Cooled multi-tubular reactors

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. 

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. 

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. 

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 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). 

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. 

Catalytic partial oxidation of o-xylene and naphthalene is performed mostly in intensively cooled multi-tubular fixed bed reactors, but systems with a fluidized bed were also developed. Typically, V20s/Ti02 catalysts with K2SO4 or A1 phosphates as promoter are used. In fixed bed reactors, the conversion of both feedstocks per pass is around 90%, and the selectivity is in the range 0.86-0.91 mol PA per mol naphthalene and 0.78 mol per mol o-xylene. (Note that the selectivity would be 100%, if only the reactions according to Eqs. (6.13.1) and (6.13.2), respectively, would take place.) The active compounds are distributed on spheres of porcelain, quartz, or silicium carbide (shell catalyst). The thickness of the shell is only around 0.2 mm, and the diffusion paths for the reactants are short. By this means, the influence of pore diffusion is small, and the unwanted oxidation of phthalic acid anhydride to CO2 is suppressed compared to a catalyst with an even distribution of active compounds where the influence of pore diffusion would be much stronger (see Section 4.5.6.3 Influence of Pore Diffusion on the Selectivity of Reactions in Series ). Thus the intrinsic reaction rates are utilized for the modeling of a technical reactor (next Section 6.13.2). 

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. 

After the pre-mixing of butane in air, the gases enter the multi-tubular reactor cooled with a mixture of nitrite and nitrate of sodivun and potassium, where each tube has a 21 mm diameter and is 3.5 or 6 metres high. The catalyst, a VPO precvursor, is tabletted into different shapes and activated to transform the VOHPO4-0.5H2O in (V0)2P207. Nowadays the most modem generation of VPO catalysts are activated outside the reactors and are equilibrated so that the metal oxide loaded in the tubes is ready to work from the first instance of the reactants feed. It then ramps up to the... 

The need to keep a concave temperature profile for a tubular reactor can be derived from the former multi-stage adiabatic reactor example. For this, the total catalyst volume is divided into more and more stages, keeping the flow cross-section and mass flow rate unchanged. It is not too difficult to realize that at multiple small stages and with similar small intercoolers this should become something like a cooled tubular reactor. Mathematically the requirement for a multi-stage reactor can be manipulated to a different form ... 

FIGURE 13.1 Types of fixed bed reactors, (a) Axial flow fixed bed reactor Up or down flow, single or multi-stage, with or without inter-stage cooling, single or multi-tubular, (b) Radial flow fixed bed reactor Radially inward or outward flow, straight or reverse flow (direction of inlet and outlet is same or opposite to each other). 

Fig. 1-3 Typical reactors (a) tubular-flow recycle reactor, (b) multi tube-flow reactor, (c) radial-flow catalytic reactor, (d) stirred-tank reactor with internal cooling, (e) loop reactor, (f) reactor with intercoolers (opposite)...

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. 

Fig. 30. Contacting patterns and contactor types for gas-liquid-solid reactors, (a) Co-current downflow trickle bed. (b) Countercurrent flow trickle bed. (c) Co-current downflow of gas, liquid, and catalyst, (d) Downflow of catalyst and co-current upflow of gas and liquid, (e) Multi-tubular trickle bed with co-current flow of gas and liquid down tubes with catalyst packed inside them coolant on shell side, (f) Multi-tubular trickle bed with downflow of gas and liquid coolant inside the tubes, (g) Three-phase fluidized bed of solids with solids-free freeboard, (h) Three-phase slurry reactor with no solids-free freeboard, (i) Three-phase fluidized beds with horizontally disposed internals to achieve staging, (j) Three-phase slurry reactor with horizontally disposed internals to achieve staging, (k) Three-phase fluidized bed in which cooling tubes have been inserted coolant inside the tubes. (1) Three-phase slurry... 

In general, temperature effects may significantly affect the performance of adsorptive processes. For this reason we have further extended previous work on the use of PSR reactors for consecutive irreversible reactions to exothermic reactions and non-isothermal conditions. In the present case we will deal with external cooling, by using a multi-tubular... 

The performance of a PSR will be bench marked against two reference cases (i) a multi-tubular cooled plug flow reactor (PFR) containing only a catalyst and cooled co- or counter currently, and (ii) a combination of a PSA vessel containing a sorbent to separate A, and a multi-tubular cooled PFR containing a catalyst and receiving the A-enriched feed from the PSA. 

Figure 4.10.7 Multi-tubular fixed bed reactor cooled by boiling water.

Figure 6.11.21 Influence of cooling temperature on the maximum axial temperature in a multi-tubular FT reactor comparison oftwo-dimensional model [Eq. (6.11.40)] and one-dimensional model [Eq. (6.11.47)] (T n = Tcooii for parameters see Table 6.11.3) (Jess and Kern, 2009).

Both reactor types are quite complicated firom a construction tube bundles in both reactor types have to be constructed to temperature variations that occur when the process is started, reactor requires a special distributor, and multi-cyclones catalyst in the bed. The tubular reactor requires a support each pipe. Nevertheless both reactor types find wide application Here we shall consider the fixed bed reactor with wall cooling, cooling causes radial temperature gradients, that in its... 

A preliminary modelling analysis involved the parametric study of a multi-tubular externally-cooled fixed-bed reactor for a generic selective oxidation process, where the catalyst load consisted of cylindrical honeycomb monoliths with washcoated square chaimels, made of highly conductive supports. In this early work, the attention was focused on the effect of catalyst design. Simulation results were generated by a steady-state, pseudo-continuous 2D monolithic reactor model, where the catalyst is regarded as a continuum consisting of a static, thermally connected solid phase... 

The chapter presents a brief overview of the current research on V205/Ti02 catalysts for o-xylene oxidation to phthalic anhydride at Clariant. Phthalic anhydride is produced in tubular, salt-cooled reactors with a capacity of about 5 Mio to per annum. There is a rather broad variety of different process conditions realized in industry in terms of feed composition, air flow rate, as well as reactor dimensions which the phthalic anhydride catalyst portfolio has to match. Catalyst active mass compositions have been optimized at Clariant for these differently realized industry processes utilizing artificial neural networks trained on high-throughput data. Fundamental pilot reactor research unravelling new details of the reaction network of the o-xylene oxidation led to an improved kinetic reactor model which allowed further optimizing of the state of the art multi-layer catalyst system for maximum phthalic anhydride yields. 

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