Instrumentation
Process
Gas Chromatography R. Villalobos Beckman Instruments, Inc. Fullerton, Calif. 92634
Process gas chromatography have been commercially available for almost 20 years. In that time they have become the most widely used process analyzer. A 1968 survey of U.S. refineries by the American Petroleum Institute (1 ) showed that chromatographs comprised fully 24% of all stream analyzers in use. This was considerably more than oxygen analyzers— the second most numerous—and 10 times as many as the number of pH systems. As in the laboratory, gas chromatography has proved to be the most versatile method to come along in the history of process analytical instrumentation. While most analysts are familiar with laboratory gas chromatographs, few are familiar with the process versions and the considerable technology that has been developed with relation to their application to automated onstream monitoring. While the similarities between laboratory and process chromatographs would be readily apparent to most analysts, the differences are perhaps more important, though less well understood. These differences are not only with regard to the appearance of the instrument itself, but the manner in which the data
are obtained, and perhaps most important, the way in which it is used. What Is a Process Gas Chromatograph? The aim in installing analyzers online is to obtain the analytical results with a speed of response that is comparable to process changes. The objective is to use the information to take corrective action. Hence, a process gas chromatograph (GC) is an instrument which has been designed to meet this objective and operates continuously on-line, automatically analyzing a flowing process stream, in a cyclic and repetitive manner. In general, such an instrument is dedicated to performing a particular analysis on a single stream, or at most, a few liquid or gas streams (multistream). Moreover, it will usually be designed to measure only one or, at most, a few components in the sample. A distinguishing characteristic of process chromatographs is that sample is transferred from the process sample point to the chromatographic column untouched by human hands. A supply of fresh sample is withdrawn continuously from the process and circulated to the sample valve, which in-
jects a small volume into the column. The sample valve, and the lines which connect it with the sample point, can be maintained hot. Therefore, the chromatograph can accept hot gaseous samples that contain large amounts of water vapor or other condensibles; samples which cannot be transported to the laboratory without drastically altering their composition. Indeed, many sampling situations which are difficult or impossible for laboratory analysis are routine for process chromatographs. A consequence of this is that the septum inlet, probably the most common component in a laboratory instrument, is unusable in a process analyzer. Instead, considerable emphasis has been placed on the development of sampling valves and column switching valves which are highly reliable. How Are Process Chromatographs Used? The uses of chromatographs in industry are varied. Table I lists some of the principal uses of on-line chromatographs. The most frequently encountered applications are for openor closed-loop process control. In open-loop control the operator makes
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1 1 , SEPTEMBER 1975 · 983 A
Table I. Principal Uses of Process Gas Chromatographs Process C o n t r o l — U s e i n f o r m a t i o n t o a d j u s t process t h r o u g h o p e n - or closed-loop control Process S t u d y — O b t a i n i n f o r m a t i o n a b o u t process t o i m p r o v e y i e l d or t h r o u g h p u t . C o r r e l a t e process variables w i t h p r o d u c t q u a l i t y Process D e v e l o p m e n t — O b t a i n inf o r m a t i o n a b o u t process c h a r a c t e r istics, as in p i l o t p l a n t s . C o r r e l a t e process variables w i t h r e a c t i o n p r o d u c t s and y i e l d s Material Balance—Use i n f o r m a t i o n t o c a l c u l a t e m a t e r i a l balance f o r process u n i t s Product Q u a l i t y Specification M o n i t o r — M o n i t o r i m p u r i t i e s in o u t g o i n g or i n c o m i n g p r o d u c t f o r c o n formance to specifications Waste Disposal M o n i t o r i n g — M o n i t o r l i q u i d or gas e f f l u e n t wastes f o r loss of v a l u a b l e p r o d u c t or f o r presence o f t o x i c c o m p o u n d s Personnel Safety-Area M o n i t o r i n g — M o n i t o r a m b i e n t air f o r presence of toxic compounds
adjustments to the process conditions based on the results of the chromatograph. In closed-loop control the chromatograph data are converted to a continuous analog signal which is input to conventional control instrumentation to control the process automatically. Components of Process Gas Chromatographs Basic elements of a process GC are shown in Figure 1. Analyzer (A). This contains all components of the analytical system— columns, sample valves, column switching valves, and detector—in a precisely thermostated oven compartment. For economy and simplicity, a single temperature zone is most frequently used. Multitemperature zone units with several columns at different temperatures have also been used, but are less common. Carrier gas flow controls, temperature controls, valve controls, and detector electronics are also located in the analyzer. The entire unit is located "on-line", as close to the sample point as possible. Supplies or carrier gas and other gases are also located in close proximity. The analyzer is usually housed in a walk-in analyzer shelter or house, as shown in Figure 2. The house provides weather protection for analyzer and sample conditioning components as well as for maintenance personnel. Several types of analyzers are often mounted in the same analyzer house. Sample Conditioning System (SH). This contains most or all of the components necessary to condition
and maintain constant flow of sample to the analyzer (e.g., pressure reduction, filters, vaporizers, flow controls, etc.) plus sample switching or selector valves for multiple stream applications and for introducing calibration standard (B). It is usually mounted below or next to the analyzer in its own heated or unheated compartment or on a flat open-plate. Some elements of the sample conditioning system, such as vaporizer, filters (F), and pressure reducers (R), may be located at the sample point (P) itself. It should be emphasized that the sample system is probably the most critical part of the entire system. If the sample is not representative and properly conditioned, the entire system will fail in its objective. Hence, the sampling system must be designed as an integral part of the chromatograph and not as an afterthought. The interested reader is referred to the monograph by Houser (2). Programmer-Controller (PC). This unit contains the program timer, power supply, signal conditioning electronics, and computer interface (where applicable). It controls all operations in the analyzer—sample injection, column switching—as well as housekeeping functions (auto zero), component gating and attenuating and data transfer to the appropriate readout channel. This unit is usually located in the control room (as much as 1,000 ft or more from the analyzer) but sometimes h r a separate room near the control room. Readout Devices (Recorders). A strip chart recorder for recording bar graph (BG) and for trend records (TR) is located in the control room.
Additionally, the system may communicate with a computer by means of priority interrupt or long-term memories (not shown). Considerations in Process Chromatograph Design The techniques and methodology used in process GC are in a general sense the same as in laboratory GC; however, a far different emphasis is placed upon the use of various methods and accessory devices. Some methods are more widely used in process—notably the use of multiple columns and column switching valves— while others are less frequently or seldom used—for example, capillary columns and temperature programming. Beyond that, process hardware bears little resemblance to its laboratory counterpart. Overall design and appearance are influenced by the following factors. Purpose. The system's purpose is to obtain information to control the process. Hence, for the majority of applications, it is usually necessary to measure only one or a few components. (Exceptions are pilot plant applications, wherein it is desirable to measure all of the components to characterize the process under study.) Location. The system is usually located in a hazardous area and requires explosion proof construction to satisfy National Electrical Code requirements (3). Analyzers and sample conditioning systems are designed to meet Class I, Groups C and D, Division 1. These are locations in which hazardous concentrations of flammable gases are present under normal conditions. Programmers and recorders are
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