Axial dispersion and flow phenomena in helically coiled tubular reactors for flow analysis and chromatography

Open tubular reactors for flow analysis and post-column reactors for chromatography are commonly designed as rather tightly coiled helices. It is shown experimentally that the dynamics of flow and axial dispersion in such coiled tubes can differ substantially from behaviour in straight tubes. The differences are almost quantitatively accounted for by the existence of secondary flow phenomena in coiled tubes. On the basis of these coiling effects, practical parameters such as sampling rate, reagent consumption, tube dimensions, pressure drop, etc. are estimated, optimized and compared with those in other systems (segmented-flow and packed-bed reactors).     

Stopped-flow injection analysis. The influence of diffusion on dispersion of normal and reverse flow injection

A relationship is derived to enable the comparison of the dispersion heights of normal and reverse flow injection analysis (FIA). A single channel flow system is employed in the absence of a chemical reaction. The stopped-flow injection method is used to probe the influence of molecular diffusion on the overall dispersion of normal and reverse FIA, which appeared to demonstrate fundamentally different diffusion behaviors. Small discrepancies are observed between the dispersion heights, which are enhanced by the stopped-flow period, especially when unmatched matrix ionic compositions of the indicator and counter solutions were involved. For these conditions, the diffusion flux rate is enhanced considerably, displaying a peak, in addition to the transient, for both methods. The influence of diffusion on the dispersion characteristics of normal and reverse FIA is discussed theoretically. Diffusion in the proposed model is postulated to oppose dispersion by convection. The latter initiates concentration gradients in the injection zone and propagates it with flow time over the dispersion zone profile. The diffusion flux then reacts in order to confine the indicator dispersion for normal FIA and to enhance it for reverse FIA. This model is consistent with the experimental results and accounts for most of the phenomena encountered. Probably owing to the influence of secondary flow phenomena, the use of coiled tubes has suppressed the effects of diffusion on the overall dispersion behavior.Part of the experimental work was performed at IMI Institute for Research and Development, Haifa, Israel.     

Extra-column dispersion of macromolecular solutes in aqueous-phase size-exclusion chromatography

A set of dextran standards was used to study the extra-column dispersion in conventional chromatographic equipment at a broad range of molecular weights, different mobile phase flow rates and connecting tube lengths and diameters. All known correlations for the tube dispersion at laminar flow, including those for short tubes, overestimated the values of the variance of the outlet concentration signal. The difference increased with the solute molecular weight and the flow rate. It was assumed that the discrepancy was due to the effect of natural convection invoked by the density differences of the injected dextran solutions and water. A suitable approximation of the relative band spreading was suggested in a form of a power function of the Reynolds and Schmidt numbers. A significant decrease of the dispersion was observed when the chromatography tubing was coiled into a circle. This decrease was successfully predicted combining the existing correlations for long coiled tubes and short straight tubes.     

Direct Determination of Glucose in Blood Serum Using Trinder's Reaction

Abstract

A very simple flow injection analysis system for direct determination of glucose in blood serum based on Trinder's reaction is described. The sera samples (15 μl) can be injected directly to the system without the deproteinization or the use of a dialyzer.

Calibration curves are linear in the range 50–400 mg/dl. The sampling frequency is 60 samples per hour. Results obtained by the proposed procedure are compared with those acquired at a local hospital using their routine glucose procedure also based on Trinder's reaction. It is shown that a better mix between sample and reagents is achieved using the single bead string reactor (SBSR).     

Influence of diameter on the degradation profile of multiwall carbon nanotubes

Nanofluid and coiled tubes have been employed as two passive methods for enhancing the heat transfer. In the present study, the turbulent flow of CuO–water nanofluid in helical and conical coiled tubes was numerically investigated with constant wall temperature through mixture model. The thermophysical properties of base fluid (water) were considered as temperature-dependent functions, while Brownian effects were adopted in thermal conductivity and dynamic viscosity of nanofluid. Simulation results were validated using experimental data for heat transfer coefficient and pressure drop in helical coiled tube for different Reynolds numbers. Four different geometries were simulated and compared. The first one was a conical coiled tube; the others were helical coiled tubes whose coil diameters were minimum, maximum, and median of the conical coiled tube pitch coil diameter. The velocity profiles indicated stronger secondary flow in conical coiled tube at a specified Dean number. The obtained results also showed higher heat transfer enhancement in the conical coiled tube in comparison with helical coiled tube with the same average pitch coil diameter. Moreover, the nanoparticle-induced heat transfer enhancement was more effective in conical coiled tube.

     

Influence of the main parameters of single-line flow-injection systems without chemical reaction on the output signal : Part 1. Dispersion Characteristics of the Main Sections of the Flow System

Single-line flow-injection systems with valve, hydrodynamic, syringe or delta-function injection of the analyte are considered. Appropriate equations for calculating the Peclet numbers of their main sections (i.e., the fore-section, the injection section, the reactor, the measurement cell, and the after-section) are described. Two types of reactor with different dispersion characteristics (i.e., straight tube and coiled tube reactors) are considered. The detector is assumed to measure one of three concentrations: (1) the mean concentration in the cross-section of the flow situated in the middle of the measurement cell; (2) the average integral concentration in the whole volume of the measurement cell; or (3) the average integral surface concentration at the inner walls of the measurement cell. Equations for calculating the Laplace transform of the surface and the integral surface concentration, and for the mean and the variance of the corresponding concentration curves are derived. The conclusions made can be used to investigate the influence of the main parameters of single-line flow-injection systems on sample throughput by simulation of the mathematical models.     

Reaction detector with three dimensional coiled open tubes in HPLC

Summary A reaction detector with non-segmented flow in open tubes as reaction track is described. To minimize peak broadening the open tubes are arranged in a three dimensional coiled structure by knitting. At low volume flow rates the h-values in these open tubes are independent of flow rate and are significantly lower than predicted by theory. The applicability of this reactor for classical colorimetric detection with corrosive reagents is shown.Part of Ph. D. Thesis, Saarbrücken 1976     

Flow injection analysis of trace hydrogen peroxide using an immobilized enzyme reactor

Summary Flow Injection Analysis of Trace Hydrogen Peroxide Using an Immobilized Enzyme Reactor Sub-part-per-billion levels of aqueous hydrogen peroxide have been determined with a flow injection analysis system employing a single bead string reactor composed of horseradish peroxidase covalently bound to an animated macroporous polymeric adsorbent with glutaraldehyde and a passive cation exchange membrane reactor to alter pH. The chemistry relies on the peroxidase mediated oxidation of nonfluorescentp-hydroxyphenylacetate to its fluorescent dimer. The advantage of the system includes rapid throughout rates (40 samples/h), excellent detection limits (0.3 ppb H₂O₂) and large dynamic range of linear response (1 ppb-1 ppm). However, the immobilized enzyme is not useful for the analysis of organic peroxides which act as inhibitors.     

Parametric optimization of heat transfer characteristics for helical coils

Journal of Thermal Analysis and Calorimetry - In this study, helically coiled tubes are used in a shell, and the effects of tube diameter, coil diameter, Reynolds number, Dean number and flow rate...     

Oscillatory control of sample dispersion in a continuous flow system

A new strategy for the instrumental control of sample dispersion in continuous flow systems is presented. The method is based on shaking a loosely held straight reactor while the sample travels through the flow injection manifold. This external disturbance yields a sample transport more similar to the plug flow type because of the changes promoted on the flow pattern. Up to a three-fold increase in peak height, a comparable reduction in peak width and a more Gaussian peak profile are observed when the signals obtained with the shaken reactor are compared with those obtained with the same reactor but static. Improvements in the analytical performance as a function of different operational variables are shown for systems with or without a chemical reaction. Analytical implications and possible uses are discussed since this strategy allows the control of dispersion by simply selecting the frequency and amplitude of oscillation.     

Particle interactions in flowing suspensions

For a fully destabilized suspension of non-Brownian praticles in laminar tube flow, the extent of orthokinetic flocculation can be calculated by classical Smoluchowski theory, using the average shear rate $\text{G}$ and the average residence time $\text{t}$. It can be shown very simply that the dimensionless quantity $\text{Gt}$ (and hence the degree of flocculation) depends only on the tube dimension and not on the flow rate. However, calculations based on this approach predict far more flocculation than is observed experimentally. There are two major reasons for the discrepancy: 1) the Smoluchowski treatment of orthokinetic flocculation neglects hydrodynamic interaction between particles, which can be introduced by a semi-empirical method due to van de Ven and Mason and this step leads collision efficiencies which are considerably less than unity and depend both on shear rate and on interparticle forces; 2) the shear rate is not uniform in the tube but varies from zero at the tube axis to a maximum value at the wall. Since the major contribution to the flow comes from regions close to the tube axis, where the shear rate is low, the simple averaging procedure considerably overestimates the degree of flocculation.From experimental measurements on the degree of flocculation of dispersions achieved by laminar flow through narrow tubes at different flow rates it is possible to draw semi-quantitative conclusions concerning particle interaction and the strength of flocs.The effect of helical winding of the tube is briefly considered and shown to give more flocculation than in a straight tube. Some experimental results for latex particles destabilized by cationic polymers flowing through straight and coiled tubes are mentioned.     

Focused-microwave-assisted reaction in flow injection spectrophotometry: a new liquid-vapor separation chamber for determination of reducing sugars in wine

The usefulness of an unsegmented liquid/vapor phases flow injection system for determining reducing sugars in wines using a focalized poly(tetrafluoroethylene) (PTFE) coiled reactor positioned at the output antenna of a domestic microwave oven at 700 W is demonstrated. In this system, sample or reference solution is mixed to 2.5 mol l(-1) sodium hydroxide and 6.8 mmol l(-1) potassium hexacyanoferrate(III) solutions previously merged in a confluence point and the reducing sugars (glucose plus fructose) were oxidized by potassium hexacyanoferrate(III) solution into the PTFE coiled reactor positioned in the microwave oven cavity. After phases separation at a proposed separation chamber (SC), the decrease of K(3)Fe(CN)(6) concentration in the liquid phase was measured at 420 nm. Using this SC, a favorable dispersion profile and a decrease of hydrodynamic pressure were observed. The flow procedure presented similar sensitivities to glucose and fructose in the experimental conditions adopted, allowing the determination of the reducing sugars content (fructose plus glucose concentrations) in the concentration range from 40 to 400 mumol l(-1) with a detection limit of 9 mumol l(-1). The RSD was 1.9% for a 120 mumol l(-1) fructose solution (n=12) and the analytical frequency was 54 h(-1). The results obtained for reducing sugars in several wines using the proposed flow system and those obtained using the Somogyi-Nelson batch procedure were in agreement at the 95% confidence level.     

Dispersion phenomena in reactors for flow analysis

Both coiled open tubular reactors and packed-bed reactors can be used in flow analysis. Band broadening and pressure drop in these reactors are discussed. Theoretical analysis shows that packed-bed reactors are to be preferred. It is shown that for a given residence time and equal band-broadening values the pressure drop over a packed-bed reactor is lower than over a coiled open tubular reactor. Rules for optimal design are given for coiled tubular reactors and packed-bed reactors. The application of both reactors is shown for the spectrophotometric determination of phosphate with a vanadomolybdate reagent yielding a yellow colour.     

Fluorimetric determination of gallium with lumogallion by flow injection analysis based on solvent extraction

Gallium in the concentration range 4 X 10^-6–8.7 X 10^-5 M is determined fluorimetrically with lumogallion by an extraction—flow injection procedure. Sensitivity and selectivity are improved compared to those obtained earlier with an entirely aqueous system. The transient phenomena occurring in extraction in a narrow teflon tube are examined by a laser excitation technique, in which the concentration profile of the gallium—lumogallion complex is measured directly without phase separation. The results show that mixing between segments plays an important role in dispersion of the sample in the extraction coil. Two extraction processes with different rates were observed in the transient signals during extraction.     

Peak shape and dispersion behavior of solutes in counter-current chromatography with a single phase

The dispersion behavior of solutes was investigated in a rotating flowing coiled tube. Potassium iodide, tartrazine, ascorbic acid, lysozyme, bovine serum albumin (BSA), and silver nanoparticle (AgNPs) samples were eluted in a coiled tube of counter-current chromatography (CCC) apparatus with a single phase. Apparent convection peaks of low-diffusivity solutes appeared in the static CCC tube, while Gaussian-like peaks showed up for the high-diffusivity solutes. When the rotation speed of the CCC apparatus was elevated, all solute peak widths became smaller, and the convection peaks of AgNPs and BSA were minimized and formed Gaussian-like peaks. The axial dispersions of all solutes were reduced owing to the higher radial mass transfer in the rotating CCC column. The same reasoning could also be used to rationalize other special band shapes encountered in two-phase CCC separations.     

Initial studies of large-bore flow-injection systems

The recent in flow injection analysis has been miniaturize the components of the system. In many industrial processes, heterogeneous turbid samples have to be analyzed; neither microconduit systems nor normal flow-injection systems with 0.5 mm i.d. tubing can be used for such samples. The effects of increasing internal diameters of coiled tubing are examined. It is shown that essential characteristics of dispersion are maintained with tubing diameters up to at least 1.3 mm.     

Axial dispersion in coiled tubular reactors : The Effect of Curvature at Low Dean Numbers

Deforming tubing (e.g., coiling) reduces axial dispersion in flow-injection systems and post-column reactors. An improvement is presented for a previous theoretical treatment of axial dispersion in coiled tubes. New experimental data are also presented for different coil radii. Changes were necessary because smaller-than-predicted dispersion improvements were observed. This contradicts earlier treatments that predict a linear dependence on the aspect ratio of the coil. Evidence is presented that this is explained by assuming a boundary layer thickness much smaller than the radius of the tube and proportional to the ratio of radial velocities. This is a special case of earlier theory and it predicts an almost curvature-independent change in dispersion. The inclusion of the critical Reynolds number, Re_c, to form a reduced parameter, (Re/Re_c), accounts for all observations, including some previously-defined empirical constants. The dispersion is found to increase as (Re/Re_c)^2/3Sc at low fluid velocities and to decrease as (Re^1/6_c/(Re/Re_c)^4/3Sc^0.08) at higher velocities. Experiments are reported for several coil radii and with a range of proteins and protein mixtures.     

Three-Dimensional Modeling of the Turbulent Plasma Jet Impinging upon a Flat Plate and with Transverse Particle and Carrier-Gas Injection

Modeling results are presented concerning the turbulent thermal plasma jet impinging normally on a substrate and with transverse injection of feedstock particles and their carrier gas from a single injection tube. The k- two-equation model is employed to model the turbulence, and particle dispersion is studied considering the interaction between the moving particles and turbulent eddies and considering the effect on particle trajectories of the random variation of the turbulent fluctuating velocities in their magnitude and direction. A well-validated three-dimensional (3-D) computer code is used in the modeling. The 3-D effects due to the carrier gas injection on the jet flow field and thus on the particle trajectories and heating histories are shown to be appreciable. The radial location of the injection tube with respect to the plasma jet is shown to be a critical parameter for the study of 3-D effects, besides the carrier-gas/plasma stream mass flux ratio. Particle dispersion considerably widens the distribution of the particle trajectories and heating histories. In addition, although pertinent swirl number is often rather small, swirling may also affect the modeling results.