A large variety of processes, few of which are exactly alike, typical unit operations share many commonalities. One of these is the requirement for accurate process analytics in order to help maximize product quality and yield and to minimize costs. Measurement of pH, ORP, conductivity, DO or turbidity may seem like small parts of a total industrial process, but these parameters can be the cause of frustration, downtime and substantial expenditure unless they are properly understood and adequately controlled.
How can plant managers and engineers ensure measuring parameters will help processes run smoothly and efficiently, and exploit the availability of process machinery? Fortunately, advanced process analytics solutions exist that not only are accurate and reliable – even in the harshest of process conditions – but that also minimize the maintenance burden by actually predicting when maintenance of a measuring point is required.
This white paper reviews issues that can occur in chemical plants when some of the above mentioned parameters are not held within desired ranges, and how cutting-edge process analytics can help avoid the problems and achieve greater plant efficiency.
Water preparation
Boiler tube failures continue to be the single largest source of forced outages in industrial-type steam generators. In most cases, corrosion occurs under deposits and scale which builds up on the boiler tubes. In addition to contributing to the corrosion process, the buildup of scale on the tubes by itself reduces heat transfer and therefore increases the overall fuel costs required to produce a given quantity of steam. Corrosion and scale buildup can be controlled by maintaining proper water chemistry.
Accurate pH control of the inlet water of the reverse osmosis unit protects membranes from damage. In the operation of ion exchangers, pH monitoring is required to ensure that no excess regeneration acids or bases contaminate the boiler water. In boiler feedwater, pH measurement is a significant challenge but essential for corrosion prevention.
Continuous in-line measurement of pH, DO and conductivity is of paramount importance for the control of corrosion and scaling and in maximizing energy efficiency.
Main processes
The quality and yield of products are the two most important aspects in individual processes. For companies to remain globally competitive therefore, individual processes must be monitored and analyzed through a specialized approach. Both quality and yield can improve significantly through in-line analysis during each individual process step.
For example, pH is a critical process parameter in various synthesis steps, such as esterification, polymerization and in the formation of certain salts. Apart from reducing chemical consumption, pH control improves the purity of the product and contributes significantly to overall cost savings. Accurate pH control can play an important role in initiating the crystallization process and in determining the crystal structure. Dissolved in-process or wastewater substances like H2S, CO2 and NH3 are potentially corrosive or harmful. Accurate pH control in the stripper ensures that these components are present in their gaseous form and so can be efficiently removed from the water.
In gas scrubbers, maintaining the correct concentration of the scrubbing agent is an absolute necessity. Depending on what agent is used, control of pH is very effective in optimizing scrubber performance.
Cooling water
Cooling water systems are vulnerable to corrosion, scaling and biological contamination, and the concentration of impurities increases over time with evaporation. Corrosion inhibitors in cooling water are effective within a certain pH range. Accurate pH control is important in the cooling tower and at various locations in the circulation. Also, typical scaling impurities tend to precipitate at high pH which is another reason for keeping tight pH control. Biological impurities grow at warm temperatures and relatively high oxygen levels. Controlled additions of biocides such as chlorine and hypochlorite compounds are possible providing reliable oxidation potential (ORP) measurements are made.