Adsorption and desorption kinetics of hydrocarbons in FCC catalysts studied using a tapered element oscillating microbalance (TEOM). Part 1: experimental measurements

An experimental procedure to measure the adsorption and desorption kinetics of hydrocarbons in fluid catalytic cracking (FCC) catalysts using a tapered element oscillating microbalance is described. It enables adsorption rates to be measured on a timescale of about . Experiments using n-hexane, n-heptane, n-octane, toluene and p-xylene were performed on both a commercial FCC catalyst and a pure rare-earth exchanged zeolite Y sample under non-reacting conditions (temperatures of 373-). Heats of adsorption for these hydrocarbons are reported. The overall adsorption and desorption kinetics are found to depend on carrier gas flowrate in cases when adsorption is strong indicating that the length of the catalyst bed can have a significant influence on the observed kinetics. However, at high carrier gas flowrates the overall adsorption and desorption kinetics do not depend on the amount of catalyst present. It is found that the rates of adsorption and desorption of the hydrocarbons studied are the same for the FCC catalyst as for the pure zeolite Y sample. This means that mass transport in the matrix component of the FCC catalyst is rapid and not a limiting step in the adsorption process for the hydrocarbons studied in this work.     

Determination of adsorption and diffusion parameters in zeolites through a structured approach

Temporal analysis of products (TAP) is a transient pulse-response technique that allows to extract kinetic information from reacting and adsorbing systems. In a previous work (Chem Eng Sci 57(2002) 1835), a detailed-transport model for the Multitrack set-up, a TAP-like system, was developed, which allows studying systems with a low bed resistance. The use of structured beds, having both a low bed resistance and small sorbent particles, is required to determine adsorption and diffusion parameters when strong adsorption and slow diffusion occurs. A method is presented to extend the range of measurable adsorption and diffusion parameters in zeolitic sorbents in the TAP technique by a structured approach. Small zeolite crystals are coated on larger non-porous glass beads. Adsorption and diffusion parameters for n-butane, SF₆ and 3-methylpentane have been determined in MFI-type zeolites. Absolute diffusivity values in the zeolite coating are estimated by using a well-defined silicalite sample as a reference to determine the effective diffusion length in the coating.Two criteria have been derived, one for the characteristic time for transport through the bed, , and one for the ratio of the latter and the characteristic diffusion time in the zeolite crystal, 0.01<α<200t_bed/(1+t_bed), which should be satisfied both to be able to determine values of the zeolite diffusivity.     

Mechanism of gas absorption enhancement in presence of fine solid particles in mechanically agitated gas-liquid dispersion. Effect of molecular diffusivity

The effect of fine particle addition in physical gas desorption and absorption with fast reaction (sulphite oxidation in the presence of a cobalt catalyst) has been studied in a stirred cell with a flat gas-liquid interface and mechanically agitated gas-liquid bubble dispersion in a wide range of stirring speeds. Activated carbon and TiO₂ were used at low loadings . The desorption was used to avoid supersaturation effect which was observed during oxygen and hydrogen absorption into liquid saturated with nitrogen. Using two gases with sufficiently different diffusivity (O₂, H₂), the effect of molecular diffusivity on the mass transfer coefficient was estimated in the form k_L∼Dⁿ, with the exponent n indicating the surface mobility accompanying the effect of the particles. The value n=2/3 indicates a fully rigid and the value 1/2 a fully mobile mass transfer interface.Chemisorption experiments confirmed that the particles do not affect mass transfer area of the agitated dispersion. After addition of particles, k_L for physical desorption from bubbles in dispersion was increased by 10-30% in water and by 20-60% in sulphate solution with decreasing agitation rate. In the stirred cell, the increases were much higher reaching 200% and 230% for water and sulphate solution, respectively. The exponent n exhibited a significant decrease in the presence of particles ranging from 10% (for dispersion in water) to 33% (for stirred cell in sulphate solution). The decrease in n encountered in dispersion indicated the transition from a partially mobile to a fully mobile surface. The reduction of k_L was interpreted through the physicochemical effect of surfactants removal from the gas-liquid interface by activated carbon particles.The results have confirmed that the mechanism of mass transfer enhancement in the presence of fine particles, based on the removal of surface contaminants from the liquid by adsorption onto the hydrophobic surface of the particles, as suggested by Kaya and Schumpe [2005. Surfactant adsorption rather than “shuttle effect”? Chemical Engineering Science 60, 6504-6510] for stirred cell, is valid also for the enhancement of absorption rate from bubbles in mechanically agitated dispersion.     

On the Langmuir-Hinshelwood formulation for zeolite catalysed reactions

The Langmuir-Hinshelwood (LH) rate expression is often used to describe the kinetics of heterogeneously catalysed reactions using zeolites. A factor in the LH expression allows for the reduction of the reaction rate with increased fractional occupancies θ_i of the individual species involved in an n-molecular reaction on the catalyst surface. Most commonly in practice the multicomponent Langmuir (MCL) approach is used for calculation of the fractional occupancies giving where the b_i are the Langmuir adsorption constants and p_i are the partial pressures in the gas phase. The LH-MCL approach is, however, thermodynamically consistent only when the saturation capacities of all the individual species in the mixture are identical to one another, or when the component loadings are small. For mixtures containing molecules with different saturation capacities, the sorption loadings are significantly affected by entropy effects, especially for high loadings within the zeolite catalyst. In the general case we need to determine the sorption loadings, and occupancies, using the Ideal Adsorbed Solution Theory. Using the gas phase isomerization of n-hexane with MFI zeolite catalyst as an illustration we demonstrate the limitations of the LH-MCL kinetics for calculation of reactor conversions. More significantly, we demonstrate how entropy effects can be exploited in simulated moving bed reactor configuration to obtain supra-equilibrium conversions and improved reaction selectivities.     

Transport of oxygen, sodium chloride, and sodium nitrate in biofilms

Diffusion coefficients of sodium chloride, sodium nitrate and oxygen were determined for heterotrophic biofilms. The biofilms were cultivated under different hydrodynamic and substrate loading conditions in tubular reactors resulting in biofilm densities between 3 and dry mass. Quantifying solute diffusion in the biofilm for these biofilms allowed to specifically evaluate the influence of biofilm density on diffusion while keeping other factors such as the type of substrate, the inoculum, and the reactor type constant. Two methods were used to measure diffusion coefficients. The two-chamber method was used to quantify the diffusion of sodium chloride and sodium nitrate. The diffusion coefficients for oxygen were measured based on oxygen concentration profiles in the biofilm measured using microelectrodes. The ratio between the diffusion coefficient in biofilm and water (f_D=D_F/D_W) was found to be lower than 1 in the majority of experiments. A clear correlation between f_D and biofilm density was found where f_D decreased with increasing biofilm density. For mean biofilm densities in the range of 10- can be approximated between 0.5 and 1. For larger densities of 20 or can be approximated as 0.8 or 0.4, respectively. For densities higher than is below 0.6. Advective transport mechanisms that would have resulted in f_D>1 can be neglected in the biofilms cultivated.     

Mechanism of enhanced gas absorption in presence of fine solid particles. Effect of molecular diffusivity on mass transfer coefficient in stirred cell

Desorption of oxygen and hydrogen from various liquids (water, 0.8 molar sodium sulphate solution) containing suspended particles of activated carbon at various solid loading was investigated. The desorption was used to avoid supersaturation effect which was observed during oxygen and hydrogen absorption into liquid saturated with nitrogen. Experiments were carried out in a stirred cell with flat gas-liquid interface at and atmospheric pressure. An increase of k_L upon addition of the particles was observed. Enhancement factor increases with increasing contact time of the particles with liquid reaching maximum steady-state value of approx. 3 after sufficiently long time (a few hours) regardless of solid loading , agitator frequency and solute gas (O₂,H₂). The results fit the correlation (e is specific power dissipated by agitator in liquid and D is molecular diffusivity of gas absorbed) with the exponent for liquids without and for the liquids with the particles. It indicates that the interface is rigid in absence of particles and hinders the motion of liquid along the interface forming boundary layer while in the presence of particles the interface is completely mobile and surface renewal proceeds according to the penetration model. These results confirm a finding of Kaya and Schumpe (2005) that the enhancement of mass transfer in the cell at the presence of hydrophobic solids is due to clean-up of the interface from surfactants by their adsorption on hydrophobic solids rather than by a “shuttle mechanism” exerted by particles with a high adsorption capacity for the transfer component.     

Determination of effective diffusion coefficient and interfacial mass transfer coefficient of bovine serum albumin (BSA) adsorption into porous polyethylene membrane by microscope FTIR-mapping study

Here, a new method for simultaneous determination of diffusion coefficient D and interfacial mass transfer coefficient (or convective mass transfer coefficient) k was proposed for bovine serum albumin (BSA) adsorption into porous polymeric membranes. The experimental data for BSA concentration at different membrane depth and different time were determined from FTIR-mapping measurements. Then the diffusion coefficient D and interfacial mass transfer coefficient k were estimated from the calculated dimensionless concentration data at different time and membrane depth by a trial-and-error method based on the diffusion equation initiated in this paper. The diffusion coefficient D and interfacial mass transfer coefficient k evaluated in this manner are respectively: and . The theoretical concentration values calculated from the determined parameters were compared with experimental reading from FTIR mappings, which showed a good agreement between them, especially for the case of a relatively long-time adsorption.     

Multidimensional NMR diffusion studies in microporous materials

The diffusional behavior of selected hydrocarbons adsorbed in NaX zeolites were investigated with pulsed magnetic field gradient (PFG) NMR methods in two different ways. First, the exchange time τ_exch of n-pentane between the interior and the exterior of the zeolite crystals was determined by two-dimensional diffusion exchange spectroscopy. The results were compared to the mean life time τ_intra as obtained by fast NMR tracer desorption method. Second, the diffusion of a propylene–propane mixture adsorbed in the zeolite was studied by Fourier transform PFG NMR, thus employing the chemical shift of the individual organic constituents as a second dimension for a selective evaluation of the PFG NMR signal decays and the subsequent extraction of corresponding diffusion coefficients. Modifications of NMR pulse sequences necessary for the application of ultra-high pulsed magnetic field gradients of up to in microporous materials are briefly discussed for both kind of experiments.     

Hydrogen generation in a Pd membrane fuel processor: Productivity effects during methanol steam reforming

Performance analyses are carried out for the palladium membrane fuel processor for catalytic generation of high purity hydrogen. The reactor model includes detailed particle-scale multi-component diffusion, multiple reversible reactions, flow, and membrane transport. Using methanol steam reforming on Cu/ZnO/Al₂O₃ catalyst as the test reaction, a systematic examination of the effects of operating and reactor design parameters on key performance metrics is presented. Single particle simulations reveal a complex interplay between nonisobaric transport and the reversible reactions (methanol reforming and decomposition, and water-gas shift), which impact overall reactor performance. An analysis of characteristic times helps to identify four different productivity controlling regimes: (i) permeation control, encountered with thick membranes and/or insufficient membrane area; (ii) catalyst pore diffusion control encountered with diffusion of reacting species in larger particles; (iii) reaction control, encountered when intrinsic catalytic rates are too low because of inadequate activity or catalyst loading; and (iv) feed control, encountered when the limiting reactant feed rate is inadequate. The simulations reveal that a maximum in the hydrogen productivity occurs at an intermediate space velocity, while the hydrogen utilization is a decreasing function of space velocity, implying a trade-off between productivity and hydrogen utilization. The locus of productivity maxima itself exhibits a maximum at an intermediate membrane surface to volume ratio, the specific value of which is dependent on the particle size, membrane thickness and reaction conditions. At moderate temperature and total pressure (, 10 bar), particles smaller than 2 mm diameter, Pd membranes with thickness less than , and membrane surface to volume ratio exceeding are needed to achieve viable productivity . A comparison between the packed-bed membrane reactor and conventional packed-bed reactor indicates a modest improvement in the conversion and productivity due to in situ hydrogen removal.     

Permeation behavior during the catalytic oxidation of CO in a Pt-loaded Y-type zeolite membrane

A Pt-loaded Y-type zeolite (Pt/NaY) membrane was prepared on the surface of a porous α-Al₂O₃ support tube by hydrothermal synthesis and then ion-exchanged with platinum. The thickness of the zeolite layer and the amount of Pt loaded were ca. and , respectively. The membrane was employed in the form of a cylindrical thin catalyst for the selective oxidation of CO in an H₂-rich mixture. A mixture of CO, O₂ and H₂ was fed to the outer surface of the membrane, and CO was selectively oxidized during its permeation through the thin layer. The permeation fluxes for H₂ and CO were determined at 423-. Permeation fluxes also calculated by means of a mathematical model using effective diffusion coefficients and reaction kinetics. The effective diffusion coefficients through the zeolite membrane were estimated from gas permeation test data, obtained at 423-, and the oxidation rates of CO were determined over a particulate catalyst that had the same composition as the Pt/NaY membrane. As a result, the diffusion coefficients of O₂, N₂ and CO were determined to be (0.7-1.0) at 423-, and the activation energies for the rate constants for CO oxidation were 61-. The predicted permeation fluxes of H₂ and CO using the mathematical model were in good agreement with the experimental data, when the oxidation selectivity of CO to H₂ was assumed to be 80% in the model calculation.     

Onsager coefficients for binary mixture diffusion in nanopores

This paper presents a critical appraisal of current estimation methods for the Onsager coefficients L₁₁, L₂₂, and L₁₂ for binary mixture diffusion inside nanopores using pure component diffusivity data inputs. The appraisal is based on extensive sets of molecular dynamics (MD) simulation data on L_ij for a variety of mixtures in zeolites (MFI, AFI, TON, FAU, CHA, DDR, MOR, and LTA), carbon nanotubes (CNTs: armchair and zig-zag configurations), titanosilicates (ETS-4), and metal-organic frameworks (IRMOF-1, CuBTC). The success of the L_ij predictions is crucially dependent on the estimates of the degree of correlations in molecular jumps for different guest-host combinations; these correlations are captured in Maxwell-Stefan approach by the exchange coefficients ?_ij. Three limiting scenarios for correlation effects have been distinguished; for each of these scenarios appropriate expressions for the L_ij are presented. For CNTs, correlation effects are dominant and the interaction factor, defined by , is close to unity. For cage-type zeolites such as LTA, CHA, and DDR with narrow windows separating cages, correlation effects are often, but not always, negligibly small and the assumption of uncoupled diffusion, i.e., α₁₂=0, is a reasonable approximation provided the occupancies are not too high. In other cases such as zeolites with one-dimensional channel structures (AFI, TON), intersecting channels (MFI), cage-type zeolite with large windows (FAU), ETS-4, CuBTC, and in IRMOF-1, it is essential to have a reliable estimation of the ?_ij; MD simulations underline the wide variety of factors that influence the ?_ij.We also highlight two situations where estimations of the L_ij fail completely; in both cases the failure is caused due to segregated adsorption. In adsorption of CO₂-bearing mixtures in LTA and DDR zeolites, CO₂ is preferentially lodged at the narrow window regions and this hinders the diffusion of partner molecules between cages. The second situation arises in MOR zeolite that has one-dimensional channels connected to side pockets. Some molecules such as methane, get preferentially lodged in the side pockets and do not freely participate in the molecular thoroughfare. Current phenomenological models do not cater for segregation effects on mixture diffusion.     

MoP/Hβ catalyst prepared by low-temperature auto-combustion for hydroisomerization of n-heptane

Molybdenum phosphide supported on Hβ zeolite (MoP/Hβ) was prepared via the low-temperature auto-combustion method, and characterized by X-ray diffraction, scanning electron microscopy, nitrogen adsorption–desorption, and temperature-programmed desorption of NH₃. The as-synthesized MoP/Hβ was employed as the bifunctional catalyst for the hydroisomerization of n-heptane, and gave a much higher activity compared to the counterpart MoP/Hβ catalyst prepared by the traditional impregnation method. The promoted effect of the low-temperature auto-combustion method on the catalytic performance is attributed to the less blockage of β pores and more strong acid sites of the as-synthesized MoP/Hβ.     

The role of structural and electronic alterations on the lithium diffusion in LixCo0.5Ni0.5O2

The mechanisms for lithium diffusion in Li_xCo_0.5Ni_0.5O₂ were investigated using the galvanostatic intermittent titration technique (GITT). Membrane electrodes prepared with poly(vinylidene fluoride) and carbon black were employed in this study. The measured Brunauer-Emmett-Teller (BET) area of the Li_xCo_0.5Ni_0.5O₂ powder was combined with the GITT data to obtain the lithium chemical diffusion coefficient (), the lithium self-diffusion coefficient (D_Li⁺) and the thermodynamic factor (Φ) as a function of Li concentration (x). All three parameters vary non-monotonically with x. A minimum in and D_Li⁺ at x=0.5, along with structural changes, suggests the formation of a lithium superlattice at that concentration. The behavior of is complex but for x<0.34 it eventually undergoes a continuous decrease due to the metallic character of Li_xCo_0.5Ni_0.5O₂. We attribute the limitation of the specific reversible capacity of Li_xCo_0.5Ni_0.5O₂ to this decrease in and to elevated electrode voltages. Li transport in Li_xCo_0.5Ni_0.5O₂ is analyzed taking the variations in the cell parameters and the oxidation states of the Ni, Co and O ions into account.     

Proteolytic cleaning of a surface-bound rubisco protein stain

We present a combined experimental and mathematical study of the proteolysis of a surface-bound rubisco protein stain. The adsorption and desorption of subtilisin A (SA) onto and from surface-bound rubisco films were found to be a strong function of the surface chemistry underlying the protein stain; the stain acted as a biosensor able to convey information about the underlying surface to the attacking protease. The apparent protease adsorption rate constants (k_a) were 0.016±0.007, 0.014±0.004 and while the apparent desorption rate constants (k_d) were 1.60±0.15, 1.05±0.02 and for hydrophobic, neutral-hydrophilic and negatively charged hydrophilic surfaces, respectively. The apparent proteolysis rate constant of surface-bound rubisco and the enzyme deactivation rate constant were estimated to be and , respectively, independent of underlying surface chemistry. The results demonstrated higher protein removal from the charged hydrophilic surface relative to the other two surfaces. Rubisco cleanability from the charged and hydrophobic surfaces increased with increasing bulk enzyme concentration (and hence surface enzyme concentration) and was better for the charged surface, perhaps reflecting the higher k_a value. Conversely, rubisco cleanability from the neutral hydrophilic surface was surprisingly insensitive to variation in bulk enzyme concentration. Overall cleaning efficiency was also substantially lower for the neutral hydrophilic surface when compared with the hydrophobic surface, even though k_a values for each surface were similar. These findings indicate that surface proteolysis is significantly impaired at low values of k_d, suggesting that enzyme mobility at the interface may be closely linked to cleaning performance. The model presented here is expected to be a useful tool in the detergent industries to screen and gauge the cleaning performance of detergent-enzyme formulations, and may also be able to facilitate the design of surface treatments that convey cleaning signals to attacking proteases.