Extraparticle diffusional effects in gel permeation chromatography. I. Theory

A systems approach is applied to the analysis of chromatogram resolution dispersion, or zone broadening, in gel permeation chromatography (GPC). Three possible sources of dispersion are considered; these are: the packed columns; the empty tubing between pump and columns, columns and detector, etc.; and the detection system, viz., the differential refractometer cell. It is shown that empty tubing can contribute significantly to the degree of dispersion and to skewness of elution curves and that this dispersion should depend on molecular weight of solute (polymer) and diameter and length of the tubing. The importance of dispersion in the empty tubing is compared with that in the packed columns and refractometer cell.     

Characterization and properties of macromolecules. VIII. A study of experimental variables in preparative-scale gel permeation chromatography

Three preparative-scale gel permeation chromatography columns were constructed and filled with Corning porous-glass packing materials. Each column was packed with a different pore-size materials, CPG 10–2000, CPG 10–350, CPG 10–120, all of which had been treated with hexamethyldisilazane. An additional preparative-scale column packed with Styragel, nominal porosity 5 × 10⁴ Å, was added to complete the column bank. Polystyrene standards were used to investigate the effects of molecular weight and sample concentration upon elution volume. A commerical polystyrene sample was fractionated using this system to study the effects of sample concentration and flow rate on fractionation efficiency. These fractions were analyzed by high-resolution analyticalscale gel permeation chromatography. Where possible, the results have been compared with similar studies that used Porasil (Spherosil) and Styragel columns.     

Change in GPC elution volume of polymers by adsorption onto porous glass as column packing

The elution volume of polystyrene or of poly(ethylene glycol) of various molecular weights was measured in both tetrahydrofuran and benzene at 40 or 60°C by means of gel permeation chromatography utilizing columns packed with crosslinked polystyrene gel or porous glass. When the polystyrene gel was used as the column packing, a relationship between log [η]M and the peak elution volume for polystyrene agreed with that for poly(ethylene glycol). Using columns packed with glass, however, the elution volume of polystyrene was smaller than that of poly(ethylene glycol) at the same log [η]M. The results are considered to be attributable to the adsorption of poly(ethylene glycol) onto the glass. The effect of the adsorption on the elution volume was evaluated by assuming a universal calibration for columns containing porous glass; the deviation of the elution volume from the universal calibration curve increased with increase in molecular weight. At lower temperatures, even polystyrene was adsorbed onto the glass. Therefore, to reduce the adsorption of polymers on glass, it is necessary to increase the temperature or use surface treatment when porous glass is used as a GPC column packing.     

Minimizing axial dispersion in narrow packed column using superhydrophobic wall

The scope of minimizing dispersion in narrow packed column using superhydrophobic (SH) wall is assessed experimentally with implications in analytical techniques such as liquid chromatography. The study includes devising a packed column (7-19 mm) with lotus leaf pasted on the inner wall and establishing a gravity driven flow through it. The flow dispersion is characterized based on the residence time distribution study of the column. The results are compared against similar flow through smooth packed column. Experimental results reveal the influence of two factors: column diameter as well as the wall features, superhydrophobic or smooth. For similar surface features, the axial dispersion reduces with decrease in column diameter due to the increase in voidage, which leads to plug flow. For the same diameter, between smooth and superhydrophobic, effects of slip in the latter reduce the dispersion significantly. Thus, the introduction of superhydrophobic narrow columns can play a crucial role in minimizing dispersion in analytical techniques.     

大型填料塔内液相分布状态的研究

本文针对填料塔内液体运动的分散方程,提出了新的边界条件,并得到描述填料塔内液体运动规律的新模型.通过实验确定了模型中的各参数.实验结果与理论模型的比较,两者吻合较好.用激光- 光导纤维测试系统测定了液体在填料层内的平均停留时间分布,作出了塔内液体的等平均停留时间曲线,并进一步分析了液体在填料塔内的流动特性.     

LDA measurements of liquid velocities in a refractive index matched packed bubble column

LDA has been used to measure liquid velocities in a small-scale bubble column, internal diameter of 50 mm, packed with glass Raschig rings, 10 and 15 mm. A mixture of benzyl-alcohol and ethyl alcohol was index matched against the packing material. A method to separate the signals from liquid and bubbles was developed. It was found that the axial time-averaged liquid velocity was lower than that obtained in empty bubble columns, and that both the time-averaged liquid velocity and the RMS value of the liquid increased for the larger packing size.     

Adsorption Isotherms of Amino Acids and Kinetic Analysis of Ion-Exchange Chromatographs by the Moment Method

Abstract

Adsorption isotherms of three amino acids (aspartic acid, glutamic acid, and glycine) were measured in a single solute or multisolutes by the batchwise method. Kinetic parameters were determined in a column packed with ion-exchange resin by the moment method. The equilibrium constants obtained by both methods were in good agreement. Experimental elution curves agreed well with calculated one.     

Theory of liquid phase dispersion in packed columns

Dispersion in the liquid phase for trickle or two-phase flow in packed columns has two important contributions, local dispersion in the packing, and axial dispersion due to the differences in liquid flow conditions between the wall and bulk regions of packing. A three-dimensional axisymmetric formulation of dispersion within the column is analysed and compared to a one-dimensional formulation in the axial co-ordinate. It is shown that the one-dimensional formulation is correct for the limit of long dispersion times. For other times the coefficient for total dispersion within the column is found to be a function of local dispersion, differences in flow between the wall and bulk regions, and dispersion time. An analytical relationship is given between the coefficient for total dispersion, and contributing factors, in local dispersion and column flows.     

Liquid dispersion,gas holdup and frictional pressure drop in a packed bubble column at elevated pressures

The gas holdup, frictional pressure drop and liquid dispersion have been investigated in a packed bubble column at elevated pressures for the air–water system. The bubble column, which had an internal diameter of 0.15 m and which was packed with 15 mm plastic Pall rings was operated in the semibatch mode. The operating pressures ranged from 0.1 to 0.66 MPa. It was found that increasing the pressure increases both the gas holdup and the dispersion coefficient. In contradiction to the results obtained from packed bubble columns fed with a continuous net flow of liquid, a maximum point of the frictional pressure drop was observed at the transition point between bubble and pulse flow region.     

A structured catalytic bubble column reactor: hydrodynamics and mixing studies

This paper reports the results of a comprehensive experimental study of the hydrodynamics and mixing in two bubble column reactors of 0.1 and 0.24 m in diameter with KATAPAK-S^® as packing material. Total gas hold up and axial dispersion coefficients were measured in the structured bubble columns and the values were compared with experimental results obtained in the same work with empty bubble columns. The results reveal that the gas hold up in structured bubble columns is practically the same as in empty bubble columns when compared at the same superficial gas velocity based on open area available for gas–liquid dispersion. The presence of the structured elements in the bubble column reactor reduces the liquid phase backmixing by one order of magnitude.     

High performance g.p.c. with crosslinked polystyrene gels: influence of particle size distribution

Spherical crosslinked polystyrene gel particles have been separated by air classification into fractions having average particle diameters in the range 10–40 μm. The particle size distributions have been shown to be narrow by Coulter Counter measurements. G.p.c. column performance improves as the average particle diameter decreases and columns packed with gel fractions having a number-average particle diameter below 20 μm give plate counts in excess of 3000 plates per foot. Plate height results as a function of eluent flow rate suggest that chromatogram broadening due to mobile phase dispersion is reduced in columns packed with spherical particles having a narrow size distribution.     

Optimization of parameters for the analysis of triglyceride by reverse phase HPLC using a UV detector at 210 nm

Factors affecting the quantitative analysis of triglycerides from natural oils were investigated on reverse-phase HPLC columns. The organic solvent used to dissolve the triglycerides greatly determined the extent to which the solute molecules interacted with the mobile phase and hydrocarbonaceous ligand. Chloroform-dissolved triglycerides resulted in better resolution and detection of the solute matrix of all the organic solvents used. Also, ethanol was superior to methanol as the polar organic modifier in the mobile-phase composition. Ballistic temperature programming significantly reduced analysis time and column temperature, when raised high enough, can reverse the elution of some solutes, Possible mechanisms of triglyceride solute retention on reverse-phase columns are advanced in conjunction with the various parameters used in the investigation. Paper Number 8705 of the Journal Series of the NC Agr. Res. Serv. (NCARS).     

精密精馏填料塔的稳态特性

本文第一部分探讨精密精馏填料塔的最佳回流比。文中提出一个关联式来估算在一定回流比下的传质单元数,并由此导出最佳回流比图。精密精馏最佳回流比的最大值约为1.4R_m。 本文第二部分探讨最佳的塔径与填料尺寸比。实验是在内径分别为20、28、32、44mm,填料层高从1020到1475mm四个填料塔中用2.5×2.5mm三角螺旋填料以正庚烷-甲基环己烷系统进行。根据实验求得的全塔浓度分布曲线,在除去塔顶及塔底两端的端效应后,得出填料层的真实分离效率（即传质单元高度或每米填料高的传质单元数）。以真实分离效率与塔径及填料尺寸比作图,所有恒气速下的曲线都呈现出最佳点,并且其比值几乎相同。对于这种填料,最佳塔径与填料尺寸比约为13—14,此值与普通填料或工业填料大致相似。     

Axial mixing and scaleup of reciprocating plate columns

Axial mixing measurements in single phase (water) flow have been taken in open-type reciprocating plate columns of diameters 25.4 and 508 mm. In the case of the smaller column, two-phase axial mixing was measured, both in the dispersed phase (water dispersed in n-heptane) and the continuous phase (with n-heptane dispersed in water). Pulse injection of a tracer solution of ammonium chloride and methanol in water was used. Under single phase conditions, the axial dispersion coefficients were found to go through a minimum as the agitation level was increased from zero. The coefficients were nearly an order of magnitude higher in the 508 mm column than in the 25,4 mm column. In two phase flow in the 25,4 mm column, the continuous phase axial dispersion coefficients also went through a minimum as agitation was increased. The dispersed phase axial dispersion coefficients decreased monotonically as agitation was increased from zero. The results of this work and previous data are used in modelling the scale-up of reciprocating plate columns by means of Pratt's simplified technique. The existing empirical scale-up equation is consistent with an assumption that continuous phase mixing increased with column diameter but dispersed phase mixing remains unchanged.     

Chromatogram broadening of proteins and dextrans in size exclusion chromatography

The mechanisms governing the broadening of experimental chromatograms for proteins and paucidisperse dextrans were studied on TSK-G2000SW and TSK-G3000SW columns. Within the conditions studied, the chromatogram variance for all solutes increased linearly with increasing effluent flow rate. As predicted by current theories of the kinetics of size exclusion chromatography, this flow rate dependence is caused mainly by slow mass transport of the solute within the stationary phase of the column. Restricted diffusion within the stationary phase was dependent upon the ratio of solute molecular size to column pore radius and was similar for both proteins and dextrans. In comparison with results for monodisperse proteins, the broader chromatograms produced by dextrans were due to sample polydispersity and not to differences in solute column spreading. Corrections for column spreading on these columns are small for the determination of integral properties of polymers but may be significant when molecular weight distributions are of interest.     

Measurement of infinite dilution activity coefficients of selected environmentally important volatile organic compounds in polydimethylsiloxane using gas - liquid chromatography

Silicon oil, chemically known as polydimethylsiloxane (PDMS), is a high boiling point solvent highly suitable for volatile organic compounds (VOCs) absorption. To use PDMS as an absorption solvent for a specific waste gas problem, it is important to determine the infinite dilution activity coefficients of the VOCs to be separated with PDMS. This work reports activity coefficients at infinite dilution of 13 VOCs in polydimethysiloxane determined by the dynamic gas liquid chromatographic technique. The measurements were carried out at various temperatures (303.15, 313.15, 323.15., 333.15, 353.15, 373.15, 393.15 and 423.15 K). Four PDMS polymers with average molecular weight ranging from 760 to 13,000 were used as solvents. A control column packed by Perkin Elmer to our specifications was used to validate the coating and packing methods. Flow rate dependence of the elution peaks was also investigated by varying it from 10–50 ml/min. Precision was improved by reproducing the results using columns with different liquid loading, thus also studying the retention mechanism. The results compare well with the data from previous work using simple headspace and UNIFAC predictions and literature values. The successful comparison gives an indication of the GLC as a rapid, simple and accurate method for studying the thermodynamics of the interaction of a volatile solute with a nonvolatile solvent.     

The Instrument Spreading Correction in GPC. II. The General Shape Function Using the Fourier Transform Method with a Nonlinear Calibration Curve

Abstract

The instrument spreading function suggested in Part I of this series is investigated for use with the Fourier transform method for generating corrected elution volume chromatograms. The instrument spreading parameters are obtained using linear theory on narrow molecular weight distribution standards, as indicated in Part I. The corrected chromatogram is then combined with a nonlinear molecular weight calibration curve which was fit with a function suggested by Yau and Malone to generate true values of the number- and weight-average molecular weights.

The instrument spreading function is shown to qualitatively and quantitatively describe the dispersion, skewing, and flattening effects ordinarily found in GPC chromatograms due to imperfect resolution by the GPC columns. The Yau-Malone function is shown to be a very useful function for fitting nonlinear molecular weight vs elution volume calibration data. Although the Fourier transform method is shown to work well with analytically generated data, it is shown that a number of numerical problems must be overcome before it can quantitatively produce corrected elution volume chromatograms. Some of these numerical problems are discussed.     

EFFECT OF STATIC MIXERS ON EXPANSION AND AXIAL DISPERSION OF LIQUID IN FLUIDIZED BEDS AT LOW REYNOLDS NUMBER

The difference of static mixers as internals on expansion and dispersion characteristics of a liquid fluidized bed was studied experimentally. The column diameter was varied between 39 and 300 mm. Fluidized bed reactors equipped with internals exhibit a high hydrodynamic stability accompanied by an increase in the operating range in terms of flow rate by a factor of 4-5 compared to the classical fluidized bed. The axial dispersion of the liquid depends on the Archimedes number, the hydraulic diameter of the static mixer and expansion. The dispersion increases with decreasing Archimedes number and increasing hydraylic diameter of the bed. Dispersion was in general reduced with bed expansion.     

Axial dispersion in a rectangular bubble column

This paper presents the results of an experimental study on the gas holdup and the liquid phase axial dispersion coefficient in a narrow packed and unpacked rectangular bubble column. In both cases the gas and liquid flow rates were varied and the data were obtained by employing standard tracer technique. The gas holdup and the axial dispersion coefficient for both the packed and unpacked columns were found to be dependent on the gas and liquid flow rates. For given gas and liquid velocities and a given packing size in the case of the packed column, the rectangular column gave significantly higher dispersion coefficients than a cylindrical column of the equivalent cross sectional area. This result agrees very well with the one predicted by the velocity distribution model. The correlations for the Peclet number, the axial dispersion coefficient, and the fluid holdup for both the unpacked and packed bubble columns are presented.     

Effective diffusivities and pore-transport characteristics of washcoated ceramic monolith for automotive catalytic converter

Using the chromatographic technique (carrier-gas: N₂; tracer-gases: He, Ar) the effective diffusivity in single-pellet string columns (SPSC) packed with porous slab particles was studied. Dispersion due to extra-column effects was eliminated via convolution of column responses for two lengths. The measurements were done for cordierite particles and for cordierite coated with alumina-based washcoat. The effective diffusion coefficients for two tracer-carrier pairs were evaluated by fitting column response (chromatographic) peaks in time-domain. Mean transport parameters were evaluated for both types of porous particles. The obtained mean transport pore radii are in reasonable agreement with pore-size distribution from mercury porosimetry.