by R. F. Wall Monsanto Chemical Co.
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INSTRUMENTATION A
W O R K B O O K
F E A T U R E
Process Analysis by Thermal Conductivity
E C
Although not so sophisticated as some of the more selective analyzers, thermal conductivity analyzers are dependable, fairly inexpensive, and easy to maintain
more selective analyzers for process control—i.e., infrared, to some extent mass spectrometry, a n d now gas chromatography—possibilities of older methods should not be overlooked. O n e of the oldest and best known, the thermal conductivity analyzer, has been widely used for m a n y years, although the selectivity depends entirely on the circumstances of application. It has been a reliable and effective analyzer for safety purposes, combustion control applications, hydrocarbon cracking operations, and a variety of analyses—in m a n y cases with excellent accuracy.
selective instruments. A recorder is not required if an indicating meter is adequate, or a spare point on almost any multipoint recorder will do. T h e installation is simple although an adequate sample system must be provided—a common requirement of all analytical instruments. M a i n tenance of the analyzer does not require special skill or extensive training; it is infrequent, resulting in low downtime. T h e best analyzer is the one that will do an adequate j o b , reliably, and with m i n i m u m maintenance. O n m a n y applications it is often possible to sacrifice a little accuracy to obtain greater reliability.
Applications
Design and Characteristics
I N THE EMPHASIS on the
newer
and
It cannot compete with the more recently developed process analyzers, for it does not have the selectivity to resolve individual components in the analysis of complex streams. However, it is adequate where circumstances provide selectivity, or where information, sufficient for control, is derived from thermal conductivity even though the accuracy of the chemical analysis is not high. T h e r e are applications that cannot justify the more expensive analyzer, even though it would do a better j o b , and a n economical thermal conductivity analysis unit will provide considerable help. Often in laboratory or in pilot plant studies, a thermal conductivity analyzer can be used to supplant laboratory analyses to advantage and make possible considerably improved control of an experimental unit. Advantages T h e analyzer is m u c h more economical t h a n the more elaborate
T h e thermal conductivity analyzer is in essence a Whcatstone bridge of either two or four temperatu rc-scnsitive resistors placed in individual cells in a common metal block. Gas diffuses into the cells from passages connected with flow paths through the block. T h e resistive elements are heated by the bridge current and cooled by the gas in which they are immersed to an equilibrium temperature dependent on the heat transport properties of the gas. T h e cell geometry is usually designed to emphasize the thermal conductivity component of heat transport. T h e resistances in opposed arms of the bridge are exposed to reference gas and to sample gas; this differential measurement tends to minimize the effects of changing pressure, temperature, and flow. T h e bridge contains resistance elements external to the cell that are adjusted to minimize effects of minor changes in temperature, pressure, and bridge current. T h e common I/EC
metal block tends to maintain all elements at the same temperature and simplifies the thermostating problem. T h e balanced bridge, plus the various factors tending to compensate for minor changes in the system, results in a very simple unit of excellent precision, and the thermal conductivity measurement is highly reproducible. Excellent evidence substantiating the precision of the thermal conductivity method is provided by one of the most selective of analyzers, the gas chromatograph. This is essentially a thermal conductivity analyzer preceded by a chromatographic separation column, so that the thermal conductivity analysis is always m a d e on the binary mixture with the carrier selected to provide a large difference in thermal conductivities. T h e gas chromatograph is specific by reason of the separation process only, and depends on precise timing for the selectivity obtained. We have observed a precision to ± C . 0 5 % with process chromatography, and believe this variation to be largely due to barometric changes in pressure. T h e most c o m m o n type of thermal conductivity analyzers measures the thermal conductivity component of the heat transport property of a gas very precisely. As thermal conductivity is a property common to all gases, this measurement will provide an accurate analysis for a component only when the sample is a binary with respect to thermal conductivity. Another type of thermal conductivity analyzer developed by Minter and Burdy [Anal. Chem. 23, 143 (1951)] of the Naval Research L a b oratory, and recently marketed as a commercial instrument by M i n e Safety Appliances, uses both convecWORKBOOK
FEATURES
69 A
I/EC
INSTRUMENTATION
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A WorKbook
Feature
RECORDER OR INDICATOR
Γ-ΑΛΛΛΑ
7®
ANNULLING RESISTORS
M/VW
M-S-A Thermatron combines thermal convection and conductivity effects for selective analysis in complex gas mixtures
tion a n d conduction cells to obtain compensation for the presence of a third thermal conductivity in the sample. As an example, they de scribe the determination of ethylene oxide in a mixture of air and carbon dioxide. T h e form of the cell used is shown in the accompanying dia gram. This type of cell m a y ap preciably increase the potential ap plications of thermal conductivity analyzers. T h e decision as to whether or not thermal conductivity is a useful method of analysis for a potential application can usually be m a d e from a knowledge of sample composi tion. Any of the several companies marketing thermal conductivity an alyzers will be willing to assist in this decision. T h e planning stage is the time for careful consideration, for marginal instrumentation almost in evitably means trouble. First esti m a t e the probability of success, a n d if the selectivity of the thermal con
ductivity method appears question able, decide on a more selective an alytical instrument. If hydrogen is present, there is not m u c h chance of analyzing for any other component of the sample stream, unless this component varies as some function of the hydrogen con centration. Carbon dioxide has a thermal conductivity that is low relative to m a n y gases, and this serves as a basis for m a n y useful analyses. T h e r m a l conductivity analyzers are a stand ard method of combustion control. T h e selectivity toward carbon di oxide is not nearly so favorable as that toward hydrogen, a n d success ful applications are more limited. Virtually any binary mixture can be analyzed very accurately by ther mal conductivity, and this class will include m a n y streams of interest in chemical and petroleum processes. Often useful analyses can be obtained on streams containing three or more
Cell block consists of four cells, two large convection cells, and two small con duction cells
FLOW PASSAGE " v T
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INDUSTRIAL AND ENGINEERING CHEMISTRY
components. I n some cases, an ac curate analysis m a y be obtained by measuring the differential thermal conductivity of a sample before a n d after the selective removal of a com ponent by physical or chemical absorption. Water vapor, ammonia, and carbon dioxide are examples of components that can often be re moved selectively. M a i n t e n a n c e of the absorption device is a disadvan tage of this technique. Possibly m a n y of these analyses that are borderline with conventional thermal conductivity analyses will be possible with the M-S-A T h e r m a t r o n described above a n d shown in schematic flow a n d wiring diagram. T h e r m a l conductivity analyzers for process use are marketed by most of the major instrument companies a n d others, and cells for those who wish to build their own by a n u m b e r of suppliers. T h e thermal conductivity analyzer is simple, reliable, highly precise, and capable of very accurate analyses under favorable circumstances. An alytically it often has rather serious limitations, particularly for the more complex streams. For the fortunate case in which it will work well, it has some definite advantages.
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