Date of Award
Doctor of Philosophy in Chemical Engineering - (Ph.D.)
Kamalesh K. Sirkar
Dana E. Knox
Crystallization was examined under a new perspective and in a flow environment much different from that available in currently used industrial devices. Three crystallization techniques were tested in the unique flow environment offered by hollow fiber devices. In addition, a new type of heat exchanger based on hollow fibers was tested as well as the potential use of porous hollow fiber devices as mixing devices. Hollow fiber devices are compact, extremely efficient on a volumetric basis, easy to scale up and control and their inherent characteristics promote the creation of homogeneous temperature and concentration conditions on a scale considerably smaller than existing industrial crystallizers without the necessity of a large energy input.
Porous hollow fiber heat exchangers (PHFHEs) were proven superior to conventional metal heat transfer equipment. For the liquid-liquid systems studied, they can transfer up to ten times more heat on a volumetric basis, achieve the same efficiency and number of transfer units at considerably smaller lengths; also, the height of a transfer unit achieved by them is 10-20 times smaller. In addition, they can transfer up to 20 times more heat at the same pumping power expenditure and need to utilize as low as 1 kPa for the achievement of one transfer unit compared to 30 kPa for metal heat exchangers. Considering their lower fixed cost, they can be considered suitable alternatives for metal and plastic heat exchangers at lower temperatures and pressures.
Solid hollow fiber cooling crystallization (SHFCC) was proved to be a promising technique for crystal size distribution control of both aqueous and organic systems. A combination of a solid hollow fiber crystallizer with a mixing device downstream was the most successful. For the aqueous potassium nitrate system, this combination provided crystal size distributions with 3-4 times smaller mean sizes compared to those mentioned in existing literature of Mixed Suspension Mixed Product Removal (MSMPR) crystallizers. In addition, 90% of the crystals produced were confined to sizes at least two times smaller, while the nucleation rates achieved were 2-3 orders of magnitude higher. Runs with aqueous paracetamol (4-acetamidophenol) solutions showed that an SHFCstatic mixer assembly can be operated successfully up to 30-40°C below the metastable zone limit, a capability not existent in industrial cooling crystallizers. This ability allows the achievement of very high nucleation rates and the decoupling of nucleation and growth, an opportunity offered currently only by impinging jet mixers for antisolvent crystallization.
Porous hollow fiber devices proved efficient mixing devices, which unlike other tubular devices offer the opportunity for substantial radial mixing and hence the production of good micromixing. By proper rating they can potentially be utilized for reaction purposes, especially for liquid-liquid reactions on a 1:1 stoichiometric ratio, a task never performed before in membrane reactors.
Porous hollow fiber emulsion crystallization (PHFEC) of a system with an immiscible solvent-antisolvent pair, salicylic acid in 1-octanol and water, encountered difficulties. While an emulsion of droplets smaller than 50 μm was obtained, crystallization at the droplet surface or interior was strongly hindered probably due to the presence of the emulsifier. The latter, although beneficial for droplet stabilization and size control, prevents contact of the solute and the antisolvent and consequently the generation of supersaturation conditions.
Polymeric hollow fiber antisolvent crystallization (PHFAC) was found to be a promising crystallization technique for miscible solvent-antisolvent pairs. When crystallization was performed at the tube side of the device, the crystal size distributions obtained for the system aqueous L-asparagine monohydrate and 2-propanol as the antisolvent were confined below 200 μm. However, prolonged operation of the membrane hollow fiber crystallizer was problematic due to pore and/or fiber blockage. The same was not true when crystallization was performed at the shell side of the device. Crystallization runs with the same system showed that, apart from stable operation, mean sizes as low as 30-40 μm can be achieved. The crystal size distribution was confined between 70 and 150 μm, a size range suitable for most pharmaceutical crystalline products and about 2-4 times smaller compared to 200-300 μm achieved in stirred crystallizers for the same system. In addition, 1-5 orders of magnitude higher nucleation rates were obtained at the same levels of supersaturation.
Zarkadas, Dimitrios, "Crystalization studies in hollow fiber devices" (2004). Dissertations. 686.