Tools and Methods for Pollution Prevention pp 205-231 | Cite as
Synthesis of Batch Processes with Integrated Solvent Recovery
Abstract
An integrated approach to the reduction of toxic waste in pharmaceutical and specialty chemical manufacturing is presented. The approach focuses on fundamental understanding and redesign of the relevant production processes.
Organic solvents are ubiquitous in the reaction and separation steps of pharmaceutical and specialty chemical processes. This typically leads to the formation of nonideal mixtures of solvents, and the resultant difficulties in separating solvent in pure form for recycle due to the presence of azeotropes. Solvent that is not recovered typically becomes toxic waste. This problem is exacerbated by the nature of manufacturing in these industries: large numbers of small volume products, short process development and product life cycles, and the use of multipurpose batch equipment.
Our approach exploits an understanding of the thermodynamic obstacles to solvent recovery created by the existence of azeotropes in a mixture to modify (or design) the mixtures formed in a process so that solvent recovery and recycling becomes feasible or is improved. We develop the notion of solvent recovery targeting:, given a stream composition, what is the sequence of pure component and azeotrope cuts that can be separated from this mixture using batch distillation, and what is the maximum feasible recovery in each cut? For an azeotropic mixture, this sequence and even the feasibility of recovering a particular pure component, is a strong function of the stream composition. An algorithm that predicts this limiting behaviour in a rapid automated manner based only on knowledge of the properties of the fixed points of the underlying nonlinear dynamic system is presented.
The synthetic component of our work observes that the presence of azeotropes imposes a structure on the overall composition space. This space is divided into a series of batch distillation regions, each of which is characterized by a different sequence of cuts achievable. Hence, we can design a stream composition by moving it from one region to another, hopefully identifying more environmentally favourable alternatives. We present a process wide geometric approach that will, given a set of candidate solvents and entrainers, choose solvents and design the mixtures formed in the process to maximize the potential for solvent recovery and recycling, subject to typical constraints such as reaction stoichiometry, solvation of reactions, etc.
Industrial examples illustrating solvent integration internal to a process, and integration across parallel processes in a multi-product facility are presented. In conclusion, outstanding issues and future directions for solvent recovery targeting are discussed.
Keywords
Pure Component Batch Process Solvent Recovery Separation Boundary Azeotropic MixturePreview
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