By MM, published on Asia Pacific Coating Journal (APCJ)
Batch reactors are the very fundamental process units used for the preparation of a vast majority of chemical raw materials, polymers, additives, intermediates, surfactants used in all sectors of industry. Examples are: paint & coatings, adhesives & sealants, construction chemicals, textile, cosmetics, pharma, personal care, etc.
A batch reactor is a much more sophisticated process machine compared to an agitated vessel or a mixers. It consists in a equipment where specific chemical transformations (syntheses) occur in a controlled way. It needs to be specifically designed as a complete assembly together with its auxiliary systems, in order to constantly achieve a perfectly controlled desired environment for the reacting mass. Therefore batch reactors for different applications present differences based on the specific reactions.
The quality of products, the reliability and ultimately the safety of a batch reactor depends upon many engineering details, regarding the internal and the external design, the ancillary equipments, the instrumentation, and, ultimately, on the capability of the control and supervision system to achieve at any moment the perfect desired process conditions for the internal reacting mass, with maximum accuracy, stability and safety, avoiding abnormal conditions, upsets, dangerous runway.
For batch reactors carrying out endothermic reactions, where a constantly controlled energy flow is necessary for the reaction to properly take place, one of the common problem is the inability of the reactor and its auxiliary systems, and/or the control system, to maintain an homogeneous and constant thermal flow throughout the reactor’s wall at the desired level. In this type of reactions it is in general unlikely that major hazards (catastrophic scenarios) can be reached. However, undesirable fluctuations of the product temperature almost always generate unpleasant consequences: unwanted side reactions if product temperature gets higher than the set point (over heating), uncompleted reaction when temperature falls lower than reaction set. These effects quite often lead to quality problems: higher viscosities, different colour, significant changes in the final physical and chemical properties, presence of solid lumps, sheets, impurities, turbidity, etc... With those problems, QC laboratory usually reject the product (causing loss of revenue, loss of productivity, impact to the environment, waste).
For batch reactors carrying out exothermic reactions, where an excess of heat is generated during the reaction and therefore must be constantly removed, the major problem is quite often the loss of temperature control. When the rate of heat generated by the chemical reaction exceeds the rate of heat removal, a positive internal feedback mechanism can occurs where the temperature of the reacting mass rises, increasing in turn the heat generation rate, thus generating off-side and uncontrolled chain reactions. When this abnormal condition happens, if no immediate actions are taken, not only the quality of the product will be irremediably effected, but also uncontrolled runaways may occurs with catastrophic consequences: damage of equipments, explosion, fire, potential injury to people, loss of contamination, loss of production, release into the environment of chemicals materials, pollution. This is why in this type of reactions it is always necessary to be able to detect every reactor upset in an early stage, and determine whether or not they may lead to dangerous conditions (hazardous scenarios). The clear determination of the type of a reactor up set is important for two main reasons. The first reason is that, if a runaway occurs and it is not detected on time, or not proper actions are taken, as we said, it may potentially lead to disasters. The second reason is based on the consideration that when a potentially dangerous condition is “detected” by the control system, immediate actions are to be taken in order to avoid that any hazardous conditions can be reached. These actions most of the time are drastic, and often may irremediably damage the product in the reactor, in order to preserve the safety of the plant and the people.
In conclusion, it appears clear now that an undetected runaway generates disasters, but a false detection lead to loss of revenue and waste.
Proper engineering design, sophisticated control and safety systems, automatic pre-warning software are necessary to guide operators in the “early stage” of possible real dangerous conditions, in order to take the proper actions, thus preserving both plant safety and production quality.
To achieve a perfect reactor control, it is necessary to operate at many levels.
- Engineering design of the reactor and its auxiliary systems
- Definition of measurable process variables and non measurable variables
- Definition of control algorithms
- Predictive algorithms
The proper engineering design guarantees that it is possible for the control system to constantly achieve the desired conditions and that is always possible to take proper necessary actions (typically: heating, cooling, vacuum , pressure relief, blow down, etc…)
The measurable process variables are used by the control system to constantly monitor the status of the reactor, detects abnormal conditions, reaction upsets, malfunctions, and consequently takes the proper corrective actions (only) before any dangerous condition may occurs. Typically, the most important variables are: reactor internal temperature, some time in multiple points, reactor pressure, thermal fluid temperature, delta T (energy balance), thermal fluid flow rate and pressure. In many applications other variables need to be acquired by the control system because necessary to fully identify the status of the reacting mass: conductivity measurement, various types of analyzers, viscometers, PH-meters, redox meters, etc..
The very basic control algorithms of a batch reactor are based on closed-loop PID controllers and on a continuous monitoring that each variable do not exceeds fixed values.
More sophisticated algorithms are also often implemented in addition to the above, to obtain a much smoother, more accurate and safer control of the batch reactor, by introducing direct correlations between measured variables. One example consists in the correlation between the variation of the internal temperature and the variation of the temperature of the heating / cooling fluid (jacket). Other correlations are also implemented between other measured process variables. These correlations have the scope to “anticipate” the future status of the reactor and therefore allow the control system (and the operators) to take proper “anticipated” actions on the controlled variables. These set of control algorithms, when properly implemented, give in general very good results, and therefore they are used in a very large variety of applications.
In some applications where very dangerous fugitive reactions take place during a production, for example when dealing with highly exothermic reactions, the process variables acquired by the control system are often correlated by more complex mathematic algorithms. One example is where the first and the second derivative of rector temperature and the derivative of the thermal fluid temperature in the jacket are related following special formula that are able to “predict” in an “early stage” trends and future reactor status in reliable way. In any case, all type of algorithms are complex and, in order to achieve reliable and robust controls, it is always necessary to study the models on the base of experience, and then validate them in laboratory.
By MM, published on Asia Pacific Coating Journal (APCJ)