The terms „process analytical chemistry" and „on-line monitoring" are often used synonymously. They are gaining increasing attention and importance in both industry and academia. The extent of agreement between the two terms is indeed very broad in that they provide sufficiently accurate and immediate information on variables that describe the state of a chemical reaction.

On-line monitoring of reactions encompasses on-stream and on-reactor application of analytical methods to monitor the chemical composition of a reaction mixture, to identify process-related chemical species, and to quantify the concentration of reaction ingredients, products and byproducts. In addition to revealing the state of the reactor, on-line analysis of physical parameters (temperature, pressure, level, density, viscosity, etc.) may also reflect the extent of a chemical reaction.

Process analytical chemistry comprises applications which supply relevant process information of interest „in-time": The time for sampling and analysis is very short compared to the overall reaction time and thus allows adequate monitoring and efficient control of the reaction. The utilization of the analytical data for control strategies makes process analytical chemistry an essential integral part of process engineering and control systems.

Methods of Data Collection

Process analytical methods may be classified as off-line, at-line, on-line, or in-line with respect to sampling, sample transport, and analysis itself (table 1). There is no clear-cut line between the different classes, and the boundaries are even moving: some of today's off-line techniques may become tomorrow's on-line techniques. An increasing number of off-line techniques have been converted into on-line methods by automated, robot-assisted withdrawal of samples from the reactor or from bypass or process streams and feeding them into off-line instruments.

On-line monitoring and control of chemical reactions contributes to guaranteeing and improving product quality and consistency, increasing the efficiency of the process and ensuring safe reactor operation by monitoring process and reactor parameters, understanding the fundamentals of the reaction itself, and saving time for the analysis and sample transport. In addition, reductions of emissions by avoiding sample withdrawal and transport as well as of costs for labor, raw materials, off-spec products and process waste can be achieved.

In most cases, purchasing and installation costs of on-line and in-line analytical instruments exceed those of off-line equipment, but as soon as the measurements required exceed a certain (relatively small) number per day, on-line analysis becomes superior to off-line analysis. However, the real savings of on-line reaction monitoring are due to improved process efficiency, lower raw materials consumption and waste generation, and, most important, the ability to manufacture high-quality products.

Application Design

Several issues have to be addressed when designing an on-line analysis application:

- Information necessary to monitor and control the process (physical parameters, chemical composition, etc.)
- Frequency of measurements with respect to the timescale of the reaction
- Average values, typical fluctuations, dynamic ranges and expected extremes of properties
- Type, precision, and response time of sensors
- Robustness of sensors and simplicity of installation
- Full automation and minimum maintenance of equipment
- Number (combination) of sensors and location of measurement
- Proper sampling and, if required, sample conditioning
- Form of data output and further handling of information
- Compatibility with process control system
- Safety precautions and possible hazards
- Costs of instrumentation and availability of trained personnel

Demands on performance characteristics of on-line analytical techniques differ for academic research applications, process development and pilot plant operation, and monitoring of industrial manufacturing processes.

Read more about this topic in Ullmann's

This article is an excerpt from the Ullmann's Encyclopedia of Industrial Chemistry which celebrates its 100^th anniversary in 2014. More about the topic can be found in the encyclopedia article on Process Systems Engineering, 5. Process Dynamics, Control, Monitoring, and Identification. More concept articles on general interest topics in industrial chemistry and chemical engineering can be found on the Ullmann's Academy homepage!