High-Pressure Gas Chromatography and Chromatography with Supercritical Fluids. III. Fluid-Liquid Chromatography

Abstract

High-boiling substances can be separated by partition chromatography, using n-pentane and isopropanol under supercritical conditions as mobile carriers. At moderate temperatures (200–250°C) the volatility of heavy substances may be enhanced by a factor of as much as 10⁴ by increasing the pressure from atmospheric to about 50 kg/cm². This enables the analysis of compounds which are too heavy for ordinary gas chromatography.

As predicted before on the basis of gas-chromatographic studies at elevated pressures, the use of supercritical mobile fluids in combination with liquid stationary phases results in an attractive separation procedure. This technique is rapid and has a high degree of flexibility. Separations according to boiling point as well as molecular type are possible.

The main features of the new chromatographic technique, for which the designation “fluid-liquid chromatography” (FLC) is proposed, are illustrated by means of several examples.     

Ultra-High-Pressure Gas Chromatography in Micro Columns to 2000 Atmospheres

Abstract

A gas-chromatographic system working at inlet pressures to 2000 atm has been constructed. Data acquired on micro columns packed with 13-μ adsorptive particles are described. Results, although rather erratic at present, show efficiencies as high as 4000 plates/ft, or roughly 40,000 plates for a 3-m column. In addition, equilibrium migration rates show a strong dependence on pressure, with variations up to the fourfold level. The relationship of the present work to previous work; the experimental system; and the role of gas nonidealities are all discussed.     

Transfer Characteristics of Packed Columns,Determined by Frontal Gas Chromatography

Abstract

A frontal-gas-chromatographic method is described for the determination of effective gas diffusion coefficients and constants of the first-order rate equation, respectively. The dependence of these parameters on particle size and degree of wetting by the stationary phase in case of supports of different origin makes it probable that the dissolution of the sorbable component can be considered as the rate-governing step in gas-liquid chromatography. Experimental data suggest that the wetting of supports in most cases does not result in a homogeneous liquid film, but rather in liquid droplets and “islands.”

The computations allow us to deduce an equation for H involving the slope at the inflexion point of the frontal chromatographic breakthrough curves, instead of the diffusion constant and the constant of the rate equation. The courses of H, measured according to the elution and frontal variant of chromatography, respectively, are similar, but both differ considerably from that computed from the van Deemter equation, when the calculation is performed by using the correct values of D _eff and k_t . A correct theory of gas-chromatographic processes ought to take into consideration the asymmetry of the elution or frontal profiles at higher flow rates.     

Nonlinear Distribution Coefficients in Gas Chromatography

Abstract

Nonlinear partition coefficients of polar solutes on coated and uncoated diatomaceous-earth supports lend themselves to mathematical treatment when proper precautions are taken to reproducibly condition the chromatographic column. A previously derived equation was used to calculate the adsorption and distribution isotherms from adsorption-desorption chromatograms whose diffuse elution curves approached the base line asymptotically. In the concentration ranges studied, the adsorption of acetone on uncoated support obeyed the Temkin isotherm equation. A plot of the amount adsorbed vs. the log of the concentration of the solute in the carrier gas gave a straight line. The adsorption of acetone on coated columns under the same conditions followed a Freundlich isotherm equation, and log-log plots of amount adsorbed vs. concentration of the solute gave straight lines. The equations for the lines obtained from the isotherm plots of the coated and uncoated columns were used to calculate the retention volumes as a function of concentration, by means of the equation V_Ng = dQ/dC. The validity of the retention volume and adsorption equations were checked by the agreement of theoretical and experimental measurements.

Within limits retention volumes may be correlated with sample size, providing the contribution of the bulk liquid phase is known. It is expected that the retention volume-sample size relationship would be more useful in low-loaded columns, where the bulk liquid-phase contribution is either small or negligible. The temperature-effect studies showed that, as expected, stronger adsorption bonds are formed at lower temperatures. The sample size-temperature studies on the inactive (RIC) support coated with tri-o-tolyl phosphate showed that the liquid phase contributed to the non-linearity of the distribution coefficient in a minor but unknown extent.     

Temperature Gradient Method for Continuous Countercurrent Gas–Liquid Chromatography

Abstract

Continuous countercurrent gas–liquid chromatography (CCGLC), where the gas and liquid phases move countercurrently and the sample can be introduced continuously, is a method suitable for separating binary sample systems. The flow ratio of the gas and liquid phases (G/L) in CCGLC should be between the partition coefficients of the two components to be separated. However, the larger the difference between the G/L and the partition coefficient of one component, the higher the separation efficiency of the other component. By shifting the partition coefficients from the point of introduction of the sample to the directions of the gas and liquid flows, we believed a more efficient separation of the components would be achieved. A separation tower with a temperature gradient was constructed to create an inclined arrangement of the partition coefficient. With this system, the separation of stereoisomers was investigated, and 99.9% purity of trans-decahydronaphthalene was achieved.     

Propagation of Signals of Finite Concentration in Gas Chromatography. I. The Quasi-Ideal Model

Abstract

A general system of partial differential equations describing the propagation of signals of finite concentrations in a chromatographic column is derived. These equations are related to the mass-balance equations for the solutes and the carrier gas. The model used assumes that there is no temperature or pressure variation at any point in the column when the signal is eluted, and that the equilibrium between stationary and mobile phases is instantaneous. It is shown that this model, which leads to a tractable set of equations, is generally valid. The solution of this system of equations gives new insight into the phenomena which are responsible for the peak deformations and broadening in preparative scale chromatography.     

Effect of processing parameters on cell morphology of polycarbonate foam

Abstract

The purpose of this research is to investigate the effect of processing parameters on the cell morphology of polycarbonate (PC) foam. In this study, foamed PC was prepared using a dynamic simulation foaming set-up. The cell morphology was compared at different temperatures, pressures, gas saturation times, pressure drop rates and shear rates. The cell morphology of foamed samples was characterised using SEM. It was found that foamed samples with better morphology could be obtained by varying mechanical properties, such as pressure, pressure drop rate and gas saturation time. Optimum temperature and shear rate for microcellular foaming of PC are presented.     

Calculation of the Pressure Drop Effect in Gas Chromatography

Abstract

Analysis by statistical moments permits the effect of pressure drop on the performance of a gas chromatography column to be understood more clearly. Through the use of the first three statistical moments, exact equations are derived for both the standard deviation and the skewness of the breakthrough curve following a pulse, input. Higher statistical moments can be derived and by their use the breakthrough curve can be determined to any desired accuracy.     

Supercritical Chromatography of Paraffins on a Molecular Sieve: Analytical and Preparative Scale

Abstract

Supercritical carbon dioxide has been used as the mobile phase in chromato-graphic investigations of higher paraffins on molecular sieve 5A at temperatures from 40 to 200°C and pressures up to 20 MPa. The pressure and temperature dependence of retention data of paraffins up to eicosane found experimentally are discussed and compared with results of model calculations. Furthermore, some physicochemical applications of the high pressure retention data seem to be possible. A pressure-programmed group separation process permitting analytical and preparative scale separations of n- and isoparaffin mixtures is outlined.     

A Gas Chromatographic Capillary Column with Negligible Pressure Drop

Abstract

Extremely high resolution can be attained with correspondingly long columns made practicable by propelling the carrier gas with a driving force other than inlet pressure.     

Gas transport in rubbery polymers

Permeability coefficients have been measured for CO₂ and CH₄ in polyethylene membranes at 20, 30, and 40°C and at applied gas pressures up to ca. 2 MPa and for CH₄ in three kinds of rubber films at 25, 30, and 35°C and at applied gas pressures up to ca. 2.4 MPa. The pressure dependence of the logarithms of permeability coefficients became linear except for the CO₂—polyethylene system, where the pressure dependence became quadratic, with a minimum at a certain value of pressure. The linear and quadratic dependences can be interpreted by a free volume model of diffusion of a gas molecule in polymers. The temperature dependence of the permeability coefficients at zero pressure difference across the polymer film for each system obeyed an Arrhenius type equation.     

Asymmetry of Peak Shapes in Linear Gas—Liquid Chromatography

Abstract

A mathematical model describing axial dispersion, interparticle mass transfer, intraparticle gas diffusion, and diffusion in a uniform thickness liquid film is used systematically to investigate the influence of intraparticle diffusivity, diffusivity in a stationary liquid phase (SLP), partition coefficient, and thickness of liquid film on the shape of the peaks in linear gas—liquid chromatography by converting Laplace transformed equations into time domain. The low diffusivities of intraparticle and/or SLP can cause the asymmetry and long tail in chromatographic peaks. A higher partition coefficient and the film thickness at low diffusivities also give skewed peaks. At a higher mass transfer rate, the peak becomes sharper. From these results a guide can be suggested to avoid the asymmetric condition of long-tailing peaks.     

Equilibrium Analysis of Cyclic Adsorption Processes: CO2 Working Capacities with NaY

Abstract

The most common application of adsorption is via pressure swing adsorption. In this type of design, the feed and regeneration temperatures are kept approximately equal, whereas the feed pressure is higher than the regeneration pressure. By exploiting the difference in the amount adsorbed at a higher pressure to the amount adsorbed at a lower pressure, a working capacity is realized. Therefore, by examining the expected (ideal) working capacity of an adsorbent, a performance characteristic can be analyzed for a pressure swing adsorption process (PSA). For this work, feed pressures up to 2.0 atm CO₂ and feed temperatures from 20°C to 200°C were investigated. These limits were chosen due to the nature of the target process: CO₂ removal from flue gas.

Carbon dioxide adsorption isotherms were determined in a constant volume system at 23°C, 45°C, 65°C, 104°C, 146°C, and 198°C, for pressures between 0.001 and 2.5 atm CO₂ with NaY zeolite. These data were fit with the temperature dependent form of the Toth isotherm. Henry's Law constants and the heat of adsorption at the limit of zero coverage were also determined using the concentration pulse method. Comparison of the Henry's Law constants derived from the Toth isotherm, and those obtained with the concentration pulse method provided excellent agreement.

By using the Toth isotherm, expected working capacity contour plots were constructed for PSA (Pressure Swing Adsorption), TSA (Temperature Swing Adsorption), and PTSA (Pressure Temperature Swing Adsorption) cycles. The largest expected working capacities were obtained when the bed was operated under a high‐pressure gradient PSA cycle, or a high thermal and pressure gradient PTSA cycle. The results also showed that certain TSA and PSA cycle conditions would result with higher expected working capacities as the feed temperature increases.     

The Effect of Temperature on Production Rate in Chromatography

Abstract

A simple theoretical model for the prediction of production rates in preparative chromatography shows that an optimum temperature exists in the case of constant mass flow rate. This result corresponds to experimental work reported by Rose et al. No optimum temperature exists in cases where a constant pressure drop is maintained at all temperatures.     

Stationary Spatial Temperature Gradient in Gas Chromatography

Abstract

Imposing an increasing stationary temperature gradient along the length of a gas chromatographic column is proposed as a technique for improving separation. Side outlet ports with control valves would allow the process to have the same advantages of programmed temperature gas chromatography without the disadvantages of temperature transients. For a simple model of gas chromatography the analysis provides expressions for temporal moments at any point along the column as a function of the temperature gradient. Reduced retention times, sharpening of peaks, and higher symmetry are predicted due to increasing the temperature gradient. The relationship to chromathermography is discussed.     

Restructuring of Chain Aggregates of Titania Nanoparticles in the Gas Phase

ABSTRACT

The in situ restructuring of chain aggregates of titania nanoparticles by heating was investigated. Titania nanoparticles (dp ? 7 nm) were prepared by thermal decomposition of titanium tetraisopropoxide vapor in a N₂ carrier gas and allowed to form aggregates with a count mean mobility diameter of 110 nm. The change in mobility diameter of the aggregates was measured to determine the fractal dimension, D_f, using two differential mobility analyzers and a condensation particle counter. The gas temperature was varied from about 20°C to 1,000°C. Aggregates that restructured at 900°C and a residence time of 5.10 sec reached a close-packed structure as observed in the electron microscope. The value of the D_f increased from 2.33 to 3.00 as the restructuring temperature increased. The activation energy for restructuring estimated from the D_f and the coordination number was 2.28 × 10^?19 J, almost an order of magnitude higher than that of metal aggregates such as silver and copper. The effect of carrier gas was studied by making a similar series of experiments using He instead of N₂. A small increase in D_f and the increase of D_f at low restructuring temperature was found with the He atmosphere. Aggregates generated at different temperatures to vary particle bond strengths showed little change in restructuring behavior.     

Gas transport in polymer membrane at temperatures above and below glass transition point

The processes of gas sorption and permeation in a polymer membrane at temperatures above and below the glass-transition point were examined using poly-4-methylpentene-1 (glass-transition temperature reported to be 40°C) as a membrane material. The permeabilities to O₂ and N₂ were independent of applied gas pressure at every temperature; the mean permeability coefficient to CO₂ increased with increasing gas pressure. The logarithm of the mean permeability coefficient to CO₂ increased linearly with gas pressure due to the plasticization effect induced by sorbed CO₂. From the sorption isotherms for CO₂ at 20 and 30°C it was judged that the glass transition was brought about by sorbed CO₂ at temperatures below the glass-transition point of the pure polymer. © 1994 John Wiley & Sons, Inc.     

Kinetic Model of Effect of a Carrier Gas on Nucleation in a Diffusion Chamber

For the first time a model of nucleation when a transport of condensing molecules to the cluster surface is determined by their diffusion through a carrier gas is proposed. The approach in use is strongly based on the microscopic theory of nucleation put forward by the author and allows an analytical representation of the cluster's concentrations through supersaturation, gas temperature, and carrier gas pressure, which is quite new. It is shown that usual conditions of experiments in diffusion cloud chambers meet the requirements of the model validity that means this model can be used to explain a mechanism of the carrier gas pressure influence on the nucleation kinetics observed in experiments.     

Drying in a Pulsed-Fluid Bed with Relocated Gas Stream

ABSTRACT

Results of studies on cooling and drying of powdery and granulated material in a pulsed-fluid bed ( PFB) system with relocated gas stream presented here are related to hydrodynamics and kinetics of these processes. Generalized results in the form of dimensionless equations can be used to determine basic process parameters such as drying time, heat transfer coefficient, pulsed-fluidzation velocity. pressure drop, etc. Results of experimental investigations, supplemented with experiments carried out in a prototype industrial equipment, can be useful for dryer scale-up and are recommended for designing.

A comparison of technical and economic parameters of these dryers with the parameters of classical fluid-bed dryers shows many advantages of PFB systems, including, among othen. reduced gas consumption, uniform bed structure and stability of final parameters of the product.

Positive estimation of the operation and technological parameters suggests that PFB dryers with relocated gas stream can be applied successfully in industrial practice.     

PROTOTYPE HIGH TEMPERATURE/HIGH PRESSURE KILN FOR THE EVALUATION OF WOOD DRYING SCHEDULES

ABSTRACT

A new laboratory kiln was developed and built to perform over a very wide range of drying conditions. For example, the dry bulb temperature can vary from 30°C to 150°C and the dew point can be adjusted between 20°C and 130°C. Obviously, with such a high level of dew point, pressures over atmospheric pressure may be induced inside the chamber. For this reason, the kiln has been designed to withstand pressure of up to 3 bars. This kiln can also perform vacuum drying.

A programmable controller allows the temperature levels to be maintained within ± 0.2°C. Because the whole kiln can be heated only through the agitated water present at the bottom of the kiln, the load temperature can be increased up to 130°C in saturated conditions, without any change of moisture content.

The kiln has various sensors attached and is capable of withstanding severe conditions (high temperature, saturated vapour and elevated pressures). At present, air and water temperatures as well as temperature at different locations within the board can be collected during the drying process. A load cell and pressure gauges are also available. The first tests performed using this equipment are presented at the end of the paper.