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Ind. Eng. Chem. Res. 2005, 44, 4042-4050
Reduction of the Residual Monomer and Volatile Organic Compounds by Devolatilization Rafael Salazar, Pedro Ilundain, Daniel Alvarez, Louis Da Cunha, Marı´a J. Barandiaran, and Jose´ M. Asua* Institute of Polymer Materials “POLYMAT” and Grupo de Ingenierı´a Quı´mica, Departamento de Quı´mica Aplicada, Facultad de Ciencias Quı´micas, The University of the Basque Country, Apartado 1072, 20080 Donostia-San Sebastia´ n, Spain
The performance of devolatilization for cleaning industrial latexes was investigated. It was found that both steam and air saturated with water were very efficient in volatile organic compound (VOC) removal, as well as in controlling the solids content. No dependence of the VOC removed fraction on the initial VOC concentration was observed. An increase in the stripping gas flow rate improved the devolatilization efficiency because the mass transfer from the aqueous phase to the gas phase, which is the controlling step of the elimination process, was enhanced. At constant pressure, an increase in the temperature led to a higher devolatilization rate because of the increase of the Henry’s law constant. No effect of the temperature when the system worked under boiling conditions was observed, likely because of the negligible change of the ratio of the Henry’s law constant over the water vapor pressure under the studied range of experimental conditions. On the other hand, it was observed that the devolatilization was not efficient enough to remove highly hydrophilic and low volatile compounds. Introduction Emulsion polymerization is the chief process used to produce a wide range of products such as paints, adhesives, binder for nonwoven fabrics, synthetic rubber, additives in paper and textiles, impact modifiers for plastic matrixes, and additives for construction materials.1 The development of this industry has been to a large extent due to environmental concerns, because of the substitution of solvent-based systems by waterborne products. The emulsion polymerization products, however, are not free from volatile organic compounds (VOCs) because the polymerization rarely proceeds to completion, and there is inevitably a level of unreacted monomer remaining in the polymer, generally in the range of a few thousand parts per million.1,2 Moreover, other VOCs result from impurities in the raw materials or byproducts formed from side reactions present in the final product. Both postpolymerization2-5 and devolatilization6-9 are used to reduce the residual monomer and VOCs from latexes.10,11 Postpolymerization consists of the addition of initiators after the end of the main polymerization process. This process is only able to reduce the residual monomer and may lead to the production of new VOCs by secondary reactions of the initiators used.12 When nonpolymerizable VOCs are present, devolatilization must be used. In the devolatilization, the latex is stripped using either steam or gas under vacuum conditions until acceptable contents of the residual monomer and VOCs are reached. The main advantage of this process is that both the monomer and VOCs can be removed. Devolatilization has been used for several decades at the industrial level. However, few scientific papers * To whom correspondence should be addressed. E-mail:
[email protected].
concerning this process have been published, and most of them were focused on poly(vinyl chloride) and styrenebutadiene rubber.13-18 The devolatilization of polymeric latexes is a complex process involving the diffusion of volatiles through the polymer particle to the particle surface, transfer from the polymer surface to the aqueous phase, diffusion through the aqueous phase, and transfer from the aqueous phase to the gas phase. Salazar et al.19 demonstrated that the mass transfer from the aqueous phase to the gas phase was the rate-determining step. Therefore, the process variables that increase the interfacial area between the aqueous phase and the gas phase, such as agitation, geometry of the sparger, or gas flow rate, would improve the devolatilization. Another way to improve the devolatilization efficiency is acting on the driving force, which is strongly affected by the thermodynamic equilibria of the VOCs between the different phases. In the devolatilization of polymeric latexes, the equilibria are expressed in terms of the partition coefficient of the different VOCs between the polymer particles and the aqueous phase (kpw) and in terms of the Henry’s law constant (H) for the partitioning of VOCs between the aqueous phase and the gas phase. In a previous work,19 the effect of the solids content on VOC removal was studied, observing that the efficiency of devolatilization was limited for compounds having high polymer particles-aqueous phase partition coefficients. The thermodynamic equilibrium is also affected by the temperature and pressure. Despite their importance, there is not a clear criterion for the selection of these variables. The system can operate under boiling conditions, with the temperature equal to the boiling point of the water at the operating pressure. This is the case of the flash operation and the devolatilization with steam as the stripping agent.14,17,20,21 When a gas saturated with water is used as the stripping agent, the system can also work at tempera-
10.1021/ie050151g CCC: $30.25 © 2005 American Chemical Society Published on Web 04/20/2005
Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005 4043
Figure 1. Scheme of the devolatilization equipment when air saturated with water is used: (1) flowmeter; (2) thermostatic bath; (3) saturators; (4) heating mantle; (5) condensation trap; (6) vacuum pump. Table 1. Initial Concentrations of VOCs in the VAc/BA/ AA Postpolymerized Latex monomers and VOCs
concn (ppm)
monomers and VOCs
concn (ppm)
VAc BA n-butanol acetaldehyde
2850
