From discontinuous to continuous : a CFD study

In a batch process, raw materials are charged before the processing and the products are discharged afterward; on the other hand, continuous processes can be conducted virtually without any interruption since both the reactants and the products flow continuously in the reactor. Normally, the fine chemical industry (polymers and pharmaceuticals) relies on batch processes for their simplicity and their flexibility as multi-purpose systems. Nevertheless, in the last few years, there is a tendency to shift to continuous processes which allow to gain several advantages over discontinuous systems, such as reduced volumes, higher productivities, automatization with simpler control, and, most importantly, constant quality of the produced chemicals. Among continuous reactors, tubular reactors guarantee a better heat exchange than continuously stirred tank reactors (CSTRs) due to higher heat transfer area-to-volume ratio. This characteristic allows tubular reactors to operate in more severe reaction conditions and to obtain a larger productivity, keeping the process safe and at a constant quality. This is generally the case when the tubular reactor operates as a plug-flow reactor (PFR). The goal of this study is to offer a series of solutions to perform in continuous reactors with a PF behavior, processes that are normally led in a discontinuous fashion.

In un processo discontinuo, i reagenti vengono caricati nel reattore e i prodotti scaricati al termine della reazione; al contrario, in un processo continuo i reagenti e i prodotti fluiscono senza interruzione all'interno del reattore. Nonostante i reattori discontinui siano ampiamente utilizzati in settori come l'industria farmaceutica, i processi continui possono garantire diversi vantaggi, come una produttività più elevata, apparecchiature di volume più ridotto e un migliore controllo sulle specifiche finali del prodotto. Tra i reattori chimici, i plug flow reactors (PFR) possono essere operati in condizioni più severe rispetto ai continuously stirred tank reactors (CSTR) grazie ad rapporto superficie-volume più elevato; inoltre, i PFR sono in grado di garantire una maggiore produttività rispetto ai CSTR. Lo scopo di questa tesi è presentare una serie di soluzioni per condurre in reattori continui, e con caratteristiche simili ad un PFR ideale, processi industriali che vengono generalmente condotti in reattori discontinui.