) ξ (Å -3 ) Ethane Ethylene The postons of the atoms of the adsorbent materal were fxed durng the smulatons. Ths mples that the sold-sold nteractons do not affect the calculatons, as they cancel out when energetcally comparng smulated confguratons. Hence, only sold-flud and flud-flud nteractons were calculated. The Lorentz-Berthelot mxng rules were used to calculate the Lennard-Jones parameters between stes of dfferent types. The smulatons requred x1 7 confguratons to reach the equlbrum. Average propertes were calculated over blocks wth 5x1 5 confguratons once the equlbrum was reached. The flud-flud potental was cut at r c = 6 σ ff as recommended by Duque and Vega [9]. 3

4 In order to compare wth prevous expermental and smulaton data on ethyl chlorde/vnyl chlorde adsorpton, the number of ethane/ethylene molecules adsorbed was converted to the amount of ethane/ethylene adsorbed on AlMePO-α n excess to what s present n the bulk. Results and Dscusson In fgure 1 the smulated sotherms for ethane wth the three dfferent models are shown at, and. The error bars are so small that they fall wthn the symbols representng the data ponts. It can be seen that the three models gve smlar results. OPLS-UA TraPPE CLJQ Fgure 1: Adsorpton sotherms of ethane n AlMePO-α at, and usng the OPLS- UA model, thetrappe model and the CLJQ model Fgure shows the smulated sotherms for ethylene wth the three dfferent models at K, and. Agan, the three models gve smlar results. OPLS-UA TraPPE CLJQ Fgure : Adsorpton sotherms of ethylene n AlMePO-α at, and usng the OPLS- UA model, thetrappe model and the CLJQ model When comparng fgures 1 and, t can be notced that the adsorpton of pure ethane s always hgher than the adsorpton of pure ethylene, so preferental adsorpton of the paraffn by AlMePO-α s also observed for ethane/ethylene mxtures. Fgure 3 compares the adsorpton of ethane and ethylene at, and usng the OPLS-UA model. It can be notced that the adsorpton of both ethane and ethylene s hgher at lower temperatures, whereas the selectvty to ethane (=paraffn) adsorpton s hgher at hgher temperatures. The optmal separaton temperature s therefore a trade-off between a hgher overall adsorpton at lower temperatures and a hgher selectvty at hgher temperatures. 4

5 . ethane ethylene ethane ethylene ethane ethylene Fgure 3: Adsorpton sotherms of pure ethane and ethylene at, and usng the OPLS-UA model The obtaned sotherms for ethane and ethylene were also compared to the sotherms determned prevously for ethyl chlorde and vnyl chlorde [1], whch are shown n fgure 4. It can be seen that the adsorpton of pure ethyl chlorde s also always hgher than the adsorpton of pure vnyl chlorde. Also, the dfference n adsorpton between ethyl chlorde and vnyl chlorde s ncreasng wth ncreasng temperature, although ths dfference seems a lttle larger compared to the dfference n adsorpton between ethane and ethylene. However, the maxmum amount of both ethyl chlorde (1.5 ) and vnyl chlorde (.9 ) adsorbed s lower than the maxmum amount of both ethane (1.4 ) and ethylene (5 ) adsorbed. Thus, both ethane and ethylene adsorb better to AlMePO-α than ethyl chlorde and vnyl chlorde. Fgure 4: Adsorpton sotherms of pure ethylene chlorde and vnyl chlorde at K (left), (mddle) and (rght) [1]. Ethyl chlorde expermental data (o), ethyl chlorde smulated results ( ), vnyl chlorde expermental data ( ), and vnyl chlorde smulated results ( ) Because AlMePO-α shows selectvty for paraffn adsorpton for both ethane/ethylene mxtures and for ethyl chlorde/vnyl chlorde mxtures, t s expected that the molecular nteracton between the materal and the paraffn s the key factor determnng the selectvty and not the sterc effects, as ethyl chlorde and vnyl chlorde are much larger than ethane and ethylene. Ths also follows from the snapshots shows n fgure 5, where the ethane, ethylene, ethyl chlorde and vnyl chlorde molecules arrange exactly n the same way n the materal AlMePO-α, wth the methyl groups of the AlMePO-α drected towards the methyl groups of ethane and ethyl chlorde. 5

6 Fgure 5: Snapshots at maxmum loadng for ethane (upper left), ethylene (below left), ethyl chlorde (upper rght) and vnyl chlorde (below rght) Conclusons The materal AlMePO-α shows selectve paraffn adsorpton for both ethane/ethylene mxtures and ethyl chlorde/vnyl chlorde mxtures. Therefore, the key factor determnng the selectvty s found to be the molecular nteracton between the methyl group of ths materal and the methyl group of the paraffn. Acknowledgements Ths work s partally funded by the Spansh government (CTQ5-96/PPQ) and the Catalan government (SGR5-88). Supercomputng tme from CESCA and MATGAS AIE s also acknowledged. We are grateful to A. Valente, Z. Ln and J.A.P. Couthno for ther contrbuton to ths work and for provdng the expermental data. References [1] Herdes, C., Valente, A., Ln, Z., Rocha, J., Coutnho, J. A. P., Medna, F., Vega, L. F., Langmur, Vol. 3, 7, p. 799 [] Herdes, C., Ln, Z., Valente, A., Coutnho, J. A. P., Vega, L. F., Langmur, Vol., 7, p. 397 [3] Schumacher, C., Gonzalez, J., Wrght, P. A., Seaton, N. A., Phys. Chem. Chem. Phys., Vol. 7, 5, p. 351 [4] Jorgensen, W. L., Madura, J. D., Swenson, C. J., J. Am. Chem. Soc., Vol. 16, 1984, p [5] Martn, M. G., Sepmann, J. I., J. Phys. Chem. B, Vol. 1, 1998, p. 569 [6] Curbelo, S., Müller, E. A., Ads. Sc. Technol., Vol. 3, 5, p. 855 [7] Vrabec, J., Stoll, J., Hasse, H., J. Phys. Chem. B, Vol. 15, 1, p. 116 [8] Hansen, J.-P., McDonald, I. R., Theory of Smple Lquds, nd ed., Academc Press: London, UK, 1986 [9] Duque, D., Vega, L. F., J. Chem. Phys., Vol. 11, 4, p