Reactor fixed-bed

A fixed-bed reactor normally consists of a cylindrical tube of assembly of tubes into which catalyst particles are loaded and contained in a fixed position. The tube, filled with the catalyst particles, may be either vertical or horizontal, depending on process circumstances and can be heated or cooled by a suitable gas or liquid medium circulated through a shell enveloping the tubes. Various designs of fixed-bed reactor are found in practice and some of these will be described briefly. The mode of 

The fixed beds of concern here are made up of catalyst particles in the range of 2-5 mm dia. Vessels that contain inert solids with the sole purpose of improving mass transfer between phases and developing plug flow behavior are not in this category. Other uses of inert packings are for purposes of heat transfer, as in pebble heaters and induction heated granular beds—these also are covered elsewhere. 

Thermal effects also are major factors in the design of reactors 

Fixed-bed reactors contain a bed of catalyst pellets (diameter 3-50 mm). The catalyst lifetime in these reactors is greater than three months. The best known design is the trickle-bed reactor [8, 10]. 

In a trickle-bed reactor the liquid flows downwards through a packed catalyst bed, while the gas can flow cocurrently or countercurrently to the liquid. The gas phase, which is present in excess, is the continuous phase. In the cocurrent trickle-bed reactor (Fig. 14-8), the gas/liquid mixture leaving the bottom of the reactor is sepa- 

Average values for the liquid flow are 10-30 m m h , and for the gas flow 300 1000 m m h . Solid-liquid separation is not necessary. Disadvantages are the poor heat removal and the occurrence of hot spots with potential instabilities. However, since the reactors are generally operated adiabatically, the relatively poor heat removal is not necessarily a problem. 

Stream formation in large-diameter reactors and wall channeling in small-diameter reactors can lower reactor performance. Often the catalyst is not fully exploited owing to incomplete wetting by the liquid and low mass-transfer rates together with low residence times within the catalyst pellets. 

Trickle-bed reactors are widely used in petrochemical hydrogenation processes and in the production of basic products. They are being used increasingly for the manufacture of fine chemicals. 

Catalytic fixed-bed reactors are the most important type of reactor for the synthesis of large-scale industrial chemicals and intermediates. Fixed-bed reactors for industrial synthesis are operated under constant (stationary) operating conditions over longer periods of time, ranging from months to years. Various reactor 

To achieve a low-pressure drop along the catalyst bed, suitable catalyst forms and arrangements, including spheres, extrudates, and hollow cylinders, as well as structured catalyst packings in the form of monoliths with parallel channels or parallel stacked plates, may be used. Typical continuous fixed-bed catalyst reactions are described in Section 15.6 (in particular, in Section 15.6.1). 

Adiabatic fixed-bed reactors on a smaller scale have a relatively simple design. An insulated stainless steel vessel or tube with a mesh at the reactor inlet and outlet to maintain the catalyst within the reactor is sufficient. If heat needs to be removed from or introduced into the catalyst bed, smaller tubes are usually chosen, which are then cooled or heated from outside by heat carriers or, in the high temperature range, by flames or hot combustion gases of homogeneous burners. Thus the reactor then has the design of a shell and tube heat-exchanger. 

Small fixed-bed (or even fluid-bed) reactors can also be used to determine reactivity and even a set of kinetic parameters with one experiment. The systems 

A distinct advantage of such a system is the suitability of high-ash char, which is sometimes not appropriate for investigation in other systems [60]. 

There is also a German standard (TGL 15388 [65]) for reactivity determination in a fixed bed, which is derived from the coke reactivity index for lump coke (CRI, ISO 18894 [66]). A mass of 5 g of 1-3 mm char is exposed to 2.51/h CO2 flow ( 95 vol%) in a reaction tube of 40-mm diameter at a temperature of 900 °C. The product gas is analyzed after 15 and 30 minutes. The reactivity value km in cm /(g s) is calculated from the gas concentrations. Typical char reactivity values for km are 10-15 for wood, 5-10 for peat, 4-8 for lignite, 1-4 for sub-bituminous coal, 0.1-1 for bituminous coals, and 0.1 for anthracite. The number has been mainly used to compare the gasifiability of fuels in fixed-bed and fluid-bed systems. 

A Chinese standard (GB/T 220-2001 [67]) applies a similar setup using 300 g of 3- to 6-mm coke grains increasing the temperature successively in 50K from 750 to 1100 °C. At each step, temperature is kept constant for 5 min and CO2 is fed and the exit gas is analyzed. Pairs of values of CO2 reduction rate and temperature are reported also revealing the relation of reaction rate to temperature. 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

Before we proceed with quantitative illustrations, let us consider the qualitative effect of external resistances on reaction rates and summarize the available information for mass- and heat-transfer coefficients (Sec. 10-2). 

CHAPTER 10 EXTERNAL TRANSPORT PROCESSES IN HETEROGENEOUS REACTIONS 

Most HDT and HDC commercial units employ FBRs. Historically, FBRs were meant for processing naphtha, kerosene, and gas oil, but they were gradually modified to handle tougher feeds such as vacuum gas oils and atmospheric/vacuum residues. They are the preferred choice of refiners due to their relative simplicity, flexibility, and ease of operation. 

Hydroprocessing reactors generally are three-phase (gas-liquid-solid) reaction systems. The gas phase is composed majorly of hydrogen, gaseous reaction products, and partially vaporized hydrocarbons the hydrocarbon feed is the liquid phase, whereas the catalyst bed is the solid phase. The only exception is naphtha HDT, which exhibits just two phases (gas-solid) as a result of the complete vaporization of the hydrocarbon. The coexistence of these three phases puts hydroprocessing FBRs 

One of the main advantages of TBRs is that liquid flow nearly approaches to plug flow and therefore they exceed the other three-phase reactors such as EBRs or SPRs in performance. They also exhibit higher ratio of catalyst-loading capacity per liquid volume. In practical terms, they are very simple in construction, require less investment, and are the most flexible with respect to the demanded throughput and reaction severity for different conversion levels. 

FIGURE 7.4 Representation of the trickle-flow regime and concentration profiles in a hydroprocessing reactor. (Adapted from Mederos, F.S. et al., Catal. Rev. Sci. Eng., 51(4), 485, 2009.) 

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Figure 2.6 Four possible arrangements for fixed-bed reactors.

Fixed-bed noncatalytic reactors. Fixed-bed reactors can be used to react a gas and a solid. For example, hydrogen sulfide can be removed from fuel gases by reaction with ferric oxide ... 

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

A fixed-bed reactor for this hydrolysis that uses feed-forward control has been described (11) the reaction, which is first order ia both reactants, has also been studied kiaeticaHy (12—14). Hydrogen peroxide interacts with acetyl chloride to yield both peroxyacetic acid [79-21-0] and acetyl peroxide... 

The original German process used either carbonyl iron or electrolytic iron as hydrogenation catalyst (113). The fixed-bed reactor was maintained at 50—100°C and 20.26 MPa (200 atm) of hydrogen pressure, giving a product containing substantial amounts of both butynediol and butanediol. Newer, more selective processes use more active catalysts at lower pressures. In particular, supported palladium, alone (49) or with promoters (114,115), has been found useful. 

Mitsui Toatsu Chemical, Inc. disclosed a similar process usiag Raney copper (74) shortiy after the discovery at Dow, and BASF came out with a variation of the copper catalyst ia 1974 (75). Siace 1971 several hundred patents have shown modifications and improvements to this technology, both homogeneous and heterogeneous, and reviews of these processes have been pubHshed (76). Nalco Chemical Company has patented a process based essentially on Raney copper catalyst (77) ia both slurry and fixed-bed reactors and produces acrylamide monomer mainly for internal uses. Other producers ia Europe, besides Dow and American Cyanamid, iaclude AUied CoUoids and Stockhausen, who are beheved to use processes similar to the Raney copper technology of Mitsui Toatsu, and all have captive uses. Acrylamide is also produced ia large quantities ia Japan. Mitsui Toatsu and Mitsubishi are the largest producers, and both are beheved to use Raney copper catalysts ia a fixed bed reactor and to sell iato the merchant market. 

Recent advances in Eischer-Tropsch technology at Sasol include the demonstration of the slurry-bed Eischer-Tropsch process and the new generation Sasol Advanced Synthol (SAS) Reactor, which is a classical fluidized-bed reactor design. The slurry-bed reactor is considered a superior alternative to the Arge tubular fixed-bed reactor. Commercial implementation of a slurry-bed design requires development of efficient catalyst separation techniques. Sasol has developed proprietary technology that provides satisfactory separation of wax and soHd catalyst, and a commercial-scale reactor is being commissioned in the first half of 1993. 

More recently, Sasol commercialized a new type of fluidized-bed reactor and was also operating a higher pressure commercial fixed-bed reactor (38). In 1989, a commercial scale fixed fluid-bed reactor was commissioned having a capacity similar to existing commercial reactors at Sasol One (39). This effort is aimed at expanded production of higher value chemicals, in particular waxes (qv) and linear olefins. 

The thermal catalytic route proposed involves heating the fresh reactant feed plus recycle up to 790°C and feeding this material into a M0S2 catalyst fixed-bed reactor operating at 0.1 MPa (1 atm). The route yields a production of H2 almost 50% higher than the decomposition of H2S route. 

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Data for the production and sales of maleic anhydride and fumaric acid ia the United States between 1979 and 1992 are shown ia Table 5. Production of maleic anhydride during this time grew - 2% on average per year. Production of fumaric acid has declined during the same period as customers have switched to the less cosdy maleic anhydride when possible. All production of maleic anhydride in the United States in 1992 was from butane-based plants which used fixed-bed reactor technology as shown in Table 6. The number of fumaric acid producers has been reduced considerably since the early 1980s with only two producers left in the United States in 1992 as shown in Table 6. Pfizer shut down its fumaric acid plant at the end of 1993. However, Bartek of Canada will start up an expanded fumaric acid faciUty to supply the North American market for both their own and Huntsman s requirements. 

Methyl-l-Pen ten e. This olefin is produced commercially by dimeriza tion of propylene in the presence of potassium-based catalysts at 150—160°C and - 10 MPa. Commercial processes utilize several catalysts, such as sodium-promoted potassium carbonate and sodium- and alurninum-promoted potassium hydroxide (12—14) in a fixed-bed reactor. The reaction produces a mixture of C olefins containing 80—85% of 4-methyl- 1-pentene. 

Some reactors are designed specifically to withstand an explosion (14). The multitube fixed-bed reactors typically have ca 2.5-cm inside-diameter tubes, and heat from the highly exothermic oxidation reaction is removed by a circulating molten salt. This salt is a eutectic mixture of sodium and potassium nitrate and nitrite. Care must be taken in reactor design and operation because fires can result if the salt comes in contact with organic materials at the reactor operating temperature (15). Reactors containing over 20,000 tubes with a 45,000-ton annual production capacity have been constmcted. 

Typically, reactors require some type of catalyst. Reactors with catalyst can be of the fixed-bed style for fiuid-bed types. Fixed-bed reactors are the most common. The feed often enters the reactor at an elevated temperature and pressure. The reaction mixtures are often corrosive to carbon steel and require some type of stainless steel alloy or an alloy liner for protection. If the vessel wall is less than 6 mm, the vessel is constmcted of all alloy if alloy is provided. Thicker reactor walls can be fabricated with a stainless overlay over a carbon steel or other lower alloy base steel at less cost than an all-alloy wall constmction. 

Process. As soHd acid catalysts have replaced Hquid acid catalysts, they have typically been placed in conventional fixed-bed reactors. An extension of fixed-bed reactor technology is the concept of catalytic distillation being offered by CR L (48). In catalytic distillation, the catalytic reaction and separation of products occur in the same vessel. The concept has been appHed commercially for the production of MTBE and is also being offered for the production of ethylbenzene and cumene. 

Among continuous reactors, the dominant system used to produce parasubstituted alkylphenols is a fixed-bed reactor holding a soHd acid catalyst. Figure 3 shows an example of this type of reactor. The phenol and alkene are premixed and heated or cooled to the desired feed temperature. This mix is fed to the reactor where it contacts the porous soHd, acid-impregnated catalyst. A key design consideration for this type of reactor is the removal of the heat of reaction. 

The predominant process for manufacture of aniline is the catalytic reduction of nitroben2ene [98-95-3] ixh. hydrogen. The reduction is carried out in the vapor phase (50—55) or Hquid phase (56—60). A fixed-bed reactor is commonly used for the vapor-phase process and the reactor is operated under pressure. A number of catalysts have been cited and include copper, copper on siHca, copper oxide, sulfides of nickel, molybdenum, tungsten, and palladium—vanadium on alumina or Htbium—aluminum spinels. Catalysts cited for the Hquid-phase processes include nickel, copper or cobalt supported on a suitable inert carrier, and palladium or platinum or their mixtures supported on carbon. 

A process for the production of DPA from phenol and ammonia has been reported (25). Typically, the reaction is carried out continuously ia a fixed-bed reactor usiag an acidic alumiaa catalyst at 300°C—420°C. The first product formed is aniline which is subsequently converted to DPA. Consequently, the reaction can be carried out to simultaneously produce DPA and aniline, ia any desired ratio, simply by varyiag the molar ratios of phenol (and aniline) ia the reactor feed stream. 

Conversions of ca 75% are obtained for propylene hydration over cation-exchange resins in a trickle-bed reactor (102). Excess Hquid water and gaseous propylene are fed concurrentiy into a downflow, fixed-bed reactor at 400 K and 3.0—10.0 MPa (30—100 atm). Selectivity to isopropanol is ca 92%, and the product alcohol is recovered by azeotropic distillation with benzene. 

Fig. 3. Multiple fixed-bed configurations (a) adiabatic fixed-bed reactor, (b) tubular fixed beds, (c) staged adiabatic reactor witb interbed beating (cooling),...

Fig. 14. Generalized map of flow regimes in downflow fixed-bed reactors using Baker plot parameters, A = ( a/ air l/ w...

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

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