Phenol-formaldehyde reaction control

The other root causes were (1) the poor understanding of the chemistry, (2) an inadequate risk analysis, and (3) no safeguard controls to prevent runaway reactions. This EPA case history also summarized seven similar accidents with phenol-formaldehyde reactions during a 10-year period (1988-1997). 

Therefore, the chapter is mainly focused on the design of model-based control approaches. Namely, a controller-observer control strategy is considered, where an observer is designed to estimate the heat released by the reaction, together with a cascade temperature control scheme. The performance of this control strategy are further improved by introducing an adaptive estimation of the heat transfer coefficient. Finally, the application of the proposed methods to the phenol-formaldehyde reaction studied in the previous chapters is presented. 

In this section, the phenol-formaldehyde reaction is introduced as a case study. This reaction has been chosen because of its kinetic complexity and its high exothermic-ity, which poses a strong challenge for modeling and control practice. The kinetic model presented here is adopted to simulate a realistic batch chemical process the identification, control, and diagnosis approaches developed in the next chapters are validated by resorting to this model. 

Because of the aforementioned circumstances, the loss of control of the phenol-formaldehyde reaction has been the cause of a number of severe incidents in chemical batch reactors during the last decades [12], These incidents have caused many injuries and, in the worst case, even fatalities among the plant operators. Other severe consequences have been the evacuation of residents in the surrounding area due to chemical contamination and a protracted stop in the plant production. 

For a safe operation, the runaway boundaries of the phenol-formaldehyde reaction must be determined. This is done here with reference to an isoperibolic batch reactor (while the temperature-controlled case is addressed in Sect. 5.8). As shown in Sect. 2.4, the complex kinetics of this system is described by 89 reactions involving 13 different chemical species. The model of the system consists of the already introduced mass (2.27) and energy (2.30) balances in the reactor. Given the system complexity, dimensionless variables are not introduced. 

The model-based controller-observer scheme requires to solve online the system of differential equations of the observer. The phenol-formaldehyde reaction model is characterized by 15 differential equations, and it is, thus, unsuitable for online computations. To overcome this problem, one of the reduced models developed in Sect. 3.8.1 can be adopted. In order to be consistent with the general form of nonchain reactions (2.27) adopted to develop the controller-observer scheme, the reduced model (3.57) with first-order kinetics has been used to design the observer. The mass balances of the reduced model are given by... 

Fig. 5.9 Safety boundaries for the phenol-formaldehyde reaction when the temperature reactor is controlled by using the model-based adaptive strategy and the PID controller...

In the case study, the adaptive model-based approach is designed on the basis of a reduced model of the phenol-formaldehyde reaction introduced in the Chap. 2. Noticeably, the results show that the model-based control scheme achieves very good performance even when a strongly simplified mathematical model of the reactive system is adopted for the design. 

Phenolics. These plastics allow the preparation of both random prepolymers, such as Baekelands A stage and true structopendant prepolymers, commonly known under the term novolaks (Figure 6). Novolaks permit one to take advantage of the newer prepolymer technology. Monomers are phenol, cresols, and formaldehyde. Molecular weights of the novolaks are between 300 and 700. Novolaks are obtained through careful selection of reaction conditions and catalysis of the phenol-formaldehyde reaction. Molecular weight, as well as the ratio of 2,2 - and 2,4 -links, can be controlled. These structural factors, studied extensively by Wood (28), have an eflFect on the physical properties of the cured polymer network. 

The resin reactor vessels and associated controls were not sufficient to minimize uncontrolled exothermic phenol-formaldehyde reactions. 

Older cook styles called for addition of phenol, formaldehyde, and water followed by alkali. Once the alkali was added, strict temperature control was the only barrier to a runaway reaction. A power or equipment failure at this point was likely to lead to disaster. Every batch made involved a struggle between the skill of the operator and capability of the equipment to control the exotherm versus the exothermic nature of the reactants. Most of the disasters that have occurred were due to utilization of this cooking method. 

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

The production of moisture resistant particleboard by treatment with a maleic anhydride -glycerol mixture and using phenol formaldehyde as the matrix material has been investigated (Fujimoto etal., 1987). Boards prepared from modified wood showed considerable improvements in modulus of elasticity and internal bond strength when compared to control boards. Composites made from aspen fibres modified with SA, MA or acetic anhydride using phenol-formaldehyde (PF) or polypropylene as binder have also been studied (Clemons etal., 1992 Rowell etal., 1993b). The reaction of wood with MA was found to proceed at a slower rate than with SA. The volume increase due to modification... 

In this section, the phenol-formaldehyde reactive system is considered as an example of identification of reduced kinetic models. The kinetic model containing 13 components and 89 reactions, developed in Sect. 2.4 to study the production of 1,3,5-methylolphenol, is too detailed and complex for control and monitoring purposes. Thus, in this section this model is referred to as detailed model, while four reduced kinetic models, based on lumped components and reactions, are developed. 

Important parameters were varied over relatively wide ranges producing particleboards with acceptable properties i.e., properties similar to those obtained using a phenol-formaldehyde resin as the control. Conditions used in both the liquid and gas phase reaction systems gave results shown by the examples in Table II. 

Copolymers of furfural with phenol or phenol-formaldehyde polymers have been available commercially for many years. Since the acid-catalyzed reaction of furfural and phenol has been difficult to control, most industrial applications involve the use of alkaline catalysts. Furfural-phenol resins are used for their alkali resistance, enhanced thermal stability, and good electrical properties compared to phenol-formaldehyde resins. 

The hydroxymethylation of phenols with formaldehyde is called the Lederer-Manasse reaction. This reaction must be carefully controlled, since it is possible for the para and both ortho positions to be substituted and for each of these to be rearylated, so that a polymeric structure 45 is produced. However, such polymers, which are of the Bakelite type (phenol-formaldehyde resins, 45), are of considerable commercial importance. 

The first fully synthetic polymer to be introduced (1909) was a phenol-formaldehyde resin known as bakelite. Although the reaction had been discovered some twenty years previously, it was only after a careful and systematic study of it that it was properly controlled to give a useful thermosetting resin. In the late 1920s two other types of thermosetting resins followed, namely urea-formaldehyde, and alkyd resins. 

It is recommended that patch tests are undertaken with the resin the patients actually handle themselves, in addition to those allergens available from commercial sources. If an individual brings their own resin, we test at 1% and 5% and in petrolatum and, if positives are identified, controls will need to be tested. I have not seen any false positives, but Kanerva et al. (1997) showed that phenol-formaldehyde resin tested at 1% and 5% in petrolatum could produce false-positive reactions on occasions. Hermal provides a Resol and Novalak resin at 5% for patch... 

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