Vincenzo Russo, Riccardo Tesser, Martino Di Serio

Department of Chemical Sciences, University of Naples “Federico II”, IT-80126 Napoli，Italy

Wies?aw Hreczuch

MEXEO, Institute of Technology, Kedzierzyn-Kozle, Poland Yongqiang Sun

China Research Institute of Daily Chemical Industry, China

Introduction

The alkoxylation reactions are generally performed in semibatch reactors,[1]also in series, in which the catalyst and the substrate (alkyl phenols, fatty alcohols or acids)are initially charged while epoxide (ethylene or propylene oxide) is added during the reaction course. This particular synthesis strategy is due to the high reactivity of alkoxides and also to the high heat involved in alkoxylation reaction. The use of semibatch reactors, however, have some drawbacks that can be summarized in the following points: (i) the reactor volume is relatively high; this aspect could represent a serious problem for safety issues due to the high quantity of alkoxide present in the reactor at a certain time; (ii) the productivity of the system is quite low for the various steps involved in a semibatch process(reactant and catalyst charge, chemical reaction, products discharge); (iii) the safety of the overall process is not optimal, fact due to possible epoxyde accumulations that could lead easily to runaways reactions.

A possible solution that could allow overcoming the mentioned drawbacks is the adoption of a continuous reactor that can be properly designed for the achievement of the desired alkoxylation degree. The shift from the traditional semibatch process to the continuous ones could really represent the start of a new era in the alkoxylation technologies. Even if a continuous process must be designed in order to operate under high pressure, this could represent a further advantage due to the absence of a vapor phase, rich in alkoxyde and more susceptible to ignition and to safety problems.In the scientific and patent literature both traditional tubular reactors and also more innovative reactor configurations, like microreactors, have been proposed for the alkoxylation reactions. These last are particularly suitable for exothermic and multiphase reactions, thanks to the high heat and mass transfer exchange.[2,3]In the present paper such proposals are examined, compared and extensively discussed, also taking into account for a possible industrial perspective in terms of productivity of different systems.

Discussion

From a literature survey of the last decades, several attempts in performing the alkoxylation process in continuous devices have been made. In Table 1, the reaction conditions adopted by different authors are summarized and compared, also in terms of system productivity expressed as the quantity of product obtained per unit of time and per unit of reactor volume.

The past research was focused on the development of a continuous alkoxylation process. The first results have been obtained by using tubular reactors, working under optimized process conditions of pressure and temperature. Therefore, even achieving high space-time yields, there are problems in the thermal control of the system. This fact has been solved by diluting the ethylene oxide concentration by splitting the feed at different points along the tubular reactor. This solution leads to drawbacks and difficulties due to the necessity of a very complex process control system, particularly in the case of high numbering-up (replication of multiple modules).

At this purpose, one of the first solutions proposed for the development of a continuous alkoxylation process is based on the use of a coiled tubular reactor suggested in the pioneering work of Umbach and Stein.[4]In theirinvestigation these authors have tested ethoxylation and propoxylation reactions in two type of tubular reactors consisting in a stainless steel tube of different diameters (6 and 9 mm) arranges as coils. With this experimental setup,the authors were able to obtain a complete conversion of alkoxydes in very short residence times, in the range 15～150 sec, which is much lower than the resident time characteristic of discontinuous processes. The system is operated under high pressure (60～100bar) in order to maintain ethylene or propylene oxide in liquid state while the temperature of the feed is quite low (60～70°C).The two reactors are designed with a very high L/D ratio(respectively 2,500 and 1,400) for a better performance in heat removal, nevertheless a rather high temperature were reached along the reactor with a characteristic profile.The achieved temperature peaks obtained with different systems tested by the authors, can be observed in Figure 1.

Table 1. Summary of the typical reaction conditions and related results for different alkoxylation processes performed in continuous seactors

As it can be seen, the maximum temperatures reached were in the range of 240～300°C. Even if only reactions with low alkoxylation degrees were tested in this investigation (2～4 moles of alkoxyde per mole of substrate), the productivities obtained resulted very interesting, giving place to a product throughput up to 100～120 kg/h that corresponds to a monthly production of 60～70 ton and with a specific productivity of 120,000 kg/(h.m3). The same concept has been developed,for sucrose based polyethers production, in the patent by Hinz et al.[5]

Figure 1. Temperature peaks for the ethoxylation of fatty alcohols

The concept of using a reactor of high L/D ratio, for maximizing the heat removal efficiency, can be further stressed by passing to microreactors and in particular microchannels reactors. At this purpose, Hubel et al.patented in 2010 the use of different microchannel devices to perform the alkoxylation of alcohols.[6]The authors stated that microdevices are characterized by a very high efficiency in both mass and heat transfer.In this way, it is possible to run the reaction in safe conditions by using microplates, whose microchannels(capillaries of a 600 μm hydraulic diameter) are coated with catalysts, where alcohol and epoxyde are mixed directly at the entrance of the plate (Figure 2). In this way, the two solutions get totally mixed and temperature peaks are avoided. The described setups allow working in different configurations, characterized by the presence of heating/cooling plates that are alternated to plates where catalyst is present. The configurations differ in how the epoxyde is fed to the reactor. In fact,it is possible to either feed the entire stream to the first plate, or feed the mentioned stream in different point of the microreactor, keeping its concentration more or less constant along the axial coordinate. The authors claim that a system like that can work in a temperature range of 50～300 °C and a pressure range of 11～800 bar, in order to keep the reaction media in liquid phase. From the different examples that the author reported, it is interesting to observe that by working with a residence time of 200 s at 190 °C and 120 bar, it is possible to achieve 99.6% conversion.

Figure 2. Microreactor designed by Hubel et al

In 2011, an interesting thesis was made by Anne-Laure Dessimoz, who tested in lab-scale microreactors for ethoxylation reactions, who performed kinetic investigations at 523 K and 50 bar,[7]claiming a productivity of 6 times higher than the conventional semi-batch reactors. The same strategy was adopted,by Rupp et al.[8,9]That studied octanol ethoxylation by using a single microchannel reactor immersed in a thermostatic bath. These authors performed an extensive experimental and modeling investigation on the possibility to continuously produce ethoxylatedoctanol,to a various degree, in a short residence time with interesting productivity results. The reactors used in this study are characterized by L/D ratio in the range 2,000～4,600 and are constituted by microchannels with diameters of respectively 250 and 876 μm. As before, the pressure was kept in the range 90～100 bar for ensure a liquid-phase reaction and the kinetic investigation of these authors covered the temperature range 130～240 °C. In these operative conditions, a residence time of 50 seconds resulted enough to reach a complete conversion of ethylene oxide and to obtain an ethoxylation degree on octanol in the range 3～9. In this particular experimental device, the maximum throughput was 0.5 cm3/min that is very low,however the productivity, defined as the amount of product obtained per hour and per m3of reactor, is on the contrary very high: 12,600 kg/(h·m3).

The problem of the low productivity of microreactors in the alkoxylation reaction has been recently faced and solved by the Microinnova Engineering GmbH, using a microstructured chemical reactor developed by the InstitutfürMikrotechnik Mainz (IMM) GmbH with innovative fabrication techniques reported in.[10]The reactor is built with the concept of modularity which allows the manufacture of different reactors according to the requirements of the process (see sketch in Figure 3). Microinnova has designed and built an alkoxylation plant with a productivity of 20 Kg/hr. This plant is now on stream and a new plant with a productivity of 200 kg/h is in assembling. The authors claim that working with their multiplates microreactor, it is possible to scale down the reaction times from 12 h to 1 min, keeping the same product characteristics, with an intensification factor of about 700.

Figure 3. Microinnovaalkoxylation plant scheme

Other kind of reactors is also present in the literature.For example, Yamada et al.[11]patented a tubular reactor constituted by different plates stirred by dynamic impellers. Both the epoxyde and initiator/catalyst mixture are fed from the bottom and the vessel is kept under pressure by an external vessel pressurized with nitrogen.The reactor has the opportunity to work also by feeding ethylene oxide in the different stirred sections. A second configuration is also shown, where the mentioned reactor is placed in a loop. The authors worked with the double metal cyanide catalysts (DMC), in particular zinc hexacyanocobaltate, patented firstly by General Tire’s in 1960s.[12]The same catalyst has been tested by other authors in loop tubular reactors,[12-19]while other kind of catalysts can be found in the literature.[20]In particular McDaniel and Reese[12-15]shown DMC catalysts lead to the production of a smaller amount of unreacted short chain alcohols, compared to the classical KOH/NaOH,when working with a series of continue stirred tank reactors (CSTR). In addition, they showed that La(PO4)catalyst lead to a narrower distribution, probably due to a different reaction mechanism than the one described by Flory, that is valid for the other catalysts. Having a narrower distribution, it is possible to produce starters,partially reacted mixtures of alcohol that contains already active catalyst. By feeding this stream to the reactor, it is possible to reduce eventual transient times necessary for the catalyst activation. Villa et al.[16-18]showed another interesting aspect of DMC catalyst that could favor their industrial application. These catalysts are thermally deactivated, fact that increases the safety of the process.By working with this catalyst, it is possible to reduce the maximum temperature that the system can reach.In Figure 3, it is possible to observe that KOH leads to a higher temperature than DMC. Moreover, starting from different initial temperatures, it is possible to reach different maximum temperatures (Figure 4). As it is evident, a slope of temperature increase is still present but very smooth. Thus, it is possible to take all the safety procedures to stop the reaction.

In 2016, Aigner et al. filed a patent claiming a new reactor technology dedicated to the continuous alkoxylation reaction of a generic substrate with active hydrogen atoms.[21]The reactor is designed as a concentric tubular reactor in which an annular space is obtained for performing the reaction continuously(see sketch in Figure 5). In this way the annular gap reaction volume (width 6.5 mm, diameter 5 inch,length 5 m) is very similar to a thin film with enhanced properties of thermal exchange and able to achieve safe operation. Such a reactor system is characterized by a L/D ratio of about 770 and is able to achieve a product throughput of 250 kg/h (7 moles of EO per mole of n-nonyl phenol) in correspondence to a residence time of roughly 160 sec and with a productivity of 22,000 kg/(h·m3). The authors claim the possibility to complete the reaction either in a separate tank, or in the same reactor by introducing a higher volume area at its bottom. In this way, products of different properties can be produced. Despite this reactor was built in a quite complex way, it open the perspective of a real industrial utilization in the field of alkoxylation technology.

Figure 4. Comparison of the maximum achievable temperature for KOH and DMC catalyst (right),DMC

Figure 5. Concentric tubular reactor in which an annular space

Conclusion

If we consider the recent findings, reviewed in this paper, a promising perspective is nowadays available for performing alkoxylation reactions in continuous modality.It has been demonstrated that specifically designed continuous reactors can furnish very good performances in terms of productivity and for ensuring safe operation in the adopted conditions. The reactors are characterized by sufficient flexibility to achieve different alkoxylation degree being, in this way, suitable for different productions. The obtained productivity is sufficiently high to guarantee,also considering reactor modularity, the possibility of useful industrial applications. Employing these emerging technologies, a new era in alkoxylation technology could start in a near future.

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