Zi Uv I U2i I

The maximum of Zi is an indication for the best location of the first sensor, while the minimum of Zi gives the best location for placement of the second sensor. For the toluene-o-xylene column this was calculated and the results are shown in Figure 8. The

Figure 8 Sensor placement based on U-vectors.

best location for the first temperature sensor is tray 25, and the best location for the second sensor is tray 28. Note that the location has changed somewhat compared to the best locations for the single composition control problem (trays 24 and 26). This is a result of the process of reducing the interaction between the vector components at the other trays.

In many columns temperature has been used to control the separation process. For binary systems at constant pressure there is a unique relationship between temperature and composition. For multicom-ponent mixtures, often a simple relationship may be found between temperature and composition. If the column pressure is not controlled tightly, temperature measurement should compensate for pressure variations:

^compensated ^measured # S(Preference Pmeasured) [13]

where S is the inverse of the slope of the vapour pressure-temperature curve at normal operating conditions.

No matter how attractive it is to control temperature rather than composition, the ultimate objective of the separation process is to control composition(s). This means that an analyser should be used to indicate the true compositions. There are, however, a number of disadvantages using analysers in control. First of all, analysers are highly sophisticated instruments and are therefore expensive and require extensive maintenance. In addition, the sampling system of analysers is prone to malfunctioning and as analysers are often used for multiple streams the response can exhibit a large dead time. This means that simple feedback control using analysers often results in poor control performance and dead time compensation techniques often have to be used to improve performance.

Analysers are well suited for use in a cascade control set-up, where the temperature (or combination of temperatures) controls one of the flows and the analyser controller resets the temperature controller setpoint. Figure 9 shows one possibility, using the reflux as manipulated variable.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment