Relation between dimensionless parameters of LC columns.

Effect of pressure on solute diffusivity, solvent viscosity and column temperature in liquid chromatography

The role of the temperature in reversed-phase high-performance liquid chromatography using pyrocarbon-containing adsorbents

We study the diffusion of small solute particles through solvent by keeping the solute-solvent interaction repulsive and varying the solvent properties. The study involves computer simulations, development of a new model to describe diffusion of small solutes in a solvent, and also mode coupling theory (MCT) calculations. In a viscous solvent, a small solute diffuses via coupling to the solvent hydrodynamic modes and also through the transient cages formed by the solvent. The model developed can estimate the independent contributions from these two different channels of diffusion. Although the solute diffusion in all the systems shows an amplification, the degree of it increases with solvent viscosity. The model correctly predicts that when the solvent viscosity is high, the solute primarily diffuses by exploiting the solvent cages. In such a scenario the MCT diffusion performed for a static solvent provides a correct estimation of the cage diffusion.

Two types of monolithic silica columns derivatized to form an ODS phase, one prepared in a fused silica capillary (SR-FS) and the other prepared in a mold and clad with an engineering plastic (poly-ether-ether-ketone) (SR-PEEK), were evaluated. The column efficiency and pressure drop were compared with those of a column packed with 5-μm ODS-silica particles and of an ODS-silica monolith prepared in a mold and wrapped with PTFE tubing (SR-PTFE). SR-FS gave a lower pressure drop than a column packed with 5-μm particles by a factor of 20, and a plate height of 20 μm at a linear velocity below 1 mm/s. SR-PEEK showed higher flow-resistance than the other monolithic silica columns, but they still showed a minimum plate height of 8-10 μm and a lower pressure drop than popular commercial columns packed with 5-μm particles. The evaluation of SR-FS columns in a CEC mode showed much higher efficiency than in a pressure-driven mode.

Polarized fluorescence photobleaching recovery for measuring rotational diffusion in solutions and membranes

Efficient desorption of selectively adsorbed N2 from air in a packed column of LiX zeolite by rapidly purging the adsorbent with an O2 enriched gas is an important element of a rapid cyclic pressure swing adsorption (RPSA) process used in the design of many medical oxygen concentrators (MOC). The amount of O2 purge gas used in the desorption process is a sensitive variable in determining the overall separation performance of a MOC unit. Various resistances like (a) adsorption kinetics, (b) column pressure drop, (c) non-isothermal column operation, (d) gas phase mass and thermal axial dispersions, and (e) gas-solid heat transfer kinetics determine the amount of purge gas required for efficient desorption of N2. The impacts of these variables on the purge efficiency were numerically simulated using a detailed mathematical model of non-isothermal, non-isobaric, and non-equilibrium desorption process in an adiabatic column. The purge gas quantity required for a specific desorption duty (fraction of total N2 removed from a column) is minimum when the process is carried out under ideal, hypothetical conditions (isothermal, isobaric, and governed by local thermodynamic equilibrium). All above-listed non-idealities (a–e) can increase the purge gas quantity, thereby, lowering the efficiency of the desorption process compared to the ideal case. Items (a–c) are primarily responsible for inefficient desorption by purge, while gas phase mass and thermal axial dispersions do not affect the purge efficiency under the conditions of operation used in this study. Smaller adsorbent particles can be used to reduce the negative effects of adsorption kinetics, especially for a fast desorption process, but increased column pressure drop adds to purge inefficiency. A particle size range of ∼300–500 μm is found to require a minimum purge gas amount for a given desorption duty. The purge gas requirement can be further reduced by employing a pancake column design (length to diameter ratio, L/D<0.2) which lowers the column pressure drop, but hydrodynamic inefficiencies (gas mal-distribution, particle agglomeration) may be introduced. Lower L/D also leads to a smaller fraction of the column volume that is free of N2 at the purge inlet end, which is required for maintaining product gas purity. The simulated gas and solid temperature profiles inside the column at the end of the rapid desorption process show that a finite gas-solid heat transfer coefficient affects these profiles only in the purge gas entrance region of the column. The profiles in the balance of the column are nearly invariant to the values of that coefficient. Consequently, the gas-solid heat transfer resistance has a minimum influence on the overall integrated N2 desorption efficiency by O2 purge for the present application.

Pressure drop effects in packed capillary column supercritical fluid chromatography

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTVariables in Perforated Plate Column Efficiency and Pressure Drop - Effect of Hole Free Area, Hole Diameter, Hole Spacing, Weir Height, and Downcomer AreaC. L. Umholtz and Matthew Van WinkleCite this: Ind. Eng. Chem. 1957, 49, 2, 226–232Publication Date (Print):February 1, 1957Publication History Published online1 May 2002Published inissue 1 February 1957https://doi.org/10.1021/ie50566a033RIGHTS & PERMISSIONSArticle Views370Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Towards elucidation of the drug release mechanism from compressed hydrophilic matrices made of cellulose ethers. I. Pulse-field-gradient spin-echo NMR study of sodium salicylate diffusivity in swollen hydrogels with respect to polymer matrix physical structure

Multiobjective optimization of simulated moving bed and Varicol processes using a genetic algorithm

This work is a theoretical and experimental investigation of the binary retention time (t step) when the disturbance is made to a chromatographic system by adding a small flow of one of the pure components. The established theory is for addition of a pulse: in this case, the retention time (t pulse) depends on the two binary isotherm gradients, and should be independent of the choice of pulse gas. From the column material balance, the value of t step also depends on the column pressure drop and perturbation gas—the value of t step should always be greater for the more-adsorbed component. The theory has been validated from results on the nitrogen–argon–5A zeolite system at 25, 54 and 81 °C. For a 50% mixture at 25 °C with a column pressure drop of 0.1 bar, the values of t step are 257 and 254 seconds for the nitrogen and argon perturbations. The values of t step are different because addition of the perturbation flow causes a very small increase in average column pressure (about 0.5 mbar), which causes the binary isotherm gradients to be measured in (slightly) different directions along the isotherm surface. The intention is to determine the value of t step for the case of a zero change in the average column pressure: experimentally, this would require a column with a zero pressure drop. The material balance shows that t step for a column with a zero pressure drop is obtained from a simple weighted function of the values of t step for the two pure-component perturbations. Accurate determination is essential because the “zero pressure drop” values are used to determine binary adsorption isotherms which are, of course, at a fixed pressure.

The self-diffusion of a monatomic solute in liquid 1-octanol and n-tetradecane was investigated by means of a molecular dynamics simulation. The diffusion coefficient of a solute as small as argon is much greater than that obtained from the hydrodynamic-based Stokes-Einstein (SE) relation, as was reported experimentally. A relaxation of the memory function of a freely diffusing solute is much faster than that of the autocorrelation function of a shear stress. However, the SE behavior is recovered when the solute is spatially fixed, and the diffusion coefficient is calculated from the force-force autocorrelation function. A relaxation of the autocorrelation function of the force also follows that of shear stress. The fast diffusion of a small solute is thus ascribed to the decoupling between the structural relaxation of solvent and the solute diffusion induced by the self-motion of the solute.

At the high pressure drop required for the fast analysis of complex mixtures, the equations for the column plate height, H, and plate duration, Q, as functions of the carrier gas velocity, u, differ substantially from the equations for the same quantities expressed via the carrier gas flow rate, F. While u as an independent pneumatic variable is more convenient for the theoretical studies, F is a more convenient as a control parameter in practical applications. Equations for H vs. u and for Q vs. u from Parts 1 and 2 are transformed here into expressions for H vs. F and Q vs. F. An efficiency-optimized flow rate (EOF) and a speed-optimized flow rate (SOF) are found. Expressions for these two quantities are considerably simpler than their velocity-based counterparts. In particular, SOF does not depend on column length, film thickness, and pressure drop.

At the high pressure drop required for the fast analysis of complex mixtures, the equations for the column plate height, H, and plate duration, Q, as functions of the carrier gas velocity, u, differ substantially from the equations for the same quantities expressed via the carrier gas flow rate, F. While u as an independent pneumatic variable is more convenient for the theoretical studies, F is a more convenient as a control parameter in practical applications. Equations for Hvs.u and for Q vs.u from Parts 1 and 2 are transformed here into expressions for H vs. F and Q vs. F. An efficiency-optimized flow rate (EOF) and a speed-optimized flow rate (SOF) are found. Expressions for these two quantities are considerably simpler than their velocity-based counterparts. In particular, SOF does not depend on column length, film thickness, and pressure drop.

Supercritical fluid chromatography, when performed on a packed column, is a powerful and fast separation technique. To enhance the number of theoretical plates (TP) available, long packed columns (>1 m) have been used successfully, despite controversy over the effect on column efficiency of the density gradient induced by the pressure drop. Peak broadening and deformation were reported, and packings with larger particle diameter than those used in liquid chromatography (10 μm instead of 3−5 μm) were advised in order to reduce the column pressure drop. Velocity gradient induced by the density gradient was reported to reduce efficiency. This paper presents the results of investigations on the influence of density gradient on the apparent efficiency obtained on a series of four 25-cm × 4.6-mm-i.d. Nucleosil C18 columns connected in series (particle size, 5 μm). Apparent column efficiency is found to vary from less than 10 000 TP to more than 100 000 TP versus the density and the density gradient. The higher the density gradient, the higher the efficiency loss. A model is presented which accounts for the effect of linear velocity and density gradients on peak broadening. It confirms that it is the linear velocity variation rather than the variation of the density which causes band broadening and allows prediction of conditions for which apparent efficiency loss occurs. To reduce the density gradient induced by column pressure drop, one can compensate for pressure gradient by a superimposed temperature gradient (multitemperature control of the mobile phase via three column ovens). It allows one to obtain the highest efficiency and to use CO2 at lower density without any loss of efficiency. When methanol is added to the CO2, no pressure drop compensation is required in order to obtain the highest apparent efficiency. As density gradient compensation via multitemperature control of the mobile phase provides higher apparent efficiency and, consequently, higher resolution than in isothermal operation, it is especially useful for separation of complex oil samples.