by R. F. Wall Monsanto Chemical Co.
I INSTRUMENTATION A
· · W O R K B O O K
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Process Analyzer Response Time Analyzer response time becomes more critical as process control techniques become more advanced
Γ ROCESS analysis has become more of a routinely used control tool as at tention has shifted to the finer de tails of application, and increased use in automatic control systems and in the more complex and intricate computer control technology is forc ing a more serious and quantitative estimate of how fast an analyzer should be. The answer lies in the process, for the rate at which analy ses are needed will be determined by how fast the process can change, and also in the attitudes and work pat terns of those using and maintaining the analysis equipment as determined by their reaction to the response characteristic of the analysis system. An analyzer that responds so slowly as to be boring or irritating to those using or maintaining it will receive little attention and poor maintenance. T h e reaction of the men is as impor tant as the reaction of the process in designing a control system. The response of an analytical con trol system is thus not a simple matter of the analyzer alone, but involves the process, the sample system, the analyzer, and a human factor, and requires a balance of these for opti mum performance to be obtained. Failure to consider any of these fac tors can lead to unsatisfactory analy tical process control system. Automatic analytical process con trol can be highly advantageous for processes that are slow and capable of satisfactory manual operation. Modern practices minimize man power, with the least possible per sonnel per process unit. Each oper ator is responsible for a great deal of equipment and observation of a large number of associated controls; time is not available for frequent adjust
ments based on reading a process analyzer, even though this is within the man's capabilities. If his atten tion is diverted by some minor emer gency, such as a cup of coffee, con trol will be neglected. Adjustments at 2-hour intervals probably should be made automatically, although this cannot be used as a rule, for it depends on the process, the number of men on shift, and problems of con trol that exist. H u m a n decision may be necessary. Adjustments at 15- to 30-minute intervals are pos sible, but such a schedule is unlikely to be strictly followed. Frequent changes of an operating variable to follow a control index can be accom plished far more efficiently by auto matic control equipment than by an operator. Failure to use automatic control techniques, where indicated, is a definite waste of manpower. Analyzer response faster than process changes can occur is required for satisfactory process control to be possible. This has long been ac cepted, and slow analyzers on fast processes have been proved unsatis factory by experience. It has been further confirmed and quantitatively defined by T. J. Williams' theoretical study of the relation between con trol accuracy and the ratio of analy sis to process time constants. Gen eralizing, the ratio of process to ana lyzer time constants should be about 2 or preferably greater for good con trol. Rather poor control is ob tained if the time constants are equal; if the analyzer response is slower than the process, the system will probably be unstable. The time constant of the analyzer must be less than that of process changes, if a stable and effective control sysI/EC
tcm is to be obtained. T h e time constant of a unit proc ess is basically determined in the design, and modern trends are to ward increased throughput, higher production per unit, and accordingly reduced time constants. The proc ess time constant is largely deter mined by what the process is and how large a plant is being built. Control should be a part of process design, for once built, controllability of a plant is not easily changed. Match ing analyzer response to process is a problem of making analysis time short enough, for changes in response characteristics of the process are usually impractical. A slow analyzer can effect excel lent automatic analytical control of a fast process, either manually or auto matically, and this is very often done. T h e principle is that the rate at which the process is permitted to change is limited through restricting the rate of change of the control pa rameters. The analyzer is thus faster than the rate of allowed process vari ation and good control is possible, even though the analyzer time con stant may exceed the natural time constant of the process. This sys tem operates very well except during process upset, when the natural time constant of the process applies and control is reduced to chaos. In the manufacture of acetylene by the par tial oxidation of natural gas, the ma jor control variable is the ratio of oxygen to natural gas, and the com position of the converter off-gas can change in a matter of seconds. Good control is obtained through holding this ratio essentially constant and changing slowly according to stream analyses.
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INSTRUMENTATION
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A Workbook Feature
It would be preferable to have the analyzer faster than the process, for then the analyzer would be much more effective during periods of process upset. However, this cascade control system has the advantage of permitting analytical control of any system, however fast, and is helpful in minimizing the effects of analyzer down time and facilitating mainte nance, for the conventional process controls can hold whenever the ana lyzer is out of service. Analysis response is determined by the response characteristics of the complete system, including process time lag between sample point and control variable, response character istics of the sample system, and re sponse of the analyzer. All three are critical, and each must be con sidered for a good process analysis control system to be developed. The composition of the process stream at the sample point must be significantly and quickly related to the desired control function. The selection of the best sample point re quires an understanding of the proc ess, and is best made with the co operation of those concerned with the design and operation of the plant. Some compromise may be advisable; the best sample point may be inacces sible, or in an unsafe or inconvenient location, making installation and maintenance impractical. Sample-Handling Systems
The sample-handling system must be carefully designed for each case, as the nature of the sample stream will dominate in determining treat ment necessary. The conditioning required to make the sample accept able by the analyzer can be a defi nite limitation to analyzer response time, or even determine whether or not an analysis is possible. Filtra tion is a common problem, with the amount of suspended material in the sample determining the filter size required for a given filter life be tween cleanings or cartridge replace ments. Doubling sample flow for faster response will require the filter size to be doubled for the same life, and the response time of the filter will remain a constant determined by the quantity of filterable material in the stream and the filter life de sired. The laer of sample lines and 76 A
other distribution components can be reduced by increasing sample flow but ordinarily not that of the filter. Careful selection of a minimum vol ume filter can help considerably, but the only answer in some cases may be an increased filter mainte nance schedule. Sometimes there is no satisfactory answer, and analysis may not be practically possible. It is then best to devise some alterna tive method of control. An undependable analytical control system can be worse than none. The best possible sample system should be devised with emphasis on reliability and maintainability. Speed of re sponse may be essential, but is sec ondary to these factors. Instrument Characteristics
The analyzer time constant is largely determined by the character istics of the instrument. In design ing an analysis system one must first select an analyzer capable of ade quate response, and then do what is reasonably possible to obtain the de sired speed of response. With opti cal analyzers, as infrared, ultraviolet, refractometer, etc., a minimum vol ume sample cell and rapid sample flow can be used, for the time re quired to flush the sample cell is usually the dominant factor in re sponse. T h e gas chromatograph, automatic titrator, and other repeti tive cycle analyzers are characterized by a relatively long analysis time de termined by the cycle of analytical operations, and require ingenuity in devising the fastest possible analyti cal scheme. With gas chromatog raphy the delay is largely the time required to obtain component sepa ration and is determined by the col umns. Use of multiple columns and careful column selection can mark edly improve the response obtainable with gas chromatographs now avail able. Developments in high speed chromatography reported by Karasek and Aycrs of Phillips, and results obtained with capillary columns, indicate that slow response is not an inherent disadvantage of gas chroma tography, but commercial availabil ity of high speed process gas chroma tographs appears to be a year or two away. The response time of a process is the yardstick against which the re sponse characteristic of an analysis
INDUSTRIAL AND ENGINEERING CHEMISTRY
system must be measured. Unfor tunately this is an area in which very little is really known. We can de scribe the steady-state behavior of a process with considerable confidence, but the dynamic behavior is of major concern in control, and knowledge of this is almost nonexistent. Studies that have been started have found the descriptive equations to be ex tremely complex, and the magnitude of data necessary and computations to obtain a description of system be havior has been proved to require automatic data handling and large computing equipment. This ap proach is active, and quantitative knowledge of the dynamic behavior of processes should be available rea sonably soon. While we have usu ally been able to get by on the basis of experience, we are often enough surprised by the unexpected response characteristics of processes to be con vinced that a quantitative engineer ing approach to dynamic process re sponse will be of significant help. Usually process sample lag can be made a minute or less. A good sam ple system always includes at least one filter to remove pipe scale and other process dirt, even though the sample may be expected to be ideally clean, and the over-all lag of filters, lines, and distribution system will usually be about 5 minutes. Analyzer response is about 2 minutes for an infrared or 5 to 15 minutes for a chromatograph. These delays are not entirely dead time and are there fore not strictly additive. Experi ence indicates total analysis response time will be about 5 to 20 minutes. The corresponding process response times required for satisfactory con trol are 10 to 40 minutes, and pref erably longer. Although informa tion on dynamic process behavior is inadequate, experience clearly proves that many ordinary processes are much faster than this, and the matter of analysis system response can be highly significant.
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