Chemical Composition[13]

(a) Physical Methods. Condensation and Fractional Vaporization. Separation of condensable vapors, generally in groups. Identification and quantitative evaluation is performed by other methods. Curves of vapor pressure vs. temperature of possible components must be known.

Fractional Distillation. Separation, identification, and quantitative evaluation of condensable hydrocarbons even in complex mixtures.

Adsorption or Absorption and Desorption (Chromatog­raphy). Separation, identification, and quantitative evalua­tion of many gases and vapors even in complex mixtures.

Diffusion. Separation of hydrogen and some isotopes.

Thermal Diffusion. Separation of some isotopes.

Electric Discharge. Separation of nomomzable gases.

(b) Chemical Methods. Selective Absorption, Separa­tion and quantitative evaluation of gases and vapors already known. There is a need for selective reagents. Quantitative analysis can be accomplished by (1) volumetric or baro­metric methods, (2) gravimetric methods, (3) titrimetric methods, (4) electrical conductivity, (5) colorimetry, or (6) calorimetry.

(c) Combustion Analysis. Fractional Combustion Sep­aration, identification, and quantitative evaluation, mainly of H2, CO, and hydrocarbons

Complete Combustion. Quantitative evaluation of H2, CO, and hydrocarbons.

(d) Absorption of Electromagnetic Radiation. Mag­netic Susceptibility. Quantitative evaluation of 02, NO, C102, and N02, not mixed with each other. Mainly used for 02

Visible. Quantitative evaluation of colored gases, not mixed with each other (N02, Cl, etc.)

Ultraviolet, Quantitative evaluation of 03,N0,C6H6, C6Hs(CH3),etc.

Infrared. Identification and selective quantitative eval­uation of CO, C02, hydrocarbons, NH3, S02 , S03, etc.

Visible Spectroscopy Identification of various sub­stances, quantitative evaluation doubtful.

Ultraviolet Spectroscopy. Identification of various substances, quantitative evaluation doubtful.

Infrared Spectroscopy. Identification and quantitative determination of H20, CO, C02, hydrocarbons, organic compounds, etc. Can detect composition of highly diluted mixtures.

Mass Spectrometry. Identification and quantitative evaluation of a large number of substances. Precise and capable of detecting composition of highly diluted mix­tures.

(e) Hydrogen Determination. The determination of hydrogen is normally the last to be performed. When all other constituents have been determined, the remaining gas can be considered as a binary mixture. Analysis for hydrogen can then be performed by one of the following methods

Sonic Analyzer. The velocity of sound, S, is related to the molecular weight of the gas through which it is propagating

image150

where R = gas constant

T = absolute temperature к = ratio of specific heats (k = cp/cv) m = molecular weight

Two sound waves of identical frequency are passed through two similar tubes, one filled with a reference gas and the other with the mixture. Different sound velocities in the two tubes result in a phase difference between the two waves reaching the ends of the tubes. This difference is used to compare the velocity of sound in the two gases. The mean molecular weight of the mixture can be derived, and the hydrogen content can thus be calculated.

Interferometry (Optical). A monochromatic light beam is split and passed through two identical tubes, one filled with a reference gas and the other with the mixture. Usually the reference gas chosen is the major constituent in the binary mixture. Because of the difference in the velocity of light in the two gases, the light beams emerge with a difference in phase and can be made to produce interference bands. The spacing of the bands is related to the relative concentrations of the components of the gas mixture and the refractive indices of the sample and the reference gas

Pa Pb nab = niy+nby

where паь = refractive index of a binary mixture (A + B) na and nj, = refractive indices of the components A and В P = pressure of the gas mixture Pa and P(, = partial pressures of the components

When the refractive index of the mixture is known, the partial pressure of one component can be deduced and hence the concentration can be estimated. Interferometers are capable of great accuracy provided they are used with skill and provided pressure and temperature corrections are applied.

Thermal Conductivity. In binary mixtures thermal conductivity can vary linearly with the concentration of one component Absolute evaluation of thermal conduc­tivity is very difficult, and normally only relative values are determined. Eor this purpose a hot wire Wheatstone bridge is used. Sample gas passes through a cell that contains a resistance wire. A second cell containing a compensating resistance is filled with a reference gas, which is usually the major constituent in the mixture. The heat-loss difference between the two arms, due to the different thermal conductivities of the gases, unbalances the bridge The bridge output must be calibrated against standard gas mixtures.

This method is highly suitable for hydrogen because the thermal conductivity of hydrogen is considerably higher than that of other gases The thermal conductivities of some gases relative to normal air at 0°C are air = 1, H2 = 7, CH4 = 1.27, CO = 0.96, and C02 = 0.59.

Diffusion. Hydrogen could be determined by taking advantage of its high diffusivity through porous dia­phragms. The method is time-consuming, but its accuracy is good.

When the analysis for hydrogen is required in a mixture containing more than two components, other methods must be used.

Combustion Analysis. If the gas mixture contains no hydrocarbons, hydrogen may be estimated by measuring the water formed by oxidation. Actually, hydrogen is usually found mixed with other hydrogenated combustible gases that also produce water on oxidation. If there are no more than two other hydrocarbons, the identity of which must be known, combustion analysis is still possible provided carbon dioxide is also estimated. This requires the solution of three equations.

Other Methods. Hydrogen is also determined by gas chromatography and mass spectrometry. Infrared spectros­copy cannot be used since hydrogen has no absorption bands in the region of the spectrum.

(f) Oxygen Determination. Paramagnetic Analyzer. A continuous stream of gas is passed through an annular tube and crossed by a transverse connection tube The latter is wound with a heating spiral, one end of which passes through a strong magnetic field. Any oxygen molecules in the gas are attracted toward the magnet more from the left side of the transverse tube than from the right. Warm molecules are less susceptible to the effect of the magnet. As a result continuous flow is established through the transverse tube. The gas flow through the transverse connection depends upon the oxygen concentration. The temperature gradient along the heater winding depends upon gas flow. Therefore, oxygen concentration is mea­sured by temperature gradient.

Electrochemical Gas Analyzer. A heated zirconium oxide tube sets up a current when there are different concentrations of oxygen in two gases that flow inside and outside the tube. The unknown gas is passed inside the tube, and the reference gas (air) is passed outside the tube. The electrical output of the tube is proportional to the logarithm of the ratio of the oxygen concentration of the two gases. The advantages of the method are that it is accurate, requires no fuel, is unaffected by high S02 or S03 concentrations, is unaffected by high C02 concentra tions, reads net 02 , and has a fast response.

(g) Summary of Methods Used for Gas Analysis. Та Ые 4.23 summarizes the various methods that may be used for gas analysis. Instrumental procedures for gas analysis tend to supersede classical laboratory methods. Laboratory methods are often used as standard references. Instruments are used as the ordinary tools of the investigation.

Eor simple analysis (C02, CO, 02, H2, and CH4), the nondispersive infrared analyzer, the magnetic oxygen ana­lyzer, and the sonic analyzer for hydrogen are suitable. For mixtures of increased complexity, chromatography is rec­ommended. It is economical and quick. It can fractionate mixtures into single or groups of components, which can be useful before applying infrared or mass spectrometry.

(h) Mass Spectrometry. The material to be studied is subjected to an ionizing process, separated according to mass by electromagnetic means, and the resulting mass spectrum is analyzed, quantitatively and qualitatively, by comparing it with the spectra of known calibrating mate­rials.

Ions are produced by four methods (1) electron bombardment, in which the unknown, if it is gaseous, is bombarded in an evacuated chamber by electrons, (2) direct emission of ions from the surface of some solid materials by heating a filament that is covered with a thin layer of the material to be analyzed, (3) the crucible method, in which materials (e. g., halides) are evaporated from a small furnace, and subsequently the vapor is ionized by electron bombardment, and (4) the spark method, in which a high-voltage spark between electrodes of the material to be analyzed yields ions of that material. Positive ions are accelerated by electric fields between a system of electrodes. Ions are focused in their passage through slits or apertures in the electrodes. The ion source in a mass

Method

Average

sample

sizet

(s. t.p.),

cm3

Average

time

required %

Average

accuracyt

Нг

о,

со,

SO,

so3

СО

Alcohols

Ethers

Aldehydes

Organic

acids

Organic

peroxides

СН„

Other

hydro­

carbons

Fractional distillation

104

6 hr

0 5

А

Gas chromatography

10

5 min—1 hr

0 1

в

В

В

В

А

А

А

А

А

А

ТА

Diffusion

200

20 mm

0 1

А

Combustion and

1 to 4 X 103

6 hr

ТА

В

А

А

А

А,

gravimetric analysis

Cambridge

10

16 hr

0 01

А

в

А

А

А2

А

А,

Modified Cambridge

10

3 hr

0 01

А

А

а2

А

А,

Simplified Cambridge

10

20 min

0 01

А

Interferometry §

4000

5 min

0 01

А

в

А

А

А

Sound velocity!

500

5 min

1

А

в

В

в

Thermal conductivity bridge!

f S

і d

0 001

А

В

В

Paramagnetic detector

f s

і d

0 05

А

Nondispersive infrared

f s

i d

0 001

А

А

А

А

в

В

А

В

Infrared spectrometry

20

20 min—1 hr

0 1

В

В

В

А

в

в

А

А

Mass spectrometry

0 01

15 min—1 hr

ТА

В

в

В

В

В

А

А

А

А

В

В

А

Special laboratory methods

А

А

А

А

А

А

А

•From G Tine, Gas Sampling and Chemical Analysis in Combustion Processes, p 86, Pergamon Press, Inc, New York, 1961 tAbbreviations i d = instrumental delay (sec)

A = particularly suitable method В = possible method Aj = undifferentiated estimation A2 = estimation of formaldehyde only T A = trace analysis

f s = analysis can be performed also on flowing streams

Подпись: PROCESS INSTRUMENTATION 107$The actual sample sizes, time consumptions, and accuracies depend upon the particular apparatus that is used and, in many instances, upon the gas to be detected §Only suitable for binary mixtures of known components

Подпись: Fig. 4.55—Conductivity—concentration curves for certain electrolytes (From D M Considine, Process Instruments and Controls Handbook, p 6 159, McGraw Hill Book Company, Inc , New York, 1957 ) Подпись:

spectrometer is a combination of the region where the ions are generated (this region usually has an electron gun to provide the electron bombardment) and the ion-accelera­ting region.

The ions are separated by one of these four basic methods a magnetic analyzer (masses separate according to their momenta in a magnetic field), a time-of-flight analyzer (ions with same kinetic energy but different masses have velocities inversely proportional to the square root of the mass and become separated if injected into a field-free “drift” region), a linear-accelerator analyzer (accelerated ions are segregated by electrostatic deflection), and an ion-resonance analyzer (ions move in a region where a radiofrequency electric field is set up at right angles to a magnetic field and, if the frequency is in resonance with the spiralling ions, the ions spiral out of the field and are collected)

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