Modular Concept of a Smart Scale Helically Coiled Tubular Reactor for Continuous Operation of Multiphase Reaction Systems

Abstract

Process intensification (PI) via microstructured devices has often been applied by research and development (R&D) and industry for a decade, as they offer a large specific surface area that enhances the mass and the heat transfer. Helically coiled tubular reactors (HCTR) in micro-scale can further increase the performance in terms of transport phenomena, as the secondary flow (Dean vortices) enhances the radial mixing along the tube. Therefore, a narrow residence time distribution (RTD) that is required for the operation of complex chemical reaction systems can be achieved for the long residence times (RTs) at laminar flow regimes In this study, the continuous precipitation of calcium carbonate (CaCO3) was investigated by using a smart scale HCTR, i.e. modular coiled flow inverter (CFI) made of polyvinyl chloride (PVC) tubes (di = 3.2 mm). Modular CFI consists of 90° bends connecting the helical coils in order to enhance the radial mixing further. For precipitation process calcium hydroxide (Ca(OH)2) solution and gaseous CO2/synthetic air mixture were contacted prior to the reactor inlet via a Y-mixer. Slug flow profile was maintained and CaCO3 was precipitated along the reactor tube. To avoid further reaction of CaCO3 with water that is saturated with CO2 (pH ≲ 8.6), which promotes the consecutive parallel reaction forming soluble calcium bicarbonate (Ca(HCO3)2), the RT of the reactor was easily varied by changing the tube length of the modular CFI. Precipitated CaCO3 particles with a conversion of ca. 90% were separated from the suspension by vacuum filtration. Influence of volumetric flow ratio of the gases (R = V̇CO2/V̇air) and the RT were investigated on the precipitation process at constant flow rates. A comparison is presented between a batch reactor and a modular CFI. Results showed that narrower particle size distribution (PSD) with median particle diameters (d50,2) around 28 μm and more uniform morphology can be achieved by using a CFI for the continuous production of the powders.