Application of a catalytic membrane reactor to catalytic wet air oxidation of formic acid

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Abstract

A series of Pt-doped, tubular ceramic membranes were prepared by the evaporation–crystallization technique. The activity of these membranes was measured in the process of liquid-phase oxidation of aqueous formic acid solutions (0.2–10 g L−1), carried out in a semi-batch three-phase reactor system by using air or oxygen. An influence of trans-membrane pressure difference, reaction temperature, catalyst loading, and re-circulation rate on the extent of formic acid removal was measured in a wide range of operating conditions. A mathematical model that captures essential physics of the process, was developed to predict concentration profiles of reactants within the membrane wall and the thickness of reaction zone. The calculations show that the productivity of membrane contactor is influenced by the concentration of dissolved oxygen in the reaction zone as well as by the molar ratio of reactants, the latter being dependent on formic acid conversion.

Introduction

Wet air oxidation (WAO) involving oxidation at high temperature (398–593 K) and pressure (2–20 MPa) conditions can be applied for the treatment of toxic and non-biodegradable effluents [1]. Operating conditions employed make WAO highly energy-consuming process and are associated with corrosion, a problem that additionally affects the economy of this technology. Severe operating conditions employed in WAO can be somewhat reduced by the introduction of heterogeneous catalysts. Nevertheless, the main limitation of catalytic wet air oxidation (CWAO) lays in the diffusion of gaseous reactant to the solid catalyst particle as well as in the catalyst recovery and leaching phenomena [2]. To surpass these limitations, a catalytic membrane reactor (CMR) was recently suggested [3], [4] as a possible alternative to conventional reactors employed in CWAO. The term CMR describes a number of different types of reactor configurations that contain a membrane. Perez and co-workers [5] proposed classification of the membrane reactors based on the role of the membrane, categorizing CMRs as extractors, distributors and contactors. According to the suggested classification, the interfacial contactor mode CMR would be the most adequate for conducting CWAO processes [3]. When operating CMR as the interfacial contactor, the membrane provides a well-defined contact region between the gas- and the liquid-phase flowing on the opposite sites of the membrane, and serves as a support for catalytically active-phase deposited on its internal structure. As the gas–liquid interface is established within the porous membrane structure, the gas-phase can be supplied directly to the catalytic region [6], consequently increasing the concentration of gaseous reactant and improving conversion rates. This alternative way to contact the reactants and catalyst is a key advantage with respect to conventional reactors, as gaseous reactant does not have to diffuse through the liquid film to reach the catalyst surface. Furthermore, this reactor type offers also a better temperature control, enables independent variation of gas and liquid flow rates, reactant concentration, and pressure in a wide range of operating conditions. Membranes used in this reactor type are not required to be permselective, and by them one can avoid the problem of catalyst recovery that frequently appears in conventional slurry reactor.

Several investigations were devoted to develop a catalytic membrane contactor enabling stable long-term operations [3], [5], [10], [12], [13], and to optimize the operating conditions for conducting CWAO process in CMR [7], [8], [9], [11]. In these studies, the positive effect of elevated trans-membrane pressure difference on the performance of catalytic membrane reactor has been already demonstrated [8], [9], [11]. It was also indicated that external and/or internal mass-transfer resistances considerably influence membrane reactor performance, and that the diffusion path of gaseous reactant (i.e., the position of gas–liquid interface within the membrane wall) is the most important parameter concerning the employability of this reactor type in practice. Results of performed liquid-phase nitrite hydrogenation [8], [9] and formic acid oxidation runs [11] have further revealed that the conversion rate is on one hand determined by diffusion of nitrite ions (formic acid) from the bulk liquid-phase through the top (filtration) and intermediate layers to the reaction zone, and on the other hand by the amount of gaseous reactant present in the reaction zone. Experiments performed on catalytic oxidation of aqueous formic acid solutions and subsequent preliminary calculations concerning the thickness of reaction zone by means of a simplified model based on mass fluxes, also showed that only a part of deposited platinum took part in the reaction [11]. In addition, it was also discovered that the performance of CMR could be further improved by the use of a static mixer installed in the inner membrane compartment, or by operating the reactor in turbulent flow regime, implying that external mass-transfer limitations might also influence the productivity of membrane reactor [9]. To obtain further insight in the behavior of Pt-doped catalytic membrane reactor, additional oxidation experiments were conducted with various initial concentrations of formic acid using both air and oxygen as oxidizing agents. The objective of this study was also to develop a comprehensive mathematical model that predicts the thickness of reaction zone along with concentration profiles of reactants within the membrane wall.

Section snippets

Ceramic membranes

Commercially available Membralox® (Pall Exekia, Bazet, France) multi-layered ceramic membranes of inner diameter 7 mm, and outer diameter 10 mm were used in this study. Tested membranes with an overall porosity of 0.4, an average weight of 27.7 g on dry basis and a total length of 250 mm, were enameled at their ends to insure smoother surface on which O-ring seals were placed. As reported elsewhere [11], membranes were made of thick coarse TiO2 coated α-Al2O3 support, two intermediate layers of

Membrane characterization

A detailed characterization of the CAT-4 catalytic membrane by H2 pulse chemisorption, SEM-BSE and EPMA can be found elsewhere [11], therefore only short summary of obtained results will be presented here. H2 pulse chemisorption measurements showed that the dispersion of Pt ensembles in tested membrane (CAT-4) was equal to 5.4%, with the average active particle diameter of about 21 nm and the metallic surface area of 0.018 m2 g−1sample. Low values of Pt dispersion might be attributed either to low

Conclusions

The results reported in this work indicate that external and/or internal mass-transfer resistances as well as resistances due to chemical reaction considerably influence the performance of catalytic membrane reactor. It was found out for the reaction studied that in the case when the concentration of organic reactant in the liquid-phase is appreciably higher with respect to the equilibrium concentration of liquid-dissolved gaseous reactant, the overall reaction rate prevails to be determined by

Acknowledgements

This work has been carried out with financial support from the European Commission, under the FP5 Contract No. EVK1-CT-2000-00073 (project WATERCATOX), and the Slovenian Ministry for Higher Education, Science and Technology (program No. P1-0152). The authors would like to thank Pall Exekia (Bazet, France), an industrial partner involved in the WATERCATOX Project, for supplying ceramic membranes used in the present study. We also wish to thank other colleagues involved in this project: Drs. E.E.

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