Mass transfer measurements probe

Class 3-Methods Based on Direct Mechanical Effects. These include the use of acoustical probes [57-71], acoustic impedance measurements [72—75], acoustic fluxmeter [76], the measurement of radiation forces [17,21,77—112], the distortion of liquid surface [ 113-115], surface cleaning, dispersive effects, emulsification [ 116-118], erosion [ 19,22,119-125], mass transfer measurements (electrochemical probe) [26,129], absorption methods [93,132], particle velocity measurements [132], and optical methods [133-141],... 

Mass transfer measurements using an electrochemical probe are still under investigation. They only allow local measurements and the system accuracy is not yet known. These methods do however allow interesting observations on acoustic streaming and standing waves conditions. 

A significant difficulty in characterizing and quantifying gas-liquid, liquid-solid, and gas-liquid-solid mixtures commonly found in bioreactor flows is that the systems are typically opaque (e.g., even an air-water system becomes opaque at fairly low volumetric gas fractions) this necessitates the use of specially designed invasive measurement probes or noninvasive techniques when determining internal flow and transport characteristics. Many of these probes or techniques were developed for a particular type of gas-liquid flow or bioreactor. This chapter first introduces experimental techniques to gauge bioreactor hydrodynamics and then summarizes gas-liquid mass transfer measurement techniques used in bioreactors. 

Ricou R, Vives C (1982) Local velocity and mass transfer measurements in molten metals using an incorporated magnet probe. Int J Heat Mass Transf 25-10 1579-1588... 

Since the mass-transfer coefficient at a micropipette is inversely proportional to its radius, the smaller the pipette the faster heterogeneous rate constants can be measured. Micrometer-sized pipettes are too large to probe rapid CT reactions at the ITIES. Such measurements require smaller (nm-sized) pipettes. Nanopipettes are also potentially useful as SECM tips (see Section IV.D) because they can greatly improve spatial resolution of that technique. The fabrication of nanopipettes was made possible by the use of a micro-processor-controlled laser pipette puller capable of puling quartz capillaries [26]. Using this technique, Wei et al. produced nanopipettes as small as 20 nm tip radius and employed them in amperometric experiments [9]. 

In Ref. 30, the transfer of tetraethylammonium (TEA ) across nonpolarizable DCE-water interface was used as a model experimental system. No attempt to measure kinetics of the rapid TEA+ transfer was made because of the lack of suitable quantitative theory for IT feedback mode. Such theory must take into account both finite quasirever-sible IT kinetics at the ITIES and a small RG value for the pipette tip. The mass transfer rate for IT experiments by SECM is similar to that for heterogeneous ET measurements, and the standard rate constants of the order of 1 cm/s should be accessible. This technique should be most useful for probing IT rates in biological systems and polymer films. 

Scanned probe microscopies (SPM) that are capable of measuring either current or electrical potential are promising for in situ characterization of nanoscale energy storage cells. Mass transfer, electrical conductivity, and the electrochemical activity of anode and cathode materials can be directly quantified by these techniques. Two examples of this class of SPM are scanning electrochemical microscopy (SECM) and current-sensing atomic force microscopy (CAFM), both of which are commercially available. 

To ensure the system is probing reactions in a kinetically controlled regime, the reaction conditions must be calculated to determine the value of the Wiesz-Prater criterion. This criterion uses measured values of the rate of reaction to determine if internal dififusion has an influence. Internal mass transfer effects can be neglected for values of the dimensionless number lower than 0.1. For example, taking a measured CPOX rate of 5.9 x 10 molcH4 s g results... 

Figure 4.13 Velocity-measuring probes based on electrode reaction controlled by mass transfer rate.

The electroresistivity probe, recently proposed by Burgess and Calder-bank (B32, B33) for the measurement of bubble properties in bubble dispersions, is a very promising apparatus. A three-dimensional resistivity probe with five channels was designed in order to sense the bubble approach angle, as well as to measure bubble size and velocity in sieve tray froths. This probe system accepts only bubbles whose location and direction coincide with the vertical probe axis, the discrimination function being achieved with the aid of an on-line computer which receives signals from five channels communicating with the probe array. Gas holdup, gas-flow specific interfacial area, and even gas and liquid-side mass-transfer efficiencies have been calculated directly from the local measured distributions of bubble size and velocity. The derived values of the disper-... 

To increase the mass transfer rate, Tokuda et al. [7] carried out normal and differential pulse voltammetry at micropipettes and extracted the rate constant values within the range from 0.009 to 0.2 cm/s for facilitated transfers of Li+, Na+, Ca2+, Sr2+, and Ba2+ to nitrobenzene (NB) with two different crown ethers (DB18C6 and DB24C8). The assumption of a = 0.5 for all IT reactions and the use of TR-drop compensation may have affected the accuracy of those results. The upper limit for the measurable rate constant was about 0.5 cm/s, too slow to probe facilitated transfer of potassium ions. 

Zhang, G. J., and Ishii, M., "Isokinetic Sampling Probe and Image Processing System for Droplet Size Measurement in Two-phase Flow", Int. J. of Heat and Mass Transfer, 38, 2019 (1995). 

The viscosity of a bioprocess liquid is an important factor, affecting mass transfer, heat transfer, and power consumption. It is affected by the concentrations and types of microorganisms, substrates, products, and solids present in the liquid, and can vary during a fermentation or separation process. Unfortunately, on-line viscosity measurements are difficult, especially when significant amounts of air bubbles and suspended solids are present, and thus viscosity probes are not in widespread use. Sensors that have been applied for viscosity determination (usually in laboratory studies) are often various types of rotational viscosimeters [42]. 

Intrinsically fast reactions can serve as probes to measure the reaction time of a transient surface process. According to equations (1) and (2), CO oxidation has a characteristic time smaller than 0.1 s above 520 K. Switching from a lean CO/O2 feed to a rich feed, and then studying the conversion of CO versus time gives the amount and the characteristic time of the oxygen released from the catalyst (See Herz (1987), Smedler et al. (1993)). In such an experiment care must be taken first with tlie water gas shift reaction when water is in the feed, and second witli the possible heat and mass transfer limitations. 

The experiments must be done, preferably at steady state, at the same conditions of temperature, pressure and concentrations (even of minor components) as in the intended process, to avoid unpredictable changes in coalescence. They must be carried out in a system of the recommended configuration, preferably at similar power per unit volume and superficial gas velocity as may be used at full scale. The agitator speed should be varied to check whether mass transfer is important and gas hold-up should be measured (e.g. by level probe or y-ray density scan). In cases involving chemical reactions, further tests are required as described in section 15.10. 

The Stanton tube is a rectangular-shaped pitot tube located very close to the boundary wall, and the mean velocity measured from this pitot tube pressure difference is directly related to the shear stress. The Preston tube is similar to the concept of the Stanton tube using a pitot static tube close to the surface, and the difference between the stagnation pressure at the center of the tube from the static pressure is related to the shear stress. The electrochemical or mass transfer probe is flush mounted with the wall, and the concentration at the wall element is maintained constant. The measurement of mass transfer rate between the fluid and the wall element is used for determination of the wall shear stress. One of the limitations of the mass transfer probe is that at very high flow rates, the mass transfer rate becomes large and it may not be possible to maintain the wall concentration constant. A detailed discussion on the above three techniques can be found in Hanratty and Campbell [1]. 

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