Page 400 - Instrumentation Reference Book 3E

Separation of gaseous mixtures 383

For certain applications there may be a choice Physical methods The most powerful physical
between the use of a highly selective detector, technique for separation of gases is mass spectro-
with relatively little pretreatment of the sample, metry, described in Chapter 16-though only
or use of a detector which responds to a wider minute quantities can be handled in that way.
range of chemical species, the sample being separ- Gases may also be separated by diffusion; for
ated into its components before it reaches the example, hydrogen may be removed from a gas
detector. In the special case of gas chromato- stream by allowing it to diffuse through a heated
graphy thLe sample is separated on the basis of the tube of gold- or silver-palladium alloy.
different times taken by each component to pass
through a tube or column packed with adsorbent. Physico-chemical methods: chromatography Gas
The outliet gas stream inay then be passed chromatography is one of the most powerfuul
through a single detector. or through more than techniques for separation of mixtures of gases or
one detector in series or switched between (in their vapor phase) volatile liquids. It is rela-
detectors to analyze several components of the tively simple and widely applicable. Mixtures of
original sample mixture. By choice of columns, permanent gases, such as oxygen, nitrogen,
operating conditions, and detectors, a gas-chro- hydrogen, carbon monoxide, and carbon dioxide
matographic analysis system may be built up to
be individually tailored to analyze several differ- can easily be separated, and when applied to
ent preselected components in a single aliquot liquids, mixtures such as benzene and cyclo-
hexane can be separated even though their boiling
taken from a gas sample. Because of its import-
ance in process analysis, gas chromatography is points differ by oniy OAK. Separation of such
mixtures by other techniques such as fractional
given particularly detailed treatment.
In addition to the analysis techniques described distillation would be extremely difficult.
in this chapiter, a number of spectroscopic methods
are given under that heading in Chapter 16, while 18.2.1 Gas chromatography
some electrochemical methods are outlined in
Chapter 17. Chromatography is a physical or physico-chemic-
a1 technique for the separation of mixtures into
their components on the basis of their molecular
distribution between two immiscible phases. One
aration of gaseous phase is normally stationary and is in a fine!y
divided state to provide a large surface area rela-
tive to volume. The second phase is mobile and
Although detectors have been developed which transports the components of the mixture over
are specific to particular gases or groups of gases, the stationary phase. The various types of chro-
for example, flammable gases or total hydrocar- matography are classified according to the par-
bons, there is often a need to separate the sample ticular mobile and stationary phases employed in
into its components, or to remove interfering each (see Chapter 15). In gas chromatography the
species, before the sample is passed to the detect- mobile phase is a gas, known as the carrier gas,
or. A non-specific detector, such as a katharom- and the stationary phase is either a granular solid
eter, may also be used to measure one component (gas-solid chromatography) or a granular solid
of a gas mixture by measuring the change in coated with a thin film of non-volatile liquid
detector response which occurs when the compon- (gas-liquid chromatography). In gas-solid chro-
ent of interest is removed from the gas mixture. matography the separation is effected on the
Methods for separating gaseous mixtures may basis of the different adsorption characteristics
be grouped under three main headings. of the components of the mixture on the solid
phase, while in gas-liquid chromatography the
separation mechanism involves the distribution
Ghemicai' reaction A simple example of chem- of the components of the mixture between the
ical separation is the use of desiccants to remove gas and stationary liquid phases. Because the
water from a gas stream. The percentage of car- components of the mixture are transported in
bon dioxide in blast furnace gas may be deter- the gaseous phase, gas chromatography is limited
mined by measuring the thermal conductivity of to separation of mixtures whose components have
the gas before and after selective removal of the significant vapor pressures, and this normally
carbon dioxide by passing the gas through soda- means gaseous mixtures or mixtures of liquids
lime. Similarly the percentage of ammonia gas in with boiling points below approximately 450 K.
a mixture of nitrogen, hydrogen, and ammonia The apparatus for gas chromatography, known
may be measured by absorbing the ammonia in as the gas chromatograph, consists of a tube or
dilute suifuric acid or a suitable solid absorbent. column to contain the stationary phase, and itself