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Process intensification is a vibrating topic in the field of chemical engineering. Its aim is to develop processes and methods with increased performance for a given chemical transformation at decreased energy consumption and waste production. Among all methods of process intensification, the microreactors are very promising (chapter I). In the case of fast gas-liquid reactions, microreactors allow to speed up the physical processes. Due to high surface to volume ratio, high mass and heat transfer performances can be achieved compared to conventional processes, leading to process intensification. In the present work (chapter III), the flow patterns and the mass transfer were studied in glass microreactors for CO2-water based systems. Volumetric mass transfer coefficients up to 9 s-1 were measured for slug flow regime in microreactors with hydraulic diameters of 400 µm. These values are at least one order of magnitude higher than for conventional contactors. Based on the experimental results, empirical correlations were developed to predict the flow regimes and the mass transfer performance. The obtained results enable a rational design of microreactors for fast gas-liquid transformations. In the case of slow chemical reactions, high pressure and temperature (high P&T) can be used to speed up the intrinsic kinetics. The microreactors offer the opportunity to perform reactions under harsh conditions due to excellent control of process parameters. The potential of unusual operating conditions (high P&T) was investigated to reduce the reaction time and increase the specific productivity of the aqueous Kolbe-Schmitt synthesis of beta-resorcylic acid (chapter IV). Based on a kinetic and thermodynamic model, the optimal operating window for the reaction was defined. The model prediction was successfully validated with a new micro-plant operated under high pressure and temperature. Moreover, a scale-up strategy was proposed using Sulzer milli static mixers. The new process allows to synthesise beta-resorcylic acid with 100% selectivity, to increase the performance by 2 orders of magnitude as compared to conventional batch process and to reduce by a factor of 4.2 the consumption of the costly reagent KHCO3. The same strategy was further applied to intensify the ethoxylation of dodecanol, an important industrial reaction (chapter V). As ethylene oxide processes have inherent safety vulnerability, an extensive calorimetric study was carried out under high P&T. For the first time, solubility and kinetic models were established up to 523K and 50 bar. Simulations were performed to predict the productivity of a new milli-plant based on Sulzer milli static mixers. It was demonstrated that, using a multi-injection reactor, the specific productivity can be increased 6 times as compared to conventional semi-batch reactors.