Pulsed-flow methods

The detection technique most commonly used with methods based on fast kinetics is photometry (either in its conventional form or with multi-detection systems such as image detectors) and, to a much lesser extent, fluorimetric and electroanalytical methods. 

The data collection system used is of paramount Importance in these methods as the detector provides a large number of data in an extremely short time thus, the collector, usually a micro- or minicomputer, must be highly responsive to time. The subsequent treatment of the collected data can be very different in nature depending on the particular aim pursued. 

This is a continuous flow mode [27] based on an adaptation of the stopped-flow assemblies originally designed to study and use reactions with fast kinetics. The essential modification with respect to the methods described above is the incorporation of a storage coil between the mixing unit and the flowcell to prevent the reacting mixture from reaching the detector in too short a time, which allows this mode to be applied to reactions attaining equilibrium in times of the order of a few seconds (or minutes). 

Normally, each turn of the coil stores the contents for one injection. The system depicted in Fig. 7.16 requires six aliquots for the coil to be flushed from the previous sample, and a seventh to be used for measurement and which is followed by another aliquot intended to isolate it from the effects of the next sample to be injected. The scheme shown in Fig. 7.16 (bottom) represents two sequential situations (1) after injection of the six flushing aliquots, three of which have by that time passed the detector unmeasured, plus the measurement aliquot (sample A) and the isolating aliquot and (2) upon injection of the next sample (B), when the previous sample has not yet been measured. The time elapsed in aliquoting, mixing and injecting the samples into the storage coil is 1.6 s. Hence, the measurement aliquot takes 9.6 s (6x1.6) to traverse the length of the coil and reach the detector. 

In this manner, Malmstadt et al. [27] applied typical stopped-flow technology to the analysis of a series of samples on the basis of reactions with normal kinetics. According to its proponents, SF/USA offers significant advantages over SFA and FIA  

---

The pulsed-flow method evolved further into the pulsed-accelerated-flow method [7], Here, the solutions have a range of flow velocities owing to the constant flow... 

H2 chemisorption. Metal dispersion was determined by H2 chemisorption performed with a pulse flow method (PulseChemisorb 2700, MICROMERITICS). The samples (0.3-1.0 g) were placed in a Pyrex reactor and heated from 20 to 300°C (heating rate of 10°C/min) in N2 flow (15 ml/min), treated at 300°C for 2 h in H2 flow (35 ml/min), kept under a stream of N2 for 1 hr to clean the surface and eventually cooled to 20°C in the same atmosphere. The ehemisorption experiments were performed at 20+/-1°C. Successive pulses of 86 ml of H2 were sent to the catalyst in a constant stream of N2 (15 ml/min) the time interval between successive pulses was 90 s. The total amount of adsorbed hydrogen was calculated from the difference between the saturation peak area and the area of the peak before saturation. From this amount metal dispersion parameters were calculated [10] (1) the percent of platinum present on the surface with respect to the total amount in Ae catalyst (Pt /Pt, %), (2) 4e catalyst surface covered by metal particles (Pt area, m Pt/g cat) and (3) the average diameter of the Pt particles on the catalyst surface, using a spherical model for the aggregates (d. A). 

In the pulse flow method, the procedure consists of the injection of a precise and well-defined gas volume (probe molecule + carrier gas) into the stream which flows through the catalyst bed held on flie fritted glass of a specially designed calorimetric cell. For each pulse, the calorimetric signal is recorded and the amount of gas which has not been retained by the catalyst is measured by a gas chromatogr h (or mass spectrometer) connected on-line to flie calorimetric cell. The major disadvantage of this technique is that the weakly chemisorbed portion of the probe gas is not held by the catalyst and gives rise to an endothermic peak of desorption which follows immediately the exothermic peak of adsorption, and thus necessitates peak deconvolution. 

Different from the secondary flows and flow channel spacer techniques, the pulsed flow method is to generate a pressure fluctuation wave in either the feed or permeate flow channel using certain oscillators. The fluctuating pressure wave can enhance the membrane filtration through reducing the boundary layer or induced instant local backflushing flow as discussed in Section 10.4.1. 

Enzyme reaction intermediates can be characterized, in sub-second timescale, using the so-called pulsed flow method [35]. It employs a direct on-line interface between a rapid-mixing device and a ESI-MS system. It circumvents chemical quenching. By way of this strategy, it was possible to detect the intermediate of a reaction catalyzed by 5-enolpyruvoyl-shikimate-3-phosphate synthase [35]. The time-resolved ESI-MS method was also implemented in measurements of pre-steady-state kinetics of an enzymatic reaction involving Bacillus circulans xylanase [36]. The pre-steady-state kinetic parameters for the formation of the covalent intermediate in the mutant xylanase were determined. The MS results were in agreement with those obtained by stopped-flow ultraviolet-visible spectroscopy. In a later work, hydrolysis of p-nitrophenyl acetate by chymotrypsin was used as a model system [27]. The chymotrypsin-catalyzed hydrolysis follows the mechanism [27] ... 

Proton inventory technique. 21.9-220 Pseudo-first-order kinetics, 16 Pulse-accelerated-flow method. 255 Pulse radiolysis, 266-268 Pump-probe technique. 266... 

Chemisorption measurements (Quantachrome Instruments, ChemBET 3000) were conducted in order to determine the metal (Co) dispersion. Therefore, the nanomaterial catalysts were reduced under a hydrogen flow (10% H2 in Ar) at 633 K for 3 h. The samples were then flushed with helium for another hour at the same temperature in order to remove the weakly adsorbed hydrogen. Chemisorption was carried out by applying a pulse-titration method with carbon monoxide as adsorbing agent at 77 K. The calculation of the dispersion is based on a molar adsorption stoichiometry of CO to Co of 1. 

A significant technical development is the pulsed-accelerated-flow (PAF) method, which is similar to the stopped-flow method but allows much more rapid reactions to be observed (1). Margerum s group has been the principal exponent of the method, and they have recently refined the technique to enable temperature-dependent studies. They have reported on the use of the method to obtain activation parameters for the outer-sphere electron transfer reaction between [Ti Clf ] and [W(CN)8]4. This reaction has a rate constant of 1x108M 1s 1 at 25°C, which is too fast for conventional stopped-flow methods. Since the reaction has a large driving force it is also unsuitable for observation by rapid relaxation methods. 

The use of magnetic resonance imaging (MRI) to study flow patterns in reactors as well as to perform spatially resolved spectroscopy is reviewed by Lynn Gladden, Michael Mantle, and Andrew Sederman (University of Cambridge). This method allows even unsteady-state processes to be studied because of the rapid data acquisition pulse sequence methods that can now be used. In addition, MRI can be used to study systems with short nuclear spin relaxation times—e.g., to study coke distribution in catalytic reactors. 

Fig. 3.2 The operation of flow methods. The distance x and the combined flow rate govern the time that elapses between mixing and when the combined solutions reach the observation, or quenching, point. In the stopped flow method, observation is made as near to the mixer as is feasible, and monitoring occurs after the solutions are stopped. In the pulsed accelerated flow method, observation is within the mixer.

Various NMR spectroscopic techniques have been applied to investigate the conversion of methanol on acidic zeolites in the low-temperature (r<523K) formation of DME and the high-temperature (T>523 K) formation of alkenes and gasoline. Techniques successfully applied were the stop-and-go method under batch reaction conditions 258,259), the pulse-quench method 113), and various flow techniques 46,49,74.207,260 263). This section is a summary of the recent progress in investigations of the mechanism of the MTO process by NMR techniques. 

The time of analysis must be very rapid. Stopped flow methods always require an inbuilt timing device, and time intervals at which analysis is made are dictated by the speed at which successive analyses can be carried out. Spectroscopic methods using pulsed radiation are very useful here because they can be both analytical and timing devices. 

A comparison between pulsed flow and conventional pulsed static calorimetry techniques for characterizing surface acidity using base probe molecule adsorption has been performed by Brown and coworkers [20, 21]. In a flow experiment, both reversible and irreversible probe adsorption occurring for each dose can be measured, and the composition of the gas flow gas can be easily modified. The AHads versus coverage profiles obtained from the two techniques were found to be comparable. The results were interpreted in terms of the extent to which NH3 adsorption on the catalyst surface is under thermodynamic control in the two methods. 

The performance of a membrane process is a function of the intrinsic properties of the membrane, the imposed operating and hydrodynamic conditions, and the namre of the feed. This chapter describes methods available to enhance performance by various techniques, mainly hydrodynamic but also chemical and physical. The focus is on the liquid-based membrane processes where performance is characterized by attainable flux, fouling control, and separation capabilities. The techniques discussed include secondary flows, flow channel spacers, pulsed flow, two-phase flow, high shear devices, electromagnetic effects, and ultrasound. 

Most of hydrodynamic methods have focused on increasing the particle back transport from the membrane-liquid interface by increasing the shear rate and the flow instability in the boundary layer. These techniques include secondary flows, spacers and inserts, pulsed flow, high shear rate devices, vibrations, and two-phase flow. The physical methods that are currently been tested to enhance filtration performance of membranes include the application of electric fields and ultrasound. 

---