Tubular Fixed-Bed Reactor

In a chemical packed-bed reactor in which a highly exothermic reaction is taking place conditions may be encountered under which, for a given set of input conditions (feed rate, temperature, concentration), the exit conversion is either high or low. To the experimental study of conditions connected with the existence of multiple steady states in packed catalytic reactors has not been paid as much attention in the past as to the study of 

In this paragraph, based on experimental observations, we are going to review a number of observed facts which may help to elucidate the lows governing the occurrence of multiple steady states in tubular packed catalytic reactors. 

The differential equations governing heat and mass transfer in tubular catalytic reactor are 

To understand the experimental observations we outline in brief the quantitative theoretical results without any analytical investigation of the governing equations (47). 

Condition 1 The necessary and sufficient condition for unicity of the steady states is B B.  

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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. 

Tubular Fixed-Bed Reactors. Bundles of downflow reactor tubes filled with catalyst and surrounded by heat-transfer media are tubular fixed-bed reactors. Such reactors are used most notably in steam reforming and phthaUc anhydride manufacture. Steam reforming is the reaction of light hydrocarbons, preferably natural gas or naphthas, with steam over a nickel-supported catalyst to form synthesis gas, which is primarily and CO with some CO2 and CH. Additional conversion to the primary products can be obtained by iron oxide-catalyzed water gas shift reactions, but these are carried out ia large-diameter, fixed-bed reactors rather than ia small-diameter tubes (65). The physical arrangement of a multitubular steam reformer ia a box-shaped furnace has been described (1). 

Approximately 45% of the world s phthaUc anhydride production is by partial oxidation of 0-xylene or naphthalene ia tubular fixed-bed reactors. Approximately 15,000 tubes of 25-mm dia would be used ia a 31,000 t/yr reactor. Nitrate salts at 375—410°C are circulated from steam generators to maintain reaction temperatures. The resultant steam can be used for gas compression and distillation as one step ia reduciag process energy requirements (100). 

For fixed-bed reactors containing rapidly deactivating catalysts, the scheduled changes ia operating variables to accommodate activity loss can have a marked effect on mn length. This is exemplified by acetylene hydrochiorination to produce vinyl chloride ia tubular fixed-bed reactors. Steel reactors,... 

Scientists from Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed-bed reactor comprising a metallic monolith [30]. The walls were coated with catalytically active material and the monolith pieces were loaded lengthwise. Corning, the world leader in ceramic structured supports, developed metallic supports with straight channels, zig-zag channels, and wall-flow channels. They were produced by extrusion of metal powders, for example, copper, fin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys [31]. An alternative method is extrusion of softened bulk metal feed, for example, aluminum, copper, and their alloys. The metal surface can be covered with carbon, carbides, and alumina, using a CVD technique [32]. For metal monoliths, it is to be expected that the main resistance lies at the interface between reactor wall and monolith. Corning... 

The catalytic reaction was carried out at 270°C and 101.3 kPa in a stainless steel tubular fixed-bed reactor. The premixed reaction solution, with a molar ratio catechol. methanol water of 1 1 6, was fed into the reactor using a micro-feed pump. To change the residence time in the reactor, the catechol molar inlet flow (Fio) and the catalyst mass (met) were varied in the range 10 < Fio <10 mol-h and 2-10 < met < 310 kg. The products were condensed at the reactor outlet and collected for analysis. The products distribution was determined quantitatively by HPLC (column Nucleosil 5Ci8, flow rate, 1 ml-min, operating pressure, 18 MPa, mobile phase, CH3CN H2O =1 9 molar ratio). 

In the feed pretreatment section oil and water are removed from the recovered or converted CCI2F2. The reactor type will be a multi-tubular fixed bed reactor because of the exothermic reaction (standard heat of reaction -150 kJ/mol). After the reactor the acids are selectively removed and collected as products of the reaction. In the light removal section the CFCs are condensed and the excess hydrogen is separated and recycled. The product CH2F2 is separated from the waste such as other CFCs produced and unconverted CCI2F2. The waste will be catalytically converted or incinerated. A preliminary process design has shown that such a CFC-destruction process would be both technically and economically feasible. 

Equations 12.7.48 and 12.7.39 provide the simplest one-dimensional mathematical model of tubular fixed bed reactor behavior. They neglect longitudinal dispersion of both matter and energy and, in essence, are completely equivalent to the plug flow model for homogeneous reactors that was examined in some detail in Chapters 8 to 10. Various simplifications in these equations will occur for different constraints on the energy transfer to or from the reactor. Normally, equations 12.7.48 and 12.7.39... 

Kinetic expressions were determined for both a tubular fixed-bed reactor containing 30 mg catalyst particles and the micro reactor coated with the catalyst particles. The coating was performed by filling the channel system with a suspension of the catalyst in 2-propanol and subsequent removal of the organic solvent by heat treatment at 500 °C [24]. 

Fig. 1.20. Autothermal syngas generation by combining simultaneous autothermal reforming in an air/oxygen-fired fixed-bed reactor (ATR) and steam reforming in a gas-heated tubular fixed-bed reactor (GHR) [32, 33].

Selective hydrocarbon oxidation reactions are characterised by both high activation energies and heats of reaction. If the desired partial oxidation products are to be safeguarded and the catalyst integrity ensured it is essential that close temperature control be maintained. In spite of the obvious attractions of the fluid bed for this purpose, mechanical considerations normally dictate that a multi-tubular fixed-bed reactor, comprising small diameter tubes between 2-4 cms. diameter, be used. 

For analysis of distributed-parameter systems, such as a tubular fixed bed reactor, numerical simulation of periodic operation at various values of control parameters is typically applied. Asymptotic models for quasisteady and relaxed steady states are valuable instruments for a substantial simplification of the original distributed-parameter system. A method allowing for... 

The objective of this work is to stress the importance of type III models, both in their two-dimensional version, proposed a decade ago (3>)>and in the recently proposed one-dimensional version (2). Although these models correctly represent the transfer phenomena in and between phases for tubular fixed-bed reactors, they have seldom been used up to now. Type II models have been included in this analysis because they have been used very frequently and it is of special importance to show that their responses may greatly deviate with respect to the response of type III models. 

Description The fresh paraffin feedstock is combined with paraffin recycle and internally generated steam. After preheating, the feed is sent to the reaction section. This section consists of an externally fired tubular fixed-bed reactor (Uhde reformer) connected in series with an adiabatic fixed-bed oxyreactor (secondary reformer type). In the reformer, the endothermic dehydrogenation reaction takes place over a proprietary, noble metal catalyst. 

Consider a tubular fixed-bed reactor accomplishing a highly exothermic gas-phase reaction. Assuming that axial dispersion can be neglected, the mass and energy balances can be written as follows and allow for radial gradients ... 

The FT process is well known and already applied on a large scale [9,10,11,12]. Currently, the two players that operate commercial Fischer-Tropsch plants are Shell and Sasol. In the Sasol and Shell plants gasification of coal and partial oxidation of natural gas, respectively, produce the syngas for the FT synthesis with well-defined compositions. Shell operates the SMDS (Shell Middle Distillate Synthesis) process in Bintulu, Malaysia, which produces heavy waxes with a cobalt catalyst in multi-tubular fixed bed reactors. Sasol in South Afirica uses iron catalysts and operates several types of reactors, of which the slurry bubble column reactor is the most versatile (i.e. applied in the Sasol Slurry Phase Distillate SSPD),... 

Ethylbenzene disproportionation was performed in a stainless-steel tubular fixed-bed reactor. Mordenites were evaluated at 150 and faujasites at 200°C at atmospheric pressure. Ethylbenzene vapour was mixed with helium and introduced into the reactor at a partial pressure of 1.33 kPa with a total flow rate of 1.81-h-i [3]. 

A tubular fixed bed reactor equipped with six independent heating zone was used to maintain an isothermality through out the reactor axis. Glass beads of comparable dimension were mixed with catalysts as diluent to avoid excessive heat generation and accumulation. Details of the flow schematics, data acquisition and system control design are given by Frycek (1934) and Megiris (1987). ... 

In the following we present an experimental set up which allows the simultaneous determination of the gas phase composition and the oxygen activity of the solid catalyst under operating conditions along a tubular fixed bed reactor. Preliminary results of the partial oxidation of acrolein to acrylic acid at an oxidic catalyst illustrate the possibilities of the procedure. 

However, the high activity together with the exothermic reaction makes reactor design critically important to ensure effective heat removal. Tubular fixed bed reactors (TFBR) are particularly vulnerable to thermal damage, so tube diameters are limited to prevent heat transfer problems. This leads to an expensive reactor as a large number of small tubes have to be used. 

The catalytic reaction was carried out at 270°C and 101.5 kPa in a stainless steel tubular fixed-bed reactor. The premixed reaction solution, with a molar ratio catechol methanol water of 1 1 6, was fed into the reactor using a micro-feed pump. Detailed description of kinetic experiments and the experimental set-up have been recently reported in [2-4]. 

The laboratory reaction system used was a conventional flow system with a tubular fixed-bed reactor as described elsewhere(18). The characteristic feature of this system is its ability to simulate various air to fuel ratios (A/F) of automotive exhaust gases using eight mass flow controllers. In this study, catalytic activity on the catalysts in simulated automotive exhaust gases was measured as a function of X, which is a normalized value of A/F by a stoichiometric one in the simulated exhaust gas, at 300°C and 420,000 h space velocity. The compositions of the simulated exhaust gases for each X are shown in Table 1. Catalytic activity was expressed as percent conversions of NOx(NO+N02), CO, and HC. 

The optimal catalyst distribution problem was studied in an adiabatic reactor (Ogunye and Ray, 197la,b). The optimal initial distribution of catalyst activity along the axis of a tubular fixed-bed reactor was examined for a class of reactivation-deactivation problems by Gryaert and Crowe (1976). A general set of simultaneous reactions was considered, quasi steady state approximation was used, and the decay of the catalyst expressed as a function of temperature, concentration and catalyst activity. The influence of various initial catalyst activity distributions upon the reactor performance was also considered. 

The laboratory reactor system was a conventional flow system with a tubular fixed-bed reactor as shown in Figure 1. A characteristic feature of this reactor system was its ability to change the feedstreams to the catalyst bed quickly so that the feedstreams can be rapidly cycled between two different gas compositions. The cycling period was varied between 0 and 2.0 seconds. 

Another class of processes where it is advantageous to keep the reactants separated from each other, except within the catalyst pores, is oxidation of light gaseous hydrocarbons (e.g., ethene, propene, butene). Conventionally these processes are carried out in multi-tubular fixed-bed reactors (see, for example, Fig. 20). Flammability considerations usually restrict the feed mixture composition. By adopting the concept of a multi-tubular cooled catalytic membrane reactor (with inclusion of heat pipes ), with reactants kept separate, we should be able to avoid any flammability constraints. 

Reaction (1) Acrylic acid is produced by catalyzed oxidation of propylene in a two-stage tubular, fixed-bed reactor system. The reactors are cooled by circulating molten heat transfer salt. The heat of reaction is used to produce steam. 

Screening experiments were carried out in a tubular fixed bed reactor with an inner diameter of 10 mm and a vertical reaction zone of 40 mm. The liquid reactant was pumped in the evaporation zone by a membrane pump. The reaction pressures were varied from 10 kPa to 0.1 MPa with a membrane vacuum pump. 

Reactor testing setup The catalyst was tested in a down-flow 3/8" OD stainless steel tubular fixed bed reactor. The charge consisted of 6.8 g of the C03O4 catalyst diluted with 18 g of quartz powder. Reduction of C03O4 should then result in a 5 g yield of cobalt black. To even out the temperature profile, in... 

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