Advanced Temperature Measurement and Control

Introduction and Overview

Temperature is a measure of a material’s internalmolecular activity. As the level of molecular activity rises, thetemperature of the material increases. Hot and cold are subjective,qualitative descriptions of a change in molecular activity

Temperature is often the most important of the commonmeasurements because it is an indicator of process streamcomposition and product quality. It would be nice if we had onlineanalyzers throughout the process but the fact of the matter is thatmost plants have infrequent, offline lab analysis at best. In thechemical industry, nearly all loops that are controlling thecomposition of a unit operation use temperature as the primarycontrolled variable. Even when online or at-line analyzers exist,these are usually relegated to monitoring and the manual trim oroptimization of temperature set points by supervisory or modelpredictive control. Temperature measurements are also essential forequipment protection and performance monitoring. Some examples ofcommon unit operations that have a critical dependence upon tight(minimum variability) temperature control are:

• Bioreactors and fermentors
• Calciners and kilns
• Chemical reactors Columns
• (e.g., absorber, distillation, stripper)
• Condensers
• Crystallizers
• Dryers
• Evaporators
• Extruders
• Furnaces
• Superheaters and desuperheaters
• Vaporizers

Over the years, the need in the process industry for moreconsistent and accurate ways to describe temperature led to theinvention of temperature- measuring devices, or sensors. Sensorsuse standard, universally recognized temperature scales. Becausethese scales rely on fixed points in nature (e.g., freezing pointof water), they provide a way to describe temperature that is bothobjective and quantitative. The four temperature scales in usetoday are Fahrenheit, Celsius (also called Centigrade), Kelvin, andRankine. In commercial applications, Fahrenheit and Celsius are themost commonly used scales.

Edition:

2

Published:

01/01/2011

Number of Pages:

334

File Size:

1 file , 5.5 MB