RU2061219C1 - Method and device for coarsening condensation nuclei - Google Patents

Abstract

FIELD: analytical methods of coarsening condensation nuclei in supersaturated vapour; devices for high-sensitive determination of admixtures in gas, for example, ecology problems, check of permeability of materials and tightness of articles. SUBSTANCE: method of coarsening condensation nuclei, supersaturated vapour of coarsening agent is produced through passing the nuclei flow into clearance between two equidistant surfaces at preset temperature difference; one of surface (having higher temperature) is coated with coarsening agent. Method is realized by means of device having chamber for supersaturation which is equipped with cooler inside which evaporator with electric heater is mounted. Chamber may be made in form of tube and cylindrical evaporator is arranged axially relative to it. EFFECT: improved quality characteristics and extended field of application of apparatus for measurement of microadmixtures in gas on base of nuclei condensation technique; device may be used as chromatographic detector possessing high selectance and super sensitivity. 3 cl, 6 dwg

Description

 The invention relates to analytical methods using the enlargement of condensation nuclei in a supersaturated vapor, and can be used in highly sensitive instruments for the analysis of microimpurities in a gas, based on the conversion of impurity molecules into aerosol particles, which are necessary for solving environmental problems, as well as for controlling the permeability of materials and tightness of products .

A known method of enlargement of the condensation nuclei (IAC) in a supersaturated steam of water, including moistening a gas sample with IAC and subsequent adiabatic expansion of the sample, leading to the formation of supersaturated steam, condensing at the nuclei with the formation of water droplets [1]
The disadvantage of this method is the frequency of sampling and analysis of samples, which does not allow to register rapid changes in the concentration of Ya. Another drawback is the use of water (for vapors of which it is impossible to achieve high supersaturations) as a working substance, which limits the bottom radius of nuclei capable of enlargement to 0.001 microns.

These disadvantages are eliminated in the method of enlargement of i.a. in which refractory diisobutyl phthalate (DIBP) is used as a working substance, and supersaturated steam is created by turbulent mixing of a cold gas stream containing i.a. with a small stream, saturated at ¹⁰⁰ ° C steam DIBP [2]
The disadvantage of this method is the high consumption of carrier gas, leading to large mass-dimensional characteristics of the equipment for its implementation, as well as the inability to enlarge the ultra-small condensation nuclei (r 10 ^-8 cm).

Closest to the proposed one is a method of enlarging condensation nuclei in a supersaturated vapor of a very non-volatile substance, created by turbulent mixing of a thermostatically controlled stream containing i.a. with a small stream of coarsening agent saturated at high temperature [3]
The use of very difficult volatile enlarging substances made it possible to enlarge the condensation nuclei of molecular sizes (r = n · 10 ^-8 cm), which in turn made it possible to develop an extremely highly sensitive and highly selective method for measuring microimpurities in a gas, based on the transformation of impurity molecules into ya. their subsequent enlargement and registration of the obtained aerosol by the optical method (the so-called method of molecular condensation nuclei (MoNC) [4]
The disadvantages of this prototype method are: the relative complexity associated with the need to accurately maintain four parameters that affect the amount of supersaturation and the stability of the results, namely, two temperatures and two gas flow rates (flow with IC and a stream saturated with steam of coarsening agent); high carrier gas consumption (1-10 l / min) caused by the turbulent mixing of cold and hot flows, which in turn gives rise to other disadvantages; large mass-dimensional characteristics and power consumption of the corresponding equipment; high aerodynamic drag (up to 200 mm water column), which limits the possibility of using low-pressure drivers of a fan-type flow; limited ability to use gas chromatographic separation of the sample (conventional chromatographic columns operate at flow rates of tens of cm ³ / min) in gas analyzers using this method; relatively low mass registration sensitivity i. K. (as well as gas impurities in the apparatus using the technique of condensation nuclei) and, accordingly, a relatively large value of the detection limit, which is associated with large losses on the walls of the mixer (up to 99%) due to the turbulent regime, as well as the fact that the registrar of condensation nuclei reacts to light concentration by light scattering of the formed aerosol and not on their mass. At the same time, when analyzing impurities in a gas sample, it is often precisely the mass detection limit that is important.

 The aim of the invention is to simplify the method of enlargement of condensation nuclei, reducing the consumption of carrier gas, weight and size characteristics, energy consumption and aerodynamic drag and reducing the detection limit of microimpurities in the gas.

 The goal is achieved by the proposed method of enlargement of the condensation nuclei, including obtaining a supersaturated steam of a very non-volatile enlarging substance between two equidistant surfaces with a given temperature difference, one of which, with a higher temperature, is covered with an enlarging substance; introducing a stream of the analyzed gas with condensation nuclei into the gap between these surfaces and condensation of supersaturated vapor on the nuclei with the formation of aerosol particles.

 The difference of the proposed method from the known one is that the production of supersaturated steam is carried out between two equidistant surfaces with a given temperature difference, one of which, with a higher temperature, is coated with an enlarging substance, while the flow of the analyzed gas is introduced into the gap between the surfaces.

The physical nature of the proposed method is as follows. In a gas stream with ya.k. flowing in the space between two different temperature surfaces, one of which is covered with an enlarging substance, a temperature gradient field and a concentration gradient field of the enlarging substance are created. Under the influence of these fields, heat transfer and mass transfer of coarsening substance vapors from the surface with a higher temperature to the surface with a lower temperature take place, the temperature changing linearly with the distance from the surface, and the concentration of saturated steam corresponding to the temperature value is exponential:
FROM $\genfrac{}{}{0ex}{}{\text{t}}{\text{s}}$ A exp (-B / t + 273)
As a result, at a certain distance from the surface, a zone is created in which the aggregate vapor is in a highly supersaturated state. In this zone, condensation of supersaturated steam takes place on the island and their enlargement with the formation of aerosol particles.

 A prototype of the proposed device can be a device for enlarging condensation nuclei in a gas stream, containing a chamber for creating a supersaturation of a hard-to-volatile enlarging substance with channels for supplying gas with condensation and aerosol cores, equipped with a cooler and connected by a heated capillary to an enlarging substance evaporator containing an electric heater.

 The disadvantages of the known device are its relative complexity, high consumption of carrier gas, large mass and dimensional characteristics, power consumption and aerodynamic drag and a relatively high mass limit for the detection of microimpurities when using the prototype device in apparatus for measuring microimpurities in gas.

 The aim of the invention is to simplify the device, reducing the consumption of carrier gas, weight and size characteristics, power consumption and aerodynamic drag and reducing the detection limit of microimpurities in the gas.

 The goal is achieved by using the proposed device for enlarging condensation nuclei in a gas stream, including a chamber made, for example, in the form of a cylindrical tube, equipped with channels for supplying a gas stream and aerosol outlet, connected to a system for creating a supersaturated vapor of a hardly volatile enlarging substance with a cooler and located inside the chamber , for example, along the axis of the tube with an evaporator made, for example, in the form of a cylinder containing an electric heater.

 The difference between the proposed device from the prototype is that the evaporator is located inside the camera.

 Another difference is that the camera is made in the form of a cylindrical tube, and the evaporator is installed along the axis of the camera and has the shape of a cylinder, inside which the heater is located.

 Figure 1-3 shows a diagram of the proposed device; in FIG. 4 section aa in figure 3; figure 5 is the same option; 6 is a graph illustrating the proposed method.

 The device consists of a chamber 1 for creating a supersaturation with a fitting 2 for the release of aerosol. A channel for supplying a gas flow with ya.k. serves as the initial portion of the tube (left). A cooler is mounted on the chamber, consisting of a thermopile 3 and a radiator 4. Inside the chamber, using a rod 5 of material with poor thermal conductivity, depending on the required number of stages of consolidation, one or two evaporators 6, 6a of porous material are impregnated with an appropriate enlarging substance. Inside the evaporator (with a two-stage enlargement inside each evaporator) there is a nichrome electric heater 7, 7a, the wires 8 from which are led out through the channel in the rod and sealed with sealant 9. The evaporator is centered along the axis of the chamber using the sleeve 10.

 In addition to the described variants of a device with cylindrical symmetry, the proposed method can be implemented in a device in which the gap is made in the form of a gap between two flat walls 11 and 12 separated by a heat insulator 13. An evaporator 14 with an electric heater 15 is installed on one wall and a cooler 16 on the opposite ( figure 3, 4, there are no channels for supplying a gas stream and the output of the aerosol having a complex shape).

The proposed method of enlargement of the condensation nuclei using the inventive device is as follows. At the input of the device serves a gas flow of 100-500 cm ³ / min with condensation nuclei (in figure 1 are conventionally shown by points a). As I.K. nuclei of two types can appear: nuclei that are close in size to the molecules obtained by photo- or thermoconversion of gas impurities and require, as a rule, two-stage enlargement, and larger nuclei, for example, atmospheric s. to. for which one level of enlargement is sufficient. In the annular gap between the evaporator 6 and the inner wall of the cooled tube 1, due to the heating of the evaporator with an enlarging substance, a concentration gradient field of this substance and a temperature gradient field are created. As a result of heat and mass transfer processes in different parts of the flow, such a ratio of temperature and vapor concentration of coarsening substance is established that at a certain distance from the center of the flow in the cylindrical volume element with a base in the form of a ring with an inner radius r and an outer radius r + Δr, a maximum supersaturation is created S coarsening substance (Fig. 5, 6). In this zone, the supersaturated vapor is condensed on the nuclei and their sizes increase. A gas stream with enlarged nuclei (shown in b in Fig. 1) is discharged from the device through the side fitting 2 and enters the aerosol nephelometer to determine the calculated concentration of the formed aerosol (in the case of a single-stage enlargement). In the case of nuclei of molecular size, the enlargement is carried out in two stages in the embodiment of the device shown in figure 2, containing two evaporators: 6 and 6a. The first of them is saturated with the so-called developing substance, the second enlarging.

 Comparative characteristics of the proposed device and the prototype device, as well as the characteristics of the apparatus for measuring microimpurities in gas using enlarging devices are shown in the table.

 As can be seen from the table, the present invention allows to simplify the device for enlarging condensation nuclei without loss of concentration sensitivity, to reduce its weight and size characteristics, power consumption and aerodynamic drag, as well as to reduce the carrier gas consumption to a value close to that used in chromatography and to reduce the detection limit gas impurities an order of magnitude. This greatly expands the possibilities of using the device.

 PRI me R 1. Measurement of metal carbonyls in air.

A sample (5 cm ³ ) of the test air with traces of carbonyl is injected into the stream of purified carrier gas (air) with a syringe. Then, the carrier gas with the sample is passed through a chromatographic column and fed to a quartz photoreactor, where, under the influence of ultraviolet radiation from medium-pressure mercury lamps, carbonyl molecules are converted into condensation nuclei. A gas stream with cores is supplied to a two-stage enlarging device of the type shown in FIG. 2, and then to a photoelectric nephelometer for measuring light scattering of the resulting aerosol. The operating parameters of the enlarging device and the results are shown below:
Carrier gas flow 300 cm ³ / min
The number of stages of enlargement 2
The first enlarging
substance (developer) Norvaline
The second enlarging
Diisobutyl
phthalate
1st temperature
evaporator 90 ^° C
2nd temperature
evaporator 70 ^° C
Cooler temperature 18 ^о С
The minimum measurable carbonyl concentration depends on the carbonyl, but no worse than 2 · 10 ^-14 mole fractions
The detection limit of carbonyl is not more than 2 · 10 ^-16 g in the sample
PRI me R 2. Determination of aluminum hexafluoroacetylacetonate in air.

The determination is carried out, as in example 1, except that for the conversion of impurity molecules into ya. use the radiation of a high-frequency electrodeless mercury lamp, and for enlargement the device with a flat slit, shown in figure 3, 4. The operating parameters of the enlarging device are shown below: Consumption of carrier gas 200 cm ³ The number of stages of enlargement I The enlarging substance Dioctyl-
oxalate Evaporator temperature 80 ^о С Cooler temperature 15 ^о С Minimum measurable concentration 10 ^-13 molar
detection limit 3 · 10 ^-14 g in
sample
Thus, from the data of the table and examples it follows that the proposed method and device have significant advantages compared with the prototype and allow with high sensitivity to determine the trace amounts in the gas. Moreover, the relatively small flow rate of the carrier gas makes it possible to combine the device of this invention with gas chromatographic separation of the sample components.

Claims (3)

 1. A method of enlarging condensation nuclei in a gas stream, including obtaining supersaturated steam of a hardly volatile enlarging substance, introducing it into the analyzed gas stream and condensing it on cores with the formation of aerosol particles, characterized in that obtaining supersaturated steam is carried out between two equidistant surfaces with a given temperature difference , one of which, with a higher temperature, is covered with an enlarging substance, while the flow of the analyzed gas is introduced into the gap between the surfaces.  2. A device for enlarging condensation nuclei in a gas stream, including a chamber equipped with channels for supplying a gas stream and aerosol outlet, connected to a system for creating a supersaturated vapor of a hardly volatile enlarging substance with a cooler and an evaporator containing an electric heater, characterized in that the evaporator is located in the chamber.  3. The device according to claim 2, characterized in that the chamber is made in the form of a cylindrical tube, and the evaporator is installed along the axis of the chamber and has the shape of a cylinder in which the heater is located.

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