Because of their better chemical resistance and fouling characteristics, plastic heat exchangers are of increasing interest for lower temperature applications. However, their lower thermal performance compared to that of metal heat exchangers has prevented their widespread use and acceptance. To overcome this constraint, polymeric hollow fiber heat exchangers (PHFHEs) are proposed as a new type of heat exchanger for lower temperature/pressure applications. In polypropylene-based PHFHEs, the overall heat-transfer coefficients achieved here, 647-1314 and 414-642 W m -2 K -1 for the water-water and ethanol-water systems, respectively, are comparable with accepted design values for metal shell-and-tube heat exchangers; further, for 20% of our water-water runs, it was higher than any value reported for plastic heat exchangers. The extremely large surface area/volume ratio of PHFHEs makes them more efficient than metal heat exchangers. Devices less than 30 cm (1 ft) long yielded efficiencies of up to 97.5%, up to 3.7 number of transfer units (NTU) and a height of a transfer unit (HTU) as low as 5 cm; the latter is 20 times less than the lower limit for shell-and-tube exchangers and 10 times less than the typical values for plate heat exchangers. PHFHEs achieve conductance/volume ratios 3-10 times higher than shell-and-tube devices accompanied by low-pressure drops, as low as 1 kPa/NTU, compared to 30 kPa/NTU for metal heat exchangers. Considering the much lower cost, weight, and elimination of metal contamination, PHFHEs can substitute metal heat exchangers on both thermal performance and economical grounds.

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Solid hollow fiber cooling crystallization (SHFCC) is a new technique suitable for crystal size distribution (CSD) control of aqueous/organic systems from bench to industrial scale. Solid hollow fiber devices possessing good temperature control and operational flexibility can serve as seed/nuclei generators, stand-alone crystallizers, or supersaturation creation devices. Operating schemes implemented include feed recycling, once-through, and solid hollow fiber crystallizer−completely stirred tank (SHFC−CST) in-series operation mode. The performance of SHFCC of aqueous KNO3 solutions was assessed against mixed suspension mixed product removal (MSMPR) crystallizer data. Feed recycling and the once-through operation modes were similar and characterized by broader CSDs and lower reproducibility due to generation of a large number of fines causing slow filtration and localized growth on the filters. SHFC−CST in-series operation proved successful yielding narrow and reproducible CSDs with mean sizes between 100 and 150 μm, 3−4 times lower than those of MSMPR crystallizers. Also, 90% of the crystals were smaller than 370 μm compared to 550−600 μm for MSMPR crystallizers. Further, the number of crystals generated per unit volume was 2−3 orders of magnitude higher. SHFC−CST in-series crystallization of salicylic acid from ethanol solutions clearly showed that, unlike other membrane hollow fiber based crystallization techniques, SHFCC does not suffer any performance loss for organic systems.

Metallic shell-and-tube heat exchangers used in thermal desalination require huge capital investments, suffer from corrosion/erosion, create heavy metal pollution, and display a large footprint. This paper has explored their potential replacement by polymeric solid hollow fiber-based heat exchangers. Using solid hollow-fibers of polypropylene (PP) (wall thickness 75 µm, outside diameter 575 µm) a number of heat exchangers were fabricated in the laboratory (heat exchange area, 195-960 cm 2 ) and at a commercial manufacturing facility (heat exchange area, 0.15-0.44 m 2 ). The heat transfer performances of these devices were studied for a hot brine (4% NaCl, ca. 80-98 °C)-cold water (8-25 °C) system as well as for a steam (101-113 °C)-cold water (8-25 °C) system; these systems are typically encountered in thermal desalination plants. Overall heat transfer coefficient values of as much as 2000 W/(m 2 K) were achieved. This is close to the limiting value imposed by the PP wall thickness, namely, 2660 W/(m 2 K). Heat exchangers built out of solid poly(ether ether ketone) (PEEK) fibers performed almost as well. Higher heat transfer coefficients were obtained by using porous asymmetric polyethersulfone hollow fibers whose internal diameter was coated by two consecutive layers of polyamide and silicone to make them impervious to moisture. A mathematical model has been developed to describe the solid hollow fiber heat exchanger performance and was proven a good predictor of heat transfer performance in such devices. Compared to metallic exchangers, these heat exchangers weigh much less, have an order of magnitude larger surface area per unit volume, and are likely to be considerably cheaper. Small polymeric heat exchange devices having an effective length less than 30.5 cm (∼1 ft.) achieve efficiencies close to 1, provide NTU values close to 4, and have HTU values as low as 5 cm. Further their conductance/volume values are as much as 2-15 times larger than metal heat exchangers. In addition, these devices have low flow pressure drops as low as 1 kPa/NTU compared to 30 kPa/NTU in conventional metal heat exchangers.

We examine here cooling crystallization of aqueous paracetamol solutions in hollow fiber devices. It is shown that a solid hollow fiber crystallizer (SHFC)-static mixer assembly can be operated successfully up to 30-40°C below the metastable zone limit. Such a capability is not existent in industrial cooling crystallizers; it allows the decoupling of nucleation and growth, an opportunity offered currently only by impinging jet mixers for antisolvent crystallization. In addition, it leads to the achievement of very high nucleation rates and hence small crystal sizes. The former were 2-4 orders of magnitude higher than values previously obtained in solid hollow fiber crystallizers for potassium nitrate and salicylic acid and reached values encountered only in impinging jet crystallization. A qualitative comparison with existing literature data showed that the SHFCstatic mixer combination confined the crystal size distribution (CSD) to smaller sizes and a narrower range. Finally, a linear relationship between the mean crystal size and the cooling medium temperature was observed. This relationship is indicative of the simplicity available in SHFCs vis-à-vis CSD control. IntroductionCrystallization and precipitation processes are often the most critical step in the separation, purification, and production of active pharmaceutical ingredients (APIs) and fine chemicals. The most important product characteristics such as crystal size distribution (CSD), crystal shape, and habit are solely determined by the operating conditions during the crystallization step (i.e., temperature, rates of supersaturation generation and depletion, mixing, and agglomeration/breakage phenomena). Good control of the above parameters can yield a product of acceptable and reproducible quality. In addition, downstream processing such as filtration and drying can benefit significantly in terms of robustness, operating simplicity, and cost-effectiveness from a well-designed and controlled crystallization process.Cooling crystallization from solution is probably the most frequently employed crystallization technique in the pharmaceutical industry. It is carried out in stirred vessels operated in a batch or continuous mode. The former is by far the predominant practice in the pharmaceutical industry; the latter is mainly employed in the production of inorganic chemicals. 1 Despite the fact that industrial crystallizers can be successfully operated, there is often considerable variability in the principal process outcome, the crystal size distribution. Moreover, they cannot in general meet the targets of a narrow CSD and a small mean crystal size, often desirable in the production of pharmaceuticals and specialty chemicals, due to imperfect mixing and the resulting non-uniform supersaturation conditions inside the crystallizer. Imperfect mixing is an inherent characteristic of industrial crystallizers whose performance is often different from that obtained at laboratory scale and frequently characterized by segregation effects. 2 Imperfect mixing is a...