(391f) Possibilities and Limitations of Static Mixers in Precipitating Environments

Static mixers are extensively applied in industrial environments to improve mixing and heat transfer. Being a continuous processing system, they offer higher productivity, safer operating conditions, reduced waste production and lower energy consumption. Static mixers have a small footprint, enable sharp residence time distributions, intensified mixing and isothermal processes and minimize byproducts in a broad range of applications in the pharmaceutical, food processing, chemical, pulp and paper, polymer synthesis, water treatment and petrochemical industries.

Fouling is a major engineering challenge in industrial chemical processes. Various types of fouling have been identified based on its diverse mechanisms: precipitation fouling, particulate fouling, corrosion fouling, chemical reaction fouling, solidification, biofouling and composite fouling. It is a complex process affected by a large number of variables. In general, fouling on a surface contains the following consecutive stages: Initiation, transport, attachment, removal and aging.

Static mixers used in harsh industrial environments (e.g., for purification of crystals via solvent exchange washing) undergo combined composite fouling, mainly determined by particulate fouling (i.e., sedimentation) and precipitation fouling (i.e., precipitation of solids out of solutions or suspensions due to the addition of an anti-solvent). The precipitation takes place either directly on a surface (i.e., static mixer) resulting in deposits that are difficult to remove or in the bulk liquid, which involves rather loose agglomerations that are easier to remove. As more depositions are created over time, the cross-sectional area is reduced, increasing the flow velocity. The depositions may be periodically removed, altering the fouling resistance progression from linear to asymptotic.

This work investigates the application of static mixers in harsh, precipitating environments. Technical opportunities and limitations of static mixer designs were evaluated for various equipment sizes and environmental conditions. It was found that, co-axial mixing of feed and anti-solvent in the static mixer by application of ultrasound resulted in the most stable process.