Multi-tubular reactors

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

Figure 6-10. An autothermal multi-tubular reactor with an internal heat exchanger.

The numbering-up concept demands achieving absolutely uniform flow equi-partition by placing special headers in front of the parallel micro channels [5]. Although this has been solved for tubes in conventional multi-tubular reactors, the expenditure for equipartition in micro channel stacks is assumed to be higher, since the small channels may have more relative differences in structural preci-... 

Multitubular falling film continuous SO3 sulfonation units, 23 547 Multi tubular reactors, 23 544 Multiunit pilot plants, 19 459 Multiuse facility design, 11 47 Multiuse operation, for fermentation, 11 47 Multivariable control, 20 701... 

Lurgi 01 Gas Chemie GmbH Phthalic anhydride O-xylene, naphthalene Multi-tubular reactor oxidizes o-xylene at high yield with maximum heat recovery for export HP steam 110 1998... 

EO is mainly produced by the direct oxidation of ethylene with air or oxygen in a packed-bed, multi tubular reactor with recycle [2]. Catalysts for EO production... 

A packed bed membrane reactor is an assembly of usually uniformly sized catalytic particles, which are randomly arranged and firmly held in position within a vessel or tube. A permeable membrane (generally tubular) is immersed within the particles or represents the tube wall of the fixed bed. The PBMR could look, for example, like a tube-in-shell or a multi-tubular reactor. Zooming in on the reaction zone the different phenomena occurring in the reactor can be described as follows ... 

Tube-to-tube interactions. The problems of velocity profile elongation and thermal runaway can be eliminated by using a multi-tubular reactor with many small-diameter tubes in parallel. Unfortunately, this may give rise to a new form of instability. Imagine a 1000-tube reactor with 999 of the tubes plugged with solid polymer ... 

Ethylbenzene and low-pressure steam (steam ratio approx. 1.2) are fed to a vaporizer, heated in heat exchangers to over 590 °C and then passed into a reactor. A multi-tubular reactor is used (tube diameter 100 to 200 mm, length 2.5 to 4.0m), and is heated by flue gas at 720 to 750 °C. The reaction product, leaving the reactor at a temperature of around 580 °C, is cooled in heat exchangers and the ethylbenzene vaporizer to 160 °C and condensed in an air cooler. 

Melting vessel 2 Vaporizer 3 Multi-tubular reactor 4 Crude anthraquinone silo 5 Melting and polymerization vessel 6 Vacuum distillation 7 Gas scrubber... 

Reactor 2 Distillation column 3 Vaporizer 4 Multi-tubular reactor 5 Gas separator 6 Distillation column... 

The reaction is carried out in the vapour phase in a multi-tubular reactor packed with a catalyst of mercuric chloride on an activated charcoal support. The reaction is highly exothermic and cooling is applied to keep the temperature at 100—180°C. The pressure used is atmospheric. The gases from the reactor are cooled and washed with aqueous sodium hydroxide to remove unreacted hydrogen chloride. The product is then liquefied by cooling to —40°C and pure vinyl chloride is obtained by fractional distillation. Provided pure reactants are used, this preparation of vinyl chloride is clean and easily accomplished. Vinyl chloride is a colourless gas (b.p. —14°C) with a pleasant, sweet odour. It is conveniently handled, under slight pressure, as a liquid, which may be stored without the addition of a polymerization inhibitor. 

For strong exothermic or endothermic reactions with an adiabatic temperature rise of several hundred degrees, the rack type reactor is not sufficient. Then, a multi-tubular reactor is used, where the catalyst is located in up to 30 000 individual tubes, the outside of which is exposed to the flow of a heat transfer medium. In many cases, cooling is provided by boiling water, and the cooling temperature can easily be controlled by the pressure. For elevated temperatures molten salts can be employed as cooling or heating medium. 

If the intrinsic chemical kinetics cannot be separated from transport processes at the conditions relevant for technical operation we speak of macrokinetics or effective apparent kinetics. Investigation of effective kinetics alone, that is, disregarding transport phenomena, sometimes also makes sense if the laboratory reactor intended for investigation of specific operating conditions is hydrodynamically similar to the technical reactor so that the transport parameters in both systems can also be assumed to be similar. For example, the response of a multi-tubular reactor (featuring up to 30 000 individual tubes) can easily be simulated in the laboratory, if a single tube is used, with which the response of the actual reactor under various conditions can be investigated. Unfortunately, this approach is in most cases not applicable, and so we have to determine all parameters of the micro- and macrokinetics. 

Formaldehyde is produced by oxidation of methanol or oxidative dehydrogenation of methanol. Oxidation of methanol (route (a) in Topic 5.3.2] is a strongly exothermic reaction (AH = -243 kj mol ) that is carried out in a pressure-less oxidation with air in a multi-tubular reactor. The reaction is catalyzed by an iron/molybde-num oxide contact, with Fe2(Mo04) being the active catalytic species. The oxidation is carried out at 350 °C with quantitative methanol conversion. The main side reaction is the total oxidation of methanol to CO2 and water. 

If the reactor were a single adiabatically operated fixed bed, the heat release would raise the temperature to 600 °C, which corresponds to an equilibrium conversion of SO2 of only 70% (Figure 6.3.4), but even this far from sufficient conversion would only be reached for an infinite residence time and reactor length. For isothermal operation, a conversion of about 98% would be possible, but this would require an expensive reactor (e.g., a multi-tubular reactor intensively cooled by a molten salt. Figure 4.10.7). 

Figure6.11.22 Runaway diagram ofa multi-tubular reactor for different tube diameters according to Figure 4.10.24 and Eqs. (4.10.47)-(4.10.49),...

For accurate modeling of a FTS multi-tubular reactor, the two-dimensional fixed... 

In the process the preheated reactants, inerts (diluent methane and recycled CO2), and promoters are fed into the multi-tubular reactor The gas stream leaving the reactor is cooled by an external heat exchanger and sent to the ethylene oxide absorber column. In this column the relatively small amounts of ethylene oxide (concentration 1-2 mol.%) are absorbed in water. A minor part of the gas leaving the top of the absorber is purged to reduce the inerts concentration (mainly CO2, argon, and methane). The rest of the gas stream is sent to the CO2 absorber unit. 

The chlorohydrin process has largely been replaced by the direct oxidation of ethylene with silver as catalyst. By-products are CO2 and water, formed by total oxidation of ethylene or EO. The reactor design is dominated by the demand for an exact temperature and selectivity control. The conversion per pass is low (about 10%) to avoid the consecutive oxidation of EO, and the unconverted ethylene is recycled. A multi-tubular reactor guarantees efficient heat transfer. 

Multi-tubular reactor for o-xylene oxidation to phthalic anhydride. Deggendorfer Werft, from Cmehlingand Brehm (1996). 

Reaction engineering aspects of cooled multi-tubular reactors have already been examined in Section 6.11 for Fischer-Tropsch synthesis, which can be simply described by a single reaction of syngas to higher hydrocarbons (at least for Co as catalyst for Fe as catalyst, this main reaction can also be used to inspect the thermal behaviour of the reactor in good approximation, see Section 6.11.1). For PA production, at least three reactions are involved (Scheme 6.13.1), and this process is a good example by which to illustrate yield and selectivity problems, which are frequently encountered in industrial practice. 

Design and Simulation of a Multi-tubular Reactor for Oxidation of o-Xylene to PA... 

To simulate such a multi-tubular reactor we use the reaction scheme given by Scheme 6.13.1, which is fairly representative for the catalytic gas-phase air oxidation of o-xylene to phthalic anhydride (Froment and Bischoff, 1990). For simplification, we assume that only CO2 (and steam) and not CO (and hydrogen) are formed as unwanted by-products. 

Figure 6.13.1 Axial profiles of temperature (tube axis) (a), o-xylene conversion (b), and selectivity to PA (c) in the multi-tubular reactor for catalytic o-xylene oxidation with different inlet temperatures [fin = Tool, two-dimensional model, Eqs. (6.13.3), (6.13.15)-(6.13.22) parameters seeTable 6.13.1].

Figure 6.13.4 Influence of internal diameter on the maximum allowable gas inlet (= wall) temperature with regard to a runaway in a multi-tubular reactor of catalytic o-xylene oxidation [determined by Eqs. (6.13.41)-(6.13.43), parameters as given by Table 6.13.1].

The strongly exothermic partial oxidation of o-xylene (and also of naphthalene) is carried out in multi-tubular reactors (with about 10000 tubes) cooled by a molten salt. To simulate the multi-tubular reactor, the two-dimensional reactor model is appropriate in order to account for the radial temperature gradient in the catalyst bed. 

The latest updates in the production from benzene include the Polynt s high load technology, which consists of the retrofitting of the existing multi-tubular reactors, introducing an improved heat exchange capacity, a new benzene evaporator and an air mixing unit and the use of a specific catalyst based on Vanadium and Molybdenum Oxides. 

---