Modeling of thermal processes in high pressure liquid chromatography (original) (raw)
Related papers
Journal of Chromatography A, 2006
The effects of viscous heat dissipation on some important HPLC parameters, such as efficiency (N) and retention factors (k), using 2.1 mm columns at pressures up to 1000 bar have been investigated from both a theoretical and experimental point of view. Two distinct experimental setups and their respective influences on non-homogenous temperature gradients within the column are described and discussed. In the first instance, a still-air column heater was used. This setup leads to approximate 'adiabatic' conditions, and a longitudinal temperature gradient is predicted across the length of the column. The magnitude of this gradient is calculated, and its occurrence confirmed with experimental measurements also indicating that no appreciable loss in efficiency occurs. Secondly, when a water bath is used to thermostat the column, a radial temperature gradient is prevalent. The extent of this gradient is estimated, and the loss in efficiency associated with this gradient is predicted and demonstrated experimentally. It is also observed that approximate adiabatic conditions can lead to floating retention factors. The implications of temperature gradients for routine HPLC analysis at ultra-high pressure are discussed.
Journal of Chromatography A, 2014
The increase of the operating pressure in Liquid Chromatography, has been one of the crucial steps toward faster and more efficient separations. In the present contribution, it was investigated if the pressure limits for narrow-bore columns (2.1 mm ID) could be increased beyond those of commercially available (1300 bar) instrumentation without performance loss. Whereas previous studies applying pressures higher than 2000 bar were limited to the use of columns with a diameter smaller or equal to 1 mm, it is a difficult feat to expand this to 2.1 mm ID given that viscous-heating effects increase according to the fifth power of the column radius. A prototype LC setup was realized, allowing to operate at pressures up to 2600 bar (260 MPa) for large separation volumes (>5 mL). The performance of an in-house-built injector was compared at 800 bar to commercially available injectors, yielding equal performance but twice the maximum pressure rating. The performance of (coupled) custom columns packed with fully porous and superficially porous particles were assessed at ultra-high-pressure conditions. Increasing the inlet pressure from 800 to 2400 bar and scaling the column length proportionally (from 150 mm to 450 mm), resulted in the theoretically expected linear increase in plate count from 20,000 to 59,000. A maximum plate number of 81,000 was realized using a 600 mm long (coupled) column at 2600 bar. Viscous-heating effects were diminished by insulating coupled columns and applying an intermediate-cooling strategy in a forced-air oven.
Journal of Chromatography A, 2007
Theoretical aspects of temperature in liquid chromatography (LC) have mostly been studied to elucidate changes in retention behavior of small and large molecules in various solvents. That temperature also plays a significant role in chromatographic performance is less known. Kinetic plots are an established tool to predict chromatographic performance in terms of speed and efficiency that can be obtained with a certain particle size at the maximum attainable column pressure. In this paper, temperature effects on mobile phase viscosity and analyte diffusion are incorporated in these plots to prove that superior performances are within experimental reach for conventional LC columns and equipment. Verification of the modified kinetic plots with experimental data points is presented.
Journal of Chromatography a, 2008
The wall temperatures of three Acquity-BEH-C 18 columns (2.1 mm × 50, 100, and 150 mm) and the temperature of the incoming eluent were maintained constant at 289 K, using a circulating water heat exchanger. The retention times and the band broadening of naphtho[2,3-a]pyrene were measured for each column as a function of the flow rate applied. Pure acetonitrile was used as the eluent. The flow rate dependence of neither elution volumes nor bandwidths can be accounted for by classical models of retention and HETP, respectively, since these models assume columns to be isothermal. Because the heat generated by friction of the eluent against the column bed increases with increasing flow rate, the column bed cannot remain isothermal at high flow rates. This heat is evacuated radially and/or longitudinally by convection, conduction, and radiation. Radial and axial temperature gradients are formed, which are maximum and minimum, respectively, when the temperature of the column wall is kept uniform and constant. The retention times that we measured match well with the values predicted based on the temperature distribution along and across the column, which we calculated and on the temperature dependence of the retention for the same column operated isothermally (i.e., at very low flow rate). The rate of band spreading varies along non-isothermal columns, so the HETP can only be defined locally. It is a function of the axial coordinate. A new contribution is needed to account for the radial thermal heterogeneity of the column, hence the radial distribution of the flow velocities, which warps the elution band. A new model, based on the general dispersion theory of Aris, allows a successful prediction of the unusually large bandwidths observed with columns packed with fine particles, operated at high flow rates, hence high inlet pressures.
Journal of Chromatography A, 2008
The wall temperatures of three Acquity-BEH-C 18 columns (2.1 mm × 50, 100, and 150 mm) and the temperature of the incoming eluent were maintained constant at 289 K, using a circulating water heat exchanger. The retention times and the band broadening of naphtho[2,3-a]pyrene were measured for each column as a function of the flow rate applied. Pure acetonitrile was used as the eluent. The flow rate dependence of neither elution volumes nor bandwidths can be accounted for by classical models of retention and HETP, respectively, since these models assume columns to be isothermal. Because the heat generated by friction of the eluent against the column bed increases with increasing flow rate, the column bed cannot remain isothermal at high flow rates. This heat is evacuated radially and/or longitudinally by convection, conduction, and radiation. Radial and axial temperature gradients are formed, which are maximum and minimum, respectively, when the temperature of the column wall is kept uniform and constant. The retention times that we measured match well with the values predicted based on the temperature distribution along and across the column, which we calculated and on the temperature dependence of the retention for the same column operated isothermally (i.e., at very low flow rate). The rate of band spreading varies along non-isothermal columns, so the HETP can only be defined locally. It is a function of the axial coordinate. A new contribution is needed to account for the radial thermal heterogeneity of the column, hence the radial distribution of the flow velocities, which warps the elution band. A new model, based on the general dispersion theory of Aris, allows a successful prediction of the unusually large bandwidths observed with columns packed with fine particles, operated at high flow rates, hence high inlet pressures.
Journal of Chromatography A, 2016
A cylindrical vacuum chamber (inner diameter 5 cm) housing a narrow-bore 2.1 mm × 100 mm column packed with 1.8 m HSS-T 3 fully porous particles was built in order to isolate thermally the chromatographic column from the external air environment. Consistent with statistical physics and the mean free path of air molecules, the experimental results show that natural air convection and conduction are fully eliminated for housing air pressures smaller than 10 −4 Torr. Heat radiation is minimized by wrapping up the column with low-emissivity aluminum-tape (emissivity coefficient = 0.03 vs. 0.28 for polished stainless steel 316). Overall, the heat flux at the column wall is reduced by 96% with respect to standard still-air ovens. From a practical viewpoint, the efficiency of the column run at a flow rate of 0.6 mL/min at a constant 13,000 psi pressure drop (the viscous heat power is around 9 W/m) is improved by up to 35% irrespective of the analyte retention. Models of heat and mass transfer reveal that (1) the amplitude of the radial temperature gradient is significantly reduced from 0.30 to 0.01 K and (2) the observed improvement in resolution power stems from a more uniform distribution of the flow velocity across the column diameter. The eddy dispersion term in the van Deemter equation is reduced by 0.8 ± 0.1 reduced plate height unit, a significant gain in column performance.
Effects of high pressure in liquid chromatography
Journal of Chromatography A, 2005
All the experimental parameters that the chromatographers are used to consider as constant (the column length and its diameter, the particle size, the column porosities, the phase ratio, the column holdup volume, the pressure gradient along the column, the mobile phase density and its viscosity, the diffusion coefficients, the equilibrium constants, the retention factors, the efficiency parameters) depend on pressure to some extent. While this dependence is negligible as long as experiments, measurements, and separations are carried out under conventional pressures not exceeding a few tens of megapascal, it is no longer so when the inlet pressure becomes much larger and exceeds 100 MPa. Equations are developed to determine the extent of the influence of pressure on all these parameters and to account for it. The results obtained are illustrated with graphics. The essential results are that (1) many parameters depend on the inlet pressure, hence on the flow rate; (2) the apparent reproducibility of parameters as simple as the retention factor will be poor if measurements are carried out at different flow rates, unless due corrections are applied to the results; (3) the influence of the temperature on the equilibrium constants should be studied under constant inlet pressure rather than at a constant flow rate, to minimize the coupling effect of pressure and temperature through the temperature dependence of the viscosity; and (4) while reproducibility of results obtained at constant pressure and flow rate will not be affected, method development becomes far more complex because of the pressure dependence of everything.
Journal of Chromatography A, 2008
The use of ultra-high pressure liquid chromatography (UHPLC) with pressures up to 1000 bar and columns packed with sub-2-m particles combined with high-temperature mobile phase conditions (up to 90 • C) is assessed according to the current available instrumentation via constrained kinetic plot equations. It is shown that the gain in separation speed, theoretically expected from high-temperature UHPLC (HT-UHPLC), is significantly reduced when taking into account the existing instrumental constraints (extracolumn band broadening, flow-rate and column length limitations). This study also shows that significant improvements could be expected on the current commercial instruments by increasing the flow-rate limit and/or using packing columns with particle size in the range 2.5-3.5 m instead of the current sub-2 m. These particles should obviously withstand very high pressure.
Thermal Science, 2021
A linearized non-isothermal general rate model is formulated and analytically solved to quantify the effects of temperature variations in fixed-bed chromatographic columns. The model contains a set of four coupled PDE accounting for energy transfer resistances, inner and outer particle-pore diffusions, and interfacial mass and axial dispersion. The Laplace transform, the eigenvalue-decomposition technique, and a conventional technique for the solutions of ODE are jointly employed for the solution of the model equations. A few numerical test studies are considered to assess the impact of system parameters on the performance of packed-bed adsorption columns. To access the range of applicability and to get the scope of the appropriateness of calculated analytical results, the numerical results are also obtained by applying a high resolution finite volume scheme. The analytical solutions obtained can be used as an invaluable tool for analyzing, optimizing, and upgrading the non-isotherm...
Journal of Chromatography A, 1997
The packing behavior of a typical 10 Ixm C~ stationary phase was studied in terms of the resultant column efficiency and capacity factor. The column-to-column reproducibility of these parameters under identical packing procedures is assessed. Correlation of these parameters and the column void volume to the column packing density is reported. Two regimes were studied; that of poor-and well-packed columns. For poorly packed columns, the column-to-column variability is high, but a concomitantly poor same-column reproducibility of measurement suggests that little statistically significant difference exists between different columns packed with the same procedure. There is also no statistically significant correlation between the column parameters and the packing densities, however, the poorest columns showed a degradation of performance after the drying procedures used to obtain the column masses. Well-packed columns showed much less degradation upon drying. For the well-packed columns, statistically significant column-to-column differences were observable, mainly due to a high same-column precision of measurement. Analysis of the results suggests that even well-packed columns are not optimally packed and that regions of high and low density coexist along the column. The results are compared to those achieved with semi-preparative columns packed with the same slurry procedure and preparative columns packed under dynamic axial compression. Poor day-to-day, same-column reproducibility (degradation) under ambient conditions was observed in conjunction with a high column-to-column variability for the semi-preparative columns.