Phenol-Formaldehyde Reactions

Strong-Acid Catalysts, Novolak Resins. PhenoHc novolaks are thermoplastic resins having a molecular weight of 500—5000 and a glass-transition temperature, T, of 45—70°C. The phenol—formaldehyde reactions are carried to their energetic completion, allowing isolation of the resin ... 

Quinone dioximes, alkylphenol disulfides, and phenol—formaldehyde reaction products are used to cross-link halobutyl mbbers. In some cases, nonhalogenated butyl mbber can be cross-linked by these materials if there is some other source of halogen in the formulation. Alkylphenol disulfides are used in halobutyl innerliners for tires. Methylol phenol—formaldehyde resins are used for heat resistance in tire curing bladders. Bisphenols, accelerated by phosphonium salts, are used to cross-link fluorocarbon mbbers. 

The initial phenol-formaldehyde reaction products may be of two types, novolaks and resols. 

Phenol-formaldehyde reactions catalyzed by zinc acetate as opposed to strong acids have been investigated, but this results in lower yields and requires longer reaction times. The reported ortho-ortho content yield was as high as 97%. Several divalent metal species such as Ca, Ba, Sr, Mg, Zn, Co, and Pb combined with an organic acid (such as sulfonic and/or fluoroboric acid) improved the reaction efficiencies.14 The importance of an acid catalyst was attributed to facilitated decomposition of any dibenzyl ether groups formed in the process. It was also found that reaction rates could be accelerated with continuous azeotropic removal of water. 

Figure 7.22 Reaction pathways for phenol-formaldehyde reactions under alkaline conditions.

Base-catalyzed phenol-formaldehyde reactions exhibit second-order kinetics [Eq. (5)]. Several alkylphenols such as cresols also follow this rate equation ... 

TABLE 7.5 Relative Positional Reaction Rates in Base-Catalyzed Phenol-Formaldehyde Reaction... 

The reaction conditions, formaldehyde-to-phenol ratios, and concentration and type of catalyst govern the mechanisms and kinetics of resole syntheses. Higher formaldehyde-to-phenol ratios accelerate the reaction rates. This is to be expected since phenol-formaldehyde reactions follow second-order kinetics. Increased hydroxymethyl substitution on phenols due to higher formaldehyde compositions also leads to more condensation products.55... 

Phenol-formaldehyde reactions, 399, 380 base-catalyzed, 400-402 Phenol-formaldehyde resins, modified, 410-411... 

Although the condensation of phenol with formaldehyde has been known for more than 100 years, it is only recently that the reaction could be studied in detail. Recent developments in analytical instrumentation like GC, GPC, HPLC, IR spectroscopy and NMR spectroscopy have made it possible for the intermediates involved in such reactions to be characterized and determined (1.-6). In addition, high speed computers can now be used to simulate the complicated multi-component, multi-path kinetic schemes involved in phenol-formaldehyde reactions (6-27) and optimization routines can be used in conjunction with computer-based models for phenol-formaldehyde reactions to estimate, from experimental data, reaction rates for the various processes involved. The combined use of precise analytical data and of computer-based techniques to analyze such data has been very fruitful. 

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). 

Phenol—formaldehyde resins, 10 408—409 Phenol—formaldehyde reactions, 18 760 Phenolic acetates, 20 45 Phenolic adhesives, 1 543—544 Phenolic antioxidants, 10 806... 

EPA 1999a. How to Prevent Runaway Reactions, Case Study Phenol-Formaldehyde Reaction Hazards. EPA 550-F99-004. U.S. Environmental Protection Agency. August. 

Occasionally, the U.S. EPA issues a process safety alert or study that is related to chemical reactivity hazards. The following incident summaries are from a Case Study on phenol-formaldehyde reaction hazards (EPA 1999a) and from an Alert urging the use of multiple data sources when developing emergency response strategies (EPA 1999b). 

Resols are obtained by phenol-formaldehyde reaction under alkaline conditions. The phenoxide anion is the reactive species ... 

The chapter ends with a case study. Four different reduced kinetic models are derived from the detailed kinetic model of the phenol-formaldehyde reaction presented in the previous chapter, by lumping the components and the reactions. The best estimates of the relevant kinetic parameters (preexponential factors, activation energies, and heats of reaction) are computed by comparing those models with a wide set of simulated isothermal experimental data, obtained via the detailed model. Finally, the reduced models are validated and compared by using a different set of simulated nonisothermal data. 

Finally, the application of some of those criteria to the phenol-formaldehyde reaction gives some interesting insights on the thermal behavior of the system and also highlights the operation limits arising from an imposed maximum allowable temperature in the reactor. 

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. 

A case study, referred to the phenol-formaldehyde reaction model developed in the previous chapters, closes the chapter. 

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. 

The kinetic model developed in Sect. 2.4 for the phenol-formaldehyde reaction belongs to a wider class of kinetic networks made up of irreversible nonchain reactions. In this section, a general form of the mathematical model for this class of reactive systems is presented moreover, it is shown that the temperature attainable in the reactor is bounded and the lower and upper bounds are computed. 

According to the approach described in [11], two alternative reduced kinetic models are proposed here to describe the phenol-formaldehyde reaction network introduced in Sect. 2.4. This approach includes, first, the selection of a general class... 

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. 

Fig. 4.11 Safety boundaries for the phenol-formaldehyde reaction according to the runaway criteria of Morbidelli and Varma (7), Thomas and Bowes (2), and for an imposed maximum allowable reactor temperature Tr,ma=98°C(5)...

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. 

The effectiveness of the proposed approach has been tested in simulation by considering a jacketed batch reactor in which the phenol-formaldehyde reaction presented in Chap. 2 takes place. The complete system of differential equations given by the 13 mass balances presented in Sect. 2.4 has been simulated in the MATLAB/SIMULINK environment. 

The authors wish to thank Prof. M. Mattei and Dr. G. Paviglianiti, who collaborated to the development of some fault diagnosis schemes presented in the sixth chapter. Moreover, the authors are grateful to their former student G. Satriano, who helped in developing the model of the phenol-formaldehyde reaction. 

The progress of the reaction was followed by monitoring the decrease in formaldehyde concentration with time. Previous studies used the hydroxylamine hydrochloride method of analysis (5 -55), but this was avoided in the current study as it requires tedious pH titrations. Instead, a colorimetric method was used that was first developed by Nash (55), involving formation of 3,5-diacetyl-1,4-dihydrolutidine, by reaction of formaldehyde with ammonia and acetyl acetone at neutral pH. The cyclic product absorbs at 412 nm with a molar extinction coefficient of 8,000 (55). Other colorimetric methods cannot be used as they all involve very strongly acidic or basic media (55), which would force the phenol-formaldehyde reaction to completion. 

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