Chemical engineering flow measurement

Particle diameter is a primary variable important to many chemical engineering calculations, including settling, slurry flow, fluidized beds, packed reactors, and packed distillation towers. Unfortunately, this dimension is usually difficult or impossible to measure, because the particles are small or irregular. Consequently, chemical engineers have become familiar with the notion of equivalent diameter of a partiele, which is the diameter of a sphere that has a volume equal to that of the particle. 

Figure 12-61D. Centrifugal compressor surge control schematic diagram shows instrumentation required when primary flow-measuring device is located in centrifugal compressor discharge line. Symbols T = temperature P = pressure A = differential across compressor outlet to inlet. See Reference 89 for a detailed discussion. (Used by permission White, M. H. Chemical Engineering, p. 54, Dec. 25,1972. McGraw-Hill, Inc. All rights reserved.)...

Washout experiments can be used to measure the residence time distribution in continuous-flow systems. A good step change must be made at the reactor inlet. The concentration of tracer molecules leaving the system must be accurately measured at the outlet. If the tracer has a background concentration, it is subtracted from the experimental measurements. The flow properties of the tracer molecules must be similar to those of the reactant molecules. It is usually possible to meet these requirements in practice. The major theoretical requirement is that the inlet and outlet streams have unidirectional flows so that molecules that once enter the system stay in until they exit, never to return. Systems with unidirectional inlet and outlet streams are closed in the sense of the axial dispersion model i.e., Di = D ut = 0- See Sections 9.3.1 and 15.2.2. Most systems of chemical engineering importance are closed to a reasonable approximation. 

Resident Time Distribution (RTD) is widely employed in the chemical engineering industry, as an analytical tool for characterizing flow dynamics within reactor vessels. RTD provides a quantitative measure of the back-mixing with in a reactor system [2]. However the cost and time involved in building and operating a pilot- or full scale reactor for RTD analysis can be economically prohibitive. As such we have implemented a numerical RTD technique through the FLUENT (ver. 6.1) commercial CFD package. 

The flow rate of fluids is a critical variable in most chemical engineering applications, ranging from flows in the process indnstries to environmental flows and to flows within the hnman body. Flow is defined as mass flow or volume flow per unit of time at specified temperatnre and pressure conditions for a given flnid. This snbsection deals with the techniques of measuring pressure, temperature, velocities, and flow rates of flowing fluids. For more detailed discussion of these variables, consult Sec. 8. Section 8 introduces methods of measuring flow rate, temperature, and pressure. This subsection bnilds on the coverage in Sec. 8 with emphasis on measurement of the flow of fluids. 

Bourne, J.R., Buerli, M. and Regenass, W. (1981) Heat Transfer and Power Measurement in stirred tanks using heat flow calorimetry. Chemical Engineering Science, 36, 347-54. 

Ed. McNaughton and the Staff of Chemical Engineering. New York McGraw-Hill. Kamath, S., Puri, V. M., Manbeck, H. B. and Hogg, R. (1993). Flow Properties of Powders Using Four Testers Measurement, Comparison and Assessment. Powder Tech., 76,277. 

Recognizing this problem, Advances in Chemical Engineering decided to organize the present thematic issue on "Characterization of Flow, Particles and Interfaces to alert the chemical engineering community to this challenging issue. We selected the following six meso-scale measurement technologies. 

J.E. Edwards, Flow measurement practice, The Chemical Engineer, May 1979, 344, 325-333. 

It is worth describing other operations where differences between the chemist s laboratory and the chemical engineer s pilot plant and plant create the need for different approaches. Pumping, flow measurement, and reactor volume measurement are a few of the more common operations deserving the chemist s attention. 

Pumps, such as piston pumps, can meter liquids into a reactor fairly precisely, but the chemical engineer uses a flow measurement device for greater precision. The most commonly used flowmeters are rotameters that are calibrated to translate the lifting of a float in a vertical slightly tapered tube (small diameter at the inlet of the flowmeter) into a measure of the amount of liquid delivered in a given timeframe. For greatest precision the rotameter is calibrated with the specific fluid being metered. Most modem rotameters are provided with a calibration plot that corresponds to performance. 

An example adapted from Verneuil, et al. (Verneuil, V.S., P. Yan, and E. Madron, "Banish Bad Plant Data, Chemical Engineering Progress, October 1992, 45-51) shows the impact of flow measurement error on misinterpretation of the unit operation. The success in interpreting and ultimately improving unit performance depends upon the uncertainty in the measurements. In Pig. 30-14, the materim balance constraint would indicate that S3 = -7, which is unrealistic. However, accounting for the uncertainties in both Si and S2 shows that the value for S3 is -7 28. Without considering uncertainties in the measurements, analysts might conclude that the flows or model contain bias (systematic) error. 

A well-known traditional approach adopted in chemical engineering to circumvent the intrinsic difficulties in obtaining the complete velocity distribution map is the characterization of nonideal flow patterns by means of residence time distribution (RTD) experiments where typically the response of apiece of process equipment is measured due to a disturbance of the inlet concentration of a tracer. From the measured response of the system (i.e., the concentration of the tracer measured in the outlet stream of the relevant piece of process equipment) the differential residence time distribution E(t) can be obtained where E(t)dt represents... 

Experimental validation of CFD results is considered a prerequisite to paving the road for widespread acceptance of CFD in the chemical engineering community, especially in connection with multiphase flow applications. The authors do not intend to give here a complete review on available measuring techniques for single-phase and multiphase flows only the more advanced techniques are briefly discussed. For an overview of the latest advances realized in noninvasive measurement of multiphase systems the interested reader is referred to Chaouki et al (1997). The available experimental techniques can be classified according to the following aspects ... 

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