Gas stream

Drying refers to the removal of water from a substance through a whole range of processes, including distillation, evaporation, and even physical separations such as with centrifuges. Here, consideration is restricted to the removal of moisture from solids and liquids into a gas stream (usually air) by heat, namely, thermal drying. Some of the types of equipment for removal of water also can be used for removal of organic liquids from solids. 

Scrubbers. Scrubbers are designed to contact a liquid with the particle-laden gas and entrain the particles with the liquid. They offer the obvious advantage that they can be used to remove gaseous as well as particulate pollutants. The gas stream may need to be cooled before entering the scrubber. Some of the more common types of scrubbers are shown in Fig. 11.2. 

As small droplets of liquid are usually still present in the gas phase, demisting secWons are required to recover the liquid mist before it is carried over" in the gas stream out of the separator. The largest liquid droplets fall out of the gas quickly under the action of gravity but smaller droplets (less than 200 microns) require more sophisticated extraction systems. 

In such a plant the gas stream passes through a series of fractionating columns in which liquids are heated at the bottom and partly vaporised, and gases are cooled and condensed at the top of the column. Gas flows up the column and liquid flows down through the column, coming into close contact at trays in the column. Lighter components are stripped to the top and heavier products stripped to the bottom of the tower. 

The method is based on the international standard ISO 4053/IV. A small amount of the radioactive tracer is injected instantaneously into the flare gas flow through e.g. a valve, representing the only physical interference with the process. Radiation detectors are mounted outside the pipe and the variation of tracer concentration with time is recorded as the tracer moves with the gas stream and passes by the detectors. A control, supply and data registration unit including PC is used for on site data treatment... 

The main sources of error which define the accuracy are counting statistics in tracer concentration measurements, the dispersion of the tracer cloud in the flare gas stream, and the stationarity of the flow during measurements. 

Phosphorus(lll) oxide is prepared by passing a slow (i.e. limited) stream of air over burning white phosphorus. A mixture of the two oxides P40(, and P40,o is thereby formed the (V) oxide can be condensed out of the emerging gas stream as a solid by passing through a U tube heated in a water bath to about 330 K the more volatile (III) oxide passes on and can be condensed in a second U trap surrounded by ice. 

Two different types of dynamic test have been devised to exploit this possibility. The first and more easily interpretable, used by Gibilaro et al [62] and by Dogu and Smith [63], employs a cell geometrically similar to the Wicke-Kallenbach apparatus, with a flow of carrier gas past each face of the porous septum. A sharp pulse of tracer is injected into the carrier stream on one side, and the response of the gas stream composition on the other side is then monitored as a function of time. Interpretation is based on the first two moments of the measured response curve, and Gibilaro et al refer explicitly to a model of the medium with a blmodal pore... 

A simple apparatus for sublimation in a stream of air or of inert gas is shown in Fig. II, 45, 3.. d is a two-necked flask equipped with a narrow inlet tube B with stopcock and a wide tube C 12-15 inm. in diameter. The latter is fitted to a sintered glass crucible and the usual adapter and suction flask E. A well-fitting filter paper is placed on the sintered glass filter plate to collect any sublimate carried by the gas stream. 

It U better to employ the special palladium catalyst which is incorporated in the Deoxo catalytic gas purifier (obtainable from Baker Platinum Limited, 52 High Holbom. London, W.C. 1). 1 his functions at the laboratory tamperature and will remove up to 1 per cent of oxygen. The water vapour formed is carried away in the gas stream and is separated by any of the common desiccants. 

P3O5 Gas streams not suitable for alcohols, amines, ketones, or amines 2 X 10-5 0.5 Not feasible... 

Sulfuric acid Air and inert gas streams 0.003-0.008 Indefinite Not feasible... 

Constant-temperature decomposition or combustion, followed by trapping and weighing the volatilized gases, requires more specialized equipment. Decomposition of the sample is conducted in a closed container, and the volatilized gases are carried by a purge-gas stream through one or more selective absorbent traps. 

The drop in pressure when a stream of gas or liquid flows over a surface can be estimated from the given approximate formula if viscosity effects are ignored. The example calculation reveals that, with the sorts of gas flows common in a concentric-tube nebulizer, the liquid (the sample solution) at the end of the innermost tube is subjected to a partial vacuum of about 0.3 atm. This vacuum causes the liquid to lift out of the capillary, where it meets the flowing gas stream and is broken into an aerosol. For cross-flow nebulizers, the vacuum created depends critically on the alignment of the gas and liquid flows but, as a maximum, it can be estimated from the given formula. 

Figure 19.7 shows a typical construction of a concentric-tube nebulizer. The sample (analyte) solution is placed in the innermost of two concentric capillary tubes and a flow of argon is forced down the annular space between the two tubes. As it emerges, the fast-flowing gas stream causes a partial vacuum at the end of the inner tube (Figure 19.4), and the sample solution lifts out (Figure 19.5). Where the emerging solution meets the fast-flowing gas, it is broken into an aerosol (Figure 19.7), which is swept along with the gas and eventually reaches the plasma flame. Uptake of sample solution is commonly a few milliliters per minute.

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

The fast-flowing narrow liquid stream has a high relative linear velocity with respect to the slower flow of the argon gas stream. This leads to breaking up the liquid stream into fast-moving droplets, which strike the impactor bead and form much smaller droplets. 

The flows of gas and liquid need not be concentric for aerosol formation and, indeed, the two flows could meet at any angle. In the cross-flow nebulizers, the flows of gas and sample solution are approximately at right angles to each other. In the simplest arrangement (Figure 19.11), a vertical capillary tube carries the sample solution. A stream of gas from a second capillary is blown across this vertical tube and creates a partial vacuum, so some sample solution lifts above the top of the capillary. There, the fast-flowing gas stream breaks down the thin film of sample... 

In one sense, the thermospray nebulizer could be considered a pneumatic device, in which a fastflowing argon gas stream is replaced by a very rapidly vaporizing flow of solvent from the sample solution. A typical arrangement of a thermospray device is shown in Figure 19.18. 

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