Flowing stream

When a pure gas flows through a channel the accompanying fall in pressure is accounted for partly by acceleration of the flowing stream and partly by momentum transfer to the stationary walls. Since a porous medium may be regarded as an assembly of channels, similar considerations apply to flow through porous media, but in the diffusional situations of principal interest here accelerational pressure loss can usually be neglected. If more than one molecular species is present, we are also interested in the relative motions of the different species, so momentum transfers by collisions between different types of molecules are also important. 

In gas-solid extractions the sample is passed through a container packed with a solid adsorbent. One example of the application of gas-solid extraction is in the analysis of organic compounds for carbon and hydrogen. The sample is combusted in a flowing stream of O2, and the gaseous combustion products are passed through a series of solid-phase adsorbents that remove the CO2 and 1T20. 

Dynamic headspace GC/MS. The distillation of volatile and semivolatile compounds into a continuously flowing stream of carrier gas and into a device for trapping sample components. Contents of the trap are then introduced onto a gas chromatographic column. This is followed by mass spectrometric analysis of compounds eluting from the gas chromatograph. 

Pitot Tubes. The fundamental design of a pitot tube is shown in Eigure 9a. The opening into the flow stream measures the total or stagnation pressure of the stream whereas a wall tap senses static pressure. The velocity at the tip opening, lA can be obtained by the Bernoulli equation ... 

Gup and Vane Anemometers. A number of flow meter designs use a rotating element kept in motion by the kinetic energy of the flowing stream such that the speed is a measure of fluid velocity. In general, these meters, if used to measure wind velocity, are called anemometers if used for open-channel Hquids, current meters and if used for closed pipes, turbine flow meters. 

Electromagnetic flow meters ate avadable with various liner and electrode materials. Liner and electrode selection is governed by the corrosion characteristics of the Hquid. Eor corrosive chemicals, fluoropolymer or ceramic liners and noble metal electrodes are commonly used polyurethane or mbber and stainless steel electrodes are often used for abrasive slurries. Some fluids tend to form an insulating coating on the electrodes introducing errors or loss of signal. To overcome this problem, specially shaped electrodes are avadable that extend into the flow stream and tend to self-clean. In another approach, the electrodes are periodically vibrated at ultrasonic frequencies. 

Tracer Type. A discrete quantity of a foreign substance is injected momentarily into the flow stream and the time interval for this substance to reach a detection point, or pass between detection points, is measured. From this time, the average velocity can be computed. Among the tracers that have historically been used are salt, anhydrous ammonia, nitrous oxide, dyes, and radioactive isotopes. The most common appHcation area for tracer methods is in gas pipelines where tracers are used to check existing metered sections and to spot-check unmetered sections. 

Differential-Temperature Thermal Flow Meters. Meters of this type inject heat into the fluid and measure the resulting temperature rise or, alternatively, the amount of power required to maintain a constant temperature differential. The power required to raise the temperature of a flowing stream by an amount AT is given by the relation ... 

Measurement by Thermal Effects. When a fine wire heated electrically is exposed to a flowing gas, it is cooled and its resistance is changed. The hot-wire anemometer makes use of this principle to measure both the average velocity and the turbulent fluctuations in the flowing stream. The fluid velocity, L, is related to the current, /, and the resistances, R, of the wire at wire, and gas, g, temperatures via... 

Fig. 2. Schematic of the fraction of chemiluminescence time profile observed in a flowing stream detector (71). See text.

Luminol-based chemiluminescence methods have also been employed for detection of analytes in flowing stream analytical techniques such capillary electrophoresis (282), flow injection analyses, and hplc (267). AppHcations of the enhanced luminol methodology to replace radioassay methods have been developed for a number of immunological labeling techniques (121,283). 

Odor. Odor is excluded in military appHcations and in the four grades specified for human breathing. The gas is tested by smelling a flowing stream or by smelling a beaker from which a Hquid sample has just evaporated. Oxygen that is produced in modem ASUs employing turbine compressors or nonlubricated piston machines is odorless. 

Subsequent to stock preparation and proper dilution, the paper furnish usually is fed to the paper machine through one or more screens or other devices to remove dirt and fiber bundles. It then enters a flow spreader which provides a uniform flowing stream and which is the width of the paper machine. The flow spreader, or manifold, discharges the slurry into a headbox, where fiber flocculation is minimised by microturbulence and where the proper pressure head is provided to cause the slurry to flow at the proper velocity through the slice and onto the moving Fourdrinier wire. 

There are many iadustrial appHcations for radioisotopes. A simple example is the determination of the thickness of some item that is being formed, or of the density of a flow stream ia a chemical process, by measuting the attenuation of or y-radiation as it passes through the material. Another... 

Minor ingredients include excess fatty acids, preservatives, and potentially other synthetic surfactants. Alternatively, mixing can be achieved through the use of in-line static mixers, with the accurate addition of the minors into a flowing stream of the wet soap. 

Mathematically speaking, a process simulation model consists of a set of variables (stream flows, stream conditions and compositions, conditions of process equipment, etc) that can be equalities and inequalities. Simulation of steady-state processes assume that the values of all the variables are independent of time a mathematical model results in a set of algebraic equations. If, on the other hand, many of the variables were to be time dependent (m the case of simulation of batch processes, shutdowns and startups of plants, dynamic response to disturbances in a plant, etc), then the mathematical model would consist of a set of differential equations or a mixed set of differential and algebraic equations. 

In addition, A denotes the difference between exit and entrance streams, and fs indicates that the term applies to all flowing streams. 

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