Multitubular fixed bed

Process Description. Reactors used in the vapor-phase synthesis of thiophene and aLkylthiophenes are all multitubular, fixed-bed catalytic reactors operating at atmospheric pressure, or up to 10 kPa and with hot-air circulation on the shell, or salt bath heating, maintaining reaction temperatures in the range of 400—500°C. The feedstocks, in the appropriate molar ratio, are vaporized and passed through the catalyst bed. Condensation gives the cmde product mixture noncondensable vapors are vented to the incinerator. 

Another process where good temperature control is essential is the synthesis of vinyl chloride by chlorination of ethylene at 200 to 300°C (392 to 572°F), 2 to 10 atm (29.4 to 147 psi), with supported cupric chloride, but a process with multitubular fixed beds is a strong competitor. 

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. 

Two basically different reactor technologies are currently in operation low temperature and high temperature. The former operates at -220 °C and 25-45 bar, employing either a multitubular, fixed bed (i.e. trickle bed) reactor or a slurry bubble column reactor with the catalyst suspended in the liquid hydrocarbon wax product. 

The oxidation of butane (or butylene or mixtures thereof) to maleic anhydride is a successful example of the replacement of a feedstock (in this case benzene) by a more economical one (Table 1, entry 5). Process conditions are similar to the conventional process starting from aromatics or butylene. Catalysts are based on vanadium and phosphorus oxides [11]. The reaction can be performed in multitubular fixed bed or in fluidized bed reactors. To achieve high selectivity the conversion is limited to <20 % in the fixed bed reactor and the concentration of C4 is limited to values below the explosion limit of approx. 2 mol% in the feed of fixed bed reactors. The fluidized-bed reactor can be operated above the explosion limits but the selectivity is lower than for a fixed bed process. The synthesis of maleic anhydride is also an example of the intensive process development that has occurred in recent decades. In the 1990s DuPont developed and introduced a so called cataloreactant concept on a technical scale. In this process hydrocarbons are oxidized by a catalyst in a high oxidation state and the catalyst is reduced in this first reaction step. In a second reaction step the catalyst is reoxidized separately. DuPont s circulating reactor-regenerator principle thus limits total oxidation of feed and products by the absence of gas phase oxygen in the reaction step of hydrocarbon oxidation [12]. 

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. 

In its present commercial operations Sasol uses two different types of FT reactors. The multitubular fixed bed (see Figure 1) produces waxes and the circulating fluidized bed (CFB, see Figure 2) produces light olefins and oils. On a cross-sectional area basis the gas throughput as well as the amount converted is much higher for fluidized- than for fixed-bed reactors. 

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. 

The main drawback of randomly packed microreactors is the high pressure drop. In multitubular fixed-bed microreactors, all channels must be packed identically, or supplementary flow resistances must be... 

Different implementations of such reactors may differ in detail. For instance, in some cases the flow to the analytical instrument is additionally controlled by a mass flow controller, so that at least during the analysis all catalysts are evaluated at exactly the same space velocity. In other setups, which will be discussed below, a truly parallelized analysis is integrated into the reactor, which allows the elimination of the multiport valve. The general features of such reactors, however, can be summarized as those of a multitubular fixed bed. Critical issues in operating such parallel reactors have been discussed by Moulijn et al. [33], who also had published one of the first prototypes of a parallel reactor, the so-called six-flow reactor [34],... 

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. 

Fixed-bed adiabatic Multitubular fixed bed (von Heyden, phthalic anhydride)... 

Equation (2) predicts that if the total pressure is increased and the gas feed rate is increased by the same factor, that is, the residence time in the catalyst bed remains the same, then the degree of conversion remains unchanged. This matched the actual experimental pilot plant findings. This means that the production rate should increase in proportion to the increase in pressure. On the basis of this prediction, new pilot plants were constructed at Sasol and tests up to 6.0 and 7.5 MPa both for the LTFT fixed-bed operations and for the HTFT fluidized bed operations, respectively, were carried out. These tests confirmed the kinetic predictions. A 4.5-MPa multitubular fixed-bed commercial reactor was... 

The traditional maleic anhydride manufacturing process involves reacting benzene with excess air. A low benzene concentration is used in order not to exceed the flammability limit of the mixture. The reaction gas mixture is passed over a multitubular, fixed bed catalyst reactor at a pressure of 0.15-0.25 MPa. In addition to maleic anhydride, the reaction produces two moles of CO2 and water. The reaction is highly exothermic, causing "hotspots" of 340-500°C. For each ton of benzene that is reacted, 27MJ of heat are generated. Molten eutectic salts circulate outside the reactor tubes to dissipate the heat. Steam is generated when the molten salts are cooled, which is used to drive air compressors (see Fig. 9.31). 

Great efforts have been devoted to development of reactor to satisfy the requirements of different GTL processes. Several reactor types are currently used for FTS. For example, reactors for FTS include the multitubular fixed-bed, gas-solid fluidized-bed, and slurry bubble column reactors (Flussain et al., 2015). The differences between these reactors are largely related to different approaches to temperature control and the choice of catalyst. 

Successful, lower capital cost than multitubular fixed bed, but fixed bed is returning in favor because of feedstock flexibility... 

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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