| dc.description.abstract | Increased focus on continuous synthesis of fine chemicals and pharmaceuticals has led to resurgence of process intensification. In this thesis we introduce a new continuous contactor. It works on the principle of impinging fine drops of one liquid, produced by a spinning disc, on thin flowing film of another liquid, on inner wall of a spinning bowl, and sweeping the mixture away from the impingement zone. The device offers the (largest) length scale of segregation itself equal to film thickness, intense mixing, and substantially reduced back-mixing.
The device is first demonstrated for the synthesis of drug nanoparticles of curcumin by using anti-solvent precipitation route. The SEM measurements confirm the device can produce spherical nanoparticles of curcumin below 100 nm in continuous mode of operation, free from choking in corresponding microfluidic operations. Experiments with the diagnostic iodide-iodate competing parallel reaction combined with a model yield time scale of micro-mixing of 4 ms. High resolution static images of the inner surface of the transparent bowl reveal perfect wetting by organic liquids and poor wetting by water as an explanation of the poor mixing observed at low rotational speeds. Several bowl geometries with tapered cylindrical wall and film flowing downward and upward are studied.
The drop formation from a spinning disc at low flow rates is studied in using high resolution static imaging by controlled duration of flash light. The images at high disc speeds also capture vivid detail, which allow primary and secondary drops to be measured separately, without ambiguity, for the first time. The size ratio of primary to the first secondary drops lies in range 2.2-2.6, and 6-9% of the incoming liquid appears as first secondary drops. The polydispersity of primary drops alone is far smaller than when all the drops are considered together. The mean size of primary drops, independent of flow rate, shows inverse dependence on disc speed raised to power one, while that of secondary drops shows much weaker dependence. Even if the surfaces of the discs of different wettability are not fully covered with liquid, the sizes of drops released are nearly same, expect at disc speeds below 1000 rpm. A chemically treated hydrophobic stainless steel disc produces distinctly large size drops, produced by flinging away of liquid rivulets beyond disc edge.
The narrowly distributed primary drops show no correlation with the large range of liquid thread lengths that produce them from at their ends. The mean size of the bulge attached to the disc shows close correlation though. The non-uniformity in circumferential distribution of droplets, which impacts the contactor performance adversely, is quantified through a new parameter named maldistribution index (MI). The discs with low contact angles such as filter paper disc exhibit more uniform distribution, than a SS disc.
The high speed measurement of time gap between two consecutive releases of primary drops from a single site showed emission to be chaotic, and for the whole disc to follow Poisson statistics. The drop formation process, with zero slip between drop and disc which is at variance with ligament mode of drop formation, however follows a well-defined sequence of stages in scaled time. A power law relation between necking time and rotational speed is proposed.
From among a large number of engineered discs with edge and surface modifications that are tried, discs with serration are found to offer significant control on mono-dispersity of size and uniformity of circumferential distribution at high speeds. A circular disc with fine grooves emerged as the best to produce a stable film on it at all rotational speeds and flow rates, with excellent circumferential distribution of drops as well.
An already established competing mixing platform created by impingement of two free liquid jets in air was studied for pattern of collisions, micro-mixing, and silver nanoparticle synthesis using a green protocol. Under similar operational conditions a spinning disc contactor yields larger and more polydispersed nanoparticles while the spinning disc spinning bowl contactor proposed here produces particles of nearly same size and polydispersity. The latter can be scaled up as well as a single unit. | en_US |
