Continuous heat-exchange reactor

N., Cabassud, M., Douglas, C., and Demissy, M. (2008) Dynamic behaviour of a continuous heat exchanger/reactor after flow failure. Int.J. Chem. React. Eng., 6 (A23), Available at http //nnnv.bepress.com/ijcre/vol6/A23. 

M. (2008) Evaluation of an intensified continuous heat-exchanger reactor for inherently safer characteristics. J. Loss Prev. Process Ind. 21 (5), 528-536. 

Figures 8.6 and 8.7 respectively show the operating temperature profile in a continuous heat-exchange reactor and in an adiabatic fixed-bed reactor. The operating line in Fig. 8.6 is based on the heat balance, t2 = to + (14—15)Ay x 100%, where to is the entrance temperature, <2 is the outlet temperature. Ay is the net value of ammonia.

A particular feature of the Chart-flo is that it was developed at the outset for applications which could encompass reactions, as well as pure heat transfer duties, in its Chart-pak and other forms (see Chapter 5). As a heat exchanger reactor the scope of application becomes very wide, including continuous chemical reactions, fuel cells and other reforming applications. The compactness (see below) implies that it could be an integral part of many intensified processes, and a study of its implications for reducing the size of absorption cycle refrigeration plant has illustrated the benefits of multi-stream, multi-pass and multi-functional use within a single module. 

Thus, the mathematic models for reactor design are also classified into continuous heat exchange bed and adiabatic one. Usually, the design of reactor adopts one-dimension quasi-homogeneous model which considers that when reactive gas passes the catalyst bed like a plug-flow, there exist no radial and axial return mixture, and microkinetics can be treated in intrinsic kinetics multiplied by an effective factor that involves the effects of transfer processes, and by an active coefficient that involves the effects of reduction, poisoning and declining etc. Macrokinetics can be... 

Regardless of the configuration of ammonia converters and their diameters, there are only two modes of heat extraction from the catalyst bed in situ heat-exchange or external heat-exchange. The former is called fixed bed with continuous heat exchange the reactor has only one bed housing all the catalysts, and is called a cold-type reactor (Fig. 8.6). The latter is called an adiabatic fixed bed reactor the reactor consists of two or more beds of catalyst, with heat exchange between the beds (Fig. 8.7). 

With a batch process, such as hot isostatic compaction (HIP), heat exchange as used in a continuous reactor is not possible, and it is common practice to provide a furnace within the pressure vessel which is thermally insulated to ensure that the temperature of the vessel does not rise above 300°C. Most HIP operations involve gas pressures in the range 70—200 MPa (10—29,000 psi) and temperatures of 1250—2000°C, occasionally 2250°C (74). The pressure vessel may have a bore diameter from 27 to 1524 mm (75) and is nearly always provided with threaded closures sealed with O-rings made of elastomer provided the temperature is low enough. 

Reaction times can be as short as 10 minutes in a continuous flow reactor (1). In a typical batch cycle, the slurry is heated to the reaction temperature and held for up to 24 hours, although hold times can be less than an hour for many processes. After reaction is complete, the material is cooled, either by batch cooling or by pumping the product slurry through a double-pipe heat exchanger. Once the temperature is reduced below approximately 100°C, the slurry can be released through a pressure letdown system to ambient pressure. The product is then recovered by filtration (qv). A series of wash steps may be required to remove any salts that are formed as by-products. The clean filter cake is then dried in a tray or tunnel dryer or reslurried with water and spray dried. 

More recent process research aimed at anionic PS is that of BASF AG. Unlike the Dow Process, the BASF process utilizes continuous linear-flow reactors (LFR) with no back-mixing to make narrow polydispersity resins. This process consists of a series alternating reactors and heat exchangers (Fig. 22). Inside the reactors, the polymerization exotherm carries the temperature from 30°C at the inlet to 90°C at the outlet. The heat exchangers then take the temperature back down to 30°C. This process, which requires no solvent, results in the formation of narrow polydispersity PS. 

Reactors may be operated batchwise or continuously, e.g. in tubular, tubes in shell (with or without internal catalyst beds), continuous stirred tank or fluidized bed reactors. Continuous reactors generally offer the advantage of low materials inventory and reduced variation of operating parameters. Recycle of reactants, products or of diluent is often used with continuous reactors, possibly in conjunction with an external heat exchanger. 

In recent years there has been a continued interest in the use of alkali metals, notably sodium and lithium, as heat exchange media in nuclear reactors and fusion systems respectively and as chemical reactants in fuel cells. This interest is reflected in the proceedings of several major conferences which are referenced in the bibliography (see p. 2.109). 

The flow diagram of the enzyme reactor for continuous production of the L-amino add is given in Figure A85. The acetyl amino add is continuously charged into the enzyme column through a filter and a heat exchanger. The effluent is concentrated and the L-amino add is crystallised. The acyl-D-amino add contained in the mother liquor is racemised by heating in a racemisation tank, and reused. 

In case of exothermic reactions, the heat-exchange capacities of the reactor allow to rapidly evacuate the heat generated by the reaction and therefore to perform a transposition of a pure batch operating mode into a continuous one. The main point is the ability to avoid, as far as possible, an initial increase of the temperature as soon as the reactants are mixed. 

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