A practical methodology is proposed to assess the feasibility of converting a batch process into a continuous one. Simple guidelines are illustrated to facilitate decision making at different stages of the evaluation, in particular the swift first decision either to proceed with or kill the idea at the early evaluation stage to avoid wasted effort. The ordered approach also provides a whole process assessment and decision making for the appropriate choice of continuous or hybrid processing mode. Three multistep processes that have been operated at kilogram scale are presented to demonstrate the application of this methodology. ■ INTRODUCTIONFine and specialty chemicals industries are looking into new ways/technologies for step-change improvements in process performance to sustain their businesses in the face of global competition, tighter safety and environmental regulations, as well as growing demands in product quality and cost efficiency. Batch and fed-batch (semibatch) processing are widely used in manufacturing because of their flexibility 1 and easy reconfigurability for multiple different sequential operations.In the pharmaceutical sector, due in part to tight regulatory control on the product and process after it is licensed, the industry tends to stick to the batch mode with reluctance to change. 2 The batch manufacturing chain consisting of multiple consecutive processes is normally contracted or segmented and carried out in different facilities. This raises the issue of quality guarantee and management of the process. Complex scheduling and sequencing of operations 3 may result in inefficient utilisation of equipment. In addition, the equipment used in batch processes is itself often inefficient; for example, stirred tank reactors suffer from mixing and heat transfer limitations.Continuous processing has been claimed to enable a promising new business model that could radically improve quality control, decrease scale-up issues and cycle time, allow for faster release of new products, increase energy efficiency, reduce waste and process inventories, develop a safer process and provide better process control. 4−8 Studies have shown that perhaps 30−50% 1,9 of the current batch processes can be operated in continuous mode and offer worthwhile benefits over the batch mode. The technology has matured as a tool that is now routinely used by many chemical synthesis laboratories and increasingly in process development and scale-up. 10 Regulators have also been encouraging the use of this technology to improve manufacturing efficiencies. 11 While isolated examples 8 have been visible for a long time, overall the implementation has been slow. 12 The industry is cautious in adopting this technology unless they can see clear techno-economic benefits. 13 The starting point for the process design in this low tonnage sector (pharmaceutical and speciality chemicals) is the presumption of a batch process. Development laboratories and protocols are usually predicated on this. A company may wish to explore a continuous ...
The effects of shear stress and shear experience within a spinneret during hollow fiber spinning on membrane morphology, gas separation performance, and thermal and mechanical properties have been experimentally determined. We purposely spun the hollow fibers using a wet phase inversion process and water as the external coagulant with the belief that the effect of gravity (elongational stress) on fiber formation can be significantly reduced and the orientation induced by shear stress within the spinneret can be frozen into the wet-spun fibers. In addition, we chose 80/20 NMP/H2O as the bore fluid with a constant bore fluid to dope fluid flow rate ratio in order to minimize the complicated coupling effects of elongational stresses, uneven internal and external solvent exchange rates, and substructure resistance on fiber formation and separation performance. Asymmetric hollow fibers for gas separation were spun from a 37% poly(ether sulfone) (PES)/N-methyl-2-pyrrolidone (NMP) dope solution using a spinneret with a L/ΔD (die length to flow channel gap) ratio of 17.5 that is much higher than the conventional spinneret. Experimental results suggest that hollow fiber membranes spun from this large L/ΔD die with high shear have a tighter molecular packing structure and therefore a higher selectivity that surpasses the intrinsic value but a lower permeance. For example, the selectivity of H2/N2 for fibers spun with high shear rate is 4-fold of the PES intrinsic value (292−307 vs 73.7). Hollow fibers spun from high shear have a lower coefficient of thermal expansion (CTE) and a higher loss modulus. Most surprisingly, we are not able to identify the nodular structure that has been observed previously in the as-cast flat membranes or at the outer skin of the hollow fibers spun from the spinneret with a small L/ΔD ratio. Clearly, the fully developed high shear stress within the spinneret has altered the thermodynamics of nodular formation, and the nodules either might not exist or become too small to be detected or deform into ambiguous elliptical shape. For the first time, we have also observed a threadlike inner skin structure in high-sheared membranes. In addition, the apparent dense layer thickness for the fiber spun with low shear is the thinnest that has ever been reported in the literature for hollow fiber membranes (450 Å).
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