Pressure drop in CIM disk monolithic columns

Pressure drop analysis in commercial CIM disk monolithic columns is presented. Experimental measurements of pressure drop are compared to hydrodynamic models usually employed for prediction of pressure drop in packed beds, e.g. free surface model and capillary model applying hydraulic radius concept. However, the comparison between pressure drop in monolith and adequate packed bed give unexpected results. Pressure drop in a CIM disk monolithic column is approximately 50% lower than in an adequate packed bed of spheres having the same hydraulic radius as CIM disk monolith; meaning they both have the same porosity and the same specific surface area. This phenomenon seems to be a consequence of the monolithic porous structure which is quite different in terms of the pore size distribution and parallel pore nonuniformity compared to the one in conventional packed beds. The number of self-similar levels for the CIM monoliths was estimated to be between 1.03 and 2.75.     

A model for the pressure drop in gas—liquid cocurrent downflow through packed beds

A model for pressure drop is proposed for gas—liquid flow through packed beds on the basis of the observed absence of radial pressure gradients and taking into consideration the structure of the bed and the physical properties of the fluids. The model divides the total voitage of the bed into internal and external voidage and appropriately distributes the total liquid holdup into internal and external holdup.Over 2500 experimental data, from the present study as well as those reported in literature, are correlated by the model with an r.m.s. deviation of less than ±9%. The significant parameters affecting the two-phase pressure drop are found to be the bed porosity, the Reynolds number, and the product of the Eötvos and the Morton numbers.     

Measurement of effective Knudsen diffusion coefficients for powder beds used in heterogeneous uptake experiments

The effective Knudsen diffusion coefficients for characteristic oxide powder beds used in heterogeneous uptake experiments have been measured using countercurrent diffusion and transient pressure drop techniques. Room-temperature thermal-velocity-normalized effective Knudsen diffusion coefficients are found to lie in the 0.15 to 0.35 microm range for magnesium silicate, aluminum oxide, and iron oxide powder beds. Measured values are compared with theoretical estimates and are consistent with low bed tortuosities (below 3) expected for media with open porosity above 0.5. The impact of uncertainties in effective diffusion coefficients on corrections of measured uptake coefficients is discussed. The value of careful uptake measurements in both the low and high sample mass limits is reinforced, as this allows uptake corrections independent of explicitly measured or estimated diffusion coefficient values. It is suggested that correction procedures requiring tortuosity values greater than 3 are suspect.     

Comparison of general rate model with a new model--artificial neural network model in describing chromatographic kinetics of solanesol adsorption in packed column by macroporous resins

Herein, two models, the general rate model taking into account convection, axial dispersion, external and intra-particle mass transfer resistances and particle size distribution (PSD) and the artificial neural network model (ANN) were developed to describe solanesol adsorption process in packed column using macroporous resins. First, Static equilibrium experiments and kinetic experiments in packed column were carried out respectively to obtain experimental data. By fitting static experimental data, Langmuir isotherm and Freundlich isotherm were estimated, and the former one was used in simulation coupled with general rate model considering better correlative coefficients. The simulated results showed that theoretical predictions of general rate model with PSD were well consistent with experimental data. Then, a new model, the ANN model, was developed to describe present adsorption process in packed column. The encouraging simulated results showed that ANN model could describe present system even better than general rate model. At last, by using the predictive ability of ANN model, the influence of each experimental parameter was investigated. Predicted results showed that with the increases of particle porosity and the ratio of bed height to inner column diameter (ROHD), the breakthrough time was delayed. On the contrary, an increase in feed concentration, flow rate, mean particle diameter and bed porosity decreased the breakthrough time.     

Optimization of synthesis procedures for structured PSA adsorbents

Structured adsorbents in the form of supported thin zeolite films may represent a competitive alternative to traditional zeolite adsorbents in form of beads or pellets used in PSA processes, due to the reduction of mass- and heat-transfer limitations typical of packed beds. Thin NaX films were grown by hydrothermal treatment using a clear solution on cordierite monoliths. Films grown by a multiple synthesis procedure were dense and uniform with a very small amount of sediments adjacent to the film, which may be an advantage in PSA applications. The CO₂ adsorption capacity and the pressure drop for the supported films were compared to those of a packed NaX bed. Although the adsorption capacity of the column filled with the structured adsorbents was 67 times lower than when the column was filled with zeolite beads, the pressure drop was 100 times lower for the structured adsorbent. The adsorption capacity can be increased by increasing the film thickness or the cell density of the monoliths without increasing the pressure drop significantly, indicating the potential advantage of structured adsorbents in PSA processes. Further investigations are needed in order to prove this hypothesis.     

添加剂对PVDF相转化过程及膜孔结构的影响

研究了PVP、PEG及LiCl 3种成孔添加剂下PVDF DMAc H2 O 添加剂体系的成膜机理 .无论那种添加剂的铸膜液相转换成膜过程中都存在凝胶分相和液液分相两种相变方式 ,在 30～ 6 0℃时凝胶分相在较低的非溶剂浓度下先于液液分相发生 ,LiCl作为添加剂较PEG、PVP对铸膜液有较强的致凝胶作用 ,成膜过程中凝胶分相段时间依PVP、PEG、LiCl的顺序延长 ,导致液液分相初始分相点处聚合物浓度增大 ,阻止了大孔结构的充分发展 .制得的膜依PVP、PEG、LiCl的顺序有效孔隙率和通量降低 ,结晶度升高 .以LiCl为添加剂制得的膜几乎不改变PVDF膜的疏水性 ,而以PVP或PEG为添加剂的膜隔水压差降低约 2 0kPa .     

Total pore blocking as an alternative method for the on-column determination of the external porosity of packed and monolithic reversed-phase columns

An alternative method to determine the interstitial void volume and the external porosity inside a packed or a monolithic column was developed. The method is based on the total blocking of the mesopores of a porous support by filling them with a hydrophobic solvent. The strong interaction of the latter with the hydrophobic coating inside the pores keeps the solvent in position during the subsequent measurements. With the pores of the stationary phase material completely inaccessible for any type of polar molecules, the method allows to perform interstitial void measurements using small molecular weight (MW) molecules instead of the large MW molecules that need to be used in inverse size exclusion chromatography (ISEC). These small MW molecules are able to penetrate every corner of the interstitial volume and therefore lead to a very accurate determination of the external porosity. Since only one type of molecules needs to be injected, the often troublesome regression analysis needed in ISEC is omitted as well. In the present contribution, the method has been applied to a packed bed and a monolithic column to investigate the optimal conditions of flow velocity, liquid compositions, and unretained marker selection. The robustness and the repeatability of the method are discussed as well.     

PREDICTION OF PREDOMINANT MECHANISMS IN THE SEPARATION OF SECONDARY DISPERSIONS IN A FIBROUS BED

The different drop capture mechanisms for secondary dispersions in fibrous beds are reviewed. A quantitative analysis showed that interception and sedimentation are the predominant mechanisms in fibrous bed coalescers. The selection of operating parameters, e.g. velocity, fibre size and drop size depends upon these mechanisms. A 3 cm deep fibrous bed of glass wool, with a fibre diameter of 25 pm, was found to capture more than 80% of the dispersed phase from an inlet secondary dispersion of 15 Mm drop size. Almost 70% of these drops were captured due to the interception mechanism.     

Novel adsorbent hollow fibres for oxygen concentration

The research examined the development of adsorbent hollow fibres as a low pressure drop structure for the production of oxygen-enriched air. The potential benefits of using a low pressure drop flexible adsorbent structure with molecular sieving properties over a bed packed with pellets include a low attrition resistance which could extend the life of the adsorbent structure. Highly macroporous, highly adsorbent loaded (up to 90 wt%) fibres were produced. By increasing adsorbent density, the separative performance and nitrogen loading were improved. The separative performance of the adsorbent fibre was found to be slightly inferior to that of the bed of smaller 0.4–0.8 mm beads, as the diffusion path length was longer in the fibres and caused increased mass transfer resistances within the macroporous structure. The pressure drop through the fibre was found to be 40 to 70 times lower than that through an equivalent packed bed of 0.4–0.8 mm beads. This experimental feasibility study has demonstrated that the novel zeolite fibre configuration shows good potential for the production of oxygen-enriched air in a low energy, short cycle time, pressure swing process. The challenges of improving the performance of the adsorbent fibres and their operating parameters are described.     

Discrete element simulation of the dynamics of adsorbents in a radial flow reactor used for gas prepurification

Radial flow reactors (RFR) are used in thermal swing adsorption (TSA) processes for gas prepurification. The aim of this work is to show the validity of the discrete element method (DEM) to simulate the effect of thermal expansion and contraction cycles occurring in such processes on the packed bed of RFR reactors. Both mono-layered and bi-layered packed beds of adsorbents are investigated. A DEM-based model of a full-scale size unit was developed, the parameters of which were calibrated by means of particle-scale experimental measurements and simple auxiliary DEM simulations. The DEM-based model used is isothermal and the thermal expansion and contraction phenomena are modelled through the displacement of the inner and outer walls of the computational domain. First, the accuracy of this model is assessed using analytical values of the static wall pressure (i.e. with no wall motion) as well as experimental measurements of the dynamic wall pressure (i.e. with wall motion) of a bi-layered bed. Next, simulation results for a few process cycles in the case of a bi-layered packed bed indicates that little mixing occurs at the interface between the two types of adsorbents. To our knowledge, this is the first time that simulation is used to investigate the behavior of the packed bed of a RFR in a TSA process. The results obtained with the proposed model show that the DEM is a valuable tool for the investigation of such slow dynamical processes, provided a careful calibration is done.     

An understanding of the porosity of residual coal/char/ash samples from an air-blown packed bed reactor operating on inertinite-rich lump coal

The thermal treatment of coal causes a development of internal porosity of the resultant char due to the changes in the coal char pores, i.e. the opening of original closed pores, the formation of new pores, and an increase in pore size of existing and newly formed pores. Furthermore, the porosity formed during de-volatilisation causes changes in pore structural characteristics such as: density, pore size distribution, total open pore volume, porosities and average pore diameter. Much research has been conducted in this area, but was mainly focused on fine particle sizes (<1 mm) and vitrinite-rich coals, particularly from the Northern hemisphere. The objective of this study was to obtain an understanding of both the macro- and micro-porosity development within the de-volatilisation zone of a packed bed consisting of lump inertinite-rich coal (75 mm × 6 mm) from the Highveld coalfield in South Africa. This was achieved by generating samples in an air-blown packed bed reactor and conducting proximate, CO₂ reactivity, mercury intrusion porosimetry, and BET CO₂ surface area analyses on the dissected coal/char/ash samples.From mercury-intrusion porosimetry results obtained for the de-volatilisation reaction zone of the reactor, it was found that although the percentage macro-porosity and average pore diameter increased by 11% and 77% respectively (which confirms pore development), that these developments do not enlarge the surface area, and thus has no significant contribution on the reactivity of the coal/char. On the other hand, the micro-pore surface area, pore volume and pore diameter were all found to increase during de-volatilisation, resulting in an increase in the coal char reactivity. The micro-porosity is thus generally responsible for the largest internal surface area during de-volatilisation, which enables increased reactivity. The CO₂ gasification reactivity (at 1000 °C) increased from 3.8 to 4.5 h⁻¹ in the first stage of de-volatilisation, and then decreased to 3.8 h⁻¹ in a slower de-volatilisation regime. This is due to the maximum pore expansion and volatile matter evolution reached at 4.5 h⁻¹, before coalescence and pore shrinkage occur with a further increase in temperature within the slower de-volatilisation region of the reactor. During de-volatilisation there is thus both an increase and decrease in reactivity which might suggest two distinct intermediate zones within the de-volatilisation zone.     

Influence of the packing heterogeneity on the performance of liquid chromatography supports

We report on a series of plate height and flow resistance data obtained via computational fluid dynamics simulations in a simplified two-dimensional (2D) mimic of real packed bed and monolithic columns. By varying the external porosity (0.4 < epsilon < 0.8) and the degree of packing randomness, a good qualitative insight in the relationship between the packing porosity and heterogeneity and the general chromatographic performance parameters is obtained, unbiased by any differences in phase retention factor k', mobile phase diffusivity or viscosity or intra-skeleton porosity. The results provide a quantitative support for the use of domain size reduced plate heights as a means to compare the performance of chromatographic beds with a different porosity, as it was found that packings with a similar degree of packing heterogeneity yield very similar domain size reduced h(min)-values, nearly completely independent of the porosity. The study also clearly shows that the presence of preferential flow paths (inevitably accompanied by the presence of more clustered regions) leads to a decrease of the flow resistance, but also leads to a strong increase of the band broadening if supports with the same porosity epsilon and the same radial width are compared. For the presently considered 2D system, the flow resistance reduction is too small to overcome the corresponding strong increase in band broadening, such that the presence of preferential flow paths always leads to an overall increase of the separation impedance.     

Characterization of some physical and chromatographic properties of monolithic poly(styrene-co-divinylbenzene) columns

Monolithic capillary columns were prepared by copolymerization of styrene and divinylbenzene inside a 200 microm i.d. fused silica capillary using a mixture of tetrahydrofuran and decanol as porogen. Important chromatographic features of the synthesized columns were characterized and critically compared to the properties of columns packed with micropellicular, octadecylated poly(styrene-co-divinylbenzene) (PS-DVB-C18) particles. The permeability of a 60 mm long monolithic column was slightly higher than that of an equally dimensioned column packed with PS-DVB-C18 beads and was invariant up to at least 250 bar column inlet pressure, indicating the high-pressure stability of the monolithic columns. Interestingly, monolithic columns showed a 3.6 times better separation efficiency for oligonucleotides than granular columns. To study differences of the molecular diffusion processes between granular and monolithic columns, Van Deemter plots were measured. Due to the favorable pore structure of monolithic columns all kind of diffusional band broadening was reduced two to five times. Using inverse size-exclusion chromatography a total porosity of 70% was determined, which consisted of internodule porosity (20%) and internal porosity (50%). The observed fast mass transfer and the resulting high separation efficiency suggested that the surface of the monolithic stationary phase is rather rough and does not feature real pores accessible to macromolecular analytes such as polypeptides or oligonucleotides. The maximum analytical loading capacity of monolithic columns for oligonucleotides was found to be in the region of 500 fmol, which compared well to the loading capacity of the granular columns. Batch-to-batch reproducibility proved to be better with granular stationary phases compared to monolithic stationary phase, in which each column bed is the result of a unique column preparation process.     

Performance of a magnetically stabilized bed reactor with immobilized yeast cells

This paper is focused on the possibility to apply the magnetic stabilization technique in bioprocessing. The feasibility of a continuous ethanol fermentation process with immobilizedSaccharomyces cerevisiae cells in a magnetically stabilized bed (MSB) was demonstrated. The fermentation processes were carried out in an external magnetic field, transverse to the fluid flow. The flexibility to change the bed expansion owing to the independent change of the fluid flow and the field intensity (the “magnetization FIRST” mode) permitted the creation of fixed beds with different particle arrangements, which affected the bed porosity, the effective fluid-particle contact area, and the mass transfer processes on the particle-fluid interface. As a result, higher ethanol concentration, ethanol production, and glucose uptake rates than in conventional packed bed reactor were reached.     

Simulation of ordered packed beds in chromatography

A computer simulation of chromatographic dispersion in an ordered packed bed of spheres is conducted utilizing a detailed fluid flow profile provided by the Lattice Boltzmann technique. The ordered configurations of simple cubic, body-centered cubic, and face-centered cubic are employed in these simulations. It is found that zone broadening is less for the fcc structure than the sc and bcc structures and less than a random packed bed analyzed in a previous study in the low flow velocity region used for experimental chromatography. The factors which contribute to the performance of the ordered pack beds are analyzed in detail and found to be dependent both on the nearest surface to surface distance and on the distribution of velocities found in the various packing geometries. The pressure drops of the four configurations are compared and contrasted with the pressure drop from monolithic columns.     

Pellicular expanded bed matrix suitable for high flow rates

A new type of expanded bed matrix with a heavy core of stainless steel covered with an agarose layer was prepared. Two bead size fractions, the smaller one (32-75 microm diameter) having a single particle core and the larger (75-180 microm diameter) with an agglomerate of stainless steel particles constituting the core, were chosen for further characterisation. The dispersion behaviour was determined both in packed bed and expanded bed modes by the retention time distribution method (RTD) and compared with the Streamline matrix (Amersham Pharmacia Biotech). The comparison turned out in favour of the new matrix. Flow rates as high as 3000 cm/h were used with the larger fraction, giving stable expanded beds with good mass transfer properties. The matrices were mechanically stable without any tendency to crack or peal, even after prolonged use.     

Effect of cooling baths on EVOH microporous membrane structures in thermally induced phase separation

The purpose of this work is to investigate the effect of cooling bath on the membrane preparation of crystalline polymer/diluent system via thermally induced phase separation (TIPS), when the cooling bath is compatible with the diluent. In this work, poly(ethylene-co-vinyl alcohol) (EVOH)/PEG300 system with water and methanol as the cooling baths was proposed. Results showed that when water was used as the cooling bath, the membrane presented an asymmetric structure consisting of a porous skin, macrovoids near the top and lacy structures near the bottom. In contrast, when cooled in the bath of methanol, it showed particulate morphology on the top surface and cellular pores near the bottom. The lacy and cellular structures were the typical structures resulted from liquid–liquid thermally induced phase separation, the novel macrovoids and particulate morphology were then supposed to be induced by the mutual diffusion between the diluent and the cooling bath. In the case of water, the diluent's outflow was comparative with the water's inflow into the membrane, so the penetrated water acted as a strong nonsolvent and induced macrovoids near the top. In the bath of methanol, the diluent's outflow was much faster than the methanol's inflow, which changed the solution composition from a liquid–liquid phase separation region to a solid–liquid phase separation region and resulted in particulate morphology near the top.     

Continuous superporous agarose beds in radial flow columns

Continuous superporous agarose beds constitute a new support material for chromatography, biocatalysis and electrophoresis. The bed consists of a single piece of agarose gel, homogeneously transected by flow-carrying pores, which easily can be varied in the range of 10-100 microm. In this work, large diameter beds (60 mm) were prepared and used in specially designed radial flow columns. The basic chromatographic properties of the beds were investigated by size-exclusion chromatography experiments. In an affinity chromatography application one bed was derivatized with Cibacron Blue 3GA and used for the purification of lactate dehydrogenase from a crude bovine heart extract. In a biotransformation application one bed was provided with immobilized beta-galactosidase and used in the production of lactose-free milk.     

Fluid dynamics in capillary and chip electrochromatography

This review is concerned with the phenomenological fluid dynamics in capillary and chip electrochromatography (EC) using high-surface-area random porous media as stationary phases. Specifically, the pore space morphology of packed beds and monoliths is analyzed with respect to the nonuniformity of local and macroscopic EOF, as well as the achievable separation efficiency. It is first pointed out that the pore-level velocity profile of EOF through packed beds and monoliths is generally nonuniform. This contrasts with the plug-like EOF profile in a single homogeneous channel and is caused by a nonuniform distribution of the local electrical field strength in porous media due to the continuously converging and diverging pores. Wall effects of geometrical and electrokinetic nature form another origin for EOF nonuniformities in packed beds which are caused by packing hard particles against a hard wall with different zeta potential. The influence of the resulting, systematic porosity fluctuations close to the confining wall over a distance of a few particle diameters becomes aggravated at low column-to-particle diameter ratio. Due to the hierarchical structure of the pore space in packed beds and silica-based monoliths which are characterized by discrete intraparticle (intraskeleton) mesoporous and interparticle (interskeleton) macroporous spatial domains, charge-selective transport prevails within the porous particles and the monolith skeleton under most general conditions. It forms the basis for electrical field-induced concentration polarization (CP). Simultaneously, a finite and -- depending on morphology -- often significant perfusive EOF is realized in these hierarchically structured materials. The data collected in this review show that the existence of CP and its relative intensity compared to perfusive EOF form fundamental ingredients which tune the fluid dynamics in EC employing monoliths and packed beds as stationary phases. This addresses the (electro)hydrodynamics, associated hydrodynamic dispersion, as well as the migration and retention of charged analytes.