An analytical solution for the analysis of zero-length-column experiments with heat effects

An analytical solution for the analysis of zero-length-column (ZLC) experiments with heat effects is developed. The model is an extension of the original one developed by Eic and Ruthven with the inclusion of the energy balance. Two additional parameters are obtained, beta = (DeltaH/C-p) (partial derivativeq/partial derivativeT)\ (c0),(T0) and alpha = (ha/C-p)(r(c)(2)/D-c). A criterion for negligible heat effects, 3L beta/alpha < 0.1, is derived from the analytical solution based on ZLC operating parameters. ZLC desorption curves in nonisothermal operation are discussed. The model reduces to the original solution of isothermal operation developed by Eic and Ruthven when heat effects are negligible. ZLC experiments with heat effects are analyzed, and trends are in good agreement with theory. Because of its simplicity, the model is a valuable tool for the analysis of ZLC experiments with heat effects

Octane upgrading of C-5/C-6 light naphtha by layered pressure swing adsorption

The performance of a layered pressure swing adsorption (PSA) process for the separation of high research octane number (HRON) paraffins from a C-5/C-6 light naphtha fraction is simulated with a detailed, adiabatic single column PSA model. A zeolite 5A layer is used for selective adsorption of the low RON linear paraffins, while a zeolite beta-layer is used to separate the intermediate RON 3MP from the H RON fraction. The effects of various independent process variables (zeolite 5A to zeolite beta ratio, purge to feed ratio, cycle time, operating temperature, and depressurization mode) on the key dependent process variables (product RON, H RON species recovery, HRON purity, and adsorbent productivity) are evaluated. It is demonstrated that an optimal zeolite 5A to zeolite beta ratio can improve the product average RON up to 1.0 point as compared to existing processes using zeolite 5A only. Moreover, process simulations demonstrated that increasing the operating temperature from 523 to 543 K results in an octane gain of 0.2 RON

Separation of branched hexane isomers using zeolite BEA for the octane improvement of gasoline pool

A sorption study of single, binary, ternary and quaternary mixtures of hexane (C6) isomers nhexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2- dimethylbutane (22DMB) was performed in commercial pellets of zeolite BETA (BEA structure), covering the temperature range between 423 K and 523 K and partial pressures up to 0.3 bar. From these data, single and multicomponent adsorption equilibrium isotherms were collected. An extended tri-site Langmuir model (TSL) was developed to interpret accurately the equilibrium data, and a dynamic adsorption model was developed and tested predicting with a good accuracy the behaviour of multicomponent fixed bed experiments. At the partial pressures studied, the sorption hierarchy in the zeolite BETA is: nHEX>3MP>23DMB>22DMB. BEA structure demonstrates a significant selectivity between C6 isomers, especially at low coverage, giving a good perspective regarding future workJosé A. C. Silva acknowledges financial support from Fundação para a Ciência e Tecnologia under project EQU/60828/2004. Patrick S. Bárcia acknowledges a FCT grant (SFRH/BD/30994/2006)

Octane Upgrading of TIP Processes by Recycling in a Layered Zeolite 5A/BETA PSA

The objective of this work consists in studying the separation mono/dibranched paraffins by cyclic adsorption process using a layered bed of zeolites 5A and Beta (Figure 1). Aspen ADSIM 2006.5 (AspenTech Inc.) was used for numerically solving an adiabatic dynamic model incorporating mass, energy and momentum balance. Model parameters were taken from experimental data reported in the literature 1, 2. Parametric studies were simulated to determine how process performance is affected by purge quantity, 5A-to-Beta ratio, repressurization/blowdown schemes and operating temperature. Figure 2 shows that a combination of zeolites 5A and Beta can produce an octane gain of 1 RON at 523 K comparatively to the conventional TIP3 by reducing the monobranched C6 fraction in the product. Another advantage of this configuration is the possibility to increase the penetration distance because zeolite Beta acts like a “barrier” to the linear alkanes desorbed from zeolite 5A during the cocurrent depressurization step. It was also demonstrated that a slight increase in temperature (20 K) results in a RON benefit of 0.2 points. Several alternatives are provided to improve the performance of the existing TIP processes with this combination of adsorbents

Adsorption equilibrium and kinetics of branched hexane isomers in pellets of BETA zeolite

Sorption equilibrium and kinetics of hexane isomers: n-hexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) were studied in commercial pellets of zeolite BETA in the form HBEA with a SUM ratio of 150, between temperatures of 423 and 523 K and partial pressures up to 0.3 bar. Four different models were used to interpret the equilibrium data, named: Langmuir, multi-site Langmuir, dual-site Langmuir and Toth. The affinity to the adsorbent measured by the Henry's constants decreases with the degree of branching, with selectivities that can reach a value of 8.8 between nHEX and 22DMB. The heats of adsorption at zero coverage decrease with the degree of branching, being: 63.4 kJ/mol for nHEX, 59.7 kJ/ mol for 3MP, 57.1 kJ/mol for 23DMB and 53.6 kJ/mol for 22DMB. However, the isosteric heat of sorption changes with coverage with a different behavior for the three isomers. Sorption kinetics studied by the Zero Length Chromatography (ZLC) technique allowed us to find the nature of controlling the diffusion mechanism (macropore or micropore); for nHEX and 3MP macropore diffusion is controlling with activation energies similar to the heats of sorption at zero coverage. For 23DMB and 22DMB, the controlling mechanism changes, being the system governed apparently by both macropore and micropore diffusion. Data from this work are also compared with those reported in literature for both zeolite BETA and silicalite. (c) 2004 Elsevier Inc. All rights reserved

Separation of light naptha by adsorption processes

In this work we studied the separation linear/mono/di-branched paraffins in a cyclic Pressure Swing Adsorption process using a layered bed of zeolites 5A and Beta (Figure 1). Zeolite Beta proved to be an efficient separator of mono-branched from di-branched paraffins [3]. Aspen ADSIM (AspenTech Inc.) was used for numerically solve an adiabatic dynamic model incorporating mass, energy and momentum balances. Model parameters were taken from experimental data obtained in our lab. The studies were performed with the objective to determine how process performance is affected by purge quantity, 5A-to-Beta ratio, repressurization/blowdown schemes and operating temperature which are typical operating parameters of PSA processes. Figure 2 shows the product average RON as a function of the zeolite 5A layer length and purge-to-feed ratio at T = 523 K, where it can be seen that RON is above 90 when the ratio of the layered bed is nearly 0.6. It was also demonstrated that a slight increase in temperature (20 K) results in a RON benefit of 0.2 points. Several alternatives are also provided to improve the performance of the existing TIP processes with this combination of adsorbents [4]

Upgrading of the light naphata fraction with zeolite beta

Multicomponent adsorption of alkanes in the C5-C6 range was investigated in a sample of zeoliteBETA. 6-components breakthrough experiments were performed showing interesting properties forthe separation of the light naphtha fraction. The lower adsorption enthalpy of C5 alkanes was explored by a temperature sensibility analysis demonstrating that at 583 K C5/C6 equimolar mixture can be separated in a single step into low and high octane number (RON) fractions. The separation 23DMB/3MP can not be achieved in the case of mixtures with the typical composition of the isomerization products, but zeolite BETA still selective for 22DMB resulting in a slight increase of the RON comparatively to the final isomerate from the conventional processes

Separation by fixed-bed adsorption of hexane isomers in zeolite BETA pellets

An experimental study of single and binary fixed-bed adsorptions of hexane isomers n-hexane (nHEX), 3-methylpentane (3MP), 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB) was performed in commercial pellets of zeolite BETA, covering the temperature range between 423 and 523 K and partial pressures up to 0.3 bar. The effect of partial pressure and temperature on the shape of the breakthrough curves was addressed. From these data, single and binary adsorption equilibrium isotherms were collected. On the basis of the analysis of sorption events at the molecular level, two different models were used to interpret, with good accuracy, the equilibrium data: dual-site Langmuir (DSL) for nHEX and 3MP and multisite Langmuir (MSL) for 23DMB and 22DMB. Thereafter, a dynamic adsorption model was developed and tested, predicting with good accuracy the behavior of the fixed-bed experiments. At the partial pressures studied, it was found that the affinity of the isomers to the zeolite is nHEX > 3MP > 23DMB > 22DMB. The selectivity between the isomers is higher at low partial pressures, decreasing as the amount adsorbed increases. The Ideal Adsorbed Solution Theory using the DSL model to describe the pure component equilibrium of nHEX and 3MP and the MSL model for the dibranched isomers 22DMB and 23DMB gives a good prediction of the mixture adsorption data

Single and multicomponent sorption of CO2, CH4 and N-2 in a microporous metal-organic framework

Single and multicomponent fixed-bed adsorption of CO2, N-2, and CH4 on crystals of MOF-508b has been studied in this work. Adsorption equilibrium was measured at temperatures ranging from 303 to 343 K and partial pressures up to 4.5 bar. MOF-508b is very selective for CO2 and the loadings of CH4 and N-2 are practically temperature independent. The Langmuir isotherm model provides a good representation of the equilibrium data. A dynamic model based on the LDF approximation for the mass transfer has been used to describe with good accuracy the adsorption kinetics of single, binary and ternary breakthrough curves. It was found that the intra-crystalline diffusivity for CO2 is one order of magnitude faster than for CH4 and N-2.This work was supported by an Award CHE 0718281 from the National Science Foundation (B.C.), the University of Texas-Pan American (UTPA) through a Faculty Research Council Award (B.C), in part by the Welch Foundation (Grant BG-0017) to the Department of Chemistry at UTPA. J.A.C.S. acknowledges financial support provided by national research grant FCT=POCTI=EQU=60828=2004 and by LSRE financing by FEDER=POCI=2010. P.S.B. acknowledges his Ph.D. scholarship by FCT (SFRH=BD=30994=2006), and L.B. acknowledges Henri Pieper Grant (Institute HEMES Gramme, Belgium) for the financial support

Modeling adsorption equilibria of xylene isomers in a microporous metal–organic framework

Single and multicomponent adsorption equilibria of xylene isomers: o-xylene (o-x), m-xylene (m-x), pxylene (p-x) and ethylbenzene (eb) was investigated on the three dimensional microporous metal–organic framework Zn(BDC)(Dabco)0.5 (BDC = 1,4-benzenedicarboxylate, Dabco = 1,4-diazabicyclo[2.2.2]-octane), MOF 1, in the range of temperatures between 398 and 448 K and partial pressures up to 0.1 bar. The equilibrium data show that a significant amount (around 34 g/100gads at 398 K) of xylene isomers can be adsorbed in MOF 1. The affinity to the adsorbent measured by the Henry’s constants to decreases in the order o-x > m-x > eb > p-x for all temperatures. The zero coverage adsorption enthalpies are all similar and range from 77.4 (eb) to 79.8 kJ/mol (o-x). The Dual-Site Langmuir model (DSL) was used for the interpretation and correlation of the experimental data. The parameters obtained from the pure component isotherms fitting were also used to predict the multicomponent equilibrium data by an extended DSL model. A good agreement was obtained between the predictions and the experimental data. It was also demonstrated that the DSL model is also capable to explain the increase in the isosteric heat of sorption with increasing coverage.José A.C. Silva acknowledges the financial support from Fundação para a Ciência e Tecnologia under Project POCI/EQU/ 60828/2004. Patrick S. Bárcia acknowledges a FCT PhD Grant (SFRH/BD/30994/2006). This work was supported by an Award CHE 0718281 from the NSF (BC)