Multitubular fixed-bed reactors

So far, consideration has been limited to chemistry physical constraints such as heat transfer may also dictate the way in which reactions are performed. Oxidation reactions are highly exothermic and effectively there are only two types of reactor in which selective oxidation can be achieved on a practical scale multitubular fixed bed reactors with fused salt cooling on the outside of the tubes and fluid bed reactors. Each has its own characteristics and constraints. Multitubular reactors have an effective upper size limit and if a plant is required which is too large to allow the use of a single reactor, two reactors must be used in parallel. 

Since in most cases the task of the heat transfer cycle is to maintain the temperature in the fixed bed within a specific range, this concept is frequently described as an isothermal fixed-bed reactor . Since isothermal reaction control does not always provide optimum selectivity or yield, the concept of heat exchangers integrated in the fixed bed is also being increasingly used to achieve specific nonisothermal temperature profiles. The most common arrangement is the multitubular fixed-bed reactor, in which the catalyst is arranged in the tubes, and... 

Figure 1. Basic types of catalytic fixed-bed reactors. A) Adiabatic fixed-bed reactor B) Multitubular fixed-bed reactor.

The multitubular fixed-bed reactor (Fig. IB) constitutes the oldest and still predominant representative of this class. The catalyst packing is located in the individual tubes of the tube bundle. The heat-transfer medium is circulated around the tube bundle and through an external heat exchanger, in which the heat of reaction is supplied or removed ( Fig. 16). Whereas with endothermic reactions circulating gas can be used as heat transfer medium, for strongly exothermic reactions exclusively liquid or boiling heat transfer media are used. Only in this way can the catalyst temperature (c.g., in the case of partial oxidations) be held in the narrow temperature range necessary for selective reaction control. 

A Stankiewicz, Advances in Modelling and Design of Multitubular Fixed-Bed Reactors , Chem Eng Tech 1989, 12, 113-130, 170-175... 

Both the partial oxidation to the product and the combustion to carbon oxides are strongly exothermic. The technical solution devised to prevent hot spots and dissipate the heat consists of a multitubular fixed bed reactor cooled by a circulating molten salts fluid. The production of steam is well above the needs of the plant and the site location is often dictated by the possibilities for heat utilization. 

An interesting variation on this is practised by Shell. In recent years, probably also helped by the sharp rise in the world oil price, there has been renewed interest in the F-T route to high quality diesel fuel. The Shell F-T plant in Malaysia which came on stream in 1993 uses multitubular fixed bed reactors to produce long chain hydrocarbon waxes by a LTFT process (over a promoted cobalt catalyst). These waxes are then selectively cracked over zeolites to give the desired shorter chain molecules suitable as diesel fuel. Since F-T hydrocarbons are predominantly linear they are not suitable for petrol engines, but are ideal for diesel. 

Some of the key attributes of the multitubular fixed-bed reactor are provided in Table 1. The corresponding attributes of the fluidized-bed reactor, which is an alternative reactor type for carrying out oxidation reactions, are also included for comparison. 

The structure of a slurry reactor is much simpler than that of a multitubular fixed-bed reactor and so it is about 45% cheaper to build. Since the slurry phase is an excellent heat exchange system, there is no need, as in the case of fixed-bed reactors, to recycle a large portion of the efffluent gas in order to obtain both a high conversion (fresh feed basis) and good temperature control. The operating cost of a slurry reactor is hence lower. If the objective is the production of high yields of FT waxes, then the slurry system appears to be a better proposition. The slurry system, however, would require an additional unit to separate the fine catalyst from the wax product. 

Note that the term accounting for effective transport in the axial direction has been neglected in this model, for the reasons given already in Sec. 11.6. This system of nonlinear second order partial differential equations was integrated by Froment using a Crank-Nicolson procedure [76,77], to simulate a multitubular fixed bed reactor for a reaction involving yield problems. 

In a multitubular fixed-bed reactor, the catalyst particles are packed into narrow tubes, grouped in bundles and enclosed in an outer shell (see Fig. 18.5). The tube bundles are immersed in water, which abstracts the heat and converts to high-pressure steam. The use of narrow tubes, high syngas velocities, and large catalyst particles ensures rapid heat exchange and minimizes exothermic temperature rise (Dry, 1996). The increased particle size of the catalyst is also necessary in order to avoid large pressure drops (Sie and Krishna, 1999), a problem encountered with... 

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