Hydroformylation reactor design

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

As outlined in Chapter 5, Section 5.2.3.2 various approaches to overcoming the low rates of the hydroformylation of long chain alkenes in aqueous biphasic systems have been proposed. Some of these, such as the use of microemulsions [24-26] or pH dependent solubility [27], have provided improvements often at the expense of complicating the separation process. Perhaps the most promising new approaches involve the introduction of new reactor designs where improved mixing allows for... 

Industrial practice often confronts the development engineer with networks that are considerably more complicated than that of cyclohexene hydroformylation in the example above. Additional simplifications may then be desirable or necessary in order to arrive at a model that remains manageable in the highly iterative applications called for in reactor design and optimization and possibly on-line process control. A useful and usually successful way of achieving such streamlining is to place all network nodes at end members or non-trace intermediates, ignoring the fact that some of them may be at trace-level intermediates [10]. 

There is still a dispute among experts as to the place in which the biphasic aqueous reaction actually takes place, although it is very probably not the bulk of the liquid but the interfacial layer between the aqueous and organic phases. In the case of aqueous biphasic hydroformylation, this question has been decided by methods of reaction modeling and comparison with experimentally proven facts, thus leading to scale-up rules and appropriate kinetic models as a basis for optimal reactor design [34]. 

By comparison between the calculated and measured pressure and heat flux vs. time curves it was shown that the site of this hydroformylation reaction could not be the bulk of the liquid. Only the assumption of a reaction in the liquid boundary layer at the gas-liquid interface gave satisfactory agreement of the data under all experimental conditions. Thus, on this basis scale-up rules for the aqueous bipha-sic hydroformylation and appropriate kinetic models can be developed for optimal reactor design. The principle of both models applied to the general equation (Eq. 10) is shown in Figure 5. 

The hydroformylation of olefins is a type of CO insertion reaction that is one of the most important industrial applications of homogeneous catalysis with transition metal complexes (208,209). Conventional industrial processes (e.g., the Oxo process) typically use either cobalt- or rhodium-based catalysts and conduct the reaction in two-phase gas-liquid reactors. Efficient transfer of the reactants from the gas phase into the liquid phase is of primary importance to minimize inherent mass transfer limitations (208). Reactor design thus focuses on optimizing this mass transfer rate by maximizing the interfacial area between phases. An SCE process eliminates this transport restriction since the hydrogen... 

Figure 6.35. Schematic design of a full-scale fluorous biphasic reactor for the hydroformylation of 1-octene...

To estimate costs for the liquid-liquid biphasic hydroformylation using ionic liquids, a process was designed for the production of 100,000 tons per year of nonanal. The use of ionic liquids in hydroformylation catalysis is a fairly new technology and exact kinetic data are scarce, thus the TOFs reported for the Rh-sulfoxantphos system [80] have been used to determine catalyst inventory and reactor dimensions. In a similar way the plant design for the SILP process for a production capacity of 100,000 tons per year of butanal has been derived based on preliminary literature results [68]. The process flow sheets for both process variations are shown in Figures 7.12 and 7.13. 

A wide variety of new approaches to the problem of product separation in homogeneous catalysis has been discussed in the preceding chapters. Few of the new approaches has so far been commercialised, with the exceptions of a the use of aqueous biphasic systems for propene hydroformylation (Chapter 5) and the use of a phosphonium based ionic liquid for the Lewis acid catalysed isomerisation of butadiene monoxide to dihydrofuran (see Equation 9.1). This process has been operated by Eastman for the last 8 years without any loss or replenishment of ionic liquid [1], It has the advantage that the product is sufficiently volatile to be distilled from the reactor at the reaction temperature so the process can be run continuously with built in product catalyst separation. Production of lower volatility products by such a process would be more problematic. A side reaction leads to the conversion of butadiene oxide to high molecular weight oligomers. The ionic liquid has been designed to facilitate their separation from the catalyst (see Section 9.7)... 

To simulate the effects of reaction kinetics, mass transfer, and flow pattern on homogeneously catalyzed gas-liquid reactions, a bubble column model is described [29, 30], Numerical solutions for the description of mass transfer accompanied by single or parallel reversible chemical reactions are known [31]. Engineering aspects of dispersion, mass transfer, and chemical reaction in multiphase contactors [32], and detailed analyses of the reaction kinetics of some new homogeneously catalyzed reactions have been recently presented, for instance, for polybutadiene functionalization by hydroformylation in the liquid phase [33], car-bonylation of 1,4-butanediol diacetate [34] and hydrogenation of cw-1,4-polybutadiene and acrylonitrile-butadiene copolymers, respectively [10], which can be used to develop design equations for different reactors. 

The crude aldehyde is fractionally distilled into n- and isobutanal in a conventional aldehyde distillation unit. The reboiler of this n/iso column is designed as a heatabsorbing falling film evaporator incorporated in the oxo reactor, thus providing a neat, efficient method of recovering heat by transferring the heat of reaction in the reactor to cold n-butanal, which subsequently heats the n/iso column. The preferred hydroformylation temperature is 110-130 °C and is therefore used for the production of process steam. Whereas other oxo processes are steam importers, the RCH/RP process including the distillation of n-/isobutanol exports steam. No special pretreatment or even purification steps are necessary for the catalyst. This reduces the environmental burden still further. 

Researchers have worked to alleviate the problems of separation and corrosion in processes such as the oxo process by designing catalysts that are confined in a separate phase from the reactants (see Section 14.2.4). A commercially successful approach for propene hydroformylation resulted from preparation of water-soluble rhodium complex catalysts by sulfonation of the phenyl rings of the triphenyl phosphine ligands. The catalyst is used in a reactor with two liquid phases the propene is concentrated in the organic phase and the catalyst in the aqueous phase near the interface. The CO -I- H2 is bubbled into a mixed reactor, and the two-phase liquid product flows to a settler the organic product flows to downstream separation devices, and the aqueous phase with the catalyst is recycled to the reactor. 

Fig. 2 General design of the biphasic hydroformylation process focal points of R D work as indicated. (1) Reactor (2) decantation (3, 4) heat recovery and steam generation (5) Rh recycle ...

The example of the mass-transfer effect in olefin hydroformylation is interesting in still another respect. If a phosphine-substituted cobalt hydrocarbonyl is used as catalyst in combination with 1-olefin as reactant, a very strong burst of reaction ensues at the reactor inlet or at start of a batch reaction (see Example 12.1). This places extreme demands on the supply of CO from the gas phase by mass transfer and can easily lead to instability and high catalyst losses unless appropriate precautions were taken in the design. 

Biphasic hydroformylation is a typical and complicated gas-liquid-liquid reaction. Although extensive studies on catalysts, ligands, and catalytic product distributions have appeared, the reaction mechanism has not been understood sufficiently and even contradictory concepts of the site of hydroformylation reaction were developed [11, 13, 20]. Studies on the kinetics of hydroformylation of olefins are not only instructive for improvement of the catalytic complexes and ligands but also provide the basic information for design and scale-up of novel commercial reactors. The kinetics of hydroformylation of different olefins, such as ethylene, propylene, 1-hexene, 1-octene, and 1-dodecene, using homogeneous or supported catalysts has been reported in the literature. However, the results on the kinetics of hydroformylation in aqueous biphasic systems are rather limited and up to now no universally accepted intrinsic biphasic kinetic model has been derived, because of the unelucidated reaction mechanism and complicated effects of multiphase mass transfer (see also Section 2.4.1.1.2). 

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