Kinetics of sorption in deactivated zeolite crystal adsorbents

doi:10.1016/S0098-1354(02)00259-4

Computers & Chemical Engineering

Volume 27, Issue 5, 15 May 2003, Pages 689-696

https://doi.org/10.1016/S0098-1354(02)00259-4 Get rights and content

Abstract

Dehydration adsorbents deactivate with time due to either pore mouth closure, window blocking, coking and/or hydrothermal decrystallization. These mechanisms of deactivation may be significant under severe hydrothermal conditions and/or in presence of reactive hydrocarbons at elevated temperatures and/or pressures. A mathematical model for predicting the uptake rates for crystal particles partially blocked either due to coking or pore mouth closure has been developed and numerical results, using the orthogonal collocation technique, have been obtained. Concentrated blockage of the crystal surface greatly reduces the mass transfer rate in comparison to uniformly distributed surface blockage.

Introduction

Zeolites are widely used for catalytic and adsorptive applications. Adsorptive applications include drying, hydrocarbon separation, separation and purification of industrial streams. One of the main industrial adsorption processes, based on 4A zeolite, is the drying of process streams. One major problem encountered in using 4A zeolite for drying is that it deactivates with time. Deactivation of 4A zeolite involves either a loss of adsorption capacity or an increase in mass transfer resistance, commonly occurring as a result of coke formation, or partial pore closure. Coke formation in zeolites results due to the presence of hydrocarbon gases or vapors (Misk et al., 2000). Partial pore closure results due to preferential closure of micropore openings in presence of water at high temperature such as 350 °C (Bülow, Struve, and Pikus, 1982, Bülow, Struve, and Rees, 1985, Kärger, Heink, Pfeifer, Rauscher, and Hoffman, 1982, Kärger, Pfeifer, Seidel, Staudte, and Gross, 1987; Kärger et al., 1988; Kärger, Pfeifer, Rauscher, & Bülow, 1981; Kärger et al., 1990; Kärger and Pfeifer, 1987, Kärger, 1982, Kondis and Dranoff, 1971). In general, both of these mechanisms may lead to the blockage of a fraction of the crystal surface, thereby decreasing the surface area available for mass transfer.

There is qualitative evidence available in the literature, which indicates that one or both of these two mechanisms is responsible for the deactivation of 4A zeolite. However, no attempt has yet been made to quantitatively model the phenomenon of deactivation or the impact of deactivation on sorption characteristics of 4A zeolite. Present work deals with the second part of the problem. A mathematical model has been developed to investigate the effect of partial surface blockage of a crystal particle of cylindrical shape in lieu of the cubic crystal for simplicity. The mathematical model is solved numerically using the orthogonal collocation technique.

Section snippets

Theoretical model for a crystalline particle

Consider a zeolite crystal particle, of cylindrical morphology, having radius r_c. At time t=0 a large step change is applied in surface concentration. Further, assume that a fraction A_b of the surface is blocked and hence the radial flux in this region of the surface is zero. The mathematical statement of this problem is $∂q ∂t = 1 r ∂ ∂r rD_{c} ∂q ∂r + 1 r^{2} ∂ ∂θ D_{c} ∂q ∂θ$ with the initial condition $q(r,θ,0)=q_{i}$ and boundary conditions $q(r,0,t)=q(r,2&0xFFFD;,t)$ $∂q ∂θ (r,0,t)= ∂q ∂θ (r,2&0xFFFD;,t)$ $lim r→0 ∫_{0}^{2&0xFFFD;} ∂q ∂r r,θ,t d θ=0$ $q r$

Numerical technique

The model , , , , , for sorption in the crystal are solved using the orthogonal collocation technique (Villadsen & Michelsen, 1978). To solve the model equations for the crystal, a sector of surface with angle Δθ*=1/2n_p was considered. The sector was chosen in such a way that it included half of one patch of the blocked area and half of one patch of adjacent exposed area with areas A_b/2n_p and (1−A_b)/2n_p, respectively. In this way the solution over the selected sector will represent that over

Results and discussion

The dimensionless adsorbed phase concentration profiles, q*, are plotted in Fig. 3. As the fractional blockage, A_b, increases diffusion to the inner core of the cylinder gets slower resulting in a low uptake rate. The uptake curves for adsorption and for a linear isotherm i.e. $λ_{s} −λ_{i} →0$ , are plotted in Fig. 4. In the figure m_t is the mass of adsorbate adsorbed at any time t* and m_∞ is that at t*→∞. It can be seen from the figure that as the blocked area A_b increases the uptake rate decreases. All

Conclusions

Pore mouth closure is modeled as complete blockage of a fraction of the crystal surface. This model can also be used for modeling the sorption in a crystal deactivated due to coking. A concentrated blockage of a fraction of a cylindrical crystal can significantly reduce the rate of mass transfer in a crystal.

Acknowledgements

The authors would like to acknowledge an anonymous referee for the comments provided on the source of the Jacobi polynomials. The authors also acknowledge King Fahd University of Petroleum & Minerals for the financial support provided for this work.

References (16)

Abramowitz, M., Stegun I. A., 1970, US NBS applied mathematics series, vol....
M. Bülow et al.
Influence of hydrothermal pretreatment on zeolitic diffusivity detected by comparative sorption kinetics and small-angle X-ray scattering investigations
Zeolites
(1982)
M. Bülow et al.
Investigation of the gaseous phase diffusion and liquid phase self-diffusion of n-decane on NaCa-A zeolites
Zeolites
(1985)
D.R. Garg et al.
Theoretical predictions of breakthrough curves for molecular sieve adsorption columns—I. Asymptotic solutions
Chemical Engineering Science
(1973)
H.W. Habgood
The kinetics of molecular sieve action. Sorption of nitrogen–methane by Linde molecular sieve 4A
Canadian Journal of Chemistry
(1958)
J. Kärger et al.
NMR evidence of the existence of surface barriers on zeolite crystallites
Zeolites
(1982)
J. Kärger et al.
Investigation of surface barriers on NaCaA type zeolites by combined application of the NMR tracer desorption method and X-ray photoelectron spectroscopy
Zeolites
(1987)
J. Kärger et al.
NMR study of mass transfer in granulated molecular sieves
American Institute of Chemical Engineering Journal
(1988)

There are more references available in the full text version of this article.

Cited by (8)

Molecular simulation and Monte Carlo study of structural-transport-properties of PEBA-MFI zeolite mixed matrix membranes for CO<inf>2</inf>, CH<inf>4</inf> and N<inf>2</inf> separation
2017, Computers and Chemical Engineering
Citation Excerpt :
These positive ions are rather loosely held and can readily be exchanged for others in a contact solution. Due to these pioneer and special properties, zeolites have been studied in numerous works (Algieri et al., 2011; Antunes et al., 2016; Khan and Loughlin, 2003; Kosinov et al., 2016; Zimmerman et al., 1997). For solution-diffusion mechanism, three assumptions can be further introduced in this process: (1) the rates of gas adsorption and desorption at the membrane interfaces are assumed to be substantially higher than the transport rate in the membrane, and thus the time for adsorption and desorption can be neglected. (
In this communication, structural-transport-properties of polyether block amide (PEBA)-MFI zeolite mixed matrix membranes (MMMs) have been investigated with molecular simulation (MS) and Monte Carlo (MC) technique. The density profile, RDF, FFV and XRD characterization have been calculated to investigate the aforementioned MMMs microstructure and also sorption and diffusion structural dependence of CO₂–CH₄ and CO₂–N₂ pairs. Sorption, diffusion, permeability and selectivity of natural gases including CO₂, CH₄ and N₂ by PEBA polymeric membrane filled with nano-zeolite (MFI type) have been examined to study the influence of MFI nanomaterial and nanomaterial loading content on the performance of the MMMs. By increase of MFI loading from 10 to 20 wt.%, permeability for CO₂, CH₄ and N₂ increases from 106.5 to 123.6, 4.75–4.74 and 1.28 to1.31, respectively. Structural analysis shows very good behavior for simulated membranes. Moreover, experiments show that simulation results for XRD and diffusivity are in good agreement with experiment results.
Template free crystallization of zeolite Rho via Hydrothermal synthesis: Effects of synthesis time, synthesis temperature, water content and alkalinity
2013, Ceramics International
Citation Excerpt :
Thus, zeolites are a subset of molecular sieves. Using molecular sieves with three-dimensional framework structures is well entrenched in applications as diverse as laundry detergents [1,2], catalysts [3–5], adsorbents [6–8], gas separations [9–11], pervaporation [12–15], membrane reactors [16–18], ion exchange [19–21], agriculture and horticulture [22,23], and is finding new applications in electronics [24], magnetism [25,26], chemical sensors [27–29], medicine [30], etc. [31–33]. Zeolites are typically synthesized under hydrothermal conditions from alkaline aqueous solutions.
Four parameters affecting organic template free hydrothermal synthesis of Rho zeolite were investigated, including synthesis time, synthesis temperature, water content, and alkalinity. XRD and SEM techniques were applied to characterize the synthesized zeolite Rho powders. The results showed that synthesis time has a higher impact on the crystal size. The crystal size increases from 1.45 to 1.90 μm by increasing synthesis time from 4 to 10 days. Synthesis time is also effective on the crystals morphology and there is an extremum in the diagram of crystallinity versus synthesis time for zeolite Rho. In addition, synthesis temperature has high impact on morphology of the crystals and can change the zeolite phases. Besides, water content is also highly effective on crystallinity of the synthesized powders and with increasing water content the crystallinity decreases dramatically. At last, increasing alkalinity increases the nucleation and decreases the crystals size. At water content of 400 mol, pollucite phase was synthesized at two ratios of Na₂O/Al₂O₃=6 and Cs₂O/Al₂O₃=0.8, and also two ratios of Na₂O/Al₂O₃=9 and Cs₂O/Al₂O₃=1.2; and for these ratios the mean crystal sizes were 0.70 and 0.40 μm, respectively, confirming the negative effect of alkalinity on the mean crystal size.
Asymptotic convergence of cubic Hermite collocation method for parabolic partial differential equation
2013, Applied Mathematics and Computation
Citation Excerpt :
By making suitable modifications the system (1-4) has been extended to mass transfer between the solid and liquid phases [3], separation of glycerol from biodiesel [4], tubular flow reactor [5–7], analysis of distillation column [8], design and development of airlift loop reactors [9], heat transfer processes in gas–solid turbulent fluidized systems [10], chromatography [11], measurement of neutron flux [12], sorption [13], enzymatic hydrolysis of racemic ibuprofen ester by lipase [14], desorption [15], modeling and design of optoelectronic devices [16], one-dimensional heat equation [17–21], nonlinear dynamic [22] and nuclear scattering problems [23].
Computationally efficient technique for weight functions and effect of orthogonal polynomials on the average
2007, Applied Mathematics and Computation
The properties of orthogonal collocation on finite elements with respect to the choice of orthogonal polynomials are studied. A simplified algorithm for the calculation of the collocation points, weight functions and discretization matrices for first and second order derivatives is presented in terms of the Lagrangian interpolation polynomial. The effect of Legendre and Chebyshev polynomials on the average value of the dependent variable is checked. It is found that the Legendre polynomial gives the better results at the centre and on the average as compare to the Chebyshev polynomial.
Removal of fulvic acids using the surfactant modified zeolite in a fixed-bed reactor
2006, Separation and Purification Technology
Modification of zeolite (clinoptilolite) surface with a quaternary ammonium, hexadecyl trimethyl ammonium (HDTMA), to improve the removal efficiency of fulvic acids (FA) in a zeolite fixed bed was investigated. The experiment consisted of modifying zeolite with HDTMA followed by adsorption and desorption of FA in the column. The effects of relevant parameters, such as HDTMA loading levels, FA flow rate and eluant concentration were examined, respectively. Optimization studies show that the surfactant modified zeolite (SMZ) bed with HDTMA loading of 120% of ECEC at a flow rate of 5 BV/h had the best performance and the volume of 23 BV of 30 vol% ethanol solution with the feed flow rate of 5 BV/h was sufficient for complete regeneration of SMZ and desorption of FA. Measurements of Zeta potential of SMZ indicate that a monolayer formation is the most viable packing that enables maximum removal of FA. FA adsorption on SMZ surfaces is largely due to the hydrophobic interaction and hydrogen bonding while FA desorption depends on hydrogen bonding. Partition and mass transfer also play an important role in the adsorption and desorption of FA.
Synthesis of 4A Zeolite from Kaolinite-type pyrite flotation tailings (KPFT)
2018, Minerals

View all citing articles on Scopus

View full text

Kinetics of sorption in deactivated zeolite crystal adsorbents

Abstract

Introduction

Section snippets

Theoretical model for a crystalline particle

Numerical technique

Results and discussion

Conclusions

Acknowledgements

Influence of hydrothermal pretreatment on zeolitic diffusivity detected by comparative sorption kinetics and small-angle X-ray scattering investigations

Zeolites

Investigation of the gaseous phase diffusion and liquid phase self-diffusion of n-decane on NaCa-A zeolites

Zeolites

Theoretical predictions of breakthrough curves for molecular sieve adsorption columns—I. Asymptotic solutions

Chemical Engineering Science

The kinetics of molecular sieve action. Sorption of nitrogen–methane by Linde molecular sieve 4A

Canadian Journal of Chemistry

NMR evidence of the existence of surface barriers on zeolite crystallites

Zeolites

Investigation of surface barriers on NaCaA type zeolites by combined application of the NMR tracer desorption method and X-ray photoelectron spectroscopy

Zeolites

NMR study of mass transfer in granulated molecular sieves

American Institute of Chemical Engineering Journal