Reactive Distillation

Reactive distillation represents the conjunction of conventional mixed-reactor technology and fractionation processes. The combination of these unit operations in a single vessel was first proposed in the 1920s (Keyes, 1932) but remained an obscurity until the 1980s. Since the 1930s, reactive distillation has been used in several specific applications but only with a homogenous catalyst. The use of heterogenous catalysts was first considered by Sennewald et al. (1971) but the potential of this development appears to have gone unrealised for another ten years.

A series of patents in the early 1980s (notably Smith, 1980a; 1980b; 1982) rejuvenated the technology by applying reactive distillation to the production of MTBE. Whereas earlier applications of reactive distillation were for specialty chemicals, MTBE was a commodity chemical and was about to enter a significant growth phase. The growth in demand for

MTBE was initiated by increasing environmental awareness and, later, boosted by legislative changes in the USA and elsewhere, but the shift from conventional technology to new reactive distillation processes was motivated by reduced capital cost and increased reactant conversion. The first reactive distillation process for MTBE production was in service by 1981 (Smith and Huddleston, 1982) and quickly gained industry acceptance.

The key developments which permitted the development of the MTBE reactive distillation process were methods for supporting small catalyst particles inside columns without creating hydraulic restrictions and for installing catalyst bales into large industrial columns. These breakthroughs were equally important for many other potential processes and initiated a significant increase in interest in reactive distillation which has continued into the 1990s.

Doherty and Buzad (1992) described three important commercial applications of reactive distillation (nylon 6,6, methyl acetate and MTBE) and referred to several other processes with the potential to benefit from reactive distillation (e.g. cumene, /er/-butyl alcohol and alkylation). In each case, the use of reactive distillation increased either the selectivity or the conversion of the limiting reactant. Design methods for equilibrium chemical reactions and kinetically controlled reactions were proposed but the modelling techniques are not rigorous and not suitable for detailed design. It is also questionable whether they would find application for preliminary design given the complexity of the interactions between reaction and separation. A useful contribution of this work was the identification of three research opportunities (areas with little or no previous work): thermodynamics; simulation; and synthesis and design.

Around the same time, DeGarmo et al. (1992) produced a similar discussion of the general merits of reactive distillation without discussing any case studies. Potential advantages of reactive distillation were described but the main contribution was the elaboration of specific hardware requirements. Unfortunately, only a superficial coverage was given to the more technical issues and important details remained buried in the patent literature. In fact, little has been published on the properties of the various catalytic packings which suggests that the companies involved in this emerging area are relying on proprietary technology to maintain a competitive advantages. Yuxiang and Xien (1992a) presented several correlations for the mass transfer coefficients in a catalytic packed bed and were able to apply those results to a reactive distillation column operating with different fluids. More recently, Subawalla et al. (1997) provided some capacity and efficiency data for a catalyst bale, indicating an increasing openness in this area.

Experimental work on reactive distillation has been limited and, so far, contributed little useful data. Flato and Hoffman (1992) and Bravo et al. (1993) are the only cases which consider etherification reactions (e.g. MTBE, ETBE, TAME, etc.). Flato and Hoffman (1992) used a bench-top column with only stripping and reactive sections to produce MTBE from separate feed streams of mixed butenes and methanol. Their column included an automatic control system but yielded results which were inconsistent with viable industrial operation. Bravo et al. (1993) conducted experiments in a taller reactive column which contained a rectifying section as well as stripping and reactive sections. They considered TAME synthesis but were unable to produce a product purity or a reactant conversion which exceeded minimum acceptable targets for commercial columns. Gonzalez et al. (1997) investigated the preparation of tert-amyl alcohol in a pilot scale reactive distillation column and produced more worthwhile results, including an assessment of the reactive packing efficiency.

The interest in reactive distillation has recently triggered a further review of the industrial uses for this technology, and there is clearly significant potential for continued development. Podrebarac et al. (1997) produced a fairly comprehensive assessment of chemical syntheses which might benefit from reactive distillation and introduced several new applications (e.g. aldol condensation, hydrolysis of epoxides and olefin oligomerization). Rock et al. (1997) also discussed some novel applications of reactive distillation (e.g. selective hydrogénation and acetylene conversion) in an overview directed at the refining sector. Walker (1997) and Nathan (1997) highlighted the impact that reactive distillation is having on multi-national companies which have traditionally been heavy users of distillation by identifying two separate research projects which are sponsored by industry conglomerates. Although the advantages of this technology have been regularly elucidated, a disregard for the potential difficulties with the operation or implementation of reactive distillation is a common deficiency in all of the articles which are cited above.

Although the reactive MTBE process has been well documented, the application of reactive distillation to ETBE synthesis has remained relatively unexplored and is the subject of considerable attention here, notably Chapters 3, 4 and 6. This example is used to develop design methods (Chapter 5) which are suitable for other reactive distillation processes as well. The difficulties associated with developing and operating reactive distillation processes due to the increased process complexity which arises from the combination of reaction and separation in a single process are also addressed (e.g. Chapters 4, 5, 7 and 8). Finally, the experimental study (Chapter 11 and 12) will provide the basis for more detailed studies of the capacity and efficiency of reactive distillation packings, thereby addressing another gap in the current literature.

2.3 ETBE Synthesis

ETBE has recently emerged as an alternative gasoline additive to MTBE (Unzelman, 19S9) ETBE has superior gasoline blending properties (Brockwell et al., 1991) and is considered partially renewable since it is produced from ethanol. The potential to utilise renewable resources increases the appeal of ETBE among environmental groups and has induced offers of substantial subsidies from some governments. It is considered that the two major obstacles which prevent the more widespread production of ETBE are: the cost of ethanol, which remains much higher than methanol in most market places; and a lack of information concerning its optimal production.

While MTBE synthesis has been researched extensively since the 1980s (e.g. Colombo et al., 1983; Rehfinger and Hoffman, 1990), the liquid-phase ETBE synthesis reaction appears not to have been studied in detail until the 1990s and data remains limited. However, several expressions for the reaction equilibrium constant are now available: Fran^oisse and Thyrion (1991); Vila et al. (1993); Cunill et al. (1993); Jensen and Datta (1995); and, Gomez et al. (1997). The agreement between the various sets of data is generally good but the thermodynamic analysis of Jensen and Datta (1995) was more rigorous than the others, and their model is preferred overall. In most cases, the UNIFAC method was used to generate the necessary estimates of thermodynamic properties. This is considered appropriate given the conformity of the predictions with the available experimental data.

Kinetic models of the ETBE reaction have been proposed by Fité et al. (1994) and Jensen and Datta (1996). These models differ significantly with respect to the methodology that was used. Jensen and Datta (1996) considered a more general case and produced a sound reaction model that is not reliant on simplifying assumptions concerning the rate limiting step (i.e. ethanol adsorption). Consequently, their reaction rate expression is valid over a wider range of compositions and temperatures and is preferred for most applications. The model is somewhat complex but simplifications can be implemented without difficulty to yield a less numerically intensive model where necessary.

The commercial production of ETBE has been discussed infrequently in the open literature. Brockwell et al. (1991) recommended a similar process to that which is used for conventional MTBE production, with the possible addition of equipment to deal with the azeotrope which forms between ethanol and water in the ethanol recovery. They did not consider the use of reactive distillation for ETBE production. Matouq et al. (1996) discussed a novel process for ETBE synthesis from low-grade alcohol using reactive distillation but the viability of their proposal appears marginal. Regular reviews of licensed processes also suggest that the process scheme which is used to produce MTBE can be applied equally to ETBE production but there is insufficient data in these reports to determine the accuracy of these claims (e.g. CDTECH, 1995; Huls AG, 1995).

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