tailieunhanh - Fundamentals of Polymer Engineering Part 5

Tham khảo tài liệu 'fundamentals of polymer engineering part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 4 Reaction Engineering of Step-Growth Polymerization INTRODUCTION 1 2 Vessels in which polymerization is carried out are called reactors they are classified according to flow conditions existing in them. Batch reactors schematically shown in Fig. are those in which materials are charged initially and polymerization is carried out to the desired time which in turn depends on the properties of the material required. We have already observed that polymerization is limited in batch reactors by equilibrium conversion. Because we wish to form polymers of high molecular weights we can overcome this limitation by applying high vacuum to the reaction mass. On application of low pressures the reaction mass begins to boil and the condensation product is driven out of the reactor as shown in Figure . Such batch reactors are called semibatch reactors. Batch and semibatch reactors are ideal when the production rate of the polymer needed is small. In larger-capacity plants continuous reactors are preferred. In these the raw materials are pumped in continuously while the products are removed at the other end. One example of these is a tubular reactor shown in Fig. . It is like an ordinary tube into which material is pumped at one end. Polymerization occurs in the tubular reactor and the product stream consists of the polymer along with the unreacted monomer. Sometimes a stirred vessel shown in Fig. is employed instead of a tubular reactor. The advantage of such a reactor is that the concentration and temperature variations 153 Copyright 2003 Marcel Dekker Inc. 154 Chapter 4 Figure Some ideal reactors. within it are removed due to vigorous stirring making it possible to control reactor conditions more easily. It may be mentioned that batch semibatch tubular and stirred-tank reactors serve as mere idealizations of actual reactors. Consider for example Copyright 2003 Marcel Dekker Inc. Reaction Engineering of Step-Growth Polymerization 155 the industrial .

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