Purge flow

A constant purge of gas through the DSC ensures that any volatile products evolved during the DSC experiment are swept away from the measuring sensor. It also ensures a nonoxidative and constant environment around the sample area, which helps in maintaining day-to-day baseline reproducibility. 

A change in the flow rate of the gas used to purge the DSC can have several effects. First, it is possible that it will change the temperature and enthalpy calibration. The magnitude of this variation will vary from one type of DSC design to another as some instruments preheat the purge gas prior to its entering the DSC cell. Second, for experiments where a volatile substance is evolved from the sample when it is heated, the DSC peak shape will be affected by the speed at which the volatile substance is removed. 

A slow decrease in the flow rate into the DSC will be observed if cylinders are used that are fitted with single-stage regulators, as the output pressure will decrease as the cylinder slowly empties. This effect may be minimized by the use of two-stage cylinder regulators or, ideally, the use of calibrated mass flow controllers, which will remove this variable from the experiment altogether. 

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Flow Sheet. Most purge-swing appHcations use two fixed-bed adsorbers to provide a continuous flow of feed and product (Fig. 16). Single beds are used when the flow to be treated is intermittent or cycHc. Because the purge flow is invariably greater than that of adsorption, purge is carried out in the down-flow direction to prevent bed lifting, and adsorption is up-flow. 

The air process has similar purity requirements to the oxygen process. The ethane content of ethylene is no longer a concern, due to the high cycle purge flow rate. Air purification schemes have been used to remove potential catalyst poisons or other unwanted impurities ia the feed. 

The concept of local age and local purging flow rate was introduced in refs. 39 and 41. These parameters were first studied in connection with CFD in refs. 32 and 42. Local age at a point is understood to mean the time that... 

TABLE 11.3 The Purging Flow Rate for the Six Regions Defined in Fig. 11.14 ... 

Davidson, L., Olsson, E. Calculation of age and local purging flow rate in rooms. Bldg. Enin-ron., vo). 22, pp. 111-127, 1987. 

Peng, S. H., Davidson, L, Towards the determination of regional purging flow rate. Build. Environ., vol. 32, pp. 513-525, 1997. 

Sandberg, M, Ventilation effectiveness and purging flow rate—A review. In Int. Symp. on Room Air Convection and Ventilation Effectiveness, Tokyo, 1992. 

Pyrrole, alkyl pyrroles, benzeneacetonitrile and benzenepropanenitrile. Pyrolyser continuous mode micro furnace pyrolysing injection system Pyrojector (SGE, Austin, Texas, USA) furnace pressure 14 psi purge flow 0.5 ml min 1 [28],... 

Figure 11.2 Py/silylation GC/MS chromatograms of aged linseed oil pyrolysed in the pre sence of HMDS, (a) Pyrogram obtained with a microfurnace pyrolyser pyrolysis temperature 600 °C furnace pressure 14 psi purge flow 0.5 ml min (b) Pyrogram obtained with a resistively heated filament pyrolyser pyrolyser interface I80°C transfer line 300°C valve oven 290°C. 1, Hexenoic acid, trimethylsilyl ester 2, hexanoic acid, trimethylsilyl ester 3, heptenoic acid, trimethylsilyl ester 4, heptanoic acid, trimethylsilyl ester 5, octenoic acid, trimethylsilyl ester 6, octanoic acid, trimethylsilyl ester 7, nonenoic acid, trimethylsilyl ester 8, nonanoic acid, trimethylsilyl ester 9, decanoic acid, trimethylsilyl ester 10, lauric acid, trimethylsilyl ester 11, suberic acid, trimethylsilyl diester 12, azelaic acid, trimethylsilyl diester 13, myristic acid, trimethylsilyl ester 14, sebacic acid, trimethylsilyl diester 15, palmitic acid, trimethylsilyl ester 16, stearic acid, trimethylsilyl ester...

Itoh (1987) These results arc achieved at high feed/purging flow rate ratios. 

Hydride generation coupled with purge and cryogenic trapping was implemented after an experimental design where purge flow, time and amount of borohydride were studied simultaneously... 

In this study the ratio of the particle sizes was set to two based on the average value for the two samples. As a result, if the diffusion is entirely controlled by secondary pore structure (interparticle diffusion) the ratio of the effective diffusion time constants (Defl/R2) will be four. In contrast, if the mass transport process is entirely controlled by intraparticle (platelet) diffusion, the ratio will become equal to unity (diffusion independent of the composite particle size). For transient situations (in which both resistances are important) the values of the ratio will be in the one to four range. Diffusional time constants for different sorbates in the Si-MCM-41 sample were obtained from experimental ZLC response curves according to the analysis discussed in the experimental section. Experiments using different purge flow rates, as well as different purge gases... 

The experimental apparatus consists of eight main parts an ultraviolet flashlamp capable of repetitive flashing at about 5 Hz, a purge flow reactor with either pinhole or molecular beam sampling, an ion source, a mass filter, an ion detector, pulse-counting electronics, computer data aquisition, and a vacuum system. A diagram of the apparatus is shown in Figure 1. 

Figure 2. Cross section of purge-flow reactor and optical arrangement.

The Axial Purge Flow. The reactor should be purged of reaction products between flashes. Although the conversion per flash is small, the large number of flashes necessary to accumulate an adequate signal would... 

Axial dispersion can affect measurements of decay and growth rates of transients of interest. In Figure 5 is sketched the concentration of a transient, initially formed as a square wave by a light pulse of uniform intensity from — L < x < L and zero elsewhere. As shown in Figure 6, at later times the profile becomes smoothed by diffusion. As the purge flow pushes reactive species past the pinhole at x = 0, the spatial dependence of the concentration becomes a time-varying concentration that will contribute to any time variation caused by kinetics. 

Purge flow should be set to >30 sec. Longer times are acceptable 1.0 min is usually used. 

With purge flow off, inject gas from the syringe into the column at a rate that is equal to or below the flow rate of the column. 

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