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INSTRUMENTATION
W O R K B O O K
F E A T U R E
by Robert Wall, Plastics Division, Monsanto Chemical Co.
Mass Spectrometry in Process Control A small, newly designed instrument may serve as the basis for a major breakthrough into true process control by mass spectrometry I HE mass spectrometer has had many applications to process problems, the . n u m b e r increasing as developments have m a d e available simpler instruments of reduced maintenance requirements. However, installation, reliability, and maintenance have been troublesome for true process control. T h e majority of the applications have been exploratory in nature, either in the pilot plant or in studies of process behavior, where the advantages of speed, precision, versatility, and resolution of complex samples have been essential to some programs and have greatly facilitated others. T h e review of mass spectrometry by Priestley and Dudenbostel in this column for February 1956 is indicative of typical applications of continuous mass spectrometry.
change from continuous to batch operation and a new method of batch-sample entry by a rotary valve. A very successful technique of gas chromatography has been a p plied to mass spectrometry. T h e electronics have been reduced to a m i n i m u m ; there are nine tubes in the complete instrument and advanced techniques of electronic construction have been utilized. Some advances in source design and operation, based on experience, should provide excellent stability and precision, as well as reduce maintenance requirements. The controls and operating techniques have also been simplified, facilitating the use of the mass spectrometer by less highly skilled personnel. T h e over-all simplification of
the instrument has resulted in a substantial reduction in price, which will make the justification considerably easier in m a n y cases. This newly designed instrument, which measures u p to analytical requirements, m a y signal the start of a much broader application of mass spectrometry, in particular for process applications where high reliability is a prerequisite. T h e installation requirements for hazardous locations remain a problem. Inert gas purging of a housing containing the instrument is possible and is facilitated by the size reduction a n d unitized assembly. However, this is never very satisfactory and in m a n y plants is not allowed—a decision most often based on some unfortunate experiences
A new, small mass spectrometer recently announced by Consolidated Electrodynamics Corp. has m a n y features that m a y enable significant advances in the applications of mass spectrometry to continuous analyses. T h e C E C 21-611 mass spectrometer, shown here, is a small laboratory instrument, yet it has m a n y features that seem directly pointed at process application. This new mass spectrometer is significantly smaller t h a n its predecessors and, in addition, it utilizes unitized construction, which will facilitate the installation of the analysis unit at the process, a n d the electronics and d a t a presentation at a remote location. T h e reduced size is the result of simplification of every unit of the instrument. T h e vacuum system has been reduced to the m i n i m u m essentials, with a single forepump serving both the sample and high v a c u u m sections. An efficient oil diffusion p u m p and baffle operate without the need for trapping. T h e sample system has also been utilized, with a simple VOL. 49, NO. 8
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AUGUST 1957
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with purged systems. Enclosure of the analysis unit in an explosion-resistant housing will be necessary to remove a major limitation. The basic work of simplifying and unitizing the construction has been done in this present instrument, and the final step to an explosionproof mass spectrometer is believed to be in progress. This mass spectrometer, with a resolution of 35 and a mass range from 2 to 80, will provide the basic data for accurate analyses of many chemical and petroleum process streams, but will be inadequate for other applications. The most serious limitation to continuous process use is the reduction of the data-to-stream analysis. The development of small, reliable, and economical equipment for direct readout and computation of the mass spectrum will be required before full use of the mass spectrometer can be made in process control. This will require advances in computer components and techniques and, until such equipment is available, process applications will be limited to those for which the mass spectrum will directly provide the analysis information' required. As a class, multicomponent hydrocarbon samples require computation, and this type of sample dominates in petroleum and petrochemical work. Simple computation is adequate and acceptable in many cases, particularly in pilot-plant and process-investigation work where skilled manpower is available. However, the full potential of process mass spectrometry will be greatly restricted until the readout is in sample-component percentages rather than in complex mass spectra. In a comparison of process mass spectrometry and process chromatography, the former has the advantage of speed of analysis. A single peak can be monitored with an instrument response of 1 second, enabling control of extremely fast processes if adequate sampling systems are devised. A multicomponent analysis may be needed for control. Several minutes are required for scanning a spectrum, and with a continuous sample system and rapidly changing process conditions, the sample at the end of a scan may be different from that at the start. This
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must be carefully considered. The speed of the mass spectrometer can be utilized for control only when a single peak provides the required information. High-speed readout-computer systems will be required for general applicability of the mass spectrometer to process control. The chromatograph analysis is representative of the stream composition at the instant the sample is taken, irrespective of the time lapse before the data is presented. The chromatograph spectrum is interpreted without computation, permitting direct analysis of many streams, which require computation with the mass spectrometer. However, a column must be developed for each application, often a compromise of resolution and analysis time must be made, and the time required to separate a large number of components tends to become very long. The gas chromatograph is not well adapted to continuous, automatic process control. The problems in applying mass spectrometry to continuous process control have been more with installation and operation than with analysis. Previously, the instruments have been small laboratory equipment and have required a small laboratory for their successful utilization. Maintenance and reliability have been a problem because of the exacting requirements placed on process analyzers. Instrument, installation, and maintenance costs have been high. The CEC 21-611 offers minor improvements in performance over preceding instruments, but, far more significantly, it presents a major advance in overcoming some of the application problems. This may result in a marked increase in continuous mass spectrometry in laboratory, pilot-plant, and continuous process control. The small mass spectrometer may prove to be a better competitor for the gas chromatograph than is generally believed possible.
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