On-stream Analysers

The coal industry has benefited greatly through the applications of nuclear techniques. Nucleonic gauges and on-stream analysers are now regularly employed for monitoring and controlling the ash and moisture content in coal and coke. Nuclear techniques make possible the on-line determinations of sulphur and nitrogen (the causes for acid rain) in coal both of these are important for pollution control. Hundreds of millions of tonnes of coal are analysed annually by this method, a process which has become routine in the coal industry. 

Table 2 Coke content on the spent Mo/ZSM-5 after 72 h on stream, analysed by TG-DSC...

The lowest layer of process control applications is described as regulatory control. This includes aU the basic controllers for flow, temperature, pressure and level. But it also includes control of product quality. Regulatory is not synonymous with basic. Regulatory controls are those which maintain the process at a desired condition, or SP, but that does not mean they are simple. They can involve complex instrumentation such as on-stream analysers. They can employ advanced techniques such as signal conditioning, feedforward, dynamic compensation, overrides, inferential properties etc. Such techniques are often described as advanced regulatory control (ARC). Generally they are implemented... 

The term open-loop unstable can also be applied to controllers that have saturated. This means that the controller output has reached either its minimum or maximum output but not eliminated the deviation between PV and SP. It can also be applied to a controller using a discontinuous on-stream analyser that fails. Such analysers continue to transmit the last measurement until a new one is obtained. If, as a result of analyser failure, no new measurement is transmitted then the controller no longer has feedback. 

The algorithm was originally developed for use when the controller scan interval (ts) is signihcant compared to the process dynamics. This makes it suitable for use if the PV is discontinuous, such as that from some types of on-stream analysers. Analysers are a major source of deadtime. They may located well downstream of the MV and their sample systems and analytical sequence can introduce a delay. An optimally tuned PID controller would then have a great deal of derivative action. However this wUl produce the spiking shown in Figure 7.6. 

The second is the use of on-stream analysers to directly measure product quality. It is not the intent of this book to cover any detail of how such analysers operate or how they should be installed or managed. Instead this chapter will focus on the use of their measurements in control strategies. 

If 4> is calculated at a high frequency, for example by the use of on-stream analyser measurements, then care must be taken to ensure that the process is at steady state. Because the dynamics of the analyser will be longer than those of the inferential, any change in the inferential will be reflected some time later in the analyser measurement. There will therefore appear to be a transient error, even if both the inferential and analyser are accurate. Alternatively, dynamic compensation can be applied. We cover this later in this chapter. 

Automatic updating using an on-stream analyser measurement is quite different from updating with laboratory results. Analysers can have a reputation of poor reliability but we describe later in the chapter techniques that prevent spurious measurements from disturbing the process or being used to update an inferential. With this measurement validation in place analysers are far less prone to random errors than the laboratory. Secondly analysers provide measurements far more frequently and so the delay introduced by filtering will be far less. 

Figure 9.12 Use of analyser to update inferential the on-stream analyser. Applying Equation (6.19) we get...

On-stream analysers measuring molecular weight are normally marketed as densitometers and so we will base the control design on specific gravity (SG), defined as... 

Some of the instrumentation, most notably on-stream analysers, can be very costly. Chromatographs are commonly used which, along with the necessary housing and sampling system, are particularly costly. Other types such as near-infra red (NIR) and nuclear mass resonance (NMR) devices can be more so and involve high ongoing support costs. It may be that we cannot economically justify such instrumentation and need to compromise on the control design. 

By most standards the column is generously instrumented - particularly with on-stream analysers. It is not the intention to suggest that all columns should be so endowed. Not all of the schemes we will design for this column are economically justifiable or applicable on all. It is assumed by this stage the control engineer needs no help in installing the necessary flow controllers so these have already been included on the products, reflux and reboiler heating fluid. 

When moving away from the top and bottom of the column, there are other issues to consider. If the components in the feed have very similar bubble points it may not be possible to identify a tray anywhere where the temperature is sufficiently sensitive to changes in composition. Figure 12.71 shows the relationship between bubble point and composition for a C3 splitter - a column separating propene from propane. A common requirement is that the propene be 99.5 % pure. The relationship shows that doubling the permitted amount of propane in the product changes the tray temperature by about 0.4 °C (0.7 °F) - a change too small to be measured acciuately by conventional instrumentation. In such a case it is unlikely that accurate composition control can only be achieved with the use of an on-stream analyser. 

Figure 12.112 shows the application of full feed rate feedforward on a column with the energy balance scheme. Either or both of the reflux-to-feed and the steam-to-feed ratios can remain with operator entered SPs or their SP can be adjusted by a higher level control (such as tray temperature, inferential or on-stream analyser). 

Feedforward on feed composition can be a valuable enhancement but may not be practical. Firstly it requires an on-stream analyser on feed. Few plant owners would install this as standard and there may not be sufficient economic justification to add it later. Secondly it may not be possible to acquire an analyser that responds quickly enough. If the change in feed composition affects tray temperatures and/or inferentials before being reported by the analyser then the feedback controller(s) will take corrective action. A delayed measurement of feed composition would then be less valuable than having no measurement. 

Success was claimed for a similar scheme based on on-stream analysers (Reference 3). Here the PV controlled by reboiler duty was defined as (HKj — LK/,) while that controlled by distillate flow was defined as the average of HKj and LKh- It is likely that analyser deadtime was large compared to the dynamics of the process and masked any differences between top and bottom of the column. 

Much of the application of on-stream analysers has been covered in general in Chapter 9. Here we focus on those issues specific to their use on distillation columns. 

Optimum operation is likely to be a function of feed composition. On many processes, susceptible to feed changes, the necessary on-stream analyser technology may not exist. The use of laboratory data introduces a delay which could cause the optimisation to be in error for many hours. 

Simple monitoring of on-stream analysers, described in Chapter 9, ensures that measurement failure does not disrapt the process and that the associated reporting tools can do much to improve their reliability and use. 

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