Wetting and separation efficiency of polypropylene packing modified by nanoparticles.

Abstract

Packed columns are one of the main unit operations used in distillation, extraction, absorption and hydrotreating due to their ease of operation, versatility and adaptability to different chemical systems. The distribution of gas and liquid over the packing material is the basis of separation. Selection of the type of packing material is a crucial step in column design as the separation efficiency of the column is dependent on the ratio of wetted area to the total available surface of packing, known as the wetting efficiency. Glass and metal packing offer superior wetting efficiency when compared to random packing fabricated from plastic. Plastic packings offer better chemical resistance in selected systems, as well as being more lightweight and cost-effective. Literature indicates that the wetting behaviour of glass, metal and polymeric substrates may be modified by applying multilayer coatings of nanoparticles. These nanoparticles are often silica based. In this work, polypropylene random packing was modified by first being treated with the Piranha solution and then being coated by silica nanoparticles that were produced via the Stober Process. The first part of this project investigates the employment of a stimulus response technique in which an inert salt tracer is injected into an inlet liquid stream, pumped by a peristaltic pump, and allowed to flow over the packing material. The packing material being investigated in this study are glass, unmodified and modified polypropylene Raschig rings. The residence time distribution, reported as the mean residence time (MRT), and exit age distribution were determined for the three types of packing used in this study. By comparison to standard distribution curves obtained from the literature, the experimental exit age distribution curves were used to estimate the wetting efficiency of the different packing. The glass and unmodified packing had a MRT of 12 seconds while the modified packing was 19 seconds. The wetting efficiencies were 0.3, 0.4 and 0.8 respectively. The increase in MRT indicates that fluid elements resided in the column packed with modified polypropylene for a longer period while the increase in wetting efficiency shows clear improvement in wettability for the modified packing. For the second part of the study the absorption performance of the different packings was investigated. A system of water and carbon dioxide was selected to be used in the study as it is a very simple, non-toxic system and performance can be analysed using the titration method. For the 280 mm and 90 mm packed height with a 16 % carbon dioxide inlet concentration, modified packing showed improvements on absorption for all liquid flowrates for up to 10.24 % and 9.36% respectively when compared to the unmodified packing. The silica nanoparticle modification to the polypropylene packing was successful as overall, it performed better than the unmodified packing in absorption performance